michael.thorne@utah.edu http://web.utah.edu/thorne

Detection of present-day slab-driven mantle flow

Michael S. ThorneEdward J. GarneroAllen K. McNamara

Heiner Igel

Dept. of Geology & Geophysics, University of UtahSchool of Earth and Space Exploration, Arizona State University

Dept. of Earth & Environmental Sciences, Ludwig Maximilians University

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Past Ultra-low velocity zone detections

McNamara et al. 2010

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SPdKS seismic probe

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• 51 high quality events• 4221 records

SPdKS data set

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• 51 high quality events• 4221 records

SPdKS data set

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PSVaxi Simulations

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PSVaxi Model Space

δVS, δVP:(-15%,-5%) (-30%,-10%)(-45%,-15%) (-5%,-5%)(-10%,-10%) (-15%,-15%)

δρ: 0%, +10%

h: 5, 7.5, 10, 12.5, 15, 20, 30 km

width: 1.5°, 3.0°, 4.5°, 6.0°, 12.0°

edge: 8.5° - 19.0° in 1.5° increments

total: 580 models

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Type A&B waveforms & ULVZ position

NORMAL

ULVZ Model:δVS = -45%δVP = -15%δρ = +10%h = 12.5 kmwidth = 3.0°

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Type A&B waveforms & ULVZ position

Type A

ULVZ Model:δVS = -45%δVP = -15%δρ = +10%h = 12.5 kmwidth = 3.0°

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Type A&B waveforms & ULVZ position

ULVZ Model:δVS = -45%δVP = -15%δρ = +10%h = 12.5 kmwidth = 3.0°

Type B

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Type A&B waveforms & ULVZ position

ULVZ Model:δVS = -45%δVP = -15%δρ = +10%h = 12.5 kmwidth = 3.0°

Type B

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Waveform Behavior

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Results: Normal, Type A & B locations

Tomo: Grand, 2002

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δVS = -45%, δVP = -15%δρ = 10% thickness =10-15 km width ≈ 200-300 km

length ≈ 800 km

Results: Mega-sizedULVZ

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Tomo: Grand, 2002

Mega-sized ULVZ location

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Tomo: Grand, 2002

Cross-Section

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Thermochemical convection modeling

• 3 distinct compositional reservoirsi. background mantleii. LLSVPiii. ULVZ

• ULVZs preferentially align along LLSVP boundaries

• ULVZs fast moving, merging ULVZs realign with LLSVPboundaries on hundred million year time scales.

McNamara et al. 2010

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Development of mega-sized ULVZ

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Conclusions

• SPdKS capable of high resolution mapping of ULVZ position and properties.

• IF LLSVP’s are thermochemical piles…

a) mega-sized ULVZ in LLSVP hole may be indicative of multiple merging piles.

b) may give rise to mantle plumes rising near interior edges of piles.

c) indeed, mega-sized ULVZ lies directly beneath the Samoa hot spot.