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Papers by Alex Sursock

Journal of Geophysical Research, 2009

Lebanon, located on a 160-km-long transpressional bend of the left-lateral Levant (Dead Sea) faul... more Lebanon, located on a 160-km-long transpressional bend of the left-lateral Levant (Dead Sea) fault system (LFS), has been the site of infrequent but large earthquakes, including one submarine, tsunamigenic event. The main objective of the Shalimar marine survey was to characterize and map active deformation offshore of Lebanon using a range of geophysical techniques, particularly seismic reflection profiling. The cruise results clearly establish the presence of young submarine thrust faults and folds and clarify the structure of this part of the Levant margin. A submarine fold belt, bounded by thrusts and lateral ramps and extending up to 30 km from the shoreline, is interpreted as the foreland thrust system of the actively growing Mount Lebanon range. There is no large fault extending into the Levant basin toward Cyprus, which indicates that thrusting only absorbs local transpression resulting from the Lebanese restraining bend. Both the Miocene and Plio-Quaternary sedimentary sequences are affected by shortening, with landward dipping blind thrusts and associated growth strata. The presence of the Messinian evaporites creates complex deformation patterns, including normal faults due both to folding accommodation and to gravity spreading, all well imaged in the seismic reflection profiles. Because the evaporite layer acts as a décollement level, shortening extends farther out seaward through a series of thrust imbricates or duplexes. The strongest shortening, observed between Beirut and Batroun, decreases toward the south between Saida and Tyre. North of Tripoli, the passive margin is not affected by Neogene deformation and is well preserved. We propose that since the Miocene, the northward propagating LFS interacted with margin structures inherited from the Mesozoic rifting phase and was deviated along the more rigid oceanic crust flooring the Levant basin, a process which led to the formation of the Lebanese restraining bend of the LFS and consequently to the offshore shortening we document here. Such coastal transpression has resulted in local ($100 km) inversion of the passive margin, which might eventually evolve into a new subduction zone.

Bulletin of the Seismological Society of America, 2007

ABSTRACT The most active seismogenic structure along the eastern shore of the Mediterranean is th... more ABSTRACT The most active seismogenic structure along the eastern shore of the Mediterranean is the Levant fault, which forms the boundary between the African and Arabian plates. In order to study the seismic history of this fault on a millennial time scale in Lebanon, we excavated a series of trenches in the Yammouneh basin, where the active trace of the fault cuts through late Pleistocene lacustrine sediments. Five trench walls across the fault zone, 4m apart from each other, were logged and mapped in 2002 and compared to an exploratory, 5m-deep trench dug in 2001. The walls expose subtabular lake beds, down to a depth of 10.5m, with 2-3m of white, shell-rich calcareous marls that overlie 6-7m of blue-grey clays, oxydized to red brown at shallow depth, that top 1-1.5m of light blue marls. The first-order sedimentary units in the 2002 trench are nearly identical to the units exposed in 2001, implying that the variations in the stratigraphy are climate-driven. The stratigraphic layers, which include finely laminated subunits, are cut and disturbed by coseismic deformation along two subparallel, 1-3m-wide, vertical fault zones, 5-10m apart. The cumulative, apparent vertical offsets of corresponding layers at the base of the deepest walls reach 1.5m. Mapping and comparing the fault splays and apparent offsets (still in progress) on all the trench walls suggests the occurrence of about 20 distinct seismic events. Radiocarbon dating of the stratigraphic sequence is underway. Preliminary dating results confirm that here, the latest earthquake along the Yammouneh fault occurred before the 14th century AD, and suggest 7 interseismic periods in the 11ky preceding that event (likely the 1202 AD earthquake). This would be consistent with a preliminary, average recurrence time of 1500-1600 years. An ongoing paleoclimatologic study of the stratigraphic log should yield more accurate insight into the late Pleistocene climate of the Middle-East, and better constraints on the ages of event horizons.

ABSTRACT The Tripoli-Roum Thrust, which is part of the Levant Fault zone, appears to take up most... more ABSTRACT The Tripoli-Roum Thrust, which is part of the Levant Fault zone, appears to take up most of the shortening perpendicular to the Yammuneh Fault, thus producing the rise of Mount Lebanon since the late Neogene. In northern Lebanon, there is clear field evidence of active and recent folding and faulting along this thrust system. Three principal faults, oriented ~NNE-SSW, cut through the recent topography north of Tripoli. These oblique right-lateral strike-slip thrust ramps deform Neogene (Vindobonian to Astian) and Quaternary sedimentary and volcanic beds. The northernmost ramp is responsible for the growth of the young, asymmetric, Borj-el-Arab anticline, which folds Quaternary beachrocks and conglomerates, and reaches the Mediterranean coastline near Aabdé. This feature (thrust and ramp-anticline) continues offshore Tripoli, north of the Palmier and Rankine islands, and is probably responsible for the asymmetric uplift of shorelines and marine-cut terraces topping the islands. Active reverse faulting along the Tripoli-Roum thrust at sea appears to be also responsible for the rise of the many paleo-seacliffs and marine terraces found up to 500m asl along the Lebanese coast between Aabdé in the North and Saida in the South. Near Tabarja, and in the islands offshore Tripoli, we interpret the lowest uplifted marine terraces and double shoreline "trottoirs" identified and mapped by P. Sanlaville, to result from recurrent coseismic uplift during two or three seismic events on the offshore thrust. The last of these events was probably that which destroyed Beyrouth in 551A.D. Shell datings of the uplifted trottoirs yield 0,5 to 0,7 mm/yr as a first estimate of the uplift rate, relative to sea level, of the hanging wall of the Tripoli-Roum thrust ramp.

