Extensional and Transtensional Continental ARC Basins: Case Studies from the Southwestern United States

Abstract

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 ...

U-Pb LA-ICP-MS GEOCHRONOLOGY AND GEOCHEMISTRY OF JURASSIC VOLCANIC AND PLUTONIC ROCKS FROM THE PUTUMAYO REGION (SOUTHERN COLOMBIA): TECTONIC SETTING AND REGIONAL CORRELATIONS

“…Early-Middle Jurassic (180-200 Ma) magmatic rocks also have been found in other segments of the Colombian Andes (FIGURE 1), including the Santander and Santa Marta massifs (Altenberger and Concha, 2005;Cardona et al, 2011), and are also common along most of the Andean Chain (Mpodozis and Ramos, 2008;Vásquez et al, 2011) Although some of the Jurassic remnants in the Colombian Andes may have been displaced from southern latitudes (Bayona et al, 2006), the existence of a major Jurassic continental scale magmatic province and its apparent link with extensional basins (Mpodozis and Ramos, 2008;Sarmiento et al, 2006;Vásquez et al, 2011) reflect the plate convergence following Pangea break-up, and the subduction of an old Pacific plate that controlled the growth of a continental magmatic arc and the contemporaneous regional scale magmatic activity (Busby, 2012;Cawood and Buchan, 2007;Cawood et al, 2009).…”

Arc and Slab-Failure Magmatism in Cordilleran Batholiths I – The Cretaceous Coastal Batholith of Peru and its Role in South American Orogenesis and Hemispheric Subduction Flip

“…Thick sequences of arc rocks erupted and deposited within a basin are common as most continental arcs form within subsiding depressions or basins on crust of average or below average thickness (Levi and Aguirre 1981;Hildebrand and Bowring 1984;Busby-Spera 1988;Busby 2012). Modern examples include the Cascades, where the volcanoes sit in a half graben; the low-standing Alaskan Peninsula where volcanoes such as Augustine sit within Cook Inlet; the Kamchatka Peninsula of eastern Russia where majestic stratovolcanoes sit in huge fault-bounded troughs; New Zealand where the Taupo zone is actively extending as calderas and stratocones form; and the Central American arc where volcanoes are aligned in a long linear depression.…”

Epithermal mineralization controlled by synextensional magmatism in the Guazapares Mining District of the Sierra Madre Occidental silicic large igneous province, Mexico

“…Removal of the slab resulted in a general southwestward migration of the arc-front magmatism towards the trench, commencing by ca. 40 Ma, in response to these FaralloneNorth American plate interactions (e.g., Coney and Reynolds, 1977;Damon et al, 1981;Ferrari et al, 1999Ferrari et al, , 2007Gans et al, 2003;Henry et al, 2010;Busby, 2012;McDowell and McIntosh, 2012;Busby, 2013).…”