Showing papers in "Geology in 1993"

Mechanical model for the tectonics of doubly vergent compressional orogens

Abstract: A mechanical model of crustal shortening and deformation driven by the relative convergence of rigid, underlying mantle plates explains many features of convergent orogens. Results based on numerical models and supported by sandbox models show that a Coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth: (1) block uplift bounded by step-up shear zones; (2) development of a low-taper wedge over the underthrusting mantle plate; and (3) development of a low-taperwedge overlying the overthrusting mantle plate and verging in the opposite direction. When modified by isostasy, basal viscous flow, surface erosion and denudation, and sedimentation, the resultant model orogens exhibit a variety of styles with characteristics in common with small, rapidly denuded orogens, large orogens with plateaus and extensional characteristics, and active subduction margins with doubly vergent accretionary wedges and deformed fore-arc basins.

Origin of high-potassium, talc-alkaline, I-type granitoids

Abstract: Many I-type granitoid magmas are generated through partial melting of older metaigneous rocks, and the compositions of such melts are broadly calc-alkaline and metaluminous. These melts are granitic to tonalitic, and result from thermal extremes in their lower crustal source regions. Data on the experimental partial melting of common crustal rocks suggest that high-K, I-type granitoid magmas can be derived only from the partial melting of hydrous, calc-alkaline to high-K calc-alkaline, mafic to intermediate metamorphic rocks in the crust. Because of their low K2O contents, metabasaltic rocks of all kinds are unsuitable as sources, and models that propose mixing of mantle-derived basaltic magmas and crustal melts are also inadequate. There is no requirement that I- type calc-alkaline magmatism be related in any way to subduction processes.

Revised plate motions relative to the hotspots from combined Atlantic and Indian Ocean hotspot tracks

Abstract: We use an updated model for global relative plate motions during the past 130 m.y. together with a compilation of bathymetry and recently published radiometric dates of major hotspot tracks to derive a plate-motion model relative to major hotspots in the Atlantic and Indian oceans. Interactive computer graphics were used to find the best fit of dated hotspot tracks on the Australian, Indian, African, and North and South American plates relative to present-day hotspots assumed fixed in the mantle. One set of rotation parameters can be found that satisfies all data constraints back to chron 34 (84 Ma) and supports little motion between the major hotspots in this hemisphere. For times between 130 and 84 Ma, the plate model is based solely on the trails of the Tristan da Cunha and Great Meteor hotspots. This approach results in a location of the Kerguelen hotspot distinct from and south of the Rajmahal Traps for this time interval. Between 115 and 105 Ma, our model locates the hotspot underneath the southern Kerguelen Plateau, which is compatible with an age estimate of this part of the plateau of 115-95 Ma. Our model suggests that the 85°E ridge between lat 10°N and the Afanasiy Nikitin seamounts may have been formed by a hotspot now located underneath the eastern Conrad rise.

Growth of normal faults: Displacement-length scaling

Abstract: The form of the scaling relation between the displacement and length of faults has been a subject of considerable controversy because of insufficient scale range and scattered data. Here we report on displacement and length data collected from well-exposed normal faults located on the Volcanic Tableland in northern Owens Valley, California. These data, which exhibit little scatter, are from a fault population that spans three orders of magnitude in fault length and were gathered in a relatively uniform lithologic and tectonic setting. With the upper cooling surface of the middle Quaternary Bishop Tuff used as a marker, the displacement distribution along individual faults can be mapped in detail. The displacement distribution profiles are consistent with a linear relation between displacement and fault length.

Timing of collision of the Sino-Korean and Yangtse cratons: U-Pb zircon dating of coesite-bearing eclogites

Abstract: The suture zone between the Sino-Korean and Yangtse cratons in central China is marked by a series of fault-bounded belts composed of crustal protoliths. In the Dabie Mountains some of these blocks contain coesite and diamond, indicating subduction to minimum depths of ∼120 km as a result of continental collision. The blocks range from the structurally lowest ultrahigh-pressure metamorphic belt and decrease in metamorphic grade upsection to the structurally highest, low greenschist facies belt. Furthermore, the faults mark large contrasts in metamorphic grade. These features form a structure that is similar to metamorphic core complexes, indicating extension as part of the exhumation process. Timing of the continental collision has previously been inferred as Late Triassic, from regional geologic relations, or Caledonian, from 40 Ar/ 39 Ar dating of the adjacent and possibly related Qinling metamorphic belt. U-Pb dating of zircon from ultra-high-pressure eclogites yields a metamorphic age of 209 ±2 Ma. This age coincides with later stages of collision and is therefore a minimum estimate of the time of collision.

