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The role of fault length, overlap and spacing in controlling extensional relay ramp fluvial system geometry

Relay ramps are integral components of normal fault systems that control sediment transport pathways in evolving rifts. We attribute differences in the geometry of fluvial systems that drain relay ramps to the scale of the ramp bounding fault segments, the spacing between segments and the amount of overlap between segments. Previous conceptual models for relay ramp geomorphological evolution have assumed that ramp fluvial catchments develop on the ramp surfaces and flow parallel to fault strike into the adjacent basin. Numerous examples exist in nature, however, that show that this is not ubiquitous. The fundamental question of what drives differences in fluvial geometry in these settings has, to date, not been fully addressed. We selected 27 relay ramps across the Basin and Range, western North America, and mapped, via GPS and remote sensing, the faults and ramp fluvial systems associated with each site. The sites represent a range of fault scales, which we define by the total outboard fault length, and a range of spacing and overlap values in order to better understand the structural controls on differences among ramp fluvial systems. Results show that the majority of a relay ramp surface drains parallel to fault strike when the outboard fault is less than about 15 km long. High overlap/spacing ratios are associated with relays along shorter (<15 km long) outboard faults, whereas lower overlap/spacing ratios are associated with relays along longer faults. Relays with lower overlap/spacing values may be more common along longer outboard faults because they survive for longer periods of time in the landscape. Our geomorphological observations can be used to predict synrift depocenter locations along segmented faults, but these observations only apply if the faults are short (<15 km long) and in early rifting stages. At longer fault lengths, ramp fluvial system geometry has no discernable relationship with any specific structural parameter.