Deep crustal expressions of exhumed strike-slip fault systems: Shear zone initiation on rheological boundaries

The formation of major exhumed strike-slip faults represents one of the most important dynamic processes affecting the evolution of the Earth's lithosphere. Detailed models of the potential initiation, their properties and architecture of orogen-scale exhumed strike-slip faults, which are often subparallel to mountain ranges, are rare. The initiation of strike-slip faults is at depth, where temperature-controlled rheological weakening mechanisms play the essential role localizing future strike-slip faults. In this review study, we highlight that in pluton- and metamorphic core complex (MCC)-controlled tectonic settings, as end-members, the initiation of strike-slip faults occurs by rheological weakening along hot-to-cold contacts deep within the crust and mantle lithosphere, respectively. These endmember processes are potential mechanisms for the initiation of orogen-scale exhumed strike-slip faults at depth result in a specific thermal and structural architecture. Similar processes guide the overall displacement and ultimately the exhumation at such deep levels. These types of exhumed strike-slip dominated fault zones expose a wide variety of mylonitic, cataclastic and non-cohesive fault rocks on the surface, which were formed at different structural levels of the crust during various stages of faulting and exhumation. Exhumation of mylonitic rocks is, therefore, a common feature of such reverse oblique-slip strike-slip faults, implying major transtensive and/or transpressive processes accompany pure strike-slip motion during exhumation. A major aspect of many exhumed strike-slip faults is their lateral thermal gradient induced by the lateral juxtaposition of hot and cold levels of the crust controlling relevant properties of such fault zones, and thus the overall fault architecture (e.g., fault core, damage zone, shear lenses, fault rocks) and its thermal structure. These properties of the overall fault architecture include strength of fault rocks, permeability and porosity, the hydrological regime, as well as the nature and origin of circulating hydrothermal fluids.