bluuebunny/crossref_metadata_2025 huggingface dataset:
Elastodynamic analysis for slow tectonic loading with spontaneous rupture episodes on faults with rate‐ and state‐dependent friction
Authors: Lapusta Nadia , Rice James R. , Ben‐Zion Yehuda , Zheng Gutuan
Published: February 2004

We present an efficient and rigorous numerical procedure for calculating the elastodynamic response of a fault subjected to slow tectonic loading processes of long duration within which there are episodes of rapid earthquake failure. This is done for a general class of rate‐ and state‐dependent friction laws with positive direct velocity effect. The algorithm allows us to treat accurately, within a single computational procedure, loading intervals of thousands of years and to calculate, for each earthquake episode, initially aseismic accelerating slip prior to dynamic rupture, the rupture propagation itself, rapid post seismic deformation which follows, and also ongoing creep slippage throughout the loading period in velocity‐strengthening fault regions. The methodology is presented using the two‐dimensional (2‐D) antiplane spectral formulation and can be readily extended to the 2‐D in‐plane and 3‐D spectral formulations and, with certain modifications, to the space‐time boundary integral formulations as well as to their discretized development using finite difference or finite element methods. The methodology can be used to address a number of important issues, such as fault operation under low overall stress, interaction of dynamic rupture propagation with pore pressure development, patterns of rupture propagation in events nucleated naturally as a part of a sequence, the earthquake nucleation process, earthquake sequences on faults with heterogeneous frictional properties and/or normal stress, and others. The procedure is illustrated for a 2‐D crustal strike‐slip fault model with depth‐variable properties. For lower values of the state‐evolution distance of the friction law, small events appear. The nucleation phases of the small and large events are very similar, suggesting that the size of an event is determined by the conditions on the fault segments the event is propagating into rather than by the nucleation process itself. We demonstrate the importance of incorporating slow tectonic loading with elastodynamics by evaluating two simplified approaches, one with the slow tectonic loading but no wave effects and the other with all dynamic effects included but much higher loading rate.
Magnetic and structural fabrics of the melange in the Shimanto accretionary complex, Okinawa Island: Implication for strain history during decollement‐related deformation
Authors: Ujiie Kohtaro , Hisamitsu Toshio , Soh Wonn
Published: February 2004

The magnetic fabrics of the melange in the Shimanto accretionary complex of Okinawa Island were obtained from the anisotropy of magnetic susceptibility (AMS) data. The magnetic fabrics show a marked contrast between the structurally lower and upper parts of the melange; the lower part records deformation in the footwall of the decollement (D1), whereas the upper part documents that in the hanging wall of the decollement (D2). In the lower part of the melange the magnetic fabrics show good correlations with S1 layer‐parallel foliation, reflecting strain geometries under layer‐perpendicular flattening. As S‐C fabrics were locally developed by heterogeneous shear during D1, some of the previous magnetic fabrics were rotated with S1. In this case, the resultant magnetic fabrics reflect “S” surfaces, and maximum AMS axes are reoriented parallel to shear directions without clear change in shape and anisotropy degree of the AMS ellipsoids. Thus “S” surfaces did not act as flattening planes but may represent the deformation through independent particulate flow and mechanical rotation. The lower part of the melange was pervasively sheared during D2, reflecting that it laid immediately above the decollement. However, D1 magnetic fabrics were not modified because shear strain was accomplished by brittle microfaults without development of penetrative strain. In contrast, penetrative strain was significant in the upper part of the melange, resulting in the magnetic fabrics recording incremental strain associated with D2. Therefore deformation mechanism and proximity to a decollement play an important role on development of magnetic fabrics in a melange.
Identifying ash flow sources with directional data: An application to the Kizilkaya ignimbrite, central Anatolia
Authors: Le Pennec Jean‐Luc
Published: February 2004

Clast shape preferred orientation or anisotropy of magnetic susceptibility (AMS) data are increasingly used to constrain the source location of ash flow tuffs. Little attention has been paid to processing of these directional data for the accurate and objective identification of the sources. This topic is the focus of this paper, as applied to locating the source of the ∼5 Ma Kizilkaya ignimbrite, central Turkey. First, geological criteria allowing a rough identification of source vent location are discussed. Second, AMS data are processed to more accurately locate the source of this widespread deposit. Hillhouse and Wells [1991] present a method which consists of determining the areal overlapping density function of the AMS “error fans” (defined by the Kmax confidence ellipse). Applications to the Kizilkaya ignimbrite show various possible source positions. However, a unique anomaly occurs in every model, suggesting a source in the Derinkuyu basin. Another method is proposed here and is named the average radial angular difference (ARAD) function. It consists of calculating the average angular difference of lines that join a given source point to the sampling sites, with the lineations corresponding to the AMS data interpreted in terms of kinematic axes. Application with the data obtained at the 44 AMS sampling sites shows that the function decreases to <35° in the Derinkuyu basin. If the analysis is narrowed to the presumed 12 most proximal sites the function provides values of <20° in the central part of the Derinkuyu basin. The identified source area occurs near the source zone of older Miocene ignimbrites and coincides with a gravity anomaly and with a basaltic shadow zone.
Segmentation in gravity and magnetic anomalies along the U.S. East Coast passive margin: Implications for incipient structure of the oceanic lithosphere
Authors: Behn Mark D. , Lin Jian
Published: February 2004

