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At least 1,423 records · Page 79Linked to original sources
USGS2012

Secular, annual, and semiannual changes in the baseline level of the earth's magnetic field at North American locations

Secular, annual, and semiannual changes were obtained from the midnight values of field on specially determined days of quiet magnetospheric conditions. From a selection of North American observatories the latitude variation of these changes was evaluated from 20° to 65°. Results for the active and quiet sun years (1958 and 1965) were compared, and changes at Tucson were traced from 1958 through 1974. The year-to-year variance in the determination of a secular change seemed to be decreased by the present analysis method in comparison to a method which uses the annual mean of hourly values. The annual horizontal field variations were largely northward in early summer and southward in early winter; quiet year amplitudes of 2 to 3 γ about doubled in the solar active year. The amplitudes of the semiannual field vectors were about one half that of the annual vectors; the directions were apparently inconsistent from year to year.

Journal of Geophysical Research Solid Earth↗
USGS2012

Electrical properties of granite with implications for the lower crust

The electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature. Minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure, structurally bound water, oxygen fugacity, and other parameters. The effect of sulfur fugacity may be important but is experimentally unconfirmed. In addition to changing the magnitude of electrical properties, the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties. With increasing temperature, changes in water content retain large, but lessened, effects on electrical properties. Near room temperature, a monolayer of water will decrease the electrical resistivity by an order of magnitude. Several weight-percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5. At elevated temperatures just below granitic melting, a few weight-percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2. Above the melting temperature (650° to 1100°C depending upon water pressure), a few weight-percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy. Remarkably, the few weight-percent water must be present as free water. Experiments with hydrated hornblende schist (with structural water) indicate an electrical resistivity very similar to that for dry granite. The implications of these results, together with the findings of deep magnetic sounding and magnetotelluric surveys, suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle.

Journal of Geophysical Research Solid Earth↗
USGS2012

The Galapagos Spreading Centre at 86° W: A detailed geothermal field study

We report here measurements of the heat flow field of the Galapagos Spreading Center on crust of age less than 1.0 m.y. The 443 measurements in an area of about 570 km 2 reveal the general planform of the geothermal flux and permit the first truly areal estimate of the near-axis conductive heat flux. The intrusion process and associated hydrothermal circulation dominate the surface heat flow pattern, with circulation apparently continuing beyond the limits of our survey. The areal average of the conductive heat flux is 7.1 ± 0.8 HFU (295 ± 33 mW/m 2 ), about one-third the heat flux predicted by plate models. The remaining heat is apparently removed by venting of hydrothermal waters at the spreading axis and through basalt outcrops and hydrothermal mounds off axis. The pattern of surface heat flux is lineated parallel to the axis and the strongly lineated topography. Sharp lateral gradients in the heat flow, greater than 10 HFU/km near escarpments and commonly expressed as high heat flow at the tops of the scarps and lower heat flow in the valleys, may indicate a local concentration of the circulation by surface fault systems and/or variable sediment thickness.

Journal of Geophysical Research Solid Earth↗
USGS2012

The accommodation of relative motion at depth on the San Andreas fault system in California

Plate motion below the seismogenic layer along the San Andreas fault system in California is generally assumed to occur by aseismic slip along a deeper extension of the fault. It is also possible that below the seismogenic layer, deformation is distributed laterally over a zone. Several observed features of the San Andreas fault in California have implications about the mode of accommodation of relative motion along the plate boundary beneath the seismogenic zone: the shallow depth of all earthquakes in California, the depth to which coseismic slip occurred during the 1906 San Francisco earthquake, the broad zone of strain accumulation, the broad heat flow anomaly, and the existence of widely separated parallel faults. The observations strongly imply that below the seismogenic zone, relative motion is distributed over a zone and occurs by inelastic flow rather than by aseismic slip on discrete fault planes. The existence of multiple faults further suggests that tractions at the base of the brittle layer are significant over time periods of years to hundreds of years.

