bluuebunny/crossref_metadata_2025 huggingface dataset:
One century of tectonic deformation along the Sumatran fault from triangulation and Global Positioning System surveys
Authors: Prawirodirdjo L. , Bock Y. , Genrich J. F. , Puntodewo S. S. O. , Rais J. , Subarya C. , Sutisna S.
Published: February 2004

An analysis combining historical triangulation and recent Global Positioning System (GPS) survey measurements in west and north Sumatra, Indonesia, reveals a detailed slip history along the central part of the Sumatran fault. The arc‐parallel components of the combined velocity field are consistent with slip rates inferred from GPS data, ranging from 23 to 24 mm/yr. Between 1.0°S and 1.3°N the Sumatran fault appears to be characterized by deep locking depths, of the order of 20 km, and the occurrence of large (Mw ≈ 7) earthquakes. The long‐term (1883–1993) strains show simple right‐lateral shear, with rates similar to GPS‐measured, 1989–1993 strain rates. Coseismic deformation due to the 1892 Tapanuli and 1926 Padang Panjang earthquakes, estimated from triangulation measurements taken before and after the events, indicates that the main shocks were significantly larger than previously reported. The 1892 earthquake had a likely magnitude of Mw ≈ 7.6, while the 1926 events appear to be comparable in size to the subsequent (M ∼ 7) 1943 events and an order of magnitude higher than previously reported.
Cadmium, indium, tin, tellurium, and sulfur in oceanic basalts: Implications for chalcophile element fractionation in the Earth
Authors: Yi Wen , Halliday Alex N. , Alt Jeff C. , Lee Der‐Chuen , Rehkämper Mark , Garcia Michael O. , Langmuir C. H. , Su Yongjun
Published: February 2004

Concentrations of S, Cd, In, Sn, and Te are reported for 80 samples of mid‐ocean ridge basalt (MORB), submarine and subaerial ocean island basalt (OIB) and submarine arc lavas. Cadmium, In, and Sn are moderately incompatible, and Te is compatible during partial melting. Cadmium is particularly uniform, consistent with a homogeneous distribution in the mantle. Tellurium is more variable (1–6 ppb) and is notably higher in Loihi, ranging up to 29 ppb, the most likely explanation for which is accumulation of Cu‐bearing sulfide. The average Cd/Dy ratio is the same (0.027) for OIB glasses, MORB glasses and the continental crust, yielding a primitive mantle Cd concentration of ∼18 ppb. Indium, despite being more volatile, is less depleted than Cd and the other very volatile chalcophile elements Pb, Bi, Tl, and Hg. From the depletion of In we deduce that core formation depleted the silicate Earth in Cd, Pb, Bi, Tl, and Hg by between factors of 5 and 10. The In depletion yields concentrations of C, S, Se, and Te in the core of C ∼1.2%, S > 2.4%, Se > 7.1 ppm, and Te > 0.89 ppm. The Moon appears to be enriched in Te relative to the silicate Earth. Either a significant fraction of the Moon was derived from a more Te‐rich body or the silicate Earth's inventory of chalcophile and siderophile elements was depleted by further terrestrial core growth after formation of the Moon.
Source process of very long period seismic events associated with the 1998 activity of Iwate Volcano, northeastern Japan
Authors: Nishimura Takeshi , Nakamichi Haruhisa , Tanaka Satoru , Sato Minemori , Kobayashi Tomokatsu , Ueki Sadato , Hamaguchi Hiroyuki , Ohtake Masakazu , Sato Haruo
Published: February 2004

We observed very long period seismic events that are associated with the 1998 activity of Iwate Volcano, northeast Japan. The events show a dominant period of 10 s and duration of 30–60 s, often with accompanying short‐period waves at the beginning and at the end of the long‐period signals. By analyzing the broadband seismograms we find that the source elongates in the east‐west direction for ∼4 km at a depth of 2 km beneath the western part of Iwate Volcano. Results of moment tensor inversions show a source mechanism of mutual deflation and inflation of two chambers located at the western and eastern edges of the source region. The source region coincides with the low seismic velocity zone detected by seismic tomography and is very close to the locations of pressure sources estimated from crustal deformation data. On the basis of these results we infer that the very long period seismic events are generated by transportation and movement of magmatic fluid (hot water and/or magma) in a shallow part of the volcano. We further present a simple source model of very long period seismic events based on one‐dimensional flow dynamics and propose a new parameter to characterize the size of very long period event: the energy flow rate, which is obtained by dividing the seismic moment by the dominant period. The energy flow rate was estimated as 3.1×1012 J/s for the event on July 29, 1998.
Distribution of slip at the northern Sumatran fault system
Authors: Genrich J. F. , Bock Y. , McCaffrey R. , Prawirodirdjo L. , Stevens C. W. , Puntodewo S. S. O. , Subarya C. , Wdowinski S.
Published: February 2004

