Sudbury Igneous Complex sublayer ores from the Levack West, Falconbridge and Strathcona mines were analyzed for their Re and Os concentrations and Os isotopic compositions. The Re-Os isotope systematics of three ores from the different mines give isochron ages of1 840 ± 60 Ma, 1770 ± 60 Ma and 1780 ± 110 Ma, suggesting that the Re-Os system became closed at the time of, or soon after the 1850 ± 1 Ma crystallization age of the complex. The Os isotopic compositions of different portions of the complex at the time of crystallization varied considerably, with initial187Os/186Os1850 ranging from 4.64 at Levack West to 7.55 at Strathcona. These heterogeneities require that the Os, and probably also the other platinum-group elements contained in the ores, were derived from at least two sources. In addition, the high initial187Os/186Os ratios indicate that the Os was derived predominantly from ancient crust. Previous studies have suggested that the complex either crystallized from a mixture of mantle-derived basaltic melt and ancient continental crust, or was derived exclusively from the fusion of ancient continental crust resulting from a meteorite impact. Results of modelling suggest that if a contemporaneous mantle-derived basaltic melt was involved in the origin of the SIC, it likely contributed < 50% of the Os to all three ores. The large percentage of ancient crust involved in the production of the ores is most consistent with an interpretation of substantial crustal fusion resulting from meteorite impact.
The fate of acetone in an outdoor model stream to which nitrate was added as a nutrient supplement was determined. The stream, in southern Mississippi, U.S.A. was 234 m long. Water was supplied to the stream by an artesian well at about 1.21 s−1, resulting in a mean water velocity of about 0.5 m min−1. Acetone was injected continuously for 26 days resulting in concentrations of 20–40 mg l−1. A nitrate solution was injected for 21 days resulting in an instream concentration of about 1.7 mg l−1 at the upstream end of the stream. Rhodamine-WT dye was used to determine the travel time and dispersion characteristics of the stream, and t-butyl alcohol was used to determine the volatilization characteristics.
Volatilization controlled the fate of acetone in the model stream. The lack of substantial bacterial degradation of acetone was contrary to expectations based on the results of laboratory degradation studies using model stream water enriched with nitrate. A possible explanation for the lack of significant degradation in the model stream may be the limited 6-h residence time of the acetone in the stream.
","language":"English","publisher":"Elsevier","doi":"10.1016/0022-1694(91)90092-V","issn":"00221694","usgsCitation":"Rathbun, R.E., Stephens, D.W., and Tai, D.Y., 1991, Fate of acetone in an outdoor model stream with a nitrate supplement, southern Mississippi, U.S.A.: Journal of Hydrology, v. 123, no. 3-4, p. 225-242, https://doi.org/10.1016/0022-1694(91)90092-V.","productDescription":"18 ","startPage":"225","endPage":"242","numberOfPages":"18","costCenters":[{"id":589,"text":"Toxic Substances Hydrology Program","active":true,"usgs":true}],"links":[{"id":224953,"rank":0,"type":{"id":24,"text":"Thumbnail"},"url":"https://pubs.usgs.gov/thumbnails/outside_thumb.jpg"}],"volume":"123","issue":"3-4","noUsgsAuthors":false,"publicationStatus":"PW","scienceBaseUri":"505a0f0fe4b0c8380cd5373e","contributors":{"authors":[{"text":"Rathbun, R. E.","contributorId":61796,"corporation":false,"usgs":true,"family":"Rathbun","given":"R.","email":"","middleInitial":"E.","affiliations":[],"preferred":false,"id":374899,"contributorType":{"id":1,"text":"Authors"},"rank":1},{"text":"Stephens, D. W.","contributorId":68335,"corporation":false,"usgs":true,"family":"Stephens","given":"D.","email":"","middleInitial":"W.","affiliations":[],"preferred":false,"id":374900,"contributorType":{"id":1,"text":"Authors"},"rank":2},{"text":"Tai, D. Y.","contributorId":59778,"corporation":false,"usgs":true,"family":"Tai","given":"D.","email":"","middleInitial":"Y.","affiliations":[],"preferred":false,"id":374898,"contributorType":{"id":1,"text":"Authors"},"rank":3}]}} ,{"id":70016494,"text":"70016494 - 1991 - Pleistocene slope instability of gas hydrate-laden sediment on the Beaufort Sea margin","interactions":[],"lastModifiedDate":"2013-02-24T19:01:35","indexId":"70016494","displayToPublicDate":"1991-01-01T00:00:00","publicationYear":"1991","noYear":false,"publicationType":{"id":2,"text":"Article"},"publicationSubtype":{"id":10,"text":"Journal Article"},"seriesTitle":{"id":2670,"text":"Marine Geotechnology","active":true,"publicationSubtype":{"id":10}},"title":"Pleistocene slope instability of gas hydrate-laden sediment on the Beaufort Sea margin","docAbstract":"In oceanic areas underlain by sediment with gas hydrate, reduction of sea level initiates disassociation along the base of the gas hydrate, which, in turn, causes the release of large volumes of gas into the sediment and creates excess pore-fluid pressures and reduced slope stability. Fluid diffusion properties dominate the disassociation process in fine-grained marine sediment. Slope failure appears likely for this sediment type on moderate slopes unless pressures can be adequately vented away from the gas hydrate base. Pleistocene eustatic-sea level regressions, likely triggered seafloor landslides on the continental slope of the Beaufort Sea and other margins where gas hydrate is present in seafloor sediment. -from Authors","largerWorkType":{"id":2,"text":"Article"},"largerWorkTitle":"Marine Geotechnology","largerWorkSubtype":{"id":10,"text":"Journal Article"},"language":"English","publisher":"Taylor and Francis","doi":"10.1080/10641199109379886","issn":"03608867","usgsCitation":"Kayen, R.E., and Lee, H., 1991, Pleistocene slope instability of gas hydrate-laden sediment on the Beaufort Sea margin: Marine Geotechnology, v. 10, no. 1-2, p. 