首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
We have studied the scaling behavior of compressional-wave velocity and density logs from an exploration borehole that extends down to about 700 m depth in the Brunswick No. 6 mining area, Bathurst Mining Camp, Canada. Using statistical methods, vertical and horizontal scale lengths of heterogeneity were estimated. Vertical scale length estimates from the velocity, density and calculated acoustic impedance are 14 m, 33 m, and about 20 m, respectively. Although the estimated scale length for the acoustic impedance implies a weak scattering environment, elastic finite difference modeling of seismic wave propagation in 2D heterogeneous media demonstrates that even this weak scattering medium can mask seismic signals from small, but yet economically feasible, massive sulfide deposits. Further analysis of the synthetic seismic data suggests that in the presence of heterogeneity, lenticular-shaped targets may only exhibit incomplete diffraction signals whereby the down-dip tails of these diffractions are mainly visible on the stacked sections. Therefore, identification of orebody generated diffractions is much easier on the unmigrated stacked sections than on migrated stacked sections. The numerical seismic modeling in 2D heterogeneous media indicates that in the presence of large horizontal, but small vertical scale lengths (structural anisotropy), identification of massive sulfide deposits is possible, but their delineation at depth requires detailed velocity modeling and processing algorithms which can handle the anisotropy.  相似文献   

2.
In an attempt to understand the structure of active faults as they emerge from bedrock into shallow semi-consolidated and unconsolidated sediments, we have recorded a comprehensive high-resolution seismic reflection/refraction data set across the Ostler Fault zone on the central South Island of New Zealand. This fault zone, which absorbs 1–2 mm/yr of compression associated with oblique convergence of the Pacific and Australian tectonic plates, consists of a series of surface-rupturing N–S trending, west-dipping reverse faults that offset a thick sequence of Quaternary glacial outwash and late Neogene fluvio-lacustrine sediments of the Mackenzie Basin. Our study focuses on a region of the basin where two non-overlapping fault segments are separated by a transfer zone. Deformation in this area is accommodated by offsets on multiple small faults and by folding in their hanging walls. The seismic data with source and receiver spacing of 6 and 3 m and nominal CMP fold of 60 was acquired along twelve 1.2 km long lines orthogonal to fault strike and an additional 1.6 km long tie-line parallel to fault strike. The combination of active deformation and shallow glacial outwash sediments results in particularly complicated seismic data, such that application of relatively standard processing schemes yields only poor quality images. We have designed a pre- and post-stack reflection/refraction processing scheme that focuses on minimising random and source-generated noise, determining appropriate static corrections and resolving contrasting reflection dips. Application of this processing scheme to the Ostler Fault data provides critical information on fault geometry and offset and on sedimentary structures from the surface to ～ 800 m depth. Our preliminary interpretation of one of the lines includes complex deformation structures with folding and multiple subsidiary fault splays on either side of a ～ 50° west-dipping primary fault plane.  相似文献   

3.
Seismic reflection data were acquired across volcanic hosted massive sulfides (VHMS) of Vihanti in order to improve the understanding of the regional geological setting. Commercially processed seismic data from Vihanti are of good quality, but reprocessing can be used to extract additional information about geological structures. Especially, careful velocity analysis influences the quality of seismic images. Differentiating reflections caused by fractures from those caused by lithological contacts is very important for exploration and geological modeling. Reflections from fracture zones known from drilling stack with lower velocity (~ 5100 m/s) compared to typical stacking velocities of the Vihanti area (> 5500 m/s). The reprocessing also indicated that fracture zones are better imaged with low frequencies due to the better overall continuity of the fault zones at scales of hundreds of meters rather than at shorter seismic wavelengths.In full stacks, long offset data can mask structure close to the surface. More detailed seismic images of the shallow subsurface emerged by preferentially stacking short offset data wherever acquisition and processing lines lay close together and were nearly straight. Long offset data remains valuable for imaging deeper structures as well as dipping reflectors. Cross-dip-analysis revealed a bright diffractor located near the base of the Vihanti volcanic basin at 1.5 km depth. The seismic data allow a geological interpretation in which the Vihanti structure has developed through significant thrust faulting and displacement of the lithological contacts. Gentle folds that were formed prior to faulting are visible as undulating reflectivity in seismic sections. The reprocessed seismic section indicates a potential deep extension of the ore-hosting altered volcanic and calc-silicate rocks previously unexplored.  相似文献   

