bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure of the Costa Rica convergent margin, offshore Nicoya Peninsula
Authors: Christeson G. L. , McIntosh K. D. , Shipley T. H. , Flueh E. R. , Goedde H.
Published: September 2002

We present the results of a seismic refraction survey conducted offshore Costa Rica near the Nicoya Peninsula. A dip profile and three strike profiles were carried out over 22 ocean bottom hydrophones and seismographs and were also recorded by land receivers. These data are used to construct a crustal structure model of the convergent margin from 20 km seaward of the Middle America Trench onto the Nicoya Peninsula. The best constrained portion of our model is the velocity at the top of the margin wedge immediately below the slope apron. Velocities increase from 3.5 to 4.2 to 4.6 km/s at distances of 10, 20, and 30–50 km landward of the trench. These velocities are higher than observed within margin wedges at other well‐studied convergent margins but lower than the velocities within the adjacent Nicoya Complex, which are ∼5.5 km/s at similar depths below the surface. We interpret the margin wedge velocities as indicating that material similar to the Nicoya Complex extends seaward to near the lower slope but that fracturing, alteration, or accretion processes have lowered the velocity of the margin wedge with respect to the Nicoya Complex. The seismic refraction data cannot constrain the exact thickness or velocity of a possible low‐velocity zone (LVZ) overlying the subducting plate; however, geologically reasonable structures are only produced with a LVZ<400 m thick. Velocities in the upper part of oceanic layer 2 are ∼3.5–4.0 km/s within the subducting slab. These velocities are unusually low for oceanic crust of this age and may correlate with a proposed highly permeable zone at the top of the subducting crust. The top of the subducted slab is well resolved, and deepens from 5 km depth at the trench to 15–16 km depth at the Nicoya Peninsula coastline. The dip angle of the subducting plate increases from 6° to 13° at a distance of ∼30 km from the trench. Interplate seismicity appears to become common ∼55 km from the trench where the plate boundary is at ∼14 km depth.
Evolution of carbonatite complexes of the Deccan flood basalt province: Stable carbon and oxygen isotopic constraints
Authors: Ray Jyotiranjan S. , Ramesh R.
Published: September 2002

The stable carbon and oxygen isotopic composition of carbonatites from three carbonatite‐alkaline complexes (Amba Dongar, Mundwara and Sarnu‐Dandali) of Deccan province, India, can be placed into two groups, primary and secondary. The primary variations indicate the mantle origin of these complexes. The source of these complexes, the Reunion plume head, was largely composed of a mantle having δ18O similar to that of mean upper mantle (5 to 8‰) but higher δ13C (>−5.5‰). The higher δ13C, particularly that of a batch of parent magma for Amba Dongar (−3.4‰), suggests the incorporation of recycled inorganic crustal carbon in these carbonatites. As in the plume model for generation of continental flood basalts and associated carbonatite‐alkaline complexes, the fluid‐rich rim of the plume head acts as the source for the carbonatites, so the above incorporation of crustal carbon could have been facilitated through the migration of CO2‐rich,13C enriched fluids (derived from ancient subducted carbonates) into the Reunion plume head. The correlated variations of δ13C and δ18O in unaltered calcite carbonatites in all three complexes are consistent with the formation of these rocks by fractional crystallization from CO2‐rich carbonate magmas, derived from parent carbonated silicate magmas through liquid immiscibility. The extreme enrichment of δ13C and δ18O in some carbonatites and metasomatic rocks is a result of postcrystallization alteration process caused by CO2‐bearing aqueous fluids. In Amba Dongar a pure magmatic fluid and/or a magmatic‐hydrothermal fluid could have caused the alteration, whereas rocks of Mundwara and Sarnu‐Dandali appear to have been altered by meteoric‐hydrothermal fluids.
Boundary layer fractionation constrained by differential information from the Kutsugata lava flow, Rishiri volcano, Japan
Authors: Kuritani Takeshi
Published: September 2002

Detailed investigation of an erupted magma with limited compositional diversity provides instantaneous information on incremental magma differentiation processes in a magma chamber. Kutsugata lava, a suitable target of such study, is a Quaternary alkali basalt (51.3–53.2 wt% in SiO2) from Rishiri Volcano, northern Japan. Despite the narrow range in the whole rock compositional variation, chemical and modal compositions of mineral phases crystallized in the magma chamber vary systematically with the whole rock composition. In the North lava (51.3–51.9 wt% in SiO2) the less differentiated portion of the Kutsugata lava, most crystals which include low‐Ni olivine and plagioclase were derived from the mushy boundary layer. The main part of the magma body was principally aphyric (<0.5 vol% crystals). Estimated chemical compositions of fractionated mineral phases during differentiation of the magma coincide with the observed compositions of low‐Ni olivine and plagioclase crystals. This indicates that the main magma was differentiated by separation of crystals grown in the mush zone. The low‐density interstitial melt is suggested to have been extracted from the floor mush zone with average crystallinity of >30 vol% by such mechanisms as compaction and compositional convection. This fractionated melt was mixed with the overlying main magma, causing differentiation of the Kutsugata magma. The average temperature of the extracted melt is 1010°C, significantly lower than 1100°C estimated for the main magma. A quantitative model of magmatic differentiation, which includes thermal and compositional evolution of a mushy boundary layer, can successfully reproduce the observed compositional trends of the North lava.
Anomalous mantle at 45°N Mid‐Atlantic Ridge?
Authors: Mello S. L. M. , Cann J. R. , Fowler C. M. R.
Published: September 2002

