Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples
Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples
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DOI：
10.1016/j.epsl.2009.08.007
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发表时间：
2009-09-30
影响因子：
5.3
通讯作者：
Searle, R. C.
中科院分区：
文献类型：
--
作者：
Morris, A.;Gee, J. S.;Searle, R. C.
大洋核杂岩通过大型拆离断层下盘的构造剥露作用，使下地壳和上地幔岩石暴露在海底。这些构造在慢速和超慢速扩张的大洋地壳中普遍存在，表明它们容纳了板块离散的重要部分。然而，大洋核杂岩中拆离断层的地下几何形态仍不清楚。相互竞争的模型包括：(a)在平面状、低角度断层上发生位移，几乎没有构造旋转；(b)由于挠曲卸载，初始陡倾断层发生旋转而逐渐变浅（“滚动枢纽”模型）。我们利用古地磁剩磁作为构造旋转的标志来解决这一争论，研究对象是大西洋中脊（MAR）亚特兰蒂斯海山大洋核杂岩中通过综合大洋钻探计划（IODP）采样获取的一段独特的1.4千米长的辉长岩下盘剖面。这些岩石包含了复杂的多极性磁化记录，与采样剖面中的蚀变和火成地层无关，推测是下盘剖面在地磁极性时标C1r.2r、C1r.1n（哈拉米洛）和C1r.1r期间逐渐冷却的结果。我们首次通过将单个岩芯块中的构造与从钻孔壁定向图像中识别出的构造相关联，将最初方位不受约束的下地壳辉长岩岩芯样本独立地重新定向到真实的地理参考框架。这使得古地磁数据能够重新定向，对构造历史施加了比仅使用古地磁倾角数据更严格的约束。对重新定向的高温反向磁化分量的分析表明，下盘围绕一条走向为011°±6°的与MAR平行的水平轴发生了46°±6°的逆时针旋转。重新定向的低温正常极性和反向极性分量表明，这种旋转大部分发生在哈拉米洛时标结束（0.99 Ma）之后。这些数据明确证实了大洋核杂岩的挠曲、滚动枢纽模型的关键预测，即大洋拆离断层以较高倾角起始，并随着位移增加旋转到现今的低角度几何形态。（C）2009 Elsevier B.V.保留所有权利。
Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the "rolling-hinge" model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46 degrees +/- 6 degrees anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011 degrees +/- 6 degrees. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases. (C) 2009 Elsevier B.V. All rights reserved.