Uplifted Pleistocene Marine Terraces at Active Margins: Modeling Reveals the Effects of Sea Reoccupation and Coseismic Uplift on Uplift Rate Calculation.

Uplifted Pleistocene marine terrace sequences are used to quantify uplift rates along active margins by knowing terrace age and elevation, and sea level (SL) position at the time of terrace formation. When terraces are undated, ages are assigned by correlating terraces at progressively higher elevations with progressively older highstands. Uplift at convergent margins can be constant over time or occur coseismically during upper plate earthquakes. We explore the formation of terrace sequences under conditions of constant and earthquake‐driven uplift by using a forward numerical model. The modeling reveals that terraces are generally abandoned at SL highstands but they are carved during all stands, depending on the time spent within the sea erosional‐depth‐range. Therefore sea reoccupation of a same platform after formation is a common occurrence that decreases with increasing uplift rates, suggesting that most platforms in nature may be in fact polygenetic. Furthermore, the model run time influences the terrace sequences: terraces formed at the beginning of longer runs constitute an 'inherited morphology' affecting subsequent sequences. When coseismic uplift is applied, the formation and preservation of terraces for a given average uplift rate depend stochastically on the coseismic displacement ‐ recurrence interval combination in relation to the SL position at the time of the earthquake. These factors significantly contribute to a higher likelihood of non‐preserved terraces along a terrace sequence, which may affect age correlation and, consequently, the resulting uplift rates. Further research is needed to explore the effect of the full seismic cycle in shaping a terrace sequence. Plain Language Summary: The topography of the upper plate in a subduction zone is subject to uplift over time. The action of the sea on the coast creates marine terraces, horizontal erosional platforms that, if uplifted, form a staircase morphology. Terrace staircase sequences are used to calculate the average uplift rate of the coast if the terrace age is known. In absence of datable material, the terrace age is inferred by assuming that terraces are preserved in chronological order of formation along the sequence. We use numerical models to explore the generation of subsequent marine terraces forming a staircase sequence on coastal margins subject to constant uplift over time or to instantaneous uplift caused by earthquakes occurring at a certain frequency (recurrence interval). We observe that: terraces are carved as long as they lie within the sea erosional depth; sea reoccupation of previously formed terraces occurs frequently, increasing with decreasing uplift rates; running the model for longer or shorter periods of time has changes morphology of the sequence; varying the earthquake recurrence interval results in different staircase morphologies even if the average uplift rate remains unchanged. The potential absence of terraces increases the chance of assigning, through cross correlation, wrong terrace ages, resulting in inexact uplift rates. Key Points: Terraces form during all sea‐level stands, and sea reoccupation is common at uplift rates ≤0.5 mm/yr; most natural terraces may be polygeneticEarthquake‐driven uplift may cause non‐preservation of individual terraces, depending on the recurrence interval ‐ coseismic uplift valuesBy keeping parameters constant and varying the model run time, terrace sequences with different morphology are created [ABSTRACT FROM AUTHOR]