SEGMENTED HANGINGWALLS AT THE BASE OF SEGMENTED MOUNTAIN-FAULTS: BASIN AND RANGE, USA

The simple regression of valley floor elevation against valley floor distance downstream identifies previously unrecognized structural features on the hangingwall of segmented faults. This regression produces autocorrelated residuals that occur as positive and negative runs identify alluvial reaches with higher or lower elevations than the regression-predicted elevation. In the Basin and Range Province of Idaho, Nevada and Wyoming, the runs on the hangingwall of segmented fault-block mountains identify: (1) subtle alluvial benches that overlie gravity basins on the underlying Bouguer anomaly surface; (2) segmented hangingwall basement blocks consisting of a half graben between basin ward extensions of fault segment boundaries: (3) extensive alluvial fan sedimentation on the benches; and (4) dissected alluvial fans along the inter-benches. In central Idaho, the alluvial benches and inter-bench segments have similar but subtle dimensions - their average relief is ~6 m and average downstream length is ~18 km. The newly recognized segmented hangingwall basement blocks average 25 km in length, 8.7 km in width, and 16 km in depth. The lateral margins of the basement blocks are the interbasin highs formed by fault tips; They extend across the valley to merge with the dip slope of adjacent fault block mountains.

I propose that hangingwall segmentation occurs during seismic collapse and post-seismic isostatic adjustments. I predict that inherited benches on alluvial valley floors and hangingwall segmentation are widespread in areas of extensional, segmented fault block mountains.