bluuebunny/crossref_metadata_2025 huggingface dataset:
Monoterpene emissions from Scots pine and Norwegian spruce
Authors: Janson Robert W.
Published: February 2008

Rates of monoterpene emissions from Scots pine (Pinus sylvestris) and Norwegian spruce (Picea abies) have been measured at four sites in Sweden with a dynamic flow chamber technique. Forest floor emissions have been made in the pine forest with the static chamber technique. Sampling was done with Tenax TA and analysis and detection by GC and ion trap detection. The compounds Δ3‐carene and α‐pinene were the predominant terpenes emitted from the crown and floor of the Scots pine forest. Alpha‐pinene was the main terpene emitted from Norwegian spruce at the sites in southern and central Sweden, while Δ3‐carene was predominant at the northern site. The relative composition of the emission of both species underwent changes in early spring and fall. Emission rates, normalized to temperature, were seen to vary diurnally with a maximum at midday, and seasonally with maxima in early May and October, and a summer maximum in June–July. The possible dependence of the emission rate on needle growth rate and other plant‐physiological processes is discussed. A higher emission rate and different relative composition of the emission was seen to occur when the vegetation was wet, as compared to dry vegetation. The emission from the pine forest floor was seen to have a composition different from that of the crown and a seasonality of the rate similar to that of the crown. The ground emission could not be explained by sources in the litter or ground vegetation alone, and it is suggested that the root system of the trees is also an emission source. The emission rate from the pine forest floor was of the order of 30% of the crown emission. The July rate of emission from the crown of Scots pine, normalized to 20°C and averaged over four sites in Sweden, was 0.8 ± 0.4 μg (gdw (grams dry weight) h)−1, and for Norwegian spruce, 0.5 ± 0.7 μg(gdw h)−1. It would seem that previous regional and global estimates of hydrocarbon fluxes to the atmosphere have used emission factors which are too high for boreal coniferous forests.
Sulfate aerosol distributions and cloud variations during El Niño anomalies
Authors: Parungo Farn , Hicks Bruce
Published: February 2008

The effects of aerosols on cloud characteristics, albedo, rainfall amount, and overall climate changes were investigated by assessing the qualitative associations and quantitative correlations between the relevant variables during El Niño‐Southern Oscillation (ENSO) perturbations. Both historical records and data from recent field measurements for the Pacific Ocean region were used for the investigation. The results show that ENSO perturbations could change sulfate aerosol production and distribution over the surveyed regions. Strong correlations were observed between condensation nucleus concentrations and sulfate aerosol concentrations, and between cloud amount and albedo. Weak but significant correlations were also observed between condensation nucleus concentrations and cloud amounts, and between sulfate aerosol concentrations and rainfall amounts. Although sulfate aerosols appeared to have a strong impact on cloud microphysics, the present data confirm that cloud dynamics play the pivotal role in control of cloud types and cloud amount in the studied regions.
Winter biotic activity and production of CO2 in Siberian soils: A factor in the greenhouse effect
Authors: Zimov S. A. , Zimova G. M. , Daviodov S. P. , Daviodova A. I. , Voropaev Y. V. , Voropaeva Z. V. , Prosiannikov S. F. , Prosiannikova O. V. , Semiletova I. V. , Semiletov I. P.
Published: February 2008

Soil CO2 emissions at latitudes near 70°N in northeastern Siberia have been measured and occur at levels high enough to account for the observed winter maximum in atmospheric CO2 concentrations in this latitude. This CO2 is produced by biological activity at the bottom of the active layer above the permafrost. Ecological and anthropological factors may play a role in stimulating this CO2 emission.
Pacific marine aerosol: 2. Equatorial gradients in chlorophyll, ammonium, and excess sulfate during SAGA 3
Authors: Clarke Antony D. , Porter John N.
Published: February 2008

In February and March 1990, measurements of aerosol physicochemistry were made during five transects across the equator between 15°N and 10°S. Marked equatorial gradients in both the aerosol NH4+/SO4= ratio and the strong surface water chlorophyll were associated with boundaries separating oligotrophic waters and regions of equatorial upwelling. Highest aerosol ammonium concentrations appeared to be unrelated to continental signatures but corresponded to regions of highest chlorophyll concentrations. Favorable aerosol chemistry, wind directions, and cruise tracks in conjunction with rapid aerosol sampling made it possible to estimate the flux of ammonia from the surface in these transition regions at about 10 μmol m−2 d−1 with possibly greater values in regions where higher chlorophyll concentrations exceeded about 0.25 mg m−3. Low NH4+/SO4= values in aerosol over oligotrophic regions with chlorophyll concentrations below 0.1 mg m−3 suggest the flux in these regions was about an order of magnitude lower. Aerosol sulfate concentrations were also generally elevated over the upwelling region but showed a less pronounced relationship to chlorophyll abundance, suggesting independent source mechanisms for ammonium and sulfate. Rapid variability in mass and number concentrations were evident in convective regions. Both depletion of larger aerosol (mass) through precipitation scavenging and an increase in the number of smaller aerosol in region of subsidence indicate the complex relationship among CN, CCN, and aerosol mass in the remote marine boundary layer.
Shifts in ice sheet temperatures
Authors: Colbeck S. C.
Published: February 2008