Mount St. Helens: Potential example of the partial melting of the subducted lithosphere in a volcanic arc

Abstract: Mount St. Helens, 50 km to the west of Mount Adams and the main Cascade volcanic chain, is only 80 km above the subducting oceanic lithosphere. The elevated temperatures off the subducting slab, because of the close proximity of the Juan de Fuca Ridge to the trench,may induce slab melting at a depth of ∼80 km. Dacites from Mount St. Helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure, i.e., relatively high AI (Al2O3 > 15% at 70% SiO2), low Y and Yb (because of garnet and amphibole stability in the source), low Sc, and high Sr and Eu. Trace element modeling of the partial melting of mid-oceanic ridge basalt (MORB) from the Juan de Fuca Ridge that yields a hornblende eclogite residue can reproduce the Mount St. Helens data (results off the model are quite distinct from data derived from the Mount Adams volcanic rocks). In contrast, Mount Adams is ∼135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate. The Cascade are has been active in its present locality, because of oblique subduction, for the past 7 m.y. The major volcanoes along the arc have existed for at least 500 ka, but Mount St. Helens has existed for <40 ka. We suggest that the subducting plate may have reached elevated temperatures, because of the approach of North America to the Juan de Fuca Ridge, at ∼40 ka, which initiated melting of the slab.

Model for episodic flow of high-pressure water in fault zones before earthquakes

Abstract: In this model for the evolution of large crustal faults, water that originally came from the country rock saturates the initially highly porous and permeable fault zone. During shearing, the fault zone compacts and water flows back into the country rock, but the flow is arrested by silicate deposition that forms very low permeability seals between the fault zone and the country rock. Because of variations in temperature and mineralogical composition and the complex structure of the fault zone, a three-dimensional network of seals is formed in the fault zone itself; thus, the high-pressure fluid is not evenly distributed. As in deep oil reservoirs, the fluid will be confined to seal-bounded fluid compartments of various sizes and porosity that are not hydraulically connected with each other or with the hydrostatic regime in the country rock. When the seal between two of these compartments is ruptured, an electrical streaming potential will be generated by the sudden movement of fluid from the high-pressure compartment to the low-pressure compartment. When the pore pressure in the two compartments reaches its final equilibrium state, the average effective normal stress across them may be lower than it was initially, and, if the two compartments are large enough, this condition may trigger an earthquake. During an earthquake, many of the remaining seals will be ruptured, and the width of the fault zone will increase by failure of the geometric irregularities on the fault. This newly created, highly porous and permeable, but now wider fault zone will fill with water, and the process described above will be repeated. Thus, the process is an episodic one, with the water moving in and out of the fault zone, and each large earthquake should be preceded by an electrical and/or magnetic signal.

Paleomagnetic constraints on timing of the Neoproterozoic breakup of Rodinia and the Cambrian formation of Gondwana

Abstract: Paleomagnetic data from East Gondwana (Australia, Antarctica, and India) and Laurentia are interpreted to demonstrate that the two continents were juxtaposed in the Rodinia supercontinent by 1050 Ma. They began to separate after 725 Ma, allowing the formation of the Pacific Ocean. The low-latitude Rapitan and Sturtian glaciations occurred during the rifting that led to continental breakup. East Gondwana remained in low latitudes for the rest of the Neoproterozoic, while Laurentia moved to polar latitudes by 580 Ma. During the Vendian, a wide Pacific Ocean separated the two continental land masses. The younger Marinoan, Ice Brook, and Varangian glaciations in the early Vendian preceded a second continental breakup in the late Vendian, causing formation of the eastern margin of Laurentia and rejuvenation of its western margin. Paleomagnetic data indicate that Gondwana was not fully assembled until the end of the Neoproterozoic, possibly as late as Middle Cambrian.

Lake-sediment record of late Holocene hurricane activities from coastal Alabama

Abstract: Coastal lake sediments contain a stratigraphically and chronologically distinct record of major hurricane strikes during late Holocene time. Frederic—a category 3 hurricane that struck the Alabama coast on the Gulf of Mexico in 1979—left a distinct sand layer in the nearshore sediments of Lake Shelby as a result of storm-tide overwash of beaches and dunes. Sediment cores taken from the center of Lake Shelby contain multiple sand layers, suggesting that major hurricanes of category 4 or 5 intensity directly struck the Alabama coast at ca. 3.2-3.0, 2.6, 2.2, 1.4, and 0.8 ka ( 14 C yr), with an average recurrence interval of ∼600 yr. The Alabama coast is likely to be struck by a category 4 or 5 hurricane within the next century.

