bluuebunny/crossref_metadata_2025 huggingface dataset:
Neogene stress field in SW Japan and mechanism of deformation during the Sea of Japan opening
Authors: Fournier Marc , Jolivet Laurent , Fabbri Olivier
Published: February 2004

We present new structural data from SW Japan and discuss the mechanism of deformation during the opening of the Sea of Japan. We studied the Miocene basins at the southern margin of the Sea of Japan (northern coast of SW Japan) and of the Median Tectonic Line (MTL) by means of fault slip analysis to determine stress field directions during the opening of the Sea of Japan in early and middle Miocene time. On the southern margin, the stress field was extensional or transtensional with a NW‐SE trending extension direction. The MTL was a normal fault in Shikoku during early middle Miocene and probably a normal fault with a left‐lateral component in central Japan before having been strained by the collisions of the Tanzawa and Izu blocks with central Japan. We combine our results with published data for the Sea of Japan area and show that early and middle Miocene stress field directions are remarkably consistent on the eastern, southern, and western margins of the Sea of Japan with σHmaxtrending NE‐SW and σHmintrending NW‐SE. The stress regime is transpressional or transtensional on the eastern and western margin and almost purely extensional on the southern margin. This stress field distribution is in agreement with the model of the opening of the Sea of Japan in an extensional transfer zone between two N‐S right‐lateral strike‐slip zones proposed by Jolivet et al. (1991). Analogue modelings and kinematic reconstructions showed that this model accounts for 20° to 30° of clockwise rotation in SW Japan, while paleomagnetic rotations reached 40° to 50° during the opening. We present structural data showing that SW Japan was strained during the rotation and did not behave as a rigid block. Our observations suggest that rotations of small blocks may have occurred. We propose that SW Japan has been sheared between the right‐lateral strike‐slip zones which bound the Sea of Japan and that the missing 10° to 30° of rotation are to be found in its internal deformation. In our model, the MTL is a normal fault with a strike‐slip component rotating in a right‐lateral shear zone.
Travel time computation in stratified anisotropic media: Generalized ray method
Authors: Shieh Chiou‐Fen
Published: February 2004

The generalized ray, combined with the system matrix, can be utilized to calculate the travel time of the elastic waves in stratified anisotropic media. The method proposed by Van der Hijden is modified to calculate the horizontal slowness (corresponding to travel time) of the Cagniard path. The accuracy of this method is verified by comparing the results with those of perturbation and ray tracing methods.
Hydrogeologic properties of a thrust fault within the Oregon Accretionary Prism
Authors: Screaton Elizabeth J. , Carson Bobb , Lennon Gerard P.
Published: February 2004

Two sets of hydrogeologic tests conducted at Ocean Drilling Program (ODP) Hole 892 on the Oregon Accretionary Prism provided the opportunity to determine hydrogeologic properties of an active accretionary prism fault zone. The first set of tests consisted of shipboard packer tests conducted during ODP Leg 146 (fall 1992), while the second set of tests were constant‐drawdown and constant‐discharge tests conducted in fall 1993 using the submersible Alvin. Pressure response during the first set of tests suggests that fractures remained open until excess fluid pressure (relative to hydrostatic) dropped below 0.315 to 0.325 MPa (λ* ∼ 0.53 to 0.54, where λ* = (pore pressure ‐ hydrostatic)/(lithostatic‐hydrostatic)). Analysis of the packer test data suggested an apparent background pressure of 0.25 MPa (λ* ∼ 0.42 to 0.50). Because the borehole had been open for 12 hours prior to the packer tests, formation pore pressures may have exceeded this value prior to drilling of the borehole. These overpressures dissipated by the time the second set of tests were conducted. One possible explanation for this decay is that the borehole may provide a vertical conduit between the overpressured zone and overlying or underlying sediments that had previously been hydraulically separated from the overpressured zone. The second set of tests were conducted at pressures (≤0.019 MPa or λ* ∼ 0.03) below that estimated to maintain open fractures and yielded transmissivities 1 to 2 orders of magnitude less than estimated for the packer tests (when fractures were open). Constraints on fluid flow rate along the fault are provided by observed displacement in a bottom‐simulating reflector (BSR) at its intersection with the fault zone. The closed‐fracture transmissivities are insufficient to produce flow rates capable of displacing the BSR; therefore open‐fracture transmissivities under conditions of elevated pore pressure are inferred to be necessary for the observed BSR displacement. In addition, calculated rates of specific discharge through the fault zone are 2 to 3 orders of magnitude lower than discharge measured at an associated seafloor vent site; fluid flow must become spatially or temporally focused as it moves up the fault zone toward the seafloor.
A comparison of volcanic edifices at the Reykjanes Ridge and the Mid‐Atlantic Ridge at 24°–30°N
Authors: Smith Deborah K. , Humphris Susan E. , Bryan Wilfred B.
Published: February 2004

