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Surface deformation in Morocco, derived from five years of global positioning system (GPS) survey observations of a 22-station network, four continuously recording GPS (CGPS) stations, and four International GNSS Service (IGS) stations in Iberia, indicates roughly southward motion (~3 mm/yr) of the Rif Mountains, Morocco, relative to stable Africa. Motion of the Rif is approximately normal to the direction of Africa-Eurasia relative motion, which is predominantly strike slip, and results in shortening of the Rif and subsequent crustal extension of the adjacent Alboran Sea region. The sense, and the N-S asymmetry of the observed deformation (i.e., no evidence for north-directed shortening in the Betic Mountains north of the Alboran Sea) cannot be easily explained in terms of crustal plate interactions, suggesting that dynamic processes below the crust are driving the recent geologic evolution of the western Mediterranean. The model that best fits the observations involves delamination and southward rollback of the African lithospheric mantle under the Alboran and Rif domains. Keywords: GPS, active tectonics, Alboran Sea, continental dynamics.

Investigation of the Bishri block, centrally positioned amid the diverse tectonic and structural zones of Syria, reveals details of the intraplate Phanerozoic development of the northern Arabian platform. The Bishri block is a broad NE-plunging inverted basin located at the NE portion of the Palmyride mountain belt where the mountains intersect the Euphrates fault system. Well and seismic data show that subsidence and sedimentation in the Bishri area was generally continuous from Carboniferous to Paleocene time, with the Bishri block pan of the extensive Palmyride-Sinjar trough. Major bounding faults and a rift-type environment are documented in the Permo-Triassic, Jurassic and Cretaceous. The present Bishri structural and topographic high has been formed through transpressive structural inversion since the Mid-Miocene; high-angle Mesozoic bounding normal faults now have net reverse offsets with a significant dextral strike-slip component. East of the Bishri block, towards the Euphrates fault system, NNW-SSE-striking normal faults exhibit less reverse movement. This deformation history correlates with the opening and closing of the nearby NeoTethys ocean that has driven the evolution of intracontinental Syria. Keywords: Palmyride mountain belt, Syria, seismic methods, tectonics, sedimentary basins.

Geological field mapping has revealed the existence of vertical strain partitioning and crustal shortening in Morocco's Middle Atlas Mountains. Geophysical information showed that the contraction has not created a significant crustal root underneath the fold belt. The apparent discrepancy in crustal thickening may be attributed to the shortening of the upper crust due to thickening and the corresponding lateral deformation of the lower crust.

A lack of dramatic surface geological structures along the Euphrates River in Syria belie a complex tectonic history revealed by newly released seismic reflection and well data. We document the intraplate Euphrates fault system, characterize the variation in structural style along its 350 km length in Syria, and infer its Mesozoic-Cenozoic tectonic and deformational history. We then relate the deformation of the Euphrates system and other proximate intraplate structures to nearby Arabian plate boundary processes in order to develop a new model for the tectonic evolution of the northern Arabian plate. Throughout most of Mesozoic time, the Euphrates area experienced minor deposition compared to the Palmyride trough to its southwest, and the Sinjar trough to its northeast. During latest Cretaceous time, however, significant sinistral transtension occurred along the length of the Euphrates fault system in Syria, with graben formation especially noteworthy in southeastern Syria. This episode was probably related to events at nearby plate boundaries, and may have reactivated a zone of weakness formed during Pan-African accretion of the Arabian plate. A Palaeogene sag basin formed over the graben system in southeastern Syria. Neogene continental collision along the northern and eastern Arabian plate boundaries caused minor reactivation of the Euphrates fault system in a dextral transpressional sense, in concert with significant inversion and the main phase of uplift of the nearby Palmyride and Sinjar mountains. Keywords: Syria, Arabian Plate, intraplate processes, seismic profiles, deformation.

Geological field observations are integrated with digital topography, LANDSAT imagery, and earthquake focal mechanisms to investigate the Middle and Late Quaternary tectonism in the intracontinental Middle Atlas mountain belt in northern Morocco. The NE-SW-trending Middle Atlas Mountains, approximately 80 km in width and about 200 km long, are part of the Atlas system of northwestern Africa and represent an inverted rift that developed into an intracratonic mountain system in the foreland of the Alpine collisional zone. The Middle Atlas is composed of two provinces, the Folded and Tabular Middle Atlas, representing the palaeo-rift and a flank of the palaeo-rift, respectively. Evidence for Late Quaternary tectonism is provided by the analysis of stream morphology in addition to geological relations. Kinematic analysis of fault-slip data and earthquake focal mechanisms demonstrate the coexistence of both extensional and compressional deformation in different areas of the Middle Atlas with a common sinistral component of slip along NE-SW-striking fault zones. Compressional features dominate the Folded Middle Atlas, whereas extension predominates in the Tabular Middle Atlas. Extension is also manifested by widespread Middle to Late Quaternary alkali volcanism. The observed kinematic variations appear to correlate with the Mesozoic palaeogeography; one possible model may involve tectonic escape. This suggests that differences in the structures inherited from the Mesozoic and Palaeozoic may influence the responses of the different regions to the Cenozoic Alpine collision between Europe and northwest Africa. Keywords: Morocco, neotectonics, intracontinental belts, kinematics, digital cartography.

Geophysical evidence has been obtained on the occurrence of lithospheric delamination beneath the Alboran Sea and the surrounding Rif and Betic mountain belts of the western Mediterranean. The most significant proof of active delamination in the sites is the occurrence of intermediate-depth tremors and their spatial distributions. A tectonic event which occurred in the early Miocene is believed to have caused instability within the lithosphere which has possibly become the driving mechanism for active delamination.

