Earthquakes and Benioff zones 1037 pattern over the wedge. Th is pattern is present at the Mentawai Ridge in the Sunda Arc (see box SW zone of subduction erosion NE and mélange formation above). Miocene (15 Ma) active volcanic arc The forearc basin extensional structures coastline Forearc basins are zones of crustal subsidence that plate boundary lie between the outer ridge and the mountain range of the volcanic arc. Th ey are generally dominated by great thicknesses of marine sediment and sedi- mentary rock that form in depths that range from abyssal to shallow. Present examples of foreaerc basins include the Sandino Basin of off shore Nica- subduction mélange ragua and the basin that lies between the Mentawai eroded until present Ridge and Sumatra. Th e former contains deposits by subduction erosion approximately 10 km thick and the latter consists of sediment 4–5 km thick that displays a general shallowing-upward sequence (Fig. 7.9). Although ~ 40 km ~ 40 km backward shift backward shift the deposits in the Sumatra example are domi- of plate boundary of volcanic arc nated by sediment derived from the adjacent arc, Present they also contain appreciable amounts of marine subsidence present coastline biogenic carbonate, especially within the younger, plate boundary shallower deposits. Forearc basins are underlain by either thinned continental or normal oceanic crust. Th e distri- oceanic crust bution of the two is related to the origin of the Miocene subduction zone. Typically new subduction zones coast line evolve at the juncture between continental and lithospheric oceanic crust, along a passive continental margin, mantle because this zone is generally structurally weak. Continental margins typically show irregular asthenosphere shapes with alternating salients and embayments; subduction zones tend to form smooth, arcuate plate boundaries. Th erefore, the margin of the up- per plate along the new plate boundary is likely to for 95 % of all earthquakes on Earth and are re- Fig. 7.16 Subduction contain embayments of oceanic crust (Fig. 7.17). ferred to as Benioff zones aft er the geophysicist, erosion at the plate mar- Hence, the type of crust in the forearc region may Hugo Benioff , who recognized and systematically gin of Costa Rica where the Cocos Plate subducts be continental, oceanic, or both in a given section. investigated such zones in the Pacifi c Ocean and beneath the Caribbean Such oceanic entrapments are called remnant or below the Sunda Arc (Benioff , 1954), or Wadati- Plate (Meschede et al., residual ocean basins. Benioff zones (in 1935 Kiyoo Wadati documented 1999). Backward shift Forearc basins on oceanic crust may feature earthquake epicenters along a down plunging (towards the backarc) of great water depth. Th e oceanic crust merges into the volcanic arc is caused by subduction erosion of thinned continental crust towards the volcanic arc. the upper plate since the If an accretionary wedge and an outer ridge develop continental crust Miocene. vo lcan at the convergent margin, the basin will be topo- ic chain cross graphically constricted and fi ll rapidly with sedi- section ment (see box). During collision, ophiolites from uction the basement of forearc basins may be accreted oceanic crust subd onto the adjacent continental crust. As opposed to continental oceanic crust subducted ophiolites from the lower plate, they do crust not experience high-pressure metamorphism un- pelagic sediments less they are scraped off and dragged down. Fig. 7.17 Development oce of a residual forearc basin Earthquakes and Benioff zones anic c rust caused by the formation Subduction zones are easy to detect because they of a subduction zone at form long, linear belts with intense earthquake ac- accretionary wedge residual basin volcanic arc an irregular continental tivity. Th ese seismically active zones are responsible margin.

Licensed to jason patton