bluuebunny/crossref_metadata_2025 huggingface dataset:
Vertical axis rotation in the Bolivian orocline, South America: 2. Kinematic and dynamical implications
Authors: Lamb Simon
Published: February 2004

An analysis is presented of the kinematic consequences and dynamical causes of paleomagnetically determined rotations about a vertical axis, documented in the central Andes, South America, between 15°S and 25°S. At these latitudes, there is a pronounced bend in the topographic and structural trends, defining the Bolivian orocline. The observed vertical axis rotations during the last 10 Myr are used, in conjunction with observations of horizontal dilatation, to calculate the finite strain and displacement field in a network of triangles spanning the Bolivian orocline, assuming that the rotations represent the rotational component of finite strain. This shows that rotation during the last 10 Myr has accommodated bending of the Bolivian Andes, increasing the curvature of the eastern margin of the Bolivian orocline by 10° to 27°, with a systematic variation in strains around the oroclinal hinge which is consistent with the observed pattern of upper Miocene to recent deformation. Vertical axis rotations, greater than that inferred for regional oroclinal bending, can be explained in terms of simple kinematic models of vertical axis rotation and reverse faulting in margin‐parallel zones of sinistral and dextral shear. The dynamical controls of vertical axis rotation are analyzed using a simple thin sheet model of viscous flow in a narrow channel. This model suggests that, in general, regional bending is controlled by a combination of along‐strike gradients of (1) boundary normal stresses at the subduction zone; (2) bulk lithospheric viscosity; (3) gravitational potential energy (buoyancy stress) contrasts; and (4) width of the fluid‐like channel. If along‐strike gradients of boundary normal stresses and viscosity are small, then the pattern of vertical axis rotation can be explained as a consequence of along‐strike gradients in buoyancy stress for a bulk lithospheric viscosity in the Bolivian Andes >7×1021 Pa s. In detail, the organization of vertical axis rotation and horizontal shortening suggests that there are both horizontal and vertical gradients of velocity and rheology in the lithosphere.
Dynamics of the India‐Eurasia collision zone
Authors: Flesch Lucy M. , Haines A. John , Holt William E.
Published: February 2004

We present simple new dynamic calculations of a vertically averaged deviatoric stress field (over a depth average of 100 km) for Asia from geodetic, geologic, topographic, and seismic data. A first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first‐order contribution of deviatoric stresses associated with stress boundary conditions. This initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere. Absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 MPa. Assuming bulk viscous behavior for the lithosphere, the magnitudes of deviatoric stresses, together with the magnitudes of strain rates inferred from Quaternary fault slip rate and GPS data, yield vertically averaged effective viscosities for Tibet of 0.5–5×1022 Pa s, compared with 1–2.5×1023 Pa s in more rigid areas elsewhere in the region. A forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field. The total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in Tibet; they are consistent with some eastward motion of Tibet and south China lithosphere relative to Eurasia; and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the Tibetan Plateau. Our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100–300 MPa (in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress).
Paleointensity determination on the Late Precambrian Tudor Gabbro, Ontario
Authors: Yu Yongjae , Dunlop David J.
Published: February 2004

Paleointensity determination by the Coe‐modified Thellier method was carried out on the ≈1100 Ma Tudor Gabbro of southern Ontario. The Tudor has a bivectorial natural remanent magnetization (NRM). The C NRM is a Grenvillian uplift thermoviscous remagnetization which overprints the A NRM, the surviving primary thermoremanent magnetization (TRM) of the Tudor intrusion. A sharp junction at ≈530°C between the A and C vectors in laboratory thermal demagnetization matches the peak metamorphic reheating temperature of 480–500°C in nature when the difference in time scales is taken into account. The unblocking temperature distributions of the A NRM and of laboratory TRM also match. A is carried by single‐domain or small pseudo‐single‐domain grains, probably magnetite rods exsolved in plagioclase or pyroxene at temperatures above 580°C. Arai plots of NRM versus partial TRM are linear from 520°C or 530°C to 580°C for 45 specimens of 19 samples from 9 sites. The 45 reliable paleointensity values from these specimens give a virtual axial dipole moment (VADM) of 4.6±0.8 × 1022 A m2 around 1100 Ma. This is about one half of the modern field intensity but is similar to the mean Cretaceous‐Cenozoic intensity and to most Archean and Proterozoic VADMs. Keweenawan rocks, whose ages are within ≈20 Myr of the Tudor Gabbro, have a VADM of 11.4×1022 A m2. The difference between the Tudor and Keweenawan VADMs is similar to differences recorded over similar time intervals in the Cretaceous and seems to represent normal variation in geomagnetic field strength throughout Earth's history.
Constraints on dike propagation from continuous GPS measurements
Authors: Segall Paul , Cervelli Peter , Owen Susan , Lisowski Mike , Miklius Asta
Published: February 2004

