Tomographic Pn velocity and anisotropy structure beneath the Anatolian plateau (eastern Turkey) and the surrounding regions

An edited version of this paper was published by the American Geophysical Union (AGU). Copyright 2003, AGU. See also: http://www.agu.org/pubs/crossref/2003.../2003GL017391.shtml; http://atlas.geo.cornell.edu/turkey/publications/Al-Lazki-et-al_2003.htmWe use Pn phase travel time residuals to invert for mantle lid velocity and anisotropy beneath northern Arabia eastern Anatolia continent-continent collision zone. The primary phase data were obtained from the temporary 29- station broadband PASSCAL array of the Eastern Turkey Seismic Experiment. These data were supplemented by phase data from available stations of the Turkish National Seismic Network, the Syrian National Seismic Network, the Iranian Long Period Array, and other stations around the southern Caspian Sea. In addition, we used carefully selected catalog data from the International Seismological Centre and the National Earthquake Information Center bulletins. Our results show that low (<8 km/s) to very low (<7.8 km/s) Pn velocity zones underlie the Anatolian plateau, the Caucasus, and northwestern Iran. Such low velocities are used to infer the presence of partially molten to absent mantle lid beneath these regions. In contrast, we observed a high Pn velocity zone beneath northern Arabia directly south of the Bitlis-Zagros suture indicating the presence of a stable Arabian mantle lid. This sharp velocity contrast across the suture zone suggests that Arabia is not underthrusting beneath the Anatolian plateau and that the surface suture extends down to the uppermost mantle. Pn anisotropy orientations within a single plate (e.g. Anatolia plate) show a higher degree of lateral variation compared to Pn velocity. Areas of coherent Pn anisotropy orientations are observed to continue across major fault zones such as the EAF zone

Structure and tectonic evolution of the Anatolian plateau in eastern Turkey

This paper was published by the Geological Society of America (GSA). Copyright 2006, GSA. See also: http://granite.geosociety.org/bookstore/default.asp?oID=0&catID=9&pID=SPE409; http://atlas.geo.cornell.edu/turkey/publications/Barazangi-et-al_2006.htmThe Cenozoic geology and the present lithospheric and upper mantle structure of the Anatolian plateau in eastern Turkey and nearby regions are the result of the final collision and suturing of the continental Arabian plate to the Turkish terranes (i.e., micro-continents). This process of collision and suturing was strongly influenced by three active structures in the region: the Caucasus mountains, the Aegean subduction zone, and the Dead Sea fault system. Understanding these three major tectonic elements are important for the development of a robust model for the formation of the Anatolian plateau. We show that the Anatolian plateau lithosphere in eastern Turkey has no lithospheric mantle, i.e., the crust floats on a partially molten asthenosphere. The average thickness of the crust in the region is approximately 45 km. The uppermost mantle beneath this crustal block strongly attenuates Sn waves and has one of the lowest Pn velocities on earth (about 7.6 km/s). The Anatolian plateau, with an average of 2 km elevation is dissected by numerous active, seismogenic faults (mostly strike-slip and some thrust type). Neogene and Quaternary volcanism with varying composition is widespread and covers more than half of the region. We argue that the northward subduction of the northern and the southern branches of the Neo-Tethyan oceanic lithosphere since the Mesozoic has resulted in the development of arc and back-arc volcanism (i.e., the Pontide and Bitlis systems) and the development of the eastern Anatolian accretionary complex that covers a large area of eastern Turkey. The northward subduction of the southern Neo-Tethys considerably thinned and weakened the overriding Eurasian plate above the descending oceanic lithosphere of the Arabian plate. The final suturing of the continental Arabian plate with the Turkish terranes in the Miocene and the continued convergence of Arabia relative to Eurasia has resulted in the shortening of the accretionary complex both in the forearc and the back-arc regions and the development of a broad zone with numerous strike-slip faults. The mobilization of the Caucasus is also partially a consequence of this convergence. The documented major episode of widespread volcanism at about 11 Ma is probably related to the breakoff of the shallowly descending oceanic segment of the Arabian lithosphere beneath eastern Turkey. The continued convergence of Arabia relative to Eurasia has resulted in the development of the North Anatolian fault (NAF) and subsequently the East Anatolian fault (EAF) in the Pliocene. At about this time, the northern segment of the Dead Sea fault (DSF) also developed in Lebanon and northwest Syria and joined the EAF to form the Anatolian - Arabian - African triple junction in the Maras region of southern Turkey. The development of these fault systems (i.e., NAF, EAF, and DSF) provided the mechanism for the tectonic escape of the Anatolian crustal block towards the Aegean arc system

