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

收敛构造板块边缘的变形经常涉及某种形式的斜辐合边界平行剪切。这种边界平行辐合分量可能被弧前、弧后的变形所吸收，或沿火山弧集中。与斜辐合有关的两种主要变形模式是1)地壳块体旋转和2)沿火山弧走滑断裂的弧前拆离-弧前银条模式。在中美洲辐合边缘的情况下，弧前切割和块体旋转的作用尚不清楚。走滑地震的线性带表明沿尼加拉瓜坳陷的右侧剪切，这是一个突出的第四纪形态和构造坳陷，从萨尔瓦多的丰塞卡湾北部和尼加拉瓜北部延伸到哥斯达黎加的加勒比海岸约600公里。该凹陷与活跃的中美洲火山带对齐，这些火山通常位于凹陷内或靠近其西南边缘。为了验证这两种变形假设，该项目利用了两个非常大的浅湖（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近期断层的图像。利用安装在湖泊客轮上的便携式地震反射系统，记录了大约1000公里的高分辨率地震数据。通过绘制湖泊活动断层模式，该项目提供了一种方法来区分所提出的变形模型。块体旋转模型需要一系列东北向的左旋断层穿过湖泊。弧前银条模式表明，活火山链与北西向的大断裂体系成线，在活火山链的右阶偏移之间发育伸展盆地。因此，根据不同的变形方式，断层系统的走向和分布应该有很大的不同。除了测试与斜向俯冲有关的变形模型外，还对尼加拉瓜坳陷的长期地质历史进行了研究。一些模式认为，坳陷主要形成于上新世—更新世（大约0 ~ 600万年前），形成于弧向伸展和板块回滚。然而，如果弧前银条模式适用于此，那么沿走滑体系右台阶的盆地发育也可能是形成尼加拉瓜坳陷的一个重要和持续的因素。在湖泊的东北和西南边缘附近正在收集数据，以调查形成盆地的正断层的存在和方向。这个项目正在帮助减轻地震灾害。湖泊内部和周围的断层对尼加拉瓜洼地的人口稠密地区构成了重大的地震危险；首都和经济中心马那瓜在过去70年里两次被地震完全摧毁，人口迅速增长，目前人口超过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工作。

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.