ABSTRACT The sources of three large (M ˜7.5) Near East earthquakes - in July, AD 551, May 1202 an... more ABSTRACT The sources of three large (M ˜7.5) Near East earthquakes - in July, AD 551, May 1202 and Nov. 1759 - remain controversial, because their mesoseismal areas overlap, straddling the three sub-parallel active faults of the Lebanese restraining bend. Paleoseismic trenching in the Yammoûneh basin yields unambiguous evidence both for slip on the Yammoûneh fault in the 12th-13th centuries AD, and for the lack of a posterior event. Only two seismic events are visible on both walls, in the uppermost 80 cm of the trench. Based on the calibrated ages of 14C samples, the latest ground-breaking earthquake occurred between AD 1008 and 1345. The only possible candidate for this event is the 1202 earthquake, since macroseismic damage for other large Near East events was clearly located either well south (AD 1033) or well north (AD 1157 and 1170) of the Beqaa. The penultimate event in the trench has a 14C-calibrated date between AD 324 - 537 and AD 802 - 1001. Such dates cannot be used to rule out that the AD 551 event took place on the Yammoûneh fault. However, the 551 event is famous for having ruined most of the seaports on the Lebanese coast and having caused a large tsunami which wiped out Beirut. The recent discovery (SHALIMAR cruise, 2003) of fresh seismic scarps related to oblique thrusting on the seafloor offshore Beirut makes it more likely that rupture of one segment of the Mount Lebanon thrust system caused the AD 551 earthquake and tsunami. Thus, we propose that each of the three earthquakes discussed originated on a distinct fault: AD 551 on the Mount Lebanon thrust, 1202 on the Yammoûneh fault, and 1759 on the Râchaïya-Serghaya fault. Our conclusion regarding the last two events is further supported by a comparison of the freshest visible seismic scarps, which indicates more recent slip on the Râchaïya-Serghaya system than on the Yammoûneh fault. Regarding the latter, the trenching results suggest that a recurring 1202-like, M ˜7.5 event might be due this century, as part of a sequence similar to that of AD 1033 - 1202, whose beginning might already have been heralded by the 1995, Mw ˜7.3 Aqaba earthquake. Clearly, a thorough re-assessment of seismic hazard in Lebanon, and on the entire Levant fault, is overdue.

North of the Hula basin, the main branch of the 1000 km-long Dead Sea transform veers eastwards, ... more North of the Hula basin, the main branch of the 1000 km-long Dead Sea transform veers eastwards, forming a 160 km-long restraining bend responsible for the uplift of Mount Lebanon. The corresponding transpression is partitioned between the offshore Mount Lebanon Thrust and the Yammouneh fault, whose Late-Pleistocene- Holocene slip rate has been estimated to be 5.1 +/- 1.3 mm/yr from offset alluvial fans. Paleoseismic trenches in the Yammouneh basin have shown that the fault produces large earthquakes (M > 7) with a recurrence time of 1127 +/- 135 years, and that the last such event occurred in AD 1202. The new study we present here is from a trench in the Jbab el-Homr basin, 20 km north of Yammouneh. The chief goal is to compare the succession of events at both sites, to obtain evidence for simultaneous or asynchronous earthquake rupture, a key question to assess seismic hazard in Lebanon. The 4 m-deep trench reveals a succession of very thin palustrine and lacustrine layers, rup...

Quaternary Science Reviews, 2010

Late Quaternary paleoclimatic changes in the Levant are difficult to extract from carbonate oxyge... more Late Quaternary paleoclimatic changes in the Levant are difficult to extract from carbonate oxygen isotope records partly because the factors controlling the terrestrial d 18 O signals are not fully understood in this region characterized by sharp climatic gradients. Here, we address this purpose by presenting the first 14 C-dated isotope record from the northern Levant. The record is based on oxygen isotopes from ostracod shells from lacustrine-palustrine deposits accumulated in a small karstic, hydrologically open basin (Yammoû neh), located on the eastern flank of Mount Lebanon. We have first constructed a composite isotopic record obtained from three different ostracod taxa. This yields an oxygen isotope record of ostracods (d ost) related to the most widespread species (Ilyocypris inermis), and converted to d 18 O values coeval with calcite precipitated in equilibrium (d C) with the lake water (d L). As with other records from the Mediterranean region, the Yammoû neh profile shows maximum and minimum d C values during the LGM and the early Holocene, respectively, and a slight late Holocene increase. In order to discuss the potential causes of the observed d 18 O values fluctuations (e.g., changes in the isotopic composition of the moisture source, temperature, precipitation minus evaporation (P-E) balance, or atmospheric circulation), we tentatively reconstruct the lake water isotope composition by correcting the d C values for lake water temperature using regional paleotemperature estimates. Inferred d L values were then corrected for the isotopic composition of the Eastern Mediterranean sea surface water (the moisture source) derived from planktonic foraminifera d 18 O values corrected for alkenone-based sea surface temperature. Our study suggests that Holocene d L fluctuations are primarily linked to changes in the sea surface water composition (source effect) amplified by enhanced inland rainfall during the early Holocene. During the LGM, low d L values at Yammoû neh are likely due to the ground-temperature effect on the rainfall isotope composition, possibly associated with a steeper altitudinal thermal gradient inland, and with changes in air mass trajectories over the Mediterranean Sea.

Geology, 2005

The sources of the May 1202 and November 1759, M 7.5 Near East earthquakes remain controversial, ... more The sources of the May 1202 and November 1759, M 7.5 Near East earthquakes remain controversial, because their macroseismal areas coincide, straddling subparallel active faults in the Lebanese restraining bend. Paleoseismic trenching in the Yammoû neh basin yields unambiguous evidence both for slip on the Yammoûneh fault in the twelfth-thirteenth centuries and for the lack of a posterior event. This conclusion is supported by comparing the freshest visible fault scarps, which imply more recent slip on the Râchaïya-Serghaya system than on the Yammoû neh fault. Our results suggest that the recurrence of an A.D. 1202-type earthquake might be due this century, as part of a sequence similar to that of A.D. 1033-1202, possibly heralded by the occurrence of the 1995 Mw 7.3 Aqaba earthquake. The seismic behavior of the Levant fault might thus be characterized by millennial periods of quiescence, separated by clusters of large earthquakes.

