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Pressure-temperature-time-deformation paths of former mid-crustal rocks, northern Monashee Complex of the southeastern Canadian Cordillera: A model of synconvergent exhumation by sequential ductile extrusion.

机译:加拿大东南部莫纳什复杂的前中地壳岩石的压力-温度-时间-变形路径:加拿大东南部山脉:序贯延性挤压的共生发掘模型。

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摘要

It is generally accepted that the high-grade rocks exposed in the southeastern Canadian Cordillera were exhumed from the middle crust by large-magnitude extension in the Paleocene-Eocene following gravitational collapse of a thickened crustal welt and/or change in far-field stress. Some models further proposed that gneiss domes of the Monashee Complex, the deepest structural level of the southeastern Canadian Cordillera, were produced by extension-assisted diapirism. However, a small number of studies also proposed that part of the exhumation occurred during convergence. This thesis aims at gaining a better understanding of the processes that led to the exhumation of former mid-crustal rocks to the surface of the Earth.;The focus of the second part shifts toward upper structural levels, and various exhumation processes are tested for the Lower Selkirk allochthon (LSa). LSa is a 10--15 km thick panel of migmatitic rocks that underlies the older lower-grade rocks of the Intermontane Belt and overlies the younger rocks of the Monashee Complex. Structural, metamorphic and geochronologic data are combined to construct a detailed Pressure-Temperature-time-deformation path (P-T-t-d) for the thrust shear zone at the base of the LSa. This path indicates that the last increments of thrust shear strain took place between ∼62 and 57 Ma, and were associated with near-isothermal decompression. A compilation of previously published data further suggests that normal shearing along the shear zone that defines the roof of the LSa also occurred in the Paleocene. Combined with several depth-time paths that point to synchronous burial of the underlying Monashee Complex, this coeval motion of two oppositely-verging shear zones bounding a migmatitic panel suggests that the LSa was exhumed to upper crustal level primarily by Paleocene syn-convergent ductile extrusion, rather than large-magnitude extension.;The four P-T-t paths constructed in the last part indicate that rocks of the Monashee cover sequence, which are now sandwiched in between the LSa above and the Monashee Complex basement below, were exhumed from a depth of ∼40 km to less than ∼14 km depth by ductile extrusion after ductile extrusion of the LSa. Fold nappes developed during extrusion, most likely induced by internal density contrast within the extruding channel. A first-order force balance analysis indicates that underthrusting of a felsic basement beneath a denser orogenic wedge should produce a lateral pressure gradient large enough to drive low-viscosity rocks, such as migmatites, up a 30° ramp.;Consequently, results of this thesis strongly argue against large-magnitude extension for exhumation of high-grade rocks of the southeastern Canadian Cordillera at the latitude of the Monashee Complex. Extension probably played a role but only for the last stages of exhumation, once the rocks were already exhumed to upper crustal levels.;The first part of the thesis focuses on the core zone of the northern dome of the Monashee Complex, Frenchman Cap. After confirming previous studies in that all gray granite dykes in basement gneiss of the dome are part of a Paleoproterozoic suite, these dykes are used as time markers to demonstrate that only the upper ∼1.5 km of the exposed 5--6 km basement section has a penetrative foliation developed during the Cordilleran orogeny. Below, strain patterns identified as Cordilleran are limited to open folds and meter-scale shear zones. Furthermore, phase equilibria forward modeling points to a maximum burial depth of ∼20 km for the deepest exposed structural level of the dome. Data presented in this first part suggest that a downward-migrating base of convergence was frozen-in in the Eocene, and preclude diapirism as a viable doming mechanism for Frenchman Cap dome (FCD).
机译:普遍认为,在地壳重力增厚和/或远场应力发生变化的重力作用下,古新世-始新世的大幅度扩展从加拿大中部东南部的中地壳中挖出了高等级岩石。一些模型进一步提出,蒙纳士综合体(加拿大东南部山脉最深的结构层)的片麻岩穹顶是由扩张辅助的远照作用所产生的。但是,少数研究也提出了部分发掘在融合过程中发生。本论文旨在更好地了解导致前中地壳岩石掘出到地球表面的过程。第二部分的重点转向较高的结构水平,并对各种掘出过程进行了测试。下Selkirk异位体(LSa)。 LSa是一块10--15 km厚的大型岩石板块,它位于Intermontane Belt的较老的低等级岩石下,并覆盖了Monashee Complex的较年轻的岩石上。结合结构,变质和地质年代学数据,为LSa底部的推力剪切带构造了详细的压力-温度-时间-变形路径(P-T-t-d)。该路径表明,最后的推力剪切应变增量发生在〜62和57 Ma之间,并且与近等温减压有关。以前发表的数据的汇编进一步表明,在古新世也发生了沿定义LSa顶部的剪切带的正常剪切。结合几个指向深埋的莫纳什群的同步埋藏的深度时间路径,界定了成片组的两个反向汇聚的剪切带的同时代运动表明,主要通过古新世同向延性球墨挤压将LSa挖掘到上地壳水平最后一部分构造的四个PTt路径表明,蒙纳士盖层序列的岩石从〜L的深度被挖出,这些岩石现在被夹在上方的LSa和下方的Monashee复杂地下室之间。 LSa延性挤压后,通过延性挤压将深度从40 km减小到约14 km以下。挤压过程中出现折叠的尿布,很可能是由挤压通道内的内部密度对比引起的。一阶力平衡分析表明,在致密的造山楔下面的长英质基底的下冲作用应产生足够大的侧向压力梯度,以将低粘度的岩石(例如辉铁矿)向上倾斜30°。论文强烈反对在Monashee Complex纬度挖掘加拿大东南部山脉山脉高品位岩石的大幅度扩展。一旦岩石已经被挖掘到上地壳水平,延伸可能只是在发掘尸体的最后阶段发挥了作用。论文的第一部分着眼于莫纳西河群北穹顶的核心区域,法国人帽。确认先前的研究后,穹顶基底片麻岩中的所有灰色花岗岩堤坝都是古元古代组的一部分,这些堤坝用作时间标记,以证明仅裸露的5--6 km基底段的上部约1.5 km具有科迪勒勒造山运动期间形成的穿透性叶状体。在下文中,识别为科迪勒兰山脉的应变模式仅限于开放褶皱和米级剪切带。此外,相平衡正演模型指出了穹顶最深的暴露结构水平,最大埋藏深度约为20 km。在第一部分中提供的数据表明,始新世冻结了向下迁移的汇聚基础,并排除了二叠纪作为法国人帽穹顶(FCD)可行的穹顶机制的可能性。

著录项

  • 作者

    Gervais, Felix.;

  • 作者单位

    Carleton University (Canada).;

  • 授予单位 Carleton University (Canada).;
  • 学科 Geology.;Petrology.
  • 学位 Ph.D.
  • 年度 2009
  • 页码 325 p.
  • 总页数 325
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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