Segmentation is a characteristic feature of seafloor spreading along the global mid‐ocean ridge system. While segmentation of active spreading centers has been the focus of much recent research, the process by which a rifted continental margin develops into a segmented mid‐ocean ridge is still poorly understood. In this study we investigate the segmentation character of the U.S. East Coast margin through a modeling study of the margin basement structure, magnetics, and gravity anomalies. The East Coast margin is of particular interest because it is one of several rifted continental margins that display thick sequences of high seismic velocity igneous crust, presumably formed during rifting. The East Coast Magnetic Anomaly (ECMA), a distinct total field magnetic high running offshore along the margin, is commonly located seaward of the thickest sections of the high‐velocity crust and displays segmentation on length scales (100–120 km) similar to the segmentation observed at the Mid‐Atlantic Ridge (MAR). Isostatic gravity anomalies were calculated by removing from free‐air gravity the predicted effects of seafloor, sediments, and a crust‐mantle model assuming local isostatic compensation. The resultant residuals show a corridor of high anomaly running along the margin, situated close to the maximum thickness of the high seismic velocity crust as determined from the two available seismic refraction lines. Reduction to the pole (R‐T‐P) of the total field magnetic anomaly shows that after the removal of skewness from the ECMA, the location of the isostatic gravity high is closely correlated to the ECMA. The isostatic gravity high is also segmented but in two distinct wave bands: 100–150 km and 300–500 km. The short‐wavelength (100–150 km) segmentation in the R‐T‐P magnetic and isostatic gravity anomalies is similar in wavelength to segmentation in magnetization and mantle Bouguer anomaly observed along the present‐day MAR. The 300–500 km segmentation in the along‐margin isostatic gravity anomaly is similar in length scale to both intermediate‐wavelength tectonic segmentation observed in the South Atlantic and variations in lithospheric strength observed along the African margin. Furthermore, two of the intermediate‐wavelength (300–500 km) isostatic gravity lows correspond to the early traces of the Kane and Atlantis fracture zones, suggesting that these two fracture zones may define boundaries of a single tectonic corridor in the North Atlantic. We hypothesize that the direct cause of the intermediate‐wavelength segmentation may be along‐margin variations in both the amount of underplated igneous crust and the strength of the lithosphere, although the relative importance of these two effects remains unresolved. Our results imply that segmentation is an important feature of margin development and that segmentation at mature oceanic spreading centers may be directly linked to segmentation during continental rifting.
Geochemistry of near‐trench intrusives associated with ridge subduction, Seldovia Quadrangle, southern Alaska
Authors: Lytwyn Jennifer , Lockhart Susan , Casey John , Kusky Timothy
Published: February 2004

An extensive belt of plutons and related dikes along the southern Alaska margin was emnplaced into the accretionary prism anomalously close to the paleotreneh and far seaward of a coeval magmatic arc. Numerous workers have attributed the near‐trench magmatism to early Tertiary subduction of the Kula‐Farallon Ridge beneath North America. We present new major, minor, and trace element data for a dike swarm in the Seldovia Quadrangle, southern Kenai Peninsula, that is part of the near‐trench Sanak‐Baranof belt. The dikes are generally calc‐alkaline and cover the entire compositional spectrum from basalts to rhyolites. The basalts and andesites of obvious mantle (I type) origin compositionally grade into dacites and rhyolites having a significant sedimentary (S type) component. Trace element modeling suggests that the dikes evolved from parental magmas, some possibly as refractory as island arc tholeiites, through assimilation of sedimentary material coupled with fractional crystallization. The dikes appear to be products of interaction between older sedimentary rocks at the base of the accretionary prism and basaltic magmas that were emplaced by the subducting ridge (slab window). I‐type basalts and basaltic andesites are products of mafic liquids that were only slightly contaminated by sedimentary material, whereas more extensive assimilation produced S‐type dacites and rhyolites. This idea is supported by the geographical distribution of dikes within the forearc. Basalts, andésites, and low‐silica dacites are found throughout the accretionary complex, whereas highsilica dacites and rhyolites only occur at some distance inboard from the paleotrench where the sedimentary column is thicker and more assimilation was likely to have taken place. We suggest that the subducting spreading center produced basaltic liquids that became more refractory closer to the paleotrench such that subducted portions of the ridge formed parental melts having the composition of island arc tholeiites. The basaltic liquids were injected into the overlying accretionary prism where variable assimilation of sedimentary material produced the wide range of dike compositions.
Impact of coating and weathering on the properties of chalk fracture surfaces
Authors: Weisbrod Noam , Nativ Ronit , Adar Eilon M. , Ronen Daniel , Ben‐Nun Adi
Published: February 2004

