Department of Geosciences

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Stability of zircon u-pb systematics in a greenschist-grade mylonite - an example from the rockfish valley fault zone, central Virginia, USA
Wayne, D. M.; Sinha, A. K. (University of Chicago Press, 1992-09)
The mid-Paleozoic, greenschist-grade Rockfish Valley Fault Zone (RVFZ) of central Virginia cuts the Grenville-aged Pedlar River Charnockite Suite (PRCS) and contains zircons that underwent brittle failure during ductile deformation. Electron microprobe analyses and scanning electron microscope (SEM) backscattered electron (BSE) imaging show that zircons from the protolith PRCS are concentrically zoned (with alternating U-Hf-rich and U-Hf-poor bands), and contain numerous radial microcracks. Zircons from the RVFZ mylonite are unzoned, fragmented, show no internal microfractures, and have low U and Hf concentrations relative to the PRCS zircons. U-Pb isotopic studies of zircons from the mylonites and from the charnockitic protolith demonstrate that no preferential Pb loss occurred in the zircons from the mylonite, and that the Pb-207/Pb-206 ages of the mylonite zircons are identical to those of the protolith zircons. The loss of primary zoning from the zircons of the RVFZ ultramylonites can be explained by the physical removal of microfractured, U-rich, alpha-damaged zircon domains as the result of brittle failure and disaggregation during mylonitization. Mechanically resistant (low-U) portions of zircon grains tended to remain intact in the mylonite. Thus, it may not always be possible to predict whether or not the zircon U-Pb system has been disturbed by mylonitization by using physical criteria (e.g., grain size reduction, obliteration of primary zoning textures) alone. Evidently, fluids present during mylonitization accomplished the hydration of primary mineral assemblages, but did not chemically interact with zircons, and their primary U-Pb and Pb-Pb ages were preserved.
Structure of the subducting Nazca plate beneath Peru
Norabuena, E. O.; Snoke, J. A.; James, D. E. (American Geophysical Union, 1994-05)
Arrival times from intermediate-depth (110-150 km) earthquakes within the region of flat subduction beneath the subandean zone and foreland basins of east-central Peru provide constraints on the geometry and velocity structure of the subducting Nazca plate. Hypocentral locations and origin times for these events were determined using observations from a 15 station digitally recording locator array deployed in the epicentral region of eastern Peru. Observed P wave arrival times for coastal stations in Peru, some 3-6-degrees from the epicenters, are up to 4 s early relative to predicted arrival times based on the best fit velocity-depth model used for hypocenter locations. These large negative time residuals appear to be the result of propagation paths which have long segments in the colder, higher-velocity subducting plate. P wave travel times were modeled for the effects of the slab using three-dimensional (3-D) ray tracing. Computed ray paths show that travel times to coastal stations for the eastern Peru events can be satisfactorily modeled with average velocities relative to the surrounding mantle 6% lower within the uppermost slab (assumed on the basis of other studies to be unconverted basaltic oceanic crust 6 km thick) and 8% higher within the cold uppermost mantle of the slab. Ray tracing for this plate model shows that P wave ''shadow zones'' can occur if the source-slab-receiver geometry results in seismic rays passing through regions in which the clip angle of the slab changes significantly. Such geometries exist for seismic waves propagating to some coastal stations from intermediate-depth earthquakes located east of the Andes. Observed first-arrival times for such cases do in fact have smaller negative residuals than those for geometries which allow for ''direct'' paths at similar distances. Modeling such arrivals as internally reflected waves propagating through the high-velocity part of the plate produces a significant improvement in the travel time residuals. For the slab velocities given above, we obtain a model thickness of approximately 36 km for the cold slab interior and a slight northwest component of dip in the region of subhorizontal subduction.
First-principles study of several hypothetical silica framework structures
Teter, D. M.; Gibbs, Gerald V.; Boisen, Monte B. Jr.; Allan, D. C.; Teter, M. P. (American Physical Society, 1995-09-15)
Several hypothetical silica structures have been generated using a simulated-annealing strategy with an ab initio based covalent-bonding potential. First-principles total-energy pseudopotential methods have been used to examine several. promising hypothetical structures and to compare their structural parameters, cohesive energies, and bulk moduli with those of low quartz;, low cristobalite, silica sodalite, and stishovite. The cohesive energies of these hypothetical structure types are found to be equivalent to those of low quartz, low cristobalite, and silica sodalite, and significantly lower than that of stishovite.
