The paleomagnetic and rock magnetic properties of 51 Jurassic basalts from Ocean Drilling Program (ODP) Hole 801C have been examined. Magnetic properties vary with lithologic composition; alkalic rocks and hydrothermally-altered tholeiites are much weaker in intensity and generally contain higher coercivity magnetic components than the older and less-altered tholeiites at the base of the hole. For the entire column, the Jurassic basalts have an average initial natural remanent magnetization (NRM) intensity of approximately 1.24 A/m and average median destructive fields (MDF) of 8.31 mT. These values and the mean Koenigsberger ratio of 1.7 are very similar to results obtained for Jurassic basalts from the Atlantic (DSDP Leg 76). The similarities suggest that the basalts of both sites and their remanence characteristics are representative of Jurassic oceanic crust.The most profound discovery in these samples was the presence of 5 inclination zones, each showing consistent positive (or negative) polarity opposite the overlying and underlying polarity bands. We interpret these to represent a record of change in polarity of the EarüYs magnetic field and, because of the large number over such a short interval (60 m) of crust, we assert that the rapid change in polarity during the Jurassic is the probable reason behind the origin of the Jurassic Quiet Zone. INTRODUCTIONSeveral attempts have been made in the past to reach Jurassic oceanic crust within the region referred to as the Jurassic Quiet Zone in order to examine the magnetic character of samples of that age. Sites 198 (25.8°N, 154.6°E) and 199 (13.5°N, 156.2°E) were terminated due to drilling complications (Heezen, MacGregor, et al., 1973) while nearby Site 585 (13.5°'N, 156.8°E) was stopped in a thickerthan-expected sequence of Lower Cretaceous turbidites (Shipboard Scientific Party, 1985a). Most disappointing was the recovery of a thick sequence of Cretaceous intrusive and extrusive rocks from Hole 462A (17°14.5'N, 165°1.9'E) in the Nauru basin, a site that lies beyond a magnetic anomaly correlated with chron M26, thus indicating a Late Jurassic age (Shipboard Scientific Party, 1981). Further drilling of Hole 462A on Leg 89 yielded similar results (Shipboard Scientific Party, 1985b). Even on the present leg, both Site 800 (21°55.4'N, 152°19.4'E) and Site 802 (12°5.8'N, 153°12.6'E) were terminated in Cretaceous volcanic rocks . The third site, Site 801 (18°38.6'N, 156°21.6'E), however, reached its intended target and finally laid to rest any doubts that the magnetic smooth zone in this region did hold, at least in part, remnants of the Middle and Late Jurassic Pacific plate.While remote sensing studies of variations of the geomagnetic field will continue to play a major role in our understanding of crustal movements and development, they can play only a partial role in our understanding of the origins of the Jurassic Quiet Zone. Only analyses of samples collected directly from the Jurassic Quiet Zone can provide answers to the questions concerning the ...

The remanent magnetizations contained in the three sedimentary sections recovered by Leg 129 are of relatively strong intensities and have notably minor amounts of secondary magnetization. Both alternating field and thermal demagnetization techniques readily separate magnetization components, resulting in remanent directions that are exceptionally single component at the higher stages of demagnetization. In all types of lithology, the magnetization is carried by a mineral with blocking temperatures below 570°C, presumably a member of the titanomagnetite series. The resulting inclinations are generally very consistent among samples from the same time periods. Substantially incomplete recovery at each site, a result of the disruptive effect on coring and recovery in mixed sediments of chert interspersed with calcareous and clayey sediments, prevented reconstruction of complete magnetostratigraphic records from these sedimentary sequences. The inclination records from the three holes provide a relatively continuous history of the paleolatitudinal motion of the oldest portion of the Pacific plate from the Campanian stage of the Late Cretaceous to the Callovian/Bathonian boundary of the Middle Jurassic; data of somewhat lessor quality were also obtained for the early Miocene through Paleocene. All data indicate that this embryonic portion of the Pacific plate was in southern paleolatitudes from the Middle Jurassic to the Late Cretaceous and probably crossed the equator between the Turonian and the late Paleocene. These data suggest that during the Late Jurassic and Cretaceous, the plate was never too far from the equator, varying between about 20°S and equatorial latitudes. If the phenomenon of inclination error is operative in these sediments, as has been claimed for many oceanic sediments, then obviously the plate was at higher southern latitudes than indicated by these data. However, qualitative estimates of paleolatitudes from nannofossil abundances (Erba, this volume) indicate paleolatitudes quite similar to those calculated from the remanent inclinations. Moreover, calculations of expected inclination error from published analyses result in paleolatitudes much farther south than predicted by Seamount paleopoles or magnetic anomaly skewness. Therefore, the paleolatitudes determined from the inclinations recorded in the sediments of this study may represent approximately the true paleopositions of the Pacific plate.

Compressional wave velocities and densities were measured for 6 basalt samples from ODP Hole 801B and 16 samples from ODP Hole 801C, a site that represents the first drilling of Jurassic-age crustal rocks in the Pacific basin. Incremental measurements, taken to a total pressure of 200 MPa, show a systematic decrease in velocity with increasing porosity and a related increase with increasing wet-bulk density. A comparison of the plot of porosity vs. compressional wave velocity with the theoretical equation from Wyllie et al. (1958) suggests this equation is inappropriate for oceanic basalts because of mineral alteration in high porosity samples. Also of interest is the dramatic change in velocity across a hydrothermal boundary. Basalts below this hydrothermal layer have a mean velocity of 6.05 km/s at 60 MPa while those above show a mean velocity of 4.55 km/s at 60 MPa. The low velocity values of the basalts above the hydrothermal deposit may be attributed to the higher porosity and composition observed in these rocks; the higher porosity is possibly the result of increased exposure to circulating seawater.

Interpretation of conventional land seismic data over a Permian-age gas field in Eastern Saudi Arabia has proven difficult over time due to low signal-to-noise ratio and limited bandwidth in the seismic volume. In an effort to improve the signal and broaden the bandwidth, newly acquired seismic data over this field have employed point receiver technology, dense wavefield sampling, a full azimuth geometry, and a specially designed sweep with useful frequencies as low as three hertz. The resulting data display enhanced reflection continuity and improved resolution. With the extension of low frequencies and improved interpretability, acoustic impedance inversion results are more robust and allow greater flexibility in reservoir characterization and prediction. In addition, because inversion to acoustic impedance is no longer completely tied to a wells-only low-frequency model, there are positive implications for exploration.

Cretaceous basalts have been recovered at several Ocean Drilling Program and Deep Sea Drilling Project sites where basement of Jurassic age was predicted. Sites 800 and 802, Leg 129, both fall in this category. We have examined the paleomagnetic properties of 25 basalt samples from Site 802 in order to establish a paleolatitude for the site at the time of basalt emplacement and to compare the results to those from Deep Sea Drilling Project Site 462. Mean natural remanent magnetization intensity for the Site 802 basalts was found to be approximately 12 A/m consistent with typical oceanic basalts. Mean stable inclination is -34.7° ± 2.2 which implies a paleolatitude of approximately 19.4°S. This is very similar to the paleolatitudes calculated for Site 462 basalts and suggests-along with similarities in geochemistry, magnetic properties, and projected age of Site 802 basalt emplacement-both contemporaneity of and a possible source link between the two sites.