bluuebunny/crossref_metadata_2025 huggingface dataset:
Propagation of rifting along the Arabia‐Somalia Plate Boundary: The Gulfs of Aden and Tadjoura
Authors: Manighetti Isabelle , Tapponnier Paul , Courtillot Vincent , Gruszow Sylvie , Gillot Pierre‐Yves
Published: February 2004

The localization and propagation of rifting between Arabia and Somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern Gulf of Aden and the Gulf of Tadjoura, using bathymetric, magnetic, seismological, and structural evidence. Large‐scale, southwestward propagation of the Aden ridge, markedly oblique to the Arabia‐Somalia relative motion vector, began about 30 Myr ago between the Error and Sharbithat ridges. It was an episodic process, with stages of rapid propagation, mostly at rates >10 cm/yr, interrupted by million year pauses on transverse discontinuities coinciding with rheological boundaries between different crustal provinces of the Arabia‐Somalia plate. The longest pause was at the Shukra‐El Sheik discontinuity (≈45°E), where the ridge tip stalled for ≈13 Myr, between ≈17 and ≈4 Ma. West of that discontinuity, rifting and spreading took place at an azimuth (≈N25°±10°E) and rate (1.2±0.3 cm/yr) different from those of the global Arabia‐Somalia motion vector (≈N39°, ≈1.73 cm/yr), implying an additional component of movement (N65°±10°E, 0.7±0.2 cm/yr) due to rotation of the Danakil microplate. At Shukra‐El Sheik, the typical oceanic ridge gives way to a narrow, WSW trending axial trough, resembling a large fissure across a shallow shelf. This trough is composed of about eight rift segments, which result from normal faulting and fissuring along N110°–N130°E trends. All the segments step to the left southwestward, mostly through oblique transfer zones with en échelon normal faults. Only two segments show clear, significant overlap. There is one clear transform, the Maskali fault, between the Obock and Tadjoura segments. The latter segment, which encroaches onland, is composed of two parallel subrifts (Iboli, Ambabbo) that propagated northwestward and formed in succession. The most recent, southwestern subrift (Ambabbo) represents the current tip of the Aden ridge. We propose a mechanical model in which the large‐scale propagation of the ridge followed a WSW trending zone of maximum tensile stress, while the small‐scale propagation of its NW trending segments was dictated by the orientation of that stress. Oblique propagation was a consequence of passive lithospheric necking of the Arabia‐Somalia plate along its narrow section, in map view, between Socotra and the kink of the Red Sea‐Ethiopian rift, above the Afar plume. Individual ridge segments oriented roughly perpendicular to plate motion, like lithospheric cracks, were forced to jump southward because of confinement within the necking zone. Self‐sustaining, plate‐scale necking may explain why the Aden ridge did not connect with the Red Sea through Bab El Mandeb but continued straight into Afar.
Crustal evolution and sedimentation history of the Bay of Bengal since the Cretaceous
Authors: Rao D. Gopala , Krishna K. S. , Sar D.
Published: February 2004

Multichannel seismic reflection, gravity, magnetic, and bathymetric investigations were made along five latitudinal profiles from the eastern shelf of India to the Andaman shelf and four NW‐SE profiles in the Western Basin off Madras (seismics on three latitudinal profiles) in the Bay of Bengal. The trend of the fracture zones, the locations of the magnetic chron 34, and the Cretaceous Magnetic Quiet Zone suggest that Greater India separated from Antarctica after a period of transform motion in the early Cretaceous, that is, about polarity chron M0 (120 Ma) and drifted northwestward. Negative gravity anomalies are associated with basement rises including the 85°E Ridge between the continental margin of India and the Ninetyeast Ridge. Magnetic reversals and northward trend of the 85°E Ridge support a hotspot origin of the ridge and its emplacement most likely after the Cretaceous quiet period. Juxtaposition of high‐amplitude hyperbolic reflections, down‐faulted continental blocks buried under thick sediments, and associated gravity and magnetic anomalies mark the boundary between continental and oceanic rocks at the foot of continental slope, about 80 km seaward of the present continental shelf edge. Eight seismic sequences, as thick as 8.5 km, overlie the early Cretaceous oceanic basement and include four unconformities (lower Eocene, upper Oligocene, upper Miocene, and upper Pleistocene) which have been correlated to the major geologic/tectonic events. We also interpret the late Cretaceous/early Tertiary features on the eastern flank of the buried 85°E Ridge as carbonate reefs. The observation of steep subduction of older (cold) Indian plate beneath the Burmese plate near the Andaman Islands suggests the Sunda Arc in this region as low to intermediate stress subduction zone.
Segmented normal faults: Correspondence between three‐dimensional mechanical models and field data
Authors: Willemse Emanuel J. M.
Published: February 2004

