T52B-01 INVITED 10:20h
Subduction and Mantle Convection in the Western Mediterranean
The western Mediterranean subduction zone (WMSz) extends from the northern Apennine to southern Spain and turns around forming the narrow and tight Calabrian and Gibraltar Arcs. The evolution of the WMSz is characterised by a first phase of orogenic wedging followed, from 30 Ma on, by trench retreat and back-arc extension. Combining geological data, tomographic images of the western Mediterranean mantle, plate kinematics, geochemical data and laboratory modelling, we describe the evolution of the WMSz during the last 35 Ma. Our reconstruction shows that the two arcs form by fragmentation of the 1500 km long WMSz in small, narrow slabs. Once formed, these two narrow slabs retreat outward, producing back-arc extension and large-scale rotation of the flanks, shaping the arcs. The Gibraltar arc first formed during the middle Miocene, while the Calabrian arc formed later, during the Late Miocene-Pliocene. Despite the different paleogeographic settings, the mechanism of rupture and backward migration of the narrow slabs presents similarities on both sides of the western Mediterranean, suggesting that the slab deformation is also driven by lateral mantle flow that is particularly efficient when slabs show a retrograde motion in a restricted (upper mantle) style of mantle convection. In particular we present evidences on the presence of lateral mantle flow around the edge of the Calabrian slab and reconstruct its path during the last 10 Ma. This has been unravel using present-day seismological constraints on the geometry of the slab (tomographic images) and on the path of mantle anisotropy, coupled with tectonic reconstruction and geochemical signature of volcanic rocks.
T52B-02 INVITED 10:35h
Subduction, convergence and the mode of backarc extension in the Mediterranean region
The Cenozoic period in the Mediterranean region was characterized by a sharp change in the subduction dynamics some 30-35 Ma ago, from generalized compressional subduction, leading to the formation of Alpine mountain belts, to extensional subduction and the formation of backarc basins, from the Alboran Sea to the Aegean Sea. Backarc extension was thus associated with collapse of the mountain belts formed before this Oligocene revolution. Sedimentary basins evolved in connection with the reactivation of thrust faults as extensional detachments or the creation of new detachments. From the Aegean Sea to the Tyrrhenian Sea and the Alboran Sea we have analysed onshore the kinematic and P-T evolution of the ductile crust exhumed by extension and the transition from ductile to brittle conditions and the relations between deep deformation and the formation of basins. We show two different types of evolution : in the eastern Mediterranean where subduction had been engaged in the Early Mesozoic, and in the Central and Western Mediterranean where it started only in the late Cretaceous or Early Cenozoic. We show that (1) the early tectonic fabric has little influence of the kinematic of the post-orogenic extension, (2) the asymmetry of extension (sense of shear along crustal-scale detachments) differs whether subduction is associated or not with convergence. We tentatively propose a scheme explaining how the interactions between the subducting slab and the mantle control the basal shear below the upper plate and the geometry and distribution of detachments and the associated sedimentary basins. The example of the Betic Cordillera and the Rif orogen, where the direction of stretching where different in the lower and the upper crust and changed through time, is discussed and we proposed a scenario of evolution from the Eocene to the Tortonian.
T52B-03 10:50h
Late Orogenic Vertical Movements in and Around the Ligurian Alps (SW Alps): Coexistence and Migration of Km-Scale Subsidence and Exhumation
The northern part of convergence zone between Europe and Africa in the central Mediterranean is composed of an intricate puzzle of different domains. They are the extensional Ligurian Sea, the mainly S-vergent Eocene orogenic wedge of the Ligurian Alps and, further to the N the western Po Plain with its pre-Messinian substratum exposed at the transition zone with the Ligurian Alps. Along a transect from the Ligurian Sea to the western Po Plain we have resolved vertical movements and horizontal deformations integrating a large amount of low temperature thermochronology (fission tracks and U-Th)/He on apatites), seismic, structural and sedimentological data. Numerical modeling work is being carried out to quantitatively test, validate and integrate the data. The most appealing and surprising result of this work is that the whole area, inclusive of the Ligurian Alps orogen, experienced kilometers-scale vertical movements (subsidence and uplift/exhumation) following major contraction. This pleads for a review of hitherto accepted tectonic scenarios of this and other orogens. During the main shortening stages, rocks of the Ligurian Alps were rapidly exhumed and eroded as recorded by the very short lag time in the sedimentary record. Little morphology had remained at ca. 30Ma when subsidence started affecting the orogen itself and the adjacent regions. Orogenic shortening had ended and Oligocene sediments seal older tectonic boundaries. At 26Ma, vertical movements in the region of the present-day orogen changed and basement rocks experienced exhumation. Subsidence continued to the N, in the region of the Po Plain. In the Ligurian Alps exhumation is ongoing. The site of maximum exhumation migrated towards the N in the subsequent millions of years affecting regions previously experiencing subsidence. In general, the reconstructed movements reflect the following pattern a) Upward and downward movements are active concurrently in different parts of the section b) Sites of maximum exhumation and subsidence migrate across the structural grain through time Similar kinematics have been proposed for the growth and late-stage evolution of other orogens such as the Carpathians. The derived pattern is incompatible with popular geodynamic models such as those invoking slab detachments.
