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USGS2012

Statistical averaging of marine magnetic anomalies and the aging of oceanic crust

Visual comparison of Mesozoic and Cenozoic magnetic anomalies in the North Pacific suggests that older anomalies contain less short-wavelength information than younger anomalies in this area. To test this observation, magnetic profiles from the North Pacific are examined from crust of three ages: 0–2.1, 29.3–33.1, and 64.9–70.3 m.y, B.P. For each time period, at least nine profiles were analyzed by (1) calculating the power density spectrum of each profile, (2) averaging the spectra together, and (3) computing a ‘recording filter’ for each time period by assuming a hypothetical seafloor model. The model assumes that the top of the source is acoustic basement, the source thickness is 0.5 km, and the time scale of geomagnetic reversals is according to Ness et al. (1980). The calculated power density spectra of the three recording filters are complex in shape but show an increase of attenuation of short-wavelength information as the crust ages. These results are interpreted using a multilayer model for marine magnetic anomalies in which the upper layer, corresponding to pillow basalt of seismic layer 2A, acts as a source of noise to the magnetic anomalies. As the ocean crust ages, this noisy contribution by the pillow basalts becomes less significant to the anomalies. Consequently, magnetic sources below layer 2A must be faithful recorders of geomagnetic reversals.

Journal of Geophysical Research Solid Earth↗
USGS2012

In situ studies of velocity in fractured crystalline rocks

A study of the effects of macroscopic fractures on P and S wave velocities has been conducted in four wells drilled in granitic rock to depths between 0.6 and 1.2 km. The effect of macroscopic fractures is to decrease both V p and V s and increase V p / V s . In wells with a relatively low density of macroscopic fractures, the in situ velocity is similar to that of saturated core samples under confining pressure in the laboratory, and there is a clear correlation between zones with macroscopic fractures and anomalously low velocities. In wells with numerous macroscopic fractures, the in situ velocity is lower than that of intact samples under pressure, and there is a correlation between the rate at which in situ velocity increases with depth and the rate at which the velocity of laboratory samples increases with pressure. Differences in in situ P wave velocity between wells cannot be explained solely by differences in the degree of macroscopic fracturing, thus emphasizing the importance of composition and microcracks on velocity. In one highly fractured well the in situ P wave velocity is essentially the same for frequencies ranging from 10 Hz to 20 kHz; this suggests that the macrofractures affect velocity similarly over a broad frequency range. Chemical alteration of rock adjacent to macroscopic fractures appears to play an important role in reducing in situ velocities. Synthetic reflection seismograms generated from the velocity logs suggest that fracture zones are one possible source of deep-crustal reflectors observed on seismic reflection profiles.

Journal of Geophysical Research Solid Earth↗
USGS2012

Investigation of internal friction in fused quartz, steel, Plexiglass, and Westerly granite from 0.01 to 1.00 Hertz at 10-8 to 10-7 strain amplitude

A detailed evaluation on the method of internal friction measurement by the stress-strain hysteresis loop method from 0.01 to 1 Hz at 10 −8 to 10 −7 strain amplitude and 23.9°C is presented. Significant systematic errors in relative phase measurement can result from convex end surfaces of the sample and stress sensor and from end surface irregularities such as nicks and asperities. Preparation of concave end surfaces polished to optical smoothness having a radius of curvature >3.6×10 4 cm reduces the systematic error in relative phase measurements to <(5.5±2.2)×10 −4 radians. The values of Q E −1 (internal friction under uniaxial compression) determined from the relative phase measurements are | Q E −1 – Q s −1 |< 2.8×10 −3 for the tool steel sample and | Q E −1 – Q s −1 |< 2.2×10 −3 for the Westerly granite sample, where Q s −1 is the internal friction of the fused quartz stress sensor under uniaxial compression. These values are consistent with those inferred from the relative modulus dispersion data also presented in this paper. The polymethyl methacrylate (PMM, trade name Plexiglass) sample shows high values of internal friction ( Q E −1 ≅5×10 −2 ) with strong frequency dependence and with a maximum in Q E −1 at ≅0.4 Hz.