Traces of many normal faults form an array of closely spaced overstepping segments. In three dimensions, fault segments may either be unconnected or link vertically or laterally into a single continuous fault surface. The slip distribution along segmented faults is complex and asymmetric, and the point of maximum slip generally is not located at the center of a segment. In relay zones between segments, slip gradients may be gentler or steeper, depending on the spatial fault arrangement. Branch points are characterized by steep slip gradients. One explanation for these observations is mechanical interaction between neighboring faults which occurs through local perturbation of the stress field. Three‐dimensional (3‐D) boundary element models show that the degree of fault interaction and hence the degree of asymmetry in the slip distribution increases with increasing fault overlap and downdip fault height and with decreasing fault spacing and Poisson's ratio. Interaction is strongest for faults with uniform shear strength and decreases if there exists a zone of greater shear strength near the tip line. This analysis provides a mechanical rationale for more frequent occurrence of overlapping segments relative to underlapping segments and for the limited range of the ratio between segment overlap and spacing along natural faults. Echelon segment configurations promote interaction, maximize the capacity to accommodate slip, and do not necessarily require a strike‐slip movement component. Model idealizations of some outcropping fault arrays and of branching/merging faults capture a wide variety of common field observations. Consistent, mechanically based 3‐D normal fault models can be obtained by combining different types of field data such as fault slip‐to‐length ratios, location of maximum slip, segment overlap‐to‐spacing ratios, and footwall uplift/hanging wall subsidence. By capitalizing on these data one can understand the mechanics of faulting, constrain the boundary conditions that govern the formation and growth of faults, and provide a rationale for interpreting normal faults in seismic surveys.
The crustal structure across the transform continental margin off Ghana, eastern equatorial Atlantic
Authors: Edwards Rosemary A. , Whitmarsh Robert B. , Scrutton Roger A.
Published: February 2004

Forward modeling of a suite of wide‐angle seismic lines across the transform continental margin at the eastern end of the Romanche fracture zone off Ghana has shown the transition from continental to oceanic crust to be confined to a narrow, 6 to 11‐km‐wide zone located at the foot of the steep continental slope. The structure of the adjacent oceanic and continental crusts has also been resolved. These results are confirmed and enhanced by gravity and magnetic models. The crust of the ocean‐continent transition zone is characterized by high velocities (5.8–7.3 km s−1), a high density (3.10 Mg m−3), and high magnetizations (1.1–1.25 A m−1). This rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin. The oceanic crust within 70 km of the margin is abnormally thin compared to normal Atlantic oceanic crust and shows an average thickness of just 4.4 km. We suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded, on at least three sides, by cold continental lithosphere. The preferred seismic model also shows a new fracture zone, 30 km southeast of the ocean‐continent transition, which is characterized by low velocities in the upper crust, a lens‐shaped layer with velocities of 7.2–7.4 km s−1 at the base of the crust, and a crustal thickness of just 3.4 km. The continental crust appears largely unaffected by the proximity of the adjacent oceanic crust. There is no evidence for underplating of the continental crust adjacent to the ocean‐continent transition zone.
Upper mantle velocity structure beneath the Tibetan Plateau from Pn travel time tomography
Authors: McNamara D. E. , Walter W. R. , Owens T. J. , Ammon C. J.
Published: February 2004

We inverted 1510 P arrival times from regional distances (333–1600 km), in and around the Tibetan Plateau to map the lateral velocity variation within the uppermost mantle. Previous studies have placed first‐order constraints on upper mantle velocities but relied on data recorded almost exclusively at stations outside of the plateau. We improve resolution by using 40 events recorded at stations within the Tibetan Plateau. We combine these data with observations obtained from the International Seismological Centre (ISC) to extend our coverage by including Pn arrivals from 85 additional plateau events, relocated in previous studies, and recorded at stations in and around the Tibetan Plateau. We use synthetic travel time data to evaluate the resolution of our data set. The observations provide good resolution to about 1° over most of the plateau and surrounding regions. Our results show average Pn velocities that are about 3% lower in the northern plateau relative to the southern plateau. These variations correlate well with major tectonic features and previous geophysical observations. In the Qiangtang terrane of the northern plateau, an area known to be inefficient for Sn propagation, Pn is slow relative to both the plateau south of the Banggong‐Nujiang suture and the tectonically stable Tarim basin north of the plateau. This is strong evidence for the existence of partial melt within the uppermost mantle beneath the northern Tibetan Plateau. However, when laboratory estimates of relationships between temperature, velocity, and attenuation are applied, a relatively small temperature variation (240° to 370°C) is required to explain our Pn velocity observations. When combined with geochemical constraints from volcanics in the northern plateau, our results strongly suggest that the mantle lid is intact beneath the northern plateau. This result would preclude tectonic models involving wholesale delamination of the mantle lithosphere in the northern Tibetan Plateau.
Paleomagnetic reconstruction of a diapir emplacement: A case study from Sedom diapir, the Dead Sea Rift
Authors: Weinberger Ram , Agnon Amotz , Ron Hagai
Published: February 2004

