Aeromagnetic and gravity investigations of the Coastal Area and Continental Shelf of Liberia, West Africa, and their relation to continental drift

USGS Publications Warehouse

Behrendt, John C.; Wotorson, Cletus S.

1970-01-01

An aeromagnetic survey has shown the existence of several basins in which magnetic basement depths are greater than 5 km on the continental shelf off Liberia. Magnetic diabase of 176 to 192 m.y. (Jurassic) in age intruding the Paleozoic (?) rocks and overlain by younger rocks onshore requires the distinction between “magnetic basement” and “basement.” Several lines of evidence suggest that the Paleozoic(?) rocks are less than 1 km thick; this implies that the diabase does not introduce a large error in depth-to-basement estimates. The dikes or their extrusive equivalents are traceable, on the basis of the magnetic data, beneath the younger sedimentary rock in the basins to the edge of the continental slope. The magnetic data also delineate a second zone of diabase dikes 90 km inland, parallel to the coast, which cross the entire country. The intrusion of the younger dikes probably coincides with rifting at the beginning of the separation of Africa and South America, and the associated magnetic anomaly zones appear to be parallel with and continuous into the anomaly bands in the Atlantic. A major northeast-trending break in the magnetic fabric intersects the coast near 9° W. and is associated with Eburnean age rocks (about 2000 m.y.) to the southeast as contrasted with Liberian-age rocks (about 2700 m.y.) to the northwest. Change in magnetic fabric direction inland from northeast to northwest in the coastal area allows recognition of a boundary between the Liberian-age rocks inland and Pan-African-age (about 550 m.y.) rocks in the coastal area northwest of about 9° 20'W. Sets of north-northwest-and west-northwest—trending faults of 1 to 2 km vertical displacement cut the Cretaceous sedimentary rocks onshore and can be traced into the offshore basins. Vertical displacements of several kilometers in the magnetic basement underlying the continental shelf suggest a pattern of block faulting all along the coast and continental shelf. Negative Bouguer anomalies exist over two Cretaceous basins in the coastal area; a negative Bouguer anomaly exists over one of the basins southwest of Monrovia, as shown by a marine traverse, suggesting that Cretaceous or younger sedimentary rocks fill these basins also. A 50 to 60 mgal positive Bouguer anomaly area exists along the coast from Sierra Leone to Ivory Coast. This anomaly correlates with mafic granulites in the Monrovia region, where the gradient is too steep to be entirely due to crustal thickening at the continental margin and may be related to tectonic activity associated with the basins. The only major break in this positive anomaly above basement rocks along the entire coast of Liberia is over granite gneiss adjacent to (and presumably underlying) the only onshore basins on the Liberian coast. Three seismic reflection profiles support the interpretation of a substantial section of sedimentary rock offshore. A suggested sequence of events indicates tectonic activity in the periods about 2700, about 2000, and about 550 m.y. B.P.; uplift and exposure of deep crustal rocks; deposition of Paleozoic sediments; intrusion of diabase dikes in inland zones; intrusion of 176 to 192 m.y.-old dikes and sills accompanying separation of Africa and South and North America; block faulting along coast and continental shelf, and active sea-floor spreading; filling of basins in Cretaceous and Tertiary(?) time; basaltic extrusion on spreading sea floor and sedimentation on continental shelf and slope.

Basement rocks of Halmahera, eastern Indonesia: Implications for the early history of the Philippine Sea

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hall, R.G.N.; Ballantyne, P.

1990-06-01

The oldest rocks known on Halmahera, eastern Indonesia, are petrologically and chemically similar to supra-subduction ophiolites and include boninitic volcanics resembling those dredged from the Marianas forearc. The age of the ophiolitic rocks is unknown; in east Halmahera they are overlain by Late Cretaceous and Eocene volcanics and associated sediments. Similar volcanics form the basement of western Halmahera. Plutonic rocks intruding the ophiolite and associated metamorphic rocks also yield Late Cretaceous to Eocene radiometric ages. The petrology and chemistry of the igneous rocks indicate an island arc origin. These rocks are locally overlain by shallow-water Eocene limestones and all aremore » overlain unconformably by Neogene sediments. The Halmahera basement rocks have many structural, petrological, and stratigraphic similarities to submarine plateaus of the southern and northern Philippine Sea and basement terranes of the eastern Philippines. The authors suggest that these similarities indicate the existence of an extensive region of Late Cretaceous and Eocene volcanism built upon probable Mesozoic ophiolitic basement. The resultant thickened crust was later fragmented by spreading in the West Philippine Sea Central Basin and backarc spreading in the Eastern Philippine Sea. It is difficult to reconcile the present distribution of these crustal fragments with a linear arc, but equally difficult to propose a simple alternative. A proto-Philippine archipelago, with short-lived arcs separated by small oceanic basins, may be the closest modern analog. The development of younger subduction zones has been influenced by the distribution of thickened crustal fragments as they have re-amalgamated since the Miocene.« less

LA-ICP-MS zircon U-Pb and muscovite K-Ar ages of basement rocks from the south arm of Sulawesi, Indonesia

NASA Astrophysics Data System (ADS)

Jaya, Asri; Nishikawa, Osamu; Hayasaka, Yasutaka

2017-11-01

The zircon U-Pb and muscovite K-Ar age from the Bantimala, Barru and Biru basement complexes in the South Arm of Sulawesi, Indonesia provide new information regarding the timing of magmatism, metamorphism and sedimentation in this region and have implications for the origin and evolution of the study area. The study area is at the juncture between the southeast margin of Sundaland and Bird's Head-Australia. The age of both the zircon U-Pb of detrital materials in the Bantimala Complex and the muscovite K-Ar of amphibolite in the Biru Complex fall in the Late Early Cretaceous (between 109 and 115 Ma), which is a similar age range to previous data for both the sedimentary and metamorphic rocks. The youngest detrital zircon in the schist samples from the Barru Complex fall into the Triassic in age (between 243 and 247 Ma). These age data indicate that the protolith of all three basement complexes were involved in the subduction system and metamorphosed in the late Early Cretaceous, but there are several differences in their deposition environment under and out of the influence of the late Early Cretaceous magmatism in the Bantimala and Barru Complexes, respectively. Felsic igneous activities are confirmed in the Late Cretaceous and the Eocene by the zircon U-Pb age of igneous rocks intruding or included as detrital fragments in three basement complexes. These dates are similar to those reported from the Meratus Complex of South Kalimantan. The detrital zircon age distributions of the basement rocks in the South Arm of Sulawesi display predominant Mesozoic (Cretaceous and Triassic) and Paleozoic populations with a small population of Proterozoic ages supporting the hypothesis that the West Sulawesi block originated from the region of the circum Bird's Head-Australian, namely the Inner Banda block. The absence of Jurassic zircon age population in the South Arm of Sulawesi suggests the division of the South Arm of Sulawesi from the Inner Banda block in early stage of rifting. Western Sulawesi is composed of several blocks separated from Inner Banda block with different histories, which is supported by the varieties of zircon population distribution in the basement rocks in the Western Sulawesi and also difference of general orientations of structural features between the Bantimala and Barru Complexes.

Paleohighs and Paleolows in the Basement Rocks of the Eastern Gulf of Mexico

NASA Astrophysics Data System (ADS)

Robinson, D.; Weislogel, A. L.

2017-12-01

The Eastern Gulf of Mexico has topography on the basement rocks composed of igneous and metamorphic rocks as well as some sedimentary rocks underneath a relatively thin salt layer with 3-6 km of topography relief. Paleohighs from south to north include Sarasota Arch, Middle Ground Arch/Southern Platform, Pensacola Arch, Conecuh Ridge Complex, Baldwin High, Wiggins Arch and Choctaw Ridge Complex. Paleolows from south to north include South Florida Basin, Tampa Embayment, Apalachicola Basin/Desoto Canyon Salt Basin, Conecuh Embayment, Manila Embayment and the Mississippi Interior Salt Basin. The topography on the basement is a result of several collisions between Laurentian and Gondwana to produce Pangea with final suturing during Pennsylvanian time and also from extension in Late Triassic to Early Cretaceous time as a result of the opening of the Gulf and rotation of Yucatan. Heterogeneities related to previous collisions may have also factored into producing these paleohighs and paleolows. A series of grabens and half-grabens, trending northeast-southwest from northwest-southeast directed extension and with the sedimentary rocks, exist on the continents and appear to be present in the offshore under the salt. We know the paleolows were depositional pathways to funnel sediments from onshore to offshore via water and wind in Jurassic and maybe Cretaceous times. Many tectonic models call for the paleohighs and paleolows to be structurally controlled; however, finding the faults called upon to control the "horst and graben" structures is challenging. We present data from several seismic studies that questions the idea that these paleohighs and paleolows are the result of horst and graben extension. Half grabens exist in the offshore with graben bounding faults northeast-southwest; however, down is to the north instead of the anticipated down to the south. Instead, the basement paleohighs and paleolows in the offshore Eastern Gulf of Mexico may be the result of preexisting lithologic and structural weaknesses in conjunction with lithospheric thinning. Some of the basement paleohighs and paleolows in the onshore are related to the buried Appalachian fold-thrust belt.

Petrographic and geochemical comparisons between the lower crystalline basement-derived section and the granite megablock and amphibolite megablock of the Eyreville B core, Chesapeake Bay impact structure, USA

USGS Publications Warehouse

Townsend, G.N.; Gibson, R.L.; Horton, J. Wright; Reimold, W.U.; Schmitt, R.T.; Bartosova, K.

2009-01-01

The Eyreville B core from the Chesapeake Bay impact structure, Virginia, USA, contains a lower basement-derived section (1551.19 m to 1766.32 m deep) and two megablocks of dominantly (1) amphibolite (1376.38 m to 1389.35 m deep) and (2) granite (1095.74 m to 1371.11 m deep), which are separated by an impactite succession. Metasedimentary rocks (muscovite-quartz-plagioclase-biotite-graphite ?? fibrolite ?? garnet ?? tourmaline ?? pyrite ?? rutile ?? pyrrhotite mica schist, hornblende-plagioclase-epidote-biotite- K-feldspar-quartz-titanite-calcite amphibolite, and vesuvianite-plagioclase- quartz-epidote calc-silicate rock) are dominant in the upper part of the lower basement-derived section, and they are intruded by pegmatitic to coarse-grained granite (K-feldspar-plagioclase-quartz-muscovite ?? biotite ?? garnet) that increases in volume proportion downward. The granite megablock contains both gneissic and weakly or nonfoliated biotite granite varieties (K-feldspar-quartz-plagioclase-biotite ?? muscovite ?? pyrite), with small schist xenoliths consisting of biotite-plagioclase-quartz ?? epidote ?? amphibole. The lower basement-derived section and both megablocks exhibit similar middleto upper-amphibolite-facies metamorphic grades that suggest they might represent parts of a single terrane. However, the mica schists in the lower basement-derived sequence and in the megablock xenoliths show differences in both mineralogy and whole-rock chemistry that suggest a more mafi c source for the xenoliths. Similarly, the mineralogy of the amphibolite in the lower basement-derived section and its association with calc-silicate rock suggest a sedimentary protolith, whereas the bulk-rock and mineral chemistry of the megablock amphibolite indicate an igneous protolith. The lower basement-derived granite also shows bulk chemical and mineralogical differences from the megablock gneissic and biotite granites. ?? 2009 The Geological Society of America.

Ages of pre-rift basement and synrift rocks along the conjugate rift and transform margins of the Argintine Precordillera and Laurentia

USGS Publications Warehouse

Thomas, William A.; Tucker, Robert D.; Astini, Ricardo A.; Denison, Rodger E.

2012-01-01

New geochronologic data from basement rocks support the interpretation that the Argentine Precordillera (Cuyania) terrane was rifted from the Ouachita embayment of the Iapetan margin of Laurentia. New data from the Ozark dome show a range of ages in two groups at 1466 ± 3 to 1462 ± 1 Ma and 1323 ± 2 to 1317 ± 2 Ma, consistent with existing data for the Eastern Granite-Rhyolite province and Southern Granite-Rhyolite province, respectively. Similarly, a newly determined age of 1364 ± 2 Ma for the Tishomingo Granite in the Arbuckle Mountains confirms previously published analyses for this part of the Southern Granite-Rhyolite province. Along with previously reported ages from basement olistoliths in Ordovician slope deposits in the Ouachita embayment, the data for basement ages support the interpretation that rocks of the Southern Granite-Rhyolite province form the margin of Laurentian crust around the corner of the Ouachita embayment, which is bounded by the Ouachita rift and Alabama-Oklahoma transform fault. In contrast, both west and east of the corner of the Ouachita embayment, Grenville-Llano basement (approximately 1325–1000 Ma) forms the rifted margin of Laurentia. New U/Pb zircon data from basement rocks in the southern part of the Argentine Precordillera indicate crystallization ages of 1205 ± 1 Ma and 1204 ± 2 Ma, consistent with previously reported ages (approximately 1250–1000 Ma) of basement rocks from other parts of the Precordillera. These data document multiple events within the same time span as multiple events in the Grenville orogeny in eastern Laurentia, and are consistent with Grenville-age rocks along the conjugate margins of the Precordillera and Laurentia. Ages from one newly analyzed collection, however, are older than those from other basement rocks in the Precordillera. These ages, from granodioritic-granitic basement clasts in a conglomerate olistolith in Ordovician slope deposits, are 1370 ± 2 Ma and 1367 ± 5 Ma. These older ages from the Precordillera are consistent with indications that the Iapetan margin in the Ouachita embayment of Laurentia truncated the Grenville front and left older rocks of the Southern Granite-Rhyolite province (1390–1320 Ma) at the rifted margin. Chronostratigraphic correlations of synrift and post-rift sedimentary deposits on the Precordillera and on the Texas promontory of Laurentia document initial rifting in the Early Cambrian. Previously published data from synrift plutonic and volcanic rocks in the Wichita and Arbuckle Mountains along the transform-parallel intracratonic Southern Oklahoma fault system inboard from the Ouachita embayment document crystallization ages of 539–530 Ma. New data from synrift volcanic rocks in the Arbuckle Mountains in the eastern part of the Southern Oklahoma fault system yield ages of 539 ± 5 Ma and 536 ± 5 Ma, confirming the age of synrift volcanism.

Inversion of Gravity Data to Define the Pre-Cenozoic Surface and Regional Structures Possibly Influencing Groundwater Flow in the Rainier Mesa Region, Nye County, Nevada.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Thomas G. Hildenbrand; Geoffrey A. Phelps; Edward A. Mankinen

2006-09-21

A three-dimensional inversion of gravity data from the Rainier Mesa area and surrounding regions reveals a topographically complex pre-Cenozoic basement surface. This model of the depth to pre-Cenozoic basement rocks is intended for use in a 3D hydrogeologic model being constructed for the Rainier Mesa area. Prior to this study, our knowledge of the depth to pre-Cenozoic basement rocks was based on a regional model, applicable to general studies of the greater Nevada Test Site area but inappropriate for higher resolution modeling of ground-water flow across the Rainier Mesa area. The new model incorporates several changes that lead to significantmore » improvements over the previous regional view. First, the addition of constraining wells, encountering old volcanic rocks lying above but near pre-Cenozoic basement, prevents modeled basement from being too shallow. Second, an extensive literature and well data search has led to an increased understanding of the change of rock density with depth in the vicinity of Rainier Mesa. The third, and most important change, relates to the application of several depth-density relationships in the study area instead of a single generalized relationship, thereby improving the overall model fit. In general, the pre-Cenozoic basement surface deepens in the western part of the study area, delineating collapses within the Silent Canyon and Timber Mountain caldera complexes, and shallows in the east in the Eleana Range and Yucca Flat regions, where basement crops out. In the Rainier Mesa study area, basement is generally shallow (< 1 km). The new model identifies previously unrecognized structures within the pre-Cenozoic basement that may influence ground-water flow, such as a shallow basement ridge related to an inferred fault extending northward from Rainier Mesa into Kawich Valley.« less

Subsurface Biodegradation in a Fractured Basement Reservoir, Shropshire, UK

NASA Astrophysics Data System (ADS)

Parnell, John; Baba, Mas'ud; Bowden, Stephen; Muirhead, David

2017-04-01

Subsurface Biodegradation in a Fractured Basement Reservoir, Shropshire, UK. John Parnell, Mas'ud Baba, Stephen Bowden, David Muirhead Subsurface biodegradation in current oil reservoirs is well established, but there are few examples of fossil subsurface degradation. Biomarker compositions of viscous and solid oil residues ('bitumen') in fractured Precambrian and other basement rocks below the Carboniferous cover in Shropshire, UK, show that they are variably biodegraded. High levels of 25-norhopanes imply that degradation occurred in the subsurface. Lower levels of 25-norhopanes occur in active seepages. Liquid oil trapped in fluid inclusions in mineral veins in the fractured basement confirm that the oil was emplaced fresh before subsurface degradation. A Triassic age for the veins implies a 200 million year history of hydrocarbon migration in the basement rocks. The data record microbial colonization of a fractured basement reservoir, and add to evidence in modern basement aquifers for microbial activity in deep fracture systems. Buried basement highs may be especially favourable to colonization, through channelling fluid flow to shallow depths and relatively low temperatures

Architecture of ductile-type, hyper-extended passive margins: Geological constraints from the inverted Cretaceous basin of the North-Pyrenean Zone ('Chaînons Béarnais', Western Pyrenees)

NASA Astrophysics Data System (ADS)

Corre, Benjamin; Lagabrielle, Yves; Labaume, Pierre; Lahfid, Abdeltif; Boulvais, Philippe; Bergamini, Geraldine; Fourcade, Serge; Clerc, Camille

2017-04-01

Sub-continental lithospheric mantle rocks are exhumed at the foot of magma-poor distal passive margins as a response to extreme stretching of the continental crust during plate separation. Remnants of the Northern Iberian paleo-passive margin are now exposed in the North-Pyrenean Zone (NPZ) and represent field analogues to study the processes of continental crust thinning and subcontinental mantle exhumation. The NPZ results from the inversion of basins opened between the Iberia and Europa plates during Albo-Cenomanian times. In the western NPZ, the 'Chaînons Béarnais' ranges display a fold-and-thrust structure involving the Mesozoic sedimentary cover, decoupled from its continental basement and associated with peridotite bodies in tectonic contact with Palaeozoic basement lenses of small size. Continental extension developed under hot thermal conditions, as demonstrated by the syn-metamorphic Cretaceous ductile deformation affecting both the crustal basement and the allochthonous Mesozoic cover. In this study, we present structural and geochemical data providing constraints to reconstruct the evolution of the northern Iberia paleo-margin. Field work confirms that the pre-rift Mesozoic cover is intimately associated to mantle rocks and to thin tectonic lenses of crustal basement. It also shows that the pre-rift cover was detached from its bedrock at the Keuper evaporites level and was welded to mantle rocks during their exhumation at the foot of the hyper-extended margin. The crust/mantle detachment fault is a major shear zone characterized by anastomosed shear bands defining a plurimetric phacoidal fabric at the top of the serpentinized mantle. The detachment is marked by a layer of metasomatic rocks, locally 20 meters thick, made of talc-chlorite-pyrite-rich rocks that developped under greenschist facies conditions. Raman Spectroscopy on Carbonaceous Materials (RSCM), performed on the Mesozoic cover reveal that the entire sedimentary pile underwent temperatures ranging between 200°C and 480°C. We show that: (i) at the site of mantle rocks exhumation, the boudinaged pre-rift sediments have undergone drastic syn-metamorphic thinning with the genesis of a S0/S1 foliation and, (ii) the Paleozoic basement has been ductilely deformed, into thin tectonic lenses that remained welded to the exhumed mantle rocks. Therefore the overall crustal rheology appears dominated by shallow levels having a ductile behavior. This rheology is related to the presence of a thick pre- and syn-rift decoupled cover acting as an efficient thermal blanket. This new geological data set highlights important characteristics of ductile-type hyper-extended passive margin that cannot be obtained from the study of seismic lines. Finally, we stress that studying field analogues represents a major tool to better understand the mechanisms of extreme crustal thinning associated with mantle exhumation and their structural inheritance during tectonic inversion.

Preliminary Isostatic Gravity Map of Joshua Tree National Park and Vicinity, Southern California

USGS Publications Warehouse

Langenheim, V.E.; Biehler, Shawn; McPhee, D.K.; McCabe, C.A.; Watt, J.T.; Anderson, M.L.; Chuchel, B.A.; Stoffer, P.

2007-01-01

This isostatic residual gravity map is part of an effort to map the three-dimensional distribution of rocks in Joshua Tree National Park, southern California. This map will serve as a basis for modeling the shape of basins beneath the Park and in adjacent valleys and also for determining the location and geometry of faults within the area. Local spatial variations in the Earth's gravity field, after accounting for variations caused by elevation, terrain, and deep crustal structure, reflect the distribution of densities in the mid- to upper crust. Densities often can be related to rock type, and abrupt spatial changes in density commonly mark lithologic or structural boundaries. High-density basement rocks exposed within the Eastern Transverse Ranges include crystalline rocks that range in age from Proterozoic to Mesozoic and these rocks are generally present in the mountainous areas of the quadrangle. Alluvial sediments, usually located in the valleys, and Tertiary sedimentary rocks are characterized by low densities. However, with increasing depth of burial and age, the densities of these rocks may become indistinguishable from those of basement rocks. Tertiary volcanic rocks are characterized by a wide range of densities, but, on average, are less dense than the pre-Cenozoic basement rocks. Basalt within the Park is as dense as crystalline basement, but is generally thin (less than 100 m thick; e.g., Powell, 2003). Isostatic residual gravity values within the map area range from about 44 mGal over Coachella Valley to about 8 mGal between the Mecca Hills and the Orocopia Mountains. Steep linear gravity gradients are coincident with the traces of several Quaternary strike-slip faults, most notably along the San Andreas Fault bounding the east side of Coachella Valley and east-west-striking, left-lateral faults, such as the Pinto Mountain, Blue Cut, and Chiriaco Faults (Fig. 1). Gravity gradients also define concealed basin-bounding faults, such as those beneath the Chuckwalla Valley (e.g. Rotstein and others, 1976). These gradients result from juxtaposing dense basement rocks against thick Cenozoic sedimentary rocks.

Colorado Potential Geothermal Pathways

DOE Data Explorer

Richard E. Zehner

2012-02-01

This layer contains the weakened basement rocks. Isostatic gravity was utilized to identify structural basin areas, characterized by gravity low values reflecting weakened basement rocks. Together interpreted regional fault zones and basin outlines define geothermal "exploration fairways", where the potential exists for deep, superheated fluid flow in the absence of Pliocene or younger volcanic units.

Gravity and magnetic study of the Pahute Mesa and Oasis Valley region, Nye County, Nevada

USGS Publications Warehouse

Mankinen, Edward A.; Hildenbrand, Thomas G.; Dixon, Gary L.; McKee, Edwin H.; Fridrich, Christopher J.; Laczniak, Randell J.

1999-01-01

Regional gravity and aeromagnetic maps reveal the existence of deep basins underlying much of the southwestern Nevada volcanic field, approximately 150 km northwest of Las Vegas. These maps also indicate the presence of prominent features (geophysical lineaments) within and beneath the basin fill. Detailed gravity surveys were conducted in order to characterize the nature of the basin boundaries, delineate additional subsurface features, and evaluate their possible influence on the movement of ground-water. Geophysical modeling of gravity and aeromagnetic data indicates that many of the features may be related to processes of caldera formation. Collapse of the various calderas within the volcanic field resulted in dense basement rocks occurring at greater depths within caldera boundaries. Modeling indicates that collapse occurred along faults that are arcuate and steeply dipping. There are indications that the basement in the western Pahute Mesa - Oasis Valley region consists predominantly of granitic and/or fine-grained siliceous sedimentary rocks that may be less permeable to groundwater flow than the predominantly fractured carbonate rock basement to the east and southeast of the study area. The northeast-trending Thirsty Canyon lineament, expressed on gravity and basin thickness maps, separates dense volcanic rocks on the northwest from less dense intracaldera accumulations in the Silent Canyon and Timber Mountain caldera complexes. The source of the lineament is an approximately 2-km wide ring fracture system with step-like differential displacements, perhaps localized on a pre-existing northeast-trending Basin and Range fault. Due to vertical offsets, the Thirsty Canyon fault zone probably juxtaposes rock types of different permeability and, thus, it may act as a barrier to ground-water flow and deflect flow from Pahute Mesa along its flanks toward Oasis Valley. Within the Thirsty Canyon fault zone, highly fractured rocks may serve also as a conduit, depending upon the degree of alteration and its effect on porosity and permeability. In the Oasis Valley region, other structures that may influence ground-water flow include the western and southern boundaries of the Oasis Valley basin, where the basement abruptly shallows.

Geochemical and isotopic study of impact melts and spherules from the Lonar impact crater, India, indicate melting of the Precambrian basement beneath the 'target' Deccan basalts

NASA Astrophysics Data System (ADS)

Chakrabarti, R.; Goderis, S.; Banerjee, A.; Gupta, R. D.; Claeys, P.; Vanhaecke, F. F.

2016-12-01

The 1.88 km diameter Lonar impact Crater, with age estimates ranging from 52 -570 ka, is located in the Buldana district of Maharashtra, India. It is an almost circular depression hosted entirely in the 65Ma old basalt flows of the Deccan Traps and is the best-known terrestrial analogue for impact craters in the Inner Solar System. Isotopic studies indicate that the basalts around Lonar correlate with the Poladpur suite, one of the mid-section volcano-stratigraphic units of the Deccan traps. Recently collected samples of the host basalt and impact melts, were analyzed for major and trace element concentrations using ICPMS, as well as for Nd and Sr isotope ratios using TIMS. Relatively more radiogenic Sr and less radiogenic Nd isotopic composition of the melt rocks compared to earlier measurements of similar rocks from Lonar are consistent with melting of the Precambrian basement beneath the Deccan basalt. Spherules ranging in size from 100 mm to 1 mm, were hand-picked under a binocular microscope from unconsolidated soil samples, collected from the south-eastern rim of the crater. Thirty-five spherule samples, screened for surface alteration using SEM were analyzed for major and trace element concentrations including PGEs using LA-ICPMS. The spherules were further classified into two groups using the Chemical Index of Alteration(CIA). Iridium and Cr concentrations of the spherules are consistent with mixing of a chondritic impactor (with 2-8% contribution) with the target rock(s). On a Nb (fluid immobile) -normalized binary plot of Th versus Cr, the composition of the spherules can be explained by mixing between the host basalt and a chondritic impactor with a definite, but minor contribution of the basement beneath Lonar, the composition of which is approximated using the average composition of the upper continental crust (UCC). Variability in the light-REE fractionation of the spherules (La/Sm(N)) can also be explained by a similar three component mixing. Overall, our geochemical data for both the melt rocks and spherules suggest mixing between the chondritic impactor, the Deccan host basalt and the basement rocks at Lonar.

Basement and cover-rock deformation during Laramide contraction in the northern Madison Range (Montana) and its influence on Cenozoic basin formation

USGS Publications Warehouse

Kellogg, K.S.; Schmidt, C.J.; Young, S.W.

1995-01-01

Two major Laramide fault systems converge in the northwestern Madison Range: the northwest-striking, southwest-vergent Spanish Peaks reverse fault and the north-striking, east-vergent Hilgard thrust system. Analysis of foliation attitudes in basement gneiss north and south of the Spanish Peaks fault indicates that the basement in thrusted blocks of the Hilgard thrust system have been rotated by an amount similar to that of the basement-cover contact. Steeply dipping, north-striking breccia zones enclosing domains of relatively undeformed basement may have permitted domino-style rotation of basement blocks during simple shear between pairs of thrusts. No hydrocarbon discoveries have been made in this unique structural province. However, petroleum exploration here has focused on basement-cored anticlines, both surface and subthrust, related to the two major Laramide fault systems and on the fault-bounded blocks of Tertiary rocks within the post-Laramide extensional basins. -from Authors

Precambrian Basement Structure Map of the Continental United States - An Interpretation of Geologic and Aeromagnetic Data

USGS Publications Warehouse

Sims, Paul K.; Saltus, Richard W.; Anderson, Eric D.

2008-01-01

The Precambrian basement rocks of the continental United States are largely covered by younger sedimentary and volcanic rocks, and the availability of updated aeromagnetic data (NAMAG, 2002) provides a means to infer major regional basement structures and tie together the scattered, but locally abundant, geologic information. Precambrian basement structures in the continental United States have strongly influenced later Proterozoic and Phanerozoic tectonism within the continent, and there is a growing awareness of the utility of these structures in deciphering major younger tectonic and related episodes. Interest in the role of basement structures in the evolution of continents has been recently stimulated, particularly by publications of the Geological Society of London (Holdsworth and others, 1998; Holdsworth and others, 2001). These publications, as well as others, stress the importance of reactivation of basement structures in guiding the subsequent evolution of continents. Knowledge of basement structures is an important key to understanding the geology of continental interiors.

Geophysical investigation of the Raton Basin

NASA Astrophysics Data System (ADS)

Cheney, R. S.

1982-05-01

This thesis correlates gravity, magnetic, and seismic data for the Raton Basin of Colorado and New Mexico. The gravity data suggest that the study area, and the region around it, is in isostatic equilibrium. The free air anomaly in the southern portion of the study area suggests lack of local compensation due to Quaternary volocanic rock. The volcanic rock thickness, calculated from the free air gravity data, is 180 m. The gravity data indicated a crustal thickness of about 45 km, and the crust thinned from west to east. A basement relief map was constructed from the Bouquer gravity data. Computer techniques were developed to calculate the depth to the basement surface and to plot a contour map of that surface. The Raton Basin magnetic map defined the same surface found on the basement relief map since the overlying sedimentary rocks have no magnetism; therefore, any magnetism present is caused by the basement rock. A seismic survey near capulin Mountain detected a high level of microseismicity that may be caused by adjustment along faults or dormant volcanic activity.

Petrographic and geochemical comparisons between the lower crystalline basement-derived section and the granite megablock and amphibolite megablock of the Eyreville-B core, Chesapeake Bay impact structure

USGS Publications Warehouse

Townsend, Gabrielle N.; Gibson, Roger L.; Horton, J. Wright; Reimold, Wolf Uwe; Schmitt, Ralf T.; Bartosova, Katerina

2009-01-01

The Eyreville B core from the Chesapeake Bay impact structure, Virginia, USA, contains a lower basement-derived section (1551.19 m to 1766.32 m deep) and two megablocks of dominantly (1) amphibolite (1376.38 m to 1389.35 m deep) and (2) granite (1095.74 m to 1371.11 m deep), which are separated by an impactite succession. Metasedimentary rocks (muscovite-quartz-plagioclase-biotite-graphite ± fibrolite ± garnet ± tourmaline ± pyrite ± rutile ± pyrrhotite mica schist, hornblende-plagioclase-epidote-biotite-K-feldspar-quartz-titanite-calcite amphibolite, and vesuvianite-plagioclase-quartz-epidote calc-silicate rock) are dominant in the upper part of the lower basement-derived section, and they are intruded by pegmatitic to coarse-grained granite (K-feldspar-plagioclase-quartz-muscovite ± biotite ± garnet) that increases in volume proportion downward. The granite megablock contains both gneissic and weakly or nonfoliated biotite granite varieties (K-feldspar-quartz-plagioclase-biotite ± muscovite ± pyrite), with small schist xenoliths consisting of biotite-plagioclase-quartz ± epidote ± amphibole. The lower basement-derived section and both megablocks exhibit similar middle- to upper-amphibolite-facies metamorphic grades that suggest they might represent parts of a single terrane. However, the mica schists in the lower basement-derived sequence and in the megablock xenoliths show differences in both mineralogy and whole-rock chemistry that suggest a more mafic source for the xenoliths. Similarly, the mineralogy of the amphibolite in the lower basement-derived section and its association with calc-silicate rock suggest a sedimentary protolith, whereas the bulk-rock and mineral chemistry of the megablock amphibolite indicate an igneous protolith. The lower basement-derived granite also shows bulk chemical and mineralogical differences from the megablock gneissic and biotite granites.

Influence of rock strength variations on interpretation of thermochronologic data

NASA Astrophysics Data System (ADS)

Flowers, Rebecca; Ehlers, Todd

2017-04-01

Low temperature thermochronologic datasets are the primary means for estimating the timing, magnitude, and rates of erosion over extended (10s to 100s of Ma) timescales. Typically, abrupt shifts in cooling rates recorded by thermochronologic data are interpreted as changes in erosion rates caused by shifts in uplift rates, drainage patterns, or climate. However, recent work shows that different rock types vary in strength and erodibility by as much as several orders of magnitude, therefore implying that lithology should be an important control on how landscapes change through time and the thermochronometer record of erosion histories. Attention in the surface processes community has begun to focus on rock strength as a critical control on short-term (Ka to Ma) landscape evolution, but there has been less consideration of the influence of this factor on landscapes over longer intervals. If intrinsic lithologic variability can strongly modify erosion rates without changes in external factors, this result would have important implications for how thermochronologic datasets are interpreted. Here we evaluate the importance of rock strength for interpreting thermochronologic datasets by examining erosion rates and total denudation magnitudes across sedimentary rock-crystalline basement rock interfaces. We particularly focus on the 'Great Unconformity', a global stratigraphic surface between Phanerozoic sedimentary rocks and Precambrian crystalline basement, which based on rock strength studies marks a dramatic rock erodibility contrast across which erosion rates should decelerate. In the Rocky Mountain basement uplifts of the western U.S., thermochronologic data and geologic observations indicate that erosion rates were high during latest Cretaceous-early Tertiary denudation of the sedimentary cover (3-4 km over 10 m.y., 300-400 m/m.y.) but dramatically decelerated when less erodible basement rocks were encountered (0.1-0.5 km over 55 m.y., 2-9 m/m.y.). Similarly, the western Canadian shield underwent multiple Phanerozoic episodes of substantial (1-4 km) sedimentary rock burial and erosion, but total Phanerozoic erosion of the crystalline basement below the Great Unconformity was no more than a few hundred meters. We use published low temperature thermochronologic dates, the LandLab landscape evolution model, and 1D thermokinematic and erosion (Pecube) models to assess whether the observed deceleration of erosion can be explained by measured variations in rock strength alone. We use these results to consider the extent to which rock strength can change the cooling history recorded by thermochronologic datasets.

A reconstruction of Proterozoic rocks in north-central New Mexico: Tectonic implications from the Proterozoic to the Cenozoic

DOE Office of Scientific and Technical Information (OSTI.GOV)

Daniel, C.G.; Karlstrom, K.E.

1993-04-01

Distinctive lithostratigraphic markers, metamorphic isobaric surfaces, major ductile thrusts and overturned folds in Early Proterozoic rocks from 4 isolated uplifts in north-central NM provide relatively firm piercing points for restoration of over 50 km of right lateral strike-slip movement along a network of N-S trending faults. In addition, the authors speculate that the Uncompahgre Group in the Needle Mts. of southern Colorado is correlative with the Hondo Group in northern NM; suggesting over 150 km of right-lateral strike slip offset has occurred across a network of N-S trending faults that includes the Picuris-Pecos fault, the Borrego fault, the Nacimiento faultmore » and others. The tectonic implications of this reconstruction span geologic time from the Proterozoic to the Cenozoic. The restoration of slip provides new insights into the structure of the Proterozoic basement in NM. Volcanogenic basement (1.74--1.72 Ga) and overlying sedimentary cover (Hondo Group) are imbricated in an originally EW- to NW-trending ductile foreland thrust and fold belt that formed near the southern margin of 1.74--1.72 basement. The authors propose that the volcanogenic basement rocks correlate with rocks of the Yavapi Province in Arizona and that the Hondo Group correlates with foreland rocks of the Tonto Basin Supergroup. Rocks south of this belt are 1.65 Ga or younger and are interpreted to belong to a separate crustal province which correlates with the Mazatzal Province in Arizona. Proterozoic ductile fault geometries suggest that the Mazatzal Province was thrust northward and resulted in imbrication of Yavapi Province basement and its siliciclastic over sequence.« less

Preliminary model of the pre-Tertiary basement rocks beneath Yucca Flat, Nevada Test Site, Nevada, based on analysis of gravity and magnetic data

USGS Publications Warehouse

Phelps, Geoffrey A.; McKee, Edwin H.; Sweetkind, D.; Langenheim, V.E.

2000-01-01

The Environmental Restoration Program of the U.S. Department of Energy, Nevada Operations Office, was developed to investigate the possible consequences to the environment of 40 years of nuclear testing on the Nevada Test Site. The majority of the tests were detonated underground, introducing contaminants into the ground-water system (Laczniak and others, 1996). An understanding of the ground-water flow paths is necessary to evaluate the extent of ground-water contamination. This report provides information specific to Yucca Flat on the Nevada Test Site. Critical to understanding the ground-water flow beneath Yucca Flat is an understanding of the subsurface geology, particularly the structure and distribution of the pre-Tertiary rocks, which comprise both the major regional aquifer and aquitard sequences (Winograd and Thordarson, 1975; Laczniak and others, 1996). Because the pre-Tertiary rocks are not exposed at the surface of Yucca Flat their distribution must be determined through well logs and less direct geophysical methods such as potential field studies. In previous studies (Phelps and others, 1999; Phelps and Mckee, 1999) developed a model of the basement surface of the Paleozoic rocks beneath Yucca Flat and a series of normal faults that create topographic relief on the basement surface. In this study the basement rocks and structure of Yucca Flat are examined in more detail using the basement gravity anomaly derived from the isostatic gravity inversion model of Phelps and others (1999) and high-resolution magnetic data, as part of an effort to gain a better understanding of the Paleozoic rocks beneath Yucca Flat in support of groundwater modeling.

Fault-Slip Data Analysis and Cover Versus Basement Fracture Patterns - Implications for Subsurface Technical Processes in Thuringia, Germany

NASA Astrophysics Data System (ADS)

Kasch, N.; Kley, J.; Navabpour, P.; Siegburg, M.; Malz, A.

2014-12-01

Recent investigations in Thuringia, Central Germany, focus on the potential for carbon sequestration, groundwater supply and geothermal energy. We report on the results of an integrated fault-slip data analysis to characterize the geometries and kinematics of systematic fractures in contrasting basement and cover rock lithologies. The lithostratigraphy of the area comprises locally exposed crystalline rocks and intermittently overlying Permian volcanic and clastic sedimentary rocks, together referred to as basement. A Late Permian sequence of evaporites, carbonates and shale constitutes the transition to the continuous sedimentary cover of Triassic age. Major NW-SE-striking fault zones and minor NNE-SSW-striking faults affect this stratigraphic succession. These characteristic narrow deforming areas (< 3 km width) build a dense network of individual fault strands with a close juxtaposition to wider (> 15 km) non-deforming areas suggesting localized zones of mechanical weakness, which can be confirmed by the frequent reactivation of single fault strands. Along the major fault zones, the basement and cover contain dominant inclined to sub-vertical NW-SE-striking fractures. These fractures indicate successive normal, dextral strike-slip and reverse senses of slip, evidencing events of NNE-SSW extension and contraction. Another system of mostly sub-vertical NNW-SSE- and NE-SW-striking conjugate strike-slip faults mainly developed within the cover implies NNE-SSW contraction and WNW-ESE extension. Earthquake focal mechanisms and in-situ stress measurements reveal a NW-SE trend for the modern SHmax. Nevertheless, fractures and fault-slip indicators are rare in the non-deforming areas, which characterizes Thuringia as a dual domain of (1) large unfractured areas and (2) narrow zones of high potential for technical applications. Our data therefore provide a basis for estimation of slip and dilation tendency of the contrasting fractures in the basement and cover under the present-day stress field, which must be taken into account for different subsurface technical approaches.

Seismic evidence of exhumed mantle rock basement at the Gorringe Bank and the adjacent Horseshoe and Tagus abyssal plains (SW Iberia)

NASA Astrophysics Data System (ADS)

Sallarès, Valentí; Martínez-Loriente, Sara; Prada, Manel; Gràcia, Eulàlia; Ranero, César; Gutscher, Marc-André; Bartolome, Rafael; Gailler, Audrey; Dañobeitia, Juan José; Zitellini, Nevio

2013-03-01

The Gorringe Bank is a gigantic seamount that separates the Horseshoe and Tagus abyssal plains offshore SW Iberia, in a zone that hosts the convergent boundary between the Africa and Eurasia plates. Although the region has been the focus of numerous investigations since the early 1970s, the lack of appropriate geophysical data makes the nature of the basement, and thus the origin of the structures, still debated. In this work, we present combined P-wave seismic velocity and gravity models along a transect that crosses the Gorringe Bank from the Tagus to the Horseshoe abyssal plains. The P-wave velocity structure of the basement is similar in the Tagus and Horseshoe plains. It shows a 2.5-3.0 km-thick top layer with a velocity gradient twice stronger than oceanic Layer 2 and an abrupt change to an underlying layer with a five-fold weaker gradient. Velocity and density is lower beneath the Gorringe Bank probably due to enhanced fracturing, that have led to rock disaggregation in the sediment-starved northern flank. In contrast to previous velocity models of this region, there is no evidence of a sharp crust-mantle boundary in any of the record sections. The modelling results indicate that the sediment overlays directly serpentinite rock, exhumed from the mantle with a degree of serpentinization decreasing from a maximum of 70-80% under the top of Gorringe Bank to less than 5% at a depth of ˜20 km. We propose that the three domains were originally part of a single serpentine rock band, of nature and possibly origin similar to the Iberia Abyssal Plain ocean-continent transition, which was probably generated during the earliest phase of the North Atlantic opening that followed continental crust breakup (Early Cretaceous). During the Miocene, the NW-SE trending Eurasia-Africa convergence resulted in thrusting of the southeastern segment of the exhumed serpentinite band over the northwestern one, forming the Gorringe Bank. The local deformation associated to plate convergence and uplift could have promoted pervasive rock fracturing of the overriding plate, leading eventually to rock disaggregation in the northern flank of the GB, which could be now a potential source of rock avalanches and tsunamis.

Cover sequence stratigraphy and structure: Salem Church basement culmination, Georgia Blue Ridge

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, L.; Tull, J.F.

The Salem Church anticline SW of Jasper, Georgia in the western Blue Ridge is roughly an oval shaped structural dome with its long axis trending NE-SW. The anticline is cored by the Grenville age Corbin Gneiss which represents allochthonous North American basement. In debate for decades has been the age and origin of several kilometers of poly-deformed cover sequence rocks which were metamorphosed to greenschist facies and were probably transported over a long distance inland after their deposition. The stratigraphy of the cover sequence exhibits rapid lithofacies changes. At most localities, the basement is overlain by a 500--600 m thickmore » coarse clastic unit sourced from the basement rocks, composed mainly of metaconglomerate, metasandstone and metadiamictite. A thin unit less than 20 m thick of sericite phyllite occurs between the basement and the coarse clastic unit along the SE limb of the anticline but pinches out to the MW. A relatively sharp stratigraphic contact occurs between quartzite unit and overlying dark colored metagreywackes and metadiamictites containing distinctive cobbles and boulders of granitic and gneissic basement rocks up to 1 meter in length. This unit is about 100 m thick in the SW but thins rapidly towards the NE. It grades up into a geographically widespread graphitic phyllite which encircles most of the anticline. Unlike the cover sequence above the corbin basement west of Waleska, Georgia, no carbonate is found in this area.« less

Implications of Eocene-age Philippine Sea and forearc basalts for initiation and early history of the Izu-Bonin-Mariana arc

NASA Astrophysics Data System (ADS)

Yogodzinski, Gene M.; Bizimis, Michael; Hickey-Vargas, Rosemary; McCarthy, Anders; Hocking, Benjamin D.; Savov, Ivan P.; Ishizuka, Osamu; Arculus, Richard

2018-05-01

Whole-rock isotope ratio (Hf, Nd, Pb, Sr) and trace element data for basement rocks at ocean drilling Sites U1438, 1201 and 447 immediately west of the KPR (Kyushu-Palau Ridge) are compared to those of FAB (forearc basalts) previously interpreted to be the initial products of IBM subduction volcanism. West-of-KPR basement basalts (drill sites U1438, 1201, 447) and FAB occupy the same Hf-Nd and Pb-Pb isotopic space and share distinctive source characteristics with εHf mostly > 16.5 and up to εHf = 19.8, which is more radiogenic than most Indian mid-ocean ridge basalts (MORB). Lead isotopic ratios are depleted, with 206Pb/204Pb = 17.8-18.8 accompanying relatively high 208Pb/204Pb, indicating an Indian-MORB source unlike that of West Philippine Basin plume basalts. Some Sr isotopes show affects of seawater alteration, but samples with 87Sr/86Sr < 0.7034 and εNd > 8.0 appear to preserve magmatic compositions and also indicate a common source for west-of-KPR basement and FAB. Trace element ratios resistant to seawater alteration (La/Yb, Lu/Hf, Zr/Nb, Sm/Nd) in west-of-KPR basement are generally more depleted than normal MORB and so also appear similar to FAB. At Site U1438, only andesite sills intruding sedimentary rocks overlying the basement have subduction-influenced geochemical characteristics (εNd ∼ 6.6, εHf ∼ 13.8, La/Yb > 2.5, Nd/Hf ∼ 9). The key characteristic that unites drill site basement rocks west of KPR and FAB is the nature of their source, which is more depleted in lithophile trace elements than average MORB but with Hf, Nd, and Pb isotope ratios that are common in MORB. The lithophile element-depleted nature of FAB has been linked to initiation of IBM subduction in the Eocene, but Sm-Nd model ages and errorchron relationships in Site U1438 basement indicate that the depleted character of the rocks is a regional characteristic that was produced well prior to the time of subduction initiation and persists today in the source of modern IBM arc volcanic rocks with Sm/Nd > 0.34 and εNd ∼ 9.0.

Distribution of oceanic and continental leads in the Arabian-Nubian Shield

USGS Publications Warehouse

Stacey, J.S.; Stoeser, D.B.

1983-01-01

New common lead data for feldspar, whole-rock, and galena samples from the Arabian-Nubian Shield, together with data from previous work, can be divided into two main groups. Group I leads have oceanic (mantle) characteristics, whereas group II leads have incorporated a continental-crustal component of at least early Proterozoic age. The group I leads are found in rocks from the Red Sea Hills of Egypt and the western and southern parts of the Arabian Shield. Group II leads are found in rocks from the northeastern and eastern parts of the Arabian Shield, as well as from the southeastern Shield near Najran. They are also found in rocks to the south in Yemen, to the east in Oman, and to the west at Aswan, Egypt. This distribution of data suggests that the Arabian-Nubian Shield has an oceanic core flanked by rocks that have developed, at least in part, from older continental material. Two mechanisms are suggested by which this older lead component could have been incorporated into the late Proterozoic rocks, and each may have operated in different parts of the Shield. The older lead component either was derived directly from an underlying early Proterozoic basement or was incorporated from subducted pelagic sediments or sediments derived from an adjacent continent. New U-Pb zircon data indicate the presence of an early Proterozoic basement southeast of Jabal Dahul in the eastern Arabian Shield. These data, together with 2,000-Ma-old zircons from the Al Amar fault zone, verify the implication of the common lead data that at least a part of the eastern Arabian Shield has an older continental basement. Because continental margins are particularly favorable locations for development of ore deposits, these findings may have important economic implications, particularly for tin, tungsten, and molybdenum exploration. ?? 1983 Springer-Verlag.

Origin of a crustal splay fault and its relation to the seismogenic zone and underplating at the erosional north Ecuador-south Colombia oceanic margin

NASA Astrophysics Data System (ADS)

Collot, J.-Y.; Agudelo, W.; Ribodetti, A.; Marcaillou, B.

2008-12-01

Splay faults within accretionary complexes are commonly associated with the updip limit of the seismogenic zone. Prestack depth migration of a multichannel seismic line across the north Ecuador-south Colombia oceanic margin images a crustal splay fault that correlates with the seaward limit of the rupture zone of the 1958 (Mw 7.7) tsunamogenic subduction earthquake. The splay fault separates 5-6.6 km/s velocity, inner wedge basement rocks, which belong to the accreted Gorgona oceanic terrane, from 4 to 5 km/s velocity outer wedge rocks. The outer wedge is dominated by basal tectonic erosion. Despite a 3-km-thick trench fill, subduction of 2-km-high seamount prevented tectonic accretion and promotes basal tectonic erosion. The low-velocity and poorly reflective subduction channel that underlies the outer wedge is associated with the aseismic, décollement thrust. Subduction channel fluids are expected to migrate upward along splay faults and alter outer wedge rocks. Conversely, duplexes are interpreted to form from and above subducting sediment, at ˜14- to 15-km depths between the overlapping seismogenic part of the splay fault and the underlying aseismic décollement. Coeval basal erosion of the outer wedge and underplating beneath the apex of inner wedge control the margin mass budget, which comes out negative. Intraoceanic basement fossil listric normal faults and a rift zone inverted in a flower structure reflect the evolution of the Gorgona terrane from Cretaceous extension to likely Eocene oblique compression. The splay faults could have resulted from tectonic inversion of listric normal faults, thus showing how inherited structures may promote fluid flow across margin basement and control seismogenesis.

Estimating the Subsurface Basement Topography of Dodge County, Wisconsin Using Three Dimensional Modeling of Gravity and Aeromagnetic Data

NASA Astrophysics Data System (ADS)

MacAlister, E.; Skalbeck, J.; Stewart, E.

2016-12-01

Since the late 1800's, geologic studies have been completed in Wisconsin in pursuit of understanding the basement topography and locating economically viable mineral resources. The doubly plunging Baraboo Syncline located in Columbia and Sauk Counties provides a classic record of Precambrian deformation. A similar buried structure is thought to exist in adjacent Dodge County based on a prominent aeromagnetic anomaly. For this study, 3-D modeling of gravity and aeromagnetic survey data was used to approximate the structure of the Precambrian basement topography beneath Dodge County, Wisconsin. The aim of the research was to determine a suitable basement topography grid using potential field data and then use this grid as the base for groundwater flow models. Geosoft Oasis Montaj GM-SYS 3D modeling software was used to build grids of subsurface layers and the model was constrained by well records of basement rock elevations located throughout the county. The study demonstrated that there is a complex network of crystalline basement structures that have been folded through tectonic activity during the Precambrian. A thick layer of iron rich sedimentary material was deposited on top of the basement rocks, causing a distinct magnetic signature that outlined the basement structure in the magnetic survey. Preliminary results reveal an iron layer with a density of 3.7 g/cm3 and magnetic susceptibility of 8000 x 10-6 cgs that is approximately 500 feet thick and ranges between elevations of -300 meters below and 400 meters above sea level. The 3-D model depths are consistent with depths from recent core drilling operations performed by the Wisconsin Geological and Natural History Survey. Knowing the depth to and structure of basement rock throughout Dodge County and Wisconsin plays an important role in understanding the geologic history of the region. Also, better resolution of the basement topography can enhance the accuracy of future groundwater flow models.

11. Photograph of a photograph in possession of Rock Island ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

11. Photograph of a photograph in possession of Rock Island Arsenal Historical Office. BASEMENT, SHOWING ORIGINAL OPEN INTERIOR PLAN. DATED APRIL 7, 1942. - Rock Island Arsenal, Building No. 56, North Avenue & East Avenue, Rock Island, Rock Island County, IL

Preliminary isostatic gravity map of the Sonoma volcanic field and vicinity, Sonoma and Napa Counties, California

USGS Publications Warehouse

Langenheim, V.E.; Roberts, C.W.; McCabe, C.A.; McPhee, D.K.; Tilden, J.E.; Jachens, R.C.

2006-01-01

This isostatic residual gravity map is part of a three-dimensional mapping effort focused on the subsurface distribution of rocks of the Sonoma volcanic field in Napa and Sonoma counties, northern California. This map will serve as a basis for modeling the shapes of basins beneath the Santa Rosa Plain and Napa and Sonoma Valleys, and for determining the location and geometry of faults within the area. Local spatial variations in the Earth's gravity field (after accounting for variations caused by elevation, terrain, and deep crustal structure explained below) reflect the distribution of densities in the mid to upper crust. Densities often can be related to rock type, and abrupt spatial changes in density commonly mark lithologic boundaries. High-density basement rocks exposed within the northern San Francisco Bay area include those of the Mesozoic Franciscan Complex and Great Valley Sequence present in the mountainous areas of the quadrangle. Alluvial sediment and Tertiary sedimentary rocks are characterized by low densities. However, with increasing depth of burial and age, the densities of these rocks may become indistinguishable from those of basement rocks. Tertiary volcanic rocks are characterized by a wide range in densities, but, on average, are less dense than the Mesozoic basement rocks. Isostatic residual gravity values within the map area range from about -41 mGal over San Pablo Bay to about 11 mGal near Greeg Mountain 10 km east of St. Helena. Steep linear gravity gradients are coincident with the traces of several Quaternary strike-slip faults, most notably along the West Napa fault bounding the west side of Napa Valley, the projection of the Hayward fault in San Pablo Bay, the Maacama Fault, and the Rodgers Creek fault in the vicinity of Santa Rosa. These gradients result from juxtaposing dense basement rocks against thick Tertiary volcanic and sedimentary rocks.

Structural investigations in the Massif-Central, France

NASA Technical Reports Server (NTRS)

Scanvic, J. Y.

1974-01-01

This survey covered the French Massif-Central (where crystalline and volcanic rocks outcrop) and its surrounding sedimentaries, Bassin de Paris, Bassin d'Aquitaine and Rhodanian valley. One objective was the mapping of fracturing and the surveying of its relationship with known ore deposits. During this survey it was found that ERTS imagery outlines lithology in some sedimentary basins. On the other hand, in a basement area, under temperature climate conditions, lithology is rarely expressed. These observations can be related to the fact that band 5 gives excellent results above sedimentary basins in France and generally band 7 is the most useful in a basement area. Several examples show clearly the value of ERTS imagery for mapping linear features and circular structures. All the main fractures are identified with the exception of new ones found both in sedimentaries and basement areas. Other interesting findings concern sun elevation which, stereoscopic effect not being possible, simulates relief in a better way under certain conditions.

Some Cenozoic hydrocarbon basins on the continental shelf of Vietnam

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dien, P.T.

1994-07-01

The formation of the East Vietnam Sea basins was related to different geodynamic processes. The pre-Oligocene basement consists of igneous, metamorphic, and metasediment complexes. The Cretaceous-Eocene basement formations are formed by convergence of continents after destruction of the Tethys Ocean. Many Jurassic-Eocene fractured magmatic highs of the Cuulong basin basement constitute important reservoirs that are producing good crude oil. The Paleocene-Eocene formations are characterized by intramountain metamolasses, sometimes interbedded volcanic rocks. Interior structures of the Tertiary basins connect with rifted branches of the widened East Vietnam Sea. Bacbo (Song Hong) basin is predominated by alluvial-rhythmic clastics in high-constructive deltas, whichmore » developed on the rifting and sagging structures of the continental branch. Petroleum plays are constituted from Type III source rocks, clastic reservoirs, and local caprocks. Cuulong basin represents sagging structures and is predominated by fine clastics, with tidal-lagoonal fine sandstone and shalestone in high-destructive deltas that are rich in Type II source rocks. The association of the pre-Cenozoic fractured basement reservoirs and the Oligocene-Miocene clastic reservoir sequences with the Oligocene source rocks and the good caprocks is frequently met in petroleum plays of this basin. Nan Conson basin was formed from complicated structures that are related to spreading of the oceanic branch. This basin is characterized by Oligocene epicontinental fine clastics and Miocene marine carbonates that are rich in Types I, II, and III organic matter. There are both pre-Cenozoic fractured basement reservoirs, Miocene buildup carbonate reservoir rocks and Oligocene-Miocene clastic reservoir sequences, in this basin. Pliocene-Quaternary sediments are sand and mud carbonates in the shelf facies of the East Vietnam Sea back-arc basin. Their great thickness provides good conditions for maturation and trapping.« less

Lithologies of the basement complex (Devonian and older) in the National Petroleum Reserve - Alaska

USGS Publications Warehouse

Dumoulin, Julie A.; Houseknecht, David W.

2001-01-01

Rocks of the basement complex (Devonian and older) were encountered in at least 30 exploratory wells in the northern part of the NPRA. Fine-grained, variably deformed sedimentary rocks deposited in a slope or basinal setting predominate and include varicolored (mainly red and green) argillite in the Simpson area, dark argillite and chert near Barrow, and widespread gray argillite. Chitinozoans of Middle-Late Ordovician and Silurian age occur in the dark argillite and chert unit. Sponge spicules and radiolarians establish a Phanerozoic age for the varicolored and gray argillite units, both of which contain local interbeds of chert-rich sandstone and silt-stone. Conglomerate and sandstone, also chert-rich but interbedded with mudstone and coal and of Early-Middle Devonian age, occur in the Topagoruk area; these strata formed in a fluvial environment. At East Teshekpuk, granite of probable Devonian age was penetrated. Brecciated, quartz-veined rock of uncertain protolith that may be part of the basement complex was encountered in the Ikpikpuk well. Seismic data indicate that angular unconformities truncate all sedimentary units of the basement complex in NPRA. Rocks correlative in age and lithofacies with the dark argillite and chert unit occur in the subsurface near Prudhoe Bay. Other argillite units in NPRA have similarities to basement rocks in the subsurface adjacent to ANWR and the Ordovician-Silurian Iviagik Group at Cape Lisburne, but lack the interbedded limestones found in the ANWR strata, and are less metamorphosed than, and compositionally distinct from, the Iviagik. The Topagoruk conglomerate and the East Teshekpuk granite resemble the Ulungarat formation and the Okpilak batholith, respectively, in the northeastern Brooks Range.

Whole-rock Pb and Sm-Nd isotopic constraints on the growth of southeastern Laurentia during Grenvillian orogenesis

USGS Publications Warehouse

Fisher, C.M.; Loewy, S.L.; Miller, C.F.; Berquist, P.; Van Schmus, W. R.; Hatcher, R.D.; Wooden, J.L.; Fullagar, P.D.

2010-01-01

The conventional view that the basement of the southern and central Appalachians represents juvenile Mesoproterozoic crust, the final stage of growth of Laurentia prior to Grenville collision, has recently been challenged. New whole-rock Pb and Sm-Nd isotopic data are presented from Meso protero zoic basement in the southern and central Appalachians and the Granite-Rhyolite province, as well as one new U-Pb zircon age from the Granite-Rhyolite province. These data, combined with existing data from Mesoproterozoic terranes throughout southeastern Laurentia, further substantiate recent suggestions that the southern and central Appalachian basement is exotic with respect to Laurentia. Sm-Nd isotopic compositions of most rocks from the southern and central Appalachian basement are consistent with progressive growth through reworking of the adjacent Granite-Rhyolite province. However, Pb isotopic data, including new analyses from important regions not sampled in previous studies, do not correspond with Pb isotopic compositions of any adjacent crust. The most distinct ages and isotopic compositions in the southern and central Appalachian basement come from the Roan Mountain area, eastern Tennessee-western North Carolina. The data set indicates U-Pb zircon ages up to 1.8 Ga for igneous rocks, inherited and detrital zircon ages >2.0 Ga, Sm-Nd depleted mantle model (TDM) ages >2.0 Ga, and the most elevated 207Pb/204Pb observed in southeastern Laurentia. The combined U-Pb geochronologic and Sm-Nd and Pb isotopic data preclude derivation of southern and central Appalachian basement from any nearby crustal material and demonstrate that Grenville age crust in southeastern Laurentia is exotic and probably was transferred during collision and assembly of Rodinia. These new data better define the boundary between the exotic southern and central Appalachian basement and adjacent Laurentian Granite-Rhyolite province. ?? 2010 Geological Society of America.

6. Photograph of a photograph in possession of Rock Island ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

6. Photograph of a photograph in possession of Rock Island Arsenal Historical Office. BASEMENT, SHOWING ORIGINAL OPEN INTERIOR FLOOR PLAN. DATED C. 1898. - Rock Island Arsenal, Building No. 62, Rodman Avenue between First & Second Streets, Rock Island, Rock Island County, IL

Geology along the Blue Ridge Parkway in Virginia

USGS Publications Warehouse

Carter, Mark W.; Southworth, C. Scott; Tollo, Richard P.; Merschat, Arthur J.; Wagner, Sara; Lazor, Ava; Aleinikoff, John N.

2017-01-01

Detailed geologic mapping and new SHRIMP (sensitive high-resolution ion microprobe) U-Pb zircon, Ar/Ar, Lu-Hf, 14C, luminescence (optically stimulated), thermochronology (fission-track), and palynology reveal the complex Mesoproterozoic to Quaternary geology along the ~350 km length of the Blue Ridge Parkway in Virginia. Traversing the boundary of the central and southern Appalachians, rocks along the parkway showcase the transition from the para-autochthonous Blue Ridge anticlinorium of northern and central Virginia to the allochthonous eastern Blue Ridge in southern Virginia. From mile post (MP) 0 near Waynesboro, Virginia, to ~MP 124 at Roanoke, the parkway crosses the unconformable to faulted boundary between Mesoproterozoic basement in the core of the Blue Ridge anticlinorium and Neoproterozoic to Cambrian metasedimentary and metavolcanic cover rocks on the western limb of the structure. Mesoproterozoic basement rocks comprise two groups based on SHRIMP U-Pb zircon geochronology: Group I rocks (1.2-1.14 Ga) are strongly foliated orthogneisses, and Group II rocks (1.08-1.00 Ga) are granitoids that mostly lack obvious Mesoproterozoic deformational features.Neoproterozoic to Cambrian cover rocks on the west limb of the anticlinorium include the Swift Run and Catoctin Formations, and constituent formations of the Chilhowee Group. These rocks unconformably overlie basement, or abut basement along steep reverse faults. Rocks of the Chilhowee Group are juxtaposed against Cambrian rocks of the Valley and Ridge province along southeast- and northwest-dipping, high-angle reverse faults. South of the James River (MP 64), Chilhowee Group and basement rocks occupy the hanging wall of the nearly flat-lying Blue Ridge thrust fault and associated splays.South of the Red Valley high-strain zone (MP 144.5), the parkway crosses into the wholly allochthonous eastern Blue Ridge, comprising metasedimentary and meta-igneous rocks assigned to the Wills Ridge, Ashe, and Alligator Back Formations. These rocks are bound by numerous faults, including the Rock Castle Creek fault that separates Ashe Formation rocks from Alligator Back Formation rocks in the core of the Ararat River synclinorium. The lack of unequivocal paleontologic or geochronologic ages for any of these rock sequences, combined with fundamental and conflicting differences in tectonogenetic models, compound the problem of regional correlation with Blue Ridge cover rocks to the north.The geologic transition from the central to southern Appalachians is also marked by a profound change in landscape and surficial deposits. In central Virginia, the Blue Ridge consists of narrow ridges that are held up by resistant but contrasting basement and cover lithologies. These ridges have shed eroded material from their crests to the base of the mountain fronts in the form of talus slopes, debris flows, and alluvial-colluvial fans for perhaps 10 m.y. South of Roanoke, however, ridges transition into a broad hilly plateau, flanked on the east by the Blue Ridge escarpment and the eastern Continental Divide. Here, deposits of rounded pebbles, cobbles, and boulders preserve remnants of ancestral west-flowing drainage systems.Both bedrock and surficial geologic processes provide an array of economic deposits along the length of the Blue Ridge Parkway corridor in Virginia, including base and precious metals and industrial minerals. However, common stone was the most important commodity for creating the Blue Ridge Parkway, which yielded building stone for overlooks and tunnels, or crushed stone for road base and pavement.

A record of spontaneous subduction initiation in the Izu-Bonin-Mariana arc

NASA Astrophysics Data System (ADS)

Arculus, Richard J.; Ishizuka, Osamu; Bogus, Kara A.; Gurnis, Michael; Hickey-Vargas, Rosemary; Aljahdali, Mohammed H.; Bandini-Maeder, Alexandre N.; Barth, Andrew P.; Brandl, Philipp A.; Drab, Laureen; Do Monte Guerra, Rodrigo; Hamada, Morihisa; Jiang, Fuqing; Kanayama, Kyoko; Kender, Sev; Kusano, Yuki; Li, He; Loudin, Lorne C.; Maffione, Marco; Marsaglia, Kathleen M.; McCarthy, Anders; Meffre, Sebastién; Morris, Antony; Neuhaus, Martin; Savov, Ivan P.; Sena, Clara; Tepley, Frank J., III; van der Land, Cees; Yogodzinski, Gene M.; Zhang, Zhaohui

2015-09-01

The initiation of tectonic plate subduction into the mantle is poorly understood. If subduction is induced by the push of a distant mid-ocean ridge or subducted slab pull, we expect compression and uplift of the overriding plate. In contrast, spontaneous subduction initiation, driven by subsidence of dense lithosphere along faults adjacent to buoyant lithosphere, would result in extension and magmatism. The rock record of subduction initiation is typically obscured by younger deposits, so evaluating these possibilities has proved elusive. Here we analyse the geochemical characteristics of igneous basement rocks and overlying sediments, sampled from the Amami Sankaku Basin in the northwest Philippine Sea. The uppermost basement rocks are areally widespread and supplied via dykes. They are similar in composition and age--as constrained by the biostratigraphy of the overlying sediments--to the 52-48-million-year-old basalts in the adjacent Izu-Bonin-Mariana fore-arc. The geochemical characteristics of the basement lavas indicate that a component of subducted lithosphere was involved in their genesis, and the lavas were derived from mantle source rocks that were more melt-depleted than those tapped at mid-ocean ridges. We propose that the basement lavas formed during the inception of Izu-Bonin-Mariana subduction in a mode consistent with the spontaneous initiation of subduction.

Polymetamorphic evolution of the granulite-facies Paleoproterozoic basement of the Kabul Block, Afghanistan

NASA Astrophysics Data System (ADS)

Collett, Stephen; Faryad, Shah Wali; Mosazai, Amir Mohammad

2015-08-01

The Kabul Block is an elongate crustal fragment which cuts across the Afghan Central Blocks, adjoining the Indian and Eurasian continents. Bounded by major strike slip faults and ophiolitic material thrust onto either side, the block contains a strongly metamorphosed basement consisting of some of the only quantifiably Proterozoic rocks south of the Herat-Panjshir Suture Zone. The basement rocks crop-out extensively in the vicinity of Kabul City and consist predominantly of migmatites, gneisses, schists and small amounts of higher-grade granulite-facies rocks. Granulite-facies assemblages were identified in felsic and mafic siliceous rocks as well as impure carbonates. Granulite-facies conditions are recorded by the presence of orthopyroxene overgrowing biotite in felsic rocks; by orthopyroxene overgrowing amphibole in mafic rocks and by the presence of olivine and clinohumite in the marbles. The granulite-facies assemblages are overprinted by a younger amphibolite-facies event that is characterized by the growth of garnet at the expense of the granulite-facies phases. Pressure-temperature (P-T) conditions for the granulite-facies event of around 850 °C and up to 7 kbar were calculated through conventional thermobarometry and phase equilibria modeling. The younger, amphibolite-facies event shows moderately higher pressures of up to 8.5 kbar at around 600 °C. This metamorphism likely corresponds to the dominant metamorphic event within the basement of the Kabul Block. The results of this work are combined with the litho-stratigraphic relations and recent geochronological dating to analyze envisaged Paleoproterozoic and Neoproterozoic metamorphic events in the Kabul Block.

5. Photograph of a photograph in possession of Rock Island ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

5. Photograph of a photograph in possession of Rock Island Arsenal Historical Office. BASEMENT, SHOWING ASSEMBLING OF ARTILLERY GUN CARRIAGES. DATED MAY 12, 1904. - Rock Island Arsenal, Building No. 108, Rodman Avenue between Third & Fourth Streets, Rock Island, Rock Island County, IL

Site Characterization for a Deep Borehole Field Test

NASA Astrophysics Data System (ADS)

Kuhlman, K. L.; Hardin, E. L.; Freeze, G. A.; Sassani, D.; Brady, P. V.

2015-12-01

The US Department of Energy Office of Nuclear Energy is at the beginning of 5-year Deep Borehole Field Test (DBFT) to investigate the feasibility of constructing and characterizing two boreholes in crystalline basement rock to a depth of 5 km (16,400 ft). The concept of deep borehole disposal for radioactive waste has some advantages over mined repositories, including incremental construction and loading, the enhanced natural barriers provided by deep continental crystalline basement, and reduced site characterization. Site characterization efforts need to determine an eligible site that does not have the following disqualifying characteristics: greater than 2 km to crystalline basement, upward vertical fluid potential gradients, presence of economically exploitable natural resources, presence of high permeability connection to the shallow subsurface, and significant probability of future seismic or volcanic activity. Site characterization activities for the DBFT will include geomechanical (i.e., rock in situ stress state, and fluid pressure), geological (i.e., rock and fracture infill lithology), hydrological (i.e., quantity of fluid, fluid convection properties, and solute transport mechanisms), and geochemical (i.e., rock-water interaction and natural tracers) aspects. Both direct (i.e., sampling and in situ testing) and indirect (i.e., borehole geophysical) methods are planned for efficient and effective characterization of these site aspects and physical processes. Borehole-based characterization will be used to determine the variability of system state (i.e., stress, pressure, temperature, and chemistry) with depth, and interpretation of material and system parameters relevant to numerical site simulation. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

13. Photograph of line drawing in possession of Engineering Plans ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

13. Photograph of line drawing in possession of Engineering Plans and Services Division, Rock Island Arsenal. BASEMENT PLAN, 1920. - Rock Island Arsenal, Building No. 90, East Avenue between North Avenue & King Drive, Rock Island, Rock Island County, IL

Constraints in the hot-dry-rock resources of the united states

USGS Publications Warehouse

Sass, John; Guffanti, Marianne; ,

1993-01-01

As with hydrothermal systems, the western U.S has higher HDR potential overall than the eastern U.S. because geothermal gradients on average are higher in the west. Nevertheless, some attractive exploration targets occur in the eastern U.S. The most favorable target in the eastern U.S. (defined here to include the Great Plains province) is one in which the heat flow from the basement rocks is higher than average, either due to heat generation from highly radioactive rocks or to a plume of hot water driven upwards from greater depths by convection, and where such basement rocks are blanketed by one or more kilometers of sedimentary material having a low thermal conductivity.

7. Photograph of a photograph in possession of Rock Island ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

7. Photograph of a photograph in possession of Rock Island Arsenal Historical Office. BASEMENT, SHOWING ASSEMBLING OF 75MM GUN CARRIAGES. DATED AUGUST 23, 1918. - Rock Island Arsenal, Building No. 110, Rodman Avenue between Fourth Street & East Avenue, Rock Island, Rock Island County, IL

Paleoproterozoic mojaveprovince in northwestern Mexico? Isotopic and U-Pb zircon geochronologic studies of precambrian and Cambrian crystalline and sedimentary rocks, Caborca, Sonora

USGS Publications Warehouse

Lang, Farmer G.; Bowring, S.A.; Matzel, J.; Maldonado, G.E.; Fedo, C.; Wooden, J.

2005-01-01

Whole-rock Nd isotopic data and U-Pb zircon geochronology from Precambrian crystalline rocks in the Caborca area, northern Sonora, reveal that these rocks are most likely a segment of the Paleoproterozoic Mojave province. Supporting this conclusion are the observations that paragneiss from the ??? 1.75 Ga Bamori Complex has a 2.4 Ga Nd model age and contains detrital zircons ranging in age from Paleo- proterozoic (1.75 Ga) to Archean (3.2 Ga). Paragneisses with similar age and isotopic characteristics occur in the Mojave province in southern California. In addition, "A-type" granite exposed at the southern end of Cerro Rajon has ca 2.0 Ga Nd model age and a U-Pb zircon age of 1.71 Ga, which are similar to those of Paleoproterozoic granites in the Mojave province. Unlike the U.S. Mojave province, the Caborcan crust contains ca. 1.1 Ga granite (Aibo Granite), which our new Nd isotopic data suggest is largely the product of anatexis of the local Precambrian basement. Detrital zircons from Neoproterozoic to early Cambrian miogeoclinal arenites at Caborca show dominant populations ca. 1.7 Ga, ca. 1.4 Ga, and ca. 1.1 Ga, with subordinate Early Cambrian and Archean zircons. These zircons were likely derived predominately from North American crust to the east and northeast, and not from the underlying Caborcan basement. The general age and isotopic similarities between Mojave province basement and overlying miogeoclinal sedimentary rocks in Sonora and southern California is necessary, but not sufficient, proof of the hypothesis that Sonoran crust is allochthonous and was transported to its current position during the Mesozoic along the proposed Mojave-Sonora megashear. One viable alternative model is that the Caborcan Precambrian crust is an isolated, autochthonous segment of Mojave province crust that shares a similar, but not identical, Proterozoic geological history with Mojave province crust found in the southwest United States ?? 2005 Geological Society of America.

basement reservoir geometry and properties

NASA Astrophysics Data System (ADS)

Walter, bastien; Geraud, yves; Diraison, marc

2017-04-01

Basement reservoirs are nowadays frequently investigated for deep-seated fluid resources (e.g. geothermal energy, groundwater, hydrocarbons). The term 'basement' generally refers to crystalline and metamorphic formations, where matrix porosity is negligible in fresh basement rocks. Geothermal production of such unconventional reservoirs is controlled by brittle structures and altered rock matrix, resulting of a combination of different tectonic, hydrothermal or weathering phenomena. This work aims to characterize the petro-structural and petrophysical properties of two basement surface analogue case studies in geological extensive setting (the Albert Lake rift in Uganda; the Ifni proximal margin of the South West Morocco Atlantic coast). Different datasets, using field structural study, geophysical acquisition and laboratory petrophysical measurements, were integrated to describe the multi-scale geometry of the porous network of such fractured and weathered basement formations. This study points out the multi-scale distribution of all the features constituting the reservoir, over ten orders of magnitude from the pluri-kilometric scale of the major tectonics structures to the infra-millimetric scale of the secondary micro-porosity of fractured and weathered basements units. Major fault zones, with relatively thick and impermeable fault core structures, control the 'compartmentalization' of the reservoir by dividing it into several structural blocks. The analysis of these fault zones highlights the necessity for the basement reservoirs to be characterized by a highly connected fault and fracture system, where structure intersections represent the main fluid drainage areas between and within the reservoir's structural blocks. The suitable fluid storage areas in these reservoirs correspond to the damage zone of all the fault structures developed during the tectonic evolution of the basement and the weathered units of the basement roof developed during pre-rift exhumation phases. Macroscopic fracture density is highly dependent on the petrographic nature of the basement, with values up to 80 frac./m in fault damage zones of crystalline rocks. Dense micro-cracks associated to major fault structures can develop porosity and permeability up to 10% and 0.1 D. In some weathered horizons, alteration can develop matrix porosity up to 40% and the permeability reaches up to 1D. This study highlights therefore that basement reservoir properties are the result of the long geodynamic evolution of such formations, and the different fault zone compartments or weathering horizons have to be considered separately for reservoir understanding.

Precambrian basement geologic map of Montana; an interpretation of aeromagnetic anomalies

USGS Publications Warehouse

Sims, P.K.; O'Neill, J. M.; Bankey, Viki; Anderson, E.

2004-01-01

Newly compiled aeromagnetic anomaly data of Montana, in conjunction with the known geologic framework of basement rocks, have been combined to produce a new interpretive geologic basement map of Montana. Crystalline basement rocks compose the basement, but are exposed only in the cores of mountain ranges in southwestern Montana. Principal features deduced from the map are: (1) A prominent northeast-trending, 200-km-wide zone of spaced negative anomalies, which extends more than 700 km from southwestern Montana's Beaverhead Mountains to the Canadian border and reflects suturing of the Archean Mexican Hat Block against the Archean Wyoming Province along the Paleoproterozoic Trans-Montana Orogen (new name) at about 1.9-1.8 Ga; (2) North-northwest-trending magnetic lows in northeastern Montana, which reflect the 1.9-1.8 Ga Trans-Hudson Orogen and truncate the older Trans-Montana Zone; and (3) Subtle northwest- and west-trending negative anomalies in central and western Montana, which represent the northernmost segment of brittle-ductile transcurrent faults of the newly recognized Mesoproterozoic Trans-Rocky Mountain fault system. Structures developed in the Proterozoic provided zones of crustal weakness reactivated during younger Proterozoic and Phanerozoic igneous and tectonic activity. For example, the Trans-Montana Zone guided basement involved thrust faulting in southwestern Montana during the Sevier Orogeny. The Boulder Batholith and associated ore deposits and the linear belt of alkaline intrusions to the northeast were localized along a zone of weakness between the Missouri River suture and the Dillon shear zone of the Trans-Montana Orogen. The northwest-trending faults of Trans-Rocky Mountain system outline depocenters for sedimentary rocks in the Belt Basin. This fault system provided zones of weakness that guided Laramide uplifts during basement crustal shortening. Northwest-trending zones have been locally reactivated during Neogene basin-range extension.

Magnetically inferred basement structure in central Saudi Arabia

USGS Publications Warehouse

Johnson, P.R.; Stewart, I.C.F.

1995-01-01

A compilation of magnetic data acquired during the past three decades for a region in central Saudi Arabia where Precambrian basement is partly exposed on the Arabian shield and partly concealed by overlying Phanerozoic strata, shows a central sector of conspicuous N-S-trending anomalies, a heterogeneous western sector of short-wavelength, high-intensity anomalies, and an eastern sector of low- to moderate-intensity broad-wavelength anomalies. Anomalies in the western and central sectors correlate with Neoproterozoic metavolcanic, metasedimentary, and intrusive rocks of the Arabian shield and are interpreted as delineating extensions of shield-type rocks down-dip beneath Phanerozoic cover. These rocks constitute terranes making up part of a Neoproterozoic orogenic belt that underlies Northeast Africa and western Arabia and it is proposed that their magnetically indicated easternmost extent marks the concealed eastern edge of the orogenic belt in central Arabia. The flat magnetic signature of the eastern sector, not entirely accounted for as an effect of deep burial, may reflect the presence of a crustal block different in character to the terranes of the orogenic belt and, speculatively, may outline a continental block that, according to some tectonic models of the region, collided with the Neoproterozoic terranes and thereby caused their deformation and tectonic accretion.

Basement diapirism associated with the emplacement of major ophiolite nappes: Some constraints

NASA Astrophysics Data System (ADS)

Andrews-Speed, C. P.; Johns, C. C.

1985-09-01

The association of basement uplifts with major ophiolite nappes in some Phanerozoic orogenic belts suggests that gravitational instability results in the local diapiric uplift of the basement following ophiolite emplacement. In previous analyses of diapirism in crustal silicate rocks, viscous behaviour of rocks has been assumed. It is argued that this assumption is not valid. An alternative analysis is offered to determine whether or not the stress would be sufficient for diapirism to occur. The negative buoyancy stress resulting from the emplacement of an ophiolite nappe 5-15 km thick onto continental basement may be in the range 10-50 MPa. If the horizontal deviatoric stress is zero, this will be the maximum principal compressive stress. After ophiolite emplacement the thermal profile through the ophiolite and the basement will relax from a saw-tooth form to an equilibrium profile. If the ophiolite is young and thick there will be a zone of ductile strain in the lower part of the ophiolite and in the upper part of the continental basement. Results from steady-state creep experiments suggest that temperatures in this zone may be high enough for a short time after ophiolite emplacement (3 Ma or more) for the rocks in this zone to deform at geologically significant strain rates (10 -14 or greater) in response to the negative buoyancy stress. A thin ophiolite or rapid erosion will result in this ductile zone being absent or too short-lived for significant strain. Aquaeous fluids may reduce the strength of brittle rocks by decreasing the effective normal stress or by encouraging pressure solution creep. Evidence suggests that the deviatoric stress across presently active faults may be as low at 10 MPa. Thus diapirism in response to ophiolite emplacement may occur through brittle strain. Gravity spreading within the ophiolite is an alternative mechanism for accommodating the gravitational instability. The critical evidence lies in the field.

Geologic Map of Prescott National Forest and the Headwaters of the Verde River, Yavapai and Coconino Counties, Arizona

USGS Publications Warehouse

DeWitt, Ed; Langenheim, V.E.; Force, Eric; Vance, R.K.; Lindberg, P.A.; Driscoll, R.L.

2008-01-01

This 1:100,000-scale digital geologic map details the complex Early Proterozoic metavolcanic and plutonic basement of north-central Arizona; shows the mildly deformed cover of Paleozoic rocks; reveals where Laramide to mid-Tertiary plutonic rocks associated with base- and precious-metals deposits are exposed; subdivides the Tertiary volcanic rocks according to chemically named units; and maps the Pliocene to Miocene fill of major basins. Associated digital files include more than 1,300 geochemical analyses of all rock units; 1,750 logs of water wells deeper than 300 feet; and interpreted logs of 300 wells that define the depth to basement in major basins. Geophysically interpreted buried features include normal faults defining previous unknown basins, mid-Tertiary intrusive rocks, and half-grabens within shallow bains.

Structural framework and hydrocarbon potential of Ross Sea, Antarctica

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cooper, A.K.; Davey, F.J.

The 400 to 1100-m deep continental shelf of the Ross Sea is underlain by three major sedimentary basins (Eastern basin, Central trough, and Victoria Land basin), which contain 5 to 6 km of sedimentary rock of Late Cretaceous(.) and younger age. An addition 6 to 7 km of older sedimentary and volcanic rocks lie within the Victoria Land basin. Eroded basement ridges of early Paleozoic(.) and older rocks similar to those of onshore Victoria Land separate the basins. The three basins formed initially in late Mesozoic time during an early period of rifting between East and West Antarctica. The Easternmore » basin is a 300-km wide, asymmetric basement trough that structurally opens into the Southern Ocean. A seaward-prograding sequence of late Oligocene and younger glacial deposits covers a deeper, layered sequence of Paleogene(.) and older age. The Central trough, a 100-km wide depression, is bounded by basement block faults and is filled with a nearly flat-lying sedimentary section. A prominent positive gravity anomaly, possibly caused by rift-related basement rocks, lies along the axis of the basin. The Victoria Land basin, unlike the other two basins, additionally contains a Paleogene(.) to Holocene rift zone, the Terror Rift. Rocks in the rift, near the axis of the 150-km wide basement half-graben, show extensive shallow faulting and magmatic intrusion of the sedimentary section. The active Terror rift and older basin structures extend at least 300 km along the base of the Transantarctic Mountains. Petroleum hydrocarbons have not been reported in the Ross Sea region, with possible exception of ethane gas found in Deep Sea Drilling Project cores from the Eastern basin. Model studies indicate that hydrocarbons could be generated at depths of 3.5 to 6 km within the sedimentary section. The best structures for hydrocarbon entrapment occur in the Victoria Land basin and associated Terror Rift.« less

Provenance of sediments from Sumatra, Indonesia - Insights from detrital U-Pb zircon geochronology, heavy mineral analyses and Raman spectroscopy

NASA Astrophysics Data System (ADS)

Liebermann, C.; Hall, R.; Gough, A.

2017-12-01

The island of Sumatra is situated at the southwestern margin of the Indonesian archipelago. Although it is the sixth largest island in the world, the geology of the Sumatra sedimentary basins and their underlying basement is relatively poorly understood in terms of their provenance. This work is a multi-proxy provenance study utilizing U-Pb detrital zircon dating by LA-ICP-MS combined with optical and Raman spectroscopy-based heavy mineral analysis. It will help to unravel the stratigraphy of Sumatra, contribute to paleogeographic reconstruction of western SE Asia, and aid a wider understanding of Sumatran petroleum plays. Thin section analyses, heavy mineral assemblages, and >3500 concordant U-Pb zircon ages, from samples acquired during two fieldwork seasons indicate a mixed provenance for Cenozoic sedimentary formations, including both local igneous sources and mature basement rocks. Characteristic Precambrian zircon age spectra are found in all analysed Cenozoic sedimentary strata. These can be correlated with zircon age populations found in Sumatran basement rocks; Neoproterozoic and Mesoproterozoic age groups are dominant (c. 500-600 Ma, c. 850-1000 Ma, c. 1050-1200 Ma). Paleoproterozoic to Archaean zircons occur as minor populations. The Phanerozoic age spectra of the Cenozoic formations are characterised by distinct Carboniferous, Permo-Triassic, and Jurassic-Cretaceous zircon populations. Permo-Triassic zircons are interpreted to come from granitoids in the Malay peninsula or Sumatra itself. Eocene to Lower Miocene strata are characterised by ultrastable heavy minerals such as zircon, tourmaline, and rutile, which together with garnet, suggest the principal sources were igneous and metamorphic basement rocks. Cenozoic zircons appear only from the Middle Miocene onwards. This change is interpreted to indicate a new contribution from a local volcanic arc, and is supported by the occurrence of unstable heavy minerals such as apatite and clinopyroxene, and the presence of volcanic quartz. The absence of an earlier volcanic contribution is surprising since subduction is widely considered to have been active from the Eocene.

Geologic map of Harrat Hutaymah, with petrologic classification and distribution of ultramafic inclusions, Saudi Arabia

USGS Publications Warehouse

Thornber, Carl R.

1990-01-01

This map shows detailed geology of the Quaternary and Tertiary volcanic deposits that comprise Harrat Hutaymah and an updated and generalized compilation of the underlying Proterozoic and Paleozoic basement rocks. Quaternary alluvial cover and details of basement geology (that is, faults, dikes, and other features) are not shown. Volcanic unit descriptions and contact relations are based upon field investigation by the author and on compilation and revision of mapping Kellogg (1984; northern half of area) and Pallister (1984; southern half of area). A single K-Ar date of 1.80 ± 0.05 Ma for an alkali olivine basalt flow transected by the Al Hutaymah tuff ring (Pallister, 1984) provides the basis for an estimated late Tertiary to Quaternary age range for all harrat volcanic units other than unit Qtr (tuff reworked during Quaternary age time). Contact relations and unit descriptions for the basement rocks were compiled from Pallister (1984), Kellogg (1984 and 1985), DuBray (1984), Johnson and Williams (1984), Vaslet and others (1987), Cole and Hedge (1986), and Richter and others (1984). All rock unit names in this report are informal and capitalization follows Saudi Arabian stratigraphic nomenclature (Fitch, 1980). Geographic information was compiled from Pallister (1984), Kellogg (1984), and Fuller (in Johnson and Williams, 1984) and from field investigation by the author in 1986. The pie diagrams on the map show the distribution and petrology of ultramafic xenoliths of Harrat Hutaymah. The pie diagrams are explained by a detailed classification of ultramafic xenoliths that is introduced in this report.

A regional view of urban sedimentary basins in Northern California based on oil industry compressional-wave velocity and density logs

USGS Publications Warehouse

Brocher, T.M.

2005-01-01

Compressional-wave (sonic) and density logs from 119 oil test wells provide knowledge of the physical properties and impedance contrasts within urban sedimentary basins in northern California, which is needed to better understand basin amplification. These wire-line logs provide estimates of sonic velocities and densities for primarily Upper Cretaceous to Pliocene clastic rocks between 0.1 - and 5.6-km depth to an average depth of 1.8 km. Regional differences in the sonic velocities and densities in these basins largely 1reflect variations in the lithology, depth of burial, porosity, and grain size of the strata, but not necessarily formation age. For example, Miocene basin filling strata west of the Calaveras Fault exhibit higher sonic velocities and densities than older but finer-grained and/or higher-porosity rocks of the Upper Cretaceous Great Valley Sequence. As another example, hard Eocene sandstones west of the San Andreas Fault have much higher impedances than Eocene strata, mainly higher-porosity sandstones and shales, located to the east of this fault, and approach those expected for Franciscan Complex basement rocks. Basement penetrations define large impedence contrasts at the sediment/basement contact along the margins of several basins, where Quaternary, Pliocene, and even Miocene deposits directly overlie Franciscan or Salinian basement rocks at depths as much as 1.7 km. In contrast, in the deepest, geographic centers of the basins, such logs exhibit only a modest impedance contrast at the sediment/basement contact at depths exceeding 2 km. Prominent (up to 1 km/sec) and thick (up to several hundred meters) velocity and density reversals in the logs refute the common assumption that velocities and densities increase monotonically with depth.

Nature of the basement of the East Anatolian plateau: Implications for the lithospheric foundering processes

NASA Astrophysics Data System (ADS)

Topuz, G.; Candan, O.; Zack, T.; Yılmaz, A.

