DEM analyses and morphotectonic interpretation: The Plio-Quaternary evolution of the eastern Ligurian Alps, Italy

Abstract

The Ligurian Alps (Italy) straddle a complex and ill-defined tectonic transition that accommodates Eurasia–Adria shortening driving concurrent growth of the western Alps and northern Apennines. We focus on the Plio-Quaternary landscape evolution of the eastern part of the Ligurian Alps with the goal of defining the active tectonics through an analysis of topography, drainage networks, and river long profiles. The Ligurian Alps have a steep, seaward-facing escarpment with topographic features reminiscent of the geomorphic markers preserved on both actively-uplifting coastlines and passive margins. For example, there are seaward-facing benches that are thought to be uplifted marine terraces common to active margins; but the drainage divide of the seaward escarpment shows clear evidence of recent northward migration into the Po foreland, a process consistent with escarpment retreat characteristic of passive or subsiding margins. This study explores the active tectonics consistent with these diverse landscape evolution pathways. Our analysis reveals that rock-type strongly throttles the development of topographic relief, drainage networks, and northward migration of the drainage divide. However, we also identify segments of the escarpment that indicate recent base level falls through an analysis of modeled channel steepness and comparison with mean hillslope gradient. The base level falls are consistent with numerous barbed tributaries that are best explained as recent stream captures. No significant correlation among the stream captures at the drainage divide, suspected marine terraces at the coast, Ligurian Sea bathymetry, and active seismicity indicates that the escarpment is the footwall of a segmented normal fault. This normal fault is embedded in a regionally subsiding margin where actively uplifting segments lead to a straightened, pinned drainage divide, whereas, relatively inactive segments permit divide migration into the footwall block. These results are broadly consistent with rock uplift increasing eastward toward Genova across the long-suspected Alps–Apennine Sestri–Voltaggio fault zone. They highlight the respective roles of legacy structures, rock-type, and localized rock uplift in constructing a landscape in an otherwise extending and subsiding crustal retrowedge of an active subduction plate boundary.