Geomorphic Insights into the Links Between Mantle Flow and Crustal Deformation in the Northern California Coast Ranges.

The tectonism that results from the migration of the Mendocino triple junction in northern California shows a systematic evolution of dramatic changes in the processes and patterns of crustal deformation. In response to this time-varying tectonism, the river systems of the northern California Coast Ranges have developed to a complex pattern. As described by the Mendocino Crustal Conveyor model (MCC), near surface deformation is driven by a combination of crustal thickening and thinning, and by spatially varying dynamic forces that arise from mantle flow. It is the dynamic topography - that is modification of the isostatic uplift driven by mantle processes - that produces the complexity of the drainage and topographic patterns in the Northern California Coast Ranges. As a transient effect, this component of the crustal deformation is not apparent in geophysically imaged crustal structure . Therefore, surface geomorphology, in the guise of the surface response to both components of tectonic uplift, can provide the necessary constraints and insight to evaluate the tectonic consequences of mantle flow associated with MTJ migration. By combining geomorphic and geophysical observations, and landscape evolution modeling, we can partition the geomorphic evolution of the Coast Ranges into the component controlled by the systematic pattern of uplift/subsidence driven by the evolution in crustal structure, and the component driven by rapidly varying effects of mantle flow into the MTJ slab window. The pattern of landscape evolution and river system development that would be produced from crustal thickening/thinning alone, is not consistent with observations. It is the added effect of the dynamic topography that results in the complex pattern of river systems in the Coast Ranges. The dynamic topography produces: (1) a topographic gradient that switches multiple times during MTJ passage from trending northwest to southeast, causing stream capture and flow reversal, (2) two major drainage divides that migrate in concert with the triple junction, generating systematic and punctuated consequences for river evolution and (3) river evolution that as a consequence of 3-D patterns in mantle influx, results in the development of a fish-hooked drainage pattern.