CENOZOIC EXHUMATION OF THE WESTERN ANTARCTIC PENINSULA:
THERMOCHRONOLOGIC RESULTS FROM NORTHERN AND SOUTHERN
GRAHAM LAND
by
William R. Guenthner
A Prepublication Manuscript Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements
for the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
2009
Abstract
Several major tectonic changes affected the Antarctic Peninsula in the Cenozoic,
including cessation of subduction south of the Bransfield strait, Scotia arc plate
kinematics, and opening of the Drake Passage. To examine the effects of these events on
the exhumational history of the peninsula, we analyzed fission track and (U-Th)/He ages
of apatite and zircon from 14 sites from Graham Land, near the northern tip of the
peninsula to just north of the Antarctic Circle. Apatite He ages in two regions along the
western coast show strongly differing ages, indicating distinct tectonic histories. South of
Anvers Island apatite He ages are relatively young, ranging from 8 to 16 Ma, whereas to
the north they are 24 to 65 Ma, with one exception at 11 Ma. In the southern region,
apatite He ages are the oldest at the southernmost site and become younger to the north,
whereas in the northern region the oldest ages are at the northernmost site and become
younger to the south. Thermal histories inferred from relative ages and closure
temperatures of multiple thermochronometers also indicate distinct histories for northeast
and southwest Graham Land. Northern sites show a late Cretaceous pulse of rapid
cooling followed by very slow cooling to the present, whereas southern sites show either
a pulse of rapid mid-Miocene cooling or roughly constant cooling to the present. These
systematic spatial trends in apatite He ages and contrasting thermal histories along the
peninsula can be related to tectonic changes along the margin during the Cenozoic. South
of Anvers Island, progressive northward opening of a slab window tracks younging
apatite He ages and an apparent pulse of rapid mid-Miocene cooling to the north. This is
consistent with a time-transgressive pulse of ~2-3 km of uplift and erosion in the upper
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plate soon after ridge-trench collision, cessation of subduction, and opening of the slab
window, presumably due to increased asthenospheric upwelling beneath the plate.
Introduction
The Antarctic Peninsula preserves a geologic record of the numerous
events that have shaped the Cretaceous-Neogene tectonic history of the greater Scotia Sea
region. Previous research on the Cenozoic tectonic history of the peninsula has focused
primarily on either 1) the sea floor record of South America-Antarctic Peninsula
separation and opening of the Drake Passage and establishment of the Antarctic
Circumpolar Current (e.g. Eagles et al., 2006; Lawver and Gahagan, 2003; Livermore et
al., 2005), or 2) the evolution of the trench and sea floor that lies parallel to the peninsular
orocline (e.g. Bart and Anderson, 1995; Larter and Barker, 1991). Another important
part of the peninsula’s tectonic evolution is the transition from active subduction to a
passive margin along its western margin. The history of this transition is well constrained
by magnetic and age records in adjacent oceanic crust, and shows a southwest to
northeast progression from Miocene to present, implying a time-transgressive opening of
a slab window over the same interval under the peninsula. Slab window opening may be
associated with asthenospheric upwelling and rock uplift, increased heat flow, and other
phenomena in the upper plate (Delong et al., 1979; Dickinson and Snyder, 1979; Furlong
and Schwartz, 2004), making the time-transgressive record of it along the Antarctic
Peninsula well suited to observing slab-window dynamics. In this study we focus on the
apatite and zircon (U-Th)/He and fission-track thermochronologic record of rocks from
the western coast of Graham Land, which record spatial-temporal patterns cooling
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associated with exhumation and possibly heat flow changes correlated with the timetransgressive cessation of subduction and slab-window opening.
Methods
We collected 14 granitoid rock samples from a transect along the west coast of
Graham Land. Extensive snow and ice cover in most locations precluded collecting in
sub-vertical transects to observe age-elevation relationships, so our constraints on cooling
rates and interpretations of erosion through time come primarily from relative ages of
multiple thermochronometers with different closure temperatures. The four
thermochronometers used in this study are: 1) apatite (U-Th)/He (apatite He) with a
closure temperature of ~62 °C (Flowers et al., 2009), 2) zircon (U-Th)/He (zircon He)
with a closure temperature of ~170-200 °C (Reiners, 2005), 3) apatite fission track
(apatite FT) with a closure temperature of ~100-120 °C (Gleadow and Duddy, 1981), and
4) zircon fission track (zircon FT) with a closure temperature of ~220-260 °C (Brandon et
al., 1998) (closure temperatures calculated for cooling rate of 10 °C /Myr). Although we
recognize the potential for advection of heat at high erosion rates to bias exhumation
histories based on multiple thermochronometers (e.g. Moore and England, 2001), this is
likely a minor issue for the relatively slow exhumation rates inferred here.
Mineral separation was performed by standard procedures involving crushing,
sieving, and magnetic and density separations. Thirteen of the 14 samples yielded
sufficient amounts of apatite and zircon grains for analysis. (U-Th)/He analyses were
performed at Arizona following standard procedures described in Reiners et al. (2004).
Two to five single-grain analyses were made on each sample, using Nd:YAG and CO2
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laser heating, cryogenic purification, and quadrupole mass spectrometry for 4He analysis,
and isotope dilution and HR-ICP-MS for U, Th, and in the case of apatite, Sm analysis.