The crustal structure of the north-eastern Gulf of Aden continental margin: insights from wide-angle seismic data

International audienceThe wide-angle seismic (WAS) and gravity data of the Encens survey allow us to determinethe deep crustal structure of the north-eastern Gulf of Aden non-volcanic passive margin.The Gulf of Aden is a young oceanic basin that began to open at least 17.6 Ma ago. Itscurrent geometry shows first- and second-order segmentation: our study focusses on theAshawq–Salalah second-order segment, between Alula–Fartak and Socotra–Hadbeen fracturezones. Modelling of theWAS and gravity data (three profiles across and three along the margin)gives insights into the first- and second-order structures. (1) Continental thinning is abrupt(15–20 km thinning across 50–100 km distance). It is accommodated by several tilted blocks.(2) The ocean–continent transition (OCT) is narrow (15 km wide). The velocity modellingprovides indications on its geometry: oceanic-type upper-crust (4.5 km s−1) and continentaltypelower crust (>6.5 km s−1). (3) The thickness of the oceanic crust decreases from West(10 km) to the East (5.5 km). This pattern is probably linked to a variation of magma supplyalong the nascent slow-spreading ridge axis. (4) A 5 km thick intermediate velocity body (7.6to 7.8 kms−1) exists at the crust-mantle interface below the thinned margin, the OCT and theoceanic crust. We interpret it as an underplated mafic body, or partly intruded mafic materialemplaced during a ‘post-rift’ event, according to the presence of a young volcano evidencedby heat-flow measurement (Encens-Flux survey) and multichannel seismic reflection (Encenssurvey). We propose that the non-volcanic passive margin is affected by post-rift volcanismsuggesting that post-rift melting anomalies may influence the late evolution of non-volcanicpassive margins

Crustal structure of the Arabian Plate: New constraints from the analysis of teleseismic receiver functions

An edited version of this paper was published by Elsevier Science. Copyright 2005, Elsevier Science. See also: http://dx.doi.org/10.1016/j.epsl.2004.12.020; http://atlas.geo.cornell.edu/SaudiArabia/publications/Al-Damegh%202005.htmReceiver functions for numerous teleseismic earthquakes recorded at 23 broadband and mid-band stations in Saudi Arabia and Jordan were analyzed to map crustal thickness within and around the Arabian plate. We used spectral division as well as time domain deconvolution to compute the individual receiver functions and receiver function stacks. The receiver functions were then stacked using the slant stacking approach to estimate Moho depths and Vp/Vs for each station. The errors in the slant stacking were estimated using a bootstrap re-sampling technique. We also employed a grid search waveform modeling technique to estimate the crustal velocity structure for seven stations. A jackknife re-sampling approach was used to estimate errors in the grid search results for three stations. In addition to our results, we have also included published receiver function results from two temporary networks in the Arabian shield and Oman as well as three permanent GSN stations in the region. The average crustal thickness of the late Proterozoic Arabian shield is 39 km. The crust thins to about 23 km along the Red Sea coast and to about 25 km along the margin of the Gulf of Aqaba. In the northern part of the Arabian platform, the crust varies from 33 to 37 km thick. However, the crust is thicker (41?53 km) in the southeastern part of the platform. There is 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, mostly based on receiver functions. The average crustal thickness for all shields is 39 km, while the average for Proterozoic shields is 40 km, and the average for Archean shields is 38 km. We found the crustal thickness of Proterozoic shields to vary between 33 and 44 km, while Archean shields vary between 32 and 47 km. Overall, we do not observe a significant difference between Proterozoic and Archean crustal thickness. We observed a dramatic change in crustal thickness along the Red Sea margin that occurs over a very short distance. We projected our results over a cross-section extending from the Red Sea ridge to the shield escarpment and contrasted it with a typical Atlantic margin. The transition from oceanic to continental crust of the Red Sea margin occurs over a distance of about 250 km, while the transition along a typical portion of the western Atlantic margin occurs at a distance of about 450 km. This important new observation highlights the abruptness of the breakup of Arabia. We argue that a preexisting zone of weakness coupled with anomalously hot upper mantle could have initiated and expedited the breakup

