bluuebunny/crossref_metadata_2025 huggingface dataset:
Whistler waves associated with the Uranian bow shock: Outbound observations
Authors: Smith Charles W. , Wong Hung K. , Goldstein Melvyn L.
Published: February 2008

The encounter of the Voyager 2 spacecraft with the Uranian planetary system led to a series of outbound crossings of the Uranian bow shock between January 27 and January 30 of 1986. Examination of magnetic field data recorded in close proximity to the shock reveals a series of whistler wave events that appear to result from processes associated with the shock. Two wave events display two separate and simultaneous wave enhancements each, one at approximately 0.2 Hz and the second more nearly at 1 Hz in the spacecraft reference frame. The actual frequencies of the waves vary with the two events. Both wave enhancements of the two events are right‐hand polarized in the spacecraft frame of reference. We have examined these wave events using high‐resolution magnetic field data and conclude that they are analogous to those whistler waves upstream of the Earth's bow shock that are driven by beams of electrons. Using observations by the plasma wave science experiment on Voyager 2 to infer the likely presence of electron beams with modest beam speeds, we present an instability analysis and show that a single electron beam with reasonable parameters can generate both of the observed whistler wave forms simultaneously. A third wave event is also seen which occurs both upstream and downstream of a shock crossing. This event displays only one relatively broad spectral enhancement in the same frequency regime and is left‐hand polarized in the spacecraft frame. We argue that this event is most likely the result of a gyrating proton distribution associated with the oblique shock. As such, it is analagous to whistler waves observed earlier during the inbound crossing of the Uranian shock.
The low‐latitude boundary layer and the boundary plasma sheet at low altitude: Prenoon precipitation regions and convection reversal boundaries
Authors: Newell Patrick T. , Burke William J. , Sánchez Ennio R. , Meng Ching‐I. , Greenspan Marian E. , Clauer C. Robert
Published: February 2008

The dayside zone of soft precipitation can be divided into four distinct types of plasma regimes, each corresponding to the respective magnetospheric source region: the cusp, the mantle, the low‐latitude boundary layer (LLBL), and the dayside extension of the BPS. Based on a detailed spectral study, including comparisons with nonsimultaneous ISEE 1 satellite LLBL data, we identify regions of LLBL‐type plasma in the DMSP data set and compare these plasma boundaries with convection reversal boundaries (CRBs) as determined by either Sondrestrom or the drift meter instrument on board the DMSP F9 spacecraft. The nine cases considered are all in the prenoon local time sector. We find that in eight of the nine cases the CRB occurs within the LLBL as expected, generally near to, but not coincident with, the equatorward edge of the LLBL‐type plasma. In our sample set, chosen for cases with latitudinally wide, easily identifiable LLBL signatures, the average latitudinal width was 1.85° magnetic latitude. The CRB, defined as the onset of steady antisunward convection, occurred about 30% of this width beyond the equatorward onset of LLBL‐type particles. The most equatorward portion of the region with LLBL‐type plasma usually had near‐zero or erratic convection and may correspond to the “stagnation region” reported from ISEE observations. The potential drop observed across the low‐altitude LLBL is roughly estimated to be typically ∼5 keV. A summary is given on how the various high‐altitude sources can be identified when plasma regions are observed at low altitude in the dayside auroral oval.
Field‐aligned current associated with a distorted two‐cell convection pattern during northward interplanetary magnetic field
Authors: Zhu L. , Schunk R. W. , Sojka J. J.
Published: February 2008

A systematic study of the influence of the ionospheric conductance on the field‐aligned current associated with a distorted two‐cell convection pattern during northward interplanetary magnetic field (IMF) has been conducted. Our modeling results indicate that the NBZ current can be associated with the distorted two‐cell convection for most of the ionospheric conductivity conditions. It is shown that the conductivity conditions related to the seasonal variation and the aurora activity can significantly influence the basic features of the field‐aligned current associated with a distorted two‐cell convection pattern. It is found that the increase of the field‐aligned current in the polar cap during the summer is mainly due to the increasing contribution from the Pedersen current, and the increase of the field‐aligned current in both the oval region and the evening‐midnight sector during the active aurora period is mainly due to the increasing contribution from the Hall current. It is also found that the conductivity change related to the solar variation has its impact mainly on the intensity of a field‐aligned current system instead of its pattern. On the basis of the modeling results it is suggested that the field‐aligned current system observed by the Magsat satellite (Iijima and Shibaji, 1987) might imply a distorted two‐cell convection pattern, and a four‐cell convection pattern is most likely to occur when the direction of the IMF is due north or very close to the north.
A family of two‐dimensional nonlinear solutions for magnetic field annihilation
Authors: Jardine M. , Allen H. R. , Grundy R. E. , Priest E. R.
Published: February 2008

