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Scraped by flat-slab subduction

Marc-André Gutscher

► To cite this version:

Marc-André Gutscher. Scraped by flat-slab subduction. Nature Geoscience, 2018, 11, pp.890-891.
10.1038/s41561-018-0270-x. insu-03685667

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Distributed under a Creative Commons Attribution 4.0 International License
they are needed to reconstruct
rapid environmental changes that cause
mass extinctions⁴. This will help confirm
that flood basalt emissions of SO₂ and
CO₂ are the main causes of the extreme
temperature swings during the end-Permian
extinction event.

Today, we are changing our environment,
in particular the carbon cycle at a speed that
is probably unprecedented in the past 252
million years⁵. The consequence could be
devastating if the trend continues unabated.
The total amount of CO₂ added during
the end-Permian extinction is estimated
at about 20,000 × 10¹⁵ g C (ref. ⁶), but was
released at a rate that is likely slower
than today⁵. Better quantification of

uncertainties of the rates of carbon and
sulfur emissions during the end-Permian
extinction could offer valuable lessons for
today's changing climate.

Black and colleagues² suggest that extreme swings in climate resulted from the different timescales of cooling from volcanic sulfur dioxide and the warming from volcanic carbon dioxide during the eruption of the Siberian Traps, and led to the end-Permian mass extinction. However, we need to continue to develop new high-resolution proxies for atmospheric CO₂ and SO₂ concentrations, precipitation, weathering and ocean circulation to understand devastating environmental changes that cause mass extinctions. ☐

Ying Cui

Department of Earth and Environmental Studies at
Montclair State University, Montclair, NJ, USA.
e-mail: cuiy@montclair.edu

Published online: 30 November 2018
https://doi.org/10.1038/s41561-018-0264-8

References

1. Erwin, D. H. *The Great Paleozoic Crisis: Life and Death in the Permian* (Columbia Univ. Press, New York, 1993).

2. Black, B. A. et al. Nat. Geosci. https://doi.org/10.1038/s41561-018-0261-y (2018).

3. Black, B. A., Hauri, E. H., Elkins-Tanton, L. T. & Brown, S. M. *Earth Planet. Sci. Lett.* **394**, 58–69 (2014).

4. Bürger, R. & Lynch, M. *Evolution* **49**, 151–163 (1995).

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6. Cui, Y., Kump, L. R. & Ridgwell, A. *Palaeogr. Palaeoclim. Palaeoclim. *387*, 176–184 (2013).

GEODYNAMICS

Scraped by flat-slab subduction

During flat subduction, material is scraped off the base of the continental mantle lithosphere, building a migrating keel. This testable mechanism for flat subduction recreates features of the Laramide orogeny.

Marc-André Gutscher

In most subduction zones, the oceanic plate descends below an overriding continental plate from a deep-sea trench and down into the mantle. The oceanic plate descends at an angle and this slab dip angle generally increases from a few degrees at the trench to 30° or more at depth. In some cases, the slab dip can reach extremes of close to vertical or horizontal. In cases of near horizontal dip, having reached 20° to 30°, the slab dip decreases and the subducting plate proceeds for several-hundred kilometres almost horizontally, sliding along the underside of the overriding plate's belly¹. Writing in *Nature Geoscience*, Axen et al.² present numerical simulations showing that during this horizontal phase, the flat slab scrapes off 20 to 50 km of the overriding continental mantle lithosphere.

An episode of flat-slab subduction
some 80 to 50 million years ago during the
Laramide orogeny has strongly affected
the modern-day landscape and geology
of the western United States3,4. Support
for Laramide flat subduction comes from
the 1,500 to 2,000 km migration of arc
magmatism inboard from the trench and
later retreat back towards the trench5.
Typically, for normal slab dip, the distance
between the volcanic arc and trench is 200 to
300 km, with the arc forming when the slab
reaches roughly 100 km depth. Therefore,

arc magmatism some 1,000 km from the
trench indicates an extremely shallow slab
dip only achievable through flat subduction.
In addition, tomographic images of the
upper mantle beneath North America
document currently subducting and fossil
slabs where the slab has resubducted⁶. When
this flat-slab phase ended, the underlying
slab peeled away and sank into the mantle.
This triggered widespread volcanic
eruptions, pyroclastic flows and ignimbrite
flare ups in the Basin and Range⁷, and
the formation of economically important
porphyry copper ore deposits.

