Richard C. Aster

Episodic zircon ages, Hf isotopic composition, and the preservation rate of continental crust

U/Pb detrital zircon ages from global modern river sediments define eight peak clusters centered at 2700, 2500, 2010, 1840, 1600, 1150, 600, and 300 Ma. These clusters extend vertically into both positive and negative e Hf (T) space and are similar to those in orogenic granitoids that correlate well with supercontinent formation. We suggest that the clusters are preservation peaks that reflect juvenile and reworked continental crust selectively preserved during continental collisions. The greatest contribution of juvenile continental crust is associated with the 1600 and 1150 Ma clusters and may reflect a change in the style of collisional orogens in the Paleoproterozoic involving thicker and stronger lithosphere. Age gaps at 2400–2200, 1400–1300, 900–650, and 185–120 Ma represent times when crustal production and recycling rates were about the same. Although some new continental growth may occur during continental collisions, supercontinent assembly does not require an increase in production rate of continental crust. Rather, we suggest that the preservation rate increases by an increased probability of capture of both new and reworked continental crust in collisional orogens.

Small-scale convection at the edge of the Colorado Plateau: Implications for topography, magmatism, and evolution of Proterozoic lithosphere

The Colorado Plateau of the southwestern United States is characterized by a bowl-shaped high elevation, late Neogene–Quaternary magmatism at its edge, large gradients in seismic wave velocity across its margins, and relatively low lithospheric seismic wave velocities. We explain these observations by edge-driven convection following rehydration of Colorado Plateau lithosphere. A rapidly emplaced Cenozoic step in lithosphere thickness between the Colorado Plateau and adjacent extended Rio Grande rift and Basin and Range province causes small-scale convection in the asthenosphere. A lithospheric drip below the plateau is removing lithosphere material from the edge that is heated and metasomatized, resulting in magmatism. Edgedriven convection also drives margin uplift, giving the plateau its characteristic bowl shape. The edge-driven convection model shows good consistency with features resolved by seismic tomography.

Three-dimensional velocity structure and hypocenter distribution in the Campi Flegrei caldera, Italy

The Campi Flegrei (Phlegraean Fields) are dominated by a Quaternary explosive calders, about 10 km in diameter. Within the caldera are numerous later eruptive vents, the last of which formed in 1538 A.D. Well documented local elevation changes of ≈ 10 m have occurred in the caldera since Roman times. Recent inflation of the central caldera began in 1968, after over 400 years of subsidence. During this time more than 2 m of localized uplift occurred, predominantly from 1980 through 1985. Microearthquakes associated with this uplift were recorded by a portable three-component digital network deployed by the University of Wisconsin and the Vesuvius Observatory from August 1983 through May 1984. Those data have been used to obtain detailed information about the velocity structure of the caldera. A best-fit homogeneous half-space model was obtained by a systematic search for optimal residual statistics. A residual-based tomographic technique was applied to isolate a low-seismicity, anomalously-high vpvs region in the central caldera, roughly coincident with the region of greatest uplift. Finally, P and S arrival times were used to simultaneously relocate 228 earthquakes and obtain a three-dimensional vp and vs model for the caldera. The results of this velocity study, considered along with drillhole findings, composite fault-plane solutions, and the space-time distribution of earthquakes, suggest that the vpvs anomaly may represent an incompetent, highly fractured volume, saturated with liquid water. Hypocenter locations indicate a zone of concentrated seismicity north of the point of highest measured uplift. An inward-dipping elliptical hypocenter pattern suggests a ring fault.

Upper mantle convection beneath the central Rio Grande rift imaged by P and S wave tomography

