bluuebunny/crossref_metadata_2025 huggingface dataset:
Thrusting and exhumation around the margins of the western Tarim basin during the India‐Asia collision
Authors: Sobel Edward R. , Dumitru Trevor A.
Published: February 2004

The Cenozoic collision between India and Asia has deformed a large part of central Asia. To the north of Tibet around the margins of the western Tarim basin, major basin‐vergent thrusting has uplifted and exhumed thick Jurassic to Neogene sedimentary sections; this presumably reflects the propagation of collision‐induced deformation into the basin. Apatite fission track data from five sections involved in this thrusting record strong late Oligocene to middle Miocene exhumation and cooling. On the northwest margin of the basin on the piedmont of the Tian Shan, a section exhumed by thrusting yields an exhumation age of 13.6±2.2 Ma (±1σ). Four Miocene sandstones from a second section 40 km to the east yield detrital source area cooling ages which decrease upsection from 25.0±3.9 to 13.1±2.2 Ma. Landsat imagery suggests that probable sediment source areas were dominated by Neogene thrusting, so these ages likely record progressive unroofing in Tian Shan thrust systems. Deformed Miocene to Pleistocene strata indicate that thrusting has continued and propagated basinward up until the present. Previously published apatite data from the Junggar basin on the northern flank of the Tian Shan yield a similar age of 24.7±3.9 Ma. On the southwest margin of Tarim on the piedmont of the western Kunlun Shan, three sections yield cooling ages of 19.8±0.9 Ma, 20.0±3.1 Ma, and roughly 20 Ma. Farther south at Kudi, previous work has yielded apatite cooling ages of 17±2 Ma and a zircon cooling age of 22±2 Ma. These similar cooling ages over a ≈250 km long belt in the western Kunlun Shan are associated with the transpressional Kumtag fault and the Main Pamir Thrust (MPT). Geologic relations within the western Kunlun Shan suggest that the MPT‐Kumtag fault system offsets the originally linear trend of the Paleozoic‐early Mesozoic Kunlun arc system by 200–300 km, accommodating much of the Neogene northward indentation of the Pamir block. We propose that the ≈20 Ma ages slightly postdate the initiation of this indentation and consequent crustal thickening. Taken together, the Tian Shan and Kunlun Shan results indicate that crustal thickening, in part accommodated by strike‐slip faulting, became the dominant mode of deformation by ≈25–20 Ma in a large region extending from the Pamir and west Kunlun Shan north to the Tian Shan.
Mineralogy of the Martian interior up to core‐mantle boundary pressures
Authors: Bertka Constance M. , Fei Yingwei
Published: February 2004

In order to determine the mineralogy of the Martian interior along a high‐temperature areotherm, multianvil experiments have been performed with a model Martian mantle composition up to 23.5 GPa. The Dreibus and Wänke [1985] Martian mantle composition yields an upper mantle that consists of olivine + clinopyroxene + orthopyroxene + garnet at pressures up to 9 GPa. Above 9 GPa, orthopyroxene is no longer present. The transition zone is marked by the appearance of γ spinel at 13.5 GPa. Up to 15 GPa, clinopyroxene and majorite coexists with β phase and/or γ spinel. By 17 GPa, clinopyroxene is entirely replaced by majorite and the modal abundance of γ spinel increases at the expense of β phase. The dominant assemblage throughout most of the transition zone is γ spinel + majorite. Two experiments completed in the perovskite stability field indicate that the lower mantle consists of Mg‐Fe silicate‐perovskite, magnesiowüstite, and majorite. CaSiO3‐perovskite is not present in these experiments. Both the presence of a Martian lower mantle, i.e., an Mg‐Fe silicate‐perovskite bearing zone, and the phase assemblage stable in the Martian lower mantle are very sensitive to the temperature profile of the interior. A low‐temperature profile may stabilize stishovite in the lower mantle or it may lead to the absence of the lower mantle because of the higher transition pressure required for forming perovskite at lower temperatures. Regardless of the temperature profile assumed, the Martian upper mantle and transition zone will account for a larger proportion of the planet's interior than is the case for the Earth's interior because of the smaller size of Mars.
The transition from brittle faulting to cataclastic flow in porous sandstones: Mechanical deformation
Authors: Wong Teng‐fong , David Christian , Zhu Wenlu
Published: February 2004

Triaxial compression experiments were conducted to investigate the inelastic and failure behavior of six sandstones with porosities ranging from 15% to 35%. A broad range of effective pressures was used so that the transition in failure mode from brittle faulting to cataclastic flow could be observed. In the brittle faulting regime, shear‐induced dilation initiates in the prepeak stage at a stress level C' which increases with effective mean stress. Under elevated effective pressures, a sample fails by cataclastic flow. Strain hardening and shear‐enhanced compaction initiates at a stress level C* which decreases with increasing effective mean stress. The critical stresses C' and C* were marked by surges in acoustic emission. In the stress space, C* maps out an approximately elliptical yield envelope, in accordance with the critical state and cap models. Using plasticity theory, the flow rule associated with this yield envelope was used to predict porosity changes which are comparable to experimental data. In the brittle faulting regime the associated flow rule predicts dilatancy to increase with decreasing effective pressure in qualitative agreement with the experimental observations. The data were also compared with prediction of a nonassociative model on the onset of shear localization. Experimental data suggest that a quantitative measure of brittleness is provided by the grain crushing pressure (which decreases with increasing porosity and grain size). Geologic data on tectonic faulting in siliciclastic formations (of different porosity and grain size) are consistent with the laboratory observations.
Seismic moment‐frequency relation for shallow earthquakes: Regional comparison
Authors: Kagan Y. Y.
Published: February 2004

