ItemLithospheric and upper mantle structure beneath northern Morocco and central SyriaSeber, Dogan (Cornell University, 1995)Northern Morocco and central Syria accommodate two of the most significant intraplate mountain belts on earth: the Atlas Mountains (High and Middle) and the Palmyride mountains, respectively. In contrast to interplate mountain belts like the Rif mountains in northern Morocco, intraplate mountain belts develop away from any plate boundaries. Hence, their formation is more difficult to explain. In this dissertation, seismological data from a recently installed digital seismic network in Morocco along with other available datasets, such as Bouguer gravity, seismic reflection, and surface geology, are analyzed in order to map the three-dimensional structure of the lithosphere and upper mantle beneath northern Morocco. Seismic data are also used in explaining some aspects of earthquake hazards in Morocco. New geodynamic models are proposed for both the Atlas and Rif mountains of northern Morocco. Teleseismic tomography results show that the lithosphere beneath the Atlas mountains is relatively thinner as evidenced by slower velocity anomalies. In contrast, beneath the Rif mountains a relatively fast upper mantle velocities are observed. Isostatic gravity anomalies show that the central High Atlas has a thick (~45 km) and isostatically compensated crust, whereas the Middle Atlas with a crustal thickness of about 30 km is not compensated, and that they are probably dynamically supported. The spatial distribution of intermediate-depth seismicity, regional seismic waveform propagation characteristics, Bouguer gravity anomalies, seismic reflection and drill hole data as well as surface geology are used to argue that the lithosphere beneath the Rif region has delaminated and it is sinking into the asthenosphere. This ongoing delamination process is proposed to have formed the Rif and Betic mountain belts around the Alboran Sea. The Palmyride intraplate mountain belt in central Syria, which shows a similar geologic history to the Atlas system of Morocco, is also studied. The upper part of the crust is mapped in central Syria beneath the Palmyrides fold-thrust belt and adjacent areas using very dense seismic refraction data. The results show that beneath the axis of the Palmyrides mountain belt a deep (~11 km) trough, formed in the Mesozoic, exists despite the Cenozoic inversion and uplift. ItemSeismicity and active tectonics of the Himalayas and Tibetan PlateauNi, James (Cornell University, 1984)Available geophysical and geological data are analyzed with additional new data to further the understanding of the fundamental tectonic processes involved in the Himalayan-Tibetan continental collision zone. Seismicity of the Himalayas suggests that at present the Indian Plate underthrusts the Himalayas as a coherent unit along a shallow detachment. The geometry of this detachment beneath the Lesser Himalayas is constrained by well data and well-determined focal depths of moderate-sized earthquakes. This detachment surface, at or near the top of the downgoing Indian plate, dips at approximately a 15 degree angle from about 10-km to 20-km depth. This result supports a model of the active tectonics of the Himalayas as "thin-skinned" and analogous to the Paleozoic tectonics of the southern Appalachian Collision Zone. New seismological observations of velocities and propagation characteristics of Pn, Sn and Lg waves beneath the Himalaya-Tibet and surrounding region can be interpreted, although not uniquely, to indicate the shallow-angle underthrusting of the Indian continental lithosphere beneath the Tibetan Plateau. The most significant observation is that, except beneath the northern part, high-frequency Sn waves propagate efficiently in the uppermost mantle beneath the Tibetan Plateau. Strong attenuation of Sn waves suggests the existence of a low-Q zone in the uppermost mantle beneath northern Tibet. Analysis of Landsat imagery and fault plane solutions of shallow crustal earthquakes in both the Tethyan Himalayas and Tibet indicate that normal faulting and east-west extension are the dominant mode of deformation occurring in the late Cenozoic time. The normal faulting is due to an east-west deviatoric tensional stress within the elevated Tethyan Himalayas and Tibet. Seismicity combined with structural elements mapped from digitally processed Landsat 3 Multispectral Scanner (MSS) data provide valuable information about neotectonic processes in the overthrusting western Himalayan blocks. The rhomboidal-shaped upper Sutlej River Basin consists of many NNE-trending fault blocks and is interpreted as a pull-apart basin. This pull-apart basin is explained as a result of oblique underthrusting of the Indian plate beneath Himalayas-Tibet. ItemLate Cenozoic tectonics of the Middle Atlas Mountains, Morocco: Continental deformation in the diffuse western Mediterranean plate boundaryGomez, Francisco (Cornell University, 1999)The Atlas Mountains of North Africa, located in the African foreland of the Alpine mountain belts, comprise a 2,000 km long Cenozoic mountain chain whose development was guided by older Mesozoic rift