Encyclopedia of Marine Geosciences DOI 10.1007/978-94-007-6644-0_102-2 # Springer Science+Business Media Dordrecht 2014 Active Continental Margin Serge Lallemand* Géosciences Montpellier, University of Montpellier, Montpellier, France Synonyms Convergent boundary; Convergent margin; Destructive margin; Ocean-continent subduction; Oceanic subduction zone; Subduction zone Definition An active continental margin refers to the submerged edge of a continent overriding an oceanic lithosphere at a convergent plate boundary by opposition with a passive continental margin which is the remaining scar at the edge of a continent following continental break-up. The term “active” stresses the importance of the tectonic activity (seismicity, volcanism, mountain building) associated with plate convergence along that boundary. Today, people typically refer to a “subduction zone” rather than an “active margin.” Generalities Active continental margins, i.e., when an oceanic plate subducts beneath a continent, represent about two-thirds of the modern convergent margins. Their cumulated length has been estimated to 45,000 km (Lallemand et al., 2005). Most of them are located in the circum-Pacific (Japan, Kurils, Aleutians, and North, Middle, and South America), Southeast Asia (Ryukyus, Philippines, New Guinea), Indian Ocean (Java, Sumatra, Andaman, Makran), Mediterranean region (Aegea, Cala- bria), or Antilles. They are generally “active” over tens (Tonga, Mariana) or hundreds (Japan, South America) of millions of years. This longevity has consequences on their internal structure, especially in terms of continental growth by tectonic accretion of oceanic terranes, or by arc magmatism, but also sometimes in terms of continental consumption by tectonic erosion. Morphology A continental margin generally extends from the coast down to the abyssal plain (see Fig. 1 and entry “▶ Morphology Across Convergent Plate Boundaries”). It typically includes a continental platform gently dipping seaward and a talus with a steeper slope down to the trench. In detail, active continental margins offer a wide spectrum of morphologies from narrow and steep ones (e.g., Porto Rico) to wide and flat ones (e.g., Mediterranean “Ridge” which is a huge evaporitic accre- tionary wedge). In general, the continental shelves bordering active margins are narrower than those *Email: [email protected] *Email: [email protected] Page 1 of 6 Encyclopedia of Marine Geosciences DOI 10.1007/978-94-007-6644-0_102-2 # Springer Science+Business Media Dordrecht 2014 Fig. 1 Schematic view of an active continental margin after Lallemand et al. (2005) along passive margins. The trench marks the deepest seaward termination of the active continental margin. It coincides with the surface trace of the subduction fault marking the boundary between the converging plates. Its depth generally ranges between 2 and 7 km depending on the age of the subducting plate (young plate like Juan de Fuca – shallow trench) and the amount of trench fill sediment (thick trench fill like Makran – shallow trench). Birth, Life, and Death The discovery of microdiamonds in two billion-year-old rocks in Canada attests to ultrahigh- pressure metamorphism compatible with subduction in the early Proterozoic (Cartigny et al., 2004). The ongoing processes responsible for the sinking of the lithosphere at that time probably differed from those at present since the Earth was hotter than today. For modern subduction zones, Seiya Uyeda, in 1984, has proposed two modes of subduction mechanisms: a forced one (Chilean type) in contrast with a spontaneous one (Mariana type), further used by many authors like Stern and Bloomer (1992). These notions should now be replaced by compressional versus extensional subduction, based on the observation of the dominant strain within the overriding plate and not on the cause of the subduction itself (Heuret and Lallemand, 2005). Indeed, the observation of nascent or young (less than 10 million years old) subduction zones in the western Pacific has shown that all of them result from ongoing collisions and plate boundaries reorganization (Lallemand et al., 2005). None of them can be simply explained by the spontaneous sinking of an old and dense oceanic lithosphere under its own weight. The main driving force for subduction, after it initiates and develops down to a depth of about 200 km, is the slab pull exerted by the excess mass of the slab with respect to the surrounding mantle (Turcotte and Schubert, 1982; Hassani et al., 1997). The mantle convection around a subduction mainly results from the dynamics of the plate subduction, not the opposite (Kincaid and Sacks, 1997). Once the subduction process is launched, it becomes stable over millions of years unless an oceanic