The role of compaction contrasts in sediments in décollement initiation in an accretionary prism

Hugues Raimbourg (1, 2) , Kohtaro Ujiie (3) , Achim Kopf (4) , Toshio Hisamitsu (5) , Yozo Hamano (2)

1 ISTO - Institut des Sciences de la Terre d'Orléans
2 JAMSTEC - Japan Agency for Marine-Earth Science and Technology
3 Université de Tsukuba = University of Tsukuba
4 MARUM - Center for Marine Environmental Sciences [Bremen]
5 Kochi Core Center

To understand how décollements develop into the pristine sedimentary succession entering subduction zones, we have performed mechanical tests on samples from the sediment column entering the Nankai accretionary prism, Japan (ODP site 1173). Both poroelastic compliance and plastic shrinkage upon application of a large effective pressure sharply decrease with depth in a ~ 100 m-thick domain in the upper section of the Lower Shikoku Basin unit, i.e. in a domain stratigraphically close to the actual location of the décollement near the toe of the prism. These property contrasts provide a potential explanation for the outward migration of the décollement into the incoming sediments. When approaching the deformation front, a given material particle is affected by an increase in stress, which has a component of vertical loading due to the deposition of overburden trench sediment, and also a component of lateral compression transmitted from the accretionary wedge. Depending on its initial mechanical state, the amount of lateral shortening in the incoming Nankai sediment column varies with depth and causes horizontal velocity gradients that concentrate into the mechanical transition zone (upper section of the Lower Shikoku Basin at appx. 450-550 m depth) into which the décollement eventually propagates. Future work has to assess the role of this plastic deformation relative to other governing factors such as friction coefficient and excess pore pressure, both at Nankai and along other active margins.

Mots clés: accretionary prism; décollement; subduction zone; compaction; mechanical testing

Domaines: Tectonique; Milieux et Changements globaux

Hugues Raimbourg, Kohtaro Ujiie, Achim Kopf, Toshio Hisamitsu, Yozo Hamano. The role of compaction contrasts in sediments in décollement initiation in an accretionary prism. Marine Geology, 2011, 282 (3-4), pp.188-200. ⟨10.1016/j.margeo.2011.02.011⟩. ⟨insu-00587331⟩