Structural, igneous,
metamorphic, sedimentological processes that occur in the
region affected by forces associated with the convergence of
two or more plates. Convergent plate boundaries are of two
fundamental types, subduction zones and collision zones.
Subduction zones are of two basic types, the first of which
being where oceanic lithosphere of one plate descends
beneath another oceanic plate, such as in the Philippines and
Marianas of the southwest Pacific. The second type of subduction
zone forms where an oceanic plate descends beneath
a continental upper plate, such as in the Andes of South
America. The southern Alaska convergent margin is particularly
interesting, as it records a transition from an ocean/continent
convergent boundary to an ocean/ocean convergent
boundary in the Aleutians.
Arcs have several different geomorphic zones defined
largely on their topographic and structural expressions. The
active arc is the topographic high with volcanoes, and the
backarc region stretches from the active arc away from the
trench, and it may end in an older rifted arc or continent. The
forearc basin is a generally flat topographic basin with shallow
to deepwater sediments, typically deposited over older
accreted sediments and ophiolitic or continental basement.
The accretionary prism includes uplifted strongly deformed
rocks that were scraped off the downgoing oceanic plate on a
series of faults. The trench may be several to 10 or more kilometers
below the average level of the seafloor in the region
and marks the boundary between the overriding and underthrusting
plate. The outer trench slope is the region from the
trench to the top of the flexed oceanic crust that forms a few-
hundred-meter-high topographic rise known as the forebulge
on the downgoing plate.
Trench floors are triangular-shaped in profile and typically
partly to completely filled with graywacke-shale turbidite
sediments derived from erosion of the accretionary
wedge. They may also be transported by currents along the
trench axis for large distances, up to hundreds or even thousands
of kilometers from their ultimate source in uplifted
mountains in the convergent orogen. Flysch is a term that
applies to rapidly deposited deep marine synorogenic clastic
rocks that are generally turbidites. Trenches are also characterized
by chaotic deposits known as olistostromes that typically
have clasts or blocks of one rock type, such as limestone
or sandstone, mixed with a muddy or shaly matrix. These are
interpreted as slump or giant submarine landslide deposits.
They are common in trenches because of the oversteepening
of slopes in the wedge. Sediments that get accreted may also
include pelagic sediments that were initially deposited on the
subducting plate, such as red clay, siliceous ooze, chert, manganiferous
chert, calcareous ooze, and windblown dust.
The sediments are deposited as flat-lying turbidite packages,
then gradually incorporated into the accretionary wedge
complex through folding and the propagation of faults
through the trench sediments. Subduction accretion is a process
that accretes sediments deposited on the underriding plate
onto the base of the overriding plate. It causes the rotation and
uplift of the accretionary prism, which is a broadly steady-state
process that continues as long as sediment-laden trench
deposits are thrust deeper into the trench. Typically new faults
will form and propagate beneath older ones, rotating the old
faults and structures to steeper attitudes as new material is
added to the toe and base of the accretionary wedge. This process
increases the size of the overriding accretionary wedge and
causes a seaward-younging in the age of deformation.
Parts of the oceanic basement to the subducting slab are
sometimes scraped off and incorporated into the accretionary
prisms. These tectonic slivers typically consist of fault-bounded
slices of basalt, gabbro, and ultramafic rocks, and rarely, partial
or even complete ophiolite sequences can be recognized.
These ophiolitic slivers are often parts of highly deformed belts
of rock known as mélanges. Mélanges are mixtures of many
different rock types typically including blocks of oceanic basement
or limestone in muddy, shaly, serpentinic, or even a cherty
matrix. Mélanges are formed by tectonic mixing of the
many different types of rocks found in the forearc, and they
are among the hallmarks of convergent boundaries.
There are major differences in processes that occur at
Andean-style v. Marianas-style arc systems. Andean-type arcs
have shallow trenches, less than 3.7 miles (6 km) deep,
whereas Marianas-type arcs typically have deep trenches
reaching 6.8 miles (11 km) in depth. Most Andean-type arcs
subduct young oceanic crust and have very shallow-dipping
subduction zones, whereas Marianas-type arcs subduct old
oceanic crust and have steeply dipping Benioff zones. Andean
arcs have back arc regions dominated by foreland (retroarc)
fold thrust belts and sedimentary basins, whereas Marianastype
arcs typically have back arc basins, often with active
seafloor spreading. Andean arcs have thick crust, up to 43.5
miles (70 km), and big earthquakes in the overriding plate,
while Marianas-type arcs have thin crust, typically only 12.5
miles (20 km), and have big earthquakes in the underriding
plate. Andean arcs have only rare volcanoes, and these have
magmas rich in SiO2 such as rhyolites and andesites. Plutonic
rocks are more common, and the basement is continental
crust. Marianas-type arcs have many volcanoes that erupt
lava low in silica content, typically basalt, and are built on
oceanic crust.
Many arcs are transitional between the Andean or continental
margin-types and the oceanic or Marianas-types, and
some arcs have large amounts of strike-slip motion. The causes
of these variation have been investigated, and it has been
determined that the rate of convergence has little effect, but
the relative motion directions and the age of the subducted
oceanic crust seem to have the biggest effects. In particular,
old oceanic crust tends to sink to the point where it has a
near-vertical dip, rolling back through the viscous mantle and
dragging the arc and forearc regions of overlying Marianastype
arcs with it. This process contributes to the formation of
back arc basins.
