STRUCTURE OF THE CARDINAL MOUNTAIN INTERPLUTON SCREEN, SIERRA NEVADA BATHOLITH, CALIFORNIA

On Split and Cardinal mountains and adjacent peaks, a thin wall rock screen is bounded by Jurassic plutons. Mapping and structural analysis indicate that wall rock deformation records tectonic strain unrelated to pluton emplacement. Contact relations suggest that the plutons were injected into gently dipping fractures to form laccoliths. The Cardinal Mountain screen comprises biotitic quartzite and semipelitic schist overlain by variably calc-silicated marble . The quartzite-schist unit contains 2-10 cm crossbeds which indicate an upright stratigraphic section where unobscured by transposition or migmatization. Lithologic characteristics suggest correlation with the Cambrian Campito and Poleta formations of the Inyo Mountains. The oldest structures are NE-vergent mesoscopic folds with a schistosity that dips 20°-50°SW. This fabric is truncated by gently dipping intrusive contacts with the overlying Tinemaha (165 Ma) and underlying Red Mountain Creek plutons. The ca. 150 Ma Independence dike swarm crosscuts all of these rocks and fabrics. Cretaceous(?) reverse-sense mylonitic shear zones strike NW with moderate to steep SW and NE dips and crosscut Independence dikes and older rocks and fabrics. The 92 Ma Lamarck granodiorite crosscuts the screen to form a contact zone 20-50 m thick within which wall rocks and dikes were transposed and boudined by flattening and Lamarck-down shear. The foregoing relations imply intrusion of the Jurassic plutons along gently dipping brittle fractures. The paucity of stoped blocks on the floor of the Tinemaha pluton and lack of xenoliths in the Red Mountain Creek pluton indicate that stoping was volumetrically unimportant. We conclude that the Jurassic plutons were emplaced as laccoliths and/or lopoliths. In spite of abundant small-scale folds, at map scale the contact between the wall rock units is smoothly curviplanar and roughly concordant with contacts that bound the screen. Fractures that localized the intrusions thus were apparently guided by wall rock lithology rather than by tectonic fabric.