Subduction and abduction zones

SEMINAR ON
SUBDUCTION AND
ABDUCTION ZONES

CONTENTS
 INTRODUCTION
 TYPES OF PLATE BOUNDARIES
 SUBDUCTION
 ABDUCTION
 CONCLUSION
 REFERENCES

INTRODUCTION
 Plate tectonics is a scientific theory that describes the large-
scale motions of Earth's lithosphere.
 The lithosphere is broken up into tectonic plates. On Earth,
there are seven or eight major plates and many minor plates.
 Where plates meet, their relative motion determines the type
of boundary: convergent, divergent, or transform.
 In geology, subduction is the process that takes place
at convergent boundaries by which one tectonic plate moves
under another tectonic plate and sinks into the mantle as the
plates converge. Regions where this process occurs are
known as subduction zones.

TYPES OF PLATE BOUNDARIES
Transform boundaries
Divergent boundaries
Convergent boundaries

SUBDUCTION
 When two oceanic plates collide, the younger of the two
plates, because it is less dense, will ride over the edge of the
older plate. [Oceanic plates grow more dense as they cool
and move further away from the Mid-Ocean Ridge]
 The older, heavier plate bends and plunges steeply through
the asthenosphere, and descending into the earth, it forms a
trench that can be as much as 70 miles wide, more than a
thousand miles long, and several miles deep.
 The Marianas Trench, where the enormous Pacific Plate is
descending under the leading edge of the Eurasian Plate, is
the deepest sea floor in the world.
 It curves northward from near the island of Guam and its
bottom lies close to 36,000 feet below the surface of the
Pacific Ocean.

ABDUCTION
Abduction is the overthrusting of continental crust by oceanic crust or
mantle rocks at a convergent plate boundary
This process is uncommon as the denser oceanic lithosphere usually
subducts underneath the less dense continental plate.
Abduction occurs where a fragment of continental crust is caught in a
subduction zone with resulting overthrusting of oceanic mafic and
ultramafic rocks from the mantle onto the continental crust.
Abduction often occurs where a small tectonic plate is caught between two
larger plates, with the crust (both island arc and oceanic) welding onto
an adjacent continent as a new terrane.

When two continental plates collide, abduction of the
oceanic crust between them is often a part of the
resulting orogeny.
Abduction is a process involving several mechanisms.
These may include up wedging, compressional
telescoping, gravity sliding or transformation of a
spreading ridge to a subduction zone.
Most abductions appear to have initiated at back-arc
basins above the subduction zones during the closing
of an ocean or an orogeny

TYPES OF ABDUCTIONS
 1 Upwedging in subduction zones
 2 Compressional telescoping onto Atlantic-type continental margins
 3 Gravity sliding onto Atlantic type continental margins
 4 Transformation of a spreading ridge to a subduction zone
 5 Interference of a spreading ridge and a subduction zone
 6 Obduction from rear-arc basin
 7 Obduction during continental collision

UPWEDGING IN SUBDUCTION ZONES
• This process is operative beneath and behind the inner walls of
oceanic trenches (subduction zone) where slices of oceanic crust
and mantle are ripped from the upper part of the descending plate
and wedged and packed in high pressure assemblages against the
leading edge of the other plate.[1]
• Weakening and cracking of oceanic crust and upper mantle is likely
to occur in the tensional regime. This results in the incorporation of
ophiolite slabs into the overriding plate.[2]
• Progressive packing of ophiolite slices and arc fragments against
the leading edge of a continent may continue over a long period of
time and lead to a form of continental accretion.

COMPRESSIONAL TELESCOPING ONTO
ATLANTIC-TYPE CONTINENTAL
MARGINS
Figure 1: A, B, C. Obduction of an ophiolite sheet by the partial
subduction of an Atlantic type continental margin. D, E, F, G.
Obduction of an arc-trench gap ophiolite sheet by the collision of an
Atlantic type continental margin with a volcanic arc. 1=Continental
crust, 2= Oceanic Crust, 3= Mantle, 4= Continental rise sediments,
5=oceanic sediments, 6= Volcanogenic sediments,
7=silicic/intermediate volcanics, 8=low-velocity zone.
The simplest form of this type of obduction may follow from the
development of a subduction zone near the continental margin
shown in figure 1A. Above and behind the subduction zone, a welt
of oceanic crust and mantle rides up over the descending plate. The
ocean, intervening between the continental margin and the
subduction zone is progressively swallowed until the continental
margin arrives at the subduction zone and a giant wedge or slice
(nappe) of oceanic crust and mantle is pushed across the
continental margin. (Fig 1B)[citation needed]
Because the buoyancy of the
relatively light continental crust is likely to prohibit its extensive
subduction, a flip in subduction polarity will occur yielding an
ophiolite sheet lying above a descending plate. (Fig 1C) [3]
If however, a large tract of ocean intervenes between the
continental margin the subduction zone, a fully developed arc and
back arc basin (Fig 1D) may eventually arrive and collide with the
continental margin. Further convergence may lead to overthrusting
of the volcanic arc assemblage (Fig 1E, 1F) and may be followed by
flipping the subduction polarity. (Fig 1G)
According to the rock assemblage as well as the complexly
deformed ophiolite basement and arc intrusions, the Coastal
Complex of western Newfoundland may well have been formed by
this mechanism.

