Backarc basin preading axis M agmatic arc Magmatic front basin Accretionary prism Outer trench high Depth (km) 0 50 00 50 400 Arc crust Lithospheric mantle Indicates feature that may or may not be present No vertical exaggeration 300 Convecting asthenosphere 500 C 00 00 Distance from trench (km) ubducting lithosphere Oceanic crust motions Plate motions Partial melt diapir Areas of melt generation Fluid pathways 0 500 C 500C 000 C 000C
~6 km (4 miles) Eruptions! ~ m c ubduction toryboard for Animation 3 4 5 6 W hat happens before the su bdu ction zone? Ocean Basin Processes ubducting Plate First (shallow) su bdu ction zone processes i ubducting Plate ome detail on the ubduction Zone Volcanic Front preading Cent er Crust Lithosphere Over-ridinging Plat e Over-riding Plate Backarc 00 C 600 C how important Mid-ocean ridge processes, then back to. how all components in motion; Give names for major features ubduction injects cold material into the mantle and causes hot mantle to flow with it. Injection of cold lithosphere chills the surrounding mantle, especially chills beneath the forearc. At greater depth, hot asthenospheric mantle is dragged down with the subducted slab, causing new hot mantle to ow in and replace it. n e m Magmatic Arc Backarc Outer Rise ubduction Zone Fresh lithospheric mantle how important deep-sea processes and hotspot volc., then back to erpentinized upper mantle due to seawater in ltration along faults caused by bending of the plate how photos of pillow basalt, dikes, gabbro, mantle peridotite Ridge Processes: Crust formation & Alteration (show clips of real vents & eruptions) b Outer Rise Processes: how how plate bends in order to go down subduction zone (forms Outer Rise) ubduction Zone Ear t h q u akes serpentinized mantle Cooking ubducted ubducted oceanic crust is rst 3 metamorphosed to blueschist; the blue ediment & Crust is caused by the mineral glaucophane. The subducted plate and sediments are rst squeezed With further subduction and () and then heated () as they sink deeper into the metamorphism, this turns to eclogite, subduction zone. At rst () water is squeezed out of a rock composed of garnet and pyroxene. fractures and pores, then mineral transformations () (metamorphism) release water from mineral lattices to make intermediate depth earthquakes (). Deep earthquakes may be caused by major changes in the mineralogy of subducted lithospheric mantle (3). The addition of water from the subducted slab s VF lowers the melting temperature of mantle Backarc rocks. The Volcanic Front (VF) marks the boundary between cold mantle beneath the forearc (too cold to melt, even with Ar c water) and mantle farther t from the trench that is hot Ar c Vo lcano es Volcanoes: a r e t yp ic a lly enough to melt when water ~00 km above is added. How & Why su bdu cted slab Pillow Basalt Dikes Gabbro ea oor preading and Hydrothermal Alteration Melting olid mantle upwelling Regions altered by hydrothermal uids (Water added) = Altered Oceanic Crust (AOC) how eart hquakes caused by bending sti plate how photos of fresh peridotite and serpentinite Backarc ubduction zones are associated with dipping zones of earthquakes, which are found as deep as 670 km (400 mi.), much deeper than for any other tectonic setting. These earthquakes can be subdivided into shallow (<00 km), intermediate (00-300 km), and deep (>300 km) earthquakes. hallow earthquakes are the most damaging, especially the megathrust earthquakes. ubducted oceanic cru st: Blu esch ist (30-60 km) o Eclo git e: (> 60 km) a f j p edimentation and Lithospheric Aging Processes (show clips of drill ship bringing up sediment cores) how di erent types of sedimentation: biogenic, dust, volcanic, meteoritic, etc. c What happens when the two plates meet? Outer forearc processes A. AP B. C. ealevel ubducting Crust TE Tsunami Backarc Young (thin) Lithosphere Old (thick) Lithosphere (~00 km/60 miles thick) how lithosphere thickening with time, show sea oor sinking with time Megath ru sts, giant earthqu akes, and tsu namis Water: Megathrust zone extends from the trench to ~30 km (0 miles) deep. Volcanic Front Mantle Wedge C What goes down must come up queezing and cooking (metamorphism) of subducted crust and sediments releases lots of water. This rises into the overlying mantle. Because of its shape, this is called the mantle wedge. ediments can also melt. u Mantle ow channels? g VF diapirs? Water and sediment melts are sweated out of the subducting plate at ~0 km (~65 miles) beneath the Volcanic Front (VF). Melting How does the water released by the subducted sediments and crust rise through downwelling mantle and reach the region near the top of the mantle where melting occurs? Does it happen by uids owing up channels, like vertical rivers? Or by diapirs of low-density sepentinite () rising like balloons from the base of the mantle wedge? However it is done, addition of water to shallow asthenosphere causes melting and forms basaltic melt, which rises to the surface and erupts. A. If thick sediments on subducting plate (>500m, 500 ) then may be scraped o to form Accretionary Prism (AP) B. ediments on subducting plate may be scraped o (subcreted C or underplated) and added to base of overriding plate C. If downgoing plate has thin sediments (<500m), then forearc crust may be faulted and carried down (tectonic erosion TE or subduction erosion) Between earthquakes, the two plates move together and the upper plate is pulled down by the lower plate. train can build up for hundreds of years. During an earthquake, upper plate breaks free and snaps back into position. This moves the water and causes a tsunami k q l O -ridge (hotspot) volcanism; ealevel d Magma ubmarine Volcano (seamount) how submarine volcano erupting, adding Ocean-island basalt and volcaniclas low earth qu akes (aka Episodic Tremor and lip; ET) No Tsunami ET zone at 30-50 km (0-36 mi.) deep. Not found in all subduction zones Backarc and co ld Mantle Wedge The mantle wedge varies a lot in temperature. The Volcanic Front marks the boundary between cold mantle beneath the forearc (too cold to melt, even with water) and mantle farther from the trench that is hot enough to melt. Mantle Peridotite plu s Water = erpentine h At slightly greater depth than the megathrust region, the plate boundary releases strain more slowly, over a few days instead of over a few seconds mantle = serpentinite mantle = p er ido t iit e W h at th e mantle makes when it melts: Basalt v Other processes in the crust change melt compositions to be richer in silica: andesit es and dacit es, which can erupt vio le n t ly w r