bluuebunny/crossref_metadata_2025 huggingface dataset:
Flexural constraints on the denudation of assymetric mountain belts
Authors: Stüwe Kurt
Published: February 2008

The regional excavation of metamorphic rocks from mid and lower crustal levels is the consequence of erosion‐driven denudation of mountain belts that are in permanent isostatic equilibrium. If the erosion process occurs at different rates in different parts of a topographically asymmetric mountain belt, then the flexural properties of the lithosphere need to be considered in order to describe the features of the isostatic rebound. Such a situation is given in plateaulike mountain ranges which are likely to erode only at their steeply sloping edges whereas erosion on top stagnates. This paper investigates the flexural isostatic response of a plate that is unloaded due to the continuous retreat of an erosional escarpment. The isostatic response is tested for two limiting model cases: (1) for a plate with a broken edge and (2) for a continuous plate with variable flexural rigidity. It is shown that for both cases the rate of topography development on the exposed plate is not linear with the retreatrate of the escarpment. This feature is utilized to predict the nature of the subsequent erosion of the newly formed topography and ultimately of the denudation of the plate. Geological limitations of the theoretical model and its relevance to plateaulike topographies in southern Africa and the eastern highlands of Australia are then discussed. In these areas the origin of the apparently cyclic rebound forces creating “stairlike” topography at their continental margins and the old age of their present surface topography has previously been unexplained. Within the presented model these features can be explained satisfactorily. Two important conclusions may be drawn: (1) The distance of flexural swells from great escarpments and the origin of subsequent stair‐like topographies may be explained by simple flexural considerations. (2) The flexural rebound forces uplifting a plate exposed by a retreating escarpment are cyclic. Excavation times of metamorphic rocks under such mountain belts may therefore be prolonged not only by the nature of erosion of their initial topography but also by the nature of the subsequent isostatic rebound and ultimately their denudation history.
High‐temperature thermodynamic properties of forsterite
Authors: Gillet Philippe , Richet Pascal , Guyot François , Fiquet Guillaume
Published: February 2008

The high‐temperature thermodynamic properties of forsterite were reviewed in the light of a new determination of the isobaric heat capacity (Cp), up to 1850K, and Raman spectroscopic measurements, up to 1150K and 10 GPa. The Cp measurements and available data on thermal expansion (α) and bulk modulus (K) show that the isochoric specific heat (Cv) exceeds the harmonic limit of Dulong and Petit above 1300 K. This intrinsic anharmonic behavior of Cv can be modeled by introducing anharmonic parameters ai = (∂lnvi/∂T)V which are calculated from the measured pressure and temperature shifts of the vibrational frequencies. These parameters are all negative, with absolute values lower for the stretching modes of the SiO4 tetrahedra (ai ≈ − 1×10−5 K−1) than for the lattice modes (ai ≈ − 2×10−5 K−1). Through the relation Cp = Cv +α2KTVT the calculated anharmonic Cv and the measured Cp are then used to determine the temperature dependences of the thermal expansion and bulk modulus of forsterite, up to 2000 K, in agreement with recent experimental results. Finally, all these data point to an inconsistency for the Grüneisen parameter of forsterite, whereby the macroscopic parameter γ= αVKT/ Cv cannot be evaluated simply at high temperature by summation of the individual isothermal mode Grüneisen parameters γiT = KT(∂lnvi/∂P).
Simulations of texture development in calcite: Comparison of polycrystal plasticity theories
Authors: Tomé C. N. , Wenk H.‐R. , Canova G. R. , Kocks U. F.
Published: February 2008

Deformation modelling based on polycrystal plasticity theory has been applied to several rocks. Recently it has been demonstrated that rate sensitivity and heterogeneous deformation can have a significant influence on the activation of slip systems and corresponding texture development. In this study previous predictions for pure shear deformation of calcite with the rigid‐plastic fully constrained Taylor theory are compared with those from a visco‐plastic Taylor theory, a relaxed Taylor theory and a visco‐plastic self‐consistent theory. In the case of calcite, texture simulations for all models are similar and conclusions based on earlier data remain valid. There are some quantitative differences and these will be discussed. A new procedure for dealing with mechanical twinning is proposed; it requires the volume fraction of twin reoriented grains to match the twinning fraction given by the twinning shears.
Effects of Canary hotspot volcanism on structure of oceanic crust off Morocco
Authors: Holik James S. , Rabinowitz Philip D. , Austin James A.
Published: February 2008

