
English: 
Hi, I’m Bob Stern, I’m Taras Gerya, and
we’re here to talk about how new subduction
zones form. Making new subduction zones is
essential for Earth's plate tectonics regime.
How this happens remains enigmatic and controversial,
and for good reason. New subduction zones
form infrequently and deep underwater, so
the process is hard to observe. We do know
that the negative buoyancy of old oceanic
lithosphere provides the primary driving force
for subduction and plate tectonics. We also
know that the oceanic lithosphere is strong
and resists the bending needed to form a new
subduction zone. In spite of these difficulties, about 1/3

English: 
hi I'm Bob Stern I'm Taras Gerya and
we're here to talk about how new
subduction zones form making new
subduction zones is essential for
Earth's plate tectonics regime how this
happens remains enigmatic and
controversial and for good reason new
subduction zones form infrequently and
deep underwater so the process is hard
to observe we do know that the negative
buoyancy of old oceanic lithosphere
provides the primary driving force for
subduction and plate tectonics we also
know that the oceanic lithosphere is
strong and resists the bending needed to
form a new subduction zone in spite of

English: 
of Earth’s convergent plate margins formed
over the past 65 million years. In this video,
we summarize some of the most important ideas
about how this happens. A detailed overview
can be found in our 2018 Tectonophysics paper
“Subduction Initiation in Nature and Models:
A Review”.
Given the strength of oceanic lithosphere
and its resistance to bending, how can new
subduction zones form? Two requirements are
a lithospheric weakness that is many hundreds
kilometers long and different lithospheric
densities on either side of the weakness.
Juxtaposition of old, dense oceanic lithosphere
with buoyant lithosphere - which can be young
oceanic lithosphere or continental lithosphere

English: 
these difficulties about one-third of
Earth's convergent plate margins formed
over the past 65 million years in this
video we summarized some of the most
important ideas about how this happens a
detailed overview can be found in our
2018 tectonophysics paper "Subduction
Initiation in Nature and Models: A Review"
given the strength of oceanic
lithosphere and its resistance to
bending how can new subduction zones
form two requirements are a
lithospheric weakness that is many hundreds
of kilometers long and different
lithospheric densities on either side of
the weakness juxtaposition of old dense
oceanic lithosphere with buoyant
lithosphere which can be young oceanic
lithosphere or continental lithosphere
is required for the asymmetry of a

English: 
- is required for the asymmetry of a sinking
plate and an overriding plate, the
essence of a subduction zone. A long lithospheric
weakness is required because the future downgoing
plate must flex to allow asthenosphere to
well up over it in one place to start the
subduction initiation process. Without these
two conditions, it is very difficult to start
a new subduction zone.
We can subdivide the causes of subduction
initiation into those that are induced or
caused by changes in plate motion and those
that are spontaneous, caused by buoyancy forces
at the subduction initiation site. Let’s discuss these two
general cases in a little more detail.
The most important way that a new subduction
zone can be induced to form is by polarity
reversal, which sometimes happens when buoyant

English: 
sinking plate and an overriding plate
the essence of a subduction zone
A long lithospheric weakness is required
because the future downgoing plate must
flex to allow asthenosphere to well up
over it in one place to start the
subduction initiation process without
these two conditions it is very
difficult to start a new subduction zone
we can subdivide the causes of
subduction initiation into those that
are induced or caused by changes in
plate motion and those that are
spontaneous caused by buoyancy forces at
the subduction initiation site let's
discuss these two general cases in a
little more detail the most important
way that a new subduction zone can be
induced to form is by polarity reversal
which sometimes happens when buoyant

English: 
lithosphere is subducted beneath an
oceanic arc when this happens the
original subduction zone can fail and
motion between the two plates may be
accommodated by formation of a new
subduction zone dipping the opposite way
in the rear arc region a good example of
this is seen in the southwest Pacific
the buoyant Ontong Java plateau was
partially subducted in late miocene time
into a South dipping subduction zone on
the north side of the Solomon arc and
the buoyant block eventually shut down
this subduction zone the Pacific and
Australian plates continued to converge
and eventually the dense old lithosphere
in the rear arc began to fail along the
lithospheric weakness near the hot
magmatic arc this eventually formed a
new north dipping subduction zone which

