bluuebunny/crossref_metadata_2025 huggingface dataset:
Lower crustal flow in an extensional setting: Constraints from the Halloran Hills region, eastern Mojave Desert, California
Authors: Kaufman Peter S. , Royden Leigh H.
Published: February 2004

Postextensional uplift and tilting of Quaternary lake sediments in the Halloran Hills, eastern Mojave desert, California, suggest that regional‐scale flow within the lower crust is occurring in response to tectonic loads created during upper crustal extension in late Miocene time. The concept that significant flow within the lower crust may occur for millions of years after the cessation of extension indicates that topographic gradients can be modified by regional‐scale flow within the lower crust and that direct constraints on lower crustal flow can be obtained by quantitative analysis of rates of postextensional surface tilting. An analytic solution, developed in this paper, combines viscous flow in the lower crust, flexural bending within the midcrust, and isostasy into an invertible linear differential equation that describes the topographic response to upper crustal extension. Assuming that there was no topographic relief across the Halloran Hills region prior to extension, inversion of modern topographic data indicates that crustal thinning above the Halloran Hills detachment increases westward with a listric geometry and that little crustal thinning has occurred east of the western flank of Clark Mountain, in good agreement with the known geometry of the fault and the location of its breakaway zone. Incorporation of Quaternary tilting data indicates that the modern viscosity beneath the Halloran Hills (assuming a 10‐km lower crustal channel) is a maximum of 1019 Pa s and that the viscosity of the lower crust has decreased by at least 1 to 2 orders of magnitude since 8 Ma. This corresponds to a temperature increase of at least 75° to 100°C at the Moho. In our opinion, the most likely source of this temperature increase is a regional‐scale thermal event within the underlying mantle and diffusion of heat upward into the lower crust. If correct, these results have important implications for the way in which crustal extension is linked to mantle heating within the Basin and Range Province.
New geomorphic data on the active Taiwan orogen: A multisource approach
Authors: Deffontaines B. , Lee J.‐C. , Angelier J. , Carvalho J. , Rudant J.‐P.
Published: February 2004

A multisource and multiscale approach of Taiwan morphotectonics combines different complementary geomorphic analyses based on a new digital elevation model (DEM), side‐looking airborne radar (SLAR), and satellite (SPOT) imagery, aerial photographs, and control from independent field data. This analysis enables us not only to present an integrated geomorphic description of the Taiwan orogen but also to highlight some new geodynamic aspects. Well‐known, major geological structures such as the Longitudinal Valley, Lishan, Pingtung, and the Foothills fault zones are of course clearly recognized, but numerous, previously unrecognized structures appear distributed within different regions of Taiwan. For instance, transfer fault zones within the Western Foothills and the Central Range are identified based on analyses of lineaments and general morphology. In many cases, the existence of geomorphic features identified in general images is supported by the results of geological field analyses carried out independently. In turn, the field analyses of structures and mechanisms at some sites provide a key for interpreting similar geomorphic features in other areas. Examples are the conjugate pattern of strike‐slip faults within the Central Range and the oblique fold‐and‐thrust pattern of the Coastal Range. Furthermore, neotectonic and morphologic analyses (drainage and erosional surfaces) have been combined in order to obtain a more comprehensive description and interpretation of neotectonic features in Taiwan, such as for the Longitudinal Valley Fault. Next, at a more general scale, numerical processing of digital elevation models, resulting in average topography, summit level or base level maps, allows identification of major features related to the dynamics of uplift and erosion and estimates of erosion balance. Finally, a preliminary morphotectonic sketch map of Taiwan, combining information from all the sources listed above, is presented.
Controls on trench topography from dynamic models of subducted slabs
Authors: Zhong Shijie , Gurnis Michael
Published: February 2004

A finite element method with constrained elements and Lagrange multipliers is used to study tectonic faults in a viscous medium. A fault, representing the interface between overriding and subducting plates, has been incorporated into a viscous flow model of a subduction zone in which both thermal buoyancy and the buoyancy associated with the phase change from olivine to spinel are included. The fault causes stress to concentrate in its vicinity, giving rise to a weak plate margin and a mobile plate if a power law rheology is used. Surface dynamic topography with either a Newtonian or a power law rheology and with typical subduction zone parameters is characterized by a narrow and deep trench and a broadly depressed back arc basin. This suggests that oceanic trenches and back arc basins over subduction zones are dynamically compensated by viscous flow. Our models show that trench depth increases with fault dip angle, slab dip angle, slab length, and age of oceanic lithosphere just prior to subduction. The influence of fault dip angle and age of lithosphere on trench depth is greater than the influence of slab dip angle and slab length. These relationships of trench depth versus subduction zone parameters explain well the statistics of observed trench depths. For those subduction zones with oceanic lithosphere on both sides of the trench, observed trench depths have been corrected for fault and slab dip angles, based on the relationships from the dynamic models. After correction to a common set of parameters, trench depth correlates linearly with age of lithosphere prior to subduction with a slope which is close to what models having high viscosities within the transition zone and lower mantle predict. Comparison between the trench depths, corrected for fault and slab dip angles, and model trench depths suggests that the resisting tangential stress on faults in subduction zones may range from 15 MPa to 30 MPa, depending on model details.
Ephemeral thermal and solute plumes formed by upwelling groundwaters near salt domes
Authors: Williams Mark D. , Ranganathan Vishnu
Published: February 2004

