bluuebunny/crossref_metadata_2025 huggingface dataset:
Distribution of active rock uplift along the eastern margin of the Tibetan Plateau: Inferences from bedrock channel longitudinal profiles
Authors: Kirby Eric , Whipple Kelin X. , Tang Wenqing , Chen Zhiliang
Published: May 2003

Current models of long‐term river incision into bedrock suggest that the local rate of differential rock uplift should exert a primary control on the gradient of channel longitudinal profiles. However, discrimination of this effect from the influence of variations in substrate erodibility, sediment flux, precipitation, and transient changes in profile shape has proved difficult in practice. Here we investigate the controls on the spatial distribution of bedrock channel gradients adjacent to the Sichuan Basin in an effort to assess the degree and nature of active deformation along this margin of the Tibetan Plateau. Analysis of river longitudinal profiles utilizing a channel steepness index (a measure of profile gradient normalized for drainage area) reveals a zone of anomalously steep channels adjacent to the topographic front of the plateau margin. Channel profiles are systematically less steep in their headwater reaches on the plateau and in their lower reaches east of the plateau margin. Comparison of steepness indices to mapped lithologic variations reveals that lithology has only a limited influence on channel gradient in this field area. We observe no systematic relationship between steepness indices and upstream drainage area; channels of all size are steeper near the plateau margin. We argue that these systematic changes are not readily explained as a consequence of increased sediment flux or of orographic precipitation. We are led to conclude that steep channel profiles along the topographic front of the plateau reflect active differential rock uplift between this region and the foreland.
Aseismic continuation of the Lesser Antilles slab beneath continental South America
Authors: VanDecar J. C. , Russo R. M. , James D. E. , Ambeh W. B. , Franke M.
Published: May 2003

We present results of travel time inversions of teleseismic P and S waves recorded at the SECaSA92 (Southeast Caribbean South America 1992) temporary broadband array in northeastern Venezuela and Trinidad. The inversions reveal the unusual structure of the southern termination of the Lesser Antilles subduction zone: A minimum 2% relatively high‐velocity anomaly trends WSW from the seismically defined Lesser Antilles slab beneath and NW of the Paria Peninsula to a point below the Venezuelan Serranía del Interior, well south of the Caribbean coast. Resolution tests utilizing actual ray geometries and densities of the source data indicate that the regional‐scale structure beneath the study area is reasonably well resolved. Thus a detached and detaching subducted South American slab appears to lie beneath continental South America. We infer that oceanic South American lithosphere has been overridden to a significant degree by continental South America. The detached slab now lying beneath continental South America was driven into its current position after detaching from the former eastward striking Mesozoic ocean‐continent passive margin as this margin entered the subduction zone. Because oceanic and continental South America are still attached without apparent relative motion between them along the Atlantic passive margin southeast of our study region, the slab must be the actively moving element during continental overriding. Thus the slab and its surrounding mantle (both Caribbean and South America) beneath northeastern South America are mobile and have moved ESE relative to the stable Guyana Shield craton.
The crustal structure of the Cocos ridge off Costa Rica
Authors: Walther Christian H. E.
Published: May 2003

The submarine Cocos ridge in the northwestern Panamá basin, a bathymetric feature more than 1000‐km long and 250–500 km broad, is about 2 km shallower than the adjacent basin. It is generally interpreted as the trace of the Galápagos hot spot. Two 127‐ and 260‐km long seismic wide‐angle sections were recorded along and across this ridge, offshore the Osa peninsula, Costa Rica. Crustal thickening is seen everywhere along the sections. On the northwestern outer ridge flank, increased thickness is exclusively attributed to the upper crust and expressed by 2‐km thick flow basalts. The Quepos plateau caps the upper crust in this area. Toward the center of the Cocos ridge, the Moho deepens from 11–12 to 21 km depth and crustal thickening is almost entirely attributed to the lower crust which makes up 80% of the crust and is three times the thickness of normal oceanic lower crust. It is homogeneously structured and the velocities which range from 6.5 km/s at the top to 7.35 km/s at the base are comparable to normal lower crust under these depth conditions and suggest no differences to a gabbroic rock composition. Similarities to the crustal velocity structure of Iceland, central Kerguelen plateau, and Broken ridge are consistent with a formation of this 13–15 Ma old Cocos ridge segment by excessive magmatism in a near‐plate boundary setting.
Dynamics of retreating slabs: 1. Insights from two‐dimensional numerical experiments
Authors: Funiciello Francesca , Morra Gabriele , Regenauer‐Lieb Klaus , Giardini Domenico
Published: May 2003

