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

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

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

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

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

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

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

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

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

offset MIN rate MAX rate T1/T2 31 ± 5 m 6.0 ± 1.0 T2/T3 52 ± 3 m 4.2 ± 0.3 T3/T4 62 ± 3 m 4.0 ± ± 0.2 Best estimate of slip rate at the eastern site… 100 m N

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

Radiocarbon age control at the western site: T2 older than 3,370 YBP T4 ~16,400 cal YBP T5 older than ~28,500 YBP T4 Loess Gravel Sample

offset MIN rate MAX rate T2/T4 60 ± 5 m 3.7 ± ± 2.0 T4/T5 115 ± 40 m 3.9 ± ± 2.5 T2 older than 3,370 YBP T4 ~cal 16,400 YBP T5 older than 28,500 YBP Rates in m/ka Best estimate of slip rate at the western site… 200 m N

Summary of slip-rates..… Ken Mu Da m/ka Deng Qin m/ka Sites < 3 km apart - spatial variation in rate unlikely Sites < 3 km apart - spatial variation in rate unlikely Isochronous terraces - no evidence for temporal variation 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?

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 = =  dZ dT d2zd2z dx 2 Many workers have exploited this description in age analyses of fault scarps, paleo-shorelines, terrace risers, etc.  t = morphologic age Photos from USGS.gov

= =  dZ dT d2zd2z dx 2 -1-D,FTCS diffusion model Approach: Determining Riser Morphologic Ages

= =  dZ dT d2zd2z dx 2 -1-D,FTCS diffusion model -multiple forward models under a range of  t (vary  - compute RMS error under a range of  t (vary  - compute RMS error Approach: Determining Riser Morphologic Ages

= =  dZ dT d2zd2z dx 2 -1-D,FTCS diffusion model -multiple forward models under a range of  t (vary  - compute RMS error under a range of  t (vary  - compute RMS error Approach: Determining Riser Morphologic Ages

= =  dZ dT d2zd2z dx 2 -1-D,FTCS diffusion model -multiple forward models under a range of  t (vary  - compute RMS error under a range of  t (vary  - compute RMS error Approach: Determining Riser Morphologic Ages

= =  dZ dT d2zd2z dx 2 -1-D,FTCS diffusion model -multiple forward models under a range of  t (vary  - compute RMS error under a range of  t (vary  - compute RMS error best fit  t for riser Approach: Determining Riser Morphologic Ages

Riser Morphologic Age at Ken Mu Da; calibration of a local  value Local  of m 2 /ka

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

Riser Morphologic Age at Deng Qin T2/T4 riser fit by  t ~49-70 m 2T2/T4 riser fit by  t ~49-70 m 2 Age range of ka (T4 14 C age ~16.4 ka)Age range of ka (T4 14 C age ~16.4 ka) Age must be kaAge must be ka Revised slip rate of m/ka

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

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

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