Sustainable transpression: An examination of strain and kinematics in deforming zones with migrating boundaries

Most orogenic belts owe their development to oblique convergence and commonly have many orogen-parallel transpressional high-strain zones. To constrain the tectonic history of orogenic belts by structural and fabric analysis of rocks, it is desirable to understand quantitatively the relationship between the boundary conditions and the resulting strain distribution and kinematics in these zones. Most current models for transpression assume homogeneous deformation confined by boundaries that are fixed to material planes. This creates a strain compatibility problem at the margins of the active deforming zone and also requires that the strain rate normal to the zone boundaries increase to implausibly high values soon after the onset of oblique convergence (transpressional motion). The latter contradicts with the observation that transpressional motion can be sustained throughout an orogeny. The assumption that zone boundaries are fixed to material planes is unrealistic. The outstanding problems of current transpressional models are resolved in this paper by allowing the zone boundaries to migrate through the rock material. The consequence of zone boundary migration for the strain field and kinematics within a transpressional high-strain zone is investigated mathematically. The implications of the modeling for fabric interpretation are discussed. The modeling makes general predictions consistent with observed planar and linear fabric patterns in natural transpressional high-strain zones. It predicts that foliations in transpressional high-strain zones are subparallel to the zone boundaries regardless of variation in the imposed boundary conditions. Lineations cluster along the great circle girdle subparallel to the average foliation. The spread of the lineations may vary from point maxima to complete girdles.