bluuebunny/crossref_metadata_2025 huggingface dataset:
Geometry and evolution of the San Andreas Fault Zone in northern California
Authors: Furlong Kevin P. , Hugo William D. , Zandt George
Published: February 2008

The evolution of the San Andreas fault system is controlled by thermal‐mechanical processes associated with the development and evolution of a narrow “slabless window” formed beneath the western edge of North America. This fault zone evolution begins after initiation of transform motion along the plate boundary with the northward migration of the Mendocino triple junction. As a consequence of initial lithospheric structure and the shallow emplacement of asthenospheric mantle, the plate boundary separating the North American and Pacific plates follows a complex three‐dimensional geometry which varies through time. Seismic velocity structure, heat flow, seismicity, surface deformation, uplift, and fault development are controlled by the evolving thermal structure in the region after triple junction passage. Thermal‐mechanical models have been used to evaluate the fault system's time‐varying three‐dimensional dynamical behavior, simulating the principal processes involved in the thermal‐mechanical evolution of the San Andreas fault system. Results from this modeling indicate that the fault system has essentially a three‐stage history. (1) In the vicinity of the Mendocino triple junction the San Andreas fault maps the eastern edge of the Pacific plate, with a broad (∼100 km) zone of asthenospheric mantle separating the Pacific and North American plates in the 25‐ to 90‐km depth range. (2) Between 37°N and ∼39°N the plate boundary (within the mantle) separating the Pacific and North American plates has developed approximately 40–60 km east of the surface trace of the San Andreas fault and lies beneath the Hayward‐Calaveras faults and associated faults. The surface trace of the plate boundary appears to be connected to the mantle segment via a lower crust subhorizontal detachment surface. This fault zone orientation produces the surface deformations observed geodetically in the region. (3) South of 37°N the surface fault again overlies the deeper plate boundary, apparently as a result of an eastward jump in the surface fault.
Crater formation and modification on the icy satellites of Uranus and Saturn: Depth/diameter and central peak occurrence
Authors: Schenk Paul M.
Published: February 2008

Depth/diameters (d/D) for fresh craters on the intermediate‐sized icy satellites of Uranus and Saturn have been determined using photoclinometry and shadow lengths, and are compared with similar measurements on the terrestrial planets. Simple bowl‐shaped craters on icy satellites, including those on Miranda for which the highest resolution data are available, are systematically 20–40% shallower than on the terrestrial planets. This pronounced difference between crater depths on icy and rocky surfaces indicates that differences in impact velocity or surface gravity are not as important as the differences in mechanical properties between ice and “rock” in controlling simple crater morphology. Experimental impact studies indicate that differences in material properties such as angle of internal friction can control crater depth. Alternatively, breccia lenses may be thicker in icy satellite simple craters. Complex craters on the icy satellites become significantly deeper with increasing crater diameters, unlike complex craters on terrestrial planets, which are nearly constant in depth. Central peaks, and hence floor rebound, are also volumetrically and morphologically more prominent than wall slumping (rim collapse) on the icy satellites. The magnitude of viscous relaxation of very large craters (e.g., Herschel and Odysseus) on the icy satellites can be estimated from extrapolation of the d/D fits at smaller crater diameters. The transition diameter from simple to complex morphology is inversely correlated with gravity, as it is on the terrestrial planets, but at significantly smaller diameters than would be expected from a simple extrapolation of the terrestrial trend. Thus crater modification, especially floor rebound, is easier to initiate on icy satellites. Estimated effective cohesions for shocked icy material are lower by a factor of 10–20 relative to rock, although estimated effective viscosities are similar. The distinct, systematic differences between crater morphology on icy and rocky worlds indicate that gross material property differences between rock and ices play a key role in crater formation and modification, but gravity is still the primary driving force.
Mountain building in the central Andes
Authors: Kono Masaru , Fukao Yoshio , Yamamoto Akihiko
Published: February 2008

The Central Andes is the middle part of the Andean chain between about 13°S and 27°S, characterized by the parallel running high mountain chains (the Western and Eastern Cordilleras) at the edges of high plateaus with a height of about 4000 m and a width of 200 to 450 km (the Altiplano‐Puna). From the examination of geophysical and geological data in this area, including earthquakes, deformation, gravity anomaly, volcanism, uplift history, and plate motion, we conclude that the continued plate subduction with domination of compressive stress over the entire arc system is the main cause of the tectonic style of the Central Andes. We propose that the present cycle of mountain building has continued in the Cenozoic with the most active phase since the Miocene, and that the present subduction angle (30°) is not typical in that period but that subduction with more shallowly dipping oceanic lithosphere has prevailed at least since the Miocene, because of the young and buoyant slab involved. This situation is responsible for the production of a broad zone of partial melt in the mantle above the descending slab. Addition of volcanic materials was not restricted to the western edge (where active volcanoes of the Western Cordillera exist) but extended to the western and central portion of the Altiplano‐Puna. The western half of the Central Andes is essentially isostatic because the heat transferred with the volcanic activities softened the crust there. In the eastern edge, the thermal effect is small, and the crust is strongly pushed by the westward moving South American plate. This caused the shortening of crustal blocks due to reverse faulting and folding in the Eastern Cordillera and Amazonian foreland. The magmatism and crustal accretion are dominant at the western end of the mountain system and decrease eastward, while the compression and consequent crustal shortening are strongest at the eastern end and wane toward west. These two processes are superposed between the two mountain chains and form high plateaus there: the Altiplano of Bolivia and Peru and the Puna of Argentina. This interpretation is supported by the observation that (1) Neogene sedimentary formations have been uplifted to high elevations without heavy distortion in the Altiplano and the Western Cordillera, (2) no significant reverse fault systems are observed on the Altiplano, (3) Neogene volcanic rocks and volcanic centers since the Miocene are not restricted to the Western Cordillera but are widely distributed over most of the Altiplano, (4) most of the Altiplano is in a zone of high heat flow values, (5) thick Paleozoic rocks are strongly folded and faulted in the Eastern Cordillera with little volcanism and no large‐scale plutonism in the Cenozoic age, (6) crustal earthquakes with reverse fault mechanisms are concentrated on the eastern flank of the Eastern Cordillera and Amazonian foreland, and (7) the crustal thickness suddenly decreases at the junction of the Eastern Cordillera and the Amazon Basin, exactly at the place of reverse earthquakes.
In situ acoustic properties of pelagic carbonate sediments on the Ontong Java Plateau
Authors: Fulthorpe Craig S. , Schlanger Seymour O. , Jarrard Richard D.
Published: February 2008

The Ontong Java Plateau, with its thick, capping sequence of Cretaceous and Cenozoic pure pelagic carbonate sediments, forms an ideal setting for the study of the acoustic properties of this lithology on an oceanic rise. Borehole logs, recorded on Deep Sea Drilling Project leg 89 at site 586, provided detailed data on in situ acoustic properties of Pleistocene to early Miocene sediments to a depth of 623 m below seafloor. Comparison of these logging results and the sonobuoy‐based results of Johnson et al. (1978) with previous laboratory measurements from the Ontong Java Plateau shows that velocity/depth functions determined from the logging and sonobuoy methods are concordant but diverge significantly from functions derived from laboratory measurements. Log densities and compressional velocities exceed those measured by laboratory techniques; the density discrepancy is strongly influenced by laboratory method. The differences between log and laboratory compressional velocities are greater than and extend to greater depths than those between densities. These differences can be attributed to reductions in the frame bulk modulus and dynamic rigidity, caused by the removal of overburden pressure in the absence of significant porosity rebound. Agreement of site 586 log velocities with velocities derived from the earlier sonobuoy measurements across the plateau argues for the interpretation that both methods measure in situ values. The disagreement between the site 586 log results and the sonobuoy results with both the empirical velocity/depth function of Carlson et al. (1986) and the empirical velocity/porosity function of Raymer et al. (1980) supports the conclusion that pelagic carbonate sediments on oceanic plateaus and rises have unique acoustic properties, primarily arising from the presence of intraparticle porosity, and should not be grouped with other oceanic lithologies in acoustic modeling studies.
Postsubduction ocean island alkali basalts in Fiji
Authors: Gill James , Whelan Peter
Published: February 2008

The character of alkalic volcanism in Fiji changed at about 3 Ma when the mode of formation of the adjacent Lau back arc basin shifted from rifting to active spreading, leaving Fiji as a remnant arc. The geochemical change occurred throughout Fiji within 500,000 years of the age of the oldest magnetic anomaly in the Lau Basin. The alkalic volcanism changed from shoshonitic during the rifting stage to ocean island basaltic (OIB) during the spreading stage. The Fijian OIBs have Nb/La ratios >1.0 and are divided into two types on the basis of the enrichment levels and ratios of incompatible elements. The less enriched type generally is older and has Dupal isotopic traits. The Fijian OIBs may reflect the introduction of sub‐Pacific mantle from Samoa beneath Fiji and the northern Lau and North Fiji basins since breakup of the arc.
Inversion of tsunami waveforms for the estimation of heterogeneous fault motion of large submarine earthquakes: The 1968 Tokachi‐oki and 1983 Japan Sea earthquakes
Authors: Satake Kenji
Published: February 2008

Waveform inversion of tsunami data provides independent and reliable information on earthquake source processes. The propagation effects of tsunamis are accounted for by computing Green's functions using a finite difference method and using the known bathymetry, which is much better constrained than the seismic velocity structure. The observed tsunami waveforms are corrected for the response of the tide gage system by using the results of in situ measurements. Using Green's functions computed from bathymetric data on a 2.5‐km grid and the observed waveforms digitized at 1‐min intervals, the spatial resolution of the faulting heterogeneity by the present method is as small as 30 km. This method is applied to the 1968 Tokachi‐oki and 1983 Japan Sea earthquakes. The slip is concentrated at the northwestern part of the fault plane for the 1968 event. For the 1983 event the slip on the fault just north of the epicenter is twice that of the other parts. These asperity distributions are similar to those inferred from seismic wave analysis. However, the recovered total seismic moments are smaller. The present method is shown to be a useful technique for estimating heterogeneous fault motion, especially for historical events with no or poor seismic records, and tsunami earthquakes in which the excitations of seismic and tsunami waves are different.
Strain effects near palmdale associated with the San Fernando Earthquake (1971)
Authors: Brunner Fritz K. , Coleman Richard
Published: February 2008

