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doi:10.2204/iodp.proc.314315316.222.2015

Results

Results of AMS measurements are shown in a series of plots against depth (Figs. F2, F3, F4, F5). Here we present results in two focus areas, the megasplay fault and the frontal thrust zone.

Megasplay fault area

At Site C0004, the results show variation in each lithostratigraphic unit (Fig. F2). The most significant feature here is that the shape parameter T shows peculiarly scattered plots in Subunit IIA and Unit III that correspond to mass wasting deposits and a fault-bounded package, respectively. In Unit I, Subunit IIB and Unit IV, positive T appears relatively dominant. The degree of anisotropy P′ first increases (roughly from 1.01 to 1.04) downward in Unit I and is distributed within the same range with no particular trend in Subunit IIA. The data from Subunit IIB, in which the core recovery was rather poor, show that the degree of anisotropy has relatively low values (from 1.00 to 1.02) in general, whereas some samples show remarkably high values (from 1.08 to 1.13). The AMS parameters from Unit III are nearly equivalent to the majority of results from Subunit IIB, and grade into an increase in Unit IV.

The inclination of K3 axes is very steep in Unit I (60°–90°) and Unit IIA (50°–80°),whereas at their boundary, which forms an unconformity, the inclination is quite gentle (10°–40°). The data below Unit IIA are dispersed. K3 inclination is no more than 75° in Unit III and ranges between 60° and 90° in Unit IV.

At Site C0008, Holes C0008A and C0008C showed trends that were similar to each other (i.e., Km, F, and P′ start increasing with depth from the middle of Subunit IA, low L, positive T, and stable axes inclinations) (Fig. F5). In Hole C0008A, P′ is stable (~1.02) in the uppermost 150 m of sediments and shifts to a much higher value (up to >1.10) deeper in the core. T has a positive value, and the inclination of the K3 axes is steep (60°–90°) throughout the hole. In the uppermost 16 m of sediments, inclination of the K3 axes is highly variable, and the biostratigraphic data suggest a discontinuity lurks somewhere between Samples 316-C0008A-1H-CC (6.805 meters core depth below seafloor [m CSF]) and 3H-CC (25.595 m CSF) (see the “Site C0008” chapter [Expedition 316 Scientists, 2009d]). Sediments from Subunit IB, which is described as a mass transport complex (see the “Site C0008” chapter [Expedition 316 Scientists, 2009d]), show signs of compaction: porosity reduction with depth, high F, high P′, positive T, and vertical K3. The higher value of Km in Subunit IB may enhance this distinct shape information. In Hole C0008C, the parameters behave synchronously with Hole C0008A with the exception of a seamless increment in the degree of anisotropy. The unstabilized inclination of the K3 axes in the uppermost 9 m of Hole C0008C also corresponds to a reported discontinuity between Samples 316-C0008C-1H-CC (5.420 m CSF) and 3H-CC (25.345 m CSF).

Frontal thrust area

The results and initial interpretations of the samples from the frontal thrust area in the accretionary prism toe are partly published in Kitamura et al. (2010), on which the following description is based.

At Site C0006, P′ starts low (1.01–1.05) and increases (maximum = 1.15) with depth (Fig. F3). The majority of the inclinations of K3 axes are steep. With respect to positive T, the magnetic ellipsoid generally tends to become more flattened as is buried more deeply, similar to Site C0008 nondeformed slope basin deposits. However, several different features reflect the distinct structural setting in this site.

For example, there is a clear gap in terms of P′ and the inclination of the K3 axes at ~400 m CSF. A major change in the trend of P′ occurs in Subunit IID (391.33 m CSF) where there is a meaningful drop of ~0.9. T shows positive values except in the two uppermost units, Unit I and Subunit IIA. The inclinations of K3 axes are also scattered in Unit I and Subunit IIA, but below that the data can be classified into a steep portion (from ~80 to 440 m CSF) and a gentle portion (below 440 m CSF). With more meticulous inspection, a stepwise decrease of inclination is observed at 405 m CSF, where the large degree of anisotropy (P′ >  1.1) decreases to lower values (P′ < 1.1).

Technical difficulties in drilling at Site C0007 led to poor recovery in the middle of the hole and below the frontal thrust (Fig. F4). Despite incomplete recovery, the results are generally consistent with the adjacent Site C0006 (which is ~800 m landward). In the uppermost 100 m of Site C0007, T and the inclination of K3 axes are not consistent; however, they do vary less than those of Site C0006. T varies between –0.86 and 0.96 at Site C0006, whereas the data from Site C0007 occupy a smaller range, from –0.42 to 0.81. The inclinations of K3 axes are gentle (<60°) rather than scattered in Unit I of Site C0007, and the sandy unit below does not show values as scattered as those found at Site C0006. In Unit III, T values are concentrated around zero, which indicates that the magnetic ellipsoid is either spherical (low P′ value) or scalene (high P′ value) (e.g., Jelinek, 1981; Borradaile, 1991).

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