Methods and criteria to evaluate brittle failure kinematics from structural data

Abstract

For the study of brittle faults, the determination of the direction, direction of movement and net displacement between the deformed rock blocks are essential; as well as lower the state of the principal stresses during deformation. In the present study, a review of the structures associated with brittle faulting is made, examining the criteria to select the appropriate method to analyze and determine the state of principal stresses at a regional scale. If there is a reference point or marker between the failed blocks, it is possible to measure their net displacement without any error. However, in metamorphic and volcanic rocks where the flat distribution is scarcer, it is necessary to resort to alternative methods to quantify the kinematics of the faults. According to modern Structural Geology, there are inverse (dynamic) methods that use numerical and graphical techniques to determine the principal stresses. On the other hand, there is the kinematic analysis which involves rotation, sample and spatial homogeneity. In the latter, a two-dimensional graph of finite deformation approximation is generated. To determine the direction and sense of displacement in any type of rock, the most useful and definitive way is the direct observation of the fault planes. Finally, the following is illustrated: a) the minor structures observable in the fault planes that define the direction of movement between the blocks, and the taking of structural data in the field; b) the selection criteria between dynamic and kinematic methods and c) the prevailing stress states during brittle deformation. For the above, examples of brittle faults from the southeast of the Sierra de Pachuca are used.