TIMING OF THICK-SKINNED EXHUMATION IN THE POTOSÍ UPLIFT, SIERRA MADRE ORIENTAL, NORTHEASTERN MEXICO

The Sierra Madre Oriental (SMO) fold-thrust belt of northeastern Mexico experienced thin-skinned deformation during Late Cretaceous to Eocene. Continued shortening resulted in thick-skinned deformation, numerous basement-cored uplifts, and exhumation of earlier thin-skinned shear zones along the orogenic trend. We focus on the timing of the transition from thin-skinned to thick-skinned deformation in the Potosí uplift of the SMO. El Alamar Fm red beds are the oldest rocks exposed in the Potosí uplift in the Galeana area of San Luis Potosí and directly underlie an exhumed thin-skinned evaporate shear zone at the base of the Jurassic Minas Viejas Fm. We present 34 new zircon (U-Th)/He ages (ZHe) from 6 samples to investigate the timing, magnitude, and spatial variation of thick-skinned exhumation in the Potosi uplift. Three El Alamar samples were collected along the N-S trend of the Potosí uplift. North to south, samples SMO1, SMO156, and SMO43 yielded mean ZHe ages of 43.8 ± 3.2 Ma (2 SE), 49.4 ± 9.1 Ma, and 59.7 ± 9.1 Ma, respectively. Although 2 of 3 El Alamar ZHe ages overlap at 2 SE, it is possible that the younging of ZHe ages to the south resulted from either the magnitude of thick-skinned exhumation in the Potosí uplift decreasing from north to south, or from exhumation initiating earlier in the south and propagating northward. Sample SMOMV from volcaniclastic sandstone in the Minas Viejas Fm yielded a mean ZHe age of 48.6 ± 3.9 Ma, whereas SMO29 from the La Primavera Member of the Minas Viejas yielded a mean ZHe age of 25.3 ± 11.6 Ma. An organic-rich shale bed in the Minas Viejas Fm yielded an indigenous %Ro of 1.92 ± 0.10 indicative of a maximum burial temperature of ~180-200 °C, which is consistent with complete resetting of ZHe ages in the underlying El Alamar Fm prior to thick-skinned exhumation. ZHe ages indicate thick-skinned exhumation in the Potosi uplift initiated by the early Eocene. This study provides new temporal constraints on the transition from thin-skinned to thick-skinned deformation in the SMO and differences between the structural evolution of the SMO and the Sevier-Laramide Orogenies. These data suggest that the Eocene transition from thin-skinned to thick-skinned deformation represents a continuum of deformation and not distinct phases.