Abstract
Abstract. The evolution of intraplate orogens is still poorly understood. Yet, it is of major importance for understanding the Earth and plate dynamics, as well as the link between surface and deep geodynamic processes. The French Massif Central is an intraplate orogen with a mean elevation of 1000 m, with the highest peak elevations ranging from 1500 to 1885 m. However, active deformation of the region is still debated due to scarce evidence either from geomorphological or geodetic and seismologic data. We focus our study on the southern part of the Massif Central, known as the Cévennes and Grands Causses, which is a key area to study the relationship between the recent geological deformation and landscape evolution. This can be done through the study of numerous karst systems with trapped sediments combined with the analysis of a high-resolution digital elevation model (DEM). Using the ability of karst to durably record morphological evolution, we first quantify the incision rates. We then investigate tilting of geomorphological benchmarks by means of a high-resolution DEM. We finally use the newly quantified incision rates to constrain numerical models and compare the results with the geomorphometric study. We show that absolute burial age (10Be∕26Al on quartz cobbles) and the paleomagnetic analysis of karstic clay deposits for multiple cave system over a large elevation range correlate consistently. This correlation indicates a regional incision rate of 83 +17/-5 m Ma−1 during the last ca. 4 Myr (Pliocene–Quaternary). Moreover, we point out through the analysis of 55 morphological benchmarks that the studied region has undergone a regional southward tilting. This tilting is expected as being due to a differential vertical motion between the northern and southern part of the studied area. Numerical models show that erosion-induced isostatic rebound can explain up to two-thirds of the regional uplift deduced from the geochronological results and are consistent with the southward tilting derived from morphological analysis. We presume that the remaining unexplained uplift is related to dynamic topography or thermal isostasy due to the Massif Central Pliocene–Quaternary magmatism. Integrating both geochronology and morphometrical results into lithospheric-scale numerical models allows a better understanding of this intraplate–orogen evolution and dynamic. We assume that the main conclusions are true to the general case of intraplate deformation. That is to say, once the topography has been generated by a triggering process, rock uplift is then enhanced by erosion and isostatic adjustment leading to a significant accumulation of mainly vertical deformation.
Highlights
For the past few decades, plate–boundary dynamics have been, to a first order, well understood
To explain the persistence through time of intraplate deformation and explain the high finite deformation that we can observe in the topography in many parts of the world as for instance the Ural Mountains in Russia, the Blue Mountains in Australia, or the French Massif Central, one needs to invoke continuous processes at the geological timescale
By analyzing a high-resolution digital elevation model (DEM) (5 m), we show that the region is affected by a southeastward regional tilting
Summary
For the past few decades, plate–boundary dynamics have been, to a first order, well understood. This is not the case for intraplate regions, where short-term (103–105 years) regional strain rates are low and the responsible dynamical processes are still in debate (e.g., Calais et al, 2010, 2016; Vernant et al, 2013; Tarayoun et al, 2017). To explain the persistence through time of intraplate deformation and explain the high finite deformation that we can observe in the topography in many parts of the world as for instance the Ural Mountains in Russia, the Blue Mountains in Australia, or the French Massif Central, one needs to invoke continuous processes at the geological timescale
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