EVOLUTION OF MARINE TERRACE PLATFORMS IN SOUTHERN CALIFORNIA: INTERPLAY BETWEEN TECTONICS, SEALEVEL AND CLIMATE MODULATED EROSIONAL PROCESSES

The coastline of Southern California from Dana Point to La Jolla is a landscape dominated by a flight of marine terraces that record continuous long-term uplift related to mantle-derived buoyancy beneath the Peninsular Ranges. To first order, marine terraces in Southern California form from the interaction of eustatic sea level change, erosion, and uplift driven by both far-field flexure and local sources due to faulting. Flights of terraces in Southern California range in width from ~ 10 km in San Diego County to less than 500 m further north, a pattern that correlates well with the strength of the underlying strata into which they are incised. Terrace strand line elevations (inner edges) can be fit to a constant uplift rate of 0.13-0.14 mm/yr, allowing their formation and subsequent erosion to be compared against numerical models of landscape evolution in the face of cycles in sea level. Previous 1D models that begin to address the mechanics of terrace formation are based on estimates of wave energy and cliff retreat assumed to be constant through time. Considerable variation in the erosion rate responsible for scribing terrace flights into the landscape is likely to occur, however, due to temporal variation in offshore bathymetry. Analysis of the modern shelf bathymetry yields insight into the effect of both channel incision on emergent platforms and oceanward shunting of sediment during sea level low stands. In addition, eolian beach ridges along the inner edges of numerous platforms in San Diego County clearly guide the development of trellis-shaped channel networks, and point to the need to incorporate more realistic topography in numerical models. For example, blowouts in beach ridges localize channel incision across them, which in turn can alter the pattern of longer-term erosion of terrace platforms. We present a suite of initial 1D models followed by 2D planform models created in CHILD (Channel-Hillslope Integrated Landscape Development) that explore how initial development and subsequent erosion of terrace platforms in Southern California occurs in response to changes in the boundary conditions outlined above.