The Mid‐Atlantic Ridge at 45°N has a ridge crest depth close to normal for slow spreading ridges and a crustal thickness of 6 km. However, the free‐air gravity over the region is higher than expected for such axial depths, and erupted basalts are enriched in incompatible trace elements and radiogenic isotopes, which could be taken to indicate the presence of a mantle plume beneath the spreading axis. But the ridge is at normal depths and the crust is of normal thickness. We model free‐air gravity across and parallel to the strike of the ridge. A density structure derived from a cooling model provides a good fit for gravity across the ridge axis. A gravity model along the ridge axis indicates that most of the gravity signal over the Azores and Reykjanes Ridge is related to thicker crust, while the gravity signal over 45°N can be modeled by a density anomaly, which extends along the spreading axis from about 43°N to 50°N and from zero age to 35 m.y. Such a mass excess can be explained by an increase in the garnet content of the mantle within the stability field of garnet peridotite. The enriched basalts occur in the same region as the mass anomaly. We interpret them as originating from cool hotspot mantle. This anomalous mantle may derive from a hotspot that existed at the North America‐Africa‐Eurasia triple junction when it was at 45°N between 59 and 26 m.y. The King's Trough Complex is the remnant of that hotspot. We consider that this cool hotspot mantle still underlies the spreading axis and contributes to crustal construction.
Seismic cycle and plate margin deformation in Costa Rica: GPS observations from 1994 to 1997
Authors: Lundgren P. , Protti M. , Donnellan A. , Heflin M. , Hernandez E. , Jefferson D.
Published: September 2002

Global Positioning System (GPS) observations in Costa Rica from 1994 to 1997 reveal a complex pattern of motion consistent with the superposition of seismic cycle and secular plate margin deformation. In the south, velocity vectors are consistent with motion of the Panama Block plus postseismic deformation following the 1991 Limon earthquake and interseismic strain due to partial locking of the Middle America Trench (MAT) thrust. In the northwest, sites west of the volcanic arc are moving to the NW as a forearc sliver. Superimposed on this sliver motion are vertical and horizontal interseismic deformations from the adjacent Nicoya segment of the MAT. We apply two different inverse methods to understand the source of the seismic strain in NW Costa Rica. We compare fault‐locking models derived using a singular value decomposition inversion with that of a simulated annealing global optimization approach. Both methods yield similar models for partial locking of the thrust interface beneath the Nicoya Peninsula. Our results define an area of nearly fully locked fault beneath the outer coast of the southern portion of the peninsula, with somewhat lower coupling beneath the northern half and with low coupling elsewhere. These initial results show the promise for detailed imaging of the locked portion of a thrust interface responsible for future large subduction zone earthquakes.
The Amurian Plate motion and current plate kinematics in eastern Asia
Authors: Heki Kosuke , Miyazaki Shin'ichi , Takahashi Hiroaki , Kasahara Minoru , Kimata Fumiaki , Miura Satoshi , Vasilenko Nikolay F. , Ivashchenko Alexei , An Ki‐Dok
Published: September 2002

We use Global Positioning System (GPS) velocity data to model eastern Asian plate kinematics. Out of 15 stations in Korea, Russia, China, and Japan studied here, three sites considered to be on the stable interior of the hypothetical Amurian Plate showed eastward velocities as fast as ∼9–10 mm/yr with respect to the Eurasian Plate. They were stationary relative to each other to within 1 mm/yr, and these velocity vectors together with those of a few additional sites were used to accurately determine the instantaneous angular velocity (Euler) vector of the Amurian Plate. The predicted movement between the Amurian and the North American Plates is consistent with slip vectors along the eastern margin of the Japan Sea and Sakhalin, which reduces the necessity to postulate the existence of the Okhotsk Plate. The Euler vector of the Amurian Plate predicts left‐lateral movement along its boundary with the south China block, consistent with neotectonic estimates of the displacement at the Qinling fault, possibly the southern boundary of the Amurian Plate. The Amurian Plate offers a platform for models of interseismic strain buildup in southwest Japan by the Philippine Sea Plate subduction at the Nankai Trough. Slip vectors along the Baikal rift, the boundary between the Amurian and the Eurasian Plates, are largely inconsistent with the GPS‐based Euler vector, suggesting an intrinsic difficulty in using earthquake slip vectors in continental rift zones for such studies.
Effects of diagenesis on magnetic mineralogy in a Jurassic claystone‐limestone succession from the Paris Basin
Authors: Moreau Marie Gabrielle , Ader Magali
Published: September 2002

