CLUSTERING AND MIGRATION OF MONOGENETIC BASALTIC FIELDS SINCE 12 MA IN THE EASTERN CALIFORNIA SHEAR ZONE, CALIFORNIA

About 12 Ma, volcanism in the Mojave Desert, California, changed from dispersed compositionally diverse (basaltic to rhyolitic) fields that migrated northward in an east-trending swath from 24-12 Ma to dispersed monogenetic basaltic fields located in the eastern California shear zone (ECSZ). Across the region, northwest- and east-striking strike-slip faults have been active since 12 Ma, but the highest total offset and slip rates on faults have consistently been within the ECSZ, defined by faults active during the last 2 Ma. Our recent literature review of 29 basaltic fields <12 Ma, detailed and reconnaissance studies of several fields, and compilation and collection of geochemistry on many fields enable evaluation of the physical and geochemical history of individual fields and comparison between fields. Sixteen fields are highly eroded remnants and 6-112 km²; 13 are moderately to well preserved and 7-106 km² with one 630 km². Compositions range from basanite through trachybasalt and basalt to basaltic andesite. Most volcanic fields are near or cut by long fault systems, but more importantly, in 10 of the 15 fields with preserved scoria cones, vents are <0.5 km from a fault, which implies a close relation of magmatism and tectonics. Comparative characteristics include distance between fields, similar trends in petrology and/or geochemical compositions, presence of mantle xenoliths, and age, but older fields have fewer types of data. Comparisons show that some nearby fields share characteristics; these characteristics are dissimilar between pairs of distant fields. Six magmatic clusters thus defined by distances to the centers of fields that are 15-60 km apart, with typical distances 20±5 km. These clusters imply that, although conditions in the mantle are non-uniform, those that influence location and processes of partial melting are locally uniform on scales of a few tens of km. The clusters had durations of 200-300 k.y., and two clusters might have been reactivated ~4 m.y and ~7 m.y after the first eruptions. Spatial patterns changed over the 12 m.y. span, with early fields forming in the north and south and the youngest in a central area from Pisgah to Amboy fields. However, the details of the time-space patterns for clusters were not simple, and the changes reflected transient changes in the mantle.