For the SE Palmdale network in California, triangulation data for the years 1938 and 1958 and trilateration data at about 1–year intervals from 1971 to 1980 are the basis for a strain analysis. A uniform shear strain accumulation over the 40–year period appears to be evident from this data set ( γ = +0.34 ± 0.18 µstrain/yr, ψ = 119° ± 10°). Previous computations of this coseismic strain change needed further qualification due to the corrupting influence of uncertainties in the network height differences because height differences are critical when triangulation data are compared with trilateration observations. As a result, the height differences of the SE Palmdale network were resurveyed by trigonometric heighting in March 1981. Using these new data, a significant coseismic strain jump related to the 1971 San Fernando earthquake is postulated in the observations. The strain jump can be identified with engineering shearing strains of Δ γ1 = +3.3 ± 1.7 µstrain and Δ γ2 = +4.9 ± 1.7 μstrain or a total shear strain jump of +(5.9±2.4) µstrain with an azimuth (ψ) of 107°±6° of a line across which right–lateral shear is a maximum. These values are in excellent agreement with those predicted by earthquake displacement models.
Spinel‐lherzolite‐bearing quaternary volcanic centers in San Luis Potosí, Mexico: 1. Geology, mineralology, and petrology
Authors: Luhr James F. , Aranda‐Gomez Jose Jorge , Pier Jean G.
Published: February 2008

The central portion of San Luis Potosí State, Mexico, contains many isolated basic alkalic volcanic centers of Quaternary age, including seven large maar complexes. These vents are divided into two geographically distinct groups, Ventura to the southwest and Santo Domingo to the northeast. The maars and other volcanic centers contain xenoliths of mantle‐derived spinel lherzolite and lower‐crustal granulites. The Santo Domingo maars also contain kaersutite megacrysts derived from kaersutite‐rich veins in spinel lherzolite. Basic volcanic rocks form a well‐defined petrologic series from strongly silica‐undersaturated olivine nephelinites and basanites rich in spinel‐lherzolite xenoliths and xenocrysts which, are dominant at Ventura, through alkali olivine basalts and basalts having little to no lherzolitic debris, which are dominant at Santo Domingo. All rocks have olivine phenocrysts with Cr‐Al spinel inclusions, titanaugite‐augite phenocrysts or microphenocrysts, and groundmass crystals of one or both Fe‐Ti oxide series. Plagioclase decreases in abundance with increasing normative ne + lc and is absent or restricted to the groundmass in the most undersaturated rocks. Normative ne + lc is also strongly correlated with the abundances of incompatible elements, among which the degree of enrichment in the suite decreases in the sequence Nb, Ta, P, La, Th, Ce, Sm, Sr, Ba, Zr, Eu, Hf, K, Ti, Yb, and Na. Based on experimental analogs and major and trace element abundances, the entire volcanic suite from San Luis Potosí can be modeled by progressive degrees of partial melting of a garnet peridotite mantle source. In this scheme, the Santo Domingo magmas were produced by relatively larger degrees of partial melting. Equally plausible, however, is formation of the San Luis Potosí magmas as mixtures of melts from two different mantle sources. In this scenario, the Ventura‐type mantle is enriched in Nb, Ta, P, La, and related elements compared with the Santo Domingo‐type mantle. In Pier et al. [this issue] we describe Sr and Nd isotopic data for these samples that demand multisource models.
Thermal infrared (2.5–13.5 μm) spectroscopic remote sensing of igneous rock types on particulate planetary surfaces
Authors: Salisbury John W. , Walter Louis S.
Published: February 2008

Fundamental molecular vibration bands are significantly diminished by scattering. Thus such bands in spectra of fine particulate regoliths (i.e., dominated by <5‐μm particles), or regoliths displaying a similar scale of porosity, are difficult to use for mineralogical or rock type identification. Consequently, other spectral features have been sought that may be more useful in spectroscopic remote sensing of composition. We find that mineralogical information is retained in overtones and combination tones of the fundamental molecular vibrations in the 3.0‐ to 7.0‐μm region, but that relatively few minerals have a sufficiently distinctive band structure to be unambiguously identified with currently available techniques. More significantly, identification of general rock type, as defined by the SCFM chemical index (SCFM = SiO2/SiO2 + CaO + FeO + MgO), is possible using spectral features associated with the principal Christiansen frequency and with a region of relative transparency between the Si‐O stretching and bending bands. However, environmental factors may affect the appearance and wavelengths of these features. Finally, prominent absorption bands may result from the presence of relatively small amounts of water, hydroxyl or carbonate, because absorption bands exhibited by these materials in the 2.7‐ to 4.0‐μm region, where silicate spectra are otherwise featureless, increase strongly in spectral contrast with decreasing particle size. Such materials are thus detectable in very small amounts in a particulate regolith composed predominantly of silicate minerals.
Mechanics of slip and fracture along small faults and simple strike‐slip fault zones in granitic rock
Authors: Martel Stephen J. , Pollard David D.
Published: February 2008

