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.

本研究详述了印度板块（Indian plate）在晚白垩世（Late Cretaceous）与第三纪（Tertiary）期间的北向运动。研究识别并厘定了印度板块北向运动速率的间断点时代，并将其与印-亚汇聚（India-Asia convergence）演化过程相对比。文中探讨了九十度海岭（Ninetyeast Ridge）成因的古纬度约束条件，并提供了有限的磁地层学细节。
研究团队对九十度海岭756、757、758站位的近1500件沉积物与基底样品开展了系统的交变场与热退磁实验，识别出原生磁化分量与多种次生磁化分量。针对758站位的原生古纬度模式，其间断点区间分别位于2.7 Ma、6.7 Ma、18.5 Ma、约53 Ma、63.5~67 Ma以及68~74.5 Ma。其中仅约53 Ma的间断点区间可可靠反映印度板块北向运动速率的减缓。基于加权线性回归线的交点，该逐渐减缓过程的起始年代被限定为55 Ma（最小年龄）。在758站位所在区域，印度板块北向运动速率从至少65 Ma至55 Ma期间的18~19.5 cm/yr，降至55 Ma至至少20 Ma期间的4.5 cm/yr。对早前深海钻探计划（Deep Sea Drilling Project, DSDP）/大洋钻探计划（Ocean Drilling Program, ODP）获取的印度板块古纬度数据的重新分析结果显示，该北向运动速率减缓的年代与前述结果相当，为53 Ma。
将本次研究获取的九十度海岭数据与喜马拉雅古地磁数据对比后可知，大印度板块（Greater India）与亚洲板块的初始接触可能已于白垩纪-第三纪界线时期形成。汇聚带与印度板块的地质记录支持以下观点：德干暗色岩（Deccan Traps）的喷溢可能源于印度板块随后发生的变形。我们认为，55+ Ma处的间断点反映了印-亚缝合作用向东渐进完成的过程。
汇聚带演化的新近纪（Neogene）阶段与758站位的磁化率（susceptibility）、天然剩余磁化强度（Natural Remanent Magnetization, NRM）以及岩石地层剖面的显著变化具有相关性。这些变化被解释为更广泛的喜马拉雅与藏南地区构造阶段的反映，且滞后于该构造阶段。研究对这些变化进行了定年与解释：17.5 Ma，伴随陆间俯冲作用（intercontinental underthrusting）启动，高喜马拉雅（Higher Himalaya）开始初始隆升；10~10.4 Ma，隆升作用加剧，西瓦里克群中段（Middle Siwaliks）沉积作用启动；8.8 Ma，沉积物输入量可能出现下降，对应纳格里组（Nagri Formation）与多克帕坦组（Dhok Pathan Formation）的地层转换；6.5 Ma，整个更广范围的喜马拉雅地区与北印度洋均出现显著构造阶段；5.1~5.4 Ma，喜马拉雅弧（Himalayan Arc）开始弧形弯曲，藏南地区伸展构造作用（extensional tectonism）启动，同时西瓦里克群上段沉积作用开始；2.5~2.7 Ma与1.9 Ma，喜马拉雅与藏南地区的隆升作用进入主要阶段，最终形成现今的高地形起伏。
758站位的底部火山灰层序与上部熔岩流层序，以及757站位的底部火山灰层序，均指示其古纬度位于南纬约50°，这支持九十度海岭起源于凯尔盖朗热点（Kerguelen hot spot）的假说。757站位玄武岩层序中持续出现的异常倾角，可能与这些玄武岩喷发时期（约58 Ma）发生的海岭向南跃迁有关。756站位的玄武岩层序指示其古纬度较低（约南纬43°），758站位部分同样带有反向极性叠加的玄武岩层序亦呈现出该特征。756站位的低古纬度特征可通过凯尔盖朗热点北侧的晚期火山活动，或阿姆斯特丹-圣保罗热点（Amsterdam-St. Paul hot spot）的影响加以解释。