We use two‐dimensional numerical experiments to investigate the long‐term dynamics of an oceanic slab. Two problems are addressed: one concerning the influence of rheology on slab dynamics, notably the role of elasticity, and the second dealing with the feedback of slab‐mantle interaction to be resolved in part 2. The strategy of our approach is to formulate the simplest setup that allows us to separate the effects of slab rheology (part 1) from the effects of mantle flux (part 2). Therefore, in this paper, we apply forces to the slab using simple analytical functions related to buoyancy and viscous forces in order to isolate the role of rheology on slab dynamics. We analyze parameters for simplified elastic, viscous, and nonlinear viscoelastoplastic single‐layer models of slabs and compare them with a stratified thermomechanical viscoelastoplastic slab embedded in a thermal solution. The near‐surface behavior of slabs is summarized by assessing the amplitude and wavelength of forebulge uplift for each rheology. In the complete thermomechanical solutions, vastly contrasting styles of slab dynamics and force balance are observed at top and bottom bends. However, we find that slab subduction can be modeled using simplified rheologies characterized by a narrow range of selected benchmark parameters. The best fit linear viscosity ranges between 5 × 1022 Pa s and 5 × 1023 Pa s. The closeness of the numerical solution to nature can be characterized by a Deborah number >0.5, indicating that elasticity is an important ingredient in subduction.
Mechanisms of extension at nonvolcanic margins: Evidence from the Galicia interior basin, west of Iberia
Authors: Pérez‐Gussinyé M. , Ranero C. R. , Reston T. J. , Sawyer D.
Published: May 2003

We have studied a nonvolcanic margin, the West Iberia margin, to understand how the mechanisms of thinning evolve with increasing extension. We present a coincident prestack depth‐migrated seismic section and a wide‐angle profile across a Mesozoic abandoned rift, the Galicia Interior Basin (GIB). The data show that the basin is asymmetric, with major faults dipping to the east. The velocity structure at both basin flanks is different, suggesting that the basin formed along a Paleozoic terrain boundary. The ratios of upper to lower crustal thickness and tectonic structure are used to infer the mechanisms of extension. At the rift flanks (stretching factor, β ≤ 2) the ratio is fairly constant, indicating that stretching of upper and lower crust was uniform. Toward the center of the basin (β ∼ 3.5–5.5), fault‐block size decreases as the crust thins and faults reach progressively deeper crustal levels, indicating a switch from ductile to brittle behavior of the lower crust. At β ≥ 3.5, faults exhume lower crustal rocks to shallow levels, creating an excess of lower crust within their footwalls. We infer that initially, extension occurred by large‐scale uniform pure shear but as extension increased, it switched to simple shear along deep penetrating faults as most of the crust was brittle. The predominant brittle deformation might have driven small‐scale flow (≤40 km) of the deepest crust to accommodate fault offsets, resulting in a smooth Moho topography. The GIB might provide a type example of nonvolcanic rifting of cold and thin crust.
Evidence for Mesozoic shear along the western Kunlun and Altyn‐Tagh fault, northern Tibet (China)
Authors: Arnaud N. , Tapponnier P. , Roger F. , Brunel M. , Scharer U. , Wen Chen , Zhiqin Xu
Published: May 2003

The strike‐slip faults of north Tibet accommodate part of the Cenozoic convergence between India and Asia. Along the Karakax valley south of Yecheng and near the Xidatan trough south of Golmud, the active traces of the Altyn‐Tagh and Kunlun faults follow narrow belts of metamorphic rocks. The deformation recorded in those mylonites is sinistral strike‐slip. Rb/Sr and 40Ar/39Ar ages of deformation from syntectonic fabrics formed at 350–400°C 120 Ma. Argon loss suggests that deformation was associated to a 250–300°C thermal pulse that lasted 5–20 Ma after the onset of movement. Unroofing occurred much later, around 25 Ma when sudden cooling suggests a component of thrusting or more likely normal faulting. The Cretaceous shear may be related to collision between the Qiantang and the Lhasa blocks. The Karakax and Xidatan shear zones may have formed a unique, continuous boundary in the Cretaceous, which was later reused by the Tertiary strike‐slip faults, leading to potentially calculable offsets along the Altyn‐Tagh fault.
Joint inversion of broadband teleseismic and interferometric synthetic aperture radar (InSAR) data for the slip history of the Mw = 7.7, Nazca ridge (Peru) earthquake of 12 November 1996
Authors: Salichon J. , Delouis B. , Lundgren P. , Giardini D. , Costantini M. , Rosen P.
Published: May 2003

The slip distribution of the 12 November 1996, Mw = 7.7, Peru earthquake is determined using broadband teleseismic waveforms, a differential SAR interferogram (interferometric synthetic aperture radar [InSAR]), and a fault parametrization allowing slip and rupture velocity to vary along the rupture plane. Both data sets are inverted jointly to limit the trade‐off between the space and time aspects of the rupture. The earthquake fault plane is located at the subduction interface; it strikes parallel to the trench and dips 30° NE. By inverting synthetic data, we show how the InSAR and teleseismic data are complementary and how the joint inversion produces a gain in the spatial and temporal resolution of the slip model, even with a SAR interferogram that covers only part of the coseismic deformation. The rupture of the 1996 Peru event initiated on the southern flank of the subducted Nazca ridge and propagated unilaterally toward the southeast (along strike) for more than 100 km at a depth between 20 and 40 km. The area of maximum slip (6–7 m) is located 50 km southeast of the hypocenter. The total seismic moment is 4.4 × 1020 N m (our joint inversion). The source time function is approximately 60 s long and presents three major pulses of moment release. The dominant one, which occurred between 30 and 45 s, does not correspond to the area of largest slip but to the rupture of a wide zone located about 100 km away from the hypocenter where slip reaches only 2–3 m. Computed coseismic coastal uplift correlates well with the location of raised marine terraces and with the topography of the coastal cordillera, suggesting that these features may be related to the repetition of 1996‐type events at the interface between the Nazca ridge and the South American plate.
Seafloor nuclear magnetic resonance assay of methane hydrate in sediment and rock
Authors: Kleinberg R. L. , Flaum C. , Straley C. , Brewer P. G. , Malby G. E. , Peltzer E. T. , Friederich G. , Yesinowski J. P.
Published: May 2003