2017-12-01

The East Anatolian Plateau (Turkey) is characterized by (1) an extensive volcanic-sedimentary cover of Neogene to Quaternary age, (2) crustal thicknesses of 42-50 km, and (3) an extremely thinned lithospheric mantle. Its basement beneath the young cover is thought to consist of oceanic accretionary complexes of Late Cretaceous to Oligocene age. The attenuated state of the lithospheric mantle and the causes of the young volcanism are accounted for by slab steepening and subsequent break-off. We present field geological, petrological and geochronological data on three basement inliers (Taşlıçay, Akdağ and Ilıca) in the region. These areas are made up of amphibolite- to granulite-facies rocks, comprising marble, amphibolite, metapelite, quartzite and metagranite. The granulite-facies domain is equilibrated at 0.7 GPa and 800 ˚C at 83 ± 2 Ma (2σ). The metamorphic rocks are intruded by subduction-related coeval gabbroic, quartz monzonitic to tonalitic rocks. Both the metamorphic rocks and the intrusions are tectonically overlain by ophiolitic rocks. All these crystalline rocks are unconformably overlain by lower Maastrichtien clastic rocks and reefal limestone, suggesting that the exhumation at the earth's surface and juxtaposition with ophiolitic rocks occurred by early Maastrichtien. U-Pb dating on igneous zircon from metagranite yielded a protolith age of 445 ± 10 Ma (2σ). The detrital zircons from a metaquartzite point to Neoproterozoic to Early Paleozoic provenance. All these data favor a more or less continuous continental substrate to the allochthonous ophiolitic rocks beneath the young volcanic-sedimentary cover. The metamorphism and coeval magmatism can be regarded as the middle- to lower-crustal root of the Late Cretaceous magmatic arc that developed due to northward subduction along the Bitlis-Zagros suture. The presence of a continental basement beneath the young cover requires that the loss of the lithospheric mantle from beneath the East Anatolian plateau have resulted from other processes of lithospheric foundering, rather than just slab steepening and break-off. This research is funded by a research grant (#114Y228) from TÜBİTAK.

Tectonics of some Amazonian greenstone belts

NASA Technical Reports Server (NTRS)

Gibbs, A. K.

1986-01-01

Greenstone belts exposed amid gneisses, granitoid rocks, and less abundant granulites along the northern and eastern margins of the Amazonian Craton yield Trans-Amazonican metamorphic ages of 2.0-2.1 Ga. Early proterozoic belts in the northern region probably originated as ensimatic island arc complexes. The Archean Carajas belt in the southeastern craton probably formed in an extensional basin on older continental basement. That basement contains older Archean belts with pillow basalts and komatiites. Belts of ultramafic rocks warrant investigatijon as possible ophiolites. A discussion follows.

Faulting, fracturing and in situ stress prediction in the Ahnet Basin, Algeria — a finite element approach

NASA Astrophysics Data System (ADS)

Beekman, Fred; Badsi, Madjid; van Wees, Jan-Diederik

2000-05-01

Many low-efficiency hydrocarbon reservoirs are productive largely because effective reservoir permeability is controlled by faults and natural fractures. Accurate and low-cost information on basic fault and fracture properties, orientation in particular, is critical in reducing well costs and increasing well recoveries. This paper describes how we used an advanced numerical modelling technique, the finite element method (FEM), to compute site-specific in situ stresses and rock deformation and to predict fracture attributes as a function of material properties, structural position and tectonic stress. Presented are the numerical results of two-dimensional, plane-strain end-member FEM models of a hydrocarbon-bearing fault-propagation-fold structure. Interpretation of the modelling results remains qualitative because of the intrinsic limitations of numerical modelling; however, it still allows comparisons with (the little available) geological and geophysical data. In all models, the weak mechanical strength and flow properties of a thick shale layer (the main seal) leads to a decoupling of the structural deformation of the shallower sediments from the underlying sediments and basement, and results in flexural slip across the shale layer. All models predict rock fracturing to initiate at the surface and to expand with depth under increasing horizontal tectonic compression. The stress regime for the formation of new fractures changes from compressional to shear with depth. If pre-existing fractures exist, only (sub)horizontal fractures are predicted to open, thus defining the principal orientation of effective reservoir permeability. In models that do not include a blind thrust fault in the basement, flexural amplification of the initial fold structure generates additional fracturing in the crest of the anticline controlled by the material properties of the rocks. The folding-induced fracturing expands laterally along the stratigraphic boundaries under enhanced tectonic loading. Models incorporating a blind thrust fault correctly predict the formation of secondary syn- and anti-thetic mesoscale faults in the basement and sediments of the hanging wall. Some of these faults cut reservoir and/or seal layers, and thus may influence effective reservoir permeability and affect seal integrity. The predicted faults divide the sediments across the anticline in several compartments with different stress levels and different rock failure (and proximity to failure). These numerical model outcomes can assist classic interpretation of seismic and well bore data in search of fractured and overpressured hydrocarbon reservoirs.

Geophysical constraints on Rio Grande rift structure and stratigraphy from magnetotelluric models and borehole resistivity logs, northern New Mexico

USGS Publications Warehouse

Rodriguez, Brian D.; Sawyer, David A.; Hudson, Mark R.; Grauch, V.J.S.

2013-01-01

Two- and three-dimensional electrical resistivity models derived from the magnetotelluric method were interpreted to provide more accurate hydrogeologic parameters for the Albuquerque and Española Basins. Analysis and interpretation of the resistivity models are aided by regional borehole resistivity data. Examination of the magnetotelluric response of hypothetical stratigraphic cases using resistivity characterizations from the borehole data elucidates two scenarios where the magnetotelluric method provides the strongest constraints. In the first scenario, the magnetotelluric method constrains the thickness of extensive volcanic cover, the underlying thickness of coarser-grained facies of buried Santa Fe Group sediments, and the depth to Precambrian basement or overlying Pennsylvanian limestones. In the second scenario, in the absence of volcanic cover, the magnetotelluric method constrains the thickness of coarser-grained facies of buried Santa Fe Group sediments and the depth to Precambrian basement or overlying Pennsylvanian limestones. Magnetotelluric surveys provide additional constraints on the relative positions of basement rocks and the thicknesses of Paleozoic, Mesozoic, and Tertiary sedimentary rocks in the region of the Albuquerque and Española Basins. The northern extent of a basement high beneath the Cerros del Rio volcanic field is delineated. Our results also reveal that the largest offset of the Hubbell Spring fault zone is located 5 km west of the exposed scarp. By correlating our resistivity models with surface geology and the deeper stratigraphic horizons using deep well log data, we are able to identify which of the resistivity variations in the upper 2 km belong to the upper Santa Fe Group sediment

Isotopic, geochemical, and temporal characterization of Proterozoic basement rocks in the Quitovac region, northwestern Sonora, Mexico: Implications for the reconstruction of the southwestern margin of Laurentia

USGS Publications Warehouse

Iriondo, A.; Premo, W.R.; Martínez-Torres, Luis M.; Budahn, J.R.; Atkinson, William W.; Siems, D.F.; Guaras-Gonzalez, B.

2004-01-01

A detailed geochemical characterization of 19 representative Proterozoic basement rocks in the Quitovac region in northwestern Sonora, Mexico, has identified two distinct Paleoproterozoic basement blocks that coincide spatially with the previously proposed Caborca and "North America" blocks. New U-Pb zircon geochronology revises their age ranges, the Caborca (1.78-1.69 Ga) and "North America" (1.71-1.66 Ga) blocks at Quitovac, and precludes a simple age differentiation between them. In addition, Grenvillian-age granitoids (ca. 1.1 Ga), spatially associated with the Caborca block have been identified at Quitovac. Nd isotopes and major- and trace-element geochemistry support the distinction of these Paleoproterozoic blocks. Granitoids of the "North America" block are characterized by depleted ??Nd values (3.4-3.9) and younger Nd model ages (1800-1740 Ma) and have lower K2O, Y, Rb, Ba, Th, REE, and Fe/Mg values than coeval rocks of the Caborca block. The Caborca block granitoids are likewise characterized by slightly less depleted ??Nd (0.6-2.6) and older Nd model ages (2070-1880 Ma). Despite the subtle differences, granitoids from both the Caborca and "North America" blocks exhibit island arc-like affinities. We propose that the Proterozoic basement rocks from the Quitovac region are an extension of the Proterozoic crustal provinces in the southwestern United States. Specifically, rocks of the Caborca block exhibit an affinity to rocks of either the Yavapai province or the Mojave-Yavapai transition zone, whereas rocks of the "North America" block have signatures similar to those of the Mazatzal province or possibly the Yavapai province of Arizona. The new isotopic ages and geochemical data do not support the existence of the Late Jurassic Mojave-Sonora megashear at Quitovac, as originally proposed. However, the Quitovac region accounts only for a small fraction of the Proterozoic basement in Sonora, so these findings do not eliminate the possibility of a megashear elsewhere in northern Sonora. Our new data create the possibility of alternative hypotheses for the distribution of Paleoproterozoic crustal provinces in southwestern North America that affect reconstructions of the original southwestern margin of Laurentia, and reduce uncertainties in the configuration, timing, and existence of the Proterozoic supercontinent, Rodinia.

Shock pressure estimation in basement rocks of the Chicxulub impact crater using cathodoluminescence spectroscopy of quartz

NASA Astrophysics Data System (ADS)

Tomioka, N.; Tani, R.; Kayama, M.; Chang, Y.; Nishido, H.; Kaushik, D.; Rae, A.; Ferrière, L.; Gulick, S. P. S.; Morgan, J. V.

2017-12-01

The Chicxulub impact structure, located in the northern Yucatan Peninsula, Mexico, was drilled by the joint IODP-ICDP Expedition 364 in April-May 2016. This expedition is the first attempt to obtain materials from the topographic peak ring within the crater previously identified by seismic imaging. A continuous core was successfully recovered from the peak ring at depths between 505.7 and 1334.7 mbsf. Uplifted, fractured, and shocked granitic basement rocks forming the peak ring were found below, in the impact breccia and impact melt rock unit (747.0-1334.7 mbsf; Morgan et al. 2016). In order to constrain impact crater formation, we investigated shock pressure distribution in the peak-ring basement rocks. Thin sections of the granitic rocks were prepared at intervals of 60 m. All the samples contains shocked minerals, with quartz grains frequently showing planar deformation features (PDFs). We determined shock pressures based on the cathodoluminescence (CL) spectroscopy of quartz. The strong advantage of the CL method is its applicability to shock pressure estimation for individual grains for both quartz and diaplectic SiO2 glass with high-spatial resolution ( 1 μm) (Chang et al. 2016). CL spectra of quartz shows a blue emission band caused by shock-induced defect centers, where its intensity increases with shock pressure. A total of 108 quartz grains in ten thin sections were analyzed using a scanning electron microscope with a CL spectrometer attached (an acceleration voltage of 15 kV and a beam current of 2 nA were used). Natural quartz single crystals, which were experimentally shocked at 0-30 GPa, were used for pressure calibration. CL spectra of all the quartz grains in the basement rocks showed broad blue emission band at the wavelength range of 300-500 nm and estimated shock pressures were in the range of 15-20 GPa. The result is consistent with values obtained from PDFs analysis in quartz using the universal stage (Ferrière et al. 2017; Rae et al. 2017). Although shock pressure gradient in the drilled section is small, the pressure slightly increases at depths of 1113.7 and 1167.0 m. The shock pressure variation could be due to dynamic perturbation of the basement rock during peak ring formation.

Late Mesoproterozoic to Early Paleozoic history of metamorphic basement from the southeastern Chiapas Massif Complex, Mexico, and implications for the evolution of NW Gondwana

NASA Astrophysics Data System (ADS)

Weber, Bodo; González-Guzmán, Reneé; Manjarrez-Juárez, Román; Cisneros de León, Alejandro; Martens, Uwe; Solari, Luigi; Hecht, Lutz; Valencia, Victor

2018-02-01

In this paper, U-Pb zircon geochronology, Lu-Hf and Sm-Nd isotope systematics, geochemistry and geothermobarometry of metaigneous basement rocks exposed in the southeastern Chiapas Massif Complex are presented. Geologic mapping of the newly defined "El Triunfo Complex" located at the southeastern edge of the Chiapas Massif reveals (1) partial melting of a metamorphic basement mainly constituted by mafic metaigneous rocks (Candelaria unit), (2) an Ediacaran metasedimentary sequence (Jocote unit), and (3) occurrence of massif-type anorthosite. All these units are intruded by undeformed Ordovician plutonic rocks of the Motozintla suite. Pressure and temperature estimates using Ca-amphiboles, plagioclase and phengite revealed prograde metamorphism that reached peak conditions at 650 °C and 6 kbar, sufficient for partial melting under water saturated conditions. Relict rutile in titanite and clinopyroxene in amphibolite further indicate a previous metamorphic event at higher P-T conditions. U-Pb zircon ages from felsic orthogneiss boudins hosted in deformed amphibolite and migmatite yield crystallization ages of 1.0 Ga, indicating that dry granitic protoliths represent remnants of Rodinia-type basement. Additionally, a mid-Tonian ( 920 Ma) metamorphic overprint is suggested by recrystallized zircon from a banded gneiss. Zircon from folded amphibolite samples yield mainly Ordovician ages ranging from 457 to 444 Ma that are indistinguishable from the age of the undeformed Motozintla plutonic suite. Similar ages between igneous- and metamorphic- zircon suggest a coeval formation during a high-grade metamorphic event, in which textural discrepancies are explained in terms of differing zircon formation mechanisms such as sub-solidus recrystallization and precipitation from anatectic melts. In addition, some amphibolite samples contain inherited zircon yielding Stenian-Tonian ages around 1.0 Ga. Lu-Hf and Sm-Nd isotopes and geochemical data indicate that the protoliths of the amphibolite have E-MORB characteristics and were derived from a depleted mantle source younger than the Rodinia-type basement. Inasmuch as similar amphibolites also occur in the Ediacaran metasedimentary rocks as dykes or lenses, Late Neoproterozoic magmatism in a rift setting is suggested. Hence, the geologic record of the El Triunfo Complex includes evidences for Rodinia assemblage, Tonian circum-Rodinia subduction, and breakup during the Late Neoproterozoic. Metamorphism, and partial melting are interpreted in terms of a convergent margin setting during the Ordovician. The results place the southern Chiapas Massif along with Oaxaquia and similar Northern Andes terranes on the NW margin of Gondwana interpreted as the extension of the Famatinian orogen that evolved during the closure of the Iapetus Ocean.

Ancient Lavas in Shenandoah National Park near Luray, Virginia

USGS Publications Warehouse

Reed, John Calvin

1969-01-01

In the Blue Ridge Province of northern Virginia, Maryland, and southern Pennsylvania, Lower Cambrian beds are underlain by a thick sequence of greenstone and interbedded sedimentary rocks known as the Catoctin Formation. An area near Luray, Va., was studied to determine the thickness of the formation, its relationship to overlying and underlying rocks, and the original nature of the lavas from which the Catoctin greenstone was derived. There the Catoctin Formation lies unconformably on granitic rocks. Its basal sedimentary layer ranges from a few inches to 150 feet in thickness and contains pebbles of underlying basement rocks. The erosion surface beneath the Catoctin is irregular, and in several places, hills as much as 1,000 feet high were buried beneath the Catoctin lavas. No important time break is indicated between the deposition of the Catoctin Formation and the overlying Cambrian sediments. The original Catoctin lavas were basaltic and were probably normal plateau basalts. Columnar joints, amygdules, sedimentary dikes, flow breccias low-dipping primary joints, and other primary structures are well preserved.

ROCK PHYSICS. Rock physics of fibrous rocks akin to Roman concrete explains uplifts at Campi Flegrei Caldera.

PubMed

Vanorio, Tiziana; Kanitpanyacharoen, Waruntorn

2015-08-07

Uplifts in the Campi Flegrei caldera reach values unsurpassed anywhere in the world (~2 meters). Despite the marked deformation, the release of strain appears delayed. The rock physics analysis of well cores highlights the presence of two horizons, above and below the seismogenic area, underlying a coupled process. The basement is a calc-silicate rock housing hydrothermal decarbonation reactions, which provide lime-rich fluids. The caprock above the seismogenic area has a pozzolanic composition and a fibril-rich matrix that results from lime-pozzolanic reactions. These findings provide evidence for a natural process reflecting that characterizing the cementitious pastes in modern and Roman concrete. The formation of fibrous minerals by intertwining filaments confers shear and tensile strength to the caprock, contributing to its ductility and increased resistance to fracture. Copyright © 2015, American Association for the Advancement of Science.

Database compilation for the geologic map of the San Francisco volcanic field, north-central Arizona

USGS Publications Warehouse

Bard, Joseph A.; Ramsey, David W.; Wolfe, Edward W.; Ulrich, George E.; Newhall, Christopher G.; Moore, Richard B.; Bailey, Norman G.; Holm, Richard F.

2016-01-08

The orignial geologic maps were prepared under the Geothermal Research Program of the U.S. Geological Survey as a basis for interpreting the history of magmatic activity in the volcanic field. The San Francisco field, which is largely Pleistocene in age, is in northern Arizona, just north of the broad transition zone between the Colorado Plateau and the Basin and Range province. It is one of several dominantly basaltic volcanic fields of the late Cenozoic age situated near the margin of the Colorado Plateau. The volcanic field contains rocks ranging in composition from basalt to rhyolite—the products of eruption through Precambrian basement rocks and approximately a kilometer of overlying, nearly horizontal, Paleozoic and Mesozoic sedimentary rocks. About 500 km3 of erupted rocks cover about 5,000 km2 of predominantly Permian and locally preserved Triassic sedimentary rocks that form the erosionally stripped surface of the Colorado Plateau in Northern Arizona.

The timing of metamorphism in the Odenwald-Spessart basement, Mid-German Crystalline Zone

NASA Astrophysics Data System (ADS)

Will, T. M.; Schulz, B.; Schmädicke, E.

2017-07-01

New in situ electron microprobe monazite and white mica 40Ar/39Ar step heating ages support the proposition that the Odenwald-Spessart basement, Mid-German Crystalline Zone, consists of at least two distinct crustal terranes that experienced different geological histories prior to their juxtaposition. The monazite ages constrain tectonothermal events at 430 ± 43 Ma, 349 ± 14 Ma, 331 ± 16 Ma and 317 ± 12 Ma/316 ± 4 Ma, and the 40Ar/39Ar analyses provide white mica ages of 322 ± 3 Ma and 324 ± 3 Ma. Granulite-facies metamorphism occurred in the western Odenwald at c. 430 and 349 Ma, and amphibolite-facies metamorphism affected the eastern Odenwald and the central Spessart basements between c. 324 and 316 Ma. We interpret these data to indicate that the Otzberg-Michelbach Fault Zone, which separates the eastern Odenwald-Spessart basement from the Western Odenwald basement, is part of the Rheic Suture, which marks the position of a major Variscan plate boundary separating Gondwana- and Avalonia-derived crustal terranes. The age of the Carboniferous granulite-facies event in the western Odenwald overlaps with the minimum age of eclogite-facies metamorphism in the adjacent eastern Odenwald. The granulite- and eclogite-facies rocks experienced contrasting pressure-temperature paths but occur in close spatial proximity, being separated by the Rheic Suture. As high-pressure and high-temperature metamorphisms are of similar age, we interpret the Odenwald-Spessart basement as a paired metamorphic belt and propose that the adjacent high-pressure and high-temperature rocks were metamorphosed in the same subduction zone system. Juxtaposition of these rocks occurred during the final stages of the Variscan orogeny along the Rheic Suture.

Testing the Mojave-Sonora megashear hypothesis: Evidence from Paleoproterozoic igneous rocks and deformed Mesozoic strata in Sonora, Mexico

USGS Publications Warehouse

Amato, J.M.; Lawton, T.F.; Mauel, D.J.; Leggett, W.J.; Gonzalez-Leon, C. M.; Farmer, G.L.; Wooden, J.L.

2009-01-01

U-Pb ages and Nd isotope values of Proterozoic rocks in Sonora, Mexico, indicate the presence of Caborca-type basement, predicted to lie only south of the Mojave-Sonora mega-shear, 40 km north of the postulated megashear. Granitoids have U-Pb zircon ages of 1763-1737 Ma and 1076 Ma, with ??Nd(t) values from +1.4 to -4.3, typical of the Caborca block. Lower Jurassic strata near the Proterozoic rocks contain large granitic clasts with U-Pb ages and ??Nd(t) values indistinguishable from those of Caborcan basement. Caborca-type basement was thus present at this location north of the megashear by 190 Ma, the depositional age of the Jurassic strata. The Proterozoic rocks are interpreted as parautochthonous, exhumed and juxtaposed against the Mesozoic section by a reverse fault that formed a footwall shortcut across a Jurassic normal fault. Geochronology, isotope geochemistry, and structural geology are therefore inconsistent with Late Jurassic megashear displacement and require either that no major transcurrent structure is present in Sonora or that strike-slip displacement occurred prior to Early Jurassic time. ?? 2009 The Geological Society of America.

A low-δ18O intrusive breccia from Koegel Fontein, South Africa: Remobilisation of basement that was hydrothermally altered during global glaciation?

NASA Astrophysics Data System (ADS)

Olianti, Camille A. E.; Harris, Chris

2018-02-01

The Cretaceous Koegel Fontein igneous complex is situated on the west coast of South Africa, and has a high proportion of rocks with abnormally low δ18O values. The rocks with the lowest δ18O values (- 5.2‰) belong to intrusive matrix-supported breccia pipes and dykes, containing a variety of clast types. The breccia rocks range in SiO2 from 44 to 68 wt% and their whole-rock δ18O values vary between - 5.2‰ and + 1.8‰. The major and trace element composition of the breccia rocks is consistent with them containing variable proportions of clasts of Cretaceous intrusive rocks and basement gneiss and the matrix being fluidized material derived from the same source as the clasts. Based on the nature of the clasts contained in the breccia, it was emplaced just prior to intrusion of the main Rietpoort Granite at 134 Ma. All components of the breccia have low δ18O value and, at least in the case of the gneiss clasts, this predates incorporation in the fluidized material. Although the early Cretaceous appears to have been a period of cold climate, it is unlikely that the δ18O values of ambient precipitation ( - 10‰) would have been low enough to have generated the required 18O-depletion. The basement gneiss was probably 2-3 km below the Cretaceous surface, minimizing the possibility of interaction with isotopically unmodified meteoric water, and there is no evidence for foundered blocks of cover rocks in the breccia. There is, therefore, no evidence for downwards movement of material. We favour a model where basement gneiss interacted with extremely 18O-depleted fluid during crustal reworking at 547 Ma, a time of global glaciation. Low-δ18O metamorphic fluids produced by dehydration melting of 18O-depleted gneiss became trapped and, as the fluid pressure increased, failure of the seal resulted in explosive upwards movement of fluidized breccia. Migration was along pre-existing dykes, incorporating fragments of these dykes, as well as the country rock gneiss.

Diverse ages and origins of basement complexes, Luzon, Philippines

DOE Office of Scientific and Technical Information (OSTI.GOV)

Geary, E.E.; Harrison, T.M.; Heizler, M.

1988-04-01

Geological field investigations and /sup 40/Ar//sup 39/Ar ages from two basement complexes in southeast Luzon document the first known occurrences of pre-Late Cretaceous age rocks in the eastern Philippines. However, individual components within the two complexes vary in age from Late Jurassic (Caramoan basement complex) to Early Cretaceous and early Miocene (Camarines Norte-Calaguas Islands basement complex). These and other data show that southeast Luzon basement complexes are genetically diverse, and they indicate that the concept of an old, autochthonous basement in the Philippines is open to question. This supports the hypothesis that the Philippine Archipelago is an amalgamation of allochthonousmore » Mesozoic and Cenozoic island-arc, ocean-basin, and continental fragments that were assembled during the Tertiary.« less

Precambrian basement geology of North and South Dakota.

USGS Publications Warehouse

Klasner, J.S.; King, E.R.

1986-01-01

Combined analysis of drill-hole, gravity and magnetic data indicates that the Precambrian rocks in the basement of the Dakotas may be divided into a series of lithotectonic terrains. On the basis of an analysis of geological and geophysical data in the Dakotas and from the surrounding states and Canada, it is shown how the exposed Precambrian rocks of the adjacent shield areas project into the study area. Brief comments are made on the tectonic implications of this study. Geological and geophysical characteristics of 11 terrains are tabulated. -P.Br.

Physical properties of sidewall cores from Decatur, Illinois

USGS Publications Warehouse

Morrow, Carolyn A.; Kaven, Joern; Moore, Diane E.; Lockner, David A.

2017-10-18

To better assess the reservoir conditions influencing the induced seismicity hazard near a carbon dioxide sequestration demonstration site in Decatur, Ill., core samples from three deep drill holes were tested to determine a suite of physical properties including bulk density, porosity, permeability, Young’s modulus, Poisson’s ratio, and failure strength. Representative samples of the shale cap rock, the sandstone reservoir, and the Precambrian basement were selected for comparison. Physical properties were strongly dependent on lithology. Bulk density was inversely related to porosity, with the cap rock and basement samples being both least porous (

Two-stage formation model of the Junggar basin basement: Constraints to the growth style of Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

He, Dengfa

2016-04-01

Junggar Basin is located in the central part of the Central Asian Orogenic Belt (CAOB). Its basement nature is a highly controversial scientific topic, involving the basic style and processes of crustal growth. Some researchers considered the basement of the Junggar Basin as a Precambrian continental crust, which is not consistent with the petrological compositions of the adjacent orogenic belts and the crust isotopic compositions revealed by the volcanic rocks in the basin. Others, on the contrary, proposed an oceanic crust basement model that does not match with the crustal thickness and geophysical characteristics of the Junggar area. Additionally, there are several viewponits, such as the duplex basement with the underlying Precambrian crystalline rocks and the overlying pre-Carboniferous folded basement, and the collaged basement by the Precambrian micro-continent block in the central part and the Hercynian accretionary folded belts circling it. Anyway, it is necessary to explain the property of basement rock, its strong inhomogeneous compositions as well as the geophysical features. In this paper, based on the borehole data from more than 300 industry wells drilled into the Carboniferous System, together with the high-resolution gravity and magnetic data (in a scale of 1:50,000), we made a detailed analysis of the basement structure, formation timing and processes and its later evolution on a basis of core geochemical and isotopic analysis. Firstly, we defined the Mahu Pre-Cambrian micro-continental block in the juvenile crust of Junggar Basin according to the Hf isotopic analysis of the Carboniferous volcanic rocks. Secondly, the results of the tectonic setting and basin analysis suggest that the Junggar area incorporates three approximately E-W trending island arc belts (from north to south: Yemaquan- Wulungu-Chingiz, Jiangjunmiao-Luliang-Darbut and Zhongguai-Mosuowan- Baijiahai-Qitai island arcs respectively) and intervened three approximately E-W trending retro-arc or inter-arc basin belts from north to south, such as Santanghu-Suosuoquan-Emin, Wucaiwan-Dongdaohaizi-Mahu (Mahu block sunk as a bathyal basin during this phase) and Fukang-western well Pen1 sag accordingly. Thirdly, the closure of these retro-arc or inter-arc basins migrating gradually toward the south led to the collision and amalgamation between the above-mentioned island arcs during the Carboniferous, constituting the basic framework of the Junggar 'block'. Fourthly, the emplacement of large-scale mantle-derived magmas occurred in the latest Carboniferous to Early Permian. For instance, the well Mahu 5 penetrate the latest Carboniferous basalts with a thickness of over 20 m, and these mantle-derived magmas consolidated the above-mentioned island arc-collaged blocks. Therefore, the Junggar basin basement mainly comprises pre-Carboniferous collaged basement, and its formation is characterized by two-stage growth model, involving the Carboniferous lateral growth of island arcs and the latest Carboniferous to Early Permian vertical crustal growth related to emplacement and underplating of the mantle-derived magmas. In the Middle Permian, the Junggar Basin is dominated by a series of stable intra-continental sag basins from west to east, such as Mahu, Shawan, western Well Pen1, Dongdaohaizi-Wucaiwan-Dajing, Fukang-Jimusaer sag lake-basins and so on. The Middle Permian (e.g., Lower Wu'erhe, Lucaogou, and Pingdiquan Formations) thick source rocks developed in these basins, suggesting that the Junggar Basin had been entered 'intra-cratonic sag' basin evolution stage. Since then, no strong thermal tectonic event could result in crust growth. The present crustal thickness of Junggar Basin is 45-52 km, which was mainly formed before the latest Early Permian. Subsequently, the Junggar Basin experienced a rapid cooling process during the Late Permian to Triassic. These events constrain the formation timing of the Junggar basin basement to be before the latest Early Permian. It is inferred that the crustal thickness of Carboniferous island arc belts and associated back-arc basins is of 30-35 km or less. The latest Carboniferous to Early Permian vertical crust growth should have a thickness of 15-20 km or more. Viewed from the deep seismic refection profile across the basin, the Junggar crust does not contain the large-scale imbricate thrust systems, but shows well-layered property. Thus, the vertical growth rate reached 0.75~1 km/Ma in the latest Carboniferous to Early Permian time, a period approximately of 20Ma. It indicates a very rapid crustal growth style which could be named as the Junggar-type vertical growth of continental crust. Its formation mechanism and geodynamic implications need to be further explored later.

Role of Precambrian compositions and fabrics in the development of foreland structures, southern Front Range, Colorado

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chase, R.B.

1985-01-01

The Front Range terminates to the south as three basement-cored uplifts located north and west of the Canon City embayment. Precambrian units consist of foliated and non-foliated granitic rocks, augen gneiss, interlayered schist and gneiss, amphibolite, quartzite, and pegmatite. Precambrian deformations include at least three phases of folding, two phases of crenulation cleavage development, and local mylonitization. Metamorphic conditions reached those of cordierite-sillimanite grade. Paleozoic and Mesozoic sediments surround and overlap the exposed uplifts to form south-plunging arches. Excellent three-dimensional exposure of structural relationships between Precambrian rocks and overlying Phanerozoic sediments is present. Deformation styles in the sedimentary cover aremore » strongly influenced by underlying Precambrian lithologies and structural orientations. Where the crystalline units are granitic, with steeply-dipping foliation or no directional fabric, uplifts are bounded by high angle faults. Some such faults show evidence of repeated movements and reversals dating back to Precambrian time. The boundary between mechanical basement and suprastructure is clearly not defined as the base of the sedimentary section. Balanced cross-sections constructed through the southern Front Range must include contemporaneous flexural folds and thrusts in Precambrian schistose and gneissic rocks as well as in Phanerozoic sedimentary layers.« less

U-Pb detrital zircon geochronology from the basement of the Central Qilian Terrane: implications for tectonic evolution of northeastern Tibetan Plateau

NASA Astrophysics Data System (ADS)

Liu, Changfeng; Wu, Chen; Zhou, Zhiguang; Yan, Zhu; Jiang, Tian; Song, Zhijie; Liu, Wencan; Yang, Xin; Zhang, Hongyuan

2018-03-01

The Tuolai Group dominates the Central Qilian Terrane, and there are different opinions on the age and tectonic attribute of the Tuolai Group. Based on large-scale geologic mapping and zircon dating, the Tuolai Group is divided into four parts: metamorphic supracrustal rocks, Neoproterozoic acid intrusive rocks, early-middle Ordovician acid intrusive rocks and middle Ordovician basic intrusive rocks. The metamorphic supracrustal rocks are the redefined Tuolai complex-group and include gneiss and schist assemblage by faulting contact. Zircon U-Pb LA-MC-ICP-MS dating was conducted on these samples of gneiss and migmatite from the gneiss assemblage, quartzite, two-mica schist and slate from the schist assemblage. The five detrital samples possess similar age spectra; have detrital zircon U-Pb main peak ages of 1.7 Ga with youngest U-Pb ages of 1150 Ma. They are intruded by Neoproterozoic acid intrusive rocks. Therefore, the Tuolai Group belonging to late Mesoproterozoic and early Neoproterozoic. With this caveat in mind, we believe that U-Pb detrital zircon dating, together with the geologic constraints obtained from this study and early work in the neighboring regions. We suggest that the formation age of the entire crystalline basement rocks of metasedimentary sequence from the Central Qilian Terrane should be constrained between the Late Mesoproterozoic and the Late Neoproterozoic, but not the previous Paleoproterozoic. The basement of the Central Qilian Terrane contains the typical Grenville ages, which indicates the Centre Qilian Terrane have been experienced the Grenville orogeny event.

LA-ICP-MS U-Pb detrital zircon study and structural observations of the Cycladic Blueschist Unit on Heraklia Island (Cyclades, Greece)

NASA Astrophysics Data System (ADS)

Laskari, Sofia; Soukis, Konstantinos; Lozios, Stylianos; Stockli, Daniel

2017-04-01

At the central and southern part of the Attic-Cycladic complex (Aegean Sea, Greece) significant exposures of the Cycladic Basement Unit orthogneisses and meta-sediments are observed. These are mainly cropping out in Paros, Naxos and Ios islands and to a much lesser extend in Sikinos Island and they comprise Variscan (granitic) orthogneisses and late Paleozoic metasediments. In this paper we present evidence of a hitherto not identified possible outcrop of the Cycladic Basement in Heraklia Island (central Cyclades). The small Heraklia Island, situated at the center of the Attic-Cycladic core complex in the Aegean, between the islands of Naxos and Ios, consists of rocks that are attributed to the Cycladic Blueschist Unit. The tectonostratigraphy of Heraklia Island includes: a) a lowermost schist sequence with interbedded lenses of felsic orthogneisses whose primary relationship is obliterated by later subduction and exhumation related shearing b) A 200m thick variegated marble sequence with sparse calk-schist intercalations, which is isoclinally folded together with 100m thick overlying quartz-mica and calc-schists schists. All rocks comprise a penetrative foliation formed by greenschist facies mineral assemblages but in the uppermost schists relics of the Eocene HP event are found in the form of glaucophane inclusions within albite porphyroblasts. A mylonitic planar fabric with a cataclastic overprint is observed at the base of the marble sequence and the roof of the underlying schists and orthogneisses. It is accompanied by a N-S stretching lineation, subparallel to isoclinal folding in all scales. Numerous kinematic indicators reveal a top-to-N sense of shear thus linking the Heraklia rocks kinematically with the crustal extensional detachment systems of both Naxos and Ios islands. LA-ICP-MS U-Pb detrital zircon study of schists and gneisses is used in order to identify provenance and to elucidate the tectonostratigrachic relationship between the lower and upper schists of the island. The lowermost schists are characterized by a Panafrican provenance and maximum deposition ages (MDA) that span from late Proterozoic in the schists to Triassic in the orthogneisses, whereas the upper quartz-mica schist sequence shows Variscan provenance and yielded a late Cretaceous maximum deposition age. Based on the above the lowermost sequence of Heraklia Island may represent a portion of the Cycladic Basement metasediments that's been intruded by felsic magmas in the Triassic. These rocks could be correlated with the carapace of the basement rocks in Ios Island.

Stratigraphy, petrography, and provenance of Archean sedimentary rocks of the Nsuze Group, Pongola Supergroup, in the Wit M'folozi Inlier, South Africa

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gamero de Villarroel, H.; Lowe, D.R.

1993-02-01

The Upper Archean Pongola Supergroup is a succession of clastic and volcanic rocks that represents the oldest relatively unmetamorphosed sedimentary sequence deposited on the basement of the 3.5-3.2 Ga-old Kaapvaal Craton. The Pongola Supergroup includes two subdivisions, the Nsuze and the Mozaan Groups. The Nsuze Group is composed of clastic rocks, minor carbonate units, and basalt. Nsuze sandstones are dominated by granite-derived sediments, and minor basaltic-derived detritus. Most Nsuze sedimentary rocks are sandstones that include both quartz-fieldspar and lithic-rich varieties. The mineralogy of Nsuze sandstones reflects the mixing of debris derived from two distinctive sources: (1) a sialic plutonic sourcemore » yielding quartz and microcline and (2) a basaltic source yielding basaltic lithic detritus and plagioclase. The most likely source rocks for the Nsuze sandstones in the Wit M'folozi Inlier were Archean granitic basement, represented by the Mpuluzi batholith, and Nsuze basaltic volcanic rocks. Both continental arc and rift settings have been proposed for the Pongola Supergroup. Nsuze sandstones show similarities to continental arc sandstone suites. However, there is no report of the existence of high standing stratovolcanoes, calc-alkaline plutonism, or contact and regional metamorphism of the intruded volcanic-sedimentary and basement rocks in the Pongola basin, features that are typically associated with continental arcs. The dominance of continent-derived detritus in the Nsuze Group argues that volcanic rocks made up a minor part of the exposed source area and that volcanism was largely restricted to the basin of deposition. Collectively, available evidence favors an intracratonic rift for the depositional setting of the Nsuze Group.« less

Hf-Nd Isotopes in West Philippine Basin Basalts: Results from International Ocean Discovery Program (IODP) Site U1438 and Implications for the Early History of the Izu-Bonin-Mariana (IBM) Subduction System

NASA Astrophysics Data System (ADS)

Yogodzinski, G. M.; Hocking, B.; Bizimis, M.; Hickey-Vargas, R.; Ishizuka, O.; Bogus, K.; Arculus, R. J.

2015-12-01

Drilling at IODP Site U1438, located immediately west of Kyushu-Palau Ridge (KPR), the site of IBM subduction initiation, penetrated 1460 m of volcaniclastic sedimentary rock and 150 m of underlying basement. Biostratigraphic controls indicate a probable age for the oldest sedimentary rocks at around 55 Ma (51-64 Ma - Arculus et al., Nat Geosci in-press). This is close to the 48-52 Ma time period of IBM subduction initiation, based on studies in the forearc. There, the first products of volcanism are tholeiitic basalts termed FAB (forearc basalt), which are more depleted than average MORB and show subtle indicators of subduction geochemical enrichment (Reagan et al., 2010 - Geochem Geophy Geosy). Shipboard data indicate that Site U1438 basement basalts share many characteristics with FABs, including primitive major elements (high MgO/FeO*) and strongly depleted incompatible element patterns (Ti, Zr, Ti/V and Zr/Y below those of average MORB). Initial results thus indicate that FAB geochemistry may have been produced not only in the forearc, but also in backarc locations (west of the KPR) at the time of subduction initiation. Hf-Nd isotopes for Site 1438 basement basalts show a significant range of compositions from ɛNd of 7.0 to 9.5 and ɛHf of 14.5 to 19.8 (present-day values). The data define a well-correlated and steep array in Hf-Nd isotope space. Relatively radiogenic Hf compared to Nd indicates an Indian Ocean-type MORB source, but the dominant signature, with ɛHf >16.5, is more radiogenic than most Indian MORB. The pattern through time is from more-to-less radiogenic and more variable Hf-Nd isotopes within the basement section. This pattern culminates in basaltic andesite sills, which intrude the lower parts of the sedimentary section. The sills have the least radiogenic compositions measured so far (ɛNd ~6.6, ɛHf ~13.8), and are similar to those of boninites of the IBM forearc and modern IBM arc and reararc rocks. The pattern within the basement suggests modest enrichment of a depleted Indian MORB source over time.

Comparative study of factors controlling the groundwater occurrence in Bir Kiseiba and Bir El Shab areas, south western desert, Egypt using hydrogeological and geophysical techniques

NASA Astrophysics Data System (ADS)

Abu Risha, U. A.; Al Temamy, A. M. M.

2016-05-01

This research presents a clear example of the significant role of basement relief on the formation of aquifers and the impact of geologic structures on groundwater occurrence. A basement relief map was constructed using the depth to basement data acquired from 20 vertical electrical soundings (VESes), 3 land magnetic profiles, and 27 drilled wells tapping the basement rocks in addition to the elevations of the basement outcrops in the area of study. The map shows three basins underlying the area. The geoelectric survey shows that these basins were formed as a result of series of step faults. The largest basin underlies El-Shab area. The medium basin underlies the area of Bir Kiseiba whereas the smallest one underlies Bir Abu El-Hussein area. The Nubian Sandstone aquifer occurs only in El-Shab basin whereas the other basins are filled completely with the confining layer of Kiseiba Formation. The depth to basement in El-Shab basin ranges from 11 m. (ves-20) to 197 m. (ves-1) m.b.g.s. The depth to basement in Kiseiba basin ranges from 20 m. (Bir Kurayim magnetic profile) to 122 m. (ves-13) m.b.g.s. The depth to basement in Abu El-Husein basin ranges from 0 (basement outcrops) to 64 m. (Abu El-Husein magnetic profile) m.b.g.s. The aquifer thickness ranges from 0 m (where the aquitard rests directly on the basement) to 153 m. (El Shab well No. 79). The aquifer is uncoformably overlain by Kiseiba Formation which represents the aquitard layer at Bir El-Shab. The thickness of the aquitard ranges from 0 (in areas covered by the Nubian Sandstone) to 120 m (ves-13). Each of the aquifer and aquitard consist of three layers. Two of the aquitard layers are water-bearing. However, the estimated transmissivity of the aquitard is very low (11.9 m2/d). The groundwater moves vertically into the overlying aquitard at Bir El-Shab and subsequently flows in concentric pattern into the surrounding areas. Faulting controls groundwater occurrence and quality. Some springs lie on the basement high associated with step faulting at the edges of El-Shab basin. An ENE low-salinity zone is associated with the basement high which separates El-Shab basin from Kiseiba basin. Focused groundwater recharge through the faults and fractures from paleo playas could be the mechanism of the formation of this anomaly. The isotope data shows local recharge of the groundwater most likely during the Pleistocene time. Two-dimension (2D) Electrical Resistivity Tomography (ERT) profiles reveal that the evaporation process has the main role in increasing the salinity of some water points. It is highly recommended to delineate the southern boundary of El-Shab basin which is expected to extend into Sudan.

Precambrian crystalline basement map of Idaho-an interpretation of aeromagnetic anomalies

USGS Publications Warehouse

Sims, P.K.; Lund, Karen; Anderson, E.

2005-01-01

Idaho lies within the northern sector of the U.S. Cordillera astride the boundary between the Proterozoic continent (Laurentia) to the east and the Permian to Jurassic accreted terranes to the west. The continental basement is mostly covered by relatively undeformed Mesoproterozoic metasedimentary rocks and intruded or covered by Phanerozoic igneous rocks; accordingly, knowledge of the basement geology is poorly constrained. Incremental knowledge gained since the pioneering studies by W. Lindgren, C.P. Ross, A.L. Anderson, A. Hietanen, and others during the early- and mid-1900's has greatly advanced our understanding of the general geology of Idaho. However, knowledge of the basement geology remains relatively poor, partly because of the remoteness of much of the region plus the lack of a stimulus to decipher the complex assemblage of high-grade gneisses and migmatite of central Idaho. The availability of an updated aeromagnetic anomaly map of Idaho (North American Magnetic Anomaly Group, 2002) provides a means to determine the regional Precambrian geologic framework of the State. The combined geologic and aeromagnetic data permit identification of previously unrecognized crystalline basement terranes, assigned to Archean and Paleoproterozoic ages, and the delineation of major shear zones, which are expressed in the aeromagnetic data as linear negative anomalies (Finn and Sims, 2004). Limited geochronologic data on exposed crystalline basement aided by isotopic studies of zircon inheritance, particularly Bickford and others (1981) and Mueller and others (1995), provide much of the geologic background for our interpretation of the basement geology. In northwestern United States, inhomogeneities in the basement inherited from Precambrian tectogenesis controlled many large-scale tectonic features that developed during the Phanerozoic. Two basement structures, in particular, provided zones of weakness that were repeatedly rejuvenated: (1) northeast-trending ductile shear zones developed on the northwest margin of the Archean Wyoming province during the Paleoproterozoic Trans-Montana orogeny (Sims and others, 2004), and (2) northwest-trending intra-continental faults of the Mesoproterozoic Trans-Rocky Mountain strike-slip fault system (Sims, unpub. data, 2003). In this report, geologic ages are reported in millions of years (Ma) and generalized ages are given in billions of years (Ga). The subdivision of Precambrian rocks used herein is the time classification recommended by the International Union of Geological Sciences (Plumb, 1991).

The kinematic evolution of the Serra Central Salient, Eastern Brazil: A Neoproterozoic progressive arc in northern Espinhaço fold-thrust belt

NASA Astrophysics Data System (ADS)

Bersan, Samuel Moreira; Danderfer, André; Lagoeiro, Leonardo; Costa, Alice Fernanda de Oliveira

2017-12-01

Convex-to-the-foreland map-view curves are common features in fold-thrust belts around cratonic areas. These features are easily identifiable in belts composed of supracrustal rocks but have been rarely described in rocks from relatively deeper crustal levels where plastic deformation mechanisms stand out. Several local salients have been described in Neoproterozoic marginal fold-thrust belts around the São Francisco craton. In the northern part of the Espinhaço fold-thrust belt, which borders the eastern portion of the São Francisco craton, both Archean-Paleoproterozoic basement rocks and Proterozoic cover rocks are involved in the so-called Serra Central salient. A combination of conventional structural analysis and microstructural and paleostress studies were conducted to characterize the kinematic and the overall architecture and processes involved in the generation of this salient. The results allowed us to determine that the deformation along the Serra Central salient occur under low-grade metamorphic conditions and was related to a gently oblique convergence with westward mass transport that developed in a confined flow, controlled by two transverse bounding shear zones. We propose that the Serra Central salient nucleates as a basin-controlled primary arc that evolves to a progressive arc with secondary vertical axis rotation. This secondary rotation, well-illustrated by the presence of two almost orthogonal families of folds, was dominantly controlled by buttress effect exert by a basement high located in the foreland of the Serra Central salient.

Provenance of Austroalpine basement metasediments: tightening up Early Palaeozoic connections between peri-Gondwanan domains of central Europe and Northern Africa

NASA Astrophysics Data System (ADS)

Siegesmund, S.; Oriolo, S.; Heinrichs, T.; Basei, M. A. S.; Nolte, N.; Hüttenrauch, F.; Schulz, B.

2018-03-01

New U-Pb and Lu-Hf detrital zircon data together with whole-rock geochemical and Sm-Nd data were obtained for paragneisses of the Austroalpine basement south of the Tauern Window. Geochemically immature metasediments of the Northern-Defereggen-Petzeck (Ötztal-Bundschuh nappe system) and Defereggen (Drauzug-Gurktal nappe system) groups contain zircon age populations which indicate derivation mainly from Pan-African orogens. Younger, generally mature metasediments of the Gailtal Metamorphic Basement (Drauzug-Gurktal nappe system), Thurntaler Phyllite Group (Drauzug-Gurktal nappe system) and Val Visdende Formation (South Alpine Basement) were possibly derived from more distant sources. Their significantly larger abundances of pre-Pan-African zircons record a more advanced stage of downwearing of the Pan-African belts and erosion of older basement when the Austroalpine terrane was part of the Early Palaeozoic Northern Gondwana passive margin. Most zircon age spectra are dominated by Ediacaran sources, with lesser Cryogenian, Tonian and Stenian contributions and subordinate Paleoproterozoic and Neoarchean ages. These age patterns are similar to those recorded by Cambro-Ordovician sedimentary sequences in northeastern Africa between Libya and Jordan, and in some pre-Variscan basement inliers of Europe (e.g. Dinarides-Hellenides, Alboran microplate). Therefore, the most likely sources seem to be in the northeastern Saharan Metacraton and the Northern Arabian-Nubian Shield (Sinai), further supported by whole-rock Sm-Nd and zircon Lu-Hf data.

Geologic Map of the Denver West 30' x 60' Quadrangle, North-Central Colorado

USGS Publications Warehouse

Kellogg, Karl S.; Shroba, Ralph R.; Bryant, Bruce; Premo, Wayne R.

2008-01-01

The Denver West quadrangle extends east-west across the entire axis of the Front Range, one of numerous uplifts in the Rocky Mountain region in which Precambrian rocks are exposed. The history of the basement rocks in the Denver West quadrangle is as old as 1,790 Ma. Along the east side of the range, a sequence of sedimentary rocks as old as Pennsylvanian, but dominated by Cretaceous-age rocks, overlies these ancient basement rocks and was upturned and locally faulted during Laramide (Late Cretaceous to early Tertiary) uplift of the range. The increasingly coarser grained sediments up section in rocks of latest Cretaceous to early Tertiary age record in remarkable detail this Laramide period of mountain building. On the west side of the range, a major Laramide fault (Williams Range thrust) places Precambrian rocks over Cretaceous sedimentary rocks. The geologic history of the quadrangle, therefore, can be divided into four major periods: (1) Proterozoic history, (2) Pennsylvanian to pre-Laramide, Late Cretaceous history, (3) Late Cretaceous to early Tertiary Laramide mountain building, and (4) post-Laramide history. In particular, the Quaternary history of the Denver West quadrangle is described in detail, based largely on extensive new mapping.

Relative chronology in high-grade crystalline terrain of the Eastern Ghats, India: new insights

NASA Astrophysics Data System (ADS)

Bhattacharya, S.; Kar, R.; Saw, A. K.; Das, P.

2011-01-01

The two major lithology or gneiss components in the polycyclic granulite terrain of the Eastern Ghats, India, are the supracrustal rocks, commonly described as khondalites, and the charnockite-gneiss. Many of the workers considered the khondalites as the oldest component with unknown basement and the charnockite-protoliths as intrusive into the khondalites. However, geochronological data do not corroborate the aforesaid relations. The field relations of the hornblende- mafic granulite with the two gneiss components together with geocronological data indicate that khondalite sediments were deposited on older mafic crustal rocks. We propose a different scenario: Mafic basement and supracrustal rocks were subsequently deformed and metamorphosed together at high to ultra-high temperatures - partial melting of mafic rocks producing the charnockitic melt; and partial melting of pelitic sediments producing the peraluminous granitoids. This is compatible with all the geochronological data as well as the petrogenetic model of partial melting for the charnockitic rocks in the Eastern Ghats Belt.

Structural heritage, reactivation and distribution of fault and fracture network in a rifting context: Case study of the western shoulder of the Upper Rhine Graben

NASA Astrophysics Data System (ADS)

Bertrand, Lionel; Jusseaume, Jessie; Géraud, Yves; Diraison, Marc; Damy, Pierre-Clément; Navelot, Vivien; Haffen, Sébastien

2018-03-01

In fractured reservoirs in the basement of extensional basins, fault and fracture parameters like density, spacing and length distribution are key properties for modelling and prediction of reservoir properties and fluids flow. As only large faults are detectable using basin-scale geophysical investigations, these fine-scale parameters need to be inferred from faults and fractures in analogous rocks at the outcrop. In this study, we use the western shoulder of the Upper Rhine Graben as an outcropping analogue of several deep borehole projects in the basement of the graben. Geological regional data, DTM (Digital Terrain Model) mapping and outcrop studies with scanlines are used to determine the spatial arrangement of the faults from the regional to the reservoir scale. The data shows that: 1) The fault network can be hierarchized in three different orders of scale and structural blocks with a characteristic structuration. This is consistent with other basement rocks studies in other rifting system allowing the extrapolation of the important parameters for modelling. 2) In the structural blocks, the fracture network linked to the faults is linked to the interplay between rock facies variation linked to the rock emplacement and the rifting event.

Geologic influence on induced seismicity: Constraints from potential field data in Oklahoma

USGS Publications Warehouse

Shah, Anjana K.; Keller, G. Randy

2017-01-01

Recent Oklahoma seismicity shows a regional correlation with increased wastewater injection activity, but local variations suggest that some areas are more likely to exhibit induced seismicity than others. We combine geophysical and drill hole data to map subsurface geologic features in the crystalline basement, where most earthquakes are occurring, and examine probable contributing factors. We find that most earthquakes are located where the crystalline basement is likely composed of fractured intrusive or metamorphic rock. Areas with extrusive rock or thick (>4 km) sedimentary cover exhibit little seismicity, even in high injection rate areas, similar to deep sedimentary basins in Michigan and western North Dakota. These differences in seismicity may be due to variations in permeability structure: within intrusive rocks, fluids can become narrowly focused in fractures and faults, causing an increase in local pore fluid pressure, whereas more distributed pore space in sedimentary and extrusive rocks may relax pore fluid pressure.

Magnetotelluric study of the Pahute Mesa and Oasis Valley regions, Nye County, Nevada

USGS Publications Warehouse

Schenkel, Clifford J.; Hildenbrand, Thomas G.; Dixon, Gary L.

1999-01-01

Magnetotelluric data delineate distinct layers and lateral variations above the pre-Tertiary basement. On Pahute Mesa, three resistivity layers associated with the volcanic rocks are defined: a moderately resistive surface layer, an underlying conductive layer, and a deep resistive layer. Considerable geologic information can be derived from the conductive layer which extents from near the water table down to a depth of approximately 2 km. The increase in conductivity is probably related to zeolite zonation observed in the volcanic rock on Pahute Mesa, which is relatively impermeable to groundwater flow unless fractured. Inferred faults within this conductive layer are modeled on several profiles crossing the Thirsty Canyon fault zone. This fault zone extends from Pahute Mesa into Oasis Valley basin. Near Colson Pond where the basement is shallow, the Thirsty Canyon fault zone is several (~2.5) kilometers wide. Due to the indicated vertical offsets associated with the Thirsty Canyon fault zone, the fault zone may act as a barrier to transverse (E-W) groundwater flow by juxtaposing rocks of different permeabilities. We propose that the Thirsty Canyon fault zone diverts water southward from Pahute Mesa to Oasis Valley. The electrically conductive nature of this fault zone indicates the presence of abundant alteration minerals or a dense network of open and interconnected fractures filled with electrically conductive groundwater. The formation of alteration minerals require the presence of water suggesting that an extensive interconnected fracture system exists or existed at one time. Thus, the fractures within the fault zone may be either a barrier or a conduit for groundwater flow, depending on the degree of alteration and the volume of open pore space. In Oasis Valley basin, a conductive surface layer, composed of alluvium and possibly altered volcanic rocks, extends to a depth of 300 to 500 m. The underlying volcanic layer, composed mostly of tuffs, fills the basin with about 3-3.5 km of relief on basement. A fault zone, related to the southern margin of the basin, appears to extend up to a depth of about 500 m. The path of groundwater encountering this fault zone is uncertain but may be either to the southwest towards Beatty or to the south towards Crater Flat.

The use of index tests to determine the mechanical properties of crushed aggregates from Precambrian basement complex rocks, Ado-Ekiti, SW Nigeria

NASA Astrophysics Data System (ADS)

Afolagboye, Lekan Olatayo; Talabi, Abel Ojo; Oyelami, Charles Adebayo

2017-05-01

This study assessed the possibility of using index tests to determine the mechanical properties of crushed aggregates. The aggregates used in this study were derived from major Precambrian basement rocks in Ado-Ekiti, Nigeria. Regression analyses were performed to determine the empirical relations that mechanical properties of the aggregates may have with the point load strength (IS(50)), Schmidt rebound hammer value (SHR) and unconfined compressive strength (UCS) of the rocks. For all the data, strong correlation coefficients were found between IS(50), SHR, UCS, and mechanical properties of the aggregates. The regression analysis conducted on the different rocks separately showed that correlations coefficients obtained between the IS(50), SHR, UCS and mechanical properties of the aggregates were stronger than those of the grouped rocks. The T-test and F-test showed that the derived models were valid. This study has shown that the mechanical properties of the aggregates can be estimated from IS(50), SHR and USC but the influence of rock type on the relationships should be taken into consideration.

Late Vendian Complexes in the Structure of Metamorphic Basement of the Fore Range Zone, Greater Caucasus

NASA Astrophysics Data System (ADS)

Kamzolkin, V. A.; Latyshev, A. V.; Vidyapin, Yu. P.; Somin, M. L.; Smul'skaya, A. I.; Ivanov, S. D.

2018-05-01

The paper presents new data on the composition, age, and relationships (with host and overlying deposits) of intrusive rocks in the basement of the Fore Range zone (Greater Caucasus), in the Malaya Laba River Basin. The evolutionary features of intrusive units located within the Blyb metamorphic complex are described. It is shown for the first time that the lower levels of this complex are, in a structural sense, outcrops of the Late Vendian basement. The basement is composed of the Balkan Formation and a massif of quartz metadiorites that intrudes it; for the rocks of this massif, ages ranging from 549 ± 7.4 to 574.1 ± 6.7 Ma are obtained for three U-Pb datings by the SHRIMP-II method. The Herzyinan magmatic event is represented by a group of granodiorite intrusions penetrating the Blyb complex on a series of faults extending along its boundary with the Main Range zone. The obtained estimate for the U-Pb age of one of the intrusions (319 ± 3.8 Ma) corresponds to the end of the Serpukhovian stage of the Early Carboniferous.

Potential anthropogenic mobilisation of mercury and arsenic from soils on mineralised rocks, Northland, New Zealand.

PubMed

Craw, D

2005-02-01

Eroded roots of hot spring systems in Northland, New Zealand consist of mineralised rocks containing sulfide minerals. Marcasite and cinnabar are the dominant sulfides with subordinate pyrite. Deep weathering and leached soil formation has occurred in a warm temperate to subtropical climate with up to 3 m/year rainfall. Decomposition of the iron sulfides in natural and anthropogenic rock exposures yields acid rock drainage with pH typically between 2 and 4, and locally down to pH 1. Soils and weathered rocks developed on basement greywacke have negligible acid neutralisation capacity. Natural rainforest soils have pH between 4 and 5 on unmineralised greywacke, and pH is as low as 3.5 in soils on mineralised rocks. Roads with aggregate made from mineralised rocks have pH near 3, and quarries from which the rock was extracted can have pH down to 1. Mineralised rocks are enriched in arsenic and mercury, both of which are environmentally available as solid solution impurities in iron sulfides and phosphate minerals. Base metals (Cu, Pb, Zn) are present at low levels in soils, at or below typical basement rock background. Decomposition of the iron sulfides releases the solid solution arsenic and mercury into the acid rock drainage solutions. Phosphate minerals release their impurities only under strongly acid conditions (pH<1). Arsenic and mercury are adsorbed on to iron oxyhydroxides in soils, concentrated in the C horizon, with up to 4000 ppm arsenic and 100 ppm mercury. Waters emanating from acid rock drainage areas have arsenic and mercury below drinking water limits. Leaching experiments and theoretical predictions indicate that both arsenic and mercury are least mobile in acid soils, at pH of c. 3-4. This optimum pH range for fixation of arsenic and mercury on iron oxyhydroxides in soils is similar to natural pH at the field site of this study. However, neutralisation of acid soils developed on mineralised rocks is likely to decrease adsorption and enhance mobility of arsenic and mercury. Hence, development of farmland by clearing forest and adding agricultural lime may mobilise arsenic and mercury from underlying soils on mineralised rocks. In addition, arsenic and mercury release into runoff water will be enhanced where sediment is washed off mineralised road aggregate (pH 3) on to farm land (pH>6). The naturally acid forest soils, or even lower pH of natural acid rock drainage, are the most desirable environmental conditions to restrict dissolution of arsenic and mercury from soils. This approach is only valid where mineralised soils have low base metal concentrations.

Synthesis of geophysical data (phase V, deliverable 55): Chapter B in Second projet de renforcement institutionnel du secteur minier de la République Islamique de Mauritanie (PRISM-II)

USGS Publications Warehouse

Finn, Carol A.; Anderson, Eric D.

2015-01-01

Aeromagnetic and radiometric data were used to map shallow Precambrian basement lithology and structure and determine the depth to magnetic basement, which in most cases, corresponds to the depth to crystalline basement of interest for mineral exploration. These depths, along with those determined from gravity data, help identify basins with hydrologic potential. In addition, the magnetic data were used to identify buried Precambrian rocks of unknown affinity.

Three decades of geochronologic studies in the New England Appalachians

USGS Publications Warehouse

Zartman, R.E.

1988-01-01

Over the past 30 years, both isotope geochronology and plate tectonics grew from infancy into authoritative disciplines in the geological sciences. The existing geochronlogy is summarized into a map and table emphasizing the temporal construction of the New England Appalachians. By using lithotectonic zones as the building blocks of the orogen, seven such zones are defined in terms of pre-, syn-, and post-assembly geologic history. The boundaries between these zones are faults in most cases, some of which may have had recurring movement to further complicate any plate-tectonic scenario. A delineation of underlying Grenvillian, Chain Lakes, and Avalonian basement is also attempted, which now can make use of isotopes in igneous rocks as petrogenic indicators to supplement the rare occurrences of basement outcrop within mobile zones of the orogen. -from Author

Hydrogeochemistry and groundwater quality assessment along Wadi Al Showat, Khamis Mushiet District, Southwest Saudi Arabia

NASA Astrophysics Data System (ADS)

Alhumidan, S. M.; Alfaifi, H. J.; Ibrahim, E. K. E.; Abdel Rahman, K.

2015-12-01

In the present study, the hydrochemistry and geologic characteristics of the shallow groundwater aquifer along Wadi Al Showat, Khamis Mushiet District, Southwest Saudi Arabia was evaluated and assessed. Along this wadi the fractured/weathered basement rocks house significant quantity of groundwater that usually used by local people for agricultural and domestic purposes. Assessing and evaluation of the quality of the groundwater in such shallow aquifers is very important; especially the groundwater is generally occurred within the fractured basement rocks at shallow depths, thus exposing the groundwater to surface or near-surface contaminants is expected. For this purpose hydrochemical and biological analysis was conducted for 25 water samples collected from the available shallow dug wells along the studied wadi. The study reveals that the groundwater quality changed due to the agriculture and urbanization practices along the wadi. The effect of domestic waste water and septic tanks was obvious. In addition, the field investigation indicates that the basement rocks in the area is dissected by two main sets of fractures that oriented in the west-northwest and east-west directions. In some places, the basement rocks is intruded by coarse-grained, quartz-rich quartzite grained monzogranite, and pegmatite veins that have a coarse-grained weathering product, therefore, they tend to develop and preserve open joint systems between the granitic blocks. These fracturing system are important from the hydrogeological point of view, as they facilitate the storage, water flow movement through them and also facilitate the vertical infiltration of the surface pollutants. These results led to a better understanding of the groundwater characteristics that is important in groundwater management in the study area.

Geochemistry of Archean metasedimentary rocks of the Aravalli craton, NW India: Implications for provenance, paleoweathering and supercontinent reconstruction

NASA Astrophysics Data System (ADS)

Ahmad, Iftikhar; Mondal, M. E. A.; Satyanarayanan, M.

2016-08-01

Basement complex of the Aravalli craton (NW India) known as the Banded Gneissic Complex (BGC) is classified into two domains viz. Archean BGC-I and Proterozoic BGC-II. We present first comprehensive geochemical study of the Archean metasedimentary rocks occurring within the BGC-I. These rocks occur associated with intrusive amphibolites in a linear belt within the basement gneisses. The association is only concentrated on the western margin of the BGC-I. The samples are highly mature (MSm) to very immature (MSi), along with highly variable geochemistry. Their major (SiO2/Al2O3, Na2O/K2O and Al2O3/TiO2) and trace (Th/Sc, Cr/Th, Th/Co, La/Sc, Zr/Sc) element ratios, and rare earth element (REE) patterns are consistent with derivation of detritus from the basement gneisses and its mafic enclaves, with major contribution from the former. Variable mixing between the two end members and closed system recycling (cannibalism) resulted in the compositional heterogeneity. Chemical index of alteration (CIA) of the samples indicate low to moderate weathering of the source terrain in a sub-tropical environment. In A-CN-K ternary diagram, some samples deceptively appear to have undergone post-depositional K-metasomatism. Nevertheless, their petrography and geochemistry (low K2O and Rb) preclude the post-depositional alteration. We propose non-preferential leaching of elements during cannibalism as the cause of the deceptive K-metasomatism as well as enigmatic low CIA values of some highly mature samples. The Archean metasedimentary rocks were deposited on stable basement gneisses, making the BGC-I a plausible participant in the Archean Ur supercontinent.

Geochemical provenance of Florida basement components

DOE Office of Scientific and Technical Information (OSTI.GOV)

Heatherington, A.L.; Mueller, P.A.; Dallmeyer, R.D.

1993-03-01

The pre-Cretaceous basement of Florida is generally considered to be exotic with respect to Proterozoic Laurentia. Paleontologic and paleomagnetic evidence have suggested a Gondwanan provenance for the Floridan basement, as either a peri-Gondwanide terrane or as a rifted block of the West African craton. The report of generally similar lithologic sequences and a record of similar Ar-Ar cooling ages in some Floridan and West African lithologic units has led to very specific correlations between these units. U-Pb, Sm-Nd, and Rb-Sr geochronologic studies as well as isotopic and elemental abundance data have been used to evaluate the validity of these correlations.more » Results indicate: (1) geochemical similarities between volcanic rocks of northeastern Florida and a Pan-African metavolcanic sequence (Niokola-Koba group) exposed in Senegal; (2) an absence of a Grenvillian-age (i.e., Laurentian) component in zircons separated from a Paleozoic Suwanee basin sandstone; and (3) whole-rock Sm-Nd and U-Pb zircon evidence for an Archean ([approximately]3.0 Ga) component in the neo-Proterozoic Osceola granitoid(s). Although silicic rocks from throughout Florida have Nd model ages (T[sub DM]) that are predominantly Grenvillian (1.1--1.4 Ga), the absence of a Grenvillian component in zircons separated from granite and sandstone suggests that the model ages represent a mixture of older and younger components. Overall, the evidence for Birimian ([approximately]2.1 Ga) and Liberian ([approximately]3.0 Ga) age components in the Florida basement are consistent with its origin as a rifted block of cratonic Gondwana. In addition to demonstrating a strong affinity between the Florida basement and cratonic West Africa/northern South America, these data provide a basis for comparison with other circum-Atlantic terranes traditionally described as Avalonian/Cadomian, etc.« less

Crustal insights from gravity and aeromagnetic analysis: Central North Slope, Alaska

USGS Publications Warehouse

Saltus, R.W.; Potter, C.J.; Phillips, J.D.

2006-01-01

Aeromagnetic and gravity data are processed and interpreted to reveal deep and shallow information about the crustal structure of the central North Slope, Alaska. Regional aeromagnetic anomalies primarily reflect deep crustal features. Regional gravity anomalies are more complex and require detailed analysis. We constrain our geophysical models with seismic data and interpretations along two transects including the Trans-Alaska Crustal Transect. Combined geophysical analysis reveals a remarkable heterogeneity of the pre-Mississippian basement. In the central North Slope, pre-Mississippian basement consists of two distinct geophysical domains. To the southwest, the basement is dense and highly magnetic; this basement is likely mafic and mechanically strong, possibly acting as a buttress to basement involvement in Brooks Range thrusting. To the northeast, the central North Slope basement consists of lower density, moderately magnetic rocks with several discrete regions (intrusions?) of more magnetic rocks. A conjugate set of geophysical trends, northwest-southeast and southwest-northeast, may be a factor in the crustal response to tectonic compression in this domain. High-resolution gravity and aeromagnetic data, where available, reflect details of shallow fault and fold structure. The maps and profile models in this report should provide useful guidelines and complementary information for regional structural studies, particularly in combination with detailed seismic reflection interpretations. Future challenges include collection of high-resolution gravity and aeromagnetic data for the entire North Slope as well as additional deep crustal information from seismic, drilling, and other complementary methods. Copyrights ?? 2006. The American Association of Petroleum Geologists. All rights reserved.

Constraints from Mesozoic siliciclastic cover rocks and satellite image analysis on the slip history of regional E-W faults in the southeast Western Desert, Egypt

NASA Astrophysics Data System (ADS)

Tewksbury, Barbara J.; Mehrtens, Charlotte J.; Gohlke, Steven A.; Tarabees, Elhamy A.; Hogan, John P.

2017-12-01

In the southeast Western Desert of Egypt, a prominent set of E-W faults and co-located domes and basins involve sedimentary cover rock as young as the early Eocene. Although earlier Mesozoic slip on faults in southern Egypt has been widely mentioned in the literature and attributed to repeated reactivation of basement faults, evidence is indirect and based on the idea that regional stresses associated with tectonic events in the Syrian Arc would likely have reactivated basement faults in south Egypt in dextral strike slip during the Mesozoic as well as the Cenozoic. Here, we present direct evidence from the rock record for the sequence of development of features along these faults. Southwest of Aswan, a small structural dome in Mesozoic Nubia facies rocks occurs where the Seiyal Fault bends northward from west to east. The dome is cut by strands of the Seiyal Fault and a related set of cataclastic deformation bands showing dominantly right lateral strike slip, as well as by younger calcite veins with related patchy poikilotopic cement. High resolution satellite image analysis of the remote southwest Kharga Valley shows a similar sequence of events: older structural domes and basins located where E-W faults bend northward from west to east, right lateral offset of domes and basins along the E-W faults, and two sets of deformation band faults that lack co-located domes and basins. We suggest that field data, image analysis, and burial depth estimates are best explained by diachronous development of features along the E-W fault system. We propose that Late Mesozoic right lateral strike slip produced domes and basins in Nubia facies rocks in stepover regions above reactivated basement faults. We further suggest that the extensively linked segments of the E-W fault system in Nubia facies rocks, plus the deformation band systems, formed during the late Eocene when basement faults were again reactivated in dominantly right lateral strike slip.

Compilation of new and previously published geochemical and modal data for Mesoproterozoic igneous rocks of the St. Francois Mountains, southeast Missouri

USGS Publications Warehouse

du Bray, Edward A.; Day, Warren C.; Meighan, Corey J.

2018-04-16

The purpose of this report is to present recently acquired as well as previously published geochemical and modal petrographic data for igneous rocks in the St. Francois Mountains, southeast Missouri, as part of an ongoing effort to understand the regional geology and ore deposits of the Mesoproterozoic basement rocks of southeast Missouri, USA. The report includes geochemical data that is (1) newly acquired by the U.S. Geological Survey and (2) compiled from numerous sources published during the last fifty-five years. These data are required for ongoing petrogenetic investigations of these rocks. Voluminous Mesoproterozoic igneous rocks in the St. Francois Mountains of southeast Missouri constitute the basement buried beneath Paleozoic sedimentary rock that is over 600 meters thick in places. The Mesoproterozoic rocks of southeast Missouri represent a significant component of approximately 1.4 billion-year-old (Ga) igneous rocks that crop out extensively in North America along the southeast margin of Laurentia and subsequent researchers suggested that iron oxide-copper deposits in the St. Francois Mountains are genetically associated with ca. 1.4 Ga magmatism in this region. The geochemical and modal data sets described herein were compiled to support investigations concerning the tectonic setting and petrologic processes responsible for the associated magmatism.

Summary of the geology and resources of uranium in the San Juan Basin and adjacent region, New Mexico, Arizona, Utah, and Colorado

USGS Publications Warehouse

Ridgley, Jennie L.; Green, M.W.; Pierson, C.T.; Finch, W.I.; Lupe, R.D.

1978-01-01

The San Juan Basin and adjacent region lie predominantly in the southeastern part of the uranium-rich Colorado Plateau of New Mexico, Arizona, Utah, and Colorado. Underlying the province are rocks of the Precambrian basement complex composed mainly of igneous and metamorphic rocks; a thickness of about 3,600 meters of generally horizontal Paleozoic, Mesozoic, and Cenozoic sedimentary rocks; and a variety of Upper Cretaceous and Cenozoic igneous rocks. Sedimentary rocks of the sequence are commonly eroded and well exposed near the present basin margins where Tertiary tectonic activity has uplifted, folded, and faulted the sequence into its present geologic configuration of basins, platforms, monoclines, and other related structural features. Sedimentary rocks of Jurassic age in the southern part of the San Juan Basin contain the largest uranium deposits in the United States, and offer the promise of additional uranium deposits. Elsewhere in the basin and the adjacent Colorado Plateau, reserves and resources of uranium are known primarily in Triassic, Jurassic, and Cretaceous strata. Only scattered occurrences of uranium are known in Paleozoic

Magnetic and gravity constraints on forearc upper crustal structure and composition, offshore northeast Japan

USGS Publications Warehouse

Finn, C.

1994-01-01

Marine magnetic and gravity data from the northeast Japan forearc offer insight to the subsurface structure, density and magnetization from which geologic interpretations and tectonic reconstructions can be made. Positive marine magnetic anomalies, on-land geology, drill hole data, and 2-1/2-dimensional models reveal that Kitakami plutons and possibly their associated volcanic rocks constitute part of the modern forearc basement and lie 100-150 km further east than previously thought. A method to create magnetization and density contrast maps was employed to produce a three-dimensional picture of the forearc basement rock properties averaged over a 14-km thickness. -Author

New zircon ages on the Cambrian-Ordovician volcanism of the Southern Gemericum basement (Western Carpathians, Slovakia): SHRIMP dating, geochemistry and provenance

NASA Astrophysics Data System (ADS)

Vozárová, Anna; Rodionov, Nickolay; Šarinová, Katarína; Presnyakov, Sergey

2017-09-01

The Southern Gemericum basement in the Inner Western Carpathians, composed of low-grade volcano-sedimentary rock complexes, constitutes a record of the polyphase Cambrian-Ordovician continental volcanic arc volcanism. These metavolcanic rocks are characterized by the enrichment in K, Rb, Ba, Th and Ce and Sm relative to Ta, Nb, Hf, Zr, Y and Yb that are the characteristic features for volcanic arc magmatites. The new SHRIMP U-Pb zircon data and compilation of previously published and re-evaluated zircon ages, contribute to a new constrain of the timing of the Cambrian-Ordovician volcanism that occurred between 496 and 447 Ma. The following peaks of the volcanic activity of the Southern Gemericum basement have been recognized: (a) mid-late Furongian at 492 Ma; (b) Tremadocian at 481 Ma; (c) Darriwilian at 464 Ma prolonged to 453 Ma within the early Upper Ordovician. The metavolcanic rocks are characterized by a high zircon inheritance, composed of Ediacaran (650-550 Ma), Tonian-Stenian (1.1-0.9 Ma), and, to a lesser extent, Mesoproterozoic (1.3 Ga), Paleoproterozoic (1.9 Ga) and Archaean assemblages (2.6 Ga). Based on the acquired zircon populations, it could be deduced that Cambrian-Ordovician arc crust was generated by a partial melting of Ediacaran basement in the subduction-related setting, into which old crustal fragments were incorporated. The ascertained zircon inheritances with Meso-, Paleoproterozoic and Archaean cores indicate the similarities with the Saharan Metacraton provenance.

Ground Truth, Magnitude Calibration and Regional Phase Propagation and Detection in the Middle East and Horn of Africa

DTIC Science & Technology

2007-09-01

consists of late Proterozoic crystalline basement overlain by Tertiary and Quaternary volcanic rocks in some places. The breakup of the Arabian Plate from...with structure directly below the crust. To investigate upper mantle structure under the Arabian Shield, measured and inverted relative travel times...Plateau, Zagros Mountains, Arabian Peninsula, Turkish Plateau, Gulf of Aqaba, Dead Sea Rift) and the Horn of Africa (including the northern part of

C-O-H-N fluids circulations and graphite precipitation in reactivated Hudsonian shear zones during basement uplift of the Wollaston-Mudjatik Transition Zone: Example of the Cigar Lake U deposit

NASA Astrophysics Data System (ADS)

Martz, Pierre; Cathelineau, Michel; Mercadier, Julien; Boiron, Marie-Christine; Jaguin, Justine; Tarantola, Alexandre; Demacon, Mickael; Gerbeaud, Olivier; Quirt, David; Doney, Amber; Ledru, Patrick

2017-12-01

Graphitic shear zones are spatially associated with unconformity-related uranium deposits that are located around the unconformity between the strata of the Paleo- to Mesoproterozoic Athabasca Basin (Saskatchewan, Canada) and its underlying Archean to Paleoproterozoic basement. The present study focuses on basement-hosted ductile-brittle graphitic shear zones near the Cigar Lake U deposit, one of the largest unconformity-related U deposits. The goal of the study is to decipher the pre-Athabasca Basin fluid migration history recorded within such structures and its potential role on the formation of such exceptional deposit. Dominantly C-O-H(-N) metamorphic fluids have been trapped in Fluid Inclusion Planes (FIPs) in magmatic quartz within ductile-brittle graphitic shear zones active during retrograde metamorphism associated with the formation of the Wollaston-Mudjatik Transition Zone (WMTZ) between ca. 1805 and 1720 Ma. Such fluids show a compositional evolution along the retrograde path, from a dense and pure CO2 fluid during the earliest stages, through a lower density CO2 ± CH4-N2 (± H2O) fluid and, finally, to a very low density CH4-N2 fluid. Statistical study of the orientation, distribution, proportion, and chemical characterization of the FIPs shows that: i) CO2 (δ13CCO2 around - 9‰ PDB) from decarbonation reactions and/or partial water-metamorphic graphite equilibrium initially migrated regionally and pervasively under lithostatic conditions at about 500 to 800 °C and 150 to 300 MPa. Such P-T conditions attest to a high geothermal gradient of around 60 to 90 °C/km, probably related to rapid exhumation of the basement or a large-scale heat source. ii) Later brittle reactivation of the shear zone at around 450 °C and 25-50 MPa favored circulation of CO2-CH4-N2(± H2O) fluids in equilibrium with metamorphic graphite (δ13CCO2 around - 14‰) under hydrostatic conditions and only within the shear zones. Cooling of these fluids and the water uptake linked to fluid-basement rock reactions led to the precipitation at around 450 °C of poorly-crystallized hydrothermal graphite. This graphite presents isotopic (δ13C - 30 to - 26‰ PDB) and morphological differences from the high-T metamorphic graphite (> 600 °C, - 29 to - 20‰ δ13C) derived from metamorphism of C-rich sedimentary material. The brittle structural reactivation and the related fluid migration and graphite precipitation were specifically focused within the shear zones and related damage zones. The brittle reactivation produced major changes in the petro-physical, mineralogical, and chemical characteristics of the structures and their damage zones. It especially increased the fracture paleoporosity and rock weakness toward the fault cores. These major late metamorphic modifications of the graphitic shear zones were likely key parameters favoring the enhanced reactivity of these basement zones under tectonic stress following deposition of the Athabasca Basin, and so controlled basinal brine movement at the basin/basement interface related to the formation of the unconformity-related uranium deposits. This relationship consequently readily explains the specific spatial relationships between unconformity-related U deposits and the ductile-brittle graphitic shear zones.

Microdeformation in Vredefort rocks; evidence for shock metamorphism

NASA Technical Reports Server (NTRS)

Reimold, W. U.; Andreoli, M. A. G.; Hart, R. J.

1988-01-01

Planar microdeformations in quartz from basement or collar rocks of the Vredefort Dome have been cited for years as the main microtextural evidence for shock metamorphism in this structure. In addition, Schreyer describes feldspar recrystallization in rocks from the center of the Dome as the result of transformation of diaplectic glass, and Lilly reported the sighting of mosaicism in quartz. These textural observations are widely believed to indicate either an impact or an internally produced shock origin for the Vredefort Dome. Two types of (mostly sub) planar microdeformations are displayed in quartz grains from Vredefort rocks: (1) fluid inclusion trails, and (2) straight optical discontinuities that sometimes resemble lamellae. Both types occur as single features or as single or multiple sets in quartz grains. Besides qualitative descriptions of cleavage and recrystallization in feldspar and kinkbands in mica, no further microtextural evidence for shock metamorphism at Vredefort has been reported to date. Some 150 thin sections of Vredefort basement rocks were re-examined for potential shock and other deformation effects in all rock-forming minerals. This included petrographic study of two drill cores from the immediate vicinity of the center of the Dome. Observations recorded throughout the granitic core are given along with conclusions.

Islas Marias Archipelago, Mexico. A Missing Piece to Reconstruct the Paleoposition of Baja California

NASA Astrophysics Data System (ADS)

Schaaf, P. E.; Pompa, V.; Hernandez, T.; Weber, B.; Solis, G.; Villanueva, D.; Perez-Venzor, J.

2011-12-01

Paleopositions for southern Baja California peninsula have yielded controversial models over the past 30 years. Mainly based on paleomagnetic data many hypotheses place Baja at lower paleolatitudes in front of southern Mexico or Central America with subsequent northward translations. Other models suggest minor, if any, northward displacements with respect to continental Mexico combined with clockwise rotations. Lithological, geochemical, and geochronological similarities for southern Baja California and Puerto Vallarta (western Mexico Pacific margin) igneous rocks seem to confirm the latter model. To further prove this model we have mapped and collected rocks from Maria Madre, the largest island of the Islas Marias archipelago, located in the mouth of the Gulf of California. In an area of only 145 square kilometers, metamorphic basements rocks (ortho and migmatitic gneisses), highly deformed metasediments, granitoids, acid to intermediate volcanic sequences, and a cover with gently folded marine sediments are exposed. The basement complex with gneisses and metasediments, including garnet-bearing paragneiss and calc silicates, as well as the granodioritic-tonalitic intrusives display an extraordinary accordance with similar units observed in the Los Cabos Block (LCB) of Baja California Sur. Furthermore, U-Pb zircon ages of 162 and 170 Ma for the basement gneisses and of 80 Ma for the granitoids have been reported also from the LCB. Additionally, upper Cretaceous intrusive ages are well known from the Puerto Vallarta batholith in Jalisco and Nayarit, mainland Mexico. Geochemical and isotopic data as well as Nd model ages confirm a magmatic consanguinity of LCB, Islas Marias, and Puerto Vallarta granitoids. The volcanic units of Maria Madre Island include ignimbrites and effusive dacitic-rhyolithic rocks, which can be correlated to the Sierra Madre Occidental province and the Comundú Formation of Baja California. Age determinations are under work to confirm this hypothesis. Finally, the sedimentary cover record several subsidence and uplift events, possibly related to the opening of the Gulf of California. Micropaleontological investigations on Neogene deposits give evidence for an above sea level position of the islands since the late Pleistocene. In summary, our new lithological, geochemical, and geochronological investigations on the Isla María Madre rock suite confirm a common magmatic arc parenthood of these units with respect to mainland Mexico at latitudes of Puerto Vallarta and with the LCB of Baja California Sur. Large paleolatitudal displacements of Baja can be discarded from these results.

Low Pressure-High Temperature Metamorphism and the Advection of Heat to the Continental Crust: A Case Study from Northwest New Guinea

NASA Astrophysics Data System (ADS)

Jost, B.; Webb, M.; White, L. T.

2017-12-01

In northwest New Guinea, Palaeozoic basement rocks forming part of the northern margin of the Australian continent are exposed in a rugged mountain range. This remote and understudied region provides a unique window into the complex Palaeozoic evolution and tectonic history of this region, which we help unravel with new field, petrographic, geochemical, and geochronological data. The basement rocks consist of extensive meta-turbidites that were subject to low pressure-high temperature metamorphism along their eastern margin. They are cross-cut by predominantly acidic granitoids. U-Pb zircon dating reveals that these granitoids intruded in three episodes in the Devonian-Carboniferous, the Carboniferous, and the Triassic. The first episode has not previously been reported in the region. The granitoids are strongly peraluminous, suggesting that partial melting of the meta-sedimentary country rock contributed to their petrogenesis (S-type). The occurrence and character of country rock xenoliths and migmatites supports this interpretation. Equilibrium thermodynamic modelling of the metapelites and the migmatites indicates that a substantial amount of heat was added to the lower and middle crust to cause partial melting and regional metamorphism at relatively low pressure. We propose repeated intrusion of hot magma as the mechanism responsible for advecting the necessary heat from the mantle. This likely occurred in an active continental margin setting during the Devonian-Carboniferous and the Triassic, possibly separated by an interval of magmatic quiescence during most of the Permian. New biostratigraphic and low-temperature thermochronological data reveal very recent Pliocene-Pleistocene uplift and unroofing of these basement rocks.

The Las Matras tonalitic trondhjemitic pluton, central Argentina: Grenvillian-age constraints, geochemical characteristics, and regional implications

NASA Astrophysics Data System (ADS)

Sato, A. M.; Tickyj, H.; Llambías, E. J.; Sato, K.

2000-12-01

The N-S trending belt with Grenvillian-age rocks developed in central western Argentina represents the basement of an allochthonous terrane derived from Laurentia during the Early Paleozoic. The Las Matras pluton (36°46‧S, 67°07‧W) is located at the southern extension of this belt in the Las Matras Block. It consists of a low-Al tonalitic to trondhjemitic facies characteristic of an arc magmatism. Isotopic studies yielded Grenvillian Rb-Sr (1212±47 Ma) and Sm-Nd (1188±47 Ma) ages which, due to the undeformed and non-metamorphosed character of the pluton, are interpreted to represent a crystallization age of around 1200 Ma. Although this age is slightly older than available dates from other exposures of the same belt, and the undeformed feature is also distinctive for Las Matras, the depleted Sr and Nd isotopic signatures of the pluton agree with those from other magmatic rocks involved in that belt. The differences found between Las Matras and the northern exposures indicate that this belt with Grenvillian-age rocks comprises regions of non-homogeneous evolution. Although the correlation of the Lower Paleozoic platform carbonates from the sedimentary cover of the Grenvillian-age basement rocks suggests the surroundings of the Southern Grenville Province (Texas and northern Mexico) as the probable detachment site for the Argentine belt, comparison of magmatic and tectonic processes involved in these basement rocks does not indicate similar evolutions. This fact can suggest an independent evolution of the Argentine belt prior to amalgamation to the Laurentian Grenville orogen.

Structural implications of an offset Early Cretaceous shoreline in northern California

USGS Publications Warehouse

Jones, D.L.; Irwin, W.P.

1971-01-01

Recognition of a nonmarine to marine transition in sedimentary rocks at Glade Creek and Big Bar in the southern Klamath Mountains permits reconstruction of the approximate position of a north-trending Early Cretaceous (Valanginian) shoreline. At the southern end of the Klamath Mountains, the shoreline is displaced 60 mi or more to the east by a west-northwest-trending fault zone. South of this fault zone the shoreline is buried at a much lower level beneath late Cenozoic rocks in the Great Valley. This large displacement probably is the result of differential movement along a system of left-lateral tear faults in the upper plate of the Coast Range thrust. The westward bulge of the Klamath arc also may have resulted from this faulting, as the amount and direction of the bulge is comparable with the displacement of the Valanginian shoreline.Basal clastic strata at both Glade Creek and Big Bar contain abundant fresh-water or brackish-water clams, many of which consist of unabraded paired valves. These are conformably overlain by Valanginian marine strata containing Buchia crassicollis solida.The position of the Valanginian shoreline beneath the Great Valley cannot be directly observed because it is buried by thick late Cenozoic deposits. However, its approximate westernmost limit must lie between the outcrop belt of marine strata on the west side of the valley and drill holes to basement on the east side, in which equivalent strata are absent.Franciscan rocks containing Valanginian fossils occur 10 mi southwest of Glade Creek, but these are deep-water marine eugeosynclinal rocks that were deposited far to the west of the shoreline. The deformation responsible for the displacement of the Valanginian shoreline and juxtaposition of the Franciscan rocks and Klamath Mountain basement rocks involved eastward under-thrusting of the Franciscan beneath the Coast Range thrust contemporaneous with differential movement along tear faults within the upper plate.

Tectonic controls on large landslide complex: Williams Fork Mountains near Dillon, Colorado

USGS Publications Warehouse

Kellogg, K.S.

2001-01-01

An extensive (~ 25 km2) landslide complex covers a large area on the west side of the Williams Fork Mountains in central Colorado. The complex is deeply weathered and incised, and in most places geomorphic evidence of sliding (breakaways, hummocky topography, transverse ridges, and lobate distal zones) are no longer visible, indicating that the main mass of the slide has long been inactive. However, localized Holocene reactivation of the landslide deposits is common above the timberline (at about 3300 m) and locally at lower elevations. Clasts within the complex, as long as several tens of meters, are entirely of crystalline basement (Proterozoic gneiss and granitic rocks) from the hanging wall of the Laramide (Late Cretaceous to Early Tertiary), west-directed Williams Range thrust, which forms the western structural boundary of the Colorado Front Range. Late Cretaceous shale and sandstone compose most footwall rocks. The crystalline hanging-wall rocks are pervasively fractured or shattered, and alteration to clay minerals is locally well developed. Sackung structures (trenches or small-scale grabens and upslope-facing scarps) are common near the rounded crest of the range, suggesting gravitational spreading of the fractured rocks and oversteepening of the mountain flanks. Late Tertiary and Quaternary incision of the Blue River Valley, just west of the Williams Fork Mountains, contributed to the oversteepening. Major landslide movement is suspected during periods of deglaciation when abundant meltwater increased pore-water pressure in bedrock fractures. A fault-flexure model for the development of the widespread fracturing and weakening of the Proterozoic basement proposes that the surface of the Williams Range thrust contains a concave-downward flexure, the axis of which coincides approximately with the contact in the footwall between Proterozoic basement and mostly Cretaceous rocks. Movement of brittle, hanging-wall rocks through the flexure during Laramide deformation pervasively fractured the hanging-wall rocks. ?? 2001 Elsevier Science B.V. All rights reserved.