We exploit quasi‐static fracture mechanics models for slip along pre‐existing faults to account for the fracture structure observed along small exhumed faults and small segmented fault zones in the Mount Abbot quadrangle of California and to estimate stress drop and shear fracture energy from geological field measurements. Along small strike‐slip faults, cracks that splay from the faults are common only near fault ends. In contrast, many cracks splay from the boundary faults at the edges of a simple fault zone. Except near segment ends, the cracks preferentially splay into a zone. We infer that shear displacement discontinuities (slip patches) along a small fault propagated to near the fault ends and caused fracturing there. Based on elastic stress analyses, we suggest that slip on one boundary fault triggered slip on the adjacent boundary fault, and that the subsequent interaction of the slip patches preferentially led to the generation of fractures that splayed into the zones away from segment ends and out of the zones near segment ends. We estimate the average stress drops for slip events along the fault zones as ∼1 MPa and the shear fracture energy release rate during slip as 5 × 102 − 2 × 104 J/m2. This estimate is similar to those obtained from shear fracture of laboratory samples, but orders of magnitude less than those for large fault zones. These results suggest that the shear fracture energy release rate increases as the structural complexity of fault zones increases.
Slip patterns in a spatially homogeneous fault model
Authors: Horowitz Franklin G. , Ruina Andy
Published: February 2008

We present a model which predicts seismological complexity even with no complexity in geometry or heterogeneity in material properties. Fault slip is numerically modeled using a Dieterich‐type rate and state variable friction law at the planar interface of two infinitely long massless elastic slabs. A constant velocity boundary condition is imposed a distance H from each interface. No geometrical, frictional, elastic, or remote loading variations are allowed in the direction parallel to the plane of the fault. In the numerical solution a periodic boundary condition is imposed, and the fault surface is divided into N subregions each of which is represented by a mathematical point. At these N points, all of which are mutually coupled by discretized two‐dimensional elasticity solutions, the friction law differential equations are numerically solved. The character of the solutions depends on model parameter values and initial conditions. Solutions are found that are periodic, quasi‐periodic, or aperiodic in time; and that are spatially homogeneous for all time, nearly homogeneous except during fast slip events, or essentially inhomogeneous for all time. For given parameter values the solutions have a qualitative character which is nearly independent of initial conditions. At any instant in time these solutions ultimately appear roughly as some superposition of those spatial sine waves which are unstable in a linearized calculation. When spatially complex, the solutions can simultaneously exhibit regions that have steady sliding, large slip rates, and local propagating creep events. Special initial conditions can generate other solutions such as steady propagating creep waves that span the whole fault. The variety of simulated slip motions and long‐term patterns of slip predicted by this spatially homogeneous nonlinear dynamical model suggests a possible role for dynamics, and not just complex geological structure, as a generator of temporal and spatial complexity in seismic phenomena.
Steady thermomechanical flow along two‐dimensional flow lines in large grounded ice sheets
Authors: Dahl‐Jensen Dorthe
Published: February 2008

A thermomechanical flow model has been developed in order to perform simultaneous calculations of the surface elevation and the distribution of stresses, velocities, and temperatures along two‐dimensional flow lines on large grounded ice sheets. In order to solve the complex system of full thermomechanical coupled equations the following approximations have been made: A coordinate scaling giving a lead order reduced model and an iteration procedure, which decouples the energy balance equation from the rest of the equations. Glen's flow law is used as the constitutive relationship between stresses and deformation in the model, and it is found that the longitudinal deviatoric stresses have a significant role in the lead order equations. At the ice divide the solution shows that the velocity, stress, and temperature distributions change rapidly. Here the basal temperature increases creating a “hot spot.” The surface strain rates increase 50% and the horizontal velocity profile is concave near the base, with an inflection point. Away from the ice divide, basal temperature increases, and in the example presented here the basal temperature reaches the pressure melting point after 2/3 of the lateral extent and basal sliding begins to influence the flow. In the ablation region it is seen that the heat produced by internal deformation is of the same order of magnitude as the geothermal heat flux from the bedrock.
Seismic detection capability at NORESS and implications for the detection threshold of a hypothetical network in the Soviet Union
Authors: Sereno Thomas J. , Bratt Steven R.
Published: February 2008

