NEWLY-DISCOVERED DIAMOND VALLEY VOLCANIC CENTER IN THE HANGINGWALL OF AN ACCOMMODATION ZONE BETWEEN THE GENOA AND GROVER HOT SPRINGS FAULTS

New mapping has revealed the presence of a major Ancestral Cascades arc volcanic center in a region previously mapped as undifferentiated undated volcanic rocks¹ that are likely Miocene or Pliocene in age (⁴⁰Ar/³⁹Ar geochronology in progress). The basal unit is the newly-discovered tuff of Diamond Valley, a dacite ignimbrite with compound cooling units and cumulate inclusions (e-probe work in progress). A breccia lens within the ignimbrite consists of blocks of ultra-welded ignimbrite of the same composition, suggestive of cannibalization of earlier-erupted ignimbrite from the walls of a subsiding caldera. A hiatus in explosive eruption is recorded by local fluvial/debris flow reworking and deposition of a wedge of andesite block-and-ash-flow tuff. Renewed explosive eruption resulted in deposition of ultrawelded dacite ignimbrite with a higher proportion of clinopyroxene and less hornblende and biotite than the earlier phase. This was followed by eruption of hornblende andesite lavas and block-and-ash-flow tuffs, fed by dikes and sills. The succession is intruded by andesites plumbed up synvolcanic faults.

The volcanic units generally dip westward toward the master faults, the ~N-S Genoa fault - one of the largest faults in Walker Lane - and the Grover Hot Springs fault. Dips fan toward these faults, and the youngest deposits (andesitic debris flow deposits) are flat lying. However, the strata thicken and thin dramatically across synvolcanic faults that have both N-S strikes (parallel to the master faults) and E-W strikes. The E-W faults record hanging wall accommodation of a right step between the Genoa and Grover Hot Springs faults. This is geomorphically expressed as the mouth of the West Fork Carson River and the E-W Diamond Valley to the east of that.

The Diamond Valley volcanic center lies along the western edge of the Walker Lane, a 100-km wide transtensional fault zone on the west margin of the Basin and Range, where large arc and rift volcanic centers of Miocene to Holocene age are sited on releasing (right) fault stepovers^2,3. The Diamond Valley volcanic center represents a further example of the controls of releasing stepovers on the development of large volcanic centers in the Walker Lane.

¹Armin et al., 1984, USGS Map I-1474.

²Busby, 2013, Geosphere, 9-5, 1147-1160.

³Busby et. al. in press, Geosphere.