We determine the parameter values for the seismic moment‐frequency relation using Flinn‐Engdahl's regionalization of global seismicity and the Harvard centroid‐moment tensor data. The earthquake size distribution is approximated by the gamma law: a version of the Gutenberg‐Richter distribution with an exponential taper at the maximum moment. There is no statistically significant variation of the β value (the analog of the b value) for all seismic regions except for the midocean ridge systems. For the latter regions β=0.93, whereas for all other zones β=0.63. The maximum moment Mxg can be statistically evaluated only for subduction zones treated as a whole, Mxg = 1021 − 2 × 1022 Newton m, which corresponds to the worldwide Mxg value. For other regions, as well as for single subduction zones, Mxg is determined by comparing the number of events in each zone with the seismic moment rate calculated on the basis of the NUVEL‐1 model of plate motion. For subduction zones, we obtain the estimate of Mxg which agrees with the statistical value, giving evidence that most tectonic deformation is released by earthquakes. We test the hypothesis that no statistically significant variation in Mxg occurs in subduction and continent collision zones; the hypothesis cannot be rejected with available data. For the midocean ridges, the Mxg estimate cannot be unambiguously determined; it is quite possible that the estimate is significantly biased because only a small part of tectonic deformation at the ridges is released by earthquakes. These results have importance in evaluating seismic risk and may also lead to developing a physical theory for earthquake generation.
Increase of electrical conductivity with pressure as an indicator of conduction through a solid phase in midcrustal rocks
Authors: Shankland T. J. , Duba A. G. , Mathez E. A. , Peach C. L.
Published: February 2004

Rocks freshly cored from depth at the German continental scientific drilling site (KTB) offer an opportunity to study transport properties in relatively unaltered samples resembling material in situ. Electrical conductivity σ was measured to 250 MPa pressure, and room temperature on 1 M NaCl‐saturated amphibolites from 4 to 5 km depth. An unexpected feature was an increase of σ with pressure P that appeared (anisotropically) in most samples. To characterize this behavior, we fitted the linear portion of log σ versus P to obtain two parameters: the slope dlogσ/dP (of order 10−3 MPa−1) and the zero‐pressure intercept σ0. Samples of positive and negative slopes behave differently. Those having negative slopes show strong correlation of σ0 with a fluid property (permeability). This behavior indicates that fluids exert the dominant control on σ0 at low pressure when σ0 is greatest, which is typical behavior observed in previous studies. In contrast, samples with positive slopes lack a correlation of σ0 with permeability, indicating that fluids are less important to positive pressure behavior. Another result is that samples of negative dlogσ/dP have uncorrelated slopes and initial conductivities. In significant contrast, samples of positive slopes have the greatest P dependence for lowest initial conductivity σ0, that is, the less fluid, the more positive dlogσ/dP. Hence positive dlogσ/dP is consistent with reconnection of solid phases into a conductive texture better resembling that of rock at depth. Detailed examination of one sample by electron probe and scanning electron microscope reveals the presence of carbon on internal cleavage surfaces in amphibole, the most abundant mineral present. Thus carbon probably dominates the reconnection, but total σ still involves fluids as well as Fe‐Ti oxides. For the KTB location it is inferred that the reason mid to deep crustal electrical conductivities modeled from geophysical measurements are so much higher than conductivities of silicates is the presence of interconnected good conductors involving films of carbon on surfaces and other solid phases.
Velocity structure from forward modeling of the eastern ridge‐transform intersection area of the Clipperton Fracture Zone, East Pacific Rise
Authors: Begnaud Michael L. , McClain James S. , Barth Ginger A. , Orcutt John A. , Harding Alistair J.
Published: February 2004

In the spring of 1994, we undertook an extensive geophysical study of the Clipperton Fracture Zone (FZ) on the fast spreading East Pacific Rise. The Clipperton Area Seismic Study to Investigate Compensation experiment (CLASSIC) included surveys to examine the deep structures associated with the fracture zone and adjacent northern ridge segment. In this paper, we report the results from five seismic profiles acquired over the eastern ridge‐transform intersection (RTI), including profiles over the RTI high, the northern ridge segment, and the eastern transform region. The travel time data for crustal phases, Moho reflections, and mantle phases were modeled using two‐dimensional ray tracing. Seismic profiles reveal that the crust is similar in thickness north and south of the Clipperton FZ, despite differences in axial topography that have previously been interpreted in terms of differences in magma supply. When compared to older crust, the northern ridge axis is characterized by lower seismic velocities and higher attenuation. In our model, a low‐velocity zone exists beneath the ridge axis, probably associated with a zone of partial melt and/or very high temperatures. Within the transform zone, we find that the southeastern trough is underlain by nearly normal crustal structure. The crust is slightly thinner than the adjacent aseismic extension but not enough to compensate for the depths of the trough. Toward the RTI, the trough is replaced by an intersection high which appears underlain by a thickened crust, and a thicker upper crustal section. Both characteristics indicate that the intersection high is a volcanic feature produced by excess volcanism at the intersection. The volcanism acts to “fill in” the transform trough, creating the thicker crust that extends under the eastern aseismic extension of the transform. Our results show that the northern ridge segment, often identified as magma‐starved, displays the crustal thickness and apparent signal attenuation characteristic of a plentiful, but perhaps episodic, magma supply.
Three‐dimensional interpretation of the KTB gravity and magnetic anomalies
Authors: Bosum W. , Casten U. , Fieberg F. C. , Heyde I. , Soffel H. C.
Published: February 2004