The surface topography of a natural fracture intersecting a chalk formation was mapped using a high‐resolution laser‐scanning device, and its roughness was evaluated using five different criteria. Two coated and two uncoated slices of the natural fracture surface were encased in flow cells and exposed to short (24 and 8 hours) interactions with percolating synthetic rainwater and industrial wastewater, followed by long drying periods (weeks). These flow experiments simulated the intermittent infiltration of surface runoff and industrial effluents typical of the study area and resulted in erosion of the fracture surfaces and modification of their roughness. The temporal variations in surface roughness were evaluated using three commonly used criteria (standard deviation from mean heights, differences between maximum and minimum heights, and the root‐mean‐square roughness) and two newly developed measures, namely, the slopes on the surfaces and a calculated roughness‐related ratio (RRR). The mean slope maps and RRR calculations were found to be good criteria for evaluating surface topography and its uniformity across the fracture surface. The coated surfaces were significantly rougher, had greater relief, more heterogeneous topography, and a larger surface area than the uncoated surfaces. The results suggest that coating and fast weathering (caused by intermittent flow events) significantly change roughness, surface area, relief, and the heterogeneity of a fracture surface. These, in turn, may affect the spatial distribution of flow paths across that fracture surface. Consequently, when exploring the properties of fracture surfaces and fracture flow, the impact of surface skin and variations of the surface properties over short timescales should be taken into account.
The influence of plate motions on three‐dimensional back arc mantle flow and shear wave splitting
Authors: Hall Chad E. , Fischer Karen M. , Parmentier E. M. , Blackman Donna K.
Published: February 2004

Both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western Pacific subduction zones. The goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting. We calculated three‐dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates. Shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow‐induced strain. Predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain. A parameter study shows that both finite strain and textural anisotropy developed within three‐dimensional, plate‐coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate. Therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere. Flow models incorporating plate motion boundary conditions for the Tonga, southern Kuril, and eastern Aleutian subduction zones produce splitting parameters consistent with observations from each region. Trench‐parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge.
Contemporary horizontal velocity and strain rate fields of the Pacific‐Australian plate boundary zone through New Zealand
Authors: Beavan John , Haines John
Published: February 2004

We have inverted velocity solutions from nine geodetic networks distributed across New Zealand to derive present‐day continuous horizontal velocity and strain rate fields at the Earth's surface throughout the country. The nine networks contain a total of 362 Global Positioning System (GPS) stations that have been observed at least twice and at least a year apart between 1991 and 1998. The model velocity field is expanded as bicubic spline interpolation functions defined within a curvilinear grid that covers the country and extends into the assumed rigid Australian and Pacific plates to west and east. The inversion jointly minimizes the magnitudes of fitted strain rates and the misfit to the observed velocity data. The spline technique allows high spatial resolution of strain rate variations, especially in regions with spatially dense GPS data. Expansion of the model velocity field on the surface of a sphere allows arbitrarily large areas to be studied. Previously known aspects of New Zealand plate boundary deformation are highlighted with improved resolution, including back‐arc extension in the Taupo Volcanic Zone, rotation of the Hikurangi forearc away from this zone in the North Island, and a band of high shear strain rate under the Southern Alps to the southeast of the Alpine fault. Our results reveal several new features, including a region of enhanced shear straining apparently associated with strike‐slip faults in the southern North Island and a band of contractional straining subparallel and well east of the Alpine fault that is similar to features found in numerical and sandbox models of continental collision.
A focused look at the Alpine fault, New Zealand: Seismicity, focal mechanisms, and stress observations
Authors: Leitner Beate , Eberhart‐Phillips Donna , Anderson Helen , Nabelek John L.
Published: February 2004