We present a family of nonlinear solutions for magnetic field annihilation in two dimensions. These solutions include fully the effects of viscosity and resistivity and are a generalization of the Sonnerup and Priest model, where an irrotational stagnation point flow carries straight field lines toward a long, thin current sheet. Here, we allow for vorticity in the inflow. When this is low, there is a unique solution for the flow and magnetic field. The current sheet adjusts its dimensions to accommodate different inflows. It is widest for a negative imposed vorticity and increases in width as the resistivity or viscosity is increased. When the imposed vorticity is large and negative, however, the solutions become nonunique, the flow pattern becomes cellular, and current sheets develop at the cell boundaries. These results, then, show that it is possible to have many more different types of inflow matched to full solutions for the current sheet than have been considered hitherto.
Observations of transverse ion acceleration in the topside auroral ionosphere
Authors: Garbe G. P. , Arnold R. L. , Moore T. E. , Kintner P. M. , Vago J. L.
Published: February 2008

Data obtained from a sounding rocket flight which reached an apogee of 927 km and passed through several auroral arcs are reported. Thermal/superthermal ions were sampled by charged particle analyzers which allowed for a rapid (≈1 s) sampling of their distribution function. During portions of the flight when the rocket was not in an energetic auroral structure, the ion data are fit to a Maxwellian function which yields the plasma parameters. Throughout the middle portion of the flight when above 700 km altitude, ion distributions having a superthermal tail were measured. These ion distributions generally coexisted with a cold thermal core distribution and peaked at pitch angles slightly greater than 90°, which identifies them as conic distributions. These ions can be modeled using a bi‐Maxwellian distribution function with a perpendicular (to B) temperature about 10 times greater than the parallel temperature of 0.15 eV. When the rocket was immersed in energetic auroral electron precipitation, two other ion distributions were observed. Transversely accelerated ions which represented bulk heating of the ambient population were observed continuously in these arcs. The characteristic perpendicular energy of the transversely bulk heated ions reached as high as 3 eV compared to typically less than 0.4 eV during nonauroral times. Cold ions flowing down the magnetic field were also continuously observed when the rocket was immersed in auroral electron precipitation and had downward speeds between 3 and 5 km/s. If one balances electric and collisional forces, these speeds translate to an electric field pointing into the atmosphere of magnitude 0.01 mV/m. A close correlation between auroral electron precipitation, measured electrostatic oxygen cyclotron waves, cold downflowing ions and transversely bulk heated ions will be shown. In addition, the ion distributions with a superthermal tail are seen to have a loose correlation with the aurora and measured H+ gyroharmonic wave structures. These observations are discussed in terms of some current theories of transverse ion energization.
Geomagnetic activity effects on the equatorial neutral thermosphere
Authors: Burrage M. D. , Abreu V. J. , Orsini N. , Fesen C. G. , Roble R. G.
Published: February 2008

The effects of geomagnetic activity on the equatorial neutral thermosphere are investigated with mass spectrometer measurements from the Atmosphere Explorer E (AE‐E) satellite and simulations generated by the National Center for Atmospheric Research thermosphere/ionosphere general circulation model (TIGCM). A study of the local time dependence of the equatorial geomagnetic storm response concentrates on a disturbed period from March 20 (day 79) to March 31 (day 90), 1979. This interval was the subject of an intense data‐gathering and analysis campaign for the Coordinated Data Analysis Workshop 6, and global TIGCM predictions are available for the specific conditions of the storm as a function of universal time. The AE‐E measurements demonstrate that significant geomagnetic storm‐induced perturbations of upper thermospheric N2 and O densities extend into the equatorial zone but are mainly restricted to the midnight/early morning sector. The qualitative features of the observations are reproduced by the TIGCM, although in general, the model simulations overestimate the storm temperature and density enhancements, primarily in the nighttime thermosphere. This suggests that either the nighttime cooling rates in the TIGCM are too small or that the specified auroral forcings of the model are too persistent.
On the form of the flow in the magnetosheath
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2008

The form of the flow field in the subsonic and transsonic magnetosheath has received little attention since the proposal of a depletion layer adjacent to the magnetopause by Zwan and Wolf. Recently, observations of density enhancements just upstream of the magnetopause which have been interpreted as slow mode compressional fronts appear to contradict the earlier suggestions. In this paper we describe the reason for the formation of an upstream compressional front standing near the magnetopause, describe its form and geometry, and explain the likely relationship between the compression region and a depletion layer. The model accounts for the variation of density and magnetic field strength with radial distance from the magnetopause and makes predictions regarding the orientation and geometry of the front.
Traveling magnetospheric convection twin vortices: Another case study, global characteristics, and a model
Authors: Glassmeier Karl‐Heinz , Heppner Christian
Published: February 2008

