Dr Brice LaCroix - The Enigma of the Nacimiento Block of Central California: Where Is the Slabless Window?

The Nacimiento block, an exhumed assemblage of an accretionary complex formed by the subduction of the Farallon oceanic plate under the western margin of North America, is enigmatic as it is affected by post-metamorphic thermal anomaly, hydrothermal activity leading to the formation of the Los Burros gold deposit, and high landslide activity.

In order to document the important vertical uplift of the area, we present a novel approach for mapping vertical uplifts in exhumed metasedimentary rocks by coupling Raman spectroscopy of carbonaceous material with (U-Th)/He thermochronometry on apatite and zircon. We apply this approach to carbonaceous metasedimentary rocks of the Franciscan subduction complex, exposed in the Nacimiento block of central California, USA, an area that records high-pressure–low-temperature metamorphism prior to entrainment within the present-day transform plate boundary We demonstrate that this approach, combined with regional structural observations, can be used to map previously unrecognized transpressive ridges and quantify the vertical crustal movement. We propose that the Nacimiento block is affected by a localized post-metamorphic thermal anomaly that corresponds to a localized transpressive deformation since ca. 25 Ma at an uplift rate of about 0.3 mm/yrs.