bluuebunny/crossref_metadata_2025 huggingface dataset:
Crustal structure of the Kolbeinsey Ridge, North Atlantic, obtained by use of ocean bottom seismographs
Authors: Kodaira Shuichi , Mjelde Rolf , Gunnarsson Karl , Shiobara Hajime , Shimamura Hideki
Published: February 2004

We present the results of a seismic survey performed around the Kolbeinsey Ridge, North Atlantic. The seismic data were acquired along three profiles: on the ridge axis (Ll), 12 km off‐ridge (L2), and perpendicular to the ridge axis (L3), The models indicate lower velocity beneath the ridge axis than in the off‐ridge area. Along L3, we obtain velocities of 2.6–3.6 km/s (upper crust), 4.8–6.0 km/s (middle crust), and 6.6–6.9 km/s (lower crust) beneath the ridge axis, while at more than 12 km off‐ridge, the model shows velocities of 3.6–4.6 km/s (upper crust), 5.6–6.4 km/s (middle crust), and 6.9–7.2 km/s (lower crust). The model of L1 also shows significant lateral variation along the axis in the lower crust. The velocity in the lower crust decreases to 6.3 km/s in the southwestern part of Ll, where the Moho depth decreases to 7.2 km. The low‐velocity structure beneath the ridge axis is interpreted to be due to high porosity in the upper crust and to higher temperatures, as well as a few percent porosity, in the middle and lower crustal material. According to this interpretation, the crustal model of L3 implies that the cracks have been sealed and the temperatures have decreased to normal in 1.2‐m.y‐old crust. The anomalous low velocity structure along Ll can be interpreted as related to the most recent injection processes or a thermal influence of the Spar Fracture Zone situated 50 km south of the profile.
Missing reversals in the geomagnetic polarity timescale: Their influence on the analysis and in constraining the process that generates geomagnetic reversals
Authors: Marzocchi W.
Published: February 2004

A major problem in defining the chronology of geomagnetic reversals is linked to the detection of short (<30 kyr) time intervals between reversals (TIBR). Published polarity timescales do not usually include the shortest TIBR; therefore the timescales are inherently incomplete. The purpose of this paper is to investigate the effects of this incompleteness on the analysis of the geomagnetic polarity timescale; the ultimate goal is to provide constraints on the Earth's core processes. The effects of inclusion/exclusion of the shortest TIBR are verified by comparing the results obtained by statistical analysis of real and synthetic series of events; for the real sequence, two basic cases are considered in which the “tiny wiggles” are attributed either to short TIBR or to paleointensity fluctuations. Particular attention is paid to the influence of measurement errors estimated for the most recently published Cenozoic timescale. By following the minimalist philosophy of Occam's razor, which is particularly suitable for studying poorly known processes, the reliability of the simplest model, i.e., the Poisson process which is symmetric in polarity, can be checked. The results indicate the plausibility of a generalized renewal process; the only regularity is relative to the long‐term trend, which is probably linked to core‐mantle coupling. In detail, a uniform exponential trend in the last 80 Myr is found for the timescale; it is not presently possible to estimate the influence of the inclusion of tiny wiggles because they are well‐resolved only in the last 30 Myr, a period in which both series are stationary. The sequences, with and without tiny wiggles, are symmetric in polarity, with no evidence of low‐dimensional chaos and memory of past configurations. The empirical statistical distribution of the TIBR departs slightly from a theoretical exponential distribution, i.e., from a Poisson process, which can be explained by a lack of short anomalies, and/or by a generating process with wear‐out properties (a more general renewal process). A real exponential distribution is sustainable only if the number of missing short TIBR in the last 30 Myr is larger than the number of tiny wiggles observed in the same period.
Effect of mantle depletion buoyancy on plume flow and melting beneath a stationary plate
Authors: Manglik A. , Christensen U. R.
Published: February 2004

We analyze a dynamical model of plume‐lithosphere interaction in two‐dimensional Cartesian geometry that takes into account the buoyancy of the depleted residue produced by melt extraction. The plume enters through the open bottom boundary at 400 km depth and leaves through the right side boundary of the model box. The viscosity is strongly pressure and temperature dependent. We use a large number of Lagrangian tracer particles to monitor progressive melting in the plume head and to track the advection of the depleted residue. The density reduction in the residue enhances small‐scale instabilities sinking from the bottom of the lithosphere into the depleted layer. Initially, the melt production rate is slightly enhanced when depletion buoyancy is taken into account. However, in the subsequent evolution, melt production rates are lowered by a factor in the range 0.5–0.65, depending on the initial thickness of the lithosphere, compared to cases without density difference of the residue. The buoyancy of the residual mantle opposes its advection away from the top of the plume. A depleted root is formed at the bottom of the lithosphere, which inhibits further thermal erosion of the plate. It forces the plume flow to stagnate at greater depth and hence reduces the melt production rate. The effect is particularly strong for a case where the plume rises beneath locally stretched lithosphere. The results are compared to the evolution of volcanism at the Cape Verde hotspot and the Kenya rift.
Gravity anomalies and spatial variations of flexural rigidity at mountain ranges
Authors: Stewart J. , Watts A. B.
Published: February 2004

