(Table T1) Clay-mineral composition of ODP Site 184-1146 sediments

The preliminary results of a comprehensive study of clay minerals (<2 µm) are presented for the upper 2 m.y. of sediments from Ocean Drilling Program Leg 184 Site 1146 from the northern margin of the South China Sea, close to the Pearl River mouth. More than 500 samples were analyzed, and four main mineral species are present: smectite, illite, chlorite, and kaolinite. On a general basis, illite + chlorite and smectite display anti-correlated behavior in relation to changes in the proportion of primary to secondary minerals in the sediment. Low-frequency and high-frequency changes are observed in the smectite/(illite + chlorite) ratio.

(Table 1) Bulk and clay mineralogy at DSDP Leg 66 Holes

During Leg 66 eight sites on an active margin transect off southern Mexico were drilled in order to determine the nature of ocean-continent transition across a subduction zone. Present outcrops of Mesozoic to Precambrian basement at the coast intruded by Mesozoic magmas within only 65 km of the Middle America Trench axis indicate truncation of the continental margin, tectonic removal of an accretionary zone, and consumption of ocean sediments and crust by subduction.

Clay mineralogy of ODP Site 131-808 sediments

Drill core recovered at Ocean Drilling Program Site 808 (Leg 131) proves that the wedge of trench sediment within the central region of the Nankai Trough comprises approximately 600 m of hemipelagic mud, sandy turbidites, and silty turbidites. The stratigraphic succession thickens and coarsens upward, with hemipelagic muds and volcanic-ash layers of the Shikoku Basin overlain by silty and sandy trench-wedge deposits. Past investigations of clay mineralogy and sand petrography within this region have led to the hypothesis that most of the detritus in the Nankai Trough was derived from the Izu-Honshu collision zone and transported southwestward via axial turbidity currents. Shipboard analyses of paleocurrent indicators, on the other hand, show that most of the ripple cross-laminae within silty turbidites of the outer marginal trench-wedge facies are inclined to the north and northwest; thus, many of the turbidity currents reflected off the seaward slope of the trench rather than moving straight down the trench axis. Shore-based analyses of detrital clay minerals demonstrate that the hemipelagic muds and matrix materials within sandy and silty turbidites are all enriched in illite; chlorite is the second-most abundant clay mineral, followed by smectite. In general, the relative mineral percentages change relatively little as a function of depth, and the hemipelagic clay-mineral population is virtually identical to the turbidite-matrix population. Comparisons between different size fractions (<2 µm and 2-6 µm) show modest amounts of mineral partitioning, with chlorite content increasing in the coarser fraction and smectite increasing in the finer fraction. Values of illite crystallinity index are consistent with conditions of advanced anchimetamorphism and epimetamorphism within the source region. Of the three mica polytypes detected, the 2M1 variety dominates over the 1M and 1Md polytypes; these data are consistent with values of illite crystallinity. Measurements of mica bo lattice spacing show that the detrital illite particles were eroded from a zone of intermediate-pressure metamorphism. Collectively, these data provide an excellent match with the lithologic and metamorphic character of the Izu-Honshu collision zone. Data from Leg 131, therefore, confirm the earlier interpretations of detrital provenance. The regional pattern of sediment dispersal is dominated by a combination of southwest-directed axial turbidity currents, radial expansion of the axial flows, oblique movement of suspended clouds onto and beyond the seaward slope of the Nankai Trough, and flow reflection back toward the trench axis.