bluuebunny/crossref_metadata_2025 huggingface dataset:
The pressure and temperature dependence of the elastic properties of polycrystal magnesiowustite
Authors: Bonczar L. J. , Graham E. K.
Published: February 2008

The compressional and shear wave velocities in a series of polycrystal samples of magnesiowustite (Mgx, Fe1‐x)YO have been measured using the pulse superposition method. Each specimen was characterized in terms of composition, stoichiometry, and porosity by chemical and microprobe analysis, and by X ray diffraction. In addition, the measurement of velocity in three perpendicular directions has demonstrated that the individual specimens were isotropic to within 1/4%. The application of empirical corrections to account for the effects of nonstoichiometry and porosity has allowed the determination of the isotropic elastic properties for the entire stoichiometric (Mg,Fe)O solid solution series. Our results for MgO (KS = 1619 ± 16 kbar, μ = 1304 ± 16 kbar) are consistent with previous determinations of these properties from single‐crystal measurements. In the case of FeO (KS = 1835 ± 31 kbar, μ = 589 ± 14 kbar), our data generally correspond to the results of previous investigations, which used both ultrasonic and static compression techniques, when corrections are applied for the effects of nonstoichiometry. Our measurements on Fe‐rich magnesiowustites have demonstrated clearly that the isotropic elastic properties, especially the bulk modulus KS, are strongly dependent on the defect structure (nonstoichiometry) of the specimens. The increasing number of vacancy clusters toward the FeYO end‐member correlates with decreasing bulk modulus and anomalously high values of (∂K /∂P)T. However, when corrections are applied for the nonstoichiometric effects, the elastic properties of the (Mg, Fe)O solid solution series behave according to the linear compliance relation proposed by Liu (1968) and Jackson et al. (1978).
The 1978 Yellowstone‐Eastern Snake River Plain Seismic Profiling Experiment: Data and upper crustal structure of the Yellowstone Region
Authors: Schilly M. M. , Smith R. B. , Braile L. W. , Ansorge J.
Published: February 2008

Eleven in‐line refraction profiles, recorded to distances of 300 km, and one azimuthal fan plot were constructed from data recorded with a 150‐station array in the Yellowstone National Park area during the 1978 Yellowstone‐Snake River Plain seismic experiment. Interpretations of the data suggest that the crustal P wave velocity model for the Yellowstone region is characterized by (1) an averaged 10‐km‐thick upper crustal layer, Vp = 6.0 km/s, (2) an average crustal velocity of 6.3 km/s, and (3) a total crustal thickness of 44 km. Velocity models are presented for profiles that emphasize the upper crust and show (1) a decrease in the depth to the top of the upper crustal crystalline basement from 5 km in southwestern Yellowstone near Island Park to 1 km at the northeast side of the Yellowstone Plateau that is interpreted as a progressive thinning of the silicic surface volcanic layer to the northeast and (2) evidence for a large lateral inhomogeneity interpreted to be a low‐velocity body, with a decrease of at least 10% in P wave velocity, located beneath the northeast corner of the Yellowstone Plateau. The low‐velocity zone coincides with a local −30‐mgal residual gravity anomaly and is located beneath part of the Sour Creek resurgent dome and part of the Hot Springs Basin, the largest hydrothermal system in Yellowstone. The low‐velocity body has a maximum depth to the top of 3 km and a minimum depth to the bottom of 9 km and may represent a zone of partial melt. In comparison to the thermally undisturbed upper crust of the surrounding Rocky Mountains the upper crust of the northeastern Yellowstone Plateau appears laterally inhomogeneous in velocity and layer thickness, suggesting effects of thermal and magma intrusion, whereas the lower crust appears relatively homogeneous.
The Mt. Hood region: Volcanic history, structure, and geothermal energy potential
Authors: Williams David L. , Hull Donald A. , Ackermann Hans D. , Beeson Marvin H.
Published: February 2008

The volcanic history of the Mt. Hood region in the last 15 m.y. shows a slow migration of six to eight volcanic centers from a position about 20 km west of Mt. Hood to the most recent volcanic center in the series. This migration was accompanied by the development of the structural setting visible in the Mt. Hood region today. Mt. Hood stands on a swell which runs north and south along the Cascade Range. Within this swell is a graben, the High Cascades graben, which is parallel to the axis of the swell and has the Hood River Fault as its eastern boundary fault. Superimposed on this graben is the Mt. Hood subsidence block. Subsidence of this block and graben is presumably related to withdrawal of magma from beneath the block and eruption of this material on top of the block to form Mt. Hood. The overall structure is one of broad uplift and narrow subsidence that is at least 5 m.y. old and presumably still active. Cutting across the entire Mt. Hood region and most of the Pacific Northwest is a set of northwest‐southeast right‐lateral strike‐slip faults. These are numerous around Mt. Hood, but the offset on any single fault is small (a few meters) and in some cases vertical. These faults are also young and indicate east‐west extension and north‐south compression. An older and probably inactive set of northeast‐southwest folds and thrust faults is evident in the region, particularly in the Miocene Columbia River Basalt Group. These may be related to a long history of north‐south compression. The volcanic history and structure are presumably a result of subduction of the Juan de Fuca plate beneath the North American plate. This subduction is oblique, leading to a substantial component of right‐lateral motion which, in turn, leads to north‐south compression in the Pacific Northwest. Crustal thinning associated with volcanism has resulted in an elevated geothermal gradient around Mt. Hood. This high gradient makes it possible to tap water in excess of 80°C within 1500 m of the surface. If these thermal waters can be produced in large volumes, they might provide a substantial amount of economically competitive energy for space heating and industrial processes, in the region adjacent to the Cascades. A simple geothermal resource calculation shows that a 1000 km2 area around Mt. Hood could produce beneficial heat which, if supplied by electricity, would require over 1000 MW for 30 years.
Lithospheric deformation within the Molucca Sea arc‐arc collision: Evidence from shallow and intermediate earthquake activity
Authors: McCaffrey Robert
Published: February 2008

