bluuebunny/crossref_metadata_2025 huggingface dataset:
Controls on erosional retreat of the uplifted rift flanks at the Gulf of Suez and northern Red Sea
Authors: Steckler Michael S. , Omar Gomaa I.
Published: February 2004

The Gulf of Suez and Red Sea rifts are currently bordered by large asymmetric uplifts that are undergoing erosion. We find that the amount and timing of erosion vary systematically along the strike of the margin and have been controlled by variations in the prerift stratigraphy. The prerift strata are composed of cliff‐forming Eocene‐Cretaceous carbonates overlying the easily eroded Cretacous‐Cambrian “Nubian” sandstone. This lithologic succession promotes scarp retreat of the sedimentary section, followed by dissection of the underlying basement. The prerift section thins from over 2000 m at the northern end of the rift to less than 400 m at its junction with the Red Sea. Thus, at the northern part of the Gulf of Suez, the Nubian sandstone is minimally exposed, and the carbonates form a scarp at the rift border fault. Farther south, undercutting of the carbonates by erosion of the sandstone has resulted in scarp retreat. The escarpment cuts diagonally away from the border fault and is over 100 km inland from the border fault at the southernmost Gulf of Suez. The amount of retreat varies inversely with the sediment thickness. Exposure and erosion of basement are initiated by the retreat of the escarpment, and the depth of erosion, as indicated by fission track ages, increases with distance from the escarpment. These observations are explained by a model in which erosion along the Gulf of Suez is initiated as rift flank uplift becomes sufficiently large to expose the friable sandstones. Undercutting of the escarpment and exhumation of basement has been propagating northward and westward for at least 20 m.y. The average rate of scarp retreat has been 6 km/m.y. and the along‐strike propagation of the erosion has been 12 km/m.y. The diachronous erosion of the the rift flanks at the Gulf of Suez highlights the importance of distinguishing between the timing of uplift and of erosion. Both thermochronometric and stratigraphic data primarily indicate the timing of erosion, which may differ significantly from the timing of the uplift that initiates it. They must be interpreted carefully to avoid erroneous conclusions about rift tectonics.
Erosional dynamics, flexural isostasy, and long‐lived escarpments: A numerical modeling study
Authors: Tucker Gregory E. , Slingerland Rudy L.
Published: February 2004

Erosional escarpments are common features of high‐elevation rifted continents. Fission track data suggest that these escarpments form by base level lowering and/or marginal uplift during rifting, followed by lateral retreat of an erosion front across tens to hundreds of kilometers. Previous modeling studies have shown that this characteristic pattern of denudation can have a profound impact upon marginal isostatic uplift and the evolution of offshore sedimentary basins. Yet at present there is only a rudimentary understanding of the geomorphic mechanisms capable of driving such prolonged escarpment retreat. In this study we present a nonlinear, two‐dimensional landscape evolution model that is used to assess the necessary and sufficient conditions for long‐term retreat of a rift‐generated escarpment. The model represents topography as a grid of cells, with drainage networks evolving as water flows across the grid in the direction of steepest descent. The model accounts for sediment production by weathering, fluvial sediment transport, bedrock channel erosion, and hillslope sediment transport by diffusive mechanisms and by mass failure. Numerical experiments presented explore the effects of different combinations of erosion processes and of dynamic coupling between denudation and flexural isostatic uplift. Model results suggest that the necessary and sufficient conditions for long‐term escarpment retreat are (1) incising bedrock channels in which the erosion rate increases with increasing drainage area, so that the channels steepen and propagate headward; (2) a low rate of sediment production relative to sediment transport efficiency, which promotes relief‐generating processes over diffusive ones; (3) high continental elevation, which allows greater freedom for fluvial dissection; and (4) any process, including flexural isostatic uplift, that helps to maintain a drainage divide near an escarpment crest. Flexural isostatic uplift also facilitates escarpment retreat by elevating topography in the vicinity of an eroding escarpment, thereby increasing channel gradients and accelerating erosion which in turn generates additional isostatic uplift. Of all the above conditions, high continental elevation is common to most rift margin escarpments and may ultimately be the most important factor.
Dislocations in inhomogeneous media via a moduli perturbation approach: General formulation and two‐dimensional solutions
Authors: Du Yijun , Segall Paul , Gao Huajian
Published: February 2004

Quasi‐static elastic dislocations in a homogeneous elastic half‐space are commonly used to model earthquake faulting processes. Recent studies of the 1989 Kalapana, Hawaii, and Loma Prieta, California, earthquakes suggest that spatial variations in elastic properties are necessary to reconcile geodetic and seismic results (Arnadottir et al., 1991; Eberhart‐Phillips and Stuart, 1992). In this paper, we use a moduli perturbation approach to investigate the effect of lateral and vertical variations in elastic properties on the elastic fields produced by dislocations. The method is simple, efficient, and in some cases leads to closed form solutions. The zero‐order solution is simply the solution for a homogeneous body. The first‐order correction for elastic heterogeneity is given by a volume integral involving the spatial variations in moduli, the displacements due to a dislocation in a homogeneous half‐space, and the half‐space Green's function. The same representation can be also used to obtain higher‐order solutions. If there are only piecewise constant variations in shear modulus, the volume integral can be reduced to a surface integral (or line integral in two‐dimensions). Comparisons with the analytical solutions for a screw dislocation in a layered medium suggest that the perturbation solutions are valid for nearly an order of magnitude variation in modulus. It is shown that a simple two‐dimensional model with both vertical and lateral variations in the elastic properties may explain a large part of the discrepancy between seismic and geodetically inferred fault depths for the 1989 Kalapana, Hawaii, earthquake.
Spherical disharmonies in the Earth sciences and the spatial solution: Ridges, hotspots, slabs, geochemistry and tomography correlations
Authors: Ray Terrill W. , Anderson Don L.
Published: February 2004

