The growth of a mountain belt forced by base-level fall: Tectonics and surface processes during the evolution of the Alborz Mountains, N Iran

Abstract:

The idea that climatically modulated erosion may impact orogenic processes has challenged geoscientists for decades, However, to date, field-based work has produced controversial results. The central-western Alborz Mts. of the Arabia-Eurasia collision zone are characterized by asymmetric precipitation superimposed on spatiotemporal changes in exhumation rates, deformation patterns, and prolonged, km-scale base-level changes. Our analysis suggests that despite the existence of a strong climatic gradient at least since 17.5 Ma, the early orogenic evolution was characterized by decoupled orographic precipitation and tectonics. In particular, faster exhumation and sedimentation along the more arid southern orogenic flank point to a north-directed accretionary flux and underthrusting of Central Iran. Conversely, from ~ 6 to 3 Ma, erosion rates along the northern orogenic flank became higher than those in the south, where they dropped to minimum values. This change occurred during a ~3-Myr-long, km-scale, base-level lowering event in the Caspian Sea. We speculate that mass redistribution processes along the northern flank of the Alborz Mts. and presumably across all mountain belts adjacent to the South Caspian Basin the Eurasian plate increased the sediment load in the basin leading to the underthusting of the Caspian Basin beneath the Alborz Mts. This underthrusting in turn triggered a new phase of northward orogenic expansion, transformed the wetter northern flank into a new pro-wedge, and led to the establishment of apparent steady-state conditions along the northern orogenic flank. Conversely, the southern mountain front became the retro-wedge and experienced limited tectonic activity. These observations overall raise the possibility that mass-distribution processes during a pronounced erosion phase driven by base-level changes may have contributed to the inferred regional plate-tectonic reorganization of the northern Arabia-Eurasia collision during the last ~ 5 Ma.