Impact of riser reconstruction on estimation of secular

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Impact of riser reconstruction on estimation of
secular variation in rates of strike-slip
faulting revisiting the Cherchen River site
along the Altyn Tagh Fault, NW China
Eric Cowgill Dept. of Geology University of
California, Davis
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Purposes of study

• Reduce epistemic uncertainties
• comparison of riser offset with inset channel
  width
• similarity of riser and tread displacements
• morphological analysis of scarp profile
• riser deflection in plan view
• diachroneity of terrace abandonment
• whether riser crests or bases yield slip vector

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• Present geomorphic observations
• determine which terrace age most closely
  approximates true riser age

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• Fluvial terrace risers erosional escarpments
  that separate two stream terraces of different
  age and elevation.
• Fill terraces- form during aggradation by
  deposition of alluvium
• Strath terraces- form during incision, cut into
  bedrock

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• Treads
• gently sloping surfaces separated by steep
  terrace risers, formed during incision

• Upper-terrace reconstruction-
• riser offset combined with age of upper tread
• assumes flow over lower tread is too weak to
  erode riser
• displacement begins upon initiation of
  upper-tread incision
• error in this reconstrucion will result in a
  slip-rate that is too slow

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• Lower-terrace reconstruction-
• riser offset combined with age of lower tread
• typically used for strath terraces, assuming all
  displacement is removed by lateral erosion and
  scarp refreshment
• displacement is accumulated only after lower
  terrace is abandoned
• error will result in slip-rates that are too fast

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Ambiguity in terrace reconstruction

• Previous studies focused on the type of terrace
  offset
• Strath
• Fill

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• Ambiguity in terrace reconstruction
• This study
• age refers to portions of the tread extending
  several hundred meters up and downstream from
  fault trace.
• age refers only to the time at which the tread
  was abandoned due to renewed incision
• this definition is thus independent upon the type
  of terrace present

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Slip rate reconstruction

• Determination of two records
• displacement record beginning with accumulation
  of offset
• age of terrace when tread is abandoned and new
  lower terrace if formed.
• Both must begin simultaneously

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Geomorphic indices for identification of upper
vs. lower terrace reconstruction

• Reduce epistemic uncertainties
• comparison of riser offset with inset channel
  width
• similarity of riser and tread displacements
• morphological analysis of scarp profile
• riser deflection in plan view
• diachroneity of terrace abandonment
• whether riser crests or bases yield slip vector

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Conclusion

• Lower-terrace reconstruction yields a maximum
  rate by assuming that stream flow on the lower
  tread completely refreshes the riser, thus offset
  does not begin untill lower surface abandonment
• Upper-terrace reconstruction provides a minimum
  rate by assuming that riser offset starts
  accumulating as soon al the riser forms
• the 6 Indices outlined are vitally important to
  determining which reconstruction to assume