bluuebunny/crossref_metadata_2025 huggingface dataset:
Introduction to Special Section: Hawaii Scientific Drilling Project
Authors: Stolper Edward M. , DePaolo Donald J. , Thomas Donald M.
Published: February 2004

Intraplate or “hot spot” volcanic island chains, exemplified by Hawaii, play an important role in plate tectonic theory as reference points for absolute plate motions, but the origin of these volcanoes is not explained by the plate tectonic paradigm [Engebretson et al., 1985; Molnar and Stock, 1987; Morgan, 1971, 1981, 1983; Wilson, 1963]. The most widely held view is that these chains of volcanoes form from magma generated by decompression melting of localized, buoyant upwellings in the mantle [Ribe and Christensen, 1994; Richards et al., 1988; Sleep, 1990; Watson and McKenzie, 1991]. These upwellings, or “plumes,” are believed to originate at boundary layers in the mantle (e.g., at the core‐mantle boundary or near the boundary at ∼670 km between the upper and lower mantle), and the cause of the buoyancy may be both compositional and thermal [Campbell and Griffiths, 1990; Griffiths, 1986; Richards et al., 1988; Watson and McKenzie, 1991]. Mantle plumes are responsible for about 10% of the Earth's heat loss and constitute an important mechanism for cycling mass from the deep mantle to the Earth's surface.
Depth‐dependent change in the focal mechanism of shallow earthquakes: Implications for brittle‐plastic transition in a seismogenic region
Authors: Iio Yoshihisa
Published: February 2004

An analysis is conducted of the focal mechanism solutions of 2150 shallow earthquakes that have occurred in the central part in the Kinki district, Japan. It is found that in regions shallower than the characteristic depth (the depth at which the frequency‐depth distribution of earthquakes is maximum), T axis plunges are widely distributed in the horizontal and vertical directions. Furthermore, the distribution of the plunges does not change with depth in the region. In regions deeper than the characteristic depth, the plunges are concentrated horizontally. That is, both strike‐slip and reverse fault events are found in regions shallower than the characteristic depth, while strike‐slip events dominate in the deeper region. It is inferred from the above results that the minimum horizontal stress has almost the same magnitude as the vertical stress in regions shallower than the characteristic depth, while in the deeper regions the minimum horizontal stress is less than the vertical stress. This conclusion implies that the minimum horizontal stress increases proportional to the depth in regions shallower than the characteristic depth but does not increase in proportion to depth in the deeper regions. It was also concluded that the brittle‐semibrittle transition could possibly occur at the characteristic depth.
Role of the Kerguelen Plume in generating the eastern Indian Ocean seafloor
Authors: Weis Dominique , Frey Frederick A.
Published: February 2004

Mid‐ocean ridge basalts (MORB) in the Indian Ocean have Sr‐Nd‐Pb isotopic characteristics that distinguish them from seafloor basalts in the Atlantic and Pacific Oceans. These differences have important implications for mantle dynamics. We discuss the isotopic variation with eruption age of seafloor basalts recovered by deep sea drilling at 10 sites in the eastern Indian Ocean ranging in age from Eocene to Late Jurassic. Except for alkalic basalts recovered from near Christmas Island in the northeast Indian Ocean, the basement lavas are tholeiitic basalts that are characterized by a wide range in incompatible element abundance ratios, such as La/Yb and Zr/Nb. Most of the tholeiitic basalts from seven sites are geochemically similar to recent Indian Ocean MORB, but the alkalic basalts and tholeiitic lavas from two other sites have isotopic and incompatible element abundance ratios similar to lavas associated with the Kerguelen Plume. Two of these three sites, however, are not close to the track of this plume. The Dupal isotopic signature (relatively high 87Sr/86Sr and high 208Pb/204Pb at a given 206Pb/204Pb) is characteristic of lavas that have been attributed to the Kerguelen Plume, i.e., the Kerguelen Archipelago, Ninetyeast Ridge, and Kerguelen Plateau. Among eastern Indian Ocean seafloor basalts, a Dupal component is apparent in basement lavas from six of the seven drill sites in the eastern Indian Ocean that range in inferred age from ∼57 to 125 Ma. The oldest (∼155 Ma) seafloor lavas recovered from the Indian Ocean, derived from a spreading center in the Argo Abyssal Plain near northwest Australia, have high 143Nd/144Nd and low 87/86Sr similar to the most depleted recent Indian MORB. Because the oldest volcanism on the Kerguelen Plateau (∼118 Ma) is the first evidence of the activity of the Kerguelen Plume, this plume is inferred to be the source of Dupal isotopic characteristics in Indian Ocean MORBs. Some recent Indian Ocean MORB are also distinctive because many have relatively low 206Pb/204Pb (<17.4). Some of the oldest (110 to 155 Ma) seafloor lavas in the eastern Indian Ocean also have relatively low 206Pb/204Pb ratios. This low 206Pb/204Pb signature predates volcanism associated with the Kerguelen Plume and may reflect a significant role for continental lithosphere as a long‐term source component for Indian Ocean MORB.
Mantle plumes, flood basalts, and thermal models for melt generation beneath continents: Assessment of a conductive heating model and application to the Paraná
Authors: Turner Simon , Hawkesworth Chris , Gallagher Kerry , Stewart Kathy , Peate David , Mantovani Marta
Published: February 2004