New continent‐wide compilations of gravity and topography data have been used to determine spatial variations in the long‐term mechanical properties of the foreland lithosphere at mountain ranges. In the southern Appalachians, a large elastic thickness Te (in the range 40–70 km) is required with variation only along the length of the mountain range. To explain the Appalachian Bouguer gravity anomaly “high”, a subsurface load and crustal thinning are necessary. For the Andes, Te varies in the range 5–85 km perpendicular and parallel to the length of the mountain range. In the Alps, Te is high in the French Alps and then decreases in the western Swiss Alps and increases toward the eastern Alps. In the Romanian Carpathians, Te variations in the range 5–20 km are found. Although the thermal state, curvature, and inelastic yielding may play some role, these variations in Te are interpreted as the result of inheritance of the thermal and mechanical properties of foreland lithosphere. Inheritance plays a role in influencing the development of the next plate tectonic event. At mountain ranges, preexisting strength variations offer some control on tectonic style, the geometry of fold and thrust belts, and the stratigraphic patterns that develop in the adjacent foreland basin. Foreland lithosphere is involved in a cycle of events that began with continental rifting and passive margin formation and ended in continental collision. Compilations of Te from continental rifts, passive margins, and foreland lithosphere show large variations. The lowest values are associated with rifts, margins, and foreland lithosphere which has recently been been involved in continental breakup. The highest values are associated with foreland lithosphere for which a long time has elapsed since the previous rifting event. We speculate that continental lithosphere undergoes some form of strength recovery.
Determination of elastic properties of very heterogeneous media with cellular automata
Authors: Küntz M. , Lavallée P. , Mareschal J. C.
Published: February 2004

A cellular automaton is used to calculate the elastic properties of very heterogeneous media. The lattice gas method (designed to model viscous flow) is applied to elastic problems through the correspondence between the steady state velocity field in an incompressible Newtonian fluid at low Reynolds number and the displacement field in an incompressible elastic solid. The cellular automaton is applied to determine the elastic properties of a matrix containing randomly distributed hard inclusions. Because the equations of equilibrium are locally solved for any distribution of the inclusion phase, the method provides a means to estimate the influence of stress field perturbations on the macroscopic properties of heterogeneous systems. The effective elastic shear modulus is calculated as a function of the concentration C of the inclusion phase (0≤C≤1) for different values of R, the rheological contrast between the inclusion and the matrix. The numerical results are compared with those obtained by various micromechanical models. Estimates given by the cellular automaton agree with the analytical solution for a double periodic triangular system of discs. However, the effective shear modulus for the cellular automaton fluid does not fall within the limits of Hashin and Shtrikman. It follows the same trend as that of the lower bound but stays below the lower bound for all concentrations. Likewise, the shear modulus for a cellular automaton fluid is also systematically lower than that predicted by the differential method, although the trends are similar.
Rupture properties of clustered microearthquakes near intersecting intraplate faults of New Madrid Seismic Zone: Implications for fault weakening
Authors: Xie Jiakang , Liu Zuyuan , Cong Lianli , Herrmann R. B. , Chiu J.‐M.
Published: February 2004

Seismic waveforms from the portable array for numerical data acquisition network were used to identify sets of 21 similar microearthquakes near Ridgely, Tennessee, where two major segments of the New Madrid Seismic Zone (NMSZ) intersect. The events are relocated with a precision of about 30 m and are found to have occurred along four subfaults that are aligned to N50°E, subparallel to the southern (Arkansas) segment of the NMSZ. The separations among the subfaults are a few hundred meters, while the span of hypocenters within each subfault is typically a few tens of meters. These subfaults are inferred from focal mechanism solutions to be oriented at high angles to N50°E trend, striking roughly north‐northwest and dipping moderately with reverse slip. This indicates that the subfaults belong to the central segment of the NMSZ and are en echelon. The similar events also cluster in time. The typical time span of events over one subfaults is about 3 weeks, while the intercluster times are a few months or more. An empirical Green's function analysis of the largest event placed a lower bound of its rupture dimension at 121±32 m, suggesting that this rupture overlapped with the smaller ruptures along the same subfault. On the basis of variations in fault zone properties previously inferred for the Ridgely junction, the rupture properties of the similar events and the result of a stress calculation, we infer that the similar events may be triggered by temporal weakening of the subfaults, possibly associated with a temporal elevation of pore fluid pressure. The nearly constant depths (∼5.7 km) and the reverse dip of the ruptures suggest that the en echelon subfaults may be intersected by subhorizontal veins, which could have been developed by episodic influx of fluid at superhydrostatic pressures.
Anisotropic scattering and travel time delay analysis in Kilauea volcano, Hawaii, earthquake coda waves
Authors: Got Jean‐Luc , Coutant Olivier
Published: February 2004