We model spatial variations in horizontal displacements of 117 geodetic sites measured during annual surveys in 1989–1996 with the Global Positioning System (GPS) as elastic strain across a locked strike‐slip fault to infer the contemporary slip rate, locking depth, and location of the Sumatran fault (SF) in northern Sumatra (1°S‐3°N). GPS‐derived slip rate estimates increase slightly northward from 23±3 mm/yr at 0.8°S to 26±2 mm/yr at 2.7°N. They agree with geologic estimates north of the equator, but at 0.5°S they are ∼10 mm/yr higher. Strain appears to be distributed asymmetrically about the fault. South of 2°N, about 5 mm/yr of shear is required within the offshore forearc, west of the fault, to achieve a closer agreement of fault locations inferred from GPS velocities with geologically identified traces of the SF. Locking depth estimates are of the order of 10–20 km. The western branch of the major fault bifurcation near 1°N slips at a rate 5 times higher than the eastern branch. The two main strands of the fault at the northwestern tip of Sumatra (5.5°N) appear to be nearly free of horizontal strain; significant slip must occur away from the two strands, probably farther east at two other geologically active branches. The Banda Aceh embayment is extruded to the northwest at a rate of 5±2 mm/yr. Within the estimated velocity uncertainties of several millimeters per year, fault‐normal deformation along the SF is insignificant. Almost strain free, the northern part of the back arc basin is part of the rigid Sunda Shelf, while the northern forearc is subjected to 8±5×10−8yr−1 of extension nearly parallel to the arc.
Tectonics and denudation adjacent to the Xianshuihe Fault, eastern Tibetan Plateau: Constraints from fission track thermochronology
Authors: Xu Ganqing , Kamp Peter J. J.
Published: February 2004

The Xianshuihe‐Xiaojiang fault system extending from eastern Tibet to central Yunnan, China, is a major left‐shear structural boundary, accommodating the clockwise rotation of crustal rocks between the Eastern Himalayan syntaxis and the South China Block. Zircon and apatite fission track (FT) data are reported for 111 samples of basement collected from both sides of the northern part of this fault and spanning 300 km across the eastern margin of the Tibetan Plateau, where its mean elevation drops from 3500 to 1500 m above sea level. The zircon FT ages define two fossil partial annealing zones at different elevations, one fossilised at circa 130 Ma, and the other at circa 21 Ma as a result of cooling probably via regional denudation. The apatite FT ages are mostly less than 25 Ma, but a few granitoids higher up on the plateau retain late Cretaceous apparent ages. Within the apatite FT data with Neogene ages, there may be several partial annealing zones, with mixture modeling identifying age components suggestive of discrete cooling phases at circa 22, 7 and 2 Ma. The first‐order pattern of the minimum amount of Neogene denudation for a section across the plateau margin immediately northeast of Xianshuihe Fault suggests that a relatively uniform 4–6 km has been eroded from the inner part of the plateau margin, increasing to 7–10 km at Kangding, where the oldest rocks (Precambrian) are exposed, and then decreasing markedly into Sichuan Basin. These new FT data combined with published FT data suggest that the present extent of the Tibetan Plateau was defined during the early Miocene.
Spectral roughness of glaciated bedrock geomorphic surfaces: Implications for glacier sliding
Authors: Hubbard Bryn , Siegert Martin J. , McCarroll Danny
Published: February 2004

A microroughness meter (MRM) was used to measure the high‐frequency roughness of a number of geomorphic surfaces in the forefield of Glacier de Tsanfleuron, Switzerland. Resulting spectral power densities are added to low‐frequency spectra, measured by electro‐optical distance meter (EDM), to generate composite roughness spectra that include almost 5 orders of magnitude of roughness in the frequency domain. These are used to define two roughness indices: a general index of bed roughness is defined as the integral of the raw, spectral power densities, and a sliding‐related index of bed roughness is defined as the integral of the spectral power densities weighted to account for the optimum dependence of glacier sliding speed on hummock wavelength. Results indicate that MRM‐measured geomorphic components vary in roughness by 3 orders of magnitude, principally depending on the surface microenvironment measured and profile orientation relative to the direction of former ice flow. Both MRM‐ and EDM‐measured roughnesses are lower parallel to the direction of former ice flow than perpendicular to it. Composite roughness spectra consequently indicate that the glacier bed is smoothed in the direction of former ice flow at all horizontal scales from 1 mm to 40 m, typically resulting in an order of magnitude decrease in sliding‐related roughness relative to that measured perpendicular to ice flow. Comparison of data from two survey sites located adjacent to, and ∼1.2 km from, the current glacier margin indicates that postglacial subaerial weathering homogenizes bedrock roughness, in particular reducing high‐frequency, flow‐orthogonal roughness. Accounting for the effect of 28% ice‐bedrock separation over one of the profiles reduces net, sliding‐dependent roughness by between 27% and 43%, depending on the transition wave number used.
Effect of lithostatic pressure and tectonic deformation on the magnetic fabric (anisotropy of magnetic susceptibility) in low‐grade metamorphic rocks
Authors: Imaz A. Gil , Pocoví A. , Lago M. , Parés J. M.
Published: February 2004

