bluuebunny/crossref_metadata_2025 huggingface dataset:
Tidal flexure of Jakobshavns Glacier, west Greenland
Authors: Lingle Craig S. , Hughes Terence J. , Kollmeyer Ronald C.
Published: February 2008

Jakobshavns Glacier, a floating outlet glacier on the West Greenland coast, was surveyed during July 1976. The vertical displacements of targets along two profiles perpendicular to the fjord wall bounding the north margin of the glacier were analyzed to determine the effect of flexure caused by tidal oscillations within the fjord. An analysis based on the assumption that vertical displacements of the glacier reflected pure elastic bending yielded the conclusion that the effective thickness of the ice (i.e., the thickness which remained unaffected by surface and basal cracking and which behaved as a continuum) was ∼160 m 2.6 km upglacier from the calving front and ∼110 m 0.6 km from the calving front. An analysis based on the more realistic assumption that observed bending reflected elastic and viscoplastic deformation yielded the conclusion that the average effective thickness of the ice was 316 ± 74 m (∼40% of the estimated 800‐m total thickness) 2.6 km from the calving front and 160 ± 48 m (∼21% of the estimated 750‐m total) 0.6 km from the calving front. A constitutive relationship appropriate for hard glide during flexure was used.
A crustal seismic refraction study in west‐central Arizona
Authors: Sinno Y. A. , Keller G. R. , Sbar Marc L.
Published: February 2008

A reversed seismic refraction profile was recorded in the southern Basin and Range Province between Parker and Globe, Arizona. Interpretation of both refracted and reflected phases indicates a two‐layered crust 23–25 km thick, and a low Pn velocity (7.67 km/s). PmP/Pn amplitude analysis on individual traces and synthetic seismogram modeling indicate that a positive velocity gradient may exist in the upper mantle. The low Pn velocity is indicative of partial melting in the upper mantle and implies that the crust is identical with the lithosphere. The heat flow data suggest that the lithosphere was about 40 km thick approximately 5 m.y. ago. On the basis of this and a negative free air gravity anomaly we propose that the lithosphere has been thinned from about 40 to 24 km over the past 5 m.y. and will probably thin and uplift further in the southern Basin and Range in Arizona.
Paleomagnetism and tectonic evolution of the Pan‐African Damara Belt, southern Africa
Authors: McWilliams M. O. , Kröner A.
Published: February 2008

Paleomagnetic results are reported from the Nosib, Otavi, and Mulden groups of the Damara Supergroup, a late Precambrian shelf sequence on the southern margin of the Congo craton in Namibia. Three magnetizations were isolated in the Nosib group samples. In order of decreasing blocking temperature they are NQ1 (n = 6 sites, λ = 28°N, ϕ = 323°E, α95 = 15°), NQ2 (n = 7 sites, λ = 51°S, ϕ = 213°E, α95 = 12°), and NQ3 (n = 13 samples, λ = 09°N, ϕ = 295°E, α95 = 13°). Overall precision of all three magnetizations upon tectonic correction suggests that they predate Pan‐African (650–450 Ma) folding. Two magnetizations were isolated in the Otavi group samples, above the Nosib in stratigraphic sequence. The DC1 component of possible prefolding age (n = 4 sites, λ = 52°S, ϕ = 186°E, α95 = 35°) has been over‐printed by the DC2 magnetization (n = 10 sites, λ = 55°S, ϕ = 044°E, α95 = 15°) of probable postfolding age. A single magnetization of probable pre‐folding age was isolated in the overlying Mulden Group samples (n = 6 sites, λ = 12°S, ϕ = 090°E, α95 = 16°). Together with previously published paleomagnetic data from Africa, the new data showed that no great relative movements have occurred between the Congo and Kalahari cratons during the interval of Pan‐African tectonism in the Damara belt (McElhinny and McWilliams, 1977). Continental collision preceded by large relative displacements and closure of a wide ocean (e.g., a Himalayan analog) is effectively ruled out for the Damara belt. We develop an alternative model consistent with the available paleomagnetic and geologic data, which invokes rifting, heating, and stretching of the lithosphere underneath the Damara belt, followed by delamination of the subcrustal lithosphere. Hot asthenospheric material rises to take the place of the detached and sinking lithospheric base, inducing subduction and interstacking of continental crust. The much thickened continental crust is partially melted at depth, intruded by synorogenic and postorogenic granites and finally uplifted and eroded to its present level of exposure. The model is compatible with plate tectonics in that the development of the Damara belt can be broadly compared with modern marginal seas, with the exception that stretching of the lithosphere was not induced by secondary convection above a downgoing slab. Possible causes for stretching are rising mantle plumes or intracontinental distortion within the pre‐Damara African plate.
Constraints on the seismic wave velocity structure beneath the Tibetan Plateau and their tectonic implications
Authors: Chen Wang‐Ping , Molnar Peter
Published: February 2008