Geologic map of the Ennis 30' x 60' quadrangle, Madison and Gallatin Counties, Montana

USGS Publications Warehouse

Kellogg, Karl S.; Williams, Van S.

1998-01-01

The Ennis 1:100,000 quadrangle lies within both the Laramide (Late Cretaceous to early Tertiary) foreland province of southwestern Montana and the northeastern margin of the middle to late Tertiary Basin and Range province. The oldest rocks in the quadrangle are Archean high-grade gneiss, and granitic to ultramafic intrusive rocks that are as old as about 3.0 Ga. The gneiss includes a supracrustal assemblage of quartz-feldspar gneiss, amphibolite, quartzite, and biotite schist and gneiss. The basement rocks are overlain by a platform sequence of sedimentary rocks as old as Cambrian Flathead Quartzite and as young as Upper Cretaceous Livingston Group sandstones, shales, and volcanic rocks. The Archean crystalline rocks crop out in the cores of large basement uplifts, most notably the 'Madison-Gravelly arch' that includes parts of the present Tobacco Root Mountains and the Gravelly, Madison, and Gallatin Ranges. These basement uplifts or blocks were thrust westward during the Laramide orogeny over rocks as young as Upper Cretaceous. The thrusts are now exposed in the quadrangle along the western flanks of the Gravelly and Madison Ranges (the Greenhorn thrust and the Hilgard fault system, respectively). Simultaneous with the west-directed thrusting, northwest-striking, northeast-side-up reverse faults formed a parallel set across southwestern Montana; the largest of these is the Spanish Peaks fault, which cuts prominently across the Ennis quadrangle. Beginning in late Eocene time, extensive volcanism of the Absorka Volcanic Supergroup covered large parts of the area; large remnants of the volcanic field remain in the eastern part of the quadrangle. The volcanism was concurrent with, and followed by, middle Tertiary extension. During this time, the axial zone of the 'Madison-Gravelly arch,' a large Laramide uplift, collapsed, forming the Madison Valley, structurally a complex down-to-the-east half graben. Basin deposits as thick as 4,500 m filled the graben. Pleistocene glaciers sculpted the high peaks of the mountain ranges and formed the present rugged topography.

Geologic map of the Ennis 30' x 60' quadrangle, Madison and Gallatin Counties, Montana, and Park County, Wyoming

USGS Publications Warehouse

Kellogg, Karl S.; Williams, Van S.

2000-01-01

The Ennis 1:100,000 quadrangle lies within both the Laramide (Late Cretaceous to early Tertiary) foreland province of southwestern Montana and the northeastern margin of the middle to late Tertiary Basin and Range province. The oldest rocks in the quadrangle are Archean high-grade gneiss, and granitic to ultramafic intrusive rocks that are as old as about 3.0 Ga. The gneiss includes a supracrustal assemblage of quartz-feldspar gneiss, amphibolite, quartzite, and biotite schist and gneiss. The basement rocks are overlain by a platform sequence of sedimentary rocks as old as Cambrian Flathead Quartzite and as young as Upper Cretaceous Livingston Group sandstones, shales, and volcanic rocks. The Archean crystalline rocks crop out in the cores of large basement uplifts, most notably the 'Madison-Gravelly arch' that includes parts of the present Tobacco Root Mountains and the Gravelly, Madison, and Gallatin Ranges. These basement uplifts or blocks were thrust westward during the Laramide orogeny over rocks as young as Upper Cretaceous. The thrusts are now exposed in the quadrangle along the western flanks of the Gravelly and Madison Ranges (the Greenhorn thrust and the Hilgard fault system, respectively). Simultaneous with the west-directed thrusting, northwest-striking, northeast-side-up reverse faults formed a parallel set across southwestern Montana; the largest of these is the Spanish Peaks fault, which cuts prominently across the Ennis quadrangle. Beginning in late Eocene time, extensive volcanism of the Absorka Volcanic Supergroup covered large parts of the area; large remnants of the volcanic field remain in the eastern part of the quadrangle. The volcanism was concurrent with, and followed by, middle Tertiary extension. During this time, the axial zone of the 'Madison-Gravelly arch,' a large Laramide uplift, collapsed, forming the Madison Valley, structurally a complex down-to-the-east half graben. Basin deposits as thick as 4,500 m filled the graben. Pleistocene glaciers sculpted the high peaks of the mountain ranges and formed the present rugged topography.

Geological Structure of the Basement of Western and Eastern Parts of the West-Siberian Plain

ERIC Educational Resources Information Center

Ivanov, Kirill S.; Erokhin, Yuriy V.; Ponomarev, Vladimir S.; Pogromskaya, Olga E.; Berzin, Stepan V.

2016-01-01

The U-Pb dating (SHRIMP-II on zircon) was obtained for the first time from the basement of the West Siberian Plain in the Western half of the region. It is established that a large part of the protolith of the metamorphic depth in the Shaim-Kuznetsov meganticlinorium contained sedimentary late- and middle-Devonian rocks (395-358 million years).…

Deciphering fluid sources of hydrothermal systems: A combined Sr- and S-isotope study on barite (Schwarzwald, SW Germany)

USGS Publications Warehouse

Staude, S.; Gob, S.; Pfaff, K.; Strobele, F.; Premo, W.R.; Markl, G.

2011-01-01

Primary and secondary barites from hydrothermal mineralizations in SW Germany were investigated, for the first time, by a combination of strontium (Sr) isotope systematics (87Sr/86Sr), Sr contents and δ34S values to distinguish fluid sources and precipitation mechanisms responsible for their formation. Barite of Permian age derived its Sr solely from crystalline basement rocks, whereas all younger barite also incorporate Sr from formation waters of the overlying sediments. In fact, most of the Sr in younger barite is leached from Lower and Middle Triassic sediments. In contrast, most of the sulfur (S) of Permian, Jurassic and northern Schwarzwald Miocene barite originated from basement rocks. The S source of Upper Rhinegraben (URG)-related Paleogene barite differs depending on geographic position: for veins of the southern URG, it is the Oligocene evaporitic sequence, while central URG mineralizations derived its S from Middle Triassic evaporites. Using Sr isotopes of barite of known age combined with estimates on the Sr contents and Sr isotopic ratios of the fluids' source rocks, we were able to quantify mixing ratios of basement-derived fluids and sedimentary formation waters for the first time. These calculations show that Jurassic barite formed by mixing of 75–95% ascending basement-derived fluids with 5–25% sedimentary formation water, but that only 20–55% of the Sr was brought by the basement-derived fluid to the depositional site. Miocene barite formed by mixing of an ascending basement-derived brine (60–70%) with 30–40% sedimentary formation waters. In this case, only 8–15% of the Sr was derived from the deep brine. This fluid-mixing calculation is an example for deposits in which the fluid source is known. This method applied to a greater number of deposits formed at different times and in various geological settings may shed light on more general causes of fluid movement in the Earth's crust and on the formation of hydrothermal ore deposits.

Coolwater culmination: Sensitive high-resolution ion microprobe (SHRIMP) U-Pb and isotopic evidence for continental delamination in the Syringa Embayment, Salmon River suture, Idaho

USGS Publications Warehouse

Lund, K.; Aleinikoff, J.N.; Yacob, E.Y.; Unruh, D.M.; Fanning, C.M.

2008-01-01

During dextral oblique translation along Laurentia in western Idaho, the Blue Mountains superterrane underwent clockwise rotation and impinged into the Syringa embayment at the northern end of the Salmon River suture. Along the suture, the superterrane is juxtaposed directly against western Laurentia, making this central Cordilleran accretionary-margin segment unusually attenuated. In the embayment, limited orthogonal contraction produced a crustal wedge of oceanic rocks that delaminated Laurentian crust. The wedge is exposed through Laurentian crust in the Coolwater culmination as documented by mapping and by sensitive high-resolution ion microprobe U-Pb, Sri, and ??Nd data for gneisses that lie inboard of the suture. The predominant country rock is Mesoproterozoic paragneiss overlying Laurentian basement. An overlying Neoproterozoic (or younger) paragneiss belt in the Syringa embayment establishes the form of the Cordilleran miogeocline and that the embayment is a relict of Rodinia rifting. An underlying Cretaceous paragneiss was derived from arc terranes and suture-zone orogenic welt but also from Laurentia. The Cretaceous paragneiss and an 86-Ma orthogneiss that intruded it formed the wedge of oceanic rocks that were inserted into the Laurentian margin between 98 and 73 Ma, splitting supracrustal Laurentian rocks from their basement. Crustal thickening, melting and intrusion within the wedge, and folding to form the Coolwater culmination continued until 61 Ma. The embayment formed a restraining bend at the end of the dextral transpressional suture. Clockwise rotation of the impinging superterrane and overthrusting of Laurentia that produced the crustal wedge in the Coolwater culmination are predicted by oblique collision into the Syringa embayment. Copyright 2008 by the American Geophysical Union.

Burial history, thermal history and hydrocarbon generation modelling of the Jurassic source rocks in the basement of the Polish Carpathian Foredeep and Outer Carpathians (SE Poland)

NASA Astrophysics Data System (ADS)

Kosakowski, Paweł; Wróbel, Magdalena

2012-08-01

Burial history, thermal maturity, and timing of hydrocarbon generation were modelled for the Jurassic source rocks in the basement of the Carpathian Foredeep and marginal part of the Outer Carpathians. The area of investigation was bounded to the west by Kraków, to the east by Rzeszów. The modelling was carried out in profiles of wells: Będzienica 2, Dębica 10K, Góra Ropczycka 1K, Goleszów 5, Nawsie 1, Pławowice E1 and Pilzno 40. The organic matter, containing gas-prone Type III kerogen with an admixture of Type II kerogen, is immature or at most, early mature to 0.7 % in the vitrinite reflectance scale. The highest thermal maturity is recorded in the south-eastern part of the study area, where the Jurassic strata are buried deeper. The thermal modelling showed that the obtained organic matter maturity in the initial phase of the "oil window" is connected with the stage of the Carpathian overthrusting. The numerical modelling indicated that the onset of hydrocarbon generation from the Middle Jurassic source rocks was also connected with the Carpathian thrust belt. The peak of hydrocarbon generation took place in the orogenic stage of the overthrusting. The amount of generated hydrocarbons is generally small, which is a consequence of the low maturity and low transformation degree of kerogen. The generated hydrocarbons were not expelled from their source rock. An analysis of maturity distribution and transformation degree of the Jurassic organic matter shows that the best conditions for hydrocarbon generation occurred most probably in areas deeply buried under the Outer Carpathians. It is most probable that the "generation kitchen" should be searched for there.

Coupling Flow & Transport Modeling with Electromagnetic Geophysics to Better Understand Crustal Permeability

NASA Astrophysics Data System (ADS)

Pepin, J.; Folsom, M.; Person, M. A.; Kelley, S.; Gomez-Velez, J. D.; Peacock, J.

2016-12-01

Over the last 30 years, considerable effort has focused on understanding the distribution of permeability within the earth's crust and its implications for flow and transport. The scarcity of direct observations makes the description of permeabilities beyond depths of about 3 km particularly challenging. Numerous studies have defined depth-decay relationships for basement permeability, while others note that it is too complex to be characterized by a general relationship. Hydrothermal modeling studies focusing on two geothermal systems within the tectonically active Rio Grande rift of New Mexico suggest that there may be laterally extensive regions of highly permeable (10-14 to 10-12 m2) basement rocks at depths ranging between 4 and 8 km. The NaCl groundwater signature, elevated fracture density, and secondary mineralization of fractured basement outcrops associated with these geothermal systems indicate that there may indeed be significant groundwater flow within the basement rocks of the rift. We hypothesize that there are extensive regions of highly permeable crystalline basement rocks at depths greater than 3 km within the Rio Grande rift. These fractured zones serve as large conduits for geothermal fluids before they ascend to shallow depths through gaps in overlying confining sediments or along faults. To test these hypotheses, we use a combination of geophysical observations and flow and transport modeling. We used electromagnetic geophysics (TEM & MT) to image resistivity in one of the hypothesized deep circulation geothermal systems near Truth or Consequences, NM. The resistivity dataset, in tandem with geochemical and thermal observations, is then used to calibrate a hydrothermal model of the system. This new calibration methodology has the potential to change the way researchers study crustal fluid flow and geothermal systems; thereby providing a tool to explore depths greater than 3 km where minimal data is available. In addition, it has the advantage of being scale-independent; meaning it can be applied to localized models just as well as continental-scale models.

Variscan tectonics in Dodecanese, Kalymnos island, Greece

NASA Astrophysics Data System (ADS)

Chatziioannou, Eleftheria; Grasemann, Bernhard; Schneider, David; Hubmann, Bernhard; Soukis, Konstantinos

2015-04-01

Kalymnos island is located in the Dodecanese, southeastern Aegean Sea, and geologically appears to be part of the external Hellenides. Pre-Alpidic basement rocks on the Dodecanese islands have been suggested to record compelling similarities with the basement rocks in Eastern Crete with respect to their lithologies and pre-Alpidic metamorphic evolution. The lithotectonic units experienced greenschist to amphibolite facies conditions during the Variscan orogeny. Whereas the rocks in Eastern Crete reveal Alpine high-pressure overprint, the Variscan basement units in the Dodecanese record no or low-grade Alpine metamorphism. A field study of basement rocks below Mesozoic limestones and dolomites in the NW part of Kalymnos near Emporios uncovered a complex history of metamorphism, folding and faulting. Three different tectonic units can be discriminated from top to bottom: a) a quartz-mica schist, b) a white-grey, fossiliferous coarse grained marble and c) a fine-grained fossiliferous blue-grey marble. In the marbles macrofossils such as brachiopods, ammonoid cephalopods (Goniatids?) and crinoids suggest a Middle-Upper Devonian deposition age (Givetian- Frasnian). Structural mapping the area resolved a dominant W-E shortening event, resulting in an overall inverted metamorphic gradient. The lowermost blue-grey marble unit is folded into large-scale upright folds, which are truncated by top-to-east overthrusting of the white-grey marble unit. Whereas deformation mechanisms in the blue-grey marble unit are dominated by dissolution-precipitation creep, the white-grey marble suffered intense crystal plastic deformation with localized high-strain mylonitic shear zones. The uppermost quartz-mica schist unit is separated from the lower units by a cataclastic phyllonitic shear zone. 40Ar/39Ar geochronological dating on white micas from the quartz-mica schists yielded cooling ages between 240 and 334 Ma indicative of Variscan cooling. Our data suggest that this part of the Dodecanese experienced intense Variscan deformation with no high-temperature Alpine overprint.

Petrogenesis of metaultramafic rocks from the Quadrilátero Ferrífero and adjacent terrains, Minas Gerais, Brazil: Two events of ultramafic magmatism?

NASA Astrophysics Data System (ADS)

da Fonseca, Gabriela Magalhães; Jordt-Evangelista, Hanna; Queiroga, Gláucia Nascimento

2018-03-01

In the worldwide known Quadrilátero Ferrífero and the adjacent terrains, southeastern Brazil, many serpentinite and soapstone quarries, and some rare bodies of metaultramafic rocks that partially preserve minerals or textures from the original igneous protolith can be found. It is not known if the protoliths and the ages of the metaultramafic rocks found in the Quadrilátero Ferrífero (and its oriental basement) and Mineiro Belt regions are the same or if they represent distinct magmatic episodes. The petrogenetic investigation, specially concerning the REE contents, aimed to gather informations about the type of magmatism and the mantle source in order to compare the metaultramafic rocks of both regions. The interpretation of the data concerning petrography, mineral chemistry and geochemistry shows that the metaultramafic rocks are similar to komatiitic peridotites, with MgO contents > 22 wt % and TiO2 < 0.9 wt %. The plot of the REE for the lithotypes found in the Quadrilátero Ferrífero shows decrease in LREE possibly reflecting the depletion of the mantle source. On the other hand the samples from the Mineiro Belt are enriched in LREE suggesting a mantle source enriched in these elements. This enrichment may have been caused by mantle metassomatism that occurred during accretion of the Paleoproterozoic magmatic arc that generated the Mineiro belt. In this paper, we therefore suggest two periods of ultramafic magmatism. The first one found in the Archean basement of the Quadrilátero Ferrífero, with a depleted mantle source. The second occurred in the Paleoproterozoic basement of the Mineiro belt, having a metassomatized mantle as source.

Geomechanical Considerations for the Deep Borehole Field Test

NASA Astrophysics Data System (ADS)

Park, B. Y.

2015-12-01

Deep borehole disposal of high-level radioactive waste is under consideration as a potential alternative to shallower mined repositories. The disposal concept consists of drilling a borehole into crystalline basement rocks to a depth of 5 km, emplacement of canisters containing solid waste in the lower 2 km, and plugging and sealing the upper 3 km of the borehole. Crystalline rocks such as granites are particularly attractive for borehole emplacement because of their low permeability and porosity at depth, and high mechanical strength to resist borehole deformation. In addition, high overburden pressures contribute to sealing of some of the fractures that provide transport pathways. We present geomechanical considerations during construction (e.g., borehole breakouts, disturbed rock zone development, and creep closure), relevant to both the smaller-diameter characterization borehole (8.5") and the larger-diameter field test borehole (17"). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

New age data on the geological evolution of Southern India

NASA Technical Reports Server (NTRS)

Taylor, Paul N.; Chadwick, B.; Friend, C. R. L.; Ramakrishnan, M.; Moorbath, Stephen; Viswanatha, M. N.

1988-01-01

The Peninsular Gneisses of Southern India developed over a period of several hundred Ma in the middle-to-late Archaean. Gneisses in the Gorur-Hassan area of southern Karnataka are the oldest recognized constituents: Beckinsale et al. reported a preliminary Rb-Sr whole-rock isochron age of 33558 + or - 66 Ma, but further Rb-Sr and Pb/Pb whole-rock isochron determinations indicate a slightly younger, though more precise age of ca 3305 Ma (R. D. Beckinsale, Pers. Comm.). It is well established that the Peninsular Gneisses constitute basement on which the Dharwar schist belts were deposited. Well-documented exposures of unconformities, with basal quartz pebble conglomerates of the Dharwar Supergroup overlying Peninsular Gneisses, have been reported from the Chikmagalur and Chitradurga areas, and basement gneisses in these two areas have been dated by Rb-Sr and Pb/Pb whole-rock isochron methods at ca 3150 Ma and ca 3000 Ma respectively. Dharwar supracrustal rocks of the Chitradurga schist belt are intruded by the Chitradurga Granite, dated by a Pb/Pb whole-rock isochron at 2605 + or - 18 Ma. These results indicate that the Dharwar Supergroup in the Chitradurga belt was deposited between 3000 Ma and 2600 Ma.

Mineral resource potential map of the James River Face Wilderness, Bedford and Rockbridge counties, Virginia

USGS Publications Warehouse

Brown, C. Erwin; Gazdik, Gertrude C.

1982-01-01

The rocks in the James River Face Wilderness are shales and quartzites that overlie a meta-igneous basement. They are folded into a large southwestward-plunging anticline that is cut off on the east and south by an extensive thrust fault that brings old basement rocks over the younger sedimentary rocks. Geochemical studies of stream sediments, soils, and rocks do not reveal any unusually high metal concentrations, but a large resource of metallurgical-grade quartzite and shale suitable for structural clay products and lightweight aggregate is in the wilderness. Antietam (Erwin) Quartzite has been quarried at three sites in the wilderness as raw material for silicon used in the manufacture of ferrosilicon. Other uses included crushed rock for concrete aggregate, road metal, and railroad ballast, and sand for cement and mortar. Potential uses include ganister for silica brick and specialty sands such as filter and furnace sand. Firing tests on samples of shale from the Harpers (Hampton) Formation show that it could be used for the manufacture of brick and as lightweight aggregate. Of marginal economic interest are heavy-mineral layers in the basal Unicoi (Weverton) Formation.

Summary of northern Atlantic coastal plain hydrology and its relation to disposal of high-level radioactive waste in buried crystalline rock; a preliminary appraisal

USGS Publications Warehouse

Lloyd, O.B.; Larson, J.D.; Davis, R.W.

1985-01-01

Interpretation of available hydrologic data suggests that some areas beneath the Coastal Plain in the States of Delaware, Maryland, New Jersey, North Carolina, and Virginia might have some potential for the disposal of nuclear waste in crystalline rock that is buried beneath the Coastal Plain sediments. The areas of major interest occur where the top of the basement rock lies between 1,000 and 4,000 feet below sea level, the aquifer(s) immediately above the basement rock are saturated with saline water, confining material overlies the saline water bearing aquifer(s), and groundwater flow in the saline water aquifer(s) can be established. Preliminary data on (1) the distribution and thickness of the lowermost aquifers and confining beds, (2) the distribution of hydraulic conductivity in the lowermost aquifers, (3) estimated hydraulic heads and inferred direction of lateral groundwater flow for 1980, and (4) the distribution of saline water and brine, indicate eastern parts of the study area relatively best meet most of the criteria proposed for sediments that would overlie any potential buried crystalline-rock disposal site.

Post-middle Miocene accretion of Franciscan rocks, northwestern California.

USGS Publications Warehouse

McLaughlin, R.J.; Kling, S.A.; Poore, R.Z.; McDougall, K.; Beutner, E.C.

1982-01-01

Deformed sedimentary rocks assigned to the Franciscan assemblage in the King Range S of Cape Mendocino, N California, are dominantly deep-water argillite and sandstone occurring as thick- to thin-bedded, locally channelized marine turbidities of arkosic to andesitic volcaniclastic composition. The King Range appears to be a displaced terrane of oceanic basement overlain by Palaeogene(?) and Neogene sedimentary and igneous rocks of continental and oceanic derivation.-Authors

Interpretation of Gravimetric and Aeromagnetic Data of the Tecoripa Chart in Southeast Sonora, Mexico.

NASA Astrophysics Data System (ADS)

Martínez-Retama, S.; Montaño-Del Cid, M. A.

2015-12-01

The Tecoripa chart H12-D64 is located southeast of the state of Sonora, México, south of Arizona. The geology is represented by sedimentary rocks of the Ordovician and Triassic, volcanic rocks of the Upper Cretaceous and Tertiary, intrusive rocks from the Upper Cretaceous- Tertiary and sedimentary rocks of the Cenozoic. In this paper a gravimetric study was conducted to determine the configuration and depth of the basement and to develop a structural model of the subsurface. For this purpose a consistent gravimetric survey in 3 profiles was conducted. To complement this study, gravimetric data obtained by INEGI (96 gravimetric stations spaced every 4000 m) that correspond to a regional survey was also used. The two sets of data were corrected and processed with the WinGLink software. The profiles were then modeled using the Talwani method. 4 Profiles corresponding to the gravimetric survey and 5 data profiles from INEGI were modeled. Aeromagnetic data from the total field of Tecoripa chart were also processed. The digital information was integrated and processed by generating a data grid. Processes applied to data consisted of reduction to the pole, regional-residual separation and upward continuations. In general, the obtained structural models show intrusive bodies associated with well-defined high gravimetric and magnetic and low gravimetric and magnetic are associated with basins and sedimentary rocks. The obtained geological models show the basement represented by volcanic rocks of the Tarahumara Formation from the Upper Cretaceous which are in contact with sedimentary rocks from the Barranca Group from Upper Cretaceous and limestones from the Middle Ordovician. Both volcanic and sedimentary rocks are intruded by granodiorite- granite with ages of the Tertiary-Oligocene. Based on the superficial geology as well as in the configuration of the basement and the obtained structural model the existence of faults with NW-SE orientation that originate Horst and Graben type structures can be inferred. The basins have depths of 2,000 to 4,000m with sedimentary fillings from the Báucarit Formation and Quaternary sediments.

Geochemistry and stratigraphic relations of middle Proterozoic rocks of the New Jersey Highlands

USGS Publications Warehouse

Volkert, Richard A.; Drake, Avery Ala

1999-01-01

Middle Proterozoic rocks of the New Jersey Highlands consist of a basement of dacitic, tonalitic, trondhjemitic, and charnockitic rocks that constitute the Losee metamorphic suite. These rocks are unconformably overlain by a layered supracrustal sequence of quartzo-feldspathic and calcareous rocks. Abundant sheets of hornblende- and biotite-bearing rocks of the Byram intrusive suite and clinopyroxene-bearing rocks of the Lake Hopatcong intrusive suite were synkinematically emplaced at about 1,090 Ma. These intrusive suites constitute the Vernon Supersuite. The postorogenic Mount Eve Granite has been dated at 1,020?4 Ma and is confined to the extreme northern Highlands.

Integrating Reflection Seismic, Gravity and Magnetic Data to Reveal the Structure of Crystalline Basement: Implications for Understanding Rift Development

NASA Astrophysics Data System (ADS)

Lenhart, Antje; Jackson, Christopher A.-L.; Bell, Rebecca E.; Duffy, Oliver B.; Fossen, Haakon; Gawthorpe, Robert L.

2016-04-01

Numerous rifts form above crystalline basement containing pervasive faults and shear zones. However, the compositional and mechanical heterogeneity within crystalline basement and the geometry and kinematics of discrete and pervasive basement fabrics are poorly understood. Furthermore, the interpretation of intra-crustal structures beneath sedimentary basins is often complicated by limitations in the depth of conventional seismic imaging, the commonly acoustically transparent nature of basement, limited well penetrations, and complex overprinting of multiple tectonic events. Yet, a detailed knowledge of the structural and lithological complexity of crystalline basement rocks is crucial to improve our understanding of how rifts evolve. Potential field methods are a powerful but perhaps underutilised regional tool that can decrease interpretational uncertainty based solely on seismic reflection data. We use petrophysical data, high-resolution 3D reflection seismic volumes, gridded gravity and magnetic data, and 2D gravity and magnetic modelling to constrain the structure of crystalline basement offshore western Norway. Intra-basement structures are well-imaged on seismic data due to relatively shallow burial of the basement beneath a thin (<3.5 km) sedimentary cover. Variations in basement composition were interpreted from detailed seismic facies analysis and mapping of discrete intra-basement reflections. A variety of data filtering and isolation techniques were applied to the original gravity and magnetic data in order to enhance small-scale field variations, to accentuate formation boundaries and discrete linear trends, and to isolate shallow and deep crustal anomalies. In addition, 2D gravity and magnetic data modelling was used to verify the seismic interpretation and to further constrain the configuration of the upper and lower crust. Our analysis shows that the basement offshore western Norway is predominantly composed of Caledonian allochthonous nappes overlying large-scale anticlines of Proterozoic rocks of the Western Gneiss Region. Major Devonian extensional brittle faults, detachments and shear zones transect those tectono-stratigraphic units. Results from structural analysis of enhanced gravity and magnetic data indicate the presence of distinct intra-basement bodies and structural lineaments at different scales and depth levels which correlate with our seismic data interpretation and can be linked to their onshore counterparts exposed on mainland Norway. 2D forward models of gravity and magnetic data further support our interpretation and quantitatively constrain variations in magnetic and density properties of principal basement units. We conclude that: i) enhanced gravity and magnetic data are a powerful tool to constrain the geometry of individual intra-basement bodies and to detect structural lineaments not imaged in seismic data; ii) insights from this study can be used to evaluate the role of pre-existing basement structures on the evolution of rift basins; and iii) the integration of a range of geophysical datasets is crucial to improve our understanding of the deep subsurface.

Petrological and geochemical compositions of beach sands of the Barton and Weaver peninsulas of King George Island, West Antarctica: implications for provenance and deglacial history

NASA Astrophysics Data System (ADS)

Lee, Y. I.; Lim, H. S.; Choi, T.

2017-12-01

We studied the provenance of beach sediments of the Baton and Weaver peninsulas of King George Island, the South Shetland Islands of West Antarctica. The studied beach sand sediments of the both peninsulas are predominantly composed of volcanic-rock fragment, followed by altered grain and plutonic rock fragment in that order. In rock fragments, the volcanic rock fragments are about four times more than the plutonic rock fragments. The median quartz-feldspar-rock fragment (Q-F-R) ratios of the beach sands of the Weaver and Barton peninsulas are Q3.4-F5.5-R99.1 and Q0.5-F2.7-R96.8, respectively. These beach sands may have been originated from basaltic andesite-andesite distributed in the ice-free areas of the Barton and Weaver peninsulas and granodiorite of the Barton Peninsula. According to the geochemistry of the beach sand sediments of the two peninsulas, most of the sand samples are interpreted as originating from intermediate rocks that have experienced little chemical weathering. Taking together the modal composition and geochemical composition of the beach sand samples, the tectonic setting of the source area is interpreted as a magmatic arc setting. This interpretation is consistent with geology of the ice-free areas of the Barton and Weaver peninsulas and the tectonic setting of King George Island. However, the sand samples of the Barton Peninsula southern beach and the Weaver Peninsula beach were not derived from basement rocks currently exposed in the ice-free areas of the corresponding peninsula, but were formerly glaciomarine sediments derived from erosion of ice-covered subglacial basement rocks and transported to the submerged glacier grounding line prior to deglaciation. Sand sediments derived from wave erosion of basement rocks of paleoshoreline might have been mixed with these glaciomarine sediments. King George Island became uplifted due to deglaciation 6,000 years ago. The studied beach sediments might have been reworked after the uplift of the King George Island to the present level. Accordingly, the studied beach sand sediments of the Barton and Weaver peninsulas are interpreted to be a palimpsest deposit comprising a mixture of originally glaciomarine sediments accumulated in the shallow fjord post the Last Glacial Maximum and some detritus supplied to the beaches since deglaciation.

Reservoir uncertainty, Precambrian topography, and carbon sequestration in the Mt. Simon Sandstone, Illinois Basin

USGS Publications Warehouse

Leetaru, H.E.; McBride, J.H.

2009-01-01

Sequestration sites are evaluated by studying the local geological structure and confirming the presence of both a reservoir facies and an impermeable seal not breached by significant faulting. The Cambrian Mt. Simon Sandstone is a blanket sandstone that underlies large parts of Midwest United States and is this region's most significant carbon sequestration reservoir. An assessment of the geological structure of any Mt. Simon sequestration site must also include knowledge of the paleotopography prior to deposition. Understanding Precambrian paleotopography is critical in estimating reservoir thickness and quality. Regional outcrop and borehole mapping of the Mt. Simon in conjunction with mapping seismic reflection data can facilitate the prediction of basement highs. Any potential site must, at the minimum, have seismic reflection data, calibrated with drill-hole information, to evaluate the presence of Precambrian topography and alleviate some of the uncertainty surrounding the thickness or possible absence of the Mt. Simon at a particular sequestration site. The Mt. Simon is thought to commonly overlie Precambrian basement granitic or rhyolitic rocks. In places, at least about 549 m (1800 ft) of topographic relief on the top of the basement surface prior to Mt. Simon deposition was observed. The Mt. Simon reservoir sandstone is thin or not present where basement is topographically high, whereas the low areas can have thick Mt. Simon. The paleotopography on the basement and its correlation to Mt. Simon thickness have been observed at both outcrops and in the subsurface from the states of Illinois, Ohio, Wisconsin, and Missouri. ?? 2009. The American Association of Petroleum Geologists/Division of Environmental Geosciences. All rights reserved.

Characterization Efforts in a Deep Borehole Field Test

NASA Astrophysics Data System (ADS)

Kuhlman, K. L.; Sassani, D.; Freeze, G. A.; Hardin, E. L.; Brady, P. V.

2016-12-01

The US Department of Energy Office of Nuclear Energy is embarking on a Deep Borehole Field Test to investigate the feasibility of constructing and characterizing two boreholes in crystalline basement rock to a depth of 5 km (16,400 ft). The concept of deep borehole disposal for radioactive waste has some advantages, including incremental construction and loading and the enhanced natural barriers provided by deep continental crystalline basement. Site characterization activities will include geomechanical (i.e., hydrofracture stress measurements), geological (i.e., core and mud logging), hydrological (i.e., packer-based pulse and pumping tests), and chemical (i.e., fluids sampled in situ from packer intervals and extracted from cores) tests. Borehole-based characterization will be used to determine the variability of system state (i.e., stress, pressure, temperature, and chemistry) with depth and interpretation of material and system parameters relevant to numerical site simulation. We explore the effects fluid density and geothermal temperature gradients (i.e., thermohaline convection) have on characterization goals in light of expected downhole conditions, including a disturbed rock zone surrounding the borehole. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Interpretation of high resolution airborne magnetic data (HRAMD) of Ilesha and its environs, Southwest Nigeria, using Euler deconvolution method

NASA Astrophysics Data System (ADS)

Olurin, Oluwaseun Tolutope

2017-12-01

Interpretation of high resolution aeromagnetic data of Ilesha and its environs within the basement complex of the geological setting of Southwestern Nigeria was carried out in the study. The study area is delimited by geographic latitudes 7°30'-8°00'N and longitudes 4°30'-5°00'E. This investigation was carried out using Euler deconvolution on filtered digitised total magnetic data (Sheet Number 243) to delineate geological structures within the area under consideration. The digitised airborne magnetic data acquired in 2009 were obtained from the archives of the Nigeria Geological Survey Agency (NGSA). The airborne magnetic data were filtered, processed and enhanced; the resultant data were subjected to qualitative and quantitative magnetic interpretation, geometry and depth weighting analyses across the study area using Euler deconvolution filter control file in Oasis Montag software. Total magnetic intensity distribution in the field ranged from -77.7 to 139.7 nT. Total magnetic field intensities reveal high-magnitude magnetic intensity values (high-amplitude anomaly) and magnetic low intensities (low-amplitude magnetic anomaly) in the area under consideration. The study area is characterised with high intensity correlated with lithological variation in the basement. The sharp contrast is enhanced due to the sharp contrast in magnetic intensity between the magnetic susceptibilities of the crystalline and sedimentary rocks. The reduced-to-equator (RTE) map is characterised by high frequencies, short wavelengths, small size, weak intensity, sharp low amplitude and nearly irregular shaped anomalies, which may due to near-surface sources, such as shallow geologic units and cultural features. Euler deconvolution solution indicates a generally undulating basement, with a depth ranging from -500 to 1000 m. The Euler deconvolution results show that the basement relief is generally gentle and flat, lying within the basement terrain.

Granulite-facies rocks in the Whatley Mill gneiss, Pine Mountain basement massif, Eastern Alabama

DOE Office of Scientific and Technical Information (OSTI.GOV)

Daniell, N.; Salpas, P.A.

1993-03-01

The Pine Mountain basement massif is a granulite terrane exposed in a tectonic window through the Inner Piedmont of western Georgia and eastern Alabama. Investigations of the westernmost extent of the massif, the Whatley Mill Gneiss, have revealed four distinct lithologies: (1) an augen gneiss, the type lithology; (2) mylonite that develops in the shear zones cutting the unit; (3) a phaneritic rock showing weak to no foliation; (4) enclaves of biotite gneiss within the weakly-foliated rock. Additionally, the weakly-foliated rock comprises two distinct phases which are in sharp contact along curved and undulating boundaries: phase 1 is a coarser-grainedmore » rock; phase 2 is a finer-grained rock of the same mineralogy as phase 1 except it contains rare hypersthene. This first recorded observation of hypersthene unequivocally confirms the granulite-facies origin of the unit. Major and trace element compositions of the phase 1 rock are identical to those of the augen gneiss. The phase 2 rock, has a distinct composition with higher SiO[sub 2] and lower incompatible trace elements than the phase 1 rock. The enclaves display a range in major elements but higher incompatible elements than the other lithologies. Geochemical and petrologic relationships leads one to interpret: (1) the weakly-foliated rock retains many of its primary igneous features including its two phases and enclaves; (2) the two phases of the weakly-foliated rock arose as a result of injection of one magma (phase 2) into a cooler, crystal mush solidifying from another magma (phase 1); (3) the enclaves represent either autoliths of xenoliths; (4) the augen gneiss arose by isochemical deformation of the phase 1 rock.« less

Nature of the Yucatan Block Basement as Derived From Study of Granitic Clasts in the Impact Breccias of Chicxulub Crater

NASA Astrophysics Data System (ADS)

Vera-Sanchez, P.; Rebolledo-Vieyra, M.; Perez-Cruz, L.; Urrutia-Fucugauchi, J.

2008-05-01

The tectonic and petrologic nature of the basement of the Yucatan Block is studied from analyses of basement clasts present in the impact suevitic breccias of Chicxulub crater. The impact breccias have been sampled as part of the drilling projects conducted in the Yucatan peninsula by Petroleos Mexicanos, the National University of Mexico and the Chicxulub Scientific Drilling Project. Samples analyzed come mainly from the Yaxcopoil-1, Tekax, and Santa Elena boreholes, and partly from Pemex boreholes. In this study we concentrate on clasts of the granites, granodiorites and quartzmonzonites in the impact breccias. We report major and trace element geochemical and petrological data, which are compared with data from the granitic and volcanic rocks from the Maya Mountains in Belize and from the Swannee terrane in Florida. Basement granitic clasts analyzed present intermediate to acidic sub-alkaline compositions. Plots of major oxides (e.g., Al2O3, Fe2O3, TiO2 and CaO) and trace elements (e.g., Th, Y, Hf, Nb and Zr) versus silica allow separation of samples into two major groups, which can be compared to units in the Maya Mountains and in Florida basement. The impact suevitic breccia samples have been affected by alteration likely related to the hydrothermal processes associated with the crater melt sheet. Cloritization, seritization and fenitization alterations are recognized, due to the long term hydrothermalism. Krogh et al. (1993) reported U-Pb dates on zircons from the suevitic breccias, which gave dates of 545 +/- 5 Ma and 418 +/- 6 Ma, which were interpreted in terms of the deep granitic metamorphic Yucatan basement. The younger date correlates with the age for the Osceola Granite and the St. Lucie metamorphic complex of the Swannee terrane in the Florida peninsula. The intrusive rocks in the Yucatan basement may be related to approx. 418 Ma ago collisional event in the Late Silurian.

Peering into the deep: Illuminating the crustal evolution of the Eucla basement and its relationship to the Albany-Fraser Orogen of southwest Australia.

NASA Astrophysics Data System (ADS)

Hartnady, Michael; Kirkland, Chris; Clark, Chris; Spaggiari, Catherine; Smithies, Hugh

2017-04-01

The Albany-Fraser Orogen is a 1200 km long east to northeasterly trending Palaeoproterozoic to Mesoproterozoic orogenic belt that defines the southern to southeastern margin of the West Australian Craton (WAC). The belt records a long and complex geological history spanning the break-up of Nuna between 2000 and 1700 Ma and amalgamation of Rodinia between 1300 and 1000 Ma. Recent geochronological, geochemical and isotopic work has shown that the Albany-Fraser Orogen formed through a protracted period of reworking of the margin of the Archean Yilgarn Craton (part of the WAC) with various additions of mantle-derived material. The Cretaceous Bight and Cenozoic Eucla Basins partially overlie the northeastern part of the Albany-Fraser Orogen and completely cover 1000 km of crystalline basement (the Eucla basement) that separates the belt from the South Australian Craton. This basement constitutes the glue between the major building blocks of Proterozoic Australia, yet, its geological history is poorly understood. New drill cores penetrating the basement have intersected interlayered granitic and gabbroic rocks that yield U-Pb zircon dates that are dissimilar to any magmatic ages from units within the adjoining Albany-Fraser Orogen, with the exception of the youngest, 1190-1125 Ma magmatic suite. In addition, mantle-like hafnium and neodymium isotopic signatures indicate that the rocks of the Eucla basement are dominated by new juvenile addition, and may represent an allochthonous terrane of oceanic heritage. New ɛHf contour maps for the Albany-Fraser Orogen and Eucla basement highlight this difference. Time-slicing the isotopic dataset reveals a pattern of Palaeoproterozoic juvenile magmatism sub-perpendicular to the present day structural grain in the belt. If this marks the presence of an older lithospheric structure then it demonstrates the power that time-constrained isotopic mapping provides for illuminating lithospheric architecture through time. This may be particularly useful for unravelling crustal evolution in regions with complex tectonic histories.

A new insight on magma generation environment beneath Jeju (Cheju) volcanic island

NASA Astrophysics Data System (ADS)

Shin, Y.; CHOI, K.; Koh, J.; Yun, S.; Nakamura, E.; Na, S.

2011-12-01

We present a Moho undulation model from gravity inversion that gives a new insight on the magma generation environment beneath Jeju (Cheju) volcanic island, Korea. The island is an intra-plate volcanic island located behind Ryukyu Trench, the collisional boundary between Eurasian plate and Philippine plate. Jeju island is a symmetrical shield volcano of oval shape (74 km by 32 km) whose peak is Hallasan (Mt. Halla: 1950m). The landform, which is closely related to the volcanism, can be divided topographically into the lava plateau, the shield-shaped Halla volcanic edifice and the monogenetic cinder cones, which numbers over 365. The basement rock mainly consists of Precambrian gneiss, Mesozoic granite and volcanic rocks. Unconsolidated sedimentary rock is found between basement rock and lava. The lava plateau is composed of voluminous basaltic lava flows, which extend to the coast region with a gentle slope. Based on volcanic stratigraphy, paleontology and geochronology, the Jeju basalts range from the early Pleistocene to Holocene in age. The mean density of the island is estimated to be very low, 2390 kg/cubic cm from gravity data analysis, which reflects the abundant unconsolidated pyroclastic sediments below the surface lava. The mean Moho depth is estimated to be 29.5 km from power spectral density of gravity anomaly, which means it has continental crust. It is noticeable that the gravity inversion indicates the island is developed above and along a swelled-up belt (ridge), several hundred meters higher than the surrounding area. The structure is also shows positive correlation with high magnetic anomaly distribution that could indicate existence of volcanic rocks. We interpret the Moho structure has a key to the magma generation: 1) the high gravity anomaly belt is formed by folding/buckling process under compressional environment, 2) it causes decrease of pressure beneath the lithosphere along the belt, and 3) it accelerates melting of basaltic magma in addition to the hot thermal structure widely distributed behind the collisional boundary.

Cover sequences at the northern margin of the Antongil Craton, NE Madagascar

USGS Publications Warehouse

Bauer, W.; Walsh, G.J.; De Waele, B.; Thomas, Ronald J.; Horstwood, M.S.A.; Bracciali, L.; Schofield, D.I.; Wollenberg, U.; Lidke, D.J.; Rasaona, I.T.; Rabarimanana, M.H.

2011-01-01

The island of Madagascar is a collage of Precambrian, generally high-grade metamorphic basement domains, that are locally overlain by unmetamorphosed sedimentary rocks and poorly understood low-grade metasediments. In the Antalaha area of NE Madagascar, two distinct cover sequences rest on high-grade metamorphic and igneous basement rocks of the Archaean Antongil craton and the Neoproterozoic Bemarivo belt. The older of these two cover sequences, the Andrarona Group, consists of low-grade metasedimentary rocks. The younger sequence, the newly defined Ampohafana Formation, consists of unmetamorphosed sedimentary rocks. The Andrarona Group rests on Neoarchaean granites and monzogranites of the Antongil craton and consists of a basal metagreywacke, thick quartzites and an upper sequence of sericite-chlorite meta-mudstones, meta-sandstones and a volcaniclastic meta-sandstone. The depositional age of the volcaniclastic meta-sandstone is constrained in age by U–Pb laser-ablation ICP-MS analyses of euhedral zircons to 1875 ± 8 Ma (2σ). Detrital zircons of Archaean and Palaeoproterozoic age represent an input from the Antongil craton and a newly defined Palaeoproterozoic igneous unit, the Masindray tonalite, which underlies the Andrarona Group, and yielded a U–Pb zircon age of 2355 ± 11 Ma (2σ), thus constraining the maximum age of deposition of the basal part of the Andrarona Group. The Andrarona Group shows a low-grade metamorphic overprint in the area near Antalaha; illite crystallinity values scatter around 0.17°Δ2Θ CuKα, which is within the epizone. The Ampohafana Formation consists of undeformed, polymict conglomerate, cross-bedded sandstone, and red mudstone. An illite crystallinity value of >0.25°Δ2Θ CuKα obtained from the rocks is typical of the diagenetic zone. Occurrences of rhyodacite pebbles in the Ampohafana Formation and the intrusion of a basaltic dyke suggest a deposition in a WSW-ENE-trending graben system during the opening of the Indian Ocean in the Upper Cretaceous, that was characterized by extensive rhyolitic to basaltic magmatism along Madagascar's eastern coast.

Precambrrian crustal evolution in the great falls tectonic zone

NASA Astrophysics Data System (ADS)

Gifford, Jennifer N.

The Great Falls Tectonic Zone (GFTZ) is a zone of northeast trending geological structures in central Montana that parallel structures in the underlying basement. U-Pb zircon and Nd isotopic data from the Little Belt Mountains (LBM) suggest that the GFTZ formed at ~1.86 to 1.80 Ga due to ocean subduction followed by collision between the Archean Wyoming Province (WP) and Medicine Hat Block (MHB). This study characterizes the GFTZ basement by geochronological and geochemical analysis of crustal xenoliths collected from Montana Alkali Province volcanics and exposed basement rock in the Little Rocky Mountains (LRM). Xenoliths collected from the Grassrange and Missouri Breaks diatremes and volcanics in the Bearpaw and Highwood Mountains have igneous crystallization ages from ~1.7 Ga to 1.9 Ga and 2.4 Ga to 2.7 Ga, and metamorphic ages from ~1.65 Ga to 1.84 Ga. Zircon Lu-Hf and whole-rock Sm-Nd data indicate that the xenoliths originated from reworked older continental crust mixed with mantle-derived components in all cases. Trace element patterns show fluid mobile element enrichments and fluid immobile element depletions suggestive of a subduction origin. Igneous ages in the LRM range older, from ~2.4 Ga to 3.2 Ga. Geochemical evidence suggests that the LRM meta-igneous units also formed in a subduction setting. Detrital zircon ages span the early Paleoproterozoic to Mesoarchean, with abundant 2.8 Ga ages. Zircon U-Pb igneous crystallization age data from xenoliths and the LRM are consistent with U-Pb zircon igneous crystallization ages from the MHB, suggesting that this segment of the GFTZ shares an affinity with concealed MHB crust. Published detrital zircon ages from the northern Wyoming Province reveal more abundant >3.0 Ga ages than the MHB or GFTZ samples. These geochronologic and geochemical data from the xenoliths and LRM samples allow for a refined model for crustal evolution in the GFTZ. Subduction under the Neoarchean to Paleoproterozoic crust of the MHB formed an igneous arc followed by metamorphism during the MHB-WP collision. Later Paleoproterozoic tectonothermal activity represents post-orogenic collapse after the terminal collision. Tectonic activity in the Cenozoic led to basement uplift and the formation of xenolith bearing volcanic units sampled for this study.

Geophysical constraints on the Virgin River Depression, Nevada, Utah, and Arizona

USGS Publications Warehouse

Langenheim, V.E.; Glen, J.M.; Jachens, R.C.; Dixon, G.L.; Katzer, T.C.; Morin, R.L.

2000-01-01

Gravity and aeromagnetic data provide insights into the subsurface lithology and structure of the Virgin River Depression (VRD) of Nevada, Utah, and Arizona. The gravity data indicate that the Quaternary and Tertiary sedimentary deposits hide a complex pre-Cenozoic surface. A north-northwest-trending basement ridge separates the Mesquite and Mormon basins, as evidenced by seismic-reflection, gravity, and aeromagnetic data. The Mesquite basin is very deep, reaching depths of 8?10 km. The Mormon basin reaches thicknesses of 5 km. Its northern margin is very steep and may be characterized by right steps, although this interpretation could change with additional gravity stations. Most of the young (Quaternary), small-displacement faults trend within 10? of due north and occur within the deeper parts of the Mesquite basin north of the Virgin River. South of the Virgin River, only a few, young, small-displacement faults are mapped; the trend of these faults is more northeasterly and parallels the basement topography and is distinct from that of the faults to the north. The Virgin River appears to follow the margin of the basin as it emerges from the plateau. The high-resolution aeromagnetic data outline the extent of shallow volcanic rocks in the Mesquite basin. The north-northwest alignment of volcanic rocks east of Toquop Wash appear to be structurally controlled because of faults imaged on seismic-reflection profiles and because the alignment is nearly perpendicular to the direction of Cenozoic extension. More buried volcanics likely exist to the north and east of the high-resolution aeromagnetic survey. Broader aeromagnetic anomalies beneath pre-Cenozoic basement in the Mormon Mountains and Tule Springs Hills reflect either Precambrian basement or Tertiary intrusions. These rocks are probably barriers to groundwater flow, except where fractured.

Geoelectrical characterisation of basement aquifers: the case of Iberekodo, southwestern Nigeria

NASA Astrophysics Data System (ADS)

Aizebeokhai, Ahzegbobor P.; Oyeyemi, Kehinde D.

2018-03-01

Basement aquifers, which occur within the weathered and fractured zones of crystalline bedrocks, are important groundwater resources in tropical and subtropical regions. The development of basement aquifers is complex owing to their high spatial variability. Geophysical techniques are used to obtain information about the hydrologic characteristics of the weathered and fractured zones of the crystalline basement rocks, which relates to the occurrence of groundwater in the zones. The spatial distributions of these hydrologic characteristics are then used to map the spatial variability of the basement aquifers. Thus, knowledge of the spatial variability of basement aquifers is useful in siting wells and boreholes for optimal and perennial yield. Geoelectrical resistivity is one of the most widely used geophysical methods for assessing the spatial variability of the weathered and fractured zones in groundwater exploration efforts in basement complex terrains. The presented study focuses on combining vertical electrical sounding with two-dimensional (2D) geoelectrical resistivity imaging to characterise the weathered and fractured zones in a crystalline basement complex terrain in southwestern Nigeria. The basement aquifer was delineated, and the nature, extent and spatial variability of the delineated basement aquifer were assessed based on the spatial variability of the weathered and fractured zones. The study shows that a multiple-gradient array for 2D resistivity imaging is sensitive to vertical and near-surface stratigraphic features, which have hydrological implications. The integration of resistivity sounding with 2D geoelectrical resistivity imaging is efficient and enhances near-surface characterisation in basement complex terrain.

The nature of the crust under Cayman Trough from gravity

USGS Publications Warehouse

ten Brink, Uri S.; Coleman, D.F.; Dillon, William P.

2002-01-01

Considerable crustal thickness variations are inferred along Cayman Trough, a slow-spreading ocean basin in the Caribbean Sea, from modeling of the gravity field. The crust to a distance of 50 km from the spreading center is only 2–3 km thick in agreement with dredge and dive results. Crustal thickness increases to ∼5.5 km at distances between 100 and 430 km west of the spreading center and to 3.5–6 km at distances between 60 and 370 km east of the spreading center. The increase in thickness is interpreted to represent serpentinization of the uppermost mantle lithosphere, rather than a true increase in the volume of accreted ocean crust. Serpentinized peridotite rocks have indeed been dredged from the base of escarpments of oceanic crust rocks in Cayman Trough. Laboratory-measured density and P-wave speed of peridotite with 40–50% serpentine are similar to the observed speed in published refraction results and to the inferred density from the model. Crustal thickness gradually increases to 7–8 km at the far ends of the trough partially in areas where sea floor magnetic anomalies were identified. Basement depth becomes gradually shallower starting 250 km west of the rise and 340 km east of the rise, in contrast to the predicted trend of increasing depth to basement from cooling models of the oceanic lithosphere. The gradual increase in apparent crustal thickness and the shallowing trend of basement depth are interpreted to indicate that the deep distal parts of Cayman Trough are underlain by highly attenuated crust, not by a continuously accreted oceanic crust.

Stress and strain patterns, kinematics and deformation mechanisms in a basement-cored anticline: Sheep Mountain Anticline, Wyoming

NASA Astrophysics Data System (ADS)

Amrouch, Khalid; Lacombe, Olivier; Bellahsen, Nicolas; Daniel, Jean-Marc; Callot, Jean-Paul

2010-02-01

In order to characterize and compare the stress-strain record prior to, during, and just after folding at the macroscopic and the microscopic scales and to provide insights into stress levels sustained by folded rocks, we investigate the relationship between the stress-strain distribution in folded strata derived from fractures, striated microfaults, and calcite twins and the development of the Laramide, basement-cored Sheep Mountain Anticline, Wyoming. Tectonic data were mainly collected in Lower Carboniferous to Permian carbonates and sandstones. In both rock matrix and veins, calcite twins recorded three different tectonic stages: the first stage is a pre-Laramide (Sevier) layer-parallel shortening (LPS) parallel to fold axis, the second one is a Laramide LPS perpendicular to the fold axis, and the third stage corresponds to Laramide late fold tightening with compression also perpendicular to the fold axis. Stress and strain orientations and regimes at the microscale agree with the polyphase stress evolution revealed by populations of fractures and striated microfaults, testifying for the homogeneity of stress record at different scales through time. Calcite twin analysis additionally reveals significant variations of differential stress magnitudes between fold limbs. Our results especially point to an increase of differential stress magnitudes related to Laramide LPS from the backlimb to the forelimb of the fold possibly in relation with motion of an underlying basement thrust fault that likely induced stress concentrations at its upper tip. This result is confirmed by a simple numerical model. Beyond regional implications, this study highlights the potential of calcite twin analyses to yield a representative quantitative picture of stress and strain patterns related to folding.

Full 40 km crustal reflection seismic datasets in several Indonesian basins

NASA Astrophysics Data System (ADS)

Dinkelman, M. G.; Granath, J. W.; Christ, J. M.; Emmet, P. A.; Bird, D. E.

2010-12-01

Long offset, deep penetration regional 2D seismic data sets have been acquired since 2002 by GX Technology in a number of regions worldwide (www.iongeo.com/Data_Libraries/Spans/). Typical surveys consist of 10+ lines located to image specific critical aspects of basin structure. Early surveys were processed to 20 km, but more recent ones have extended to 40-45 km from 16 sec records. Pre-stack time migration is followed by pre-stack depth migration using gravity and in some cases magnetic modeling to constrain the velocity structure. We illustrate several cases in the SE Asian and Australasian area. In NatunaSPAN™ two generations of inversion can be distinguished, one involving Paleogene faults with Neogene inversion and one involving strike slip-related uplift in the West Natuna Basin. Crustal structure in the very deep Neogene East Natuna Basin has also been imaged. The JavaSPAN™ program traced Paleogene sediments onto oceanic crust of the Flores Sea, thus equating back arc spreading there to the widespread Eocene extension. It also imaged basement in the Makassar Strait beneath as much as 6 km of Cenozoic sedimentary rocks that accumulated Eocene rift basins (the North and South Makassar basins) on the edge of Sundaland, the core of SE Asia. The basement is seismically layered: a noisy upper crust overlies a prominent 10 km thick transparent zone, the base of which marks another change to slightly noisier reflectivity. Eocene normal faults responsible for the opening of extensional basins root in the top of the transparent layer which may be Moho or a brittle-ductile transition within the extended continental crust. Of particular significance is the first image of thick Precambrian basins comprising the bulk of continental crust under the Arafura Sea in the ArafuraSPAN™ program. Four lines some 1200 km long located between Australia and New Guinea on the Arafura platform image a thin Phanerozoic section overlying a striking Precambrian basement composed of sedimentary and burial metamorphosed sedimentary rock that we divide into two packages on the basis of seismic character. The upper is 8-15 km of undeformed late Precambrian sediments the top of which ties Eocambrian rocks in wells in offshore New Guinea. This package appears to correlate to the Wessel Group in northern Australia. The lower package is composed of 10-15 km of strongly bedded, presumably burial metamorphosed rocks that make up the bulk of the lower crust. These may equate to any of a number of northern Australian Mesoproterozoic basins. This lower package offlaps ‘pods’ of seismically transparent basement (?Paleoproterozoic or Archean) that make up at most the lowermost 15 km of the 40 km PSDM line. Both Precambrian packages appear to be craton-margin sedimentary wedges, the younger overlapping the older. The SE extent of the lowermost package is deformed in a thrust system which may mark the event that detached it from its original underlying oceanic or transitional crust during cratonization. The SPAN programs are important new data sets to clarify and in some cases solve outstanding problems in basin architecture and tectonic evolution.

Architecture of ductile-type passive margins: Geological constraints from the inverted Cretaceous basin of the North-Pyrenean Zone (`Chaînons Béarnais', Western Pyrenees)

NASA Astrophysics Data System (ADS)

Corre, B.; Lagabrielle, Y.; Labaume, P.; Lahfid, A.; Boulvais, P.; Bergamini, G.; Fourcade, S.; Clerc, C. N.; Asti, R.

2017-12-01

Subcontinental lithospheric mantle rocks are exhumed at the foot of magma-poor distal passive margins as a response to extreme stretching of the continental crust. The North-Pyrenean Zone (NPZ) exposes remnants of such extremely stretched paleo-passive margin that represent field analogues to study the processes of continental crust thinning and mantle exhumation. The NPZ results from the inversion of basins opened between the Iberia and Europa plates during Albo-Cenomanian times. The Chaînons Béarnais belt displays a fold-and-thrust structure involving the Mesozoic sedimentary cover associated with peridotite bodies in tectonic contact with Paleozoic basement lenses of small size. Continental extension developed under hot thermal conditions, as demonstrated by the syn-metamorphic Cretaceous ductile deformation affecting both the crustal basement and the Mesozoic cover. In this study, we present structural and geochemical data providing constraints to reconstruct the evolution of this paleo-margin. Field work confirms that the Mesozoic cover is intimately associated with mantle rocks and thin tectonic lenses of middle crust. Micro-structural studies show that the greenschist facies ductile deformation in the crust produced a mylonitic foliation which is always parallel to the crust/mantle contact. The crust/mantle detachment fault is a major shear zone characterized by anastomosed shear bands. It also shows that the pre-rift cover was detached from its bedrock at the Keuper evaporites level and was welded to mantle rocks during their exhumation at the foot of the margin. We show that: (i) the boudinaged pre-rift sediments have undergone drastic syn-metamorphic thinning with the genesis of a S0/S1 foliation and, (ii) the Paleozoic basement has been ductilely deformed, into thin tectonic lenses that remained welded to the exhumed mantle rocks. The ductile behavior is related to the presence of a thick pre- and syn-rift cover acting as an efficient thermal blanket. This new geological data set highlights important characteristics of ductile-type hyper-extended passive margin. Finally, we stress that studying field analogues represents a major tool to better understand the mechanisms of crustal thinning associated with mantle exhumation and their structural inheritance during tectonic inversion.

Database for Regional Geology, Phase 1: A Tool for Informing Regional Evaluations of Alternative Geologic Media and Decision Making

DOE Office of Scientific and Technical Information (OSTI.GOV)

Perry, Frank Vinton; Kelley, Richard E.; Birdsell, Suzanne M.

Reported is progress in the following areas: Phase 1 and 2 websites for the regional geology GIS database; terrane maps of crystalline basement rocks; inventory of shale formations in the US; and rock properties and in-situ conditions for shale estimated from sonic velocity measurements.

The role of basement inheritance faults in the recent fracture system of the inner shelf around Alboran Island, Western Mediterranean

NASA Astrophysics Data System (ADS)

Maestro-González, A.; Bárcenas, P.; Vázquez, J. T.; Díaz-Del-Río, V.

2008-02-01

Fractures associated with volcanic rock outcrops on the inner shelf of Alboran Island, Western Mediterranean, were mapped on the basis of a side-scan sonar mosaic. Absolute maximum fracture orientation frequency is NW SE to NNW SSE, with several sub-maxima oriented NNE SSW, NE SW and ENE WSW. The origin of the main fracture systems in Neogene and Quaternary rocks of the Alboran Basin (south Spain) appears to be controlled by older structures, namely NE SW and WNW ESE to NW SE faults which cross-cut the basement. These faults, pre-Tortonian in origin, have been reactivated since the early Neogene in the form of strike-slip and extensional movements linked to the recent stress field in this area. Fracture analysis of volcanic outcrops on the inner continental shelf of Alboran Island suggests that the shelf has been deformed into a narrow shear zone limited by two NE SW-trending, sub-parallel high-angle faults, the main orientation and density of which have been influenced by previous WNW ESE to NW SE basement fractures.

LOFT/FET complex. Construction view of abutment footings for arches of ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

LOFT/FET complex. Construction view of abutment footings for arches of hangar (TAN-629). Tunnels between basement of hangar and control building (TAN-630) had to fit between arches. (Note concrete work taking place at hole at lower edge of view. This photo may document unexpected bubble in underlying lava rock. It was dumped full of concrete and a footing made. Source: Interview with John DeClue). Date: December 19, 1957. Photographer: Jack L. Anderson. INEEL negative no. 57-6203 - Idaho National Engineering Laboratory, Test Area North, Scoville, Butte County, ID

Boninite-like intraplate magmas from Manihiki Plateau require ultra-depleted and enriched source components

PubMed Central

Golowin, Roman; Portnyagin, Maxim; Hoernle, Kaj; Hauff, Folkmar; Gurenko, Andrey; Garbe-Schönberg, Dieter; Werner, Reinhard; Turner, Simon

2017-01-01

The Ontong Java and Manihiki oceanic plateaus are believed to have formed through high-degree melting of a mantle plume head. Boninite-like, low-Ti basement rocks at Manihiki, however, imply a more complex magma genesis compared with Ontong Java basement lavas that can be generated by ∼30% melting of a primitive mantle source. Here we show that the trace element and isotope compositions of low-Ti Manihiki rocks can best be explained by re-melting of an ultra-depleted source (possibly a common mantle component in the Ontong Java and Manihiki plume sources) re-enriched by ≤1% of an ocean-island-basalt-like melt component. Unlike boninites formed via hydrous flux melting of refractory mantle at subduction zones, these boninite-like intraplate rocks formed through adiabatic decompression melting of refractory plume material that has been metasomatized by ocean-island-basalt-like melts. Our results suggest that caution is required before assuming all Archaean boninites were formed in association with subduction processes. PMID:28181497

New Constraints on the Extent of Paleoproterozoic and Archean Basement in the Northwest U.S. Cordillera

NASA Astrophysics Data System (ADS)

Brewer, R. A.; Vervoort, J.; Lewis, R. S.; Gaschnig, R. M.; Hart, G.

2008-12-01

The Laurentian basement west of the Wyoming craton in southwest Montana and northern Idaho has been interpreted as a collage of Archean and Proterozoic terranes which accreted to the North American craton and incorporated into Laurentia at ~ 1.86 Ga [1]. This basement and the geometry of the Archean and Proterozoic crust are poorly understood due to coverage by metasediments of the Belt-Purcell Supergroup and are further obscured by Mesozoic magmatism (Idaho Batholith, sensu lato). Exposures of the basement are rare but have been documented in a few regions including the Priest River Complex in northern Idaho and the Sevier fold and thrust belt just northwest of the Wyoming craton in the Great Falls tectonic zone (Foster et al. 2006). New ages and isotopic data from orthogneisses in north-central Idaho provide evidence for previously undocumented exposures of both Paleoproterozoic and Archean basement that may place important constraints on the reconstruction of Laurentia and its tectonic setting. The orthogneisses analyzed in this study (all previously mapped as deformed Cretaceous plutons) fall into two distinct age groups of 1.86 Ga and 2.67 Ga. The zircons from both the Archean and Proterozoic rocks have simple systematics. The zircons from three Archean samples have ɛHf(i) values of 2.4 ± 2.1, 3.8 ± 1.8, and 5.2 ± 3.5 (average values based on 6 individual zircon Hf analyses per sample). Zircons from the Paleoproterozoic gneisses have different but internally consistent ɛHf(i) values of -8.0 ± 0.9 and -0.6 ± 1.4. In contrast, both Hf and Nd whole rock data are highly scattered in these samples especially in the Archean samples in which ɛHf(i) varies from -25 to +21 and ɛNd(i) varies from -8 to +11. These extreme values are implausible for initial compositions and indicate open system behavior in both Lu-Hf and Sm-Nd in the whole rocks. The zircons, in contrast, appear to be closed to significant Hf mobility on the scale of the laser analyses. The data from this project provides evidence for new exposures of Paleoproterozoic and Archean basement in North Central Idaho. The presence of Archean basement in this region suggests unrecognized complexities in the Selway Terrane possibly obscuring the boundary between this and the older Priest River block [1]. The positive ɛHf(i) values of the Archean gneisses at ~ +4 are consistent with depleted mantle compositions in the Late Archean and suggest that this is juvenile crust derived from the mantle at ~ 2.67 Ga. On the other hand, the Paleoproterozoic gneisses, with ɛHf(i) values of ~ -1 and -8, clearly represent the products of reworked pre-existing crust. The whole-rock Lu-Hf and Sm-Nd isotopic data from both the Paleoproterozoic and (certainly) the Archean samples clearly record a major isotopic disturbance. The timing of this disturbance in the Archean rocks is not yet clear but may have happened during Paleoproterozoic magmatism (1860 Ga), widespread metamorphism in the Mesoproterozoic (1.1 Ga; [2]), or tectonic thickening and magmatism in the Cretaceous (~ 80 Ma). [1] D.A. Foster et al., Can. J. Earth Sci., 43, 1601 (2006). [2] J.D. Vervoort, et al., Geol. Soc. Amer. Abstr, 36 (2005).

Geochemical Specific Characters of the Oil and the Origin of the Oil and Gas Fields

NASA Astrophysics Data System (ADS)

Gottikh, Rimma; Pisotskiy, Bogdan; Plotnikova, Irina

2010-05-01

It is generally assumed that the fluid regime of the basement of ancient platforms is not associated with that of the sedimentary cover. This assumption is mainly due to the substantial time gap between the formation of the crystalline and sedimentary rocks as well as the evolutionary differences between the thermal regime of the interior and the redox potentials of fluid systems. The presence of loosely aggregated zones filled with salt-water solutions, oil or gas in the upper basement is explained by downward fluid flows from sedimentary rocks through tectonic faults into the disintegrated crystalline rocks. The formation of such zones is believed to be due to the crustal stratification due to Earth's pulsation, periodic variations of its rotational rate, hydrogenic deconsolidation, burial of the post-Early Proterozoic disintegration zones, etc. This pattern suggests that the matter and energy exchange between the Earth's spheres in the late stages of the platform development could only take place with the help of magmatic melts and the associated fluids during the tectonomagmatic cycles of the Earth's crust transformation. Gas and liquid hydrocarbon components mainly occur in crystalline basement rocks of ancient platforms penetrated to a depth of more than 3000 m due to deep degassing processes. The traces of the upward migration of fluids are sealed in the geological sequence, including the sedimentary cover, within secondary inclusions of rocks and minerals. The fluids are complex, reduced, multicomponent systems that transport lithophilous, chalcophilous and siderophilous elements. The presence of microelements in the bituminous phase of inclusions indicates that metals mainly occur in the complexes containing organic ligands. During the evolution of the fluid systems under new pressure and temperature conditions, low-solubility substances were separated out of the fluid to form hard bitumen, and the lighter components migrated into the overlying fractured and porous rocks. The high metal content of carbonaceous substances and their compositional variations governed by homogenisation temperatures of the inclusions suggest that they are not the products of the decomposition of oil fields. The constant presence of uranium in the fluid and its differentiation products allows the tracing of the systems' migration ways from the crystalline basement to oil-saturated reservoir zones of the sedimentary cover The known geochemical properties of bitumen and oil - high platinum content, specific distributions of rare earth elements, that are not characteristic of the upper crust formations, as well as 143Nd/144Nd and 87Sr/86Sr isotopic compounds, which are out of balance with the organic matter of sedimentary rocks - suggest that hydrocarbons are accumulated in the presence of cooling high-alkalinity mafite-ultramafite intrusions. This logically corresponds to the distribution of seismic anomalies and magnetic and gravity fields in the consolidated crust below the various petroleum fields (for example, South Tatarstan and Nepsky arches of the Romashkino and Verkhne-Chonskoye oil fields). The acquired geochemical and thermodynamic characteristics of the reduced fluids and their differentiation products from the crystalline basement and the sedimentary cover of the southern Siberian and eastern East European platforms indicate that these were formed outside of the sedimentary cover and that the migration was directed upwards. The analysis of the magmatic evolution on platforms reveals its alkaline trend due to the impeded degassing of magmatic sources at depth and the inflow of new doses of alkaline fluids or melts into them. Further evolution of the zones of partial melting of the substratum led, in the authors' view, to the generation of oil-forming fluids and their transportation into the Earth's upper crust. Their interaction with the surrounding rocks in turn led to the formation of oil accumulations. Thus, oil is the product of the interaction of deep, reduced fluids. Oil, graphite of the Archaean crystalline complexes and hard bitumens are interrelated elements of the evolution of deep, high-enthalpy systems. These large-scale reduced palaeofluid phenomena are obviously related to geodynamic and tectonomagmatic processes. The source of these fluid systems, their impact on the geological environment and its consequences can be determined through additional integrated geochemical studies using the isotopes of heavy elements and through the correlation of the observed potential fields with the structure of the consolidated crust and the sedimentary cover for the identification of geodynamic processes in geophysically inhomogeneous zones of the geological medium.

Limiting the Magnitude of Potential Injection-Induced Seismicity Associated With Waste-Water Disposal, Hydraulic Fracturing and CO2 Sequestration

NASA Astrophysics Data System (ADS)

Zoback, Mark

2017-04-01

In this talk, I will address the likelihood for fault slip to occur in response to fluid injection and the likely magnitude of potentially induced earthquakes. First, I will review a methodology that applies Quantitative Risk Assessment to calculate the probability of a fault exceeding Mohr-Coulomb slip criteria. The methodology utilizes information about the local state of stress, fault strike and dip and the estimated pore pressure perturbation to predict the probability of the fault slip as a function of time. Uncertainties in the input parameters are utilized to assess the probability of slip on known faults due to the predictable pore pressure perturbations. Application to known faults in Oklahoma has been presented by Walsh and Zoback (Geology, 2016). This has been updated with application to the previously unknown faults associated with M >5 earthquakes in the state. Second, I will discuss two geologic factors that limit the magnitudes of earthquakes (either natural or induced) in sedimentary sequences. Fundamentally, the layered nature of sedimentary rocks means that seismogenic fault slip will be limited by i) the velocity strengthening frictional properties of clay- and carbonate-rich rock sequences (Kohli and Zoback, JGR, 2013; in prep) and ii) viscoplastic stress relaxation in rocks with similar composition (Sone and Zoback, Geophysics, 2013a, b; IJRM, 2014; Rassouli and Zoback, in prep). In the former case, if fault slip is triggered in these types of rocks, it would likely be aseismic due the velocity strengthening behavior of faults. In the latter case, the stress relaxation could result in rupture termination in viscoplastic formations. In both cases, the stratified nature of sedimentary rock sequences could limit the magnitude of potentially induced earthquakes. Moreover, even when injection into sedimentary rocks initiates fault slip, earthquakes large enough to cause damage will usually require slip on faults sufficiently large that they extend into basement. This suggests that an important criterion for large-scale CO2 sequestration projects is that the injection zone is isolated from crystalline basement rocks by viscoplastic shales to prevent rupture propagation from extending down into basement.

K-Ar geochronology of basement rocks on the northern flank of the Huancabama deflection, Ecuador

USGS Publications Warehouse

Feininger, Tomas; Silberman, M.L.

1982-01-01

The Huancabamba deflection, a major Andean orocline located at the Ecuador-Peru border, constitutes an important geologic boundary on the Pacific coast of South America. Crust to the north of the deflection is oceanic and the basement is composed of basic igneous rocks of Cretaceous age, whereas crust to the south is continental and felsic rocks of Precambrian to Cretaceous age make up the basement. The northern flank of the Huancabamba Deflection in El Oro Province, Ecuador, is underlain by Precambrian polymetamorphic basic rocks of the Piedras Group; shale, siltstone, sandstone, and their metamorphosed equivalents in the Tahuin Group (in part of Devonian age); concordant syntectonic granitic rocks; quartz diorite and alaskite of the Maroabeli pluton; a protrusion of serpentinized harzburgite that contains a large inclusion of blueschist-facies metamorphic rocks, the Raspas Formation, and metamorphic rocks north of the La Palma fault. Biotite from gneiss of the Tahuin Group yields a Late Triassic K-Ar age (210 ? 8 m.y.). This is interpreted as an uplift age and is consistent with a regional metamorphism of Paleozoic age. A nearby sample from the Piedras Group that yielded a hornblende K-Ar age of 196 ? 8 m.y. was affected by the same metamorphic event. Biotite from quartz diorite of the mesozonal Maroabeli pluton yields a Late Triassic age (214 ? 6 m.y.) which is interpreted as an uplift age which may be only slightly younger than the age of magmatic crystallization. Emplacement of the pluton may postdate regional metamorphism of the Tahuin Group. Phengite from politic schist of the Raspas Formation yields an Early Cretaceous K-Ar age (132 ? 5 m.y.). This age is believed to date the isostatic rise of the encasing serpentinized harzburgite as movement along a subjacent subduction zone ceased, and it is synchronous with the age of the youngest lavas of a coeval volcanic arc in eastern Ecuador. A Late Cretaceous K-Ar age (74.4 ? 1.1 m.y.) from hornblende in amphibolite north of the La Palma fault shows that rocks there are distinct from the superficially similar rocks of the Tahuin Group to the south. Biotite from schist in the Eastern Andean Cordillera yields an Early Eocene age (56.6 ? 1.6 m.y.). Metamorphic rocks in the northern part of the Eastern Andean Cordillera are Cretaceous in age and were metamorphosed in part in early Tertiary time. They are unrelated to and were metamorphosed later than any of the diverse rocks exposed on the northern flank of the Huancabamba Deflection.

Field evidences for a Mesozoic palaeo-relief through the northern Tianshan

NASA Astrophysics Data System (ADS)

Gumiaux, Charles; Chen, Ke; Augier, Romain; Chen, Yan; Wang, Qingchen

2010-05-01

The modern Tianshan mountain belt, located in Central Asia, offers a natural laboratory to study orogenic processes linked with convergent geodynamical settings. Most of the previous studies either focused on the Paleozoic evolution of the range - subductions, arc accretions and continental collision - or on its Cenozoic intra-continental evolution linked with the India-Asia collision. At first order, the finite structure of this range obviously displays a remarkable uprising of Paleozoic "basement" rocks - as a crustal-scale ‘pop-up' - surrounded by two Cenozoic foreland basins. The present-day topography of the Tianshan is traditionally related to the latest intra-continental reactivation of the range. In contrast, the present field study of the northern Tianshan brings new and clear evidences for the existence of a significant relief, in this area, during Mesozoic times. The investigation zone is about 250 km long, from Wusu to Urumqi, along the northern flank of the Tianshan where the rivers deeply incised the topography. In such valleys, lithologies and structural relationships between Paleozoic basement rocks, Mesozoic and Cenozoic sedimentary series are particularly well exposed along several sections. Jurassic series are mostly characterized by coal-bearing, coarse-grained continental deposits. Within intra-mountain basins, sedimentary breccias, with clasts of Carboniferous basement rocks, have been locally found at the base of the series. This argues for the presence of a rather proximal palaeo-relief of basement rocks along the range front and the occurrence of proximal intra-mountain basins, during the Jurassic. Moreover, while a major thrust is mostly evoked between Jurassic deposits and the Paleozoic units, some of the studied sections show that the Triassic to Jurassic sedimentary series can be followed from the basin to the range. In these cases, the unconformity of the Mesozoic series on top of the Carboniferous basement has been locally clearly identified quite high in the mountain range or even, surprisingly, directly along the northern Tianshan "front" itself. Combining available information from geological maps, field investigations and numerous drilling wells, regional-scale cross-sections have been built. Some of them show "onlap" type deposit of the Triassic to Jurassic clastic sediments on top of the Paleozoic basement that was thus significantly sloping down to the North at that time. Our study clearly evidences, at different scales, the existence of a major palaeo-relief along the northern Tianshan range during Mesozoic, and particularly during Jurassic times. Such results are compatible with previous fission tracks and sedimentology studies. From this, the Tianshan's uplift and the movements associated with along its northern front structures, which are traditionally assigned to its Cenozoic reactivation, must be reduced. These new results question on the mode and timing of reactivation of structures and on the link between topography and intra-continental collisional settings.

The Amazon-Laurentian connection as viewed from the Middle Proterozoic rocks in the central Andes, western Bolivia and northern Chile

USGS Publications Warehouse

Tosdal, R.M.

1996-01-01

Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Peru?? from the Arequipa-Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171??20 Ma and 1158??12 Ma; U-Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U-Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite- and granulite-facies metamorphic mineral assemblages (???1080 Ma metamorphism; U-Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Bele??n Schist in northern Chile generally have present-day low 206Pb/204Pb ( 15.57), and elevated 208Pb/204Pb (37.2 to 50.7) indicative of high time-averaged Th/U values. The Middle Proterozoic rocks in general have higher presentday 206Pb/204Pb values than those of the Early Proterozoic rocks of the Arequipa massif (206Pb/204Pb between 16.1 and 17.1) but lower than rocks of the southern Arequipa-Antofalla craton (206Pb/204Pb> 18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa-Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa-Antofalla craton overlap those of the Amazon craton, thereby supporting a link between these cratons and seemingly precluding part of the Arequipa-Antofalla craton from being a detached fragment of another craton such as eastern Laurentia, which has been characterized by a different U/Pb history. Pb isotopic compositions for the Arequipa-Antofalla craton are, furthermore, distinct from those of the Proterozoic basement in the Precordillera terrane, western Argentina, indicating a Pb isotopic and presumably a tectonic boundary between them. The Pb isotopic compositions for the Precordillera basement are similar to those of eastern Laurentia, and support other data indicating that these rocks are a detached fragment of North America. Finally, the distinct Pb isotopic evolution history of the Arequipa-Antofalla craton and eastern Laurentia require minor modification to tectonic models linking eastern North America-Scotland to the oroclinal bend in western South America.

Late Neoproterozoic basement rocks of Meatiq area, Central Eastern Desert, Egypt: Petrography and remote sensing characterizations

NASA Astrophysics Data System (ADS)

Hassan, Safaa M.; El kazzaz, Yahiya A.; Taha, Maysa M. N.; Mohammad, Abdullah T.

2017-07-01

Meatiq dome is one of the mysteries of the basement rocks in Central Eastern Desert (CED) of Egypt. Its mode of formation, and tectonic evolution are still controversial and not fully understood. Satellite remote sensing is a powerful tool for geologic applications, especially in inaccessible regions of the Earth's surface. In this study, three proposed Landsat-8 band ratios (6/2, 6/7, (6/4*4/3)), (6/7, 6/4, 4/2), and (7/5, 7/6, 5/3) are successfully used for detailed geological mapping of the different lithological rock units exposed in Meatiq dome area in the CED. Landsat-8 Principal component (PC) images is also used for refinement the boundaries between the widely-exposed rock units in the study area. Fourteen spectral bands of Advanced Space borne Thermal Emission and Reflection Radiometer (ASTER) data are successfully used to emphasize the distribution of some rock forming minerals (i.e. muscovite, quartz, ferrous oxides, ferrous silicates and hydroxyl-bearing minerals) in the lithological rock units of Meatiq dome area. ASTER muscovite index (B7/B6) and quartz index (B14/B12), ferrous iron index (B5/B3), ferrous silicates index (B5/B4), mafic index (B12/B13) and hydroxyl-bearing minerals index ((B7/B6)*(B4/B6)) discriminate muscovite bearing rocks, Granitoids, and other felsic rocks, amphibolite and other mafic rocks. The proposed image processing methods effectively discriminates between four granitic varieties existed in Meatiq area. They are namely; Abu Ziran, Ariki, Fawakhir and Atalla Plutons. This study reveals that the applied data of ASTER and Landsat-8 enhanced images produced a modified geological map with well emphasized rock units which are verified with field observations, and petrographic study.

Unroofing history of Late Paleozoic magmatic arcs within the ``Turan Plate'' (Tuarkyr, Turkmenistan)

NASA Astrophysics Data System (ADS)

Garzanti, E.; Gaetani, M.

2002-07-01

Stratigraphic, sedimentologic and petrographic data collected on the Kizilkaya sedimentary succession (Western Turkmenistan) demonstrate that the "Turan Plate" consists in fact of an amalgamation of Late Paleozoic to Triassic continental microblocks separated by ocean sutures. In the Kizilkaya area, an ophiolitic sequence including pyroxenite, gabbro, pillow basalt and chert, interpreted as the oceanic crust of a back-arc or intra-arc basin, is tectonically juxtaposed against volcaniclastic redbeds documenting penecontemporaneous felsic arc magmatism (Amanbulak Group). A collisional event took place around ?mid-Carboniferous times, when oceanic rocks underwent greenschist-facies metamorphism and a thick volcaniclastic wedge, with pyroclastic rocks interbedded in the lower part, accumulated (Kizilkaya Formation). The climax of orogenic activity is testified by arid fanglomerates shed from the rapid unroofing of a continental arc sequence, including Middle-Upper Devonian back-reef carbonates and cherts, and the underlying metamorphic and granitoid basement rocks (Yashmu Formation). After a short period of relative quiescence, renewed tectonic activity is indicated by a conglomeratic sequence documenting erosion of a sedimentary and metasedimentary succession including chert, sandstone, slate and a few carbonates. A final stage of rhyolitic magmatism took place during rapid unroofing of granitoid basement rocks (Kizildag Formation). Such a complex sequence of events recorded by the Kizilkaya episutural basin succession documents the stepwise assemblage of magmatic arcs and continental fragments to form the Turan microblock collage during the Late Paleozoic. Evolution of detrital modes is compatible with that predicted for juvenile to accreted and unroofed crustal blocks. The deposition of braidplain lithic arkoses in earliest Triassic time indicates that strong subsidence continued after the end of the volcanic activity, possibly in retroarc foreland basin settings. The occurrence of transgressive coquinas yielding endemic ammonoids ( Dorikranites) characteristic of the whole Caspian area suggests proximity to the southern margin of the newly formed Eurasian continent in the late Early Triassic. The Late Triassic Eo-Cimmerian Orogeny caused only mild tilting and rejuvenation of the underlying succession in the study area. Only at this time were the Turan blocks, a series of Indonesian-type terranes comprised between the Mashad Paleo-Tethys Suture in the south and the Mangyshlak belt in the north, finally incorporated into the Eurasian landmass.

Stratigraphy, structure and regional correlation of eastern Blue Ridge sequences in southern Virginia and northwestern North Carolina: an interim report from new USGS mapping

USGS Publications Warehouse

Carter, Mark W.; Merschat, Arthur J.

2014-01-01

The contact between eastern Blue Ridge stratified rocks above Mesoproterozoic basement rocks is mostly faulted (Gossan Lead and Red Valley). The Callaway fault juxtaposes Ashe and Lynchburg rocks above Wills Ridge Formation. Alligator Back Formation rocks overlie Ashe and Lynchburg rocks along the Rock Castle Creek fault, which juxtaposes rocks of different metamorphism. The fault separates major structural domains: rocks with one penetrative foliation in the footwall, and pin-striped recrystallized compositional layering, superposed penetrative foliations, and cleavage characterize the hanging wall. These relationships are ambiguous along strike to the southwest, where the Ashe and Alligator Back formations are recrystallized at higher metamorphic grades.

Gondwanan/peri-Gondwanan origin for the Uchee terrane, Alabama and georgia: Carolina zone or Suwannee terrane(?) and its suture with Grenvillian basement of the Pine Mountain window

USGS Publications Warehouse

Steltenpohl, M.G.; Mueller, P.M.; Heatherington, A.L.; Hanley, T.B.; Wooden, J.L.

2008-01-01

The poorly known, suspect, Uchee terrane occupies a critical tectonic position with regard to how and when peri-Gondwanan (Carolina) and Gondwanan (Suwannee) terranes were sutured to Laurentia. It lies sandwiched between Laurentian(?) continental basement exposed in the Pine Mountain window and adjacent buried Gondwanan crust of the Suwannee terrane. The Uchee terrane has been proposed as both a septum of Piedmont rocks that once was continuous across the erosionally breached Pine Mountain window or part of the Carolina zone. To help resolve this issue, we conducted U-Pb (SHRIMP-RG) (sensitive high-resolution ion microprobe-reverse geometry) zircon studies and whole-rock isotopic analyses of principal metasedimentary and metaplutonic units. U-Pb ages for zircons from the Phenix City Gneiss suggest igneous crystallization at ca. 620 Ma, inheritance ca. 1000 to ca. 1700 Ma, and a ca. 300 Ma (Alleghanian) overprint recorded by zircon rims. Zircons from the metasedimentary/metavolcaniclastic Moffits Mill Schist yield bimodal dates at ca. 620 and 640 Ma. The 620 to 640 Ma dates make these rocks age-equivalent to the oldest parts of the Carolina slate belt (Virgilina and Savannah River) and strongly suggest a Gondwanan (Pan-African and/or Trans-Brasiliano) origin for the Uchee terrane. Alternatively, the Uchee terrane may be correlative with metamorphic basement of the Suwannee terrane. The ca. 300 Ma overgrowths on zircons are compatible with previously reported 295 to 288 Ma 40Ar/39Ar hornblende dates on Uchee terrane rocks, which were interpreted to indicate deep tectonic burial of the Uchee terrane contemporaneous with the Alleghanian orogeny recorded in the foreland. Temperature-time paths for the Uchee terrane are similar to that of the Pine Mountain terrane, indicating a minimum age of ca. 295 Ma for docking. In terms of tectono-metamorphic history of the Uchee terrane, it is important to note that no evidence for intermediate "Appalachian" dates (e.g, Acadian or Taconian) has been reported. This younger history, together with the ages of metaigneous rocks and evidence for pre-Grenville basement, suggests the Uchee terrane is likely of Gondwanan origin and may he related to Carolina zone terranes that accreted during the Alleghanian orogeny. ?? 2008 Geological Society of America.

Laramide structure of the central Sangre de Cristo Mountains and adjacent Raton Basin, southern Colorado

USGS Publications Warehouse

Lindsey, D.A.

1998-01-01

Laramide structure of the central Sangre de Cristo Mountains (Culebra Range) is interpreted as a system of west-dipping, basement-involved thrusts and reverse faults. The Culebra thrust is the dominant structure in the central part of the range; it dips 30 -55?? west and brings Precambrian metamorphic base-ment rocks over unmetamorphosed Paleozoic rocks. East of the Culebra thrust, thrusts and reverse faults break the basement and overlying cover rocks into north-trending fault blocks; these boundary faults probably dip 40-60?? westward. The orientation of fault slickensides indicates oblique (northeast) slip on the Culebra thrust and dip-slip (ranging from eastward to northward) movement on adjacent faults. In sedimentary cover rocks, east-vergent anticlines overlie and merge with thrusts and reverse faults; these anticlines are interpreted as fault-propagation folds. Minor east-dipping thrusts and reverse faults (backthrusts) occur in both the hanging walls and footwalls of thrusts. The easternmost faults and folds of the Culebra Range form a continuous structural boundary between the Laramide Sangre de Cristo highland and the Raton Basin. Boundary structures consist of west-dipping frontal thrusts flanked on the basinward side by poorly exposed, east-dipping backthrusts. The backthrusts are interpreted to overlie structural wedges that have been emplaced above blind thrusts in the basin margin. West-dipping frontal thrusts and blind thrusts are interpreted to involve basement, but backthrusts are rooted in basin-margin cover rocks. At shallow structural levels where erosion has not exposed a frontal thrust, the structural boundary of the basin is represented by an anticline or monocline. Based on both regional and local stratigraphic evidence, Laramide deformation in the Culebra Range and accompanying synorogenic sedimentation in the western Raton Basin probably took place from latest Cretaceous through early Eocene time. The earliest evidence of uplift and erosion of a highland is the appearance of abundant feldspar in the Late Cretaceous Vermejo Formation. Above the Vermejo, unconformities overlain by conglomerate indicate continued thrusting and erosion of highlands from late Cretaceous (Raton) through Eocene (Cuchara) time. Eocene alluvial-fan conglomerates in the Cuchara Formation may represent erosion of the Culebra thrust block. Deposition in the Raton Basin probably shifted north from New Mexico to southern Colorado from Paleocene to Eocene time as movement on individual thrusts depressed adjacent segments of the basin.

Carboniferous and older carbonate rocks: Lithofacies, extent, and reservoir quality: Chapter CC in The oil and gas resource potential of the Arctic National Wildlife Refuge 1002 area, Alaska

USGS Publications Warehouse

Dumoulin, Julie A.

