Magnetostratigraphy of Paleogene and upper Cretaceous sediments from Broken Ridge

Broken Ridge represents a fragment of the oceanic Kerguelen-Heard platform, constructed at high southern latitudes in the middle Cretaceous and rifted apart during the middle Eocene (43-45 Ma). The approximately 1400-m section of prerift sediments on Broken Ridge preserves a polarity sequence that spans the middle Eocene to Upper Cretaceous (Chrons C18/C20 to C34), including a 500-m continuous sequence from the lower Eocene (C23R) to the Maestrichtian/Campanian boundary (C32R). The polarity record, together with biostratigraphic data, provides a well-constrained time framework for interpreting the history of Broken Ridge. Inclinations in the lower portion of the section are generally 5°-10° shallower than the expected geocentric axial dipole inclination. Comparison of inclination and porosity changes with stratigraphic depth suggests the importance of compaction in generating these shallow inclinations. Changes in the magnetic fabric, reflected in the anisotropy of anhysteretic remanence, provide additional support for this interpretation

Magnetic petrology of ODP Leg 121 holes

Given the importance of the inversion of seamount magnetic anomalies, particularly to the motion of the Pacific plate, it is important to gain a better understanding of the nature of the magnetic source of these features. Although different in detail, Ninetyeast Ridge is composed of submarine and subaerial igneous rocks that are similar to those found at many seamounts, making it a suitable proxy. We report here on the magnetic petrology of a collection of samples from Ninetyeast Ridge in the Indian Ocean. Our purpose is to determine the relationship between primary petrology, subsequent alteration, and magnetic properties of the recovered rocks. Such information will eventually lead to a more complete understanding of the magnetization of seamounts and presumably improvements in the accuracy of anomaly inversions. Three basement sites were drilled on Ninetyeast Ridge, with recovery of subaerial basalt flows at the first two (Sites 756 and 757) and submarine massive and pillow flows at the final one (Site 758). The three sites were distinctly different. Site 756 was dominated by ilmenite. What titanomagnetite was present had undergone deuteric alteration and secondary hematite was present in many samples. The magnetization was moderate and stable although it yielded a paleolatitude somewhat lower than expected. Site 757 was highly oxidized, presumably while above sea level. It was dominated by primary titanomagnetite, which was deuterically altered. Secondary hematite was common. Magnetization was relatively weak but quite stable. The paleolatitude for all but the lowermost flows was approximately 40° lower than expected. Site 758 was also dominated by primary titanomagnetite. There was relatively little oxidation with most primary titanomagnetite showing no evidence of high-temperature alteration. No secondary hematite was in evidence. This site had the highest magnetization of the three (although somewhat low relative to other seamounts) but was relatively unstable with significant viscous remanence in many samples. Paleolatitude was close to the expected value. It is not possible, at present, to confidently associate these rocks with specific locations in a seamount structure. A possible and highly speculative model would place rocks similar to Site 757 near the top of the edifice, Site 756 lower down but still erupted above sea level, and Site 758 underlying these units, erupted while the seamount was still below sea level

Depth/Age of changes in ODP Site 121-758 (Table 1)

Magnetic susceptibility, remanence and lithostratigraphic profiles for the Neogene-Quaternary sequence at Site 758 (ODP Leg 121) on the northern Ninetyeast Ridge show distinct changes, dated from biostratigraphy and detailed magnetostratigraphy, at 17.5, 10.4-10.0, 8.8, 6.5, 5.4-5.1, 2.7-2.5, 1.9, and 1.2-1.1 Ma. These magnetic and lithologic changes appear to reflect changes in the supply and character of terrigenous material from the Himalayan-Tibetan region resulting from changes in gradient of the Ganges, Brahmaputra and probably the ancient Indus drainage systems. The sedimentary changes can be correlated with changes in uplift-sensitive markers such as the oceanic 87Sr/86Sr ratio and monsoonal induced upwelling, but not clearly so with sealevel variations. We interpret these sedimentary changes, therefore, to primarily reflect changes in the tectonic evolution of the Himalayan-Tibetan region. The changes in the distal marine sedimentary record of the northern Ninetyeast Ridge are compared with isotopic control on the timing of Himalayan-Tibetan tectonic phases and magnetostratigraphic control on their reflection in the proximal Siwalik molasse record. This comparison indicates that the distal Ninetyeast Ridge record can be used to detail and to place minimal age constraints on tectonic phases in the wider Himalayan region and on evolution of the proximal molasse sequence, with a time lag determined for the four earliest changes at less than 1 m.y. The changes at 17.5 Ma and 5.4-5.1 Ma can be interpreted in terms of the causative chain: enhanced plate motion -> uplift and sedimentation change -> climatic change, supporting arguments that the Late Cainozoic global climatic deterioration is driven by uplift of large plateaus such as the Himalayan-Tibetan region and the Western Cordillera

