RAPID AND PUNCTUATED VERTICAL STRAIN ALONG THE LEADING EDGE OF THE CENTRAL CALIFORNIA COAST RANGE: A MULTI-PROXY APPROACH

The Santa Lucia range of central California has exhumed faster and more than many transpressional reaches along the main San Andreas fault system, yet its bounding fault system, the San Gregorio-Hosgri fault (SGHF), is not well understood. The history of vertical strain is examined using a multi-proxy approach with zircon and apatite (U-Th)/He thermochronometry, deformed marine terraces, geomorphic metrics of erosion, and basin analysis.

Cooling ages indicate rapid late Miocene cooling along the entire 90 km southwest flank of the range; a ~6 Ma onset is most consistent with available constraints, but a range of ~5–10 Ma is permissible. Exhumation rate is strongly correlated with proximity to the SGHF: within a few km of the fault apatite rates are ~0.35–1.1 mm/yr and zircon rates are ~0.2–0.7 mm/yr. Rates from both are <0.2 mm/yr beyond ~7 km NE of the fault or SW across its trace. Deformed marine terraces are 3–5 times higher NE of the SGHF and decay to regional values over a ~5 km-wide zone; elevations drop rapidly SW across the fault. SE of the SGHF-Oceanic fault junction the terrace uplift pattern reverses and suggests that the Oceanic fault transfers vertical strain inland. At a range-wide scale, geomorphic metrics of erosion correlate well with exhumation and suggest that the range is perhaps near long-term equilibrium between uplift and erosion. Normalized channel steepness is highest NE of the SGHF and decays with distance from the fault, even after controlling for lithology.

Each of these data sets show a strikingly similar pattern of vertical strain and together indicate that uplift and exhumation has been focused along the NE side of the SGHF since the late Miocene. From a longer-term perspective, analysis of ~70 m.y. of basin fill in the Santa Lucia range suggests that the post-Miocene event is the most recent in a cyclic pattern of uplift and subsidence which began with crustal restructuring that exhumed mid-crustal rocks in the late Cretaceous. In this light, the accumulation of vertical strain along this portion of the plate boundary appears to be a persistent theme and may be symptomatic of the dynamics of highly oblique plate boundaries. We posit that the vertical cycling results from strong rheologic contrasts created by the emplacement of schist beneath a more-rigid Salinian lid.