Alpha ejection corrections followed the approaches of Farley (2002) for apatite, and
Hourigan et al. (2005) for zircon. Fission track analyses were made at Arizona and Yale,
following methods described by Thomson and Ring (2006). Grains were irradiated at the
Oregon State University Triga Reactor, Corvallis, USA and IRMM540R and IRMM541
glasses were used to monitor neutron fluence; zeta calibration factors (Hurford and
Green, 1983) were 356.8±10.3 (IRMM540R apatite) and 121.3±2.6 (IRMM541 zircon).
Results
Tables 1 and 2 shows FT and He data, including weighted mean ages and
standard errors of replicate single-grain He ages, and central FT ages (Galbraith and
Laslett, 1993). Figure 1 shows the spatial distribution of apatite He ages, which range
from 8-64 Ma. The most striking feature of Figure 1 is that all apatite He ages south of
Anvers Island are Neogene (8-16 Ma), whereas, with one exception, all ages north of
Anvers Island are Paleogene (24-65 Ma). The one exception is sample Murray1, with an
apatite He age of 11 Ma. Another notable pattern in Figure 1 is that ages become
younger with decreasing distance from Anvers Island. South of Anvers Island, He ages
decrease systematically to the north, from 16 to 7.8 Ma, whereas north of it (with the
exception of Murray1) ages decrease systematically to the south, from 65 to 24 Ma.
Hawkes (1981) also drew a distinction between the regions southwest and northeast of
Anvers Island, based on on-shore structural architecture inferred from off-shore fracture
zones.
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Thermal histories interpreted from the relative ages and closure temperatures of
each thermochronometer also show regional differences (Fig. 2). We identify three
distinct types of cooling path; each one is restricted to either the northeastern or
southwestern regions. The first type involves rapid cooling at ~80 Ma, followed by slow
cooling to the present, and is restricted to the northeastern region (Andvord1,
Wilhelmina2, Murray1, and Roque1). The second type shows approximately steady
cooling from the oldest thermochronometric age to the present, and is restricted to the
southwestern region (PaulingA, Bellue1, and Petermann1). The third type involves an
initial phase of slow cooling, followed by a pulse of rapid cooling (~15 Ma), and is only
observed for one sample (Py1), also in the southwestern region. The thermal histories of
two samples (Roque2 and Tuxen1) lack constraints from apatite He ages, and cannot be
adequately classified. The three remaining samples, Lahille West, Palmer1, and Hope3,
have apatite FT ages older than their corresponding zircon He age. These discrepancies
cannot be explained, at least in any straightforward way, by typical culprits such as parent
nuclide zonation, mineral inclusions, or analytical errors, and we do not have a
satisfactory explanation.
Discussion
We seek to explain three primary observations of the thermochronologic data: 1)
young apatite He ages south of Anvers Island (Fig. 1); 2) decreasing apatite He ages
towards Anvers Island from both the south and north; and 3) the distinct thermal histories
of samples south and north of Anvers Island (slow Cretaceous to modern cooling to the
north, and relatively rapid cooling to the south).
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One potential tectonic explanation relates to the history of ridge-trench migration
along the peninsula. Larter and Barker (1991) established the age of sea floor magnetic
anomalies adjacent to the remnant Antarctic-Phoenix subduction zone and inferred the
timing of collision between ridge crest segments of the Antarctic-Phoenix plate ridge
boundary and the trench on the Pacific margin of the Peninsula. As first described by
Dickinson and Snyder (1979), ridge collision and the subsequent cessation of subduction
leads to development of a lithospheric slab window beneath the upper plate. The
northward younging trend of trench-adjacent sea floor is evidence that slab window
opening proceeded in a similar fashion. In the present day configuration, the boundary
between the active subduction zone and passive margin lies along the Hero Fracture
Zone. South of this fracture zone, the sea floor ages preserve a history of ridge cresttrench collision with the youngest sea floor age adjacent to the currently passive trench
dating these events. From these observations, Larter and Barker (1991) deduced that
ridge crest collision was nearly orthogonal to the trench, occurred in a step-wise fashion
as each section of the spreading ridge was offset by fracture zones, and became
progressively younger from the southern part of the peninsula towards the Hero Fracture
Zone. Spatial-temporal patterns of alkalic basalts from both trench proximal (Hole and
Larter, 1993) and on-land locations (Hole, 1988) have also been interpreted in the context
of northward slab window development along the Peninsula. We propose that the
progressive northward opening of a slab window beneath the southwestern region of
Graham Land was accompanied by a time-transgressive northward progression of rock
uplift and erosional exhumation, possibly accompanied by increased basalt heat flow.
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Replacement of cold subducting lithospheric slab with hot sub-slab asthenosphere
beneath the overriding plate in the slab window may have several upper-plate
manifestations. These include changes in the volume, location, and geochemistry of arc
magmatism (Cole and Basu, 1992; Dickinson, 1997; Hole, 1988; Hole and Larter, 1993);
increased heat flux at the base of the upper plate (Delong et al., 1979; Iwamori, 2000);
and initiation of rock uplift (Lock et al., 2006; Ramos, 2005). Mechanisms that cause
rock uplift include increases in shortening rates at the time of ridge-trench collision
(Ramos, 2005), isostatic responses to lower crustal thickening (Cloos, 1993), dynamic
responses to mantle flow (Dickinson and Snyder, 1979), or a combination of isostasy and
dynamic flow (Furlong and Govers, 1999).