Proton nuclear magnetic resonance (NMR) of liquid water can be used to indirectly detect and assay methane hydrate in opaque earth materials. It is also potentially useful for quantifying pore size control of hydrate formation, and for estimating in situ hydraulic permeability of hydrate‐affected earth formations. The method is quantitative, nondestructive, and volumetrically averaging over a spatially selected region. In the first controlled validation of this measurement, we have used NMR to measure changes in the liquid water content of sediment and rock resulting from methane hydrate formation. Hydrates were artificially formed at the seafloor by introducing methane into tubes containing sediments or rock saturated with seawater; after several weeks the samples were visited by a remotely operated vehicle on which NMR equipment was mounted. Reduction of the NMR signal indicated hydrate had formed in the sample within a well‐defined measurement volume. Hydrate content determined by NMR quantitatively predicted the volume of methane gas evolved during subsequent dissociation.
The Central Basin Spreading Center in the Philippine Sea: Structure of an extinct spreading center and implications for marginal basin formation
Authors: Okino Kyoko , Fujioka Kantaro
Published: May 2003

Newly acquired marine geophysical data along the Central Basin Spreading Center (CBSC), the extinct spreading axis of the West Philippine Basin (WPB), display along‐axis variations of spreading style. We have described and analyzed the tectonic spreading fabric and segmentation patterns along a 1000‐km‐long section of the fossil ridge between 126°00′ and 133°30′E. “Slow‐spreading features” (deep rift valleys and nodal basins, rough abyssal hills on the ridge flanks, and mantle Bouguer anomaly lows beneath segment centers) are observed in the eastern CBSC. In contrast, “fast‐spreading” features (overlapping spreading centers, volcanic axial ridges, and smooth abyssal hill fabric) are identified along the western CBSC. We attribute the large morphological and geophysical variations along the CBSC to higher melt supply in the western region caused by high mantle temperature and/or mantle heterogeneity, which may be related to a relatively small‐scale mantle plume forming the oceanic plateaus located in the WPB. Another prominent feature of the CBSC is the development of a deep valley oblique to the spreading fabric. Reconstructions of the plate boundary geometry through time, using abyssal hill geometry as well as other measurements, provide evidence for a later stage of amagmatic extension (i.e., reactivation) along the CBSC after the formation of the main basin. This stage is dominated by tectonic deformation with minor amounts of volcanism (mainly located in eastern segments), resulting in the observed surface brittle deformation distributed within a broad zone of ductile deformation.
Stress orientations in northern and central California: Evidence for the evolution of frictional strength along the San Andreas plate boundary system
Authors: Provost Ann‐Sophie , Houston Heidi
Published: May 2003

We determined orientations of principal stresses around the San Andreas fault (SAF) system in the greater San Francisco Bay Area and regions farther north along the strike‐slip plate boundary. Stress orientations, as well as a ratio between principal stress magnitudes, were determined by inversions of ∼6000 earthquake fault plane solutions, divided into ∼100 groups based on the spatial distribution of seismicity with respect to major regional fault strands (i.e., the SAF, the Hayward‐Rodgers Creek‐Maacama fault zone, and the Calaveras‐Green Valley‐Bartlett Springs fault zone). The stress orientations, while spatially variable, reveal several features; to describe them, it is useful to distinguish between groups of events occurring on and off the major fault strands. First, for off‐fault groups, the angle between major fault strands and the maximum horizontal compression SH decreases systematically to the north. This contrasts with the high angles of SH (∼80°) found immediately adjacent to, as well as farther from, the creeping segment of the SAF in central California by Provost and Houston [2001]. Second, for on‐fault groups, the angle that SH makes with major fault strands changes little along strike, averaging 50° to 55° in the creeping segment, the Bay area, and the northernmost part of our study area. Third, as in the vicinity of the creeping segment of the SAF in central California, the majority of both the off‐fault groups, and the on‐fault groups, are in a strike‐slip, rather than thrust, tectonic regime. Finally, anomalous east‐west SH orientations are seen in the vicinity of Sutter Buttes. In the north, multiple strands of the strike‐slip fault system have accumulated little slip, dip relatively shallowly and are composed of short, complex en échelon segments, suggesting that they originated as thrust faults in the accretionary prism associated with the Farallon subduction and have been subsequently reactivated in a strike‐slip sense following the northward passage of the Mendocino triple junction. Our results, together with the geological context, suggest that the fault system is mechanically stronger with a greater effective frictional strength in the northern portion than in the creeping section of the SAF, with the Bay Area in an intermediate state. We interpret this situation to result from the evolution of the plate boundary toward lower effective frictional strength with increasing slip.
Subduction factory 2. Are intermediate‐depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?
Authors: Hacker Bradley R. , Peacock Simon M. , Abers Geoffrey A. , Holloway Stephen D.
Published: May 2003

