Structural Geology 3443 Ch' 17 Convergence

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Structural Geology (3443)Ch. 17 Convergence
Collision
Department of Geology University of Texas at
Arlington
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Ch. 17 Collision
Convergent boundaries are the most complex of the
plate boundaries.

• The essential elements are
• Outer swell
• Subduction Zone
• Trench
• Trench/slope break
• Accretionary prism
• Forearc Basin
• Volcanic Arc and associated regional metamorphism

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Ch. 17 Converging Margins
The outer swell (not shown) is an upward bulge
caused by the flexural rigidity as the plate
bends down.
It may have normal faults due to the bending
extension of the outer curve.
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Ch. 17 Converging Margins
The trench is the depression in the ocean floor
where the plate turns downward
The subduction zone is the thrust fault along
which the two plates move. Most of the seismic
activity occurs in the subduction zone, called
the Wadati-Benioff zone.
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Ch. 17 Converging Margins
The accretionary prism is where sediment and
other crustal material brought along by the
descending plate is scrapped off and transferred
to the overriding plate.
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Ch. 17 Converging Margins
The trench/slope break is a ridge at the top of
the accretionary prism, formed as the prism is
pushed up by the material accumulating from the
downgoing slab.
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Ch. 17 Converging Margins
The forearc basin is bounded by the trench-slope
break and the volacnic arc. It accumulates
sediment mostly from the arc.
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Ch. 17 Converging Margins
The arc itself consists of intermediate to felsic
volcanic rocks at the surface with plutonic
intrusions beneath. Source is from partial melts
in the downgoing slab and upper plate. volatiles
in the suducted plate reduce the melting
temperature.
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Ch. 17 Converging Margins
The back arc region may be under extension or
compression depending on the relative plate
motions.
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Ch. 17 Converging Margins
Because the subducting plate is sinking, is tends
to rollback if the plate velocity is slow
enough.
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Ch. 17 Converging Margins
The dip of the subduction zone is determined by
the sinking rate (how cool the plate is) and the
plate velocity. Subduction zones range from
shallow to steep.
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Ch. 17 Converging Margins
Subduction zones have the deepest earthquakes
because the sinking lithosphere remains
relatively cool (it takes a long time to heat
up). Because it is cool it remains brittle.
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Ch. 17 Converging Margins
Subduction zones also have unusual low T High P
metamorphic rocks, called blueschists due to the
presence of the glaucophane amphibole. Note that
at 100km depth the Temp is normally about 800C,
but in the slab it is 3-400C.
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Ch. 17 Converging Margins
Earthquakes in subducted slabs are produced by
both layer parallel extension and compression
depending on depth. About ½ way down, many plates
are being extended.
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Ch. 17 Accretionary Prisms
Accretionary prisms form by incorporating
sediment and oceanic crust from the lower plate
into the upper plate.
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Ch. 17 Accretionary Prisms
The accretionary prism is so deformed that little
remains of sedimentary layering in parts of it. A
zone of intense deformation, called a tectonic
melange (left), are common in accretionary
prisms. A melange is usually difficult to tell
from a submarine slide or large debris flow
called an Olistostrome.
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Ch. 17 Accretionary Prisms
Material in the accretionary prism comes from
oceanic sediment and crystalline crust, and
includes mud, radiolarian chert, turbidites,
basalt and gabbro. Larger objects from the ocean
floor like seamounts may also end up accreted
onto the upper plate.
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Ch. 17 Accretionary Prisms
Structure of the wedge consists of thrust faults,
isoclinal folds, and mélange formed by
underthrusting or underplating oceanic material
and trench sediment. As the wedge builds up
