Crustal structure of the Ruby Mountains and southern Carlin Trend region, Nevada, from magnetotelluric data     

Philip E. Wannamaker  William M. Doerner 《Ore Geology Reviews》2002,21(3-4)

We have collected about 150 magnetotelluric (MT) soundings in northeastern Nevada in the region of the Ruby Mountains metamorphic core complex uplift and southern Carlin mineral trend, in an effort to illuminate controls on core complex evolution and deposition of world-class gold deposits. The region has experienced a broad range of tectonic events including several periods of compressional and extensional deformation, which have contributed to the total expression of electrical resistivity. Most of the soundings reside in three east–west profiles across increasing degrees of core uplift to the north (Bald Mountain, Harrison Pass, and Secret Pass latitudes). One short cross-line was also taken to assess an east–west structure to the north of the northern profile. Model resistivity cross-sections were derived from the MT data using a 2-D inversion algorithm, which damps departures of model parameters from an a priori structure. Geological interpretation of the resistivity combines previous seismic, potential field and isotope models, structural and petrological models for regional compression and extension, and detailed structural/stratigraphic interpretations incorporating drilling for petroleum and mineral exploration. To first order, the resistivity structure is one of a moderately conductive, Phanerozoic sedimentary section fundamentally disrupted by intrusion and uplift of resistive crystalline rocks. Late Devonian and early Mississippian shales of the Pilot and Chainman Formations together form an important conductive marker sequence in the stratigraphy and show pronounced increases in conductance (conductivity–thickness product) from east to west. These increases are attributed to graphitization caused by Elko–Sevier era compressional shear deformation and possibly by intrusive heating. The resistive crystalline central massifs adjoin the host stratigraphy across crustal-scale, steeply dipping fault zones. The zones provide pathways to the lower crust for heterogeneous, upper crustal induced, electric current flow. Resistive core complex crust appears steeply bounded under the middle of the neighboring grabens and not to deepen at a shallow angle to arbitrary distances to the west. The numerous crustal breaks imaged with MT may contribute to the low effective elastic thickness (T_e) estimated regionally for the Great Basin and exemplify the mid-crustal, steeply dipping slip zones in which major earthquakes nucleate. An east–west oriented conductor in the crystalline upper crust spans the East Humboldt Range and northern Ruby Mountains. The conductor may be related to nearby graphitic metasediments, with possible alteration by middle Tertiary magmatism. Lower crustal resistivity everywhere under the profiles is low and appears quasi one-dimensional. It is consistent with a low rock porosity (<1 vol.%) containing hypersaline brines and possible water-undersaturated crustal melts, residual to the mostly Miocene regional extension. The resistivity expression of the southern Carlin Trend (CT) in the Pinon Range is not a simple lineament but rather a family of structures attributed to Eocene intrusion, stratal deformation, and alteration/graphitization. Substantial reactivation or overprinting by core complex uplift or Basin–Range extensional events seems likely. We concur with others that the Carlin Trend may result in part from overlap of the large Eocene Northeast Nevada Volcanic Field with Precambrian–Paleozoic deep-water clastic source rocks thickening abruptly to the west of the Pinon Range, and projecting to the north–northwest.  相似文献   

Isotopic partitioning between scallop shell calcite and seawater: effect of shell growth rate     

Richard Owen  Hilary KennedyChristopher Richardson 《Geochimica et cosmochimica acta》2002,66(10):1727-1737

The relationship between molluscan shell growth rate and skeletal δ¹⁸O and δ¹³C was investigated in a detailed field study for the scallop, Pecten maximus. Seasonal variation in shell growth rate was found to be a governing factor influencing shell δ¹⁸O and δ¹³C. At low shell growth rates, shell δ¹⁸O were more positive (of the order +0.4‰) and δ¹³C more negative (up to −2‰) as compared with predicted values for precipitation of inorganic calcite in isotopic equilibrium with seawater. The deviations in δ¹⁸O were hypothesized as reflecting possible differences in solution carbonate chemistry at the site of mineralization in the extrapallial fluid as compared with that of the external seawater medium. The deviations in shell δ¹³C were consistent with incorporation of isotopically depleted respiratory ¹³C (i.e., a metabolic effect). A trend toward more depleted shell δ¹⁸O and δ¹³C values occurred at higher shell growth rates, with negative δ¹⁸O values as compared with predicted equilibrium at shell growth rates above 0.13 mm per day. These simultaneous negative deviations in skeletal δ¹⁸O and δ¹³C were interpreted as resulting from a kinetic effect. The implications for environmental reconstruction from molluscan isotopic records are discussed in light of a model of isotopic behavior based on the findings of the study.  相似文献   

G-Probe-1 - An International Proficiency Test for Microprobe Laboratories - Report on Round 1 : February 2002 (TB-1 Basaltic Glass)     

