IMPLICATIONS OF A 102 MA ALABAMA HILLS GRANITE FOR DEXTRAL OFFSET IN OWENS VALLEY, CA AND THE ORGANIZATION OF SIERRAN MAGMATISM

The Alabama Hills (AH) of CA expose a Jurassic volcanic complex and a Cretaceous pluton (AH Granite; AHG) east of the Sierra Nevada batholith (SNB) near Lone Pine, CA. New U-Pb CA-TIMS zircon geochronology for the AHG yields ages from 102.8-102.2 Ma. These dates are older than the previously suggested age of 85 Ma, which implied a link between the AH and the adjacent 90-83 Ma Whitney Intrusive Suite (WIS). This connection is no longer tenable. Instead, the dates suggest a link to rocks tens of kilometers NW. Whole rock samples of the AHG yield ⁸⁷Sr/⁸⁶Sr_i = 0.7058-0.7089 and εNd_t= -2.73 to -3.03 and extend the range of isotopic ratios for ~102 Ma plutons in the SNB.

Because there is no temporal connection between the AHG and the WIS, there is no requirement that the block be derived from the adjacent range front. Correlation of the highest density portion of the Independence Dike Swarm (IDS) between the AH and the SNB is consistent with 25-55 km of dextral offset between the Sierran range front and the AH (imprecise due to the obliquity with which the IDS intersects Owens Valley). However, the AH are not cut by the 83 Ma Golden Bear dike. This requires that the block be offset either <10 km or >28 km. We note that, if offset of the AH block is <10 km, the 102 Ma granite lies along a SE-NW trend of ~102 Ma plutons stretching from the AH NW to the ~102 Ma Dinkey Creek pluton in the central part of the SNB. Likewise, the Jurassic volcanic complex exposed in the AH is aligned with comparable rocks of the Oak Creek and Goddard pendants. Additional work is necessary to distinguish between the options of <10 km and >28 km offset, but because the offset estimate on the basis of density of the IDS is so imprecise, and the other features line up fairly well, we suggest that minimal dextral offset (<10 km) between the SNB and the AH is more likely than >28 km offset.

The new data for the AH suggest that Cretaceous magmatism in the SNB at ~102 Ma was organized obliquely to magmatism immediately before and after. Jurassic magmatism and the 98-83 Ma intrusive suites are elongated parallel to the batholith (N20ºW) and have limited isotopic variability. In contrast, ~102 Ma rocks define a trend of N60ºW and are isotopically diverse, ranging from juvenile isotopic ratios near the AH, to crustal values in the Shaver Intrusive Suite to the NW.