There is increasing use of statistical correlations between geophysical fields and between geochemical and geophysical fields in attempts to understand how the Earth works. Typically, such correlations have been based on spherical harmonic expansions. The expression of functions on the sphere as spherical harmonic series has many pitfalls, especially if the data are nonuniformly and/or sparsely sampled. Many of the difficulties involved in the use of spherical harmonic expansion techniques can be avoided through the use of spatial domain correlations, but this introduces other complications, such as the choice of a sampling lattice. Additionally, many geophysical and geochemical fields fail to satisfy the assumptions of standard statistical significance tests. This is especially problematic when the data values to be correlated with a geophysical field were collected at sample locations which themselves correlate with that field. This paper examines many correlations which have been claimed in the past between geochemistry and mantle tomography and between hotspot, ridge, and slab locations and tomography using both spherical harmonic coefficient correlations and spatial domain correlations. No conclusively significant correlations are found between isotopic geochemistry and mantle tomography. The Crough and Jurdy (short) hotspot location list shows statistically significant correlation with lowermost mantle tomography for degree 2 of the spherical harmonic expansion, but there are no statistically significant correlations in the spatial case. The Vogt (long) hotspot location list does not correlate with tomography anywhere in the mantle using either technique. Both hotspot lists show a strong correlation between hotspot locations and geoid highs when spatially correlated, but no correlations are revealed by spherical harmonic techniques. Ridge locations do not show any statistically significant correlations with tomography, slab locations, or the geoid; the strongest correlation is with lowermost mantle tomography, which is probably spurious. The most striking correlations are between mantle tomography and post‐Pangean subducted slabs. The integrated locations of slabs correlate strongly with fast areas near the transition zone and the core‐mantle boundary and with slow regions from 1022–1284 km depth. This seems to be consistent with the “avalanching” downwellings which have been indicated by models of the mantle which include an endothermic phase transition at the 670‐km discontinuity, although this is not a unique interpretation. Taken as a whole, these results suggest that slabs and associated cold downwellings are the dominant feature of mantle convection. Hotspot locations are no better correlated with lower mantle tomography than are ridge locations.
Influence of eruption parameters on the thermofluid dynamics of collapsing volcanic columns
Authors: Neri Augusto , Dobran Flavio
Published: February 2004

A complex two‐phase flow model was employed to study the influence of eruption parameters on the thermofluid dynamics of collapsing volcanic columns. The physical model accounts for thermal and mechanical nonequilibrium between the gas and solid phases and for mixing of water vapor leaving the vent with the pressure‐ and temperature‐stratified atmosphere. The effects of particle collisions and pressure were modeled by a kinetic theory of granular flow. Depending on the eruption, the simulations were performed from several to tens of minutes after the beginning of the eruptions. Initial conditions for simulations involved different vent diameters, exit velocities, and temperatures of the two phases, particle diameters, and water vapor contents of the flow mixture. After an initial period of fountain building, characterized by column collapse and formation of pyroclastic flows and material recycling from the collapsed column into the fountain, the simulated columns developed different structures, producing stationary or oscillatory behaviors which depend on the geometry and physical parameters of the gas and pyroclasts at the vent. Such behaviors of the collapsed columns may strongly influence the dynamics of pyroclastic flows, causing variations of the flow properties and strong pulsations of their mass flow rates. The application of spectral analysis to timewise distributions of some properties at different points in the fountains and pyroclastic flows permitted identification of the main frequencies associated with the nonstationary column phenomena. The columns at the transition between the collapsing and plinian behaviors developed characteristic suspended flows that spread radially at the fountain height level for several kilometers from the vent and their heads intermittently collapsed due to the changing atmospheric dynamics above and below the suspended flows. The results from computer simulations of collapsing volcanic columns are consistent with previous simple and complex modeling approaches, laboratory experiments, and field observations. The results also show that the thermofluid dynamics of collapsing volcanic columns may produce very complex depositional structures in pyroclastic flows and should therefore provide important clues in the interpretations of field deposits associated with explosive volcanic eruptions.
Geophysical response of flood basalts from analysis of wire line logs: Ocean Drilling Program Site 642, Vøring volcanic margin
Authors: Planke Sverre
Published: February 2004