Analysis of over 6400 km of multichannel seismics (MCS) and 50 sonobuoy reflection and refraction experiments reduced both in the domain of X‐T and tau‐p shows that a region within the Jurassic Quiet Zone off Morocco underwent dramatic changes as a result of the passage of the lithosphere over the Canary hotspot commencing approximately 60 Ma. A seismic unit (UCF), interpreted as volcanic in origin, is observed within the sediments in a region characterized by a broad bathymetric swell. It shows diffractions from its upper surface and an internally chaotic seismic facies and pinches out between sedimentary units of continuous, subparallel facies. A velocity inversion is noted between the UCF (4.7km/s) and the underlying sediment (3.1 km/s). The UCF is time transgressive; it lies near the Cretaceous/s Tertiary boundary in the northern portion of the study area and is younger to the south. Kinematic studies of the movement of the Canary hotspot relative to Africa show that the hotspot first appeared off NW Africa about 60 Ma and was located beneath oceanic crust in the region where the UCF is observed. Depth‐to‐basement measurements in areas not effected by the hotspot show a consistent linear trend of increased depth with age. In areas effected by the hotspot the thermal rejuvenation is evident as basement depths shoal with increased proximity to the present hotspot. The reheating of the crust resets the thermal age of the lithosphere with many of the properties of crust of a younger age. Subsidence curves of the reheated crust off Morocco show good correlation to subsidence curves of other reheated crust on a global basis. A zone characterized by high crustal velocities, (7.1–7.4 km/s) and greater crustal thicknesses (by ∼1–2 km) is observed in an area that corresponds to the bathymetric swell, the region of the UCF, and the reelevated basement. The high velocities and increased crustal thickness are interpreted to be the result of underplating and assimilation of existing oceanic crust caused by the Canary thermal anomaly. The presence of high crustal velocities coupled with a thickened crustal section has been noted on various passive margins of the world. They have generally been attributed to the thermal processes associated with continental rifting. Off Morocco, we believe that similar, thermally induced phenomena have occurred but that here; the heat anomaly was the midplate volcanism associated with the Canary hotspot.
Reflection polarity of the midcrustal surrency bright spot beneath southeastern Georgia: Testing the fluid hypothesis
Authors: Pratt Thomas L. , Hauser Ernest C. , Hearn Thomas M. , Reston Timothy J.
Published: February 2008

A small reflection seismic experiment with an explosive source was conducted in southeastern Georgia to determine the polarity of an unusually strong midcrustal reflector, the Surrency Bright Spot (SBS), which was found at a depth of approximately 16 km during earlier COCORP profiling in the region. In addition to being very bright, the SBS is notable for being unusually flat and horizontal for about half of its 4 km length. As these characteristics are similar to those of fluid‐caused reflections at shallow depths, it has been suggested that the SBS may be caused by in situ midcrustal fluids. If caused by fluid enclosed in fracture porosity in solid rock, the reflection would be expected to exhibit a negative polarity from the top of the porous zone. The vibroseis data, however, gave ambiguous results with regard to polarity due to the limited bandwidth and inherent uncertainties about the phase of the source signal. The new experiment consisted of four dynamite shots, each recorded at three receiver stations by Seismic Group Recorders (SGR) borrowed from Amoco Production Company. Comparison of the dynamite records with geophone polarity tests indicate that the SBS is characterized by a positive reflection coefficient at its top. This result itself does not negate the fluid hypothesis—a fluid‐fluid interface could cause the positive reflection as well as the ‘flat‐spot’ nature of the reflector. Waveform modeling shows that the SBS is caused by two or more thinly‐spaced reflectors. A relatively high‐impedance layer about 120 m thick provided a good match to the observed dynamite data, but requires a lower boundary having a slightly smaller reflection coefficient than the upper boundary (0.7 versus 1.0). A ∼150 m thick porous zone model also provided a relatively good fit to the observed dynamite data, but in this model the polarity test requires that the initial SBS reflection (of positive polarity) be caused by a fluid‐fluid interface within the porous zone and that the top of the porous zone be relatively non‐reflective. Though these observations do not preclude the fluid hypothesis, they certainly make the high‐impedance model the simpler of the two alternatives presented here. The additional constraints imposed by the modeling also suggest that the SBS is more complex than these simple, two‐interface models. In either of these cases wavelet tuning contributes in part to the unusually large amplitude of the SBS reflection.
Evidence for relative motions between the Indian and Australian Plates during the last 20 m.y. from plate tectonic reconstructions: Implications for the deformation of the Indo‐Australian Plate
Authors: Royer Jean‐Yves , Chang Ted
Published: February 2008

We use plate tectonic reconstructions to establish whether motions between India and Australia occurred since chron 18 (43 Ma). We test the Africa/Antarctica/Australia/India plate circuit closure at chrons 5 (10 Ma), 6 (21 Ma) and 13 (36 Ma) using a compilation of magnetic anomalies and fracture zone traces from the Southeast, Southwest, Central Indian and the Carlsberg ridges. Additional reconstructions at chrons 23 (55 Ma) and 26 (61 Ma) are used to estimate the overall motion between India and Australia. Relative motions between the Indian and Australian plates are estimated using the plate circuit India → Africa → Australia. A new statistical approach, based on spherical regression analyses, is used to assess the uncertainty of the “best‐fitting” finite rotations from the uncertainties in the data. The uncertainty in a rotation is described by a covariance matrix directly related to the geometry of the reconstructed plate boundary, to the distribution and estimated errors of the data points along it. Our parameterization of the rotations allows for simple combination of the rotation uncertainties along a plate circuit path. Results for chron 5 are remarkably consistent with present‐day kinematics in the Indian Ocean, except that the Arabian and Indian plates are found to be separate plates. Comparisons of the motions between the Indian and African plates across the Carlsberg Ridge with that between the Australian and African plates across the Central Indian Ridge evidence a significant counterclockwise rotation of the Australian plate relative to the Indian plate about a pole located in the Central Indian Basin. The determinations are consistent for chrons 26, 13, 6 and 5. Determination at chron 23 is different but questionable due to the small number of available data. We propose two alternative solutions that both predict convergence within the Wharton and Central Indian basins and extension in the vicinity of the Chagos‐Laccadive Ridge. The first solution assumes that all the deformation in the equatorial Indian Ocean started 7 Ma ago as found by Ocean Drilling Program Leg 116. Hence all the determinations at different times represent the total motion between India and Australia. The averaged India/Australia Euler vector (chrons 5, 6, 13, and 26: 11.1°S, 78.0°, ω=3.54°) lies within the Central Indian Basin and yields a N‐S contraction of 46±52 km at 85°, and 80±63 km at 90°. However, the difference of the India/Australia Euler vectors at chrons 5 and 6 suggests that the India/Australia convergence started between 10 and 21 Ma, following the continent‐continent collision of India with Asia in the Early Miocene. The second averaged solution (chrons 6, 13, and 26: 5.2°S, 74.3°E, ω=5.93°) predicts a total N‐S contraction of 123±73 km at 85°E, and 178±91 km at 90°E. Both models are compatible with the deformation pattern observed in the equatorial Indian Ocean.
Deformation of the Central Andes (15°–27°S) derived from a flow model of subduction zones
Authors: Wdowinski Shimon , O'Connell Richard J.
Published: February 2008