A detailed analysis of high‐resolution side‐scan sonar data shows that the volcanic edifices built within the axial zones of the slow spreading (20 mm/yr) Reykjanes Ridge and the slow spreading (25 mm/yr) Mid‐Atlantic Ridge are remarkably similar. In fact, individual seamounts, hummocks (rounded mounds), and hummocky ridges at the Reykjanes Ridge have nearly identical morphological counterparts at the Mid‐Atlantic Ridge (MAR), arguing that commonly eruptive conditions are similar at the two ridges. The construction of these volcanic edifices, typical of slow spreading ridges, occurs at the Reykjanes Ridge despite its proximity to the Iceland hot spot and the influence of the hot spot on other characteristics of the ridge axis, such as its overall shape (an axial high north of about 59°N), its shallow axial depths, and its thicker crustal section. The volume of magma that goes into constructing volcanic edifices at each of 12 segments of the MAR and four regions of the Reykjanes Ridge is calculated using multibeam bathymetry data. Our results indicate that edifice volume per unit seafloor area, equivalent to a uniform thickness over the area of interest, is larger at the MAR than at the Reykjanes Ridge, on average. Taking into account spreading rate, we then estimate B, the average building rate per unit length along the axis, which is also larger at the MAR. We conclude that the same volcanic edifices build the crust at the Reykjanes Ridge and MAR, but that their emplacement is more frequent at the MAR. To explain our results and reconcile them with those of Magde and Smith (1995) who documented that seamounts are taller and more abundant at the Reykjanes Ridge than at the MAR, we consider the controls on diking and eruption within the axial zones of the two ridges. We suggest that the combination of slower spreading rate and the oblique geometry of spreading at the Reykjanes Ridge leads to fewer eruptions which are more widely dispersed across the axial zone than at the MAR. If this hypothesis is correct, the proportion of seamounts modified by subsequent eruptions will be less. This might also explain the taller heights of the seamounts if there is less overlap and infilling between edifices.
Microearthquake characteristics and crustal velocity structure at 29°N on the Mid‐Atlantic Ridge: The architecture of a slow spreading segment
Authors: Wolfe Cecily J. , Purdy G. M. , Toomey Douglas R. , Solomon Sean C.
Published: February 2004

We report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the Mid‐Atlantic Ridge segment at 29°N and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes. The seismic velocity structure is obtained from two‐dimensional (2‐D) and three‐dimensional (3‐D) tomographic inversions of travel times from shots along an axial refraction line. Inversion solutions indicate that the velocity structure in the lower crust is heterogeneous, with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high. The thickness of the lower crust at the segment end is asymmetric across axis, with thinner crust beneath the inside corner. The indicated variations in crustal thickness are consistent with those inferred from mantle Bouguer gravity anomalies. The microearthquakes located along axis during the 41‐day recording period cluster in three separate along‐axis regions: (1) the southern segment end near 28°55′N, (2) the central along‐axis topographic high at 29°11′N, near and north of the Broken Spur hydrothermal vent field, and (3) a region midway between, beneath a volcano near 29°02′N. The greatest level of microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset. This pattern of off‐axis microearthquake activity, and the cross‐axis asymmetry in crustal thickness at the segment end, support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions. Anomalous focal mechanisms for microearthquakes beneath the along‐axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center, in contrast, may be the result of volcanic and hydrothermal processes, such as magma movement or thermal stresses generated near cooling plutons. A comparison of microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature, whereas microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust. Along axis, well‐resolved focal depths determined with a 3‐D velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center. These observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state.
Experimental study of the influence of stress, temperature, and strain on the dynamic recrystallization of Carrara marble
Authors: Rutter Ernest H.
Published: February 2004

Carrara marble has been deformed experimentally at temperatures ranging between 500° and 1000°C, at confining pressures of 200 and 300 MPa, and up to very large strains in extension and compression, in order to study the microstructural and mechanical property changes associated with dynamic recrystallization. Microstructural studies were made by optical microscopy on ultrathin sections, and automatic grain size measurement techniques were used. When the temperature is sufficiently high (≈ 600°C) and the stresses are high enough for deformation twinning to occur, twin boundary migration is a powerful recrystallization mechanism that does not modify the grain size. At stress levels too low to activate twinning, recrystallization occurs in two stages: the formation of nuclei by grain boundary bulging and subgrain rotation recrystallization in the grain boundary regions, followed by a second stage of grain boundary migration recrystallization to a larger grain size that eventually overprints the entire rock volume. The recrystallization process requires larger prestrains at the lower temperatures. The size to which migration‐recrystallized grains grow seems to be limited by the stress and grain size dependent twinning field boundary or by the boundary between the dislocation creep and grain size sensitive flow mechanisms. Within the range of experimental observations, dynamically recrystallized grain size does not depend on strain rate or temperature. Separate empirical stress versus grain size relations are presented for rotation and migration recrystallization that may be used for palaeopiezometry. No unequivocal experimental evidence of weakening resulting from recrystallization to finer grain sizes at laboratory strain rates was found. However, extrapolation of the experimental data to low, natural strain rates suggests that weakening following recrystallization may occur in nature.
Early Paleozoic Island arc accretion to the North China craton and the Shang Dan fault zone: A major paleoplate boundary in eastern Asia
Authors: Lerch M. F. , Xue F. , Kröner A. , Todt W.
Published: February 2004

