Tomographic Imaging of the Crust and Upper Mantle in the

Southern Great Basin - Interpretation

Glenn Biasi

University of Nevada Reno

Seismological Laboratory

Method and Source Data

• Data – Relative teleseismic travel-time delays

• Invert by projecting delays on the ray path

• Preferred regional model 450x450x300 km – 15x15 km blocks of variable thickness.

• Local model: 4.5x4.5 km blocks

Inversion: SIRT, with back-azimuth binning and model updates damped inversely to the hit count.

Fit explains 75% of the model RMS (0.23 sec.); model slowness rms: 0.95 sec/km.

90 stations, >7700 travel-time delays

Station Coverage

90 stations total

Portable stations supplement analog and digital coverage around Yucca Mountain

Note station coverage north, east of the TM/SC Caldera Complex

Velocity Scaling Relationships

• Factors: Temperature, composition (degree of depletion), melt fraction, and degree of hydration

• UM near solidus – temperature variations likely are small < 100 degrees C, <1-2% dVp

• Composition: A small effect. (Schutt and Lesher, 2005) dVp < 0.7% for depletion < 20% (cpx out); to 2.5% at 40%.

• Velocity very sensitive to retained melt fraction F (Hammond and Humphreys, 2000): –3.6% dlnVp/dF

• Hydration – Subsolidus: OH in UM decreases velocity, shear strength– Hypersolidus: water escapes to melt -> velocity increase at low

retained melt fractions. Volcanism dries, cools, source -> farther from solidus. Melt retention decrease velocities.

Resolution – Point anomaly Reconstruction

Blocks at 50-70 km reconstructed at ~30%; 40% at 90-120 km

Map view (NS, EW) resolution excellent.

70-90 km

• Slight depression in velocity evident connecting Crater Flat to Thirsty Mountain.

• 2-5 Ma centers concentrated in 0 dV ring.

Shallow Mantle

YM is on the velocity contrast

<5 Ma basalts concentrate at 1/2% to -1/2% dVp

50-70 km: TM high velocities become clearer; structure divides the Death Valley-Pancake Range lineation.NE low velocity trend clear, south halfamagmatic

Regional Tomographic Interpretations

• TM/SC caldera complex caps a depleted hydrous melt ascent column.

• 2-3% high velocities due to dehydration, cooling, melt loss.

• Basaltic volcanism follows the margins of the larger root. Model: residual volatiles +/- heat trigger volcanism. Waning volumes reflect depletion of volatiles (e.g., Crater Flat).

• Large NE-trending low-velocity interpreted as hydrated, warm, sub-solidus. – Melt (>1%) should have some surface expression– Low melt fractions and dry would not explain spatial correlation

with shattered zone in Miocene tuffs and Paleozoic rocks.

Detailed Model

• 4.5x4.5 km blocks, 80 km total model depth.

• Station corrections from refraction velocity models.

• Shallow regional model is consistent at similar wavelengths.

• Interpret for spatial patterns; locally amplitudes may be excessive.

Local Model

Red line bounds areas with crossing raycoverage. CAF: base of the Calico Hills; LC: Lathrop Cone; LSM: Little Skull Mntn

Velocity amplitude plot clipped at +-3% - in crust variation can be silica content

Local Model, 20-45 km depths

-- The ESF is underlain by higher thanbackground velocity crust.-- Resolvable structure separates ESF from North CF cone.-- NNE trend through Crater Flat – UM weak zone

-- Shallow mantle depth low velocities S,SE of LSM may be crustal in origin.-- Suggested Moho topographyeast of the Bare Mountain fault.

Local Model, 45-80 km

Shallow mantle not homogeneousSource depth for Crater Flat, Lathrop inferred at 50-60 km at velocity increase.

North-South Profiles, West of YM

Source areas suggested at 50-60 km

Melt, water extraction leaves above background velocities

- High velocity crust, upper mantle under YM-Geometry suggests intrusion or metamorphic-grade increase, Timber Mntn source-Low vel. in S. half of Jackass Flats -Anomaly B source not clear.

Interpretations and Conclusions - Root

• P-wave tomography provides the third dimension – the upper mantle today – with useful resolution.

• Root structure separates volcanism north and south of Timber Mountain.

