bluuebunny/crossref_metadata_2025 huggingface dataset:
Theory for the propagation of tidally induced pore pressure variations in layered subseafloor formations
Authors: Wang Kelin , Davis Earl E.
Published: February 2004

Tidally induced pore pressure variations below the seafloor depend on the elastic moduli and transport properties of the pore fluid and formation. Hence observations of pore pressure variations, in conjunction with model predictions, can provide important constraints on these formation properties. In this paper, we study the propagation of tidally induced pore pressure variations in a layered poroelastic medium. We derive an analytic solution and use the solution to investigate the effects of various parameters, in particular, the bulk modulus of the formation, the bulk modulus of the pore fluid, and the formation permeability. Specific examples are considered that include the typical continuous depth variation of properties that occurs through normal sediment consolidation and imbedded layers of contrasting properties. Diffusive propagation of tidal pressure variations from the seafloor depends on the hydraulic diffusivity. The depth limit of diffusive propagation scales with the inverse square root of permeability and the period of the signal; for typical fine‐grained marine sediments the depth scale at tidal frequencies is only a few meters. Any internal contrast in elastic properties, due to the presence of free gas for example, can give rise to large instantaneous pressure changes across a layer boundary, which in turn can induce diffusive propagation of signals above and below the interface. Long‐term pressure records from sealed deep‐ocean boreholes that included tidal signals are considered in light of the model results. In Ocean Drilling Project (ODP) Hole 857D on the Juan de Fuca Ridge, the observed attenuation of the seafloor tidal signal to 15% is consistent with the relatively low compressibility of the hydrothermally indurated section intersected by the open part of the borehole and with the high compressibility of the hot formation fluid. In ODP Hole 892B in the Cascadia accretionary prism, the attenuation to 55% and several degree phase lead of the formation tidal signal are probably the result of the open part of the hole being connected to an overlying interval bearing a few percent free gas via a high‐permeability fault zone.
Evolution and strength of Pacific fracture zones
Authors: Kruse Sarah E. , McCarthy Mary C. , Brudzinski Michael R. , Ranieri M. Elaine
Published: February 2004

Previous studies have shown that Pacific fracture zones are strong in some locations, sustaining the stresses associated with differential subsidence across a locked fault, while at other locations they show signs of weakness, acting as preferential conduits for volcanism or supporting anomalously low shear stresses. We find that half of all Geosat crossings of central Pacific fracture zones are inconsistent with a flexurally maintained scarp. We test two hypotheses for the origin of such anomalous crossings: (1) anomalous structures formed at the transform fault during times of changes in plate motions, and preserved at “strong” fracture zones; and (2) anomalous structures representing a posttransform response of the fracture zone to subsequent tectonic activity. We find that three‐quarters of the anomalous crossings occur over the parts of fracture zones that formed during or immediately subsequent to times of changes in spreading directions. With the exception of several locations overprinted by hot spot volcanism, these same crossings show no obvious correlation with regional tectonic processes. These observations suggest that most anomalous fracture zone topography is inherited from the transform fault and is not a product of subsequent activity, consistent with the hypothesis that fracture zones generally remain strong throughout their lifetimes.
A Barremian‐Aptian (Early Cretaceous) North American paleomagnetic reference pole
Authors: McEnroe Suzanne A.
Published: February 2004

Four intrusive complexes from southern Maine, containing rocks ranging in composition from olivine gabbro to granodiorite, were sampled to determine an Early Cretaceous reference pole for the North American plate. Sampling included the Lebanon diorite, the Tatnic Complex, Alfred Complex, and the Cape Neddick Complex dated radiometrically by the 40Ar/39Ar or K‐Ar method on biotite between 125 and 120 Ma. The majority of samples have unblocking temperatures between 550°C and 580°C and Curie temperatures between 550°C and 570°C, indicating that the dominant remanence carrier is titanomagnetite, which crystallized from the magma or was produced by deuteric oxidation alteration during post‐magmatic cooling. Titanomagnetite occurs as discrete C1‐C3 grains, as exsolution or oxidation‐exsolution lamellae in the silicates, as a product of magmatic reactions forming magnetite‐ilmenite symplectites with pyroxene or olivine, or as ultrafine dust. Hemo‐ilmenite is present as a minor component in the Lebanon Diorite, and, in this case, the remanence direction carried by the hemo‐ilmenite is indistinguishable from the titanomagnetite. Detailed petrographic observations and rockmagnetic experiments demonstrate that, despite the coarse average grain size of the oxides in these rocks, which might lead one to expect unstable magnetic remanence, the magnetic remanence in most samples is extremely stable and caused by single‐domain or pseudo‐single‐domain behavior in magnetite containing fine oxidation‐exsolution lamellae of ilmenite. The Tatnic and Alfred intrusions are of normal polarity (D = 333°, I = 62°, pole 70.5°N, 210.3°E, A95 = 3.7°, N = 24 sites); the Cape Neddick and Lebanon intrusions are of reversed polarity (D = 161°, I = −57°, pole 73.7°N, 185.7°E, A95 = 5.0°, N = 17 sites). Together these intrusions yield a mean paleomagnetic pole at 72.2°N, 198.9°E, A95 = 3.3°, N = 41 sites. The well‐characterized reversed polarity recorded in the precisely dated Cape Neddick Complex may represent the best age for the magnetic polarity chron M0r, the last reversal before the onset of the Cretaceous normal superchron at the Barremian‐Aptian stage boundary.
Shear wave velocity structure in North America from large‐scale waveform inversions of surface waves
Authors: Alsina D. , Woodward R. L. , Snieder R. K.
Published: February 2004