New thermal‐petrologic models of subduction zones are used to test the hypothesis that intermediate‐depth intraslab earthquakes are linked to metamorphic dehydration reactions in the subducting oceanic crust and mantle. We show that there is a correlation between the patterns of intermediate‐depth seismicity and the locations of predicted hydrous minerals: Earthquakes occur in subducting slabs where dehydration is expected, and they are absent from parts of slabs predicted to be anhydrous. We propose that a subducting oceanic plate can consist of four petrologically and seismically distinct layers: (1) hydrated, fine‐grained basaltic upper crust dehydrating under equilibrium conditions and producing earthquakes facilitated by dehydration embrittlement; (2) coarse‐grained, locally hydrated gabbroic lower crust that produces some earthquakes during dehydration but transforms chiefly aseismically to eclogite at depths beyond equilibrium; (3) locally hydrated uppermost mantle dehydrating under equilibrium conditions and producing earthquakes; and (4) anhydrous mantle lithosphere transforming sluggishly and aseismically to denser minerals. Fluid generated through dehydration reactions can move via at least three distinct flow paths: percolation through local, transient, reaction‐generated high‐permeability zones; flow through mode I cracks produced by the local stress state; and postseismic flow through fault zones.
Receiver functions in the western United States, with implications for upper mantle structure and dynamics
Authors: Gilbert Hersh J. , Sheehan Anne F. , Dueker Kenneth G. , Molnar Peter
Published: May 2003

Investigations into mechanisms driving surface tectonics commonly search for mantle sources, but few observations constrain flow in the upper mantle and transition zone. Here variations in the upper mantle discontinuities at 410 km and 660 km below the western United States are revealed through mapping depths of compressional‐to‐shear wave conversions recorded by broadband seismometers. The resulting image exhibits 20 and 30 km of topography on the 410‐ and 660‐km discontinuities, similar depth variations to those seen where subducting slabs of lithosphere reach the transition zone. The pattern of discontinuity topography imaged here does not correlate with the surface tectonics of the Rocky Mountains, Colorado Plateau, or Basin and Range Province, providing no support for upward and downward flow at transition zone depths controlling surface topography and deformation in this region, at least at scale lengths of a few hundred kilometers. Furthermore, we find no clear correlation between the depths of the 410‐ and 660‐km discontinuities. Undulations on the surfaces of both discontinuities appear to be spaced at distances of ∼800 km. If the topography were due only to lateral temperature differences, such differences would be comparable to those where slabs sink and might suggest separate convective flow above and below the transition zone. Alternatively, the topography may reflect lateral variations in composition. Variations in the sharpness of converted phases across the region offer some support for compositional heterogeneity, but the lack of a correlation between sharpness and depth casts doubt on this explanation for the variations in depth.
Geomagnetic secular variation 1995–2000
Authors: Cain Joseph C. , Mozzoni David T. , Ferguson Bruce B. , Ajayi Olufunto
Published: May 2003

A study was done to determine the difficulty of obtaining a field model for 1995 using magnetic observations taken since that epoch. Ørsted vector data taken from 1999 to 2001 in conjunction with scalar data from all surface ship surveys and secular variation data from magnetic observatories taken since 1995 were utilized. Spherical harmonic models were constructed for the interval from 1995 through early 2001 using selected, magnetically quiet, nighttime intervals. Corrections were made for Dst variations assuming a constant, i.e., ratio (0.28). The data were corrected for magnetic anomalies using Magsat coefficients (m102389) above n = 21. In order to compute secular change data, some observatory hourly values were reduced to remove the annual variations, which ranged up to 40 nT at a few stations. Coefficients were adjusted up to n = 21 in spatial terms, n = 13 in linear secular variation terms, and n = 12 in parabolic secular variation terms. There was also a 30 nT, n = 1 external term. Component maps of this model (f052101) truncated to n = 10 were compared with the IGRF2000 and generally found to be within 20 nT at the surface except for a 110 nT difference in the northern polar region. The model agreed with the OIFM at satellite altitude to within a few nanoteslas except for a few tens of nanoteslas at the poles. Model differences at 1995 from the IGRF1995 peaked over 800 nT in the region west of South America. However, statistical estimates in this region and epoch show model errors up to several hundred nanoteslas. Use of a linear model or one truncated to lower degree is seen to reduce these differences and maximum errors at 1995, but at the expense of less accuracy elsewhere. Besides the large differences from the 1995 IGRF it was also noted that there are a number of areas where the field changes require parabolic coefficients, even over this short a time span.
Geodynamic implications of paleomagnetic data from Tertiary sediments in Sakhalin, Russia (NW Pacific)
Authors: Weaver Richard , Roberts Andrew P. , Flecker Rachel , Macdonald David I. M. , Fot'yanova Lidiya M.
Published: May 2003