We study the S wave coda of microearthquake multiplets (earthquakes showing similar seismograms) recorded on the Kilauea volcano, and we analyze the direction of emission for wavelets composing the coda. Our data set consists of 71 small magnitude, well‐relocated, clustered events that occurred in an area of about 1 km2 in the southern flank of Kilauea, recorded at hypocentral distances of 10 to 20 km. For each station, we apply a cross‐spectral moving‐window technique to each pair of multiplet records to estimate the travel time delays due to source location offsets. These delays are computed for consecutive windows, starting at the P wave arrival time and ending at 4 times the S wave travel time. They are next used to estimate the slowness vector for each window along the seismograms. Our analysis shows (1) that scattered waves are emitted anisotropically along some preferred directions as late as 4 times the S wave travel time, and (2) that for some stations, the azimuthal plane rotates during the seismogram with a clear travel time delay signature. Our analysis shows that travel time delay variation in the coda can be purely due to geometrical propagation effects. This conclusion is of special interest when trying to use travel time delays to infer temporal changes in the crust. Our results do not support a coda diffracted isotropically in the crust, in the magnitude range and at the hypocentral distances we investigated.
Crustal implications of bedrock geology along the Trans‐Alaska Crustal Transect (TACT) in the Brooks Range, northern Alaska
Authors: Moore Thomas E. , Wallace Wesley K. , Mull Charles G. , Adams Karen E. , Plafker George , Nokleberg Warren J.
Published: February 2004

Geologic mapping of the Trans‐Alaska Crustal Transect (TACT) project along the Dalton Highway in northern Alaska indicates that the Endicott Mountains allochthon and the Hammond terrane compose a combined allochthon that was thrust northward at least 90 km in the Early Cretaceous. The basal thrust of the combined allochthon climbs up section in the hanging wall from a ductile shear zone in the south through lower Paleozoic rocks of the Hammond terrane and into Upper Devonian rocks of the Endicott Mountains allochthon at the Mount Doonerak antiform, culminating in Early Cretaceous shale in the northern foothills of the Brooks Range. Footwall rocks north of the Mount Doonerak antiform are everywhere parautochthonous Permian and Triassic shale of the North Slope terrane rather than Jurassic and Lower Cretaceous strata of the Colville Basin as shown in most other tectonic models of the central Brooks Range. Stratigraphic and structural relations suggest that this thrust was the basal detachment for Early Cretaceous deformation. Younger structures, such as the Tertiary Mount Doonerak antiform, deform the Early Cretaceous structures and are cored by thrusts that root at a depth of about 10 to 30 km along a deeper detachment than the Early Cretaceous detachment. The Brooks Range, therefore, exposes (1) an Early Cretaceous thin‐skinned deformational belt developed during arc‐continent collision and (2) a mainly Tertiary thick‐skinned orogen that is probabty the northward continuation of the Rocky Mountains orogenic belt. A down‐to‐the‐south zone of both ductile and brittle normal faulting along the southern margin of the Brooks Range probably formed in the mid‐Cretaceous by extensional exhumation of the Early Cretaceous contractional deformation.
Composition of the crust in the Grenville and Appalachian Provinces of North America inferred from VP/VS ratios
Authors: Musacchio Gemma , Mooney Walter D. , Luetgert James H. , Christensen Nikolas I.
Published: February 2004

We use the ratios between P and S wave velocities (VP/VS), derived from seismic refraction data, to infer the composition of the crust in the Grenville and the Appalachian Provinces of North America. The crust exhibits VP/VS increasing with depth from 1.64 to 1.84; there is a clear distinction between the Grenville Province (average VP/VS=1.81) and the Appalachian Province (average VP/VS=1.73) which persists at all depths. The boundary between these provinces is east dipping extending for 100 km east of the Champlain thrust. In the Appalachian Province the increase in VP/VS ratios with depth from 1.67 to 1.74±0.02 may reflect a normal decrease of silica content in the continental crust. In the Grenville Province beneath the Central Granulite Terrane, an anomalous VP/VS ratio of 1.82±0.02 is observed extending to a depth of 10 km; this correlates with the abundance of Ca‐plagioclase in the Marcy Anorthosite. At greater depth (15–20 km), where seismic lamination and high electrical conductivity is observed, VP/VS is 1.84±0.02 and correlates with the Tahawus Complex, a layered mafic intrusion. Within the 25‐km‐thick lower crust of the Grenville Province the VP/VS is 1.84±0.02 and P‐velocity is 7.0±0.1 km/s, which are typical for plagioclase‐bearing rocks (gabbro‐norite). The high VP/VS ratio in the Grenville Province has not been reported in crust of any other age. Since the Grenville Province contains 75% of the world's known anorthosites, high VP/VS ratio is related to high plagioclase. We suggest that the composition of the Grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid‐Proterozoic.
Backscattered coherent noise and seismic reflection imaging of the oceanic crust: An example from the rift valley of the Mid‐Atlantic Ridge at 23°N
Authors: Calvert A. J.
Published: February 2004