A combination of a closely spaced surface gravity survey with a high‐resolution helicopter aeromagnetic survey as well as borehole gravity and magnetometer measurements allowed a detailed three‐dimensional (3‐D) modeling of the anomalies at the KTB drill site. The models could be constrained by new evidence from a 3‐D seismic survey and by structural geology and petrophysical data from drill cores and cuttings. The source body for the positive gravity anomaly consist of high‐density metabasite. The vertical derivative of the Bouguer anomaly does not resemble the aeromagnetic anomaly in all areas, indicating that parts of the metabasites are more or less nonmagnetic. Surprisingly and confirming the observation in other deep drill holes into continental crystalline basement rocks, pyrrhotite is the dominant magnetic mineral below a depth of about 300 m. Magnetite mainly occurs in the depth intervals 360–520 m and 7300–7900 m. The lower interval causes the anomalous vertical gradient of 60 nT/km for the geomagnetic field. The occurrence of strongly magnetic minerals in the borehole down to about 3000 m correlates with the lithology, while in the deeper parts it is more related to fissures and fault zones where chemical processes (reduction/oxidation) are active.
NLOADF: A program for computing ocean‐tide loading
Authors: Agnew Duncan Carr
Published: February 2004

The loading of the Earth by the ocean tides produces several kinds of signals which can be measured by geodetic technique. In order to compute these most accurately, a combination of global and local models of the ocean tides may be needed. The program NLOADF convolves the Green functions for loading with ocean tide models using a station‐centered grid with fixed dimensions, making it easy to combine different ocean models without overlap in the convolution. The program computes all the quantities of interest (gravity, displacement, tilt, and strain) and includes the case where measurements are made beneath the surface of the ocean.
Melange‐forming processes in the development of an accretionary prism: Evidence from fission track thermochronology
Authors: Hasebe Noriko , Tagami Takahiro , Nishimura Susumu
Published: February 2004

To give thermotectonic constraints on melange‐forming processes and to understand the significance of melange in the evolution of accretionary complexes, fission track analysis was carried out on samples collected from sandstone blocks in melange matrix and coherent sandstone sequences adjacent to melange units in the Cretaceous to Miocene Shimanto accretionary complex, southwest Japan, which is one of the typical accretionary complexes with an accumulation of geological information. Nineteen zircon ages and 15 track length distributions from melanges indicate with some local variation that materials with higher maximum paleotemperatures are exposed at the rear part of the prism, as is the case of the coherent units. Two apatite ages are contemporaneous with previously reported apatite ages of ∼10 Ma obtained from coherent units in the region, emphasizing the regional cooling through the apatite closure temperature of ∼100°C at that time. There is no evidence for substantial relative motion of the melanges with respect to the surrounding coherent units; melange formation is explained by processes that exclude large vertical transport (i.e., 5 km) of melange forming materials. These thermotectonic characteristics observed in the Shimanto accretionary complex are maintained during subsequent local internal deformation caused by the successive growth of the wedge.
Geodetic measurements of horizontal strain near the White Wolf fault, Kern County, California, 1926–1993
Authors: Bawden Gerald W. , Donnellan Andrea , Kellogg Louise H. , Dong Danan , Rundle John B.
Published: February 2004

The White Wolf fault, located north of the Big Bend segment of the San Andreas fault, is the NE‐SW trending, left lateral‐oblique reverse fault responsible for the Ms=7.8 1952 Kern County earthquake. We combined Global Positioning System (GPS) measurements with historical triangulation and trilateration data to determine changes in the strain rate over 7 decades (1926–1993). We reanalyzed the historical geodetic data and calculated an elevated preseismic (1926–1952) maximum shear strain rate
of 0.62±0.16 μstrain/yr across the White Wolf fault. The maximum shear strain rate decreased with distance toward the Garlock fault to 0.09±0.08 μstrain/yr. In the decade following the earthquake (1952–1963), the near fault
was high (0.85±0.23μstrain/yr), and decreased to 0.23±0.13 μstrain/yr across the Garlock fault. In 1993, we resurveyed many of the same monuments with GPS receivers to estimate fault‐crossing and off‐fault strain rates for the preceding 30 years. Across the White Wolf fault, the maximum shear strain rate
dropped to 0.19±0.07 μstrain/yr. The azimuths of the maximum principal strain rates (ϕ) for the 1963–1993 epoch rotate from a fault normal orientation (−57°±15°) across the White Wolf fault to 11°±3°E across the Garlock fault.
Across‐arc variations of isotope and trace element compositions from Quaternary basaltic volcanic rocks in northeastern Japan: Implications for interaction between subducted oceanic slab and mantle wedge
Authors: Shibata Tomoyuki , Nakamura Eizo
Published: February 2004