Growing evidence suggests that there is more than one type of continental flood basalt (CFB). Many CFB, such as the Deccan, were probably derived by decompression melting of asthenospheric peridotite in a mantle plume resulting in high eruption rates (∼1 km3 yr−1) and geochemical signatures of uncontaminated basalt which are similar to ocean island basalts. However, several geochemical studies have concluded that other CFB were derived from the subcontinental lithospheric mantle. This requires that melting occurs via heating of lithospheric peridotite above a mantle plume instead of melting within the plume itself. The feasibility of this model is tested with a simple one‐dimensional, time‐dependent thermal model. The results show that 1–2 km thick CFB may be derived from the lithospheric mantie without melting occurring within the underlying mantle plume if (1) the mantle potential temperature of the plume is between 1380 and 1580°C; (2) the lithospheric mantle is composed of volatile‐enriched peridotite; (3) the overlying lithosphere is >100 km thick; (4) eruption occurs over 10–15 Myr; and (5) melting occurs over an area similar to the surface distribution of basalt. Small volumes of alkalic basalt may precede the eruption of tholeiites and if higher plume temperatures prevail, or extension/thermal erosion leads to lithospheric thicknesses ≤100 km, melts from the plume will rapidly dominate those from the lithosphere and eruption rates will increase (magmatic underplating may reduce the strength of the lithosphere sufficiently to initiate or focus rifting if the lithosphere is already under tension). These predictions can be used to discriminate between different CFB and are illustrated by application to the Paraná CFB using published data along with new geochemical data from a borehole through the thickest section of basalt.
Isotopic and paleomagnetic constraints on the Mesozoic tectonic evolution of south China
Authors: Gilder Stuart A. , Gill James , Coe Robert S. , Zhao Xixi , Liu Zhongwei , Wang Genxian , Yuan Kuirong , Liu Wenlong , Kuang Guodun , Wu Haoruo
Published: February 2004

In order to better constrain the paleogeographic evolution of south China we measured Sm‐Nd and Rb‐Sr isotopic compositions for 23 Mesozoic granites that crop out throughout the area. Tightly grouped neodymium depleted mantle model ages (1.4 ± 0.3 Ga) suggest the region is underlain by relatively homogeneous Proterozoic crust and fail to define crustal provinces. Neither the isotopic nor geologic data suggest that a Mesozoic suture exists. However, granites possessing anomalously high Sm (>8 ppm) and Nd (>45 ppm) concentrations, relatively high initial epsilon neodymium (−4 to −8), and high but variable initial87Sr/86Sr (0.759 to 0.713) form a northeast trending zone that coincides with two prominent Mesozoic basins. Southeast of the zone lie the majority of Mesozoic intrusives and Upper Triassic to Lower Cretaceous extensional basins found in south China. Mesozoic paleomagnetic poles are well clustered northwest of the zone. Pre‐Cretaceous poles southeast of it are discordant with respect to those from the northwest. The only recognized tectonostratigraphic terrane in south China lies southeast of the zone. The terrane is bordered by a northeast trending sinistral fault that was active in the Mesozoic. Other faults in south China have similar attitudes, ages, and sense of shear. Together, the observations suggest that the Mesozoic tectonic regime in south China consisted of strike‐slip activity plus concomitant rifting as terranes or fragments of similar crust were transported north along sinistral faults. The zone, defined by the granites enriched in Nd and Sm, demarcates displaced terranes to the southeast from relatively stable land to the northwest.
Distribution of heat‐producing elements in the upper and middle crust of southern and west central Arizona: Evidence from the core complexes
Authors: Ketcham Richard A.
Published: February 2004

The amount and distribution of crustal heat production are a central component of all estimates of continental thermal structure, yet they remain in most cases an assumption rather than a constraint. This study utilizes the Catalina and Harquahala metamorphic core complexes as large exposures of the recent (<30 m.y. ago) upper and middle crust of the Arizona Basin and Range to gather primary heat production data. Samples were measured for their heat production, and their positions in the predetachment crust were reconstructed by restoring all major deformation associated with core complex formation, including subsequent block faulting. The restored part of the Catalina core complex spans a depth range from 3 to 13 ± 1 km, and the Harquahala core complex encompasses a depth range from the surface to 14 ± 3 km. Mylonitization probably has little systematic effect on the distribution of heat production in these core complexes. The depth distributions of heat production obtained in this study do not follow a smooth or systematic function but rather reflect a primary control exerted by local structural or magmatic history. Variogram analysis indicates that the scale of autocorrelation of heat production is variable but in most cases under 10 km. In each core complex, the amount of heat production observed plus that inferred to reside in the deeper crust is approximately 30 ± 5 μW/m3, roughly 50% higher than previous estimates. The difference can cause overestimates of 75–300°C in deep crustal temperatures if surface heat flow is assumed to be known. The use of a temperature‐ and pressure‐dependent function for thermal conductivity instead of a constant value causes an upward revision in estimated average geothermal gradients of approximately 2.5–4°C/km.
Wide‐angle, three‐component seismic reflection profiling of the crust along the East Coast Gravity High, southern Appalachians, using quarry blasts
Authors: Hawman Robert B.
Published: February 2004