125-141, https://doi.org/10.1080/10641199109379886.","startPage":"125","endPage":"141","numberOfPages":"17","costCenters":[],"links":[{"id":223327,"rank":0,"type":{"id":24,"text":"Thumbnail"},"url":"https://pubs.usgs.gov/thumbnails/outside_thumb.jpg"},{"id":268176,"type":{"id":10,"text":"Digital Object Identifier"},"url":"https://dx.doi.org/10.1080/10641199109379886"}],"volume":"10","issue":"1-2","noUsgsAuthors":false,"publicationStatus":"PW","scienceBaseUri":"505a7c52e4b0c8380cd79907","contributors":{"authors":[{"text":"Kayen, R. E.","contributorId":14424,"corporation":false,"usgs":true,"family":"Kayen","given":"R.","email":"","middleInitial":"E.","affiliations":[],"preferred":false,"id":373711,"contributorType":{"id":1,"text":"Authors"},"rank":1},{"text":"Lee, H.J.","contributorId":96693,"corporation":false,"usgs":true,"family":"Lee","given":"H.J.","email":"","affiliations":[],"preferred":false,"id":373712,"contributorType":{"id":1,"text":"Authors"},"rank":2}]}} ,{"id":70016937,"text":"70016937 - 1991 - Coupled surface-water and ground-water model","interactions":[],"lastModifiedDate":"2012-03-12T17:18:50","indexId":"70016937","displayToPublicDate":"1991-01-01T00:00:00","publicationYear":"1991","noYear":false,"publicationType":{"id":24,"text":"Conference Paper"},"publicationSubtype":{"id":19,"text":"Conference Paper"},"title":"Coupled surface-water and ground-water model","docAbstract":"In areas with dynamic and hydraulically well connected ground-water and surface-water systems, it is desirable that stream-aquifer interaction be simulated with models of equal sophistication and accuracy. Accordingly, a new, coupled ground-water and surface-water model was developed by combining the U.S. Geological Survey models MODFLOW and BRANCH. MODFLOW is the widely used modular three-dimensional, finite-difference, ground-water model and BRANCH is a one-dimensional numerical model commonly used to simulate flow in open-channel networks. Because time steps used in ground-water modeling commonly are much longer than those used in surface-water simulations, provision has been made for handling multiple BRANCH time steps within one MODFLOW time step. Verification testing of the coupled model was done using data from previous studies and by comparing results with output from a simpler four-point implicit open-channel flow model linked with MODFLOW.","conferenceTitle":"Proceedings of the 1991 National Conference on Irrigation and Drainage","conferenceDate":"22 July 1991 through 26 July 1991","conferenceLocation":"Honolulu, HI, USA","language":"English","publisher":"Publ by ASCE","publisherLocation":"New York, NY, United States","isbn":"0872628116","usgsCitation":"Swain, E.D., and Wexler, E.J., 1991, Coupled surface-water and ground-water model, Proceedings of the 1991 National Conference on Irrigation and Drainage, Honolulu, HI, USA, 22 July 1991 through 26 July 1991, p. 330-336.","startPage":"330","endPage":"336","numberOfPages":"7","costCenters":[],"links":[{"id":225039,"rank":0,"type":{"id":24,"text":"Thumbnail"},"url":"https://pubs.usgs.gov/thumbnails/outside_thumb.jpg"}],"noUsgsAuthors":false,"publicationStatus":"PW","scienceBaseUri":"5059fc8be4b0c8380cd4e2e2","contributors":{"authors":[{"text":"Swain, Eric D. 0000-0001-7168-708X edswain@usgs.gov","orcid":"https://orcid.org/0000-0001-7168-708X","contributorId":1538,"corporation":false,"usgs":true,"family":"Swain","given":"Eric","email":"edswain@usgs.gov","middleInitial":"D.","affiliations":[{"id":27821,"text":"Caribbean-Florida Water Science Center","active":true,"usgs":true}],"preferred":true,"id":374916,"contributorType":{"id":1,"text":"Authors"},"rank":1},{"text":"Wexler, Eliezer J.","contributorId":99963,"corporation":false,"usgs":true,"family":"Wexler","given":"Eliezer","email":"","middleInitial":"J.","affiliations":[],"preferred":false,"id":374917,"contributorType":{"id":1,"text":"Authors"},"rank":2}]}} ,{"id":70016944,"text":"70016944 - 1991 - Numerical assessment of a landfill compliance limit","interactions":[],"lastModifiedDate":"2024-03-19T22:30:36.280643","indexId":"70016944","displayToPublicDate":"1991-01-01T00:00:00","publicationYear":"1991","noYear":false,"publicationType":{"id":2,"text":"Article"},"publicationSubtype":{"id":10,"text":"Journal Article"},"seriesTitle":{"id":3825,"text":"Groundwater","active":true,"publicationSubtype":{"id":10}},"title":"Numerical assessment of a landfill compliance limit","docAbstract":"The PLASM and Random Walk ground-water flow and contaminant transport models were used to assess the potential impact of various proposed regulatory compliance distances on landfill siting. Contaminant transport modeling was performed for 16 generalized geological sequences representative of hydrogeological conditions over an estimated 90 to 95 percent of Illinois. Results of this modeling indicate that about 50 percent of the state would be hydrogeologically suitable for landfilling of nonhazardous wastes if the compliance distance was 100 feet. With a compliance distance of 500 feet, about 55 percent of the state would be hydrogeologically suitable. This work demonstrates the utility of computer modeling in the development of regulations governing landfill siting.
Event record sections, which are constructed by plotting seismograms from many closely spaced earthquakes recorded on a few stations, show multiple free-surface reflections (PP, PPP, PPPP) of the P wave in the Imperial Valley, California. The relative timing of these arrivals is used to estimate the strength of the P-wave velocity gradient within the upper 5 km of the sediment layer. Consistent with previous studies, a velocity model with a value of 1.8 km/sec at the surface increasing linearly to 5.8 km/sec at a depth of 5.5 km fits the data well. The relative amplitudes of the P and PP arrivals are used to estimate the source depth for the aftershock distributions of the Elmore Ranch and Superstition Hills main shocks. Although the depth determination has large uncertainties, both the Elmore Ranch and Superstition Hills aftershock sequences appear to have similar depth distribution in the range of 4 to 10 km.
