PALEOMAGNETISM

Pass-through magnetometer measurements were taken on all split-core archive sections. Sediment cores were measured at 5-cm intervals. Coherent basalt pieces that could be oriented unambiguously with respect to the top were measured at 1-cm intervals. Pass-through magnetic susceptibility measurements were taken on all unsplit core sections at 4-cm intervals.

In order to isolate the characteristic remanent magnetization (ChRM), cores were subjected to alternating-field (AF) cleaning. The number of AF demagnetization steps and peak-field intensity varied depending on lithology, the natural remanent magnetization (NRM) intensity, and the amount of time available. On average, sediment half-cores were demagnetized using three AF steps in addition to the measurement of NRM. The basalt half-cores were demagnetized using a minimum of six AF steps. The maximum applied field ranged between 25 and 50 mT. We analyzed the results in Zijderveld and stereoplot diagrams; where possible, we calculated the ChRM direction using principal component analysis (Kirschvink, 1980). Examples of the AF demagnetization of sediment and basalt samples are shown in Figure F32.

We recovered 1.47 m of sediments, consisting of 1.45 m of dark brown ferruginous claystone overlying a 2-cm-thick chalk layer. The NRM intensity of the claystone is high for sediments and has a mean value of 4.3 × 10^-2 A/m. The progressive downward increase in magnetic susceptibility of the claystone from 1 × 10^-3 SI at 365.5 mbsf to 4 × 10^-3 SI immediately above basement probably results from the decrease in bioturbation with depth observed in the core. The magnetization and susceptibility of the 2-cm chalk layer were not determined. AF demagnetization of the claystone with peak fields of 15 mT was effective in isolating the ChRM (Fig. F32A). Although the sediments were somewhat disturbed by drilling and the magnetic inclinations obtained were not very consistent, we were able to identify the polarity of the ChRM. At 365.7 mbsf we observed a downward transition from negative to positive (i.e., reversely magnetized) inclination values. The inclinations become negative again at 366.5 mbsf. Because the chalk layer underlying the claystone contains Aptian microfossils (see "Biostratigraphy") and the basalts beneath the chalk are all normally magnetized, we have tentatively correlated this ~70-cm-thick reversely magnetized layer with the M''-1r'' (ISEA) Subchron within the Cretaceous Normal Superchron (Fig. F33). Because this subchron is thought to be short in duration and the claystone to have accumulated slowly, this correlation cannot be made with confidence. Accurate magnetic inclinations obtained from shore-based studies of discrete samples from this layer will enable us to document more carefully the presence or absence of this reversely magnetized interval.

The basaltic basement recovered from Hole 1187 consists almost entirely of pillow lavas (Fig. F34). In the 100.9 m of basalt recovered (average recovery 74.3%), we distinguished a minimum of 147 pillows (see "Igneous Petrology"). As with other Leg 192 sites, the pillow lavas proved ideal for paleomagnetic studies, and ChRM directions could easily be defined using AF demagnetization and principal component analysis (Fig. F32B). The ChRM direction, NRM intensity, magnetic susceptibility, Koenigsberger ratio, and median destructive field (MDF) for all coherent basalt pieces longer than 15 cm for which a reliable ChRM direction could be defined are listed in Table T6. For coherent pieces longer than 50 cm, we list data for every ~25 cm.

The massive interiors of large pillows have slightly higher magnetic susceptibility than the more fine-grained pillow margins (see "Physical Properties"). These fine-scale variations are not apparent on the downhole plot of NRM intensity and MDF (Fig. F34), which displays more widely spaced depth intervals.

The magnetic inclination is negative for all 252 ChRM determinations (Table T6), indicating normal polarity for all basalt cores. The normal-polarity magnetization is consistent with the Aptian biostratigraphic age of the thin chalk layer immediately overlying basalt basement (see "Biostratigraphy"), indicating lava emplacement during the Cretaceous Normal Superchron sometime after M0 (~121 Ma) but before the M''-1r'' Subchron at ~115 Ma (Fig. F33). The downhole variation of ChRM inclinations is shown in Figure F34. Note that we do not observe any evidence for significant rotation (i.e., >30°) of pillows after they cooled below their magnetic blocking temperatures. We were unable to group the ChRM inclinations into paleomagnetic units at Site 1187 as we did at other Leg 192 sites (see "Paleomagnetism" in the "Site 1185" chapter). The reason is that we had too few inclination values for each of the large number of cooling units to adequately define individual paleomagnetic units. Thus, we have used the statistics of Kono (1980) on the entire data set to obtain a mean inclination of -35.2° (N = 252, ₉₅ = 0.9°, k = 107, and angular standard deviation [ASD] = 7.8°) and a corresponding paleolatitude of 19.4°S. Whereas we feel confident that the mean inclination value provides a reasonable estimate of the geomagnetic field during basalt emplacement, we note that the ASD is ~5° lower than that expected for the paleolatitude and age (McFadden et al., 1991). The lower than expected ASD value indicates that some of the measurements are not independent but sample the same paleosecular variation. Shore-based studies on discrete samples are necessary for a more precise definition of the paleomagnetic units and their mean inclination.

Because the basalt composition in Hole 1187 is very similar to that obtained for the upper igneous units at Site 1185 (see "Igneous Petrology"), we have compared the mean magnetic inclinations at both sites. The mean inclination at Site 1187 is essentially the same as that obtained from paleomagnetic units corresponding to the upper igneous units at Site 1185 (see Table T11 in the "Site 1185" chapter).