INTEGRATING DATA FROM LOW AND HIGH STRAIN DOMAINS TO EVALUATE PARTITIONING AND CONSTRAIN 3-D KINEMATICS

Structural geologists commonly focus on shear zones as a record of the movement history of major terrane boundaries. The assumption that shear zones are bound by rigid walls is implicit in this approach. In a system with rigid walls, the shear-zone rocks provide the only record of kinematics. However, examples like the San Andreas fault system are more generally represented by both low and high strain domains, where high strain domains (fault or shear zones) are bound by or wrap around lower strain domains. In these heterogeneous systems, the shear zones walls are not rigid, but are lower strain domains that may record a different style of deformation (e.g., distributed faulting or folding) than the high strain zones. This type of strain partitioning occurs from the microstructural to the plate-boundary scale at all crustal levels. Recognition of partitioning is critical if we are to distinguish transpressional margins from other boundary types that produce strike-slip zones through processes such as extrusion.

Previous workers have shown that the 3-D kinematics of a partitioned system can be quantified if the strain magnitudes and deformation paths of both low and high strain domains are known. This type of analysis is not possible in most naturally deformed systems, where both exposure and the record of deformation history are limited. A more general, semi-quantitative approach requires an evaluation of structural symmetry. As most geological materials are lithologically heterogeneous, they display competence contrast. Deformation of a heterogeneous system will therefore produce spatial variations in stress and strain, and adjacent competence domains will record different components of the bulk flow field. We can use these variations to determine bulk kinematics by integrating information regarding: 1) the symmetry and deformation history of structures within adjacent low (competent) and high strain (incompetent) domains, and 2) the overall symmetry produced by the 3-D shapes and distribution of these domains. The geometry and internal structure of low strain domains are therefore critical in determining bulk kinematics, and potentially allow us to distinguish between different plate tectonic settings, such as transpression versus extrusion.