THE EARLY MIOCENE (24-19 MA) MAGMATIC BELT OF SOUTHERN CALIFORNIA AND WEST-CENTRAL ARIZONA: ORIGIN, CHARACTERISTICS, AND TECTONIC SIGNIFICANCE (Invited Presentation)

The early Miocene magmatic belt of southern California as defined here is a narrow WNW-trending, 275 km long x 50 km wide band of 24 to 19 Ma volcanic rocks that extends from west central Arizona to the western Mojave Desert. This magmatic belt defies most petro-tectonic explanations in light of its age (post-subduction), geochemistry (calc-alkaline), orientation (orthogonal to the plate boundary), and relationship to local tectonism (predating local extension). I summarize aspects of this magmatic belt and speculate as to its causes and influence on the development of the of the NA-Pac transform margin.

Eruptive centers in the Whipple/Mopah, Clipper, Bristol, Calico/Newberry, and Soledad Mountains (to name a few) erupted copious amounts of basalt to rhyolite lavas and subordinate pyroclastic rocks, forming dome fields and thick lava stacks. The original vent edifices are gone and present exposures are biased towards accumulations of long runout lavas and pyroclastic debris in early Miocene lowlands that have been uplifted along younger faults. Extensive E-W to NW-trending dike swarms suggest mainly fissure eruptions and that the early Miocene landscape consisted of extensive coalescing lava plains punctuated by silicic dome fields. The progression of local eruptive and tectonic events throughout this magmatic belt seems to follow a consistent pattern: voluminous early (24 to 20) volcanic eruptions were followed by a brief episode of modest to severe extensional faulting and tilting (~20 to 19 Ma), locally followed by “capping” basalts. The total volume of erupted magma and magnitude of local extension generally diminishes westward.

The ultimate cause of this magmatic activity likely reflects localized asthenospheric upwelling, into either a slab-free window or a narrow band of lithospheric delamination. The consequence appears to have been a thermal weakening and extensional failure of the overlying previously-thickened crust. This early Miocene crustal thinning and development of an extensive system of favorably oriented steep NW-trending normal faults across the entire breadth of southern California set the stage to favor the development of an exceptionally wide NA-PAC transform boundary at this latitude.