","language":"English","publisher":"Seismological Society of America","doi":"10.1785/BSSA0810020508","usgsCitation":"Mori, J., 1991, Estimates of velocity structure and source depth using multiple P waves from aftershocks of the 1987 Elmore Ranch and Superstition Hills, California, earthquakes: Bulletin of the Seismological Society of America, v. 81, no. 2, p. 508-523, https://doi.org/10.1785/BSSA0810020508.","productDescription":"16 p.","startPage":"508","endPage":"523","numberOfPages":"16","costCenters":[],"links":[{"id":225192,"rank":1,"type":{"id":24,"text":"Thumbnail"},"url":"https://pubs.usgs.gov/thumbnails/outside_thumb.jpg"}],"country":"United States","state":"California","geographicExtents":"{\n \"type\": \"FeatureCollection\",\n \"features\": [\n {\n \"type\": \"Feature\",\n \"properties\": {},\n \"geometry\": {\n \"coordinates\": [\n [\n [\n -115.37424013022881,\n 33.234666306800605\n ],\n [\n -115.37424013022881,\n 32.83924814821901\n ],\n [\n -114.875450495058,\n 32.83924814821901\n ],\n [\n -114.875450495058,\n 33.234666306800605\n ],\n [\n -115.37424013022881,\n 33.234666306800605\n ]\n ]\n ],\n \"type\": \"Polygon\"\n }\n }\n ]\n}","volume":"81","issue":"2","noUsgsAuthors":false,"publicationDate":"1991-04-01","publicationStatus":"PW","scienceBaseUri":"505a0af9e4b0c8380cd524e6","contributors":{"authors":[{"text":"Mori, J.","contributorId":24923,"corporation":false,"usgs":true,"family":"Mori","given":"J.","email":"","affiliations":[],"preferred":false,"id":374954,"contributorType":{"id":1,"text":"Authors"},"rank":1}]}} ,{"id":70016954,"text":"70016954 - 1991 - Origin of late dolomite cement by CO2-saturated deep basin brines: evidence from the Ozark region, central United States","interactions":[],"lastModifiedDate":"2024-01-23T12:12:47.389089","indexId":"70016954","displayToPublicDate":"1991-01-01T00:00:00","publicationYear":"1991","noYear":false,"publicationType":{"id":2,"text":"Article"},"publicationSubtype":{"id":10,"text":"Journal Article"},"seriesTitle":{"id":1796,"text":"Geology","active":true,"publicationSubtype":{"id":10}},"title":"Origin of late dolomite cement by CO2-saturated deep basin brines: evidence from the Ozark region, central United States","docAbstract":"Studies of fluid inclusions in regionally extensive late dolomite cement (LDC) throughout the Ozark region show that CO2 effervescence was widespread during dolomite precipitation. On the basis of quantitative analyses of inclusion fluids, reaction-path modeling shows that LDC with trace amounts of sulfides can be deposited by effervescence of a CO2-saturated basin brine as it migrates to shallower levels and lower confining pressures. This precipitation mechanism best explains occurrences of LDC in the Ozark region and may account for LDC found in sedimentary basins worldwide.
In 1983, the City of Indianapolis, Indiana, completed construction of advanced wastewater treatment (AWT) systems to enlarge and upgrade its existing Belmont Road and Southport Road secondary treatment plants. A nonparametric statistical procedure, a modified form of the Wilcoxon-Mann-Whitney rank-sum test, was used to test for trends in water quality at two upstream and two downstream sites on White River and at the two treatment plants. Results comparing the pre- (1978-1980) and post- (1983-1988) AWT periods show statistically significant improvements in the quality of the treated effluent and of the White River downstream from the plants. Water quality at sites upstream from the city was relatively constant during the period of study. Total ammonia (as N) decreased 14.6 mg/L and BOD5 (five-day biochemical oxygen demand) decreased 10 to 19 mg/L in the two effluents. Total ammonia in the river downstream from the plants decreased 0.8 to 1.9 mg/L and BOD5 decreased 2.3 to 2.5 mg/L. Nitrate (as N) increased 14.5 mg/L in the plant effluents and 2.0 to 2.4 mg/L in the river because of in-plant nitrification. Dissolved oxygen concentration in the river increased about 3 mg/L because of reduced oxygen demand for nitrification and biochemical oxidation processes.
","language":"English","publisher":"American Water Resources Association","doi":"10.1111/j.1752-1688.1991.tb01475.x","issn":"00431370","usgsCitation":"Crawford, C.G., and Wangsness, D.J., 1991, Effects of advanced wastewater treatment on the quality of White River, Indiana: Water Resources Bulletin, v. 27, no. 5, p. 769-779, https://doi.org/10.1111/j.1752-1688.1991.tb01475.x.","productDescription":"11","startPage":"769","endPage":"779","onlineOnly":"N","additionalOnlineFiles":"N","costCenters":[{"id":346,"text":"Indiana Water Science Center","active":true,"usgs":true}],"links":[{"id":223371,"rank":0,"type":{"id":24,"text":"Thumbnail"},"url":"https://pubs.usgs.gov/thumbnails/outside_thumb.jpg"},{"id":267707,"type":{"id":10,"text":"Digital Object Identifier"},"url":"https://dx.doi.org/10.1111/j.1752-1688.1991.tb01475.x"}],"country":"United States","state":"Indiana","city":"Indianapolils","otherGeospatial":"Upper White River Basin","geographicExtents":"{\n \"type\": 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40.16418235037417\n ],\n [\n -85.16481399536133,\n 40.17283991040108\n ]\n ]\n ]\n }\n }\n ]\n}","volume":"27","issue":"5","noUsgsAuthors":false,"publicationDate":"2007-06-08","publicationStatus":"PW","scienceBaseUri":"505a067de4b0c8380cd51286","contributors":{"authors":[{"text":"Crawford, Charles G. 0000-0003-1653-7841 cgcrawfo@usgs.gov","orcid":"https://orcid.org/0000-0003-1653-7841","contributorId":1064,"corporation":false,"usgs":true,"family":"Crawford","given":"Charles","email":"cgcrawfo@usgs.gov","middleInitial":"G.","affiliations":[{"id":451,"text":"National Water Quality Assessment Program","active":true,"usgs":true},{"id":37947,"text":"Upper Midwest Water Science Center","active":true,"usgs":true}],"preferred":true,"id":373706,"contributorType":{"id":1,"text":"Authors"},"rank":1},{"text":"Wangsness, David J.","contributorId":81475,"corporation":false,"usgs":true,"family":"Wangsness","given":"David","email":"","middleInitial":"J.","affiliations":[],"preferred":false,"id":373707,"contributorType":{"id":1,"text":"Authors"},"rank":2}]}} ,{"id":70016488,"text":"70016488 - 1991 - Wave groupiness variations in the nearshore","interactions":[],"lastModifiedDate":"2023-09-08T16:35:34.265026","indexId":"70016488","displayToPublicDate":"1991-01-01T00:00:00","publicationYear":"1991","noYear":false,"publicationType":{"id":2,"text":"Article"},"publicationSubtype":{"id":10,"text":"Journal Article"},"seriesTitle":{"id":1262,"text":"Coastal Engineering","active":true,"publicationSubtype":{"id":10}},"title":"Wave groupiness variations in the nearshore","docAbstract":"This paper proposes a new definition of the groupiness factor, GF, based on the envelope of the incident-wave time series. It is shown that an envelope-based GF has several important advantages over the SIWEH-based groupiness factor, including objective criteria for determining the accuracy of the envelope function and well-defined numerical limits.
