RHEOLOGICAL HETEROGENEITY, STRAIN PARTITIONING, SHEAR STRESS COUPLING, AND KINEMATICS IN PORPHYROBLASTIC SCHISTS

Strain partitions in polymineralic rocks owing to inherent contrasts in competency and mechanical anisotropy among the constituent minerals. As a result, different portions of the rock will accommodate different proportions of the bulk flow field (e.g. vorticity partitioning). In addition, shear stress coupling at the boundaries of relatively large rigid grains, such as porphyroblasts, will partly determine their kinematic response to the bulk flow. This is important because the orientation of porphyroblasts and/or their inclusion trails are commonly used to: (a) estimate shear strain rates and flow vorticity, or in contrast (b) reconstruct the geometrical evolution of deformed regions. The first approach generally assumes perfect stress coupling and homogeneous flow; whereas the other assumes effectively no stress coupling and flow partitioning. We doubt the veracity of either approach. To address this, we evaluated porphyroblast kinematics from naturally deformed porphyroblastic schists through optical, scanning electron microscopy, and numerical experiments.

Acadian aged deformation in south-central Maine resulted in multiple generations of temporally distinct fabrics. The latest deformation event was marked by dextral non-coaxial flow associated with the early development of the Norumbega fault system. Staurolite porphyroblast microstructures suggest that fragments of boudinaged staurolite have rotated relative to one another, and electron backscatter diffraction provides an exact measure of the relative rotation. Kinematic reconstruction of fragmented staurolite indicates complex rotational behavior, with magnitudes of relative rotation up to ~30-40°. These observations require shear stress coupling at the porphyroblast-matrix boundary; however, the degree of coupling is difficult to directly quantify. Finite element numerical experiments were employed to assess clast-matrix coupling, the dependence of coupling on the distribution of relatively incompetent material, and the effects of these parameters on bulk viscous strength. Initial results suggest that the bulk viscosity of a clast-matrix mixture is strongly dependent on the relative spatial proximity of competent and incompetent phases.