Segmentation of plate coupling, fate of subduction fluids, and modes of arc magmatism in Cascadia, inferred from magnetotelluric resistivity
Segmentation of plate coupling, fate of subduction fluids, and modes of arc magmatism in Cascadia, inferred from magnetotelluric resistivity
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DOI：
10.1002/2014gc005509
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发表时间：
2014-11
期刊：
影响因子：
3.7
通讯作者：
P. Wannamaker;Rob L. Evans;P. Bedrosian;M. Unsworth;Virginie Maris;R. Mcgary
中科院分区：
文献类型：
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作者：
P. Wannamaker;Rob L. Evans;P. Bedrosian;M. Unsworth;Virginie Maris;R. Mcgary
在卡斯卡迪亚俯冲系统中采集了五个大地电磁（MT）剖面，并使用二维和三维非线性反演进行了转换，以产生深度为200 km的电阻率横截面。板块耦合、俯冲流体演化和沿着卡斯卡迪亚长度的弧岩浆活动模式的明显变化在电阻率结构中得到了清楚的表达。卡斯卡迪亚北方和南方海岸下相对较高的渗透率与推测的板块锁定程度升高相关，并表明流体和沉积物缺乏的条件。相比之下，俄勒冈州中北部海岸结构从板块界面到近海浅层具有很强的传导性，这与板块锁定不良和可能存在俯冲沉积物有关。在所有剖面上，低电阻率流体化带从弧以西100 km处开始，在35-40 km的板片深度处发育，并被解释为代表俯冲板片的俯冲变质流体释放。流体上升到弧前莫霍面水平，有时更浅，因为弧接近。这些区域开始靠近低频地震群，表明流体控制了向稳定滑动的过渡。根据北方和南部卡斯卡迪亚弧段，低上地幔热膨胀率是一致的通量熔融以上的板加上可能的深对流弧背上涌向弧。在卡斯卡迪亚中部，伸展变形被解释为分离上地幔熔体导致底侵和低渗透率在莫霍面下地壳水平低于弧和附近的弧后。低温到高温的地幔楔过渡位于弧的稍微向沟的方向。
Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2‐D and 3‐D nonlinear inversion to yield electrical resistivity cross sections to depths of ∼200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid‐ and sediment‐deficient conditions. In contrast, the north‐central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low‐resistivity fluidized zones develop at slab depths of 35–40 km starting ∼100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low‐frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and nearby backarc. The low‐ to high‐temperature mantle wedge transition lies slightly trenchward of the arc.