Tectonic Control on Drainage Evolution in Broken Forelands: Examples From NW Argentina

Abstract

AbstractPlanform drainage patterns in orogens reflect the initial control of local structures, resulting in strike‐parallel longitudinal drainage, and the subsequent influence of regional slopes that result from crustal thickening and reorganizing rivers into strike‐perpendicular transverse drainage. However, in broken forelands, out‐of‐sequence range uplift along steep faults results in significant basin‐range relief, suggesting that such regions may not follow the transition from strike‐parallel to strike‐perpendicular drainage and require a distinct model of drainage evolution. Using the broken foreland of NW Argentina as an example, we describe modern and paleo‐topography, planform drainage patterns, and drainage evolution using geomorphic observations and a synthesis of existing thermochronologic, structural, and sedimentologic data. We find three distinct drainage patterns in NW Argentina: (a) major long‐lived, strike‐parallel longitudinal rivers, (b) outlets localized between along‐strike faults, and (c) sharp changes in flow direction that developed as rivers passively deflected around growing structures and top‐down drainage reorganization (e.g., divide overtopping). These drainage patterns formed during Oligocene‐Miocene basin fragmentation and reflect a strong tectonic control. Rivers, unable to incise into rapidly uplifting bedrock, instead deflected and formed major longitudinal rivers that flow along ranges and through structural gaps, and frequently reorganized through top‐down processes. Sustained high relief between basins and ranges suggest that longitudinal drainages are entrenched and unlikely to reorganize into transverse rivers. These drainage patterns and their evolution are distinct from the expected longitudinal‐transverse progression in thin‐skinned and thick‐skinned tectonic wedges. We therefore suggest the fluvial evolution of broken forelands may be more comparable to extensional landscapes.