Experimental investigation of first-order kinematics and fault patterns in upper crust affected by sequential phases of variable shortening directions

Obliquely convergent plate margins and continental collision zones, such as the Andes and Tibet, respectively, are often characterized by crustal-scale, rhomb-shaped basins enveloped by transpression zones. The geometries and sizes of such basins, typically evident as low-strain upper-crustal domains, and their transpressive margins are highly variable. Using two-layer analogue experiments, crudely scaled to length and strength of viscous lower and brittle upper continental crust, and horizontally shortened by an inclined piston, we explored the deformation patterns of model upper crust. Specifically, we were interested in the evolution of displacement vector fields, vertical-axis rotations and strain gradients, collectively monitored by digital image correlation, resulting from consecutive phases of shortening with variable directions. In all experiments, we observed the formation of high-amplitude ramp anticlines at the piston, followed respectively by a distinct zone of rhomb-shaped deformation domains and a low-strain zone of rather distributed deformation. As somewhat expected, orthogonal shortening tended to generate symmetric deformation domains enveloped by conjugate model transpression zones, increasing in spacing with distance to the piston. By contrast, highly oblique convergence of the piston generated transpression zones, the geometry and kinematics of which adhered to sets of Riedel shears. Most importantly, the geometry of deformation domains and deformation kinematics change according to the sequence of imposed shortening directions. An added value of the experiments is that one of the lateral margins is unconstrained, allowing shortened material to escape laterally, thereby mimicking indenter tectonics.