bluuebunny/crossref_metadata_2025 huggingface dataset:
Contemporary vertical surface displacements in Yellowstone National Park
Authors: Pelton J. R. , Smith R. B.
Published: February 2008

Relative vertical displacements and average relative vertical velocities have been determined for bench marks located throughout Yellowstone National Park (YNP) by comparing geodetic leveling measurements made in 1923 (second order), 1936 (third order), 1941 (third order), 1955 (third order), 1960 (first order), and 1975–1977 (first order). The 1923–1975 data indicate uplift greater than 400 mm (7.7 mm/yr) of an area approximately 15–20 km wide and at least 40 km long which is centered along the northeast trending axis of the 600,000‐year‐old Yellowstone caldera; the maximum 1923–1975 vertical displacement in this area is 726 mm (14.0 mm/yr). The existence of a well‐defined and untilted Yellowstone Lake terrace implies that the 1923–1975 average vertical velocities cannot have been maintained for more than a few hundred years. The 1955–1977 data from central YNP indicate that the northeastern caldera half has risen relative to the southwestern half at an average velocity of about 8 mm/yr; a similar bias is present in the 1923–1975 data but it is less pronounced. The 1923–1960 and 1960–1975 data from northwestern YNP, and a 200 mm depression in the 1923–1975 data near Norris Junction, may be interpreted in terms of deformation associated with the 1959 Hebgen Lake earthquake and the 1975 Yellowstone Park earthquake. A composite profile of the 1936–1977 and 1941–1977 average vertical velocity data which skirts the edge of the Yellowstone caldera in northeastern YNP is most easily explained as the result of random leveling error and/or bench mark instabilities. The apparent youth of the 1923–1975 Yellowstone uplift and its close association with the Yellowstone caldera suggest that the most likely cause of the uplift is a recent (not more than 500 years ago) influx of molten material into the upper crust beneath the caldera.
Structural and heat flow implications of infrared anomalies at Mt. Hood, Oregon, 1972–1977
Authors: Friedman Jules D. , Williams David L. , Frank David
Published: February 2008

Surface thermal features occupied an area of approximately 9700 m2 at Mt. Hood in April 1973 and 9200 m2 in September 1977, as determined by aerial infrared line scan surveys. The distribution of thermal anomalies below the summit of Mt. Hood suggests structural control by a fracture system and a brecciated zone peripheral to a young hornblende‐dacite plug dome and by a concentric fracture system associated with the development of the present crater. We estimate a heat discharge of 5 to 10 MW from the extent and temperature of the thermal areas, including a heat loss of 2–4 MW via conduction, diffusion, evaporation, and radiation to the atmosphere, and a somewhat less certain loss of 3–6 MW via fumarolic mass transfer and surface advection. Additional heat is transferred to groundwater. The first part of the estimate is based on two‐point models for differential radiant exitance and differential geothermal flux from heat balance of the ground surface and on a shallow temperature probe traverse across the Devils Kitchen fumarole field. Alternate methods for estimating volcanogenic flux that assume a quasi steady state heat flow yield values in the 5‐ to 12‐MW range. The estimated heat loss due to cooling of the 220±150 year old dacite plug dome is insufficient to account for the heat flux at the fumarole field. The heat source is judged to be deep seated.
A constitutive model describing dilatancy and cracking in brittle rocks
Authors: Moss W. C. , Gupta Y. M.
Published: February 2008

A micromechanical constitutive model, based on the responses of sliding cracks and elliptic cracks, is used to simulate the inelastic deformation of brittle rock. In addition to the usual incremental equations, the model has the following features: (1) activation equations that include the effects of friction and relate the far‐field stresses to sliding and elliptic crack openings and lengths, (2) a stress intensity analysis of the sliding cracks that determines how much the cracks may grow, and (3) expressions for the crack strains that are constructed directly from the amount of crack sliding, opening, growth and the number of sliding and elliptic cracks in a given region. Numerical simulations of uniaxial and triaxial loading experiments on Westerly granite were compared with experimental data to determine the validity of the constitutive model. These comparisons show that most of the uniaxial stress data and the loading portion of the triaxial data can be simulated quantitatively. The unloading portion of the triaxial data cannot be simulated properly, suggesting that a nonfrictional mechanism, not included in our model, may dominate the response in this region.
Mars: Near‐infrared spectral reflectance of surface regions and compositional implications
Authors: McCord Thomas B. , Clark Roger N. , Singer Robert B.
Published: February 2008

