Deformation, veins, fluid inclusions and the tectonic evolution of the Valle de Bravo Cretaceous rocks, Estado de México, Mexico

Abstract

In the Valle de Bravo area, central Mexico, two assemblages of cretaceous rocks are exposed. Both units present very low grade metamorphism: (1) a meta-sedimentary rock assemblage (EMS), and (2) a meta-volcanic rock assemblage (EMV). These rocks are part of the Ixtapan de la Sal–Teloloapan volcano-sedimentary metamorphosed Mesozoic sequence. The history of deformation of the two assemblages is represented by three sub-horizontal shortening events (D1, D2 y D3), that display similar NE-SW orientation, but alternating tectonic transport direction. D1 has a general vergence toward 043°, whereas D2 toward 218°, and D3 toward 045°. The three events have been recognized in different localities in Guerrero and Michoacan states, and they represent important shortening episodes at regional scale.

Event D1 exhibits ductile deformation at grain scale, while D2 is brittle-ductile, and D3 presents strictly brittle characteristics. We propose that D1 is the peak metamorphic/deformational event and that D2 and D3 occurred during exhumation of the rocks. During D1 and D2, two generations of veins (V1 and V2, respectively) were developed. Abundance of the veins has been related directly to the penetrativity of deformation; which suggests that the mobilization of soluble material (mainly by pressure-solution/reprecipitation) was the effective mechanism of deformation at the microscopic scale. This soluble material was deposited in veins in later stages of the deformation event and was more intense during D1. This is demonstrated by the analysis of microstructures in thin section, by outcrop observations and by the comparative-quantitative determination of density of veins through image analysis.

In addition to the earlier V1 and V2 veins, late veins (V3) were also identified, and are associated dominantly to normal faults that cut D3 structures. Petrography and microthermometric analyses on fluid inclusions were done on quartz and calcite crystals of the three generations of veins in a 15 km2 area in the EMS. The average homogenization temperatures were: 250 °C for V1, between 167 and 202 °C for V2 and 220 °C for V3. Corresponding salinities obtained were: between 6.1 y 7.4 (V1), 5.2 (V2), and between 2.6 and 4.6 (V3) (weight % of NaCl equivalent). The data obtained in V1 and V2 are in agreement with a progressive exhumation of the rocks in the studied area, while the temperatures measured in V3 indicate that hotter fluids circulated along normal faults and associated vertical fractures after D3.