Regional seismic wave propagation (Lg and Sn) and Pn attenuation in the Arabian Plate and surrounding regions

An edited version of this paper was published by Blackwell Publishing. Copyright 2004, Blackwell Publishing. See also: http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-246X.2004.02246.x; http://atlas.geo.cornell.edu/MiddleEastNorthAfrica/publications/Al-Damegh2004.htmContinuous recordings of 17 broadband and short period digital seismic stations from a newly established seismological network in Saudi Arabia, along with digital recordings from the broadband stations of the GSN, MEDNET, GEOFON, a temporary array in Saudi Arabia, and a temporary short period stations in Oman, were analyzed to study the lithospheric structure of the Arabian plate and surrounding regions. The Arabian plate is surrounded by a variety of types of plate boundaries: continental collision (Zagros belt and Bitlis suture), continental transform (Dead Sea fault system), young sea floor spreading (Red Sea and Gulf of Aden), and oceanic transform (Owen fracture zone). Also, there are many intraplate Cenozoic processes such as volcanic eruptions, faulting, and folding that are taking place. We used this massive waveform database of more than 6200 regional seismogram to map zones of blockage, inefficient, and efficient propagation of the Lg and Sn phases in the Middle East and East Africa. We observed Lg blockage across the Bitlis suture and Zagros fold and thrust belt, corresponding to the boundary between the Arabian and Eurasian plates. This is probably due to a major lateral change in the Lg crustal wave-guide. We also observed inefficient Lg propagation along the Oman mountains. Blockage and inefficient Sn propagation is observed along and for a considerable distance to the east of the Dead Sea fault system and in the northern portion of the Arabian plate (south of the Bitlis suture). These mapped zones of high Sn attenuation, moreover, closely coincide with extensive Neogene and Quaternary volcanic activity. We have also carefully mapped the boundaries of the Sn blockage within the Turkish and Iranian plateaus. Furthermore, we observed Sn blockage across the Owen fracture zone and across some segments of the Red Sea. These regions of high Sn attenuation most probably have anomalously hot and possibly thin lithospheric mantle (i.e., mantle lid). A surprising result is the efficient propagation of Sn across a segment of the Red Sea; an indication that active sea floor spreading is not continuous along the axis of the Red Sea. We also investigated the attenuation of Pn phase (QPn) for 1-2 Hz along the Red Sea, Dead Sea fault system, within the Arabian shield, and in the Arabian platform. Consistent with the Sn attenuation, we observed low QPn values of 22 and 15 along the western coast of the Arabian plate and along the Dead Sea fault system, respectively, for a frequency of 1.5 Hz. Higher QPn values on the order of 400 were observed within the Arabian shield and platform for the same frequency. Our results based on Sn and Pn observations along the western and northern portions of the Arabian plate 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

The crustal structure of the East Anatolian plateau (Turkey) from receiver functions

An edited version of this paper was published by the American Geophysical Union (AGU). Copyright 2003, AGU. See also: http://www.agu.org/pubs/crossref/2003.../2003GL018192.shtml; http://atlas.geo.cornell.edu/turkey/publications/Zor-et-al_2003.htmThe crustal structure of the Anatolian plateau in Eastern Turkey is investigated using receiver functions obtained from the teleseismic recordings of a 29 broadband PASSCAL temporary network, i.e., the Eastern Turkey Seismic Experiment [ETSE]. The S-wave velocity structure was estimated from the stacked receiver functions by performing a 6-plane layered grid search scheme in order to model the first order features in the receiver functions with minimum trade-off. We found no significant crustal root beneath the western portion of the network, but there is some evidence of crustal thickening in the northern portion of the network. We found an average crustal thickness of 45 km and an average crustal shear velocity of 3.7 km/s for the entire eastern Anatolian plateau. Within the Anatolian plateau we found evidence of a prominent low velocity zone where the crust thickness is approximately 46 km. These results suggests that the 2 km high topography across the Anatolian plateau is dynamically supported because most of the plateau appears to be isostatically under-compensated. Also, there appears to be a region of thin crust at the easternmost edge of the Anatolian plateau that may be a relic from the accretion of island arcs to the Eurasian plate

