INSIGHTS INTO STRUCTURE WITHIN A SUBDUCTION-ZONE COMPLEX FROM SIMPLE, CURVILINEAR MAGNETIC ANOMALIES IN THE COASTAL BELT OF THE FRANCISCAN COMPLEX, NORTHERN CALIFORNIA

Magnetic anomalies provide surprising structural detail within the Coastal Belt, the westernmost, youngest, and least metamorphosed part of the Franciscan Complex. Although the Coastal Belt consists almost solely of arkosic greywacke and minor shale, of mainly Eocene age, new aeromagnetic data indicate that it is pervasively marked by long, narrow and regularly spaced magnetic anomalies. These anomalies arise from relatively simple tabular bodies containing magnetic basalt or greywacke confined mostly to the top couple of km, even though metamorphic grade indicates these rocks had been more deeply buried, at depths of 5-8 km. This implies surprisingly uniform uplift of these rocks. The basalt (and associated Cretaceous limestone) occurs largely in the northern part of the Coastal Belt; the greywacke is recognized only in the southern Coastal Belt and is magnetic because it contains andesitic grains. The magnetic grains were not derived from the basalt, and thus require a separate source. The anomalies form simple patterns that can be related to folding and faulting within the Coastal Belt. This apparent simplicity belies complex structure mapped at outcrop scale, a contrast that implies the relatively simple tabular magnetic bodies are internally deformed, fault-bounded slabs. One mechanism that might explain thin basalt-bearing layers of wide areal extent is peeling up of oceanic crust into the accretionary prism, controlled by porosity and permeability contrasts produced by alteration in the upper part of the subducting slab. It is not clear, however, how such a mechanism might apply to the fault-bounded layers containing magnetic greywacke, nor is the source of the magnetic greywacke evident. We propose that structural domains defined by differing magnetic anomaly trend, wavelength, and source, reflect local plate interactions as the Mendocino Triple Junction migrated north, a hypothesis that should be tested by more detailed structural studies.