Deformational Corridors along the San Andreas Plate Boundary: Evidence from Lithospheric Depths to the Surface

With the systematics migration of the Mendocino Triple Junction (MTJ), the San Andreas plate boundary forms within lithosphere transitioning from a convergent (subduction) to translation tectonic regime.  How that transition occurs, and what crustal/lithospheric deformation is associated with the fundamental plate boundary change has not been well understood. Through the combined analysis of a detailed 3-D lithospheric structure in the vicinity of the MTJ (from seismic tomography) in conjunction with geodetic data, seismicity, regional thermochronology, surficial geomorphic characteristics, and observed heat flow we are able to delineate two distinct deformational corridors defining the tectonics of plate boundary transition. A well recognized consequence of MTJ migration is the development of a slab window in its wake. Our seismic tomographic imagery helps us define the extent of that slab window - in particular ion western boundary. We are also able to image a fragment of former Farallon plate (which we term the Pioneer fragment), now accreted to the Pacific plate,  that has migrated with the MTJ, that also has served as the western boundary of the slab window. Geodetic data indicates the primary lithospheric-scale plate boundary structure forms along that Pioneer - slab window transition. The result is two distinctive corridors with quite discordant tectonic histories that lie on either side of the nascent plate boundary.

The Pioneer Corridor, which bounds the San Andreas Plate boundary on the west has experienced a coupled burial/erosion sequence as the MTJ migrates. This involves rapid rates of burial (Eel River Basin) followed by a short lived, but extremely rapid unroofing (~ 10 mm/yr) followed by subsequent, but slow exhumation. This results in major changes in observed surface heat flow, a complex pattern of low-T thermochronolgy ages, and a relatively subdued landscape (except in the region of rapid exhumation).

The Mendocino Crustal Conveyor (MCC) Corridor overlies the slab window, bounding the San Andreas boundary on the east. It has a distinctly different tectonic history involving a sequence of crustal thickening (uplift) followed by crustal thinning, with a complex lower bounding thermal evolution . The result is a quite different thermal-chronologic history, a variation in heat flow consistent with the crustal evolution, and a much more rugged landscape reflecting the long-lived uplift/exhumation history driven by slab-window processes.

Although the development of the San Andreas in the wake of the MTJ is oftentimes thought to be a tectonically simple process. This analysis indicates a very discordant history recorded in the thermal and surficial data of the two corridors bounding the nascent plate boundary.