Analysis of wire line logs at Ocean Drilling Program Site 642, the deepest scientific hole yet drilled in an offshore large igneous province (LIP), reveals the main features of the extrusive complex constructed during formation of a volcanic margin. A characteristic cyclic log response images tholeiitic lavas and thin tuffs corresponding to the seaward dipping reflectors in the seismic record. Flows are 0.6–18.5 m thick, while most sediments are <l m thick. Porosity dependent sonic, density, neutron porosity and resistivity logs respond to gently decreasing porosity in upper 5–7 m of the flow and rapid porosity increase near the flow base. Velocity and density top and bottom gradients are largely independent of flow thickness, with velocity gradients of 400 to 600 s−1 and −1000 to −2000 s−1, respectively. A positive correlation between flow thickness and mean velocity simply reflects increased proportion of massive basalt in thicker flows. There is also an overall increase in mean fine‐grained basalt velocity with depth with a gradient of 0.8±0.3 s−1. Flow velocity‐density relations show a downward velocity strengthening related to a decrease in high aspect ratio voids. High velocities near the flow base correlate with high, >2000 Ω‐m, focused resistivity kicks. The large flow‐scale variations in physical properties show that representative core measurements must be densely sampled. Both on flow and sequence scales Site 642 log responses appears typical for the North Atlantic Volcanic Province and other LIPs but differs from logs in deep ocean crustal holes. Principal differences in log responses reflect mode of crustal emplacement and alteration. Hence Site 642 forms a reference hole for extrusive constructions on oceanic LIPs.
Tertiary structural evolution of the Gangdese Thrust System, southeastern Tibet
Authors: Yin An , Harrison T. Mark , Ryerson F. J. , Wenji Chen , Kidd W. S. F. , Copeland Peter
Published: February 2004

Structural and thermochronological investigations of southern Tibet (Xizang) suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost Tibetan plateau since late Oligocene. Two thrust systems are documented in this study: the north dipping Gangdese system (GTS) and the younger south dipping Renbu‐Zedong system (RZT). West of Lhasa, the Gangdese thrust juxtaposes the Late Cretaceous forearc basin deposits of the Lhasa Block (the Xigaze Group) over the Tethyan sedimentary rocks of the Indian plate, whereas east of Lhasa, the fault juxtaposes the Late Cretaceous‐Eocene, Andean‐type arc (the Gangdese batholith) over Tethyan sedimentary rocks. Near Zedong, 150 km southeast of Lhasa, the Gangdese thrust is marked by a >200‐m‐thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks. A major south dipping backthrust in the hanging wall of the Gangdese thrust puts the Xigaze Group over Tertiary conglomerates and the Gangdese plutonics north of Xigaze and west of Lhasa. A lower age bound for the Gangdese thrust of 18.3±0.5 Ma is given by crosscutting relationships. The timing of slip on the Gangdese thrust is estimated to be 27–23 Ma from 40Ar/39Ar thermochronology, and a displacement of at least 46±9 km is indicated near Zedong. The age of the Gangdese thrust (GT) is consistent with an upper age limit of ∼24 Ma for the initiation of movement on the Main Central thrust. In places, the younger Renbu‐Zedong fault is thrust over the trace of the GT, obscuring its exposure. The RZT appears to have been active at circa 18 Ma but had ceased movement by 8±1 Ma. The suture between India and Asia has been complexly modified by development of the GTS, RZT, and, locally, strike‐slip and normal fault systems.
Deconvolving Ocean Drilling Program temperature logging tool data to improve borehole temperature estimates: Chile Triple Junction
Authors: Sawyer Dale S. , Bangs Nathan L. , Golovchenko Xenia
Published: February 2004

We present a technique for correcting borehole fluid temperature observations made by the Ocean Drilling Program (ODP) with the Lamont temperature logging tool (TLT), for the effects of the slow temperature response of one of its sensors. TLT data have been recorded in many ODP boreholes, but, perhaps partly because of tool response effects, the data have only rarely been used. It has been shown that a continuous temperature log is the convolution of a tool response function and the temperature history experienced by the tool. We use temperature data from ODP Leg 141 to estimate the tool response function of the TLT. We then use Wiener filter theory to design a decon volution operator to remove the effect of the tool response from the recorded data. We apply the deconvolution operator to the data from Leg 141, assess the effectiveness of the deconvolution technique, and extrapolate the resulting borehole fluid temperatures to estimate the equilibrium geotherm at the two sites considered. The geothermal gradient in the accretionary wedge near the Chile Triple Junction increases with depth. This suggests that the thermal environment is not steady state, that fluid flow is transporting heat, or, most likely, both. The average heat flow in the accretionary wedge near the Chile Triple Junction is higher over the subducting Chile Ridge axis than over subducting young oceanic crust near the ridge axis.
Shear wave velocity and attenuation from pulse‐echo studies of Berea sandstone
Authors: Green Douglas H. , Wang Herbert F.
Published: February 2004