Journal of Geophysical Research Solid Earth↗
USGS2012

Crustal and upper mantle structure of the northern and central Sierra Nevada

Teleseismic data were recorded within the Sierra Nevada to look for lateral variations in the upper mantle. The data were collected at both temporary and permanent stations, and P wave residuals were computed. After correcting the P residual data for crustal and topographic effects, there is still a variation of as much as 0.5-0.6 s from the north end of the Sierra Nevada to Mono Lake, located east of the central part of the range. In addition, there are significant variations in travel time patterns, depending on the azimuth of wave arrivals. Two simple modeling approaches have been used to infer the upper mantle velocity structure from the observed variations in travel time. In the first, it is assumed that the velocity variations are distributed throughout a depth range of about 60–160 km. One model that fits the data indicates a north-to-south trend in upper mantle P velocities from about 7.9 km/s at the north end of the Sierra to 7.6 km/s near Mono Lake. Superimposed is a west-to-east trend from 7.85 km/s under the Sierra Nevada crest to 7.7 km/s east of Lake Tahoe. In the second modeling approach, a fixed velocity contrast is assumed between the lithosphere and asthenosphere with variations in travel time resulting from variations in lithospheric thickness. One model, consistent with the observations, indicates a thinning of the lithosphere from 110 km at the north end of the Sierra to 60 km near Mono Lake.

Journal of Geophysical Research Solid Earth↗
USGS2012

The use of MAGSAT data to determine secular variation

A combined spatial and secular variation model of the geomagnetic field, labeled M061581, is derived from a selection of Magsat data. Secular variation (SV) data computed from linear fits to midnight hourly values from 19 magnetic observatories were also included in the analysis but were seen to have little effect on the model. The SV patterns from this new model are compared with those from the 1980 IGRF and with those for 1970 computed by the DGRF and with the 1960 patterns computed using the GSFC(12/66) model. Most of the features of the M061581 are identical in location and level with those of the 1980 IGRF. Together they confirm that the reversals in sign of field change seen over Asia and North America between 1965 and 1975 are reverting to the pre-1965 states. The M061581 model gives −32 nT/yr for the dipole decay rate, larger than the 70% increase already reported since 1965. This abnormally high value is interpreted as being a defect of the model because it appears to result from a much larger (−100 nT/yr) drop in field over the polar regions not indicated by the 1980 IGRF. This north polar decrease is shown to be of external origin as the result of a combination of the seasonal effect of the north polar ionospheric (counterclockwise) afternoon Sq p cell increasing in intensity from the beginning (November 1979) to the end (June 1980) of the Magsat data collection period, coupled with an enhancement of its effect as the orbit lowered from the 350– 550 km initial altitudes to near 200 km just prior to burnup. This experiment indicates that secular variation can be obtained from satellite data for intervals of less than a full year if corrections can be made for seasonal effects and that ‘annual snapshots’ of the field by a satellite would allow easy and accurate models of secular change without the use of any surface data.

Journal of Geophysical Research Solid Earth↗
USGS2012

The 1979 Homestead Valley earthquake sequence, California: Control of aftershocks and postseismic deformation