ABSTRACT The most active seismogenic structure along the eastern shore of the Mediterranean is th... more ABSTRACT The most active seismogenic structure along the eastern shore of the Mediterranean is the left-lateral strike-slip Levant fault, plate boundary between Arabia and Africa. To this day there is consensus neither on its present slip rate and segmentation, nor on the exact size and frequence of the earthquakes it generates. Between latitudes 33\textsuperscript{o} N and 35\textsuperscript{o} N in Lebanon, the Levant fault's trace veers eastwards by 24\textsuperscript{o}, forming a 160km-long restraining bend, responsible for the uplift of Mount Lebanon (3083m). Most of the resulting transpressive deformation is partitioned between two main structures: the offshore Tripoli-Beirut thrust and the Yammouneh strike-slip fault, whose degree of seismogenic activity has been questioned (various estimations of its modern slip rate range from 0 to 8mm/yr). Using aerial photographs, satellite images, topographic maps and field observations, we mapped and measured left-laterally offset alluvial fans and gullies along the Yammouneh fault. The measured offsets range from less than 10m to about 3km. To constrain the slip rate of the Yammouneh fault, limestone cobbles were sampled on three alluvial fans, each offset by 40--50m. The concentrations of \textsuperscript{36}Cl\ and stable Chlorine in the 48 samples were measured by accelerator mass spectrometry at the LLNL-CAMS. The first results suggest an average slip rate of 5--10mm/yr along the Yammouneh fault in the last 8,000 years.

ABSTRACT We present the results of the first paleoseismic study of the Yammoneh fault, the main o... more ABSTRACT We present the results of the first paleoseismic study of the Yammoneh fault, the main on-land segment of the Levant fault system within the Lebanese restraining bend. A 75-m-long trench was excavated in the dried-up Yammoneh paleo-lake, where the fault cuts though a rich and detailed sequence of marls and clays. Within this exceptional stratigraphy, first-order variations appear to reflect climate change at centennial and millennial scales. The lake beds are offset and deformed in a 2-m-wide zone coinciding with the mapped fault trace. 10 to 14 events are identified, going back more than 12+/-1 kyr. Reliable time constraints on 8 of these events constrain the mean seismic return time to 11+/-1.7~centuries, implying that this fault slips in infrequent but large (M~7.5) earthquakes. Our results also provide conclusive evidence that the latest event at this site was the great AD 1202 historical earthquake, and suggest that the Yammoneh fault might be the source of a less well-known event circa AD 350. This raises the possibility of a M>7 event occurring on this fault in the coming century, a scenario which should be taken into account for hazard assessment. Combining our findings with previous paleoseismic data from the Zebadani valley implies that the parallel faults bounding the Beqaa release strain in events of comparable frequencies but significantly different magnitudes. Finally, our results contribute to document the time-clustering of large events on the Levant fault system at millennial time scales, such as that of the 11th-12th centuries.

The Lebanese Restraining Bend (LRB), principal irregularity along the left-lateral Dead Sea Trans... more The Lebanese Restraining Bend (LRB), principal irregularity along the left-lateral Dead Sea Transform (DST), is a ~25? clockwise inflexion of the Yammouneh Fault (Y.F.), through-going branch of the DST between Galilee and Syria. This is a region of high and broad relief, with the highest mountain range of the Levant (Mt-Lebanon, 3100m asl), whose east flank is truncated by the fault. Transpressional shortening is mostly taken up by a large, partly submarine thrust system ? the Mount Lebanon Thrust (MLT) ? that dips beneath the west flank of the range and is responsible for its growth and uplift. This flank is marked by the steeply west-dipping Lebanese Flexure, which warps a thick sequence of Mesozoic limestones, separating the uplifted, deeply incised, Mt Lebanon mega-anticline core from the flat narrow, coastal stretch. Such west-vergent, asymmetric folding is the surface expression of the crustal-scale MLT ramp, which extends for ~ 120 km from south to north. A well-developed foreland fold and thrust belt exists in northern Lebanon, and smaller-scale thrusting is observed along the southeastern flank of the range. Fieldwork helps constrain the onset and evolution of shortening and uplift. There is quantitative evidence that shortening started around 15-20My ago in the south, but not much before 5Ma in the north. Geological observations in central Lebanon suggest a Mid- to Late- Miocene age for the Flexure, with much of the mountain building postdating 13Ma. The bulk of uplift and largest amount of folding postdate the Late-Miocene (10-7Ma). At Kousba, the Flexure is of late-Miocene-Pliocene age, with much of the growth between the Messinian and Lower Pleistocene (7-2Ma). Folding, uplift and faulting along the northern foreland belt started after the end of the Miocene (5Ma), and continues today. The Qalhat, Tourbol, Bebnine-Miniara and Aabdeh ramp-anticlines thus provide a clear example of progressive stepping and migration of thrust ramps into a foreland basin. The timing of mountain building is in keeping with the inference that the Dead Sea Transform propagated northwards across the LRB, and implies that its main "Syrian" branch did not accumulate much offset prior to 5Ma.

ABSTRACT The dense grid of 6-channel seismic profiles, complemented by 25 high-resolution profile... more ABSTRACT The dense grid of 6-channel seismic profiles, complemented by 25 high-resolution profiles, shot offshore Lebanon during the SHALIMAR cruise provides exceptional insight into the stratigraphy of the top 3.5 km of sediments, and into the evolution of Miocene to Quaternary tectonic deformation. The topmost sediments are Plio-Quaternary turbidites, reaching a maximum thickness of 1000-1300m near shore between Beyrouth and Batroun, from an average of 400-600m north of Tripoli, south of Saida, and farther west in the Levantine abyssal plain. Two strong reflectors mark the top and base of the seismically transparent Messinian evaporite layer, which thins landwards from a maximum thickness of 1500-1900m. Deeper down, regularly bedded horizons likely represent Miocene carbonates, and stronger reflectors below, Eocene and Late Cretaceous limestone. There is no trace of reactivation of the passive, Mesozoic Levantine basin margin north of Tripoli. The whole sediment sequence is essentially undeformed, with the evaporite layer pinching out at the base of the continental slope. A rough erosion surface related to the Messinian emersion event, unconformably draped by the Plio-Quaternary sequence, reaches down to 1700m depth. By contrast, strong shortening affects Messinian and younger sediments between Tripoli and Saida. Steeply east-dipping thrust faults mark the base of the steep continental slope. Offshore Jounieh, folding of the turbidites and underlying sediments extends as far as 30 km from the coast. Up to 4 rows of large west-vergent anticlines, 4-7km wide, underlain by mostly blind thrust ramps are observed; extensional faulting affects the turbidites above the hinges of these fault-bend folds. The Plio-Quaternary growth of the anticlines, synchronous with offshore sedimentation, has dammed onlapping units of turbidites into broad and deep piggy-back synclines. The base of the Messinian evaporites is offset by east-dipping thrust ramps, and diapirs have risen in the pinched cores of several anticlines. South of a NNW-striking lateral ramp system, the submarine region between Saida and Sour shows much less evidence of Neogene shortening. The 90km-long, thin-skinned, fold and thrust belt observed between Saida and Tripoli therefore represents the Miocene-Quaternary foreland thrust-wedge linked with the growth of Mt Lebanon. The offshore décollements and thrust ramps likely root into a steep crustal ramp plunging beneath the coastal flexure. Overall, the amount of shortening in the last 15 Ma may be on order of a few tens of kilometers.