The pulse‐echo spectral‐ratio technique has been adapted to the determination of ultrasonic shear wave attenuation in sandstone at variable states of saturation and pressure. The method can measure shear attenuation coefficients in the range 0.5 dB/cm to 8 dB/cm to within ±0.5 dB/cm. For the Berea sandstone, this range corresponds to values of the shear quality factor (Qs) between 10 and 100. Spectra of Qs show that between 600 and 1100 kHz, Qs decreases with frequency, particularly at high pressures (up to 70 MPa). Ultrasonic shear wave attenuation in a 90% water‐saturated sample was intermediate between that for dry samples and the relatively high attenuation in fully saturated rock. Strong pressure dependence is seen in the shear attenuation for all saturation states, indicating a dominant role of dissipation mechanisms operating within open and compliant cracks. Substantial shear attenuation remains at the highest effective pressure applied to the saturated sample, which may be due to a more “global” fluid‐flow loss mechanism. Scattering losses, as described by weak scattering theories for compressional waves, do not appear to be dominant at these frequencies.
Silicate‐perovskite and majorite signature komatiites from the Archean Abitibi Greenstone Belt: Implications for early mantle differentiation and stratification
Authors: Xie Qianli , Kerrich Robert
Published: February 2004

Three komatiite‐tholeiite sequences from the Archean Abitibi Southern Volcanic Zone (SVZ), which are separated by major terrane boundaries, show systematic differences in composition, rare earth element (REE) patterns and high field strength element (HFSE) /REE fractionation. Komatiites from Tisdale Township are Al‐undepleted, with MgO = 12–24%, Al2O3/TiO2= 13–17, CaO/Al2O3= 1–1.3, and high Ni (722–1275 ppm) and Cr (1875–2820 ppm) contents. Mg‐tholeiites and Fe‐tholeiites are spatially associated with komatiites. Both komatiites and Mg‐tholeiites have flat REE patterns [(La/Yb)n= 0.6–1.2, and Nb/Nb*, Zr/Zr*, and Hf/Hf* ≈ 1 (calculated using primitive mantle normalized values and adjacent REEs in the incompatibility sequence of elements in mid‐ocean ridge basalt; MORB), suggesting that they may have formed by high‐degree partial melting of primitive mantle materials with olivine as the main liquidus phase at shallow depths (<300 km). Munro Township komatiites are Al‐undepleted, with MgO = 22–30%, Al2O3/TiO2= 17–20, CaO/Al2O3= 1.0–1.2, and high Ni (831–1590 ppm) and Cr (1250–2760 ppm) contents. They have light rare earth element (LREE) depleted patterns [(La/Yb)n= 0.2–0.4], and Nb/Nb*, Zr/Zr*, and Hf/Hf* > 1, consistent with having been derived from Mg‐perovskite enriched sources at depths >700 km below the transition zone, whereas spatially associated tholeiites have (La/Yb)n= 1–2, and Nb/Nb*, Zr/Zr*, and Hf/Hf* ≈ 1, suggesting a shallow undepleted mantle source, distinct from the komatiites. Komatiites and tholeiites from Boston Township are characterized by MgO = 11–29%, Al2O3/TiO2= 4.5–5.4, CaO/Al2O3= l.4–2.5, high Ni (203–3420 ppm) and Cr (194–1965 ppm) contents; Al, Sc, Y, and heavy rare earth element (HREE) depletion, and pronounced negative normalized Zr and Hf anomalies (Zr/Zr* and Hf/Hf* < 1), in accord with either partial melting of a majorite depleted mantle source or majorite fractionation during magma formation in the depth range of 300–600 km. The fractionated HFSEs and REEs of Munro ‐ and Boston‐area komatiites indicate that these Archean komatiites may have preserved some signatures of early mantle differentiation, which so far has only been inferred based on high‐pressure experimental studies and theoretical predictions. Further geochemical studies of Archean komatiites, especially precise multiple trace element analysis, combined with improved measurements of partition coefficients over a wide pressure range, will provide better understanding on early mantle differentiation, stratification, and the transition zone.
Dynamic basis of volcanic spreading
Authors: Borgia Andrea
Published: February 2004

The established models of the structure and dynamics of a “typical” Hawaiian volcano lead inevitably to a paradox: finite element viscoelastic calculations, using TECTON [Melosh and Raefsky, 1980, 1981], predict that a compressional stress field should characterize their upper flanks; instead, extension is observed. The paradox is solved by postulating that volcanic evolution is determined by feedback processes related to growth and spreading of the volcanic edifice. Many examples of the process of spreading are documented in the geologic literature on volcanoes of a very broad size range and very different geologic settings. The process is similar across many orders of magnitude in size: the deformation of the substratum related to magma intrusion during lava dome building events is at the lower end of the scale; the spreading of seafloor at oceanic ridges is at the higher end of the scale. Volcanoes may evolve from small constructs into huge shield volcanoes and perhaps into ocean plates. Key elements for volcanic spreading are (1) the existence of a weak basal layer and (2) a sufficiently high mass and magma influx to drive the process. As the mass of a volcano grows, it passes through five phases (which may overlap, repeat, or be omitted): building, compressing, thrusting, intruding, and spreading. During the building phase the mass of a volcano does not significantly affect the stress field of the substratum; primitive magmas are likely to be erupted. As the mass increases, the volcano subsides and becomes characterized by a compressional stress field which inhibits any further intrusion of magma into the volcanic edifice; magma may differentiate in the crust, allowing for large caldera‐forming eruptions to occur. In the next phase, thrusting begins on a decollement usually located at the foot of the edifice; while the stress field on the upper slopes of the volcano remains compressional, the stress at the base, close to the center, becomes extensional, allowing the magma to intrude and differentiate at the base of the edifice; a basal intrusive complex begins to grow; explosive eruptions may be expected. Finally, spreading of this complex creates an extensional stress field on the upper flanks of the volcano, confining compression to the lower slopes of the edifice; the extension allows primitive magma to erupt again at the summit. The process of spreading seems to have a fundamental influence on the structural and magmatic evolution of volcanoes. At every scale, this process results from coupling and feedback between the gravitational and thermal fields. Thus studying this process may improve our understanding of the relation between structural dynamics and magma evolution and of the origin and evolution of volcanic seismicity and eruptions.
The influence of room temperature deformation on porosity and permeability in calcite aggregates
Authors: Zhang Shuqing , Cox Stephen F. , Paterson Mervyn S.
Published: February 2004

