Mass Wasting at the Easternmost Cyprus Arc,
Off Syria, Eastern Mediterranean
E. Tahchi, R. Urgeles, C. Hübscher, and J. Benkhelil
Abstract The seafloor topography at the easternmost deformation front between
the African and the Anatolian plate off Syria is dominated by the Latakia Ridge,
which obliquely intersects the Syrian margin. In this study, we investigate postMiocene depositional processes of this topographically intricate area and their relationship with mass-wasting phenomena by means of bathymetric, multi-channel
seismic reflection and sediment sub-bottom profiler data. Northward of the Latakia
Ridge, the Latakia Slope is characterized by steep scarps of up to 500 m height.
The Pliocene-Quaternary strata are truncated by the scarps, which are located in
the upward prolongation of normal fault planes. Some scarps are from erosion or
non-deposition as a consequence of contour currents. Evidence for recent active
tectonics is also present in the Latakia Ridge. A basement outcrop along the crest
of the northern Latakia Ridge presumably reflects the transtensional faulting of
this easternmost section of the African-Anatolian deformation front. The western
side of the northern Latakia Ridge shows evidence of more cohesive slumping,
probably owing to the overconsolidated nature of the sediment. Here a potential
future slide of 11 km3 associated with a rotational fault has been identified. The
sedimentary and tectonic setting has resulted in frequent mass wasting. Abundant
scars and debris flow-like deposits have been observed on the flanking slopes of the
Latakia canyon and the Syrian Margin. The Latakia canyon is fed by several tributary canyons which are incised into the Syrian Slope. Steep slopes, high sediment
accumulation rates and active strike-slip tectonics appear to have a fundamental
E. Tahchi () and R. Urgeles
Departament d’Estratigrafia, Paleontologia i Geociències Marines, Facultat de Geologia,
Universitat de Barcelona, Martí i Franquès, s/n, 08028 Barcelona, Catalonia, Spain
e-mail: etahchi@univ-perp.fr
C. Hübscher
Institut für Geophysik, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany
J. Benkhelil
IMAGES, University of Perpignan Via Domitia, Perpignan 66860 Cedex, France
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
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E. Tahchi et al.
role in submarine mass-failure initiation. Mass-failure characteristics indicate that
geohazards may exist from subsequent potential tsunami generation.
Keywords Syrian Margin • seafloor bathymetry • mass failure • contour
current • debris-flow deposits
1
Introduction
The collision between the African and Eurasian plates produced the most striking
tectonic feature in the eastern Mediterranean Sea, the Cyprus Arc. The Latakia
Ridge (LR) is the eastern morphological continuity of the Cyprus Arc, it intersects
obliquely the northern part of the Syrian coast and controls the complex submarine
morphology of the Syrian Margin (SM) to the north (Fig. 1). Previous studies
focused mainly on the plate tectonic evolution of this intricate realm (Sage and
Letouzey 1990; Vidal et al. 2000; Robertson 1998). However, swath sounder data
(bathymetry and backscatter imagery), high-resolution seismic-reflection, and
high-frequency sediment subbottom profiler (CHIRP) data collected during the
BLAC cruise (2003) revealed a clear line of evidence that the morphostructure of
the LR reflects not only active tectonics, but also mass wasting and contourite
deposition (Benkhelil et al. 2005).
Seismic data revealed abundant active faulting on the SM (Fig. 3 of Benkhelil
et al. 2005). The western flank of the LR revealed Pliocene-Quaternary reflections
truncated at the present seafloor. The question arose, whether the truncated surfaces
represent slump or fault scarps, or whether the truncation results from erosion or
non-deposition due to contour currents.
The aims of this study are to identify and illustrate recent depositional processes
on the SM with special emphasis on mass wasting and drift deposition. Slope failures
will be identified and characterized in order to understand their triggering mechanisms and pre-conditioning factors to characterize geohazard from submarine
landslides.
2
Geological Setting
The present-day eastern Mediterranean Sea represents the last remnants of the
Neotethys Ocean, which evolved through a complex rifting that took place in the
Mesozoic (e.g. Moores et al. 1984; Robertson 1998). The northward subduction of
the African plate started before the early Miocene (Eaton and Robertson 1993).
