Subcretionary tectonics: Linking variability in the expression of subduction along the Cascadia forearc
Subcretionary tectonics: Linking variability in the expression of subduction along the Cascadia forearc
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DOI：
10.1016/j.epsl.2020.116724
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发表时间：
2021-01-06
影响因子：
5.3
通讯作者：
Levander, Alan
中科院分区：
文献类型：
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作者：
Delph, Jonathan R.;Thomas, Amanda M.;Levander, Alan
许多地震和其他地球物理现象沿着卡斯卡迪亚前弧形走向显示出显著的横向非均质性。在控制孕震行为、位场测量、形态/构造特征和地震构造之间的沿走向相关性方面，盖覆板块和下行板块都发挥了主导作用，但两者之间可能存在显著的反馈。在这项研究中，我们对接收函数数据进行了三维速度校正，并与最近发表的横波速度模型一起解释了由此产生的不连续模型，以了解相关的沿走向变化之间可能的因果关系。我们的不连续性模型表明，弧前地壳在接近地幔楔形角时增厚，然后向岩浆弧逐渐变薄，这可能是由于物质从下行板块向上覆板块的底部吸积。在前弧的北部和南部，这种“地下”物质的特征是厚的(类似于10公里)异常低的剪切波速度带。与低速带有关的厚度、高内反射率和低布格重力特征可能表明，这种地下物质主要由(变质)沉积物质组成，这些物质是在地质时间尺度上通过连续的俯冲事件侵位的。此外，异常低的速度和空间相关性与高的非火山震动(NVT)密度和短的慢滑复发间隔表明这些地区是流体丰富的地区。尽管控制NVT和缓慢滑动的流体的一阶变化可能是由于下行板块的渗透性差异(从其应力状态和实验室内地震活动的分布推断)，但这些地下封套可能代表了下地壳中厚的、垂直不透水的区域，进一步加强了这种相关性。沉降量的变异性解释了沿着卡斯卡迪亚边缘的折返和抬升模式，以及在地质时间尺度上所产生的弧前地形，可能受某种组合的板块界面几何/流变学和覆盖板块结构的控制。(C)2020爱思唯尔B.V.保留所有权利。
A number of seismic and other geophysical phenomena exhibit significant lateral heterogeneity along the strike of the Cascadia forearc. Both the overriding and downgoing plate have been invoked to play the dominant role in controlling along-strike correlations between seismogenic behavior, potential field measurements, morphological/tectonic characteristics, and seismic structure; however, significant feedbacks likely exist between the two. In this study, we apply a 3D velocity correction to receiver function data and interpret the resulting discontinuity model alongside a recently published shearwave velocity model to understand the possible causative relationships between correlative along-strike variations. Our discontinuity model indicates that the forearc crust thickens as it approaches the mantle wedge corner before progressively thinning toward the magmatic arc, likely due to the basal accretion of material from the downgoing plate to the overriding plate. In the northern and southern portions of the forearc, this "subcreted" material is characterized by thick (similar to 10km) anomalously low shear-wave velocity zones. The thickness, high internal reflectivity, and low Bouguer gravity signatures associated with the low velocity zones likelyindicate that this subcreted material is composed of dominantly (meta)sedimentary material that has been emplaced through successive subcretion events over geologic timescales. Furthermore, the anomalously low velocities and spatial correlation with high non-volcanic tremor (NVT) density and short slow slip recurrence intervals indicate that these regions are fluid-rich. While first-order variations in the fluids that control NVT and slow slip likely result from differences in the permeability of the downgoing slab as inferred from its stress state and the distribution of intraslab seismicity, these subcreted packages likely represent thick, vertically-impermeable regions in the lower crust that further accentuate this correlation. Variability in the amount of subcretion explains patterns of exhumation and uplift along the Cascadia margin and the resulting forearc topography over geologic timescales, and is likely controlled by some combination plate interface geometry/rheology and overriding plate architecture. (C) 2020 Elsevier B.V. All rights reserved.