U/Pb ZIRCON AGES OF CONGLOMERATE COBBLES, LATE CRETACEOUS-PALEOGENE GUALALA BASIN, CALIF.; IMPLICATIONS FOR PROVENANCE AND SAN ANDREAS FAULT OFFSET HISTORY.
SCHOTT, Ronald C., JOHNSON, Clark M., Department of Geology & Geophysics, University of Wisconsin-Madison, Madison, WI 53706

The Gualala Basin (GB) lies 180 km north of San Francisco, and is the northernmost on-land exposure west of the San Andreas Fault Zone (SAFZ). Clasts display a wide variety of lithologies, some of which are sparsely represented in the Mesozoic rocks of the western Cordilleran batholiths, which may shed light on the offset history of the northern SAFZ. An early estimate of ~600 km of dextral offset on the northern SAFZ was based on correlation of GB conglomerate cobbles with a basement terrane at Eagle Rest Peak (ERP) in the "tail" of the southern Sierra Nevada (Ross et al., 1973, USGS Jour. Res.). Currently accepted right lateral offset of ~465 km along the northern SAFZ (~315 km from the central portion of the SAFZ and ~150 km from the merging San Gregorio-Hosgri fault) fails to juxtapose the GB with the ERP basement terrane. Moreover, the original correlation has recently been called into question (James et al., 1993, in GSA Memoir 178).

We present new U/Pb zircon ages of conglomerate cobbles at Gualala which point to recognized source terranes such as the Salinian Block (SB) or southern Sierra Nevada (SSN), but which also indicate other sources, such as the western Mojave Desert (WMD). Granitoid cobbles (undeformed granite and granodiorite and weakly foliated tonalite) from both the Late Cretaceous Gualala Fm. and Eocene German Rancho Fm. yield concordant and normally discordant zircon fractions with 206Pb/238U ages between 95 Ma and 107 Ma. These ages, discordance patterns, and rock types are compatible with derivation from either the SB or the SSN. More mafic cobbles (quartz-hornblende gabbro) from the Late Cretaceous section thus far yield zircon fractions concordant at 144 +/- 1 Ma and normally discordant with 206Pb/238U ages between 140 Ma and 143 Ma; a source terrane for these rocks is not apparent. Two distinctive garnet-bearing granitoid cobbles from the Eocene German Rancho Fm. yield concordant ages at 135 +/- 0.5 Ma (foliated garnet-bearing biotite tonalite) and 129 +/- 1 Ma (garnet-bearing leuco-trondhjemite). Garnet-bearing granitoids of this age and composition are not found in either the SB or SSN; similar rocks in the WMD are a potential source for these cobbles, although age relations in the WMD are uncertain.

Identification of source terranes for the Gualala cobbles, and fitting these into the framework of known Cordilleran batholiths, requires a combination of geochronology and elemental and isotopic chemistry (see also Johnson et al., this meeting).