N‐S trending right‐lateral strike‐slip faults, which were active in the Tertiary, transect Sakhalin, Russia, while Mesozoic forearc and accretionary rocks testify to an earlier period of subduction. Several kinematic models have been proposed for the region, but the details required to constrain these models, such as the timing of the transition from subduction to strike‐slip tectonics in Sakhalin, are still unknown. Even first‐order tectonic features, such as the boundaries of the plates with which Sakhalin evolved during the Tertiary, are poorly known. Paleomagnetic results from around Sakhalin were obtained to constrain the geodynamic evolution of the region. Comparison of paleomagnetic inclination data with the apparent polar wander paths for the Eurasian, Pacific, and North American Plates suggests that Sakhalin probably evolved with the North American Plate, although a history including the Eurasian Plate cannot be ruled out. Paleomagnetic declination data suggest that significant clockwise vertical axis rotation has occurred in Sakhalin since the mid‐Paleocene. It is likely that this rotational deformation was accommodated by Tertiary activity on right‐lateral strike‐slip faults, which may be associated with the opening of the Japan Sea, Tatar Strait, and Kuril Basin. These data contradict a published kinematic model for eastern Sakhalin, where counterclockwise vertical axis rotations were predicted for Neogene basins in the East Sakhalin Mountains. Agreement is better, however, with published paleomagnetic data from southern Sakhalin, where clockwise vertical axis rotations were documented.
A revised dislocation model of interseismic deformation of the Cascadia subduction zone
Authors: Wang Kelin , Wells Ray , Mazzotti Stephane , Hyndman Roy D. , Sagiya Takeshi
Published: May 2003

CAS3D‐2, a new three‐dimensional (3‐D) dislocation model, is developed to model interseismic deformation rates at the Cascadia subduction zone. The model is considered a snapshot description of the deformation field that changes with time. The effect of northward secular motion of the central and southern Cascadia forearc sliver is subtracted to obtain the effective convergence between the subducting plate and the forearc. Horizontal deformation data, including strain rates and surface velocities from Global Positioning System (GPS) measurements, provide primary geodetic constraints, but uplift rate data from tide gauges and leveling also provide important validations for the model. A locked zone, based on the results of previous thermal models constrained by heat flow observations, is located entirely offshore beneath the continental slope. Similar to previous dislocation models, an effective zone of downdip transition from locking to full slip is used, but the slip deficit rate is assumed to decrease exponentially with downdip distance. The exponential function resolves the problem of overpredicting coastal GPS velocities and underpredicting inland velocities by previous models that used a linear downdip transition. A wide effective transition zone (ETZ) partially accounts for stress relaxation in the mantle wedge that cannot be simulated by the elastic model. The pattern of coseismic deformation is expected to be different from that of interseismic deformation at present, 300 years after the last great subduction earthquake. The downdip transition from full rupture to no slip should take place over a much narrower zone.
Northeastward growth and uplift of the Tibetan Plateau: Magnetostratigraphic insights from the Guide Basin
Authors: Pares Josep M. , Van der Voo Rob , Downs Will R. , Yan Maodu , Fang Xiaomin
Published: May 2003

Most of the evidence for the uplift of the Tibetan Plateau comes from its southern and central parts. Although the northern rim has been less studied, it may greatly contribute to the understanding of the mechanism and timing of the uplift. Recent studies on the northeastern part of the plateau suggest that the uplift can largely be explained by Cenozoic thrusting and folding linked to the movement of the Altyn Tagh and Kunlun faults and that diffuse shortening might be a realistic way to explain the thickening of the continental interior. Nevertheless, the age of sediments that record the tectonic processes is poorly known. The terrigenous formations in the Guide–Gonghe Basin, NE Tibet, show several phases of deformation and include thick deposits of conglomerates, interbedded with sandstones and siltstones that are thought to be the result of an increased sediment flux. Dating of these gravel deposits with magnetostratigraphic and faunal studies has permitted the precise timing of the basin infilling and its tectonic evolution to be established. Our results indicate that the onset of conglomerate accumulation occurs in the Pliocene, an observation that is in agreement with those of the Linxia Basin, just east of the Plateau. If the high sedimentation rates (∼22 cm/kyr) in the Guide Basin are mostly due to tectonism, then significant uplift occurred as late as Pliocene.
A ∼580 kyr paleomagnetic record from the sub‐Antarctic South Atlantic (Ocean Drilling Program Site 1089)
Authors: Stoner J. S. , Channell J. E. T. , Hodell D. A. , Charles C. D.
Published: May 2003

We report geomagnetic directional paleosecular variation, relative paleointensity proxies and oxygen isotope data from the upper 88 m composite depth (mcd) at South Atlantic Ocean Drilling Program (ODP) Site 1089 (40°56.2′S, 9°53.64′E, 4620 m water depth). The age model is provided by high‐resolution oxygen isotope stratigraphy, augmented by radiocarbon dates from the upper 8 mcd of nearby piston core RC11‐83. Mean sedimentation rates at Site 1089 are in the range of 15 to 20 cm/kyr. Two intervals during the Brunhes Chron, at ∼29.6 mcd (∼190 ka) and at ∼48 mcd (∼335 ka), have component magnetization directions with positive (reverse polarity) inclination; however, the excursional directions are heavily overprinted by the postexcursional field. Magnetite is the dominant carrier of magnetic remanence, and occurs in the pseudosingle‐domain (PSD) grain size. An additional higher‐coercivity magnetic carrier, characterized by low unblocking temperatures (<350°C), is assumed to be authigenic pyrrhotite. A decrease in magnetization intensity down core is mirrored by a reduction in pore water sulfate, indicating diagenetic reduction of magnetite. Despite down‐core changes in magnetic mineralogy, normalized intensity records from Site 1089 are comparable with high‐resolution paleointensity records from the North Atlantic (e.g., ODP Sites 983 and 984). Sediment properties and sedimentation patterns within the Cape (Site 1089) and Iceland (Sites 983 and 984) Basins are distinctly different at both millennial and orbital timescales and therefore preclude lithologic variability from being the source of this correlation. Variations in normalized intensity from Site 1089 therefore appear to reflect changes in global‐scale geomagnetic field intensity.
Tonga Ridge and Lau Basin crustal structure from seismic refraction data
Authors: Crawford Wayne C. , Hildebrand John A. , Dorman LeRoy M. , Webb Spahr C. , Wiens Douglas A.
Published: May 2003

