bluuebunny/crossref_metadata_2025 huggingface dataset:
On inclined features in the Iceland Research Drilling Project core
Authors: Hall James M. , Walls Charles , Hall Swaran L.
Published: February 2008

Precise dip and direction of dip measurements are reported for 148 dike contacts, 126 bedding features in volcaniclastics, and 30 planar, upper surfaces of secondary mineral infillings in vesicles in flows from a 1919‐m drill core from Reydarfjordur, eastern Iceland. The drill core extends from about 1.6 to 3.6 km beneath the original surface of the crustal section. Lava flow dips and directions of dips are reported for a 750‐m exposed section situated directly above and in continuity with the drill core section. While dips and within‐unit differences in dip direction appear to be accurately represented by the measurements, absolute directions are only known inaccurately. This is a consequence of the use of the paleomagnetic method of obtaining dip direction for which both high site latitude and tectonic rotation contribute to poorly constrained results. The main results of the study are the commonplace irregular form of dike contacts, such that the drill hole often entered and exited from the same side of a dike, evidence for the presence of only one population of minor intrusions and the ability to recognize where several sections of intrusives in the drill core belong to the same dike. The irregular form of the dikes prevents a reliable estimate of change of dilation with depth from being obtained. It is clear, however, that a sheeted dike complex does not occur in the upper 4 km of the crust in the area. Measurement of bedding features in the volcaniclastics shows them to be considerably dispersed, particularly in dip direction. Tectonic tilt in the crustal section increases from 11° at sea level to an extrapolated value of close to 17° at 4 km crustal depth. Corresponding lava deposition rates increase from 1620 to 2580 m/m.y., respectively. Tectonic tilting was probably complete by the time of deposition of the last components of the secondary mineralization event in the vesicles of lava flows. The optimum conditions for this type of tectonic investigation where measurements made on vertical drill core are used are high‐speed diamond drilling and original magnetization at middle to low but not equatorial latitudes.
Cometary and meteorite swarm impact on planetary surfaces
Authors: O'Keefe John D. , Ahrens Thomas J.
Published: February 2008

The velocity flow fields, energy partitioning, and ejecta distributions resulting from impact of porous (fragmented) icy cometary nuclei with silicate planetary surfaces at speeds from 5 to 45 km/s are different than those resulting from the impact of solid ice or silicate meteorites. The impact of 1 g/cm3 ice spheres onto an atmosphereless anorthosite planetary surface induces cratering flows that appear similar to those induced by normal density anorthosite meteorite impact. Both of these impactors lead to deep transient crater cavities for final crater diameters less than ∼1 to ∼10 km and for escape velocities ≲105cm/s. Moreover the fraction of internal energy partitioned into the planetary surface at the cratering site is 0.6 for both ice and anorthosite impactors at 15 km/s. As the assumed density of the hypothetical cometary nucleus or fragment cloud from a nucleus decreases to 0.01 g/cm3, the fraction of the impact energy partitioned into planetary surface internal energy decreases to less than 0.01, and the flow field displays a toroidal behavior in which the apparent source of the flow appears to emanate from a disc or ringlike region rather than from a single point, as in the explosive cratering case. The edges of the crater region are in several cases depressed and flow downward, whereas the center of the crater region is uplifted. Moreover, the resultant postimpact particle velocity flow in some cases indicates the formation of concentric ridges, a central peak, and a distinct absence of a deep transient cavity. In contrast, transient cavities are a ubiquitous feature of nearly all previous hypervelocity impact calculations. The calculations of the flow fields for low density (0.01 g/cm3) impactors exhibited surface interface (comet‐planet) instabilities. These are attributed to both the Rayleigh‐Taylor and Helmholtz instability conditions, and we believe that these occur in all flows involving volatile low‐density (0.01 g/cm3) projectiles. It is speculated that these hydrodynamic instabilities can give rise to concentric rings in the inner crater region in large‐scale impacts on planetary surfaces, although other mechanisms for their production may also act. The ejecta mass loss versus planetary escape velocity was computed, and these results imply that the critical escape velocity, at which as much material is lost as is being accreted from a planet, ranges from 1.2 to 2.75 km/s for encounter speeds of 5 to 15 kms/s, with cometary impactors having a density of 0.01 to 1 g/cm3. These values compare to 0.83 and 1.5 km/s for silicate impactors, thereby indicating that it is more difficult for volatiles than silicates to be accreted onto objects with escape velocities similar to the Moon, Mercury, and Mars. For objects with escape velocities in the 0.1 to 1 km/s range the accretional efficiency for silicate and various porosity ices are similar, whereas for objects with escape velocities <0.1 km/s the accretional efficiency of icy impactors becomes significantly lower than for silicate impactors.
Seismic structure of Superior‐Churchill Precambrian Boundary Zone
Authors: Delandro Wanda , Moon Wooil
Published: February 2008

Two reversed refraction/wide‐angle reflection profiles obtained across the Nelson Front by CONCRUST(77) group are interpreted in detail. The results indicate that the Moho beneath the boundary zone between the Superior and Churchill provinces dips down toward the west and there exists a complicated lateral seismic structure in the upper crustal and midcrustal depths. The ray tracing results also indicate that there may exist a zone of negative velocity gradient along the boundary zone at the depth range of 4–13 km.
Heat flow in the Gulf Coastal Plain
Authors: Smith Douglas L. , Dees William T.
Published: February 2008

