1
Results of Prior NSF Support
NSF Grant EAR-9614970, The Recent Structural Morphologic and Dynamic Evolution of the Eastern
Tibetan Plateau: A Multi-Disciplinary Study of the Process in Continental Deformation, $1,275,622 from
5/1/97 to 5/31/01, awarded to B.C. Burchfiel, L.H. Royden, K.X. Whipple and R.W. King, M.I.T. This
study has produced a new interpretation for the mode of continental deformation in the region of eastern
Tibet. We conducted geological field observations, established a GPS network of ~40 stations and a
broadband array of 25 seismometers,, identified and obtained preliminary age dates on a low-relief, preuplift surface that caps much of eastern Tibet, and developed a quantitative model for large scale
continental deformation to place our results in a geodynamic framework. Geodynamic modeling suggests
that many features of Tibet result from continental deformation where the lower crust is very weak and
undergoing regional scale flow. Our interpretation is that lower crust along the eastern plateau margin is
thickened by evacuation of lower crust from beneath the central plateau, with little shortening of the
upper crust. This is reflected in the uplift history of the eastern plateau and incision of Tibet by major
rivers. We have obtained the first ages for uplift of the eastern plateau (8-10 Ma). Many publications have
resulted from this study; a selected - to fit on one page - subset is below (* indicates MIT student author).
Northrup*, C. J., Royden, L. H., and Burchfiel, B. C., l995, Motion of the Pacific plate relative to Eurasia and its
potential relation to Cenozoic extension along the eastern margin of Eurasia: Geology, v. 23, p. 673-768.
Burchfiel, B. C., Chen., Z., Liu, Y., Royden, L. H., 1995, Tectonics of the Longmen Shan and adjacent regions:
Inter. Geol. Rev., v. 37, 661-736.
Royden, L.H., 1996, Coupling and Decoupling of Crust and Mantle in Convergent Orogens: Implications for Strain
Partitioning in the Crust, J. Geophys. Res., v. 101, 17, 679-17,705.
King, R. W., Shen*, F., Burchfiel, B. C., Chen, Z., Li, Y., Liu, Y., Royden, L. H., Wang*, E., Zhang, X., and Zhao,
J., 1997, Geodetic measurement of crustal
motion in southwest China: Geology, v. 25, 179-182.
Royden, L. H., Burchfiel, B. C., King, R. W., Chen, Z., Shen*, F., and Liu, Y., 1997, Surface deformation and lower
crustal flow in eastern Tibet: Science, v. 276, 788-790.
Wang*, E., and Burchfiel, B.C., 1997, Interpretation of Cenozoic tectonics in the right-lateral accommodation zone
between the Ailao Shan Shear zone and the Eastern Himalayan syntaxis: Inter. Geol. Rev., v. 39, 191-219.
Wang*, E., Burchfiel, B.C., Royden, L.H., Chen, L., Chen, J., and Li, W., 1998, Late Cenozoic XianshuiheXiaojiang and Red River fault systems of southwestern Sichuan and central Yunnan, China: Geol. Soc. Am.
Spec. Pap. 327, 108 pp.
Whipple, K.X., Kirby*, E., and Brocklehurst*, S. H., l999, Geomorphic limits to climate-induced increases in
topogrpahic relief: Nature, v. 410, 39-43.
Wang*, E., and Burchfiel, B.C., 2000, Late Cenozoic to Recent deformation in SW Sichuan and adjacent Yunnan,
China, and its role in formation of the SE part of the Tibetan plateau: Geol. Soc. Am. Bull.. v. 112, 413-423.
Kirby*, E., Whipple, K. X., Burchfiel, B. C., Tang, W., Berger, G., Sun, Z., and Chen, Z., 2000, Quaternary
deformation along the eastern margin of the Tibetan Plateau: Tectonic and topographic evolution of the Min
Shan, Sichuan Provice, China: Geol. Soc. Amer. Bulletin. v. 112, 375-393.
Chen, Z., Burchfiel, B. C., Liu, Y., King, R. W., Royden, L. H., Tang, W., Wang*, E.,
Zhao, J., and Zhang, X.,
2000, GPS measurements from eastern Tibet and their
implcations for India/Eurasia intracontinental
deformation: Jour. Geophys. Res., v. 105, p 16,215-16,227.
Clark*, M.K. and Royden, L.H., 2000, Topographic Ooze: Building the eastern margin of Tibet by lower crustal
flow, Geology, v. 28, 703-706.
Shen* F., Royden, L. H., and Burchfiel, B. C., 2001, Large scale crustal deformation of the Tibetan plateau: Jour.
Geophys. Res.v. 106, p. 6793-6816.
Kirby*, E., Reiners, P., Krol, M., Whipple, K.X., Farley, K., Liu, Y., Tang, W. and Chen, B., Later Cenozoic uplift
and landscape evolution along the eastern margin of the Tibetan plateau: Inferences from 40Ar/39Ar and (UTh)/He Thermochronology, Tectonics, submitted.
Burchfiel, B. C., and Wang*, E., 2002, Northwest-trending, middle Cenozoic, left-lateral fautls in southern Yunnan,
China, and their tectonic significance: Jour. Structural Geol., v. 25/5, p. 781-792
Clark*, M. K., Schoenbohm*, L. M., Royden, L. H., Whipple, K. X., Burchfiel, B. C., Zhang, X., Tang, W., Wang,
E., and Chen, L., 2002, Surface uplift, tectonics and erosion of the Eastern Tibet from large-scale drainage
patterns: Tectonics, p. 1-14.
Clark*, M., Bush, J., and Royden, L., submitted, Crustally-derived dynamic topography produced near strength
heterogeneities in the continental crust: An application of the Hele Shaw Cell for modeling weak lower crustal
flow, JGR.
Clark*, M.K., House, M.A., Royden, L.H., Burchfiel, B.C., Whipple, K.X., Zhong, X. and Tang, W., submitted,
Late Cenozoic Uplift of Southern Tibet, Science.
Schoenbaum*, L.M., Whipple, K.X., Burchfiel, B.C., and Chen, L., in press, Geomorphic constraints on surface
uplift, exhumation, and plateau growth in the Red River region, Yunnan Province, China, Tectonics.
2
Introduction
Within the major oceanic subduction systems, the interaction of dense subducting lithosphere
with the surrounding mantle controls the geometry of the descending slab, the motion of the trench with
respect to the adjacent plates, and the directions and rates of global plate motions. Because complex
feedback systems between plate motions and subduction zones exist on a global scale, such plate
boundaries must be understood by studies of mantle convection at a global (or nearly global) scale.
Within the Mediterranean region, where subduction boundaries are typically less than a thousand
kilometers in length (Fig. 1a), subduction probably exerts little to no effect on the motions of the major
tectonic plates, implying that feedback system between subduction and global plate motions is negligible.
Thus subduction processes within the Mediterranean can be studied at a regional scale.
This proposal is for study of the active Hellenic subduction system, located in the east-central
Mediterranean region and sandwiched between the slowly converging African and Eurasian plates.
Because the Hellenic subduction system exhibits both spatial and temporal variability in subduction rate,
it offers an excellent opportunity to quantify the relationships among subduction rate, trench retreat,
density of subducted lithosphere and deformation of the over-riding lithosphere. It also offers an excellent
opportunity to study the dynamic interaction between subducting lithosphere and the surrounding mantle.
Subduction Systems of the Mediterranean Region
The Hellenic system is one of several Miocene to Recent subduction systems within the
Mediterranean region that exhibit thrust faulting and subduction along arcuate belts with low topographic
relief (Figs. 1 and 2, McKenzie, 1972, 1978; Le Pichon, 1982; other examples include the Carpathian and
Apennine subduction systems; Royden 1983; Lonergan and White, 1977; Malinverno and Ryan, 1986,
Jolivet and Faccenna, 2000). These subduction systems can be distinguished from thrust belts like the
European Alps by the presence of widespread extension within the upper plate lithosphere and by the
migration, or “retreat”, of the trench away from the rigid part of the over-riding plate (Fig. 3). They also
share a number of distinguishing geological features, such as little involvement of crystalline basement in
the thrust complex, low topographic relief and anomalously deep foreland basins.
Subduction along these young retreating boundaries has been generally oblique to the direction of
Europe-Africa convergence, so that, unlike the European Alps, the relationship of these subduction
systems to the overall convergence between Africa and Eurasia is not immediately obvious. Royden
(1993ab) has suggested an evolutionary sequence whereby these subduction systems evolve naturally
from continental collision along irregularly shaped continental boundaries. She concluded that the
primary driving mechanism for retreating subduction is gravity acting on negatively buoyant subducted
lithosphere. Similar conclusions have been reached by other authors (e.g. Le Pichon, 1982, Malinverno
and Ryan, 1986, Lonergan and White, 1977; Faccenna et al., 2001), although the importance of slab pull
in driving deformation is controversial (e.g. Dewey and Sengor, 1979; Le Pichon and Angelier, 1979;
Mercier et al., 1979, Platt and Vissers, 1989; Jackson, 1994; Meijer and Wortel, 1997; Hatzfeld et al.,
1997).
A direct consequence of this hypothesis is that subduction at retreating subduction boundaries
will occur only when the subducting plate has a sufficiently high density to drive the subduction (e.g.
Faccenna et al, 2001). Subduction ends when large volumes of buoyant material enter the subduction
system. Within this conceptual framework, differences in the evolution and duration of retreating
subduction systems are largely attributed to the size and configuration of the deep water regions,
underlain by dense lithosphere, that are available for subduction.
Within the Mediterranean region there is a close correspondence between the density (water
depth) of lithosphere that enters these subduction boundaries and the occurrence of active subduction
(Fig. 1; Royden, 1993ab). Nevertheless, the length and time scales over which subduction rate, geometry
and slab density are related and the details of slab-mantle interactions in these systems remain poorly
understood. This proposal is aimed at addressing these general issues through a detailed analysis of the
young Hellenic subduction system.
3
Active Tectonics of the Hellenic System
Over the past decades Greece has been the focus of intense study by the earth science community.
The classic papers of McKenzie (1972, 1978), Dewey and Sengor (1979), Angelier et al. (1982) and Le
Pichon (1982) were published more than two decades ago. Since that time much new data has come to
light but our understanding of the regional tectonics and driving forces for deformation has changed little.
Although much excellent work has been done in recent years, most of the observational studies have been
undertaken piecemeal or are aimed at local problems and the results are dispersed among a plethora of
publications.
Arguably, the most significant advance in Hellenic geology has come in the refinement of the
rates of active deformation as determined by GPS measurements throughout Greece and adjacent regions
(e.g. Clarke et al., 1998; McClusky et al., 2000). Precise dating and quantitative determinations of
paleotemperatures and pressures associated with metamorphic events has led to a more precise
description of metamorphic and extensional events and their spatial and temporal distribution. Indeed, the
large volume of relatively new data that is available from Greece now makes it possible to formulate
highly specific hypotheses about the dynamics of the system, and to test these hypotheses quantitatively
through a targeted and tightly coordinated plan of research. This would not be possible in an area where
the geology is not well known.
Along these lines, we have drawn mainly on published data in constructing the following
description of the tectonic setting and the development of the Hellenic system. However, many of the
correlations that we highlight in this discussion are our own and come about only through the broad
perspective of dynamic and kinematic processes that involve the entire Hellenic system. Many of these
correlations have not, to our knowledge, been discussed elsewhere in the literature. This is particularly the
case for the correlations between subduction rate, subduction zone geometry and the character of
subducted lithosphere, in space and in time, which form the underpinnings for our proposed study.
Active Tectonics of the Subduction System
The Hellenic thrust belt involves faulting and folding of sedimentary rocks along a length of
approximately 1000 km, from the southern Mediterranean northward through the Ionian Sea to the
southeastern Adriatic Sea (Figs. 1 and 4). (We confine most of the discussion of the Hellenic Arc system
to Greece because data from Albania are difficult to obtain and not required for this study.) We
distinguish northern and southern segments of the Hellenic thrust belt that are dextrally discontinuous
along the west-southwest trending Kephalonia Transform (e.g. Dewey and Sengor, 1979; Finetti, 1982;
Kahle and Muller, 1993; Kahle et al., 1995). GPS data show that the rate of convergence across the
southern segment of the belt is ~40 mm/yr (Fig. 5, Clarke et al., 1998; McClusky et al., 2000).
Convergence across the northern segment of the Hellenides is ~5 mm/yr relative to Africa and ~10 mm/yr
relative to southern peninsular Italy. Onland, the transition from fast to slow subduction rates occurs
across a broad zone that is approximately continuous with the Kephalonia Transform (e.g. Goldsworthy et
al., 2003; Armijo et al., 1996; Roberts and Jackson, 1991).