1999-01-01

Carboniferous and older carbonate rocks are potential hydrocarbon reservoir facies for four plays in the 1002 area of the Arctic National Wildlife Refuge. These rocks include several units in the pre-Carboniferous basement and the Carboniferous Lisburne Group. Data from exploratory wells west of the 1002 area, outcrops south of the 1002 area, seismic lines, and well logs are synthesized herein to infer carbonate lithofacies, extent, and reservoir character beneath the northeastern Arctic coastal plain.A chiefly shallow-water basement carbonate succession of Late Proterozoic through Early Devonian age (Katakturuk Dolomite, Nanook Limestone, and Mount Copleston Limestone) is interpreted to be present beneath much of the south-central 1002 area; it reaches 3,700 m thick in outcrop and is the primary reservoir for the Deformed Franklinian Play. A more heterogeneous lithologic assemblage of uncertain age forms basement in the northwestern part of the 1002 area; well data define three subunits that contain carbonate intervals 5- 50 m thick. These strata are prospective reservoirs for the Undeformed Franklinian Play and could also be reservoirs for the Niguanak- Aurora Play. Regional lithologic correlations suggest a Cambrian-Late Proterozoic(?) age for subunits one and two, and a slightly younger, later Cambrian-Silurian age for subunit three. Seismic and well data indicate that subunit one overlies subunit two and is overlain by subunit three. The Mississippian and Pennsylvanian Lisburne Group, a predominantly carbonate platform succession as much as 1 km thick, is projected beneath the southernmost part of the 1002 area and is a potential reservoir for the Ellesmerian Thrust-belt and Niguanak-Aurora Plays.Carbonate rocks in the 1002 area probably retain little primary porosity but may have locally well developed secondary porosity. Measured reservoir parameters in basement carbonate strata are low (porosity generally ≤ 5%; permeability ≤ 0.2 md) but drill-stem tests found locally reasonable flow rates (4,220-4,800 bpd) and, in the Flaxman Island area, recovered gas and condensate from these rocks. The Lisburne Group has produced up to 50,000 bbl of oil/ day from the Lisburne field at Prudhoe Bay. Reservoir parameters of the Lisburne in northeastern Alaska range from low (porosities ≤ 5% in most limestones) to good (porosities average 6.5-10% in some dolostones). Reservoir quality in Carboniferous and older carbonate strata in the 1002 area should be greatest where these rocks are highly fractured and (or) truncated by the Lower Cretaceous Unconformity.

Petrography and geochemistry of modern river sediments in an equatorial environment (Rwenzori Mountains and Albertine rift, Uganda) - Implications for weathering and provenance

NASA Astrophysics Data System (ADS)

Schneider, Sandra; Hornung, Jens; Hinderer, Matthias; Garzanti, Eduardo

2016-05-01

In hot-humid equatorial climate chemical weathering may be so strong that provenance signatures may be largely lost and even detritus derived from crystalline basement rocks reduced to quartzose sand. We tested this hypothesis in western Uganda, where stable plateau areas contrast with the active tectonic setting of the Albertine Rift (western branch of the East African Rift System, EARS), culminating in the strongly exhumed fault block of the > 5000 m high Rwenzori Mountains. In this setting, sediments derived from similar types of basement rocks including gneiss, schist, amphibolite, metasediments and granites can be traced from rapidly eroding high-altitude areas to low-altitude areas undergoing prolonged weathering. Sand and mud carried by 51 rivers overall in these two contrasting landscapes were sampled to study how and to what extent detrital modes are modified by the selective loss of unstable detrital minerals. Sediments generated in the high-relief Rwenzori Mountains show abundant feldspar (up to 32%) and rock fragments (up to 52%), which together with low SiO2/Al2O3 ratio and composition close to the Upper Continental Crust (UCC standard) reflect erosion in weathering-limited conditions. In the central Rwenzoris, low Th/Sc and Zr/Sc ratios, weak negative Eu anomaly, lower LaN/YbN values, and heavy-mineral assemblages with hornblende and epidote reflect the lithology of source rocks in the Buganda-Toro-Greenstone Belt. In contrast, sediments produced on the low-relief plateau have quartz content up to 98% and higher SiO2/Al2O3 ratio. Systematic loss of mobile elements is indicated by high chemical weathering indices CIA, PIA and WIP. However, provenance from metamorphic basement rocks is still indicated by heavy-mineral assemblages dominated by epidote and amphibole, whereas provenance from granitic rocks is revealed by high Th/Sc and Zr/Sc ratio, negative Eu anomaly and higher LaN/YbN values. We conclude that first-cycle sediments generated in high-relief areas preserve the original imprint of parent lithologies even in very humid equatorial climate. In low-relief areas, although weathering processes have proceeded over millions of years turning basement-derived detritus into an almost pure quartzose resistate, provenance signals are not erased entirely, and can be still retrieved from the residual heavy-mineral suite and relative abundance of high-field-strength trace elements.

Map showing structure of the Mississippi Valley Graben in the vicinity of New Madrid, Missouri

USGS Publications Warehouse

Wheeler, Russell L.; Rhea, Susan; Dart, Richard L.

1994-01-01

This is one of a series of five seismotectonic maps of the seismically active New Madrid area in southeast Missouri and adjacent parts of Arkansas, Kentucky, and Tennessee (table 1). We cannot legibly show all the seismotectonic data on a single map, therefore each of the five maps in this series groups a different type of related information. Rhea and others (1994) summarized the background and purpose of the seismotectonic map folio. The different types of data shown on this map are described in table 2. Except for a few exposed faults, all structures shown on the map are in Paleozoic sedimentary rocks of the midcontinent or underlying metamorphic and igneous basement rocks of presumed Precambrian age (Dart, 1992; Muehlberger, 1992). Edge of Mississippi Embayment, as shown on the map, marks the contact between gently dipping, exposed Paleozoic rocks to the northwest (Anderson and others, 1979) and unconformably overlying, flat or gently dipping Mesozoic and Cenozoic strata of the embayment to the southeast.

Petrographic and chemical reconnaissance study of some granitic and gneissic rocks near the San Andreas fault from Bodega Head to Cajon Pass, California

USGS Publications Warehouse

Ross, Donald C.

1972-01-01

This petrographic and chemical study is based on reconnaissance sampling of granitic and related gneissic rock in the California Coast and Transverse Ranges. In the Coast Ranges, granitic rocks are restricted to an elongate belt, the Salinian block, between the San Andreas and Sur-Nacimiento fault zones. These rocks have a considerable compositional range, but are dominantly quartz monzonite and granodiorite. Moist of the Salinian block seems to be a structurally coherent basement block of chemically related granitic rocks. However, on both the east and the west sides of the block, gneiss crops out in abundance; these rocks may be structurally separate from the main part of the Salinian block. In the Transverse Ranges, the granitic and related rocks are dominantly of granodiorite composition, and in many areas granitic and gneissic rocks are intimately intermixed.Chemically the rocks of the California Coast and Transverse Ranges are somewhat intermediate in character between those of the east-central part of the Sierra Nevada batholith and those of the western part of the Sierra Nevada batholith and the southern California batholith. Probably the closest similarity is to the east-central Sierra Nevada rocks, but the rocks of the Coast and Transverse Ranges are somewhat higher in Al2O3 and lower in K2O than Sierran rocks of the comparable SiO2 content.Granitic basement rocks of the Salinian block are now anomalously sandwiched between Franciscan terranes. The petrographic and chemical data are compatible with the concept that the Salinian rocks were originally part of the great batholithic belt along the west coast, which is exemplified by the Sierra Nevada hatholith. It also seems most likely that the Salinian block was transported from somewhere south of the Sierra Nevada batholith by large-scale right-lateral movement along the San Andreas fault zone.

Continental rupture and the creation of new crust in the Salton Trough rift, Southern California and northern Mexico: Results from the Salton Seismic Imaging Project

NASA Astrophysics Data System (ADS)

Han, Liang; Hole, John A.; Stock, Joann M.; Fuis, Gary S.; Kell, Annie; Driscoll, Neal W.; Kent, Graham M.; Harding, Alistair J.; Rymer, Michael J.; González-Fernández, Antonio; Lázaro-Mancilla, Octavio

2016-10-01

A refraction and wide-angle reflection seismic profile along the axis of the Salton Trough, California and Mexico, was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting. From the northern Salton Sea to the southern Imperial Valley, the crust is 17-18 km thick and approximately one-dimensional. The transition at depth from Colorado River sediment to underlying crystalline rock is gradual and is not a depositional surface. The crystalline rock from 3 to 8 km depth is interpreted as sediment metamorphosed by high heat flow. Deeper felsic crystalline rock could be stretched preexisting crust or higher-grade metamorphosed sediment. The lower crust below 12 km depth is interpreted to be gabbro emplaced by rift-related magmatic intrusion by underplating. Low upper mantle velocity indicates high temperature and partial melting. Under the Coachella Valley, sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement. Mafic rock does not exist at 12-18 km depth as it does to the south, and a weak reflection suggests Moho at 28 km depth. Structure in adjacent Mexico has slower midcrustal velocity, and rocks with mantle velocity must be much deeper than in the Imperial Valley. Slower velocity and thicker crust in the Coachella and Mexicali valleys define the rift zone between them to be >100 km wide in the direction of plate motion. North American lithosphere in the central Salton Trough has been rifted apart and is being replaced by new crust created by magmatism, sedimentation, and metamorphism.

Continental rupture and the creation of new crust in the Salton Trough rift, southern California and northern Mexico: Results from the Salton Seismic Imaging Project

USGS Publications Warehouse

Han, Liang; Hole, John A.; Stock, Joann M.; Fuis, Gary S.; Kell, Annie; Driscoll, Neal W.; Kent, Graham M.; Rymer, Michael J.; Gonzalez-Fernandez, Antonio; Aburto-Oropeza, Octavio

2016-01-01

A refraction and wide-angle reflection seismic profile along the axis of the Salton Trough, California and Mexico, was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting. From the northern Salton Sea to the southern Imperial Valley, the crust is 17-18 km thick and approximately one-dimensional. The transition at depth from Colorado River sediment to underlying crystalline rock is gradual and is not a depositional surface. The crystalline rock from ~3 to ~8 km depth is interpreted as sediment metamorphosed by high heat flow. Deeper felsic crystalline rock could be stretched pre-existing crust or higher grade metamorphosed sediment. The lower crust below ~12 km depth is interpreted to be gabbro emplaced by rift-related magmatic intrusion by underplating. Low upper-mantle velocity indicates high temperature and partial melting. Under the Coachella Valley, sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement. Mafic rock does not exist at 12-18 depth as it does to the south, and a weak reflection suggests Moho at ~28 km depth. Structure in adjacent Mexico has slower mid-crustal velocity and rocks with mantle velocity must be much deeper than in the Imperial Valley. Slower velocity and thicker crust in the Coachella and Mexicali valleys define the rift zone between them to be >100 km wide in the direction of plate motion. North American lithosphere in the central Salton Trough has been rifted apart and is being replaced by new crust created by magmatism, sedimentation, and metamorphism.

Relationship between high- and low-grade Archean terranes: Implications for early Earth paleogeography

NASA Technical Reports Server (NTRS)

Eriksson, K. A.

1986-01-01

The Western Gneiss Terrain (WGT) of the Yilgarn Block, Western Australia was studied. The WGT forms an arcuate belt of Archean gneisses that flank the western margin of the Yilgarn Block. In general the WGT is composed of high-grade orthogneisses and paragneisses which contain supracrustal belts composed largely of siliciclastic metasediments and subordinate iron formation. The platformal nature of the metasedimentary belts and lack of obvious metavolcanic lithologies contrasts with the composition of typical Yilgarn greenstones to the east. Radiometric data from WGT rocks indicates that these rocks are significantly older than Yilgarn rocks to the east (less than 3.3 Ga) and this has led to the suggestion that the WGT represents sialic basement to Yilgarn granite-greenstone belts. The Mount Narryer region exposes the northernmost occurrence of high-grade metasediments within the WGT and consists of quartz-rich clastic metasediments at upper amphibolite to granulite grade. Most occurrences of supracrustal rocks in this region comprise isolated lenses within the gneissic basement. However, at Mount Narryer a unique sequence of metaclastics with preserved bedding provide an unusual window into the parentage of similar supracrustal bodies in this region.

Permeability in fractured rocks from deep geothermal boreholes in the Upper Rhine Graben

NASA Astrophysics Data System (ADS)

Vidal, Jeanne; Whitechurch, Hubert; Genter, Albert; Schmittbuhl, Jean; Baujard, Clément

2015-04-01

Permeability in fractured rocks from deep geothermal boreholes in the Upper Rhine Graben Vidal J.1, Whitechurch H.1, Genter A.2, Schmittbuhl J.1, Baujard C.2 1 EOST, Université de Strasbourg 2 ES-Géothermie, Strasbourg The thermal regime of the Upper Rhine Graben (URG) is characterized by a series of geothermal anomalies on its French part near Soultz-sous-Forêts, Rittershoffen and in the surrounding area of Strasbourg. Sedimentary formations of these areas host oil field widely exploited in the past which exhibit exceptionally high temperature gradients. Thus, geothermal anomalies are superimposed to the oil fields which are interpreted as natural brine advection occurring inside a nearly vertical multi-scale fracture system cross-cutting both deep-seated Triassic sediments and Paleozoic crystalline basement. The sediments-basement interface is therefore very challenging for geothermal industry because most of the geothermal resource is trapped there within natural fractures. Several deep geothermal projects exploit local geothermal energy to use the heat or produce electricity and thus target permeable fractured rocks at this interface. In 1980, a geothermal exploration well was drilled close to Strasbourg down to the Permian sediments at 3220 m depth. Bottom hole temperature was estimated to 148°C but the natural flow rate was too low for an economic profitability (<7 L/s). Petrophysics and reservoir investigations based on core analysis revealed a low matrix porosity with fracture zones spatially isolated and sealed in the sandstone formations. Any stimulation operation was planned and the project was abandoned. The Soultz-sous-Forêts project, initiated in 1986, explored during more than 30 years the experimental geothermal site by drilling five boreholes, three of which extend to 5 km depth. They identified a temperature of 200° C at 5 km depth in the granitic basement but with a variable flow rate. Hydraulic and chemical stimulation operations were applied in order to increase the initial low permeability by reactivating and dissolving sealed fractures in basement. The productivity was considerably improved and allows geothermal exploitation at 165° C and 20 L/s. Recent studies revealed the occurrences of permeable fractures in the limestones of Muschelkalk and the sandstones of Buntsandstein also. For the ongoing project at Rittershoffen, two deep boreholes, drilled down to 2.7 km depth target a reservoir in the sandstones of Buntsandstein and in the granitic basement interface. The thermal, hydraulic and chemical stimulations of the first well lead the project to an economic profitability with a temperature of 170° C and an industrial flow rate of 70 L/s. The deep sedimentary cover and the top of the granitic basement are the main target of the geothermal project in the URG. Permeability of fractured rocks after drilling operations or stimulation operations demonstrates the viability of French industrial deep geothermal projects in the URG was also confirmed by several geothermal projects in Germany that target the similar sediments-basement interface (Landau and Insheim) or the deep Triassic sediments (Bruchsal and Brühl). In France, future geothermal projects are planned in particular in Strasbourg suburb to exploit the permeability of deep-seated fractured sediment-basement interface.

Magnetotelluric imaging of anisotropic crust near Fort McMurray, Alberta: implications for engineered geothermal system development

NASA Astrophysics Data System (ADS)

Liddell, Mitch; Unsworth, Martyn; Pek, Josef

2016-06-01

Viability for the development of an engineered geothermal system (EGS) in the oilsands region near Fort McMurray, Alberta, is investigated by studying the structure of the Precambrian basement rocks with magnetotellurics (MT). MT data were collected at 94 broad-band stations on two east-west profiles. Apparent resistivity and phase data showed little variation along each profile. The short period MT data detected a 1-D resistivity structure that could be identified as the shallow sedimentary basin underlain by crystalline basement rocks to a depth of 4-5 km. At lower frequencies a strong directional dependence, large phase splits, and regions of out-of-quadrant (OOQ) phase were detected. 2-D isotropic inversions of these data failed to produce a realistic resistivity model. A detailed dimensionality analysis found links between large phase tensor skews (˜15°), azimuths, OOQ phases and tensor decomposition strike angles at periods greater than 1 s. Low magnitude induction vectors, as well as uniformity of phase splits and phase tensor character between the northern and southern profiles imply that a 3-D analysis is not necessary or appropriate. Therefore, 2-D anisotropic forward modelling was used to generate a resistivity model to interpret the MT data. The preferred model was based on geological observations of outcropping anisotropic mylonitic basement rocks of the Charles Lake shear zone, 150 km to the north, linked to the study area by aeromagnetic and core sample data. This model fits all four impedance tensor elements with an rms misfit of 2.82 on the southern profile, and 3.3 on the northern. The conductive phase causing the anisotropy is interpreted to be interconnected graphite films within the metamorphic basement rocks. Characterizing the anisotropy is important for understanding how artificial fractures, necessary for EGS development, would form. Features of MT data commonly interpreted to be 3-D (e.g. out of OOQ phase and large phase tensor skew) are shown to be interpretable with this 2-D anisotropic model.

The Ajo Mining District, Pima County, Arizona--Evidence for Middle Cenozoic Detachment Faulting, Plutonism, Volcanism, and Hydrothermal Alteration

USGS Publications Warehouse

Cox, Dennis P.; Force, Eric R.; Wilkinson, William H.; More, Syver W.; Rivera, John S.; Wooden, Joseph L.

2006-01-01

Introduction: The Ajo porphyry copper deposit and surrounding Upper Cretaceous rocks have been separated from their plutonic source and rotated by detachment faulting. Overlying middle Cenozoic sedimentary and volcanic rocks have been tilted and show evidence for two periods of rotation. Following these rotations, a granitic stock (23.7?0.2 Ma) intruded basement rocks west of the Ajo deposit. This stock was uplifted 2.5 km to expose deep-seated Na-Ca alteration.

Can the Metamorphic Basement of Northwestern Guatemala be Correlated with the Chuacús Complex?

NASA Astrophysics Data System (ADS)

Cacao, N.; Martens, U.

2007-05-01

The Chuacús complex constitutes a northward concave metamorphic belt that stretches ca. 150 km south of the Cuilco-Chixoy-Polochic (CCP) fault system in central and central-eastern Guatemala. It represents the basement of the southern edge of the Maya block, being well exposed in the sierra de Chuacús and the sierra de Las Minas. It is composed of high-Al metapelites, amphibolites, quartzofeldspathic gneisses, and migmatites. In central Guatemala the Chuacús complex contains ubiquitous epidote-amphibolite mineral associations, and local relics of eclogite reveal a previous high-pressure metamorphic event. North of the CCP, in the Sierra de Los Cuchumatanes area of western Guatemala, metamorphic rocks have been considered the equivalent of the Chuacús complex and hence been given the name Western Chuacús group, These rocks, which were intruded by granitic rocks and later mylonitized, include chloritic schist and gneiss, biotite-garnet schist, migmatites, and amphibolites. No eclogitic relics have been found within metamorphic rocks in northwestern Guatemala. Petrographic analyses of garnet-biotite schist reveal abundant retrogression and the formation of abundant zeolite-bearing veins associated with intrusion. Although metamorphic conditions in the greenschist and amphibolite facies are similar to those in the sierra de Chuacús, the association with deformed intrusive granites is unique for western Guatemala. Hence a correlation with metasediments intruded by the Rabinal granite in the San Gabriel area of Baja Verapaz seems more feasible than a correlation with the Chuacús complex. This idea is supported by reintegration of the Cenozoic left-lateral displacement along the CCP, which would place the metamorphic basement of western Guatemala north of Baja Verapaz, adjacent to metasediments intruded by granites in the San Gabriel-Rabinal area.

Provenance of sediments from Sumatra, Indonesia

NASA Astrophysics Data System (ADS)

Liebermann, Christof; Hall, Robert; Gough, Amy

2017-04-01

The island of Sumatra is situated at the south-western margin of the Indonesian archipelago. Sumatra is affected by active continental margin volcanism along the Sunda Trench, west of Sumatra as a result of active northeast subduction of the Indian plate under the Eurasian plate. Exposures of the Palaeozoic meta-sedimentary basement are mainly limited in extent to the northeast-southwest trending Barisan Mountain chain. The younger Cenozoic rocks are widespread across Sumatra, but can be grouped into structurally subdivided 'fore-arc', 'intramontane', and 'back-arc' basins. However, the formation of the basins pre-dates the current magmatic arc, thus a classical arc-related generation model can not be applied. The Cenozoic formations are well studied due to hydrocarbon enrichment, but little is known about their provenance history. A comprehensive sedimentary provenance study of the Cenozoic formations can aid in the wider understanding of Sumatran petroleum plays, can contribute to palaeographic reconstruction of western SE Asia, and might help to simplify the overall stratigraphy of Sumatra. This work represents a multi-proxy provenance study of sedimentary rocks from the main Cenozoic basins of Sumatra, alongside sediment from present-day river systems. The project refines the provenance in two ways: first, by studying the heavy mineral assemblages of the targeted formations, and secondly, by U-Pb detrital zircon dating using LA-ICP-MS to identify the age-range of the potential sediment sources. Preliminary U-Pb zircon age-data of >1500 concordant grains (10% discordant cut-off), heavy mineral compositions, and thin section analysis from two fieldwork seasons indicate a mixed provenance model, with a proximal igneous source, and mature basement rocks. An increase of the proximal signature in Lower-Miocene strata indicated by the occurrence of unstable heavy mineral phases such as apatite, and clinopyroxene suggests a major change of the source at the Oligocene-Miocene boundary. This can be interpreted as a pulse in the uplift of the Barisan Mountains. The presence of volcanic quartz in thin section supports this hypothesis. On the contrary, older sedimentary strata are characterised by ultra-stable heavy minerals such as zircon, tourmaline, and rutile; the presence of garnet in both pre-, and post-uplift affected strata indicates a contribution from metamorphic basement rocks, either from the local Sumatran basement or the Malay-Peninsula. Detrital zircon ages as old as Archean are present in all sedimentary formations; a prominent Triassic age group can be correlated with the Main Range Province granitoids reported from the Malay-Peninsula. It is noteworthy that zircon age spectra from Sumatra lack some diagnostic age groups commonly found in central- and western SE Asia, such as Cretaceous ages, correlated with igneous rock in the Schwaner Mountains, SW Borneo. The analysis of modern river sands suggests that the current sedimentary fluvial systems are mainly sourced from the recent Barisan-related volcanic arc. Zircon age patterns of the modern river sands resemble the populations found in the sedimentary strata, whereas, the heavy mineral composition is highly diluted by the recent igneous sources.

Rock Magnetic Study of IODP/ICDP Expedition 364 Site M0077A Drill Cores: Post-Impact Sediments, Impact Breccias, Melt, Granitic Basement and Dikes

NASA Astrophysics Data System (ADS)

Fucugauchi, J. U.; Perez-Cruz, L. L.; Rebolledo-Vieyra, M.; Tikoo, S.; Zylberman, W.; Lofi, J.

2017-12-01

Drilling at Site M0077 sampled post-impact sediments overlying a peak ring consisting of impact breccias, melt rock and granitoids. Here we focus on characterizing the peak ring using magnetic properties, which vary widely and depend on mineralogy, depositional and emplacement conditions and secondary alterations. Rock magnetic properties are integrated with Multi-Sensor Core Logger (MSCL) data, vertical seismic profile, physical properties, petrographic and chemical analyses and geophysical models. We measure low-field magnetic susceptibility at low- and high-frequencies, intensity and direction of natural remanent magnetization (NRM) and laboratory-induced isothermal (IRM) and anhysteretic (ARM) magnetizations, alternating-field demagnetization of NRM, IRM and NRM, susceptibility variation with temperature, anisotropy of magnetic susceptibility, hysteresis and IRM back-field demagnetization. Post-impact carbonates show low susceptibilities and NRM intensities, variable frequency-dependent susceptibilities and multivectorial remanences residing in low and high coercivity minerals. Hysteresis loops show low coercivity saturation magnetizations and variable paramagnetic mineral contents. Impact breccias (suevites) and melt rock show higher susceptibilities, low frequency-dependent susceptibilities, high NRM, ARM and IRM intensities and moderate ARM intensity/susceptibility ratios. Magnetic signal is dominated by fine-grained magnetite and titanomagnetites with PSD domain states. Melt rocks at the base of impactite section show the highest susceptibilities and remanence intensities. Basement section is characterized by low susceptibilities in the granites and higher values in the dikes, with NRM and ARM intensities increasing towards the base. The high susceptibilities and remanence intensities correlate with high seismic velocities, density and decreased porosity and electrical resistivity. Fracturing and alteration account for the reduced seismic velocities, density and magnetic properties in the basement section. Site M0077 is in a horizontal gradient high within the semi-circular gravity low in the crater central zone. Correlation with MSCL logs and petrographic and chemical data will allow further detailed characterization of peak ring units.

Hydrothermal origin of oil and gas reservoirs in basement rock of the South Vietnam continental shelf

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dmitriyevskiy, A.N.; Kireyev, F.A.; Bochko, R.A.

1993-07-01

Oil-saturated granites, with mineral parageneses typical of hydrothermal metasomatism and leaching haloes, have been found near faults in the crystalline basement of the South Vietnam continental shelf. The presence of native silver, barite, zincian copper, and iron chloride indicates a deep origin for the mineralizing fluids. Hydrothermally altered granites are a new possible type of reservoir and considerably broaden the possibilities of oil and gas exploration. 15 refs., 22 figs., 1 tab.

Dillon cutoff-Basement-involved tectonic link between the disturbed belt of west-central Montana and the overthrust belt of extreme southwestern Montana

NASA Astrophysics Data System (ADS)

O'Neill, J. Michael; Schmidt, Christopher J.; Genovese, Paul W.

1990-11-01

The front of the Cordilleran fold and thrust belt in western Montana follows the disturbed belt in the north, merges with the southwest Montana transverse zone in the west-central part of the region, and in southwestern Montana is marked by a broad zone characterized by complex interaction between thrust belt structures and basement uplifts. The front margin of the thrust belt in Montana reflects mainly thin-skinned tectonic features in the north, an east-trending lateral ramp that curves southwest in the central part into the Dillon cutoff, an oblique-slip, thick-skinned displacement transfer zone that cuts through basement rocks of the Lima recess, and a zone of overlap between thin- and thick-skinned thrusts in extreme southwestern Montana. The transverse ramp and basement-involved thrust faults are controlled by Proterozoic structures.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Delhal, J.; Lay, C.; Ledent, D.

Ten Sr/Rb apparent ages obtained on minerals separated from rocks of the Kasai (South Congo) basement are interpreted as giving the age of three major events of the geological history of this basement. From North to South the area studied can be subdivised into three major portions: the Dibaya-type basement, the intermediate region, and the Luiza-type basement. A first metamorphism in the Luiza basement appears to be at least 3,300 m.y. old. It is followed by a charnockitization and a migmatitization. The latter phenomenon appears to be identical with the granitization of the Dibaya basement which occurred at circa 2,700more » m.y. Later, an important cataclasis with pegmatitic intrusions affected the Dibaya basement. The pegmatites are dated at circa 2,100 m.y. This is also the age of the metamorphism and orogeny of the Luiza metasedimentary series which forms part of the intermediate region. This orogeny is therefore considered responsible for the above mentioned widespread cataclasis and activity. Three dated orogenic cycles are therefore superposed in this part of the Kasai basement; at least one younger, undated, cycle (the Lulua cycle) is recognized in the same general area. These preliminary results will be used as a foundation for a more complete dating program based not only on further Sr/Rb ages but also on ages obtained by other methods. An adequate nomenclature will be adopted as a result of the planned detailed studies. (auth)« less

Petrographic and geochemical data for Cenozoic volcanic rocks of the Bodie Hills, California and Nevada

USGS Publications Warehouse

du Bray, Edward A.; John, David A.; Box, Stephen E.; Vikre, Peter G.; Fleck, Robert J.; Cousens, Brian L.

2013-04-23

Petrographic and geochemical data for Cenozoic volcanic rocks of the Bodie Hills, California and Nevada // // This report presents petrographic and geochemical data for samples collected during investigations of Tertiary volcanism in the Bodie Hills of California and Nevada. Igneous rocks in the area are principally 15–6 Ma subduction-related volcanic rocks of the Bodie Hills volcanic field but also include 3.9–0.1 Ma rocks of the bimodal, post-subduction Aurora volcanic field. Limited petrographic results for local basement rocks, including Mesozoic granitoid rocks and their metamorphic host rocks, are also included in the compilation. The petrographic data include visual estimates of phenocryst abundances as well as other diagnostic petrographic criteria. The geochemical data include whole-rock major oxide and trace element data, as well as limited whole-rock isotopic data.

RIO Tinto Faulted Volcanosedimentary Deposits as Analog Habitats for Extant Subsurface Biospheres on Mars: A Synthesis of the MARTE Drilling Project Geobiology Results

NASA Technical Reports Server (NTRS)

Fernandez-Remolar, D. C.; Prieto-Ballesteros, O.; Rodriquez, N.; Davila, F.; Stevens, T.; Amils, R.; Gomez-Elvira, J.; Stoker, C.

2005-01-01

Geochemistry and mineralogy on Mars surface characterized by the MER Opportunity Rover suggest that early Mars hosted acidic environments in the Meridiani Planum region [1, 2]. Such extreme paleoenvironments have been suggested to be a regional expression of the global Mars geological cycle that induced acidic conditions by sulfur complexation and iron buffering of aqueous solutions [3]. Under these assumptions, underground reservoirs of acidic brines and, thereby, putative acidic cryptobiospheres, may be expected. The MARTE project [4, 5] has performed a drilling campaign to search for acidic and anaerobic biospheres in R o Tinto basement [6] that may be analogs of these hypothetical communities occurring in cryptic habitats of Mars. This Rio Tinto geological region is characterized by the occurrence of huge metallic deposits of iron sulfides [7]. Late intensive diagenesis of rocks driven by a compressive regimen [8] largely reduced the porosity of rocks and induced a cortical thickening through thrusting and inverse faulting and folding. Such structures play an essential role in transporting and storing water underground as any other aquifers do in the Earth. Once the underground water reservoirs of the Ro Tinto basement contact the hydrothermal pyrite deposits, acidic brines are produced by the release of sulfates and iron through the oxidation of sulfides [9].

Unraveling multiple provenance areas using sandstone petrofacies and geochemistry: An example in the southern flank of the Golfo San Jorge Basin (Patagonia, Argentina)

NASA Astrophysics Data System (ADS)

Limarino, Carlos Oscar; Giordano, Sergio Roberto

2016-03-01

The aim of this paper is to study the provenance of Late Cretaceous sandstones deposited along the south flank of the Golfo San Jorge Basin. For this purpose, detrital modes of three hundred thirty-seven sandstone samples collected in the Mina del Carmen, Bajo Barreal, and Cañadón Seco Formations were studied in ten oil fields. According to the modal composition of the sandstones, six petrofacies were defined allowing the identification of not only principal, but also secondary provenance areas. The QVM and VQM petrofacies are more than 20% metamorphic, sedimentary, and polycrystalline quartz clasts (Lm + Ls + Qpg > 20%), evidencing a secondary signal of basement supply masked by a predominant volcanic provenance. The petrofacies VP and VF are characterized by Lm + Ls + Qpg <20% and more than 20% total feldspar (Pm + Om >20%.), which indicate a supply of sediment from volcanic terrains and scarce derivation of materials from basement rocks. Based on the plagioclase/k-feldspar ratio, the VF petrofacies is interpreted to be dominated by the supply of sand grains from the Andean volcanic-arc, while VP is supposed have originated through the erosion of intermediate volcanic rock outcroppings in the Macizo del Deseado. Finally, both the VQ and QV petrofacies show Lm + Ls + Qpg <20% and Pm + Om<20%, indicating a provenance of volcanic areas coupled with minor contributions from basement rocks. During the Late Cretaceous, the Golfo San Jorge Basin underwent a sag phase that was characterized by very scarce volcanism and tectonic activity. Although these conditions did not favor defined patterns in the vertical stacking of petrofacies, the sandstones exhibit remarkable changes in their regional distribution, which were determined by the paleogeography of the basin and differences in basement composition within the source areas. Finally, a paleogeographic model for sediment circulation in the basin is proposed. This model recognizes the main fluvial dispersal trends that flowed northwest to southeast and transported large amounts of volcanic clasts (associated with petrofacies VF-VQ). To the extent that rivers flowed eastward, a secondary supply from the Precambrian basement, which were composed of low-to high-grade metamorphic rocks, was also important (petrofacies association VQM and QVM). The southwestern area of the basin is dominated by VP petrofacies that record the supply of plagioclase-rich volcanic clasts. This petrofacies likely corresponds to the erosion of Jurassic volcanic units that crop out in the Macizo del Deseado.

High-resolution seismic reflection/refraction images near the outer margin of the Chesapeake Bay impact crater, York-James Peninsula, southeastern Virginia

USGS Publications Warehouse

Catchings, R.D.; Saulter, D.E.; Powars, D.S.; Goldman, M.R.; Dingler, J.A.; Gohn, G.S.; Schindler, J.S.; Johnson, G.H.

2001-01-01

Powars and Bruce (1999) showed that the Chesapeake Bay region of southeastern Virginia was the site of an asteroid or comet impact during the late Eocene, approximately 35 million years ago (Fig. 1). Initial borehole and marine seismic-reflection data revealed a 90-km-diameter impact structure, referred to as the Chesapeake Bay Impact Crater (CBIC), that lies buried beneath the southern Chesapeake Bay and surrounding Virginia Coastal Plain (Powars and Bruce, Figs. 1b). Stratigraphic correlations among a series of boreholes suggest that the impact disrupted basement rock and the overlying Cretaceous through middle Eocene deltaic and marine sediments. The CBIC truncates important regional sedimentary aquifer systems and possibly caused differential flushing of connate seawater. Therefore, the CBIC affects the present-day ground-water quantity and quality in the rapidly growing Hampton Roads region of southeastern Virginia. Impact-generated faults in the basement rock may be the sources of small-to-moderate earthquakes that have been occurred around the perimeter of the impact structure over the past few hundred years (Johnson et al., 1998). Powars and Bruce (1999) suggest that 150 m to 490 m of relatively undisturbed, post-impact Coastal-Plain sediments overlie the impact-disrupted sediments and basement rocks west of Chesapeake Bay. Their interpretation of marine seismic data, released from Texaco and Exxon, revealed a central 38-km-wide, 1.6-km-deep disrupted zone in the basement rocks (inner basin), which is surrounded by a 21- to 31-km-wide, 1- km-deep annular trough. Steep rim escarpments surround these features, which they mapped regionally as the outer and inner margins (rims) of the CBIC (Fig. 1b). The outer margin is a slumped terrace zone that has a 120- to 305-m-high gullied escarpment and varies in width from 0.8 to 3.2 km. However, the geographic bounds of the CBIC, its effects on the regional aquifer systems, and the distribution of impact generated faults and fractures in basement are not well determined. To better determine some of the unknowns associated with the CBIC, we conducted a 350-m-long, high-resolution seismic reflection and refraction survey, referred to here as the CBIC-1 seismic survey, on the York-James Peninsula in June 1999. In particular, we attempted to: better define the outer margin of the CBIC, understand lateral variations in the stratigraphic sequence, help assess potential hazards associated with regional seismicity, and determine acquisition parameters needed for shallow-depth seismic imaging in the Chesapeake Bay area.

Evidence for synchronous thin-skinned and basement deformation in the Cordilleran fold-thrust belt: the Tendoy Mountains, southwestern Montana

NASA Astrophysics Data System (ADS)

McDowell, Robin John

1997-01-01

The Tendoy Mountains contain the easternmost thin-skinned thrust sheets in the Cordilleran fold-thrust belt of southwestern Montana, and are in the zone of tectonic overlap between the Rocky Mountain foreland and the Cordilleran fold-thrust belt. The three frontal thrust sheets of the Tendoy Mountains are from north to south, the Armstead, McKenzie, and Tendoy sheets. Near the southeastern terminus of the Tendoy thrust sheet is a lateral ramp in which the Tendoy thrust climbs along strike from the Upper Mississippian Lombard Limestone to lower Cretaceous rocks. This ramp coincides with the southeastern side of the Paleozoic Snowcrest trough and projection of the range-flanking basement thrust of the Blacktail-Snowcrest uplift, suggesting either basement or stratigraphic control on location of the lateral ramp. Axes of major folds on the southern part of the Tendoy thrust sheet are parallel to the direction of thrust transport and to the trend of the Snowcrest Range. They are a result of: (1) Pre-thrust folding above basement faults; (2) Passive transportation of the folds from a down-plunge position; (3) Minor reactivation of basement faults; and (4) Emplacement of blind, sub-Tendoy, thin-skinned thrust faults. The Tendoy sheet also contains a major out-of-sequence thrust fault that formed in thick Upper Mississippian shales and created large, overturned, foreland-verging folds in Upper Mississippian to Triassic rocks. The out-of-sequence fault can be identified where stratigraphic section is omitted, and by a stratigraphic separation diagram that shows it cutting down section in the direction of transport. The prominent lateral ramp at the southern terminus of the Tendoy thrust sheet is a result of fault propagation through strata folded over the edge of the Blacktail-Snowcrest uplift.

Geologic summary of the Appalachian Basin, with reference to the subsurface disposal of radioactive waste solutions

USGS Publications Warehouse

Colton, G.W.

1962-01-01

The Appalachian basin is an elongate depression in the crystalline basement complex< which contains a great volume of predominantly sedimentary stratified rocks. As defined in this paper it extends from the Adirondack Mountains in New York to central Alabama. From east to west it extends from the west flank of the Blue Ridge Mountains to the crest of the Findlay and Cincinnati arches and the Nashville dome. It encompasses an area of about 207,000 square miles, including all of West Virginia and parts of New York, New Jersey, Pennsylvania, Ohio, Maryland, Virginia, Kentucky, Tennessee, North Carolina, Georgia, and Alabama. The stratified rocks that occupy the basin constitute a wedge-shaped mass whose axis of greatest thickness lies close to and parallel to the east edge of the basin. The maximum thickness of stratified rocks preserved in any one part of the basin today is between 35,000 and 40,000 feet. The volume of the sedimentary rocks is approximately 510,000 cubic miles and of volcanic rocks is a few thousand cubic miles. The sedimentary rocks are predominantly Paleozoic in age, whereas the volcanic rocks are predominantly Late Precambrian. On the basis of gross lithology the stratified rocks overlying the crystalline basement complex can be divided into nine vertically sequential units, which are designated 'sequences' in this report. The boundaries between contiguous sequences do not necessarily coincide with the commonly recognized boundaries between systems or series. All sequences are grossly wedge shaped, being thickest along the eastern margin of the basin and thinnest along the western margin. The lowermost unit--the Late Precambrian stratified sequence--is present only along part of the eastern margin of the basin, where it lies unconformably on the basement complex. It consists largely of volcanic tuffs and flows but contains some interbedded sedimentary rocks. The Late Precambrian sequence is overlain by the Early Cambrian clastic sequence. Where the older sequence is absent, the Early Cambrian sequence rests on the basement complex. Interbedded fine- to coarse-grained noncarbonate detrital rocks comprise the bulk of the sequence, but some volcanic and carbonate rocks are included. Next above is the Cambrian-Ordovician carbonate sequence which consists largely of limestone and dolomite. Some quartzose sandstone is present in the lower part in the western half of the basin, and much shale is present in the upper part in the southeast part of the basin. The next higher sequence is the Late Ordovician clastic sequence, which consists largely of shale, siltstone, and sandstone. Coarse-grained light-gray to red rocks are common in the sequence along the eastern side of the basin, whereas fine-grained dark-gray to black calcareous rocks are common along the west side. The Late Ordovician clastic sequence is overlain--unconformably in many places--by the Early Silurian clastic sequence. The latter comprises a relatively thin wedge of coarse-grained clastic rocks. Some of the most prolific oil- and gas-producing sandstones in the Appalachian basin are included. Among these are the 'Clinton' sands of Ohio, the Medina Sandstones of New York and Pennsylvania, and the Keefer or 'Big Six' Sandstone of West Virginia and Kentucky. Conformably overlying the Early Silurian clastic sequence is the Silurian-Devonian carbonate sequence, which consists predominantly of limestone and dolomite. It also contains a salt-bearing unit in the north-central part of the basin and a thick wedge of coarse-grained red beds in the northeastern part. The sequence is absent in much of the southern part of the basin. Large volumes of gas and much oil are obtained from some of its rocks, especially from the Oriskany Sandstone and the Huntersville Chert. The Silurian-Devonian carbonate sequence is abruptly overlain by the Devonian clastic sequence--a thick succession of interbedded shale, mudrock, siltstone, and sandstone. Colors range f

Age and isotopic systematics of Cretaceous borehole and surface samples from the greater Los Angeles Basin region: Implications for the types of crust that might underlie Los Angeles and their distribution along late Cenozoic fault systems

USGS Publications Warehouse

Premo, Wayne R.; Morton, Douglas M.; Kistler, Ronald W.

2014-01-01

Nine U-Pb zircon ages were determined on plutonic rocks sampled from surface outcrops and rock chips of drill core from boreholes within the greater Los Angeles Basin region. In addition, lead-strontium-neodymium (Pb-Sr-Nd) whole-rock isotopic data were obtained for eight of these samples. These results help to characterize the crystalline basement rocks hidden in the subsurface and provide information that bears on the tectonic history of the myriad of fault systems that have dissected the Los Angeles region over the past 15 m.y. Seven of the nine samples have U-Pb ages ranging from 115 to 103 Ma and whole-rock Pb-Sr-Nd isotopic characteristics that indicate the crystalline basement underneath the greater Los Angeles Basin region is mostly part of the Peninsular Ranges batholith. Furthermore, these data are interpreted as evidence for (1) the juxtaposition of mid-Cretaceous, northern Peninsular Ranges batholith plutonic rocks against Late Cretaceous plutonic rocks of the Transverse Ranges in the San Fernando Valley, probably along the Verdugo fault; (2) the juxtaposition of older northwestern Peninsular Ranges batholith rocks against younger northeastern Peninsular Ranges batholith rocks in the northern Puente Hills, implying transposition of northeastern Peninsular Ranges batholith rocks to the west along unrecognized faults beneath the Chino Basin; and (3) juxtaposition of northern Peninsular Ranges batholith plutonic rocks against Late Cretaceous plutonic rocks of the Transverse Ranges along the San Jose fault in the northern San Jose Hills at Ganesha Park. These mainly left-lateral strike-slip faults of the eastern part of the greater Los Angeles Basin region could be the result of block rotation within the adjacent orthogonal, right-lateral, Elsinore-Whittier fault zone to the west and the subparallel San Jacinto fault zone to the east. The San Andreas fault system is the larger, subparallel, driving force further to the east.

PBF Reactor Building (PER620). Construction view shows native lava rock ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

PBF Reactor Building (PER-620). Construction view shows native lava rock surrounding basement excavation and general complexity of planning required to build the PBF. A three-inch low-pressure air line protrudes from wall just below left center. Date: February 21, 1967. Photographer: Larry Page. INEEL negative no. 67-1125 - Idaho National Engineering Laboratory, SPERT-I & Power Burst Facility Area, Scoville, Butte County, ID

Strides in Preservation of Malawi's Natural Stone

NASA Astrophysics Data System (ADS)

Kamanga, Tamara; Chisenga, Chikondi; Katonda, Vincent

2017-04-01

The geology of Malawi is broadly grouped into four main lithological units that is the Basement Complex, the Karoo Super group, Tertiary to Quaternary sedimentary deposits and the Chilwa Alkaline province. The basement complex rocks cover much of the country and range in age from late Precambrian to early Paleozoic. They have been affected by three major phases of deformation and metamorphism that is the Irumide, Ubendian and The Pan-African. These rocks comprise gneisses, granulites and schists with associated mafic, ultramafic, syenites and granite rocks. The Karoo System sedimentary rocks range in age from Permian to lower Jurassic and are mainly restricted to two areas in the extreme North and extreme Alkaline Province - late Jurassic to Cretaceous in age, preceded by upper Karoo Dolerite dyke swarms and basaltic lavas, have been intruded into the Basement Complex gneisses of southern Malawi. Malawi is endowed with different types of natural stone deposits most of which remain unexploited and explored. Over twenty quarry operators supply quarry stone for road and building construction in Malawi. Hundreds of artisanal workers continue to supply aggregate stones within and on the outskirts of urban areas. Ornamental stones and granitic dimension stones are also quarried, but in insignificant volumes. In Northern Malawi, there are several granite deposits including the Nyika, which is the largest single outcrop occupying approximately 260.5 km2 , Mtwalo Amazonite an opaque to translucent bluish -green variety of microcline feldspar that occurs in alkali granites and pegmatite, the Ilomba granite (sodalite) occurring in small areas within biotite; apatite, plagioclase and calcite. In the Center, there are the Dzalanyama granites, and the Sani granites. In the South, there are the Mangochi granites. Dolerite and gabbroic rocks spread across the country, treading as black granites. Malawi is also endowed with many deposits of marble. A variety of other igneous, metamorphic and sedimentary rocks are also used as dimension stones. Discovery and preservation of more natural stone deposits through research is essential in the country .Natural stone preservation has not only the potential to generate significant direct and indirect economic benefits for Malawi but also to preserve its heritage .

Magnetic and gravity gradiometry framework for Mesoproterozoic iron oxide-apatite and iron oxide-copper-gold deposits, southeast Missouri, USA

USGS Publications Warehouse

McCafferty, Anne E.; Phillips, Jeffrey; Driscoll, Rhonda L.

2016-01-01

High-resolution airborne magnetic and gravity gradiometry data provide the geophysical framework for evaluating the exploration potential of hidden iron oxide deposits in Mesoproterozoic basement rocks of southeast Missouri. The data are used to calculate mineral prospectivity for iron oxide-apatite (IOA) ± rare earth element (REE) and iron oxide-copper-gold (IOCG) deposits. Results delineate the geophysical footprints of all known iron oxide deposits and reveal several previously unrecognized prospective areas. The airborne data are also inverted to three-dimensional density and magnetic susceptibility models over four concealed deposits at Pea Ridge (IOA ± REE), Boss (IOCG), Kratz Spring (IOA), and Bourbon (IOCG). The Pea Ridge susceptibility model shows a magnetic source that is vertically extensive and traceable to a depth of greater than 2 km. A smaller density source, located within the shallow Precambrian basement, is partly coincident with the magnetic source at Pea Ridge. In contrast, the Boss models show a large (625-m-wide), vertically extensive, and coincident dense and magnetic stock with shallower adjacent lobes that extend more than 2,600 m across the shallow Precambrian paleosurface. The Kratz Spring deposit appears to be a smaller volume of iron oxides and is characterized by lower density and less magnetic rock compared to the other iron deposits. A prospective area identified south of the Kratz Spring deposit shows the largest volume of coincident dense and nonmagnetic rock in the subsurface, and is interpreted as prospective for a hematite-dominant lithology that extends from the top of the Precambrian to depths exceeding 2 km. The Bourbon deposit displays a large bowl-shaped volume of coincident high density and high-magnetic susceptibility rock, and a geometry that suggests the iron mineralization is vertically restricted to the upper parts of the Precambrian basement. In order to underpin the evaluation of the prospectivity and three-dimensional models, an extensive statistical summary of density and apparent magnetic susceptibility measurements is presented that includes data on several hundred samples taken from the deposits, altered wall rocks, and unaltered country rocks.

Aeromagnetic maps of the Uinta and Piceance Basins and vicinity, Utah and Colorado

USGS Publications Warehouse

Grauch, V.J.S.; Plesha, Joseph L.

1989-01-01

In order to understand the evolution of sedimentary basins, it is important to understand their tectonic setting. In a U.S. Geological Survey (USGS) study of the Uinta and Piceance basins in Utah and Colorado, this understanding is approached through characterization of subsurface structure and lithology of a large region encompassing the basins. An important tool for interpreting these subsurface features is aeromagnetic data. Aeromagnetic anomalies represent variations in the strength and direction of the Earth's magnetic field that are produced by rocks containing a significant number of magnetic minerals (commonly magnetite). The shape and magnitude of an anomaly produced by one body of rock are complexly related to the amount of magnetic minerals present, the magnetic properties of those minerals (determined by a number of factors, including the history of the rock), and the shape of the rock body. In the study area, only crystalline basement rocks and volcanic rocks are likely to contain enough magnetic minerals to produce anomalies; sedimentary rocks and metasediments are generally so poor in magnetic minerals that their magnetic effects cannot be detected by the types of surveys presented in this report. Patterns of anomalies on aeromagnetic maps can reveal not only lithologic differences related to magnetite content, but structural features as well, such as faults that have juxtaposed crystalline rocks against sedimentary rocks, and upwarps of crystalline basement underlying sedimentary sequences. Tectonic features of regional extent may not become apparent until a number of aeromagnetic surveys have been compiled and plotted at the same scale. Commonly the compilation involves piecing together data from surveys that were flown at different times and have widely disparate flight specifications and data reduction procedures. The data may be compiled into a composite map, where all the pieces are plotted onto one map without regard to the differences in flight elevation and datum, or they may be compiled into a merged map, where all survey data are analytically reduced to a common flight elevation and datum, and then digitally merged at the survey boundaries. The composite map retains the original resolution of all survey data, but computer methods to enhance or model regional features crossing the survey boundaries cannot be applied. On the other hand, these computer methods can be applied to the merged data, but the resolution of the data may be somewhat diminished. This report presents both composite and merged aeromagnetic maps for a large region that includes the Uinta Basin in Utah and the Piceance basin in Colorado (fig. 1).

Middle Proterozoic age for the Montpelier Anorthosite, Goochland terrane, eastern Piedmont, Virginia

USGS Publications Warehouse

Aleinikoff, J.N.; Horton, J. Wright; Walter, M.

1996-01-01

Uranium-lead dating of zircons from the Montpelier Anorthosite confirms previous interpretations, based on equivocal evidence, that the Goochland terrane in the eastern Piedmont of Virginia contains Grenvillian basement rocks of Middle Proterozoic age. A very few prismatic, elongate, euhedral zircons, which contain 12-29 ppm uranium, are interpreted to be igneous in origin. The vast majority of zircons are more equant, subangular to anhedral, contain 38-52 ppm uranium, and are interpreted to be metamorphic in origin. One fraction of elongate zircon, and four fragments of a very large zircon (occurring in a nelsonite segregation) yield an upper intercept age of 1045 ?? 10 Ma, interpreted as the time of anorthosite crystallization. Irregularly shaped metamorphic zircons are dated at 1011 ?? 2 Ma (weighted average of the 207Pb/206Pb ages). The U-Pb isotopic systematics of metamorphic titanite were reset during the Alleghanian orogeny at 297 ?? 5 Ma. These data provide a minimum age for gneisses of the Goochland terrane that are intruded by the anorthosite. Middle Proterozoic basement rocks of the Goochland terrane may be correlative with those in the Shenandoah massif of the Blue Ridge tectonic province, as suggested by similarities between the Montpelier Anorthosite and the Roseland anorthosite. Although the areal extent of Middle Proterozoic basement and basement-cover relations in the eastern Piedmont remain unresolved, results of this investigation indicate that the Goochland terrane is an internal massif of Laurentian crust rather than an exotic accreted terrane.

Environmental Impact Assessment of Dumpsite: Case Study from southwestern Saudi Arabia

NASA Astrophysics Data System (ADS)

Alfaifi, H. J.; Alhumidan, S. M.; Kahal, A. Y.; Abdel Rahman, K.; Al-Qadasi, B.

2017-12-01

The dumpsite is underlain by highly fractured Precambrian basement complex of metamorphosed igneous and sedimentary rocks. Minor Tertiary, Quaternary basalts and Quaternary alluvial deposits overlie the basement rocks. Structurally, the area is affected by intersected series of north-to northwest trending faults. Hydrogeological setting of the study area is characterized by shallow groundwater aquifers in the fractured and weathered basement rocks. Moreover, the area exposes heavy rains especially during summer seasons, which may accelerate the transferring of contaminated water to the neighbouring valleys and low land. At present, the residential and Khamis Mushait new industrial zone are situated close to the dumpsite. The main objective of this study is to assess the leachate intrusion and groundwater contamination in the urban area of Khamis Mushait. Geophysical and geochemical techniques have been successfully applied in the assessment of environmental impact of dumpsites globally. Near-surface geophysical investigations such as Seismic refraction tomography, Schlumberger vertical electrical soundings (VES) and ground magnetic survey have been conducted to detect the controlling structures and lithological variation of the dumpsite. In addition, four water samples from hand dug wells and two surface water samples were collected from and around dumpsite. These water samples were analysed geochemically to inspect the presence of heavy metals, salts (sulphates, nitrates and chlorides), radioactive elements and physically to assess pH, TDS, DO, salinity, total hardness, turbidity, electrical conductivity and temperature. Results of VES illustrate low resistivity zones (≤ 30 Ohm-m) due to conductive leachate from dumpsite while seismic models and ground magnetic intensity map delineated fractures beneath the weathered basement layer which may provide pathways for the contaminants. The physico-chemical analysis of the collected groundwater samples revealed that there are considerable impacts of dumpsite leachate in the shallow groundwater. pH values of the representative samples indicate its unsuitability for human consumption. Leachate flow direction is oriented NNW-SSE and follows the similar flow pattern as deduced from hydrogeological investigation.

Geophysical Studies Based on Gravity and Seismic Data of Tule Desert, Meadow Valley Wash, and California Wash Basins, Southern Nevada

USGS Publications Warehouse

Scheirer, Daniel S.; Page, William R.; Miller, John J.

2006-01-01

Gravity and seismic data from Tule Desert, Meadow Valley Wash, and California Wash, Nevada, provide insight into the subsurface geometry of these three basins that lie adjacent to rapidly developing areas of Clark County, Nevada. Each of the basins is the product of Tertiary extension accommodated with the general form of north-south oriented, asymmetrically-faulted half-grabens. Geophysical inversion of gravity observations indicates that Tule Desert and Meadow Valley Wash basins are segmented into subbasins by shallow, buried basement highs. In this study, basement refers to pre-Cenozoic bedrock units that underlie basins filled with Cenozoic sedimentary and volcanic units. In Tule Desert, a small, buried basement high inferred from gravity data appears to be a horst whose placement is consistent with seismic reflection and magnetotelluric observations. Meadow Valley Wash consists of three subbasins separated by basement highs at structural zones that accommodated different styles of extension of the adjacent subbasins, an interpretation consistent with geologic mapping of fault traces oblique to the predominant north-south fault orientation of Tertiary extension in this area. California Wash is a single structural basin. The three seismic reflection lines analyzed in this study image the sedimentary basin fill, and they allow identification of faults that offset basin deposits and underlying basement. The degree of faulting and folding of the basin-fill deposits increases with depth. Pre-Cenozoic units are observed in some of the seismic reflection lines, but their reflections are generally of poor quality or are absent. Factors that degrade seismic reflector quality in this area are rough land topography due to erosion, deformed sedimentary units at the land surface, rock layers that dip out of the plane of the seismic profile, and the presence of volcanic units that obscure underlying reflectors. Geophysical methods illustrate that basin geometry is more complicated than would be inferred from extrapolation of surface topography and geology, and these methods aid in defining a three-dimensional framework to understand groundwater storage and flow in southern Nevada.

Maps showing geology, structure, and geophysics of the central Black Hills, South Dakota

USGS Publications Warehouse

Redden, Jack A.; DeWitt, Ed

2008-01-01

This 1:100,000-scale digital geologic map details the complex Early Proterozoic granitic rocks, Early Proterozoic supracrustal metamorphic rocks, and Archean crystalline basement of the Black Hills. The granitic rocks host pegmatite deposits renowned for their feldspar, mica, spodumene, and beryl. The supracrustal rocks host the Homestake gold mine, which produced more than 40 million ounces of gold over a 125-year lifetime. The map documents the Laramide deformation of Paleozoic and Mesozoic cover rocks; and shows the distribution of Laramide plutonic rocks associated with precious-metals deposits. Four 1:300,000-scale maps summarize Laramide structures; Early Proterozoic structures; aeromagnetic anomalies; and gravity anomalies. Three 1:500,000-scale maps show geophysical interpretations of buried Early Proterozoic to Archean rocks in western South Dakota and eastern Wyoming.

Proterozoic crustal evolution of the Eucla basement, Australia: Implications for destruction of oceanic crust during emergence of Nuna

NASA Astrophysics Data System (ADS)

Kirkland, C. L.; Smithies, R. H.; Spaggiari, C. V.; Wingate, M. T. D.; Quentin de Gromard, R.; Clark, C.; Gardiner, N. J.; Belousova, E. A.

2017-05-01

The crystalline basement beneath the Cretaceous to Cenozoic Bight and Eucla Basins, in Western Australia has received comparatively little attention even though it lies on the eastern margin of one of the most mineral resource endowed regions on the planet. This basement is characterized by a complex geological evolution spanning c. 2 billion years, but paucity of outcrop and younger basin cover present a daunting challenge to understand the basement geology. In this work the composition of the unexposed Proterozoic crystalline basement to the Bight and Eucla Basins is investigated through zircon Hf isotopes and whole rock geochemistry from new drillcore samples. This region includes two geophysically defined basement entities: The Madura Province, containing: 1) c. 1478 Ma Sleeper Camp Formation, which has variable isotopic signatures including evolved values interpreted to reflect reworking of rare slivers of hyperextended Archean crust, 2) 1415-1389 Ma Haig Cave Supersuite, with mantle-like isotope values interpreted as melting of subduction-modified N-MORB source, and 3) 1181-1125 Ma Moodini Supersuite, with juvenile isotopic signatures interpreted to reflect mixed mafic lower-crustal and asthenospheric melts produced at the base of thinned crust. The Coompana Province, to the east of the Madura Province, has three major magmatic components: 1) c. 1610 Ma Toolgana Supersuite, with chemical and isotopic characteristics of primitive arc rock, 2) c. 1490 Ma Undawidgi Supersuite, with juvenile isotope values consistent with extensional processes involving asthenospheric input and 3) 1192-1140 Ma Moodini Supersuite, with strong isotopic similarity to Moodini Supersuite rocks in the Madura Province. This new isotopic and geochemical data shows that the Madura and Coompana regions together represent a huge tract of predominantly juvenile material. Magma sources recognised, include; 1) depleted mantle, producing MORB-like crust at c. 1950 Ma, but also contributing to younger magmatism; 2) recycled c. 1950 Ma crust reworked in primitive arcs and in intra-plate settings and; 3) minor evolved material representing fragments of hyperextended continent. The observed isotopic evolution pattern is comparable to that of other central Australian Proterozoic provinces, including the Musgrave Province, the northern margin of the Gawler Craton, and components within the Rudall Province. Linking these isotopic signatures defines the Mirning Ocean, and its subducted and underplated equivalents. In a global context we suggest c. 1950 Ma crust production reflects the onset of ordered oceanic spreading centres, which swept juvenile crustal fragments into Nuna.

European and Middle-East ferroan hydrothermal dolomites: lessons learnt with respect to crustal dynamics, fluid circulations and rock-fluid interactions

NASA Astrophysics Data System (ADS)

Nader, Fadi Henri; Gasparrini, Marta; Bachaud, Pierre

2016-04-01

Classical case studies of hydrothermal dolostones, which are known worldwide to provide excellent reservoirs for ores and hydrocarbons, often illustrate the presence of iron-rich dolomite phases. The world-class hydrothermal dolostones from the Basque-Cantabrian Basin (Northern Spain) exemplify the initiation of high temperature dolomitization (at about 200°C), with significant amount of ferroan dolomite phases (including up to 2% FeO). These dolomites are believed to be responsible for the pervasive replacement of the original limestone rocks - they are followed by non-ferroan dolomite phases. The associated fluids are supposed to have interacted with basement rocks, and travelled from deep-seated sources along major fault pathways. The geochemical traits of such fluids are also typically similar to, and probably associated with, mineralization fluids (e.g. Pb-Zn, MVT). In the Middle East, several observed dolostones show, on the contrary, a later phase of ferroan dolomite cements which occlude the inter-crystalline porosity of earlier non-ferroan matrix dolomites. Dolomitization occurred under increasingly higher temperatures (from 50 to 100°C) during burial. Here, the origin of iron-rich fluids and conditions of precipitation of associated dolomites do not necessarily involve interactions with basement rocks, but rather a relative Fe-enrichment with further reducing settings. Based on previous research projects concerning a variety of dolostones from Europe and the Middle-East, this contribution presents observational, analytical and computational results focused on ferroan dolomites. Recent numerical geochemical modelling emphasized the physico-chemical pre-requisites for crystallizing ferroan rather than non-ferroan dolomites (and vice-versa), allowing better understanding of related diagenetic processes. Besides, important larger-scale information on the crustal fluid circulations are demonstrated to be intimately associated to the parent-fluids sources and the conditions of mineral precipitation. By adopting this approach, ferroan dolomites are no longer considered simply as accessory diagenetic phases. They rather provide significant clues for understanding crustal dynamics and the impacts of evolving rock-fluid interactions on carbonate reservoir properties which are essential for ore and hydrocarbon exploitation, and for underground storage and gas sequestration.

Geologic Map of the Eastern Three-Quarters of the Cuyama 30' x 60' Quadrangle, California

USGS Publications Warehouse

Kellogg, Karl S.; Minor, Scott A.; Cossette, Pamela M.

2008-01-01

The map area encompasses a large part of the western Transverse Ranges and southern Coast Ranges of southern California. The San Andreas fault (SAF) cuts the northern part of the map. The area south of the SAF, about 80 percent of the map area, encompasses several distinct tectonic blocks bounded by major thrust or reverse faults, including the Santa Ynez fault, Big Pine fault (and structurally continuous Pine Mountain fault), Tule Creek fault, Nacimiento fault, Ozena fault, Munson Creek fault, Morales fault, and Frazier Mountain Thrust System. Movement on these faults is as old as Miocene and some faults may still be active. In addition, the Paleocene Sawmill Mountain Thrust south of the SAF and the Pastoria Thrust north of the SAF place Cretaceous and older crystalline rocks above Pelona Schist (south of the SAF) and Rand Schist (north of the SAF). South of the SAF, each tectonic block contains a unique stratigraphy, reflecting either large-scale movement on bounding faults or different depositional environments within each block. On Mount Pinos and Frazier Mountain, intrusive and metamorphic rocks as old as Mesoproterozoic, but including voluminous Cretaceous granitoid rocks, underlie or are thrust above non-marine sedimentary rocks as old as Miocene. Elsewhere, marine and non-marine sedimentary rocks are as old as Cretaceous, dominated by thick sequences of both Eocene and Cretaceous marine shales and sandstones. Middle Miocene to early Oligocene volcanic rocks crop out in the Caliente Hills (part of Caliente Formation) and south of Mount Pinos (part of the Plush Ranch Formation). Fault-bounded windows of Jurassic Franciscan Complex ophiolitic rocks are evident in the southwest corner of the area. North of the SAF, marine and non-marine sedimentary rocks as old as Eocene and Miocene volcanic rocks overlie a crystalline basement complex. Basement rocks include Cretaceous intrusive rocks that range from monzogranite to diorite, and Jurassic to late Paleozoic intrusive and metamorphic rocks. The Jurassic to late Paleozoic intrusive rocks include diorite, gabbro, and ultramafic rocks, and the metasedimentary rocks include marble, quartzite, schist, and gneiss.

A continuous record of tectonic evolution from 3.5 Ga to 2.6 Ga in Swaziland and northern Natal

NASA Technical Reports Server (NTRS)

Hunter, D. R.; Wilson, A. H.; Versfeld, J. A.; Allen, A. R.; Smith, R. G.; Sleigh, D. W. W.; Groenewald, P. B.; Chutter, G. M.; Preston, V. A.

1986-01-01

The approx. 3.5 Ga-old bimodal suite underlying an extensive area in southwestern Swaziland comprises the oldest-dated sialic rocks in the Kaapvaal structural province. The suite consists of leucocratic, layered tonalitic-trondhjemitic gneisses and amphibolites characterized by the effects of repeated high strains. This suite is considered to represent a sialic basement on which metavolcanic and metasedimentary rocks, now preserved as scattered greenstone remnants, accumulated. Direct evidence to confirm this temporal relationship is lacking, but structural data from the Dwalile, Assegaai and Commondale areas indicate that (1) the bimodal gneisses experienced a complex structural history prior to the first recognizable deformation in the supracrustal rocks (i.e., D1 in the supracrustals is equivalent to Dn + 1 in the gneisses) and (2) scattered remnants of the Dwalile rocks infolded with the bimodal suite structurally overlie the gneisses and are preserved in synformal keels. Significant proportions of metaquartzites and metapelites are present in the Assegaai greenstone sequence, the presence of which implies the existence of felsic crust in the source area from which these sediments were derived, a conclusion that is consistent with the structural data.

Geologic map of the Kechumstuk fault zone in the Mount Veta area, Fortymile mining district, east-central Alaska

USGS Publications Warehouse

Day, Warren C.; O’Neill, J. Michael; Dusel-Bacon, Cynthia; Aleinikoff, John N.; Siron, Christopher R.

2014-01-01

This map was developed by the U.S. Geological Survey Mineral Resources Program to depict the fundamental geologic features for the western part of the Fortymile mining district of east-central Alaska, and to delineate the location of known bedrock mineral prospects and their relationship to rock types and structural features. This geospatial map database presents a 1:63,360-scale geologic map for the Kechumstuk fault zone and surrounding area, which lies 55 km northwest of Chicken, Alaska. The Kechumstuk fault zone is a northeast-trending zone of faults that transects the crystalline basement rocks of the Yukon-Tanana Upland of the western part of the Fortymile mining district. The crystalline basement rocks include Paleozoic metasedimentary and metaigneous rocks as well as granitoid intrusions of Triassic, Jurassic, and Cretaceous age. The geologic units represented by polygons in this dataset are based on new geologic mapping and geochronological data coupled with an interpretation of regional and new geophysical data collected by the Alaska Department of Natural Resources, Division of Geological and Geophysical Surveys. The geochronological data are reported in the accompanying geologic map text and represent new U-Pb dates on zircons collected from the igneous and metaigneous units within the map area.

Subduction of Proterozoic to Late Triassic continental basement in the Guatemala suture zone: A petrological and geochronological study of high-pressure metagranitoids from the Chuacús complex

NASA Astrophysics Data System (ADS)

Maldonado, Roberto; Ortega-Gutiérrez, Fernando; Ortíz-Joya, Guillermo A.

2018-05-01

Many continental subduction complexes contain abundant granitic rocks coexisting with minor volumes of eclogite-facies rocks. Characterization of granitic protoliths is crucial to decipher the origin of subducted continental crust, whereas knowledge of its metamorphic evolution is required to constrain the mechanisms of burial and exhumation. In this work we present geochronological and petrological evidence that demonstrate the occurrence of a subducted Proterozoic to Late Triassic granitic basement in the Chuacús complex of central Guatemala. Metagranitoids exposed in this area are interlayered with eclogite and other high-pressure rocks, and their structure is considerably variable due to strain partitioning during deformation. Laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon data from two ferroan metagranites yield protolith crystallization ages of ca. 1.1 Ga and their trace-element abundances suggest an origin related to intraplate magmatism, while a high-silica, peraluminous metagranite is dated at 1.0 Ga and was probably originated by partial melting of a high-grade continental crust. On the other hand, two megacrystic to augen metagranitoids yield protolith crystallization ages of ca. 224 Ma, which are identical within errors to the protolith age of hosted eclogitic metabasites. Their high incompatible trace element abundances together with the observed spatial-temporal relationships with mafic protoliths suggest that Late Triassic bimodal magmatism in the Chuacús complex was probably originated in a within-plate setting. Regardless of their age or structure, the studied metagranites preserve evidences for high-pressure metamorphic equilibration, such as the occurrence of Ca-rich garnet (XCa up to 0.52) in association with phengite (Si contents of up to 3.4 pfu) and rutile. The integration of Zr-in-rutile thermometry and phengite barometry allows the peak metamorphic conditions to be constrained at 640-680 °C and 13 kbar. This pressure-temperature estimate indicates that metagranitoids underwent high-pressure metamorphism but equilibrated at significantly lower pressures than associated eclogite-facies rocks, and, therefore, they do not necessarily share a common high-pressure metamorphic evolution. The new data show that the Chuacús complex in the study area represents a Proterozoic (1.1-1.0 Ga) to Late Triassic (220 Ma) continental basement that was subducted, and consequently metamorphosed under high-pressure conditions, during the Cretaceous evolution of the North America-Caribbean plate boundary.

Geology and Geochemistry of Magmatic Rocks from the Southern Part of the Kyushu-Palau Ridge in the Philippine Sea

NASA Astrophysics Data System (ADS)

Lelikov, E. P.; Sedin, V. T.; Pugachev, A. A.

2018-03-01

The paper reports the results of a geochemical study of volcanogenic rocks from the southern part of the Kyushu-Palau Ridge. Volcanic structures, such as plateaulike rises, mountain massifs, and single volcanoes, are the major relief-forming elements of the southern part of the Kyushu-Palau Ridge. They are divided into three types according to the features of the relief and geological structure: shield, cone-shaped, and dome-shaped volcanoes. The ridge was formed on oceanic crust in the Late Mesozoic and underwent several stages of evolution with different significance and application of forces (tension and compression). Change in the geodynamic conditions during the geological evolution of the ridge mostly determined the composition of volcanic rocks of deep-mantle nature. Most of the ridge was formed by the Early Paleogene under geodynamic conditions close to the formation of oceanic islands (shield volcanoes) under tension. The island arc formed on the oceanic basement in the compression mode in the Late Eocene-Early Oligocene. Dome-shaped volcanic edifices composed of alkaline volcanic rocks were formed in the Late Oligocene-Early Miocene under tension. Based on the new geochemical data, detailed characteristics of volcanic rocks making up the shield, cone-shape, and dome-shape stratovolcanoes resulting in the features of these volcanic edifices are given for the first time. Continuous volcanism (with an age from the Cretaceous to the Late Miocene and composition from oceanic tholeiite to calc-alkaline volcanites of the island arc type) resulting in growth of the Earth's crust beneath the Kyushu-Palau Ridge was the major factor in the formation this ridge.

Seismic amplification within the Seattle Basin, Washington State: Insights from SHIPS seismic tomography experiments

USGS Publications Warehouse

Snelson, C.M.; Brocher, T.M.; Miller, K.C.; Pratt, T.L.; Trehu, A.M.

2007-01-01

Recent observations indicate that the Seattle sedimentary basin, underlying Seattle and other urban centers in the Puget Lowland, Washington, amplifies long-period (1-5 sec) weak ground motions by factors of 10 or more. We computed east-trending P- and S-wave velocity models across the Seattle basin from Seismic Hazard Investigations of Puget Sound (SHIPS) experiments to better characterize the seismic hazard the basin poses. The 3D tomographic models, which resolve features to a depth of 10 km, for the first time define the P- and S-wave velocity structure of the eastern end of the basin. The basin, which contains sedimentary rocks of Eocene to Holocene, is broadly symmetric in east-west section and reaches a maximum thickness of 6 km along our profile beneath north Seattle. A comparison of our velocity model with coincident amplification curves for weak ground motions produced by the 1999 Chi-Chi earthquake suggests that the distribution of Quaternary deposits and reduced velocity gradients in the upper part of the basement east of Seattle have significance in forecasting variations in seismic-wave amplification across the basin. Specifically, eastward increases in the amplification of 0.2- to 5-Hz energy correlate with locally thicker unconsolidated deposits and a change from Crescent Formation basement to pre-Tertiary Cascadia basement. These models define the extent of the Seattle basin, the Seattle fault, and the geometry of the basement contact, giving insight into the tectonic evolution of the Seattle basin and its influence on ground shaking.

U-Pb SHRIMP-RG zircon ages and Nd signature of lower Paleozoic rifting-related magmatism in the Variscan basement of the Eastern Pyrenees

USGS Publications Warehouse

Martinez, F.J.; Iriondo, A.; Dietsch, C.; Aleinikoff, J.N.; Peucat, J.J.; Cires, J.; Reche, J.; Capdevila, R.

2011-01-01

The ages of orthogneisses exposed in massifs of the Variscan chain can determine whether they are part of a pre-Neoproterozoic basement, a Neoproterozoic, Panafrican arc, or are, in fact, lower Paleozoic, and their isotopic compositions can be used to probe the nature of their source rocks, adding to the understanding of the types, distribution, and tectonic evolution of peri-Gondwanan crystalline basement. Using SHRIMP U-Pb zircon geochronology and Nd isotopic analysis, pre-Variscan metaigneous rocks from the N??ria massif in the Eastern Pyrenean axial zone and the Guilleries massif, 70km to the south, have been dated and their Nd signatures characterized. All dated orthogneisses from the N??ria massif have the same age within error, ~457Ma, including the Ribes granophyre, interpreted as a subvolcanic unit within Caradocian sediments contemporaneous with granitic magmas intruded into Cambro-Ordovician sediments at deeper levels. Orthogneisses in the Guilleries massif record essentially continuous magmatic activity during the Ordovician, beginning at the Cambro-Ordovician boundary (488??3Ma) and reaching a peak in the volume of magma in the early Late Ordovician (~460Ma). Metavolcanic rocks in the Guilleries massif were extruded at 452??4Ma and appear to have their intrusive equivalent in thin, deformed veins of granitic gneiss (451??7Ma) within metasedimentary rocks. In orthogneisses from both massifs, the cores of some zircons yield Neoproterozoic ages between ~520 and 900Ma. The age of deposition of a pre-Late Ordovician metapelite in the Guilleries massif is bracketed by the weighted average age of the youngest detrital zircon population, 582??11Ma, and the age of cross-cutting granitic veins, 451??7Ma. Older detrital zircons populations in this metapelite include Neoproterozoic (749-610Ma; n=10), Neo- to Mesoproterozoic (1.04-0.86Ga; n=7), Paleoproterozoic (2.02-1.59Ga; n=5), and Neoarchean (2.74-2.58Ga; n=3). Nd isotopic analyses of the N??ria and Guilleries orthogneisses yielded negative ??Nd values ranging between -2.1 and -5.2 at 450Ma, the same as Ediacaran sediments from northwestern Iberia. We interpret these slightly negative ??Nd values as a signature of Panafrican arc rocks, mixed with an older crustal component as indicated by the inherited and detrital zircon ages we analyzed. The crustal level in which Ordovician melting occurred has not been recognized and the absence of pre-Neoproterozoic basement is a striking feature of the southern part of the Variscan Chain. ?? 2011 Elsevier B.V.

Westernmost Tian Shan (Uzbekistan): Magmatism and Exhumation

NASA Astrophysics Data System (ADS)

Abdulhameed, Sanaa; Ratchbacher, Lothar; Gagala, Lukasz; Jonkheere, Raymond

2014-05-01

The westernmost segment of the Tian Shan comprises the Ghissar-Alai Range of Tajikistan and Uzbekistan; its southwestern promontory contains the Baysunta crystalline massif. This orocline extends northwest of the Pamir and southeast of the Turan platform and forms the (north) western margin of the intra-orogenic Afghan-Tajik basin that was inverted during the India-Asia collision, mainly as a result of the gravitational collapse of the Pamir Plateau. The area contains Paleozoic slope and shelf clastics mantling crystalline basement rocks, altogether intruded by massif granitoids; it hides the cryptic Late Paleozoic South Ghissar suture. In Uzbekistan, the crystalline basement rocks of the westernmost Tien Shan are involved in the folding and thrusting of the Jurassic to Neogene sediments of the Afghan-Tajik basin, spectacularly proving thick-skinned deformation and demonstrating basement involvement below the Jurassic evaporate décollement underneath the Afghan-Tajik basin. We sampled the crystalline basement rocks of the Tian Shan of Uzbekistan to constrain the formation of the enigmatic Baysunta block and date the crystallization and high-grade metamorphism of the granitoids and associated metamorphic rocks of the Ghissar range; we employed U-Pb zircon geochronology. To time the deformation and exhumation of the fold-and thrust belt of the westernmost Afghan-Tajik basin, we used apatite fission-track thermochronology. Concordant U-Pb crystallization ages of zircons in the orthogneiss and paragneiss comprise 620 to 300 Ma; the Neoproterozoic ages imply a correlation of the Baysunta block with the Garm crystalline massif of the central Ghissar-Alai range of northeastern Tajikistan. The youngest zircon crystallization ages from granitoids are ~220 Ma, revealing enigmatically young magmatism, post-dating the last known collision event by >50 Ma. Together with 270-240 Ma meta-basaltic dykes and stocks in Tajikistan, they may trace a regional post-orogenic delamination event. The apatite fission-track thermochronology suggests a two-phase exhumation history. Exhumation/cooling within the age range of ~17 and 4 Ma and clustering around 10 Ma date slip along the thick-skinned thrusts. This demonstrates the impact of the India-Asia collision on the edge of the Turan platform, far northwest of the western edge of the collision zone. Ages from the northwestern edge of the study area cover 197 to 69 Ma; they trace a fossil partial annealing zone. They show that the Tian Shan along the margin of the Turan platform was never covered by sedimentary rocks >3 km thick.

Reconnaissance electrical surveys in the Coso Range, California

NASA Astrophysics Data System (ADS)

Jackson, Dallas B.; O'Donnell, James E.

1980-05-01

Telluric current, audiomagnetotelluric (AMT), and direct current (dc) methods were used to study the electrical structure of the Coso Range and Coso geothermal area. Telluric current mapping outlined major resistivity lows associated with conductive valley fill of the Rose Valley basin, the Coso Basin, and the northern extension of the Coso Basin east of Coso Hot Springs. A secondary resistivity low with a north-south trend runs through the Coso Hot Springs-Devil's Kitchen geothermal area. The secondary low in the geothermal area, best defined by the 7.5-Hz AMT map and dc soundings, is caused by a shallow conductive zone (5-30 ohm m) interpreted to be hydrothermally altered Sierra Nevada basement rocks containing saline water of a hot water geothermal system. This zone of lowest apparent resistivities over the basement rocks lies within a closed contour of a heat flow anomaly where all values are greater than 10 heat flow units.

Relations between extensional tectonics and magmatism within the Southern Oklahoma aulacogen

NASA Technical Reports Server (NTRS)

Mcconnell, D. A.; Gilbert, M. C.

1985-01-01

Variations in the geometry, distribution and thickness of Cambrian igneous and sedimentary units within southwest Oklahoma are related to a late Proterozoic - early Paleozoic rifting event which formed the Southern Oklahoma aulacogen. These rock units are exposed in the Wichita Mountains, southwest Olkahoma, located on the northern margin of a Proterozoic basin, identified in the subsurface by COCORP reflection data. Overprinting of the Cambrian extensional event by Pennyslvanian tectonism obsured the influence of pre-existing basement structures and contrasting basement lithologies upon the initial development of the aulacogen.

Mid to late Devonian back-arc rift basins in the Brooks Range, AK, and across the Arctic: a possible paleogeographic piercing point for Arctic reconstructions

NASA Astrophysics Data System (ADS)

Hoiland, C. W.; Miller, E. L.; Hourigan, J. K.

2013-12-01

The westernmost Brooks Range, Alaska, is underlain by basement of probable Baltic or Timanian affinity (e.g. Miller et al., 2011; Amato et al., 2009), while the eastern Brooks Range is underlain by Laurentian affinity basement (e.g. Strauss et al., 2013). A post-Timanian and pre-Mississippian suture or contact is thus required based on continuity of late Devonian and younger strata across the Brooks Range (e.g. Dumoulin et al., 2002). This inferred juxtaposition has been proposed as the distal and diachronous (though possibly non-collisional) continuation of the Caledonian orogen (e.g. Moore et al., 2012) but the actual location and character of this suture within basement rocks of the Brooks Range remain speculative. New laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb single grain detrital zircon (DZ) geochronology of basement rocks from the Cosmos Hills, Slate Creek, and Wiseman regions suggest that metamorphic rocks in these regions are Devonian, not pre-Devonian. New SHRIMP-RG analyses of the Kogoluktuk orthogneiss (Cosmos) (zircon: 383 Ma × 5 Ma, 2-sigma errors, consistent with Dillon et al. 1980) revealed no inherited cores from which to infer basement affinity. DZ spectra from metasedimentary and metavolcanic wall rock contain youngest detrital zircon populations with ages (390 Ma) just barely older than the cross-cutting intrusive age, providing tight bracketing of depositional age. These zircon ages are noticeably younger than Caledonian magmatic ages (430-420 Ma) suggesting deposition in a volcanically and tectonically active setting (likely extensional) as originally suggested by Hitzman et al (1986). Zircon spectra (Cosmos) contain notable amounts of "Timanian" age zircons (c. 700-550 Ma), and a spread of zircons from 1-2 Ga (including 1.5-1.6 Ga ages of the Laurentian "magmatic gap', e.g. Grove et al. 2008) more typical of derivation from Baltic rather than Laurentian sources. East in the Wiseman and Slate Creek localities, the detrital signature becomes characteristically Laurentian, with a notable absence of Timanian and "magmatic gap" ages. A youngest age population of 390 Ma still provides a maximum depositional age, but minimum age is poorly constrained. The coarse and feldspathic nature of many of these intercalated volcanic and clastic sequences suggests a proximal provenance, thus serving as a proxy for local pre-Devonian basement ages and affinity. We might, therefore, infer a non-Laurentian basement for the AACM at least as far east as the Cosmos Hills but not further east than the Wiseman region. These Devonian-age volcanic/rift basins may be related to slab roll-back and induced backarc rifting that occurred obliquely across a 'Caledonian' suture, possibly in response to global plate re-organization. Rifting, accompanied by bimodal volcanism (the Ambler Sequence), may have aided the removal and translation of peri-Baltic terranes to a position outboard of the proto-Cordilleran margin ('Northwestern Passage' of Colpron & Nelson, 2009). Further correlations might be drawn with the Sakmarian-Magnitogorsk arcs of the pre-Uralian margin of Europe. These Devonian backarc rift sequences - more widespread than previously thought - may serve as critical additional tie-points for paleogeographic reconstructions of the Arctic.

Geochemical, microtextural and petrological studies of the Samba prospect in the Zambian Copperbelt basement: a metamorphosed Palaeoproterozoic porphyry Cu deposit.

NASA Astrophysics Data System (ADS)

Master, Sharad; Mirrander Ndhlovu, N.

2015-04-01

Ever since Wakefield (1978, IMM Trans., B87, 43-52) described a porphyry-type meta-morphosed Cu prospect, the ca 50 Mt, 0.5% Cu Samba deposit (12.717°S, 27.833°E), hosted by porphyry-associated quartz-sericite-biotite schists in northern Zambia, there has been controversy about its origin and significance. This is because it is situated in the basement to the world's largest stratabound sediment-hosted copper province, the Central African Copperbelt, which is hosted by rocks of the Neoproterozoic Katanga Supergroup. Mineralization in the pre-Katangan basement has long played a prominent role in ore genetic models, with some authors suggesting that basement Cu mineralization may have been recycled into the Katangan basin through erosion and redeposition, while others have suggested that the circulation of fluids through Cu-rich basement may have leached out the metals which are found concentrated in the Katangan orebodies. On the basis of ca 490-460 Ma Ar-Ar ages, Hitzman et al. (2012, Sillitoe Vol., SEG Spec. Publ., 16, 487-514) suggested that Samba represents late-stage impregnation of copper mineralization into the basement, and that it was one of the youngest copper deposits known in the Central African Copperbelt. If the Samba deposit really is that young, then it would have post-dated regional deformation and metamorphism (560-510 Ma), and it ought to be undeformed and unmetamorphosed. The Samba mineralization consists of chalcopyrite and bornite, occurring as disseminations, stringers and veinlets, found in a zone >1 km along strike, in steeply-dipping lenses up to 10m thick and >150m deep. Our new major and trace element XRF geochemical data (14 samples) show that the host rocks are mainly calc-alkaline metadacites. Cu is correlated with Ag (Cu/Ag ~10,000:1) with no Au or Mo. Our study focused on the microtextures and petrology of the Samba ores. We confirm that there is alteration of similar style to that accompanying classical porphyry Cu mineralization, including potassic (biotite+sericite+ quartz), propylitic (clinozoisite+chlorite+saussuritized plagioclase), phyllic (sericite+quartz+ pyrite+hydromuscovite/illite) and argillic (kaolinite+chlorite+dolomite) alteration. The clays were identified with XRD. All the rocks show penetrative deformational textures and fabrics. Our textural studies show that phyllic zone pyrite crystals have quartz-rich pressure shadows, and they predate all phases of deformation. Similarly, in the potassic zone, fracture-controlled biotite stringers in particular orientations are deformed, and partly replaced by chlorite, again showing their pre-deformational, pre-metamorphic origin. Copper sulfide-bearing quartz veinlets are deformed. Many of the alteration assemblages containing biotite or sericite have been deformed into crenulated schists, showing that they were formed early in the deformation history. Coupled with the dating of a Samba metavolcanic rock at 1964±12 Ma (Rainaud et al., 2005, JAES, 42, 1-31), we regard the Samba deposit as a metamorphosed Palaeoproterozoic porphyry-type Cu deposit, which has undergone deformation, and retrograde metamorphism of its alteration assemblages, during the Neoproterozoic Lufilian Orogeny, followed by post-tectonic cooling, which occurred throughout the Copperbelt at about 480±20 Ma. Samba, together with the Mkushi deposits, is part of a long-lived (>100 Ma) Palaeoproterozoic porphyry-Cu province in the Zambian Copperbelt basement, and ore genetic theories for the Copperbelt mineralization must now seriously take this into account.

Numerical Models of Pore Pressure and Stress Changes along Basement Faults due to Wastewater Injection: Applications to Potentially Induced Seismicity in Southern Kansas

NASA Astrophysics Data System (ADS)

Koltermann, C.; Hearn, E. H.

2015-12-01

As hydrocarbon extraction techniques that generate large volumes of wastewater have come into widespread use in the central United States, increased volumes have been injected into deep disposal wells, with a corresponding dramatic increase in seismicity rates. South-central Kansas is of particular scientific interest because fluid injection rates have recently increased due to renewed gas and oil production from the Mississippi Lime Play, and the local seismicity is being monitored with a seismometer network deployed by the USGS. In addition, since only a small percentage of injection wells seem to induce seismicity, it is important to characterize contributing factors. We have developed groundwater flow models using MODFLOW-USG to (1) assess hydrogeologic conditions under which seismicity may be triggered, for cases in which wastewater is injected into sedimentary strata overlying fractured crystalline basement rock and to (2) explore the possible relationship between wastewater injection and the November 2014 M 4.8 Milan, Kansas earthquake. The USG version of MODFLOW allows us to use unstructured meshes, which vastly reduces computation time while allowing dense meshing near injection wells and faults. Our single-well test model has been benchmarked to published models (Zhang et al., 2013) and will be used to evaluate sensitivity pore pressures and stresses to model parameters. Our south Kansas model represents high-rate injection wells, as well as oil and gas wells producing from the Arbuckle and overlying Mississippian formations in a 40-km square region. Based on modeled pore pressure and stress changes along the target fault, we will identify conditions that would be consistent with inducing an earthquake at the Milan hypocenter. Parameters to be varied include hydraulic properties of sedimentary rock units, crystalline basement and the fault zone, as well as the (poorly resolved) Milan earthquake hypocenter depth.