Reconstructing the emplacement path of a diapiric sequence is relatively complex, in part because the stratigraphic (top versus bottom) sense of the sequence can be obscured by the complex structure. Paleomagnetic study of the diapiric sequence may reveal simultaneously three characteristics of it: (1) the stratigraphic sense, (2) the three‐dimensional rotation path of the tilted sequence, and (3) the magnetic polarity of the sequence. We apply paleomagnetism to reconstruct the emplacement path of the Sedom diapir in the Dead Sea Rift, Israel. This application resolves the ambiguity regarding the geometrical orientation of the thickest intrarift stratigraphic section (2 km) exposed in the Dead Sea region. We here report the natural remanent magnetization for 172 oriented samples (mainly dolomite and siltstone) from seven localities in the steeply inclined beds of the Sedom diapir. A fold test, as well as other field and laboratory tests, shows that the characteristic remanent magnetization (ChRM) component of the rocks was acquired before the tilting of the Sedom sequence, whereas the low‐coercivity magnetic component of probably chemical remanent magnetization origin was acquired after the tilting. This low‐coercivity (secondary) component, likely acquired in the Brunhes Normal chron, records up to 40° young counterclockwise rotation about a vertical axis. The ChRM and secondary components (together with the bedding plane) were restored to their expected (original) position by rotating them about horizontal, vertical, or inclined axes to get the complete three‐dimensional rotation path. The resulting structure of the southern Sedom diapir is composed of a thin western “salt wall” that is juxtaposed against a thick eastern salt wall. In the northern part of the mountain apparently only the thick eastern salt wall is exposed.
Stratigraphic and structural implications of conodont and detrital zircon U—Pb ages from metamorphic rocks of the Coldfoot terrane, Brooks Range, Alaska
Authors: Moore Thomas E. , Aleinikoff John N. , Harris Anita G.
Published: February 2004

New paleontologic and isotopic data from the Emma Creek and Marion Creek schists of the Coldfoot terrane, Arctic Alaska superterrane, central Brooks Range, suggest Devonian and possibly younger ages of deposition for their sedimentary protoliths. Conodonts from marble of the Emma Creek schist, intruded by a roughly 392 Ma orthogneiss, are late Lochkovian (early Early Devonian, between about 408 and 396 Ma) and Silurian to Devonian at two other locations. Spherical to oblong detrital zircons from quartz‐mica schist of the overlying Marion Creek schist yield mostly discordant U—Pb data suggestive of provenance ages of 3.0, 2.0–1.8, and 1.5–1.4 Ga; however, several euhedral grains of zircon from Marion Creek quartz‐mica schist have concordant U—Pb ages from 370 to 360 Ma. The Marion Creek schist in our study area therefore is at least 26 m.y. younger than the Emma Creek schist. The age data imply that the protolith of the Emma Creek schist is age correlative with Devonian carbonate rocks in the Hammond and North Slope terranes, whereas the Marion Creek schist is age correlative with Upper Devonian and Lower Mississippian clastic sedimentary rocks of the Endicott Group in the Endicott Mountains terrane and shale and carbonate units in the De Long Mountains and Sheenjek River terranes. Consequently, tectonic models restoring the entire Coldfoot terrane beneath partly or wholly coeval rocks of the Hammond, Endicott Mountains, De Long Mountains, and Sheenjek River terranes of the Arctic Alaska superterrane require revision. Alternative reconstructions, including restoration of the Coldfoot terrane inboard of the Endicott Mountains terrane or outboard of the De Long Mountains and Sheenjek River terranes are plausible but require either larger amounts of shortening than previously suggested or indicate problematic facies relations.
Generation and maintenance of pore pressure excess in a dehydrating system 2. Theoretical analysis
Authors: Wong Teng‐Fong , Ko Suz‐Chung , Olgaard David L.
Published: February 2004

Fluid is released by dehydration reactions during prograde metamorphism. If the dilation of the pore space is insufficient to provide storage for all the released fluid, then pore pressure excess is generated. Whether the excess can be maintained over long duration hinges on the hydraulic transport properties of the rock. Motivated by recent experimental and microstructural observations, we developed a theoretical model which incorporates dehydration and porosity production rates as source terms in the hydraulic diffusion equation. The permeability was assumed to be sensitively dependent on the porosity. The finite difference technique was used to analyze the generation and maintenance of pore pressure excess for several types of boundary conditions of importance in laboratory and crustal scales. Analytic estimates of the pore pressure anomaly were also obtained. The model is in reasonable agreement with experimental observations on dehydration‐induced weakening and transient buildup of pore pressure in a nominally drained sample. It provides hydrogeological constraints on the development of pore pressure excess in metamorphic and tectonic settings. The maintenance of a nearly lithostatic pore pressure requires the permeability to be below a critical value which increases with increasing dehydration rate and thickness of the dehydrating layer, and with decreasing porosity production rate. If these constraints are not met, the pore pressure excess can only occur as a transient pulse, the amplitude of which may approach lithostatic for sufficiently large dehydration rate and layer thickness, or sufficiently low permeability.
Oblique strike‐slip faulting of the central Cascadia submarine forearc
Authors: Goldfinger Chris , Kulm LaVerne D. , Yeats Robert S. , McNeill Lisa , Hummon Cheryl
Published: February 2004