Using this new GF, the variability of incident wave groupiness in the field is examined both temporally, in unbroken waves at a fixed location, and spatially, in a cross-shore array through the surf zone. Contrary to previous studies using the SIWEH-based GF, results suggest that incident wave groupiness may not be an independent parameter in unbroken waves; through a wide range of spectral shapes, from swell to storm waves, the groupiness did not vary significantly. As expected, the groupiness decreases rapidly as waves break through the surf zone, although significant wave height variability persists even through a saturated surf zone. The source of this inner surf zone groupiness is not identified; however, this observation implies that models of long wave generation must account for nonsteady radiation stress gradients landward of some narrow zone near the mean breakpoint.
","language":"English","publisher":"Elsevier","doi":"10.1016/0378-3839(91)90024-B","usgsCitation":"List, J.H., 1991, Wave groupiness variations in the nearshore: Coastal Engineering, v. 15, no. 5-6, p. 475-496, https://doi.org/10.1016/0378-3839(91)90024-B.","productDescription":"22 p.","startPage":"475","endPage":"496","numberOfPages":"22","costCenters":[{"id":678,"text":"Woods Hole Coastal and Marine Science Center","active":true,"usgs":true}],"links":[{"id":223326,"rank":1,"type":{"id":24,"text":"Thumbnail"},"url":"https://pubs.usgs.gov/thumbnails/outside_thumb.jpg"}],"volume":"15","issue":"5-6","noUsgsAuthors":false,"publicationStatus":"PW","scienceBaseUri":"505bcf94e4b08c986b32e9a2","contributors":{"authors":[{"text":"List, Jeffrey H. 0000-0001-8594-2491 jlist@usgs.gov","orcid":"https://orcid.org/0000-0001-8594-2491","contributorId":174581,"corporation":false,"usgs":true,"family":"List","given":"Jeffrey","email":"jlist@usgs.gov","middleInitial":"H.","affiliations":[{"id":678,"text":"Woods Hole Coastal and Marine Science Center","active":true,"usgs":true}],"preferred":true,"id":373704,"contributorType":{"id":1,"text":"Authors"},"rank":1}]}} ,{"id":70014931,"text":"70014931 - 1991 - Mineral remains of early life on Earth? On Mars?","interactions":[],"lastModifiedDate":"2023-11-16T17:13:36.268713","indexId":"70014931","displayToPublicDate":"1991-01-01T00:00:00","publicationYear":"1991","noYear":false,"publicationType":{"id":2,"text":"Article"},"publicationSubtype":{"id":10,"text":"Journal Article"},"seriesTitle":{"id":1800,"text":"Geomicrobiology Journal","active":true,"publicationSubtype":{"id":10}},"title":"Mineral remains of early life on Earth? On Mars?","docAbstract":"The oldest sedimentary rocks on Earth, the 3.8‐Ga Isua Iron‐Formation in southwestern Greenland, are metamorphosed past the point where organic‐walled fossils would remain. Acid residues and thin sections of these rocks reveal ferric microstructures that have filamentous, hollow rod, and spherical shapes not characteristic of crystalline minerals. Instead, they resemble ferric‐coated remains of bacteria. Modern so‐called iron bacteria were therefore studied to enhance a search image for oxide minerals precipitated by early bacteria. Iron bacteria become coated with ferrihydrite, a metastable mineral that converts to hematite, which is stable under high temperatures. If these unusual morphotypes are mineral remains of microfossils, then life must have evolved somewhat earlier than 3.8 Ga, and may have involved the interaction of sediments and molecular oxygen in water, with iron as a catalyst. Timing is constrained by the early in fall of planetary materials that would have heated the planet's surface.
Because there are no earlier sedimentary rocks to study on Earth, it may be necessary to expand the search elsewhere in the solar system for clues to any biotic precursors or other types of early life. Evidence from Mars shows geophysical and geochemical differentiation at a very early stage, which makes it an important candidate for such a search if sedimentation is an important process in life's origins. Not only does Mars have iron oxide‐rich soils, but its oldest regions have river channels where surface water and sediment may have been carried, and seepage areas where groundwater may have discharged. Mars may have had an atmosphere and liquid water in the crucial time frame of 3.9–4.0 Ga. A study of morphologies of iron oxide minerals collected in the southern highlands during a Mars sample return mission may therefore help to fill in important gaps in the history of Earth's earliest biosphere.
","language":"English","publisher":"Taylor & Francis","doi":"10.1080/01490459109385985","usgsCitation":"Iberall-Robbins, E., and Iberall, A.S., 1991, Mineral remains of early life on Earth? On Mars?: Geomicrobiology Journal, v. 9, no. 1, p. 51-66, https://doi.org/10.1080/01490459109385985.","productDescription":"16 p.","startPage":"51","endPage":"66","numberOfPages":"16","costCenters":[],"links":[{"id":224120,"rank":1,"type":{"id":24,"text":"Thumbnail"},"url":"https://pubs.usgs.gov/thumbnails/outside_thumb.jpg"}],"volume":"9","issue":"1","noUsgsAuthors":false,"publicationStatus":"PW","scienceBaseUri":"505a57afe4b0c8380cd6de29","contributors":{"authors":[{"text":"Iberall-Robbins, E.","contributorId":18515,"corporation":false,"usgs":true,"family":"Iberall-Robbins","given":"E.","email":"","affiliations":[],"preferred":false,"id":369640,"contributorType":{"id":1,"text":"Authors"},"rank":1},{"text":"Iberall, A. S.","contributorId":25302,"corporation":false,"usgs":true,"family":"Iberall","given":"A.","email":"","middleInitial":"S.","affiliations":[],"preferred":false,"id":369641,"contributorType":{"id":1,"text":"Authors"},"rank":2}]}} ,{"id":70016484,"text":"70016484 - 1991 - Pumping tests in non-uniform aquifers - The linear strip case","interactions":[],"lastModifiedDate":"2025-03-06T16:58:48.257499","indexId":"70016484","displayToPublicDate":"1991-01-01T00:00:00","publicationYear":"1991","noYear":false,"publicationType":{"id":2,"text":"Article"},"publicationSubtype":{"id":10,"text":"Journal Article"},"seriesTitle":{"id":2342,"text":"Journal of Hydrology","active":true,"publicationSubtype":{"id":10}},"title":"Pumping tests in non-uniform aquifers - The linear strip case","docAbstract":"Many pumping tests are performed in geologic settings that can be conceptualized as a linear infinite strip of one material embedded in a matrix of differing flow properties. A semi-analytical solution is presented to aid the analysis of drawdown data obtained from pumping tests performed in settings that can be represented by such a conceptual model. Integral transform techniques are employed to obtain a solution in transform space that can be numerically inverted to real space. Examination of the numerically transformed solution reveals several interesting features of flow in this configuration. If the transmissivity of the strip is much higher than that of the matrix, linear and bilinear flow are the primary flow regimes during a pumping test. If the contrast between matrix and strip properties is not as extreme, then radial flow should be the primary flow mechanism. Sensitivity analysis is employed to develop insight into the controls on drawdown in this conceptual model and to demonstrate the importance of temporal and spatial placement of observations. Changes in drawdown are sensitive to the transmissivity of the strip for a limited time duration. After that time, only the total drawdown remains a function of strip transmissivity. In the case of storativity, both the total drawdown and changes in drawdown are sensitive to the storativity of the strip for a time of quite limited duration. After that time, essentially no information can be gained about the storage properties of the strip from drawdown data. An example analysis is performed using data previously presented in the literature to demonstrate the viability of the semi-analytical solution and to illustrate a general procedure for analysis of drawdown data in complex geologic settings. This example reinforces the importance of observation well placement and the time of data collection in constraining parameter correlation, a major source of the uncertainty that arises in the parameter estimation procedure.
