HR: 1340h
AN: S13B-1041    [Abstracts]
TI: Cenozoic Crustal Extension and Regional Fault Interaction in Southern Arizona Revealed Through Seismic Reflection Investigations
AU: * Wagner, F H
EM: fwagner@geo.arizona.edu
AF: University of Arizona, Dept. of Geosciences 1040 E 4th St, Tucson, AZ 85721 United States
AU: Johnson, R A
EM: johnson@geo.arizona.edu
AF: University of Arizona, Dept. of Geosciences 1040 E 4th St, Tucson, AZ 85721 United States
AB: Recent reprocessing and interpretation of industry seismic reflection data collected in the 1970's around southern Arizona have illuminated subsurface features related to Cenozoic crustal extension. Within the northern Tucson Basin, the Catalina detachment fault dips at 23-35 degrees to the southwest off of the western flank of the Catalina-Rincon Metamorphic Core Complex. Imaged on the detachment surface are broad corrugations on the scale of tens of kilometers, parallel to the extension direction, evident down to several seconds two-way travel time in the data. In the southern portion of the basin, the Santa Rita fault extends northwest from the western flank of the Santa Rita Mountains and dips 15-20 degrees and is cut by the Catalina detachment beneath the central Tucson Basin. This cross-cutting relationship is problematical in that the Santa Rita fault shows late Quaternary movement and also is oriented nearly perpendicular to extension directions in the region. We propose that the Santa Rita fault represents contemporaneous upper-plate deformation in the detachment system, and that recent movement inferred on the fault is simply a reactivation of this older fault surface. West of the Tucson basin, separated from it by the Sierrita Mountains, is Altar Valley. Seismic data reveal that Altar Valley is a half graben with a large-displacement normal fault bounding the western margin of the basin. This basin-bounding fault dips steeply toward the east from the eastern flank of the Baboquivari Mountains and appears to dip less steeply with increasing depth. The seismic data image multiple fault splays that merge in basement crystalline rocks into a main, shallowly eastward-dipping surface at around four seconds two-way travel time (approximately 10 km). This surface appears to anastomose with other surfaces imaged in the basement rocks and suggests that regional deformation has been distributed through discrete deformation zones. In a more regional context, the subsurface relationships between the Catalina detachment, the Santa Rita fault and the Baboquivari fault are intriguing. One aspect of interest is the unusual orientation of the Santa Rita fault, which is oblique to the direction of major extension in the area. The Santa Rita fault is located between areas of differing amounts of crustal extension, with a zone to the north of extreme crustal extension on the order of 100% or more across the Catalina / Rincon metamorphic core complex, and a zone to the immediate south that has experienced moderate crustal extension of perhaps 30%. Consequently, the nature and orientation of the Santa Rita fault could be the result of mountain-range-scale block rotations transferring strain between extensional domains. Also of interest is the relationship between the Catalina detachment system and the Baboquivari fault. The projections of these two fault systems suggest the two may interact at mid-crustal levels, perhaps joining into a common regional detachment. This places the Sierrita Mountains in the hanging-wall block between these two fault systems. Indeed, deformation within the Sierrita's seems to support contemporaneous movement of the Baboquivari fault and the Catalina detachment.
UR: http://www.geo.arizona.edu/$\sim$fwagner/
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 8109 Continental tectonics--extensional (0905)
DE: 8110 Continental tectonics--general (0905)
DE: 7205 Continental crust (1242)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting