INSIGHTS INTO THE FORMATION OF THE COTTONWOOD CANYON FAULT IN THE CATALINA SCHIST

Exhumed subduction zone terranes record physical and petrologic heterogeneities that develop during subduction and exhumation. The Cottonwood Canyon fault is in a coherent amphibolite unit near the boundary between this amphibolite and amphibolite facies mélange, both of which are in the Catalina Schist, Santa Catalina Island, California. This study focuses on the relationships between the nearby units and the mélange, which is associated with subduction zone processes. The mélange is thought to have formed by mixing mafic and ultramafic protoliths during subduction to produce large blocks in a fine-grained matrix, though the mixing processes themselves are poorly understood. The fault itself is exposed for approximately 10 m. Field observations suggest that the fault rock is mineralogically different from the host amphibolite, which brings into question the origin of the fault rock. The fault permits geochemical and petrographic assessments of the formation dynamics of the mélange as the fault rock may have been derived from the fine-grained mélange matrix, the coherent amphibolite, or both. Geochemical heterogeneity and deformation microstructures in the fault imply that it experienced displacement both parallel and perpendicular to its boundaries, ultimately forming a fault rock with compositions intermediate between the coherent host amphibolite and mélange matrix. Contrasting rock fabrics also indicate a period of retrograde metamorphism and the subsequent crystallization of pumpellyite, which may imply an influx of water into the system. Thus, the following chain of events for fault formation is proposed: 1) amphibolite crystallization; 2) deformation, shearing of amphibolite, and flow of mélange matrix into the fault; 3) retrograde metamorphism. Future work involving the deformational tendencies of the mélange matrix and the role of water in this system will enable further comprehension of mélange dynamics.