Reflectance spectra (0.65–2.50 μm) are presented for 11 Martian areas. The spectral resolution is ∼1½% and the spatial resolution is 1000–2000 km. These are the first high‐quality spectrophotometric data at these wavelengths for regions on the surface. Spectral features previously observed are confirmed and better defined, and a number of important spectral properties are characterized for the first time. The spectra show water ice absorptions in the 1.5‐ and 2.0‐μm regions, which if due to surface frost, would imply the presence of 1 to 2 mg/cm2 H2O. However, other studies have shown that the presence of an unprotected surface frost in late morning is unlikely. Water ice is stable at night even at the equator and might persist until late morning when the air is well undersaturated if it is intimately dispersed in weathering products, especially if clays are present. The dark region spectra indicate about 4 times less water ice than seen in bright regions. Since some bright material is present in dark regions there may be no water ice associated with the dark materials themselves. This tends to confirm that weathering products (thought to be more abundant in bright regions) are involved in the mechanism that allows temporary persistance of unstable water ice. The presence of weak 2.3‐μm features in many of the spectra are consistent with the presence of hydroxylated magnesium‐rich minerals such as sheet silicates (serpentine, talc, and magnesian smectites) or amphiboles (anthophyllite). The apparent absence of a 2.2‐μm absorption implies that montmorillonite may not be a major component of the Martian regolith. Many of the spectra also show an apparent absorption at 1.9 μm, in the wing of the 2.0‐μm Martian atmospheric CO2 absorption, which would indicate bound molecular water. Observed dark regions have distinctive near‐infrared spectral shapes, previously not well determined, which are characteristic of thin semi‐transparent alteration coatings overlying dark unaltered rock. Previously observed ferrous‐ and ferric‐iron absorptions in the 1‐μm region are better defined by these new data. Clinopyroxene (augite) is definitely present, but olivine is not spectrally apparent.
South Kitakami Region: An allochthonous terrane in Japan
Authors: Saito Yasuji , Hashimoto Mitsuo
Published: February 2008

The pre‐Cretaceous geology of the Japanese Islands may be interpreted as an accretion complex in which many terranes of several origins are juxtaposed and a number of allochthonous blocks are involved. The South Kitakami and Abukuma regions of northeast Japan, and probably also Silurian and Devonian blocks in the Kurosegawa fault zone and the border fault zone of the Hida metamorphic terrane, are the allochthonous blocks. The stratigraphic sequence and paleobiofacies of the regions are similar to each other, and their lithologic characters are of continental affinity, but, however, are quite different from those of the other terranes having oceanic characters. The South Kitakami and Abukuma regions have been fragments of a lost continent, less remotely situated to Gondwana during the middle Paleozoic, and may have drifted and incorporated with the other part of the Japanese Islands before late Mesozoic to early Tertiary igneous activity.
Paleomagnetism and Mesozoic tectonics of the Seven Devils Volcanic Arc in northeastern Oregon
Authors: Hillhouse John W. , Grommé C. Sherman , Vallier Tracy L.
Published: February 2008

Upper Triassic volcanogenic rocks of the Seven Devils Group and Huntington Formation of Brooks (1979b) in northeastern Oregon record two episodes of magnetization. The older magnetization, which predates Late Jurassic deformation of the volcanic arc terrane comprising the Seven Devils Group, was probably acquired in Late Triassic time. The inclinations of both normal and reversed polarity magnetizations indicate that the Triassic rocks originated approximately 18° from the paleoequator. The results are consistent with paleomagnetic data from similar Triassic rock assemblages on Vancouver Island and in south central Alaska. We interpret these assemblages as displaced fragments of Wrangellia, a once coherent late Paleozoic volcanic arc and early Mesozoic volcanic plateau that was torn apart and carried north by plate motion along the continental margin. We interpret the younger magnetization, which fails the fold test, as a widespread thermochemical overprint caused by Late Jurassic and Early Cretaceous plutonism. This magnetization yields a paleomagnetic pole at 60°N, 295°E (α95 = 7°), and is similar to the pole obtained from plutons that intrude the Seven Devils Group. The angular difference between the pole from the plutons and the Late Jurassic‐Early Cretaceous reference pole for stable North America suggests that the Seven Devils volcanic arc has undergone a large clockwise rotation (66°±21°). The rotation may reflect the last stages of the arc's accretion to the continental margin or later large‐scale block movements in the Pacific Northwest.
Contrast of metamorphic and structural histories across the work channel lineament, Coast Plutonic Complex, British Columbia
Authors: Crawford Maria Luisa , Hollister Lincoln S.
Published: February 2008

A prominent topographic lineament along the western margin of the Coast Plutonic Complex of southeast Alaska and adjacent parts of British Columbia separates terranes of contrasting metamorphic histories where it enters British Columbia near Prince Rupert as the Work Channel lineament. The western terrane preserves a regional medium‐high pressure metamorphic facies series, increasing from chlorite grade in the west to kyanite + muscovite migmatite near the lineament. The metamorphic, structural and age relations of the western terrane are consistent with a model of crustal thickening accompanied by westward transport of higher over lower grade metamorphic units during the Jurassic. This terrane was uplifted and cooled by Cretaceous time (∼85 Ma). East of Work Channel lineament, high temperature, relatively low pressure assemblages are overprinted onto high pressure assemblages. The low pressure metamorphic assemblages developed in the Eocene during rapid uplift (2 mm/yr) of rocks which initially may have been metamorphosed at the same time and along the same PT gradient as those of the western terrane. Structures along the Work Channel lineament suggest vertical motion during uplift of the eastern terrane and after metamorphism of the western terrane. Because the metamorphic discontinuity across the lineament is 3–5 kbar at about 700°, the lineament in the Prince Rupert area marks a major crustal break.
Earthquake focal mechanisms and the intraplate setting of the Bermuda Rise
Authors: Nishenko S. P. , Kafka A. L.
Published: February 2008