Changes in permeability and porosity during shortening deformation of Carrara marble and hot‐pressed calcite aggregates were measured under high pressure at room temperature using argon as pore fluid. At effective pressures of 30 and 50 MPa, the permeability of Carrara marble increased by up to 2 orders of magnitude with less than 2% strain during which the connected porosity increased by only 0.005. The permeability increased more slowly with further strain up to 18%, during which the connected porosity increased by a further 0.05 to 0.06. At effective pressures of 100 MPa to 200 MPa, these effects were much less marked. In hot‐pressed calcite aggregates, deformed at an effective pressure of 50 MPa, the permeability increased by about 2 orders of magnitude after about 12% strain and an increase in connected porosity of about 0.03. Microstructural studies indicate that, in the coarse‐grained Carrara marble specimens, both transgranular and grain boundary cracks are present after room temperature deformation. For a given strain, the average length and the linear density of transgranular cracks decrease with increasing effective pressure. In fine‐grained, hot‐pressed calcite aggregates, dilatancy is mainly due to opening of grain boundary cracks. The very marked increase in permeability with small strain at low effective pressure can be correlated with the proliferation of connected microcracks of relatively large apertures, deduced on the basis of theoretical models.
The Marquesas archipelagic apron: Seismic stratigraphy and implications for volcano growth, mass wasting, and crustal underplating
Authors: Wolfe Cecily J. , McNutt Marcia K. , Detrick Robert S.
Published: February 2004

Multichannel seismic lines, sonobuoy and gravity data across the Marquesas Islands are used to study volcano growth, island mass wasting, and crustal underplating at island chains with overfilled moats. The Marquesas bathymetry reflects the changing thickness of the sedimentary infill rather than the basement topography. The moat contains two major regions of differing seismic stratigraphy: (1) the moat edges, where a unit of continuous layered reflectors is present containing minor lenses of chaotic diffractors and, (2) the central moat, where the deep moat basins are overfilled by an acoustically opaque unit of discontinuous reflectors of up to 2 km thickness, in places capped by a ponded unit. Plate flexure models require broad underplating of the crust by low‐density (crustal?) material at the Marquesas Islands to explain the depth to volcanic basement and gravity observations. The seismic velocities and seismic stratigraphy, as well as the general structure of the islands and surrounding seafloor, indicate the apron is mostly debris from island mass wasting. Reflectors of the outermost moat generally onlap the flexural arch in the lower section and offlap and overfill it in the upper section. In the central moat, reflectors change shape from concave up in the lower section to convex in the upper section. Three‐dimensional diffusion models of sedimentation, which incorporate a time‐dependent seafloor deflection from progressive island loading and vary sediment influx as islands are formed and mass waste, suggest that three main factors make the moat stratigraphy at the Marquesas different from Hawaii: (1) the Marquesas moat is overfilled, while the Hawaiian moat is underfilled, (2) sediment diffusivities are lower at the Marquesas, and (3) the Marquesas islands are separated by deep sedimentary basins, in contrast to Hawaii, where islands are separated by a shallow ridge. The lower sediment diffusivity at the Marquesas may reflect a larger proportion of “blocky”, massive material in the central Marquesas moat or alternatively a change in the dominant process of sediment transport. While there is similar sediment supply for a given along‐moat distance at both the Marquesas and Hawaii, the underfilled moat at Hawaii is apparently a consequence of greater moat volumes due to the larger size of the Hawaiian volcanoes, and possibly variations in underplating, that load the plate. The difference in sediment/edifice ratios is likely related to the larger eruption rates at Hawaii and different styles of volcano construction between Hawaii and the Marquesas.
Magnetospheric and ionospheric signals in magnetic observatory monthly means: Electrical conductivity of the deep mantle
Authors: McLeod Malcolm G.
Published: February 2004