Isotopic compositions of Pb, Sr, and Nd and concentrations of trace elements were determined for Quaternary island arc basaltic rocks from northeastern Japan. Sr and Pb isotopic ratios decrease, and Nd isotopic ratios increase from the volcanic front toward the back arc. The isotopic compositions nearest the back arc side are nearly identical to those of mid‐ocean ridge basalt (MORB). The high field strength elements and heavy rare earth elements show homogeneous and MORB‐like characteristics. These observations indicate that the mantle wedge beneath northeastern Japan originally had a MORB‐type mantle composition that was homogeneous across the arc. Pb isotope compositions show a mixing relationship between mantle wedge and oceanic sediments reflecting the introduction of subduction component into the mantle wedge, Across‐arc isotopic variations were caused by interaction between MORB‐type mantle wedge and the subducting slab, and the amount of subduction component correlates with the depth to the slab. The isotopic compositions of subduction component are expressed by bulk mixing of 15 wt % of oceanic sediment and 85 wt % of altered MORB. Inversion analyses of isotopic compositions using two‐component mixing relationships show that the Sr/Nd and Pb/Nd ratios in subduction component decrease with increasing depth to the slab, while the Sr/Pb ratio is nearly constant. These changes can be explained only by a preferential discharge of the elements into the wedge mantle associated with continuous dehydration of the subducting slab. The present study further demonstrates that a very wide range of isotopic and elemental compositions in island arc magmas is a consequence of the interaction between subducting slab and mantle wedge without the involvement of an oceanic island basalts component, and the slab can carry water and supply a subduction component as a fluid to the overlying mantle wedge to depths exceeding 150 km.
Details of stress directions in the Alaska subduction zone from fault plane solutions
Authors: Lu Zhong , Wyss Max , Pulpan Hans
Published: February 2004

We used the cumulative misfit method to divide a data set with heterogeneous stress orientations into subsets with homogeneous stress directions. We propose that slope changes of the cumulative misfit as a function of earthquake number pinpoint the boundaries of homogeneous stresses. Using a synthetic data set of 50 fault plane solutions, composed of two halves corresponding to two incompatible stress tensors, we tested the validity and efficacy of the cumulative misfit method. Our acceptance criteria for results of stress tensor inversions are (1) the directions must be well constrained, that is, the 95% confidence regions of the greatest and least principal stresses do not overlap and (2) there must be evidence for homogeneity in the sample, that is, the average misfit of the inversion is less than 6°. We estimated stress directions in the Alaska Wadati‐Benioff Zone (WBZ) using 470 fault plane solutions, which we determined for earthquakes with ML∼3, and 67 published fault plane solutions for earthquakes with Ms∼5. We succeeded in subdividing the data set of the small and large earthquakes into 25 and 3 subsets, with the average misfit ranging from 3.2° to 5.5°. The average misfits of the subsets we accept as satisfying the assumption of homogeneity are smaller than the average misfit of the overall data set (F∼10°) by factors of 2 to 3. We estimated the stress fields at two scales along the Alaska WBZ. The stress directions measured by the large earthquakes (Ms∼5) were homogeneous over large volumes, with extension downdip and the direction of greatest compression along strike. This unusual orientation of the greatest principal stress is attributed to the bend of the slab under central Alaska, which generates compressive stresses along strike. The stress orientations revealed by small earthquakes, in contrast, exhibited a great deal of heterogeneity as a function of space, although their general trend confirms the overall stress directions obtained from the large events. We propose that the ratio of the dimensions of the stress field sensed by earthquakes to the rupture dimensions is about 20 to 50. The estimated stress directions of the crustal earthquakes corresponded to the following mechanisms: (1) strike‐slip faulting with the greatest principal stress oriented N‐S near Fairbanks and (2) thrusting with the greatest principal stress oriented NW‐SE near Mount McKinley.
Spectral analyses of the KTB sonic and density logs using robust nonparametric methods
Authors: Jones Alan G. , Holliger Klaus
Published: February 2004

We use robust techniques to estimate power spectra, coherences and transfer functions of the German Continental Deep Drilling Program (KTB) sonic and density logs and lithologically defined subsets thereof. Our results confirm the overall 1/wavelength‐decay of the power spectra inferred by parametric analyses, but provide superior resolution and nonparametric estimates of errors and statistical significance. We demonstrate the absence of any statistically meaningful coherence between the velocity logs from the main and pilot holes, suggesting a spatially quasi‐isotropic upper crustal velocity structure. Also, there is little coherence between the physical and caliper logs, indicating that disturbances introduced by breakouts and uneven relief of the borehole wall mostly contribute to the uncorrelated portions of the velocity logs. Coherence between the gamma and physical logs is weak to absent, indicating that the observed velocity and density fluctuations are dominated by the physical state of the rocks rather than by their petrological composition. Attempts to derive Poisson's ratio, and its variation with wavelength, from the relationship between the shear and compressional velocity logs met with limited success, but imply that caution should be exercised when comparing Poisson's ratio derived from laboratory studies on samples representative of a region to crustal‐scale seismic determinations. Our preferred interpretation is that fluctuations in the physical logs in the intermediate wavelength range (∼10–150 m) are dominated by cracks and their level of fluid saturation. At larger wavelengths (>50–150 m) the effects of the petrology becomes more significant as shown by changes in slope of the power spectra and the emerging coherence between the Vp and the gamma logs.
P and S deep velocity structure of the Hellenic area obtained by robust nonlinear inversion of travel times
Authors: Papazachos C. , Nolet G.
Published: February 2004

