CONTROLS ON SEDIMENT SUPPLY FROM RELAY ZONE CATCHMENTS ALONG EXTENSIONAL FAULT SYSTEMS AND THE IMPLICATIONS FOR RIFT BASIN STRATIGRAPHY

The structural evolution of linked extensional fault systems impacts the size of footwall catchments, the location and the persistence of sediment entry points from the footwall into the hanging-wall basin. We aim to understand better the controls on sediment volumes and spatial variations in erosion rates in these settings to shed light on observed variations in rift basin sedimentation. Locations where faults link, breached relay zones, set the relative uplift rate and thus local base level where a river emerges from the footwall. Changes in base level drive catchment incision in the footwall and produce variations in the amount and calibre of sediment entering the hangingwall basin, particularly if the incision rate is sufficient to drive a transition from diffusive to landslide-dominated hill-slopes. Furthermore, the locus of tectonic activity may shift laterally through time, as extension localizes, so that early-syn-rift sedimentary deposits, rather than bedrock, are uplifted and incised. Observations of rift flank erosion indicate that, where footwall uplift exposes bedrock, the pattern of incision is consistent with predictions of the detachment-limited (DL) fluvial erosion law, whereas when syn-rift sediments are reworked, the pattern of incision is more consistent with predictions of the transport-limited (TL) erosion law. We use a landscape evolution model (CHILD), which includes a catchment uplifted and back-tilted by an active normal fault, to quantify differences in erosion rate in space and time and total volumes of eroded material for each model. When the relative uplift rate across the fault is increased there is an instantaneous increase in sediment volume supplied to the hanging wall in the TL model whereas the DL model predicts a significant lag time (10⁵-10⁶ years) when the supply does not change and may even decrease due to drainage capture in the headwaters. The stratigraphic response to these variations in sediment supply are investigated and compared to syn-tectonic rift sedimentation in the Gulf of Evia, Greece. Our study reveals that the combined effects of fault array evolution and fluvial incision processes operating in relay zone catchments lead to different footwall geomorphologies, as well as differences in sediment supply and depositional patterns in basin stratigraphy.