Rock magnetic properties of ODP Leg 134 sites@en

During Leg 134, the influence of ridge collision and subduction on the structural evolution of island arcs was investigated by drilling at a series of sites in the collision zone between the d'Entrecasteaux Zone (DEZ) and the central New Hebrides Island Arc. The DEZ is an arcuate Eocene-Oligocene submarine volcanic chain that extends from the northern New Caledonia Ridge to the New Hebrides Trench. High magnetic susceptibilities and intensities of magnetic remanence were measured in volcanic silts, sands, siltstones, and sandstones from collision zone sites. This chapter presents the preliminary results of studies of magnetic mineralogy, magnetic properties, and magnetic fabric of sediments and rocks from Sites 827 through 830 in the collision zone. The dominant carrier of remanence in the highly magnetic sediments and sedimentary rocks in the DEZ is low-titanium titanomagnetite of variable particle size. Changes in rock magnetic properties reflect variations in the abundance and size of titanomagnetite particles, which result from differences in volcanogenic contribution and the presence or absence of graded beds. Although the anisotropy of magnetic susceptibility results are difficult to interpret in terms of regional stresses because the cores were azimuthally unoriented, the shapes of the susceptibility ellipsoids provide information about deformation style. The magnetic fabric of most samples is oblate, dominated by foliation, as is the structural fabric. The variability of degree of anisotropy (P) and a factor that measures the shape of the ellipsoid (q) reflect the patchy nature of deformation, at a micrometer scale, that is elucidated by scanning electron microscope analysis. The nature of this patchiness implies that deformation in the shear zones is accomplished primarily by motion along bedding planes, whereas the material within the beds themselves remains relatively undeformed.在第134航次(Leg 134)期间，研究团队通过在德恩特卡斯托带(d'Entrecasteaux Zone, DEZ)与中部新赫布里底岛弧(central New Hebrides Island Arc)之间的碰撞带内的一系列站位开展钻探，探究了洋脊碰撞与俯冲作用对岛弧构造演化的影响。德恩特卡斯托带是一条弧形的始新世-渐新世(Eocene-Oligocene)海底火山链，其延伸范围从新喀里多尼亚北部海岭(New Caledonia Ridge)直至新赫布里底海沟(New Hebrides Trench)。研究人员在碰撞带站位采集的火山粉砂、砂、粉砂岩和砂岩样品中，测得较高的磁化率与剩余磁化强度。本章呈现了碰撞带内827至830站位的沉积物与岩石的磁性矿物学(magnetic mineralogy)、磁学性质及磁组构(magnetic fabric)研究的初步结果。在德恩特卡斯托带的强磁性沉积物及沉积岩中，剩余磁化的主要载体为粒径不均的低钛钛磁铁矿(low-titanium titanomagnetite)。岩石磁性特征的变化反映了钛磁铁矿颗粒的丰度与粒径的差异，这一差异源于火山物质贡献量的不同以及粒序层(graded beds)的有无。尽管由于岩心方位未定向，磁化率各向异性(anisotropy of magnetic susceptibility)结果难以通过区域应力进行解释，但磁化率椭球(susceptibility ellipsoids)的形状可提供变形样式(deformation style)相关的信息。多数样品的磁组构为扁椭球形(oblate)，以面理(foliation)为主，这与构造组构(structural fabric)的特征一致。各向异性度(P)与表征椭球形状的因子(q)的变化，反映了微米尺度下变形的斑块性特征，这一点可通过扫描电子显微镜(scanning electron microscope)分析得以阐明。这种斑块性的本质表明，剪切带(shear zones)内的变形主要通过沿层理面(bedding planes)的滑移实现，而层内的物质本身则保持相对未变形的状态。