ANALOG MODELING OF "THICK-SKIN" CRUSTAL DEFORMATION DURING CONTINENTAL COLLISION

We investigate the role of topography and pre-existing faults in the basement on the development of a fold and thrust belt with a demonstration sandbox model. A moveable wall is set at 40 cm from the back of the model and colored sand (i.e., cover rocks) is deposited in alternating 0.5 cm layers to a thickness of 3.0 cm on either a flat basement (control) or a ~.64 cm layer of plywood with faults cut at 45° and 60° angles to create basement topography (e.g., hinge zone). The advancing wall produces faults and folds in the cover. Structures are photographed from the side and above the model. We measure, digitally from photos, fault dip, fault spacing, and interlimb fold angle with respect to bulk shortening. Results of flat basement experiments are similar to published models. With shortening, older thrusts steadily 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 compaction. Experiments with faulted basement also result in a fold and thrust belt but with more thrust faults (6 vs 4) and a fault wedge that is vertically thicker and horizontally shorter. Thrust dips increase then decrease (e.g., 34° to 69°, ending at 60°) due to induced fault bend folding. The klippe fault plane induces more extensive internal wedge deformation with larger, more penetrating, backthrusts that produce a doubly vergent wedge. Eventually, a “klippe” is thrust over the cover. Seismic data and mapping in the Blue Ridge Province suggest that basement blocks (klippes) were thrust over part of the Valley and Ridge sedimentary cover during the Alleghanian Orogeny (see Hopson et al., 1989). Our model reproduces the klippe structures seen in the Blue Ridge Province and suggests topography on the basement significantly alters the structural style of deformation in the advancing thrust wedge.