We derived a new model for the P and S velocity structure of the lithosphere in the subduction area of the Hellenic arc and the Aegean Sea from the inversion of travel times of local events. The inversion technique applied is nonlinear, since three‐dimensional ray tracing is incorporated. At the same time, an appropriate preconditioning of the final linearized system is used in order to reduce ray density effects on the results. The P‐S coherency is controlled by an additional damping of the VP/VS ratio. The study focuses mainly on the structure of the Hellenic subduction in the southern Aegean region. Interesting features and details of the subducted slab can be recognized in the final tomographic images. On the western part, the shallower section of the slab is dipping at a very small angle (≃10°). This changes to approximately 25° for the deeper part of the subduction, resulting in a prominent kink at a depth of about 70 km, which is in accordance with the general characteristics of the associated Benioff zone. The smaller eastern part of the slab is not resolved with the same detail, but a clearly steeper subduction zone can be recognized. Moreover, detailed information about the crustal thickness variations are inferred from the velocity structure and correlate very well with the existing results from refraction experiments. Finally, the results confirm the recent suggestion concerning the existence of a low‐velocity crustal layer at shallow depths (≃10–15 km) in the accretionary prism of the Alpine belt.
Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)
Authors: De Astis Gianfilippo , La Volpe Luigi , Peccerillo Angelo , Civetta Lucia
Published: February 2004

Petrological and geochemical data are reported for volcanic rocks from Vulcano island. The subaerial volcanism (120 ka to present) built up a NW‐SE elongated composite structure, affected by two intersecting multistage calderas. Volcanics older than 20 ka consist mostly of high‐K calc‐alkaline (HKCA) to shoshonitic (SHO) mafic rocks. These magmas interacted significantly with the continental crust, which generated variable Sr isotopic ratios (0.70412–0.70520). However, a major role was also played by input of parental liquids into the magma chamber, which prevented further evolution of the magmas. HKCA, SHO, and potassic (KS) rocks formed from 20 to 8 ka, display a much larger range of SiO2 (from shoshonites to rhyolites) and higher concentrations of incompatible elements with respect to the previous stage. Sr isotopic ratios show small variations (0.70448–0.70486). Mixing of silicic and mafic liquids and fractional crystallization processes (FC) were the main evolutionary processes during this stage. Volcanics younger than 8 ka consist of SHO and leucite‐bearing KS mafic rocks, with abundant intermediate and silicic products. Mafic and intermediate rocks display similar incompatible element abundances and Sr isotopic ratios as the previous stage volcanics, whereas higher 87Sr/86Sr (0.70494–0.70583) are observed in some rhyolites. These products originated from a complex interplay of FC, crustal assimilation, and magma mixing processes. The most mafic rocks show increasing incompatible element abundances, Rb/Sr, Rb/Ba, Mg/Al, Mg/Ca, and a decrease in large ion lithophile to high field strength element ratios, passing from older HKCA‐SHO to the younger SHO‐KS volcanics. These variations suggest a shifting of magma sources from a slightly metasomatized asthenosphere (fertile peridotite) to a more strongly metasomatized lithospheric mantle (residual peridotite). Time‐related petrological and geochemical variations have been used to develop a model for the evolution of the Vulcano plumbing system.
Global Positioning System measurements of present‐day crustal movements in the Arabia‐Africa‐Eurasia plate collision zone
Authors: Reilinger R. E. , McClusky S. C. , Oral M. B. , King R. W. , Toksoz M. N. , Barka A. A. , Kinik I. , Lenk O. , Sanli I.
Published: February 2004

We present and interpret Global Positioning System (GPS) measurements of crustal motions for the period 1988–1994 at 54 sites extending east‐west from the Caucasus mountains of southern Russia, Georgia, and Armenia to the Aegean coast of Turkey and north‐south from the southern edge of the Eurasian plate (Pontus block) to the northern edge of the Arabian platform. Viewed from a Eurasia‐fixed reference frame, sites on the northern Arabian platform move N38±13°W at 20±3 mm/yr, roughly consistent with the velocity implied by NUVEL 1A circuit closure (N23±7°W at 24±2 mm/yr). The motion of Arabia appears to be transferred directly to the region of Turkey north of the suture. However, eastern Turkey is characterized by distributed deformation while central/western Turkey is characterized by coherent plate motion involving westward displacement and counterclockwise rotation of the Anatolian plate. Internal deformation within the central part of the Anatolian plate is less than 2 mm/yr. The Anatolian plate is decoupled from Eurasia along the right‐lateral, strike‐slip North Anatolian fault (NAF). This different response in eastern and western Turkey to the collision of Arabia may result from the different boundary conditions, the Hellenic arc forming a “free” boundary to the west and the Asian continent and oceanic lithosphere of the Black and Caspian Seas forming a resistant boundary to the north and east. We derive a best fitting Euler vector for Anatolia‐Eurasia motion of 29.2±0.8°N, 32.9±0.4°, 1.3±0.1°/m.y. The mapped surface trace of the NAF corresponds well to a small circle about this pole. The new Euler vector implies an upper bound for NAF slip rate of 30±2 mm/yr (i.e., assuming all relative motion is accommodated along the NAF). Using the NUVEL 1A Euler vector for Arabia‐Eurasia and the GPS Euler vector for Anatolia‐Eurasia, we determine an Arabia‐Anatolia Euler vector of 31±2°N, 45±2°E, 0.9±0.1 °/m.y. and an upper bound on the East Anatolian fault slip rate of 15±3 mm/yr. The Aegean Trough region of western Turkey deviates significantly from coherent plate rotation. In addition to rotating with Anatolia, this region shows roughly N‐S extension at a rate of 14±5 mm/yr. Taken together with satellite laser ranging results along the Hellenic arc, the contemporary pattern of deformation indicates increasing motions toward the arc, suggesting that the westward displacement and counterclockwise rotation of Anatolia is driven both by “pushing” from the Arabian plate and by “pulling” or basal drag associated with the foundering African plate along the Hellenic subduction zone.
Brittle failure of columnar saline ice under triaxial compression
Authors: Gratz E. T. , Schulson E. M.
Published: February 2004