A number of intraplate earthquakes occurring in the western North Atlantic Ocean are located near the perimeter of the Bermuda rise. Focal mechanisms and depths of two earthquakes, November 24, 1976 (mb 5.1; M0 = 2.96 × 1023 dyne cm) and March 24, 1978 (mb 6.1; M0 = 3.58 × 1025 dyne cm), were determined using Rayleigh wave amplitude data in the period range 20–50 s. The 1978 earthquake occurred approximately 380 km southwest of Bermuda, near magnetic anomaly M4 (≈118 m.y. B.P.). The focal mechanism for the 1978 event is of thrust type and has nodal planes striking 340°. The depth of this event is 6 km below the seafloor, near the local depth to Mono. The strike of the fault planes does not parallel the trends of either fracture zones (300°) or magnetic lineations (035°) in the area. The fault planes do, however, parallel the strike of a magnetic gradient in the epicentral area. The 1976 earthquake occurred approximately 300 km northeast of Bermuda, near Muir seamount. The depth of this event is 10 km below the seafloor. The available data are suggestive of one nodal plane striking between 320° and 340° and nearly parallel to the trend of Muir seamount and other volcanic features in the region. In contrast to the 1978 event, the 1976 earthquake appears to exhibit a significant component of strike slip motion. P axes of both mechanisms are subparallel to the direction of absolute plate motion for North America. We suggest, however, that strain release in the Bermuda rise area is not occurring along major fracture zones or topography parallel to seafloor spreading anomalies but rather on smaller‐scale structures. The stresses induced by variations of crustal thickness may be responsible for triggering intraplate seismicity in this region.
A rock magnetic study of the lower massive sandstone, Moenkopi Formation (Triassic), Gray Mountain Area, Arizona
Authors: Larson E. E. , Walker T. R.
Published: February 2008

This paper describes the results of rock magnetic and petrographic studies of the Lower Massive sandstone of the Moenkopi Formation, north central Arizona, which heretofore has been considered to be a reliable magnetostratigraphic unit. The rock‐magnetic studies included stepwise thermal demagnetization of samples of intraformational mudstone clasts and a deformed shale lens, both of which occur within the lower part of the sandstone, and stepwise thermal and selective destructive demagnetization (SDD) of samples of the sandstone containing black sand laminae composed largely of opaque grains. In samples both from the claystone clasts and deformed shale lens two secondary components, one of modern normal polarity and one of Triassic normal polarity, were unambiguously identified, and a third CRM component of Triassic reversed polarity was tentatively identified. There is no evidence in samples of either the claystone clasts or deformed shale lens of a remanence acquired prior to deposition or to deformation, respectively. SDD, by means of which the small‐scale homogeneity of remanence within samples of the Lower Massive sandstone containing black sand laminae could be assessed, shows that each sample primarily contains a heterogeneous three‐dimensional mixture of Triassic normal and reversed components. The Triassic normal direction that is characteristic of the Lower Massive sandstone is a vector resultant of almost balanced proportions of the two nearly antiparallel components, averaging about 55 to 60% normal and 45 to 40% reversed. The local variability in both direction and intensity of remanence within and between individual black sand laminae confirms that acquisition of the remanence took place over a geologic time interval that spanned at least two and probably several polarity intervals and that, on a small scale, the remanence was sporadically acquired more or less randomly, probably in response to local conditions in the interstitial physicochemical environment that affected the growth of authigenic hematite. No primary component of remanence acquired at or soon after deposition could be identified. The remanence in the black sand‐bearing Lower Massive sandstone is largely carried by the black sand grains which, as determined from reflected‐light petrographic studies, are primarily martite (67%) and altered ilmenite grains (20%) composed of complex intergrowths of hematite and iron‐titanium minerals. These data strongly suggest that these grains, deposited initially as unaltered to only moderately altered magnetite and ilmenite, subsequently underwent oxidation during which they acquired their remanence.
The Iceland Research Drilling Project
Authors: Fridleifsson Ingvar B. , Gibson Ian L. , Hall J. M. , Johnson H. Paul , Christensen N. I. , Schmincke Hans‐Ulrich , Schönharting Gunther
Published: February 2008

This issue of the Journal of Geophysical Research contains the collected results of the initial studies of a 3‐km vertical section of Icelandic crust. The material described in these papers was largely presented at a meeting of the investigators working in this project held in Reykjavík, Iceland, from May 13 to May 15, 1980

Iceland presents a very well exposed example of crust formed at an accretional plate margin, the crest of the Mid‐Atlantic Ridge. The desirability of a detailed study of a long vertical section of Icelandic crust by deep drilling has been expressed in proposals and recommendations of scientists and international scientific committees since the early 1960's. A formal proposal with descriptions of several alternative drill sites was presented in 1975 [Working Group on Deep Crustal Drilling in Iceland, 1975], but difficulties in financing a drill hole that would penetrate into crustal layer 3 (Vp = 6.5 km/s) delayed implementation of the recommendations.
Volcaniclastic rocks of the Reydarfjordur drill hole, eastern Iceland: 2. Alteration
Authors: Viereck Lothar G. , Griffin Brendon J. , Schmincke Hans‐Ulrich , Pritchard Richard G.
Published: February 2008