The Alpine fault is the Pacific‐Australian plate boundary in the South Island of New Zealand. This study analyzes 195 earthquakes recorded during the 6 month duration of the Southern Alps Passive Seismic Experiment (SAPSE) in 1995/1996 and two ML 5.0 earthquakes and aftershocks in 1997, which occurred close to the central part of the Alpine fault. Precise earthquake locations are derived by simultaneous inversion for hypocenter parameters, a one‐dimensional velocity model, and station corrections. Together with focal mechanisms calculated using a first motion and amplitude ratio method, these results provide a picture of the seismotectonics in the central South Island over a 6 month period. Moment tensor inversions of three earthquakes provide an independent means of comparison to the focal mechanisms derived using the amplitude/first motion method. To validate our observations over time, we compare the SAPSE seismicity with the seismicity recorded by the New Zealand National Seismic Network (NZNSN) and a local network at Lake Pukaki east of the Southern Alps (6 months versus 8 years). Our study indicates that the Alpine fault releases elastic strain seismically from the surface down to 10–12 km depth between Milford Sound in the south and the Hope fault in the north. The seismicity rate of the Alpine fault is low but comparable to locked sections of the San Andreas fault, with large earthquakes expected. Seismicity decreases north of Bruce Bay at the Alpine fault and within a triangular region along the Alpine fault located between the Hope and Porters Pass fault zones. We interpret this as the result of deformation distributed on the Alpine fault and the Hope and Porters Pass fault zones. The base of the seismogenic zone is fairly uniform at 12 km ± 2 km over large parts of the South Island. The high Alps region has a shallower base of the seismogenic zone, indicating localized elevated temperatures east of the Alpine fault. Most of the study region deforms under a uniform stress field with a maximum principal horizontal shortening direction of 110°–120°, similar to geodetic observations and plate motions. This confirms that the region is not undergoing strain partitioning. The earthquake data show that the deformation away from the Alpine fault is distributed on mainly NNE trending thrust faults and strike‐slip transfer faults with a maximum seismogenic depth of 12 km.
Low‐latitude and Southern Hemisphere origin of Anisian (Triassic) bedded chert in the Inuyama area, Mino terrane, central Japan
Authors: Ando Akifumi , Kodama Kazuto , Kojima Satoru
Published: February 2004

Paleomagnetic and paleontologic studies of Middle Triassic to earliest Jurassic radiolarian bedded chert in the Inuyama area, southern Mino terrane of central Japan, have shown that the chert was deposited in an equatorial region with a paleolatitude between 10°N and 10°S, and in particular, the lower to middle Anisian chert depositional basin was situated in the Southern Hemisphere. The chert has four characteristic components of remanent magnetization. The highest blocking temperature component, carried by hematite, shows positive fold and reversal tests and is regarded as the primary remanent magnetization acquired just after deposition. The other three components are interpreted as a present field overprint and two postfolding remagnetizations. The absolute, tilt‐corrected inclinations of the primary component are less than 20° for all the samples of Middle Triassic to earliest Jurassic age, with the mean inclination of the lower to middle Anisian chert being 11.0°±4.3°. Paleomagnetic polarity reversals observed for the lower to middle Anisian chert are well correlated with those of the European Triassic sections, under paleontologic age constraints using radiolarians and conodonts, provided that the Anisian chert in Inuyama was deposited in the Southern Hemisphere. This conclusion illuminates the paleogeographic evolution of the chert‐bearing accretionary complexes not only in Japan but along the eastern margin of Asia as well.
Passive roof thrusting and forelandward fold propagation in scaled brittle‐ductile physical models of thrust wedges
Authors: Bonini Marco
Published: February 2004

A set of scaled analogue models was performed to investigate the role of a décollement layer in the structural style of fold‐and‐thrust belts. Silicone putty (SGM 36) has been employed to represent the ductile décollement, while frictional material (quartz sand) has been used to simulate the brittle behavior of roof and floor sequences, situated above and below the décollement, respectively. In order to explore a wide combination of strength profiles, models have been deformed under five distinct rates of compression (0.15, 0.3, 0.45, 0.6, and 1.5 cm h−1) employing four different thicknesses for the silicone layer (0.1, 0.2, 0.4, and 0.8 cm). Experimental results indicate that roof sequences exhibit two distinct styles of deformation: (1) passive roof duplex (PRD) geometry [e.g., Banks and Warburton, 1986] and (2) outward propagation of folding (OFP) along the décollement [e.g., Davis and Engelder, 1985]. In PRD geometry the roof sequence remains relatively “stationary” being underthrust by a wedge‐shaped floor duplex, while in OFP, displacement along the floor thrusts is transferred horizontally along the décollement layer. Experiments suggest the occurrence of a genetic relationship between shear stresses (at the base of the roof sequence τb and within the ductile layer τd) and the development of PRD or OFP geometry. For a given strain rate the deformation pattern of roof sequences depends upon the τb/τd ratio as well as the absolute τb and τd values. The transition field between OFP and PRD styles has been approximated by curvilinear regression of transition‐style experimental data points, with a minimum value of τd, varying between about 112 and 160 Pa, to enter the PRD field. Syntectonic sedimentation in the molasse basin is found to inhibit development of folding into the foreland but to promote PRD style. Experiments simulate many of the principal characteristics of fold‐and‐thrust belts developed above a basal evaporite décollement, as well as the development of passive roof duplexes at mountain fronts. Shear stress values inferred for many fold‐and‐thrust belts correlate well the deformation fields predicted by the present experimental study, allowing to propose a simple model describing the mechanical behavior of natural roof sequences underlain by a décollement layer.
A proposed method for locating the critical region of a future earthquake using the critical earthquake concept
Authors: Yang Wenzheng , Vere‐Jones David , Ma Li
Published: February 2004

