QUANTIFYING COMBINED DISTRIBUTED AND DISCRETE DEFORMATION

Deformation within the earth's crust occurs as both wide areas of distributed strain (distributed deformation) and areas of localized deformation (discrete deformation). Although the description depends on the scale of observation, faults are generally considered to be discrete deformation. However, adjacent to major faults there is also distributed deformation, often in the form of folding. The faults of the San Andreas fault system and the en echelon folding adjacent to these faults are evidence of the simultaneous activity of both discrete and distributed deformation.

One can evaluate these styles of deformation together using a displacement-based approach, following the analysis of Wojtal (1989). This method was designed to characterize the bulk strain accommodated by faulting. Using measurements of geologically available displacements, one can plot directional displacement vs. an arbitrary spatial coordinate axis. From these data, one can calculate the inverse finite strain ellipse and consequently the magnitude and orientation of the finite strain axes. Distributed deformation - either homogeneous or heterogeneous - can also be characterized by the same methodology. If a strain is measured directly, it is first returned to displacement values on separate directional displacement vs. spatial coordinate axes plots. The distributed and discrete components are both combined on these plots, and finite strain is calculated.

This method is calibrated using published experimental models of transpressional deformations. Because the boundary conditions of these experiments are known, we are able to characterize the errors and uncertainties of this approach.