I'd like to learn about the relationship between the foliation occurrence of the accretionary wedge in fore-arc and the direction of subduction.
Forget the environment and think only about the forces. Take a book and fol it. The sheets are the foliation. Along the fold you have the fold axis. So, perpendicular to the axis direction you have the direction of the force you used to fold the book. The dip direction of the planes of the foliation is the direction of the plate`s movement. You can also think that the fold axis is inthe perpendicular direction to the force that made the fold and it is the direction that the plane moved from. That means. The direction of subduction is perpendicular to the direction of the fold axis, and it is paralelel to the direction to where the foliation dips. The direction has two sides, N-S, E-W, NW-SE, SW-NE. To know if it came from South to North or from North to South, you must measure the fold`s axial plane which is then called VERGENCE. The meaning of vergence in modern structural geology usage is unclear. Vergence has been used to indicate the direction in which an antiform or synform is inclined or overturned. It is very similar to the fold´s axial plane. But in this case, the side the vergenge dips is the side from where the force came, what is the side the subducting plate took and pushed the non subducting plate.
Hi,
I am afraid that using the foliation orientation is not enough to deduce the direction of subduction. The foliation may have been rotated as a consequence of later events, or obliterated by a new phase of deformation that not necessarily coincides with the subduction direction (e.g. exhumation).
While it is true that many foliations form parallel to folds (i.e. in the X-Y plane of the strain ellipsoid), that is not always the case (e.g. they may develop in a shear zone), and a careful examination of the relations between large-scale folds and foliation is needed to ascertain that. Furthermore, fold axis orientation is a poor constraint on the direction of subduction, as they may rotate due to superimposed shear strain, and their orientation can be anything from normal to parallel to the tectonic transport. Again, a careful structural analysis of the fabrics is needed to ascertain this issue.
Ideally, I would look for a combination of metamorphic facies indicators (if you are dealing with those rocks), mapping and a detailed structural analysis of foliations, fold axes, lineations, and shear sense indicators (either non-coaxial or coaxial).
These estimations are often very difficult, and a lot of data and effort is usually needed. There are several books or articles where this is further developed, I would reccommend you Structural Geology by Fossen, Microtectonics by Passchier, or articles such as:
Trotet, F., Jolivet, L., Vidal, O. (2001). Tectonometamorphic evolution of Syros and Sifnos islands (Cyclades, Greece). Tectonophysics, 338, 179-206.
Laurent, V., Jolivet, L., Roche, V., Augier, R., Scaillet, S., Cardello, G.L. (2016). Strain localization in a fossilized subduction channel: Insights from the Cycladic Blueschist Unit (Syros, Greece). Tectonophysics, 672-673, 150-169.
Those are the ones I'm more familiar with.
Kind regards,
Manuel
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