Approximately 50 Myr ago, the triple junction of the Kula-Farallon-North America plates converged with the continental margin, causing ridge subduction and the formation, accretion and translation of two oceanic plateaus. We investigate the effects of this tectonic configuration on the exhumation of southern Wrangellia terrane on southern Vancouver Island since the Eocene. We report late Cretaceous to late Oligocene (85.4 to 23.3 Ma) apatite fission track ages (AFT) and, for the first time, Oligocene to early Miocene (36.6 to 14.0 Ma) apatite (UTh)/He ages (AHe) for 16 bedrock samples of Wrangellia. The thermal history modelling of these ages for 13 samples reveals variable cooling patterns between regions. Samples close to the major faults of a fold and thrust belt show accelerated cooling (4–5 °C/Myr) during the Eocene. In the central area, the modelled cooling rates have been slow and generally uniform throughout the Cenozoic (0.5–1.5 °C/Myr), whereas samples from the west coast yielded very slow cooling (<0.5 °C/Myr) from 70 to 30 Ma, followed by moderate cooling (1.5–3 °C/Myr) since. Combining ages, fission track length and thermal history models in this and previous studies, we interpret the moderate-accelerated exhumation of the fold and thrust belt in the Eocene to be a response to oroclinal bending following oceanic plateau accretion. The exhumation pattern of the western side of southern Wrangellia is linked to the ongoing Cascadian Subduction zone ca.30 Ma. This exhumation pattern also supports a hypothesis that all crust of southern Wrangellia was all overlain by sedimentary strata in Eocene before ∼50 Ma, and that an accretionary complex of the Pacific Rim terrane was partly the outboard equivalent of these strata. In the southern Wrangellia, no exhumation response to the Miocene oroclinal bending associated with formation of the Olympic mountains is observed.

中文翻译：

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挖掘对海洋高原吸积和口斜弯曲的响应：加拿大温哥华岛南部 Wrangellia terrane 的低温热年代学研究


大约 50 Myr 前，库拉-法拉隆-北美板块的三重交界处与大陆边缘汇合，导致脊俯冲和两个海洋高原的形成、吸积和平移。我们研究了自始新世以来这种构造配置对温哥华岛南部南部 Wrangellia 地层挖掘的影响。我们报道了晚白垩世至晚渐新世（85.4 至 23.3 马）磷灰石裂变轨道年龄 （AFT），并首次报道了渐新世至中新世早期（36.6 至 14.0 马）磷灰石 （UTh）/He 年龄 （AHe） 的 16 个 Wrangellia 基岩样品。13 个样品的这些年龄的热历史模型揭示了区域之间不同的冷却模式。靠近褶皱带和逆冲带主要断层的样品在始新世期间显示出加速冷却 （4–5 °C/Myr）。在中部区域，模拟的冷却速率在整个新生代一直很慢且大致均匀（0.5-1.5 °C/Myr），而来自西海岸的样品从 70 到 30 马 的冷却非常缓慢（<0.5 °C/Myr），随后是适度冷却（1.5-3 °C/Myr）。结合本研究和先前研究中的年龄、裂变轨迹长度和热历史模型，我们将始新世褶皱和逆冲带的中度加速挖掘解释为对海洋高原吸积后口斜弯曲的响应。兰格利亚南部西侧的挖掘模式与正在进行的卡斯卡迪亚俯冲带有关，约30 马。这种挖掘模式也支持一个假设，即在 ∼50 马 之前，南部 Wrangellia 的所有地壳都被始新世的沉积地层覆盖，并且环太平洋地层的增生复合体部分相当于这些地层的外侧。 在南部的 Wrangellia，没有观察到与奥林匹克山脉形成相关的中新世 oroclinal 弯曲的挖掘反应。