In the middle to upper Miocene, the continued convergence led to the evolution of
fold-thrust belts north of the Cyprus Arc, e.g. the Gelendzhic Rise and Misis Kyrenia
Zone (Fig. 1) (Hall et al. 2005b). At the end of the Miocene, the compressional
Mass Wasting at the Easternmost Cyprus Arc, Off Syria, Eastern Mediterranean
325
Fig. 1 Bathymetric isocontour map (contour interval = 50 m) and shaded bathymetry map of
the study area showing the morphology of the seafloor and the major canyons off the Syrian
margin. Boxed is the geological setting of the study area and sketch of the BLAC survey and its
track lines
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E. Tahchi et al.
regime changed into a sinistral strike-slip dominated regime as a consequence of
the eastward shift of the Aegean-Anatolian micro-plate (Jackson and Mc-Kenzie
1988; Hall et al. 2005a; Hall et al. 2005b). The sea-level drop of approximately
1500 m during the Messinian (Hsü et al. 1973) caused drain-age channels to incise
canyons and valleys into the continental shelf (Garfunkel et al. 1979). After that
major event, the sediment cover deposited on the eastern Cyprus Arc was laterally
continuous and seismically well stratified (Hall et al. 2005b).
3
Data and Methods
The BLAC survey carried out aboard the R/V Le Suroît (October-November 2003)
covered an area of 20,000 km2 and collected more than 5000 km of geophysical data
between Cyprus and the Syrian coast (Fig. 1). The hull-mounted EM300 multibeam
swath-sounder system was used for bathymetry and backscatter imagery. The
multi-channel Seismic-reflection MCS equipment consisted of two GI-guns (45
and 75 in3) and a 300 m long six-channel streamer. Shots were fired every 12.5 s at
a ship velocity of 8 km, resulting in a shot distance of 50 m. Consequently, the CDP
coverage was three and the CDP-spacing was 25 m. NMO-correction was carried
out with a NMO-velocity of 1500 m/s. The 3.5 kHz sub-bottom profiler (CHIRP)
data post-processing included mainly frequency filtering and travel-time de-pendent
amplitude amplification.
4
Results
The study area consists of four dominant features, which are termed the SM,
Latakia Slope (LS), the LR, and the Inner Latakia Basin (ILB). The stratigraphic
interpretation of the seismic profiles follows the nomenclature of Hall et al. (2005a;
2005b). U1 is the Pliocene-Quaternary sediment succession and is locally divided
into subunits U1–1 to U1–3 (Fig. 3). U2 denotes strata deposited during the
Messinian low-stand (MTDs and evaporites), and U3 labels older strata. “M” flags
the Messinian unconformity separating sequences U1 and U2 in the basin, or U1
from U3 when Messinian deposits are absent on the ridge.
4.1
Syrian Margin
South of Latakia, the SM exhibits numerous canyons with east-west orientation
perpendicular to the Syrian slope. The canyon flanks have an average elevation of
350 m above the canyon floor and are affected by fresh regular gullies (Fig. 2). They
drain into the lower part of the Latakia Canyon at 1400 m of water depth (Fig. 1).
Mass Wasting at the Easternmost Cyprus Arc, Off Syria, Eastern Mediterranean
327
Fig. 2 Geophysical profile across the northernmost active canyon (El Sin Canyon) of the southern Syrian continental slope. C/C: Change Course. tr: Truncation
Slope failures on the canyons flanks are well observed on the shaded bathymetry
map and seismic data (Figs.1 and 2).
The Latakia Canyon runs north-south and parallel to the LR and is considered
the marine prolongation of the Nahr El Kabir valley in the hinterland. It receives
the other canyons from the SM and broadens significantly west of the El Sin canyon, where it is covered by marine sediments deposits (Figs. 1 and 2). The Latakia
Canyon is incised into U1 (Fig. 2). The lower U1 already fills the incision left by
the Messinian desiccation beneath the present-day location of the Latakia canyon.
4.2
Latakia Slope
The northern segment of the SM and the LR is called the LS and extends from
35°23′ N to 35°32′ N (Fig. 1). The slope angles of the LS vary between 8° and 12°.
The LS is characterized by two escarpment chains on top of each other. The upper
chain comprises a concave escarpment of up to 250 m height facing the ILB to the
328
Fig. 3 Geophysical profile
across the Latakia ridge and
the Inner Latakia basin.
C/C: Change Course.
tr: Truncation
Fig. 4 Geophysical profile
across the ILB and the LS.
OLB: Outer Latakia Basin.
tr: Truncation
E. Tahchi et al.
Mass Wasting at the Easternmost Cyprus Arc, Off Syria, Eastern Mediterranean
329
west (Fig. 3). The lower chain includes a more than 500 m high escarpment that
reaches down to the basin floor of the ILB (Fig. 4). The escarpments truncate the
U1 strata (Fig. 3). Most of the escarpments are correlated with fault planes which
can be traced into the seismic basement. A ca. 400 m-thick pile of PlioceneQuaternary sediments (U1) rests on the footwall block on the LS, and the sediment
succession is bounded to the east by a fault plane and to the west by the truncational
surface (Fig. 3).