The early Paleozoic tectonic evolution of part of the Qinling‐Dabie‐Shandong orogenic belt is reconstructed on the basis of the results of field mapping, geochemical, and geochronological studies. The Qinling orogenic belt separates the Archean‐Proterozoic North China craton (NCC) from the South China craton (SCC, also Yangtze craton). The study area is located about 90 km southwest of the city of Xi'an. Three lithologically distinctive zones, separated from each other by fault zones, can be distinguished. Passive continental margin deposits of the NCC (zone 1) have εNd(t) values between −6.16 and −12.54 and TDM ages between 1.29 and 2.20 Ga. They are separated from an early Paleozoic island arc sequence (zone 2) by the sinistral Chenche fault. Mafic volcanic lithologies in the southern part of zone 1 are associated with the development of a passive continental margin in late Proterozoic time. Similiar lithologies of zone 2 have geochemical characteristics of an oceanic island arc. Accretion was accompanied by a strong deformation event between early Ordovician and late Silurian time. Magmatic intrusions in zone 2 and the southern part of zone 1 are associated with a short‐lived magmatic arc that developed above a north dipping subduction zone on the modified continental margin of the NCC in late Silurian to early Devonian time. The εNd(t) values of felsic intrusions from the northern part of zone 2 decrease in a northern direction from 1.78 to −1.05. Mafic to intermediate intrusions in the southern part of zone 2 have positive εNd(t) values from 1.28 to 2.04 as well as low 87Sr/86Sr(i) values between 0.705 and 0.706 indicating an increasing mantle component toward the south. Low‐grade metamorphic pelites are separated from the island arc sequence of zone 2 by a ≤2 km wide Shang Dan shear zone. Isotopic studies show strong similarities between a granite sample from zone 3 and samples from the SCC. 87Sr/86Sr(i) values increase from 0.709 to 0.713, whereas εNd(t) values decrease from −1.90 to −5.34 in a southern direction. The shear zone that separates zone 2 from 3 is of fundamental importance and marks the boundary between the NCC and the SCC. The results presented in this paper document a previously unknown early Paleozoic tectonic evolution of the Qinling orogenic belt and delineate one of the fundamental paleoplate boundaries in eastern Asia.
Dynamics of a zone of four parallel faults: A deterministic model
Authors: Senatorski Piotr
Published: February 2004

We consider a two‐dimensional model for quasi‐static evolution of four parallel faults. The model is based on the physics of continuum, with slips along the faults, long‐range interactions, a distribution of barriers and asperities, a velocity dependent friction, healing and reactivation of cohesive forces, and a mutual causality between faulting and tectonic forces all taken into account. Both the long‐term time series of seismic activity and details of seismic and aseismic crustal deformations during each phase of the earthquake cycle are produced by the model. It is shown that the system whose evolution is described by a set of deterministic differential equations may exhibit periodic or quite irregular (despite its assumed symmetry) behavior, depending on model parameters.
Transponder altimetry: Precise height measurements over land
Authors: Denys Paul , Birks Andrew , Cross Paul , Powell John , Bürki Beat
Published: February 2004

A novel experiment, coordinated by the University of Newcastle and Rutherford Appleton Laboratory, was conducted to demonstrate the feasibility of measuring relative height differences using transponder altimetry. This technique used the radar altimeter onboard the first European Remote‐sensing Satellite, ERS‐1, in conjunction with active ground‐based microwave transponders. Two transponders were placed 127 km apart, one in the foothills close to Venice and the second, 1000 m higher in the Austrian Alps. The sites were chosen to coincide with the ground track of ERS‐1 and also to be within the altimeter calibration zone which provided reliable ERS‐1 orbits. Coincident observations at both transponder sites were made for a total of five overpasses. These measurements were subsequently compared with a height difference determined using the Global Positioning System (GPS). The experiment has estimated the precision of the transponder derived height difference by differencing the observed and computed transponder ranges at both transponder sites. The precision of one transponder‐derived height difference, based on four of the recorded passes, is estimated as ±3.5 cm and the standard deviation of the mean is ±1.7 cm. This implies that the standard deviation of one transponder range at a single site is ±2.5 cm. The precision of the height difference derived from the GPS network was ±3.1 cm and thus the ground‐based transponders have performed, as a height difference measurement system, to a comparable accuracy to that of GPS. Ground‐based transponders do, however, have the significant practical advantage of being able to be operated in a simpler, automatic, and possibly more economical mode of operation. Data collection takes place onboard the space vehicle and power requirements can be made such that the equipment could be left unattended for many months. This makes the equipment especially suitable for hostile environments, for example, measuring the vertical precursory motion of volcanoes and for situations where long‐period observations are needed such as the monitoring of subsidence or sea level changes.
Thermal and unroofing history of a thick, tilted Basin‐and‐Range crustal section in the Tortilla Mountains, Arizona
Authors: Howard Keith A. , Foster David A.
Published: February 2004