We combine observations of group and phase velocity dispersion of Rayleigh waves, of the waveform of a long‐period PL phase, of Pn and Sn velocities from unreversed refraction profiles using earthquakes, and of teleseismic S‐P travel time residuals to place bounds on the seismic wave velocity structure of the crust and upper mantle under Tibet. From surface wave measurements alone, the Tibetan crustal thickness can be from 55 to 85 km, with corresponding uppermost mantle shear wave velocities of about 4.4 to 4.9 km/s, respectively. The Pn and Sn velocities were determined to be 8.12±0.06 and 4.8±0.1 km/s, respectively, using travel time data at Lhasa from earthquakes in and on the margins of Tibet. Combining these results, the crustal thickness is most likely to be between 65–80 km with an average shear wave velocity in the upper crust less than 3.5 km/s. A synthesis of one PL waveform does not provide an additional constraint on the velocity structure but is compatible with the range of models given above. In contrast to observations obtained for eight earthquakes in the Himalaya, measurements of both teleseismic S and P wave arrival times for nine earthquakes within Tibet show unusually large intervals between P and S compared with the Jeffreys‐Bullen Tables. Thus the Pn and Sn velocities apparently do not reflect high velocities in the mantle to a great depth beneath Tibet. From the dependence of the seismic velocities of olivine on pressure and temperature and from the similarity of the measured Pn and Sn velocities beneath Tibet and beneath shields and platforms, the velocities at the Moho beneath Tibet are compatible with the temperature being 250°–300° higher than beneath shields and platforms, i.e., 750°C if the temperature beneath the platforms is close to 500°C. Such a temperature could reach or exceed the solidus of the lower crust. Simple one‐dimensional heat conduction calculations suggest that the volcanic activity could be explained by the recovery of the geotherm maintained by a mantle heat flux of about 0.9 HFU at the base of the crust. If the distribution of radioactive heat production elements were not concentrated at the top of the crust, radioactive heating could also contribute significantly to the recovery of the geotherm and thus lower the required mantle heat flow. Thus the idea of a thickened crust in response to horizontal shortening is compatible both with these data and with these calculations.

Appendices are available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐004; $1.00. Payment must accompany order.
Velocity‐density properties of the lithosphere from three‐dimensional modeling at the Geysers‐Clear Lake Region, California
Authors: Oppenheimer David H. , Herkenhoff Ken E.
Published: February 2008

The analysis of travel time delays from 94 teleseisms recorded at The Geysers‐Clear Lake, California, region reveals a broad area of delay centered at Mount Hannah and encompassing The Geysers geothermal field. A three‐dimensional velocity inversion of the delays indicates that the source of the delays is a low‐velocity body largely within the top 30 km of the lithosphere. Velocity decreases of as much as 20% are observed in the top 15 km near Mount Hannah and diminish with depth. No significant velocity fluctuations are observed below 60 km. The block model of velocity perturbations resulting from the teleseismic residual inversion is used to predict the observed Bouguer gravity field by solving for the relation between velocity and density perturbation. Comparison between the predicted gravity and observed gravity indicates that the same body responsible for the teleseismic delays accounts for the long‐wave‐length 30‐mGal Bouguer gravity minima at Mount Hannah. The results of the velocity‐density modeling suggest that the material beneath Mount Hannah and The Geysers may be partially molten and somewhat more silicic than the surrounding medium.

Supplemental table is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐006; $1.00. Payment must accompany order.
Majorite: Vibrational and compressional properties of a high‐pressure phase
Authors: Jeanloz Raymond
Published: February 2008

Majorite, the garnet‐structured high‐pressure phase of pyroxene, has been characterized by infrared (IR) spectroscopy, and X ray diffraction between 0 and 8 GPa (80 kbar). In this structure, three out of four (Mg, Fe) are in eightfold coordination and one out of four Si is in sixfold coordination. The IR spectrum indicates the existence of large tetrahedral distortions, probably associated with the small cation radii (significant anion‐anion interactions). For a sample corresponding approximately to (Mg0.79Fe0.21)SiO3 the lattice parameter and density are a = 1151.4 (±0.1) pm and ρ = 3.737 (±0.001))Mg/m3. Hydrostatic compression measurements yield a zero‐pressure bulk modulus and its pressure derivative of K0 = 221 (±15) GPa and K0′ ∼ 4.4 (±4.8). The composition‐dependent volume and bulk modulus in the system pyrope‐almandine‐majorite are well constrained by the present data; for majorite, these are substantially smaller and larger, respectively, than predicted by systematics for garnets. Elasticity systematics largely fail to account for the moduli of garnets, although Anderson's seismic equation of state provides a satisfactory correlation. In transforming to higher pressure phases beyond majorite (with substantially higher densities), pyroxene does not exhibit significant increases in bulk sound velocity. Transformation to majorite probably occurs within the upper mantle and further phase transitions in pyroxene may be difficult to observe seismologically.
Shock compression measurements of single‐crystal forsterite in the pressure range 15–93 GPa
Authors: Syono Yasuhiko , Goto Tsuneaki , Sato Jun‐ichi , Takei Humihiko
Published: February 2008