The January 1997 East Rift Zone eruption on Kilauea volcano, Hawaii, occurred within a network of continuous Global Positioning System (GPS) receivers. The GPS measurements reveal the temporal history of deformation during dike intrusion, beginning ∼8 hours prior to the onset of the eruption. The dike volume as a function of time, estimated from the GPS data using elastic Green's functions for a homogeneous half‐space, shows that only two thirds of the final dike volume accumulated prior to the eruption and the rate of volume change decreased with time. These observations are inconsistent with simple models of dike propagation, which predict accelerating dike volume up to the time of the eruption and little or no change thereafter. Deflationary tilt changes at Kilauea summit mirror the inferred dike volume history, suggesting that the rate of dike propagation is limited by flow of magma into the dike. A simple, lumped parameter model of a coupled dike magma chamber system shows that the tendency for a dike to end in an eruption (rather than intrusion) is favored by high initial dike pressures, compressional stress states, large, compressible magma reservoirs, and highly conductive conduits linking the dike and source reservoirs. Comparison of model predictions to the observed dike volume history, the ratio of erupted to intruded magma, and the deflationary history of the summit magma chamber suggest that most of the magma supplied to the growing dike came from sources near to the eruption through highly conductive conduits. Interpretation is complicated by the presence of multiple source reservoirs, magma vesiculation and cooling, as well as spatial variations in dike‐normal stress. Reinflation of the summit magma chamber following the eruption was measured by GPS and accompanied a rise in the level of the Pu'u O'o lava lake. For a spheroidal chamber these data imply a summit magma chamber volume of ∼20 km3, consistent with recent estimates from seismic tomography. Continuous deformation measurements can be used to image the spatiotemporal evolution of propagating dikes and to reveal quantitative information about the volcanic plumbing systems.
Crustal evolution over the last 2 m.y. at the Mid‐Atlantic Ridge OH‐1 segment, 35°N
Authors: Hosford Allegra , Lin Jian , Detrick Robert S.
Published: February 2004

We present the crustal and mantle velocity structure along the strike of the eastern rift mountains at 35°N on the Mid‐Atlantic Ridge. These results were obtained by an inversion of ∼1800 Pg/Pn and ∼450 PmP travel times and by gravity modeling. As commonly observed at slow spreading mid‐ocean ridges, thicker crust (9 km) occurs at the segment midpoint, while thinner crust (7 km) is found toward the segment ends. This along strike variation occurs primarily in the lower crust, which is 7 km thick at the segment center and 4–6 km thick at the segment ends. In contrast, the thickness of the upper crust is relatively constant along strike. At the segment ends, relatively low velocities extend for 10–15 km along strike and from the seafloor to 4 km depth. These low velocities may indicate an attenuated melt supply and/or fracturing and alteration within the shallow to mid‐crust. Directly beneath a cluster of three seamounts at the segment center is a region of relatively high velocity (+0.5 km/s) in the mid‐crust. This feature may correspond to a frozen magma chamber that fed the overlying volcanoes. A synthesis of these results with those from two companion experiments along the rift valley and the conjugate flank provide a detailed record of crustal accretion and evolution at this segment. Specifically, the crustal velocity structures of each flank are nearly identical, and they exhibit a thinner and 16% faster upper crust than is observed on axis. The lower crust is remarkably similar in all three settings, except for a low‐velocity body on axis, which is interpreted as a partially molten zone. The maximum crustal thickness is also similar in all three profiles, but north of the segment center, zero‐age crust is nearly 4 km thinner than beneath the eastern flank and 2 km thinner than beneath the western flank. These differences may indicate that segment‐centered mantle upwelling varies on a timescale of ∼2 m.y.
Random sequences of lithologies exposed on the Mid‐Atlantic Ridge
Authors: Mitchell Neil C.
Published: February 2004

Magmatic, tectonic, and sedimentary processes at slow spreading ridges are known to create complex distributions of rock types at the surface that do not reflect the classical model for oceanic crust with horizontally stratified lithologic layers comprising extrusives, sheeted dikes, and gabbro overlying mantle rocks. A database of seafloor lithology observed in >100 submersible dives is analyzed in order to characterize this complexity. As a way of summarizing the relations between units exposed on rock slopes, the transitions between different lithologies are counted to create transition probability tables, which represent the relative likelihood of passing between different rock types. They reveal that extrusives exposed on Atlantic rock slopes crop out above dikes, serpentinite, and gabbro, in order of decreasing, though with similar, likelihood. Tables derived from the occurrences of the different rock types in dredge hauls are similar to the tables derived from dive observations, so the dredges record a similar heterogeneity. It is suggested that slope failure of gabbroic and ultramafic escarpments, implied by sedimentary breccias in off‐axis drill sites, may contribute to the heterogeneity, in addition to the magmatic and tectonic processes that have been discussed extensively by previous workers. Further analysis of the tables reveals that the net effect of tectonic, magmatic, and sedimentary processes is to produce a distribution of rock types that is indistinguishable from a random distribution. This result is derived from the immediate transitions between units and therefore applies to the finescale structure. If oceanic crust is also heterogeneous on a larger scale, however, it could have more general implications for resolving different crustal models. For example, the presence of a sheeted dike layer is critical evidence for continuous seafloor spreading, but its presence or absence in any one location may be an artifact of the heterogeneity or discontinuous exposure. Methods will therefore be needed to characterize large‐scale heterogeneity so that the significance of these large‐scale observations can be assessed.
CO2 degassing and energy release at Solfatara volcano, Campi Flegrei, Italy
Authors: Chiodini G. , Frondini F. , Cardellini C. , Granieri D. , Marini L. , Ventura G.
Published: February 2004

In the present period of quiescence, the Solfatara volcano, 1 km far from Pozzuoli, releases 1500 t d−1 of hydrothermal CO2 through soil diffuse degassing from a relatively small area (0.5 km2). This amount of gas is comparable to that released by crater plume emissions of many active volcanoes. On the basis of the CO2/H2O ratio measured in high‐temperature fumaroles inside the degassing area, we computed a total thermal energy flux of 1.19×1013 J d−1 (138 MW). Most of this energy is lost by shallow steam condensation and transferred to the atmosphere through the hot soil of the degassing area. The thermal energy released by diffuse degassing at Solfatara is by far the main way of energy release from the whole Campi Flegrei caldera. It is 1 order of magnitude higher than the conductive heat flux through the entire caldera, and, during the last 20 years, it was several times higher than the energy associated with seismic crises and ground deformation events. It is possible that changes in the energy flux from a magma body seated underneath Solfatara and/or argillification processes at relatively shallow depths determine pressurization events in the hydrothermal system and consequently ground deformation and shallow seismic swarms, as recorded during the recent episodes of volcanic unrest centered at Pozzuoli.
Fractional restrengthening in simulated fault gouge: Effect of shear load perturbations
Authors: Karner Stephen L. , Marone Chris
Published: February 2004

Laboratory friction experiments are important for understanding fault restrengthening (healing) between failure events. To date, studies have focused mainly on time and velocity dependence of friction for small perturbations about conditions for steady state sliding. To investigate healing under a wider range of conditions, as appropriate for the interseismic period and dynamic rupture on seismogenic faults, we vary shear load for holds τhold, hold time th, load point velocity V, and initial gouge layer thickness T0. We shear layers of granular quartz in a biaxial testing apparatus at room temperature and humidity. In addition to conventional slide‐hold‐slide (CSHS) healing tests, we perform tests in which shear stress is rapidly reduced prior to each hold. Identical slip histories are used in all experiments. Our CSHS tests show time‐dependent healing, where Δμ is the difference between peak static friction and prehold sliding friction, consistent with previous work. For a given th we find a systematic increase in peak static strength and Δμ with decreasing τhold (for th = 100 s, Δμ = 0.007 for CSHS tests and 0.05 for τhold = 0 tests). Significantly, healing tests at zero shear stress show decreasing static frictional yield strength with increasing th; thus we observe time‐dependent weakening in this case. We vary initial layer thickness (0.5–3 mm) and find greater healing for thicker layers. Numerical simulations using rate and state friction laws show that neither the Dieterich nor Ruina evolution laws predict our experimentally observed healing rates for the full range of conditions studied. Our results have significant implications for the mechanics of deformation within granular media. We present a micromechanical model based on stress chains, jamming, and time‐dependent unjamming of sheared granular layers. As applied to earthquakes, our data indicate that coseismic stress drop is expected to have an important effect on fault healing rates and static yield strength.
Permeability of Wilcox shale and its effective pressure law
Authors: Kwon Ohmyoung , Kronenberg Andreas K. , Gangi Anthony F. , Johnson Brann
Published: February 2004

The permeability of illite‐rich shale from the Wilcox formation has been measured as a function of effective pressure for bedding‐parallel flow of 1 M NaCl pore fluid. Permeability k decreases from ∼300×10−21 m2 to 3×10−21 m2 as effective pressure Pe is increased from 3 to 12 MPa; these values confirm that shales form effective barriers to fluid transport in sedimentary strata over extended geologic times. The variation of k with Pe for Wilcox shale is given by k = k0 [1 − (Pe/P1)m]3, where P1 = 19.3 (±1.6) MPa and m = 0.159 (±0.007). The value of k0 for Wilcox shale is of the order of 10−17 m2 and may vary among samples by as much as 70%. Effective pressure is given in terms of the external confining pressure Pc and internal pore pressure Pp by Pe = Pc − χPp, where χ = 0.99 (±0.06). While our measurements yield χ = ∼1 for shale with a clay content of ∼45%, others have reported χ values for clay‐bearing sandstones that rise from ∼0.75 to 7.1 with increasing clay content (from 0 to 20%). The trends between χ and clay content revealed by these comparisons imply that the value of χ depends upon the relative distributions of compliant clay minerals and other stiffer minerals. These values of χ also suggest that effective pressures within interbedded sandstones and shales may differ, even at the same equilibrium Pc and Pp conditions.
A model for sorted circles as self‐organized patterns
Authors: Kessler M. A. , Murray A. B. , Werner B. T. , Hallet B.
Published: February 2004

Sorted circles emerge as self‐organized patterns from a laterally uniform active layer that becomes laterally sorted as frost heave deforms the interface between a stone layer and an underlying soil layer. In a three‐dimensional, cellular model of the active layer, cyclic freezing and thawing drives transport of stone and soil particles by (1) addition of ice particles representing soil expansion by frost heave, (2) removal of ice particles representing soil consolidation during thawing, (3) addition of void particles (a discrete abstraction of soil compressibility) representing soil expansion by water absorption, (4) removal of void particles representing compaction and desiccation of underlying soil by frost heave, (5) relaxation of surface morphology by soil creep and stone avalanche, and (6) vertical sorting of stones and soil by illuviation. These transport processes give rise to sorted circles, which are characterized by a mean spacing of 3.6 m, a 2.4 m wide soil domain surrounded by a 1.0 m wide, 0.3 m high annulus of stones, and a 750 year period of circulation in the soil domain, all consistent with measured characteristics of sorted circles in western Spitsbergen. In the model, instabilities on the stonesoil interface grow upward as soil plugs by drawing in soil from the surrounding subsurface soil layer; soil plugs develop into sorted circles as they contact the ground surface, simultaneously elevating an encircling annulus of stones. Sorted circles are dynamically maintained by circulation within the stone and soil domains. Initiation of soil plugs is driven by a positive feedback in which frost heave near the stone‐soil interface pushes soil toward more compressible soil regions, where the soil layer is thicker. The lateral component of these frost‐heave‐induced displacements is not reversed during thaw because soil consolidation (as ice‐rich soil melts and drains) and soil expansion (as desiccated and compacted soil hydrates) displace soil vertically. Further development of soil plugs and sorted circles is determined by an interplay between this positive feedback and amplitude dependent negative feedbacks that result from decoupling of the freezing front from the stone‐soil interface. Parameters outside the range in which sorted circles form can result, for example, in stone islands and labyrinthine patterns. The initial wavelength of perturbations on the stone‐soil interface is accurately predicted using a linear stability analysis, but increase in this wavelength through time reflects the nonlinearities that control the spacing of soil plugs and sorted circles, namely, interactions and mergers between neighboring forms.
High‐resolution bathymetric surveys using scanning sonars: Lava flow morphology, hydrothermal vents, and geologic structure at recent eruption sites on the Juan de Fuca Ridge
Authors: Chadwick William W. , Scheirer Daniel S. , Embley Robert W. , Johnson H. Paul
Published: February 2004

The CoAxial and Cleft segments of the Juan de Fuca Ridge have isolated, chronic, high‐temperature, and focused hydrothermal vent sites. Both segments also have experienced recent volcanic eruptions which produced extensive, ephemeral, low‐temperature, and diffuse hydrothermal venting. To study the geologic setting of these sites, high‐resolution bathymetric surveys at eight locations on the CoAxial and Cleft segments were collected between 1993 and 1999. Two 675‐kHz scanning sonar systems were used, Mesotech on the submersible Alvin and Imagenex on the remotely operated vehicle Jason. The bathymetry from these surveys can be gridded at a scale of 2–4 m and contoured at 1 m and thus can resolve many fine‐scale features on the seafloor that are indistinguishable in multibeam bathymetry collected at the sea surface. Bathymetric data at this resolution are particularly useful for identifying geologic features related to diking, faulting, and lava flow emplacement. For example, the high‐resolution bathymetric maps show that submarine fissure eruptions that form pillow lavas last long enough to become localized and to produce point source constructs along their length, and their extrusion rate is low enough that no significant drainback occurs. In contrast, lobate sheet flows are formed by short‐lived, high‐effusion rate eruptions in which no localization of output occurs along the eruptive fissure, and inflation is quickly followed by drainback, resulting in extensive collapse features. However, if the process of submarine lava flow inflation occurs at a slower rate and over a longer period of time, it can create lava rises up to 25 m high with distinctive structure and morphology. The scanning sonar data also show that fissures and grabens have formed or reactivated where dikes approach the surface adjacent to recent eruptive sites. The fine‐scale bathymetry establishes that all the hydrothermal vent sites studied at the CoAxial and Cleft segments are located along prominent volcanic or tectonic extensional structures which provide the physical pathway for fluids from the subsurface to the seafloor. Furthermore, the fine‐scale morphology of recent lava flows can be used as a qualitative indication of eruption duration.
Steady state deformation of the Coso Range, east central California, inferred from satellite radar interferometry
Authors: Wicks Charles W. , Thatcher Wayne , Monastero Francis C. , Hasting Michael A.
Published: February 2004

Observations of deformation from 1992 to 1997 in the southern Coso Range using satellite radar interferometry show deformation rates of up to 35 mm yr−1 in an area ∼10 km by 15 km. The deformation is most likely the result of subsidence in an area around the Coso geothermal field. The deformation signal has a short‐wavelength component, related to production in the field, and a long‐wavelength component, deforming at a constant rate, that may represent a source of deformation deeper than the geothermal reservoir. We have modeled the long‐wavelength component of deformation and inferred a deformation source at ∼4 km depth. The source depth is near the brittle‐ductile transition depth (inferred from seismicity) and ∼1.5 km above the top of the rhyolite magma body that was a source for the most recent volcanic eruption in the Coso volcanic field [Manley and Bacon, 2000]. From this evidence and results of other studies in the Coso Range, we interpret the source to be a leaking deep reservoir of magmatic fluids derived from a crystallizing rhyolite magma body.
Stretching and mixing of viscous blobs in Earth's mantle
Authors: du Vignaux Nicolas Merveilleux , Fleitout Luce
Published: February 2004