4.3
Latakia Ridge
The LR extends from 34°45′ N to 35°25′ N (Fig. 1). It subdivides the SM into a
northern and southern segment. The eastern flank of the LR is characterized by
gullied escarpment with a mean height of 350 m and a slope gradient decreasing
northward from 10° to 4° (Fig. 1). The western flank of the LR shows a more regular and smooth topography with slope gradients about 4°. Several escarpments
about 100 m height are oblique or parallel to the LR summit (Fig. 1).
On Fig. 5, a scarp corresponds to a surficial rotational slump. On top of the LR
and west of the outcrop, an erosional moat channel is present that coincides with a
fault. The outcrop is clearly correlated with an active fault directly affecting the
present-day sea-floor in correlation with hydrodynamic forces. East of the outcrop,
the U1-strata are bent and abruptly truncated close to the seafloor (Fig. 5).
The shaded bathymetry map reveals an 18 km long channel that extends parallel
the regional contours at the base of the western side of the LR (Figs. 5 and 6).
The U1-strata are truncated within the channel (Fig. 5). Faults are present beneath
the channel and they reach down to the seismic basement.
Fig. 5 Geophysical profile across the western flank of the Latakia ridge. C/C: Change Course.
tr: Truncation
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Fig. 6 Synthetic shaded map of the recent and fossil sedimentary processes over the Syrian
margin deduced from bathymetry and MCS data. The mass wasting areas are bordered with a
dashed white line including their general movement. Boxed is the back-scattering in the tributary canyon axis (40% transparency)
4.4
Inner Latakia Basin
The ILB west of the LS and LR is an elliptical basin located at 1500 m of water
depth that throughout shows a smooth topography. On its eastern side, at the foot
wall of the LS, sediment failure have accumulated and older sediment deposits are
identified in the U1 sediment fill (Fig. 4). On the southern side of the ILB, the
uppermost Pliocene-Quaternary sequence U1–1 shows regular interbedded layers
Mass Wasting at the Easternmost Cyprus Arc, Off Syria, Eastern Mediterranean
331
onlapping older strata of U1–2 (Fig. 3). Some sediment waves are located at the
southern edge of the basin (Fig. 6). The sediment-wave field covers an area of about
32 km2. In the central ILB, the Pliocene-Quaternary sediment succession has a
maximum thickness of 0.8 s TWT (>600 m). Some faults reach from within U1–1
down to the seismic basement (Fig. 3).
5
5.1
Interpretation and Discussion
Submarine Slope Failures
In several areas along the LS, the upper reflections from the U1 unit are truncated
close and up to the seafloor. The most prominent truncational surface is on the LS,
where a gullied escarpment of more than 500 m height is present (Fig. 4). There are
several processes that might explain the abrupt termination of reflections by truncation, e.g. by post-depositional slumping, faulting or erosion. Mass wasting on
submarine slopes may leave a slump scarp behind, which truncates strata from the
gliding plane up to the seafloor. Mass failures could be therefore suggested to
explain the escarpment facing the eastern side of the ILB in Fig. 4. The presence of
debris-flow deposits, as imaged from seismic reflection and chirp profiles, downslope the LS interbedded within the Pliocene-Quaternary sequence show that it is a
recurrent process that has also taken place recently.
The Latakia Canyon, which lies in the prolongation of the Nahr El Kabir River,
shows interbedded slump like sediment body visible on the multichannel seimic
data of Fig. 2. The failures that reach the La-takia Canyon are not recent, since the
younger event is covered by younger sediments and is reincised by the canyon (Fig. 2).
The east-west striking canyon walls of the SM reveal also slope failures shown for
instance in Fig. 6. Materials from these canyon walls and heads form blocks of
uncon-solidated sedi-ment that have moved down-slope, were deposited on the
canyon floor (high back-scattering) and were subsequently reworked by turbidity
currents and transported into the lower Latakia Canyon (low back-scattering).
Relatively large and recent debris flow deposits are also present on the west-east
striking canyons interfluves (outlined in dashed white line on Fig. 6). An area of ca.
12 km2 on the interfluves between El Sin Canyon and Daisseh Canyon and an area
of ca. 25 km2 south of Daisseh Canyon are covered with debris flows deposits leaving scours on the upper slope. On the opposite side and along the eastern flank of
the LR, other scours are also abundantly present just above the gullies.
5.2
Drift Deposits
Alternatively, the truncational surface may be just apparent. The term “apparent
truncation” describes reflection terminations at a depositional limit (non-deposition)
or thin-ning below seismic resolution (Emery and Myers 1996). Those depositional
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limits may characterize for instance levees of turbidite channel-levee systems (e.g.
Hübscher et al. 1997) or contourite drifts (Faugères et al. 1999).
