Paleomagnetic Correlation of Late Miocene-Pliocene Basalt Flows in the Northwestern Basin and Range: Documenting Timing of Faulting, Volcanism and Vertical-axis Rotation in Surprise Valley, Northeastern California

Abstract:

Examining the temporal and spatial relationship between basaltic volcanism and extensional faulting can help delineate the influence of faulting on the location of source vents and distribution of subsequent lava flows. Young extensional environments offer a unique opportunity to resolve the interdependence of faulting and volcanism as faults have not yet been buried and precise ages can be acquired on volcanic rocks. The Larkspur Hills, located along the northwestern margin of the Basin and Range in northeastern California preserve flow-on-flow basalt sequences exposed in the footwalls of numerous normal faults. The eruption of these 3 to 8 Ma flows was coeval with the development and progression of extensional faulting, but the detailed relationship was not clear. Although several flows have previously been dated using ⁴⁰Ar/³⁹Ar geochronology, individual flows cannot easily be identified in outcrop or geochemically. Paleomagnetic analyses have allowed us to differentiate individual flows possessing unique remanence directions, and to correlate flow sequences. Nearly 1,300 paleomagnetic core samples from 167 flows were collected within the Larkspur Hills. Generalized magnetostratigraphy and detailed remanent magnetic directions were analyzed in conjunction with the existing geochronology to correlate flows and constrain the relative timing of faulting and volcanism. Results indicate that volcanic activity occurred during four distinct episodes between 3 to 8 Ma, and suggest that contemporaneous faulting likely influenced the distribution of subsequent flows by producing topographic barriers or pathways to the flows. In addition, when compared to the expected reference direction for stable North America, the paleomagnetic data indicate that the region has undergone 11.9 ± 4.5˚ of clockwise rotation. This result agrees favorably with studies that have inferred appreciable rotation of our study area associated with clockwise rotation of the Oregon coastal block.