TECTONICS/Convergent Plate Boundaries and Accretionary Wedges 315 Figure Tectonic erosion by the relief of the oceanic basement is a common process where sediment cover on the subducting plate is thin Scarps originally formed by normal faulting at the mid ocean ridge are enhanced, or a new one is created by normal faulting in the outer part of the trench as the oceanic lithosphere flexes downward into the subduction zone The tops of these scarps collide with the toe of the accretionary wedge or with the leading edge of the crystalline fore arc of the overriding plate, forcing the decollement to jump upward to the level of the top of the scarp, thereby transferring some of the material that was in the wedge to the subducting plate The surface of the wedge is steepened by the removal of material from its toe, inducing slumps into the trench of material that is subsequently accreted and swept into the subduction zone by the basement scarps The subduction of seamounts produces a similar, although more severe, process of tectonic erosion that is effective in thicker sediment cover, but is geographically more localized Copyright Graham Westbrook Erosion from the Effects of High Fluid Pressure Obduction The need to invoke this second type of mechanism is presented by those subduction zones where it is clear that tectonic erosion is or has been active, yet no features of basement topography appear to be responsible The fluid pressure ratio, l, is greatest at the top of any body of rock in which pore water is connected This makes the uppermost rocks weakest and most liable to failure and displacement As zones of high pressure are driven to migrate upward, progressive failure and displacement remove material from (tectonically erode) the section through which the highpressure pore fluid migrates, until it dissipates or escapes to the surface As subduction is continually feeding sediments with a high water content beneath wedges of accreted sediment or the basement of the overriding plate, the potential for this type of mechanism to operate is always present, if pore water expelled from the sediment cannot escape easily through the overlying wedge There is evidence for two possible variants of this process, the first more general, the second more specific: Sometimes, features of oceanic basement are not subducted, but are sheared off against the crystalline crust of the overriding plate, leading to obduction of part of the oceanic crust This appears to occur most commonly where asperities exist in the oceanic crust that is being subducted, such as seamounts or the ridges flanking transform faults The Tres Montes Peninsula on the coast of southern Chile, close to the Chile triple junction, was probably brought about by this process Very large-scale obduction of ocean crust to create ophiolite complexes is associated with continent/ continent collision or continent/island-arc collision and may also be a consequence of the closing of back-arc basins following a change in plate motions, with young buoyant ocean crust thrust onto the adjacent plate, as exemplified by the Rocas Verdes complex of southern Chile In ‘stoping’, high-pressure fluid causes disaggregation of the base of the wedge The disaggregated rock is incorporated into a shear zone melange and is subducted (Figure 10) Shear zone melanges are exhibited by the exhumed deeper parts of old accretionary wedges that can be observed on land, such as the Franciscan wedge in California In the process of reactivation and upward migration of detachment surfaces, high-pressure fluid weakens the upper surfaces more than the lower ones Rock beneath the reactivated surfaces is subducted (Figure 10) Seismic images of duplexes within the subducting section have provided the indication that this process occurs Oblique Subduction There are several convergent margins, including the eastern end of the Aleutian island arc, Sumatra, the north-west United States (Cascadia), and the northern Pacific margin of Colombia, where the direction of convergence is oblique; in these cases, as well as convergence between the two plates in the direction orthogonal to the plate boundary, there is a component of motion parallel to the plate boundary At these margins, the directions of thrust-fault outcrops and of fold axes, which form at right angles to the direction of compression, run subparallel to the margin, not at right angles to the direction of convergence of the two plates The reason for this is that the displacement between the accretionary wedge and the subducting plate is partitioned into a component orthogonal to