233-3 Influence of Geologic Setting on Elevated Shear Strength Along Active Continental Margins

Abstract:

Sediments along active continental margins frequently exhibit elevated shear strength compared to those on passive margins, a phenomenon traditionally attributed to seismic strengthening. However, the precise influence of localized geologic setting on this strength elevation remains poorly quantified. Using undrained shear strength measurements collected using automated vane shear (AVS), penetrometer (PEN), and torvane (TOR) devices across > 70 drill sites from 10 Integrated Ocean Drilling Program and International Ocean Discovery Program (IODP) expeditions included in the LIMS with Lithology (LILY) database and 13 Ocean Drilling Program (ODP) expeditions, we evaluated the spatial variability of shear strength across different geologic settings on active margins (specifically the incoming plate, frontal prism, forearc slope and basin, rear-arc and back-arc region). Our findings confirm that active margin sediments generally display higher shear strength than passive margin sediments, though the rate and magnitude of this strengthening vary significantly by geological setting. Along subduction zones, the highest shear strength increase occurs within the frontal prism and, to a lesser extent, in the fore-arc region. Conversely, sediments on the incoming plate and back-arc basin of these active settings typically exhibit shear strengths more comparable to passive margin sediments, except where localized overconsolidation has been induced by landslides or tectonic uplift. This pattern aligns with expectations that the frontal region of subduction zones may be influenced by lateral compression, whereas the back-arc region is typically dominated by an extensional regime. Overall, these results highlight that while seismic strengthening may contribute to enhanced sediment strength on active margins, local geologic setting and stress state exert primary controls over the evolution of undrained shear strength across ocean basin sediments.

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