First differences of magnetic observatory monthly means for 1963–1982 were analyzed using techniques of spherical harmonic analysis and power spectral analysis. The external source signal is shown to be primarily zonal in geomagnetic coordinates. Prominent peaks are present in the power spectrum at frequencies of 1.0 cycle/yr and 2.0 cycles/yr. The annual signal is largest on the degree 2 external zonal spherical harmonic, while the semiannual signal is largest on the degree 1 and degree 3 external zonal spherical harmonics. The presence of the semiannual signal on odd‐degree spherical harmonics and of the annual signal on even‐degree spherical harmonics was predicted from symmetry considerations and the annual cycle of solar inclination. These signals are all modulated by the sunspot frequency and its harmonics. The degree 1 term is believed to be due mainly to magnetopause and ring currents while the degree 2 and degree 3 terms are believed to be due mainly to ionospheric currents. The degree 1 external zonal harmonic has a continuous spectrum in addition to the semiannual spectral peak. A corresponding degree 1 internal term is due to electromagnetic induction. The degree 1 continuous spectrum is useful for study of the electrical conductivity of the deep mantle. A global geomagnetic response function consistent with a mantle conductivity of about 10 S/m at the core‐mantle boundary has been derived.
Mid‐Cretaceous paleomagnetic results from Marie Byrd Land, West Antarctica: A test of post‐100 Ma relative motion between East and West Antarctica
Authors: DiVenere V. J. , Kent D. V. , Dalziel I. W. D.
Published: February 2004

As part of the tripartite, United States ‐ United Kingdom ‐ New Zealand, 1990–1991 South Pacific Rim International Tectonics Expedition, oriented samples were collected for paleomagnetic analysis from mid‐Cretaceous (circa 100 Ma) intrusive rocks at sampling localities across 350 km of the Ruppert and Hobbs Coast area of Marie Byrd Land, West Antarctica. Paleomagnetic results are presented along with several lines of evidence, including a positive tilt test based on the attitude of circa 117 Ma volcanic rocks that the circa 100 Ma rocks intrude, which argue that these results are a representative estimate of the mid‐Cretaceous magnetic field in Marie Byrd Land (MBL). The new circa 100 Ma mean south pole (224.1°E/75.7°S, A95 = 3.8°, N = 19 site means) is concordant with other West Antarctic results of similar age implying that at least Marie Byrd Land, Thurston Island and the Antarctic Peninsula have not experienced any paleomagnetically resolvable relative motion since the mid‐Cretaceous. However, the poles from these Pacific‐bordering blocks of West Antarctica are significantly offset from a synthetic apparent polar wander path that was produced for the East Antarctic craton, implying relative movement between East Antarctica and Pacific West Antarctica since about 100 Ma. Though the paleomagnetic estimate for east‐west Antarctic relative motion may be reconciled with geologic estimates for extension in the Ross Sea at the extremes of the error envelope, the best paleomagnetic estimate of relative motion suggests a larger amount of total extension between East and West Antarctica (MBL) than previously suspected. Both estimates call for several hundreds of kilometers of post‐100 Ma displacement between East Antarctica and the Pacific‐bordering blocks of West Antarctica.
Sensitivity of convection with an endothermic phase change to the form of governing equations, initial conditions, boundary conditions, and equation of state
Authors: Ita Joel , King Scott D.
Published: February 2004

Recent convection calculations have demonstrated that an endothermic phase transition can greatly decrease the vertical flow through the transition in a convecting system, in some cases leading to a layered flow. Using reasonable estimates of both the Rayleigh number and Clapeyron slope of the spinel to perovskite plus magnesiowüstite phase change, these results suggest that the 670‐km phase change has a strong effect on mantle convection. This so‐called “dynamic layering” phenomenon is further investigated with a compressible finite element code using a two‐dimensional, Cartesian geometry. We find a weak sensitivity of the pattern of flow to the form of the equations, considering Boussinesq, extended Boussinesq, and anelastic compressible forms of the governing equations, assuming that the thermodynamic properties (thermal expansivity, heat capacity, and latent heat) remain constant. The pattern of flow, however, depends strongly on the initial conditions, boundary conditions and equation of state. We compare the simple equation‐of‐state formulations used in previous work with a self‐consistent equation of state based on Debye and Birch‐Murnaghan finite strain theory under a Mie‐Grüneisen formulation. A thermal expansion coefficient that decreases monotonically with depth and is unaffected by changes in phase or temperature greatly enhances dynamic layering. This trend is reversed when the temperature, pressure, and phase dependence of thermodynamic properties such as thermal expansivity, entropy, and heat capacity is introduced. At moderate Rayleigh numbers, the pattern of the flow is strongly influenced by the pattern of the initial condition (i.e., the location of upwellings and downwellings); however, it is not sensitive to the thickness of the initial thermal boundary layers. The sensitivity of the flow to the pattern of the initial condition can potentially bias mass fluxes, especially for moderate Rayleigh number calculations.
The relationship between Late Cenozoic tectonic events and stress field and basin development in northeast Japan
Authors: Sato Hiroshi
Published: February 2004

