bluuebunny/crossref_metadata_2025 huggingface dataset:
An episode of seafloor spreading and associated plate deformation inferred from crustal fluid pressure transients
Authors: Davis E. E. , Wang K. , Thomson R. E. , Becker K. , Cassidy J. F.
Published: February 2004

Three‐year records of crustal fluid pressures and temperatures at four Ocean Drilling Program (ODP) sites on the northern Juan de Fuca Ridge and eastern ridge flank reveal a broad range of variations that include hydrologic transients that are contemporaneous with earthquakes along the ridge axis, the Nootka transform fault, and within the Juan de Fuca plate. One example of such a transient is the response to what is inferred to be a seafloor spreading event that triggered a swarm of earthquakes near the Endeavour ridge segment, beginning with a MW = 4.6 event on June 8, 1999. The largest transients were observed at ODP Sites 1024 and 1025 located 25.6 and 33.5 km east of the Endeavour axis. Pressures rose coseismically with the first earthquake of the swarm by roughly 1.6 and 1 kPa, continued to rise to maxima of 3.2 and 2.0 kPa within hours, then decayed to half the peak levels in 1 and 2 days at the two respective sites. A small (∼0.2 kPa) response of the same sign followed by a decay over 100 days was observed at Site 1027 situated 101 km east of the axis, and a similarly small response but of opposite sign was observed at Site 857 in Middle Valley, located along strike of the Endeavour segment roughly 50 km to the north of the earthquake swarm. The pressure transients are inferred to reflect a combination of the instantaneous internal plate deformation associated with extension at the ridge axis and lateral water flow in the crust following strain‐induced fluid pressure gradients. The rate at which the transients dissipate constrains the regional‐scale permeability of the upper igneous crust to be of the order of 10−10–10−9 m2. Instantaneous strain calculated from the initial amplitude of the transients ranges from 8×10−9 at the most distal site to 1.7×10−7 at the most proximal. The magnitude of regional strain is much larger than that which would result from the simultaneous earthquake, and we conclude that the first and all subsequent earthquakes of the swarm, and the crustal strain responsible for the hydrologic transients, are the consequence of a dominantly aseismic spreading “event” involving ∼12 cm of dilatation at the ridge axis. There were no clear indications of associated magmatic activity; hence episodic spreading may take place outside the influence of either dike injection or seismic rupture. Given the excellent sensitivity of pore pressure to strain, we anticipate that this simple observational technique can be applied to the investigation of seismic and aseismic deformation in a variety of tectonically active settings.
Gas‐saturated crystallization and degassing in large‐volume, crystal‐rich dacitic magmas from the Altiplano‐Puna, northern Chile
Authors: Schmitt Axel K.
Published: February 2004

Melt inclusions hosted in quartz from large‐volume ignimbrites and related lava flows from the late Neogene to Pleistocene Altiplano‐Puna Volcanic Complex, northern Chile, record the magmatic volatile evolution and constrain conditions of magma storage. Glasses from pristine and rehomogenized inclusions have high‐Si rhyolitic compositions (average SiO2=77.5 wt%). Their host rocks range from dacite to rhyodacite (SiO2=63.9–72.5 wt%) and have a high abundance of phenocrysts (33–55%). Infrared spectroscopic analysis of inclusions from pumice samples typically yielded H2O contents between 3.0 and 4.0 wt% and relatively low and more variable CO2 contents <400 ppm. Increasing H2O contents were found in a series of successively trapped inclusions, and bubble‐free inclusions tend to have lower CO2 contents with increasing H2O. Inclusions from lava samples have lower but constant H2O contents of 2.0±0.3 wt% and CO2 close to the detection limit (∼10 ppm). Incompatible trace elements with high affinities to partition into a fluid phase (e.g., B, Cl) show minor enrichment in melt inclusions, whereas compatible trace elements (e.g., Sr, Ba) became strongly depleted due to feldspar‐dominated crystallization. Variations in H2O and CO2 contents as well as concordant preeruptive pressures inferred from volatile solubility and Al‐in‐hornblende barometry (150±50 MPa) also indicate volatile saturation and upper crustal magma storage for the ignimbrite magmas. The melt inclusion record is consistent with near‐isobaric cooling from ∼830° to 780°C (inferred from mineral thermometry) under gas‐saturated conditions in shallow (4–6 km deep) reservoirs. Model calculations for overpressures generated by closed‐system crystallization and gas exsolution either result in premature failing of the magma chamber walls or require the presence of large volumes of highly compressible magmatic foam. As an alternative, open‐system degassing of these magmas prior to their explosive and effusive eruption is proposed.
Characterization of the San Jacinto fault zone near Anza, California, by fault zone trapped waves
Authors: Li Yong‐Gang , Vernon Frank L.
Published: February 2004

