After measuring the natural remanent magnetization (NRM), all sections of the archive half of the core were partially demagnetized using alternating-field (AF) magnetization up to 30 mT in increments of 5 mT at 5-cm intervals to remove magnetic overprints. Two oriented discrete samples were routinely collected from each section of the working half of the core primarily for shore-based analysis of the anisotropy of magnetic susceptibility. Additional measurements of polarity and basic magnetic character of selected discrete samples were used to aid in the interpretation of the archive long-core magnetization record. Most of the discrete samples were demagnetized up to 60 mT in 5-mT increments to permit principal component analysis. Rock magnetic experiments were conducted to identify the magnetic minerals at Site 1174.
The majority of NRM inclinations for all sections are strongly biased toward steep inclinations of ~60°-80°. The steep inclinations observed are interpreted as magnetic overprints acquired during drilling; a problem identified on many previous Deep Sea Drilling Project (DSDP) and ODP legs. These overprints were successfully removed by AF demagnetization at 30 mT (see "Paleomagnetism" in the "Site 1173" chapter). Stable magnetic remanent inclinations were measured after AF demagnetization. These inclinations provide information about middle Miocene to Holocene magnetic polarity changes and were used in conjunction with the standard geomagnetic polarity time scale (GPTS) of Cande and Kent (1995) to date the sediments.
Declinations obtained from rotated archive-half core pieces after AF demagnetization aided in core orientation corrections and structural analysis such as fracture fabric analysis (see "Structural Geology").
The sediments of Units I and II in Hole 1174A, consisting of sandy and silty turbidites, show stable paleomagnetic remanence after AF demagnetization. All recovered sediment from Hole 1174A is included in the Brunhes Chron (0-0.78 Ma) based on the predominantly normal polarity inclinations. In contrast to these stable normal polarity inclinations, three reversed polarity inclinations were observed at 29.5 mbsf (Section 4H-5, 10 cm), 51.95 mbsf (Section 7H-1, 5 cm), and 61.58 mbsf (Section 8H-4, 5 cm) (Fig. F26). Intensity values are high in both the reversed and normal polarity inclinations. Zijderveld analysis (Fig. F27) suggests that these short reversal events show the possibility of geomagnetic polarity changes during short geomagnetic excursions.
Inclination changes in Hole 1174B were compared with the GPTS of Cande and Kent (1995). However, noisy inclination changes from 544.7 (Section 43R-1, 110 cm) to 685.95 mbsf (Section 57R-6, 15 cm) complicate the identification of reversed polarity events. Magnetic intensity of this zone shows a distinct low value. Further rock magnetic analysis will also be needed in order to identify reasons for the scattered inclinations.
The paleomagnetic remanence shows characteristic magnetic intensity zones similar to those of Hole 1173A (Fig. F28). Multisensor track (MST) susceptibility values also follow a similar pattern (see "Physical Properties"). Based on these characteristic intensity and susceptibility changes, the sediments can be divided into four distinct zones (Fig. F28). High magnetic intensities (intensity Zone [IZ] 1) range from 0 to 553.25 mbsf (Section 44R-1, 35 cm). The termination of this zone does not correspond with a lithostratigraphic boundary (Fig. F28). There is a sudden decrease of magnetic intensity that extends to 763.1 mbsf (Section 65R-6, 60 cm). This low-intensity zone (IZ 2) with a slightly high intensity subzone (IZ 2´) was also observed at Hole 1173A. From 763.1 to 941.7 mbsf (Section 85R-6, 60 cm), intensities increase slightly (IZ 3). Below this, intensities return to very low values for the remainder of the hole (IZ 4).
Rock-magnetic experiments were conducted to identify the magnetic minerals of the four major intensity zones (Fig. F29). Thermal demagnetization of multicomponent isothermal remanent magnetization (Lowrie, 1990) was used as the primary means of identifying magnetic minerals. For these experiments, orthogonally applied fields of 1.0, 0.3, and 0.1 T were used to generate the isothermal remanent magnetization (IRM) components. The samples were then demagnetized using 15 thermal steps from 50° to 650°C (Fig. F29). Greigite was determined to be the main magnetic carrier in IZ 1, identified by a saturation isothermal remanent magnetization (sIRM) of 300 mT and an unblocking temperature (Tub) of ~400°C. IZ 2 is believed to contain both greigite and magnetite, as indicated by a smooth IRM acquisition curve that reaches an sIRM value of ~200 and 800 mT, and the identification of two Tub values of ~600°C. Low-intensity values in this zone indicate that greigite is probably the dominant carrier of magnetism. IZs 3 and 4 are determined to contain magnetite as indicated by the Tub values of 500°-600°C and a narrow sIRM curve point of 300 mT. Similar IRM curves that slightly increase up to ~800 mT were observed for both IZs 3 and 4, indicating that a small amount of greigite is also present, although the high intensities of IZs 3 and 4 suggest that magnetite is probably the dominant magnetic carrier.