A two‐step nonlinear and linear inversion is carried out to map the lateral heterogeneity beneath North America using surface wave data. The lateral resolution for most areas of the model is of the order of several hundred kilometers. The most obvious feature in the tomographic images is the rapid transition between low velocities in the tectonically active region west of the Rocky Mountains and high velocities in the stable central and eastern shield of North America. The model also reveals smaller‐scale heterogeneous velocity structures. A high‐velocity anomaly is imaged beneath the state of Washington that could be explained as the subducting Juan de Fuca plate beneath the Cascades. A large low‐velocity structure extends along the coast from the Mendocino to the Rivera triple junction and to the continental interior across the southwestern United States and northwestern Mexico. Its shape changes notably with depth. This anomaly largely coincides with the part of the margin where no lithosphere is consumed since the subduction has been replaced by a transform fault. Evidence for a discontinuous subduction of the Cocos plate along the Middle American Trench is found. In central Mexico a transition is visible from low velocities across the Trans‐Mexican Volcanic Belt (TMVB) to high velocities beneath the Yucatan Peninsula. Two elongated low‐velocity anomalies beneath the Yellowstone Plateau and the eastern Snake River Plain volcanic system and beneath central Mexico and the TMVB seem to be associated with magmatism and partial melting. Another low‐velocity feature is seen at depths of approximately 200 km beneath Florida and the Atlantic Coastal Plain. The inversion technique used is based on a linear surface wave scattering theory, which gives tomographic images of the relative phase velocity perturbations in four period bands ranging from 40 to 150 s. In order to find a smooth reference model a nonlinear inversion based on ray theory is first performed. After correcting for the crustal thickness the phase velocity perturbations obtained from the subsequent linear waveform inversion for the different period bands are converted to a three‐layer model of S velocity perturbations (layer 1, 25–100 km; layer 2, 100–200 km; layer 3, 200–300 km). We have applied this method on 275 high‐quality Rayleigh waves recorded by a variety of instruments in North America (IRIS/USGS, IRIS/IDA, TERRAscope, RSTN). Sensitivity tests indicate that the lateral resolution is especially good in the densely sampled western continental United States, Mexico, and the Gulf of Mexico.
Peak factors of Mexican accelerograms: Evidence of a non‐Gaussian amplitude distribution
Authors: Gusev A. A.
Published: February 2004

To study statistical properties of the near‐source radiation field, 35 horizontal accelerograms were selected, recorded at distances R < 110 km from five M > 7 Mexican earthquakes. For each one, the maximum‐amplitude segment, assumedly containing direct S waves, was selected by an automatic procedure. Then two parameters were determined for this segment: the peak to rms amplitude ratio, or peak factor PF, and the slope K of the dependence of PF on log duration. These two values were compared to the corresponding values expected for a simplest stochastic record model, of a segment of the stationary Gaussian process. For about 40% of the records, both parameters show clear (above 2σ) deviations from this model. Another record model was also tested, of quasi‐stationary Gaussian process (with time‐varying rms amplitude). With this end, the original records were “stationarized,” that is, modified to attain nearly constant rms amplitude, and then analyzed in the same manner as the original ones. However, for a distinct fraction of records, deviations persisted. The type of the deviations is characteristic for a “heavy‐tailed” amplitude distribution, with the enhanced probability of large peaks. The degree of expression of this phenomenon seems to decay with distance. This observation suggests that source radiation, as it is generated at the fault, often is inherently non‐Gaussian (heavy‐tailed), but because of the scattering and multipathing, it quickly loses this property. One can expect this picture not to be specific for Mexican earthquakes only. From the viewpoint of stochastic strong motion simulation, the results imply that the PF value calculated from the stationary Gaussian model can underestimate average peak acceleration, on the average, by some 25%. In an analysis aimed at the recurrence of rare peaks, the underestimation may be even larger: among 35 records studied, five have peak factors more than 50% above the values expected for the stationary Gaussian model.
Major crustal rotations in the Andean margin: Paleomagnetic results from the Coastal Cordillera of northern Chile
Authors: Randall Darren E. , Taylor Graeme K. , Grocott John
Published: February 2004

