Distinguishing Tectonic Mechanisms of Extension and Forearc Translation near the Central American Volcanic Arc by High-Resolution Seismic Profiling in Lakes Nicaragua and Managua

会聚构造板块边缘的变形经常涉及某种形式的边界平行剪切，这归因于斜向会聚。这种平行于边界的会聚分量可能被弧前、弧后的变形所吸收，或沿沿着火山弧集中。与斜向会聚有关的两种主要变形模式是：1）地壳块体旋转和2）沿火山弧沿着走滑断层作用引起的弧前拆离--弧前条模型。在中美洲会聚边缘的情况下，弧前滑动和块体旋转的作用并不清楚。走滑地震的线性带表明，沿沿着尼加拉瓜凹陷的右旋剪切，尼加拉瓜凹陷是一个突出的第四纪地貌和构造凹陷，从萨尔瓦多的丰塞卡湾北方和尼加拉瓜北方延伸到哥斯达黎加的加勒比海沿岸约600 km。该凹陷与中美洲活火山带对齐，这些活火山通常位于凹陷内或西南边缘附近。为了检验这两个变形假说，该项目利用两个非常大的浅水湖（尼加拉瓜和马那瓜）的存在，以获取湖泊沉积物和使这些沉积物变形的最近断层的图像。利用安装在湖中客轮上的便携式地震反射系统，正在记录大约1 000公里的高分辨率地震数据。通过绘制湖泊中的活动断层模式，该项目提供了一种区分拟议变形模型的方法。块体旋转模型需要一系列的东北向，左横向断层穿越湖泊。弧前银模型表明，一个主要的西北走向的断层系统与活动火山链对齐，伸展盆地将在链中的右步进偏移之间发展。因此，断层系统的方向和分布应根据变形模式而有显著差异。除了测试与斜向俯冲有关的变形模式外，还正在调查尼加拉瓜凹陷的长期地质历史。一些模型表明，该凹陷主要形成于上新世-更新世时期（约0至600万年前），是通过与板片回滚相关的弧形正向伸展形成的。然而，如果弧前长条模型适用于这里，那么盆地的发展，在右台阶沿着走滑系统也可能是一个重要的和持续的因素，在形成尼加拉瓜凹陷。目前正在湖泊东北和西南边缘附近采集数据，以调查形成盆地的正断层的存在和方向，该项目有助于减轻地震灾害。尼加拉瓜的首都和经济中心马那瓜在过去的70年里两次被地震完全摧毁，现在人口迅速增长，超过100万。该项目更好地确定了活动断层的位置，从而有助于指导减轻地震灾害的努力。

Deformation at convergent tectonic plate margins frequently involves some form of boundary-parallel shearing attributed to oblique convergence. This boundary-parallel component of convergence may be absorbed by deformation in the forearc, backarc, or concentrated along the volcanic arc. Two of the major deformation modes associated with oblique convergence are 1) crustal block rotation and 2) forearc detachment by strike-slip faulting along the volcanic arc - the forearc sliver model. The roles of forearc slivering and block rotation are not clear in the case of the Central American convergent margin. A linear zone of strike-slip earthquakes indicates right-lateral shearing along the Nicaragua depression, a prominent Quaternary morphologic and structural depression, which extends approximately 600 km from the northern Gulf of Fonseca in El Salvador and northern Nicaragua to the Caribbean coast of Costa Rica. The depression is aligned with the belt of active Central American volcanoes, which are generally within the depression or near its southwest margin. To test the two deformation hypotheses, this project exploits the existence of two very large, shallow lakes (Nicaragua and Managua), to acquire images of the lake sediments and the recent faults that deform these sediments. Using a portable seismic reflection system mounted on passenger ferries in the lakes, roughly 1000 km of high-resolution seismic data is being recorded. By mapping the active fault patterns in the lakes this project provides a way to distinguish between the proposed deformation models. The block rotation model requires a series of northeast-oriented, left-lateral faults crossing the lakes. The forearc sliver model suggests that a major northwest-oriented fault system is aligned with the active volcanic chain and that extensional basins will develop between right-stepping offsets in the chain. Thus there should be a significant difference in fault system orientation and distribution depending on the mode of deformation.In addition to testing deformation models related to oblique subduction, the longer term geologic history of the Nicaragua depression is also being investigated. Some models suggest that the depression formed largely in Plio-Pleistocene times (approximately 0 to 6 million years ago) by arc-normal extension associated with slab rollback. However, if the forearc sliver model applies here, then basin development at right-step-overs along the strike slip system may also be an important and ongoing factor in forming the Nicaragua depression. Data is being acquired near both the northeast and southwest margins of the lakes to investigate the presence and orientation of basin-forming normal faults.This project is helping with seismic hazard mitigation. Faults in and around the lakes pose a major seismic hazard for the populated areas of the Nicaragua depression; the capital city and economic hub of Managua has been completely destroyed twice in the last 70 years by earthquakes and has a rapidly growing, present-day population of over one million inhabitants. This project better defines locations of active faulting and thus helps guide seismic hazard mitigation efforts.