The coseismic slip and geometry of the March 15, 1979, Homestead Valley, California, earthquake sequence are well constrained by precise horizontal and vertical geodetic observations and by data from a dense local seismic network. These observations indicate 0.52±0.10 m of right-lateral slip and 0.17±0.04 m of reverse slip on a buried vertical 6-km-long and 5-km-deep fault and yield a mean static stress drop of 7.2±1.3 MPa. The largest shock had M S = 5.6. Observations of the ground rupture revealed up to 0.1 m of right-lateral slip on two mapped faults that are subparallel to the modeled seismic slip plane. In the 1.9 years since the earthquakes, geodetic network displacements indicate that an additional 60±10 mm of postseismic creep took place. The rate of postseismic shear strain (0.53±0.13 μrad/yr) measured within a 30×30-km network centered on the principal events was anomalously high compared to its preearthquake value and the postseismic rate in the adjacent network. This transient cannot be explained by postseismic slip on the seismic fault but rather indicates that broadscale release of strain followed the earthquake sequence. We have calculated the postearthquake stress field caused by the modeled coseismic slip. We assume that failure is promoted when the sum of the shear stress plus 0.75 times the faultopening stress increases. Most aftershocks concentrate at points where the stresses are enhanced by 0.3 MPa (3 bars) or more; aftershocks are nearly absent where postearthquake stresses decrease by 0.3–0.5 MPa. Isolated off-fault clusters of aftershocks that locate at one fault length from the rupture plane are explainable by this hypothesis. We find that ground rupture and postseismic creep take place where near-surface stresses are calculated to increase within the preexisting fault zones. Two patches that extend 4 km from both ends of the seismic fault exhibited neither aftershocks nor measurable postseismic creep. The sensitivity of aftershocks and ground rupture to changes in stress that are less than 5% of the earthquake stress drop demonstrates that the region around the earthquakes was within a few percent of its failure threshold before the main shocks. The preearthquake stress field and the stress required for failure must also have been nearly uniform.

Journal of Geophysical Research Solid Earth↗
USGS2012

Petrology and comparative thermal and mechanical histories of clasts in breccia 62236

Lunar breccia 62236 contains large lithic fragments of troctolite, norite, and anorthosite. The mafic phases, olivine, inverted pigeonite, and augite, fill interstitial areas between larger plagioclases and appear to be cumulate phases with extensive adcumulus growth. Pyroxene compositional homogeneity indicates that cation exchange during cooling was limited to an area of about 1 mm. Primary augite and pigeonite both contain 30 μm-wide lamellae of the other along ‘001’. Pigeonite inverted to orthopyroxene without retaining any crystallographic orientation and subsequently exsolved fine lamellae of augite on (100). Profiles across orthopyroxene-augite interfaces obtained in the analytical transmission electron microscope show an increase of ∼5% wollastonite in augite within 0.5 μm of the interface, suggesting that short-range cation exchange continued to temperatures below 500°C. The entire sample has undergone heterogeneous shock deformation. Shock melting of the troctolitic clast suggests pressures of 200–300 kb and well-developed basal twinning in augite from the norite clasts suggests pressures of 50–300 kbars. The present evidence indicates that 62236 contains parts of a slowly cooled microlayered adcumulate that has been heterogeneously shocked several times and combined into the present breccia.

Journal of Geophysical Research Solid Earth↗
USGS2012

Local gravity anomalies produced by dislocation sources

Rundle (1978) and Walsh and Rice (1979) have shown that the change in the vertical component of gravity is proportional to uplift for a spherical source of dilatation and for slip on an infinitely long dip-slip fault. In the first case, no free air gravity anomaly is produced and in the second case no Bouguer gravity anomaly. Gravity anomalies due to other dislocation sources in three dimensions are as follows: For strike-slip faulting the ratio of the gravity change to uplift depends upon position; however, the gravity change contours are roughly similar to those corresponding to a zero free air gravity anomaly. Nor is the ratio constant for dip-slip faulting except for the two special cases of dip slip on a vertical fault and horizontal slip on a horizontal fault, neither of which produce a Bouguer anomaly. The Bouguer anomaly produced by an open horizontal crack is the same as would be produced had the material within the crack been mined out without deforming the solid. If the horizontal crack were filled with material of density equal to the host rock (a good approximation to sill formation), no Bouguer anomaly is produced. For cracks of other inclinations the ratio of gravity change to uplift is not constant. Thus, dilatancy, in general, does not correspond to the absence of a free air anomaly, as might be suggested by the special case of a spherical source of dilatation. For two-dimensional models a cylindrical source of dilatation produces no free air gravity anomaly, dip-slip faulting produces no Bouguer anomaly, and open cracks produce a Bouguer anomaly equal to that which would be produced had the material within the crack been mined out without deforming the solid. A two-dimensional crack filled with material of density equal to that of the host rock would produce no Bouguer anomaly. Jachens et al. (1983) have reported temporal changes in gravity, elevation, and area strain along the San Andreas fault in southern California such that the Bouguer anomaly apparently remains unchanged and the uplift-to-strain ratio is about −100 km. Several dislocation mechanisms are proposed that fulfill these constraints, but these mechanisms appear to be rather contrived and are not regarded as satisfactory explanations.