Forty‐four new heat flow values from panhandle Florida, southwestern Georgia, southern Alabama, Mississippi, and Louisiana range from 13 mW/m2 (0.3 HFU) (1 HFU = 1 μcal cm−2 s−1 = 41.8 mW/m2) in Florida and Alabama to 88 mW/m2 (2.1 HFU) in northern Louisiana. In general, the values depict the eastern Gulf Coastal Plain as an area with low to normal continental heat flow that increases from east to west. Anomalously low heat flow (13–33 mW/m2) (0.3–0.8 HFU) characterizes north‐central Florida, southwestern Georgia, and southern Alabama, while in northern Louisiana, 13 measurement sites yield values ranging from 50 mW/m2 (1.2 HFU) to 88 mW/m2 (2.1 HFU). Distinctive disruptions in the temperature‐depth profiles, attributed to convective effects within aquifers, are limited in magnitude and extent and do not alter the geothermal gradient computations. The region of relatively high heat flux in northern Louisiana is coincident with an area underlain by Cenozoic alkalic igneous rocks of the Monroe Uplift, and an abnormal concentration of radiogenic heat sources within that body is postulated as the cause of the thermal anomaly.
Subsidence of the Paris Basin
Authors: Brunet Marie‐Françoise , Le Pichon Xavier
Published: February 2008

We discuss the main results of a systematic quantitative analysis of the subsidence of the Paris basin made on 100 wells. Backstripping techniques with decompaction as well as sea level and water depth corrections are used to derive tectonic subsidence curves (in the absence of water and sediment loading) assuming local as well as regional compensation. A best fitting sea level variations curve is derived for the deepest central part on the basis of the resulting tectonic subsidence curve. The adopted amplitude of the regression since Upper Cretaceous is 330 m. The spatial and temporal distribution of tectonic subsidence is reconstructed and discussed. It is shown that rigidity of the lithosphere (with an adopted 50‐km flexural parameter) plays a significant role. These results are then quantitatively interpreted in terms of the lithospheric stretching model. This interpretation is reasonable provided it is made in terms of volumetric subsidence over the whole basin, because of the thermal conduction and the rigidity low‐pass filtering effects. The subsidence can be explained if the axial zone was thinned during Permo‐Triassic by a factor of about 1.3 with respect to the adjacent lithosphere. It is shown that either the initial altitude was more than 500 m above sea level or the lower part of the lithosphere was lost at the beginning of the Permo‐Triassic phase of extension, resulting in a rapid uplift of several hundreds of meters. The similarity of the Permo‐Triassic initiating extensional phase with active continental rifting is shown.
Subduction seismicity and tectonics in the Lesser Antilles Arc
Authors: Stein Seth , Engeln Joseph F. , Wiens Douglas A. , Fujita Kazuya , Speed Robert C.
Published: February 2008

We have studied the mechanisms of 17 earthquakes along the Lesser Antilles subduction zone to examine a site where very old lithosphere subducts at a slow convergence rate. No large thrust earthquakes occurred during the 1950–1978 study period; the three large (magnitude seven) events are all normal faults. One is a normal faulting event seaward of the trench. Its aftershock sequence includes strike slip events on differently oriented faults, probably due to lateral block motion in response to the main shock. A second indicates extension within the slab at depth. These observations suggest that subduction in this region is primarily decoupled and aseismic unless the time interval studied is unrepresentative. The third normal fault earthquake occurred within the upper plate with fault planes perpendicular to the arc and trench. This unusual geometry may represent a flexural response to the subduction of the Barracuda Ridge, a major bathymetric high with uncompensated excess mass at depth which seems analogous to flanking ridges found along some Mid‐Atlantic Ridge fracture zones. Thus, the Barracuda Ridge is not buoyant and does not affect Benioff zone dip. Strike slip faulting occurs at depth in the subduction zone along a concentration normal to the arc and may indicate a fossil fracture zone. There is no direct evidence in the shallow seismicity for the hypothetical North America‐South America‐Caribbean triple junction though some of the oceanic intraplate seismicity is consistent with such a boundary.
A kinematic thermal history of the Earth's mantle
Authors: Langan Robert T. , Sleep Norman H.
Published: February 2008

Kinematic convection models of the earth's thermal history are computed assuming that subducted slabs, along with some entrained material, sink all the way to the core‐mantle boundary. The temperatures within the lithosphere and within the mantle away from sinking slabs are computed assuming that the return flow from the core‐mantle boundary to the surface can be considered as slow, vertical advection. In the models, a drop in asthenospheric temperature occurs near the end of the Archean when subducted material first returns to the surface. This drop is about 200°C if the subducted material does not mix thermally with the mantle on its way down and if only a thin thermal layer exists at the core‐mantle boundary. We consider the possibility that this temperature drop is an explanation for the transition between Archean and Proterozoic tectonic styles. An implication of the kinematic scheme is that the asthenospheric temperature increases gradually during the Archean until the cooler material reaches the surface. After the Archean, the thermal evolution is similar to a well‐mixed model. A difficulty with the convection scheme, as well as with any temperature profile in the mantle, which is basically adiabatic, is that excessively low temperatures in the core are implied. If a 100‐km thick thermal boundary layer at the core‐mantle boundary is included, then higher core temperatures occur, but there is a smaller drop in asthenospheric temperature at the end of the Archean.
Stress magnitudes in natural rock salt
Authors: Carter Neville L. , Hansen Francis D. , Senseny Paul E.
Published: February 2008