The brittle compressive failure of cubes of columnar saline ice was studied under triaxial (proportional) loading where the ratio of normal stresses (σ11:σ22:σ33:) was varied in a systematic manner using a true multiaxial loading system. The strain rate in the direction of the greatest applied compressive stress was 6×10−3 s−1 and the temperature was −10°C. The failure surface is faceted and reveals three regimes: (1) of lower across‐column confinement where the along‐column confinement has no significant effect on the major across‐column stress at failure, but where the minor across‐column stress raises the strength; (2) of higher across‐column confinement where along‐column confinement now raises the major across‐column stress at failure; and (3) of predominantly along‐column loading where the along‐column failure stress increases in proportion to the smaller of the two across‐column confining stresses. Each regime shows a high sensitivity of strength to the appropriate confining stress. The behavior is explained in terms of the frictional crack sliding‐wing crack mechanism of brittle compressive failure.
Estimating tectonic stress rate and state with Landers aftershocks
Authors: Gross Susanna , Kisslinger Carl
Published: February 2004

A new method of analyzing the location and timing of aftershocks that makes possible estimates of the rate at which faults in the area are being loaded with stress has been developed. This new method is an application of a theory of aftershocks proposed by Dieterich [1994]. The stress step from the mainshock triggers the aftershocks, which release energy stored by tectonic loading and the mainshock, and the tectonic stressing rate is responsible for the background activity. Thus a comparison of the number of aftershocks with the rate of background activity as a function of stress step can provide an estimate of tectonic stress rate. This method requires analysis of a catalog which includes background activity as well as aftershocks. The method has been applied to the aftershocks of the 1992 Landers, California, earthquake. The tectonic stress rate estimated for the Mojave block is ≈1–2 Pa/d, which corresponds to a repeat time for large events of ≈14,000 years, and suggests that much of the observed strain in the Mojave is anelastic. The southern San Andreas fault system is found to have a stress fate of ≈100 Pa/d, which corresponds to a repeat time of ≈270 years and is consistent with primarily elastic strain accommodation. Inversions for the best fitting background stress state are consistent with fits of focal mechanisms and support an effective friction coefficient near 0.6. Fits to the aftershock decay provide estimates of the Dieterich friction parameter AD ranging from ≈0.02, if lithostatic pore fluids are present, to ≈0.0003 if they are not.
Integrated log interpretation in the German Continental Deep Drilling Program: Lithology, porosity, and fracture zones
Authors: Pechnig Renate , Haverkamp Susanne , Wohlenberg Jürgen , Zimmermann Günter , Burkhardt Hans
Published: February 2004

Well logs, aquired in the two scientific drill holes of the German Continental Deep Drilling Program (KTB), provide continuous records of physical and chemical data of the metamorphic rocks penetrated. The 4‐km‐deep pilot hole was almost completely cored, enabling the well logs to be calibrated with regard to rock composition and structural features derived from laboratory analysis of cores. The observed relationships were transferred to the 9101 m deep, nearly uncored, main hole to reproduce in detail the lithology and to estimate physical properties from the logs. Synthetic lithological profiles were constructed for the pilot hole and the main hole by applying the electrofacies concept adapted to the crystalline environment. These profiles provide information on lithostratigraphy, alteration, cataclastic overprint, and petrogenetic features. Cross‐hole correlations of these profiles reveal identical rock sequences for large sections of the drilled, metamorphic basement in both holes, in which the primary differences between the protoliths are largely preserved. Multivariate statistical methods were used to determine porosity depth functions from log responses. Linear as well as multilinear regression yielded continuous porosity profiles for both boreholes. Factor analysis was used to extract a parameter interpreted as a fluid and fracture indicator. Comparison of the porosity profiles with lithological information from log, core, and cuttings data revealed two different origins of increased porosity. Rock porosity and permeability are not only related to discrete planar discontinuities such as faults and fractures but also to more extensive zones of intense rock alteration where considerable matrix porosity occurs.
Buoyant mantle upwelling and crustal production at oceanic spreading centers: On‐axis segmentation and off‐axis melting
Authors: Barnouin‐Jha Kopal , Parmentier E. M. , Sparks David W.
Published: February 2004

