Collaborative Research: Forearc Extension in a Strongly Coupled Subduction System, Northern Chile

对智利北部前弧的伸展结构的研究为了解俯冲的纳斯卡大洋板块和南美大陆板块之间的耦合性质提供了新的见解。这种耦合的性质不仅具有科学价值，而且具有巨大的实际意义，因为这个板块边界是地球上一些最大地震的原因。每隔100到150年，就会在不同的地区发生8级以上的地震。进行这项研究的地区上一次发生大地震是在130年前。尽管板块边界似乎在大地震之间被强烈锁定，并在此期间被强烈弹性压缩，但位于板块间耦合带上方的沿海科迪勒拉的地质特征仅显示出与平行于板块会聚方向的水平伸展有关的特征。这种伸展既表现为北向正断层，也表现为一套壮观的张性断裂，由于独特的干燥环境，可以相当完整地绘制出它们的地图。伸展构造可能与大地震期间或之后的弹性回弹或粘弹性松弛有关，与地震时的震动有关，与板块边界的俯冲侵蚀有关，也可能与向下延伸到距海岸仅75公里处8公里深的海沟的过陡峭的大陆斜坡有关。该项目通过结合以下几个方面来评估这些因素的相对贡献：(1)野外工作与地质年代学相结合，以建立正断层的运动学模型，以得出伸展的强度和年龄以及滑脱的深度；(2)研究张性断裂所代表的伸展的来源、分布和强度；(3)差分干涉侧孔雷达，以记录弧前的垂直运动，包括潜在的沿海悬崖的早期坍塌和撤退；以及(4)有限元模拟，以评估沿海陡坡和大陆斜坡的地形对区域应力场的作用。

Work on the extensional structures of the forearc of northern Chile is providing new insight into the nature of coupling between the subducted Nazca oceanic plate and the overriding South American continental plate. The nature of this coupling is not only of scientific interest but of tremendous practical importance as this plate boundary is responsible for some of the greatest earthquakes on Earth. Every 100 to 150 years a magnitude 8+ earthquake occurs in various segments. The area in which the research is being carried out last had a major earthquake 130 years ago. Although the plate boundary appears to be strongly locked between great earthquakes and is strongly elastically compressed during that time, the geology of the Coastal Cordillera, which overlies the zone of interplate coupling, displays only features associated with horizontal extension parallel to the plate convergence direction. This extension is manifest both as north-striking normal faults and as a spectacular suite of tension fractures which, due to the uniquely dry environment can be mapped quite completely. The extensional structures may be related to elastic rebound or visco-elastic relaxation during or following great earthquakes, to shaking during earthquakes, to subduction erosion at the plate boundary, or perhaps to an overly steep continental slope going down to the oceanic trench at 8 km depth just 75 km offshore. This project is assessing the relative contributions of these factors by combining: (1) field work with geochronology to construct kinematic models of normal faults to yield magnitude and age of extension as well as depth to detachment; (2) study of the origin, distribution, and magnitude of extension represented by the tension fractures; (3) differential interferometric side aperture radar to document vertical motions in the forearc, including, potentially, incipient coastal escarpment collapse and retreat; and (4) finite element modeling to evaluate the role of the topography of the Coastal Escarpment and the continental slope on the regional stress field.