STRAIN GRADIENTS IN ATTACHMENT ZONES DEVELOPED IN WRENCH, TRANSPRESSION, AND TRANSTENSION ZONES

Classical transpression fabrics are characterized by vertical foliation. However, in regions that have undergone oblique convergence, it is common to find a range of foliation orientation, including flat-lying fabrics. The cause of this phenomenon is explored by considering the attachment between the ductile and the brittle crust in wrench, transpression, and transtension zones. An attachment zone couples two subhorizontal rheological layers that undergo different displacement fields. In wrenching, attachment zones allow the transition from continuous deformation (simple shearing) at depth to discrete faulting of relatively rigid blocks of upper crust. Strain in these attachment zones is controlled by a component of wrench shearing and a component of horizontal shearing associated with the differential displacement of finite-width rigid blocks. Strain modeling of wrench attachments predicts high lateral and vertical strain gradients and foliation patterns showing gothic-arch anticlines and funnel-shaped synclines. Lineations are shallowly plunging and oriented close to the direction of wrenching. Shear sense reverses across the vertical axial surfaces of synclines and anticlines. In transpressional and transtensional attachments developed during low-angle oblique convergence or divergence, the pattern of foliation and lineation is similar to that produced in wrench attachments. Transpressional attachments display gradients in the shape of the finite strain ellipsoid, from flattening at the base, as expected, to strongly constrictional beneath the rigid blocks, owing to the increased effect of the horizontal shear component. Conversely, transtensional attachments show constriction at the base changing to flattening beneath the rigid blocks. This change in shape of the finite strain ellipsoid should be one of the most robust criteria to identify transpression and transtension attachments. In general, the coaxial flow that characterizes transpressional and transtensional systems decreases upward through attachment zones, due to the increased role of the horizontal simple shear in the finite vorticity. These strain and kinematic gradients are a robust result of attachment modeling and can be used as indicators of attachments developed in wrench, transpression, or transtension.