TECTONICS/Convergent Plate Boundaries and Accretionary Wedges 313 Figure Fluid flow and modes of expulsion of fluid from an accretionary wedge The sources of fluid are pore water expelled by compaction from sediment subducted beneath the accretionary wedge and sediment accreted to the wedge, and dehydration of hydrous minerals such as smectite, as temperature increases with increasing depth of subduction Copyright Graham Westbrook mud-rich layers that are accreted into the wedge or subcreted beneath it Mud volcanoes also appear to be created by the fluidization and entrainment of mud by water driven to the surface by tectonic expulsion, and this mode of formation is characteristic of mud volcanoes created in front of accretionary wedges on the ocean floor, or behind them in fore-arc basins The migration of methane-containing pore water through the sediments of an accretionary wedge as it compacts creates methane hydrate in the sediments occupying the first few hundred metres depth range beneath the seabed (see Petroleum Geology: Gas Hydrates) This occurs because this region lies within the stability field for methane hydrate (which is a solid clathrate formed from water and methane, in which the methane molecules are held within a cage of water molecules in an approximately 1:6 ratio) The hydrate stability field generally exists in Earth’s major oceans in water depths greater than about 300 m, and is favoured by increasing pressure and decreasing temperature Consequently, most of the sediments beneath continental margins are in the hydrate stability field down to the depth at which, because of the increase of temperature with depth, the geotherm crosses the stability boundary for hydrate Beneath this boundary, methane can be present as free gas, in which case the boundary creates a seismic reflection because the presence of only a very small amount of free gas (less than 1% of the pore space is enough) reduces the seismic velocity of P waves significantly The polarity of the reflection is negative, opposite to that of the seabed, because of the decrease in velocity beneath it, and is most clearly visible on seismic reflection sections where it cuts across the reflections produced by sedimentary bedding (Figure 8) This reflection is widespread in accretionary wedges, and is usually termed a bottom-simulating reflection (BSR) because its shape, to the first order, mimics that of the seabed, which, because of its nearly uniform temperature, controls the shape of the isotherms beneath it Uplift of the seabed produced by the thickening of the wedge continually moves the base of the zone containing hydrate upward out of hydrate stability field, causing hydrate to dissociate and release free gas that produces the BSR Because the depth of the BSR below the seabed is controlled by the geothermal gradient, mapping the depth of the BSR has been used to map variations in heat flow from accretionary wedges, which is influenced by tectonic thickening and fluid flow The tectonic expulsion of methanerich pore water and the dissociation of hydrate to free gas caused by uplift results in methane hydrate and BSRs being widespread in accretionary wedges, whereas they occur only rarely in the sediments of passive continental margins Tectonic Erosion at Subduction Zones The inner walls of trenches of arcs (e.g., the Mariana arc, Tonga arc, and South Sandwich arc) not have significant accretionary wedges Those that occur