The Neotethyan Arabian necking zone exposed at the SE Oman mountains: field evidence and consequences

The Late Cretaceous Oman Mountains are generally assumed to result from obduction followed by the inversion of the mid-Permian- to Triassic Neotethyan rifted margin. However, the key rift-related crustal features, such as a necking zone or hyper-extended rift domains remain inferred and poorly described so far. In this study, we investigate the tectono-stratigraphic record of the eastern part of the Oman Mountains where the exposed Tonian (Neoproterozoic) crystalline basement outcrops together with the pre- to syn-obduction sedimentary record in the Ja’alan massif area. The description of these units together with subsurface data enables to describe the former Arabian necking zone. The Ja’alan massif itself and the Arabian platform to the southwest represent the former proximal margin domain. It is characterized by the eroded basement sealed by post-obduction continental to shallow marine sediments. In contrast, the north-eastern side of the massif is flanked by Permian-Mesozoic deep marine post-rift sediments (Batain Group) equivalent to the Hawasina thrust sheet in the Oman Mountains. These two endmember paleogeographic units are separated by a major N20 dipping top-to-the-NE normal fault with dip-slip kinematics (slikensides with striae, S/C-fabric). The damage zone of this fault is characterized by a cataclastic and a gouges fault zone, overlain by slope facies with syn-kinematic polymictic mega-breccias reworking the adjacent basement. The breccias are grading finer upwards, contain conglomerate and sandstone interbeds interpreted as to slope-environment turbiditic channel deposits. This exhumation and rift-related record is unconformably covered by the post-obduction sequence affected by a late Cenozoic E/W-directed low-amplitude shortening. The intensity of shortening is increasing toward the NW leading to reactivate the Arabian Necking zone as a ramp for the Hawasina thrust system. Based on these observations, we propose a new geodynamic model showing that the final stage of the obduction result from the inversion of the former Arabian necking zone with significant impacts on the evaluation of (1) the shortening rates accommodated and (2) the former architecture of the Arabian Tethyan rifted margin. As the belt never recorded a mature continent-continent collision, we think that the Oman study case could significantly help to investigate the dynamics of hyper-extended rifted margins inversion at an early orogenic stage.