Shock compression experiments have been performed on pure synthetic single‐crystal forsterite (Mg2SiO4) in the pressure range 15–93 GPa using a newly installed two‐stage light gas gun. The values of the Hugoniot elastic limit (HEL) vary with the shock propagation directions with respect to the crystallographic orientations, reflecting a notable elastic anisotropy in the olivine crystal. The largest value of HEL up to 12 GPa is observed in the shock direction parallel to [010]. In the hydrostatic regime, shock data in the shock velocity‐particle velocity (Us‐up) plane are segmented into two parts by a transient plateau starting at around Us = 8.4 km/s, indicating a phase transition. For the shock data of the low‐pressure olivine phase below about 50 GPa, a linear fit with the equation Us = C0 + sup has yielded parameters of C0 = 6.26 km/s and s = 1.12. The value of C0 is close to the bulk sound velocity of forsterite. Murnaghan‐Birch fit of shock data for the low‐pressure olivine phase incorporated with the static compression data by Olinger (1977) has resulted in K0 = 132 GPa and K0′ = 3.4, which are in reasonable agreement with the ultrasonic data. The close correspondence of the present shock data to the observation of shock residual effects in olivines is noteworthy. The appearance of melt zone or diaplectic glass observed above about 50 GPa seems to correspond to the onset of the phase transition inferred in the present measurements.
Crustal structure of the Sunda Forearc Region west of central Sumatra from gravity data
Authors: Kieckhefer R. M. , Moore G. F. , Emmel F. J. , Sugiarta W.
Published: February 2008

Free‐air gravity anomalies near Nias Island include a −100 mGal low centered 10–20 km landward of the trench axis and a +80 mGal high over the outer arc ridge but also show a few large anomalies unrelated to topography. A −90 mGal low is found on the structural high between the Banyak Islands and Sumatra, north of Nias. An 80 mGal rise (at a gradient of 8 mGal/km) is modeled by a near‐surface body of high‐density material (oceanic crust?) 4 km wide and tens of kilometers long. This slab may be exposed on the southwest coast of Nias, where ultramafic bodies have been mapped, and its northerly trend (compared with northwesterly bathymetric trends) may be related to other obliquely trending structures near here, to a proposed strike slip fault which crosses Nias, and/or to the basement high topped by the Batu Islands. A −30 mGal free‐air low over the forearc basin can be modeled best if pre‐Miocene mélange or continental crust underlies the basin, and it seems unlikely that the basin can be underlain by a large thickness of oceanic crust.
Heterogeneous alteration of the upper oceanic crust: Correlation of rock chemistry, magnetic properties, and O isotope ratios with alteration patterns in basalts from site 396B, DSDP
Authors: Böhlke J. K. , Honnorez J. , Honnorez‐Guerstein B.‐M. , Muehlenbachs K. , Petersen N.
Published: February 2008

Mineralogy, chemistry, and some magnetic properties of altered basaltic rocks from DSDP Site 396B are rather simply related to one another but differ dramatically with the nature of the igneous material and with the sharp alteration zonation within individual basalt fragments. Low‐temperature alteration of the major igneous minerals appears to have been primarily a function of oxidation, which affects titano‐magnetite pervasively (gray interiors) and olivine locally (brown zones). Destruction of olivine in the pillow basalts released Mg++, SiO2, Co, and Ni to the circulating solution and resulted in a calculated 9–10% decrease in density of the most altered (brown) zones in the rock. Seawater phosphate was incorporated in secondary ferric oxides replacing the olivine. In the massive basalts, all of the SiO2 and much of the Mg from olivine recombined as smectite, and the inferred rock density is almost unchanged. In almost all of the crystalline rocks, primary Ca, Na, Fe, Ti, and Al were immobile, owing to the metastability of plagioclase and clinopyroxene. The filling of cracks by smectite, Fe‐Mn oxides, phillipsite, and calcite consumed elements from various sources and was probably more important than basalt alteration for some chemical fluxes. Values of δ18O for the altered rocks are roughly 1–3‰ higher than their fresh equivalents, owing to the addition of 18O‐rich secondary oxides, smectites, and lesser carbonates and phillipsite. The highest values are associated with the smectite‐rich samples. However δ18O is not a good indicator of the degree of alteration because of strong leaching in the most altered zones of the pillow basalts. Pervasive oxidation of titanomagnetite to titanomaghemite has caused all samples to have high Curie temperatures, indicating up to about 70% oxidation. With the destruction of olivine in the brown zones, strong oxidation has caused breakdown of titanomaghemite to more stable nonmagnetic oxides, hence a drop in saturation intensity. These reactions have proceeded further in the pillow basalts than in the massive basalts. Freshly created oceanic crust consisting of various basaltic materials with abundant void space and (or) interstitial sediments is clearly heterogeneous; this primary heterogeneity is accentuated by alteration.
Focal depths and moment tensor representations of shallow earthquakes associated with the Great Sumba Earthquake
Authors: Fitch Thomas J. , North Robert G. , Shields Michael W.
Published: February 2008