Geochemical observations made by previous workers indicate that rapid upward advection of hot water has occurred along the flanks of some salt domes in the Northern Gulf of Mexico Basin and in the Danish Central Trough. The observations are based on pore fluid chemistry and on high temperatures of formation of metal sulfides and other diagenetic minerals near the crests of salt domes. Some of the diagenetic minerals formed at 40°C to 80°C above the ambient temperature of the host rocks. Numerical simulations of coupled heat and dissolved salt transport show that thermal perturbations in excess of 60°C can be repeatedly produced near the crest of a salt dome by focused dewatering of a thick strongly overpressured sedimentary interval around the dome. In our simulations, dewatering was triggered and stopped by cyclically changing the permeability at the salt‐sediment interface. The thermal plumes dissipated within 100 kyr after dewatering ceased. Stratigraphically shallow brine plumes were also formed by dewatering and these gravitationally unstable plumes dissipated more slowly by sinking within 1 m.y. Simulations showed that free thermohaline convection can also generate thermal perturbations near the crest of a dome. In this mode of upwelling, thermal perturbations reached 30°C when the vertical permeability of the hydrostatically pressured interval was 10−14 m2 (10 mdarcy). Because such permeabilities seem large for the hydrostatically pressured interval as a whole in the Northern Gulf of Mexico Basin, the formation of high‐temperature metal sulfide deposits near the crests of salt domes seems more likely to be due to dewatering of the overpressured interval with very strong focusing of expelled waters along the flanks of salt domes.
On the relationship between dike width and magma viscosity
Authors: Wada Yutaka
Published: February 2004

On the basis of measurements of bulk rock and phenocryst chemical compositions, phenocryst content, and estimation of water content and magma temperature, the magma viscosity of 44 dikes from Oki‐Dozen and Tango, southwest Japan, and Ocros, Peruvian Andes, was estimated to examine a relationship between viscosity and dike width. The results show that magma viscosity increases with dike width: mafic magmas with viscosities of 101–102 Pa s form dikes l m wide, while felsic magmas with viscosities of 106–107 Pa s form dikes 100 m wide. This indicates that magma viscosity as well as magma driving pressure and host rock stiffness are important factors in explaining the variation of dike width. Because magma viscosity is a measure of internal friction, the magmatic force required to widen the fracture during hydrofracturing is proportional to viscosity. On the basis of the range of chemical composition of the studied samples, dike widths formed by highly viscous felsic magma should be of the order of 100 m maximum. Furthermore, the observed basaltic dike width variations from various tectonic settings indicate that the effective factor on driving pressure of dike magma may be not magma pressure but compressive stress. On the other hand, dikes derived from low‐viscosity basaltic magma and exceeding 100 m width, may be formed by extremely high magma pressure associated with flood basalt volcanism. This is supported by (1) the observation that a rifting event with decreased compressive stress was not always associated with the volcanism and (2) the inferred effusion rates for this type of volcanism are anomalously high, suggesting increased magma pressure. In addition, the active magma generation and the decreased host rock stiffness by locally anomalous thermal conditions like a hotspot phenomenon may assist flood basalt volcanism or emplacement of anomalously wide mafic dikes.
The role of plastic deformation in normal loading and unloading cycles
Authors: Yoshioka Naoto
Published: February 2004

Using three‐dimensional topography data of a rough surface, a computer simulation was performed to investigate the effect of plastic deformation on normal loading and unloading cycles. A micromechanical model for elastic‐plastic deformation was proposed and incorporated in the simulation. The model includes the effect of elastic recovery observed in laboratory indentation hardness tests. The results of the simulation show that hysteresis and permanent sets typically observed in laboratory experiments are caused by plastic deformation. Even if the physical properties of the contacting material are of hard rock and the change in surface roughness after the loading and unloading cycles is small, the amount of hysteresis and permanent sets is significantly large. Plastic deformation plays an important role in contact mechanics of rocks.
Rapid three‐dimensional hypocentral determination using a master station method
Authors: Zhou Hua‐wei
Published: February 2004