Journal of Geophysical Research Solid Earth↗
USGS2012

Strain accumulation across the Denali fault in the Delta River canyon, Alaska

Deformation along the Denali fault in the Delta River canyon was determined from geodetic surveys in 1941/1942, 1970, 1975, and 1979. The data were best for the 1975–1979 interval; in that period the average strain accumulation was essentially pure right lateral shear at a rate of 0.6 ± 0.1 μrad/a (a is years) (engineering shear) across a vertical plane striking N87°E. The plane of maximum shear is rotated about 30° counterclockwise from the local strike of the Denali fault but closely coincides with the strike of a major linear segment of the fault that begins 50 km farther west. The deformation between 1941/1942 and 1970 is consistent with a similar rate of strain accumulation if one removes the coseismic strain step contributed by the 1964 Alaska earthquake. The 1970–1975 deformation is poorly defined owing to uncertainties in the 1970 survey, but the strain accumulation during that period is certainly much less than during the 1975–1979 interval. The 1975–1979 strain accumulation is interpreted by means of a dislocation model which suggests that the Denali fault in the vicinity of the Delta River Canyon behaves as a leaky transform fault (i.e., a source of spreading as well as lateral slip). The block south of the Denali fault appears to be moving westward (parallel to the strike of the Denali fault west of the 147th meridian) at the rate of about 20 mm/a relative to the North American plate. Because the linear segment of the Denali fault east of the 147th meridian (along which the geodetic network is located) strikes N63°W, accommodation of the westward motion of the southern block requires some spreading within the fault zone as well as right lateral slip on the fault.

Journal of Geophysical Research Solid Earth↗
USGS2012

Variable rates of late Quaternary strike slip on the San Jacinto fault zone, southern California.

3 strike slip displacements of strata with known approximate ages have been measured at 2 locations on the San Jacinto fault zone. Minimum horizontal offset between 5.7 and 8.6km in no more than 0.73Myr NE of Anza indicates 8-12 mm/yr average slip rate since late Pleistocene time. Horizontal slip of 1.7m has been calculated for the youngest sediment of Lake Cahuilla since its deposition 271- 510 yr BP. The corresponding slip rate is 2.8-5.0 mm/yr. Right lateral offset of 10.9m measured on a buried stream channel older than 5060 yr BP but younger than 6820 yr BP yields average slip rates for the intermediate time periods, 400 to 6000 yr BP of 1-2 mm/yr. The rates of slip suggest a relatively quiescent period from about 4000 BC to about 1600 AD.-from Author

Journal of Geophysical Research↗
USGS2012

Tectonic setting for ophiolite obduction in Oman

The Samail ophiolite is part of an elongate belt in the Middle East that forms an integral part of the Alpine mountain chains that make up the northern boundary of the Arabian-African plate. The Samail ophiolite represents a portion of the Tethyan ocean crust formed at a spreading center of Middle Cretaceous age (Cenomanian). During the Cretaceous spreading of the Tethyan Sea, Gondwana Land continued its dispersal, and the Arabian-African plate drifted northward about 10°. These events combined with the opposite rotation of Eurasia and Africa initiated the closing of the Tethyan during the Late Cretaceous. At the early stages of closure, downwarping of the Arabian continental margin combined with the compressional forces of closure from the Eurasian plate initiated obduction of the Tethyan oceanic crust along preexisting transform faults, and still hot oceanic crust was detached along oblique northeast dipping thrust faults. Amphibolites developed at the base of the detached hot peridotite as it was thrust southward over oceanic volcanic and sedimentary rocks. Plate configurations combined with palinspastic reconstructions show that subduction and attendant large-scale island arc volcanism did not commence until after the Tethyan sea began to close and after the Samail ophiolite was emplaced southward across the Arabian continental margin. The Samail ophiolite nappe now rests upon a melange consisting mainly of pelagic sediments, volcanics, and detached fragments of the basal amphibolites which in turn rest on autochthonous shelf carbonates of the Arabian platform. Laterites and conglomerates with reworked laterites on the eroded upper surface of the ophiolite indicate a period of emergence prior to the deposition of shallow water Maestrichtian carbonates. Following emplacement (Eocene) of the Samail ophiolite, the Tethyan oceanic crust began northward subduction, and active arc volcanism started just north of the present Jaz Murian depression in Iran.