Subgrain sizes have been determined for Avery Island domal salt that was deformed experimentally at pressures in the range 3.4 to 20.7 MPa, temperatures from 100° to 200°C, and constant mean principal stress differences of from 2.0 to 20.3 MPa. The subgrain size decreases systematically with increasing stress, independent of temperature, according to the relation d = 190 σ−1, which is in substantial agreement with the work of others on single crystals and poly crystals of synthetic rocksalt. For two‐stage creep experiments it is observed that while subgrain size decreases upon increasing the stress, it does not increase upon lowering the stress. Thus the stresses deduced for seven naturally deformed bedded, domal, and anticlinal rocksalts, 0.5–1.1 MPa, reflect maximum stress differences experienced. The results are applied briefly to considerations of steady state flow during the growth of salt diapirs.
Tharsis volcanoes: Separation distances, relative ages, sizes, morphologies, and depths of burial
Authors: Whitford‐Stark J. L.
Published: February 2008

Network analysis indicates the separation distances of the Tharsis volcanoes to be 700 to 90 km; this is considered to be the thickness of the Tharsis thermal lithosphere or ‘tectosphere’ which has thickened with time. Although as a whole the Tharsis volcanoes exhibit a random distribution, preferential alignments emerge when the history of the region is divided into separate periods. The preferential alignments mimic those of fractures in the surrounding terrain. Photogeologic and morphometric data indicate that the volcanoes fall into four distinct classes: montes, tholi, paterae, and a class whose sole representative is Alba Patera. The volcanoes have been buried by lava up to about 4 km in thickness. The sizes of the volcanoes directly correlate with their separation distances from their nearest neighbors. It is concluded that the greater the lithosphere thickness, the larger will be the volcano and the greater its separation distance from its nearest neighbor.
Syrtis major: A low‐relief volcanic shield
Authors: Schaber Gerald G.
Published: February 2008

Viking orbiter images and earth‐based radar elevation data have been used to document the volcanic geology of Syrtis Major and to reevaluate its physiographic setting. These data indicate that Syrtis Major is actually a very low relief, simple shield with a central caldera complex; arcuate segments of concentric grabens at the shield summit define the limits of a 280‐km‐diameter depression that is thought to have been created by foundering of a magma chamber. Lava flows with well‐defined scarps are associated with two summit calderas; fissures and lava tubes are recognized on the shield flanks. The lava flows were deposited on volcanic ridged plains that do not display flow scarps. Compressional stresses acting on the shield were apparently greatest following the deposition of the ridged‐plains unit but continued at a somewhat reduced rate after deposition of the younger lavas. The extremely low‐relief shield of the Syrtis Major as well as other volcanic plains of intermediate age in the eastern hemisphere of Mars is in striking contrast to the giant shields of Tharsis Montes. Such different types of volcanic landforms may result from changes with times in the composition, differentiation history, eruption rates, and eruption temperatures of surface igneous rocks, thus, directly reflecting the thermal and chemical evolution of the interior. The low relief on the shields may also reflect a substantially thinner crust in these regions relative to Tharsis. The physiographic evidence and radar elevation data confirm the fact that Syrtis Major is a ‘planum’ (plateau) rather than a ‘planitia’ (low plain), as published maps of Mars now portray the feature. In addition, present evidence does not support an earlier suggestion that an impact basin predated volcanism at Syrtis Major.
Ice sculpture in the Martian outflow channels
Authors: Lucchitta Baerbel K.
Published: February 2008

Many landforms in Martian outflow channels have characteristics that suggest sculpture by glaciers, ice streams, or ice sheets. Viking Orbiter and terrestrial satellite images were examined at similar resolution to compare features of the Martian outflow channels to features produced by the movement of ice on earth. Many resemblances were found. They include the anastomoses, sinuosities, and U‐shaped cross profiles of valleys; hanging valleys; and linear scour marks on valley walls, grooves and ridges on valley floors, and streamlining of bedrock highs. The question of whether ice could have moved in the Martian environment is investigated. Since gradients on Martian channel floors are extremely low or even reversed, a certain thickness of ice is required to initiate glacial motion. If ice filled the Martian channels to the level of interior plateaus, flow could have occurred, even under present climatic conditions, although warmer climates in the past would have been more favorable. Additionally, if the channel fluids were brines, ice might have been wet based and would have moved readily by slipping over the ground. It is envisioned that springs or small catastrophic outbursts discharged fluids from structural outlets or chaotic terrains. The fluids built icings that may have grown into substantial masses and eventually flowed like glaciers down preexisting valleys. Alternatively, the fluids may have formed rivers or floods that formed ice jams and consolidated into icy masses in places where obstacles blocked their flow. These masses of ice or slush were probably episodic and temporary, but where they occurred within the channels, they may have produced landforms that are characteristic of glacial sculpture on earth.
Present wind activity on Mars: Relation to large latitudinally zoned sediment deposits
Authors: Thomas Peter
Published: February 2008

