Geomechanical Controls on Fracture Density in Andean Fold and Thrust Belts
In Andean Fold and Thrust Belts, progressive hierarchical shearing and mechanical stratigraphy play a significant role in the distribution of fracture density. This conclusion is based on outcrop studies of Anticlines in Suesca (Colombia) and Abra-del-Condor (Bolivia), and complemented with observations of fracture systems using satellite images from Venezuela, Bolivia, and Chile.
At Suesca, fracture patterns and density are described from selected pavement outcrops. High-density fracture corridors occur at different positions within the fold. At each location, significant contrast in fracture spacing is observed, varying from 0.3 to 1 m in high-density areas, and from to 3 to 20 m in low-density areas. Tilted fractures in the forelimb indicate that fracturing started at an early stage of folding. Variations in fracture orientation indicate rotations of the stress field across the structure. Local variations in fracture density are the result of increasing shear deformation along dominant fracture sets.
At Abra-del-Condor, faults and joints occur at different scales in a hierarchical fashion as a consequence of progressive shearing. The first generation is an orthogonal set of joints, symmetric to the fold axis. Fractures and subsidiary faults oblique to the fold axis are interpreted to be the result of shearing along the pre-existing orthogonal joint sets. Linkage of these small faults facilitated the formation of larger faults with significant strike-slip offset. These outcrops also show a hierarchical correspondence between fracture spacing and stratigraphic architecture, with a first-order relationship between the mean spacing of joints, small faults and intermediate faults and the thickness of their confining stratigraphic sequences.
Fracture patterns with hierarchical abutting relationships indicating progressive shearing also occur in flat lying strata in Tepuis North of Duida-Marahuaca National Park (Venezuela) and in massive granitic rocks at Rennell Islands (Patagonia, Chile). Similarly, satellite images of pavement surfaces along the Chaunaca Anticline (Bolivia) present clear evidences of progressive shearing in a hierarchical fashion. Similar to Suesca and Abra-del-Condor outcrops, these images show that progressive shearing is a dominant process controlling the distribution of fault and fractures in the Andean Cordillera.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013