SUMMARY OF SCIENTIFIC RESULTS

Evolution of the Accretionary Prism
Sites 1175, 1176 and 1178 are among a handful of DSDP and ODP sites to have penetrated completely through slope sediments into an underlying accretionary prism (Figs. 8B, 11). They are, therefore, important for refining models of trench-slope evolution. The upper facies unit at all three sites fits into conceptual models of a trench-slope basin or slope apron, at least for cases where there is isolation from influx of coarse terrigenous sediment. Resedimentation of muddy material by submarine slumps and mudflows, perhaps triggered by seismogenic activity, contributed to high rates of sedimentation (200—3000 m/m.y.).

One of the unexpected discoveries of Leg 190 is the lack of change in sediment composition or lithofacies across the interpreted contact between trench-slope basin and accretionary prism, as deduced from seismic reflection data. Strata throughout the slope to prism transition at Sites 1175 and 1176 include abundant sand turbidites and muddy gravel. One way to explain this paradox would be through frontal offscraping of a trench fan that had been fed by a transverse sediment conduit (e.g., throughgoing submarine canyon). Evidently, coarse siliciclastic material was diverted away from the slope basin relatively early in its development, as canyons were rerouted upslope by uplift and tilting of the prism. This change in the sediment-dispersal network resulted in an overall upward-thinning and upward-fining megasequence within the slope basin. Regardless of the ambiguity in the exact position of its basal unconformity, the slope basin at Sites 1175 and 1176 must have developed within the last 2 m.y. This time constraint means that a phenomenal 40-km-wide accretionary prism has been added to the Nankai margin over that same time period.

Interpretations of results from Site 1178 are even more problematic because of inconclusive biostratigraphic control. One option is to place the fundamental lithotectonic boundary between a mud-dominated slope apron and sandy accretionary prism at a major facies break (~200 mbsf). If this interpretation is correct, then the contact is probably a slump surface. The underlying prism strata probably were accreted during the late Miocene to early Pliocene, prior to the frontal accretion at Sites 1175 and 1176. Even though the timing of accretion remains uncertain, it is clear that strata below 200 mbsf display many effects of intense deformation (e.g., fractures, incipient cleavage, and steeply dipping beds). Their facies character also matches that of the axial trench wedge and outer trench wedge, and there is good sedimentologic evidence for thrust repetition of the axial trench—wedge facies with slices at 199—411 mbsf and below 564 mbsf. These distinguishing features of both sedimentology and structural geology have important implications for interpretation of kindred rock units in such ancient subduction complexes as the Shimanto Belt of southwest Japan.