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Snapshot of
the final state of a compressional "experiment" in a
numerical sandbox. Assemblage has deformed by displacement
of left-hand wall to the right, compressing the system and
causing slip along the basal decollement surface. Particle
configurations show cumulative displacements of particles
resulting final wedge geometry. Displacement vectors
indicate portions of the system which displaced within the
last increment of strain. Displacement gradients denote
discontinuities, i.e., instantaneous fault surfaces; fore
thrusts and back thrusts can be observed, as well as the
advancing deformation front and protothrust zone. Average
particle differential stresses (darker colors) increase as
the wedge grows, and principal stress orientations (vectors)
rotate to become subhorizontal once the deformation front
progresses past them.
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Abstract
Submarine accretionary prisms are composed largely of
poorly consolidated sediments, particularly near the toe.
Field and laboratory observations have shown that these
sediments can display both brittle and ductile behavior,
depending on local stress conditions, stress history, and
physical properties of the sediment. This complexity in
deformation behavior of accreted sediments has stymied many
attempts to develop numerical models for the study of
accretionary processes; continuum models may capture the
overall geometry of the prism, but cannot reproduce the
complex structure and evolution of natural prisms.
A numerical technique known as the distinct element
method (DEM) provides a way to simulate accretionary prisms
as discontinuous systems, e.g., assemblages of particles
that interact individually with eachother to generate the
behavior of the whole. Because particles obey simple
physical laws of interaction, the technique defines rather
than relies on the constitutive behavior of the assemblage.
In this "numerical sandbox", it becomes possible to explore
relationships among sediment properties, local stress
conditions, deformation mode, and prism structure. As a test
of the feasibility of this technique, a series of DEM
simulations were conducted using several thousand particles
within compressional boundaries. These simulations
qualitatively reproduce the behavior and geometries of
natural prisms. Depending on material strength, the prism
grows by smooth advance of the deformation front (ductile
deformation) or by formation and propagation of discrete
frontal thrust faults (brittle deformation); broad folds
form above thrust faults; out-of-sequence thrusts displace
faults and strata within the prism. Estimates of prism taper
show variations with internal and basal friction in
approximate agreement with critical Coulomb wedge theory.
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