A case study of a transient geomagnetic field variation event associated with a traveling magnetospheric convection twin vortex is presented. The characteristics of this event are different from those of other presented cases, as the sense of rotation of the associated ionospheric current system is reversed, that is, it exhibits upward (downward) field‐aligned currents in the tailward (dayside) part of the vortex structure. Other features are as usually observed of such transients. In particular, a clear tailward motion at high latitudes can be deduced from local as well as global magnetic field observations. At low dayside latitudes, magnetic field variations similar to sudden impulse (SI) variations are recorded. Almost simultaneous onset and no westward propagation are observed, too. As possible source mechanisms of such transient events, localized magnetic field reconnection as well as pressure pulses at the dayside magnetopause are discussed. If the switch‐on of magnetic reconnection can be regarded as equivalent to an equivalent current flowing against the existing magnetopause current and if this countercurrent is spatially localized, an Alfvén wave is generated with downward (upward) field‐aligned current flow in the tailward (dayside) part of the wave, as observed for some of the reported transients. However, the SI‐like behavior of the event studied in this paper, its reversed sense of rotation, and the vortex associated field‐aligned current density excludes such localized magnetic reconnection as a possible source mechanism and argues in favour of the existence of pressure pulses at the magnetopause. It is shown that a spatially localized compression or dilatation of the dayside magnetopause is also associated with the onset of a current flowing parallel or antiparallel to the magnetopause current, respectively. The additional current associated with compression gives rise to a system of upward and downward field‐aligned currents in the tailward and dayside part of the system, much as observed for the event analysed. As the pressure pulse is moving along the magnetopause, the generated Alfvén wave pattern is that of an Alfvén wing structure, with the ground‐magnetic transient representing the passing by of the first ionospheric reflection site. Such an Alfvén wing structure may also explain recent observations of multiple twin vortex systems.
Magnetosheath plasma precipitation in the polar cusp and its control by the interplanetary magnetic field
Authors: Woch J. , Lundin R.
Published: February 2008

Magnetosheath particle precipitation in the polar cusp region is studied based on Viking hot plasma data obtained on meridional cusp crossings. Two distinctively different regions are commonly encountered on a typical pass. One region is characterized by high‐density particle precipitation, with an ion population characterized by a convecting Maxwellian distribution. Typical magnetosheath parameters are inferred for the spectrum of the source population. The spectral shape of the ion population encountered in the second region suggests that here the magnetosheath ions have been energized by about 1 keV, corresponding to an ion velocity gain of about twice the magnetosheath Alfvén velocity. The location of the region containing the accelerated plasma is dependent on the IMF Bz component. For southward IMF the acceleration region is bounded by the ring current population on the equatorward side and by the unaccelerated magnetosheath plasma precipitation on the poleward side. For northward IMF the region is located at the poleward edge of the region with unaccelerated precipitation. The accelerated ion population is obviously transported duskward (dawnward) for a dawnward (duskward) directed IMF. These observations are interpreted as evidence for plasma acceleration due to magnetopause current sheet disruptions/ merging of magnetospheric and interplanetary magnetic flux tubes.
Further investigation of the CDAW 7 substorm using geosynchronous particle data: Multiple injections and their implications
Authors: Reeves G. D. , Kettmann G. , Fritz T. A. , Belian R. D.
Published: February 2008

A substorm which occurred on April 24, 1979 was the subject of the CDAW 7 workshop and several in‐depth studies of ISEE and ground‐based data. In this paper we continue the analysis by looking in detail at geosynchronous energetic particle data from the Los Alamos charged particle analyzer instruments. Observations show three distinct injection events during this substorm. We have used a drift shell tracing technique to determine the time and location on the geosynchronous drift shell of each injection region. We then compare the timing of events on the ground, in the tail and at geosynchronous orbit. The first injection is very closely associated with observations from ISEE and ground‐based measurements which have been interpreted as the signatures of plasmoid formation in the near‐Earth tail. The later two injections occurred while ISEE 1 and 2 were in the lobes following the exit of the plasmoid downtail. All three injections are calculated to have originated in the same local time sector in which the ISEE spacecraft were located and also to have overlapped one another in local time extent. These observations show that the spatial and temporal evolution of the substorm is more dynamic than previous studies have implied. We have considered possible interpretations of the more complete set of observations the context of the both the traditional near‐Earth neutral line model and the inner magnetospheric current disruption model. Either model can provide an interpretation of the data but neither by itself, provides a completely satisfactory explanation. We suggest a new synthesis of these two models which provides a simple interpretation for all the observations. We propose that reconnection in the tail initiates the substorm but diverts much of the cross‐tail current around the reconnection region enhancing the near‐Earth cross‐tail current. This creates an unstable situation and current disruptions dipolarize the field in three steps creating the three injections.
Can economic porphyry copper mineralization be generated by a typical calc‐alkaline melt?
Authors: Cline Jean Schroeder , Bodnar Robert J.
Published: February 2008