Journal of Geophysical Research Solid Earth↗
USGS2012

Magnetic effects of maghemitization of oceanic crust

Both theoretical considerations and available experimental results indicate that magnetic effects of maghemitization are strongly dependent on the grain size of the originally unoxidized titanomagnetite. Maghemitization of single‐domain titanomagnetite results in a decrease in coercivity, an increase in susceptibility, and a large decrease in Q ratio. Maghemitization of multidomain titanomagnetite results in an increase in coercivity, a decrease in susceptibility, and no large changes in Q ratio. Single‐domain titanomagnetite is probably resistant to the development of a chemical remanent magnetization (CRM), whereas multidomain titanomagnetite can acquire a CRM during maghemitization. The behavior of pseudo‐single‐domain titanomagnetite, which is the main carrier of remanence in submarine extrusive rocks, is investigated by comparing the magnetic properties of the French‐American Mid‐Ocean Undersea Study (FAMOUS) (less than 0.1 m.y. old) and the Leg 37 (3.5 m.y. old) pillow basalts recovered from the Mid‐Atlantic Ridge near 37°N. Combining electron microprobe analyses, Curie temperature measurements, and cell edge determinations, we find that the FAMOUS rocks are already oxidized ( z = 0.38), possibly as a result of some high‐temperature maghemitization during cooling of the magma. Comparison with the more highly oxidized ( z = 0.7) Leg 37 pillow basalts indicates that low‐temperature maghemitization of such rocks does not result in appreciable changes of coercivity and susceptibility, although the Q ratio does decrease and CRM seems to be acquired. Such a CRM could account for the anomalously low magnetic inclinations observed at most of the Leg 37 sites.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

An interpretation of episodic slip on the Calaveras fault near Hollister, California

Data from a precision multi-wavelength distance measuring (MWDM) instrument, located near Hollister, California, have been analyzed in terms of strike-slip faulting in the region covered by the network. Four episodes of deformation that are readily identifiable in the MWDM data for the year following September 1975 can be modeled as slip on the Calaveras fault. Creepmeter data tend to support this interpretation, since the creep events and the observed episodes of deformation appear to be correlated in a temporal sense. Lower bounds on the moment for each episode of slip can be determined from the MWDM data by the technique of linear programing. The results indicate that aseismic slip is the dominant mechanism for strain release, since the combined moment of 1.16 × 10 24 dyn cm for the four episodes far exceeds the moment of all of the earthquakes that occurred during the year. Creepmeter data and geologic evidence that are taken in conjunction with the lower bound of the moment indicate that the depth of aseismic slip for some of the episodes of fault slip could extend to below the greatest depth of earthquake hypocenters for this region of central California.

Journal of Geophysical Research Solid Earth↗
USGS2012

Geodetic analysis of reservoir depletion at the Geyser steam field in northern California

Reservoir depletion at the Geysers from 1974 to 1977 is evident in measured changes in gravity, surface strain, and pore pressure drainage. The drainage area increased about 20%, the maximum gravity decrease was about −120 μGal, and the maximum elevation change was about 6 cm during this period. Since the net mass withdrawal is known, it may be combined with the gravity change to estimate a drainage volume. The maximum drainage volume is 25 km 3 . Because the depth of the caprock and extent of the pore pressure drainage is known, this volume limit implies that no significant mass withdrawal occurs below a depth of 4 km. The ratios of surface elevation changes to horizontal contraction coinciding with the drainage area imply an equant drainage geometry, assuming that fluid production produces negative dilatation. Using the same cylindrical geometry used to model the gravity, negative dilatational strain rates of 4 to 5 × 10 −5 /yr were found to produce the observed surface displacements. The likelihood of boiling in the system to produce steam combined with apparent large bulk moduli (from seismic velocities) and small pore pressure declines suggest that most of the reservoir contraction is due to cooling.

Journal of Geophysical Research Solid Earth↗
USGS2012

Strain accumulation in southern California, 1973-1980

Frequent surveys of seven trilateration networks in southern California over the interval 1973–1980 suggest that a regional increment in strain may have occurred in 1978–1979. Prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north-south compression. This secular trend was interrupted sometime in 1978–1979 by an increment in both north-south and east-west extension in five of the seven networks. The onset of this change appears to have occurred first in the networks farthest south. The changes occurred without any unusual seismicity within the networks, but the overall seismicity in southern California was unusually low prior to and has been unusually high since the occurrence. The average principal strain rates for the seven networks in the 1973–1980 interval are 0.17 μstrain/yr north-south contraction and 0.08 μstrain/yr east-west extension. Although the observed increment in strain could be related to unidentified systematic error in the measuring system, a careful review of the measurements and comparisons with three other measuring systems reveal no appreciable cumulative systematic error.