A simple viscous flow model of a subduction zone is used to calculate the deformation within continental lithosphere above a subducting slab. Our formulation accounts for two forces that dominate the deformation in the overriding lithosphere: tectonic forces and buoyancy forces. The tectonic forces arise from the subduction of oceanic lithosphere and produce minor extension in the lithosphere near the trench and compression farther inland. Buoyancy forces can induce horizontal pressure gradients due to variations in crustal thickness, which cause lithospheric extension where the crust is thick. Numerical solutions, obtained by using a finite element technique, are compared with observations from the central Andes (15°–27°S). The model predicts the observed deformation pattern of extension in the forearc, compression in the Western Monocline (corresponding to magmatic activity), extension in the Altiplano, compression in the Eastern Monocline and Subandes, and no deformation in the Brazilian Shield. By comparing the calculated solutions with the large‐scale tectonic observations we are able to evaluate the forces that govern the deformation in the central Andes. The approximately constant subduction velocity in the past 26 m.y. suggests that the rate of crustal shortening in the Andes has decreased with time due to the thickening of the crust.
Nd‐Sr isotopic and geochemical characteristics of the southern Adamello (northern Italy) intrusives: Implications for crustal versus mantle origin
Authors: Kagami Hiroo , Ulmer Peter , Hansmann Werner , Dietrich Volker , Steiger Rudolf H.
Published: February 2008

Among the Periadriatic Tertiary calc‐alkaline intrusions in northern Italy, the Adamello batholith displays the most convincing evidence for a mafic parentage. This is particularly the case for the southern Adamello, where minor ultramafic to gabbroic bodies form the margins of several distinct and dominantly tonalitic plutons. Isotopic analyses are presented for the Val Fredda and Blumone ultramafic‐gabbroic to tonalitic‐leucoquartzdioritic suites, for the more felsic rock types of the spatially predominant Re di Castello group, as well as for a lamprophyre. This dike of picrobasaltic composition is thought to represent the parental magma. Field relations and a large body of petrographic, geochemical and mineral chemical data point to a cogenetic origin for all of these rocks and to magmatic differentiation with fractional crystallization and accumulation as the main evolutionary mechanism. This is compatible with the isotopic results for the Blumone series and the mafic rocks of Val Fredda with εNd = +3.8 to +1.5 and εSr = −9.1 to −4.6. A common parental magma similar to the lamprophyre (εNd = +3.1, εSr = −4.4) also seems plausible. The isotopically homogeneous felsic lithologies of Val Fredda (εNd = +0.1 to −0.3, εSr = +2.9 to +0.7), structurally intimately associated with their mafic counterpart, indicate crustal contamination incurred prior to their differentiation. For the Re di Castello group, progressing crustal affinity of isotopic compositions (εNd of −1.9 to −4.9 and εSr of +16.5 to +40.9) with chemical differentiation implies large‐scale interaction with a granitoid crustal component. Attempts at quantifying this process reveal individual characteristics and evolution for each pluton. The data indicate extraction of this component from a source in the lower crust with time‐integrated low Rb/Sr, εSr approximating +50 and εN close to −6 corresponding to a 1000 Ma crustal residence age. If a two‐stage evolution is assumed, formation of a gabbroic reservoir from the above parental melt would require uptake of some 4–12 wt % of granitoid component by combined assimilation/fractional crystallization, whereas subsequent evolution to tonalitic compositions would necessitate admixing of 40–70 wt % of granitoid partial melt. Assimilation thus mainly occurs during evolution from gabbroic to tonalitic compositions, when the mafic magma is ponded near the mantle‐crust boundary and where homogenization prior to resuming buoyant ascent is assured.

