bluuebunny/crossref_metadata_2025 huggingface dataset:
Localization of listric faults at thrust fault ramps beneath the Great Salt Lake Basin, Utah: Evidence from seismic imaging and finite element modeling
Authors: Mohapatra Gopal K. , Johnson Roy A.
Published: February 2004

Reflection seismic data from the Great Salt Lake Basin, Utah, show that the major basin‐bounding normal faults decrease in dip from ∼60° at the surface to ∼10°–20° at depths as shallow as 4–6 km. This rapid decrease in fault dip at depths shallower than the brittle‐ductile transition zone in the Basin and Range Province suggests an explanation other than a gradual change of rheology and stress orientations with depth. Using a dense grid of seismic data, gravity data, borehole data, and published geologic information from islands in the lake, we constrain the position of the Sevier age Willard thrust and a footwall imbricate and show their reactivation as normal faults during Tertiary extension. In the absence of surface geologic information, we use available subsurface information from the lake to draw an analogy with the Ogden duplex in the Wasatch Front, where Cenozoic normal faulting was superimposed on an earlier Sevier age thrust regime to give rise to listric normal faults. Our interpretations are consistent with finite element modeling results, which demonstrate that extensional slip on preexisting thrust ramps leads to the formation of energetically favored synthetic normal faults, some of which may merge with the thrust ramp and obtain listric geometries. Further slip on these listric faults gives rise to secondary synthetic and antithetic faults resulting in hanging wall grabens.
Elastic wave velocities of forsterite and its β‐spinel form and chemical boundary hypothesis for the 410‐km discontinuity
Authors: Fujisawa Hideyuki
Published: February 2004

A method for measuring elastic wave velocities at pressures up to 15 GPa has been developed. Velocities of forsterite and β‐spinel were measured using a piston‐cylinder apparatus with a pulse‐echo‐overlap method for pressures up to 4 GPa and using a split‐sphere apparatus with a pulse‐transmission technique for higher pressures. To validate the results, volume compression, incompressibility, and rigidity were estimated from measured travel times, and comparison was made between the present estimates and data from X ray, Brillouin, and acoustic measurements. The Vp and K values for β‐spinel are 3 and 8% higher than previous measurements at 10 GPa, respectively. Velocity jumps in Vp and Vs due to olivine to β‐spinel transition were about 1.34 and 0.80 km s−1, respectively, very different from seismological observations of velocity jumps at the 410‐km discontinuity, which were concentrated around 0.45 km s−1 for the P wave and 0.22 km s−1 for the S wave. Velocity jumps in mantle peridotite were calculated using the experimental results, and comparison was made between these estimates and seismological observations at the 410‐km discontinuity. The evidence does not support the phase‐change hypothesis, and a chemical boundary was proposed as the origin of the 410‐km discontinuity. An eclogitic mineral assemblage might be an alternative for the composition of the transition layer.
Axial summit trough of the East Pacific Rice 9°–10°N: Geological characteristics and evolution of the axial zone on fast spreading mid‐ocean ridge
Authors: Fornari Daniel J. , Haymon Rachel M. , Perfit Michael R. , Gregg Tracy K. P. , Edwards Margo H.
Published: February 2004

The nature and morphological characteristics of axial summit troughs on fast (∼90–130 mm/yr−1full spreading rate) and superfast spreading (>130 mm/yr−1) mid‐ocean ridge crests reflect the time‐integrated effects of long‐term magmatic cycles, short‐term volcanic episodicity, and the tensional stress regime imposed on young ocean crust. Two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid‐ocean ridge crests. (1) Narrow axial troughs, <∼200 m wide and <∼15 m deep, are termed axial summit collapse troughs and are interpreted to be generated by shallow collapse of lava flow surfaces over zones of primary eruptive fissures and subsurface lava conduits (such as channels and tubes). Axial summit collapse trough position, physical characteristics, and associations with hydrothermal venting on the ridge crest strongly suggest a primary volcanic/magmatic origin for the trough. (2) Wider, fault‐bounded axial troughs (>∼300–2000 m wide and ∼30–100 m deep) on the East Pacific Rise crest are classified as axial summit graben. The dimensions of axial summit graben, as well as the morphological and structural character of their walls and floors, suggest a primary tectonic origin. An axial summit graben may contain a nested axial summit collapse trough, implying that processes responsible for these endemic features may be linked. Near‐bottom, side‐looking sonar and observational data collected using the towed vehicleArgo Iand submersibleAlvinhave been used to characterize the axial summit trough of the fast spreading East Pacific Rise between 9° and 10°N. A four‐stage model is presented for the evolution of this axial summit collapse trough, as well as for other well‐studied portions of the East Pacific Rise crest from 21°N to ∼20°S. We propose that the transition from a narrow, surface collapse‐dominated axial trough to a broader, fault‐bounded graben is controlled by the relative importance of diking, volcanism, hydrothermal cooling, and tectonism along a ridge segment over time periods <104years.
Constraints on a buoyant model for the formation of the axial topographic high on the East Pacific Rise
Authors: Eberle Michael A. , Forsyth Donald W. , Parmentier E. M.
Published: February 2004