We estimate here a geothermal gradient of only 17 ± 5°C km−1 for the tilted Grayback fault block in southeastern Arizona when extension began ∼25 Ma. This gradient is lower than preextension gradients estimated elsewhere in the Basin and Range, is only about 50% of typical gradients in the Basin and Range today, and needs to be accounted for in models of continental extension. The Grayback block exposes a 12‐km‐thick crustal section of Proterozoic and Cretaceous granitoids, which was tilted 90° during extension between 25 and 15 Ma. Zircon fission‐track ages decrease structurally downward (westward) across the block and were all within a zone of partial track annealing prior to tilting and quenching. The zircon age gradient suggests that the 220°–240°C isotherm migrated downward 5–6 km during Paleogene erosion and regional cooling. Apatite fission‐track ages decrease westward from ∼83 Ma in the structurally highest crystalline rocks to ∼24 Ma at ∼6‐km paleodepth and then to ∼15 Ma another 6 km farther west. Track‐length analysis confirms that apatites above the break in slope in age at ∼5.7‐km paleodepth resided in a zone of partial annealing prior to tilting, and deeper apatites record rapid cooling upon tilting and unroofing beginning ∼25 Ma. At that time the 110 ± 10°C isotherm determined by the depth at which tracks in apatite were fully erased was at a basement paleodepth of ∼5.7 km, and the 220 ± 30°C isotherm as estimated from zircon data resided at a pretilting basement depth of ∼12.15 km. From consistent values of paleogeothermal gradient for two depth intervals we estimate the pretilt gradient was 17 ± 5°C km−1. From 25 to 15 Ma the rotating Grayback block cooled rapidly as higher, westward moving blocks unroofed it tectonically at a rate of ∼1 km m.y.−1.
Observations concerning the vigor of hydrothermal circulation in young oceanic crust
Authors: Davis Earl E. , Chapman David S. , Forster Craig B.
Published: February 2004

A detailed suite of seafloor heat flow measurements and seismic reflection profiles has been completed in a young (circa 1 Ma) area on the eastern flank of the Juan de Fuca Ridge that is characterized by unusually smooth basement topography and uniform sediment cover. Measurements spaced nominally 100 m apart along one 5‐km‐long line segment define a coherent pattern of heat flow variation. The profile exhibits a series of four maxima and minima with an average half‐wavelength of 600 m and an amplitude of variation of 35 mW m−2, roughly 15% of the average background heat flow of 270 mW m−2. The heat flow variations are uncorrelated with local basement topography or sediment thickness variations and may reflect cellular convection in the extrusive layer of the igneous oceanic crust. This layer, which is bounded above by low‐permeability sediments and below by low‐permeability intrusive rocks, is imaged locally along a multichannel seismic reflection profile and estimated to be about 600 m thick. Temperatures estimated at the top of this layer by extrapolation of the seafloor heat flow measurements average roughly 40°C and vary between adjacent heat flow maxima and minima by only about 7 K. These observations, together with a series of numerical simulations of hydrothermal circulation in a confined, permeable upper crustal layer, provide quantitative insights into the convection process. Values of upper crustal permeability used in the simulations ranged from below the critical value required for convective instability to roughly 2 orders of magnitude above. Results show the amplitudes of lateral seafloor heat flow variability, upper basement temperature variability, and fluid‐pressure variability to be strongly dependent on permeability. For example, the amplitude of heat flow variations is predicted to increase rapidly above critical conditions, from zero at a subcritical permeability (k) of 2 × 10−14 m2 to 75 mW m−2 at k = 5 × 10−14 m2 and to a maximum of 90 mW m−2 at k = 8 × l0−14 m2. Predicted variability then falls roughly as k−1/2, reaching the level observed in the Juan de Fuca Ridge flank study area at a permeability of 2 × 10−12 m2. Although a value at near‐critical conditions is also allowed by the results, the higher value is probably correct, for it is known from other observations that hydrothermal circulation persists in crust of much greater age, despite the effect of chemical alteration, which reduces permeability, and thermal aging, which reduces buoyancy driving forces. The value of average permeability thus estimated for the upper oceanic crust is more than 1 order of magnitude greater than values determined in deep‐ocean boreholes. The borehole values may be lower because they are representative of older and/or more highly altered crust or because they do not correctly represent the permeability at the full scale of the convective system.
Timing and slip for prehistoric earthquakes on the Superstition Mountain Fault, Imperial Valley, southern California
Authors: Gurrola Larry D. , Rockwell Thomas K.
Published: February 2004

