TIMING AND DEVELOPMENT OF AN OROGEN-PARALLEL LINEATION AND OF FRONTAL THRUSTING IN THE SOUTHERN CORDILLERAN FOLD-THRUST BELT, NEW YORK MOUNTAINS, CALIFORNIA

Complexly faulted and folded Paleozoic and Mesozoic metasedimentary rocks and Mesozoic metavolcanic rocks lie within a roof pendant of the Cretaceous Teutonia batholith in the New York Mountains of the eastern Mojave Desert, California. New detailed mapping, strain analysis, and U-Pb geochronology of cross-cutting igneous rocks provide important constraints on the kinematics and timing of ductile flow and low-angle faulting. Cambrian to Triassic rocks lie in low-angle fault contact - across the here named Sagamore Canyon thrust - above a previously undated sequence of Mesozoic metasedimentary and metavolcanic rocks. New U-Pb SHRIMP dating of zircon from the metavolcanic rock yields an age of 100.6 ± 1.0 Ma, confirming an earlier interpretation that the metavolcanic rocks are correlative with the Delfonte volcanic rocks found 30 km to the north in the eastern Mescal range, beneath the Keaney-Mollusk Mine thrust. The Sagamore Canyon thrust cuts earlier east-verging folds (F1), associated axial planar foliation (S1), and N-trending stretching lineation (L1) subparallel to the fold hingelines. The thrust is cut by the Mid Hills monzogranite. Ion microprobe U-Pb analyses of zircon from the Mid Hills monzogranite, yields an age of 90.0 ± 1.3 Ma. Detailed mapping and strain analysis within the metavolcanic unit show that the early deformation records constrictional strain, requiring either a period of orogen-parallel, synconvergent, gravitational relaxation, or transpression. The early constrictional strain is overprinted by a flattening strain, associated with the emplacement of the laccolith–like Live Oak Canyon pluton, in which the maximum stretch (L2) is subhorizontal and tangential to the margin of the pluton. All plutons and structures, including other low-angle faults of equivocal contractional origin, are cut by 80 ± 4 Ma (TIMS, U-Pb zircon) E-striking porphyry granodiorite dikes. These ages and relations tightly constrain the sequence of thrust burial and thermal weakening of volcanic rocks, followed by motion on the Sagamore Canyon thrust, to be between 101-89 Ma. Extension, manifest as the Pinto shear zone in the southern New York Mountains and widespread in the larger region, was ongoing by 74 Ma.