RAMP DIP AND THE STEADY-STATE TOPOGRAPHY OF FAULT-BEND FOLDS

Provided with structural data or geomorphic surfaces on a growing fold (e.g., uplifted, undissected pre-growth surfaces and deformed fluvial terraces), one can use models of fault-related fold growth to estimate the underlying fault geometry. Folds that have achieved a topographic steady-state, on the other hand, offer fewer direct lines of geomorphic evidence from which to interpret fault geometry. To identify indicators of fault geometry in steady-state landscapes, we investigate the role that the dip of the underlying fault ramp plays on the steady-state, macroscale morphology of a fluvially eroded fault-bend fold using CHILD, a landscape evolution model. Ramp dips are varied from 25° to 60°, yielding vertical to horizontal velocity ratios ranging from 0.47 to 1.73. We simplify the experiments by assuming uniform fluvial erosivity, a unit stream power rule for erosion in detachment-limited fluvial channels with no critical shear stress for detachment, and linear diffusion on hillslopes. Furthermore, slip rate is held the same in all simulations. Results show that at steady-state, all of the mountain ranges exhibit a topographic front at the up-dip end of the fault ramp and an opposite front at the intersection of the land surface and the backlimb’s lower active axial surface. As ramp angle increases, the mean width of the range decreases and average erosion rate increases. Geophysical relief and erosion rates on the up-dip side of the range are greater than the down-dip side, consistent with the inclined path of bedrock advection. The relative widths of the two sides of the range are, however, less sensitive to ramp dip. Range symmetry instead results from high fluvial erosivity and/or low fault slip rates. A number of relatively symmetrical, real fault-bend folds, associated with ramps dipping between 30° and 60°, are argued to be in topographic or flux steady-state. Although our model does not capture all of the geomorphic processes that are likely to occur in tectonically active landscapes (e.g., landslides and debris flows) and it probably overemphasizes the role of fluvial erosion near drainage divides, its consistency with real examples does suggest that the morphology of steady-state fault-bend folds is linked only weakly to the dip of the ramp.