Double Subduction |
By Jennifer BromleyUniversity of Maine, USAERS 416 Structural Geology 2006 |
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| Figure 1. A cross-section of double subduction zone of the Molucca Sea that shows the collision of the Halmahera and the Sangihe arcs. The figure illustrates that the Halmahera arc in the east has been completely over-ridden by the Sangihe arc in the west and ophilotie complexes have been exposed from the Sangihe fore-arc basin on the Talaud islands. (Hall, 2000) |
Abstract |
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Subduction
of dense ocean lithosphere is a crucial process because it
provides the primary driving force for plate tectonics. Once the edge
of an oceanic plate is turned down,
it will sink down into more ductile and less dense asthenosphere
below. The falling of the oceanic slab exerts a strong downward
pull force and is the most influential force in subduction. Subduction
leads to volcanism and the largest earthquakes on Earth, for example,
the 9.2 magnitude earthquake in Alaska, 1964. Therefore, subduction is
of first order significance in the understanding if the evolution of
the Earth. Subduction at both ends of oceanic lithosphere can result in
the complete removal of an ocean and collision of the over-riding
continental plates. This is currently happening to Molucca Sea Plate and
the double subduction is clearly marked by two oceanic
arcs. In this special case of subduction little is known about the flow
that is generated in the asthenosphere, which is important because it
will drive the motions of surrounding plates. In order to investigate
the flow patterns on the mantle an analogue experiment was established
with oceanic lithosphere represented as weight silicon putty that
subducted into corn syrup. Borax flubber represented continental
lithosphere and was placed on top of the putty at either end. Both
sides of the putty were allowed to sink into the corn syrup, which was
seeded with many tapioca pearls to track the motions in the syrup. The
densities of the silicon putty and corn syrup were scaled to represent
density driven sinking that occurs in the natural world. The
results show that the shear strain and rotation are both concentrated
in the the corner flow regions and around the edges of the subducted
slab. Volumetric strain indicates where particles flow around the edge
of the down going slab because of the corn syrup is an uncompress able
fluid. The analogue model of double subduction shows that there is a
change from two regions of corner flow to wholesale vertical
displacement when subduction ends and the slab falls away. This model
gives an incite into the current flow fields in the Molucca sea.
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| Abstract |
Subduction background |
Molucca
Sea |
Experimental Setup |
Scaling | Results | Conclusion |
References |