Roles of the Mendocino Transform, Vizcaino Block, and Onshore King Range Terrane in Evolution of the Northern San Andreas Fault System and Its Associated Slab Windows

Abstract:

We integrate recent seismic reflection, geochemical and radiometric age data from basalts and sedimentary rocks along the Mendocino Transform (MT) and Gorda Escarpment, with basalt ages and biostratigraphy from the Miocene King Range terrane (KRT) of the Franciscan Complex, to better link the onshore and offshore geology and clarify how the northernmost San Andreas Fault (SAF) evolved. The MT extends eastward from the Gorda Ridge spreading center, along the S side of the Gorda Plate, to the edge of the North American plate (NAP) and separates the Cascadia subduction zone to the north, from the modern SAF to the south. Between 127.5º W and the shoreline, the MT and Mendocino Ridge (MR) align with the N side of the S-tilted Vizcaino structural block (VB), a remnant of NAP captured by the Pacific plate ~12 Ma, when the MT was 480 km S of its present location. The modern SAF bounds the NE-side of the VB. The SW side of the VB is bounded at the base of the continental slope by the proto-San Andreas fault (PSAF), where extinct remnants of the Pacific-Farallon ridge (PFR) interacted with the paleosubduction margin to form an incipient transform and several microplates, now part of the Pacific plate. Capture of the VB resulted from inboard breaking of the MT with a jump of the PSAF to the modern SAF. Dated ~20-12 Ma basaltic rocks from the MR between ~125º-128º W may be partly exhumed slab window underplating that formed beneath the VB during breakup of the PFR along the PSAF. High Fe and Ti relative to Mg in MR and KRT basalts, suggest eruption near ridge-transform intersections and perhaps, intratransform spreading.

Onshore, high KRT relief aligns with the MR offshore. The KRT was assembled ~16-15 Ma (basalt K-Ar age; biostratigraphy); followed by its complex deformation and zeolitic metamorphism, indicating subduction to 5-8 km depth ~15-14 Ma and thermal metamorphism ~13.8 Ma (K-Ar age; vitrinite reflectance). The thermal overprint sets the KRT apart from adjacent accreted rocks. Similar MR and KRT age relations and ridge-transform intersection-related chemistry is consistent with collision of the PFR with the PSAF margin and development of a slab window beneath the VB and subducted KRT. This leads us to propose that the KRT is the NE corner of the VB, recently obducted onto the continental margin after northward translation with the Pacific plate.