Volcaniclastic rocks of basaltic to rhyolitic composition, interlayered with basaltic to icelanditic lava flows in a 1918‐m drill hole in eastern Iceland are altered to zeolite‐ and pumpellyite‐prehnite facies assemblages. Major alteration minerals are chlorites, illite‐chlorite mixed minerals, illites, K‐feldspar, laumontite, quartz, and sphene; others include julgoldite and secondary allanite. Smectites as well as mordenite and heulandite are restricted to the upper 900 m. Epidote and minor pumpellyite, prehnite, wairakite, and anhydrite are restricted to the core below 900 m. The boundary between zeolite‐ and prehnite‐pumpellyite facies is marked by the appearance of chlorite‐epidote‐quartz assemblages at around 900 m. The typical paragenesis prehnite‐pumpellyite‐epidote‐chlorite occurs near the base of the drill hole at 1855 m. Textures indicate generally prograde metamorphic conditions. The broad compositional spectrum, ranging from basalt to rhyolite, abundance of vitric fragments, and high initial porosity and permeability of the volcaniclastic rocks were major factors governing alteration. The site of growth (crystal, shard, vesicle, pore, and vein) of a secondary phase strongly influences composition, habit, amount, paragenesis, and time of crystallization. For example, sphene and, to a lesser degree, epidote are less rich in Fe and better crystallized in plagioclase than in the adjacent matrix. The different mineral assemblages developed within plagioclase crystals through the section most clearly reflect the metamorphic changes. The abundance of vitric rhyolitic clasts (shards and pumice) favored K. Al, Si‐enriched fluids from which phases like illite and adularia crystallized. These phases were not found in interlayered basalt flows (Mehegan et al., this issue). Low Ca activity is also indicated by the occurrence of wairakite and pumpellyite rather than prehnite, which is common in the basalts. Secular changes during metamorphism in pore fluid composition are shown by zonations in mineral compositions. For example, the depletion of epidote and juldoldite‐pumpellyite in Fe with time appears to reflect decreasing ƒo2. Bulk rock chemical changes are more intense in the volcaniclastic rocks compared to the basalt lavas. H2O+, Fe3+/Fe2+ and, in part, CO2 values are higher in the upper part of the hole, reflecting the higher porosity and glass content of the clastic rocks. They decrease markedly downhole, approaching values of the lava flows due to more pervasive metamorphic alteration and thus similar mineral assemblages and physical properties.
Oxygen isotope ratios of the Icelandic crust
Authors: Hattori Keiko , Muehlenbachs Karlis
Published: February 2008

Oxygen isotope ratios of hydrothermally altered basalts from depths of up to ∼3 km are reported from three localities in Iceland: International Research Drilling Project (IRDP) core at Reydarfjordur, eastern Iceland (Tertiary age); drill cuttings from Reykjavik (Plio‐Pleistocene age); and Holocene drill cuttings from the active Krafla central volcano. Whole rock samples from these three localities have δ18O values averaging +3.9±1.3, +2.4±1.1, and −7.7±2.4‰, respectively. The observed values in the deeper samples from Krafla are as low as the values for any rocks previously reported. There seems to be a slight negative gradient in δ18O with depth at the former two localities and a more pronounced one at Krafla. Oxygen isotope fractionations between epidote and quartz and those between calcite and fluid suggest that the basalts were altered at temperatures of 300°–400°C. Low δD and δ18O of epidote and low δ34S of anhydrite indicate that the altering fluids in all three areas originated as meteoric waters and have undergone varied ‘oxygen shifts.’ Differences in the 18O shift of the fluids are attributed to differences in hydrothermal systems; low water/rock ratios (≤0.1) are found in the IRDP core and the Reykjavik hole, but high water/rock ratios (>5) at Krafla. The convective hydrothermal activity, which is probably driven by silicic magma beneath the central volcanoes, has caused strong subsolidus depletion of 18O in the rocks. The primary‐magmatic δ18O value of the rocks in the Tertiary IRDP core was about +3.9‰, which is lower than that obtained for fresh basalt from other places. Such exceptionally low δ18O magmas are common in Iceland and may occur as the result of oxygen isotope exchange with or assimilation of altered rocks that form a thick sequence beneath the island due to isostatic subsidence.
Thermal origin of the Hawaiian swell: Heat flow evidence and thermal models
Authors: Von Herzen R. P. , Detrick R. S. , Crough S. T. , Epp D. , Fehn U.
Published: February 2008

The lithospheric reheating hypothesis for the origin of the Hawaiian swell has been tested by heat flow measurements along the swell between Hawaii and Midway. Ninety five measurements at eight carefully selected sites show that the heat flux on the swell increases systematically from near‐normal values (52±2 mW m−2) near Hawaii to values near Midway (59±2 mW m−2) about 20–25% higher than normal for lithosphere of that age (110 m.y.). This heat flow pattern is qualitatively consistent with a deep‐seated thermal anomaly created in a lithospheric plate moving across a fixed heat source. Modeling of the heat flow and bathymetry data on the Hawaiian swell strongly suggest that the swell is caused by a rapid heating (within <5 m.y.) of the lower part (>40‐ to 50‐km depth) of normal 90‐m.y.‐ old lithosphere over a relatively broad (500–1000 km) region near Hawaii. This model is also consistent With gravity and geoid anomalies associated with the swell, which require a low‐density region in the lower lithosphere, and with flexure studies of seamounts in the Hawaiian chain, which indicate that the rigidity of the lithosphere is not significantly reduced by the reheating.