Precambrian domains in Lithuania: evidence of terrane tectonics

NASA Astrophysics Data System (ADS)

Skridlaite, Grazina; Motuza, Gediminas

2001-09-01

The West Lithuanian Granulite (WLG) and East Lithuanian domains (ELD) form the Proterozoic basement of Lithuania and can be distinguished on the basis of differing structural patterns, lithologies, and evolutionary histories. They are juxtaposed along the Mid-Lithuanian Suture Zone (MLSZ). In the WLG, the main lithotectonic complexes comprise felsic and intermediate, mostly metasedimentary granulites in the south-west and mafic metaigneous granulites in the north-east. The former are interpreted as marine metapelites, while most of the mafic ones have been derived from island-arc tholeiites. These rock complexes trend NW-SE and are marked by contrasting gravity and magnetic anomalies. NE- and E-W-striking faults and shear zones complicate the potential-field patterns. Sets of NW-trending anomalies also extend from Lithuania across the Baltic Sea to south-central Sweden and indicate that the WLG complexes continue into the Baltic/Fennoscandian Shield. Voluminous anatectic granites alternate with the metapelites, whereas the mafic granulites occur together with enderbites and charnockites. In the ELD, the main structures produce strong, NNE-SSW-oriented gravity and magnetic anomalies which trend parallel to the Belarus-Baltic Granulite Belt (BBG) and other terranes situated still farther east. The ELD is composed of metasedimentary rocks interpreted as one-time graywackes, shales and dolomites accumulated in continental-margin arc and shallow-water basinal environments. Amphibolites and gabbros with MORB and IAT characteristics, and voluminous granitoids are also present. The coexistence of juvenile mafic rocks with continental-margin and shelf sediments suggests an oceanic back-arc setting. The two Lithuanian basement domains display contrasting metamorphic histories that suggest separate developments before the eventual amalgamation. In the WLG, the metapelites indicate peak metamorphism at high temperatures (up to 850-900°C) and moderate pressures (8-10 kbar). This was followed by cooling and reheating, and then an uplift event. Repeated magmatic underplating accompanied the metamorphism. In the ELD, in contrast, the rocks have been subjected to comprehensive metamorphism under moderate, amphibolite-facies conditions. That metamorphism, however, was not uniform throughout. The metasediments in the east have recorded pressures similar to those in the neighbouring BBG (7-8 kbar) but lower temperatures (650-680°C), while in the central and western parts of the ELD, metamorphism occurred at ca. 480-580°C with pressures increasing from 3-4 kbar in the centre, to 6 kbar close to the western boundary. Reheating to 700°C due to a ca. 1.5-Ga magmatic event is characteristic. The MLSZ, which separates the two Lithuanian basement domains from each other, is a N-S-oriented, ca. 30-50 km wide, westward-plunging crustal discontinuity marked by magnetic and gravity highs, mafic and felsic intrusions, and sheared rocks. Crustal thicknesses change from 42-44 km in the west to 50 km in the eastern side of the Zone, which also truncates a crustal low-velocity layer characteristic of the WLG. The amalgamation of the WLG and ELD along the MLSZ occurred at ca. 1.71-1.66 Ga, after which time both domains were affected by the same post-kinematic, anorogenic magmatism ca. 1.58-1.45 Ga ago. That event and related shearing were responsible for some ultimate refragmentation of the Lithuanian basement terranes.

Geology of Joshua Tree National Park geodatabase

USGS Publications Warehouse

Powell, Robert E.; Matti, Jonathan C.; Cossette, Pamela M.

2015-09-16

The database in this Open-File Report describes the geology of Joshua Tree National Park and was completed in support of the National Cooperative Geologic Mapping Program of the U.S. Geological Survey (USGS) and in cooperation with the National Park Service (NPS). The geologic observations and interpretations represented in the database are relevant to both the ongoing scientific interests of the USGS in southern California and the management requirements of NPS, specifically of Joshua Tree National Park (JOTR).Joshua Tree National Park is situated within the eastern part of California’s Transverse Ranges province and straddles the transition between the Mojave and Sonoran deserts. The geologically diverse terrain that underlies JOTR reveals a rich and varied geologic evolution, one that spans nearly two billion years of Earth history. The Park’s landscape is the current expression of this evolution, its varied landforms reflecting the differing origins of underlying rock types and their differing responses to subsequent geologic events. Crystalline basement in the Park consists of Proterozoic plutonic and metamorphic rocks intruded by a composite Mesozoic batholith of Triassic through Late Cretaceous plutons arrayed in northwest-trending lithodemic belts. The basement was exhumed during the Cenozoic and underwent differential deep weathering beneath a low-relief erosion surface, with the deepest weathering profiles forming on quartz-rich, biotite-bearing granitoid rocks. Disruption of the basement terrain by faults of the San Andreas system began ca. 20 Ma and the JOTR sinistral domain, preceded by basalt eruptions, began perhaps as early as ca. 7 Ma, but no later than 5 Ma. Uplift of the mountain blocks during this interval led to erosional stripping of the thick zones of weathered quartz-rich granitoid rocks to form etchplains dotted by bouldery tors—the iconic landscape of the Park. The stripped debris filled basins along the fault zones.Mountain ranges and basins in the Park exhibit an east-west physiographic grain controlled by left-lateral fault zones that form a sinistral domain within the broad zone of dextral shear along the transform boundary between the North American and Pacific plates. Geologic and geophysical evidence reveal that movement on the sinistral faults zones has resulted in left steps along the zones, resulting in the development of sub-basins beneath Pinto Basin and Shavers and Chuckwalla Valleys. The sinistral fault zones connect the Mojave Desert dextral faults of the Eastern California Shear Zone to the north and east with the Coachella Valley strands of the southern San Andreas Fault Zone to the west.Quaternary surficial deposits accumulated in alluvial washes and playas and lakes along the valley floors; in alluvial fans, washes, and sheet wash aprons along piedmonts flanking the mountain ranges; and in eolian dunes and sand sheets that span the transition from valley floor to piedmont slope. Sequences of Quaternary pediments are planed into piedmonts flanking valley-floor and upland basins, each pediment in turn overlain by successively younger residual and alluvial surficial deposits.

Cenozoic volcanic geology and probable age of inception of basin-range faulting in the southeasternmost Chocolate Mountains, California

DOE Office of Scientific and Technical Information (OSTI.GOV)

Crowe, B.M.

1978-02-01

A complex sequence of Oligocene-age volcanic and volcaniclastic rocks form a major volcanic center in the Picacho area of the southeasternmost Chocolate Mountains, Imperial County, California. Basal-volcanic rocks consist of lava flows and flow breccia of trachybasalt, pyroxene rhyodacite, and pyroxene dacite (32 My old). These volcanic rocks locally overlie fanglomerate and rest unconformably on pre-Cenozoic basement rocks. South and southeast of a prominent arcuate fault zone in the central part of the area, the rhyolite ignimbrite (26 My old) forms a major ash-flow sheet. In the southwestern part of the Picacho area the rhyolite ignimbrite interfingers with and ismore » overlain by dacite flows and laharic breccia. The rhyolite ignimbrite and the dacite of Picacho Peak are overlapped by lava flows and breccia of pyroxene andesite (25 My old) that locally rest on pre-Cenozoic basement rocks. The volcanic rocks of the Picacho area form a slightly bimodal volcanic suite consisting chiefly of silicic volcanic rocks with subordinate andesite. Late Miocene augite-olivine basalt is most similar in major-element abundances to transitional alkali-olivine basalt of the Basin and Range province. Normal separation faults in the Picacho area trend northwest and north parallel to major linear mountain ranges in the region. The areal distribution of the 26-My-old rhyolite ignimbrite and the local presence of megabreccia and fanglomerate flanking probable paleohighs suggest that the ignimbrite was erupted over irregular topography controlled by northwest- and north-trending probable basin-range faults. These relations date the inception of faulting in southeasternmost California at pre-26 and probably pre-32 My ago. A transition of basaltic volcanism in the area is dated at 13 My ago. 9 figures, 2 tables.« less

Gravity, magnetic, and physical property data in the Smoke Creek Desert area, northwest Nevada

USGS Publications Warehouse

Tilden, Janet E.; Ponce, David A.; Glen, Jonathan M.G.; Chuchel, Bruce A.; Tushman, Kira; Duvall, Alison

2006-01-01

The Smoke Creek Desert, located approximately 100 km (60 mi) north of Reno near the California-Nevada border, is a large basin situated along the northernmost parts of the Walker Lane Belt (Stewart, 1988), a physiographic province defined by northwest-striking topographic features and strike-slip faulting. Because geologic framework studies play an important role in understanding the hydrology of the Smoke Creek Desert, a geologic and geophysical effort was begun to help determine basin geometry, infer structural features, and estimate depth to Pre-Cenozoic rocks, or basement. In May and June of 2004, and June of 2005, the U.S. Geological Survey (USGS) collected 587 new gravity stations, more than 160 line-kilometers (100 line-miles) of truck-towed magnetometer data, and 111 rock property samples in the Smoke Creek Desert and vicinity in northwest Nevada, as part of an effort to characterize its hydrogeologic framework. In the Smoke Creek Desert area, gravity highs occur over rocks of the Skedaddle Mountains, Fox Range, Granite Range, and over portions of Tertiary volcanic rocks in the Buffalo Hills. These gravity highs likely reflect basement rocks, either exposed at the surface or buried at shallow depths. The southern Smoke Creek Desert corresponds to a 25-mGal isostatic gravity low, which corresponds with a basin depth of approximately 2 km. Magnetic highs are likely due to granitic, andesitic, and metavolcanic rocks, whereas magnetic lows are probably associated with less magnetic gneiss and metasedimentary rocks in the region. Three distinctive patterns of magnetic anomalies occur throughout the Smoke Creek Desert and Squaw Creek Valley, likely reflecting three different geological and structural settings.

Initiation and evolution of the Arabia-Eurasia collision in the Caucasus region constrained by detrital zircon geochronology

NASA Astrophysics Data System (ADS)

Tye, A. R.; Niemi, N. A.

2016-12-01

The Greater Caucasus (GC) mountain range is composed of thrust sheets of Paleozoic (Pz) - Mesozoic (Mz) flysch. Crystalline basement is exposed in the western part of the range, but not in the eastern. Detrital zircon ages from Eocene - recent foreland strata to the south of the western GC in Georgia suggest sediment sourcing from GC basement or Pz strata since Eocene time, requiring significant exhumation prior to or coincident with the onset of Arabia-Eurasia collision 30 Ma. We sampled foreland basin sedimentary rocks and modern river sands whose catchment areas together span the potential source rocks exposed in the western Greater Caucasus (GC) in Georgia. We find that GC basement rocks and lower Pz strata contain a diagnostic 450 Ma zircon population that is absent from the upper Pz and Mz sedimentary strata that are exposed throughout most of the range. These 450 Ma zircons are from an unknown source with an age distinct from the Hercynian ( 300 Ma) and Pan-African ( 600 Ma) orogens. Despite their absence in late Pz and Mz strata, the 450 Ma zircons are prevalent in Eocene - recent foreland basin deposits, whose ages were determined biostratigraphically [1]. Paleocurrent directions also indicate a GC source for Eocene strata [2], necessitating early Cenozoic exposure of GC basement or Pz strata. Exposing GC basement or Pz strata during Eocene time requires erosional removal of >3500 m of Mesozoic and late Paleozoic strata [1]. The detrital zircon age observations suggest that erosional removal of these strata took place prior to the initiation of the Arabia-Eurasia collision at 30 Ma and well before the ongoing episode of rapid GC exhumation and erosion from 5 Ma - present. Foreland basin detrital zircon ages also reveal a lack of input from Late Cretaceous to Paleogene volcanism of the Adjara-Trialet zone. This finding is consistent with the existence of a Paleogene ocean basin between the Greater Caucasus and Lesser Caucasus wide enough to prevent transport of sand sized sediments from one side to the other. References[1] Gamkrelidze, P. D., & I. R. Kakhazdze (1959), K-38-VII, Min. Geol. Min. Prot. USSR. [2] Vincent, S. J., et al. (2007). Terra Nova, 19(2), 160-166. [3] Avdeev, B., & Niemi, N. A. (2011). Tectonics, 30(2), TC2009[4] Nalivkin, D. V. (1976). Geologic Map of the Caucasus, 1:500000.

Peninsular terrane basement ages recorded by Paleozoic and Paleoproterozoic zircon in gabbro xenoliths and andesite from Redoubt volcano, Alaska

USGS Publications Warehouse

Bacon, Charles R.; Vazquez, Jorge A.; Wooden, Joseph L.

2012-01-01

Historically Sactive Redoubt volcano is an Aleutian arc basalt-to-dacite cone constructed upon the Jurassic–Early Tertiary Alaska–Aleutian Range batholith. The batholith intrudes the Peninsular tectonostratigraphic terrane, which is considered to have developed on oceanic basement and to have accreted to North America, possibly in Late Jurassic time. Xenoliths in Redoubt magmas have been thought to be modern cumulate gabbros and fragments of the batholith. However, new sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages for zircon from gabbro xenoliths from a late Pleistocene pyroclastic deposit are dominated by much older, ca. 310 Ma Pennsylvanian and ca. 1865 Ma Paleoproterozoic grains. Zircon age distributions and trace-element concentrations indicate that the ca. 310 Ma zircons date gabbroic intrusive rocks, and the ca. 1865 Ma zircons also are likely from igneous rocks in or beneath Peninsular terrane basement. The trace-element data imply that four of five Cretaceous–Paleocene zircons, and Pennsylvanian low-U, low-Th zircons in one sample, grew from metamorphic or hydrothermal fluids. Textural evidence of xenocrysts and a dominant population of ca. 1865 Ma zircon in juvenile crystal-rich andesite from the same pyroclastic deposit show that this basement has been assimilated by Redoubt magma. Equilibration temperatures and oxygen fugacities indicated by Fe-Ti–oxide minerals in the gabbros and crystal-rich andesite suggest sources near the margins of the Redoubt magmatic system, most likely in the magma accumulation and storage region currently outlined by seismicity and magma petrology at ∼4–10 km below sea level. Additionally, a partially melted gabbro from the 1990 eruption contains zircon with U-Pb ages between ca. 620 Ma and ca. 1705 Ma, as well as one zircon with a U-Th disequilibrium model age of 0 ka. The zircon ages demonstrate that Pennsylvanian, and probably Paleoproterozoic, igneous rocks exist in, or possibly beneath, Peninsular terrane basement. Discovery of Pennsylvanian gabbro similar in age to Skolai arc plutons 500 km to the northeast indicates that the Peninsular terrane, along with the Wrangellia and Alexander terranes, has been part of the Wrangellia composite terrane since at least Pennsylvanian time. Moreover, the zircon data suggest that a Paleoproterozoic continental fragment may be present in the mid-to-upper crust in southern Alaska.

A reassessment of the Archean-Mesoproterozoic tectonic development of the southeastern Chhattisgarh Basin, Central India through detailed aeromagnetic analysis

NASA Astrophysics Data System (ADS)

Sridhar, M.; Ramesh Babu, V.; Markandeyulu, A.; Raju, B. V. S. N.; Chaturvedi, A. K.; Roy, M. K.

2017-08-01

We constrained the geological framework over polydeformed Paleoproterozoic Sonakhan Greenstone Belt and addressed the tectonic evolution of Singhora basin in the fringes of Bastar Craton, central India by utilizing aeromagnetic data interpretation, 2.5D forward modelling and 3D magnetic susceptibility inversions. The Sonakhan Greenstone Belt exposes volcano-sedimentary sequences of the Sonakhan Group within NNW-SSE to NW-SE trending linear belts surrounded by granite gneisses, which are unconformably overlain by sedimentary rocks of Chhattisgarh Basin. The orientations of aeromagnetic anomalies are coincident with geological trends and appear to correlate with lithology and geologic structure. Regional magnetic anomalies and lineaments reveal both NNW-SSE and NE-SW trends. Prominent E-W trending linear, high amplitude magnetic anomalies are interpreted as the Trans-Chhattisgarh Aeromagnetic Lineament (TCAL). NW-SE trending aeromagnetic signatures related to Sonakhan Greenstone Belt extends below the Singhora sedimentary rocks and forms the basement in the west. The analysis suggests that TCAL is a block fault with northern block down-thrown and affected the basement rocks comprising the Sonakhan Greenstone Belt and Samblapur Granitoids. The episode of faulting represented by the TCAL is pre-Singhora sedimentation and played a vital role in basin evolution. The basement configuration image generated by estimates of depth to magnetic basement suggests a complex pattern of NNE-SSW to NE-SW trending depressions separated by a linear N-S trending basement ridge. It is inferred from the 3D magnetic susceptibility inversion that the thickness of sediments is more towards the eastern basin margin and the N-S ridge is a manifestation of post sedimentary faulting. Results of 2.5D modelling of a WNW-ESE profile across the Singhora Basin combined with results from 3D inversion suggest suggests the basin subsidence was controlled by NE-SW trending regional faults in an active system. The basin geometry evolved by E-W block faulting overprinted by NE-SW trending pre- to syn-depositional normal faults generating NE-SW depression, which are affected by N-S trend post-sedimentary faulting. Though the present work relates the basin evolution with the initiation of rift basin, it warrants further work to establish the deformation within the basin pertaining to the proximal thrust and uplift along the craton fringe.

Cross sections showing stratigraphic and depositional lithofacies of upper Cambrian rocks and the relation of lithofacies to potential for Mississippi Valley-type mineralization in the Harrison 1° x 2° quadrangle, Missouri and Arkansas (folio of the Harrison 1 degree by 2 degrees quadrangle, Missouri and Arkansas)

USGS Publications Warehouse

Hayes, Timothy S.; Palmer, James R.; Pratt, Walden P.; Krizanich, Gary; Whitfield, John W.; Seeger, Cheryl M.

1997-01-01

These cross sections are the fifth publication in a folio of maps of the Harrison 1° x 2° quadrangle, Missouri and Arkansas, prepared under the Conterminous United States Mineral Assessment Program (CUSMAP). Previously published maps in this folio relate to the geochemistry of the subsurface carbonate rocks (Erickson and others, 1989), the geophysics of the basement terranes (McCafferty and others, 1989), the sedimentary rocks and mineralization of the Caulfield district (Hayes and others, 1992), the mineral resource potential of the quadrangle (Pratt and others, 1993), and the bedrock geology of the quadrangle (Middendorf and others, 1994 and in press). A final set of maps showing locations of known Mississippi Valley-type deposits and occurrences relative to Late Cambrian shaly lithofacies and other shales in the Harrison and adjoining quadrangle is in preparation (Palmer and Hayes, in press).

Petrology and chemistry of Jebel Tanumah complex, Khamis Mushayt, Southern Arabian shield, Saudi Arabia

NASA Astrophysics Data System (ADS)

Nassief, M. O.; Ali, H. M.; Zakir, F. A.

The mafic intrusive complex at Jebel Tanumah is located 15 km north-west of Khamis Mushayt in the southern Arabian Shield and includes olivine-bearing gabbro as well as amphibole-diopside-hornblende gabbro cumulates. These rocks have been generally metamorphosed to upper greeenschist-lower amphibolite facies. Fourteen white rock silicate analyses indicate that the majority of the rocks are calc-alkaline to tholeiitic in composition. The two major structural units in the Khamis Mushayt region identified by Coleman consist of the basement complex of Asir Mountains and the younger metamorphic rocks. Syntectonic granitic rocks intruded the antiforms characterizing the younger rocks whereas the lower parts of the synforms are intruded by post-tectonic intrusions of layered gabbros such as the one studied at Jebel Tanumah.

PBF Reactor Building (PER620). Camera faces south along west wall. ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

PBF Reactor Building (PER-620). Camera faces south along west wall. Gap between native lava rock and concrete basement walls is being backfilled and compacted. Wire mesh protects workers from falling rock. Note penetrations for piping that will carry secondary coolant water to Cooling Tower. Photographer: Holmes. Date: June 15, 1967. INEEL negative no. 67-3665 - Idaho National Engineering Laboratory, SPERT-I & Power Burst Facility Area, Scoville, Butte County, ID

Estimating the reactivation potential of existing fractures in subsurface granitoids from outcrop analogues and in-situ stress modeling: implications for EGS reservoir stimulation with an example from Meiningen (Thuringia, Central Germany)

NASA Astrophysics Data System (ADS)

Ustaszewski, Kamil; Kasch, Norbert; Siegburg, Melanie; Navabpour, Payman; Thieme, Manuel

2014-05-01

The southwestern part of Thuringia (central Germany) hosts large subsurface extents of Lower Carboniferous granitoids of the Mid-German Crystalline Rise, overlain by an up to several kilometer thick succession of Lower Permian to Mid-Triassic volcanic and sedimentary rocks. The granitic basement represents a conductivity-controlled ('hot dry rock') reservoir of high potential that could be targeted for economic exploitation as an enhanced geothermal system (EGS) in the future. As a preparatory measure, the federal states of Thuringia and Saxony have jointly funded a collaborative research and development project ('Optiriss') aimed at mitigating non-productivity risks during the exploration of such reservoirs. In order to provide structural constraints on the fracture network design during reservoir stimulation, we have carried out a geometric and kinematic analysis of pre-existing fracture patterns in exposures of the Carboniferous basement and Mesozoic cover rocks within an area of c. 500 km2 around the towns of Meiningen and Suhl, where granitic basement and sedimentary cover are juxtaposed along the southern border fault of the Thuringian Forest basement high. The frequency distribution of fractures was assessed by combining outcrop-scale fracture measurements in 31 exposures and photogrammetric analysis of fractures using a LIDAR DEM with 5 m horizontal resolution and rectified aerial images at 4 localities. This analysis revealed a prevalence of NW-SE-trending fractures of mainly joints, extension veins, Permian magmatic dikes and subordinately brittle faults in the Carboniferous granitic basement, which probably resulted from Permian tectonics. In order to assess the reactivation potential of fractures in the reservoir during a stimulation phase, constraints on the current strain regime and in-situ stress magnitudes, including borehole data and earthquake focal mechanisms in a larger area, were needed. These data reveal a presently NW-SE-trending maximum horizontal stress SHmax and a strike-slip regime (Heidbach et al. 2008). In-situ stress magnitudes at a reservoir depth of 4.5 km were calculated assuming hydrostatic pore pressures and frictional equilibrium along pre-existing fractures. Our estimates allow predicting that NW-SE-trending fractures in the reservoir would probably be reactivated as dilational veins during stimulation. In order to ensure that the stimulated rock volume is as large as possible and injected fluids circulate along newly-formed fractures rather than other pre-existing fractures, hydraulic fracturing at reservoir depth should follow a well trajectory parallel to the minimum horizontal stress Shmin, i.e. subhorizontal and NE-SW-oriented. References: Heidbach, O., et al., 2008, World Stress Map database release 2008, doi:10.1594/GFZ.WSM.Rel2008.

Thermochronological Record of a Jurassic Heating-Cooling Cycle Within a Distal Rifted Margin (Calizzano Massif, Ligurian Alps)

NASA Astrophysics Data System (ADS)

Seno, S.; Decarlis, A.; Fellin, M. G.; Maino, M.; Beltrando, M.; Ferrando, S.; Manatschal, G.; Gaggero, L.; Stuart, F. M.

2017-12-01

The aim of the present study is to analyse, through thermochronological investigations, the thermal evolution of a fossil distal margin owing to the Alpine Tethys rifting system. The studied distal margin section consists of a polymetamorphic basement (Calizzano basement) and of a well-developed Mesozoic sedimentary cover (Case Tuberto unit) of the Ligurian Alps (NW Italy). The incomplete reset of zircon (U-Th)/He ages and the non-reset of the zircon fission track ages during the Alpine metamorphism indicate that during the subduction and the orogenic stages these rocks were subjected to temperatures lower than 200 ºC. Thus, the Alpine metamorphic overprint occurred during a short-lived, low temperature pulse. The lack of a pervasive orogenic reset, allowed the preservation of an older heating-cooling event that occurred during Alpine Tethys rifting. Zircon fission-track data indicate, in fact, that the Calizzano basement records a cooling under 240 °C, at 156 Ma (early Upper Jurassic). This cooling followed a Middle Jurassic syn-rift heating at temperatures of about 300-350°C, typical of greenschist facies conditions occurred at few kilometres depth, as indicated by stratigraphic and petrologic constraints. Thus, in our interpretation, major crustal thinning likely promoted high geothermal gradients ( 60-90°C/km) triggering the circulation of hot, deep-seated fluids along brittle faults, causing the observed thermal anomaly at shallow crustal level.

Proterozoic evolution of the western margin of the Wyoming craton: Implications for the tectonic and magmatic evolution of the northern Rocky Mountains

USGS Publications Warehouse

Foster, D.A.; Mueller, P.A.; Mogk, D.W.; Wooden, J.L.; Vogl, J.J.

2006-01-01

Defining the extent and age of basement provinces west of the exposed western margin of the Archean Wyoming craton has been elusive because of thick sedimentary cover and voluminous Cretaceous-Tertiary magmatism. U-Pb zircon geochronological data from small exposures of pre-Belt supergroup basement along the western side of the Wyoming craton, in southwestern Montana, reveal crystallization ages ranging from ???2.4 to ???1.8 Ga. Rock-forming events in the area as young as ???1.6 Ga are also indicated by isotopic (Nd, Pb, Sr) signatures and xenocrystic zircon populations in Cretaceous-Eocene granitoids. Most of this lithosphere is primitive, gives ages ???1.7-1.86 Ga, and occurs in a zone that extends west to the Neoproterozoic rifted margin of Laurentia. These data suggest that the basement west of the exposed Archean Wyoming craton contains accreted juvenile Paleoproterozoic arc-like terranes, along with a possible mafic underplate of similar age. This area is largely under the Mesoproterozoic Belt basin and intruded by the Idaho batholith. We refer to this Paleoproterozoic crust herein as the Selway terrane. The Selway terrane has been more easily reactivated and much more fertile for magma production and mineralization than the thick lithosphere of the Wyoming craton, and is of prime importance for evaluating Neoproterozoic continental reconstructions. ?? 2006 NRC Canada.

Pre-Cenozoic basement rocks of the Proto-Philippine Sea Plate: Constraints for the birthplace of the Izu-Bonin-Mariana Arc

NASA Astrophysics Data System (ADS)

Tani, K.; Ishizuka, O.; Horie, K.; Barth, A. P.; Harigane, Y.; Ueda, H.

2016-12-01

The Izu-Bonin-Mariana Arc is widely regarded to be a typical intra-oceanic arc, with the oceanic Pacific Plate subducting beneath the Philippine Sea Plate, an evolving complex of active and inactive arcs and back-arc basins. However, little is known about the origin of the proto-Philippine Sea Plate, which existed along with the Pacific Plate at the time of subduction initiation in the Eocene. To investigate the crustal structures of the proto-Philippine Sea Plate, we conducted manned-submersible and dredge surveys in the Daito Ridges and the Kyushu-Palau Ridge. The Daito Ridges comprise the northwestern Philippine Sea Plate along with what are regarded as remnants of the proto-Philippine Sea Plate. Submersible observations and rock sampling revealed that the Daito Ridges expose deep crustal sections of gabbroic, granitic, metamorphic, and ultra-mafic rocks, along with volcanic rocks ranging from basalt to andesite. Mesozoic magmatic zircon U-Pb ages have been obtained from the plutonic rocks, and whole-rock geochemistry of the igneous rocks indicates arc origins. Furthermore, mafic schist collected from the Daito Ridge has experienced amphibolite facies metamorphism, with phase assemblages suggesting that the crust was thicker than 20 km at the time. Similar amphibolite-facies metamorphic rocks with Proterozoic zircons have been recovered in the southern Kyushu-Palau Ridge, indicating that such distinctively older basement rocks exist as isolated tectonic blocks within the present Philippine Sea Plate. These finds show that the parts of the Daito Ridges and Kyushu-Palau Ridge represent developed crustal sections of the Pre-Cenozoic arc that comprises part of the proto-Philippine Sea Plate, and, together with the tectonic reconstruction of the proto-Philippine Sea Plate (Deschamps and Lallemand 2002, JGR), they suggest that subduction of the Izu-Bonin-Mariana Arc initiated at the continental margin of the Southeast Asia.

Lithospheric-folding-based understanding on the origin of the back-arc basaltic magmatism beneath Jeju volcanic island, Korea

NASA Astrophysics Data System (ADS)

Yun, S.; Shin, Y.; CHOI, K.; Koh, J.; Nakamura, E.; Na, S.

2012-12-01

Jeju Island is an intraplate volcanic island located at the eastern margin on the East Asia behind the Ryukyu Trench, the collisional/subduction boundary between the Eurasian plate and Philippine Sea plate. It is a symmetrical shield volcano, having numerous monogenetic cinder cones, over 365, on the Mt. Halla volcanic edifice. The basement rock mainly consists of Precambrian gneiss, Mesozoic granite and volcanic rocks. Unconsolidated sedimentary rock is found between basement rock and surface lava. The lava plateau is composed of voluminous basaltic lava flows, which extend to the coast region with a gentle slope. Based on the evidence obtained from volcanic stratigraphy, paleontology, and geochronology, the age of the Jeju basalts ranges from the early Pleistocene to Holocene(Historic). The alkaline and tholeiitic basalts exhibits OIB composition from intraplate volcanism which is not associated with plate subduction, while the basement xenolith contained in the volcanic rock indicates that there were volcanic activities associated with the Mesozoic plate subduction. The Geochemical characteristics have been explained with the plume model, lithospheric mantle origin, and melting of shallow asthenosphere by the rapid change of stress regimes between the collision of the India-Eurasia plates and subduction of the Pacific plate, while there has not been any geophysical investigation to disclose it. Compression near collisional plate boundaries causes lithospheric folding which results in the decrease of pressure beneath the ridge of the fold while the pressure increases beneath trough. The decompression beneath lithosphere is likely to accelerate basaltic magmatism along and below the ridge. We investigate the subsurface structure beneath Jeju volcanic island, South Korea and its vicinity and propose an alternative hypothesis that the basaltic magma beneath the island could be caused by episodic lithospheric folding. Unlike the prevailing hypothesis of the intraplate basaltic magmatism that requires extending lithosphere, ours can explain how the basaltic magma could be generated at the back-arc regions without the extension. A schematic diagram illustrating the magma formation beneath Arc and Back-arc regions due to the lithospheric folding: Basaltic magma could be generated at upper mantle beneath ridge of the lithospheric fold by decompression and pre-existing high temperature.

Numerical analysis of temperature distribution due to basement radiogenic heat production, St. Lawrence Lowlands, eastern Canada

NASA Astrophysics Data System (ADS)

Liu, Hejuan; Giroux, Bernard; Harris, Lyal B.; Mansour, John

2017-04-01

Although eastern Canada is considered as having a low potential for high-temperature geothermal resources, the possibility for additional localized radioactive heat sources in Mesoproterozoic Grenvillian basement to parts of the Palaeozoic St. Lawrence Lowlands in Quebec, Canada, suggests that this potential should be reassessed. However, such a task remains hard to achieve due to scarcity of heat flow data and ambiguity about the nature of the basement. To get an appraisal, the impact of radiogenic heat production for different Grenville Province crystalline basement units on temperature distribution at depth was simulated using the Underworld Geothermal numerical modelling code. The region south of Trois-Rivières was selected as representative for the St. Lawrence Lowlands. An existing 3D geological model based on well log data, seismic profiles and surface geology was used to build a catalogue of plausible thermal models. Statistical analyses of radiogenic element (U, Th, K) concentrations from neighbouring outcropping Grenville domains indicate that the radiogenic heat production of rocks in the modelled region is in the range of 0.34-3.24 μW/m3, with variations in the range of 0.94-5.83 μW/m3 for the Portneuf-Mauricie (PM) Domain, 0.02-4.13 μW/m3 for the Shawinigan Domain (Morin Terrane), and 0.34-1.96 μW/m3 for the Parc des Laurentides (PDL) Domain. Various scenarios considering basement characteristics similar to the PM domain, Morin Terrane and PDL Domain were modelled. The results show that the temperature difference between the scenarios can be as much as 12 °C at a depth of 5 km. The results also show that the temperature distribution is strongly affected by both the concentration of radiogenic elements and the thermal conductivity of the basement rocks. The thermal conductivity in the basement affects the trend of temperature change between two different geological units, and the spatial extent of thermal anomalies. The validity of the results was assessed by comparing the modelled temperature and heat flow data with the available experimental data. The overall agreement is good, although some discrepancies appear at some wells. Hence, detailed investigations are needed to obtain a more reliable estimate of temperature distribution at a local scale.

Geologic history of the Neogene “Qena Lake” developed during the evolution of the Nile Valley: A sedimentological, mineralogical and geochemical approach

NASA Astrophysics Data System (ADS)

Philobbos, Emad R.; Essa, Mahmoud A.; Ismail, Mustafa M.

2015-01-01

Siliciclastic and carbonate sediments were laid down in southern Wadi Qena and around the Qena Nile bend (Middle Egypt) in a lacustrine-alluvial environment which dominated a relatively wide lake, the "Qena Lake" that interrupted the Nile course during the Neogene time. These sediments are represented mainly by the oldest dominantly lacustrine chocolate brown mudstones of the Khuzam Formation that accumulated nearer to the center of that lake (now forming a 185 m terrace above sea level), overlain by the dominantly lacustrine carbonates and marls of the Durri Formation which accumulated during semi-arid conditions, mainly nearer to the periphery of the lake (now forming 170, 180 and 185 m terraces a.s.l. in the studied sections). The water level of the "Qena Lake" reached 240 m. above sea level, as indicated by the maximum carbonate elevation reached in the region. Finally fanglomerates of the Higaza Formation with its chert and limestone conglomerates accumulated during torrential periods at higher elevations (forming 240, 300 and 400 m terraces a.s.l.). These three formations accumulated in this particular area before and during the unroofing of the basement rocks of the Eastern Desert, west of the watershed. According to the known Early Miocene initial development of the Nile Valley, beside the occurrence of similar deposits of Oligocene age along the eastern side of the basement range, the earlier known Pliocene age given for these sediments in the Qena area is here questioned. It might belong to earlier Miocene?-Pliocene times. As the basement rocks of the Eastern Desert were still covered by Cretaceous-Paleogene sedimentary rocks while the Khuzam, Durri and Higaza Formations were accumulating in the Qena Lake region, it is believed, contrary to the belief of some authors, that the basement rocks of the Eastern Desert were not the source of these sediments. The carbonate petrographic study, beside the X-ray, and the11 major oxides and 22 trace elements analyses, all point to that the mudrock sediments of the oldest Neogene Khuzam and Durri Formations of the "Qena Lake" phase were carried out and entered the area of southern Wadi Qena and around the Qena Nile bend mainly from the south. The intermediate igneous rocks of southern Egypt and northern Sudan were the main source areas. Additional contributions had possibly come from the weathering of the non-marine to brackish Cretaceous (pre-Campanian) shales of southern Egypt. Accumulation of conglomerates with mixed igneous and sedimentary clasts followed (forming 7 terraces in Wadi Qena, ranging from 240 m in the north to 140 m a.s.l. in the south), constituting the newly introduced Late Pliocene formation; El Heita Formation. These conglomerates were mainly drained from the then exposed basement rocks of the middle parts of Wadi Qena, and cut through the older Neogene sediments. Later on, after the lake became connected to the northern parts of the Nile Valley, the lake water level was lowered to 180 m a.s.l., and another lake with this lower level was formed (Isawiyya Lake). With the successive lowering of water level the younger well known Issawia, Qena, Abbassia and Dandara Formations accumulated successively; nearer to, and within, the present Nile Valley.

Petrology, geochemistry and LA-ICP-MS U-Pb geochronology of Paleoproterozoic basement rocks in Bangladesh: An evaluation of calc-alkaline magmatism and implication for Columbia supercontinent amalgamation

NASA Astrophysics Data System (ADS)

Hossain, Ismail; Tsunogae, Toshiaki; Tsutsumi, Yukiyasu; Takahashi, Kazuki

2018-05-01

The Paleoproterozoic (1.7 Ga) basement rocks from Maddhapara, Bangladesh show a large range of chemical variations (e.g. SiO2 = 50.7-74.7%) and include diorite, quartz diorite, monzodiorite, quartz monzonite and granite. The pluton overall displays metaluminous, calc-alkaline orogenic suite; mostly I-type suites formed within subduction-related magmatism. The observed major elements show general trends for fractional crystallization. Trace element contents also indicate the possibility of a fractionation or assimilation; explain the entire variation from diorite to monzonite, even granite. The pluton may have evolved the unique chemical features by a process that included partial melting of calc-alkaline lithologies and mixing of mantle-derived magmas, followed by fractional crystallization, and by assimilation of country rocks. The pluton shows evidence of crystal fractionation involving largely plagioclase, amphibole and possibly biotite. Some of the fractionated magmas may have mixed with more potassic melts from distinct parts of the continental lithosphere to produce granites and/or pegmatites. New geochronological results of granitic pegmatite (1722 ± 10 Ma) are indisputably consistent with diorite and tonalite and those data showing credible geochronological sequence (i.e., diorite - tonalite - granitic pegmatite). Identical Paleoproterozoic age (1.7 Ga) with distinctive magmatism of the Maddhapara basement rocks have agreeable relationship with the CITZ, India. The consistent magmatism is also common in the Transamazonian of South America, Trans-Hudson orogeny in North America, Bohemian Massif and the Svecofennian, Poland, have identified the sequential growth of the continent through the amalgamation of juvenile terrains, succeeded by a major collisional orogeny. Such Paleoproterozoic subduction-related orogens in Australia have similar counterparts in Antarctica and other part of the world. These types of Paleoproterozoic magmatism dominantly contributed to assemble, amalgamation and breakup of the enormous Columbia supercontinent.

Meteoroid mayhem in Ole Virginny: Source of the North American tektite strewn field

USGS Publications Warehouse

Poag, C. Wylie; Powars, David S.; Poppe, Lawrence J.; Mixon, Robert B.

1994-01-01

New seismic reflection data from Chesapeake Bay reveal a buried, 85-km-wide, 1.5-2.0-km-deep, peak-ring impact crater, carved through upper Eocene to Lower Cretaceous sedimentary strata and into underlying pre-Mesozoic crystalline basement rocks. A polymictic, late Eocene impact breccia, composed mainly of locally derived sedimentary debris (determined from four continuous cores), surrounds and partly fills the crater. Structural and sedimentary characteristics of the Chesapeake Bay crater closely resemble those of the Miocene Ries peakring crater in southern Germany. We speculate that the Chesapeake Bay crater is the source of the North American tektite strewn field.

Meteoroid mayhem in Ole Virginny: source of the North American tektite strewn field

USGS Publications Warehouse

Poag, C.W.; Powars, D.S.; Poppe, L.J.; Mixon, R.B.

1994-01-01

New seismic reflection data from Chesapeake Bay reveal a buried, 85-km-wide, 1.5-2.0-km-deep, peak-ring impact crater, carved through upper Eocene to Lower Cretaceous sedimentary strata and into underlying pre-Mesozoic crystalline basement rocks. A polymictic, late Eocene impact breccia, composed mainly of locally derived sedimentary debris (determined from four continuous cores), surrounds and partly fills the crater. Structural and sedimentary characteristics of the Chesapeake Bay crater closely resemble those of the Miocene Ries peak-ring crater in southern Germany. It is speculated that the Chesapeake Bay crater is the source of the North American tektite strewn field. -Authors

Tectonic significance of precambrian apatite fission-track ages from the midcontinent United States

USGS Publications Warehouse

Crowley, K.D.; Naeser, C.W.; Babel, C.A.

1986-01-01

Apparent apatite fission-track ages from drill core penetrating basement on the flank of the Transcontinental Arch in northwestern Iowa range from 934 ?? 86 to 641 ?? 90 Ma. These ages, the oldest reported in North America, record at least two thermal events. The 934 Ma age, which is synchronous with KAr ages in the Grenville Province and many KAr whole-rock and RbSr isochron ages from the Lake Superior region, may document basement cooling caused by regional uplift and erosion of the crust. The remaining fission-track ages are products of a more recent thermal event, relative to the age of the samples, which raised temperatures into the zone of partial annealing. Heating may have occurred between the Middle Ordovician and Middle Cretaceous by burial of the basement with additional sediment. It is estimated that burial raised temperatures in the part of the basement sampled by the core to between 50 and 75??C. These temperature estimates imply paleogeothermal gradients of about 20??C/km, approximately two and one-half times present-day values, and burial of the basement by an additional 2-3 km of sediment. ?? 1986.

State-of-the-Art for Assessing Earthquake Hazards in the United States. Report 19. The Evidence for Reservoir-Induced Macroearthquakes.

DTIC Science & Technology

1982-06-01

pore pressures are dissipating. 232. The question of deep fluid communication is unresolved. Koyna is situated in flow basalt known as the Deccan Traps ...The trap rock formation is about 1200 m thick near Koyna. The basalt flows are irregular and at the damsite seven flows have been mapped. Some of the...ranges from 0 to about 30 km but is generally 2 to 8 km in depth. This places the bulk of the seismicity below the trap rock in a basement rock of unknown

ARNOLD MESA ROADLESS AREA, ARIZONA.

USGS Publications Warehouse

Wolfe, Edward W.; McColly, Robert A.

1984-01-01

Geologic geochemical, and aeromagnetic investigations and a survey of mines and prospects in the Arnold Mesa Roadless Area, Arizona, provide little evidence for the occurrence of mineral or energy resources. Buried Proterozoic basement rocks are possible hosts of porphyry-type copper and massive sulfide deposits but the thick cover of Paleozoic sedimentary rocks and upper Cenozoic volcanic rocks precluded assessment of this possibility. Chemistry and temperature of spring and well waters suggest that a geothermal resource may exist near the eastern margin of the roadless area, but the anomaly has not been tested by drilling and this resource remains unverified. No other energy resources were identified.

Tectonics of the North American Cordillera near the Fortieth Parallel

USGS Publications Warehouse

King, P.B.

1978-01-01

The North American Cordillera near the Fortieth Parallel consists of the following tectonic units: 1. (A) To the east is a reactivated cratonic area, in the Southern Rocky Mountains and Colorado Plateau, in which the supracrustal rocks (Cambrian to Cretaceous) were broadly deformed during the late Cretaceous-Paleocene Laramide orogeny, and the Precambrian basement was raised in folds of wide amplitude. 2. (B) West of it is a miogeosynclinal belt, in the eastern Great Basin, in which a thick sequence of Paleozoic carbonates and related deposits was thrust eastward along low-angle faults during the middle to late Cretaceous Sevier orogeny. The miogeosyncline is the downwarped western margin of the original North American continent, and its rocks accumulated on Precambrian basement. 3. (C) Beyond is a eugeosynclinal belt, in the western Great Basin, in which Paleozoic graywackes, cherts, and volcanics were thrust easteastward along low-angle faults during several Paleozoic orogenies - the mid-Paleozoic Antler orogeny which produced the Roberts thrust on the east, and the end-Paleozoic Sonoma orogeny which produced the Golconda thrust farther west. The Paleozoic eugeosynclinal rocks accumulated on oceanic basement. They are overlapped from the west by Triassic and Jurassic shelf deposits, which pass westward into eugeosynclinal deposits. 4. (D) A volcanic island-arc belt existed on the sites of the Sierra Nevada in Paleozoic and early Mesozoic time, which produced thick bodies of sediments and volcanics. During the mid-Mesozoic Nevadan orogeny these were steeply deformed and thrust westward over subduction zones, and were intruded by granitic rocks that rose from the upper mantle to form great batholiths. 5. (E) West of the Sierra Nevada, in the Great Valley, is a great sedimentary embankment of later Mesozoic flysch or turbidite, largely younger than the supracrustal rocks of the Sierra Nevada and the Nevadan orogeny. It was formed of the erosional products of the supracrustal and granitic rocks of the Sierra Nevada. 6. (F) This sequence is, in turn, thrust westward over the Mesozoic Franciscan terrane of the Coast Ranges, which forms the westernmost belt of the Cordillera, and which is being treated in other papers in this symposium. The net effect of the prolonged events that produced the Cordillera in this segment has been the addition of successive tectonic belts to the North American continent at the expense of the Pacific Ocean basin during Phanerozoic time. ?? 1978.

To evaluate ERTS-1 data for usefulness as a geological sensor in the diverse geological terranes of New York State

NASA Technical Reports Server (NTRS)

Isachsen, Y. W. (Principal Investigator)

1972-01-01

The author has identified the following significant results. In the present imagery, obtained during the full foliage of summer and fall, the greatest amount of spectral geology is displayed in the Adirondack region where bedrock geology is strongly linked to topography. Of the four spectral bands imaged, band 5 and 7 provide the most geological information. The boundary between the basement rocks of the Adirondack Dome and the surrounding Lower Paleozoic rocks is well delineated except in the Northwest Lowlands and along parts of the eastern Adirondacks. Within the basement complex, the most prominently displayed features are numerous north-northeast trending faults and topographic lineaments, and arcuate east-west valleys developed in some of the weaker metasedimentary rocks. The majority of the faults and lineaments shown on the geologic map of New York appear in the ERTS-1 imagery. In addition, many new linears were detected, as well as a number of anomalous curvilinear elements, some circular in plan and measuring up to 25 km in diameter, which do not bear any clear relationship to mapped geological contacts. The possibility that it is an astrobleme will be investigated after snow melts in the spring.

Trace-element composition of Chicxulub crater melt rock, K/T tektites and Yucatan basement

NASA Technical Reports Server (NTRS)

Hildebrand, A. R.; Gregoire, D. C.; Attrep, M., Jr.; Claeys, P.; Thompson, C. M.; Boynton, W. V.

1993-01-01

The Cretaceous/Tertiary (K/T) boundary Chicxulub impact is the best preserved large impact in the geologic record. The Chicxulub crater has been buried with no apparent erosion of its intracrater deposits, and its ejecta blanket is known and is well preserved at hundreds of localities globally. Although most of the molten material ejected from the crater has been largely altered, a few localities still preserve tektite glass. Availability of intra- and extracrater impact products as well as plausible matches to the targeted rocks allows the comparison of compositions of the different classes of impact products to those of the impacted lithologies. Determination of trace-element compositions of the K/T tektites, Chicxulub melt rock, and the targeted Yucatan silicate basement and carbonate/evaporite lithologies have been made using instrumental neutron activation analysis (INAA) and inductively coupled plasma mass spectrometry (ICP-MS). Some sample splits were studied with both techniques to ensure that inter-laboratory variation was not significant or could be corrected. The concentration of a few major and minor elements was also checked against microprobe results. Radiochemical neutron activation analysis (RNAA) was used to determine Ir abundances in some samples.

Basement control of structure in the Gettysburg rift basin, Pennsylvania and Maryland

NASA Astrophysics Data System (ADS)

Root, Samuel I.

1989-09-01

Jurassic faulting formed the 93 km long Gettysburg basin as an extensional half graben paralleling the basement structural grain. Preserved in the basin are rift-related Carnian to Rhaetian strata that were tilted 20-30° NW into a SE dipping, listric normal fault at the northwest border of the basin. Vertical displacement on the border fault approaches 10 km. The border fault developed parallel to the trend of the terminal Paleozoic Alleghenian South Mountain cleavage of the Blue Ridge basement along 80% of its extent. However, it is only roughly parallel to discordant to dip of the cleavage. Relationship of cleavage and later border faulting may be the result of persistent reactivation of the original Appalachian continental margin. Local complex structures in the half graben are related to reactivation of two subvertical, pre-Mesozoic faults that transect basement structural grain (cleavage) at a large angle. The northern Shippensburg fault was reactivated during basin normal faulting, offsetting the border fault in a right-lateral sense by 3.5 km and forming within the basin a fold and a fault sliver of basement. The southern Carbaugh-Marsh Creek fault was not reactivated, but is the locus of a 20°-30° change of trend of both the basement cleavage and later border fault. However, two large, NW trending, left-lateral wrench faults, antithetic to the Carbaugh-March Creek fault, developed here offsetting the border fault and forming en echelon folds and horst blocks of basement rock within the basin.

The morphology and nature of the East Arctic ocean acoustic basement

NASA Astrophysics Data System (ADS)

Rekant, Pavel

2017-04-01

As the result of the thorough interpretation and cross-correlation of the large seismic dataset (>150000 km and >600 seismic lines), the depth structure map of the acoustic basement was constrained. Tectonic framework, basement surface morphology and linkage of the deep basin structures with shelves ones, was significantly clarified based on the map. It becomes clear that most morphostructures presently located within deep-water basin are tectonically connected with shelf structures. Acoustic basement contains a number of pre-Cambrian, Caledonian and Mesozoic consolidated blocks. The basement heterogeneity is highlighted by faults framework and basement surface morphology differences, as well thickness and stratigraphy of the sediment cover. The deepest basins of the East Arctic - Hanna Trough, North Chukchi and Podvodnikov Basins form a united mega-depression, wedged between pre-Cambrian continental blocks (Chukchi Borderland - Mendeleev Rise - Toll Saddle) from the north and the Caledonian deformation front from the south. The basement age/origin speculations are consistent with paleontological and U-Pb zircon ages from dredged rock samples. Most of morphological boundaries in the modern Arctic differ considerably from the tectonic framework. Only part of the Arctic morphostructures is constrained by tectonic boundaries. They are: eastern slope of the Lomonosov Ridge, continental slope in the Laptev Sea, upper continental slope in the Podvodnikov Basin, southern slope of the North Chukchi Basin and borders of the Chukchi Borderland. The rest significant part of modern morphological boundaries are caused by sedimentation processes.

A new interpretation of seismic tomography in the southern Dead Sea basin using neural network clustering techniques

NASA Astrophysics Data System (ADS)

Braeuer, Benjamin; Bauer, Klaus

2015-11-01

The Dead Sea is a prime location to study the structure and development of pull-apart basins. We analyzed tomographic models of Vp, Vs, and Vp/Vs using self-organizing map clustering techniques. The method allows us to identify major lithologies by their petrophysical signatures. Remapping the clusters into the subsurface reveals the distribution of basin sediments, prebasin sedimentary rocks, and crystalline basement. The Dead Sea basin shows an asymmetric structure with thickness variation from 5 km in the west to 13 km in the east. Most importantly, we identified a distinct, well-defined body under the eastern part of the basin down to 18 km depth. Considering its geometry and petrophysical signature, this unit is interpreted as a buried counterpart of the shallow prebasin sediments encountered outside of the basin and not as crystalline basement. The seismicity distribution supports our results, where events are concentrated along boundaries of the basin and the deep prebasin sedimentary body. Our results suggest that the Dead Sea basin is about 4 km deeper than assumed from previous studies.

Geophysical setting of western Utah and eastern Nevada between latitudes 37°45′ and 40°N

USGS Publications Warehouse

Mankinen, Edward A.; McKee, Edwin H.; Tripp, Bryce; Krahulec, Ken; Jordan, Lucy

2009-01-01

Gravity and aeromagnetic data refine the structural setting for the region of western Utah and eastern Nevada between Snake and Hamlin Valleys on the west and Tule Valley on the east. These data are used here as part of a regional analysis. An isostatic gravity map shows large areas underlain by gravity lows, the most prominent of which is a large semi-circular low associated with the Indian Peak caldera complex in the southwestern part of the study area. Another low underlies the Thomas caldera in the northeast, and linear lows elsewhere indicate low-density basin-fill in all major north-trending graben valleys. Gravity highs reflect pre-Cenozoic rocks mostly exposed in the mountain ranges. In the Confusion Range, however, the gravity high extends about 15 km east of the range front to Coyote Knolls, indicating a broad pediment cut on upper Paleozoic rocks and covered by a thin veneer of alluvium. Aeromagnetic highs sharply delineate Oligocene and Miocene volcanic rocks and intracaldera plutons associated with the Indian Peak caldera complex and the Pioche–Marysvale igneous belt. Jurassic to Eocene plutons and volcanic rocks elsewhere in the study area, however, have much more modest magnetic signatures. Some relatively small magnetic highs in the region are associated with outcrops of volcanic rock, and the continuation of those anomalies indicates that the rocks are probably extensive in the subsurface. A gravity inversion method separating the isostatic gravity anomaly into fields representing pre-Cenozoic basement rocks and Cenozoic basin deposits was used to calculate depth to basement and estimate maximum amounts of alluvial and volcanic fill within the valleys. Maximum depths within the Indian Peak caldera complex average about 2.5 km, locally reaching 3 km. North of the caldera complex, thickness of valley fill in most graben valleys ranges from 1.5 to 3 km thick, with Hamlin and Pine Valleys averaging ~3 km. The main basin beneath Tule Valley is relatively shallow (~0.6 km), reaching a maximum depth of ~1 km over a small area northeast of Coyote Knolls. Maximum horizontal gradients were calculated for both long-wavelength gravity and magnetic-potential data, and these were used to constrain major density and magnetic lineaments. These lineaments help delineate deep-seated crustal structures that separate major tectonic domains, potentially localizing Cenozoic tectonic features that may control regional ground-water flow.

The furnace in the basement: Part 1, The early days of the Hot Dry Rock Geothermal Energy Program, 1970--1973

DOE Office of Scientific and Technical Information (OSTI.GOV)

Smith, M.C.

1995-09-01

This report presents the descriptions of the background information and formation of the Los Alamos Scientific Laboratory Geothermal Energy Group. It discusses the organizational, financial, political, public-relations,geologic, hydrologic, physical, and mechanical problems encountered by the group during the period 1970--1973. It reports the failures as well as the successes of this essential first stage in the development of hot dry rock geothermal energy systems.

Field Demonstration of Propane Biosparging for In Situ Remediation of N-Nitrosodimethylamine (NDMA) in Groundwater

DTIC Science & Technology

2015-12-01

and metamorphic rocks that make up the Sierra Nevada Mountain Range. Aerojet is underlain by fluvial and marine sedimentary deposits ranging in...age from Cretaceous to Recent. These sedimentary deposits unconformably overlie Jurassic-aged metamorphic basement rocks that dip to the west. These...BSW-1. Figure 5-3. Bromide concentrations in PMW-2, PMW-3 ( used as injection well), and PMW-4 during 14-day tracer study. Figure 5-4. Darcy

Precambrian accretionary history and phanerozoic structures-A unified explanation for the tectonic architecture of the nebraska region, USA

USGS Publications Warehouse

Carlson, M.P.

2007-01-01

The Phanerozoic history in Nebraska and adjacent regions contains many patterns of structure and stratigraphy that can be directly related to the history of the Precambrian basement rocks of the area. A process is proposed that explains the southward growth of North America during the period 1.8-1.6 Ga. A series of families of accretionary events during the Proterozoic emplaced sutures that remained as fundamental basement weak zones. These zones were rejuvenated in response to a variety of continental stress events that occurred during the Phanerozoic. By combining the knowledge of basement history with the history of rejuvenation during the Phanerozoic, both the details of Proterozoic accretionary growth and an explanation for the patterns of Phanerozoic structure and stratigraphy is provided. ?? 2007 The Geological Society of America. All rights reserved.

Basement structure based on gravity anomaly in the northern Noto peninsula, Central Japan

NASA Astrophysics Data System (ADS)

Mizubayashi, T.; Sawada, A.; Hamada, M.; Hiramatsu, Y.; Honda, R.

2012-12-01

Upper crustal block structures are usually defined by using surface information, such as geological and morphological data. The northern Noto Peninsula, central Japan, is divided into four geological block structures from tectonic geomorphologic perspectives (Ota and Hirakawa, 1979). This division is based on the surface crustal movement. To image the geological blocks three-dimensionally, it is necessary to construct a subsurface structure model. Gravity survey can clarify the detailed subsurface structure with dense gravity measurement. From the detailed Bouguer anomalies in the northwestern Noto Peninsula, Honda et al. (2008) suggested that the rupture size of the 2007 Noto Hanto earthquake was constrained by the geological block structures. Hiramatsu et al. (2008) also suggested the active faults on the seafloor, such as the source fault of the 2007 Noto Hanto earthquake plays a major role for the formation of the geological block structures. In this study, we analyze subsurface density structure based on the Bouguer anomaly and estimate the distribution of basement depth in the northern Noto Peninsula. We focus the relationship among the basement depth, the block structures and the active faults on the seafloor and discuss the block movement in the northern Noto Peninsula. We compiled the data measured and published previously (Gravity Database of Southwest Japan, 2001; Geological survey of Japan, 2004; Geographical survey institute of Japan, 2006; The Gravity Research Group in Southwest Japan, 2001; Komazawa and Okuma, 2010; Hokuriku electric power Co. Ltd., undisclosed) and calculated Bouguer anomaly in the northern Noto Peninsula. Based on this Bouguer anomaly, we analyzed subsurface density structures along 13 northeastern-southwestern profiles and 35 northwestern-southeastern profiles with the interval of 2 km using the two dimensional Talwani's method (Talwani et al., 1959). In the analysis, we assumed a density structure with four layers: basement (density is 2670kg/m3), Neocene volcanic rock (density is 2400kg/m3, or 2550kg/m3), Neocene sedimentary rock (density is 2200kg/m3), and Quaternary sedimentary rock (density is 1800kg/m3, or 1500kg/m3) (Honda et al., 2008). To compare our basement model to the geological block structures, we focus on a transition zone of the basement depth. We recognize that two of three geological block boundaries correspond to the transition zones. These boundaries also correspond to the boundary of active fault segments on the seafloor. Therefore, based on the relationship between the source fault of the 2007 Noto Hanto earthquake and the geological block, we suggest that the movement of those geological blocks is possibly controlled by the corresponding active fault segments. However, we find that the other block boundary doesn't correspond to the transition zone.

Reconnaissance geologic map of the Loreto and part of the San Janier quadrangles, Baja California Sur, Mexico

USGS Publications Warehouse

McLean, Hugh

1988-01-01

The Loreto area of Baja California Sur, Mexico, contains a diverse association of igneous, sedimentary, and metasedimentary rocks exposed in the foothills and arroyos between the Sierra La Giganta and Gulf of California. The Loreto area was selected for this study to examine the possible relation of the marine rocks to the opening of the Gulf of California, and to determine the stratigraphic and structural relations between basement rocks composed of granitic and prebatholithic rocks and overlying Tertiary (mainly Miocene) sedimentary and volcanic rocks, and by a sequence of Pliocene marine and nonmarine sedimentary rocks. The Pliocene marine rocks lie in a structural depression informally called here, the Loreto embayment. This geologic map and report stem from a cooperative agreement between the U.S. Geological Survey and the Consejo de Recursos Minerales of Mexico that was initiated in 1982.

Cobalt—Styles of deposits and the search for primary deposits

USGS Publications Warehouse

Hitzman, Murray W.; Bookstrom, Arthur A.; Slack, John F.; Zientek, Michael L.

2017-11-30

Cobalt (Co) is a potentially critical mineral. The vast majority of cobalt is a byproduct of copper and (or) nickel production. Cobalt is increasingly used in magnets and rechargeable batteries. More than 50 percent of primary cobalt production is from the Central African Copperbelt. The Central African Copperbelt is the only sedimentary rock-hosted stratiform copper district that contains significant cobalt. Its presence may indicate significant mafic-ultramafic rocks in the local basement. The balance of primary cobalt production is from magmatic nickel-copper and nickel laterite deposits. Cobalt is present in several carbonate-hosted lead-zinc and copper districts. It is also variably present in Besshi-type volcanogenic massive sulfide and siliciclastic sedimentary rock-hosted deposits in back arc and rift environments associated with mafic-ultramafic rocks. Metasedimentary cobalt-copper-gold deposits (such as Blackbird, Idaho), iron oxide-copper-gold deposits, and the five-element vein deposits (such as Cobalt, Ontario) contain different amounts of cobalt. None of these deposit types show direct links to mafic-ultramafic rocks; the deposits may result from crustal-scale hydrothermal systems capable of leaching and transporting cobalt from great depths. Hydrothermal deposits associated with ultramafic rocks, typified by the Bou Azzer district of Morocco, represent another type of primary cobalt deposit.In the United States, exploration for cobalt deposits may focus on magmatic nickel-copper deposits in the Archean and Proterozoic rocks of the Midwest and the east coast (Pennsylvania) and younger mafic rocks in southeastern and southern Alaska; also, possibly basement rocks in southeastern Missouri. Other potential exploration targets include—The Belt-Purcell basin of British Columbia (Canada), Idaho, Montana, and Washington for different styles of sedimentary rock-hosted cobalt deposits;Besshi-type VMS deposits, such as the Greens Creek (Alaska) deposit and the Ducktown (Tennessee) waste and tailings; andKnown five-element vein districts in Arizona and New Mexico, as well as in the Yukon-Tanana terrane of Alaska; and hydrothermal deposits associated with ultramafic rocks along the west coast, in Alaska, and in the Appalachian Mountains.

Pyrenean hyper-extension : breaking, thinning, or stretching of the crust ? A view from the central north-Pyrenean zone

NASA Astrophysics Data System (ADS)

de Saint Blanquat, Michel; Bajolet, Flora; Boulvais, Philippe; Boutin, Alexandre; Clerc, Camille; Delacour, Adélie; Deschamp, Fabien; Ford, Mary; Fourcade, Serge; Gouache, Corentin; Grool, Arjan; Labaume, Pierre; Lagabrielle, Yves; Lahfid, Abdeltif; Lemirre, Baptiste; Monié, Patrick; de Parseval, Philippe; Poujol, Marc

2017-04-01

The geology of the North Pyrenean Zone in the central Pyrenees allows for the observation in the field of the entire section of the Pyrenean rift, from the mantle to the crust and the Mesozoic cover (pre, syn and post rift). The good knowledge we have of the pre-Alpine history of the Pyrenees allows us to properly constrain the Alpine geological evolution of the pre-Triassic rocks which record both Variscan and Alpine orogenic cycles. The mantle outcrop as kilometric to centimetric fragments of peridotite dispersed within a carbonate metamorphic breccia. The study of peridotite serpentinisation shows several events of low-temperature serpentinisation, in contact with seawater. In some locallities, we can observe a mixture of fragments of variously serpentinized peridotites. This suggests a tectonic context where fragments of peridotites from different structural levels were sampled more or less synchronously. The granulitic basement is characterized by a Variscan syndeformational HT event (300-280 Ma). So far we have not found any trace of a Cretaceous HT event (> 500°C). On the other hand, the basement is affected by a regional metasomatism that began during the Jurassic and became more spatially focused with time until it was restricted to the Pyrenean rift during the Aptien, Albian and Cenomanian. The talc-chlorite metasomatism (120-95 Ma) shows an evolution from a static toward a syn-deformation hydrothermal event, under a more or less normal geothermal gradient. Extensional deformation is recorded by the reworking of several inherited low-angle Variscan tectonic contacts, but also by dispersed high-angle extensional shear zones formed under greenshist conditions. The metamorphic Mesozoic cover of the basement massifs, which constitute the so-called Internal Metamorphic Zone, is an allochtonous unit made of lenses of Mesozoic rocks enclosed into the breccia, which locally contains peridotite and basement clasts. The Mesozoic metamorphic carbonates show a first phase of syn-metamorphic (450-600°C, P < 2 kb) ductile deformation, and subsequent phases of folding and fracturing. The datation of neoformed minerals give a 108-85 Ma time span for the metamorphism. We interpret this breccia as an abandonment breccia which marks the emergence of the main detachment. The basal contact of the Mesozoic cover has a complex 3D geometry traced by Triassic evaporites. It corresponds to a major pre- and synorogenic polyphased tectonic contact. All these data show a geometrically complex hyper-extended rift where the crust was not stretched under a high geothermal gradient but thinned by the tectonic extraction of relatively thin lenses and perhaps cut by high angle low-grade shear zones. The 3D geometry, as well as the strain records and the breccia lithologies strongly suggest a non-cylindricity for the exhumation process, probably within a transtentional system.

Subduction or obduction of continental crust in the northern Norwegian Caledonides? An example from the Nordmannvik Nappe

NASA Astrophysics Data System (ADS)

Faber, Carly; Stünitz, Holger; Jeřábek, Petr; Gasser, Deta; Konopásek, Jiří; Kraus, Katrin

2017-04-01

The debate about how and why continental crust is subducted is ongoing (Ingalls et al., 2016). This work uses the tectonmetamorphic history of a the Nordmannvik nappe in the northern Scandinavian Caledonides to discuss mid- to lower-crustal processes involved in the subduction of continental crust during the Caledonian Orogeny. The Nordmannvik Nappe, together with the underlying Kåfjord and Vaddas nappes, constitutes the Reisa Nappe Complex (RNC). The RNC overlies continental rocks of the Kalak Nappe Complex (KNC), and a clear oceanic suture between Baltican basement, the KNC and the RNC is missing. The RNC consists mainly of paragneisses of mostly unknown depositional age. Rare fossils in the Vaddas Nappe indicate that it at least partly consists of Ordovician-Silurian (>460 Ma) metasediments (Binns and Gayer, 1980). Both the Nordmannvik and Vaddas Nappes were intruded by gabbroic melt around 439 Ma at 9 kbar (c. 30 km) (Getsinger et al., 2013). Therefore, the host and intrusive rocks were already buried to positions far deeper than oceanic crust prior to nappe stacking. Nordmannvik nappe rocks show at least two distinct metamorphic fabrics; 1) an early high-grade kyanite-present migmatitic fabric and 2) a pervasive mylonitic fabric. Based on microstructural observations and pseudosection modeling these two fabrics are estimated to have formed at 770-800 °C and 9.4-11 kbar and 580-630 °C and 8-9.8 kbar, respectively. The presence of sillimanite in garnet cores (confirmed by Raman spectra) and garnet core compositions also suggest that an earlier, less well constrained, history exists with metamorphism around 815 °C and 8.7 kbar, similar to that recognized in the KNC, where it is dated to be pre-Caledonian. The lack of ocean floor rocks between the Nordmannvik Nappe and the Baltica basement suggests that the Nordmannvik Nappe and nappe units below were fairly proximal to Baltica prior to the Caledonian Orogeny. Their position below the Lyngen Nappe (Iapetus ocean floor) indicates they may even have been the leading edge of a pre-Scandian Baltica continent connected to Baltica-proper underneath an extensional but continental basin hosting Vaddas and Kåfjord sediments. If this is the case it may explain the lack of UHP Baltica basement rocks in northern Norway, commonly seen in the mid- and southern- Caledonian segments. The rheological weakening as a result of partial melting in these fertile rocks at the Baltica continent edge may have caused them to be obducted rather than subducted beyond c. 40 km depth. Binns, R.E., and Gayer, R.A., 1980. Silurian or Upper Ordovician fossils at Guoolasjav'ri Troms, Norway, Nature, 284, 53-55 Getsinger, A.J., Hirth, G., Stünitz, H., and Georgen, E.T., 2013. Influence of water on rheology and strain localization in the lower continental crust, Geochemistry, Geophysics, Geosystems, 14, 2247-2264 Ingalls, M., Rowley, D.B., Currie, B., and Colman, A.S. 2016. Large-scale subduction of continental crust implied by India-Asia mass balance calculation, Nature Geoscience 9, 848-853, doi:10.1038/ngeo2806

Inheritance, Variscan tectonometamorphic evolution and Permian to Mesozoic rejuvenations in the metamorphic basement complexes of the Romanian Carpathians revealed by monazite microprobe geochronology

NASA Astrophysics Data System (ADS)

Săbău, Gavril; Negulescu, Elena

2014-05-01

Monazite U-Th-Pb chemical dating reaches an acceptable compromise between precision and accuracy on one side, and spatial resolution and textural constraints on the other side. Thus it has a powerful potential in testing the coherence of individual metamorphic basement units, and enabling correlations among them. Yet, sensitivity and specificity issues in monazite response to thermotectonic events, especially in the case of superposed effects, remain still unclear. Monazite dating at informative to detailed scale in the main metamorphic basement units of the Carpathians resulted in complex age spectra. In the main, the spectra are dominated by the most pervasive thermal and structural overprint, as checked against independent geochronological data. Post-peak age resetting is mostly present, but statistically subordinate. Resetting in case of superposed events is correlated with the degree of textural and paragenetic overprinting, inheritances being always indicated by more or less well-defined age clusters. The lack of relict ages correlating with prograde structural and porphyroblast zonation patterns is indicative for juvenile formations. Age data distribution in the Carpathians allowed distinction of pre-Variscan events, syn-metamorphic Variscan tectonic stacking of juvenile and reworked basement, post-Variscan differential tectonic uplift, as well as prograde metamorphic units ranging down to Upper Cretaceous ages. In the South Carpathians, the Alpine Danubian domain consists of several Variscan and Alpine thrust sheets containing a metamorphic complex dominated by Upper Proterozoic to Lower Cambrian metamorphic and magmatic ages (Lainici-Păiuş), and several complexes with metamorphic overprints ranging from Carboniferous to Lower Permian. Any correlation among these units, as well as geotectonic models placing a Lower Paleozoic oceanic domain between pre-existing Lainici-Păiuş and Drăgşan terranes are precluded by the age data. Other basement of the South Carpathians contain lower Paleozoic or older units intruded by Ordovician granitoids, imbricated with juvenile Variscan slivers, the structural sequence differing in individual basement complexes. So, in the Leaota Massif the lowermost term of the sequence is prograde Variscan, tectonically overlain by reworked lower Paleozoic gneisses, supporting thrust sheets with very low- to low-grade Variscan schists. In the Făgăraş Massif a lower Paleozoic (Cumpăna) complex bearing a strong Variscan overprint, straddles Variscan juvenile rocks, and the lowermost visible structural level is assumed by upper Carboniferous to Permian juvenile medium-grade metamorphic schists. In the Lotru Metamorphic Suite of the Alpine Getic Nappe, the Variscan stacking is overprinted by post-orogenic differential uplift, documented by the correlation among younging ages, structural and metamorphic low-pressure overprints, recording often higher metamorphic temperatures. The most spectacular structure is Upper Jurassic in age, contains high-grade metamorphic rocks and peraluminous anatectic granitoids, is outlined by a deformed boundary evolving from ductile to brittle regime during cooling, and induces a thermal overprint in the neighbouring rocks. In the basement units thrust over the Getic Nappe, the Sibişel unit yielded Permian prograde peak metamorphic ages and Triassic post-peak overprints, while an adjacent gneissic unit (Laz) delivered an exclusively Cretaceous age pattern. Unexpectedly young metamorphic ages resulted also for the East Carpathians and the Apuseni Mountains. While most of the ages obtained so far correspond to Variscan retrogression of older basement units, the lowermost structural unit of the infra-Bucovinian nappe system in the East Carpathians yielded Upper Cretaceous metamorphic ages in apparently monometamorphic medium-grade schists. In the Apuseni Mountains, schists of the Baia de Arieş Unit display an Upper Jurassic age spectrum, corresponding to a clearly prograde medium-grade event. The ages recorded not only question some of the currently accepted correlations among basement units, but urge to reconsideration of the way in which the basement-cover relationships are interpreted and extrapolated.

Basin formation and Neogene sedimentation in a backarc setting, Halmahera, eastern Indonesia

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hall, R.; Nichols, G.J.

1991-03-01

It has been proposed that basins in backarc setting form in association with subduction by thinning of continental crust, backarc spreading in oceanic crust, compression, or trapping of pieces of oceanic plate behind an arc. The Halmahera basin in eastern Indonesia developed in a backarc setting but does not fall into these categories; it formed by subsidence of thickened crust made up of imbricated Mesozoic-Paleogene arc and ophiolite rocks. Halmahera lies at the western edge of the Philippine Sea Plate in a complex zone of convergence between the Eurasian margin, the oceanic plates of the West Pacific, and the Australian/Indianmore » Plate to the south. The basement is an imbricated complex of Mesozoic to Paleogene ophiolite, arc, and arc-related rocks. During the Miocene this basement complex formed an area of thickened crust upon which carbonate reef and reef-associated sediments were deposited. The authors interpret this shallow marine region to be similar to many of the oceanic plateaus and ridges found within the Philippine Sea Plate today. In the Late Miocene, convergence between the Philippine Sea Plate and the Eurasian margin resulted in the formation of the Halmahera Trench to the west of this region of thickened crust. Subduction of the Molucca Sea Plate caused the development of a volcanic island arc. Subsidence in the backarc area produced a broad sedimentary basin filled by clastics eroded from the arc and from uplifted basement and cover rocks. The basin was asymmetric with the thickest sedimentary fill on the western side, against the volcanic arc. The Halmahera basin was modified in the Plio-Pleistocene by east-west compression as the Molucca Sea Plate was eliminated by subduction.« less

Tectonics of Precambrian basement along the Pacific margin of Antarctica and relation to western North America

DOE Office of Scientific and Technical Information (OSTI.GOV)

Goodge, J.W.; Hansen, V.L.; Walker, N.W.

1993-02-01

High-grade metamorphic rocks of the Precambrian Nimrod Group (NG) constitute one of few cratonal basement exposures in the Transantarctic Mountains. These rocks represent an outlier of the East Antarctic craton, evolved as part of Gondwana and pre-Gondwana (Rodinia) supercontinents. Despite pervasive, high-strain ductile deformation at T [>=] 650 C, they preserve petrologic and geochronologic evidence of an earlier history. Sm-Nd model ages from several NG lithologies, including that of a [approximately]1.7 Ga orthogneiss, range from about 2.7--2.9 Ga; these ages reflect both sedimentary and magmatic derivation from Archean crust. Individual detrital zircon U-Pb ages (about 1.7--2.6 Ga) from NG quartzitesmore » indicate clastic input from Archean to Paleoproterozoic source terrains. The Sm-Nd and U-Pb ages are reminiscent of both the Yavapai-Mazatzal (1.6--1.8 Ga) and Wyoming (> 2.5 Ga) provinces in western North America. U-Pb ages from syn-tectonic metaigneous and pelitic NG tectonites indicate that this basement complex was re-worked by the major ductile deformation in latest neoproterozoic to Early Cambrian time. Supracrustal assemblages that lie outboard of the Nimrod craton include Neoproterozoic graywacke, impure carbonate, and minor mafic volcanics (Beardmore Group), and Cambrian to Lower Ordovician carbonate and siliciclastic rocks (Byrd Group). Neoproterozoic ([approximately]750 Ma) rifting along the proto-Pacific margin of East Antarctica is reflected by deposition of Beardmore turbidites and coeval mafic magmatism. Latest Neoproterozoic to early Paleozoic orogenesis occurred along a left-oblique convergent plate margin of East Antarctica is reflected by deposition of Beardmore turbidites and coeval mafic magmatism.« less

Variability over time in the sources of South Portuguese Zone turbidites: evidence of denudation of different crustal blocks during the assembly of Pangaea

NASA Astrophysics Data System (ADS)

Pereira, M. F.; Ribeiro, C.; Vilallonga, F.; Chichorro, M.; Drost, K.; Silva, J. B.; Albardeiro, L.; Hofmann, M.; Linnemann, U.

2014-07-01

This study combines geochemical and geochronological data in order to decipher the provenance of Carboniferous turbidites from the South Portuguese Zone (SW Iberia). Major and trace elements of 25 samples of graywackes and mudstones from the Mértola (Visean), Mira (Serpukhovian), and Brejeira (Moscovian) Formations were analyzed, and 363 U-Pb ages were obtained on detrital zircons from five samples of graywackes from the Mira and Brejeira Formations using LA-ICPMS. The results indicate that turbiditic sedimentation during the Carboniferous was marked by variability in the sources, involving the denudation of different crustal blocks and a break in synorogenic volcanism. The Visean is characterized by the accumulation of immature turbidites (Mértola Formation and the base of the Mira Formation) inherited from a terrane with intermediate to mafic source rocks. These source rocks were probably formed in relation to Devonian magmatic arcs poorly influenced by sedimentary recycling, as indicated by the almost total absence of pre-Devonian zircons typical of the Gondwana and/or Laurussia basements. The presence of Carboniferous grains in Visean turbidites indicates that volcanism was active at this time. Later, Serpukhovian to Moscovian turbiditic sedimentation (Mira and Brejeira Formations) included sedimentary detritus derived from felsic mature source rocks situated far from active magmatism. The abundance of Precambrian and Paleozoic zircons reveals strong recycling of the Gondwana and/or Laurussia basements. A peri-Gondwanan provenance is indicated by zircon populations with Neoproterozoic (Cadomian-Avalonian and Pan-African zircon-forming events), Paleoproterozoic, and Archean ages. The presence of late Ordovician and Silurian detrital zircons in Brejeira turbidites, which have no correspondence in the Gondwana basement of SW Iberia, indicates Laurussia as their most probable source.

High-quality heat flow determination from the crystalline basement of the south-east margin of North China Craton

NASA Astrophysics Data System (ADS)

Jiang, Guangzheng; Tang, Xiaoyin; Rao, Song; Gao, Peng; Zhang, Linyou; Zhao, Ping; Hu, Shengbiao

2016-03-01

Very few of heat flow data have come from the crystalline basement in the North China Craton but rather from boreholes in the sedimentary cover of oil-gas basins. Explorations for hot dry rock (HDR) geothermal resources and porphyry gold deposits in eastern China offer now valuable opportunities to study the terrestrial heat flow in the crystalline basement. In this study, we obtained continuous temperature logs from two boreholes (the LZ borehole with a depth of 3471 m and the DR borehole with a depth of 2179 m) located in the south-east margin of the North China Craton. The boreholes have experienced long shut-in times (442 days and 261 days for the LZ borehole and DR borehole, respectively); thus, it can be expected that the temperature conditions have re-equilibrated after drilling and drill-mud circulation. Rock thermal conductivity and radiogenic heat production were measured for 68 crystalline rock samples from these two boreholes. The measured heat-flow density was determined to be 71.8 ± 2.3 mW m-2 (for the LZ borehole) and 91.5 ± 1.2 mW m-2 (for the DR borehole). The heat flow for the LZ borehole is close to the value of 75 mW m-2 determined in the Chinese Continental Scientific Drilling main hole (CCSD MH), both being in the Sulu-Dabie orogenic belt and thus able to verify each other. The value for the DR borehole is higher than the above two values, which supports former high heat-flow values determined in the Bohai Bay Basin.

The Najd Fault System of Saudi Arabia

NASA Astrophysics Data System (ADS)

Stüwe, Kurt; Kadi, Khalid; Abu-Alam, Tamer; Hassan, Mahmoud

2014-05-01

The Najd Fault System of the Arabian-Nubian Shield is considered to be the largest Proterozoic Shear zone system on Earth. The shear zone was active during the late stages of the Pan African evolution and is known to be responsible for the exhumation of fragments of juvenile Proterozoic continental crust that form a series of basement domes across the shield areas of Egypt and Saudi Arabia. A three year research project funded by the Austrian Science Fund (FWF) and supported by the Saudi Geological Survey (SGS) has focused on structural mapping, petrology and geochronology of the shear zone system in order to constrain age and mechanisms of exhumation of the domes - with focus on the Saudi Arabian side of the Red Sea. We recognise important differences in comparison with the basement domes in the Eastern desert of Egypt. In particular, high grade metamorphic rocks are not exclusively confined to basement domes surrounded by shear zones, but also occur within shear zones themselves. Moreover, we recognise both exhumation in extensional and in transpressive regimes to be responsible for exhumation of high grade metamorphic rocks in different parts of the shield. We suggest that these apparent structural differences between different sub-regions of the shield largely reflect different timing of activity of various branches of the Najd Fault System. In order to tackle the ill-resolved timing of the Najd Fault System, zircon geochronology is performed on intrusive rocks with different cross cutting relationships to the shear zone. We are able to constrain an age between 580 Ma and 605 Ma for one of the major branches of the shear zone, namely the Ajjaj shear zone. In our contribution we present a strain map for the shield as well as early geochronological data for selected shear zone branches.

Geophysical Framework Investigations Influencing Ground-Water Resources in East-Central Nevada and West-Central Utah

USGS Publications Warehouse

Watt, Janet T.; Ponce, David A.

2007-01-01

A geophysical investigation was undertaken as part of an effort to characterize the geologic framework influencing ground-water resources in east-central Nevada and west-central Utah. New gravity data were combined with existing aeromagnetic, drill-hole, and geologic data to help determine basin geometry, infer structural features, estimate depth to pre-Cenozoic basement rocks, and further constrain the horizontal extents of exposed and buried plutons. In addition, a three-dimensional (3D) geologic model was constructed to help illustrate the often complex geometries of individual basins and aid in assessing the connectivity of adjacent basins. In general, the thirteen major valleys within the study area have axes oriented north-south and frequently contain one or more sub-basins. These basins are often asymmetric and typically reach depths of 2 km. Analysis of gravity data helped delineate geophysical lineaments and accommodation zones. Structural complexities may further compartmentalize ground-water flow within basins and the influence of tectonics on basin sedimentation further complicates their hydrologic properties. The horizontal extent of exposed and, in particular, buried plutons was estimated over the entire study area. The location and subsurface extents of these plutons will be very important for regional water resource assessments, as these features may act as either barriers or pathways for groundwater flow. A previously identified basement gravity low strikes NW within the study area and occurs within a highly extended terrane between the Butte and Confusion synclinoria. Evidence from geophysical, geologic, and seismic reflection data suggests relatively lower density plutonic rocks may extend to moderate crustal depths and rocks of similar composition may be the source of the entire basement gravity anomaly.

Thermochronology and tectonics of the Leeward Antilles: Evolution of the southern Caribbean Plate boundary zone

USGS Publications Warehouse

van der Lelij, Roelant; Spikings, Richard A.; Kerr, Andrew C.; Kounov, Alexandre; Cosca, Michael; Chew, David; Villagomez, Diego

2010-01-01

Tectonic reconstructions of the Caribbean Plate are severely hampered by a paucity of geochronologic and exhumation constraints from anastomosed basement blocks along its southern margin. New U/Pb, 40Ar/39Ar, apatite fission track, and apatite (U-Th)/He data constrain quantitative thermal and exhumation histories, which have been used to propose a model for the tectonic evolution of the emergent parts of the Bonaire Block and the southern Caribbean Plate boundary zone. An east facing arc system intruded through an oceanic plateau during ~90 to ~87 Ma and crops out on Aruba. Subsequent structural displacements resulted in >80°C of cooling on Aruba during 70–60 Ma. In contrast, exhumation of the island arc sequence exposed on Bonaire occurred at 85–80 Ma and 55–45 Ma. Santonian exhumation on Bonaire occurred immediately subsequent to burial metamorphism and may have been driven by the collision of a west facing island arc with the Caribbean Plate. Island arc rocks intruded oceanic plateau rocks on Gran Roque at ~65 Ma and exhumed rapidly at 55–45 Ma. We attribute Maastrichtian-Danian exhumation on Aruba and early Eocene exhumation on Bonaire and Gran Roque to sequential diachronous accretion of their basement units to the South American Plate. Widespread unconformities indicate late Eocene subaerial exposure. Late Oligocene–early Miocene dextral transtension within the Bonaire Block drove subsidence and burial of crystalline basement rocks of the Leeward Antilles to ≤1 km. Late Miocene–recent transpression caused inversion and ≤1 km of exhumation, possibly as a result of the northward escape of the Maracaibo Block.

Finite strain analysis of metavolcanics and metapyroclastics in gold-bearing shear zone of the Dungash area, Central Eastern Desert, Egypt

NASA Astrophysics Data System (ADS)

Kassem, Osama M. K.; Abd El Rahim, Said H.

2014-11-01

The Dungash gold mine area is situated in an EW-trending quartz vein along a shear zone in metavolcanic and metasedimentary host rocks in the Eastern Desert of Egypt. These rocks are associated with the major geologic structures, which are attributed to various deformational stages of the Neoproterozoic basement rocks. Field geology, finite strain and microstructural analyses were carried out and the relation-ships between the lithological contacts and major/minor structures have been studied. The R f/ϕ and Fry methods were applied on the metavolcano-sedimentary and metapyroclastic samples from 5 quartz veins samples, 7 metavolcanics samples, 3 metasedimentary samples and 4 metapyroclastic samples in Dungash area. Finite-strain data show that a low to moderate range of deformation of the metavolcano-sedimentary samples and axial ratios in the XZ section range from 1.70 to 4.80 for the R f/ϕ method and from 1.65 to 4.50 for the Fry method. We conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. Furthermore, the contact between principal rock units is sheared in the Dungash area under brittle to semi-ductile deformation conditions. In this case, the accumulated finite strain is associated with the deformation during thrusting to assemble nappe structure. It indicates that the sheared contacts have been formed during the accumulation of finite strain.

Isotopic composition of Pb in ore deposits of the Betic Cordillera, Spain; origin and relationship to other European deposits

USGS Publications Warehouse

Arribas , Antonio; Tosdal, Richard M.

1994-01-01

The Betic Cordillera in southern Spain is a complex Alpine fold belt that resulted from the Cretaceous through Cenozoic collision of Africa with Europe. The region is illustrative of one of the characteristics of the Alpine-Mediterranean orogen: the occurrence over a limited area of mineral deposits with a wide variety of host rocks, mineralization ages, and styles. The metamorphic basement in the Betic zone is characterized by a nappe structure of superimposed tectonostratigraphic units and consists of lower Paleozoic to Lower Triassic clastic metasedimentary rocks. This is overlain by Middle to Upper Triassic platform carbonate rocks with abundant strata-bound F-Pb-Zn-(Ba) deposits (e.g., Sierra de Gador, Sierra Alhamilla). Cretaceous to Paleogene subduction-related compression in southeastern Spain was followed by Miocene postcollisional extension and resulted in the formation of the Almeria-Cartagena volcanic belt and widespread hydrothermal activity and associated polymetallic mineralization. Typical Miocene hydrothermal deposits include volcanic-hosted Au (e.g., Rodalquilar) and Ag-rich base metal (e.g., Cabo de Gata, Mazarron) deposits as well as complex polymetallic veins, mantos, and irregular replacement bodies which are hosted by Paleozoic and Mesozoic metamorphic rocks and Neogene sedimentary and volcanic rocks (e.g., Cartagena, Sierra Almagrera, Sierra del Aguilon, Loma de Bas).Lead isotope compositions were measured on sulfide samples from nine ore districts and from representative fresh samples of volcanic and basement rock types of the region. The results have been used to evaluate ore-forming processes in southeastern Spain with emphasis on the sources of metals. During a Late Triassic mineralizing event, Pb was leached from Paleozoic clastic metasedimentary rocks and incorporated in galena in strata-bound F-Pb-Zn-(Ba) deposits ( 206 Pb/ 204 Pb = 18.332 + or - 12, 207Pb/ 204 Pb = 15.672 + or - 12, 208 Pb/ 204 Pb = 38.523 + or - 46). The second episode of mineralization was essentially contemporaneous (late Miocene) throughout the region and did not involve remobilization of less radiogenic Triassic ore Pb. Lead isotope data indicate a dominantly Paleozoic metasedimentary source for polymetallic vein- and manto-type deposits that formed by hydrothermal circulation through the Betic basement, driven by Miocene intrusions ( 206 Pb/ 204 Pb = 18.747 + or - 20, 207 Pb/ 204Pb = 15.685 + or - 9, 208 /Pb/ 204 Pb = 39.026 + or - 37). Lead in Au-(Cu-Te-Sn) ores is isotopically indistinguishable from that of the calc-alkalic volcanic host ( 206 Pb/ 204 Pb = 18.860 + or - 9, 207 Pb/ 204 Pb = 15.686 + or - 8, 208 Pb/ 204 Pb = 38.940 + or - 27). In contrast, the Pb in volcanic-hosted Pb-Zn-Cu-(Ag-Au) veins was derived from Paleozoic metamorphic and Miocene volcanic rocks ( 206 Pb/ 204 Pb = 18.786 + or - 5, 207 Pb/ 204 Pb = 15.686 + or - 2, 208 Pb/ 204 Pb = 38.967 + or - 9).A comparison of the Pb isotope data from southeastern Spain with published data from selected Pb-Zn deposits in southern Europe (including Les Malines, L'Argentiere, and the Alpine, Iglesiente-Sulcis, and Montagne Noire districts) indicates the importance of a metasedimentary basement as a common source of ore Pb.

Broadband Magnetotelluric Investigations of Crustal Resistivity Structure in North-Eastern Alberta: Implications for Engineered Geothermal Systems

NASA Astrophysics Data System (ADS)

Liddell, M. V.; Unsworth, M. J.; Nieuwenhuis, G.