The stretching and mixing of blobs more viscous than the surrounding mantle is an important issue: The subducted lithospheric material at depth might be more viscous than the surrounding mantle, and its ability to mix affects its properties as a geochemical reservoir. In order to study the effect of the viscosity ratio λ (viscosity of the blob to the viscosity of the surrounding mantle) on the deformation we perform three types of two‐dimensional numerical simulations: a pure shear simulation, a simulation of steady state convection, and a simulation of time‐dependent convection. In a pure shear flow the inverse of the rate of deformation of a viscous blob is, to a first approximation, a linear function of the ratio λ/AR, where AR is the aspect ratio of the blob. For a viscous blob in a steady convection cell a periodic deformation is superimposed on the long‐term deformation. The period corresponds to an overturn time along the streamline followed by the blob. The strains of this periodic deformation obey the laws found for pure shear. In a time‐dependent flow, more representative of the mantle flow, we find that the average distance a* between two tracers situated within a viscous blob obeys the relationship ln(a*/a0)=At/(1+λ)2 and that it is independent of the aspect ratio of the blob. These two results are well explained by a simple model of successive strains in arbitrary directions. The viscosity contrast controls the mixing efficiency, and it has an effect much larger than predicted by earlier studies. Blobs of matter more viscous than the average mantle can therefore persist unmixed in the mantle over Earth's history.
Volcano monitoring using the Global Positioning System: Filtering strategies
Authors: Larson Kristine M. , Cervelli Peter , Lisowski Michael , Miklius Asta , Segall Paul , Owen Susan
Published: February 2004

Permanent Global Positioning System (GPS) networks are routinely used for producing improved orbits and monitoring secular tectonic deformation. For these applications, data are transferred to an analysis center each day and routinely processed in 24‐hour segments. To use GPS for monitoring volcanic events, which may last only a few hours, real‐time or near real‐time data processing and subdaily position estimates are valuable. Strategies have been researched for obtaining station coordinates every 15 min using a Kalman filter; these strategies have been tested on data collected by a GPS network on Kilauea Volcano. Data from this network are tracked continuously, recorded every 30 s, and telemetered hourly to the Hawaiian Volcano Observatory. A white noise model is heavily impacted by data outages and poor satellite geometry, but a properly constrained random walk model fits the data well. Using a borehole tiltmeter at Kilauea's summit as ground‐truth, solutions using different random walk constraints were compared. This study indicates that signals on the order of 5 mm/h are resolvable using a random walk standard deviation of 0.45 cm/√h. Values lower than this suppress small signals, and values greater than this have significantly higher noise at periods of 1–6 hours.
P‐V‐Vp‐Vs‐T measurements on wadsleyite to 7 GPa and 873 K: Implications for the 410‐km seismic discontinuity
Authors: Li Baosheng , Liebermann Robert C. , Weidner Donald J.
Published: February 2004

The compressional (P) and shear wave (S) velocities for Mg2SiO4 wadsleyite have been measured to 7 GPa and 873 K using simultaneous ultrasonic interferometry and in situ X‐ray diffraction techniques. From the velocity measurements we obtained the pressure and temperature derivatives for the elastic shear (G) and adiabatic bulk (KS) moduli, (∂G/∂P)T=1.5(1), (∂G/∂T)P=−0.017(1) GPa/K, KS=173(2) GPa, (∂Ks/∂P)T=4.2(1), and (∂Ks/∂T)P=−0.012(1) GPa/K; for the P and S waves, we obtained (∂Vs/∂P)T=0.021(1) (km/s)/GPa, (∂Vs/∂T)P=−0.035(2) (km/s)/K, (∂VP/∂P)T=0.065(2) (km/s)/GPa, and (∂VP/∂T)P = −0.038(2) (km/s)/K (values in parentheses are standard deviations, e.g., 1.5(1)=1.5±1). Independent equation of state analysis of P‐V‐T data provided an estimation of the temperature dependence for the isothermal bulk modulus of (∂KT/∂T)P=−0.022(12) GPa/K and thermal expansion (α=a+bT) coefficients of a=2×10−5 K−1 and b=2.5×10−8 K−2. Using these data along with elastic properties for other mantle phases, a velocity‐depth profile for a pyrolite model to 670 km depth is constructed using a finite strain method along a 1673 K adiabat. In the transition zone the pyrolite model has a smaller gradient between 410 and 660 km than the body wave models from synthetic waveform analyses but converges with the seismic profiles at the bottom of the transition zone just above the 660‐km discontinuity. The pyrolite model has velocity jumps of 6.9% and 7.9% for P and S waves, respectively, over a thickness of ∼10 km for the phase transformation from olivine to wadsleyite, which is in good agreement with short‐period P wave reflection data and a recent fine structure model (C4) for both the velocity jumps and the thickness of the 410‐km seismic discontinuity.
Oligo‐Miocene magnetostratigraphy and rock magnetism of the Xishuigou section, Subei (Gansu Province, western China) and implications for shallow inclinations in central Asia
Authors: Gilder Stuart , Chen Yan , Sen Sevket
Published: February 2004

