【作者】 于大勇；

【导师】 王良书； 赵大鹏；

【作者基本信息】 南京大学 ， 构造地质学， 2014， 博士

【摘要】 地震波各向异性是地球内部应变状态及动力学过程的“示踪剂”,层析成像是进行三维尺度深部结构研究的有效手段,P波各向异性层析成像方法能同时获取深部三维尺度的P波各向异性和波速结构。本文采用P波各向异性层析成像方法对日本中部和青藏东北缘地区进行了研究。结果表明：日本中部地区岛弧火山下方及地幔楔内可见大量低速、高泊松比异常,很多低速异常在深部都起源于俯冲的太平洋板块和菲律宾海板块。推测是由于板块在俯冲过程中发生脱水作用引起上伏地幔部分熔融造成的。岛弧地壳、菲律宾海板块及太平洋板块上部都具有较强的各向异性。俯冲带地幔楔的顶部与底部存在较强的各向异性层,而地幔楔内部各向异性较弱,这可能是由于地幔楔内不同构造部位的变形机制不同造成的。日本中部地区地幔楔的快波方向总体上在弧前地区平行于海沟,在弧后地区垂直于海沟。原因可能与弧前相对富水区橄榄石发育B型晶格优势方位,弧后相对贫水区发育橄榄石A型晶格优势方位,加之地幔流的发生有关。日本中部东海地区下方,俯冲的菲律宾海板块表现为一个明显的低角度北倾高速异常体,并在其最北端,俯冲角度陡然增大。快波方向为NEE至NE-SW方向,与菲律宾海板块形成时的洋底扩张方向一致。表明菲律宾海板块可能保存了其“化石”各向异性。约60～120公里深度,处于弧前区且具有不同俯冲倾向的太平洋板块,快波方向分别平行于其各自俯冲的日本海沟和伊豆-小笠原海沟；而到100～120公里深度以下,太平洋板块基本都处于弧后区,快波方向也转变为分别垂直于日本海沟和伊豆-小笠原海沟。经过与中生代海底磁异常条带及太平洋板块内应力方向的对比,认为太平洋板块内部的“化石”各向异性己被板块内的应力改造。青藏东北缘地区地壳内波速异常存在明显的横向差异,各向异性在地壳内广泛存在。自浅至深各向异性幅度逐渐增大。快波方向大多与断裂和山体等构造线的走向接近,表明在地壳深度内,形状优势方位是大多数各向异性的成因。松潘—甘孜块体内,夹在龙日坝断裂带和龙门山断裂带之间的龙门山次级块体下地壳25～35公里左右深度可能存在一个厚度不大的高速异常体,具有与松潘—甘孜块体一致的NW-SE快波方向,自龙门山次级块体下地壳低角度逆冲进四川盆地西缘下地壳,其前锋已经到达四川盆地西部龙泉山断裂下方25公里左右深度,构成松潘—甘孜向四川盆地逆冲的深部逆冲构造。秦岭造山带内佛坪反向推覆构造花岗岩带在层析成像结果中表现为108°E附近自浅至深都具有N-S向快波方向的低速异常；武当块体连同扬子北缘的黄陵背斜—神农架群在25公里之下,表现为具有复杂多变快波方向、呈近圆柱状的低速异常,可能与武当块体本身的穹窿构造有关。华北中部造山带下地壳至上地幔为低速异常,快波方向总体上为NE-SW,与该区域断裂走向一致。其低速异常可能是由华北岩石圈拆沉造成下部软流圈物质上涌引起的,NE-SW的快波方向与区域性断裂走向一致,表明形状优势方位仍是该区域各向异性的主要成因。利用各向异性层析成像方法对日本中部和青藏东北缘的研究同时表明在数据充分的情况下,P波各向异性层析成像方法是进行地壳上地幔结构和各向异性研究的有效手段。更多还原

