RAMAN THERMOMETRY REVEALS UNSUSPECTED TRANSPRESSIONAL VERTICAL UPLIFT IN THE NACIMIENTO BLOCK OF CENTRAL CALIFORNIA

Transpressional deformation is a common feature of continental transform plate boundaries that is potentially both a response to oblique convergence and a reflection of the localization of strain along contrasts in lithospheric strength. Because regions of localized transpression infrequently generate syn-deformational sedimentary records, low-temperature thermochronometry is a widely used tool to reconstruct the initiation age, rate, and magnitude of transpression-driven vertical exhumation. Such studies are frequently undertaken along active restraining bends, where the development of significant topographic relief serves as a proxy for the identification of transpressive uplift. However, in cases where the topographic expression of transpression is minimal, either due to low uplift rates or rapid erosional removal of transpressional topography, or the cessation of active transpressional uplift, identification and quantification of such deformation may be challenging.

We present a novel approach for mapping the spatial extent of vertical uplift in a subduction-related tectonic prism or tectonic mélange based on quantitative mapping of the distribution of peak temperature by Raman spectroscopy of carbonaceous materials coupled with (U-Th)/He thermochronometry on apatite and zircon. We apply this approach to the Nacimiento block of central California, which is composed of carbonaceous metasediments of the Franciscan subduction complex that experienced HP-LT metamorphism prior to entrainment within the present-day transform plate boundary. We reveal the extent and magnitude of previously unrecognized exhumation gradients, which, when combined with regional structural observations, can be used to quantify vertical crustal motion associated with regional transpression. We propose that the Nacimiento block was affected by a post-metamorphic thermal anomaly since ca. 25 Ma that corresponds with a localized transpressive event having an uplift rate of about 0.3 mm/yr.