Lithospheric structure of the Chaco and Parana Basins of South America from surface-wave inversion
Snoke, J. A.; James, D. E. (American Geophysical Union, 1997-02)
Surface-wave data from a portable broadband array have been used to invert for the velocity structure of the crust and upper mantle beneath the Chaco and Parana Basins of central South America. The upper-mantle velocity structure beneath the Parana Basin is cratonic in character, whereas that beneath the Chaco Basin is tectonic or asthenospheric in character. The surface-wave analysis used ;broadband recordings from a subset of a 14-station array deployed in a roughly east-west sawtooth arrangement along 20 degrees S latitude, with a total EW aperture of similar to 1,400 km. Results from receiver-function analysis, as well as direct P-wave regional travel-time data, were used in the inversions to help constrain Moho depths and crust and upper-mantle velocities. S-wave structure for the intracratonic Parana Basin was determined using interstation phase and group velocities for Rayleigh waves (fundamental and first higher mode) and Love waves (fundamental mode only) based on seven events with paths which traverse the eastern Parana Basin and one event with a path across the western Parana Basin. The average Moho depth in the eastern Parana Basin is similar to 42 km. The high-velocity upper-mantle lid has a maximum S-wave velocity of 4.7 km/s, with no resolvable low-velocity zone to at least 200 km depth. This cratonic velocity structure indicates the presence of a lithospheric root beneath the Parana Basin despite emplacement of the Parana plume. The limited data from the western Parana Basin are consistent with a homogeneous upper-mantle structure throughout the Parana Basin. Waveform inversion of fundamental-mode and first-higher-mode Rayleigh waves from a single subandean event was used to obtain estimates for pure-path dispersion along propagation paths through the Chaco Basin and the western half of the Parana Basin. The data were partitioned to isolate the partial-path contribution of the phase and group velocities for the Chaco Basin. The phase and group velocities from this somewhat sparse data set were inverted to obtain: a velocity-depth model for the Chaco Basin. The distinguishing features of the Chaco model consist of a rather shallow Moho depth, 32 km, and low (''asthenospheric'') upper-mantle S-wave velocities, about 4.2 km/s, with velocity increasing only slightly to about 4.3 km/s at 150 km depth.
Location of the southern edge of the Gorda slab and evidence for an adjacent asthenospheric window: Results from seismic profiling and gravity
Beaudoin, B. C.; Hole, John A.; Klemperer, S. L.; Trehu, A. M. (American Geophysical Union, 1998-12)
As the Mendocino Triple Junction migrates northward along the California margin it is widely presumed to leave a "slab-free" or "asthenospheric" window in its wake. A 250-km-long south-north seismic refraction-reflection profile crossing the transition from transform to subduction regimes allows us to compare and contrast crust and upper mantle of the North American margin before and after it is modified by passage of the Mendocino Triple Junction. From the seismic data we have determined that (1) the crust is laterally homogeneous in velocity to a depth of 20 km (interpreted by us as Franciscan complex), (2) below 20 km depth the crust is characterized by velocities of greater than or equal to 7.0 km/s for the southern half of the profile and by velocities of less than or equal to 7.0 km/s for the northern half, (3) regions of high reflectivity in the crust occur below similar to 20 km depth throughout the profile, and (4) the North American crust is thickest (similar to 35 km) in the center of the profile and thins to similar to 25 km at either end. From the gravity data we have determined that (1) asthenospheric densities (3.2 g/cm(3)) occur subjacent to the North American crust in the center of the profile, and (2) a wedge of lithospheric mantle density material (greater than or equal to 3.2 g/cm(3)) is required on the southern end of the profile. We interpret these combined results to indicate that our profile crosses the southern edge of the Gorda plate and that directly adjacent to this edge is an asthenospheric window with overlying mafic rocks in the crust. These mafic rocks and a reforming lithospheric mantle increase in thickness southward.