【Abstract】 Seismic anisotropy is an indicator of stress state and geodynamics of the Earth’s interior. Seismic tomography is a powerful method to map out the structural heterogeneities in Earth. P-wave anisotropic tomography can determine the anisotropies and the structural heterogeneities at the same time. Central Japan and northeastern margin of Tibetan Plateau were studied by using P-wave anisotropic tomography method and the results can be summarized as follows:Beneath central Japan, lots of low-velocity (low-V) and high Poisson’s ratio anomalies are visible beneath the arc volcanoes and within the mantel wedge and most of them origin from the subducting Pacific slab (PAC) and Philippine Sea slab (PHS). The partial melting of the upper mantle initialed by the dehydration of the underlain slab during subduction may be responsible for it. Anisotropy distribute widely beneath central Japan except for the central portion of the mantle wedge. The anisotropic mantle layer may exist on the top and the bottom of the mantle wedge. Partial melts and the domination of diffusion creep deformation which both weaken the olivine lattice-preferred orientation (LPO) may be responsible for the relatively isotropic central portion of mantle wedge. The fast-velocity directions (FVDs) of the mantle wedge beneath central Japan are generally trench-parallel in the fore-arc area and trench-normal in the back-arc area, which can be explained by the LPO of olivine changing from B-type under the fore-arc area to A-type under the back-arc area with the variation of water content and the occurrence of mantle flow. The subducting PHS slab beneath Tokai is revealed as a high-velocity (high-V) anomaly descending gently with relatively larger anisotropic amplitudes than the surroundings and with NEE to NE-SW FVDs which are consistent with the spreading direction of the PHS during its formation. The NEE to NE-SW FVDs of the PHS are inferred as the fossil anisotropy of the PHS. The subduction of the PHS beneath Tokai may have no influences on the anisotropy kept in the PHS slab. The anisotropy of the upper portion of the Pacific slab is revealed in this study. The FVDs of Japan slab and Izu-Bonin slab are sub-parallel to the Japan Trench and Izu-Bonin Trench respectively from～60to120km and change to trench-normal below100-120km. After comparisons with the Mesozoic magnetic anomaly lineations and the principle stresses in the Pacific slab, we propose that the fossil FVDs of the Pacific slab beneath central Japan were already rebuilt by the principle stresses in the slab.Obvious lateral velocity heterogeneities and widely distributed anisotropies manifest in the crust of northeastern margin of Tibetan Plateau. Along with the depth increasing, the anisotropic magnitude increases. Most of FVDs are sub-parallel to the strikes of tectonic lines (e.g., faults and mountains), which indicates the shape-preferred orientations (SPOs) are responsible for the anisotropies. Beneath Longmenshan sub-block, a thin high-V anomaly with NW-SE FVDs which is the same with FVDs of Songpan-Ganzi block has thrust into the west margin of Sichuan Basin block with low angles and has reached the lower crust at about25km depth beneath Longquanshan fault. It forms a thrust structure from Songpan-Ganzi block to Sichuan Basin block in crustal scale. The low-V anomaly nearby108°E with N-S FVDs corresponds to the Foping reverse thrust tectonic granite belt of Qinling orogen.The cylindrical low-V anomaly with variant FVDs manifests beneath the Wudang terrain and adjacent Huangling anticline and Shennongjia Group below25km depth may be attributed to the dome structure of Wudang terrain. Significant low-V anomalies with NE-SW FVDs are visible at lower crust to uppermost mantle depth beneath trans-North China orogen. Low-V anomaly may be related to the upwelling of asthenospheric material initiated by the delamination of the lithosphere of North China Craton. The NE-SW FVDs share the same orientation with regional normal faults, which indicates the SPOs are still the main reasons for the anisotropies beneath trans-North China orogen.The anisotropic tomography studies of central Japan and northeastern margin of Tibetan Plateau also indicate that with sufficient data the anisotropic tomography method is an effective way to study structures and anisotropies of crust and upper mantle.更多还原