bluuebunny/crossref_metadata_2025 huggingface dataset:
Origin of thick, high‐velocity igneous crust along the U.S. East Coast Margin
Authors: Kelemen Peter B. , Holbrook W. Steven
Published: February 2004

Recent seismic results on the U.S. East Coast continental margin show that the zone between rifted continental and normal oceanic crust consists of thick (up to 25 km), high seismic velocity (νp of 7.2–7.3 km s−1) crust, interpreted as mafic igneous rocks emplaced during Triassic/Jurassic continental rifting. The total volume of igneous rocks in this zone, which we call the East Coast Margin Igneous Province (ECMIP), may be as much as 2.7 × 106 km3, placing the ECMIP among the world's large igneous provinces. We constrain the composition and origin of the thick, igneous crust by using a compilation of laboratory measurements to predict P wave velocities for rocks with the compositions of liquids produced by partial melting of mantle rocks. The high‐velocity crust was produced from partial melting of mantle peridotite, with smaller melt fractions (<10%) but at higher average pressures (≥2.0 GPa) than beneath normal mid‐ocean ridges. This requires higher than normal asthenospheric potential temperatures during rifting and a lid of lithosphere above upwelling asthenosphere to limit the minimum pressure of melting. Production of thick igneous crust at small melt fractions requires that the vertical flux of asthenosphere during rifting exceeded the lateral flux of lithosphere due to extension; that is, mantle “upwelling” was more rapid than lithospheric “spreading.” Thick igneous crust is strongly asymmetrical, extending up to 2000 km along the margin but only for about 80–100 km seaward. The rapid seaward transition to oceanic crust with normal thickness and seismic velocity implies that the thermal anomaly and relatively rapid upwelling lasted for only 5–8 m.y. Moreover, there is no crustal thickness anomaly in the Central Atlantic, in contrast to the North Atlantic where the influence of the Iceland plume created thick crust in a belt spanning the ocean from Greenland to the Faeroes Islands. These factors seem to preclude formation of thick igneous crust in response to a deep‐seated mantle plume. The ECMIP may have formed when high upper mantle temperatures induced asthenospheric upwelling. Magmatism and seafloor spreading dissipated the thermal anomaly in the upper mantle, after which normal oceanic crust formed along the Mid‐Atlantic Ridge.
The dynamic rupture process of the 1993 Kushiro‐oki earthquake
Authors: Ide Satoshi , Takeo Minoru
Published: February 2004

We have constructed a dynamic rapture model of the 1993 Kushiro‐oki earthquake (Mw= 7.6) that is consistent with the observed seismograms. This earthquake occurred beneath the city of Kushiro, Japan, at a depth of about 100 km, and the aftershocks are limited to a horizontal plane between a double seismic zone in the subducting slab. Excellent near‐field strong‐motion records are available. In our analysis, a kinematic inversion is followed by a dynamic crack calculation, and this pair of operations is repeated until convergence to the data is obtained. Two different conditions concerning the dynamic stress drop were considered in the dynamic calculation, and a dynamic model was constructed for each condition. The difference in these conditions has little effect on the results. The final dynamic model shows that this earthquake occurred with a localized high stress drop of about 40 MPa in the subducting slab, which indicates that the region between the double seismic zone is strong enough to sustain such a high stress. One of the two high strength regions in the final model lies near the upper plane of the double seismic zone and represents a barrier due to a substantial change of stress state in the slab. In the region where the initial breaks occurred 5 s before the main rupture, both the strength and the stress drop are smaller than in the adjacent region. This implies that before the main rupture the accumulated stress had already been released in the area of the initial breaks. The aftershocks tend to occur near the high strength area; in some of them, thePwave first‐motions are opposite to those of the main shock.
On the robustness of elastic thickness estimates obtained using the coherence method
Authors: Macario Ana , Malinverno Alberto , Haxby William F.
Published: February 2004

In order to uniquely determine the elastic thickness of the lithosphere, Te, from gravity and topography data, the coherence method explicitly assumes that surface and subsurface loads are statistically uncorrelated. In many realistic cases (e.g., mountain building) this assumption is likely to be violated. We present the results of Monte Carlo simulations designed to measure the bias and variance of Te estimates when the degree of correlation, R, between fractal surface and subsurface loads is not null. In this study, R denotes the linear correlation coefficient (Pearson's coefficient), and surface and subsurface loads are self‐affine (fractal). Recording to our results, when there is no correlation between the loads (R = 0), the estimated
is in excellent agreement with the simulated Te although its accuracy decreases for increasing plate stiffness. We also find that there is a strong likelihood of an upward bias in
estimates if the dimensions of the study area are not adequate to fully resolve the longest wavelengths. This might partially explain why Te values for previous continental studies obtained using the coherence method are often higher than Te values obtained using conventional forward modeling techniques. As the degree of correlation increases, we observe a clear downward bias in the estimated
. When surface and subsurface loads are correlated by only a small amount, the fit between the computer‐simulated and predicted coherences is still good for the longest wavelengths within the diagnostic wavebands. In light of these limitations, we use the coherence method to reassess Te estimates for the Alps. Our results indicate that the flexural behavior of the lithosphere to loading does not change significantly along the fold/thrust belt. The similarity between our Te results for the western (30–39 km) and eastern Alps (33–40 km) does not support the hypothesis that Te and the horizontal radius of curvature of mountain belts are correlated.
Identifying coseismic subsidence in tidal‐wetland stratigraphic sequences at the Cascadia subduction zone of western North America
Authors: Nelson Alan R. , Shennan Ian , Long Antony J.
Published: February 2004