","language":"English","publisher":"Elsevier","doi":"10.1016/0022-1694(91)90132-2","issn":"00221694","usgsCitation":"Butler, J., and Liu, W., 1991, Pumping tests in non-uniform aquifers - The linear strip case: Journal of Hydrology, v. 128, no. 1-4, p. 69-99, https://doi.org/10.1016/0022-1694(91)90132-2.","productDescription":"31 p.","startPage":"69","endPage":"99","numberOfPages":"31","costCenters":[],"links":[{"id":223276,"rank":1,"type":{"id":24,"text":"Thumbnail"},"url":"https://pubs.usgs.gov/thumbnails/outside_thumb.jpg"}],"volume":"128","issue":"1-4","noUsgsAuthors":false,"publicationStatus":"PW","scienceBaseUri":"505a9034e4b0c8380cd7fbcd","contributors":{"authors":[{"text":"Butler, J.J. Jr.","contributorId":12194,"corporation":false,"usgs":true,"family":"Butler","given":"J.J.","suffix":"Jr.","email":"","affiliations":[],"preferred":false,"id":373694,"contributorType":{"id":1,"text":"Authors"},"rank":1},{"text":"Liu, W.Z.","contributorId":99301,"corporation":false,"usgs":true,"family":"Liu","given":"W.Z.","email":"","affiliations":[],"preferred":false,"id":373695,"contributorType":{"id":1,"text":"Authors"},"rank":2}]}} ,{"id":70016891,"text":"70016891 - 1991 - Identification of dissolved-constituent sources in mine-site ground water using batch mixing","interactions":[],"lastModifiedDate":"2013-02-19T11:53:22","indexId":"70016891","displayToPublicDate":"1991-01-01T00:00:00","publicationYear":"1991","noYear":false,"publicationType":{"id":2,"text":"Article"},"publicationSubtype":{"id":10,"text":"Journal Article"},"seriesTitle":{"id":3718,"text":"Water Resources Bulletin","printIssn":"0043-1370","active":true,"publicationSubtype":{"id":10}},"title":"Identification of dissolved-constituent sources in mine-site ground water using batch mixing","docAbstract":"Batch-mixing experiments were used to help identify lithologic and mineralogic sources of increased concentrations of dissolved solids in water affected by surface coal mining in northwestern Colorado. Ten overburden core samples were analyzed for mineral composition and mixed with distilled water for 90 days until mineral-water equilibrium was reached. Between one day and 90 days after initial contact, specific conductance in the sample mixtures had a median increase of 306 percent. Dissolved-solids concentrations ranged from 200 to 8,700 mg/L in water samples extracted from the mixtures after 90 days. Mass-balance simulations were conducted using the geochemical models BALANCE and WATEQF to quantify mineral-water interactions occurring in five selected sample mixtures and in water collected from a spring at a reclaimed mine site. The spring water is affected by mineral-water interactions occurring in all of the lithologic units comprising the overburden. Results of the simulations indicate that oxidation of pyrite, dissolution of dolomite, gypsum, and epsomite, and cation-exchange reactions are the primary mineral-water interactions occurring in the overburden. Three lithologic units in the overburden (a coal, a sandstone, and a shale) probably contribute most of the dissolved solids to the spring water. Water sample extracts from mixtures using core from these three units accounted for 85 percent of the total dissolved solids in the 10 sample extracts. Other lithologic units in the over-burden probably contribute smaller quantities of dissolved solids to the spring water.","largerWorkType":{"id":2,"text":"Article"},"largerWorkTitle":"Water Resources Bulletin","largerWorkSubtype":{"id":10,"text":"Journal Article"},"language":"English","publisher":"American Water Resources Association","doi":"10.1111/j.1752-1688.1991.tb03116.x","issn":"00431370","usgsCitation":"Clark, G.M., and Williams, R.S., 1991, Identification of dissolved-constituent sources in mine-site ground water using batch mixing: Water Resources Bulletin, v. 27, no. 1, p. 93-100, https://doi.org/10.1111/j.1752-1688.1991.tb03116.x.","startPage":"93","endPage":"100","numberOfPages":"8","costCenters":[],"links":[{"id":225086,"rank":0,"type":{"id":24,"text":"Thumbnail"},"url":"https://pubs.usgs.gov/thumbnails/outside_thumb.jpg"},{"id":267708,"type":{"id":10,"text":"Digital Object Identifier"},"url":"https://dx.doi.org/10.1111/j.1752-1688.1991.tb03116.x"}],"volume":"27","issue":"1","noUsgsAuthors":false,"publicationDate":"2007-06-08","publicationStatus":"PW","scienceBaseUri":"505a3826e4b0c8380cd61470","contributors":{"authors":[{"text":"Clark, Gregory M. gmclark@usgs.gov","contributorId":1377,"corporation":false,"usgs":true,"family":"Clark","given":"Gregory","email":"gmclark@usgs.gov","middleInitial":"M.","affiliations":[{"id":343,"text":"Idaho Water Science Center","active":true,"usgs":true}],"preferred":true,"id":374776,"contributorType":{"id":1,"text":"Authors"},"rank":1},{"text":"Williams, Robert S. Jr.","contributorId":95207,"corporation":false,"usgs":true,"family":"Williams","given":"Robert","suffix":"Jr.","email":"","middleInitial":"S.","affiliations":[],"preferred":false,"id":374777,"contributorType":{"id":1,"text":"Authors"},"rank":2}]}} ,{"id":70016478,"text":"70016478 - 1991 - Geochemistry of a Tertiary sedimentary phosphate deposit: Baja California Sur, Mexico","interactions":[],"lastModifiedDate":"2013-01-20T20:38:05","indexId":"70016478","displayToPublicDate":"1991-01-01T00:00:00","publicationYear":"1991","noYear":false,"publicationType":{"id":2,"text":"Article"},"publicationSubtype":{"id":10,"text":"Journal