Precise locations and more detailed petrography of the samples is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document 91B‐002; $2.50. Payment must accompany order.
Subsurface imaging of the Garlock Fault, Cantil Valley, California
Authors: Louie John N. , Qin Jizeng
Published: February 2008

Imaging of the Garlock fault from seismic reflection data tests tectonic models for the Mojave Desert region of southern California. Models developed from geologic and geodetic evidence of fault movement rates disagree on whether the Garlock fault should dip north or south. Such models are further at odds with focal mechanism and dislocation analyses consistent with a vertical strike‐slip fault. Our analysis demonstrates a nonvertical, southward dip for the Garlock fault. Consortium for Continental Reflection Profiling Mojave line 5 collected seismic reflection data across the Garlock in Cantil Valley, where the 3‐km‐deep Cantil pull‐apart basin has developed between the southwest and east branches of the fault. We analyze the reflection data for evidence of the fault's structure to 5 km depths. Garlock fault plane reflections on unprocessed shot gathers merge with the first arrival as the source progresses to the surface trace of the east branch, explicitly tracing the Garlock fault as a seismic reflector from depth to the mapped contemporary fault trace. The apparent velocity of these reflections gives a 37°±10° south dip for the east branch of the fault. A prestack Kirchhoff sum migration images the east branch reflector with a 45° south dip. Our migration process fully accounts for the bending of seismic rays through the strong lateral velocity variations in Cantil basin. The south dip of the Garlock's east branch together with basement steps that decrease to the south suggest that Cantil basin developed from a 0.4–1 mm/yr component of extension normal to the fault. Development of the basin by detachment provides a mechanism to widen it as left‐lateral motion lengthens it, allowing the pull‐apart to maintain the globally constant length‐to‐width ratio of 3. Detachment by the Garlock at Cantil Valley similarly requires 0.4–1 mm/yr of dextral strike slip on a northwest striking fault such as the Helendale that cuts the Mojave between the Garlock and Pinto Mountain faults. Such motion is consistent with regional tectonic models.
Strain accumulation along the Denali Fault at the Nenana River and Delta River Crossings, Alaska
Authors: Savage J. C. , Lisowski M.
Published: February 2008

Surveys of trilateration networks across the Denali fault at the Nenana River in 1982, 1984, and 1988 and at the Delta River in 1975, 1979, 1982, and 1984 indicate a minor (0.10±0.04 μstrain/yr) northeastward uniaxial extension. The component of right‐lateral shear‐strain accumulation across the fault is not significant at the two‐standard‐deviation level. At the Delta River network the strain accumulation rate decreases rapidly with distance from the fault, but evidence for a similar decrease with distance from the fault is lacking at the Nenana River network. The strain accumulation rates inferred from trilateration are consistent with the very long baseline interferometry (VLBI) measurement reported by Ma et al. (1990) and support their contention that significant right‐lateral shear is not accumulating along the Denali fault at the present time. Savage et al. (1981) had earlier concluded erroneously that preliminary geodetic measurements at the Delta River network demonstrated right‐lateral shear strain accumulation. The absence of significant right‐lateral deformation across the Denali fault in the 1975–1988 interval is in marked contrast with the abundant geomorphic evidence for Holocene right‐lateral secular slip at the rate of 10–20 mm/yr on the Denali fault in this sector.
Variations in heat flow across the Goban Spur and Galicia Bank continental margins
Authors: Louden K. E. , Sibuet J.‐C. , Foucher J.‐P.
Published: February 2008

We present the results of 44 new heat flow stations which were taken in 1984 and 1989 in profiles across the Goban Spur and Galicia Bank continental margins (NW Atlantic Ocean). Simple extensional models indicate that the heat flow across both these Early Cretaceous rifted margins should increase from values of 45–50 mW/m2 over oceanic crust to 65–80 mW/m2 on the continents. The rate of this increase should help to constrain the mechanism (simple versus pure shear) by which the upper, more radiogenic continental crust is thinned. Measurements across Goban Spur increase from values of 40–45 mW/m2 over oceanic crust to 50–55 mW/m2 near the continental shelf. They follow the predicted trend for pure‐shear rifting, but only if the value of upper crustal radiogenic heating is low (1–2 μW/m3). Otherwise, they would require the upper crust to thin more rapidly than the total crustal thickness, as with a lower plate, simple‐shear margin. Measurements across Galicia Bank show a very different pattern, with similar values over oceanic crust but much lower values (30–35 mW/m2) nearer land. This is difficult to reconcile with any simple, single rifting event but is more compatible with an origin as a pure‐shear or lower plate rather than upper plate margin. We also note that oceanic values of heat flow require asthenospheric temperatures 100°C lower than normal for both margins. This indicates that the triple junction in existance between these margin segments during the breakup of Iberia, Europe, and North America was not the site of a major mantle plume.
An estimate of the absolute stress tensor in Kaoiki, Hawaii
Authors: Wyss M. , Glllard D. , Liang B.
Published: February 2008