The detection threshold of a network including NORESS‐type arrays in the Soviet Union is estimated using the frequency‐dependent attenuation of Pn and Lg observed at NORESS to normalize the threshold of individual stations. The normalization begins with a parameterization of the magnitude and range dependence of the NORESS Pn and Lg spectra in terms of seismic moment and apparent attenuation. A relationship between these spectral amplitudes and the time‐domain amplitudes used in signal detection at NORESS is then determined. This allows an estimate of NORESS detection capability from the spectral parameterization and a prediction of the Pn detection capability for regions with different wave propagation characteristics. A comparable prediction is not possible for Lg because of dispersion and the non‐stationarity of pre‐Lg noise. Instead, an empirical relationship for Lg detectability based on observed temporal amplitudes is determined. To validate the results for NORESS, predictions based on our model are compared with observed detection statistics and with results obtained by Ringdal (1986), who compared detections at NORESS to bulletins produced by local seismic networks. The simulations represent the detection capability of a network of NORESS‐type arrays, 20 within and 13 outside the Soviet Union. The 90% ML threshold for detecting three Pn phases for events in the Soviet Union is estimated to be between 2.4 and 2.7 if the frequency‐dependent attenuation and noise throughout the network are the same as those observed at NORESS. This threshold is reduced by 0.1 ML if Pn Q is 50% higher in the Soviet Union than estimated for Scandinavia, and is increased by 0.2–0.3 ML if Pn Q is 50% lower. The ML thresholds are reduced by 0.2–0.3 if the detection criteria include Lg as well as Pn phases. While these simulations are carefully normalized by the actual detection capability of a prototype station for test ban treaty monitoring, observations of attenuation and noise inside the Soviet Union are needed to determine how they actually differ from those at NORESS. Also, it must be noted that the NORESS detection capability estimates (and therefore the simulations) are for average conditions and there are regular variations that must be considered when estimating treaty monitoring capability.
Derivation of the complete Gutenberg‐Richter magnitude‐frequency relation using the principle of scale invariance
Authors: Rundle John B.
Published: February 2008

The purpose of this paper is to show that the various observational parameters characterizing the statistical properties of earthquakes can be related to each other. The fundamental postulate which is used to obtain quantitative results is the idea that the physics of earthquake occurrence scales as a power law, similar to properties one often sees in critical phenomena. When the physics of earthquake occurrence is exactly scale invariant, b = 1, and it can be shown as a consequence that earthquakes in any magnitude band Δm cover the same area in unit time. This result therefore implies the existence of a “universal” covering interval τT, which is here called the “cycle interval.” Using this idea, the complete Gutenberg‐Richter relation is derived in terms of the fault area ST, which is available to events of any given size, the average stress drop ΔσT for events occurring on ST, the interval τT for events of stress drop ΔσT to cover an area ST, and the scaling exponent α, which is proportional to the b value. Observationally, the average recurrence time interval for great earthquakes, or perhaps equivalently, the recurrence interval for characteristic earthquakes on a fault segment, is a measure of the cycle interval τT. The exponent α may depend on time, but scale invariance (self similarity) demands that α = 1. It is shown in the appendix that the A value in the Gutenberg‐Richter relation can be written in terms of ST, τT, ΔσT, and the parameter α. The b value is either 1 or 1.5 (depending on the geometry of the fault zone) multiplied by α.
Heat flow data and vertical groundwater movement, examples from southwestern Virginia
Authors: Reiter Marshall , Costain John K. , Minier Jeffrie
Published: February 2008

In the present study we show that estimates of vertical specific discharge are often obtainable from plots of conducted heat flow Q versus temperature T. Q‐T plots allow for variations in thermal conductivity across a zone of flow, whereas temperature gradient G versus T plots assume constant conductivity. It is demonstrated that the two different plots can provide quite different estimates of vertical specific discharge. By considering vertically convected heat flow across a zone, better estimates of regional (geologically representative) heat flow can be made. Agreement between corrected heat flow values, conducted heat flows calculated near fluid flow exit boundaries, and regional values suggests that the steady state flow model used is reasonably representative, although the fluid flow depth interval is typically difficult to estimate confidently. The effects of small continuous well bore flow can be removed if continuity in the temperature log results; otherwise, it is most difficult to estimate specific discharge as confirmed by unrepresentative corrected heat flow data.
Mechanics of graben formation in crustal rocks: A finite element analysis
Authors: Melosh H. J. , Williams C. A.
Published: February 2008