English: 
lithosphere is subducted beneath an oceanic
arc. When this happens, the original subduction
zone can fail and motion between the two plates
may be accommodated by formation of a new
subduction zone dipping the opposite way in
the reararc region. A good example of this
is seen in the South West Pacific. The buoyant Ontong-Java Plateau was partially subducted in Late Miocene
time into a south-dipping subduction zone on the
north side of the Solomon arc, and the buoyant
block eventually shut down this subduction
zone. The Pacific and Australian plates continued
to converge and eventually the dense, old
lithosphere in the reararc began to fail along
the lithospheric weakness near the hot magmatic
arc. This eventually formed a new, north-dipping
subduction zone, which continues to operate
today.

English: 
continues to operate today new
subduction zones can also form
spontaneously without any compression if
there is a long enough lithospheric weakness
and a large enough density
contrast on either side of it there are
three ways for this to happen
transform or fracture zone collapse
flume margin collapse and maybe
margin collapse let's consider these one
at a time
transform or fracture zone collapse is
simple transforms and fracture zones are
profound lithospheric weaknesses that
are often long and often juxtaposed
oceanic lithosphere of different ages in
thus densities this happened in the
western Pacific when much of the western
margin of the Pacific plate collapsed
into a West dipping subduction zone
evidence for how and when this new
subduction zone formed is preserved in

English: 
New subduction zones can also form spontaneously,
without any compression, if there is a long
enough lithospheric weakness and a large enough
density contrast on either side of it. There
are three ways for this to happen: Transform
or fracture zone collapse, plume margin
collapse, and maybe passive margin collapse.
Let’s consider these one at a time.
Transform or fracture zone collapse is simple.
Transforms and fracture zones are profound
lithospheric weaknesses that are often long
and often juxtapose oceanic lithospheres of
different ages and thus densities. This happened
in the Western Pacific when much of the western
margin of the Pacific plate collapsed into
a west-dipping subduction zone. Evidence for

English: 
the forearc and inner trench wall of the
EZ Bonin Mariana or IBM convergent
margin over the past two decades we have
figured out how this subduction zone
formed and tested these ideas in 2014 by
seafloor drilling before about 52
million years ago there was a long
fracture zone separating young oceanic
lithosphere to the west from much older
oceanic lithosphere of the Pacific plate
to the east the old lithosphere began to
sag allowing asthenosphere from beneath
the younger plate to flow over it this
led to seafloor spreading above the
sinking older plate forming the modern
Izu Bonin Mariana forearc at first
pure asthenospheric partial melts
indistinguishable from mid-ocean ridge

English: 
how and when this new subduction zone formed
is preserved in the forearc and inner trench
wall of the Izu-Bonin-Mariana or IBM convergent
margin. Over the past two decades we have
figured out how this subduction zone formed
and tested these ideas in 2014 by seafloor drilling.
Before about 52 million years ago, there was
a long fracture zone separating young oceanic
lithosphere to the west from much older oceanic
lithosphere of the Pacific plate to the east.
The old lithosphere began to sag, allowing
asthenosphere from beneath the younger plate
to flow over it. This led to seafloor spreading
above the sinking older plate, forming the
modern Izu-Bonin-Mariana forearc. At first, pure asthenospheric partial melts indistinguishable from MORB

English: 
were erupted, but after a few million years fluids
rising from the sinking plate reached the
region of melt generation and melts with the
telltale geochemical signature of subduction-related
magmas were generated. As the flexing Pacific
plate sank deeper, it eventually became easier
for the slab to slide down-dip instead of
sinking vertically, marking the transition
from subduction initiation to true subduction.
The start of true subduction pulled the Pacific
plate to the west, causing the bend in the
Emperor-Hawaii seamount chain. For the Izu-Bonin-Mariana
system, it took about 7 million years for
the transition from lithospheric collapse
to true subduction to occur. There are other
examples of this mode of subduction initiation,
for example as represented by the 3000 km
long Late Cretaceous ophiolite belt that stretches