The stress tensor orientation was calculated by inversion of 81 fault plane solutions of M = 3.5 ± 0.6 earthquakes located in an area 6 by 16 km at depths of 7 ± 2 km. This crustal volume is situated on a straight line between the summits of the active volcanoes Kilauea and Mauna Loa. The orientation of the greatest principal stress was found to be near horizontal and in the line connecting the two volcanoes. This is further evidence supporting the model that magmatic expansion within the feeding conduits of these volcanoes is the source of stress that causes earthquakes in southern Hawaii. These earthquakes are tectonic earthquakes in the sense that they occur in a brittle elastic crust at distances of a few to several tens of kilometers from the volcanoes, and that they are not directly associated with the opening of cracks by intrusions. We propose that this model leads to the corollary that the shear stresses (τ) responsible for earthquake failure in the shallow Hawaiian crust are approximately 3 ± 2 MPa, and that the pore pressure of ground‐water (p) in the hypocentral volume must be under near lithostatic pressure. This result is obtained by arguing that the greatest principal stress (σ1) is equal to the magma pressure in the volcanic feeder pipe, but cannot exceed the least principal stress (σ3) by more than the tensile strength of the crust, because otherwise hydrofracture would occur, opening a crack against the least principal stress. Given the difference between the greatest and least principal stress, the fact that the overburden at the 7 km hypocentral depth equals the intermediate principal stress and the ratio R = (σ1 − σ2)/(σ1 − σ3) = 0.4, which is obtained in the inversion of fault plane solutions, we estimate that σ1 = 202 MPa, σ2 = 200 MPa, σ3 = 196 MPa, τ = 3 MPa, and σn = 199 MPa, with uncertainties on the order of several MPa. It follows that for any coefficients of friction larger than 0.15, the pore fluid pressure necessary for faulting on the observed fault planes has to be larger than 90% of the lithostatic pressure. These considerations suggest that many earthquakes represent fault ruptures under low ambient shear stresses in the order of the average observed stress drop of 3 MPa.
The Patagonian Orocline: New paleomagnetic data from the Andean magmatic arc in Tierra del Fuego, Chile
Authors: Cunningham W. Dickson , Klepeis Keith A. , Gose Wulf A. , Dalziel Ian W. D.
Published: February 2008

The Hardy Formation is a 1300‐m‐thick succession of Upper Jurassic‐Lower Cretaceous volcaniclastic sedimentary rocks interbedded with lava flows on Hoste Island at the southernmost tip of South America (55.5°S, 291.8°E). The strata are gently folded and metamorphosed to the prehnite‐pumpellyite grade. A well‐defined characteristic direction of magnetization, carried by magnetite, was readily identified in 95 samples from seven sites. At a given site, the directions group slightly better without structural correction. However, the means of the seven sites cluster better without tilt correction at the 99% significance level, implying that the magnetization postdates the folding event. It is most likely that the magnetization was acquired during the mid‐ to Late Cretaceous Andean orogeny that involved the folding and emplacement of the Patagonian Batholith. The fact that all samples are normally magnetized supports this age assignment. The pole position of 42.9°N, 156.6°E, α95=3.3° implies that the sampling area has rotated counterclockwise relative to cratonic South America by 90.1±11.9° with no significant flattening of inclination (F=1.9 ± 3.7°). Geologic considerations indicate that the rotation involved the entire Andean magmatic arc in Tierra Del Fuego. The results support interpretation of the Hardy Formation as part of the Andean magmatic arc deposited on the Pacific side of the Late Jurassic‐Early Cretaceous Rocas Verdes marginal basin. Oroclinal bending of the arc in southernmost South America accompanied inversion of the marginal basin and the development of a Late Cretaceous‐Cenozoic left‐lateral transform system (South America‐Antarctica) that later developed into the North Scotia Ridge.
Seismicity and detection/location threshold in the Southern Great Basin Seismic Network
Authors: Gomberg Joan
Published: February 2008

A spatially varying model of the detection/location capabilities of the Southern Great Basin seismic network (SGBSN) has been derived that is based on simple empirical relations and statistics. This permits use of almost all the catalog data gathered; instead of ignoring data that are below the threshold of completeness, a spatially varying threshold model is developed so that subregions having lower completeness levels than the network as a whole can be outlined and the completeness level of each sub‐region determined. Such a model is required to unambiguously identify regions that are aseismic due to natural processes rather than to limited detection and/or location capabilities. Accounting for spatial variations in detection/location threshold is also important for studies in which magnitude‐frequency distributions are interpreted in terms of source scaling properties. The characteristics of the spatial distribution of earthquakes, where earthquake clusters and aseismic regions locate, appear to be stable at all magnitude levels so that inferences about where strain is being accommodated will be the same whether numbers of earthquakes or strain estimated from seismic moments are examined. For the southern Great Basin region these principal characteristics include clusters at the northern end of the Furnace Creek fault and in the Pahranagat Shear Zone, and a relatively large number of earthquakes in the northern and southeastern portions of the Nevada Test Site. These clusters cover regions much larger than the surface projections of any of the mapped faults. The extent to which seismicity is induced by nuclear testing is unclear. The predominantly aseismic regions include the area west of the Death Valley/Furnace Creek fault system and an almost complete absence of events at Yucca Mountain. Finally, a considerable number of isolated events in the SGBSN catalog cannot be correlated with mapped faults.
Rock‐buffered fluid‐rock interaction in deformed quartz‐rich turbidite sequences, eastern Australia
Authors: Gray David R. , Gregory Robert T. , Durney David W.
Published: February 2008