Trenches excavated across the Superstition Mountain fault in the Imperial Valley, California, have exposed evidence for four prehistorical earthquakes preserved in displaced lacustrine stratigraphy associated with ancient Lake Cahuilla. The presence of shoreline peat accumulations along with abundant detrital charcoal allows for high‐precision age determination of some stratigraphic units, thereby providing constraints on the timing of three of the paleoearthquakes. These three events occurred within a 480‐ to 820‐year interval during the past 1200 years. The most recent earthquake (event 1) occurred during a fluvial phase of deposition between A.D. 1440–1637, immediately prior to the inundation of the Cahuilla basin at about A.D. 1480 and 1660. A channel margin was offset 2.2 +0.4/−0.15 m in this rupture, suggesting an earthquake with a magnitude ≥7. The penultimate event (event 2) also occurred during fluvial deposition after A.D. 1280 but before another lakestand at A.D. 1440–1640. Lateral slip could not be resolved for event 2. However, based on juxtaposition of dissimilar units and the amount of deformation produced by this event, it is presumed that this was also a large earthquake. The timing of event 3 is constrained to have occurred between about A.D. 820 and 1280. This event is represented by several fractures and small displacements that rupture up to a distinct stratigraphic level or event horizon. Slip was not resolved for this event. Finally, the timing of event 4 is very poorly constrained to between A.D. 964 and 4670 B.C. Undoubtedly, many events may have occurred during this period. Notably, the past three earthquakes occurred within a period of less than 820 years, and it has been over 350 years since the last earthquake.
Surface deformation due to a strike‐slip fault in an elastic gravitational layer overlying a viscoelastic gravitational half‐space
Authors: Yu Ting‐To , Rundle John B. , Fernández Jose
Published: February 2004

We extend a technique previously used to model surface displacements resulting from thrust faulting in an elastic‐gravitational layer over a viscoelastic‐gravitational half‐space to the case of strike‐slip faulting. The method involves the calculation of the Green's functions for a strike‐slip point source contained in an elastic‐gravitational layer over a viscoelastic‐gravitational half‐space. The correspondence principle of linear viscoelasticity is applied to introduce time dependence. The resulting Green's functions are then integrated over the source region to obtain the near‐field displacements. Several sample calculations are presented involving 90° and 30° dipping faults and ruptures completely and partially through the elastic layer. We also illustrate the time dependent deformation due to a buried fault. Results show that the use of a viscoelastic half‐space underlying an elastic layer introduces a long wavelength component into the deformation field [Cohen and Kramer, 1984], even in cases of non vertical strike‐slip fault and inclusion of gravitational effect, that cannot be modeled by purely elastic techniques. Calculations have shown that vertical postseismic displacement is insignificant and that the horizontal movement is about the same magnitude as the coseismic strike‐slip displacement. The inclusion of gravity affects the horizontal displacement due to vertical strike‐slip faulting in far field and the vertical displacement for dipping strike‐slip faulting in near‐field. The computed results have been fit to the Global Positioning System measurements of the Landers earthquake taken shortly after the main shock, assuming a relaxation time of the order of days. This relaxation time is considerably shorter than times of the order of years to decades found in previous studies. The major differences between this detailed three‐dimensional and simplified two‐dimensional model are the decay of magnitude in displacement field and the distinct displacement pattern in the regions beyond the fault tip. The displacement field due to the cyclic earthquakes was constructed by considering the finite fault length and inclusion of gravity. It is found that the displacement field is dominated by the plate motion in the case of short recurrence time. On the other hand, a “looping” and migrating pattern in the displacement field is found in the case of very long recurrence time, which is not seen in those 2D simplified models.
Basalts from the Central Pacific Basin: Evidence for the origin of Cretaceous igneous complexes in the Jurassic western Pacific
Authors: Janney Philip E. , Castillo Paterno R.
Published: February 2004

Studies of marine magnetic anomalies suggest that oceanic crust of Jurassic age underlies the Nauru, East Mariana and northwestern Central Pacific basins of the west central Pacific Ocean. However, the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program have only recovered basalts of Cretaceous age from these basins, indicating either that large areas of the Jurassic western Pacific are covered by Cretaceous intraplate igneous complexes or that Cretaceous ocean crust is present in these areas. We present chemical and isotopic data on basalts and dolerites recovered by DSDP Leg 17 from the Central Pacific Basin (CPB). Drilling in the predicted Jurassic‐age portion of the CPB recovered Late Albian (100–105 Ma) tholeiitic pillow basalts at Site 169 and Late Cretaceous alkalic dolerite sills at Sites 170 and 169 above the extrusives. Early Cretaceous crust was recovered from Site 166. The Site 169 tholeiites are LREE depleted but slightly enriched in highly incompatible elements relative to normal mid‐ocean ridge basalt (MORB), giving them trace element ratios similar to MORB erupted near hot spots. The Sr, Nd, and Pb isotopic compositions of the tholeiites (87Sr/86Sri = 0.70341–0.70348; εNd(t) = +6.2–6.4; 206Pb/204Pbmeas = 18.63–18.68) overlap with MORB from the Indian Ocean, but fall outside of the Sr and Nd isotopic ranges for Pacific MORB. The Site 169 tholeiites are compositionally almost identical to basalts from the Nauru and East Mariana basins and are isotopically similar to some Ontong Java Plateau basalts. Site 166 crustal lavas are similar to normal‐MORB from the East Pacific Rise. Chemical and isotopic data for the Site 169 tholeiites are consistent with an origin at a spreading center contaminated with EM I‐type plume materials, probably from the Ontong Java plume head. Based on geochemical and geophysical data from the region, we propose that the Site 169 tholeiites, as well as basalts from the Nauru and East Mariana basins, were created at a system of short‐lived mid‐Cretaceous spreading centers extending from the East Mariana Basin into the northwestern Central Pacific Basin, and that rifting of Jurassic crust was initiated as a result of the rapid formation of Ontong Java Plateau. The Sites 169 and 170 sills appear to have been intruded as a result of near‐ridge, non‐hot spot volcanism similar to that producing young seamounts in the eastern Pacific today. However, the intrusion of these sills, as well as their HIMU (high μ) isotopic affinity, may have been influenced by nearby mantle plumes.
Effects of strongly variable viscosity on three‐dimensional compressible convection in planetary mantles
Authors: Tackley Paul J.
Published: February 2004