Article"},"seriesTitle":{"id":1213,"text":"Chemical Geology","active":true,"publicationSubtype":{"id":10}},"title":"Geochemistry of a Tertiary sedimentary phosphate deposit: Baja California Sur, Mexico","docAbstract":"The San Gregorio Formation in Baja California Sur, a phosphate-enriched sedimentary unit of late Oligocene to early Miocene age, has been analyzed in two areas (La Purisima and San Hilario) for its chemical composition (major oxides, Cu, Cd, Cr, Co, V, and rare-earth elements - REE) and isotopic composition (??18O and ??13C). A detrital and a marine component were determined from major oxides. The detrital component consists of an unaltered volcanic-ash fraction and a terrigenous clay-silt fraction. The marine component, which accumulated initially as biogenic and hydrogenous material, is now present as opal-A, opal-CT, CaCO3, organic matter, and an authigenic phosphate fraction, mostly pelletal and composed of the carbonate-fluorapatite mineral francolite. The minor elements have been partitioned into these components by assuming a constant composition for the two detrital fractions. The composition of the marine component of minor elements can then be interpreted by assuming that the stoichiometry of the original accumulating organic matter was equal to that of modern plankton. The Cu and Cd contents in the marine component of all rocks require that the seawater-derived fractions of these two metals were supplied to the seafloor solely by organic matter. Enrichments of Cr and V at both sites required an additional marine input. On the basis of their geochemistry in the modern ocean, Cr and V could have precipitated, or been adsorbed, onto settling particles from an O2 minimum zone in which the O2 content was low enough to promote denitrification rather than oxygen respiration. An enrichment of the REE, now within the apatite fraction, resulted from their adsorption onto particulates also in the O2 minimum zone and to the dissolution and alteration of biogenic phases (predominantly silica) within the sediment. Co and Fe2O3 show no enrichment above a detrital contribution. The ??18O-values of apatites from the La Purisima site are heavier than those of apatites from the San Hilario site, whereas the ??13C-values show the opposite trend. One possible interpretation of these variations is that ??18O reflects seawater values and ??13C sediment pore water values. This interpretation suggests that upwelling rates and primary productivity within the water column were greater at La Purisima, an interpretation that is corroborated by a greater abundance of apatite measured in outcrop at La Purisima. The Ce anomalies of the phosphate-enriched samples also differ between the two sites, indicating that they also recorded water masses, similar to the ??18O-values. ?? 1991.","largerWorkType":{"id":2,"text":"Article"},"largerWorkTitle":"Chemical Geology","largerWorkSubtype":{"id":10,"text":"Journal Article"},"language":"English","publisher":"Elsevier","doi":"10.1016/0009-2541(91)90075-3","issn":"00092541","usgsCitation":"Piper, D., 1991, Geochemistry of a Tertiary sedimentary phosphate deposit: Baja California Sur, Mexico: Chemical Geology, v. 92, no. 4, p. 283-316, https://doi.org/10.1016/0009-2541(91)90075-3.","startPage":"283","endPage":"316","numberOfPages":"34","costCenters":[],"links":[{"id":266077,"type":{"id":10,"text":"Digital Object Identifier"},"url":"https://dx.doi.org/10.1016/0009-2541(91)90075-3"},{"id":223224,"rank":0,"type":{"id":24,"text":"Thumbnail"},"url":"https://pubs.usgs.gov/thumbnails/outside_thumb.jpg"}],"volume":"92","issue":"4","noUsgsAuthors":false,"publicationStatus":"PW","scienceBaseUri":"505a16eae4b0c8380cd552ef","contributors":{"authors":[{"text":"Piper, D.Z.","contributorId":34154,"corporation":false,"usgs":false,"family":"Piper","given":"D.Z.","email":"","affiliations":[],"preferred":false,"id":373678,"contributorType":{"id":1,"text":"Authors"},"rank":1}]}} ,{"id":70016475,"text":"70016475 - 1991 - Modelling the petrogenesis of high Rb/Sr silicic magmas","interactions":[],"lastModifiedDate":"2013-01-20T20:37:08","indexId":"70016475","displayToPublicDate":"1991-01-01T00:00:00","publicationYear":"1991","noYear":false,"publicationType":{"id":2,"text":"Article"},"publicationSubtype":{"id":10,"text":"Journal Article"},"seriesTitle":{"id":1213,"text":"Chemical Geology","active":true,"publicationSubtype":{"id":10}},"title":"Modelling the petrogenesis of high Rb/Sr silicic magmas","docAbstract":"Rhyolites can be highly evolved with Sr contents as low as 0.1 ppm and Rb Sr > 2,000. In contrast, granite batholiths are commonly comprised of rocks with Rb Sr < 10 and only rarely > 100. Mass-balance modelling of source compositions, differentiation and contamination using the trace-element geochemistry of granites are therefore commonly in error because of the failure to account for evolved differentiates that may have been erupted from the system. Rhyolitic magmas with very low Sr concentrations (???1 ppm) cannot be explained by any partial melting models involving typical crustal source compositions. The only plausible mechanism for the production of such rhyolites is Rayleigh fractional