This paper presents the results of a local earthquake survey in the Molucca Sea arc‐arc collision zone of eastern Indonesia. Although intermediate depth earthquakes define zones dipping to the west beneath the Sangihe arc and to the east beneath the Halmahera volcanic arc, only shallow activity (less than 60 km depth) occurred beneath the central part of the collision zone where the majority of activity is observed. The concentration of earthquake foci in the 10‐ to 50‐km depth range in a limited region beneath the Talaud‐Mayu Ridge suggests that convergence between the arcs proceeds by shortening within basement of the intervening Molucca Sea plate rather than by slip along shallow dipping planes between the arcs and the subducted slabs. Published focal mechanism solutions suggest that high‐angle reverse faulting beneath the Talaud‐Mayu Ridge generates almost all of the seismic energy release within the collision zone. The predominance of shallow reverse and strike slip faulting at the axis of the bilaterally subducting Molucca Sea lithosphere suggests that stresses are not due to bending alone and have a large horizontal regional component perpendicular to the island arcs. The geometry and state of stress within the Molucca Sea lithosphere requires an external driving force for the convergence between the two arcs and a strong degree of coupling between the island arcs and the subducting Molucca Sea plate, at least at the present stage of collision.
Steady state volcanism: Evidence from eruption histories of polygenetic volcanoes
Authors: Wadge G.
Published: February 2008

Some volcanoes erupt magma at average rates which are constant over periods of many years, even though this magma may appear in a complex series of eruptions. This constancy of output is tested by construction of a curve of cumulative volume of erupted magma, which is linear for steady state volcanism, and whose gradient defines the steady state rate Qss. The assumption is made that Qss is the rate at which magma is supplied to these polygenetic volcanoes. Five general types of eruptive behavior can be distinguished from the cumulative volume curves studied. These types are interpreted in terms of a simple model of batches of magma rising buoyantly through the crust and interacting with a small‐capacity subvolcanic magma reservoir. Recognition of previous steady state behavior at a volcano may enable the cumulative volume curve to be used empirically as a constraint on the timing and volume of the next eruption. The steady state model thus has a limited predictive capability. With the exception of Kilauea (Qss = 4 m3 s−1 ) all the identified steady state volcanoes have values of Qss of a few tenths of one cubic meter per second. These rates are consistent with the minimum flux rates required by theoretical cooling models of batches of magma traversing the crust. The similarity of these Qss values at a variety of volcanoes (producing basalt, andesite, and dacite magmas) in very different tectonic settings suggests that the common factors of crustal buoyancy forces and the geotherm‐controlled cooling rates control the dynamics of magma supply through the crust. Long‐term dormancy at active volcanoes may be a manifestation of the steady state accumulation of magma in large crustal reservoirs, a process that complements the intermittent periods of steady state output at the surface. This possibility has several implications, the most important of which is that it provides a constraint on the supply rate of new magma to the bases of plutons.
Lithospheric flexure at fracture zones
Authors: Sandwell David , Schubert Gerald
Published: February 2008

Bathymetric profiles across six major fracture zones (FZ's) in the North Pacific are used to demonstrate the absence of vertical slip on the fossil fault planes. The scarp heights on these FZ's are constant with age and equal to the initial vertical offsets at the ridge‐transform fault‐FZ intersections. Because of the frozen‐in scarp and the differential subsidence of lithosphere far from the FZ, the lithosphere bends in the vicinity of the FZ. This flexure results in a characteristic ridge‐trough topographic FZ signature. The flexural amplitude, which is the difference between the scarp height and the overall change in depth across the FZ, increases with age. Good fits to the bathymetric profiles across the Mendocino and Pioneer FZ's are obtained by modelling the topography as the flexure of a thin elastic plate with an age‐dependent effective elastic thickness. Results of the modelling indicate that the base of the elastic lithosphere is approximately defined by the 450°C isotherm. Maximum bending stresses at FZ's are on the order of 100 MPa, substantially less than the stresses encountered at subduction zones. Because the Mendocino and Pioneer FZ's are separated by less than a flexural wavelength, they are elastically coupled.
Array refraction profiles and crustal models of the Canada Basin
Authors: Baggeroer Arthur B. , Falconer Robin
Published: February 2008