Numerical simulation of chlorine and copper partitioning between a crystallizing melt and exsolving aqueous fluids indicates that “typical” calc‐alkaline magmas contain sufficient copper, chlorine, and water to produce economic porphyry copper mineralization. Neither an elevated copper content in the magma nor an additional large volume of magma are required to provide metals or volatiles. The most important variables that determine the volume of melt necessary to produce sufficient copper are the degree of compatible behavior displayed by copper, the ratio of initial water in the melt to the water saturation level, and the initial chlorine/water ratio of the melt. The absolute values for initial water in the melt and water content at saturation are relatively unimportant in determining the required melt volume. The bulk salinity of the exsolved fluid may vary from < 2.0 wt.% NaCl to saturation levels (84 wt.% NaCl at 700°C) indicating that boiling is not necessary to produce high salinity brines. At appropriate P‐T‐XNaCl conditions the magmatic aqueous fluid separates into a saline liquid, which transports most of the copper, and a low‐salinity vapor. The salinities of the two immiscible phases are governed by the P‐T conditions, while the bulk fluid salinity determines the mass fractions of liquid and vapor formed. Pressure quenching causes rapid crystallization of the aplitic groundmass in porphyritic rocks associated with copper mineralization and significantly reduces the amount of chlorine and copper partitioning to the aqueous fluid. This results in abrupt and possibly large reductions in fluid salinity and causes copper to become concentrated in the melt. As copper is transported from the melt by the earliest exsolving fluids in deep (2.0 kbar) systems and by late exsolving fluids in shallow (0.5 kbar) systems, the relative timing of pressure quenching/aplite formation and fluid transport of copper from the melt can vary significantly in systems produced under different confining pressures. Model results incorporating petrologic constraints determined for Yerington, Nevada, are in good agreement with observed mineralization.
Effect of pressure, temperature, and composition on lattice parameters and density of (Fe,Mg)SiO3‐perovskites to 30 GPa
Authors: Mao H. K. , Hemley R. J. , Fei Y. , Shu J. F. , Chen L. C. , Jephcoat A. P. , Wu Y. , Bassett W. A.
Published: February 2008

High‐pressure, high‐temperature properties of MgSiO3, (Fe0.1Mg0.9)SiO3, and (Fe0.2Mg0.8)SiO3 perovskites have been investigated using a newly developed X ray diffraction technique involving monochromatic synchrotron radiation. The first direct measurements of unit cell distortions and equation‐of‐state parameters of the orthorhombic perovskite as functions of composition and simultaneous high pressure and high temperature were obtained. The experiments were conducted under hydrostatic pressure up to 30 GPa, into the stability field of the perovskite. The results demonstrate that the perovskite is elastically anisotropic, with the lattice parameter b being 25% less compressible than a and c. Under increasing pressures the orthorhombic perovskite is distorted further away from the ideal cubic structure in agreement with theoretical predictions. The 298‐K isothermal equations of state of the three perovskites are indistinguishable within the uncertainty limits of the experiment. The zero‐pressure bulk modulus KT0 = 261 (±4) GPa with its pressure derivative KT0′ = 4 is close to that determined in previous static high pressure measurements. The thermal expansion obtained from the high P ‐ T experiments are consistent with previous measurements carried out at zero pressure but shows a strong volume dependence. The temperature derivative of the isothermal bulk modulus at constant pressure (∂KT/∂T)p is −6.3(±0.5)×10−2 GPa/K. Analyses of the high‐temperature data give a value for the Anderson‐Grüneisen parameter δT of 6.5–7.5, which is significantly higher than that used in recent lower mantle models.
An Early Pliocene thermal disturbance of the main central thrust, central Nepal: Implications for Himalayan tectonics
Authors: Copeland Peter , Harrison T. Mark , Hodges Kip V. , Maruéjol Patricia , Le Fort Patrick , Pecher Arnaud
Published: February 2008

Since the beginning of the collision between India and Asia at about 50 Ma, the convergence in the Himalaya has largely been taken up along major thrust zones. In this study, samples of the Lesser Himalaya Formations, up to 10 km below the Main Central Thrust (MCT), and the Greater Himalaya Sequence, up to 12 km above the MCT, have been analyzed by the 40Ar/39Ar and U‐Pb methods to investigate the thermal history of the MCT. The ages can be summarized as follows: (1) The youngest ages from muscovites (3.1 Ma), biotites (3.4 Ma), and hornblendes (4.1 Ma) all come from within 1 km of the MCT, (2) there is a marked asymmetry of ages between the footwall and the hanging wall of the MCT; the maximum mica age in the hanging wall (Greater Himalaya Sequence) is 13 Ma, whereas a muscovite 5 km below the MCT, in the footwall, has an age spectrum with a gradient from 400 to 1400 Ma, (3) five K‐feldspars from −6.2 to 11.9 km above the MCT all give minimum ages in the range 3.0–6.4 Ma, and (4) a 206Pb/238U age on a brannerite from the Greater Himalayan Sequence is 4.8 Ma. Structural and petrologic observations preclude the possibility that this age‐distance distribution reflects faulting within the Greater Himalaya Sequence and production of the necessary thermal energy by shear heating requires unrealistically high shear stresses. Infiltration of hot fluids through the MCT zone appears to be the best hypothesis to explain these data. Simple numerical simulations, which account for heat transfer by advection within the fluid flow zone and by conduction outside it, indicate that the observed age distribution could have been produced by infiltration of hot fluids through the MCT zone at circa 5 to 4 Ma within the following range of conditions: the fluids heated rocks to temperatures in the range 470 to 510°C for less than 1 million years in a region narrower than the entire MCT zone. The temperature of the thermal disturbance inferred from the 40Ar/39Ar data is consistent with petrologic data from albitic alteration assemblages in the Greater Himalaya Sequence and the U‐Pb age of the brannerite is similar to the youngest 40Ar/39Ar hornblende age. This scenario, hydrothermal heating culminating at about 4 Ma, is similar to the model of Le Fort (1981) for the generation of the High Himalayan leucogranites in the Late Oligocene ‐ Early Miocene by dehydration of the footwall rocks of an active thrust. In the younger instance, we interpret that fluids derived from the footwall rocks of the Main Boundary Thrust migrated upward through the Lesser Himalaya Formations and were subsequently channeled along the MCT, producing the thermal disturbance.
Some problems on detection of earthquake precursors by means of continuous monitoring of crustal strains and tilts
Authors: Takemoto Shuzo
Published: February 2008