The role of lithostatic pressure and strain in low‐grade metamorphic cover rocks is analyzed through the anisotropy of magnetic susceptibility (AMS) of Jurassic and Cretaceous siltstones and limestones from the Cameros Basin (north Spain). For rocks initially characterized by precleavage magnetic fabric the main change in the magnetic fabric produced by the increase of lithostatic pressure is the increment in the degree of anisotropy (Pj). In rocks affected by syncleavage deformation the magnetic fabric depends on the mechanism of internal deformation involved in fabric development. In siltstones, where cleavage is correlated with synkinematic recrystallization of phyllosilicates and rigid‐body rotation of grains, the magnetic fabric is coherent with the internal deformation determined by strain analysis on deformed nodules. On the contrary, in limestones we observe a complete reorganization of the magnetic fabric probably due to the discontinuous character of the cleavage formation mechanism. Detailed analysis of the β parameter (angle between the bedding pole and the minimum susceptibility axis parallel to the cleavage pole) indicates that for rocks characterized by syncleavage magnetic fabric the different variation pattern of Tj and Pj parameters is closely related to the deformation mechanisms responsible for the fabric development.
Influence of water on plastic deformation of olivine aggregates: 1. Diffusion creep regime
Authors: Mei S. , Kohlstedt D. L.
Published: February 2004

The influence of water on diffusion creep of olivine aggregates was investigated by performing high‐temperature creep experiments under both hydrous and anhydrous conditions. Deformation experiments were conducted on fine‐grained samples using a gasmedium apparatus at confining pressures of 100 to 450 MPa and temperatures between 1473 and 1573 K. Water was supplied by the dehydration of talc, which occurs near 1075 K. Water fugacities of ∼85 to 520 MPa were obtained by varying the confining pressure under water‐saturated conditions. Under both hydrous and anhydrous conditions deformation was dominated by grain boundary diffusion. At a water fugacity of ∼300 MPa, samples crept ∼5 times faster than those deformed under anhydrous conditions at similar differential stresses and temperatures. Within the range of water fugacity investigated, the strain rate is proportional to water fugacity to the 0.7 to 1.0 power, assuming values for the activation volume of 0 to 20×10−6 m3/mol, respectively. We propose the following point defect model to explain this water‐weakening effect: In going from an anhydrous to a hydrous environment the charge neutrality condition changes from [FeMe•] = 2[VMe″] to [FeMe•] = [HMe′]. As a consequence, for olivine aggregates the concentration of silicon interstitials, the rate of silicon diffusion, and therefore the rate of diffusion creep increase systematically with increasing water fugacity (i.e., OH concentration).
Active deformation in the Bolivian Andes, South America
Authors: Lamb Simon
Published: February 2004

A method is described for determining the horizontal field of velocity and velocity gradients in the Bolivian Andes in the vicinity of the Bolivian orocline, using the following data: (1) space‐geodetic measurements of crustal motions; (2) the style, distribution, and rate of late Miocene‐Quaternary folding and faulting; and (3) paleomagnetic measurements of rigid body rotations about a vertical axis. These data were analyzed in a network of 26 triangles which spanned the Bolivian Andes by solving the velocity gradient compatibility equations between adjacent triangles, subject to the input data constraints. This yielded 120–140 linear equations to constrain 108 unknowns, making the problem moderately overdetermined. A series of experiments were carried out with different combinations of the input data to determine the best fit, in the least squares sense, field of Quaternary horizontal velocities and velocity gradients. The azimuths of velocity vectors in the high Andes are subparallel to both the relative plate convergence vector and the symmetry axis of the Bolivian orocline, with magnitudes relative to stable South America in the range 7–17 mm yr−1, accommodating up to 30% of the relative plate convergence. However, the azimuths of velocities along the range front are nearly orthogonal to the range, with a component of distributed range‐parallel dextral or sinistral shear within the Eastern Cordillera and Altiplano. The horizontal velocity field requires bending about a vertical axis of both limbs of the Bolivian orocline. The field of horizontal dilatation, assuming homogeneous deformation of the entire crustal thickness, requires vertical velocities of material points up to 2.5 mm/yr in the sub‐Andean zone, decreasing to <0.1 mm/yr farther east in the Eastern Cordillera and Altiplano. The direction of maximum gradients of buoyancy stress, determined from the field of gradients of appropriate combinations of velocity gradients, using a thin sheet model of viscous flow, are broadly parallel to the direction of maximum topographic gradients in the Eastern Cordillera, providing support for the idea that the bulk behaviour of the Bolivian lithosphere is fluid‐like. A comparison of these gradients with buoyancy stress gradients calculated from crustal thickness contrasts, suggest that the bulk lithospheric viscosity is in the range 0.5–1.0×1022 Pa s; gradients of velocity gradients along the eastern margin of the Bolivian Andes can be interpreted in terms of marked gradients of viscosity, with viscosity increasing toward the Brazilan shield in the cratonic core of South America. Buoyancy stresses calculated from crustal thickness contrasts, calibrated with the crustal thicknesses at the changeovers from thrust to strike‐slip or normal faulting, suggest that the average lithosphericscale deviatoric stresses are in the range 7–15 MPa.
Crustal structure of the southeast Greenland margin from joint refraction and reflection seismic tomography
Authors: Korenaga J. , Holbrook W. S. , Kent G. M. , Kelemen P. B. , Detrick R. S. , Larsen H.‐C. , Hopper J. R. , Dahl‐Jensen T.
Published: February 2004