[1] We present models for upper mantle P and S velocity structure beneath a southwestern United States transect extending from near the center of the Colorado Plateau across the Rio Grande rift to the Great Plains. The models were derived from travel times of compressional and shear seismic phases recorded by the La Ristra passive seismic array deployed from July 1999 to May 2001. Large variations in seismic velocity (up to 8% in S and 5% in P) are seen across the transect in the upper 200 km of the mantle. Seismically slow mantle underlies the Rio Grande rift and Jemez lineament and relatively slow mantle is seen beneath the Navajo volcanic field within the Colorado Plateau. The relative variations of P and S velocity imply that high temperatures are the primary cause of the slow mantle although a small amount of partial melting or hydration cannot be ruled out. Sharp boundaries in mantle seismic velocity are coincident with boundaries of Proterozoic structural trends implying that ancient lithospheric structure exerts a control on the tectonic and magmatic activity in the region. Weaker seismic variations are imaged below 200 km depth with three southeastward dipping structures in our images. Two of the structures have fast seismic anomalies, beneath the central Colorado Plateau and the Great Plains respectively, and the third has anomalously slow seismic wave speed. We interpret the western deep seismic anomaly to be foundering Farallon slab and the slow anomaly just to the east as upwelling mantle possibly associated with volatile release from the sinking Farallon slab. Beneath the Great Plains, there is also downwelling in the upper mantle. The combination of upwelling in the west and downwelling in the east are likely part of an upper mantle convection cell that may include entrained lithosphere from beneath the rift. INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 8121 Tectonophysics: Dynamics, convection currents and mantle plumes; 8180 Tectonophysics: Evolution of the Earth: Tomography; KEYWORDS: convection, Rio Grande rift, Colorado Plateau

An Automatic, Adaptive Algorithm for Refining Phase Picks in Large Seismic Data Sets

We have developed an adaptive, automatic, correlation- and clustering- based method for greatly reducing the degree of picking inconsistency in large, digital seismic catalogs and for quantifying similarity within, and discriminating among, clusters of disparate waveform families. Innovations in the technique include (1) the use of eigenspectral methods for cross-spectral phase estimation and for providing subsample pick lag error estimates in units of time, as opposed to dimensionless relative scaling of uncertainties; (2) adaptive, cross-coherency-based filtering; and (3) a hierarchical waveform stack correlation method for adjusting mean intercluster pick times without compromising tight intracluster relative pick estimates. To solve the systems of cross-correlation lags we apply an iterative, optimized conjugate gra- dient technique that minimizes an L1-norm misfit. Our repicking technique not only provides robust similarity classification-event discrimination without making a priori assumptions regarding waveform similarity as a function of preliminary hypocenter estimates, but also facilitates high-resolution relocation of seismic sources. Although knowledgeable user input is needed initially to establish run-time parameters, sig- nificant improvement in pick consistency and waveform-based event classification may be obtained by then allowing the programs to operate automatically on the data. The process shows promise for enhancing catalog reliability while at the same time reducing analyst workload, although careful assessment of the automatic results is still important.

Mantle-driven dynamic uplift of the Rocky Mountains and Colorado Plateau and its surface response: Toward a unified hypothesis

The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that significant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6–10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500–1000 m may account for ∼25%–50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.

Imaging the seismic structure of the crust and upper mantle beneath the Great Plains, Rio Grande Rift, and Colorado Plateau using receiver functions

Received 20 October 2004; accepted 2 March 2005; published 28 May 2005. [1] The seismic structure of the crust and upper mantle of the southwestern United States is examined using receiver functions calculated from teleseismic arrivals recorded in the Colorado Plateau–Rio Grande Rift–Great Plains Seismic Transect (LA RISTRA) experiment. We apply receiver function estimation and filtering methods developed by Wilson and Aster (2005) to produce receiver functions with decreased sensitivity to noise and deconvolutional instability. Crustal thickness and Vp/Vs ratios are estimated using both direct and reverberated P-to-S receiver function modes. We apply regularized receiver function migration methods to produce a multiple-suppressed image of the velocity discontinuity structure of the subsurface. Our results show that crustal thickness averages 44.1 ± 2.3 km beneath the Great Plains (GP) and 45.6 ± 1.1 km beneath the Colorado Plateau (CP). Crustal thinning beneath the Rio Grande Rift (RGR) is broadly symmetric about the rift axis, with the thinnest crust (35 km) located directly beneath the rift axis, suggesting a pure shear stretched lithosphere beneath the RGR. We also observe a prominent northwest dipping discontinuity, ranging from 65 to 85 km deep beneath the CP, and possible subcrustal discontinuities beneath the GP. These discontinuities, along with recent xenolith data, are consistent with preserved ancient lithospheric structures such as relict suture zones associated with Proterozoic subduction. We observe an upper mantle discontinuity at 220–300 km depth that may correlate with similar discontinuities observed beneath eastern North America. We also observe relatively flat discontinuities at 410 and 660 km depth, indicating there is not a large-scale thermal anomaly beneath the RGR at these depths.

