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

对智利北部前弧伸展构造的研究为了解纳斯卡洋板块与上覆南美大陆板块之间的耦合性质提供了新的认识。这种耦合的性质不仅具有科学意义，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，一次8级以上的地震就会在不同的地区发生。正在进行这项研究的地区上一次发生大地震是在130年前。尽管板块边界在大地震期间似乎被强烈锁定，并且在此期间受到强烈的弹性压缩，但位于板块间耦合带之上的沿海科迪勒拉的地质情况只显示出与板块辐合方向平行的水平伸展特征。这一延伸既表现为向北的正断层，也表现为一组壮观的张性裂缝，由于独特的干燥环境，这些张性裂缝可以完全绘制出来。伸展构造可能与大地震期间或之后的弹性反弹或粘弹性松弛有关，与地震期间的震动有关，与板块边界的俯冲侵蚀有关，也可能与过于陡峭的大陆斜坡向下延伸到距海岸仅75公里的8公里深的海沟有关。本项目通过以下方法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，建立正断层的运动学模型，得出伸展的幅度和年龄以及剥离的深度；(2)研究以张性裂缝为代表的伸展成因、分布和幅度；(3)差分干涉侧孔径雷达记录弧前的垂直运动，包括潜在的早期海岸陡崖崩塌和退缩；(4)采用有限元模拟方法评价海岸断崖地形和大陆斜坡地形对区域应力场的影响。

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.