Paleomagnetic analyses of Mesozoic lavas and dike swarms from the northern Chilean Coastal Cordillera, between 25.4°S and 26.4°S, reveal a clockwise rotation of about 42°. Magnetizations from lava flows of andesitic‐basaltic composition of the Middle Jurassic La Negra Formation pass both fold and reversal tests and are interpreted as prefolding remanences. Five dike swarms of Middle Jurassic to Early Cretaceous age yield similar directions to that obtained from the La Negra Formation. Four of the five swarms have mixed polarity, suggesting that they too carry a primary or very early remanence. The structural setting of the dikes suggests that they have not suffered any substantial tilting about nonvertical axes since acquisition of the remanence. The clockwise rotation of the area is believed to have been the consequence of transpressional deformation of mid–Late Cretaceous age, post‐100 Ma, associated with abandonment of the Jurassic–Early Cretaceous magmatic arc in this region and its eastward migration to form a new mid–Late Cretaceous magmatic arc in the former back arc region. This younger arc is located east of the Coastal Cordillera and lies in the Central Valley region. The clockwise sense of rotation is consistent with other paleomagnetic data from northern Chile and southern Bolivia, south of the Arica Deflection in the Andean margin, although it is the largest yet reported. To the north of the Arica Deflection, paleomagnetic studies report counterclockwise rotations, and several large‐scale models have been proposed to explain the overall pattern of rotations. Models include oroclinal bending of an originally straight margin, differential shortening across the margin at a preexisting bend which is subsequently tightened by the passive rotation of the limbs of the bend, and distributed shear throughout the margin as a consequence of oblique convergence at a preexisting bend. In contrast to these models, several workers have argued that rotation is better explained in terms of localized in situ rotations. We review these models in light of our results and present a domino‐type model with blocks bounded by left‐lateral faults and rotating clockwise in response to mid‐Late Cretaceous transpression within a crustal scale shear zone. This is consistent with the observed strike‐slip fault systems identified in the Coastal Cordillera.
Mantle anisotropy beneath northwest Pacific subduction zones
Authors: Fouch Matthew J. , Fischer Karen M.
Published: February 2004

To assess the location, strength, and orientation of seismic anisotropy in the southern Kuril, Japan, and Izu‐Bonin subduction zones, we analyzed shear wave splitting in local S phases and teleseismic SKS phases. Fast directions from these phases are roughly parallel to the direction of absolute Pacific plate motion (∼WNW) in Izu‐Bonin, roughly parallel to the strike of the trench (∼NNE) in central Honshu, and roughly N in the southern Kuril back arc in the vicinity of Sakhalin Island. Assuming that the orientation and strength of anisotropy does not vary with depth, modeling of splitting times from local S and teleseismic phases recorded at Sakhalin Island requires that the maximum depth of anisotropy lies between 480 km and 950 km. In contrast, splitting times for local S phases from events in Izu‐Bonin rule out anisotropy in the transition zone. All the data are consistent with a model in which the lower transition zone (520–660 km) and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone (410–520 km), possibly due to preferred orientation of β spinel. Assuming that splitting in the upper mantle is produced by preferred orientation of olivine, the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones. The relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow, but that flow correlated with upper plate deformation must also exist.
Coupling and decoupling of crust and mantle in convergent orogens: Implications for strain partitioning in the crust
Authors: Royden Leigh
Published: February 2004

Topographic relief, strain partitioning and crust‐mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust. This result is obtained by solving analytically for basally driven three‐dimensional flow within an idealized crust with a laterally invariant, depth‐dependent Newtonian viscosity. Two end‐member flow states are shown to occur. In the first, crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust. In the second, upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust. This model is applied to deformation of continental crust above “plate‐like” mantle that undergoes oblique convergence across a narrow “suture.” If no significant low‐viscosity zone develops within the crust, crustal deformation is coupled to motion of the underlying mantle, and a triangular mountain range develops. Surface shortening occurs at the foot of the mountains, normal to the trend of the range, while strike slip and extension are partitioned along the axis of the orogen. If a low‐viscosity zone is initially absent but develops during crustal thickening, a steep‐sided, flat‐topped plateau forms. Crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau. Surface shortening normal to the plateau occurs at the foot of the plateau, while surface extension normal to the plateau occurs at the plateau edge. If a low‐viscosity zone is present in the lower crust prior to convergence, a wide orogen with low topographic relief develops. Crustal deformation is decoupled from the mantle except at the edges of the deforming region, and surface shortening is partitioned uniformly across the belt. This study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen. Similar morphology and strain partitioning can be observed in many real orogenic belts, including: the Eastern Alps, the Tibetan plateau, and the Lachlan (Tasman) fold belt.
Anisotropy in central part of inner core
Authors: Song Xiaodong
Published: February 2004

