Over the past four years, studies of the kinematics and mechanics of trench-parallel translation and rotation of crustal blocks within plates that are being obliquely underthrust during subduction have demonstrated that oblique convergence is sometimes partitioned into two components, one directed more orthogonal to the local strike of a trench than the local plate convergence vector, and the other directed parallel to local strike of the trench. The former, convergent component is accommodated through subduction. The latter, margin-parallel component gives rise to shear tractions that can cause block rotations and associated extension, strike-slip motion, and shortening within the upper plate. Assuming that oblique subduction was as common in the geologic past as it is in the present, it has undoubtedly played an important role in the disassembly and reamalgamation of continental margins through translation and rotation of allochthonous (e.g. displaced) terranes.
Direct kinematic evidence for partitioning of obliquely directed convergence comes from studies of shallow-focus subduction zone earthquakes [ Jarrard, 1986; McCaffrey, 1991; DeMets, 1992; McCaffrey, 1992; Yu et al., 1993], which demonstrate that the horizontal slip directions from these earthquakes are commonly oriented between the direction normal to the local trace of a trench and that predicted by plate motion models such as NUVEL-1 (Fig. 3). Independent studies have demonstrated that the shear tractions imposed by the margin-parallel component of the plate vector deform the the upper plate. Geodetically-measured velocities of sites in northwestern Ecuador, located above the obliquely subducting Nazca plate, have significant trench-parallel components in the direction predicted by a simple partitioning model [ Freymueller et al., 1993]. England and Wells [1991] demonstrate that paleomagnetically determined rotations of the Columbia River Basalt group decrease exponentially as a function of distance from the Cascadia subduction zone, and suggest that the tangential component of Juan de Fuca-North America motion [ Wilson, 1993b] caused the observed rotations. Seismic reflection, sidescan sonar, and bathymetric surveys of the continental shelf above the subducting Juan de Fuca plate suggest that the tangential component of motion also results in folding and clockwise rotation of fault-bounded blocks [ Goldfinger et al., 1992].
The factors that control partitioning of oblique convergence are also being investigated. For instance, observed convergence obliquities are not strongly correlated with the magnitude of the angular discrepancy between predicted and observed slip directions, suggesting that partitioning is determined by other factors [ McCaffrey, 1992; McCaffrey, 1994]. Variability in the bulk rheological behavior of forearcs may influence partitioning---forearcs that behave elastically exhibit a lower degree of partitioning than those that behave plastically [ McCaffrey, 1994]. Beck [1993] further proposes that geometrical irregularities in the trace of a subduction zone can impede or permit along-arc translation of coastal slivers due to the presence or absence of a buttress at the leading edge of such slivers.