Inversion of refraction/wide‐angle reflection travel times for two‐dimensional velocity structure along the axis of the East Coast Gravity High in the Carolina Terrane of Georgia indicates much higher crustal P wave velocities than previously modeled. Thirty‐nine timed quarry blasts recorded with a 20‐element array of three‐component recorders at 330‐m spacings provide reversed and overlapping coverage along the profile which crosses Consortium for Continental Reflection Profiling Georgia line 5. Ray paths are concentrated in high‐grade rocks of the Charlotte and Kiokee Belts. Shot gathers show strong first arrivals, Moho reflections, and numerous reflections from within the crust. Shear wave arrivals are particularly strong, with prominent direct arrivals (Sg) and reflections from the Moho (SmS) observed for all quarries. Three alternative models show average crustal P wave velocities of 6.5–6.6 km/s and Moho depths between 37 and 39 km. P wave velocities increase from 6.0–6.2 km/s at 1 km depth to 6.7–6.8 km/s at a first‐order discontinuity at a depth of 24–26 km. Alternative models for the lower crust feature both a slight increase in velocity and velocity reversals at the 25‐km discontinuity, with velocities steadily increasing to between 6.9 and 7.5 km/s at the base of the crust. Low average Vp/Vs ratios for the crust (1.70–1.75) estimated from travel time ratios for SmS and PmP reflections suggest that the high compressional‐wave velocities derived by inversion are associated with rocks of high metamorphic grade and granitic to intermediate, not mafic, average composition over most of the crust. The low average ratios can also be explained in part by the presence of a layer of low P wave velocity in the midcrust, possibly associated with a package of quartz‐rich platform and rift‐basin sediments overthrust late in the Alleghanian orogeny by the Carolina Terrane allochthon, as suggested by earlier workers. Crustal models incorporating the new estimate for crustal thickness beneath the Carolina Terrane show that the long‐wavelength component of the gravity gradient in Georgia can be explained in terms of crustal thickness variations; aeromagnetic data indicate that density variations responsible for local steepening of the gradient are confined largely to the lower half of the crust.
Morphology and physical properties of Fontainebleau sandstone via a tomographic analysis
Authors: Coker David A. , Torquato Salvatore , Dunsmuir John H.
Published: February 2004

We present a study of the morphology and bulk physical properties of a Fontainebleau sandstone via an X ray tomographic analysis. Synchrotron‐based X ray tomographic techniques provide us with a high‐resolution (7.5 μm), three‐dimensional digitized representation of the sandstone that leaves the sample intact and unaltered. To estimate a wide spectrum of bulk properties of the Fontainebleau sandstone specimen, we extract from this image a number of different correlation functions that statistically characterize the pore‐space morphology and relevant pore‐space length and time scales. These statistical measures are obtainable from lineal, plane, and/or volume measurements and include the porosity, specific surface, two‐point and three‐point probability functions, lineal‐path function, chord‐length distribution function, pore‐size distribution function, and coarseness. The pore‐size distribution function, in particular, contains a certain level of connectedness information and accordingly can only be obtained from a three‐dimensional representation of the sample. Many bulk properties of the sandstone, such as the mean survival time τ (obtainable from Nuclear Magnetic Resonance relaxation studies), fluid permeability k, effective electrical and thermal conductivities, and effective elastic moduli, can be estimated using the aforementioned statistical correlation functions. Specifically, the electrical conductivity (or, equivalently, the formation factor F), mean survival time, and fluid permeability are determined using rigorous bounds. The mean survival time and fluid permeability are also found using direct simulation techniques and cross‐property relations, respectively. One such cross‐property relation for k depending on τ and F gives a permeability estimate that is within a factor of 2 of the experimental result.
Uplift of the Colorado Plateau due to lithosphere attenuation during Laramide low‐angle subduction
Authors: Spencer Jon E.
Published: February 2004

The Colorado Plateau is blanketed by Phanerozoic marine and nonmarine strata as young as Cretaceous that are now exposed at elevations of about 2 km. Crustal thickening due to magmatism and horizontal crustal shortening was far less than necessary to cause this uplift, which is commonly attributed to the consequences of mantle lithosphere thinning and heating. The Colorado Plateau and the midcontinent region around Iowa consist of Precambrian bedrock overlain by a similar amount of Paleozoic platformal strata, and thus both regions once had similar lithospheric buoyancy. Mesozoic sedimentation increased the crustal thickness and lithospheric buoyancy of the Colorado Plateau relative to the midcontinent region. Backstripping calculations yield elevation without these sediments and lead to a calculated elevation difference between the two areas of about 1200 m, which represents unexplained plateau uplift. Review of constraints on uplift timing finds little support for a late Cenozoic uplift age and allows early to middle Cenozoic uplift, which is consistent with uplift mechanisms related to low‐angle subduction that ended in the middle Cenozoic. Finite element heat flow calculations of low‐angle subduction and lithosphere attenuation, using a range of initial lithosphere thicknesses and degree of attenuation, indicate that required uplift can result from tectonic removal of about 120 km of mantle lithosphere from an initially 200‐km‐thick lithosphere. This allows for partial preservation of North American mantie lithosphere with its distinctive isotopic signature in some late Cenozoic volcanic rocks and is consistent with normal Pn velocities in the uppermost mantle beneath the plateau.
Progress toward a stochastic rock mechanics model of engineered geothermal systems
Authors: Willis‐Richards J. , Watanabe K. , Takahashi H.
Published: February 2004

