Fore arcs in an intra- oceanic basin.
What and where is a forearc?
Fore-arc is the crust which lies between the Volcanic arc and the trench (FIG.1). The forearc traditionally is understudied for the following reasons:
- On continental-intraoceanic subduction zones the forearc is covered by accretionary wedge material.
- Though most IOAS are erosive and have no accretionary forearc, they are kilometers below sea level and hard to acess.
- When noted it was assumed to be a slice of N-MORB which was trapped between the trench and Volcanic front.
Fore-arc is the crust which lies between the Volcanic arc and the trench (FIG.1). The forearc traditionally is understudied for the following reasons:
- On continental-intraoceanic subduction zones the forearc is covered by accretionary wedge material.
- Though most IOAS are erosive and have no accretionary forearc, they are kilometers below sea level and hard to acess.
- When noted it was assumed to be a slice of N-MORB which was trapped between the trench and Volcanic front.
So what made some geologist's change their minds and begin to believe that forearc crust was made by a different process?
Scientific dive missions to the inner wall of the trench collected samples of exposed oceanic arc rock ( FIG.2). The Peridotite exposed by erosion, was low in Aluminium oxide and calcium oxide ( Fig.3) compared to MOR or abyssal Peridotite. MOR is formed from the direct melt of fertile unalterered asthenospheric mantle or Llherzolite, whilst this Fore arc Peridodite was formed from the melting of Harzburgite( FIG.4). Harzburgite is part of a series, in which the original lherzolite undergoes melt depletion characteristic of an super high temperature and/of high volatile environment.
Scientific dive missions to the inner wall of the trench collected samples of exposed oceanic arc rock ( FIG.2). The Peridotite exposed by erosion, was low in Aluminium oxide and calcium oxide ( Fig.3) compared to MOR or abyssal Peridotite. MOR is formed from the direct melt of fertile unalterered asthenospheric mantle or Llherzolite, whilst this Fore arc Peridodite was formed from the melting of Harzburgite( FIG.4). Harzburgite is part of a series, in which the original lherzolite undergoes melt depletion characteristic of an super high temperature and/of high volatile environment.
Who cares about forearcs anyway? Why does this matter?
The Forearc is the section of the subduction setting which is most likely to survive a later collision events ( Most is consumed into the subduction zone). So this is what we will see obducted onto to the continent and evidence that a subduction setting even existed. The type of Peridotite will show how depleted the mantle was and presence of Boninites indicates an intraoceanic setting ( Stern 2012)
The Forearc is the section of the subduction setting which is most likely to survive a later collision events ( Most is consumed into the subduction zone). So this is what we will see obducted onto to the continent and evidence that a subduction setting even existed. The type of Peridotite will show how depleted the mantle was and presence of Boninites indicates an intraoceanic setting ( Stern 2012)