Bathymetry and gravity data from four geophysical surveys along the 15°–19°S superfast spreading section of the East Pacific Rise are used to study formation of the axial topographic high. After removing a best fitting thermal model the averaged, residual, axial topography is ∼265 m high and ∼15 km wide. The residual gravity shows no evidence for shallow isostatic compensation of this topography suggesting deep‐rooted compensation or dynamic support. Previous models postulate that the axial high is compensated by low densities in a zone of partial melting within the upper mantle. We model the physical properties of the lithosphere and asthenosphere required to match the residual gravity and bathymetry using a nonlinear inversion, assuming the high is uplifted by buoyancy forces from low‐density mantle. Deep‐rooted buoyancy forces cannot create a narrow axial high unless they occur in a low‐viscosity conduit surrounded by high‐viscosity mantle. This high‐viscosity region could be created by depletion and extraction of all water during the melting process. We find that the viscosity of the asthenosphere beneath the melt conduit must also be lower than the viscosity of the residual mantle surrounding the conduit. Our most reasonable model has a viscosity of 1020 Pa s in the residual mantle surrounding a melt conduit which extends 50 km below the seafloor and a viscosity of 1016.6 Pa s in the asthenosphere below the melt conduit. The viscosity of the melt conduit must be about 1015 Pa s, or about 105 times lower than the surrounding residual mantle, with a density contrast equivalent to the retention of a small fraction of melt (5%). Because higher melt concentrations are probably required to create even small changes in viscosity and the viscosity of the undepleted mantle is probably higher than 1016.6 Pa s, we conclude that it is unlikely that the axial topographic high is formed by buoyancy forces in the crust and mantle.
Viscoelastic coupling model of the San Andreas Fault along the Big Bend, southern California
Authors: Savage J. C. , Lisowski M.
Published: February 2004

The big bend segment of the San Andreas fault is the 300‐km‐long segment in southern California that strikes about N65°W, roughly 25° counterclockwise from the local tangent to the small circle about the Pacific‐North America pole of rotation. The broad distribution of deformation of trilateration networks along this segment implies a locking depth of at least 25 km as interpreted by the conventional model of strain accumulation (continuous slip on the fault below the locking depth at the rate of relative plate motion), whereas the observed seismicity and laboratory data on fault strength suggest that the locking depth should be no greater than 10 to 15 km. The discrepancy is explained by the viscoelastic coupling model which accounts for the viscoelastic response of the lower crust. Thus the broad distribution of deformation observed across the big bend segment can be largely associated with the San Andreas fault itself, not subsidiary faults distributed throughout the region. The Working Group on California Earthquake Probabilities [1995] in using geodetic data to estimate the seismic risk in southern California has assumed that strain accumulated off the San Andreas fault is released by earthquakes located off the San Andreas fault. Thus they count the San Andreas contribution to total seismic moment accumulation more than once, leading to an overestimate of the seismicity for magnitude 6 and greater earthquakes in their Type C zones.
40Ar‐39Ar thermochronology of the southern Gawler Craton, Australia: Implications for Mesoproterozoic and Neoproterozoic tectonics of East Gondwana and Rodinia
Authors: Foster David A. , Ehlers Karin
Published: February 2004

40Ar‐39Ar thermochronology of the southern Gawler Craton, Australia, reveals a protracted history of slow cooling, with local crustal heating, rapid cooling, and fault reactivation, between ∼1700 and ∼700 Ma. Hornblendes from most of the southern part of the Craton, including the Kalinjala Mylonite Zone, yield ages ∼1580–1610 Ma, with younger more discordant samples showing evidence of retrogression. These results indicate that the region cooled below ∼500°C ∼100 m.y. after the Kimban Orogeny and coincident with the Gawler Range‐Hiltaba Suite magmatism, to the north, perhaps associated with postorogenic extension. Biotite and K‐feldspar data indicate that regional cooling after this time was more gradual, with a few exceptions. Locally, in the southeastern Eyre Peninsula, rapid cooling at ∼1425 Ma is indicated by nearly concordant hornblende and biotite ages and may record a phase of regional extension. Other exceptions include areas of lower grade Archean rocks that cooled to <350°C by ∼1700–1640 Ma. Greenschist facies reactivation of shear zones occurred at ∼1100 Ma, during the Musgravian Orogeny, on the basis of biotite single‐grain data and retrogressed amphiboles. K‐feldspar age spectra record cooling at 700–800 Ma following regional intrusion of the Gairdner Dike Swarm and/or denudation during rifting along the Pacific margin of East Gondwana and breakup of Rodinia. Tectonothermal events revealed by these results, previous geochronological, and geological studies and available paleomagnetic data are comparable to other cratons and mobile belts in Australia, East Gondwana, and Laurentia, providing evidence that southeastern Australia and North America were adjacent during much of Proterozoic time.
Quantitative measure of the variation in fault rheology due to fluid‐rock interactions
Authors: Blanpied M. L. , Marone C. J. , Lockner D. A. , Byerlee J. D. , King D. P.
Published: February 2004