We performed detailed rock magnetic and geochemical analyses on a previously published Early Jurassic magnetostratigraphic section. The results improve our understanding of acquisition and preservation processes of magnetization in a series of alternating claystones and limestones. The main carrier of magnetization is magnetite. Anhysteretic remanent magnetization (ARM) varies by a factor of 40 and the ARM variations are linked to magnetite grain size. Comparison of magnetic, and geochemical data shows that when carbonate content is high (>30%) and δ13C ≈ 0‰, magnetite is characterized by small grain sizes, whereas when carbonate content is low (<20%), magnetite is coarse‐grained. It appears that the oxidation of organic matter by sulfate reduction controls both δ13C and magnetite grain size. H2S produced during sulfate reduction causes partial dissolution of magnetite grains, with the finest magnetite grains (those that best record the magnetic signal) being dissolved first. Despite this partial dissolution, both the direction and polarity of the original remanent magnetization are preserved.
Effects of normal stress vibrations on frictional healing
Authors: Richardson Eliza , Marone Chris
Published: September 2002

We conducted laboratory experiments to study frictional healing and the effects of normal stress vibrations on healing. The experiments were carried out using a servo‐controlled double‐direct shear apparatus on 10 cm × 10 cm blocks separated by a 3 mm‐thick gouge layer of fine‐grained (grain size of 75–212 μm) quartz powder. We performed slide‐hold‐slide tests in which sliding surfaces were driven at a constant velocity, halted for a given interval, then restarted at the prior driving velocity. Healing varied systematically with cumulative displacement, and by conducting several sets of identical slide‐hold‐slides we calibrated and removed these effects. Forward modeling of the healing and relaxation curves using the rate‐ and state‐dependent friction laws shows that a displacement‐dependent increase in the parameter b can account for our observations. To study the effects of vibration, we varied the mean normal stress of 25 MPa during holds by double amplitudes ranging from 1 to 13 MPa at a frequency of 1 Hz. Vibrations increased rates and magnitudes of frictional relaxation and healing, most likely due to increased gouge compaction. These effects increased with increasing amplitude of vibration. We performed normal stress step tests and used the results to model the vibrational slide‐hold‐slide tests. Rate‐ and state‐dependent constitutive laws cannot adequately describe the behavior we observed experimentally because they neglect gouge compaction. Mechanisms such as normal force oscillations may explain faster fault healing rates than would be predicted by standard laboratory measurements at constant stress.
Precise relocations and stress change calculations for the Upland earthquake sequence in southern California
Authors: Astiz Luciana , Shearer Peter M. , Agnew Duncan C.
Published: September 2002

We relocate earthquakes that occurred near the 1988 (ML = 4.7) and the 1990 (ML = 5.5) Upland, California, earthquakes to map the fault geometry of the poorly defined San Jose fault and to test the static stress triggering hypothesis for this sequence. We adopt the L1 norm, waveform cross‐correlation method of Shearer [1997] to obtain precise relocations for 1573 events between 1981 and 1997 in the Upland area. To limit computation time, we only perform waveform cross correlation on 60 of the nearest neighbors of each relocated event. Our final relocations show two linear features. The first is imaged by the locations of the initial month of aftershocks of the 1988 Upland earthquake, which delineate a fault with a dip angle of ∼45° between 7 and 9 km depth, consistent with the mainshock focal mechanism. The second linear feature is a plane dipping at about 74° from 2 to 9 km depth, which is illuminated by both the 1988 and 1990 Upland sequences, in agreement with the inferred location of the San Jose fault at depth. However, below 9 km the event locations become more diffuse, giving rise to two different interpretations of the fate of the San Jose fault at depth. One possibility is that the fault shallows at depth, consistent with our relocations but not with the focal mechanism of a ML = 4.7 deep aftershock. Alternatively, the fault may be offset at depth by the more shallow dipping fault strand broken during the 1988 earthquake. Using these inferred fault geometries, we compute stress changes resulting from slip during the mainshocks to test whether the relocated aftershocks are consistent with the hypothesis that more aftershocks occur where the change in static Coulomb failure stress is positive (on faults optimally oriented for failure). This requires an extension of previous models of changes in the failure stress to three dimensions and arbitrary fault orientation. We find that patterns of change in Coulomb failure stress differ little between the different fault geometries: all are nearly symmetric about the fault and so do not match the aftershock distribution, in which most of the off‐fault events occur to one side of the fault plane.
Crustal rheology and faulting at strike‐slip plate boundaries: 1. An analytic model
Authors: Roy Mousumi , Royden Leigh H.
Published: September 2002

This paper investigates how rheologic stratification within the crust affects the formation and long‐term evolution of fault systems at a strike‐slip plate boundary. We present an analytic model of deformation at a strike‐slip plate margin within a two‐layer viscoelastic crust, with fixed shear modulus but varying viscosity in each layer. Faulting is represented by static elastic dislocations of fixed, shallow depth extent, imposed at a high critical stress threshold for fracture of a new fault and a lower one for sliding on a preexisting fault. To drive crustal deformation, we impose basal velocity boundary conditions at the Moho representing a narrow zone of high shear in the mantle. In this study we restrict attention to deformation at the surface, where simple analytic solutions exist for velocities and stresses. Our results suggest that when a primarily elastic/brittle upper crust is underlain by a low‐viscosity lower crust, the deformation zone in the upper crust widens in time. At steady state the surface width of the deformation zone may be significantly greater than the prescribed, narrow mantle shear zone. The long‐term width of the deformation zone increases with the viscosity contrast within the crust and with the thickness of the low‐viscosity lower crust. Widening of the deformation zone is accompanied by the fracture of new faults in the upper crust, leading to the formation of a system of parallel strike‐slip faults surrounding the plate boundary. For fixed plate velocity and fracture criteria, we find that the width of the brittle fault network within the total (distributed) deformation zone is primarily governed by strength properties of the upper crust and by the viscosity contrast within the crust.
Mapping asperities by minima of local recurrence time: San Jacinto‐Elsinore fault zones
Authors: Wyss Max , Schorlemmer Danijel , Wiemer Stefan
Published: September 2002

