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11.
This paper explores how changes in sea level and biome distribution may have affected the habitats occupied by hunter‐gatherers in East Asia. Using a model‐based reconstruction of changing sea level from the Last Glacial Maximum (LGM) to present day, our analysis reveals that the exposure of a large continental shelf during the LGM sea level lowstand created a wealth of wooded, estuarine, and coastal biomes that could have been exploited intensively by Late Pleistocene hunter‐gatherers. Models explaining hunter‐gatherer subsistence changes and migrations in this period should take into account the large area that has been lost to rising sea level since the LGM. 相似文献
12.
Ahmed Salem Sheona Masterton Simon Campbell J. Derek Fairhead Jade Dickinson Colm Murphy 《Geophysical Prospecting》2013,61(5):1065-1076
Full Tensor Gravity Gradiometry (FTG) data are routinely used in exploration programmes to evaluate and explore geological complexities hosting hydrocarbon and mineral resources. FTG data are typically used to map a host structure and locate target responses of interest using a myriad of imaging techniques. Identified anomalies of interest are then examined using 2D and 3D forward and inverse modelling methods for depth estimation. However, such methods tend to be time consuming and reliant on an independent constraint for clarification. This paper presents a semi‐automatic method to interpret FTG data using an adaptive tilt angle approach. The present method uses only the three vertical tensor components of the FTG data (Tzx, Tzy and Tzz) with a scale value that is related to the nature of the source (point anomaly or linear anomaly). With this adaptation, it is possible to estimate the location and depth of simple buried gravity sources such as point masses, line masses and vertical and horizontal thin sheets, provided that these sources exist in isolation and that the FTG data have been sufficiently filtered to minimize the influence of noise. Computation times are fast, producing plausible results of single solution depth estimates t hat relate directly to anomalies. For thick sheets, the method can resolve the thickness of these layers assuming the depth to the top is known from drilling or other independent geophysical data. We demonstrate the practical utility of the method using examples of FTG data acquired over the Vinton Salt Dome, Louisiana, USA and basalt flows in the Faeroe‐Shetland Basin, UK. A major benefit of the method is the ability to quickly construct depth maps. Such results are used to produce best estimate initial depth to source maps that can act as initial models for any detailed quantitative modelling exercises using 2D/3D forward/inverse modelling techniques. 相似文献
13.
Ti-in-zircon thermometry: applications and limitations 总被引:11,自引:5,他引:11
Bin Fu F. Zeb Page Aaron J. Cavosie John Fournelle Noriko T. Kita Jade Star Lackey Simon A. Wilde John W. Valley 《Contributions to Mineralogy and Petrology》2008,156(2):197-215
The titanium concentrations of 484 zircons with U-Pb ages of ∼1 Ma to 4.4 Ga were measured by ion microprobe. Samples come
from 45 different igneous rocks (365 zircons), as well as zircon megacrysts (84) from kimberlite, Early Archean detrital zircons
(32), and zircon reference materials (3). Samples were chosen to represent a large range of igneous rock compositions. Most
of the zircons contain less than 20 ppm Ti. Apparent temperatures for zircon crystallization were calculated using the Ti-in-zircon
thermometer (Watson et al. 2006, Contrib Mineral Petrol 151:413–433) without making corrections for reduced oxide activities (e.g., TiO2 or SiO2), or variable pressure. Average apparent Ti-in-zircon temperatures range from 500° to 850°C, and are lower than either zircon
saturation temperatures (for granitic rocks) or predicted crystallization temperatures of evolved melts (∼15% melt residue
for mafic rocks). Temperatures average: 653 ± 124°C (2 standard deviations, 60 zircons) for felsic to intermediate igneous
rocks, 758 ± 111°C (261 zircons) for mafic rocks, and 758 ± 98°C (84 zircons) for mantle megacrysts from kimberlite. Individually,
the effects of reduced or , variable pressure, deviations from Henry’s Law, and subsolidus Ti exchange are insufficient to explain the seemingly low
temperatures for zircon crystallization in igneous rocks. MELTs calculations show that mafic magmas can evolve to hydrous
melts with significantly lower crystallization temperature for the last 10–15% melt residue than that of the main rock. While
some magmatic zircons surely form in such late hydrous melts, low apparent temperatures are found in zircons that are included
within phenocrysts or glass showing that those zircons are not from evolved residue melts. Intracrystalline variability in
Ti concentration, in excess of analytical precision, is observed for nearly all zircons that were analyzed more than once.
