Upper-Plate Deformation of Late Pleistocene Marine Terraces in the Trinidad, California coastal area, southern Cascadia subduction zone

Forming at sea level, uplifted shore platforms serve as long-term geodetic markers. The spatial distribution and elevation of marine terrace sequences offer insight to regional tectonics. In the Trinidad coastal area, active tectonic processes reflect upperplate deformation above the southern extent of the Cascadia subduction megathrust. A set of five uplifted and deformed late Pleistocene marine terraces is preserved in the Trinidad, California region and provides an opportunity to analyze regional uplift, folding and faulting. Using lidar imagery embedded within a geospatial information system (GIS), I employ a surface classification model (SCM) that identifies uplifted marine terraces on the basis of their micro topographical characteristics, low slope and low roughness. The SCM-based identification of marine terraces both supplements and verifies existing field mapping. Age assignments for the five marine terraces, which range from 80 ka to less than 500 ka, are based on paleo sea cliff geomorphology and soil development trends. Specifically, the steepest, highest and most prominent paleo sea cliff, which is associated with terrace number three, is correlated to the long-duration sea level high stand centered at 125 ka. Based on these age assignments, the average maximum uplift rates in the Trinidad coastal area are ~1.1 m/ky, and the average long term uplift rate diminishes westward to about 0.5-0.6 m/ky on the downthrown side of the Trinidad fault. Based on analysis of deformation using the high-resolution lidar imagery of the marine terraces, the Trinidad hanging wall anticline represents a fault propagation fold that ceased to be active when the associated reverse fault, the Trinidad fault, daylighted to the surface approximately 80-100 ka. The distribution of knickpoints on longitudinal profiles of coastal streams that trend across the Trinidad fault and the Trinidad anticline are consistent with uplift of the anticline within the hanging wall of the Trinidad fault, which caused base level fall of coastal stream outlets and prompted knickpoint migration up these streams. Overall, both the hanging wall and the footwall of the Trinidad fault show long term positive rock uplift, which implies that the Trinidad anticline and fault are contained within the hanging wall of a deeper structure. Therefore, the Trinidad fault likely splays off the Cascadia subduction zone megathrust or off a deeper thrust fault that splays off the megathrust.