The Effect of Microstructural and Rheological Heterogeneity on Porphyroblast Kinematics and Bulk Strength in Porphyroblastic Schists

Porphyroblast are relatively large, strong minerals that grow in rocks when they are subjected to heat and pressure (i.e. metamorphism). Due to their chemical makeup, clay rich rocks commonly develop porphyroblasts. And, while porphyroblasts are strong, clay minerals or micas are generally thought to be relatively weak. Therefore, porphyroblastic rocks are commonly composed of a mixture of strong and weak minerals. This thesis investigates the role of such mixtures of minerals, how they relates to the bulk strength of these rocks, and their effect on the rotational behavior of porphyroblasts during mountain building events. My research has shown that when subjected to pressure porphyroblasts interact mechanically with the surrounding minerals and as a result rotate. Prior theoretical descriptions have suggested that this rotation should be predictable and relate to the large scale pressures. My work suggests that the rotation of staurolite porphyroblasts from the Appleton Ridge Formation, south-east Maine, is complex in 3-D and does not match ideal theoretical descriptions. The observed complexity is largely attributed to variation in initial mineral shape and orientation as well as the proximity of weak mica layers. It is difficult, from these observations, to directly quantify the relative influence of different distributions of weak and strong minerals on bulk strength and the rotational behavior of porphyroblasts. I, therefore, developed numerical models wherein I varied the relative distribution and strength of weak ‘mica’ domains relative to a large, strong central inclusion (i.e., a porphyroblast). My models indicate that the strength of synthetic rocks is dependent on the relative proximity of the weak and strong domains. Bulk weakening of the consistently occurs at discrete thresholds regardless of the spatial proximity of weak and strong domains. My models also indicate that rotation of the strong domain is dependent on the proximity of weak and strong domains. This rotation dependence was, however, more sensitive to the proximity weak and strong domains then the bulk strength and occurred at different combinations of parameters than were required for bulk weakening. This indicates that, although both porphyroblast rotation and bulk strength are dependent on the distribution of weak and strong minerals, the relative effects on each are not equal. This information is critically important for the way in which geologists read the rock record, interpret measurements, and can be used to help resolve the progression of mountain building events.