Seismic reflection surveys shot over oceanic crust are contaminated by source‐generated coherent noise that limits the identification and interpretation of subbasement reflections. Coherent noise arrivals are generated around the seismic line by scattering from the upper surface of the basaltic layer and by backscattering of turning waves from the underside of the same interface; both types of arrival may be enhanced by conventional common midpoint (CMP) stacking. Some turning arrivals also exhibit negative moveout in a CMP gather due to the increase of crustal velocity with depth. In unsedimented areas, coherent arrivals scattered from the top of the seafloor can be aligned by dip moveout, with or without a subsequent zero‐offset migration prestack, and suppressed, but turning arrivals will usually remain. Turning arrivals generated by linear features in the seafloor, such as fault scarps, appear on a stack section as nearly linear coherent arrivals, the apparent dip of which is related to the features' angle to the seismic line. After migration, these turning wave arrivals can extend close to the topography that produced them and continue to late times, leading to the possibility that such arrivals might be incorrectly interpreted as intrusions that crosscut, but do not offset, the Moho. Careful identification of crustal reflections on a seismic profile along the MARK area rift valley indicates that the Snake Pit hydrothermal field marks the northern limit of identifiable crustal reflections, at least some of which appear to be related to magma injection. The recent extensive volcanism of the northern MARK area may well be a result of faulting caused by the stong northward growth of the southern spreading center, permitting melt to rise more easily to shallow crustal levels.
Färoe‐Iceland Ridge Experiment 2. Crustal structure of the Krafla central volcano
Authors: Brandsdóttir Bryndís , Menke William , Einarsson Páll , White Robert S. , Staples Robert K.
Published: February 2004

The seismic velocity structure of the Krafla central volcano is characterized by large variations in compressional velocity. A 40 km wide high‐velocity dome extends from the lower crust (11–14 km depth) beneath the volcano narrowing upward. A magma chamber sits at its top near 3 km depth. It is defined by both 0.2–0.3 s compressional wave delays and shear wave shadowing to be 2–3 km N‐S, 8–10 km E‐W, and 0.7–1.8 km thick. The near‐surface structure (uppermost 2.5 km) of the Krafla caldera is approximately flat‐lying, with only minor lateral heterogeneities. The crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities. Shear waves, reflected from a 19 km deep Moho, are clearly visible for some paths through the crustal volume below the magma chamber, even though the more shallow diving S waves are severely attenuated. The midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near‐solidus temperatures. The Krafla central volcano plays a major role in crustal genesis along the plate boundary. The high‐velocity dome, in our view, represents crust generated in and around the magma chamber, which has subsequently been advected to greater depths.
Seismic anisotropy in the eastern United States: Deep structure of a complex continental plate
Authors: Barruol Guilhem , Silver Paul G. , Vauchez Alain
Published: February 2004

We have analyzed shear wave splitting recorded by portable and permanent broadband and long‐period stations located in the eastern United States. Teleseismic shear waves (SKS, SKKS, and PKS) were used to retrieve the splitting parameters: the orientation of the fast wave polarization plane ϕ and the delay time δt. In total, 120 seismic events were processed, allowing for more than 600 splitting measurements. Within the Appalachians, stations located in the western (external) part are characterized by δt≈1s and ϕ trending N50°– 70°E in the south and central regions and N30°–40°E in the north, closely following the trend of the orogenic belt in these areas. The transition region between north and central is characterized by δt≈1–1.3 s and by E‐W trending ϕ that are at a high angle to the regional geologic trend. Measurements at two stations located in the eastern (internal) part of the belt indicate very weak anisotropy. The large‐scale pattern of anisotropy is not consistent with that predicted for simple asthenospheric flow beneath the plate. Splitting along the southern and eastern margins of the continent is consistent with that expected for Grenvillian deformation, an alternative model of asthenospheric flow around the cratonic keel cannot be ruled out. Within the cratonic core, the correlation between δt and lithospheric thickness suggests a lithospheric anisotropy. Smaller‐length‐scale variations also argue for a significant contribution of lithospheric structures. The fabric responsible for shear wave splitting may have formed during tectonic episodes that affected the eastern United States, i.e., the Grenville and Appalachian orogenies and the subsequent rifting of the North Atlantic Ocean. Our observations in the western Appalachians suggest that the anisotropy may be preserved since the Grenvillian orogeny. The absence of detectable splitting in the two stations in the eastern Appalachians is attributed to the igneous intrusions related to the Atlantic rifting. The measurements in the transition between the northern and central southern Appalachians, constitute an intriguing anomaly, whose E‐W ϕ have little obvious relation to the regional surface geology. We suggest two possible causes: (1) the local dominance of asthenospheric flow, motivated by the proximity of a pervasive low‐velocity anomaly and (2) lithospheric deformation in a transcontinental strike‐slip fault zone active during the Appalachian collision.
Origin and nature of crystal reflections: Results from integrated seismic measurements at the KTB superdeep drilling site
Authors: Harjes H.‐P. , Bram K. , Dürbaum H.‐J. , Gebrande H. , Hirschmann G. , Janik M. , Klöckner M. , Lüschen E. , Rabbel W. , Simon M. , Thomas R. , Tormann J. , Wenzel F.
Published: February 2004