A systematic investigation into the effects of temperature dependent viscosity on three‐dimensional compressible mantle convection has been performed by means of numerical simulations in Cartesian geometry using a finite volume multigrid code, with a factor of 1000–2500 viscosity variation, Rayleigh numbers ranging from 105–107, and stress‐free upper and lower boundaries. Considerable differences in model behavior are found depending on the details of rheology, heating mode, compressibility, and boundary conditions. Parameter choices were guided by realistic Earth models. In Boussinesq, basally heated cases with viscosity solely dependent on temperature and stress‐free, isothermal boundaries, very long wavelength flows (∼25,000 km, assuming the depth corresponds to mantle thickness) with cold plumes and hot upwelling sheets result, in contrast to the upwelling plumes and downwelling sheets found in small domains, illustrating the importance of simulating wide domains. The addition of depth dependence results in small cells and reverses the planform, causing hot plumes and cold sheets. The planform of temperature‐dependent viscosity convection is due predominantly to vertical variations in viscosity resulting from the temperature dependence. Compressibility, with associated depth‐dependent properties, results in a tendency for broad upwelling plumes and narrow downwelling sheets, with large aspect ratio cells. Perhaps the greatest modulation effect occurs in internally heated compressible cases, in which the short‐wavelength pattern of time‐dependent cold plumes commonly observed in constant‐viscosity calculations completely changes into a very long wavelength pattern of downwelling sheets (spaced up to 24,000 km apart) with time‐dependent plumelike instabilities. These results are particularly interesting, since the basal heat flow in the Earth's mantle is usually thought to be very low, e.g., 5–20% of total. The effects of viscous dissipation and adiabatic heating play only a minor role in the overall heat budget for constant‐viscosity cases, an observation which is not much affected by the Rayleigh number. However, viscous dissipation becomes important in the stiff upper boundary layer when viscosity is temperature dependent. This effect is caused by the very high stresses occurring in this stiff lid, typically 2 orders of magnitude higher than the stresses in the interior of the domain for the viscosity contrast modeled here. The temperature in the interior of convective cells is highly sensitive to the material properties, with temperature‐dependent viscosity and depth‐dependent thermal conductivity strongly increasing the internal temperature, and depth‐dependent viscosity strongly decreasing it. The sensitivity of the observed flow pattern to these various complexities clearly illustrates the importance of performing compressible, variable‐viscosity mantle convection calculations with rheological and thermodynamic properties matching as closely as possible those of the Earth.
Tectonic stress within the New Madrid seismic zone
Authors: Grana Jeffrey P. , Richardson Randall M.
Published: February 2004

Refraction data indicate a significant high‐density rift pillow beneath the New Madrid seismic zone. We present results of linear and nonlinear viscoelastic finite element modeling to determine whether support of the rift pillow may contribute significantly to the total present‐day stress field, and we consider the implications for intraplate seismicity. These models were run for a loading time of 100 m.y. to account for relaxation and transfer of stress since the last reactivation of the rift in the mid‐Mesozoic. Results indicate that the nonlinear viscoelastic model with rheological stratification based on composition and temperature agrees well with the observed deformation within the seismic zone and with estimates of regional stress magnitudes. The model predicts a maximum compression of 30–40 MPa above the rift pillow in the center of the rift axis. If the magnitude of local compression predicted by the nonlinear model produces the inferred clockwise rotation of the order of 10°–30° in the direction of SHmax (maximum horizontal compression) near the rift axis, the magnitude of regional compression is a factor of 1 to 2 times the magnitude of local compression and consistent with an origin due to ridge push forces. The addition of the local stress associated with the rift pillow, however, results in an approximately 30% reduction in the resolved maximum horizontal shear stress. Thus, while the stress associated with the rift pillow can rotate the stress field into an orientation favorable for failure, reduction in the resolved shear stress requires a separate mechanism for strength reduction. Results of the modeling indicate that stresses from the load of the rift pillow may still be present in the upper crust even after 100 m.y. and may still play a role in present‐day deformation and seismicity of the New Madrid seismic zone. Local stress fields of significant tectonic magnitudes may also occur around other ancient rift pillows and help explain the observed correlation between intraplate seismicity and failed rift zones.
Paleomagnetism of upper Miocene ignimbrites at the Puna: An analysis of vertical‐axis rotations in the Central Andes
Authors: Somoza Rubén , Singer Silvia , Coira Beatriz
Published: February 2004

