Physical and geochemical architecture and age of the Pliocene Bicycle Lake basalt, southeastern Fort Irwin, California

Abstract

The informally named Bicycle Lake basalt forms a volcanic field in southeastern Fort Irwin, California, disrupted by three east-striking faults and linked cross-faults of the Eastern California Shear Zone, and its distribution provides a framework for assessing volcanic field development, groundwater resources, and fault offsets. Previous geochronologic studies yielded ages ranging from ~2.9-5.6 Ma, and a new cycle of dating reported herein yields a more robust age of ~4.55 Ma. No vents or dikes have been identified for the basalt. No interstratified sedimentary rocks have been identified, not even aeolianites, implying that the field was monogenic and short-lived. Thickness of the field varies from 1 m at distal edges to as much as 20-35 m (projected in cross-section) in the central part of the field. The field consists of lava flows <8 m thick, and the relative abundance of flows <2 m thick and 3-8 m thick varies across the field, as does the interstratification of <3 m thick cosets of <1 m thick flows. Most flows are basaltic andesite with minor basalt. Within stratigraphic sequences, variations in major oxides and trace element geochemistry indicate interstratification of basalt with basaltic andesite, but also different compositions of interstratified basaltic andesite. The local abundance of basalt or different compositions of basaltic andesite varies. The distribution of compositions suggests minor changes in the composition of magma erupted and minor sector partitioning of flow paths during growth of the field. The basalt has been deformed as a result of transpressional stress transferred from the Coyote Lake to Bicycle Lake faults, which formed NW- striking reverse faults and faulted kink folds along with broad arching of the basalt. Restoring separation along the Coyote Canyon and Bicycle Lake faults indicates some flows of the basalt field traveled at least 10 km from their inferred source.