Topographic modulation of fault kinematics in the Himalaya and Tibet

Throughout the Himalaya and Tibet, moderate- to high-elevation strike-slip faults undergo extensional stepovers where they cross higher topography though to be pre-existing. Related seismological data have long been explained by an influential orogen-scale cross-section model where high-elevation normal faulting and low-elevation thrust faulting result from laterallyinvariant horizontal tectonic stress and vertical stress that varies with topography; however, this model cannot incorporate strike-slip faulting or ~10 km wavelength topography. Therefore I introduce a 3D elastic model describing the modulation of fault kinematics by shorterwavelength topographic stress, and show how this may tightly constrain the tectonic stress field. I then calculate the topographic stress field on the Western Nepal Fault System, and use topographic stresses and observed fault kinematics to invert for the tectonic stress field. The results yield a maximum tectonic compression of 0–0.2 ρgz and minimum tectonic compression of -0.1–0.1 ρgz, and reproduce kinematics from normal, strike-slip and thrust faults and earthquakes in and around western Nepal, including the 2015 Gorkha earthquake. This demonstrates that where vertical and a horizontal principal stress are near equal, 1-10 km scale variations in topography can change fault kinematics, and that pre-existing topography can influence the location of subsequent faults and stepovers.

Transcript

1. Plate convergence, topography and faulting in the Himalaya and Tibet Richard Styron Global Earthquake Model Foundation Pavia, Italy / Los Gatos, CA USA With help from Mike Taylor Eric Hetland, Mike Murphy
2. India Tibet (China) Nepal Pakistan 37 mm/yr
3. India Tibet (China) Nepal Pakistan
4. India Nepal Tibet (China)
5. India Nepal Tibet (China) Karakoram Fault (KF) Western Nepal Fault System (WNFS)
6. Styron et al., 2011 Geosphere Models for Himalayan and south Tibetan deformation
7. • Dextral slip on KF, through suture zone to east Styron et al., 2011 Geosphere e.g. Tapponnier et al., 1982
8. • Range- parallel normal faulting • Sinistral slip on KF, WNFS • E-W contraction within Tibet Styron et al., 2011 Geosphere e.g. Klootwijk et al., 1985
9. • E-W extension of Himalaya and Tibet • N-S extension of Tibet • No role of KF Styron et al., 2011 Geosphere e.g., Copley and McKenzie, 2007
10. • Range- parallel extension in Himalaya (Tibetan extension unrelated) • Dextral slip on KF, WNFS Styron et al., 2011 Geosphere McCaffrey and Nabelek, 1998
11. Himalayan range-parallel GPS velocities Styron et al., 2011 Geosphere • ~3 cm/yr range- parallel extension • Range-parallel component near zero where convergence is perpendicular
12. Himalayan range-parallel GPS velocities Styron et al., 2011 Geosphere • Dextral shear across KF, WNFS • Shortening perpendicular to range Range-parallel Range-perpendicular
13. • Dextral shear across KF, WNFS • N-S shortening in central Tibet
14. • Range- parallel extension in Himalaya (Tibetan extension unrelated) • Dextral slip on KF, WNFS Styron et al., 2010 Geosphere
15. Himalaya as subduction zone analog • Himalaya shows upper plate deformation similar to subduction zones • May serve as a good laboratory for convergence dynamics • Oblique convergence • Slab rollback • Some obvious differences (no volcanism) Ave Lallemant and Oldow 2000 Geology
16. New WNFS mapping • Ongoing mapping over past decade • WNFS thought to be oblique splay faults • Stepovers at (older?) high topography
17. Gurla Mandhata, 7694 m Himalaya Karakoram Fault Gurla Mandhata detachment
18. New WNFS mapping • Ongoing mapping over past decade • WNFS thought to be oblique splay faults • Stepovers at (older?) high topography Murphy et al., 2014 Nature Geoscience
22. 6000 5000 4000 3000 2000 1000 0 thrust strike-slip normal thrust to s.s. s.s. to normal Elevation of faults in Tibet Elevation of fault-type transitions n = 106 n = 120 n = 142 n = 31 n = 46
23. Thrust Faulting Normal Faulting Strike-Slip Faulting Modified from Styron et al., 2015 Nature Geoscience
24. An old idea at the orogen scale (isostatic support) Molnar and Lyon-Caen, 1988 GSA Special Papers
25. Apply to local scale, elastic support?
26. Simple explanatory model: Concepts • Fault kinematics determined by local stress field (+ pre-existing weak planes) • Topographic stress (M) varies locally (w/ topography), tectonic stress (T) varies more broadly • Therefore topography can influence fault kinematics even over short distances
27. Simple explanatory model: Validation and Use • Can’t directly test: Don’t know Tectonic stress • Topographic stress can/will be approximated • Instead, calculate M, resolve on faults, and find T that matches deformation observations • Get tectonic stress as a result • Validate results against new data