At least nine WNW trending left‐lateral strike‐slip faults have been mapped on the Oregon‐Washington continental margin using sidescan sonar, seismic reflection, and bathymetric data, augmented by submersible observations. The faults range in length from 33 to 115 km and cross much of the continental slope. Five faults offset both the Juan de Fuca plate and North American plates and cross the plate boundary with little or no offset by the frontal thrust. Left‐lateral separation of channels, folds, and Holocene sediments indicate active slip during the Holocene and late Pleistocene. Offset of surficial features ranges from 120 to 900 m, and displaced subsurface piercing points at the seaward ends of the faults indicate a minimum of 2.2 to 5.5 km of total slip. Near their western tips, fault ages range from 300 ka to 650 ka, yielding late Pleistocene‐Holocene slip rates of 5.5±2 to 8.5±2 mm/yr. The geometry and slip direction of these faults implies clockwise rotation of fault‐bounded blocks about vertical axes within the Cascadia forearc. Structural relationships indicate that some of the faults probably originate in the Juan de Fuca plate and propagate into the overlying forearc. The basement‐involved faults may originate as shears antithetic to a dextral shear couple within the slab, as plate‐coupling forces are probably insufficient to rupture the oceanic lithosphere. The set of sinistral faults is consistent with a model of regional deformation of the submarine forearc (defined to include the deforming slab) by right simple shear driven by oblique subduction of the Juan de Fuca plate.
Multistory duplexes with forward dipping roofs, north central Brooks Range, Alaska
Authors: Wallace Wesley K. , Moore Thomas E. , Plafker George
Published: February 2004

The Endicott Mountains allochthon has been thrust far northward over the North Slope parautochthon in the northern Brooks Range. Progressively younger units are exposed northward within the allochthon. To the south, the incompetent Hunt Fork Shale has thickened internally by asymmetric folds and thrust faults. Northward, the competent Kanayut Conglomerate forms a duplex between a floor thrust in Hunt Fork and a roof thrust in the Kayak Shale. To the north, the competent Lisburne Group forms a duplex between a floor thrust in Kayak and a roof thrust in the Siksikpuk Formation. Both duplexes formed from north vergent detachment folds whose steep limbs were later truncated by south dipping thrust faults that only locally breach immediately overlying roof thrusts. Within the parautochthon, the Kayak, Lisburne, and Siksikpuk‐equivalent Echooka Formation form a duplex identical to that in the allochthon. This duplex is succeeded abruptly northward by detachment folds in Lisburne. These folds are parasitic to an anticlinorium interpreted to reflect a fault‐bend folded horse in North Slope “basement,” with a roof thrust in Kayak and a floor thrust at depth/These structures constitute two northward tapered, internally deformed wedges that are juxtaposed at the base of the allochthon. Within each wedge, competent units have been shortened independently between detachments, located mainly in incompetent units. The basal detachment of each wedge cuts upsection forward (northward) to define a wedge geometry within which units dip regionally forward. These dips reflect forward decrease in internal structural thickening by forward vergent folds and hindward dipping thrust faults.
Injection‐induced earthquakes and crustal stress at 9 km depth at the KTB deep drilling site, Germany
Authors: Zoback Mark D. , Harjes Hans‐Peter
Published: February 2004

A fluid injection‐induced seismicity experiment was conducted in the German Continental Deep Drilling Program (KTB) main borehole at 9.1 km depth (in situ temperature of 260°C) to extend knowledge about stress magnitudes and brittle faulting to depths and temperatures approaching the brittle‐ductile transition. Almost 400 microearthquakes were induced at an average depth of 8.8 km by injection of KBr/KCl brine into a ∼70 m open hole section near the bottom of the borehole. Although most focal plane mechanisms were poorly constrained due to the very small size of the induced earthquakes, several different clusters of microearthquakes with distinct mechanisms were defined. Most of the microearthquakes for which focal plane mechanisms were determined were strike‐slip events with a NNW trending P axis, essentially parallel to the direction of maximum horizontal compression observed in the borehole. The largest induced earthquake, M 1.2, occurred 18 hours after injection was started. This event was a strike‐slip/reverse faulting event which also had a NNW trending P axis. Utilization of a precise relative location technique indicates that many of microearthquakes occurred relatively far (50–100 m) from the well bore. Modeling of the pore pressure disturbance caused by injection suggests that many of the earthquakes were induced by extremely small pore pressure perturbations (<1 MPa) less than 1% greater than the ambient, approximately hydrostatic pore pressure at depth. Thus it is apparent that there are critically stressed, permeable fault zones in the crust, even at great depth and temperature. A frictional analysis of the focal plane mechanisms of the induced microearthquakes indicates that fault slip is consistent with the stress magnitudes and orientations determined in situ at depths to 7.7 km in the borehole and relatively high coefficients of friction (∼0.6–0.7) reported by Brudy et al. [this issue]. This and the observation that very small pore pressure perturbations were able to trigger seismicity appear to confirm the hypothesis that “Byerlee's law” (i.e., that differential stresses in situ are limited by the frictional strength of well‐oriented, preexisting faults) is valid to great crustal depth and that the crust is in brittle failure equilibrium at depths and temperatures approaching the brittle‐ductile transition, even in this relatively stable intraplate area.
Magnetostratigraphy of the Late Cretaceous to Eocene Sverdrup Basin: Implications for heterochroneity, deformation, and rotations in the Canadian Arctic archipelago
Authors: Tarduno John A. , Cottrell Rory D. , Wilkison Sarah L.
Published: February 2004