The relation of present Martian winds to large latitudinally zoned sediment deposits has been investigated using global wind streak data and mapping of large sand and dust deposits. Dune sand deposits occur primarily in three latitude belts: north polar (74°–85°N), low latitude (5°N–20°S), and south polar (40°–80°S). Comparison with wind streak data shows the high‐latitude dunes to be in areas of seasonally reversing winds. The present winds can form latitudinal dune belts from a variety of initial dune distributions, including uniform distribution and a polar source. The presence of dune sand within the polar layered deposits, the erosional state of the deposits, and the present surface wind flow away from the poles indicate that both polar dune concentrations have been derived from erosion of the layered deposits. The low‐latitude dunes are topographically confined in canyons and craters; they are probably subject to long‐term reversal of orientations with climate cycles.
Two Mars years of surface changes seen at the Viking landing sites
Authors: Guinness Edward A. , Leff Craig E. , Arvidson Raymond E.
Published: February 2008

Color and brightness changes have occurred at both Viking Landing sites during the first two Mars years of the Lander mission. The contrast between adjacent soil units decreased, and the surface generally became brighter and redder. These changes are attributed to the deposition of a layer of bright, red dust that is most likely only several micrometers in thickness. Analysis of the spectrophotometric properties of the new dust deposited at the Viking Lander 1 site suggests that the new deposits are the same as the bright, red material that only partially covered the site at the beginning of the mission. Certain areas of the Viking 1 site also exhibited an increase in contrast during the second year. The contrast increase is interpreted to be due to the partial removal of the recently deposited dust cover by high velocity winds. In addition, a small soil slump occurred near the base of a boulder located about 25 m from Viking Lander 1. No obvious erosion has been detected at the Viking 2 site. However, periodic removal of the dust cover at the Viking 2 site is implied by the abundance of rocks at the site.
Chemical composition of Martian fines
Authors: Clark Benton C. , Baird Alex K. , Weldon Ray J. , Tsusaki Donald M. , Schnabel Lorraine , Candelaria Magell P.
Published: February 2008

Of the 21 samples acquired for the Viking X ray fluorescence spectrometer, 17 were analyzed to high precision. Compared to typical terrestrial continental soils and lunar mare fines, the Martian fines are lower in Al, higher in Fe, and much higher in S and Cl concentrations. Protected fines at the two lander sites are almost indistinguishable, but concentration of the element S is somewhat higher at Utopia. Duricrust fragments, successfully acquired only at the Chryse site, invariably contained about 50% higher S than fines. No elements correlate positively with S, except Cl and possibly Mg. A sympathetic variation is found among the triad Si, Al, Ca; positive correlation occurs between Ti and Fe. Sample variabilities are as great within a few meters as between lander locations (4500 km apart), implying the existence of a universal Martian regolith component of constant average composition. The nature of the source materials for the regolith fines must be mafic to ultramafic.
Microaftershock survey of the 1978 Bermuda Rise earthquake
Authors: Nishenko S. P. , Purdy G. M. , Ewing J. I.
Published: February 2008

On March 24, 1978, a magnitude 6.0 intraplate earthquake occurred 380 km southwest of Bermuda near magnetic anomaly M4 (≈118 m.y.B.P.). A catalog of seismicity for the Bermuda rise indicates that this is an area of significant intraplate seismicity in the western North Atlantic Ocean. The fault plane solution for the 1978 event is of thrust type and strikes 340°, in an intermediate direction to the trends of major fracture zones (300°) and abyssal hill topography (035°) in the area. The P axis of this mechanism is nearly horizontal and trends 259°, subparallel to the absolute plate motion vector for North America. Aftershock activity was detected teleseismically for approximately 8 months after March 24, and the entire sequence is best described as a prolonged mainshock‐aftershock series. During June 18–28, 1978, we conducted a microaftershock survey of the area using ocean bottom hydrophones and recorded 250 events (0 < mb < 2). These smaller events are located in a tight cluster 5–10 km northeast of the NEIS mainshock location. SP‐P data from microaftershocks recorded during this survey indicate a relatively narrow band of activity (about 10 km wide), smaller than would be inferred based on NEIS aftershock locations alone. Many of the recorded microaftershocks exhibited prominent water wave phases, which have been used to constrain further the epicentral locations. The depths of these events, while not well deter mined, appear to be shallow, i.e., less than 5 km below the seafloor. Estimates of the b value, determined from two instruments for the 10‐day recording period, are 1.10 ±0.30 to 1.50 ±0.60. Overall, both the fault plane solution and subsequent aftershock locations of the 1978 event suggest that faulting did not occur along a major oceanic fracture zone but, rather, along a smaller‐scale feature within the M11‐M4 spreading discontinuity. The spatial clustering of seismicity near the perimeter of the rise suggests that variations of crustal thickness associated with the Bermuda rise may be responsible for triggering intraplate seismicity in this region.
Numerical modeling of intraplate deformation: Simple mechanical models of continental collision
Authors: Vilotte J. P. , Daignières M. , Madariaga R.
Published: February 2008

