Tectonic Implications of Late Cenozoic Volcanism in the Eastern Mojave Desert
Jessey, David R.1, Reynolds, Robert E.2, Michalka, Leianna L.1, and Baltzer, Suzanne M.1
(1) Geological Sciences Dept., California State Polytechnic University, Pomona, CA, (2) LSA Associates, Riverside, CA
ABSTRACT
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,
related to detachment, and a younger north-
south directed event