Canada Basin acoustic reverberation experiment (CANBARX) was an ice camp located at 74°N, 154°W in the Canada Basin. The CANBARX marine geophysical experiments successfully tested the use of both multichannel arrays and tethered ocean bottom seismometers on the drifting ice pack for seismic refraction studies in the ice covered regions of the Arctic Ocean. The seismic refraction line shot indicates a crustal structure with approximately 4.3 km of sediment overlying an oceanic basement and a 9.8–10.7 km depth beneath the seafloor to the Moho. Both the CANBARX data and that of the other nearby refraction lines in the Canada Basin indicate an unusual oceanic basement in which layer 3 is either exceptionally thin or absent. After correcting for sediment loading and crustal radioactivity, the Parsons and Sclater age versus depth curve suggests an age between 90 and 130 m.y. B.P. is suggested. This range is consistent with recent magnetic studies and heat flow data.
Magnetostratigrahy of the exposed lava section east of the IRDP drill hole in Reydarfjordur, eastern Iceland
Authors: Helgason Johann
Published: February 2008

Magnetic polarity directions (VGP) of 154 exposed Miocene basalt lavas from Reydarfjordur, eastern Iceland, can be used successfully for correlation with the IRDP drill core. Three partly synchronous lava sections east of the drill site are predominantly normally magnetized and are stratigraphically of the same age as lavas dated (K‐Ar) at about 9 to 10.5 m.y. (McDougall et al., 1976). The age range and magnetic properties of these lavas, the landward equivalents to marine magnetic anomaly 5 (epoch 9), suggest that several reversely magnetized lavas in one of the investigated sections were remagnetized by a 10 m thick dyke. The data presented suggest that in fresh lavas with low Curie temperatures, relatively thick dykes (about 10 m) may cause thermal overprinting of the host rocks to a distance from the dyke equal to its thickness. These results are in contrast with the generally accepted thesis that dykes affect host rocks only up to about 1/5 of their thickness. No systematic decrease in magnetic intensity with depth was observed for a 0.9 km thick vertical lava section in Holmatindur, about 13.5 km east of the drill site. It is suggested that the heterogeneity of the lava pile in terms of magnetic intensity reflects primarily the distinct geochemistry and/or grain size of magnetic phases of individual lavas.
Constraints on the structure of the Tibet Plateau from pure path phase velocities of Love and Rayleigh waves
Authors: Romanowicz Barbara A.
Published: February 2008

Pure path phase velocities of Love and Rayleigh waves across the Tibet Plateau are determined for the period range 30–90 s and for two opposite directions of propagation along the length of the Plateau. The method used is a two‐event method, similar in principle to the classical two‐station method, where the roles of stations and events are interchanged. The success of this method is due to a favorable geometry, whereby events located at each end of the plateau align to within 1° of a great circle path with one WWSSN station at teleseismic distance to the east and one to the west. Body wave modeling is used to constrain depth and source mechanism of the events in order to calculate the corresponding source phases for surface waves. Phase velocities are determined to within ±0.02 km/s or better for Rayleigh waves and ±0.007 km/s for Love waves. The constraints added by these data to the crust and upper mantle structure beneath Tibet are discussed in the light of previous studies. The best fitting model has a 65 km thick crust, with low average crustal velocities and an Sn velocity of 4.65 km/s. The low velocity zone is limited to the depth range 100–150 km. The interesting result of this study is that both Love and Rayleigh wave data can be matched to within 1% in phase velocity by a unique isotropic model. This points to a similarity with a stable region upper mantle structure such as that of the Canadian Shield. Finally, additional Rayleigh wave phase velocity data are used to compare propagation on different paths in the vicinity of Tibet. The Tarim Basin, to the north of Tibet, stands out as having a distinct crustal structure, more similar to an average “tectonic region” structure. Moreover, evidence is presented for the existence of a very low velocity region at the western end of Tibet, which correlates well with a pronounced gravity low observed in that area.
Magnetic mineralogy of continental deposits, San Juan Basin, New Mexico, and Clark's Fork Basin, Wyoming
Authors: Butler Robert F.
Published: February 2008