We study a model of continental collision in which one of the continents acts as a rigid die indenting the other plate which flows as an incompressible viscoplastic medium. We consider two extreme cases of plane deformation: (1) plane strain which corresponds to an infinitely thick lithospheric plate, and (2) plane stress corresponding to a very thin plate. Deformation of the lithosphere, a thick plate, should be intermediate between those extremes. We found that the flow in the plane strain case is quite similar to that obtained by slip line, theory. The plane stress results are quite different, since in this case most of the plate shortening is taken up by the thickening of the lithosphere. We also explored the role of boundary conditions on the flow, in particular, the role of the side walls containing the flow of the lithosphere. In the case of a free lateral boundary the main feature is a flow of matter toward this free wall and a S‐like pattern for the horizontal stress field. For a rigid wall, on the other hand, the plane strain and the plane stress results are quite different. In the first case, there is a large return flow on the sides of the punch, the material being extruded along the only free surface available. In the plane stress case, the return flow disappears, and the material displaced by the penetration of the die tends to thicken the plate. The role of a nonlinear constitutive relation is studied for power law creep. As the power of the flow limit increases, the flow retains its general features, but the deformation localizes creating sharper contrasts between high and low strain rate areas; in plane stress, the effect of nonlinearity is to increase the contrasts in vertical motion. Available data for Asia are discussed in the light of the new results.
Theoretical prediction of phase relationships in planetary mantles
Authors: Wood Bernard J. , Holloway John R.
Published: February 2008

Calorimetric and phase equilibrium data have been used to establish an internally consistent thermodynamic data base for the silica‐undersaturated part of the system Na2O‐FeO‐CaO‐MgO‐Al2O3‐SiO2. This data base has been used to calculate the stability fields of assemblages involving the phases: olivine (ol), orthopyroxene (opx), clinopyroxene (cpx), spinel (sp), plagioclase (plag) and garnet (gt) at high pressures and temperatures. In the six‐component system all these phases are solid solutions; their mixing properties were also obtained from phase equilibrium and calorimetric data. In the model system CaO‐MgO‐Al2O3‐SiO2 (CMAS), the stable assemblages are ol‐opx‐cpx‐plag at pressures less than 8 kb at 1300°C and less than 6 kb at 900°C. Above these pressures, spinel replaces plagioclase, until pressures of 16 kb at 1300° and 14 kb at 900°C are reached. At higher pressures, garnet lherzolite (ol+opx+cpx+gt) is stable. Stable phase assemblages have also been calculated for sodium and iron‐bearing compositions postulated as possible Martian Mantles. The McGetchin‐Smyth composition would not contain orthopyroxene and a low‐pressure assemblage of plag+ol+cpx+sp would be replaced by gt+ol+cpx+sp at pressures above 13 kb. In contrast, the Morgan‐Anders composition has stability fields of plagioclase Iherzolite (P < 8 kb) and garnet lherzolite (P > 8 kb) and a small field of spinel Iherzolite at about 6 kb at temperatures below 900°C.
Volumetric analysis of complex lunar craters: Implications for basin ring formation
Authors: Hale W. S. , Grieve Richard A. F.
Published: February 2008

The transition in hypervelocity impact structures from complex craters to basins is defined by a transition in uplifted central features from peaks to peak rings. Volumetric and morphologic analyses of the central peaks in complex lunar craters are combined with subsurface data from terrestrial complex impact structures in an effort to characterize the crater to basin transition. Central peak volumes in lunar craters with rim diameters (Drc) < 51 km follow a power‐law relationship to apparent crater volume. In craters with Drc > 80 km, central peak volume is relatively reduced and a second power‐law relationship is defined. The relative reduction in peak volume in craters with Drc > 80 km is associated with a relative decrease in peak height, and is accompanied by the development of a concentric zone of enhanced floor roughening. Studies of terrestrial complex impact structures suggest that the amount of uplifted material, as judged from its depth of origin, continues to increase with increasing rim diameter. This suggests that a redistribution of uplifted material away from a centralized peak may occur in the larger craters. The volumetric and morphologic changes described occur over a rim diameter range of 51–80 km, far below the previously proposed range for the crater to basin transition (Drc = 140–175 km). Evidence is presented to support a crater to basin transition beginning at Drc = 51–80 km, characterized by a relative reduction in central peak volume and the development of rings of floor roughening, which are suggested to be precursors of peak ring development.
Orientation and origin of ridges in the Lunae Palus—Coprates region of Mars
Authors: Maxwell Ted A.
Published: February 2008

Numerous wrinkle ridges similar in appearance to lunar mare ridges occur on volcanic plains units surrounding the Tharsis region of Mars, where they are oriented circumferential to the center of the plateau. By analogy with lunar mare ridges and from morphologic evidence such as the smooth slopes of arches, these features are interpreted to result primarily from compressional folding of the volcanic plains, although the uppermost ridge may represent faulting of the near surface materials. East of the Tharsis region, ridges are best developed in the Lunae Palus and Coprates quadrangles, where they can be used to indicate the degree of compressional stress occurring there. Comparison of ridge orientations in these quadrangles to a Tharsis center at 1°N, 122°W indicates that more than 60% of the total length of ridges is orthogonal to a normal from this center. Non‐orthogonal ridges are seen to have general northerly orientations after removal of the Tharsis orthogonal population. Trends of graben in surrounding terrain indicate that non‐orthogonal ridges are not greatly affected by regional scale structures. Comparison of the frequency of orthogonal and non‐orthogonal ridges further suggests that loading due to the Tharsis plateau was effective in producing compressional features as far as 4000 km from the center of the load; additional sources for compressional stress are necessary to account for ridge systems at greater distances from the Tharsis plateau.
Large‐scale Quaternary detachments in Ventura Basin, southern California
Authors: Yeats Robert S.
Published: February 2008