The regional tectonic stress field, basin development, and crustal deformation of the NE Japan arc in the interval between 32 Ma to the Quaternary can be synthesized based on dike, vein, and fault orientation data, as well as on the compilation of the regional geology. An extensional stress field became prevalent from 32 Ma, and major normal faulting started at 25–20 Ma, which resulted from back arc rifting. Normal faulting, trending nearly parallel to the arc, propagated from the present Japan Sea coast to the forearc side, following the trenchward migration of the main volcanic field. From 20 to 15 Ma, normal faults with an oblique trend to the arc developed due to the counterclockwise rotation of NE Japan. The rapid clockwise rotation of SW Japan since 16 Ma produced a NW‐SE directed transtensional stress regime in the NE Japan arc. Due to crustal stretching associated with this pull‐apart movement, the back arc side of the NE Japan arc subsided rapidly to middle bathyal environments. After the termination of the opening of the Japan Sea at about 14 Ma, a neutral stress regime prevailed, which included phases of both weak extension and compression. Lithospheric cooling eventually led to thermal subsidence of the back arc region, and igneous underplating caused uplift of the axial zone of the volcanic arc. The increase in velocity of the westward motion of the Pacific plate at around 4 Ma produced strong compression across the arc, reactivating most of the Miocene normal faults, and uplifting the volcanic arc. The greatest crustal shortening occurred in areas that were stretched the most in the Miocene within the volcanic arc, which implies tectonic inversion.
Modeling of dynamic uplift, denudation rates, and thermomechanical consequences of erosion in isostatically compensated mountain belts
Authors: Zhou Shaohua , Stüwe Kurt
Published: February 2004

In this paper we present a simple one‐dimensional model to examine the interplay between dynamic uplift and denudation in response to simultaneous surface erosion and lithospheric deformation under the assumption that the surface elevation of a mountain belt is isostatically compensated and the lithosphere undergoes vertically homogeneous deformation. We also discuss the dynamics of isostatically compensated mountain belts in relation to surface erosion, based on a ‘Brace‐Goetze’ lithospheric model consisting of a quartz‐dominated crust and an olivine‐dominated upper mantle. The mechanical strength of such a lithosphere is governed by a combination of brittle failure and power law or Dorn law creep. Assuming a simple linear dependence of erosion rate on elevation, we find that contrary to previous suggestions, the effect of high denudation rates on the mechanical strength of the lithosphere does not involve significant weakening of the lithosphere.
Submarine lavas from Mauna Kea Volcano, Hawaii: Implications for Hawaiian shield stage processes
Authors: Yang Huai‐Jen , Frey Frederick A. , Garcia Michael O. , Clague David A.
Published: February 2004

The island of Hawaii is composed of five voluminous shields but only the youngest, active and well‐exposed shields of Mauna Loa and Kilauea have been studied in detail. The shield lavas forming Kohala, Hualalai and Mauna Kea are largely covered by postshield lavas with geochemical characteristics that differ from the shield lavas. In order to determine the geochemical characteristics of the Mauna Kea shield which is adjacent to the Kilauea and Mauna Loa shields, 12 Mauna Kea shield basalts dredged from the submarine east rift were analyzed for major and trace element contents and isotopic (Sr, Nd, and Pb) ratios. The lavas are MgO‐rich (11 to 20%), submarine erupted, tholeiitic basalts, but they are not representative of crystallized MgO‐rich melts. Their whole rock and mineral compositions are consistent with mixing of an evolved magma, <7% MgO, with a magma containing abundant olivine xenocrysts, probably disaggregated from a dunitic cumulate. At a given MgO content, some of the Mauna Kea whole rocks have lower abundances of CaO and higher abundances of incompatible elements. The evolved melt component in these lavas reflects significant fractionation of plagioclase and clinopyroxene and in some cases even the late crystallizing phases orthopyroxene and Fe‐Ti oxide. Although these Mauna Kea lavas are not isotopically homogeneous, in general their Sr, Nd and Pb isotopic ratios overlap with the fields for lavas from Loihi and Kilauea volcanoes. Lavas from these three shields define the low 87Sr/86Sr, high 206Pb/204Pb, and low Zr/Nb ranges for Hawaiian shield lavas, and they may contain a larger proportion of a component with high 238U/204Pb. Most of these Mauna Kea shield lavas, however, are distinguished from Kilauea and Loihi tholeiitic basalts by having lower La/Ce and La/Yb. These data provide additional evidence that each Hawaiian shield has distinctive geochemical characteristics.
Young mafic back arc volcanic rocks as indicators of continental lithospheric delamination beneath the Argentine Puna Plateau, central Andes
Authors: Kay Suzanne Mahlburg , Coira Beatriz , Viramonte Jose
Published: February 2004