Philip J. Potts  Michael Thompson  Stephen Wilson 《Geostandards and Geoanalytical Research》2002,26(2):197-235

Results are presented for round one of a new international proficiency testing programme designed for microprobe laboratories involved in the routine analysis of silicate minerals. The sample used for this round was TB-1, a basaltic glass fused and prepared by the USGS. Thirty nine laboratories contributed data to this round, the majority of major element results being undertaken by EPMA and the majority of trace elements by LA-ICP-MS. Assigned values were derived from the median of results produced by nine selected laboratories that analysed powdered material by conventional ICP-MS, INAA and XRF techniques using bulk powders of the sample. Submitted microprobe results were evaluated using a target precision calculated using the Horwitz function, adopting the same criteria as those used for "applied" geochemistry laboratories in the companion GeoPT proficiency testing programme for laboratories involved in the routine bulk analysis of silicate rocks. An evaluation of results from participating microprobe laboratories indicated that overall, data were compatible with this precision function. A comparison between the performance of bulk and microprobe techniques used in the analysis of the basaltic glass showed remarkably good agreement, with significant bias only observed for the major oxide MgO.  相似文献   

Relationships between tissue concentrations of chlorinated hydrocarbons (polychlorinated biphenyls and chlorinated pesticides) and antioxidative responses of marine mussels, Perna viridis     

C. C. C. Cheung  G. J. Zheng  P. K. S. Lam  B. J. Richardson 《Marine pollution bulletin》2002,45(1-12)

Marine mussels, Perna viridis, were transplanted from a reference site to various polluted sites around Hong Kong. After 30 d of exposure, antioxidative responses in the gills and hepatopancreas and tissue concentrations of chlorinated hydrocarbons [polychlorinated biphenyls (PCBs) and chlorinated pesticides (CPs)] were determined for individual mussels. Glutathione S transferase (GST) and glutathione (GSH) were positively correlated with tissue PCB concentrations. Only one of the enzymatic antioxidants, glutathione peroxidase (GPx), showed significant response to tissue PCB. No significant correlation was found between tissue concentrations of chlorinated hydrocarbons and other enzymatic antioxidants (superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR) and NADPH DT-diaphorase (DT-d). Oxidative stress, measured as thiobarbituric acid reactive substances, was correlated with chlorinated pesticide concentrations in tissues. This study demonstrated a correlation between GST/GSH and chlorinated hydrocarbons. The apparent lack of correlation between trace organic pollutants and some of the enzymatic antioxidants may be due to the inhibitory effects caused by these chemicals. The above results suggest that more investigations are needed before these enzymes can be used as biomarkers.  相似文献   

Patch use and exploratory movements of a resident houbara bustard in northern Saudi Arabia     

Yolanda van Heezik  Philip J. Seddon 《Journal of Arid Environments》2002,50(4):683

  相似文献   

Field Survey and Numerical Simulations: A Review of the 1998 Papua New Guinea Tsunami     

Patrick J. Lynett  Jose C. Borrero  Philip L.-F. Liu  Costas E. Synolakis 《Pure and Applied Geophysics》2003,160(10-11):2119-2146

— The Papua New Guinea (PNG) tsunami of 1998 is re-examined through a detailed review of the field survey as well as numerous numerical computations. The discussion of the field survey explores a number of possible misinterpretations of the recorded data. The survey data are then employed by a numerical model as a validation tool. A Boussinesq model and a nonlinear shallow water wave (NLSW) model are compared in order to quantify the effect of frequency dispersion on the landslide-generated tsunami. The numerical comparisons indicate that the NLSW model is a poor estimator of offshore wave heights. However, due to what appears to be depth-limited breaking seaward of Sissano spit, both numerical models are in agreement in the prediction of maximum water elevations at the overtopped spit. By comparing three different hot-start initial profiles of the tsunami wave, it is shown that the initial shape and orientation of the tsunami wave is secondary to the initial displaced water mass in regard to prediction of water elevations on the spit. These numerical results indicate that agreement between numerical prediction of runup values with field recorded values at PNG cannot be used to validate either a NLSW tsunami propagation model or a specific landslide tsunami hot-start initial condition. Finally, with the use of traditional tsunami codes, a new interpretation of the PNG runup measurements is presented.  相似文献   

Mesozoic thermal evolution of the southern African mantle lithosphere     

David R. Bell  Mark D. Schmitz  Philip E. Janney   《Lithos》2003,71(2-4):273-287

The thermal structure of Archean and Proterozoic lithospheric terranes in southern Africa during the Mesozoic was evaluated by thermobarometry of mantle peridotite xenoliths erupted in alkaline magmas between 180 and 60 Ma. For cratonic xenoliths, the presence of a 150–200 °C isobaric temperature range at 5–6 GPa confirms original interpretations of a conductive geotherm, which is perturbed at depth, and therefore does not record steady state lithospheric mantle structure.