We analyze friction data from two published suites of laboratory tests on granite in order to explore and quantify the effects of temperature (T) and pore water pressure (Pp) on the sliding behavior of faults. Rate‐stepping sliding tests were performed on laboratory faults in granite containing “gouge” (granite powder), both dry at 23° to 845°C [Lockner et al., 1986], and wet (Pp = 100 MPa) at 23° to 600°C [Blanpied et al., 1991, 1995]. Imposed slip velocities (V) ranged from 0.01 to 5.5 μm/s, and effective normal stresses were near 400 MPa. For dried granite at all temperatures, and wet granite below ∼300°C, the coefficient of friction (μ) shows low sensitivity to V, T, and Pp. For wet granite above ∼350°, μ drops rapidly with increasing T and shows a strong, positive rate dependence and protracted strength transients following steps in V, presumably reflecting the activity of a water‐aided deformation process. By inverting strength data from velocity stepping tests we determined values for parameters in three formulations of a rate‐ and state‐dependent constitutive law. One or two state variables were used to represent slip history effects. Each velocity step yielded an independent set of values for the nominal friction level, five constitutive parameters (transient parameters a, b1, and b2 and characteristic displacements Dc1 and Dc2), and the velocity dependence of steady state friction ∂μss/∂ ln V = a‐b1−b2. Below 250°, data from dry and most wet tests are adequately modeled by using the “slip law” [Ruina, 1983] and one state variable (a = 0.003 to 0.018, b = 0.001 to +0.018, Dc ≈ 1 to 20 μm). Dried tests above 250° can also be fitted with one state variable. In contrast, wet tests above 350° require higher direct rate dependence (a = 0.03 to 0.12), plus a second state variable with large, negative amplitude (b2 = −0.03 to −0.14) and large characteristic displacement (Dc2 = 300 to >4000 μm). Thus the parameters a, b1, and b2 for wet granite show a pronounced change in their temperature dependence in the range 270° to 350°C, which may reflect a change in underlying deformation mechanism. We quantify the trends in parameter values from 25° to 600°C by piecewise linear regressions, which provide a straightforward means to incorporate the full constitutive response of granite into numerical models of fault slip. The modeling results suggest that the succeptibility for unstable (stick‐slip) sliding is maximized between 90° and 360°C, in agreement with laboratory observations and consistent with the depth range of earthquakes on mature faults in the continental crust.
Magnetic signature of upper plate structures and subducting seamounts at the convergent margin off Costa Rica
Authors: Barckhausen Udo , Roeser Hans A. , von Huene Roland
Published: February 2004

Offshore the Pacific margin of Costa Rica the Caribbean Plate converges with the subducting Cocos Plate along the Middle America Trench. The tectonics of both plates have been studied on the basis of reflection seismic, gravimetric, and magnetic data and swath‐mapping. The magnetic data were acquired by GEOMAR, Kiel, Germany, during cruise SO‐76 of R/V Sonne off Costa Rica in 1992. Two different magnetic anomaly patterns are observed. Off Nicoya Peninsula the linear magnetic anomalies trend about N50°E and can be traced from the ocean basin to the landward end of the profiles. To the southeast of a major structural break trending parallel to Fisher Seamount and the associated ridge the strike of the magnetic anomalies is N70°E. In the area southeast of Fisher Seamount the magnetic signatures of numerous seamounts are superimposed upon the linear anomalies. We interpret the magnetic lineations as chrons 5C to 6B (16–22.5 Ma). Magnetic modeling demonstrates a significant magnetization (∼1 A/m) of the margin wedge in the continental slope, a structural unit known from seismic studies. This is not compatible with the previously discussed origin of the continental slope as an accretionary wedge. A composition of the margin wedge similar to the ophiolitic rocks found onshore on Nicoya Peninsula appears likely. A number of morphological furrows observed in the landward trench slope opposite the seamount covered domain of the oceanic crust coincide with magnetic anomalies. Three‐dimensional magnetic modeling demonstrates that these anomalies can be explained well by subducting seamounts. This implies that the seamounts remain intact, at least magnetically, for a long time interval during the subduction process. Thus they may indeed be important seismogenic asperities.
Frictional heating on faults: Stable sliding versus stick slip
Authors: Brown Stephen R.
Published: February 2004

Laboratory experiments were performed where heat generation was measured during both stable sliding and stick‐slip friction. For a given average slip velocity, stick‐slip sliding produces heat at a considerably lower rate than stable sliding without appealing to high pore pressure or lubrication. These experiments show that faults can be apparently both “strong” when slip initiates and “weak” during rapid sliding, suggesting that different mechanisms operate under the two sliding conditions. If these results apply to the Earth, then all seismogenic faults should produce less heat than creeping fault zones. Possible explanations are that during rapid sliding, in the slip portion of a stick‐slip cycle, either the friction coefficient or the normal stress is reduced by a factor up to about 7. Comparison with other experimental results suggests the reduction of normal stress by interface separation waves is the most likely explanation.
The excitation and characteristic frequency of the long‐period volcanic event: An approach based on an inhomogeneous autoregressive model of a linear dynamic system
Authors: Nakano Masaru , Kumagai Hiroyuki , Kumazawa Mineo , Yamaoka Koshun , Chouet Bernard A.
Published: February 2004

