4. Site 11491

Shipboard Scientific Party2

BACKGROUND AND OBJECTIVES

In the same way as Site 801 is a reference site for Jurassic oceanic crust and overlying sediments being subducted into the Mariana Trench, Site 1149 is a reference site for Cretaceous oceanic crust and sediments being transported into the Izu-Bonin subduction zone. As discussed in the "Leg 185 Summary" chapter, the geochemical compositions of volcanic rocks erupted in the two arc systems show significant differences in key elements and, in particular, in Pb isotope compositions and Ba abundance. To understand the geochemical budgets in these arc systems, it is essential to characterize the input into the system via the subducting plate. This requires quantifying the geochemical composition of the sediments and the upper oceanic crust from which the bulk of the elements contributed from the subducting plate to the arc are derived. There is no continuously sampled section of the sediment in the Nadezhda Basin of the west Pacific Ocean that feeds the Izu-Bonin arc. In contrast sediment input to the Mariana arc has been relatively well sampled at Ocean Drilling Program (ODP) Sites 800, 801, and 802 (see Fig. F1 in the "Site 801" chapter and Fig. F2 in the "Leg 185 Summary" chapter). The volcanic basement in the Nadezhda Basin has been reached only once, at Deep Sea Drilling Project (DSDP) Site 197 (Fig. F1) where only 1 m of basalt was recovered. Site 1149 was thus selected from available seismic data at a position as close as possible to the Izu-Bonin Trench (see "Tectonic Setting and Magnetic Anomalies"). The site was chosen based on the roughness of the basement, which indicated an absence of younger igneous intrusions in the lowermost sediments. In addition, the site chosen has a well-defined sedimentary sequence undisturbed by faulting or other structural complexities in the seismic stratigraphy.

Sampling and geochemical analysis of the recovered sediments and basement altered at low temperatures will be coupled with reconstruction of the cored (recovered and not recovered) lithologies by use of cored samples and logging. This will permit the calculation of a bulk geochemical composition for Cretaceous Pacific crust and its sediment cover being subducted beneath the Izu-Bonin arc. These results will be compared with those for the crustal and sedimentary sections being subducted in the Mariana arc.

Stratigraphic columns for DSDP sites in this region of the western Pacific are given in Figure F2. These are compared with a highly simplified section of the actual sequence drilled at Site 1149. The total sediment recovered in these four DSDP drill holes in Mesozoic Pacific crust is <40 m; none recovered more than 15 m. Drilling in all of the holes ran into difficulties penetrating the Mesozoic chert, chalk, and clay horizons that are found below ~200 m of pelagic sediment. These horizons are evident as strong reflectors beginning at 150-180 mbsf on seismic profiles in the Site 1149 area (see Fig. F3). The top of basement was interpreted to be located at ~400-420 mbsf. The well-defined sediment stratigraphy at Site 1149, in particular the Mesozoic section, permits

  1. Testing of the Early Cretaceous paleomagnetic time scale. The site is on the older half of magnetic Anomaly M11 (Nakanishi et al., 1992) (see "Tectonic Setting and Magnetic Anomalies"), which should correlate to a basement age of ~132 Ma, corresponding to the late Valanginian stage of the Early Cretaceous on the Channell et al. (1995) time scale. Coring the basement and sediment would test the proposed new time scale of Channell et al. (1995).
  2. Determining Cretaceous paleolatitude history. As indicated in Figure F7 in the "Leg 185 Summary" chapter, Site 1149 may have formed at ~5S, drifted south to 10S in its early history, and then gradually drifted north, crossing the paleoequator as the Pacific plate accelerated its northward motion at ~85-90 Ma. The site may thus provide constraints on the mid-Cretaceous carbonate compensation depth (CCD) and equatorial productivity zones.
  3. Characterizing the pelagic sedimentation history and expected volcanic ashes in the upper 200-m section at Site 1149, which provide a complementary record to that recovered during Leg 124.

The location of Site 1149 is ~100 km east of the Izu-Bonin Trench (Fig. F1) where the westernmost extremity of the Pacific plate bends into the subduction zone and the plate surface is broken by normal faults (Fig. F4). The detailed geology and geophysics of the site are given in "Seismic Stratigraphy".

1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

Ms 185IR-104

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