Xenoliths from both Archean and Proterozoic terranes record conductive limb temperatures characteristic of a “cratonic” geotherm (40 mW m⁻²), indicating cooling of Proterozoic mantle following the last major tectonothermal event in the region at 1 Ga and the probability of thick off-craton lithosphere capable of hosting diamond. This inference is supported by U–Pb thermochronology of lower crustal xenoliths [Schmitz and Bowring, 2003. Contrib. Mineral. Petrol. 144, 592–618].

The entire region then suffered a protracted regional heating event in the Mesozoic, affecting both mantle and lower crust. In the mantle, the event is recorded at 150 Ma to the southeast of the craton, propagating to the west by 108–74 Ma, the craton interior by 85–90 Ma and the far southwest and northwest by 65–70 Ma. The heating penetrated to shallower levels in the off-craton areas than on the craton, and is more apparent on the southern margin of the craton than in its western interior. The focus and spatial progression mimic inferred patterns of plume activity and supercontinent breakup 30–100 Ma earlier and are probably connected.

Contrasting thermal profiles from Archean and Proterozoic mantle result from penetration to shallower levels of the Proterozoic lithosphere by heat transporting magmas. Extent of penetration is related not to original lithospheric thickness, but to its more fertile character and the presence of structurally weak zones of old tectonism. The present day distribution of surface heat flow in southern Africa is related to this dynamic event and is not a direct reflection of the pre-existing lithospheric architecture.  相似文献   

Climatic and volcanic forcing revealed in a 50,000-year diatom record from Lake Massoko, Tanzania     

Philip Barker  David Williamson  Franoise Gasse  Elisabeth Gibert 《Quaternary Research》2003,60(3):368-376

The interactions between climatic and volcanic forcing on diatom communities contained in a 50,000-year sedimentary sequence from Lake Massoko, Tanzania, were examined. At the century scale, 19 discrete tephra inputs to the lake isolated the sedimentary nutrient supply and shifted the diatom communities to those tolerant of low phosphorus levels, whereas at the millennial scale, diatom-inferred shifts in precipitation–evaporation based on conductivity optima and diatom life-form ratios were broadly similar to lake-level reconstructions from Lake Rukwa, Lake Malawi, and others in the region. Some fluctuations of Lake Massoko are consistent with the precession-driven changes in insolation, but the major climate shifts do not relate directly to orbital forcing of summer insolation south of the equator and show more consistency with records from the equatorial and northern tropics that receive rainfall from the passing of the intertropical convergence zone. Sea surface temperatures are strongly correlated to multimillennial-scale climate patterns over this region of Africa.  相似文献   

Seismic source study of the Racha-Dzhava (Georgia) earthquake from aftershocks and broad-band teleseismic body-wave records: an example of active nappe tectonics   总被引：1，自引：0，他引：1  

H. Fuenzalida  L. Rivera  H. Haessler  D. Legrand  H. Philip  L. Dorbath    D. McCormack  S. Arefiev  C. Langer  A. Cisternas 《Geophysical Journal International》1997,130(1):29-46

  相似文献   

Calculating the two-dimensional magnetotelluric Jacobian in finite elements using reciprocity   总被引：6，自引：0，他引：6  

Patricia Pastana de Lugão  Philip E. Wannamaker 《Geophysical Journal International》1996,127(3):806-810

To speed up the calculation of the field Jacobian for 2-D magnetoteliuric inversion using finite elements, the principle of electromagnetic reciprocity is applied. The governing relationship for the Jacobian of the field along strike is obtained by differentiating the Helmholtz equation with respect to the resistivity of each region in the finite-element mesh. The result is a similar Helmholtz equation for the Jacobian, with new sources distributed over all nodes within the parameter medium. However, according to the principle of electromagnetic reciprocity, the roles of sources and receivers are interchangeable. Utilizing reciprocity, the field values obtained from the original forward problem and for new unit sources imposed at the receivers are then utilized in the calculation of the Jacobian by simple multiplication and summation with finite-element terms at each rectangle in the mesh. For the auxiliary (across-strike) fields, the Jacobian terms are obtained by solving source vectors loaded with parabola coefficients used in the approximation to Maxwell's equations. Jacobian terms for the apparent resistivity ( p _a), the impedance phase (φ) and the vertical magnetic field ( K _zy) are then calculated utilizing the parallel- and auxiliary-field Jacobians. Comparison of Jacobian values obtained from reciprocity calculations and by differencing two forward solutions show that the reciprocity method is accurate and can be used to decrease the number of calculations required to obtain sensitivities by one to two orders of magnitude.  相似文献