The anisotropy in the central part of the inner core has been difficult to study because of the paucity of PKP data at antipodal distances, especially for those paths running nearly north‐south (polar paths). In this paper, we systematically search through the International Seismological Center (ISC) picks of PKIKP travel times for polar paths at near‐antipodal distances reported recently by Su and Dziewonski [1995]. Corresponding seismograms were obtained from the Worldwide Standardized Seismographs Network (WWSSN) film chip library at Lamont‐Doherty Earth Observatory. By using seismograms, we can impose quality control on the data, measure differential times (AB‐DF in this case) by cross correlation and use long‐period waveforms if available. Nonpolar paths at near‐antipodal distances are assembled as well, to address biases from lateral variation in the lowermost mantle on AB‐DF times. Our results show large AB‐DF residuals of 2 to 8 s for the polar paths at near‐antipodal distances. Moreover, the absolute DF travel times for the polar paths exhibit fast anomalies of the same magnitude relative to nonpolar paths, while AB times show no obvious difference between polar and nonpolar paths. These observations are consistent with an axisymmetric anisotropy of ∼3% around the spin axis throughout the bulk of the inner core, with the possibility of even stronger anisotropy near Earth's center. This degree of anisotropy is still greater than what the most recent models based on normal modes would indicate.
Geologic and topographic controls on fast flow in the Laurentide and Cordilleran Ice Sheets
Authors: Marshall Shawn J. , Clarke Garry K. C. , Dyke Art S. , Fisher David A.
Published: February 2004

Ice streams are fast flowing arteries which play a vital role in the dynamics and mass balance of present‐day ice sheets. Although not fully understood, fast flow dynamics are intimately coupled with geologic, topographic, thermal, and hydrologic conditions of the underlying bed. These are difficult observables beneath contemporary ice sheets, hindering elucidation of the processes which govern ice stream behavior. For past ice sheets the problem is antithetic. Geologic evidence of former ice streams exists, but spatial and temporal histories are uncertain; however, detailed knowledge of bed geology and topography is available in many places. We take advantage of this information to compile terrain characteristics relevant to fast flow dynamics in the Laurentide and Cordilleran Ice Sheets. Using seed points where fast flowing Wisconsinan ice has been geologically inferred, discriminant analysis of a suite of North American geologic and topographic properties yields a concise measure of ice‐bed coupling strength. Our analysis suggests that the interior plains and continental shelf regions of North America have low basal coupling relative to areas of variable relief or exposed bedrock in the Cordillera and on the Canadian Shield. We conclude that the interior plains and continental shelves are both topographically and geologically predisposed to large‐scale basal flows (i.e., ice streams or surge lobes). This result holds independent of whether the mechanism of fast flow is sediment deformation or decoupled sliding over the bed.
The relative importance of plate‐driven and buoyancy‐driven flow at mid‐ocean ridges
Authors: Kincaid Chris , Sparks David W. , Detrick Robert
Published: February 2004

The dynamical interaction between three‐dimensional (3‐D) buoyant flow and plate‐driven mantle flow beneath a mid‐ocean ridge is examined using a combination of laboratory and numerical experiments. In a unique laboratory setup a layer of strongly temperature‐dependent viscous fluid is heated from below and cooled from above to drive thermal convection. Forced, plate‐driven flow is modeled by dragging mylar sheeting in opposite directions across the fluid surface. Uniform viscosity 3‐D numerical models have been designed to simulate the laboratory runs, to provide additional information on the flow, and to attempt to isolate the effects of variable viscosity. In one type of experiment we modeled buoyancy on the scale of the partial melting region with a linear heat source beneath the spreading axis. In a second set of experiments the entire base of the tank is heated in order to investigate the interaction between plate‐driven flow and larger‐scale (upper mantle) convection. The pattern of segment‐scale (∼100–150 km) convection beneath a spreading center is found to be a strong function of the spreading rate and the Rayleigh number (Ra) of the buoyant flow. Purely two‐dimensional (2‐D) flow exists only in the case of low Ra (∼105) and faster spreading rates (>4–6 cm/yr half rate). Three dimensionality is strongly enhanced during transient pulses of upwelling. Convection on the scale of the upper mantle can contribute significant long‐wavelength spatial (∼600–1000 km) and temporal (20–40 m.y.) variability in upwelling rates and temperatures at spreading centers and may provide an alternative model for plume‐ridge interaction. For a given Ra the upwelling near the spreading axis can essentially be described by three regimes: weakly 3‐D at the slow spreading rates, strongly 3‐D at slow to intermediate rates, and 2‐D at fast spreading rates. The regime boundaries shift toward higher plate velocities with increasing Ra. Comparison of the laboratory and numerical experiments indicates that temperature‐dependent viscosity may strengthen the position of focused upwelling centers and cause a sharper transition between 2‐D and 3‐D upwelling patterns. Taken together, these results suggest that buoyancy‐driven, dynamic flow is an important element in the geodynamics of mid‐ocean ridges.
Thermoremanent magnetization in monodomain monoclinic pyrrhotite Fe7S8
Authors: Menyeh A. , O'Reilly W.
Published: February 2004