Using the critical point concept and extending Bowman's idea of critical earthquake, we develop an intersecting circle method to locate the critical region. A simulation check shows that this method is effective in finding a given critical region. We selected several real cases from New Zealand and China and used this method to find the critical regions before the occurrence of large earthquakes. The result shows that this method is valid for detecting a critical region and the epicenter of mainshock might be in the critical region.
Crustal velocity field of southwest Japan: Subduction and arc‐arc collision
Authors: Miyazaki Shin'ichi , Heki Kosuke
Published: February 2004

We investigate crustal deformation in southwest Japan over a 3‐year period revealed by a permanent dense Global Positioning System (GPS) array. Southwest Japan is a part of the Amurian Plate, a microplate moving about 10 mm/yr toward the east with respect to the Eurasian Plate. It overrides the Philippine Sea Plate at the Nankai Trough and collides with the northeast Japan arc in the central part of Japan. In this paper we first derive GPS site velocities relative to the stable part of the Amurian Plate in order to isolate signals of crustal deformation caused by the subduction and/or the collision. The velocity field has a conspicuous feature indicating the interseismic elastic loading by the Philippine Sea Plate slab at the Nankai Trough, characterized by the northwestward movements of points throughout the studied area. Their amplitudes are the largest at the Pacific coast and decay toward the Japan Sea coast with a subtle systematic shift of azimuths. A model assuming an elastic half‐space, the convergence rate at the Nankai Trough based on a refined Euler vector, and the strength of the coupling inferred from a thermal model, could explain the velocity field in the western part of the studied area to a large extent. Those in the eastern part systematically deviate from them, and the residual components there show east‐west shortening and north‐south extension. This may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast Japan. This suggests that the collision between southwest and northeast Japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block. A velocity contrast was found across the Median Tectonic Line, the largest inland active fault in Japan, but the current permanent GPS network is not dense enough for us to discuss its coupling depth.
High‐resolution geological record of historic earthquakes in the Dead Sea basin
Authors: Ken‐Tor Revital , Agnon Amotz , Enzel Yehouda , Stein Mordechai , Marco Shmuel , Negendank Jorg F. W.
Published: February 2004

A 2000 year paleoseismic record of the Dead Sea area was recovered from a lacustrine sedimentary section. The section is being exposed at the Ze'elim Terrace on the shores of the Dead Sea due to the fast retreat of the lake. The section consists of laminated detrital and chemical (mainly aragonite) sediments that were deposited in the Holocene paleo‐Dead Sea. Eight layers in the section show deformed sedimentary structures and are identified as seismites. Their chronology was determined by radiocarbon dating on organic remains. The seismite ages are well correlated with the historically documented earthquakes of 64 and 31 B.C. and 33, 363, 1212, 1293, 1834 and 1927 A.D. The few historically documented earthquakes that have no correlatives in the Ze'elim seismite record occurred in times of sedimentary hiatuses at this site (e.g., 749 A.D.). Based on modern analogues and the association of similar disturbed layers with syndepositional faults, the Ze'elim Terrace seismites indicateM>5.5 earthquakes. The average recurrence interval is estimated as ∼100–300 years and represents slip events on different faults in the Dead Sea area. The Ze'elim section provides a unique opportunity to correlate two independent and extensive data sets, the historical and sedimentary records. This study opens the way for better understanding of spatial and temporal distribution of earthquakes along the Dead Sea Transform and elsewhere.
Time‐dependent inversion study of the slow thrust event in the Nankai trough subduction zone, southwestern Japan
Authors: Ozawa Shinzaburo , Murakami Makoto , Tada Takashi
Published: February 2004

We estimate time variation of interplate coupling between the subducting Philippine Sea and the overriding continental plates along the western part of the Nankai trough, Japan, during the period between 1996 and 1999. After subtracting annual components and average annual velocities, deviation from steady crustal deformation in the Shikoku and Kyushu islands is examined. Around 15 GPS stations facing the Bungo channel detected the following three‐stage ground displacements of up to 3 cm in 1997, with a duration of ∼1 year: (1) increased displacement rate associated with the 1996 Hyuga‐nada earthquakes; (2) an increasingly rapid anomalous period of deformation from June 1997 to the end of 1997; and (3) a slowdown period and return to normal rates of deformation after the end of 1997. Linearized least squares locate the slip area of this slow event near the plate boundary beneath the Bungo channel. The estimated slip is opposite the motion of the Philippine Sea plate. These results support the hypothesis that the observed anomalous caistal deformation is due to a rebound process of the Philippine Sea plate. Time‐dependent inversion analysis shows the following slip history. During the initial period of increased displacement rate, a visible slip area is resolved beneath the coastal area of southwestern Shikoku. In the next stage of accelerated displacements, the slip area expanded southwestward to the Bungo channel beginning around June 1997, when postseismic slip of the 1996 Hyuga‐nada earthquakes almost ended. After this accelerated period, the estimated thrust slip ends with time.
Geomagnetic field behavior before and after the Kauai reverse‐normal polarity transition
Authors: Bogue Scott W.
Published: February 2004