crystallization involving substantial volumes of cumulates. A variety of methods for modelling the differentiation of magmas with extremely high Rb/Sr is discussed. In each case it is concluded that the bulk partition coefficients for Sr have to be large. In the simplest models, the bulk DSr of the most evolved types is modelled as > 50. Evidence from phenocryst/glass/whole-rock concentrations supports high Sr partition coefficients in feldspars from high silica rhyolites. However, the low modal abundance of plagioclase commonly observed in such rocks is difficult to reconcile with such simple fractionation models of the observed trace-element trends. In certain cases, this may be because the apparent trace-element trend defined by the suite of cognetic rhyolites is the product of different batches of magma with separate differentiation histories accumulating in the magma chamber roof zone. ?? 1991.","largerWorkType":{"id":2,"text":"Article"},"largerWorkTitle":"Chemical Geology","largerWorkSubtype":{"id":10,"text":"Journal Article"},"language":"English","publisher":"Elsevier","doi":"10.1016/0009-2541(91)90051-R","issn":"00092541","usgsCitation":"Halliday, A.N., Davidson, J., Hildreth, W., and Holden, P., 1991, Modelling the petrogenesis of high Rb/Sr silicic magmas: Chemical Geology, v. 92, no. 1-3, p. 107-114, https://doi.org/10.1016/0009-2541(91)90051-R.","startPage":"107","endPage":"114","numberOfPages":"8","costCenters":[],"links":[{"id":479714,"rank":1,"type":{"id":41,"text":"Open Access External Repository Page"},"url":"https://doi.org/10.1016/0009-2541(91)90051-r","text":"External Repository"},{"id":266076,"type":{"id":10,"text":"Digital Object Identifier"},"url":"https://dx.doi.org/10.1016/0009-2541(91)90051-R"},{"id":223271,"rank":0,"type":{"id":24,"text":"Thumbnail"},"url":"https://pubs.usgs.gov/thumbnails/outside_thumb.jpg"}],"volume":"92","issue":"1-3","noUsgsAuthors":false,"publicationStatus":"PW","scienceBaseUri":"505a5c78e4b0c8380cd6fd0e","contributors":{"authors":[{"text":"Halliday, A. N.","contributorId":87663,"corporation":false,"usgs":true,"family":"Halliday","given":"A.","email":"","middleInitial":"N.","affiliations":[],"preferred":false,"id":373670,"contributorType":{"id":1,"text":"Authors"},"rank":1},{"text":"Davidson, J.P.","contributorId":16123,"corporation":false,"usgs":true,"family":"Davidson","given":"J.P.","email":"","affiliations":[],"preferred":false,"id":373668,"contributorType":{"id":1,"text":"Authors"},"rank":2},{"text":"Hildreth, W. 0000-0002-7925-4251","orcid":"https://orcid.org/0000-0002-7925-4251","contributorId":100487,"corporation":false,"usgs":true,"family":"Hildreth","given":"W.","affiliations":[],"preferred":false,"id":373671,"contributorType":{"id":1,"text":"Authors"},"rank":3},{"text":"Holden, P.","contributorId":26075,"corporation":false,"usgs":true,"family":"Holden","given":"P.","email":"","affiliations":[],"preferred":false,"id":373669,"contributorType":{"id":1,"text":"Authors"},"rank":4}]}} ,{"id":70016702,"text":"70016702 - 1991 - Fractal patterns of fractures in granites","interactions":[],"lastModifiedDate":"2023-12-09T01:16:28.18703","indexId":"70016702","displayToPublicDate":"1991-01-01T00:00:00","publicationYear":"1991","noYear":false,"publicationType":{"id":2,"text":"Article"},"publicationSubtype":{"id":10,"text":"Journal Article"},"seriesTitle":{"id":1427,"text":"Earth and Planetary Science Letters","active":true,"publicationSubtype":{"id":10}},"title":"Fractal patterns of fractures in granites","docAbstract":"Fractal measurements using the Cantor's dust method in a linear one-dimensional analysis mode were made on the fracture patterns revealed on two-dimensional, planar surfaces in four granites. This method allows one to conclude that:
(1)|The fracture systems seen on two-dimensional surfaces in granites are consistent with the part of fractal theory that predicts a repetition of patterns on different scales of observation, self similarity. Fractal analysis gives essentially the same values of D on the scale of kilometres, metres and centimetres (five orders of magnitude) using mapped, surface fracture patterns in a Sierra Nevada granite batholith (Mt. Abbot quadrangle, Calif.).
(2)|Fractures show the same fractal values at different depths in a given batholith. Mapped fractures (main stage ore veins) at three mining levels (over a 700 m depth interval) of the Boulder batholith, Butte, Mont. show the same fractal values although the fracture disposition appears to be different at different levels.
(3)|Different sets of fracture planes in a granite batholith, Central France, and in experimental deformation can have different fractal values. In these examples shear and tension modes have the same fractal values while compressional fractures follow a different fractal mode of failure. The composite fracture patterns are also fractal but with a different, median, fractal value compared to the individual values for the fracture plane sets. These observations indicate that the fractal method can possibly be used to distinguish fractures of different origins in a complex system.
It is concluded that granites fracture in a fractal manner which can be followed at many scales. It appears that fracture planes of different origins can be characterized using linear fractal analysis.
