THE ALABAMA HILLS AND DEXTRAL OFFSET IN OWENS VALLEY, CA

The Alabama Hills block (AHB) is located in Owens Valley, CA, adjacent to the Sierra Nevada batholith (SNB), and is significant for understanding the Mesozoic and Cenozoic tectonic history of the region. Correlations of the Jurassic Independence dike swarm suggest the AHB may be dextrally offset from the SNB by up to 55 km, whereas bulk U-Pb zircon geochronology of plutonic rocks in the AHB and SNB allows as little as 0 km of lateral offset between the two since 85 Ma. Offset markers in both the AHB and SNB correlate with markers exposed in the Coso Range to the southeast across Owens Valley. Therefore, assessing connections between the AHB and SNB is important for determining the total offset across Owens Valley.

We present single-grain zircon U-Pb CA-ID-TIMS geochronology and Sr and Nd whole rock isotopic geochemistry to test links between the AHB and SNB. Four samples of the Alabama Hills Granite yield ⁸⁷Sr/⁸⁶Sr_i = 0.7052–0.7058, εNd_i = -3.1 to -2.4, and ages of ~102 Ma. These data demonstrate the Alabama Hills Granite is distinct from the Late Cretaceous Whitney Intrusive Suite. Instead, we suggest a connection between the Alabama Hills Granite and mid-Cretaceous leucogranites with similar ages and geochemistry ~10 km north on the Sierra Nevada crest. This group of plutons, which we refer to as the intrusive suite of Kearsarge, is characterized by ages from 103–101.5 Ma, ⁸⁷Sr/⁸⁶Sr_i = 0.7048–0.7058, and εNd_i = -4.5 to -2.4.

We propose that the AHB has moved less than ~10 km dextrally relative to the SNB since assembly of the Kearsarge suite ca. 103 Ma. Suggestions of greater displacement between the AHB and SNB are less certain owing to the low angle at which the Independence dike swarm strikes across them. If there is little offset between the AHB and SNB, the AHB is a suitable piercing point for assessing offset between the SNB across Owens Valley. The Independence dike swarm is dextrally offset ~75 km between the AHB and the Coso Range to the southeast. The Cactus Flat pluton in the Coso Range is chemically and temporally similar to plutons of the Kearsarge suite, and also supports 65–75 km of dextral offset since the mid-Cretaceous. Offset markers of the AHB agree with other independent markers that suggest ~65 km of dextral offset across Owens Valley since the cessation of Sierran magmatism ca. 83.5 Ma.