The temporal and spatial patterns of sedimentation in the Sverdrup Basin provide clues to how deformation in the Canadian Arctic accommodated Late Cretaceous‐Eocene relative motion between Greenland and North America. Although the sediments contain a rich assemblage of mammal and megafloral fossils, dating of the sequence has been controversial. Some work suggests a dramatic faunal and floral heterochroneity with species appearing in the Arctic 2–18 m.y. prior to their appearance at lower latitudes. To obtain a chronostratigraphic framework for these sediments, a 2.6‐km section of the Eureka Sound Group and Kanguk Formation on western Axel Heiberg Island was sampled for magnetostratigraphy. After removal of a pervasive modern field overprint with thermal and alternating field demagnetization, a characteristic remanent magnetization (ChRM) is isolated. Despite high directional dispersion, the ChRMs form 11 distinct polarity intervals which can be correlated to chrons 34 to 24r. This correlation indicates that some of these sediments are ∼10 m.y. younger than thought previously, reducing the need for large‐scale heterochroneity. Sedimentation rates derived from the magnetostratigraphy suggest that an increase in basin subsidence is recorded near the middle of the section sampled. A similar pattern has been reported from the Eureka Sound Group exposed on Ellesmere Island. We interpret this increased sedimentation as a response to crustal flexure caused by lithospheric loading during the middle Paleocene (C26r). The loading may be related to a blind thrust system to the west of Axel Heiberg Island that marks compression between North America and Greenland driven by rapid seafloor spreading in the Labrador Sea. The new data, together with prior results, indicate that most of the Cretaceous Canadian Arctic archipelago has undergone a counterclockwise vertical axis rotation. The new data are more consistent with this rotation being related to events during chron 26r, rather than marking block rotations associated with the terminal Eocene phases of Eurekan deformation.
Basal conditions on Rutford Ice Stream, West Antarctica, from seismic observations
Authors: Smith A. M.
Published: February 2004

A seismic reflection profile, perpendicular to the ice flow direction has been acquired on Rutford Ice Stream, Antarctica. An interpretation of both the amplitude and phase of the seismic reflections from the ice‐bed interface has been made to investigate the properties of the sub‐ice material. The interpretation assumes that acoustic impedance can be used to imply subglacial sediment porosity. Multiple reflections from the ice‐bed interface on a number of seismic wide‐angle lines allowed a calibration of the reflection coefficient at the bed. This enabled the acoustic impedance of the bed material to be calculated from the seismic reflection data. Mean acoustic impedance in the different sections of the bed ranges from 2.70×106 kg m−2 s−1 to 4.31×106 kg m−2 s−1. Almost three quarters of the ice stream bed at this site appears to be saturated, deforming sediments. The rest of the bed is probably also saturated sediments, but they are not deforming to any significant degree. Ice flow could include a combination of subglacial deformation and basal sliding. Localized regions which support disproportionately high amounts of basal shear stress may also occur. From the seismic data, it is not possible to apportion relative amounts of restraint to these different processes. There does not appear to be any correlation between the different sections identified on the ice stream bed with either satellite images or nearby surface velocity data. A feature which is interpreted as a subglacial drumlin is seen on the seismic section.
Methane hydrate stability in pore water: A simple theoretical approach for geophysical applications
Authors: Dickens Gerald R. , Quinby‐Hunt Mary S.
Published: February 2004

Geophysicists have recently expressed an interest in understanding how pore water composition affects CH4 hydrate stability conditions in the marine environment. It has previously been shown in the chemical engineering literature that CH4 hydrate stability conditions in electrolyte solutions are related to the activity of water (aw). Here we present additional experimental data in support of this relationship and then use the relationship to address issues relevant to geophysicists. Pressure and temperature conditions of CH4 hydrate dissociation were determined for 10 solutions containing variable concentrations of Cl−, SO42− Br−, Na+, K+, Mg2+, NH4+, and Cu2+. The reciprocal temperature offset of CH4 hydrate dissociation between the CH4‐pure water system and each of these solutions (and for other electrolyte solutions in literature) is directly related to the logarithm of the activity of water (lnaw). Stability conditions for CH4 hydrate in any pore water system therefore can be predicted simply and accurately by calculating lnaw. The effect of salinity variation and chemical diagenesis on CH4 hydrate stability conditions in the marine environment can be evaluated by determining how these processes affect lnaw of pore water.
Microseismicity evidence for subduction of the Caribbean plate beneath the South American Plate in northwestern Venezuela
Authors: Pérez Omar J. , Jaimes Martha A. , Garciacaro Emilio
Published: February 2004

