EVOLUTION OF THE EARLY MESOZOIC CORDILLERAN ARC, MOJAVE DESERT TO EAST-CENTRAL SIERRA NEVADA, CALIFORNIA

Field relations, geochemistry, and zircon ²⁰⁶Pb/²³⁸U ages in the central and southern Cordillera provide benchmarks for testing models of the timing and vergence of subduction along the continental margin. California arc and backarc assemblages provide a record of ~100 m.y. of early Mesozoic eastward subduction beneath SW North America. Early Mesozoic intrusive suites and coeval volcanic sections developed across a diverse continental margin arc framework. Jurassic plutons in the Mojave Desert arc segment intruded Proterozoic craton basement and cratonal to proximal miogeoclinal sedimentary cover sequences. In the northern Mojave Desert plutons intruded Paleozoic deep-water sediments of the allochthonous El Paso terrane. In several Mojave ranges, coeval volcanic cover sequences characterized by silicic ignimbrites and lavas are preserved. Zircon age distributions in Mojave arc assemblages imply continuous magmatism from mid-Early to Late Jurassic time, ca. 190 to 149 Ma. Jurassic arc rocks in the east Sierra arc segment are not as deeply exhumed and record magmatic assemblages composed of lavas, breccias, and silicic tuffs in several pendants preserved within the coeval eastern Sierra Nevada batholith. Compared to the Mojave segment, this part of the arc was emplaced into more distal miogeoclinal sedimentary rocks and allochthonous outermost shelf and slope sedimentary units imbricated in the Permian to Early Triassic Sierra Nevada – Death Valley thrust belt. Zircon age distributions in east Sierra arc rocks imply continuous magmatism from mid-Early to early Late Jurassic time, at ca. 180 to 161 Ma, and waning magmatism as young as 149 Ma. Regional age compilations suggest the Jurassic arc in both the Mojave Desert and east Sierra segments was coextensive with a Permo-Triassic arc developed ca. 251 to 203 Ma; thus a region-wide magmatic lull from about 210 to 190 Ma separated two early Mesozoic arc pulses. Triassic to Late Jurassic backarc strata parallel observations from the arc in peak and lull ages. The Triassic pulse is matched by ages from the Upper Triassic Chinle Formation and the presence of tuffaceous sandstones in that unit. Similarly, Upper Jurassic strata contain abundant tuffaceous sandstones and zircon populations that record the Jurassic pulse of arc volcanism.