Exhumation history of the Orocopia Schist and related rocks in the Gavilan Hills area of southeasternmost California

The Gavilan Hills area of southeasternmost California exposes three distinctly different crystalline rock packages in a postmetamorphic, E–W elongated dome. Structurally deepest is the relatively high-pressure, eugeoclinal Orocopia Schist, which underlies the low-angle Chocolate Mountains fault. Above the schist are gneisses derived from midto lower-crustal levels of the Mesozoic Cordilleran magmatic arc. The gneisses, in turn, are separated by the Gatuna fault from low-grade metasedimentary and metavolcanic rocks of the Winterhaven Formation. The Chocolate Mountains fault was originally thought to be a SW-dipping subduction thrust along which an exotic continental sliver was sutured to North America. Recent workers, however, have proposed that it is a late fault responsible for exhumation of the Orocopia Schist and that it places no constraints on burial history. The latter interpretation is supported by the presence in the schist of two distinct structural fabrics, an older one presumably related to underthrusting, and a younger one attributed to exhumation. The older fabric is preserved in schist away from the Chocolate Mountains fault and is associated with a NNE–SSW-trending lineation that formed during prograde metamorphism to lowermost amphibolite facies. The younger fabric is best developed within 100 m structurally of the Chocolate Mountains fault and is characterized by discrete shear zones, greenschist-facies retrogression, and E–W-trending lineations. Lineations with similar orientation also occur in gneiss adjacent to both the Chocolate Mountains and Gatuna faults and in the Winterhaven Formation. This Jacobson, C.E. , Grove, M. , Stamp, M. M. , Vucic, A. , Oyarzabal, F.R. , Haxel, G.B. , Tosdal, R.M. , and Sherrod, D.R. , 2002, Exhumation history of the Orocopia Schist and related rocks in the Gavilan Hills area of southeasternmost California, in Barth, A. , ed., Contributions to Crustal Evolution of the Southwestern United States: Boulder, Colorado, Geological Society of America Special Paper 365, p. 129–154. *E-mail: cejac@iastate.edu C.E. Jacobson et al. 130 observation, combined with interpretation of outcrop patterns, suggests that the Gatuna fault, which was previously considered a steep, shallow-level fault of Miocene age, is a low-angle structure that accommodated relatively high-temperature deformation similar to that recorded by the Chocolate Mountains fault. Both faults may have been synchronously active. However, because the Gatuna fault exhibits more intense brittle overprinting of early mylonitic fabrics and greater structural excision than the Chocolate Mountains fault, it is indicated to have been more recently active and the more important of the two in exhuming the schist and overriding gneiss to shallow crustal levels. Thermal history results based upon Ar/Ar analysis of hornblende, muscovite, biotite, and K-feldspar and previous apatite fission track measurements reveal a twostage exhumation history that we relate to slip along the Chocolate Mountains and Gatuna faults, respectively. An initial phase of rapid cooling occurred from 60 Ma to 44 Ma. Younger and more discordant Ar/Ar ages recorded by hornblendes from the schist (52–57 Ma) relative to the gneiss (59–64 Ma) confirm postpeak metamorphic juxtaposition of the two units along the Chocolate Mountains fault in a manner consistent with normal faulting. Coincidence of muscovite Ar/Ar ages between the Orocopia Schist and gneiss implies that this juxtaposition occurred by 48 2 Ma and indicate that the Chocolate Mountains fault is a Laramide-age structure. Biotite ages ranging from 45 to 31 Ma reveal that the initial exhumation phase was followed by protracted residence of the schist and gneiss in the middle crust at 350 C. Kfeldspars record a second period of rapid exhumation from 28 to 24 Ma, which we correlate with the brittle phase of movement on the Gatuna fault. This second phase of exhumation is considered to reflect an early stage of the middle Tertiary extensional event that is widespread in southeastern California and southwestern Arizona. Localized disruption of the Chocolate Mountains fault that juxtaposed structurally deeper schist against gneiss at the eastern end of the Gavilan Hills probably also occurred at this time.