Minima of the local recurrence time, TL(M), are correlated with the locations of historical main shocks in the San Jacinto (SJ)‐Elsinore (E) fault zones of southern California. We define TL(M) as the recurrence time calculated from the local b and a values of the frequency‐magnitude distribution (earthquakes within 6 and 10 km from each point in the SJ and E fault zones, respectively). We propose that anomalously low TL(M) values map asperities. Out of six historic main shocks (1899–1968), five ruptured substantial parts of four asperities mapped by our method, which uses solely the modern earthquake catalog (M ≥ 1.2, 1981.0–1998.8). We estimate the probability for this correlation to occur at random as ∼10−2 to 10−3, depending on the scenario chosen. It appears that the minima in TL(M) are tied to minima in b values more than to maxima in a values. Thus locations of low b values can also be used as an indication where asperities may be located and where main shocks are most likely. The Anza gap, a segment of the San Jacinto fault with low a values, is mapped as an asperity by our method. The heterogeneity in b values is strong throughout the study area. The b values range from 0.5 to 1.8 over distances of 10 km. Volumes with approximately constant b values have radii of r = 8±6 km. Our results suggest that detailed mapping of b values and local recurrence times in volumes with a few kilometers radius reveals important information about the local seismic hazard, hitherto untapped by conventional seismicity analyses.
P‐V‐T equation of state of periclase from synchrotron radiation measurements
Authors: Dewaele Agnès , Fiquet Guillaume , Andrault Denis , Hausermann Daniel
Published: September 2002

The volume of periclase (MgO) has been measured by monochromatic X‐ray diffraction in a laser‐heated diamond anvil cell up to a pressure of 53 GPa and a temperature of 2500 K. The X‐ray source was synchrotron radiation at the European Synchrotron Radiation Facility (Grenoble, France). In addition to laser heating, the use of argon as a pressure transmitting medium provided quasi‐hydrostatic conditions in the cell assembly. In order to take thermal pressure effect into account, pressure was measured using an internal pressure calibrant (platinum). By analysis of the experimental P‐V‐T data set the following parameters were obtained: at ambient temperature,K′0= 3.94 ± 0.2 whenK0is fixed to 161 GPa (with a Birch‐Murnaghan equation of state); under high temperature, α(P= 0,T) = (3.0 + 0.0012T) × 10−5K−1; (∂KT/∂T)P= −0.022(3) GPa K−1. The quasi‐harmonic Debye model appears to describe correctly the temperature dependence of the volume at high pressure within experimental errors, with the following parameters: θD0= 800 K, γ0= 1.45 (Grüneisen parameter under ambient conditions), andq= 0.8 ± 0.5.
Coupling of long oceanic waves in the Pacific Ocean and the rotating elastic Earth during the 1986–1987 El Niño
Authors: Kakuta Chuichi , Tsubokawa Tsuneya , Iwadate Kenzaburo
Published: September 2002

We investigate interannual coupling between the rotating elastic Earth and the ocean during the 1986–1987 El Niño. We find the E‐W component of the ground tilt at the Esashi Earth Tides Station up toward the east about 50 milliarc seconds (mas), a rise of sea level about 5 cm at Chichijima and Oshima in the northwestern Pacific Ocean, and an increase of ∼1 cm in the very long baseline interferometry (VLBI) baseline distance between Kashima (Japan) and Kauai (Hawaiian Islands) as well as the residual length of day (LOD) changes of ∼50 μs during the 1986–1987 El Niño. The residual LOD is derived from elimination of changes in atmospheric angular momentum. Our discussions are based on the coupling between torsional motions of the elastic Earth and reflection of the eastward traveling equatorial Kelvin waves at the eastern boundary of the Pacific Ocean. These equatorial waves exchange the momentum and the axial and zonal component of vorticity with the elastic Earth. During the 1986–1987 El Niño, reflection of the Kelvin wave is expected to excite torsional motions and to couple with differential zonal rotation in the elastic Earth and to generate anomalies of ∼1.6 μs in LOD. The differential zonal rotation due to the torsional motion can produce the same order of the magnitude of changes in the VLBI baseline distance between Kashima and Kauai. Variations of the E‐W component of the tilt cannot be explained by torsional motion. We infer that the midlatitude Rossby waves are generated in the central Pacific Ocean. This conclusion is consistent with sea level changes at Chichijima and Oshima and changes of the tilt at Esashi.
Deep crustal heterogeneity along and around the San Andreas fault system in central California and its relation to the segmentation
Authors: Nishigami Kin'ya
Published: September 2002