structures. This dissertation examines one component, the Middle Atlas Mountains of Morocco, using geological and geophysical data to constrain the tectonic development of the mountain chain. These results are then placed in the broader context of regional deformation. The NE-SW trending Middle Atlas Mountains are obliquely oriented within the late Cenozoic regional stress field, resulting in deformation partitioned into strike-slip faulting and thrust-related folding. Kinematic analyses of fault-slip data and earthquake focal mechanisms demonstrate that compressional deformation dominates the Folded Middle Atlas, whereas strike-slip faulting, with possible horizontal extension, predominates in the Tabular Middle Atlas. Geological field observations, digital topography, LANDSAT imagery, and seismicity provide evidence for recent tectonics in the Middle Atlas. In the central Middle Atlas, cross-section balancing across the 20 km wide fold belt demonstrates about 4.7 km of Cenozoic horizontal shortening producing 800 m of structural relief. Other constraints on crustal thickening suggest a discrepancy between contraction and thickening. One possible explanation involves partitioning crustal deformation with depth: The upper crust shortens by thickening (faulting and folding), whereas the lower crust deforms laterally. At the northern extent of the mountain chain, the Guercif Basin developed where the Middle Atlas abut the Rif thrust belt. Similar timing of extensional deformation and proximity with the Rif, suggest that the Guercif Basin has been influenced by Rif tectonics. Stratal relations demonstrate that uplift of the Middle Atlas is a late Cenozoic phenomenon. In Morocco, shortening of the High and Middle Atlas Mountains accommodated 20-45% of the total African-Eurasian plate convergence since the Early Miocene. The diffuse plate boundary comprises large, relatively rigid crustal blocks (Moroccan Meseta, High Plateau, and Saharan Platform) bounded by narrow deformable zones (the Atlas). In this context, the Middle Atlas can be interpreted as an accommodation zone resulting from differential movements between two large crustal blocks impinging on stable Africa. The Atlas Mountains exemplify the possible structural influence of inherited crustal weaknesses in a diffuse plate boundary such as the western Mediterranean region. ItemBalanced cross sections, seismic stratigraphy, and structural interpretation of the intracontinental Palmyride fold belt, SyriaChaimov, Thomas (Cornell University, 1991)The Palmyride fold belt in central Syria is the result of Late Mesozoic and Cenozoic inversion of a Late Paleozoic and Mesozoic intraplate trough located within the northern Arabian platform. Detailed analysis of available seismic reflection profiles from the Palmyrides reveals the Late Mesozoic to present transpressive structures of the Palmyrides and clarifies the timing and magnitude of such deformation within the belt. Uplift of the Mesozoic Palmyride trough began in the Late Cretaceous, rejuvenated in the Middle Eocene, and culminated in the period from the Miocene to present. Each of the three episodes of deformation was temporally associated with a distant (~300 km) Arabian plate margin tectonic event as follows: (1) Late Cretaceous collision between the northern and eastern margin of the Arabian plate and a microplate or island arc; (2) Middle Eocene incipient faulting of the Red Sea/Dead Sea fault system; and (3) Miocene to present shortening of the Arabian-Eurasian plate collision zone along the Bitlis/Zagros suture in Turkey and Iran. Despite this repeated tectonism, only 20-25 km of shortening accumulated in the southwestern, most strongly deformed sector of the belt, diminishing to only a few kilometers 400 km along strike to the northeast. And although Triassic evaporites form local detachment surfaces, there has been no large-scale lateral transport of Mesozoic and Cenozoic rocks over Paleozoic rocks in the Palmyrides of Syria. Rather, deep structures in Paleozoic rocks appear to be in general concord with structures in overlying Mesozoic and Cenozoic rocks. ItemCrustal evolution of the northern Arabian Platform beneath the Syrian Arab RepublicBest, John (Cornell University, 1991)Newly released geological and geophysical data from the Syrian Arab Republic are used to document the geological history of the northern Arabian platform in the Middle East. The primary observation of this synthesis is the focusing of various phases of Phanerozoic deformation (Mesozoic rifting and Cenozoic transpression) along strike of a proposed Proterozoic suture that has acted as a long-lived zone of crustal weakness. This deformation zone is presently manifested by the intracontinental Palmyride mountain belt, an inverted rift, trending NE-SW through central Syria. The geological history recognized for the northern Arabian platform is similar in many respects to that of the southern Arabian platform, including: (1) Proterozoic convergence and cratonization, (2) minor Cambrian extension, (3) a relatively stable Paleozoic margin of Gondwanaland marked by predominantly clastic deposition, (4) eastward tilting of the Arabian plate in the