HR: 0800h
AN: T41E-1270    [Abstracts]
TI: Evidence for Early Crustal Thickening, Poly-phase Oligo-Miocene Extension, and Footwall Rotation at the Sierra Mazatan Metamorphic Core Complex, Sonora, Mexico
AU: * Wong, M S
EM: mwong@umail.ucsb.edu
AF: Dept. of Geological Sciences, UC Santa Barbara, Santa Barbara, CA 93106
AU: Gans, P B
EM: gans@geol.ucsb.edu
AF: Dept. of Geological Sciences, UC Santa Barbara, Santa Barbara, CA 93106
AB: The Sierra Mazatan metamorphic core complex records large-magnitude extension in the Mexican portion of the southern Basin and Range. We present new geologic and thermochronologic evidence that documents the pre-extensional history and the tectonic exhumation of this core complex. The geological evolution of Sierra Mazatan provides important insights into the extensional history of northwestern Mexico as well as models of core complex formation. The shallowly (10-$15\deg$) west dipping detachment fault that unroofed the core complex extends $>$50 km north from Sierra Mazatan and exhumed the southern portion of the Aconchi batholith. The fault likely extends both further north and south, but its trace is covered and its full length is unknown. Rare schistose intervals in the eastern footwall contain kyanite overprinted by sillimanite, indicating that at least local crustal thickening, likely of Sevier age, occurred in Sonora and may have played an important role in localizing core complex-style extension. New $^{40}$Ar/$^{39}$Ar thermochronologic data reveal two distinct pulses of rapid footwall cooling that support a poly-phase unroofing history. Based on these data, an early pulse of slip on the detachment fault occurred from 25-23 Ma followed by later slip event from 21-15 Ma. $^{40}$Ar/$^{39}$Ar geochronology of moderately NE dipping volcanic flows interbedded with hanging wall sediments yield ages ranging from 17.7 to 15.3 Ma, supporting the timing of the last slip event and demonstrating syn-extensional basin development during that time. The unconformable deposition of a 12.4 Ma ignimbrite on the footwall brackets the end of major slip. Total slip on the detachment fault was likely 15-20 km based on the amount of rapid footwall cooling (likely 200-$300\deg$C) and the apparent offset of correlative hanging wall and footwall sequences. Multiple lines of evidence support a steep initial dip for the presently low-angle detachment fault. Thermochronologic data indicate a $>$$300\deg$C temperature difference across 15.5 km of the footwall in the slip direction at 21 Ma, implying 20-$45\deg$ of eastward footwall tilt depending on the assumed paleo-geothermal gradient (15-$30\deg$C/km). Geologic data also strongly support significant footwall tilt. Sedimentary sequences east of the granitic footwall consistently dip 40-$55\deg$ NE and footwall dikes generally dip 45-$75\deg$ WSW, suggesting $\sim$$45\deg$ of eastward footwall tilting. Accounting for this footwall tilt would restore the detachment fault to a steeper initial dip of 40-$60\deg$. This study demonstrates that at least some detachment faults related to core complexes formed at steep initial dips. Early crustal thickening, poly-phase extension, and steep initial dips for detachment faults may be common in the evolution of many Cordilleran metamorphic core complexes.
DE: 8010 Fractures and faults
DE: 8109 Continental tectonics--extensional (0905)
DE: 8110 Continental tectonics--general (0905)
DE: 5475 Tectonics (8149)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting