Abrupt changes in crustal structure beneath the Coast Ranges of northern California - developing new techniques in receiver function analysis

New approaches to receiver function analyses at a suite of broad-band stations in the Coast Ranges of northern California identify rapid changes in crustal thickness accompanied by melt accumulation in the lower crust. These processes are associated with the northward migration of the Mendocino triple junction through the area over the past ∼8 Myr. By simultaneously solving for crustal thickness, Poisson's ratio and Moho strike and dip, we facilitate the application of receiver function analyses in areas of crustal complexity where data sets are sparse. We also introduce a straightforward approach to calculating the Vp/Vs velocity ratio of the seismogenic zone, which is used here to aid in the interpretation of results from the receiver function analysis. Results are consistent with triple junction related crustal modifications occurring in the plate motion direction in a narrow swath of crust between the Máacama and Bartlett Springs Faults. Crustal thicknesses decrease from over 35 km thick in Round Valley, south of the current triple junction location, to 20 km thick just 100 km further south, near Clear Lake. Major crustal thinning is localized to a narrow area of the crust around Redwood Valley, where the Moho dips approximately 17° to the northeast. This thinning may also be restricted to the lower crust rather than being evenly distributed throughout the crustal column. Coincident with these changes, melts are inferred in the lower crust from low velocity zones and elevated values of Poisson's ratio (σ ≥ 0.32) at two seismic stations. These melts occur in the same area as melt bodies suggested in other seismological studies. Together these results provide the first detailed description of the crustal modifications—and the spatial extent of that deformation—associated with the passage of the Mendocino triple junction.