NEW AGE INFORMATION FOR THE GENESEE-REWARD STOCK, TAYLORSVILLE AREA, CALIFORNIA: IMPLICATIONS FOR THE TIMING OF NORTHEAST-VERGENT THRUST FAULTING IN THE NORTHERN SIERRA AND KETTLE ROCK TERRANES, NORTHERN SIERRA NEVADA

Several northwest-striking, northeast-vergent, Mesozoic thrust faults, including the Taylorsville fault, have been identified in the northernmost Sierra Nevada. Some of these faults duplicate stratigraphy within the northern Sierra (i.e., Grizzly Mountain fault zone) or Kettle Rock terranes (i.e., Cool Springs shear zone), while others form the complicated structural boundaries of the terranes (i.e., Taylorsville fault, Ward Creek thrust, Austrian-Syndicate thrust). Many of these fault zones are heavily mineralized and contained economicaly viable copper ore deposits.

The age of these structures was long assumed to be Jurassic. Recently, samples from distal-facies, subaerial tuff within fluvial rocks of the Trail Fm, near the stratigraphic top of the Kettle Rock terrane yielded overlapping Barremian U-Pb ages of 129.4 +/- 2.0 Ma and 127.2 +/- 3.1 Ma (Christe, 2010). This data constrained the age of the northeast-vergent thrusting that juxtaposed the Kettle Rock and northern Sierra terranes to be Early Cretaceous or younger; unrelated to the Late Jurassic Nevadan orogeny. This study focused on provding a more precise younger age limit for this faulting. The previously undated Genesee-Reward stock, a small composite diorite to tonalite intrusive body which cuts the Wards Creek thrust separating the northern Sierra and Kettle Rock terranes was sampled for zircon. The stock is important in that it intrudes lower Cretaceous clastic rock of the Kettle Rock terrane and Triassic carbonate and clastic rock of the northern Sierra terrane on either side of the Wards Creek thrust. The zircon analysis, conducted via LA-ICPMS at the Arizona Laserchron Center (n=28), yielded an Early Cretaceous, weighted mean U-Pb age of 116.9 +/- 1.1 Ma (Aptian). The new age determined for the undeformed stock constrains the contractile episode of northeast-vergent thrusting in the northernmost Sierra Nevada to a short interval (approx. 10 m.y.) between the upper Barremian and lower Aptian, after which, emplacement of granites associated with the Sierra Nevada batholith took place. The timing of this contractile event in the Taylorsville area is similar to the age of Early Cretaceous deformation to the west along the fault boundary separating the Feather River peridotite and the northern Sierra terrane (Shoo Fly complex) Carlson (2005).