28. • Total stress = Topographic + Tectonic + Lithostatic stress • S = M + T + L • Topographic stress M is 6-component, highly variable • T is horizontal (Txx, Txy, Tyy -> Tmax, Tmin, azimuth) • increases linearly w/ depth • no lateral variation Simple explanatory model: Stress approximations
29. Calculating topographic stress fields • Convolve solutions for point-source stresses with DEM • Correct for effects of slope, irregular surface boundary condition • From Liu and Zoback, 1992 point load F v surface horizontal stress σ xx vertical stress σ zz elastic half- space topography F v Convolved vertical stress Styron and Hetland, 2015 JGR
30. Calculating topographic stress on faults ‣ Fault geometry contoured at depth ‣ 6 stress tensor components interpolated onto fault at each point ‣ τ, σn calculated at each point using local geometry (strike, dip) schematic
31. Making 3D Faults ‣ Fault geometry from field measurements, thermal modeling, inference ‣ Points of assumed elevation mapped, triangulated ‣ Rakes from field measurements and extrapolation
32. Fault kinematics modulated by topographic stresses? ‣ Normal segments of WNFS strongly loaded down-dip by topography ‣ Moderate vertical stress under strike-slip segments ‣ Gorkha slip patch loaded in thrust sense
33. Bayesian tectonic stress inversions: Data ‣T inversion only uses WNFS fault data ‣Dogari/Tibrikot, Gurla Mandhata ‣Compared (validated) with other datasets ‣Gorkha coseismic slip model (Galetzka et al. 2015) ‣Pre-Gorka ISC focal mechanisms ‣Will incorporate more faults in future
34. Bayesian tectonic stress inversions: Priors ‣Tectonic stresses considered horizontal, linearly increasing with depth ‣Maximum (Tmax), Minimum (Tmin), azimuth of Tmax ‣Set (uniform) priors for each: ‣Tmax: 0–2.0 * ρgz ‣Tmin: -1 –1 * Tmax ‣azimuth: 0–180°/360°
35. Bayesian tectonic stress inversions: Likelihood calculation and posterior sampling • Likelihoods based on goodness of fit between shear stress rake and coseismic slip rake • Posterior sampled proportional to likelihood
36. • Tmax ~0.1 * ρgz • (higher values eliminate normal slip) • Tmin slightly negative • Az Tmax same as Indian convergence direction Bayesian tectonic stress inversions: Results (Posteriors)
37. Stress observations vs. predictions
41. Himalaya and Wenchuan stresses 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 T’ max T’ min a 0.0 0.0 0.2 0.4 0.6 0.8 1.0 T’ min /T’ max b 0° 45° 90° 135° 180° 225° 270° 315° 100 200 300 400 500 c 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0. μ d Tmin 2.0 2.0 1.0 Tmax 1 Styron and Hetland, 2015 JGR
42. Wenchuan slip vs stress • High normal stress under 4 km relief may have locally suppressed earthquake slip • Earthquake-wise topography/stress reequilibration? Styron and Hetland, 2015 JGR
43. Lower WNFS predictions • Low-elevation segments may have reverse component • Segments may splay off of megathrust • Subaerial analog for deep trench forearc sliver-bounding faults Dextral- Normal Dextral- Reverse? Murphy et al., 2014 Nature Geoscience
44. Topographic stresses and faulting • Variable topographic stress and uniform tectonic stress can modulate fault kinematics on local-regional scales • Topographic stress effects may be important at timescales from seconds to millions of years • Topographic stress and fault interaction can help us constrain tectonic stress
45. Himalaya as subduction zone analog (II) • Elevation-fault type relationships seen in many subduction zone upper plates • Strain partitioning and forearc sliver-bounding faults better exposed in Himalaya • Tectonic stresses may be similar (???) • Erosion and air vs. water densities very different
46. Further thoughts: Stress equilibrium • Orogenic processes are balance between tectonic and gravitational forces • Equilibrium is dynamic, local • Re-equilibration processes operate on range of spatial and temporal scales
47. Further thoughts: Tectonics<->Topography<->Erosion • Feedback loops between tectonics, topography and erosion are important • Smaller-scale, non-Nature-Paper feedback loops may be cumulatively very important • Fault localization, fluvial geomorph, earthquake slip, … Beaumont et al., 2001 Nature
48. Conclusions • Himalaya deforms internally due to variably-oblique convergence • Topographic stresses modulate fault kinematics in Himalaya and Tibet • Himalaya may serve as accessible analog for subduction zones in many respects
49. Acknowledgements and thank-yous • Collaborators: Mike Taylor, Eric Hetland, Mike Murphy • Conference organizers • Spanish ham, sausage and cheese makers