Kinematics and a balanced and restored cross-section across the toe of the eastern Nankai accretionary prism

Abstract

Seismic profiles across the eastern Nankai accretionary prism show evidence for diffuse deformation through stratal thickening and uplift of the accreting sediment package, thought to reflect the combination of small-scale ductile and brittle strains evident within drill cores. Using a kinematic solution based on changes in stratal thickness and porosities, diffuse strains are estimated for a transect across the eastern Nankai accretionary prism toe, in the vicinity of ODP Site 808. Calculated element displacements are used to reconstruct the undeformed configuration of the prism toe, providing a new method for balancing and restoring deformation in accretionary prisms. The results of this analysis demonstrate a heterogeneous distribution of strain within the prism toe, which appears to correlate with the distribution of brittle deformation structures in drill cores. The greatest vertical tectonic thickening and horizontal shortening estimates are obtained within the deepest sediments, which also display abundant brittle shears. Shallower sediments exhibit high volume loss and lower horizontal shortening, and in drill cores display very few deformation structures. This spatially variable strain distribution may result from inferred high pore pressures near the frontal thrust and décollement inducing a brittle overprint of previous ductile strains.