Numerical experiments are used to examine the effect of buoyant mantle flow on melt generation near spreading centers. Buoyancy results from (1) the depletion of residual mantle in Fe relative to Mg (mantle‐depletion buoyancy), (2) the presence of low‐density melt (melt‐retention buoyancy), and (3) thermal expansion (thermal buoyancy). Thermal buoyancy drives off‐axis convective rolls that develop closer to the spreading axis at slower spreading rates. Melt‐retention buoyancy drives focused upwelling into isolated melting centers. With increasing spreading rate, the amplitude of on‐axis crustal segmentation decreases, the melting region being essentially two‐dimensional at full spreading rates greater than 100 km/m.y. These numerical experiments predict centers of mantle upwelling that are more widely spaced than the actual wavelength of inferred crustal thickness variations. This discrepancy indicates that short wavelength segmentation of slow spreading centers requires some process not included in our models of mantle flow. At fast spreading rates, when the lithosphere is thinner, melt‐retention buoyancy gives rise to isolated, focused centers of off‐axis upwelling and melt production. Off‐axis melting centers begin to form at about 300 km from the axis, with a characteristic along‐axis spacing of 200 km. Successive off‐axis melting centers are shifted along axis, so that crustal thickness far from the axis is increased at a 100 km wavelength.
Characterizing the Mid‐Faeroe Ridge using seismic velocity measurements
Authors: Hughes Stephen , Barton P. J. , Harrison D. J.
Published: February 2004

The character of the basement and overlying sedimentary sequences in the vicinity of the Mid‐Faeroe Ridge is investigated through a synthesis of seismic velocity measurements. A finite difference tomographic algorithm was applied to a wide‐aperture seismic profile acquired across the center of the Faeroe Basin to produce a velocity model free from a priori artifacts. A series of parameterizations and smoothing constraints were run to assess the uniqueness of the final model. Uncertainties in the velocity structure were estimated at ±5%. Comparison of the in situ velocity of the Mid‐Faeroe Ridge with deterministic lithology‐velocity data extracted from nearby boreholes strongly implies that Tertiary to Cretaceous shale and sandstone sequences extend to depths of 4.5 km. The occurrence of pre‐Cretaceous sedimentary lithologies at 4.5–8.0 km depth is supported by in situ velocities of 4.0–4.5 km/s which are best explained by well‐consolidated sediments and are incompatible with likely constituents of the basement. Evidence for a major structural discordance across the southeastern flank of the Mid‐Faeroe Ridge is provided by a prominent fault whose location demarcates a lateral velocity variation of ∼0.5 km/s. A rapid increase of the in situ velocity at 8 km depth is strongly suggestive of a transition into the basement, whose velocity (5.5 km/s) is comparable with high‐pressure laboratory measurements of basalts from the Faeroe Islands. This basaltic basement in the center of the Faeroe Basin lies above a Moho depth of 18±3 km which suggests cumulative stretching factors of about 3.0 and implies substantial igneous activity. We conclude that in the region of the Mid‐Faeroe Ridge the prerift (Lewisian?) basement has been highly attenuated, pervasively intruded, and possibly wholly replaced by basaltic material.
Precise point positioning for the efficient and robust analysis of GPS data from large networks
Authors: Zumberge J. F. , Heflin M. B. , Jefferson D. C. , Watkins M. M. , Webb F. H.
Published: February 2004

Networks of dozens to hundreds of permanently operating precision Global Positioning System (GPS) receivers are emerging at spatial scales that range from 100to 103km. To keep the computational burden associated with the analysis of such data economically feasible, one approach is to first determine precise GPS satellite positions and clock corrections from a globally distributed network of GPS receivers. Then, data from the local network are analyzed by estimating receiver‐specific parameters with receiver‐specific data; satellite parameters are held fixed at their values determined in the global solution. This “precise point positioning” allows analysis of data from hundreds to thousands of sites every day with 40‐Mflop computers, with results comparable in quality to the simultaneous analysis of all data. The reference frames for the global and network solutions can be free of distortion imposed by erroneous fiducial constraints on any sites.
Application of the LI‐COR CO2 analyzer to volcanic plumes: A case study, volcán Popocatépetl, Mexico, June 7 and 10, 1995
Authors: Gerlach T. M. , Delgado H. , McGee K. A. , Doukas M. P. , Venegas J. J. , Cárdenas L.
Published: February 2004

Volcanic CO2 emission rate data are sparse despite their potential importance for constraining the role of magma degassing in the biogeochemical cycle of carbon and for assessing volcanic hazards. We used a LI‐COR CO2 analyzer to determine volcanic CO2 emission rates by airborne measurements in volcanic plumes at Popocatépetl volcano on June 7 and 10, 1995. LI‐COR sample paths of ∼72 m, compared with ∼1 km for the analyzer customarily used, together with fast Fourier transforms to remove instrument noise from raw data greatly improve resolution of volcanic CO2 anomalies. Parametric models fit to background CO2 provide a statistical tool for distinguishing volcanic from ambient CO2. Global Positioning System referenced flight traverses provide vastly improved data on the shape, coherence, and spatial distribution of volcanic CO2 in plume cross sections and contrast markedly with previous results based on traverse stacking. The continuous escape of CO2 and SO2 from Popocatépetl was fundamentally noneruptive and represented quiescent magma degassing from the top of a magma chamber ∼5 km deep. The average CO2 emission rate for January‐June 1995 is estimated to be at least 6400 t d−1, one of the highest determined for a quiescently degassing volcano, although correction for downwind dispersion effects on volcanic CO2 indicates a higher rate of ∼9000 t d−1. Analysis of random errors indicates emission rates have 95% confidence intervals of ∼±20%, with uncertainty contributed mostly by wind speed variance, although the variance of plume cross‐sectional areas during traversing is poorly constrained and possibly significant.
High μ (HIMU) ocean island basalts in southern Polynesia: New evidence for whole mantle scale recycling of subducted oceanic crust
Authors: Kogiso Tetsu , Tatsumi Yoshiyuki , Shimoda Gen , Barsczus Hans G.
Published: February 2004