The crustal structure across the Tonga‐Lau arc‐back arc system from the Lau Ridge to the Pacific Plate (178°–170°W, 18°19°S) is modeled, using data from an 840‐km‐long air gun refraction line over 19 ocean bottom seismometers and one land station. The data reveal that the Pacific Plate crust is 5.5 km thick, with a velocity structure similar to that found at the present‐day East Pacific Rise (EPR). Beneath Tonga Ridge, an intermediate velocity layer (6–7 km/s) is up to 7.5 km thick and has a velocity‐depth distribution similar to andesitic rocks found in continental crust. The crust is abnormally thin (4 km) at the boundary between the Tonga Ridge and the Lau Basin. At the east end of Lau Basin, the crust is 5.5–6.5 km thick and resembles crust formed at the EPR except for a thicker sheeted‐dike section (2–3 km) and thinner lower crust (2 km). The Lau Basin crust thickens to 7–8 km near the Central Lau Spreading Center (CLSC), mostly through thickening of the lower crust. The crust thickens again to 8.5–9.5 km at 50 km west of the CLSC, mostly through thickening of the midcrust. In the thick westernmost section, the crustal structure is uniform even though one part of this section formed through extension of arc‐type crust while the rest was created at an oceanic spreading center. The relative homogeneity of these rocks suggests that their petrology may be dominated by postemplacement magmatic infilling from a mantle source west of the spreading center.
Multiple‐fault rupture of theM7.1 Hector Mine, California, earthquake from fault zone trapped waves
Authors: Li Yong‐Gang , Vidale John E. , Oglesby David D. , Day Steven M. , Cochran Elizabeth
Published: May 2003

We studied the complex multiple‐faulting pattern of the 40‐km‐long rupture zone of the 1999M7.1 Hector Mine, California, earthquake with fault zone trapped waves generated by near‐surface explosions and aftershocks, and recorded by linear seismic arrays deployed across the surface rupture. The explosion excited trapped waves, with relatively large amplitudes at 3–5 Hz and a long duration ofScoda waves, are similar to those observed for aftershocks but have lower frequencies and travel more slowly. Three‐dimensional finite difference simulations of fault zone trapped waves indicate a 75‐ to 100‐m‐wide low‐velocity and low‐Qzone (waveguide) along the rupture surface on the Lavic Lake fault (LLF) in the Bullion Mountains. TheSvelocity within the waveguide varies from 1.0 to 2.5 km/s at depths of 0–8 km, reduced by ∼35–45% from the wall rock velocity, andQis ∼10–60. The pattern of aftershocks for which we observed trapped waves shows that this low‐velocity waveguide has two branches in the northern and southern portions of the rupture zone, indicating a multiple‐fault rupture at seismogenic depth. North of the Bullion Mountains, although only the rupture segment on the northwest LLF broke to the surface, a rupture segment on a buried fault also extended ∼15 km in the more northerly direction from the main shock epicenter. To the south, the rupture on the LLF intersected the Bullion fault (BF) and bifurcated. The rupture on the south LLF extended ∼10 km from the intersection and diminished while there was minor rupture on the southeast BF, which dips to the northeast and disconnects from the LLF at depth. Thus the analysis of fault zone trapped waves helps delineate a more complex set of rupture planes than the surface breakage, in accord with the complex pattern of aftershock distribution and geodetic evidence that the Hector Mine event involved several faults which may also rupture individually. Our simulations of dynamic rupture using a finite element code show that generic models are able to produce the general features of the northern part of the rupture, including slips on subparallel fault segments. The models indicate that such a faulting pattern is physically plausible and consistent with observations.
Duration‐amplitude distribution of volcanic tremor
Authors: Benoit John P. , McNutt Stephen R. , Barboza Vilma
Published: May 2003

The duration‐amplitude distribution of volcanic tremor was examined in eight volcanoes and one geothermal area. An exponential model, implying a scale‐bound source process, is found to be a better fit to the data than a power law (scale invariant) model. The exponential model well describes tremor associated with magmatic and phreatic eruptions, shallow and deep source regions, and geothermal sources. We tested the exponential model described by: , where d is the duration of tremor greater than or equal to a particular amplitude DR, dt is the total duration of tremor, and the inverse of λ is the characteristic amplitude of the distribution. λ−1 takes on values between 0.003 and 7.7 cm2. Our results show that the characteristic amplitude for eruptive tremor is greater than for noneruptive tremor, that for deep tremor is greater than for shallow tremor, and that for tremor associated with magmatic eruptions is greater than for tremor associated with phreatic eruptions. The exponential scaling of tremor suggests that tremor is not simply composed of a series of low‐frequency events closely spaced in time. Further, the exponential scaling requires the source to be scale bound; the amplitude variations of tremor are distributed about a constant characteristic amplitude. We propose that exponential scaling of tremor amplitude is caused by fixed source geometry driven by variable excess pressures. The exponential scaling of tremor demonstrates that tremor source processes are fundamentally different from those for earthquakes.
Active deformation in the Mediterranean from Gibraltar to Anatolia inferred from numerical modeling and geodetic and seismological data
Authors: Jiménez‐Munt I. , Sabadini R. , Gardi A. , Bianco G.
Published: May 2003

