The estimation of diffuse strains in the toe of the western Nankai accretionary prism: A kinematic solution

Abstract

Arcward changes in the vertical spacing of seismically defined stratigraphic reflectors and sediment porosities determined from drill cores and seismic interval velocities suggest a compact kinematic solution for estimating diffuse deformation within accretionary prism toes. The method involves the solution of the Lagrangian form of the conservation of mass equation for the two‐dimensional finite deformation gradient field within discrete deforming domains. We apply this technique to the unfaulted protothrust zone (PTZ) along a western transect across the Nankai accretionary prism, in the vicinity of Deep Sea Drilling Project sites 582 and 583. The results yield horizontal shortening estimates of about 1.7 km, or about 20%. This shortening appears to be accommodated through different strain responses depending on the consolidation state of the sediments at the time of accretion. Shallow sediments respond to horizontal shortening dominantly by expelling pore fluids, producing volume strains up to 30%, while deeper sediments accommodate horizontal shortening dominantly through vertical extension reaching 70% in the footwall of the frontal thrust. Horizontal shear strains within the PTZ are not well resolved by this solution due to uncertainties in the porosity distribution, but they generally appear to be quite small.

Fore‐arc deformation controls frequency‐size distribution of megathrust earthquakes in subduction zones

“…Close to the trench, predictions of our models may be less quantitative. Here, diffuse (ductile) strain resulting from consolidation during accretion of porous, water‐rich sediments to the frontal wedge may accommodate up to 10–20 percent of strain prior to localization [e.g., Morgan et al , 1994; Henry et al , 2003]. In the model, strain localization at the updip limit of the seismogenic zone and the development of peripheral shear zones occurs at strains about an order of magnitude lower and more likely corresponds to consolidated sediments and crystalline rocks of the brittle crust.…”

Upward delamination of Cascadia Basin sediment infill with landward frontal accretion thrusting caused by rapid glacial age material flux

“…In addition to tectonic shortening estimated from the seismically imaged fault structures at many margins, a significant amount of tectonic strain is accumulated by diffuse deformation during frontal accretion of a sequence of trench sediment [ Bangs and Shipley , 1999; Morgan et al , 1995]. The processes and related deformation mechanisms were studied in detail in ODP cores.…”

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

“…Although the larger part of this porosity decrease is probably the result of loading by overlying trench turbidites (~100 m-thick in 1177 and~550 m-thick in site 582), there may be a contribution of tectonic shortening, but it has not yet been estimated. Unfortunately, in their kinematic model, Morgan et al (1994) also assume that in the domain seaward of the deformation front only vertical compaction operates, preventing the quantification of horizontal shortening there.…”