For almost 10 years the KTB superdeep drilling project has offered an excellent field laboratory for adapting seismic techniques to crystalline environments and for testing new ideas for interpreting seismic reflections in terms of lithological or textural properties of metamorphic rock units. The seismic investigations culminated in a three‐dimensional (3‐D) reflection survey on a 19×19 km area with the drill site at its center. Interpretation of these data resulted in a detailed, structural model of the German Continental Deep Drilling Program (KTB) location with dominant, steep faults in the upper crust. The 3‐D reflection survey was part of a suite of seismic experiments, ranging from wide‐angle reflection and refraction profiles to standard vertical seismic profiles (VSP) and more sophisticated surface‐to‐borehole observations. It was predicted that the drill bit would meet the most prominent, steeply dipping, crustal reflector at a depth of about 6500–7000 m, and indeed, the borehole penetrated a major fault zone in the depth interval between 6850 and 7300 m. This reflector offered the rare opportunity to relate logging results, reflective properties, and geology to observed and modeled data. Post‐Variscan thrusting caused cataclastic deformation, with partial, strong alterations within a steeply dipping reverse fault zone. This process generated impedance contrasts within the fault zone on a lateral scale large enough to cause seismic reflections. This was confirmed by borehole measurements along the whole 9.1 km deep KTB profile. The strongest, reflected signals originated from fluid‐filled fractures and cataclastic fracture zones rather than from lithological boundaries (i.e., first‐order discontinuities between different rock types) or from texture‐ and/or foliation‐induced anisotropy. During the interpretation of seismic data at KTB several lessons were learned: Conventional processing of two‐dimensional (2‐D) reflection data from a presite survey showed predominantly subhorizontal layering in the upper crust with reflectivity striking in the Variscan direction. Drilling, however, revealed that all rock units are steeply dipping. This confirms that surface common depth point (CDP) seismics strongly enhances subhorizontal reflectivity and may thus produce a very misleading crustal image. Although this was shown for synthetic examples earlier, the KTB provides the experimental proof of how crucial this insight can be.
Coherence of signals from magnetometers on parallel paths
Authors: Parker Robert L.
Published: February 2004

During a recent marine magnetic survey of the Juan de Fuca Rise, two magnetometers were towed near the seafloor, one about 300 m above the other. To understand how to interpret the records, we investigate a simple statistical model: two magnetometers moving on parallel paths above a statistically stationary source, with known spectrum. Magnetometers on paths normal to perfectly lineated magnetic anomalies will measure signals that have unit coherence at all wavelengths. Departure of the system from this ideal state can be diagnosed by a lower coherence, and something about the across‐track structure can be learned from the shape of the coherence spectrum. We calculate the power and cross spectra of the profile signals in terms of the two‐dimensional power spectrum of the field just above the source region; hence we obtain the coherence and phase spectra. For the special case of a white source spectrum we find surprisingly high coherences. A set of inequalities between the spectral estimates is derived and can be used to check the consistency of the measured signals with the model assumptions. The theory is applied to a magnetic traverse of the Juan de Fuca Rise when two near‐bottom magnetometers were deployed. The key results are these: in the wavelength range above about 1 km the observed coherency is substantially higher than that from the disordered field model, consistent with the highly lineated structures observed at the surface over all ocean ridge systems. On scales between 500 m and 1 km the coherence falls to levels indistinguishable from those given by an isotropic flat spectrum, implying that on these scales there is little or no across‐track lineation. This finding means that the resolution of paleomagnetic field behavior based on seafloor data in this area is no better than 36,000 years.
U.S. mid‐Atlantic margin structure and early thermal evolution
Authors: Lizarralde Daniel , Holbrook W. Steven
Published: February 2004

Marine seismic experiments have revealed that the U.S. east coast rifted margin crustal structure is dominated by voluminous volcanics emplaced during continental breakup. We present results from seismic data recorded onshore during the 1990 EDGE mid‐Atlantic onshore/offshore seismic experiment that extend the margin crustal profile westward 180 km across the coastal plain and place the offshore results into a broader tectonic context. The onshore/offshore crustal model defines the crustal thinning profile across the margin, enabling calculation of total extension based on direct measurements of crustal thickness. We find that the crust beneath the coastal plain is largely unextended, has a uniform thickness of ∼35 km, shows no evidence for magmatic additions of sufficient volume to affect seismic velocity, and that the total half extension of the crust is 30 to 40 km, less than previous estimates by a factor of 3 to 4. We also present results of depth migration of the offshore vertical incidence and wide‐angle seismic data that yield the most complete and accurate image to date of reflectivity across the U.S. east coast margin. We use this image to analyze margin subsidence by exploiting the stratigraphy of the subaerially extruded basalts beneath the sediments. We find that (1) margin subsidence is consistent with a petrogenetic model for initial seafloor‐spreading magmatism involving high average melting pressures (3.5–4.0 GPa) and potential temperatures (>1500°C) for a present‐day thermal lid thickness of ∼225 km; (2) it is difficult to explain margin subsidence with a present‐day lid thickness of much less than ∼150 km; and (3) rapid subsidence following breakup suggests the initially hot mantle underlying the rifting margin cooled very quickly, implying a limited and finite initial distribution of hot mantle, rather than a continuous upwelling or a ubiquitously warm upper mantle.
A sensitivity study of three‐dimensional spherical mantle convection at 108 Rayleigh number: Effects of depth‐dependent viscosity, heating mode, and an endothermic phase change
Authors: Bunge Hans‐Peter , Richards Mark A. , Baumgardner John R.
Published: February 2004