• Hydrous root may explain unique regional isotopic characteristics and extensive metasomatism.– Not a simple subduction enrichment– Provides hydrous alternative to extreme melting temperatures of Wang,

Smith, et al. (2002). – Apparent ancient lithospheric Nd, Sr signature may originate with

enrichment source – perhaps water collected on the 410.• Dry, strong root may protect crust from extension, extensional

faulting• Near-vertical alignment precludes post-12 Ma (post-15 Ma?) crustal

displacement relative to mantle

Interpretations and Conclusions – Location of Basaltic Centers

• Quaternary volcanic centers overlie edges of TM/SC U.M. structure – Not spatially random.

• Outward diffusion, possible upward mobility of water leads to hydrous “halo”.

• Melting occurs locally where additional water lowers melting point. – Melt volume intrinsically limited by water incompatibility and loss

with melt. – Appears compatible with polybaric melting hypothesis - water in

the garnet lherzolite field is lost to small melt fractions but picks up garnet melting signature. Shallower melting increases FeO and SiO2.

• Image suggests a shallow source for the basalts of Buckboard Mesa. Petrology may support this inference.

Melting Depth and Mantle Heterogeneity

• Melting depth appears imaged in Crater Flat, Yucca Mountain areas.– Melt, water removal increases velocities– Should be compared with petrologic estimates – May be directly testable with Vp/Vs ratios

• Deep crust and shallow mantle are not homogeneous beneath Yucca Mountain or Crater Flat. – PVHA impact on spatial probabilities of future activity.– Shallow high velocity under Yucca Mountain – No special low velocities in Crater Flat (c.f., Evans interpretation).

• NNW tectonic trend through W Crater Flat follows mantle low Vp lineation from Crater Flat to Thirsty Mountain

• Lowest velocity upper mantle interpreted as warm, subsolidus, partially water saturated.– Low velocity extends to eastern edge of images.– West half is amagmatic, north and east are covered with shattered

Miocene and older rocks. Apparently no Quaternary basalt.

Ray Angles in the Upper Mantle

• Core phases provide lateral constraint

• Near tele’s constrain depth

• Rays are traced in model but angles change little in the U.M.

• Most rays at 300 km sample 100 to 170 km from the station

Composition

• Schutt and Humphreys (2005): – spinel lherzolite (shallowest mantle) dVp <

0.7% until cpx exhausted, can reach 2.5% for 40% melt extraction.

– garnet lherzolite (>2-3 Gpa) dVp < 0.5% for depletion to 40%

– dVp/dVs > 1.8 is diagnostic of melt presence.

• Result: little of dVp is due to modal change from depletion.

Velocity Sensitivity to Melt and Hydration

• Hammond and Humphreys (JGR, 2000) – velocity decreases sharply with increasing melt fraction in realistic geometries

– 1% melt, 3.6% dVp, 7.9% dVs Dry melting only.– Melt interconnects at small fractions (<1%, maybe 0.5% or less)– Lowest dVp < ~4% - suggests a 1% upper bound on melt fraction in area imaged

by upper mantle tomography– H&H model: 2% melt, ~10% decrease in velocity for (T-Ts)=10 degrees C– dln(Vp)/dln(Vs) (ratio of percent dV’s) diagnostic of melt presence.

• Karato and Jung (1998) velocity increases when small fraction partial melt is present

– Water incompatible, partitions into melt– Water in melt = water not in crystals– Water-free crystal matrix faster as a result.– Limited to low melt fractions – Connectivity at low melt fraction promotes melt loss; residuum higher velocity

after melt removal.– Could explain neutral velocities under modern caldera (Long Valley)

120-150 km

• Timber Mountain root is well defined

• Low-velocity region is wider and lower amplitude.

150-180 km

• Little changed from 120-150 km layer.

180-210 km depth

• Root is 1-2% above average velocity; 2-3% with approximate amplitude correction

• Root is ~30 km north and 0-15 km east of surface edifice – no strong crustal shear.

210-250 km depth

• At 210-250 km the root is ~95 km EW by 105 km NS.

• Shallowest maximum depth to the base of the root is over 200 km.

• Slight north plunge from Moho to this layer.

250-300 km depth

• NE plunge begins

300-350 km depth

• High velocity body remains coherent

• Amplitudes max at base of model indicate that the true source could be deeper.

350-400 km

• Amplitude of NE structure and lack of this body in realistic synthetic models indicates high velocities are real, if ill-resolved.

Squeezing: Estimating Required Model Depth

• Method: decrease model depth until misfit suffers and/or imaging artifacts or unphysical amplitudes result

• Amplitude vs. model depth: deeper => smaller average block amplitude.

• RMS misfit vs. model depth: deeper models fit better

• Spatial distributions of structure unaffected by model depth.

Squeezing models