From Gibraltar to Anatolia, the active tectonics in the Mediterranean is studied by means of an integrated approach based on geophysical, geodetic, and seismological methodologies. The aim of this study is to gain a deep insight into the kinematics and dynamics of the crustal and lithospheric processes affecting the Mediterranean. Major tectonic processes, such as continental collision and subduction, characterize this region, which marks a broad transition zone between the African/Arabian and Eurasian plates. A thin‐shell finite element approach allows us to simulate the deformation pattern in the Mediterranean, from 10°W to 40°E and from 30° to 50°N. The global plate motion model NUVEL‐1A is used to account for the convergence, while the relative velocities of the overriding and subduction plates are obtained from another family of models. These models simulate the effects of the negatively buoyant density contrasts of the subducted lithosphere on the horizontal velocity at the surface. A systematic comparison between model results and the seismic strain rates obtained from the National Earthquake Information Center catalog, the geodetic velocity field and strain resulting from GPS, satellite laser ranging, and very long baseline interferometry analyses and the World Stress Map, indicate that Africa/Arabia versus Eurasia convergence and subduction in the Aegean Sea and Calabrian Arc are the major tectonic mechanisms controlling the deformation style in the Mediterranean. It is shown that in order to carry into coincidence the modeled and the seismic strain rate patterns and the geodetically retrieved strain rate tensors, a deep subduction in the Aegean Arc must be included in the modeling.
Subduction erosion and basal friction along the sediment‐starved convergent margin off Antofagasta, Chile
Authors: von Huene R. , Ranero C. R.
Published: May 2003

Subduction erosion is commonly associated with strong interplate coupling and a consequent abrasion of the upper plate. Northern Chile is an often cited example of a strongly coupled erosional margin. Its crystalline basement is inferred to form a strong upper plate, the trench axis contains little detectable sediment, and the subducting lower plate has a high‐relief horst‐graben topography. With little water‐rich sediment to reduce interplate friction, the high relief of an igneous ocean crust thrust beneath continental basement should generate high friction interplate abrasion. However, a prestack depth‐migrated seismic record images slope debris that collects in a frontal prism. This debris, including ∼30% pore fluid, fills subducting grabens and is subsequently incorporated into an ∼1.5‐km‐thick interplate reflective layer. The subduction zone thrust passes through the upper part of this layer. Interplate seismicity and taper analyses indicate basal friction at levels that are common in sedimented convergent margins. The continued growth of lower plate grabens after subduction probably accommodates upper plate material, a process that erodes the upper plate. Erosion is aided by weakening of the upper plate rock framework beneath the continental slope. This erosion undermines the upper plate and tips it seaward thereby steepening the continental slope which induces midslope gravity tectonics. Despite sediment starvation, a frontal prism constructed of remolded slope debris elevates pore pressure to reduce interplate friction. Coeval erosion and prism building control the size of the frontal prism. Processes other than high friction abrasion best explain subduction erosion along northern Chile.
Three‐dimensional modeling of Mount Vesuvius with sequential integrated inversion
Authors: Tondi Rosaria , de Franco Roberto
Published: May 2003

A new image of Mount Vesuvius and the surrounding area is recovered from the tomographic inversion of 693 first P wave arrivals recorded by 314 receivers deployed along five profiles which intersect the crater, and gravity data collected in 17,598 stations on land and offshore. The final three‐dimensional (3‐D) velocity model presented here is determined by interpolation of five 2‐D velocity sections obtained from sequential integrated inversion (SII) of seismic and gravity data. The inversion procedure adopts the “maximum likelihood” scheme in order to jointly optimize seismic velocities and densities. In this way we recover velocity and density models both consistent with seismic and gravity data information. The model parameterization of these 2‐D models is chosen in order to keep the diagonal elements of the seismic resolution matrix in the order of 0.2–0.8. The highest values of resolution are detected under the volcano edifice. The imaged 6‐km‐thick crustal volume underlies a 25 × 45 km2 area. The interpolation is performed by choosing the right grid for a smoothing algorithm which prepares optimum models for asymptotic ray theory methods. Hence this model can be used as a reference model for a 3‐D tomographic inversion of seismic data. The 3‐D gravity modeling is straightforward. The results of this study clearly image the continuous structure of the Mesozoic carbonate basement top and the connection of the volcano conduit structure to two shallow depressions, which in terms of hazard prevention are the regions through which magma may more easily flow toward the surface and cause possible eruptions.
Weichselian temperatures from geothermal heat flow data
Authors: Kukkonen Ilmo T. , Jõeleht Argo
Published: May 2003