Major elements, trace elements, and Pb isotopic compositions were determined for ocean island basalts (OIBs) from Polynesia in the southern Pacific in order to document the chemical characteristics of OIB sources and to understand their origin. High μ (HIMU: μ=238U/204Pb) basalts, which have distinctly high Pb isotopic ratios, have systematically different compositions from non‐HIMU basalts; HIMU basalts are more enriched in Fe2O3*, MnO, and CaO and more depleted in SiO2, K2O, P2O5, Ni and incompatible trace elements than non‐HIMU, except for Nb. Major element characteristics of HIMU basalts suggest that the HIMU source is more fertile, i.e., more enriched in a basaltic component, than non‐HIMU sources. This is consistent with the suggestion that subducted oceanic crust may contribute to the formation of the HIMU reservoir. Relative depletion of incompatible trace elements in HIMU is consistent with involvement of sedimentary components in non‐HIMU sources. However, enrichment of Nb relative to other incompatible elements in HIMU cannot be explained by simple addition of the crustal component nor partial melting processes in the upper mantle, implying that lower mantle processes may contribute to the formation of the HIMU source.
Distribution of porosity in a section of upper oceanic crust exposed in the Troodos Ophiolite
Authors: Gillis Kathryn M. , Sapp Kimberly
Published: February 2004

The porosity structure of a section of old oceanic crust was characterized by a detailed study of the volcanic sequence of the Troodos Ophiolite. The distribution and abundance of porosity was compared to the amount of secondary minerals in order to calculate how porosity created by crustal accretion is modified by alteration as a section of crust ages. Eight sites with one dominant lithology and contrasting alteration histories were studied. Two types of porosity were quantified: macroscopic (macrofractures, vesicles/vugs, ± interpillow zones) and total (macroscopic and laboratory‐measured microscopic porosity). Crustal accretion produced a vertically zoned porosity structure due to differences between pillows that formed during the main and waning stages of crustal construction. Mean initial macroscopic porosity ranges from 14 to 17% in uppermost pillows and from 6 to 10% in the underlying pillows and massive flows. Macroscopic porosity was reduced by 10–11% where alteration was most pervasive (i.e., ≤250 m of the paleoseafloor). Beneath this zone, pillow and flow outcrops showed decreases of <1–3% and 2–5%, respectively. The combined effects of volcanic morphology and alteration resulted in no systematic variation with depth in the final macroscopic porosity (4–8%). The greatest decrease in macroscopic porosity was localized where abyssal hill topography or volcanic edifices and sedimentation rates facilitated the growth of the seafloor weathering zone, which comprises ∼10% of the volcanic pile. Thus in modern ocean basins alteration would have the greatest effect on crustal porosity in areas of rough topography, where crests of abyssal hills remain unsedimented for long periods of time. Porosity data derived from Deep Sea Drilling Project/Ocean Drilling Program cores and borehole logs show similar relationships, which implies that the evolution of upper crustal porosity is controlled by the local environment and that the evolution of upper crustal porosity cannot be uniquely described by seismic data.
Evidence from Ocean Drilling Program Leg 149 mafic igneous rocks for oceanic crust in the Iberia Abyssal Plain ocean‐continent transition zone
Authors: Seifert Karl E. , Chang Cheng‐Wen , Brunotte Dale A.
Published: February 2004

Leg 149 of the Ocean Drilling Program explored the ocean‐continent transition (OCT) on the Iberia Abyssal Plain and its role in the opening of the Atlantic Ocean approximately 130 Ma. Mafic igneous rocks recovered from Holes 899B and 900A have Mid‐Ocean Ridge Basalt (MORB) trace element and isotopic characteristics indicating that a spreading center was active during the opening of the Iberia Abyssal Plain OCT. The Hole 899B weathered basalt and diabase clasts have transitional to enriched MORB rare earth element characteristics, and the Hole 900A metamorphosed gabbros have MORB initial epsilon Nd values between +6 and +11. During the opening event the Iberia Abyssal Plain OCT is envisioned to have resembled the central and northern parts of the present Red Sea with localized spreading centers and magma chambers producing localized patches of MORB mafic rocks. The lack of a normal ocean floor magnetic anomaly pattern in the Iberia Abyssal Plain means that a continuous spreading center similar to that observed in the present southern Red Sea was not formed before spreading ceased in the Iberia Abyssal Plain OCT and jumped to the present Mid‐Atlantic Ridge.
Introduction to Special Section: The KTB Deep Drill Hole
Authors: Haak Volker , Jones Alan G.
Published: February 2004

When we pose the question “What do we want to know about the Earth?”, the answer is “Everything!” We are demanding, we must be demanding. Our life, and the very survival of our species, depends on our understanding of the Earth. As long as we do not know when earthquakes will happen, when volcanoes will erupt, and how water circulates or energy is stored or minerals are concentrated, our life is a matter of chance. Fortunately, we are curious creatures, and our curiousity leads us to observe, measure, and interpret our surroundings.