The Kephalonia Transform coincides with a dramatic change in the lithosphere entering the
Hellenic subduction system (Fig. 1b). Lithosphere north of the Kephalonia Transform is continental with
crust ~25 km thick. This crust is overlain by sediments deposited in shallow water since Triassic time
(e.g. Jacobshagen et al., 1978). The modern water depths are between 0 and 1 km. This lithosphere has a
lower average density than the underlying mantle because its water depth is less than the ~2.5 km depth
of typical zero-age oceanic lithosphere. Lithosphere south of the Kephalonia Transform underlies the
deep water Ionian Sea and is thought to be oceanic in character [Makris, 1985; de Voogd et al., 1992]. Its
age is unknown but is probably early Mesozoic and ~8 km of sediments overlie the basement. Water
depths in the Ionian Sea are between 3 and 4 km, with maximum depths of ~5 km along parts of the
Hellenic trench. This lithosphere has a higher average density than the underlying mantle because its
water depth is greater than the ~2.5 km depth of typical zero-age oceanic lithosphere.
4
In the northern Hellenides convergence is confined to a narrow zone located west of the coast of
mainland Greece; a bathymetric trench is not present here (Finetti and Morelli, 1972, 1973; Baker et al,
1997). Convergence along the southern Hellenides involves shortening of sedimentary rocks across a
wide region of the Mediterranean sea floor, reaching from the Hellenic trench to the east side of the
Mediterranean ridge (e.g. Kopf et al., 2003, Fig. 4) These folded and thrust-faulted sedimentary rocks
appear to be detached from the underlying oceanic basement. Seismic and gravity data indicate that the
basal detachment is ~12-15 km deep at the trench (Hirn et al, 1997; Clément et al., 2000). Deformation
outboard of the trench is thin-skinned, involving only a veneer of sedimentary rocks and subduction to
depth of the oceanic basement occurs only beneath the Hellenic trench.
If we consider the shallow expression of the Hellenic subduction boundary to coincide with the
zone where basement rocks are subducted to depth, then the Kephalonia Transform forms a pronounced
discontinuity in the Hellenic subduction boundary (e.g. Dewey and Sengor, 1979). In the northern
Hellenides the subduction boundary is a narrow northwest-striking zone of convergence located west of
the Greek coast. In the southern Hellenides, the subduction boundary coincides with the northwesttrending Hellenic trench for hundreds of kilometers until the subduction boundary bends eastward
towards Crete. Between the northern and southern Hellenides, the subduction boundary, or at least its
shallow expression, is discontinuous by ~100 km along a right-stepping discontinuity that currently
displays right-slip displacements (Sachpazi et al., 2000; Kahle et al., 1995). This discontinuity
corresponds closely to the transition from shallow to deep water lithosphere and to the change in
convergence rate from ~10 to ~40 mm/yr (Fig. 1b).
The Hellenic slab exhibits a Bennioff zone to ~150 km depth (Papazachos et al., 2000 and
references contained therein). Seismic tomography reveals a high-velocity slab-like structure dipping 45°
to the northeast and north, extending to at least 600 km depth and perhaps into the lower mantle (Fig. 6,
Spakman et al., 1993; van der Hilst et al., 1997; Karason, 2000). Gravity data indicate that the slab has an
anomalously high density relative to the surrounding mantle. Gravity anomalies over the Aegean Sea are
50-100 mg more positive than expected from point-wise compensation of the Aegean and consistent with
a dense slab dipping 45° northeast up to depths of ~250 km (Royden, 1993a; Tsokas and Hansen, 1997).
Large, regional-scale isostatic gravity anomalies are not present along the northern portion of the
Hellenides (Moretti and Royden, 1988). These observations suggest that the anomalously dense mass
associated with the southern portion of the subducted lithosphere is not present, or is greatly reduced,
along the northern portion of the subduction boundary. This is consistent with low-density lithosphere
currently entering the subduction boundary in the northern Hellenides and high-density lithosphere
entering the subduction boundary in the southern Hellenides.
Active Tectonics of the Upper Plate: Central Hellenic Shear Zone and Aegean Domain
The continental lithosphere above the Hellenic subduction system includes the emergent regions
of mainland Greece, the Peloponnesus, Crete, westernmost Turkey and the submerged region of the
Aegean Sea. GPS data show that the central and southern Aegean domains, Crete, and some of the
Peloponnesus, are currently behaving as an almost rigid block with little internal deformation (Fig. 7,
McClusky et al., 2000). Earthquake data show little seismic deformation of this region, with intense
seismicity along its boundaries (Fig. 6).
GPS velocities show that the northern boundary of the Aegean block is a broad east-northeasttrending zone of shear which we shall refer to as the Central Hellenic Shear Zone. It is approximately
continuous with the Kephalonia Transform to the west and with the North Aegean Trough and North
Anatolian Fault to the east. The displacement rate across the Central Hellenic Shear Zone is ~24 mm/yr in
a N45°E direction, giving oblique right slip across the zone as a whole (Figs. 5 and 7). Estimating the
trend of the Central Hellenic Shear Zone at ~N60°E, this resolves into ~22 mm/yr of right-slip parallel to
the zone and ~9 mm/yr of extension perpendicular to it.
The distribution of active deformation through the Central Hellenic Shear Zone is reasonably well
known from GPS, earthquake locations and mapping of active fault breaks (e.g. Goldsworthy et al., 2003
5
and papers cited therein.) Within and adjacent to the Gulf of Corinth, active tectonic structures have been
studied extensively but other areas are more poorly known (Freyberg, 1973; Jackson and McKenzie,
1983; Armijo et al., 1996). Earthquake focal mechanisms indicate mainly normal faulting on east-west
trending faults with a few events showing right slip on northeast-trending faults or mixtures of the two
modes (Fig. 6). Surface ruptures along active faults indicate that extension is largely accommodated on
east-west to northwest-trending faults, resulting in water depths of ~900 m in the Central Gulf of Corinth.
Extension has been accompanied by rapid uplift on the south side of the gulf where Pliocene-Quaternary
sedimentary rocks are present at elevations up to 1200-1600 m (Philipson, 1893, Keraudren and Sorel,
1987).
GPS data within and adjacent to the Central Hellenic Shear Zone show two primary zones of
deformation, the Gulf of Corinth and the Gulf of Evoikos (Clarke et al., 1997; Goldsworthy et al., 2003).
These data indicate little active rotation between the Peloponnesus and mainland Greece, but ~7°/m.y.
clockwise rotation of Boetia (the region between the Gulf of Corinth and the Gulf of Evia, Fig. 4). These
data, which are locally accurate to ±3 mm/yr, are not sufficient to discriminate finer details within the
Central Hellenic Shear Zone. Small rates of displacement (and earthquakes) also occur on active faults
north of the Central Hellenic Shear Zone, but their total contribution to Aegean-Eurasia velocities is small
(Figs. 5 and 6).
Within the Aegean Sea, the northern boundary of the Aegean block is more narrowly defined,
corresponding a region of right-slip and extension in the North Aegean Trough (e.g. Taymaz et al., 1991;
Roussos and Lyssimachou, 1991). Here Pliocene-Quaternary subsidence has resulted in a deep water
basin with water depths of ~1 km as compared to the shallow water in surrounding parts of the northern
Aegean. Earthquakes with right-slip focal mechanisms also occur in the north Aegean south of the North
Aegean Trough, but GPS displacement rates across this zone are small.
The GPS velocity field shows that the Aegean domain is bordered to the east by a broad
northwest-trending zone of extension and left-slip in western Turkey, which we refer to as the West
Anatolian Shear Zone. The displacement rate across this zone is ~18 mm/yr in a direction N15°E (Figs. 5
and 7). Estimating the trend of this broad deformation zone at ~N50°W, this resolves into ~13 mm/yr of
left-slip parallel to the zone and ~11 mm/yr of extension perpendicular the zone. This zone is poorly
studied. Little is known in detail about the geologic structures that accommodate active displacement
although Quaternary extensional structures have been identified.
East of the West Anatolian Shear Zone, much of Anatolia behaves as a rigid block moving
eastward at ~20-25 mm/yr relative to Eurasia (Fig. 5). This lithospheric block is bounded to the north by
the dextral North Anatolian Fault, a well-defined structure that accommodates almost all of the motion of
Anatolia relative to Eurasia. It ends to the east in the collision zone between Arabia and Eurasia and
extends westward into the northern Aegean Sea, where it merges with the northern boundary of the
Aegean block. Total right-slip displacement on the North Anatolian Fault is estimated at ~85 km (Sengor,
1979).
The Young Volcanic Arc
Pliocene-Quaternary arc volcanoes form an arcuate chain located approximately 200 km north
and east of the Hellenic trench (Fytikas et al., 1984). These volcanic centers lie above lithosphere
subducted along the southern segment of the Hellenides (Figs. 4 and 8). No Pliocene-Quaternary arc
volcanoes are present above lithosphere subducted along the northern segment of the Hellenic trench,
with the possible exception of the northernmost volcano in the chain, which lies within the Central
Hellenic Shear Zone.
Cenozoic (Mainly Late) Evolution of the Hellenic Arc System
Data that constrain the active deformation of a tectonic system give only a snapshot of the
system’s current state. Because dynamic processes operate in space and in time, information about how a
tectonic system has evolved is needed in the search for governing processes. Here we summarize
geological data that constrain when and how the southern Hellenic subduction system became
6
distinguished from the northern Hellenic system and how these changes are related to the character of the
subducted lithosphere and the rate of subduction (We summarize the most important relationships and
chronology at the end of this section so that the bored reader can skip all but the summary section.)
The Hellenic Thrust Belt and Subduction System
North- to northeast subduction along the Hellenic belt has been continuous since Jurassic time
(e.g. Mercier et al., 1975, Jacobshagen, 1978; Papanikolaou, 1993). From early Tertiary to late Miocene
time, subduction resulted in stacking of the external Hellenic thrust sheets by west and south-vergent
thrust faulting. Most of these thrust sheets consist of sedimentary cover stripped from its basement while
the underlying crust and lithosphere were subducted beneath the Hellenides (Burchfiel, 1980). Thrusting
progressed from internal to external zones, presumably paralleling the southward advance of the volcanic
arc and subduction boundary (Fig. 8).
A roughly delineated boundary separates the southern segment of the external Hellenides,
exposed on the Peloponnesus, from the northern segment of the external Hellenides, exposed on mainland
Greece. This zone coincides approximately with the Central Hellenic Shear Zone but is more narrowly
defined. Some tectonic units, such as the Pindos nappe, are present north and south of this zone and
continue across it. Other units of the southern Hellenides can be correlated with similar units in the
northern Hellenides, but display differences in paleogeographic, sedimentary and/or metamorphic facies
(for example, the Olympus and Mani marbles). Still other units, like the Parnassos unit, exist only on one
side of this boundary zone. Hence this geologic boundary has been a persistent feature during the
evolution of the Hellenic system. Because the Central Hellenic Shear Zone is approximately coincident
with this older transverse zone of displacement, the task of identifying young structures that
accommodate recent offset is complicated by older deformations that have occurred along approximately
the same zone.
Subducted Lithosphere: Oceanic or Continental?
Beginning in late Eocene time, northeast-dipping subduction beneath the external Hellenides
involved subduction of oceanic lithosphere (the Pindos zone, subducted until late Eocene time);
subduction of continental lithosphere overlain by shallow-water carbonates (the Tripolis-GavrovoOlympus zones, subducted in late Eocene to Oligocene time); subduction of continental lithosphere
overlain by deep-water carbonates (the Ionian-Mani zones, subducted in Oligocene to earliest Miocene
time; the Ionian zone is not to be confused with the modern Ionian Sea); and subduction of continental
lithosphere overlain by shallow-water carbonates (the Paxos zone, sometimes confusingly called the preApulian zone, subducted beginning in early Miocene time). (For a review see Jacobshagen et al., 1978.)
Thus subduction of continental crust occurred throughout the Hellenides from Oligocene to Late Miocene
time. In the northern Hellenides, subduction of Paxos zone continental lithosphere continues today.
A widespread blueschist event in occurred Oligocene time, when rocks of the Ambelakia and
Arna units were metamorphosed and thrust westward over the continental crust of the Olympus/Mani
zone. These high-pressure metamorphic rocks are present along almost the entire portion of the external
Hellenides. They were emplaced in late Oligocene to earliest Miocene time and occur above the
Olympus/Mani units and below the Tripolis and Pindos units (e.g. Schermer, 1990; Bassias and Triboulet,
1994).
