Collaborative Research: Thermomechanical Models of Forearc Deformation at the Cascadia Subduction Zone

关于卡斯卡迪亚俯冲带主动变形的弧前楔体的宽度存在很大分歧。狭窄的楔形解释认为，大陆架下方有一个强大的支撑，将楔形变形限制在约50公里宽的陆坡范围内。此视图与货架平坦且变形相对较慢这一事实相一致。陆架支撑的强度通常归因于楔形背部较老的吸积岩的岩化特征。另一种解释是，主动变形楔形变宽约150-225公里，由卡斯卡迪亚海沟的向海变形锋和俄勒冈-华盛顿海岸山脉、奥运会和温哥华岛岛山脉东侧的向陆变形锋划定。弧前高峰处地形坡度的变化代表了楔形构造收敛的反转。在这个模型中，覆盖板块的相对较强的岩石圈地幔代表了一个深层的平躺的后盾。地幔支撑的较大强度使得楔形变形既包括增生的沉积岩，也包括俯冲带较老的地壳盖层(例如Silitez玄武岩和新月玄武岩)。这一模型解释了弧前高度沿整个卡斯卡迪亚边缘的发展，无论当地的地壳地质情况如何，都具有显着的一致性。弧前高地上到处可见活动的永久隆升，速率最快(约0.8公里/英里)。发生在奥林匹克山脉。PI提出了一项为期两年的研究，将使用热机械模型来测试卡斯卡迪亚的宽楔形假说。考虑到最近的地震和大地测量研究提供了有关弧前结构和短期变形的详细信息，以及最近的热年代学、地貌和地质研究提供了有关边缘的奥林匹克和科瓦利斯地段的长期变形和隆起的当地信息，这一时机是这项工作的理想时机。这项拟议的工作将研究PI假设最有可能失败的三个问题：1)如何使搁板在活跃变形的楔形物中保持平坦和相对不变形？在奥运会上，海沟坡度、陆架和弧前高地都被堆积的沉积岩覆盖，因此楔形强度的变化似乎不太可能解释。PI将测试楔形的陆架部分是通过2到3公里厚的陆架盆地中的沉积来稳定的想法。2)是什么原因导致俯冲带厚厚的构造盖层抬升并褶皱成今天观察到的弧前高地？热机械模拟将允许PI‘s确定延性流动在控制弧前高的生长中的作用。他们还将探索盖子的抬升和折叠是否可以仅通过正面吸积作用发生，或者是否需要底侵作用。3)岩石强度分布对楔体变形形态有何影响？卡斯卡迪亚边缘包括软的吸积沉积物、较老的岩化吸积沉积和由较老的火成岩组成的构造盖层(例如，Siletz、新月、Wrangellia地体)。利用现实的本构关系，PI将确定这些单元如何在更强大的地幔背景下在上方和向大海变形。这些面向过程的研究将为建立完整的热力学模型提供基础，以测试宽弧前楔体的长期演化是否与卡斯卡迪亚前弧的已知构造演化一致。这将允许PI测试在面对沉积物通量的巨大变化时，楔形是否会保持稳定的演变。这一研究将有助于更真实地了解卡斯卡迪亚弧前的热结构和长期速度场。这一信息对于提高卡斯卡迪亚俯冲带孕震带宽度的分辨率至关重要。

There is much disagreement about the width of the actively deforming forearc wedge at theCascadia subduction zone. The narrow wedge interpretation maintains that the continental shelf is underlain by a strong backstop that limits wedge deformation to the ~50 km wide continental slope. This view is compatible with the fact that the shelf is flat and appears to deform relatively slowly. The strength of the shelf backstop is usually attributed to the more lithified character of older accreted rocks within the back of the wedge. The alternative interpretation is that actively deforming wedge is some 150 to 225 km wide, and is delimited by a seaward-vergent deformation front at the Cascadia trench and a landward-vergent deformation front at the east flank of the Oregon-Washington Coast Ranges, the Olympics and the Vancouver Island Insular Range. The change in topographic slope at the crest of this forearc high represents a reversal in structural vergence in the wedge. In this model, the relatively strong lithospheric mantle of the overriding plate represents a deep-seated flat-lying backstop. The greater strength of the mantle backstop allows wedge deformation to involve both accreted sedimentary rocks and the older crustal lid of the subduction zone (e.g. Silitez and Crescent basalts). This model accounts for the development of the forearc high along the entire length of the Cascadia margin with remarkable uniformity irrespective of local crustal geology. Active permanent uplift is recognized everywhere along the forearc high, with the fastest rates (~0.8 km/m.y.) occurring in the Olympic Mountains. The PI's propose a 2 year study that will use thermomechanical modeling to test the wide wedge hypothesis at Cascadia. The timing is ideal for this work given recent seismic and geodetic studies that provide detailed information about the structure and short-term deformation of the forearc, and recent thermochronologic, geomorphic, and geologic studies that provide local information about long-term deformation and uplift across the Olympics and Corvallis sectors of the margin. The proposed work will examine 3 issues where the PI's hypothesis is most likely to fail: 1) How is the shelf able to remain flat lying and relatively undeformed within an actively deformed wedge? In the Olympics, the trench slope, shelf, and forearc high are all underlain by accreted sedimentary rocks, so variations in wedge strength seems an unlikely explanation. The PI's will test the idea that the shelf part of the wedge is stabilized by deposition in shelf basins, which are 2 to 3 km thick. 2) What causes the thick structural lid of the subduction zone to uplift and fold into the forearc high observed today? Thermomechanical modeling will allow the PI's to determine the role of ductile flow in controlling the growth of the forearc high. They will also explore if uplift and folding of the lid can occur by frontal accretion alone, or if underplating is required. 3) How is the pattern of wedge deformation influenced by the distribution of rock strength? The Cascadia margin includes soft accreted sediments, older lithified accreted sediments, and a structural lid of older igneous rocks (e.g., Siletz, Crescent, Wrangellia terranes). Using realistic constitutive relationships, the PI's will determine how these units deform above and seaward of a much stronger mantle backstop.These process-oriented studies will provide the basis for building a full thermomechanical model to test if the long-term evolution of a wide forearc wedge is consistent with the known tectonic evolution of the Cascadia forearc. This will allow the PI's to test if the wedge will retain a steady evolution in the face of large changes in sediment fluxes. This research will contribute towards a more realistic understanding of the thermal structure and long-term velocity field within the Cascadia forearc. This information is essential for improving resolution of the width of the seismogenic zone for the Cascadia subduction zone.