Tidal‐wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimentologic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive as well as active continental margins. Tidal‐wetland stratigraphy at many Cascadia estuaries differs little from that at similar sites on passive‐margin coasts where stratigraphic sequences form through nonseismic processes unrelated to coseismic land level changes. Methods developed through study of similar stratigraphic sequences in Europe provide a framework for investigating the Cascadia estuarine record. Five kinds of criteria must be evaluated when inferring regional coastal subsidence due to great plate boundary earthquakes: the suddenness and amount of submergence, the lateral extent of submerged tidal‐wetland soils, the coincidence of submergence with tsunami deposits, and the degree of synchroneity of submergence events at widely spaced sites. Evaluation of such criteria at the Cascadia subduction zone indicates regional coastal subsidence during at least two great earthquakes. Evidence for a coseismic origin remains equivocal, however, for the many peat‐mud contacts in Cascadia stratigraphic sequences that lack (1) contrasts in lithology or fossils indicative of more than half a meter of submergence, (2) well‐studied tsunami deposits, or (3) precise ages needed for regional correlation. Paleoecologic studies of fossil assemblages are particularly important in estimating the size of sudden sea level changes recorded by abrupt peat‐mud contacts and in helping to distinguish erosional and gradually formed contacts from coseismic contacts. Reconstruction of a history of great earthquakes for the Cascadia subduction zone will require rigorous application of the above criteria and many detailed investigations.
Temporal and spatial variations in crustal accretion along the Mid‐Atlantic Ridge (29°–31°30′N) over the last 10 m.y.: Implications from a three‐dimensional gravity study
Authors: Pariso Janet E. , Sempéré Jean‐Christophe , Rommevaux Céline
Published: February 2004

We have conducted a three‐dimensional gravity study of the Mid‐Atlantic Ridge near the Atlantis Transform to study the evolution of accretionary processes at this slow‐spreading center over the last 10 m.y. We have removed from the free‐air gravity anomaly the gravity contribution of the density contrast at the seafloor and the gravity contribution of the lateral density variations associated with the cooling of the lithosphere. The resulting residual gravity anomaly exhibits substantial variation along and across the ridge axis. The residual gravity anomaly can be accounted for by variations in crustal thickness of up to 3 km. For the first two segments south of the Atlantis Transform, the midportions of the segments have been associated with thick crust and the segment discontinuities have been associated with thin crust for the last 10 m.y., suggesting the segment discontinuities act as long‐term boundaries in the delivery of melt to the individual segments. In contrast, our calculations indicate that for the segments north of the fracture zone, thick crust is associated with the midportions of segments and thin crust is associated with segment discontinuities only in crust less than ∼3 m.y. This result suggests that focused mantle upwelling has only recently developed north of the fracture zone. The onset of focused mantle upwelling at approximately 2–3 m.y. may be related to a change in the spreading direction which occurred between magnetic anomalies 5 and 3 (Figure 1) and resulted in changes in the geometry of the plate boundary north of the fracture zone. Cross sections of crustal thickness extracted along the midpoint traces of paleosegments show that, for a few segments, up to 2 km of gradual crustal thinning is observed. We suggest that the “apparent” crustal thinning is a result of lateral changes in mantle density associated with buoyant upwelling not predicted by the passive flow model used in our study. Variations in computed crustal thickness are observed across axis in all of the paleosegments in our study area, but are not correlated between individual segments. If these computed crustal thickness variations are due to temporal variations in melt production, this implies that there is little interdependence in the amount of melt supplied to adjacent segments.
The meaning of “mean F”: Clarifying the mean extent of melting at ocean ridges
Authors: Plank Terry , Spiegelman Marc , Langmuir Charles H. , Forsyth Donald W.
Published: February 2004

In a recent paper, Forsyth (1993) concludes that fractional melting leads to unexpected relationships between the degree of melting (F), crustal thickness, and the depth of melting beneath mid‐ocean ridges. Specifically, he suggests that a commonly cited rule of thumb, that 10% mean melting of a 60‐km column of mantle leads to 6 km of crustal thickness (Klein et al., 1991; Langmuir et al., 1992), is incorrect for fractional melting of the mantle. Here we show that the rule of thumb remains valid for Langmuir et al.'s definition of mean F and that confusion has arisen because there has been disagreement on the definition of mean F. Plank and Langmuir (1992) have defined mean F as the ratio of the mass flux of melt added to the oceanic crust to the mass flux of mantle entering the melting region; Forsyth (1993) has defined mean F as the average degree of melting of all pooled melt increments, with degree calculated at the last point of chemical equilibration. We show here that both definitions of mean F are valid conceptually and mathematically, clarify the differences between them, show how they relate differently to observables such as crustal thickness and crustal composition, and propose nomenclature to clarify usage in the future (FB for Plank and Langmuir's bulk melt fraction and Fv for Forsyth's mean value).
Damage mechanisms in stressed rock from acoustic emission
Authors: Shah Ketan R. , Labuz Joseph F.
Published: February 2004