Focal depths in the range 8 to 24 km below the seafloor were determined for seven of the larger shocks associated with the Great Sumba earthquake of August 19, 1977. Depth control was achieved by modeling digitized long‐period P waves recorded by the World‐Wide Standard Seismic Network. If the base of the rupture at the time of the mainshock were no greater than the depth of the deepest aftershock, the stress drop would be nearly 500 bars. Amplitude and first motion data from the P waves were inverted for point source moment tensors. The inversion of Rayleigh and Love wave spectra computed from digital seismograms recorded by the Seismic Research Observatories (SRO) and the Abbreviated Seismic Research Observatories (ASRO) broadened the source study to include earthquakes with moments as small as 1024 dyn cm: too small for either traditional first motion solutions or P wave modeling. The inversion procedure was applied to data in the passband from 30 to 120 s. The propagation phase for the Love and the Rayleigh waves was calibrated with signals from two earthquakes for which the source phase was inferred from the P wave solutions. Agreement with the corresponding P wave solutions is nearly the same whether or not the smaller eigenvalues of the inner product matrix are retained in the inversions. The double couple components generally account for more than two thirds of the total moment. Normal faulting along an east to northeasterly strike characterizes the mechanisms in the principal zone of aftershock activity. Strike slip mechanisms consistent with horizontal compression normal to the arc prevail in the delayed aftershock zone located 200 km northwest of the mainshock. Of the earthquakes subjected to this surface wave analysis the foreshock stood out as having felt a significantly higher apparent stress, perhaps as much as one half an order of magnitude higher than the others. These results suggest that accurate source parameters for moderate to small magnitude earthquakes can be determined from surface waves recorded by the SRO and ASRO. In particular, the mechanisms of small magnitude foreshocks could be obtained in this way.
Heat‐flow determination in three DSDP boreholes near the Japan Trench
Authors: Burch Thomas K. , Langseth Marcus G.
Published: February 2008

The first deep borehole determinations of temperature gradients and heat flow on the landward wall of the Japan Trench and forearc were made on IPOD DSDP leg 57. These heat flow values are based on temperature logs corrected to equilibrium, using a detailed model of the drilling disturbance. Heat flow values on a deeply submerged marine terrace, landward of the trench slope break are 28 and 32 mW m−2. A measurement in the midslope terrace basin on the landward wall of the trench yielded a value of 22 mW m−2. The results are in good agreement with earlier seafloor measurements and indicate that most of the forearc area is characterized by heat flow about one half of that over oceanic lithosphere seaward of the trench. Our observations indicate only a small increase of heat flow from the trench to the volcanic arc, in agreement with thermal models, which suggests that the subduction of the relatively cold oceanic plate continues to dominate the temperature structure for distances of up to 250 km landward of the trench. The temperature profile in the borehole on the midslope terrace indicates possible vertical flow of pore waters. Hundreds of conductivity determinations were made using a new technique.
Petrogenesis of oceanic andesites
Authors: Maaløe Sven , Petersen Tom Svane
Published: February 2008

The andesites and cogenetic rocks of the intraoceanic island arc systems vary in silica content from 49–50% SiO2 to about 76% SiO2. The most primitive compositions, having contents of about 50% SiO2, are the basaltic andesites, which are primary compositions generated by partial melting of the uppermost section of the subducted oceanic crust. The oceanic andesites cannot have assimilated material from the continental crusts, but their petrogenesis bears on the origin of the continental crust. The fractionation of andesitic magmas occurs at relatively low pressures, within the stability range of plagioclase. Basaltic andesites fractionate mainly orthopyroxene + plagioclase, while andesites fractionate clinopyroxene + orthopyroxene + plagioclase. The petrogenesis of the continental andesites is more complex, as the primary andesites contain sedimentary material from the continental crusts.
Radioelement concentrations in the deep profile through Precambrian basement of the Vredefort structure
Authors: Hart R. J. , Nicolaysen L. O. , Gale N. H.
Published: February 2008

The Vredefort basement granite in South Africa forms the core of an updomed and overturned sequence of Precambrian strata. Slawson (1976) proposed that the granite core has also been overturned, exposing to view a thick section through the Archean crust, increasing in depth toward the center of the dome. Profiles across the Vredefort basement provide an opportunity for studying the distribution of U, Th, and K with depth in the upper and middle crust. Detailed radioelement concentration profiles across the granitic basement showed that (1) the central part of the basement core is depleted in Th and U relative to the perimeter, and the concentration of Th and U falls off regularly from the basement margin inward, (2) the distribution of the radioelements and the distribution of heat production with depth over the outer 8 km are consistent with an exponential depth function, (3) the felsic rocks of the central basement are characterized by high K/U and Th/U ratios and the ratios K/U, K/Th, and Th/U increase from the margin inward, and (4) mafic granulites, which occur in the central regions of the dome, have ‘normal’ K/U, K/Th, and Th/U ratios. High‐grade metamorphic environment resulted in the depletion of both U and Th relative to K in the greater proportion of the felsic rocks from the central regions of the Vredefort basement. Depletion of U was far greater than that of Th. The normal Th/U, K/U, and K/Th ratios in mafic rocks, in close proximity to the felsic rocks, suggest that mineral assemblages have a strong control on the behavior of the radioelements during regional metamorphism in the Vredefort basement.
Present‐day tectonics of the southeastern Caribbean and northeastern Venezuela
Authors: Pérez Omar J. , Aggarwal Yash P.
Published: February 2008