Appendices A‐C are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J82‐005; $1.00. Payment must accompany order.
Lake St. Martin impact structure, Manitoba, Canada: Magnetic anomalies and magnetizations
Authors: Coles R. L. , Clark J. F.
Published: February 2008

A ground magnetic survey has further delineated the previously known intense (1000 nT) negative anomaly at the centre of the Lake St. Martin impact structure, Manitoba. Shocked gneiss in the central uplift has a strong reverse remanent magnetization whose direction is similar to that found after alternating field demagnetization of melt rock specimens from within the impact structure. Shocked gneiss is interpreted as the source of the central negative anomaly. The direction of the remanence is consistent with a Permo‐Triassic age for the impact event. The magnetization in the shocked gneiss is interpreted as a chemical remanent magnetization carried by hematite formed during an extensive alteration of mafic silicates, occurring immediately after the impact.
Magnetic effects of maghemitization of Plio‐Pleistocene marine sediments, northern California
Authors: Kodama Kenneth P.
Published: February 2008

A magnetic study conducted on reversed polarity, Plio‐Pleistocene marine sediments from the Rio Dell formation suggests that maghemitization occurring in the source area may have enhanced their paleomagnetic reliability. In a section characterized by variable amounts of magnetic overprinting, some of the reliable horizons had natural remanent magnetization (NRM) intensities nearly 1 order of magnitude greater than other reliable and all the unreliable horizons from the Rio Dell. These high‐NRM horizons had high anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM) intensities, suggesting a greater concentration of magnetic minerals. Thermal demagnetization data indicate that maghemite is the magnetic carrier in the high‐NRM horizons. ARM versus χ and Lowrie‐Fuller tests show that these horizons have magnetic grains dramatically smaller than magnetite‐bearing horizons. Contraction cracking during maghemitization may have reduced the effective grain size of the maghemite‐bearing horizons. The small magnetic grain size of the maghemite increases these horizons' coercivity and viscous acquisition coefficient but not their JVRB. Sediment grain size analyses show that the magnemite‐bearing horizons have the smallest sediment grains and unreliable magnetite‐bearing horizons have the largest grains. Assuming hydrodynamic sorting of the magnetic oxides, these data suggest that the finest‐grained magnetic minerals are the most susceptible to oxidation. Maghemite occurs in horizons 1.5–0.9 m.y. old. Reduced chemical and increased mechanical weathering during the Pleistocene glaciations could have increased the concentration of magnetic iron oxides in these sediments and caused their maghemitization in the provenance area.
On Venus impact basins: Viscous relaxation of topographic relief
Authors: Solomon Sean C. , Stephens Stuart K. , Head James W.
Published: February 2008

The question of whether impact basins dating from the time of heavy bombardment of the inner solar system are preserved on the surface of Venus is of paramount importance for an understanding of the geological processes that have shaped that planet. Two types of features have been suggested as candidates for impact basins on Venus: (1) large, quasi‐circular topographic depressions and (2) large, quasi‐circular regions of low radar backscatter surrounded by terrain of greater radar backscatter and generally displaying little topographic relief. As a guide to the expected character of an impact basin on Venus, we have calculated the viscous relaxation with time of the topographic relief of such a basin, under the assumption that the surface temperature of the planet has been at or near its presently high value since the time of basin formation. The viscous relaxation model includes the effects of a decrease in effective viscosity with depth and the partial to complete isostatic compensation of initial topography. We conclude that an impact basin several hundred kilometers or greater in diameter and 3 b.y. old or greater in age should have negligible topographic relief at present. This conclusion does not exclude the hypothesis that quasi‐circular features of low radar backscatter but little topographic relief may be relaxed remnants of ancient impact basins. Large quasi‐circular depressions on Venus, such as Atalanta Planitia, however, cannot exceed a few hundred million years in age; otherwise their initial topographic relief would be excessive. Such large depressions are thus likely to be geologically young and to have resulted from processes other than impact. An additional process must therefore have operated on Venus to create large, roughly circular areas of lowland terrain. One possibility is that these topographic features are analogs on Venus to terrestrial platform basins produced by lithospheric extension and thermal subsidence.
Paleomagnetic study of Devonian rocks from Ste. Cécile‐St. Sébastien Region, Quebec Appalachians
Authors: Seguin M. K. , Rao K. V. , Pineault R.
Published: February 2008

