An integrated geophysical approach for imaging of the Semail ophiolite

The Semail ophiolite, a thick thrust sheet of Late Cretaceous oceanic crust and upper mantle, was obducted onto the previously rifted Arabian continental margin in the Late Cretaceous, and now forms part of the United Arab Emirates (UAE)-Oman mountain belt. A deep foreland basin along the west and SW margin of the mountains developed during the obduction process, as a result of flexure due to loading of the ophiolite and underlying thrust sheets. Structural and compositional complexities (e.g., presence of thick sand dunes, relatively shallow high-velocity and dense ophiolite structure) have made geophysical imaging of the sub-ophiolite and mid-lower crustal structure particularly challenging.

A combination of active and passive-source seismic techniques, potential field modelling and surface geological mapping are used to constrain the stratigraphy, velocity structure and crustal thickness beneath the UAE-Oman mountains and its bounding basins. Depth-migrated multichannel seismic-reflection profile data are integrated in the modeling of traveltimes from long offset reflections and refractions, which are used to resolve the crustal thickness and velocity structure along two E-W onshore/offshore transects in the UAE. Additionally, we apply receiver function and virtual deep seismic sounding methods to distant earthquake data recorded along the two transects to image crustal thickness variations. Seismic and geological constraints from the transects have been finally used to model gravity and magnetic anomaly data along two coincident profiles.

Geophysical methods define the Semail ophiolite as a high-velocity, high density, > 15 km thick body dipping to the east. The western limit of the ophiolite is defined onshore by the Semail thrust while the eastern limit extends several km offshore, where it is defined seismically by a ~40–45° normal fault. Emplacement of the ophiolite has probably flexed down a previously rifted continental margin, thus contributing to subsidence of flanking sedimentary basins. The new crustal thickness model presented in this work provides evidence that a crustal root is present beneath the Semail ophiolite, suggesting that folding and thrusting during the obduction process may have thickened the pre-existing crust by 16 km.