G. Schreurs¹, R. Hänni¹ & P. Vock²

¹Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland
schreurs@geo.unibe.ch
²Institute of Diagnostic Radiology, Inselspital, CH-3010 Bern

Abstract

We performed a series of analogue experiments to investigate the development and evolution of transfer zones in fold and thrust belts. Experiments were done within the investigation field of a spiral computerized tomography (CT) scanner. This new-generation scanner revolves around the model and allows a 4-D analysis of the deforming model by generating time-lapse 3-D volumetric images. Computer visualisation techniques were used to create animations on the basis of CT images and to analyse 3-D geometry and kinematics of our models in detail. Granular materials were used to simulate the brittle behaviour of sediments, whereas a viscous Newtonian material was used to model the ductile flow of rock salt or other evaporites. Models showed a marked difference in structural evolution between domains that were underlain or not underlain by a thin viscous layer, representing brittle-viscous and brittle rheological domains, respectively. In the brittle domain, closely spaced and dominantly forward propagating thrusts formed a narrow and high fold-and-thrust belt. In the brittle-viscous domain, the thrust belt was wider and lower, and there was no consistent vergence of thrusting and folding. Transfer zones formed in the transition zone between the two domains. Location and orientation of these transfer zones are directly related to the geometry of the boundary between basal viscous layer and adjacent brittle layers. A lateral ramp formed where this boundary was initially parallel to the shortening direction, whereas an oblique ramp formed where this boundary was oblique. Both lateral and oblique ramps have a shallow dip and dip angles that change along strike. Thrusts in the brittle domain may propagate laterally into the brittle-viscous domain resulting in out-of-sequence thrusting. Experiments show that the location and orientation of transfer zones in nature may be controlled by rheological changes in the basal detachment.