Showing papers in "Geology in 1996"

Fault zone architecture and permeability structure

Abstract: Fault zone architecture and related permeability structures form primary controls on fluid flow in upper-crustal, brittle fault zones. We develop qualitative and quantitative schemes for evaluating fault-related permeability structures by using results of field investigations, laboratory permeability measurements, and numerical models offlow within andnearfaultzones.Thequalitativeschemecomparesthepercentageofthetotalfaultzone width composed of fault core materials (e.g., anastomosing slip surfaces, clay-rich gouge, cataclasite,andfaultbreccias)tothepercentageofsubsidiarydamagezonestructures(e.g., kinematically related fracture sets, small faults, and veins). A more quantitative scheme is developed to define a set of indices that characterize fault zone architecture and spatial variability.Thefaultcoreanddamagezonearedistinctstructuralandhydrogeologicunits that reflect the material properties and deformation conditions within a fault zone. Whether a fault zone will act as a conduit, barrier, or combined conduit-barrier system is controlled by the relative percentage of fault core and damage zone structures and the inherent variability in grain scale and fracture permeability. This paper outlines a frameworkforunderstanding,comparing,andcorrelatingthefluidflowpropertiesoffaultzones in various geologic settings.

Secular variation in seawater chemistry: An explanation for the coupled secular variation in the mineralogies of marine limestones and potash evaporites over the past 600 m.y.

Abstract: Secular changes in the mineralogies of marine nonskeletal limestones and potash evaporites occur in phase on a 100–200 m.y. time scale such that periods of “aragonite seas” are synchronized with MgSO4 evaporites and periods of “calcite seas” with KCl evaporites. It is proposed that these coupled changes are the result of secular variation in seawater chemistry controlled primarily by fluctuations in the mid-ocean ridge hydrothermal brine flux, which in turn have been driven by fluctuations in the rate of ocean crust production. Quantitative predictions based on this hypothesis yield secular variation in limestone and potash evaporite mineralogies that closely match the observed variation over the past 600 m.y., providing strong support for the thesis that seawater chemistry, rather than remaining constant, has oscillated significantly over geologic time.

Slip-tendency analysis and fault reactivation

Abstract: Slip-tendency analysis is a new technique that permits rapid assessment of stress states and related potential fault activity. The tendency of a surface to undergo slip in a given stress field depends on its frictional characteristics (primarily controlled by rock type) and the ratio of shear to normal stress acting on the surface, here defined as slip tendency (determined by orientation of the surface within the stress field). An interactive computer tool displays the stress tensor in terms of its associated slip-tendency distribution and the relative likelihood and direction of slip on surfaces of all orientations. The technique provides easy visualization and rapid evaluation of stress in terms of its potential for causing slip on individual faults or fault populations for use in seismic-risk and fault-rupture–risk assessment, exploration for high-risk and earthquake-prone blind faults, selection of likely earthquake focal mechanism solutions, and for use in analysis of compatibility of geologic structures.

Marine pore-water sulfate profiles indicate in situ methane flux from underlying gas hydrate

Abstract: Marine pore-water sulfate profiles measured in piston cores are used to estimate methane flux toward the sea floor and to detect anomalous methane gradients within sediments overlying a major gas hydrate deposit at the Carolina Rise and Blake Ridge (U.S. Atlantic continental margin). Here, sulfate gradients are linear, implying that sulfate depletion is driven by methane flux from below, rather than by the flux of sedimentary organic matter from above. Thus, these linear sulfate gradients can be used to quantify and assess in situ methane flux, which is a function of the methane inventory below.

