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Structural and metamorphic evolution of the Orocopia Schist and related rocks, southern California: Evidence for late movement on the Orocopia fault
Author(s) -
Jacobson Carl E.,
Dawson M. Robert
Publication year - 1995
Publication title -
tectonics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.465
H-Index - 134
eISSN - 1944-9194
pISSN - 0278-7407
DOI - 10.1029/95tc01446
Subject(s) - schist , geology , lineation , crenulation , geochemistry , mylonite , metamorphic rock , gneiss , shear zone , cataclastic rock , seismology , petrology , fault (geology) , tectonics
The Pelona, Orocopia, and Rand Schists (POR schists) of southern California and southwesternmost Arizona are late Mesozoic or early Tertiary subduction complexes that underlie Precambrian to Mesozoic continental basement along the low‐angle Vincent‐Chocolate Mountains (VCM) fault system. The VCM faults are often considered to be remnants of the original subduction zone, but recent work indicates that many have undergone substantial postsubduction reactivation. In the Orocopia Mountains, for example, the Orocopia Schist exhibits an exceptionally complex structural and metamorphic history due to multiple periods of movement along the Orocopia fault. Structures in the schist include isoclinal folds with axial‐planar schistosity, open‐to‐tight folds that fold schistosity, penetrative stretching lineations, and crenulation lineations, all of which show a nearly 360° range in trend. Folds and lineations that trend approximately NE–SW occur throughout the schist and are thought to be part of an early phase of deformation related to subduction. Folds of this orientation show no consistent vergence. Folds and lineations that trend approximately NW–SE are concentrated near the Orocopia fault and are interpreted to have formed during exhumation of the schist. The NW–SE trending folds, and shear indicators in late‐stage mylonite at the top of the schist, consistently verge NE. The exhumation event culminated in emplacement of the schist against brittlely deformed upper plate. Exhumation of the Orocopia Schist was accompanied by retrograde replacement of garnet, biotite, epidote, and calcic amphibole by chlorite, calcite, and sericite. Matrix amphibole has a lower Na/Al ratio than amphibole inclusions in albite, consistent with a late‐stage decrease in pressure. As NE vergence in the Orocopia Mountains is associated with exhumation of the schist, the NE movement along other segments of the VCM fault may also be late and therefore have no bearing on the facing direction of the VCM subduction zone, contrary to past interpretations.