We present a method to quantify the source excitation function and characteristic frequencies of long‐period volcanic events. The method is based on an inhomogeneous autoregressive (AR) model of a linear dynamic system, in which the excitation is assumed to be a time‐localized function applied at the beginning of the event. The tail of an exponentially decaying harmonic waveform is used to determine the characteristic complex frequencies of the event by the Sompi method. The excitation function is then derived by operating an AR filter constructed from the characteristic frequencies to the entire seismogram of the event, including the inhomogeneous part of the signal. We apply this method to three long‐period events at Kusatsu‐Shirane Volcano, central Japan, whose waveforms display simple decaying monochromatic oscillations except for the beginning of the events. We recover time‐localized excitation functions lasting roughly 1 s at the start of each event and find that the estimated functions are very similar to each other at all the stations of the seismic network for each event. The phases of the characteristic oscillations referred to the estimated excitation function fall within a narrow range for almost all the stations. These results strongly suggest that the excitation and mode of oscillation are both dominated by volumetric change components. Each excitation function starts with a pronounced dilatation consistent with a sudden deflation of the volumetric source which may be interpreted in terms of a choked‐flow transport mechanism. The frequency and Q of the characteristic oscillation both display a temporal evolution from event to event. Assuming a crack filled with bubbly water as seismic source for these events, we apply the Van Wijngaarden‐Papanicolaou model to estimate the acoustic properties of the bubbly liquid and find that the observed changes in the frequencies and Q are consistently explained by a temporal change in the radii of the bubbles characterizing the bubbly water in the crack.
An analysis of the ice‐sheet surface and subsurface topography above the Vostok Station subglacial lake, central East Antarctica
Authors: Siegert Martin J. , Ridley Jeffrey K.
Published: February 2004

Radio‐echo sounding (RES) and satellite radar‐altimeter data were analyzed in order to identify the surface and subsurface topography around the largest known subglacial lake, at Vostok Station, East Antarctica (named here as Lake Vostok). In doing so, a data set was established from which a qualitative description of the flow of ice across the lake was developed. Patterns of ice flow were constructed through consideration of (1) the ice‐sheet surface from the satellite altimeter and (2) internal layering from RES. It is concluded that Lake Vostok influences the dynamics of the overriding ice. Specifically, although the flow of ice across the lake is dominated by the general eastward flow of the grounded ice sheet, a southward velocity component, caused by an ice‐shelf‐type ice‐flow mechanism, is also present over the subglacial lake. A further 67 smaller subglacial lakes exist within central regions of the Antarctic Ice Sheet, occupying 5–10% of the ice‐sheet base. Subglacial lakes may therefore exert a significant control on ice dynamics within central Antarctica.
Oxygen isotope ratios in authigenic magnetites from the Belden Formation, Colorado
Authors: Ripperdan Robert L. , Riciputi Lee R. , Cole David R. , Elmore R. Douglas , Banerjee Sanjay , Engel M. H.
Published: February 2004

Authigenic magnetites from the Belden Formation at Bassam Park, Colorado, were analyzed for their 18O/16O composition using secondary ion mass spectrometry. The δ18O values of magnetite‐rimmed pyrite (MagPy) grains averaged −8.5‰ (versus VSMOW). Small (25–50 μm) iron oxide grains had a range of δ18O values from −9.7‰ to −5.5‰, consistent with a variably hematized magnetite population having a δ18O value of around −9‰ for magnetite. The δ18O results suggest that the magnetite rims and grains formed in contact with a water having δ18O of near 0‰ or less, implying a meteoric or connate source rather than a highly evolved orogenic or basinal fluid. This supports paleomagnetic results that suggest the formation of authigenic magnetites under Mesozoic burial conditions at some Belden Formation localities, rather than in response to orogenic fluid migration driven by Early Tertiary tectonic uplift. The δ18O results also support the observation that chemical remanent magnetizations related to authigenic magnetite formation cannot be universally applied to the dating of fluid migration events associated with orogenic deformation.
Morphologic dating of scarps formed by repeated slip events along the San Andreas Fault, Carrizo Plain, California
Authors: Arrowsmith J. Ramón , Rhodes Dallas D. , Pollard David D.
Published: February 2004

Morphologic dating of fault scarps determines late Cenozoic fault activity by comparing observed topographic profiles with those determined using a calibrated hillslope development model. We postulate that the material transport rate along the profile is a function only of local slope and is transport‐limited. Morphologic dating of hillslopes bounded by continuously dropping boundaries or cut by continuously slipping faults is used to determine the material transport rate constant, the time since the downdrop was initiated, or the fault began to slip. We calibrated the hillslope development model on the southwest facing scarp southeast of Wallace Creek along the San Andreas Fault (SAF) in the Carrizo Plain, California. The scarp has been exposed by right‐lateral offset of a southeast sloping shutter ridge located on the southwest side of the SAF and by vertical offset related to secondary deformation. We assume that all the observed offset occurs after initial exposure of the scarp by passage of the shutter ridge. Forward modeling of profile development yielded a κ (mass diffusivity) of 8.6±0.8 m2 kyr−1. Normal fault slip rates were determined for two graben‐bounding faults in the Northern Elkhorn Hills in the southeastern Carrizo Plain by applying the calibrated κ to the degradation of the scarps to determine the scarp age. One fault scarp began to form about 12 kyr ago and the other about 63 kyr ago. Those ages and estimates of the dip slip along the faults result in slip rates of 1–2 mm yr−1.
Thermochemistry and phase equilibria of hydrous phases in the system MgO‐SiO2‐H2O: Implications for volatile transport to the mantle
Authors: Bose Kunal , Navrotsky Alexandra
Published: February 2004

