Along 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. The trend 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.