The entire temperature gradient in an ice sheet can be shifted by processes that occur just below the surface. Energy absorbed close to the surface flows back to the surface which steepens the average temperature gradient near the surface and shifts the entire temperature profile to higher values. For a constant density this shift equals Â/α2k, where Â is the average annual strength of the energy source, α is its exponential decay rate with depth, and k is the thermal conductivity. Various sources are considered as are density profiles and transients. While some sources can increase the 10‐m temperature in ice sheets, air currents flowing through snow can either heat or cool the snow at depth and can dominate all other processes. The relative contributions of the different mechanisms must be evaluated for any particular site.
A simulation of the Cerro Hudson SO2 cloud
Authors: Schoeberl Mark R. , Doiron Scott D. , Lait Leslie R. , Newman Paul A. , Krueger Arlin J.
Published: February 2008

An isentropic trajectory model is used to simulate the evolution of the southern hemisphere SO2 cloud associated with the eruption of Cerro Hudson. By matching the parcel trajectories with total ozone mapping spectrometer SO2 retrievals, the principal stratospheric injection region is determined to be between 11 and 16 km in altitude. This region is characterized by weak wind shears and is located just poleward of the subtropical jet in the outer fringe of the stratospheric polar vortex. The lack of wind shear in the injection region explains the slow zonal dispersal of the SO2 cloud which was still clearly observed 19 days after the eruption. The trajectory model simulation of the SO2 cloud shows good agreement with observations for 7 days after the eruption. Using the potential vorticity and potential temperature estimates of the initial eruption cloud, the cloud position relative to the polar night jet is shown to be nearly fixed up to September 2, 1991, which was as long as the cloud was observed. This result suggests that the lower stratospheric polar and mid‐latitude regions are nearly isolated from each other during the late August period.
Diurnal cycle of O3and monoterpenes in a coniferous forest: Importance of atmospheric stability, surface exchange, and chemistry
Authors: Johansson C. , Janson R. W.
Published: February 2008

A one‐dimensional model of turbulent diffusion and chemistry has been applied to evaluate measurements of the diurnal variation of O3and monoterpenes in a coniferous forest in Sweden. The role of photochemical, meteorological, and surface exchange processes was investigated by studying days characteristic of neutral and stable/unstable conditions, respectively. Atmospheric stability, mixing height, and eddy diffusivities were estimated from acoustic soundings (SODAR) measurements. The atmospheric concentrations and surface exchange rates of O3, monoterpenes and NOxwere taken from measurements at the site, using chamber techniques for the flux measurements. The chemical scheme applied in the model included 45 reactions representative of a clean air situation with rather small impact of anthropogenic hydrocarbon emissions. The model calculations showed the predominant role of turbulent mixing in the observed diurnal variation of O3and monoterpenes. The reaction with OH accounted for 50–60% of the terpene loss over a 24‐hour period with a well‐mixed boundary layer and about 40% when nighttime conditions were stable. Increasing the background O3concentration to 80–90 ppbv increased the relative importance of the terpene + O3reaction to equal that of the OH reaction. The terpene chemical lifetime was found to be shortest, less than 1 hour, under neutral nighttime conditions when NO3concentrations were high. Under stable nighttime conditions, the lower boundary layer became chemically less active due to the depletion of O3and NO3. Terpene reactions were found to be significant nighttime sinks for O3and NO3under stable conditions and during periods or at sites of nocturnal terpene emission rates greater than 150μg/m2h. An increase in NOxconcentrations from 0.5 ppbv to 1 and then 5 ppbv, changed O3concentrations first up and then down, by not more than 1 ppbv. At terpene emission rates of 50–100 μg (m2h)−1, the terpene + OH reaction caused less than 5% of the OH loss near the surface, while at a rate of 1500 μg (m2h)−1, it accounted for up to 30–40% of OH loss. The impact decreased rapidly with height. The daytime terpene+NO3reaction was found to be a minor pathway for NOxin comparison to the OH + NO2reaction.
Tropical deforestation: Modeling local‐ to regional‐scale climate change
Authors: Henderson‐Sellers A. , Dickinson R. E. , Durbidge T. B. , Kennedy P. J. , McGuffie K. , Pitman A. J.
Published: February 2008

A tropical deforestation experiment has been conducted in which the tropical moist forest throughout the Amazon Basin and SE Asia has been replaced by scrub grassland in a version of the National Center for Atmospheric Research Community Climate Model (Version 1), which also incorporates a mixed layer ocean and the Biosphere‐Atmosphere Transfer Scheme. In both regions we find a smaller temperature increase than did all other previous experiments except that of Henderson‐Sellers and Gornitz (1984); indeed, temperatures decrease in some months. On the other hand, we find larger runoff decreases and a larger difference between the changes in evaporation and precipitation than all earlier experiments indicating a basin‐wide decrease in moisture convergence. Disturbances in South America extend beyond the region of land‐surface change causing temperature reductions and precipitation increases to the south of the deforested Amazon. Changes to the surface climate in the deforested area take between 1 to 2 years to become fully established although the root zone soil moisture is still decreasing in year 3 and the variability of soil moisture and total cloud amount continue increasing throughout the 6‐year integration. Besides temperature and precipitation, other fields show statistically significant alterations, especially evaporation and net surface radiation (both decreased).
Time‐space model of summer ozone distribution around Ravenna
Authors: Fortezza F. , Strocchi V. , Bonasoni P. , Georgiadis T. , Giovanelli G.
Published: February 2008