Previous work carried out in Japan to search for earthquake precursors by means of continuous monitoring of crustal strains and tilts was reexamined. We find no evidence of reliable precursors observed simultaneously with more than two instruments or at more than two observation sites. There is a fair chance to detect earthquake precursors if strain meters and tiltmeters are distributed at many closely spaced sites in and around the source region, but observations with strain meters and tiltmeters have not been executed at distances less than twice the source dimension. We thus propose the following relation as a criterion of earthquake prediction at the present stage: M = 1.96 log r + 3.84, where M is the magnitude of an earthquake and r (kilometers) corresponds to 2 times the radius of the source dimension. Precursory strain and tilt changes may be detected within the distance of r, as revealed by data of geodetic surveys. Then we examined whether this criterion can be revised by employing reliable data obtained from laser strain meters installed in a deep tunnel at the Amagase station, Kyoto, Japan. During the period of 32 months from January 1, 1987, to August 31, 1989, there occurred three moderate earthquakes exceeding magnitude 4 within 30 km of the strain meter, but we could not detect anomalous strain changes associated with earthquakes. Strain data obtained from laser strain meters at Amagase do not correlate with surface meteorological changes, such as atmospheric and temperature changes and rainfalls, but are affected by the changes in the groundwater table around the observation tunnel. A search of tidal strain data from the laser strain meters at Amagase reveals no evidence of anomalous changes in tidal signals during the 32 months of observation. The limit of detection for changes in the tidal strain amplitudes are 5% for the M2 constituent during the 32‐month averaging period. Judging from these facts, it would be difficult to detect earthquake precursors by employing strain meters and tiltmeters at distances of more than twice the earthquake source dimension. In searching for precursory changes, one must take into account the effect of groundwater flow, even when strain meters and tiltmeters are installed in a deep tunnel to avoid the surface meteorological effects.
Spatially heterogeneous strength in thrust fault zones
Authors: Erickson S. Gregg , Wiltschko David V.
Published: February 2008

Deformation in the hanging wall of the Lewis thrust fault at Marias Pass, northwest Montana, is heterogeneous in space, with regions dominated by either large‐scale contraction or extension faults. The distribution of mesoscopic structures within the two regions also differs. The density of fractures is highest in the extensional region, whereas the density of solution seams is highest in the contractional regime. This spatial heterogeneity of structures at the base of the Lewis thrust sheet may result from spatially heterogeneous fault zone strength. In analytical models of an elastic thrust sheet overlying a fault represented by spatial heterogeneity in shear traction or displacement, stress magnitudes and principal directions vary with position within the model thrust sheet. With the application of a Mohr‐Coulomb criterion, contraction and extension faults initiate within localized regions at the base of the model thrust sheet. Spatial variations in mean stress at the base of the thrust sheet predicted by the models may explain both the greater density of solution seams in the contractional region than in the extensional region and the greater density of mesoscopic faults and veins in the extensional region than in the contractional region of the Marias Pass exposure. The spatial heterogeneity of pressure solution and cataclasis in response to variable mean stress within a fault zone of heterogeneous strength may facilitate thrust sheet movement relative to movement over a fault zone of uniform strength, because a decrease in mean stress enhances cataclasis and an increase in mean stress maintains a constant shear stress during pressure solution.
Early to Mid‐Tertiary inverted metamorphic gradient under the Colorado Plateau: Evidence from eclogite xenoliths in ultramafic microbreccias, Navajo Volcanic Field
Authors: Helmstaedt H. H. , Schulze D. J.
Published: February 2008