We installed three 350‐m‐long seismic arrays, each array consisting of 12 three‐component stations, across the Coyote Creek fault (CCF), Clark Valley fault (CVF), and Buck Ridge fault (BRF) of the San Jacinto fault zone (SJFZ) near Anza, California, to record fault zone trapped waves from microearthquakes. We observed trapped waves with relatively large amplitudes and long duration at stations close to the fault traces for earthquakes occurring within the fault zone. The coda‐normalized amplitude spectra of trapped waves showed peaks at 4–7 Hz, which decreased sharply with the distance from the fault trace. Observations and three‐dimensional finite difference simulations of trapped waves revealed low‐velocity and low‐Q waveguides on these active faults with the width of 75–100 m in which shear velocities are reduced by 25–30% from wall rock velocities and Q values are 40–90 at depths between the surface and 18 km. The locations of earthquakes for which we observed trapped waves delineate the most seismically active fault strands of the SJFZ in a region with complicated slip planes near Anza. The low‐velocity waveguides inferred from trapped waves extend 15 to 20 km in the length on these active faults and are segmented by the fault discontinuities. The waveguide on the BRF dips south westward to connect the waveguide on the CVF, which dips northeastward. This waveguide extends at the seismogenic depth through Anza slip gap to another low‐velocity waveguide on the Casa Loma fault (CLF), which has been delineated in our previous study of the SJFZ using trapped waves [Li et al., 1997]. The waveguide on the CCF in Coyote Mountain is nearly vertical and disconnected from the CLF at the south edge of Anza gap. We interpret the low‐velocity waveguides on these active strands to partly result from recent prehistoric significant earthquakes on them and evaluate the future earthquake in the Anza region
How to relate body wave and surface wave anisotropy?
Authors: Montagner Jean‐Paul , Griot‐Pommera Daphné‐Anne , Lavé Jérôme
Published: February 2004

Seismic anisotropy is one of the most efficient geological and geodynamical tools for understanding the dynamics of the Earth. Upper mantle anisotropy is evident in seismic data sets for the last 30 years primarily from surface wave dispersion curves and body wave SKS data. We demonstrate in this paper that surface wave and body wave derived anisotropy can be explained by the same anisotropic parameters (L, Gc, Gs) in the simplest case of a horizontal fast symmetry axis. One application of this method is to display the SKS delay time and the corresponding azimuth, which can be derived from surface wave global (with a lateral resolution of around 2000 km) and regional (lateral resolution of 350 km) anisotropy tomography. A global‐scale comparison is disappointing since there are only a few areas where both data types are correctly retrieved at the same spatial scale (same lateral resolution). The anisotropy is well resolved in oceanic basins from surface waves, whereas most measurements of SKS splitting are available below continents. However, there is a good agreement between surface wave and body wave anisotropy in regions where large‐scale processes (primarily tectonic) are taking place, such as the western United States and in central Asia. In particular, we show that, at high frequency (> 1Hz), the observed SKS delay time and azimuth depend on the order in which S waves propagate through the layers. A comparison between synthetic and observed SKS phases in central Asia highlights the varying sensitivity of SKS waves at depth. We can obtain a good correlation between synthetic and observed SKS by considering anisotropy only between 80 and 200 km depth in the tomographic model, rather than in the entire upper mantle. However, it is more difficult to explain the discrepancy between synthetic and observed SKS under the eastern United States, where a large‐scale coherent SKS anisotropy is present. Such results call for more sophisticated SKS waveform modeling and for enhanced lateral resolution in anisotropic tomographic models.
Recent eruptions on the CoAxial segment of the Juan de Fuca Ridge: Implications for mid‐ocean ridge accretion processes
Authors: Embley R. W. , Chadwick W. W. , Perfit M. R. , Smith M. C. , Delaney J. R.
Published: February 2004

The 1993 seismic swarm and volcanic eruption on the CoAxial segment of the Juan de Fuca Ridge was the first verified mid‐ocean ridge accretion event monitored by the National Oceanic and Atmospheric Administration/U.S. Navy Sound Surveillance System (SOSUS). Ambiguity in the location of the initial seismicity resulted in an uncertainty as to whether the dike intruded from the summit and north rift zone of the adjacent Axial Volcano or whether it arose locally within the CoAxial segment. However, analyses of multibeam, side‐scan sonar, towed camera, submersible, and geochemical data show that the CoAxial segment is morphologically, structurally, and geochemically distinct from the north rift zone of Axial Volcano. There is no geologic or geochemical evidence that dike injections from Axial Volcano have extended north of 46°18'N in the past, whereas the 1993 eruption site is at 46°31'N. Furthermore, all seafloor manifestations of the 1993 dike injection lie along the central neovolcanic zone of the CoAxial segment. Analyses of repeat SeaBeam surveys combined with seafloor observations show that two other eruptions of approximately the same volume as the 1993 eruption occurred along the CoAxial segment in the 1981–1991 interval. These three diking events may have relieved decades of accumulated stress over ∼35 km or more of this segment. The spatial pattern and hydrothermal history of the 1993 event is consistent with a dike with a significant lateral component of injection.
Internal stress field at Mount Vesuvius: A model for background seismicity at a central volcano
Authors: De Natale Giuseppe , Petrazzuoli Stefano M. , Troise Claudia , Pingue Folco , Capuano Paolo
Published: February 2004