The time‐space distribution of photochemical ozone during the summer along the western coast of the upper Adriatic Sea is strongly influenced by small‐scale transport phenomena, especially land and sea breezes (Giovanelli et al., 1985). When these events involve the movement of air masses rich in primary and secondary pollutants across one or more lines of physical discontinuities, such as that between land and sea or across the inshore strip (flat, predominantly farmland) lying between seaboard and city, they can determine irregular, varying ozone distribution patterns at ground level in areas only a few kilometers apart. An empirical model was developed to characterize these local differences in ozone distribution over the entire area. The method makes it possible to compare the data recorded at the various sites and times by “standardizing” parameters for meteorological and local‐diffusion conditions as well as for source emission values by processing the data from a series of measurements taken at several coastal sites, both inshore and offshore, during the summers 1980–1985. The resulting space‐time models show, despite the limited number of monitor stations, “local” differences in the processes of production, transport, and spread of photochemical ozone in the area studied. The final distribution of the mean daily evolution (MDE) of ozone throughout the greater Ravenna area is reported and discussed.
Total electron scattering and electronic state excitations cross sections for O2, CO, and CH4
Authors: Kanik I. , Trajmar S. , Nickel J. C.
Published: February 2008

Available electron collision cross section data concerning total and elastic scattering, vibrational excitation, and ionization for O2, CO, and CH4 have been critically reviewed, and a set of cross sections for modeling of planetary atmospheric behavior is recommended. Utilizing these recommended cross sections, we derived total electronic state excitation cross sections and upper limits for dissociation cross sections, which in the case of CH4 should very closely equal the actual dissociation cross section.
Kinetics of crystal growth in a terrestrial magma ocean
Authors: Solomatov Viatcheslav S. , Stevenson David J.
Published: February 2008

The problem of crystal sizes is one of the central problems of differentiation of a terrestrial magma ocean and it has been an arbitrary parameter in previous models. The crystal sizes are controlled by kinetics of nucleation and crystal growth in a convective magma ocean. In contrast with crystallization in magma chambers, volcanic lavas, dikes, and other relatively well studied systems, nucleation and crystallization of solid phases occur due to the adiabatic compression in downward moving magma (adiabatic “cooling”). This problem is solved analytically for an arbitrary crystal growth law, using the following assumptions: convection is not influenced by the kinetics, interface kinetics is the rate controlling mechanism of crystal growth, and the adiabatic cooling is sufficiently slow for the asymptotic solution to be valid. The problems of nucleation and crystal growth at constant heat flux from the system and at constant temperature drop rate are shown to be described with similar equations. This allows comparison with numerical and experimental data available for these cases. A good agreement was found. When, during the cooling, the temperature drops below the temperature of the expected solid phase appearance, the subsequent evolution consists of three basic periods: cooling without any nucleation and crystallization, a short time interval of nucleation and initial crystallization (relaxation to equilibrium), and slow crystallization due to crystal growth controlled by quasi‐equilibrium cooling. In contrast to previously discussed problems, nucleation is not as important as the crystal growth rate function and the rate of cooling. The physics of this unusual behavior is that both the characteristic nucleation rate and the time interval during which the nucleation takes place are now controlled by a competition between the cooling and crystallization rates. A probable size range for the magma ocean is found to be 10−2− 1 cm, which is close to the upper bound for the critical crystal size dividing fractional and nonfractional crystallization discussed elsewhere in this issue. Both the volatile content and pressure are important and can influence the estimate by 1–2 orders of magnitude. Different kinds of Ostwald ripening take place in the final stage of the crystal growth. If the surface nucleation is the rate‐controlling mechanism of crystal growth at small supercooling, then the Ostwald ripening is negligibly slow. In the case of other mechanisms of crystal growth, the crystal radius can reach the critical value required to start the fractional crystallization. It can happen in the latest stages of the evolution when the crystals do not dissolve completely and the time for the ripening is large.
Velocity distributions of hydrogen atoms and hydroxyl radicals produced through solar photodissociation of water
Authors: Wu C. Y. Robert , Chen F. Z.
Published: February 2008

