Paleomagnetic and Rock Magnetic Properties of Jurassic Quiet Zone Basalts, Hole 801C . . .

Abstract

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 ...

Downhole magnetic measurements of ODP Hole 801C: Implications for Pacific oceanic crust and magnetic field behavior in the Middle Jurassic

“…We agree with this interpretation and suggest that it is supported by the remainder of the magnetic logging data below 587 mbsf. Thus the interval from 550 to 583 mbsf is mainly reversely magnetized and the interval below 583 mbsf is normally magnetized (Figures 4a and 4b), which agrees with the polarities assigned to the paleomagnetic samples from those intervals [ Wallick and Steiner , 1992]. We extend the polarity estimation analysis of Ito et al [1995] to the entire downhole log record.…”

“…Downhole magnetic anomaly field for the (a) vertical and (b) horizontal magnetic field components, (c) polarity estimates based on the sign of the ratio of the horizontal to vertical components with a total field vector magnitude threshold of 2000 nT (red circles indicate normal polarity, blue circles indicate reverse polarity), and the polarity unit summary from previous work and this study, (d) calculated apparent inclination (black dots), where red dots indicate normal polarity and blue dots are reverse polarity estimates, (e) shipboard rock core NRM measurements of continuous core from Leg 185 ( http://www.oceandrilling.org) and discrete sample measurements for Leg 129 [ Wallick and Steiner , 1992], (f) diagrammatic version of computed inclination with red arrows (left‐pointing) representing normal polarity and blue arrows (right‐pointing) representing reverse polarity, and (g) lithologic sequence identification for reference. The red dashed line indicates the depth limit of the previous work of Ito et al [1995].…”

“…We used an analytic approach to model the effects of a borehole field due to a sequence of thin horizontal layers of magnetized material [ Daly and Tabbagh , 1988]. We ran two models each with a constant magnetization: one was at 5 A/m compatible with the NRM values reported by Wallick and Steiner [1992] and the other was at 10 A/m simulating stronger zones of magnetization (Figure 9). We used a smoothed version of the apparent inclination data with the same geometry for the strike of the rotation axis and declination as described earlier.…”

Deep‐tow magnetic anomaly study of the Pacific Jurassic Quiet Zone and implications for the geomagnetic polarity reversal timescale and geomagnetic field behavior

“…Paleomagnetic and downhole logging data are available from the 474‐m basalt section cored at Hole 801C. Both data sets imply six reversals in the section [ Wallick and Steiner , 1982; Steiner , 2001; Tivey et al , 2005]. Four polarity changes are found within the upper, 150‐m, off axis lava sequence while two polarity changes are found within the lower lava sequence, which was presumably accreted at or near the spreading axis.…”

Neotectonic Controls on Petroleum Accumulations, Offshore China

“…As part of the Eurasian plate, the eastern China block was later subducted by the Pacific plate, inducing the Yanshanian orogeny. The timing of this subduction has been estimated from magnetic anomalies and began no earlier than Late Jurassic (Wallick and Steiner, 1992). Due to the subduction of the Kula/Pacific ridge at about 45 Ma, convergence of the Pacific plate changed from a NNW to a more westerly direction (Hilde et al, 1977).…”