We propose a model to explain the background seismicity occurring at Somma‐Vesuvius in its present, mostly quiescent period. A finite element procedure has been used to simulate the stress field due to gravitational body forces in an axisymmetric volcano characterized by a central high‐rigidity anomaly. Results emphasize the important effect of axial high‐rigidity, which concentrates at its borders stresses resulting from the gravitational load of the volcanic edifice, as well as external (regional) stresses. The joint effect of the gravitational loading and of the presence of the anomaly produces stresses very close to or above the critical rupture threshold. The observed spatial concentrations of seismicity and moment release correlate well with peaks of computed maximum shear stress. Seismicity is then interpreted as due to small stress perturbations concentrated around the high‐rigidity core and added to a system already close, to the failure threshold. This model can explain the widely observed occurrence of background seismicity at central volcanoes worldwide.
Velocity field in Asia inferred from Quaternary fault slip rates and Global Positioning System observations
Authors: Holt W. E. , Chamot‐Rooke N. , Le Pichon X. , Haines A. J. , Shen‐Tu B. , Ren J.
Published: February 2004

We perform a joint inversion of Quaternary strain rates and 238 Global Positioning System (GPS) velocities in Asia for a self‐consistent velocity field. The reference frames for all geodetic velocity observations are determined in our inversion procedure. India (IN) moves relative to Eurasia (EU) about a pole of rotation at (29.78°N, 7.51°E, 0.353° Myr−1), which yields a velocity along the Himalaya within India that is ∼73–76% of the magnitude of the IN‐EU NUVEL‐1A velocity and a vector azimuth that is 8–10° clockwise of NUVEL‐1A IN‐EU vector azimuth. Relative to Eurasia, south China moves at 9–11 mm/yr in the direction 110–120° with a pole position (64.84°N, 156.74°E, 0.12° Myr−1). Amurian block motion has a pole position in a similar location but at a slower rate (64.61°N, 158.23°E, 0.077° Myr−1) and most of the Amurian‐Eurasia motion is accommodated by extension across Lake Baikal. Tarim Basin moves relative to Eurasia about a pole of rotation at (39.24°N, 98.2°E, −0.539° Myr−1) and ∼16–18 mm/yr of shortening is accommodated across the west central Tien Shan. There is distributed E‐W extension throughout both southern and north central Tibet. Within southern Tibet, between the longitudes of 77°E to 92°E, the deformation field accommodates ∼16–19 mm/yr of E‐W extension. We compare predicted seismic moment rates with those observed in this century in Asia. Total observed seismic moment rates within the entire area of central and east Asia (2.2×107 km2) in this century are 2.26±0.7×1020 N m yr−1 as compared with a predicted total rate of 2.03±0.066×1020 N m yr−1. Comparisons between observed and predicted moment rates within 42 subregions reflect the generally unstable process of inferring long‐term seismic moment rates from a catalog of limited duration (94 years). An observation period of ∼10,000 years would be required to reduce uncertainties in observed seismic moment rate to the same size as the uncertainties in model tectonic moment rates, inferred from the joint inversion of GPS and Quaternary rates of strain. We show that in general, a better correlation with model tectonic moment rate is inferred from the seismicity catalog by considering the numbers of earthquakes above a cutoff magnitude (mb ≥ 5.0, for the period January 1, 1965, to January 1, 1999).
An ab initio study of hydrogen in forsterite and a possible mechanism for hydrolytic weakening
Authors: Brodholt John P. , Refson Keith
Published: February 2004

Even small amounts of water can profoundly effect the physical properties of minerals. In olivine <1 H in every 1000 unit cells acts to increase creep rates of dunite by ∼ 2 orders of magnitude. Although the mechanism for this is not known, it is not unreasonable to suggest that it is in some way related to an increase in the point defect population. In order to understand this better we have performed ab initio pseudopotential calculations within the generalized gradient approximation on protonic defects in Mg2SiO4 forsterite. Three mechanisms for incorporating protons are considered: (1) interstitial, (2) binding at cation vacancies, and (3) binding at silicon vacancies. Assuming the existence of both Si and Mg vacancies, on energetic considerations, protons will initially populate Si vacancies until there are three protons in the vacancy. At this point, the addition of one more proton (to make a hydrogarnet substitution) is energetically unfavourable in comparison to populating the Mg vacancy, and the next proton will enter the Mg site. Interstitial protons will be very rare. Since it is >2 eV more favourable to put the first proton into the Si vacancy than the magnesium site, the presence of water will certainly act to increase the population of silicon vacancies. In fact, in the presence of water the energy required to form a Si vacancy is perhaps less than that to form an Mg vacancy. This is in stark contrast to dry olivine where Si vacancies are many eV less favourable. If creep is rate limited by the diffusion of the slowest species, silicon in olivine, then increasing the Si vacancy concentration could provide a mechanism for hydrolytic weakening.
Decoupled flexure in the South Pyrenean Foreland
Authors: Waltham D. , Docherty C. , Taberner C.
Published: February 2004

Complex subsidence patterns during the mid‐late Eocene of the easternmost South Pyrenean foreland are well explained by a decoupled flexure model in which upper crustal buckling is accommodated by ductile flow in the mid‐lower crust while, simultaneously and independently, the lower lithosphere flexes above a ductile asthenosphere. The resulting predicted subsidence values agree with observation to well within the estimated uncertainties. The resulting lithospheric effective elastic thickness (EET) estimate (10±3 km) is low but similar to values obtained in the South Pyrenean foreland by other authors. The separate EET obtained for the upper crust (1.5±0.8 km) also agrees with tentative values given by other workers.
Fine structure of the rupture zone of the April 26 and 27, 1997, Northridge aftershocks
Authors: Venkataraman Anupama , Mori Jim , Kanamori Hiroo , Zhu Lupei
Published: February 2004