Magnetostratigraphy of 222 remanent directions together with late Oligocene to early Miocene mammal and charophyte paleontology suggest that 2179 m of the Xishuigou section (Subei, Gansu Province, China) were deposited from ∼26 to ∼19 Ma. Stratigraphic patterns of bulk susceptibility, anisotropy of magnetic susceptibility parameters, and natural and anhysteretic remanent magnetization intensities demonstrate that (1) faulting does not significantly affect the record, (2) sediment deposition was relatively continuous, (3) sediment source changed around 23 Ma, and (4) rapid uplift near Subei occurred at 21 Ma. Subei rotated 27°±5° counterclockwise with respect to the 20 Ma pole from the Eurasian synthetic apparent polar wander path. Folding and rotation of the section took place after 19 Ma. The paleolatitude of Subei is 14° less than at present and 19°±3° less than predicted from the reference pole. Both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing, a phenomenon which has likely affected other paleomagnetic data from central Asia.
Creep of dry clinopyroxene aggregates
Authors: Bystricky Misha , Mackwell Stephen
Published: February 2004

We have determined diffusional and dislocation creep rheologies for clinopyroxenite Ca1.0Mg0.8Fe0.2Si2O6 under dry conditions by deforming natural and hot‐pressed samples at confining pressures of 300–430 MPa and temperatures of 1100°–1250°C with the oxygen fugacity buffered by either nickel‐nickel oxide or iron‐wüstite powders. The coarse‐grained natural Sleaford Bay clinopyroxenite yielded a stress exponent of n = 4.7 ± 0.2 and an activation energy for creep of Q = 760 ± 40 kJ mol−1, consistent with deformation in the dislocation creep regime. The strength of the natural clinopyroxenite is consistent with previous high‐temperature measurements of dislocation creep behavior of Sleaford Bay clinopyroxenite by Kirby and Kronenberg [1984] and Boland and Tullis [1986]. Fine‐grained clinopyroxenite was prepared from ground powders of the natural clinopyroxenite. Hot‐pressed samples were deformed under similar conditions to the natural samples. Mixed‐mode deformation behavior was observed, with diffusional creep (n = 1) at lower differential stresses and dislocation creep (with n and Q similar to those of the natural samples) at higher differential stresses. Within the dislocation creep field the predried hot‐pressed samples generally yielded creep rates that were about an order of magnitude faster than the natural samples. Thus, even at the highest differential stresses, a component of strain accommodation by grain boundary diffusion was present in the hot‐pressed samples. Optical and electron microscope investigations of the deformation microstructures of the natural and hot‐pressed samples show evidence for mechanical twinning and activation of dislocation slip systems. When extrapolated to geological conditions expected in the deep crust and upper mantle on Earth and other terrestrial planets, the strength of dry single‐phase clinopyroxene aggregates is very high, exceeding that of dry olivine‐rich rocks.
Hydrothermal alteration in a modern suprasubduction zone: The Tonga forearc crust
Authors: Banerjee Neil R. , Gillis Kathryn M.
Published: February 2004

An extensive suite of hydrothermally altered basalts, gabbros, and plagiogranites was recovered from the trench‐facing slope of the Tonga forearc. The tectonic setting, lithology, and geochemistry of these samples make them a unique collection for comparison with suprasubduction zone (SSZ) ophiolites. Petrography, mineral chemistry, and geothermometry are used to constrain the metamorphic evolution of ocean crust formed in a modern SSZ setting. Seawater‐derived hydrothermal fluids first penetrated the lower crust along grain boundaries and microscopic fracture networks at temperatures >800°C. As the plutonic sequence cooled, amphibole progressively replaced the mafic phases, followed by chlorite and epidote below ∼550°C. Basalts record peak alteration temperatures up to 773°C; however, most were altered at lower temperatures typical of mid‐ocean ridge (MOR) volcanic sequences [Kelman, 1998]. Epidosites formed by pervasive alteration of basalt and plagiogranite at greenschist facies conditions and at high water‐rock ratios. Pervasively altered gabbros and basalts display evidence of late cataclastic deformation and/or contain veins that is likely related to a later tectonic event such as trench rollback. The range of alteration temperatures and mineral assemblages in basalts and gabbros are similar to those described from both SSZ ophiolites and MORs. However, the degree of alteration in basalts and the presence of epidosites in the Tonga collection are most similar to alteration characteristics in SSZ ophiolites. The initiation of high‐temperature brittle deformation in the absence of ductile deformation suggests that the Tonga forearc crust was constructed in a magma‐rich environment, similar to fast spreading MORs.
Near‐field transpressive deformation along the San Andreas fault zone in southern California, based on exhumation constrained by (U‐Th)/He dating
Authors: Spotila James A. , Farley Kenneth A. , Yule J. Douglas , Reiners Peter W.
Published: February 2004

