In this manuscript, we present the results of a physical properties investigation carried out on basaltic cores recovered from the four Leg 192 basement sites, focusing on the relationship between physical properties and alteration in basalts. Variations in physical properties in the Leg 192 basement sites closely resemble each other and reflect the amount of alteration and vein formation in the basement basalts. Pwave velocities, magnetic susceptibilities, and densities for the dense massive basalts are higher than those of more altered and heavily veined basalts. Porosity-dependent alteration is observed at Leg 192 basement sites: P-wave velocity displays a general decrease with increasing loss on ignition and potassium content. These trends are consistent with trends documented for typical alteration of oceanic crust and suggest that basalt alteration is largely responsible for the variation of the physical properties exhibited by rocks at Leg 192 basement sites. Our physical property data support the conclusion that only low-temperature seawater-mediated alteration occurred in the lava flows of the Ontong Java Plateau (OJP). This lack of higher-temperature hydrothermal alteration is consistent with the idea that the OJP basement sites are far from their eruptive vents. 1Zhao, X., Antretter, M., Kroenke, L., Riisager, P., and Hall, S., 2004. Relationships between physical properties and alteration in basement rocks from the Ontong Java Plateau. In Fitton, J.G., Mahoney, J.J., Wallace, P.J., and Saunders, A.D. (Eds.), Proc. ODP, Sci. Results, 192, 1–33 [Online]. Available from World Wide Web: . [Cited YYYYMM-DD] 2Center for Study of Imaging and Dynamics of the Earth, Institute of Geophysics and Planetary Physics, University of California, Santa Cruz CA 95064, USA. Correspondence author: xzhao@es.ucsc.edu 3Institut fur Geophysik, University of Munchen, Theresienstrasse 41, D80333 Munchen, Germany. 4Institute of Geophysics, University of Hawaii, Honolulu HI 96822, USA. 5Danish Lithosphere Centre, Oster Voldgade 10, DK-1350 Copenhagen K, Denmark. 6Department of Geosciences, University of Houston, Houston TX 77204-5007, USA. Initial receipt: 30 December 2003 Acceptance: 6 July 2004 Web publication: 14 September 2004 Ms 192SR-109 X. ZHAO ET AL. PHYSICAL PROPERTIES AND ALTERATION 2
Paleomagnetic study of Miocene basalts and baked sediments from Velay Oriental, France, yields a paleomagnetic pole with coordinates 84.1°N, 171.2°E (A 95 = 8.6°, K = 29, N = 11). The pole is supported by a positive reversal test, and it corresponds well with the apparent polar wander path for Eurasia for 10 Ma. Paleosecular variation, estimated as the angular standard deviation of the VGP distribution 14.9° +6.5°/−3.5°, is close to expected, suggesting that the paleomagnetic pole represents a time‐averaged field. Moreover, five new 40 Ar/ 39 Ar plateau ages, falling between 9.20 and 13.55 Ma, were obtained. Thellier paleointensity experiments were carried out on both basalts and baked sediments. Almost all basalts are chemically unstable during the laboratory heatings and of the 36 preselected samples only three yield usable results. In contrast, the baked sediments are ideal for Thellier experiments with 11 out of 13 samples giving reliable paleointensity estimates. Rock magnetic experiments show that the main ferromagnetic minerals in the baked sediments are fine‐grained magnetite and hematite with a large proportion of superparamagnetic grains. In addition to the standard partial thermoremanent magnetization (pTRM) check, used in Thellier experiments to detect chemical/crystalline alteration, we introduce a second “pTRM tail” check designed to test the basic prerequisite of the method, namely, the equality of unblocking and blocking temperatures. For the baked sediments, unblocking and blocking temperatures are found to be identical, which indicates that the remanence is carried by single‐domain grains.