4.
The Narmada–Son Lineament (NSL) Zone is the second most important tectonic feature after Himalayas, in the Indian geology. Magnetotelluric (MT) studies were carried out in the NSL zone along a 130 km long NNE-SSW trending profile. The area of investigation extends from Edlabad (20°46′16″; 75°59′05″) in the South to Khandwa (21°53′51″; 76°18′05″) in the North. The data shows in general the validity of a two-dimensional (2D) approach. Besides providing details on the shallow crustal section, the 2D modeling results resolved four high conductive zones extending from the middle to deep crust, spatially coinciding with the major structural features in the area namely the Gavligarh, Tapti, Barwani-Sukta and Narmada South faults. The model for the shallow section has brought out a moderately resistive layer (30–150 Ω m) representing the exposed Deccan trap layer, overlying a conductive layer (10–30 Ω m) inferred to be the subtrappean Gondwana sediments, the latter resting on a high resistive basement/upper crust. The Deccan trap thickness varies from around a few hundred meters to as much as 1.5 km along the traverse. A subtrappean sedimentary basin like feature is delineated in the northern half of the traverse where a sudden thickening of subtrappean sediments amounting to as much as 2 km is noticed. The high resistive upper crust is relatively thick towards the southern end and tends to become thinner towards the middle and northern part of the traverse. The lower crustal segment is conductive over a major part of the profile. Considering the generally enhanced heat flow values in the NSL region, coupled with characteristic gravity highs and enhanced seismic velocities coinciding with the mid to lower crustal conductors delineated from MT, presence of zones of high density mafic bodies/intrusives with fluids, presumably associated with magmatic underplating of the crust in the zone of major tectonic faults in NSL region are inferred.  相似文献   

5.
Explosion deep seismic sounding data sections of high quality had been obtained with RV Meteor in the Reykjanes Iceland Seismic Project (RRISP77 [Angenheister, G., Gebrande, H., Miller, H., Goldflam, P., Weigel, W., Jacoby, W.R., Pálmason, G., Björnsson, S., Einarsson, P., Pavlenkova, N.I., Zverev, S., Litvinenko, I.V., Loncarecic, B., Solomon, S., 1980. Reykjanes Ridge Iceland Seismic Experiment (RRISP 77). J. Geophys. 47, 228–238]) which close an information gap near 62°N. Preliminary results were presented by Weigel [Weigel, W., 1980. Aufbau des Reykjanes Rückens nach refraktionsseismischen Messungen. In: Weigel, W. (Ed.), Reykjanes Rücken, Island, Norwegischer Kontinentalrand. Abschlusskolloquium, Hamburg zur Meteor-Expedition, vol. 45. DFG, Bonn, pp. 53–61], and here we report on the data and results of interpretation. Clear refracted phases to 90 km distance permit crustal and uppermost mantle structure to be modelled by ray tracing. The apparent P-wave velocities are around 4.5, 6–6.5, 7–7.6 and 8.2–8.7 km/s, but no wide-angle reflections have been clearly seen. Accompanying sparker reflection data reveal thin sediment ponds in the axial zone and up to 400 m thick sediments at 10 Ma crustal age. Ray tracing reveals the following model below the sediments: (1) a distinct, 1–2 km thick upper crust (layer 2A) with Vp increasing with age (to 10 Ma) from <3.4 to 4.9 km/s and with a vertical gradient of 0.1–0.2 km/s/km, (2) a lower crust or layer 3 beginning at depths of 2 (axis) to 4 km (10 Ma age) below sea level with 6.1–6.8 km/s and similar vertical gradients as above, (3) the lower crust bottoms at 5.2–9.5 km depth below sea level (0–10 Ma) with a marked discontinuity, underneath which (4) Vp rises from about 7.5–7.8 km/s (0–10 Ma) with a positive vertical gradient of, again, 0.1–0.2 km/s/km such that 8 km/s would be reached at 12 km and deeper near the axis. Our preferred interpretation is that the mantle begins at the distinct discontinuity (“Moho”), but a deeper “Moho” of Vp ≈ 8 km/s cannot be excluded. From Iceland southward to 60°N several experiments show a decrease of crustal thickness from 14 to 8 km. Velocity trends with age across the ridge reflect cooling and filling of cracks, and thickness trends probably suggest volcanic productivity variations as previously suggested.Gravity inversion concentrates on a profile across the ridge with the above seismic a priori information; with 0.2–0.5 km depth uncertainty it leads to a good fit (±2.5 mGal where seismic data exist). Best fitting densities are (in kg/m3) for sediments, 2180; upper crust, 2450–2570; lower crust, 2850–2940; mantle lithosphere, 3215–3240 with a deficit for an asthenospheric wedge of no more than −100 kg/m3. The morphological ridges and troughs superimposed on the SE ridge flank are partly correlated, partly anti-correlated with the Bouguer anomaly and suggest that variable crustal density variations accompany the morphology variations.  相似文献   

