messages from the deep: reviewing seismic evidence for deep mantle slabs thermochemical piles

Messages From the Deep: Reviewing Seismic Evidence for

Deep Mantle Slabs Thermochemical Piles Post-Perovskite Phase Transition Ultra-Low Velocity Zones

Messages From the Deep: Reviewing Seismic Evidence for

Deep Mantle Slabs Thermochemical Piles Post-Perovskite Phase Transition Ultra-Low Velocity Zones

June 21, 2006 5th Annual COMPRES MeetingJune 21, 2006 5th Annual COMPRES Meeting

Ed Garnero Arizona State Univ. Dept. of Geological Sciences

Ed Garnero Arizona State Univ. Dept. of Geological Sciences

and Uncertainties

Multidisciplinary research conducted in collaboration with:

Avants, Megan (UCSC) Ford, Sean (UCB) Hernlund, John (IPGP) Hutko, Alex (UCSC) Igel, Heiner (U Munich) Lay, Thorne (UCSC) Manga, Michael (UCB)

Multidisciplinary research conducted in collaboration with:

Avants, Megan (UCSC) Ford, Sean (UCB) Hernlund, John (IPGP) Hutko, Alex (UCSC) Igel, Heiner (U Munich) Lay, Thorne (UCSC) Manga, Michael (UCB)

McNamara, Allen (ASU) Rokosky, Juliana (UCSC) Rost, Sebastian (U Leeds) Schmerr, Nick (ASU) Thomas, Christine (U

Liverpool) Thorne, Mike (U Alaska) Williams, Quentin (UCSC)

McNamara, Allen (ASU) Rokosky, Juliana (UCSC) Rost, Sebastian (U Leeds) Schmerr, Nick (ASU) Thomas, Christine (U

Liverpool) Thorne, Mike (U Alaska) Williams, Quentin (UCSC)

-- Today --

Some Seismo Truths: Important modeling uncertainties/trade-offs Outlook: possible things to come

Recent results and interpretations Focus on deep mantle ‘high resolution’ work Draw connections to global scales/processes inferred from long wavelength studies

-- Today --

Some Seismo Truths: Important modeling uncertainties/trade-offs Outlook: possible things to come

Recent results and interpretations Focus on deep mantle ‘high resolution’ work Draw connections to global scales/processes inferred from long wavelength studies

Isaacs, Oliver, Sykes [1969]

Why care? …We’d like to better understand:

Mode/style of mantle convection Depth extent, nature of subduction Source of hot spot magma

Mantle H2O budget/cycle

Transition zone structure, dynamics Nature, structure of fluid and solid cores Thermal evolution/budget of deep interior

Why care? …We’d like to better understand:

Mode/style of mantle convection Depth extent, nature of subduction Source of hot spot magma

Mantle H2O budget/cycle

Transition zone structure, dynamics Nature, structure of fluid and solid cores Thermal evolution/budget of deep interior

Seismology Report CardSeismology Report Card

Structural feature Evidence Constrained?

Transition zone layering/topography

Deep mantle heterogeneity

Deep mantle “piles”

D” Vs discontinuity/layering

D” Vp discontinuity/layering

D” anisotropy

Ultra-low velocity zone

CMB topography

Transition zone layering/topography

Deep mantle heterogeneity

Deep mantle “piles”

D” Vs discontinuity/layering

D” Vp discontinuity/layering

D” anisotropy

Ultra-low velocity zone

CMB topography

B+B+ CC

A+A+ B-B-

CC C-C-

BBA+A+

D-D- D-D-

BB C-C-

AA CC

FF FF

Five reasons for bad “Constraints” gradesFive reasons for bad “Constraints” grades

Seismology Report CardSeismology Report Card

1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)

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Five reasons for bad “Constraints” gradesFive reasons for bad “Constraints” grades

1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)

Seismology Report CardSeismology Report Card

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Time

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Five reasons for bad “Constraints” gradesFive reasons for bad “Constraints” grades

1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)

Seismology Report CardSeismology Report Card

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Five reasons for bad “Constraints” gradesFive reasons for bad “Constraints” grades

1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)

2) Modeling trade-off between discontinuity location & isotropic heterogeneity

1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)

2) Modeling trade-off between discontinuity location & isotropic heterogeneity

Seismology Report CardSeismology Report Card

Am

plit

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New arrival!

