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

两个板块碰撞的板块边界上的运动，如俯冲带或大陆碰撞带，通常是正面的。在某些情况下，碰撞更加倾斜，这种情况建立了一个复杂的地壳变形模式。有些变形是由断层调节的，断层允许地壳块体相互滑动，有些变形是由褶皱和断层的发育调节的，褶皱和断层同时具有垂直和水平的运动成分。加勒比海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国家大学的科学家组成的研究小组正在海地的Azuei湖进行地震反射和沉积物取心研究，以确定该地区的断层是如何响应倾斜板块碰撞而移动的。这一地区之所以特别引人关注，是因为该研究区域正好位于2010年1月12日海地7.0级地震断裂带的东部，这场灾难性的地震摧毁了海地首都太子港及周边地区。除了揭示沿这一板块边界的运动是如何适应的之外，这项研究还将为有意义地评估这一人口稠密地区面临的地震危险提供一些关键信息。该项目将通过以下方式促进其他预期的社会成果：教育专业学生参与项目；通过研究生和本科生培训，培养多元化、具有全球竞争力的STEM人才队伍；以及与海地科学家的密切合作。首先，加勒比板块相对于北美板块以每年约2厘米的速度向东北偏东移动，这意味着两个板块之间存在一个斜向辐合。这一斜向辐合部分是由两条东西向的左旋转换断层和一条西北-东南向的褶皱冲断带之间的运动分割而形成的，横跨伊斯帕尼奥拉岛。令人惊讶的是，2010年Mw7.0级海地地震并没有沿着Enriquillo-Plantain Garden断层（南转换断层）的明确轨迹破裂。相反，滑移发生在与走滑断层相邻的一条未预料到的盲斜逆冲断层上。沿伊斯帕尼奥拉岛的其他地方，走滑断层和逆冲断层之间可能会发生类似的危险相互作用。特别是位于海地首都太子港以东约30公里处的Azuei湖，被Enriquillo-Plantain Garden断层一分为二，并被海地褶皱和冲断带的北部和东部所束缚。该项目采用高分辨率多通道地震反射测量和CHIRP亚底剖面技术，对走滑和逆冲褶皱构造在Azuei湖下方交汇的地方进行成像，并确定它们之间的时空关系。Azuei湖是一个浅湖，位于2010年震中以东约60公里处，被Enriquillo-Plantain Garden断层穿过，也被海地褶皱冲断带所包围。为期两周的调查将产生高分辨率多通道地震反射剖面的密集网格，预计穿透深度至少在湖床以下几百米。同时获取CHIRP海底剖面；这将提供更高的地层分辨率（亚米），但渗透率较低（高达20米）。此外，还将用一系列短沉积物岩心对地震地层学进行实地考察。这些岩心将被放置在地震数据表明更深（更古老）的地震层与湖床相交的地方。沉积物样品将使用古地磁测量和放射性同位素方法相结合的方法来确定年代，以建立阿祖湖的全新世年代地层学。对地震反射和CHIRP剖面进行仔细的地层分析，将产生三维断层、褶皱和层序边界的数字表示。结合沉积物岩心的地层分析，该项目将表征两组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.