A SURPRISINGLY UNDEFORMED REMNANT OF THE JURASSIC VOLCANIC ARC IN THE NORTHERN SIERRA NEVADA: MOUNT TALLAC ROOF PENDANT, DESOLATION WILDERNESS AREA, EL DORADO COUNTY, CALIFORNIA

Recent geologic mapping, strain analysis, and paleomagnetic analysis in the vicinity of Grass Lake suggest that the southernmost Mount Tallac metamorphic roof pendant experienced remarkably little deformation since deposition. Here, exposures of Middle Jurassic metavolcaniclastic strata in the Tuttle Lake Formation (maximum age ~170 Ma based upon fossils in underlying Sailor Canyon Fm.) are cut by four sets of andesitic dikes, which are in turn cut by the Keiths Dome quartz monzonite pluton (ca. 159-160 Ma based upon an unpublished TIMS age and contact relationships elsewhere). Conglomeratic strata within the Tuttle Lake Fm. are gently folded into a broad upright anticline that is cut by an array of brittle/ductile faults and joints. Faults with offsets of less than 10 m, strike NW-SE, dip steeply SW, and have predominantly sinistral senses of motion. Strata are also cut by at least four joint sets; some of which are hydrothermally altered. A 3D outcrop-scale analysis of petrofabrics at 22 locations within the Tuttle Lake Fm reveals a range of very weak fabric intensities (E = 0.15 - 0.79) and symmetries (v = -0.43 - 0.71), and weakly oblate to prolate shapes (k = values 0.28 - 3.05). When considered with their generally subhorizontal NNE-SSW long (X) axes trends and subvertical short (Z) axes orientations, these fabrics appear to be consistent with deposition and compaction. Analysis of paleomagnetic data from 106 cores prepared from seven different andesitic dike samples define virtual geographic poles that strongly correlate with the North American polar wander path at 150 Ma. These data suggest the Tallac pendant experienced very little vertical axis rotation since the emplacement of the dikes. Thus, it is reasonable to assume that the southernmost Tallac pendant accumulated little deformation and may offer an unprecedented opportunity to examine near-surface environments within a Jurassic submarine intra-arc basin along the margin of western north America. The lack of deformation is significant because remnants of Jurassic host rocks within the Sierra Nevada are often strongly deformed by the emplacement of granitoid plutons during the Cretaceous. Indeed, the southernmost Tallac pendant may effectively provide a low-strain endmember to the range of strains measured among Jurassic Sierra Nevadan pendants.