The calculated velocity distributions of hydrogen atoms and hydroxyl radicals produced through solar photodissociation of gaseous water molecules are presented. Under collisionless conditions, the calculation has been carried out using (1) the most recently available absolute partial cross sections for the production of H and OH through photodissociation of H2O from its absorption onset at 1860 Å down to 500 Å, (2) the newly available vibrational and rotational energy distributions of both the excited and ground state OH photofragments, (3) the calculated cross sections for the total dissociation processes, and (4) the integrated solar flux in 10 Å increments from 500 to 1860Å in the continuum regions and the specific wavelength and flux at the bright solar lines, e.g., the H Lyα, Lyβ, Lyγ, O VI, C III, He I. Because of the lack of data in several dissociation processes or in certain spectral regions, we can only obtain upper and lower bound velocity distributions. In terms of quantum yields, the lower bound case is set by assuming that all the undetected neutral products are something other than H and OH fragments and an upper bound is set by assuming the undetected neutral products are all H and OH fragments. In terms of internal energy distributions of the OH fragments the upper bound is set by neglecting them and a lower bound is set by taking the available or the best estimated values. The calculated results show that the H atoms and the OH radicals produced exhibit multiple velocity groups. Since most of the current cometary modeling uses a single velocity of 20 km/s associated with the photodissociation of H2O, the present results may be useful in interpreting the many peaks observed in the velocity distributions of the H Lyα and Hα of comets.
Iunconsistencies in Accumulation Rates of Black Sea Sediments Inferred from Records of Laminae and 210Pb
Authors: Crusius J. , Anderson R. F.
Published: February 2008

Recently‐published estimates for the age of the unit 1‐unit 2 contact in Black Sea sediments based on accelerator mass spectrometry (AMS) 14C measurements [Jones, 1990; Calvert et al., 1991] appear to be older than those based on the previously published chronology based on lamina couplets [Degens et al., 1980; Hay, 1988] by a factor of 2 to 3. To help reconcile the differences, we compare sediment accumulation rates based on the 210Pb method with estimates based on lamina counts for two cores from the Black Sea abyssal plain. Accumulation rates estimated using the 210Pb technique have varied little over the last 150 years from the averages of 55 and 50 g m−2 yr−1 at stations in the western and eastern basins, respectively. These values are about a factor of 2 lower than accumulation rates derived by counting lamina couplets over the dated intervals. Close examination of the laminae suggests that the discrepancy exists both because it is difficult to count the very fine laminae and because a complete couplet is not deposited every year. In order to provide a useful stratigraphic horizon for future investigators studying sedimentary records of the Black Sea, we estimate the age of a distinct black marker horizon which can be easily identified across the entire abyssal plain to be 150±8 years (deposited in 1838±8 A.D.).
Seismic and geologic evidence for Early Paleogene deepwater circulation in the western North Atlantic
Authors: Mountain Gregory S. , Miller Kenneth G.
Published: February 2008

Previous studies of paleogeography, global climate, and foraminiferal stable isotopes have suggested that Paleogene deepwater circulation was significantly different from that of today. Extracting details of deepwater history from the western North Atlantic has been hampered by Oligocene erosion and the lack of ideally located drill holes. We report the discovery of a seismic disconformity on the SW Bermuda Rise that we have tied to a late Paleocene erosional event in Deep Sea Drilling Project (DSDP) boreholes from this region. Distribution of this new seismic marker, Reflector Ab, indicates that it was formed by cyclonically circulating bottom water entering the western North Atlantic from the south. We attribute this vigorous deep circulation to a Southern Ocean source, analogous to the effects of modern Antarctic Bottom Water; this interpretation is supported by isotopic comparisons that show the Paleocene Southern Ocean was enriched in 13C relative to the Pacific. We distinguish Reflector Ab from the overlying Reflector Au on the basis of superposition and ages obtained at DSDP sites and piston cores; using one of the cores, we have estimated that the youngest age for Reflector Au is earliest Oligocene (36.6–34.0 Ma), while its oldest age has been previously established as late Eocene. Laminated sediments beneath Reflector Ab are enriched in organic carbon, indicating that the early Paleocene deep western North Atlantic was occasionally anoxic. These low oxygen levels may have been the result of sluggish deep circulation and/or low‐latitude production of Warm Saline Deep Water (WSDW). Sediments immediately above Reflector Ab show that deposition resumed on the western Bermuda Rise (Site 387) by the latest Paleocene to earliest Eocene (Biochron NP10; 57.2±0.6 Ma). While deposition resumed on the central Bermuda Rise (Site 386) during the late Paleocene, erosion/nondeposition continued across much of the Bermuda Rise. Widespread resumption of pelagic deposition in the early Eocene is consistent with δ13C evidence for reduced Southern Ocean influence at this time. However, lower Eocene sediments record well‐oxygenated conditions, and δ13C and δ18O comparisons indicate that there were two early Eocene deepwater sources: the Southern Ocean and WSDW (Pak and Miller, this issue).
Interhemispheric moisture transport in the Indian Ocean summer monsoon: Data‐model and model‐model comparisons
Authors: Clemens Steven C. , Oglesby Robert J.
Published: February 2008

