The nature of deformation in the toe of the Nankai accretionary prism based on sedimentary fabric analyses and numerical strain calculations

Seismic profiles across the eastern Nankai accretionary prism show evidence for diffuse deformation through stratal thickening, decrease in seismic coherence, and uplift of the accreting sediment package. This distributed deformation is thought to reflect a combination of ductile and brittle strains not resolvable at the seismic scale. The contributions of these strains to the deformation state within the accretionary prism have not yet been adequately quantified. In this work, two novel approaches to redress this situation are presented.;A kinematic analysis based on the numerical solution of the Lagrangian conservation of mass equation is presented for estimating diffuse deformation, using changes in seismic reflector spacing and sediment porosity along two transects across the Nankai accretionary prism. Calculations along an unfaulted portion of a western transect demonstrates that the strain response depends on the consolidation state of the sediment at the time of accretion, with the most consolidated sediment displaying high vertical extension and low volume strain, and the most porous sediment experiencing high volume strain and little vertical extension. The results also suggest the presence of a "finite strain front" arcward of the inferred deformation front, which may be related to the propagation of the decollement beneath the PTZ.;Diffuse strains calculated along an eastern transect across the prism toe, near ODP Site 808, demonstrate a heterogeneous distribution of strain related to both consolidation state and proximity to thrust faults. Sediments within the trailing edges of thrust sheets show up to 40% horizontal shortening, compared to about 15-20% in the leading edges of the thrust sheets. The trailing edge sediments also show relatively high vertical extension and low volume strain. This spatially variable strain distribution may reflect increased pore pressures inducing brittle failure and nearly constant volume distortion in the footwalls of the thrust faults, while uplifted sediments may preserve a ductile strain history.;The method of X-ray pole figure goniometry is used to determine the clay-mineral fabric in sediments collected from Hole 808C within the Nankai prism toe. Total finite ductile strains are estimated assuming a relationship between grain preferred orientation and strain known as the March Model. The resulting strains suggest up to 10% lateral shortening within the accreting sediments and a nearly uniaxial strain state within the subducting sediments. Vertical strains are lower than expected, especially for the deepest samples. This may be due to bioturbation during primary deposition, or to the breakdown of the March model at high strains. The results also suggest that sediment response may vary with position, based on reversals in depth trends for strains.