The intensity and stability of thermoremanent magnetization (TRM) acquired by fine particles (<1µm) of synthetic monoclinic pyrrhotite were determined. A specific TRM (magnetization per unit inducing field) of 32 with a median destructive field of 36 kA m−1 was observed. The median destructive field of saturation isothermal remanence is 42 kA m−1 and the coercive force is 62 kA m−1. The intensity of TRM has been compared to that predicted by the classical monodomain model adapted to the magnetic symmetry of pyrrhotite. If the optically determined particle size is incorporated into the model, the agreement between model and observation is poor (a factor of 100 difference); if a grain size (∼0.05 µm) inferred from the measurement of coercive force is adopted, model and observation are in agreement within experimental error.
Acoustic emission, microstructure, and damage model of dry and wet sandstone stressed to failure
Authors: Zang Arno , Wagner Christian F. , Dresen Georg
Published: February 2004

Twenty‐three uniaxial compression tests were performed on dry and wet Flechtingen sandstone from Germany. Compressive strength of wet core is 60% of the strength of dry core. Before fracture, the transverse P wave speed drops by 13% and the pulse amplitude by 22% for wet and 37% for dry cores. Accumulated strain energy doubles for dry core. Acoustic emissions (AE) are detected with 10 sensors for 19 cores. AE activity starts at 84% of the fracture strength of wet cores (55 MPa) and at 91% of the strength of dry cores (87 MPa). The ratio of located to recorded AE is 0.37 for dry and 0.13 for fully wet cores. AE hypocenter patterns document the development of two opposite fracture cones. The negative slope of cumulative AE‐amplitude frequency distribution drops by 50% before failure in dry cores. The slope of the wet core drops and recovers. Energy discrimination of AE detected by a broadband sensor resolves different stages of damage and captures the onset of the dilatant throughgoing macrofracture. Using the analogy to visible light microfracturing events are separated into high‐energy short pulses (blue AE) and low‐energy pulses with long duration times (red AE). Blue AE are explained by intragranular grain breakage, red AE by multiple stick slip on crack planes or grain boundaries. Deformed cores show highly fractured calcite cement and mostly intact quartz grains. The stochastic damage model for brittle composites developed highlights that microfracturing of the sandstone is controlled by the amount and distribution of the weak mineral (calcite).
Melt conduits in a viscous porous matrix
Authors: Richardson C. N. , Lister J. R. , McKenzie D.
Published: February 2004

Although melting is inferred to start at depths of 120 km or more in the mantle, little is known about the style of melt transport from the source region to the surface. Geochemical observations require the transport of melt from depth to be fast, and that melt is not in chemical equilibrium with the matrix through which it passes. We present a model which allows fast transport by an open melt conduit in a viscous permeable matrix and discuss the possibility of chemical isolation in such a conduit. We show that disturbances on the conduit surface propagate slowly upwards as solitary waves and cause melt to flow between the conduit and the surrounding porous matrix.
Three‐dimensional dynamical model of continental rift propagation and margin plateau formation
Authors: Dunbar John A. , Sawyer Dale S.
Published: February 2004

We investigate the dynamics of rift propagation through continental lithosphere and the resulting differential extension along passive margins using a full three‐dimensional finite element implementation of the yield stress envelope model. In its initial configuration the three‐dimensional model represents a 1000 km by 1000 km area of continental lithosphere. An initial weak trend representing a suture zone or other preexisting weakness within the continental lithosphere extends 500 km from one model edge to the center of the model and terminates. The model region beyond the weak trend is uniform. Model conditions are varied by changing the thickness and composition of the crust, the geotherm and extension rate, the strength of the initial weakness and the dimensions of the preweakened and uniform regions. For each model, breakup nucleates at a point on the weak trend in response to an applied constant extension rate. The break propagates rapidly along the weak trend and then slows as it crosses the uniform region. This produces a locked zone in which breakup is delayed, resulting in an increase in the width of the model margin analogous to offsets associated with margin plateaus. The total extension required to tear across the 1000‐km length of the model is 210 km for 40‐km‐thick crust, 400 km for 45‐km‐thick crust, and 710 km for 50‐km‐thick crust. Smaller increases in total extension occur for elevated geotherms and reduced extension rates. This indicates that prior extension estimates based on subsidence, which exceed 600 km for margin plateaus, can be explained by rift propagation in three dimensions. Differential extension along the model margin is linearly related to the length of the locked zone. The slope of the line, which we call δ, is a function of the rheological properties of the locked zone and reflects the extent to which continental lithosphere within the locked zone resists rift propagation. Model calculations for crustal thickness from 35 km to 50 km and compositions from quartz diorite to granite predict a range in δ from 0.2 to 1.3. We compare δ values computed from our model to those estimated from the subsidence of Bonavista Platform and Rockall Plateau conjugate margins in the north Atlantic basin and Georges Bank and Blake Plateau and their conjugate margins in the central Atlantic basin. These results indicate that the differential extension associated with these locked zones, which ranges from 170 km to 550 km, can be predicted in terms of their initial crustal thickness and their length along the margins.
The effect of deglaciation on mantle melting beneath Iceland
Authors: Jull M. , McKenzie D.
Published: February 2004