A master station (MS) method is presented in this paper to rapidly determine hypocenters in three‐dimensional (3‐D) heterogeneous velocities. An equal differential time (EDT) surface is defined as the collection of all spatial points that satisfy the time difference between two arrivals, which can be two picks at two stations or two different phase picks at one station. The EDT surface is independent of the origin time and will contain the hypocenter. For an event with J arrivals, there are (J‐1) independent EDT surfaces. The MS method determines the hypocenter that satisfies two types of constraints: to be traversed by most EDT surfaces and to yield minimum travel time residual statistics. The statistics include both the residual variance and the amplitude of the origin time error. The combined use of the EDT surfaces and residual statistics allows for a unique determination of the hypocenter and origin time using different types of phase arrivals. In principle, only three arrivals are minimally required to constrain a unique solution if three different stations are used. For a 3‐D velocity model, the EDT surfaces and the residual statistics can be computed efficiently using a reference file created by Moser's (1991) ray tracing method. An illustration of the MS method is given for 27 small events that occurred in southern California, using a P wave velocity model modified from that of Magistrale et al. (1992). The average misfit between the bulletin hypocenters and the new solutions is 3.8 km. If a 3‐D velocity model is accurate, the MS method can be a viable means of hypocenter determination.
Hunting for azimuthal anisotropy beneath the Pacific Ocean region
Authors: Yu Yang , Park Jeffrey
Published: February 2004

Lateral variations in azimuthal anisotropy cause significant waveform anomalies in long‐period surface waves (ƒ < 15 mHz, T ≳ 70 s), as a result of coupling between fundamental branch Rayleigh and Love waves. These anomalies, termed “quasi‐Love” (QL) waves, have elliptical polarization, arrive slightly behind the Love wave but prior to the Rayleigh wave, and are observed on many propagation paths in the Pacific Ocean region. Our observations of quasi‐Love waves indicate the existence of strong lateral anisotropic gradients in the western Pacific Ocean, in particular, near Hawaii and seaward of the Tonga‐Kermadec, Kurile and Marianas‐Izu‐Bonin subduction zones. A lack of quasi‐Love generation beneath the Phillipine Sea suggests weak azimuthal anisotropy in the region. In the southwest Pacific Ocean, the long‐period quasi‐Love waveforms recorded at station SNZO (South Karori, New Zealand) can be fit well with a simple anisotropic Earth model with a 90° rotation in the orientation of the fast P velocity axis near the boundary between the Indo‐Australian and the Pacific plates. In this model, 6% P wave velocity anisotropy from the Moho to 210 km depth, with a NNE‐SSW fast direction (roughly parallel to the plate boundary), extends 500–1000 km east of the Tonga‐Kermadec trench, where it shifts to a WNW‐ESE orientation (parallel to fracture zones in the southernmost Pacific plate). This solution is nonunique, however, as a similar lateral variation of 2% S wave anisotropy at asthenospheric depths (100–300 km) generates similar waveform anomalies. We attribute the laterally varying anisotropy to either (1) flow variations in the asthenosphere, consistent with lateral shear detected in deep Tonga‐Kermadec seismicity, (2) variations in fossil spreading direction in the Cretaceous long normal polarity interval, (3) the disturbance of fossil anisotropy caused by the passage of the Louisville Ridge hotspot, and (4) lithospheric compression associated with continental collision along the Alpine Fault. The apparent location of the Love‐to‐Rayleigh “scatterer” favors options 1 and 3. Gradients in azimuthal anisotropy are inferred in the northwestern Pacific, associated with one or more of these mechanisms. A quasi‐Love wave observed on a path from Alaska to PPT (Papeete, Tahiti) is bandlimited to ƒ > 8 mHz, suggesting lateral variations with wavelength twice the spacing of fracture zones in the north central Pacific.
Flexural rigidity of the Basin and Range‐Colorado Plateau‐Rocky Mountain transition from coherence analysis of gravity and topography
Authors: Lowry Anthony R. , Smith Robert B.
Published: February 2004

Stochastic inversion for flexural loads and flexural rigidity of the continental elastic layer can be accomplished most effectively by using the coherence of gravity and topography. However, the spatial resolution of coherence analysis has been limited by use of two‐dimensional periodogram spectra from very large (>105 km2) windows that generally include multiple tectonic features. Using a two‐dimensional spectral estimator based on the maximum entropy method, the spatial resolution of flexural properties can be enhanced by a factor of 4 or more, enabling more detailed analysis at the scale of individual tectonic features. This new approach is used to map the spatial variation of flexural rigidity along the Basin and Range transition to the Colorado Plateau and Middle Rocky Mountains physiographic provinces. Large variations in flexural isostatic response are found, with rigidities ranging from as low as 8.7×1020 N m (elastic thickness Te = 4.6 km) in the Basin and Range to as high as 4.1×1024 N m (Te = 77 km) in the Middle Rocky Mountains. These results compare favorably with independent determinations of flexural rigidity in the region. Areas of low flexural rigidity correlate strongly with areas of high surface heat flow, as is expected from the contingence of flexural rigidity on a temperature‐dependent flow law. Also, late Cenozoic normal faults with large displacements are found primarily in areas of low flexural rigidity, while deformation fronts of Mesozoic/Tertiary overthrusts occur 0 to 100 km east of the low‐rigidity region. The highest flexural rigidity is found within the Archean Wyoming craton, where evidence suggests that deeply rooted cratonic lithosphere may play a role in determining the distribution of tectonism at the surface.
A historical review of metamorphic fluid flow
Authors: Ferry John M.
Published: February 2004