Site 1174 magnetostratigraphy is based on polarity changes determined by measuring the inclination of the archive half of the core after AF demagnetization at 30 mT. Many middle Miocene to Pleistocene magnetic polarity records were identified using biostratigraphic datums (calcareous nannofossils; see "Biostratigraphy") and correlated to the GPTS of Cande and Kent (1995) (Fig. F30). The identified chrons and subchrons are given in Table T13.
A magnetic polarity change from normal to reversed at 544.7 mbsf (Section 190-1174B-42R-7, 25 cm) is interpreted as the Brunhes/Matuyama Chron boundary dated at 0.78 Ma (Cande and Kent, 1995). The Matuyama Chron (0.780-2.581 Ma) is interpreted to occur at 543.15-685.95 mbsf (Section 190-1174B-57R-6, 15 cm). Although this section of the hole is characterized by a predominantly reversed polarity, only a few normal events are identifiable during the Matuyama Chron (see "Hole 1174B").
The Gauss Chron (2.581-3.58 Ma), interpreted to occur at 685.95-727.85 mbsf (Section 190-1174B-62R-1, 135 cm), is characterized by a change to a predominantly normal polarity. The magnetic polarity change at 727.85-802.12 mbsf (Section 190-1174B-69R-7, 75 cm) is interpreted as the Gilbert Chron. The termination of the Gilbert Chron and the beginning of Chron C3 is identified at 802.12 mbsf (Section 190-1174B-69R-7, 75 cm). The polarity boundary at 890.15 mbsf (Section 190-1174B-79R-1, 85 cm) is interpreted as the beginning of Chron C4A. A normal polarity inclination interval observed from 901.40 (Section 190-1174B-80R-2, 90 cm) to 947.15 mbsf (Section 85R-1, 5 cm) is identified as Chron C5n.
Comparison of the magnetostratigraphy and biostratigraphy is shown in Figure F31. A marked change of sedimentation rate from 69.83 to 4.07 cm/k.y. occurs at 544.7 mbsf (~1 Ma) within the upper Shikoku Basin facies. A second change from 4.07 to 3.53 cm/k.y. occurs at 793.92 mbsf within the lower Shikoku Basin facies (Fig. F31).
Clear geomagnetic records were identified at both Sites 1173 and 1174. Clearly definable chron boundaries include the Brunhes/Matuyama (0.78 Ma) and Matuyama/Gauss (2.581 Ma). The Gauss/Gilbert (3.58 Ma) and Chron C4/C5 (9.74 Ma) boundaries were also identified but were not as clearly defined.
Correlation between Sites 1173 and 1174 was also performed using zones and peaks of high and low intensity. The first distinct low-intensity peak at 578.49 mbsf in Hole 1174B is correlated to a low-intensity peak at 179.29 mbsf in Hole 1173A (Section 190-1173A-20H-1, 115 cm) in the upper part of lithostratigraphic Unit II (Fig. F32). The top of another low-intensity peak at 763.19 mbsf can be correlated to 391.99 mbsf in Hole 1173A (Section 190-1173A-42X-3, 15 cm). Therefore, sediments in the upper part of Site 1174 (lithostratigraphic Unit I to the top of Unit III) correspond to the upper part of lithostratigraphic Unit II in Hole 1173A.
This comparison of magnetic intensity peaks, zones, and magnetostratigraphic results suggests that the upper part of Hole 1174A, consisting mainly of trench turbidites of the Nankai Trough, is about three times thicker than the equivalent sediments in Hole 1173A. In contrast, the thickness of the upper and lower Shikoku Basin facies is similar to that of Hole 1173A.
The relationship between Site 1174 and Site 808 previously drilled during Leg 131 (Taira, Hill, Firth, et al., 1991) can be clarified by magnetostratigraphy (Fig. F32). At both sites a very long normal polarity interval identified as the Brunhes Chron was observed in the trench turbidites.
Site 1174 magnetostratigraphy around the décollement shows the décollement occurring within time-equivalent sedimentary horizons from 5.894 to 6.567 Ma (Chron C3An). The magnetostratigraphic comparison of Sites 1174 and 808 suggests that the décollement zone at both sites represents the same horizon.