Paleoseismic Trenching Reveals Late Quaternary Kinematics of the Leech River Fault: Implications for Forearc Strain Accumulation in Northern Cascadia
Paleoseismic Trenching Reveals Late Quaternary Kinematics of the Leech River Fault: Implications for Forearc Strain Accumulation in Northern Cascadia
复制标题
DOI：
10.1785/0120200204
复制
发表时间：
2021-04
影响因子：
3
通讯作者：
N. Harrichhausen;K. Morell;C. Regalla;S. Bennett;L. Leonard;Emerson M. Lynch;E. Nissen
中科院分区：
文献类型：
--
作者：
N. Harrichhausen;K. Morell;C. Regalla;S. Bennett;L. Leonard;Emerson M. Lynch;E. Nissen
新的古地震槽探表明，加拿大温哥华岛南部利奇河断层在晚第四纪发生了倾斜右旋滑动，并限制了北方卡斯卡迪亚的永久性弧前变形。全球导航卫星系统向北的速度和横跨奥林匹克山脉、胡安·德富卡海峡（JDF）和格鲁吉亚南部海峡的地震活动性从南到北减少，已被用作加拿大西南部向北迁移的南部前弧和刚性北方后挡之间的逆冲断层作用造成的永久性南北地壳缩短的证据。然而，以前的古地震研究表明晚第四纪斜向右滑动的西西北走向的弧前断层北部的奥林匹克山脉和格鲁吉亚海峡南部是更一致的弧前变形模型，调用造山弯曲和（或）向西挤压的奥林匹克山脉。为了帮助评估应变进一步北跨越海峡的JDF，我们提出了从两个新的古地震沟槽挖掘跨越利奇河断层的结果。在最东端的好望角海沟，我们记录了一个垂直断层带和一个广泛的背斜变形冰碛。断层和未变形冰碛物中的冰碛物碎屑方向的比较显示了沉积后断层冰碛物旋转的证据，表明了走滑剪切。在地表破裂过程中，张开型裂缝的方向与右旋滑动和已发表的区域SHmax方向一致。垂直分离和沿沿着一条断层的陡崖崩积层的形成也表明存在倾滑分量。在冰碛物和陡崖崩积物中的放射性碳定年表明，在9.4±3.4 ka时发生了一次地表破裂地震。在好望角海沟中推断的斜向右滑动方向与在其他区域性西北偏西走向断层上观察到的滑动运动学一致，并表明这些构造不通过逆冲断层作用容纳显著的南北缩短。
New paleoseismic trenching indicates late Quaternary oblique right-lateral slip on the Leech River fault, southern Vancouver Island, Canada, and constrains permanent forearc deformation in northern Cascadia. A south-to-north reduction in northward Global Navigation Satellite System velocities and seismicity across the Olympic Mountains, Strait of Juan de Fuca (JDF), and the southern Strait of Georgia, has been used as evidence for permanent north–south crustal shortening via thrust faulting between a northward migrating southern forearc and rigid northern backstop in southwestern Canada. However, previous paleoseismic studies indicating late Quaternary oblique right-lateral slip on west-northwest-striking forearc faults north of the Olympic Mountains and in the southern Strait of Georgia are more consistent with forearc deformation models that invoke oroclinal bending and(or) westward extrusion of the Olympic Mountains. To help evaluate strain further north across the Strait of JDF, we present the results from two new paleoseismic trenches excavated across the Leech River fault. In the easternmost Good Hope trench, we document a vertical fault zone and a broad anticline deforming glacial till. Comparison of till clast orientations in faulted and undeformed glacial till shows evidence for postdeposition faulted till clast rotation, indicating strike-slip shear. The orientation of opening mode fissuring during surface rupture is consistent with right-lateral slip and the published regional SHmax directions. Vertical separation and the formation of scarp-derived colluvium along one fault also indicate a dip-slip component. Radiocarbon charcoal dating within offset glacial till and scarp-derived colluvium suggest a single surface rupturing earthquake at 9.4±3.4 ka. The oblique right-lateral slip sense inferred in the Good Hope trench is consistent with slip kinematics observed on other regional west-northwest-striking faults and indicates that these structures do not accommodate significant north–south shortening via thrust faulting.