Subsidence of a volcanic basin by flexure and lower crustal flow: The eastern Snake River Plain, Idaho
Subsidence of a volcanic basin by flexure and lower crustal flow: The eastern Snake River Plain, Idaho
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火山盆地因弯曲和下地壳流动而沉降：爱达荷州东部斯内克河平原
DOI：
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发表时间：
1998
期刊：
影响因子：
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通讯作者：
D. Rodgers
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文献类型：
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作者：
N. McQuarrie;D. Rodgers
东蛇河平原（ESRP）是一个线性火山盆地，许多工作者解释为反映了晚新生代北美在黄石热点上的迁移。这个火山区相对于黄石公园的热沉降已经被几个工作者记录下来，但没有人描述过与邻近的盆地和山脉区有关的沉降。本文记录了沿ESRP西北边缘沿着的地壳挠曲，使用挠曲来模拟盆地下方密集载荷的尺寸，并提供了支持密度驱动沉降和远离盆地的下地壳流动的证据。中生代褶皱枢纽和新近纪火山岩产状反映了ESRP附近的地壳挠曲。褶皱枢纽的形成具有近水平倾伏和垂直于ESRP的趋势，但现在在ESRP附近显示出高达20°-25°的向南倾伏。我们提出了一个等值线图的平等褶皱暴跌近端的ESRP显示弯曲大致平行，并延伸10-20公里的平均边缘的ESRP。挠曲剖面表明，相对于盆地和山脉，ESRP沉降量最小;沉降范围为4.5至8.5 km。构造等高线图和已公布的地震和重力数据被用来开发和约束挠曲沉降模型。这些模型表明，弯曲的地壳非常弱（弯曲参数为4-10公里），解释为ESRP的高热流的结果。假设沉降是由ESRP下方的致密地壳层的就位引起的，那么在以前的地震勘测中确定的中地壳“岩床”太宽，可能太薄，无法产生测量的弯曲。新的尺度包括17-25公里的厚度和40-50公里的半宽，这将岩床的边缘置于ESRP的边缘之下。ESRP岩床的尺寸基于下地壳的均衡补偿，因为软流圈的补偿需要30+ km的不合理的岩床厚度，而且ESRP地震、重力和热流数据支持下地壳补偿。密度驱动的下地壳流远离ESRP建议，以适应沉降和维持均衡平衡。沉降的时间受到ESRP勘探威尔斯井，其中6.6马在1.5公里的深度的韵律表明大多数沉降发生之前，他们的定位，并通过强烈的空间相关性之间的暴跌轮廓和第四纪火山裂谷带。两个过程解释有助于负载包括一个广泛的中地壳镁铁质负载安置在1000万，这提供了热源的初始流纹岩火山作用的ESRP，并继续，本地化的负载与第四纪玄武岩的岩脉和岩床。大范围的10 Ma岩浆活动和沉降与黄石热点的简单时间侵入迁移相冲突，表明需要修改热点范式。
The Eastern Snake River Plain (ESRP) is a linear volcanic basin interpreted by many workers to reflect late Cenozoic migration of North America over the Yellowstone hotspot. Thermal subsidence of this volcanic province with respect to Yellowstone has been documented by several workers, but no one has characterized subsidence with respect to the adjacent Basin and Range Province. This paper documents crustal flexure along the northwest edge of the ESRP, uses flexure to model the dimensions of a dense load beneath the basin, and presents evidence in support of density‐driven subsidence and lower crustal flow away from the basin. Crustal flexure adjacent to the ESRP is reflected by the attitudes of Mesozoic fold hinges and Neogene volcanic rocks. Fold hinges formed with a subhorizontal plunge and a trend perpendicular to the ESRP but now show a southward plunge near the ESRP of as much as 20°–25°. We present a contour map of equal fold plunges proximal to the ESRP that shows flexure is roughly parallel to and extends 10–20 km north of the average edge of the ESRP. Flexural profiles indicate the minimum amount of ESRP subsidence, with respect to the Basin and Range; subsidence ranges from 4.5 to 8.5 km. The structural contour map and published seismic and gravity data were used to develop and constrain flexural subsidence models. These models indicate the flexed crust is very weak (flexural parameter of 4–10 km), interpreted to be a result of the high heat flow of the ESRP. Assuming subsidence was induced by emplacement of a dense crustal layer beneath the ESRP, a midcrustal “sill” identified in previous seismic surveys is too wide and probably too thin to produce the measured flexure. New dimensions include a thickness of 17–25 km and a half width of 40–50 km, which place the edge of the sill beneath the edge of the ESRP. The dimensions of the ESRP sill are based on isostatic compensation in the lower crust because compensation in the asthenosphere requires an unreasonable sill thickness of 30+ km and because ESRP seismic, gravity, and heat flow data support lower crustal compensation. Density‐driven lower crustal flow away from the ESRP is proposed to accommodate subsidence and maintain isostatic equilibrium. Timing of subsidence is constrained by ESRP exploratory wells, where 6.6 Ma rhyolites at a depth of 1.5 km indicate most subsidence occurred prior to their emplacement, and by strong spatial correlations between plunge contours and Quaternary volcanic rift zones. Two processes interpreted to contribute to the load include an extensive midcrustal mafic load emplaced at ∼10 Ma, which provided the heat source for the initial rhyolitic volcanism on the ESRP, and continuing, localized loads from dikes and sills associated with Quaternary basalts. Widespread ∼10 Ma magmatism and subsidence conflicts with simple time‐transgressive migration of the Yellowstone hotspot, indicating a need for revision of the hotspot paradigm.