Within the southern Hellenides, the Paxos zone continental lithosphere has been completely
subducted and dense oceanic lithosphere of the modern Ionian Sea is currently entering the trench. The
time at which Ionian lithosphere entered the southern Hellenic subduction zone is not well constrained by
geological data, but is late Miocene or younger based on the youngest exposed thrust sheets of the
external Hellenides. In the northwestern Peloponnesus, along the west coast of mainland Greece and in
the Ionian islands to the west of mainland Greece, upper Miocene (Messinian) sedimentary rocks are
overthrust by the outer thrust sheets of the Hellenides. These upper Miocene rocks comprise the
uppermost part of the thick sedimentary sequence of the Paxos zone, reflecting deposition in shallow
water conditions beginning with platform carbonate deposition in Triassic time. This implies that shallow
7
water continental lithosphere was subducted beneath the southern Hellenides as late as 6-8 Ma,
constraining the oceanic lithosphere of the Ionian Sea to have entered the southern trench no earlier than
~8 Ma (or perhaps slightly earlier if overthrusting of these occurred within the thrust belt after they were
stripped from their continental basement.)
Continuity of Thrust Belt
The Hellenic thrust belt was continuous from northern to southern Hellenides from Eocene
through Late Miocene time as indicated by the largely continuous frontal thrusts of the Pindos zone (late
Eocene) and thrust faults of late Miocene (Messinian) age (Fig. 4). The time at which the modern
discontinuity developed in the Hellenic subduction boundary is not well dated but sediments deposited in
east-west trending depocenters unconformably overlie the late Miocene thrust faults on islands within the
Kephalonia Transform zone (e.g. Zakinthos). Marine seismic reflection data reveal areas of Pliocene
normal faulting in the western part of the Kephalonia Transform region, with rapid subsidence from
shallow water to more than 1 km water depth (Monopolis and Bruneton, 1982; Kamberis et al., 1996).
Thus structures that disrupt the late Miocene thrust front were active by Pliocene time.
Paleosubduction: Rates, Magnitude and Timing
Throughout Cenozoic time, the down-going lithosphere beneath the Hellenides has been part of
the African plate. Thus the rate of subduction beneath the Hellenides must be equal to the rate of trench
migration toward Africa and we can equate subduction rate with the rate of trench migration and/or the
rate of slab roll-back. Paleosubduction rates and magnitudes are difficult to determine precisely. Faccenna
et al. (2003) estimate a subduction rate slightly in excess of 10 mm/yr for Oligocene to Recent. Several
other independent estimates of subduction rate can also be made:
(1) A series of volcanic arcs exist within the Aegean domain (e.g. Fytikas et al., 1984; Papanikalaou,
1993; and references contained therein). These arc progress from north to south, advancing ~300 km from
Oligocene to recent time (Fig. 8), and until Late Miocene time were erupted above both northern and
southern segments of the Hellenic subduction system. The rate of advance of the arc gives an estimate of
the rate of subduction, although this is a lower bound because previously erupted arcs may have been
moved southward due to later extension within the Aegean domain.
The late Miocene volcanic arc is ~150 km north and east of the modern volcanic arc and ~150 km
south of the Oligocene arc. Comparing the position of the Oligocene volcanic arc to that of the Late
Miocene arc, yields ~150 km of subduction from Oligocene to Late Miocene time. Dividing by a time
interval of ~20 m.y., yields an average subduction rate of ~8 mm/yr from 25-6 Ma. This applies to the
northern and southern Hellenides and is similar to the modern convergence rate across the northern
Hellenides.
Taking the difference in position between the late Miocene and the modern volcanic arcs (~150
km) and dividing by 6 m.y. we obtain an average subduction rate of 25 mm/yr for the southern Hellenides
in Pliocene-Quaternary time. This is slower than the modern convergence rate of 40 mm/yr across the
southern Hellenides, but shows that subduction rates in the southern Hellenides increased markedly in late
Miocene to Pliocene time. Alternatively, dividing the 150 km distance between the late Miocene and
modern arcs by the modern subduction rate of 40 mm/yr yields ~4 m.y, indicating that the subduction rate
increased beneath the southern Hellenides at or before 4 Ma.
(2) The magnitudes and rates of subduction along the Hellenic trench can also be estimated from
overthrusting of the relatively autochthonous basement beneath the Hellenides. In the eastern thrust belt,
windows of the Olympus and Mani marbles give and the age of emplacement of the overlying thrust
sheets of ~25-30 Ma. Rocks in the westernmost part of the Hellenides that correlate with the Olympus and
Mani marbles (in the Gavrovo and Ionian zones) were overthrust at ~6-8 Ma. The distance between these
internal basement windows and the external thrust faults, measured perpendicular to the trend of the
Hellenides, is ~150-200 km. Dividing by the intervening time of 17-25 m.y. yields an average shortening
rate of 6-12 mm/yr. This is a minimum rate for subduction because the relatively autochthonous basement
exposed in the eastern Peloponnesus and at Olympus has probably been detached and transported toward
8
the foreland, but these results are at least consistent with the estimates of subduction rate determined from
the same period.
(3) If the modern discontinuity in the Hellenic subduction boundary across the Kephalonia Transform
corresponds to a difference in subduction between the northern and southern Hellenides, then the
subduction zone in the southern Hellenides has advanced 100 km further towards the foreland than the
subduction zone in the northern Hellenides. Because thrust sheets emplaced in the external Hellenides
from late Eocene to late Miocene time are continuous from the northern to southern Hellenides, the
additional subduction beneath the Southern Hellenides was probably post late Miocene. Dividing 100 km
by 6-8 m.y., yields a difference in average subduction rate between the northern and southern Hellenides
of 12-17 mm/yr. Adding to this current rate of 5-10 mm/yr in the northern Hellenides, we arrive at an
average subduction rate for the southern Hellenides of 17-27 mm/yr for Pliocene-Quaternary time. This is
similar to the rate of subduction estimated for the southern Hellenides from advance of the volcanic arc.
Late Cenozoic Evolution of the Hellenic Upper Plate
Constraints on the relative rates of displacement beneath the northern and southern Hellenides
can be obtained by examining the deformation of upper plate lithosphere along the Central Hellenic Shear
Zone (Fig. 4). Because this zone separates the northern Hellenides from the southern Hellenides, the
timing and magnitude of shear along this boundary should correlate with the relative rates of trench
retreat. We correlate the initiation of this shear zone with the incipient disruption of the Hellenic
subduction boundary into northern and southern segments. In this section we summarize briefly what is
currently known about displacement within this broad shear zone, the timing of initiation, and the tectonic
features that predate the formation of the Central Hellenic Shear Zone.
Since at least Eocene time, extension within the upper plate of the Hellenic subduction system has
occurred along dominantly arc-parallel structures, producing a series of back-arc basins with extension
generally progressing from more internal to more external zones (Fig. 4, Mercier et al., 1989;
Papanikolaou, 1993). (We use the term “back-arc extension” loosely, to indicate regional-scale extension
of upper plate lithosphere. Extension may occur within and in front of the volcanic arc, but occurs largely
behind the arc of the thrust belt and/or accretionary prism.) Extensional structures are preserved as
regional-scale low-angle detachment faults that separate high-grade rocks in their footwalls from
unmetamorphosed and highly extended hanging wall rocks. Such structures include the Early Miocene
Strymon detachment system in northern Greece, the Middle Miocene detachment fault at Mt. Olympus,
and the Middle to Late Miocene shear zones of the Central Aegean (Lister et al., 1984; Schermer, 1993;
Dinter and Royden, 1993; Forster and Lister, 1999).
In Late Miocene (Tortonian) time, extension resulted in normal faulting and subsidence of the
Cretan Basin, forming an arc-parallel zone of subsidence north of the island of Crete (e.g. Jongsma et al.,
1977). Subsidence of the Cretan Basin occurred in the hanging wall(s) of gently north to northeast
dipping detachment faults exposed on the islands of Kythera (Papanikolaou and Danamos, 1991) and
Crete (Fassoulas et al., 1994; Jolivet et al., 1996; Ring et al., 2001). Our recent mapping reveals that this
zone of extensional faulting also extends northwestwards along the eastern Peloponnesus to the Gulf of
Corinth (Figs. 4 and 9). The fault system is a low-angle extensional feature dipping 15° to 35° northeast
and is similar to many of the extensional detachment systems mapped in the Basin and Range Province
(Fig. 10, Skourtsos, 2002).
This Late Miocene extensional complex contains the youngest arc-parallel extensional structures
of regional extent within the Hellenic system. It is approximately coeval with the last thrusting and
subduction events prior to disruption of the Hellenic subduction system by the Kephalonia Transform and
Central Hellenic Shear Zone. No younger regional-scale arc-parallel extensional structures exist in the
Aegean domain. Thus a fundamental change in the tectonics of the Aegean region occurred in late
Miocene or early Pliocene time. This is reflected by termination of arc-parallel extension on the Parnon
detachment system and initiation of arc-disruption by faulting along the Central Hellenic Shear Zone.
9
In Late Miocene or Pliocene time, younger east-west trending faults formed within the Central
Hellenic Shear Zone and its offshore continuations to the east and west. Many of these cross-cut and
disrupt the Hellenic thrust system (Figs. 4, 9 and 11). Within the Central Hellenic Shear Zone, the Gulf of
Corinth has been extensively studied (e.g. Freyberg, 1973; Collier and Dart, 1991; Briole et al., 2000; Ori,
1989; Poulimenos, 1993; Leeder et al., 2002), as has the late Miocene Megara basin exposed near Corinth
(e.g. Bentham et al., 1991). Other cross-cutting structures, dominantly extensional and with large
displacement are fairly poorly known. These include the northwest-trending Beotikos-Kifissos
detachment fault, which is no longer active but has a hanging wall basin containing probable PlioceneQuaternary, possibly latest Miocene, sediments. However, the initiation age, cessation age and magnitude
of extension across this detachment are not well known. To the northeast faults bounding the Gulf of Evia
and the northeastern (Aegean) margin of Evia are active (Roberts and Jackson, 1991). The age of
initiation of extension in the Gulf of Evia is not known but seismic data indicate that the crust has been
thinned to ~18 km beneath the gulf (Makris, 1977), suggesting significant extension over a period of
several million years. East of the Central Hellenic Shear Zone, the dominantly dextral North Aegean
Trough and the North Anatolian Fault also have a predominantly Pliocene age of initiation (Armijo, 1999,
2000; Taymaz et al., 1998; Barka and Kadinsky-Cade, 1988; Westaway, 1994) .
GPS data show that the modern deformation zone along the North Anatolian Fault continues
westward through the Central Hellenic Shear Zone and into the Kephalonia Transform (Kahle et al., 1995,
2000; McClusky et al., 2000). However, the nearly continuous exposure of internal Hellenic thrust sheets
(especially the front of the Pindos nappe) indicate that a hypothesized 100 km of dextral displacement
cannot be accommodated by a localized right slip within the Central Hellenic Shear Zone. Instead,
structures that are present within the Central Hellenic Shear Zone are dominantly extensional and eastwest to west-northwest-trending. Reconciliation of the continuous nature of older thrust structures within
the Central Hellenic Shear Zone with significant amounts of dextral shear across the zone remains an
important problem.
Collisional Events in Eastern Turkey
In eastern Turkey the continental lithospheres of Arabia and Eurasia collided at ~12 Ma (e.g.
Sengor et al., 1985). Here collisional and post-collisional convergence have resulted in significant crustal
thickening and the development of high topography throughout eastern Turkey (Fig. 1a). These events
probably contribute to the driving forces for the westward motion of Anatolia and perhaps for
southwestward motion of the Aegean region (for example, McKenzie, 1972). In proposing to study the
relationship of Hellenic subduction to deformation and motion of the greater Aegean region, we do not
assume that the collisional tectonics of eastern Turkey have no effect on motion or deformation in the
Aegean region. Indeed, it is likely that the events in eastern Turkey have some influence on the current
motion of the Aegean block. However, GPS data show that the Aegean domain is moving nearly as fast
relative to Anatolia (~18 mm/yr) as Anatolia is moving relative to Eurasia (~22 mm/yr). The Aegean
domain also moves faster relative to Eurasia (~30 mm/yr) and in a different direction than Anatolia. This
indicates that there are other important driving forces for the modern motion of the Aegean domain.
