Intra-oceanic Arc Vs. Andean-type Arc
Fig.1
Figure 1: Andean-type Arc Vs. Intra-Oceanic Arc (from Stern, 2004). This is a very simplified model for both arc types. (A) Andean-type arc where oceanic lithosphere is subducted under continental crust. Note the accretionary prism and the relatively high sediment flux in addition to the thicknesses of the upper and lower crust. (B) Intra-oceanic arc where oceanic crust subducts under oceanic crust. Even though this diagram suggests that mostly mafic lavas erupt, it is known that more bimodal magmastism occurs. Generally there is no accretionary wedge and erision is much more common.
What exactly are intra-oceanic arcs?
Intra-oceanic arcs are magmatic arcs developed within ocean basins, built on oceanic derived crust. This oceanic crust is formed at either mid-ocean ridges or back-arc spreading centres, crust forming part of an oceanic plateau, crust formed by accretion of oceanic sediments in a subduction zone fore-arc or earlier intra-oceanic arc material. Figure 1 displays the locations of modern intra-oceanic subduction systems. IOASs tend to form where older oceanic crust is subducted under younger oceanic crust. Relatively older oceanic crust is negatively buoyant. This causes the subducting plate to sink vertically in addition to subducting down-dip which causes roll-back of the trench and associated arc. This situation favours the development of convergent plate margins within the oceanic realm and thus IOASs. Subduction initiation is further discussed here
Figure 1:Locations of modern intra-oceanic subduction systems (Mollweide projection). Trenches associated
with these systems are marked by barbed lines. Other plate boundaries are shown as thin solid lines.
Intra-oceanic subduction systems marked by numbers enclosed in circles are: 1, MacQuarie; 2,
Tonga-Kermadec; 3, Vanuatu (New Hebrides); 4, Solomon; 5, New Britain; 6, Halmahera; 7, Sangihe; 8,
Ryukyu; 9, Mariana; 10, Izu-Ogasawara (Bonin); 11, Aleutian; 12, Lesser Antilles; 13, South Sandwich. The
Halmahera and Sangihe arcs are shown in their Neogene configuration; they are now in collision.
The Importance of Intra-oceanic Arc Systems:
Most of what is known about convergent margins can be attributed to Andean-type subduction zones as they are more exposed and easily accessible. Because intra-oceanic arc systems are predominantly submerged under water, much less is known about them. However, as intra-oceanic convergent margins are far-removed from terrestrial sources, of sediments fore-arc architecture is largely exposed. This lack of sediments allows subduction processes to be more easily studied as sampling can become more direct. This includes fluids released during subduction which vent through the fore-arc.
Consequently the composition and structure of the earliest subduction-related rocks can be utilized to further understand how subduction zones form, where a greater understanding of how the crust evolved can also be established.
Consequently the composition and structure of the earliest subduction-related rocks can be utilized to further understand how subduction zones form, where a greater understanding of how the crust evolved can also be established.