New paleomagnetic results from 4 m.y. old lava flows from Kauai, Hawaii, suggest that strong poloidal field is associated with an unusual state of the geodynamo that follows attempts at polarity reversal (successful or not). The new data comprise 50 paleomagnetic sites from superposed lava flows occurring just below and above the Kauai reverse‐normal polarity transition. A composite record of 45 distinct field determinations was constructed by combining sites that record similar ancient field and correlating them to previously published results from Kauai. Of the 45 data, 25 include paleointensity estimates derived from double‐heating experiments. A comparison of the composite record from Kauai with two similar data sets from volcanic sequences shows that field variability (in direction, intensity, or both) can change substantially across a polarity transition. These changes, however, do not appear to be systematic in sign or magnitude. The only feature common to all three records is high field strength in the posttransitional interval, interpreted here as a transient phenomenon associated with the reversal process.
Rheological evolution during extension at nonvolcanic rifted margins: Onset of serpentinization and development of detachments leading to continental breakup
Authors: Pérez‐Gussinyé Marta , Reston Tim J.
Published: February 2004

Within the continent‐ocean transition several nonvolcanic rifted margins exhibit a zone of partially serpentinized peridotites which continue under the thinned and faulted continental crust and are thought to represent subcontinental lithosphere serpentinized by contact with water. As water in sufficient volumes can only come from the surface, we suggest that a major condition for the onset of serpentinization is the embrittlement of the entire crust during progressive extension and hence the development of active crustal penetrating faults acting as fluid conduits. We investigate this possibility by modeling the rheological evolution of the lithosphere during extension and hence determining at what stretching factors the lower crust enters the brittle regime for a variety of different strain rates and lower crustal rheologies. Using an initial thermal structure appropriate for nonvolcanic margins, we find that the entire crust becomes brittle at stretching factors of between ∼3 and 5, depending on the strain rate. This compares well with the thickness of the crust observed just landward of the onset of partially serpentinized peridotites west of Iberia. The predicted thickness of the serpentinized peridotites (depth of the thermal limit of serpentinite stability beneath the crust‐mantle boundary) also compares reasonably well with their observed thickness. As serpentinites are characterized by low friction coefficients, we suggest that the onset of mantle serpentinization controls the development of decollements at the crust‐mantle boundary such as the S and H reflectors west of Iberia (leading to crustal separation). The development of thick serpentinites probably contributes to the weakening of the upper lithosphere and hence the localization of final breakup.
Flow of variably fluidized granular masses across three‐dimensional terrain: 1. Coulomb mixture theory
Authors: Iverson Richard M. , Denlinger Roger P.
Published: February 2004

Rock avalanches, debris flows, and related phenomena consist of grain‐fluid mixtures that move across three‐dimensional terrain. In all these phenomena the same basic forces govern motion, but differing mixture compositions, initial conditions, and boundary conditions yield varied dynamics and deposits. To predict motion of diverse grain‐fluid masses from initiation to deposition, we develop a depth‐averaged, three‐dimensional mathematical model that accounts explicitly for solid‐ and fluid‐phase forces and interactions. Model input consists of initial conditions, path topography, basal and internal friction angles of solid grains, viscosity of pore fluid, mixture density, and a mixture diffusivity that controls pore pressure dissipation. Because these properties are constrained by independent measurements, the model requires little or no calibration and yields readily testable predictions. In the limit of vanishing Coulomb friction due to persistent high fluid pressure the model equations describe motion of viscous floods, and in the limit of vanishing fluid stress they describe one‐phase granular avalanches. Analysis of intermediate phenomena such as debris flows and pyroclastic flows requires use of the full mixture equations, which can simulate interaction of high‐friction surge fronts with more‐fluid debris that follows. Special numerical methods (described in the companion paper) are necessary to solve the full equations, but exact analytical solutions of simplified equations provide critical insight. An analytical solution for translational motion of a Coulomb mixture accelerating from rest and descending a uniform slope demonstrates that steady flow can occur only asymptotically. A solution for the asymptotic limit of steady flow in a rectangular channel explains why shear may be concentrated in narrow marginal bands that border a plug of translating debris. Solutions for static equilibrium of source areas describe conditions of incipient slope instability, and other static solutions show that nonuniform distributions of pore fluid pressure produce bluntly tapered vertical profiles at the margins of deposits. Simplified equations and solutions may apply in additional situations identified by a scaling analysis. Assessment of dimensionless scaling parameters also reveals that miniature laboratory experiments poorly simulate the dynamics of full‐scale flows in which fluid effects are significant. Therefore large geophysical flows can exhibit dynamics not evident at laboratory scales.
Genesis of Mariana shoshonites: Contribution of the subduction component
Authors: Sun Chih‐Hsien , Stern Robert J.
Published: February 2004