We investigated the rupture geometry of two Mw ∼4.5 earthquakes (April 26, 1997, and April 27, 1997) that occurred on the western edge of the aftershock zone of the 1994, Mw 6.7, Northridge earthquake. Both events have thrust mechanisms with a steep plane dipping ∼75°SE and a shallow plane dipping ∼45°NE. An empirical Green's function deconvolution followed by a waveform inversion was used to determine the slip distribution of the two events. The inversion results show that the steep plane fits the data slightly better than the shallow plane. The background seismicity (aftershocks of the 1994 Northridge event) in the epicentral region shows the existence of a north dipping fault plane, similar to that of the 1971 San Fernando earthquake. However, the spatial trend of the two April 1997 events and their aftershocks reveals tightly clustered seismicity on a steep plane dipping south. Relative relocation of the April 26 and 27 events shows that the April 27 event ruptured ∼1.4 km N70°E of the April 26 event and at a slightly shallower depth, i.e., almost along strike and on a steep plane dipping south. These observations suggest that the steep plane is the fault plane of both events. The two events ruptured on a plane which is almost perpendicular to the trend of the regional background seismicity. Thus the seismogenic structure beneath the Transverse Ranges exhibits complexity on scales of a few kilometers. The rupture area for both the April 26 and 27 events is ∼l km2 with a stress drop of at least 20 to 30 bars.
Volcanic plume electrification: Experimental investigation of a fracture‐charging mechanism
Authors: James M. R. , Lane S. J. , Gilbert J. S.
Published: February 2004

Although ashfall from particulate volcanic plumes is known to be highly electrically charged, little is known about the charging mechanism. We describe experiments designed to investigate the particle charges generated from the fracture of pumice. Small silicate particles were produced in the laboratory during collisions between two samples cut from pumice clasts. The net charge magnitudes detected on these particles are similar to those previously measured on ashfall from volcanic plumes (∼10−5 to 10−6 C kg−1). This net charge is also shown to be the result of a small imbalance between the sums of individual particle charges of both polarities, which are up to several orders of magnitude larger than the net charge. The magnitude of both the net and single polarity specific charges were only weakly affected by changes of relative humidity, but single polarity charges increased steadily with increasing sample impact velocities. The dominant charging process during the experiments was that of material fracture. The charging mechanism is thus interpreted to be fractoemission (the release of nuclear particles from fresh crack surfaces) occurring during the production of the silicate particles. This implies that the electrification of volcanic plumes could be the result of brittle fragmentation of magma or pumice clasts within the upper regions of the conduit and in the jet region of the plume.
A fossil, serpentinization‐related hydrothermal vent, Ocean Drilling Program Leg 173, Site 1068 (Iberia Abyssal Plain): Some aspects of mineral and fluid chemistry
Authors: Beard James S. , Hopkinson Laurence
Published: February 2004

The basement at Site 1068, Ocean Drilling Program (ODP) Leg 173 (serpentinized peridotite in fault contact with overlying amphibolite‐clast‐dominated sedimentary and tectonic breccias) is host to a hydrothermal system rooted in serpentinization reactions occurring at depth. The serpentinite grades downward from cataclasites at the fault, through brecciated, recrystallized, tochilinite‐bearing serpentinite, to awaruite‐bearing massive, mesh‐textured serpentinite. Andradite is common throughout and is a major sink for iron. The breccias are similarly zoned, from tectonized rocks near the fault upward into sedimentary breccias. Mg‐silicate vein assemblages and rodingitized amphibolite clasts near the fault give way to calcite veins and nonpervasive albite‐chlorite alteration upsection. Marcasite (± pyrrhotite at the fault) is the sulfide phase and occurs only in the tectonic breccias. Fe oxides are magnetite near the fault and hematite and ferric oxyhydroxides upsection. The zonation reflects mixing of seawater with a fluid whose composition (low fO2, fS2 Si, CO2, high Ca, Fe, Ca/Mg, pH) is controlled by serpentinization reactions. The deepest serpentinites have strongly reduced mineral assemblages that are unusual in a totally serpentinized peridotite. This probably reflects equilibration with a fluid derived from ongoing serpentinization at depth. The upper serpentinites, on through the mineral sequences seen in the breccias reflect increasing input from seawater upsection. Increased fO2 and fS2 stabilizes increasingly S‐ and O‐rich assemblages. Calcite (and ferric oxide) precipitation decreases pH, stabilizing marcasite. Relative to mid‐ocean ridge hydrothermal systems, fluids in serpentinite‐hosted hydrothermal systems are poor in S and rich in Mg and are unlikely to host large sulfide ore deposits.
The great March 25, 1998, Antarctic Plate earthquake: Moment tensor and rupture history
Authors: Henry C. , Das S. , Woodhouse J. H.
Published: February 2004