Reduction of CO2 during serpentinization of olivine at 300 °C and 500 bar

Abstract: CO 2 reduction processes occurring during experimental serpentinization of olivine at 300 °C and 500 bar confirm that ultramafic rocks can play an important role in the generation of abiogenic hydrocarbon gas. Data reveal that conversion of Fe(II) in olivine to Fe(III) in magnetite during serpentinization leads to production of H 2 and conversion of dissolved CO 2 to reduced-C species including methane, ethane, propane, and an amorphous carbonaceous phase. Hydrocarbon gases generated in the process fit a Schulz-Flory distribution consistent with catalysis by mineral reactants or products. Magnetite is inferred to be the catalyst for methanization during serpentinization, because it has been previously shown to accelerate Fischer-Tropsch synthesis of methane in industrial applications involving mixtures of H 2 and CO 2 . The carbonaceous phase was predominantly aliphatic, but had a significant aromatic component. Although this phase should ultimately be converted to hydrocarbon gases and graphite, if full thermodynamic equilibrium were established, its formation in these experiments indicates that the pathway for reduction of CO 2 during serpentinization processes is complex and involves a series of metastable intermediates.

Carbon isotopes and the rise of atmospheric oxygen

Abstract: New data for the isotopic composition of carbon in carbonate sediments deposited between 2.6 and 1.6 Ga indicate that the value of δ13C in these sediments underwent a very large positive excursion between 2.22 and 2.06 Ga. A reassessment of the earlier δ13C data for carbonate sediments shows that this excursion was probably worldwide, and that it was preceded and followed by several hundred million years during which the δ13C of carbonate sediments differed little from that of modern carbonates. The large δ13C excursion between 2.22 and 2.06 Ga was probably related to an abnormally high rate of organic carbon deposition, which generated an abnormally high rate of O2 production. We estimate that the total excess O2 produced during the excursion was between 12 and 22 times the present atmospheric O2 inventory. The δ13C data therefore suggest that the O2 content of the atmosphere increased very significantly between 2.22 and 2.06 Ga. This inference is supported strongly by several other lines of evidence.

Alternate interpretation of the Messinian salinity crisis: Controversy resolved?

Abstract: A high-resolution chronology of the Mediterranean Messinian salinity crisis is proposed. Two types of evaporite deposition may be distinguished: those in marginal areas vs. those in basinal ones. Their diachroneity is deduced from the stratigraphic relationships linking these evaporites to a major Messinian erosional surface, A two-step model is proposed for the evolution of the salinity crisis through time. During the first phase (from 5.75 to 5.60 Ma), the deposition of marginal evaporites took place in response to a modest sea-level fall; in the second interval (from 5.60 to 5.32 Ma), the Mediterranean basin became isolated. During this later period, the deposition of basinal evaporites and the cutting of the Mediterranean canyons took place.

Evaporitic-source model for igneous-related Fe oxide-(REE-Cu-Au-U) mineralization

Abstract: We propose that many igneous-related Fe oxide-rich (REE-Cu-Au-U-bearing) deposits form by hydrothermal processes involving evaporitic ligand sources, either coeval salars or older evaporites. These deposits are abundant in both Phanerozoic and Proterozoic extensional continental and continent-margin settings. They commonly form in global arid zones, but they also occur where magmatism is superimposed upon older evaporites. Magmatic compositions exert only second-order control, mainly on alteration mineralogy and on element abundances. Hot S-poor brines generated by interaction with evaporitic materials are consistent with geologic settings and help rationalize the distinctive element enrichments (siderophile, lithophile) and hydrothermal alteration (sodic, locally alkaline) found in these systems. This model contrasts with immiscible oxide melt and magmatic-hydrothermal origins commonly proposed for these deposits, although all three mechanisms can occur. 31 refs., 3 figs., 1 tab.

