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Exploring uncertainties in millennial slip rates along the eastern Kunlun fault, NE Tibet

Exploring uncertainties in millennial slip rates along the eastern Kunlun fault, NE Tibet. Nathan Harkins PSU Geosciences. Eric Kirby and Xuhua Shi. SCEC Joint Fault System History / SoSAFE Workshop 1/31/2008. Motivation: Terrace risers as slip markers.

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Exploring uncertainties in millennial slip rates along the eastern Kunlun fault, NE Tibet

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  1. Exploring uncertainties in millennial slip rates along the eastern Kunlun fault, NE Tibet Nathan Harkins PSU Geosciences Eric Kirby and Xuhua Shi SCEC Joint Fault System History / SoSAFE Workshop 1/31/2008

  2. Motivation: Terrace risers as slip markers Bounding terrace ages provide a range of allowable rates

  3. Motivation: Terrace risers as slip markers Bounding terrace ages provide a range of allowable rates Many workers assume riser age = age of lower bounding terrace

  4. Motivation: Terrace risers as slip markers More than 50% of slip rate estimates in Tibet from offset terraces

  5. Motivation: Terrace risers as slip markers Data from Cowgill et al. 2006; Meriaux et al. 2004; Van Der Woerd et al. 2002 Rates in m/ka Many of these sites possess wide ranges of allowable rates

  6. Motivation: Terrace risers as slip markers Data from Cowgill et al. 2006; Meriaux et al. 2004; Van Der Woerd et al. 2002 Many of these sites possess wide ranges of allowable rates Explore uncertainties, reduce rage of allowable rates

  7. Eastern Kunlun fault field site: Flights of offset terrace risers at 2 locations ~3 km from one another Eastern Site Western Site Corona high altitude photography

  8. N 100 m The eastern slip-rate site (Ken Mu Da): View direction Corona high altitude photography -Well preserved terrace flight on east side of channel -Single fault trace

  9. Radiocarbon age control at the eastern site: T4 surface Loess Gravel Sample T1 ~5,000 cal YBP T3 ~12,500 cal YBP T4 ~15,400 cal YBP

  10. offsetMIN rateMAX rate T1/T2 31 ± 5 m 6.0 ± 1.0 T2/T3 52 ± 3 m4.2 ± 0.3T3/T4 62 ± 3 m4.0 ± 0.3 4.9 ± 0.2 Best estimate of slip rate at the eastern site… N 100 m

  11. The western slip-rate site (Deng Qin): View direction N 200 m Corona high altitude photography -Broad terrace treads (>400 m wide) -Two fault strands

  12. Radiocarbon age control at the western site: T4 Loess Gravel Sample T2 older than3,370 YBP T4 ~16,400 cal YBP T5older than ~28,500 YBP

  13. N 200 m Best estimate of slip rate at the western site… T2 older than 3,370 YBP T4 ~cal 16,400 YBPT5older than 28,500 YBP offsetMIN rateMAX rate T2/T4 60 ± 5 m3.7 ± 0.4 17.2 ± 2.0T4/T5 115 ± 40 m3.9 ± 1.2 7.2 ± 2.5 Rates in m/ka

  14. Summary of slip-rates..… Deng Qin2.7-19.2 m/ka Ken Mu Da3.9-5.1 m/ka • Sites < 3 km apart - spatial variation in rate unlikely • Isochronous terraces - no evidence for temporal variation Best estimate of slip rate along here is 4-5 m/ka What can morphologic ages of risers reveal about uncertainties?

  15. Morphologic Ages of Scarps (risers) Scarps in alluvium decay over time Decay can be described as a diffusive flux of material down a topographic gradient d2z dZ = dx2 dT kt= morphologic age Many workers have exploited this description in age analyses of fault scarps, paleo-shorelines, terrace risers, etc. Photos from USGS.gov

  16. Approach: Determining Riser Morphologic Ages d2z dZ = dx2 dT -1-D,FTCS diffusion model

  17. Approach: Determining Riser Morphologic Ages d2z dZ = dx2 dT -1-D,FTCS diffusion model -multiple forward models under a range of kt (varyk) -compute RMS error

  18. Approach: Determining Riser Morphologic Ages d2z dZ = dx2 dT -1-D,FTCS diffusion model -multiple forward models under a range of kt (varyk) -compute RMS error

  19. Approach: Determining Riser Morphologic Ages d2z dZ = dx2 dT -1-D,FTCS diffusion model -multiple forward models under a range of kt (varyk) -compute RMS error

  20. Approach: Determining Riser Morphologic Ages d2z dZ = dx2 dT -1-D,FTCS diffusion model -multiple forward models under a range of kt (varyk) -compute RMS error best fit kt for riser

  21. Riser Morphologic Age at Ken Mu Da; calibration of a local k value Local k of 3.2-5.4 m2/ka

  22. Diachronous Riser Ages across fault trace Preferential riser ‘protection’ or ‘exposure’ to erosion due to fault motion

  23. Riser Morphologic Age at Deng Qin • T2/T4 riser fit by kt ~49-70 m2 • Age range of 9.1-21.9 ka (T4 14C age ~16.4 ka) • Age must be 9.1-16.4 ka Revised slip rate of 3.4-7.1 m/ka

  24. T2 T4 Significance Site history at Deng Qin revealed Age of upper terrace appropriate

  25. Significance Contrasting modes of terrace age vs. riser offset records at adjacent sites

  26. Significance/ Conclusions Deformed terraces important slip-markers globally Uncertainty in appropriate model of riser age vs. fault offset can lead to wide ranges in allowable rates Lower terrace age not always appropriate Consideration of riser morphologic age can help reduce uncertainties

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