HOW INSIGHTS INTO CORDILLERAN EXTENSIONAL TECTONICS HAVE REVOLUTIONIZED OUR UNDERSTANDING OF COLLISIONAL OROGENESIS

A revolution in our understanding of continental extensional tectonics occurred in the late 1970’s and early 1980’s with an appreciation of the important role of low-angle normal faulting in the North American Cordillera. Much of the foundational work in this realm can be traced to the efforts of Clark Burchfiel, Greg Davis, and their numerous graduate students working in the western United States. While the core concepts that developed through their research initiatives naturally had an influence on studies of other extensional landscapes worldwide, it would have been hard to predict forty years ago how impactful these concepts would be on understanding the deep structure of collisional orogens. Working in the Himalayan-Tibetan orogenic system in the late 1980’s, Burchfiel and colleagues identified a family of low-angle extensional faults exposed near the boundary between the Tibetan Plateau and the active Himalayan orogenic wedge with remarkable similarities to the regionally important low-angle detachments identified earlier in Cordilleran metamorphic core complexes. Since that time, these structures – collectively referred to as the South Tibetan Detachment System (STDS) – have been studied by numerous groups of international researchers. Through their work, the STDS had been mapped for well over 1000 km along strike. Its continuity and displacement suggest that the STDS has been as fundamentally important to the evolution of the Himalayan-Tibetan orogenic system as more familiar contractional fault systems found in the orogenic wedge. The existence of the STDS has influenced many subsequent models for other large, hot contractional orogenic systems (e.g., the Grenville orogen and the Caledonides). Ironically, these concepts, developed in the Himalaya but based on insights from the North American Cordillera, have also influenced interpretations of Mesozoic decoupling horizons with extensional kinematics within the deeper parts of the Sevier orogen as exposed in the Basin and Range.