Journal of Geophysical Research Solid Earth↗
USGS2012

Deformation of clinopyroxenite: Evidence for a transition in flow mechanisms and semibrittle behavior

A systematic suite of constant strain rate experiments was performed on a vacuum-dried, high-purity, fine-grained clinopyroxenite using NaCl and NaF as confining media in a Griggs-type piston-cylinder apparatus. The experiments were carried out over a range of temperatures from 400° to 1100°C, strain rates from 10 −3 to 10 −7 s −1 , and confining pressures from 170 to 1990 MPa. At T = 600°C and = 1.1 × 10 −5 s −1 , three modes of deformation occur with increasing confining pressure: (1) Macroscopic faulting associated with low strength and stress drops, (2) stable microfracturing and plastic deformation associated with pressure-dependent strength, and (3) plastic deformation (mechanical twinning and 〈001〉 slip) with high strengths which are insensitive to pressure variations. In experiments at P = 1500 MPa, within this high-pressure plastic mode, two regimes of flow are clearly defined. At low to intermediate temperatures and high strain rates, flow strengths are insensitive to changes in strain rate and temperature. Optical and transmission electron microscope observations indicate that plastic strain is accomplished by mechanical twinning on (100) and (001) and by {100}〈001〉 slip. In contrast, at high temperatures and low strain rates the flow stress is strongly dependent on temperature and strain rate. Specimens deformed in this regime show evidence of recovery, multiple slip, and recrystallization; and plastic strain is much more homogeneous. The flow data within each regime can be satisfactorily fit to thermally activated power laws. In the low-temperature regime n (the stress exponent) = 83 ± 16 and E * (the activation energy for flow) = 220 ± 40 kJ/mol. We believe that these parameters reflect flow dominated by the kinetics of dislocation glide associated with mechanical twinning and (100)〈001〉 slip. In the high-temperature regime, n = 5.3 ± 1.1 and E * = 380 ± 30 kJ/mol. These parameters describe creep by multiple slip accompanied by increased rates of diffusion and recovery.

Journal of Geophysical Research Solid Earth↗
USGS2012

Permeability of fault gouge under confining pressure and shear stress

The permeability of both clay-rich and non-clay gouges, as well as several pure clays, was studied as a function of confining pressures from 5 to 200 MPa and shear strain to 10. Permeability ranged over four orders of magnitude, from around 10 −22 to 10 −18 m 2 (1 darcy = 0.987 × 10 −12 m 2 ). The lowest values were characteristic of the montmorillonite-rich and finer grained non-clay gouges. Illite, kaolinite, and chlorite had intermediate permeabilities, while the highest values were typical of the serpentine and coarser grained non-clay gouges. Grain size was an important factor in determining permeability, particularly for the clay-rich samples. The coarse grained gouges were the most permeable and decreased in permeability after shearing. Conversely, the fine grained gouges had characteristically lower permeabilities that did not vary significantly after various amounts of shearing. The permeabilities of the non-clay samples were not significantly different than those of the clays. Therefore, comminuted rock flours can be equally as effective in reducing the flow of water as the characteristically low permeability clay gouges. The strengths of the samples were quite variable. The non-clay gouges were consistently the strongest, with yield points (beginning of nonelastic behavior) around 850 MPa, while montmorillonite had an anomalously low strength in relation to all the other gouges at 250 MPa. Strength of the saturated samples under drained (low pore pressure) conditions did not correlate with high or low permeability. However, the low permeabilities of these gouges could be a factor in the measured low shear stresses along fault regions if excess pore pressures were created as a result of shearing or compaction, and this pressure was unable to dissipate through a thick section of the material.