6.
《Marine pollution bulletin》2012,64(5-12):195-200
Submarine groundwater discharge (SGD) on the reef flat of Bolinao, Pangasinan (Philippines) was mapped using electrical resistivity, 222Rn, and nutrient concentration measurements. Nitrate levels as high as 126 μM, or 1–2 orders of magnitude higher than ambient concentrations, were measured in some areas of the reef flat. Nutrient fluxes were higher during the wet season (May–October) than the dry season (November–April). Dissolved inorganic nitrogen (DIN = NO3 + NO2 + NH4) and soluble reactive phosphorus (SRP) fluxes during the wet season were 4.4 and 0.2 mmoles m−2 d−1, respectively. With the increase population size and anthropogenic activities in Bolinao, an enhancement of SGD-derived nitrogen levels is likely. This could lead to eutrophic conditions in the otherwise oligotrophic waters surrounding the Santiago reef flat.  相似文献   

7.
Where they are preserved, landforms that have been truncated and offset by past fault movements provide potentially valuable quantitative data that can be used to estimate slip rates. At such locations, it is important to investigate the fault zone in sufficient detail to understand how displacements are accommodated on individual fault strands. At a site along a northern section of the Alpine fault zone on the South Island of New Zealand, surface mapping of a series of faulted river terraces and channels has revealed a complicated and poorly understood paleoearthquake history. We have acquired high-resolution 2- and 3-D ground-penetrating radar (GPR) data over a large area (~ 500 × 500 m) of the terraces to map along-strike changes in shallow (<20 m) fault zone morphology. By identifying distinct reflection patterns within the topographically migrated 3-D GPR volumes and extrapolating them to the longer and more widely spaced GPR profiles, we determined the subsurface extent of two main structural/depositional facies that were juxtaposed by three left-stepping en-echelon fault strands. Two regions of warped strata are interpreted to result from transpressive folding between the overlapping strands, where displacement is transferred from one fault to the next. We suggest that diffuse deformation between the overlapping fault tips results in anomalously low estimates for horizontal and vertical fault displacements of some geomorphic features.  相似文献   

