7 Subduction zones, island arcs and active continental margins ubduction zones are created when two litho- Th e third type of convergent plate boundary spheric plates move against each other and represent the active continental margins where S one of the two plates descends under the other oceanic litho sphere is subducted beneath conti- through the process of subduction. However, only nental litho sphere without a marine basin behind oceanic litho sphere is able to sink deeply into the the volcanic arc; rather, the arc is built directly on Earth’s mantle to become reincorporated there. the adjacent continent. Th e continental margin is Continental crustal material is generally too light connected directly to the hinterland, although a to be subducted to great depth. Th e interaction of shallow marine basin may exist behind the volcanic the subduction zone and the asthenosphere of the arc. Examples for active continental margins are mantle generates the melts that rise to feed the the Andes, SE Alaska, and the western and central volcanism typical of island arcs and active conti- Sunda Arc that includes Sumatra and Java. nental margins. Th e forth type of convergent margin occurs Subduction zones are critical to the dynamics of along zones of continent-continent collision. If the Earth because they represent the essential driv- two continental masses collide during continuous ing force behind the movement of plates. Moreover, subduction, they eventually merge. Telescoping of magmatism initiated by subduction is responsible the two plates and the buoyancy of the subducting for the creation of continental crust through a se- continent eventually leads to a standstill of subduc- ries of complex processes. Th e continental crustal tion within the collision zone. Th e oceanic part material generated in this fashion has a low specifi c of the subducting plate tears off and continues to weight and remains at the outer rind of the Earth drop down, a process referred to as " slab breakoff ". and is not reintegrated into the mantle. Without this Continent-continent collisions ultimately result in light-weight continental crust, which forms high the formation of mountain ranges like the Hima- topographic features on the Earth, our planet would layas or the Alps. Fig. 7.1 Convergent have a completely diff erent face. Th e total length plate margins of the Earth of global subduction zones sums to more than Structure of plate margin systems with characterized by ensimatic 55,000 km, a length only slightly shorter than the subduction zones island arcs (underlain by total length of the mid-ocean ridges (60,000 km). Systems of convergent plate boundaries are char- oceanic crust), ensialic island arcs (underlain by Four types types of convergent plate boundaries acterized by a distinct topographic and geologic continental crust), and ac- are recognized (Figs. 7.1, 7.2): subdivision. Although the plate boundary itself is tive continental margins. Th e fi rst type occurs when ocean lithosphere is subducted below other ocean litho sphere ("intra- oceanic subduction zone") to create a volcanic Kamchatka Alaska island arc system built on oceanic crust ("ensi- Nankai Alëutian Kuril Cascade matic island arc"; sima – artifi cal word fi rst used Ryukyu Japan by Wegener made from silicon and magnesium Izu-Bonin Lesser to characterize ocean fl oor and Earth’s mantle). Mariana Antilles Examples for intra-oceanic, ensimatic island arc Philippine Central America systems include the Mariana Islands in the Pacifi c Solomon and the Lesser Antilles in the Atlantic. Vanuatu Sunda Banda Peru Th e second type occurs where oceanic litho- Tonga Chile sphere is subducted beneath continental litho sphere Kermadec South and an island arc underlain by continental crust Sand- forms (" ensialic island arc"; sial – silicon and alu- wich minum for continental crust). Th e island arc of this system is separated from the continent by a marine basin underlain by oceanic crust. Examples for is- active continental margin ensimatic island arc (deep sea trench <8 km) land arc systems underlain by continental crust are ensialic island arc ensimatic island arc (deep sea trench >8 km) the Japanese Islands and the eastern Sunda Arc. W. Frisch et al., Plate Tectonics, DOI 10.1007/978-3-540-76504-2_7, © Springer-Verlag Berlin Heidelberg 2011 Licensed to jason patton<[email protected]> 792 Subduction zones, island arcs and active continental margins Ensimatic island arc: Mariana Islands only represented by a line at the surface, commonly volcanic arc within a deep sea trench, a zone several hundreds Philippine backarc basin Mariana Trench Sea Plate of kilometers wide is formed by processes which Pacific Plate are related to subduction. Th e volcanic zone above the subduction zone, in many cases expressed as an lithospheric mantle island arc, is the dominating element of this