Thrust sequences and the significance of lowangle, outofsequence faults in the northernmost Moine Na

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Thrust sequences and the significance of
low-angle, out-of-sequence faults in the
northernmost Moine Nappe and Moine Thrust Zone,
NW Scotland

• R.E. Holdsworth, R.A. Strachan, G.I. Alsop, C.J.
  Grant R.W. Wilson
• Journal of the Geological Society
• Volume 163, part 5. September 2006

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Purpose of the study?

• The Moine Thrust Zone preserves a
  foreland-propagating sequence of thrusting, which
  is common to most thrust belts
• Current models and interpretations of thrusting
  along the Moine Thrust Zone are controversial
• Remapping of the northernmost Moine Thrust zone
  showed evidence for an out-of-sequence structure,
  the Lochan Riabhach Thrust (LRT)
• Holdsworth et. al. proposed a new interpretation
  of the LRT as an extensional fault that formed
  between thrust sequences in the Moine Nappe

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Significance of the Moine Thrust

• Its a geological sanctuary for structural
  studies
• The discovery of duplex thrust geometry was made
  by Peach and Horne as they mapped (1883-1896)
  what is now the Moine thrust system
• Peach and Horne (Peach et al., 1907) carried out
  phenomenal structural mapping of duplexes and
  imbricates here
• And it was here where mylonites were described
  for the first time ever!

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Geological sketch map of the lowermost Moine
Nappe and upper Moine Thrust Zone between Loch
Hope and Creagan Meall Horn.
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Current models

• The most widely accepted interpretation of the
  evolution of the Moine Thrust Zone and overlying
  Moine Nappe is a foreland-propagating deformation
  sequence of thrusting and folding with associated
  mylonitization and regional metamorphosis.
• Regionally, foreland propagation of thrusts
  appears to have occurred synchronously with
  exhumation so that earlier ductile thrusts in the
  east are superceded and carried passively in the
  hanging walls of increasingly brittle, later
  thrusts to the west during a single phase of
  WNW-directed Caledonian overthrusting.

Sketch cross-sections showing characteristic roof
thrust geometries for (a) foreland-propagating or
piggyback thrusting sequences, and (b) overstep
thrusting sequences
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Evidence for out-of-sequence displacement along
the Lochan Riabhach Thrust

• The LRT is relatively planar, low angle (dips
  between 5 and 35), laterally continuous fault
  that post-dates major folds in the MTZ
• It cross-cuts thrust faults in both its
  hanging wall and
  footwall
• It forms a structural base to the Moine
  Thrust Zone
  mylonites, separating the
  imbricates below, and
  it is associated
  with a major change in ductile
  strain
  intensity from hanging wall to footwall.
• Unlike the ductile Moine Thrust, the LRT is a
  brittle structure. However, where exposed, the
  LRT is found with a few millimeters of gouge that
  could not have formed at the same metamorphic
  grade as the mylonites in its hanging wall or the
  plastically deformed rocks in its footwall.

FW
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• The observations made during the remapping of the
  Moine Thrust Zone contradict earlier proposals of
  synchronous movement sequences that predict
  localized development of out-of-sequence,
  overstep thrusts with differing parts of the same
  thrust plane being either in or out of
  sequence (Butler 2004, and Butler et al., 2007).
• The general foreland-propagating thrust sequence
  is present, but Holdsworth et al. also recognized
  the extensional, out-of-sequence LRT that excises
  the preexisting stratigraphy of the Mone Thrust
  Zone, placing earlier formed mylonites up against
  later formed brittle imbricates.

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A new speculative model of thrusting and
low-angle extensional faulting sequences in the
MTZ
Foreland
Thrust wedge

• foreland-propagating thrusting begins
• (b) foreland-propagating thrusting stops due to
  collapse of the frontal part of the thrust sheet
  (triggered by either internal weakening of the
  thrust wedge or basal decollement)
• (c) once the collapse ceases, foreland-propagating
  thrusting resumes forming a layer of lower
  imbricates and new Sole Thrust (ST)

Out-of-sequence shearing
Extension occurs
Extension occurs
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Conclusions

• The Moine Thrust is deformed by thrust-related
  folds that developed in its footwall mylonites,
  thus demonstrating that the foreland-propagating
  pattern of ductile deformation established in the
  Moine Nappe persisted downward into the mylonite
  belt
• The base of the mylonite belt is defined by a
  brittle fault, the Lochan Riabhach Thrust, that
  formed out-of-sequence, cutting through
  preexisting folds and thrusts in its footwall
• There is no unambiguous solution to whether the
  LRT resulted from shortening, cutting gently
  across steeper parts of the thrust pile, or
  extensional collapse (modeled by Holdsworth et
  al.)
• Butlers synchronous thrusting model is
  appropriate in some areas of the Moine Thrust
  Zone, but cannot account for the development of
  the LRT
• This study suggests that regionally developed
  fault rock asymmetry across thrusts can be used
  to detect the presence of out-of-sequence
  structures within other ancient thrust belts