8.
Two-dimensional crustal velocity models are derived from passive seismic observations for the Archean Karelian bedrock of north-eastern Finland. In addition, an updated Moho depth map is constructed by integrating the results of this study with previous data sets. The structural models image a typical three-layer Archean crust, with thickness varying between 40 and 52 km. P wave velocities within the 12–20 km thick upper crust range from 6.1 to 6.4 km/s. The relatively high velocities are related to layered mafic intrusive and volcanic rocks. The middle crust is a fairly homogeneous layer associated with velocities of 6.5–6.8 km/s. The boundary between middle and lower crust is located at depths between 28 and 38 km. The thickness of the lower crust increases from 5–15 km in the Archean part to 15–22 km in the Archean–Proterozoic transition zone. In the lower crust and uppermost mantle, P wave velocities vary between 6.9–7.3 km/s and 7.9–8.2 km/s. The average Vp/Vs ratio increases from 1.71 in the upper crust to 1.76 in the lower crust.The crust attains its maximum thickness in the south-east, where the Archean crust is both over- and underthrust by the Proterozoic crust. A crustal depression bulging out from that zone to the N–NE towards Kuusamo is linked to a collision between major Archean blocks. Further north, crustal thickening under the Salla and Kittilä greenstone belts is tentatively associated with a NW–SE-oriented collision zone or major shear zone. Elevated Moho beneath the Pudasjärvi block is primarily explained with rift-related extension and crustal thinning at ∼2.4–2.1 Ga.The new crustal velocity models and synthetic waveform modelling are used to outline the thickness of the seismogenic layer beneath the temporary Kuusamo seismic network. Lack of seismic activity within the mafic high-velocity body in the uppermost 8 km of crust and relative abundance of mid-crustal, i.e., 14–30 km deep earthquakes are characteristic features of the Kuusamo seismicity. The upper limit of seismicity is attributed to the excess of strong mafic material in the uppermost crust. Comparison with the rheological profiles of the lithosphere, calculated at nearby locations, indicates that the base of the seismogenic layer correlates best with the onset of brittle to ductile transition at about 30 km depth.We found no evidence on microearthquake activity in the lower crust beneath the Archean Karelian craton. However, a data set of relatively well-constrained events extracted from the regional earthquake catalogue implies a deeper cut-off depth for earthquakes in the Norrbotten tectonic province of northern Sweden.  相似文献   

9.
Reflection seismic data were acquired along a c. 23 km long profile over the Pärvie Fault system with a nominal receiver and source spacing of 20 m. An hydraulic breaking hammer was used as a source, generating signals with a penetration depth of about 5–6 km. Steeply dipping reflections from the end-glacial faults are observed, as well as sub-horizontal reflections. The location and orientation of the reflections from the faults agree well with surface geological observations of fault geometries. Reflections from a potential fourth end-glacial fault is observed further to the east along the profile. The more sub-horizontal reflections may originate from gabbroic bodies within the granitic basement or from deeper lying greenstones. Our results indicate that the end-glacial faults dip at moderate to steep dips down to at least 2–3 km depth, and possibly continue at this dip to depths of 6 km. This result has significant implications for determining the state of stress required to activate the faults in the past and in the future.  相似文献   

10.
Inversion of local earthquake travel times and joint inversion of receiver functions and Rayleigh wave group velocity measurements were used to derive a simple model for the velocity crustal structure beneath the southern edge of the Central Alborz (Iran), including the seismically active area around the megacity of Tehran. The P and S travel times from 115 well-located earthquakes recorded by a dense local seismic network, operated from June to November 2006, were inverted to determine a 1D velocity model of the upper crust. The limited range of earthquake depths (between 2 km and 26 km) prevents us determining any velocity interfaces deeper than 25 km. The velocity of the lower crust and the depth of the Moho were found by joint inversion of receiver functions and Rayleigh wave group velocity data. The resulting P-wave velocity model comprises an upper crust with 3 km and 4 km thick sedimentary layers with P wave velocities (Vp) of ～5.4 and ～5.8 km s?1, respectively, above 9 km and 8 km thick layers of upper crystalline crust (Vp ～6.1 and ～6.25 km s?1 respectively). The lower crystalline crust is ～34 km thick (Vp ～ 6.40 km s?1). The total crustal thickness beneath this part of the Central Alborz is 58 ± 2 km.  相似文献   

11.
The optical properties and light climate in the deep and extremely acid Lake Caviahue have been studied in order to better understand its characteristics and the possible influence upon the phytoplankton community. The absorption coefficients for the dissolved fraction were maximal in the ultraviolet (UV) region and the water absorption spectra showed a shoulder around 300 nm, which was attributed to the concentration of Fe(III). No radiation was detected in the water column below 360 nm. The depth of the 1% incident radiation was dependent of wavelength, showing its maximum of 13.3 m at 565 nm, compared to 1.7 m and 4.8 m at 400 nm and 700 nm, respectively. Phytoplankton biomass was low and showed an almost constant profile with depth despite the relative darkness of the water column. Optical climate of Lake Caviahue is not typical of high elevation lakes but is more similar to low elevation shallow lakes of the Andean region. The chemical composition of the water, mainly Fe oxidation state and concentration, is the responsible for the high attenuation of the UV radiation (UVR). Living organisms are protected of UVR because Lake Caviahue waters are a shield against UV-B.  相似文献   