Geometry and scaling relations of a population of very small rift-related normal faults

Abstract: Exceptionally well exposed normal faults within the Solite Quarry of the Dan River rift basin range in length from a few millimetres to a few metres and are possibly the smallest visible faults studied to date Displacement is greatest at or near the center of isolated faults and decreases toward the fault tips Relay structures form between closely overlapping faults The distribution of fault sizes in the study area follows a power-law (fractal) relation, and the maximum observed displacement scales linearly with fault length The new fault data extend the global data set to more than eight orders of magnitude of fault length and indicate that there is no significant change in displacement geometry and the linear length-displacement scaling relation between small and large faults

Explicit treatment of inheritance in dating depositional surfaces using in situ 10Be and 26Al

Abstract: We describe a new strategy for dating depositional landscape surfaces using in situ-produced cosmogenic radionuclides (CRNs) that removes the complication of nuclide inheritance by clasts prior to depositon. two amalgamated samples, each consisting of 30 clasts, one from the surface and one from a fixed depth in the subsurface, constrain this CRN inheritance and date the surface. The inheritance may be used to estimate minimum exhumation rates and maximum transport times within the geomorphic system. We test the technique using {sup 10}Be and {sup 26}Al to date the third (FR3) of five terraces along the Fremont River, Utah, and the third (WR3) of 15 along the Wind River, Wyoming. Whereas effective ages based solely upon the surface samples yield 118 -138 ka (FR3) and 93 ka (WR3), the subsurface samples reveal that inheritance accounts for 22% (WR3) to 43% (FR3) of the total CRN concentration. Taking this into account yeilds terrace ages of nearly 61-81 ka for FR3 and nearly 67-76 ka for WR3. We explore the dependence of age estimates on the accumulation history of the terrace silt caps. 22 refs., 3 figs., 1 tab.

Evolution patterns of radiolaria and organic matter variations: A new approach to identify sea-level changes in mid-Cretaceous pelagic environments

Abstract: The relation between sea-level changes, plankton productivity, and evolution, as well as the occurrence of anoxic sediments, provides an interesting avenue of paleooceanographic research. In this context, we examined mid-Cretaceous radiolarian faunas, carbonate isotopic and organic matter type data of Deep Sea Drilling Project and Ocean Drilling Program cores from the North Atlantic, and samples from outcrops of the western Tethys from central and northern Italy. Former studies indicate that an expansion of the oxygen minimum zone caused plankton extinctions at the Cenomanian-Turonian boundary. An expanded oxygen minimum zone would destroy deeper habitats of planktic foraminifera, causing the extinction of deeper dwelling forms. Although this model is well established for the Cenomanian-Turonian boundary, not much is known about the causes of extinctions and radiations during the entire mid-Cretaceous (Aptian-Turonian). We demonstrate that the dimension of the oxygen minimum zone, which depends on the relative sea level and the corresponding nutrient supply, causes the complex pattern of evolution and radiation of planktic protozoa and the sedimentation of black shales in the mid-Cretaceous. This new depositional model allows correlation of micropaleontologic data and different types of black shales in the pelagic realm within a sequence stratigraphic framework.

Age of Etendeka flood volcanism and associated intrusions in southwestern Africa

Abstract: Detailed 40Ar/39Ar laser step-heating analyses of mineral separates from five volcanic units in Namibia and Angola and five intrusions in Namibia yield important geochronological data for the Etendeka igneous province. Ten plateau dates on plagioclase, hornblende, and biotite between 131.7 ± 0.7 and 132.3 ± 0.7 Ma were obtained, and a late syenite from the Messum intrusive complex yielded a slightly younger hornblende plateau date of 129.3 ± 0.7 Ma. Magnetostratigraphy of the volcanic rocks in three sections up to 700 m thick, laterally spanning more than 100 km, suggests that the flows record only two geomagnetic polarity reversals. Precise temporal coincidence with the Parana flood volcanic province in South America indicates that Etendeka volcanism does not represent a significantly younger phase of magmatism that migrated from northwest to southeast over 10 m.y., as has recently been proposed. The duration of intrusive activity was at least 2–3 m.y. longer than recorded by volcanism, and its total duration awaits further constraints.

