NEW CONSTRAINT OF POST-SUBDUCTION THERMAL ANOMALY: INSIGHTS FROM THE NACIMIENTO BLOCK, CENTRAL CALIFORNIA

One of the most significant stages in the geodynamics of the Coastal Range of California is the passage from subduction to transform motion between the Pacific and North American plates. The subduction of shallow asthenosphere is thought to trigger unusual heat flow, anomalous volcanism, and hydrothermal activities. Past post-subduction thermal overprints have been described in the Franciscan group from the Northern and Central California Coastal Ranges. In Central California, the Nacimiento block, which consists of the Franciscan accretionary complex, is affected by a post-subduction thermal anomaly, which is hypothesized to be associated with hydrothermal activity and formation of a concurrent gold deposit: the Los Burros deposit. Although both the thermal anomaly and the Los Burros deposit seem spatially correlated, the regional peak temperature distribution is still poorly constrained. In this contribution we combine regional structural analysis and Raman Spectroscopy on Carbonaceous Material (RSCM) thermometer on 50 samples from the Nacimiento block and present a new peak-temperature distribution map. Our results show an important thermal anomaly (up to 340˚C) in the vicinity of Cape San Martin/Los Burros. Within the thermal anomaly the regional structures are oriented E-W, which significantly contrasts with the regional fabrics generally oriented N315˚-N350˚. This drastic orientation change suggests a counterclockwise rotation of about 50˚- 75˚ compared to the regional fabrics. We infer that the observed rotation of the regional fabrics along the regional thermal anomaly highlights the presence of a transpressional uplift developed between the San Gregorio Hozgri Fault (SGHF) and the Nacimiento Fault during dextral movement. This would explain the exposure of previously deeply buried and consequently warmer rocks along this apparent thermal anomaly.