Small (1–10 km wide) grabens are ubiquitous tectonic structures that have been observed on the surfaces of all the terrestrial planets and satellites. In spite of their frequent occurrence, there is no fundamental understanding of how they form, other than that they are characteristic of extensional terranes. This paper examines the mechanics of the initial stages of graben formation and demonstrates that the configuration of a pair of antithetically dipping normal faults (a graben) is the energetically most favorable fault configuration in elastic‐brittle rocks subjected to pure extension. A corollary to this hypothesis is that the occurrence of synthetic normal faults is indicative of a stress field other than purely extensional, or of nonhomogeneous characteristics of the rock. The analysis proceeds by computing the stress field in the vicinity of a single initial normal fault using a two‐dimensional finite element model. It is shown that slip on this fault substantially lowers the proximity to failure of the surrounding rocks except for two small regions, one near the surface of the downthrown block and the other near the fault tip. The region on the downthrown block is located a horizontal distance from the initial fault slightly less than the fault's depth. The proximity to failure actually increases here, and a secondary fault develops. An antithetic secondary fault would link this region with the other failure zone near the fault tip. To demonstrate the preference for antithetic secondary faults more rigorously, we show that this fault configuration results in a smaller total strain energy than the synthetic configuration. In addition, a two‐layer model is used to show that the graben's width is only weakly dependent upon the presence of mechanically distinct layers in the crust. Both weak‐over‐strong and strong‐over‐weak configurations are investigated. The depth of the initial fault and the thickness of the top layer are varied, and the width of the resulting graben is evaluated by computing the distance between the initial fault and the region of highest proximity to failure near the surface. This analysis indicates that the depth of the initial fault is the major factor controlling graben width.
Pleistocene deformation of a portion of the Southern Cascadia Forearc: Prairie Creek Formation, northern California
Authors: Kelsey Harvey M. , Trexler James H.
Published: February 2008

The Prairie Creek Formation, a sedimentary unit of limited outcrop extent (150 km2) on the southern Cascadia forearc in northern California, provides geologic data that constrain the style of tectonic activity in this portion of the forearc during the time interval 4(?) Ma to about 0.5 Ma. The formation records deposition of approximately 550 m of mostly fluvial sediment during a period of subsidence of the forearc. The formation was deposited by a large braided river, most likely an ancestral Klamath River, that prograded into the forearc. Sedimentation mainly consisted of sand and well‐rounded conglomeratic alluvium. However, within the lower half of the section, there occur several thin horizons of coarse angular conglomerate of a distinctive lithology. The only likely provenance for this anomalous conglomerate is fault scarps intermittently exposed near the area of deposition. The tectonic setting of this portion of the southern Cascadia forearc in late Pliocene or early Pleistocene through mid‐Pleistocene was therefore one of general forearc subsidence with intermittent uplift due to faulting. Intercalation of sand and gravel braidplain deposits in the fluvial section suggests periodic changes in the net subsidence rate. Subsidence culminated in a brief period of submergence followed shortly by uplift to as much as 500 m in the late Pleistocene. The transition to uplift records a significant change in tectonic regime. This change may be due to the deformational effects of either or both of the following tectonic processes: northward encroachment of the Mendocino triple junction during the late Pleistocene, or a late Pleistocene change in the direction and/or rate of convergence of the Juan de Fuca plate relative to the overriding North American plate.
Stratigraphy and sedimentology of the Mid‐Cretaceous deposits of the Yukon‐Koyukuk Basin, west central Alaska
Authors: Nilsen Tor H.
Published: February 2008

The northeast trending Yukon‐Koyukuk basin of west central Alaska consists of two subbasins, the Kobuk‐Koyukuk subbasin to the north and east and the Lower Yukon subbasin to the southwest. The subbasins are separated by an arcuate Lower Cretaceous volcanic pile, the Hogatza trend, which is thought to be an accreted volcanic arc. The oldest part of the sedimentary fill of the subbasins consists of Valanginian to lower Albian(?) volcaniclastic rocks deposited on the flanks of the Hogatza trend. Following subsidence of the Hogatza trend, mid‐Cretaceous clastic sedimentary strata of mainly Albian and Cenomanian age, and possibly as thick as 8000 m, were shed into the basin; these deposits were derived from surrounding uplands or borderlands in the Seward Peninsula to the west, the Brooks Range to the north, and the Ruby geanticline to the southeast. These mid‐Cretaceous basin fill deposits can be divided into four main facies: (1) basin margin conglomerate facies, chiefly alluvial fan deposits that were transported basinward and rest in part unconformably on the surrounding uplands; (2) shelf facies, chiefly cross‐stratified and hummocky cross‐stratified sandstone deposited by wave‐generated currents on a shelf that rimmed the basin on its western and northern margins; (3) deltaic facies, chiefly sandstone and shale deposited in delta plain and delta front environments on a large constructional delta that prograded westward from the eastern basin margin across both subbasins and across the subsided southern part of the Hogatza trend; and (4) turbidite facies, chiefly interbedded sandstone and shale deposited as elongate deep‐sea fans and related deep‐sea clastic systems by flows that transported sediment to the axial parts of both subbasins, northeastward in the Lower Yukon subbasin and eastward to southward in the Kobuk‐Koyukuk subbasin. Sedimentation appears to have ended in the Santonian, followed by uplift, folding, and faulting of the basin fill. Less deformed, lower Tertiary nonmarine volcanic and volcaniclastic rocks unconformably overlie the more highly deformed Cretaceous strata.
Thermoviscous remanent magnetization of oceanic lithosphere inferred from its thermal evolution
Authors: Arkani‐Hamed Jafar
Published: February 2008