We present results from a combined multichannel seismic reflection (MCS) and wideangle onshore/offshore seismic experiment conducted in 1996 across the southeast Greenland continental margin. A new seismic tomographic method is developed to jointly invert refraction and reflection travel times for a two‐dimensional velocity structure. We employ a hybrid ray‐tracing scheme based on the graph method and the local ray‐bending refinement to efficiently obtain an accurate forward solution, and we employ smoothing and optional damping constraints to regularize an iterative inversion. We invert 2318 Pg and 2078 PmP travel times to construct a compressional velocity model for the 350‐km‐long transect, and a long‐wavelength structure with strong lateral heterogeneity is recovered, including (1) ∼30‐km‐thick, undeformed continental crust with a velocity of 6.0 to 7.0 km/s near the landward end, (2) 30‐ to 15‐km‐thick igneous crust within a 150‐km‐wide continent‐ocean transition zone, and (3) 15‐ to 9‐km‐thick oceanic crust toward the seaward end. The thickness of the igneous upper crust characterized by a high‐velocity gradient also varies from 6 km within the transition zone to ∼3 km seaward. The bottom half of the lower crust generally has a velocity higher than 7.0 km/s, reaching a maximum of 7.2 to 7.5 km/s at the Moho. A nonlinear Monte Carlo uncertainty analysis is performed to estimate the a posteriori model variance, showing that most velocity and depth nodes are well determined with one standard deviation of 0.05–0.10 km/s and 0.25–1.5 km, respectively. Despite significant variation in crustal thickness, the mean velocity of the igneous crust, which serves as a proxy for the bulk crustal composition, is surprisingly constant (∼7.0 km/s) along the transect. On the basis of a mantle melting model incorporating the effect of active mantle upwelling, this velocity‐thickness relationship is used to constrain the mantle melting process during the breakup of Greenland and Europe. Our result is consistent with a nearly constant mantle potential temperature of 1270–1340°C throughout the rifting but with a rapid transition in the style of mantle upwelling, from vigorous active upwelling during the initial rifting phase to passive upwelling in the later phase.
Mount Etna dense array local earthquake P and S tomography and implications for volcanic plumbing
Authors: Laigle Mireille , Hirn Alfred , Sapin Martine , Lépine Jean‐Claude , Diaz Jordi , Gallart Josep , Nicolich Rinaldo
Published: February 2004

Inversion for the three‐dimensional velocity structure of Mount Etna is performed with a data set of arrival times of P and S waves of local earthquakes from temporary dense arrays of three‐component seismographs. A high‐Vp body revealed by the original tomography without nearby stations is confirmed, and its image is sharpened using new velocity constraints provided by refraction data. Synthetic tests of Vp and Vp/Vs and comparison with an independent artificial source tomography with a fundamentally different geometry consistently calibrate the significance threshold of the resolution indicators. The trustworthy part of the image shows a high‐Vp body centered under the southern part of Valle del Bove above the 6 km below sea level deep basement, which extends towards sea level and may be rooted in or through the crust. It has a large contrast of over 1 km/s with the surrounding sediments and sharp lateral limits and can thus be regarded as made of intrusive material of magmatic origin. The massive high‐Vp body is heterogeneous in Vp/Vs. The regions inside it where Vs is relatively low can then be suspected of containing a proportion of melt or be fractured and act as pressure links or transport zones. Such features may be structurally linked and appear to be activated in eruptive phenomena. By taking into account the heterogeneities in structure and physical state retrieved by seismic tomography a succession of seismic events, deformational episodes, and geochemical variation in lavas can be discussed with respect to the well‐observed eruptions.
Subsolidus evolution and alteration of titanomagnetite in ocean ridge basalts from Deep Sea Drilling Project/Ocean Drilling Program Hole 504B9 Leg 83: Implications for the timing of magnetization
Authors: Shau Y.‐H. , Torii M. , Horng C.‐S. , Peacor D. R.
Published: February 2004

Magnetic minerals in six samples of oceanic basalts of the transition zone and upper sheeted dikes from Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) Hole 504B, Leg 83, were studied by methods of rock magnetism and transmission electron microscopy (TEM). TEM observations showed that the magnetic mineral in these basalts is end‐member magnetite (TMO) of extremely fine‐grain size (30–100 nm) primarily in the range of pseudosingle‐domain magnetite, consistent with the rock magnetic properties including hysteresis parameters, Curie temperature, and low‐temperature measurements (Verwey transition). Magnetite formed by two different processes: (1) oxidation‐“exsolution,” true exsolution, and hydrothermal alteration, and (2) oxidation‐exsolution, a second stage of oxidation‐exsolution, and hydrothermal alteration. The primary titanomagnetite (TM60‐70) that crystallized from the melt thus evolved to end‐member magnetite coexisting with titanite (sphene), kassite, ulvöspinel (TM ∼ 87), and ilmenite on a submicroscopic scale. On the basis of the formation mechanisms of the magnetic carrier, the primary titanomagnetite (TM ∼ 60) with Curie temperature of ∼180°C did not acquire thermoremanent magnetization (TRM) in these basalts. Instead, the Ti‐bearing magnetite (TM ∼ 10–20) that formed as oxidized or exsolved lamellae acquired its first thermal chemical remanent magnetization (CRM) at ∼ 500–400°C during subsolidus cooling. Upon the onset of hydrothermal alteration the recrystallized end‐member magnetite acquired a second CRM. The natural remanent magnetization of the basalts from the transition zone and upper sheeted dikes is therefore characteristic of CRMs that were acquired when titanomagnetite altered, in part, to magnetite during subsolidus cooling and hydrothermal alteration close to the ridge axis.
Active deformation of Taiwan from GPS measurements and numerical simulations
Authors: Hu Jyr‐Ching , Yu Shui‐Beih , Angelier Jacques , Chu Hao‐Tsu
Published: February 2004