Mid‐Tertiary ultramafic microbreccia diatremes from the Navajo volcanic field in the Four Corners area of the Colorado Plateau contain an ultramafic xenoliths suite in which lower‐temperature, prograde‐metamorphic eclogites are associated with higher‐temperature garnet clinopyroxenites that were retrograded and hydrated to chlorite eclogite. Assemblages and directions of metamorphic reactions are consistent with the model of a Laramide age inverted metamorphic gradient established as prograde reactions in a dehydrating oceanic slab caused hydration and cooling in the overlying, subcontinental upper mantle wedge. Comparison with an inverse metamorphic gradient in the Pelona schists, inferred to represent an exposed part of the same subduction system in southwestern Arizona and southern California, confirms the shallow dip of the Laramide subduction zone predicted by the regional distribution of subduction‐related volcanic rocks.
Large‐scale crust formation and lithosphere modification beneath Middle to Late Cenozoic calderas and volcanic fields, western North America
Authors: Johnson Clark M.
Published: February 2008

Over 500,000 km3 of intermediate‐ to silicic‐composition ash flow tuffs and lavas were erupted from large‐volume volcanic fields in western North America during the middle to late Cenozoic. Of the commonly used isotope systems, Nd isotope data provide the best constraint on the proportions of crust and mantle components in the tuffs; all tuffs that have been studied contain a major, often dominant, mantle component. The proportion of mantle to crustal components is best constrained for ash flow tuffs that were erupted on Precambrian crust. There is no simple correlation between inception of extensional faulting with development of calderas, although all calderas formed in or adjacent to regions that ultimately underwent crustal extension to some degree. Similarly, there are no first‐order correlations between the proportion of mantle‐derived component in the silicic magmas and the tectonic setting. The common thread to all caldera complexes is that they are generated by large fluxes of mantle‐derived basaltic magmas. Detailed isotopic and petrologic studies of several caldera complexes indicate that the silicic magmas were fundamentally derived by fractional crystallization of mantle‐derived magmas, accompanied by assimilation of continental crust. Cenozoic caldera‐related magmatism in western North America represents a major episode of crustal growth and hybridization; crustal growth rates in the Cenozoic may rival those that occurred in the Proterozoic during initial crust formation. New results of petrologic and isotopic studies of multicyclic caldera complexes in the northern Rio Grande rift region, in addition to several other areas of the western United States, confirm previous models that predict large changes in O, Sr, and Nd isotope compositions of the crust during injection of mantle‐derived basaltic magmas. Continued injection of basaltic magmas results in an increase in the mean density of the crust, which may be reflected in upward movement of the locus of silicic magma evolution in the crust; such changes may be monitored in areas underlain by ancient crust using Pb isotope ratios if the crust is stratified in U/Pb ratios. If accompanied by assimilation, crystallization of mantle‐derived magmas that stall near the crust‐mantle boundary will return crustal components to the mantle in the form of mafic and ultramafic cumulates, in addition to producing a petrologically complex crust‐mantle boundary. This model can explain many geophysical characteristics of the crust and upper mantle in western North America and supports the conclusions of many recent studies of lower crustal xenoliths, which propose recent magmatic underplating in tectonically active regions.
Isotopic composition of Oligocene mafic volcanic rocks in the Northern Rio Grande Rift: Evidence for contributions of ancient intraplate and subduction magmatism to evolution of the lithosphere
Authors: Johnson Clark M. , Thompson Ren A.
Published: February 2008

Mafic lavas erupted during initiation of regional extension at 26 Ma in the northern Rio Grande rift were derived from at least two isotopically distinct mantle sources. One is characterized by 87Sr/86Sr = 0.70495, εNd = −4, 206Pb/204Pb = 18.2, and the other by 87Sr/86Sr = 0.7044, εNd = 0, 206Pb/204Pb = 18.2. The low‐εNd value source (MANTLE 1) is interpreted to largely reflect the isotopic compositions of the lithospheric mantle in the region. Isotopic compositions of Cenozoic mafic lavas and Proterozoic rocks are used to constrain models for evolution of the lithosphere. The low εNd values of the MANTLE 1 component were probably produced by evolution of a light rare earth element‐enriched upper mantle or input of low‐εNd material during development of the lithosphere in the Early Proterozoic. The εNd values and 206Pb/204Pb ratios in evolved rocks are as low as −7 and 17.3, respectively, indicating interaction with lower crust that had εNd ≤ −12 and 206Pb/204Pb ≤ 17.0. Initial 87Sr/86Sr ratios both increase and decrease slightly in evolved rocks, indicating interaction with lower crust that had Sr isotope ratios that were generally similar to those of the mantle. The εNd value of the modern lithospheric mantle beneath the northern Rio Grande rift is >18 units lower than the projected modern values of the asthenospheric mantle from which the Proterozoic crust was originally derived. The higher‐εNd value source (MANTLE 2) may reflect mixing of asthenosphere and lithosphere components. The mantle source for most late Cenozoic mafic lavas in the northern Rio Grande rift region lies at 207Pb/204Pb ratios that are significantly higher than the 206Pb/204Pb – 207Pb/204Pb array defined by Proterozoic crust in the region. Although the crustal array is also displaced to higher 207Pb/204Pb ratios as compared to the northern hemisphere oceanic mantle array, indicating incorporation of Archean Pb during crust formation, the still higher 207Pb/204Pb ratios in the Rio Grande rift mantle source is interpreted to reflect continued input of Archean Pb into the developing Early Proterozoic lithospheric mantle after crust formation. These relations are strong evidence that the lithosphere became stabilized shortly after the major crust formation events in the Early Proterozoic.
Evolving geographic patterns of Cenozoic magmatism in the North American Cordillera: The temporal and spatial association of magmatism and metamorphic core complexes
Authors: Lee Armstrong Richard , Ward Peter
Published: February 2008