GRAVITY SLIDING ONTO ATLANTIC TYPE
CONTINENTAL MARGINS
• This concept involves the progressive uplift of
an actively spreading oceanic ridge, the
detachment of slices from the upper part of
the lithosphere and the subsequent gravity
sliding of these slices onto the continental
margin. This concept was advocated by
Reinhardt for the emplacement of the Semail
Complex in Oman and argued by Church and
Church and Stevens for the emplacement of
the Bay of Islands sheet in western
Newfoundland.

TRANSFORMATION OF A SPREADING RIDGE TO A
SUBDUCTION ZONE
• Many ophiolite complexes were emplaced as thin hot
obducted sheets of oceanic lithosphere shortly after
their generation by plate accretion. The change from a
spreading plate boundary to a subduction plate
boundary may result from rapid rearrangement of
relative plate motion. Transform moving from pure
strike slip to oblique compression fields could become
subduction zones, the side with the higher, hotter,
thinner lithosphere riding over the lower, colder
lithosphere. This mechanism would lead to obduction
of ophiolite complex if it occurred near a continental
margin.

INTERFERENCE OF A SPREADING
RIDGE AND A SUBDUCTION ZONE
• Figure 2: Various possible geometric relationships
resulting from the progressive interaction of a
subduction zone and a spreading ridge. 1=continental
crust, 2=oceanic crust, 3=mantle, 4=oceanic
sediments, 5=volcanogenic sediments,
6=silicic/intermediate volcanics, 7=low-velocity zone
• In the situation where a spreading approaches a
subduction zone, the ridge collides with the subduction
zone, at which time there will develop a complex
interaction of subduction related tectonic sedimentary,
and spreading related tectonic igneous activity. The left
over ridge may either subduct or ride upward across
the trench onto arc trench gap and arc terranes as a
hot ophiolite slice. These two mechanisms are shown
in figure 2 B and C.[10]
Two examples of this interaction
of a ridge colliding into a trench are well documented.
The first one is the progressive diminution of the
Farallon plate off California. Ophiolite obduction by the
above proposed mechanism would not be expected as
the two plates share a dextral transform boundary.
However, the major collision of the Kula/Pacific plate
with the Alaskan/Aleutian resulted in the initiation of
subduction of the Pacific plate beneath Alaska, with no
sign of either obduction or indeed any major
manifestation of a ridge being “swallowed”.[11]

ABDUCTION FROM REAR-ARC BASIN
• Dewey and Bird [12]
suggested that a common form of ophiolite obduction is related
to the closure of rear-arc marginal basins and that, during such closure by
subduction, slices of oceanic crust and mantle may be expelled onto adjacent
continental forelands and emplaced as ophiolite sheets. In the high heat-flow
region of a volcanic arc and rear-arc basin the lithosphere is particularly thin. This
thin lithosphere may preferentially fail along gently dipping thrust surface if a
compressional stress is applied to the region. Under these circumstances a thin
sheet of lithosphere may become detached and begin to ride over adjacent
lithosphere to finally become emplaced as a thin ophiolite sheet on the adjacent
continental foreland.[13]
This mechanism is a form of plate convergence where a
thin, hot layer of oceanic lithosphere is obducted over cooler and thicker
lithosphere.

ABDUCTION DURING CONTINENTAL
COLLISION
• Figure 3: Obduction of oceanic, arc-trench gap, ophiolite
sheets by the progressive convergence, collision, and
suturing of two continents. 1=continental crust,
2=oceanic crust, 3=mantle, 4=continental rise
sediments, 5=oceanic sediments, 6=volcanogenic
sediments, 7=silicic/intermediate volcanics, 8=exo-
geosynclinal flysch and molasse, 9=low velocity zone.
• Figure 3 shows the sequence of events illustrating the
obduction model following continental collision. As an
ocean is progressively trapped in between two colliding
continental lithospheres, the rising wedges of oceanic
crust and mantle rise are caught in the jaws of the
continent/continent vise and detach and begin to move
up the advancing continental rise. Continued
convergence may lead to the overthrusting of the arc-
trench gap and eventually overthrusting of the
metamorphic plutonic and volcanic rocks of the volcanic
arc. (Fig 3D) Following total subduction of oceanic tract,
continuing convergence may lead to a further sequence
of intra-continental mechanisms of crustal shortening.
This mechanism is thought to be responsible for the
various ocean basins of the Mediterranean region. The
Alpine belt is believed to register a complex history of
plate interactions during the general convergence of the
Eurasian plate and African plates.

Subduction and abduction zones

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