Magnetic concentrates were obtained from nine bulk samples from the Late Cretaceous through middle Paleocene continental sedimentary section in the San Juan Basin, New Mexico, and from two bulk samples from the late Paleocene and early Eocene section in the Clark's Fork Basin, Wyoming. Strong‐field thermomagnetic (JS‐T) curves of almost all the San Juan Basin concentrates show only a single Curie temperature in the 175°–195°C range, indicating that the dominant ferrimagnetic mineral is detrital titanomagnetite (Fe3−xTixO4) of composition 0.51 ≤ x ≤ 0.54. The Clark's Fork Basin concentrates exhibited Curie temperatures of 200°C and 580°C, indicating a mixture of x = 0.5 titanomagnetite and magnetite, respectively. No evidence of hematite (or other ferric oxides or oxyhydroxides) was observed in the JS‐T data. Isothermal remanent magnetization (IRM) acquisition curves were determined for 56 samples from the Clark's Fork Basin and 114 samples from the San Juan Basin. Although dominated by IRM acquired in magnetizing fields of ≤300 mT, additional IRM was acquired in magnetizing fields from 300 to 700 mT. The IRM acquired above 300 mT is attributed to hematite (or other ferric oxides or oxyhydroxides) and is thought to be the result of minor oxidation, probably during recent weathering. These results indicate that IRM acquisition behavior may be used to monitor the hematite content in continental sedimentary rocks and may indicate stratigraphic intervals within which the natural remanent magnetization (NRM) could contain significant chemical remanence overprints. Although minor hematite content is indicated, the detailed sampling of IRM acquisition behavior for the San Juan Basin and Clark's Fork Basin sedimentary sequences did not reveal any significant correlation of IRM behavior with polarity of the characteristic NRM. These data thus support the previous conclusion, based on paleomagnetic data, that the characteristic NRM is a depositional remanence which provides a valid recording of the geomagnetic polarity sequence during deposition of these continental sedimentary sequences.
Constraints on the structure of the East Pacific Rise from gravity
Authors: Lewis Brian T. R.
Published: February 2008

Gravity data across the East Pacific Rise at 9°N and 12°N (the ROSE area) are characterized by a high of 12–20 mGal with a width of about 20 km. It is shown that this axial gravity high is not adequately explained by uncompensated constant density topography or an isostatically compensated thermally expanded ridge crest. It can be adequately explained by Airy compensation with a depth of compensation of at least 20 km or by elastic plate compensation with a flexural rigidity between 1018 and 1020 N m. The significance of the depth and flexural rigidity parameters is that in both cases they result in a gravity field from the compensation whose amplitude is below the data resolution for wavelengths less than about 50 km. That is, the gravity data do not require compensation of the rise axis for wavelengths less than about 50 km. This suggests that these wavelengths are either supported mechanically or by other forces (dynamic). The mechanical interpretation is not consistent with partial melt under the axis, indicating that dynamic forces may be important in supporting the rise axis. The same conclusion is arrived at from the thermal model. The thermal model can be made to fit the gravity data if we allow departures from isostasy and the addition of a dike‐like body about 2 km wide in the crust having a positive density contrast of about 0.25 Mg m−3. This geometry is consistent with a seismically determined low velocity zone in the crust in the ROSE area. In this model the nonhydrostatic pressures must be supported dynamically. A general conclusion of this study (for the portions of the East Pacific Rise studied here) is that the axial topography is not buoyantly supported by low density material in the crust. Therefore, either the crust under the rise axis mechanically supports the axial topography or a mass excess at the thermally expanded rise axis is supported by other forces, such as plate driving forces. The latter option is favored on the basis of heat flux and seismic data.
Elasticity of pyroxenes: Effects of composition versus crystal structure
Authors: Weidner Donald J. , Vaughan Michael T.
Published: February 2008

The variation of elastic moduli among five pyroxenes exhibiting three crystal structures is generally small. The variations that do exist are related to structural differences. C11 is most sensitive to compositional changes on the basis of a mechanical structural analog. Its small variation among these pyroxenes suggests that the compositional range reflected by these samples (enstatite, bronzite, diopside) does not affect the elastic moduli. C33 is most sensitive to structural perturbations. The large variation of C33 (20%) implies that the small variations in crystal structure dominate the variations in elastic moduli.
Magnetic changes accompanying the thermal decomposition of nontronite (in air) and its relevance to Martian mineralogy
Authors: Moskowitz B. M. , Hargraves R. B.
Published: February 2008

Thermal treatment of nontronite in air, for long periods at 700°C or short periods at 900°C, results in destruction of the nontronite structure, a distinct reddening in color, and a spectacular increase in magnetic susceptibility and saturation magnetization (up to 4.4 Am2 /Kg). Magnetic property measurements and calculations suggest that the magnetism is due to the presence of ultrafine particles of α or γFe2O3; the precise identity has not yet been resolved. The highly magnetic thermally treated nontronite is amorphous to X rays consistent with an ultrafine grain size. Prolonged heating results in the growth of αFe2O3, as suggested by X ray, IR, and magnetic measurements. Reflectivity spectra of a sample heated for 1 hour at 900°C indicates, in addition to αFe2O3, the presence of an opaque, magnetite‐like phase. Given the composition of the starting material, and the experimental conditions, the presence of Fe3O4, is unlikely; the cause of this opacity is as yet unknown. Thermally treated nontronite has chemical, color and magnetic properties akin to those found by Viking on Mars. These results favor an origin for the fine grained Martian surface material by repeated impacts into an Fe‐rich smectite‐charged regolith (Weldon et al., 1980).
Synthesis of molecules by irradiation in silicates
Authors: Bibring J. P. , Langevin Y. , Rocard F.
Published: February 2008