There are several observations that sediment deposition on the LR occurred
under the influence of contour currents. The entire western flank of the LR is characterized by truncation (Fig. 5). The southern side of the ILB shows absence of
thick mass waste deposits downslope the LR (Fig. 5). The activity of a contour current at the western flank of the LR is corroborated by the presence of the about
18 km long moat channel between the Ugarit Complex and the LR and the sediment
wave field south of the ILB (Figs. 5 and 6). The U1-strata within the channel are at
least apparently truncated (Fig. 5). Therefore, along the entire western LR, the U1
sediment succession can be considered as an elongated over-consolidated drift
deposit. The drift deposits take place on steep slope of the western flank of the LR.
Persisting supply of sediment with newer sediment load might lead to over sedimentation and destabilization of the overconsolidated sediment deposits. Erosional
currents can also lead to over-steepening and weaken the sediment deposits by
eroding its slip plane. A weak plane induces drift deposit material to breaks off in
a slide. For in-stance in Fig. 5, the rotational slump observed of a volume of 11 km3
is considered as a drift sediment deposition showing interbedded regular stratification and it is underlined by faults. Truncations are also identified on the upper and
lower edges of the rotational slump. Therefore, the contour current may be considered as triggering mechanism and/or as preconditioning factor for mass wasting.
5.3
Mass Wasting and Tectonics
Since faults affect directly the seafloor, active tectonics has to be assumed. The most
recent phase in the area is strike slip faulting. Along the crest of the LR, an
elongated basement ridge has been uplifted (Figs. 5 and 6). The uplift of the outcropping ridge is clearly indicated by the upward bending and eroded strata east of
the ridge. Here transtension and transpression along restraining and releasing fault
bends is interpreted for the uplift. The divergent reflection pattern of the ILB deposits (Fig. 3) suggests sedi-mentation during continuous subsidence. Considering
both the subsidence of the ILB and the high-angle faults at the western flanks of the
LR and the LS, respectively, an extensional regime west of the LR is interpreted.
The high-angle normal fault plane within the U1-strata on the LS (Fig. 3) points
towards a causative correlation between the slump and the fault. It is likely that the
slump represents an incipient failure stage. The entire volume of this slide has been
estimated at about 10 km3, indicating that the scarps are mainly result of faulting
(Fig. 3). Here the scarps are eroded by secondary processes (e.g. contour current).
Some smaller slump aprons are present further north on the LS. West of Latakia,
a slump body of 0.2 km3 rests on the uppermost sediments of the ILB (Fig. 6).
The subbottom data prove that the slumping occurred quite recently, since no
draping sediment is present.
Some of the faults bound the scars that constitute the upper part of the slip plane
for different slope failures (Fig. 3), and thus they might have a role in pre-weakening
Mass Wasting at the Easternmost Cyprus Arc, Off Syria, Eastern Mediterranean
333
the headwall area of the observed failures prior to their initiation. In addition, some
of these faults might have produced seismic ground motions of sufficient intensity
(even with relatively low earthquake magnitudes) that could have triggered some of
the observed slope failures (Fig. 5).
6
Conclusions
Mass wasting occurred frequently in the entire survey area. Between the SM, the
LR and the LS several slump scarps, scars and deposits have been identified.
The SM is clearly characterized by recent sedimentation activity that buried some
of the mass wasting units into the Latakia Canyon deposits and makes it poorly
detectable on the present-day seafloor (Figs. 2 and 6). The southern part of the
Syrian continental slope on its lower part is also characterized by mass wasting as
well as the lower Latakia Canyon which is a sort of catchments area of the neighboring instabilities originating from the SM and the LR. The walls of the east-west
striking canyons, which guide the sediment transport from the shelf into the Latakia
Canyon, reveal also slope failures.
Recent and continuous debris flows have also been identified on the LS, leaving
slump scarps behind, which truncates strata from the gliding plane down to the
seafloor (Fig. 4). Most of the observed scarps do not result from slumping or faulting. The edgewise termination of Pliocene-Quaternary strata at the escarpments of
the LR results mainly from non-deposition as a consequence of contour currents
and not from erosion or faulting. The contour currents caused moat channels by
erosion or non-deposition (Fig. 5) and they are also considered as triggering mechanism and/or as preconditioning factor for mass wasting.
Northwest of the LR in the ILB, continuous and ongoing extension has been
observed. According to the presented data, a potential rotational slump of 11 km3 is
present atop an angular footwall block on the western flank of the LR (Fig. 5) and
another slump of 10 km3, rest on a high-angle fault on the LS (Fig. 3). Other smaller
slumps happened recently on the LS, and draped down the ILB basin floor (Fig. 6).
Acknowledgements The authors acknowledge D. Piper and D. Tappin for their constructive
detailed review on the manuscript. This research was supported by a Marie Curie Intra European
Fellowship, PIEF-GA-2008–219188, within the 7th European Community Framework
Programme.
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