THE ROLE OF BASEMENT TOPOGRAPHY ON DEVELOPMENT OF FOLD AND THRUST BELTS: RESULTS FROM SANDBOX MODELS

We investigate the role of “basement” topography and pre-existing faults in extended crust formed during continental break-up on the development of fold and thrust belts formed during continental collision using a demonstration sandbox model. A moveable rigid wall represents the advancing continent lithosphere. Continental “basement” is modeled by rigid plywood. The overlying passive margin sedimentary section or “cover rocks” is deposited as colored sand in alternating 0.5 cm layers to a thickness of 2.0 cm on either a flat basement (control) or a raised layer of plywood with listric faults cut at angles to represent the “hinge zone” of the extended continental margin. The advancing wall produces faults and folds in the overlying cover. We measure, digitally from sequential photos, fault dip, fault spacing, and inter-limb fold angle with respect to progressive bulk shortening. Flat basement experiments yield results similar to published models. With increased shortening, older thrusts rotate (dips from 34° to 64°) as the wedge grows. Internal deformation of the wedge is minor - back thrusts are rare. Younger thrusts are considerably longer in length due to an “effective ramp” formed from sand compaction. Experiments with basement topography also form a fold and thrust belt but with more thrust faults and a vertically thicker and horizontally shorter fault wedge. Thrust dips first increase then decrease due to induced fault bend folding from basement ramps. We also see more extensive internal wedge deformation with larger, penetrating, back-thrusts producing a doubly vergent wedge. Basement fault blocks are thrust over the cover. This analog model is similar to the Blue Ridge Province, where basement blocks (klippes) are thrust over Valley and Ridge cover during the Alleghanian Orogeny (Hopson et al., 1989). Our model suggests basement topography significantly alters the structural style of deformation in the advancing thrust wedge.