Very long period oscillations of Mount Erebus Volcano

as the signal decays. VLP scalar moments, up to � 5� 10 11 N m, exceed SP moments by an order of magnitude or more, suggesting distinct, though genetically related, SP and VLP source mechanisms. We conclude that VLP signals arise from excitation of a quasi-linear resonator that is intimately associated with the conduit system and is excited by gravity and inertial forces associated with gas slug ascent, eruption, and magma recharge. VLP signal stability across hundreds of eruptions spanning 5 years, the persistence of the lava lake, and the rapid posteruptive lava lake recovery indicate a stable near-summit magma reservoir and VLP source process. INDEX TERMS: 4544 Oceanography: Physical: Internal and inertial waves; 7280 Seismology: Volcano seismology (8419); 8414 Volcanology: Eruption mechanisms; 8419 Volcanology: Eruption monitoring (7280); KEYWORDS: Strombolian, very long period, volcano seismology

Crust and upper mantle shear wave structure of the southwest United States: Implications for rifting and support for high elevation

[1] Surface wave phase velocities from 29 earthquakes are used to map the shear velocity structure to � 350 km depth across the 950-km-long Rio Grande Rift Seismic Transect Experiment (LA RISTRA) seismic array in the southwest United States. Events from a range of back azimuths minimize the effects of multipathing. The resulting velocity model reveals a transition in lithospheric thickness from 200 km in the Great Plains to 45–55 km beneath the Rio Grande Rift, thickening beneath the Colorado Plateau to 120–150 km. The upper mantle low-velocity signature of the rift is roughly twice the width of its surface morphology. An asthenospheric low-velocity channel underlies the region west of the Great Plains and extends to 300 km depth. This channel is likely the result of warm mantle infill behind the sinking Farallon plate. Buoyant forces within this channel are sufficient to support much of the high elevation of the rift and plateau. No evidence for a deep mantle source is found beneath the rift, implying that present rifting is not driven by deep mantle upwelling. Velocities from 55 to 90 km beneath the rift axis are 10% slower than beneath the Great Plains, consistent with small amounts of partial melt. Low velocities extend to 200–300 km depth on either side of the rift but not directly beneath it, forming an inverted-U shape. This feature may reflect mantle that has cooled through vertical advection in a subadiabatic environment. Upwelling may be reinforced by small-scale convection caused by variations in lithospheric thickness and shallow mantle temperatures. INDEX TERMS: 7205 Seismology: Continental crust (1242); 7255 Seismology: Surface waves and free oscillations; 8109 Tectonophysics: Continental tectonics—extensional (0905); 8120 Tectonophysics: Dynamics of lithosphere and mantle—general; KEYWORDS: surface waves, continental rifting, upper mantle structure

Seismic and acoustic observations at Mount Erebus Volcano, Ross Island, Antarctica, 1994-1998

Volcanic activity at Mount Erebus is dominated by eruptive activity within a phonolitic summit lava lake. Common eruption styles range from passive degassing to Strombolian explosions, which typically occur several times daily, and occasionally in swarms of up to 900 per day. Shallow explosions, although generally the result of steady exsolution of volatiles from depth, can be triggered by surficial input of H 2O through mass wasting of rock, snow and ice from the crater walls. Broadband observations of Strombolian explosions document very-long-period (VLP) signals with strong spectral peaks near 20, 12 and 7 s, which are polarized in the vertical/radial plane. These signals precede lava lake surface explosions by ,1.5 s, are highly repeatable, and persist for up to 200 s. First motions indicate a deflationary source, with any precursory inflation being below the ,30 s passband of our instruments. Particle motions suggest a VLP source residing up to 800 m below the lava lake surface; however, this depth could be exaggerated by near-field radial tilt. Seismic and acoustic signals associated with lava lake explosions commonly show evidence for multiple bubble bursts in corresponding complexity features resulting from varying time delays and relative sizes of superimposed bursts. A systematic decrease in seismic/acoustic ratio for smaller surface explosions suggests that either the seismic energy from the smallest, shallowest bubble bursts experiences much greater seismic attenuation than energy arising from larger events which may involve a deeper, less attenuative portion of the magma column, and/or that the shallowest layer is seismically isolated from deeper parts of a stratified magma column, which are not excited by the smallest explosions due to sharp impedance contrasts across distinct layers. Tremor at Erebus is uncommon, with only a few isolated instances identified in five years of monitoring. Some tremor events are nearly monochromatic, and some exhibit numerous gliding harmonic spectral lines. q 2000 Elsevier Science B.V. All rights reserved.