Migrating ridge-trench collisions and a northward-opening slab window would be
expected to produce a northward progression of rock uplift south of Anvers Island, the
modern boundary between active subduction and the passive margin. Apatite He ages do
show a northward younging that parallels that of sea floor ages (Fig. 3, Larter and Barker,
1991). In addition, the abrupt change in apatite He ages from Py1 to Andvord1 matches
well with the position of the South Anvers Fracture Zone. The tectonic reconstructions in
figure 4 further emphasize this geographical correlation. As ridge crest-trench collision
diachronically progressed, sites in the southwest passed through the closure temperature
for apatite He at times that roughly coincided with the spatially equivalent collision
event.
Model t-T Histories
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By interpreting the cooling trends for our samples in the context of a slab
window, we can construct a schematic plot of the t-T histories for the northeastern and
southwestern regions since the Cretaceous (Fig. 5a). At ~80 Ma, samples now at the
surface in the northeast were exhumed to shallower depth than those now at the surface in
the southwest, and both regions experienced roughly similar exhumation rates until the
Miocene. In the Miocene, opening of the slab window and ensuing asthenospheric
upwelling in the southwest would have produced uplift and erosional exhumation. This
phase of recent rapid cooling may have also been accompanied by an increase in basal
heat flux. In order to estimate the amount of slab window-induced exhumation
necessary to produce the spatial patterns in apatite He ages and thermal histories, we
inversely modeled t-T histories for 9 samples (those with apatite He ages and logical
cooling paths) using the HeFTy software package (Ketcham, 2005). As shown below,
our analysis demonstrates that, whereas increased exhumation rates due to slab window
activity are of seemingly primary importance, some amount of increased basal heat flux
is also required to explain the data.
We followed an iterative approach to modeling the cooling ages with HeFTy
whereby we determined constraints for the inverse model of a given sample by
attempting to both match the modeled cooling trend with the observed cooling trend and
maximize the number of viable cooling paths (see appendix for method details). For the
northeastern sites (Roque1, Murray1, Wilhelmina2, and Andvord1), HeFTy generated
numerous viable t-T paths from our constraints (Fig. 5b), demonstrating that our
interpretations of cooling trends for this domain are reasonable.
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Results from the models of the southwestern sites (PaulingA, Bellue1,
Petermann1, Palmer1, and Py1) depict a more complex picture (Fig. 5b). From the
forward modeling we determined that exhumation of ~2 to 3 km in a late phase of erosion
could adequately explain the combined data. However, of the five samples, only
PaulingA showed viable paths using an inverse model of 2 to 3 km of exhumation at the
time of ridge crest-trench collision. Most required some degree of non-monotonic
reheating. Results from another series of forward models show that for the 2 km
exhumation end member, a basal temperature increase of ~400 °C (a 42 °C/km
geothermal gradient) is needed, and for the 3 km exhumation end member, a basal
temperature increase of ~100 °C (a 28 °C/km geothermal gradient) is needed. The time
interval over which this reheating takes place was also obtained from forward modeling
and ranges ~5 Myr after ridge crest-trench collision. This time and temperature range,
when translated into constraints in the inverse models, gives viable cooling paths for the
four remaining samples. Various scenarios for the rate at which both exhumation and
reheating take place are possible, however the consensus appears to be that exhumation
rates increase following initial slab window development. One caveat to these data is
that HeFTy cooling paths for the sample Palmer1 were constructed by disregarding the
incongruous zircon He age and should therefore be interpreted with some caution.
Despite this, the general modeled pattern of cooling in the southwestern domain consists
of a phase of relatively slow cooling from 80 Ma to the time of ridge crest-trench
collision, followed by either 2 to 3 km of exhumation at an increased rate (PaulingA), or
2 to 3 km of exhumation at an increased rate with a 100° to 400 °C increase in basal heat
flux (Bellue1, Petermann1, Palmer1, and Py1).
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We recognize that the thermal and erosional histories derived from a combination
of forward and inverse HeFTy models are not unique solutions but rather provide a
viability tests for the hypotheses of increased exhumation rates at about the time of ridgetrench collision. However, these models also suggest that explaining the relatively young
ages in the southwest with less than ~2 km of erosional exhumation would require a
transient heating pulse producing geothermal gradients in excess of ~60 °C/km, and
unrealistically large increases in basal heat flux.
Outstanding Issues and Complexities
Although a model of slab window induced uplift and increased heat flux accounts
for the general apatite He age patterns in both domains, a few subtleties require closer
examination. Notably, whereas ridge crest-trench collision occurred north of the South
Anvers Fracture Zone, apatite He ages at spatially equivalent sites are older than the neartrench sea floor ages and therefore do not record a response to a slab window (Fig. 3).
Once again, the distribution of sea floor ages documented by Larter and Barker (1991)
offer an explanation. Shortly after the 10 Ma collision of the ridge crest segment between
the North and South Anvers Fracture Zone, the remaining vestiges of the Phoenix plate
underwent clockwise rotation (Fig. 4). Because of the young, and therefore relatively
buoyant, oceanic lithosphere involved in subduction, rotation may have been an
expression of large intra-plate contrasts in the buoyancy of the subducting slab (Cloos,
1993). Such buoyancy contrasts could have led to a break in the down-dip portion of the
slab. This slab break left behind young plate segments that were difficult to subduct and
their tectonic resilience possibly shielded the upper plate from an influx of asthenospheric
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material, thus preventing the opening of a window. Regardless of cause, changes to the
subduction zone geometry took place north of Anvers Island as evidenced by sea floor
ages. Any alteration of this geometry could have also influenced the thermochronologic
record of the slab window.