The spreading ridge on Iceland shows large variations in eruption rate over the last 10,000 years. An increase of about 30 times the steady state value, between 10,000 and 8000 years ago, coincides with the disappearance of ice at the end of the last ice age. We examine the possibility that deglaciation caused this increase by modeling the effect on melt generation of the removal of an axisymmetric ice sheet from a spreading ridge. Our calculations take into account the influence of both a nonhydrostatic stress field and viscous flow. The results show that the average melting rate is increased by about 30 times its steady state value when a 2‐km‐thick ice sheet melts in 1000 years. The effect of the glacial cycle of loading and unloading is to cause a nonlinear modulation in the melt production. The factor of 30 increase in melt generation during the 1‐kyr unloading can occur only because melt generation is reduced for about 60 kyr after the ice load is applied. The total volume of magma that can be produced from the deglaciation of Iceland is about 3100 km3. The concentrations of the light rare earth elements in the melt produced by deglaciation near the center of the ice sheet are about 15% less than those in melts produced by steady state melting. Transport time for the melt to reach the surface is not yet well constrained but is likely to be less than ∼1 kyr.
A model for the kinetic control of quartz dissolution and precipitation in porous media flow with spatially variable permeability: Formulation and examples of thermal convection
Authors: Bolton Edward W. , Lasaga Antonio C. , Rye Danny M.
Published: February 2004

We present the formulation and model results of kinetically controlled quartz dissolution and precipitation in a two‐dimensional heterogeneous permeable medium. The quartz matrix is modeled as a partially occluded spherical close pack, with dissolution and precipitation occurring on the exposed faces of the grains. This formulation yields larger permeabilities and lower surface area to fluid volume ratios in regions of larger grain radii, and thus allows us to investigate the influence of crack‐like regions on the flow and silica exchange. We use the kinetic data for quartz dissolution of Rimstidt and Barnes [1980]. Thermal convection results indicate that channelizing of fluid flow in high‐permeability zones is enhanced in the transient regime by buoyancy effects arising from the advection of heat. The highly permeable zones are most out of chemical equilibrium, owing to their lower surface areas and to more rapid advection. Porosity changes are most pronounced in regions of high surface area often downstream of high‐permeability zones. Oscillatory convection is observed, accompanied by saturation state reversals peripheral to the high‐permeability zones. Such internally generated oscillatory regimes provide a mechanism for quartz zonation down to the nanometer scale. When the thermal forcing is strong enough, recurrent plumes emanate from the thermal boundary layers and plunge through the high‐permeability zones. When departures from equilibrium become significant, we observe some regions of undersaturated upwelling fluid moving down temperature, and regions of oversaturated downwelling fluid moving up temperature, both cases opposing the conventional wisdom based on equilibrium.
Upper mantle shear structure beneath the Colorado Rocky Mountains
Authors: Lee Duk‐Kee , Grand Stephen P.
Published: February 2004

A tomographic inversion of teleseismic shear wave delays has been performed using data collected in the 1992 Program for Array Seismic Studies of the Continental Lithosphere Rocky Mountain Front experiment. The shear wave residuals used as data were corrected for known crustal, sediment and topographic differences and thus were sensitive to mantle shear velocity variations beneath the Colorado array. Large differences in shear wave travel time are observed across Colorado and into western Kansas with a maximum difference of near 5 sec between the central Rockies in Colorado and a station in western Kansas. The shear wave residuals were inverted for shear wave velocity variations in the upper 350 km of the mantle beneath Colorado and Kansas. Large differences in mantle shear velocity are found between the Rockies and the Kansas Great Plains. The average shear velocity in the upper 200 km is about 9% greater beneath Kansas than beneath the Rockies. At greater depths, lateral variations are far smaller and not as well correlated with surface tectonics. In a qualitative sense, there appear to be three mantle structures present beneath the study area. Beneath the Rockies the upper mantle is extremely slow. Both to the west and east of the Rockies we find intermediate mantle shear velocity. There is then an abrupt increase in mantle shear velocity near the Colorado‐Kansas border. We interpret the abrupt increase in mantle shear velocity near the Colorado‐Kansas border to mark the boundary of cratonic lithosphere. The slow velocity zone beneath the Rockies appears to be upwelled asthenosphere as is seen beneath the Rio Grande Rift to the south. To match the seismic contrast between the mantle beneath Kansas and the Colorado Rockies requires a temperature contrast of at least 350° C as well as partial melt beneath the Rockies. The hot mantle beneath the Rockies implies that the mantle density is low and the high topography of the Rockies is at least partially due to buoyant mantle.
Episodic unroofing of the Kohistan Batholith, Pakistan: Implications from K‐feldspar thermochronology
Authors: Krol Michael A. , Zeitler Peter K. , Copeland Peter
Published: February 2004