Using a two‐dimensional distinct element model, we evaluate the relationships between plate kinematics and present‐day deformation in Taiwan where active collision occurs. In particular, the distribution of velocity fields calculated in our models is compared with the actual velocity field revealed by the most recent geodetic data (GPS) obtained in Taiwan and the surrounding islands of the Philippine Sea plate and the Eurasian shelf. The main aim of this paper is to produce a mechanically consistent 2‐D model that accounts for the observed velocity field taken as whole, within the limits of acceptable rheological parameters and reasonable boundary displacement conditions. We evaluate how the active deformation of Taiwan is influenced by the presence of strong and weak zones such as the structural highs in the foreland and subduction zones with accretionary prisms, respectively, major mechanical discontinuities such as the main fault zones in the mountain belt, and the opening of the Okinawa Trough. Particular attention is paid to the role of preexisting discontinuities since the presence of mechanical decoupling along major faults strongly affects the distribution of the velocity and stress patterns. We show that despite parameter uncertainties, several tectonic factors (the presence of the strong Kuanyin and Peikang highs in contrast to the weak subduction zone to the south, the “weak” active regional shear zones, and the opening of the back arc Okinawa Trough) concur to provide an acceptable mechanical model for this regional deformation. These sources are related not only to the geometry of the plate boundary, the direction of plate convergence, and the shape of the Chinese margin but also to the presence of major zones of relative weakness and mechanical decoupling such as the Longitudinal Valley fault zone and the western thrust belt of Taiwan.
A simple model of flow patterns in overpressured sedimentary basins with heat transport and fracturing
Authors: L'Heureux Ivan , Fowler Anthony D.
Published: February 2004

Overpressure zones in sedimentary basins are defined as sediments where the fluid pore pressure is substantially higher than hydrostatic. Chief among the causes for the formation of overpressure is the rapid deposition of low‐permeablity sediments so that compaction and dewatering are inhibited. This results in the support of the overlying material in part by the fluid, rather than by grain to grain contact. Many overpressure zones also have associated geothermal anomalies whereby the geotherms are hotter than normal. In this paper, we present a one‐dimensional model of overpressure generation and dissipation, with hydraulic fracture. The fluid flow is coupled to the fluid temperature field and to the motion of the solid sediment matrix. Compacted coordinates are introduced to decouple the fluid velocity and temperature from the solid matrix velocity. Using a maximum sedimentation rate of 1 km Myr−1 and a hydraulic conductivity at the top of the layer equal to 5×10−12 m s−1, 3 km of partially compacted sediment are deposited in 5 Myr, generating an overpressure of 0.81 of the lithostatic pressure at the bottom of the sediment layer. An undercompaction of 19% relative to normally compacted sediment is developed at a final depth of 3.66 km by the end of sedimentation. Dissipation of the overpressure takes place on a timescale of the order of 40 Myr. The equilibrium temperature generates thermal gradients in excess of 38°C km−1 at shallow depths. Fast sedimentation rate and high permeability in the fractured state favor oscillating behavior and propagation of fracture waves. Overpressure zones may constitute a proximal source of hydrothermal fluids for the genesis of ore deposits in sedimentary basins.
Three‐dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9°30'N
Authors: Dunn Robert A. , Toomey Douglas R. , Solomon Sean C.
Published: February 2004

The seismic structure of the crust and shallow mantle beneath the East Pacific Rise near 9°3O′N is imaged by inverting P wave travel time data. Our tomographic results constrain for the first time the three‐dimensional structure of the lower crust in this region and allow us to compare it to shallow crustal and mantle structure. The seismic structure is characterized by a low‐velocity volume (LVV) that extends from 1.2 km depth below the seafloor into the mantle. The cross‐axis width of the LVV is narrow in the crust (5–7 km) and broad in the mantle (∼18 km). Although the width of the top of the LVV is similar to previous estimates, its narrow shape at lower crustal depths and its significant widening in the mantle are previously unknown features of the rise velocity structure. In the rise‐parallel direction the LVV varies in magnitude such that the lowest velocities are located between two minor rise axis discontinuities near 9°28′N and 9°35′N. From the seismic results we estimate the thermal structure and melt distribution beneath the rise. The thermal structure suggests that heat removal is relatively efficient throughout the crust yet inefficient at Moho and mantle depths. Estimates of the melt distribution indicate that magma accumulates at two levels in the magmatic system. One is at the top of the magmatic system and is capped by the shallow melt lens detected by seismic reflection surveys; the other is within the Moho transition zone and topmost portion of the mantle. The highest melt fractions occur within the upper reservoir, whereas the lower reservoir contains a lower melt fraction distributed over a broader area. By volume, however, there may be up to 40% more melt in the lower reservoir than in the upper reservoir. Along‐axis variations in crustal melt content are similar to those in the mantle, supporting the hypothesis that the mantle, midway between the 9°28′N and 9°35′N devais, is presently delivering greater amounts of melt to the lower crust than to regions immediately to the north or south. We see no evidence (from seismic anisotropy) for diapiric mantle flow, suggesting that solid‐state flow and melt migration are decoupled in the shallow mantle. Our results are not compatible with models that require a large, segment‐scale redistribution of melt within the crust. Instead, our results imply that crustal magma chambers are replenished at closely spaced intervals along the rise.
Crustal and upper mantle structure beneath the Corinth rift (Greece) from a teleseismic tomography study
Authors: Tiberi Christel , Lyon‐Caen Hélène , Hatzfeld Denis , Achauer Ulrich , Karagianni E. , Kiratzi A. , Louvari E. , Panagiotopoulos D. , Kassaras I. , Kaviris G. , Makropoulos K. , Papadimitriou P.
Published: February 2004