Initiation of subduction and the generation of slab melts in western and eastern Mindanao, Philippines

Abstract: Adakite, found in both the eastern and western parts of Mindanao Island, Philippines, is a rare rock type, characterized by low heavy rare earth elements and Y contents together with high Sr/Y ratios, and is considered to be the result of the melting of young subducted oceanic crust, which leaves an eclogite residue. Pliocene-Quaternary adakites from western Mindanao (Zamboanga Peninsula) are probably derived from the melting of the young Miocene Sulu Sea crust, which is currently subducting beneath Zamboanga. Associated Nb-enriched basalts are thought to come from mantle metasomatized through interaction with adakitic melts. In eastern Mindanao, Pliocene-Quaternary cones and plugs of typical adakitic composition mark the trace of the Philippine fault in Surigao and north Davao. The underlying Philippine Sea crust is of Eocene age and therefore cannot melt under normal subduction thermal conditions. Thermal models indicate that melting at the start of subduction can occur. Subduction of the Philippine Sea plate began 3 to 4 Ma beneath eastern Mindanao and probably accounts for the presence of adakites along the Philippine fault.

Pedogenesis and paleoclimate: Interpretation of the magnetic susceptibility record of Chinese loess-paleosol sequences

Abstract: In the Chinese loess-paleosol sequences from Luochuan, Haanxi province, China, variations in the magnetic susceptibility have been accepted as an excellent proxy for paleoclimate, and the standard interpretation is that climatic processes have enhanced the magnetic susceptibility of the paleosols beyond the base-line level of the loess. By using mineral magnetic properties and a soil-chemistry extraction procedure, we show that most of the magnetic susceptibility signal in both the paleosols and the loess is due to pedogenesis. In effect, loess deposition and pedogenesis are competing processes at all times, and the presence of a paleosol simply indicates that the latter process was predominant. These results imply that any interpretation of the paleoclimate record of loess-paleosol sequences must recognize the pedogenic nature of the magnetic susceptibility signal and that the focus of future research must be on the critical role of climate in the development of the paleosols.

Dike transport of granitoid magmas

Abstract: Thermal and fluid-dynamical analyses suggest that for viscosities and density contrasts spanning the range considered typical for many calc-alkalic granitoids, dike ascent is a viable mechanism for the transport of large volumes of granitoid melt through the continental crust. We present calculations showing that a granitoid melt with calculated viscosity of the order of 10 6 Pa ⋅ s and a density contrast between magma and crust of 200 kg/m 3 can be transported 30 km through the crust in ∼1 month, corresponding to a mean ascent velocity of 1 cm/s. Using analysis modified from numerical studies of the flow of basaltic magmas in dikes, we also present an expression that allows the calculation of the critical (minimum) dike or fault width required for granitic magma to ascend without freezing. For all reasonable estimates of Cordilleran granitoid viscosity and density contrast, the critical dike width is determined to be between ∼2 and 7 m. Calculated peak batholith-filling rates are orders of magnitude greater than mean cavity-opening rates based on estimated fault slippage, which is consistent with chemical evidence for intermittent supply of magma pulses.

High-level surfaces, plateau uplift, and foreland development, Bolivian central Andes

Abstract: Newly dated Tertiary strata in the Bolivian central Andean plateau and synthesis of the Tertiary record in the adjacent Subandean fold-thrust belt constrain the age of deformation in both regions. Age of deformation within the plateau is determined by dated crosscutting relations associated with a regionally extensive high-level surface known as the San Juan del Oro surface. New 40 Ar- 39 Ar dates on undeformed strata above the high-level surface preclude significant upper-crustal shortening within the Eastern Cordillera after 10 Ma. Tertiary strata within the adjacent Subandean region demonstrate that formation of the fold-thrust belt occurred after 10 Ma. On the basis of these data, we propose a two-stage model of late Cenozoic Andean growth that links plateau uplift to the development of the fold-thrust belt. In the first stage, early plateau uplift occurred in response to widespread compressional deformation of the plateau (Eastern Cordillera and Altiplano). During the second stage, beginning after 10 Ma, upper-crustal deformation within the plateau terminated, and the Subandean fold-thrust belt developed. Crustal-scale eastward thrusting along the eastern margin of the Eastern Cordillera drove Subandean folding and thrusting; the Eastern Cordillera served as the "bulldozer" for the deforming Subandean wedge.

