Investigating the role of rift inheritances during early orogeny in the Oman belt

Preceding mature continental collision, early orogeny results from the complex interaction between subduction and a continental margin. The lithospheric architecture of former rifted margins representing the pre-orogenic setting of Alpine belts has been drastically redefined by two decades of offshore/onshore investigations. Although being generally assumed, the importance and the way rift inheritances drive early orogenic processes remain poorly documented as pervasive deformations overprint this record in collisional belts.

This study therefore focuses on the Oman belt that deformed a former Permian-Triassic rift segment but escaped from late collisional deformations. Based on a tectono-stratigraphic review further supported by surface and subsurface observations, we identified and mapped pre-orogenic rift domains across the belt. From this analysis, we show that rift domains were coherently sampled in each alpine units and their bounding alpine structures correspond to former limits of rift domains. From outer to internal belt, they successively correspond to 1) the Arabian shelf ahead of the Hawasina frontal thrust, 2) the crustal necking zone and its basin in the Sumeini-Hawasina units, 3) the former distal margin made of a continental ribbon as the Jabal Al Akhdar-Saih Hatat unit and a continent-ocean transition as the Oman exotics. The overlying Semail ophiolite corresponds to the obducted Cretaceous oceanic lithosphere.

Accounting for HP metamorphism distribution that documents continental subduction and the timing of tectonic inversion of rift domains, we propose that early orogeny consisted of 2 phases preceding the tertiary mature continental collision (i.e. "Zagros phase"). First, the Semail obduction occurred coevally with the continental subduction of distal domains. Deformation propagated and stopped with the inversion of the necking zone while HP units were uplifted and exhumed. At that time, tectonic activity migrated back to the Makran subduction zone where the remaining oceanic lithosphere was consumed.

From this study, we propose that the maturity of an alpine-type orogen depends on the former rift domains getting involved into orogenic processes. We further speculate that such a rift-related orogenic maturity may drive far field interactions at plate tectonics scale.