Four telemetered seismic arrays were operated in northeastern Venezuela during the summer of 1979. An analysis of the data collected has resulted in accurate locations for about 100 microearthquakes and four new focal mechanism solutions. On the basis of these new data, an in‐depth analysis of teleseismic data, and geologic evidence, we propose a new plate tectonic framework for the southeastern Caribbean. In this model, underthrusting of the Atlantic seafloor along the Lesser Antilles is extended to the southeast of Trinidad. The subducted slab is shown to dip northwesterly beneath Trinidad and the Caribbean Sea, penetrating depths of at least 150 km. This subduction appears to terminate abruptly in the vicinity of the Los Bajos‐El Soldado fault zone, which trends WNW‐NW and is located in the Gulf of Paria west of Trinidad. Geologic evidence shows right‐lateral strike slip (RLSS) motion in this fault zone, probably initiated in the late Pliocene. This WNW‐NW trending fault zone joins up with the E‐W trending El Pilar‐Casanay fault system in northeastern Venezuela, which also moves right laterally. The El Pilar fault is apparently offset by NNW trending faults that exhibit RLSS motion as well as normal faulting. This complex motion on an en echelon series of faults is interpreted to be the result of WNW motion of South America relative to the Caribbean plate. In our interpretation, the E‐W component of relative motion is accommodated by RLSS motion on E‐W faults and results in normal faulting on NNW‐SSE faults. The smaller component of N‐S convergence is reflected by RLSS motion on NNW trending faults and is accommodated by some internal deformation. The timing of the initiation of RLSS motion on the Los Bajos fault zone and the known total amount of displacement on it as well as other geologic indicators lead us to suggest that this complex mode of strike slip motion began only a few m.y. ago, and that the average rate of relative plate motion in this region is probably ≥0.9 ± 0.2 cm/yr.
Heat transfer in the oceanic crust of the Brazil Basin
Authors: Langseth Marcus G. , Herman Bruce M.
Published: February 2008

A detailed geothermal survey has been made on the western flank of the Mid‐Atlantic Ridge in the Brazil Basin where the crust is 20 m.y. old. The area is generally covered by a 150‐ to 200‐m thick layer of foraminiferal ooze, but basement outcrops are numerous. Sedimentary temperature profiles indicate anomalously low temperatures at the base of the sedimentary layer, which results from cold bottom water penetrating laterally into the upper igneous crust and absorbing much of the geothermal heat. Net influx rates of 10−8 m s−1 are implied by a simple aquifer model. A negative pressure drop across the sedimentary layer is required by this interpretation. Refraction studies indicate low seismic velocities (3.3 to 4.5 km s−1) near the top of layer 2, and a large velocity gradient with depth. The velocity structure in the upper kilometer of the crust is related to the porosity, from which it is inferred that permeabilities there are high. Many of the 52 temperature profiles show a significant decrease in gradient with depth. The curvature can be attributed to one or a combination of the following causes: (1) upward advection of pore water at rates of 10−7 s−1, (2) bottom water temperature variations, (3) variation of conductivity with depth, and (4) experimental errors. Interpretation of curvature in terms of an advective model leads to results hard to reconcile with present knowledge of pressure variations and permeabilities of the oceanic crust.
Possible earthquake migration along the Western European Plate Boundary
Authors: Limond William Q. , Recq Maurice
Published: February 2008

We have observed an apparent migration of seismic events along the North Atlantic Ridge from Jan Mayen Island to the Azores and eastward, culminating in the large earthquake and associated aftershocks in the Goringe Bank area in 1969. The migration velocity is about 10° of latitude in 9 to 12 months, about 3 to 10 km/d. Tests performed on normalized data show the migration to be statistically significant with a t‐statistic equalling that for aftershocks, swarms, etc. occurring in an area. We believe this is the first reported earthquake migration along a spreading ridge system and one of the first with such high velocities. We hypothesize that the migration could involve creep migration along transform faults, since reported phenomena with velocities as high as this have involved creep along strike slip faults.
Paleomagnetism and Mesozoic‐Cenozoic paleocontinental maps
Authors: Briden J. C. , Hurley A. M. , Smith A. G.
Published: February 2008

A method of making paleocontinental reconstructions based on ocean floor magnetic anomaly data and land‐based paleomagnetic data is briefly described, and its limitations are assessed. Synthetic polar wander paths for the period 20–200 Ma are presented for Australia, Antarctica, India, Africa, Eurasia, North America, and South America. Comparison is made with observed paths, except for Antarctica, for which the data are too sparse. There are no significant differences between observed and synthetic paths. A plot of inclination anomaly against paleolatitude shows a large scatter that limits the precision of the paleogeographic grid on continental reassemblies. The absence of any systematic trend to this scatter with time suggests that the field has had the same average behavior over the past 180 Ma and that large‐scale expansion of the earth cannot have occurred in this period. South America has undergone the least change in position relative to the poles in the past 160 Ma.
Displacement of surface monuments: Horizontal motion
Authors: Wyatt Frank
Published: February 2008

