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首页> 外文期刊>Geological Society of America Bulletin >The Chulitna terrane of south-central Alaska: A rifted volcanic arc caught between the Wrangellia composite terrane and the Mesozoic margin of North America
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The Chulitna terrane of south-central Alaska: A rifted volcanic arc caught between the Wrangellia composite terrane and the Mesozoic margin of North America

机译：阿拉斯加中南部的丘利特纳（Chulitna）地貌：裂谷火山弧夹在兰格利亚（Wrangellia）复合地层和北美中生代边缘之间

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The Chulitna terrane is an enigmatic, fault-bounded package of mid-Paleozoic to early Mesozoic sedimentary rocks and vol-canics that are exotic to North America and show no clear affinity with any of the other allochthonous terranes in southern Alaska. Geologic mapping and geochemical analyses of Triassic strata from the Chulitna terrane indicate Triassic rifting of a Devonian volcanic arc and development of an asymmetric basin that was later deformed in the Cretaceous within the broadly denned suture between the Wrangellia composite terrane and the Mesozoic margin of North America. The structure of the region consists of an overturned synclinorium that verges southeast, Triassic strata in the core of the syncline, and basement rocks exposed in the hanging walls of a series of out-of-syncline thrusts on the southeastern, upright limb. The basement of the complex consists of ser-pentinite, vesicular flows of basaltic andesite with minor pillow lavas, and volcanic debris with blocks of andesite and minor Devonian red chert From southeast to northwest, there are systematic changes in the facies associated with Triassic strata that overlie this basement. To the southeast, the basement is unconformably overlain by bedded sedimentary breccias that consist of subangular clasts of basement lithologies, including red chert, serpentinite, and andesite. Further to the northwest, in the hanging wall of a more rearward thrust, the red-bed strata consist of conglomerate intermixed with siltstone and cross-bedded sandstone. In the northwest on the overturned limb of the syncline, the red beds are composed of siltstone and sandstone interflngered with limestone. There is an overall increase in the thickness of the red-bed unit from southeast to northwest. On the northwestern limb of the syncline, the red beds overlie a 1500-m-thick basalt sequence with columnar jointed flows and pillow basalts interbedded with limestone. Geochemical analyses of clinopyroxene phe-nocrysts in the basalts show similarities with island-arc basalts, suggesting that volcanism was related to early rifting of an oceanic arc. A marine sandstone unit overlies the entire red-bed package along a flooding surface and thins to the southeast. These results are consistent with Triassic deposition within a rifted volcanic arc, an upland erosion surface to the southeast, and increasing thickness of the Triassic section toward a basin-bounding fault in the northwest. This basin was initially ~18 km wide prior to accretion along the Mesozoic margin of North America, which resulted in southeast-vergent folding and 30% shortening.
机译：丘利特纳地块是一个由断层带界的，由古生界到中生代早期的沉积岩和火山岩组成的断层边界包裹，这些沉积岩和火山岩对北美洲很陌生，并且与阿拉斯加南部的任何其他异源地层都没有明显的亲和力。 Chulitna地层的三叠纪地层的地质图和地球化学分析表明，泥盆纪火山弧的三叠纪裂谷和不对称盆地的发育，后来在白垩纪的Wrangellia复合地层和北美中生代界之间的广泛缝线中变形。 。该区域的结构包括一个向东南倾斜的向斜斜斜线，向斜线核心处的三叠纪地层和暴露在东南直立肢体一系列不向斜线冲断的悬挂壁中的基底岩石。该复合物的基底由蛇纹岩，玄武质安山岩的小泡流和较小的枕状熔岩组成，以及由安山岩和较小的泥盆纪红石块组成的火山碎屑。从东南到西北，与三叠纪地层相关的相系统地变化，覆盖这个地下室。在东南部，基底不规则地覆盖着层状沉积角砾岩，这些沉积角砾岩由基底岩性的角下碎屑组成，包括红石，蛇纹岩和安山岩。再往西北，在向后推力更大的悬挂壁中，红层地层由砾岩与粉砂岩和交叉层状砂岩混合而成。在西北向斜向倾斜的四肢上，红层由粉砂岩和砂岩与石灰岩夹杂组成。从东南到西北，红层单元的厚度总体上增加了。在向斜线的西北边缘，红色的床层覆盖着厚度为1500 m的玄武岩层序，柱状节理流和枕形玄武岩均与石灰岩互层。玄武岩中斜辉石山玄武岩的地球化学分析显示出与岛弧玄武岩的相似之处，这表明火山作用与海洋弧的早期裂开有关。一个海洋砂岩单元沿着洪水表面覆盖整个红层包裹，并向东南变薄。这些结果与裂谷火山弧内的三叠纪沉积，东南部的高地侵蚀表面以及西北向盆地边界断层的三叠纪剖面厚度的增加是一致的。在沿中生代边缘增生之前，该盆地最初约18 km宽，导致东南向褶皱，缩短了30％。

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