Estimating the age of formation of lakes: An example from Lake Tanganyika, East African Rift system

Abstract: Age estimates for ancient lakes are important for determining their histories and their rates of biotic and tectonic evolution. In the absence of dated core material from the lake`s sedimentary basement, several techniques have been used to generate such age estimates. The most common of these, herein called the reflection seismic-radiocarbon method (RSRM), combines estimates of short-term sediment-accumulation rates derived from radiocarbon-dated cores and depth-to-basement estimates derived from reflection-seismic data at or near the same locality to estimate an age to basement. Age estimates form the RSRM suggest that the structural basins of central Lake Tanganyika began to form between 9 and 12 Ma. Estimates for the northern and southern basins are younger (7 to 8 Ma and 2 to 4 Ma, respectively). The diachroneity of estimates for different segments of the lake is equivocal, and may be due to erosional loss of record in the northern and southern structural basins or to progressive opening of the rift. The RSRM age estimates for Lake Tanganyika are considerably younger than most prior estimates and clarify the extensional history of the western branch of the East African Rift system. 31 refs., 3 figs., 1 tab.

Anoxic events in the late Frasnian—Causes of the Frasnian-Famennian faunal crisis?

Abstract: Carbon isotope data collected from five Frasnian-Famennian boundary sections in central Europe show two positive δ 13 C excursions in the late Frasnian (∼367 Ma). Both anomalies coincided with the deposition of the bituminous lower and upper Kellwasser horizons. The carbon isotope patterns indicate two phases of enhanced burial and subsequent recycling of organic carbon. A maximum formation of warm saline waters on the subtropical to tropical epicontinental shelves during late Frasnian transgressive episodes may have induced oceanic oxygen deficits. Variations in the C org burial rates may have resulted in changes in the CO 2 concentrations in the oceans and in the atmosphere, culminating in global climatic changes. We propose that the repeated co-occurrences of sea-level fluctuations, anoxic conditions, and global climatic changes during the late Frasnian would affect especially the subtropical to tropical shallow-water communities and reef ecosystems, which were severely affected during the Frasnian-Famennian faunal crisis.

Late Cenozoic extension in northeastern Greece: Strymon Valley detachment system and Rhodope metamorphic core complex

Abstract: The Strymon Valley detachment system accommodated at least 25 km of extension at the northern margin of the Aegean extensional province from middle Miocene through early Pliocene time. The primary detachment surface is exposed as a gently southwest-dipping low-angle normal fault—similar to those widely known in core complex settings of the U.S. Basin and Range province—for more than 150 km along strike, from the Aegean Sea coast in northeastern Greece northward into the Struma Valley of southern Bulgaria. Hanging-wall rocks in the Strymon Valley detachment system moved in a S53°W direction relative to the footwall, resulting ultimately in the tectonic unroofing of the Rhodope metamorphic core complex. Several segments of the Strymon Valley detachment have previously been interpreted as thrust faults formed during a Miocene compressional event. Our results negate the principal evidence for this event and imply instead that extensional deformation has characterized the northern Aegean region since at least middle Miocene time. Displacement ceased on the Strymon Valley detachment with the late Pliocene development of the Strymon and Drama basins. We propose that those basins, together with the other principal basins of the northern Aegean region, are subsiding above an active northeast-dipping, extensional detachment zone that forms a unified kinematic system with the western offshore continuation of the dextral North Anatolian strike-slip fault, the North Aegean trough. If this hypothesis is correct, then the North Aegean trough, the western strands of the North Anatolian fault, and the principal modern depocenters of the northern Aegean region may all have originated no earlier than late Pliocene time.

Plutonism and the origin of metamorphic core complexes

Abstract: The formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension. Pulses of ductile deformation have taken place during short-lived thermal events initiated by the heat input from intruded plutons, sills, or dikes. Such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes. Fast cooling inferred from 40 Ar/ 39 Ar apparent ages may have taken place after periods of magma arrival, and need not be the result of rapid erosional or tectonic denudation. Heterogeneity of 40 Ar/ 39 Ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills (or sill swarms). Under such circumstances, apparently isothermal decompression paths may actually link pressure-temperature points set during periods of transient mineral growth during and after periods of igneous activity.