This proposal is aimed at understanding the extent to which the evolution and modern setting of
the Aegean domain, particularly along its western margin, can be understood in light of coeval events
along the Hellenic trench and in the underlying subduction zone. The possible role of continental collision
in eastern Turkey, and the westward extent of its influence, is something that we will continue to reassess
throughout the proposed study.
Summary of Key Points
Key points, on which we base our current interpretation and working hypothesis for the Hellenic
subduction system, can be abstracted from the previous sections and include:
The modern subduction rates and the density of the lithosphere currently entering the Hellenic
subduction zone change abruptly across the Kephalonia Transform, a dextral discontinuity in the
subduction boundary with an apparent displacement of ~100 km. North of the Kephalonia Transform
10
lithosphere entering the subduction boundary is continental and subduction rates are 5-10 mm/yr. South
of the Kephalonia Transform lithosphere entering the trench is oceanic and subduction rates are 40
mm/yr. The Kephalonia Transform also correlates with the apparent density of subducted lithosphere to
several hundred kilometers depth, with a denser slab occurring beneath the southern Hellenides.
The Kephalonia Transform continues eastward to a broad zone of dextral shear in central Greece,
the Central Hellenic Shear Zone. This shear zone currently exhibits extensional (at ~11 mm/yr) and right
slip (at ~22 mm/yr) components of displacement. It continues eastward to the North Anatolian Fault,
which has approximately 100 km of total right slip at a modern rate of ~25 mm/yr. The modern volcanic
arc exists only above lithosphere subducted beneath the southern Hellenides.
Late Cenozoic Development of the Hellenic System: The average rate of subduction beneath the
Hellenides appears to have been more-or-less constant from 25-8 Ma, at approximately 5-10 mm/yr.
Lithosphere entering the subduction zone over this time period was continental, with shallow water sea
floor entering the subduction boundary since 15-20 Ma. Over this time period the Hellenic thrust front
appears to have been a continuous from northern to southern Hellenides and the Kephalonia Transform
and the Central Hellenic Shear Zone did not yet exist.
Subduction of continental lithosphere at 5-10 mm/yr continues unchanged to present time. But,
beginning sometime during the interval 4-8 Ma, the subduction rate in the southern Hellenides increased
rapidly to its current rate of 40 mm/yr. This is approximately the time of entry of oceanic lithosphere into
the Hellenic trench, although this can only be constrained to be post 6-8 Ma. East-west trending faults
within the Kephalonia Transform were active by Pliocene time while east-west trending faults of the
Central Hellenic Shear Zone first became active in latest Miocene to Pliocene time. This is the same time
as initiation of subsidence of the North Aegean Basin and right-slip along the North Anatolian Fault.
Together, the Kephalonia Transform, the Central Hellenic Shear Zone and the North Aegean Basin
constitute a system of Pliocene-Quaternary faults that disrupt arc-parallel thrust and extensional features
of the older Hellenic system. Their initiation was coeval with the cessation of arc-parallel extension in the
south-central Aegean domain.
Interpretation and Working Hypothesis
To our knowledge, the correlations between events that we have outlined in this proposal are
largely new and not discussed in the literature. They indicate a fundamental reorganization of the Hellenic
subduction system between latest Miocene and Pliocene time. Reorganization involved the subduction
boundary, the subducted slab and the lithosphere of the over-riding plate. It was approximately
coincident, in space and time, with entry of Ionian oceanic lithosphere into the Hellenic trench.
Our current interpretation of these data is that the entry of dense oceanic lithosphere into the
southern segment of the Hellenic subduction boundary is largely responsible for the post-Miocene
evolution of the Hellenic system. Our working hypothesis is that the negative buoyancy of this dense
oceanic slab has brought about an increase in the rate of subduction and trench migration in the southern
Hellenides beginning in latest Miocene or early Pliocene time. This has caused an additional 100 km of
subduction and trench roll back in the southern Hellenides, as compared to the northern Hellenides,
producing ~100 km of dextral offset along the Kephalonia Transform, the Central Hellenic Shear Zone,
and the North Aegean Trough. If this interpretation is correct, then the Hellenic system provides us with
an unparalleled opportunity to study the dynamic interactions that occur within subduction systems.
We propose to test this interpretation, and its dynamic implications, through a focused set of
interdisciplinary studies. Before proceeding to our proposed research plan, we present two preliminary
examples how one might connect kinematics and dynamics within the Hellenic system. Both examples
are highly speculative and have not been tested or refined. Nevertheless, they are useful preliminaries to a
discussion of the proposed project and demonstrate how various components of our multi-component
research plan will be needed to understand the evolution and physical processes operating within this
dynamic system.
Preliminary Kinematic Model for the Central Hellenic Shear Zone
11
One component of our working hypothesis is that right shear across the Central Hellenic shear
zone accommodates differential subduction rates between the northern and southern Hellenic subduction
zones. Structures that might accommodate right-shear across the Central Hellenic shear zone are not
obvious, a feature that has long prevented tectonicists from extending the North Anatolian Fault Zone as a
narrow feature through mainland Greece. Since the advent of GPS measurements, it is clear that active
displacement of ~25 mm/yr does traverse the Central Hellenic Shear Zone. Goldsworthy et al. (2003)
have suggested that this occurs through a combination of extensional faulting and block-rotation within
the shear zone. Similarly, if ~100 km of Pliocene-Quaternary displacement is to be accommodated on the
dominantly extensional faults of the Central Hellenic Shear Zone, then rotational features are probably of
great importance (e.g. McKenzie and Jackson, 1983) However, it is also clear that many currently active
faults were not active in Pliocene time, and that there are large Pliocene extensional structures that are not
active. Hence the location of faulting has changed with time. An important part of our proposed study will
be to document how faulting and rotation of crustal fragments in the Central Hellenic Shear Zone have
evolved through time.
Fig. 12 contains a highly speculative reconstruction showing how 100 km of overall right-shear
might have been accommodated by known extensional structures within the Central Hellenic Shear Zone.
We begin with the present configuration of the Hellenides, including the modern subduction boundaries,
the Pindos (late Eocene) thrust front, and two zones of late Miocene thrusting. We subdivide the Central
Hellenic shear zone into four major blocks separated from one another by known zones of PlioceneQuaternary extension, including the well studied extensional system in the Gulf of Corinth and the poorly
known extensional structures of the Beotikos-Kifissos normal fault system and normal faults in the Gulf
of Evia.
Based on our hypothesis that the 100 km dextral discontinuity in the subduction boundary is due
to 100 km of additional trench roll-back of the southern Hellenic trench, we restore the southern trench
segment, relative to the northern trench segment, by moving it 100 km northeast (without rotation). This
aligns the northern and southern subduction boundaries as we hypothesize they might have been in late
Miocene time. Holding mainland Greece fixed, the Central Hellenic Shear Zone is reconstructed by
allowing extension across the accommodating fault zones to increase to the southeast, producing
clockwise rotation of crustal fragments within the shear zone. The Ionian islands (Zakinthos and
Kephalonia) move with the adjacent crustal blocks as indicated by the color scheme in Fig. 12, except that
they close up against the west coast of Greece.
The result is ~100 km of westward motion in the southern Peleponnesus and progressively lesser
amounts of translation for regions north and east. This reconstruction produces clockwise rotation of
crustal blocks within the Central Hellenic shear zone, with ~30° clockwise rotation of the Peloponnesus
and progressively lesser amounts in crustal blocks to the east. We note that GPS data show little rotation
of the Peloponnesus relative to mainland Greece. However, paleomagnetic data indicate approximately
30° of clockwise rotation of the southern Peloponnesus since about 5 Ma (and more in the Ionian islands,
Kissel and Laj, 1988; Kissel et al., 1989, Duermeijeret al., 1999, 2000). This is consistent with the 30°
clockwise bend in the overall trend of the Pindos and late Miocene thrust fronts as they extend south from
mainland Greece into the Peloponnesus.
This reconstruction is highly speculative and almost certainly incorrect in many details, but some
features of the reconstruction are encouraging. First, it is possible to accommodate 100 km of right shear
through the Central Hellenic Shear Zone by predominantly extensional deformation. Second, clockwise
rotation of fragments brings the Pindos and late Miocene thrust fronts into parallelism across the Central
Hellenic Shear Zone and produces thrust faults that maintain an approximately constant distance from the
trench for a distance of ~500 km along strike. Third, and most important, it produces a hypothesis that can
be tested in the field through geologic mapping, dating and paleomagnetic studies.
Preliminary Geodynamic Model
12
An integral part of our proposed work is theoretical investigation of how subducted slabs interact
with the surrounding mantle. For slabs with a large component of trench retreat and slab roll-back, this is
a fundamentally 3D problem because flow occurs around the subducting lithosphere and stress on the slab
is probably a function of the along-strike length of the subduction zone. This is perhaps one reason why
few self-consistent trench models have been incorporated into studies of mantle dynamics. The few
works that do include a trench whose motion is computed from the mantle flow, and not specified a
priori, include numerical (for example Zhong and Gurnis, 1995; Becker et al., 1999; Tan and Gurnis;
2003;) and analogue models (Faccenna et al., 2001; Funiciello et al., 2003; Bellahsen et al., 2003).
This section contains a preliminary example of how slab density may control subduction
geometry and rate. Other models are described in the section on Proposed Study. This example addresses
the case of no overall plate convergence so that subduction rate and upper plate extension rate are equal
(Fig. 3, right side); the subducted lithosphere sinks through the surrounding mantle due to its negative
buoyancy. We treat the subducting slab as a thin sheet with zero flexural rigidity (Fig. 13). Slab length
does not change in a down-dip direction but stress is transmitted longitudinally along the slab. Buoyancy
is proportional to the density difference between the slab and surrounding mantle, integrated through the
slab. Buoyancy is also linearly proportional to water depth above the slab prior to its entry into the
subduction zone, thus water depth can be used as a proxy for buoyancy. Water depths of 0 km indicate
buoyant lithosphere, 2.5 km depths (mid-ocean ridge depth) indicate neutrally buoyant lithosphere, and
6.5 km depths indicate very negatively buoyant lithosphere. Slab buoyancy is allowed to vary as a
function of position to simulate oceanic and continental regions.
Forces that act on the slab are (1) vertical forces proportional to the buoyancy of slab, (2) forces
normal to the slab due to viscous flow in the surrounding mantle, and (2) shear forces along the slab due
to viscous flow in the surrounding mantle. The crux of the model is how to specify the viscous stresses
that act on the slab, which will be the subject of much of our proposed theoretical work. Here we use a
simple Stokes relation for motion of a rigid body through a viscous fluid of viscosity µ. Thus shear stress
is proportional to µ/λ times the slab-parallel velocity of the plate in a lower mantle reference frame,
where λ is some length scale that characterizes the dimensions of viscous flow in the surrounding mantle.
Normal stress is proportional to 2µ/λ times the slab-normal velocity of the plate in the same reference
frame.
For a known slab geometry and buoyancy distribution, the velocity of the slab is computed by
balancing the horizontal and vertical forces on the slab:
-
!(Ccos !)
+ qs cos ! + qn sin != 0
!s
!(Csin !)
- qs sin ! + qn cos ! + Fg = 0
!s
(1)
where C is the internal compressional force that acts longitudinally along the slab, qs and qn are the
viscous shear and normal stresses due to flow in the surrounding mantle, θ is the dip of the subducted
slab, s is position along the slab and Fg is the downward force related to the buoyancy contrast between
slab and mantle. These equations can be combined to eliminate C, and qs and qn can be replaced by terms
proportional to components of slab velocity.
Fig. 14 shows steady-state geometries and rates of subduction (trench roll-back) for three
uniformly buoyant lithospheric slabs. Rates of subduction vary from 10 to 70 mm/yr, increasing with the
pre-subduction water depth of the slab. The angle of subduction increases with increasing slab buoyancy,
so that the densest slab is subducted at the shallowest angle. The latter is not realistic. It is a function of
how viscous stresses are applied to the shallow slab and we have found that slab dip is highly sensitive to
the normal stress applied to the shallowest parts of the slab.
Slab geometry and rate change when lithosphere of variable density enters the subduction zone.
When buoyant continental material of large extent enters the trench, the rate of subduction drops from 70
mm/yr to <10 mm/yr in 5-10 m.y (Fig. 15). Slab angle steepens to nearly 90° and continental material is
subducted to depths of ~300 km. If the continental block is only 200 km wide (approximately the largest
13
block that can be subducted) slab behavior is nearly identical to that of the large continent for 10 m.y. but,
as dense oceanic lithosphere enters the trench behind the continental block, the rate of subduction
increases to its initial value over 20-30 m.y.