Exchange of energy and mass between the ocean and the atmosphere is an integral component of Indian Ocean summer‐monsoon dynamics. Most previous general circulation model simulations of the monsoon have prescribed sea surface temperature (SST), thus severely constraining atmosphere‐ocean energy exchanges. Here we compare model physics, based on both prescribed and computed SST simulations, to observed climate links between northern hemisphere monsoon strength and the cross‐equatorial transport of moisture (latent heat) from the southern subtropical Indian Ocean. For the computed SST case, stronger southern hemisphere southeast trades and colder southern subtropical SSTs are associated with increased cross‐equatorial moisture flux. Increased moisture flux induces strengthened monsoon circulation via increased condensational heating and decreased sea level pressure over Asia. For the prescribed SST case, stronger southeast trades and colder southern subtropical SSTs are also associated with increased cross‐equatorial moisture flux and, hence, strengthened monsoon circulation. However, in this case, the increased moisture flux is induced by strengthened cross‐equatorial winds associated with subsidence over the cold southern hemisphere SSTs (i.e., increased sea level pressure in the southern subtropics). Thus the model physics associated with changes in monsoon strength are very different for the prescribed and computed SST simulations. Only the computed SST simulation replicates observed meteorological relationships.
Jurassic and Cretaceous paleomagnetic data from the Southern Alps (Italy)
Authors: Channell J. E. T. , Doglioni C. , Stoner J. S.
Published: February 2008

Jurassic and Cretaceous paleomagnetic data from 46 sites in the Venetian Alps, Trento Plateau and SE Lombardian Basin have been compiled. Nine of these site mean directions have been previously published. The sites can be divided into four age groups (Senonian, Hauterivian/Barremian, Kimmeridgian/Tithonian, and Callovian/Oxfordian). For three of the age groups, positive regional fold tests indicate (at the 99% confidence level) that the characteristic magnetization components predate the deformation. Structural mapping in the Venetian Alps allowed us to assign sites to individual thrust sheets, and we observe no significant relative rotation of thrust sheets in the nappe pile. Indeed, we observe remarkable consistency in mean direction throughout the entire sampling area for each of the four age groups. The Senonian and Hauterivian/Barremian pole positions lie close to coeval poles on African apparent polar wander paths (APWPs). This result is consistent with previous analyses of middle and Late Cretaceous data from the Venetian Alps. The new Jurassic poles deviate significantly from recently published African APWPs, reflecting either the poor definition of the African APWP in the Late Jurassic or about 20° anticlockwise rotation of the Southern Alps relative to Africa during Early Cretaceous time. The former of the two possibilities is favored by a new Late Jurassic/Early Cretaceous pole from North America, and by Early Permian paleomagnetic data from the Southern Alps which are consistent with coeval African data, implying that the movement of the Southern Alps was similar to that of Africa since Early Permian.
Upper Jurassic‐Lower Cretaceous basinal strata along the Cordilleran Margin: Implications for the accretionary history of the Alexander‐Wrangellia‐Peninsular Terrane
Authors: McClelland William C. , Gehrels George E. , Saleeby Jason B.
Published: February 2008

Upper Jurassic and Lower Cretaceous basinal strata are preserved in a discontinuous belt along the inboard margin of the Alexander‐Wrangellia‐Peninsular terrane (AWP) in Alaska and western Canada, on the outboard margin of terranes in the Canadian Cordillera accreted to North America prior to Late Jurassic time, and along the Cordilleran margin from southern Oregon to southern California. Nearly all of the basinal assemblages contain turbiditic strata deposited between Oxfordian and Albian time. Arc‐type volcanic rocks and abundant volcanic detritus in many of the assemblages suggest deposition within or adjacent to a coeval arc complex. On the basis of the general similarities between the basinal sequences, we propose that they record involvement of the AWP in the Late Jurassic‐Early Cretaceous evolution of the Cordilleran margin. A geologically reasonable scenario for the accretion of the AWP includes (1) Middle Jurassic accretion to the Cordilleran margin, in particular the Stikine and Yukon‐Tanana terranes, in a dextral transpressional regime, (2) Late Jurassic‐Early Cretaceous overall northward translation of the AWP and evolution of a series of transtensional basins within a complex dextral strike‐slip system along the Cordilleran margin, and (3) mid‐Cretaceous structural imbrication of the AWP and inboard terranes that either terminated or resulted in a change in the character of deposition in the marginal basins. Mid‐Cretaceous deformation along the inboard margin of the AWP was broadly synchronous with contractional deformation throughout the Cordillera and most likely due to changes in subduction zone parameters along the Cordilleran margin, outboard of the AWP, rather than collision of the AWP.
Kinematics and timing of three superposed extensional systems, east central Idaho: Evidence for an Eocene tectonic transition
Authors: Janecke Susanne U.
Published: February 2008