T52B-04 11:05h
Rapid Lateral Transition Between Crustal and Llithospheric Folding In Post-Collisional Orogenic Setting: An Example From The SE Carpathians Foreland
Post-collisional basin evolution studies demonstrate significant differential vertical motions along and across the Carpathians arc. On the overall, subsidence takes place in the SE foreland associated with coeval uplift of the neighbouring Carpathians orogenic wedge and the northern foredeep. These features are associated at lithospheric levels with coeval, late-stage slab-pull resulting from thermal re-equilibration of an inherited slab in a system locked during the Late Miocene collision. The apparent coincidence in time of these differential movements acting at all depth levels demonstrate a complex 3D mechanism accommodating coeval different types of deformation in localised places along the coupled orogen - foredeep - backarc basin. In this respect, intra-plate folding due to the Pliocene to Quaternary inversion of the locked Carpathians - foreland system appears to play a more important role than hitherto assumed. In the central-northern part of the East Carpathians this folding reflects a lithospheric wavelength, larger than the scale of the local system. In the southern part of the East Carpathians, the collision has favoured during the Pliocene - Quaternary significant crustal folding with km amplitudes both in subsidence taking place in the foreland and uplift in the adjacent thin-skinned units. In the back-arc domain, the differential vertical movements associated with the overall compressional stress regime in the corridor between the SE Carpathians foreland and the easternmost Pannonian basin suggest a significant role for a crustal folding mechanisms with large scale effects in the neotectonic, climate and surface processes, previously not taken into account. Analysis of the vertical movements amplitudes indicates a gradual decrease from kilometres magnitude in the Carpathians foreland to hundreds of meters near the Pannonian basin, with a time migration of the folding null-point.
T52B-05 11:20h
Passive-active margin inversion along the Levant plate-boundary: Subduction birth and growth of Mt Lebanon.
Evidence obtained during the SHALIMAR cruise clarifies the Miocene to Present tectonic evolution of the Lebanese segment of the Arabia/Africa plate boundary. A 90 km-long, 30 km-wide submarine fold and thrust belt deforms Neogene carbonates, Messinian evaporites and Plio-Quaternary turbidites offshore Mount Lebanon. The fold belt faces the broadest ($\approx$ 40 km) and highest ($\approx$ 3100 m) part of the mountain, and the steepest continental slope of the Levantine Basin margin, underwater continuation of the great Lebanese flexure. It is simply interpreted as a thin-skinned, foreland migrating thrust wedge, typical of active mountain piedmonts in the Alpine ranges of Eurasia. This belt is limited to the north by the Aabd\'e oblique thrust-ramp, and to the south by a left-stepping, offshore branch of the Roum lateral ramp. That neither of these ramps, or other faults cutting Miocene to Quaternary deposits extend farther into the Basin towards Cyprus indicates that the arcuate Mt Lebanon Thrusts and the Yammouneh fault, with which they connect north of Koubayate and south of Marjayoun, form a local, closed system. As they plunged down under the Lebanese flexure to merge at depth with the fault, the thrusts have governed the growth and rise of the mountain. Since the Mid-Miocene ($\approx$ 15 Ma), this thickening wedge of continental crust was extruded upwards, due to complete slip-partitioning in the Lebanese transpressive bend. Geological sections suggest that the total amount of shortening onshore and offshore is a few tens of kilometers. That the thickening offshore sediments are decoupled from the Levantine basin's basement implies that the mantle lithosphere and $\approx$10km-thick crust of that basin subduct beneath Mount Lebanon. The 25$\deg$CW Lebanese restraining bend and correlative inversion of the Levant passive margin into an active margin only offshore Lebanon result from the 180$\deg$ inversion of tectonic stresses between the Cretaceous and Miocene and propagation of the Dead Sea Transform. The NE-SW striking normal faults observed in Mt Lebanon and the Palmyrides are also clear in the Levant basin from the SHALIMAR bathymetric, gravity and magnetic data. They formed during the NW-SE late Jurassic extension event that led to the final opening of the Levant basin. As it propagated northwards in the Miocene, the left-lateral Dead Sea fault was likely deviated by one prominent Jurassic fault zone, the Damietta-Beyrouth line. This zone marks the southeastern margin of the deepest part of the Basin, with particularly thin (oceanic?) crust. The resulting clockwise swing of the fault, where it reached nearest to the basin margin, naturally led to fault perpendicular shortening, and to successful, runaway underthrusting of dense lithosphere under Mt Lebanon. This illustrates a mechanism for triggering subduction at passive margins reactivated by strike-slip faulting.
