|Title:||Exhumation of the Orocopia Schist and crystalline hangingwall rocks of southeastern California: Relative roles of erosion, synsubduction tectonic denudation, and middle Cenozoic extension|
|Authors:||C.E. Jacobson, M. Grove, A. Vucic, and J. Pedrick|
|Publication:||Geol. Soc. Am. Spec. Pap., v. 419, p. 1‐37.|
The Orocopia Schist of the Orocopia Mountains is part of the regionally extensive Pelona‐Orocopia‐Rand Schist terrane, which is generally interpreted as a relict subduction complex underplated beneath southern California and southwestern Arizona during the latest Cretaceous‐early Cenozoic Laramide orogeny. The schist in the Orocopia Mountains forms the lower plate of the Orocopia Mountains detachment fault and has an exposed structural thickness of ⁓1.5 km. Prograde meta morphism occurred in the albite‐epidote amphibolite facies, although the upper half of the section exhibits a strong greenschist‐facies retrograde overprint. A mylonite zone just a few meters thick is present at the top of the schist. The upper plate of the Orocopia Mountains detachment fault is divided into one mappable unit consisting of Proterozoic gneiss widely intruded by 76 Ma leucogranite and a second unit dominated by anorthosite‐syenite with minor amounts of leucogranite. Both the leucogranite‐gneiss and anorthosite‐syenite units are locally cut by faults that may be genetically related to the Orocopia Mountains detachment fault. None of the rocks in the upper plate exhibit evidence of ductile deformation related to movement on the detachment fault. 40Ar/39Ar analysis of the schist yielded total gas ages of 54‐50 Ma for hornblende, 52‐34 Ma for muscovite, 33‐14 Ma for biotite, and 25‐24 Ma for K‐feldspar. A single apatite fission track sample yielded an age of 16 Ma. The above results, combined with multidiffusion domain (MDD) analysis of K‐feldspar, indicate two major episodes of cooling: one beginning at ca. 52‐50 Ma, the other starting at ca. 24‐22 Ma. The early Cenozoic phase of cooling is attributed to subduction refrigeration combined with erosional and tectonic denudation. The greenschist‐facies retrogression of the schist probably occurred at this time. The middle Cenozoic cooling event is thought to be the result of normal‐sense slip on the Orocopia Mountains detachment fault. The thin mylonite at the top of the schist probably formed in association with this structure. The early and middle Cenozoic events each appear to have contributed substantially to the 30‐35 km of total exhumation required to bring the schist from its maximum depth of underthrusting to the surface. Most 40Ar/39Ar ages from the upper plate fall into the following ranges: 76‐69 Ma for hornblende, 75‐56 Ma for biotite, and 78‐42 Ma for K‐feldspar. One apatite fi ssion track age of 27 Ma was obtained from the anorthosite‐syenite unit. MDD thermal histories for K‐feldspar vary signifi cantly with structural position, implying the presence of at least one major structural break within the upper plate. The distinctly old ages for the upper plate compared to the schist indicate that the former was exhumed to relatively shallow crustal levels by latest Cretaceous to early Cenozoic time. The upper plate was juxtaposed against the schist in the earliest Miocene by slip on the Orocopia Mountains detachment fault. The two‐stage cooling and exhumation history for the Orocopia Schist in the Orocopia Mountains is virtually identical to that inferred recently for the Gavilan Hills to the southeast based upon a similar thermochronologic analysis. Combined with preliminary investigations of several additional bodies of schist exposed along the Chocolate Mountains anticlinorium of southeastern California and southwestern Arizona, these data provide strong evidence for a major middle Cenozoic extensional event throughout the region. The inferred middle Cenozoic extensional faults are folded by the Chocolate Mountains anticlinorium. This contradicts a recent model for erosional unroofi ng of the Orocopia Schist, which predicts that the Chocolate Mountains anticlinorium developed primarily during the early Cenozoic.