WHAT CONTROLS THE REGULAR SURFACE GEOMETRY OF DIFFUSELY DEFORMING CONTINENTAL REGIONS?

The 800 km-wide northern Basin and Range province is a type example of a diffusely deforming continental region. Modern basin-and-range extension started in mid-Miocene time and resulted in formation of regularly spaced (c. 30 to 40 km) tilted upper crustal blocks. The typical vertical relief between range crests and basins is about 1.5 to 2 km, the basin fill typically ranges from 1 to 3 km in thickness, the amount of footwall exhumation of most mountain ranges is typically more than 4 km (e.g., Stockli, 2000), and the present-day extensional strain rate is c. 10 nstr (e.g., Bennett et al., 1999). It is difficult, however, to identify the parameters that control this regular topographic behavior, because regions of diffuse continental deformation form in a variety of crustal, structural, and climatic settings. Comparison of the Basin and Range province with the western Anatolian extensional province, Turkey, indicates that most potentially significant parameters are similar between the two regions: Moho depth (c. 30 km), fault spacing (c. 20 to 40 km), throw (> 4 km), and inherited crustal heterogeneity. However, both, strain rate (McClusky et al. 2001) and erosion/sedimentation rates are significantly higher in western Turkey than in the Basin and Range province. The high deposition rates in the turkish grabens result in a paucity of triangular facets and wine glass canyons. Furthermore, rivers in the turkish grabens remove sediments through their hanging wall basins to the Aegean sea, such that an isostatic imbalance may eventually lead to irregular graben morphology and deformation rates (Westaway, 1994). In the northern Basin and Range province, however, all basins are internally drained which may have resulted in a more regularly developed, isostatically balanced topography.