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Geomorphic expression of midcrustal extension in convergent orogens

Channel orientation, gradient, drainage basin spacing, drainage patterns, and alluvial fan sedimentology define a set of geomorphic observables for fingerprinting modes of extensional tectonism in convergent settings. Anomalous, finely textured trellis drainage patterns in the Alps are spatially coincident with the low‐angle Simplon and Brenner detachments. The detachment footwalls are characterized by numerous high‐angle brittle faults, which are etched out by low‐order stream gullies and channels. We quantify the relationship between these faults and watershed‐scale geomorphology using high‐resolution digital topography to extract channel and basin metrics. Topographic data are complemented by a field data set of channel gradients and the integrated erosional response of the watersheds preserved in valley bottom alluvial fans. In footwall regions adjacent to the detachments, channels are steeper, more closely spaced, have statistically coherent orientations, and feed coarse, steep alluvial fans. Drainage spacing increases in the footwall away from the detachment, and the trellis pattern diminishes. In the detachment hanging walls, which typically lack high‐angle brittle faults, there is a poor correlation between channel steepness, density, or orientation, and hanging wall alluvial fans tend to be relatively larger, gentler, and debris‐flow dominated. Comparisons are made to the Ticino dome region of the Alps as well as to the Pioneer core complex of central Idaho. In both cases, our analysis suggests high‐angle brittle faulting of the footwall is particularly well preserved in the region adjacent to a detachment. These results are consistent with a subvertical simple shear model for footwall extrusion as well as annealing or a lack of reactivation of brittle structures in the footwall distal to the rolling hinge.