Amplification of shoreline response to sea-level change by back-tilted subsidence

Shorelines move in response to the balance of geodynamic processes acting on sedimentary basins; thus the stratigraphic record of shoreline migration is an important tool for reconstructing climate, tectonic, and eustatic histories from ancient deposits. Here we test whether subsidence geometry influences shoreline migration in response to sea-level change by comparing two physical experiments conducted in the Experimental EarthScape (XES) basin. The experiments had similar sediment supply, subsidence rate, and sinusoidal sea-level cycles, but one experiment had a fore-tilted subsidence profile, where subsidence rates increased with distance from the sediment source (similar to a passive-margin setting) and the other had a back-tilted subsidence profile, where maximum subsidence was close to the sediment source with subsidence rates decreasing downstream. In the recent back-tilted experiment, decreasing subsidence rates downstream resulted in a tendency for shoreline regressions to self-amplify during base-level fall, whereas increasing subsidence rates upstream caused a rapid shoreline retreat during base-level rise, causing amplified shoreline fluctuations during sea-level cycles compared to the previous fore-tilted experiment. These results indicate that the spatial pattern of subsidence in a basin has a significant effect on shoreline migration in response to eustatic cycles. Shorelines in back-tilted basins are substantially more sensitive to changes in relative sea level than comparable coastlines in passive-margin settings, all else being equal.