Fault and fracture systems in a fold and thrust belt:: An example from Bolivia

This paper presents an outcrop-analog study of fractures in low-porosity sandstones in the Subandean thrust belt. We analyze the evolution of fault and joint systems in these sandstones, quantify their density along the structural trend, and identify the main factors controlling their variability. We show that 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, one parallel and the other perpendicular to the bedding azimuth. Shearing along these joints transformed them into small faults and created new sets of fractures, oblique to the bedding attitude. Linkage of these small faults facilitated the formation of larger faults with significant strike-slip offset. Shearing along bedding planes created subvertical splay joints that induced the formation of conjugate normal faults In. this thrust belt, subordinate strike-slip and normal faults are concomitant products of compressive deformation. This study documents a hierarchical correspondence between spacing of structural heterogeneities and stratigraphic architecture. We measured spacings of joints and outcrop-scale faults along the backlimb of the Abra del Condor anticline. We subdivided the structural discontinuities into four main groups: joints, small faults, intermediate faults, and fault zones. Spacing of joints, small faults, and intermediate faults has a lognormal distribution, whereas spacing of fault zones shows a normal distribution. The mean of these distributions is about the same as the thickness of the confining stratigraphic intervals. Therefore, spacing and dimensions of joints and faults have a first-order relationship to the thickness of the confining stratigraphic sequences.