HR: 08:05h
AN: T21D-01 INVITED    [Abstracts]
TI: Extensional Tectonic Events in the Basin and Range of Western North America
AU: * Axen, G J
EM: gaxen@ees.nmt.edu
AF: Earth & Environmental Sci., New Mexico Tech, Socorro, NM 87801,
AB: Following late Proterozoic rifting and Paleozoic passive-margin development, western North America (W NAm) was dominated by subduction, accretion, retroarc thrust-belt activity, and lateral terrain translations, followed by widespread extension. Extension began with metamorphic core complex (MCC) formation in an Eocene back-arc setting in the NW US and SW Canada, and MCCs rapidly spread south to E Nevada and W Utah, probably triggered by slab collapse following low-angle Laramide subduction. In late Oligocene-early Miocene time, extension occurred in Chihuahua, Mexico, and spread west along with west-migrating arc volcanics, but "jumped" across the unextended Sierra Madre Occidental. MCC extension in the southern Basin and Range (Arizona, New Mexico, and Sonora, Mexico) began at this time and rapidly propagated NW, to SE California and S Nevada, meeting the northern belt by ~20 Ma near Las Vegas, NV. This propagation was probably in response to slab rollback and a migrating slab window. The Rio Grande rift also formed at this time in cratonal crust, so extended crust surrounded the strong Colorado Plateau on the W, S and E. Subsequent extension in Arizona and the Rio Grande Rift was relatively minor but rapid extension in Nevada and E California continued on steeply to gently dipping faults forming half graben basins with tilted footwall ranges and local MCCs. Both WNW- and ESE- striking dextral and sinistral faults, respectively, formed in conjunction. During this phase, subduction ceased, volcanism switched from calcalkaline to bi-modal alkaline, the dextral San Andreas fault system and Gulf of California dextral-oblique rift evolved, and the regional extension direction rotated CW. Lithospheric rupture in W NAm finally occurred during rapid (>300 km in <6 m.y.) transtension in the Gulf of California, along the mid- Miocene volcanic arc, probably in response to basal traction along the continental margin and increased strain rates between the strong Sierra Madre and Peninsular Ranges batholithic belts. This phase of extension left significant lateral variation in Moho depth and degree of continental extension prior to lithospheric rupture.
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8109 Continental tectonics: extensional (0905)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting