The Mantle Transition Zone: Seismic Properties, Deep Subduction, Earthquakes, and Petrology

1. The Mantle Transition Zone: Seismic Properties, Deep Subduction, Earthquakes, and Petrology Wang-Ping Chen University of Illinois, Urbana-Champaign [Green, ’01]

2. Outline • Why the mixture of topics? • Forego comprehensive review • Work toward an integration, relating these topics • Preview of overall theme • Intricate interplays between temperature and petrology • Current issues on deep slab-penetration and mantle convection/mixing • Compare Tonga-Fiji and Izu-Bonin: • Counter-intuitive • Extremely fast subduction of cold slab results in impounding of buoyant slab in transition zone

3. Introspection/Re-interpretation • Low VP in the lower mantle • High temperature? • High density? • van der Hilst [’04] • Perspective on Trampert et al. [’04] • Scale of DVPnot reported

4. A picture is worth a thousand words – If the image is valid, clear, and correctly interpreted • Deep slab-penetration advanced by travel-time tomography • Issues and assumptions • High seismic wave speed implies low temperature? • Ignores petrologic effects (mineralogy, composition, volatiles, kinetics) • Amplitude of anomalies (and color-scale) • ± 0.5% in lower mantle (except near the base) • ± 2-3% in upper mantle and transition zone • Watch out for color-saturation • Inconsistency among images (with similar data and techniques)

5. 2 pix [Grand et al., 1997] DVS

6. Tonga-Fiji: Examples of Travel-Time Tomography [Deal et al., 1999]

7. Some Fundamental Issues • Mass imbalance • Fast subduction of cold slab but only modest anomalies in the lower mantle • Tonga subduction zone • Rate of convergence > 200 mm/yr (from GPS) • Slab older than 100 Ma • 65% of world’s deep seismicity • Best example of lower mantle anomaly • Caribbean anomaly (remnant of Farallon plate?) • Thermal window for deep slab penetration? [Grand et al., 1997]

8. Some Fundamental Issues (Cont.) • Incomplete mantle mixing • Primordial component of mantle • Transition zone “water filter” hypothesis [Bercovici and Karato, ‘03] • Tolstikhin-Hofmann hypothesis • Origin of deep earthquakes • Double couple • Sudden slip over faults • Kinematics similar to shallow earthquakes • Brittle failure requires unrealistic shear stress at depth • Strength is limited by ductile flow at high temperatures • Potential mechanisms • “Embrittlement” (fluid/fluid-like materials) • Transformational faulting (phase change)

9. Natural Laboratory: Seismicity

10. Tonga-Fiji Region: Seismicity • Wadati-Benioff zone • The usual inclined zone of seismicity • Consistency in fault plane solutions • The “toe” • Complexity in fault plane solutions • Outboard earthquakes • A separate zone • Lack of pattern in fault plane solutions

11. Experimental Configuration(Simplified)

12. Seismic Profiles • Triplicate waveforms • Basis for recognizing major discontinuities • Principles • Aperture of profiles • Variations in focal depth (50 - 670 km) • Variations in epicentral distance (1200 - 2500 km)

13. Waveform Modeling • Match absolute timing, relative timing, amplitudes • Isotropic medium: WKBJ method • Anisotropic medium: ANRAY and reflectivity 660-km disc 410-km disc 50 Reduced Time (s) 20 3000 1000 Distance (km)

14. Fore-Arc • Geologic baseline before subduction • Similar to average Earth models (i.e., iasp91) • Laterally homogeneous and isotropic over 1000 km • Anomalies under back-arc must be a consequence of subduction

15. Seismic Anisotropy • Definition • Variations of seismic wave speeds along different directions of propagation or polarization • Shear-wave birefringence (splitting) • Anisotropy in the Mantle • Usually concentrates above 200 km • Typically null anisotropy in transition zone • New observations in the TZ: Split-time up to 3 s

16. Lateral Variations of Anisotropy

17. Focal Depth: ~ 150 km ~ 590 km ~ 660 km Sector I: Strong Birefringence

18. Modeling Birefringence

19. Sectors II & III: Null Birefringence

20. Tonga-Fiji Region: Transition Zone Structures • 3D rendition of seismicity • Radial seismic anisotropy • Associated only with zone of outboard earthquakes • Petrofabric (VSH > VSV)

21. Lateral Variations in Wave Speeds • Compare fore-arc, source zone of outboard earthquakes, and the rest of the back-arc

22. Waveforms: Sector I • Similar to fore-arc • NO anomalies associated with outboard earthquakes

23. Waveforms: Sector II • High VP and VS in the TZ adjacent to outboard earthquakes

24. Magnitude of Anomalies

25. Tonga-Fiji Region: Transition Zone Structures (II) • Lack of high VP and VSwithin zone of outboard earthquakes • Seismicity indicates low temperature • Global patterns of deep seismicity and zones of recent convergence • Low strength of rocks at high temperatures • Otherwise mantle earthquakes away from subduction zones and sluggish mantle convection • Limiting temperature for seismicity in the mantle • 600 to 800C [Chen and Molnar, 1983; Molnar et al., 1979; Wiens and Stein, 1983]

26. Tonga-Fiji Region: Transition Zone Structures (III) Region I: Coldest Core • Effect of low temperature to raise V counteracted by petrologic anomalies (volatiles, meta-stable phases, melts?) • Contrast in anisotropy • Sharp boundary for VP and VS(3% over 25 km) • High VP and VS surrounding outboard earthquakes • Gradual decrease in magnitude outward • Cold “aureole”, yet too warm to maintain petrologic anomaly Region II: Cold Aureole Region III: Assimilated Slab

