EXTENSIONAL AND TRANSTENSIONAL CONTINENTAL ARC BASINS: LESSONS FROM THE JURASSIC ARC, SOUTHWEST CORDILLERA (Invited Presentation)

Prior to 1988¹, the role of extension and transtension in preserving continental arc successions was not recognized. Arcs formed on continental crust were referred to as “Andean-type”, leading to the notion that they are high-standing regions of uplift². Our work in the southwest Cordillera demonstrated that the Early to Middle Jurassic arc formed a sink, not a barrier, for craton-derived eolian quartz arenites, ponding them to greater thicknesses, and more continuously through time, than their equivalents on the Colorado Plateau (e.g. Wingate, Navajo, Entrada sandstones). Other diagnostic features of extensional arcs include preservation of multi-kilometer-thick sections that span little time; syndepositional normal faults; giant continental calderas; high-level intrusions; local subsidence below sea level¹; rapid extensional unroofing of plutons³; and penetration of meteoric water to mid-crustal levels⁴.

Late Jurassic sinistral strike-slip intra-arc basins formed along the axis of earlier extensional-arc basins as the Gulf of Mexico opened and North America migrated rapidly northward, from the horse latitudes to temperate latitudes⁵. Volcanism changed from “dry” to “wet”, causing more widespread dispersal of ashes. Basins were downdropped at releasing bends or stepovers, in close proximity to uplifts along restraining bends or stepovers, with coeval normal and reverse 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^5,6. 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. The Independence dike swarm was emplaced under a sinistral transtensional regime in the arc axis, over a distance of 600 km, and eruptive equivalents were locally preserved in basins³.

¹Busby 1988, Geology 16 (1121-1125)

²Hamilton, 1969, GSAB 80 (2409-2430); Burchfiel and Davis, 1972, AJS 272 (97-118)

³Schermer & Busby, 1994, GSAB 106 (767-790)

⁴Solomon & Taylor 1991, Geochem. Soc. SP (3) 449-462

⁵Busby et al., 2005, GSA SP 393 (359-376)

⁶Busby & Bassett 2007, BV 70 (85-103)