Mantle convection is influenced simultaneously by a number of physical effects: brittle failure in the surface plates, strongly variable viscosity, mineral phase changes, and both internal heating (radioactivity) and bottom heating from the core. Here we present a systematic study of three potentially important effects: depth‐dependent viscosity, an endothermic phase change, and bottom versus internal heating. We model three‐dimensional spherical convection at Rayleigh Ra=108 thus approaching the dynamical regime of the mantle. An isoviscous, internally heated reference model displays point‐like downwellings from the cold upper boundary layer, a blue spectrum of thermal heterogeneity, and small but rapid time variations in flow diagnostics. A modest factor 30 increase in lower mantle viscosity results in a planform dominated by long, linear downwellings, a red spectrum, and great temporal stability. Bottom heating has the predictable effect of adding a thermal boundary layer at the base of the mantle. We use a Clapeyron slope of γ=−4 MPa °K−1 for the 670 km phase transition, resulting in a phase buoyancy parameter of P=−0.112. This phase change causes upwellings and downwellings to pause in the transition zone but has little influence on the inherent time dependence of flow and only a modest reddening effect on the heterogeneity spectrum. Larger values of P result in stronger effects, but our choice of P is likely already too large to be representative of the mantle transition zone. Combinations of all three effects are remarkably predictable in terms of the single‐effect models, and the effect of depth‐dependent viscosity is found to be dominant.
Gravity and magnetic studies of the subglacial Grímsvötn volcano, Iceland: Implications for crustal and thermal structure
Authors: Gudmundsson Magnús T. , Milsom John
Published: February 2004

The highly active, subglacial, Grímsvötn volcano is associated with a 20‐mGal Bouguer gravity high and a broad magnetic low. The magnetic low roughly coincides with the gravity high, but short‐wavelength magnetic highs are superimposed on the low, especially along caldera rims. Forward models of the gravity and magnetic fields show variable magnetization (0–12 A m−1) in the uppermost 1–2 km of the crust, probably reflecting differences in the proportions of crystalline rocks and hyaloclastites and variations in hydrothermal alteration. The models are consistent with, but do not require, a 10‐km3 molten magma chamber in the shallow crust beneath the main caldera. Between 1.5 and 4 km depth, a dense body (ρ = 2.95–3.0 Mg m−3) with a volume of 400 km3 is located, probably composed of gabbro. Modeling suggests that this intrusive body is nonmagnetic, which may imply that its temperature is at or above the Curie point. The distribution of geothermal activity at the glacier bed is consistent with heat extraction from this dense, hot intrusive body; it may be an important source for the presently observed geothermal heat output of 2000–4000 MW at Grímsvötn. This high heat transfer rate is difficult to reconcile with estimates of long‐term heat production in an Icelandic rift zone volcano. The heat flux may be a transient phenomenon, caused by enhanced vertical permeability due to a recent caldera collapse and subsequent heat mining of the intrusive body.
Tertiary diachronic extrusion and deformation of western Indochina: Structural and 40Ar/39Ar evidence from NW Thailand
Authors: Lacassin Robin , Maluski Henri , Leloup P. Hervé , Tapponnier Paul , Hinthong Chaiyan , Siribhakdi Kanchit , Chuaviroj Saengathit , Charoenravat Adul
Published: February 2004

The Wang Chao and Three Pagodas fault zones cut the western part of the Indochina block and run parallel to the Red River Fault. Evidence of intense ductile left‐lateral shear is found in the Lansang gneisses, which form a 5 km wide elongated core along the Wang Chao fault zone. Dating by 40Ar/39Ar shows that such deformation probably terminated around 30.5 Ma. The Wang Chao and Three Pagodas faults offset the north striking lower Mesozoic metamorphic and magmatic belt of northern Thailand. 40Ar/39Ar results suggest that this belt suffered rapid cooling in the Tertiary, probably around 23 Ma. These results imply that the extrusion of the southwestern part of Indochina occurred in the upper Eocene‐lower Oligocene. It probably induced rifting in some basins of the Gulf of Thailand and in the Malay and Mekong basins. In the Oligo‐Miocene, the continuing penetration of India into Asia culminated with the extrusion of all of Indochina along the Ailao Shan‐Red River fault. This occurred concurrently with the onset of E‐W extension more to the south. Plotting in a geographical reference frame the diachronic time spans of movement on left‐lateral faults east and southeast of Tibet implies that the northward movement of the Indian indenter successively initiated new strike‐slip faults located farther and farther north along its path.
A model of rift basin evolution constrained by first‐order stratigraphic observations
Authors: Contreras Juan , Scholz Christopher H. , King Geoffrey C. P.
Published: February 2004