Fig. 16 shows the same model applied to a simulation of the Hellenic subduction system. We
begin with steady-state subduction of low density lithosphere at 10 mm/yr. At 8 Ma, the water depth of
lithosphere entering the trench is increased by 2.5 km to simulate entry of oceanic lithosphere into the
southern Hellenic trench in late Miocene time. By 0 Ma the rate of subduction has increased to about ~40
mm/yr. If we compare this to the case where there is to change in the buoyancy of subducted material at 8
Ma (simulating the northern Hellenides), model results show approximately 100 km of additional
subduction. Model results also show that entry of the dense lithosphere into the subduction system only
affects the subducted slab to depths of ~200 km. At greater depth the model slab geometries are identical
from northern to southern Hellenides.
These preliminary results are approximately consistent with the time at which oceanic lithosphere
entered the southern Hellenic trench, with the inferred 8 m.y. time interval for the increase of the southern
Hellenic subduction rate from 10 to 40 mm/yr, and with the apparent 100 km dextral displacement of the
subduction boundary between northern and southern Hellenides. Although much remains to be done to
improve this preliminary geodynamic model of subduction, these first results are encouraging and help to
guide our proposed plan of research.
Proposed Study
We propose a multidisciplinary study of the Hellenic subduction system aimed at better
understanding the dynamics of the subducting lithosphere, its interaction with the surrounding mantle,
and its connection to surface deformation and tectonics. Better documentation of the magnitudes, rates
and timing of the various events within the young Hellenic system will enable us to better constrain these
dynamic interactions and the length and time scales over which they occur.
The proposed study is an international collaborative effort conducted jointly with colleagues from
the University of Athens (Professor Dimitrios Papanikalaou), Athens Technical University (Professors
Charis Biliris and Dimitrios Paradisos), and the University of Rome (Professor Claudio Faccenna and Dr.
Francesca Funicello). Each of these scientists has had extensive past collaboration with at least one of the
PI’s.
The proposed work has three basic components, In brief, they include (1) Field and laboratory
studies directed largely at constraining young and active near-surface kinematics through the Central
Hellenic Shear Zone (and adjacent regions), including structural and stratigraphic mapping,
paleomagnetic studies of crustal rotation, isotopic dating and analysis, and GPS. (2) Passive seismologic
studies aimed at imaging the geometry and character of the shallow (<200 km) subducted slab beneath the
northern and southern Hellenides. (3) Theoretical studies (modeling and analog) of the kinematics and
dynamics of subduction systems like the Hellenic arc.
Field Geological Studies
The answers to many of the questions that arise as part of this proposal can be obtained only
through detailed field studies. Establishing the age of initiation, temporal evolution and total displacement
through the shear zone is crucial to understanding when and how the southern Hellenic subduction system
became differentiated from the northern Hellenic system. Within a program of structural and stratigraphic
mapping, closely coordinated with paleomagnetic studies, isotopic dating and GPS measurements, the
questions that need to be answered include:
How much right-lateral shear has occurred across the Central Hellenic shear zone, how is it
accommodated, today and in the past, and how does it connect westward to the Kephalonia Transform
and eastwards to the North Aegean Trough and the North Anatolian Fault?
If overall shear is accommodated largely by extensional structures, how many extensional structures
exist? Is displacement on these structures consistent with the GPS velocity field?
14
How much rotation of crustal fragments has taken place within the Central Hellenic Shear Zone and
what role did it play in accommodating right-lateral shear?
When did the present day pattern of deformation begin? How has it evolved through time?
What is the deeper crustal response to the shallow crustal deformation? How is it related to the
underlying slab geometry?
Pliocene-Quaternary Structures in the Central Hellenic Shear Zone
Published work and our preliminary field studies indicate that most of the young and active
structures in the Central Hellenic Shear Zone are normal faults and associated hanging wall sedimentary
basins. These normal faults trend northwest (in the east) to east-west (in the west) and are mainly
confined to the Central Hellenic Shear Zone, although a few structures extend north and south of the shear
zone proper. Within the Central Hellenic Shear Zone, the major graben structures bounding the Gulf of
Corinth have been studied extensively (e.g. Briole et al., 2000). Other important extensional structures
within the Central Hellenic shear zone have not been well studied. These include the northeast-dipping
Beotikos-Kifissos normal fault and active extensional faults that bound the Gulf of Evia (Figs. 4 and 911).
The Gulf of Evia and the Evia Basin, located west and east of the island of Evia, are bounded by
east-dipping normal faults. Some of these faults are exposed on land on the west of the gulf, but east of
Evia the structures are mostly under water (Jackson, and McKenzie, l988; Armijo et al., 1996). We
propose to map selected regions along the Beotikos-Kifissos and Evoikos fault systems to constrain the
timing and magnitude of extension. Hanging wall strata are poorly dated, but are generally mapped as
Pliocene-Quaternary. Some hanging wall units contain lignite and floral fossils that should yield
reasonable age constraints on these young normal fault systems. We also propose isotopic dating of
volcanic detritus in the hanging wall basins.
Our preliminary mapping has shown that in addition to the major graben structures, numerous
smaller extensional faults and related basins form a complex interrelated pattern of extensional
deformation within the Central Hellenic Shear Zone. These structures have not generally been recognized
as extensional and are not active. However, they appear to have accommodated some of the earliest
extension within the Central Hellenic Shear Zone. For example, in the northern Peloponnesus, near
Feneos, we have identified an extensional system that in map pattern displays curved fault traces (Fig.
17). The fault patterns in this area are complex and successive generations of faults intersect one another.
The faults appear to be listric in cross section and have hanging wall sediment accumulations. Hanging
wall units are not aligned and the detached blocks have rotated around vertical and horizontal axes and
relative to one another. The presence of syn-tectonic and post-tectonic sediments is important as they
appear to be Pliocene-Quaternary in age and constrain the age of this extensional faulting.
We have identified similar, but as yet unmapped, structures in several areas in the northern
Peloponnesus and in areas immediately north of the Gulf of Corinth. These young structures are not large
but are relatively easy to recognize because they are well expressed in the topography as small valleys
and basins of deposition bounded by arcuate mountain fronts. We propose systematic mapping of such
structures within the Central Hellenic Shear Zone, beginning with the Feneos region in the northern
Peloponnesus and extensional structures north of the Gulf of Corinth. These faults are important because
they will constrain the timing of early deformation within the Central Hellenic Shear Zone. These
structural studies will be augmented by paleomagnetic measurements of block rotations and isotopic
dating of hanging wall and overlap sedimentary assemblages.
Interaction of Central Hellenic Shear Zone Structures with Older Hellenic Structures
The young faults of the Central Hellenic Shear Zone cut older structures that are part of a regional
system of arc-parallel thrust and extensional faults. These arc-parallel faults formed as the Hellenic thrust
belt and associated extension migrated to the southwest until late Miocene time (Fig. 4). Our current
hypothesis is that initiation of the Central Hellenic shear zone marks a major change in the tectonic
regime of the Hellenic orogen as arc-parallel features became inactive and were cut by faults of the
15
Central Hellenic Shear Zone. To investigate this hypothesis we propose to map the arc-parallel
extensional faults that immediately predate the Central Hellenic Shear Zone.
Along the eastern side of the Peloponessus we have identified a previously unknown low-angle
arc-parallel extensional fault system of regional extent, referred to as the Parnon detachment system,
which parallels the older thrust units (Fig. 10). Normal faults within this detachment system displace the
older thrust sheets and bring higher structural units down onto lower structural units, thinning or
eliminating units within the original thrust complex. Within the upper plate of the Parnon detachment
system high-angle faults can be shown to terminate downward at the basal low-angle fault contact. This
detachment system is typical of the low-angle faults that are widespread throughout the Hellenic orogen
(Fig. 4). We propose detailed mapping to constrain the age of the latest movements along the Parnon
detachment system. Preliminary age control indicates that the Parnon detachment system may be as
young as late Miocene as it appears to be the continuation of the dated late Miocene detachment on the
island of Kythera (Papanikolaou and Danamos, l991).
Particularly important will be mapping in the area where northwest trending faults of the Parnon
detachment system and similar faults are intersected by younger east-west trending faults of the Central
Hellenic shear zone. This occurs in the northeastern Peloponnesus where young east-west trending faults
cut and in places reactivated older extensional structures of the Parnon detachment system. For example,
the marine Megara basin is exposed in the northeastern Peloponnesus. Our mapping indicates that it may
be a hanging wall basin to the Parnon detachment system. It contains lignite-bearing deposits that are
tilted ~20° to the northeast and cut by east-west trending normal faults. These data suggest that the Parnon
detachment was active here until late Miocene time (6-10 Ma). We propose to map key portions of the
northern part of the Parnon detachment system and its hanging wall deposits to better constrain the
geometry, direction of motion and age of faulting. Where the Parnon detachment system exposes lowgrade Mani and higher grade Arna metamorphic rocks in the footwall, we will collect samples for
U/Th/He radiometric dating to constrain the time of exhumation.
A detailed knowledge of older structures in the vicinity of the Central Hellenic shear zone will
also be helpful in constraining the younger displacements within and across the shear zone. The older
system of thrust faults and superposed regional detachment faults can be recognized in mainland Greece
and in the Peloponnesus. However, the trend of the external belt appears to be rotated ~30° clockwise
across the Central Hellenic shear zone (Figs. 4 and 12). We propose to test the continuity of these and
other major structures across the Central Hellenic shear zone through more detailed field correlation of
structure and stratigraphy than is now available, and to test the interpretation of clockwise rotation
through mapping and paleomagnetic sampling.
These structural and stratigraphic observations will be integrated with isotopic (and
paleontologic) dating of hanging wall basin fill, and with paleomagnetic measurements to provide a
temporal record of extension and rotation within the Central Hellenic Shear Zone. These results will be
compared to estimates of overall displacement and rotation from regional lines of evidence and with
seismic images of the subducted lithosphere to yield a regional picture of deformation from the surface
into the mantle.
Paleomagnetic Studies
Paleomagnetic studies in the early 1980s were among the first to recognize that rotation occurred
during the deformation in central Greece and the Peloponnesus from at least mid-Tertiary time to the
present. Kissel and coworkers demonstrated that a large region west of the Aegean, extending from the
Peloponnesus northward through Greece to Albania, experienced two apparent pulses of clockwise
rotation around a pole located in Albania, with 25° of rotation in the Middle Miocene, and an additional
25° since early Pliocene time (Kissel et al., 2003; see also Sperenza et al. 1995; Duermeijeret al., 1999,
2000 and references contained therein). This zone of rotation extends across the Kefalonia Transform, the
Central Hellenic Shear Zone, and other large-scale tectonic structures, showing no obvious relationship to
them. Paleomagnetic data from Kissel et al. (2003) from within the Central Hellenic Sheer Zone show
16
that superimposed on this large-scale clockwise rotation is an additional 25° of clockwise rotation of
Evia, apparently of post-Miocene age (and in approximately the same region that GPS data show
undergoing rapid clockwise rotations today). This additional rotation was not found further east on the
island of Skyros in the westernAegean. The authors interpreted this to be a reflection of rotation of faultbounded blocks within the Central Hellenic Shear Zone related to slip along the surrounding faults.
These data are in partial agreement with our preliminary kinematic model of this region (Fig. 12)
but there is still much that remains to be understood. First, Kissel et al.’s (2003) rotational data were from
only two blocks and it is possible that differential rotations occur within subregions of the Central
Hellenic Shear Zone. A recent study by Mattei et al. (in press) that sampled young sedimentary rocks
south of the Gulf of Corinth found evidence for both clockwise and anticlockwise rotation in the western
part of the Central Hellenic Shear Zone, although they the uncertainty on these rotations is extremely
large. Second, Kissel et al. (2003) were not able to date precisely the observed rotations at sites in the
Central Hellenic Shear Zone. Finally, the origin and nature of the large scale rotation of this part of
Greece and its connection to the past and present tectonic structures remains largely a mystery, even after
20 years of paleomagnetic work. However, it seems significant that the second (post-Miocene) phase of
rotation roughly coincides with the transition from subduction of continental to oceanic lithosphere in the
Southern Hellenides. We are thus encouraged to pursue more extensive paleomagnetic work in the
Central Hellenic Shear Zone and surrounding regions.
We propose to conduct densely distributed sampling of all the crustal blocks (in so far as we can
identify them) in the Central Hellenic Shear Zone so as to reconstruct a continuous record of the
magnitude and orientation of progressive rotations (and the related geological structures) throughout the
region. Our approach will be to conduct combined field mapping, paleomagnetic and geochronological
work to obtain precise time constraints on these rotations. One of our aims will be to determine how
closely linked in time are the onset of faulting in the Central Hellenic Shear Zone, the onset of the second
clockwise rotational phase noted by Kissel et al. (2003), and the entry of dense oceanic lithosphere into
the southern Hellenic trench. A close correspondence in timing would clearly implicate a connection
between these processes.