Hot dry rock geothermal energy extraction experiments in artificial reservoirs created by hydraulic stimulation in naturally fractured crystalline rocks have been undertaken in several countries over the last 20 years. The experiments have had mixed results in terms of fluid recovery, system impedance, and heat extraction. Numerical models have not yet delivered a generally agreed understanding of the processes and sensitivities involved in reservoir creation and circulation. In this paper a two‐dimensional fracture network model is described, which attempts to address the problems of both reservoir creation and circulation using rock mass characterization and in situ stress data as the primary inputs with a view to encapsulating our present understanding of how such systems work. The model is a prototype for a three‐dimensional version currently under development and is intended mainly for engineering sensitivity studies. The basis of the model lies in approximations of fracture mechanical behavior drawn from the rock mechanics literature, a very simplified analysis of the operative physical processes, and mapping of the connectivity of fracture networks to a fine resolution regular grid. Taken together, these permit the approximate resolution of what is normally a supercomputer problem on a personal computer. The model is applied to field data gathered at Fenton Hill, New Mexico; Hijiori, Yamagata Prefecture, Japan; and Rosemanowes, Cornwall, England.
Testing crustal melting models for the origin of flood rhyolites: A Nd‐Pb‐Sr isotopic study of the Tertiary Davis Mountains volcanic field, west Texas
Authors: Cameron K. L. , Parker D. F. , Sampson D. E.
Published: February 2004

Flood rhyolites are lavas that have areal extents similar to large ignimbrites. The Davis Mountains volcanic field differs from most other Cenozoic volcanic fields in the southwestern United States in that a major volume of the silicic rocks erupted there are flood rhyolites rather than ignimbrites. A comparative isotopic study was made of a major flood rhyolite unit, the Star Mountain, and mafic to silicic rocks from three other magma systems. The entire range in Nd isotopic compositions found in the volcanic field is relatively small, with εNd values ranging from about +2 in a basalt to between +1.0 and −1.5 in the silicic rocks including the Star Mountain rhyolite. Pb isotope ratios of the Star Mountain lavas (206Pb/204Pb ≈ 17.9) are considerably higher than those of xenoliths of Proterozoic deep crustal granulites from the region (<17.4). Two types of crustal sources appear viable for the origin of the flood rhyolites. First, if the Star Mountain samples represent near primary melts from a crustal source, then that source must have been Cenozoic in age and silicic in composition. Second, if the Star Mountain samples represent highly fractionated magmas, then the crustal source could have been mafic, but it could not have experienced the Proterozoic granulite facies metamorphism known to have affected the basement. The overall isotopic trends of the volcanic rocks are qualitatively consistent with the hypothesis that all the silicic rocks including the flood rhyolites evolved from mantle‐derived basalts with variable, but generally small, amounts of crustal contamination.
High‐temperature alteration of oceanic gabbros by seawater (Hess Deep, Ocean Drilling Program Leg 147): Evidence from oxygen isotopes and elemental fluxes
Authors: Lecuyer Christophe , Reynard Bruno
Published: February 2004

Two major stages of high‐temperature water rock interactions have been identified in gabbros of oceanic layer 3 exposed in the Hess Deep Rift Valley at the East Pacific Rise (2°N, 101°W). The 154 m of plutonic rocks cored at site 894G show mineralogies and textures that suggest that they represent the roof of an approximate 1 Myr magma chamber. A late stage of hydrothermal alteration produced mineral assemblages typical of lower amphibolite ‐ upper greenschist facies conditions. The hydration of the gabbros occurred locally through the development of well‐crystallized green amphiboles. Significant chemical fluxes were associated with the fluid flow and precipitation‐dissolution mechanisms. Oxygen isotope data suggest an earlier cryptic stage of hydrothermal alteration of the Hess Deep gabbros at temperatures above 500°C. Hydration was limited with only minute amphibole lamellae in pyroxenes that trapped Cl, Na, and minor K from seawater. Ca‐plagioclases remained stable and preserved their magmatic cationic compositions and zoning patterns. We propose that this high‐temperature event lowered the δ18O of the gabbros by isotopic exchange with a hydrothermal fluid through solid state oxygen diffusion. Although it is unclear whether the 18O depletion of Hole 894G gabbros is representative of the entire lower oceanic crust, it is nevertheless thought to contribute to the oxygen isotope buffering of the oceans. During this stage, water rock ratios calculated using Sr isotopes are underestimated (0.1 – 0.5) because of insignificant diffusion of this cation in plagioclases. Better estimates of water rock ratios are provided by oxygen isotopes (0.2 – 1) which are important in unraveling the cooling history of the oceanic crust.
Fast deformation processes and eruptive activity at Mount Etna (Italy)
Authors: Bonaccorso Alessandro , Ferrucci Fabrizio , Patanè Domenico , Villari Letterio
Published: February 2004

