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

A crustal transect across the Oman Mountains on the eastern margin of Arabia

Publisher's version archived with permission from publisher. http://www.gulfpetrolink.net/publication/geoarabia.htmThe unique tectonic setting of the Oman Mountains and the Semail Ophiolite, together with ongoing hydrocarbon exploration, have focused geological research on the sedimentary and ophiolite stratigraphy of Oman. However, there have been few investigations of the crustal-scale structure of the eastern Arabian continental margin. In order to rectify this omission, we made a 255-km-long, southwesterly oriented crustal transect of the Oman Mountains from the Coastal Zone to the interior Foreland via the 3,000-m-high Jebel Akhdar. The model for the upper 8 km of the crust was constrained using 152 km of 2-D seismic reflection profiles, 15 exploratory wells, and 1:100,000- to 1:250,000-scale geological maps. Receiver-function analysis of teleseismic earthquake waveform data from three temporary digital seismic stations gave the first reliable estimates of depth-to-Moho. Bouguer gravity modeling provided further evidence of depths to the Moho and metamorphic basement. Four principal results were obtained from the transect. (1) An interpreted mountain root beneath Jebel Akhdar has a lateral extent of about 60 km along the transect. The depthto- Moho of 41 to 44 km about 25 km southwest of Jebel Akhdar increased to 48 to 51 km on its northeastern side but decreased to 39 to 42 km beneath the coastal plain farther to the northeast. (2) The average depth to the metamorphic basement was inferred from Bouguer gravity modeling to be 9 km in the core of Jebel Akhdar and immediately to the southwest. A relatively shallow depth-to-basement of 7 to 8 km coincided with the Jebel Qusaybah anticline south of the Hamrat Ad Duru Range. (3) Based on surface, subsurface, and gravity modeling, the Nakhl Ophiolite block extends seaward for approximately 80 km from its most southerly outcrop. It has an average thickness of about 5 km, whereas ophiolite south of Jebel Akhdar is only 1 km thick. The underlying Hawasina Sediments are between 2 and 3 km thick in the Hamrat Ad Duru Zone, and 2 km thick in the Coastal Zone. (4) Southwest of Jebel Akhdar, reactivated NW-oriented strike-slip basement faults that deformed Miocene to Pliocene sediments were inferred from the interpretation of seismic reflection profiles

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

Radiometric Surveys for Detection of Uranium in Dhofar Region, Sultanate of Oman

The Proterozoic basement of the Dhofar region of the Sultanate of Oman might be considered as a potential source of Uranium mineralization like other Proterozoic basins of the world, which collectively contribute 30%-40% to the global production.  Uranium exploration assumes great importance in this part of the world and therefore geochemical and geophysical exploration for Uranium was initiated.  Initial, random sampling of the study area with Gamma ray spectrometry as well as in situ XRF (Xray Florescence) analysis indicated the presence of   around 30-40 ppm of Uranium. Since Gamma ray spectrometry results in the calculation of the heat produced during radioactive decay of Potassium (K), Uranium (U) and Thorium (Th) within the rocks in the study area, measurements were acquired along eleven traverses of varying length from 100-250 m at a traverse interval of 20 m and sampling interval of 10 m. Individual plots of the concentrations  of radioelements such as U, K and Th have shown favorable anomalous radioactive sources. Besides this, Uranium to Thorium (eU/eTh) and Uranium to Potassium (eU/K) ratio plots have been presented and the alteration zones associated with Uranium mineralization through all traverses have been identified. Further, the generated composite ternary image based on the combination of K, Th and U from Gamma ray spectrometry determines different relative amounts of radioelements and paves the way for deciphering the level of radioactivity in the study area. However, there seems to be no presence of strong Uranium anomalies in the near surface of the limited study area. As the study area is heavily faulted, the depth to concealed  subsurface fault structures  is  estimated to be around  64 m based on Hartley spectral analysis of total magnetic anomaly (line-6).

Upper mantle anisotropy of Southeast Arabia passive margin [Gulf of aden northern conjugate margin], Oman

International audienceIn this study, we used data recorded by two consecutive passive broadband deployments on the Gulf of Aden northern margin, Dhofar region, Sultanate of Oman. The objective of these deployments is to map the young eastern Gulf of Aden passive continental margin crust and upper mantle structure and rheology. In this study, we use shear-wave splitting analysis to map lateral variations of upper mantle anisotropy beneath the study area. In this study, we found splitting magnitudes to vary between 0.33 and 1.0 s delay times, averaging about 0.6 s for a total of 17 stations from both deployment periods. Results show distinct abrupt lateral anisotropy variation along the study area. Three anisotropy zones are identified: a western zone dominated by NW-SE anisotropy orientations, an eastern zone dominat- ed with NE-SW anisotropy orientations, and central zone with mixed anisotropy orientations similar to the east and west zones. We interpret these shorter wavelength anisotropy zones to possibly represent fossil lithospheric mantle anisotropy. We postulate that the central anisotropy zone may be representing a Proterozoic suture zone that separates two terranes to the east and west of it. The anisotropy zones west and east were being used indicative of different terranes with different upper mantle anisotropy signatures

Magmatism at continental passive margins inferred from ambient-noise phase-velocity in the Gulf of Aden

Non-volcanic continental passive margins have traditionally been considered to be tectonically and magmatically inactive once continental breakup has occurred and seafloor spreading has commenced. We use ambient-noise tomography to constrain Rayleigh-wave phase-velocity maps beneath the eastern Gulf of Aden (eastern Yemen and southern Oman). In the crust, we image low velocities beneath the Jiza-Qamar (Yemen) and Ashawq-Salalah (Oman) basins, likely caused by the presence of partial melt associated with magmatic plumbing systems beneath the rifted margin. Our results provide strong evidence that magma intrusion persists after breakup, modifying the composition and thermal structure of the continental margin. The coincidence between zones of crustal intrusion and steep gradients in lithospheric thinning, as well as with transform faults, suggests that magmatism post breakup may be driven by small-scale convection and enhanced by edge-driven flow at the juxtaposition of lithosphere of varying thickness and thermal age