GPS Measurements
To determine present-day crustal motions in southern Greece, we will build on the alreadyextensive GPS observations in this region (Fig. 18). Most of the high-precision data have been collected
by our collaborators at the National Technical University of Athens working with scientists at MIT, ETHZurich, and a consortium of universities in the UK. The continuous GPS stations of the Hellenic Arc
network and many of the survey-mode stations of the Peloponnesus and Aegean have velocity
uncertainties at the level 1-2 mm/yr, as shown in Figs. 5 and 7 and can resolve strain and rotations at a
sufficient to test our hypotheses. Current estimates of velocities from the 66 stations in the Central Greece
Network [Clarke et al., 1997] have uncertainties of 3-4 mm/yr but may be improved with additional
measurements made by the UK consortium through 2000 (expected release date for the newer data is
highly uncertain and no earlier than 2005).
In order to tie the UK network to the MIT analysis and to refine estimates of strain in the Central
Hellenic Shear Zone, we propose to make additional measurements at 10 stations within the Central
Hellenic Shear Zone in years 1, 3 and 5 of our study. Our MIT colleagues R. Reilinger and S. McClusky
have been funded to carry out additional measurements in the southern Peloponnesus and Crete in 2004
and 2007. These new measurements will be south of the Central Hellenic Shear Zone but will be useful in
constraining the detailed velocity field of the Peloponnesus relative to northern Greece. Thus we
anticipate that the very modest GPS effort proposed here will be highly leveraged and will tie the
independently measured UK network to the reference frame of the greater Aegean region and the existing
large regional GPS data set at MIT.
17
We will perform the analysis of the GPS data at MIT using the GAMIT/GLOBK software [King
and Bock, 2003; Herring, 2003] as described in McClusky et al. [2000], combining the new observations
with the full set of data acquired for the eastern Mediterranean since 1989.
Isotopic Studies
We propose a program of radiogenic isotopic and U-Pb and U-Th-He studies to support our field
mapping within the Central Hellenic Shear Zone and adjacent regions. Dating the youngest movement on
the extensional faults of the south-central Aegean domain, such as the Parnon detachment system of the
eastern Peloponnesus, is necessary to establish the time at which arc-parallel extensional features were
still active within the Aegean extensional domain. Dating of the oldest motions on the east-west to
northwest trending extensional faults of the Central Hellenic Shear zone is important to establish the time
at which arc-disruption began. It is conceivable that the two sets of structures overlap in time, recording a
gradual transition from arc-parallel extension to arc-disruption and the termination of extension in the
south-central Aegean domain.
Biostratigraphic dates are available for a few parts of the Late Miocene to Present sedimentary
sections of Central Greece, particularly where young marine sediments have been uplifted and exposed in
areas adjacent to the Aegean Sea, the Gulf of Corinth, and on islands within the Ionian and Aegean Seas.
These data are sufficient to suggest a late Miocene age for the Parnon detachment system (dated in one
place) and Pliocene-Quaternary ages for faults within the Central Hellenic Shear Zone. Unfortunately,
much of the sections are non marine and tight age control is largely lacking, except for the young (<500
kyr) marine terraces along the south side of the Gulf of Corinth and a few other scattered ages. Within the
Central Hellenic Shear Zone, the important Beotikos-Kifissos detachment system is largely undated, as
are the older displacements on the active Gulf of Evia. Dating of the onset and cessation of extension in
these areas, as well as the older, inactive normal faults near the Gulf of Corinth, will be a high priority for
our proposed study of the Central Hellenic Shear Zone. In so far as possible, we aim to date pre-, syn- and
post-kinematic deposits associate with faulting in this area, so that in conjunction with our structural
mapping it will be possible to reconstruct the magnitude and locus of extension within the shear zone and
how it has evolved with time.
We propose two primary dating techniques to achieve this objective. First, footwall rocks for the
Parnon detachment system and for some of the extensional faults in the Central Hellenic shear zone
expose the low grade metamorphic rocks of the Mani, Arna and base Tripolis (Tyros beds) units. (The
Arna is higher grade than the Mani or Tyros units, but the metamorphism is Oligocene and not related to
extensional unroofing.) Paleotemperature estimates in the structurally deepest Mani unit are ~250°C,
while the structurally highest Tyros beds show low greenschist facies metamorphism (Figs. 10 and 11).
We propose to date the unroofing ages of these footwall rocks using U/Th/He geochronolgy. We have
budgeted funds to do these analyses either at MIT or Cal Tech. The MIT lab is running but given the
back log we could easily switch to Cal Tech if necessary.
Second, we propose to date hanging wall basin sediments, and overlap assemblages, using U/Pb
geochronology on volcanic detritus within the basin fill. Although thick ash beds are relatively rare, the
presence of an active arc approximately 100 km to the east has provided a volcanic contribution to these
sedimentary sections. Thus we propose to generate a detailed chronology by the dating of thin beds of
volcanic ash using zircons. Bowring has shown that precise dates may be obtained on Miocene and even
younger rocks and we will get around the problem of having to find fresh igneous minerals for Ar-Ar
geochronology.
Radiogenic isotopic studies will be to examine temporal changes in arc magmatism. We will use
a full suite of Nd, Sr, and Pb isotopes to evaluate whether there is a contribution from subducted
continental lithosphere and whether we can detect when it became involved in arc magmas.
Proposed Seismic Studies
A two-stage, passive seismic investigation is proposed to image the subducted Mediterranean slab
in the 0-250 km depth range, to the north and south of the Central Hellenic Shear Zone. Previous seismic
18
analyses in the region have imaged the general outline of the slab, but the available data coverage and
employed methodologies have not yielded the geometrical resolution necessary for the geodynamic
modeling purposes of this study. Furthermore, most previous studies have focused on the southern portion
of the Hellenic subduction system, leaving the northern part poorly constrained
In this experiment, appropriate array design and application of new high-resolution teleseismic
imaging techniques will provide improved resolution of the slab beneath northern and southern
Hellenides and a means of detecting the transition from subducted continental to subducted oceanic
lithosphere. Other goals include providing constraints on devolatilization and hydrous reactions in the
slab and mantle wedge, and the coupling between the slab, the overlying accretionary prism and the
Aegean lithosphere. These new geometrical constraints will be directly applicable to geodynamic
simulations of the Hellenic subduction system. In the next section we provide a brief overview of past
seismological work in the study area and then proceed to the specific objectives and methods of the
proposed experiment.
Previous seismological results
For more than 30 years the eastern Mediterranean has been the focus of relentless seismological
scrutiny. A variety of seismic approaches have been employed to image the subsurface structure of the
Hellenic subduction system. The studies range from local investigations of the upper crust through
seismic reflection and refraction profiles to large-scale imaging of the subducted slab from global
tomography. We can divide seismic images of the Hellenic system into three distinct depth domains:
shallow (0-30 km), intermediate (30-180 km) and deep (>180 km).
In the shallow domain, marine reflection and refraction profiles west of the trench support the
idea that the crust beneath the Ionian Sea is oceanic in origin, with an average thickness of 8 km, overlain
by up to 8-10 km of sediment [Makris, 1985; de Voogd et al., 1992; Kopf et al., 2003]. East of the trench,
a series of local marine and land-based seismic experiments in the Ionian Islands show the subduction
décollement as a shallow-dipping discontinuity at ~12 km depth [Hirn et al, 1997; Clément et al., 2000;
Sachpazi et al., 2000]. Landward to the east, seismic signal generated in the shallow domain is dominated
by the structure of the continental crust and Moho topography. Travel-time tomography and receiver
functions show that the continental Moho reaches depths in excess of 40 km beneath the Peloponnesus
and mainland Greece [Makris, 1985; Papazachos and Nolet, 1997; Tiberi et al., 2000; Marone et al.,
2003]. The crust thins to the northeast, reaching thicknesses of ~30 km beneath the Gulf of Evia and the
central Aegean Sea.
The seismic signal in the intermediate depth domain is dominated by interactions with the cold
subducted lithosphere, which exhibits higher than average wavespeeds. Independent tomographic results
using local and teleseismic events image the slab as a tabular anomaly dipping shallowly to the northeast
beneath the Peloponnesus, at an angle of ~10-12° [Papazachos and Nolet, 1997; Tiberi et al., 2000]. The
20 km slab thickness obtained by Papazachos and Nolet (1997) is surprisingly thin for a lithospheric
plate, but their results may not be well constrained for sub-horizontal geometries. Both studies detect a
sudden increase in subduction angle at ~70 km depth beneath the Gulf of Corinth. Below that point, the
slab dips ~30° to a depth of 150-170 km below the western Aegean Sea. The distribution of earthquake
foci outlines a diffuse Wadati-Benioff zone extending to depths <180 km (Papazachos et al., 2000), with
an increase in subduction angle at ~90-100 km beneath the northeast Peloponnesus. This kink in the slab
is deeper than that imaged tomographically, but this is reasonable because the Wadati-Benioff zone does
not necessarily represent the top of the slab. Anisotropy in the mantle beneath the Hellenic subduction
system has been investigated through SKS splitting [Schmid et al, 2003] showing small splitting times
and poorly constrained polarization directions. Tomographic inversion of Pn waves by Hearn [1999]
shows a clear preferential alignment of the fast axis parallel to the arc.
19
Below 180 km depth, the geometry of the Hellenic subduction system is exclusively constrained
by regional and global tomography [Spakman et al., 1993; van der Hilst et al., 1997]. Models show the
subducted slab as a fast wavespeed anomaly that dips steeply to the northwest and extends to a depths of
~1000 km (Fig. 6).
Objectives of the Proposed Seismic Experiment
To test geodynamic models of the Hellenic subduction system, robust constraints are required for
the geometry and internal structure of the subducted slab and for slab interactions with the overlying
mantle wedge. Although the southern slab segment has been clearly detected in previous seismic
investigations, its finer structure at intermediate depth remains poorly constrained. The proposed seismic
experiment is designed to improve resolution in this depth range and address the following questions:
a) what are the position and geometry of the subduction décollement (i.e., top of the slab)?
b) what is the thickness of the subducted slab?
c) how are earthquake foci distributed with respect to the slab (i.e., at the décollement, in the subducted
crust or mantle?)
d) is there evidence for devolatilization of the slab and association hydration of the mantle wedge?
e) what morphological features distinguish the southern Hellenic subduction system, where oceanic
crust is being subducted, from the northern Hellenic subduction system, where continental crust
enters the trench?
f)
is there a change in signature of the subducted crust with depth associated with the inferred transition
from oceanic to continental composition?
g) why is there a sudden increase in subduction angle near 70 km depth? Is this related to eclogitization
or to the transition from oceanic to continental subducted crust?
We propose deployment of two dense arrays of three-component, broadband seismometers to the
north and south of the Central Hellenic Shear Zone (Figs. 19 and 20). Our rationale for two lines is to
image and compare the distinct morphologies of the slab in the two regimes of the Hellenic subduction
system. The total duration of the experiment will be 3 years, with deployments of 1.5 years each for the
southern and northern lines. Each line will comprise 40 stations with average station spacing of 7.5 km.
Two-Dimensional GRT Inversion
The array configuration is primarily devised for the 2D Generalized Radon Transform (GRT)
Inversion, a teleseismic imaging/migration approach recently developed at the University of British
Columbia [Bostock and Rondenay, 1999; Bostock et al., 2001]. This method is designed to image
complex discontinuities (rapid transitions in material properties) beneath dense broadband seismic arrays
using the scattered wavefield comprised in the teleseismic P-coda. In contrast to many previous migration
approaches that yield images of subsurface properties analogous to scattering potential, this method
directly inverts for P-wave and S-wave velocity perturbations and density. The 2D GRT inversion is a
powerful tool to image structure at intermediate depth, in subduction zone settings and stable continental
areas (Fig. 21). For example, images obtained in the Cascadia subduction zone clearly delineate the
geometry of the subducted oceanic crust, a change in subduction angle at ~40 km depth, and a disruption
near that inflection point that has been interpreted as devolitization of the crust and
hydration/serpentinization of the mantle wedge [Rondenay et al., 2001; Bostock et al., 2002]. The
example shown in Fig. 21 clearly demonstrates that this method will image currently unresolved structure
of the Hellenic subduction system.