We use broadband body and mantle wave data to study the 1998 Antarctic intraplate earthquake. The centroid moment tensor (CMT) has a large non‐double‐couple component. There exist two pure double‐couple constrained solutions that fit the data almost equally well. The frequent practice of taking the “best double‐couple” gives a far from optimal solution. We use P and SH body waves to determine the rupture parameters of the first and larger of the two observed subevents. The best rupture plane, with strike 96°, dip 69°, and rake −18°, is compatible with only one of the two CMT solutions: strike 96°, dip 64°, rake −23°, centroid location (63.1°S, 148.4°E, 10 km depth), centroid time 0313:02 UT, and M0 = 1.3 × 1021 N m (Mw = 8.0). The first subevent is a simple, primarily westward propagating ∼140‐km rupture, of ∼45‐s duration, with average velocity ≳3 km s−1; it has a seismic moment of 1.2×1021 N m (Mw = 8.0), with 75% of its moment released between 10 and 27 s, and a stress drop of ∼240 bars. The rupture is physically bounded by two fracture zones at 147.5°E and 150°E. The second subevent lasted from 70 to 90 s on a fault extending from 210 to 270 km west of the epicenter, with a moment of 0.3–0.6×1021 N m (Mw = 7.6–7.8). This is a spectacular example of dynamic stress triggering over a 100‐km separation distance with a time delay of ∼40 s. The complex pattern of aftershocks is primarily controlled by preexisting fracture zones on the ocean floor.
A mantle flow mechanism for the late Paleozoic subsidence of the Paraná Basin
Authors: Pysklywec Russell N. , Quintas Marcia C. L.
Published: February 2004

The Carboniferous‐Permian stage in the evolution of the Paraná Basin is characterized by the long‐wavelength deposition of up to ∼2.5 km of sediment in the continental interior. The tectonics of the region during this time were dominated by convergence and active subduction along the nearby Panthalassan margin of Gondwana. While subsidence in the basin is clearly related to the major tectonic events at the plate margin, there is a distinct delay between the onset of subduction and the initiation of basin deposition. This, and the nature of basin stratigraphy, suggests that the large‐scale subsidence may have been the dynamic response of the lithosphere to mantle flow associated with the penetration of accumulated slab material through the phase change at 660 km depth. We present numerical experiments of mantle convection which show that subducting slabs can stagnate at the 660 km depth phase change and intermittently penetrate through. The associated (negative) dynamic topography remains at relatively low amplitudes as the subducting material pools and then increases rapidly to a maximum during the phase of penetrative flow. We find that the predicted delay time between initial descent of the slab and the maximum surface topography corresponds with evidence from the geological record. Several models of sedimentation based on the evolving dynamic topography are presented, and a simulation generated using infill rates independently estimated for the basin provides a close fit to the amplitude and horizontal length scale of material accumulation in the Paraná Basin.
Fault rocks and past to recent fluid characteristics from the borehole survey of the Nojima fault ruptured in the 1995 Kobe earthquake, southwest Japan
Authors: Ohtani Tomoyuki , Fujimoto Koichiro , Ito Hisao , Tanaka Hidemi , Tomida Naoto , Higuchi Takayuki
Published: February 2004

The mineralogy, fluid inclusions, and distribution of fault rocks of the Nojima fault were examined in the core recovered from a borehole drilled by the Geological Survey of Japan (GSJ) 12 months after the 1995 Kobe (Hyogo‐ken Nanbu) earthquake (MJMA = 7.2) in southwest Japan. The borehole was drilled across a slipped portion of the fault to a depth of 746.7 m. Nearly continuous coring between 152.2 and 746.7 m recovered granodiorite protolith, porphyry dikes, and fault‐related rocks. The fault zone was intersected at 426.2 m and is characterized by a greater intensity of brittle deformation and/or hydrothermal alteration than typical host granodiorite. The fault core consists of three types of fault gouge and occurs at the depth range of 623.1 to 625.3 m. The fault‐normal thicknesses of the fault core and the fault zone are 0.3 m and >46.5 m, respectively. Three types of hydrothermal alteration are recorded by mineral assemblages and fluid inclusions. The first type is characterized by chloritization of mafic minerals at >200°C and occurred prior to the fault activity during the intrusion and cooling of the granodiorite. The second type occurred during faulting and is recorded by zeolite mineralization at <200°C. The third type is recorded by carbonate mineralization related to recent fluid flow. Although most of the second type of alteration occurred prior to the third type, repeated mineralization is recorded by mutually crosscutting relationships between zeolite and carbonate veins and between zeolite vein and carbonate‐precipitated fault gouge. This may record repeated changes in fluid chemistry within the fault zone in connection with the seismic cycle. Although the Nojima fault slipped in the 1995 earthquake, ancient fault‐related textures and mineral alteration are well preserved in the fault rocks.
Factors controlling normal fault offset in an ideal brittle layer
Authors: Lavier Luc L. , Buck W. Roger , Poliakov Alexei N. B.
Published: February 2004

