Shear Induced Mineralogical, Textural and AMS Variations in black shale: a study case in the Monterrey Salient, Northern Mexico.

María Isabel Vidal Reyes, Posgrado en Ciencias de la Tierra, Instituto de Geología, Universidad Nacional Autónoma de México
Elisa Fitz Diaz, Instituto de Geología, Universidad Nacional Autónoma de México
Cecilia Irene Caballero Miranda, Instituto de Geofísica, Universidad Nacional Autónoma de México
Bernardo García Amador, Instituto de Geofísica, Universidad Nacional Autónoma de México

The Monterrey salient is located in the northeastern portion of the Mexican Fold and Thrust Belt (MFTB), in northern Mexico. This element shows a kilometer-scale train of upright buckle folds across an N-S-oriented section. These folds involve a succession of carbonates detached over Middle Jurassic evaporites. The Nuncios fold-system is found at the northernmost front of the salient consists of two anticlines (Los Muertos and San Blas) separated by the Sierra Urbano syncline (SSU). Along the limbs of SSU, bed-parallel shear related to flexural folding is observed. The shear is localized on an Early Cretaceous black-shale horizon and shows opposite kinematics on both fold limbs, consistently with flexural slip. A strong anastomosing fabric, with abundant vertical calcite fibers, makes a clear difference between the shear zone and the host rock.

In this study we documented and quantified mineralogical and textural variations resulting from localized shear in black shale, in order to better understand the feedback relations between deformation, fluid-rock interactions and fluid flow during folding. We systematically collected 62 samples across a 20 meters long section centered on the shear zone on the southern limb of the SSU. Different observational and analytical techniques, including: Petrography, SEM textural and mineralogical characterization, XRD analyses, Illite Crystallinity Index determination, Anisotropy of Magnetic Susceptibilty (AMS), and organic and inorganic total carbon (TIC- TOC) analyses, were carried on samples from the shear zone and less deformed black shale. In sheared rocks pressure-solution and solution-transfer were dominant deformation processes, as evidenced by the abundant calcite-quartz fibers and insoluble-minerals domains. An increase abundance and grain size of illite, quartz and calcite, as well as in oxides and sulfides; and a decrease in magnetic fabric and organic matter grain size is observed in the shear zone compared to the host rocks. Such textural and mineralogical anomalies within the shear zone are due to a strong fluid-rock interaction, which was promoted by localized fluid flow. This suggests that the shear zone is a conduit for aqueous fluids parallel to bedding, but it is at the same time a barrier for fluids flow oblique or transversal to bed-parallel shear zones. The observed flow pattern shows a mechanism for fluid compartmentalization during folding.


Shear Zones