We report here the results of a tomographic lithospheric study in the area of the Corinth and Evvia rifts (Greece), designed to constrain the mechanism of continental extension. Sixty seismological stations were deployed in the area for a period of 6 months, and 177 teleseismic events were recorded by more than five stations and gave more than 2000 travel time residuals (P and PKP phases), which were inverted to image the velocity structure down to 200 km depth. We use both a linear and a nonlinear method to invert the data set. The main result is a long‐wavelength positive velocity anomaly located in the upper mantle, which is interpreted as the subducted African lithosphere. The subducted lithosphere is well defined from ∼7O km depth down to 200 km. From synthetic tests as well as from the amplitude of the anomaly (more than +7%) we conclude that the subduction continues below 200 km. In addition, a second positive velocity anomaly of about +4% from the surface down to 40 km depth, located north of the Gulf of Corinth, has been found. This is interpreted as the result of a crustal thinning of several kilometers (∼5 km), shifted to the north from the Gulf of Corinth and trending obliquily NW‐SE. We suggest that this crustal thinning is mainly related to the Miocene widespread extension in the Aegean and that the Quaternary Corinth rift initiated where the crust was already thinned. The different styles of deformation of the eastern and western part of the rift are consistent with this interpretation. No clear velocity anomaly can be related to the Evvia rift.
Earthquake cycle, fault zones, and seismicity patterns in a rheologically layered lithosphere
Authors: Lyakhovsky Vladimir , Ben‐Zion Yehuda , Agnon Amotz
Published: February 2004

We study the coupled evolution of earthquakes and faults in a model consisting of a seismogenic upper crust governed by damage rheology over a viscoelastic substrate. The damage rheology has two types of functional coefficients: (1) a “generalized internal friction” separating states associated with material degradation and healing and (2) damage rate coefficients for positive (degradation) and negative (healing) changes. The evolving damage modifies the effective elastic properties of material in the upper crust as a function of the ongoing deformation. This simulates the creation and healing of fault systems in the upper seismogenic zone. In addition to the vertically averaged thin sheet approximation we introduce a Green function for three‐dimensional elastic half‐space for the instantaneous component of deformation. The formulation accounts in an internally consistent manner for evolving deformation fields, evolving fault structures, aseismic energy release, and spatiotemporal seismicity patterns. These developments allow us to simulate long histories of crustal deformation and to study the simultaneous evolution of regional earthquakes and faults for various model realizations. To focus on basic features of a large strike‐slip fault system, we first consider a simplified geometry of the seismogenic crust by prescribing initial conditions consisting of a narrow damage zone in an otherwise damage‐free plate. For this configuration, the model generates an earthquake cycle with distinct interseismic, preseismic, coseismic, and postseismic periods. Model evolution during each period is controlled by a subset of physical properties, which may be constrained by geophysical, geodetic, rock mechanics, and seismological data. In the more generic case with a random initial damage distribution, the model generates large crustal faults and subsidiary branches with complex geometries. The simulated statistics depend on the space‐time window of the observational domain. The results indicate that long healing timescale, τh, describing systems with relatively long memory, leads to the development of geometrically regular fault systems and the characteristic frequency‐size earthquake distribution. Conversely, short τh (relatively short memory) leads to the development of a network of disordered fault systems and the Gutenberg‐Richter earthquake statistics. For intermediate values of τh the results exhibit alternating overall switching of response from periods of intense seismic activity and the characteristic earthquake distribution to periods of low seismic activity and Gutenberg‐Richter statistics.
Dislocation and diffusion creep of synthetic anorthite aggregates
Authors: Rybacki E. , Dresen G.
Published: February 2004