Rapid diversification of planktonic foraminifera in the tropical Pacific (ODP Site 865) during the late Paleocene thermal maximum

Abstract: The planktonic foraminiferal generaMorozovellaandAcarininarapidly (in ;10 k.y.) diversified during the late Paleocene thermal maximum (LPTM), giving rise to such morphotypes asM. allisonensis(new species),M. africana, andA. sibaiyaensis. Single-specimen isotopicanalysisconfirmsthatM.allisonensisandA.sibaiyaensisarerestrictedtotheLPTM carbon isotope excursion recorded at Ocean Drilling Program Site 865 (equatorial Pacific Ocean). The short-lived (50 to several 100 k.y.) ‘‘excursion’’ taxa attest to the ephemeral effectsoftheLPTMonthecalcareousplankton.Single-specimenoxygenisotopedatashow that evolution of M. allisonensis and A. sibaiyaensis was accompanied by migration to deeper water depths. AncestralM. velascoensisandA. soldadoensiswere extremely rare or absent during the early stages of the LPTM, but immigrated back into the study area to coexist with their descendants in later LPTM horizons. Photosymbiosis may have facilitatedthemorozovellidandacarininiddiversificationsduringtheoligotrophicconditionsof the LPTM.

Crustal-thickness variations in the central Andes

Abstract: We estimated the crustal thickness along an east-west transect across the Andes at lat 20°S and along a north-south transect along the eastern edge of the Altiplano from data recorded on two arrays of portable broadband seismic stations (BANJO and SEDA). Waveforms of deep regional events in the downgoing Nazca slab and teleseismic earthquakes were processed to isolate the P-to-S converted phases from the Moho in order to compute the crustal thickness. We found crustal-thickness variations of nearly 40 km across the Andes. Maximum crustal thicknesses of 70–74 km under the Western Cordillera and the Eastern Cordillera thin to 32–38 km 200 km east of the Andes in the Chaco Plain. The central Altiplano at 20°S has crustal thicknesses of 60 to 65 km. The crust also appears to thicken from north (16°S, 55–60 km) to south (20°S, 70–74 km) along the Eastern Cordillera. The Subandean zone crust has intermediate thicknesses of 43 to 47 km. Crustal-thickness predictions for the Andes based on Airy-type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation. In contrast, at the boundary between the Eastern Cordillera and the Subandean zone and in the Chaco Plain, the crust is thinner than predicted, suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere. With additional constraints, we conclude that the observation of Airy-type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting Nazca plate and the cratonic lithosphere of the Brazilian craton.

Possible eastward extension of Chinese collision belt in South Korea: The Imjingang belt

Abstract: Structural, petrological, and geochronological data from the middle Korean peninsula indicate that the Qinling-Dabie-Sulu collisional belt of east-central China crosses the Yellow Sea and extends into the Imjingang belt. The Yeoncheon complex, first identified as the western Imjingang belt, comprises primarily north-dipping metamorphic sequences: (1) the northern Jingok unit, consisting of Barrovian-type metapelites, and (2) the southern Samgot unit, consisting of calc-silicate and amphibolitic rocks. South-vergent structures with reverse-sense shearing are dominant in the Jingok unit, whereas late normal-sense shearing is pervasive in the Samgot unit and the deformed granitoid to the south. These structural patterns are interpreted to correspond to extensional deformation associated with uplift following compression in a collisional belt. Pressure-temperature ( P-T ) estimates from the amphibolites suggest a high-P amphibolite-facies metamorphism (8–13 kbar and 630–790 °C), possibly evolving from eclogite facies conditions along a clockwise P-T path. Sm-Nd and Rb-Sr geochronological data suggest that the amphibolites emplaced in Late Proterozoic time were metamorphosed during Permian-Triassic time.

