Forearcs
Forearc is the one section of the IOAC which is most likely to survive a later continental collision or continental- oceanic subduction ( discussed in Forearcs). You may see:
Forearc is the one section of the IOAC which is most likely to survive a later continental collision or continental- oceanic subduction ( discussed in Forearcs). You may see:
- Boninites and High magnesium andesites
- Basalts
- Blocks Harzburgite (Ultramafic periodite which is high in olivine and orthopyroxene but low in clinopyroxene, See FIG. X ) sitting in an serpentinite matrix from mud volcanoes. Blocks in the serpentinite muds can be metamorphosed to the greenschist or Blueschist facies.
- Harzburgite
Accretion vs. non-accretion
Studies done on modern intra-oceanic arc systems shows that most lack accretion. This means that there is a lack of accumulation of sediment 'scraped off' as the underlying plate subducts (Draut & Clift, 2012) This means that accretionary complexes are uncommon as the sediments arriving at the trenches are subducted along with the underlying plate. However there are two exceptions. The Lesser Antilles and Aleutian arcs both have relatively high sediment inputs and accretionary complexes have formed. Tectonic erosion vs. accretion is an important control on the ultimate survival of material from the trench, forearc, arc massif, intra-arc basins, and backarc basins (Draut & Clift, 2012). Consequently, how well an ancient arc terrane preserves evidence for tectonic processes such as subduction of seismic ridges and seamounts, oblique plate convergence, and arc rifting is also related to accretion vs. non-accretion (Draut & Clift, 2012).
Figure 2: Schematic cartoons showing the two basic types of active margin: tectonically accreting and tectonically eroding from Draut and Clift (2012). (A) Accretionary margins are characterized by forearc regions comprising thrusted and penetratively deformed trench and oceanic sediments that often develop mud diapirism and volcanism due to sediment over-pressure. Gas-hydrate zones also are commonly associated with structures in the accretionary wedge. (B) Erosive plate margins, such as Tonga, are marked by steep trench slopes composed of volcanic, plutonic and mantle rocks. Sedimentary rocks typically are limited to the forearc basin, where they may be faulted but are not strongly sheared as in an accretionary wedge. In the Mariana arc serpentinite mud volcanism occurs.