Over 1100 microearthquakes with body wave magnitude mb<4 have been located in western Venezuela and the southwestern Caribbean region since the installation in 1980 of the Venezuelan Seismological Array, together with 120 events of mb≥4, one of them with surface wave magnitude Ms∼6. This tectonically complex region is part of the boundary between the Caribbean and the South American plates. The main seismically active feature inland in western Venezuela is the northeast striking, 600‐km long, 100‐km wide, right‐lateral strike‐slip Boconó fault zone along the Venezuelan Andes. About 80% of the earthquakes located in the entire region in the period 1980‐mid‐1995 have occurred on this fault zone, at focal depths <20 km. Microearthquake activity at lower rates also occurs northwest of the Venezuelan Andes, both in the continental and Caribbean sea regions. Part of this activity takes place at depths down to ∼150 km. Northwest oriented seismicity depth profiles show the existence of a Benioff zone dipping to the southeast beneath northwestern Venezuela and northern Colombia. This indicates the presence of a northeast striking, southeast dipping subducted slab of the Caribbean plate beneath the South American plate. Hypocentral locations show that the northeastern end of this subduction occurs northwest of the Curaçao‐Aruba region, in the vicinity of a northwest trending, right‐lateral strike‐slip fault zone that joins up with the northeastern end of the Boconó fault zone. This latter place turns out to be the western end of the east‐west striking San Sebastián fault along the Venezuelan coast.
Radial profile of mantle viscosity: Results from the joint inversion of convection and postglacial rebound observables
Authors: Mitrovica Jerry X. , Forte Alessandro M.
Published: February 2004

We present new inferences of the radial profile of mantle viscosity that simultaneously fit long‐wavelength free‐air gravity harmonics associated with mantle convection and a large set of decay times estimated from the postglacial uplift of sites within previously glaciated regions (Hudson Bay, Arctic Canada, and Fennoscandia). The relative sea level variation at these latter sites is constrained by age‐height pairs obtained by geological survey, rather than the subjective trends which are commonly used in glacial isostatic adjustment (GIA) studies. Our viscosity inferences are generated using two approaches. First, we adopt a relative viscosity profile which is known to provide a good fit to the free‐air gravity harmonics and determine an absolute scaling which yields a best fit to the GIA decay time constraints. Second, we perform an iterative, nonlinear, joint inversion of the two data sets. In both cases our inferred profiles are characterized by a significant increase of viscosity (∼2 orders of magnitude), with depth, to values of ∼1022 Pa s in the bottom half of the lower mantle. The new viscosity profiles are shown to satisfy constraints based on the postglacial uplift of both Fennoscandia (the classic Haskell [1935] number) and Hudson Bay which have commonly been invoked to argue for an isoviscous mantle. Furthermore, the models are used to predict a set of long‐wavelength signatures of the GIA process. These include predictions of GIA‐induced variations in (1) the length‐of‐day over the late Holocene period; (2) the Earth's precession constant and obliquity over the last 2.6 Myr; and (3) the present‐day zonal harmonics of the geopotential, Jl(l≤7). The predictions (1) and (3) bound the late Holocene (and ongoing) mass flux between the large polar ice sheets (Greenland and Antarctic) and the global oceans to small values (≤0.4 mm/yr equivalent eustatic sea level rise).
Dynamics of gas‐driven eruptions: Experimental simulations using CO2‐H2O‐polymer system
Authors: Zhang Youxue , Sturtevant B. , Stolper E. M.
Published: February 2004

We report exploratory experiments simulating gas‐driven eruptions using the CO2‐H2O system at room temperature as an analog of natural eruptive systems. The experimental apparatus consists of a test cell and a large tank. Initially, up to 1.0 wt% of CO2 is dissolved in liquid water under a pressure of up to 735 kPa in the test cell. The experiment is initiated by suddenly reducing the pressure of the test cell to a typical tank pressure of 10 kPa. The following are the main results: (1) The style of the process depends on the decompression ratio. There is a threshold decompression ratio above which rapid eruption occurs. (2) During rapid eruption, there is always fragmentation at the liquid‐vapor interface. Fragmentation may also occur in the flow interior. (3) Initially, the top of the erupting column ascends at a constant acceleration (instead of constant velocity). (4) Average bubble radius grows as t2/3. (5) When viscosity is 20 times that of pure water or greater, a static foam may be stable after expansion to 97% vesicularity. The experiments provide several insights into natural gas‐driven eruptions, including (1) the interplay between bubble growth and ascent of the erupting column must be considered for realistic modeling of bubble growth during gas‐driven eruptions, (2) buoyant rise of the bubbly magma is not necessary during an explosive volcanic eruption, and (3) CO2‐driven limnic eruptions can be explosive. The violence increases with the initial CO2 content dissolved in water.
The March 9, 1994 (Mw 7.6), deep Tonga earthquake: Rupture outside the seismically active slab
Authors: McGuire Jeffrey J. , Wiens Douglas A. , Shore Patrick J. , Bevis Michael G.
Published: February 2004

