3D Structural Analysis of the Oman Ophiolite and the Lithosphere of the Eastern Arabian Continental Margin
The Oman Mountains along the eastern Arabian passive continental margin are geologically a highly complicated region. As part of the Peri-Arabic obduction along the convergence zone between the Arabian and Eurasian plate, oceanic crustal and mantle rocks were emplaced on the Arabian continent in Mid Cretaceous. The Semail ophiolite in the Oman Mountains covers an area of about 700x140km². Additional tectonic processes lead to the circumstance that every geological strata of the ophiolite is exposed at the surface, which makes it one of the best preserved and studied ophiolite in the world in case of geological field investigations. However, the vertical extent of the Oman ophiolite as well as the structure and thickness of the underlying continental lithosphere has thus far been geophysical imaged by only a few local 2D studies. To fully understand the obduction process and the formation of today’s topography of the Oman Mountains, information on the present state of the Arabian continental lithosphere is needed. In this work, a 3D model of the lithosphere beneath the eastern Arabian continental margin will be estimated. For that reason, we operated a temporary broadband seismic network with 40 instruments for continuous, passive seismic registration from October 2013 to February 2016. The dataset is complemented by data of 18 permanent broadband seismometers, operated by the Oman Earthquake Monitoring centre, the Dubai Seismic Network and the Global Seismographic Network. A newly implemented approach for inter-station phase velocity measurements was applied to compute Rayleigh and Love wave fundamental mode phase velocities in a period range of 2–40 s from ambient seismic noise cross correlations. This approach utilized the measured phases of the cross-correlation functions and performed a grid search to fit them to synthetic phases, calculated via the Bessel function of third kind. Azimuthal anisotropic Rayleigh and Love wave phase velocity maps are calculated, which show velocity anomalies across the eastern Arabian continental margin as a function of period. We then simultaneously inverted Rayleigh and Love wave local dispersion curves for shear wave velocity over depth by using a novel implementation of a radially anisotropic, probabilistic 1D inversion. The resulting 1D models are combined to construct the 3D model of isotropic shear wave velocity and radial anisotropy to a depth of 55km beneath the eastern Arabian continental margin. In addition, we calculated P- and S-receiver functions using our continuous Oman data and 286 teleseismic events from the ISC catalogue, which occurred during the time of the station deployment, to image different crustal and mantle discontinuities like the Moho and the mantle transition zone. A newly implemented migration approach led to depth information from P- and S-receiver functions. We are able to present the first 3D model of isotropic and radially anisotropic shear wave velocity of the eastern Arabian continental margin. It provides information on the lateral and vertical extent of the ophiolite and confirms previous works on 2D modelling of the ophiolite structure. Moreover, the 3D model clearly features the different properties of the strongly deformed crustal structure of the Oman Mountains, especially west- and eastwards of the Semail Gap. We present a map for the crustal thickness across the entire region, which is mostly consistent to the obtained results on receiver functions. The crustal thickness decreases from west to east and features a localized thickening beneath the Oman Mountains which is most prominent in the Jebel Akhdar. In combination with azimuthal anisotropy, the 3D model substantiate previous works on post-obductional extension leading to the exhumation of once subducted and partly metamorphosed rocks in the Saih Hatat and Jebel Akhdar window. Azimuthal anisotropy in the upper crust would support a compressional regime, causative for the uplift of the Oman Mountains, whereas azimuthal anisotropy in the lower crust points to the importance of an extensional phase in the Oman Mountains. Despite that the timing of this extensional phase has still to be identified, a reinvestigation of the question about the formation of the Oman Mountains seems to be required. By evaluating our results on the present state structure of the continental lithosphere, clear differences between the areas west- and eastwards of the Semail Gap are observable. In combination with differences in the lateral thrusting of the ophiolite onto the Arabian continent, we are able to propose properties of the continental crust which are beneficial for an obduction, namely a relatively large deformability and crustal thickness in combination with a rather flat subduction during obduction. The new model provides considerable insight in the lithospheric structure of the eastern Arabian continental margin and can serve as a starting point for revisiting its geodynamic evolution.
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