Journal of Geophysical Research Solid Earth↗
USGS2012

Modification of wave-cut and faulting-controlled landforms

From a casual observation that the form of degraded fault scarps resembles the error function, this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms. The morphologic observations can be analyzed either in the form of one or more cross-strike elevation profiles or in the form of the slope-offset plot, a point plot of maximum scarp slope versus scarp offset. Working with either or both of these data representations for nine geologic structures, which range in age from 3 to 400 ka B.P. and in offset from 1 to 50 m, we apply analytical solutions for the vertical initial value scarp, the vertical continuous offset scarp, and the finite slope, initial value scarp. The model calculations are intrinsically ambiguous, yielding as the final answer only the product κ t (in the case of the initial value problem) or the product κ A −1 (in the case of the repeated faulting problem); here t is the age of a single scarp-forming event, 2 A is the vertical slip rate, and κ is the “mass diffusivity.” A single profile across three sea cliffs along the Santa Cruz, California, coast is analyzed as three separate initial value problems. A reasonably constrained age for the sea cliff standing above the Highway 1 platform returns κ = 11 GKG (1 GKG = 1 m 2 /ka). With this κ, we can date the two older sea cliffs. In fact, we do the converse: age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same κ as for the lower sea cliff. We treat a single profile of the Raymond fault in Pasadena/San Marino in terms of the repeated faulting problem; for it the uplift rate of R. Crook and others yields κ = 16 GKG. The very substantial preexisting offset across the Raymond fault must have been buried/leveled some 230 ka B.P., when the modern topography began to form. Our analysis of the Lake Bonneville shoreline scarps reveals a dependence of κ t on 2a, suggestive of nonlinear modification processes. This appearance is treated with the finite slope initial value scarp model to determine κ=1.1 GKG for the Lake Bonneville shoreline scarps. The suggestion of M. N. Machette that approximately 100,000-year-old, meter-high scarps are “unobservable” in weakly consolidated alluvial terranes of the Basin and Range and Rio Grande Rift Valley provinces can be formulated as κ ≳ 1 GKG. The coincidence between this inequality and the Lake Bonneville shoreline κ is striking, and it suggests that the value of κ = 1 GKG may be generally applicable, as a good first approximation, to the modification of alluvial terranes within the semiarid regions of the western United States. The Lake Bonneville shoreline κ is the basis for dating four sets of fault scarps in west-central Utah. The Drum Mountains fault scarps can be modeled in several different circumstances, but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3.6 to 5.7 ka B.P. The younger age is associated with quite low initial slope angles (25°). The other three sets of fault scarps show no evidence for finite initial value slopes. Fault scarps along the eastern base of the Fish Springs Range are very young, 3 ka B.P. We estimate the age of fault scarps along the western flank of the Oquirrh Mountains to be 32 ka B.P., which meets the weak geologic constraint that they be older than the Lake Bonneville shoreline. Fault scarps along the northeastern margin of the Sheeprock Mountains are even older, 53 ka B.P. An intriguing consequence of our single-event analysis of these scarps is that an 11.5-m offset occurred in a single earthquake.

