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Australia: The Land Where Time Began |
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Scraped by flat bed Subduction
Material that is scraped off the base of the continental mantle
lithosphere builds a migrating keel. Features of the Laramide Orogeny
are recreated by this testable mechanism for flat subduction.
The oceanic plate descends below an overriding continental plate from a
deep-sea trench and down into the mantle in most subduction zones. The
oceanic plate descends at an angle, and the dip of the slab usually
increases from a few degrees at the trench to 30o
or more at depth. The slab dip can reach extremes of nearly vertical or
horizontal. In cases where the dip is almost horizontal, after reaching
20o to 30o, there is decrease of the slab dip and
the subducting plate proceeds for several hundred kilometres almost
horizontally, sliding along the underside of the overriding plate
(Gutscher, 2000). Axen et al.
(2018) present numerical simulations that show that during the
horizontal phase, the flat slab scrapes off 20 to 50 km of the
overriding continental mantle lithosphere.
The landscape and geology of the western United States has been strongly
affected by an episode of flat-slab subduction that occurred during the
Laramide Orogeny about 80 – 50 Ma (Liu & Currie, 2016; Dickenson &
Snyder, 1978). Migration of arc
magmatism 1,500-2,000 km in extent inboard from the trench and later
retreat back towards the trench provides support for Laramide flat
subduction. The distance between the volcanic arc and the trench of 200
to 300 km is typical for normal slab dip, with the arc forming when the
slab reaches a depth of about 100 km. Arc magmatism about 1,000 km from
the trench, therefore, indicates a slab dip that is extremely shallow
that is achieved only by flat subduction. Also, tomographic images of
the upper mantle beneath North America document currently subducting and
fossil slabs where the lab has resubducted (Sigloch et al., 2008).
Following the end of this flat slab phase, the underlying slab peeled
away and sank into the mantle. This triggers widespread volcanic
eruptions, pyroclastic flows and ignimbrite flare ups in the Basin and
Range (Humphreys 1995), and the formation of porphyry copper deposits
that are economically important.
Numerical simulations of the initiation and evolution of flat subduction
have been presented by Axen et al.
(Axen et al., 2018). They found that the flattened slab advances and
scrapes the base of the overriding plate. The mantle lithosphere that is
scraped off accumulates at the nose of the advancing flat slab, in the
form of a bulldozed keel to fill the asthenospheric wedge. Convection in
the asthenospheric wedge is inhibited by this filling, where water would
typically have been released from the descending slab which would cause
partial melting. Thus, arc magmatism ends. The model of Axen et
al. is specifically applied
to the Laramide Orogeny of the Late Cretaceous in North America and show
that it can recreate features such as block fault uplift in the
overriding continental plate.
In modern times flat subduction is relatively rare, being observed in
about 10% of the subduction zones in the world, such as Peru and central
Chile. There has long been debate about the causes of flat subduction.
It has been contributed to a combination of factors such as slab
buoyancy, kinematics of plates and dynamics of the asthenosphere that
combine to produce favourable conditions (Gutscher, 2000; Dickinson &
Snyder, 1978; van Hunen, 2002). Flat slab subduction beneath North
America has previously been simulated by numerical models (Bird, 1988),
though some of the key observations were difficult to reproduce. E.g.,
there is a belt of compressional structures, block fault uplifts, the
age of which progresses from the Colorado Plateau in the west to the
high plains, such as the Big Horn Range of South Dakota to the east.
Previously, these were attributed to compressional deformation
throughout the overlying portion of the upper plate that was driven by
basal shear that was exerted from the flat slab below. The lack of
compressional deformation above the flat portion of the slab, which has
been demonstrated by active normal faulting that was observed in field
studies in the cordillera Blanca in Peru (Margirier et al., 2016),
proved difficult to reconcile with the basal shear model (Gutscher,
2000).
The simulations by Axen et al.
(Axen et al., 2018) actually predict that behind the compressional
deformation belt that is observed and modelled there is a zone in which
extensional stress is elevated in the upper plate. Examination of stress
field indicators above flat slab regions can test this prediction.
Included among these in situ
measurements, e.g., are borehole elongations during breakouts, field
observations and analysis of earthquake focal mechanisms. Above flat
slab zones in Central and Southern Peru initial assessment of the focal
measurements for crustal earthquakes reveals a low level of seismicity
in general, with 4 normal faulting mechanisms out of 6. Ongoing
extension is indicated by focal mechanisms and mapped normal faults and
they are therefore supportive of the predictions of the model by Axen
(Axen et al., 2018). A large-scale seismology survey was carried out
above the Peruvian flat slab which succeeded in imaging the deep slab,
where deep subduction occurs (Scire, 2016), and the impact on the upper
plate (Fitzcarrald Arch) uplift was also explored and explained
tentatively by basal shear (Bishop et al., 2018). Gutscher suggested
that this interpretation may need to be reconsidered in light of the
Axen et al. study. The
interior portion of the upper plate could be targeted by future passive
seismological work and direct images of the bulldozed keel be obtained.
An elegant explanation has been offered by Axen et
al. (Axen et al., 2018) of
the observations that are specific to the Laramide Orogeny of western
North America. Though beyond this regional significance, the mechanism
that is proposed of a scraped continental mantle lithosphere and
regional extension may be important to flat subduction zones more
generally, at the present as well as in the past. The Peruvian Andes of
the present may yet prove to be an analogue for the Basin and Range of
50-60 Ma.
Gutscher, M.-A. (2018). "Scraped by flat-slab subduction." Nature
Geoscience 11(12): 890-891. |
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |