bluuebunny/crossref_metadata_2025 huggingface dataset:
Paleogeographic and tectonic evolution of southern Tibet since Middle Cretaceous time: NEw paleomagnetic data and synthesis
Authors: Achache José , Courtillot Vincent , Xiu Zhou Yao
Published: February 2008

During the second French‐Chinese paleomagnetic campaign in Tibet, in the summer of 1981, we sampled the middle Cretaceous red beds of the Takena formation and the Cenozoic volcanics of the Lingzizong formation. A detailed analysis of about 400 specimens of red beds reveals a stable component of normal magnetization N in the north of the Lhasa block (north of the Nyainqentanglha range) and two stable components in the south (LT, HT) where the red beds are more strongly folded and are unconformably overlain by the Lingzizong volcanics. All three components are carried by hematite. Blocking temperatures range from 400° to 600° C for LT and up to the Néel temperature of hematite for both N and HT. LT is shown to postdate part of the folding of the Takena, whereas N and HT both indicate a positive fold test at the 99% probability level. In addition, a small (less than 10% of the natural remanent magnetization) antiparallel component is superimposed in most samples and may be consistent with a model of long‐term acquisition of a chemical remanent magnetization for the red beds. Analysis of the volcanics reveals a single stable component with mixed polarities, carried either by hematite or magnetite. The paleomagnetic directions indicate that (1) the Lhasa block stood at 12.5°±3° N by Upper Cretaceous and at 13.5°±6.5° N in the Paleocene‐Eocene, thus confirming the limited displacement of the margin of Eurasia between Albian‐Aptian (deposition of the red beds) and Eocene (onset of the India/Asia collision). These values imply crustal shortening by as much as 1900±850 km north of the block (since the Upper Cretaceous) and by 1400±1200 km south of the block (since the onset of the collision), (2) Southern Tibet formed a single tectonic unit by middle Cretaceous time, and (3) differential rotations are observed within the block and are related to the collision. These rotations are discussed in the frame of a recent propagating extrusion tectonics model. This study confirms the close paleogeographic relationship between Indochina (Khorat plateau) and Southern Tibet prior to the India/Asia collision.
Paleomagnetism of Line Islands Seamounts: Evidence for Late Cretaceous and Early Tertiary volcanism
Authors: Sager W. W. , Keating B. H.
Published: February 2008

Seamount magnetic anomaly studies have been conducted on eight volcanoes in the Line Islands seamount chain. The magnetic anomalies of these seamounts were inverted to obtain paleomagnetic information. Four of these gave paleomagnetic pole positions consistent with other Pacific Cretaceous pole positions. Of these, three have reliable radiometric ages ranging from 72 to 85 Ma. The remaining four seamounts gave geomagnetic poles that agree with other Eocene‐Early Oligocene Pacific paleomagnetic data, and one has a reliable radiometric age of 39 Ma. All of the apparently Eocene volcanoes occur in the central Line Islands between 8°N and 3°S. We suggest that the Eocene volcanic event was approximately coincident with the profound change in Pacific plate motion, recorded by the bend in the Hawaiian‐Emperor seamount chain, that occurred at approximately 43 Ma. This volcanism may have resulted from the large change in intraplate stress accompanying the change of plate motion. The change in stress might have pulled apart a zone of weakness caused by the original formation of the Line Islands. Detailed paleomagnetic modeling of the Line Islands seamounts was necessary to obtain good results, as seven of the eight seamounts appear to be inhomogeneously magnetized to some extent. Despite these inhomogeneities the high goodness‐of‐fit parameters and the excellent agreement of these results with other Pacific paleomagnetic data suggest that the paleopoles derived from these seamounts are reliably determined.
Lithospheric flexure and thermal anomalies
Authors: McNutt Marcia K.
Published: February 2008

Plots of effective elastic thickness, Te, of the oceanic lithosphere as a function of age when elastically deformed show a general increase in Te with the square root of age, although there is much scatter. Some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks, which makes Te for more sharply bent plates underestimate the true depth Tm to the rheological boundary at the base of the high strength mechanical lithosphere. After converting effective values Te to true thicknesses Tm, it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones. From several other lines of evidence, such as depth anomalies, seamount subsidence, and heat flow data, it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere. The reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature. This dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere. Since the mechanical plate thickness depends principally upon the temperature at one depth, specifically the depth at which the temperature reaches 550–600°C (depending on strain rate), flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating, compared to quantities dependent on depth integrals of temperature, such as swell height and subsidence. The thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first ∼10 Ma after loading if initially reheating is confined to depths close to and below the 550°C isotherm. The flexural data combined with depth anomaly information for Hawaii point to extremely rapid reheating localized in the lower lithosphere. Rejuvenation in the Society Islands, on the other hand, is compatible with slower methods of heat injection involving the entire lithospheric column.
Geotectonic significance of gneissic amphibolites from the Vema Fracture Zone, equatorial Mid‐Atlantic Ridge
Authors: Honnorez Jose , Mével Catherine , Montigny Raymond
Published: February 2008

A large collection of gneissic amphibolites was recovered by two close dredge hauls from the deepest part of the north facing slope of the transverse ridge forming the south wall of the Vema Fracture Zone. Serpentinites and various types of gabbroic rocks ranging from undeformed slightly uralitized gabbros and norites to flaser and mylonitic gabbros were associated with the gneissic amphibolites (in the shallowest of the dredge hauls). Petrological studies indicate that the amphibolites were derived from similar gabbroic rocks that reequilibrated under stress in the conditions of the amphibolite facies. On the other hand, the associated meta‐gabbros display sequences of secondary minerals, indicating complex cooling and cataclastic histories without reaching metamorphic equilibrium. We tentatively suggest that the gneissic amphibolites and associated metagabbros formed in a vertical shear zone generated in oceanic layer 3 by tectonism associated with the Vema Fracture Zone. Hydrothermal circulation of seawater along the highly permeable shear zone was activated by magmatic intrusions. K/Ar dating suggests, within relatively large analytical uncertainties, that the amphibolite metamorphism took place 10 m.y. ago, i.e., at a time when the dredging sites were located in the vicinity of the spreading center.
New data on the composition, structure, and properties of Venus rock obtained by Venera 13 and Venera 14
Authors: Surkov Yu. A. , Barsukov V. L. , Moskalyeva L. P. , Kharyukova V. P. , Kemurdzhian A. L.
Published: February 2008

A study of the composition, structure, and properties of Venusian rocks was carried out by Venera 13 and Venera 14. The main structural‐morphological types of provinces of the Venus surface were chosen for the study. The chemical and mineralogical composition of the rock at the Venera 13 landing site shows that it belongs to a group of weakly differentiated melanocratic alkaline gabbroids; at the Venera 14 landing site, the rock is similar to oceanic tholeiitic basalts. A most interesting property of the surface rocks is the great number of the strata that were visible in panoramas of the landing sites. Measurements of their physical and mechanical properties show that Venusian rocks have friable, weakly cemented porous structures. Analysis of information from Venera 8, 9, 10, 13 and 14 led to the conclusion that the most likely global process responsible for the formation of the present rock is volcanic eruption. Moreover, due to the high gas content of basaltic melts, eruptions are of an explosive character leading to the formation of pyroclastic rocks. Summing up data obtained by Venera enabled us to compare Venus with the earth, Mars, and the moon, to point out some regular global features in the evolution of these bodies, and to suggest some new problems for investigation.
Melting, ablation, and vapor phase condensation during atmospheric passage of the Bjurbole meteorite
Authors: Rietmeijer Frans J. M. , MacKinnon Ian D. R.
Published: February 2008

A detailed study of the Bjurbole fusion crust using scanning electron microscopy (SEM) and energy dispersive analysis (EDS) shows that filamentary crystals and ablation spheres may form on the meteoroid surface. Filamentary crystals, hollow spheres, and porous regions of the surface point to a period of intense vapor phase activity during atmospheric passage. Filamentary crystals can be divided into three categories on the basis of bulk composition and morphology. Two types of filamentary crystals are vapor phase condensation products formed during atmospheric entry of the meteoroid. The other type forms by the interaction of seawater with the fusion surface. The density and composition of ablation spheres varies with the flight orientation of the meteorite. The size range and composition of iron‐nickel spheres on the surface of Bjurbole are similar to spheres collected in the stratosphere. A comparison of stratospheric dust collections with meteorite surfaces may provide further insight into the mechanisms of meteoroid entry into planetary atmospheres.
Iridium emissions from Kilauea Volcano
Authors: Olmez I. , Finnegan D. L. , Zoller W. H.
Published: February 2008

During May 1983, gas and particulate samples were collected at the cooling vents of Kilauea volcano in Hawaii. Three vents on the southeast rift zone were sampled using base‐treated filter packs to absorb acidic gases and Teflon filters for particles. The samples were analyzed for 40 elements by nondestructive neutron activation analysis. As with other volcanoes, the chalcophilic and volatile elements were enriched by up to seven orders of magnitude relative to the erupted basalt. Unlike the case of other volcanoes, iridium was observed to be highly enriched (105 fold) at the two higher temperature vents, and the degree of enrichment appears to be related to both high temperatures and high fluorine content of the gases. Estimates of the emission rates of Ir during eruptions of Kilauea based on the measurements reported here and those of the Hawaiian Volcano Observatory yield an emission rate of about 3 g Ir per 106 m3 of magma. This rate amounts to only about 0.3% of the Ir present in the magma that is being released by the volcano. This previously unidentified Ir source to the atmosphere and ocean may have a significant influence on the geochemical cycle of Ir on the earth. The uniqueness of Kilauea in its emissions of Ir is probably related to the deep source of magma, which contains significant levels of Ir (0.32 ppb) and high levels of the halogens F and Cl.
A dynamical model of lithosphere extension and sedimentary basin formation
Authors: Houseman Gregory , England Philip
Published: February 2008

Convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses. This possibility is investigated assuming homogeneous, biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle, power law, or Dorn law behavior in the mantle. For a given geotherm and for rheologies based on current laboratory data, the lithosphere responds to convection‐induced uplift in one of three ways, depending primarily on the magnitude of the driving stresses. These regimes are as follows: (1) When the driving stress exceeds a threshold value, the resulting strain rates are high enough (greater than about 3×10−16 s−1) that the lithosphere strains almost isothermally, its effective rheology is strain thinning, and it undergoes accelerating extension. The magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm (characterized here by the Moho temperature) and varies from about 1 km for a Moho temperature of 700°C to around 3 km for a Moho temperature of 550°C; an uncertainty of at least 100°C is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates. (2) At stress levels due to less than about half of the critical uplift, extensional strain is negligible. (3) At intermediate stress levels, moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension.
Rifting of old oceanic lithosphere
Authors: Mammerickx J. , Sandwell D.
Published: February 2008

Geophysical data from five regions in the Pacific and Indian oceans reveal that long distance (>400 km) spreading center jumps have occurred in the past. The present‐day seafloor morphology is used to develop a scenario for a spreading center jump. The major events are (1) thinning and weakening of the lithosphere at the future rifting site, (2) rifting of the weakened lithosphere (during rifting, the crack is filled from above by normal faulting and wedge subsidence; viscous upwelling fills the crack from below), (3) spreading at the rift site results in a ridge bounded by two troughs (spreading ceases at the dying spreading center, resulting in a deep central graben surrounded by flexural ridges; periods of slow spreading at both spreading centers produce rough topography), (4) ageing and cooling that produce a general deepening of the abandoned spreading ridge and also reduce the thermal contrast across the fossil rifting site. The new spreading center develops into a normal spreading rift. The major topographic expressions apparent in the seafloor today are the deep trough of the abandoned spreading center and the proximal and distal troughs which formed when the emerging spreading center bisected the fossil rifting site. The proximal trough (nearer the new spreading ridge) and the distal trough (farther from the new ridge) are first‐order topographic features, 100–1000 km long and 300 km wide, resembling fracture zones with which they are often confused. They share with fracture zones the characteristic of bringing together fragments of lithosphere of different ages, but unlike fracture zones they are generally parallel to magnetic lineations.
The Azores‐Gibraltar Plate Boundary: Focal mechanisms, depths of earthquakes, and their tectonic implications
Authors: Grimison Nina L. , Chen Wang‐Ping
Published: February 2008

We have analyzed the focal mechanisms and depths of 10 moderately sized earthquakes along the Azores‐Gibraltar plate boundary by a variety of methods including formal inversion of the waveform and amplitude of teleseismic P and SH waves, first motion readings, and the identification of depth phases. Our data, together with a compilation of results reported for very large events from the past 30 years and for two recent events in 1983, place new constraints on the present‐day deformation of this boundary separating the Eurasian and African plates. The slow (∼2 to 3 mm/yr) divergent plate motion near the Azores Islands at the western end of the boundary appears to be very similar to that of a typical ridge‐transform‐ridge configuration. A currently aseismic segment with presumed transcurrent motion separates the western end from a broad zone of ocean‐ocean convergence to the east. This convergence zone is characterized by scattered seismicity, complex bathymetry, and large positive geoid and gravity anomalies. The dispersed locations of the earthquakes and the lack of consistency in the orientation of their nodal planes and slip vectors all suggest that a single major plate boundary is not present there. We note that a regional stress field of north‐northwest‐south‐southeast compression can be inferred from the consistent P axis directions. The focal depths for earthquakes in this region reach a maximum of 50±5 km beneath the seafloor. These events are among the deepest oceanic earthquakes which are apparently unrelated to a Benioff zone and their maximum depths are approximately limited by the position of the 600°C isotherm. We interpret the regional stress field and the focal depths as the result of the interaction of two sections of strong, cold oceanic lithosphere with nearly identical thermal structure. As the plate boundary crosses the passive continental margins of Iberia and Africa, the zone of ocean‐ocean convergence becomes that of continental collision. Two unusually deep events, one at 100 and another at 640 km, have occurred beneath southern Spain since the occurrence of the deep Spanish earthquake of 1954 in the same area. The configuration of a seismic upper crust and uppermost mantle straddling an aseismic lower crust resembles the distribution of focal depths in other areas of recent continental convergence. Based on the distribution of focal depths along the entire plate boundary and recent reconstructions of the history of relative motions between Eurasia and Africa, the very deep events beneath southern Spain are likely to have occurred within a detached piece of lithosphere which sank to its present depth during the current phase of plate convergence.
Upper mantle structure from teleseismic P wave arrivals in Washington and northern Oregon
Authors: Michaelson Caryl A. , Weaver Craig S.
Published: February 2008

Teleseismic P wave travel time residuals are used to detect lateral velocity heterogeneities in the upper mantle beneath Washington and northern Oregon. The results of an inversion for three‐dimensional velocity variations resolves an east dipping high‐velocity zone that we interpret as the subducting Juan de Fuca plate. The plate is characterized by 3–8% higher velocities than those in the surrounding upper mantle. Inversion of the travel time data and ray trace modeling indicate that the plate extends to a depth of 200–300 km. The plate dips at a moderate angle of 45° to the east‐northeast beneath the central Washington Cascade Range north of Mount Rainier, with 5% faster velocities than the surrounding upper mantle. Beneath the North Cascade Range of Washington, the plate strikes to the northwest and has 6–8% faster velocities than the upper mantle to the west. South of 47°N, beneath the Cascade Range in southern Washington and northern Oregon, the plate dips steeply to the east and has 3–4% faster velocities than the surrounding upper mantle. Based on changes in the geometry and velocity structure of the subducted Juan de Fuca plate east of about 123°W, we propose that the subducted slab is segmented into three sections beneath Washington and northern Oregon.
Yield estimates of nevada test site explosions obtained from seismic Lg waves
Authors: Nuttli Otto W.
Published: February 2008

A methodology is presented for determining the yield of underground nuclear explosions from Lg wave amplitudes. The methodology is applied to Nevada Test Site (NTS) explosions, for which the data from short‐period, vertical component analog seismographs at three stations are used to develop calibration curves for unsaturated material and water‐saturated rock source conditions. The latter curves are found to provide reasonably accurate estimates of the yields of explosions in other areas of the United States and in the French Sahara, suggesting that they may be applicable to all continental areas. If so, they also can provide an estimate of the bias of mb(P) magnitudes between different continental sites. For example, the Lg data from NTS explosions indicate a 0.31±0.02 magnitude unit bias between NTS and eastern North America, similar to the approximately 0.33 unit bias found between western and eastern North America previously by use of earthquake data.
Holocene secular variation in western North America: Paleomagnetic record from Fish Lake, Harney County, Oregon
Authors: Verosub Kenneth L. , Mehringer Peter J. , Waterstraat Paul
Published: February 2008

An 8.9‐m composite section of lake sediment from Fish Lake, Steens Mountain, southeastern Oregon, has provided a high‐resolution record of Holocene geomagnetic secular variation in western North America. The section is composed of overlapping core segments with stratigraphic control based on six tephra layers and other thin, distinct bands and with age control from 37 radiocarbon dates. The 455 paleomagnetic samples possess a strong and stable magnetic remanence which is carried by pseudo‐single‐domain magnetite. Paleomagnetic records for individual core segments show low scatter and high serial correlation, and where the segments overlap, there is excellent agreement between records. The azimuthal orientation of the composite section was recovered from the known direction of the geomagnetic field at the time of the climactic eruption of Mount Mazama tephra which is present in Fish Lake. Both internal and external evidence, including the correspondence with an archaeomagnetic record from the southwestern United States, support the conclusion that the Fish Lake record represents the history of geomagnetic field behavior during at least the last 10,000 years B.P. Analysis of that record reveals that the average geomagnetic direction can not be distinguished from that of a geocentric axial dipole field and that the overall circularity of the field was clockwise but the detailed behavior was quite complex. Angular standard deviations of both field directions and virtual geomagnetic poles differ significantly from the values predicted from theoretical models of secular variation but are consistent with those of other Holocene records. When the record from Fish Lake is compared to the proposed type curve for east central North America, there is excellent agreement for the past 6000 years, provided the curves are offset by about 400 years. This offset corresponds to a rate of westward drift of about 0.062°/yr. Beyond about 6000 radiocarbon years ago, good agreement between the two curves occurs when there is no offset, suggesting that the field was standing rather than drifting during this time.
Finite strain calculations of continental deformation: 2. Comparison with the India‐Asia Collision Zone
Authors: England Philip , Houseman Gregory
Published: February 2008

Numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness, of stress and strain rate, and of latitudinal displacements that may be compared with observations in the India‐Asia collision zone. A simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations, provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2×1013 (±5 × 1012) N m−1 in regions in front of the indenter. Under these conditions, the calculated deformation shows accommodation of convergence primarily by crustal thickening, to produce a plateau in front of the indenter. Palaeomagnetic data from India and Tibet, and the observed distribution of topography, suggest that much of the post‐Eocene convergence of India with Asia has been taken up by deformation within Asia that involved crustal thickening. The principal difference between calculation and observation is the absence from the calculated strain rate fields of east‐west extension of the plateau in front of the indenting boundary. The calculations show that once such a plateau is formed, the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings. Seismically determined regional strain rates exhibit a similar distribution, with the Tibetan plateau straining at about one quarter the rate of the Tien Shan and Ningxia‐Gansu regions. Calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the Mongolia‐Baikal, Tien Shan, Tibet, and Ningxia‐Gansu regions of Asia, to within the uncertainty of the latter. The vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a Theologically stratified continental lithosphere obeying laboratory‐determined flow laws. We suggest that the thin viscous sheet model, described in this paper and its companion, gives a simple and physically plausible description of the observed deformation in central Asia; in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening, with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established.
Chemistry of the Apollo 12 highland component
Authors: Laul J. C.
Published: February 2008

Based on the chemical data for 30 elements for 17 highland fragments (3–35 mg) from 6 suites of Apollo 12 coarse fines, 11 chemical highland groups (6 with negative Eu anomalies and 5 with positive Eu anomalies) are identified in the Apollo 12 fragments. The highland chemical groups are represented by a range of petrographic types. Among the chemical groups four are newly recognized. One group is very high‐K (VHK) KREEP‐La at 520× (chondrite) with a negative Eu anomaly at 50×. The other three new chemical groups have positive Eu anomalies and these are as follows: a poikilitic/granulitic rock with La at 130×; anorthosite with La at 30X and Eu at 90×, and anorthosite with La at 12× and Eu at 25×. The Apollo 12 highland suite is dominated by high‐K KREEP and is similar to the Apollo 14 highland suite. The presence of high‐K KREEP explains the relatively high LIL contents in the Apollo 12 soils. The plutonic suite data appear to show a trend in Eu anomaly versus longitude, i.e., eastern (Apollo 11) and western (Apollo 12 and 14) plutonic (ANT) rocks, as suggested by Warren et al. [1981]. Several parent magmas are suggested to explain the wide variety of plutonic and other highland suites observed at the Apollo 11 and 12 sites.
Mineralogy of Yamato 791073 with reference to crystal fractionation of the Howardite parent body
Authors: Takeda Hiroshi
Published: February 2008

Shock textures and mineral chemistry indicate that Yamato 791073 is a type of meteorite intermediate between diogenite and cumulate eucrite and is similar to the Yamato 75032‐type achondrites. The presence of vesicles in shock melt glass and swirly glasses of plagioclase composition as in the suevite in the matrix indicates that it is the most intensely shocked sample among this group of achondrites. Pyroxene compositions in Y791073 fall in a limited range from Mg/(Mg + Fe) = 0.7 to 0.6 and are compatible with the continuum of rock types produced presumably by differentiation on the howardite parent body. The chemical variation of pyroxene is wider than that of the monomict Y75032 group. In addition to Fe‐rich diogenite‐like pyroxene (Ca2Mg66Fe31) common in Y75032, Y791073 contains clasts of pyroxene texturally like those in Moore County. The plagioclase glass with An89 may originally have been associated with this pyroxene as a cumulate eucrite. Such clasts are uncommon in the Y75032‐type achondrites, Y791073 also contains a very differentiated Si‐ and K‐rich clast. Y791073 shows a more polymict nature than other Y75032‐type achondrites, but the lithic variation more limited than normal howardites suggests that it sampled two or three layers of the layered crust or pluton produced by the crystal fractionation.
The geology and geomorphology of the Venus surface as revealed by the radar images obtained by Veneras 15 and 16
Authors: Barsukov V. L. , Basilevsky A. T. , Burba G. A. , Bobinna N. N. , Kryuchkov V. P. , Kuzmin R. O. , Nikolaeva O. V. , Pronin A. A. , Ronca L. B. , Chernaya I. M. , Shashkina V. P. , Garanin A. V. , Kushky E. R. , Markov M. S. , Sukhanov A. L. , Kotelnikov V. A. , Rzhiga O. N. , Petrov G. M. , Alexandrov Yu. N. , Sidorenko A. I. , Bogomolov A. F. , Skrypnik G. I. , Bergman M. Yu. , Kudrin L. V. , Bokshtein I. M. , Kronrod M. A. , Chochia P. A. , Tyuflin Yu. S. , Kadnichansky S. A. , Akim E. L.
Published: February 2008

A region‐by‐region condensed description of almost all of the area that was radar‐photographed by Veneras 15 and 16 is presented. Using some generalizations, the diversity of terrain was reduced to a discrete set from which a geological‐morphological map was constructed. The predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types: ridge‐and‐band, patchy rolling plain, dome‐and‐butte plain, smooth plain, and high smooth plain. Stratigraphically, the ridge‐and‐band plains are the oldest and the smooth plains are the youngest. The stratigraphic position of the other types is yet to be determined. Large sections of the plains show similarities to the mare‐type basaltic plains of the moon, Mercury, and Mars. Other types of terrain are combinations of ridges and grooves in various patterns: linear parallel, orthogonal, diagonal or chevron‐like, and chaotic. In some places the ridge‐and‐groove terrain is Stratigraphically below the plain material, but in other places it appears to be plain material that has been subsequently deformed. Near the eastern and western boundaries of Ishtar Terra large (several hundred kilometers in diameter) ring‐like features can be seen that are named coronae or ovoids. Evidence of tectonic deformation and the presence of flow‐like patterns support their designation as volcano‐tectonic features. Beta Regio seems to be an uplifted plain showing evidence of rifting and volcanism. All types of terrain are sparesely peppered with craters of obvious impact morphology. Their average density gives the plain an age range of 0.5 to 1×109 years. The fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking, at least for the last 0.5 to 1×109 years. No evidence for water‐erosion‐sedimentation processes has been found. The tectonic activity of Venus has no equivalent on the moon, Mercury, or even Mars, and can be compared only with that of the Earth. Intensive horizontal deformation, previously known only on Earth, occurs on Venus, but in a characteristic Venusian style.
Geodesy by radio interferometry: Studies of the forced nutations of the Earth: 2. Interpretation
Authors: Gwinn Carl R. , Herring Thomas A. , Shapiro Irwin I.
Published: February 2008

We demonstrate that earth nutation measurements made with very long baseline interferometry are of sufficient accuracy to be sensitive to the properties of the core‐mantle boundary. The retrograde nutation with annual frequency is particularly sensitive, since this frequency is closest to that of the free core nutation, a nutational normal mode which produces relative motion of the core and mantle. Our nutation measurements imply a deviation of the amplitude of the retrograde annual nutation from its value as calculated by Wahr and by Sasao et al. If this deviation is interpreted as the effect of a departure of the core‐mantle boundary from its hydrostatic figure, then the observed amplitude is consistent with a core‐mantle boundary that has a second zonal harmonic deviation from the hydrostatic equilibrium figure, with the peak‐to‐valley deviation being 490±110 m. The part of the retrograde annual nutation out of phase with the driving torques yields an upper limit on the kinematic viscosity at the surface of the fluid core of 0.54 m2/s (99.5% confidence limit).
An estimate of the effective elastic thickness of the Cape Verde Rise
Authors: Young Robert , Hill Ian A.
Published: February 2008

Application of the response function technique to the load of the Cape Verde Archipelago on the Cape Verde Rise suggests the effective elastic thickness of the rise is only 15±3 km. This is thinner than would be expected, for 130 Ma crust, from previous observations worldwide which suggest a value of 30±10 km. This thinning is considered to be substantial evidence that the lithosphere of the Cape Verde Rise has undergone thermal rejuvenation. Comparison of this study with previous work at Hawaii suggests that reduction of the effective elastic thickness over swells with large depth anomalies is related to plate motion over the mantle heat source.
Gravity anomalies and sea surface heights derived from a combined GEOS 3/Seasat altimeter data set
Authors: Rapp Richard H.
Published: February 2008

Gravity anomalies and sea surface heights have been computed on a 0.125° grid in the ocean areas from a combined GEOS 3/Seasat altimeter data set. The basic estimation procedure used least squares collocation where model covariance functions are tailored to individual areas through altimeter residual variance scaling. Preliminary tests led to production prediction procedures using a reference model defined by a set of potential coefficients complete to degree 180. Comparisons of the predicted anomalies with ship‐derived values showed agreement varying from ±9 to ±30 mGal. No correction to the altimeter‐implied sea surface heights was made for sea surface topography effects. The maximum anomaly predicted is 396 mGal near Hawaii and the most negative anomaly is −361 mGal over the Puerto Rican Trench. The actual resolution of the predicted quantities was estimated to be about 0.19° based on a power spectrum analysis. The resolution is limited by our data selection process which uses 300 data points from a thinned altimeter data set for the prediction of the 0.125° in a 3° × 3° block with one data selection and matrix inversion. The predicted data are used to compute 104 potential coefficient spectrums using flat earth approximations developed by Forsberg. The spectra were classified in terms of smooth, mild, or rough areas. The great majority (78) of the spectra were in the smooth classification. Good agreement was found from spectra computed from topographically reduced land data and the mean ocean data, up to degree 800. We found that the spectra decayed as l−3.6 as compared to the decay of l−3 implied by the Kaula model. The gridded data was used to compute 1° × 1° and 0.5° × 0.5° mean values. The 1° mean anomalies were compared to terrestrial data where an rms difference of ±7 mGal was found in comparing 10,139 values. These new values have allowed us to identify 1° mean anomalies (based on terrestrial estimates) that are in substantial error.
Seismicity and fault kinematics through the Eastern Transverse Ranges, California: Block rotation, strike‐slip faulting and low‐angle thrusts
Authors: Nicholson Craig , Seeber Leonardo , Williams Patrick , Sykes Lynn R.
Published: February 2008

Hypocentral locations and first‐motion data from the southern California seismic network are used to infer the present pattern of deformation along the southern San Andreas fault. The study area lies between the San Jacinto fault and Desert Hot Springs and includes the San Bernardino Mountains and San Gorgonio Pass. This region has some of the deepest earthquakes observed anywhere along the entire San Andreas system, exhibits a complex surface geology characterized by both right‐ and left‐lateral faults, has high topographic relief as a result of recent uplift, and is a potential site for the nucleation of a great earthquake. Although this area is unusually seismogenic, little activity can be directly associated with major throughgoing faults. Seismicity is also generally absent in the upper 5 km. The predominant style of faulting above 10–12 km is oblique slip with a large reverse component. The spatial distribution of relocated hypocenters and first‐motion data suggests the presence of a system of left‐slip faults striking northeast. This pattern of faulting, in conjunction with an unusual set of normal and reverse focal mechanisms, is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right‐lateral shear. At depths greater than about 10 km, seismicity defines a wedge‐shaped volume undergoing pervasive internal deformation on a combination of strike‐slip and low‐angle thrust faults. Velocity structures determined from earthquake arrival times suggest a low‐velocity zone at about 10 km below the San Bernardino Mountains but not below the San Jacinto Mountains. This is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the Mill Creek—Mission Creek branch of the San Andreas fault and the maximum depth of seismicity seen north of that fault. The low‐velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust San Bernardino Mountains are allochthonous. The present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right‐lateral displacements.
Field relations between dikes and joints: Emplacement processes and paleostress analysis
Authors: Delaney Paul T. , Pollard David D. , Ziony Joseph I. , McKee Edwin H.
Published: February 2008

Sedimentary rocks intruded by Tertiary mafic dikes on the Colorado Plateau typically display systematic dike‐parallel joints. These closely spaced joints occur only near dikes, their spacing commonly increasing with distance from the dike contacts. Igneous breccias along some of these joints indicate that the joints are not younger than the dikes and, therefore, did not form during cooling. Field relations are best explained if the joints form in host rocks beyond the dike tip, becoming juxtaposed against the dike with continued propagation: an interpretation supported by previously reported descriptions of ground surface cracks formed near eruptive fissures and of microcracks formed near the tips of larger extension cracks during laboratory experiments. Tensile stress generated by magmatic pressure is sufficient to fracture host rocks beyond dike tips. The tensile maxima are located on either side of the dike plane, beyond the tip. The stresses increase in magnitude closer to the tip, thus explaining the greater abundance of joints near the dike plane. Noting that dikes may parallel regional joint sets, we contrast (1) emplacement along older joints oriented arbitrarily with respect to the principal stress directions acting at the time of intrusion and (2) emplacement along self‐generated fractures propagated in a plane perpendicular to the least compressive stress direction. Magma can invade along older joints if magmatic pressure exceeds the horizontal stress acting across the joint plane. This situation is most common if the horizontal principal stress difference is small compared to the magmatic driving pressure or if joints are nearly perpendicular to the direction of least compressive regional stress. Magma must advance by filling self‐generated fractures if older, suitably oriented, joints are absent.
Introduction to Special Section on Open Magmatic Systems
Authors: Hildreth Wes , Grove Timothy L. , Dungan Michael A.
Published: February 2008

The idea that magmatic systems are open to intermittent gains and losses of mass and energy is hardly new. Eruptive, conductive, and hydrothermally convective losses are obvious, and both wall rock assimilation and mixing of discrete magma batches are petrological ideas with long and distinguished conceptual lineages. New ideas of the last decade are largely outgrowths of the mounting evidence that mixing and assimilation are ubiquitous, that heterogeneity and mixing in mantle source regions are common, and that few igneous rock suites are likely to reflect strictly closed‐system fractionation of single magma batches.

The evidence has accumulated quickly on many fronts, owing in part to the wider availability of high‐precision analytical tools and the development of sophisticated methods for modeling the wealth of chemical and isotopic data. Detailed studies of zoned ash flow sheets, zoned and layered plutons, and macroscopically mixed igneous rocks have made petrologists aware that dynamic and nonequilibrium aspects of magma transport play important roles in the evolution of many magmatic systems. Technological advances have permitted routine experimentation over a wide range of pressures, providing kinetic data and phase equilibrum constraints essential to understanding both mantle source processes and magmatic evolution in crustal reservoirs. Finally, the plate tectonic synthesis and seafloor sampling programs have respectively provided the impetus for a detailed assessment of geochemical heterogeneity in a dynamic mantle and an important avenue for accomplishing that assessment.
Open magma systems: Trace element and isotopic constraints
Authors: Leeman W. P. , Hawkesworth C. J.
Published: February 2008

Open system modification of magmas during their storage and transport through the mantle and crust is now widely recognized (or suspected) as masking many primary features of these liquids. Nevertheless, magmas derived from the mantle and the crust and from specific tectonic settings possess distinctive trace element signatures which presumably are imposed by source compositions or specific petrogenetic processes that also control isotopic compositions. We briefly summarize evidence to indicate the scale and importance of regional isotopic variations in the mantle and then discuss subsequent open system modifications of magmas during their ascent. Specific case studies of continental flood basalts (Deccan, Snake River Plain), volcanic arc magmas (Andes), and other intraplate magmatic suites (Kane Springs Wash and Yellowstone eruptive centers) are discussed to illustrate different styles of trace element/isotope variations. These examples demonstrate the sorts of constraints provided by combined trace element and isotopic studies concerning mixing and assimilation processes that lead to modification of evolving magmas, as opposed to compositional variations inherited from magma source regions. These examples further illustrate effects of source mixing and magma‐crust or magma‐fluid interactions. Despite uncertainties in compositions of assimilant or mixing end‐members, trace element/isotopic compositional vectors can provide useful constraints on the nature of such end‐members and on the dominant petrogenetic processes involved. Obviously, this approach is most successful when applied to discrete eruptive centers where the magmas are most likely to be cogenetic.
Upper crustal structure beneath the Jemez Mountains Volcanic Field, New Mexico, determined by three‐dimensional simultaneous inversion of seismic refraction and earthquake data
Authors: Ankeny L. A. , Braile L. W. , Olsen K. H.
Published: February 2008

The upper crustal structure beneath the Jemez Mountains volcanic field (JVF), New Mexico, was determined using a simultaneous inversion of earthquake and seismic refraction travel time data. One‐, two‐, and three‐dimensional inversions were sequentially applied to derive a detailed velocity model of the upper 10 km of the crust beneath the JVF. Travel time observations from six shot points recorded at as many as 72 temporary seismograph stations and from 27 earthquakes recorded at from 6 to 12 network stations were simultaneously inverted to determine both compressional wave velocity structure and hypocentral parameters. The velocity model was described by a grid of velocity values (unknowns in the inversion procedure) with velocities between grid points given by linear interpolation. Ray tracing through two‐dimensional slices of the three‐dimensional model was performed to calculate theoretical travel times for each source‐receiver pair. A damped least squares iterative inversion was used to derive the velocity model and hypocenters from the differences between observed and theoretical travel times. The relocation of the earthquakes resulted in changes in epicentral location of up to 16 km, although most events were moved less than 7 km. Hypocentral depths of the relocated events ranged from sea level to 17 km, averaging about 8 km. The final three‐dimensional model provided a good fit to the travel time observations. The velocity model displays a prominent low‐velocity, approximately cylindrically shaped body beneath the Valles caldera within the JVF. The velocity anomaly is defined by a lateral decrease in velocity from about 6.0 km/s to as low as 5.6 km/s. The low‐velocity body has a diameter of about 15 km. We interpret the velocity anomaly beneath the Valles caldera to be the result of the combined effects of a silicic intrusion in the upper crust and velocity reduction caused by elevated temperature.
Early basin and range development in Trans‐Pecos Texas and adjacent Chihuahua: Magmatism and orientation, timing, and style of extension
Authors: Henry Christopher D. , Price Jonathan G.
Published: February 2008

Magmatism during early Basin and Range extension in Trans‐Pecos Texas consisted of widespread but volumetrically minor alkali basalts (24–17 Ma), an alkalic mafic to intermediate composition stratovolcano (28–27 Ma), and a dominantly rhyolitic, possibly bimodal, caldera complex in adjacent Chihuahua (31–28 Ma). This magmatism contrasted sharply with the much more voluminous and compositionally more diverse Eocene‐Oligocene magmatism. The age and orientation of dikes emplaced during this magmatic episode have been used to determine the orientation of least principal stress, σ3. An ENE orientation of σ3, similar to early extension throughout the Basin and Range province, can be well documented by 28 Ma and possibly as early as 31 Ma. Faulting, consisting almost exclusively of movement along high‐angle normal faults, was underway by 23 Ma. Some initial extension was taken up along preexisting WNW trending structures, oblique to ENE oriented σ3. The least principal stress probably changed to WNW as it did elsewhere in the Basin and Range province, but the timing of this change is not well constrained in Texas. ENE and NE striking silver‐copper‐lead veins in Precambrian and Cretaceous red beds may have formed during the later Basin and Range extension. The present Basin and Range topography of this region is the result of movement along high‐angle and dominantly NNW trending faults established early under σ3 oriented ENE and reactivated after σ3 changed to WNW.