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Description
Seismic refraction studies were conducted on the Cretaceous La Posta granodiorite pluton, San Diego, California, to determine the depths of weathering across three microclimatic zones; Mediterranean hot summer, semi-arid hot, and arid. Data have been resolved as three layers. Layer-1 consists of surficial La Posta derived soils, alluvium , colluvium, grus, and spheriodally weathered boulders. Layer-2 includes an upper surface defined by the top of the water table and represents a transition from highly weathered boulders and intervening grus to fractured blocks of granite with slightly weathered intervening grus with depth. Layer-3 represents unweathered granodiorite. The top of Layer-3 coincides with the terminal depths of the weathering front. A Type-N Schmidt Hammer was used on bedrock outcrops in an attempt to constrain mechanical properties and aid in seismic interpretation . Schmidt hammer data were found to be highly variable and it was determined that strict testing conditions should be followed and more data were needed for good correlation to seismic data. The weathering mantle atop the pluton was developed from the latest Cretaceous to recent time. Seismic refraction lines indicate that the thinnest regolith (shallowest weathering front) lies in the wettest and coolest microclimate (Mediterranean hot summer) while the thickest regolith lies in a drier and hotter arid microclimate. The westward shallowness of the weathering front resulted from long-term evolution culminating within a weathering-limited regime that developed over the last 8 Ma as the Salton trough evolved. Uplift of western regions contributed to increased rates of erosion and mass wasting relative to soil development in the western part of the pluton while the same processes occurring within the eastern areas were not as effective.