plate boundary system. We use arc as shorthand for the spreading zone terms volcanic arc or magmatic zone. Th e arc is the asthenosphere oceanic crust point of reference for the convergent boundary that is usually divided into three parallel zones: from the trench to the arc is the forearc zone, the arc zone Ensialic island arc: Japan comprises the magmatic belt, and the region behind Sea of Japan Japan the arc is the backarc zone (Fig. 7.3). Th is generally Eurasian Pacific Plate Plate (backarc basin) Mt. agreed upon subdivision of three parts turned out to Fuji be practical in order to describe the complex struc- tures of convergent plate boundary systems. Deep-sea trenches form the major topographic continental crust expression at convergent plate boundaries. Th ese accretionary wedge deep, narrow furrows surround most of the Pacifi c Rim and small portions of the rims that surround the Indian and Atlantic oceans. Oceanic litho sphere is bent downward under the margin of the "upper Active continental margin: Andes Andes high plateau active volcanoes plate" and dives into the asthenosphere. Th e result is an elongated deep trench that is located between South American Nazca Plate retroarc the abyssal plains and the border of the upper plate. Peru-Chile Plate Trench foreland basin Th e deepest trenches, with water depths of about 11,000 m, are known from the Challenger and the Vitiaz deep (named aft er an English research vessel and a Russian researcher) in the southern Mariana trench. Water depths of more than 10,000 m are also known from the Kurile, Izu-Bonin, Philippine and Tonga- Kermadec trenches (Fig. 7.1). Convergent plate boundaries are responsible for the greatest diff erences of relief on the Earth’s Continent-continent collision: Himalayas Eurasian Plate surface. Diff erences in altitude of 10 km between the deep sea trench and volcanoes of the magmatic Indo-Australian Plate arc are not unusual. A relief of 14,300 m across a distance of less than 300 km can be observed Himalayas between Richards Deep (–7636 m) and Llullaillaca India Tibetan Plateau (+6723 m) in the Chilean Andes, the highest active continental crust volcano on Earth. In a transect from the trench onto the upper lithospheric mantle plate, the following morphological features gener- ally occur (Fig. 7.3). Th e landward side of the deep asthenosphere sea trench is part of the upper plate and consists of a slope with an average steepness of several degrees. slab breakoff In front of the Philippine Islands the angle exceeds 8° where a rise from –10,500 m to –200 m occurs over a distance of 70 km. Th e outer ridge follows Fig. 7.2 Examples of diff erent types of plate margins behind the slope. In most cases, the ridge remains with subduction zones. The island arc of the Marianas substantially below sea level; however, in several developed on oceanic crust, that of Japan on continental cases, islands emerge above sea level ( Sunda Arc: crust. The volcanic zone of the Andes is built on the South American continent ( active continental margin). The Mentawai; Lesser Antilles: Barbados). Th e outer collision of two continents produces a mountain range ridge is not always distinctive. Next in the transect, like the Himalayas – subduction wanes, leading to slab directly in front of the volcanic arc, lies the forearc breakoff . basin, another prominent morphological element. Licensed to jason patton<[email protected]> Spontaneous and forced subduction: Mariana- and Chile-type subduction 937 forearc region magmatic backarc region arc outer forearc ocean volcanic arc backarc basin continent ridge basin volcanic front Abukuma-type metamorphism abyssal plain deep sea trench accretionary wedge 0 oceanic crust 50 lithospheric mantle subducted sediments high-pressure metamorphism 100 asthenosphere [km] +100 free-air gravity [mGal] -100 200 heat flow [mW/m2] 100 0 Collectively, the deep sea trench, outer ridge, Fig. 7.3 Structure of a plate margin system with sub- and forearc basin comprise the forearc region. Th e duction zone and ensialic island arc. Gravity and heat fl ow distance between the plate boundary and magmatic data show typical pair of negative and positive anomaly. 1 Gal (galilei) = 1 cm/s2(unit of acceleration). zone has a width of between 100 and 250 km. Th is 1 mGal = 10-3 Gal. mW/m2 = milliwatt per square meter. region is also called the arc- trench-gap, a magmatic gap that with very few exceptions is void of magmat- ic activity. Low temperatures in the crust are caused Spontaneous and forced subduction: by the coolness of the subducting plate underneath Mariana- and Chile-type subduction and prevent the formation of magma by melting or Subduction zones can be subdivided into two the rise of magma from deeper sources. types, based on characteristics of the subducted The volcanic arc, with an average width of plate (Uyeda and Kanamori, 1979).