Experimental investigation of the role of lower-crustal strength on upper-crustal deformation in obliquely convergent tectonic settings

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. The strength of the viscously deforming lower crust may significantly influence deformation patterns of the upper crust. For this reason, we tested two viscous materials as lower-crustal analogues with regard to the deformational behavior of the model upper crust, induced by a piston driven obliquely into the materials. For modelling a weak lower crust, Laponite, a low-viscosity gel, was used. By contrast, Polydimethylsiloxane (PDMS), a silica polymer, mixed with corundum grains was used for modelling a strong lower crust. Digital image correlation of the top surfaces of experiments allowed us to monitor the evolution of displacement vector fields, vertical-axis rotations and strain gradients. In all experiments, we observed the formation of high-amplitude ramp anticlines above the inclined piston, followed respectively by a well-defined zone of rhomb-shaped deformation domains and a low-strain zone of distributed deformation. In experiments using low-viscosity Laponite mixtures, ramp anticlines with unrealistically large amplitudes formed above the piston. Moreover, rhomb-shaped domains appeared more elongate than in experiments with PDMS, regardless of shortening directions. Apart from the viscosity of modal lower crust, we note that the distance between the piston and the back wall of the modelling box influences deformation patterns significantly, a most important result to be considered in physical experiments.