Four maps are presented here that show the location and extent of magmatic fields between eastern Alaska and northern Mexico during the successive time intervals of 55–40, 40–25, 25–10, and 10–0 Ma, and four others show the distribution of metamorphic core complexes during the same Cenozoic time intervals. The maps are based on U.S. Geological Survey and Canadian Cordilleran data bases contining about 6000 isotopic dates and extensive literature review. For nearly 60 Ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate‐composition magmatic fields. The association is suggestive of a close link between magmatism and core complex formation, namely that magma directly and indirectly lowers the strength of the crust. Magmatism thus controls the location and timing of core complex formation. The stresses responsible may be inherited from Mesozoic crustal thickening, locally created by uplift and magmatic thickening of the crust, and imposed by the global pattern of plate motions and driving forces. Since the Miocene, rates of magmatism, extension, and core complex formation have declined. The modern Basin and Range province is not a suitable model for the situation that existed during major magmatic culminations. The singular event of early Miocene time, the merging of two large magmatic fields, extinguishing the Laramide magmatic gap, explains several disconnected observations: the hyperextension episode of the Colorado River corridor, rapid reorientation of stress patterns across much of western North America, and subsequent rapid tectonic movements in California. Magma‐triggered breakup of western North America lithosphere coincided with development of the San Andreas transform system. Thermal destruction of the Laramide magmatic gap created a California “microplate” about 22 Ma ago that moved rapidly away from North America. Thus two plate tectonic processes, thermal destruction of the lithosphere “bridge” and northward growth of a transform system, interacted to produce Miocene and later tectonic patterns and events.
Modeling ocean tides with and without assimilating data
Authors: Zahel Wilfried
Published: February 2008

When reviewing the application of semiempirical and free ocean tide models to the computation of semidiurnal, diurnal and long‐period tides, it is recognized that the basin dominating features of the computed oscillation systems have in general proven to be compatible with the relevant data available. In particular, free tidal models moreover allow, together with the results from investigations of free oscillations and of tides in idealized ocean basins, to determine the dependence of tidal regimes on specific constituents of tidal dynamics, e.g., on loading and self‐attraction. As the computational results are not as precise as required for, for example, geophysical and astronomical applications, a procedure has been developed permitting to utilize also the increasing amount of information from pelagic data for improving the mapping of ocean tides by numerical models. Using a spectral model, test calculations have first been performed for an idealized ocean basin with well‐determined free and forced oscillation properties. Reference is made in detail to a realistic ocean tide model which includes loading and self‐attraction using a convolution integral and which permits the assimilation of data by minimizing a functional made up by the residuals of data and of equations of momentum. Consistent fields of sea surface elevation and currents, with mass precisely conserved, have been obtained for the M2 tide in the world ocean. Comparisons are drawn between these fields and those having been obtained by applying that free model which represents a special case of the model considering information from data. The gain of the tidal oscillation system in basin wide realism due to assimilating pelagic harmonic constants is estimated and found promising.
Active faults and state of stress in the central part of the Trans‐Mexican Volcanic Belt, Mexico 1. The Venta de Bravo Fault
Authors: Suter M. , Quintero O. , Johnson C. A.
Published: February 2008

The central part of the Trans‐Mexican volcanic belt is being deformed by seismically active, west‐east striking normal faults that cause pronounced scarps. Structural and seismic along‐strike discontinuities allow the following segmentation: (1) The western segment, the area between Morelia and the Los Azufres volcanic center, is characterized by ≤ 25‐km‐long fault scarps that define the Cuitzeo graben, and the Santa Ana Maya horst which has a relief ≤ 850 m with respect to the Cuitzeo graben. The faults cut several Quaternary scoria cones as well as alluvium. (2) The central neotectonic segment extends between Maravatio in the west and Altamirano volcano in the east. The most prominent structure of this segment is the 45‐km‐long Venta de Bravo fault with a relief ≤ 300 m. The February 22, 1979,mb= 5.3 Maravatio earthquake occurred on this fault. The late Quaternary vertical slip rate along the easternmost part of the Venta de Bravo fault is estimated at 2 mm/yr. The lateral extension of the segment is defined by the limits of the Venta de Bravo fault and the eastern termination of the Cuitzeo graben. (3) The eastern segment is made up of the Acambay graben. The limit between the central and eastern segments is defined by an extensional jog between the Venta de Bravo and Pastores faults, which indicates a left‐lateral strike‐slip component. The most recent rupture of this segment occurred in the November 19, 1912,Ms= 6.9 Acambay earthquake that caused vertical displacements ≤ 50 cm along the faults flanking the graben. Stress determinations based on Quaternary scoria cone alignments, the focal mechanism of the 1979 earthquake, and the inversion of striation measurements indicate a stress field withSv>SENE>SNNW. The fault striations as well as the focal mechanism indicate a minor but consistent left‐lateral strike‐slip component. The observed stress and deformation can be explained by the superposition of two sources of stress. Isostatically compensated surface loads related to the high elevation of the Trans‐Mexican volcanic belt probably cause horizontal deviatoric tension and the observed crestal normal faults which strike parallel to the axis of the volcanic belt. The observed left‐lateral strike‐slip component, on the other hand, can be explained by compressional far‐field stresses caused by loads at the boundary between the Cocos and North American plates; the orientation of the faults with respect to the direction of plate convergence is a prefered orientation for left‐lateral slip along the observed faults. Alternatively, the observed left‐lateral strike‐slip component may accommodate trench‐parallel motion of crustal blocks in southwestern Mexico.
The 1988 and 1990 upland earthquakes: Left‐lateral faulting adjacent to the central transverse ranges
Authors: Hauksson Egill , Jones Lucile M.
Published: February 2008

