310 TECTONICS/Convergent Plate Boundaries and Accretionary Wedges Figure The process of accretion at the toe of an accretionary wedge Commonly, but not invariably, the turbidites deposited in the trench form most of the accreted material The accreted materials separate from the overlain pelagic and hemipelagic muddy sediments along a detachment surface (decollement) in the upper part of the sediment cover; the decollement is where loading from turbidite deposition has increased pore fluid pressure to a value nearing that of the lithostatic pressure imposed by the weight of the overlying sediments, weakening them and making them prone to failure by shearing The thrust faults that detach the individual thrust slices that form the accreted section originate in the decollement, which propagates ahead of the toe of wedge The accreted section always includes the youngest turbidites; these are deposited in the trench but not on older accreted thrust slices, which have been uplifted out of the zone of deposition Consequently, the stratigraphy of each successively accreted thrust slice contains younger turbidites Within each thrust slice, the sediments upward and landward, but the overall stratigraphy of the wedge are younger becomes younger seaward Superimposed on the accreted sediments of the wedge is a drape of hemipelagic sediment, undiluted by turbidites, and the age of the base of this drape is youngest seaward The drape is also deformed by the deformation of the wedge as it thickens, with the oldest part of the drape sequence being more deformed than the youngest Copyright Graham Westbrook slope-drape and slope-basin sediments In frontal accretion, thrusts propagating from a decollement in a weak, overpressured horizon at the toe of the wedge divide the overlying section into thrust slices, which become added to the toe of the wedge (Figure 3) The level of the decollement is commonly in the upper part of the pelagic–hemipelagic sequence on the subducting plate, which has been overpressured by the deposition of turbidites above it in the trench The age of the accreted sediment changes with time, giving the wedge a characteristic tectonostratigraphy Each thrust slice is youngest upward and landward, but the sequence of successively accreted thrust slices has the youngest thrust being seaward and downward It is this characteristic stratigraphy that can be used to identify ancient accretionary wedges, such as the Ordovician–Silurian wedge of the Southern Uplands of Scotland In wedge growth by subcretion, sediment is added to the base of the accretionary wedge by the formation of duplexes at a ramp where the decollement changes level These propagate successively forward because the work required to continue to move the wedge up a ramp becomes greater than that needed to propagate displacement along the lower decollement and generate a new ramp In the process, the energy in the sediment between the ramps is transferred from the subducting plate to the accretionary wedge (Figure 5) It has also been suggested that the formation of a zone of tectonic melange along the decollement enables material from the subducting plate to be added to the accretionary wedge, but this can also operate in the opposite sense Accretionary wedge growth can occur when landward force imparted by the subducting lithosphere increases with increases in wedge width, pushing the wedge backward into the fore-arc basin and forming thrusts that incorporate fore-arc basin sediment into the wedge (Figure 5) In the mechanism involving slope-drape and slope-basin sediments, deposits directly onto the wedge are deformed by the continual deformation of the wedge beneath as it strives to maintain its critical taper The sediment forming the slope drape is usually hemipelagic, but in some cases,