Deep seismic structure of the Tonga subduction zone: Implications for mantle hydration, tectonic erosion, and arc magmatism
Deep seismic structure of the Tonga subduction zone: Implications for mantle hydration, tectonic erosion, and arc magmatism
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DOI：
10.1029/2011jb008434
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发表时间：
2011-10-13
影响因子：
3.9
通讯作者：
Papenberg, Cord
中科院分区：
文献类型：
--
作者：
Contreras-Reyes, Eduardo;Grevemeyer, Ingo;Papenberg, Cord
我们提出了第一个详细的2D地震层析成像的沟外隆起，前和后弧的汤加俯冲带。研究区域位于路易斯维尔热点轨迹和覆盖的印度-澳大利亚板块之间碰撞以北约100公里处，类似于汤加海沟80 Ma的太平洋板块俯冲。在外隆区，上大洋板块普遍断裂，最有可能的水化表现为伸展弯曲相关的断层，巨大的地垒和地堑结构，以及地壳和地幔速度的降低。所提出的二维速度模型显示，最上层地幔的速度类似于7.3 km/s，比地幔橄榄岩的典型速度低10%（类似于30%的地幔蛇纹石化）。在该模型中，汤加岛弧地壳厚度在7至20公里之间，速度是典型的弧型火成岩基底，最高和最低地壳速度分别为3.5和7.1公里/秒。然而，在海沟内侧斜坡之下，低速区（4.0-5.5 km/s）的存在表明，外弧前可能是流体饱和的，变质的，并由于前缘和基底侵蚀而破裂而解体。路易斯维尔岭的俯冲很可能加速了构造侵蚀，导致地壳变薄和外前弧沉降。外弧前的伸展表现为（1）向沟倾斜的正断层和（2）存在一个巨大的陡坎（类似于2 km的偏移量和几百公里长），表明最外弧前块体的重力塌陷。此外，俯冲板片和覆盖弧壳之间的接触只有20公里宽，地幔楔的特点是类似于7.5公里/秒的低速，这表明上地幔蛇纹化或存在熔融冻结在地幔。
We present the first detailed 2D seismic tomographic image of the trench-outer rise, fore- and back-arc of the Tonga subduction zone. The study area is located approximately 100 km north of the collision between the Louisville hot spot track and the overriding Indo-Australian plate where similar to 80 Ma old oceanic Pacific plate subducts at the Tonga Trench. In the outer rise region, the upper oceanic plate is pervasively fractured and most likely hydrated as demonstrated by extensional bending-related faults, anomalously large horst and graben structures, and a reduction of both crustal and mantle velocities. The 2D velocity model presented shows uppermost mantle velocities of similar to 7.3 km/s, similar to 10% lower than typical for mantle peridotite (similar to 30% mantle serpentinization). In the model, Tonga arc crust ranges between 7 and 20 km in thickness, and velocities are typical of arc-type igneous basement with uppermost and lowermost crustal velocities of similar to 3.5 and similar to 7.1 km/s, respectively. Beneath the inner trench slope, however, the presence of a low velocity zone (4.0-5.5 km/s) suggests that the outer fore-arc is probably fluid-saturated, metamorphosed and disaggregated by fracturing as a consequence of frontal and basal erosion. Tectonic erosion has, most likely, been accelerated by the subduction of the Louisville Ridge, causing crustal thinning and subsidence of the outer fore-arc. Extension in the outer fore-arc is evidenced by (1) trenchward-dipping normal faults and (2) the presence of a giant scarp (similar to 2 km offset and several hundred kilometers long) indicating gravitational collapse of the outermost fore-arc block. In addition, the contact between the subducting slab and the overriding arc crust is only 20 km wide, and the mantle wedge is characterized by low velocities of similar to 7.5 km/s, suggesting upper mantle serpentinization or the presence of melts frozen in the mantle.