Low‐temperature thermochronometry reveals that a narrow crustal sliver trapped within strands of the San Andreas fault zone in southern California has experienced recent, rapid exhumation. Eight apatite (U‐Th)/He ages from a 1‐km‐relief section along Yucaipa Ridge in the San Bernardino Mountains range from 1.4 to 1.7 Ma. The minimal change in age with elevation implies exhumation of ∼5–7 mm yr−1, sustained for at least several hundred thousand years. Three titanite helium ages from the ridge are much older, ranging from 57 to 82 Ma. These show a steep gradient with elevation, representing either an exhumed, partial retention zone or slow cooling through much of the Tertiary. These data imply that a total exhumation of ∼3 to 6 km has occurred since 1.8 Ma. It is uncertain whether this exhumation terminated as early as 1 Ma or has continued up to the present at a decelerated rate. We surmise that this exhumation represents rock uplift in the absence of major surface uplift, in that it kept pace with tectonic uplift as the narrow fault block maintained steady state relief. The record of sedimentation in adjacent basins is consistent with the implied magnitude of erosion. Such rapid, large‐magnitude exhumation within the strands of the San Andreas fault zone is important for models of transpressional tectonics. It is consistent with a strain partitioning model which predicts that pure shear dominated fault zones experience significant vertical strain. However, it is inconsistent with a stress‐partitioning model which predicts that fault zone weakness limits pure shear deformation to the borderlands of the master strike‐slip fault. In addition, a concentration of secondary contraction within the fault zone may require modification of coupling models between strong upper mantle and brittle upper crust via the weak lower crust. These models predict that transpressional deformation will either be uniformly distributed across the plate boundary or be limited to the far‐field borderlands, rather than concentrated in the near field. Alternatively, the exhumation of Yucaipa Ridge may have been driven by the nearby restraining bend in the San Andreas fault at San Gorgonio Pass, in which case it represents local fault geometry rather than accommodation of far‐field plate motion.
Regional electrical resistivity structure of the southern Canadian Cordillera and its physical interpretation
Authors: Ledo Juanjo , Jones Alan G.
Published: February 2004

The regional geoelectric crustal structure of the southern and central Canadian Cordillera of western Canada is interpreted from the inversion of magnetotelluric data along five profiles crossing the physiographic morphogeological belts, with emphasis on the Intermontane and Omineca Belts. Decomposition of the tensor impedance response estimates demonstrates that large‐scale regional structures can be reasonably approximated along each profile as two‐dimensional, with dominant geoelectric strikes of either −30° or +15°, depending on profile location. These profile‐specific strike directions are consistent with a local clockwise rotation of crustal structures in the southern Intermontane and Omineca Belts suggested by others based on paleomagnetic data and palinspastic reconstructions. Comparing the resistivity models derived from two‐dimensional inversions of the distortion‐corrected data along each profile allows us to construct an orogen‐scale three‐dimensional resistivity model for southern British Columbia. Generally, the model shows a resistive upper crust overlying a conductive lower crust. The resistivity of the lower crust beneath the Intermontane Belt is independent of latitude and is similar for all profiles. In stark contrast, a 2 orders of magnitude variation in lower crustal resistivity is observed along strike in the Omineca Belt, with higher conductivities to the south in the region of Eocene extension and lower conductivities to the north in the unextended part of the belt. Such spatial association has noteworthy implications for the cause of lower crustal resistivity in active, or recently active, young regions. Our preferred interpretation of the observed lower crustal resistivities with other geophysical data is in terms of fluids, with brines dominating for the most part but partial melt possible at the base of the crust in specific localities. We attribute the along‐strike variation in the Omineca Belt mostly to variation in fluid content and interconnectivity, with the lowermost crust of the southern Omineca Belt being partially molten. This physical state difference is a consequence of degree of extension and implies that mantle‐derived fluids are important for lower crustal resistivity.
Inversion for parameters of tensile earthquakes
Authors: Vavryčuk Václav
Published: February 2004

The tensile source model generalizes the shear source model by assuming that the slip vector can be arbitrarily oriented with respect to the fault and is not constrained to lie within the fault plane. The proposed inversion for the parameters of tensile sources is based on the evaluation of the isotropic (ISO), compensated linear vector dipole (CLVD), and double‐couple (DC) components in seismic moment tensors. The most significant parameters inverted are the λ/μ ratio at the fault (denoted as the κ parameter) and the inclination α of the slip vector from the fault. The κ parameter is significant for discriminating noisy moment tensors of shear earthquakes from those of tensile earthquakes. The inclination α can be accurately determined from the DC component in the moment tensor because the DC component rapidly decreases with increasing α. For example, the inclination of 20° causes DC being ∼50–60% only. The inversion is applied to earthquakes which occurred in January 1997 in West Bohemia, Czech Republic. It is shown that some of these earthquakes display tensile faulting. The κ parameter is ∼0.1. The inclination of the slip from the fault attains values of up to 20°. This inclination is a result of tensile traction and reduced shear traction along the fault and high‐fluid pressure in the region.
Three‐dimensional normal faulting models of the Valles Marineris, Mars, and geodynamic implications
Authors: Schultz Richard A. , Lin Jian
Published: February 2004

Inversion of Martian topography, using a three‐dimensional boundary element model, permits revised estimates of fault dip angle, depth of faulting, paleogeothermal gradient, and extensional strain in the Valles Marineris region. The major normal faults dip at 40°–55° to depths of ∼60–75 km with comparatively small footwall uplifts (<500 m for throws to 10 km), implying that paleogeothermal gradients during faulting were ∼10 K km−1 or less, assuming relatively rapid strain rates appropriate to the extending Martian crust. Accumulation of dip‐slip offsets along the main normal faults likely was associated with anticlinal flexure and deformation of preexisting strata within the troughs. The magnitudes of the predicted vertical and horizontal displacements outside the troughs, as well as extensional strains, are spatially variable, depending on both cross‐ and along‐strike position relative to the troughs. The predicted displacements and strains attain maximum values at the troughs and decay to background values at cross‐strike distances of ∼250 km, corresponding to 3–4 times the depth of faulting, with an average province‐wide strain of 4–15%. By implication, the aggregate strain field surrounding Tharsis is a highly variable spatial and temporal composite of the inhomogeneous strain fields, each associated and scaling in size with an individual graben, from the smallest structures through the largest Valles Marineris troughs.
Ascent and decompression of viscous vesicular magma in a volcanic conduit
Authors: Massol Hélène , Jaupart Claude , Pepper Darrell W.
Published: February 2004