English: 
basalt were erupted but after a few
million years
fluids rising from the sinking plate
reached the region of melt generation
and melts with the tell-tale geo
chemical signature of subduction related
magmas were generated as the flexing
Pacific plate sank deeper it eventually
became easier for the slab to slide down
dip instead of sinking vertically
marking the transition from subduction
initiation to true subduction the start
of true subduction pulled the Pacific
plate to the west causing the bend in
the
Emperor Hawaii seamount chain for the Izu
Bonin Mariana system it took about
seven million years for the transition
from lithospheric collapse to true
subduction to occur there are other
examples of this mode of subduction
initiation for example as represented by
the 3000 kilometer long Late Cretaceous
ophiolite belt that stretches from

English: 
from Cyprus in the west through Syria and
Turkey into Iraq and Iran, terminating in
the Semail ophiolite of Oman. Another example
is the Late Jurassic ophiolite belt of western
North America that includes the Coast Range
ophiolite of California.
Plume-induced subduction initiation, or PISI,
is another way to spontaneously form a new
subduction zone. Only the broad heads of strong
mantle plumes when they first reach the base
of the lithosphere can penetrate oceanic lithosphere
to create large enough density contrasts and
long enough lithospheric weaknesses that can
collapse into a new subduction zone. The best
PISI example is found around the margins of
the Late Cretaceous Caribbean Large Igneous

English: 
Cyprus in the West through Syria and
Turkey into Iraq and Iran terminating in
the samail ophiolite of Oman another
example is the Late Jurassic ophiolite
belt of Western North America that
includes the coast range ophiolite of
California plume induced subduction
initiation or PISI is another way to
spontaneously form a new subduction zone
only the broad heads of strong mantle
plumes when they first reached the base
of the lithosphere can penetrate oceanic
lithosphere to create large enough
density contrasts and long enough
lithospheric weaknesses that can
collapse into a new subduction zone the
best PISI example is found around the
margins of the Late Cretaceous Caribbean

English: 
large igneous province or CLIP about 100
million years ago the plume head for
what is now the Galapagos hotspot
reached the base of the Farallon plate
oceanic lithosphere this old lithosphere
collapsed around the western and
southern margins of the plume head
creating new subduction zones including
the currently active subduction zone
beneath Central America evidence of CLIP
PISI is preserved in Late Cretaceous
volcanic sequences that transition from
older oceanic plateau lavas to younger
arc lavas without a significant
stratigraphic break a second example of
PC is found in the northwestern United
States where the arrival of the
Yellowstone plume head
in Eocene time caused lithospheric
collapse on its western margin resulting
in upper plate extension to form the
Siletzia terrane in what is now the

English: 
Province, or CLIP. About 100 million years
ago, the plume head for what is now the Galapagos
hotspot reached the base of Farallon Plate
oceanic lithosphere. This old lithosphere
collapsed around the western and southern
margins of the plume head, creating new subduction
zones including the currently active subduction
zone beneath Central America. Evidence of
CLIP PISI is preserved in Late Cretaceous
volcanic sequences that transition from older
oceanic plateau lavas to younger arc lavas without a significant stratigraphic break
A second example of PISI is found in the north western United States, where
the arrival of the Yellowstone plume head
in Eocene time caused lithospheric collapse
on Its western margin, resulting in upper
plate extension to form the Siletzia terrane
in what is now the Cascadia forearc and core

English: 
complexes in what is now the reararc. Soon
after subduction began, the descending slab
cut off the rising plume head. It took 30
million years for North America and the Cascadia
subduction zone to migrate west sufficiently to
allow the plume to reform and rise to the
surface, to resume as the Columbia River Flood
Basalts.
Finally, there is the question of passive
continental margins and whether or not these
collapse to form new subduction zones. This
is implicit in the Wilson Cycle, which describes
the opening and closing of oceans over a few
hundred million years. In spite of this, there
are no documented examples of a passive margin
collapsing into a subduction zone in Mesozoic
or Cenozoic time. After 180 million years,
none of the passive margins on either side

English: 
Cascadia forearc and core complexes in
what is now the reararc soon after
subduction began the descending slab cut
off the rising plume head and it took 30
million years for North America and the Cascadia
subduction zone to migrate west
sufficiently to allow the plume to
reform and rise to the surface to resume
as the Columbia River flood basalts
finally there is the question of passive
continental margins and whether or not
these collapse to form new subduction
zones this is implicit in the Wilson
cycle which describes the opening and
closing of oceans over a few hundred
million years in spite of this there are
no documented examples of a passive
margin collapsing into a subduction zone
in Mesozoic or Cenozoic time after 180
million years none of the passive
margins on either side of the Atlantic
have collapsed to form any subduction