12.
The Abrolhos Bank is an area of high ecological, socio-economic importance and harbour the richest and most-extensive coral reefs in the South Atlantic. Here we report the discovery of shallow (12–25 m depth) reef complex with ten large biogenic structures, intermediate between the typical mushroom-shaped pinnacles of the northern Abrolhos Bank (17°–18° S) and the small patch reefs found on the central/southern coast of the Espírito Santo State (19°–20° S). The newly discovered reefs harbour a relatively rich and abundant reef community, with 73 fish and 14 benthic cnidarian species, including endangered and commercially important ones. We discuss on urgent needs of properly mapping and understanding the ecological functioning of this reef system. Information provided here is a baseline for future impact evaluations, particularly considering the recent worst environmental disaster of Brazil from a dam collapse in Doce river that affected the region.  相似文献   

13.
The site of Bonneval (France) exhibits a 6 m thick stratigraphical sequence in which Middle and Lower Palaeolithic artefacts were discovered. The stratigraphy of the site consists of an alternation of loess-like silt layers deposited along glacial stages, and paleosoil horizons corresponding to the vegetal stabilization of the area during interglacials. These layers hence provide past environmental informations and can be used as stratigraphical markers and compared with well dated sequences of Northern France. Eight sediment samples from this Middle and Upper Pleistocene sequence were dated using the recuperated optically stimulated luminescence (Re-OSL) signal of quartz grains. The middle-size fraction (40–63 μm) shows a good reproducibility of the OSL signals which allows the application of a multiple-aliquot additive-dose approach for De determination. For all samples, the growth curves are linear up to an accumulated dose of 3000 Gy. The Re-OSL ages range from ～100 to ～450 ka and are in good agreement with the independent chronological information; they establish when handaxe industries have been made during the Middle and Upper Pleistocene in the Paris Basin.  相似文献   

14.
In this study an analytical solution was developed to predict steady radially-symmetric percolation rates from an aquifer underlain by a variable thickness aquitard. The solutions consider an aquitard with constant thickness and with radial-symmetrically increasing thickness outward from the center. The solution was used to predict the percolation rate from a peat layer around a bedrock outcrop in the James Bay Lowland near the De Beers Victor diamond mine. In this case the marine sediment layer limited the direct connection between the peat layer and the bedrock as an aquitard. Our zero order solution with constant marine sediment thickness showed the best fit to the steady state water level data of June 2012. It was found that the enhanced recharge around bioherms (i.e., at rates greater than the regional average of 0.7 mm/day) will only occur in marine sediments less than 4.3 m thick, for extreme depressurization of 30 m.  相似文献   

15.
A new IRSL dataset is presented for the age and setting of a critical Late Glacial Maximum tephra isochron marker. The rhyolitic tephra, known as the Kawakawa Tephra, occurs as a 14 cm thick layer within a 5.9 m thick loess section overlying alluvial gravels in the Rangitikei River valley, SW North Island of New Zealand. Ages range from 21 at the base to 5 ka near the top of the loess and bracket an age of 17.0 ± 2.2 for the tephra. The new IRSL ages are in agreement with published and unpublished luminescence ages from other localities of loess, sand and ash above and below the tephra and of the tephra itself, that indicate an age of ca. 19 ka for the Kawakawa Tephra. This age is considerably younger than the generally accepted 14C 27.1 ka cal yrs BP age of the Kawakawa Tephra and highlights an unresolved discrepancy between the two dating systems.  相似文献   