Mean inclination results of ODP Leg 121 samples (Table 3)

This study details the Late Cretaceous and Tertiary northward movement of the Indian plate. Breaks in India's northward movement rate are identified, dated, and correlated with the evolution of the India-Asia convergence. Paleolatitudinal constraints on the origin of Ninetyeast Ridge are discussed, and limited magnetostratigraphic detail is provided. Nearly 1500 sediment and basement samples from Sites 756, 757, and 758 on Ninetyeast Ridge were studied through detailed alternating field and thermal demagnetization. Primary and various secondary magnetization components were identified. Breakpoint intervals in the primary paleolatitude pattern for common-Site 758 were identified at 2.7, 6.7,18.5, about 53, 63.5-67, and 68-74.5 Ma. Only the breakpoint interval at about 53 Ma reliably reflects a reduction in India's northward movement rate. The onset of this probably gradual slowdown was dated at 55 Ma (minimal age) based on the intersection of weighted linear regression lines. At the location of common-Site 758, northward movement slowed from 18-19.5 cm/yr (from at least 65 to 55 Ma) to 4.5 cm/yr (from 55 to at least 20 Ma). Reanalysis of earlier DSDP/ODP paleolatitude data from the Indian plate gives a comparable date (53 Ma) for this reduction in northward velocity. Comparison of our Ninetyeast Ridge data and Himalayan paleomagnetic data indicates that the initial contact of Greater India and Asia may have already been established by Cretaceous/Tertiary boundary time. The geological record of the convergence zone and the Indian plate supports the notion that the Deccan Traps extrusion may have resulted from the ensuing deformation of the Indian plate. We interpret the breakpoint at 55+ Ma to reflect completion of the eastward progressive India-Asia suturing process. Neogene phases in the evolution of the convergence zone were correlated with significant changes in the susceptibility, NRM intensity, and lithostratigraphic profile of Site 758. These changes are interpreted to reflect and postdate tectonic phases in the evolution of the wider Himalayan and southern Tibetan region. The changes were dated and interpreted as follows: 17.5 Ma, initial uplift of the Higher Himalaya following initiation of intercontinental underthrusting; 10-10.4 Ma, increased uplift and onset of Middle Siwaliks sedimentation; 8.8 Ma, probable reduction in influx corresponding with the Nagri Formation to Dhok Pathan Formation changeover; 6.5 Ma, major tectonic phase evident throughout the wider Himalayan region and northern Indian Ocean; 5.1-5.4 Ma, onset of oroclinal bending of the Himalayan Arc, of extensional tectonism in southern Tibet, and of Upper Siwalik sedimentation; 2.5-2.7 and 1.9 Ma, major phases of uplift of the Himalayan and Tibetan region culminating in the present-day high relief. The basal ash sequence and upper flow sequence of Site 758 and the basal ash sequence of Site 757 indicate paleolatitudes at about 50°S. These support a Kerguelen hot spot origin for Ninetyeast Ridge. Consistently aberrant inclinations in the basalt sequence of Site 757 may be related to a southward ridge jump at about the time (58 Ma) that these basalts were erupted. The basalt sequence of Site 756 indicates a lower paleolatitude (about 43°S), as do parts of the basalt sequence of Site 758 which also have reversed polarity overprints. The low paleolatitudes for Site 756 may be explained by late-stage volcanism north of the Kerguelen hot spot or the influence of the Amsterdam-St. Paul hot spot