The three‐dimensional distribution of scatterers in the crust along and around the San Andreas fault system in central California is estimated using an inversion analysis of coda envelopes from local earthquakes. I analyzed 3801 wave traces from 157 events recorded at 140 stations of the Northern California Seismic Network. The resulting scatterer distribution shows a correlation with the San Gregorio, San Andreas, Hayward, and Calaveras faults. These faults seem to be almost vertical from the surface to ∼15 km depth. Some of the other scatterers are estimated to be at shallow depths, 0–5 km, below the Diablo Range, and these may be interpreted as being generated by topographic roughness. The depth distribution of scatterers shows relatively stronger scattering in the lower crust, at ∼15–25 km depth, especially between the San Andreas fault and the Hayward‐Calaveras faults. This suggests a subhorizontal detachment structure connecting these two faults in the lower crust. Several clusters of scatterers are located along the San Andreas fault at intervals of ∼20–30 km from south of San Francisco to the intersection with the Calaveras fault. This part of the San Andreas fault appears to consist of partially locked segments, also ∼20–30 km long, which rupture during M6–7 events, and segment boundaries characterized by stronger scattering and stationary microseismicity. The segment boundaries delineated by the present analysis correspond with those estimated from the slip distribution of the great 1906 San Francisco earthquake, and from the fault geometry as reported by the Working Group on California Earthquake Probabilities [1990], although the segment boundaries along the San Andreas fault in and around the San Francisco Bay area are still uncertain.
Statistical physics of earthquakes: Comparison of distribution exponents for source area and potential energy and the dynamic emergence of log‐periodic energy quanta
Authors: Main Ian G. , O'Brien Gareth , Henderson Jeremy R.
Published: September 2002

We investigate the relationship between the size distribution of earthquake rupture area and the underlying elastic potential energy distribution in a cellular automaton model for earthquake dynamics. The frequency‐rupture area distribution has the form n(S) ∼Sτ exp (−S/So) and the system potential energy distribution from the elastic Hamiltonian has the form n(E) ∼Ev exp (−E/θ), both gamma distributions. Here n(S) reduces to the Gutenberg‐Richter frequency‐magnitude law, with slope b ∼τ, in the limit that the correlation length ξ, related to the characteristic source size So, tends to infinity. The form of the energy distribution is consistent with a statistical mechanical model with l degrees of freedom, where v = (l–2)/2 and θ is proportional to the mean energy per site
. We examine the effect of the local energy conservation factor β and the degree of material heterogeneity (quenched disorder) on the distribution parameters, which vary systematically with the controlling variables. The inferred correlation length increases systematically with increasing material homogeneity and with increasing β. The thermal parameter θ varies systematically between the leaf springs and the connecting springs, and is proportional to
as predicted. For heterogeneous faults, τ ∼1 stays relatively constant, consistent with field observation, and S0 increases with increasing β or decreasing heterogeneity. In contrast, smooth faults produce a systematic decrease in τ with respect to β and So remains relatively constant. For high β approximately log‐periodic quanta emerge spontaneously from the dynamics in the form of modulations on the energy distribution. The output energy for both types of fault shows a transition from strongly quasi‐periodic temporal fluctuations for strong dissipation, to more chaotic fluctuations for more conservative models. Only strongly heterogeneous faults show the small fluctuations in energy strictly required by models of self‐organized criticality.
Gravity constraints on the structure and volcanic evolution of Tenerife, Canary Islands
Authors: Ablay Giray J. , Kearey Philip
Published: September 2002

A detailed gravity study of Tenerife constrains the internal structure of the island and its active magma system. The maximum Bouguer anomaly (2636 gu, 1 gu = 10−6 m s−2 = 0.1 mGal) occurs over the SW part of the postshield Las Cañadas edifice (LCE), while smaller highs occur over the eroded shield volcanoes. The main high extends NE and NW over the adjacent Dorsal and Santiago rift zones. A deep low occurs over the northern LCE and Teide volcano. The power spectrum of the anomalies suggests shallow causative structures (< ∼3 km), inferred to belong to the island's dense core. A model computed from the anomalies uses core and carapace densities of 2900 and 2360 kg m−3, respectively, except beneath the main high, where the core requires a higher density (3210 kg m−3). This part of the body is interpreted as a buried mafic volcano of substantial size, the Boca Tauce volcano, containing an old mafic plutonic complex. The enigmatic Las Cañadas caldera that truncates the LCE shows strong asymmetry. Dense materials occupy the southern part whereas low‐density materials occupy the northern part to 2–3 km depth, consistent with a vertical collapse origin. High‐density materials in the south are related to the intracaldera extension of the Dorsal rift zone. The boundary between subcaldera domains is interpreted as the buried head of the giant Icod landslide valley. The Icod landslide appears to have modified the caldera, truncating its northern side, and to have subsequently controlled the location of the active Pico Teide‐Pico Viejo vent systems. Voluminous chaotic deposits lying off northern Tenerife, previously associated with the formation of the caldera and Icod valley, are better interpreted as being derived from older mass wasting events. The southern LCE has been buttressed against lateral failure by the buried Boca Tauce volcano.
Characteristics and consequences of flow in the lower crust
Authors: McKenzie Dan , Nimmo Francis , Jackson James A. , Gans P. B. , Miller E. L.
Published: September 2002