The origin of the rotations detected paleomagnetically in the Central Andes is controversial. Tectonic models proposed to explain it involve Late Cenozoic oroclinal bending (the Bolivian Orocline), and/or small‐block rotations driven by oblique subduction. In this paper, we report paleomagnetic data from upper Miocene ignimbrites from the Puna, in the presumed southern limb of the orocline. These rocks showed high unblocking temperature and high coercivity magnetization. The resulting pole position (latitude 85.7°S, longitude 80.5°E, A95 = 7.9°, K = 22, N = 17) indicates that the study area has not been significantly affected by vertical‐axis rotation since late Miocene onward. This implies that the whole Puna has not undergone significant regional rigid‐body rotation during the same time span. A farther analysis based on Neogene paleomagnetic data from 31 Andean localities distributed from 11°S to 31°S shows the presence of rotated and unrotated areas. Systematic counterclockwise rotations in Perü and northern Bolivia are observed, whereas clockwise rotations are present in southern Bolivia, northern Chile, and perhaps northwestern Argentina. No systematic rotations are found farther south. The overall time‐spatial distributions of these Neogene paleomagnetic data suggest that if orogenic bending occurred, it must have taken place before the middle Miocene. Thus the oroclinal hypothesis cannot explain the rotations observed in middle Miocene rocks or younger ones, which also show orocline‐like declination anomalies. We suggest that small‐block rotations driven by distributed shear may be a single process that can account for the whole rotations in the Central Andes. It is hypothesized that horizontal shear in the Andean crust could be controlled by ancient structures.
Magnetic potential and magnetization contrasts of Earth's lithosphere
Authors: Arkani‐Hamed J. , Dyment J.
Published: February 2004

An inversion technique is developed in order to transform a global scalar magnetic anomaly map into a global magnetic susceptibility contrast map which delineates the magnetic characteristics of the lithosphere. The inversion involves two stages. In the first stage, a scalar magnetic anomaly map is transformed into a magnetic potential map. This stage requires no simplifying assumption, and it provides a means to upward/downward continue global scalar magnetic anomaly maps. In the second stage, the potential map is inverted into a magnetic susceptibility contrast map of the lithosphere based on the assumption that the magnetization is of induced origin. The magnetization of the continental crust that gives rise to satellite magnetic anomalies is largely of induced origin. It is also shown that the magnetization direction of the oceanic lithosphere does not differ significantly from the direction of the present core field, except in a few limited areas, supporting the assumption made about the induced magnetization of the lithosphere. The technique is applied to the scalar magnetic anomaly map of Earth derived using POGO and Magsat data. The resulting susceptibility contrasts directly correlate with geological features and better delineate small‐scale features due to the enhancement of the higher‐degree harmonics compared to the lower‐degree ones.
Rheology of water ices V and VI
Authors: Durham William B. , Stern Laura A. , Kirby Stephen H.
Published: February 2004

We have measured the mechanical strength (σ) of pure water ices V and VI under steady state deformation conditions. Constant displacement rate compressional tests were conducted in a gas apparatus at confining pressures from 400 < P < 800 MPa, temperatures from 209 < T < 270 K, and strain rates 7 × 10−7 <
< 7 × 10−4 s−1. Most of the results fit to an empirical flow law of the form
= A σn exp {−(E* + PV*)/RT}, where the four material constants A, n, E*, and V* are (for
in inverse seconds and P and σ in megapascals) 1023.0, 6.0, 136 kJ/mol, and 29 cm3/mol, respectively, for ice V; and 106.7, 4.5, 66 kJ/mol, and 11 cm3/mol, respectively, for ice VI. Ice VI may weaken to a mechanism of higher E* at T > 250 K. Ices V and VI are thus rheologically distinct but by coincidence have approximately the same strength under the conditions chosen for these experiments. To avoid misidentification, these tests are therefore accompanied by careful observations of the occurrences and characteristics of phase changes. One sample each of ice V and VI was quenched at pressure to metastably retain the high‐pressure phase and the acquired deformation microstructures; X ray diffraction analysis of these samples confirmed the phase identification. Surface replicas of the deformed and quenched samples suggest that ice V probably deforms largely by dislocation creep, while ice VI deforms by a more complicated process involving substantial grain size reduction through recrystallization.
Origin and depositional environment of clastic deposits in the Hilo drill hole, Hawaii
Authors: Beeson M. H. , Clague D. A. , Lockwood J. P.
Published: February 2004