The Izu‐Bonin‐Mariana arc contains a unique group of shoshonitic volcanoes from along the magmatic front of this intraoceanic arc. Shoshonites are greatly enriched in incompatible elements compared to lavas typically found in primitive arc settings but have fractionations of lithophile (LIL) and high‐field strength (HFSE) incompatible elements characteristic of convergent margin magmas and thus are characterized by an unusually large “subduction component.” New geochemical and isotopic data for Izu‐Bonin‐Mariana shoshonites and related rocks are presented and interpreted to examine the origin of these enrichments. Enrichments are associated with distinctive isotopic compositions, including the most radiogenic Pb (206Pb/204Pb ∼ 19.47) and least radiogenic Nd (εNd ∼ 5.6) from along the magmatic front of the arc. Despite highly elevated concentrations of fluid‐mobile lithophile elements in the lavas, the similarity of diagnostic element ratios (e.g., Ba/La, Pb/Ce, and U/Th) to those in mid‐ocean ridge basalts and ocean island basalts indicates little role for fluid‐induced elemental fractionation in the generation of these shoshonites. Modeling isotopic data allows up to 6% subducted sediments to be involved, but oxygen isotopic evidence limits this to <3%. Low‐P fractionation explains most of the chemical variations observed in these shoshonites. Removal of <2% Ti‐rich phases could fractionate HFSE from LIL, indicating an important role for low‐P fractionation. Although many features of these shoshonites are consistent with a greater role for subducted sediments, such a role is not accompanied by an unequivocal and universal signal in both isotopic compositions and trace element abundances and fractionations. This signifies a large role for both equilibration of these melts with mantle and for low‐pressure fractionation.
Nonlinear pressure diffusion in a porous medium: Approximate solutions with applications to permeability measurements using transient pulse decay method
Authors: Liang Yan , Price Jonathan D. , Wark David A. , Watson E. Bruce
Published: February 2004

Transient pulse decay has been widely used to measure permeability of tight rocks and synthetic materials. When the pore fluid is a gas (e.g., dry air, Ar, or N2) as used in a gas permeameter, the pressure diffusion equation governing the pulse decay problem is nonlinear due to a pressure‐dependent gas compressibility and molecular slippage effect (also known as the Klinkenberg effect). To simplify data analysis in permeability measurement using a gas permeameter, an approximate solution to the nonlinear diffusion equation was obtained using a regular perturbation method. This solution, which is similar to the original exponential solution of Brace et al. [1968] for a case when the compressibility of the pore fluid is a constant, is valid in the limit when the volume of the interconnected pore fluid is much smaller than the volume of the upstream reservoir. Applications of the approximate solution to laboratory measured pulse decay data show that the estimated sample permeability can be overestimated by as much as a factor of two if the transient gas pressure decay experiment is conducted at low pressures and if molecular slippage is not taken into account. The molecular slippage can be effectively eliminated if the pulse decay measurement is conducted at a mean pressure at least 5 times higher than the Klinkenberg slip factor, which is on the order of 1 bar for texturally equilibrated marble and quartzite used in the permeability study of Wark and Watson [1998].
On fluid flow and mineral alteration in fractured caprock of magmatic hydrothermal systems
Authors: Xu Tianfu , Pruess Karsten
Published: February 2004

Geochemical evolution in hydrothermal fractured rock systems occurs through a complex interplay of multiphase fluid and heat flow and chemical transport processes.Building on previous work, we present here simulations of reactive hydrothermal flow that include (1) detailed fracture‐matrix interaction for fluid, heat, and chemical constituents, (2) gas‐phase participation in multiphase fluid flow and geochemical reactions, (3) the kinetics of fluid‐rock chemical interaction, and (4) heat effects on thermophysical and chemical properties and processes. The present study uses, as an example, water and gas chemistry data as well as caprock mineral composition from the hydrothermal system in Long Valley Caldera (LVC), California. The flow system studied is intended to capture realistic features of fractured magmatic hydrothermal systems. The purpose of this “numerical experiment” is to gain useful insight into process mechanisms such as fracture‐matrix interaction, liquid‐gas phase partitioning, and conditions and parameters controlling water‐gas‐rock interactions in a hydrothermal setting. Simulation results indicate that almost all CO2 is transported through the fracture. Cooling and condensation results in an elevated CO2 partial pressure. The CO2 is the dominant gas‐phase constituent close to the land surface. Close to the heat source, dissolution dominates over precipitation. Away from the heat source precipitation dominates because chemical constituents, transported from the bottom, precipitate in a lower‐temperature environment. Mixing with cold meteoric water enhances mineral dissolution and precipitation effects. The rock alteration pattern is sensitive to reaction kinetics. The predicted alteration of primary rock minerals and the development of secondary mineral assemblages are generally consistent with field observations in the LVC. The observed sequence of argillic alteration in the LVC consists of an upper zone with smectite and kaolinite (in the lower‐temperature region), a lower illite zone, and an intermediate mixed illite and smectite zone. The sequence is reasonably well reproduced in the numerical simulation. In addition, calcite and chlorite precipitation in the hot region coincides with the observations. Using the reactive geochemical transport model, we have successfully simulated spatial distribution of the three argillic alteration zones.
Modeling consolidation and dewatering near the toe of the northern Barbados accretionary complex
Authors: Stauffer Philip , Bekins Barbara A.
Published: February 2004