Recognition of the importance of fluid flow in the process of metamorphism was an outgrowth of efforts by petrologists over the last 50–60 years to understand the mineralogical evolution of metamorphic rocks. Mineralogical, chemical, and isotopic data are now routinely used to identify where fluid has flowed in metamorphic terranes, to measure the amount of fluid, to constrain the direction of flow relative to temperature and pressure gradients and lithologic contacts, and to determine the age of flow. Fluid may flow through a static, interconnected network of microscopic tubes at grain corners only under special combinations of mineralogy, fluid composition, pressure, and temperature. Fluid flow typically is restricted to hydraulic fractures whose formation and maintenance require dynamic processes such as increasing temperature, active de volatilization reactions, and/or deformation. Hydraulic fracture flow is heterogeneous in both space and time. Metamorphic fluid transport may be driven by density differences between rock and fluid, by density variations in fluid generated from temperature gradients, by deformation, and by surface tension. Metamorphic fluid flow plays a significant role in heat and mass transfer in Earth's crust and in the mechanisms and rates of deformation.
On the derivation of coseismic displacement fields using differential radar interferometry: The Landers earthquake
Authors: Zebker Howard A. , Rosen Paul A. , Goldstein Richard M. , Gabriel Andrew , Werner Charles L.
Published: February 2004

We present a map of the coseismic displacement field resulting from the Landers, California, June 28, 1992, earthquake derived using data acquired from an orbiting high‐resolution radar system. We achieve results more accurate than previous space studies and similar in accuracy to those obtained by conventional field survey techniques. Data from the ERS 1 synthetic aperture radar instrument acquired in April, July, and August 1992 are used to generate a high‐resolution, wide area map of the displacements. The data represent the motion in the direction of the radar line of sight to centimeter level precision of each 30‐m resolution element in a 113 km by 90 km image. Our coseismic displacement contour map gives a lobed pattern consistent with theoretical models of the displacement field from the earthquake. Fine structure observed as displacement tiling in regions several kilometers from the fault appears to be the result of local surface fracturing. Comparison of these data with Global Positioning System and electronic distance measurement survey data yield a correlation of 0.96; thus the radar measurements are a means to extend the point measurements acquired by traditional techniques to an area map format. The technique we use is (1) more automatic, (2) more precise, and (3) better validated than previous similar applications of differential radar interferometry. Since we require only remotely sensed satellite data with no additional requirements for ancillary information, the technique is well suited for global seismic monitoring and analysis.
Thermal and chemical convection in planetary mantles
Authors: Dupeyrat L. , Sotin C. , Parmentier E. M.
Published: February 2004

Melting of the upper mantle and extraction of melt result in the formation of a less dense depleted mantle. This paper describes series of two‐dimensional models that investigate the effects of chemical buoyancy induced by these density variations. The range of Rayleigh numbers and boundary conditions are appropriate to convection in Earth's upper mantle. A tracer particles method has been set up to follow as closely as possible the chemical state of the mantle and to model the chemical buoyant force at each grid point. Each series of models provides the evolution with time of magma production, crustal thickness, surface heat flux, and thermal and chemical state of the mantle. First, models that do not take into account the displacement of plates at the surface of Earth demonstrate that chemical buoyancy has an important effect on the geometry of convection. A depleted layer ∼100 km thick forms at the top of the mantle. This light stagnant layer reduces the heat transfer and widens the aspect ratio of the cells. Consequently, the mantle cools down more slowly. The flow field is strongly modified by the chemical forces which yield displacements of the partial melting zones and periodic volcanism. Then models include horizontal motion of plates 5000 km wide. Recycling of crust is taken into account. For a sufficiently high plate velocity which depends on the thermal Rayleigh number, the cell's size is strongly coupled with the plate's size. Plate motion forces chemically buoyant material to sink into the mantie. Then the positive chemical buoyancy yields upwelling as depleted mantle reaches the interface between the upper and the lower mantle. This process is very efficient in mixing the depleted and undepleted mantle at the scale of the grid spacing since these zones of upwelling disrupt the large convective flow. At low spreading rates, zones of upwelling develop quickly, melting occurs, and the model predicts intraplate volcanism by melting of subducted crust. At fast spreading rates, depleted mantle also favors the formation of these zones of upwelling, but they are not strong enough to yield partial melting. Their rapid displacement toward the ridge contributes to faster large‐scale homogenization.
Fourier transform IR‐determination of intragranular water content in quartzites experimentally deformed with and without added water in the ductile deformation field
Authors: den Brok Bas , Meinecke Jörg , Röller Klaus
Published: February 2004