High-resolution digital topography (three arc-second grid) for most of Tibet provides new information to characterize the relief of the highest and largest plateau on Earth. The arid to semiarid central and northern part of the plateau interior has low relief (average slopes of approximately 5 degrees over 250 m windows) and a mean elevation of 5023 m above sea level. At moderate wavelengths of approximately 100 km, relief is approximately 1 km or less for most of Tibet, as opposed to the much higher relief of up to 6 km on the plateau edges, where glacial and fluvial dissection is greater because of higher levels of precipitation. The only faults manifesting significant topographic relief are the relatively small scale, generally north-trending graben systems, primarily in southern Tibet, and several large-scale fault systems near the edges of Tibet. The flatness of Tibet implies that (1) there has been little deformation (especially shortening) of the uppermost crust north of the graben systems during the late Cenozoic, and (2) shallow crustal isostatic compensation has been acting to level the surface of the plateau.

The northern extension of the Dead Sea transform fault in southern Lebanon bifurcates into several faults that cross Lebanon from south to north. The main strand, the Yammouneh fault, marks the boundary between the Levantine (eastern Mediterranean) and Arabian plates and separates the western mountain range (Mount Lebanon) from the eastern mountain range (Anti-Lebanon). Bouguer gravity contours in Lebanon approximately follow topographic contours; i.e., positive Bouguer anomalies are associated with the Mount Lebanon and Anti-Lebanon ranges. This suggests that the region is not in simple isostatic compensation. Gravity observations based on 2.5-dimensional modeling and other available geological and geophysical information have produced the following interpretations. (1) The crust of Lebanon thins from approximately 35 km beneath the Anti-Lebanon range, near the Syrian border, to approximately 27 km beneath the Lebanese coast. No crustal roots exist beneath the Lebanese ranges. (2) The depth to basement is approximately 3.5-6 km below sea level under the ranges and is approximately 8-10 km beneath the Bekaa depression. (3) The Yammouneh fault bifurcates northward into two branches; one passes beneath the Yammouneh Lake through the eastern part of Mount Lebanon and another bisects the northern part of the Bekaa Valley (i.e., Mid-Bekaa fault). The Lebanese mountain ranges and the Bekaa depression were formed as a result of transtension and later transperssion associated with the relative motion of a few crustal blocks in response to the northward movement of the Arabian plate relative to the Levantine plate.

This study examines the crustal structure of the Palmyrides and the northern Arabian platform in Syria by two- and three-dimensional modeling of the Bouguer gravity anomalies. Results of the gravity modeling indicate that (1) western Syria is composed of at least two different crustal blocks, (2) the southern crustal block is penetrated by a series of crustal-scale, high-density intrusive complexes, and (3) short-wavelength gravity anomalies in the southwest part of the mountain belt are clearly related to basement structure. The crustal thickness in Syria, as modeled on the gravity profiles, is approximately 40 [+ or -] 4 km, which is similar to crustal thicknesses interpreted from refraction data in Jordan and Saudi Arabia. The different crustal blocks and large-scale mafic intrusions are best explained, though not uniquely, by Proterozoic convergence and suturing and early Paleozoic rifting, as interpreted in the exposed rocks of the Arabian shield. These two processes, combined with documented Mesozoic rifting and Cenozoic transpersion, compose the crustal evolution of the northern Arabian platform beneath Syria.

We examine the structure and evolution of the Ghab basin that formed on the active, yet poorly understood, northern segment of the Dead Sea transform fault system. The basin formed in Plio-Quaternary time at a complex step-over zone on the fault. Subsidence occurred along cross-basin and transform-parallel faults in two asymmetric depocentres. The larger depocentre in the south of the basin is asymmetric towards the east, the margin along which most active transform displacement apparently occurs. The Syrian Coastal Ranges, located directly west of the Ghab basin, are a consequence of Late Cretaceous and Cenozoic regional compression, heavily modified by the Dead Sea fault system and Ghab basin formation. We prefer a model whereby the Dead Sea fault system in northwest Syria developed in Plio-Quaternary time, consistent with previously proposed models of two-phase Dead Sea fault system movement and Red Sea spreading. Keywords: Dead Sea Rift, Syria, pull-apart basins, seismic methods, tectonics.

The magnitudes and timing of deformation in the intracontinental Atlas Mountains of northern Africa suggest that the Atlas Mountains have been an integral part of the African-Eurasian plate-boundary zone in the western Mediterranean during the Cenozoic. Shortening of the Moroccan Atlas has accommodated 17%-45% of the total African-Eurasian plate convergence since the early Miocene, whereas the majority of the plate convergence is accommodated in the Rif-Betic-Alboran region. Although the latter underwent other geodynamic processes, as demonstrated by extension of the Alboran Sea contemporaneous with plate convergence, shortening directions in the Atlas are generally consistent with ongoing plate convergence and show no influence of these additional processes. In the framework of plate tectonics, the western Mediterranean region, including the Atlas system, should be regarded as a diffuse plate boundary in which the Atlas Mountains comprise narrow deformable zones bounding larger, relatively rigid crustal blocks. The deformable zones reflect the influence of crustal structures inherited from a major early Mesozoic episode of intracontinental rifting in the Atlas. Keywords: Mediterranean, Cenozoic, plate tectonics, crustal shortening, Morocco.

The High Atlas Mountains of Morocco formed over a major intracontinental rift system when African and European plates converged during the Cenozoic. A geophysical-geological cross section across the central High Atlas Mountains to understand better how intracontinental mountain belts are formed. This allows the study of the kinematic history of the mountain belt and how strain is partitioned. The results show the huge difference between the evolution and current structural architecture of the mountain belt, compared to mountain belts associated with plate boundary processes.