Fluid- and melt-related enrichment in the subarc mantle: Evidence from Nb/Ta variations in island-arc basalts

Abstract: The single most distinctive feature of volcanic rocks from convergent-margin settings is a marked depletion of the high field strength elements (HFSE) Nb, Ta, and Ti relative to large ion lithophile and light rare earth elements when compared with basalts from mid-oceanic ridges (MORB) and the oceanic islands. A major impediment to a better understanding of this problem has been a lack of high-quality data for the HFSE (particularly Nb and Ta) that occur in very low concentrations in most volcanic rocks from convergent-margin settings. We report new analyses of Nb and Ta for a suite of island-arc volcanic rocks as well as some sea-floor sediments. Our data show that Nb/Ta values for relatively depleted island-arc volcanic rocks are similar to MORB and essentially chondritic (Nb/Ta ∼ 17), whereas more potassic arc volcanics have substantially higher Nb/Ta values (up to 33). We interpret these high values as due to modification of the subarc mantle source by silicic melts derived from the subducting slab, whereas enrichment of the source regions of the less potassic arc rocks involved a slab-derived fluid.

Synchronous emplacement of Ferrar and Karoo dolerites and the early breakup of Gondwana

Abstract: Constraints on the timing of Karoo and Ferrar continental flood-basalt magmatism in Africa and Antarctica, respectively, are critical to understanding the relationship of the Karoo and Ferrar to mantle plumes, subduction, and the initial breakup of Gondwana. Although recent work has shown that Ferrar magmas were emplaced over a short interval (<1 m.y.), the timing of magmatism within the Karoo and its relationship to the Ferrar have been problematic. New zircon and baddeleyite U-Pb ages on Ferrar (183.6 ± 1.0 Ma) and southern Karoo (183.7 ± 0.6 Ma) dolerites demonstrate that part of Karoo magmatism occurred during the rapid emplacement of Ferrar magmas. A mantle plume is thought to have been important in the genesis of the Karoo province, whereas lithospheric extension, perhaps related to subduction, has been invoked for Ferrar magmatism. The new ages now suggest that Ferrar and southern Karoo magmatism were related to a single mantle thermal anomaly and rifting event. This event may have produced local rift basins and caused rotation of microblocks in west Antarctica several million years before the breakup of east and west Gondwana.

Diversity of iron and silica precipitation by microbial mats in hydrothermal waters, iceland : implications for precambrian iron formations

Abstract: Direct examination of microbial mats from Icelandic hot springs with transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed a consortium of bacterial cells in varying stages of mineralization Differences in observed mineralogy largelyreflectdifferencesinthechemistryofthehydrothermalwatersSilica-richspheroids formedepicellularlyoncellwallsandsurroundingsheathsandcapsulesofmicroorganisms and,insomecases,intracellularlywhenpresumablythecell(s)hadlysedCommonly,these precipitateswereobservedcoalescingtoformamatrixofamorphoussilicathatcompletely encapsulated the cells and/or replaced their cytoplasmic material However, in other cells, the precipitates were composed of amorphous granules made exclusively of iron and silica inapproximatelyequalproportionsAtonelocality,thebacteriaformedseveralepicellular iron minerals, ranging from iron-mineralized capsules tofine-grained spheroids of amorphous ferric hydroxide and acicular aggregates of goethite The complete encrustation of bacterial cells by silica, iron, or a combination of both may greatly enhance their preservation potential, such that these mineralized microorganisms may conceivably represent future microfossils Thus, we may be witnessing contemporaneous biomineralization processes that are similar to those of the geologic past, particularly with regard to the origin of some Precambrian banded iron formations

