In the Framework of Deformation in the Nankai Trough

Abstract

Drilling data and processed reflection profiles permit a quantitative description of deformation across the toe of the accretionary prism behind the Nankai Trough. Deformation begins in a 5-km-wide protothrust zone, where trench-fill strata are tectonically thickened and uplifted, both by diffuse mechanisms and along 60°N dipping seismic discontinuities. This zone is bounded arcward by the frontal thrust, which displays the characteristics of a bedding-plane step thrust, including a 30° ramp above which a hanging-wall anticline has formed. A balanced section of the fault zone shows 280 m of horizontal shortening, except along the uppermost flat section, where the thrust merges with a sediment apron built largely of material slumped from the thrust toe. Both the frontal thrust slice and the one behind it show an arcward increase in diffuse deformation and internal, now inactive thrusts of large (> 1 km) displacement. Cumulative deformation becomes greater toward the arc until the section loses seismic coherence. Total horizontal shortening estimated from structural geometries across the seawardmost 15 km of the accretionary prism is estimated as near 8 km. The style of deformation in this setting is that of foreland thrust belts, not recumbent folding as previously suggested. Thrusts initially form at intervals near 3 km, but some quickly deactivate, resulting in compound thrust sheets 5 to 7 km in width, each having a wedge-shaped cross section.Modeling of the distribution of deformation with the methods of continuum mechanics produces results very similar to those from the structural analysis. The instantaneous distribution of deformation permitted by this approach indicates that one third of the subduction rate is absorbed within the protothrust zone and three fourths within the 15 km of prism toe that was analyzed. The remaining fraction of the instantaneous deformation is distributed across the lower trench slope in the form of continued arcward rotation of strata and isoclinal folding, as observed in emergent sections of the prism along the south coast of Shikoku. The continuum relationships, together with constraints on the age of deformation resulting from DSDP Leg 87, generate a subduction rate of 2 cm/yr. or less.

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

“…In this paper, we have demonstrated the application of this analysis using the protothrust zone along the western transect across the Nankai prism. The results improve upon previous balancing efforts [Karig, 1985] by permitting spatially varying porosity distributions and by resolving finer heterogeneities in the deformation field.…”

“…The results of our kinematic analysis demonstrate that diffuse strain can contribute significantly to the deformation in the toes of accretionary prisms. We have documented about 1.7 krn, or about 20%, horizontal shortening in the PTZ, a somewhat higher estimate than that of Karig [1985] It is interesting to note that the occurrence of dipping seismic discontinuities throughout the PTZ (Figure 2) does not correlate directly with the distribution of diffuse horizontal shortening (Figure 8a). This suggests that the two features express distinct modes of deformation.…”

“…At the seismic scale, the toes of accretionary prisms resemble continental fold and thrust belts composed of imbricate thrust faults which accommodate large horizontal displacements. However, mass balancing attempts across the toes of several prisms have demonstrated that between 20 and 50% of the necessary shortening cannot be accounted for by displacement along seismically resolvable faults [Karig, 1985;Brown et al, 1990]…”

Underplating of mélange evidenced by the depositional ages: U–Pb dating of zircons from the Shimanto accretionary complex, southwest Japan

“…where, x is the distance from the deformation front, z is the thickness of the accretionary prism at distance x , and b p is a constant for the porosity–depth relationship in an accretionary prism (Karig 1986). The thickness of the accretionary prism is assumed to increase linearly as a function of distance from the deformation front such that…”

“…When we put b 0 = 1.80 (Hamilton 1976), b p = 0.65 (Karig 1986), and x = 35 km (using the underplating depth of the Nankai prism of Park et al . (2002b), z d and z are 1200 m (thickness of offscraping trench‐filling sediments at the deformation front at the Nankai Trough from Park et al .…”