Measurements of tectonic deformation depend on both accurate instrumentation and adequate coupling of the apparatus to the earth's crust. Existing techniques, capable of resolving the signals of interest (strain rates of 10 n∈/yr), are mainly observatory based. The limitation in the baselength of these instruments (∼1 km) results in a requirement that the reference monuments be exceptionally stable (10 μm/yr). However, records from Piñon Flat Observatory, California, show that the actual horizontal displacements for massive near surface monuments, emplaced in competent, weathered granite, are of the order of 50 μm/yr. The low noise level of the strain measurements at this site indicates that this magnitude of monument displacement is abnormally small. Until high‐accuracy geodetic techniques are developed, sophisticated monuments (or monument monitoring devices) will be necessary to record faithfully continuous crustal deformation.
Design of coils for alternating field demagnetization
Authors: Kono Masaru
Published: February 2008

In alternating field demagnetizers, it is required that as large a uniform magnetic field as possible be produced at the sample space by using a given power supply. To calculate the strength of the magnetic field inside alternating field coils to any desired accuracy, a computer oriented algorithm is developed. Next, the shape of alternating field coils that produce large peak fields is investigated. Although there is a shape of coils that gives absolute maximum magnetic field, it is found that multilayer solenoids are usually satisfactory for maximum field as well as field uniformity requirements. The process of designing an alternating field coil is formulated and some convenient figures and tables are presented.
Heat flow in the north‐central Colorado Plateau
Authors: Bodell John Michael , Chapman David S.
Published: February 2008

We report new heat flow measurements at 25 evenly distributed sites in the north‐central Colorado Plateau. Heat flow values computed for these new sites and one previously published site range from 43 to 116 mW m−2 but fall into the following distinct subsets related to physiographic and tectonic elements within the Plateau: (1) heat flow of 51 mW m−2 (12 sites; s.d. 6) in the San Rafael Swell and Green River Desert which constitute the core of the Colorado Plateau at this latitude, (2) heat flows of 69 mW m−2 (5 sites; s.d. 10) and 88 mW m−2 (4 sites; s.d. 19) in successive parallel north‐south bands approaching the Wasatch Plateau to the west but still 80 km east of the Basin and Range physiographic boundary, (3) heat flow of 64 mW m−2 (5 sites; s.d. 2) along the Salt Anticline trend which strikes northwest in the region of Moab, Utah. Heat flow results for the entire Colorado Plateau have been reexamined in view of our new results, and the overall pattern supports the concept of a low heat flow ‘thermal interior’ for the plateau surrounded by a periphery some 100 km wide having substantially higher heat flow. Average heat flow in the thermal interior is about 60 mW m−2 compared to 80–90 mW m−2 in the periphery. This regional heat flow pattern supports a model of Tertiary lithospheric thinning under the Colorado Plateau whereby the plateau is still in transient thermal response and a 15–20 m.y. lag between uplift and corresponding surface heat flow anomaly is to be expected. The position of the heat flow transition between our interior and peripheral regions in the northwest plateau is roughly consistent with lateral warming and weakening of the Colorado Plateau lithosphere initiated at the Basin and Range boundary some 20 m.y. ago.
Transport of radon from fractured rock
Authors: Schery S. D. , Gaeddert D. H. , Wilkening M. H.
Published: February 2008

The transport of 222Rn from fractured rock has been studied in an abandoned mine. Pressure‐induced flow (both natural and artificial) is quite important and can easily cause an order of magnitude increase in the instantaneous transport of radon into the tunnel airspace compared to that due to flowfree diffusion. Permeability studies indicate that large‐scale cracks of surface density of the order of several cracks per square meter dominate flow. In first order, effective flux due to natural pressure variation follows an approximate dP/dt dependence. In higher order, there exists an enhancement of time‐averaged flux by typically a factor of 2 due to natural pressure variation. Mathematical modeling indicates that the relation between pressure and the strength and time dependence of the radon transport is difficult to explain solely with conventional models for semi‐infinite homogeneous porous media. A model of cul‐de‐sac chambers is proposed to account for some of this dependence.
The Cenozoic Denali Fault System and the Cretaceous accretionary development of southern Alaska
Authors: Csejtey Béla , Cox Dennis P. , Evarts Russell C. , Stricker Gary D. , Foster Helen L.
Published: February 2008

The juxtaposition of disparate geologic terranes in southern Alaska has been previously interpreted to be mainly the result of several hundred kilometers of right lateral offset along the Denali fault system in Cenozoic time. Recent geologic investigations in the Healy quadrangle strongly suggest that Cenozoic horizontal displacements of such magnitude along the Denali fault system do not exist. In the Healy quadrangle, isograds and metamorphic facies boundaries of an early Late Cretaceous metamorphic belt trend across the Cenozoic McKinley strand of the Denali system without significant horizontal offsets. The present geologic makeup of most of southern Alaska is primarily the result of the Talkeetna superterrane, consisting of the previously assembled Peninsular terrane and Wrangellia, colliding with and subsequently being thrust upon the Yukon‐Tanana and Nixon Fork terranes of the ancient North American continent in about middle Cretaceous time. The leading edge of the Talkeetna superterrane faces a wide, complexly deformed zone that contains numerous northwestward thrust miniterranes tectonically intermixed with Jurassic and Cretaceous flysch. The flysch is interpreted to have been deposited mostly in the narrowing and subsequently collapsed oceanic basin between the converging continental blocks. The postcollisional Denali fault system developed in Cenozoic time across the already accreted continental margin, in eastern Alaska along an older, Cretaceous suture.
Rotation of the southernmost Sierra Nevada, California
Authors: Kanter Lisa R. , McWilliams Michael O.
Published: February 2008

