A laboratory model of subduction zone anisotropy
A laboratory model of subduction zone anisotropy
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DOI：
10.1016/s0012-821x(98)00211-8
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发表时间：
1998-12
影响因子：
5.3
通讯作者：
J. Buttles;P. Olson
中科院分区：
文献类型：
--
作者：
J. Buttles;P. Olson
太平洋俯冲带附近的分裂地震剪切波测量表明，在俯冲板块下的海侧地幔中，快速极化方向主要是与海沟平行的，而在弧后盆地下的地幔楔中，快速极化方向是变化的。人们普遍认为，快极化方向的方位角决定了橄榄石晶体a轴的平均取向。在这里，我们使用实验室实验来确定俯冲板附近上地幔中橄榄石a轴的方向。利用悬浮在粘性流体中的小圆柱体（晶须），开发了蠕变过程中橄榄石A轴运动的实验室模拟。研究了静止上覆板块下的俯冲、变倾角和真实的回滚和下倾板块运动学。板回滚产生强大的板平行须对准所有倾角检查。楔形的排列有三层：下行板块上方的倾斜平行排列层，上覆板块下方的亚水平排列层，以及中间方向的核心。随倾角的增大，覆板和楔芯下方的对中量减小。结果表明，向海侧地幔的橄榄石a轴取向受板块回滚量的控制，而地幔楔中的橄榄石a轴取向与板块倾角有关。
Measurements of split seismic shear-waves near Pacific subduction zones show that fast polarization directions are primarily trench-parallel in the seaward-side mantle beneath a subducting slab and variable in the mantle wedge beneath back arc basins. It is generally accepted that the azimuth of the fast polarization direction defines the average orientation of the crystallographic a-axis in olivine. Here we use laboratory experiments to determine the orientation of the olivine a-axis in the upper mantle in the vicinity of a subducting slab. A laboratory analog for olivine a-axis motion during creep deformation is developed using small cylinders (whiskers) suspended in a viscous fluid. Subduction beneath a stationary overriding plate, variable dip angles and realistic rollback and down-dip plate kinematics are examined. Plate rollback produces strong plate-parallel whisker alignments for all dip angles examined. Alignment in the wedge occurs in three layers: a layer of dip-parallel alignment above the downgoing plate, a layer of subhorizontal alignment beneath the overriding plate, and a core of intermediate orientations. The amount of alignment beneath the overriding plate and the wedge core decreases with increasing dip angle. Our results indicate that olivine a-axis orientation in the seaward-side mantle is controlled by the amount of slab rollback, and that orientations in the mantle wedge depend upon slab dip angle.