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

对于卡斯卡迪亚俯冲带活跃变形前弧楔块的宽度，目前存在很大的分歧。窄楔解释认为，大陆架下部有一个强大的支撑，将楔变形限制在约50公里宽的大陆斜坡上。这个观点与大陆架是平的并且看起来变形相对缓慢的事实是一致的。陆架支撑的强度通常归因于楔体背面较老的增生岩石的岩化特征。另一种解释是，活跃变形的楔体宽约150至225公里，由卡斯卡迪亚海沟的向海向变形锋和俄勒冈-华盛顿海岸山脉、奥林匹克山脉和温哥华岛岛山脉东翼的向陆向变形锋划分。在这个弧前高地的峰顶，地形坡度的变化代表了楔形构造辐合的反转。在这个模型中，上覆板块相对强大的岩石圈地幔代表了一个深埋的平坦支撑。更大强度的地幔支撑使得楔形变形既包括沉积岩石，也包括俯冲带的旧地壳盖（如硅利特斯和新月玄武岩）。该模型不考虑当地的地壳地质情况，以显著的均匀性，解释了沿整个卡斯卡迪亚边缘弧前高地的发育。活跃的永久抬升在弧前高地随处可见，速度最快的是奥林匹克山脉（约0.8公里/英里）。PI提出了一项为期两年的研究，将使用热力学模型来测试卡斯卡迪亚的宽楔假说。考虑到最近的地震和大地测量学研究提供了关于弧前构造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关于边缘的奥林匹克和科瓦利斯板块长期变形和隆起的局部信息，这个时间点对这项工作来说是理想的。提议的工作将检查PI的假设最有可能失败的3个问题：1)在一个主动变形的楔子中，架子如何能够保持平躺和相对不变形？在奥运会上，海沟斜坡、陆架和弧前高地都被沉积的沉积岩所覆盖，所以楔形强度的变化似乎不太可能解释。极地考察队将测试这样一种观点，即楔的大陆架部分是由2至3公里厚的大陆架盆地中的沉积稳定下来的。2)是什么原因导致俯冲带的厚构造盖隆升并褶皱成今天观测到的弧前高地？热力学模型将允许PI确定韧性流在控制弧前高地生长中的作用。他们还将探索是否抬升和折叠的盖子可以发生单独的正面增生，或者如果底板是必需的。3)岩石强度分布对楔体变形模式有何影响？卡斯卡迪亚边缘包括软沉积、较老的岩化沉积和较老的火成岩构造盖（如Siletz、Crescent、Wrangellia地体）。使用现实的本构关系，PI将决定这些单位在更强的地幔支撑之上和海上是如何变形的。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，以检验宽前弧楔体的长期演化是否与已知的卡斯卡迪亚前弧的构造演化相一致。这将允许PI测试楔形是否会在面对沉积物通量的巨大变化时保持稳定的演变。这项研究将有助于更现实地认识卡斯卡迪亚前弧的热结构和长期速度场。这些信息对于提高卡斯卡迪亚俯冲带发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 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.