accretionary wedge

Structurally complex parts of subduction
zone systems, accretionary wedges are formed on the
landward side of the trench by material scraped off from the
subducting plate as well as trench fill sediments. They typically
have wedge-shaped cross sections and have one of the most
complex internal structures of any tectonic element known
on Earth. Parts of accretionary wedges are characterized by
numerous thin units of rock layers that are repeated bynumerous thrust faults, whereas other parts or other wedges
are characterized by relatively large semi-coherent or folded
packages of rocks. They also host rocks known as tectonic
mélanges that are complex mixtures of blocks and thrust
slices of many rock types (such as graywacke, basalt, chert,
and limestone) typically encased in a matrix of a different
rock type (such as shale or serpentinite). Some accretionary
wedges contain small blocks or layers of high-pressure lowtemperature
metamorphic rocks (known as blueschists) that
have formed deep within the wedge where pressures are high
and temperatures are low because of the insulating effect of
the cold subducting plate. These high-pressure rocks were
brought to the surface by structural processes.
Accretionary wedges grow by the progressive offscraping
of material from the trench and subducting plate, which constantly
pushes new material in front of and under the wedge
as plate tectonics drives plate convergence. The type and style
of material that is offscraped and incorporated into the
wedge depends on the type of material near the surface on
the subducting plate. Subducting plates with thin veneers of
sediment on their surface yield packages in the accretionary
wedge dominated by basalt and chert rock types, whereas
subducting plates with thick sequences of graywacke sediments
yield packages in the accretionary wedge dominated by
graywacke. They may also grow by a process known as
underplating, where packages (thrust slices of rock from the
subducting plate) are added to the base of the accretionary
wedge, a process that typically causes folding of the overlying
parts of the wedge. The fronts or toes of accretionary wedges
are also characterized by material slumping off of the steep
slope of the wedge into the trench. This material may then be
recycled back into the accretionary wedge, forming even
more complex structures. Together, the processes of offscraping
and underplating tend to steepen structures and rock layers
from an orientation that is near horizontal at the toe of
the wedge to near vertical at the back of the wedge.
The accretionary wedges are thought to behave mechanically
somewhat as if they were piles of sand bulldozed in
front of a plow. They grow a triangular wedge shape that
increases its slope until it becomes oversteepened and
mechanically unstable, which will then cause the toe of the
wedge to advance by thrusting, or the top of the wedge to collapse by normal faulting. Either of these two processes can
reduce the slope of the wedge and lead it to become more stable.
In addition to finding the evidence for thrust faulting in
accretionary wedges, structural geologists have documented
many examples of normal faults where the tops of the wedges
have collapsed, supporting models of extensional collapse of
oversteepened wedges.
Accretionary wedges are forming above nearly every
subduction zone on the planet. However, these accretionary
wedges presently border open oceans that have not yet closed
by plate tectonic processes. Eventually, the movements of the
plates and continents will cause the accretionary wedges to
become involved in plate collisions that will dramatically
change the character of the accretionary wedges. They are
typically overprinted by additional shortening, faulting, folding,
and high-temperature metamorphism, and intruded by
magmas related to arcs and collisions. These later events,
coupled with the initial complexity and variety, make identification
of accretionary wedges in ancient mountain belts difficult,
and prone to uncertainty.
Further Reading
Kusky, Timothy M., and Dwight C. Bradley. “Kinematics of Mélange
Fabrics: Examples and Applications from the McHugh Complex,
Kenai Peninsula, Alaska.” Journal of Structural Geology 21, no.
12 (1999): 1,773–1,796.
Kusky, Timothy M., Dwight C. Bradley, Peter Haeussler, and Susan
M. Karl. “Controls on Accretion of Flysch and Mélange Belts at
Convergent Margins: Evidence from The Chugach Bay Thrust
and Iceworm Mélange, Chugach Terrane, Alaska.” Tectonics 16,
no. 6 (1997): 855–878.
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