Cenozoic crustal extension in east central Idaho began about 50 Ma and continues at present. Three distinct episodes characterize one of the longest intervals of Cenozoic extension yet documented in the North America Cordillera. Crosscutting relationships between NE striking normal faults and volcanic rocks, regional dike trends, and slickenline data indicate NW‐SE extension during peak Eocene volcanism about 49–48 Ma (episode 1). NE striking normal faults, with at most a few kilometers of offset, formed in an intraarc setting during rapid NE subduction of oceanic plates under the Pacific Northwest. North to NNW striking and west dipping normal faults, with offsets up to 10–15 km, formed during a younger middle Eocene to Oligocene basin‐forming event (episode 2). This newly documented episode was the most important extensional event in east central Idaho and began during the waning phases of Challis volcanism. WSW‐ENE to SW‐NE extension during episode 2 was nearly perpendicular to the extension direction during episode 1 and perpendicular to the grain of the Idaho‐Montana fold and thrust belt. The flip in extension direction between episode 1 and episode 2 is tightly constrained by 40Ar/39Ar age determinations to have taken place at the end of Eocene Challis magmatism about 46–48 Ma. I infer that plate boundary forces controlled the geometry of normal faults and dikes during episode 1, whereas internal stresses within previously thickened crust drove major SW to WSW directed extension during episode 2. A drop in convergence rates between the North American and Farallon plates between 59 Ma and 42 Ma (Stock and Molnar, 1988) may coincide with the onset of gravitational spreading during episode 2 and may also explain the abrupt end of Eocene magmatism in the Pacific Northwest. Miocene and younger SW dipping Basin and Range faults (episode 3) extended the region in a NE‐SW direction. Although faults formed during episode 2 and episode 3 are not parallel, slickenlines indicate only small changes in slip vector trends, suggesting little rotation of the extension direction in east central Idaho since 46 Ma.
Late Paleozoic Rifting in northern Pakistan
Authors: Pogue Kevin R. , DiPietro Joseph A. , Khan Said Rahim , Hughes Scott S. , Dilles John H. , Lawrence Robert D.
Published: February 2008

Metasedimentary rocks exposed in the eastern Peshawar basin and the southern Swat region of northern Pakistan provide evidence for late Paleozoic continental rifting. The onset of extensional tectonics in the Early Carboniferous is indicated by north derived clasts in the Jafar Kandao Formation eroded from thermally induced uplifts of parts of the formerly passive margin of Gondwana. Rift highlands were eroded until they were inundated during the Middle Carboniferous. Renewed uplift accompanied the eruption of basaltic lava flows during the Early Permian. Uplift along south dipping, northeast striking normal faults during the Carboniferous was accompanied by alkaline magmatism represented by the Shewa‐Shahbazgarhi and Warsak porphyries and Koga syenite. Geochemistry of basaltic flows (now amphibolites) and intrusions associated with Permian uplift is similar to the coeval Panjal volcanics of northwestern India and indicates rift zone magmatism. Postrifting thermal subsidence led to the deposition of Upper Triassic marine carbonate rocks which unconformably overlie the rift basalts. A similar tectonic history in central Afghanistan suggests continuity between the two regions prior to the opening of the Neo‐Tethys.
Gold butte crustal section, South Virgin Mountains, Nevada
Authors: Fryxell Joan E. , Salton Gillian G. , Selverstone Jane , Wernicke Brian
Published: February 2008

The Gold Butte block consists of a complex of Proterozoic medium‐ to high‐grade metamorphic and plutonic rocks, unconformably overlain by steeply east dipping Paleozoic sedimentary rocks on its east side. This complex is one of the largest continuous exposures of crystalline rocks in the southwestern United States that lie structurally beneath a steeply tilted section of Phanerozoic strata, and it may represent part of an intact Proterozoic‐Miocene crustal section reaching paleodepths of approximately 15 km. The section was exposed during tectonic denudation in Miocene time (at about 15 Ma) and lies directly adjacent to the Colorado Plateau to the east. Proterozoic and Tertiary geological features support the hypothesis that the block represents a relatively intact crustal section. A large exposure of Proterozoic rapakivi granite, the Gold Butte Granite (circa 1.45 Ga), crops out over most of the crystalline complex and displays strong zoning in phenocryst abundance, which increases toward the top of a 10‐ to 12‐km‐thick magma chamber. The wallrocks of the granite are predominantly garnet‐bearing gneisses (circa 1.7 Ga), which display increasing pressures of final equilibration westward away from the basal Phanerozoic unconformity. Thermobarometry of gneisses immediately beneath the unconformity indicate pressures of 2–3 kbar, whereas those 12–13 km west of the unconformity yield pressures of 5–6 kbar, suggesting the crustal section was established by 1.7 Ga. The style of deformation displayed by denudation structures also varies from east to west, with brittle faulting only in eastern portions of the block and retrograde metamorphism, mylonitization, and chlorite brecciation of the garnet gneiss observed in western areas.
The tectonic history of the southwestern UNited States and Sonora, Mexico, during the past 100 M.Y.
Authors: Gastil Gordon , Wracher Mike , Strand Gene , Lee Kear Lora , Eley Don , Chapman David , Anderson Camille
Published: February 2008

