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

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Presentation on theme: "The Mantle Transition Zone: Seismic Properties, Deep Subduction, Earthquakes, and Petrology Wang-Ping Chen University of Illinois, Urbana-Champaign [Green,"— Presentation transcript:

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 V P in the lower mantle –High temperature? –High density? van der Hilst [’04] Perspective on Trampert et al. [’04] Scale of  V P not 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] VSVS

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 ( km) Variations in epicentral distance ( km)

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

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

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 (V SH > V SV )

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 V P and V S in the TZ adjacent to outboard earthquakes

24 Magnitude of Anomalies

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

27 Metastable OlivineHydros PhasesPartial Melt Stability in Transition Zone Favorable Induced by dehydration Trigger for Earthquakes Transformational faulting Embrittlement 3% reduction in V P 60% of olivine- polymorph in  - phase ~2.2 wt % water~2% melt (~0.04 wt% water) Corresponding reduction in V SH Consistent Too much (~6%) SH-SV splittingPlausible, fossil fabric Minor effect by themselves Oriented magma pockets (SPO) No pattern in Focal Mechanisms Self-stressInconsistent, need other localized source of stress Inconsistent, need regional stress for SPO Buoyancy of slab2%, but only in TZ1%, buoyant in the upper mantle ~0% (depending on compressibility) 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 ObservationsRegional Stress (Slab-Pull)Localized Stress Numerous seismicity in sub- horizontal slabs Small to not present, thus not necessary Only alternative, thus both necessary and sufficient Great pressure at depthSlab pull too small to be sufficient, ~0.5% negative buoyancy Only alternative, thus necessary and sufficient Down-dip compression from depths of 100 to 700 km, but gap in seismicity near 300 km InsufficientNot present? 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 V P and V S (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) [Modifying Green, ’01] Stage 1: “Toe” Stage 2b: “Anchor” Stage 2a: Buoyant remnant

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

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 Mg 2 SiO 4 ? –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, 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, Wu, L.-R., and W.-P. Chen, Anomalous aftershocks of deep earthquakes, Geophys. Res. Lett., 26, 1977–1980, 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, Wu, L.-R., and W.-P. Chen, Rupture of the large (M W 7.8), deep earthquake of 1973 beneath the Japan Sea with implications for seismogenesis, Bull. Seismol. Soc. Am., 91, 102–111, Chen, W.-P., and M. R. Brudsinzski, Evidence for a large-scale remnant of subducted lithosphere beneath Fiji, Science, 292, , 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: /2002JB002012, 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: /2002GL016330, 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: /2003JB002613, Chen, W.-P., and Z.-H. Yang, Earthquakes beneath the Himalayas and Tibet: Evidence for strong lithospheric mantle, Science, 304, , Brudzinski, M.R., and W.-P. Chen, Earthquakes and strain in sub-horizontal slabs, J. Geophys. Res., in press, 2005.

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