Extensional collapse of the northern Taiwan mountain belt

Abstract: As an active collision zone between the Luzon arc and the China continental margin, the Taiwan mountain belt, particularly its south-central part, is undergoing crustal shortening and rapid uplift. The northern part of the orogen is, however, subjected to crustal stretching and rifting as a result of flipping of subduction from the northwest-facing Luzon arc system to the south-facing Ryukyu arc system. Taiwan and its neighboring southern Ryukyu used to be part of the rifted China continental margin before the collision started in late Miocene time. Engineered by the northwest movement of the Philippine Sea plate in the past 5 m.y., the north-south–trending Luzon arc obliquely overrode the northeast-southwest–trending continental margin and tectonized it into a collision orogen that progressively enlarged and migrated from northeast to southwest. Following the southwest-propagating collision, the north-dipping Philippine Sea plate, which was subducting beneath the Ryukyu arc, also extended westward and caused flipping of subduction in the northern part of the collision orogen. In consequence, the orogen lost the compressive support by the colliding plates and became subjected to lithospheric stretching induced by the trench suction. The collision orogen collapsed as a result of crustal thinning, and its central part foundered into a deep submerged basin. A whole spectrum of active tectonic scenarios, ranging from orogenic buildup by collision to orogenic collapse by rifting and subsidence, can be readily observed in the Taiwan-Ryukyu area and provides a vivid example for the process of orogenic collapse.

Seasonal oxygen depletion in continental-shelf waters of Louisiana: Historical record of benthic foraminifers

Abstract: A strong spring and summer oxygen depletion is induced in nearshore bottom waters of the Louisiana continental shelf by density stratification and by the carbon flux from phytoplankton production, which, in turn, is related to the nutrient load of the Mississippi and Atchafalaya rivers. In an attempt to read the historical record of this shelf hypoxia during the past two centuries, we compared the stratigraphic signals of benthic foraminifera (as reflected in a relative-dominance index for two common species of Ammonia and Elphidium ) in 210 Pb-dated cores, and we found evidence of an overall rise in oxygen stress (in intensity or duration), especially in the past 100 yr. This implies a progressive increase in the influence of river-borne nutrients, particularly anthropogenically influenced nitrates. Judging by our results, foraminiferal indices based on appropriate species ratios should prove useful in testing hypotheses about long-term environmental stresses, including eutrophication and paleohypoxia, on other marine shelves.

Cenozoic deep-sea benthic foraminifers: Tracers for changes in oceanic productivity?

Abstract: From late middle Eocene through earliest Oligocene, high-latitude regions cooled, and by the end of the period, continental ice sheets existed in Antarctica. Diversity of planktonic microorganisms declined, and modern groups of terrestrial vertebrates originated. Coeval faunal changes in deep-sea benthic foraminifers have been related to cooling of deep waters and increased oxygenation. Cooling, however, occurred globally, whereas species richness declined at high latitudes and not in the tropics. The late Eocene and younger lower-diversity, high-latitude faunas typically contain common Epistominella exigua and Alabaminella weddellensis , opportunistic phytodetritus-exploiting species that indicate a seasonally fluctuating input of organic matter to the sea floor. We speculate that the species-richness gradient and increase in abundance of phytodetritus-exploiting species resulted largely from the onset of a more unpredictable and seasonally fluctuating food supply, especially at high latitudes.

Genesis of the two main types of peraluminous granitoids

Abstract: Two-mica monzogranites to leucogranites and biotite-rich, cordierite-bearing tonalites to monzogranites form two distinct groups of peraluminous granitoids. They can be distinguished by mineral and rock associations and by the variation of their peraluminosity during differentiation. Except for the rare muscovite-bearing granitoids produced by extreme fractionation or local contamination of metaluminous magmas, the majority of peraluminous granitoids are produced by partial melting of crustal rocks. Production of either muscovite-bearing granites or biotite-rich, cordierite-bearing granitoids does not depend only on the nature of the sources, but is also controlled by the physical parameters of partial melting and consequently by the way anatexis of a thickened crust is enhanced. Biotite-rich, cordierite-bearing granitoids are generated where mantle-derived magmas are injected into or have underplated crustal rocks; two-mica granites are generated where thickened crust is affected by major crustal shears or thrusts. Correlations between differentiation and peraluminosity indicate the dominant role of either restite unmixing or fractional crystallization in the production and evolution of the various types of peraluminous granitoids.