The seismic and deformation patterns observed on Mount Etna before and during the 1991–1993 eruption, the third largest since the seventeenth century in terms of lava volume, are consistent with the regional tectonic framework of eastern Sicily. The pattern of the stress field acting on the intermediate and lower crust was defined at the local scale by focal mechanisms of microearthquakes occurring at depths between 10 and 25 km beneath the volcano. They provide evidence for a strike‐slip compressional stress regime with the maximum compressive component acting approximately N‐S. The fault plane solutions and the spatial and temporal distribution of seismicity indicate that usually sinistral shear ruptures occur along approximately NE‐SW trending fault zones, while dip‐slip ruptures affect approximately NNW‐SSE trending fault zones. The latter include the avenues along which magma ascended during the 1991–1993 eruption. Seismic observations indicate a local inversion of the stress field acting on the upper crust (depth < ∼10 km) underlying Mount Etna, which was initiated less than 2 months before the eruptive event and disappeared with its end. This is consistent with a local tensile regime that favored the magma ascent through the shallow crust. The events preceding the eruption and accompanying its onset (tilt anomalies and seismic swarms) and those occurring shortly after its beginning (a mainshock‐aftershock seismic sequence and associated coseismic tilts) provide important evidence for understanding the dynamics of the two main volcano‐tectonic structures (NE‐SW and NNW‐SSE trending fault zones) and associated intrusive mechanisms on Etna. The shape and location of the eruption‐feeding dike have been modeled from ground deformation data. This approximately NNW‐SSE modeled dike, the seismicity, and the position of the fractures are consistent with the regional stress field characterized by σ1 oriented approximately N‐S. The geophysical data presented and analyzed in the present paper strongly suggest an overall regional tectonic control as well as an active role for the intruding magma in the dynamics of the volcano.
Optical constants of minerals derived from emission spectroscopy: Application to quartz
Authors: Wenrich Melissa L. , Christensen Philip R.
Published: February 2004

The objective of this study was to develop a method for determining the optical constants (i.e., the real (n) and imaginary (k) indices of refraction) of single‐crystal, uniaxial minerals using emission, rather than reflection, observations as a step toward deriving n and k from particulate minerals. The vibrational, midinfrared, emissivity spectra of polished quartz crystals were obtained by measuring the thermal radiance from planar surfaces, parallel and perpendicular to the optic axis. From these oriented spectra the ordinary ray was directly measured, and the extraordinary ray was mathematically derived. The n and k values for quartz were obtained by application of the Fresnel laws of reflection to the ordinary‐ and extraordinary‐ray spectra of the polished surfaces. Comparison of theoretical spectra, obtained through classical Lorentz‐Lorenz dispersion theory, to experimental spectra enabled new oscillator parameters for quartz to be determined. Construction of the ordinary‐ray spectrum required seven oscillators defined by frequencies (ν, converted to wavenumbers) of 1215 cm−1, 1161 cm−1, 1067 cm−1, 795 cm−1, 694 cm−1, 449 cm−1, and 393 cm−1; strengths (4πρ) of 0.03, 0.007, 0.67, 0.11, 0.01, 0.815, and 0.4; and damping terms (γ) of 0.4, 0.007, 0.008, 0.0115, 0.01, 0.012, and 0.012, respectively. Construction of the extraordinary‐ray spectrum required six oscillators defined by frequencies (converted to wavenumbers) of 1222 cm−1, 1074 cm−1, 776 cm−1, 509 cm−1, 494 cm−1, and 364 cm−1; strengths of 0.011, 0.67, 0.10, 0.015, 0.7, and 0.68; and damping terms of 0.15, 0.008, 0.011, 0.016, 0.014, and 0.019, respectively. These results were compared to the previously published results of Spitzer and Kleinman that required an additional oscillator (seven total) to define the extraordinary‐ray spectrum. Regardless of the difference in number of required oscillators, the spectra of n and k for the two studies agree within the experimental error. The results confirm that emission spectroscopy is a viable means for determining the optical constants of uniaxial minerals using the technique described in this study.
Terrestrial heat flow in the Sirt Basin, Libya, and the pattern of heat flow across northern Africa
Authors: Nyblade Andrew A. , Suleiman I. S. , Roy Robert F. , Pursell Benjamin , Suleiman A. S. , Doser Diane I. , Keller G. Randy
Published: February 2004

We report 66 new heat flow and 24 new heat production measurements from the Sirt Basin, a late Jurassic‐Miocene sedimentary depression in north central Libya underlain by late Proterozoic basement. Heat flow determinations were made using bottom hole temperatures from oil wells and thermal conductivity measurements from drill core and cuttings; heat production measurements come from core samples of basement rock. Heat flow is fairly uniform throughout the basin, with a mean of 72 ± 9 (s. d.) mW m−2. It is not clear if heat flow from the Sirt Basin is elevated as a consequence of its origin as a late Mesozoic rift. The difference between the mean basin heat flow and the global mean heat flow from tectonically undisturbed late Proterozoic terrains (55 ± 17 mW m−2) is 17 mW m−2, but this difference lies within the uncertainties associated with these mean heat flow estimates. If heat flow from the Sirt Basin is elevated, it could be caused by enhanced crustal heat production and need not be attributed to thermal alteration of the lithosphere related to basin formation. Mean crustal heat production is 3.9 ± 2.1 μW m−3, 1/2 to 3 times greater than surface heat production in other Proterozoic terrains in Africa. From west to east, the pattern of heat flow across northern Africa is characterized by high (80–110 mW m−2) heat flow throughout most of northwestern Africa, normal to perhaps slightly elevated heat flow in the Sirt Basin, low to normal (35–55 mW m−2) heat flow in Egypt inboard of the Red Sea, and high heat (75–100 mW m−2) flow along the Red Sea. High heat flow near the Red Sea and in northwestern Africa along the Mediterranean coast can be readily attributed to Cenozoic tectonic activity, but high heat flow in the Paleozoic Sahara basins of southern Algeria is harder to understand within the tectonic framework of northern Africa. A possible explanation, advanced previously, is that elevated heat flow in the Sahara basins arises from a regional thermal anomaly within the north African lithosphere. If that explanation is correct, then the heat flow distribution in the Sirt Basin and in Egypt away from the Red Sea suggests that the postulated lithospheric thermal anomaly does not extend beyond the Sahara basins to the east.
Large (1000 to 4000 km) northward movements of tectonic domains in the northern Cordillera, 83 to 45 Ma
Authors: Irving E. , Wynne P. J. , Thorkelson D. J. , Schiarizza P.
Published: February 2004