ABSTRACT The M>7 earthquake that struck the shore of Lebanon in 551 AD was associated with... more ABSTRACT The M>7 earthquake that struck the shore of Lebanon in 551 AD was associated with a large tsunami that destroyed Beyrouth and other seaports, and thus likely originated offshore. Onshore in the north, the Tripoli thrust, whose 70m-high cumulative scarp cuts the city in half, is responsible for the Plio-Quaternary growth and current uplift of the Turbol-Qualhat anticline. Similarly, the growth of the Aabdé anticline, and the tilt and emergence of the Ramkine islands group result from slip on the Aabdé thrust. We have inferred both thrusts to continue at sea. The EM300 multibeam bathymetric data obtained during the SHALIMAR cruise establishes the existence of the active, submarine Mount Lebanon thrust system, which connects the Tripoli and Aabdé thrusts with the Roum fault. Between Beyrouth and Tripoli the Levantine margin shows its steepest near-shore bathymetric gradient (100 to 1500 m in

Throughout history, the eastern shore of the Mediterranean has been repeatedly shaken by large ea... more Throughout history, the eastern shore of the Mediterranean has been repeatedly shaken by large earthquakes, but no such earthquake has occurred in the past 165 years. Although the most active seismogenic structure of the region is the Levant fault system (LFS), which forms the boundary between the African and Arabian plates, there is still no consensus on its present slip

The objectives of the SHALIMAR cruise were to study recent deformation of the Mediterranean seafl... more The objectives of the SHALIMAR cruise were to study recent deformation of the Mediterranean seafloor west of Mt Lebanon. We collected multibeam bathymetry and back-scatter images, reflection seismic profiles - surface and deep-towed, 3.5 kHz echo-sounder data, gravity and magnetic data over an 80 km-wide zone offshore the entire Lebanese coast. The bathymetry reveals a very steep slope between Beyrouth and Batroun, with a water depth of 1500 m only 5 km offshore. Between Saida and Tripoli (33.5N to 34.5N), both the bathymetry and seismic lines show a series of ramp anticlines affecting Plio-Quaternary deposits and a seismically transparent layer with variable thickness corresponding to the Messinian evaporites (5.6 Ma). This fold-and-thrust belt is the offshore expression of shortening related to the formation of Mt Lebanon. It is limited westwards by a prominent fold front 30 km from shore. Some ramps appear to be submarine continuations of faults documented ashore (e.g., Aabde and Tripoli thrusts). The strikes of fold axes are consistent with WNW-ESE shortening and slip-partitioning along the 30° Lebanese bend of the Levant fault. North of Tripoli and south of Saida, the continental margin displays a wider shelf (20 km) and gentler slope. In the south, at 1200-1500 m depth small, closely spaced, NE-trending scarps attest to young, distributed dip-slip faulting, although deformation is much less than north of Beyrouth. At the southern extremity of our survey, NW-trending normal fault scarps roughly aligned with the Mt Carmel-Haifa fault vanish 50 km offshore in the Levant basin. Bouguer gravity anomalies, estimated by removing from free-air gravity data the effect of bathymetry, display a very steep gradient between Beyrouth and Tripoli, marking the passage from thickened Mt Lebanon crust to thin crust in the Levant basin. Gravity anomalies also outline large, NE-SW trending steps between the basin crust and thinned continental crust near and south of Beyrouth. The particularly large negative anomalies in the north likely reflect flexure of the lithosphere due to subduction under Cyprus and underthrusting beneath Lebanon. The SHALIMAR data suggest that local compression induced by the bend in the Dead Sea Transform reactivated faults of the Mesozoic Levant margin into lateral ramps and thrusts whose evolution may lead to subduction. It is possible that the shorter distance between the transform and margin in central Lebanon (35 km) than in Syria (65 km) is one measure of continental shortening. That the oceanic lithosphere is strong may explain why the Carmel and Roum faults did not extend into the Levant basin.

Earth and Planetary …, 2004

The most active seismogenic structure along the eastern shore of the Mediterranean is the N-S-tre... more The most active seismogenic structure along the eastern shore of the Mediterranean is the N-S-trending left-lateral Levant Fault System (LFS), the plate boundary between Arabia and Africa. In Lebanon, it forms a 160-km-long restraining bend responsible for the uplift of Mount Lebanon. The resulting transpression is partitioned between the offshore Tripoli-Roûm thrust and the Yammoûneh strike-slip fault. There are few quantitative constraints on the Quaternary slip rate along the LFS. Here we present a direct estimate of the~25-ka mean slip rate on the Yammoûneh fault. Mapped offsets of alluvial fans at two sites~50 km apart on the eastern flank of Mount Lebanon range between 24F2 and 80F8 m. About 30 limestone cobbles sampled on these fans yield in situ cosmogenic 36 Cl exposure ages mostly between 6 and 27 ka. A statistical assessment of offsets versus ages provides bounds on the Late Pleistocene-Holocene slip rate on the fault: 3.8-6.4 mm/yr. These results are consistent with long-term geological inferences, confirming that the Yammoûneh fault is the main strike-slip branch of the LFS in Lebanon. They illustrate both the potential and the difficulties of using in situ cosmogenic 36 Cl dating of limestone-clast fan deposits for deciphering tectonic and geomorphic processes in the Mediterranean.