","language":"English","publisher":"Elsevier","doi":"10.1016/0012-821X(91)90234-9","issn":"0012821X","usgsCitation":"Velde, B., Dubois, J., Moore, D., and Touchard, G., 1991, Fractal patterns of fractures in granites: Earth and Planetary Science Letters, v. 104, no. 1, p. 25-35, https://doi.org/10.1016/0012-821X(91)90234-9.","productDescription":"11 p.","startPage":"25","endPage":"35","numberOfPages":"11","costCenters":[],"links":[{"id":224650,"rank":1,"type":{"id":24,"text":"Thumbnail"},"url":"https://pubs.usgs.gov/thumbnails/outside_thumb.jpg"}],"volume":"104","issue":"1","noUsgsAuthors":false,"publicationStatus":"PW","scienceBaseUri":"505a13a4e4b0c8380cd54708","contributors":{"authors":[{"text":"Velde, B.","contributorId":41600,"corporation":false,"usgs":true,"family":"Velde","given":"B.","affiliations":[],"preferred":false,"id":374254,"contributorType":{"id":1,"text":"Authors"},"rank":1},{"text":"Dubois, J.","contributorId":39529,"corporation":false,"usgs":true,"family":"Dubois","given":"J.","email":"","affiliations":[],"preferred":false,"id":374253,"contributorType":{"id":1,"text":"Authors"},"rank":2},{"text":"Moore, D.","contributorId":105307,"corporation":false,"usgs":true,"family":"Moore","given":"D.","affiliations":[],"preferred":false,"id":374256,"contributorType":{"id":1,"text":"Authors"},"rank":3},{"text":"Touchard, G.","contributorId":85046,"corporation":false,"usgs":true,"family":"Touchard","given":"G.","email":"","affiliations":[],"preferred":false,"id":374255,"contributorType":{"id":1,"text":"Authors"},"rank":4}]}} ,{"id":70016468,"text":"70016468 - 1991 - Use of a single-bowl continuous-flow centrifuge for dewatering suspended sediments: effect on sediment physical and chemical characteristics","interactions":[],"lastModifiedDate":"2024-03-28T00:14:53.430863","indexId":"70016468","displayToPublicDate":"1991-01-01T00:00:00","publicationYear":"1991","noYear":false,"publicationType":{"id":2,"text":"Article"},"publicationSubtype":{"id":10,"text":"Journal Article"},"seriesTitle":{"id":1924,"text":"Hydrological Processes","active":true,"publicationSubtype":{"id":10}},"title":"Use of a single-bowl continuous-flow centrifuge for dewatering suspended sediments: effect on sediment physical and chemical characteristics","docAbstract":"The use of a single-bowl continuous-flow centrifuge (CFC, Sharples-Pennwalt Model AS-12) for dewatering suspended sediment from large volumes of river water is evaluated. Sediment-recovery efficiency of 86-91 per cent is comparable to that of other types of CFC units. The recovery efficiency is limited by the particle-size distribution of the feed water and by the limiting particle diameter that is retained in the centrifuge bowl. The limiting particle diameter, using the parameters for this study (bowl radius = 10.5cm; bowl length = 71.1 cm; rotational velocity = 16000 r min−1; flow rate = 2 L min−1, and an assumed hydrated particle density = 1.7 gm cm−3), is 370 nm. There seems to be no particle-size fractionation within the centrifuge bowl—the median particle size was the same at the top as at the bottom. Particle electrophoretic mobility plays some role in fractionation of particles within the centrifuge. The mobility ranged from −1.19 to −2.01 × 10−8 m2 V−1 s−1, which is typical of clays coated with organic matter, the charge of which is partially neutralized by divalent cations and iron. Contamination by trace metals and organics is minimized by coating all surfaces that come in contact with the sample with either FEP or PFA Teflon and using a removable FEP Teflon liner in the centrifuge bowl. Because of the physical and chemical factors affecting particle fractionation within the centrifuge, care must be exercised in interpreting the environmental consequences of particles collected by continuous-flow centrifugation.
This study examines the relationship between the distribution of small earthquakes (ML≤4.3) and mechanisms of strain accumulation and relaxation in an area with long repeat times between large events, the Southern Great Basin Seismic Network (SGBSN) region. The Great Basin is a unique continental extensional province characterized by normal and strike-slip faulting, high heat flow, crust of thin to normal thickness, and high elevations. The SGBSN is operated to provide data to address suitability issues pertaining to Yucca Mountain, Nevada which is being evaluated as a potential site for a national mined geologic nuclear waste repository. Suitability issues include estimation of the probability of occurrence of future damaging earthquakes, the characterization of the mechanisms that drive hydrologic flow, and the identification of fractures (faults) that might act as flow conduits or barriers. This study attempts to explain the distribution of small earthquakes in terms of spatial variations in the shear strain field; where strain concentrates there should be a greater number of small earthquakes. Strain field models are constructed under the assumption that long term fault behavior perturbs an otherwise uniform strain field. These strain field models are then interpreted with regard to the regional tectonics and site suitability issues. Modeling results provide one possible explanation of why earthquake clusters cover regions much larger than the surface projections of any of mapped major faults; clusters in a wide band along and extending northeast of the northern half of the Furnace Creek fault may correspond to elevated shear strains along the fault and a broad cluster in the Pahranagat Shear Zone may be associated with shear strain arising from a distribution of smaller localized faults. The relatively large number of small earthquakes in the southern and eastern portions of the Nevada Test Site is consistent with the strain field models. A minimum in shear strain at Yucca Mountain is predicted by all models consistent with an almost total lack of earthquakes observed there. The region to the west of the Death Valley/Furnace Creek fault system, the portion of the study area with the most active deformation but few small earthquakes, is an area of low shear strain. A possible reason for this is that the fault configuration in the area is optimal for accommodating regional deformation via large earthquakes or creep. While there is also a relative lack of earthquakes at Yucca Mountain, this may be indicative of a lack of accumulating strain energy and thus, a lower potential for a large earthquake.
Resistivity structure to about 25 km depth is defined from two-dimensional modeling of 29 magnetotelluric (MT) soundings (0.002–5 Hz) that traverse 280 km of the southwestern Colorado Plateau, transition zone, and Basin and Range provinces in Arizona. From the surface to 5 km depth, the MT model suggests structural relationships between low-resistivity sedimentary and volcanic rocks (50–300 ohm m) and high-resistivity granitic and gneissic basement (500–9000 ohm m). In the Basin and Range province, the MT model and a seismic reflection section show a generally consistent distribution of supracrustal rocks that have relatively low to moderate resistivity (MT) and relatively strong, locally coherent reflectivity. The supracrustal zone defined by these physical properties is inferred to be composed of upper plate rocks above a middle Tertiary detachment fault system which has been mapped in surrounding ranges. Some low-angle fault zones inferred from seismic reflections to extend into high resistivity basement below the supracrustal rocks are not resolved by the MT model. A low-resistivity zone with a conductance of 500 S or more is modeled in the crust at an average depth of about 15 km in the Basin and Range province and transition zone and may deepen below the southwestern part of the Colorado Plateau. In the Basin and Range province, the top of the low resistivity may correspond to a reflective layer with a 6-s two-way-travel time. This deep low-resistivity zone might be caused by a small fraction of connected hydrous solutions or silicic melts.
A review of existing geophysical information and new data presented in this special section indicate that major changes in crustal properties between the Basin and Range and Colorado Plateau occur in, or directly adjacent to, the region defined as the Arizona Transition Zone. Although this region was designated on a physiographic basis, studies indicate that it is also the geophysical transition between adjoining provinces. The Transition Zone displays anomalous crustal and upper mantle seismic properties, shallow Curie isotherms, high heat flow, and steep down-to-the-plateau Bouguer gravity gradients. Seismic and gravity studies suggest that the change in crustal thickness, from thin crust in the Basin and Range to thick crust in the Colorado Plateau, may occur as a series of steps rather than a planar surface. Anomalous P wave velocities, high heat flow, shallow Curie isotherms, and results of gravity modeling suggest that the upper mantle is heterogeneous in this region. A relatively shallow asthenosphere beneath the Basin and Range and Transition Zone contrasted with a thick lithosphere beneath the Colorado Plateau would be one explanation that would satisfy these geophysical observations.