Abstract

A comparative study of the geology of the early Tertiary western United States and the modern Tibetan and Turkish–Iranian Plateaux indicates that these three regions, situated in the hinterland of major orogenic belts, share a common taphrogenic evolutionary path. Both the Tibetan and Turkish–Iranian Plateaux have high mean elevations above sea level (>5 km and 2.0–2.5 km, respectively), have experienced fast uplift and widespread unroofing during the last 10–8 Ma, and are undergoing extension nearly perpendicular to the plate convergence direction. Successive accretionary and collisional events accompanied by significant shortening and underplating during the formation of these plateaux resulted in the generation of overthickened continental crust (>50 km) that is currently in the process of extending. A two-stage magmatism, characterized by widespread calc-alkaline volcanism followed and/or overlapped by bimodal (basaltic to rhyolitic) volcanism, occurs synchronously with crustal extension and thinning. The plateaux in general have an internal (closed) drainage system and are drained by antecedent rivers near their edges. The early Tertiary western US Cordillera displayed similar geological and geomorphic features to those observed in the Tibetan and Turkish–Iranian Plateaux in the aftermath of a prolonged Mesozoic orogeny and underwent an orogenic collapse starting around mid-Tertiary time. A N-trending crustal welt with a maximum thickness of 60 km and an elevation around >3 km formed a highland with alpine flora in the hinterland of the Cordilleran orogenic belt and separated internal drainage basins and saline lakes on the east from major flood plains of antecedent river systems on the west. This highland subsequently became the locus of crustal extension, metamorphic core complex formation, and wide-spread calcalkaline volcanism ('ignimbrite flare-up'). This early stage of orogenic collapse was followed by the Basin and Range extension, characterized by block faulting and bimodal volcanism, starting around 18–16 Ma. The Basin and Range extension and associated subsidence resulted in development of the Great Basin with a mean elevation of 1.5 km above sea level and a crustal thickness of 30 km. Similarities in the post-orogenic evolutionary paths of these three regions indicate that continental lithosphere reacts to overthickening in a consistent way through large-scale crustal remobilization and extensional collapse, regardless of the mode and nature of its orogenic build-up.

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