EXTENSIONAL SHEAR ZONES, OROGENIC COLLAPSE, AND PALEOTOPOGRAPHY RECORD

Orogenic crust is commonly exhumed beneath extensional detachment systems that separate upper crustal units, including syn-extension basins, and deep crustal lithologies, including high-grade metamorphic rocks and migmatite domes. Frequently, the associated fault and detachment systems also represent shallowly dipping fluid pathways that separate surface (meteoric) fluids from deep metamorphic fluids. Orogenic detachment systems are crustal discontinuities where strain localized, heat was exchanged by the convective flow of surface fluids, and transient geotherms generated extreme metamorphic gradients. Moreover, deformation and recrystallization within detachments result in the preservation of microstructures that inform thermomechanical properties, and stable isotope geochemical records that inform the paleotopographic context in which detachments evolved. Our work initially concentrated on the North American Cordillera and was later applied to other orogens, some older (Variscan belt), and some younger (Himalaya). This latter study greatly benefited from Micah Jessup’s generosity with his sample collection and from his vision of the value that detailed structural sections represent in such tectonically significant shear zones as the South Tibetan detachment. We are very grateful for this collaboration.

Taken collectively, this work on extensional shear zones has illuminated not just the process of orogenic collapse. We better understand quantitatively the thermomechanics of orogens through the analysis of microstructures developed in quartz and other minerals at temperatures ~ 300-500 °C. In some cases, we can reconstruct the geochemical composition of rainwater (meteoric fluid) that infiltrated the Earth’s surface and permeated hydrous minerals (biotite, muscovite, hornblende) that were (re)crystallizing dynamically during shearing. In this case, shear zones provide unique insight into the elevation of accretionary or collisional orogens prior to their collapse.