The Oman-United Arab Emirates ophiolite has been used extensively to document the geological processes that form oceanic crust. The geometry of the ophiolite, its extension into the Gulf of Oman, and the nature of the crust that underlies it are, however, unknown. Here, we show the ophiolite forms a high velocity, high density, >15 km thick east-dipping body that during emplacement flexed down a previously rifted continental margin thereby contributing to subsidence of flanking sedimentary basins. 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°, east-dipping, normal fault. The fault is interpreted as the southwestern margin of an incipient suture zone that separates the Arabian plate from in situ Gulf of Oman oceanic crust and mantle presently subducting northwards beneath the Eurasian plate along the Makran trench.

[1] We use seismicity generated from the Erua earthquake cluster (a consistently active area of seismicity about 20 km to the west of Mount Ruapehu) over the last 12 years to study seismic anisotropy in the Ruapehu region. In particular, we search for changes associated with two minor phreatic eruptions on the 4th of October 2006 and the 25th of September 2007. The seismicity rate, magnitude of completeness, focal mechanisms and b-value of the cluster are also examined to investigate whether the characteristics of the seismicity changed over the duration of the study. The hypocenters were relocated, which revealed a westward dip in the shallow seismicity. Shear wave splitting results revealed a decrease in delay time in the 2006-2007 period and a significant variation in the fast shear wave polarization in the same time period. The b-value also increased significantly from 1.0 ± 0.2 in 2004 to a peak of 1.8 ± 0.2 in 2007, but no other parameters were found to vary significantly over this time period. We attribute these changes to an increase in pore-fluid pressure in the Erua region due to fluid movement and suggest that this fluid movement may be associated with the eruptions in 2006 and 2007.

<p>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.</p><p>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.</p><p>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.</p>