UPPER-PLATE DEFORMATION OF LATE PLEISTOCENE MARINE TERRACES IN THE TRINIDAD (CALIFORNIA) COASTAL AREA THROUGH GIS-BASED ANALYSIS OF LIDAR IMAGERY AND SOIL DEVELOPMENT TRENDS

Forming at sea level, uplifted wave-cut 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 upper-plate deformation above the southern extent of the Cascadia subduction megathrust. A set of four 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), we employ a surface classification model (SCM) developed by Bowels and Cowgill (2012) that identifies uplifted marine terraces on the basis of their micro topographical characteristics, low slope and low roughness. We supplement LiDAR-derived elevations with elevations derived from kinematic GPS. The SCM approach is verified through field mapping; the mapped extent of the terraces is then used to define regional tilts, folding and faults that deform and offset the terrace surfaces.

Using degree of soil development as a means to correlate wave cut platforms along the coastline and assess relative age, we assign the most likely marine-oxygen-isotope stage 5a, 5c, 5e and 7 ages to the four well preserved marine terrace surfaces. Using ages and the magnitudes of uplift relative to sea level at times of terrace platform formation, we document surface uplift rates that are variable in a shore parallel sense and across tectonic structures. Faults are identified based on surficial deformation and by the distribution of knickpoints on longitudinal stream profiles that trend across structure. Based on the strike and style of these faults, they may be faults that splay off the megathrust at depth.