Volcaniclastic units cored at depths of about 87, 164, 178, 226, and 246 m below sea level and carbonate units located between depths of 27 and 53 m below sea level in the Hilo drill core were found to be deposited at or near sea level. Four of these units are hydroclastic deposits, formed when subaerially erupted Mauna Loa lava flows entered the ocean and fragmented to produce quenched, glassy fragments during hydrovolcanic explosions. Ash units 24 and 26, at 178 m depth, accumulated at sea level in a freshwater bog. They contain pyroxenes crystallized from tholeiitic magma that we infer erupted explosively at the summit of Kilauea volcano. Two carbon‐rich layers from these ashes have a weighted average radiocarbon age of 38.6 ± 0.9 ka; the ashes probably correlate with the oldest and thickest part of the Pahala ash. Ash unit 44, at the transition from Mauna Kea to Mauna Loa lava flows, was probably nearly 3.2 m thick and is inferred to be equivalent to the lower thick part of the composite Homelani ash mapped in Hilo and on the flanks of Mauna Kea. The age of this part of Homelani ash is between 128 ± 33 and 200 ± 10 ka; it may have erupted subglacially during the Pohakuloa glacial maxima on Mauna Kea. Beach sand units 12 and 22 were derived from nearby Mauna Loa and Mauna Kea lava flows. The middle of beach sand unit 38 was derived mainly from lava erupted near the distal end of the subaerial east rift zone of Kilauea volcano; these sands were transported about 33 km northwest to Hilo Bay by prevailing longshore currents. Combined age, depth, and sea level markers in the core allow us to determine that lava flow recurrence intervals averaged one flow every 4 kyr during the past 86 kyr and one flow every 16 kyr between 86 and 200 ka at the drill site and that major explosive eruptions that deposit thick ash in Hilo have occurred only twice in the last 400 kyr. These recurrence intervals support the moderate lava flow hazard zonation (zone 3) for coastal Hilo previously determined from surficial mapping.
Scaled experiments of volcanic spreading
Authors: Merle Olivier , Borgia Andrea
Published: February 2004

Experiments were conducted to study the spreading of volcanic constructs. Volcanoes are simulated by a sand cone, and the volcanic substratum is simulated by a sand layer (brittle substratum) overlying a silicone layer (ductile substratum). Similarity conditions between natural volcanoes and experimental prototypes led to the definition of dimensionless π numbers. Experiments determine π values which predict whether or not spreading takes place. Of particular importance are the ratio between the thickness of the brittle substratum and the height of the volcano (π2) and the brittle/ductile ratio of the substratum (π3). π2 indicates that the volcano must be large enough to “break” the substratum before spreading occurs, whereas π3 controls the style of deformation. During spreading, these dimensionless numbers change with time, reaching values that tend toward those observed for stable configurations. Experimental values are compared with those from well‐constrained natural examples. It is found that an essential requirement for volcanic spreading is the presence of a low‐viscosity layer within the substratum. Flow of the weak layer away from the excess load is responsible for the spreading. The overlying edifice displays radial intersecting grabens, due to concentric stretching, dissected summit areas; concentric zones of thrusts and folds form in the substratum around the edifice, and diapirs of the ductile substratum rise within the fault zones.
Marine controlled‐source electromagnetic sounding: 2. The PEGASUS experiment
Authors: Constable S. , Cox C. S.
Published: February 2004

The marine controlled‐source electromagnetic sounding method developed over the past 15 years at Scripps Institution of Oceanography employs a towed seafloor electric dipole transmitter of moment 4 × 104 Am and multiple free‐vehicle seafloor electric field recorders. A survey of 40 Ma normal oceanic lithosphere in the northeast Pacific using frequencies of 0.25 to 24 Hz and synchronous stacking of 0.25‐ to 12‐hour‐duration detected signals at transmitter‐receiver ranges between 5 and 95 km. One‐dimensional electrical conductivity structure is recovered from the data using the Occam process of nonlinear regularized inversion. Repeated inversion of a model terminated with an essentially infinite conductor or resistor demonstrates that the maximum depth of inference for this experiment is about 30 km, well into the upper mantle, with bounds placed on conductivity to depths of 60 km. Structure shallower than about 1 km is comparable to that obtained by a similar experiment on the East Pacific Rise and by borehole logging, with a sharp increase in resistivity at depths of 600–800 m, although strictly our experiment is sensitive only to integrated square root of conductivity, or total attenuation, in the surface layers. The lower crust and upper mantle has a resistivity between 2 and 7 × 104 Ω m and a transverse resistance of at least 109 Ω m2, suggesting at most 0.3% volume fraction of free water in the lower crust and some form of conductivity enhancement over mineral conductivity in the uppermost mantle. Although resolution is weak, below 30 km our data are compatible with a dry olivine model of mantle conductivity‐temperature.