Provenance of Synorogenic Foreland Basin Strata in Southwestern Montana Requires Revision of Existing Models for Laramide Tectonism: North American Cordillera
Provenance of Synorogenic Foreland Basin Strata in Southwestern Montana Requires Revision of Existing Models for Laramide Tectonism: North American Cordillera
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DOI：
10.1029/2019tc005944
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发表时间：
2020-02-01
期刊：
影响因子：
4.2
通讯作者：
Pearson, D. M.
中科院分区：
文献类型：
--
作者：
Garber, K. L.;Finzel, E. S.;Pearson, D. M.
Laramide省的特点是通过基底拱的增长，前陆盆地分割。尽管这种构造样式在美国西部边缘沿着变化不定，但普遍认为这种构造样式是在坎帕尼亚早期（与80 Ma相似）开始的。这与北美西部的平板俯冲有关，但平板的程度和原因仍有争议，需要更好地限制Laramide变形时间的区域变化。我们提出了新的砾岩碎屑成分，砂岩岩相，碎屑锆石U-Pb年代学数据，从上白垩纪比弗黑德集团在蒙大拿州西南部，这表明前坎帕尼亚的历史基底参与变形。在沉积的早期阶段（类似于88-83马），两个独立的沉积体系从莱姆希海盆和远端冲断席的西部以及古生代地层侵蚀了挖出来的黑尾雪顶拱东部的沉积物。我们的数据为砾石通过科迪勒拉古河流的纵向输送提供了第一个确凿的证据，该古河流连接着爱达荷州中东部的沉积物来源和蒙大拿州西南部和怀俄明州西北部的沉积中心。此外，古生代地层的侵蚀到这个时候，需要Blacktail-Snowcrest拱挖出之前，类似于88马，以消除中生代覆盖层。后期（73-66 Ma）的沉积物通量完全来自于向西的前陆扩张褶皱冲断带。这些结果表明，Laramide风格的变形在蒙大拿州西南部启动之前，其他地方沿着边缘，需要修改现有的模型Laramide构造作用。
The Laramide province is characterized by foreland basin partitioning through the growth of basement arches. Although variable along the western U.S. margin, the general consensus is initiation of this structural style by the early Campanian (similar to 80 Ma). This has been linked to flat-slab subduction beneath western North America, but the extent and cause for a flat slab remain debated, invoking the need for better constraints on the regional variations in timing of Laramide deformation. We present new conglomerate clast composition, sandstone petrographic, and detrital zircon U-Pb geochronologic data from the Upper Cretaceous Beaverhead Group in southwestern Montana that suggest a pre-Campanian history of basement-involved deformation. During the early stages of deposition (similar to 88-83 Ma), two separate depositional systems derived sediment from the Lemhi subbasin and distal thrust sheets to the west as well as Paleozoic strata eroding off the exhuming Blacktail-Snowcrest arch to the east. Our data provide the first conclusive evidence for the longitudinal transport of gravel via Cordilleran paleorivers connecting sediment sources in east central Idaho to depocenters in southwestern Montana and northwestern Wyoming. Furthermore, erosion of Paleozoic strata by this time requires that the Blacktail-Snowcrest arch was exhuming prior to similar to 88 Ma in order to remove the Mesozoic overburden. Later (similar to 73-66 Ma) sediment flux was entirely from the foreland-propagating fold-thrust belt to the west. These results suggest that Laramide-style deformation in southwestern Montana preceded initiation elsewhere along the margin, requiring revision of existing models for Laramide tectonism.