We investigate the rupture process of the March 9, 1994, Mw 7.6 deep Tonga earthquake and its relationship to the background seismicity of the subducted Tonga slab. Variations in observed P and S wave pulse duration indicate that the rupture propagated to the NNE and extended well beyond the background seismicity. We inverted 47 P and SH waveforms, including regional broadband waveforms from the Southwest Pacific Seismic Experiment, using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane. The results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10–20 km beyond the bounds of the previous seismic activity. A significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region. Rupture along the near‐vertical NNE striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near‐horizontal nodal plane. This result, combined with the planar alignment of aftershocks and the general NNE directivity of the waveforms, provides strong evidence that the rupture occurred on the near‐vertical plane. Thermal modeling of the Tonga slab indicates that the rupture terminated in material about 200°C warmer than the temperature that normally limits the occurrence of smaller earthquakes. Additionally, aftershocks seem to be suppressed in the outer regions of the rupture, which contain about half of the moment release but only 1 of the 15 well‐located aftershocks. We suggest that slabs may be composed of an inner cold core, where seismic rupture initiates and small earthquakes occur, and a thermal “halo” of warmer material, which can sustain rupture and only a few aftershocks. The mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core; nucleation may occur through a process limited to the cold core such as transformational faulting, and propagation through the warmer material may occur through ductile faulting or plastic instabilities. Isolated deep earthquakes in other subduction zones, such as the 1994 Bolivia event, may occur almost completely within the warmer zone, accounting for the lack of background seismicity and the dearth of aftershocks.
Nongravitational effects and the LAGEOS eccentricity excitations
Authors: Métris Gilles , Vokrouhlický David , Ries John C. , Eanes Richard J.
Published: February 2004

A model for explaining the anomalous eccentricity excitations of the orbit of the LAGEOS I satellite is considered. It is suggested that two phenomena are responsible for the major part of these residuals: (1) about 1.3% mismodeling of the LAGEOS I surface mean radiation pressure coefficient, and (2) the Yarkovsky‐Schach thermal effect with a properly modeled evolution of the satellite spin axis. Minor influences are also attributed to the asymmetric reflectivity of the satellite surface and the asymmetric thermal emissivity of the Earth. For these cases we give a suitable and compact formulation for the long‐term orbit analysis. It is argued that contrary to the case of the LAGEOS I along‐track residuals adjustment, which does not allow the determination of a particular combination of the Yarkovsky‐Schach effect parameters, the combined analysis of the along‐track residuals and the eccentricity excitations allows for their decorrelation. The resulting value of the Yarkovsky‐Schach amplitude is estimated to be approximately 241 pm/s2, more than twice as large as given in previous works. In view of the unexplained part of the eccentricity residuals, we discuss the possible importance of several other nongravitational effects (Yarkovsky thermal effects due to Earth's heating, electromagnetic effects, mismodeling of the transitions through Earth's penumbra), concluding that none of them is quantitatively a good candidate.
Three‐dimensional VP and VP/VS Velocity Models of the Los Angeles basin and central Transverse Ranges, California
Authors: Hauksson Egill , Haase Jennifer S.
Published: February 2004

We use P and S arrival times from 5225 earthquakes and 53 explosions, recorded by the Southern California Seismographic Network, to invert for the three‐dimensional P‐ velocity (VP) and the P and S‐velocity ratio (VP/ VS) in the central Transverse Ranges and the Los Angeles basin. To model long‐wavelength features of the velocity structure, we invert for the models by interpolating models determined successively from a sparse, medium, to a dense grid, with 40, 20, and 10 km spacing of horizontal grid nodes. Layers of grid nodes are placed at depths of 1, 4, 6, 8, 12, 16, and 20 km. The data variance decreased about 80% in the gradational inversion. Ample data from the 1994 Northridge and other earthquake sequences, the rich background seismicity, and the dense station distribution along with controlled sources made the model well resolved, except along the edges, to the southwest in the offshore region, and at depths greater than 20 km. The VP model images the shape of the Los Angeles and east Ventura basins down to depths of 8 and 12 km. Three low‐velocity areas at 1 km depth in the Los Angeles basin that coincide with recent sediment depositional areas are also imaged. The north edge of the Peninsular Ranges, the Santa Monica, and the San Gabriel Mountains, form discontinuous high‐velocity ridges extending to depths of 20 km. The high VP/VS ratios in the near surface are consistent with high pore fluid pressures in the basin sediments. At depth beneath the east Ventura basin the high VP/VS and high VP suggest the presence of ophiolitic assemblages or mid‐Miocene volcanics. In contrast, a body with normal VP and low VP/VS that is identified in the upper crust beneath Santa Monica Bay may be a fragment of quartz‐rich continental crust. The VP model of both the Ventura and the Los Angeles basins shows features that suggest deformation of the hanging wall or basin closure. At seismogenic depths of 16 km the hypocenters of moderate‐sized and large earthquakes are located within or adjacent to high‐VP‐velocity bodies. In most cases these high‐velocity bodies form the upper block, consistent with shortening of the lower crust as described in thick‐skinned tectonic interpretations.
Marine gravity anomaly from Geosat and ERS 1 satellite altimetry
Authors: Sandwell David T. , Smith Walter H. F.
Published: February 2004