Cenozoic. The important difference in the evolution of the northern platform from the southern platform occurs during the Mesozoic with the development of the Levantine margin in the eastern Mediterranean and the Palmyride rift in the continental interior. The intracontinental Palmyride mountain belt is the result of Late Cretaceous-present inversion of the Palmyride rift. Reactivation of rift boundary faults occurred in response to transpressive movement along basement-controlled strike-slip faults. The belt is divided into three provinces based on changes in structural style: the south Palmyride fold belt characterized by narrow, en enchelon ridges, the Bishri, and Bilas blocks expressed as broad, antiformal structures. The mountain belt may be characterized by thin-skinned deformation in the south fold belt and thick-skinned deformation in the northern and eastern provinces. ItemLithospheric structure of the western United States and the Tibetan Plateau: Implications on their mechanism of upliftBeghoul, Mohammed Noureddine (Cornell University, 1991)This dissertation seeks to determine the upper mantle structure and the mechanisms responsible for the Cenozoic uplift of the high terranes located near plate boundaries: the Tibetan Plateau and western North America. The upper mantle structure is determined using the first P arrivals obtained from the International Seismological Centre (ISC) at regional distances (2 - 22 degrees). In the first chapter a methodology is presented for computing mantle lid Pn velocities using ISC data together with a detailed error analysis. Application of this algorithm to Colorado Plateau yields an average Pn velocity of 8.12 +/- 0.09 km/s. This value is higher than the one reported in the literature but similar to that beneath stable midcontinent regions. We use this Pn value and the Cenozoic history of the plateau to constrain the mode of uplift. In chapter two, using the same techniques, we confirm the lower Pn velocity beneath the Basin and Range Province and show the presence of about 4% intrinsic azimuthal Pn velocity anisotropy in the mantle lid beneath the Basin and Range. The direction of high velocity coincides with the direction of present-day extension in the Basin and Range Province (i.e., NW - SE). We show that this anisotropy is the result of Cenozoic extension rather than a cumulative signature of older tectonic events. In chapter three, a modified version of the algorithm and detailed mapping of Sn attenuation allow the determination of mantle lid thickness beneath the western United States and Tibet. We show that the mantle lid thickness beneath the southern 2/3 of Tibet ranges from 135-165 km thick. This value is similar to the one we find for the Great Plains. The deep structure, Cenozoic uplift, and various other geophysical and geological data of these two high terranes are consistent with the subduction of flat slabs beneath them. The continental Indian Plate is still beneath the southern 2/3 of Tibet, but the oceanic Farallon Plate has already been delaminated from the overriding North American Plate. ItemTectonic evolution of the Atlas Mountains, North AfricaBeauchamp, Weldon (Cornell University, 1998)The Atlas Mountains of North Africa are one of the largest intracontinental mountain belts in the world. Despite the size of this orogen, the basic kinematic and tectonic evolution of the Atlas Mountains has previously not been well understood. These mountains formed hundreds of kilometers from active plate margins. The formation of the Atlas Mountains was greatly influenced by a previous Mesozoic intracontinental rift system. This rift system spanned half of the African continent and was larger in breadth than the Red Sea. This study set out to synthesize existing data and studies of the Atlas Mountains and integrate these data with new geological, geophysical and remote sensing data. The construction of a tectonic map was undertaken to define the tectonic units and terraines of North Africa. The delineation of these regions allow for the study of how they have interacted during the kinematic evolution of the Atlas system. Geological field work was undertaken to study the kinematics of inversion tectonics and to construct a balanced geological-geophysical transect. The transect suggests shortening across the orogen (36 km) was achieved by thrusting along detachments at several levels in the upper crust. Syn-rift and post-rift sedimentary rocks were uplifted by the reactivation of Synrift normal faults and newly formed thin-skinned thrust faults. A restoration of the deformed cross section indicates the original Atlas rift basin was approximately 113 kilometers wide. Shortening across the High Atlas Mountains resulted in a partitioning of strain, with the greatest magnitude of shortening occurring along the margins of the High Atlas Mountains. The partitioning of strain may involve the transfer of shortening from the margins at shallow depths, to the mid-lower crust in the central region of the orogen. Thrusting in the High Atlas Mountains is bivergent, with thrusts dipping to the south along the northern margin, and northward dipping faults to the south. The presence