To test whether the weakening effect of added water in ductile deformation experiments on quartzite at high P‐T conditions is associated with measurable changes in intragranular water content (cw) we determined Fourier transform infrared (FTIR) spectra of single grains (grain size 150–250 μm IR spot size ∼70 μm) in samples of Dongelberg quartzite experimentally deformed in a Griggs solid‐medium deformation apparatus at a temperature of 800°C, a confining pressure of ∼1200 MPa, and a strain rate of ∼10−7 s−1, both with and without ∼0.4 wt % added water. The added water caused a weakening effect of a factor ≥9. Microstructural observations indicate that the dominant deformation mechanism in the weakened samples was solution‐precipitation creep. The FTIR measurements showed that the starting material has a broad variation in cw as well as in the type of OH absorption. The cw of individual grains range from <200–2900 molar ppm H/Si (measurement resolution: 100–1000 molar ppm H/Si; standard error of average cw values 300–900 molar ppm H/Si). The experimentally deformed samples, whether deformed with or without added water, showed a broadly similar variation in cw with similar types of OH absorption. Relatively “dry” grains (<100–500 molar ppm H/Si) were still present in the samples deformed with added water, as well as relatively “wet” grains (>3000 molar ppm H/Si) in samples deformed without added water. We were unable to demonstrate any significant changes in average cw after either of the treatments. Apparently, the water weakening effects seen in our experiments occurred without measurable changes in cw.
Transdomain thermoremanent magnetization
Authors: Dunlop David J. , Newell Andrew J. , Enkin Randolph J.
Published: February 2004

Transdomain thermoremanent magnetization (TRM) is produced when thermally activated transitions between different domain structures become blocked during cooling. This paper investigates transdomain remanence in the pseudo‐single‐domain size range (<1 μm) in magnetite. We use a one‐dimensional micromagnetic model of single‐domain (SD), two‐domain (2D) and three‐domain (3D) structures to determine the energies of these states, the energy barriers between them, blocking temperatures for SD ↔ 2D and 2D ↔ 3D transitions, and relaxation times for the resulting TRMs. Energy barriers are very high at 20°C, from 2000kT to >6000kT for SD ↔ 2D transitions. Transdomain viscous remanent magnetization (VRM) will not occur, even on geological timescales, by a one‐dimensional excitation such as edge nucleation of a domain wall. Transdomain blocking temperatures, at which energy barriers fall to 25kT–60kT, are ≥553°C for SD ↔ 2D and ≥574°C for 2D ↔ 3D transitions. There are two separate blocking temperatures, e.g. TB1 for SD ↔ 2D and TB2 for 2D ↔ SD transitions. Usually, only the higher of the two has practical significance because the favored (lower energy) state is already 100% populated at this temperature. Our theory is the first to make quantitative predictions of transition paths, relaxation times, and blocking temperatures for transdomain TRM. It is also quite robust. Relaxing the one‐dimensional constraint and introducing crystal defects would make it easier to nucleate domains, but energy barriers and blocking temperatures would not be reduced greatly. Our principal conclusion, that only the lowest energy state at blocking is significantly populated, is a fundamental consequence of the Boltzmann statistics of equilibrium states and is unaffected by the details of transitions between states. Grain interactions may be responsible for the multiplicity of states observed in large titanomagnetite grains following replicate TRM experiments.
Effect of finite extension rate on melt generation at rifted continental margins
Authors: Bown Jonathan W. , White Robert S.
Published: February 2004

We describe a new model for melt generation by mantle decompression during uniform pure shear extension of continental lithosphere at finite rates. As the duration of rifting increases, conductive heat loss from the up welling mantle causes the amount of melt generated beneath a rift to decrease. Melt generation is strongly dependent on rift duration because most melt is generated in the region near the top of the upwelling asthenosphere, where conductive cooling is greatest when the lithosphere is extended at a finite rate. Whereas instantaneous thinning of the lithosphere by a factor of 5 generates more than 2 km of melt as the underlying normal temperature mantle decompresses, rifting over a period of 15 m.y. reduces the melt generation to zero as heat is lost vertically by conduction. If the underlying mantle is initially abnormally hot, at 1400°C or 100°C hotter than normal, then the thickness of melt generated during a 15‐m.y. period of rifting is reduced to only half of the 8.5 km produced by instantaneous stretching. Lateral heat conduction to the adjacent unextended lithosphere further reduces the mantle melting. Temporal variation in the rate of extension also affects melting. We compile results from the “nonvolcanic” rifted continental margins of the North Atlantic and show that they exhibit little synrift magmatism, despite extreme thinning of the continental crust. If rifting had occurred instantaneously, several kilometers thickness of melt should have been generated by such large degrees of lithospheric thinning. We show that rifting lasted typically 15–25 m.y., which from the results of our model is sufficient to suppress completely melt generation from normal potential temperature asthenosphere. The production of abnormally thin oceanic crust immediately adjacent to the rifted continental margins may also reflect significant cooling of the asthenospheric mantle during the long period of continental extension prior to breakup.
Crustal structure across the Brunswick magnetic anomaly, offshore Georgia, from coincident ocean bottom and multi‐channel seismic data
Authors: Lizarralde Daniel , Holbrook W. Steven , Oh Jinyong
Published: February 2004