Using paleomagnetically derived estimates of latitudinal displacement, within the northern Cordillera we identify two major tectonic domains that were active during Late Cretaceous through Eocene time (roughly 83 to 45 Ma). At 100 to 90 Ma, the Interior Domain (comprising much of interior British Columbia, central Yukon, and eastern and central Alaska) was situated in the latitudes of Oregon and northern California, and the Coast Domain (southeastern Alaska, much of the Coast Ranges and islands of British Columbia, and the Cascade Mountains of Washington) was in latitudes similar to those of northern Mexico. Subsequently, both domains moved northward, reaching their present positions before 45 Ma. Within each domain, displacement estimates from northern locations are greater than those from southern locations, as if the domains had been elongated in an orogen‐parallel sense. Evidently, during the latest Cretaceous through Eocene, the Cordillera was a huge dextral shear system, which (83 to 65 Ma) carried the Coast Domain about 2000 km northward until it combined with the Interior Domain and then (65 to 45 Ma) drove the combined domains an additional 1000 to 2000 km north northwestward, spreading them out along the continental margin. Following previous workers, we ascribe these motions to the rapid oblique subduction of the oceanic Kula Plate beneath the continental western margin of the North American Plate, and derive a model based, in part, on comparisons with the oblique subduction of the Nazca Plate beneath the central Andes. Between the Interior Domain and the craton, geologic evidence in the north has been interpreted as indicating displacements comparable in magnitude to paleomagnetic estimates; in the south, the record has been obscured by Eocene extension. Between the Interior and Coast Domains, geological relationships are complex, and most current geologically based interpretations propose that post‐mid‐Cretaceous relative motions between them did not exceed a few hundred kilometers, whereas paleomagnetic evidence indicates 2000 km. Our reading of the geologic evidence is that it neither requires nor precludes the very large displacements inferred from paleomagnetic observations. Hence we argue that although the paleomagnetically derived displacements far exceed those proposed by geologic interpretations, they are not inconsistent with the geologic evidence itself.
High‐temperature deformation of calcite single crystals
Authors: Wang Z.‐C. , Bai Q. , Dresen G. , Wirth R. , Evans B.
Published: February 2004

To investigate the high‐temperature plasticity of calcite, we performed creep experiments in a 0.1‐MPa, dead‐load creep rig at temperatures T = 690°–880°C, oxygen fugacities fO2 = 10−20 − 10−2 MPa, and a constant CO2 fugacity fCO2 ≈ 10−1 MPa. Single crystals from four different sources were compressed along

. Applied stress, σ, was controlled at 5 to 75 MPa to yield strain rates, ε°, between 10−7 and 10−4 s−1. Inductive coupled plasma mass spectrometry analysis indicated that Mn is the major impurity in the four crystals; concentrations varied between 0.002 and 0.06 wt %. Two deformation regimes were revealed: a power law regime and a power law breakdown regime. At low stress (σ ≤ 30 MPa), steady state creep data yielded a stress exponent of ∼3.5 ± 0.5. Optical microscopy showed few twins. Transmission electron microscopy revealed that f
and c
were the major slip systems. The majority of dislocations in deformed samples are curved, suggesting that dislocation climb is the rate‐limiting step within the low stress regime. Three deformation mechanisms with different flow laws operated. For the samples with Mn concentration <200 ppm, the oxygen fugacity exponent, m, is 0 and the activation energy for creep, Q, is 305 kJ/mol. For the samples with higher Mn concentration (≥600 ppm), creep depends on both fO2 and temperature. A deconvolution of the fO2 ‐ ε° data and the ε° ‐ 1/T data yielded that, at low fO2 and low T, m = 0 and Q = 200 kJ/mol, while at high fO2 and/or high T, m = 1/6 and Q = 400 kJ/mol. At similar temperature and stresses, samples with low Mn concentration deform substantially slower than those with high Mn concentration. Appararently, variations in Mn concentration affect creep through the charge neutrality conditions governing point defect structure. Based on the mechanical data, published diffusion data and point defect chemistry calculations, three rate‐controlling mechanisms for creep are proposed. At high stress (σ ≥ 30 MPa), deformation rate is much more sensitive to stress, indicating power law creep down break. The microstructure showed increased activity of dislocation glide, cross slip, and twinning. The value of the stress separating the two deformation regimes is about 10−3 when normalized by the shear modulus, in agreement with those for some other geological materials.
The dynamics of plume‐ridge interaction: 2. Off‐ridge plumes
Authors: Ribe N. M.
Published: February 2004

