Collaborative Research: Interactions Between Transpressional Structures at the North American-Caribbean Plate Boundary: Geophysical Imaging Beneath Lake Azuei, Haiti

沿着两块板块碰撞的板块边界(如俯冲或大陆碰撞带)的运动通常是迎头而行的。在某些情况下，碰撞是更倾斜的，这种情况建立了复杂的地壳形变模式。一些变形是由允许地壳块体相互滑动的断层来调节的，而一些变形是通过具有垂直和水平运动分量的褶皱和断层的发展来调节的。加勒比海板块和北美板块之间的边界就是这样一个例子。一个由罗德岛大学、纽约城市大学和海地大学的科学家组成的研究小组正在海地的阿祖伊湖进行地震反射和沉积物取心研究，以确定该地区的断层如何在板块倾斜碰撞后移动。这一地区特别令人感兴趣，因为研究区域位于2010年1月12日海地7.0级地震破裂带以东，那次灾难性事件摧毁了首都太子港及周边地区。除了阐明沿板块边界的运动是如何适应的，这项研究还将提供一些必要的关键信息，以有意义地评估这一人口稠密地区面临的地震危险。该项目将通过以下方式推动其他预期的社会成果：教育专业学生参与该项目；通过研究生和本科生培训发展一支多样化的、具有全球竞争力的STEM劳动力队伍；以及与海地科学家的密切合作。这种斜向会聚在伊斯帕尼奥拉岛上的部分原因是两条东西向左旋转换断层和一条走向西北-东南的褶皱冲断带之间的运动分割。令人惊讶的是，2010年海地7.0级地震并没有沿着恩里基洛-芭蕉园断层的清晰痕迹破裂，恩里基洛-芭蕉园断层是南部的转换断层。相反，滑动发生在一个意想不到的盲目斜冲断层上，该断层毗邻走滑断层。走滑和逆冲断层之间类似的危险相互作用预计也会在伊斯帕尼奥拉的其他地方发生。特别是，阿祖伊湖位于海地首都太子港以东约30公里处，被恩里基洛-芭蕉园断层一分为二，北部和东部以海地褶皱冲断带为界。该项目采用高分辨率多道地震反射测量和Chirp海底剖面，以成像走滑构造和逆冲褶皱构造在阿祖伊湖下相遇的地方，并将确定它们之间的时空关系。阿祖伊湖是一个浅水湖，位于2010年震中以东约60公里处，由恩里基洛-芭蕉园断层穿过，也以海地褶皱冲断带为界。为期两周的调查将产生高分辨率多道地震反射剖面的密集网格，预计穿透湖床以下至少几百米。将同时采集Chirp海底剖面；这些剖面将提供更高的地层分辨率(亚微米)，但穿透度较低(最高可达20米)。此外，地震地层学将用一系列短沉积岩心进行地面勘察。这些岩心将被安置在地震数据显示较深(较老的)地震层位与湖床相交的地方。将结合古地磁测量和放射性同位素方法测定沉积物样品的年代，以建立阿祖伊湖的全新世年代地层学。对地震反射和啁啾剖面进行仔细的地层学分析，将得到断层、褶皱和层序界面的三维数字表示。结合沉积岩心的地层分析，该项目将描述两套倾斜构造之间的空间和时间关系。

Motion along plate boundaries where two plates collide, such as subduction or continental collision zones, is typically head-on. In some cases, the collisions are more oblique, a situation that sets up a complex pattern of crustal deformation. Some deformation is accommodated by faults that allow crustal blocks to slide by each other and some deformation is accommodated by development of folds and faults with both a vertical and horizontal component of motion. The boundary between the Caribbean and North American plates is such a case. A research team of scientists from the University of Rhode Island, City University of New York, and University d'Etat de Haiti is carrying out a seismic reflection and sediment coring study on Lake Azuei, Haiti, in order determine how the faults in this region are moving in response to oblique plate collision. This region is of particular interest because the study area lies just to the east of the rupture zone of the January 12, 2010 magnitude 7.0 Haiti earthquake, the catastrophic event that devastated the capital Port-au-Prince and surrounding regions. In addition to shedding light on how motion along this plate boundary is accommodated, this study will provide some critical information needed for a meaningful assessment of the seismic hazards facing this densely populated region. The project will advance other desired societal outcomes through: involvement of education major students in the project; development of a diverse, globally competitive STEM workforce through graduate and undergraduate student training; and close collaboration with Haitian scientists.To a first order, the Caribbean plate is moving east-northeastward at about 2 cm/yr relative to the North American plate, implying an oblique convergence between the two plates. This oblique convergence is partly accommodated across the island of Hispaniola by the partitioning of motion between two east-west left-lateral transform faults and a fold-and-thrust belt that strikes northwest-southeast. Surprisingly, the 2010 Mw7.0 Haiti earthquake did not rupture along the well-defined trace of the Enriquillo-Plantain Garden Fault, the southern transform fault. Instead, slip occurred on an unsuspected blind oblique thrust fault that abuts the strike-slip fault. Similar hazardous interactions between strike-slip and thrust faults are expected elsewhere along Hispaniola. In particular, Lake Azuei, located about 30 km east of Haiti's capital city of Port-au-Prince, is bisected by the Enriquillo-Plantain Garden Fault and is bounded to the north and east by the Haiti fold-and-thrust belt. This project employs a high-resolution multichannel seismic reflection survey and CHIRP sub-bottom profiling to image both strike-slip and thrust-fold structures where they meet beneath Lake Azuei and will determine the spatial and temporal relations between them. Lake Azuei, a shallow lake located about 60 km east of the 2010 epicenter, is crossed by the Enriquillo-Plantain Garden Fault and is also bounded by the Haiti fold-and-thrust belt. A two-week survey will produce a dense grid of high-resolution multichannel seismic reflection profiles with an expected penetration of at least a few hundred meters below the lake bed. CHIRP subbottom profiles will be acquired concurrently; these will provide an even higher stratigraphic resolution (sub-meter) but with a lower penetration (up to 20 m). In addition, the seismic stratigraphy will be ground-truthed with a series of short sediment cores. These cores will be sited where seismic data indicate that deeper (older) seismic horizons intersect the lake bed. Sediment samples will be dated using a combination of paleomagnetic measurements and radioisotopic methods in order to establish a Holocene chronostratigraphy for Lake Azuei. The careful stratigraphic analysis of the seismic reflection and CHIRP profiles will result in a digital representation of faults, folds, and sequence boundaries in three-dimensions. Together with the stratigraphic analysis of the sediment cores, this project will thus characterize the spatial and temporal relations between the two sets of oblique structures.