We present results from a coincident wide‐angle and deep‐penetrating, multichannel reflection seismic experiment conducted on the continental shelf of the Southeast Georgia Embayment. Over 5000 air gun shots were recorded by a 6‐km‐long, towed streamer and six ocean bottom seismic instruments along a ∼250‐km, north‐south profile crossing the Brunswick magnetic anomaly (BMA). These data indicate a transition in seismic properties across the BMA, including higher seismic velocities south of the BMA, particularly in the upper crust (7–15 km depth), and a transition from reflective to transparent crust from north to south. We interpret this transition to indicate an increased mafic content in crust south of the BMA. Magnetic modeling based on our seismic results indicates that the BMA may be explained as an edge effect anomaly of the more mafic upper crust. We suggest that the increased mafic content toward the south is due to rift‐related mafic intrusion. An alternative interpretation is discussed in which the seismic transition across the BMA is related to an Alleghanian suture. Athough there is no direct indication in the data for the association of the BMA and an Alleghanian suture along our profile, such an association cannot be ruled out. The southern portion of the profile crosses the margin hinge zone, where we observe seaward dipping reflectors similar to those observed along the northern Blake Plateau Basin and Carolina Trough hinge zones, which have been interpreted as volcanic sequences. Our seismic data include no evidence for very high velocity (>7.0 km/s) mid‐crust to lower‐crust inboard of the hinge zone such as is observed in the Carolina Trough outboard of the hinge zone. This indicates that the volume of emplaced mafic material changes dramatically across the hinge zone and suggests that rift‐related magmatic processes along the margin were highly focused.
Strain accumulation near Yucca Mountain, Nevada, 1983–1993
Authors: Savage J. C. , Lisowski M. , Gross W. K. , King N. E. , Svarc J. L.
Published: February 2004

Over the decade 1983–1993 the U.S. Geological Survey has measured the deformation of a 50‐km‐aperture trilateration network centered on Yucca Mountain, the proposed disposal site for high‐level nuclear waste in the United States. The network was surveyed in 1983, 1984, and 1993. The average annual principal strain rates are 0.010±0.020μstrain/yr N90°W±24° and −0.009±0.021 μstrain/yr N00°E±24°, indicating no significant strain accumulation. The southeast corner of the network was disturbed on June 29, 1992, by the Little Skull Mountain earthquake (Ms = 5.4), the epicenter of which is about 20 km southeast of the Yucca Mountain site. Using the seismically determined fault plane (dip 54°S55°E), we find that 0.580±0.075 m of normal slip on a 5‐km square rupture surface at a depth of about 8 km provides a good fit to the observed deformation in the southeast corner of the network. The inferred seismic moment is (4.4±0.6)×1017 N m, which compares well with the observed seismic moment of 4.1×1017 N m.
Subducted slabs and the geoid: 1. Numerical experiments with temperature‐dependent viscosity
Authors: King Scott D. , Hager Bradford H.
Published: February 2004

One of the most powerful constraints on mantle viscosity comes from the correlation of the long‐wavelength (degree 4–9) geoid with that predicted by a density model for subducted slabs. The effect of lateral variations of viscosity should be most pronounced at subduction zones, due to the strong effect of temperature on viscosity. An idealized slab model with temperature‐dependent viscosity is considered, with various lateral and vertical viscosity structures, using a two‐dimensional finite element formulation. The viscosity parameterization affects the amplitude of the long‐wavelength geoid anomaly but not the sign of the correlation between the geoid and density anomalies. Depth‐dependent viscosity models with a high‐viscosity lithospheric layer do not completely match the temperature‐dependent (laterally varying) results, suggesting that the rheology of the slab does have an effect on the long‐wavelength surface topography and geoid; however, this affect is minor, suggesting that the radial models of mantle viscosity inferred from surface observables are correct to first order. In contrast, the short‐wavelength features are dramatically affected by the rheology of the region surrounding the downwelling. Further study of the shorter‐wavelength geoid over subduction zones may provide better insight into subduction zone processes.
Why are there no clockwise rotations along the North Anatolian Fault Zone?
Authors: Platzman E. S. , Platt J. P. , Tapirdamaz C. , Sanver M. , Rundle C. C.
Published: February 2004

Mechanical analysis of lithospheric deformation associated with an intracontinental transform fault suggests that it should be associated with a pattern of vertical axis rotations governed by the length of the fault, its displacement, and the mechanical properties of the lithosphere. Paleomagnetic results from two transects across the North Anatolian Fault Zone indicate that there have been no measurable clockwise rotations on a regional scale associated with Neogene dextral strike‐slip motion along the fault. The data come from two groups of volcanic rocks that have been dated by K‐Ar whole rock geochronology and have yielded ages ranging from 45.3±3.1 m.y. to 41.8±1.3 m.y. (Eocene) and 21.7±1.5 m.y. to 7.5±0.2 m.y. (Miocene). The absence of clockwise rotations suggests one of the following explanations. (1) In contrast to the assumptions underlying current mechanical analyses, deformation in the upper crust may not directly reflect the flow in the lithosphere and is localized along a single fault trace. (2) Deformation is distributed, but rotations have been prevented by the presence of elongate fault blocks subparallel to the trace of the main fault. (3) Displacement along the North Anatolian Fault may be too small or the rheologic properties of the lithosphere such that rotations about vertical axes cannot be resolved. Instead of the predicted clockwise rotations, measured declinations show that there has been approximately 30° of anticlockwise rotation of both the Eocene and Miocene volcanic rocks. This suggests that no significant rotation occurred between Eocene and Miocene times and that the anticlockwise rotation occurred either immediately before or concurrent with the Plio‐Pleistocene slip on the North Anatolian Fault.
The Magallanes and Deseado fault zones: Major segments of the South American‐Scotia transform plate boundary in southernmost South America, Tierra del Fuego
Authors: Klepeis Keith A.
Published: February 2004

