Abstract

FITZ-DIAZ, Elisa et al. Deformation, veins, fluid inclusions and the tectonic evolution of the Valle de Bravo Cretaceous rocks, Estado de Mexico, Mexico. Rev. mex. cienc. geol [online]. 2008, vol.25, n.1, pp.59-81. ISSN 2007-2902.

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, D₂yD₃), 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 andMichoacan 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 D₁and 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/re-precipitation) 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 ofmicrostructures 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 °Cfor V₁, between 167 and 202 °Cfor V₂ and 220 °Cfor V₃. Corresponding salinities obtained were: between 6.1 y 7.4 (V₁), 5.2 (V₁), and between 2.6 and 4.6 (V₃) (weight % ofNaCl 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.

Keywords : folds; deformation; veins; microthermometry; exhumation; Cretaceous; Valle de Bravo; Mexico.

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