NEW GEOCHRONOLOGY HIGHLIGHTS THE EVOLUTION OF AN ACTIVE MARGIN IN THE NORTHERN SIERRA NEVADA, CALIFORNIA

New geochronology in the northern Sierra Nevada was applied to three significant chapters in the region’s geologic history: Paleozoic paleogeography, Mesozoic plutonism, and Cenozoic volcanism. These new ages located in the Portola 30’x60’ quadrangle add important data to the northern Sierra’s long history as an active margin.

The Paleozoic Shoo Fly Complex consists of metasedimentary units that were part of an accretionary complex along a Devonian arc offshore of western Laurentia. U-Pb dating on detrital zircons in the Shoo Fly’s Lang sequence yielded major peaks at ~1250 to 1020 Ma. The Culbertson Lake allochthon, which overthrusts the Lang sequence, yielded major peaks at ~1910 to 1820 Ma. These data are consistent with both units having a provenance along the northern Laurentian margin, with peak age distributions indicating that sediments came from different parts of the margin. This provenance required southward transport of the Shoo Fly sediments until they reached their pre-accretionary position offshore of western Laurentia.

U/Pb zircon geochronology on plutons northeast of the Mohawk Valley Fault, which were previously mapped as Jurassic, shows that some of these plutons are Cretaceous. A highly magnetic diorite did not yield zircons, but a granodiorite mapped as part of the same unit yielded a U/Pb age of 108.1 ± 2.0 Ma. The lithologic difference between these plutons, supported by a prominent magnetic anomaly over the diorite, indicates the diorite is a separate unit, and its mafic composition is more typical of Jurassic plutons. Cretaceous granodiorite plutons are more extensive farther east, as observed elsewhere, with ages between 92 and 107 Ma.

During the Eocene, rollback of the Farallon slab led to westward migration of volcanism across Nevada, with the ancestral Cascades reaching the northern Sierra around 15 Ma. New ⁴⁰Ar/³⁹Ar ages of basalt and andesite from volcanic remnants west of the Mohawk Valley Fault range from 3.8 to 4.7 Ma. These ages and trace element geochemistry are consistent with rocks from the ancestral Cascades, though variations in some trace element concentrations may be related to melting of different lithologies in the Sierran crust. Undated but similar volcanic rocks farther west may indicate the Cascade arc was in its current position before northward migration of the Mendocino Triple Junction through this area around 3 Ma.