Variable extents of glaciation along the Antarctic Peninsula are another
mechanism that could potentially cause differential exhumation. Changes to the
Chemical Index of Alteration of the sedimentary record on James Ross Island suggests
that the Antarctic Peninsula, like much of West Antarctica, was heavily glaciated by the
early Oligocene (Dingle and Lavelle, 1998), and continental shelf stratigraphy from the
peninsula shows that sea ice extent—and possibly on-land glacial activity—was episodic
and locally variable throughout the Miocene (Bart and Anderson, 1995). Localized
exhumation due to differential glaciation may account for anomalous apatite He ages in
otherwise systematic trends along the orocline—Murray1 for example—and we cannot
dismiss the possibility that other sites were likewise affected.
In the southwestern domain the difference between the apatite He age and its
geographically equivalent sea floor age varies from ~6 to 0 My (Fig. 3). This may mean
that the timing and rate of uplift and exhumation due to slab window opening was
variable. Part of this time lag may be the result of delay in reheating at the depth of
present exposure. Furthermore, without other types of constraints we cannot robustly
constrain rates of exhumation during the inferred uplift-erosion episodes. Future
thermochronologic studies in the area that focus on ages from strategically located
vertical transects may be able to address this issue.
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Thermochronologic approaches to constraining upper plate responses to the
opening of a slab window may be relevant in other settings. We have shown spatial and
temporal correlations in exhumation patterns with the tectonic evolution of the trench
margin. More focused sampling could potentially reveal exhumation rates through time
(e.g., with vertical transects), providing a more complete understanding of these
correlations. Because our ability to make connections between regional
thermochronology and ridge crest-trench collision is a consequence of detailed sea floor
analysis, our methods may offer geodynamic insights in areas with similarly well-defined
margin histories and trench parallel topography (e.g. Furlong and Schwartz, 2004). In
this manner, thermochronology provides quantitative analyses of near surface processes
that may open a window into asthenospheric dynamics.
Conclusion
Thermochronologic data from western Graham Land reveal two domains with
distinct Cenozoic thermal histories, corresponding to their response to tectonic transitions
associated with a time-transgressive shift from subduction to passive margin and opening
of a slab window beneath the region. Apatite He ages show systematic trends along the
orocline, with younger ages in the southwestern region and a gradual younging towards
the latitude of Anvers Island, just north of which ages are significantly older. In addition,
samples from the northeastern region show rapid cooling in the late Cretaceous followed
by very slow cooling to the present, whereas the southwestern samples show either steady
cooling since the late Cretaceous or slow Paleogene cooling followed by rapid Neogene
cooling. Although tectonic segmentation of the Antarctic Peninsula has been previously
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suggested (Hawkes, 1981; Johnson and Swain, 1995), we present new data that both
demonstrates differential exhumation across peninsular domains, and documents the late
Cenozoic interactions of the Antarctic and remnant Phoenix plates. Principally, we
suggest that the opening of a slab window in the southwestern domain caused 2-3 km of
exhumation at an increased rate due to rock uplift associated with asthenospheric
upwelling. However, we also cannot rule out some contribution from increased heat flux
to the observed cooling histories in the southwestern domain. These techniques could be
applied to other regions with comparable plate margins in an attempt to better understand
lithosphere scale processes.
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Figure Captions
Figure 1: Geologic map of the northern Antarctic Peninsula with sample locations,
names, and apatite He ages. Ages are reported as weighted mean ages of three to six
single-grain aliquots with corresponding 2 weighted errors. Solid black line denotes
inferred domain boundary.
Figure 2: Time-temperature plots for all 13 sites from both the northeastern and
southwestern domains. The three panels are divided by geographic location: the top
panel show all northeastern sites, the middle panel shows all sites south of the Biscoe
Fracture Zone, and the bottom panel shows all sites between the Biscoe Fracture Zone
and the S. Anvers Fracture Zone. Each point corresponds to a different
thermochronologic system: apatite and zircon (U-Th)/He or apatite and zircon fission
track. (U-Th)/He ages are reported as weighted mean ages with 2 standard errors, while
fission track ages are central ages quoted with 2 errors. Closure temperatures and
corresponding error bars were calculated using Mark Brandon’s CLOSURE computer
program. Closure temperature error was estimated based upon the full range of closure
temperature obtained from an estimated range of possible cooling rates through the
pertinent interval.
Figure 3: Apatite He age trend (in black) and ridge crest age trend (in gray) along-strike
of western Graham Land. Sea floor ages are after Larter and Barker (1991). Solid
rectangles indicate times of oblique ridge crest-trench collision. Inset shows trends for
only the southwestern domain.
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Figure 4: Antarctic Peninsula subduction zone at three different times based on sea floor
data of Larter and Barker (1991). Ages in white boxes are approximate apatite He ages
for various sites and are displayed only if the system has passed through its closure
temperature for a given time. Solid black rectangles with arrows symbolize actively
spreading ridges, solid black triangles symbolize active subduction, white triangles
symbolize an inactive subduction zone. Ridge crest ages are displayed along the passive
subduction zone in the final time box. Note the rotation of ridge crests at 10 Ma.
Figure 5: A) Schematic diagram of exhumation along strike of the Antarctic Peninsula.
Dashed line under the topography represents the apatite (U-Th)/He closure isotherm.