forming steep slopes, normal faults landslides
may occur.
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Ch. 17 Accretionary Prisms
A bivergent wedge may occur if the overlying
plate bends down (fig b) so that accretionary
material can be thrust over it forming a
retrowedge
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Ch. 17 Accretionary Prisms
Accretionary prisms can be described using
critical taper theory. The idea is that
material accumulating against a backstop will
develop into a wedge, and the slope and length of
the wedge remains in dynamic equilibrium.
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Ch. 17 Accretionary Prisms
If the slope becomes too steep, two things can
happen Erosion and landsliding (a) Lengthening
of the wedge (b) If the slope is too shallow,
the wedge will thicken by underplating (c)
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Ch. 17 Forearc Basins
The Forearc Basin is where sediment coming from
the volcanic arc is trapped by the accretionary
prism ridge. In the seismic line the volcanic
arc is on the left and the prism to the right,
off the line. The line is vertically exaggerated.
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Ch. 17 Volcanic Arcs
The volcanic arc is produced when fluids are
driven from the subducting plate as it heats up.
These fluids rise into the overlying
asthenosphere and reduce the melting point
producing magma which rises buoyantly.
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Ch. 17 Volcanic Arcs
The volcanic arc magma tends to be intermediate
in composition (diorite/andesite) with fluids
leading to explosive eruptions, and large,
composite volcanoes.
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Ch. 17 Volcanic Arcs
All volcanic arcs are dominated by intermediate
andesitic volcanic rocks. But those on oceanic
crust like the Kuril arc tend to have more mafic
volcanic rocks.
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Ch. 17 Volcanic Arcs
The Andes, however, tend to have more felsic
compositions because the continental crust may be
partially melted by the rising magmas.
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Ch. 17 Volcanic Arcs
The distance between the trench and the arc
(arc-trench gap) depends on the dip of the
subduction zone.
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Ch. 17 Back Arc Regions
The area behind the volcanic arc can be either in
extension, compression or neither. The
controlling factors are the convergence rate and
the rollback rate.
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Ch. 17 Back Arc Regions
In the figure, the sinking plate is rolling away
at a slower rate (Vrb) than the overriding plate
velocity (Vor). This maintains a compressive
state behind the arc and a highly coupled margin,
which may result in folds and thrust faults.
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Ch. 17 Back Arc Regions
If the rollback rate (Vrb) is faster than the
upper plate velocity (Vor), then the margin is
uncoupled and extension and a back arc basin may
form.
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Ch. 17 Back Arc Regions
Even with extension caused by rollback, some
buoyant rise of magma is necessary to produce a
back arc basin.
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Ch. 17 Back Arc Regions
Backarc or marginal basins are commonly
associated with oceanic arcs. They have a
spreading center and generate new oceanic crust.
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Ch. 17 Back Arc Regions
The Japan Sea is a marginal basin formed behind
the Japan volcanic arc. It is no longer active.
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Ch. 17 Back Arc Regions
A stable backarc occurs when Vrb Vor
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Ch. 17 Accretion and collision
Collision occurs across a converging plate margin
when two masses that are too light to sink meet
at a subduction zone. The boundary between the
two masses is called a suture zone
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Ch. 17 Accretion and collision
Collision can be complicated because the
convergence can be frontal or oblique. Buoyant
masses have irregular boundaries with
promontories and recesses that produce different
patterns of collision and deformation.
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Ch. 17 Accretion and collision
In most cases, a passive continental margin
collides with an active one. This places
shelf/deltaic sediments next to the accretionary
prism and igneous/metamorphic rocks on the active
margin.
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Ch. 17 Accretion and collision