Calorimetric, phase equilibrium and in situ equation of state data for antigorite, talc, and dense hydrous magnesium silicate (DHMS) phases have been obtained to construct a self‐consistent thermodynamic data set that ensures internal consistency, well‐behaved extrapolation and physically reasonable values of ΔH°, S°, V°, K, K′ and α for talc, antigorite, and hydrous phase A. Used in conjunction with temperature profiles of subducting oceanic lithosphere, the calculated phase equilibria show that there is no barrier to subducting substantial amounts of water to depth of 400–600 km in colder slabs, since the slab can remain in the stability field of hydrous phases throughout its descent. Once water is brought to these depths, it can probably diffuse into hotter mantle regions via hydrous β‐Mg2SiO4 or by other mechanisms, indicating that the transition zone is unlikely to be dry.
Response time of glaciers as a function of size and mass balance: 1. Theory
Authors: Bahr David B. , Pfeffer W. Tad , Sassolas Christophe , Meier Mark F.
Published: February 2004

A simple interpretation of the traditional definitions of glacier and ice sheet response time (e.g., thickness divided by mass balance rate,
) suggests that larger glaciers respond more slowly than small glaciers to a perturbation in climate. However, with reasonable choices for mass balance behavior, a scaling analysis shows that the response time of valley glaciers decreases as a function of increasing size when other variables are held constant. In essence, this is because larger valley glaciers push further into the ablation zone, and ablation increases more rapidly than the thickness (so that
gets smaller). Ice sheets have different mass balance regimes than valley glaciers, and as they grow larger, the ablation does not increase faster than the thickness. Therefore, as ice sheets grow in surface area, the response time increases. For both ice sheets and valley glaciers, the response time also depends on a mass balance index, which is defined as the slope of the balance curve as a function of horizontal distance along the glacier surface (rather than elevation). The response time decreases as the balance index increases, so for valley glaciers, an increase in the balance index and an increase in glacier size have opposite effects on the response time. The balance index is typically larger for smaller valley glaciers. Therefore a small glacier will typically respond faster than a large glacier, but this quicker response is because of mass balance considerations and not because of the dynamic characteristics of the glacier arising from its small size, as often assumed.
Landsliding and the evolution of normal‐fault‐bounded mountains
Authors: Densmore Alexander L. , Ellis Michael A. , Anderson Robert S.
Published: February 2004

Much of the tectonic and climatic history in high‐relief regions, such as the mountains of the western U.S. Basin and Range province, is contained in the morphology of hillslopes, drainage networks, and other landforms that range in scale from 10−1 to 101km. To understand how these landforms evolve, we have developed a numerical landscape evolution model that combines a detailed tectonic displacement field with a set of physically based geomorphic rules. Bedrock landsliding, long recognized as a significant geomorphic process in mountainous topography, is for the first time explicitly included in the rule set. In a series of numerical experiments, we generate synthetic landscapes that closely resemble mountainous topography observed in the Basin and Range. The production of realistic landscapes depends critically on the presence of bedrock landslides, and landsliding yields rates of long‐term erosion that are comparable in magnitude to those of fluvial erosion. The erosive efficiency of bedrock landsliding implies that hillslopes may respond very quickly to changes in local base level and that fluvial erosion is the rate‐limiting process in steady state experimental landscapes, Our experiments generate power law distributions of landslide sizes, somewhat similar to both field and laboratory observations. Thus even a simple model of bedrock landsliding is capable of quantitatively reproducing mountainous topography and landslide distributions and represents a significant step forward in our understanding of the evolution of normal‐fault‐bounded ranges.
Mapping and modeling of radial fracture patterns on Venus
Authors: Koenig Elissa , Pollard David D.
Published: February 2004

Synthetic aperture radar images indicate numerous radial fracture systems on Venus. The formation of these systems has been attributed either to surface deformation related to subsurface dike emplacement or to fracturing caused by domical uplift above an ascending magma plume. We evaluate the dike emplacement hypothesis by investigating the relationship between the process of subsurface dike emplacement and the geometric attributes of radial fracture patterns, assuming the surface fracture patterns correspond to the patterns of dikes about a magma chamber below the volcanic summit. We select a radial fracture system for mapping and model its formation using a two‐dimensional analysis which idealizes the chamber as a pressurized hole in an elastic plate subject to a remote biaxial stress field. Radial fractures correspond to maximum principal stress trajectories, and the pattern of these trajectories is controlled primarily by four variables: the chamber radius, the magma pressure, the remote differential stress, and the remote principal stress direction. We find that stress trajectories also depend on the remote mean stress and that the location of isotropic points in the trajectory pattern can be used to interpret the mean stress state. These models are used to estimate the remote loading conditions under which dikes were emplaced. The mapped fracture pattern is consistent with dikes emplaced under remote differential stress conditions ranging from 2 to 8 MPa, calculated over a range of possible emplacement depths from 1 to 9 km. This is consistent with regional stress states on Earth.
The thermal structure of collisional orogens as a response to accretion, erosion, and radiogenic heating
Authors: Huerta Audrey D. , Royden Leigh H. , Hodges Kip V.
Published: February 2004

