19. Extensional and Transtensional Continental ARC Basins: Case Studies from the Southwestern United States
- Cathy Busby1 and
- Antonio Azor2
Published Online: 30 JAN 2012
Copyright © 2012 Blackwell Publishing Ltd
Tectonics of Sedimentary Basins: Recent Advances
How to Cite
Busby, C. J. (2011) Extensional and Transtensional Continental ARC Basins: Case Studies from the Southwestern United States, in Tectonics of Sedimentary Basins: Recent Advances (eds C. Busby and A. Azor), John Wiley & Sons, Ltd, Chichester, UK. doi: 10.1002/9781444347166.ch19
Department of Earth Science, University of California, Santa Barbara CA 93106, USA
Departamento de Geodinámica, Universidad de Granada, Campus de Fuentenueva, s/n, 18071 Granada, Spain
- Published Online: 30 JAN 2012
- Published Print: 30 DEC 2011
Print ISBN: 9781405194655
Online ISBN: 9781444347166
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 the western margin of North and South America. Late Triassic to Middle 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 subsided deeply at high rates, locally below sea level. These basins 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 formed along the axis of the early-stage low-lying extensional continental arc basins, due to the opening of the Gulf of Mexico, 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 differed markedly from early-stage basins by forming atop a deeply 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 ∼12 Ma, 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 Plateau, resulting in microplate capture. The Sierra Nevada microplate was born, with its trailing edge in the axis of the Ancestral Cascades arc. The late-stage transtensional continental arc is characterized by siting of major volcanic centers at transtensional fault stepovers. Each transtensional pulse produced an unconformity, followed by a magmatic pulse. Lithosphere-scale pull-apart structures tapped deep melts, producing “flood andesite” eruptions. Close proximity of uplifts and basins resulted in preservation of huge landslide deposits, and ancient E-W drainage systems coming off the Nevadaplano were deranged into N-S drainage systems along the new plate boundary.
The stratigraphic and structural aspects of continental arcs have been neglected relative to geochemical and geophysical aspects; however, these features are important for providing constraints on the tectonic evolution of such regions.