Fault locking, block rotation and crustal deformation in the Pacific Northwest
Fault locking, block rotation and crustal deformation in the Pacific Northwest
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DOI：
10.1111/j.1365-246x.2007.03371.x
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发表时间：
2007-06
影响因子：
2.8
通讯作者：
R. Mccaffrey;A. Qamar;R. King;R. Wells;G. Khazaradze;C. Williams;C. Stevens;J. Vollick;P. Zwi
中科院分区：
文献类型：
--
作者：
R. Mccaffrey;A. Qamar;R. King;R. Wells;G. Khazaradze;C. Williams;C. Stevens;J. Vollick;P. Zwi
我们解释了全球定位系统（GPS）测量在美国西北部和加拿大西部的邻近地区，以描述地壳块体的相对运动，锁定在断层和永久变形与胡安德富卡和北美板块之间的收敛。为了估计海洋Juan de Fuca和Explorer板块以及几个大陆地壳块体的角速度，我们将GPS速度与海底扩展速率、地震滑动矢量方位角和断层滑动方位角和速率一起反演。我们还确定故障的程度，要么蠕动抗震，或者，锁定在块边界故障。卡斯卡迪亚俯冲逆冲断层主要锁定在近海，除了在俄勒冈州中部，锁定延伸到内陆。俄勒冈州和华盛顿西南部的大部分地区相对于北美洲以0.4-1.0百万年-1的速率顺时针旋转。在过去的十年中，没有剪切或伸展沿着喀斯喀特火山弧发生在毫米/年的水平，这表明从步行者巷和俄勒冈州以南的加州东部剪切带向北延伸的剪切变形在很大程度上是由俄勒冈州的块体旋转所容纳的。由GPS速度得到的垂直轴旋转速率与由古地磁下降异常估计的垂直轴旋转速率基本一致，表明10-15 Ma的旋转速率相对稳定。在靠近海岸的俄勒冈州海岸山脉内显示出额外的永久性右旋剪切。太平洋西北部的板块旋转并不会导致地壳物质的净向西向流动--地壳只是在旋转而不是逃逸。在温哥华岛上，辐合强度小于俄勒冈州和华盛顿，海岸的收缩应变与胡安·德富卡-北美运动更为一致。当温哥华岛和南部海岸山脉相对于北美以块状方式移动时，GPS速度拟合得明显更好。俄勒冈州、华盛顿西部和温哥华岛地壳块体的相对运动表明，普吉特海湾地区永久缩短的速率为4.4 ± 0.3 mm yr-1，这种类型的缩短会导致上板块地震。这种缩短很可能分布在几个断层上，但仅凭GPS数据无法确定断层上滑动的划分。从门多西诺三联点以南的大陆内部主要剪切变形到其以北主要块体旋转的转变，类似于从剪切到俯冲的其他转变中构造样式的变化。这种相似性表明，在俯冲带附近的地壳块体旋转增强，可能是由于较低的阻力。
SUMMARY We interpret Global Positioning System (GPS) measurements in the northwestern United States and adjacent parts of western Canada to describe relative motions of crustal blocks, locking on faults and permanent deformation associated with convergence between the Juan de Fuca and North American plates. To estimate angular velocities of the oceanic Juan de Fuca and Explorer plates and several continental crustal blocks, we invert the GPS velocities together with seafloor spreading rates, earthquake slip vector azimuths and fault slip azimuths and rates. We also determine the degree to which faults are either creeping aseismically or, alternatively, locked on the block-bounding faults. The Cascadia subduction thrust is locked mainly offshore, except in central Oregon, where locking extends inland. Most of Oregon and southwest Washington rotate clockwise relative to North America at rates of 0.4–1.0 ° Myr–1. No shear or extension along the Cascades volcanic arc has occurred at the mm/yr level during the past decade, suggesting that the shear deformation extending northward from the Walker Lane and eastern California shear zone south of Oregon is largely accommodated by block rotation in Oregon. The general agreement of vertical axis rotation rates derived from GPS velocities with those estimated from palaeomagnetic declination anomalies suggests that the rotations have been relatively steady for 10–15 Ma. Additional permanent dextral shear is indicated within the Oregon Coast Range near the coast. Block rotations in the Pacific Northwest do not result in net westward flux of crustal material—the crust is simply spinning and not escaping. On Vancouver Island, where the convergence obliquity is less than in Oregon and Washington, the contractional strain at the coast is more aligned with Juan de Fuca—North America motion. GPS velocities are fit significantly better when Vancouver Island and the southern Coast Mountains move relative to North America in a block-like fashion. The relative motions of the Oregon, western Washington and Vancouver Island crustal blocks indicate that the rate of permanent shortening, the type that causes upper plate earthquakes, across the Puget Sound region is 4.4 ± 0.3 mm yr–1. This shortening is likely distributed over several faults but GPS data alone cannot determine the partitioning of slip on them. The transition from predominantly shear deformation within the continent south of the Mendocino Triple Junction to predominantly block rotations north of it is similar to changes in tectonic style at other transitions from shear to subduction. This similarity suggests that crustal block rotations are enhanced in the vicinity of subduction zones possibly due to lower resisting stress.