PALEOMAGNETIC ANALYSIS OF MIDDLE JURASSIC SIERRA NEVADAN DIKES IN THE SOUTHERNMOST MOUNT TALLAC ROOF PENDANT, DESOLATION WILDERNESS AREA, ELDORADO COUNTY, CALIFORNIA

Sierra Nevadan metamorphic roof pendants record important parts of the volcanic history of western North America often obscured by subsequent plutonic activity and erosion. The Mount Tallac pendant – one of the largest and most well exposed of these arc remnants – provides an isolated glimpse of the near-surface environment within the Jurassic volcanic arc. It is primarily composed of Middle Jurassic volcaniclastic strata in the Sailor Canyon (166-163 Ma fossil age) and Tuttle Lake Formations. The southernmost Tuttle Lake Fm. is crosscut by andesitic dikes and the Keiths Dome quartz monzonite pluton. Contact relationships elsewhere suggest the Keiths Dome pluton is Late Jurassic (158-148 Ma). We collected paleomagnetic data from 106 cores prepared from seven different andesitic dike samples. Analysis the magnetic mineralogy of one of the dikes indicates the presence of single domain magnetite, hematite, and goethite. Alternating field experiments (up to 200 mT) indicate that these samples are characterized by one or two component systems, with the high coercivity component decaying towards the origin. Multiple cores from the same sample display similar orientations for the high coercivity component (α₉₅ 3-14˚). Six of seven samples analyzed define virtual geographic poles that scatter about the expected direction and the locality mean falls directly on the North American polar wander path at 150 Ma. These data suggest the Mount Tallac pendant experienced very little vertical axis rotation since the emplacement of the dikes. Geologic mapping and strain analyses in the area indicate that bedding within the Tuttle Lake Fm. is nearly horizontal and has accumulated little to no measurable penetrative strain. Together the geologic mapping, strain analysis, and paleomagnetic analysis suggest that the Mount Tallac roof pendant has experienced remarkably little deformation since deposition, despite being enveloped by the ascending granitic plutons of the Cretaceous Sierra Nevada Batholith. Thus, the southernmost Tallac pendant is an ideal location for studying the often-cryptic aspects of the evolution of western margin of North America during the Jurassic.