To better understand the phenomena leading to the failure of rock, unconfined compression experiments on Charcoal granite specimens were performed with the monitoring of acoustic emission (AE). Localization in the form of spalling near the free surface was detected by the concentration of hypocenters. The AE locations, which look random in space and time before localization, actually were clustered and have fractal structure in either space or time. After localization, the fractal dimensions reduced significantly in a certain range of distance and time. The analysis of interaction between spatial and temporal clustering revealed the size of clusters in both space and time. The cluster sizes may be related to the intrinsic properties of the rock. The seismic moment tensor was evaluated through a deconvolution technique to obtain the source mechanism and orientation of each AE event. The dominant mode of failure from AE was sliding on inclined planes, although a significant number of source planes were parallel to the loading axis, while the growth of cracks perpendicular to the loading axis was inhibited. This preferential growth of microcracks is related to a tensorial measure of damage and is used to study stress‐induced anisotropy.
The effect of change in thermal properties on the propagation of a periodic thermal wave: Application to a snow‐buried rocky outcrop
Authors: Gray J. M. N. T. , Morland L. W. , Colbeck S. C.
Published: February 2004

The propagation of a periodic thermal wave into snow is significantly altered by the presense of a shallow rock interface because of the large difference in thermal properties of the two media. The temperature distribution is modeled using classical heat conduction equations subject to a periodic diurnal or seasonal surface heat flux condition, jump conditions at the interface, and insulating conditions in the far‐field. The natural length scale (skin depth) over which order unity changes in temperature occur is proportional to the square root of the timescale of the surface temperature variations. If the interface lies close to or within the skin depth then large temperature gradients can be sustained in the snow before temperature oscillations are forced through to the underlying rock. These features are explained by an analytic one‐dimensional periodic solution. A numerical algorithm is constructed to solve for the temperature around plane two‐dimensional rock geometries. The results show that during a period of atmospheric cooling the presence of a buried rocky outcrop increases the snow temperature and temperature gradients simultaneously to produce very favorable conditions for crystal growth and avalanche formation.
More thoughts on convergent crustal plateau formation and mantle dynamics with regard to Tibet
Authors: Lenardic A. , Kaula W. M.
Published: February 2004

Evidence suggests that some 10 m.y. ago a large portion of thickened mantle lithosphere below the Tibetan Plateau was rapidly removed. Removal appears to have been abrupt and delayed from the time of substantial thickening. Fluid dynamical models of convective thermal boundary layer instability have had difficulties explaining abrupt lithospheric removal long after initial thickening. Such models have implied that either removal should be synchronous with thickening or should not occur at all, due to boundary layer stabilization resulting from high lithospheric viscosity. We present simple thermal/chemical boundary layer convection models that suggest a compromise between previous end‐member results. The models allow for delayed instability of a thickened thermal boundary layer under geologically reasonable parameter conditions. Instability delay relies on high viscosity of thickened mantle lithosphere stabilizing it against free convective removal and on lateral extent of thickened crust exceeding that of mantle lithosphere. The latter condition leads to a peripheral region of thickened crust and relatively unthickened mantle lithosphere and an interior plateau region of both thickened crust and mantle lithosphere. If the thermal resistance and/or the rate of internal heat production of crust is greater than that of mantle, then mantle lithosphere at the peripheral region can be thermally eroded to the point that the base of the crust comes into contact with bulk mantle. This creates a peripheral weak zone that allows thickened mantle lithosphere of the interior plateau region to become mechanically detached and sink into bulk mantle. The instability is akin to delamination, as originally defined, and its delay time is roughly the thermal diffusion time across the thinnest portion of mantle lithosphere.
Crustal volume balance during the India‐Eurasia collision and altitude of the Tibetan Plateau: A working hypothesis
Authors: Westaway Rob
Published: February 2004

A working hypothesis is developed for the two stages of evolution of the Tibetan plateau during the continental collision between India and Eurasia. The first phase was dominated by crustal shortening and thickening in what is now Tibet, while Indochina was expelled southeastward, away from Eurasia. At the end of this phase at 17 Ma, the estimated Tibetan crustal thickness was ∼51 km. The second phase has been dominated by thickening of Tibet by the influx of crustal material from India, with no associated crustal shortening. The crustal thickening that is inferred to have occurred by this process is only feasible because leakage of crust from beneath the plateau is restricted by neighboring Archean cratons that block lower crustal flow. This leakage, to the east and south from Tibet to beneath Yunnan, is estimated as ∼20–30% of the influx at present but was much smaller at the start of this second phase of deformation. It is suggested that at ∼11 Ma the force exerted against the Tibetan upper crust by this flowing material first exceeded the threshold that this brittle layer could resist and Tibet began to extend eastward. Any satisfactory working hypothesis for the Tibetan plateau must explain its dimensions, its ∼17 Ma ending of shortening and ∼11 Ma start of extension and volcanism, the eastward slip sense on its active normal and strike‐slip faults, and the ratios of their displacements to present‐day slip rates, which imply that their slip rates have gradually increased. This working hypothesis accounts for these features and predicts a smooth and continuing increase in altitude.
Paleoseismology and seismic hazards evaluations in extensional volcanic terrains
Authors: Smith Richard P. , Jackson Suzette M. , Hackett William R.
Published: February 2004