Subducted, detached, and torn slabs beneath the Greater Caucasus

© 2014 Published by Elsevier Ltd. The Greater Caucasus Mountains contain the highest peaks in Europe and define, for over 850. km along strike, the leading edge of the second-largest active collisional orogen on Earth. However, the mechanisms by which this range is being constructed remain disputed. Using a new database of earthquake records from local networks in Georgia, Russia, and Azerbaijan, together with previously published hypocenter locations, we show that the central and eastern Greater Caucasus Mountains are underlain by a northeast-dipping zone of mantle seismicity that we interpret as a subducted slab. Beneath the central Greater Caucasus (east of 45°E), the zone of seismicity extends to a depth of at least 158. km with a dip of ~40°NE and a slab length of ~130-280. km. In contrast, beneath the western GC (west of 45°E) there is a pronounced lack of events below ~50. km, which we infer to reflect slab breakoff and detachment. We also observe a gap in intermediate-depth seismicity (45-75. km) at the western end of the subducted slab beneath the central Greater Caucasus, which we interpret as an eastward-propagating tear. This tear coincides with a region of minimum horizontal convergence rates between the Lesser and Greater Caucasus, as expected in a region of active slab breakoff. Active subduction beneath the eastern Greater Caucasus presents a potentially larger seismic hazard than previously recognized and may explain historical records of large magnitude (M 8) seismicity in this region

The Thickness of the Mantle Lithosphere and Collision-Related Volcanism in the Lesser Caucasus

The Lesser Caucasus mountains sit on a transition within the Arabia–Eurasia collision zone between very thin lithosphere (<100 km) to the west, under Eastern Anatolia, and a very thick lithospheric root (up to 200 km) in the east, under western Iran. A transect of volcanic highlands running from NW to SE in the Lesser Caucasus allows us to look at the effects of lithosphere thickness variations on the geochemistry of volcanic rocks in this continental collision zone. Volcanic rocks from across the region show a wide compositional range from basanites to rhyolites, and have arc-like geochemical characteristics, typified by ubiquitous negative Nb–Ta anomalies. Magmatic rocks from the SE, where the lithosphere is thought to be thicker, are more enriched in incompatible trace elements, especially the light rare earth elements, Sr and P. They also have more radiogenic ⁸⁷Sr/⁸⁶Sr, and less radiogenic ¹⁴³Nd/¹⁴⁴Nd. Across the region, there is no correlation between SiO₂ content and Sr–Nd isotope ratios, revealing a lack of crustal contamination. Instead, ‘spiky’ mid-ocean ridge basalt normalized trace element patterns are the result of derivation from a subduction-modified mantle source, which probably inherited its subduction component from subduction of the Tethys Ocean prior to the onset of continent–continent collision in the late Miocene. In addition to the more isotopically enriched mantle source, modelling of non-modal batch melting suggests lower degrees of melting and the involvement of garnet as a residual phase in the SE. Melt thermobarometry calculations based on bulk-rock major elements confirm that melting in the SE must occur at greater depths in the mantle. Temperatures of melting below 1200°C, along with the subduction-modified source, suggest that melting occurred within the lithosphere. It is proposed that in the northern Lesser Caucasus this melting occurs close to the base of the very thin lithosphere (at a depth of ∼45 km) as a result of small-scale delamination. A striking similarity between the conditions of melting in NW Iran and the southern Lesser Caucasus (two regions between which the difference in lithosphere thickness is ∼100 km) suggests a common mechanism of melt generation in the mid-lithosphere (∼75 km). The southern Lesser Caucasus magmas result from mixing between partial melts of deep lithosphere (∼120 km in the south) and mid-lithosphere sources to give a composition intermediate between magmas from the northern Lesser Caucasus and NW Iran. The mid-lithosphere magma source has a distinct composition compared with the base of the lithosphere, which is argued to be the result of the increased retention of metasomatic components in phases such as apatite and amphibole, which are stabilized by lower temperatures prior to magma generation

Crustal and upper mantle structure of Oman and the Northern Middle East

Copyright 2003, Ali Al-Lazki. See also: http://atlas.geo.cornell.edu/oman/publications/Al-Lazki_dissertation.htmThis 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