Middle Jurassic to Early Cretaceous tectonic evolution of the western Klamath Mountains and outboard Franciscan assemblages, northern California–southern Oregon, USA
Middle Jurassic to Early Cretaceous tectonic evolution of the western Klamath Mountains and outboard Franciscan assemblages, northern California–southern Oregon, USA
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DOI：
10.1130/2021.0062(04
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发表时间：
2021-09
期刊：
影响因子：
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通讯作者：
A. Chapman;D. Yule;W. Schmidt;Todd A. LaMaskin
中科院分区：
文献类型：
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作者：
A. Chapman;D. Yule;W. Schmidt;Todd A. LaMaskin
克拉马斯山脉省和邻近的方济各会俯冲复合体（北方加利福尼亚-俄勒冈州南部）共同包含了世界级的大陆岩石圈俯冲相关增长和稳定的档案。从中侏罗世到早白垩世，北美山脉的这些关键元素明显扩张，显然是由于构造增生和大陆弧加裂谷相关的岩浆增加。这次实地考察的目的有两个：展示该地区大陆生长的岩石记录，并讨论尚未解决的区域地质问题。后者包括：（1）中生代造山作用的程度（例如，锡斯基尤和内华达事件加上方济各会吸积的开始）是由大陆或海洋碎片的碰撞与板块运动的变化驱动的，（2）生长是否涉及“手风琴构造”，（和相关的边缘弧）反复打开和关闭，或者是由大量北美外来物质的增生所驱动，以及（3）康诺利山片岩的起源，一个复合的低品位单元占据了克拉马斯中部的一个神秘的结构窗口，与东倾的逆冲岩席区域结构“规则”不一致。我们分别断言：（1）如果碰撞驱动造山作用，则缺少必要的外来物质（我们不能排除这些物质通过俯冲和/或走滑断层作用被移走的可能性）;（2）约瑟芬蛇绿岩包裹体和Galice组填充盆地的打开和关闭明显发生在北美附近;内部的康诺利山片岩域相当于最古老的碎屑方济亚基（南叉山片岩），因此代表俯冲边缘内侧>100 km的海沟组合，据推测是在一个以前未被认识的浅角俯冲阶段。总之，这些关系表明，克拉马斯山脉和邻近的方济各会复杂的伸缩弧和弧前上板块域的一个动态中生代俯冲带，其中下行的海洋板块采取了各种各样的轨迹进入地幔。我们推测，下行板块包含交替道的光滑和致密与粗糙和浮力岩石圈，前者滑入地幔（促进板卷回滚和上板的延伸）和后者增强基底牵引（驱动上板压缩和板变浅）。在西太平洋，类似复杂的会聚背景的现代快照非常丰富，澳大利亚板块在新几内亚和邻近岛屿群下的俯冲可能提供了最好的模拟。
The Klamath Mountains province and adjacent Franciscan subduction complex (northern California–southern Oregon) together contain a world-class archive of subduction-related growth and stabilization of continental lithosphere. These key elements of the North American Cordillera expanded significantly from Middle Jurassic to Early Cretaceous time, apparently by a combination of tectonic accretion and continental arc– plus rift-related magmatic additions. The purpose of this field trip is twofold: to showcase the rock record of continental growth in this region and to discuss unresolved regional geologic problems. The latter include: (1) the extent to which Mesozoic orogenesis (e.g., Siskiyou and Nevadan events plus the onset of Franciscan accretion) was driven by collision of continental or oceanic fragments versus changes in plate motion, (2) whether growth involved “accordion tectonics” whereby marginal basins (and associated fringing arcs) repeatedly opened and closed or was driven by the accretion of significant volumes of material exotic to North America, and (3) the origin of the Condrey Mountain schist, a composite low-grade unit occupying an enigmatic structural window in the central Klamaths—at odds with the east-dipping thrust sheet regional structural “rule.” Respectively, we assert that (1) if collision drove orogenesis, the requisite exotic materials are missing (we cannot rule out the possibility that such materials were removed via subduction and/or strike slip faulting); (2) opening and closure of the Josephine ophiolite-floored and Galice Formation–filled basin demonstrably occurred adjacent to North America; and (3) the inner Condrey Mountain schist domain is equivalent to the oldest clastic Franciscan subunit (the South Fork Mountain schist) and therefore represents trench assemblages underplated >100 km inboard of the subduction margin, presumably during a previously unrecognized phase of shallow-angle subduction. In aggregate, these relations suggest that the Klamath Mountains and adjacent Franciscan complex represent telescoped arc and forearc upper plate domains of a dynamic Mesozoic subduction zone, wherein the downgoing oceanic plate took a variety of trajectories into the mantle. We speculate that the downgoing plate contained alternating tracts of smooth and dense versus rough and buoyant lithosphere—the former gliding into the mantle (facilitating slab rollback and upper plate extension) and the latter enhancing basal traction (driving upper plate compression and slab-shallowing). Modern snapshots of similarly complex convergent settings are abundant in the western Pacific Ocean, with subduction of the Australian plate beneath New Guinea and adjacent island groups providing perhaps the best analog.