Journal of Geophysical Research, 2009

Lebanon, located on a 160-km-long transpressional bend of the left-lateral Levant (Dead Sea) faul... more Lebanon, located on a 160-km-long transpressional bend of the left-lateral Levant (Dead Sea) fault system (LFS), has been the site of infrequent but large earthquakes, including one submarine, tsunamigenic event. The main objective of the Shalimar marine survey was to characterize and map active deformation offshore of Lebanon using a range of geophysical techniques, particularly seismic reflection profiling. The cruise results clearly establish the presence of young submarine thrust faults and folds and clarify the structure of this part of the Levant margin. A submarine fold belt, bounded by thrusts and lateral ramps and extending up to 30 km from the shoreline, is interpreted as the foreland thrust system of the actively growing Mount Lebanon range. There is no large fault extending into the Levant basin toward Cyprus, which indicates that thrusting only absorbs local transpression resulting from the Lebanese restraining bend. Both the Miocene and Plio-Quaternary sedimentary sequences are affected by shortening, with landward dipping blind thrusts and associated growth strata. The presence of the Messinian evaporites creates complex deformation patterns, including normal faults due both to folding accommodation and to gravity spreading, all well imaged in the seismic reflection profiles. Because the evaporite layer acts as a décollement level, shortening extends farther out seaward through a series of thrust imbricates or duplexes. The strongest shortening, observed between Beirut and Batroun, decreases toward the south between Saida and Tyre. North of Tripoli, the passive margin is not affected by Neogene deformation and is well preserved. We propose that since the Miocene, the northward propagating LFS interacted with margin structures inherited from the Mesozoic rifting phase and was deviated along the more rigid oceanic crust flooring the Levant basin, a process which led to the formation of the Lebanese restraining bend of the LFS and consequently to the offshore shortening we document here. Such coastal transpression has resulted in local ($100 km) inversion of the passive margin, which might eventually evolve into a new subduction zone.

Bulletin of the Seismological Society of America, 2007

ABSTRACT The most active seismogenic structure along the eastern shore of the Mediterranean is th... more ABSTRACT The most active seismogenic structure along the eastern shore of the Mediterranean is the Levant fault, which forms the boundary between the African and Arabian plates. In order to study the seismic history of this fault on a millennial time scale in Lebanon, we excavated a series of trenches in the Yammouneh basin, where the active trace of the fault cuts through late Pleistocene lacustrine sediments. Five trench walls across the fault zone, 4m apart from each other, were logged and mapped in 2002 and compared to an exploratory, 5m-deep trench dug in 2001. The walls expose subtabular lake beds, down to a depth of 10.5m, with 2-3m of white, shell-rich calcareous marls that overlie 6-7m of blue-grey clays, oxydized to red brown at shallow depth, that top 1-1.5m of light blue marls. The first-order sedimentary units in the 2002 trench are nearly identical to the units exposed in 2001, implying that the variations in the stratigraphy are climate-driven. The stratigraphic layers, which include finely laminated subunits, are cut and disturbed by coseismic deformation along two subparallel, 1-3m-wide, vertical fault zones, 5-10m apart. The cumulative, apparent vertical offsets of corresponding layers at the base of the deepest walls reach 1.5m. Mapping and comparing the fault splays and apparent offsets (still in progress) on all the trench walls suggests the occurrence of about 20 distinct seismic events. Radiocarbon dating of the stratigraphic sequence is underway. Preliminary dating results confirm that here, the latest earthquake along the Yammouneh fault occurred before the 14th century AD, and suggest 7 interseismic periods in the 11ky preceding that event (likely the 1202 AD earthquake). This would be consistent with a preliminary, average recurrence time of 1500-1600 years. An ongoing paleoclimatologic study of the stratigraphic log should yield more accurate insight into the late Pleistocene climate of the Middle-East, and better constraints on the ages of event horizons.

ABSTRACT The Tripoli-Roum Thrust, which is part of the Levant Fault zone, appears to take up most... more ABSTRACT The Tripoli-Roum Thrust, which is part of the Levant Fault zone, appears to take up most of the shortening perpendicular to the Yammuneh Fault, thus producing the rise of Mount Lebanon since the late Neogene. In northern Lebanon, there is clear field evidence of active and recent folding and faulting along this thrust system. Three principal faults, oriented ~NNE-SSW, cut through the recent topography north of Tripoli. These oblique right-lateral strike-slip thrust ramps deform Neogene (Vindobonian to Astian) and Quaternary sedimentary and volcanic beds. The northernmost ramp is responsible for the growth of the young, asymmetric, Borj-el-Arab anticline, which folds Quaternary beachrocks and conglomerates, and reaches the Mediterranean coastline near Aabdé. This feature (thrust and ramp-anticline) continues offshore Tripoli, north of the Palmier and Rankine islands, and is probably responsible for the asymmetric uplift of shorelines and marine-cut terraces topping the islands. Active reverse faulting along the Tripoli-Roum thrust at sea appears to be also responsible for the rise of the many paleo-seacliffs and marine terraces found up to 500m asl along the Lebanese coast between Aabdé in the North and Saida in the South. Near Tabarja, and in the islands offshore Tripoli, we interpret the lowest uplifted marine terraces and double shoreline "trottoirs" identified and mapped by P. Sanlaville, to result from recurrent coseismic uplift during two or three seismic events on the offshore thrust. The last of these events was probably that which destroyed Beyrouth in 551A.D. Shell datings of the uplifted trottoirs yield 0,5 to 0,7 mm/yr as a first estimate of the uplift rate, relative to sea level, of the hanging wall of the Tripoli-Roum thrust ramp.

