The geomorphologic responses to hinge migration in the faultrelated folds in the Southern Tunisian A

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The geomorphologic responses to hinge migration
in the fault-related folds in the Southern
Tunisian Atlas by Riadh Ahmadi, Jamel Ouali,
Eric Mercier, Jean-Louis Mansy, Brigitte
Van-Vliet Lanoë, Patrick Launeau, Farhat Rhekhiss
and Silvain Rafini

• By Miguel Cruz

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Background

• Kinematic fold models imply hinge migration
• Traces of hinge migration were hard to find,
  possibly because of flexural slip
• Shoulder shaped growth strata are good evidence
  for hinge migration
• Fig. 1. Comparison between hinge migration and
  hinge rotation (synclinal hinge case). (a)
  Kinematic evolution. (b) Structural context and
  location of the deformation (inspired from
  Saint-Bezar et al., 1999). (c) Recording by
  growth strata (Rafini and Mercier, 2002). (d)
  Example of expected geomorphological features
  for the limb rotation case, the slope has the
  same tilting age in each point, the erosion
  evolves regularly near the theoretical
  long-profile asymptotic shape. In the limb
  migration case, the tilted slope is younger
  toward the base, so that the knick-point is
  continuously uplifted, entraining under-erosion
  in the recently outcropping areas (inspired from
  Seeber and Gronitz, 1983, Merrits and Hesterberg,
  1994, Whipple and Tucker, 1999 and Burbank and
  Anderson, 2001). (Ahmadi et al. 2006)

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Setting

• Anticlines in the frontal Southern Tunisian Atlas
• Folding is still active
• Mostly fault-propagation folds
• Morphological responses to migrating synclinal
  hinges are investigated in this paper
• Fig. 2. Geologic map of studied area (with
  location of figures and photos). lClower
  Cretaceous uCupper Cretaceous E-PPaleocene
  and Eocene N-QNeogene and Quaternary RRecent.
  
• (Ahmadi et al. 2006)

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• Kinematic models for fault propagation folds
  predict that 3 of the 4 hinges should migrate

Fig. 4. Kinematic evolution of fault-propagation
fold. Due to an increase in shortening, both
syncline hinges ((1) and (4)) migrate and new
material from near-by plains become deformed and
uplifted toward fold slopes. One of the anticline
hinges (2), migrates and transfers material from
the upper anticline flat to the back-limb. The
fourth hinge (3) remains practically fixed during
fold development.
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• Erosion has removed most post Cretaceous series
• Structural features are preserved by the upper
  Cretaceous Abiod Fm. Limestone
• (Ahmadi et al., 2006)

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Observations Interpretations

• Mainly tectonics climate control erosion
• Pediments are incised by younger Pleistocene
  alluvial splays
• Pediments are deformed by a shoulder that offsets
  youngest beds by 10m and oldest beds by 30m
• Suggests that the southern hinge migrated
  recently
• (Ahmadi et al., 2006)

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• Perched canyons and less eroded lower surfaces
  suggest uplift occurred only recently.
• (Ahmadi et al., 2006)

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• Lowest areas have less developed less mature
  drainage systems, indicating more recent uplift
  of lower areas caused by hinge migration followed
  by incision erosion
• Fig. 7. (a) SPOT 4 view of Jebel Sehib. (b)
  Drainage network established on the main
  structural relief massive Abiod limestone
  formation. (c) Structural surface incision map
  (in white) of the Abiod Formation roof. (Ahmadi
  et al., 2006)

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Conclusion

• The kinematic evolution of the fault-propagation
  folds in this area are shown to exhibit hinge
  migration based on geomorphological observations.
• This demonstrates that hinge migration during
  folding does occur.