We study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two‐dimensional elastic‐plastic (brittle) layer floating on an inviscid fluid. The yield stress of the layer is the sum of the layer cohesion and its frictional stress. Faults are initiated by a small plastic flaw in the layer. We get finite fault offset when we make fault cohesion decrease with strain. Even in this highly idealized system we vary six physical parameters: the initial cohesion of the layer, the thickness of the layer, the rate of cohesion reduction with plastic strain, the friction coefficient, the flaw size and the fault width. We obtain two main types of faulting behavior: (1) multiple major faults with small offset and (2) single major fault that can develop very large offset. We show that only two parameters control these different types of faulting patterns: (1) the brittle layer thickness for a given cohesion and (2) the rate of cohesion reduction with strain. For a large brittle layer thickness (>22 km with 44 MPa of cohesion), extension always leads to multiple faults distributed over the width of the layer. For a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening: a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault. When the offset is very large, the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes.
Paleomagnetism of Siluro‐Devonian sequences, NE Spain
Authors: Tait J. A. , Bachtadse V. , Dinarès‐Turell J.
Published: February 2004

The paleogeography of northeastern Spain is addressed in a detailed paleomagnetic study of Siluro‐Devonian boundary sediments from the Catalan Coastal Ranges (CCR) at Santa Creu d'Olorda, Barcelona, and the eastern Pyrenees at Baèn. The sequences studied are all well dated biostratigraphically and have undergone only very minor deformation, mainly during the late Carboniferous, and extremely low grades of metamorphism (anchizone or less). After stepwise thermal demagnetisation of samples and principal component analysis of the data, up to two paleomagnetic directions can be clearly identified after removal of a present‐day magnetic overprint. The first component (labeled B) is identified only in samples from the CCR with intermediate unblocking temperature spectra (300°–400°C). An overall mean direction of 187°/−03°, semi angle of cone of 95% confidence α95 = 9.0°, and precision parameter k = 43.1 in situ, and 195°/03° α95 = 24.4°, and k = 7.1 (51 samples at seven sites) after bedding correction is obtained for B. The exact age of this secondary component is uncertain, but when compared to published paleomagnetic data, a late Carboniferous to Early Triassic age could be inferred. A stable endpoint direction (labeled C) can be identified between 400° and 600°C in samples from both regions. For the CCR it is southwesterly directed with intermediate inclination values in situ, and yields a mean direction of 208°/48°, α95 = 12.8° and k = 28.4 (six sites) after bedding correction. It passes the fold test, is considered to be primary in origin, and is thus indicative of the Siluro‐Devonian paleofield direction in the CCR. In the Pyrenean samples the C direction is northwesterly directed in situ, with a mean direction of 240°/51°, α95 = 6.5° and k = 86.5 (seven sites) after bedding correction. Although there is a clear improvement in grouping of these site mean directions, this is not significant at the 95% level of confidence. Combining the site mean directions obtained for component C from both regions yields an overall mean direction of 224°/51°, α95 = 8.5° and k = 24.6 (13 sites) after bedding correction. The improvement in grouping is significant at the 99% level of confidence at 90% unfolding, reinforcing the interpretation that this is a primary magnetisation. Adopting the reversed polarity option, the inclination value obtained translates into paleolatitudes of 30°S, placing NE Spain in the same latitudinal belt as the Armorican Massif and the Barrandian terrane of the Bohemian Massif (i.e., the Armorican Terrane Assemblage) in latest Silurian/earliest Devonian times.
Size dependence of hysteresis properties of small pseudo‐single‐domain grains
Authors: Newell Andrew J. , Merrill Ronald T.
Published: February 2004

The Day plot (Mrs/Ms versus Hcr/Hc) is widely used by paleomagnetists to estimate the size of ferromagnetic grains and classify them as single‐domain (SD), pseudo‐single‐domain (PSD,) or multidomain (MD). How reliable is this plot? To find out, a numerical micromagnetic model is used to calculate hysteresis loops as a function of grain size for two grain shapes (cube and cuboid with X = 1.5Y = 1.4Z). Magnetocrystalline anisotropy is ignored. The average Mrs/Ms is calculated for a collection of randomly oriented grains: In the elongated grain it drops from 0.4 to 0.06 over a negligible size range, almost missing the usual PSD range altogether. Other hysteresis parameters, Hc, Hcr, and χ0, can only be calculated for a grain at a time. This is done for two magnetic field directions (close to the longest axis and close to the shortest axis). The single‐grain values of Hcr/Hc depend strongly on field direction, but it is clear that the average jumps rapidly from SD to MD values. There are large, rapid fluctuations in Hcr and χ0 associated with changes in remanent states. However, these fluctuations may not be apparent when averaged over a broad size range. This may explain why Hcr and χ0 depend weakly on grain size in real samples. In the size range studied (L ≤ 0.25 μm), hysteresis parameters do not represent a typical grain size. Instead, they depend strongly on the size distribution.
Magnetic measurements of glass from Tikal, Guatemala: Possible tektites
Authors: Senftle F. E. , Thorpe A. N. , Grant J. R. , Hildebrand A. , Moholy‐Nagy H. , Evans B. J. , May L.
Published: February 2004