Three Devonian rock units were collected for paleomagnetic study from the Ste. Cécile‐St. Sébastien region located in the Gaspé‐Connecticut Valley synclinorium of Québec Appalachians: 15 sites (48 samples) from the metasediments (subgreenschist facies‐siltstones, sandstones) of the Compton Formation (early Middle to late Early Devonian age based on fossil plants) which were tightly folded during Middle Devonian Acadian orogeny; 14 sites (61 samples) from the Ste. Cécile‐St. Sébastien granitic body (362 m.y., K‐Ar age) which intruded the metasediments; 14 sites (62 samples) from the hornfels created subsequent to the emplacement of the intrusive. Detailed thermal cleaning of the metasediments yielded a mean remanence direction characterized by linear vector trajectories, after tilt correction, at D = 209°, I = +20°, k = 30, α95 = 7.2° with unblocking temperature spectrum, 300°–550°C, carried probably by magnetite. The predeformational origin of this remanence is tested at 99% confidence level as well as antipodal normal and reverse polarity directions. The corresponding paleomagnetic pole, MS, falls at 28°N, 77°E (dp, dm = 6°, 11°) in agreement with the recently reported poles which are mostly Early Devonian in age and derived from the southeastern margin of the presumed proto‐Atlantic ocean. AF demagnetization of granites resulted in a univectorial mean remanence at: (1)D = 332°, I = − 39°k = 31 for N = 5 sites in fine‐grained granite (FG) unit; (2) D = 313°, I = −25°; K = 19 for N = 9 sites in medium‐grained granite (MG) unit; and (3) D = 319°, I = −30°, K = 17 for all the total (N = 14) granite (G) sites. AF demagnetization of hornfels revealed a post intrusive mean remanence direction, HF, at D = 346°, I = −22°, K = 8 for N = 14 sites. The individual site‐mean remanence directions (with coercivity spectra: 10–60 mT) from both granites and hornfels suffer from a large declination spread of about 60° but the confidence ovals for the FG and HF remanences overlap. This indicates that the FG and HF remanences are indistinguishable from each other and therefore are proximal in time and probably originated during Middle to Late Devonian period. Interpretation of this kind is speculative in the absence of a convincing evidence for a two (FG younger than MG) or three phase intrusive history of the pluton. Since the pole positions of G (18°N, 150°E), MG (18°N, 158°E), FG (18°N, 137°E) and HF (32°N, 125°E) spread over 40° in longitude, these data cannot confirm or negate the possibility of a Mid‐Paleozoic transcurrent motion proposed for northern Appalachians. We regard only the pole, MS, with probable late Early or early Middle Devonian age as reliable for the apparent polar path construction.
Mineral chemistry of ultramafic cumulates from the North Arm Mountain Massif of the Bay of Islands ophiolite: Evidence for high‐pressure crystal fractionation of oceanic basalts
Authors: Elthon D. , Casey J. F. , Komor S.
Published: February 2008

The mineral chemistry of ultramafic cumulates forming the basal portions of the plutonic section of the North Arm Mountain massif, Bay of Islands ophiolite complex, is not consistent with crystal‐liquid fractionation of a primitive mid‐ocean ridge basalt at low pressures. The appearance of clinopyroxene and orthopyroxene having very high Mg numbers [100 × Mg/(Mg + Fe)= 85 to 92] and the absence of plagioclase as early fractionating phases coprecipitating with forsteritic olivine (Fo86–92) suggest that moderate‐ to high‐pressure phase relationships are applicable. We propose on the basis of mineral chemistry, crystallization sequences, and structural relationships that the basal ultramafic cumulates on the North Arm massif represent the products of high‐pressure (>10 kbar) polybaric crystal fractionation of primary basaltic melts generated within the mantle beneath a mid‐ocean spreading center. These basal ultramafic cumulates were probably precipitated due to in situ crystallization processes along the margin of a vertical conduit extending downward from a crustal level magma chamber to mantle depths that exceed 30 km. After their formation, the cumulates were transported to the near surface along the axis of upwelling and divergence as the lithosphere was separating beneath a mid‐ocean ridge.
Marine geology of the Hess Rise: 2. Basement morphology, sediment thickness, and structural geology
Authors: Kroenke Loren W. , Nemoto Kenji
Published: February 2008

The basement morphology and sediment thickness of the Hess Rise, an oceanic plateau in the central North Pacific, have been mapped on the basis of seismic reflection profiles. The acoustic stratigraphy on and around the rise is correlated with the lithostratigraphy at Deep Sea Drilling Project sites 464, 310, 465, and 466. A total sediment isopach chart of the rise reveals small‐scale departures from the expected sedimentary pattern (thick sediment in shallow areas; thin sediment in deep areas). Sediment‐filled basement depressions result from mass transport; thin sediment (<50 m) occurs on steep scarps, basement ridges, and areas affected by bottom currents. A pre‐Senonian sediment isopach chart shows a thickening from less than 50 m to more than 250 m of sediment from the northeast to the southwest. This trend seems explainable only in terms of the time‐transgressive nature of seafloor formed at a mid‐ocean ridge. The axial trend of the rise (N30°W) parallels nearby Mesozoic magnetic lineations and seems to be isochronous as deduced from the Deep Sea Drilling Project data. The Hess Rise began developing in late Aptian time along a segment of the Pacific‐Farallon Ridge. Important events in the history of the rise are late‐stage volcanism on the southern margin of the rise along the Mendocino Fracture Zone, tectonism and volcanism about 85 Ma that resulted in a major regional unconformity (reflector C), and another period of tectonism and volcanism between 65 and 43 Ma that coincided with the formation of the Emperor Seamounts and created structural benches on the western side of the rise. A significant change in the paleoenvironment that apparently occurred around the Paleogene‐Neogene boundary (∼25–20 Ma) caused pronounced changes in the depositional environment and resulted in another major regional unconformity (reflector A).
Analysis of states of stress between provinces of constant stress
Authors: McGarr A.
Published: February 2008