At the toe of the northern Barbados accretionary complex, temperature and pore water chemistry data indicate that fluid flow is channeled along the décollement and other shallow thrust faults. We examine mechanisms that may prevent consolidation and maintain high permeability over large sections of the décollement. High‐resolution bulk density data from five boreholes show that the décollement is well consolidated at some sites while other sites remain underconsolidated. Underconsolidated décollement behavior is associated with kilometer‐scale negative‐polarity seismic reflections from the décollement plane that have been interpreted to be fluid conduits. We use a coupled fluid flow/consolidation model to simulate the loading response of a 10‐km‐long by 680‐m‐thick slice of sediment as it enters the accretionary complex. The simulations capture 185 ka (5 km) of subduction, with a load function representing the estimated effective stress of the overriding accretionary prism (3.8° taper angle). Simulation results of bulk density in the décollement 3.2 km arcward of the deformation front are compared with observations. The results show that persistent high pore pressures at the arcward edge of the simulation domain can explain underconsolidated behavior. The scenario is consistent with previous modeling results showing that high pore pressures can propagate intermittently along the décollement from deeper in the complex. Simulated seaward fluxes in the décollement (1–14 cm yr−1) lie between previous estimates from modeling studies of steady state (<1 cm yr−1) and transient (>1 m yr−1) flow. Maximum simulated instantaneous fluid sources (2.5×10−13 s−1) are comparable to previous estimates. The simulations show minor swelling of incoming sediments (fluid sources ∼−3×10−15 s−1) up to 3 km before subduction that may help to explain small‐scale shearing and normal faulting proximal to the protodécollement.
Relationships between pahoehoe surface units, topography, and lava tubes at Mauna Ulu, Kilauea Volcano, Hawaii
Authors: Byrnes Jeffrey M. , Crown David A.
Published: February 2004

Lava flow field development at Mauna Ulu was analyzed by characterizing pahoehoe surface units and their distribution relative to pre‐Mauna Ulu topography and the main lava tube system. Four pahoehoe surface units were identified in the field and described on the basis of color, surface texture, and morphology: broad, flat sheets (unit I), networks of interconnected glassy‐surfaced toes (unit II), late stage breakout lobes of viscous toes (unit III), and irregular surfaces exhibiting meter‐scale roughness, which typically occur as channels (unit IV). The distribution of these units was mapped on high‐resolution aerial photographs using an automated supervised classification technique; Geographical Information Systems (GIS) analyses utilized digitized ∼6 m (20 foot) contour interval topography of the pre‐Mauna Ulu surface and the mapped lava tube network to assess the influence of topography and lava tubes on the emplacement of surface flows. The four pahoehoe units represent variations in emplacement conditions, on the basis of the various flow regimes (sheet, toe, and channel) and surface textures (smooth/glassy and rough) displayed. These surface units show a limited correlation to pre‐Mauna Ulu topography based on their mean underlying slopes. The higher flow rates indicated by the channelized surfaces of unit IV are spatially correlated with higher (22.2°) mean underlying slopes relative to those of the sheets of unit I (14.2°) and the toe networks representing units II (15.2°) and III (15.9°). The distribution of the four units does not appear to be directly related to their proximity to the largest scale of lava tubes, suggesting two possible scenarios: the main lava tubes do not significantly affect surface unit emplacement within the study area and/or these tubes do not preferentially emplace any of the four units identified in this study.
Observation of growing correlation length as an indicator for critical point behavior prior to large earthquakes
Authors: Zöller Gert , Hainzl Sebastian , Kurths Jürgen
Published: February 2004

We test the critical point concept for earthquakes in terms of the spatial correlation length. A system near a critical point is associated with a diverging correlation length following a power law time‐to‐failure relation. We estimate the correlation length directly from an earthquake catalog using single‐link cluster analysis. Therefore we assume that the distribution of moderate earthquakes reflects the state of the regional stress field. The parameters of the analysis are determined by an optimization procedure, and the results are tested against a Poisson process with realistic distributions of epicenters, magnitudes, and aftershocks. A systematic analysis of all earthquakes with M≥6.5 in California since 1952 is conducted. In fact, we observe growing correlation lengths in most cases. The null hypothesis that this behavior can be found in random data is rejected with a confidence level of more than 99%. Furthermore, we find a scaling relation log R∼0.7M (log〈ξmax〉 ∼ 0.5M), between the mainshock magnitude M and the critical region R (the correlation length 〈ξmax〉 before the mainshock), which is in good agreement with theoretical values.