The Ventura basin is an east‐north‐east trending trough in the California Transverse Ranges which records major Quaternary detachment faulting at three levels. The earliest thrusting occurred along weak siltstone interbeds in a sequence dominated by competent basin‐plain turbidite sandstone. Because sedimentation continued during thrusting, the age, rate, and direction of thrusting can be worked out. Faulting began 1.3 m.y. ago and ceased 0.65 m.y. ago, with a maximum slip rate of 2.8 mm/yr to the southeast. The fault set moved up a 45° ramp and ended as a blind thrust. The ramp had topographic expression on the seafloor, diverting turbidites around the ramp and preserving ash beds along with other hemipelagic sediments on its crest. Following the end of deposition 0.2 m.y. ago, the competent basin‐plain turbidites underwent flexural slip folding over an incompetent Miocene sequence dominated by shale; underlying competent Paleogene strata were not folded. The south flank of the Ventura Avenue anticline tilted at 3.4 μrad/yr, the anticlinal crest rose at a rate of 15–16 mm/yr decelerating to 4.3–5.2 mm/yr, and the anticline and an adjacent syncline shortened at a rate of 20 mm/yr. The high rate of folding in the Ventura Avenue oil field resulted in overpressured sandstone reservoirs and oil‐water interfaces which have not had time to reach gravity equilibrium. The Red Mountain, San Cayetano, and Santa Susana faults mark the surface expression of a seismically active midcrustal detachment which produced convergence across the Ventura basin at rates as high as 23 mm/yr. Total convergence across the eastern San Cayetano fault near Fillmore is 11,600±2000 m in the last million years.
Episodic rifting and volcanism at Krafla in north Iceland: Growth of large ground fissures along the plate boundary
Authors: Hauksson Egill
Published: February 2008

The rifting of the plate boundary in north Iceland forms new ground fissures and reactivates old ones. Continuous growth of fissures is observed in the crust above a magma chamber beneath the Krafla caldera as it responds to changes in subterranean magma pressure. Rapid episodes of fissure formation and reactivation are observed in conjunction with magma intrusions at depth along the rift zone that constitutes the plate boundary between the North American and Eurasian plates. The Krafla caldera contains several fissures that open during periods of uplift lasting 100 to 300 days and that close incompletely in subsidence episodes lasting several hours to 3 weeks. The average rate of opening differs from one period to another, ranging from 0.1 to 0.5 mm/d. Fissure closing and average closing rates also vary from one subsidence episode to another, ranging from minimal values of 2.1 mm and 2.3 mm/d to maximal values of 49 mm and 18 mm/d, respectively. During the two intrusion episodes of September 1977 and March 1980 the magma migrated toward the south with an average velocity 0.6 and 0.5 m/s, respectively. In the September 1977 episode some fissures in the rift zone widened with a rate greater than 190 mm/d, whereas during the March episode that probably resulted in a thinner intrusion the maximum widening rate was only 50 mm/d. The data suggest that the magnitude of a subsidence episode is determined at least partly by the available excess magma pressure in the chamber. To explain the extensive growth of open fissures and normal faulting in the rift zone, it is necessary to assume that in addition to the intrusion of the main central dike at depth, many small dikes are intruded at shallow depths (less than 1 km).
Heat flow: An Acknowledgment to R. P. Von Herzen
Authors: Anderson R. N.
Published: February 2008

Oceanography, and marine geophysics in particular, has always been as much a science of exploration as of experimentation. Even our most carefully planned and supposedly controlled experiments as often as not result in serendipitous but new physical insights into how the earth works. Of late, the profession has been dominated more and more by the invention of new technology, for example, ways of seeing deeper into the earth with greater and greater resolution. Satellite measurements of gravity, multichannel seismic profiling, the deep diving submersible Alvin, and remote survey systems such as Seamark, Deeptow, Gloria, and Angus come quickly to mind.
Velocity and attenuation anisotropy in deep‐sea carbonate sediments
Authors: Kim Dae‐Choul , Katahara Keith W. , Manghnani Murli H. , Schlanger Seymour O.
Published: February 2008

Laboratory studies have been carried out to determine the causes of velocity and attenuation anisotropy in pelagic carbonate sediments from Deep Sea Drilling Project site 288 on the Ontong‐Java Plateau. Compressional velocity Vp was measured under independently controlled pore pressure and confining pressure. Velocity anisotropy for compressional waves (horizontal velocity > vertical velocity) is found to decrease with increasing effective pressure, i.e., with closure of pores. Previous studies have suggested that velocity anisotropy in pelagic carbonate sequences is due largely to preferred vertical orientation of calcite c axes. The present results indicate that preferred horizontal orientation of grain contacts and flat pores is also a significant cause of the observed anisotropy for many samples. Significant anisotropy in ultrasonic attenuation for compressional (Qp−1) and shear (Qs−1) waves is observed such that the losses are greatest for propagation modes that can generate relative motion across horizontal grain contacts or flat horizontal pores. The results are discussed in terms of the possible mechanisms of dissipation.
On the theory of electromagnetic induction in the Earth by ocean currents
Authors: Chave Alan D.
Published: February 2008

