Coupled spatial variations in precipitation and longterm erosion rates across the Washington Cascade

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Coupled spatial variations in precipitation and
long-term erosion rates across the Washington
Cascades
Presented by Gretta and Julie

• By Peter Reiners, Todd Ehlers, Sara Mitchell,
  David Montgomery
• Perspective by Peter Molnar

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Tectonics (Nature) vs. Climate (Nurture)
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Thermochronometry
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Fission Tracks (U-Th)/He Apatite
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Northern Washington Cascades

• Deep seated bed rock (granitoid plutonic and
  gneiss) uplift and resultant erosion
• Summits high as 2.7 km
• Local relief is 1.2 to 1.8 km
• Orographic rain shadow to the east
• Westward mean annual precipitation 4 m/yr
• Eastward mean annual precipitation 0.2 m/yr
• Absence of obvious active structures to produce
  late cenozoic rock uplift

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Calculated Erosion Rates
h elevation from sea level z total depth ?
h-h z-z
? exhumation rate z effective depth
hlocal mean elevation
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Average erosion rates 0.10 km/Myr
0.02-0.04 km/Myr
0.33 km/Myr
If these rates persisted for the last 10-15 my,
then 3-5 km of of rock have been removed from
the midslope and only 0.5-1 km from other
regions!!!!!
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Other influences?

• Possible broad arching or folding could lead to
  higher uplift rates (higher erosion rates at
  crest)
• Stream-power indices (doesnt correlate with
  precipitation)
• Glacial erosion

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Reiner et al Conclusions

• Long-term erosion rate pattern across the range
  is primarily controlled by precipitation pattern.

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The effect of tectonics and climate on erosion3
other case studies

• Himalayas and Taiwan Orogen
• Thermochronometric methods
• Apatite Fission
• Tracks
• 40Ar/39Ar Dating
• Different time
• scales- 14C dating
• (Molnar, 2003)

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3D perspective of Himalayas

• (Burbank et al, 2003)

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Apatite Fission Track

• Greater erosion in Greater Himalayas-lesser in
  the Lesser
• No difference in Greater Himalaya
• Precipitation plays no role in erosion
• Rapid erosion and isostatic compensation
• (Burbank et al, 2003)

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40Ar/39Ar

• Large differences in erosion between Greater
  Himalaya and Lesser Himalaya
• A few km in Lesser since 40-50Mya
• 10 km in Greater since 10Mya
• Rock in Greater moved up relative to Lesser
• (Wobus, et al., 2003)

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Different time scalesErosion Rates

• a) Fluvial suspended b) Bedrock strath
  incision c) Exhumation rates from
• Sediment rates
  apatite fission track
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  ages
• (Dadson, et al, 2003)

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Erosion rates and controls

• Stream Power erosion rates vary with the stress
  a flowing stream exerts on its bed
• ? ?gQS/w
• ? water density
• g acceleration due to gravity
• Q water discharge
• S channel slope
• w channel width
• Recent seismicity and precipitation from typhoons

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Conclusion

• Erosion rates and stream power
• Large rainfall variations
• Large contrast in stream power
• Geomorphology research moves away from
  observation and toward theory