HR: 16:15h
AN: T44A-02    [Abstracts]
TI: Rapid Exhumation of the Rand Schist: Constraints From Natural Garnet Diffusion Couples
AU: * Chapman, A D
EM: alan@gps.caltech.edu
AF: Caltech, GPS Division, 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Saleeby, J B
EM: jason@gps.caltech.edu
AF: Caltech, GPS Division, 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Luffi, P I
EM: pluffi@rice.edu
AF: Caltech, GPS Division, 1200 E. California Blvd., Pasadena, CA 91125, United States
AB: Intensive study of the Rand schist (hereafter referred to as the "schist") of southern California demonstrate convincingly that it formed in the Late Cretaceous by subduction of Cordilleran eogeoclinal detritus along a shallow dipping segment of the Farallon plate. Comparatively little is known about the exhumation history of the schist. Recent thermochronologic and thermobarometric analyses suggest that, in some localities, the schist was deposited, underplated beneath western Sierra Nevada batholithic assemblages (the "upper plate"), and exhumed with the upper plate from 30 - 40 km depths in less than 3 million years. Such a rapid cycling interval implies exhumation rates in excess of 5 mm/yr. To corroborate these existing data on the exhumation rate of the schist, we exploit diffusional annealing of garnet zonation. Some garnets from the schist of the San Emigdio Mountains were broken during late stages of their growth. Overgrowth along broken margins resulted in natural diffusion couples ca. 25 μm inward from the rims, where peak metamorphic temperatures reached 610 °C. A maximum time interval, representing exhumation prior to diffusion cessation, of 1 Ma was estimated from the degree of annealing of the originally sharp compositional step. Similar time intervals were calculated from narrow (10 μm) retrograde zones at the rims of compositionally homogenized garnets belonging to the upper plate. Garnets from the schist conspicuously lack retrograde zonation patterns, likely due to exhumation rates exceeding those of retrograde diffusion. Both garnet-based geospeedometry and thermochronometric studies indicate that the schist was exhumed at rates comparable to lateral tectonic plate motions (> 5 mm/yr). This conclusion, in the context of field-based analyses suggestive of west-directed ductile transport of the schist relative to the upper plate during exhumation, reveals a profound Late Cretaceous extensional event. These results yield important insight into the timing of Late Cretaceous schist ascent, in addition to the mechanisms necessary for such a rapid process.
DE: 1140 Thermochronology
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3652 Pressure-temperature-time paths
DE: 3660 Metamorphic petrology
DE: 8104 Continental margins: convergent
SC: Tectonophysics [T]
MN: 2007 Fall Meeting