Long- and Short-term Landscape Evolution of the Carpathian Bend Zone – Linking Low-Temperature Thermochronology with Geomorphometric Analyses

The Carpathian Bend Zone is an orocline in the Southeastern Carpathians that links different segments of the Carpathian arc and represents a structurally unique sector of the mountain range. The region experienced Cretaceous to Miocene thick- and thin-skinned nappe stacking as well as post-collisional shortening and out-of-sequence thrusting. Unlike in many other places, these nappe stacks were not overprinted by subsequent back arc extension. In addition to this tectonic inheritance, the bend zone hosts the most seismically active region in Europe, characterized by persistent deep seismicity referred as “seismic nest”. This reflects deep-seated processes that are only partially expressed in the upper crust and are partially manifested through surface uplift and landscape reorganization rather than upper crust faulting.

 These factors lead to preserved nappe stacks and ongoing landscape evolution driven by recent uplift. Previous studies aiming to quantify exhumation and uplift rates have so far been limited to regions north and south of the Bend Zone, leaving this key segment poorly constrained. This study aims at closing this knowledge gap by investigating if long-term and short-term uplift rates are comparable. Furthermore, it collates these data with preexisting rates from other segments along the orogen to reveal local differences in exhumation patterns.

To investigate long-term exhumation, six sandstone samples were analyzed using apatite (U-Th)/He thermochronology. Additionally, geomorphometric analyses such as river longitudinal profiles, knickpoints, and χ-maps were used to study topographic evidence of recent uplift and assess drainage divide migration and equilibrium conditions. Furthermore, river terraces were mapped and their relative elevations above the modern riverbed were used to estimate since the Early Pleistocene. By correlating terrace elevations with known dated levels from nearby regions, constraints were placed on the timing of Quaternary incision and rock uplift.

The Apatite (U-Th)/He dates show a variable amount of thermal overprint. Two samples yield (U-Th)/He dates younger than their stratigraphic ages while four samples show dispersed dates older as well as younger than the corresponding Miocene stratigraphic ages. This indicates partial resetting, an inherited thermal history from the grains’ sources, and limited post-Miocene burial. Therefore, assuming a geothermal gradient of 30 °C/km, a maximum amount of approximately 2.7 km of burial since the Middle Miocene can be presumed. The geomorphic signals consistently indicate active uplift within the Bend Zone, particularly in areas where structural controls induce sharp knickpoints and asymmetry in watershed geometry. The average rock uplift rate indicated by river terraces is 1.1 mm/yr since the Middle Pleistocene. In combination with estimated exhumation rates derived from the thermochronological data, a overall stable landscape surface within the Bend Zone is proposed for the last 2.5 Ma. Overall, our results indicate that the Bend Zone has been characterized by low long-term exhumation rates since the mid Miocene and higher uplift rates during the Quaternary.