Journal of Geophysical Research Solid Earth↗
USGS2012

Deep volcanic tremor and magma ascent mechanism under Kilauea, Hawaii

Deep harmonic tremor originating at depths around 40 km under Kilauea was studied using records accumulated since 1962 at the Hawaii Volcano Observatory of the U.S. Geological Survey. The deep source of the tremor was determined by onset times and confirmed by the relative amplitude across the island-wide network of seismometers. The period of tremor was conclusively shown to be determined by the source effect and not by the path or station site effect because the period would change considerably in time but maintained uniformity across the seismic net during the tremor episode. The tremor appeared to be primarily composed of P waves. We interpret the observed period and amplitude in terms of the stationary crack model of Aki et al. (1977) and find that the seismic moment rates for deep tremors are considerably larger than those for shallow-tremors suggesting more vigorous transport for the former. We propose a kinematic source model which may be more appropriate for deep tremor. According to this model, a measurable quantity called ‘reduced displacement’ is directly proportional to the rate of magma flow. A systematic search for deep tremor episodes was made for the period from 1962 through 1979, and the amplitude, period, and duration of the tremor were tabulated. We then constructed a cumulative reduced-displacement plot over the 18-year period. The result shows a generally steady process which does not seem to be significantly affected by major eruptions and large earthquakes near the surface. The total magma flow estimated from the reduced displacement is however, one order of magnitude smaller than that estimated by Swanson (1972). It may be that most channels transport magma aseismically, and only those with strong barriers generate tremor.

Hawaii↗
USGS2012

Magnetic signals from the core of the earth and secular variation

An oscillating, radial magnetic dipole source was assumed to exist in the core of the earth, 100 km beneath the core-mantle boundary. As an approximation, electromagnetic propagation was assumed in the core in lieu of hydromagnetic propagation, which could not be used because of unknown internal fields.Using Debye potentials, the radial and horizontal components of the surface fields were calculated using various assumed conductivity parameters in the core and in the mantle. It is concluded that most spherical harmonic models of the earth's magnetic field do not include enough terms to describe properly the field of core sources with periods of tens of years. These short-period variations are especially important in describing the secular variation. Because of this, a proper description of the secular variation requires more spherical harmonic terms than are required for the field itself. Inadequate representation of short-period variations in spherical harmonic models may contribute to the rapid deterioration of predictive models. Alternatives to spherical harmonic analysis for secular variation should be investigated; regardless of the method used, a much greater spatial distribution of high quality secular variation data is needed.

Journal of Geophysical Research Solid Earth↗
USGS2012

Quaternary and geodetically measured crustal movements in the Tokai District, Central Honshu, Japan

Geodetic measurements made in this century demonstrate that modern movements correlate well with deformation that has occurred during the past 2 m.y. Both show effects of subduction and of the Quaternary collision of Izu Peninsula with Central Honshu. Westward from Suruga Bay, deformation is greatest near the coast, decreases markedly towards the uplifting Akaishi Mountains, and then increases again in the conjugate block-faulted terranes of Chubu district. Quaternary faults, historic earthquakes, and the distribution of horizontal shear straining since ∼1880 all reflect this deformation pattern. Since 1900 the west coast of Suruga Bay has undergone secular subsidence, horizontal contraction and tilting towards Suruga Trough, all typical of the strain buildup process that occurs landward of subduction zones. However, futher westward, active mountain building is occuring, and the Akaishi Ranges are rising 3 mm/a. Unlike the episodic aseismic uplift of the Transverse Ranges of southern California, movements here have been relatively uniform. It seems likely that an increase in the tilt rate of the Suruga coast recognized since ∼1973 is part of a long-term readjustment to the great 1944 Tonankai earthquake. Data show that a monotonic increase has occurred since at least 1949, and similar effects are seen adjacent to the 1944 rupture zone over 200 km southwest along the Nankai Trough.