The theory of electromagnetic induction is extended to the case where the driving electric currents must be considered explicitly. Differential equations for independent poloidal magnetic (PM) and toroidal magnetic (TM) modes are derived directly from the governing quasistatic form of the Maxwell equations. They are applied to induction by motional source fields in the ocean by deriving Green functions for the two modes in a constant depth ocean. PM and TM modes are shown to behave in very different ways as a function both of the ocean velocity field dynamics and of the electrical conductivity structure of the earth. TM modes are closely associated with the Coriolis force deflection of water currents and with coastline effects that limit the source currents to the ocean basin. PM modes are induced by nondivergent electric currents that are fully contained within the ocean. Surface gravity waves and a Kelvin wave are examined in detail. The Kelvin wave result of Larsen (1968) is reevaluated, and because the upper lithosphere was modeled as an insulator, significant errors in his magnetic induction values, caused by neglect of the TM mode, are revealed. The sensitivity of the TM mode magnetic field to lithospheric electrical conductivity suggests the use of tidal induced electromagnetic fields to probe the earth's conductivity structure.
Magma reservoir subsidence mechanics: Theoretical summary and application to Kilauea Volcano, Hawaii
Authors: Ryan Michael P. , Blevins James Y. K. , Okamura Arnold T. , Koyanagi Robert Y.
Published: February 2008

An analytic model has been developed for the prediction of the three‐dimensional deformation field generated by the withdrawal of magma from a sill‐like storage compartment during an intrusion or eruption cycle. The model is based on the work of Berry and Sales (1961, 1962) and predicts the vertical displacement components over the areal plane. Model parameters are the depth of burial h, the intrusion half width a, the intrusion half length b, the thickness of the magmatic interior at the moment of melt withdrawal tm, and the planform aspect ratio ξ = a/b. The products of the model include areal deformation maps. Systematic variation in model parameters within the context of Kilauea Volcano, Hawaii, have revealed that circular and elliptical deformation patterns result from the collapse of draining rectilinear intrusions at depth. Moreover, the geometric parameters of a storage compartment may interact in complex ways to produce similar deformation patterns. The model has been applied to Kilauea Volcano for three periods of pronounced summit subsidence: (1) 1921–1927 (bracketing the steamblast eruptive phases of 1924); (2) June 1972 to December 1972, and (3) December 1972 to May 1973. Application of the model requires the simultaneous optimization of five predicted deformation features with respect to field measurements and the derivative deformation maps: (1) the vertical displacement maxima; (2) the vertical displacement gradients over the areal plane, (3) the lateral extent of the deformation field, (4) the aspect ratio of the subsidence pattern, and (5) the strike of the major axis of the deformation field. The constrained geometries and volumes of the inferred collapsed storage cavities for each period are (1) 1921–1927: depth ≅ 3 km, a ≅ 1500 m, b ≅ 4500 m, tm ≅ 20 m, V 540×106 m3, (2) June 1972 to December 1972: depth ≅ 3.3 km, a ≅ 600 m, b ≅ 2000 m, tm ≅ 1 m, V ≅ 4.8×106 m3, and (3) December 1972 to May 1973: depth ≅ 2.2 km, a ≅ 500 m, b ≅ 1612 m, tm ≅ 1 m, V ≅ 3.2×106 m3. For 2 and 3, calculated magmatic thicknesses tm happen to be in the range (3.48–0.15 m) of measurements for sill‐like bodies in deeply dissected Hawaiian shield volcanoes. The fits obtained between calculated and observed deformation patterns allow quantification of the location, overall dimensions, orientation, and volume of the discrete, still molten, interior of sill‐like compartments from which magma is tapped during eruption or intrusion.
Teleseismic detection in the Aleutian Island Arc
Authors: Habermann R. E.
Published: February 2008

Recently it has become apparent that teleseismic detection has decreased substantially in many regions of the world. The major decrease was related to the closure of the VELA arrays in the United States during the late 1960's. This detection decrease has been recognized in South and Central America, Mexico, the Kuriles, the Caribbean, Tonga, and the New Hebrides. In this paper the effect of the closure of these arrays on the reporting of events in the Aleutian Island Arc is examined. In the Aleutians, the detection history is complicated by the short‐term installation of a local network on and near Amchitka Island during the early 1970's. The temporal coincidence of the installation of this network and the closure of the VELA arrays delayed the detection decrease in the central Aleutians until the Amchitka network was closed in early 1973. Reporting in the eastern Aleutians was unaffected by the installation of the Amchitka network. In that region the detection decreased between 1968 and 1970, the time of the closure of the VELA arrays. New techniques have been developed which make it possible to determine the effect of station installation or closure on the reporting in some regions. These techniques rely on plots which show the distribution of an observed seismicity rate change in the magnitude domain. These plots make it possible to recognize probable detection changes and to determine quantitatively magnitude cutoffs which avoid these changes. The magnitude level at which these cutoffs are made is termed the minimum magnitude of homogeneity (mmin h). The reporting of events with mb≤4.6 in the Aleutians decreased substantially during the mid‐1970's, so mmin h in this region is 4.7. This is different from the magnitude of completeness (mmin c) which is mb = 5.0±0.1. If one is interested in examining seismicity rates for changes which may be precursors to earthquakes, then awareness of detection‐related changes and magnitude cutoffs which avoid these changes are essential for meaningful results.
Petrology and physical properties of granites from the Illinois Deep Hole in Stephenson County
Authors: Lidiak Edward G. , Denison Roger E.
Published: February 2008