The 600‐km‐long Magallanes fault zone and the newly discovered 100‐km‐long Deseado fault zone represent large but little understood segments of the South American‐Scotia (SAM‐SCO) transform on the South American continent (∼50–56° S latitude). Kinematic analyses of fault populations near Mount Hope on Tierra del Fuego indicate that these fault zones have accommodated sinistral strike‐slip motion with a probable component of normal slip on vertical‐subvertical faults during Cenozoic time. Regional geological data and overprinting relationships between the strike‐slip/oblique‐slip faults and mid‐Cretaceous to Oligocene contractional structures suggest that sinistral motion in the Mount Hope segments of the two fault zones occurred significantly since ∼60 Ma and dominantly since ∼30 Ma with the most recent episode of activity occurring during the Quaternary. Results from mapping show 20–25 km of cumulative sinistral separation of a thrust contact across a well‐exposed segment of the Magallanes fault zone north and east of Mount Hope. Although the exact amount of displacement accommodated by this segment of the Magallanes fault zone remains uncertain, the 20–25 km of cumulative sinistral separation combined with an estimate of 3 km of vertical separation is consistent with small magnitudes of displacement of the order of tens of kilometers. This result apparently contrasts with predictions based on plate reconstructions that large amounts of strike‐slip displacement (>200 km) have been accommodated internally within southernmost South America since early Cenozoic time. A low magnitude of displacement along the Mount Hope segment of the Magallanes fault zone suggests that (1) strike‐slip motion has been partitioned across the continent since about 30 Ma and/or (2) the Mount Hope segment of the Magallanes fault zone represents a young part of the modern plate boundary that migrated to its present location during the last few million years.
Equations of state and thermodynamic properties of enstatite pyroxenes
Authors: Angel R. J. , Hugh‐Jones D. A.
Published: February 2004

The equations of state of end‐member MgSiO3 low‐clinoenstatite and orthoenstatite have been measured by single‐crystal X ray diffraction to pressures in excess of 8.5 GPa. The volume variation with pressure of low‐clinoenstatite (space group P21/c) is described by a third‐order Birch‐Murnaghan equation of state with the parameters V0 = 31.292(8) cm3 mol−1, KT0 = 111.1(3.3) GPa, and KT0′ = 6.6(1.1). At pressures in excess of 7 GPa the low‐clinoenstatite inverts to a high‐clinoenstatite phase (space group C2/c), with a volume reduction of 0.798(14) cm3 mol−1. This transformation is reversible but with significant hysteresis at room temperature; the low‐clinoenstatite is recovered on decompression to below 5.3 GPa. The volume variation of orthoenstatite (space group Pbca) displays a discontinuity at ∼4 GPa which corresponds to a change in the internal mechanism of compression. The equations of state fitted to data below and above 4 GPa have the parameters: P < 4 GPa, V0 = 31.354(8) cm3 mol−1, KT0 = 95.8(3.0) GPa, and KT0′ = 14.9(2.0) and P > 4 GPa, V0 = 31.23(10) cm3 mol−1, KT0 = 122.8(16.5) GPa, and KT0′ = 5.6(2.9). From these data, and the slopes of the equilibrium phase boundaries, estimates of ΔS, ΔH, and ΔV of the transformations between the three enstatite polymorphs are calculated.
The application of Hart's state variable description of inelastic deformation to Carrara marble at T < 450°C
Authors: Covey‐Crump S. J.
Published: February 2004

An attempt has been made to determine if the material parameters in constitutive equations for inelastic deformation which fully accommodate the deformation history dependence of inelastic properties can be evaluated with sufficient accuracy at elevated confining pressures for such equations to be of interest in characterizing geological materials. Accordingly, a large number of constant displacement rate and load relaxation experiments (or variants thereof) have been conducted on Carrara marble at 200 MPa confining pressure in the temperature range 120 to 400°C, and the results have been interpreted in terms of Hart's state variable description of inelastic deformation. The observed mechanical behavior conforms to Hart's description, with a changeover at about 250°C from deformation rate controlled by dislocation glide controlled processes (as represented by concave upward () load relaxation curves), to rate control by strongly thermally activated processes (as represented by concave downward relaxation curves). All relaxation curves generated at a given temperature may be superposed by a translation in fixed direction in the () plane. This defines a master relaxation curve which represents what any individual relaxation curve would look like if it could be determined over a sufficiently wide range of strain rates. For Concave downward relaxation curves the master curve extends over 9 orders of magnitude of strain rate and, in consequence, substantially eases the problem of extrapolating the laboratory data to geological strain rates. The success of Hart's description in accounting for the mechanical behavior means that it is possible to avoid the steady state approximation both in the experimental evaluation of inelastic flow laws and in the application of those flow laws in geophysical modeling problems. Moreover, it provides a simple method of quantifying the mechanical state of a material and thereby is of potential significance for the problem of correlating microstructural observations with mechanical properties.
Present‐day crustal deformation in southern California
Authors: Johnson Hadley O. , Agnew Duncan Carr , Wyatt Frank K.
Published: February 2004

