DYNAMICS OF THE POST-SUBDUCTION THERMAL ANOMALY OF THE NACIMIENTO BLOCK REVEALED BY RSCM AND (U-TH/HE) THERMOCHRONOLOGY

One of the most significant stage in the geodynamic 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 Northern and Central coastal Range of California. In Central California, the Nacimiento block, which consists to Franciscan accretionary complex, is affected by a post-subduction thermal anomaly, which is hypothesized to be associated with hydrothermal activity and formation of contemporaneous Gold deposit.

In this contribution we combine regional structural analysis to Raman Spectroscopy on Carbonaceous Material (RSCM) thermometer on 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 highlights the presence of a transpressional uplift developed between the SGHF and the Nacimiento Fault during dextral movement, that would explain the exposure more deeply buried and consequently warmer rocks along this apparent thermal anomaly. In order to document the uplift dynamic of this area, we also present new (U-Th)/He dataset on both apatite and zircon from samples collected across the thermal anomaly.