Extensional structures in volcanic terrains are the surface expression of shallow dike intrusion and can be misinterpreted as structures associated with major tectonic faults. Dike‐induced structures can be distinguished from their tectonic counterparts by their association with cogenetic volcanic rocks and by several geometric relationships. Tensile fissures with little or no vertical displacement, fissure swarms, flexural monoclines, and normal faults are commonly symmetrically distributed about a central eruptive fissure, sometimes forming a graben above shallow dikes. The structures typically occur within broad zones, not narrow belts near a main fault zone, reflecting their origin by repeated dike injection. Colluvial wedges containing records of single large earthquakes generally do not form; instead, fault scarps with several‐meter vertical displacements in volcanic bedrock may reflect the cumulative effects of many decimeter‐scale displacement events from several dike injection episodes. The mechanism of dike intrusion and the nature of cointrusive seismicity have important implications for determination of the maximum magnitude and recurrence of earthquakes in extensional volcanic terrains. Observational seismicity from volcanic rift zones worldwide suggests the maximum magnitudes of dike‐induced earthquakes are 3.8 ± 0.8. Earthquakes are generally small to moderate because downdip extents of faults and fissures are controlled by the depth to the top of the associated dike (usually <5 km), permitting only small rupture areas. Also, rupture and displacement on faults and fissures migrate incrementally at about the velocity of propagating dikes (0.5 m/s) as dike dilation stresses the zone above and ahead of the dike. Earthquake recurrence is tied to recurrence of volcanic cycles based on the geochronology of the associated volcanic materials. Based on these concepts, innovative approaches have led to estimates of the maximum magnitude and recurrence for magma‐induced seismicity in eastern Snake River Plain (ESRP) volcanic rift zones. The most conservative approach uses surface length, fault width (downdip extent), and rupture areas of normal faults and fissures produced by dike injection to estimate a maximumM5.5 earthquake for an episode of rift zone volcanism. Chronology of volcanic rocks suggests annual probabilities of 10−4to 10−5for volcanic rift zones near Idaho National Engineering Laboratory facilities. Probabilistic assessments show that ground motion hazard due to volcanic rift zone seismicity is lower than the hazard from other earthquake sources. This is because of the relatively low magnitudes and long recurrence intervals of cointrusive earthquakes in volcanic rift zones.
Petrogenesis of alkaline basalts from Socorro Island, Mexico: Trace element evidence for contamination of ocean island basalt in the shallow ocean crust
Authors: Bohrson Wendy A. , Reid Mary R.
Published: February 2004

Postcaldera mafic volcanism on Socorro Island, Mexico, an island located in the eastern Pacific Ocean on a segment of a mid‐ocean ridge spreading center abandoned at ∼3.5 Ma, dominantly comprises alkalic basalt, hawaiite, and mugearite with subordinate mildly alkalic‐transitional basalt. Most major element data are consistent with differentiation of alkalic basalt to hawaiite and mugearite by up to 50% fractionation of plagioclase+clinopyroxene+olivine±Fe−Ti oxides. High‐field strength element abundances are consistent with this interpretation, and the narrow range in Zr/Nb suggests that parental magmas formed by similar degrees of partial melting of a relatively homogeneous mantle source. Sr and Nd isotopic ratios exhibit relatively narrow ranges (0.7031–0.7032, 0.5128–0.5130, respectively). Despite this, a subset of the alkalic basalts has negative Ce anomalies and abundances of P2O5, Ba, Y, and some rare earth elements in excess of those predicted by fractional crystallization models; these chemical features require open‐system processes. The observed heterogeneities are perhaps best explained by assimilation of ocean crustal components by basalt in a shallow magma reservoir. Regional mantle heterogeneity is evident from elemental and isotopic data for mildly alkalic‐transitional basalt from Socorro Island, mugearite from San Benedicto Island, and submarine basalts from near Socorro Island. Evidence presented here indicates that compositional variations in ocean island basalts can be introduced by ocean crustal contamination and underscores the importance of considering this source of chemical heterogeneity in basalts before characterizing associated mantle.
Mantle plumes and flood basalts
Authors: White R. S. , McKenzie D.
Published: February 2004