We have studied the molecular synthesis induced by the implantation of C and H ions in SiO2 by in situ infrared spectroscopy. We have observed the infrared CO and CO2 bands; the relative concentrations of CO and CO2 depend on the C and H fluences; the bands have characteristics different from those of the corresponding gaseous species. This enables us to identify unambiguously CO2 synthesized by solar wind irradiation in lunar fines.
High paleointensities of the geomagnetic field from thermomagnetic studies on Rift Valley pillow basalts from the Mid‐Atlantic Ridge
Authors: Prévot Michel , Mankinen Edward A. , Grommé Sherman , Lecaille Alain
Published: February 2008

Nineteen pillow basalts dredged within the rift valley of the Mid‐Atlantic Ridge at36.8°N were studied by the Thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor. Previously reported fission track ages are 2,000 to 6,000 years for the youngest rocks (mainly olivine basalts) and 10,000 to 100,000 years for the others (mainly plagioclase basalts and pyroxene basalts). All but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates: stability of the direction of NRM during its thermal demagnetization, constant ratio of NRM/TRM (natural remanent magnetization to thermoremanent magnetization) over 50% or more of the original NRM intensity (80 to 94% for 11 specimens), and reproducibility of low‐temperature partial TRM (PTRM). However, strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in Curie temperature (15 to 80°C) during the paleointensity experiments below 250°C, notwithstanding the linearity of the NRM‐TRM plot in this temperature interval. This alteration, probably due to low‐temperature oxidation of the specimens, seems typical of young pillow basalts and may result in paleointensity estimates which are too high. This result shows that excellence of remanence tests (NRM‐TRM linearity and PTRM stability) does not ensure the absence of chemical changes during the Thellier experiments and therefore the validity of the paleointensity obtained. The assumption that reliable paleointensities are obtained when the Curie point increase is 10°C or less led to the selection of five of our specimens, all from the youngest group. Their mean paleointensity is 64.2±20.5 μT (standard deviation) and the corresponding virtual axial dipole moment (VADM) is 11.5±3.7× 1022 A m2. Given the variations of the VADM of the Earth's magnetic field over the last 6,000 years, as established from archeomagnetic studies, our paleointensity results suggest that the latest eruptions on the inner floor of the Rift Valley at 36.8°N occurred 1,500±1,000 years ago.
Paleomagnetism of carboniferous intrusions in North Carolina
Authors: Barton Colleen , Brown Laurie
Published: February 2008

Paleomagnetic samples of two late Paleozoic plutons were collected from the eastern Piedmont Province in North Carolina. Results from the early Carboniferous (326±27 m.y.) Lilesville pluton, located in the Carolina Slate Belt, indicate a pole position similar to that of the stable North American craton for the lower Carboniferous. Five sites inthis gabbro/granite complex give a pole of 40°N, 134°E (α 95 = 2.9). The late Carboniferous 282±6 m.y.) Churchland pluton, located in the Charlotte Belt, gives a pole of 34°N, 126°E α 95 = 16.3) which correlates well with other North American poles for the late Carboniferous. Observations made by Kent and Opdyke (1978) on rocks from the eastern edge of the northern Appalachians suggest gross lateral displacement between the mid‐Devonian and late Carboniferous in the late Paleozoic. This paleomagnetic information from the eastern Piedmont region implies that the area of the Carolina Slate Belt and Charlotte Belt was associated with the North American platform for the entire Carboniferous period. The Slate Belt rocks, although geologically a southern counterpart to some of the Avalonian rocks from the northeastern edge of North America, do not display a paleomagnetic history suggestive of great displacement.
Magnetic properties of three segments of the Mid‐Atlantic Ridge at 37°N: Famous, Narrowgate, and Amar: AMAR 2
Authors: Van Wagoner Nancy A. , Johnson H. Paul
Published: February 2008

During the AMAR 1978 Expedition, over 200 separate basalt samples were collected by Alvin and dredge from the Narrowgate and AMAR segments of the Mid‐Atlantic Ridge near 37°N to determine the origin of reversely magnetized rocks previously recognized in the FAMOUS rift area to the north and to study the variations in rock magnetic properties that occur along the strike of the Mid‐Atlantic Ridge. Magnetic intensity determinations of rocks collected in the FAMOUS area indicate that intensity decays with time by a factor of 5 in less than 106 years. Rocks with low intensities have high Curie temperatures (Tc) indicating that low‐temperature oxidation of the magnetic minerals takes place soon after the basalts are extruded and is pervasive in crust older than 106 years. Magnetic intensity (Jnrm) of the uppermost oceanic crust can be expressed as a function of age (t) by the expression J = 5.82t−1/3. By measuring Jnrm of the surface rocks from the three valley segments, the relative ages of the surface rocks from the FAMOUS, AMAR, and Narrowgate areas can be estimated to be 1×104 years, 3×104 years, and 4×104 years, respectively. One exception is Mt. Mars, located in the center of the Narrowgate area that has high Jnrm values (84 × 10−3 emu/cm3) and low Tc values (165°C), suggesting that this is an area where volcanic activity is becoming reactivated following a period of quiescence. Of the 55 oriented samples collected, eight are reversely magnetized. Although three of the reversed samples have Curie temperatures above 300°C and may have undergone self‐reversal, the other five reversed samples have magnetic properties similar to normally magnetized rocks and may have been misoriented or extruded during a short reversed event within the Brunhes epoch. Because most of the surficial rock samples are normally magnetized, it is suggested that the previously recognized reversed zones, which appear in a deep‐tow magnetometer profile that crossed the Narrowgate area, are located at depth and are the result of the spreading axis moving into older Matuyama crust that is now covered by more recent volcanism.
Surface deformation associated with a dipping hydrofracture
Authors: Davis Paul M.
Published: February 2008