Synthetic fine‐grained anorthite aggregates were deformed at 300 MPa confining pressure in a Paterson‐type gas deformation apparatus. Creep tests were performed at temperatures ranging from 1140 to 1480 K, stresses from 30 to 600 MPa, and strain rates between 2×10−6 and 1×10−3 s−1. We prepared samples with water total contents of 0.004 wt % (dry) and 0.07 wt % (wet), respectively. The wet (dry) material contained <0.7 (0.2) vol % glass, associated with fluid inclusions or contained at triple junctions. The arithmetic mean grain size of the specimens varied between 2.7±0.1 μm for the dry material and 3.4±0.2 μm for wet samples. Two different creep regimes were identified for dry and wet anorthite aggregates. The data could be fitted to a power law. At stresses >120 MPa we found a stress exponent of n = 3 irrespective of the water content, indicating dislocation creep. However, the activation energy of wet samples is 356±9 kJ mol−1, substantially lower than for dry specimens with 648±20 kJ mol−1. The preexponential factor is log A = 2.6 (12.7) MPa−n s−1 for wet (dry) samples. Microstructural observations suggest that grain boundary migration recrystallization is important in accommodating dislocation creep. In the low‐stress regime we observed a stress exponent of n = 1, suggesting diffusion creep. The activation energies for dry and wet samples are 467 ±16 and 170 ± 6 kJ mol−1, respectively. Log A is 12.1 MPa−n μmm s−1 for the dry material and 1.7 MPa−n μmm s−1 for wet anorthite. The data show that the strengths of anorthite aggregates decrease with increasing water content in both the dislocation and diffusion creep regimes. A comparison of the creep data of synthetic plagioclase from this study with published data for feldspar, olivine, and quartz indicates a linear relationship between activation energy and log A similar to the suggested compensation law for diffusion in silicates.
Viscoelastic thermal stress in cooling basalt flows
Authors: Lore Jason , Gao Huajian , Aydin Atilla
Published: February 2004

Polygonal fractures commonly form in basaltic lava flows during cooling of the molten lava. A transient thermomechanical model is constructed to apply the temperature history in a cooling basalt flow as a boundary condition in a mechanical model of viscoelastic stress in cooling basalt. Nearly all past modeling of stress in cooling basalt has treated the material as purely elastic below a well‐defined glass transition temperature. An important distinction of this model is that the viscosity of the material is allowed to be temperature‐dependent, and the effects of viscoelastic relaxation are included. On the basis of this research, faster cooling and higher strain rates lead to markedly higher effective glass transition temperatures, consistent with previous observations by researchers. The total thermal strain in the system is constant, yet the percentage of strain that is not relaxed by viscous flow varies with position relative to the cooling surface. This elastic strain partition is the portion of the total strain that creates stress in the material, and it is directly dependent on cooling rate. Thus a genetic link between cooling rate and magnitude of stress is identified.
Hydrodynamic response to strike‐ and dip‐slip faulting in a half‐space
Authors: Ge Shemin , Stover S. Chereé
Published: February 2004

Field observations have shown strong coupling between earthquake‐induced stress‐strain fields and subsurface hydrodynamics, reflected by water level change in wells and stream flow fluctuations. Various models have been used in an attempt to interpret the coseismic fluctuations in groundwater level, predict water table rise in the event of an earthquake, and explain stream flow variations. However, a general model integrating earthquake‐induced stress‐strain fields, coseismic pore pressure generation, and postseismic pore pressure diffusion is still lacking. This paper presents such a general framework with which one can approach the general problem of postseismic pore pressure diflusion in three dimensions. We first use an earthquake strain model to generate the stress‐strain field. We then discuss the linkage coupling stress and strain with pore pressure and present an analytical solution of time‐dependent pore pressure diffusion. Finally, we use two examples, a strike‐slip and a dip‐slip fault, to demonstrate the application of the analytical model and the effects of earthquakes on fluid flow. The application to the two fault systems shows that the diffusion time is shorter than conventional estimates, which are based on a diffusivity and a length scale. We find that the diffusion time is predominately a function of the diffusivity of the system, while the length scale influences the magnitude of the initial pore pressure. A diffusion time based on the diffusivity and a length may be misleading because significant localized flow occurs in complex three‐dimensional systems. Furthermore, the induced patterns of a pore pressure change resemble the strain field when shear stress effects are neglected but are significantly modified when shear stresses are included in the coupling relation. The theoretical basis of this work is developed assuming a single episode dislocation. However, the methodology and the results can be readily applied to studying pore pressure conditions after multifaulting events by simple superposition.
Compositional and metamorphic controls on velocity and reflectivity in the continental crust: An example from the Grenville Province of eastern Québec
Authors: Hurich C. A. , Deemer S. J. , Indares A. , Salisbury M.
Published: February 2004

We present velocity and density data for a suite of gabbros and metagabbros over a range of metamorphic grades, from subgreenschist to the eclogite facies. Most of these rocks have a mesocratic protolith with overlapping bulk chemistry ranges providing an opportunity to investigate the systematics of mineralogy and physical properties across a wide range of metamorphic grades with minimal compositional variation. In addition, more mafic gabbros in the eclogite facies were also investigated in order to take into account variations in the extent of transformation of gabbro to eclogite controlled by bulk chemistry. The physical properties data are used as the basis for a first‐order crustal model composed of granitic, anorthositic, and gabbroic components. The objective of constructing the crustal model is to evaluate the relative contribution of compositional variation and metamorphism to the velocity, density, and reflectivity structure of the continental crust. The model suggests that in the upper crust, if fracture porosity is neglected, composition dominates the velocity and density structure, while in the middle and lower crust, composition and metamorphism play equally important roles. Simulations designed to predict potential reflection coefficient size distributions for the various bulk chemistries and mineralogies in the crustal model indicate step increases in the potential for large reflection coefficients that are dominantly controlled by metamorphic grade. Step increases in reflectivity are predicted between the lower and upper amphibolite facies and between incompletely and completely eclogitized metagabbros in the eclogite facies. We also consider several alternative mixing models which account for the effects of one of the three lithologic components occurring as isolated bodies rather than perfectly mixed with the other two components. These models indicate a reduction of the potential for large reflection coefficients by as much as 50%, emphasizing the importance of the degree of mixing for crustal reflectivity.
Lithospheric structure of the Costa Rican Isthmus: Effects of subduction zone magmatism on an oceanic plateau
Authors: Sallarès Valentí , Dañobeitia Juan José , Flueh Ernst R.
Published: February 2004