Bivalve skeletons record sea-surface temperature and δ18O via Mg/Ca and 18O/16O ratios

Abstract: Paleotemperatures have been widely deduced from skeletal 18 O/ 16 O ratios, but these are also dependent on salinity. Without an independent measure of salinity, 18 O/ 16 O ratios cannot provide accurate data on past temperature and climate. We grew marine mollusks (bivalves) in the field while real-time data on local seawater temperature and chemistry (Mg, Ca, δ 18 O, and salinity) were gathered. Here we show that for Mytilus trossulus, skeletal Mg/Ca ratios provide an accurate measure of temperature and that weekly sea-surface temperatures may be estimated with an apparent accuracy of approximately ± 1.5 °C. Thus, with analyses of both Mg/Ca and δ 18 O from the same specimen, it is possible to determine seawater temperature and δ 18 O.

Catastrophic collapse of the volcanic island of Hierro 15 ka ago and the history of landslides in the Canary Islands

Abstract: Landslides play an important role in the evolution of many volcanic islands, producing huge fields of blocky volcanic debris on their submarine slopes. Sidescan sonar images presented in this paper provide evidence for a large debris avalanche, El Golfo, on the northern flank of Hierro Island in the Canary Islands. Angular blocks, as much as 1.2 km across and 200 m high, cover the debris avalanche surface. El Golfo avalanche is related to both the Canary debris flow and a volcaniclastic turbidite found in the Madeira abyssal plain 600 km west of the Canaries. Dating of the turbidite and the failure scarp onshore indicates that the failure probably occurred between 13 and 17 ka. There appears to be a general correlation between volcaniclastic turbidites in the abyssal-plain sequence and landslides in the Canaries during the past 750 ka. Tentatively, this correlation suggests that seven major landslides have affected the Canaries in that time.

Staircase 40/Ar39Ar spectra of fine-grained white mica: Timing and duration of deformation and empirical constraints on argon diffusion

Abstract: Staircase 40 Ar/ 39 Ar spectra of 2 to 6 μm white micas in ductile carbonate mylonites (Morcles-Doldenhorn nappe, Switzerland) record ages between 32 and 13 Ma. These ages are within independent geochronologic and biostratigraphic time limits for the beginning and ending of deformation of this fold-thrust nappe. We interpret these data to record protracted white mica neocrystallization and growth during low-temperature ductile deformation, such that the differences between the maximum and minimum ages in the 40 Ar/ 39 Ar spectra record the time span over which mica formed and grew. We have modeled the argon concentrations in these micas, taking into account the thermal history of the nappe, mica growth, and diffusional loss of argon. Our results indicate that available experimental data underestimate the activation energy for argon diffusion in muscovite, and that the retentivity of argon in white mica is greater than generally thought.

Field evidence for rapid downstream fining of river gravels through selective transport

Abstract: Doubts have been expressed about the ability of either abrasion or sorting to explain strong downstream fining of river gravels. We describe pronounced fining over a short distance in a Scottish river that has no human disturbance or lateral input of water and sediment. Measured abrasion rates are far too small to explain observed downstream fining, but bed-load trap measurements and the dispersion of magnetic tracer pebbles in six subreaches both show sorting during transport. The observed downstream fining is also simulated well by a numerical sediment routing model using hydraulic and transport laws consistent with our field measurements. The geomorphological cause of the strong fining is slope reduction above a local base-level control. In this common situation the development of downstream fining is part of the river9s tendency to minimize downstream variation in bed-load transport rates, and can proceed far more rapidly than the major aggradation otherwise required for equilibration.