The area between the Colorado Plateau on the north, New Mexico and Chihuahua to the east, and southern and Baja California to the southwest has undergone a complicated thermal and tectonic history climaxing in medial Cretaceous time and manifested in a sequence of events which continue to the present time, independent of plate boundary transform faulting. This sustained sequence of events was initiated by subduction‐related magmatic emplacement, largely between 120 and 50 Ma, beginning in the west and sweeping eastward across the region. The emplacement of these rocks generated a large isostatic welt which began to elevate in late medial Cretaceous time in the west central portion of the area and spread outward until in Oligocene time the west coast of southern California was emergent almost to the continental escarpment. Both erosional and tectonic unloading accompanied elevation, with the axis of the welt achieving its maximum elevation in earliest Cenozoic time when north central Sonora may have had elevations of the order of 5000 m. Body forces caused the welt to relax laterally, producing thrust and nappe structures which were westward vergent in the west and eastward vergent in the east. To the east these structures are included in what is called the Larimide Orogeny; to the west they include such features as the Santa Rosa mylonite belt and related structures. As with the magmatic emplacement, the gravitational spreading began in the west. During the Eocene, additional rebound in response to deep erosion and tectonic unroofing began to expose midcrustal rocks within the core area of the welt, and subhorizontal detachment began along the brittle‐ductile boundary. During Oligocene and early Miocene time, renewed isostatic uplift resulted from rising magma, and continued erosional and tectonic denudation spawned both brittle and ductile extension, hydrothermal alteration, and mineral deposition. From mid‐Miocene to the present time the central portion of the area has subsided, while the marginal mountain ranges have elevated.
The kinematic history of the Pennine Nappes east of the Lepontine Dome: Implications for the tectonic evolution of the Central Alps
Authors: Ring Uwe
Published: February 2008

The Pennine zone of the eastern Central Alps (South, Middle, and North Pennine from top to bottom of sequence) underwent a complex structural and strain history. A first phase (D1) is characterized by E‐W oriented stretching and is of Cretaceous/lower Tertiary age. D1 structures affected most, but not all, of the South and Middle Pennine units, were probably obliterated in the North Pennine Adula nappe, and are absent in the basement nappes underneath the Adula nappe. Further deformation led to N/NW directed nappe movement (D2), followed by incipient strike‐slip motion along the Periadriatic line (late D2). Subsequently, large‐scale folding (D3) affected mainly the nappe fronts. In the Adula nappe F3 folds verge towards the foreland, whereas the overlying units were backfolded towards the hinterland. The D2/D3 events were accompanied by a complex strain pattern that is best seen in the Adula nappe and includes plane to prolate strains in the upper frontal part and oblate strains in both the lower frontal part and the interior of the nappe. D3 backfolding was associated with the beginning of backthrusting at the Periadriatic line and was accompanied and/or followed by rapid uplift in the Lepontine dome and subsequent unroofing of the latter which led to E/NE directed extensional movements (D4) in the eastern Central Alps. The D1 event is related to the anticlockwise rotation of the Adriatic plate relative to Europe. This caused dextral transpression between the Adriatic and the European plates and partitioning of the deformation into convergence and strike‐slip. It is suggested that the North Pennine nappes below the Adula nappe were situated along a major strike‐slip fault and escaped this event. D2 resulted from continental collision between Adria and Europe. D2 displacement directions are subperpendicular to the Alpine arc and are best approximated by WNW/NW oriented motion of Adria. The structures and the strain pattern of D3 are interpreted as the result of gravitational force. Late D2 strike‐slip deformation at the Periadriatic line, D3 backthrusting at the southern steep belt, and the unroofing stage (D4) are generally attributed to dextral transpression between Adria and Europe. This approach implies that the Alpine evolution of the Central Alps was largely governed by tangential convergence between the Adriatic and the European plates.
Neotectonic development of western Nicaragua
Authors: Weinberg Roberto Ferrez
Published: February 2008

This study presents a reconstruction of the Neotectonic evolution of western Nicaragua by means of structural analysis of outcrop scale joints, faults and bedding planes integrated with macroscale structures. The results indicate three clearly separable deformation phases which are closely related to specific volcanic events and account for the major tectonic features of Nicaragua. The first deformation phase was a Late Miocene to Early Pliocene shortening that produced the Pacific Plain anticlines and controlled the contemporaneous extrusion of the El Coyol Group volcanic rocks. The second phase, active between the Pliocene and the Pleistocene, resulted in extensional structures such as the 500‐km‐long and 50‐km‐wide NW‐SE trending Nicaragua Depression. This phase was accompanied by a gradual trenchward migration of the volcanic activity responsible for the formation of the Las Sierras Group. The third deformation phase is still active and is responsible for the present shallow focus‐seismic activity associated with N‐S trending volcanic vents and the N‐S Managua pull‐apart Graben. The Neotectonic development of western Nicaragua, as interpreted here, is closely related to the contemporaneous evolution of the neighboring areas of Guatemala (south of the Jocotan Fault), Honduras, El Salvador and Costa Rica (north of the Valle Central). The Valle Central of Costa Rica represents the southern boundary of this structurally homogeneous domain and may indicate the southern boundary of the Neogene Chortis Block. Changes in deformation style and the trenchward migration of volcanism which occurred during the Pliocene may indicate an increase in the subduction angle of the Cocos Plate and may also be correlated to Pliocene shortening of Jamaica and Hispaniola.
Structural evolution of an arc‐basin: The Gravina Belt in central southeastern Alaska
Authors: Haeussler Peter J.
Published: February 2008