In some places, there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening. In order to better understand the processes involved, we investigate the behavior of a fluid layer over a fluid half‐space to see how such a system responds to the deformation of its upper and lower boundaries. This simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet. The changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically. The most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front, which may cause a topographic step above it at the Earth's surface. Deformation within the channel will be principally by simple shear. The clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur. The smoothing of crustal thickness contrasts leads to differential vertical motions, and is thus a method by which substantial tilting can occur without faulting. This differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the Basin and Range province of the western United States. It is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults.
Paleointensity studies of a lava succession from Jilin Province, northeastern China: Evidence for the Blake event
Authors: Zhu Rixiang , Pan Yongxin , Coe Robert S.
Published: September 2002

Paleomagnetic results are presented from a volcanic sequence in Tianchi, Jilin Province, northeastern China. Five basalt lava flows can be identified at the sampling site with ages from 210±4 to 88±15 kyr B.P. Rock magnetic analyses and reflected light microscopy indicate that magnetite with a significant fraction of pseudo‐single‐domain grains is dominant in these lava flows. A single stable characteristic remanence can be thermally isolated above 150–200°C. Intermediate direction is preserved in lava 3 at an age of ∼123 kyr corresponding to the Blake event, and its virtual geomagnetic pole is located in eastern Asia and Australia. From a total of 121 specimens used for paleointensity determinations by the Thellier method in an argon atmosphere, 34 specimens yielded reliable results with low within‐flow scatter. The results indicate that a significant intensity low is coincident with the Blake event.
Transport processes in the Kara Sea
Authors: McClimans Thomas A. , Johnson Donald R. , Krosshavn Marit , King Steven E. , Carroll JoLynn , Grenness Øivind
Published: September 2002

Field data from late summer cruises in 1993–1996 and results from a rotating laboratory model study are used to infer transport routes for contaminants in the Kara Sea on the western Siberian coast. The data show a circulation pattern that differs significantly from published concepts and contains a richer structure. Both the inflowing coastal water from the west and the modified Atlantic Water from the Barents Sea and the Arctic Ocean in the north affect the water properties of the East Novaya Zemlya Trough through mixing, intrusion, and possible transformation during winter convection. Much of the current structure can be explained as a result of topographical steering of the throughflows. Weak summer winds of variable direction affect only the upper layer, whereas persistent strong winds may affect long‐term circulation patterns and transports. The main outflow routes are north to the Santa Anna Trough and northeast to the Voronin Trough. The outflowing surface water and ice to the north are expected to join the inflowing Polar Water from the Arctic Ocean that forms the Persey Current into the Barents Sea. Deeper outflows are expected to follow the continental slope to the east in the Arctic Ocean. The laboratory model has been validated to earlier current measurements to the north and south of Novaya Zemlya and shows several of the features observed in the newer field measurements. This suggests that the dynamic similitude and boundary forcing of the model are good. Two notable exceptions are the westward transport of drifting buoys to the north of Novaya Zemlya and the late winter hydrography to the north of the Ob and Yenisey estuaries. Both are probably due to persistent winds. The conclusions from the measured and modeled currents are supported by measurements of 137Cs in the seawater and sediments.
Comparison of laboratory data with a viscous two‐layer model of wave propagation in grease ice
Authors: Newyear Karl , Martin Seelye
Published: September 2002

We compare laboratory measurements of wave propagation and attenuation in grease ice with two viscous fluid models. The first is the classic infinite depth, constant viscosity one‐layer model. The second is a new finite depth two‐layer model developed by Keller in which the upper layer (grease ice) is a constant viscosity, immiscible fluid overlying a denser but inviscid lower layer (seawater). In our comparison of these models with the laboratory results, we use the viscosity v as a free parameter and search for that v which gives a best fit to the data. The results of this calculation show that the two‐layer model, with a grease ice viscosity 2 − 3 × 104 that of seawater, provides the best agreement of data with theory. Because the 0.1 m thickness of our laboratory grease ice is comparable to thicknesses observed in the polar oceans, the Keller model with this viscosity value should be directly applicable to the field interaction of waves and grease ice.
Modeling tidal currents beneath Filchner‐Ronne Ice Shelf and on the adjacent continental shelf: Their effect on mixing and transport
Authors: Makinson Keith , Nicholls Keith W.
Published: September 2002