Remanent magnetization patterns of the oceanic lithosphere determined on the basis of the thermal evolution of the lithosphere, while taking into account both thermal and viscous magnetizations, give rise to magnetic anomalies similar to those observed by Magsat over the Cretaceous quiet zones of the Atlantic Ocean and by marine magnetic surveys over anomaly 33. Viscous magnetization acquired during a given core field polarity period is not removed completely by that acquired in the next period of opposite polarity because of cooling of the lithosphere and the strong temperature dependence of viscous magnetization. The magnetization pattern of layer 2A resembles that of the standard seafloor spreading magnetic model, consisting of normally and reversely magnetized zones separated by vertical boundaries, and produces symmetric magnetic anomalies. The magnetization pattern of the middle crust is similar to that of layer 2A for high magnetic blocking temperature models, but it is slightly skewed with respect to layer 2A for low magnetic blocking temperature models. The magnetization pattern of the lower crust is strongly skewed relative to that of layer 2A and has diffuse boundaries with pronounced slopes between normally and reversely magnetized zones. It produces magnetic anomalies with appreciable skewness, suggesting that observed skewness of marine magnetic anomaly 33 arises from the magnetization of this layer. The magnetization pattern of the upper mantle is appreciably skewed and almost devoid of small‐scale variations. It produces strongly skewed magnetic anomalies at satellite altitudes which may explain the observed skewness of Magsat anomalies over the Cretaceous quiet zones of the Atlantic Ocean.
Elastic properties of contacting surfaces under normal and shear loads: 2. Comparison of theory with experiment
Authors: Yoshioka Naoto , Scholz Christopher H.
Published: February 2008

A contact theory for both normal and shear behavior of contacting surfaces under elastic and no‐slip conditions was examined by laboratory experiments using hollow cylindrical granite samples upon which surfaces of different roughnesses were prepared. The surfaces were placed in contact in a torsional‐loading apparatus, and both their normal closure and deformation in shear were measured. To compare with the theoretical prediction, the shear experiments were done under very low shear stress assuming no‐slip at the real contacts: we measured shear stiffness of the contacting surfaces under various normal stresses up to 35 MPa. The theoretical predictions, based on profilometry data from the surfaces, generally show quantitative agreement with the mechanical measurements: the normal joint closure increases nonlinearly with normal stress and shows larger deformation for the rougher surface. The shear stiffness of contacting surfaces also increases nonlinearly with normal stress and is larger for the smoother surface. A poorer fit was obtained for the smooth surface which seems due to the limitation of resolution in our topography measurements and minor anelastic behavior which occurred at the real contacts. The micromechanical approach presented here may open a way to physical understanding shear contact properties, such as shear wave propagation across interfaces and friction.
Broadening of seismogram envelopes in the randomly inhomogeneous lithosphere based on the parabolic approximation: southeastern Honshu, Japan
Authors: Sato Haruo
Published: February 2008

Analyzing horizontal component seismograms of small earthquakes with intermediate hypocentral distances in southeastern Honshu, Japan, we found that time widths of seismogram envelopes around direct S waves are much longer than source duration times estimated from their magnitudes. The time lags of the maximum amplitude and the half maximum after the peak were measured from the onset of the direct S wave arrival from band‐pass‐filtered seismograms from 2 to 32 Hz. Even though there is considerable scatter, both the time lags are found to increase with increasing hypocentral distance up to 305 km. We hypothesize that pulse shape broadens and the maximum amplitude is reduced after propagating through the random structure of the lithosphere. The parabolic approximation theoretically predicts that the long‐wavelength component of velocity inhomogeneities compared with the wavelength of seismic waves produces diffraction fluctuations and makes seismogram envelopes broaden with increasing travel distance in the saturated regime. Supposing a Gaussian autocorrelation function for the randomness and an empirical frequency dependent attenuation, we propose a formula for the temporal change in the power spectral density of seismic waves. Applying this formula to the observed data, we statistically evaluated the scale of random inhomogeneities: the mean square fractional velocity fluctuation was estimated to be 10−3 times the correlation distance a in kilometers.
Stability and equation of state of CaSiO3‐Perovskite to 134 GPa
Authors: Mao H. K. , Chen L. C. , Hemley R. J. , Jephcoat A. P. , Wu Y. , Bassett W. A.
Published: February 2008

