INTEGRATED GRAVITY AND SEISMIC INVESTIGATION OVER THE JABAL HAFIT STRUCTURE: IMPLICATIONS FOR BASEMENT CONFIGURATION OF THE FRONTAL FOLD-AND-THRUST BELT OF THE NORTHERN OMAN MOUNTAINS

M. Y. Ali^1*, M. Sirat^1,2 and J. Small¹

¹The Petroleum Institute, PO Box 2533, Abu Dhabi, UAE.

² current address: Schlumberger Oilfield Eastern Ltd, PO Box 9056, East Ahmadi, 61001, Kuwait.

* author for correspondence: mali@pi.ac.ae

A gravity survey was conducted over and around Jabal Hafit, located on the border between the United Arab Emirates (UAE) and Oman on the western edge of the northern Oman Mountains, as part of a study to investigate the subsurface structures and sedimentary sequences of the area. This new data, together with outcrop geology, well data and measurements of physical properties of rock samples, was integrated with a new interpretation of reprocessed commercial seismic reflection profiles recorded across the Jabal Hafit anticline.

We recognize five major tectonostratigraphic sequences in the seismic profiles: Mesozoic shelf carbonates; Upper Cretaceous foreland (primarily Fiqa Formation); the Hawasina allochthon; and Upper Cretaceous to Lower Tertiary, and Upper Tertiary sequences. The seismic sections suggest that Jabal Hafit represents a backfolded anticline associated with a steep east-vergent thrust that probably lies above a blind thrust fault along the base of the foreland basin sequence at depth. The lack of onlapping and thinning in the Lower Tertiary sequence suggests that the initiation of folding may have started after the Oligocene-Miocene and corresponds to the beginning of the collision of Arabia and central Iran along the Zagros suture in Iran. In addition, the seismic sections suggest a pop-up structure beneath the Jabal Hafit structure at the base of the foreland basin sequence. The structure is bounded by high-angle listric reverse faults that cut downward through the Mesozoic shelf carbonates and are interpreted to be related to the inversion of deep structures.

NNW to SSE trending positive residual gravity anomalies of up to +8 mGal occur on the Jabal Hafit and Al-Ain anticlines. These positive gravity highs are attributed to uplifted basement structures which probably resulted from the reactivation of deep-seated fault blocks. The residual Bouguer anomaly values decline gradually to the east and west of Jabal Hafit probably due to the deepening of the basement. A large negative gravity anomaly of <-5 mGal is observed east of Jabal Hafit and coincides with a syncline which caused thickening of the Upper Cretaceous and Tertiary sequences.

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