Journal of Geophysical Research Solid Earth↗
USGS2012

Laboratory studies of volcanic jets

The study of the fluid dynamics of violent volcanic eruptions by laboratory experiment is described, and the important fluid-dynamic processes that can be examined in laboratory models are discussed in detail. In preliminary experiments, pure gases are erupted from small reservoirs. The gases used are Freon 12 and Freon 22, two gases of high molecular weight and high density that are good analogs of heavy and particulate-laden volcanic gases; nitrogen, a moderate molecular weight, moderate density gas for which the thermodynamic properties are well known; and helium, a low molecular weight, lowdensity gas that is used as a basis for comparison with the behavior of the heavier gases and as an analog of steam, the gas that dominates many volcanic eruptions. Transient jets erupt from the reservoir into the laboratory upon rupture of a thin diaphragm at the exit of a convergent nozzle. The gas accelerates from rest in the reservoir to high velocity in the jet. Reservoir pressures and geometries are such that the fluid velocity in the jets is initially supersonic and later decays to subsonic. The measured reservoir pressure decreases as the fluid expands through repetitively reflecting rarefaction waves, but for the conditions of these experiments, a simple steady-discharge model is sufficient to explain the pressure decay and to predict the duration of the flow. Density variations in the flow field have been visualized with schlieren and shadowgraph photography. The observed structure of the jet is correlated with the measured pressure history. The starting vortex generated when the diaphragm ruptures becomes the head of the jet. Though the exit velocity is sonic, the flow head in the helium jet decelerates to about one-third of sonic velocity in the first few nozzle diameters, the nitrogen head decelerates to about three-fourths of sonic velocity, while Freon maintains nearly sonic velocity. The impulsive acceleration of reservoir fluid into the surrounding atmosphere produces a compression wave. The strength of this wave depends primarily on the sound speed of the fluid in the reservoir but also, secondarily with opposite effect, on the density: helium produces a relatively strong atmospheric shock while the Freons do not produce any optically observable wave front. Well-formed N waves are detected with a microphone far from the reservoir. Barrel shocks, Mach disks, and other familiar features of steady underexpanded supersonic jets form inside the jet almost immediately after passage of the flow head. These features are maintained until the pressure in the reservoir decays to sonic conditions. At low pressures the jets are relatively structureless. Gas-particle jets from volcanic eruptions may behave as pseudogases if particle concentrations and mass and momentum exchange between the components are sufficiently small. The sound speed of volcanic pseudogases can be as large as 1000 m s −1 or as small as a few tens of meters per second depending on the mass loading and initial temperature. Fluids of high sound speed produce stronger atmospheric shock waves than do those of low sound speed. Therefore eruption of a hot gas lightly laden with particulates should produce a stronger shock than eruption of a cooler or heavily laden fluid. An empirical expression suggests that the initial velocity of the head of supersonic volcanic jets is controlled by the sound speed and the ratio of the density of the erupting fluid to that of the atmosphere. The duration of gas or pseudogas eruptions is controlled by the sound speed of the fluid and the ratio of reservoir volume to vent area.

Journal of Geophysical Research Solid Earth↗
USGS2012

Catastrophic isotopic modification of rhyolitic magma at times of caldera subsidence, Yellowstone Plateau Volcanic Field

The Yellowstone Plateau volcanic field has undergone repeated eruption of rhyolitic magma strongly depleted in 18 O. Large calderas subsided 2.0, 1.3, and 0.6 Ma ago, on eruption of ash flow sheets that represent at least 2500, 280, and 1000 km 3 of zoned magma. More than 60 other rhyolite lavas and tuffs permit reconstruction of the long-term chemical and isotopic evolution of the silicic system. Narrow δ 18 O ranges in the ash flow sheets contrast with wide δ 18 O variations in postcaldera lavas of the first and third caldera cycles. Earliest postcollapse lavas are 3 to 6‰ lighter than the preceding ash flow sheets. The O 18 depletions were short-lived events that immediately followed caldera subsidence; hundreds of cubic kilometers of magma were drastically 18 O depleted and thousands were depleted by 1–2‰. Sequences of postcaldera lavas record partial recovery toward precaldera δ 18 O values; secular trends between collapse events thus reflect gradual reenrichment of the roofmost magma in δ 18 O. Much of the subcaldera reservoir was affected, because lavas that erupted as far apart as 115 km reflect the same pattern of depletion and partial recovery. Contemporaneous extracaldera rhyolites have the highest δ 18 O values in the volcanic field and show no effects of the repeated depletions. Sr and Pb isotope ratios of intracaldera rhyolites jump to more radiogenic values at times of caldera formation and show a longterm zigzag pattern like that of δ 18 O. Although some contamination by foundering roof rocks seenis to be required, water was probably the predominant contaminant. Even if roof rocks had been strongly depleted in O 18 before engulfment, their assimilation would have been far from sufficient to account for the large O 18 shift. The low- O 18 lavas contain no xenocrysts and show no trace element or phenocryst evidence of massive contamination. Their Fe-Ti-oxide temperatures indicate no cooling relative to the caldera-forming ash flow magma, and their whole-rock, glass, and phenoeryst chemistry suggests compositional continuity with the ash flow sequence. Oxygen exchange between the magma and a mass of low-O 18 water greatly exceeding solubility limits may require (1) recurrent explosive activity to sustain access and mixing of water with the magma and (2) convection of the magma reservoir to prevent local saturation.

