Gravity anomalies and receiver functions across the Hajar mountains in the United Arab Emirates

The Hajar mountains are a 700 km long mountain belt situated on the north-eastern margin of the Arabian continent. This mountain belt is made up of a series of stacked thrust sheets that record a passive margin to deep ocean crustal sequence and an obducted slab of oceanic crust and upper mantle known as the Semail ophiolite. These ophiolite nappes are perhaps the largest, best exposed, and most intensively studied in the world. However, their thickness, internal structure, and offshore extent are still poorly constrained. We utilize passive seismic and onshore/offshore gravity anomaly data sets to construct a 160 km long 2D gravity profile of the crust and upper mantle, stretching from the foreland basin in the Arabian Gulf, across the Hajar mountains and out into the rifted margin in the Gulf of Oman. Constraints on the foreland basin and the post-obduction Tertiary rifted margin are provided by new and existing seismic reflection and refraction data, while constraints on the lower crust are provided by receiver functions calculated across our seismic network. Moho conversions are observed at a depth of 33 km in the foreland, but are conspicuously absent across the rest of the network. A strong mid crustal conversion is also observed in the foreland at 12 km depth, and we interpret this be a transition from the foreland sediments to basement. A second strong, reverse polarity (slow to fast), mid crustal conversion is observed beneath the ophiolite in the mountains to the east. The depth of this conversion increases from 10 km on the eastern side of the mountains to 18 km on the western coastline. We interpret this to be a transition from the obducted ophiolite to underlying basement. Gravity anomalies increase from -110 mGal in the foreland, up to 170 mGal across the ophiolite in the eastern mountains, and then decreases to -10 mGal offshore in the Gulf of Oman. We successfully model these anomalies with: a 12 km thick low density sedimentary sequence in the foreland; a 10-15 km thick high density unit comprising the obducted ophiolite nappes; and a 10 km thick sedimentary sequence overlying a thinned continental or oceanic crust in the Gulf of Oman.