Extensional and transtensional continental arc basins: case studies from the southwestern United States

Extensional and transtensional continental arc basins preserve very thick, continuous sequences and are an important contributor to the growth of continents; therefore, it is important to understand how they evolve. In this chapter, I describe four continental arc basin types, using Mesozoic to Cenozoic case studies from the SW US: (1) early-stage, low-lying extensional; (2) early-stage, low-lying transtensional; (3) late-stage, highstanding extensional; and (4) late-stage high-standing transtensional. During the breakup of Pangea in early Mesozoic time, the paleo-Pacific Ocean basin was likely composed of very large, old, cold plates; these rolled back during subduction to produce subsidence and extension in the upper plate, particularly along the thermally-weakened arc that formed along thewesternmargin of North and South America. Late Triassic toMiddle Jurassic early-stage low-lying extensional continental arc basins of the SW US were floored by supracrustal rocks, showing that uplift did not precede magmatism, and they subsideddeeply athigh rates, locally belowsea level. Thesebasins formed a sink, rather than a barrier, for craton-derived sediment. They were characterized by abundant,widespread, large-volume silicic calderas,whose explosive eruptions buried fault scarps and horst blocks, resulting in a paucity of epiclastic debris in the basins. In Late Jurassic time, early-stage low-lying transtensional continental arc basins formedalong the axis of the early-stage low-lying extensional continental arc basins, due to the opening of the Gulf ofMexico, which resulted in oblique subduction. Basins were downdropped at releasing bends or stepovers, in close proximity to uplifts along restraining bends or stepovers (referred to as “porpoising”), with coeval reverse and normal faults. Uplift events produced numerous large-scale unconformities, in the form of deep paleocanyons and huge landslide scars, while giant slide blocks of “cannibalized” basin fill accumulated in subsiding areas. Erosion of pop-up structures yielded abundant, coarse-grained epiclastic sediment. Silicic giant continental calderas continued to form in this setting, but they were restricted to symmetrical basins at releasing stepovers. In Cretaceous to Paleocene time, the arc migrated eastward under a contractional strain regime, due to shallowing of the progressively younger subducting slab. This produced a broad, high plateau, referred to as the “Nevadaplano” because of its similarity to the modern Altiplano of the Andean arc. Then, in Eocene to Miocene time, volcanism migrated westward due to slab rollback or steepening, producing latestage high-standing extensional continental arc basins. Late-stage extensional continental arc basins were similar to early-stage extensional arc basins in having “supervolcano” silicic caldera fields, but they were restricted to areas of thickest crust (along the crest of the Nevadaplano), rather than forming everywhere in the arc. Late-stage basins differedmarkedly fromearly-stage basins by forming atop adeeply eroded substrate, with eruptive products funneled through canyons carved during the preceding phase of crustal shortening. These basins lack marine strata, and stood too high above the rest of the continent to receive sediment from the craton. At 12Ma, E-W extension was replaced by NW-SE transtension, corresponding to a change from more westerly motion to more northerly motion of the Pacific plate relative to the Colorado Tectonics of Sedimentary Basins: Recent Advances, First Edition. Edited by Cathy Busby and Antonio Azor. 2012 Blackwell Publishing Ltd. Published 2012 by Blackwell Publishing Ltd.