16.
《Continental Shelf Research》2007,27(3-4):475-488
Across a limited depth range (5–10 m) on many continental shelves, the dominant sediment size changes from sand to mud. This important boundary, called the sand–mud transition (SMT), separates distinct benthic habitats, causes a significant change in acoustic backscatter, represents a key facies change, and delimits more surface-reactive mud from less surface-reactive sand. With the goal of improving dynamical understanding of the SMT, surficial sediments were characterized across two SMTs on the Adriatic continental shelf of Italy. Geometric mean diameter, specific surface area (SSA), mud fraction (<63 μm) and heavy metal concentrations were all measured. The SMT related to the Tronto River is identified between 15 and 20 m water depth while the SMT associated with the Pescara River varies between 15 and 25 m water depth. The sediment properties correlate with a new, process-based sedimentological parameter that quantifies the fraction of the sediment in the seabed that was delivered as flocs. These correlations suggest that floc dynamics exert strong influence over sediment textural properties and metal concentrations. Relative constancy in the depth of the SMT along this portion of the margin and its lack of evolution over a period during which sediment input to the margin has dramatically decreased suggest that on the Adriatic continental shelf energy is the dominant control on the depth of the SMT.  相似文献   

17.
In the present study, the uppermost 3 cm of muddy, seabed sediment was collected from a deep-sea sediment core, drilled from a water depth of 4 km, near the deepest site of the Mediterranean Sea, outside Pylos, Greece. The core was divided into 7 layer samples, each 3–4 mm thick, in order to get an independent age assessment for each one using luminescence dating; from polymineral coarse grains in the range 30–60 μm. Between 11 and 22 aliquots were measured for each sample using the optically stimulated luminescence, single-aliquot regenerative-dose analysis. The estimated mean equivalent dose values had an uncertainty less than 3% and resulted in optical ages ranging from 3.5 to approximately 5 ka, with uncertainties lying between 5.5 and 7.4%. Multiple-aliquot, thermoluminescence-based, additive-dose, total bleaching approach provided equivalent doses with typical errors of 10–15% and ages in the range of 3.6 and 9.4 ka, with uncertainties up to 17%. Single-aliquot optical ages are shown to be relatively successful, due to their consistency with AMS 14C radiocarbon ages, obtained from Planktonic foraminifera from the same core. Luminescence dates for the topmost 1.5 cm indicate a substantial mix and post burial disturbance of the surface sediment. Below the topmost 1.5 cm, both luminescent approaches indicate ages which increase smoothly with depth. The concordant OSL and TL age estimates for the sample O5, in conjunction with specific luminescence properties and its major-element geochemical chemistry content, suggest that it was heated during the Santorini volcanic eruption. Deposition rates of 8.6–18.9 cm over 10 ka below the sample O5, provided by optical ages, exhibit an excellent level of agreement with the accumulation rate of 7–18 cm over 10 ka at the sea bottom, already reported for the site under study.  相似文献   

18.
Detailed acoustic mapping have been carried out in the Århus Bay in order to establish the general Late Weichselian and Holocene stratigraphy, and to map the gas related acoustic blanking.The mapping results show that the oldest seismic unit is glacial till, probably related to the latest glacial advance in the region. The glacial till is covered by late-glacial ice-lake clay and silt reaching a thickness of up to 10 m. In the deeper part of the bay, early Holocene organic material and peat has been recorded in a few cores. A thin seismic unit is observed, which probably represents an early Holocene lowstand period, when most of the Århus Bay was dry land. The three upper seismic units are related to the Holocene transgression of the region representing different hydrographical conditions. The lowermost unit (Marine unit 1) partly drapes the basin area with clay sediments and partly shows prograding sandy coastal deposits around glacial ridges. Marine shells from this unit date back to 8700 cal. years BP which are the oldest marine shells found south of the threshold in the northern Great Belt. The next unit (Marine unit 2) consists in general of mud to sandy mud, which cover most of the western central part of the Århus Bay and in some places reach the present seabed in areas of erosion or non-deposition. The distribution of the youngest seismic unit (mud, Marine unit 3) is confined to the sub-recent to recent sedimentation basins in the eastern central part of the area. Acoustical blanking shows that the methane production takes place in the Holocene marine sediments. A map of the distribution and depth to free methane in the muddy sediments has been produced. Combined information from the different seismic equipment used allowed a mapping of the distribution and depth to free gas in the intervals 0.5–2, 2–4 and >4 m. The map shows that acoustic blanking is found in the central part of Århus Bay about 4 m below the seabed. In areas with high sedimentation rate, the acoustic blanking is found closer to the sediment surface and in selected key stations, pore-water chemistry have documented that depth to acoustical blanking is comparable to the methane saturation depth.Barotropic induced inflow dominates the present current system in the semi-enclosed Århus Bay. The inflow events create turbulence in the outer eastern parts of the bay, followed by high sedimentation rates. The recent situation is reflected in the seabed sediments in the eastern part of the bay, which are characterised as soft sandy mud deposits, with gas bubbles close to the seabed.Seismic investigations have not previously been used to identify recent sedimentation areas and the most vulnerable areas in respect to possible escape of methane and toxic hydrogen-sulphide. The results of the present survey show that future monitoring must be focused in shallow gas areas in the eastern-most part of Århus Bay.  相似文献   