Closely spaced satellite altimeter profiles collected during the Geosat Geodetic Mission (∼6 km) and the ERS 1 Geodetic Phase (8 km) are easily converted to grids of vertical gravity gradient and gravity anomaly. The long‐wavelength radial orbit error is suppressed below the noise level of the altimeter by taking the along‐track derivative of each profile. Ascending and descending slope profiles are then interpolated onto separate uniform grids. These four grids are combined to form comparable grids of east and north vertical deflection using an iteration scheme that interpolates data gaps with minimum curvature. The vertical gravity gradient is calculated directly from the derivatives of the vertical deflection grids, while Fourier analysis is required to construct gravity anomalies from the two vertical deflection grids. These techniques are applied to a combination of high‐density data from the dense mapping phases of Geosat and ERS 1 along with lower‐density but higher‐accuracy profiles from their repeat orbit phases. A comparison with shipboard gravity data shows the accuracy of the satellite‐derived gravity anomaly is about 4–7 mGal for random ship tracks. The accuracy improves to 3 mGal when the ship track follows a Geosat Exact Repeat Mission track line. These data provide the first view of the ocean floor structures in many remote areas of the Earth. Some applications include inertial navigation, prediction of seafloor depth, planning shipboard surveys, plate tectonics, isostasy of volcanoes and spreading ridges, and petroleum exploration.
Potassic mafic magmatism in the Kigluaik gneiss dome, northern Alaska: A geochemical study of arc magmatism in an extensional tectonic setting
Authors: Amato Jeffrey M. , Wright James E.
Published: February 2004

The 90 Ma Kigluaik pluton forms the core of a granulite facies gneiss dome and is part of an extensive belt of Cretaceous plutons in northern Alaska and Russia. The dome consists of Precambrian to Paleozoic gneisses metamorphosed during a pre‐120 Ma high‐pressure event and a Late Cretaceous high‐temperature event. The pluton is essentially bimodal in composition. A “mafic root” consists of granodiorite, quartz monzodiorite, tonalite, and diorite. This unit appears to be cut by gabbroic dikes or lenses and is overlain by a 1‐km‐thick “felsic cap” of biotite granite. Separating these units is a zone of mafic enclaves, with pillow shapes and crenulate margins, within a granodiorite matrix. The mafic root is depleted in Nb and enriched in K and other large‐ion lithophile elements, including the light rare earth elements; the felsic cap is geochemically similar but with higher concentrations of incompatible elements. Sr and Nd isotopic compositions near bulk earth suggest that a mantle‐derived parental magma fractionated and assimilated crustal rocks during its history. The mafic root has trace element characteristics similar to Early Cretaceous arc‐related volcanic rocks and Late Cretaceous potassic plutons in the Yukon‐Koyukuk Basin of Alaska. It is also similar to other high‐K mafic plutons formed above continental subduction zones. The Cretaceous tectonic setting of the Seward Peninsula was likely characterized by a continental arc that was being fragmented during extension, probably as the result of rollback of the north dipping subducting slab during Late Cretaceous time.
Improving local earthquake locations using the L1 norm and waveform cross correlation: Application to the Whittier Narrows, California, aftershock sequence
Authors: Shearer Peter M.
Published: February 2004

Experiments with different earthquake location methods applied to aftershocks of the October 1, 1987, Whittier Narrows earthquake in California (ML=5.9) suggest that local event locations can be greatly improved through the use of the L1 norm, station corrections and waveform cross correlation. The Whittier Narrows sequence is a compact cluster of over 500 events at 12 to 18 km depth located within the dense station coverage of the Southern California Seismic Network (SCSN), a telemetered network of several hundred short‐period seismographs. SCSN travel time picks and waveforms obtained through the Southern California Earthquake Center are examined for 589 earthquakes between 1981 and 1994 in the vicinity of the mainshock. Using a smoothed version of the standard southern California velocity model and the existing travel time picks, improved location accuracy is obtained through use of the L1 norm rather than the conventional least squares (L2 norm) approach, presumably due to the more robust response of the former to outliers in the data. A large additional improvement results from the use of station terms to account for three‐dimensional velocity structure outside of the event cluster. To achieve greater location accuracy, waveforms for these events are resampled and low‐pass filtered, and thePandSwave cross‐correlation functions are computed at each station for every event pair. For those events with similar waveforms, differential times may be obtained from the cross‐correlation functions. These times are then combined with the travel time picks to invert for an adjusted set of picks that are more consistent than the original picks and include seismograms that were originally unpicked. Locations obtained from the adjusted picks show a further improvement in accuracy. Location uncertainties are estimated using a bootstrap technique in which events are relocated many times for sets of picks in which the travel time residuals at the best fitting location are used to randomly perturb each pick. Improvements in location accuracy are indicated by the reduced scatter in the residuals, smaller estimated location errors, and the increased tendency of the locations to cluster along well‐defined fault planes. Median standard errors for the final inversion are 150 m in horizontal location and 230 m in vertical location, although the relative locations within localized clusters of similar events are better constrained. Seismicity cross sections resolve the shallow dipping fault plane associated with the mainshock and a steeply dipping fault plane associated with aML=5.3 aftershock. These fault planes appear to cross, and activity began on the secondary fault plane prior to the large aftershock.