2013-12-01

Greenhouse gas emissions from hydrocarbon consumption produce profound changes in the global climate, and the implementation of alternative energy sources is needed. The oilsands industry in Alberta (Canada) is a major producer of greenhouse gases as natural gas is burnt to produce the heat required to extract and process bitumen. Geothermal energy could be utilized to provide this necessary heat and has the potential to reduce both financial costs and environmental impacts of the oilsands industry. In order to determine the geothermal potential the details of the reservoir must be understood. Conventional hydrothermal reservoirs have been detected using geophysical techniques such as magnetotellurics (MT) which measures the electrical conductivity of the Earth. However, in Northern Alberta the geothermal gradient is relatively low, and heat must be extracted from deep inside the basement rocks using Engineered Geothermal Systems (EGS) and therefore an alternative exploration technique is required. MT can be useful in this context as it can detect fracture zones and regions of elevated porosity. MT data were recorded near Fort McMurray with the goal of determining the geothermal potential by understanding the crustal resistivity structure beneath the Athabasca Oilsands. The MT data are being used to locate targets of significance for geothermal exploration such as regions of low resistivity in the basement rocks which can relate to in situ fluids or fracture zones which can facilitate efficient heat extraction or het transport. A total of 93 stations were collected ~500m apart on two profiles stretching 30 and 20km respectively. Signals were recorded using Phoenix Geophysics V5-2000 systems over frequency bands from 1000 to 0.001 Hz, corresponding to depths of penetration approximately 50m to 50km. Groom-Bailey tensor decomposition and phase tensor analysis shows a well defined geoelectric strike direction that varied along the profile from N60°E to N45°E. Inversion of the data reveals the low resistivity sedimentary rocks of the Western Canadian Sedimentary Basin overlying a highly resistive Pre-Cambrian crystalline basement. The basement rocks have strong indications of being electrically anisotropic. Groom-Bailey and phase tensor azimuths are stable and consistent across both frequency and distance but display large phase tensor skew values (indicating 3D structure) and small induction vectors (indicating a lack of lateral structure). This type of anisotropy is unique because of its apparent widespread nature and the number of sites we have to constrain the anisotropic characteristics. These results can help to guide future geothermal development in Alberta as detailed information of the host rock resistivity structure can aid any EGS development.

Crustal recycling through intraplate magmatism: Evidence from the Trans-North China Orogen

NASA Astrophysics Data System (ADS)

He, Xiao-Fang; Santosh, M.

2014-12-01

The North China Craton (NCC) preserves the history of crustal growth and craton formation during the early Precambrian followed by extensive lithospheric thinning and craton destruction in the Mesozoic. Here we present evidence for magma mixing and mingling associated with the Mesozoic tectonic processes from the Central NCC, along the Trans-North China Orogen, a paleo suture along which the Eastern and Western Blocks were amalgamated at end of Paleoproterozoic. Our investigations focus on two granitoids - the Chiwawu and the Mapeng plutons. Typical signatures for the interaction of mafic and felsic magmas are observed in these plutons such as: (1) the presence of diorite enclaves; (2) flow structures; (3) schlierens; (4) varying degrees of hybridization; and (5) macro-, and micro-textures. Porphyritic feldspar crystals show numerous mineral inclusions as well as rapakivi and anti-rapakivi textures. We present bulk chemistry, zircon U-Pb geochronology and REE data, and Lu-Hf isotopes on the granitoids, diorite enclaves, and surrounding basement rocks to constrain the timing of intraplate magmatism and processes of interaction between felsic and mafic magmas. Our LA-ICP-MS zircon U-Pb data show that the pophyritic granodiorite was emplaced at 129.7 ± 1.0 Ma. The diorite enclaves within this granodiorite show identical ages (128.2 ± 1.5 Ma). The basement TTG (tonalite-trondhjemite-granodiorite) gneisses formed at ca. 2.5 Ga coinciding with the major period of crustal accretion in the NCC. The 1.85 Ga age from zircons in the gabbro with positive Hf isotope signature may be related to mantle magmatism during post-collisional extension following the assembly of the Western and Eastern Blocks of the NCC along the Trans-North China Orogen. Our Hf isotope data indicate that the Neoarchean-Paleoproterozoic basement rocks were derived from complex sources of both juvenile magmas and reworked ancient crust, whereas the magma source for the Mesozoic units are dominantly reworked basement rocks. Our study provides a window to intraplate magmatism triggered by mantle upwelling beneath a paleosuture in the North China Craton.

Geophysical observations on northern part of Georges Bank and adjacent basins of Gulf of Maine

USGS Publications Warehouse

Oldale, R.N.; Hathaway, J.C.; Dillon, William P.; Hendricks, J.D.; Robb, James M.

1974-01-01

Continuous-seismic-reflection and magnetic-intensity profiles provide data for inferences about the geology of the northern part of Georges Bank and the basins of the Gulf of Maine adjacent to the bank.Basement is inferred to be mostly sedimentary and volcanic rocks of Paleozoic age that were metamorphosed and intruded locally by felsic and mafic plutons near the end of the Paleozoic Era. During Late Triassic time, large fault basins formed within the Gulf of Maine and probably beneath Georges Bank. The fault basins and a possible major northeast-trending fault zone beneath the northern part of the bank probably formed as a result of the opening Atlantic during the Mesozoic. Nonmarine sediments, associated with mafic flows and intrusive rocks, were deposited in the fault basins as they formed. The upper surface of the Triassic and pre-Triassic rocks that comprise basement is an unconformity that makes up much of the bottom of the Gulf of Maine. Depth to the basement surface beneath the gulf differ greatly because of fluvial erosion in Tertiary time and glacial erosion in Pleistocene time. Beneath the northern part of Georges Bank the basement surface is smoother and slopes southward. Prominent valleys, cut before Late Cretaceous time, are present beneath this part of the bank.Cretaceous, Tertiary, and possibly Jurassic times were characterized by episodes of coastal-plain deposition and fluvial erosion. During this time a very thick wedge of sediment, mostly of Jurassic(?) and Cretaceous ages, was deposited on the shelf. Major periods of erosion took place at the close of the Cretaceous and during the Pliocene. Fluvial erosion during the Pliocene removed much of the coastal-plain sedimentary wedge and formed the Gulf of Maine.Pleistocene glaciers eroded all but a few remnants of the coastal-plain sediments within the gulf and deposited a thick section of drift against the north slope of Georges Bank and a thin veneer of outwash on the bank. Marine sediments were deposited in the basins of the Gulf of Maine during the retreat of the last ice and the postglacial rise in sea level.

Influence of inherited structures on the growth of basement-cored ranges, basin inversion and foreland basin development in the Central Andes, from apatite fission-track and apatite Helium thermochronology.

NASA Astrophysics Data System (ADS)

Zapata, S.; Sobel, E. R.; Del Papa, C.; Jelinek, A. R.; Muruaga, C.

2017-12-01

The Central Andes in NW of Argentina is part of a long-lived subduction zone, active since the Paleozoic. This region experienced several tectonic cycles; each of which created an unique set of structures and may have reactivated preexisting structures. These inherited structures may exert a first-order control over the different foreland deformational styles observed along the strike in the Central Andes. Our study area is located between 26°S and 28°S on the transition between the broken foreland (Santa Barbara system), which expresses a combination of thin-skin and thick-skin styles, and the Sierras Pampeanas, which is deform in a thick-skin style. The Cumbres Calchaquies range and the associated Choromoro Basin are located in the northern part of the study area, and are the southern expression of the Santa Barbara system. Published thermochronology data suggest that the rocks from the basement experienced Late Cretaceous and Late Miocene exhumation; the associated sedimentary rocks within the Choromoro basin experienced Paleogene and Late Miocene deformational phases. In contrast, the Sierra Aconquija range, located immediately south on the transition to the Sierras Pampeanas (thick skin) foreland basin, exhibit larger amounts of Miocene exhumation and lack of Cretaceous exhumation; the associated sedimentary rocks from the Tucuman basin have not been deformed since the Cretaceous. Our goal is to understand the evolution of the structural blocks and the structures responsible for the along strike changes in foreland basin deformational styles and their relation with inherited structures from previous tectonic cycles. We are obtaining new apatite U-Th/He and fission track data to reconstruct the thermal history of the basement, accompanied by U-Pb geochronology and stratigraphy to constrain the evolution of the associated sedimentary basins. Preliminary results combined with published data suggest that inherited structures within the study area have evolved through different tectonic cycles, controlling the thicknes and the geometry of the sediments within the Mesozoic rift basin, the Miocene amount of exhumation in the basement-cored ranges and the deformation style of the associated foreland basins.

Geomorphological and Geoelectric Techniques for Kwoi's Multiple Tremor Assessment

NASA Astrophysics Data System (ADS)

Dikedi, P. N.

2017-12-01

This work epicentres on geomorphological and geoelectric techniques for multiple tremor assessment in Kwoi, Nigeria. Earth tremor occurrences have been noted by Akpan and Yakubu (2010) within the last 70 years, in nine regions in Nigeria; on September 11,12,20,22, 23 and 24, 2016, additional earth tremors rocked the village of Kwoi eleven times. Houses cracked and collapsed, a rock split and slid and smoke evolved at N9027''5.909''', E800'44.951'', from an altitude of 798m. By employing the Ohmega Meter and Schlumberger configuration, four VES points are sounded for subsurface structure characterisation. Thereafter, a cylindrical steel ring is hammered into the ground at the first point (VES 1) and earth samples are scooped from this location; this procedure is repeated for other points (VES 2, 3 and 4). Winresist, Geo-earth, and Surfer version 12.0.626 software are employed to generate geo-sections, lithology, resistivity profile, Iso resistivity and Isopach maps, of the region. Outcome of results reveal some lithological formations of lateritic topsoil, fractured basement and fresh basement; additionally, results reveal 206.6m, 90.7m, 73.2m and 99.4m fractured basement thicknesses for four points. Scooped samples are transferred to the specimen stage of a Scanning Electron Microscope (SEM). SEM images show rounded inter-granular boundaries—the granular structures act like micro-wheels making the upper crustal mass susceptible to movement at the slightest vibration. Collapsed buildings are sited around VES1 location; samples from VES 1 are the most well fragmented sample owing to multiple microfractures—this result explains why VES 1 has the thickest fractured basement. Abrupt frictional sliding occurs between networks of fault lines; there is a likelihood that friction is most intense at the rock slide site on N9027'21.516'' and E800'44.9993'', VES 1 at N9027'5.819'' and E8005'3.1120'' and smoke sites—holo-centres are suspected below these locations. The presence of borehole facilities and quarry activities around the region serve as artificial causal factors of these tremors.

Dating paleo-seismic faulting in the Taiwan Mountain Belt

NASA Astrophysics Data System (ADS)

Lo, C. H.; Wu, C. Y.; Chu, H. T.; Yui, T. F.

2017-12-01

In-situ 40Ar/39Ar laser microprobe dating was carried out on the Hoping pseudotachylite from a mylonite-fault zone in the metamorphosed basement complex of the active Taiwan Mountain Belt to determine the timing of the responsible earthquake(s). The dating results, distributed between 3.2 to 1.6 Ma with errors ranging 0.2 1.1 Ma, were derived from a combination of two Ar isotopic system end-members with inverse isochron ages of 1.55±0.05 and 2.87±0.07 Ma, respectively. Fault melt was found mixed with ultracataclasis in petrographical observations, therefore the older inverse isochron end-member may be attributed to the relic wall rock Ar isotopic system contained in micro-breccia as published 40Ar/39Ar mylonitization ages from 4.1 to 3.0 Ma. Without significant Ar loss expected, the young 1.6 Ma end-member represents the Ar isotopic system and age of the exact pseudotachylite. Seismic faulting therefore occurred during basement rock exhumation in the Taiwanese hinterland.

Deformation associated with the Ste. Genevieve fault zone and mid-continent tectonics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schultz, A.; Baker, G.S.; Harrison, R.W.

1992-01-01

The Ste. Genevieve fault is a northwest-trending deformation zone on the northeast edge of the Ozark Dome in Missouri. The fault has been described as a high-angle block fault resulting from vertical uplift of Proterozoic basement rocks, and also as a left-lateral, strike-slip or transpressive wrench fault associated with the Reelfoot rift. Recent mapping across the fault zone documents significant changes in the style of deformation along strike, including variations in the number and the spacing of fault strands, changes in the orientation of rocks within and adjacent to the fault zone, and changes in the direction of stratigraphic offsetmore » between different fault slices. These data are inconsistent with existing Ste. Genevieve models of monoclinal folding over basement upthrusts. Mesoscopic structural analysis of rocks in and near the fault zone indicates highly deformed noncylindrical folds, faults with normal, reverse, oblique, and strike-slip components of movement, and complex joint systems. Fabric orientation, calcite shear fibers, and slickensides indicate that the majority of these mesoscopic structures are kinematically related to left-lateral oblique slip with the southwest side up. Within the fault zone are highly fractured rocks, microscopic to coarse-grained carbonate breccia, and siliciclastic cataclasite. Microscopic deformation includes twinning in carbonate rocks, deformation banding, undulose extinction, and strain-induced polygonization in quartz, tectonic stylolites, extension veining, microfractures, and grain-scale cataclasis. Data are consistent with models relating the Ste. Genevieve fault zone to left-lateral oblique slip possibly associated with New Madrid tectonism.« less

New Mexico structural zone - An analogue of the Colorado mineral belt

USGS Publications Warehouse

Sims, P.K.; Stein, H.J.; Finn, C.A.

2002-01-01

Updated aeromagnetic maps of New Mexico together with current knowledge of the basement geology in the northern part of the state (Sangre de Cristo and Sandia-Manzano Mountains)-where basement rocks were exposed in Precambrian-cored uplifts-indicate that the northeast-trending Proterozoic shear zones that controlled localization of ore deposits in the Colorado mineral belt extend laterally into New Mexico. The shear zones in New Mexico coincide spatially with known epigenetic precious- and base-metal ore deposits; thus, the mineralized belts in the two states share a common inherited basement tectonic setting. Reactivation of the basement structures in Late Cretaceous-Eocene and Mid-Tertiary times provided zones of weakness for emplacement of magmas and conduits for ore-forming solutions. Ore deposits in the Colorado mineral belt are of both Late Cretaceous-Eocene and Mid-Tertiary age; those in New Mexico are predominantly Mid-Tertiary in age, but include Late Cretaceous porphyry-copper deposits in southwestern New Mexico. The mineralized belt in New Mexico, named the New Mexico structural zone, is 250-km wide. The northwest boundary is the Jemez subzone (or the approximately equivalent Globe belt), and the southeastern boundary was approximately marked by the Santa Rita belt. Three groups (subzones) of mineral deposits characterize the structural zone: (1) Mid-Tertiary porphyry molybdenite and alkaline-precious-metal deposits, in the northeast segment of the Jemez zone; (2) Mid-Tertiary epithermal precious-metal deposits in the Tijeras (intermediate) zone; and (3) Late Cretaceous porphyry-copper deposits in the Santa Rita zone. The structural zone was inferred to extend from New Mexico into adjacent Arizona. The structural zone provides favorable sites for exploration, particularly those parts of the Jemez subzone covered by Neogene volcanic and sedimentary rocks. ?? 2002 Published by Elsevier Science B.V.

Inherited weaknesses control deformation in the flat slab region of Central Argentina

NASA Astrophysics Data System (ADS)

Stevens, A.; Carrapa, B.; Larrovere, M.; Aciar, R. H.

2015-12-01

The Sierras Pampeanas region of west-central Argentina has long been considered a geologic type-area for flat-slab induced thick-skinned deformation. Frictional coupling between the horizontal subducting plate and South American lithosphere from ~12 Ma to the present provides an obvious causal mechanism for the basement block uplifts that characterize this region. New low temperature thermochronometry data show basement rocks from the central Sierras Pampeanas (~ longitude 66 ̊ W) including Sierras Cadena de Paiman, Velasco and Mazan retain a cooling history of Paleozoic - Mesozoic tectonics events. Results from this study indicate that less than 2 km of basement has been exhumed since at least the Mesozoic. These trends recorded by both apatite fission track (AFT) and apatite helium (AHe) thermochronometry suggest that recent Mio-Pliocene thick-skinned deformation associated with flat-slab subduction follow inherited zones of weakness from Paleozoic terrane sutures and shear zones and Mesozoic rifting. If a Cenozoic foreland basin exisited in this region, its thickness was minimal and was controlled by paleotopography. Pre-Cenozoic cooling ages in these ranges that now reach as high as 4 km imply significant exhumation of basement rocks before the advent of flat slab subduction in the mid-late Miocene. It also suggests that thick-skinned deformation associated with flat slab subduction may at least be facilitated by inherited crustal-scale weaknesses. At the most, pre-existing zones of weakness may be required in regions of thick-skinned deformation. Although flat-slab subduction plays an important role in the exhumation of the Sierras Pampeanas, it is likely not the sole mechanism responsible for thick-skinned deformation in this region. This insight sheds light on the interpretation of modern and ancient regions of thick-skinned deformation in Cordilleran systems.

The White Nile as a source for Nile sediments: Assessment using U-Pb geochronology of detrital rutile and monazite

NASA Astrophysics Data System (ADS)

Be'eri-Shlevin, Yaron; Avigad, Dov; Gerdes, Axel

2018-04-01

Basement terranes exposed at the headwaters of the White Nile include Archean-Paleoproterozoic rocks of the Congo Craton, whose northern sectors were severely reworked during Neoproterozoic orogeny. New U-Pb analyses of detrital rutile and monazite from early Quaternary to Recent coastal quartz sands of Israel, at the northeast extension of the Nile sedimentary system, yield mostly late Neoproterozoic ages, with a dominant peak at ca. 600 Ma. While derivation from the reworked sectors of the Craton cannot be negated, the absence of pre-Neoproterozoic rutile and monazite indicates that the detrital contribution from the Congo cratonic nuclei into the main Nile was insignificant. The near absence of White Nile basement-derived heavy minerals from the Nile sands arriving at the Eastern Mediterranean may be explained by a number of factors such as relatively minor erosion of the Cratonic basement nuclei during the Quaternary, late connection of the White Nile to the main Nile system with a possibility that northern segments connected prior to more southerly ones, and a long-term effective sediment blockage mechanism at the mouth of White Nile. Likewise, our previous study demonstrated that Nile sands display a detrital zircon U-Pb-Hf pattern consistent with significant recycling of NE African Paleozoic sediments. It is thus plausible that any detrital contribution from White Nile basement rocks was thoroughly diluted by eroded Paleozoic sediments, or their recycled products, which were likely the greatest sand reservoir in the region. This study adds to previous studies showing the advantage of a multi mineral U-Pb geochronology strategy in constraining sediment provenance patterns.

Thermal history of the Pan-African basement under the Jurassic Marib-Shabwa Basin, Yemen

NASA Astrophysics Data System (ADS)

Rice, A. Hugh N.; Schneider, David; Veeningen, Resi; Grasemann, Bernhard; Decker, Kurt

2013-04-01

Pan-African tectonism within the Arabian Nubian Shield in Yemen is very poorly known. New drill-cores from the Marib-Shabwa Basin (Habban oil field) from central Yemen penetrated 600 m into the pre-Jurassic crystalline basement, providing a unique opportunity to extend our understanding of Pan-African events in Yemen. The cores were obtained some 80 km NE of the exposure limit of the Al Bayda Terrane, which lies SE of Sana'a. This terrane, which has no direct correlative in the ANS further north in Saudi Arabia, comprises deformed greenschist facies acid to basic volcanic rocks later witnessing acid to basic magmatism and has been previously interpreted as a Pan-African island arc complex with a basement component. Ophiolite fragments are common, both within the terrane and at its margins (sutures). To the north lies the Abas Gneiss Terrane and to the south the Al Mahfid Gneiss Terrane; both consist of older pre-Pan-African crystalline basement rocks. Geochemistry of a red, undeformed granite from the drill core indicates an A-type composition. LA-ICPMS U-Pb analysis of granite zircons gave two concordant age populations: 628.3 ± 3.1 Ma (large & small zircons) and 604.9 ± 2.0 Ma (intermediate sized zircons). The former age is interpreted as the time of crystallization, within the range of other A-type Younger Granites in the ANS, and the latter age as constraining lower temperature dissolution-reprecipitation of zircon, due to hydrothermal fluids or melt remobilization. Nd Tdm model ages for two granite samples from the drill core both gave ages of 1.24 Ga, within the range of the Al Bayda Terrane (1.2-2.5 Ga) and outside the range of the adjacent Palaeoproterozoic gneissic terranes (1.7-2.3 Ga, Abas Gneiss Terrane; 1.8-3.0 Ga, Al Mahfid Gneiss Terrane). Thus it seems certain that the Al Bayda Terrane extends at least 80 km to the NE of its present surface exposure. Rb-Sr biotite ages from the granite indicate closure through ~300°C at 593 Ma, indicating fast cooling either as a result of near-surface conditions of emplacement or rapid exhumation. Zircon (U-Th)/He cooling ages (~180°C) are constrained to the Early Carboniferous. The youngest (Cenozoic) set of veins contain pyrite, (saddle) dolomite and calcite. Pyrite sulphur isotopes, the occurrence of saddle dolomite and calcite fluid inclusions provide strong evidence that the Pan-African basement was not re-heated to above 150°C after the last stage of deformation; this places some constraint on the thickness of the sedimentary cover that was deposited.

New U-Pb zircon geochronology of the Choma-Kalomo Block (Zambia) and the Dete-Kamativi Inlier (Zimbabwe), with implications for the extent of the Zimbabwe Craton.

NASA Astrophysics Data System (ADS)

Glynn, Sarah; Wiedenbeck, Michael; Master, Sharad; Frei, Dirk

2015-04-01

The Choma-Kalomo Block is a north-east trending, Mesoproterozoic terrane located in southern Zambia. It is composed of as yet undated gneissic basement with a high-grade metamorphosed supracrustal metasedimentary sequence, which is intruded by hornblende granites and gneisses of the Choma-Kalomo Batholith, that is dated between ca. 1.37 and 1.18 Ga. Our new zircon U-Pb age data on metasedimentary rocks of the Choma-Kalomo Block identifies samples of different ages, with slightly different provenances. The oldest metasedimentary rock is a muscovite-biotite schist, which has only Palaeoproterozoic detrital zircons, the two age clusters around 2.03-2.02 Ga and 1.8-1.9 Ga, correspond to the ages of granitic intrusion, and metamorphism, in the Magondi Mobile Belt on the western side of the Archaean Zimbabwe Craton. The second sample is a garnetiferous paragneiss, which contains both Palaeoproterozoic (2.04 Ga), and Mesoproterozoic zircons, ca. 1.36 Ga, derived from the granites of the Choma-Kalomo Batholith. The third sample is a biotite-muscovite schist, in which the detrital zircon ages fall into four separate clusters: ca. 3.39 Ga, ca. 2.7-2.6 Ga, ca. 2.1-1.7 Ga (with a peak at ca. 1.18 Ga), and 1.55 - 1.28 Ga. The Archaean zircons in this sample are derived from the Zimbabwe Craton, while the Palaeoproterozoic samples come from the Magondi belt, and the youngest zircons come from both phases of the Choma-Kalomo Batholith. A possible connection between the Choma-Kalomo Block and the Dete-Kamativi Inlier - some 150 km to the south-east in western Zimbabwe - has been proposed on the basis of similarities in the nature of their Sn-Ta-muscovite pegmatite mineralisation. The Dete-Kamativi Inlier, which is part of the Magondi Mobile Belt, is a window into Palaeoproterozoic north-east trending belts of deformed and metamorphosed supracrustal rocks. By dating localities which we suspect form the basement to the surrounding younger sediments, along with selected pegmatites from within the inlier itself; we have concluded that the Choma-Kalomo Block and Dete-Kamativi Inlier are, in fact, coeval. Preliminary results for a number of these granites and gneisses give ages between 2.05 and 2.02 Ga; correlating well with the 2.03-2.02 Ga ages of detrital zircons from the Choma-Kalomo Block. While these basement rocks are not Archaean in age, we have identified Archaean aged zircons in both the Choma-Kalomo Block and the Dete-Kamativi Inlier, making them the Western most occurrences of Archaean granitoids, implying that the Zimbabwe Craton extends much further west under the Magondi Belt than previously thought.

The mineralogical, chemical, and chronological characteristics of the crystalline Apollo 16 impact melt rocks

NASA Technical Reports Server (NTRS)

Reimold, W. U.; Reimold, J. N.

1984-01-01

A comparative review of mineralogical, chemical, and chronological data on crystalline Apollo 16 impact melt rocks is presented. The use of such data to identify distinct impact melt complex is discussed, and 22 distinct impact melt bodies are identified. The recently detected group of feldspathic microporphyritic (FM) melt rocks was tested for chemical and isotopic homogeneity; instrumental neutron activation analysis and new Rb-Sr isotopic whole rock data indicate that FMs were probably not derived from a single impact melt sheet, but might be representative of the Descartes basement. Stratigraphical and chronological concepts for the geological development of the landing site are discussed, and a model is presented for the formation of the Cayley Plains and the Descartes formation.

Structure of the Tucson Basin, Arizona from gravity and aeromagnetic data

USGS Publications Warehouse

Rystrom, Victoria Louise

2003-01-01

Interpretation of gravity and high-resolution aeromagnetic data reveal the three-dimensional geometry of the Tuscson Basin, Arizona and the lithology of its basement. Limited drill hole and seismic data indicate that the maximum depth to the crystalline basement is approximately 3600 meters and that the sedimentary sequences in the upper ~2000 m of the basin were deposited during the most recent extensional episode that commenced about 13 Ma. The negative density contrasts between these upper Neogene and Quaternary sedimentary sequences and the adjacent country rock produce a Bouguer residual gravity low, whose steep gradients clearly define the lateral extent of the upper ~2000m of the basin. The aeromagnetic maps show large positive anomalies associated with deeply buried, late Cretaceous-early Tertiary and mid-Tertiary igneous rocks at and below the surface of the basin. These magnetic anomalies provide insight into the older (>13 Ma) and deeper structures of the basin. Simultaneous 2.5-dimensional modeling of both gravity and magnetic anomalies constrained by geologic and seismic data delineates the thickness of the basin and the dips of the buried faults that bound the basin. This geologic-based forward modeling approach to using geophysical data is shown to result in more information about the geologic and tectonic history of the basin as well as more accurate depth to basement determinations than using generalized geophysical inversion techniques.

High-resolution gravity and seismic-refraction surveys of the Smoke Tree Wash area, Joshua Tree National Park, California

USGS Publications Warehouse

Langenheim, Victoria E.; Rymer, Michael J.; Catchings, Rufus D.; Goldman, Mark R.; Watt, Janet T.; Powell, Robert E.; Matti, Jonathan C.

2016-03-02

We describe high-resolution gravity and seismic refraction surveys acquired to determine the thickness of valley-fill deposits and to delineate geologic structures that might influence groundwater flow beneath the Smoke Tree Wash area in Joshua Tree National Park. These surveys identified a sedimentary basin that is fault-controlled. A profile across the Smoke Tree Wash fault zone reveals low gravity values and seismic velocities that coincide with a mapped strand of the Smoke Tree Wash fault. Modeling of the gravity data reveals a basin about 2–2.5 km long and 1 km wide that is roughly centered on this mapped strand, and bounded by inferred faults. According to the gravity model the deepest part of the basin is about 270 m, but this area coincides with low velocities that are not characteristic of typical basement complex rocks. Most likely, the density contrast assumed in the inversion is too high or the uncharacteristically low velocities represent highly fractured or weathered basement rocks, or both. A longer seismic profile extending onto basement outcrops would help differentiate which scenario is more accurate. The seismic velocities also determine the depth to water table along the profile to be about 40–60 m, consistent with water levels measured in water wells near the northern end of the profile.

3. AERIAL VIEW, LOOKING SOUTH, OF BUILDING 371 BASEMENT UNDER ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

3. AERIAL VIEW, LOOKING SOUTH, OF BUILDING 371 BASEMENT UNDER CONSTRUCTION. THE BASEMENT HOUSES HEATING, VENTILATION, AND AIR CONDITIONING EQUIPMENT AND MECHANICAL UTILITIES, THE UPPER PART OF THE PLUTONIUM STORAGE VAULT AND MAINTENANCE BAY, AND SMALL PLUTONIUM PROCESSING AREAS. THE BASEMENT LEVEL IS DIVIDED INTO NEARLY EQUAL NORTH AND SOUTH PARTS BY THE UPPER PORTION OF THE PLUTONIUM STORAGE VAULT. (10/7/74) - Rocky Flats Plant, Plutonium Recovery Facility, Northwest portion of Rocky Flats Plant, Golden, Jefferson County, CO

Basement Basalts from IODP Site 1438, Amami-Sankaku Basin: Implications for Sources and Melting Processes during Subduction Initiation in the Izu-Bonin-Mariana System

NASA Astrophysics Data System (ADS)

McCarthy, A. J.; Hickey-Vargas, R.; Yogodzinski, G. M.; Ishizuka, O.; Hocking, B.; Bizimis, M.; Savov, I. P.; Kusano, Y.; Arculus, R. J.

2016-12-01

IODP Expedition 351 Site 1438 is located in the Amami-Sankaku basin, just west of the Kyushu-Palau Ridge (KPR), a remnant of the early Izu-Bonin-Mariana (IBM) volcanic arc. 150 meters of basement basalt were drilled beneath 1460 m of volcaniclastic sediments and sedimentary rock. The age range inferred for these basalts is 51-52 Ma, close to the 48-52 Ma age of basalts associated with subduction initiation in the IBM forearc (forearc basalts or FABs). Site 1438 basement basalts form several distinct subunits, all relatively mafic (MgO = 6-14 %; Mg# = 51-83). Non-fluid-mobile incompatible trace element patterns are profoundly depleted. Sm/Nd (0.34-0.43) and Lu/Hf (0.18-0.37) reach values higher than most normal MORBs while La/Yb (0.31-0.98) and Ti/V (15.8-27.0) are lower. These features are shared with basalts drilled just west of the KPR at ODP Site 1201 and DSDP Site 447, and many FABs. Abundances of fluid-mobile incompatible elements vary together and are correlated with subunits defined by flow margins and rock physical properties, suggesting control by post-eruptive seawater alteration rather than varying inputs of subduction fluids. Hf-Nd isotopes for Site 1438 basement basalts range from (present-day) ɛNd of 7.0 to 9.5 and ɛHf of 14.5 to 19.8 in a well-correlated array. Their more radiogenic Hf-isotope character could indicate an Indian-type MORB source, however, basalts with ɛHf >16.5, are more radiogenic than many Indian MORB. Pb isotope data will help distinguish differing mantle source domains and origins for fluid-mobile elements. Overall, the combined geochemical data indicate that the mantle source of basement basalts in drill sites west of the KPR (1438, 1201, 447) are closely similar to those for FAB, and that as a group, these rocks are more depleted than more than 90% of global MORB. Our interpretation is that both IBM forearc basalts and basalts from drill sites immediately west of the KPR formed by melting of the same uniquely depleted mantle source during subduction initiation. Melting may have been promoted by rapid decompression and by flux melting with a solute-poor hydrous subduction fluid. These basalts were erupted over a broad area in an extensional setting, which later narrowed as subduction and the subduction-related IBM volcanic arc became established.

Impact of climate changes during the last 5 million years on groundwater in basement aquifers.

PubMed

Aquilina, Luc; Vergnaud-Ayraud, Virginie; Les Landes, Antoine Armandine; Pauwels, Hélène; Davy, Philippe; Pételet-Giraud, Emmanuelle; Labasque, Thierry; Roques, Clément; Chatton, Eliot; Bour, Olivier; Ben Maamar, Sarah; Dufresne, Alexis; Khaska, Mahmoud; Le Gal La Salle, Corinne; Barbecot, Florent

2015-09-22

Climate change is thought to have major effects on groundwater resources. There is however a limited knowledge of the impacts of past climate changes such as warm or glacial periods on groundwater although marine or glacial fluids may have circulated in basements during these periods. Geochemical investigations of groundwater at shallow depth (80-400 m) in the Armorican basement (western France) revealed three major phases of evolution: (1) Mio-Pliocene transgressions led to marine water introduction in the whole rock porosity through density and then diffusion processes, (2) intensive and rapid recharge after the glacial maximum down to several hundred meters depths, (3) a present-day regime of groundwater circulation limited to shallow depth. This work identifies important constraints regarding the mechanisms responsible for both marine and glacial fluid migrations and their preservation within a basement. It defines the first clear time scales of these processes and thus provides a unique case for understanding the effects of climate changes on hydrogeology in basements. It reveals that glacial water is supplied in significant amounts to deep aquifers even in permafrosted zones. It also emphasizes the vulnerability of modern groundwater hydrosystems to climate change as groundwater active aquifers is restricted to shallow depths.

Delineation of tectonic provinces of New York state as a component of seismic-hazard evaluation

USGS Publications Warehouse

Fakundiny, R.H.

2004-01-01

Seismic-hazard evaluations in the eastern United States must be based on interpretations of the composition and form of Proterozoic basement-rock terranes and overlying Paleozoic strata, and on factors that can cause relative movements among their units, rather than Phanerozoic orogenic structures, which may be independent of modern tectonics. The tectonic-province concept is a major part of both probabilistic and deterministic seismic-hazard evaluations, yet those that have been proposed to date have not attempted to geographically correlate modern earthquakes with regional basement structure. Comparison of basement terrane (megablock) boundaries with the spatial pattern of modern seismicity may lead to the mechanically sound definition of tectonic provinces, and thus, better seismic-hazard evaluation capability than is currently available. Delineation of megablock boundaries will require research on the many factors that affect their structure and movement. This paper discusses and groups these factors into two broad categories-megablock tectonics in relation to seismicity and regional horizontal-compressive stresses, with megablock tectonics divided into subcategories of basement, overlying strata, regional lineaments, basement tectonic terranes, earthquake epicenter distribution, and epeirogeny, and compressive stresses divided into pop-ups and the contemporary maximum horizontal-compressive stress field. A list presenting four to nine proposed research topics for each of these categories is given at the end.

Petrophysics Features of the Hydrocarbon Reservoirs in the Precambrian Crystalline Basement

NASA Astrophysics Data System (ADS)

Plotnikova, Irina

2014-05-01

A prerequisite for determining the distribution patterns of reservoir zones on the section of crystalline basement (CB) is the solution of a number of problems connected with the study of the nature and structure of empty spaces of reservoirs with crystalline basement (CB) and the impact of petrological, and tectonic factors and the intensity of the secondary transformation of rocks. We decided to choose the Novoelhovskaya well # 20009 as an object of our research because of the following factors. Firstly, the depth of the drilling of the Precambrian crystalline rocks was 4077 m ( advance heading - 5881 m) and it is a maximum for the Volga-Urals region. Secondly, petrographic cut of the well is made on core and waste water, and the latter was sampled regularly and studied macroscopically. Thirdly, a wide range of geophysical studies were performed for this well, which allowed to identify promising areas of collector with high probability. Fourth, along with geological and technical studies that were carried out continuously (including washing and bore hole redressing periods), the studies of the gaseous component of deep samples of clay wash were also carried out, which indirectly helped us estimate reservoir properties and fluid saturation permeable zones. As a result of comprehensive analysis of the stone material and the results of the geophysical studies we could confidently distinguish 5 with strata different composition and structure in the cut of the well. The dominating role in each of them is performed by rocks belonging to one of the structural-material complexes of Archean, and local variations in composition and properties are caused by later processes of granitization on different stages and high temperature diaphthoresis imposed on them. Total capacity of reservoir zones identified according to geophysical studies reached 1034.2 m, which corresponds to 25.8% of the total capacity of 5 rock masses. However, the distribution of reservoirs within the cut is uneven. The manifestation of reservoir properties of crystalline rocks and their gas content is to a high degree connected with those parts of the cut of the well that are represented by Bolshecheremshanskaya series of rocks. The analysis of the distribution of reservoir intervals that were identified in the well section # 20009 according to the geophysical studies showed that they tend to coincide with the intervals of intensive secondary changes and rock breaking, or with contacts of series of rocks or thick layers of rocks that differ greatly in physical and mechanical properties. About half of the potential reservoir zones are characterized by explicit, well-defined fractures, which was determined according to core and wastewater samples, as well as with the help of caliper gauge. The rocks of a Bolshecheremshanskaya series were more exposed to the repeated impact of the parallel processes (mylonitization, diaphtoresis, migmatization, etc.), or they were simply more affected by these processes, and that led to the characteristic distribution of collector areas, temperature and gas anomalies along the borehole cut. The presence of Bolshecheremshanskaya quartz series in the material composition of the rocks caused, firstly, increased amount of fractures, and secondly, the preservation of the porous-cavernous space frame within the superimposed secondary processes.

High-Resolution Aeromagnetic Survey To Image Shallow Faults, Poncha Springs and Vicinity, Chaffee County, Colorado

USGS Publications Warehouse

Grauch, V.J.S.; Drenth, Benjamin J.

2009-01-01

High-resolution aeromagnetic data were acquired over the town of Poncha Springs and areas to the northwest to image faults, especially where they are concealed. Because this area has known hot springs, faults or fault intersections at depth can provide pathways for upward migration of geothermal fluids or concentrate fracturing that enhances permeability. Thus, mapping concealed faults provides a focus for follow-up geothermal studies. Fault interpretation was accomplished by synthesizing interpretative maps derived from several different analytical methods, along with preliminary depth estimates. Faults were interpreted along linear aeromagnetic anomalies and breaks in anomaly patterns. Many linear features correspond to topographic features, such as drainages. A few of these are inferred to be fault-related. The interpreted faults show an overall pattern of criss-crossing fault zones, some of which appear to step over where they cross. Faults mapped by geologists suggest similar crossing patterns in exposed rocks along the mountain front. In low-lying areas, interpreted faults show zones of west-northwest-, north-, and northwest-striking faults that cross ~3 km (~2 mi) west-northwest of the town of Poncha Springs. More easterly striking faults extend east from this juncture. The associated aeromagnetic anomalies are likely caused by magnetic contrasts associated with faulted sediments that are concealed less than 200 m (656 ft) below the valley floor. The faults may involve basement rocks at greater depth as well. A relatively shallow (<300 m or <984 ft), faulted basement block is indicated under basin-fill sediments just north of the hot springs and south of the town of Poncha Springs.

The thermal environment of Cascadia Basin

NASA Astrophysics Data System (ADS)

Johnson, H. Paul; Hautala, Susan L.; Bjorklund, Tor A.

2012-07-01

Located adjacent to the NE Pacific convergent boundary, Cascadia Basin has a global impact well beyond its small geographic size. Composed of young oceanic crust formed at the Juan de Fuca Ridge, igneous rocks underlying the basin are partially insulated from cooling of their initial heat of formation by a thick layer of pelagic and turbidite sediments derived from the adjacent North American margin. The igneous seafloor is eventually consumed at the Cascadia subduction zone, where interactions between the approaching oceanic crust and the North American continental margin are partially controlled by the thermal environment. Within Cascadia Basin, basement topographic relief varies dramatically, and sediments have a wide range of thickness and physical properties. This variation produces regional differences in heat flow and basement temperatures for seafloor even of similar age. Previous studies proposed a north-south thermal gradient within Cascadia Basin, with high geothermal flux and crustal temperatures measured in the heavily sedimented northern portion near Vancouver Island and lower than average heat flux and basement temperatures predicted for the central and southern portions of the basin. If confirmed, this prediction has implications for processes associated with the Cascadia subduction zone, including the location of the "locked zone" of the megathrust fault. Although existing archival geophysical data in the central and southern basin are sparse, nonuniformly distributed, and derived from a wide range of historical sources, a substantial N-S geothermal gradient appears to be confirmed by our present compilation of combined water column and heat flow measurements.

Petroleum geology and resources of the Amu-Darya basin, Turkmenistan, Uzbekistan, Afghanistan, and Iran

USGS Publications Warehouse

Ulmishek, Gregory F.

2004-01-01

The Amu-Darya basin is a highly productive petroleum province in Turkmenistan and Uzbekistan (former Soviet Union), extending southwestward into Iran and southeastward into Afghanistan. The basin underlies deserts and semideserts north of the high ridges of the Kopet-Dag and Bande-Turkestan Mountains. On the northwest, the basin boundary crosses the crest of the Karakum regional structural high, and on the north the basin is bounded by the shallow basement of the Kyzylkum high. On the east, the Amu-Darya basin is separated by the buried southeast spur of the Gissar Range from the Afghan-Tajik basin, which is deformed into a series of north-south-trending synclinoria and anticlinoria. The separation of the two basins occurred during the Neogene Alpine orogeny; earlier, they were parts of a single sedimentary province. The basement of the Amu-Darya basin is a Hercynian accreted terrane composed of deformed and commonly metamorphosed Paleozoic rocks. These rocks are overlain by rift grabens filled with Upper Permian-Triassic rocks that are strongly compacted and diagenetically altered. This taphrogenic sequence, also considered to be a part of the economic basement, is overlain by thick Lower to Middle Jurassic, largely continental, coal-bearing rocks. The overlying Callovian-Oxfordian rocks are primarily carbonates. A deep-water basin surrounded by shallow shelves with reefs along their margins was formed during this time and reached its maximum topographic expression in the late Oxfordian. In Kimmeridgian-Tithonian time, the basin was filled with thick evaporites of the Gaurdak Formation. The Cretaceous-Paleogene sequence is composed chiefly of marine clastic rocks with carbonate intervals prominent in the Valanginian, Barremian, Maastrichtian, and Paleocene stratigraphic units. In Neogene time, the Alpine orogeny on the basin periphery resulted in deposition of continental clastics, initiation of new and rejuvenation of old faults, and formation of most structural traps. A single total petroleum system is identified in the Amu-Darya basin. The system is primarily gas prone. Discovered gas reserves are listed by Petroconsultants (1996) at about 230 trillion cubic feet, but recent discoveries and recent reserve estimates in older fields should increase this number by 40 to 50 trillion cubic feet. Reserves of liquid hydrocarbons (oil and condensate) are comparatively small, less than 2 billion barrels. Most of the gas reserves are concentrated in two stratigraphic intervals, Upper Jurassic carbonates and Neocomian clastics, each of which contains about one-half of the reserves. Reserves of other stratigraphic units?from Middle Jurassic to Paleogene in age?are relatively small. Source rocks for the gas are the Lower to Middle Jurassic clastics and coal and Oxfordian basinal black shales in the east-central part of the basin. The latter is probably responsible for the oil legs and much of the condensate in gas pools. Throughout most of the basin both source-rock units are presently in the gas-window zone. Traps are structural, paleogeomorphic, and stratigraphic, as well as a combination of these types. The giant Dauletabad field is in a combination trap with an essential hydrodynamic component. Four assessment units were identified in the total petroleum system. One unit in the northeastern, northern, and northwestern marginal areas of the basin and another in the southern marginal area are characterized by wide vertical distribution of hydrocarbon pools in Middle Jurassic to Paleocene rocks and the absence of the salt of the Gaurdak Formation. The other two assessment units are stratigraphically stacked; they occupy the central area of the basin and are separated by the regional undeformed salt seal of the Gaurdak Formation. The largest part of undiscovered hydrocarbon resources of the Amu-Darya basin is expected in older of these assessment units. The mean value of total assessed resources of the Amu-Darya basin is estimated

Late Cretaceous tectonothermal evolution of the southern Lhasa terrane, South Tibet: Consequence of a Mesozoic Andean-type orogeny

NASA Astrophysics Data System (ADS)

Dong, Xin; Zhang, Ze-ming; Klemd, Reiner; He, Zhen-yu; Tian, Zuo-lin

2018-04-01

The Lhasa terrane of the southern Tibetan Plateau participated in a Mesozoic Andean-type orogeny caused by the northward subduction of the Neo-Tethyan oceanic lithosphere. However, metamorphic rocks, which can unravel details of the geodynamic evolution, are rare and only exposed in the south-eastern part of the Lhasa terrane. Therefore, we conducted a detailed petrological, geochemical and U-Pb zircon geochronological study of the late Cretaceous metamorphic rocks and associated gabbros from the Nyemo inlier of the southern Lhasa terrane. The Nyemo metamorphic rocks including gneisses, schists, marbles and calc-silicate rocks, experienced peak amphibolite-facies contact metamorphism under P-T conditions of 3.5-4.0 kbar and 642-657 °C with a very high geothermal gradient of 45-50 °C/km, revealing a distinct deflection from the steady-state geotherm during low-pressure metamorphism. Inherited magmatic zircon cores from the metamorphic rocks yielded protolith ages of 197-194 Ma, while overgrowth zircon rims yielded metamorphic ages of ca. 86 Ma. Whole-rock chemistry and zircon Hf isotopes suggest that the protoliths of the gneisses and schists are andesites and tuffs of the early Jurassic Sangri Group, which were derived from a depleted mantle source of a continental arc affinity. The coeval intimately-associated gabbro (ca. 86 Ma) crystallized under P-T conditions of 3.5-5.3 kbar and 914-970 °C, supplying the heat flux high enough to cause the contact metamorphism of the Sangri Group rock types. We propose that the intrusion of the gabbro and a simultaneous pressure increase of up to 4.0 kbar, which is related to crustal thickening due to crustal overthrusting and the intrusion of mafic material, resulted in the late Cretaceous metamorphism of the early Jurassic Sangri Group during an Andean-type orogeny. Furthermore the Nyemo metamorphic rocks, which have previously been considered to represent slivers of the Precambrian metamorphic basement of the Lhasa terrane, are late Cretaceous metamorphic supracrustal rocks.

The Pongola structure of southeastern Africa - The world's oldest preserved rift?

NASA Technical Reports Server (NTRS)

Burke, K.; Kidd, W. S. F.; Kusky, T. M.

1985-01-01

Rocks of the Pongola Supergroup form an elongate belt in the Archean Kaapvaal Craton of southern Africa. Because these rocks exhibit many features that are characteristic of rocks deposited in continental rifts, including rapid lateral variations in thickness and character of sediments, volcanic rocks that are bimodal in silica content, coarse, basement derived conglomerates and thick sequences of shallow water sedimentary facies associations, it is suggested that the Pongola Supergroup was deposited in such a rift. The age of these rocks (approximately 3.0 Ga) makes the Pongola structure the world's oldest well-preserved rift so far recognized, and comparison of the Pongola Rift with other rifts formed more recently in earth history reveals striking similarities, suggesting that the processes that formed this rift were not significantly different from those that form continental rifts today.

A 'Propagating' Active Across-Arc Normal Fault Shows Rupture Process of the Basement: the Case of the Southwestern Ryukyu Arc

NASA Astrophysics Data System (ADS)

Matsumoto, T.; Shinjo, R.; Nakamura, M.; Kubo, A.; Doi, A.; Tamanaha, S.

2011-12-01

Ryukyu Arc is located on the southwestern extension of Japanese Island-arc towards the east of Taiwan Island along the margin of the Asian continent off China. The island-arc forms an arcuate trench-arc-backarc system. A NW-ward subduction of the Philippine Sea Plate (PSP)at a rate of 6-8 cm/y relative to the Eurasian Plate (EP) causes frequent earthquakes. The PSP is subducting almost normally in the north-central area and more obliquely around the southwestern area. Behind the arc-trench system, the Okinawa Trough (OT) was formed by back-arc rifting, where active hydrothermal vent systems have been discovered. Several across-arc submarine faults are located in the central and southern Ryukyu Arc. The East Ishigaki Fault (EIF) is one of the across-arc normal faults located in the southwestern Ryukyu Arc, ranging by 44km and extending from SE to NW. This fault was surveyed by SEABAT8160 multibeam echo sounder and by ROV Hyper-Dolphin in 2005 and 2008. The result shows that the main fault consists of five fault segments. A branched segment from the main fault was also observed. The southernmost segment is most mature (oldest but still active) and the northernmost one is most nascent. This suggests the north-westward propagation of the fault rupture corresponding to the rifting of the southwestern OT and the southward retreat of the arc-trench system. Considering that the fault is segmented and in some part branched, propagation might take place episodically rather than continuously from SE to NW. The ROV survey also revealed the rupture process of the limestone basement along this fault from the nascent stage to the mature stage. Most of the rock samples collected from the basement outcrop were limestone blocks (or calcareous sedimentary rocks). Limestone basement was observed to the west on the hanging wall far away from the main fault scarp. Then fine-grained sand with ripple marks was observed towards the main scarp. Limestone basement was observed on the main scarp and on the footwall. These suggest that basically the both sides are composed of the same material, that the whole study area is characterised by Ryukyu limestone exposure and that the basement was split by the across-arc normal fault. Coarse-grained sand and gravels/rubbles were observed towards and on the trough of the fault. On the main scarp an outcrop of limestone basement was exposed and in some part it was broken into rubbles. These facts suggest that crash of the basement due to rupturing is taking place repeatedly on the scarp and the trough. The observed fine-grained sand on the hanging wall might be the final product by the process of the crash of the limestone basement.

Geologic map of the Harvard Lakes 7.5' quadrangle, Park and Chaffee Counties, Colorado

USGS Publications Warehouse

Kellogg, Karl S.; Lee, Keenan; Premo, Wayne R.; Cosca, Michael A.

2013-01-01

The Harvard Lakes 1:24,000-scale quadrangle spans the Arkansas River Valley in central Colorado, and includes the foothills of the Sawatch Range on the west and Mosquito Range on the east. The Arkansas River valley lies in the northern end of the Rio Grande rift and is structurally controlled by Oligocene and younger normal faults mostly along the west side of the valley. Five separate pediment surfaces were mapped, and distinctions were made between terraces formed by the Arkansas River and surfaces that formed from erosion and alluviation that emanated from the Sawatch Range. Three flood deposits containing boulders as long as 15 m were deposited from glacial breakouts just north of the quadrangle. Miocene and Pliocene basin-fill deposits of the Dry Union Formation are exposed beneath terrace or pediment deposits in several places. The southwestern part of the late Eocene Buffalo Peaks volcanic center, mostly andesitic breccias and flows and ash-flow tuffs, occupy the northeastern corner of the map. Dated Tertiary intrusive rocks include Late Cretaceous or early Paleocene hornblende gabbro and hornblende monzonite. Numerous rhyolite and dacite dikes of inferred early Tertiary or Late Cretaceous age also intrude the basement rocks. Basement rocks are predominantly Mesoproterozoic granites, and subordinately Paleoproterozoic biotite gneiss and granitic gneiss.

Oil/source rock correlations in the Polish Flysch Carpathians and Mesozoic basement and organic facies of the Oligocene Menilite Shales: Insights from hydrous pyrolysis experiments

USGS Publications Warehouse

Curtis, John B.; Kotarba, M.J.; Lewan, M.D.; Wieclaw, D.

2004-01-01

The Oligocene Menilite Shales in the study area in the Polish Flysch Carpathians are organic-rich and contain varying mixtures of Type-II, Type-IIS and Type-III kerogen. The kerogens are thermally immature to marginally mature based on atomic H/C ratios and Rock-Eval data. This study defined three organic facies, i.e., sedimentary strata with differing hydrocarbon-generation potentials due to varying types and concentrations of organic matter. These facies correspond to the Silesian Unit and the eastern and western portions of the Skole Unit. Analysis of oils generated by hydrous pyrolysis of outcrop samples of Menilite Shales demonstrates that natural crude oils reservoired in the flysch sediments appear to have been generated from the Menilite Shales. Natural oils reservoired in the Mesozoic basement of the Carpathian Foredeep appear to be predominantly derived and migrated from Menilite Shales, with a minor contribution from at least one other source rock most probably within Middle Jurassic strata. Definition of organic facies may have been influenced by the heterogeneous distribution of suitable Menilite Shales outcrops and producing wells, and subsequent sample selection during the analytical phases of the study. ?? 2004 Elsevier Ltd. All rights reserved.

Laboratory electrical resistivity analysis of geologic samples from Fort Irwin, California: Chapter E in Geology and geophysics applied to groundwater hydrology at Fort Irwin, California

USGS Publications Warehouse

Bloss, Benjamin R.; Bedrosian, Paul A.; Buesch, David C.

2015-01-01

Correlating laboratory resistivity measurements with geophysical resistivity models helps constrain these models to the geology and lithology of an area. Throughout the Fort Irwin National Training Center area, 111 samples from both cored boreholes and surface outcrops were collected and processed for laboratory measurements. These samples represent various lithologic types that include plutonic and metamorphic (basement) rocks, lava flows, consolidated sedimentary rocks, and unconsolidated sedimentary deposits that formed in a series of intermountain basins. Basement rocks, lava flows, and some lithified tuffs are generally resistive (≥100 ohm-meters [Ω·m]) when saturated. Saturated unconsolidated samples are moderately conductive to conductive, with resistivities generally less than 100 Ω·m, and many of these samples are less than 50 Ω·m. The unconsolidated samples can further be separated into two broad groups: (1) younger sediments that are moderately conductive, owing to their limited clay content, and (2) older, more conductive sediments with a higher clay content that reflects substantial amounts of originally glassy volcanic ash subsequently altered to clay. The older sediments are believed to be Tertiary. Time-domain electromagnetic (TEM) data were acquired near most of the boreholes, and, on the whole, close agreements between laboratory measurements and resistivity models were found. 

Florida: A Jurassic transform plate boundary

USGS Publications Warehouse

Klitgord, Kim D.; Popenoe, Peter; Schouten, Hans

1984-01-01

Magnetic, gravity, seismic, and deep drill hole data integrated with plate tectonic reconstructions substantiate the existence of a transform plate boundary across southern Florida during the Jurassic. On the basis of this integrated suite of data the pre-Cretaceous Florida-Bahamas region can be divided into the pre-Jurassic North American plate, Jurassic marginal rift basins, and a broad Jurassic transform zone including stranded blocks of pre-Mesozoic continental crust. Major tectonic units include the Suwannee basin in northern Florida containing Paleozoic sedimentary rocks, a central Florida basement complex of Paleozoic age crystalline rock, the west Florida platform composed of stranded blocks of continental crust, the south Georgia rift containing Triassic sedimentary rocks which overlie block-faulted Suwannee basin sedimentary rocks, the Late Triassic-Jurassic age Apalachicola rift basin, and the Jurassic age south Florida, Bahamas, and Blake Plateau marginal rift basins. The major tectonic units are bounded by basement hinge zones and fracture zones (FZ). The basement hinge zone represents the block-faulted edge of the North American plate, separating Paleozoic and older crustal rocks from Jurassic rifted crust beneath the marginal basins. Fracture zones separate Mesozoic marginal sedimentary basins and include the Blake Spur FZ, Jacksonville FZ, Bahamas FZ, and Cuba FZ, bounding the Blake Plateau, Bahamas, south Florida, and southeastern Gulf of Mexico basins. The Bahamas FZ is the most important of all these features because its northwest extension coincides with the Gulf basin marginal fault zone, forming the southern edge of the North American plate during the Jurassic. The limited space between the North American and the South American/African plates requires that the Jurassic transform zone, connecting the Central Atlantic and the Gulf of Mexico spreading systems, was located between the Bahamas and Cuba FZ's in the region of southern Florida. Our plate reconstructions combined with chronostratigraphic and lithostratigraphic information for the Gulf of Mexico, southern Florida, and the Bahamas indicate that the gulf was sealed off from the Atlantic waters until Callovian time by an elevated Florida-Bahamas region. Restricted influx of waters started in Callovian as a plate reorganization, and increased plate separation between North America and South America/Africa produced waterways into the Gulf of Mexico from the Pacific and possibly from the Atlantic.

U-Pb geochronology and Hf-Nd isotope compositions of the oldest Neoproterozoic crust within the Cadomian orogen: new evidence for a unique juvenile terrane

NASA Astrophysics Data System (ADS)

Samson, S. D.; D'Lemos, R. S.; Blichert-Toft, J.; Vervoort, J.

2003-03-01

New U-Pb dates, combined with Nd and Hf isotopic data, from rocks within the Port Morvan area of the Baie de St Brieuc region of Brittany identify a unique portion of the Neoproterozoic Cadomia terrane. Two gneisses near Port Morvan yielded U-Pb dates of 754.6±0.8 Ma and 746.0±0.9 Ma, ages that are more than 130 Myr older than the oldest units formed during the main phase of early Cadomian magmatism. Two trondhjemite boulders from the monogenetic facies of the Cesson conglomerate yielded identical ages of 665.2±0.5 Ma and 665.5±0.7 Ma, and a cobble from the polygenetic facies yields a 207Pb- 206Pb date of 637±2 Ma. Individual detrital zircons from a sandstone associated with the Cesson conglomerates yield concordant U-Pb dates ranging from 650±3 Ma to 624.1±0.6 Ma. Initial ɛNd values for the rocks in this region range from +5.0 to +6.6, indicative of a substantial input from depleted mantle. Initial ɛHf values determined on zircons from these Neoproterozoic rocks, including the detrital zircons, range from +6.7 to +14.5, consistent with the Nd isotopic results. Maximum initial ɛHf values for two 2 Ga Icartian gneisses, considered basement to Cadomia, average +8.4 and +8.7. In contrast to the results of the Port Morvan rocks, 616-608 Ma syn-tectonic intrusions from Normandy and the British Channel Islands all have negative initial ɛNd values (-10.4 to -8.3) consistent with significant contamination by ancient crust such as the 2 Ga gneisses. The oldest arc-related magmas should have interacted most extensively with Cadomian basement, buffering younger mantle-derived magmas that were generated in subsequent magmatic episodes. The rocks within the Port Morvan region are thus inconsistent as examples of the earliest Cadomian intrusions as they show no evidence of interaction with 2 Ga basement. Instead, the older ages and mantle-like isotopic composition of these rocks suggest they are part of an independent terrane that formed prior to, and independently from, the Cadomian arc. Possible terrane-scale structural boundaries have recently been identified, including the newly recognized Port Morvan thrust fault and the NW-dipping Main Cadomian thrust.

Genetic Aspects of Gold Mineralization at Some Occurrences in the Eastern Desert of Egypt

NASA Astrophysics Data System (ADS)

Abd El Monsef, M.; Slobodník, M.; Salem, I. A.

2012-04-01

The Eastern Desert of Egypt is well known as a gold-mining area since ancient times, there're more than 95 gold deposits and occurrences spread the whole area covered by the basement rocks of Precambrian age. The basement rocks of the Eastern Desert of Egypt constitute the Nubian Shield that has formed a continuous part of the Arabian-Nubian Shield before the opening of Red Sea (Oligocene-Early Miocene). Commonly, the system of gold-bearing quartz veins in the Eastern Desert is clearly structural controlled related to brittle-ductile shear zones that mostly developed during late deformational stages of the evolution history for basement rocks in the Eastern Desert. This running study principally aims to contribute the mineral resource potential of the gold deposits in Egypt, so particularly Fatira, Gidami and Atalla occurrences have been involved into a comprehensive study based on field, structural, mineralogical, geochemical and genetic investigations. It is intended to better understanding for the characteristics, distribution controls, conditions and age of mineralization in relation to the age of the hosting rocks intrusion to find if there're genetic links between the gold mineralization and the evolution of the host intrusive complex. Several authors suggested that the gold mineralization was related to the intrusion of the (postorogenic) Younger granites. Other authors interpret these deposits as products of hydrothermal activity induced either by metamorphism or cooling effects of early Paleozoic magmatism or as combined metamorphic/magmatic episodes. The prime focus will be directed to the ore itself and the associated hydrothermal alteration zones based on detailed maps and well-distributed samples network and geochemical anomalies distribution. The laboratory studies included microscopic examination (reflecting and transmitting microscopy) to allow for determination of the hosting rocks types and mineralogical changes related to the gold mineralization in each area and revealing the ore mineralogy and the ore textures, geochemical analyses (including rare earth elements) are to be used in order to determine the tectonic setting and magmatic evolution of the host intrusions, scanning electron microscope, microprobe analysis, stable isotopes and fluid inclusions will serve as a new part of this study in detection of the origin and the physico-chemical conditions (P-T condition) for the gold precipitation, Age dating of the host intrusion and mineralization will be based on K-Ar for dating potassium-bearing minerals in fresh host rocks and hydrothermal mineral phases.

Petrology and geochronology of Mesoproterozoic basement of the Mount Rogers area of southwestern Virginia and northwestern North Carolina: Implications for the Precambrian tectonic evolution of the southern Blue Ridge province

USGS Publications Warehouse

Tollo, Richard P.; Aleinikoff, John N.; Dickin, Alan P.; Radwany, Molly S.; Southworth, C. Scott; Fanning, C. Mark

2017-01-01

Results from new geologic mapping, SHRIMP U-Pb geochronology, and petrologic studies indicate that Mesoproterozoic basement in the northern French Broad massif near Mount Rogers consists of multiple, mostly granitic plutons, map- and outcrop-scale xenoliths of pre-existing crustal rocks, and remnants of formerly overlying meta-sedimentary lithologies. Zircon and titanite ages demonstrate that these rocks collectively record nearly 350 m.y. of tectonic evolution including periods of igneous intrusion at ca. 1190 to 1130 Ma (Early Magmatic Suite) and ca. 1075 to 1030 Ma (Late Magmatic Suite) and three episodes of regional metamorphism at ca. 1170 to 1140, 1070 to 1020, and 1000 to 970 Ma. The existence of ca. 1.3 Ga age crust is indicated by (1) orthogranofels of ca. 1.32 Ga age in a map-scale xenolith, (2) inherited zircons of ca. 1.33 to 1.29 Ga age in Early Magmatic Suite plutons, and (3) ca. 1.36 to 1.30 Ga age detrital zircons in meta-sedimentary lithologies. Mineral assemblages developed in amphibolites and granofelses indicate that metamorphism during both Mesoproterozoic episodes occurred at upper amphibolite- to lower granulite-facies conditions. Syn-orogenic Early Magmatic Suite plutons emplaced at ca. 1190 to 1145 Ma are characterized by high-K, variably magnesian, dominantly calc-alkalic compositions, and have trace-element characteristics indicative of continental-arc magmatic origin involving melting of thick continental crust. In contrast, ca. 1140 Ma age quartz syenite displays A-type features indicating derivation from depleted crustal sources with increased mantle input during waning stages of regional contraction. Plutons of the compositionally bimodal Late Magmatic Suite include (1) ca. 1060 Ma meta-granite with geochemical characteristics transitional between silicic rocks of arc systems and post-collisional granites of A-type lineage, and (2) ca. 1055 Ma monzodioritic rocks with A-type compositional characteristics that likely reflect derivation from fertile, mafic sources in the lower crust. Collectively, these data suggest that Mesoproterozoic rocks of the study area preserve evidence of multiple orogenic episodes that likely involved continental-arc development and deformation at ca. 1150 Ma followed by crustal thickening at ca. 1060 Ma. Field relations and geochronologic data indicate that regional uplift and sedimentation occurred at ca. 1100 Ma between the two episodes of overlapping magmatism and orogenesis. The nature and timing of Mesoproterozoic events recorded in basement rocks of the study area illustrate significant differences in the lithologic assemblages and geologic history preserved by Mesoproterozoic basement of the adjacent Shenandoah and French Broad massifs, suggesting that the Blue Ridge massifs occupied different locations within the regional Grenville-age orogen until about 1070 Ma when the effects of Ottawan-age tectonics began to affect both areas. The near ubiquity of Ottawan-age orogenic activity recorded in Mesoproterozoic rocks of the Blue Ridge, other Appalachian inliers such as the New Jersey Highlands, and the Grenville province of Canada, including the Adirondacks, suggests that these formerly disparate terranes were amalgamated to form a common, regional orogen by this time.

Time evolution of a rifted continental arc: Integrated ID-TIMS and LA-ICPMS study of magmatic zircons from the Eastern Srednogorie, Bulgaria

NASA Astrophysics Data System (ADS)

Georgiev, S.; von Quadt, A.; Heinrich, C. A.; Peytcheva, I.; Marchev, P.

2012-12-01

Eastern Srednogorie in Bulgaria is the widest segment of an extensive magmatic arc that formed by convergence of Africa and Europe during Mesozoic to Tertiary times. Northward subduction of the Tethys Ocean beneath Europe in the Late Cretaceous gave rise to a broad range of basaltic to more evolved magmas with locally associated Cu-Au mineralization along this arc. We used U-Pb geochronology of single zircons to constrain the temporal evolution of the Upper Cretaceous magmatism and the age of basement rocks through which the magmas were emplaced in this arc segment. High precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) was combined with laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) for spatial resolution within single zircon grains. Three tectono-magmatic regions are distinguished from north to south within Eastern Srednogorie: East Balkan, Yambol-Burgas and Strandzha. Late Cretaceous magmatic activity started at ~ 92 Ma in the northernmost East Balkan region, based on stratigraphic evidence and limited geochronology, with the emplacement of minor shallow intrusions and volcanic rocks onto pre-Cretaceous basement. In the southernmost Strandzha region, magmatism was initiated at ~ 86 Ma with emplacement of gabbroic to dioritic intrusions and related dikes into metamorphic basement rocks that have previously been overprinted by Jurassic-Lower Cretaceous metamorphism. The Yambol-Burgas region is an extensional basin between the East Balkan and the Strandzha regions, which broadens and deepens toward the Black Sea further east and is filled with a thick pile of marine sediments and submarine extrusive volcanic rocks accompanied by coeval intrusions. This dominantly mafic magmatism in the intermediate Yambol-Burgas region commenced at ~ 81 Ma and produced large volumes of potassium-rich magma until ~ 78 Ma. These shoshonitic to ultrapotassic basaltic to intermediate magmas formed by differentiation of ankaramitic (high Ca) parental melts, produced from partial remelting of amphibole clinopyroxenites upon interaction with subduction-modified mantle wedge melts, according to earlier petrological studies. This peak of dominantly extrusive activity in the Yambol-Burgas region extended into the Strandzha region further south, in the form of numerous tholeiitic, calc-alkaline and high-K intrusions emplaced in the same time period between 81 and 78 Ma. Granitic rocks from exposed basement of Eastern Srednogorie zone are dated as Permian/Carboniferous (~ 275-300 Ma). Zircons with similar ages occur in Upper Cretaceous rocks from the East Balkan and Strandzha regions, indicating local incorporation as xenocrysts. In contrast, magmatic rocks from the intermediate Yambol-Burgas region contain mostly Ordovician (~ 460 Ma) or older inherited zircons, suggesting a either a different basement history or, more likely, a different level of magma storage and crustal assimilation. Integrating these geochronological results with a synthesis of the regional geology, we propose a two-stage geodynamic evolution for the Eastern Srednogorie segment of the Tethyan arc. The earlier stage of normal arc magmatism was driven by a southward slab retreat, which formed the ~ 92 Ma calc-alkaline to high-K shallow intrusions and volcanics in the north (East Balkan), 87-86 Ma old tholeiitic and calc-alkaline intrusions in the south (Strandzha), and the voluminous 81-78 Ma old gabbroic to granitic intrusions with predominantly calc-alkaline to high-K composition throughout the Strandzha region. This stage continued westward into the Central Srednogorie zone, where the southward younging of calc-alkaline magmatism correlates well with an increased input of primitive mantle melts, indicating asthenospheric incursion into a widening mantle wedge as a result of slab roll-back. The second stage proceeded in the Eastern Srednogorie zone only, where more extreme extension associated with the opening of the Black Sea back-arc basin led to the formation of an intra-arc rift in the Yambol-Burgas region, which now separates the East Balkan region from the Strandzha region. In this extensional environment, crustal thinning lead to decompression and increased heat flow, facilitating large-scale melting of lower crustal rocks and the formation of 81-78 Ma magmas. The unusual calcic composition of the parent magmas, their isotopic character and distinct xenocrystic population are consistent with a component of re-melting of hydrous lower-crustal cumulates, which probably formed in part during the first stage of the evolving arc.

Now and Long Ago at Gale Crater, Mars Illustration

NASA Image and Video Library

2016-12-13

This pair of drawings depicts the same location on Mars at two points in time: now and billions of years ago. The location is in Gale Crater, near the Red Planet's equator. Since August 2012, NASA's Curiosity Mars rover mission has been investigating rock layers in the crater floor and in the crater's central peak (Mount Sharp) for information recorded in the rocks about ancient environmental conditions and how they changed over time. Slide 1 shows a present-day snapshot of the northern half of Gale Crater. North is to the left. The underlying basement is the crust of Mars that forms the crater's rim (left) and central peak (right). About 3.5 billion years ago, rivers brought sediment into the crater, depositing pebbles where the river was flowing more quickly, sand where the river entered a standing body of water in the center of the basin, and silt within this lake. Lake level rose over time as the sediments built up. Eventually they were buried by dry dust. These sediments later turned into the conglomerate, sandstone, mudstone, and duststone rocks that Curiosity has found. Wind then carved the stack of deposits into the present shape of a mountain, which Curiosity is climbing as approximately shown. The basement rock fractured during the initial impact that formed the crater, and the later sediments fractured as they were buried. Slide 2 shows a snapshot in time when a lake was present in the crater. As on Earth, Martian lakes were the surface expression of a much larger lake and groundwater system. Spaces between grains and in fractures were saturated with water at levels below the water table (dashed blue line). This groundwater circulated due to gravity and the topography within and around the crater. In this case, groundwater pressurized under the nearby Martian highlands may have flowed into the crater, where it would be less confined. Groundwater also flowed downward from the lake. As the groundwater circulated, it drove chemical reactions that dissolved some minerals and precipitate others. Habitable environments in ancient Gale Crater -- identified during Curiosity's first year on Mars --expanded both in space and time beyond just the lakes. They extend throughout the subsurface where groundwater was present, and extended in time well after the lakes disappeared, when groundwater continued to circulate through the buried and fractured sediments. The unannotated figure and an animated gif are available at http://photojournal.jpl.nasa.gov/catalog/PIA21255

Yakataga fold-and-thrust belt: Structural geometry and tectonic implications of a small continental collision zone

NASA Astrophysics Data System (ADS)

Wallace, Wesley K.

Collision of the Yakutat terrane with southern Alaska created a collisional fold-and-thrust belt along the Pacific-North America plate boundary. This southerner fold-and-thrust belt formed within continental sedimentary rocks but with the seaward vergence and tectonic position typical of an accretionary wedge. Northward exposure of progressively older rocks reflects that the fold-and-thrust belt forms a southward-tapered orogenic wedge that increases northward in structural relief and depth of erosion. Narrow, sharp anticlines separate wider, flat-bottomed synclines. Relatively steep thrust faults commonly cut the forelimbs of anticlines. Fold shortening and fault displacement both generally increase northward, whereas fault dip generally decreases northward. The coal-bearing lower part of the sedimentary section serves as a detachment for both folds and thrust faults. The folded and faulted sedimentary section defines a regional south dip of about 8°. The structural relief combined with the low magnitude of shortening of the sedimentary section suggest that the underlying basement is structurally thickened. I propose a new interpretation in which this thickening was accommodated by a passive-roof duplex with basement horses that are separated from the overlying folded and thrust-faulted sedimentary cover by a roof thrust with a backthrust sense of motion. Basement horses are ˜7 km thick, based on the thickness between the inferred roof thrust and the top of the basement in offshore seismic reflection data. This thickness is consistent with the depth of the zone of seismicity onshore. The inferred zone of detachment and imbrication of basement corresponds with the area of surface exposure of the fold-and-thrust belt within the Yakutat terrane and with the Wrangell subduction zone and arc farther landward. By contrast, to the west, the crust of the Yakutat terrane has been carried down a subduction zone that extends far landward with a gentle dip, corresponding with a gap in arc magmatism, anomalous topography, and the rupture zone of the 1964 great southern Alaska earthquake. I suggest that, to the east, detachment and imbrication of basement combined with coupling in the fold-and-thrust belt allowed the delaminated dense mantle lithosphere to subduct with a steeper dip than to the west, where buoyant Yakutat terrane crust remains attached to the subducted lithosphere. According to this interpretation, the Wrangell subduction zone is lithosphere of the Yakutat terrane, not Pacific Ocean lithosphere subducted beneath the Yakutat terrane. The Pacific-North America plate boundary would be within the northern deformed part of the Yakutat terrane, not along the boundary between the undeformed southern part of the Yakutat terrane and oceanic crust of the Pacific Ocean. The plate boundary is an evolving zone of distributed deformation in which most of the convergent component has been accommodated within the fold-and-thrust belt south of the northern boundary of the Yakutat terrane, the Chugach-St. Elias thrust fault, and most of the right-lateral component likely has been accommodated on the Bagley Icefield fault just to the north.

Integrated Potential-field Studies in Support of Energy Resource Assessment in Frontier Areas of Alaska

NASA Astrophysics Data System (ADS)

Phillips, J. D.; Saltus, R. W.; Potter, C. J.; Stanley, R. G.; Till, A. B.

2008-05-01

In frontier areas of Alaska, potential-field studies play an important role in characterizing the geologic structure of sedimentary basins having potential for undiscovered oil and gas resources. Two such areas are the Yukon Flats basin in the east-central interior of Alaska, and the coastal plain of the Arctic National Wildlife Refuge (ANWR) in northeastern Alaska. The Yukon Flats basin is a potential source of hydrocarbon resources for local consumption and possible export. Knowledge of the subsurface configuration of the basin is restricted to a few seismic reflection profiles covering a limited area and one well. The seismic profiles were reprocessed and reinterpreted in preparation for an assessment of the oil and gas resources of the basin. The assessment effort required knowledge of the basin configuration away from the seismic profiles, as well as an understanding of the nature of the underlying basement. To extend the interpretation of the basin thickness across the entire area of the basin, an iterative Jachens-Moring gravity inversion was performed on gridded quasi-isostatic residual gravity anomaly data. The inversion was constrained to agree with the interpreted basement surface along the seismic profiles. In addition to the main sedimentary depocenter interpreted from the seismic data as having over 8 km of fill, the gravity inversion indicated a depocenter with over 7 km of fill in the Crooked Creek sub-basin. Results for the Crooked Creek sub-basin are consistent with magnetic and magnetotelluric modeling, but they await confirmation by drilling or seismic profiling. Whether hydrocarbon source rocks are present in the pre-Cenozoic basement beneath Yukon Flats is difficult to determine because extensive surficial deposits obscure the bedrock geology, and no deep boreholes penetrate basement. The color and texture patterns in a red-green-blue composite image consisting of reduced-to-the-pole aeromagnetic data (red), magnetic potential (blue), and basement gravity (green) highlight domains with common geophysical characteristics and, by inference, lithology. The observed patterns suggest that much of the basin is underlain by Devonian to Jurassic oceanic rocks that probably have little or no potential for hydrocarbon generation. The coastal plain surficial deposits in the northern part of ANWR conceal another frontier basin with hydrocarbon potential. Proprietary aeromagnetic and gravity data were used, along with seismic reflection profiles, to construct a structural and stratigraphic model of this highly deformed sedimentary basin for use in an energy resource assessment. Matched-filtering techniques were used to separate short-wavelength magnetic and gravity anomalies attributed to sources near the top of the sedimentary section from longer-wavelength anomalies attributed to deeper basin and basement sources. Models along the seismic reflection lines indicate that the primary sources of the short-wavelength anomalies are folded and faulted sedimentary beds truncated at the Pleistocene erosion surface. In map view, the aeromagnetic and gravity anomalies produced by the sedimentary units were used to identify possible structural trapping features and geometries, but they also indicated that these features may be significantly disrupted by faulting.

New evidence for an old idea: Geochronological constraints for a paired metamorphic belt in the central European Variscides

NASA Astrophysics Data System (ADS)

Will, T. M.; Schmädicke, E.; Ling, X.-X.; Li, X.-H.; Li, Q.-L.

2018-03-01

New geochronological data reveal a prolonged tectonothermal evolution of the Variscan Odenwald-Spessart basement, being part of the Mid-German Crystalline Zone in central Europe. We report the results from (i) secondary ion mass spectrometry (SIMS) U-Pb dating of zircon, rutile and monazite, (ii) SIMS zircon oxygen isotope analyses, (iii) laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) zircon Lu-Hf isotope analyses and, (iv) LA-ICPMS zircon and rutile trace element data for a suite of metamorphic rocks (five amphibolite- and eclogite-facies mafic meta-igneous rocks and one granulite-facies paragneiss). The protoliths of the mafic rocks formed from juvenile as well as depleted mantle sources in distinct tectonic environments at different times. Magmatism took place at a divergent oceanic margin (possibly in a back-arc setting) at 460 Ma, in an intraoceanic basin at ca. 445 Ma and at a continental margin at 329 Ma. Regardless of lithology, zircon in eclogite, amphibolite and high-temperature paragneiss provide almost identical Carboniferous ages of 333.7 ± 4.1 Ma (eclogite), 329.1 ± 1.8 to 328.4 ± 8.9 Ma (amphibolite), and 334.0 ± 2.0 Ma (paragneiss), respectively. Rutile yielded ages of 328.6 ± 4.7 and 321.4 ± 7.0 Ma in eclogite and amphibolite, and monazite in high-temperature paragneiss grew at 330.1 ± 2.4 Ma (all ages are quoted at the 2σ level). The data constrain coeval high-pressure eclogite- and high-temperature granulite-facies metamorphism of the Odenwald-Spessart basement at ca. 330 Ma. Amphibolite-facies conditions were attained shortly afterwards. The lower plate eclogite formed in a fossil subduction zone and the upper plate high-temperature, low-pressure rocks are the remains of an eroded Carboniferous magmatic arc. The close proximity of tectonically juxtaposed units of such radically different metamorphic conditions and thermal gradients is characteristic for a paired metamorphic belt sensu Miyashiro (1961). Thus, the Odenwald-Spessart basement represents the first recognised paired metamorphic belt in the European Variscides.

Geochemistry of contaminated komatiites from the Umburanas greenstone belt, Bahia State, Brazil

NASA Astrophysics Data System (ADS)

Menezes Leal, Angela Beatriz de; Santos, André Luis Dias; Bastos Leal, Luiz Rogério; Cunha, José Carlos

2015-08-01

The late Archaean Umburanas greenstone belt (UGB) is located in the São Francisco Craton, southwest of Bahia State, Brazil. The lower unit of UGB comprises basal komatiite lavas and tholeiitic basalts intercalated with felsic volcanic rocks. The regional crystalline basement rocks, the Gavião block, predominantly consist of granitic, granodioritic and migmatitic gneiss along with tonalite-trondhjemite-granodiorite (TTG) associations. Petrographic studies of UGB komatiites reveal characteristic spinifex igneous texture although primary mineralogy is rarely preserved. Based on textural relationships, komatiites are divided into cumulate, spinifex, and massive types. The MgO content varies in the range 31.5-40.4 wt%. The MgO-SiO2 negative correlation in komatiites suggests olivine fractionation trend. The UGB komatiites are of Al-undepleted type, characterized by Al2O3/TiO2 (21-48) ratio, enriched in highly incompatible LILE relative to moderately incompatible HFSE and distinct negative Nb, Sr and Eu anomalies. Also shows depletion of light rare earths, convex-downward rare earth patterns typically not observed in komatiites world-wide, and primitive mantle normalized Gd/Yb (1.03-1.23) and La/Sm (2.36-4.99) ratios. The negative Eu anomaly is attributed to the circulation of H2O-rich fluid, whereas the negative Nb and Sr anomalies are attributed to contamination from granitic basement rocks of the Gavião block. The UGB komatiites are most likely derived from adiabatic decompressional melting of a mantle plume. The melting took place at liquidus temperatures in the range 1572-1711 °C, which is consistent with mantle-plume origin invoked for several other komatiites in Archaean greenstone belts elsewhere. The melts were more likely generated at a depth shallower than 100 km (pressure < 2.5 GPa) where garnet was absent in the source mineralogy. Geochemical characteristics suggest contamination of primary melts with granitic basement rocks either during ascent of melt or during emplacement of magma in a continental basin setting. Greenschist to low-T amphibolite facies metamorphism at ˜2Ga may also have played a role in modifying the original komatiite petrography and composition.

Origins and evolution of rhyolitic magmas in the central Snake River Plain: insights from coupled high-precision geochronology, oxygen isotope, and hafnium isotope analyses of zircon

NASA Astrophysics Data System (ADS)

Colón, Dylan P.; Bindeman, Ilya N.; Wotzlaw, Jörn-Frederik; Christiansen, Eric H.; Stern, Richard A.

2018-02-01

We present new high-precision CA-ID-TIMS and in situ U-Pb ages together with Hf and O isotopic analyses (analyses performed all on the same grains) from four tuffs from the 15-10 Ma Bruneau-Jarbidge center of the Snake River Plain and from three rhyolitic units from the Kimberly borehole in the neighboring 10-6 Ma Twin Falls volcanic center. We find significant intrasample diversity in zircon ages (ranges of up to 3 Myr) and in δ18O (ranges of up to 6‰) and ɛHf (ranges of up to 24 ɛ units) values. Zircon rims are also more homogeneous than the associated cores, and we show that zircon rim growth occurs faster than the resolution of in situ dating techniques. CA-ID-TIMS dating of a subset of zircon grains from the Twin Falls samples reveals complex crystallization histories spanning 104-106 years prior to some eruptions, suggesting that magma genesis was characterized by the cyclic remelting of buried volcanic rocks and intrusions associated with previous magmatic episodes. Age-dependent trends in zircon isotopic compositions show that rhyolite production in the Yellowstone hotspot track is driven by the mixing of mantle-derived melts (normal δ18O and ɛHf) and a combination of Precambrian basement rock (normal δ18O and ɛHf down to - 60) and shallow Mesozoic and Cenozoic age rocks, some of which are hydrothermally altered (to low δ18O values) by earlier stages of Snake River Plain magmatism. These crustal melts hybridize with juvenile basalts and rhyolites to produce the erupted rhyolites. We also observe that the Precambrian basement rock is only an important component in the erupted magmas in the first eruption at each caldera center, suggesting that the accumulation of new intrusions quickly builds an upper crustal intrusive body which is isolated from the Precambrian basement and evolves towards more isotopically juvenile and lower-δ18O compositions over time.

Geochemical compositions of Neoproterozoic to Lower Palaeozoic (?) shales and siltstones in the Volta Basin (Ghana): Constraints on provenance and tectonic setting

NASA Astrophysics Data System (ADS)

Amedjoe, Chiri G.; Gawu, S. K. Y.; Ali, B.; Aseidu, D. K.; Nude, P. M.

2018-06-01

Many researchers have investigated the provenance and tectonic setting of the Voltaian sediments using the geochemistry of sandstones in the basin. The shales and siltstones in the basin have not been used much in the provenance studies. In this paper, the geochemistry of shales and siltstones in the Kwahu Group and Oti Group of the Voltaian Supergroup from Agogo and environs in the southeastern section of the basin has constrained the provenance and tectonic setting. Trace element ratios La/Sc, Th/Sc and Cr/Th and REEs sensitive to average source compositions revealed sediments in the shales and siltstones may mainly be from felsic rocks, though contributions from old recycled sediments and some andesitic rock sediments were identified. The felsic rocks may be granites and/or granodiorites. Some intermediate rocks of andesitic composition are also identified, while the recycled sediments were probably derived from the basement metasedimentary rocks. The enrichment of light REE (LaN/YbN c. 7.47), negative Eu anomalies (Eu/Eu* c. 0.59), and flat heavy REE chondrite-normalized patterns, denote an upper-continental-crustal granitic source materials for the sediments. Trace-element ternary discriminant diagrams reveal passive margin settings for sediments, though some continental island arc settings sediments were also depicted. Mixing calculations based on REE concentrations and modeled chondrite-normalized REE patterns suggest that the Birimian basement complex may be the source of detritus in the Voltaian Basin. REEs are more associated with shales than siltstones. On this basis chondrite-normalized REE patterns show that shale lithostratigraphic units may be distinguished from siltstone lithostratigraphic units. The significant variability in shales elemental ratios can therefore be used to distinguish between shales of the Oti Group from that of the Kwahu Group.

Homogeneous impact melts produced by a heterogeneous target?. Sr-Nd isotopic evidence from the Popigai crater, Russia

NASA Astrophysics Data System (ADS)

Kettrup, B.; Deutsch, A.; Masaitis, V. L.

The 35.7 ± 0.2 Ma old Popigai crater, Siberia, with a diameter of about 100 km is one of the best preserved large terrestrial impact structures. The heterogeneous target at the impact site consists of Archean to Lower Proterozoic metamorphic rocks of the crystalline basement, Upper Proterozoic quartzites and other clastic deposits, as well as Cambrian to Cretaceous clastic sediments and sedimentary rocks, including carbonate rocks. Moreover, Proterozoic and Permo-Triassic dolerite dykes are found in the target area. We report major element, Sr and Nd isotope data for 13 of these target rocks and for various types of impactites. The 15 analysed impactite samples include tagamites (impact melt rocks), suevites and impact glass from small veins. Furthermore, two impact breccias and two impact glass-coated gneiss bombs were analysed. We discuss the relation of these impactites to the target lithologies, and evaluate on the basis of literature data the relation of microkrystites (and associated microtektites) in Upper Eocene sediments to the Popigai event. The impactites have SiO 2 abundances ranging from 59 to 66 wt.% and show significant variations in the content of Fe, Ca, and Ti. They have present day 87Sr/ 86Sr ratios between 0.7191 and 0.7369. Their Sr model ages T SrUR range from 1.9 to 2.3 Ga. The 143Nd/ 144Nd ratios for the impactite samples cluster between 0.5113 and 0.5115. The Nd model ages T NdCHUR range from 1.9 to 2.1 Ga. In an ɛ CHUR(Nd)-ɛ UR(Sr) diagram, the impactites and Upper Eocene microkrystites (and associated microtektites) plot in a field delimited by Popigai target lithologies. The impactites are restricted to the field of crystalline basement rocks and Upper Proterozoic quartzites, but they show different isotopic signatures in different crater sectors. Impactites and Upper Eocene microkrystites plot in different, only partly overlapping clusters. The leucocratic microkrystites and microtektites have a higher affinity to the post-Proterozoic rocks in the target area than the impactites. Seemingly, the melanocratic microkrystites originated mostly from crystalline basement. This data alignment supports the assumption that Popigai is the source crater for all three types of ejecta. For the first time, clear relations are established of the geochemically variable Upper Eocene microkrystites and associated microtektites to specific target lithologies at Popigai crater. Finally, the observed range in Sr and Nd isotope parameters determined for impact melt lithologies that originated during the Popigai event show a much higher variability than known from other craters. This result indicates that mixing of impact melt which later formed tagamite sheets and glass particles in different impact breccias, was incomplete at the time of ejecta dispersal.

Stratigraphy of Slick Rock district and vicinity, San Miguel and Dolores Counties, Colorado

USGS Publications Warehouse

Shawe, Daniel R.; Simmons, George C.; Archbold, Norbert L.

1968-01-01

The Slick Rock district covers about 570 square miles in western San Miguel and Dolores Counties, in southwestern Colorado. It is at the south edge of the salt-anticline region of southwestern Colorado and southeastern Utah and of the Uravan mineral belt.Deposition of Paleozoic sedimentary rocks in the district and vicinity was principally controlled by development of the Paradox Basin, and of Mesozoic rocks by development of a depositional basin farther west. The Paleozoic rocks generally are thickest at the northeast side of the Paradox Basin in a northwest- trending trough which seems to be a wide graben in Precambrian igneous and metamorphic basement rocks; Mesozoic rocks generally thicken westward and southwestward from the district.Sedimentary rocks rest on a Precambrian basement consisting of a variety of rocks, including granite and amphibolite. The surface of the Precambrian rocks is irregular and generally more than 2,000 feet below sea level and 7,000-11,000 feet below the ground surface. In the northern part of the district the Precambrian surface plunges abruptly northeastward into the trough occupying the northeast side of the Paradox Basin, and in the southern part it sags in a narrow northeasterly oriented trough. Deepening of both troughs, or crustal deformation in their vicinity, influenced sedimentation during much of late Paleozoic and Mesozoic time.The maximum total thickness of sedimentary rocks underlying the district is 13,000 feet, and prior to extensive erosion in the late Tertiary and the Quaternary it may have been as much as about 18,000 feet. The lower 5,000 feet or more of the sequence of sedimentary rocks consists of arenaceous strata of early Paleozoic age overlain by dominantly marine carbonate rocks and evaporite beds interbedded with lesser amounts of clastic sediments of late Paleozoic age. Overlying these rocks is about 4,500 feet of terrestrial clastic sediments, dominantly sandstone with lesser amounts of shale, mudstone, siltstone, and conglomerate, of late Paleozoic and Mesozoic age. Above these rocks is as much as 2,300 feet of marine shale of late Mesozoic age. Perhaps about 5,000 feet of clastic sedimentary rocks, dominantly sandstone and in part shale, of late Mesozoic and early Cenozoic age, overlay the older rocks of the district prior to late Cenozoic erosion...Outside the Slick Rock district the Mancos Shale is overlain by dominantly terrestrial sandstone, mudstone, and coaly beds of the Mesaverde Group of Late Cretaceous age, and younger units such as the Wasatch and Green River Formations of Tertiary age, which once may have extended across the district. These units, totaling possibly 5,000 feet in thickness, were removed by erosion following middle Tertiary uplift of the Colorado Plateau.Igneous rocks of Tertiary age crop out in only one small area in the district, but they are intruded extensively in the Mancos Shale east of the district, and, as shown by deep oil test wells, appear to be intruded widely in the Paradox Member of the Hermosa Formation in the southern part of the district and southeast of the district. Andesite porphyry occurs in a dike on Glade Mountain, microgranogabbro and microgranodiorite occur in thin sills east of the district, and rocks of similar composition form thick sills in the subsurface. All are similar chemically to igneous rocks in the San Juan Mountains southeast of the district and probably were the result of a specific igneous episode. They were intruded most likely during the Miocene.Surficial deposits of Quaternary age include glacial till, terrace gravels, alluvial fans, landslide debris, loess, other soil, alluvium, colluvium, and talus. On Glade Mountain, glacial till of probable early Pleistocene age merges westward with terrace gravels that are correlative with terrace gravels which lie on an old weathered surface of Mancos Shale farther west on the rim of the Dolores River Canyon.

