Strain and fabric analyses based on porphyroclast interaction

Two-dimensional computer models of the interaction of elliptical clasts in a variety of kinematic conditions are used to study how clast imbrication and shape fabric development relate to strain and strain history. The models provide templates, for a known clast density and clast ellipticity, that correlate clast imbrication, angle of collision and clast shape fabric to strain and kinematics of deformed rocks. Modeling also shows that the mode of clast interaction plays a significant role in the accumulation of imbricated clasts and the development of shape fabric. Upon collision between clasts, the trains formed can remain fixed or rotate. If they rotate, imbricated clasts can become independent when they are no longer forced together by the flow. Based upon the mode of clast and train rotation (using the March rotation of passive lines and the Jeffery rotation of rigid particles as end-members), three models are defined in an attempt to simulate the conditions of clast interaction under varying conditions. The March-fixed train, March-rotating train and Jeffery-rotating train models simulate solid-state, intermediate and magmatic conditions, respectively. Modeling shows that the two March models produce similar results, but the Jeffery model of train rotation always produces a lower proportion of imbricated clasts and a much weaker shape fabric. Application of the modeling to the study of a syntectonic granite in the Sierra Nevada suggests that deformation was dominated by simple shear. Some K-feldspar imbrication and fabric development took place initially in the magmatic stage, and culminated in the solid state to produce a large proportion of imbricated K-feldspar porphyroclasts which define a strong shape fabric. This type of analysis provides a rationale to determine syn-vs post-emplacement deformation in the granite.