Terraces response to different uplift modes at subduction margins: a forward modelling approach

Marine terraces are preserved along the coast when their uplift rate overcomes the rate of sea level increase. Generally, if the relative sea level history is known, elevation and age of marine terraces can be used to quantify the average uplift rate.

At subduction margins, large-scale topography of the fore-arc is the result of complex subduction mechanisms. The existence of uplifted marine terraces along fore-arc coastal areas indicates that the topography is subject to long-term permanent uplift. However, it is yet not known when this permanent uplift is accommodated. Geodetic observations show that, of the total deformation occurring during the megathrust earthquake cycle, only a minimal part (<10-20%) is translated into permanent vertical deformation of the topography. Additionally, particularly high uplift rates (~1 mm/yr) of fore-arcs observed geodetically, or geologically using uplifted marine terraces, suggest the existence of uplift transients or pulses that seem to reflect earthquake clustering on upper plate faults lasting 10 to 100 kyrs, while underplating cycles deduced from field observations and derived from numerical models occur at time scales from 0.5 to 6 Myrs.

We use numerical models to investigate whether different uplift styles are reflected in the geometry of the marine terraces sequences. In particular, we aim at spotting the occurrence of diagnostic patterns representative of different uplift ‘modes’: constant uplift rate, uplift by earthquake pulses (permanent uplift only), or uplift resulting from interseismic and coseismic vertical displacements. The results show that the variability of the terrace staircase morphology subject to different uplift modes increases with the earthquake recurrence time. Preliminary comparison with natural case studies displaying an analogue variability confirms our argument.