ABSTRACT The sources of three large (M ˜7.5) Near East earthquakes - in July, AD 551, May 1202 an... more ABSTRACT The sources of three large (M ˜7.5) Near East earthquakes - in July, AD 551, May 1202 and Nov. 1759 - remain controversial, because their mesoseismal areas overlap, straddling the three sub-parallel active faults of the Lebanese restraining bend. Paleoseismic trenching in the Yammoûneh basin yields unambiguous evidence both for slip on the Yammoûneh fault in the 12th-13th centuries AD, and for the lack of a posterior event. Only two seismic events are visible on both walls, in the uppermost 80 cm of the trench. Based on the calibrated ages of 14C samples, the latest ground-breaking earthquake occurred between AD 1008 and 1345. The only possible candidate for this event is the 1202 earthquake, since macroseismic damage for other large Near East events was clearly located either well south (AD 1033) or well north (AD 1157 and 1170) of the Beqaa. The penultimate event in the trench has a 14C-calibrated date between AD 324 - 537 and AD 802 - 1001. Such dates cannot be used to rule out that the AD 551 event took place on the Yammoûneh fault. However, the 551 event is famous for having ruined most of the seaports on the Lebanese coast and having caused a large tsunami which wiped out Beirut. The recent discovery (SHALIMAR cruise, 2003) of fresh seismic scarps related to oblique thrusting on the seafloor offshore Beirut makes it more likely that rupture of one segment of the Mount Lebanon thrust system caused the AD 551 earthquake and tsunami. Thus, we propose that each of the three earthquakes discussed originated on a distinct fault: AD 551 on the Mount Lebanon thrust, 1202 on the Yammoûneh fault, and 1759 on the Râchaïya-Serghaya fault. Our conclusion regarding the last two events is further supported by a comparison of the freshest visible seismic scarps, which indicates more recent slip on the Râchaïya-Serghaya system than on the Yammoûneh fault. Regarding the latter, the trenching results suggest that a recurring 1202-like, M ˜7.5 event might be due this century, as part of a sequence similar to that of AD 1033 - 1202, whose beginning might already have been heralded by the 1995, Mw ˜7.3 Aqaba earthquake. Clearly, a thorough re-assessment of seismic hazard in Lebanon, and on the entire Levant fault, is overdue.

North of the Hula basin, the main branch of the 1000 km-long Dead Sea transform veers eastwards, ... more North of the Hula basin, the main branch of the 1000 km-long Dead Sea transform veers eastwards, forming a 160 km-long restraining bend responsible for the uplift of Mount Lebanon. The corresponding transpression is partitioned between the offshore Mount Lebanon Thrust and the Yammouneh fault, whose Late-Pleistocene- Holocene slip rate has been estimated to be 5.1 +/- 1.3 mm/yr from offset alluvial fans. Paleoseismic trenches in the Yammouneh basin have shown that the fault produces large earthquakes (M > 7) with a recurrence time of 1127 +/- 135 years, and that the last such event occurred in AD 1202. The new study we present here is from a trench in the Jbab el-Homr basin, 20 km north of Yammouneh. The chief goal is to compare the succession of events at both sites, to obtain evidence for simultaneous or asynchronous earthquake rupture, a key question to assess seismic hazard in Lebanon. The 4 m-deep trench reveals a succession of very thin palustrine and lacustrine layers, rup...

Quaternary Science Reviews, 2010

Late Quaternary paleoclimatic changes in the Levant are difficult to extract from carbonate oxyge... more Late Quaternary paleoclimatic changes in the Levant are difficult to extract from carbonate oxygen isotope records partly because the factors controlling the terrestrial d 18 O signals are not fully understood in this region characterized by sharp climatic gradients. Here, we address this purpose by presenting the first 14 C-dated isotope record from the northern Levant. The record is based on oxygen isotopes from ostracod shells from lacustrine-palustrine deposits accumulated in a small karstic, hydrologically open basin (Yammoû neh), located on the eastern flank of Mount Lebanon. We have first constructed a composite isotopic record obtained from three different ostracod taxa. This yields an oxygen isotope record of ostracods (d ost) related to the most widespread species (Ilyocypris inermis), and converted to d 18 O values coeval with calcite precipitated in equilibrium (d C) with the lake water (d L). As with other records from the Mediterranean region, the Yammoû neh profile shows maximum and minimum d C values during the LGM and the early Holocene, respectively, and a slight late Holocene increase. In order to discuss the potential causes of the observed d 18 O values fluctuations (e.g., changes in the isotopic composition of the moisture source, temperature, precipitation minus evaporation (P-E) balance, or atmospheric circulation), we tentatively reconstruct the lake water isotope composition by correcting the d C values for lake water temperature using regional paleotemperature estimates. Inferred d L values were then corrected for the isotopic composition of the Eastern Mediterranean sea surface water (the moisture source) derived from planktonic foraminifera d 18 O values corrected for alkenone-based sea surface temperature. Our study suggests that Holocene d L fluctuations are primarily linked to changes in the sea surface water composition (source effect) amplified by enhanced inland rainfall during the early Holocene. During the LGM, low d L values at Yammoû neh are likely due to the ground-temperature effect on the rainfall isotope composition, possibly associated with a steeper altitudinal thermal gradient inland, and with changes in air mass trajectories over the Mediterranean Sea.

Geology, 2005

The sources of the May 1202 and November 1759, M 7.5 Near East earthquakes remain controversial, ... more The sources of the May 1202 and November 1759, M 7.5 Near East earthquakes remain controversial, because their macroseismal areas coincide, straddling subparallel active faults in the Lebanese restraining bend. Paleoseismic trenching in the Yammoû neh basin yields unambiguous evidence both for slip on the Yammoûneh fault in the twelfth-thirteenth centuries and for the lack of a posterior event. This conclusion is supported by comparing the freshest visible fault scarps, which imply more recent slip on the Râchaïya-Serghaya system than on the Yammoû neh fault. Our results suggest that the recurrence of an A.D. 1202-type earthquake might be due this century, as part of a sequence similar to that of A.D. 1033-1202, possibly heralded by the occurrence of the 1995 Mw 7.3 Aqaba earthquake. The seismic behavior of the Levant fault might thus be characterized by millennial periods of quiescence, separated by clusters of large earthquakes.

