How detachments connect shallow and deep crust during mass redistribution in orogens

The thermal, physical, and chemical processes of detachment faults profoundly influence the dynamics of continental lithosphere far beyond the fault zones. The conditions and timing of deformation in detachment fault zones are therefore important to investigate in order to evaluate how these faults are dynamically linked to tectonic processes over a wide range of spatial scales, laterally and vertically.

Detachments are exhuming structures in which the amount of exhumation accommodated by a particular fault ranges from a few to tens of kilometers. This magnitude depends primarily on the total extension, its spatial distribution/localization, and the buoyancy of the exhumed crust. Exhumation-related deformation is accompanied by (hydro)thermal processes that may be recorded in the composition and zoning of minerals such as quartz and micas, particularly in lithologies such as quartzites that may preserve a diachronous record of deformation in incompletely-overprinted domains. These minerals provide pressure-temperature-time-deformation information, as well as serving as geochemical tracers of syn-tectonic fluid-rock interaction. Excellent examples are the detachment-footwall quartzites of metamorphic core complexes (mcc) in the North American Cordillera. Results of integrated microstructure, thermobarometry, geochemistry, and thermochronology studies track the conditions and timing of deformation during exhumation and cooling. In cases of detachment faults bounding exhumed deep crust, footwall rocks display a sharp metamorphic gradient caused by a combination of thinning and shearing. Metamorphic conditions and paths may reflect exhumation trajectories rather than maximum temperature/depth; this is supported by numerical models that predict that rocks from similar pre-extension depths can be exhumed during extension to create an apparent progressive metamorphic sequence from detachment faults into mcc footwalls.

Integrated studies from nature and numerical experiments also give insights at a larger scale, indicating that regions of thickened continental crust flow towards regions of thinner crust, driving contraction in the latter. Formation of detachment faults may be driven actively by extension of the lithosphere and/or by gravitational crustal flow away from the orogenic core and towards the foreland, where coeval thrusting may occur. In this case also, pressure-temperature-time-deformation studies coupled with geochemistry provide insights into the mechanisms, conditions, and timing/rates of mass redistribution in orogens. The significance of this phenomenon is indicated by the prevalence in orogens of coeval domains of extension (detachment faulting / metamorphic core complexes) and contraction (fold-and-thrust belts).