Journal of Geophysical Research Solid Earth↗
USGS2012

Modeling crater topography and albedo from monoscopic Viking orbiter images: 1. Methodology

A new photoclinometric technique for extraction of topographic data from single planetary images is presented that overcomes many previous limitations of photoclinometry. The procedure fully compensates for oblique viewing geometry prevalent in spacecraft images. Albedo variations have been one of the most serious obstacles in the application of photoclinometry to planetary surfaces. This problem is overcome in the topographic solution by simultaneously utilizing brightness data from a pair of profiles; both segments are assumed to have the same topographic and albedo variations along their lengths. Profile directions are chosen where the orientation of downslope or upslope is obvious, thus resolving a major ambiguity in photoclinometry. This requirement is particularly easy to satisfy for craters and not very difficult for many irregular features. An additional procedure is presented that eliminates even the requirement of topographic symmetry along the pair of profiles. If two profiles have the same relief but their shapes are very different, another method can be used in an iterative process to derive topographic profiles; however, this procedure does, require that the albedo not vary along the profiles. Test results indicate that both procedures have an accuracy and precision of approximately 2° for slopes of typical bowl-shaped craters, which translates to approximately 5% for depths.

Journal of Geophysical Research Solid Earth↗
USGS2012

Propagation and linkage of oceanic ridge segments

We have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates. The ridge-propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other. The ridge-propagation force increases as ridge ends approach but then declines sharply as the ends pass, so ridge segments may overlap somewhat. The sign of the path factor changes as ridge ends approach and pass, so the overlapping ridge ends may diverge and then converge following a hook-shaped path. The magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting. For different loading conditions simulating ridge push, plate pull, and ridge suction, we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated. For all conditions, the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap; thus crust in this zone may fracture and weaken in preparation for the formation of a through-going transform fault. The calculated shear planes rotate toward an orientation about 90° from the strike of ridges as the ends pass, thus favoring the orthogonal arrangement of ridges and transforms. The magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated. The mean stress is tensile in the interaction zone, so basins may form there, except in the case of ridge push loading. The planes across which the maximum tension acts are oblique to ridges, thus favoring obliquely oriented normal faults bounding the transform valley.

Journal of Geophysical Research Solid Earth↗
USGS2012

Interpretation of gravity data in a complex volcano-tectonic setting, southwestern Nevada