Quartz vein 18O/16O ratios across a 500 km transect through the Lachlan fold belt of southeastern Australia are remarkably uniform (±1–1.5 permil) at both local (centimeter to meter) and regional (over 104 km2) scales. They define isotopic zones that correlate with the tectonic divisions of the Cambrian through Devonian (dominantly Ordovician) quartz‐rich turbidites determined by regional mapping. From west to east these divisions are (1) Stawell, δ18O=14.7±1.1 (47 samples); (2) Bendigo‐Ballarat, δ18O=17.5±1.3 (204 samples); (3) Melbourne, δ18O=19.0±1.6 (80 samples); (4) Tabberabbera, δ18O=16.3±1.9 (12 samples); and (5) Omeo, δ18O=14.4±1.0 (26 samples). Isotope profiles across zone boundaries, and intrazone fault zones particularly within the Bendigo‐Ballarat zone, show steps indicating abrupt changes across faults with little or no evidence of fluid mixing within the fault zones. δ18O values of veins are insensitive to relative age, type of vein, and immediate host rock lithologies. The δ18O values of coexisting vein and host rocks show nonequilibrium relationships which can be explained in terms of rock buffering under conditions of low fluid/rock ratios (water/rock ≪ 1). Limited D/H determinations on fluid inclusions fall mainly in the range −70 and −100, with one value as low as −140. These low deuterium values when considered in the context of paleolatitude may have been inherited from deutrium‐depleted detrital minerals and do not necessarily require the direct penetration of meteoric fluids to midcrustal depths. The work suggests that vein formation is possible in regions where the integrated water/rock (w/r) ratio is very low (w/r ≪ 1) as long as a pervasive fluid phase is present. This fluid appears to achieve a steady state isotopic composition on scales of hundreds of meters, and once a quasi steady state has been reached, the isotopic compositions of the resultant quartz veins are rather insensitive to the diachronous and nonisothermal conditions under which vein growth occurs. Advective cycling of the fluid on this scale is by episodic dilatancy “pumping” in fracture networks associated with localized faulting.
Mantle layering from ScS reverberations: 1. Waveform inversion of zeroth‐order reverberations
Authors: Revenaugh Justin , Jordan Thomas H.
Published: February 2008

This is the first paper in a four‐part sequence that examines the nature of mantle layering required by the multiple‐ScS phases and internal reflections observed within the reverberative interval of SH‐polarized seismograms. Here we present path‐averaged estimates of the two‐way travel times through the crust+mantle (τScS) and crust alone (τM), the whole mantle quality factor (QScS) and the reflection coefficient at the M discontinuity (R0(zM)) obtained by iterative waveform inversion of ScS reverberations. We show that QScS estimates derived from direct waveform inversion can be biased significantly if reverberations originating within the crust are ignored, especially in continental regions where the crust is thick. Applying our technique, which explicitly models crustal reverberations, we substantially expand the geographical coverage of τScS and QScS measurements available from previous studies. The τScS data correlate well with whole mantle travel times predicted by the Harvard tomographic models. Both the τScS data and the QScS data support the hypothesis that variations in velocity and attenuation structure between old continents and oceans persist to depths of several hundred kilometers. In particular, the value of QScS derived for paths crossing stable continent is very high (280±30 at 30 mHz) compared with that for mature ocean basin (189±16), consistent with the thick‐plate model of cratons requiring that differences in thermal structure extend to several hundred kilometers. The crustal thicknesses implied by the observations of path‐averaged τM are consistent with those inferred from short‐range refraction data, gravity measurements, and surface topography. The path‐averaged reflectivity of the M discontinuity is also tectonically correlated, becoming brighter as the percentage of continental crust encountered along the path increases. This result implies that, on average, the impedance at the base of the continental crust is significantly lower than at the base of the oceanic crust.
Stress magnitude estimates from earthquakes in oceanic plate interiors
Authors: Govers R. , Wortel M. J. R. , Cloetingh S. A. P. L. , Stein C. A.
Published: February 2008

We propose a method to estimate stress magnitudes in oceanic plate interiors from focal depths and focal mechanisms. Using a depth‐dependent rheology, we show it is possible to estimate the differential stress (σ1–σ3), averaged over some reference lithospheric thickness. The resolving power of the method is investigated by evaluating the effect of uncertainties in parameters that are involved in the analysis. We apply the method to the Central Indian Ocean, where intraplate seismicity is high. From well‐studied earthquakes we estimate differential stresses of the order of hundreds of rnegapascals. This result is consistent with the high level of stress that was found from numerical model calculations by Cloetingh and Wortel (1985, 1986). From the few intraplate events in the Pacific plate, we also estimate differential stresses in this area.
Atmospheric pumping: A mechanism causing vertical transport of contaminated gases through fractured permeable media
Authors: Nilson R. H. , Peterson E. W. , Lie K. H. , Burkhard N. R. , Hearst J. R.
Published: February 2008

