The Coast Range Orogeny of California: transpression or oblique subduction?

The San Andreas Fault (SAF) is one of the most studied examples of a transition from subduction to a transform plate boundary. Has subduction terminated? Here we compile existing geophysics and new G-plates reconstructions to show the history and magnitude of subduction of vestiges of the Farallon Plate beneath North America. Furthermore, we explore a new geodynamic model demonstrating the feasibility of a hot, viscous Coast Range.

Continental transform settings with oblique relative plate motions are commonly characterized by shortening or extension in addition to their predominantly strike-slip motion. It is commonly held that uplift of the Coast Range was caused by shortening during oblique convergence between the Pacific plate and the Sierra Nevada microplate. Many have described the uplift of the Coast Range as a wholesale shortening of the entire lithosphere, implying a vise-like deformation (Argus and Gordon, 2001). In contrast, Braun and Beaumont (1995) proposed a wedge-style deformation driven by basal traction as developed in subduction zones. An understanding geometry of the plate interface at depth is central to these two geodynamic models. New studies support prior geophysical work that reveal a continuous dipping slab beneath coastal California, which extends as far east as the Sierra Nevada (Jiang et al., 2018). This favors a model whereby slow subduction accommodates a significant fraction of plate convergence.

In addition to geometry, the thermal structure of the plate interface should strongly influence strain accommodation. Prior to the initiation of the SAF, the proximity of Farallon-Pacific ridge crest produced subduction of hot, young oceanic lithosphere. Some researchers proposed that the divergence between the subducted parts of the Farallon and Pacific would cease to create oceanic lithosphere and would result in a "slab window" with hot asthenosphere directly beneath the overriding plate (Dickinson and Snyder, 1979). Such a model neglects the influence of lateral strength of an oceanic lithosphere, which could result in continued subduction, spreading and magmatism at the subjacent ridge. Subduction of a young oceanic plate may produce viscous softening of the overriding plate. We propose that the broad Coastal uplift is driven by basal traction against a hot, weak upper plate.