Crustal deformation associated with hydrofracture is modeled by a dipping rectangular dislocation beneath the surface of an elastic half space. The Burger's vector is taken normal to the rectangular surface. Analytical expressions for vertical displacements are obtained from integration of Volterra's equation by using Mindlin's point force solutions for the elastic halfspace. In the limits of 0° and 90° dip, the results agree with those obtained by Maruyama (1964) for Poisson's ratio 0.25. Expressions for arbitrary Poisson's ratio are given. For elongate dislocations the displacement field agrees with curves given by Pollard and Holzhausen (1979) for two‐dimensional, near surface fractures at various dips. At dip = 0°, a deep square dislocation gives displacements indistinguishable from those of Sun's (1969) model of a horizontal, penny shaped crack beneath the surface of an elastic half space. Modifying his result for shallow depths shows that they correspond to a dislocation of semi‐elliptical cross‐section with the surface displacement field asymptotically approaching a semi‐elliptical shape as depth decreases to zero. Similarly, the surface displacement field of the square dislocation approaches that of its rectangular cross‐section. Nonlinear inversion of surface tilt fields, associated with hydrofracture of deep boreholes, gives estimates of fracture geometry, position, and orientation along with their uncertainties. Two examples are presented, one of a near horizontal and one of a near vertical fracture.
Minimal upper mantle temperature variations consistent with observed heat flow and plate velocities
Authors: Kaula William M.
Published: February 2008

The momentum equations applied to 5° block means are integrated from the observed surface plate velocities downward to a depth of 280 km, assuming no lateral heterogeneities in density or viscosity. It is assumed that 85% of the global heat production QG = 0.85 × 4.0 ×1013 W comes from below 280 km and that at this level the transfer is fully convective. A temperature field 7 is inferred at depth 280 km by minimizing the quantity∫|T− T0|n dS + nλ{QG−pC∫(T−T0)υrdS}, where the integrals are over the sphere, ρ is density, C is heat capacity, υr is radial velocity, T0 is a prescribed mean, and λ is a Lagrangian multiplier. Norms n, ranging from 1.5 to 2.5, are tried. The intervening temperature fields are then inferred, integrating the energy equation downward by using the previously calculated velocity field. This integration is subject to the limitations that the derivative of the temperature with respect to depth is everywhere sufficient to attain the fully convecting temperature, but never less than adiabatic. A surface heat flow based on observations plus age and tectonic setting is used. The principal inferences are: (1) the greatest lateral variations in temperature, ∼1000°C, occur with the top 20 km; (2) the greatest advection, ∼200°C/m.y., occurs within the top 20 km; (3) below 50 km, the greatest departures of temperature from the mean are negative “tongues,” reaching an extreme of about −800°C at depth 100 km; (4) below 50 km, heat transfer becomes more convective than conductive; (5) at the fully convecting level, 280 km, temperature variations are at least ±180°C about the mean. The principal defect in the entire calculation is unrealistically low temperatures arising from unrepresentatively low surface heat flows. The principal defect of the model probably arises from the assumption that all heat transfer at a depth of 280 km is representable by 5° means in velocity and temperature.
A proposed origin for palimpsests and anomalous pit craters on Ganymede and Callisto
Authors: Croft Steven K.
Published: February 2008

Palimpsests and anomalous pit craters are two classes of craters found on the icy satellites Ganymede and Callisto that have no obvious counterparts among the crater populations found on rocky planets. The distinct characteristics of palimpsests and anomalous pit craters in morphology, morphometry, and population statistics, which differentiate them from the more numerous ‘normal’ craters and basins on the icy satellites, also preclude their origin as normal craters that have nearly vanished through viscous relaxation. An alternate origin is proposed: palimpsests are formed when the flow of material during the modification stage of crater formation is dominated by 'wet' fluid flow as opposed to ‘dry’ granular flow, which forms normal craters; conditions of ‘wet’ modification flow occur when the volume of melt remaining in the crater attains a volume comparable to the volume of the transient crater. By use of an impact model that includes: (1) plausible impact velocity distributions, (2) crater scaling in ice, (3) shock melting in ice, (4) thermal profiles as a function of time in Ganymede, and (5) cratering flow field constraints, conditions for wet modification flow are found to occur for sufficiently large impacts or high‐velocity impactors. The range of normal crater‐palimpsest transition diameters and the morphologic characteristics inferred from the impact model are consistent with the observed characteristics of palimpsests. Thus normal craters are inferred to result from the majority of impactors which have impact velocities near the average of 15 km/s, anomalous pit craters (inferred to be transitional forms) from intermediate velocity impactors at about 30 km/s, and palimpsests by impactors travelling in excess of about 40 km/s. The thermal state of the crust at the time of impact is a second‐order effect, but sufficient to account for the difference in palimpsest population between Ganymede and Callisto if Ganymede once had a liquid mantle while Callisto remained unmelted and undifferentiated or solidified prior to the formation of the presently observed crater population.
Ancient crustal components in the Fra Mauro breccias
Authors: Shervais John W. , Taylor Lawrence A. , Laul J. C.
Published: February 2008