Thermal models of collisional orogens generally predict temperature structures that are much cooler than those recovered by thermobarometric studies. Here we demonstrate that high‐temperature, low‐pressure metamorphism and the development of inverted geotherms within collisional belts may be the result of accretion and erosion acting on crust enriched with heat‐producing elements. A new two‐dimensional finite difference model, described here, incorporates the subduction of lithosphere with heat‐producing material in the upper crust, accretion of crustal material from the subducting plate to the upper plate, and surface erosion of the upper plate. These processes result in the development of a wedge of heat‐producing material within the upper plate. The rate of heat production within the wedge and maximum depth of the wedge are the most important parameters controlling the magnitude of upper plate temperatures. Our model yields inverted upper plate geotherms when heat production rates exceed 0.75 μW/m3 and the heat‐producing wedge extends to a depth greater than 35 km. Temperatures in excess of 500°C at depths of 20–30 km are computed when heat production rates are greater than ∼1.75 μW/m3 and the wedge extends to a depth >50 km. Other processes, such as shear heating, fluid flow, or mantle delamination, need not be invoked to explain geologic evidence of high temperatures or inverted thermal gradients in collisional systems.
Numerical models of magma chambers in the Oman ophiolite
Authors: Chenevez Jérôme , Machetel Philippe , Nicolas Adolphe
Published: February 2004

Long standing studies conducted in the Oman ophiolite have led to models of fast oceanic spreading centers which in some aspects contradict the results of geophysical studies carried out along the East Pacific Rise (EPR); this is true in particular for the shape, size and internal magmatic flow of magma chambers, thus justifying numerical modeling. Our modeling, founded on the equations of fluid mechanics, is based on a two‐dimensional finite difference method that takes into account high viscosity contrasts at the walls of the magma chamber, boundary conditions deduced from the geophysical data of the EPR, and geological observations from the Oman ophiolite. The model predicts a magma chamber shape consistent with field studies. Whether the magma feeding is from the top by the melt lens or from the bottom by basaltic sills, it has only a weak influence on the shape of the chamber. The viscosity of the magma plays a crucial role for the setting of convection in the chamber. According to geological observations which argue against the existence of convective flow patterns, the viscosity of the gabbroic mush within the chamber must exceed 1014–1015 Pa s, depending on the oceanic plate velocity. The flow pattern in the magma chamber and the associated foliation pattern rely critically on whether the underlying upwelling mantle is passive (foliations oriented inward) or active (foliations oriented outward). The active structural analysis in the Oman model is supported by ophiolite data.
A fractal model for physical properties of porous rock: Theoretical formulations and application to elastic properties
Authors: Spangenberg Erik
Published: February 2004

Besides the mineralogical composition and porosity the elastic properties of porous rock are strongly influenced by the rock microstructure (pore shapes, pore size distribution, grain‐to‐grain contacts, etc.). Quantitative microstructural models can provide an important contribution to achieve a fundamental understanding of the relationship between microscopic rock structures and macroscopic rock properties which is crucial for the interpretation of seismic velocity data. The conceptual model presented here provides the possibility of considering different geometries of the pore canals as well as the influences of different grain‐to‐grain contacts. The main difference to other structural rock models is the use of a fractal approach for the modeling of the grain‐to‐grain contacts. This approach results in a discrete pore size distribution and an enlarged internal surface. The contact conditions in the model are characterized by a contact parameter which varies between 1 for a pore free rock and 0 for a suspension. The application of this model to the elastic properties allows the calculation of velocities as a function of porosity and degree of contact. The model may be either isotropic concerning the velocities or anisotropic, up to an orthorhombic anisotropy. Model calculations concerning the influence of contact cementation on the elastic velocities are in good agreement with other experimental and theoretical investigations. When combining the model withGassmann's[1951] theory, it is possible to derive and compare the predictions for high‐ and low‐frequency velocities in the case of fluid saturation for rocks with different microstructures.
Role of faults, nonlinear rheology, and viscosity structure in generating plates from instantaneous mantle flow models
Authors: Zhong Shijie , Gurnis Michael , Moresi Louis
Published: February 2004

