RAPID EXHUMATION OF THE RAND SCHIST: CORROBORATIVE EVIDENCE FROM GARNET-BASED GEOSPEEDOMETRY, THERMOCHRONOMETRY, AND FIELD STUDIES

Intensive study of the Rand schist (hereafter referred to as the “schist”) of southern California demonstrate convincingly that it formed in the Late Cretaceous by subduction of Cordilleran eogeoclinal detritus along a shallow dipping segment of the Farallon plate. Comparatively less is understood regarding the exhumation history of the schist. Recent thermochronologic and thermobarometric analyses suggest that, in some localities, the schist was deposited, underplated beneath western Sierra Nevada batholithic assemblages (the “upper plate”), and exhumed with the upper plate from 30 – 40 km depths in less than 3 million years. Such a rapid cycling interval implies exhumation rates in excess of 5 mm/yr.

We exploit diffusional annealing of garnet zonation and place our result in the context of existing data on the exhumation rate of the schist. Some garnets from the schist of the San Emigdio Mountains were broken during late stages of their growth. Overgrowth along broken margins resulted in natural diffusion couples ~ 25 μm inward from the rims, where peak metamorphic temperatures of 610 °C were calculated. A maximum time interval, representing exhumation prior to diffusion cessation, of 0.5 Ma was estimated from the degree of annealing of the originally sharp compositional step. Similar time intervals were calculated from narrow (10 μm) retrograde zones at the rims of compositionally homogenized garnets belonging to the upper plate. Garnets from the schist conspicuously lack retrograde zonation patterns, likely due to exhumation rates exceeding those of retrograde diffusion.

Both garnet-based geospeedometry and thermochronometric studies indicate that the schist was exhumed at rates comparable to lateral tectonic plate motions (> 1 cm/yr). This observation reveals a profound Late Cretaceous extensional event. We hypothesize that rollback of the Farallon plate from shallow to steeper trajectories provided the tectonic driving mechanism for such Late Cretaceous extension, and facilitated schist and upper plate exhumation. These results yield important insight into the timing of Late Cretaceous ascent of the schist and the upper plate, in addition to the mechanisms necessary for such a rapid process.