Quantification of soil production and downslope creep rates from cosmogenic 10Be accumulations on a hillslope profile

Abstract: Average soil transport rates over a period of ∼3500 yr on a convex soil-mantled hillslope have been quantified using a mass-balance model that incorporates the soil concentration of the cosmogenic isotope 10 Be. The 10 Be model results support the assumption used in most geomorphic models that the soil creep rate is proportional to surface gradient. The predicted diffusion coefficient is 360 ±55 cm 3 ⋅ yr -1 ⋅ cm -1 contour length and the average rate of soil production is 0.026 ±0.007 cm/yr. Within the uncertainty of this technique, the data do not reject G. K. Gilbert9s hypothesis that some hillslopes may exist in a condition of dynamic equilibrium with a uniform soil production rate. However, the model does not require an assumption of dynamic equilibrium and may be an approach that uniquely allows the quantification of a local soil-production rate law.

Statistical inevitability of Horton's laws and the apparent randomness of stream channel networks

Abstract: The remarkably regular geometric relations observed in stream networks have been widely interpreted as evidence of a distinctive structure that reflects particular geomorphic processes. These relations have also been interpreted as evidence that stream networks are topologically random, formed by the laws of chance. Neither of these inferences is justified. The oft-cited geometric properties are not specific to particular kinds of stream networks or to topologically random networks; instead, they describe virtually all possible networks. They therefore compel no particular conclusion about the origin or structure of stream networks.

Synchronous anatexis, metamorphism, and rapid denudation at Nanga Parbat (Pakistan Himalaya)

Abstract: The Nanga Parbat-Haramosh massif of the western Himalaya is a north-trending half-window of Indian crust that provides spectacular exposures of Precambrian basement gneisses that have been overprinted by Himalayan metamorphism. We report here petrologic data and U/Pb dates on zircon and monazite which document that Nanga Parbat gneisses underwent a Pliocene-Pleistocene episode of high-grade metamorphism and anatexis during an interval in which the Nanga Parbat massif was undergoing rapid denudation at mean rates of ∼5 mm/yr. We speculate that by initiating decompression melting, this denudation may be at least partly responsible for the anatexis and high-grade metamorphism.

Accretion and indentation tectonics at the southern edge of the Kaapvaal craton during the Kibaran (Grenville) orogeny

Abstract: A comparison of the lithologic, geochronologic, and kinematic features of the ∼1Ga orogens in southern Africa (Namaqua-Natal belt) and the Heimefrontfjella (East Antarctica) shows that the three areas originally constituted a contiguous belt that evolved within a prolonged, consistently northeast-oriented stress regime (African azimuths). An early northeast- or southwest-directed thrusting event has been identified along the entire belt. A later episode of transcurrent shearing can account for both the emplacement of extensive late-tectonic granitoid plutons in Natal and for the development of the Koras and Sinclair basins in Namaqualand and Namibia. The geometry and sense of movement of the later shear zones are functions of the varying orientation of the adjacent Archean cratonic margin and a prolonged period of northeast-directed plate convergence. Consequently, the craton is interpreted as a southwest-directed indenter during the Grenville orogeny.

Zonation patterns of skarn garnets: Records of hydrothermal system evolution

Abstract: Chemically zoned skarn garnets provide a continuous record of hydrothermal processes in lower Paleozoic sedimentary rocks within the contact aureole around the Drammen granite in the Oslo rift, southern Norway. Major and trace element zonation profiles, the latter obtained using a scanning high-resolution proton microprobe, reveal early infiltration-controlled growth of relatively grossular rich garnets, the major and trace elements compositions being buffered by local mineral dissolution. Subsequent rapid, epitaxial growth of andradite-rich garnet on grossular-rich cores marks the onset of vigorous and focused fluid flow along high-permeability zones. During this later stage, the hydrothermal fluid composition was to a large extent externally controlled, and the andradite precipitating from these fluids was characterized by high As and W contents. The zonation patterns of the andradite-rich garnets record at least five intermittent growth periods, with rapid andradite precipitation from fluid batches with high f O 2 , and progressively decreasing As and W contents. Thin layers, poor in Fe, As, and W, but relatively high in Al and Mn, represent periods of slow growth rates between the major pulses of hydrothermal fluids. The marked rimward decrease in the As and W contents of the garnets may reflect influx of meteoric waters or exhaustion of these elements in the hydrothermal fluid reservoir caused by boiling-controlled distillation processes at depth.