Arrows denote exhumation, opening of the slab window, or bending of this isotherm
towards the surface due to basal heat flux. Plots directly beneath each time slice
represent the expected t-T history for a sample in either the northeastern or southwestern
region, given the tectonic conditions. B) Results from HeFTy inverse models. Panels are
arranged in the same format as discussed in Fig. 2B and each shows the cumulative
envelops for all samples in a given geographic location. Opacity of the envelopes is
based upon whether the envelope contains acceptable paths (goodness of fit greater than
.05, lighter shade) or good paths (goodness of fit greater than .5, darker shade). See
Ketcham (2005) for more details on HeFTy statistical methods.
Appendix
HeFTy methods
Our inverse modeling in HeFTy consisted of applying time and temperature
constraint boxes to a Monte Carlo simulation with 10,000 iterations. Sample from sites
in the northeastern domain were constrained by boxes at 87-75 Ma by 300-180 °C, 85-55
Ma by 120-20 °C, 40-15 Ma by 80-20 °C, and a final point at 0 Ma and 0 °C.
Samples from sites southwest of Anvers Island were constrained in a slightly
different fashion. We retained the highest temperature/oldest time constraint from the
northeastern samples, however, because we were interested in examining the geodynamic
effects of a slab window on cooling trends from these southwest sites, revised model
constraints consisted of a specific depth-dependent temperature range placed at the time
interval of spatially equivalent ridge crest-trench collision (16.5 Ma for PaulingA and
Bellue1; 13.5 Ma for Petermann1, Py1, and Palmer1). For example, in order to model the
16
16
effect of 2 km exhumation on PaulingA, a temperature corresponding to a depth of 2 km
would be calculated and that temperature constraint positioned at 16.5 Ma. To this end,
we constructed a simple crustal geotherm using the equation for steady-state, depthdependent temperature with an additional term for radiogenic heat production (Turcotte
and Schubert, 2002). For variables, we chose values of 25 km2 * Myr-1 for thermal
diffusivity, 1 °C for heat production, 750 °C for basal temperature, and 30 km for layer
thickness (corresponding to a geothermal gradient of 25 °C/km). Then, we determined
the appropriate range of exhumation from a series of forward model experiments in
which we held the time constraint constant but varied the temperature. Once this range
was established, constraint boxes were designed accordingly. If the model did not
produce viable paths from these boxes (i.e. re-heating was necessary), then additional
boxes were constructed through a trial-and-error, forward modeling approach.
17
17
55°W
60°W
65°W
70°W
60°S
Upper Mesozoic and Cenozoic Magmatic Arc
Hope3
AHe:
Mesozoic Metamorphosed Accretionary Prism
64.7 ± 1.3 Ma
Roque1
AHe:
Back-arc Sedimentary Basin
34.6 ± 1.7 Ma
Recent Extensional Volcanics
Wilhelmina2
AHe:
Murray1
AHe:
10.9 ± 0.7 Ma
29.3 ± 0.7 Ma
Andvord1
AHe:
23.9 ± 0.8 Ma
Py1
AHe:
Palmer1
AHe:
7.9 ± 0.3 Ma
7.8 ± 0.2 Ma
Petermann1
AHe:
11.1 ± 0.9 Ma
Bellue1
AHe:
9.6 ± 0.3 Ma
PaulingA
AHe:
15.7 ± 1.6 Ma
67°S
18
0
20
40
60
80
Temperature (°C)
100
120
140
Hope3
Roque1
Roque2
Murray1
Wilhelmina2
Andvord1
160
180
200
220
240
260
280
300
0
20
40
60
80
Temperature (°C)
100
120
140
160
Tuxen1
180
Lahille West
200
Bellue1
220
PaulingA
240
260
280
300
0
20
40
60
80
Temperature (°C)
100
120
140
160
180
200
220
Palmer1
Py1
Petermann1
240
260
280
300
100
90
80
70
60
50
40
30
20
10
0
Time (Ma)
19
18
Biscoe FZ
16
PaulingA
70
Age (Ma)
60
14
S. Anvers FZ
Hope3
12
N. Anvers FZ
Petermann1
10
Bellue1
8
Palmer1
Py1
50
Age (Ma)
6
40
4
-10
40
SW
90
140
190
240
290
340
Distance Along Strike (km)
NE
Roque1
Wilhelmina2
30
Andvord1
20
Biscoe FZ
S. Anvers FZ
PaulingA
10
N. Anvers FZ
Petermann1
Bellue1
Murray1
C FZ
Hero FZ
Palmer1
Py1
0
-10
SW
90
190
290
390
490
590
690
NE
Distance Along Strike (km)
20
HFZ
13.5 Ma
10.0 Ma
HFZ
CFZ
CFZ
NAFZ
NAFZ
SAFZ
BFZ
SAFZ
~65 Ma
~35 Ma
~65 Ma
~35 Ma
~11 Ma
~29 Ma
~24 Ma
BFZ
~29 Ma
~24 Ma
~11 Ma
~10 Ma
~16 Ma
~16 Ma
N
0
N
0
100 Km
100 Km
Present
HFZ
CFZ
Bransfield Strait Rift
NAFZ
5.5 Ma
3.1 Ma
SAFZ
BFZ
6.0 Ma
5.6 Ma
10.0 Ma
13.5 Ma
14.5 Ma
~8 Ma
~11 Ma
~35 Ma
~65 Ma
~11 Ma
~29 Ma
~24 Ma
~8 Ma
16.5 Ma
~10 Ma
~16 Ma
N
0
100 Km
21
A.
Slab Window
0
0
0
20
20
20
40
40
40
60
60
120
140
160
180
200
220
240
260
280
80
Temperature (°C)
100
100
120
140
160
180
200
220
240
260
280
70
60
50
40
30
20
10
0
300
100
100
120
140
160
180
200
220
240
260
280
90
80
70
Time (Ma)
60
50
40
30
20
10
300
100
0
90
Time (Ma)
B.
