DEFORMATION HISTORY OF SIERRA DE CATORCE, SAN LUIS POTOSí, MéXICO

The Sierra de Catorce (SC, Range of Catorce) includes some of the westernmost exposures of the Mexican Fold-Thrust Belt (MFTB) and at the same time is one of the easternmost Basin and Range features (horst) in central Mexico. This physiographic feature has a maximum elevation of 3 km above the sea level and resulted from Late Cretaceous-Paleogene contraction and subsequent Neogene extension affecting this region. Rocks exposed across the SC include Triassic clastic units, unconformably covered by a succession of Jurassic volcaniclastic rocks, in turn covered by fluvial deposits, which are overlaid by Late Jurassic-Cretaceous carbonates, and intruded by Eocene quartz monzonitic dikes. Along an E-W cross-section, the SC shows an anticlinorium like structure cut by normal faults dipping to the valleys on both sides.

The Triassic units show ductile E-W shortening as suggested by a pervasive, continuous, planar, N-S trending cleavage developed at very low-grade metamorphic conditions. Such first generation of cleavage is associated with folds with vertical axes. The Jurassic units also show a pervasive, sub vertical pencil cleavage associated with spaced open folds, which is also trending N-S, and were formed in the anchizone. The Jurassic units’ cleavage is disrupted by a regional detachment localized at the base of the carbonate succession (Zuloaga Fm.), which separates cleavage dominated shortening in the Jurassic clastic units from fold-dominated shortening in the upper Cretaceous carbonates. Above the detachment zone, folds have axial planes oriented N20E 60 SW with a dominant vergence towards the East.

All shortening directions are nearly co-axial. The Triassic/Jurassic unconformity indicates there are at least two phases of deformation in the rocks of SC, and one of them is probably Triassic-Early Jurassic in age. However, ages of younger structures are not well established. We applied Ar-Ar illite dating to folds and to the detachment zone in order to determine the age of shortening affecting the Cretaceous carbonates. We also use apatite and zircon U-Th/He thermochronology to test if the last important exhumation of SC occurred during the formation of the MFTB (Late Cretaceous-Paleogene) or during Basin and Range extension in the region (Oligocene).