THE CRUSTAL STRUCTURE OF THE NORTHERN CALIFORNIA COAST RANGES FROM RECEIVER FUNCTIONS

The crust of Northern California has undergone a process of ephemeral thickening and thinning as a direct result of the northward migration of the Mendocino triple junction along the coast of California over the past 15Ma. The deformation is driven not by compression, but instead by a viscous coupling between the base of the North American plate and the south edge of the Gorda plate, based on the Mendocino Crustal Conveyor model.

We can use the seismologic tool of receiver functions at stations distributed through the Coast Ranges to produce snapshots of this deformation process, allowing us to image the response of the crust to the triple junctions' passage. Detailed analyses, focusing on the local crustal structure from receiver functions and also inferring the change in structure between these stations can help us to identify where the major deformation takes place and how it is manifested in the crust. We find that rather than occurring gradually over a broad zone, the deformation is localized, potentially to a narrow area around station FREY (in Redwood Valley). Furthermore, rather than the thickening and thinning being evenly distributed throughout the crustal column, it seems restricted to the lower parts of the crust closer to where the coupling takes place.

Here we present a detailed receiver function analysis at three stations in the Coast Ranges, demonstrating how we can constrain major crustal characteristics through these studies. We find Moho depths varying from ~35km at station CVLO (Covelo/Round Valley) in the north to only 20km less than 100km further south; dips on the Moho on the order of 10º, predominantly in a northeasterly direction around station FREY; high Poisson's Ratio's and low velocity zones that infer melt in the lower crust. These observations can in turn help us to address our questions, key to understanding the evolution of the crust and the formation of a major plate boundary through Northern California.