Evidence in pre–2.2 Ga paleosols for the early evolution of atmospheric oxygen and terrestrial biota

Abstract: The loss of Fe from some pre–2.2 Ga paleosols has been considered by previous investigators as the best evidence for a reduced atmosphere prior to 2.2 Ga. I have examined the behavior of Fe in both pre–and post–2.2 Ga paleosols from depth profiles of Fe 3+ Ti, Fe 2+ Ti, and ΣFe/Ti ratios, and Fe 3+ Ti vs. Fe 2+ Ti plots. This new approach reveals a previously unrecognized history of paleosols. Essentially all paleosols, regardless of age, retain some characteristics of soils formed under an oxic atmosphere, such as increased Fe 3+ Ti ratios from their parental rocks. The minimum oxygen pressure ( P O 2 ) for the 3.0–2.2 Ga atmosphere is calculated to be about 1.5% of the present atmospheric level, which is the same as that for the post–1.9 Ga atmosphere. The loss of ΣFe, common in paleosol sections of all ages, was not due to a reducing atmosphere, but to reductive dissolution of ferric hydroxides formed under an oxic atmosphere. This reductive dissolution of ferric hydroxides occurred either (1) after soil formation by hydrothermal fluids or (2) during and/or after soil formation by organic acids generated from the decay of terrestrial organic matter. Terrestrial biomass on the early continents may have been more extensive than previously recognized.

Repeated volcanic disasters in Prehispanic time at Popocatépetl, central Mexico: Past key to the future?

Abstract: The Holocene eruptive history of Popocatepetl volcano is characterized by recurrent voluminous Plinian eruptions every 1000 to 3000 yr, the most recent of which destroyed human settlements. Major eruptions occurred between 3195 and 2830 B.C., 800 and 215 B.C., and A.D. 675 and 1095. The three eruptions followed a similar pattern and started with minor ash fall and ash flows. The eruptions reached their peak with a main Plinian pulse that produced deposition of a pumice fall, the emplacement of hot ash flows, and finally extensive mudflows. Each time the area of devastation had become repopulated, before being devastated once again. During the last eruption several settlements, including Cholula (a major urban center), were inundated by lahars. A scenario of the possible recurrence of an eruption of similar magnitude, which would have disastrous consequences for the now highly populated areas around Popocatepetl, should be considered seriously in any volcano emergency contingency plan. This is especially important because more than one million people are living within a radius of 35 km around the volcano (the outskirts of Mexico City are at a distance of 40 km), and Popocatepetl resumed emitting ash on December 21, 1994, after decades of dormancy.

Earliest part of Earth's stratigraphic record: A reappraisal of the >3.7 Ga Isua (Greenland) supracrustal sequence

Abstract: Investigations of the Isua supracrustal rocks in West Greenland allow us to identify the protoliths and alteration history for most Isua rocks. The protoliths consisted of alternating basalt and banded iron formation. This sequence was invaded by dunitic sills, metamorphosed, and later intruded by felsic gneisses. Pervasive carbonation and K metasomatism produced a sequence of lithologies, mimicking those found in modern platform deposits. However, the protoliths could have originated in a purely oceanic environment with no sialic detrital components. The Isua sequence probably consists of several tectonic panels, some of which are cut by >3810 Ma felsic intrusive rocks.

Mechanical model for subduction-collision tectonics of Alpine-type compressional orogens

Abstract: Alpine-type orogens are characterized by three distinct convergent tectonic phases: subduction with deformation that has primarily single vergence, a transition from subduction to collision, and continental collision with double vergence. Although the Cenozoic history of the European Alps has additional complexities, a mechanical explanation for these three phases would provide the necessary crustal-scale framework in which to develop an understanding of the smaller-scale processes. We present results from a simple numerical model, which explain the mechanics of these three phases as a consequence of the changing buoyancy of the lithosphere subducted beneath the orogen. The development and exhumation of a subduction complex, suture zone, and basement nappe stack (Piemont suture, Penninic Nappes); the presence of a crustal-scale back fold and thrust (Insubric Line); and uplift of basement on the pro- (European) side of the orogen (external basement massifs) may be explained as a simple consequence of changing dynamics during the transition from subduction to collision.