Young basalt fields of the Mojave Desert

Basalt, a mafic volcanic rock common in mid-ocean islands and in several continental settings, is melted from upper mantle rocks in many cases and thus provides information on mantle conditions. Basalt lava fields, some decorated with cinder cones, are scattered around the Mojave Desert. Only a few basalt fields have been well studied, so we undertook a compilation of basalt fields that are younger than ~12 Ma to examine space-time patterns. Cima volcanic field is unique in having eruptions that span ~7.5 MY, including the youngest eruption in the Mojave Desert at ~12 ka. Other fields probably erupted over short timespans of decades to hundreds of years based on analogy with modern eruptions, with few exceptions. We find that all basalt fields except Cima are restricted to the active eastern California shear zone, and many lie on active faults, indicating a direct relation between faulting and volcanism. Area and volume of lava is greatest for those fields associated with dextral faults, which may be attributed to less shear stress across those faults as compared to sinistral faults. Xenolith-bearing basalts that include chunks of mantle and deep crustal rocks are known in a few locations from the eastern San Bernardino Mountains to Cima and have a wide range in age.