A variety of geophysical and geochemical evidence indicates that ascending mantle plumes can interact with ocean ridges located up to 1400 km away. I investigate the dynamics of this interaction using a simple model in which a point source with volume flux Q (analogous to a plume “stem”) releases buoyant fluid into a viscous corner flow driven by the divergence of rigid surface plates with thickness ∼(kx/U)1/2, where U is the half spreading rate. The point source is located at a distance xp from the ridge, and ridge migration is neglected. The buoyant fluid forms a thin sublithospheric layer whose thickness S(x, y) satisfies a nonlinear advection‐diffusion equation describing the balance of advection by the corner flow, buoyancy‐driven “self‐spreading,” flow toward the ridge along the sloping base of the lithosphere, and continuous accretion into the lithosphere. Numerical solutions of this equation yield scaling laws for the lateral extent W (“waist width”) of plume material along the ridge, the fraction R of the plume flux that crosses the ridge, and the maximum value of xp beyond which interaction ceases. The sloping base of the lithosphere has only a minor (few tens of percent) influence on these quantities, which are determined principally by the balance of advection and self‐spreading. An extension of the model to include plume‐induced lithospheric thinning shows that this process increases the waist width by an amount of order 10%. Finally, the model provides a new explanation for the observation that plumes interact primarily with ridges that are migrating away from them, rather than toward them.
Anisotropy‐induced coupling in borehole acoustic modes
Authors: Norris Andrew N. , Sinha Bikash K.
Published: February 2004

The guided wave modes of a circular borehole in a weakly anisotropic formation are composed of linear superpositions of the associated modes for an isotropic formation. At moderate frequencies the major modes of concern are the quasi‐Stoneley and quasi‐flexural modes. These guided modes in anisotropic formations can be estimated from a perturbation analysis in terms of the unperturbed solutions for an isotropic formation. When the formation anisotropy is of monoclinic or lower symmetry, the normal and shear stresses become functions of both normal and shear strains through some additional anisotropic constants that are not present in materials with orthorhombic or higher symmetry. These additional elastic constants cause a coupling between the Stoneley and flexural modes. Under these circumstances, an on‐axis monopole or dipole source excites both modes. Coupling coefficients account for the excitation of quasi‐flexural motion by a monopole source, and of the quasi‐Stoneley mode by a dipole. A transversely isotropic (TI) formation with its symmetry axis obliquely inclined with the borehole exhibits monoclinic symmetry in its rotated constants referred to the borehole axis. The monoclinic symmetry of the surrounding formation in such cases causes a coupling between the Stoneley and flexural modes. Computational results show that a borehole inclined at an angle of 60° from the symmetry axis of Austin chalk, a slow TI medium, exhibits coupling between the Stoneley and qSV‐polarized flexural mode acceleration amplitudes of the order of 20 dB or less in the frequency range of interest. A similar obliquely inclined borehole in Bakken shale, a fast TI formation, exhibits a far weaker coupling between the Stoneley and qSV‐polarized flexural modes. The stronger coupling in the case of Austin chalk is a result of relatively large anisotropic constants together with close proximity of the Stoneley and qSV‐polarized flexural dispersions. On the other hand, weaker coupling in Bakken shale is caused by relatively small anisotropic constants and a large separation between the Stoneley and qSV‐polarized flexural dispersions in the moderate frequency range of interest.
Estimation of geomagnetic transfer functions from nonuniform magnetic fields induced by the equatorial electrojet: A method to determine static shifts in magnetotelluric data
Authors: Stoerzel Andreas
Published: February 2004

Local conductivity inhomogeneities in the near‐surface commonly shift magnetotelluric (MT) apparent resistivity curves in magnitude by a frequency independent factor (static shift). Proper interpretation of MT data requires detection and compensation of these static distortions. A variation of the Z/H method applied to single site recordings of magnetic variations of the equatorial electrojet (EEJ) produces static‐free geomagnetic transfer functions in the same period range as MT impedances. Numerical modeling demonstrates that a comparison of these undistorted geomagnetic transfer functions with distorted MT impedances at periods between 10 and 100 seconds determines the magnitude of static shifts. By using a priori information on the morphology of the EEJ field and removing regional two‐dimensional (2‐D) and three‐dimensional (3‐D) effects from the magnetic data, the single‐site Z/H method provides a reasonable estimate of the static shift in MT data from northeast Brazil. Critical factors in determining MT static shifts with this method are (1) good control on the morphology of the EEJ field and on the ratio of the regional uniform field to the nonuniform EEJ field at the magnetic equator, (2) a model reproducing the source field, (3) information on the distance between the MT site and the EEJ center, and (4) removal of regional 2‐D and 3‐D effects from the magnetic data.
Waves of bubbles in basaltic magmas and lavas
Authors: Manga Michael
Published: February 2004