A depth‐averaged tidal model has been applied to the southern Weddell Sea. The model domain covers the southern continental shelf, including the ocean cavity beneath Filchner‐Ronne Ice Shelf. Reasonable agreement with the available current meter data has been achieved. Our results confirm that in areas with shallow water and large topographic gradients, tidal oscillations with peak velocities up to 1 m s−1 play a significant role in the vertical mixing and transport of water masses. The estimated energy dissipation beneath Filchner‐Ronne Ice Shelf due to surface friction is 25 GW, approximately 1% of the world's total tidal dissipation. Tidally induced Lagrangian residual currents converging at the ice front, an area of strong mixing, draw together water masses from the continental shelf and sub‐ice shelf cavity. The model indicates that Lagrangian residual currents have fluxes of up to 250,000 m3 s−1, and speeds of over 5 cm s−1 along the ice front, with over 350,000 m3 s−1 being exchanged between the sub‐ice shelf cavity and adjacent continental shelf. These currents are particularly efficient in ventilating the sub‐ice shelf cavity within 150 km of Ronne Ice Front. Such strong tidal mixing will significantly modify the properties of water masses that flow through this region, particularly to the west of Berkner Island. The model predictions indicate that tidal processes strongly influence the oceanographic conditions in the vicinity of Ronne Ice Front. Shipborne observations along the ice front support many of the model predictions concerning the effect of tides on the hydrography.
Heat budget of snow‐covered sea ice at North Pole 4
Authors: Jordan Rachel E. , Andreas Edgar L , Makshtas Aleksandr P.
Published: September 2002

The Russian drifting station North Pole 4 (NP‐4) was within 5° latitude of the North Pole from April 1956 to April 1957. We use a wide‐ranging set of snow and meteorological data collected at 3‐hourly intervals on NP‐4 during this period to investigate energy and mass transfer in the snow, sea ice, and atmospheric surface layer in the central Arctic. SNTHERM, a one‐dimensional energy and mass balance model, synthesizes these diverse NP‐4 data and thereby yields energetically consistent time series of the components of the surface heat budget. To parameterize the sensible heat flux during extremely stable stratification, we replace the usual log‐linear stability function with the “Dutch” formulation and introduce a windless coefficient in the bulk parameterization. This coefficient provides sensible heat transfer at the surface, even when the mean wind speed is near zero, and thereby prevents the surface temperature from falling to unrealistically low values, a common modeling problem when the stratification is very stable. Several other modifications to SNTHERM introduce procedures for creating a realistic snowpack that has continuously variable density and is subject to erosion and wind packing. The NP‐4 data provide for two distinct simulations: one on 2‐year ice and one on multiyear ice. We validate our modeling by comparing simulated and observed temperatures at various depths in the snow and sea ice. Simulations for both sites show the same tendencies. During the summer, the shortwave radiation is the main term in the surface heat budget. Shortwave radiation also penetrates into the snow and causes a subsurface temperature maximum that both the data and the model capture. During the winter, the net longwave balance is the main term in the surface heat budget. The snow and sea ice cool in response to longwave losses, but the flux of sensible heat from the air to the surface mitigates these losses and is thus nearly a mirror image of the emitted longwave flux.
Annual validation of significant wave heights of ERS‐1 synthetic aperture radar wave mode spectra using TOPEX/Poseidon and ERS‐1 altimeter data
Authors: Bauer Eva , Heimbach Patrick
Published: September 2002

Significant wave heights retrieved globally during 1994 from low bit rate imagette spectra of the synthetic aperture radar (SAR) operating in the intermittent wave mode (SWM) onboard the first European Remote Sensing (ERS‐1) satellite (Hsswm) are validated using independent, buoy‐validated satellite altimeter data from TOPEX/Poseidon (Hstop) and ERS‐1 (Hsers).Hsswmis retrieved using the extended inversion algorithm of the fully nonlinear wave‐to‐SAR spectral integral transform [Hasselmann et al., 1996]. The statistical comparison shows that globally retrievedHsswmagree remarkably well with collocated data of both altimeters. Histograms of globalHsswmare well approximated by the universal lognormal distribution function. A small but systematic underestimation ofHsswmwith respect to both altimeters by 0.1 m is noticed. This bias is small compared to the uncorrelated root‐mean‐square (rms) deviation of 0.5 m. The underestimation originates from underestimations of the high sea states dominated by wind sea. The entire range of swell waves ofHsswm, however, corresponds very closely toHstop. Thus the high accuracy of SWM‐retrieved swell wave heights substantiates their suitability for model validation and for operational wave data assimilation. To achieve also reliable estimates of the high sea states, the combined analysis of wave and wind data from both modeling and observation is the most promising approach.
Ventilation of Red Sea Water with respect to chlorofluorocarbons
Authors: Mecking Sabine , Warner Mark J.
Published: September 2002

Chlorofluorocarbon (CFC) measurements were made in the northwestern Indian Ocean in August‐September 1995 as part of the World Ocean Circulation Experiment Hydrographic Program Section I1. The data in the Gulf of Aden section from this expedition provide information on the mixing processes that affect Red Sea Water (RSW) as it flows from its source into the open northwestern Indian Ocean. Combining measurements by Rhein et al. [1997] with the outcome from a two‐end‐member mixing model results in an estimate of 65–70% saturation for the CFCs in the RSW end‐member at Bab el Mandeb. A boundary condition that may be more applicable to ocean models is given by the CFC content of RSW after its initial diapycnal descent along the continental slope in the western Gulf of Aden. Entrainment of water containing lower amounts of CFCs produces RSW that has CFC saturations of 20–50% once RSW has settled at its depth of neutral buoyancy. Apparent pCFC ages at Bab el Mandeb and after the separation of RSW from the bottom are 12–13 and 18–27 years, respectively. As RSW continues eastward through the Gulf of Aden as an isopycnal flow, horizontal eddy diffusion plays an important role in further diluting RSW, which is characterized by a maximum in salinity and a positive anomaly in CFC concentrations. Horizontal eddy diffusion parameters estimated from tracer conservation in a box model of the central and eastern Gulf of Aden range from 0.2 × 104 to 2.0 × 104 m2 s−1. As other observations show, eddy diffusion is presumably enhanced in this part of the ocean.
A note on the vertical scales of temperature and salinity and their signature in dynamic height in the western Pacific Ocean: Implications for data assimilation
Authors: Maes Christophe
Published: September 2002

