Eocene dike orientations across the Washington Cascades in response to a major strike-slip faulting episode and ridge-trench interaction
Eocene dike orientations across the Washington Cascades in response to a major strike-slip faulting episode and ridge-trench interaction
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华盛顿喀斯喀特始新世堤坝方向对主要走滑断层事件和山脊-海沟相互作用的响应
作者：
R. Miller;Kathleen I. Bryant;Brigid A. Doran;M. Eddy;Franco P. Raviola;Nick Sylva;P. Umhoefer
美国华盛顿州的北部喀斯喀特山脉保留了异常的浅层到中地壳的始新世张转记录，其特征是右旋走滑断层、岩墙群和岩体侵入、快速非海相沉积、以及北喀斯喀特结晶核心部分地区的延性流和快速冷却。在脊-海沟相互作用与板片窗口形成过程中发生了转变，并且在约 100 年前的 Siletzia 海洋高原碰撞后不久，发生了板片的回滚和断裂。 50马。堤坝群侵入约 1250 平方公里的区域。 49.3 Ma 和 44.9 Ma 以及 1500 多个测量的岩脉的方向与地质年代学数据相结合，提供了区域应变场的重要快照。镁铁质蒂纳韦堤坝是最南端、数量最多的堤坝。它们走向 NE（平均值 = 036°），平均厚度约为 15 m。在北部，流纹岩到玄武岩岩脉在 49.3-46.5 Ma 之间空间重叠，主要是花岗闪长岩岩体，但它们通常早于附近的岩体约 10.5 Ma。 500 k.y.六个堤域中的五个的平均方位范围为 010° 至 058°； W-NW 至 NW 走向的堤坝是一个区域的特征，在其他一些区域中也有少量发现。总体而言，所有始新世岩脉的平均走向为035°，平均延伸方向（305°–125°）与北喀斯喀特造山带的走向（~320°）倾斜。在穿过 Teanaway 群的一条 >7 公里长的样带中，堤防延伸达到约 45%，在穿过其他群的较短样带中，范围从约 5% 到局部约 79%，这相当于至少约 12 公里的延伸。
主要的北东走向岩脉与北北西走向（~355-320°）区域走滑断层的右旋运动相一致。一些W-NW-至NW-走向的岩墙可能受到预先存在的断层、剪切裂缝和叶理的影响，并且可能在同时存在NE-和较小的W-NW-走向的岩墙的一群中，受到岩墙就位引起的主应力轴转换的影响。或者，W-NW-到NW-走向的堤坝可能反映了较年轻的区域应变场，如大约。 NE 走向的岩脉的 49.3–47.5 Ma U-Pb 锆石年龄比少数 W-NW 至 NW 走向的岩脉年龄要老。在一种情况下，北东走向的岩墙在应变主要反映右旋走滑断层的时间段内侵入，而较年轻的岩墙记录了向更多弧法向延伸的转变。当岩浆活动迁移到北喀斯喀特核心以西的南北向带中时，堤坝运动结束，这标志着祖先喀斯喀特弧的开始。
The northern Cascade Mountains in Washington (USA) preserve an exceptional shallow to mid-crustal record of Eocene transtension marked by dextral strike-slip faulting, intrusion of dike swarms and plutons, rapid non-marine sedimentation, and ductile flow and rapid cooling in parts of the North Cas- cades crystalline core. Transtension occurred during ridge-trench interaction with the formation of a slab window, and slab rollback and break-off occurred shortly after collision of the Siletzia oceanic plateau at ca. 50 Ma. Dike swarms intruded a ≥1250 km2 region between ca. 49.3 Ma and 44.9 Ma, and orientations of more than 1500 measured dikes coupled with geochronologic data provide important snapshots of the regional strain field. The mafic Teanaway dikes are the southernmost and most voluminous of the swarms. They strike NE (mean = 036°) and average ~15 m in thickness. To the north, rhyolitic to basaltic dikes overlap spatially with 49.3–46.5 Ma, mainly granodioritic plutons, but they typically predate the nearby plutons by ca. 500 k.y. The average orientations of five of the six dike domains range from 010° to 058°; W-NW– to NW–striking dikes characterize one domain and are found in lesser amounts in a few other domains. Overall, the mean strike for all Eocene dikes is 035°, and the average extension direction (305°–125°) is oblique to the strike (~320°) of the North Cascades orogen. Extension by diking reached ~45% in one >7-km-long transect through the Teanaway swarm and ranged from ~5% to locally ~79% in shorter transects across other swarms, which corresponds to a minimum of ~12 km of extension.
The dominant NE-striking dikes are compatible with the dextral motion on the N- to NW-striking (~355–320°) regional strike-slip faults. Some of the W–NW- to NW-striking dikes were arguably influenced by pre-existing faults, shear fractures, and foliations, and potentially in one swarm where both NE- and lesser W-NW–striking dikes are present, by a switch in principal stress axes induced by dike emplacement. Alternatively, the W-NW– to NW-striking dikes may reflect a younger regional strain field, as ca. 49.3–47.5 Ma U-Pb zircon ages of the NE-striking dikes are older than those of the few dated W-NW– to NW-trending dikes. In one scenario, NE-striking dikes intruded during an interval when strain mainly reflected dextral strike-slip faulting, and the younger dikes record a switch to more arc-normal extension. Diking ended as magmatism migrated into a N-S–trending belt west of the North Cascade core that marks the initiation of the ancestral Cascade arc.