FIELD RELATIONS AND AGE OF LATE CENOZOIC VOLCANIC UNITS INSET WITHIN THE MID-UPPER SAN JOAQUIN RIVER DRAINAGE, CA

Late Cenozoic volcanic units draped over Mesozoic and older basement rocks are important markers of tectonic-geomorphic evolution of the Sierra Nevada of California. Whereas most of these volcanic rocks cap interfluves and pre-date late Cenozoic stream incision, a few units are inset into the major canyons. Inset volcanic rocks crop out in San Joaquin River canyon, the largest drainage of the central Sierra, and their presence has been used to estimate a minimum age for onset of late Cenozoic incision in that drainage. Because interpretations for the drainage hinge on the absolute age of these volcanic units, accurate dates are of the utmost importance. The original dates (3.4-3.6 Ma) that figured prominently in the evaluation of uplift timing for this drainage are K/Ar ages over four decades old. Subsequent sampling and later K/Ar studies have now expanded the age range to 2.2-3.9 Ma. We have resampled several of these volcanic units near the confluences of the North, Middle, and South Forks of the San Joaquin River. Samples have been submitted for Ar/Ar incremental heating analyses. Paleomagnetic analysis of these flows provides additional constraints to the geochronologic analysis, and possibly refines the age estimates. Field investigations suggest that the interpretation that the lowest elevation remnants mark the paleocanyon bottom at the time of eruption, as previously interpreted, may be too simplistic for it is difficult to find remnants for which the lowest elevation caps stream gravels on a low-gradient surface. Accordingly, the lowest elevation of any given volcanic remnant probably represents the maximum elevation of the paleo-San Joaquin River channel bottom at the time of eruption, the amount of incision beneath the remnant being the maximum incision since emplacement. Present whole rock XRF compositional data shows no correlatable units, a result corroborated by paleomagntic analysis. In addition, paleomagnetic results, all of normal polarity, constrain lava emplacement to the Gauss normal chron (2.6-3.6 Ma). Ar/Ar age dates, currently in process, are expected to fall within this period.