The relationship between the vertical structures of ocean temperature and salinity and their concurrent signature in surface dynamic height anomalies are investigated along the 165°E transequatorial section in the western Pacific Ocean. The data are from conductivity‐temperature depth casts made during 28 oceanographic cruises mainly conducted during the 1984–1992 time period as part of the TOGA program. The analysis is based on empirical orthogonal functions that isolate the primary modes of variability, and some possible physical interpretations are suggested. The results show that the first six modes, explaining roughly 80% of the variance, display distinct vertical scales and exhibit coherent and observable signatures in dynamic height which reveal complex patterns in latitude and time. In addition, a method has been developed to reconstruct the temperature and salinity profiles from their signature in the surface dynamic height anomaly using the dominant EOFs. The method is new in the sense that no recourse is made to T–S relations. Residual errors in the vertical after reconstruction are lower than the intrinsic variability, and the method can successfully reproduce the variability at ENSO timescales. The residual error in dynamic height anomalies is lower than 1 dyn. cm in the equatorial belt, and of the order of 2–4 dyn. cm in the subtropics. The results demonstrate that sea level observations may be translate into temperature and salinity anomalies at depth. The implications for data assimilation in ocean models are discussed.
Current variability at the Pacific entrance of the Indonesian Throughflow
Authors: Kashino Yuji , Watanabe Hidetoshi , Herunadi Bambang , Aoyama Michio , Hartoyo Djoko
Published: September 2002

Current variability at the Pacific entrance of the Indonesian Throughflow is investigated using direct current and hydrographic measurements. Two moorings with three current meters (depths of 350, 550, and 1050 m) and one conductivity‐temperature‐depth profiler (260 m) were deployed at 4°1′N, 127°31′E and 3°11′N, 128°27′E between Talaud Islands and Morotai Island (Indonesia) from February 1994 to June 1995. Data from four hydrographic surveys conducted mainly between Mindanao and New Guinea from 1994 to 1996 are also used. The onset of a strong northwestward flow was observed at the southern mooring during boreal winter. In contrast, a southwestward flow containing salty South Pacific water was observed there during boreal summer. This current pattern change matched monsoon change around the mooring sites, suggesting that this variability is a seasonal signal in this region. This current change may occur because of the meridional shift of the Halmahera Eddy associated with an enlargement/diminishment of the Mindanao Dome. Our observation result during summer (the southwestward flow with the South Pacific water at the southern mooring) suggests that the Maluku Sea is one of the eastern routes of the Indonesian Throughflow. The current data also revealed that intraseasonal variability occurs in 50‐day oscillations. Because the coherence between wind variability in the tropics with a period of 40–50 days (Madden‐Julian Oscillation) and current variability with this period are >0.4, it is possible that the 50‐day oscillation in the ocean current is induced by wind variability associated with the Madden‐Julian Oscillation. The ocean eddy activity with an intrinsic period in this region may also be related to this 50‐day oscillation.
Eddies in the eastern Gulf of Alaska from TOPEX/POSEIDON altimetry
Authors: Meyers Steven D. , Basu Sujit
Published: September 2002

Eddy activity in the Gulf of Alaska (GOA) is evidenced to fluctuate with the El Niño‐Southern Oscillation (ENSO) cycle. Altimetry from TOPEX/POSEIDON is used to study eddy formation along a single descending track in the far eastern GOA from late 1992 through late 1998. The track is sampled between 45°N and 60°N, where it is roughly parallel to the eastern boundary. The geoid and mean currents are removed from the data through extraction of the temporal mean at each point. The resulting anomalies are passed through a matched filter designed to detect Gaussian signals embedded in noise. The filter is optimized to detect eddies with amplitude ≳10 cm and diameters roughly 50–350 km. Both anticyclonic and cyclonic structures are examined. The distribution of eddies in space is similar for both anticyclonic and cyclonic eddies, with most occurring between 50°N and the northern limits of the GOA. Few eddies of either sign were found south of 50°N. Anticyclonic eddies are stronger and have a broader distribution of amplitudes (typically 5–30 cm) compared to cyclonic features, which are more symmetrically distributed over 5–15 cm. The detection of weak (≃15 cm) eddies is sensitive to the filter threshold, but detection of larger‐amplitude eddies is robust. The average amplitude of anticyclonic eddies increases following the 1993, 1994/1995, and 1998 El Niño events. Eddy amplitude appears to vary with ENSO strength: the strongest eddies detected form in early 1998. These results support previous numerical studies of eddy development in the eastern GOA.