High pore pressures and porosity at 35 km depth in the Cascadia subduction zone
High pore pressures and porosity at 35 km depth in the Cascadia subduction zone
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DOI：
10.1130/g31649.1
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发表时间：
2011-05-01
期刊：
影响因子：
5.8
通讯作者：
Audet, Pascal
中科院分区：
文献类型：
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作者：
Peacock, Simon M.;Christensen, Nikolas I.;Audet, Pascal
在南温哥华岛下方25-45 km深度的Cascadia俯冲带，转换远震波显示出一个类似于5 km厚的向陆倾层，异常高Vp/Vs平均为2.35 +/- 0.10 (2 sigma)，解释为Juan de Fuca板块的洋壳俯冲。这一层是推断的锁定发震带的下倾，位于间歇性震颤和滑动区域。在200 MPa围压和高孔隙压力条件下结晶岩样的实验室速度测量表明，Vp/Vs随充液孔隙度的增加而增加。观察到的高Vp/Vs值最好的解释是在接近静岩压力下的孔隙流体，孔隙度为2.7% ~ 4.0%。根据胡安·德·富卡地壳和地幔俯冲过程中变质流体产生的合理速率(约10(-4)m(3)/(m(2) yr)计算，如此大量的流体大约需要1立方米才能积聚起来。因此，在这些深度处，板界面的渗透率必须非常低，类似于10（-24）到10(-21)m(2)，或者多孔层的渗透率必须小于3 × 10(-20) m(2)。
In the Cascadia subduction zone, beneath southern Vancouver Island at 25-45 km depth, converted teleseismic waves reveal an similar to 5-km-thick landward-dipping layer with anomalously high Vp/Vs averaging 2.35 +/- 0.10 (2 sigma), interpreted as subducted oceanic crust of the Juan de Fuca plate. This layer is observed downdip of the inferred locked seismogenic zone, in the region of episodic tremor and slip. Laboratory velocity measurements of crystalline rock samples made at 200 MPa confining pressure and elevated pore pressures demonstrate that Vp/Vs increases with increasing fluid-filled porosity. The observed high Vp/Vs values are best explained by pore fluids under near lithostatic pressure in a layer with a high porosity of 2.7%-4.0%. Such large volumes of fluid take similar to 1 m.y. to accumulate based on reasonable rates of metamorphic fluid production of similar to 10(-4) m(3)/(m(2) yr) in subducting Juan de Fuca crust and mantle. Accordingly, the permeability of the plate interface at these depths must be very low, similar to 10(-24) to similar to 10(-21) m(2), or the porous layer must have a permeability < 3 x 10(-20) m(2).