Initially homogeneous suspensions of bubbles in basaltic magmas and lava flows may become unstable and form rising waves or layers of bubbles. We derive a set of model equations for the two‐phase (bubble‐liquid) system and present the results of a linear stability analysis and numerical simulations. Periodic vesicle layers are preserved in Columbia River flows with a spacing of ∼1 m; the prediction of the stability analysis combined with a simple model for solidification of the lava suggests that vesicle layers may represent waves of bubbles formed within the flow. The spacing of layers is determined by a balance between the growth of bubbles and hydrodynamic self‐diffusion. Vesicular layers preserved in Columbia River flows may thus form by hydrodynamic processes and do not require the repeated injection of fresh volatile‐rich magma or a periodic nucleation of bubbles. Waves of bubbles may also develop in magma chambers and conduits and could play a role in eruption dynamics.
Tectonics of the Antarctic‐Scotia plate boundary near Elephant and Clarence Islands, West Antarctica
Authors: Klepeis Keith A. , Lawver Lawrence A.
Published: February 2004

Over 5000 km of new bathymetric data collected from near the northern Antarctic Peninsula (60°S–63.5°S latitude, 53.5°W–63°W longitude) show the morphology of an irregular segment of the Antarctic‐Scotia plate boundary and nearby Shetland microplate. The irregular plate boundary is formed by an oblique intersection (>70°) of the sinistral transpressional Shackleton fracture zone (SFZ) and the sinistral transtensional South Scotia Ridge transform (SSR) near Elephant (EI) and Clarence (CI) Islands. Mapped boundaries of the Shetland microplate include the South Shetland Trench and the volcanic rift axis of Bransfield Strait marginal basin. Bathymetric data, single‐channel seismic reflection profiles, and Geosat/ERS 1 free air gravity data show a southeast trending fault zone on the northeast side of a prominent ridge in the SFZ. The fault zone is defined by scarps that affect ocean floor sediments, fault‐bounded subbasins, rotated sedimentary layers, angular unconformities, linear gravity trends, and transtensional followed by contractional deformation. Southeast of a termination of the SFZ ridge at the South Shetland Trench, the fault zone subdivides into segments displaying steep scarps (up to 23°) and canyons on the northeast margin of the EI platform. These features become east‐west trending nearer to the western SSR. South of the islands, southwest trending extensional or transtensional fault zones disrupt the Bransfield Strait volcanic rift axis. These data suggest that (1) recent (<4 Ma) changes in the configuration of the Antarctic plate near the Antarctic Peninsula caused a segment of the SFZ transform to adjust to a more stable, rectilinear geometry with the SSR transform, and (2) diffuse transtension resulting from current Antarctic‐Scotia relative motion is dissecting the Shetland microplate near EI and CI and transferring slivers of the Scotia plate onto the Antarctic plate.
The effect of lower crustal flow on continental extension and passive margin formation
Authors: Hopper John R. , Buck W. Roger
Published: February 2004

The great variety of styles of continental extension may reflect different crustal thickness and thermal states of continental lithosphere during the initiation of rifting. To investigate how these and other factors affect rifting and the development of passive continental margins, we develop a simplified model of lithospheric extension. We consider the evolution of extensional deformation for a three‐layer model lithosphere bounded laterally by much stronger lithosphere. The cold part of the crust and mantle are treated as thin brittle/plastic layers. The lower crust is approximated as a thin viscous channel. Each brittle/plastic layer can extend in only one location determined by the strength of the layer, shear of the lower crust, and buoyancy forces related to both crustal thickness variations and thermally induced density differences. The lower crust flows in response to crustal thickness variations and is sheared when the loci of extension for the two brittle/plastic layers are horizontally offset, a situation we term shear decoupling. As in previous studies, we see three distinct patterns, or modes, of extensional deformation that occur under different sets of model parameters: the core complex mode, the wide rift mode, and the narrow rift mode. Shear decoupling occurs only in cases with a crustal rheology at the weak end of the spectrum of laboratory estimated values. We are aware of no observations that require that the upper crust and upper mantle strain at laterally displaced positions. We show that for large magnitudes of extension there can be transitions between modes as inferred for some highly extended continental areas. Predicted patterns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins, including very wide and asymmetric margins.
Worldwide wavelet analysis of geomagnetic jerks
Authors: Alexandrescu Mioara , Gibert Dominique , Hulot Gauthier , Le Mouël Jean‐Louis , Saracco Ginette
Published: February 2004

Following an earlier study which gives the principles of the method and an example of application to the eastern component of the magnetic field in the European region [Alexandrescu et al., 1995], detection and characterization of geomagnetic jerks using wavelet analysis is generalized to any horizontal component of the field and to a worldwide distribution (involving 97 locations) of observations. This allows for a systematic and global search for such events within the twentieth century and makes it possible to unravel a number of intriguing properties associated with them. Whereas our first study only reveals five such events in Europe, we can now state that seven and only seven events have apparently occurred throughout the world during the present century. Two (1969 and 1978) are unquestionably of global extent, three (1901, 1913, and 1925) being possibly of similar extent, while the remaining two (1932 and 1949) are not seen everywhere at the Earth's surface. We confirm our early result that the events are more singular than previously thought, with a “regularity” systematically closer to 1.5 than to 2, and a common mean value of about 1.6. Furthermore, the 1969 and 1978 events display a two‐step spatio‐temporal behavior consisting of an “early arrival” in the northern hemisphere, a “late arrival” in the southern hemisphere, and a time lag between the two arrivals of the order of a couple of years. We were also able to show that the 1969 and 1978 events tend to at least partially balance each other. The extent to which this is true remains to be assessed, mainly because our method, although already providing some information about the geometry of the events, does not yet allow the proper recovery of their intensities.