Paleomagnetism and tectonics of the Crescent Formation, northern Olympic Mountains, Washington
Paleomagnetism and tectonics of the Crescent Formation, northern Olympic Mountains, Washington
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华盛顿州奥林匹克山北部新月地层的古地磁和构造
DOI：
10.1029/93jb00709
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发表时间：
1993
影响因子：
--
通讯作者：
D. Engebretson
中科院分区：
文献类型：
--
作者：
A. C. Warnock;R. Burmester;D. Engebretson
在北奥运山始新世下新月组枕状玄武岩中发现了稳定的预褶皱磁化。新月形组的弯曲露头模式已成为几项不成功的研究的目标，以测试oroc斜弯曲。这项研究的成功部分是由于开发了一种小直径电芯钻，用于对玄武岩枕的断裂边缘进行取样。热退磁在34个取样点中的12个点产生了580°C的稳定终点（在其余点中普遍存在较大的点内散射）。在可接受的样点中，样点内散点较小，层理倾斜校正显著降低了样点间的散点。新取样的枕状玄武岩的平均古磁极（北纬86.4°，东经170.0°，A95=16.5°）与北美洲始新世早期至中期的磁极没有明显区别。结合前人在奥林匹斯山脉东部及其附近的上新月组陆基玄武岩的观测结果，发现古磁极（北纬80.7°，东经192.0°，A95 =8.0°，N=46）的地域性更强，没有明显的旋转（0.8°±14.4°）或向极移动（−3.6±8.5）。磁矿物学分析表明，剩余物为早期剩余物，可能为原生剩余物。因此，该极对于该地区的构造解释应该是有效的。经层理倾斜校正后的虚拟地磁极的圆形分布支持了北新月组由于奥林匹克核地体沉积物以圆顶状的方式上升而经历变形的假设。对部分向西开放的圆顶的侵蚀可能产生了新月组露头模式中所见的曲率。最北端的海岸山脉没有明显的旋转，与南部的净顺时针旋转形成鲜明对比。不同之处在于，奥林匹克山脉以东的南北走向走滑断层调节了由斜辐合驱动的向北旋转平移（古地磁上不明显，但地质上很重要）。
A stable prefolding magnetization has been discovered in pillow basalts of the Eocene lower Crescent Formation of the northern Olympic Mountains. The curved outcrop pattern of the Crescent Formation has been the target of several unsuccessful studies to test for oroclinal bending. The success of this study is due, in part, to the development of a small-diameter electric core drill for sampling the fractured rims of basalt pillows. Thermal demagnetization produced stable endpoints by 580°C in 12 of the 34 sites sampled (large within-site scatter was common in the remaining sites). Among the accepted sites, within-site scatter was small and correction for bedding tilt significantly reduced the scatter between sites. The mean paleomagnetic pole for newly sampled pillow basalts (86.4° north latitude, 170.0° east longitude, A95=16.5°) is indistinguishable from the early to middle Eocene pole expected for North America. Including previous results from sites in subaerial basalt of the upper Crescent Formation in and near the easterm Olympic Mountains results in a more regional paleomagnetic pole (80.7° north latitude, 192.0° east longitude, A95 =8.0°, N=46) that shows no significant rotation (0.8° ± 14.4°) or poleward displacement (−3.6 ± 8.5). Analysis of the magnetic mineralogy suggests that the remanence is early, probably primary. The pole, therefore, should be valid for tectonic interpretation of the region. A circular distribution of virtual geomagnetic poles after correction for bedding tilt supports the hypothesis that the northern Crescent Formation experienced deformation due to the rise, in a domelike fashion, of the sediments of the Olympic Core terrane. Erosion of a partial dome open to the west could have produced the curvature seen in the outcrop pattern of the Crescent Formation. The lack of significant rotation of the northernmost Coast Ranges contrasts with the net clockwise rotation seen to the south. The difference could be that irrotational northward translation (paleomagnetically insignificant yet geologically important), driven by oblique convergence, was accommodated by the north-south trending strike-slip faults to the east of the Olympic Mountains.