Structural, paleogeographic, and magmatic trends of the Sierra Nevada batholith swing from a characteristic NNW trend in the northern and central parts of the batholith to a nearly E‐W trend in the Tehachapi and San Emigdio mountains. Paleomagnetic results from eight sites in the upper Cretaceous (80–86 m.y.) Bear Valley Springs pluton in the Tehachapi Mountains yield a mean direction D = 022°, I = 56° (k = 97, α95 = 5.6°). This mean direction is significantly different from the expected late Cretaceous field direction for this locality and suggests that the pluton has undergone a clockwise rotation of 45° ± 14°, in accord with the change in structural trend in that area. Lower to middle Miocene volcanic and sedimentary rocks of the Kinnick Formation overlie the plutonic rocks and yield a mean direction D = 354°, I = 64°. A paleomagnetic pole derived from this mean direction is not significantly different from the expected Miocene pole for North America. This constrains the time of rotation of the pluton to be between about 80 and 20 m.y., probably predating establishment of the present San Andreas transform in this area. We suggest that the observed rotation is part of a large oroclinal bend affecting to different degrees the entire southern Sierra Nevada. The driving mechanism for such a bending may be right‐lateral shear along a proto‐San Andreas transform, for which there is now a significant body of evidence.
Induced seismicity and style of deformation at Nurek Reservoir, Tadjik SSR
Authors: Keith C. M. , Simpson D. W. , Soboleva O. V.
Published: February 2008

Induced seismicity is occurring at the 315‐m‐high Nurek dam in Tadjikistan, USSR. An intense burst of seismicity accompanied the second stage of filling of the reservoir from 120 to 200 m during August to December 1976. Hypocentral locations and fault plane solutions determined using data from a 10‐station telemetered network surrounding the reservoir show the activity to be concentrated on a series of narrow, planar fault zones near the central basin and on more diffuse zones along the upstream edges of the reservoir. The activity extends to depths of 8 km and appears to be confined within the upper plate of the Ionakhsh thrust, a listric fault which outcrops 1 km north of the dam. The dominant mode of deformation in the post‐Paleozoic sediments of the Tadjik Depression is thrusting. Fault plane solutions for the induced seismicity show short segments of strike slip and thrust faulting for the most intense activity near the central basin and normal and thrust faulting along the upstream edges of the reservoir. None of the fault zones delineated by the induced seismicity corresponds to faults mapped at the surface, while none of the surface faults mapped in the reservoir area has shown an increase in seismicity. The spatial distribution of seismicity within the active zones is in general agreement with that expected from models of the changes in strength associated with reservoir loading. The temporal distribution of seismicity shows a migration outward from the center of the reservoir, probably associated with diffusion of pore pressure.
Comment [on “Possible forms of seismic anisotropy of the uppermost mantle under oceans” by George E. Backus]
Authors: Crampin Stuart
Published: February 2008

Over 15 years ago, Backus (1965) demonstrated approximate equations for the velocity variations of body waves in weakly anisotropic solids. The P wave equation is widely quoted (Aki and Richards (1980) and many others) as modeling P wave variations over all planes in all anisotropic solids. As there appears to be revival of interest in anisotropy, it is worthwhile to note a minor limitation to the application of the P wave equation and to draw attention to some similar linear equations for shear waves, which combined with the P wave equation are very successful in modeling the velocity variations of anisotropic solids.
Modes of mantle convection and the removal of heat from the Earth's interior
Authors: Spohn T. , Schubert G.
Published: February 2008

Earth thermal histories based on two‐layer and whole‐mantle parameterized convection models have been explored to determine the influence of the mode of mantle convection on the content and distribution of mantle radioactive heat sources and on the temperatures and heat transfer properties of the mantle. Two‐layer mantle convection thermally insulates the lower mantle and is less efficient at heat removal than whole mantle convection. Consequently, two‐layer mantle convection requires a lower mantle depleted of radioactive heat sources contrary to the undepleted lower mantles of recent geochemical models. Whole‐mantle convection removes 3 times more primordial heat from the earth's interior and 6 times more heat from the core in 4.5 billion years than does two‐layer convection. The average mantle volumetric heat generation rates for both modes of convection are comparable to that of a potassium‐depleted chondrite. Thus no potassium is necessary in the core to balance the observed present‐day surface heat flux. However, most of the radioactives are concentrated in the upper mantle for two‐layer convection. Model geotherms for whole‐mantle convection compare well with independent estimates of temperature at various depths, including the core‐mantle boundary, while the lower mantle is too hot for two‐layer convection models. Thermal histories of the upper mantle in two‐layer models are similar to those of whole mantle models. Thermal histories for the lower mantle in two‐layer models are similar to those of the core in whole mantle convection models. This is because the lower mantle is thermally insulated by the upper mantle if it convects separately, just as the core is thermally insulated by the whole mantle. The differences between whole and two‐layer mantle convection are mostly due to the thermal insulation of the lower mantle and the smaller efficiency of heat removal of the upper mantle compared to that of the whole mantle, which is much larger in volume. A whole‐mantle convection model reconciling the need for distinct geochemical reservoirs with the physics of mantle convection is presented.
The moho as a phase change: A test of the hypothesis
Authors: Mareschal J.‐C. , Gangi A. F. , Lamping N. L.
Published: February 2008