New 40Ar/39Ar and U‐Pb mineral ages from plutonic rocks help constrain the thermal and tectonic evolution of the Kohistan island arc terrane following its collision with India in early Tertiary time. Kohistan has experienced a prolonged tectonomagmatic history extending from the Early Cretaceous through the Tertiary. Thermal histories derived from multi diffusion domain analyses of K‐feldspar within the Kohistan batholith reveal rapid cooling events (70–140°C/m.y.) distributed through space and time. The cooling histories show a systematic variation along the length of the batholith suggesting that Kohistan experienced differential unroofing. An episode of rapid cooling in the middle Eocene is recognized in western Kohistan, whereas rapid cooling occurs substantially later, during the middle and late Miocene, in eastern Kohistan. Rapid cooling in western Kohistan might have been caused by postemplacement cooling of hot magma against cold country rocks at relatively shallow crustal levels. Within the NW region of eastern Kohistan, rapid cooling at 13–12 Ma is recorded in K‐feldspars ∼60 km from the contact with the Nanga Parbat‐Haramosh Massif (NPHM), whereas cooling does not occur until 11 Ma only 2.5 km from the contact. This temporal and spatial variation in cooling histories may record differential unroofing in response to the development and propagation of the NPHM structure beneath Kohistan. The NPHM has experienced rapid cooling and unroofing over the last 10 m.y., and our results are consistent with this mid–late Miocene event.
Small‐scale models of oceanic transform zones
Authors: Mauduit Thomas , Dauteuil Olivier
Published: February 2004

Previous studies about oceanic transform zones reveal major morphological and structural differences that cannot be explained by simple factors such as spreading rates or offsets. In order to determine how kinematic and mechanic parameters control the deformation pattern, we perform small‐scale modeling using sand and silicone putty as analog material of brittle and ductile layers of oceanic lithosphere, respectively. The three dimensional depth geometry of these layers follows the thermal structure of the ridge‐transform system. The structure of the deformed transform zone is very sensitive to input parameters, specially to the offset. The obliquity of the deformed zone with the extension direction (i.e., spreading direction in a natural system) depends on the offset and on the brittle layer thickness; for greater thickness or a shorter offset, the transform domain widens and the components of extension become significant. For smaller lithosphere thickness or larger offset, the transform domain becomes narrow with steeper walls and mainly undergoes strike‐slip deformation. Such variability in possible controls on the offset length may help explain the structure complexity observed within the oceanic transform zones. A comparison with 28 natural systems indicates the same relationship of the offset with the angle between the principal transform displacement zone (PTDZ) and spreading direction. A relationship between offset and width is clearly established for Mid‐Atlantic Ridge (MAR) transforms, as on analog models. A similar relationship is observed for the East Pacific Rise (EPR) transforms, but with greater variabilty. This difference is interpreted as resulting either from greater complexity of transform zones on EPR than on MAR making it difficult to estimate some parameters, from different mechanical behaviors between slow and fast spreading systems, or from higher rheological complexity of natural systems than our model.
Diurnal/semidiurnal polar motion excited by oceanic tidal angular momentum
Authors: Chao B. F. , Ray R. D. , Gipson J. M. , Egbert G. D. , Ma C.
Published: February 2004

The axial component of the oceanic tidal angular momentum (OTAM) has been demonstrated to be responsible for most of the diurnal and semidiurnal variations in Earth's rotational rate. In this paper we study the equatorial components of OTAM and their corresponding effects on the orientation of Earth's rotational axis, or polar motion. Three ocean tide models derived from TOPEX/Poseidon satellite altimetry are employed to predict the polar motion excited by eight major diurnal/semidiurnal tides (Q1,O1,P1, K1, N2, M2, S2, K2). The predictions are compared with geodetic measurements of polar motion from both long‐term observations and during the intensive campaign Cont94. The prograde diurnal and prograde and retrograde semidiurnal periods are treated, whereas the retrograde diurnal polar motion is not treated (because it cannot be observed directly and uniquely.) The comparison shows generally good agreement, with discrepancies typically within 10–30 micro‐arc‐ seconds for the largest tides. The eight tides collectively explain nearly 60% of the total variance in subdaily polar motion during Cont94. This establishes the dominant role of OTAM in exciting the diurnal/semidiurnal polar motion and paves the way for detailed studies of short‐period nontidal polar motion. The present accuracy, however, is inadequate to shed light on the prograde diurnal polar libration.
Geophysical study of the East Pacific Rise 15°N–17°N: An unusually robust segment
Authors: Weiland Charles M. , Macdonald Ken C.
Published: February 2004