80
70
60
50
40
30
Time (Ma)
0
20
40
60
80
Temperature (°C)
80
100
120
140
160
180
200
220
240
260
280
300
0
20
40
60
80
Temperature (°C)
90
100
120
140
160
180
200
220
240
260
280
300
0
20
40
60
80
Temperature (°C)
300
100
60
80
Temperature (°C)
80
Temperature (°C)
Slab Window
100
120
140
160
180
200
220
240
260
280
300
100
90
80
70
60
50
Time (Ma)
40
30
20
10
0
22
20
10
0
TABLE 1. APATITE (U-Th)/He DATA AND ZIRCON (U-Th)/He DATA
Sample
Name
Sample
Type
Hope3aB
Hope3aC
Hope3aD
Hope3aE
Hope3aF
Hope3
Ap
Ap
Ap
Ap
Ap
Ap
Roque1aA
Roque1aB
Roque1aC
Roque1aE
Roque1
Ap
Ap
Ap
Ap
Ap
Murray1aA
Murray1aB
Murray1aC
Murray1
Ap
Ap
Ap
Ap
Wilhelmina2aA
Wilhelmina2aB
Wilhelmina2aC
Wilhelmina2
Ap
Ap
Ap
Ap
Palmer1aA
Palmer1aF
Palmer1aC
Palmer1aD
Palmer1
Ap
Ap
Ap
Ap
Ap
Py1aA
Py1aB
Py1aC
Ap
Ap
Ap
Lat.
(°S)
Long.
(°W)
Mass Halfwidth
(μg)
(μm)
U
(ppm)
Th
(ppm)
4
Sm
He
(ppm) (nmol/g)
1.98
0.57
0.95
1.36
1.54
41.18
49.04
70.67
42.88
46.92
35.44
38.60
64.89
34.04
47.54
261.38
246.50
344.51
255.81
283.41
41
28
36
38
42
12.032
9.921
17.351
11.516
14.536
Ft
0.67
0.53
0.60
0.63
0.68
Corr. Age Analyt. ± Weight. Mean Age Std. ±
(Ma)
(2)
(Ma)
(2)
67.1
59.6
62.1
65.8
68.0
2.4
2.9
3.1
3.2
3.2
63 24.475 057 3.386
1.32
1.46
0.80
0.64
36
40
34
31
5.80
7.49
5.91
6.47
9.38
10.94
18.37
11.37
129.06
145.66
131.02
86.82
0.794
1.691
0.793
0.650
0.62
0.64
0.57
0.57
29.0
47.9
24.9
22.7
40
38
32
22.23
12.04
13.20
55.41
30.51
36.81
419.83
412.88
474.91
1.228
0.863
0.886
0.64
0.65
0.58
9.9
12.4
12.6
37
34
41
26.31
34.17
41.46
67.57
39.18
83.14
598.19
484.64
645.69
4.424
4.080
6.520
0.65
0.60
0.66
29.3
28.8
29.7
44
34
29
36
78.43
6.16
43.19
40.54
111.13
15.48
77.93
74.06
206.37
252.16
166.77
138.29
3.139
0.232
1.410
1.493
0.70
0.60
0.53
0.61
8.0
7.0
7.9
7.9
43
38
44
50.75
33.80
47.50
102.73
86.47
106.01
105.88
116.30
191.33
2.046
1.324
2.124
0.63
0.63
0.67
8.0
7.2
8.1
10.9
0.7
29.3
0.8
7.9
0.3
0.3
2.2
0.9
0.7
64 46.428 063 35.392
1.35
0.99
2.07
1.7
1.2
1.3
1.2
64 33.672 061 54.244
2.14
0.82
0.64
1.12
34.6
0.9
1.1
2.6
64 21.319 061 36.698
1.17
1.02
1.29
1.3
3.1
2.8
4.4
4.6
63 31.505 058 59.851
1.11
1.53
0.57
64.7
0.4
0.4
0.4
23
TABLE 1. (CONTINUED)
Py1
Ap
Andvord1aA
Andvord1aB
Andvord1aC
Andvord1aE
Andvord1aF
Andvord1
Ap
Ap
Ap
Ap
Ap
Ap
Petermann1aB
Petermann1aC
Petermann1aE
Petermann1
Ap
Ap
Ap
Ap
Bellue1aA
Bellue1aB
Bellue1aC
Bellue1
Ap
Ap
Ap
Ap
PaulingAa1
PaulingAa2
PaulingAa3
PaulingA
Ap
Ap
Ap
Ap
Hope3zA
Hope3zB
Hope3zC
Hope3
Zrc
Zrc
Zrc
Zrc
Roque1zA
Roque1zB
Roque1zC
Roque1zD
Roque1zE
Roque1
Zrc
Zrc
Zrc
Zrc
Zrc
Zrc
64 52.604 063 35.392
0.80
0.89
1.05
0.75
0.51
32
35
36
34
27
30.41
34.15
16.79
11.07
10.02
25.31
33.91
20.42
15.43
11.87
267.24
262.74
196.41
229.19
188.12
2.952
4.506
1.281
1.198
0.924
0.60
0.62
0.63
0.60
0.51
24.8
31.8
17.3
24.6
25.8
37
34
32
8.47
6.61
6.60
22.02
21.38
24.51
328.08
309.50
523.33
0.557
0.371
0.489
0.61
0.60
0.57
12.1
9.5
12.1