The expected results will address many of our objectives, including the geometry of the
décollement, the distribution of earthquakes in the slab (obtained by relocating foci and mapping them
onto the imaged sections with additional constraints from tomography as described below), hydrous
reactions in the crust and mantle wedge, morphological differences between north and south subduction
regimes, and possible down-dip transition from an oceanic to a continental slab in the southern
20
subduction system (to be determined from changes in crustal/slab thickness and signature). Our capacity
to differentiate between dehydration of subducted crust and the transition from oceanic to continental
subducted crust will rely on establishing morphological differences between the northern and southern
regimes (where continental and oceanic crust, respectively, are entering the subduction zone) and on the
characteristic signatures associated with eclogitization of subducted crust and hydration of the mantle
wedge (determined from GRT images and additional geophysical observables discussed below).
The resolution afforded by 2D GRT inversion has been thoroughly assessed on theoretical
grounds [Bostock et al., 2001], through synthetic tests [see Fig. 21; Shragge et al., 2001], and as a
function of experimental geometry [Rondenay et al., 2002]. These results ensure that the method will
accurately resolve structure such as subducted crust down to a depth of at least 200 km, with a maximum
dip resolution ranging from 85° near the surface, to 45° at 200 km depth (dependent on event distribution
and array aperture), and a volume resolution of ~5 km (based on the high cut-off frequency of 0.3 Hz
chosen to avoid scattering from surface topography). Note that 7.5 km station spacing and 1.5 year
duration for each array are needed for comprehensive sampling of scatterers in the model, so as to yield
accurate material property perturbations [Rondenay et al., 2002].
We need adequate event distribution to illuminate slab structure from updip (WNW) and downdip
directions. A plot of world seismicity over 1.5 years (01/2000-06/2002) for events of magnitude ≥5.7 and
teleseismic epicentral distances 30°<∆<100° shows that this condition will be met (Fig. 20). We must also
validate the assumption that 2D imaging is appropriate for the Hellenic subduction system. The
applicability of the 2D assumption can be tested from the forecasted event distribution (Fig. 20) in the
context of slab geometry (dashed line in Fig. 19). For the Hellenic subduction system, the structure must
extend linearly for a minimum of ~100 km on either side of the imaging plane. Results from previous
tomographic studies [e.g., Papazachos and Nolet, 1997] and placement of the proposed array across a
relatively straight portion of the crescent-shaped subduction complex validate this second condition. Note
that the imaging plane does not have to be exactly parallel to the array; data may be projected onto any
vertical plane oblique to the array, allowing us to optimize model geometry for the 2D assumption.
The validity of images obtained in the proposed study will be thoroughly assessed by generating
synthetic datasets for these models. For this purpose, we will use the finite-difference scheme of Hicks
[2003], which allows for arbitrary source and receiver positions – a geometrical condition that prevails in
all teleseismic experiments.
Additional Complementary Analyses
To better constrain the structure of the Hellenic subduction system and the possible occurrence of
hydrous reactions or partial melt, data from our temporary dense arrays will be augmented with data from
permanent stations of the Greek National Seismic Network (Fig. 19) and other temporary arrays [e.g.,
Papazachos and Nolet, 1997; Papazachos et al., 2000]. Travel-time tomography using local and
teleseismic events will be used in conjunction with the 2D GRT inversion to obtain new images of long
wavelength (i.e., volumetric) P- and S-wavespeed anomalies in the Hellenic subduction system, with 2D
GRT results projected as constraints onto the model space of the tomography. This will reduce smearing
of anomalies in the tomographic model and yield a more robust assessment of the thickness of the
subducted slab at intermediate depth. Tomographic results will also be used to obtain estimates of the
Vp/Vs ratio (or δlnVs/δlnVp) and bulk sound velocity throughout the Hellenic subduction system. These
seismic observables will help constrain the local influence of temperature, fluids and partial melt on
mantle wedge rheology [e.g., Karato, 1993, 2001; Christensen, 1996; Wiens and Smith, 2001]. Seismic
attenuation, another important rheological indicator, may be inverted for using P and S spectra from local
events [see, e.g., Tsumura et al., 2000].
Earthquake foci located near the imaged profiles will be relocated using these new tomographic
models and mapped onto the slab structure imaged by 2D GRT inversion. These results will yield
improved constraints on the distribution of earthquake foci with respect to the subduction décollement
and the subducted Moho. Shear-wave splitting will be performed on teleseismic and, perhaps, regional
21
events. Teleseismic SKS splitting measurements will be used to verify whether, with an increase in data
coverage and number of stations, more consistent estimates of the anisotropic parameters than those of
Schmid et al [2003] can be obtained for the mantle underlying the Hellenic subduction system. If
possible, splitting measurements will be conducted on the waveforms of large local events, although these
seldom occur in the western Hellenic subduction system. Indeed, there have only been four events with
magnitude >4.0 and focal depth >60 km over the past 3 years (2000-2003). As a consequence, local
events may only loosely constrain anisotropy of the mantle wedge.
Modeling of Subduction Zone Dynamics
There has been much work on the interplay of surface plate motions and mantle flow (see recent
reviews by King, 2001, and Christensen, 2001), but most existing models are restricted to 2D and
prescribe, rather than calculate, the trench positions through time. Exceptions include numerical studies
by Zhong, Gurnis and coworkers (e.g. Zhong and Gurnis, 1995, and Gurnis et al., 2000) and laboratory
work by Faccenna and coworkers (see below). Here we propose to develop and investigate a variety of
subduction models with self-consistent trench motions, where slab migration and trench retreat arise as a
natural result of slab/mantle rheology and density, rather than being specified a priori. This problem is
fundamentally three dimensional because once the end of the slab has descended to the base of the upper
mantle, flow is constricted around the end of the slab and slab roll-back occurs only through lateral flow
of mantle material around the unconfined edges of the slab. Thus we propose a three pronged
investigative approach (purely viscous, coherent slab, and analog), using 2D and 3D analyses, to provide
maximum insight into this difficult problem in 3D fluid flow.
Within the context of the Hellenic system, constraints from geology and geodesy (past and
present-day kinematics) and tomography (density structure at depth) will enable us to evaluate the role of
slab dynamics in plate boundary evolution. Our geodynamic studies will proceed hand-in-hand with field
studies as it is our experience that theoretical results are invaluable for guiding observational efforts as
well as for interpreting the results of the observations.
Coherent Slab Models (led by PI Royden)
The preliminary geodynamic model presented earlier in this proposal is perhaps the most
simplistic slab model that can be constructed. It has the advantage of simplicity and of being semianalytical, but is lacking a well-grounded exploration of the viscous stresses on the subducted slab. We
propose to expand this approach to incorporate explicitly the effects of viscous flow within the
surrounding mantle. The intrinsic behavior of the slab, and the way in which stresses are transmitted
along the slab, are satisfactorily managed by treating the slab as a thin, flexurally competent sheet.
Equation 1 is easily modified to incorporate flexural behavior of the slab, and we currently have working
models for a range of slab rheologies, including time and depth-dependent slab properties. Variable slab
buoyancy and variable densities within of the upper plate lithosphere and “fore-arc” region are also easily
computed and incorporated into our existing models.
The crucial issue is determining how viscous stresses are applied to the subducted slab by the
surrounding mantle. Proper analysis of these stresses involves computation of fluid flow within the
mantle as a function of slab geometry and velocity, which in turn are controlled by slab density and
mantle viscosity. This problem is complex because toroidal flow around the subducting slab is the
dominant mode of mantle flow when a slab rolls back through the surrounding mantle. (This may be
different from the dynamics of subduction when the convergence rate and subduction rate are
comparable, so that the slab may descend through the mantle without a significant slab-normal
component of motion).
We propose to solve the Stokes equation in 3D for mantle surrounding a subducting slab with
prescribed geometry and velocity conditions at the slab surface (using the method of Successive OverRelaxation on a finite-difference grid; for this computation finite element techniques are not needed). We
will develop a self-consistent, steady-state model of subduction in 3D. In particular, for a specified slab
width, density, rheology and side boundary conditions there is a unique slab geometry such that the
22
buoyancy and viscous forces acting on the slab sum to zero and the slab is in steady-state. We propose to
use an iterative approach to find this unique geometry, so that although the mantle flow field will be
instantaneous, we will use successive numerical approximations of slab geometry to converge on the
final, consistent, steady-state slab geometry. This in itself will be an important outcome because the
controls on 3D slab geometry in subduction boundaries are currently not well understood. Questions that
we hope to address include:
What is the 3D steady-state geometry of subducted lithosphere? What is the flow field in the mantle?
What are the viscous stresses acting on the slab? How do these results depend on slab width, density
and flexural properties?
While the problem of slab roll-back is inherently three-dimensional, solution of the Stokes
equation in 3D is probably not feasible at every time-step during non steady-state evolution. But, because
non-steady-state behavior is crucial to understanding the Hellenic, and many other, subduction systems,
we will attempt to extract some simple “rules” from the 3D analysis that will enable us to make
meaningful computations in 2D. We anticipate that viscous stress will be a function of velocity, slab
width, distance from slab edge, depth, viscous layering in the mantle, etc. With the answers to these and
other questions, we hope that we will be able to make useful, reasonably accurate models of subduction,
trench migration and slab geometry with 2D numerical studies. Our goal is to understand how slab
density, and its variability, affect the subduction process, how flow is distributed within the mantle, and
the length and time scales over which slab density affects subduction dynamics. These studies will be
informed by, and conducted in parallel with, purely viscous numerical models and 3d analog models, as
described below.
Purely Viscous Flow Models (led by PI Becker)
Alongside the coherent-slab approach, we propose to study subduction dynamics and trench
motion with purely viscous numerical models. The overall goals are similar to those described for the
coherent slab modeling. The proposed work involves adapting existing fluid models for the Hellenic
subduction zone to study the effect of lithospheric density variations (oceanic/continental material) flow
constrainment, 3-D background currents, and viscosity stratification on back-arc motion, slab shapes and
margin evolution.
We build upon the numerical modeling experience of Becker, who has previously worked with
Faccenna's group in Italy. Our studies to date compare 2D viscous computations with Faccenna's 3D
analog models (Becker et al., 1999) and use the results to interpret the tectonic evolution of the Central
Mediterranean subduction zone (Faccenna et al., 2001). Currently Becker is involved in a follow-up study
of trench migration for simplified, but dynamically consistent, 2D subduction models (Enns et al., in
prep.). By way of example, Fig. 22 shows dense oceanic lithosphere sinking within a mantle whose
(Newtonian) viscosity increases by a factor of 50 at 660 km (cf. Hager et al., 1985). The trench is allowed
to move freely. Two sets of mechanical side-boundary conditions illustrate the importance of flow
constrainment, which is only poorly explored by 2D models. Our results show that trench migration and
slab morphology vary dramatically depending on how the slab and the lower mantle interact.
We intend to build on these general results and adapt this model to our study region in the
Hellenides, with the inclusion of density variations in the subducting lithosphere. We plan to analyze
similarities and differences between these purely viscous models and the coherent-slab models described
above, in order to determine how the existence of a competent slab influences the dynamics of mantle
flow, and to what extent the rheology of the slab itself plays an important role.
Laboratory models of subduction clearly show that 3D currents around the slab are important for
the shape of streamlines in the mantle, affecting slab arcuity, morphology, and episodicity in trench
motion (e.g. Funiciello et al., 2003; Kincaid and Griffith, 2003). Thus we will strive to explore 3D
viscous models numerically. Toward this end, Becker has begun collaboration with F. Funiciello (U
Roma TRE), C. Piromallo (INGV, Rome), and Faccenna. This study focuses on the large scale temporal
23
evolution of mantle structure in the whole Mediterranean region as it results from the complex collision
process between the African and European plates.
For 3D flow, we use the parallelized finite element code CITCOM (Moresi and Solomatov, 1999;
Zhong et al., 2000) obtained from the Caltech geoframework.org site. We have begun study of isolated
viscous, thermal slabs for evolving, prescribed surface plate motions (cf. Tan et al., 2002). We plan to
construct a regional, high resolution model for the Hellenic setting and incorporate the new constraints
that will become available during the study, with close collaboration with the observational team. The
methods that will be developed by Becker and coworkers will be useful for both the Hellenic
implementation, and the parallel, complementary effort on the large scale structure of the region with the
Italian group.
