Late Quaternary incision rates in the Vésubie catchment area (Southern French Alps) from in situ-produced³⁶Cl cosmogenic nuclide dating: Tectonic and climatic implications

Abstract

We have estimated recent river incision rates using the in situ-produced 36 Cl cosmogenic nuclide concentrations. The target site consists of a~25 m high vertical profile along a polished river cliff located in Jurassic limestones in the Vésubie catchment area, in the southern French Alps. The 36Cl exposure ages of the sampled river polished surface range from 3 to 14 ka, i.e., after the Last Glacial Maximum. Our data suggest as a first approximation a linear age/height relationship and lead to a mean incision rate of 2.2 mm a À1 over the last 14 ka. More precisely, incision rates are characterized by two peaks reaching~2 and 4-5 mm a À1 at 4-5 ka and 11-12 ka, respectively, separated by a period experiencing a lower incision rate (~1 mm a À1). A chi-plot of the river longitudinal profile suggests that on the long term, the river is close to equilibrium conditions with a concavity index of 0.475. The evolution of the Vésubie River longitudinal profile over a time period of 2 Ma based on the stream power law of river incision was then modeled with varying erodibility coefficients and uplift rates ranging from 0.5 to 2 mm a À1. The best fitting models yield erodibility coefficient values ranging from 2.5 to 9.0 × 10 À6 m À0.475 a À1 for the considered uplift rates. For long-term uplift rates lower than 2 mm a À1 , an increase of the erodibility coefficient during the last 16 ka, with two peaks at 11-12 and 4-5 ka, is necessary to precisely match the observed incision rates and is interpreted as resulting from recent climatic changes. These variations do not strongly affect the general shape of the river profile and suggest that the measured short-term incision rate is dominated by a climatic signal, which does not preclude the possible role of tectonic uplift.

Deglaciation history at the Alpine‐Mediterranean transition (Argentera‐Mercantour, SW Alps) from 10Be dating of moraines and glacially polished bedrock

“…The study area is located in the Argentera-Mercantour massif (southwest Alps) along the upper Tinée Valley (Figure 1) where climatic fluctuations, and in particular the chronology of glacier retreat since the LGM, have been investigated by several studies (Bigot-Cormier et al, 2005;Sanchez et al, 2010a;Darnault et al, 2012;Saillard et al, 2014;Brisset et al, 2015;Rolland et al, 2017). 394 Y. ROLLAND ET AL.…”

“…Figure 12). Subsequent installation of modern denudation and fluvial incision in low valleys is also constrained at that time period by CRE dating of polished river surfaces, which highlight onset of fluvial erosion at 19-22 ka, and transient erosional events involving incision rates >10 mm yr À1 at~14 and~11 ka (Saillard et al, 2014;Rolland et al, 2017;Petit et al, 2017). Subsequent installation of modern denudation and fluvial incision in low valleys is also constrained at that time period by CRE dating of polished river surfaces, which highlight onset of fluvial erosion at 19-22 ka, and transient erosional events involving incision rates >10 mm yr À1 at~14 and~11 ka (Saillard et al, 2014;Rolland et al, 2017;Petit et al, 2017).…”

Active transpressional tectonics in the Andean forearc of southern Peru quantified by10Be surface exposure dating of an active fault scarp

“…In compressional forearc settings, near‐surface fault expressions are often blind, associated with folded Quaternary sediments or abrupt changes in topography. In turn, river incision rates can be interpreted as fault uplift rates [ Hall et al , ; Alvarado et al , ], but this requires taking into account the effects of both climate variations and transient processes that remain mostly unknown [e.g., Saillard et al , ; Rolland et al , ]. Globally, only a few cases exist for which precise fault slip rates have been successfully determined by direct dating of a fault scarp [e.g., Jackson et al , ].…”

“…Previous studies have argued that records of tectonic activity are limited to Andean forearc landscape features of southern Peru [e.g., Sébrier et al, 1985]; however, more recent studies have not only shown that active structures exist and accommodate relief growth but have also suggested that rates based on dated offset geomorphic features should also be considered in models of recent forearc deformation [e.g., Hall et al, 2012]. fault uplift rates [Hall et al, 2012;Alvarado et al, 2014], but this requires taking into account the effects of both climate variations and transient processes that remain mostly unknown [e.g., Saillard et al, 2014;Rolland et al, 2017]. Globally, only a few cases exist for which precise fault slip rates have been successfully determined by direct dating of a fault scarp [e.g., Jackson et al, 2002].…”

Quantifying river incision into low‐relief surfaces using local and catchment‐wide 10Be denudation rates

“…Although there are a few case studies that reported river incision rates, such data stem either from steep bedrock gorges Saillard et al, 2014), from high-relief areas that have attained threshold hillslopes (Himalaya; Burbank et al, 1996;Taiwan;Schaller et al, 2005), or from regions that were completely covered by ice during the last glaciation (Reusser et al, 2004). Unfortunately, there currently seems to be no region where both an accurate incision rate as well as local and catchment-wide 10 Be denudation rates are available.…”