Recent studies have indicated the existence of crustal provinces or domains within which the state of stress shows little apparent horizontal variation. Between provinces the state of stress may undergo substantial changes, often over remarkably short distances. As analyzed here, the transition in stress state between adjacent domains is caused by shear tractions acting on the base of the elastic‐brittle layer and is mediated by a stress field within a zone between the provinces that can be determined by solving the equations of force equilibrium and stress compatibility. The component of horizontal stress oriented perpendicular to the provincial boundary changes smoothly across the transition zone from one province to the other, whereas the horizontal stress oriented parallel to the boundary shows smaller but discontinuous changes. The horizontal component of shear stress is unlikely to change from one province to another because of the implausible nature of the required basal traction; thus, in this regard, states of stress in adjacent domains appear to be coupled. The coincidence of a number of stress transition zones with boundaries defined by heat flow suggests that in many cases the basal tractions are the result of thermally induced mass transport below the elastic‐brittle layer in the direction of increasing horizontal compressive stress. This analysis provides a quantitative relationship between changes in the stress field measured in the upper portion of the crust and shear stresses acting on the base of the elastic‐brittle layer.
On fluid reserves and the production of superheated steam from fractured, vapor‐dominated geothermal reservoirs
Authors: Pruess K. , Narasimhan T. N.
Published: February 2008

Vapor‐dominated geothermal reservoirs produce saturated or superheated steam, and vertical pressure gradients are close to vapor static. These observations have been generally accepted as providing conclusive evidence that the liquid saturation must be rather small (<50%) in order that liquid may be nearly immobile. This conclusion ignores the crucial role of conductive heat transfer mechanisms in fractured reservoirs for vaporizing liquid flowing under two‐phase conditions. We have developed a multiple interacting continuum method (MINC) for numerically simulating two‐phase flow of a homogeneous fluid in a fractured porous medium. Application of this method to reservoir conditions representative of The Geysers, California, and results from an analytical approximation show that, for matrix permeability less than a critical value (≈2.5 to 5 microdarcies), the mass flux of water from the matrix to the fractures will be continuously vaporized by heat transported due to conduction. This gives rise to production of superheated steam even when the matrix has nearly full liquid saturation. Simple estimates also show that heat‐driven steam/water counterflow can maintain a nearly vapor static vertical pressure profile in the presence of mobile liquid water in a reservoir with low vertical matrix permeability. The implication of these findings is that the fluid reserves of vapor‐dominated geothermal reservoirs may be larger by a factor of about 2 than has generally been believed in the past.
New radar‐derived topography for the northern hemisphere of Mars
Authors: Downs G. S. , Mouginis‐Mark P. J. , Zisk S. H. , Thompson T. W.
Published: February 2008

Earth‐based radar altimetry data for the northern equatorial belt of Mars (6°S–23°N) have recently been reduced to a common basis corresponding to the 6.1‐mbar reference surface. A first look at these data indicates that the elevations of Tharsis, Elysium, and Lunae Planum are lower (by 2–5 km) than has been suggested by previous estimates. These differences show that the required amount of tectonic uplift (or constructional volcanism) for each area is less than has been previously envisioned. Atmospheric or surficial conditions are suggested which may explain the discrepancies between the radar topography and elevations measured by other techniques. The topographies of Chryse Planitia, Syrtis Major, and Valles Marineris are also described.
Evolution of the Tharsis Province of Mars: The importance of heterogeneous lithospheric thickness and volcanic construction
Authors: Solomon Sean C. , Head James W.
Published: February 2008

The Tharsis province of Mars is a broad region characterized by anomalously elevated topography, a positive free‐air gravity anomaly, and extensive volcanic and tectonic activity. The evolution of this region has spanned up to 4 b.y. of Martian history. The traditional explanation of the Tharsis province is that uplift of the lithosphere caused by a thermal, chemical, or dynamical anomaly in the Martian mantle or crust led to lithospheric fracturing and to the later volcanic emplacement of thin plains units and large shields. By this explanation the majority of the topographic anomaly is due to uplift. The stress field predicted for lithospheric uplift, however, does not match the generally radial trends of observed extensional fractures. Strictly thermal or compositional explanations for the support of the Tharsis rise encounter problems with the magnitude and duration of the required lateral variations in mantle density. For these reasons, we propose a new model for the origin and evolution of the Tharsis province which emphasizes that the topography of Tharsis may be produced largely by construction, rather than uplift. The model is based on the premise that the elastic lithosphere of Mars was laterally heterogeneous early in Martian history; such heterogeneity is discernible later in Martian history from the variable tectonic response of the lithosphere to local surface loads. Stress due to both global and local causes was concentrated in zones of thin lithosphere, including at least a portion of the Tharsis region. As a result, fracturing was also concentrated in such zones and favored localization of volcanism by providing access to the surface for mantle‐derived magma. The heating associated with volcanism maintained the lithosphere locally thin, so that further fracturing and volcanism were concentrated in the same area. The Tharsis rise was thus built primarily by volcanic construction. Most visible tectonic features, by this model, were produced by the response of the lithosphere to loading, not by uplift. Early in the history of Tharsis the lithosphere responded to volcanic loading by nearly local isostatic compensation, while later additional loads have been partly supported by the finite strength of the globally thick elastic lithosphere of Mars. This mechanism for the evolution of Tharsis led to permanent topographic and gravity highs and a greatly thickened crust in the Tharsis region. A major advantage of the model is that no anomalous dynamical or chemical properties need to be sustained in the Martian mantle beneath Tharsis for billions of years. The mantle beneath Tharsis would thus play a passive rather than an active role in the regional volcanic and tectonic activity, much like the role of the mantle beneath major midocean ridges on earth. This and other models for the origin and evolution of the Tharsis province can be further tested by establishing the detailed chronology of tectonic features in the region.
Further evidence for a mass movement origin of the Olympus Mons aureole
Authors: Lopes Rosaly , Guest J. E. , Hiller K. , Neukum G.
Published: February 2008

