Geophysical Imaging of an Obduction Plate Margin, Oman

Project Investigators

Stephan Thiel (PhD Candidate)

Associate Professor Graham Heinson

Project Collaborators

Professor Dave Gray (The University of Melbourne)

Bob Gregory (Southern Methodist University, Texas)

Project Summary

Understanding the mechanism of obduction (emplacement of oceanic crust onto continental margins) is fundamental to our knowledge of plate tectonic processes, earth evolution and dynamics. This project will integrate geophysical imaging methods, constrained by structural geological mapping, applied to the best exposure of obducted oceanic crust on the planet, the Samail Ophiolite in Oman. Magnetotellurics, seismic refraction and reflection, and potential field data will be used to define physical properties of crust and upper mantle, particularly to delineate major plate-boundary shear-zones. Integrated, these methods will resolve debate on roles of margin underthrusting versus oceanward subduction in the obduction process.

The primary hypothesis tested in this project is whether obduction simply involves (a) continent-vergent, piggy-back style overthrusting of the margin by the ocean basinal facies and ophiolite nappe (Figure 1 A or B), or (b) underthrusting directed toward the Arabian continental shield (Figure 1 C).

Figure 1 . Three possible tectonic scenarios allowing a mid-crustal geometry where the margin rocks are back-folded and/or thrust towards the former Tethy's ocean at the time of ophiolite emplacement (UP: upper plate, LP: lower plate. BLACK: oceanic lithosphere; GREY: continental crust). (A) Buoyancy-driven crustal slice inducing back-folding/thrusting (after Chemenda et al. 1996, fig.8); (B) Retrocharriage model after the Alps (e.g. Roedder, 1977, fig. 3); (C) Underthrusting towards the margin incorporating tiered-detachments and buoyant ascent of lower plate blocks (after Gregory et al. 1998, fig. 8; Gray et al. 2000, fig. 7).

With seed funding from the University of Melbourne and the University of Adelaide, a pilot study for the magnetotelluric work in this proposal was carried out in January 2005. Figure 2 shows location of 28 magnetotelluric sites. Each site was occupied with a long-period magnetotelluric system, developed in Adelaide, for two-days. Orthogonal components of electric field and three-component magnetic fields were collected at a rate of 1 Hz. Measurements exhibited a significant distortion effects, but were generally of good quality, with <5% errors. Preliminary 2D smooth inversion modelling (Rodi and Mackie, 2001) carried out in the field of TE mode measurements (electric fields measured parallel to the dominant strike of the ophiolite and the Arabian margin is shown in Figure 3.

Figure 2 . Location of the pilot MT survey sites (black circles) collected in January 2005.

Figure 3 . Preliminary 2D inversion of MT data TE mode apparent resistivity (to rms ~ 2). The dashed line shows a low resistivity zone that extends under the Jabal Abyad mountains that may reflect conduction along the Hatat schist, which is highly deformed basement in the core region of the large-scale, NE-facing Saih Hatat fold-nappe.