27. The Check List

28. Andean Seduction Zones • N3: Moderate-dipping slab • Classic down-dip extension • N2: Sub-horizontal slab • Incoherent P/T-axis • No regional stress field

29. Global Study of Seismic Strain • Two populations • Down-dip compression or extension, if • Slab dip > 20 • Down-dip comp. > sin(20) of total slab-pull • No clear pattern of strain, if • Slab dip < 20 • Vanishing down-dip comp. < 1/3 of total slab-pull • Localized, self-stress is both necessary and sufficient for generating deep earthquakes

30. Tonga-Fiji Region: Synthesis • Detached remnant of slab juxtaposed over active Wadati-Benioff zone • Buoyancy • Amount of reduction in VP and VS (2-3%) requires lots of meta-stable olivine (60% of olivine) or volatiles • Both are buoyant • Self-limiting buoyancy for meta-stable olivine • Petrologic anomalies are likely trigger of deep earthquakes • Large-scale remnant of slabs alleviate problems in mass imbalance and mantle mixing

31. Evolution of Buoyant Slab • Toe of WBZ as the predecessor of detached slab • The missing link between WBZ and detached slab • Detached slab • Anchored by dense leading edge where petrologic anomaly dissipates with rising temperature • Partially assimilated slab remnant • Aseismic • Petrologic anomaly dissipated • Attenuated thermal anomaly remains • Rests directly above the 660-km discontinuity (e.g., N. Philippine Sea anomaly) Stage 2a: Buoyant remnant Stage 2b: “Anchor” Stage 1: “Toe” [Modifying Green, ’01]

32. N. Philippine Sea Anomaly • Null Birefringence • Moderate increase in VP and VS (~1.5%) • Aseismic • Resting on 660-km discontinuity [Tseng and Chen, ‘04] [van der Hilst and Seno, ‘93]

33. Conclusions • Extremely fast subduction of cold slab results in impounding of buoyant slab in transition zone • Unified interpretation for observed seismic anisotropy, lateral variations in seismic wave speeds, and outboard earthquakes • Help resolving paradox in mass imbalance and perplexing patterns of seismicity • Slab penetration into the lower mantle may require special thermal window

34. Proposed Experiments in Tonga-Fiji • CAVASCOPE Array • Deep earthquakes • How deep is the deepest? • Still in the TZ? • The nature of the 660-km discontinuity • Topography • Compare with lab. data on pure Mg2SiO4? • Density contrast • High contrast (Japan Sea): 8.1±0.8% • Contrast in seismic anisotropy • Detached slab in the TZ • Toe of Wadati-Benioff Zone – detached slab in the making? • Complete mapping of cold aureole surrounding outboard earthquakes • Connection between two detached slab-remnants? [Tseng and Chen, 2004]

35. Collaborators • Michael R. Brudzinski (Miami Univ., Ohio) • Tai-Lin (Ellen) Tseng (UIUC) • Robert L. Nowack (Purdue) • Robert Pillet (Institut de Recherche pour le Développement, New Caledonia) • Bor-Shouh Huang (Institute of Earth Sciences, Taiwan) • Special thanks to • Harry Green, Steve Kirby • Jay Bass, Chu-Yung Chen, Jennie Jackson, Jie Li, Holger Hellwig, Stas Sinogeikin

36. Sources of materials not explicitly cited are mainly from the following articles: Brudzinski, M. R., W.-P. Chen, R. L. Nowack, and B.-S. Huang, Variations of P-wave speeds in the mantle transition zone beneath the Northern Philippine Sea, J. Geophys. Res., 102, 11,815–11,827, 1997. Nowack, R. L., E. Ay, W.-P. Chen, and B.-S. Huang, A seismic profile of the upper mantle along the southwestern edge of the Philippine Sea plate using short-period array data, Geophys. J. Int., 136, 171–179, 1999. Wu, L.-R., and W.-P. Chen, Anomalous aftershocks of deep earthquakes, Geophys. Res. Lett., 26, 1977–1980, 1999. Brudzinski, M. R., and W.-P. Chen, Variations of P-wave speeds and outboard earthquakes: Evidence for a petrologic anomaly in the mantle transition zone, J. Geophys. Res., 105, 21,661–21,682, 2000. Wu, L.-R., and W.-P. Chen, Rupture of the large (MW 7.8), deep earthquake of 1973 beneath the Japan Sea with implications for seismogenesis, Bull. Seismol. Soc. Am., 91, 102–111, 2001. Chen, W.-P., and M. R. Brudsinzski, Evidence for a large-scale remnant of subducted lithosphere beneath Fiji, Science, 292, 2475-2479, 2001. Brudzinski, M. R., and W.-P. Chen, A petrologic anomaly accompanying outboard earthquakes beneath Fiji-Tonga: Corresponding evidence from broadband P and S waveforms, J. Geophys. Res., 108(B6), 2299 (19 pp.), doi:10.1029/2002JB002012, 2003. Chen, W.-P. and Brudzinski, M.R., Seismic anisotropy in the mantle transition zone beneath Fiji-Tonga, Geophys. Res. Lett., 30,1682 (4 pp.), doi:10.1029/2002GL016330, 2003. Tseng, T.-L., and W.-P. Chen, Contrasts in seismic waves speeds and density across the 660-km discontinuity beneath the Philippine and the Japan Seas, J. Geophys. Res., 109, (12 pp.), B04302, doi:10.1029/2003JB002613, 2004. Chen, W.-P., and Z.-H. Yang, Earthquakes beneath the Himalayas and Tibet: Evidence for strong lithospheric mantle, Science, 304, 1949-1952, 2004. Brudzinski, M.R., and W.-P. Chen, Earthquakes and strain in sub-horizontal slabs, J. Geophys. Res., in press, 2005.