Journal of Geophysical Research Solid Earth↗
USGS2012

Gravity model and structural implications of the Goddard Pendant, Sierra Nevada, California

A subsurface model for the Goddard pendant is constructed from a residual gravity high of about 7 mGal over the pendant. The model, which is the simplest and most geologically reasonable possibility, shows a metamorphic block that tapers with depth and extends about 3.5 km below the surface. The structures in the Goddard pendant are similar in style and orientation to those in other Sierra Nevada pendants, indicating that the country rock was neither deformed nor rotated during pluton emplacement. Consequently, emplacement must have been a passive rather than a forceful process. The pendant itself represents a piece of country rock trapped between plutons which are dome shaped in cross section.

Journal of Geophysical Research Solid Earth↗
USGS2012

Minor and trace element geochemistry of volcanic rocks dredged from the Galapagos spreading center: Role of crystal fractionation and mantle heterogeneity

A wide range of rock types (abyssal tholeiite, Fe-Ti-rich basalt, andesite, and rhyodacite) were dredged from near 95°W and 85°W on the Galapagos spreading center. Computer modeling of major element compositions has shown that these rocks could be derived from common parental magmas by successive degrees of fractional crystallization. However, the P 2 O 5 /K 2 O ratio averages 0.83 at 95°W and 1.66 at 85°W and implies distinct mantle source compositions for the two areas. These source regions also have different rare earth element (REE) abundance patterns, with [La/Sm] EF = 0.67 at 95°W and 0.46 at 85°W. The sequence of fractionated lavas differs for the two areas and indicates earlier fractionation of apatite and titanomagnetite in the lavas from 95°W. The mantle source regions for these two areas are interpreted to be depleted in incompatible (and volatile?) elements, although the source region beneath 95°W is less severely depleted in La and K. Incompatible trace element abundances in 26 samples are used to infer that the range of Fe-Ti-rich basalt from 85°W represents 19 to 35% residual liquid following crystal fractionation of a mineral assemblage of plagioclase, clinopyroxene, and lesser olivine. The most highly differentiated samples have also had less than 1% titanomagnetite removed. Most samples from 85°W can be related to a common parental magma that contained approximately 9 wt % FeO*, 1 wt % TiO 2 , and had an Mg number (Mg# = 100 Mg/(Mg + Fe 2+ )) of about 65. Although the samples from 95°W cannot all be derived from a common parental magma, the inferred parental magmas may have been derived by varying degrees of partial melting of a common source. The fractionation sequence consists of two parts: an initial iron enrichment trend followed by a silica enrichment trend. We interpret the trace element data to indicate that the most iron rich lavas represent about 32% residual liquid derived by crystal fractionation of plagioclase, clinopyroxene, and lesser olivine from a parental magma with an Mg number of about 66. The silica enrichment trend results from crystallization of titanomagnetite and some apatite. Fractionation of pigeonite, which is a minor phase in the major element models, cannot be distinguished from clinopyroxene fractionation by using trace elements.

Journal of Geophysical Research Solid Earth↗
USGS2012

The Redskin granite: Evidence for thermogravitational diffusion in a Precambrian granite batholith

The 1000-Ma-old composite Pikes Peak batholith consists largely of medium- to coarse-grained biotite granite but is intruded by several smaller granite plutons. Two of these, the Tarryall Mountains batholith and the Redskin Granite (Hawley, 1969; Hawley and Wobus, 1977), are here interpreted as cupolas atop the main mass of the Pikes Peak batholith. Chemical analysis for 38 elements in rocks from the Redskin Granite, the Tarryall Mountains batholith, and the biotite granite of the Pikes Peak batholith support the contention that the compositions of the smaller plutons are unlikely to have evolved from that of the main mass of the batholith by crystal fractionation. It is believed that these compositions arose through liquid-state fractionation processes, possibly the convection-aided thermogravitational model of Shaw et al. (1976). It is important to recognize that the chemical signature of this fractionation process can be discerned in holocrystalline rocks as well as in their glassy equivalents because the process may be instrumental in the formation of some kinds of lithophile ore deposits.

Journal of Geophysical Research Solid Earth↗