Mechanism of folding in Subduction zones. DHSGU M.Tech

1. Dr Harisingh Gour Vishwavidyalaya
Sagar, Madhya Pradesh
Under Guidance of:
Dr. Gaurav Kumar Singh
Department of Applied Geology
Presented by:
Rahul Vishwakarma
M.Tech Ist Sem
Reg. no. Y22251046

2. Contents
1. Different Mechanism of Fold development.
• Buckling
• Flexural flow (Flexural slip and Flexural shear)
• Bending
• Passive Folding
2. Subduction Zones
3. Mechanism of Folding in Subduction Zone
4. Conclusion
5. References

3. Different Mechanisms of Fold development
 Buckling
 Fold wavelength- Thickness ratio (L/H)
constant.
 Homogenous material deformed under
same physical condition.
 Class 1 B Fold
 Layer parallel compression.
 L = 2πt (µL/6µM)1/3
…………..
Source: http://youtu.be/-_4BYWgSu60

4. …………..
Beds with lesser thickness Buckles more
source: https://youtu.be/p5b0eTSxbD4

5.  FLEXURAL FLOW
(FLEXURAL SLIPAND
FLEXURAL SHEAR)
 Flexural slip implies slip along
layer interfaces.
 Maintains bed thickness so-
Class 1B fold.
 Maximum slip occurs at the
inflection points and dies out
towards the hinge line
…………..
source: https://youtu.be/p5b0eTSxbD4

6.  BENDING
• Unlike buckle folds (force acts parallel to layer).
• Bending occurs when force acts across layers.
…………..
source: https://youtu.be/p5b0eTSxbD4

7.  Bending Folds between boudins.
 Fault-bend folds as bending folds
…………..
 Forceful intrusion of magma or salt can also bend roof layers.
source:
http://youtube.com/watch
?v=zou-
RkGaNro&feature=share
source: https://youtu.be/p5b0eTSxbD4

8.  Passive Folding
 Produced by accentuation of pre-existing
curvature in layers by more or less homogenous
strain.
 Homogeneity of strain is not essential.
…………..
source: https://youtu.be/p5b0eTSxbD4

9. Subduction Zones
 Oceanic lithosphere and some
continental lithosphere is recycled
into Earth’s mantle at convergent
boundaries.
 Dense lithospheric plate subducts
into less dense lithospheric plate.
 Possible because the cold oceanic
lithosphere is slightly denser than
the underlying asthenosphere
…………..
Source: http://youtube.com/shorts/8enMk23DN2M?feature=share

10. …………..
Source: http://youtu.be/if2SJq_sSIA

11. Mechanism of Folding in Subduction zone
 Lithospheric bending of subducting slabs was traditionally
considered to be accommodated by orthogonal flexure.
 Can possibly accommodated through simple shear deformation
parallel to slab.
 When the fold is generated through orthogonal flexure, the
difference of longitude of lines is accomplished by a
combination of pure-shear extension of the outer arc and pure-
shear compression of the inner arc
…………..

12. Orthogonal flexure: fold
geometry is generated by
extension of lines above and
compression below NS (neutral
surface), LO: longitude along
NS which remains constant, Lu:
longitude along the top of the
slab which suffers elongation,
Ld: longitude along the bottom
of the slab which suffers
shortening, hO: depth of NS.
…………..
Source: I. Romeo & J.A. Alvarez-Gomez. Lithospheric folding by flexural slip in subduction zones as source
for reverse fault intraslab earthquakes. scientific report (2018)

13.  Neutral surface separates the domain of extension and compression.
 The extension suffered by outer arc is given by:
eu = (Lu – Lo)/Lo
Lu = βr & Lo = β(r – ho)
therefore, eu = ho/(r – ho)
 The compression suffered by inner arc given by:
ed = (Ld - Lo)/Lo
Ld = β(r – h) & Lo = β(r – ho)
therefore, ed = (ho – h)/(r – ho)
…………..

14. The bending strain is
instantaneous when the
slab enters the curved
zone, and there is no
strain until the slab
unbends again by
instantaneous strain.
…………..
Source: I. Romeo & J.A. Alvarez-Gomez. Lithospheric folding by flexural slip in subduction zones as source
for reverse fault intraslab earthquakes. scientific report (2018)

15.  Flexural flow: fold geometry
is generated by conservation of
longitudes of lines parallel to
the slab (L) but allowing
simple shear strain between
these lines characterized by a
shear angle (ψ).
 Markers are curves
progressively inclined during
deformation.
…………..
Source: I. Romeo & J.A. Alvarez-Gomez. Lithospheric folding by flexural slip in subduction zones as source
for reverse fault intraslab earthquakes. scientific report (2018)

16.  The simple-shear model considers that there is no extension or
compression of lines parallel to the lithosphere, but shear slip between
them is allowed. The amount of shear deformation is defined by the
shear angle (ψ).
Ψ = arctan (Lu - Ld / h)
Combined with Lu=β r and Ld=β (r−h), it gives:
Ψ = arctan (β)
 The only parameter that controls the amount of deformation in the
simple-shear model is the dip of the flexed lithosphere.
…………..

17. Strain increases progressively with
β increase.
…………..
Source: I. Romeo & J.A. Alvarez-Gomez. Lithospheric folding by flexural slip in subduction zones as source
for reverse fault intraslab earthquakes. scientific report (2018)

18. Conclusion
 In nature we found different types of Fold Mechanisms which
developes different kind of Folds in different regimes.
 Lithospheric bending of subducting slabs was traditionally considered
to be accommodated by orthogonal flexure.
 Can possibly accommodated through simple shear deformation
parallel to slab.
 Flexural slip as a mechanism of slab bending in subduction zone
provides a satisfactory explanation for the Folding can possibly
accommodated.
…………..

19. References
 The Techniques of Modern Structural Geology: JOHN G. RAMSAY and
M.I. HUBER. Kinematics of folding(238-268)
 I. Romeo & J.A. Alvarez-Gomez. Lithospheric folding by flexural slip in
subduction zones as source for reverse fault intraslab earthquakes.
scientific report (2018) 8:1367|DOI:10.1038/s41598-018-19682-7
 http://Britannica.com/science/fold
 http://en.m.wikipedia.org
 discover subduction zones
http://www.geo.cornell.edu/hawaii/220/PRI/PRI_PT_subduction.html
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20. Thank You
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