THE INFLUENCE OF HIGH-ANGLE BRITTLE FAULTING ON SUBSEQUENT LANDSCAPE DEVELOPMENT IN EXTENSIONAL TERRANES

Two regions of synorogenic extension in the Alps provide the setting for an investigation of how deep brittle faulting is expressed in the modern topography. The Brenner and Simplon low-angle faults developed during constrictional deformation from ca. 30 to <10 Ma. In both areas, rolling hinge footwall uplift was largely accommodated by subvertical simple shear. This history is expressed by abundant, closely spaced, high-angle brittle faults with offset ranging from mm to 10s of meters. Faulting occurred at depths ranging from 25 to 3 km. These faults are most abundant within 10-15 km of the detachments. Our analysis focuses upon an anomalous trellis drainage pattern that appears to be tied to these faults in both footwalls.

For the Brenner footwall, stream long profiles subparallel to the detachment trace are convex proximal to the detachment and become progressively more concave and indistinguishable from hanging wall drainages with distance in the footwall. The drainages near the detachment are smaller because of the overriding influence of the closely spaced, high-angle brittle faults. The drainages stay small and never pass into the elaboration stage of basin development. In the Simplon footwall, the influence of the brittle faults is also manifest as an increase in drainage density. These densities are consistent with numerous small basins that have not evolved beyond the elongation stage.

These data are supported by field observations indicating that drainages developed on the hanging walls vs. footwalls deliver sediment to the piedmont in different ways. In general, hanging wall alluvial fans are larger, more gentle, and have a longer residence time in the landscape than the small, steep footwall fans. We attribute these differences to drainage basin shape, which again is controlled by the presence (footwalls) or absence (hanging walls) of the high-angle brittle faults. We conclude that extension accomplished by subvertical simple shear leads to these distinctive geomorphic features. Where these geomorphic features are largely absent, such as in the western U.S., flexural failure is the inferred dominant mechanism for footwall uplift.