Using High Resolution Aeromagnetic Data to Map Pervasive Folding in the Lithologically Indistinct Franciscan Coastal Belt

We use high-resolution aeromagnetic data to map magnetic bodies of graywacke of limited exposure that are either interbedded or structurally emplaced within broader areas of non-magnetic graywacke within the Franciscan Complex Coastal belt in northern California, which is bounded by the San Andreas Fault on the west and the Franciscan Complex Central belt on the south and east. Previous work has not extensively subdivided the Coastal belt because of the poor exposure and the fact that the exposed lithology is primarily graywacke indistinguishable in outcrop and hand sample and is thus difficult to map in the field. A hand-held magnetic susceptibility meter, however, in combination with thin-section analysis, reveals that some Coastal belt graywackes are magnetic. The thin-section analysis shows that the magnetic samples have a significant component of andesitic grains, whereas the non-magnetic samples do not. Further, the locations of these magnetic rocks correspond to elongate regions of high magnetic intensity (magnetic anomalies) kilometers to tens of kilometers in length. Previous 2D modeling showed that the bodies of magnetic graywacke can be modeled as a folded sheet, with antiformal limbs near or exposed at the surface and synformal limbs reaching a depth of about 1 km. Locations of edges of magnetic source bodies can be extracted from their magnetic anomalies. Near surface, steeply dipping edges lie beneath local maxima in the horizontal gradient of the magnetic potential surface. The edges are demarcated by locating discrete points along the local maxima. We connected these points, using an algorithm with a specific set of parameters, to delineate the edges of the magnetic graywacke bodies. Together with the previous 2D modeling, the anomalies and their edges show that the Coastal belt contains antiformal structures 5 to 20 km in length and 1.5 km in width, with a wavelength approximately 1.5 km. The modal direction of elongation is oriented approximately 50 degrees NW, sub parallel to the surface trace of the San Andreas Fault (trending 40 degrees NW in the area), which it intersects at an acute angle. The elongated structures in the relatively lithologically indistinct Coastal belt closely resemble structures defined by terrane fault boundaries and magnetic anomalies (produced mainly by tabular ophiolitic sources) in the adjacent lithologically heterogeneous Central belt. The distributions of mean vector direction and dispersion of magnetically-defined edges of the two datasets are very similar, and the global mean direction and dispersion agree to within 2 degrees. Many edges defined by the magnetic data are coincident with or continue the trend of mapped faults that form the structural boundaries between the Coastal belt and the Central belt terranes. The similarity of the pervasive structure in the Coastal belt evident from the aeromagnetic data with structure in the adjacent Central belt suggests that the structures within the two terranes are similar. This not only implies that structure in the Central belt can be used to inform further structural interpretations in the Coastal belt, but also suggests that the structural pattern seen in both terranes is the result of a younger tectonic overprinting that postdates the amalgamation of the two terranes.