Geodetic Constraints on Arc Dynamics and Coupling on the Plate Interface in the Lesser Antilles

沿着倾斜收敛的板块边界，相对板块运动矢量经常被划分为弧法向分量和弧平行分量。正、平行分量的完全分离导致弧前形成弧平行逆冲褶皱，火山弧形成弧平行走滑断裂。因此，前弧相对于覆盖板块的内部进行平移。然而，两个组件的部分分离更常见，特别是当耦合发生在板界面上时。此外，沿板块边界的辐合倾角的变化可以产生弧平行的剪应力梯度，从而产生弧前的内部变形。弧平行延伸或压缩都可能发生。加勒比海东北角提供了一个极好的机会，利用全球定位系统（GPS）大地测量学来检查应变划分、沿板块界面的耦合以及缓慢俯冲带的弧形平行延伸。加勒比-北美相对板块运动为~20毫米/年。在小安的列斯群岛，板块边界的走向由北西向西向大安的列斯群岛东部转变。辐合矢量的ENE方位角预测马提尼克岛的接近正常俯冲和波多黎各的接近走滑运动。小安的列斯群岛以东的浅层逆冲地震产生的水平滑动方向位于预测辐合方向和俯冲正线之间，表明存在一定的分割。重点放在小安的列斯群岛北部有四个原因。首先，PI自1994年以来收集了该地区的GPS数据，并制定了加勒比参考框架。其次，小安的列斯群岛南部的gps速度与加勒比海相对于北美在这些位置的运动预测在误差范围内一致，这意味着覆盖的板块很少或没有变形。相比之下，一个简单的弹性半空间模型表明，小安的列斯群岛北部的初步GPS大地测量结果与沿板块界面的耦合一致。第三，小安的列斯群岛北部的岛链由两个同心岛链组成，与南部的岛屿相比，这些岛链提供了更接近海沟的地面暴露，并且弧的横向距离更大。第四，在小安的列斯群岛北部，横贯弧的ne向构造具有弧平行伸展的成分。此外，个别岛屿上的火山中心以与弧平行的速度迁移，其速度相当于弧平行的收敛分量。最后，利用GPS大地测量法测量了东西向大安的列斯群岛东部沿ns向构造的东西向伸展。加勒比海东北部相对缓慢的板块运动需要较长的GPS时间序列来产生稳健的速度，以评估潜在的弧平行应变梯度。因此，这项五年建议以1994年以来在加勒比东北部收集的数据为基础。如果在10年或更长时间的时间尺度上无法检测到小安的列斯群岛北部岛屿相对于加勒比板块的运动——鉴于我们的初步数据集，这种情况不太可能发生——这些地点将用于为加勒比参考框架的制定提供额外的约束。建议的工作包括：GPS数据的收集和处理；噪声分析，这在加勒比海等运动缓慢的地方尤为重要；以及结果的建模。需要解决的问题是：斜滑是否在缓慢俯冲带（即小安的列斯群岛北部）被划分为海沟平行和正常成分？如果发生划分，它是否取决于收敛的倾斜度？小安的列斯群岛北部沿板块界面的耦合程度如何？圆弧平行延伸的幅度是否足够用GPS大地测量法检测？如果是这样，它的强度是否会随着俯冲倾角的增加而增加？弧平行滑动和/或弧平行伸展的速率是否与小安的列斯群岛北部火山中心的迁移速率一致？这对深部岩浆过程的动力学意味着什么？确定大安的列斯群岛东部和小安的列斯群岛的加勒比板块角前缘是否由相对于加勒比内陆移动的不同块体组成，也将限制加勒比东北部的地震危险，这是一个研究很少的地区，特别容易发生地震海啸。

PROJECT SUMMARYAlong obliquely convergent plate boundaries, the relative plate motion vector frequently is partitioned into arc-normal and arc-parallel components. Complete separation of the normal and parallel components leads to arc-parallel thrusts and folds in the forearc and arc-parallel strike-slip faults in the volcanic arc. The forearc thus translates relative to the interior of the overriding plate. Partial separation of the two components, however, is more common, particularly when coupling occurs across the plate interface. In addition, changes in obliquity of convergence along the plate boundary can create arc- parallel gradients in shear stress to produce internal deformation of the forearc. Either arc-parallel extension or compression may occur. The northeastern Caribbean corner affords an excellent opportunity to use Global Positioning System (GPS) geodesy to examine strain partitioning, coupling along the plate interface, and arc-parallel extension in a slow subduction zone. Caribbean-North American relative plate motion is ~20 mm/yr. Thetrend of the plate boundary changes from NNW in the Lesser Antilles to EW along the eastern Greater Antilles. The ENE azimuth of the convergence vector predicts near normal subduction at Martinique and near strike-slip motion at Puerto Rico. Horizontal slip directions derived from shallow-thrust earthquakes east of the Lesser Antilles lie between the predicted convergence direction and the normal to the subduction trace, suggesting that some partitioning occurs. The focus is on the northern Lesser Antilles for four reasons. First, The PI's have collected GPS data in the region since 1994 and have developed a Caribbean reference frame. Second, GPS-derived velocities for the southern Lesser Antilles agree within error with those predicted for Caribbean motion relative to North America at those locations, implying little or no deformation of the overriding plate. In contrast, a simple elastic half-space model shows that preliminary GPS geodetic results from the northern Lesser Antilles are consistent with coupling along the plate interface. Third, the northern Lesser Antilles arc consists of two concentric island chains that provide subaerial exposure closer to the trench and over a greater arc transverse distance than do the islands in the south. Fourth, NE-trending structures transverse to the arc in the northern Lesser Antilles accommodate a component of arc-parallel extension. In addition, volcanic centers on individual islands have migrated parallel to the arc at rates equivalent to those of the arc-parallel component of convergence. Finally, EW-oriented extension along NS-trending structures has been measured by GPS geodesy for the EW-trending eastern Greater Antilles. The slow relative plate motion in the northeastern Caribbean requires long GPS time-series to yield robust velocities to assess potential arc-parallel strain gradients. This five-year proposal, therefore, builds on data collected in the northeastern Caribbean since 1994.If no motion of the islands of the northern Lesser Antilles relative to the Caribbean plate can be detected over temporal scales of a decade or longer a scenario that is unlikely given our preliminary dataset these sites will be used to provide additional constraints for the Caribbean reference frame formulation. The proposed work includes: collection and processing of GPS data; noise analysis, which is particularly important where motion is slow such as the Caribbean; and modeling of results. The questions to be addressed are: Is oblique slip partitioned into trench parallel and normal components in slow subduction zones, i.e. the northern Lesser Antilles? If partitioning occurs, does it depend upon obliquity of convergence? What is the degree of coupling along the plate interface in the northern Lesser Antilles? Is arc-parallel extension of sufficient magnitude for detection by GPS geodesy? If so, does it increase in magnitude as obliquity of subduction increases? Are the rates of arc-parallel slip and/or arc-parallel extension consistent with rates of migration of volcanic centers within the northern Lesser Antilles? What are the implications of this for the dynamics of deep magmatic processes? Determining whether the leading edge of the Caribbean plate corner in the eastern Greater Antilles and the Lesser Antilles is composed of distinct block(s)that move relative to the Caribbean interior also will constrain seismic hazard in the northeastern Caribbean, an area that is little studied and particularly at risk for seismogenic tsunamis.