Geo-Thermochronometric Insights on the Cycladic Basement and Cycladic Blueschist Unit Contact in the Southern Cyclades, Ios Island, Greece

NASA Astrophysics Data System (ADS)

Flansburg, M. E.; Stockli, D. F.; Poulaki, E. M.; Soukis, K. I.

2017-12-01

The North Cycladic Detachment System, the West Cycladic Detachment System, and the Naxos-Paros Detachment accommodated large-scale Oligo-Miocene exhumation in the backarc of the retreating Hellenic subduction zone. While bivergent detachment faults in the northern and western Cyclades are either contained within the Cycladic Blueschist Unit (CBU) or at the CBU-Upper Unit interface, the sheared contact between the CBU and the underlying Cycladic Basement in the southern Cyclades (Ios) has been debated for over 30 years, largely due to the ambiguous coexistence of both top-to-the-N and top-to-the-S shear sense indicators and a lack of robust timing information. Reliable chronostratigraphic and thermal history constraints allow us to test whether the contact is a low-angle normal fault-possibly part of a larger detachment system-or the South Cycladic Thrust by placing absolute ages on deformation, determining older over younger relationships or vice versa, and quantifying possible differential exhumation during Cenozoic extension. Zircon U-Pb dating for the granitic Basement core of Ios gave Carboniferous-Permian age and shows that surrounding Basement metasedimentary units can be divided into two groups based on detrital zircon signatures. An older group of metasedimentary rocks have maximum depositional ages (MDAs) ranging from 450 Ma to 354 Ma and predate the intrusions, and late Permian Basement paragneisses are younger than the intrusions and likely originally deposited unconformably on the older units. Samples from the CBU in northern Ios yielded MDAs ranging from Mid-Jurassic to Late Cretaceous and appear to be repeated due to either thrusting or subduction accretion and exhibit older over younger relationships. MDA data from mapped CBU at the southern end of Ios yielded Ordovician to Permian ages, calling into question their assignment as CBU, while also revealing older over younger relationships. Zircon (U-Th)/He ages for the Basement and the CBU on Ios are 9-14 Ma and do not exhibit any differential cooling-suggesting that they were juxtaposed prior to Miocene detachment faulting and exhumed together in response to top-to-the-N detachment faulting. This is supported by the fact that both units experienced Eocene subduction metamorphism as evidenced by 60-45 Ma metamorphic zircon rims.

Cretaceous crust beneath SW Borneo: U-Pb dating of zircons from metamorphic and granitic rocks

NASA Astrophysics Data System (ADS)

Davies, L.; Hall, R.; Armstrong, R.

2012-12-01

Metamorphic basement rocks from SW Borneo are undated but have been suggested to be Palaeozoic. This study shows they record low pressure 'Buchan-type' metamorphism and U-Pb SHRIMP dating of zircons indicates a mid-Cretaceous (volcaniclastic) protolith. SW Borneo is the southeast promontory of Sundaland, the continental core of SE Asia. It has no sedimentary cover and the exposed basement has been widely assumed to be a crustal fragment from the Indochina-China margin. Metamorphic rocks of the Pinoh Group in Kalimantan (Indonesian Borneo) are intruded by granitoid rocks of Jurassic-Cretaceous age, based on K-Ar dating, suggesting emplacement mainly between 130 and 80 Ma. The Pinoh metamorphic rocks have been described as a suite of pelitic schists, slates, phyllites, and hornfelses, and have not been dated, although they have been correlated with rocks elsewhere in Borneo of supposed Palaeozoic age. Pelitic schists contain biotite, chlorite, cordierite, andalusite, quartz, plagioclase and in some cases high-Mn almandine-rich garnet. Many have a shear fabric associated with biotite and fibrolite intergrowth. Contact metamorphism due to intrusion of the granitoid rocks produced hornfelses with abundant andalusite and cordierite porphyroblasts. Granitoids range from alkali-granite to tonalite and contain abundant hornblende and biotite, with rare white mica. Zircons from granitoid rocks exhibit sector- and concentric- zoning; some have xenocrystic cores mantled by magmatic zircon. There are four important age populations at c. 112, 98, 84 and 84 Ma broadly confirming earlier dating studies. There is a single granite body with a Jurassic age (186 ± 2.3 Ma). Zircons from pelitic metamorphic rocks are typically euhedral, with no evidence of rounding or resorbing of grains; a few preserve volcanic textures. They record older ages than those from igneous rocks; U-Pb ages are Cretaceous with a major population between 134 and 110 Ma. A single sample contains Proterozoic and Phanerozoic zircons. The metamorphic rocks from SW Borneo are not an ancient core to the island as previously assumed. We propose that extensive arc volcanism produced fine grained volcanogenic sediments during the Early Cretaceous deposited on, or reworking, older crust. These sediments were subjected to low pressure 'Buchan-type' metamorphism soon after deposition. Magmatism continued into the Late Cretaceous, resulting in contact metamorphism.

NASA Astrophysics Data System (ADS)

2014-12-01

The crystalline basement rocks of Ethiopia were traditionally described as one system of regional aquiclude. This attribution was made disregarding variations in groundwater occurrence and potential which often times is promising in some geologic settings. Systematic studies addressing their genesis and spatial variations are lacking. Based on a thorough review of existing data and field observations, this work has shown that the genesis of basement aquifers is the result of complex interplay between the present/past climate and geomorphic processes which are tectonically controlled. It thus follows that the groundwater occurrence and the type of aquifer exhibit important contrasts on the surfaces of crystalline basement terrains of Ethiopia. Three coherent zones have been identified in this work based on their genesis, thickness of regolith, mechanisms of flow and storage properties: (a) in Western Ethiopia the aquifer is characterized by a vertical profile of fractured low to high grade bedrocks mantled by thick weathering profiles leading to high groundwater storage but low hydraulic conductance, (b) in Northern Ethiopia the weathered mantle is stripped to negligible thickness; groundwater occurs in high conducting but low storage fractured low grade bedrocks, (c) in the Borena lowlands (the southern basement region, the occurrence of groundwater is associated with wadi beds. The orientations of wadi beds follow regional fractures. These fractures control groundwater flow regime and enhance preferential weathering of bedrocks. The presence of alluvial sediments (mostly derived from gneiss and inselbergs of gneisses and granites) over the weathered mantle, facilitates infiltration into the weathered mantle and fractured bedrocks underneath. This enhances groundwater storage and movement both in the regolith and fractured bedrock. Elsewhere outside the wadi beds, duri crusts limit vertical recharge and groundwater availability to the bedrock; aquifers are of intermediate type with regard to hydraulic properties. Potential remnants of weathered mantle are still visible but contribute little to groundwater flow. It is therefore suggested here that more comprehension about groundwater in crystalline basement rocks of Ethiopia could be gained given the comparison is made based on the genesis of the aquifers as related to tectonics and climate induced stripping and deep weathering history.

Fault tectonics and earthquake hazards in the Peninsular Ranges, Southern California. [including San Diego River, Otay Mts., Japatul Valley, Barrett Lake, Horsethief Canyon, Pine Valley Creek, Pine Creek, and Mojave Desert

NASA Technical Reports Server (NTRS)

Merifield, P. M. (Principal Investigator)

1975-01-01

The author has identified the following significant results. Thin sections of rock exposed along the San Diego River linear were prepared and determined to be fault breccia. Single band and ratio images of the western Mojave Desert were prepared from the multispectral scanner digital tapes. Subtle differences in color of soil and rock are enhanced on the ratio images. Two north-northeast trending linears (Horsethief Canyon and Pine Valley Creek) and an east-west linear (Pine Creek) were concluded to have resulted from erosion along well-developed foliation in crystalline basement rocks.

The western Wabigoon Subprovince, Superior Province, Canada: Archean greenstone succession in rifted basement complex

NASA Technical Reports Server (NTRS)

Edwards, G. R.; Davis, D. W.

1986-01-01

The Wabigoon Subprovince, interposed between the predominantly metasedimentary-plutonic and gneissic English River and Quetico Subprovinces to the north and south respectively, exposed Archean greenstone and granitoid rocks for a strike length of greater than 700 km. Based on predominating rock types, the western part of the subprovince is divided into two terrains: the northern Wabigoon volcano-sedimentary and pluonic terrain (NWW) and the Wabigoon Diapiric Axis terrain (WDA). Both the NWW and WDA are described according to volcanic sequence, geological faults, chemical composition and evolutionary history.

The Alberhill and other clay deposits of Temescal Canyon, Riverside County, California

USGS Publications Warehouse

Daviess, Steven Norman; Bramlette, M.N.

1953-01-01

Clay is mined in open pits by several companies in the Alberhill district, and the refractory clays of relatively high alumina sediment are used largely for fire brick. The Alberhill Coal and Clay Company is the largest operator and has produced a little over 2,000,000 tons of clay, of which nearly half was the refractory type. The clay occurs at the contact of the lower Tertiary and the Mesozoic basement complex. The weathered surface of basement rocks includes much clay of high iron and low alumina content, and the better clay occurs in the basal Tertiary sediments. The clay deposits vary rather abruptly in thickness and quality, and only local lenses contain workable deposits. Structural deformation makes dips of 10 to 20 degrees common and the clay strata therefore pitch under excessive overburden in short distances. Extensive deposits of thick alluvial fan deposits cover the clay-bearing strata over most of the area, and add to the overburden problems. The apparent lack of clay deposits of good quality that would total several million tons of ore, and the geological conditions that would make exploration and mining difficult and expensive make this district unpromising.

Hydrogeology and simulation of groundwater flow in fractured-rock aquifers of the Piedmont and Blue Ridge Physiographic Provinces, Bedford County, Virginia

USGS Publications Warehouse

McCoy, Kurt J.; White, Bradley A.; Yager, Richard M.; Harlow, George E.

2015-09-11

A steady-state groundwater-flow simulation for Bedford County was developed to test the conceptual understanding of flow in the fractured-rock aquifers and to compute a groundwater budget for the four major drainages: James River, Smith Mountain and Leesville Lakes, Goose Creek, and Big Otter River. Model results indicate that groundwater levels mimic topography and that minimal differences in aquifer properties exist between the Proterozoic basement crystalline rocks and Late Proterozoic-Cambrian cover crystalline rocks. The Big Otter River receives 40.8 percent of the total daily groundwater outflow from fractured-rock aquifers in Bedford County; Goose Creek receives 25.8 percent, the James River receives 18.2 percent, and Smith Mountain and Leesville Lakes receive 15.2 percent. The remaining percentage of outflow is attributed to pumping from the aquifer (consumptive use).

Geologic Map of the Clark Peak Quadrangle, Jackson and Larimer Counties, Colorado

USGS Publications Warehouse

Kellogg, Karl S.; Ruleman, Chester A.; Shroba, Ralph R.; Braddock, William A.

2008-01-01

The Clark Peak quadrangle encompasses the southern end of the Medicine Bow Mountains and the northernmost end of the Mummy Range. The Continental Divide traverses the map area and Highway 14 cross the Divide at Cameron Pass, in the southeastern corner of the map. Approximately the eastern half of the map, and a few areas to the west, are underlain by Early Proterozoic plutonic and metamorphic rocks. Most of these basement rocks are part of the ~1,715 Ma Rawah batholith, composed mostly of pinkish, massive to moderately foliated monzogranite and granodiorite intruded by numerous, large pegmatite- aplite bodies. The metamorphic rocks, many of which form large inclusions in the granitic rocks of the Rawah batholith, include biotite-hornblende gneiss, hornblende gneiss, amphibolite, and biotite schist. The crystalline basement rocks are thrust westward along the Medicine Bow thrust over a sequence of sedimentary rocks as old as the Upper Permian Satanka Shale. The Satanka Shale, Middle and Lower Triassic Chugwater group, and a thin sandstone tentatively correlated with the Lower Jurassic and Upper Triassic Jelm Formation are combined as one map unit. This undivided unit is overlain sequentially upward by the Upper Jurassic Sundance Formation, Upper Jurassic Morrison Formation, Lower Cretaceous Dakota Group, Upper and Lower Cretaceous Benton Group, Upper Cretaceous Niobrara Formation, and the Eocene and Paleocene Coalmont Formation. The Late Cretaceous to early Eocene Medicine Bow thrust is folded in places, and several back thrusts produced a complicated thrust pattern in the south part of the map. Early Oligocene magmatism produced rhyolite tuff, dacite and basalt flows, and intermediate dikes and small stocks. A 40Ar/39Ar date on sanidine from one rhyolite tuff is ~28.5 Ma; a similar whole-rock date on a trachybasalt is ~29.6 Ma. A very coarse, unsorted probably pre-Quaternary ridge-top diamicton crops out in the southern part of the quadrangle. Numerous glacial deposits (mostly of Pinedale age), rock glaciers, block-slope deposits, landslide deposits, talus deposits, fan deposits, colluvium, and alluvium comprise the surficial deposits of the map area.

Enhanced Preliminary Assessment Report: Old Bridge Army Housing Units, Old Bridge, New Jersey

DTIC Science & Technology

1989-11-01

overlain by the Old Bridge (or Magothy ) aquifer. The basement rock in Middlesex County consists of basalt, sandstone, and shale of Triassic age. The...Woodbury Clay and Merchantville formations form a confining layer above the Magothy aquifer; the thickness of this confining layer is less than 100 feet

PBF Reactor Building (PER620). Aerial view of early construction. Camera ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

PBF Reactor Building (PER-620). Aerial view of early construction. Camera facing northwest. Excavation and concrete placement in two basements are underway. Note exposed lava rock. Photographer: Farmer. Date: March 22, 1965. INEEL negative no. 65-2219 - Idaho National Engineering Laboratory, SPERT-I & Power Burst Facility Area, Scoville, Butte County, ID

Geoscience Awareness in Nigeria--A Preliminary Study

ERIC Educational Resources Information Center

Okunlola, Olugbenga A.

2012-01-01

Nigeria (total land area of 923,768 km2 ) is underlain by a crystalline Precambrian basement, Jurassic granites, and Cretaceous to Recent sedimentary rocks, and is prone in places to environmental degradation and geohazards. The country hosts approximately 34 different mineral types in about 855 locations with considerable oil and gas reserves.…

Aleutian terranes from Nd isotopes

NASA Technical Reports Server (NTRS)

Kay, R. W.; Kay, S. M.; Rubenstone, J. L.

1986-01-01

Nd isotope ratios substantiate the identification of oceanic crustal terranes within the continental crustal basement of the Aleutian island arc. The oceanic terranes are exposed in the westernmost Aleutians, but to the east, they are completely buried by isotopically distinct arc-volcanic rocks. Analogous oceanic terranes may be important components of the terrane collages that comprise the continents.

Site Simulation in Teaching Archaeology: A Hands On Approach.

ERIC Educational Resources Information Center

Rice, Patricia C.

An indoor simulated archaeology site for use in a college level introductory archaeology course is described. Housed in the basement of a building on campus, the site simulates an eight-layered French rock shelter. Layers contain "remains" of a microband of Neanderthals, a Lower and Upper Aurignacian group, an Upper Perigordian group, Magdalenian…

Geologic map of the Basque-Cantabrian Basin and a new tectonic interpretation of the Basque Arc

NASA Astrophysics Data System (ADS)

Ábalos, B.

2016-11-01

A new printable 1/200.000 bedrock geological map of the onshore Basque-Cantabrian Basin is presented, aimed to contribute to future geologic developments in the central segment of the Pyrenean-Cantabrian Alpine orogenic system. It is accompanied in separate appendixes by a historic report on the precedent geological maps and by a compilation above 350 bibliographic citations of maps and academic reports (usually overlooked or ignored) that are central to this contribution. Structural scrutiny of the map permits to propose a new tectonic interpretation of the Basque Arc, implementing previously published partial reconstructions. It is presented as a printable 1/400.000 tectonic map. The Basque Arc consists of various thrust slices that can expose at the surface basement rocks (Palaeozoic to Lower Triassic) and their sedimentary cover (uppermost Triassic to Tertiary), from which they are detached by intervening (Upper Triassic) evaporites and associated rocks. The slice-bounding thrusts are in most cases reactivated normal faults active during Meso-Cenozoic sedimentation that can be readily related to basement discontinuities generated during the Hercynian orogeny.

Remarkable isotopic and trace element trends in potassic through sodic Cretaceous plutons of the Yukon-Koyukuk Basin, Alaska, and the nature of the lithosphere beneath the Koyukuk terrane

USGS Publications Warehouse

Arth, Joseph G.; Criss, Robert E.; Zmuda, Clara C.; Foley, Nora K.; Patton, W.W.; Miller, T.P.

1989-01-01

During the period from 110 to 80 m.y. ago, a 450-km-long magmatic belt was active along the northern margin of Yukon-Koyukuk basin and on eastern Seward Peninsula. The plutons intruded Upper Jurassic(?) and Lower Cretaceous volcanic arc rocks and Cretaceous sedimentary rocks in Yukon-Koyukuk basin and Proterozoic and lower Paleozoic continental rocks in Seward Peninsula. Within Yukon-Koyukuk basin, the plutons vary in composition from calc-alkalic plutons on the east to potassic and ultrapotassic alkalic plutons on the west. Plutons within Yukon-Koyukuk basin were analyzed for trace element and isotopic compositions in order to discern their origin and the nature of the underling lithosphere. Farthest to the east, the calc-alkalic rocks of Indian Mountain pluton are largely tonalite and sodic granodiorite, and have low Rb (average 82 ppm), high Sr (>600 ppm), high chondrite-normalized (cn) Ce/Yb (16–37), low δ18O (+6.5 to +7.1), low initial 87Sr/86Sr (SIR) (0.704), and high initial 143Nd/144Nd (NIR) (0.5126). These rocks resemble those modelled elsewhere as partial melts and subsequent fractionates of basaltic or gabbroic metaigneous rocks, and may be products of melting in the deeper parts of the Late Jurassic(?) and Early Cretaceous volcanic arc. Farthest to the west, the two ultrapotassic bodies of Selawik and Inland Lake are high in Cs (up to 93 ppm), Rb (up to 997 ppm), Sr, Ba, Th, and light rare earth elements, have high (Ce/Yb)cn (30, 27), moderate to low δ18O (+8.4, +6.9), high SIR (0.712, 0.710), and moderate NIR (0.5121–0.5122). These rocks resemble rocks of Australia and elsewhere that were modelled as melts of continental mantle that had been previously enriched in large cations. This mantle may be Paleozoic or older. The farthest west alkalic pluton of Selawik Hills is largely monzonite, quartz monzonite, and granite; has moderate Rb (average 284 ppm), high Sr (>600 ppm), high (Ce/Yb)cn (15–25), moderate δ18O (+8.3 to +8.6), high SIR (0.708–0.712), and moderate NIR (0.5121–0.5122). These rocks may be the product of interaction of magma derived from old continental mantle and magma derived from old continental crust. Plutons between eastern and western extremes show completely gradational variations in the concentration of K and Rb and in the isotopic compositions of Sr, Nd, and O. These plutons probably originated either by melting in a mixed source composed of a Paleozoic or older continental section (mantle + crust) overlain by Mesozoic mafic arc rocks, or by mixing of ultrapotassic to potassic magmas from continental sources (mantle + crust), and tonalitic magmas from arc sources. We infer from these results that the northwest portion of Yukon-Koyukuk basin is underlain by a substantial continental basement of Paleozoic or greater age. This basement probably thins out to the east. There is no geochemical evidence for continental basement east of about longitude 157°, or along a belt of at least 50 km width flanking Ruby Geanticline as far to the southwest as about longitude 161°. These areas are probably underlain by oceanic and Mesozoic arc rocks.

Combining conventional and thermal drilling in order to increase speed and reduce costs of drilling operations to access deep geothermal resources

NASA Astrophysics Data System (ADS)

Rossi, Edoardo; Kant, Michael A.; von Rohr, Philipp Rudolf; Saar, Martin O.

2017-04-01

The exploitation of deep geothermal resources for energy production relies on finding cost effective solutions to increase the drilling performance in hard rocks. Conventional rotary drilling techniques, based on mechanical rock exportation, result in high rates of drilling tool wearing, causing significant costs. Additionally, rotary drilling results in low drilling speeds in the typically hard crystalline basement rocks targeted for enhanced geothermal energy utilization technologies. Furthermore, even lower overall drilling rates result, when considering tripping times required to exchange worn drill tools. Therefore, alternative drilling techniques, such as hammering, thermal drilling, plasma drilling, and jetting processes are widely investigated in order to provide cost-effective alternatives to conventional drilling methods. A promising approach, that combines conventional rotary and thermal drilling techniques, is investigated in the present work. Here, the rock material is thermally weakened before being exported by conventional cutters. Heat is locally provided by a flame, which moves over the rock surface, heat-treating the material. Besides reducing the rock strength, an in-depth smoothening effect of the mechanical rock properties is observed due to the thermal treatment. This results in reduced rates of drill bit wearing and higher rates of penetration, which in turn decreases drilling costs significantly, particularly for deep-drilling projects. Due to the high heating rates, rock-hardening, commonly observed at moderate temperatures, can be avoided. The flame action can be modelled as a localized, high heat transfer coefficient flame treatment, which results in orders of magnitude higher heating rates than conventional oven treatments. Therefore, we analyse rock strength variations after different maximum temperatures, flame-based heating rates, and rock confinement pressures. The results show that flame treatments lead to a monotonous decrease of rock strength with temperature. This is different from oven treatments, where an initial increase of strength is typically observed, followed by a steep decrease upon further (slow) oven-heating. Thus, the weakening of sandstone and granite samples due to flame treatments indicates the feasibility of a combined mechanical-thermal drilling system. These results suggest that the new combined method enables improved rates of penetration in hard rocks while reducing the rate of drill tool wear. We also present possible implementations of this combined drilling system in the field. From field test results, advantages and limitations of the proposed new technology are presented, with an emphasis on accessing geothermal energy resources in crystalline basement rocks.

Stratigraphy and Mesozoic–Cenozoic tectonic history of northern Sierra Los Ajos and adjacent areas, Sonora, Mexico

USGS Publications Warehouse

Page, William R.; Gray, Floyd; Iriondo, Alexander; Miggins, Daniel P.; Blodgett, Robert B.; Maldonado, Florian; Miller, Robert J.

2010-01-01

Geologic mapping in the northern Sierra Los Ajos reveals new stratigraphic and structural data relevant to deciphering the Mesozoic–Cenozoic tectonic evolution of the range. The northern Sierra Los Ajos is cored by Proterozoic, Cambrian, Devonian, Mississippian, and Pennsylvanian strata, equivalent respectively to the Pinal Schist, Bolsa Quartzite and Abrigo Limestone, Martin Formation, Escabrosa Limestone, and Horquilla Limestone. The Proterozoic–Paleozoic sequence is mantled by Upper Cretaceous rocks partly equivalent to the Fort Crittenden and Salero Formations in Arizona, and the Cabullona Group in Sonora, Mexico.Absence of the Upper Jurassic–Lower Cretaceous Bisbee Group below the Upper Cretaceous rocks and above the Proterozoic–Paleozoic rocks indicates that the Sierra Los Ajos was part of the Cananea high, a topographic highland during the Late Jurassic and Early Cretaceous. Deposition of Upper Cretaceous rocks directly on Paleozoic and Proterozoic rocks indicates that the Sierra Los Ajos area had subsided as part of the Laramide Cabullona basin during Late Cretaceous time. Basal beds of the Upper Cretaceous sequence are clast-supported conglomerate composed locally of basement (Paleozoic) clasts. The conglomerate represents erosion of Paleozoic basement in the Sierra Los Ajos area coincident with development of the Cabullona basin.The present-day Sierra Los Ajos reaches elevations of greater than 2600 m, and was uplifted during Tertiary basin-and-range extension. Upper Cretaceous rocks are exposed at higher elevations in the northern Sierra Los Ajos and represent an uplifted part of the inverted Cabullona basin. Tertiary uplift of the Sierra Los Ajos was largely accommodated by vertical movement along the north-to-northwest-striking Sierra Los Ajos fault zone flanking the west side of the range. This fault zone structurally controls the configuration of the headwaters of the San Pedro River basin, an important bi-national water resource in the US-Mexico border region.

Differentiating pedogenesis from diagenesis in early terrestrial paleoweathering surfaces formed on granitic composition parent materials

USGS Publications Warehouse

Driese, S.G.; Medaris, L.G.; Ren, M.; Runkel, Anthony C.; Langford, R.P.

2007-01-01

Unconformable surfaces separating Precambrian crystalline basement and overlying Proterozoic to Cambrian sedimentary rocks provide an exceptional opportunity to examine the role of primitive soil ecosystems in weathering and resultant formation of saprolite (weathered rock retaining rock structure) and regolith (weathered rock without rock structure), but many appear to have been affected by burial diagenesis and hydrothermal fluid flow, leading some researchers to discount their suitability for such studies. We examine one modern weathering profile (Cecil series), four Cambrian paleoweathering profiles from the North American craton (Squaw Creek, Franklin Mountains, Core SQ-8, and Core 4), one Neoproterozoic profile (Sheigra), and one late Paleoproterozoic profile (Baraboo), to test the hypothesis that these paleoweathering profiles do provide evidence of primitive terrestrial weathering despite their diagenetic and hydrothermal overprinting, especially additions of potassium. We employ an integrated approach using (1) detailed thin-section investigations to identify characteristic pedogenic features associated with saprolitization and formation of well-drained regoliths, (2) electron microprobe analysis to identify specific weathered and new mineral phases, and (3) geochemical mass balance techniques to characterize volume changes during weathering and elemental gains and losses of major and minor elements relative to the inferred parent materials. There is strong pedogenic evidence of paleoweathering, such as clay illuviation, sepic-plasmic fabrics, redoximorphic features, and dissolution and alteration of feldspars and mafic minerals to kaolinite, gibbsite, and Fe oxides, as well as geochemical evidence, such as whole-rock losses of Na, Ca, Mg, Si, Sr, Fe, and Mn greater than in modern profiles. Evidence of diagenesis includes net additions of K, Ba, and Rb determined through geochemical mass balance, K-feldspar overgrowths in overlying sandstone sections, and K-feldspars with reaction rims in weathered basement. The sub-Cambrian paleoweathering profiles formed on granite are remarkably similar to modern weathering profiles formed on granite, in spite of overprinting by potassium diagenesis. ?? 2007 by The University of Chicago. All rights reserved.

Geochemistry of hydrothermal vent fluids and its implications for subsurface processes at the active Longqi hydrothermal field, Southwest Indian Ridge

NASA Astrophysics Data System (ADS)

Ji, Fuwu; Zhou, Huaiyang; Yang, Qunhui; Gao, Hang; Wang, Hu; Lilley, Marvin D.

2017-04-01

The Longqi hydrothermal field at 49.6°E on the Southwest Indian Ridge was the first active hydrothermal field found at a bare-rock ultra-slow spreading mid-ocean ridge. Here we report the chemistry of the hydrothermal fluids, for the first time, that were collected from the S zone and the M zone of the Longqi field by gas-tight isobaric samplers by the HOV "Jiaolong" diving cruise in January 2015. According to H2, CH4 and other chemical data of the vent fluid, we suggest that the basement rock at the Longqi field is dominantly mafic. This is consistent with the observation that the host rock of the active Longqi Hydrothermal field is dominated by extensively distributed basaltic rock. It was very interesting to detect simultaneously discharging brine and vapor caused by phase separation at vents DFF6, DFF20, and DFF5 respectively, in a distance of about 400 m. Based on the end-member fluid chemistry and distance between the vents, we propose that there is a single fluid source at the Longqi field. The fluid branches while rising to the seafloor, and two of the branches reach S zone and M zone and phase separate at similar conditions of about 28-30.2 MPa and 400.6-408.3 °C before they discharge from the vents. The end-member fluid compositions of these vents are comparable with or within the range of variation of known global seafloor hydrothermal fluid chemical data from fast, intermediate and slow spreading ridges, which confirms that the spreading rate is not the key factor that directly controls hydrothermal fluid chemistry. The composition of basement rock, water-rock interaction and phase separation are the major factors that control the composition of the vent fluids in the Longqi field.

Detrital zircon U-Pb geochronology of Cambrian to Triassic miogeoclinal and eugeoclinal strata of Sonora, Mexico

USGS Publications Warehouse

Gehrels, G.E.; Stewart, John H.

1998-01-01

One hundred and eighty two individual detrital zircon grains from Cambrian through Permian miogeoclinal strata, Ordovician eugeoclinal rocks, and Triassic post-orogenic sediments in northwestern Sonora have been analyzed. During Cambrian, Devonian, Permian, and Triassic time, most zircons accumulating along this part of the Cordilleran margin were shed from 1.40-1.45 and 1.62-1.78 Ga igneous rocks that are widespread in the southwestern United States and northwestern Mexico. Zircons with ages of approximately 1.11 Ga are common in Cambrian strata and were apparently shed from granite bodies near the sample site. The sources of 225-280 Ma zircons in our Triassic sample are more problematic, as few igneous rocks of these ages are recognized in northwestern Mexico. Such sources may be present but unrecognized, or the grains could have been derived from igneous rocks of the appropriate ages to the northwest in the Mojave Desert region, to the east in Chihuahua and Coahuila, or to the south in accreted(?) arc-type terranes. Because the zircon grains in our Cambrian and Devonian to Triassic samples could have accumulated in proximity to basement rocks near their present position or in the Death Valley region of southern California, our data do not support or refute the existence of the Mojave-Sonora megashear. Ordovician strata of both miogeoclinal and eugeoclinal affinity are dominated by >1.77 Ga detrital zircons, which are considerably older than most basement rocks in the region. Zircon grains in the miogeoclinal sample were apparently derived from the Peace River arch area of northwestern Canada and transported southward by longshore currents. The eugeoclinal grains may also have come from the Peace River arch region, with southward transport by either sedimentary or tectonic processes, or they may have been shed from off-shelf slivers of continents (perhaps Antarctica?) removed from the Cordilleran margin during Neoproterozoic rifting. It is also possible that the Ordovician eugeoclinal strata are far traveled and exotic to North America.

The basement of the Mount Athos peninsula, northern Greece: insights from geochemistry and zircon ages

NASA Astrophysics Data System (ADS)

Himmerkus, F.; Zachariadis, P.; Reischmann, T.; Kostopoulos, D.

2012-09-01

The Mount Athos Peninsula is situated in the south-easternmost part of the Chalkidiki Peninsula in northern Greece. It belongs to the Serbo-Macedonian Massif (SMM), a large basement massif within the Internal Hellenides. The south-eastern part of the Mount Athos peninsula is built by fine-grained banded biotite gneisses and migmatites forming a domal structure. The southern tip of the peninsula, which also comprises Mount Athos itself, is built by limestone, marble and low-grade metamorphic rocks of the Chortiatis Unit. The northern part and the majority of the western shore of the Mount Athos peninsula are composed of highly deformed rocks belonging to a tectonic mélange termed the Athos-Volvi-Suture Zone (AVZ), which separates two major basement units: the Vertiskos Terrane in the west and the Kerdillion Unit in the east. The rock-types in this mélange range from metasediments, marbles and gneisses to amphibolites, eclogites and peridotites. The gneisses are tectonic slivers of the adjacent basement complexes. The mélange zone and the gneisses were intruded by granites (Ierissos, Ouranoupolis and Gregoriou). The Ouranoupolis intrusion obscures the contact between the mélange and the gneisses. The granites are only slightly deformed and therefore postdate the accretionary event that assembled the units and created the mélange. Pb-Pb- and U-Pb-SHRIMP-dating of igneous zircons of the gneisses and granites of the eastern Athos peninsula in conjunction with geochemical and isotopic analyses are used to put Athos into the context of a regional tectonic model. The ages form three clusters: The basement age is indicated by two samples that yielded Permo-Carboniferous U-Pb-ages of 292.6 ± 2.9 Ma and 299.4 ± 3.5 Ma. The main magmatic event of the granitoids now forming the gneiss dome is dated by Pb-Pb-ages between 140.0 ± 2.6 Ma and 155.7 ± 5.1 Ma with a mean of 144.7 ± 2.4 Ma. A within-error identical age of 146.6 ± 2.3 Ma was obtained by the U-Pb-SHRIMP method. This Late Jurassic age is also known from the Kerdillion Unit and the Rhodope Terrane. The rather undeformed granites are interpreted as piercing plutons. The small granite stocks sampled have Late Cretaceous to Early Tertiary ages of 66.8 ± 0.8 Ma and 68.0 ± 1.0 Ma (U-Pb-SHRIMP)/62.8 ± 3.9 Ma (Pb-Pb). The main accretionary event was according to these data in the Late Jurassic since all younger rocks show little or no deformation. The age distribution together with the geochemical and isotopic signature and the lithology indicates that the eastern part of the Mount Athos peninsula is part of a large-scale gneiss dome also building the Kerdillion Unit of the eastern SMM and the Rhodope Massif. This finding extends the area of this dome significantly to the south and indicates that the tectonic boundary between the SMM and the Rhodope Massif lies within the AVZ.

Exploring the deep, ancient hydrogeosphere within Precambrian crystalline rocks using noble gases

NASA Astrophysics Data System (ADS)

Warr, O.; Sherwood Lollar, B.; Fellowes, J.; Sutcliffe, C. N.; McDermott, J. M.; Holland, G.; Mabry, J.; Ballentine, C. J.

2016-12-01

Serpentinization is a key long-term water-rock interaction occurring within isolated fractures in Precambrian crystalline rocks and is a significant source of global H2 production. Highly saline fracture fluids, containing in-situ produced dissolved gases (e.g. percent level He, abiogenic CH4 and mM H2), have revealed microbial ecosystems isolated from the surface photosphere for millions of years. Noble gases can provide crucial physical and temporal constraints on these serpentinizing and life-supporting environments via radiogenic-derived fluid residence times, while also providing evidence of isolation. New noble gas data is presented here from four locations on the Canadian Shield. Kidd Creek Mine in Ontario, where fluids with a mean residence time ≥ 1.1 Ga were identified in 2013, was revisited with resampling of the waters from 2.4 km bls (below land surface), and new samples collected from 2.9 km bls. The study was also expanded to include two mines from Sudbury, Ontario at 1.7 (Mine 1) and 1.4 (Mine 2) km bls. The radiogenic excesses within the fluids were greatest for the 2.9 km Kidd Creek samples and provided an average residence time of 1.6 Ga. Consistent with our hypothesis, the resampling of the 2.4 km fluids (80 months after the original study) reveal significantly reduced residence times (1.1 Ga to 390 Ma) due to stress-induced opening of younger, though nonetheless old, fractures. This is supported by recent sulphur isotope, and 2H & 18O data. Additional hydrogeological constraints are provided by the 129Xe & 136Xe data, which suggest distinct fracture networks feed the 2.4 km, and the 2.9 km systems. Fracture fluids in the Sudbury Basin were targeted to investigate the influence of a later 1.8 Ga bolide impact which formed major fractures in the underlying basement. As hypothesised the fluids in the Sudbury Archean basement are younger than those at Kidd Creek, with mean residence times of 313 and 544 Ma for Mine 1 and 2 respectively. Our results demonstrate that ancient fracture fluids in the Precambrian crust represent a previously under-investigated groundwater domain and H2 source. With mean residence times of 0.3-1.6 Ga, they provide an opportunity to explore an unprecedented ancient component of the Earth's hydrogeosphere.

Sodium storage in deep paleoweathering profiles beneath the Paleozoic-Triassic unconformity

NASA Astrophysics Data System (ADS)

Thiry, M.; Parcerisa, D.; Ricordel-Prognon, C.; Schmitt, J.-M.

2009-04-01

A major sodium accumulation has been recognized for long and by numerous authors in the Permo-Triassic salt deposits (Hay et al., 2006). Beside these basinal deposits, important masses of sodium were stored on the continents within deep palaeoweathering profiles in form of albite. Indeed, wide surfaces and huge volumes of granito-gneissic basements of the Hercynian massifs are albitized from North-Africa up to Scandinavia. These albitized rocks have usually been considered as related to tardi-magmatic metasomatic processes (Cathelineau 1986; Petersson and Eliasson 1997). Geometrical arrangement and dating of these alterations point out that these albitizations, or at least a part of them, developed under low temperature subsurface conditions in relation with the Triassic palaeosurface (Ricordel et al., 2007; Parcerisa et al., 2009). Petrology The albitized igneous rocks show a strong alteration with pseudomorphic replacement of the primary plagioclases into albite, replacement of primary biotite by chlorite and minor precipitation of neogenic minerals like albite, chlorite, apatite, haematite, calcite and titanite. Albitized rocks are characterized by their pink coloration due to the presence of minute haematite inclusions in the albite. The development and distribution of the albitization and related alterations above the unaltered basement occurs in three steps that define a vertical profile, up to 100-150 m depth. 1) In the lower part of the profile, albitization occurs within pink-colored patches in the unaltered rock, giving a pink-spotted aspect to the rock. 2) In the middle part of the profile, rocks have an overall pink coloration due to the albitization of the primary Ca-bearing igneous plagioclases. Usually, this facies develops in a pervasive manner, affecting the whole rock, but it may also be restricted to joints, giving a sharp-pink coloration to the fracture wall. 3) Finally, the top of the profile is defined by the same mineral paragenesis as in the pink stage, with an increase in the amount and size of sericite and hematite inclusions. The latter causes the red coloration of the altered rocks. Regional layout Regional distribution of the alterations which affect the Carboniferous igneous and volcanic formations beneath the Jurassic sedimentary cover lead to associate these alterations to the Triassic unconformity. Besides, albitized facies show generally both topographic and regional arrangements, with more altered facies occurring in the mountain highs and in the external parts of the massifs and unaltered facies occurring in the river valleys and in the central parts of the massifs. Moreover, the haematite associated with these albitized basement rocks has been dated from Early Trias by means of paleomagnetism (Ricordel et al, 2007). From this layout and dating, it is deduced that albitization is related to the development of a deep weathering profile (up to 150 m deep) during a long-lasting exposure of the Triassic erosional unconformity (regolith). Geochemistry and paleoenvironmental setting It has to be highlighted that, this alteration may not behave like an "ordinary" weathering profile and occurred under unusual, or at least very specific, geological settings. The scale of the profiles (over 100 m depth) relates this alteration rather to a groundwater environment. The weak mobility of most chemical elements may point to a groundwater with very low outflows and deep water table. This may occur in very subdued landscape and in arid climatic conditions. It has also to be pointed that this alteration may have lasted for several 10's of Ma. Albite formation at low temperature may be envisioned consequently in alkaline, confined waters with sufficient concentrations of sodium and silica. Early attempts of modeling (Schmitt, 1994) have also indicated that a high Na+/K+ ratio is as well probably required. Petrographic data also indicate an import of sodium by the weathering solutions, without any clear enrichment in potassium. The Na+ enrichment is most likely linked with the peculiar geochemical setting of the Triassic environment where for instance halite moulds are very common in transgressive epicontinental deposits. The leaching of such salts, the role of salty marine aerosols, or a periodic/episodic contribution of seawater or evaporative solutions may be equally invoked. Mass balance Taking into account the surpergene origin of albitization and its widespread development on the Paleozoic basement rocks (from Morocco to Scandinavia) means that high amounts of Na+ have been stored in the deep paleoweathering profiles of the Triassic continents. This sodium storage in weathering profiles has to be taken in consideration in addition to the major sodium chloride accumulation in the basins during the Permo-Triassic times. Further investigations are needed to demonstrate the extent of these paleoweathering profiles and then to estimate the amount of this continental sodium storage. References Cathelineau M (1986) The hydrothermal alkali metasomatism effects on granitic rocks: Quartz dissolution and related sub-solidus changes. Jour. Petrol., 27: 945-965. Hay, W.W.; Migdisov, A.; Balukhovsky, A.N.; Wold, C.N.; Flogel, S., Soding, E. (2006) Evaporites and the salinity of the ocean during the Phanerozoic: Implications for climate, ocean circulation and life. Palaeogeography, Palaeoclimatology, Palaeoecology, 240/1-2: 3-46. Parcerisa D., Thiry M., Schmitt J.-M. (2009) Albitisation related to the Triassic unconformity in igneous rocks of the Morvan Massif (France), International Journal of Earth Sciences, DOI: 10.1007/s00531-008-0405-1. Petersson J, Eliasson T (1997) Mineral evolution and element mobility during episyenitization (dequartzification) and albitization in the postkinematic Bohus granite, southwest Sweden. Lithos, 42: 123-146. Ricordel C, Parcerisa D, Thiry M, Moreau M-G, Gómez-Gras D (2007) Triassic magnetic overprints related to albitization in granites from the Morvan massif (France). Palaeogeography Palaeoclimatology Palaeoecology, 251: 268-282. Schmitt JM (1994) Geochemical modelling and origin of the Triassic albitized regolith in southern France. 14th International Sedimentological Congress, Recife, Brazil. Abstracts book S8: 19-21.

Ocean Drilling: Forty Years of International Collaboration

NASA Astrophysics Data System (ADS)

Smith, Deborah K.; Exon, Neville; Barriga, Fernando J. A. S.; Tatsumi, Yoshiyuki

2010-10-01

International cooperation is an essential component of modern scientific research and societal advancement [see Ismail-Zadeh and Beer, 2009], and scientific ocean drilling represents one of Earth science's longest-running and most successful international collaborations. The strength of this collaboration and its continued success result from the realization that scientific ocean drilling provides a unique and powerful tool to study the critical processes of both short-term change and the long-term evolution of Earth systems. A record of Earth's changing tectonics, climate, ocean circulation, and biota is preserved in marine sedimentary deposits and the underlying basement rocks. And because the ocean floor is the natural site for accumulation and preservation of geological materials, it may preserve a continuous record of these processes.

26. EXCAVATION OF EAST (FRONT) BASEMENT WELL AND DRAINAGE SYSTEM, ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

26. EXCAVATION OF EAST (FRONT) BASEMENT WELL AND DRAINAGE SYSTEM, WITH ARCHED ENTRY INTO BASEMENT UNDER FRONT ENTRY IN BACKGROUND, LOOKING NORTH (NOTE GALLETING IN BRICK FOUNDATION) - Belair, Tulip Grove Drive, Belair-at-Bowie, Bowie, Prince George's County, MD

Polyphase vein mineralization in the Fennoscandian Shield at Åkerlandet, Järvsand, and Laisvall along the erosional front of the Caledonian orogen, Sweden

NASA Astrophysics Data System (ADS)

Saintilan, Nicolas J.; Stephens, Michael B.; Spikings, Richard; Schneider, Jens; Chiaradia, Massimo; Spangenberg, Jorge E.; Ulianov, Alexey; Fontboté, Lluís

2017-08-01

The Åkerlandet, Järvsand, and Laisvall deposits in Sweden are calcite-fluorite-sulfide vein deposits and occurrences located close to the current erosional front of the Caledonian orogen and hosted by crystalline basement rocks in the Fennoscandian Shield. At Laisvall, basement-hosted veinlets occur beneath Ediacaran to Cambrian sandstones that host a strata-bound Pb-Zn deposit. The mineralized fractures at Åkerlandet and Järvsand occur along fault systems oriented N-S to NNW-SSE. Veins or veinlets strike NNW-SSE and NW-SE at Åkerlandet, NNE-SSW at Järvsand, and NNW-SSE and NNE-SSW to NE-SW at Laisvall. At Åkerlandet and Järvsand, fractures acted as conduits for hydrothermal fluids of variable composition and formed during separate tectonic events. At Åkerlandet, the fault zone with NNW-SSE strike shows kinematic indicators consistent with NE-SW bulk horizontal extension. At Järvsand, the calcite-fluorite-galena veins formed along R-Riedel shears related to the host N-S to NNW-SSE fault system. The kinematic indicators are consistent with NW-SE bulk horizontal extension, similar to the extensional deformation during the later part of the Caledonian orogeny (Silurian to Devonian). At Åkerlandet, adularia-quartz deposition was followed by sphalerite ± galena and finally by precipitation of fluorite and calcite. 40Ar-39Ar thermochronology of a single adularia sample did not yield a well-defined plateau age but the gas released at higher temperatures suggests an early Tonian (980 to 950 Ma) crystallization age, i.e., during the later part of the Sveconorwegian orogeny, although the data do not exclude other less likely interpretations. Previous fluid inclusion microthermometry and geochronological studies and new petrographic and geochemical results suggest that sphalerite ± galena mineralization formed from saline, relatively oxidizing, moderate-temperature, and slightly acidic hydrothermal fluids, either during the Ediacaran or the Middle Ordovician. Metals and H2S were derived from local basement rocks. Based on petrographic evidence, rare earth element composition, and S, C, and O isotope data, fluorite and calcite precipitated under near neutral and relatively reducing conditions. Occurrence of solid bitumen in veins at Åkerlandet and C and O isotope data of calcite at Åkerlandet and in the Laisvall basement veinlets suggest that the precipitation of calcite and fluorite was triggered by interaction of hot and evolved hydrothermal fluids (87Sr/86Sr = 0.718-0.732) with organic matter. Structural, petrographic, and geochemical data at Laisvall suggest that the basement structures hosting calcite-fluorite ± pyrite veinlets were utilized in the Middle Ordovician as the plumbing system for the oxidizing, slightly acidic, metal-bearing brines that caused the economic Pb-Zn mineralization in the overlying sandstones.

Igneous sills record far-field and near-field stress interactions during volcano construction: Isle of Mull, Scotland

NASA Astrophysics Data System (ADS)

Stephens, T. L.; Walker, R. J.; Healy, D.; Bubeck, A.; England, R. W.; McCaffrey, K. J. W.

2017-11-01

Sill emplacement is typically associated with horizontally mechanically layered host rocks in a near-hydrostatic far-field stress state, where contrasting mechanical properties across the layers promote transitions from dykes, or inclined sheets, to sills. We used detailed field observations from the Loch Scridain Sill Complex (Isle of Mull, UK), and mechanical models to show that layering is not always the dominant control on sill emplacement. The studied sills have consistently shallow dips (1°-25°) and cut vertically bedded and foliated metamorphic basement rocks, and horizontally bedded cover sedimentary rocks and lavas. Horizontal and shallowly-dipping fractures in the host rock were intruded with vertical opening in all cases, whilst steeply-dipping discontinuities within the sequence (i.e. vertical fractures and foliation in the basement, and vertical polygonal joints in the lavas) were not intruded during sill emplacement. Mechanical models of slip tendency, dilation tendency, and fracture susceptibility for local and overall sill geometry data, support a radial horizontal compression during sill emplacement. Our models show that dykes and sills across Mull were emplaced during NW-SE horizontal shortening, related to a far-field tectonic stress state. The dykes generally accommodated phases of NE-SW horizontal tectonic extension, whereas the sills record the superposition of the far-field stress with a near-field stress state, imposed by emplacement of the Mull Central Volcano. We show that through detailed geometric characterisation coupled with mechanical modelling, sills may be used as an indication of fluctuations in the paleostress state.

The Influence Of Hydrothermal Alteration And Weathering On Rock Magnetic Properties Of Granites From The Eps-1 Drilling (soultz-sous-forÊts / France)

NASA Astrophysics Data System (ADS)

Just, J.; Schleicher, A.; Kontny, A.; de Wall, H.

The EPS-1 drilling in Soultz-sous-Forêts (Rhinegraben, France) recovered a core pro- file of Tertiary to Permo-Mesozoic sediments deposited on a Variscan granitic base- ment. Magnetic susceptibility (k) measurements on the core material revealed a con- tinous increase from the basement/cover boundary (kmean 0.4 x 10-3 SI) into the magnetite-bearing granite (kmean 13 x 10-3 SI) over a depth range of 1417 U 1555 m. Rock magnetic and mineralogic studies were performed for the fresh granite, the hydrothermally altered granite near a fault zone and the altered granite from the fossil land surface near the basement/cover boundary. The decrease in susceptibility can be correlated with a gradual decomposition of magnetite to hematite and an alteration of the matrix minerals feldspars, biotite and hornblende to clay minerals and carbon- ates. Along with this transition, characteristic rock magnetic signatures can be dis- criminated for different degrees of alteration. While temperature-dependent magnetic susceptibility k(T)-curves in fresh granites indicate a typical multidomain magnetite course with good reversibility, different types of irreversible courses are observed for the altered granite. However, hematite could not be identified in the k(T)-curves. Al- tered granite shows relatively weak magnetic behaviour in AF-demagnetisation exper- iments, untypical for hematite. The alteration of the fresh granite also causes a change in magnetic fabric parameter, especially of the anisotropy factor. The magnetic min- eralogy from the altered granite in respect to the changes in rock magnetic properties will be discussed.

Geological indicators for impact: The anomalous case of the Vredefort structure, South Africa

NASA Technical Reports Server (NTRS)

Antoine, L. A. G.; Reimold, W. U.

1988-01-01

The Vredefort Dome is located within and almost central to the Witwatersrand basin in its presently known extent. It exposes a central Archean granite core which is surrounded by a collar of supracrustal rocks. These collar rocks outline a strong polygonal geometry. The Archean core is comprised of two concentric zones, the Outer Granite Gneiss (OGG), and the core central Inlandsee Leucogranofels (ILG). The rocks of the inner core display granulite facies metamorphism, while the OGG is in amphibolite facies. The inner core is believed from recent drill hole information to be underlain by mafic and ultramafic gneisses, the extent of which cannot be assessed at present. A fairly broad zone of charnockites separates the OGG and ILG domains. This zone is characterized by a high concentration of pseudotachylite and ductile shearing. Whereas a number of other domical structures are located within or surrounding the Witwatersrand basin, the Vredefort structure is anomalous, in that it has: a partly polygonal geometry; extensive alkali intrusives in the northwestern sector; granophyre dykes (ring-dykes peripheral to the contact collar-basement and NW-SE or NE-SW trending dykes within the Archean basement); contact metamorphism of the collar supracrustal rocks; the overturning of collar supracrustals in the northern sectors; deformation phenomena widely regarded as representing shock metamorphism (pseudotachylite, (sub)planar microdeformation features in quartz, shatter cones and occurrences of high-P quartz polymorphs); a positive 30 mgal gravity anomaly; and high amplitude magnetic anomalies. Recent geophysical, structural and petrological evidence pertinent for the identification of the processes that led to the formation of the Vredefort structure are summarized.

The upper crust laid on its side: tectonic implications of steeply tilted crustal slabs for extension in the basin and range

USGS Publications Warehouse

Howard, Keith A.

2005-01-01

Tilted slabs expose as much as the top 8–15 km of the upper crust in many parts of the Basin and Range province. Exposures of now-recumbent crustal sections in these slabs allow analysis of pre-tilt depth variations in dike swarms, plutons, and thermal history. Before tilting the slabs were panels between moderately dipping, active Tertiary normal faults. The slabs and their bounding normal faults were tilted to piggyback positions on deeper footwalls that warped up isostatically beneath them during tectonic unloading. Stratal dips within the slabs are commonly tilted to vertical or even slightly overturned, especially in the southern Basin and Range where the thin stratified cover overlies similarly tilted basement granite and gneiss. Some homoclinal recumbent slabs of basement rock display faults that splay upward into forced folds in overlying cover sequences, which thereby exhibit shallower dips. The 15-km maximum exposed paleodepth for the slabs represents the base of the brittle upper crust, as it coincides with the depth of the modern base of the seismogenic zone and the maximum focal depths of large normal-fault earthquakes in the Basin and Range. Many upended slabs accompany metamorphic core complexes, but not all core complexes have corresponding thick recumbent hanging-wall slabs. The Ruby Mountains core complex, for example, preserves only scraps of upper-plate rocks as domed-up extensional klippen, and most of the thick crustal section that originally overlay the uplifted metamorphic core now must reside below little-tilted hanging-wall blocks in the Elko-Carlin area to the west. The Whipple and Catalina Mountains core complexes in contrast are footwall to large recumbent hanging-wall slabs of basement rock exposing 8-15 km paleodepths that originally roofed the metamorphic cores; the exposed paleodepths require that a footwall rolled up beneath the slabs.

Comparison of hydrothermal activity between the Adriatic and the Red Sea rift margins

NASA Astrophysics Data System (ADS)

Ball, Philip; Incerpi, Nicolò; Birkle, Peter; Lacsamana, Elizabeth; Manatschal, Gianreto; Agar, Susan; Zhang, Shuo; Borsato, Ron

2017-04-01

Detailed field studies, and access to high-quality seismic reflection and refraction data have led to an improved understanding of the architecture and evolution of magma poor and magma rich margins. Associated with the spatial-temporal evolution of the rift, it is evident that there are evolving, extensive, fluid-rock interactions due to the infiltration of fluids within the sediment, basement and lithospheric mantle. Key questions therefore arise: What are the different fluid-rock reactions that can be typed to different geodynamic stages of the rift evolution? What are their compositions and how do they interact with their environment (basement, sediments, evaporites, hydrosphere, and magmatism)? What are the implications for the evolution of the margin rheology, thermal structure, depositional environments/organic matter maturity, and reservoir quality? The Adriatic paleo-rifted margin is preserved in both SE Switzerland and northern Italy. The field exposures provide a unique opportunity to study the fluid flow history of a hyperextended magma poor extensional margin. Analysis of breccias, cement veins and replacement minerals reveal that the margin records a complex, long-lasting history of dolomitization, calcification and silicification during the Jurassic rifting. The Red Sea by contrast is a young rifted margin. It differs from the paleo-Adriatic margin by several characteristics: volcanism is more evident, and syn-tectonic sediments, including evaporites (halite and anhydrite) are thicker. Several core and fluid samples are available from both onshore and offshore wells, which reveal rift-related hydrothermal alteration. In addition, we find evidence for the presence of an extreme dynamic hydraulic system with infiltration of surface water into sub-salt units during Late Pleistocene. In this study we present results from petrographic and geochemical analysis of basement and sedimentary rocks from Adriatic field-derived samples and core/subsurface fluid samples for the Eastern Red Sea margin. The results are presented using rift domain interpretations, thereby enabling the simple comparison of the observed hydrothermal alteration within a first-order (spatial temporal) rift geodynamic framework.

Igneous activity, metamorphism, and deformation in the Mount Rogers area of SW Virginia and NW North Carolina: A geologic record of Precambrian tectonic evolution of the southern Blue Ridge Province

USGS Publications Warehouse

Tollo, Richard P.; Aleinikoff, John N.; Mundil, Roland; Southworth, C. Scott; Cosca, Michael A.; Rankin, Douglas W.; Rubin, Allison E.; Kentner, Adrienne; Parendo, Christopher A.; Ray, Molly S.

2012-01-01

Mesoproterozoic basement in the vicinity of Mount Rogers is characterized by considerable lithologic variability, including major map units composed of gneiss, amphibolite, migmatite, meta-quartz monzodiorite and various types of granitoid. SHRIMP U-Pb geochronology and field mapping indicate that basement units define four types of occurrences, including (1) xenoliths of ca. 1.33 to ≥1.18 Ga age, (2) an early magmatic suite including meta-granitoids of ca. 1185–1140 Ma age that enclose or locally intrude the xenoliths, (3) metasedimentary rocks represented by layered granofels and biotite schist whose protoliths were likely deposited on the older meta-granitoids, and (4) a late magmatic suite composed of younger, ca. 1075–1030 Ma intrusive rocks of variable chemical composition that intruded the older rocks. The magmatic protolith of granofels constituting part of a layered, map-scale xenolith crystallized at ca. 1327 Ma, indicating that the lithology represents the oldest, intact crust presently recognized in the southern Appalachians. SHRIMP U-Pb data indicate that periods of regional Mesoproterozoic metamorphism occurred at 1170–1140 and 1070–1020 Ma. The near synchroneity in timing of regional metamorphism and magmatism suggests that magmas were emplaced into crust that was likely at near-solidus temperatures and that melts might have contributed to the regional heat budget. Much of the area is cut by numerous, generally east- to northeast-striking Paleozoic fault zones characterized by variable degrees of ductile deformation and recrystallization. These high-strain fault zones dismember the terrane, resulting in juxtaposition of units and transformation of basement lithologies to quartz- and mica-rich tectonites with protomylonitic and mylonitic textures. Mineral assemblages developed within such zones indicate that deformation and recrystallization likely occurred at greenschist-facies conditions at ca. 340 Ma.

Structure and development of the Southeast Georgia Embayment and northern Blake Plateau: Preliminary analysis

USGS Publications Warehouse

Dillon, William P.; Paull, Charles K.; Buffler, Richard T.; Fail, Jean-Pierre

1979-01-01

Multichannel seismic reflection profiles from the Southeast Georgia Embayment and northern Blake Plateau show reflectors that have been correlated tentatively with horizons of known age. The top of the Cretaceous extends smoothly seaward beneath the continental shelf and Blake Plateau, unaffected at the present shelf edge. A reflector inferred to correspond approximately to the top of the Jurassic section onlaps and pinches out against rocks below. A widespread smooth reflector probably represents a volcanic layer of Early Jurassic age that underlies only the northwestern part of the research area. A major unconformity beneath the inferred volcanic layer is probably of Late Triassic or Early Jurassic age. This unconformity dips rather smoothly seaward beneath the northern Blake Plateau, but south of a geological boundary near 31°N, it has subsided much more rapidly, and reaches depths of more than 12 km. Development of the continental margin north of the boundary began with rifting and subsidence of continental basement in the Triassic. An episode of volcanism may have been due to stresses associated with a spreading center jump at about 175 million years ago. Jurassic and Cretaceous deposits form an onlapping wedge above the inferred early Jurassic volcanics and Triassic sedimentary rocks. During Cenozoic times, development of Gulf Stream flow caused a radical decrease in sedimentation rates so that a shelf that was much narrower than the Mesozoic shelf was formed by progradation against the inner edge of the stream. South of the 31°N geological boundary, the basement probably is semi-oceanic and reef growth, unlike that in the area to the north, has been very active at the outer edge of the plateau.

Tertiary plate tectonics and high-pressure metamorphism in New Caledonia

USGS Publications Warehouse

Brothers, R.N.; Blake, M.C.

1973-01-01

The sialic basement of New Caledonia is a Permian-Jurassic greywacke sequence which was folded and metamorphosed to prehnite-pumpellyite or low-grade greenschist facies by the Late Jurassic. Succeeding Cretaceous-Eocene sediments unconformably overlie this basement and extend outwards onto oceanic crust. Tertiary tectonism occurred in three distinct phases. 1. (1) During the Late Eocene a nappe of peridotite was obducted onto southern New Caledonia from northeast to southwest, but without causing significant metamorphism in the underlying sialic rocks. 2. (2) Oligocene compressive thrust tectonics in the northern part of the island accompanied a major east-west subduction zone, at least 30 km wide, which is identified by an imbricate system of tectonically intruded melanges and by development of lawsonite-bearing assemblages in adjacent country rocks; this high-pressure mineralogy constituted a primary metamorphism for the Cretaceous-Eocene sedimentary pile, but was overprinted on the Mesozoic prehnite-pumpellyite metagreywackes. 3. (3) Post-Oligocene transcurrent faulting along a northwest-southeast line (the sillon) parallel to the west coast caused at least 150 km of dextral offset of the southwest frontal margin of the Eocene ultramafic nappe. At the present time, the tectonics of the southwest Pacific are related to a series of opposite facing subduction (Benioff) zones connected by transform faults extending from New Britain-Solomon Islands south through the New Hebrides to New Zealand and marking the boundary between the Australian and Pacific plates. Available geologic data from this region suggest that a similar geometry existed during the Tertiary and that the microcontinents of New Guinea, New Caledonia and New Zealand all lay along the former plate boundary which has since migrated north and east by a complex process of sea-floor spreading behind the active island arcs. ?? 1973.

Map of normal faults and extensional folds in the Tendoy Mountains and Beaverhead Range, Southwest Montana and eastern Idaho

USGS Publications Warehouse

Janecke, S.U.; Blankenau, J.J.; VanDenburg, C.J.; VanGosen, B.S.

2001-01-01

Compilation of a 1:100,000-scale map of normal faults and extensional folds in southwest Montana and adjacent Idaho reveals a complex history of normal faulting that spanned at least the last 50 m.y. and involved six or more generations of normal faults. The map is based on both published and unpublished mapping and shows normal faults and extensional folds between the valley of the Red Rock River of southwest Montana and the Lemhi and Birch Creek valleys of eastern Idaho between latitudes 45°05' N. and 44°15' N. in the Tendoy and Beaverhead Mountains. Some of the unpublished mapping has been compiled in Lonn and others (2000). Many traces of the normal faults parallel the generally northwest to north-northwest structural grain of the preexisting Sevier fold and thrust belt and dip west-southwest, but northeastand east-striking normal faults are also prominent. Northeaststriking normal faults are subparallel to the traces of southeast-directed thrusts that shortened the foreland during the Laramide orogeny. It is unlikely that the northeast-striking normal faults reactivated fabrics in the underlying Precambrian basement, as has been documented elsewhere in southwestern Montana (Schmidt and others, 1984), because exposures of basement rocks in the map area exhibit north-northwest- to northwest-striking deformational fabrics (Lowell, 1965; M’Gonigle, 1993, 1994; M’Gonigle and Hait, 1997; M’Gonigle and others, 1991). The largest normal faults in the area are southwest-dipping normal faults that locally reactivate thrust faults (fig. 1). Normal faulting began before middle Eocene Challis volcanism and continues today. The extension direction flipped by about 90° four times.

The formation of peak rings in large impact craters.

PubMed

Morgan, Joanna V; Gulick, Sean P S; Bralower, Timothy; Chenot, Elise; Christeson, Gail; Claeys, Philippe; Cockell, Charles; Collins, Gareth S; Coolen, Marco J L; Ferrière, Ludovic; Gebhardt, Catalina; Goto, Kazuhisa; Jones, Heather; Kring, David A; Le Ber, Erwan; Lofi, Johanna; Long, Xiao; Lowery, Christopher; Mellett, Claire; Ocampo-Torres, Rubén; Osinski, Gordon R; Perez-Cruz, Ligia; Pickersgill, Annemarie; Poelchau, Michael; Rae, Auriol; Rasmussen, Cornelia; Rebolledo-Vieyra, Mario; Riller, Ulrich; Sato, Honami; Schmitt, Douglas R; Smit, Jan; Tikoo, Sonia; Tomioka, Naotaka; Urrutia-Fucugauchi, Jaime; Whalen, Michael; Wittmann, Axel; Yamaguchi, Kosei E; Zylberman, William

2016-11-18

Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust. Copyright © 2016, American Association for the Advancement of Science.

Microbially mediated alteration of crystalline basalts as identified from analogical reactive percolation experiments

NASA Astrophysics Data System (ADS)

Moore, Rachael; Ménez, Bénédicte; Stéphant, Sylvian; Dupraz, Sébastien; Ranchou-Peyruse, Magali; Ranchou-Peyruse, Anthony; Gérard, Emmanuelle

2017-04-01

Alteration in the ocean crust through fluid circulation is an ongoing process affecting the first kilometers and at low temperatures some alteration may be microbially mediated. Hydrothermal activity through the hard rock basement supports diverse microbial communities within the rock by providing nutrient and energy sources. Currently, the impact of basement hosted microbial communities on alteration is poorly understood. In order to identify and quantify the nature of microbially mediated alteration two reactive percolation experiments mimicking circulation of CO2 enriched ground water were performed at 35 °C and 30 bar for 21 days each. The experiments were performed using a crystalline basalt substrate from an earlier drilled deep Icelandic aquifer. One experiment was conducted on sterile rock while the other was conducted with the addition of a microbial inoculate derived from groundwater enrichment cultures obtained from the same aquifer. µCT on the experimental basaltic substrate before and after the reactive percolation experiment along with synchrotron radiation x-ray tomographic microscopy and the mineralogical characterization of resulting material allows for the comparative volumetric quantification of dissolution and precipitation. The unique design of this experiment allows for the identification of alteration which occurs solely abiotically and of microbially mediated alteration. Experimental results are compared to natural basaltic cores from Iceland retrieved following a large field CO2 injection experiment that stimulated microbial activity at depth.

Strain-dependent partial slip on rock fractures under seismic-frequency torsion

NASA Astrophysics Data System (ADS)

Saltiel, Seth; Bonner, Brian P.; Ajo-Franklin, Jonathan B.

2017-05-01

Measurements of nonlinear modulus and attenuation of fractures provide the opportunity to probe their mechanical state. We have adapted a low-frequency torsional apparatus to explore the seismic signature of fractures under low normal stress, simulating low effective stress environments such as shallow or high pore pressure reservoirs. We report strain-dependent modulus and attenuation for fractured samples of Duperow dolomite (a carbon sequestration target reservoir in Montana), Blue Canyon Dome rhyolite (a geothermal analog reservoir in New Mexico), and Montello granite (a deep basement disposal analog from Wisconsin). We use a simple single effective asperity partial slip model to fit our measured stress-strain curves and solve for the friction coefficient, contact radius, and full slip condition. These observations have the potential to develop into new field techniques for measuring differences in frictional properties during reservoir engineering manipulations and estimate the stress conditions where reservoir fractures and faults begin to fully slip.

Interpretation of shallow crustal structure of the Imperial Valley, California, from seismic reflection profiles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Severson, L.K.

1987-05-01

Eight seismic reflection profiles (285 km total length) from the Imperial Valley, California, were provided to CALCRUST for reprocessing and interpretation. Two profiles were located along the western margin of the valley, five profiles were situated along the eastern margin and one traversed the deepest portion of the basin. These data reveal that the central basin contains a wedge of highly faulted sediments that thins to the east. Most of the faulting is strike-slip but there is evidence for block rotations on the scale of 5 to 10 kilometers within the Brawley Seismic Zone. These lines provide insight into themore » nature of the east and west edges of the Imperial Valley. The basement at the northwestern margin of the valley, to the north of the Superstition Hills, has been normal-faulted and blocks of basement material have ''calved'' into the trough. A blanket of sediments has been deposited on this margin. To the south of the Superstition Hills and Superstition Mountain, the top of the basement is a detachment surface that dips gently into the basin. This margin is also covered by a thick sequence sediments. The basement of the eastern margin consists of metamorphic rocks of the upper plate of the Chocolate Mountain Thrust system underlain by the Orocopia Schist. These rocks dip to the southeast and extend westward to the Sand Hills Fault but do not appear to cross it. Thus, the Sand Hills Fault is interpreted to be the southern extension of the San Andreas Fault. North of the Sand Hills Fault the East Highline Canal seismicity lineament is associated with a strike-slip fault and is probably linked to the Sand Hills Fault. Six geothermal areas crossed by these lines, in agreement with previous studies of geothermal reservoirs, are associated with ''faded'' zones, Bouguer gravity and heat flow maxima, and with higher seismic velocities than surrounding terranes.« less

The Subduction of an Exhumed and Serpentinized Magma-Poor Basement Beneath the Northern Lesser Antilles Reveals the Early Tectonic Fabric at Slow-Spreading Mid-Oceanic Ridges

NASA Astrophysics Data System (ADS)

Marcaillou, B.; Klingelhoefer, F.; Laurencin, M.; Biari, Y.; Graindorge, D.; Jean-Frederic, L.; Laigle, M.; Lallemand, S.

2017-12-01

Multichannel and wide-angle seismic data as well as heat-flow measurements (ANTITHESIS cruise, 2016) reveal a 200x200km patch of magma-poor oceanic basement in the trench and beneath the outer fore-arc offshore of Antigua to Saint Martin in the Northern Lesser Antilles. These data highlight an oceanic basement with the following features: 1/ Absence of any reflection at typical Moho depth and layer2/layer3 limit depths. 2/ High Velocity Vp at the top (>5.5 km/s), low velocity gradient with depth (<0.3 s-1) and no significant velocity change at theoretical Moho depth. 3/ Anomalously low heat-flow (40±15mW.m-2) compared to the central Antilles and to theoretical values for an 80 Myr-old oceanic plate suggesting the influence of deep hydrothermal circulation. 4/ Two sets of reflections dipping toward the paleo mid-Atlantic ridge and toward the Vidal Transform Fault Zone respectively. These highly reflective planes sometimes fracture the top of the basement, deforming the interplate contact and extend downward to 20km depth with a 20° angle. We thus propose that a large patch of mantle rocks, exhumed and serpentinized at the slow-spreading mid-Atlantic Ridge 80 Myr ago, is currently subducting beneath the Northern Lesser Antilles. During the exhumation, early extension triggers penetrative shear zones sub-parallel to the ridge and to the transform fault. Eventually, this early extension generates sliding along the so-called detachment fault, while the other proto-detachment abort. Approaching the trench, the plate bending reactivates these weak zones in normal faults and fluid pathways promoting deep serpentinisation and localizing tectonic deformation at the plate interface. These subducting fluid-rich mechanically weak mantle rocks rise questions about their relation to the faster slab deepening, the lower seismic activity and the pervasive tectonic partitioning in this margin segment.

Resolving the fault systems with the magnetotelluric method in the western Ilan plain of NE Taiwan

NASA Astrophysics Data System (ADS)

Chang, P. Y.; Chen, C. S.

2017-12-01

In the study we attempt to use the magnetotelluric (MT) surveys to delineate the basement topography of the western part of the Ilan plain. The triangular plain is located on the extension part of the Okinawa Trough, and is thought to be a subsidence basin bounded by the Hsueshan Range in the north and the Central Range in the south. The basement of the basin is composed of Tertiary metamorphic rocks such as argillites and slates. The recent extension of the Okinawa Trough started from approximately 0.1 Ma and involved ENE- and WSW-trending normal faults that may extended into the Ilan plain area. However, high sedimentation rates as well as the frequent human activities have resulted in unconsolidated sediments with a thickness of over 100 meters, and caused the difficulties in observing the surface traces of the active faults in the area. Hence we deployed about 70 MT stations across the southwestern tip of the triangular plain. We also tried to resolve the subsurface faults the relief variations of the basement with the inverted resistivity images, since the saturated sediments are relatively conductive and the consolidated rocks are resistive. With the inverted MT images, we found that there are a series of N-S trending horsts and grabens in addition to the ENE-WSW normal fault systems. The ENE-WSW trending faults are dipping mainly toward the north in our study area in the western tip of the Ilan plain. The preliminary results suggest that a younger N-S trending normal fault system may modify the relief of the basement in the recent stage after the activation of the ENE-WSW normal faults. The findings of the MT resistivity images provide new information to further review the tectonic explanations of the region in the future.

Origin of the ca. 50 Ma Linzizong shoshonitic volcanic rocks in the eastern Gangdese arc, southern Tibet

NASA Astrophysics Data System (ADS)

Liu, An-Lin; Wang, Qing; Zhu, Di-Cheng; Zhao, Zhi-Dan; Liu, Sheng-Ao; Wang, Rui; Dai, Jin-Gen; Zheng, Yuan-Chuan; Zhang, Liang-Liang

2018-04-01

The origin of the Eocene shoshonitic rocks within the upper part of the extensive Linzizong volcanic succession (i.e., the Pana Formation) in the Gangdese arc, southern Tibet remains unclear, inhibiting the detailed investigations on the crust-mantle interaction and mantle dynamics that operate the generation of the coeval magmatic flare-up in the arc. We report mineral composition, zircon U-Pb age and zircon Hf isotope, whole-rock element and Sr-Nd-Hf isotope data for the Pana Formation volcanic rocks from Pangduo, eastern Gangdese arc in southern Tibet. The Pana volcanic rocks from Pangduo include basalts, basaltic andesites, and dacites. SIMS and LA-ICPMS zircon U-Pb dating indicates that the Pangduo dacites were erupted at 50 ± 1 Ma, representing the volcanic equivalent of the coeval Gangdese Batholith that define a magmatic flare-up at 51 ± 1 Ma. The Pangduo volcanic rocks are exclusively shoshonitic, differing from typical subduction-related calc-alkaline volcanic rocks. The basalts have positive whole-rock ƐNd(t) (+1.7) and ƐHf(t) (+3.8) with high Zr abundances (121-169 ppm) and Zr/Y ratios (4.3-5.2), most likely derived from the partial melting of an enriched garnet-bearing lithospheric mantle that was metasomatized by subduction-related components with input from asthenosphere. Compared to the basalts, similar trace elemental patterns and decreased whole-rock ƐNd(t) (-3.5 to -3.3) and ƐHf(t) (-2.5 to -1.6) of the basaltic andesites can be attributed to the input of the ancient basement-derived material of the central Lhasa subterrane into the basaltic magmas. The coherent whole-rock Sr-Nd-Hf isotopic compositions ((87Sr/86Sr)i = 0.7064-0.7069, ƐNd(t) = -6.0 to -5.2, ƐHf(t) = -5.6 to -5.0) and varying zircon ƐHf(t) (-6.0 to +4.1) of the dacites can be interpreted by the partial melting of a hybrid lower crust source (juvenile and ancient lower crust) with incorporation of basement-derived components. Calculations of zircon-Ti temperature and whole-rock zircon saturation temperature of the dacites, and clinopyroxene crystallization temperature of the basalts suggest that the Pangduo volcanic rocks are most likely derived from the high-temperature melting of the lithosphere (including lithospheric mantle and overlying continental crust) as a result of the slab breakoff of the Neo-Tethyan oceanic lithosphere.

Archean Arctic continental crust fingerprints revealing by zircons from Alpha Ridge bottom rocks

NASA Astrophysics Data System (ADS)

Sergeev, Sergey; Petrov, Oleg; Morozov, Andrey; Shevchenko, Sergey; Presnyakov, Sergey; Antonov, Anton; Belyatsky, Boris

2015-04-01

Whereas thick Cenozoic sedimentary cover overlapping bedrock of the Arctic Ocean, some tectonic windows were sampled by scientific submarine manipulator, as well as by grabbing, dredging and drilling during «Arctic-2012» Russian High-Arctic expedition (21 thousands samples in total, from 400-km profile along Alpha-Mendeleev Ridges). Among others, on the western slope of Alpha Ridge one 10x10 cm fragment without any tracks of glacial transportation of fine-layered migmatitic-gneiss with prominent quartz veinlets was studied. Its mineral (47.5 vol.% plagioclase + 29.6% quartz + 16.6% biotite + 6.1% orthoclase) and chemical composition (SiO2:68.2, Al2O3:14.9, Fe2O3:4.44, TiO2:0.54, MgO:2.03, CaO:3.13, Na2O:3.23, K2O:2.16%) corresponds to trachydacite vulcanite, deformed and metamorphozed under amphibolite facies. Most zircon grains (>80%) from this sample has an concordant U-Pb age 3450 Ma with Th/U 0.8-1.4 and U content of 100-400 ppm, epsilon Hf from -4 up to 0, and ca 20% - ca 3.3 Ga with Th/U 0.7-1.4 and 90-190 ppm U, epsilon Hf -6.5 to -4.5, while only 2% of the grains show Proterozoic age of ca 1.9 Ga (Th/U: 0.02-0.07, U~500 ppm, epsilon Hf about 0). No younger zircons were revealed at all. We suppose that magmatic zircon crystallized as early as 3450 Ma ago during acid volcanism, the second phase zircon crystallization from partial melt (or by volcanics remelting) under amphibolite facies metamorphism was at 3.3 Ga ago with formation of migmatitie gneisses. Last zircon formation from crustal fluids under low-grade metamorphic conditions was 1.9 Ga ago. There are two principal possibilities for the provenance of this metavolcanic rock. The first one - this is ice-rafted debris deposited by melted glacial iceberg. However, presently there are no temporal and compositional analogues of such rocks in basement geology of peri-oceanic regions, including Archean Itsaq Gneiss Complex, Lewisian Complex and Baltic Shield but these regions are far from the places of nowadays iceberg formation. Moreover, no Grenvillian-age zircons were revealed in studied sample. The nearest areas of Paleo- and Mesoarchean rocks appearance are deeply inland territories like Acasta Gneiss Complex, Uivak Gneisses, the Superior Province, Anabar and Aldan Shields, Northern China craton (Anshan complex), thus, a only possibility to bring some fragments of basement rocks from these areas to oceanic coast is transcontinental river transportation. The second possibility - this gneissic clast has a local provenance and has undergone a submarine weathering, shallow marine avalanche, proximal transportation by alongside ocean flows, tidal waves abrasion, and as so, reflects local bedrock geology, i.e. adjacent Alpha Ridge rock composition. Additional confirmation of this can be seen in a wide distribution of Qu-sandstones with Paleo-Mezoarchean zircons, and finds of similar allochtonous zircons in dolerites along Alpha-Mendeleev Ridge profile. The studied fragment is a unique evidence for the possible existence of Paleoarchean continental crust within the submarine Alpha-Mendeleev Ridge in Arctic Ocean.

Structural analysis of Precambrian rocks at the Hot Dry Rock Site at Fenton Hill, New Mexico

DOE Office of Scientific and Technical Information (OSTI.GOV)

Burns, K.L.; Potter, R.M.

1995-01-01

The subcrop of basement rock at Fenton HIll comprises Precambrian gneiss, schist, amphibolite, pegmatite, and granitoids with affinities in metamorphic and structural history to surface outcrops in the Tusas and Picuris Ranges. Televiewer measurements of structures were analyzed by taking advantage of the spatial continuity of foliations. Folds in the foliation are predominantly conical forms due to interference between structures formed in F2 and F3 tectonic events. Field observations of outcrops in the Picuris Range show that the fractures are predominantly an X-T network controlled by the lithological layering, and statistical evidence indicates that this layer-controlled network persists to depthmore » at Fenton Hill.« less

The south-central United States magnetic anomaly

NASA Technical Reports Server (NTRS)

Hinze, W. J.; Braile, L. W. (Principal Investigator); Starich, P. J.

1984-01-01

The South-Central United States Magnetic Anomaly is the most prominent positive feature in the MAGSAT scalar magnetic field over North America. The anomaly correlates with increased crustal thickness, above average crustal velocity, negative free air gravity anomalies and an extensive zone of Middle Proterozoic anorogenic felsic basement rocks. Spherical dipole source inversion of the MAGSAT scalar data and subsequent calculation of reduced to pole and derivative maps provide constraints for a crustal magnetic model which corresponds geographically to the extensive Middle Proterozoic felsic rocks trending northeasterly across the United States. These felsic rocks contain insufficient magnetization or volume to produce the anomaly, but are rather indicative of a crustal zone which was disturbed during a Middle Proterozoic thermal event which enriched magnetic material deep in the crust.

Proximity of Wastewater Disposal and Hydraulic Fracturing to Crystalline Basement Affects the Likelihood of Induced Seismicity in the Central and Eastern United States

NASA Astrophysics Data System (ADS)

Brudzinski, M.; Skoumal, R.; Currie, B.

2016-12-01

Over the past decade, the dramatic rise in seismicity in the central and eastern US has been attributed to industry operations associated with wastewater injection and hydraulic fracturing. While most of the observed seismicity has occurred in sedimentary basins that have experienced overall increases in oil and gas development (e.g. the Anadarko and Ft. Worth basins), other basins with similar activity (e.g. the Williston and northern Appalachian basins) have experienced very little, if any, induced seismicity. While hydro-geomechanical modeling indicates that induced seismicity may be related to the proximity of critically stressed faults in the crystalline basement, recent studies have found fluid injection rate to be the dominant factor controlling induced seismicity. To test these interpretations we evaluated water disposal and well completion records from the Appalachian, Illinois, and Williston basins, and compared them with induced seismic sequences identified through seismic template matching of all cataloged earthquakes in these regions. Our results indicate a strong correspondence between induced seismic events and the proximity of subsurface wastewater injection/hydraulic fracturing targets to crystalline basement rocks. For example, in the northern Appalachian Basin, of the >20 identified induced seismic sequences, all but two were associated with injection/completion targets located at depths within 1 km of the basement. In parts of the basin where target intervals are at depths >1 km from basement, induced events have been recorded only in proximity to basement-involved faults. In addition, in the Williston Basin most disposal interval/hydraulic fracturing targets are >1 km above the crystalline basement which may explain the lack of induced seismic events in the region despite high rate fluid injection. Collectively, the results of our investigation suggest that proximity to basement is an important variable in considering the likelihood of induced seismicity associated with wastewater disposal and hydraulic fracturing. This has important implications regarding induced-seismic risk assessment related to the siting of new disposal wells and/or the production of hydrocarbon from near-basement reservoirs.

Structure of the active rift zone and margins of the northern Imperial Valley from Salton Seismic Imaging Project (SSIP) data

NASA Astrophysics Data System (ADS)

Livers, A.; Han, L.; Delph, J. R.; White-Gaynor, A. L.; Petit, R.; Hole, J. A.; Stock, J. M.; Fuis, G. S.

2012-12-01

First-arrival refraction data were used to create a seismic velocity model of the upper crust across the actively rifting northern Imperial Valley and its margins. The densely sampled seismic refraction data were acquired by the Salton Seismic Imaging Project (SSIP) , which is investigating rift processes in the northern-most rift segment of the Gulf of California extensional province and earthquake hazards at the southern end of the San Andreas Fault system. A 95-km long seismic line was acquired across the northern Imperial Valley, through the Salton Sea geothermal field, parallel to the five Salton Butte volcanoes and perpendicular to the Brawley Seismic Zone and major strike-slip faults. Nineteen explosive shots were recorded with 100 m seismometer spacing across the valley and with 300-500 m spacing into the adjacent ranges. First-arrival travel times were picked from shot gathers along this line and a seismic velocity model was produced using tomographic inversion. Sedimentary basement and seismic basement in the valley are interpreted to be sediment metamorphosed by the very high heat flow. The velocity model shows that this basement to the west of the Brawley Seismic Zone is at ~4-km depth. The basement shallows to ~2-km depth in the active geothermal field and Salton Buttes volcanic field which locally coincide with the Brawley Seismic Zone. At the eastern edge of the geothermal field, the basement drops off again to ~3.5-km depth. The eastern edge of the valley appears to be fault bounded by the along-strike extension of the Sand Hills Fault, an inactive strike-slip fault. The seismic velocities to the east of the fault correspond to metamorphic rock of the Chocolate Mountains, different from the metamorphosed basement in the valley. The western edge of the valley appears to be fault bounded by the active Superstition Hills Fault. To the west of the valley, >4-km deep valley basement extends to the active Superstition Hills Fault. Basement then shallows westward towards exposures of granitic basement in the Superstition Mountains. The basin between the Superstition Mountains and Coyote Mountains is ~2 km deep.

Continental Subduction: Mass Fluxes and Interactions with the Wider Earth System

NASA Astrophysics Data System (ADS)

Cuthbert, S. J.

2011-12-01

Substantial parts of ultra-high pressure (UHP) terrains probably represent subducted passive continental margins (PCM). This contribution reviews and synthesises research on processes operating in such systems and their implication for the wider Earth system. PCM sediments are large repositories of volatiles including hydrates, nitrogen species, carbonates and hydrocarbons. Sediments and upper/ mid-crustal basement are rich in incompatible elements and are fertile for melting. Lower crust may be more mafic and refractory. Juvenile rift-related mafic rocks also have the potential to generate substantial volumes of granitoid melts, especially if they have been hydrated. Exposed UHP terrains demonstrate the return of continental crust from mantle depths, show evidence for substantial fluxes of aqueous fluid, anatexis and, in entrained orogenic peridotites, metasomatism of mantle rocks by crust- derived C-O-H fluids. However, substantial bodies of continental material may never return to the surface as coherent masses of rock, but remain sequestered in the mantle where they melt or become entrained in the deeper mantle circulation. Hence during subduction, PCM's become partitioned by a range of mechanisms. Mechanical partitioning strips away weaker sediment and middle/upper crust, which circulate back up the subduction channel, while denser, stronger transitional pro-crust and lower crust may "stall" near the base of the lithosphere or be irreversibly subducted to join the global mantle circulation. Under certain conditions sediment and upper crustal basement may reach depths for UHPM. Further partitioning takes place by anatexis, which either aids stripping and exhumation of the more melt-prone rock-masses through mechanical softening, or separates melt from residuum so that melt escapes and is accreted to the upper plate leading to "undercrusting", late-orogenic magmatism and further refinement of the crust. Melt that traverses sections of mantle will interact with it causing metasomatism and refertilisation. Partitioning also takes place by solid-fluid and melt-fluid partitioning. Dehydration may take place both during subduction and exhumation, and fluxes between dehydrating and hydrating rock masses influence the internal fluid budget of the orogen (essential for eclogitisation and densification of mafic lithologies). Ascending granitic melts advect dissolved water to shallow levels, or even the atmosphere. Irreversible subduction of PCM sediment carries water plus nitrogen species to the deeper mantle. Decarbonation of voluminous PCM carbonates depends on thermal regime and may release a pulse of CO2 to the atmosphere, but is limited in colder subduction zones hence transferring large volumes of carbon to the deep mantle. This may ultimately be mobilised by melting or dissolution to form fluid media for diamond formation.

Kinematic vicissitudes and the spatial distribution of the alteration zone related to the Byobuyama fault, central Japan. (Implication; Influence of another faults.)

NASA Astrophysics Data System (ADS)

Katori, T.; Kobayashi, K.

2015-12-01

The central Japan is one of the most concentrated area of active faults (Quaternary fault). These are roughly classified into two orthogonally-oriented fault sets of NE-SW and NW-SE strikes. The study area is located in Gifu prefecture, central Japan. In there, the basement rocks are composed mainly of Triassic-Jurassic accretionary prism (Mino belt), Cretaceous Nohi Rhyolite and Cretaceous granitic rocks. Miocene Mizunami G. and Pliocene-Pleistocene Toki Sand and Gravel F. unconformably cover the basement rocks. The Byobuyama fault, 32 km in length, is NE-SW strike and displaces perpendicularly the Toki Sand and Gravel F. by 500 m. The northeastern terminal of the fault has contact with the southern terminal of the Atera fault of NW-SE strike and offset their displacements each other. It is clear that the activity of the Byobuyama fault plays a role of the development of the complicated fault geometry system in the central Japan. In this study, we performed a broad-based investigation along the Byobuyama fault and collected samples. Actually, we observed 400 faults and analyzed 200 fault rocks. Based on these results, we obtained the following new opinion. 1. The Byobuyama fault has experienced following activities that can be divided to 3 stages at least under different stress field. 1) Movement with the sinisterly sense (preserved in cataclasite zone). 2) Dextral movement (preserved in fault gouge zone). 3) Reverse fault movement (due to the aggressive rise of mountains). In addition, the change from Stage 2 to Stage 3 is a continuous. 2. There is a relationship between the distance from the trace of the Byobuyama fault and the combination of alteration minerals included in the fault rocks. 3. In the central part of the Byobuyama fault (CPBF), fault plane trend and combination of alteration minerals shows specific features. The continuous change is considered to mean the presence of factors that interfere with the dextral movement of the Byobuyama fault. What is considered as one of the factors is the effect of the fault zone adjacent, especially the Atera fault. CPBF is located just southeast extension of the Akou fault, NW-SE strike. We think that this extension reaches up to CPBF. Based on the above, we make a presentation about interaction of two faults from the point of view of kinematic vicissitudes and alteration process.

Upper-crustal structure beneath the strait of Georgia, Southwest British Columbia

USGS Publications Warehouse

Dash, R.K.; Spence, G.D.; Riedel, M.; Hyndman, R.D.; Brocher, T.M.