We discuss the geological, geophysical, and petrological observations that constrain the nature of mantle convection in plumes, and show how theoretical models of mantle plumes have developed over the past three decades. The large volumes of lava emplaced in geologically short periods as flood basalts are generated mainly by decompression melting of abnormally hot mantle brought to the base of the lithosphere by plumes. We present new results from the application of McKenzieand O'Nions' (1991) rare earth element inversion scheme to the geochemistry of flood basalts to infer the amount of mantle melting and the depth interval over which it occurred. Our survey covers flood basalts from the Archaean to the Tertiary. The abundance of geochemical data from the Siberian Traps (250 Ma) and from the Keweenawan (1095 Ma) enables us to infer the temporal evolution of mantle melting in these two flood basalt provinces. We find that three quarters of the samples from flood basalts can be modeled by single‐stage melting of asthenospheric mantle. The remainder require enrichment by small amounts of metasomatic melts emplaced in the lithospheric mantle by an earlier phase of minor (0.3%) melting at depths where garnet is stable. The mantle melting responsible for flood basalts starts at depths of 110 km or more beneath the surface, and is consistent with enhanced mantle potential temperatures of 1450–1550°C. Melting continues to depths of 70–30 km. At least some lithospheric thinning is required to explain both the geochemistry of the melts and the high rate of generation of flood basalts.
A comparison of historical and paleoseismicity in a newly formed fault zone and a mature fault zone, North Canterbury, New Zealand
Authors: Cowan Hugh , Nicol Andrew , Tonkin Phillip
Published: February 2004

The timing of large Holocene prehistoric earthquakes is determined by dated surface ruptures and landslides at the edge of the Australia‐Pacific plate boundary zone in North Canterbury, New Zealand. Collectively, these data indicate two large (M> 7) earthquakes during the last circa 2500 years, within a newly formed zone of hybrid strike‐slip and thrust faulting herein described as the Porter's Pass‐to‐Amberley Fault Zone (PPAFZ). Two earlier events during the Holocene are also recognized, but the data prior to 2500 years are presumed to be incomplete. A return period of 1300–2000 years between large earthquakes in the PPAFZ is consistent with a late Holocene slip rate of 3–4 mm/yr if each displacement is in the range 4–8 m. Historical seismicity in the PPAFZ is characterized by frequent small and moderate magnitude earthquakes and a seismicity rate that is identical to a region surrounding the structurally mature Hope fault of the Marlborough Fault System farther north. This is despite an order‐of‐magnitude difference in slip rate between the respective fault zones and considerable differences in the recurrence rate of large earthquakes. The magnitude‐frequency distribution in the Hope fault region is in accord with the characteristic earthquake model, whereas the rate of large earthquakes in the PPAFZ is approximated (but overpredicted) by the Gutenberg‐Richter model. The comparison of these two fault zones demonstrates the importance of the structural maturity of the fault zone in relation to seismicity rates inferred from recent, historical, and paleoseismic data.
In situ X ray observation of high‐pressure phase transitions of MgSiO3 and thermal expansion of MgSiO3 perovskite at 25 GPa by double‐stage multianvil system
Authors: Kato Takumi , Ohtani Eiji , Morishima Hideaki , Yamazaki Daisuke , Suzuki Akio , Suto Minoru , Kubo Tomoaki , Kikegawa Takumi , Shimomura Osamu
Published: February 2004

A double‐stage multianvil system was developed to conduct in situ X ray diffraction study at high pressures. The system generates pressures and temperature above 25 GPa and 1200°C and has been used in conjunction with intense synchrotron radiation to study the phase transitions of MgSiO3 and the thermal expansion of MgSiO3 perovskite. At pressures between 20 and 30 GPa, we observed a series of phase transitions from spinel+stishovite through ilmenite to perovskite. Spinel+stishovite transforms into ilmenite at 20.0±1.0 GPa, and ilmenite transforms into perovskite at 24.0±0.5 GPa at 800°C. The phase boundary between ilmenite and perovskite is estimated to be P (GPa) = (24.0±0.5) ‐ (0.0025±0.0025)(T‐800)(°C) by combining the present results and previous constraints on the slope of the phase boundary from the thermodynamic properties. Measured unit cell parameters of MgSiOs perovskite indicate that the average volumetric thermal expansion coefficient of MgSiOs perovskite over 25°–1200°C is 2.0±0.4×10−5 K−1 at 25 GPa. These results provide the phase stability relation of MgSiO3 and the thermal equation of state of MgSiO3 perovskite at pressures corresponding to the boundary between the upper and lower mantle. However, uncertainties involved in extrapolating to the actual mantle temperatures do not allow evaluation of the chemical structure near the 670 km discontinuity in the mantle.
Free thermohaline convection beneath allochthonous salt sheets: An agent for salt dissolution and fluid flow in Gulf Coast sediments
Authors: Sarkar Alok , Nunn Jeffrey A. , Hanor Jeffrey S.
Published: February 2004

