Illite growth along a detachment zone and folds of the Sierra de Catorce, San Luis Potosí, Mexico.
Rodrigo Gutiérrez-Navarro, Postgraduate in Earth Sciences, UNAM
Elisa Fitz-Diaz, Instituto de Geologia, UNAM
The Sierra de Catorce (SC) range is located in the Mesa Central in northeastern Mexico, includes the westernmost exposures of the Mexican Fold and Thrust Belt (MFTB) and easternmost structures of the Basin and Range province in Mexico. On an E-W oriented cross-section, through the N-S trending SC, Mesozoic strata were shaped in an anticlinorium-like structure. We present data from structural analysis, optical microscopy, Scanning Electron Microscopy (SEM), characterization of clay minerals with X-ray diffraction (XRD), and illite Ar-Ar dating. These data allow us a better understanding of illite growth during folding and thrusting, and a more accurate interpretation of the age of shortening affecting Cretaceous units on the western most exposures of the MFTB.
The Mesozoic strata in SC include: 1) a Triassic succession of siliciclastic sandstone and shale showing ductile E-W shortening by folding, with vertical axes and a pervasive, continuous, planar, N-S trending cleavage developed at very low-grade metamorphic conditions. 2) A Jurassic succession of volcaniclastic succession interpreted as "Nazas Arc" in central Mexico, which also shows a pervasive, sub-vertical pencil N-S trending cleavage associated with open folds, which according to illite crystallinity data was formed in the anchizone. This cleavage is also present in red siltstone and continental conglomerates deposited on top of the Nazas arc rocks. 3) A Late Jurassic shale carbonate succession on top of these units developed a strong bed parallel foliation and lineation that in thin section shows a mylonite texture with S-C microstructures that suggest a direction of transport to the east. This deformation localization zone separates cleavage dominated shortening in the Jurassic clastic units below, from fold-dominated shortening in the upper Cretaceous carbonates above it. 4) Layers of Early Cretaceous basinal carbonates above the detachment zone were shortened by folding, and the folds are in general asymmetrical, verging to the NE. 5) Finally, Late Cretaceous turbidites also show fold with a similar geometry as those in the Cretaceous carbonates, but with a more intense associate cleavage.
After the mesoscopic analysis, three samples were selected to apply 40Ar-39Ar illite dating; two samples were collected in the limbs of two mesoscopic folds, one from Early Cretaceous limestone and one from Late Cretaceous turbidities (Samples 2 and 3), and the third from a clay rich, highly deformed zone in the detachment zone localized in the Jurassic limestone (Sample 1). For illite age interpretation, we used illite XRD characterization, illite polytype quantification, SEM textural observations and analysis and stepwise incremental heating of three fractions of each sample (<2 µm, 0.2-1µm and <0.05µm) in light of recent theory on Ar-Ar degasification of fine grain particles. Based on these analyses we determined that the populations of illite in the samples were formed in the following ranges: Sample 1: 69-74 ± 1 Ma, Sample 2: 54-92 ± 1 Ma and sample 3: 54-75 ± 1 Ma. Overall, these data indicate that the deformation observed in SC was accumulated over ~30 Ma, with folding starting about 92 Ma, intense folding and faulting between 74 and 65 Ma, and refolding at about 55 Ma.