Obduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a convergent plate boundary, such as closing of an ocean or a mountain building episode. This process is uncommon because the denser oceanic lithosphere usually subducts underneath the less dense continental plate.
Obduction 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. Obduction 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, obduction of the oceanic crust between them is often a part of the resulting orogeny.
Most obductions appear to have initiated at back-arc basins above the subduction zones during the closing of an ocean or an orogeny.
Characteristic rocks
The characteristic rocks of obducted oceanic lithosphere are the ophiolites.
Ophiolites are an assemblage of oceanic lithosphere rocks that have
been emplaced onto a continent. This assemblage consists of deep-marine
sedimentary rock (chert, limestone, clastic sediments), volcanic rocks (pillow lavas, glass, ash, sheeted dykes and gabbros) and peridotite (mantle rock).
Types of obductions
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.
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.
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
The
simplest form of this type of obduction may follow from the development
of a subduction zone near the continental margin. 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.
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.
If however, a large tract of ocean intervenes between the
continental margin the subduction zone, a fully developed arc and back
arc basin may eventually arrive and collide with the continental margin.
Further convergence may lead to overthrusting of the volcanic arc
assemblage and may be followed by flipping the subduction polarity.
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 as ophiolites. This concept was advocated by Reinhardt for the emplacement of the Semail Ophiolite complex in Oman and argued by Church and Church and Stevens for the emplacement of the Bay of Islands
sheet in western Newfoundland. This concept has subsequently been
replaced by hypotheses that advocate subduction of the continental
margin beneath oceanic lithosphere.
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.
A transform fault may also become a subduction zone, with 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
In
the situation where a spreading ridge 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.
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”.
Obduction from rear-arc basin
Dewey and Bird 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.
This mechanism is a form of plate convergence where a thin, hot layer of
oceanic lithosphere is obducted over cooler and thicker lithosphere.
Obduction during 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.
Following total subduction of an 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.
Examples
There
are many examples of oceanic crustal rocks and deeper mantle rocks that
have been obducted and exposed at the surface worldwide. New Caledonia is one example of recent obduction. The Klamath Mountains of northern California contain several obducted oceanic slabs. Obducted fragments also are found in Oman, the Troodos Mountains of Cyprus, Newfoundland, New Zealand, the Alps of Europe, the Shetland islands of Unst and Fetlar, and the Appalachians of eastern North America.