Two earthquakes (ML=4.6 and ML=5.2) occurred at almost the same location in Upland, southern California, in June 1988 and February 1990 and had similar strike‐slip focal mechanisms with left‐lateral motion on a northeast striking plane. The focal mechanisms and aftershock locations showed that the causative fault was the San Jose fault, an 18‐km‐long concealed fault that splays west‐southwest from the frontal fault of the central Transverse Ranges. Left‐lateral strike‐slip faults adjacent to the frontal faults may play an important role in the deformation of the Transverse Ranges and the Los Angeles basin as suggested by these Upland earthquakes, the left‐lateral strike‐slip 1988 ( ML=4.9) Pasadena earthquake on the Raymond fault, 30 km to the west of Upland, and scattered background seismicity along other active left‐lateral faults. These faults may transfer slip away from part of the frontal fault toward the south. Alternatively, these faults could represent secondary faulting related to the termination of the northwest striking right‐lateral strike‐slip faults to the south of the range front. The 1988 and 1990 Upland earthquakes ruptured abutting or possibly overlapping segments of the San Jose fault. The edges of the overlapping aftershock zones, which are sharply defined, together with background seismicity, outline a 14‐km‐long aseismic segment of the San Jose fault. The 1988 mainshock originated at 9.5 km depth and caused aftershocks between 5 and 12 km. In contrast, the 1990 mainshock focus occurred at the top of its aftershock zone, at 5 km, and caused aftershocks down to 13 km depth. These deep aftershocks tapered off within 2 weeks. The rate of occurrence of aftershocks in magnitude‐time space was the same for both sequences. The state of stress reflected in the focal mechanisms of the aftershocks is identical to that determined from background activity and did not change with time during the aftershock sequence. The constant stress state suggests that the 1988 and 1990 events did not completely release all the stored slip on that segment of the fault. The presence of 14 km of unbroken fault, the abrupt temporal termination of deep aftershocks, and the constant stress state all suggest that a future moderate‐sized earthquake (ML=6.0–6.5) on the San Jose fault is possible with a rupture length of at least 14 km and possibly 18 km.
Hypocenter constraint with regional seismic data: A theoretical analysis for the natural resources defense council network in Kazakhstan, USSR
Authors: Li Yingping , Thurber Clifford H.
Published: February 2008

Source depth and epicenter determination remains a simple but key method for discrimination between earthquakes and nuclear explosions. At near‐regional distances, a variety of secondary body wave arrivals can be observed that may be useful for constraining source depth and epicenter. In this paper, we study the source location problem using regional arrival time and arrival azimuth data from a sparse seismic network. A theoretical analysis of the source location inverse problem provides estimates of the stability and uncertainty of the solution for a particular suite of data as a function of epicenter and depth, as well as evaluating the importance of each datum to the solution. An empirical analysis with actual data complements the theoretical analysis. Our approach is applied to the three‐station Natural Resources Defense Council‐Soviet Academy of Sciences network in Kazakhstan. The theoretical analysis indicates that reasonable constraint on depth and epicenter is possible whenever three distinct phases (P, Pn, Pg, PzP, and PmP, or equivalent S phases) are observed at two or three stations. Our theoretical analysis for two‐station locations shows that the epicenter constraints are extremely poor for sources along the line passing through the two observing stations. The use of arrival azimuth in two‐station locations results in improvement in epicenter constraint for events occurring in the vicinity of the line connecting the two stations, but there is no significant improvement elsewhere. An empirical analysis of data from two of the September 1987 chemical explosions and data from an explosion in an active mining area supports the theoretical analysis, showing that two‐station locations (both epicenter and depth) are quite reliable. Furthermore, the theoretical analysis also indicates that, even with data from a single observing station, satisfactory results are obtained if three or more phases plus arrival azimuth are available.