Brittle deformation in Miocene rocks of the Morelia-Cuitzeo region, Michoacán: implications on the local geothermal system

Abstract

Brittle deformation is accommodated by faults and fractures, which affect the upper crust of the Earth. The distribution of brittle deformation and the arrangement of faults and fractures are little known variables, although they influence the rock permeability and are important to understand the strain concentration. Our work explores the strain distribution and the fault arrangement on Miocene andesitic rocks in the Morelia-Cuitzeo area through a quantitative strain analysis at the mesoscopic scale. Two fracture families (F1 and F2) affect the rocks in the study area, and they are related to two brittle deformation events (D1 and D2) occurred between the Miocene and the Recent. The F1 fractures have an NNW-SSE strike and accommodate ~2.5 % of a ENE-WSW extension. The F2 fractures cut the F1 fractures, have an ENE-WSW strike, and accommodate ~7 % of a ~N-S extension. Both facture families have an arrangement of fracture clusters and average fracture intensity of 17 fractures/meter. The fracture intensity behavior is related to the rock type and not to the amount of strain. The finite strain (extension) of the F1 and F2 fractures has a heterogeneous distribution. This behavior contributes significantly to secondary porosity associated with F1 and F2 fractures. The strain distribution, and the quantity and arrangement of the fractures suggest that the Miocene rocks may be the permeable package of a geothermal system in the Cuitzeo Lake area.