Interplay between oblique strike-slip faulting and fold-perpendicular extension during the growth of the Jebel Qusaybah inversion anticline, at western tip of the Salakh Arch, Oman
Storti, Fabrizio Balsamo, Fabrizio Clemenzi, Luca Mozafari, Mahtab Al-Kindy, M.H.N Solum, John Tueckmantel, Christian Swennen, Rudy Taberner, Conxita
Geometry and growth of normal faults location:Geological Society of London date:23-25 June 2014
We performed a structural and petrographical-geochemical study of deformation structures exposed in the Jabal Qusaybah anticline, at the western termination of the Salakh Arc, a major salient at the toe of the Oman Mountains thrust wedge. The results of our work have been integrated with reflection seismic data which indicate that the anticline developed by positive inversion of an extensional fault zone that formed in Turonian times (i.e., during deposition of the Natih A formation). The deformation pattern characterizing the doubly-plunging E-W trending Jabal Qusaybah anticline includes NE-SW left-lateral strike-slip and N-S extensional fault zones. Petrographical features, 13C and 18O signatures of differentiated calcite vein fills, and the geometric, kinematic, and overprinting relations of calcite veins indicate that folding and faulting occurred coevally during the growth of the anticline. The central sector of the fold crest is characterized by N-S striking extensional fault zones subperpendicular to the fold axis and abutting the NE-SW left-lateral strike-slip zones. Both strike-slip and extensional fault zones show widespread evidence for substantial dilation in the form of infilling by large columnar calcite crystals and aggregates. This is particularly outstanding along the N-S extensional fault zones where the magnitude of fault core dilation frequently exceeds one meter and the size of euhedral calcite crystals can be larger than 20 cm. A plausible kinematic pathway that could explain the coexistence of such unusual synfolding noncylindrical deformation patterns is motion of the underlying Ara evaporites in contraction under a foredeep-parallel tapering overburden wedge of sedimentary and/or tectonic origin. This geometry would add a component of south-eastward extrusion to the evolution of the Salakh Arc, thus triggering NE-SW left-lateral strike-slip faulting, and would have favoured longitudinal axial bulging of the anticline accompanied by generalized dilation in the central region of the crest. This work illustrates how complex deformation patterns can be in fault-related anticlines that develop near or within curved segments of major thrusts. Conventional templates for folding-related fracturing do not predict the occurrence of either the N-S extensional or NE-SW strike slip fault systems. Similar atypical fault and fold patterns could be expected in structures at the toe of thrust wedges, and so should be incorporated into conceptual models of those structures. We consequently stress the importance of studying appropriate field analogues of fault-related folds constrained using a multidisciplinary structural and petrographical-geochemical approach. Understanding the timing of the fault, fold and fracture system growth would not have been possible without this integrated approach.