plateau, an island arc, or a continental block reaches Page 2 of 6 Encyclopedia of Marine Geosciences DOI 10.1007/978-94-007-6644-0_102-2 # Springer Science+Business Media Dordrecht 2014 the subduction zone. If this occurs, subduction may stop in the collision zone and jump to another place where a new oceanic subduction may develop (Taiwan, New Guinea, Philippines, Indonesia, etc.). In a few cases, continental subduction prevails for tens of millions of years before plate reorganization (Himalaya, Weissel et al., 1980; Shemenda, 1992). Tectonic Activity Earthquakes The subduction process generates large stresses within both converging plates as well as along the subduction interface. Part of the stress is intermittently released seismically (seismic cycle). The largest earthquakes are produced by the so-called subduction earthquakes, i.e., the seismic expres- sion of mega-ruptures along the frictional part of the plate interface, also called “seismogenic zone” (M9.0 2011 Tohoku earthquake, M9.2 2004 Sumatra earthquake, etc.). These particular earthquakes dissipate at least 85 % of the total seismic energy released in the world (Scholz, 1990). The reason is that the subduction interfaces are the largest faults on Earth (up to several hundred km, even 1,000 + km long) accommodating the largest slips (up to several tens of meters). Kanamori (1977) has shown that the seismic moment for great earthquakes is proportional to the faulted area and the slip on the fault. In addition to the subduction earthquakes, intraplate earthquakes may also occur either in the overriding plate, especially if compressive or extensive stress is transmitted from the subduction zone, or the downgoing plate (commonly called slab when it subducts into the mantle). The intraslab earthquakes include (Fig. 2) the flexural earthquakes caused by plate bending near the trench, the intermediate earthquakes (down to about 300 km) resulting from the down-dip stress (often extensional as a result of the slab pull) or the unbending processes necessary to unfold the slab as it penetrates into the mantle, and the deep earthquakes (down to the discontinuity between the upper and lower mantle at 660–670 km). For further details, see also the following entries: “▶ Subduction,”“▶ Earthquake,” and “▶ Seismogenic Zone.” Volcanism Most subduction zones are marked by a volcanic arc located at a distance between 100 and 300 km landward from the trench and 110 Æ 20 km above the subducting slab. Arc magmatism results from the dehydration of the crust and overlying sediment carried by the downgoing oceanic plate. Such dehydration occurs at various depths depending on pressure/temperature conditions, but those occurring below the convective mantle wedge trigger melting and metasomatism of the mantle and subsequent rise of magmas. The explosivity of the volcanism is attributed to the high volatile content in the magma, especially H2O. Calc-alkaline, potassic calc-alkaline, and shoshonitic series characterize the arc volcanoes lying on continental crust. For further details, see entry “▶ Island Arc Volcanism, Volcanic Arcs.” Terrane Collision/Accretion and Mountain Building Oceanic plates may carry seamounts, plateaus, active or fossil island arcs, and active or fossil spreading centers. Moreover, they may be attached to continents. Thus, it follows logically that during long period of activity of an oceanic subduction, collisions with buoyant features carried by the subducting plate occur. Collisions may be frontal like the Ontong-Java plateau with the Solomon arc or oblique like the Philippine Mobile Belt (Lallemand, 1999). Collisions along active continental margins often give rise to the accretion of crustal slivers (e.g., Izu-Bonin arc accretion in Japan; Tamura et al., 2010) or even large-size exotic terranes of oceanic or continental affinities such as the Page 3 of 6 Encyclopedia of Marine Geosciences DOI 10.1007/978-94-007-6644-0_102-2 # Springer Science+Business Media Dordrecht 2014 Fig. 2 Global view of an oceanic subduction zone with an old slab sinking at a high rate in order to get deep earthquakes after Lallemand et al. (2005). The isotherms are deflected along the top of the cold descending slab. Phase changes occur at various temperature/pressure (depth) conditions. This is why they occur at different depths in the slab. Different types of earthquakes (shallow, intermediate, and deep) and stable minerals are described. Variations in the mean density of the slab is indicated in the left column as the main phase transitions occur Caribbean (Antilles) or the Okhotsk (Russia) plateaus, the Panama-Choco (Colombia, Panama) or the Halmahera (Philippines) arcs, and the Qiantang, the Lhasa, or the South China continental blocks (e.g., Lallemand et al., 1998;