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

关于卡斯卡迪亚俯冲带活动变形的弧前楔的宽度，存在很大分歧。狭义楔形解释认为，大陆架下面有一个强大的后挡板，将楔形变形限制在约50公里宽的大陆坡。这种观点与架子是平的并且看起来变形相对缓慢的事实是一致的。陆棚挡块的强度通常归因于楔体后部较老的增生岩石的更石化的特征。另一种解释是，积极变形楔是约150至225公里宽，并界定了一个向海的变形锋在卡斯卡迪亚海沟和一个向陆的变形锋在东翼的俄勒冈州-华盛顿海岸山脉，奥运会和温哥华岛岛屿山脉。在这个弧前高点顶部的地形坡度的变化代表了楔体中构造聚散的逆转。在这个模型中，相对强大的岩石圈地幔的覆盖板块代表了一个深层次的平躺的支持。地幔后挡的强度更大，使得楔状变形既涉及增生沉积岩，也涉及俯冲带较老的地壳盖（如Silitez和Crescent玄武岩）。这个模型占弧前高沿着卡斯卡迪亚边缘的整个长度的发展显着的均匀性，无论当地的地壳地质。沿弧前高压沿着到处都有活跃的永久性隆升，其速率最快（~0.8 km/m.y.）发生在奥林匹克山脉。PI提出了一项为期2年的研究，该研究将使用热机械建模来测试卡斯卡迪亚的宽楔形假设。鉴于最近的地震和大地测量研究提供了有关前弧结构和短期变形的详细信息，以及最近的热年代学，地貌学和地质学研究提供了有关奥林匹克和科瓦利斯边缘长期变形和隆起的局部信息，这项工作的时机是理想的。拟议的工作将检查PI的假设最有可能失败的3个问题：1）如何使搁架能够在主动变形的楔形物内保持平放和相对不变形？在奥林匹克运动会中，海沟斜坡、陆棚和弧前高地都是由增生的沉积岩构成的，因此楔强度的变化似乎是一个不太可能的解释。PI将检验这样一种观点，即楔形体的陆架部分是通过陆架盆地的沉积而稳定的，陆架盆地的厚度为2至3公里。2)是什么原因导致俯冲带的厚构造盖隆起并折叠成今天观察到的弧前高压？ 热力学模型将允许PI的，以确定在控制弧前高压的增长的韧性流动的作用。他们还将探讨是否隆起和折叠的盖子可以发生由正面增生单独，或者如果底侵是必需的。3)岩石强度分布对楔体变形的影响如何？卡斯卡迪亚边缘包括软增生沉积物、较老的岩化增生沉积物和较老的火成岩的结构盖（例如，锡莱茨、新月花、弗兰格利亚花）。使用现实的本构关系，PI的将确定这些单位如何变形以上和向海的一个更强大的地幔backstop.These过程为导向的研究将提供基础，建立一个完整的热力学模型来测试，如果长期演变的广泛的弧前楔是一致的卡斯卡迪亚弧前已知的构造演化。这将使PI的测试，如果楔将保持稳定的演变，面对沉积物通量的巨大变化。这项研究将有助于对卡斯卡迪亚前弧的热结构和长期的速度场更现实的理解。这些信息对于提高卡斯卡迪亚俯冲带孕震区宽度的分辨率至关重要。

There is much disagreement about the width of the actively deforming forearc wedge at the Cascadia 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.