Analysis of the ridge, graben, and fracture patterns on the aureole material surrounding Olympus Mons lends further support to an origin by gravity sliding of the outer flanks of the shield. The patterns provide evidence of the nature of sliding, especially in the last stages of emplacement. No part of the slide need have traveled the whole extent of the aureole. For the slides to occur on a shallow slope, reduction of shear strength of the materials involved is required, possibly by melting of ground ice. Triggering mechanisms could be volcanically related marsquakes and possibly stresses induced on the surrounding area by the growing Olympus Mons volcano.
Surface properties of ancient cratered terrain in the northern hemisphere of Mars
Authors: Zimbelman James R. , Greeley Ronald
Published: February 2008

The hilly and cratered material of Scott and Carr [1978], interpreted to be the oldest extensively exposed surface of Mars, has been examined where it is covered by high‐resolution (16–62 km2) infrared data from the Viking IRTM. This allows spatial variations in observed properties to be associated directly with geologic features and an assessment to be made of the contribution to the remotely determined properties from the ancient rock unit. The measured nighttime temperatures at 20 and 11 μm indicate that from 0 to 20% of the surface consists of high thermal inertia ‘blocks’ (assumed to be I = 30 × 10−3 cal cm−2 s½ K−1) surrounded by lower thermal inertia ‘soil’ (I = 1.5 to 7.5). Block abundances are typically ∼9% within the Arabia low thermal inertia region (I = 1.5 to 3.0) identified from global mapping and increase away from Arabia, particularly to the south. Geologic features crossed by the IRTM data generally show no distinguishable difference from the regional properties with the exceptions of dark patch material and craters near 27°N, 338°W. The dark patch material, typically within craters, has block abundances (5–20%) and soil thermal inertias (5.0–7.0 for an albedo of 0.15) distinctly different from the Arabia material; nonunit emissivity for this material would make the block abundances upper limits and the soil thermal inertias lower limits. The effective grain diameter of the dark patch material (160 to 360 μm) is indicative of sand‐sized particles capable of saltation. Two craters near 27°N, 338°W have slightly higher soil thermal inertias (∼3.5, corresponding to ∼70 μm) and lower block abundances (0–5%) than the surrounding Arabia material, but no prominent dark patches are associated with them. Imaging data from within and around the Arabia region indicate that subdued surface morphology is not always associated with low thermal inertias. The extensive mantling of northern hemisphere locations of ancient cratered terrain by fine‐grained material does not allow thermal measurements to be directly related to rock unit properties, but less mantling may be present in southern hemisphere locations of this material.
Galapagos Hot Spot‐SPreading Center System: 2. 87SR/86 SR and large ion lithophile element variations (85°W‐101°W)
Authors: Verma Surendra Pal , Schilling Jean‐Guy
Published: February 2008

Thirty eight samples of basalts dredged from the Galapagos spreading center (GSC) between 85°W and 101.5°W longitudes have been analyzed for K, Rb, Cs, Ba, and Sr contents as well as for 87Sr/86sr ratios. Basalts between 85°W and 89°W segment of the GSC range from 0.7025 to 0.7028 in 87Sr/86sr and those between 95.5°W and 101.5°W range from 0.7026 to 0.7027. These basalts are characterized by LREE‐depleted patterns. Basalts from 89°W to 95.5°W segment having slightly LREE‐depleted to LREE‐enriched patterns ((La/Sm)E.F. between 0.8 and 2.4) range from 0.7027 to 0.7031 in 87Sr/86Sr. The longitudinal 87Sr/86Sr variation shows a maximum at about 92°W, where the GSC is nearest to Darwin Island. The alkali and alkaline earth elements as well as several element ratios also show maxima in their longitudinal profiles, but these occur at about 91°W. The apparent shift of 87Sr/86Sr maximum with respect to that of the trace elements may be related to differences in the degree of partial melting. The highest 87Sr/86Sr from the GSC is only slightly lower than the highest 87Sr/86Sr for tholeiitic basalts from the Galapagos Archipelago. Several possible hypotheses have been examined to account for these geochemical and Sr isotopic variations. In the light of the presently available data, the most promising explanation seems to be a binary mixing of hot spot (or plume‐type) material with the source of ‘normal’ mid‐ocean ridge basalts and resulting rift propagation. Both end‐members seem to be heterogeneous, and therefore it is at present difficult to put severe constraints on this binary mixing. Nevertheless, the flow pattern of mantle material from the hot spot does not appear to be radial but is preferentially channeled along the 91°W and 92.5°W fracture zones toward the rift.