Much of the variation in processes that occur in convergent
margin arcs can be attributed to the relative convergence
vectors between the overriding and underriding plates. In this
kinematic approach to modeling convergent margin processes,
the underriding plate may converge at any angle with the
overriding plate, which itself moves toward or away from the
trench. Since the active arc is a surface expression of the 68-
mile (110-km) isobath on the subducted slab, the arc will
always stay 110 kilometers above this zone. The arc therefore
separates two parts of the overriding plate that may move
independently, including the frontal arc sliver between the arc
and trench, that is kinematically linked to the downgoing
plate, and the main part of the overriding plate. Different relative
angles of convergence between the overriding and
underriding plate determine whether or not an arc will have
strike-slip motions, and the amount that the subducting slab
rolls back (which is age-dependent) determines whether the
frontal arc sliver rifts from the arc and causes a back arc
basin to open or not. This model helps to explain why some
arcs are extensional with big back arc basins, others have
strike-slip dominated systems, and others are purely compressional
arcs. Convergent margins also show changes in these
vectors and consequent geologic processes with time, often
switching from one regime to the other quickly with changes
in the parameters of the subducting plate.
The thermal and fluid structure of arcs is dominated by
effects of the downgoing slab, which is much cooler than the
surrounding mantle and serves to cool the forearc. Fluids
released from the slab as it descends past 110 kilometers aid
partial melting in the overlying mantle and form the magmas
that form the arc on the overriding plate. This broad thermal
structure of arcs results in the formation of paired metamorphic
belts, where the metamorphism in the trench environment
grades from cold and low-pressure at the surface to
cold and high-pressure at depth, whereas the arc records low
and high-pressure high-temperature metamorphic facies
series. One of the distinctive rock associations of trench environments
is the formation of the unusual high-pressure lowtemperature
blueschist facies rocks in paleosubduction zones.
The presence of index minerals glaucophane (a sodic amphibole),
jadeite (a sodic pyroxene), and lawsonite (Ca-zeolite)
indicate low temperatures extended to depths of 20–30 kilometers
(7–10 kilobars). Since these minerals are unstable at
high temperatures, their presence indicates they formed in a
low temperature environment, and the cooling effects of the
subducting plate offer the only known environment to maintain
such cool temperatures at depth in the Earth.
Forearc basins may include several-kilometer-thick accumulations
of sediments that were deposited in response to
subsidence induced by tectonic loading or thermal cooling of
forearcs built on oceanic lithosphere. The Great Valley of California
is a forearc basin that formed on oceanic forearc crust
preserved in ophiolitic fragments found in central California,
and Cook Inlet in Alaska is an active forearc basin formed in
front of the Aleutian and Alaska range volcanic arc.
The rocks in the active arcs typically include several different
facies. Volcanic rocks may include subaerial flows,
tuffs, welded tuffs, volcaniclastic conglomerate, sandstone,
and pelagic rocks. Debris flows from volcanic flanks are common,
and there may be abundant and thick accumulations of
ash deposited by winds and dropped by Plinian and other
eruption columns. Volcanic rocks in arcs include mainly calcalkaline
series, showing an early iron enrichment in the melt,
typically including basalts, andesites, dacites, and rhyolites.
Immature island arcs are strongly biased toward eruption at
the mafic end of the spectrum and may also include tholeiitic
basalts, picrites, and other volcanic and intrusive series. More
mature continental arcs erupt more felsic rocks and may
include large caldera complexes.
Back arc or marginal basins form behind extensional
arcs or may include pieces of oceanic crust that were trapped
by the formation of a new arc on the edge of an oceanic
plate. Many extensional back arcs are found in the southwest
Pacific, whereas the Bering Sea between Alaska and Kamchatka
is thought to be a piece of oceanic crust trapped during
the formation of the Aleutian chain. Extensional back arc
basins may have oceanic crust generated by seafloor spreading,
and these systems very much resemble the spreading centers
found at divergent plate boundaries. However, the
geochemical signature of some of the lavas shows some subtle
and some not-so-subtle differences, with water and volatiles
being more important in the generation of magmas in back
arc supra-subduction zone environments.
Compressional arcs such as the Andes have tall mountains,
reaching heights of more than 24,000 feet (7,315 m)
over broad areas. They have rare or no volcanism but much
plutonism and typically have shallow dipping slabs beneath
them. They have thick continental crust with large compressional
earthquakes, and show a foreland-style retroarc basin in
the back arc region. Some compressional arc segments do not
have accretionary forearcs but exhibit subduction erosion during
which material is eroded and scraped off the overriding
plate and dragged down into the subduction zone. The Andes
show some remarkable along-strike variations in processes and
tectonic style, with sharp boundaries between different segments.
These variations seem to be related to what is being
subducted and plate motion vectors. In areas where the downgoing
slab has steep dips, the overriding plate has volcanic
rocks; in areas of shallow subduction there is no volcanism.
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