The stability and P‐V equation of state of CaSiO3 have been investigated using in situ diamond‐anvil X ray diffraction techniques to 134 GPa, a pressure equivalent to that at the core‐mantle boundary. Samples were heated by YAG laser at each pressure increment at high pressures to accelerate phase transitions. X ray diffraction measurements were carried out at 300 K using both energy‐dispersive synchrotron and sealed‐tube film techniques. Quenched CaSiO3‐perovskite was observed to remain metastable close to 0.1 MPa, and to convert rapidly to an amorphous phase on pressure release. The simple cubic perovskite phase of CaSiO3 was found to be the stable phase for all lower mantle pressure conditions. All 47 P‐V data points were used to obtain a third‐order Birch‐Murnaghan equation of state with zero‐pressure parameters: unit cell volume V0 = 45.37±(0.08) Å3, density ρ0 = 4.252(±0.008) Mg/m3, and bulk modulus K0 = 281(±4) GPa, with an assumed bulk modulus pressure K0′ = 4. These parameters are close to those of (Mg0.88Fe0.12)SiO3‐perovskite and to those inferred by the Preliminary Reference Earth Model for the lower mantle. Hence, CaSiO3 must be considered an invisible component, in terms of density and bulk modulus constraints, in the lower mantle. Mantle composition models with both high and low calcium content can satisfy existing seismological constraints for the lower mantle.
High‐frequency seismic observations in eastern Kazakhstan, USSR, with emphasis on chemical explosion experiments
Authors: Given Holly K. , Tarasov Nikolai T. , Zhuravlev Vladimir , Vernon Frank L. , Berger Jonathan , Nersesov Igor L.
Published: February 2008

Two temporary three‐station seismic networks, deploying surface and 100‐m borehole high‐frequency seismometers, of the order of 200 km from the Kazakh test site in the USSR and the Nevada test site in the United States are discussed, with emphasis on chemical explosion experiments. Seismograms attained from the detonation of three buried explosions (10 t, 20 t, 10 t) in eastern Kazakhstan at distances between 156 and 637 km are examined in the frequency band of 1–80 Hz. Observed signal‐to‐noise (S/N) ratios were high, reaching a maximum of 400 for Pg waves and 200 for Lg waves. Good signal‐to‐noise levels persisted to high frequencies; S/N = 2 at about 50 Hz for Lg waves about 250 km from the source, and at about 14 Hz at 680 km distance. For Pg waves, S/N = 2 at about 50 Hz 270 km from the source. Shapes of displacement amplitude spectra were similar, characterized by a broad maximum in signal‐to‐noise levels between 4–8 Hz, and a decay at higher frequencies (e.g. above 10 Hz) of about f−3.5 − f−4.1 for Lg waves, and f−3.1 − f−4.5 for Pg, unconnected for distance. Magnitudes estimated from Lg time domain amplitudes for the 10 t explosion are between 2.8 and 3.3, depending on the magnitude relation used. Spectral characteristics are used to put some constraint on Lg Q. Pg Q is poorly constrained by the data. A similar experiment in southern Nevada showed much lower Pg and Lg signal‐to‐noise levels above 1 Hz, although Kazakh and Nevada absolute noise levels are comparable.
Mixing and the distribution of heterogeneities in a chaotically convecting mantle
Authors: Kellogg L. H. , Turcotte D. L.
Published: February 2008

Chemical heterogeneities are introduced into the mantle by subduction of oceanic lithosphere and possibly by subduction of sediments and delamination of the continental lithosphere. These heterogeneities are blended into the mantle by convective mixing. Chemically distinct heterogeneities are stirred into the surrounding mantle matrix by convective shear, which deforms them, increasing the area of contact between an heterogeneity and the matrix. Final homogenization is accomplished by diffusion, which brings centimetersized heterogeneities into chemical equilibrium with the matrix. Numerical models of mantle convection suggest that it is chaotic. To study the effect of a chaotic flow on mixing, we have generated a space‐filling, two‐dimensional flow using the Lorenz equations in the chaotic regime as a driver. We specify a two‐mode expansion of the stream function, resulting in a flow which oscillates smoothly between one and two cells. Particle paths are chaotic in time and space. Mixing is very rapid; the mixing time for heterogeneities with an initial size of 6 km is 240 m.y. for layered mantle convection; the corresponding mixing time for whole mantle convection is 960 m.y. The source region for mid‐ocean ridge basalts has a “marble cake” structure; it is made up of material which has been processed through ridges, subducted, stretched, and thinned and is either partially or wholly homogenized. The incompletely mixed subducted oceanic crust forms bands of eclogite in the peridotite matrix. These bands range in scale from 6 km to a few centimeters.
Year‐to‐year instability of the Mars south polar cap
Authors: Jakosky Bruce M. , Haberle Robert M.
Published: February 2008

Models of the Mars polar cap energy balance suggest that the south cap has two possible configurations at the current epoch. It can be covered all year by CO2 frost, as was observed by the Viking spacecraft, or the CO2 frost can disappear at midsummer to expose an underlying surface. The difference between the two states depends on the role of energy conducted into the subsurface: If the cap was exposed the previous year, conducted energy will cause less CO2 frost to condense in winter, and the frost will disappear completely in summer; if the cap was covered the previous year, there will be no conducted energy, and it will stay covered again. Subtle effects in the atmosphere can perturb the cap energy balance sufficiently to cause the cap to jump between states. This behavior might provide an explanation for the large southern summer water vapor abundances observed in 1969. It also explains the observed differences between the north and the south residual caps as resulting in part from a difference in the role in the annual energy balance of energy conducted into the subsurface.