Texturally pristine clasts preserve primary petrographic relationships and mineral compositions, yielding insights into igneous processes of the early lunar crust that cannot be gained from highly shocked and brecciated ‘chemically pristine’ samples. The use of texture as a prime criterion allows for expansion of the data base derived solely from chemical criteria, and provides complementary data. Texturally pristine clasts from the Apollo 14 site studied here include anorthosite, troctolites, gabbronorites, and basalts. Alkali anorthosites are plagioclase orthocumulates and may form by flotation in Mg‐suite plutons. Ferroan anorthosite was cataclastically deformed and metamorphosed to granulite facies. Troctolites include both 01 + Plg and 01 + En + Plg cumulates. Major and trace element analyses of two troctolites reveal ‘eastern’ geochemical affinities that contrast other ‘western’ troctolites. Gabbronorites are Plg + Plg ± Sp cumulates whose parent magmas may range from high‐Al to intermediate‐Ti mare basalt. At least three varieties of mare basalt are found at Apollo 14: high‐Al, low‐Ti; low‐Al, intermediate‐Ti; and low‐Al, Ti VHK basalt. VHK (Very High Potassium) basalt is a new variety indigenous to Apollo 14.
Cooling rates for glass containing lunar compositions
Authors: Fang C. Y. , Yinnon H. , Uhlmann D. R.
Published: February 2008

Cooling rates required to form glassy or partly‐crystalline bodies of 14 lunar compositions have been estimated using a previously introduced, simplified model. The calculated cooling rates are found to be in good agreement with cooling rates measured for the same compositions. Measurements are also reported of the liquidus temperature and glass transition temperature for each composition. Inferred cooling rates are combined with heat flow analyses to obtain insight into the thermal histories of samples 15422,14162, 15025, 74220,74241,10084,15425, and 15427. The critical cooling rates required to form glasses of 24 lunar compositions, including the 14 compositions of the present study, are suggested to increase systematically with increasing ratio of total network modifiers/total network formers in the compositions. This reflects the importance of melt viscosity in affecting glass formation.
Monitoring velocity variations in the crust using earthquake doublets: An application to the Calaveras Fault, California
Authors: Poupinet G. , Ellsworth W. L. , Frechet J.
Published: February 2008

We present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet, or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates. We compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal. A moving‐window analysis of the entire seismograms, including the coda, gives δ(t), the difference in arrival times versus running time along the seismogram. The time resolution of the method is an order of magnitude better than the digitization interval. The δ(t) technique is illustrated with a pair of microearthquakes, M = 1.7 and 2.0, that occurred before and after the Coyote Lake, California, earthquake (M = 5.9) of August 6, 1979, and on the same segment of the Calaveras fault that ruptured during the earthquake. The coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet, so that its seismogram appears as a stretched version of the earlier event. We interpret this systematic variation in δ(t) along the coda as a change in the average S velocity in the upper crust in the time interval between the two doublets. S wave velocities appear to have decreased by 0.2% in an oblong region 5–10 km in radius at the south end of the aftershock zone.
Patterns and timing of Late Quaternary faulting in the Great Basin Province and relation to some regional tectonic features
Authors: Wallace Robert E.
Published: February 2008

Large‐scale surface faulting events, each tens of kilometers long and involving more than a meter of displacement, of late Quaternary age have not been uniformly distributed but have been concentrated in subprovincial and smaller belts and areas within the Great Basin province. Furthermore, faulting apparently has not been uniformly distributed over time. The average recurrence interval of large‐scale faulting events on individual faults in the province is measured in thousands or tens of thousands of years. In contrast to such long recurrence intervals, in the central Nevada and eastern California seismic belts, large‐scale faulting events have occurred during the last 111 years in an apparently coherent, sequential belt‐filling pulse of activity in which events were separated by only a few years or a few decades. The questions are, where are other pulses of large‐scale faulting likely to occur and what controls the localization of such belts and areas characterized by higher rates of faulting? Two examples of tectonic subprovinces that may play a role in delimiting smaller zones of faulting are the Black Rock‐Carson Sink zone of extension and the central Nevada downwarp. The central Nevada seismic belt lies along the dividing line between these two subprovinces, and the northwest margin of the Black Rock‐Carson Sink zone of extension coincides with the margin of a fingerlike belt of dense faulting in northwestern Nevada.
Near‐surface in situ stress: 1. Strain relaxation measurements along the San Andreas Fault in southern California
Authors: Sbar Marc L. , Richardson Randall M. , Flaccus Christopher , Engelder Terry
Published: February 2008