Two main basement granitoid types have been identified in core samples from the Illinois deep hole project. The main variety is a medium‐ to coarse‐grained porphyritic biotite granite with phenocrysts of microcline perthite and less commonly quartz and sodic plagioclase in a matrix of these minerals and biotite, muscovite, fluorite, magnetite, ilmenite, zircon, hornblende, apatite, sphene, monazite, rutile, and clinopyroxene (relict). The texture is typically hypidiomorphic gradational to recrystallized xenomorphic. The second main granitoid, which occurs in the upper part of hole UPH 3, is a fine‐grained granoblastic to lepidoblastic gneissic granite that is distinct from and possibly older than the nonfoliated granite. The textures of both rocks have been modified by a mild cataclastic shearing that has partially recrystallized the more susceptible mineral phases. Thin fracture planes that crosscut the earlier foliations are common. Microprobe analyses indicate that biotites in the gneissic granite are chemically distinct from those in the granite. Biotites in the gneissic granite have higher Fe/Fe + Mg ratio, FeO, and Al2O3 and lower MgO and SiO2. Reflection microscopy and microprobe analyses indicate that the oxide phases in the two rocks are also different. The oxides in the granite are magnetite and ilmenite, whereas hematite and pseudobrookite occur in the gneissic granite. The biotite and Fe‐Ti oxide data represent additional evidence in support of the fact that the granite and gneissic granite are distinct rocks and probably not part of a continuous comagmatic sequence. Major element chemical analyses indicate that the granites have affinities to anorogenic rapakivi granites. The granites in the deep holes are high in SiO2, alkalis (Na2O+K2O), F, FeO/MgO; low in Al2O3, FeO, Fe2O3, TiO2, MnO, and P2O5; and slightly low in MgO and CaO. Magnetic susceptibility and density measurements correlate generally well with magnetic susceptibility and density logs. Magnetic susceptibilities range from 20 to 17,000 micro SI units; higher values correlate mainly with greater amounts of Fe‐Ti oxides and biotite; lower values with gneissic rocks and muscovite content. Density measurements range from 2.595 to 2.685 Mg/m3; denser rocks correlate positively with higher amounts of biotite and Fe‐Ti oxides and inversely with microcline perthite content.
Isotopic composition of uranium and thorium in crystalline rocks
Authors: Rosholt John N.
Published: February 2008

The 238U‐234U‐230Th‐232Th system has been investigated in 84 silicic crystalline rocks obtained from drill cores, surface, and near‐surface samples in California, Wyoming, Colorado, and Illinois. Results of these analyses displayed on ternary diagrams with apexes for 238U, 234U, and 230Th indicate five predominant geochemical processes that affected uranium in the rock: (1) bulk uranium leaching where 238U and 234U were removed with little or no fractionation; (2) preferential 234U leaching by alpha recoil displacement (234U recoil loss) with lesser 238U loss; (3) 234U recoil loss with little or no 238U loss; (4) uranium assimilation where both 238U and 234U were added with present‐day 234U/238U activity ratios varying from 0.8 to 1.2; and (5) addition of 234U and 230Th by daughter emplacement processes (234U + 230Th recoil gain). Evidence for the existence of 234U and 230Th recoil gain in rocks is the most important finding of this investigation. Radioactive disequilibrium occurs in the majority of rocks analyzed where 234U recoil loss is the predominant process associated with incipient weathering; U assimilation and 234U+230Th recoil gain occur under conditions of substantial water penetration along fractures and into weathered zones in the rocks. Relatively unfractured and petrographically fresh rocks from the UPH‐3 drill hole in northern Illinois are closest to being in radioactive equilibrium for any suite of rocks included in this study, and they demonstrate that equilibrium during the last 0.5 m.y. can be maintained over a substantial vertical distance where there has been little or no movement of water in the basement rock. The 238U‐234U‐230Th system can be a sensitive indicator of geologically recent U mobility and rock/water interaction both in petrographically fresh core samples and in ‘sealed’ fracture zones.
Sampling of brine in cores of Precambrian granite from northern Illinois
Authors: Couture Rex A. , Seitz Martin G. , Steindler Martin J.
Published: February 2008

The composition of groundwater in deep‐seated rocks is of great interest, both geochemically and in connection with the disposal of radioactive waste. However, sampling of deep‐seated groundwater is expensive and often difficult. We describe here a simple technique for the elution of pore fluid (groundwater) from cores of igneous rocks, and we describe brine eluted from a core of Precambrian granite from Northern Illinois drill hole UPH‐3. The brine is predominantly NaCl and CaCl2, with a Ca/Na mole ratio of about 0.05 to 0.1. From analysis of soluble chloride in a crushed sample the chloride concentration in the pore water is estimated as 2.3 M. The brine is similar to brine in the Illinois Basin, and it was probably derived from the overlying sediments.