Basinward migration of Jurassic salt in the U.S. Gulf of Mexico has resulted in the emplacement of large allochthonous salt sheets into shallow Miocene to Holocene sediments. Although comparatively little direct information is available on the environment below these salt bodies, it is reasonable to suppose that the formation of dense brines by dissolution of the base of these sheets may induce free thermohaline pore fluid convection within the sediments below. We derived equations which make it possible to quantitatively estimate rates of dissolution of these subsurface salt structures. From these calculations and by geologically realistic numerical simulations it can be shown that free convection beneath allochthonous salt sheets has the potential for being a significant mechanism for both salt dissolution and mass transport, even if the underlying sediments have permeabilities as low as 10−17 m2 (0.01 mD). The calculated maximum Darcy fluxes and rates of salt dissolution rapidly increase with sediment permeability. When the vertical permeability of the underlying sediment is 10−17 m2 (0.01 mD), salt is dissolved from the base of the sheet at an average rate of 3–5 m m.y.−1 The corresponding fluid velocities are such that over a 10 m.y. period the integrated fluid flux in the underlying sediments would be ∼104 m3 m−2. By comparison, integrated fluid flux for compactive expulsion is <103 m3 m−2. Thus, for the offshore sediments of the Gulf of Mexico, thermohaline convection beneath an allochthonous salt sheet is a significant driving mechanism for fluid flow with potentially important implications for heat and mass transport, diagenesis, and salt tectonics.
Low‐angle normal faults and seismicity: A review
Authors: Wernicke Brian
Published: February 2004

Although large, low‐angle normal faults in the continental crust are widely recognized, doubts persist that they either initiate or slip at shallow dips (<30°), because (1) global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30° and (2) Andersonian fault mechanics predict that normal faults dipping less than 30° cannot slip. Geological reconstructions, thermochronology, paleomagnetic studies, and seismic reflection profiles, mainly published in the last 5 years, reinforce the view that active low‐angle normal faulting in the brittle crust is widespread, underscoring the paradox of the seismicity data. For dip‐slip faults large enough to break the entire brittle layer during earthquakes (Mw ∼ 6.5), consideration of their surface area and efficiency in accommodating extension as a function of dip θ suggests average recurrrence intervals of earthquakes R' ∝ tan θ, assuming stress drop, rigidity modulus, and thickness of the seismogenic layer do not vary systematically with dip. If the global distribution of fault dip, normalized to total fault length, is uniform, the global recurrence of earthquakes as a function of dip is shown to be R ∝ tan θ sin θ. This relationship predicts that the frequency of earthquakes with nodal planes dipping between 30° and 60° will exceed those with planes shallower than 30° by a factor of 10, in good agreement with continental seismicity, assuming major normal faults dipping more than 60° are relatively uncommon. Revision of Andersonian fault mechanics to include rotation of the stress axes with depth, perhaps as a result of deep crustal shear against the brittle layer, would explain both the common occurrence of low‐angle faults and the lack of large faults dipping more than 60°. If correct, this resolution of the paradox may indicate significant seismic hazard from large, low‐angle normal faults.
Shock temperature and melting in iron sulfides at core pressures
Authors: Anderson William W. , Ahrens Thomas J.
Published: February 2004

The temperatures of shock‐compressed FeS and FeS2 in the pressure ranges 125–170 GPa and 100–244 GPa, respectively, are reported and used to constrain the melting curves and thermodynamic properties to core pressures. A fit of the Lindemann law parameters corresponding to the usual functional form for the lattice Grüneisen parameter gives γL = 1.17 ± 0.13 and nL = 0.5 ± 0.5 for the high‐pressure phase of FeS at ρ = 5340 kg/m3 and γL = 2.18 ± 0.32 and nL = 1.6 ± 0.7 for FeS2 at ρ = 5011 kg/m3. The entropies of fusion are ∼203 J kg−1 K−1 for FeS at 120 GPa and ∼180 J kg−1 K−1 for FeS2 at 220 GPa. We find that the melting temperature of FeS is 3240±200 K, 4210 ± 700 K, and 4310 ± 750 K at 136 GPa, 330 GPa, and 360 GPa, respectively. For FeS2, the melting temperatures are 3990 ± 300 K, 5310 ± 700 K, and 5440 ± 750 K, respectively, for the same pressures. The electronic specific heat for FeS is given by Ce = β0 (ρ0/ρ)γe with β0 = 0.25 ± 0.10 J kg−1 K−2 and γe = 1.34 for ρ0 = 5340 kg/m3 for the high‐pressure solid phase and β0 ≈ 0.05 J kg−1 K−2 and γe = 1.34 for ρ0 = 5150 kg/m3 for the liquid phase. For FeS2, there is no detectable electronic contribution, and the lattice specific heat is only 67% of the Dulong‐Petit limit, possibly implying tight S‐S binding in S2 units. A reexamination of all shock wave melting data for Fe indicates these approximately agree, but they do not resolve the disagreement between the extrapolated static diamond anvil cell data sets. Fe should melt at ∼6600 K at 243 GPa and 6900 ± 750 K at 330 GPa (the pressure of the inner core‐outer core boundary). Because the FeS melting curve falls well below that of FeS2, FeS may eventually undergo peritectic melting at high pressures, while FeS2 melts congruently.
Continuous GPS observations across the Hayward Fault, California, 1991–1994
Authors: King N. E. , Svarc J. L. , Fogleman E. B. , Gross W. K. , Clark K. W. , Hamilton G. D. , Stiffler C. H. , Sutton J. M.
Published: February 2004

