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| Titel |
Forward Versus Back Thrusts in Accretionary Wedges: Effects of Rheology and Thickness of the Décollement Layer |
| VerfasserIn |
Garrett Ito, Jean-Arthur Olive, Gregory Moore, Marc-Andre Gutscher, Jonathan Weiss |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250130070
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| Publikation (Nr.) |
 EGU/EGU2016-10261.pdf |
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| Zusammenfassung |
| The mechanical processes that control whether major thrusts in accretionary wedges verge
forward toward the foreland, versus backward toward the hinterland has long been a topic of
debate. Whereas forethrusts are the most common major thrusts, the importance of the
globally rare back thrusts has recently been highlighted given their prominence along
the Cascadia margin off of the NW coast of North America as well as along the
Andaman-Sumatra subduction zone, in the rupture area of the great 2004 earthquake.
We address this problem using 2-D numerical models that use a finite-difference,
particle-in-cell method with a viscoelastic-plastic rheology for simulating thrusting in
accretionary wedges. Simulations of a weak frictional décollement confirm prior
numerical and analogue modeling studies in that they predict lower wedge tapers
and repeated sequences of doubly verging conjugate thrusts. A forward dipping
backstop was shown in prior laboratory experiments to promote backthrusting, and
our results confirm that backthrusting occurs near the backstop but as the wedge
widens away from the backstop forethrusts become dominant. Other laboratory
experimental studies have found that a non-brittle, viscously deforming décollement
can promote backthrusting. Our numerical models show that if the viscosity of the
décollement layer η is too high, such that the stress scale, ηU/H (where U is the
convergence rate and H is the décollement layer thickness), is comparable to the
frictional strength at the base, then forethrusts dominate. For ηU/H less than the
basal frictional strength, doubly verging faults are prominent over a wide range
of décollement layer thicknesses. Only for cases with relatively low ηU/H and
décollement layer thicknesses H that are 25-33% of the thickness of the whole, incoming
sediment layer do backthrusts dominate. Thus backthrusting appears to require unusual
rheological properties of the deepest sediments, which is consistent with the rarity of
backthrusting, but is a prediction that requires further testing with seismic and geologic
observations. |
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