36
34
34
28.96
54.52
22.09
61.30
102.27
42.41
180.98
198.44
250.78
1.613
2.178
1.373
0.62
0.60
0.61
11.1
8.5
12.9
32
39
27
12.75
3.55
17.59
33.43
18.34
42.04
33.08
121.00
17.53
1.147
0.331
1.038
0.59
0.63
0.50
17.3
12.2
13.9
43
37
43
220.02
262.41
316.19
195.09
199.88
266.55
0
0
0
67.411 0.76
83.446 0.71
93.736 0.73
61.4
70.3
62.2
63 31.505 058 59.851
38
42
37
41
36
270.30
789.13
972.74
331.10
639.55
142.31
254.97
367.46
152.72
264.12
0
0
0
0
0
93.266
261.520
375.083
124.766
224.756
0.73
0.77
0.74
0.76
0.74
77.6
74.3
88.8
82.1
79.7
0.9
9.6
0.3
15.7
1.6
64.1
1.8
80.1
1.4
3.0
3.5
3.1
63 24.475 057 3.386
2.96
5.08
3.63
5.54
4.40
11.1
2.1
3.6
4.1
66 31.339 065 52.458
4.65
2.21
2.88
0.8
0.7
0.4
1.0
66 17.673 065 52.458
0.87
1.06
0.36
23.9
1.3
1.5
2.7
65 10.542 064 8.419
1.30
0.83
0.69
0.2
1.8
1.6
1.3
2.5
4.1
64 53.418 062 32.652
1.16
0.80
0.66
7.8
3.3
3.3
4.0
2.9
2.9
24
Roque2zA
Roque2zB
Roque2zC
Roque2
Zrc
Zrc
Zrc
Zrc
Murray1zA
Murray1zB
Murray1zC
Murray1zD
Murray1zE
Murray1
Zrc
Zrc
Zrc
Zrc
Zrc
Zrc
Wilhelmina2zA
Wilhelmina2zB
Wilhelmina2zC
Wilhelmina2zD
Wilhelmina2zE
Wilhelmina2
Zrc
Zrc
Zrc
Zrc
Zrc
Zrc
Palmer1zA
Palmer1zB
Palmer1zC
Palmer1
Zrc
Zrc
Zrc
Zrc
Py1zB
Py1zC
Py1
Zrc
Zrc
Zrc
Andvord1zA
Andvord1zB
Andvord1zC
Andvord1zD
Andvord1zE
Andvord1
Zrc
Zrc
Zrc
Zrc
Zrc
Zrc
2.74
8.19
4.78
39
47
41
TABLE 1.
362.47
531.65
226.18
(CONTINUED)
201.36
0
184.66
0
137.78
0
162.212 0.73
185.506 0.79
104.619 0.76
100.2
75.1
98.3
4.3
3.2
4.2
63 32.901 058 58.169
2.37
6.31
3.43
2.50
1.79
39
53
42
38
33
115.64
126.88
171.67
126.71
101.13
113.46
142.85
210.88
108.65
122.85
0
0
0
0
0
41.272
54.395
83.593
40.158
39.546
0.72
0.79
0.74
0.72
0.69
74.8
79.4
94.2
67.6
81.9
40
34
46
41
35
499.83
1428.35
407.83
942.69
1489.93
230.82
992.25
212.66
411.21
938.85
0
0
0
0
0
179.374
424.183
171.798
351.440
529.332
0.74
0.71
0.78
0.76
0.72
80.3
66.2
89.0
81.6
79.6
53
43
40
321.72
275.45
170.72
413.98
1219.76
560.59
0
0
0
16.708 0.79
14.849 0.72
8.465 0.71
9.4
6.7
7.3
34
47
602.73
155.87
905.20
87.85
0
0
50.446 0.69
10.847 0.78
16.6
14.7
64 53.418 062 32.652
33
38
35
43
36
477.08
757.49
1074.94
928.76
1386.14
210.44
291.49
344.75
311.16
519.01
0
0
0
0
0
149.465
249.796
431.477
342.886
547.239
0.72
0.72
0.72
0.78
0.72
73.1
77.1
95.4
80.6
92.9
77.5
1.4
7.6
0.2
15.7
0.4
82.6
0.7
0.6
0.6
64 52.604 063 35.392
3.10
2.59
3.04
8.67
2.86
1.3
0.5
0.3
0.5
64 46.428 063 35.392
1.95
5.96
77.2
3.2
2.5
3.6
3.3
3.0
64 33.672 061 54.244
6.08
3.09
2.79
2.2
3.0
3.2
3.8
2.4
2.9
64 21.319 061 36.698
3.62
2.70
5.65
5.80
3.03
87.8
3.0
3.1
3.7
3.2
3.4
25
Petermann1zA
Petermann1zB
Petermann1zC
Petermann1zD
Petermann1zE
Petermann1
Zrc
Zrc
Zrc
Zrc
Zrc
Zrc
Tuxen1zA
Tuxen1zB
Tuxen1zC
Tuxen1
Zrc
Zrc
Zrc
Zrc
Lahille WestzA
Lahille WestzB
Lahille West
Zrc
Zrc
Zrc
Bellue1zA
Bellue1zB
Bellue1zC
Bellue1zD
Bellue1
Zrc
Zrc
Zrc
Zrc
Zrc
1.53
1.82
2.01
2.93
3.20
32
34
34
40
39
TABLE 1.