As the barometer falls, gases are drawn upward out of the permeable Earth into the atmosphere. Conversely, a rising barometer pushes air downward. In a homogeneous permeable medium, these cyclical gas motions are piston‐like and nearly reversible, so they contribute only modestly to the net transport of contaminant gases. In a fractured permeable medium, however, the fractures will generally serve as breathing passages for all of the gas‐filled porosity, greatly increasing the amplitude and nonuniformity of vertical motions. The resulting transport process may be orders of magnitude more significant than molecular diffusion, according to the theoretical analysis presented here. Analytical solutions are first derived for the sinusoidal pressure response of a medium containing identical vertical fractures equally spaced by slabs of permeable matrix material. These solutions are then used to constrain the relationship between fracture aperture and fracture spacing, based on field comparisons between surface and subsurface pressure variations. The final phase of the analysis addresses the diffusive and advective transport of an inert trace gas which is carried by an oscillatory flow along a fracture having permeable walls. A maximum rate of transport is predicted to occur for an intermediate fracture spacing which is typically a few meters.
Long‐period seismic radiation from the May 23, 1989, Macquarie Ridge Earthquake: Evidence for coseismic slip in the mantle?
Authors: Anderson Helen J. , Zhang Jiajun
Published: February 2008

Long‐period seismic source parameters of the May 23, 1989, Macquarie Ridge earthquake are determined using a surface wave inversion procedure that incorporates detailed source‐time functions obtained from shorter‐period body waves. The seismic source model obtained using this method is consistent with observations of both body waves and long‐period Rayleigh waves from the earthquake. The Macquarie Ridge earthquake rupture has a centroid time (28 s) and a right‐lateral strike‐slip fault mechanism with a rake of 175° on a vertical fault plane striking N38°E. This mechanism is consistent with P wave first motions of the event Inversions performed for various earth models demonstrate that the choice of surface wave attenuation model, in particular, affects the estimates of centroid depth and seismic moment significantly. Allowing for uncertainty in attenuation, the long‐period Rayleigh waves (periods from 150 to 300 s) indicate that the Macquarie Ridge earthquake had a seismic moment of 1.9±0.2 × 1021 N m and a corresponding centroid depth of 21±6 km. The static stress drop calculated using the depth and seismic moment is 0.7±0.5×107 Pa (70±50 bars). The centroid depth, combined with the lack of resolvable directivity of the earthquake rupture, suggests that significant slip occurred beneath the Moho, which has a maximum depth of about 16 km in the epicentral region. The mantle slip component may have preferentially radiated long‐period seismic energy given the shallow centroid depths and low moments determined for the event from the shorter‐period body wave observations.
Gravity field error analysis: Applications of global positioning system receivers and gradiometers on low orbiting platforms
Authors: Schrama Ernst J. O.
Published: February 2008

The concept of a Global Positioning System (GPS) receiver as a tracking facility and a gradiometer as a separate instrument on a low orbiting platform offers a unique tool to map the Earth's gravitational field with unprecedented accuracies. The former technique allows determination of the spacecraft's ephemeris at any epoch to within 3–10 cm; the latter permits the measurement of the tensor of second‐order derivatives of the gravity field to within 0.01–0.0001 Eötvös units depending on the type of gradiometer. An analytical technique is described and applied for an error analysis of gravity field parameters from gradiometer and GPS observation types. Results are discussed for the configuration proposed on Aristoteles, indicating that a “GPS only” solution may be computed up to degree and order 85, whereas a combined GPS/gradiometer experiment may result in an acceptable solution up to degree and order 240.
Piezomagnetic fields of screw dislocation fault models
Authors: Banks Philip O. , Stuart William D. , Liu Shu‐Wang
Published: February 2008

An analytical solution is obtained for the piezomagnetic field generated by a single screw dislocation and its image, representing an infinitely long vertical fault with uniform strike slip that cuts the upper or lower portion of a half‐space. The solution is readily extendable to other models of strike‐slip faults based on vertical arrays of screw dislocations. For reasonable values of the relevant physical constants the field is predicted to be generally below current levels of detectability. This result is a consequence of the symmetry of piezomagnetization around a screw dislocation, which inhibits regions of intense piezomagnetism near the dislocation from contributing to the external field.
Source time functions of large Mexican subduction earthquakes, morphology of the Benioff Zone, age of the plate, and their tectonic implications
Authors: Singh S. K. , Mortera F.
Published: February 2008

We study source parameters of large, shallow Mexican subduction zone earthquakes (95°W to 106°W) which occurred between 1928 and 1986 by modeling the P waves recorded on Galitzin‐Wilip seismograph in DeBilt (DBN), Holland. For post‐1962 events the source parameters retrieved from DBN seismograms alone agree well with those obtained from long‐period World‐Wide Standardized Seismograph Network records, giving us confidence in our results for pre‐1962 events. All earthquakes are shallow (H∼10 to 20 km). With few exceptions the sources in Oaxaca (95°W to 99°W) are very simple. To the northwest of 99°W they are simple as well as complex. The ratio of surface wave to body wave seismic moment (Mos/MoP), which is a measure of long‐ to short‐period radiation, is smaller in Oaxaca (∼ 1.5±0.5) than in the regions northwest of 99°W (∼3.1±1.3). These results suggest a change in the plate interface characteristics near 99°W. The sharp change in the rupture mode and the intersection of the O'Gorman Fracture Zone (OFZ) with the trench occur near 99°W. Two strike‐slip events offshore, close to OFZ, suggest a segmentation of the subducting plate near 99°W. The age of the plate near the trench in Oaxaca is not well known; it is possible that it does not increase continuously from northwest to southeast in the region but jumps across 99°W. If so, then the older age of the subducted plate southeast of 99°W may be the cause of the distinct rupture mode of the Oaxaca earthquakes. The length of the Benioff zone, which is greatest below Oaxaca ( ≈ 400 km) and decreases toward the northwest, can be explained by the correlation between the length of the subducted slab and the product of the lithosphere age and convergence rate. The relative complexity of sources, the weaker background seismicity, and the lesser number of aftershocks northwest of Oaxaca may be explained by a stronger interface coupling resulting from subduction of younger oceanic slabs (∼5 to 13 m.y. old) in this region. This, however, explains neither larger Mos/MoP values northwest of Oaxaca nor the low stress drop estimates obtained from the analysis of near‐field strong‐motion data for the Michoacan earthquake of 1985, both of which indicate weaker coupling of the interface. Thus the issue of whether subduction of very young plates (≤ 10 m.y. old) results in strong or weak coupling remains unsolved from the presently available Mexican data.
Monte Carlo simulation of errors in the anisotropy of magnetic susceptibility: A second‐rank symmetric tensor
Authors: Lienert Barry R.
Published: February 2008