inner

Five reasons for bad “Constraints” gradesFive reasons for bad “Constraints” grades

1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)

2) Modeling trade-off between discontinuity location & isotropic heterogeneity

3) Globally, only very long wavelength structure is retrievable, which involves significant smearing, and may not accurately depict physics of interior

1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)

2) Modeling trade-off between discontinuity location & isotropic heterogeneity

3) Globally, only very long wavelength structure is retrievable, which involves significant smearing, and may not accurately depict physics of interior

Seismology Report CardSeismology Report Card

Five reasons for bad “Constraints” gradesFive reasons for bad “Constraints” grades

1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)

2) Modeling trade-off between discontinuity location & isotropic heterogeneity

3) Globally, only very long wavelength structure is retrievable, which involves significant smearing, and may not accurately depict physics of interior

4) Only fraction of a percent of the globe has been probed at “high resolution”, and hence projection of results to global scales is conjecture

1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)

2) Modeling trade-off between discontinuity location & isotropic heterogeneity

3) Globally, only very long wavelength structure is retrievable, which involves significant smearing, and may not accurately depict physics of interior

4) Only fraction of a percent of the globe has been probed at “high resolution”, and hence projection of results to global scales is conjecture

Seismology Report CardSeismology Report Card

~ 2502 km

~ 200 x 700 km~ 1002 km

Five reasons for bad “Constraints” gradesFive reasons for bad “Constraints” grades

1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)

2) Modeling trade-off between discontinuity location & isotropic heterogeneity

3) Globally, only very long wavelength structure is retrievable, which involves significant smearing, and may not accurately depict physics of interior

4) Only fraction of a percent of the globe has been probed at “high resolution”, and hence projection of results to global scales is conjecture

5) Still using 1-D techniques to get 3-D answers….

1) Poor constraints regarding where on seismic raypath observables occur (travel time delays, extra arrivals, shear wave splitting, etc)

2) Modeling trade-off between discontinuity location & isotropic heterogeneity

3) Globally, only very long wavelength structure is retrievable, which involves significant smearing, and may not accurately depict physics of interior

4) Only fraction of a percent of the globe has been probed at “high resolution”, and hence projection of results to global scales is conjecture

5) Still using 1-D techniques to get 3-D answers….

Seismology Report CardSeismology Report Card

Messages From the Deep: Reviewing Seismic Evidence for Deep Mantle Slabs,

Thermochemical Piles, Post-Perovskite Phase Transition, Ultra-Low Velocity Zones

Messages From the Deep: Reviewing Seismic Evidence for Deep Mantle Slabs,

Thermochemical Piles, Post-Perovskite Phase Transition, Ultra-Low Velocity Zones

A Mineral Physicist’s Guide to Disbelieving Seismologists

A Mineral Physicist’s Guide to Disbelieving Seismologists

IgnoringEmbracingOstracizingCanonizingEnslaving

::

IgnoringEmbracingOstracizingCanonizingEnslaving

::

Unparalleled seismic network seismometer populations (e.g., NSF-funded EarthScope’s USArray)

Enables technique refinement/developmentPermits structural retrieval at smaller scale lengths

Unparalleled seismic network seismometer populations (e.g., NSF-funded EarthScope’s USArray)

Enables technique refinement/developmentPermits structural retrieval at smaller scale lengths

Seismology Report Card: Grades Rapidly Improving !Seismology Report Card: Grades Rapidly Improving !

Better computational capabilities

2- and 3-D wave propagation computations doableWe are approaching capabilities of benchmarking solution structures

Better computational capabilities

2- and 3-D wave propagation computations doableWe are approaching capabilities of benchmarking solution structures

Advances in mineral physics, geodynamics, & geochemistry

Provides significant guidance of our research targets and goals

Advances in mineral physics, geodynamics, & geochemistry

Provides significant guidance of our research targets and goals

Recent Seismic ResultsRecent Seismic Results

Some short seismic modeling vignettes relating to:

-Slabs in the lower mantle

-Post-perovskite phase transition

-Ultra-low velocity zone

-Deep mantle piles?