English: 
zones India is flanked by passive
margins but these have not collapsed to
form a subduction zone instead it is
easier for India to continue converging
with Asia as it has been doing for 50
million years
there is a strong density contrast
between India continental lithosphere
and flanking oceanic lithosphere so the
problem must be a lack of a long enough
lithospheric weakness there may be a
first example of passive margin collapse
on the southwest margin of Iran where a
new subduction zone formed in late
Cretaceous time this was a passive
margin for most of Mesozoic time what
lithospheric weakness allowed this
margin to collapse
recently a distinctive feature along
this margin the Sanandaj-Sirjan Zone
was reinterpreted as a Late
Jurassic

English: 
of the Atlantic have collapsed to form any
subduction zones. India is flanked by passive
margins but these have not collapsed to form
a subduction zone; instead it is easier for
India to continue converging with Asia, as
it has been doing for 50 million years. There is
a strong density contrast between India continental
lithosphere and flanking oceanic lithosphere,
so the problem must be a lack of a long-enough
Iithospheric weaknesses. There may be a first
example of passive margin collapse on the
south west margin of Iran, where a new subduction
zone formed in Late Cretaceous time. This
was a passive margin for most of Mesozoic
time, what lithospheric weakness allowed this
margin to collapse? Recently, a distinctive
feature along this margin, the Sanandaj-Sirjan
Zone, was reinterpreted as a Late Jurassic

English: 
magmatic rift. This may have served as the
weakness to allow the south west Iran passive margin
to collapse into a subduction zone. In contrast,
evaluation of subduction initiation risk along
American Atlantic margins by Nikolaeva et
al. (2011) showed that only the southern part
of the Argentine-Brazilian margin may be at
high risk due to its thinned lithosphere,
while other Atlantic margins of South and
North America are stable under the present
geodynamic conditions. Further studies
are needed to address the Wilson Cycle paradox.
We are making good progress in figuring out
how and why new subduction zones form but
our understanding is far from complete and
many controversies persist. We understand

English: 
magmatic rift this may have served as the
weakness to allow the southwest Iran
passive margin to collapse into a
subduction zone
in contrast evaluation of subduction
initiation risk along American Atlantic
margins by Nikolaeva et al (2011) showed that
only the southern part of the Argentine
Brazilian margin may be at high risk due
to its thin lithosphere while other
Atlantic margins of South and North
America are stable under the present
geodynamic conditions further studies
are needed to address the Wilson cycle
paradox we are making good progress in
figuring out how and why new subduction
zones form but our understanding is far
from complete and many controversies

English: 
persist we understand that is difficult
to form a new subduction zone except
where contrasts in lithospheric density
occur across weaknesses that are at
least many hundred kilometers long we
now have a much better understanding of
what oceanic forearcs are made of and
where most ophiolites form and we now
appreciate that the Wilson cycle is not
as simple as we teach it to our students
advances in our understanding of how new
subduction zones form also leads to
clearer thinking about how the first
subduction zone required to start global
plate tectonics might have begun if
you'd like to learn more about
subduction Initiation please read some
of the references listed at the end of
this video it is now clear that
subduction initiation is a promising
line of research that brings geoscientists together requiring field

English: 
that it is difficult to form a new subduction
zone except where contrasts in lithospheric
density occur across weaknesses that are at
least many hundreds km long. We now have a
much better understanding of what oceanic
forearcs are made of and where most ophiolites
form, and we now appreciate that the Wilson Cycle
is not as simple as we teach it to our students.
Advances in our understanding of how new subduction
zones form also leads to clearer thinking about
how the first subduction zone required to
start global plate tectonics might have begun.
If you’d like to learn more about subduction
initiation, please read some of the references
listed at the end of this video. It is now clear
that subduction initiation is a promising
line of research that brings geoscientists
together, requiring field studies, igneous

English: 
geochemistry, geochronology, mineral physics,
and geodynamic modeling. We invite you to
join us to help develop this emerging research
field.

English: 
studies, igneous geochemistry,  geochronology,
mineral physics and geo dynamic modeling
we invite you to join us to help develop
this emerging
research field
you