We report an analysis of geothermal heat flow density (HFD) in the Fennoscandian Shield and East European Platform showing a systematic variation with depth. The HFD data (1352 values) averaged in 25 m depth intervals and in 1° × 2° latitude‐longitude areas increase from ∼35–40 mW m−2 in the first 500 m to ∼45 mW m−2 at 1000 m, finally reaching ∼50 mW m−2 between 1000 and 3000 m and deeper. We attribute this variation to long‐term climatic changes in ground surface temperatures (GST) during the Weichselian (late Pleistocene) glaciation and the Holocene. Monte Carlo inversion was applied for determining ground surface temperature histories during the past 100,000 years, and the results indicate the lowest GST at time of the Last Glacial Maximum (∼20,000 years B.P.), followed by an average warming of 8.0 ± 4.5° at ∼10,000 years B.P.
The effect of temperature, the nature of the pore fluid, and subyield differential stress on the permeability of phyllosilicate‐rich fault gouge
Authors: Faulkner D. R. , Rutter E. H.
Published: May 2003

Elevated temperatures and subyield deviatoric stresses were applied to oriented natural samples of clay‐bearing fault gouge under pressure in order to establish their effects on the water and argon permeability under conditions closer to those found at depth in fault zones. It was found that the application of subyield differential stress normal to the fault plane resulted in a permeability reduction by a factor of ∼2. Argon permeability showed a constant rate of decrease with increasing temperature up to 150°C and limited time‐dependent compaction at temperature. More substantial time‐dependent compaction at elevated temperature with water pore fluid reduced the permeability by a factor of 3 over ∼6 days. When water‐wet, water permeability increased with temperature up to ∼80°C, and then reduced at higher temperatures. This behavior is interpreted to be due to a combination of thermal compaction and the destabilization of structured water films coating pore throats at elevated temperatures. Increasing the pore fluid pressure did not appear to play an important role in stabilizing water films in pore throats. Based on these results, the reduction of permeability due to blocking of pore throats by structured water films is unlikely to play a role in the deeper part of fault zones. However, in the shallow crust this permeability reduction mechanism may be responsible for a significant lowering of the permeability. This has important implications for fluids that may be traveling up large faults or for the modeling of fluid flow around waste repositories, particularly when these may create temperature perturbations by their presence.
Determination of the complete elastic stiffnesses from ultrasonic phase velocity measurements
Authors: Mah Marko , Schmitt Douglas R.
Published: May 2003

The complete set of 21 elastic stiffnesses of a composite material are found from ultrasonic measurements of the phase velocity anisotropy. Quasi‐P and quasi‐S wave phase speeds at a variety of incident angles within a number of differing planes through the material are obtained using the τ‐p plane wave decomposition technique. The 779 individual phase velocities were inverted, under no a priori presumptions about the symmetry or orientation of the material, to provide all the stiffnesses. These show that the material has nearly orthorhombic symmetry, as is expected from its texture. This orthorhombic character was further apparent in a number of bootstrap tests of the inversion that assumed differing levels of symmetry from triclinic to orthorhombic and using various subsets of the measured phase velocities. However, the present analysis does not account for the effects of wave speed dispersion evident in the observed waveforms. This dispersion is particularly severe for the in‐plane q‐S polarization and is possibly a consequence of the fine‐layered structure of the material.
Subparallel dipping heterogeneities in the mid‐lower mantle
Authors: Kaneshima Satoshi , Helffrich George
Published: May 2003

Data from western U.S. short‐period seismic networks show anomalous later phases which we interpret as S‐to‐P converted waves from heterogeneities in the mid‐lower mantle (depths of ∼1000–1850 km) northeast of the Mariana subduction zone. The geometry of the objects is mapped by a two‐step approach: (1) computing the S‐to‐P scattering likelihood with the arrival time, slowness, and azimuth of the later arrivals measured by array techniques relative to direct P waves, and (2) by direct use of the waveforms using semblance. At least three subparallel objects are detected in the lower mantle study region, with the dips of 30° to 40° southward and with a lateral extent larger than 500 km by 300 km. The elastic properties associated with the heterogeneous objects appear to change within several kilometers, considering their high efficiency at converting short‐period waves (wavelengths of 5–10 km). Previous seismic tomography studies have not predicted significant velocity anomalies around the heterogeneities. This is probably because they are below the resolution limit of present methods. The three major heterogeneities are separated from each other nearly 300 km vertically, and significant anomalies seem absent between each object. Nevertheless, semblance cross sections with a finer grid size (0.1° × 0.1° × 10 km) of the clearest intermediate object (at 1400–1600 km) show a hint that the object is not a single interface but is actually composed of double interfaces nearly 50 km apart. Purely thermal anomalies of kilometers scale must be transient features in the mantle and thus are highly unlikely to be the cause of the heterogeneities. They should rather represent either sharp chemical variations in major element composition or sharp changes in preferred orientations of anisotropic lattice of the lower mantle minerals. Thermal anomalies, however, may be superimposed on such variations because of heat generation either by concentrated radioactive nuclei or by viscous shear heating due to deformations. A plausible tectonic interpretation of these dipping objects is that they represent remnants of subducted lithospheric plates which preceded the current subduction of the Pacific plate. Although we cannot make conclusive arguments on what the mechanism of creating such heterogeneities from these observations alone, we suggest that they may represent stretched, thinned, and duplexed rocks within chaotic mantle flows.