19.
We present sub-crystal-scale 238U–230Th zircon ages and 238U–230Th–226Ra plagioclase ages of bulk mineral separates from the Holocene (2.0–2.3 ka) eruptions of the Rock Mesa (RM) and Devil's Hills (DH) rhyolites at South Sister volcano, Oregon. We link these age data with sub-crystal trace-element analyses of zircon and plagioclase to provide insight into the subvolcanic system at South Sister, as an example of a small-volume continental arc volcano. Our results document the presence of coeval yet physically-distinct regions within the magma reservoir and constrain the timescales over which these heterogeneities existed. Zircons from the RM and DH dominantly record ages from 20 to 80 ka, with some grains recording ages > 350 ka, whereas plagioclase records 230Th–226Ra ages of 2.3–6.8 ka (RM) and 4.0–9.6 ka (DH-3) and a 238U–230Th age of 10 ± 34 ka (DH-3). We interpret zircons with ages < 350 ka as antecrysts inherited from a longer lived upper-crustal magma reservoir from which the rhyolites were generated, based on crystallization ages coeval with earlier periods of silicic volcanism at South Sister, the undersaturated nature of the RM and DH magmas with respect to zircon, and Ti-in-zircon temperatures consistent with low-temperature (< 815 °C) crystallization. In contrast, plagioclase ages are near the eruption age and dominantly preserve information about the recent (< 10 ka), higher-temperature evolution of the host magmas. Although zircon and plagioclase record different crystallization ages, each phase crystallized over the same time period in the RM compared to DH rhyolites. Linking these crystal age data with sub-crystal trace-element analyses demonstrates that zircon and plagioclase have distinct trace-element characteristics between eruptions, which require that the RM and DH crystals (and therefore magmas) were derived from distinct regions that had evolved independently for > 50 ka within a heterogeneous magmatic system and coexisted as physically-distinct, dominantly-liquid bodies prior to eruption. Thus, we favor a model where rhyolites are generated in independent batches by accumulation of evolved liquids in a heterogeneous, largely crystalline reservoir. Similarities in crystal age and chemical data to that at other young silicic systems (e.g., Mount St. Helens, Okataina Caldera Complex) suggest that this model may be more generally applicable to silicic magmas.  相似文献   

20.
Contamination with As, Cd and Hg, their spatial and temporal distribution are reported from the coastal wetland sediments of the northern Beibu Gulf, South China Sea. The content of As, Cd, Hg and TOC in surface sediments is 8.1 ± 5.8 μg g?1, 0.08 ± 0.14 μg g?1, 0.034 ± 0.028 μg g?1 and 0.45 ± 0.39%, respectively. The mean sedimentation rates are 0.93–1.37 cm year?1 during 1920s to 2008 determined by 210Pb and 137Cs dating in three cores. The vertical profiles of As, Cd and Hg content in the cores retrieved from Qin and Nanliu River estuaries show increasing trends during 1985–2008 due to anthropogenic impact caused by local economic development. Locally the surface sediments have potential ecological risk of As to benthos according to the NOAA sediment quality guidelines.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号