This regional gravity study, based on an irregular 2-km data grid, was conducted during the past few years at Yucca Mountain, southern Nye County, Nevada, as part of a program to locate a suitable repository for high-level nuclear waste. About 100 surface rock samples, three borehole gamma-gamma logs, and one borehole gravity study provide excellent density control. A nearly linear increase in density of 0.26 g/cm 3 per kilometer of depth is indicated in the thick tuff sequences that underlie the mountain. Isostatic and 2.0-g/cm 3 Bouguer corrections were applied to the observed gravity values to remove regional gradients and topographic effects, respectively. The Bare Mountain gravity high, with an isostatic anomaly maximum of 48 mGal, is connected with a greater gravity high over the Funeral Mountains, to the southwest; together, these highs result from a continuous block of dense, metamorphosed Precambrian and Paleozoic rocks that stretches across much of the Walker Lane from the east edge of Death Valley to Bare Mountain. The Calico Hills gravity high appears more likely to originate from a northeast trending buried ridge of Paleozoic rocks that extends southwestward beneath Busted Butte, 5 km southeast of the proposed repository, where two- and three-dimensional modeling indicates that the pre-Cenozoic rocks lie less than 1000 m beneath the surface. Tuff, at least 4000 m thick, fills a large steep-sided depression in the pretuff rocks beneath Yucca Mountain and Crater Flat. The gravity low and the thick tuff section lie within a large collapse area that includes the Crater Flat-Timber Mountain-Silent Canyon caldera complexes. Gravity lows in Crater Flat itself are interpreted to coincide with the source areas of the Prow Pass Member, the Bullfrog Member, and the Tram Member of the Crater Flat Tuff; these source areas add nearly 350 km 2 to the previously recognized extent of the local caldera complexes. Southward extension of the broad gravity low associated with Crater Flat into the Amargosa Desert is evidence for sector graben-type collapse segments related to the formation of the Timber Mountain caldera and superimposed on the other volcanic and extensional structures within Crater Flat.

Journal of Geophysical Research Solid Earth↗
USGS2012

Lu-Hf constraints on the evolution of lunar basalts

Very low Ti basalts and green glass samples from the moon show high Lu/Hf ratios and low Hf concentrations. Low-Ti lunar basalts show high and variable Lu/Hf ratios and higher Hf concentrations, whereas high-Ti lunar basalts show low Lu/Hf ratios and high Hf concentrations. KREEP basalts have constant Lu/Hf ratios and high but variable Hf concentrations. Using the Lu-Hf behavior as a constraint, we propose a model for the mare basalts evolution. This constraint requires extensive crystallization of the primary lunar magma ocean prior to formation of the lunar mare basalt sources and the KREEP basalts. Mare basalts are produced by the melting of the cumulate rocks, and KREEP basalts represent the residual liquid of the magma ocean. Lu and Hf concentrations and the Hf isotopic data of lunar rocks suggest that assimilation cannot be accepted as a major process to explain the diversity of the lunar mare basalts. The urKREEP hypothesis is also unnecessary. Both high- and low-Ti basalts show enough iron enrichment to be regarded as melting products of the last stage cumulate rocks from the lunar magma ocean. The KREEP basalts are also rich in iron and may be regarded as the final, residual liquid left after the crystallization of the major portion of the primary lunar magma ocean.

Journal of Geophysical Research Solid Earth↗
USGS2012

Lu-Hf and Sm-Nd evolution in lunar mare basalts

Lu-Hf and Sm-Nd data for mare basalts combined with Rb-Sr and total REE data taken from the literature suggest that the mare basalts were derived by small (≤10%) degrees of partial melting of cumulate sources, but that the magma ocean from which these sources formed was light REE and Hf-enriched. Calculated source compositions range from lherzolite to olivine websterite. Nonmodal melting of small amounts of ilmenite (≤3%) in the sources seems to be required by the Lu/Hf data. A comparison of the Hf and Nd isotopic characteristics between the mare basalts and terrestrial oceanic basalts reveals that the εHf/εNd ratios in low-Ti mare basalts are much higher than in terrestrial oceanic basalts. The results are qualitatively consistent with the hypothesis that terrestrial basalt sources are partial melt residues whereas mare basalt sources are cumulates. Alternatively, the results may imply that the terrestrial mantle has evolved in two (or more) stages of evolution, and that the net effect was depletion of the mantle during the first ∼1–3 b.y. followed by enrichment during the last 1–2 b.y.; or simply that there is a difference in Lu-Hf crystal-liquid partitioning (relative to Sm-Nd between the lunar and terrestrial mantles.

Journal of Geophysical Research Solid Earth↗