T52B-06 11:35h
Seismotectonics of the Cyprus Arc and Dead Sea Fault Zone: Eastern Mediterranean
This presentation is concerned with the seismotectonics of Cyprus Arc, Dead Sea Fault Zone, and surrounding regions that are prominent features in the collision zone between the African and Arabian plates accommodating the relative motion between the plates, and provides us with a relatively simple pattern of deformation. Seismotectonics of the Eastern Mediterranean region is evolved as a result of the interaction between the African, Arabian and Eurasian plates. Most of the seismic activity are concentrated and released along the major tectonic boundaries: Dead Sea Transform Fault, Hellenic and Cyprus Arcs, the East and North Anatolian Faults and Bitlis-Zagros suture zone. The Cyprus Arc extends from the Antalya Bay to the Gulf of Iskenderun (SE Turkey), eventually reaching East Anatolian Fault Zone. The nature of relative motion and the geometry of Cyprus Arc, and Dead Sea Fault Zone have been closely examined. We have studied the tectonic characteristics of a comprehensive seismicity occurred in the region during the last 15 years, based on newly retrieved and obtained source mechanisms and rupture histories of $\sim$15 earthquakes, M$_w$$>$5.3, by using waveform inversion of teleseismic long-period P-, SH-, and broad-band P-waveforms recorded in the distance range of 30-90$\deg$, for which signal amplitudes were large enough, with synthetic waveforms. Seismograms are generated by combining direct (P, S) and reflected (pP and sP or sS) phases from a point source embedded in a given velocity structure. The solutions were also constrained by P-wave first motion polarities of near-field stations, and InSAR data where available. Source mechanisms exhibit the characteristics and structural complexities associated with strike-slip, thrust and normal faulting as a result of ongoing crustal deformation. The character of the faulting along the Dead Sea Fault Zone exhibits mainly left-lateral strike-slip faulting with normal component that is in a good agreement with the geology and neotectonic features of the region. In the Gulf of Aqaba region, focal depth of earthquakes become shallower and scarcely reaches to the lower crust. Focal depth distribution exhibits the thinning of the seismogenic zone in the south that is probably related to the increase in the heat flow distribution towards the Red Sea. At the southwestern part of Cyprus, most of the mechanisms show right-lateral strike-slip faulting with thrust component, but some earthquakes with shallow focal depths confirm the normal faulting with strike-slip component.
T52B-07 11:50h
Normal and transcurrent convergence along the Cyprian Arc
The Cyprian Arc forms the plate boundary between the Anatolian plate in the north and the Nubian and Sinai plates in the south. We examine the tectonic setting and seismic activity along the arc in light of new geodetic studies indicating relative NE-SW plate motions across the arc. The first order tectonic variations are determined by the arc's geometry. The eastern arc, oriented sub-parallel to relative motion, is dominated by transcurrent tectonism. The western arc is oriented almost normal to relative plate motion and is subjected to convergent processes. Variations in the level and depth of seismic activity along the western arc suggest that the northwestern section of the arc represents a subduction boundary, whereas the southeastern section represents a collision boundary. The two tectonic domains of the western arc are separated by a NE-SW trending tear fault, which produces large earthquakes, such as the MW=6.8, 1996, Paphos Earthquake. We compare the geometrically similar Cyprian and Hellenic arcs and find significant differences in the rate, direction and type of convergence across the two arcs. The Hellenic Arc is subjected mainly to subduction, whereas the shorter Cyprian Arc is subjected to subduction, collision and transcurrent tectonic processes.