Do forethrust ramps propagate up-section from the tip line of a detachment or do they initiate within the deforming strata, connecting later with the detachment? To address this question, we acquired cross-sectional digital images of a sand layer undergoing shortening, in a sandbox analog model. Analysis of a succession of images, using particle-image velocimetry (PIV), defines spatial and temporal variations in strain and vorticity in the sand layer during shortening. Results show that the foreland edge of layer-parallel shortening in the sand layer defines the location of the detachment's tip line, and that initiation of a new forethrust ramp begins within the layer above the detachment, well to the hinterland of the detachment tip. Once initiated, the ramp grows both up-dip and down-dip until it links to the detachment to define a new branch line. As the forethrust ramp grows, the position of the tip line of the active detachment migrates to the hinterland, so that previously active portions of the detachment stop slipping. Once the ramp intersects the detachment, displacement on the detachment to the foreland of the new branch line ceases temporarily, while slip on the ramp accommodates all shortening.
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