Geosciences Newsletter, Fall 2001
(Virginia Tech, 2001)
The Fall 2001 newsletter from the Department of Geosciences.
Geological Sciences Newsletter, Spring 2001
(Virginia Tech, 2001)
The Spring 2001 newsletter from the Department of Geosciences.
Crustal structure beneath southern Africa and its implications for the formation and evolution of the Kaapvaal and Zimbabwe cratons
Nguuri, T. K.; Gore, J.; James, D. E.; Webb, S. J.; Wright, C.; Zengeni, T. G.; Gwavava, O.; Snoke, J. A.; Kaapvaal Seismic Group (American Geophysical Union, 2001-07-01)
The formation of Archean crust appears to involve processes unique to early earth history. Initial results from receiver function analysis of crustal structure beneath 81 broadband stations deployed across southern Africa re veal significant differences in the nature of the crust and the crust-mantle boundary between Archean and post-Archean geologic terranes. With the notable exception of the collisional Limpopo belt, where the crust is thick and the Moho complex, the crust beneath undisturbed Archean craton is typically thin (similar to 35-40 km), unlayered, and characterized by a strong velocity contrast across a relatively sharp. Moho. This crustal structure contrasts markedly with that beneath post-Archean terranes and beneath Archean regions affected by large-scale Proterozoic events (the Bushveld complex and the Okwa/Magondi belts), where the crust tends to be relatively thick (similar to 46-50 km) and the Moho is complex.
Geosciences Newsletter, Fall 2002
(Virginia Tech, 2002)
The Fall 2002 newsletter from the Department of Geosciences.
Geological Sciences Newsletter, Spring 2002
(Virginia Tech, 2002)
The Spring 2002 newsletter from the Department of Geosciences.
Crustal thicknesses in SE Brazilian Shield by receiver function analysis: Implications for isostatic compensation
Assumpcao, M.; James, D.; Snoke, A. (American Geophysical Union, 2002-01)
[1] The Brazilian Lithosphere Seismic Project (BLSP, a joint project by University of Sao Paulo and Carnegie Institution, 1992-1999) operated more than 20 temporary broadband stations in the southeastern Brazilian shield. The area, a transect 1000 km long and 300 km wide, covers different geological provinces: the Precambrian Sao Francisco craton, the adjacent Brasiliano (700-500 Ma) fold belts, and the Parana basin of Paleozoic origin. Crustal thicknesses were estimated for 23 sites using receiver functions. For each station, receiver functions were stacked for different sets of earthquakes according to azimuth and distance. The P-to-S Moho converted phase was clearly identified at most sites. Crustal thicknesses were estimated using an average crustal P wave velocity of 6.5 km/s. Poisson's ratio of 0.23 (Vp/Vs = 1.70) was used for the Sao Francisco craton and adjacent fold belt (based on travel times from small, local earthquakes) and 0.25 was used for the Parana basin and coastal belt. Crustal thicknesses ranged from 35-47 km. Although there is a clear inverse correlation between topography and Bouguer gravity anomalies in the study area, Moho depths show the opposite pattern from that expected: areas of low topography and less negative Bouguer anomalies, such as the Parana basin, have thicker crust (40-47 km) compared with the high elevation areas of the craton and fold belt (37-43 km). Two hypothesis are proposed to explain the data: (1) A lower density, by 30-40 kg/m(3), in the lithospheric mantle under the Archean block of the Sao Francisco craton relative to the Proterozoic lithosphere is responsible for maintaining the high elevations in the plateau area. Relatively low density and high P wave velocity are compatible with a depleted (low FeO) composition for the Archean lithosphere. (2) Alternatively, if the density contrasts between Archean and Proterozoic lithospheres are smaller than the values above, then the crust beneath the Parana basin must be more dense than that of the craton. Higher crustal density and high Poisson's ratio would be consistent with magmatic underplating in the lower crust beneath the Parana basin, as inferred from other studies.
