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

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

Pn tomographic imaging of mantle lid velocity and anisotropy at the junction of the Arabian, Eurasian, and African plates

An edited version of this paper was published by Blackwell Publishing in Geophysical Journal International. Copyright 2004, Blackwell Publishing. See also: http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-246X.2004.02355.x; http://atlas.geo.cornell.edu/MiddleEastNorthAfrica/publications/Al-Lazki2004.htmThe interaction of the Arabian plate with the Eurasian plate has played a major role in building the young mountain belts along the Zagros-Bitlis continent-continent collision zone. Arabia's northward motion is considered to be the primary driving force behind the present-day westerly escape of the Anatolian plate along the North and East Anatolian fault zones as well as the formation of the Turkish and the Iranian plateaux. In this study we mapped Pn-wave velocity and anisotropy structures at the junction of the Arabian, Eurasian and African plates in order to elucidate the upper-mantle dynamics in this region. Pn is a wave that propagates within the mantle lid of the lithosphere and is often used to infer the rheology and fabric of the mantle lithosphere. Applying strict selection criteria, we used arrival times of 166 000 Pn phases to invert for velocity and anisotropy in the region. Using a least-squares tomographic code, these data were analyzed to solve simultaneously for both velocity and azimuthal anisotropy in the mantle lithosphere. We found that most of the continental regions in our study area are underlain by low Pn velocity structures. Broad-scale (~500 km) zones of low (<8 km s-1) Pn velocity anomalies underlie the Anatolian plate, the Anatolian plateau, the Caucasus region, northwestern Iran and northwestern Arabia, and smaller scale (~200 km), very low (<7.8 km s-1) Pn velocity zones underlie southern Syria, the Lesser Caucasus, the Isparta Angle, central Turkey and the northern Aegean Sea. The broad-scale low-velocity regions are interpreted to be hot and unstable mantle lid zones, whereas very low Pn velocity zones are interpreted to be regions of no mantle lid. The low and very low Pn velocity zones in eastern Turkey, northwestern Iran and the Caucasus region may be associated with the latest stage of intense volcanism that has been active since the Late Miocene. The low Pn velocity zones beneath the Anatolian plate, eastern Turkey and northwestern Iran may in part be a result of the subducted Tethyan oceanic lithosphere beneath Eurasia. We also found a major low-velocity zone beneath northwestern Arabia and the Dead Sea fault system. We interpret this anomaly to be a possible extension of the hot and anomalous upper mantle of the Red Sea and East Africa rift system. High Pn velocities (8.1-8.4 km s-1) are observed to underlie the Mediterranean Sea, the Black Sea, the Caspian Sea, and the central and eastern Arabian plate. Observed Pn anisotropy showed a higher degree of lateral variation than did the Pn velocity structure. Although the Pn anisotropy varies even in a given tectonic region, in eastern Anatolia very low Pn velocity and Pn anisotropy structures appear to be coherent

Petrogenesis of early cretaceous carbonatite and ultramafic lamprophyres in a diatreme in the Batain Nappes, Eastern Oman continental margin