The stratigraphy of many nonmarine rift basins records a common sedimentological history consisting of a relatively abrupt increase of water depth followed by a slower shoaling upward trend. We explore in this paper the origin of these sequences through a three‐dimensional physical model that incorporates the affects of fault growth, flexure, erosion, sedimentation, and isostasy. Fault growth is assumed to occur in a self‐similar manner and is treated as dislocations inducing flexure in a thin elastic plate. Topography generated by faulting and flexure is allowed to undergo diffusion to simulate erosive and sedimentary processes. Isostatic response to the redistribution of mass results in additional flexure of the thin plate. The model is tested using two models of fault growth, and we compare the numerical results with the stratigraphy observed in nonmarine rifts. Basins generated using a constant‐lengthening‐rate mode of fault growth fail to reproduce the stratigraphy of continental rift basins. We find that the commonly observed stratigraphic succession of lacustrine to fluvial conditions requires the displacement rate of the bounding fault to decrease with time, consistent with opening at a relatively constant areal extension rate.
A successful rock magnetic technique for correctng paleomagnetic inclination shallowing: Case study of the Nacimiento Formation, New Mexico
Authors: Kodama K. P.
Published: February 2004

The accuracy of an inclination shallowing correction technique was tested on the remanence of the Paleocene Nacimiento Formation. The correction assumes that changes in paleomagnetic inclination during deformation will be directly related to changes in remanence anisotropy. The remanence anisotropy is also a function of the magnetic anisotropy of the individual magnetic grains. The Nacimiento Formation was selected for this test because its inclination is shallow by 7° to 8° and previous paleomagnetic studies indicate that it has a primary remanence with a well‐constrained age and that magnetic overprints are minimal. Tectonics are an unlikely explanation for its shallow inclination. These conditions allow the inclination‐corrected remanence to be compared to a well‐defined paleomagnetic pole to determine the accuracy of the technique. The characteristic remanence from 20 Nacimiento sites was isolated by alternating field and thermal demagnetization and was corrected using the anisotropy of anhysteretic remanence measured for three to five samples per site. Individual particle anisotropy was determined by drying a magnetic separate/epoxy mixture in a 35–50 mT magnetic field. Laboratory compaction of disaggregated Nacimiento material was also used to yield an effective individual magnetic particle anisotropy. The two techniques resulted in similar values for individual particle anisotropy and similar inclination shallowing corrections. Both corrections were successful, and the inclination‐corrected formation mean direction was indistinguishable from the direction predicted by North America's Paleocene paleopole. The inclination shallowing correction technique reported here should be applied routinely in paleomagnetic studies, provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes (e.g., compaction).
Arc‐like mid‐ocean ridge basalt formed seaward of a trench‐forearc system just prior to ridge subduction: An example from subaccreted ophiolites in southern Alaska
Authors: Lytwyn Jennifer , Casey John , Gilbert Susan , Kusky Timothy
Published: February 2004

The dismembered Resurrection Peninsula and Knight Island ophiolites, south‐central Alaska, contain pillow basalts interbedded with flysch, indicating that these ophiolites may have formed along an oceanic spreading center proximal to a continental margin and may be relicts of early Tertiary subduction of the Kula‐Farallon ridge beneath North America. New major, minor and trace element data for Resurrection and Knight Island pillow basalts and sheeted dikes support a near‐trench origin and ridge subduction model. The rare earth element abundances are broadly similar to transitional (T‐type) mid‐ocean ridge basalt, but extended trace element patterns show high field strength elemental characteristics and alkali enrichments more typical of calc‐alkaline basalts. The lavas and dikes exhibit variable trace element abundances that cannot be related to a common parental magma composition and appear to indicate multiple sources. The multiple parental magmas can be modeled through mixing of compositionally diverse, near‐instantaneous melts derived through near‐fractional (critical) melting of variably depleted mantle source within a subaxial melting column. The unusual calc‐alkaline overprint may reflect variable assimilation of continentally derived, flyschoid sedimentary material from a nearby accretionary prism below which an asthenospheric window opened as the result of ridge subduction. Delivery of the arc‐like assimilant products to the ridge axis in front of the trench may occur by buoyancy driven flow of melt upward and laterally along the base of the accretionary prism. Flow would occur within a channelized zone corresponding to the asthenospheric window (or melting column) that opened during ridge subduction below the accretionary prism. East of Knight Island, metabasic pillow lavas, breccias, and dikes within the Valdez/Orca accretionary complex are compositionally similar to refractory island‐arc tholeiites and are possibly products of parental liquids formed through inefficient mixing of near‐instantaneous melts derived primarily from depleted mantle sources. The Valdez/Orca metabasalts may have been emplaced directly into the accretionary prism forearc environment as refractory melts generated near the top of the asthenospheric window and were possibly associated with the same Kula‐Farallon ridge system that produced the Resurrection and Knight Island ophiolites.
Chemical plumes from low‐temperature hydrothermal venting on the eastern flank of the Juan de Fuca Ridge
Authors: Wheat C. Geoffrey , Mottl Michael J. , Baker Edward T. , Feely Richard A. , Lupton John E. , Sansone Francis J. , Resing Joseph A. , Lebon Geoff T. , Becker Nathan C.
Published: February 2004

