Tracking tectonic versus lithological impacts on sediment generation in catchments

Hillslope and channel processes in upland catchments combine to erode landscapes and release sediments. This underpins the genetic link between sediment generation and landscape drivers, such as tectonics and lithology, mediated by hillslope and channel processes; however, this link has yet to be thoroughly analysed from a process-based framework. Here, we focus on two well-constrained catchments in terms of tectonics and lithology in the Gulf of Corinth, Greece, to track how tectonic and lithological impacts on sediment grain size are translated through channel and hillslope processes. Topographic analysis reveals that both tectonic forcing and lithological variations can be translated into topography through bi-directional hillslope-channel couplings. In catchment 1, normal faulting initiated >600 ka is manifested by steepened hillslopes and concentrated mass wasting downstream of knickpoints. In catchment 2, which is perturbed only by active faulting <100 ka, the stronger and mass-wasting-prone bedrock steepens hillslopes and triggers pervasive mass wasting. Combined with the observation that mass wasting produces coarser grains, our data therefore show that both tectonic and lithological forcing are expressed in sediment grain size at the hillslope scale. However, if the bedrock is friable, tectonically induced coarsening of hillslope sediments can be erased by intense abrasion after they reach river channels. This is well-illustrated in catchment 1, where sandstone-siltstone-dominated tributaries do not export coarse sediments, despite intensive mass wasting driven by knickpoints. In contrast, lithologically controlled coarsening of hillslope sediments is preserved in catchment 2, as the sediments in this catchment are resistant to abrasion. In both catchments, selective transport filters out hillslope sediments coarser than the threshold for entrainment, but its impact attenuates rather than obliterates the forcing imprinted in coarse sediments. This non-obliteration effect arises because coarse sediment input itself can increase the entrainment threshold by influencing channel steepness. In short, our study demonstrates the central role of hillslope and channel processes in transmitting tectonics and lithology into sediment grain size.