STRUCTURAL AND KINEMATIC RELATIONSHIPS SUGGESTING A GENETIC LINKAGE BETWEEN THE MONTEREY PLATE AND THE TRANSVERSE RANGES UPPER MANTLE HIGH SEISMIC WAVE SPEED ANOMALY, SOUTHERN CALIFORNIA

The profound tectonic features that developed during the mid-Cenozoic impingement of the Pacific-Farallon spreading ridge into the southern CA convergent margin can be linked in a coherent structural-kinematic progression. At ca. 30 Ma the Monterey plate nucleated along the Pacific-Farallon ridge as it impinged into the CA margin, with a 25-30° clockwise shift in spreading direction parallel to Pacific plate motion. Spreading along the Monterey ridge was accommodated by northwest Pacific plate motion, and 200-250 km of southeast directed subduction beneath the CA margin until ca. 19 Ma when the young ridge ceased spreading. The Monterey plate was then coupled to the Pacific plate driving the divergence of the young plate away from the CA margin. The 19-12 Ma phase of divergence inverted the Monterey subduction megathrust into a lithosphere scale normal fault, which by upper plate traction drove transrotational rifting of the Transverse Ranges, whose NE-SW strain gradient was controlled by an asperity in the convergent margin belt that was inherited from latest Cretaceous segmentation of the Farallon slab. The southern CA inner Borderland core complex developed in the wake of transrotational rifting, wherein the (Franciscan) Catalina schist was denuded from beneath the western margin of the Peninsular Ranges batholith. Transrotation was accommodated to the north by the N-S fold-thrust closure of the Santa Maria basin, and progressively to the south by NW-SE striking right-slip faults that overprinted the core complex. By ca. 6 Ma western Transverse Ranges transrotational motion took on a N-S convergence component, buttressed against the collapsed Santa Maria basin. The deeper subducted portion of the Monterey plate could not be drug out of the mantle, and thus between 12-8 Ma it necked off along its high strain gradient zone beneath the outer core complex and transrotational belt. This necking event liberated the Monterey plate to translate northwards along the Hosgri fault, to its current position in the central CA offshore. The necked off portion of the plate subsequently began foundering to form the Transverse Ranges mantle anomaly. Various deformation geometries for its necking and subsequent foundering are explored in the context of the various tomographic images published for the anomaly.