I have used Monte Carlo perturbations of synthetic tensors to evaluate the Hext/Jelínek elliptical confidence regions for anisotropy of magnetic susceptibility (AMS) eigenvectors. When the perturbations are 33% of the minimum anisotropy, both the shapes and probability densities of the resulting eigenvector distributions agree with the elliptical distributions predicted by the Hext/Jelínek equations. When the perturbation size is increased to 100% of the minimum eigenvalue difference, the major axis of the 95% confidence ellipse under estimates the observed eigenvector dispersion by about 10°. The observed distributions of the principal susceptibilities (eigenvalues) are close to being normal, with standard errors that agree well with the calculated Hext/Jelínek errors. The Hext/Jelínek ellipses are also able to describe the AMS dispersions due to instrumental noise and provide reasonable limits for the AMS dispersions observed in two Hawaiian basaltic dikes. I conclude that the Hext/Jelínek method provides a satisfactory description of the errors in AMS data and should be a standard part of any AMS data analysis.
Conduction model for the thermal influence of lithic clasts in mixtures of hot gases and ejecta
Authors: Marti J. , Diez‐Gil J. L. , Ortiz R.
Published: February 2008

The thermal influence of lithic clasts in mixtures of hot gases and ejecta produced during explosive eruptions is an important aspect of the dynamics of eruption columns and pyroclastic flows. In order to study this aspect we present a heat conduction model, which considers both the temperature distributions of the lithic clasts and the exchange of thermal energy between the lithic clasts and other constituents (gases and juvenile pyroclasts). The model quantifies the thermal energy of the system by means of a characteristic energy function that describes the energy absorbed by each lithic clast. Using this energy function and considering some hypothetical arrangements of lithic clasts, it is possible to calculate, by means of heat balance, the temperature variations in mixtures of hot gases and ejecta as a function of time, volume fraction of particles to gas, and thermal diffusivity, initial temperature, diameter, and volume fraction of lithic clasts. Theoretical results show that in an eruption a volume fraction of 12% of cool lithic clasts (20–100°C) whose size is less than 5 cm will cause the mixture of hot gases and ejecta to cool by 300°C. We suggest therefore that the thermal effects of lithic clasts should be considered in numerical models of eruption columns and pyroclastic flows.
Structure of the crust and upper mantle beneath the Great Valley and Allegheny Plateau of eastern Pennsylvania 2. Gravity modeling and migration of wide‐angle reflection data
Authors: Hawman Robert B. , Phinney Robert A.
Published: February 2008

Linear inversion of τ(p) composites for three different profiles suggests that the lower crust beneath eastern Pennsylvania consists of material with high averaged P wave velocities (6.8–7.5 km/s) compared with 6.4–7.1 km/s for most crustal models derived for surrounding areas within the Appalachians and the Grenville Province of the Canadian Shield. Interpretations of these velocities in terms of rock type are limited by the low resolving power of the data and by the nonuniqueness in inferring rock type from P wave velocity alone. The high averaged P wave velocities could be associated with substantial volumes of mafic rock, perhaps emplaced by underplating, or with high‐grade metamorphic assemblages of intermediate composition and sedimentary origin, similar to those found in the Ivrea Zone in northern Italy. Because of the sparseness of the S wave data, averaged shear wave velocities are not well enough resolved over most depth ranges to distinguish among the different compositional models. Computations of gravity anomalies for two‐dimensional models using various linear velocity‐density relations, however, suggest that at least part of the crust beneath the Scranton Gravity High is mafic. Moreover, if the entire basement section beneath this anomaly consists of mafic material, then the gravity studies indicate that the lower crust beneath the Great Valley must be mafic as well. Migrated shot gathers show a concentration of reflectors at depths between 35 and 45 km, a result that is controlled in part by the limited range of offsets used, but that is also consistent with recent observations of a highly reflective lower crust. Reflections that are migrated to depths within the uppermost mantle could be multiples or primary reflections from upper mantle layering. Although the data are rather sparse, the interpretation of these later arrivals as primary reflections is supported by their small ray parameters, which suggests that they are not converted phases, and by their large relative amplitudes, which suggests that they are not multiples. Layering within the upper mantle could be in the form of alternating high‐ and low‐velocity zones, perhaps related to emplacement of eclogites or garnet granulites in the uppermost mantle during the process of underplating.