Heat flow, seismic, and gravity data are used to determine the pressure and temperature at the crust/mantle boundary and at the Conrad seismic velocity discontinuity (where the compressional wave velocity jumps to 6.5 km/s or more) for 12 sites located in the central United States or in the High Plains region. If these discontinuities correspond to phase changes in thermodynamic equilibrium, the pressure and temperature are related by the integrated Clausius‐Clapeyron equation. An ‘empirical Clausius‐Clapeyron’ equation is determined as the best fitting line to the pressures and temperatures obtained at the different sites. Two thermal models of the crust corresponding to different assumptions on the variation of the thermal conductivity, yield as an integrated Claussius‐Clapeyron equation P = (0.085T + 6.2) kbar and P = (0.095T + 4.8) kbar for the Moho (where T is the temperature in degrees Celsius). The correlation coefficient between the temperatures and pressures is too low to give strong support to the hypothesis that the Moho in this region corresponds to a phase change in equilibrium; however, the authors favor the hypothesis because the pressures and temperatures are within the range experimentally determined by Ito and Kennedy [1970] for the transformation from garnet‐granulite to plagioclase‐eclogite.
Seismic Q of the lunar upper mantle
Authors: Nakamura Yosio , Koyama Junji
Published: February 2008

We have determined the frequency dependence of anelastic attenuation for both P and S waves in the upper mantle of the moon in the frequency range of roughly 3 to 8 Hz. The method used is a combination of a single‐station method, in which Q−1 is obtained as an integral with an unknown constant, and a multiple‐station method, the result of which is used to determine the integration constant. The Q determined for P waves (Qp) appears to decrease with increasing frequency, though it is not significantly different from a constant Q, while the Q for S waves (Qs) increases with frequency, becoming significantly greater than Qp at high frequencies. All Q values remain greater than 4000 within this frequency range. Qs above 5 Hz is approximately proportional to the 0.7th power of frequency. The high Q for shear waves at high frequencies suggests that compressional heat loss may be a dominant dissipation mechanism. Some uncertainties in the absolute Q values remain because the detailed velocity structure of the lunar upper mantle is largely unknown.
Paleothermometry of the Sydney Basin
Authors: Middleton M. F. , Schmidt P. W.
Published: February 2008

Evidence from overprinting of magnetizations of Late Permian and Mesozoic rocks and from the rank of Permian coals and Mesozoic phytoclasts (coal particles) suggests that surface rocks in the Sydney Basin, eastern Australia, have been raised to temperatures of the order of 200°C or higher. As vitrinite reflectance, an index of coal rank or coalification, is postulated to vary predictably with temperature and time, estimates of the paleotemperatures in the Sydney Basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin. These estimates give maximum paleotemperatures of present day surface rocks in the range 60–249°C, depending on factors such as location in the basin, the thickness of the sediment eroded, and the maximum paleogeothermal gradient. Higher coal rank and, consequently, larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the Tasman Sea on the basin's eastern margin. A theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperatures in the Sydney Basin. This estimate places the maximum paleotemperature in the range 250–300°C along the coastal region. Both coalification and thermal activation of magnetization models provide strong evidence of elevated paleo‐temperatures, which in places exceed 200°C, and the loss of sediment thicknesses in excess of 1 km due to erosion.
Cosmogenic nuclides in recently fallen meteorites: Evidence for galactic cosmic ray variations during the period 1967–1978
Authors: Evans J. C. , Reeves J. H. , Rancitelli L. A. , Bogard D. D.
Published: February 2008

Cosmogenic radionuclides were measured on 48 fragments of 24 meteorites which fell between 1967 and 1978. Nondestructive gamma counting techniques were used to obtain data on 7Be, 46Sc, 48V, 51Cr, 54Mn, 56Co, 57Co, 58Co and 60Co on at least some of the samples. Sodium 22 and 26Al measurements are reported on all 48 samples. In addition, new rare gas data and exposure ages are reported for the meteorites Guibga, Gorlovka, Dhajala, Louisville, Acapulco, Jilin, Kabo, Alta‐Ameen, and Canon City. The cosmogenic radioisotope and rare gas data are interpreted in terms of a time dependent modulation of galactic cosmic rays spanning one full 11 year sun spot cycle. Special attention is given to the data on 22Na, 46Sc, 54Mn, and 48V with either 26Al or 22Ne/21Ne used to provide a shielding correction. The shielding normalized data using the 26Al method appear to correlate well with calculated production rates scaled against the Deep River neutron monitor. The data for the four isotopes are consistent with a production rate variation of a factor of 2.5–3 between solar maximum and solar minimum for sun spot cycle 20. These data demonstrate that the production rates of cosmic ray‐produced nuclides in meteorites vary considerably according to modulation by the 11‐year solar cycle and support the concept that variations of solar‐modulated, cosmic ray flux of similar magnitude have occurred over much longer time periods.