Certain glass nodules found at the archeological site at Tikal, Guatemala, have been considered as possible tektites. To test this possibility, magnetic studies have been made on three of the glass specimens. These specimens are similar to tektites, both visually and also because they contain very little Fe3+ as detected by Mössbauer spectroscopy. The magnetic Curie constants are similar in magnitude to those found for normal tektites but show some variation from point to point in the same specimen. This variation reflects an inhomogeneity in the iron concentration. The Fe2+ calculated from the Curie constants accounts for most of the iron. The temperature‐independent component of the total dc magnetic susceptibility is several times higher than that found in tektites from other strewn fields. The high values can be explained if the glass contains metallic iron spherules with Fe in the parts per million range and/or a ferromagnetic component which does not saturate in a low magnetic field. The magnetic properties resemble those of Muong Nong type tektites and suggest that the Tikal glass specimens are tektites of the Muong Nong type.
Fracture spacing and orientation distributions for two‐dimensional data sets
Authors: Ryan Jenny‐Lynn , Lonergan Lidia , Jolly Richard J. H.
Published: February 2004

A new, automated method is developed for the measurement and display of fracture spacing and orientation from two‐dimensional (2‐D) fracture maps. Separate histograms of fracture spacing are calculated at different orientations through the 2‐D data set. The histograms for all orientations are plotted together, reducing the biasing effects associated with one‐dimensional (1‐D) sampling of a fracture population. Quantitative measurements of fracture strike anisotropy and fracture density are incorporated in the plot. No prior measurements are needed of parameters such as fracture spacing, orientation, or width. The data do not have to be preprocessed into a numerical vector format, saving time and reducing user bias. The method retains the relative frequency of different ranges of fracture spacings, which are lost in other fracture frequency measures such as fracture intensity/density. Thus the presence of any dominant (rather than average) fracture spacing and fracture clustering is highlighted. The method provides a quantitative, statistically unbiased representation of fracture orientation and spacing and is illustrated on synthetic and real fracture data sets at a range of scales (from meters to tens of kilometers). Its application to a seismic energy scattering problem is also discussed.
Dilatancy, compaction, and failure mode in Solnhofen limestone
Authors: Baud Patrick , Schubnel Alexandre , Wong Teng‐fong
Published: February 2004

Failure mode is intimately related to porosity change, and whether deformation occurs in conjunction with dilatation or compaction has important implications on fluid transport processes. Laboratory studies on the inelastic and failure behavior of carbonate rocks have focused on the very porous and compact end‐members. In this study, experiments were conducted on the Solnhofen limestone of intermediate porosity to investigate the interplay of dilatancy and shear compaction in controlling the brittle‐ductile transition. Hydrostatic and triaxial compression experiments were conducted on nominally dry samples at confining pressures up to 435 MPa. Two conclusions can be drawn from our new data. First, shear‐enhanced compaction can be appreciable in a relatively compact rock. The compactive yield behavior of Solnhofen limestone samples (with initial porosities as low as 3%) is phenomenologically similar to that of carbonate rocks, sandstone, and granular materials with porosities up to 40%. Second, compactive cataclastic flow is commonly observed to be a transient phenomenon, in that the failure mode evolves with increasing strain to dilatant cataclastic flow and ultimately shear localization. It is therefore inappropriate to view stress‐induced compaction and dilatancy as mutually exclusive processes, especially when large strains are involved as in many geological settings. Several theoretical models were employed to interpret the micromechanics of the brittle‐ductile transition. The laboratory data on the onset of shear‐enhanced compaction are in reasonable agreement with Curran and Carroll's [1979] plastic pore collapse model. In the transitional regime, the Stroh [1957] model for microcrack nucleation due to dislocation pileup can be used to analyze the transition from shear‐enhanced compaction to dilatant cataclastic flow. In the brittle faulting regime the wing crack model provides a consistent description of the effect of grain size on the onset of dilatancy and brittle faulting.
Integrated geological and geophysical studies in the SG4 borehole area, Tagil Volcanic Arc, Middle Urals: Location of seismic reflectors and source of the reflectivity
Authors: Ayarza P. , Juhlin C. , Brown D. , Beckholmen M. , Kimbell G. , Pechnig R. , Pevzner L. , Pevzner R. , Ayala C. , Bliznetsov M. , Glushkov A. , Rybalka A.
Published: February 2004

Near‐vertical incidence reflection seismic data acquired in the Tagil Volcanic Arc (Middle Urals) show the upper crust to be highly reflective. Two intersecting seismic lines located near the ongoing ∼5400 m deep SG4 borehole show that the main reflectivity strikes approximately N‐S and dips ∼35°–55° to the east. Prominent reflections intercept the borehole at ∼1000, ∼1500, 2800–2900, ∼3400, and between ∼4000 and 5400 m, which correspond to intervals of low velocity/low density/low resistivity. The surface projections of these reflections lie parallel to the strike of magnetic anomaly trends. Multioffset vertical seismic profile (VSP) data acquired in the SG4 borehole show a seismic response dominated by P to S reflected converted waves from the moderately east dipping reflectivity and from a set of very steep east dipping reflectors not imaged by the surface data. Modeling of the VSP data constrains the depth at which reflectors intercept the borehole and suggests that the P to S conversions are best explained by low‐velocity porous intervals rather than higher‐velocity mafic material. The most prominent east dipping reflection on the surface seismic data is only imaged on VSP shots that sample the crust closer to the E‐W seismic line. This discrepancy between the VSP and the surface seismic data is attributed to rapid lateral changes in the physical properties of the reflector. Surface and borehole data suggest that the low‐velocity/low‐density/low‐resistivity intervals are the most important source of reflectivity in the SG4 borehole area, although lithological contrasts may also play a role. Drill cores from the these zones contain hydrothermal alteration minerals indicating interaction with fluids. Tectonic criteria suggest that they might represent imbricated fracture zones often bounding different lithologies and/or intrusions. Some of them might also represent high‐porosity lava flows or pyroclastic units, common in island arc environments.
D″ as a transition in the heterogeneity spectrum of the lowermost mantle
Authors: Cormier Vernon F.
Published: February 2004