The effects of laterally homogeneous mantle electrical conductivity have been included in steady. Using an extensive set of precise geodetic measurements, we have developed a detailed picture of present‐day deformation rates in southern California. This large set of measurements, amounting to nearly 2000 repeated distance measurements over the period 1973 to 1991, comes from the U.S. Geological Survey's Geodolite trilateration program, involving their combined Anza, Joshua Tree, and Salton networks. Building on previous results from these data, we are able to present the deformation field as estimates of the rate of horizontal strain accumulation in small four‐station subnetworks of the overall 89‐station network. Using this technique, the spatial details of the 18‐year average strain rate field can be determined. By correlating these spatial details with the tectonics of the region we are able to understand better how deformation is partitioned across this highly complex margin between the Pacific and North American tectonic plates. Some of the more interesting findings of this study are that (1) the vast majority of strain rate estimates show a pattern of nearly pure shear as would be expected in a transcurrent environment, (2) the fastest accumulation of surface strain in southern California is along the San Jacinto Fault west of the Salton Sea, not along the San Andreas Fault, (3) strain accumulation rate along the length of the San Jacinto Fault increases toward the southeast as the fault enters the Imperial Valley, (4) a large area near the southern end of the Salton Sea, where the San Andreas Fault meets the Brawley Seismic Zone, is undergoing areal dilatation, which is in part consistent with the formation of crust at a spreading center, and (5) deformation at the transition zone between the San Andreas Fault and the Eastern California Shear Zone also appears to be the result of crustal spreading.
Tomographic imaging of the shallow crustal structure of the East Pacific Rise at 9°30′N
Authors: Toomey Douglas R. , Solomon Sean C. , Purdy G. M.
Published: February 2004

Compressional wave travel times from a seismic tomography experiment at 9°30′N on the East Pacific Rise are analyzed by a new tomographic method to determine the three‐dimensional seismic velocity structure of the upper 2.5 km of oceanic crust within a 20×18 km2 area centered on the rise axis. The data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber. A careful analysis of source and receiver parameters, in conjunction with an automated method of picking P wave onsets and assigning uncertainties, constrains the prior uncertainty in the data to 5 to 20 ms. The new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models. The nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function, horizontal and vertical smoothing constraints, and prior model assumptions; all constraints applied to model perturbations are normalized to remove bias. We use the tomographic method to reject the null hypothesis that the axial seismic structure is two‐dimensional. Three‐dimensional models reveal a seismic structure that correlates well with cross‐ and along‐axis variations in seafloor morphology, the location of the axial summit caldera, and the distribution of seafloor hydrothermal activity. The along‐axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle‐derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length. The seismic data are compatible with a 300‐ to 500‐m‐thick thermal anomaly above a midcrustal melt lens; such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years. Asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis. These anomalies may be the result of off‐axis magmatism; alternatively, the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling.
Partial melting of apatite‐bearing charnockite, granulite, and diorite: Melt compositions, restite mineralogy, and petrologic implications
Authors: Beard James S. , Lofgren Gary E. , Sinha A. Krishna , Tollo Richard P.
Published: February 2004

Melting experiments (P = 6.9 kbar, T = 850–950°C, NNO < ƒ02 < HM) were done on mafic to felsic charnockites, a dioritic gneiss, and a felsic garnet granulite, all common rock types in the Grenville basement of eastern North America. A graphite‐bearing granulite gneiss did not melt. Water (H2O+ = 0.60 to 2.0 wt %) is bound in low‐grade, retrograde metamorphic minerals and is consumed during the earliest stages of melting. Most melts are water‐undersaturated. Melt compositions range from metaluminous, silicic granodiorite (diorite starting composition) to peraluminous or weakly metaluminous granites (all others). In general, liquids become more feldspathic, less silicic, and less peraluminous and are enriched in FeO, MgO, and TiO2 with increasing temperature. Residual feldspar mineralogy controls the CaO, K2O, and Na2O contents of the partial melts and the behavior of these elements can be used, particularly if the degree of source melting can be ascertained, to infer some aspects of the feldspar mineralogy of the source. K‐feldspar, a common restite phase in the charnockite and granulite (but not the diorite) should control the behavior of Ba and, possibly, Eu in these systems and yield signatures of these elements that can distinguish source regions and, in some cases, bulk versus melt assimilation. Apatite, a common restite phase, is enriched in rare earth elements (REE), especially middle REE. Retention of apatite in the restite will result in steep, light REE‐enriched patterns for melts derived from the diorite and charnockites.