However, there is no systematic change in Ti content from core to rim, or correlation with zoning, age, U content, Th/U ratio,
or concordance in U-Pb age. Thus, it is likely that other variables, in addition to temperature and , are important in controlling the Ti content of zircon. The Ti contents of igneous zircons from different rock types worldwide
overlap significantly. However, on a more restricted regional scale, apparent Ti-in-zircon temperatures correlate with whole-rock
SiO2 and HfO2 for plutonic rocks of the Sierra Nevada batholith, averaging 750°C at 50 wt.% SiO2 and 600°C at 75 wt.%. Among felsic plutons in the Sierra, peraluminous granites average 610 ± 88°C, while metaluminous rocks
average 694 ± 94°C. Detrital zircons from the Jack Hills, Western Australia with ages from 4.4 to 4.0 Ga have apparent temperatures
of 717 ± 108°C, which are intermediate between values for felsic rocks and those for mafic rocks. Although some mafic zircons
have higher Ti content, values for Early Archean detrital zircons from a proposed granitic provenance are similar to zircons
from many mafic rocks, including anorthosites from the Adirondack Mts (709 ± 76°C). Furthermore, the Jack Hills zircon apparent
Ti-temperatures are significantly higher than measured values for peraluminous granites (610 ± 88°C). Thus the Ti concentration
in detrital zircons and apparent Ti-in-zircon temperatures are not sufficient to independently identify parent melt composition.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
14.
No terrestrial planet formation simulation completed to date has considered the detailed chemical composition of the planets produced. While many have considered possible water contents and late veneer compositions, none have examined the bulk elemental abundances of the planets produced as an important check of formation models. Here we report on the first study of this type. Bulk elemental abundances based on disk equilibrium studies have been determined for the simulated terrestrial planets of O’Brien et al. [O’Brien, D.P., Morbidelli, A., Levison, H.F., 2006. Icarus 184, 39-58]. These abundances are in excellent agreement with observed planetary values, indicating that the models of O’Brien et al. [O’Brien, D.P., Morbidelli, A., Levison, H.F., 2006. Icarus 184, 39-58] are successfully producing planets comparable to those of the Solar System in terms of both their dynamical and chemical properties. Significant amounts of water are accreted in the present simulations, implying that the terrestrial planets form “wet” and do not need significant water delivery from other sources. Under the assumption of equilibrium controlled chemistry, the biogenic species N and C still need to be delivered to the Earth as they are not accreted in significant proportions during the formation process. Negligible solar photospheric pollution is produced by the planetary formation process. Assuming similar levels of pollution in other planetary systems, this in turn implies that the high metallicity trend observed in extrasolar planetary systems is in fact primordial. 相似文献
15.
Pre-seismic, co-seismic and post-seismic displacements associated with the Bhuj 2001 earthquake derived from recent and historic geodetic data 总被引:1,自引:0,他引:1
Sridevi Jade M. Mukul I. A. Parvez M. B. Ananda P. D. Kumar V. K. Gaur R. Bendick R. Bilham F. Blume K. Wallace I. A. Abbasi M. Asif Khan S. Ulhadi 《Journal of Earth System Science》2003,112(3):331-345
The 26th January 2001 Bhuj earthquake occurred in the Kachchh Rift Basin which has a long history of major earthquakes. Great
Triangulation Survey points (GTS) were first installed in the area in 1856–60 and some of these were measured using Global
Positioning System (GPS) in the months of February and July 2001. Despite uncertainties associated with repairs and possible
reconstruction of points in the past century, the re-measurements reveal pre-seismic, co-seismic and post-seismic deformation
related to Bhuj earthquake. More than 25 Μ-strain contraction north of the epicenter appears to have occurred in the past
140 years corresponding to a linear convergence rate of approximately 10 mm/yr across the Rann of Kachchh. Motion of a single
point at Jamnagar 150 km south of the epicenter in the 4 years prior to the earthquake, and GTS-GPS displacements in Kathiawar
suggests that pre-seismic strain south of the epicenter was small and differs insignificantly from that measured elsewhere
in India. Of the 20 points measured within 150 km of the epicenter, 12 were made at existing GTS points which revealed epicentral
displacements of up to 1 m, and strain changes exceeding 30 Μ-strain. Observed displacements are consistent with reverse co-seismic
slip. Re-measurements in July 2001 of one GTS point (Hathria) and eight new points established in February reveal post-seismic
deformation consistent with continued slip on the Bhuj rupture zone. 相似文献
16.
Jade Star Lackey John W. Valley Hans J. Hinke 《Contributions to Mineralogy and Petrology》2006,151(1):20-44
Peraluminous granitoids provide critical insight as to the amount and kinds of supracrustal material recycled in the central
Sierra Nevada batholith, California. Major element concentrations indicate Sierran peraluminous granitoids are high-SiO2 (68.9–76.9) and slightly peraluminous (average molar Al2O3/(CaO + Na2O + K2O)=1.06). Both major and trace element trends mimic those of other high-silica Sierran plutons. Garnet (Grt) in the peraluminous
plutons is almandine–spessartine-rich and of magmatic origin. Low grossular contents are consistent with shallow (<4 kbar)
depths of garnet crystallization. Metasediments of the Kings Sequence commonly occur as wallrocks associated with the plutons,
including biotite schists that are highly peraluminous (A/CNK=2.25) and have high whole rock (WR) δ18O values (9.6–21.8‰, average=14.5±2.9‰, n=26). Ultramafic wallrocks of the Kings–Kaweah ophiolite have lower average δ18O (7.1±1.3‰, n=9). The δ18O(WR) of the Kings Sequence is variable from west to east. Higher δ18O values occur in the west, where quartz in schists is derived from marine chert; values decrease eastward as the proportion
of quartz from igneous and metamorphic sources increases. Peraluminous plutons have high δ18O(WR) values (9.5–13‰) consistent with supracrustal enrichment of their sources. However, relatively low initial 87Sr/86Sr values (0.705–0.708) indicate that the supracrustal component in the source of peraluminous magmas was dominantly altered
ocean crust and/or greywacke. Also, plutons lack or have very low abundances (<1% of grains) of inherited zircon (Zrc) cores.