Recent Seismic ResultsRecent Seismic Results

200-800 km depth

Upper mantle, transition zone structure in the central Pacific

400-1000 km depth

Slab detection fromseismic reflections

between

2800-2900 km depth

Ultra-low velocityzone structure

2400-2800 km depth

D” discontinuitytopography

2400-2800 km depth

D” fine-scalelayering

Recent Seismic ResultsRecent Seismic Results

200-800 km depth

Upper mantle, transition zone structure in the central Pacific

400-1000 km depth

Slab detection fromseismic reflections

between

2800-2900 km depth

Ultra-low velocityzone structure

2400-2800 km depth

D” discontinuitytopography

2400-2800 km depth

D” fine-scalelayering

Long wavelength suggestion:Some slabs continue to CMB.

dVs: Grand [2002]Hutko, Lay, Garnero, Revenaugh [Nature, 2006]

Fine-Scale D” Structure Beneath the Cocos PlateFine-Scale D” Structure Beneath the Cocos Plate

Thomas, Garnero, Lay [JGR, 2004]

200-300 km above CMB

0.5-3% velocity increase

consistent with onset of post- perovskite phase

A few % discontinuous dVs increase is consistent with the post-perovskite phase transition

Lay et al. [EOS, 2005]Lay et al [PEPI, 2004]

e.g., Murakami et al. [Science, 2004]

Observations:

Fine-Scale D” Structure Beneath the Cocos PlateFine-Scale D” Structure Beneath the Cocos Plate

Hutko, Lay, Garnero, Revenaugh [Nature, 2006]

Results: Vertical Step in D” Discontinuity:Height of D” increases by 100 km over >200 km horizontally

Hutko, Lay, Garnero, Revenaugh [Nature, 2006]

Fine-Scale D” Structure Beneath the Cocos PlateFine-Scale D” Structure Beneath the Cocos Plate

After Crampin [1981]

D” anisotropy

After Crampin [1981]

D” anisotropy

D” anisotropy

Rokosky, Lay, Garnero [EPSL, 2006, in press]

The apparent step in the The apparent step in the D” layer coincides with a D” layer coincides with a change in D” anisotropy change in D” anisotropy parametersparameters

S, SdiffS, SdiffScSScS

Garnero, Maupin, Lay, Fouch [Science, 2004]Maupin,Garnero,Lay,Fouch [JGR, 2005]

Hutko et al. [Nature, 2006, in press]

Recent Seismic ResultsRecent Seismic Results

200-800 km depth

Upper mantle, transition zone structure in the central Pacific

400-1000 km depth

Slab detection fromseismic reflections

between

2800-2900 km depth

Ultra-low velocityzone structure

2400-2800 km depth

D” discontinuitytopography

2400-2800 km depth

D” fine-scalelayering

Raw array data: Fiji EQ recordedon the Canadian Yellowknife Array

Rost,Garnero,Williams [in prep., 2006]

Array processed data:Each trace = stack at a different Incoming angle to the YKA array

Rost,Garnero,Williams [in prep., 2006]

Back projecting along determinedback azimuth and slowness of eachprecursor permits estimation of reflection location that matches differential time between precursor and the direct PP wave

Rost,Garnero,Williams [in prep., 2006]

Rost,Garnero,Williams [in prep., 2006]

Recent Seismic ResultsRecent Seismic Results

200-800 km depth

Upper mantle, transition zone Structure in the central Pacific

400-1000 km depth

Slab detection fromseismic reflections

between

2800-2900 km depth

Ultra-low velocityzone structure

2400-2800 km depth

D” discontinuitytopography

2400-2800 km depth

D” fine-scalelayering

Williams and Garnero [Science, 1996]

Ultra-low velocity zones at Earth’s core mantle boundaryUltra-low velocity zones at Earth’s core mantle boundary

ULVZ Thickness

(km)

Thorne and Garnero [JGR, 2004]

Not detected globally

Isolated anomalous zones

Best attempt at global coverage ( ~40 % )

Thickness depends of several assumptions

Uncertainties in global ULVZ details quite large

Ultra-low velocity zones at Earth’s core mantle boundaryUltra-low velocity zones at Earth’s core mantle boundary

Rost,,Garnero,Williams,Manga [Nature, 2005]

Best-fit model properties:

Thickness : 8.5 (1) km DVP : -8 (2.5) % DVS : -25 (4) % D : +10 (5) %

Ultra-low velocity zones at Earth’s core mantle boundaryUltra-low velocity zones at Earth’s core mantle boundary