Bathymetric, side‐scan sonar, magnetic and gravity data from the East Pacific Rise (EPR) between 15° and 17°N are used to establish the spreading history and examine melt delivery to an unusually robust spreading segment. The axial ridge between the Orozco transform fault (15°30′N) and the 16°20′N overlapping spreading center (OSC) has an average elevation of 2300 m which is 300 m shallower than typical EPR depths, and its cross‐sectional area is double the average value for the northern EPR. The total opening rate is 86 km/Myr, but the inflated segment appears to have spread faster to the east by more than 20% since 0.78 Ma. The orientation of magnetic isochrons and lineaments in the side‐scan sonar indicates a ∼3° counterclockwise rotation of the spreading direction since 1.8 Ma (C2) and reflects a change in the Pacific‐Cocos plate motion. The side‐scan lineaments also show that the percentage of inward facing faults (83%) and the spacing between faults (1.5 km) are consistent with the spreading rate dependence shown by Carbotte and Macdonald [1994]. However, the mean fault length (4.8 km) is 1.5 km shorter than expected for the spreading rate and suggests that extensive off‐axis volcanism has draped the faults. Gravity analysis shows that the inflated segment has a ∼12‐mGal bull's eye shaped low in residual mantle Bouguer anomaly. We offer several possible end‐member models for the anomaly, including a prism of 10% partial melt in the mantle and lower crust or a crustal thickness anomaly of 2.25 km. Kinematic modeling that is based on structure and magnetic data suggests that two large magmatic pulses occurred at approximately 0.8 Ma and 0.3 Ma and have reshaped the plate boundary geometry and inflated the segment.
Seismicity in the outer rise–forearc region and configuration of the subducting lithosphere with special reference to the Japan Trench
Authors: Kao Honn , Chen Wang‐Ping
Published: February 2004

Along nine subduction zones where the largest (Mw or Ms ≥ 7.6) normal‐faulting earthquakes have occurred beneath the outer rise, there is an apparent correlation between a large curvature of the subducting slab and regions of low background seismicity in the forearc. In six cases, a gentle inner trench is also observed. Taking advantage of the excellent coverage of teleseismic stations and extensive previous studies using local data, we investigate the nature of this spatial correlation between seismicity and bathymetry along the Honshu forearc in some detail. An aseismic region in the forearc lies directly west of the rupture zone of the 1933 Sanriku earthquake (Mw = 8.4), a great normal‐faulting earthquake that most likely has ruptured the entire brittle portion of the lithosphere in the outer rise. Along this particular cross section of the Japan trench, large to moderate‐sized interplate earthquakes span a short interval of depths between approximately 14 and 31 km, about half the interval observed along an adjacent profile to the south (12–45 km). The aseismic portion of the plate interface (between depths of approximately 30 and 45 km) has persisted for over 100 years. The occurrence of lithospheric normal‐faulting earthquakes in the outer rise region indicates that bending moment within the subducting slab, limited by yield strength of the lithosphere, has nearly reached an asymptotic value (moment saturation). At this state, the curvature of the plate can increase with little increase in bending moment. As such, any stresses transmitted across the plate interface will be relaxed by a permanent increase in the plate's curvature and little elastic strain can accumulate across the interplate thrust zone to produce large earthquakes. Thus the saturation of bending moment in the subducting lithosphere offers a straightforward interpretation for the observed correlation between seismicity in the outer rise–forearc region and configuration of the subducting lithosphere.
Full waveform ultrasonic transmission seismograms: A fast new method for the determination of physical and sedimentological parameters of marine sediment cores
Authors: Breitzke M. , Grobe H. , Kuhn G. , Müller P.
Published: February 2004

Detailed information about the sediment properties and microstructure can be provided through the analysis of digital ultrasonic P wave seismograms recorded automatically during full waveform core logging. The physical parameter which predominantly affects the elastic wave propagation in water‐saturated sediments is the P wave attenuation coefficient. The related sedimentological parameter is the grain size distribution. A set of high‐resolution ultrasonic transmission seismograms (∼50–500 kHz), which indicate downcore variations in the grain size by their signal shape and frequency content, are presented. Layers of coarse‐grained foraminiferal ooze can be identified by highly attenuated P waves, whereas almost unattenuated waves are recorded in fine‐grained areas of nannofossil ooze. Color‐encoded pixel graphics of the seismograms and instantaneous frequencies present full waveform images of the lithology and attenuation. A modified spectral difference method is introduced to determine the attenuation coefficient and its power law α = kfn. Applied to synthetic seismograms derived using a “constant Q” model, even low attenuation coefficients can be quantified. A downcore analysis gives an attenuation log which ranges from ∼700 dB/m at 400 kHz and a power of n ≅ 1–2 in coarse‐grained sands to few decibels per meter and n ≤ 0.5 in fine‐grained clays. A least squares fit of a second degree polynomial describes the mutual relationship between the mean grain size and the attenuation coefficient. When it is used to predict the mean grain size, an almost perfect coincidence with the values derived from sedimentological measurements is achieved.