Exceptional fossil record: Distribution of soft-tissue preservation through the Phanerozoic

Abstract: Preservation of soft-bodied fossil biotas (Konservat-Lagerstaten) that preserve traces of volatile nonmineralized tissues (readily degraded by bacteria) are not evenly spaced through geologic time. When compared to outcrop area, exceptional faunas appear to be over-represented in the Cambrian and Jurassic. These concentrations in time correspond to particular environments, indicating that controls on the distribution of exceptional faunas may have operated on a global scale. The reduction in the number of exceptional faunas after the Cambrian may reflect the evolution and diversification of deep bio- turbators. Specific conditions favoring stagnation and episodic burial were required to ensure preservation in younger rocks.

Free gas at the base of the gas hydrate zone in the vicinity of the Chile triple junction

Abstract: At Ocean Drilling Program Site 859 in the vicinity of the Chile triple junction, the source of the bottom simulating reflection (BSR) at the base of the gas hydrate layer has, for the first time, been logged to reveal the nature of the impedance contrasts producing the reflection. We estimate from the P-wave velocity (V[sub P]) that hydrate occupies no more than 18% of the pore space just above the BSR and is not concentrated enough to cause the reflections. The BSR is caused by a sharp drop in V[sub P], and presumably density, from [approximately]1950 to 1600 m/s (on average) within an 8 m interval. Seismic modeling of wave form and amplitude vs. offset of the BSR at Site 860 indicates that the BSR is produced by a 12 m interval with low V[sub P] and shear-wave velocities that are consistent with small quantities of free gas ([approximately] 1% of pore space) in the interval. 22 refs., 5 figs.

Scratching the surface: estimating dimensions of large igneous provinces

Abstract: A study of five major basaltic provinces, including oceanic plateaus, volcanic passive margins, and continental flood basalts, shows that they are voluminous constructions of extrusive igneous rock underlain by intrusive rock. Crustal thickness ranges from 20 to 40 km, and lower crust is characterized by high (7.0-7.6 km/s) seismic velocities. Volumes and emplacement rates derived for two oceanic plateaus, the Ontong Java and Kerguelen-Broken Ridge, reveal short-lived pulses of increased global crustal production and suggest an origin involving the lower mantle. The Ontong Java rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid-ocean ridge system. Despite the importance off large igneous provinces in studies of mantle dynamics and the global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features.

Temperature dependence of silicate weathering in nature: How strong a negative feedback on long-term accumulation of atmospheric CO2 and global greenhouse warming?

Abstract: Estimation of the temperature dependence of natural feldspar weathering in two catchments at different elevations yields an apparent Arrhenius activation energy of 18.4 kcal/mol (77.0 kJ/mol), much higher than most laboratory values. This finding supports recent suggestions that hydrolytic weathering of silicate minerals may consume carbonic acid and thereby remove atmospheric carbon dioxide more rapidly with increasing temperature than previously thought. This result provides a stronger negative feedback on long-term greenhouse warming than has been assumed in most models of global carbon cycling. The present estimate was determined from the ratio of feldspar weathering rates (determined by geochemical mass balance) in the southern Blue Ridge Mountains of North Carolina, United States. Temperature (a function of elevation) is the only factor that differs between the two catchments; parent rock type, aspect, hillslope hydrology, and vegetation type and successional stage are the same in both.

Vesiculation of basaltic magma during eruption

Abstract: Vesicle size distributions in vent lavas from the Pu9u9O9o-Kupaianaha eruption of Kilauea volcano are used to estimate nucleation and growth rates of H 2 O-rich gas bubbles in basaltic magma nearing the earth9s surface (≤120 m depth). By using well-constrained estimates for the depth of volatile exsolution and magma ascent rate, nucleation rates of 35.9 events ⋅ cm -3 ⋅ s -1 and growth rates of 3.2 x 10 -4 cm/s are determined directly from size-distribution data. The results are consistent with diffusion-controlled growth as predicted by a parabolic growth law. This empirical approach is not subject to the limitations inherent in classical nucleation and growth theory and provides the first direct measurement of vesiculation kinetics in natural settings. In addition, perturbations in the measured size distributions are used to examine bubble escape, accumulation, and coalescence prior to the eruption of magma.

