Drainage evolution in broken forelands: A case study from NW Argentina

Abstract:

Despite variable lithology, climate, and tectonic setting, many orogens and associated forelands are characterized by broadly similar drainage patterns and drainage evolution. These regions are drained by local structure-parallel longitudinal rivers during initial convergence. Continued uplift results in the formation of a regional taper across the orogenic wedge, encouraging the development of a slope-parallel transverse drainage network that expands through cannibalization of uplifted longitudinal drainages. In contrast, in ‘broken forelands’, where deformation is characterized by out-of-sequence uplifts of basement-cored ranges along steep reverse faults, an inconsistent regional taper may result in entrenched longitudinal drainage patterns and therefore require a new model of drainage evolution.

We assess the unique drainage evolution of broken forelands by examining modern and paleo-drainage patterns in NW Argentina (23-28°S). We characterize the modern topography and drainage of the region and utilize published thermochronology, sedimentology, and structural data to assess regional paleo-drainage evolution. Our analysis suggests the region is characterized by major longitudinal drainages established during the fragmentation of the foreland, long-lived structurally-controlled drainage outlets, and frequent passive drainage reorganization. Furthermore, NW Argentina has a complex regional topography; rather than a consistent regional taper, swath profiles document multiple local minima and maxima, corresponding to the large intermontane basins and basement-cored uplifts, respectively. Given the significant elevation differences between intermontane basins, containing major longitudinal rivers, and basement-cored ranges, drained by transverse channels, we propose that longitudinal drainages are not at risk of stream capture and the region is likely unable to transition to a regional, transverse drainage system. We suggest that, unlike thin and thick-skinned fold and thrust belts, drainage patterns in broken forelands are more closely related to basin and range systems, characterized by both distinct drainage patterns and evolution.