Paleomagnetic measurements from oceanic basaltic rock cores are important for tectonic studies because they give site paleolatitude and can be used to trace past plate motions. Such data are especially significant for Pacific plate studies because the plate is almost entirely covered with water, so few traditional paleomagnetic data are available. Moreover, Pacific paleomagnetic data are desirable for global tectonic studies as a result of the plate's rapid motion, large area, and unparalleled record of drift relative to the hotspots.
Because fully oriented paleomagnetic data from outcrops are rare, the apparent polar wander path (APWP) for the Pacific plate is mostly constrained by data from cores, analyses of magnetic lineation asymmetry (skewness), and inversions of magnetic anomalies over seamounts (e.g., Gordon, 1983; Cox and Gordon, 1984; Sager and Pringle, 1988; Petronotis and Gordon, 1999; Sager and Koppers, 2000). Limitations for each of these data types make interpretation and synthesis of an APWP more challenging.
Seamount magnetic anomaly inversions are the most abundant data, but may suffer the most systematic bias (e.g., Sager et al., 1993). Seamount paleomagnetic poles are calculated with the assumption that the anomaly results from a magnetization that was entirely acquired at the time of formation and is homogeneous throughout the seamount. Although these are adequate approximations for many seamounts, neither assumption is likely to be strictly true (Sager et al., 1993). The result is scatter in paleomagnetic pole positions caused by magnetization inhomogeneities and a shift of the pole toward or away from the present geomagnetic pole (depending on seamount polarity), resulting from the contribution from induced or viscous magnetization (Sager et al., 1993; Sager and Koppers, 2000). Accurate dating of seamounts is also problematic; consequently, only a small number of seamount paleopoles are useful for APWP determination (Sager and Koppers, 2000).
Paleopoles calculated from lineation skewness data are fewer in number but provide greater time resolution because the Cretaceous and Cenozoic magnetic reversal timescale is relatively well dated. Skewness data cannot be determined for the Cretaceous Long Normal Superchron, so there is a gap of ~35 m.y. during the mid-Cretaceous for which no skewness poles can be determined. In addition, skewness data suffer from a systematic error, known as "anomalous skewness," the source of which is still unclear (Cande, 1978; Petronotis et al., 1992). With coeval anomalies from two or more different lineation sets, anomalous skewness can be estimated with the assumption that this factor is constant for anomalies of the same age (Larson and Sager, 1992; Petronotis et al., 1992). Although this may reduce the bias caused by anomalous skewness, there are poorly understood differences in pole positions estimated from skewness data and other data types (e.g., Petronotis et al., 1994).
Many paleomagnetic measurements have been made on Deep Sea Drilling Program (DSDP) and Ocean Drilling Program (ODP) cores taken from the Pacific plate. These data also have limitations imposed by acquisition and geologic factors. Virtually all such data give only the magnetic inclination because the coring tool rotates around a vertical axis and azimuthal orientation of the core is rarely possible. This lack of azimuthal orientation results in a lack of magnetic declination data, and as a result only the paleolatitude of the site can be determined (Peirce, 1976; Cox and Gordon, 1984). Thus, determination of a paleopole usually requires combining coeval paleolatitude data from widely separated sites (Peirce, 1976; Cox and Gordon, 1984).
An additional problem for sedimentary core paleomagnetic data is that the fidelity of the paleoinclination may be in doubt (Gordon, 1990; Tarduno, 1990). Although many sediment core paleolatitude data are consistent with coeval paleomagnetic data (Peirce, 1976; Sager and Pringle, 1988), some display a systematic error termed "inclination shallowing." This bias appears to be caused by compaction or the adherence of magnetic particles to clay particles, and its result is to cast doubt on the accuracy of sedimentary core paleomagnetic data.
Basalt flows are thought to be reliable magnetic field recorders if enough flows can be sampled to average secular variation. Unfortunately, few DSDP and ODP holes have penetrated more than several meters into basaltic basement, so few sites have sufficient data to do so. Even when numerous flows are sampled, secular variation averaging may be in doubt because multiple flows can be extruded in a short time and therefore not all flows are independent measurements of the magnetic field. One of the deepest basalt sections was drilled from Hole 433C into the top of Suiko Seamount. This section recovered 65 flows, which produced only ~20-30 groups considered magnetically independent (Kono, 1980; Cox and Gordon, 1984). Although paleolatitudes have been determined with fewer numbers of units, the results may have large uncertainties because of insufficient averaging (Cox and Gordon, 1984; Sager, 2002). Nevertheless, even imprecise paleolatitude data are important because basalt core paleolatitudes can be combined with coeval data and weighted according to their uncertainties to calculate paleopoles (Gordon and Cox, 1980; Cox and Gordon, 1984).
During Leg 191, Hole 1179D (41.08°N, 159.96°E) was bored ~100 m into basaltic rocks located on the abyssal plain northwest of Shatsky Rise (Fig. F1). Because the hole is located on magnetic Anomaly M8 (Shipboard Scientific Party, 2001), the implied age of the igneous crust is ~129 Ma using the geomagnetic polarity reversal timescale (Gradstein et al., 1994). Coring recovered 49.9 m of mostly fresh, aphyric basalt flows and pillows, comprising 48 flow units divided by changes in texture, composition, or cooling boundaries (Shipboard Scientific Party, 2001). Igneous units in the sequence were grouped into three sections based on the abundance of olivine: (1) Group I, olivine-poor basalt (flows 1-8; 367.9-396.4 meters below seafloor [mbsf]); (2) Group II, olivine-free basalt (flows 8-24; 396.4-438.8 mbsf); and (3) Group III, olivine-rich basalt (flows 24-48; 438.8-471.4 mbsf). With 48 igneous flow units cored, the Site 1179 basalts provide an excellent opportunity to determine an Early Cretaceous paleolatitude for the Pacific plate. Here we report a study of the paleomagnetic characteristics of this igneous section, determine a paleolatitude, and discuss tectonic implications. Because of the location of Site 1179 on Anomaly M8 (the source of which is a block of reversed polarity crust), we expected the basalt sample magnetization to be reversed polarity. Other paleomagnetic studies from the western Pacific indicate that the Pacific plate has drifted 30°-40° northward since the mid-Cretaceous, so we expected that the Site 1179 crust was emplaced slightly north of the equator. Our results are consistent with these hypotheses.