We present palaeomagnetic inclinations from Late Cretaceous-Early Tertiary deep-water carbonates obtained during ODP Leg 192 drilling at the Ontong Java Plateau (OJP) for the following periods: magnetochrons C33n (73.6-79.1 Ma), C32 (71.1-73.6 Ma), C27r-C31n (61.3-68.7 Ma) and C25-C26 (55.9-60.9 Ma). Compaction-induced inclination shallowing is considered to be negligible for the OJP sediments examined here because: (i) their palaeomagnetic inclinations are in excellent accord with those recently obtained from underlying OJP basement rocks, (ii) the studied sediments have anisotropy of anhysteretic remanent magnetization fabrics that appear poorly correlated with individual characteristic remanent magnetization inclinations and (iii) in the few cases where we observe a significant difference between our new OJP sedimentary palaeomagnetic data and data from other parts of the Pacific Plate, obtained mainly from seamount magnetization studies and skewness analyses, the sedimentary inclinations are not systematically lower, and therefore, cannot be explained in terms of inclination shallowing. Combining our new data with existing OJP palaeomagnetic data we obtain an internally consistent data set that we interpret to indicate northward motion of the plateau from its formation at similar to 120 Ma until 55 Ma. At 120 Ma the central plateau was located at 24 degrees +/- 4 degrees S. Approximately 45 Myr later (similar to 76 Ma) the plateau is located near 21 degrees S. Our data, therefore, indicate that the OJP was either stationary or exhibited a slow northward latitudinal drift during this interval. In contrast, from similar to 76 Ma until 68 Ma the plateau moved rapidly through approximately 10 degrees of latitude to similar to 10 degrees S. These intervals of slow and rapid motion track similar motions previously proposed for the entire Pacific Plate during the Cretaceous but extends the interval of slow motion into the Late Cretaceous. From 68 Ma until 56 Ma the plateau was again almost latitudinally stationary before moving slowly northward. More direct palaeomagnetic data are necessary to better define Pacific Plate motions during this time interval.
The 1996 summer season saw continued petroleum geological activities in the Disko–Nuussuaq area, onshore West Greenland. These took the form of a geological field project led by the Geological Survey of Denmark and Greenland (GEUS), and continued commercial exploration by grønArctic Energy Inc. (grønArctic). In the second year of their licence, grønArctic carried out an airborne geophysical programme early in 1996 and drilled a c. 3 km deep exploration well on Nuussuaq, GRO#3, in the late summer (Fig. 1). Although the detailed results from grønArctic’s exploration are confidential (apart from the information made available at conferences and in press releases), it is evident that knowledge of the Nuussuaq Basin has greatly increased in recent years and that the basin has considerable exploration potential of its own (see Christiansen et al., 1995b, 1996a). The activities by GEUS and the exploration by grønArctic will significantly improve the understanding of the petroleum system of the basin; available data from the 1996 activities have shed light on the types and distribution of oils, source rocks and potential reservoir units.
A palaeomagnetic study comprising the directional results from 289 individual lava flows, sampled along eight sections in the Palaeocene basalts of West Greenland, is reported. The eight individual sections are correlated using lithostratigraphical marker horizons to form a single composite profile. Generally, the lithological correlation is in good agreement with the record of geomagnetic secular variation. The total composite palaeomagnetic profile represents a stratigraphic thickness of 1.6 km through the Vaigat Formation, which is the lowermost of the two volcanic formations formed during the main stage of plateau volcanism. Only two polarity zones are found in the composite profile, suggesting a very short duration for the West Greenland main plateau-building volcanism. 40Ar/39Ar dates support a high extrusion rate and also indicate that the lower normal polarity zone is Chron C27n and that the upper reverse polarity zone is Chron C26r. The C27n–C26r transition is fully recorded along one of the sections (Nuusap Qaqqarsua), with intermediate directions covering a 200 m thick succession of lavas. A combined palaeomagnetic, field and geochemical study along this profile showed good agreement; that is, geochemically and geologically derived single magmatic events show groupings of the palaeomagnetic directions. Supposing a duration for the geomagnetic transition of 5000 years, the eruption frequency during this period was as high as one flow every 80 years.
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