Concentrated strain within plate margins and a significant toroidal component in global plate motion are among the most fundamental features of plate tectonics. A significant proportion of strain in plate margins is accommodated through motion on major tectonic faults. The decoupling influence of faulted plate margins primarily results from history‐dependent lithospheric deformation rather than from instantaneous stress‐weakening rheologies. For instantaneous mantle flow models, we argue that faults should be treated as preexisting mechanical structures. With models incorporating preexisting faults, a power law rheology with an exponent of 3, and slab pull and ridge push forces, we demonstrate that nonlinear interaction between weak faults and this power law rheology produces plate‐like motion. Our models show that in order to produce plate‐like motion, the frictional stress on faults needs to be small and the asthenosphere viscosity should be much weaker than that of lithosphere. While both plateness and the ratio of toroidal to poloidal velocities are reduced with increasing fault coupling, the viscosity contrast between the lithosphere and asthenosphere only influences plateness. This shows that both diagnostics, plateness and the ratio of toroidal to poloidal velocities, are necessary to characterize plate motion. The models demonstrate that weak transform faults can guide plate motion. This guiding property of transform faults and the decoupling of thrust faults result in oblique subduction where the strike of subducted slabs is oblique to transform faults. Subducted slabs beneath a dipping fault produce oceanic trench and fore bulge topography and principal stresses consistent with subduction zone observations.
Further structural constraints and uncertainties of a thin laterally varying ultralow‐velocity layer at the base of the mantle
Authors: Garnero Edward J. , Helmberger Donald V.
Published: February 2004

Constraints and uncertainties are presented for modeling of an ultralow‐velocity zone layer (ULVZ) at the base of Earth's mantle using an SKS wave with small segments of P wave diffraction at the SKS core entry and exit locations, called SPdKS. Source or receiver effects are ruled out as causes for the SPdKS anomalies used to map ULVZ structure, since systematic SPdKS‐SKS travel time moveout behavior is present in profiles of recordings of a given earthquake at many seismographic stations and also for many events recorded at one station. The southwest Pacific region produces strong variability in observed SPdKS/SKS amplitude ratios (compared to synthetic seismograms), which geographically corresponds to an anomalous ULVZ region. Accurate determination of absolute ULVZ thicknesses requires knowledge of, in addition to magnitude of P wave velocity (Vp) reduction in the layer, the magnitude of S wave velocity (VS) reduction and density (ρ) perturbation (if any). Synthetic seismogram experiments demonstrate several key points regarding uncertainties and constraints in modeling ULVZ structure: (1) thicker layers (up to 300 km thick) with mild reductions (e.g., −2.5 to −5.0%) cannot reproduce the anomalous SPdKS behavior seen in the data; (2) for ULVZ layers less than 10 km thick, strong trade‐offs exist between discontinuous velocity reductions and linear gradient reductions over a thicker zone; (3) uncertainties preclude precise determination of magnitude of δVP and δVS reductions, as well as the δVS:δVP ratio; (4) large density increases within the ULVZ (e.g., up to 60% and more) can efficiently broaden and delay the peak of the energy that we identify as SPdKS for models with strong velocity reductions in the layer; (5) models with extreme Q reductions in the ULVZ can affect SPdKS waveforms, and dampen spurious ringing energy present in Sd waveshapes due to the ULVZ; and (6) the minimum required Vp reduction for the most anomalous data (around −10%) trades off with thinner ULVZ structures containing larger velocity reductions (with possible density increases as well).
Crust and upper mantle P wave velocity structure beneath Valles Caldera, New Mexico: Results from the Jemez teleseismic tomography experiment
Authors: Steck Lee K. , Thurber Clifford H. , Fehler Michael C. , Lutter William J. , Roberts Peter M. , Baldridge W. Scott , Stafford Darrik G. , Sessions Robert
Published: February 2004

New results are presented from the teleseismic component of the Jemez Tomography Experiment conducted across Valles caldera in northern New Mexico. We invert 4872 relative P wave arrival times recorded on 50 portable stations to determine velocity structure to depths of 40 km. The three principle features of our model for Valles caldera are: (1) near‐surface low velocities of −17% beneath the Toledo embayment and the Valle Grande, (2) midcrustal low velocities of −23% in an ellipsoidal volume underneath the northwest quadrant of the caldera, and (3) a broad zone of low velocities (−15%) in the lower crust or upper mantle. Crust shallower than 20 km is generally fast to the northwest of the caldera and slow to the southeast. Near‐surface low velocities are interpreted as thick deposits of Bandelier tuff and postcaldera volcaniclastic rocks. Lateral variation in the thickness of these deposits supports increased caldera collapse to the southeast, beneath the Valle Grande. We interpret the midcrustal low‐velocity zone to contain a minimum melt fraction of 10%. While we cannot rule out the possibility that this zone is the remnant 1.2 Ma Bandelier magma chamber, the eruption history and geochemistry of the volcanic rocks erupted in Valles caldera following the Bandelier tuff make it more likely that magma results from a new pulse of intrusion, indicating that melt flux into the upper crust beneath Valles caldera continues. The low‐velocity zone near the crust‐mantle boundary is consistent with either partial melt in the lower crust or mafic rocks without partial melt in the upper mantle. In either case, this low‐velocity anomaly indicates that underplating by mantle‐derived melts has occurred.
A two‐dimensional tomographic study of the Clipperton transform fault
Authors: Van Avendonk Harm J. A. , Harding Alistair J. , Orcutt John A. , McClain James S.
Published: February 2004

