Nucleation of Ramps in Fold-thrust Belts: Insight from Quantitative Analysis of Sandbox Models

Pragnyadipta Sen, University of Illinois at Urbana-Champaign
Stephen Marshak, University of Illinois at Urbana-Champaign
Saad Haq, Purdue University

Faults in thin-skinned fold-thrust belts typically display a stair-step geometry, consisting of flats oriented parallel to bedding, linked by ramps that cut up-section across bedding. The process of fault growth leading to the establishment of ramp-flat geometry remains unclear, and a key question remains: Do ramps initiate at the detachment tip and grow up-dip from the tip into the strata above the detachment, or do they initiate within the strata and propagate both up-dip and down-dip until they link to the detachment? To answer this question, we used particle-imaging velocimetry (PIV) to document the evolutionary relationship between detachment growth and ramp growth in a sandbox model in cross-section. In the sand box (60 cm wide by 150 cm glass walled box) the model geometry consisted of a 4 cm thick sand-pack of sub-rounded quartz sand (internal friction angle φ = 32˚) sifted on to a mylar sheet (µbasal = 0.44). Convergence was initiated by pulling the mylar below a fixed back wall. A series of high-resolution digital images were taken recording the development of individual thrusts in the developing wedge. These images were analyzed using image correlation algorithms (i.e., PIV LAB and MATLAB) to determine the displacement field in the model. Our results show that ramps initiate within the stratigraphic layer above the ramp tip, not at the detachment tip, that they grow up-dip and down dip, and that the down-dip end of the ramp links to the detachment at a point that lies to the hinterland of the detachment's tip line. Displacement parallel to the ramp decreases from the point at which the ramp initiates to each tip of the ramp. Once the ramp intersects the detachment, displacement on the detachment to the foreland of the thrust ramp slows as the displacement gets partitioned between the ramp and the detachment. When the vertical load on the ramp becomes great enough to cause it to lock, a new ramp initiates to the foreland.