Tasks before us include incorporation of a tracer advection scheme for the parallel version of
CITCOM (for compositional advection) and inclusion of a regional finite element flow model in global
circulation calculations. The latter will be of importance to the wider community of geodynamicists. The
problem of embedding regional, high resolution computations within a consistent treatment of global, 3D,
earth like flow also arises in other subjects like the study of regional seismic anisotropy (cf. CSEDI
proposal by D. Blackman, UCSD). We intend to explore prescribing velocity boundary conditions from a
Hager and O'Connell (1981) semi-analytical calculation to a regional CITCOM mesh, and use the global
version of CITCOM with mesh refinement in the Hellenic arc.
Analog Modeling
Analog, or laboratory, modeling is a useful tool for gaining insight into complex 3D process,
including subduction and plate interactions. Faccenna and co-workers in Rome have been conducting
analog experiments of subduction systems for several years, with particular application to the
Mediterranean subduction systems. These experiments are usually performed inside a large tank, using a
visco-elastic material like silicone putty (rhodorsil gomme) to simulate the lithosphere and honey or
glucose syrup to simulate a viscous mantle. The viscosity and density of the simulated slab and mantle
can be varied over a large range to simulate the depth and/or temperature dependent rheology in the
mantle and zones of varying buoyancy in the slab. Because almost any boundary conditions, density and
geometry can be imposed on the slab, this method is highly flexible and complementary to numerical
modeling efforts. The main issue is to ensure that the scaling from tank experiments to numerical models
is carried out correctly, and that the experimental set-up scales appropriately with the earth.
Much of Faccenna previous work has focused on the way subduction is initiated (Faccenna et al.,
1999), on the evolution of the slab as it descends into the upper mantle (Becker et al., 1999) and on the
way the slab interacts with the 660-km discontinuity (Faccenna et al., 2001; Funiciello et al., 2003). The
latter experiment shows that the dynamics, geometry and rate of subduction depend on the details of slabmantle interaction. So far, most of our experiments assume a constant density contrast between the
subducting slab and the mantle. However, in the context of this proposal we have examined the role of
buoyancy in slab dynamics has been tested with a preliminary simulation of a continental plate and a
buoyant continental block entering trench (Figs. 23 and 24). These results are qualitatively consistent with
the preliminary subduction model shown in Figs. 14 and 15 and provide encouragement that there will be
a strong connection between theoretical and analog models of subduction of variable density slabs.
Becker and Faccenna have previously collaborated, combining numerical and analog approaches
to achieve a better understanding of subduction initiation and evolution and we will build on this history
of collaboration. Our proposed study of the Hellenic subduction system, and of the subduction process in
general, will proceed alongside the ongoing study of Becker, Faccenna, Funicello and Piromallo, into the
temporal evolution of mantle structure within the whole Mediterranean region. We expect strong
interactions between the Italian group, Becker and Royden, with an MIT PhD student to be co-advised by
Royden and Becker.
Management Plan
The proposed study is an international collaborative effort conducted jointly between U.S.
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investigators and European colleagues from Greece and Italy. Our European colleagues will supply
funding for their portions of the proposed work, but the bulk of the costs and the overall management of
the research plan will lie with the MIT group. The lead PI (Leigh Royden) and two of the CoI's (Clark
Burchfiel and Bob King) have much experience in organizing and carrying out large, multidisciplinary
studies in conjunction with foreign colleagues. In particular, these investigators have a long history of
successful participation in CD-funded research program in eastern Tibet, with Burchfiel as lead PI (see
Results of Prior NSF Funding). The study proposed here is of similar scope, duration and funding level to
the east Tibetan project, although logistics in Greece will be considerably less difficult. We note that it is
relatively unusual to have all (but one, Thorsten Becker) of the US investigators come from the same
institution. However, the MIT earth science department is large and diverse, providing an unusually broad
range of expertise within a single department. Our experience has been that, by having most of the
primary US investigators at MIT, we are able to achieve a high level of interaction and coordination
among PI’s due to proximity and daily interactions.
Collaborative Workshop/School in Greece, Years 1, 3 and 5
In order to maximize our collaborative efforts and coordinate the scientific and logistical aspects
of the proposed work, we plan to hold a two-week workshop in Greece in years 1, 3 and 5. This workshop
will function partly as a school to teach our Greek collaborators (and ourselves) about new ideas,
techniques and scientific approaches that are relevant to this project and to tectonics studies in general. It
is important that these workshops be held in Greece because it will enable many Greek geoscientists to
attend all or part of the workshop and will be easily accessible to our Italian colleagues. We particularly
aim to include Greek and Italian graduate students, some of whom will be associated with our study, but
some of whom will not be. We intend to have all of the US investigators and students associated with the
project attend each workshop, so that the workshop will also serve as a venue for the exchange of
scientific culture between US, Greek and Italian students as well as the more senior scientists.
We believe that these workshops will be most effective if held in a relatively isolated
environment where the participants will be away from their usual daily activities and can spend time
discussing science outside of the formal workshop hours. We have been offered the use of conference and
lodging facilities in a small village outside of Kalamata (in the Peloponnesus) by the vice Mayor of
Kalamata. This is within a half-day's drive of Athens and will be easily accessible to all participants. We
estimate approximately 20-30 participants for the workshop, the numbers varying depending on whether
we are discussing general scientific issues in tectonics (more participants) or the details of our
collaborative study (fewer participants). Travel to the workshop and lodging for participants will be
funded by each of the collaborative groups, so that we need only to fund travel and lodging for the US
investigators. Travel costs will be highly leveraged as many of the participants will be traveling to Greece
for field work at the same time.
Roles of Principle Investigators and Foreign Collaborators
As lead PI, Leigh Royden will be responsible for coordinating all aspects of the proposed work
(see above). In addition, she will conduct field geologic studies in conjunction with Clark Burchfiel and
Dimitrios Papanikalaou and modeling studies of subduction zone dynamics in conjunction with Thorsten
Becker, Claudio Faccenna and Francesco Funicello. She will also take the lead in tectonic analyses and
integration of data for the Hellenic region. Royden has had extensive experience with the tectonics of
Mediterranean subduction systems, conducted numerous modeling studies of continental deformation and
dynamics, and, although not primarily a field geologist, has conducted and supervised field geologic
studies in Greece, Tibet and elsewhere.
Clark Burchfiel will conduct geological field mapping studies, and together with Leigh Royden
and Dimitris Papanikalaou, be responsible for supervising student geologic mapping. He will also be
involved in tectonic analysis of the Hellenic system. Burchfiel has a long history of international field
mapping and regional tectonic analysis. He has worked in the Mediterranean/Balkan region off and on for
over 30 years.
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Robert King will be responsible for GPS measurements and analysis. He has worked extensively
with existing GPS data from the Aegean region.
Stephane Rondenay will be responsible for the proposed seismic arrays, processing of seismic
data, and integration of regional observations with global tomographic data. He has successfully
conducted a similar experiment in the Cascadia subduction boundary. Rondenay is a first-time NSF
investigator.
Ben Weiss will be responsible for laboratory analysis of paleomagnetic samples. He has much
laboratory experience. Jointly with Royden and Burchfiel, he will be responsible for citing and collecting
the paleomagnetic samples.
Sam Bowring will be responsible for geochronology, collecting samples in coordination with
Royden and Burchfiel, and for all U/Pb laboratory analyses. He is the director of the U/Pb facility at MIT
and has done extensive high resolution U/Pb dating as well as isotopic characterization of arc magmas.
Thorsten Becker will be responsible for dynamic modeling of subduction zones, along with Leigh
Royden and Claudio Faccenna. He is a post-doctoral fellow at Scripps Institute of Oceanography and has
written papers on the viscous dynamics of subduction zone roll back and trench migration, with particular
reference to the Mediterranean subduction systems. Becker, the single PI not at MIT, will be closely
integrated into the project through joint theoretical work with Leigh Royden and co-supervision of a PhD
student at MIT. Becker is a first-time NSF investigator.
Foreign Collaborators
Dimitrios Papanikalaou is our principle counterpart in Greece. He is Professor of Geology at the
University of Athens and the leading tectonician in Greece. Until September, 2002, he was the Secretary
of Civil Protection within the Greek government, prior to which he was director of the Greek National
Marine Research Center and head of the Earthquake Planning Office. He is primarily a field and
structural geologist. He will involve (and fund) graduate and undergraduate students from the University
of Athens in our field research, particularly the structural and stratigraphic mapping and tectonic analysis.
Charis Biliris and Dimitrios Paradisos are Professors of Geodesy at the National Technical
University in Athens. They are involved in virtually all GPS work in Greece. They have successfully
collaborated with King and other MIT scientists on past geodetic studies.
Professors Claudio Faccenna and Massimo Mattei and Dr. Francesca Funicello are our Italian
collaborators from the University of Rome, Three. Faccenna and Funicello will be responsible for analog
models of subduction systems, an extension of their ongoing work in this area. These analog experiments
will be conducted with strong coordination with the theoretical studies of Becker and Royden. Mattei will
be responsible for conducting paleomagnetic sampling and measurements in collaboration with the US
PI’s. Faccenna is a structural geologist and tectonician with extensive knowledge of the Mediterranean
region. Funicello has a background in numerical modeling of subduction at the ETH. Mattei is a
paleomagnetist with prior experience working in the young sedimentary rocks of central Greece.
Multi-Institutional Budget
MIT
Scripps
Year 1
$480,803
$22,751
Year 2
$440,035
$23,786
Year 3
$488,861
$24,873
Year 4
$436,310
$29,979
Year 5
$547,109
$31,376
Total
$2,393,178
$132,765
In addition to the funds requested in these proposals, our foreign counterparts will provide
resources for the proposed work. Papanikolaou will provide resources to fund himself and Greek
graduate students for field and structural mapping. He will also provide funds and personnel for the
routine maintenance of the seismic network. Faccenna will provide funding for analog and theoretical
modeling for himself and Funicello and will host project visitors in Rome for joint theoretical and analog
work. Mattei will provide additional resources for paleomagnetic fieldwork and measurements. Our
Greek counterparts will provide conference facilities for our bi-annual workshops (see below). Scientists
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from each country will fund themselves and their students for travel to and accommodation at these
workshops.
Education and Human Resources
Women are reasonably well represented among U.S. college students studying science, but at
every subsequent step along the educational corridor the percentage of women scientists drops. Within
US universities, the percentage of earth science faculty who are women is ~12%. At in the earth sciences
at MIT, 78% of the undergraduates are female, 42% of the PhD students are female, but only 8% of the
faculty are female. Many female scientists cite the choice between career and family as a reason for
opting out. The lead PI of this proposal, Leigh Royden, provides an example for the next generation of
female scientists that women can succeed as faculty in top research universities without giving up
children and family. More than half her former PhD students have been women; many of whom are now
faculty in other institutions.
A recent survey has shown that MIT PhD students hold more faculty positions in U.S.
universities than PhD's from any other department in the country. This speaks to the quality of the
doctoral education at MIT and the quality of the students who choose MIT for graduate work. This
proposal seeks funding for four PhD students over a 5-year period, which will fund these students from
start to finish of their graduate careers. Thus the proposed study will, if funded, contribute to the scientific
infrastructure of the U.S. by educating the next generation of research faculty.
The proposed study will also feed back into the graduate and undergraduate curriculum at MIT.
For example, unfunded collaborations between Leigh Royden and Dimitrios Papanikalaou led to a spring
lecture series on the geology of Greece and an MIT-supported two-week field trip to Greece for graduate
and undergraduate students in June of 2003. Undergraduates were fully funded by the department while
graduate students each contributed $200 toward expenses. Events like this are instrumental in drawing
undergraduates into the earth sciences The same unfunded work in Greece has provided for two senior
undergraduate thesis projects supervised by Leigh Royden, one completed in June ’03 and one currently
underway (both female students).
The proposed study is for an internationally collaborative study, involving senior geoscientists
and graduate students from the U.S., Greece and Italy. We plan a series of three workshops in Greece
throughout our 5-year study period. Although a crucial component of the workshops will be coordination
of the research efforts and results, much of the workshop will be devoted to discussion and lectures on
new scientific ideas, approaches and techniques. We will involve Greek scientists and students outside of
our immediate collaborators so that the workshop will become part school, part research effort. Indeed,
one of the best aspects of the MIT student trip to Greece was the interaction of U.S. and Greek students
throughout the trip, with a stimulating exchange of scientific and cultural ideas. Many of the US and
Greek students are still in email contact as a result of the friendships that developed during the field trip.
The Vice Mayor of Kalamata (southern Peloponnesus) has offered us conference facilities in a
secluded village on the outskirts of Kalamata because he feels strongly about building a high level of
scientific and cultural events within the city and surrounding areas. We anticipate that these workshops
will help to disseminate recent scientific advances to an international group of scholars and students, and
will make for strong collegial ties between U.S., Italian and Greek scientists.
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