Broadband SH and P wave fields are pseudospectrally modeled in three forms of the heterogeneity spectrum of the mantle, each having a transition in the power spectrum of heterogeneity in the lowermost mantle. With D″ defined as a zone of increased heterogeneity, low‐pass and high‐pass filters of models of the heterogeneity spectrum demonstrate that the thickness of D″ inferred from an incident seismic wave field can depend on the dominant wavelength of that wave field. A model of D″ heterogeneity that agrees with the coda power between S and ScS and between P and PcP is a simple linear extrapolation of the spectrum of heterogeneity power versus depth determined from global tomography to wavenumbers corresponding to short‐period body waves. The negligible decay in the power spectrum of heterogeneity across a band of wavenumbers from 0.0001 to 1 km−1 is consistent with a pervasive entrainment of small scale chemical heterogeneities by convective motions. The lack of strong scattering attenuation in observed ScS and PcP waveforms is used to establish upper bounds in this wavenumber band of 3% and 1.5% on the root‐mean‐square variations of S and P velocity, respectively. Some realizations of the preferred model of the heterogeneity spectrum generate an SdS waveform from scattering by an elongated, high velocity anomaly, roughly parallel to the core‐mantle boundary. The high topography of similarly shaped anomalies needed to account for the amplitude of narrow angle scattering observed from D″ argues against such anomalies having the high‐density perturbation expected for a chemically distinct or phase‐transformed layer.
Wave front healing and the evolution of seismic delay times
Authors: Nolet Guust , Dahlen F. A.
Published: February 2004

Using a simple Gaussian beam solution to the one‐way scalar wave equation, we derive analytical expressions for the evolution of phase and group delay after a wave passes through a Gaussian‐shaped heterogeneity of half width L. As a function of distance χ, there are two clearly separated regimes, depending upon the wavelength λ of the wave. In regime I, when χ/L ≪ πL/λ, the absolute magnitude of the phase delay decreases approximately linearly with χ, and the anomaly does not widen appreciably except by developing small sidelobes with delays of opposite sign. Tomographie inversions of such delays will be damped but are theoretically well posed. In regime II, when χ/L ≫ πL/λ, the absolute delay decreases toward zero as 1/χ, most markedly on the ray itself, and the cross‐path shape of the wave front bears little resemblance to the original anomaly. Tomographic inversions of delay times in this regime are ill posed. Group delay times show a similar behavior in the two regimes. Although their rate of decrease with distance is slower in regime I, they develop more disturbing sidelobe behavior off the central ray. The effects of wave front healing for surface waves traveling in two dimensions are less severe than those for body waves in three dimensions; as a result, surface wave inversions will commonly be in regime I. Short‐period body wave group delays are also in regime I; nevertheless, the damping of delays in this regime is likely to contribute significantly to the scatter of observed travel time anomalies. Tomographie inversions of long‐period body waves, which fall at the limit of regime I, or even in regime II, face perceptible limitations in theoretical resolving power. Finally, we show that there is an asymmetry in the evolution of positive versus negative travel time anomalies.
Thermochronology of the Torlesse accretionary complex, Wellington region, New Zealand
Authors: Kamp Peter J. J.
Published: February 2004

The Torlesse Complex comprises several Mesozoic accretionary prism complexes together forming continental basement over large parts of New Zealand. This study focuses on the thermal history of relatively low grade graywacke rocks exposed in a transect in southern North Island that crosses the structural grain of the Torlesse Complex, including its older and younger parts. Zircon fission track (FT) ages for the Late Triassic Rakaia Terrane, which is the most inboard of the accretionary complexes, are partially annealed, some possibly reset, and may indicate early Cretaceous (134±10 Ma) cooling from maximum temperatures (Tmax), probably related to imbrication of younger complexes of the Pahau Terrane. Numerical modeling of the zircon FT ages and published 40ArA/39Ar muscovite and biotite ages for the Rakaia Terrane suggest Tmax values of 265–310°C and exhumation from depths of 10–12 km. The rocks underlying the Aorangi Range and involving the youngest accretionary complex have experienced much lower Tmax values of ≤210° and ≥110°C, bracketed by reset apatite FT ages and detrital zircon FT ages. The occurrence of a circa 100 Ma component of zircon FT ages in both the weakly and highly indurated rocks beneath the Aorangi Range, as well as in remnants of an overlying Albian accretionary slope basin (Whatarangi Formation), imply multistorey accretion and incorporation of sediment into the youngest prism. This circa 100 Ma zircon FT age component also places a maximum age on the termination of Mesozoic subduction beneath the New Zealand region. The occurrence of reset apatite FT ages across the whole of the Wellington transect indicates that at least 4 km of exhumation occurred during the late Miocene.