In September 1991, the U. S. Geological Survey began continuous operation of two permanent Global Positioning System (GPS) sites near the Hayward fault. We use two and one half years of data from an 8‐km baseline to investigate GPS processing strategies, errors in the time and frequency domains, and the uncertainties of rates of change calculated from such data. Experiments with session lengths show that at least 6 hours of data should be used to obtain a precision of 2 to 4 mm. Experiments with broadcast and improved orbits show that the broadcast orbit is sufficient for this short baseline. Single‐frequency solutions have larger rms scatter; for scenarios made mostly in daylight, ionospheric delay systematically shortens the baseline length by 2.4 parts per million for L1 and 4.0 parts per million for L2. For the dual‐frequency results, the rms scatter about the best‐fitting straight line is 2.1 mm for baseline length, 2.2 mm for north, 2.9 mm for east, and 11.2 mm for vertical. For Winton relative to Chabot, the rates of change are −0.6±0.1 mm/yr in length, 2.8±0.1 mm/yr in north, −8.1±0.2 mm/yr in east, and −8.1±0.6 mm/yr in vertical. The baseline rate of change is consistent with right‐lateral shear across the Hayward fault but is not consistent with the −3.5 mm/yr predicted by the model of Lienkaemper et al. (1991). The large westward and downward motion of Winton relative to Chabot may be due to monument instability. Power spectra appear white at high frequencies; the estimated standard deviations for periods shorter than 5 days are 1.2 mm for length and north, 1.9 mm for east, and 5.5 mm for vertical. Power spectral density increases only slightly as frequency decreases from 0.2 to 0.03 cycle per day, and there is no distinct corner. In particular, no characteristic 1/ƒ2 signature of random walk monument noise emerges from the white noise. Estimated autocorrelations for length, north, east, and vertical fall to about 0.1 for lag times shorter than 10 days and fluctuate about zero for lag times longer than about 25 days. Because the time series is short, it is possible that random walk monument noise exists but is undetectable in the estimated power spectral density and autocorrelation functions. We use the estimated full covariance matrix to calculate the standard deviations of the baseline rate of change for various sampling schemes. The theoretical standard deviation of dL/dt determined from 1 year of daily observations is 0.28 times that determined from 1 year of annual observations. We can obtain a similar uncertainty from 2 years of measurements made every 30 days and better results from 5 years of annual measurements. However, daily measurements allow the detection and correction of offsets that sometimes occur with equipment or firmware changes.
Late Quaternary coseismic uplift events on the Huon Peninsula, Papua New Guinea, deduced from coral terrace data
Authors: Ota Yoko , Chappell John
Published: February 2004

Up to six regressive terraces occur on the Holocene raised reef tract and up to 15 occur on late Pleistocene raised reef tracts along 40 km of coastline at Huon Peninsula, Papua New Guinea. We suggest that the regressive terraces represent repeated episodic uplift caused by great earthquakes. Ages of Holocene coseismic uplift events are established by radiocarbon dating; the late Pleistocene events are bracketed by U series ages of the raised reef tracts on which they occur. The mean recurrence interval of great earthquakes that caused the uplift events is 970 to 1165 years in the Holocene and probably the same in the late Pleistocene; the interval ranged from about 200 to 1900 years. The uplift rate increases parallel to the coast from northwest to southeast, and the amplitude of coseismic uplifts generally increases similarly, although some events produced uplift with little shore‐parallel tilting. The mean amplitude of coseismic uplifts throughout the study area is ∼3 m in for both Holocene and late Pleistocene sequences. Large, late Quaternary landslides are numerous, and some probably were triggered by the great earthquakes that caused coseismic uplift. There appears to be no continuum between historical large earthquakes at Huon Peninsula of magnitudes >7 that produced no or only minor uplift, and the great earthquakes represented by meter‐scale coseismic uplifts and very large landslides. Two tectonic subregions are recognized, which were uplifted together by some Holocene events but not by others. There is no surface trace of Holocene faulting between the subregions, and a buried fault is thought to separate them.
Thermodynamic properties of α‐quartz, coesite, and stishovite and equilibrium phase relations at high pressures and high temperatures
Authors: Akaogi Masaki , Yusa Hitoshi , Shiraishi Kimiko , Suzuki Toshihiro
Published: February 2004

Isobaric heat capacities of α ‐quartz, coesite, and stishovite were measured by differential scanning calorimetry at 183–703 K. The heat capacity data of coesite represent a significant revision of the previous data. The heat capacities and entropies were also calculated using Kieffer's model. By high‐temperature solution calorimetry, transition enthalpy for the β‐quartz‐coesite at 979 K was measured to be 1.27±0.39 kJ/mol. The enthalpies for the α‐quartz‐coesite and coesite‐stishovite transitions at 298 K were measured to be 3.40±0.56 and 33.62±1.01 kJ/mol, respectively, by differential drop‐solution calorimetry. The enthalpy of the coesite‐stishovite transition obtained in this study is about 10–15 kJ/mol smaller than those in the previous studies. Phase relations in SiO2 at high pressures and high temperatures were calculated using these new thermodynamic data. The calculated boundary for the α‐quartz‐coesite transition is consistent with those determined experimentally. For the coesite‐stishovite transition, the calculated boundary having a slope of 2.5±0.3 MPa/K disagrees with that by Yagi and Akimoto but is generally consistent with that by Zhang et al.'s new in situ X ray diffraction study.
Thermal infrared directional emissivity of powdered quartz
Authors: Wald Andrew E. , Salisbury John W.
Published: February 2004

