TRANSFORM MARGIN BASALTIC MAGMATISM AND THE EASTERN CALIFORNIA SHEAR ZONE

The Pacific-North American transform plate boundary expanded from ~28 Ma onward; its widening slab window under the continental margin underwent several tectonic transitions: 1) ~12 Ma the continental margin began to fail in dextral shear across a wide area, 2) ~6 to 5.3 Ma the transform completed jumps east into this shear zone, forming the Gulf of California and San Andreas fault, which had a nascent bend that placed the western Mojave Desert into transpression, and 3) ~3.3 Ma accelerating uplift of the central Transverse Ranges signaled increasing compression at the bend in the SAF. Changes in magmatism accompanied some transitions: 1) 16-12 Ma eruption of low Ti sodic subalkalic basalt to rhyolite in small slab-window fields from the Channel Islands to the Mojave Desert and south to Baja California changed to 2) 12-0 Ma high-Ti sodic basalt eruptions in the NW Mojave Desert and continued low Ti basalt to rhyolite fields to the north and south. Magmatic event 2 in the Mojave was confined to the transpressional part of the eastern California shear zone, which is the zone of highest shear rate in a wide soft plate margin. From 12-8 Ma eruptions were scattered in the northern and southern parts of the Mojave; 5.4-3.9 Ma eruptions shifted toward the center; and <2.1 Ma eruptions focused in a narrow segment. Geochemically, <12 Ma Mojave sodic basalts range from basanite to subalkalic high Ti sodic basaltic andesite, and total alkalis negatively correlate with SiO2, an unusual trend. Individual fields share this trend, and higher SiO2 rocks do not show increased crustal inheritance. These atypical Mojave basalts changed in eruptive volume with time: volume increased ~5-3.9 Ma, followed by quiescence until a small pulse at ~2 Ma and higher volume eruptions that resumed ~750 ka from the Rodman Mountains to Bristol Lake. Fourteen fields, several of them large, lie on or near dextral faults; 7 small fields lie on sinistral faults. Vents mainly lie within 0.5 km of faults. Entrained sheared mantle xenoliths indicate rapid magma ascent from a deformed mantle. We propose that the atypical Mojave basalts formed by shear melting, along with rapid transport of magma along faults, and while melting was perhaps aided by melt and metasomatic fluids derived from deeper levels, it fundamentally differed from decompression melt basalts associated with mantle upwelling in the slab window under transtension.