Mongolian plateau: Evidence for a late Cenozoic mantle plume under central Asia

Abstract: The 2500 x 700 km Mongolian plateau (average elevation 2000 m) is situated between the Altai orogen and the Siberian craton and occupies much of Mongolia and Transbaikalia in Russia. The plateau is characterized by (1) basin and range topography and two major domes(Hentai, 600 x 300 km, and Hangai, 800 x 550 km), where altitudes reach 3905 m; (2) lithosphere that is thinner than adjacent areas (minimum ∼50 km); (3) elevated heat flow (up to 120 mW/m2); (4) dominantly alkaline basaltic volcanism in the form of cones, lava fields, and volcanic plateaus mostly of Miocene-Quaternary age, and (5) rifts, including Baikal (main evolution in the Pliocene-Quaternary), Tunka (Oligocene-early Miocene), and Hobsogol (Pliocene-Quaternary). Existing models explain these features in terms of diapiric upwelling of a mantle asthenolith below the main rifts and/or as a long-distance effect of the India-Asia collision. We propose that the late Cenozoic uplift of the whole Mongolian plateau and associated rifting, magmatism, high heat flow, and lithospherec thinning are not externally driven by the India-Asia collision, but are the expression of the interaction of a mantle plume with overlying lithosphere. Some rifts link and interact with major strike-slip faults, such as the Bolnai. Such faults may be the major expression of the India-Asia collision in this region.

Siliciclastic sequence stratigraphic patterns in foreland, ramp-type basins

Abstract: Stratal patterns within shelf depositional sequences are dependent on tectonically controlled subsidence rates and their regional patterns. In active tectonic basin settings, regional subsidence patterns can be very different from passive continental margin settings, resulting in substantial modifications of the basic sequence stratigraphic model. In ramp-type foreland basins, on the tectonically active side, subsidence rates decrease seaward in contrast with passive continental margins, where the opposite subsidence pattern exists. Analysis of the interplay between eustasy and subsidence suggests the existence of two tectono-stratigraphic zones, occurring proximally and distally with respect to the basin margin. Zone A is defined as the region within which the rate of subsidence always exceeds the rate of eustatic fall. Consequently, relative sea level rises continuously during a eustatic cycle, albeit at varying rates. Zone B is defined as the region within which the rate of eustatic fall periodically exceeds the rate of subsidence, resulting in an interval of relative sea-level fall during a eustatic cycle. On the tectonically active sides of foreland basins, zone A lies on the landward side of the basin margin, proximal to the orogenic belt, and zone B lies seaward off zone A, away from the orogenic belt where subsidence rates are lower. On passive continental margins, because of the opposite subsidence patterns, zone A lies seaward of zone B, where subsidence rates are greater. The location of the shoreline relative to these zones determines the stacking patterns and stratal discontinuities within a depositional sequence. If the shoreline remains in zone A, then only type 2 sequence boundaries will occur; if sufficient sediment flux is available, allowing the shoreline to prograde into zone B, then type 1 sequence boundaries can occur. In foreland basins, these type 1 sequence boundaries would become type 2 sequence boundaries updip in zone A. The pattern of seaward-decreasing subsidence on the tectonically active sides of foreland basins results in characteristic longitudinal facies and stratal patterns. When the two zones occur, the updip region of the basin proximal to the orogenic belt is characterized by nearly continuous nonmarine deposition, albeit at varying rates. The downdip region is characterized by forced regressions and deposition primarily of lowstand and transgressive systems tracts. A transitional region containing deposits of all three systems tracts commonly occurs between the updip and downdip areas.

Accretion of Archean oceanic plateau fragments in the Abitibi, greenstone belt, Canada

Abstract: The arc-arc collision model is often applied to the evolution of Archean greenstone belts. This model predicts concordant stratigraphic relations among the various lithologies evolving generally from komatiitic-tholeiitic affinities toward calc-alkalic affinities. New geological data from the Malartic composite block of the Abitibi greenstone belt indicate that this volcanic pile, rather than representing a conformable sequence, is composed of four distinct mafic lithotectonic domains that are overlain by a calc-alkalic sequence. The mafic domains are thought to be examples of accreted Archean oceanic plateau material that were deformed during their collage state. The younger calc-alkalic sequence represents extension-related volcanism that erupted through the ac- creted and deformed plateau material, possibly as a consequence of ridge subduction in a regime of oblique convergence. The relations described here may represent a common feature of Archean terranes.