We report evidence for chemical anomalies in the water column from low‐temperature ridge‐flank hydrothermal venting. During cruises in 1992 and 1994, samples were taken from the water column for trace metals, nutrients, dissolved gases, and particles near each of three basaltic outcrops overlying 3.5 m. y. old crust on the eastern flank of the Juan de Fuca Ridge in Cascadia Basin. The water column above one of these outcrops, Baby Bare, which rises about 70 m above a flat turbidite plain, was the most thoroughly sampled. Thermal, chemical (Mn, Fe, δ(3He)%, CH4, and O2), and particulate anomalies in the water column confirm the existence of (1) early diagenesis of organic matter in seafloor sediment which produces a flux of dissolved metals and nutrients to bottom seawater, (2) hydrothermal emissions which are both focused (spring‐like) and diffuse, and (3) resuspension of sediment by turbulent flow of tidal currents about a topographical high. On the basis of data from the water column and thermal and chemical pore water data from 46 piston and gravity sediment cores near and on Baby Bare (FlankFlux 90 and 92), we constrain the composition of seawater in basement and thus the composition of spring‐like water. Given this composition, no measurable dissolved silica or phosphate hydrothermal anomalies are expected in the water column.
Extension across a divergent plate boundary, the Eastern Volcanic Rift Zone, south Iceland, 1967–1994, observed with GPS and electronic distance measurements
Authors: Jónsson Sigurjón , Einarsson Páll , Sigmundsson Freysteinn
Published: February 2004

The average spreading rate in south Iceland, 19 mm/yr, is distributed over two parallel rift zones. We measured a Global Positioning System (GPS) network of 42 stations in the eastern zone in 1994. This network consists of stations measured with GPS in 1986 and 1992, stations in a 60 km long distance profile measured several times since 1967, and a few new stations. The 1994 GPS data were processed using the Bernese software version 3.5, and the average position uncertainties are about 3 mm in horizontal components and about 8 mm in the vertical component for baselines up to 100 km. Comparison with results of former GPS campaigns gives a uniform extension, steady in time, perpendicular to the spreading axis. A strain rate of 0.12±0.01 μstrain/yr is observed across the 100 km wide network, or an extension of about 12 mm/yr. Minor deformation is observed in direction parallel to the spreading axis. Observations along the distance profile, which lies across the rift zone, gave a significant contraction during the period 1967–1977 but gave an extension during 1977–1994. This extension is about 138±47 mm and is mainly accommodated in the western part of the profile. The observed extension across the rift zone during 1986–1994 can be simulated with a simple model of infinitely long dike intrusions into an elastic layer overlying a viscous layer. This model is not able to simulate observed contraction along the distance profile 1967–1977. The observed irregularities along the western part of the distance profile coincide in space and time with volcanic and tectonic unrest near the Hekla volcano. The disturbances are probably caused by some common underlying process leading to crustal deformation, eruptions, and earthquakes.
Three‐dimensional structure of asthenospheric flow beneath the Southeast Indian Ridge
Authors: West Brian P. , Wilcock William S. D. , Sempéré Jean‐Christophe , Géli Louis
Published: February 2004

Both geophysical and geochemical evidence suggests the presence of along‐axis asthenospheric flow toward the Australian‐Antarctic Discordance (AAD) beneath the Southeast Indian Ridge (SEIR). We use a three‐dimensional, finite‐volume formulation of viscous flow to investigate the structure of asthenospheric motion beneath the SEIR. Our results show that simple continental separation in either a constant‐ or variable‐viscosity mantle without horizontal temperature gradients is unable to reproduce the inferred asthenospheric flow velocities and observed geographic distribution of the “Indian” and “Pacific” upper mantle isotopic provinces. The presence of a cooler, more viscous mantle directly beneath the AAD is necessary to reproduce observed constraints. High viscosities beneath the AAD induce significant along‐axis flow beneath the neighboring SEIR that advects warmer material over the cooler, more viscous mantle. In passive flow models, a temperature anomaly of about 300°C at a 400‐km depth is required. Simulations that include the effects of buoyancy forces reduce the required temperature anomaly to 100°–200°C, a result in good agreement with other estimates of the regional temperature anomaly. These models also match observed near‐axis variations in residual depth and crustal thickness. In both passive and buoyant simulations, the presence of high‐viscosity (cooler) upper mantle beneath the AAD results in reduced upwelling, consistent with low extents of decompressional melting inferred from geochemical and geophysical constraints. Along‐axis flow acts to subdue temperature variations within the melting region relative to the deeper mantle and results in a temperature inversion in the subaxial asthenosphere. This effect may also reduce the variations in geochemical parameters such as Na8.0 and Fe8.0 with axial depth below those observed in global correlations.