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Description
Mylonites are ubiquitous features of the Earth's lithosphere and are found in all varieties of fault-related environments and hence a thorough understanding of the processes associated with their development is important. Mylonitization is a process by which rock materials change shape and form as they respond to imposed stress fields without losing cohesion. Such a process is commonly associated with relatively high temperatures, high pressures, low strain rates, and the presence of a fluid phase that acts as a hydrolytic weakening agent. Two different methods are commonly used to quantify rock mass and volume change during mylonitization. The first employs a deterministic approach and commonly involves "eye-balling" best fit lines to bivariate plots of elemental composition in mylonitic versus non-mylonitic samples. Uncertainties in mass and volume change cannot be assessed easily using this method. In contrast, the second method is probabilistic and thus allows the assignment of uncertainties in resulting data. In order to test the validity of a probabilistic approach for assessing mass and volume change in mylonitic shear zones, I have evaluated 5 well-known shear zones using the probabilistic approach. Geochemical data for these shear zones were extracted from the literature and were evaluated using the overlapping cones method of L. Baumgartner and S. Olsen. Using a probabilistic approach, as much as 38% ± 4%, 43% ± 11 %, 79% ± 7%, and 22% ± 6% of the rock volume was lost during the formation of the Brevard shear zone, the Waterman Hills detachment, the Rector Branch thrust, and the Broken Hill shear zone, respectively. In contrast, the results for the Lewisian complex indicate that no change in rock volume was associated with mylonitization. The results of my probabilistic analysis of the five shear zones were consistent with previous interpretations and indicate that the probabilistic approach is robust and appropriate for use in evaluating changes in rock mass and volume during mylonitization. The Cretaceous Scove Canyon shear zone is located in the Peninsular Ranges of southern California. Within the study area, protomylonites are characterized by a northwest-striking, northeast-dipping S-C mylonitic fabric that in present-day field coordinates indicate a down-to-the-east normal sense of shear. Microstructural analysis suggests that formation of the Scove Canyon protomylonite occurred at high temperatures and low strain rates. As documented in the first part of my thesis, mylonitization under such conditions is often accompanied by rock mass and volume gains and losses. However, the results of my analysis of the Scove Canyon shear zone using a probabilistic approach, indicate that in some cases mylonitization can occur without a loss of rock mass or volume. Several authors have attributed changes in rock mass and volume to focused or multi-pass fluid flow during the mylonitization process. In the case of the Scove Canyon example, and by analogy the shear zones of the Lewisian complex, mylonitization appears to have occurred within a relatively closed chemical system. This result may imply that the Scove Canyon protomylonites formed in a relatively dry, fluid-absent environment. Thus, results of work presented here, consistently indicate that mylonitization varies in complexity, and that generalizations about such processes are difficult, if not impossible to make. Nevertheless, it seems clear that in the presence of a fluid phase, rnylontization can result in a major redistribution of elemental and rock mass within the interior of the Earth, but that the presence of such a phase is not required to produce a mylonite.