During eruption, lava domes and flows may become unstable and generate dangerous explosions. Fossil lava‐filled eruption conduits and ancient lava flows are often characterized by complex internal variations of gas content. These observations indicate a need for accurate predictions of the distribution of gas content and bubble pressure in an eruption conduit. Bubbly magma behaves as a compressible viscous liquid involving three different pressures: those of the gas and magma phases, and that of the exterior. To solve for these three different pressures, one must account for expansion in all directions and hence for both horizontal and vertical velocity components. We present a new two‐dimensional finite element numerical code to solve for the flow of bubbly magma. Even with small dissolved water concentrations, gas overpressures may reach values larger than 1 MPa at a volcanic vent. For constant viscosity the magnitude of gas overpressure does not depend on magma viscosity and increases with the conduit radius and magma chamber pressure. In the conduit and at the vent, there are large horizontal variations of gas pressure and hence of exsolved water content. Such variations depend on decompression rate and are sensitive to the “exit” boundary conditions for the flow. For zero horizontal shear stress at the vent, relevant to lava flows spreading horizontally at the surface, the largest gas overpressures, and hence the smallest exsolved gas contents, are achieved at the conduit walls. For zero horizontal velocity at the vent, corresponding to a plug‐like eruption through a preexisting lava dome or to spine growth, gas overpressures are largest at the center of the vent. The magnitude of gas overpressure is sensitive to changes of magma viscosity induced by degassing and to shallow expansion conditions in conduits with depth‐dependent radii.
Crustal deformation at the oblique spreading Reykjanes Peninsula, SW Iceland: GPS measurements from 1993 to 1998
Authors: Hreinsdóttir Sigrún , Einarsson Páll , Sigmundsson Freysteinn
Published: February 2004

In 1993 and 1998 a 38‐point GPS network was surveyed on the Reykjanes Peninsula, SW Iceland. According to the NUVEL‐1A plate motion model the spreading rate of the North American and Eurasian plates in SW Iceland is 18.9±0.5 mm/yr toward N102.7°±1.1°E, highly oblique to the plate boundary. Instead of oblique spreading, the measurements indicate left‐lateral shear strain accumulation parallel to the Reykjanes Peninsula seismic zone (∼N76°E) at the rate of about εyx ≈ −0.2 μstrain/yr (tensor shear strain). Subsidence is generally observed toward the seismic zone. A local maximum subsidence of 60 mm was measured in the Svartsengi geothermal area. Subsidence in this area has previously been detected with geodetic measurements and is considered a result of geothermal usage. Expansion was observed in the area of the Hengill triple junction. This is probably a result of magma accumulation beneath! mount Hrómundartindur, as previous seismic and geodetic measurements have indicated. Using a simple screw dislocation model, we fit the majority of the data. Assuming a left‐lateral shear zone at depth along the seismic zone, we estimate locking depth of ∼6.5 km and deep slip rate of ∼16.5 mm/yr. The maximum left‐lateral displacement predicted by the screw dislocation model, 11.85±0.06 mm/yr, is consistent with the observed value of 11.9±0.5 mm/yr. If the Hengill area is excluded, little extension is observed across the peninsula. The discrepancy between the NUVEL‐1A oblique spreading and the observed transcurrent motion is thought to be caused by lack of magma intrusion into the crust during this time period.
An experimental study into the rheology of synthetic polycrystalline coesite aggregates
Authors: Renner J. , Stöckhert B. , Zerbian A. , Röller K. , Rummel F.
Published: February 2004

Coesite has been found as a relic in ultrahigh pressure metamorphic (UHPM) crust worldwide and is expected to play a major role in the mechanical behavior of continental crust at UHPM conditions. We performed triaxial compression tests on synthetic polycrystalline coesitite in a solid medium apparatus at confining pressures of 3.1 to 3.7 GPa, temperatures of 700° to ∼1160°C, and strain rates betweeen 6×10−7 and 1×10−3 s−1. The problem of the limited stress resolution of the solid medium apparatus was addressed by applying two extreme friction corrections that yield lower and upper bounds to the differential stress. The correlation between the mechanical data and the microstructural record of the deformed samples, as a function of temperature and imposed strain rate, is consistent with deformation by dislocation creep. We deduced parameters of a power law as n ≈ 3±1 and Q ≈ 275±50 kJ mol−1. Extrapolation of the experimental data to natural conditions cannot be constrained by comparison with natural microstructures, due to the lack of preserved coesite other than as single crystal inclusions. Nevertheless, the extrapolation indicates a low strength (of order 10 MPa) for natural strain rates at typical UHPM conditions. Absent deformation of the UHPM Brossasco granite (Dora Maira Massif, Western Alps) thus implies low stresses; deformation must have been localized in very weak shear zones during burial and exhumation.