Thermal infrared directional emissivity of quartz was measured for several particle size ranges and packing fractions. The samples measured were (1) a polished quartz slab; (2) a 75–250 μm powdered quartz sample, cleansed of clinging fines; (3) a 0–75 μm powdered quartz sample, sifted into a fairy castle structure; and (4) the same sample compressed to minimize porosity. The spectra of the particulate samples showed a strong dependence on exitance angle, particle size and packing fraction. In addition, thermal gradient effects significantly affected the measured emissivity of the fine, sifted sample. The measured directional emissivity was modeled by first using Mie theory to calculate single‐particle scattering properties of a quartz spheres of appropriate size at a single wavelength and radiative transfer theory to calculate the flux reflected from an optically thick, plane‐parallel “atmosphere” composed of particles with these scattering properties. For the 75–250 μm sample, close packing of individual particles was accounted for by subtracting the diffraction contribution to the scattering cross section. These calculations are repeated wavelength‐by‐wavelength to determine the spectral directional hemispherical reflectance of the quartz sample. Kirchhoff s law was then used to obtain spectral directional emissivity. This model, which uses optical constants derived from widely used oscillator parameters for quartz, reproduces the directional emissivity spectrum of the large powdered quartz to better than 10%. However, model calculations for 0–75 μm particle size quartz were less successful. This could be due to the several approximations used in the model, or to a possible error in the oscillator parameters. More successful calculations based on different optical constants suggest that the widely used oscillator parameters may well be wrong.
Kinetically limited isotope exchange in a shallow level normal fault, Mineral Mountains, Utah
Authors: Barnett D. E. , Bowman J. R. , Bromley C. , Cady C.
Published: February 2004

We have conducted a stable isotope and geochemical study of the Corral Canyon fault zone, a shallow‐level, low‐angle normal fault on the west flank of the Mineral Mountains in southwest Utah. Our stable isotope and geochemical data document the infiltration of local meteoric water during brittle deformation and hydrothermal alteration of the shear zone during Tertiary Basin and Range extensional faulting. Within the fault zone, 18O depletion correlates with progressive hydrothermal alteration and cataclasis, demonstrating that the processes of fluid infiltration, hydrothermal alteration, fracturing, comminution of grains, and 18O exchange took place concurrently. Interpretation of our data using a model of combined fluid flow and kinetically limited isotope exchange leads us to conclude that the exposed fault zone was infiltrated by isotopically unevolved fluids from the wall rock adjacent to the fault zone and that an important component of fluid flow in this fault was between the wall rock and the fault zone rather than strictly within the fault plane. We propose that the rate of isotopic alteration in shallow level fault zones is enhanced by comminution and recrystallization of grains during deformation, so that fluids in the wall rock, because of the relatively coarser grain size, remain relatively unevolved until they infiltrate the fault zone where they rapidly become isotopically evolved due to exchange with cataclasized grains. We present a general model for calculating the duration of isotopic alteration in flow systems characterized by kinetically‐limited isotope exchange. Calculated exchange durations for the Corral Canyon fault zone are short, on the order of thousands to a few hundreds of thousands of years, compared to the likely duration of deformation of millions of years. The geologically short duration of isotope exchange leads us to suggest that the fluid flow responsible for the isotopic alteration was episodic, of short duration, and possibly related to discrete seismic events. This result corroborates diverse lines of evidence that support a model of alternating fluid flow and permeability sealing by hydrothermal precipitation within fault zones.
Stoneley and flexural modes in pressurized boreholes
Authors: Sinha Bikash K. , Kostek Sergio , Norris Andrew N.
Published: February 2004

The propagation of Stoneley and flexural waves in a fluid‐filled borehole is adequately described by the linear equations of elasticity. However, when the borehole fluid is pressurized either due to the hydrostatic head at a given depth or with the aid of packers at the wellhead, both the fluid and the surrounding formation are subjected to biasing stresses. Under this situation, wave propagation along the borehole is described by the equations of motion for small dynamic fields superposed on a bias. The resulting formulation allows us to study the influence of a change in the fluid pressure on the Stoneley and flexural mode dispersion curves. Since the biasing stresses in the surrounding formation exhibit a radial decay away from the borehole, it is expedient to employ a perturbation technique to calculate changes in the borehole Stoneley and flexural wave dispersion curves as a function of hydrostatic pressure change in the fluid. A key advantage of this perturbation technique is that it separates contributions of the acoustoelastic effect due to the borehole fluid and that due to the formation. Insofar as the fluid nonlinear properties at a given pressure and temperature are known, the model provides a procedure for estimating the acoustoelastic coefficient of the formation for the borehole Stoneley and flexural wave velocities for a given change in the fluid pressure. The formation acoustoelastic coefficient can be expressed as a fractional change in the acoustic wave velocity caused by a unit change in the borehole pressure. Computational results show that acoustoelastic coefficients for the Stoneley and flexural waves are larger for formations with higher degree of nonlinearity which is typically associated with poorly consolidated rocks.