The spatial distribution of some major and trace element and isotopic characteristics of backarc Plio‐Quaternary basaltic to high‐Mg andesitic (51% to 58% SiO2) lavas in the southern Puna (24°S to 27°S) of the Central Andean Volcanic Zone (CVZ) reflect varying continental lithospheric thickness and the thermal state of the underlying mantle wedge and subducting plate. These lavas erupted from small cones and fissures associated with faults related to a change in the regional stress system in the southern Puna at ≈ 2 to 3 Ma. Three geochemical groups are recognized: (1) a relatively high volume intraplate group (high K; La/Ta ratio <25) that occurs over a thin continental lithosphere above a gap in the modern seismic zone and represents the highest percentage of mantle partial melt, (2) an intermediate volume, high‐K calc‐alkaline group ( La/Ta ratio >25) that occurs over intermediate thickness lithosphere on the margins of the seismic gap and behind the main CVZ and represents an intermediate percentage of mantle partial melt, and (3) a small‐volume shoshonitic group (very high K) that occurs over relatively thick continental lithosphere in the northeast Puna and Altiplano and represents a very small percentage of mantle partial melt. Mantle‐generated characteristics of these lavas are partially overprinted by mixing with melts of the overlying thickened crust as shown by the presence of quartz and feldspar xenocrysts, negative Eu anomalies (Eu/Eu < 0.90; most < 0.80), and radiogenic Sr (> 0.7055) and Pb and nonradiogenic Nd ( εNd < −0.4) isotopic ratios. Mixing calculations show that the lavas generally contain more than 20% to 25% crustal melt. The eruption of the intraplate group mafic lavas, the change in regional stress orientation, and the high elevation of the southern Puna are suggested to be the result of the late Pliocene mechanical delamination of a block (or blocks) of continental lithosphere (mantle and possibly lowermost crust). The loss of this lithosphere resulted in an influx of asthenosphere that caused heating of the subducting slab and yielded intraplate basic magmas that produced extensive melting at the base of the thickened crust. Heating of the subducting slab led to formation of the seismic gap and trenchward depletion of the slab component. Backarc calc‐alkaline group lavas erupted on the margins of this delaminated block, whereas shoshonitic group lavas erupted over a zone of relatively thick nondelaminated lithosphere to the north.
Structure of a continental strike‐slip fault from deep seismic reflection: Walls Boundary fault, northern British Caledonides
Authors: McBride John H.
Published: February 2004

Reprocessing of four offshore deep reflection profiles across the Walls Boundary strike‐slip fault, postulated as the northward continuation of the Great Glen fault over the Shetland platform, provides useful constraints on the lithospheric structure of a major continental transform and some insight into the manner in which strike‐slip faults deform. The reprocessing was aimed at delineating complex structure and reducing noise contamination and consisted of analyses of diffractions and amplitude variations and experimentation with seismic migration. The results demonstrate a highly variable crustal and uppermost mantle structure for the Walls Boundary fault along more than 150 km of its length. Six key observations can be made from this study: (1) as middle and lower crustal reflections approach the fault, they are truncated by, and do not continue across, the downward projection of the fault; (2) trends in reflection structure of the crust and/or uppermost mantle differ markedly from one side of the fault to the other; (3) diffractions typically are concentrated at the level of the Moho discontinuity directly beneath the fault; (4) after seismic migration, steeply dipping reflectors are imaged in places directly beneath the fault in the lower crust; (5) in one location, the Moho reflection is deflected across the fault into a narrow synform or “keel” structure which is exactly bisected by the downward projection of the fault; and (6) in a separate location, a prominent upper mantle reflector is truncated across the fault. A vertical or steeply dipping structure is deduced for the fault in much of the crust and perhaps parts of the uppermost mantle; however, more diverse reflecting structures are interpreted for the lower crust, such as the Moho deflection and steeply dipping reflectors directly beneath the fault that may have been related to a localized component of transpression and resultant crustal thickening. The presence of diffractions at the level of the Moho discontinuity where the downward projection of the fault intersects it suggests points of structural “roughness” that may be related to deformation of the Moho by late motion along the fault. Integrating the interpretation of the reflection data with previous geological studies implies that the age of the Moho deflection is post Early Cretaceous but that the dipping structure adjacent to and cut off by the fault is probably Caledonian (Silurian‐Early Devonian). Although the disparate structure of the crust on opposite sides of the fault supports geologic interpretations of large‐scale displacement, a major step in the Moho discontinuity directly beneath the Walls Boundary fault cannot be substantiated from the seismic data.
Crustal structure of a nonvolcanic rifted margin east of Newfoundland
Authors: Reid Ian D.
Published: February 2004

A detailed seismic refraction study was undertaken of the rifted continental margin southeast of the Grand Banks of Newfoundland, along a preexisting deep multichannel seismic reflection profile. The primary objectives were to determine the topography of Moho across the margin, to detect any evidence for crustal underplating, and to determine the position of the continent‐ocean boundary. Data were obtained from nine ocean bottom seismometers deployed along the profile, with an array of six 1000‐inch3 air guns as the source. Two additional refraction lines were shot parallel to structure over the continental slope. The resultant velocity model is well constrained and consistent with the observed gravity. The crust thins abruptly beneath the continental slope to a major structural transition identified as the continent‐ocean boundary. There is no significant magmatic underplating of the thinned continental crust. Seaward of the transition, the seismic velocity structure is characterized by a 50‐km‐wide zone of intermediate velocity (7.2–7.6 km/s). Beyond this is a thin (3–5 km) crust with low velocity (4.5–5 km/s) and high gradient, over mantle with a velocity of 7.7 km/s increasing to 8.0 km/s. The zone of intermediate velocity is interpreted to be highly serpentinized mantle material beneath a very thin crust, suggesting little or no magmatic activity during and immediately after rifting. This interpretation is consistent with results from adjacent and conjugate margins.