Strain relaxation stress measurements were made in the Mojave Desert southeast of Palmdale, California, at two sites during the summers of 1979 and 1980, using the U.S. Bureau of Mines technique to depths of about 30 m. The field data and finite element modeling studies demonstrate that thermally induced stress dominates the results obtained in the upper 6 m. At depths greater than 6 m the average orientation for the horizontal maximum compressive stress at these sites is N22°W±9° at 2 km south of the San Andreas fault and N13°W±2° at 20 km north of the fault. These azimuths compare favorably with the average of N21°W determined with nearby hydrofracture stress measurements (Zoback et al, 1980). Savage et al. (1981) also found a NNW orientation for the maximum shortening from a geodetic network with a 15‐km aperature in the Palmdale area. The fact that essentially the same orientation is recovered by three different techniques which sample to different depths and over different areal extents argues strongly for a contemporary tectonic origin for the stress. Finite element models of the San Andreas fault in southern California develop a stress field similar to that observed regionally (∼15°E for the maximum compressive stress) away from the fault when displacements corresponding to relative motion between lithospheric plates are applied on the boundaries of the models. Near the fault, however, the model principal stresses are rotated counterclockwise similar to those measured near Palmdale, demonstrating the influence of the faults on the principal stress orientations.
Factors affecting seismic moment release rates in subduction zones
Authors: Peterson Eric T. , Seno Tetsuzo
Published: February 2008

The amount of shallow seismic activity in subduction zones varies greatly from region to region. We quantify this seismicity by calculating seismic moment release rates and seismic slip rates for 24 subduction zones. To calculate the moment release, we sum the seismic moment for all interplate thrust‐type events with surface wave magnitude Ms≥7.0, occurring from 1904 to 1980. We present a time history plot of the seismic moment release for each subduction zone; these exhibit the differences in the seismic release patterns. For subduction zones where the time window of our study is not representative, the total moment release is corrected using information on repeat times. The moment release rates are compared with various subduction parameters in order to determine which factors influence the degree of coupling. These parameters include the age of the subducting lithosphere, absolute velocities of the upper and subducting plates, convergence velocity, and length, maximum depth, and dip of the Wadati‐Benioff zone. The moment release rate decreases as the age of the subducting lithosphere increases, when the zones belonging to a single subducting plate are considered. This age versus moment release relation is consistent for the zones in which the Pacific, Cocos, Philippine Sea, and Indian plates are subducting. The moment release rates for the subduction zones in which the Pacific plate is subducting are much higher than for zones of other plates with similar age. The age versus moment release relation holds among the zones which belong to one subducting plate; however, zones with similar ages but belonging to different plates do not have the same moment release rates. This suggests that within a single plate the age is the dominating factor affecting the strength of seismic coupling but that each plate as a whole has a characteristic moment release budget. Zones with retreating upper plates tend to have lower moment release rates. The moment release rate does not increase with convergence velocity; no simple relationship was found between these two parameters. The moment release rate depends most clearly on the age of the subducting lithosphere and the absolute velocity of the upper plate. These are the two independent subduction zone parameters among the variables considered. The other variables depend on these two parameters.
Origin of Guyots: The Beagle to Seabeam
Authors: Menard H. W.
Published: February 2008

Thermal subsidence would have drowned ancient volcanic islands (DAVI). Ancient volcanoes, if active at present rates in the main ocean basins, would have produced 1.5–3 times the observed number of DAVI. Shallow‐water indicators have been collected from 11% of guyots, and another 3% appear to be drowned barrier reefs or atolls on reflection profiles. A minimum of 65% of DAVI lie downdrift from hot spots that generate volcanic islands. The DAVI have the same 3 to 4‐km relief as the islands would have if truncated. Nonetheless hypotheses of a constructional origin of guyots continue to be proposed. Relative to these hypotheses are the following: (1) all four large, active submarine volcanoes that have been surveyed lack flat tops, and (2) many small, deep, inactive, relatively flat‐topped volcanoes are constructional forms but none with more than 2 km of relief are known to exist. Thus it is probable that all guyots with more than 2 km of relief are DAVI, but the possible existence of larger constructional forms is not excluded. There is no evidence that guyots large enough to be of interest in studying the epeirogenic history of midplate swells are not DAVI.
Petrology of Apollo 11 regolith breccias
Authors: Simon S. B. , Papike J. J. , Shearer C. K.
Published: February 2008

Petrographic and mineral chemical data for 16 Apollo 11 regolith breccias show that: (1) the regolith breccias differ from soil 10084 with respect to (a) agglutinate content, (b) glass population, (c) plagioclase compositions, and (d) proportions of high‐K mare and low‐K mare basalt components; (2) the A‐11 breccias and soil have highland components that are similar both in abundance and petrology; and (3) lunar regolith breccias provide a better comparison with howardites than do lunar soils. The data and observations are consistent with formation of the regolith breccias from immature soil. It appears that little or no highland material has been added to the Tranquillitatis regolith since the formation of the breccias.