From the marine refraction data recorded on five instruments during the Clipperton Area Seismic Survey to Investigate Compensation (CLASSIC) experiment in 1994 we construct a compressional velocity model for a 108 km long profile across the Clipperton transform. We apply a new seismic tomography code that alternates between ray tracing and linearized inversions to find a smooth seismic velocity model that fits the observed refraction travel times. The solution to the forward ray‐tracing problem is a hybrid of the graph (or shortest path) method and a ray‐bending method. The inversion is performed with least squares penalties on the data misfit and first derivatives of the seismic structure. Starting with a one‐dimensional compressional velocity model for oceanic crust, the misfit in the normalized travel time residuals is reduced by 96%, decreasing the median travel time residual from 110 to 25 ms. The compressional velocity structure of the Clipperton transform is characterized by anomalously low velocities, about 1.0 km/s lower than average, beneath the median ridge and parallel troughs of the transform domain. The low compressional velocities can be explained by an increased porosity due to fracturing of the oceanic crust. We found crustal thicknesses of 5.6–5.9 km under the transform fault to produce the best fit of the PmP phase arrivals and Pg/Pn crossovers. Since the crust is not thin beneath the transform parallel troughs and the velocity anomaly is not confined to the median ridge, we find uplift by serpentinite diapirs unlikely as an explanation for the relief of the median ridge. A median ridge that is the result of brittle deformation due to compression across the transform domain is, however, compatible with our results. The upper crust is thicker to the north of the transform than to the south, which is likely a consequence of the contrast in temperature structure of these two spreading segments.
Rock deformation and fracturing processes due to nonlinear shock waves propagating in hyperthermal fluid pressurized domains
Authors: Natale G. , Salusti E. , Troisi A.
Published: February 2004

An analytical model for rock deformation and fracturing processes due to nonlinear waves of temperature and pressure and evolving with particularly large velocities in a fluid‐saturated rock over a hyperthermal aquifer is here discussed. As in classical studies by Rice and Cleary [1976], McTigue [1986], and also Bonafede [1991] and Natale and Salusti [1996], the upper fluid‐saturated matrix is considered homogeneous, thermoelastic, and isotropic in a one‐dimensional formulation. As regards the nonlinear wave generation mechanism, at the boundary between the two horizons the hot fluid from below is forced upward by a pressure gradient, thus giving rise to strong pressure and temperature perturbations. In such a context, in order to schematize wave‐induced rock deformation and fracturing processes we assume a continuous temperature and pressure dependence of model parameters such as fluid diffusivity and thermal expansivity of the rock and fluid. The solution we obtain is a strong shock wave corresponding to nonlinear fluid migration mechanisms, in turn enhanced by the flow through the induced fractures in the overlying rock. It has to be stressed that once such a process starts off, it may be amplified and also may produce rather catastrophic behavior in the natural system. This model essentially provides a theoretical tool for interpreting such geophysical effects, in particular for volcanic complexes such as the Rabaul caldera, Long Valley caldera, Campi Flegrei, the island of Vulcano, or similar kinds of geological system.
Strain constraint on the source of the alleged Varotsos‐Alexopoulos‐Nomicos (VAN) precursor of the 1995 Aigion earthquake (Greece)
Authors: Pinettes Patrick , Bernard Pascal , Artru Juliette , Blum Pierre‐Antoine , Verhille René , Milas Paraskevas , Veis George
Published: February 2004

We present the data continuously monitored at the Institut de Physique du Globe de Paris/National Technical University of Athens Galaxidi geophysical observatory before the 1995 Aigion earthquake (Greece) and analyze their implications for the source of the alleged Varotsos‐Alexopoulos‐Nomicos (VAN) precursor (seismic electric signals (SES)) of this earthquake. On April 30, 1995, an electrotelluric anomaly of 10 mV km−1 was observed at the VAN station of Volos, located 150 km NE of Aigion. The VAN group attributed it to the preparation for the 1995 Aigion earthquake. The same day, the N‐S tiltmeter of the Galaxidi observatory, located 30 km NE of Aigion, did not detect any tilt above its noise level, 7.5 × 10−8 rad. Assuming all conditions which provide the greatest electric signal at Volos but the smallest local deformation, we model the characteristics of the source that is required to generate the SES observed at Volos with a N‐S tilt at Galaxidi compatible with our observations. We suppose, as is usually done for possible earthquake precursors, that the electrotelluric signal results from streaming potentials induced in response to the pore fluid flow accompanying an aseismic creep event near the hypocenter of the upcoming earthquake. We also suppose that the electrical structure between Aigion and Volos is composed of a favorable two‐dimensional (2‐D) horizontal conductive layer at midcrustal depth, excluding a local amplification at Volos (the SES appeared with comparable ΔV/L values on both the short and long measurement lines), as well as a highly speculative ad hoc 1‐D channel between Aigion and Volos (as proposed by VAN). With these assumptions, we find that a coupling coefficient greater than 500 V MPa−1 would explain the reported anomaly. Typical values of this coefficient measured in the laboratory are of the order of 100 mV MPa−1 The source of the SES of April 30, 1995, is hence very unlikely to have been located in the hypocentral zone of the 1995 Aigion earthquake, whatever its actual link with the earthquake.