Average δ18O(Zrc) is 7.9‰ in peraluminous plutons, a higher value than in coeval metaluminous plutons (6–7‰). Diorites associated with
peraluminous plutons also have high δ18O(Zrc), 7.4–8.3‰, which is consistent with the diorites being derived from a similar source. Magmatic garnet has variable
δ18O (6.6–10.5‰, avg.=7.9‰) due to complex contamination and crystallization histories, evidenced by multiple garnet populations
in some rocks. Comparison of δ18O(Zrc) and δ18O(Grt) commonly reveals disequilibrium, which documents evolving magma composition. Minor (5–7%) contamination by high δ18O wallrocks occurred in the middle and upper crust in some cases, although low δ18O wallrock may have been a contaminant in one case. Overall, oxygen isotope analysis of minerals having slow oxygen diffusion
and different times of crystallization (e.g., zircon and garnet), together with detailed textural analysis, can be used to
monitor assimilation in peraluminous magmas. Moreover, oxygen isotope studies are a valuable way to identify magmatic versus
xenocrystic minerals in igneous rocks.
Electronic Supplementary Material Supplementary material is available for this article at 相似文献
17.
Analysis of strength and moduli of jointed rocks 总被引:1,自引:0,他引:1
This paper deals with two aspects of jointed rock mass behavior, first the finite element modeling of a jointed rock mass as an equivalent continuum, second the comparison of empirical strength criteria of a jointed rock mass. In finite element modeling the jointed rock properties are represented by a set of empirical relationships, which express the properties of the jointed medium as a function of joint factor and the properties of the intact rock. These relationships have been derived from a large set of experimental data of tangent elastic modulus. It is concluded that equivalent continuum analysis gives the best results for both single and multiple jointed rock. The reliability of the analysis depends on the estimation of joint factor, which is a function of the joint orientation, joint frequency and joint strength.Empirical strength criteria for jointed rocks, namely Hoek and Brown, Yudhbir et al., Ramamurthy and Arora, Mohr–Coulomb have been incorporated in a nonlinear finite element analysis of jointed rock using the equivalent continuum approach, to determine the failure stress. The major principal stress at failure, obtained using Ramamurthy's criteria, compares very well with experimental results. 相似文献
18.
19.
Impact of internal waves on the acoustic field at a coastal station off Paradeep,east coast of India
Internal Wave (IW) characteristics and the impact of IW on acoustic field have been studied utilizing the hourly time series
of temperature and salinity data collected at a coastal site off Paradeep (north Bay of Bengal) during 24–25 October 2008.
The IW characteristics, viz. period (t
per
), velocity (C
vel
), wavelength (L), and wave numbers (k), are found to be 2.133–34.72 h, 0.135 km h−1, 0.37–6.2 km and 2.70–0.16 cycles km−1, respectively. The semi-diurnal tidal forces are predominant than diurnal as well as at other frequencies and its contribution
is about 64% towards the total potential energy (E
0 = 3.34 J m−2). Sound velocity perturbations with space and time in the presence of IW field are examined from Garrettt-Munk (GM) model.
Transmission loss anomaly for optimized source-receiver configuration at the depth of 53 m and range of 9 km has been computed
from acoustic modelling. The loss in the acoustic transmission is found to be 38.4 dB in the presence of low-frequency IW
field. 相似文献
20.
B. Sridevi T.V. Ramana MurtyY. Sadhuram M.M.M. RaoK. Maneesha S. Sujith KumarP.L. Prasanna 《Estuarine, Coastal and Shelf Science》2010
Internal waves (IW) are identified off Bhimilipatnam, east coast of India, from the time series CTD (hourly interval) and thermistor chain data (2 min interval) collected during 23–25 Feb 2007. The measurements were carried out at 94 m water depth on the continental shelf edge. These data sets are used to describe the characteristics of IW and their impact on acoustic fields. Garrett and Munk (GM) model has been used to predict the characteristics of low frequency (LF) IW with space and time. Active IW are seen in the layers 54 m–94 m with a velocity of 0.548 km h−1 and the wavelengths of the order of 0.03 km–21.8 km. The model could capture the IW features in the thermocline region accurately than at the bottom. This could be due to the limitation of the model which considers linearity. High frequency IW observed at the bottom could be due to the advection of tidal currents over the shallow irregular bottom in the presence of stratification. The study emphasizes linear IW rather than transient non-linear waves induced by tidal interaction with topography. 相似文献