of preexisting structural geometries such as accommodation zones, fault ramps, fault relays and en echelon faulting formed by rift processes will have an effect on subsequent compressional stress fields generated by plate convergence and other tectonic processes. Superposed folding which is disharmonic may in fact be a unique characteristic to inverted rift systems that result in intracontinental mountain belts. ItemTectonic evolution of Syria Interpreted from Integrated Geophysical and Geological AnalysisBrew, G. (Cornell University, 2001-01)Using a variety of geophysical and geological data, the Phanerozoic tectonic evolution of Syria has been interpreted. The study is inspired by the diverse styles of tectonic deformation within Syria generated by long-lived proximity to active plate boundaries. The work is also relevant to hydrocarbon exploration. The availability of seismic reflection and refraction profiles, wells, and other resources made this research possible. Three studies focused on specific areas of Syria are presented. The first is a seismic refraction interpretation along a north-south profile in eastern Syria. The results show that metamorphic basement depth (and hence Paleozoic thickness) in southeast Syria is greater, by >2 km, than that in the northeast. The next study interprets the structure and tectonics in northeast Syria. During Late Paleozoic and Mesozoic time northeast Syria was an extension of the Palmyride trough. In the Maastrichtian, regional extension opened the Abd el Aziz and Sinjar graben that were structurally inverted in the Late Cenozoic to form the present topography. The third study concerns the Ghab Basin in western Syria. This 3.4 km deep Plio- Quaternary pull-apart basin suggests that the Dead Sea Fault System has only been active in Syria since the end of the Miocene in accordance with a two-phase model of Red Sea opening. The final study integrates the previous interpretations with new work to provide a tectonic evolutionary model that shows the Phanerozoic development of all Syria. This model is closely tied to stratigraphic data that improve the interpretation of many tectonic events, and put the results into a paleogeographical context. The model shows how specific deformation episodes within Syria have been penecontemporaneous with regional plate tectonic events. The Late Paleozoic / Mesozoic northeast trending Palmyride / Sinjar trough formed across central Syria in response to Permo-Triassic opening of the NeoTethys Ocean. Proximal subduction in the NeoTethys created the Late Cretaceous Euphrates Fault System and Abd El Aziz / Sinjar graben in eastern Syria. Late Cretaceous to Late Miocene collisions and shortening along the northern Arabian margin caused platform-wide structural inversion, uplift, and shortening. This compression continues today under the influence of Arabia / Eurasia convergence. ItemLithospheric Structure of the Arabian Plate and Surrounding RegionsAl-Damegh, Khaled (2004-05)Continuous waveform recording from a newly established broadband seismic network in Saudi Arabia, in addition to data produced by other stations in the region, were used to map regional seismic wave propagation (Lg and Sn) and Pn attenuation. Moreover, crustal thickness in the Arabian plate was also estimated based on receiver function analysis. Zone blockage and inefficient Sn propagation is observed along and to the east of the Dead Sea fault system and in the northern portion of the Arabian plate (south of the Bitlis suture)., We observed Sn blockage across some segments of the Red Sea. These regions of high Sn attenuation have anomalously hot and possibly thin lithospheric mantle (i.e., mantle lid). Consistent with our Sn attenuation findings, we also observed low Qpn along the western portion of the Arabian plate and along the Dead Sea fault system. Our results imply the presence of a major anomalously hot and thinned lithosphere in these regions that may be caused by the extensive upper mantle anomaly that appears to span most of east Africa and western Arabia. These mapped zones of high attenuation closely coincide with an extensive Neogene and Quaternary volcanic activity. We found that the average crustal thickness of the Arabian shield is 39 km. The crust thins to about 23 km along the Red Sea coast and to about 25 km along the Gulf of Aqaba. We observed a dramatic change in crustal thickness between the topographic escarpment of the Arabian shield and the shorelines of the Red Sea. We compared our results in the Arabian shield to nine other Proterozoic and Archean shields that include reasonably well-determined Moho depths. We do not observe a significant difference between Proterozoic and Archean crustal thickness. Our observations show that the transition from oceanic to continental crust along the Red Sea margin occurs over a relatively short distance compared to a typical west Atlantic continental margin. We argue that the anomalous nature of the Red Sea margin may be one of the consequences of the presence of a mega plume that extends from the core-mantle boundary into the upper mantle beneath east Africa, the Red Sea, and the western portion of the Arabian plate. In