5 to 30 vol.% melt

no spreading along CMB

trapped intercumulus liquid

incompatible-element

enriched liquid

crystals are initially overgrown

and trap residual

requires large overlying thermal anomaly

downward percolation of melt

correlation to dynamic instabilities/upwellings

probably a fixed base for mantle upwellings

Conceptual model possibility

Rost, Garnero, Williams, Manga [Nature, 2005]

Ultra-low velocity zones at Earth’s core mantle boundaryUltra-low velocity zones at Earth’s core mantle boundary

Recent Seismic ResultsRecent Seismic Results

200-800 km depth

Upper mantle, transition zone structure in the central Pacific

400-1000 km depth

Slab detection fromseismic reflections

between

2800-2900 km depth

Ultra-low velocityzone structure

2400-2800 km depth

D” discontinuitytopography

2400-2800 km depth

D” fine-scalelayering

Lay, Hernlund, Garnero, Thorne [Science, 2006, in review]Avants, M., T. Lay, S. Russell, and E.J. Garnero [JGR, 2006]Avants, M., T. Lay, and E.J. Garnero [GRL, 2006]

dVs [Grand, 2002]

Central Pacific D” LayeringCentral Pacific D” Layering

Lay, Hernlund, Garnero, Thorne [Science, 2006, in review]

Central Pacific D” LayeringCentral Pacific D” Layering

Bin1 Bin2 Bin3

Central Pacific D” LayeringCentral Pacific D” Layering

~ 1000 km

~ 50

0 km

Lay, Hernlund, Garnero, Thorne [Science, 2006, in review]

Recent Seismic ResultsRecent Seismic Results

200-800 km depth

Upper mantle, transition zone structure in the central Pacific

400-1000 km depth

Slab detection fromseismic reflections

between

2800-2900 km depth

Ultra-low velocityzone structure

2400-2800 km depth

D” discontinuitytopography

2400-2800 km depth

D” fine-scalelayering

Schmerr and Garnero[2006, JGR, in press]

SS waves and precursors

Probing the Transition Zone Regionally: Central Pacific Probing the Transition Zone Regionally: Central Pacific

Schmerr and Garnero[2006, JGR, in press]

- Thinning of the TZ ( ~ 15 km average)

- 410 disc slightly depressed

- 660 disc upwarped

Probing the Transition Zone Regionally: Central Pacific Probing the Transition Zone Regionally: Central Pacific

Thorne, Garnero, Grand [2004, PEPI]

Slabs: at least to 1000 km

Step in D” discontinuity:consistent w/ slab piling/folding

Very localized ULVZ:dense, partial melt,base of upwelling

D” stratification:LLSVP, pPv, ULVZ

Thinnedtransition

zone

What about chemically distinct piles in the deep mantle?

Pacificanomaly

Africananomaly

South pole

Caribbeananomaly

corecore

mantle

Sharp top

Ni and Helmberger [2003,2005, Science, EPSL]Ford, Garnero,McNamara [2006, JGR]

Sharp “edges” to low velocities inferred from seismic waveforms

Thorne, Garnero, Grand, PEPI., 2004

red: lowest velocities for S20RTS

green: strongest lateral VS gradients

Indirect evidence from global tomographyIndirect evidence from global tomography

Model: Grand 2002Iso-velocity contour: - 0.7%Hotspots:

Deep mantle shear velocity and hot spots

Thorne, Garnero, Grand [2004, PEPI]

Summing upSumming up

Topic More constrained Less constrainedDeep mantle heterogeneity

Long wavelength dVs patterns

Short scale structure, Vp

Deep mantle anisotropy It exists, it laterally varies

Strength, depth distribution, geometry

D” discontinuity dVs reflector strength, location

Sharpness, height above CMB, dVp disc, deeper disc assoc. w/ pPv phase

ULVZ It exists, it laterally varies

Except for one spot: internal properties, geographical distribution, sharpness

Deep mantle slabs Existence in upper half of lower mantle

Structure/behavior in lower half of the mantle

Deep mantle piles Abrupt transition between low velocities and mantle

Density, internal structure

Today: lots of seismically imaged short scale details in just a few spots of the volume of the interior

“there are known unknownsand unknown unknowns”

Point: Where we’re afforded the ability to image in great detail, richness in complexity is apparent

Garnero [Ann. Rev. , 2000]Garnero, Maupin, Lay, Fouch [Science, 2004]

garnero@asu.eduhttp://garnero.asu.edu