Axen and colleagues² present numerical
simulations for the initiation and evolution
of flat subduction. They find that the
flattened slab advances and scrapes the
base of the overriding plate. This scraped-
off mantle lithosphere accumulates at the
nose of the advancing flat slab, in the form
of a bulldozed keel (Fig. 1b), and fills the
asthenospheric wedge. This filling inhibits
convection in the asthenospheric wedge,
where water would typically be released
from the downgoing slab causing partial
melting. Thus, arc magmatism ends. They
specifically apply their model to the Late
Cretaceous Laramide orogeny in North
America and show that it can recreate
features such as block fault uplift in the
interior of the overriding continental plate.

Flat subduction is relatively rare in
modern times, only observed in about 10%
of the world's subduction zones, such as
Peru and Central Chile. The causes of flat
subduction have long been debated. Previous
studies attributed it to a combination
of factors such as slab buoyancy, plate
kinematics and asthenospheric dynamics
that can combine to produce favourable
conditions¹,⁴,⁸. Numerical models have
previously simulated flat-slab subduction
below North America⁹, but some of the key
observations were difficult to reproduce.
For instance, there is a belt of compressional
structures — block fault uplifts — that
progress in age from the Colorado Plateau in
the west to the high plains (such as the Big
Horn range, South Dakota) in the east. These
were previously attributed to compressional
deformation throughout the overlying
portion of the upper plate driven by basal
shear exerted from the flat slab below (Fig. 1a).
However, the lack of compressional
deformation above the flat portion of the slab,
as demonstrated by active normal faulting
observed in field studies in the Cordillera
Blanca in Peru¹⁰, was difficult to reconcile
with the basal shear model (Fig. 1a)¹.

Behind the observed and modelled
compressional deformation belt, the
simulations by Axen et al.² actually predict
a zone of elevated extensional stress in the
<figure><img src="image_3.png" /></figure>

**Fig. 1** | Less basal shear, more scraping in flat subduction. **a,b,** Compressional features of flat subduction, such as the uplifted block faults and transpression, were previously explained by basal shear from the undersliding plate¹ (a). However, Axen et al.² present numerical simulations that instead suggest that accumulated material from the continental mantle lithosphere forms a bulldozed keel. This keel drives a belt of compressional deformation far inboard of the trench and results in extension above the flat slab (b). Green circles represent crustal seismicity in the upper plate, along the subduction interface, and within the down-going slab.

upper plate. This prediction can be tested by examining stress field indicators above flat-slab regions. These include in-situ measurements, for example, elongation of boreholes during breakouts, field observations and analysis of earthquake focal mechanisms. Initial assessment of the focal mechanisms for crustal earthquakes above flat-slab zones in Central and Southern Peru reveals a low level of seismicity in general, with four normal faulting mechanisms out of a total of six. The focal mechanisms and mapped normal faults indicate ongoing extension and are therefore supportive of the predictions made by the Axen et al. model². A large-scale seismology survey above the southern edge of the Peruvian flat slab succeeded in imaging the deep slab, where resubduction occurs¹¹, and the impact on the upper plate (Fitzcarrald Arch) uplift was also

explored and tentatively explained by basal shear¹². This interpretation may need to be reconsidered in light of the Axen et al. study. Future passive seismological work here could target the interior portion of the upper plate and offer direct images of the bulldozed keel.

Axen et al.² offer an elegant explanation of the observations specific to the Laramide orogeny of western North America. But, beyond this regional significance, the proposed mechanism of a scraped continental mantle lithosphere and regional extension may be important to flat subduction zones more generally, today and in the past. The modern Peruvian Andes may yet prove to be an analogue for the Basin and Range of 50 to 60 million years ago. ☐

Marc-André Gutscher

Univ. Brest/French National Centre for Scientific

Research, Laboratoire Géosciences Océan, Plouzané, France.

e-mail: gutscher@univ-brest.fr

Published online: 26 November 2018

https://doi.org/10.1038/s41561-018-0270-x

References

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