Cataclastic Flow: A Means for Ensuring Ductility Within the Elastico-Frictional Regime

Cataclastic flow accommodates ductile deformation by satisfying the von Mises criterion within the upper crust (i.e., elastico-frictional regime). Recent work, from portions of the Sevier fold-thrust belt that have deformed primarily within the elastico-frictional regime, reveals that cataclastic flow can be sub-divided into two types: matrix- and block controlled. Both types may operate simultaneously within the same deforming material, but their activity can vary spatially, temporally and across scales. Established concepts and definitions for cataclastic flow, however, are chiefly based on matrix-controlled cataclastic flow. In matrix-controlled cataclastic flow, clasts are surrounded by a fine-grained matrix and are not constrained by adjacent clasts, and are therefore able to undergo large displacements and rotate independently. Because of this, matrix-controlled cataclastic flow can result in a significant amount of deformation and even give rise to foliations. In block-controlled cataclastic flow, however, a matrix does not support the blocks/clasts. Because the fracture-bound blocks are constrained by adjacent blocks, if one block moves, the surrounding blocks also have to move. Although blocks cannot independently rotate or move large distances in block-controlled cataclastic flow, small amounts of slip on a large number of fractures can result in significant amounts of deformation. Because the clasts do not independently rotate, the fracture network pattern is not disrupted. As a result, the fracture sets used to accommodate cataclastic flow are well preserved. Because of this, the development of fracture networks in rocks accommodated by block-controlled cataclastic flow is examined in order to better understand how cataclastic flow, in general, operates. Cataclastic flow is re-examined in terms of first principles in order to develop a more inclusive definition. To clarify the kinematics of the process, a mechanical analog model for cataclastic flow is presented. In addition, stress Mohr diagrams for ductile materials, such as those deforming by cataclastic flow, are re-evaluated. These models and revised definitions are tested against natural data from cataclasized rocks in terms of: (1) energy budget calculations, (2) fracture development forward modeling and (3) three-dimensional block (clast) shape evolution.