addition, the site where the sea-floor spreading of the Red Sea occurred was a Proterozoic suture and a zone of weakness. These observations combined may explain the relatively abrupt breakup of the Arabian plate and the anomalous nature of the Red Sea margin. ItemCrustal and upper mantle structure of Oman and the Northern Middle EastAl-Lazki, Ali (Cornell University, 2003-01)This dissertation focuses on studying the crustal structure on the southeast margin and foreland of Arabia in Oman, and upper mantle rheology and structure at the zone of interaction between the Arabian, Eurasian, and African plates (Figure 1.1). At the center of the study area, the Arabian plate is bounded in the east by the Indian plate along the Owen and Murray Transform Fault zones, in the northeast and north it is bounded by the Eurasian plate along the ZagrosBitlis Suture zones, and in the west, northwest, and southwest it is bounded by the African plate along the Dead Sea Fault, the Red Sea, and the Gulf of Aden (Figure 1.1). Northwest of Arabia, the Hellenic and the Cyprean arcs define the convergence boundary between the African plate and the Anatolian plate in eastern Mediterranean Sea (Figure 1.1). One of the most important events throughout geologic history of the region is the closure of the NeoTethys ocean. It began in Early Cretaceous along the eastern and northeastern boundaries of the Arabian-Africa Plate and lasted to Pliocene times (Sengor and Yilmaz, 1981). Ophiolite emplacement is a process that commonly accompanied the closure and subduction of the NeoTethys ocean. At present day a belt of NeoTethyan ophiolites follows the suture zone between the Arabian-Eurasian plate boundary and farther west within the Anatolian plate (Figure 1.1). While at the north and northeast boundaries of the Arabian plate the closure of the NeoTethys and final suturing processes have concluded and resulted in the building of the Iranian-Anaoltian plateaus, at the southeast Arabian plate boundary, a piece of the NeoTethys oceanic lithosphere (Semail Ophiolites) was emplaced in the late Cretaceous, but the closure process is still ongoing by subducting the remnant basin of Oman at the Makran Subduction zone (Figure 1.1). At a later stage, the opening of the Red Sea and Gulf of Aden is thought to have occurred episodically (Hempton, 1987). An initial phase occurring in the period Middle-Late Eocene and a later phase occurred in the Early Pliocene (~14.5 Ma) (Hempton, 1987). This separation of Arabia from Africa accommodated by the left lateral Dead Sea Fault System is thought to be responsible for the reorganization of relative plate motions in the Anatolian Plateau (Eurasian plate) (Sengor and Yilmaz, 1981). In early Pliocene, continued N-S convergence between Arabia and Eurasia resulted in the extrusion of an Anatolian plate along the North Anatolian Fault (NAF) and the East Anatolian Fault (EAF) zones (Bozkurt, 2001). The Anatolian plate's westward escape is converging along the Hellenic and Cyprean subduction zones, where Africa's oceanic lithosphere is being subducted. Chapter two of this dissertation presents a detailed study of the crustal structure along 255 km long transect that includes the hinterland, the mountains, and the foreland of Oman. The main objective of this study is to investigate the crustalscale structure of the eastern Arabian margin, across the 3,000 meters high Oman Mountains. Various geophysical and geological data are used to model the crustal thickness along the transect. We used exploration seismic and well data to constrain the upper 78 km of the sedimentary column, receiver function to infer Moho depth along the transect, and gravity modeling to constrain Moho lateral variations and infer a basement depths along the transect. Furthermore, integrated geological and geophysical data shed valuable information about the processes that accompanied the Semail Ophiolite emplacement. Chapter three focuses on the young continent-continent collision zone between northern Arabia and Eurasia along the Bitlis-Zagros Suture zone. We use Pn tomography to further our knowledge about the mantle lithosphere rheology and structure and its contribution to lithosphere dynamics at the young Bitlis-Zagros continent-continent collision zone. Pn velocities higher than 8 km/s are used to infer stable mantle lid, while Pn velocities less than 8 km/s are used to infer mantle lid instability. Chapter four presents evidence on upper mantle rheology using Pn velocity and structure and using Pn anisotropy at the junction of the Arabian, Eurasian, and African plates. This research looks at the larger scale picture of the three plates' interactions and use Pn velocity and anisotropy to contrast regions underlain by stable mantle lid from those unstable and to investigate uppermost mantle processes. This study, also, focuses on regions underlain by small scale (< 200 km) very low Pn velocity anomalies that indicate thinned to absent mantle lid. This study compares Pn velocity with Sn attenuation map of the region. It also compares observed Pn azimuthal anisotropy with shear wave SKS polarization anisotropy to infer asthenospheric mantle deformation.