ABSTRACT The most active seismogenic structure along the eastern shore of the Mediterranean is th... more ABSTRACT The most active seismogenic structure along the eastern shore of the Mediterranean is the left-lateral strike-slip Levant fault, plate boundary between Arabia and Africa. To this day there is consensus neither on its present slip rate and segmentation, nor on the exact size and frequence of the earthquakes it generates. Between latitudes 33\textsuperscript{o} N and 35\textsuperscript{o} N in Lebanon, the Levant fault's trace veers eastwards by 24\textsuperscript{o}, forming a 160km-long restraining bend, responsible for the uplift of Mount Lebanon (3083m). Most of the resulting transpressive deformation is partitioned between two main structures: the offshore Tripoli-Beirut thrust and the Yammouneh strike-slip fault, whose degree of seismogenic activity has been questioned (various estimations of its modern slip rate range from 0 to 8mm/yr). Using aerial photographs, satellite images, topographic maps and field observations, we mapped and measured left-laterally offset alluvial fans and gullies along the Yammouneh fault. The measured offsets range from less than 10m to about 3km. To constrain the slip rate of the Yammouneh fault, limestone cobbles were sampled on three alluvial fans, each offset by 40--50m. The concentrations of \textsuperscript{36}Cl\ and stable Chlorine in the 48 samples were measured by accelerator mass spectrometry at the LLNL-CAMS. The first results suggest an average slip rate of 5--10mm/yr along the Yammouneh fault in the last 8,000 years.

ABSTRACT We present the results of the first paleoseismic study of the Yammoneh fault, the main o... more ABSTRACT We present the results of the first paleoseismic study of the Yammoneh fault, the main on-land segment of the Levant fault system within the Lebanese restraining bend. A 75-m-long trench was excavated in the dried-up Yammoneh paleo-lake, where the fault cuts though a rich and detailed sequence of marls and clays. Within this exceptional stratigraphy, first-order variations appear to reflect climate change at centennial and millennial scales. The lake beds are offset and deformed in a 2-m-wide zone coinciding with the mapped fault trace. 10 to 14 events are identified, going back more than 12+/-1 kyr. Reliable time constraints on 8 of these events constrain the mean seismic return time to 11+/-1.7~centuries, implying that this fault slips in infrequent but large (M~7.5) earthquakes. Our results also provide conclusive evidence that the latest event at this site was the great AD 1202 historical earthquake, and suggest that the Yammoneh fault might be the source of a less well-known event circa AD 350. This raises the possibility of a M>7 event occurring on this fault in the coming century, a scenario which should be taken into account for hazard assessment. Combining our findings with previous paleoseismic data from the Zebadani valley implies that the parallel faults bounding the Beqaa release strain in events of comparable frequencies but significantly different magnitudes. Finally, our results contribute to document the time-clustering of large events on the Levant fault system at millennial time scales, such as that of the 11th-12th centuries.

The Lebanese Restraining Bend (LRB), principal irregularity along the left-lateral Dead Sea Trans... more The Lebanese Restraining Bend (LRB), principal irregularity along the left-lateral Dead Sea Transform (DST), is a ~25? clockwise inflexion of the Yammouneh Fault (Y.F.), through-going branch of the DST between Galilee and Syria. This is a region of high and broad relief, with the highest mountain range of the Levant (Mt-Lebanon, 3100m asl), whose east flank is truncated by the fault. Transpressional shortening is mostly taken up by a large, partly submarine thrust system ? the Mount Lebanon Thrust (MLT) ? that dips beneath the west flank of the range and is responsible for its growth and uplift. This flank is marked by the steeply west-dipping Lebanese Flexure, which warps a thick sequence of Mesozoic limestones, separating the uplifted, deeply incised, Mt Lebanon mega-anticline core from the flat narrow, coastal stretch. Such west-vergent, asymmetric folding is the surface expression of the crustal-scale MLT ramp, which extends for ~ 120 km from south to north. A well-developed foreland fold and thrust belt exists in northern Lebanon, and smaller-scale thrusting is observed along the southeastern flank of the range. Fieldwork helps constrain the onset and evolution of shortening and uplift. There is quantitative evidence that shortening started around 15-20My ago in the south, but not much before 5Ma in the north. Geological observations in central Lebanon suggest a Mid- to Late- Miocene age for the Flexure, with much of the mountain building postdating 13Ma. The bulk of uplift and largest amount of folding postdate the Late-Miocene (10-7Ma). At Kousba, the Flexure is of late-Miocene-Pliocene age, with much of the growth between the Messinian and Lower Pleistocene (7-2Ma). Folding, uplift and faulting along the northern foreland belt started after the end of the Miocene (5Ma), and continues today. The Qalhat, Tourbol, Bebnine-Miniara and Aabdeh ramp-anticlines thus provide a clear example of progressive stepping and migration of thrust ramps into a foreland basin. The timing of mountain building is in keeping with the inference that the Dead Sea Transform propagated northwards across the LRB, and implies that its main "Syrian" branch did not accumulate much offset prior to 5Ma.

ABSTRACT The dense grid of 6-channel seismic profiles, complemented by 25 high-resolution profile... more ABSTRACT The dense grid of 6-channel seismic profiles, complemented by 25 high-resolution profiles, shot offshore Lebanon during the SHALIMAR cruise provides exceptional insight into the stratigraphy of the top 3.5 km of sediments, and into the evolution of Miocene to Quaternary tectonic deformation. The topmost sediments are Plio-Quaternary turbidites, reaching a maximum thickness of 1000-1300m near shore between Beyrouth and Batroun, from an average of 400-600m north of Tripoli, south of Saida, and farther west in the Levantine abyssal plain. Two strong reflectors mark the top and base of the seismically transparent Messinian evaporite layer, which thins landwards from a maximum thickness of 1500-1900m. Deeper down, regularly bedded horizons likely represent Miocene carbonates, and stronger reflectors below, Eocene and Late Cretaceous limestone. There is no trace of reactivation of the passive, Mesozoic Levantine basin margin north of Tripoli. The whole sediment sequence is essentially undeformed, with the evaporite layer pinching out at the base of the continental slope. A rough erosion surface related to the Messinian emersion event, unconformably draped by the Plio-Quaternary sequence, reaches down to 1700m depth. By contrast, strong shortening affects Messinian and younger sediments between Tripoli and Saida. Steeply east-dipping thrust faults mark the base of the steep continental slope. Offshore Jounieh, folding of the turbidites and underlying sediments extends as far as 30 km from the coast. Up to 4 rows of large west-vergent anticlines, 4-7km wide, underlain by mostly blind thrust ramps are observed; extensional faulting affects the turbidites above the hinges of these fault-bend folds. The Plio-Quaternary growth of the anticlines, synchronous with offshore sedimentation, has dammed onlapping units of turbidites into broad and deep piggy-back synclines. The base of the Messinian evaporites is offset by east-dipping thrust ramps, and diapirs have risen in the pinched cores of several anticlines. South of a NNW-striking lateral ramp system, the submarine region between Saida and Sour shows much less evidence of Neogene shortening. The 90km-long, thin-skinned, fold and thrust belt observed between Saida and Tripoli therefore represents the Miocene-Quaternary foreland thrust-wedge linked with the growth of Mt Lebanon. The offshore décollements and thrust ramps likely root into a steep crustal ramp plunging beneath the coastal flexure. Overall, the amount of shortening in the last 15 Ma may be on order of a few tens of kilometers.

