Crustal Thicknessand Geodynamicsof the Western U.S.(NEW!!! Brought to You By EarthScope)

Anthony R. LowryDepartment of Geology, Utah State Universitytony.lowry@usu.edu

Thanks Also To:Philip Crotwell, University of South CarolinaJon Kirby, Curtin University, Perth, AustraliaMarta Pérez-Gussinyé, CSIC Barcelona, SpainChristine Puskas, University of UtahJoel Rackham, Utah State UniversityBob Smith, University of UtahChris Swain, Curtin University, Perth, Australia

Topography

Gravity

Seismic (Moho)

Heat Flow

3D Viscosity

• Just a bit on Isostatic Analysis

• Where Geodynamics (& Rheology) come into play…

• Crustal Thickness estimation from EarthScope Transportable Array (TA) seismometers

• Some likely geodynamical implications of western U.S. crustal thickness…

Isostasy

LocalIsostasy

€

gdzcolumn 1

∫ = ρgdzcolumn 2

∫

FlexuralIsostasy

Balance of vertical stressin a fluid “asthenosphere”

Equilibrium of horizontal and vertical stress for an elastic plate (or “lithosphere”) over a fluid “asthenosphere”

Isostatic Analysis:

Uses easily available data (gravity, topography)

Parameterized by flexural rigidity ( rheology)

Provides information about loads ( mass flux processes)

Topography &/or Bathymetry

Gravity or Geoid

Effective Elastic Thickness (Te)

Loads Mass Flux Processes:

Gravity & Topography reflect a complicated mix of all massflux processes… But if we can separate the loads from theirisostatic response, it narrows the field of candidate processes.

Surface Loads Erosion Deposition Fault Displacements Volcanic Construction

Subsurface Loads Thermal Variations Lithologic Variations Crustal Thickness (Lower Crustal Flow)

METHOD:Using equations for observed

topography h and geoid Nincluding:

the definition of surface load finite amplitude geoid calculation flexure of a thin elastic shell over a self-gravitating, viscous sphere

Then search for Te (& perhapsother parameters) that minimizethe difference between observed

& predicted coherence

Or equivalently, that minimize coherence of the load fields

EXAMPLE The Tharsis Rise, Mars:

Martian Topography:• Hemispheric “crustal dichotomy”• Tharsis rise ~ 5000 m elevation 20% of surface area

Martian Geoid:• 2000 m Tharsis anomaly (largest in solar system!)

Controversial Nature of Tharsis Rise:

Volcanic Construction?( Surface Load!)

Thermal/chemical buoyancy of a single

mantle plume?

[e.g., Willemann & Turcotte, 1982;Solomon & Head, 1982]

[e.g., Sleep & Phillips, 1979; Harder& Christensen, 1996; Harder, 2000]

Probably some combination of both!

“BEST ESTIMATE”

(i.e., minimum loadcorrelation)

• ~ 17 km volcanic extrusives

• ~ 12 km flexural deflection

• Small (~ 5%) internal load

Flexural Rigidityreflectsrheology, and hencedistributions of intraplate seismicity

Reflects both the geotherm and composition…

Te and Flow Rheology

A Geodynamical Application: Flat Slab Subduction

Many studiesemphasize buoyancy ofthe down-goingslab and/orvelocity of theover-riding upperplate as controlson subductiongeometry

But these are poorly correlatedwith slab dip inSouth America

Correlation of South American flat slab subduction with high Te near the trench suggeststhickness of viscous upper plate may controlsubductiongeometry!

Isostasy illuminates geodynamics…

The Problem:Recovery from synthetic data, from Pérez-Gussinyé et al. [2007]:

Kirby & Swain [G3, 2008] found poor recovery where sub-sampled wavelet load fields exhibit chance correlation

E.g., EARS receiver function estimates of crustal thickness [Crotwell & Owens, SRL, 2005]

http://www.seis.sc.edu/ears/index.html

Possible Solution:Use seismic datato independentlyconstrain internalmass fields

EarthScope sampling is promising, but

Receiver function compilations such as EARSare prone to outliers & errors

Receiver Function Estimates ofCrustal Thickness:

P Ps

Delay Time

• Deconvolve source-time function to get impulse response of phases converted at impedance boundaries

• Delay time between phase arrivals depends on crustal thickness and P- & S-velocity

• EARS uses iterative time-domain deconvolution [Ligorria & Ammon, BSSA, 1999]: well-suited to automation

P PsCrustMantle

P Ps PpPs

PpSsPsPs

Contribution of crustal thickness Hversus Vp/Vs ratio K to delay time isambiguous…

Resolve using reverberations, which have differing sensitivity to H and K

P Ps PpPs PpSs PsPs

Ps

PsPs

PpSs&

PpPs

H-K parametersthat predict the observed phase delay times intersect ata point inparameterspace

P Ps PpPs

PpSsPsPs

H–K Stacking: [Zhu & Kanamori, JGR, 2000]

Ps

PsPs

PpSs&

PpPs

Method stacksobserved amplitudes atdelay times predicted for each phase, for all earthquakes.

Max stackamplitude should reveal true crustal thickness &Vp/Vs ratio.

H–K Stacking: [Zhu & Kanamori, JGR, 2000]

P Ps PpPs

PpSsPsPs

(EARS H–K stack for station COR)

The Moho is not the only lithospheric impedancecontrast… And crustalthickness is not constant

EARS can yield extreme crustal thickness or unlikely changes over short distances.

The Problem:

(EARS H–K stack for station Y35)

Despite outliers, H & K have properties consistent with a fractal surface…

CrustalThickness

H

Vp/VsRatioK

Station TA.P10A (Central Nevada)

The semivarianceproperties can beused to estimatea “most likely” crustal thickness and Vp/Vs ratio byoptimal interpolationfrom nearby sites.

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Station TA.P10A (Central Nevada)Can also model gravity predictedby estimates…

And find a “most likely” model with uncertainties.

12

12

Station TA.S16A (Central Utah)Gravity ModelLikelihood Filter

Optimal Interp.Likelihood Filter

Combined

Unlikely stack amplitude maxima aredownweighted using likelihood statistics

Station TA.P10A (Central Nevada)

Method removes outliers, provides a mapthat makes sense (with some exceptions)

The reliable (i.e.western) parts ofthe map confirm some already-known aspectsof western U.S.geodynamics &also suggest a few new interpretations…

EARS-derived Old refraction data

The reliable (i.e.western) parts ofthe map confirm some already-known aspectsof western U.S.geodynamics &also suggest a few new interpretations…

And interesting patterns show up in Vp/Vs

Raw EARS estimates Weighted stack estimates

Perhaps the most intriguingimplicationsarise from the gravity modelinghowever…