Widths of imbricate thrust blocks and the strength of the front of accretionary wedges and fold-and-thrust belts
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2021-12-03
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Abstract
Besides the large-scale wedge shape itself, the most prominent structural feature of accretionary wedges and foldand-
thrust belts is the common pattern of imbricate thrust faults. This study illuminates the fundamental mechanical
processes and material properties controlling the width of the crustal blocks bounded by major thrusts
using analytical solutions of stress as well as two-dimensional finite-difference models. The numerical models
predict that the initial width w0 of a thrust block is set when that block first forms at the very front of the wedge.
The width is found to subsequently decreases approximately in proportion to the mean horizontal strain needed
for an ideally triangular-shaped Coulomb wedge with a critical taper. Block width is proportional to the thickness
H of the incoming, accreting sediment. A key quantity that influences the normalized initial block width w0/H is
the distance L forward of the frontal thrust needed for the net horizontal force from shear on the base of the
incoming sediment to balance the net force on the frontal thrust. It is within this distance where stress in the
incoming sediment is substantially elevated and thus where the new frontal thrust forms. Results show that L/H
and, correspondingly, w0/H increase with increasing sediment friction angle ϕ, cohesive strength C0 and porefluid
pressure ratio λ, and decrease with increasing basal friction angle ϕb and basal dip β. Normalized width
is sensitive to ϕ and relatively insensitive to ϕb and λ. Results for submarine and subaerial wedges follow the
same scaling law. The scaling law relates the observables, w0/H and β, to the material properties, ϕ, ϕb, λ, and
therefore provides a theoretical relation that can be used independent of, or together with critical Coulomb
wedge theory (CWT) to constrain these properties.
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Ito, G. and G.F. Moore (2021) Widths of imbricate thrust blocks and the strength of the front of accretionary wedges and fold-and-thrust belts. Tectonophys., 799, https://doi.org/10.1016/j.tecto.2020.228704
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