ABSTRACT The M>7 earthquake that struck the shore of Lebanon in 551 AD was associated with... more ABSTRACT The M>7 earthquake that struck the shore of Lebanon in 551 AD was associated with a large tsunami that destroyed Beyrouth and other seaports, and thus likely originated offshore. Onshore in the north, the Tripoli thrust, whose 70m-high cumulative scarp cuts the city in half, is responsible for the Plio-Quaternary growth and current uplift of the Turbol-Qualhat anticline. Similarly, the growth of the Aabdé anticline, and the tilt and emergence of the Ramkine islands group result from slip on the Aabdé thrust. We have inferred both thrusts to continue at sea. The EM300 multibeam bathymetric data obtained during the SHALIMAR cruise establishes the existence of the active, submarine Mount Lebanon thrust system, which connects the Tripoli and Aabdé thrusts with the Roum fault. Between Beyrouth and Tripoli the Levantine margin shows its steepest near-shore bathymetric gradient (100 to 1500 m in

Throughout history, the eastern shore of the Mediterranean has been repeatedly shaken by large ea... more Throughout history, the eastern shore of the Mediterranean has been repeatedly shaken by large earthquakes, but no such earthquake has occurred in the past 165 years. Although the most active seismogenic structure of the region is the Levant fault system (LFS), which forms the boundary between the African and Arabian plates, there is still no consensus on its present slip

The objectives of the SHALIMAR cruise were to study recent deformation of the Mediterranean seafl... more The objectives of the SHALIMAR cruise were to study recent deformation of the Mediterranean seafloor west of Mt Lebanon. We collected multibeam bathymetry and back-scatter images, reflection seismic profiles - surface and deep-towed, 3.5 kHz echo-sounder data, gravity and magnetic data over an 80 km-wide zone offshore the entire Lebanese coast. The bathymetry reveals a very steep slope between Beyrouth and Batroun, with a water depth of 1500 m only 5 km offshore. Between Saida and Tripoli (33.5N to 34.5N), both the bathymetry and seismic lines show a series of ramp anticlines affecting Plio-Quaternary deposits and a seismically transparent layer with variable thickness corresponding to the Messinian evaporites (5.6 Ma). This fold-and-thrust belt is the offshore expression of shortening related to the formation of Mt Lebanon. It is limited westwards by a prominent fold front 30 km from shore. Some ramps appear to be submarine continuations of faults documented ashore (e.g., Aabde and Tripoli thrusts). The strikes of fold axes are consistent with WNW-ESE shortening and slip-partitioning along the 30° Lebanese bend of the Levant fault. North of Tripoli and south of Saida, the continental margin displays a wider shelf (20 km) and gentler slope. In the south, at 1200-1500 m depth small, closely spaced, NE-trending scarps attest to young, distributed dip-slip faulting, although deformation is much less than north of Beyrouth. At the southern extremity of our survey, NW-trending normal fault scarps roughly aligned with the Mt Carmel-Haifa fault vanish 50 km offshore in the Levant basin. Bouguer gravity anomalies, estimated by removing from free-air gravity data the effect of bathymetry, display a very steep gradient between Beyrouth and Tripoli, marking the passage from thickened Mt Lebanon crust to thin crust in the Levant basin. Gravity anomalies also outline large, NE-SW trending steps between the basin crust and thinned continental crust near and south of Beyrouth. The particularly large negative anomalies in the north likely reflect flexure of the lithosphere due to subduction under Cyprus and underthrusting beneath Lebanon. The SHALIMAR data suggest that local compression induced by the bend in the Dead Sea Transform reactivated faults of the Mesozoic Levant margin into lateral ramps and thrusts whose evolution may lead to subduction. It is possible that the shorter distance between the transform and margin in central Lebanon (35 km) than in Syria (65 km) is one measure of continental shortening. That the oceanic lithosphere is strong may explain why the Carmel and Roum faults did not extend into the Levant basin.

Earth and Planetary …, 2004

The most active seismogenic structure along the eastern shore of the Mediterranean is the N-S-tre... more The most active seismogenic structure along the eastern shore of the Mediterranean is the N-S-trending left-lateral Levant Fault System (LFS), the plate boundary between Arabia and Africa. In Lebanon, it forms a 160-km-long restraining bend responsible for the uplift of Mount Lebanon. The resulting transpression is partitioned between the offshore Tripoli-Roûm thrust and the Yammoûneh strike-slip fault. There are few quantitative constraints on the Quaternary slip rate along the LFS. Here we present a direct estimate of the~25-ka mean slip rate on the Yammoûneh fault. Mapped offsets of alluvial fans at two sites~50 km apart on the eastern flank of Mount Lebanon range between 24F2 and 80F8 m. About 30 limestone cobbles sampled on these fans yield in situ cosmogenic 36 Cl exposure ages mostly between 6 and 27 ka. A statistical assessment of offsets versus ages provides bounds on the Late Pleistocene-Holocene slip rate on the fault: 3.8-6.4 mm/yr. These results are consistent with long-term geological inferences, confirming that the Yammoûneh fault is the main strike-slip branch of the LFS in Lebanon. They illustrate both the potential and the difficulties of using in situ cosmogenic 36 Cl dating of limestone-clast fan deposits for deciphering tectonic and geomorphic processes in the Mediterranean.