DEXTRAL OFFSET ACROSS OWENS VALLEY: A VIEW FROM THE ALABAMA HILLS TO THE COSO RANGE

New single-grain CA-ID-TIMS zircon U-Pb data for four samples from the Alabama Hills Granite in Owens Valley, eastern CA, range from 103.1–101.8 Ma, nearly 20 Ma older than previous bulk TIMS U-Pb zircon data for the pluton suggested (85 Ma; Chen and Moore, 1982). Additional new CA-ID-TIMS data for the Bullfrog leucogranite on the Sierra Nevada crest indicate crystallization ca. 101.5 Ma, in rough agreement with Chen and Moore's (1982) bulk zircon geochronology. The new data preclude interpreted links between the Alabama Hills pluton and adjacent Whitney Intrusive Suite. The data also highlight: 1) the importance of using modern dating techniques designed to mitigate Pb-loss in zircon; 2) the need to reanalyze plutons dated by legacy geochronology methods in order to accurately assess their significance in the batholith.

New isotopic and geochronologic data indicate a connection between the Alabama Hills Granite and the Bullfrog and Independence leucogranites, referred to here as the Kearsarge intrusive suite. The suite is characterized by zircon crystallization ages from 103–101.5 Ma, ⁸⁷Sr/⁸⁶Sr_i = 0.7048–0.7058, εNd_i = -2.4 to -4.5, and distinctive trace element geochemistry. Field relationships, legacy geochronologic and radiogenic isotope data, and modern δ¹⁸O_zircon data (Lackey et al., 2008) suggest the Dragon, Diamond, and Sardine plutons, plus rhyolitic tuffs in the Oak Creek metavolcanic pendant, could also be correlative with the Kearsarge suite.

Recognition of the Kearsarge suite leads us to suggest the Alabama Hills block has moved less than ~10 km dextrally relative to the batholith since the Independence dike swarm intruded ca. 151 Ma, and thus the block is a suitable piercing point for measuring offset across Owens Valley. This puts the 65–75 km of offset between the highly-dilated central zone of the Independence dike swarm between the Alabama Hills and Coso Range (Bartley et al., 2007) in agreement with other offset markers, including Devonian submarine deposits and the Golden Bear and Coso dikes (Stevens et al., 1997; Kylander-Clark et al., 2005). Restoring 65–75 km of dextral offset also expands the Kearsarge suite, aligning the silicic Cactus Flat pluton of the Coso Range with the temporally and chemically similar Alabama Hills, Bullfrog, and Independence plutons (Kylander-Clark et al., 2005).