Triple folded surface morphology of Neoproterozoic rocks (Jabal Akhdar Dome, Oman Mountains) – Insights into buttressing effects and regional tectonics

Abstract

• Two Pre-Permian deformation events affected Neoproterozoic rocks of NE Oman. • An inherited horst controls the deformation style of Neoproterozoic rocks. • The Jabal Akhdar is a prime example for studying buttressing effect. • Structural geology controls the morphology of a regional-scale carbonate surface. • The Shuram-Khufai boundary is an unconformity . Northern Oman was obducted by allochthonous rocks, including the Semail Ophiolite, during the Cretaceous, which generally hampers field investigations on autochthonous rocks. However, central parts of the Jabal Akhdar Dome provide insights into sub-allochthonous deformation patterns from northern Oman because autochthonous Arabian rocks are exposed. Field survey and satellite image analysis reveal triple-folded autochthonous Neoproterozoic rocks. In the Neoproterozoic succession, carbonates are more resistant to erosion than the under- and overlaying siliciclastics and shape present-day morphology in the superbly exposed surface of the Hat Plateau, where anticlines and synclines coincide with ridges and troughs, respectively. Previously unrecognized F1 folds are NNE-verging, overturned and tight with amplitudes of 10–100 s of meters, have ESE-oriented fold axes, and display a gently to moderately SSW-dipping penetrative axial-plane cleavage. F1 structures are re-folded by open-tight upright kilometric F2 folds, with NE/NNE-oriented fold axes and a penetrative NE/NNE-striking sub-vertical axial-plane cleavage. A younger ∼WNW-trending broad anticline (F3) exhibits a widely spaced sub-vertical ESE-striking axial-plane cleavage. The F2 deformation style is heterogeneous. In the West, the NE/ENE-oriented F2 folds are ∼3 km in amplitude and gently plunging. In the East, the NNE-oriented folds are <1 km in amplitude and non-plunging. The change ensues abruptly along a NNE-oriented zone at the western end of the Hat Plateau. The pre-existing NNE-oriented western flank of the Makarem-Mabrouk High/Horst in the subsurface controlled this change, exerting a buttressing effect with less deformation in the East.