• The ideal anatomy of an orogen produced by
  continental collision consists of
• foreland basin
• foreland fold/thrust belt
• Suture
• internal metamorphic/igneous zone
• retro (or back arc) fold/thrust belt.

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Ch. 17 Accretion and collision
The foreland basin is caused by isostatic
subsidence as the lithosphere is tectonically
loaded by thrust sheets of the fold thrust
belt. The subsidence is usually filled by fluvial
sediments derived from both the craton and the
thrust belt.
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Ch. 17 Accretion and collision
Thrusting can extend into the foreland basin over
time causing it to migrate toward the craton.
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Ch. 17 Accretion and collision
The fold/thrust belt is mostly thin-skinned and
only deforms the former passive margin shelf
sediments and not the basement. However, basement
faults from the buried rift zone are usually
reactivated near the suture.
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Ch. 17 Accretion and collision
The suture is usually recognized by the presence
of ophiolites (oceanic crust and their
metamorphic equivalents), mélange from the
accretionary prism, and blueschist metamorphic
rocks.
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Ch. 17 Accretion and collision
The internal metamorphic zone consists of
metamorphosed sediments of the accretionary prism
and the forearc basin, volcanics from the former
arc and their plutonic equivalents.
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Ch. 17 Accretion and collision
Metamorphism ranges from greenschist in the outer
part of the metamorphic zone to amphibolite and
above in the inner part. Deformation is usually
extreme with isoclinal folds, cleavage, lineation
shear zones.
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Ch. 17 Accretion and collision
The Retro fold/thrust belt reactivates the back
arc basin (if it was extensional), or the earlier
thrust belt (if the back arc was under
compression).
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Ch. 17 Accretion and collision
The collision produces high elevations and deep
roots formed by thickened continental crust
lithosphere. Under gravitational forces, the
elevated regions may undergo extensional collapse
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Ch. 17 Accretion and collision
In addition, the lower portion of the thickened
lithosphere may delaminate and sink into the
asthenosphere. Hotter asthenosphere taking its
place may partially melt the lower crust
producing post orogenic granites which intrude
the collision zone.
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Ch. 17 Interior plate deformation
Continental collision can lead to deformation in
the interior of the plates, far from the
collision zone. These interior stresses usually
reactivate old faults and zones of weakness.
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Ch. 17 Interior plate deformation
Indias collision with Asia has reactivated
numerous faults, many are strike slip, and some
are thrusts.
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Ch. 17 Accretion and collision
This has lead to the notion of escape tectonics
where an indenter (India) activates slip lines
causing large blocks to escape by moving East
along subduction zones in the Pacific.
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Ch. 17 Interior plate deformation
These escape blocks include Southern China and
Indochina
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Ch. 17 Interior plate deformation
In the US, the Carboniferous collision of Africa
and North America produced no escape blocks,
but did reactivate many older faults.
These basement faults produced isolated uplifts
during the Pennsylvanian called the ancestral
Rockies because the deformation style was
similar to that of the more recent Rocky
Mountains.
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Ch. 17 Interior plate deformation
At least some of these reactivated faults were
created during the Late Proterozoic - Early
Cambrian rifting that dismembered Rodinia, a
supercontinent that began to form 1.2 by ago.
The Southern Oklahoma Aulacogen was part of this
Pennsylvanian deformational event
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Ch. 17 Terranes and Crustal Accretion
Continents are not the only buoyant fragments
that will not subduct. Island arcs, small
continental fragments like Madagascar
(micro-continents) , oceanic basaltic plateaus
are all examples. When these smaller features
arrive at a subduction zone, they become accreted
and sutured onto the larger continent causing it
to grow in size.
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Ch. 17 Accretion and collision
The diagram below shows a theory of Appalachian
development. At least two islands were accreted
to North America before collision with Africa in
Late Paleozoic.
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Ch. 17 Accretion and collision
The map shows Western Newfoundland which
consists of terrains (Gondwana) accreted onto
North America (Laurentia) during the Paleozoic.
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Ch. 17 Accretion and collision
Western North America also consists of accreted
terrains that have been added during Phanerozoic
time
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Ch. 17 Accretion and collision
The collision of India with Eurasia is another
example of accretion. The subduction has nearly
halted although crustal earthquakes are still
frequent.
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Ch. 17 Accretion and collision
The structure has large thrust faults. The two
continents are separated by the Tsangbo/Indus
suture zone.
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Ch. 17 Accretion and collision
Rifting, Diverging, converging, collision and
suturing are collectively known as the Wilson
Cycle, which can be repeated over 100s of
millions of years.