L tectonites

Rocks with a pure linear fabric, or L tectonites, often indicate nearly perfect constrictional deformation. This paper assimilates published data, models, and interpretations to understand the forcing mechanisms that can form L tectonites. Most noncoaxial kinematic geometries that can result in constrictional deformation involve vorticity-parallel shortening. Local variations in external boundary conditions that localize components of constriction include releasing and restraining bends in shear zones, linear channels in shear zone boundaries, intersections between shear zones, and foliation triple points between ballooning diapirs. Internally, L tectonites are often localized in fold hinge zones, and rheologic variations partition constriction into discrete domains. st common external kinematic framework that can form L tectonites involves simultaneous transport-perpendicular shortening in two directions. Hence, large domains of L>S and L tectonites are a common feature of orogen-parallel elongation. In every case, external variations in boundary conditions and/or internal variations in structural setting and rheology localize constriction to form L tectonites. External boundary conditions are important in density-driven vertical tectonics. Elsewhere, internal variations in structural setting and rheology are more important. The most common are the formation of L tectonites in fold hinge zones and in compositionally homogeneous rocks while heterogeneous rocks accommodate constriction by folding.