305.47
363.15
151.34
166.74
170.66
(CONTINUED)
217.03
0
104.48
0
111.26
0
140.44
0
111.06
0
53.341
57.000
43.478
35.964
31.336
0.67
0.69
0.70
0.73
0.73
41.1
39.3
65.0
45.4
40.4
2.1
2.0
4.0
1.8
1.7
65 10.542 064 8.419
5.12
43
118.49
212.73
0
34.511
0.76
49.8
1.5
1.72
33
68.38
66.81
0
16.916
0.69
54.3
1.4
2.09
29
124.63
175.64
0
32.831
0.67
54.3
1.3
65 16.997 064 06.863
4.16
2.27
42
35
228.02
651.99
402.59
673.09
0
0
43.387 0.75
150.168 0.70
33.1
48.7
66 17.673 065 52.458
49
42
46
60
607.72
299.44
142.60
422.51
314.45
162.64
161.16
249.53
0
0
0
0
118.536
74.616
43.294
98.187
0.78
0.75
0.76
0.83
41.4
54.5
58.4
45.4
0.9
52.3
1.3
37.4
1.0
48.3
0.9
1.2
2.0
65 32.084 064 25.015
5.14
3.66
4.18
15.51
42.8
1.6
2.1
2.1
1.7
26
TABLE 2. APATITE AND ZIRCON FISSION TRACK DATA*
Track Density
(x 106 tr cm-2)
Sample
Name
Sample
Type
Number of
Grains
Lat.
(°S)
Long.
(°W)
s(Ns)
i(Ni)
d(Nd)
Age
Dispersion
Central Age
(Ma ± 2)
(P2, %)
Hope3
Ap
20
63 24.475
057 3.386
1.807 (380) 5.748 (1209) 1.558 (4862) <0.01 (>99)
86.8 ± 11.6
Hope3
Zrc
20
63 24.475
057 3.386
4.719 (2686) 1.527 (869) 0.4848 (3026) 1.36 (76.8)
90.3 ± 8.8
Roque1
Ap
20
63 31.505
058 59.851
0.2784 (45)
1.578 (255) 1.395 (4354) <0.01 (>99)
43.8 ± 14.4
Roque1
Zrc
20
63 31.505
058 59.851 7.807 (2485) 2.507 (798) 0.4677 (2919) 4.65 (45.1)
87.6 ± 8.8
Murray1
Ap
20
64 21.319
061 36.698 0.4971 (121) 1.742 (424) 1.512 (4717) <0.01 (>99)
76.5 ± 16.6
Murray1
Zrc
20
64 21.319
061 36.698 3.833 (1122) 1.493 (437) 4.799 (2995)
5.35 (47.6)
74.4 ± 9.6
Wilhelmina2
Ap
20
64 33.672
061 54.244 0.8551 (341) 2.492 (994) 1.325 (4136) <0.01 (>99)
80.6 ± 11.4
Wilhelmina2
Zrc
20
64 33.672
061 54.244 12.93 (2462) 4.303 (819) 0.4604 (2873) 0.38 (79.1)
83.4 ± 8.2
Palmer1
Ap
20
64 46.428
063 35.392 0.4881 (145) 5.669 (1684) 1.488 (4644) <0.01 (>99)
22.8 ± 4.2
Palmer1
Zrc
20
64 46.428
063 35.392 2.271 (1634) 2.785 (2004) 0.4775 (2980) <0.01 (>99)
23.6 ± 2.0
Py1
Ap
20
64 52.604
063 35.392
2.564 (48)
4.214 (789) 1.419 (4427) <0.01 (>99)
15.4 ± 4.6
Py1
Zrc
20
64 52.604
063 35.392 3.136 (2204) 2.056 (1445) 0.4702 (2934) 1.45 (62.4)
43.4 ± 3.8
Andvord1
Ap
20
64 53.418
062 32.652 0.4617 (119) 1.951 (503) 1.605 (5007) <0.01 (>99)
67.4 ± 14.4
Andvord1
Zrc
20
64 53.418
062 32.652 21.58 (3610) 7.055 (1180) 0.4897 (3056) 5.52 (38.9)
89.8 ± 8.2
Petermann1
Ap
20
65 10.542
064 8.419
0.2062 (48)
1.731 (403) 1.442 (4499) <0.01 (>99)
30.6 ± 9.6
Petermann1
Zrc
20
65 10.542
064 8.419
3.714 (758)
2.984 (609) 0.4726 (2950) <0.01 (>99)
35.6 ± 4.2
Bellue1
Ap
3
66 17.673
065 52.458
0.3924 (12)
1.831 (56)
1.581 (4935) <0.01 (69.0)
60.2 ± 38.4
Bellue1
Zrc
20
66 17.673
065 52.458 7.686 (3631) 2.828 (1336) 0.4873 (3041) 0.64 (63.8)
79.8 ± 6.8
PaulingA
Ap
20
66 31.339
065 52.458 0.2087 (159) 1.108 (844) 1.465 (4572) <0.01 (>99)
49.1 ± 9.0
PaulingA
Zrc
3
66 31.339
065 52.458
7.090 (177)
2.283 (57) 0.4751 (2965) <0.01 (91.1)
88.9 ± 27.6
*Analyses performed by external detector method using 0.5 for the 2/4 geometry correction factor. Ages calculated using dosimeter glass: CN5 with CN5 = 356.1 ± 15.3. P2 is the probability of obtaining a 2 value for v degrees of freedom where v is number of crystals – 1.
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