Allochthonous carbonatite and ultramafic lamprophyre occur in a diatreme at the beach of the Asseelah village, northeastern Oman. The diatreme consists of heterogeneous deposits dominated by 'diatreme facies' pyroclastic rocks. These include aillikite and carbonatite, which intrude late Jurassic to early Cretaceous cherts and shales of the Wahra Formation within the Batain nappes. Both rock types are dominated by carbonate, altered olivine, Ti-Al-phlogopite and Cr-Al-spinel and contain varying amounts of apatite and rutile. The carbonatite occur as fine-grained heterolithic breccias with abundant rounded carbonatite xenoliths, glimmerite and crustal xenoliths. The aillikite consists of pelletal lapilli tuff with abundant fine-grained carbonatite autoliths and crustal xenoliths, which resemble those in the carbonatite breccia. The aillikite and carbonatite are characterized by low SiO₂ (11-24 wt%), MgO (9.5-12.4 wt%) and K₂O (&lt;0.3 wt%), but high CaO (18-22 wt%), Al₂O₃ (4.75-7.04 wt%), Fe₂O₃tot (8.7-13.8 wt%) and loss-on-ignition (24-30 wt%). Higher CaO, Fe₂O₃total, Al₂O₃, MnO, TiO₂, P₂O₅ and lower SiO₂ and MgO content distinguish carbonatite from the aillikite. The associated carbonatite xenoliths and autoliths have intermediate composition between the aillikite and carbonatite. Mg number is variable and ranges between 58 and 66 in the carbonatite, 66 and 72 in the aillikite and between 48 to 64 in the carbonatite autoliths and xenoliths. The Asseelah aillikite, carbonatite, carbonatite xenoliths and autoliths overlap in most of their mineral parageneses, mineral composition and major and trace element chemistry and have variable but overlapping Sr, Nd and Pb isotopic composition, implying that these rocks are related to a common type of parental magma with variable isotopic characteristics. The Asseelah aillikite, carbonatite and carbonatites xenoliths are LREE-enriched and significantly depleted in HREE. They exhibit similar smooth, subparallel REE pattern and steep slopes with (La/Sm)n of 6-10 and relative depletion in heavy rare earth elements (Lu = 3-10 chondrite). Initial ⁸⁷Sr/⁸⁶Sr ratios vary from 0.70409 to 0.70787, whereas initial ¹⁴³Nd/¹⁴⁴Nd ratios vary between 0.512603 and 0.512716 (εNdi between 2.8 and 3.6). ²⁰⁶Pb/²⁰⁴Pbi ratios vary between 18.4 and 18.76, ²⁰⁷Pb/²⁰⁴Pbi ratios vary between 15.34 and 15.63, whereas ²⁰⁸Pb/²⁰⁴Pbi varies between 38.42 and 39.05. Zircons grains extracted from the carbonatite have a mean age of 137 ± 1 Ma (95% confidence, MSWD = 0.49). This age correlates with large-scale tectonic events recorded in the early Indian Ocean at 140-160 Ma. Geochemical and isotopic signatures displayed by the Asseelah rocks can be accounted for by vein-plus-wall-rock model of Foley (1992) wherein veins are represented by phlogopite, carbonate and apatite and depleted peridotite constitutes the wall-rock. The carbonatite and aillikite magmatism is probably a distal effect of the breaking up of Gondwana, during and/or after the rift-to-drift transition that led to the opening of the Indian Ocean.28 page(s

Aspects of U-Th fractionation in Tertiary limestones and calcretes of Dhofar, southern Oman

This paper presentsfindings from a comprehensive geochemical and geophysical re-examination of knownradiogenic anomalies in Tertiary limestones and (sub-)recent calcretes of southwestern Dhofar in the Sultanate ofOman. U-Th-enrichments seem to be associated with deep-rooted fault systems that cross-cut Corg-rich shales atdepths of some 800–1000 m, which generally show elevated gamma-ray levels in southern Oman and act as theinitial geochemical trap. Metals and radiogenic elements, such as K (max 1945 ppm), U (max 44 ppm), and Th(max 26 ppm) mobilised from these rocks and emplaced higher up in the faults must have constituted radiogeniclineaments at and near the surface (observed in a different but difficult to access location). However, successiveweathering partially obscured such anomalies through further re-mobilisation/-mineralisation processes withinthe calcretes that also enriched Sr and V. In these carbonates, uranium correlates positively with Sr but not verywell with V, while thorium shows moderate positive correlations with Sr and V. Both U and Th are also notpresent (i.e., below the detection limits of a few ppm) in a second sample group that represents backgroundconditions. Being much more immobile than U, Th remained closer to the original western fault positions of theexamined site (outlined by magnetics and VLF-EM during the surveys), while uranium moved down-dip over theplateau and through underlying sub-horizontal strata towards the eastern fault system. Here, supergene Sr-enriched calcite preferentially incorporated U, thus reflecting the observed U-Th fractionation

The first record of allochthonous kimberlite within the Batain Nappes, Eastern Oman

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Seismogenic zones in eastern Turkey

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.../2003GL018023.shtml; http://atlas.geo.cornell.edu/turkey/publications/Turkelli-et-al_2003.htmA 29-station temporary broadband PASSCAL network was operated from late October 1999 to August 2001 in eastern Turkey in order to decipher the geodynamics of one of the youngest continent-continent collision zones in the world. This paper focuses on the hypocentral distribution of local earthquakes located during the operation of the network and provides new insights into the active faulting in the Anatolian plateau. A total of 1165 earthquakes were located and classified into four different categories based on the reliability of the locations as established by the data coverage. The accuracy of the locations ranked in the best two categories is estimated to be less than approximately 5 km. The results show that seismic activity in Eastern Turkey is higher than previously documented and there were no subcrustal earthquakes beneath the Arabian- Eurasian collision zone or beneath the Anatolian plateau during our deployment. This result suggests no or very little underthrusting of the Arabian plate beneath Eurasia. Our results also suggest that the North Anatolian Fault zone extends farther toward the southeast, well beyond the Karliova triple junction, and that a number of unmapped active, seismogenic faults exist in the region. We also observed a possible difference in the seismogenic thickness of the East Anatolian fault zone (EAFZ) and the North Anatolian fault zone (NAFZ)