Messinian or Pleistocene valley incision within the Southern Alps

Abstract

Many of the valleys on the southern slope of the Alps are over-deepened, having bedrock valley floors well below sea level. This has typically been attributed to incision that occurred during the Messinian Salinity Crisis (MSC) when sea level dropped by hundreds of meters, leading to incision of many of the margins of the Mediterranean. We reassess this interpretation by documenting the correct elevation of the valley floor of the Adige river, one of the major valleys draining the Southern Alps, and by estimating the vertical motion of that valley floor since the end of Messinian incision. We re-evaluated the bedrock incision in the Adige valley using existing borehole data and seismic profiles. We estimate the vertical post-Messinian uplift using thermochronometric data that reveal the removed rock mass and then infer the expected isostatic uplift. These data are combined to reconstruct paleo-river gradients and to test viability of incision profiles. We find that the erosive surfaces in the drill holes restore to a paleo-elevation well below estimates of the Messinian Salinity Crisis (MSC) sea level. Restored valley gradients are often reversed compared to todays river gradients, as the uplift correction is higher upstream. A Messinian age of the erosional unconformities within the Alps can therefore be excluded based on the current best estimates of Messinian Mediterranean sea level and post-Messinian rock uplift. Pleistocene glacial erosion left a major signature on the geomorphology of the valleys, which is documented by glacially over-deepended valleys in the northern Alps. These valleys are not influenced by the Messinian sea-level drawdown. Therefore, it is suggested that the over-deepened valleys on the southern slope of the Alps are also glacial in origin.

Hints on the Late Miocene Evolution of the Tonale-Adamello-Brenta Region (Alps, Italy) Based on Allochtonous Sediments From Raponzolo Cave

“…In this paleo-landscape, the valleys separating different geological domains (Adamello, Tonale, Brenta) were likely not as deeply entrenched as today, allowing an efficient transfer of sediment from the north-west to south-east across major tectonic lineaments, partially by fluvial systems and occasionally by glaciation events. This observation is consistent with recent studies (Winterberg et al, 2020) showing that major river valleys in inner parts of the Southern Alps (i.e., Adige Valley) were not all yet deeply carved during the Messinian sea-level drawdown.…”

“…These clues open intriguing questions that could be refined by future studies, especially if other allochtonous sediments would be identified in other caves of Brenta. For example a Miocene fluvial-glacial interconnection between different areas of the Alps (Brenta-Adamello-Tonale) suggest that the Messinian Salinity Crisis sea-level drop and related accelerated fluvial erosion may not have reached the internal most elevated region of the Southern Alps (Winterberg et al, 2020).…”

Active Faulting in Lake Constance (Austria, Germany, Switzerland) Unraveled by Multi-Vintage Reflection Seismic Data

“…This is generally in good agreement with observations in other perialpine lakes north of the Alps (e.g., Finckh et al, 1984;Preusser et al, 2010;Fabbri et al, 2018). While the Southern Alpine deep incision (Finckh, 1978) may be caused either by subglacial activity (Winterberg et al, 2020) or by the Messinian drawdown of base level (e.g., Cazzini et al, 2020), overdeepenings north of the Alps are primarily attributed to a glacial origin (Preusser et al, 2010). During the Last Glacial Maximum (LGM), the Rhine Glacier's ice elevation above the Lake Constance area was at ∼1,000 m a.s.l (Bini et al, 2009), indicating an ice thickness of ∼1,500 m. During the Quaternary, several glacial advances coupled with the erosive power in the subglacial domain created overdeepenings and shaped the perialpine realm (Preusser et al, 2010;Reber and Schlunegger, 2016;Magrani et al, 2020).…”

Reconstructing the Gorte and Spiaz de Navesele Landslides, NE of Lake Garda, Trentino Dolomites (Italy)

“…The study site is located within the NNE-SSW-oriented Lake Garda valley. This valley is linked to the tectonic evolution of the Giudicarie fault [32,33], while its depth is related to the Messinian sea level drop [34][35][36]. Lake Garda and the lower Sarca Valley are affected by present-day tectonic deformation [19,37,38], and earthquakes with an equivalent magnitude of M e = 5 are common in the region [39].…”

“…The aim of our study was to clarify whether the Gorte deposits were formed during a single event or during several events, as proposed by [28,29]. We used a multi-method approach combining field surveys supported by remote imagery, cosmogenic 36 Cl exposure dating and numerical runout modeling to reconstruct the large slope failures and the emplacement dynamics. Our new data can be compared with nearby isotopically dated deposits, for example, the Marocche di Dro [16], Lavini di Marco [30], Varini [31] and Molveno [12] rock avalanches (Figure 2), which allows an assessment of the most important preconditioning factors and possible triggers in a regional context.…”