Constraints on the S wave velocity structure in a continental shield from surface wave data: Comparing linearized least squares inversion and the direct search Neighbourhood Algorithm
Snoke, J. A.; Sambridge, M. (American Geophysical Union, 2002-05)
[1] In their study of upper mantle structure beneath the Parana Basin of SE Brazil, Snoke and James [1997] concluded, on the basis of a linearized least squares inversion (LLSI) of surface wave dispersion data, that a strong (5% contrast) low-velocity zone (LVZ) beginning at a depth less than 150 km was not required to fit the data. They were unable to establish a quantitative estimate, however, on the maximum depth at which such a LVZ could be resolved by their data. Sambridge [1999a, 1999b] has introduced the Neighbourhood Algorithm (NA), a direct search method for nonlinear inversion which can be tuned to extract information from an ensemble of models in addition to finding a single best fit model. Applying NA to the Brazilian dispersion data quantifies the statistics of the ensemble of models classified as "acceptable'' based on a data misfit criterion and a smoothness constraint. The NA best fit model is not significantly different from the LLSI best fit model, but the analysis of the ensemble of models provides new insights regarding how well constrained the model is. Synthetics runs show that for this data set, our modeling procedures could resolve a strong LVZ that began at a depth of 120 km but could not rule out such an LVZ beginning at a depth of 180 km.
Geosciences Newsletter, Fall 2003
(Virginia Tech, 2003)
The Fall 2003 newsletter from the Department of Geosciences.
Geological Sciences Newsletter, Spring 2003
(Virginia Tech, 2003)
The Spring 2003 newsletter from the Department of Geosciences.
Fluorinert as a pressure-transmitting medium for high-pressure diffraction studies
Varga, T.; Wilkinson, A. P.; Angel, R. J. (AIP Publishing, 2003-10)
Fluorinert is a liquid pressure-transmitting medium that is widely used in high-pressure diffraction work. A systematic study of five different fluorinerts was carried out using single-crystal x-ray diffraction in a diamond-anvil cell in order to determine the pressure range over which they provide a hydrostatic stress state to the sample. It was found that none of the fluorinerts studied can be considered hydrostatic above 1.2 GPa, a lower pressure than reported previously. (C) 2003 American Institute of Physics.
Geosciences Newsletter, Spring 2004
(Virginia Tech, 2004)
The Spring 2004 newsletter from the Department of Geosciences.
Geosciences Magazine, Fall 2004
(Virginia Tech, 2004)
The Fall 2004 magazine from the Department of Geosciences.
Geosciences Magazine, Spring 2005
(Virginia Tech, 2005)
The Spring 2005 magazine from the Department of Geosciences.
General rules for predicting phase transitions in perovskites due to octahedral tilting
Angel, Ross J.; Zhao, J.; Ross, Nancy L. (American Physical Society, 2005-07-08)
Recent experiments on several oxide perovskites reveal that they undergo tilt phase transitions to higher-symmetry phases on increasing pressure and that dT(c)/dP < 0, contrary to a general rule previously proposed for such zone-boundary transitions. We show that the negative slope of the phase boundary is a consequence of the octahedra in these perovskites being more compressible than the extra-framework cation sites. Conversely, when the octahedra are stiffer than the extra-framework cation sites, the phase transition temperatures increase with increasing pressure, dT(c)/dP > 0.
Cell adhesion of Shewanella oneidensis to iron oxide minerals: Effect of different single crystal faces
Neal, Andrew L.; Bank, Tracy L.; Hochella, Michael F. Jr.; Rosso, Kevin M. (American Institute of Physics, 2005-12-30)
The results of experiments designed to test the hypothesis that near-surface molecular structure of iron oxide minerals influences adhesion of dissimilatory iron reducing bacteria are presented. These experiments involved the measurement, using atomic force microscopy, of interaction forces generated between Shewanella oneidensis MR-1 cells and single crystal growth faces of iron oxide minerals. Significantly different adhesive force was measured between cells and the (001) face of hematite, and the (100) and (111) faces of magnetite. A role for electrostatic interactions is apparent. The trend in relative forces of adhesion generated at the mineral surfaces is in agreement with predicted ferric site densities published previously. These results suggest that near-surface structure does indeed influence initial cell attachment to iron oxide surfaces; whether this is mediated via specific cell surface-mineral surface interactions or by more general interfacial phenomena remains untested. (C) 2005 American Institute of Physics.

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