TECTONIC IMPLICATIONS OF LATE CENOZOIC VOLCANISM IN THE EASTERN MOJAVE DESERT

Late Cenozoic volcanism in the eastern Mojave Desert, near Baker California, spans 13 million years. The earliest event from 12.8 to 13.0 Ma resulted in the emplacement of hypabyssal sills of rhyolitic to trachydacitic composition. This was followed at 12.1 Ma by flows of trachyandesite (pyroxene andesite), approximately 15 km to the west in the Halloran hills. The most recent period of activity began about 7.0 Ma and has continued to Recent time. The result has been trachybasalts (hawaiites) and basaltic trachyandesites (mugearites) of the Cima volcanic field. This study analyzed over 125 rock samples from the three areas for 21 major, minor and trace elements. The data define a trend of high K₂O+Na₂O volcanic rocks showing minimal decease in alkalis with decreasing silica content over time. Trace element analyses of the pyroxene andesites and many of the basalts reveal that crustal interaction has played an important role in their genesis.

It is suggested that Cenozoic volcanism began during the initial stages of late Miocene detachment and has continued to Recent time, as upwelling asthenosphere has occupied the void created by the thinning lithosphere. The compositional variations reflect progressively deeper melting, from shallow crustal rhyolite and trachydacite to lower crustal pyroxene andesite to lithospheric and asthenospheric mantle basalts. The pattern of volcanism also indicates that during the Late Cenozoic the eastern Mojave has undergone clockwise rotation at an average rate of 5 mm/year. This rotation appears to consist of two distinct events; a period of nearly east-west motion from 10 to 13 Ma, perhaps related to detachment, and a younger north-south directed event from the Present to 7 Ma, a consequence of dextral shear.