The upper Middle Jurassic‐Lower Cretaceous Gravina belt lies along the eastern margin of the Alexander terrane in southeastern Alaska. This group of turbidites and mafic to intermediate volcanic rocks was deformed during mid to Late Cretaceous time during the closing of a basin of unknown size between the Alexander terrane on the west and the Stikine terrane to the east. Therefore structures of Gravina belt rocks largely reflect the final accretion and subsequent transport of the Alexander terrane. Six geologic transects across the central Gravina belt (southern Mitkof Island to northern Douglas Island) define a structural history that includes (1) syndepositional soft‐sediment folding and faulting, possibly in conjunction with block tilting and extension; (2) tight to isoclinal folding or thrust faulting, with formation of a slaty cleavage (S1) striking ∼330°; (3) local coaxial refolding with formation of crenulation cleavage (S2); (4) development of domainal crenulation folds and cleavage (S3) oriented at a large angle to the margin of the belt; (5) intrusion of tonalitic plutons around 90 Ma; and (6) right‐lateral strike‐slip displacement on faults oriented ∼330°. Finite strain measurements on sedimentary rocks suggest the belt was at least twice its present width, normal to the foliation, before deformation. Subhorizontal margin‐parallel fold axes, margin‐parallel slaty cleavage, and margin‐perpendicular stretching lineations suggest orthogonal contraction of the Gravina basin, assuming that oblique plate convergence was not decoupled along strike‐slip faults. After contractional deformation, strike‐slip faults indicate dextral displacement (probably of the order of several tens of kilometers) of the Alexander terrane with respect to the terranes to the east. Domainal crenulation folds and cleavages at a high angle to the margin of the belt suggest that the change in convergence directions occurred while the rocks (presently at the surface) could still plastically deform. This kinematic interpretation of structures is consistent with changes in plate motions [Engebretson et al., 1985]. Before 100 Ma, the convergence directions between the Kula and North America plates were at a high angle to the continental margin, whereas after 100 Ma, convergence directions were at a small angle to the continental margin. In addition, after 100 Ma, the Kula‐North America, and not the Farallon‐North America, convergence direction is most compatible with a N‐S principal paleostress orientation derived from inversion of strike‐slip fault data. This relationship suggests that it may have been the Kula plate that drove northward transport of the Alexander terrane along the margin of North America.
Subduction initiation at passive margins: The Scotian Basin, eastern Canada as a potential example
Authors: Erickson S. Gregg , Arkani‐Hamed Jafar
Published: February 2008

Thermal evolution modeling based on the geometry of the Scotian Basin, eastern Canada, and analysis of the shear resistance associated with lithospheric faulting are used to investigate the initiation of subduction at passive margins. Previous studies have demonstrated the difficulty of subduction initiation at mature passive margins. However, unique features along a passive margin may facilitate subduction initiation. Subduction may initiate along a part of the margin (1) by extensional or strike‐slip deformation rather than shortening, (2) by deformation of the weaker continental lithosphere rather than oceanic lithosphere, (3) by heating and weakening of oceanic lithosphere due to lateral heat transfer from adjacent continental lithosphere, and (4) by heating and weakening of the lithosphere due to the passage of a mantle plume beneath the margin. The values of shear resistance under these conditions are still higher than those calculated for ridge push, indicating that subduction initiation at passive margins is difficult even with these weakening mechanisms. However, pulses of compression associated with plate reorganization may be enough to trigger subduction at such passive margins.
Segmentation, fragmentation and jostling of cratonic basement in and near Äspö, southeast Sweden
Authors: Munier R. , Talbot C. J.
Published: February 2008

This paper summarizes studies of the structural geology in and around the island of Äspö before the excavation of an underground rock laboratory there to explore the problems of storing spent nuclear fuel in Swedish bedrock. Each of the major reorganizations of the plate boundaries around the Baltic shield are recognizable as distinctive fabrics or structures in the 1.7 Ga Proterozoic cratonic basement exposed at Äspö. This rock mass segmented along ductile shear zones, while in the lower crust, fragmented along much the same zones as it rose into the upper crust and has been jostled mainly by repeated reactivation of the same brittle fracture zones ever since. The first strains were ductile and penetrative and shared by all the rocks of the region. Subsequent strains became increasingly localized to the boundaries of relatively unstrained blocks. Ductile gneiss zones that segmented the crust at amphibolite facies narrowed to mylonites and then faults as the rock mass passed through the ductile‐brittle transition in greenschist facies at ≈1.4 Ga. Since they have been in the cool upper crust, many of the same shear zones have widened to brittle fracture zones. Fracture infills of decreasing age consist of different combinations of quartz, epidote, Fe‐oxihydroxides, chlorite, prehnite, and calcite. Fragmentation (generation of new fractures) generally gave way to jostling (reactivation of preexisting fractures) in successive episodes of brittle intracratonic strain at Äspö. The regional fracture pattern on all scales consists of different combinations of the same five fracture sets. By the end of Palaeozoic times, the Äspö bedrock had accumulated fracture zones with a sufficiently wide range of orientations that subsequent strains involved more jostling than fragmentation. Plate tectonic and glacial strains of Sweden anticipated in the next few million years are also likely to occur along long‐established zones of weakness that are already available for study.