We present the results of a multidisciplinary geophysical study, conducted to investigate the lithospheric structure of the Costa Rican Isthmus. The physical properties of the lithosphere are resolved by three‐dimensional (3‐D) simultaneous inversion of velocity anomalies and hypocenter parameters using local earthquakes and 2‐D forward modeling of onshore and offshore seismic refraction and gravity data. According to our results, the northern half of the Costa Rican Isthmus is constituted by a ∼40‐km‐thick crust, with a 6‐ to 7‐km‐thick oceanic crust subducting under it. The uppermost level of the basement and most of the marginal wedge show intermediate velocities and high densities, in good agreement with those described for flood basalts. The midlevel shows velocities and densities representative of oceanic crust. The bottommost level (20–40 km) shows high velocities and densities, typical of mafic rocks, and the upper mantle displays anomalously low densities and velocities. Intracrustal heterogeneities at intermediate wavelengths are indicated by prominent velocity anomalies. These results are consistent with a basement beneath the Costa Rican Isthmus being part of the Caribbean plateau, originated at 85–90 Ma with the onset of the Galápagos hotspot. The upper level corresponds to the flood basalts extruded during this phase, and it includes most of the marginal wedge. The second level represents the preexisting oceanic crust. The mafic lower crust, intracrustal heterogeneities, and anomalous upper mantle are interpreted to be built up by underplating, intrusion, and crystallization of basaltic melts, formed under the influence of subducting lithosphere dehydration.
Basaltic lava domes, lava lakes, and volcanic segmentation on the southern East Pacific Rise
Authors: White Scott M. , Macdonald Ken C. , Haymon Rachel M.
Published: February 2004

Meter‐scale DSL‐120 sonar mapping and coregistered Argo II photographic observations reveal changes in eruptive style that closely follow the third‐order structural segmentation of the ridge axis on the southern East Pacific Rise, 17°11′–18°37′S. Near segment ends we observe abundant basaltic lava domes which average 20 m in height and 200 m in basal diameter and have pillow lava as the dominant lava morphology. The ubiquity of pillow lava suggests low effusion rate eruptions. The abundance of lava domes suggests that the fissure eruptions were of sufficient duration to focus and produce a line of volcanic edifices. Near segment centers we observe fewer but larger lava domes, voluminous drained and collapsed lava lakes, and smooth lobate and sheet lava flows with very little pillow lava. The abundance of sheet flows suggests that high effusion rate eruptions are common. Fewer lava domes and large lava lakes suggest that fissure eruptions do not focus to point sources. This pattern was observed on eight third‐order ridge segments suggesting that a fundamental volcanic segmentation of the ridge occurs on this scale. The third‐order segment boundaries also correlate with local maxima in the seismic axial magma chamber reflector depth throughout the study area and decreased across‐axis width of the region of seismic layer 2A thickening along the one segment where sufficient cross‐axis seismic lines exist. The geochemically defined magmatic segment boundaries in the study area match the locations of our volcanic segment boundaries, although rock sampling density is not adequate to constrain the variation across all the third‐order volcanic segments that we identify. These observations suggest that variation in the processes of crustal accretion along axis occurs at a length scale of tens of kilometers on superfast spreading (>140 km/Myr full rate) mid‐ocean ridges.
Models of isostatic and dynamic topography, geoid anomalies, and their uncertainties
Authors: Panasyuk Svetlana V. , Hager Bradford H.
Published: February 2004

Knowledge of the dynamic topography at Earth's surface caused by sublithospheric density contrasts would provide crucial constraints on models of mantle dynamics. We calculate global models of this dynamic topography by subtracting from the observed topography estimates of the topography caused by assumed isostatically compensated density variations in the crust, the oceanic lithosphere, and the continental tectosphere. We also calculate the isostatic geoid anomaly that would result from these compensated near‐surface density anomalies. Because there is controversy about the thermal structure of old oceanic lithosphere, we investigate two models of the cooling of the oceanic lithosphere (half‐space and plate) and calculate the effect of each on the inferred dynamic topography. Peak amplitudes of dynamic topography from sublithospheric sources expanded to spherical harmonic degree 6 are approximately ±1 km. We estimate the uncertainties in the calculated dynamic topography and geoid by combining the errors in the data and systematic errors. To obtain the spectral coefficients of the fields, we use weighted least squares inversions. The calculated dynamic topography arising from sublithospheric sources, residual geoid, and errors associated with these fields can be used in studies of mantle dynamics.