2007-01-01

We present a new three-dimensional (3-D) P-wave velocity model for the upper-crustal structure beneath the Strait of Georgia, southwestern British Columbia based on non-linear tomographic inversion of wide-angle seismic refraction data. Our study, part of the Georgia Basin Geohazards Initiative (GBGI) is primarily aimed at mapping the depth of the Cenozoic sedimentary basin and delineating the near-surface crustal faults associated with recent seismic activities (e.g. M = 4.6 in 1997 and M = 5.0 in 1975) in the region. Joint inversion of first-arrival traveltimes from the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS) and the 2002 Georgia Basin experiment provides a high-resolution velocity model of the subsurface to a depth of ???7 km. In the southcentral Georgia Basin, sedimentary rocks of the Cretaceous Nanaimo Group and early Tertiary rocks have seismic velocities between 3.0 and 5.5 km s-1. The basin thickness increases from north to south with a maximum thickness of 7 (??1) km (depth to velocities of 5.5 km s-1) at the southeast end of the strait. The underlying basement rocks, probably representing the Wrangellia terrane, have velocities of 5.5-6.5 km-1 with considerable lateral variation. Our tomographic model reveals that the Strait of Georgia is underlain by a fault-bounded block within the central Georgia Basin. It also shows a correlation between microearthquakes and areas of rapid change in basin thickness. The 1997/1975 earthquakes are located near a northeast-trending hinge line where the thicknesses of sedimentary rocks increase rapidly to the southeast. Given its association with instrumentally recorded, moderate sized earthquakes, we infer that the hinge region is cored by an active fault that we informally name the Gabriola Island fault. A northwest-trending, southwest dipping velocity discontinuity along the eastern side of Vancouver Island correlates spatially with the surface expression of the Outer Island fault. The Outer Island fault as mapped in our seismic tomography model is a thrust fault that projects directly into the Lummi Island fault, suggesting that they are related structures forming a fault system that is continuous for nearly 90 km. Together, these inferred thrust faults may account for at least a portion of the basement uplift at the San Juan Islands. ?? 2007 The Authors Journal compilation ?? 2007 RAS.

Preliminary aeromagnetic anomaly map of California

USGS Publications Warehouse

Roberts, Carter W.; Jachens, Rober C.

1999-01-01

The magnetization in crustal rocks is the vector sum of induced in minerals by the Earth’s present main field and the remanent magnetization of minerals susceptible to magnetization (chiefly magnetite) (Blakely, 1995). The direction of remanent magnetization acquired during the rock’s history can be highly variable. Crystalline rocks generally contain sufficient magnetic minerals to cause variations in the Earth’s magnetic field that can be mapped by aeromagnetic surveys. Sedimentary rocks are generally weakly magnetized and consequently have a small effect on the magnetic field: thus a magnetic anomaly map can be used to “see through” the sedimentary rock cover and can convey information on lithologic contrasts and structural trends related to the underlying crystalline basement (see Nettleton,1971; Blakely, 1995). The magnetic anomaly map (fig. 2) provides a synoptic view of major anomalies and contributes to our understanding of the tectonic development of California. Reference fields, that approximate the Earth’s main (core) field, have been subtracted from the recorded magnetic data. The resulting map of the total magnetic anomalies exhibits anomaly patterns related to the distribution of magnetized crustal rocks at depths shallower than the Curie point isotherm (the surface within the Earth beneath which temperatures are so high that rocks lose their magnetic properties). The magnetic anomaly map has been compiled from existing digital data. Data obtained from aeromagnetic surveys that were made at different times, spacings and elevations, were merged by analytical continuation of each set onto a common surface 305 m (1000 ft) above terrain. Digital data in this compatible form allows application of analytical techniques (Blakley, 1995) that can be used to enhance anomaly characteristics (e.g., wavelength and trends) and provide new interpretive information.

Carboniferous - Early Permian magmatic evolution of the Bogda Range (Xinjiang, NW China): Implications for the Late Paleozoic accretionary tectonics of the SW Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Wali, Guzalnur; Wang, Bo; Cluzel, Dominique; Zhong, Linglin

2018-03-01

The Late Paleozoic magmatic evolution of the Bogda Range (Chinese North Tianshan) is important for understanding the accretionary history of the Central Asian Orogenic Belt. We investigated the Carboniferous and Lower Permian volcanic and sedimentary sequences of the Daheyan section, southern Bogda Range, and present new zircon U-Pb ages and whole-rock geochemical data for the volcanic rocks. One Carboniferous rhyolite is dated at 298 ± 8 Ma; a Permian basalt yielded many Proterozoic zircon xenocrysts, and its maximum age (∼297 Ma) is constrained by the detrital zircon ages of the sandstone that stratigraphically underlies it. These volcanic rocks belong to calc-alkaline series. We further synthesize previous geochronological, geochemical and isotopic data of magmatic and sedimentary rocks in the Bogda Range. The available data indicate that the magmatism occurred continuously from 350 Ma to 280 Ma. A comprehensive analysis allows us to propose that: (1) the Carboniferous to Early Permian magmatic rocks of the Bogda Range generally show consistent arc-type features; (2) increasing mantle input through time suggests intra-arc extension in a supra-subduction zone; (3) the localized occurrence of Early Permian alkaline pillow basalts and deep water sediments close to the major shear zone advocate a transtensional crustal thinning during the transition from Carboniferous convergence to Early Permian transcurrent tectonics; (4) occurrence of a large number of Proterozoic zircon xenocrysts in the Late Paleozoic magmatic rocks, and Proterozoic detrital zircons in the coeval clastic sediments suggest a continental or transitional basement of the Bogda Arc; (5) subduction in the Bogda area terminated prior to the deposition of Middle Permian terrestrial sediments.

Investigations on Local Seismic Phases and Modeling of Seismic Signals

DTIC Science & Technology

1993-10-31

basement is 1 km. The water table, wt , is from Doty and Thordarson (1983). It separates the dry, DT, and the wet, WT, tuff levels. Above these volcanic...regional variations of t*(f) in the United States. Geophys. J. R. astr. Soc. 82 , 125-140 Doty, G. C. and W. Thordarson , 1983. Water table in rocks of

Rock strength measurements on Archaean basement granitoids recovered from scientific drilling in the active Koyna seismogenic zone, western India

NASA Astrophysics Data System (ADS)

Goswami, Deepjyoti; Akkiraju, Vyasulu V.; Misra, Surajit; Roy, Sukanta; Singh, Santosh K.; Sinha, Amalendu; Gupta, Harsh; Bansal, B. K.; Nayak, Shailesh

2017-08-01

Reservoir triggered earthquakes have been occurring in the Koyna area, western India for the past five decades. Triaxial tests carried out on 181 core samples of Archaean granitoids underlying the Deccan Traps provide valuable constraints on rock strength properties in the Koyna seismogenic zone for the first time. The data include measurements on granite gneiss, granite, migmatitic gneiss and mylonitised granite gneiss obtained from boreholes KBH-3, KBH-4A, KBH-5 and KBH-7 located in the western and eastern margins of the seismic zone. Salient results are as follows. (i) Increase of rock strength with increasing confining pressure allow determination of the linearized failure envelopes from which the cohesive strength and angle of internal friction are calculated. (ii) Variable differential stresses at different depths are the manifestations of deformation partitioning in close association of fault zone(s) or localized fracture zones. (iii) Fractures controlled by naturally developed weak planes such as cleavage and fabric directly affect the rock strength properties, but the majority of failure planes developed during triaxial tests is not consistent with the orientations of pre-existing weak planes. The failure planes may, therefore, represent other planes of weakness induced by ongoing seismic activity. (iv) Stress-strain curves confirm that axial deformation is controlled by the varying intensity of pre-existing shear in the granitoids, viz., mylonite, granite gneiss and migmatitic gneiss. (v) Frequent occurrences of low magnitude earthquakes may be attributed to low and variable rock strength of the granitoids, which, in turn, is modified by successive seismic events.

Basement thrust sheets in the Clearwater orogenic zone, central Idaho and western Montana

NASA Astrophysics Data System (ADS)

Skipp, Betty

1987-03-01

The Clearwater orogenic zone in central Idaho and western Montana contains at least two major northeast-directed Cordilleran thrust plates of Early Proterozoic metasedimentary and metaigneous rocks that overrode previously folded Middle Proterozoic rocks of the Belt basin in Cretaceous time. The northeastward migration of the resultant thickened wedge of crustal material combined with Cretaceous subduction along the western continental margin produced a younger northern Bitterroot lobe of the Idaho batholith relative to an older southern Atlanta lobe. Eocene extensional unroofing and erosion of the Bitterroot lobe has exposed the roots of the thick Cordilleran thrust sheets.

Modelling the gravity and magnetic field anomalies of the Chicxulub crater

NASA Technical Reports Server (NTRS)

Aleman, C. Ortiz; Pilkington, M.; Hildebrand, A. R.; Roest, W. R.; Grieve, R. A. F.; Keating, P.

1993-01-01

The approximately 180-km-diameter Chicxulub crater lies buried by approximately 1 km of sediment on the northwestern corner of the Yucatan Peninsula, Mexico. Geophysical, stratigraphic and petrologic evidence support an impact origin for the structure and biostratigraphy suggests that a K/T age is possible for the impact. The crater's location is in agreement with constraints derived from proximal K/T impact-wave and ejecta deposits and its melt-rock is similar in composition to the K/T tektites. Radiometric dating of the melt rock reveals an age identical to that of the K/T tektites. The impact which produced the Chicxulub crater probably produced the K/T extinctions and understanding the now-buried crater will provide constraints on the impact's lethal effects. The outstanding preservation of the crater, the availability of detailed gravity and magnetic data sets, and the two-component target of carbonate/evaporites overlying silicate basement allow application of geophysical modeling techniques to explore the crater under most favorable circumstances. We have found that the main features of the gravity and magnetic field anomalies may be produced by the crater lithologies.

Southwest USA Exhumation History Recorded Below the Great Unconformity

NASA Astrophysics Data System (ADS)

Heizler, M. T.; Karlstrom, K. E.

2002-05-01

The Southwestern USA Precambrian terranes preserve a long and variable exhumation history that can be tracked using thermochronological methods. This exhumation history is controlled on two interrelated scales. At first order, it is recognized that 1.7 to 1.4 Ga mid-crustal (10 km, 2-4 kbar) rocks were ultimately exhumed and reside below unconformities of variable age. In Arizona, Mesoproterozoic Apache Group and Neoproterozoic Supergroup sedimentary rocks lie directly on basement and thus indicate exhumation of some regions relatively soon following the 1.4 Ga events. In the Rocky Mountains of Colorado and the Rio Grande rift uplifts of New Mexico, basement is generally overlain by Cambrian to Mississippian strata. The unconformities are useful markers of net exhumation; however do not reveal a time-integrated path. Using published, and hundreds of new 40Ar/39Ar analyses of hornblende, muscovite, biotite and K-feldspar, and a growing U/Pb accessory mineral thermochronology database, we are extracting exhumation information with great detail. The thermochronological data continue to support the claim that relatively low net exhumation occurred following 1.7 to 1.6 Ga accretion of volcanic arc terranes to the southern margin of Laurentia. Mid-crustal (2-4 kbar) rocks stabilized soon after accretion, whereas in some regions like the Upper Granite Gorge, Grand Canyon deeper (6 kbar) metamorphic terranes decompressed to 3 kbar before stabilization. The cooling history of these mid-crustal rocks post 1.65 Ga remains somewhat unknown. Overall slow-cooling models (550 \\deg C to 300 \\deg C from 1.7 to 1.4 Ga) require high geothermal gradients in order to maintain 10 km deep rocks at high temperatures for 100's of Ma. Alternatively, isobaric cooling models to more normal geothermal gradients (i.e. 25 \\deg C/km) at ca. 1.65 Ga require later (1.4 Ga) thermal pulses and/or Mesoproterozoic vertical displacements to explain highly discordant thermochronological data. Either model supports relatively normal crustal thickness during 1.8-1.6 Ga arc accretion. In the Grand Canyon, cooling and exhumation are recorded at 1.4 Ga and 1.25 Ga. 1.4 Ga tectonism is shown by a sharp contrast in mica ages (1.4 vs. 1.6 Ga) across the 96-mile shear zone. The 1.25 Ga denudation is required by 1.25-1.30 Ga K-feldspar argon ages from basement that is unconformably overlain by a 1253 Ma volcanic ash horizon in the Unkar Group sediments. Arizona transition zone rocks of similar metamorphic pressure yield highly variable argon ages and indicate that small differences in exposed paleodepth can yield significantly different data. This is supported by cooling ages that systematically decrease with increase in paleodepth in the Gold Butte block, NV. Combined, the thermochronological data from the SW USA record a complex exhumation history that is characterized by discrete block uplift superimposed on an overall billion year erosional history that brings mid-crustal rocks to the surface a variable times.

4. VIEW LOOKING WEST FROM BASEMENT UNDER OPEN CONCOURSE ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

4. VIEW LOOKING WEST FROM BASEMENT UNDER OPEN CONCOURSE - CEILING OF BRICK ARCHES; WOOD PANELS MARK LOCATION OF ORIGINAL GLASS PRISM SKYLIGHTS REPLACED WITH CONCRETE - Pennsylvania Railroad Station, Open Concourse & Concourse Roof Extension, 1101 Liberty Avenue, Pittsburgh, Allegheny County, PA

Timing and Mechanisms of Exhumation in West Central Sulawesi, Indonesia

NASA Astrophysics Data System (ADS)

Hennig, J.; Hall, R.; Watkinson, I. M.; Forster, M.

2012-12-01

New U-Pb and 40Ar/39Ar ages from basement and intrusive rocks from NW Sulawesi record Neogene deformation, much younger than expected, and rapid exhumation. The unusual K-shape of Sulawesi reflects a complex tectonic history in the convergent zone between the Australian, Eurasian and Philippine Sea plates. The Neck is only a few tens of kilometres wide but includes mountains up to 2.5 km high, separating the 2 km deep Gorontalo Bay from similar depths of the Makassar Straits. It represents the Mesozoic-Cenozoic Sundaland continental margin and includes numerous granitoid intrusions. Little is known about the basement protoliths, timing of deformation or causes of magmatic activity. New models propose an important role for extension, associated with rollback of the Banda and North Sulawesi subduction zones. The major NNW-trending Palu-Koro strike-slip fault exhumes ultra high-pressure rocks and granitoids and may be related to North Sulawesi subduction. Work in progress on central Sulawesi's granitic basement orthogneisses shows that zircons dated by U-Pb LA-ICPMS contain Proterozoic inherited cores, and Devonian, Permo-Triassic and Jurassic zircon populations, which suggest an Australian-derived terrane. Basement rocks of the Palu Metamorphic Complex (PMC) were also thought to have Permo-Triassic protoliths and were previously suggested to represent the upper plate of a late Mesozoic subduction zone. Schistose rocks of the PMC have a complex history of metamorphism, crystal growth and deformation. Aluminium silicate porphyroblasts were interpreted as the product of contact metamorphism around granitic intrusions. However, pre-kinematic cordierite, andalusite porphyroblasts and muscovite pseudomorphs after staurolite in the complex indicate a regional high temperature-low pressure metamorphic event. The schists are strongly mylonitized, and overprinted by an S-C fabric recording several generations of biotite and some muscovite growth. 40Ar/39Ar thermochronology reveals a Pliocene cooling age. Further dating of biotites, white mica and amphiboles from schists and amphibolite intercalations is ongoing to determine the history of mylonitic deformation. Temperature-age plots using U-Pb zircon dating, and 40Ar/39Ar and (U-Th)/He geochronological techniques on biotites and apatites from granitic rocks, define thermal histories for the intrusions. Granites from the Neck and the mountain range west of the Palu-Koro Fault have approximately Late Miocene crystallisation ages as indicated by LA-ICPMS and 40Ar/39Ar cooling ages of 7.20 ± 0.05 Ma and 6.41 ± 0.06 Ma. Late-stage exhumation started in the Neck during the Pliocene (AHe: 2.9 ± 0.2 Ma). Erosion rates determined by (U-Th)/He ages can help estimate the amount of sediment input into adjacent deep basins. Age-elevation plots and modelling suggest exhumation rates of 0.75 (-0.16/+0.27) mm/a, which results in a calculated amount of c. 2 km of continental crust that has been removed in the last 3 Myr. We suggest magmatism, metamorphic core complex exhumation, and subsidence of Gorontalo Bay are all related to crustal thinning due to extension driven by subduction rollback.

Tectonic framework of Turkish sedimentary basins

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yilmaz, P.O.

1988-08-01

Turkey's exploration potential primarily exists in seven onshore (Southeast Turkey platform, Tauride platform, Pontide platform, East Anatolian platform, Interior, Trace, and Adana) basins and four offshore (Black Sea, Marmara Sea, Aegean Sea, and Mediterranean Sea) regional basins formed during the Mesozoic and Tertiary. The Mesozoic basins are the onshore basins: Southeast Turkey, Tauride, Pontide, East Anatolian, and Interior basins. Due to their common tectonic heritage, the southeast Turkey and Tauride basins have similar source rocks, structural growth, trap size, and structural styles. In the north, another Mesozoic basin, the Pontide platform, has a much more complex history and very littlemore » in common with the southerly basins. The Pontide has two distinct parts; the west has Paleozoic continental basement and the east is underlain by island-arc basement of Jurassic age. The plays are in the upper Mesozoic rocks in the west Pontide. The remaining Mesozoic basins of the onshore Interior and East Anatolian basins are poorly known and very complex. Their source, reservoir, and seal are not clearly defined. The basins formed during several orogenic phases in mesozoic and Tertiary. The Cenozoic basins are the onshore Thrace and Adana basins, and all offshore regional basins formed during Miocene extension. Further complicating the onshore basins evolution is the superposition of Cenozoic basins and Mesozoic basins. The Thrace basin in the northwest and Adana basin in the south both originate from Tertiary extension over Tethyan basement and result in a similar source, reservoir, and seal. Local strike-slip movement along the North Anatolian fault modifies the Thrace basin structures, influencing its hydrocarbon potential.« less

The Olympic Dam copper-uranium-gold deposit, Roxby Downs, South Australia

DOE Office of Scientific and Technical Information (OSTI.GOV)

Roberts, D.E.; Hudson, G.R.T.

1983-08-01

The Olympic Dam copper-uranium-gold deposit appears to be a new type of strata-bound sediment-hosted ore deposit. It is located 650 km north-northwest of Adelaide in South Australia and was discovered in 1975. It has an areal extent exceeding 20 km/sup 2/ with vertical thicknesses of mineralization up to 350 m. The deposit is estimated to contain in excess of 2,000 million metric tons of mineralized material with an average grade of 1.6 percent copper, 0.06 percent uranium oxide, and 0.6 g/metric ton gold. The deposit occurs in the basement beneath 350 m of unmineralized, flat-lying Adelaidean (late Proterozoic) to Cambrianmore » sediments in the Stuart shelf region of South Australia. The host rocks of the deposit are unmetamorphosed and are probably younger than 1,580 m.y. The deposit is spatially related to coincident gravity and magnetic anomalies and the intersection of west-northwest- and north-northwest-trending lineaments. The Proterozoic sediments comprising the local basement sequence are predominantly sedimentary breccias ranging from matrix-poor granite breccias to matrix-rich polymict breccias containing clasts of a variety of rock types. This sequence is over 1 km thick and has been divided into two main units--the Olympic Dam Formation and the Greenfield Formation. The Olympic Dam Formation has five members, three of which are matrix rich. The Greenfield Formation has three members, the lower two being very hematite rich while the upper has a significant volcanic component. Pervasive hematite, chlorite, and sericite alteration of varying intensity affects all the basement sequence.« less

Exhumation of Basement-cored Uplifts: Example of the Kyrgyz Range Quantified with Apatite Fission-track Thermochronology

NASA Technical Reports Server (NTRS)

Sobel, Edward R.; Oskin, Michael; Burbank, Douglas; Mikolaichuk, Alexander

2005-01-01

The Kyrgyz Range, the northernmost portion of the Kyrgyzstan Tien Shan, displays topographic evidence for lateral propagation of surface uplift and exhumation. The highest and most deeply dissected segment lies in the center of the range. To the east, topography and relief decrease, and preserved remnants of a Cretaceous regional erosion surface imply minimal amounts of bedrock exhumation. The timing of exhumation of range segments defines the lateral propagation rate of the range-bounding reverse fault and quantifies the time and erosion depth needed to transform a mountain range from a juvenile to a mature morphology. New apatite fission-track (AFT) data from three transects from the eastern Kyrgyz Range, combined with published AFT data, demonstrate that the range has propagated over 110 km eastwards over the last 7-11 Myr. Based on the thermal and topographic evolutionary history, we present a model for a time-varying exhumation rate driven by rock uplift and changes in erodability and the time scale of geomorphic adjustment to surface uplift. Easily eroded, Cenozoic sedimentary rocks overlying resistant basement control early, rapid exhumation and slow surface upliftrates. As increasing amounts of resistant basement are exposed, exhumation rates decrease while surface uplift rates are sustained or increase, thereby growing topography. As the range becomes high enough to cause ice accumulation and develop steep river valleys, fluvial and glacial erosion become more powerful and exhumation rates once again increase. Independently determined range-noma1 shortening rates have also varied over time, suggesting a feedback between erosional efficiency and shortening rate.

Magnetic Properties of Three Impact Structures in Canada

NASA Astrophysics Data System (ADS)

Scott, R. G.; Pilkington, M.; Tanczyk, E. I.; Grieve, R. A. F.

1995-09-01

Magnetic anomaly lows associated with the West Hawk Lake (Manitoba), Deep Bay (Saskatchewan) and Clearwater Lakes (Quebec) impact structures, are variable in lateral extent and intensity, a characteristic shared with most impact structures [1]. Drill core from the centres of these structures provides a unique opportunity to ground truth the causes of the reduction in magnetic field intensity in impact structures. Magnetic susceptibility and remanent magnetization levels have been found to be well below regional levels in melt rocks, impact breccias, fractured/shocked basement rocks in the central uplifts, and post-impact sediments. Deep Bay, formed in Pre-Cambrian paragneisses, is a complex crater with a submerged central uplift. It has been extensively infilled with non-magnetic black shales of Cretaceous age [2]. An airborne magnetic low of about 100 nT is associated with the Deep Bay structure. Below the shales and along the rim of the structure are highly brecciated country rocks with variable amounts of very fine rock flour. Susceptibility and remanent magnetization are both weak due to extensive alteration in the brecciated rocks. Alteration of the brecciated rocks, and the effect of several hundred meters of non-magnetic sedimentary infill, both contribute to the magnetic low. West Hawk Lake, a simple crater, was excavated in metavolcanic and metasedimentary rocks of the Superior Province [3], and has a ground magnetic low of about 250 nT. As with Deep Bay, West Hawk Lake has been infilled with dominantly non-magnetic sediments. Brecciation and alteration are extensive, with breccia derived from greenschist-facies meta-andesite displaying slightly higher susceptibilities and remanent magnetizations than breccia derived from the more felsic metasediments. Brecciation has effectively randomized magnetization vectors, and subsequent alteration resulted in the destruction of magnetic phases. These two factors contribute to the magnetic low over this structure. The Clearwater Lakes impact structures are two complex craters formed in Archean retrograde granulite facies rocks [4]. Clearwater West, at 36 km diameter, has an annular ring of islands and a shallowly submerged central uplift. Clearwater East, at 26 km diameter, has a more deeply submerged central uplift. The structures are characterised by highly oxidized melt rock and melt- breccia lenses exposed at the surface. Shocked crystalline basement rocks and minor amounts of breccia and melt rock occur in the central uplifts [5]. Despite relatively little alteration at depth, these rocks exhibit both susceptibilities and remanent magnetizations well below the regionally high values. The Clearwater rocks also contain a thermoremanent reversed magnetization, acquired at the time of impact, and characteristic of the Permo-Carboniferous Reversed Polarity Superchron. The magnetization is carried by titanomagnetite in Clearwater West, and both magnetite and pyrrhotite in Clearwater East. This reversed magnetization contributes to the magnetic low, but cannot account for all of it. The intense airborne magnetic low (> 500 nT) requires a significant contribution from the shocked basement at depth, produced by either alteration of magnetic phases along fractures, or reduction in magnetic properties by lower shock levels away from the point of impact [6]. References: [1] Pilkington M. and Grieve R. A. F. (1992) Rev. Geophys., 30, 161-181. [2] Innes M. J. S. et al. (1964) Publ. Dom. Obs. Ottawa, 31, 19-52. [3] Halliday I. and Griffin A. A. (1967) J. Roy. Astron. Soc. Can., 61, 1-8. [4] Simonds C. H. et al. (1978) LPS IX, 2633-2658. [5] Hische R. (1994) Unpublished Ph.D. thesis, Munster. [6] Pohl J. (1994) 3rd Intl. Wkshp., ESF Network Impact Cratering and Evol. of Planet Earth, Shockwave Behavior in Nature and Expt., Progr. Abstr., 51.

Isostatic Gravity Map with Geology of the Santa Ana 30' x 60' Quadrangle, Southern California

USGS Publications Warehouse

Langenheim, V.E.; Lee, Tien-Chang; Biehler, Shawn; Jachens, R.C.; Morton, D.M.

2006-01-01

This report presents an updated isostatic gravity map, with an accompanying discussion of the geologic significance of gravity anomalies in the Santa Ana 30 by 60 minute quadrangle, southern California. Comparison and analysis of the gravity field with mapped geology indicates the configuration of structures bounding the Los Angeles Basin, geometry of basins developed within the Elsinore and San Jacinto Fault zones, and a probable Pliocene drainage network carved into the bedrock of the Perris block. Total cumulative horizontal displacement on the Elsinore Fault derived from analysis of the length of strike-slip basins within the fault zone is about 5-12 km and is consistent with previously published estimates derived from other sources of information. This report also presents a map of density variations within pre-Cenozoic metamorphic and igneous basement rocks. Analysis of basement gravity patterns across the Elsinore Fault zone suggests 6-10 km of right-lateral displacement. A high-amplitude basement gravity high is present over the San Joaquin Hills and is most likely caused by Peninsular Ranges gabbro and/or Tertiary mafic intrusion. A major basement gravity gradient coincides with the San Jacinto Fault zone and marked magnetic, seismic-velocity, and isotopic gradients that reflect a discontinuity within the Peninsular Ranges batholith in the northeast corner of the quadrangle.

Ages of the Xinghuadukou Group in the Erguna Block, NE China

NASA Astrophysics Data System (ADS)

Liu, X.; Hou, W.

2016-12-01

The Xinghuadukou group is outcropped in the Erguna block (EB) of NE China, which is an important component of the eastern segment of the Central Asian Orogenic Belt (CAOB). This group was previously classified as Paleoproterozoic in age. However, recent studies reported Paleozoic ages from the meta-volcanic rocks, Paleoproterozoic to Neoarchean detrital zircon ages from the meta-sedimentary rocks and Neoproterozoic ages from the granitoids. The tectonic affinity of the EB is still debated. In order to clarify the aforementioned issues, 19 samples were collected from the Xinghuadukou group from the Mohe region in NE China. All samples underwent gneiss facies metamorphism, including two-mica granitic gneiss and quartz biotite gneiss. Based on the protolith discrimination diagram of Si—(al+fm)-(c+alk) system, 7 samples originated from sedimentary rocks and the other 12 of igneous origin. The orthogneiss samples were plotted as diorite, granodiorite and granite respectively in TAS, showing felsic character (SiO2 57% - 74%). One orthogneiss and one paragneiss samples were chosen to conduct the LA-ICP-MS U-Pb zircon age analysis. Apart from one zircon with the age of 742 Ma shows evident metamorphic rim, all zircons from the orthogneiss show euhedral to subhedral prismatic shape and typical concentric or oscillatory structure indicating the igneous origin. The concordant age of 2478±26 Ma was generated, indicating the existence of the near Archean basement of the EB. The detrital zircons from the paragneiss produced age populations cluster at 0.6, 0.8, 1.9, 2.6 and 2.7 Ga, lacking of the Grenville event age. The youngest zircon age is 395 Ma, taken as the maximum depositional age of the sedimentary protolith. According to the new data obtained, it is suggested that the Xinghuadukou group comprises the early Paleoproterozoic granite-gneiss, which proves the granitic basement of the Erguna block. The sedimentary rocks formed overlying the basement during the early Paleozoic and underwent metamorphism afterward. Compared with the detrital zircon ages profile of S. Siberia, NCC, Tarim, Mongolia and NE Gondwana, the EB is suggested most likely affiliated with NE Gondwana. Therefore, this study prefers a tectonic scenario that the EB drifted from the NE Gondwana and joined the CAOB tectonic domain during the Paleozoic.

Basement hall under the northeast part of the building. Live ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

Basement hall under the northeast part of the building. Live animal cages and dissection rooms are to the right. Note concrete footings. - San Bernardino Valley College, Life Science Building, 701 South Mount Vernon Avenue, San Bernardino, San Bernardino County, CA

From crustal thinning to mantle exhumation: what the Pyrenean breccia formations tell us.

NASA Astrophysics Data System (ADS)

Clerc, C.; Chauvet, A.; Lagabrielle, Y.; Reynaud, J.-Y.; Boulvais, P.; Bousquet, R.; Lahfid, A.; Vauchez, A.; Mahé, S.

2012-04-01

Several formations with various breccia types occur in Mesozoic basins disseminated along the North Pyrenean fault, on the northern flank of the French Pyrenees. Due to their location along the Iberia-Europa plate boundary, the North Pyrenean breccia formations represent complex archives documenting the tectonic and sedimentary evolution of the Pyrenean realm during the Aptian-Albian period. In particular, the North Pyrenean breccia formations have recorded the main stages of crustal thinning, continental break-up and mantle exhumation, which occurred along the North Pyrenean Zone (NPZ). We will review the main sedimentary, structural, metamorphic and geochemical characters of these breccias, based on new field investigations conducted in both the Western and Eastern Pyrenées (Agly, Aulus, Moncaup-St Béas and Urdach localities). Based on our new founding, we re-intrepret the significance of the breccia formations in the light of the most recent models developed for the pre-orogenic evolution of the Pyrenees. In several places and mostly close to the contact between Paleozoic basement and Mesozoic cover, we systematically recognized the following three types of breccias: i) Semi-ductile syn-metamorphic breccias resulting from the boudinage of silicic or dolomitic beddings in ductily deformed marbles. ii) Cataclastic breccias disturbing the neighbouring host rocks and displaying a relatively monogenetic character. These tectonic breccias result from the disruption of the Mesozoic metamorphic platform under cooling conditions. They are dominated by cataclastic levels mainly located in the Triassic and Liassic weaker levels, iii) Polymictic sedimentary breccias, which composition is dominated by clasts of Mesozoic metasediments. Locally, close to subcontinental mantle bodies, the sedimentary breccias include numerous clasts of ultramafic and/or crustal basement rocks. Such breccias are the witness of the disruption of the sedimentary cover of the North Pyrenean Zone massifs followed by clastic sedimentation in a context of hyper-extended crust and mantle exhumation. Improving the knowledge of the formation of the different types of breccia exposed all along the Northern Pyrenees brings important hints to decipher the tectonic history responsible for the formation of the metamorphic basins and the exhumation (and reworking) of deep crustal and mantle rocks in the NPZ.

Burial and exhumation of the Eisenhower Range, Transantarctic Mountains, based on thermochronological, sedimentary rock maturity and petrographic constraints

NASA Astrophysics Data System (ADS)

Prenzel, J.; Lisker, F.; Elsner, M.; Schöner, R.; Balestrieri, M. L.; Läufer, A. L.; Berner, U.; Spiegel, C.

2014-09-01

The Eisenhower Range is a N-S trending mountain range in the Transantarctic Mountains (TAM) adjacent to the NW Ross Sea Embayment. New AFT and apatite (U-Th-Sm)/He (AHe) data from vertical basement profiles supplemented by paleotemperature and pressure estimates derived from Beacon sandstones provide new quantitative results on regional burial evolution and first regional constraints on basin inversion and exhumation processes. AFT ages between 32 ± 2 and 259 ± 18 Ma and AHe ages of 37 ± 3-173 ± 16 Ma correlate positively with sample elevations. Thermal history modeling of these data and complementary thermal indications detect heating of the paleosurface on the Eisenhower Range to temperatures ≥ 80 °C subsequent to Ferrar magmatism, and constrain Late Eocene rapid cooling. Regression of modeled paleotemperatures against sample elevations refers to a high Jurassic (~ 45 °C/km) and a moderate Cretaceous-Eocene (28 ± 8 °C/km) geothermal gradient. The texture of Beacon sandstones supports strong mechanical compaction that requires a higher overburden than preserved in the stratigraphic record. Modeled paleotemperatures and pressures suggest basement burial that increases from Late Jurassic (0.7-1.1 km) to Eocene (1.8-2.1 km). The overburden comprises 0.7-1.1 km cumulative Beacon/Ferrar rocks and 0.7-1.4 km of post-Ferrar sediments. Rapid cooling of the whole sample suite between ~ 35 and 30 Ma implies fast erosion of the post-Ferrar sediments and (re-) exposure of underlying magmatic rocks. Subsequent differential sample cooling to present-day surface temperature infers ongoing exhumation by glacial incision enhanced by isostatic response to basin inversion. Decreasing amounts of exhumation from the coast (> 3 km) toward the interior (1.5-2.2 km) point to backstepping incision along the fault controlled Priestley Glacier. Substantial exhumation of the Eisenhower Range since the Late Eocene is hence triggered by both tectonic and climatic factors, superimposed by considerable lithological influence during the initial exhumation stage.

New Tracers of Gas Migration in the Continental Crust

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kurz, Mark D.

2015-11-01

Noble gases are exceptional tracers in continental settings due to the remarkable isotopic variability between the mantle, crust, and atmosphere, and because they are inert. Due to systematic variability in physical properties, such as diffusion, solubility, and production rates, the combination of helium, neon, and argon provides unique but under-utilized indices of gas migration. Existing noble gas data sets are dominated by measurements of gas and fluid phases from gas wells, ground waters and hot springs. There are very few noble gas measurements from the solid continental crust itself, which means that this important reservoir is poorly characterized. The centralmore » goal of this project was to enhance understanding of gas distribution and migration in the continental crust using new measurements of noble gases in whole rocks and minerals from existing continental drill cores, with an emphasis on helium, neon, argon. We carried out whole-rock and mineral-separate noble gas measurements on Precambrian basement samples from the Texas Panhandle. The Texas Panhandle gas field is the southern limb of the giant Hugoton-Panhandle oil and gas field; it has high helium contents (up to ~ 2 %) and 3He/4He of 0.21 (± 0.03) Ra. Because the total amount of helium in the Panhandle gas field is relatively well known, crustal isotopic data and mass balance calculations can be used to constrain the ultimate source rocks, and hence the helium migration paths. The new 3He/4He data range from 0.03 to 0.11 Ra (total), all of which are lower than the gas field values. There is internal isotopic heterogeneity in helium, neon, and argon, within all the samples; crushing extractions yield less radiogenic values than melting, demonstrating that fluid inclusions preserve less radiogenic gases. The new data suggest that the Precambrian basement has lost significant amounts of helium, and shows the importance of measuring helium with neon and argon. The 4He/40Ar values are particularly useful in demonstrating helium loss because all the data falls well below the production ratio.« less

Preliminary isostatic residual gravity anomaly map of Paso Robles 30 x 60 minute quadrangle, California

USGS Publications Warehouse

McPhee, D.K.; Langenheim, V.E.; Watt, J.T.

2011-01-01

This isostatic residual gravity map is part of an effort to map the three-dimensional distribution of rocks in the central California Coast Ranges and will serve as a basis for modeling the shape of basins and for determining the location and geometry of faults within the Paso Robles quadrangle. Local spatial variations in the Earth\\'s gravity field, after accounting for variations caused by elevation, terrain, and deep crustal structure reflect the distribution of densities in the mid- to upper crust. Densities often can be related to rock type, and abrupt spatial changes in density commonly mark lithological or structural boundaries. High-density rocks exposed within the central Coast Ranges include Mesozoic granitic rocks (exposed northwest of Paso Robles), Jurassic to Cretaceous marine strata of the Great Valley Sequence (exposed primarily northeast of the San Andreas fault), and Mesozoic sedimentary and volcanic rocks of the Franciscan Complex [exposed in the Santa Lucia Range and northeast of the San Andreas fault (SAF) near Parkfield, California]. Alluvial sediments and Tertiary sedimentary rocks are characterized by low densities; however, with increasing depth of burial and age, the densities of these rocks may become indistinguishable from those of older basement rocks.

Structural and petrographic constraints on the stratigraphy of the Lapataia Formation, with implications for the tectonic evolution of the Fuegian Andes

NASA Astrophysics Data System (ADS)

Cao, Sebastián J.; Torres Carbonell, Pablo J.; Dimieri, Luis V.

2018-07-01

The structure of the Fuegian Andes central belt is characterized by a first phase of peak metamorphism and ductile deformation, followed by a brittle-ductile thrusting phase including juxtaposition of different (first phase) structural levels; both related to the closure and inversion of the Late Jurassic-Early Cretaceous Rocas Verdes basin. The second phase involved thrust sheets of pre-Jurassic basement, as well as Upper Jurassic and Lower Cretaceous units from the volcanic-sedimentary fill of the basin. Rock exposures in the Parque Nacional Tierra del Fuego reveal a diversity of metamorphic mineral assemblages, dynamic recrystallization grades and associated structures, evidencing a variety of protoliths and positions in the crust during their orogenic evolution. Among the units present in this sector, the Lapataia Formation portrays the higher metamorphic grade reported in the Argentine side of the Fuegian Andes, and since no precise radiometric ages have been established to date, its stratigraphic position remains a matter of debate: the discussion being whether it belongs to the pre-Jurassic basement, or the Upper Jurassic volcanic/volcaniclastic initial fill of the Rocas Verdes basin. The mapping and petrographic/microstructural study of the Lapataia Formation and those of undoubtedly Mesozoic age, allow to characterize the former as a group of rocks with great lithological affinity with the Upper Jurassic metamorphic rocks found elsewhere in the central belt of the Fuegian Andes. The main differences in metamorphic grade are indebted to its deformation at deeper crustal levels, but during the same stages than the Mesozoic rocks. Accordingly, we interpret the regional structure to be associated with the stacking of thrust sheets from different structural levels through the emplacement of a duplex system during the growth of the Fuegian Andes.

Apatite fission-track thermochronometric constraints on the exhumation and evolution of the southeastern Indian (Tamil Nadu) passive margin and the role of structural inheritance

NASA Astrophysics Data System (ADS)

De Grave, Johan; Glorie, Stijn; Singh, Tejpal; Van Ranst, Gerben; Nachtergaele, Simon

2017-04-01

After rifting from Gondwana in the Late Jurassic - Early Cretaceous, and subsequent opening of the Indian Ocean basin, the continental margins of India developed into typical passive margins. Extensional tectonic forces and thermal subsidence gave rise to the formation of both on-shore and off-shore basins along the southeastern passive margin of the Indian continent, along the Tamil Nadu coast. There, basins such as the Cauvery and Krishna-Godavari basin, accumulated Meso- and Cenozoic (Early Cretaceous to recent) detrital sediments coming off the rifted blocks and the Tamil Nadu hinterland. In places, deep rift basins have accumulated up to over 3000 m of sediments. The continental basement of Tamil Nadu is chiefly composed of metamorphic rocks of the Archean to Palaeoproterozoic Eastern Dharwar Craton and the coeval Southern Granulite Terrane (e.g. Peucat et al., 2013). Several crustal scale shear zones crosscut this assemblage and at least some are considered to represent Gondwanan sutures (Santosh et al., 2012). Smaller, younger granitoid plutons intrude the basement at several locations and most of these are of Late Neoproterozoic age (Glorie et al., 2014). In this work metamorphic basements rocks and the younger granitoids were sampled for a apatite fission-track (AFT) thermochronometric study. A North-South profile from Chennai to Thanjavur mainly transects the Salem block of the Southern Granulite Terrane, and crosscuts several crustal scale shear zones, such as the Cauvery, Salem-Attur and Gangavalli shear zones. Apatites from over 30 samples were used in this study. AFT ages all range between about 190 and 120 Ma (Jurassic - Early Cretaceous). These mainly represent the slow, shallow exhumation of the basement during the rift and early drift phase of the Indian plate from Gondwana. AFT mean track lengths vary between 11 and 13 µm and are typical of slowly exhumed basement. Thermal history modelling (using the QTQt software by Gallagher, 2012) confirms that internal regions of fault blocks experienced a slow and steady cooling to ambient temperatures throughout the Meso-Cenozoic, while younger samples, mainly positioned closeby or inside the shear zones, additionally record a more moderate to rapid cooling since the Early Cenozoic.

22. INTERIOR VIEW, BASEMENT UNDER NORTH ROOM OF MAIN BLOCK, ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

22. INTERIOR VIEW, BASEMENT UNDER NORTH ROOM OF MAIN BLOCK, VIEW OF NORTHWEST WALL SHOWING CORBELING BASE OF FIRST FLOOR CHIMNEY BLOCK WITH STOVE-PIPE HOLE, AND MORTISE AND TENON FRAMING FOR HEARTH BED - Clifton Farm, Off Baker Road, Frederick, Frederick County, MD

Subseafloor processes in mid-ocean ridge hydrothennal systems

NASA Astrophysics Data System (ADS)

Alt, Jeffrey C.

Convective circulation of seawater through oceanic crust at mid-ocean ridges (MOR) and on ridge flanks has wide-ranging effects on heat transport, the chemical and isotopic compositions of ocean crust and seawater, mineralization of the crust, and on the physical properties of oceanic basement. Submarine hydrothermal systems remove about 30% of the heat lost from oceanic crust [Selater et al., 1981; Stein and Stein, 1994], and chemical and isotopic exchange between seawater and basement rocks exerts important controls on the composition of seawater [Edmond et al., 1979a; Thompson, 1983]. The composition of altered crust is also changed and, when subducted, this altered crust can contribute to chemical and isotopic heterogeneities in the mantle [Zindler and Hart, 1986] and may affect the compositions of volcanic rocks in island arcs [Perfit et al., 1980; Tatsumi, 1989]. Mineralization of ocean crust occurs where metals, leached from large volumes of altered crust at depth, are concentrated at or near the surface by hydrothermal circulation [Hannington, 1995]. Hydrothermal alteration of magnetic minerals may affect the source of marine magnetic anomalies [Pariso and Johnson, 1991], and the formation of secondary minerals influences the density, porosity, and seismic velocity structure of the crust [Wilkens et al., 1991; Jacobson, 1992].

Structure and Evolution of the Central Andes of Peru

NASA Astrophysics Data System (ADS)

Gonzalez, L.; Pfiffner, O. A.

2009-04-01

Three major units make up the Andes in Peru: (1) The Western Cordillera consists of the Cretaceous Coastal Batholith intruding Jurassic to Cretaceous volcaniclastics (Casma group) in the west, and a fold-and-thrust belt of Mesozoic sediments in the east. Eocene and Miocene volcanics (Calipuy group and equivalents) overly all of these rock types. (2) The Central Highland contains a folded Paleozoic-Mesozoic sedimentary sequence overlain by thick Quaternary deposits. A major fault puts Neoproterozoic basement rocks of the Eastern Cordillera next to these units. (3) In the Eastern Cordillera, Late Paleozoic clastic successions unconformably overly folded Early Paleozoic sediments and a Neoproterozoic basement in the east. Permian (locally Triassic) granitoids intruded these units and were affected by folding and thrusting. In the core of the Eastern Cordillera, Early Cretaceous overly Early or Late Paleozoic strata. To the west, a thrust belt of Paleozoic to Cenozoic strata forms the transition to the foreland of the Brasilian shield. The most external part of this thrust belt involves Pliocene sediments and is referred to as Subandine zone. The Coastal Batholith is internally undeformed. The adjacent fold-and-thrust belt to the east is characterized by tight, nearly isoclinal upright folds with amplitudes of up to 1000 m. At the surface only Cretaceous rocks are observed. Using balancing techniques, a detachment horizon at the base of the Lowermost Cretaceous (Goyallarisquizga group - Oyon Formation) can be proposed. Further east, folds are more open, asymmetric and east verging, Jurassic sediments appear in the cores of the anticlines. The abrupt change in style from upright tight folding in the west to more open folding in the east is explained by a primary difference in the depositional sequence, most probably associated with synsedimentary faulting. The overlying volcanics of the Calipuy group and equivalents are, in turn, only slightly folded. In the Northern part of the Western Cordillera, near Huaraz, a vertical fault puts a Late Miocene to Early Pliocene batholith (Cordillera Blanca) in direct contact to Miocene volcanics (Calipuy group, Cordillera Negra). The structure of the Central Highlands is characterized by relatively open folds in the Paleozoic to Mesozoic strata. Overlying Quaternary deposits are tilted and locally even folded. Eocene to Miocene undeformed granitoids intrude these structures. A swarm of NNW-SSE striking and steeply dipping faults separate the Eastern Cordillera from the Highlands. Some of these faults suggest block faulting. However, near Huancayo a clear indication of strike-slip motion could be found. The Neoproterozoic basement rocks and the Early Paleozoic sediments are unconformably overlain by Late Paleozoic sediments which in turn are folded. Within the Subandine zone, the structural style is characterized by east directed imbricate thrusting. The thrust faults cut down into the crystalline basement going west, suggesting a detachment within upper crustal crystalline basement rocks. In the Central Peruvian Andes, compressional deformation events progressed from west to east. Early Cretaceous plutons of the coast batholith intruded folded Jurassic to Early Cretaceous volcaniclastic rocks of the Casma group and suggest an Early Cretaceous phase of shortening in the Pacific coastal area of the Western Cordillera (referred to as Mochica phase in the literature). Within the Western Cordillera, a major phase of pre-Eocene erosion removed a substantial amount of the tight upright folds. The youngest strata folded are of Late Cretaceous to Early Paleocene age (Red Beds). The overlying volcanics are slightly younger (middle Eocene) and bracket the tight folding, referred to as Inca phase, to Late Paleocene to Early Eocene times. This is corroborated by Eocene to Miocene granitic intrusions in the adjacent fold-and-thrust belt. Still younger deformations, referred to as Quechua Phase, produced gentle folds within the Eocene volcanics. Vertical motions in the Cordillera Blanca juxtaposed a Late Miocene-Pliocene batholith to Late Miocene volcanics. These movements are post-Pleistonce in age and still active. In the Central High Zone, even Pleistocene deposits were tilted and locally folded. Timing of the steeply dipping faults bordering the Eastern Cordillera is more difficult to assess. Cretaceous strata in tectonic contact with Neoproterozoic basement indicate a Cenozoic age. But within the fold-and-thrust belt of the Subandine zone in the east, youngest strata affected by thrusting are progressively younger toward the east. They suggest thrust propagation ranging from Oligocene to Pliocene age. These young thrust faults were responsible for the uplift of the Central Highland to their present elevation.

Late Cretaceous remagnetization of Proterozoic mafic dikes, southern Highland Mountains, southwestern Montana: A paleomagnetic and 40Ar/39Ar study

USGS Publications Warehouse

Harlan, S.S.; Geissman, J.W.; Snee, L.W.; Reynolds, R.L.

1996-01-01

Paleomagnetic results from Early Proterozoic metabasite sills and Middle Proterozoic diabase dikes from the southern Highland Mountains of southwestern Montana give well-defined, dual-polarity magnetizations that are statistically identical to those from a small Late Cretaceous pluton that cuts the dikes. The concordance of paleomagnetic directions from rocks of three widely separated ages indicates that the Proterozoic rocks were remagnetized, probably during Late Cretaceous time. Paleomagnetic, rock magnetic, and petrographic observations from the metabasite and diabase samples indicate that remanence is carried primarily by low-Ti magnetite. Combining virtual geomagnetic poles from metabasite sills, diabase dikes, and the Late Cretaceous pluton, we obtain a paleomagnetic pole at 85.5??N, 310.7??E (K = 19.9, A95 = 9.1??, N = 14 sites) that is similar to a reference pole from the 74 Ma Adel Mountain Volcanics of western Montana. Biotite and hornblende 40Ar/39Ar isotopic dates from host basement geneiss and a hornblende from a remagnetized metabasite sill yield ages of ca. 1800 Ma; these dates probably record cooling of the southern Highland Mountains following high-grade metamorphism at 1.9-1.8 Ga. The gneiss and metabasite age spectra show virtually no evidence of disturbance, indicating that the basement rocks were never heated to temperatures sufficient to cause even partial resetting of their argon systems. Thus, the overprint magnetization of the Highland Mountains rocks is not a thermoremanent magnetization acquired during conductive cooling of nearby Late Cretaceous plutons. Remagnetization of the metabasite sills and diabase dikes was probably caused by localized thermochemical and thermoviscous effects during circulation of Late Cretaceous hydrothermal fluids related to epithermal mineralization. The absence of significant disturbance to the 40Ar/39Ar age spectrum from the remagnetized metabasite hornblende indicates that some secondary magnetizations may go unrecognized and undated, even if 40Ar/39Ar dating is applied.

Detrital zircon ages in Korean mid-Paleozoic meta-sandstones (Imjingang Belt and Taean Formation): Constraints on tectonic and depositional setting, source regions and possible affinity with Chinese terranes

NASA Astrophysics Data System (ADS)

Han, Seokyoung; de Jong, Koen; Yi, Keewook

2017-08-01

Sensitive High-Resolution Ion Microprobe (SHRIMP) U-Th-Pb isotopic data of detrital zircons from mature, quartz-rich meta-sandstones are used to constrain possible tectonic affinities and source regions of the rhythmically layered and graded-bedded series in the Yeoncheon Complex (Imjingang Belt) and the correlative Taean Formation. These metamorphic marine turbidite sequences presently occur along the Paleoproterozoic (1.93-1.83 Ga) Gyeonggi Massif, central Korea's main high-grade metamorphic gneiss terrane. Yet, detrital zircons yielded highly similar multimodal age spectra with peaks that do not match the age repartition in these basement rocks, as late (1.9-1.8 Ga) and earliest (∼ 2.5 Ga) Paleoproterozoic detrital modes are subordinate but, in contrast, Paleozoic (440-425 Ma) and Neoproterozoic (980-920 Ma) spikes are prominent, yet the basement essentially lacks lithologies with such ages. The youngest concordant zircon ages in each sample are: 378, 394 and 423 Ma. The maturity of the meta-sandstones and the general roundness of zircons of magmatic signature, irrespective of their age, suggest that sediments underwent considerable transport from source to sink, and possibly important weathering and recycling, which may have filtered out irradiation-weakened metamorphic zircon grains. In combination with these isotopic data, presence of a low-angle ductile fault contact between the Yeoncheon Complex and the Taean Formation and the underlying mylonitized Precambrian basement implies that they are in tectonic contact and do not have a stratigraphic relationship, as often assumed. Consequently, in all likelihood, both meta-sedimentary formations: (1) are at least of early Late Devonian age, (2) received much of their detritus from distant (reworked) Silurian-Devonian and Early Neoproterozoic magmatic sources, not present in the Gyeonggi Massif, (3) and not from Paleoproterozoic crystalline rocks of this massif, or other Korean Precambrian basement terranes, and (4) should be viewed as independent tectonic units that had sources not exposed in Korea. A thorough literature review reveals that the Yeoncheon Complex and the Taean Formation were potentially sourced from the Liuling, Nanwan and Foziling groups in the Qinling-Dabie Belt, which all show very similar detrital zircon age spectra. These immature middle-late Devonian sandstones were deposited in a pro-foreland basin formed as a result of the aborted subduction of the South Qinling Terrane below the North Qinling Terrane, which was uplifted and eroded during post-collision isostatic rebound. The submarine fans where the mature distal turbiditic Yeoncheon and Taean sandstones were deposited may have constituted the eastern terminal part of a routing system originating in the uplifted and eroded middle Paleozoic Qinling Belt and adjacent part of the foreland basin.

Geophysical Characterization and Structural Model of the Santa ROSALÍA Aquifer, Sonora, MÉXICO

NASA Astrophysics Data System (ADS)

Martínez-Retama, S.; Montaño-Del Cid, M. A.

2017-12-01

The main objective of this work was to determine the morphology and depth of the basement, as well as the elaboration of a structural model for the Santa Rosalía aquifer, from the processing and interpretation of gravimetric and aeromagnetic data and its correlation with the Geology of the area. The study area is located in the central portion of the State of Sonora, Mexico. In general, the geology of the site is characterized by sedimentary, igneous and metamorphic rocks whose ages vary from the Precambrian to Recent. Chronologically, the geology of the study area consists of igneous and metamorphic rocks of Precambrian age, considered as a metamorphic complex. The Paleozoic is represented by a sequence of prebatolytic rocks. This sequence is intruded by rocks of the Upper Cretaceous. The Triassic-Jurassic periods consist of arenaceous units of the Barranca Group. The Cretaceous is constituted by the Tarahumara Formation, as well as granite bodies. The Quaternary is composed of alluvial deposits, which are overlain by sediments of Recent. In this work a gravimetric survey was performed, registering a total of 7 profiles. In addition, measured data from the National Institute of Statistics and Geography (INEGI) were used. The aeromagnetic study was carried out with data from the Mexican Geological Service (SGM). In order to reduce the ambiguity in the modeling process, a rock sampling was taken from the study area and its density and magnetic susceptibility were measured. Finally, two-dimensional models of gravimetric and magnetic profiles were made to obtain the structural model of the study area. The geological-structural models obtained show gravimetric anomalies (low)associated with sedimentary basins with depths of 800 m to 1,500 m., indicating the most susceptible áreas to water storage. The basement is represented by volcanic and granite rocks that are in contact with Paleozoic sedimentary rocks (Limestone) and in some areas with volcanic rocks of the Tarahumara Formation. In these models two types of sliding tectonic events were interpreted. In the first one a system of low-angle normal faulting related to the distensive event Basin and Range was interpreted. In the second, a series of high- angle normal faults, which form Horst and Grabens structures related to the opening of the Gulf of California were modeled.

The thrust belt in Southwest Montana and east-central Idaho

USGS Publications Warehouse

Ruppel, Edward T.; Lopez, David A.

1984-01-01

The leading edge of the Cordilleran fold and thrust in southwest Montana appears to be a continuation of the edge of the Wyoming thrust belt, projected northward beneath the Snake River Plain. Trces of the thrust faults that form the leading edge of the thrust belts are mostly concealed, but stratigraphic and structural evidence suggests that the belt enters Montana near the middle of the Centennial Mountains, continues west along the Red Rock River valley, and swings north into the Highland Mountains near Butte. The thrust belt in southwest Montana and east-central Idaho includes at least two major plates -- the Medicine Lodge and Grasshopper thrust plates -- each of which contains a distinctive sequence of rocks, different in facies and structural style from those of the cratonic region east of the thrust belt. The thrust plates are characterized by persuasive, open to tight and locally overturned folds, and imbricate thrust faults, structural styles unusual in Phanerozoic cratonic rocks. The basal decollement zones of the plates are composed of intensely sheared, crushed, brecciated, and mylonitized rocks, the decollement at the base of the Medicine Lodge plate is as much as 300 meters thick. The Medicine Lodge and Grasshopper thrust plates are fringed on the east by a 10- to 50-kilometer-wide zone of tightly folded rocks cut by imbricate thrust fauls, a zone that forms the eastern margin of the thrust belt in southwest Montana. The frontal fold and thrust zone includes rocks that are similar to those of the craton, even though they differ in details of thickness, composition, or stratigraphic sequence. The zone is interpreted to be one of terminal folding and thrusting in cratonic rocks overridden by the major thrust plates from farther west. The cratonic rocks were drape-folded over rising basement blocks that formed a foreland bulge in front of the thrust belt. The basement blocks are bounded by steep faults of Proterozoic ancestry, which also moved as tear faults during thrusting, and seem to have controlled the curving patterns of salients and reentrants at the leading edge of the thrust belt. Radiometric and stratiographic evidence shows that the thrust belt was in its present position by about 75 million year go.

Active accommodation of plate convergence in Southern Iran: Earthquake locations, triggered aseismic slip, and regional strain rates

NASA Astrophysics Data System (ADS)

Barnhart, William D.; Lohman, Rowena B.; Mellors, Robert J.

2013-10-01

We present a catalog of interferometric synthetic aperture radar (InSAR) constraints on deformation that occurred during earthquake sequences in southern Iran between 1992 and 2011, and explore the implications on the accommodation of large-scale continental convergence between Saudi Arabia and Eurasia within the Zagros Mountains. The Zagros Mountains, a salt-laden fold-and-thrust belt involving ~10 km of sedimentary rocks overlying Precambrian basement rocks, have formed as a result of ongoing continental collision since 10-20 Ma that is currently occurring at a rate of ~3 cm/yr. We first demonstrate that there is a biased misfit in earthquake locations in global catalogs that likely results from neglect of 3-D velocity structure. Previous work involving two M ~ 6 earthquakes with well-recorded aftershocks has shown that the deformation observed with InSAR may represent triggered slip on faults much shallower than the primary earthquake, which likely occurred within the basement rocks (>10 km depth). We explore the hypothesis that most of the deformation observed with InSAR spanning earthquake sequences is also due to shallow, triggered slip above a deeper earthquake, effectively doubling the moment release for each event. We quantify the effects that this extra moment release would have on the discrepancy between seismically and geodetically constrained moment rates in the region, finding that even with the extra triggered fault slip, significant aseismic deformation during the interseismic period is necessary to fully explain the convergence between Eurasia and Saudi Arabia.

Cenozoic pull-apart basins in southwest Montana

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ruppel, E.T.

1991-06-01

Faults and fault zones bounding the mountain ranges of southwest Montana commonly have been described as normal faults, and the region has been considered to be a northern extension of the Basin and Range. New geologic mapping suggests, however, that Cenozoic movements along most of the zones of steep faults in southwest Montana and in east-central Idaho have been strike-slip, and the intermontane basins appear to be pull-aparts. The principal fault zones trend about north, northwest, east, and north-northeast; the north-trending zones are Cenozoic in age, but the others are of Archean ancestry and are rooted in basement rocks. Thesemore » faults break the region into rhomboidal mountain blocks separated by broad basins with parallel sides. The basins are as much as 5,000 m deep, and their floors are deeply indented by centers of subsidence wherre they are crossed by major fault zones. The basins are floored by Archean or Proterozoic rocks and are filled with tuffaceous sedimentary rocks of late Oligocene to late Miocene age. The Big Hole basin and the smaller basins in upper Grasshopper Creek and Horse Prairie are interpreted to be pull-aparts between zones of east-trending right-lateral faults. The cratonic basins farther east in southwest Montana are interpreted to be basement-floored openings between mountain blocks that have been separated by subcrustal flow to the northwest. The interpretations suggest that significant accumulations of oil or gas are not likely to be found in this region.« less

Radiolytic hydrogen production in basaltic basement of the South Pacific Gyre

NASA Astrophysics Data System (ADS)

Dzaugis, M. E.; Spivack, A. J.; Dunlea, A. G.; Murray, R. W.; Kelley, K. A.; D'Hondt, S. L.

2013-12-01

Water radiolysis is the decomposition of water molecules due to interactions with ionizing radiation from the natural decay of radioactive elements, such as uranium (238U, 235U), thorium (232Th) and potassium (40K). This abiotic process produces electron donors (e.g., H2) and acceptors (e.g., O2) that microorganisms can metabolize for energy. Although water radiolysis has been examined in deep continental crust (Lin et al., 2005) and marine sediment (Blair et al., 2007), it has not been rigorously addressed in oceanic basement. The submarine depth to which life extends on Earth, and the potential for life in basaltic aquifers of other worlds (such as Mars and Europa), may depend on radiolytic production of electron donors and acceptors. In order to quantify the extent to which water radiolysis occurs in the subseafloor basaltic basement, we (i) quantified radioactive element concentrations of basement samples from Integrated Ocean Drilling Program (IODP) Expedition 329 and (ii) developed a quantitative model of H2 production by water radiolysis in the basement aquifer. Modeling radiolytic production of H2 in oceanic basement is difficult because the basement is a heterogeneous environment. Microscale changes in physical properties and chemical composition cause microscale variation in water radiolysis within the basement. During radioactive decay, alpha particles, beta particles and gamma rays are emitted, each with a spectrum of characteristic energies. The distance over which radiation is attenuated depends on the kind of radiation (alpha, beta or gamma), initial energy, and the absorbing material. These properties and the concentration of radioactive elements provide the basis for our preliminary model. We are using inductively coupled plasma emission spectroscopy (ICP-ES), mass spectrometry (ICP-MS) and laser ablation (LA ICP-MS) to map variation in radioelement concentrations from phase to phase (e.g., across successive alteration halos to unaltered rock). The last step in our model combines (i) the rate at which radiation energy is transferred to the water with (ii) published H2 yields per rate of energy transfer.

Lead isotope compositions of Late Cretaceous and early Tertiary igneous rocks and sulfide minerals in Arizona: Implications for the sources of plutons and metals in porphyry copper deposits

USGS Publications Warehouse

Bouse, R.M.; Ruiz, J.; Titley, S.R.; Tosdal, R.M.; Wooden, J.L.

1999-01-01

Porphyry copper deposits in Arizona are genetically associated with Late Cretaceous and early Tertiary igneous complexes that consist of older intermediate volcanic rocks and younger intermediate to felsic intrusions. The igneous complexes and their associated porphyry copper deposits were emplaced into an Early Proterozoic basement characterized by different rocks, geologic histories, and isotopic compositions. Lead isotope compositions of the Proterozoic basement rocks define, from northwest to southeast, the Mojave, central Arizona, and southeastern Arizona provinces. Porphyry copper deposits are present in each Pb isotope province. Lead isotope compositions of Late Cretaceous and early Tertiary plutons, together with those of sulfide minerals in porphyry copper deposits and of Proterozoic country rocks, place important constraints on genesis of the magmatic suites and the porphyry copper deposits themselves. The range of age-corrected Pb isotope compositions of plutons in 12 Late Cretaceous and early Tertiary igneous complexes is 206Pb/204Pb = 17.34 to 22.66, 207Pb/204Pb = 15.43 to 15.96, and 208Pb/204Pb = 37.19 to 40.33. These Pb isotope compositions and calculated model Th/U are similar to those of the Proterozoic rocks in which the plutons were emplaced, thereby indicating that Pb in the younger rocks and ore deposits was inherited from the basement rocks and their sources. No Pb isotope differences distinguish Late Cretaceous and early Tertiary igneous complexes that contain large economic porphyry copper deposits from less rich or smaller deposits that have not been considered economic for mining. Lead isotope compositions of Late Cretaceous and early Tertiary plutons and sulfide minerals from 30 metallic mineral districts, furthermore, require that the southeastern Arizona Pb province be divided into two subprovinces. The northern subprovince has generally lower 206Pb/204Pb and higher model Th/U, and the southern subprovince has higher 206Pb/204Pb and lower model Th/U. These Pb isotope differences are inferred to result from differences in their respective post-1.7 Ga magmatic histories. Throughout Arizona, Pb isotope compositions of Late Cretaceous and early Tertiary plutons and associated sulfide minerals are distinct from those of Jurassic plutons and also middle Tertiary igneous rocks and sulfide minerals. These differences most likely reflect changes in tectonic setting and magmatic sources. Within Late Cretaceous and early Tertiary igneous complexes that host economic porphyry copper deposits, there is commonly a decrease in Pb isotope composition from older to younger plutons. This decrease in Pb isotope values with time suggests an increasing involvement of crust with lower U/Pb than average crust in the source(s) of Late Cretaceous and early Tertiary magmas. Lead isotope compositions of the youngest porphyries in the igneous complexes are similar to those in most sulfide minerals within the associated porphyry copper deposit. This Pb isotope similarity argues for a genetic link between them. However, not all Pb in the sulfide minerals in porphyry copper deposits is magmatically derived. Some sulfide minerals, particularly those that are late stage, or distal to the main orebody, or in Proterozoic or Paleozoic rocks, have elevated Pb isotope compositions displaced toward the gross average Pb isotope composition of the local country rocks. The more radiogenic isotopic compositions argue for a contribution of Pb from those rocks at the site of ore deposition. Combining the Pb isotope data with available geochemical, isotopic, and petrologic data suggests derivation of the young porphyry copper-related plutons, most of their Pb, and other metals from a hybridized lower continental crustal source. Because of the likely involvement of subduction-related mantle-derived basaltic magma in the hybridized lower crustal source, an indiscernible mantle contribution is probable in the porphyry magmas. Clearly, in addition

Integrating seismic reflection and geological data and interpretations across an internal basement massif: The southern Appalachian Pine Mountain window, USA

USGS Publications Warehouse

McBride, J.H.; Hatcher, R.D.; Stephenson, W.J.; Hooper, R.J.

2005-01-01

The southern Appalachian Pine Mountain window exposes 1.1 Ga Grenvillian basement and its metasedimentary Paleozoic(?) cover through the allochthonous Inner Piedmont. The issue of whether the crustal block inside the window was either transported above the master Appalachian (late Alleghanian) de??collement or is an autochthonous block that was overridden by the de??collement has been debated for some time. New detrital zircon geochronologic data from the cover rocks inside the window suggest this crustal block was derived from Gondwana but docked with Laurentia before the Alleghanian event. Reprocessed deep seismic reflection data from west-central Georgia (pre- and poststack noise reduction, amplitude variation analysis, and prestack depth migration) indicate that a significant band of subhorizontal reflections occurs almost continuously beneath the window collinear with the originally recognized de??collement reflections north of the window. A marked variation in the de??collement image, from strong and coherent north of the window to more diffuse directly beneath the window, is likely a partial consequence of the different geology between the Inner Piedmont and the window. The more diffuse image beneath the window may also result from imaging problems related to changes in topography and fold of cover (i.e., signal-to-noise ratio). Two alternative tectonic models for the Pine Mountain window can partially account for the observed variation in the de??collement reflectivity. (1) The Pine Mountain block could be truncated below by a relatively smooth continuation of the de??collement. The window would thus expose an allochthonous basement duplex or horse-block thrust upward from the south along the Late Proterozoic rifted continental margin. (2) The window represents localized exhumation of autochthonous basement and cover along a zone of distributed intrabasement shearing directly beneath the window. Either model is viable if only reflector geometry is considered; model (1) is favored if both geometry and kinematics of Blue Ridge-Piedmont thrust sheet emplacement are incorporated. In either model, the southern margin of the window merges to the west with the Iapetan early Alleghanian Central Piedmont suture, which juxtaposes North American-affinity Piedmont rocks to the north and exotic Panafrican rocks of the Carolina (Avalon) terrane to the south. Immediately south of the window, this suture dips southward and merges in the lower crust with the late Alleghanian suture joining the Appalachians with Gondwana. ?? 2005 Geological Society of America.

Concepts for diamond exploration in "on/off craton" areas—British Columbia, Canada

NASA Astrophysics Data System (ADS)

Simandl, George J.

2004-09-01

The tectonic setting of British Columbia (BC) differs from classic diamond-bearing intracratonic regions such as the Northwest Territories and South Africa. Nevertheless, several diamond occurrences have been reported in BC. It is also known that parts of the province are underlain by Proterozoic and possibly Archean basement. Because the continents of today are composites of fragments of ancient continents, it is possible that some of the regions underlain by old crystalline basement in eastern British Columbia were associated with a deep crustal keel. The keel may have predated the break-up of the early Neoproterozoic supercontinent called Rodinia and was preserved possibly until the Triassic. Some of these old continental fragments may have been displaced relative to their position of origin and dissociated from their keel, or the keel may have since been destroyed. Such fragments represent favourable exploration grounds in terms of the "Diamondiferous Mantle Root" model (DMR model) if they were intersected by kimberlites or lamproites prior to displacement or destruction of their underlying deep keel. Therefore, extrapolation of fragments of the diamond-bearing Precambrian basement from the Northwest Territories or Alberta to BC provides a sufficient reason for initiating reconnaissance indicator mineral surveys. The "Eclogite Subduction Zone" model (ES model) predicts formation of diamonds at lower pressure (i.e., depth) than required by the DMR model in convergent tectonic settings. Although not proven, this model is supported by thermal modeling of cold subduction zones and recent discoveries of diamonds in areas characterized by convergent tectonic settings. If the ES model is correct, then the parts of BC with a geological history similar to today's "cold" subduction zones, such as Honshu (Japan), or to continental collision zones, such as Kokchetav massif (Kazakhstan) and the Dabie-Sulu Terrane (east central China), may be diamondiferous. The terranes where geological evidences suggest an ultrahigh pressure (UHP) metamorphic event followed by rapid tectonic exhumation (which could have prevented complete resorption of diamonds on their journey to the surface) are worth investigating. If UHP rocks were intercepted at depth by syn- or post-subduction diamond elevators, such as kimberlites, lamproites, lamprophyres, nephelinites or other alkali volcanic rocks of deep-seated origin, the diamond potential of the area would be even higher.

Geologic setting of the proposed Fallon FORGE Site, Nevada: Suitability for EGS research and development

USGS Publications Warehouse

Faulds, James E.; Blankenship, Douglas; Hinz, Nicholas H.; Sabin, Andrew; Nordquist, Josh; Hickman, Stephen H.; Glen, Jonathan; Kennedy, Mack; Siler, Drew; Robinson-Tait, Ann; Williams, Colin F.; Drakos, Peter; Calvin, Wendy M.

2015-01-01

The proposed Fallon FORGE site lies within and adjacent to the Naval Air Station Fallon (NASF) directly southeast of the town of Fallon, Nevada, within the large basin of the Carson Sink in west-central Nevada. The site is located on two parcels that include land owned by the NASF and leased and owned by Ormat Nevada, Inc. The Carson Sink in the vicinity of the Fallon site is covered by Quaternary deposits, including alluvial fan, eolian, and lacustrine sediments. Four wells penetrate the entire Neogene section and bottom in Mesozoic basement. Late Miocene to Quaternary basin-fill sediments are 0.5 to >1 km thick and overlie Oligocene-Miocene volcanic and lesser sedimentary rocks. The volcanic section is 0.5 to 1.0 km thick and dominated by Miocene mafic lavas. The Neogene section rests nonconformably on heterogeneous Mesozoic basement, which consists of Triassic-Jurassic metamorphic rocks intruded by Cretaceous granitic plutons. The structural framework is dominated by a gently west-tilted half graben cut by moderately to steeply dipping N- to NNEstriking normal faults that dip both east and west. Quaternary faults have not been observed within the proposed FORGE site. Fallon was selected for a potential FORGE site due to its extensional tectonic setting, abundance of available data, existing infrastructure, and documented temperatures, permeability, and lithologic composition of potential reservoirs that fall within the ranges specified by DOE for FORGE. Since the early 1970s, more than 45 wells have been drilled for geothermal exploration within the area. Four exploration wells within the FORGE site are available for use in the project. Several additional wells are available for monitoring outside the central FORGE site within the NASF and Ormat lease area, including numerous temperature gradient holes. There is an existing, ten-station micro-seismic earthquake (MEQ) array that has been collecting data since 2001; the MEQ array can be expanded to encompass the entire Fallon project. The well data indicate that a sizeable area (~4.5 km2 ) has adequate temperatures in crystalline basement but lacks sufficient permeability within the proposed FORGE site. There are two possible, competent target formations in Mesozoic basement for stimulation in the FORGE project area: 1) Jurassic felsic metavolcanic rocks/and or metaquartzite; and 2) Cretaceous granitic intrusions. These units make up at least 3 km3 in the project area and have target temperatures of ~175-215o C. The abundant well data and detailed geophysical surveys (e.g., gravity, MT, and seismic reflection) provide significant subsurface control for the site and will permit development of a detailed 3D model. The documented temperatures, low permeability, and basement lithologies, as well as abundant available data facilitate development of a site dedicated to testing and improving new EGS technologies and techniques, thus making Fallon an ideal candidate for FORGE.

Paleoproterozoic Keulik-Kenirim Ore-Bearing Gabbro-Peridotite Complex, Kola Region: A New Occurrence of Ferropicritic Magmatism

NASA Astrophysics Data System (ADS)

Smolkin, V. F.; Lokhov, K. I.; Skublov, S. G.; Sergeeva, L. Yu.; Lokhov, D. K.; Sergeev, S. A.

2018-03-01

Comprehensive research of ore-bearing differentiated intrusions of the Keulik-Kenirim structural unit, which represents a fragment of the Paleoproterozoic Pechenga-Varzuga Belt, has been carried out for the first time. The intrusions are subvolcanic by type and lenticular in shape, nearly conformable and steeply dipping. They are made up of peridotite, olivine and plagioclase pyroxenites, and gabbro metamorphosed under amphibolite facies conditions along with host basic volcanics. All intrusive rocks are enriched in TiO2 and FeO. Sulfide Cu-Ni mineralization is represented by disseminated, pocket, and stringer-disseminated types, which are clustered in the peridotitic zone as hanging units and bottom lodes. The Ni content in disseminated ore is estimated at 0.45-0.55 wt % and 1.15-3.32 wt % in ore pockets; the Cu grades are 0.17-0.20 and 0.46-5.65 wt %, respectively. To determine the age of intrusions and metamorphism of intrusive and volcanic rocks, various isotopic systems have been used: Sm-Nd (TIMS) in rock and U-Pb (SIMS SHRIMP) and Lu-Hf (LA-ICP-MS) in zircon. Conclusions on the origin of zircons are based on concentrations of trace elements including REE therein and Hf-Nd correlation in zircons and rocks. The U-Pb system of zircons reflects episodes of igneous rock formation (1982 ± 12 Ma) and their postmagmatic transformation (1938 ± 20 Ma). The last disturbance of the U-Pb isotopic system occurred 700 and 425 Ma. Xenogenic zircons dated from 3.17 to 2.65 Ga have been revealed in the studied samples. These zircons were captured by magma from the Archean basement during its ascent. The intrusions were emplaced synchronously with economic ore formation in the Pechenga ore field (1985 ± 10 Ma). The peak metamorphism of intrusive rocks under amphibolite facies conditions is recorded at 40 Ma later. The differentiated intrusions of the Keulik-Kenirim structural unit are close in their internal structure, mineralogy, and geochemistry, as well as in age and features of related Cu-Ni mineralization to ore-bearing intrusions of the Pechenga ore field, which are derivatives of ferropicritic (ferriferous) magmatism.

The effect of topography and rock type on soil cation contents and stream solute and phosphorus concentrations of streams in the southwestern Brazilian Amazon basin.

NASA Astrophysics Data System (ADS)

Biggs, T. W.; Dunne, T.; Holmes, K.; Martinelli, L. A.

2001-12-01

Topography plays an important role in determining soil properties, stream solute concentrations and landscape denudation rates. Stallard (1985) suggested that catchment denudation rates should depend on soil thickness. Areas with low slopes are limited by the rate of transport of sediment, and typically contain thick soils that prevent interaction of stream waters with underlying bedrock [Stallard 1985]. Steep areas typically have thin soils, but a lower hydrologic residence time that may prevent soil water from coming into thermodynamic equilibrium with the soil-rock complex. In a survey of streams in the Brazilian Amazon basin, Biggs et al. (2001) found that stream solute concentrations correlate with soil cation contents in the humid tropics, but the mechanism underlying the correlation has not been determined. We combine chemical analyses of water samples from ~40 different streams with soil surveys, geology maps, and a 100m resolution DEM to examine the relationship between topography, rock type, soil cation contents, and stream solute concentrations in the Brazilian Amazon state of Rondônia. The basins are all more than 60% forested at the time of stream sampling and lie on granite-gneiss rocks, tertiary sediments, or sandstone. The catchment-averaged slope correlates positively with both soil cation contents and stream concentrations of P, Na, Ca, Mg, K, Si, ANC, and pH. Though we have no data about the relationship between soil depth and average slope, we assume an inverse correlation, so the data demonstrates that thick soils yield lower solute concentrations. Stream concentrations of Ca, Mg, ANC and pH reach a maximum at intermediate average slopes (3 degrees), suggesting that denudation rates may increase with slope up to a maximum, when the catchment becomes limited by the weathering rate of the basement rock. Catchments on mica-schists or mafic rocks have low average slopes and higher concentrations of Ca, Mg, Si, ANC, and pH than catchments on granite-gneiss, tertiary sediments or sandstone.

U-Pb ID-TIMS zircon ages of TTG gneisses of the Aravalli Craton of India

NASA Astrophysics Data System (ADS)

Chauhan, Hiredya; Saikia, Ashima; Kaulina, Tatiana; Bayanova, Tamara; Ahmad, Talat

2015-04-01

The crystalline basement of the Aravalli Craton is a heterogeneous assemblage dominated by granitic gneisses and granites with sporadic occurrences of amphibolites and dismembered sedimentary enclaves (Upadhyaya et al., 1992). This assemblage is known to have experienced multiple deformation and metamorphic events followed by emplacement of voluminous granites and basaltic dykes. Based on Sm-Nd whole rock data on the basement Mewar orthogneisses of Jhamarkotra region (Gopalan et al., 1990) and Pb/Pb ages of zircon from Gingla Granites which intrudes the basement (Wiedenbeck et al., 1996), it has been inferred that the whole magmatic episode leading to the formation of the basement spanned from 3300 to 2400 Ma and that the Aravalli cratonic block had broadly stabilized by 2500 Ma on which the younger Aravalli and Delhi Supergroup unconformably deposited. However, no comprehensive age data on the basement gneisses from the study area spanning the entire magmatic episode is available. This work attempts to provide a time frame work for evolution of the basement gneisses of the Aravalli Craton. We present here U-Pb zircon ages from the Precambrian basement TTG gneisses of the Aravalli Craton of north western India. Pb and U were measured on multicollector Finnigan-MAT 262 mass spectrometer. The temperatures of measurements were 1300°C for Pb and 1500°C for U. Pb isotope ratios were corrected for mass fractionation with a factor of 0.10% per amu, based on repeat analyses of the standard NBS SRM 982. The U analyses were corrected for mass fractionation with a factor of 0.003% per amu, based on repeat analyses of the NBS U 500 standard. Reproducibility of the U-Pb ratios was determined from the repeated analysis of standard zircon IGFM-87 (Ukraine) and taken as 0.5% for 207Pb/235U and 206Pb/238U ratios, respectively, at 95% confidence level. All calculations were done using the programs PBDAT and ISOPLOT (Ludwig 1991, 2008). Four zircon fractions corresponding to four zircon types from UD-16 sample yield a U-Pb discordant age of 2680±30 Ma. Two zircon fractions from UD-17 sample show discordant 207Pb/206Pb ages of 2506 and 2577 Ma. Zircons in our samples have moderate to high U contents (180-770 ppm) with low Th/U ratios (0.2-0.5) in the sample UD-16, characteristic for magmatic zircons from TTG rocks. Thus the obtained age of 2680±30 Ma is interpreted as an age of magmatic crystallization of tonalites. Gopalan, K. et al., (1990): Precambrian Res., 48, 287-297. Ludwig, K.R. (1991): PBDAT program. US. Geol. Surv. Open-file report 88-542, 38 p. Ludwig, K. R. (2008): Isoplot/Ex, version 3.6, Berkeley Geochronology Center, Special Publication no. 4. Upadhyaya, R. et al., (1992): Current Sci., 62(2): 87-92. Wiedenbeck, M. et al., (1996): Chem Geol. 129: 325-340.

3D Model of the San Emidio Geothermal Area

DOE Data Explorer

James E. Faulds

2013-12-31

The San Emidio geothermal system is characterized by a left-step in a west-dipping normal fault system that bounds the western side of the Lake Range. The 3D geologic model consists of 5 geologic units and 55 faults. Overlying Jurrassic-Triassic metasedimentary basement is a ~500 m-1000 m thick section of the Miocene lower Pyramid sequence, pre- syn-extensional Quaternary sedimentary rocks and post-extensional Quaternary rocks. 15-30º eastward dip of the stratigraphy is controlled by the predominant west-dipping fault set. Both geothermal production and injection are concentrated north of the step over in an area of closely spaced west dipping normal faults.

Aeromagnetic data, processing, and maps of Fort Irwin and vicinity, California: Chapter I in Geology and geophysics applied to groundwater hydrology at Fort Irwin, California

USGS Publications Warehouse

Langenheim, V.E.; Jachens, Robert C.; Buesch, David C.

2014-01-01

Aeromagnetic data help provide the underpinnings of a hydrogeologic framework for Fort Irwin by locating inferred structural features or grain that influence groundwater flow. Magnetization boundaries defined by horizontal-gradient analyses coincide locally with Cenozoic faults and can be used to extend these faults beneath cover. These boundaries also highlight the structural grain within the crystalline rocks and may serve as a proxy for fracturing, an important source of permeability within the generally impermeable basement rocks, thus mapping potential groundwater pathways through and along the mountain ranges in the study area.

Basement thrust sheets in the Clearwater orogenic zone, central Idaho and western Montana ( USA).

USGS Publications Warehouse

Skipp, B.

1987-01-01

The Clearwater orogenic zone in central Idaho and W Montana contains at least 2 major NE-directed Cordilleran thrust plates of Early Proterozoic metasedimentary and metaigneous rocks that overrode previously folded Middle Proterozoic rocks of the Belt basin in Cretaceous time. The northeastward migration of the resultant thickened wedge of crustal material combined with Cretaceous subduction along the W continental margin produced a younger N Bitterroot lobe of the Idaho batholith relative to an older S Atlanta lobe. Eocene extensional unroofing and erosion of the Bitterroot lobe has exposed the roots of the thick Cordilleran thrust sheets.-Author

Evidence for large-magnitude, post-Eocene extension in the northern Shoshone Range, Nevada, and its implications for Carlin-type gold deposits in the lower plate of the Roberts Mountains allochthon

USGS Publications Warehouse

Colgan, Joseph P.; Henry, Christopher D.; John, David A.

2014-01-01

The northern Shoshone and Toiyabe Ranges in north-central Nevada expose numerous areas of mineralized Paleozoic rock, including major Carlin-type gold deposits at Pipeline and Cortez. Paleozoic rocks in these areas were previously interpreted to have undergone negligible postmineralization extension and tilting, but here we present new data that suggest major post-Eocene extension along west-dipping normal faults. Tertiary rocks in the northern Shoshone Range crop out in two W-NW–trending belts that locally overlie and intrude highly deformed Lower Paleozoic rocks of the Roberts Mountains allochthon. Tertiary exposures in the more extensive, northern belt were interpreted as subvertical breccia pipes (intrusions), but new field data indicate that these “pipes” consist of a 35.8 Ma densely welded dacitic ash flow tuff (informally named the tuff of Mount Lewis) interbedded with sandstones and coarse volcaniclastic deposits. Both tuff and sedimentary rocks strike N-S and dip 30° to 70° E; the steeply dipping compaction foliation in the tuffs was interpreted as subvertical flow foliation in breccia pipes. The southern belt along Mill Creek, previously mapped as undivided welded tuff, includes the tuff of Cove mine (34.4 Ma) and unit B of the Bates Mountain Tuff (30.6 Ma). These tuffs dip 30° to 50° east, suggesting that their west-dipping contacts with underlying Paleozoic rocks (previously mapped as depositional) are normal faults. Tertiary rocks in both belts were deposited on Paleozoic basement and none appear to be breccia pipes. We infer that their present east tilt is due to extension on west-dipping normal faults. Some of these faults may be the northern strands of middle Miocene (ca. 16 Ma) faults that cut and tilted the 34.0 Ma Caetano caldera ~40° east in the central Shoshone Range (

Petroleum geology and resources of the North Ustyurt Basin, Kazakhstan and Uzbekistan

USGS Publications Warehouse

Ulmishek, Gregory F.

2001-01-01

The triangular-shaped North Ustyurt basin is located between the Caspian Sea and the Aral Lake in Kazakhstan and Uzbekistan and extends offshore both on the west and east. Along all its sides, the basin is bounded by the late Paleozoic and Triassic foldbelts that are partially overlain by Jurassic and younger rocks. The basin formed on a cratonic microcontinental block that was accreted northward to the Russian craton in Visean or Early Permian time. Continental collision and deformation along the southern and eastern basin margins occurred in Early Permian time. In Late Triassic time, the basin was subjected to strong compression that resulted in intrabasinal thrusting and faulting. Jurassic-Tertiary, mostly clastic rocks several hundred meters to 5 km thick overlie an older sequence of Devonian?Middle Carboniferous carbonates, Upper Precambrian massifs and deformed Caledonian foldbelts. The Carboniferous?Lower Permian clastics, carbonates, and volca-basement is at depths from 5.5 km on the highest uplifts to 11 nics, and Upper Permian?Triassic continental clastic rocks, pri-km in the deepest depressions. marily red beds. Paleogeographic conditions of sedimentation, Three total petroleum systems are identified in the basin. the distribution of rock types, and the thicknesses of pre-Triassic Combined volumes of discovered hydrocarbons in these sysstratigraphic units are poorly known because the rocks have been tems are nearly 2.4 billion barrels of oil and 2.4 trillion cubic penetrated by only a few wells in the western and eastern basin feet of gas. Almost all of the oil reserves are in the Buzachi Arch areas. The basement probably is heterogeneous; it includes and Surrounding Areas Composite Total Petroleum System in 2 Petroleum Geology, Resources?North Ustyurt Basin, Kazakhstan and Uzbekistan the western part of the basin. Oil pools are in shallow Jurassic and Neocomian sandstone reservoirs, in structural traps. Source rocks are absent in the total petroleum system area; therefore, the oil could have migrated from the adjacent North Caspian basin. The North Ustyurt Jurassic Total Petroleum System encompasses the rest of the basin area and includes Jurassic and younger rocks. Several oil and gas fields have been discovered in this total petroleum system. Oil accumulations are in Jurassic clastic reservoirs, in structural traps at depths of 2.5?3 km. Source rocks for the oil are lacustrine beds and coals in the continental Jurassic sequence. Gas fields are in shallow Eocene sandstones in the northern part of the total petroleum system. The origin of the gas is unknown. The North Ustyurt Paleozoic Total Petroleum System stratigraphically underlies the North Ustyurt Jurassic system and occupies the same geographic area. The total petroleum system is almost unexplored. Two commercial flows of gas and several oil and gas shows have been tested in Carboniferous shelf carbonates in the eastern part of the total petroleum system. Source rocks probably are adjacent Carboniferous deep-water facies interpreted from seismic data. The western extent of the total petroleum system is conjectural. Almost all exploration drilling in the North Ustyurt basin has been limited to Jurassic and younger targets. The underlying Paleozoic-Triassic sequence is poorly known and completely unexplored. No wells have been drilled in offshore parts of the basin. Each of three total petroleum systems was assessed as a single assessment unit. Undiscovered resources of the basin are small to moderate. Most of the undiscovered oil probably will be discovered in Jurassic and Neocomian stratigraphic and structural traps on the Buzachi arch, especially on its undrilled off-shore extension. Most of the gas discoveries are expected to be in Paleozoic carbonate reservoirs in the eastern part of the basin.

29. Basement under central corridor. Shaft on right actuates cross ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

29. Basement under central corridor. Shaft on right actuates cross over valve. Shaft at left operates main flood valve to admit water into the bed. - Lake Whitney Water Filtration Plant, Filtration Plant, South side of Armory Street between Edgehill Road & Whitney Avenue, Hamden, New Haven County, CT

REACTIVITY MEASUREMENT FACILITY, UNDER CONSTRUCTION OVER MTR CANAL IN BASEMENT ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

REACTIVITY MEASUREMENT FACILITY, UNDER CONSTRUCTION OVER MTR CANAL IN BASEMENT OF MTR BUILDING, TRA-603. WOOD PLANKS REST ON CANAL WALL OBSERVABLE IN FOREGROUND. INL NEGATIVE NO. 11745. Unknown Photographer, 8/20/1954 - Idaho National Engineering Laboratory, Test Reactor Area, Materials & Engineering Test Reactors, Scoville, Butte County, ID

Aeromagnetic maps with geologic interpretations for the Tularosa Valley, south-central New Mexico

USGS Publications Warehouse

Bath, G.D.

1977-01-01

An aeromagnetic survey of the Tularosa Valley in south-central New Mexico has provided information on the igneous rocks that are buried beneath alluvium and colluvium. The data, compiled as residual magnetic anomalies, are shown on twelve maps at a scale of 1:62,500. Measurements of magnetic properties of samples collected in the valley and adjacent highlands give a basis for identifying the anomaly-producing rocks. Precambrian rocks of the crystalline basement have weakly induced magnetizations and produce anomalies having low magnetic intensities and low magnetic gradients. Late Cretaceous and Cenozoic intrusive rocks have moderately to strongly induced magnetizations. Precambrian rocks produce prominent magnetic anomalies having higher amplitudes and higher gradients. The Quaternary basalt has a strong remanent magnetization of normal polarity and produces narrow anomalies having high-magnetic gradients. Interpretations include an increase in elevation to the top of buried Precambrian rock in the northern part of the valley, a large Late Cretaceous and Cenozoic intrusive near Alamogordo, and a southern extension of the intrusive rock exposed in the Jarilla Mountains. Evidence for the southern extension comes from a quantitative analysis of the magnetic anomalies..

Initiation of long-term coupled microbiological, geochemical, and hydrological experimentation within the seafloor at North Pond, western flank of the Mid-Atlantic Ridge

USGS Publications Warehouse

Edwards, K.J.; Backert, N.; Bach, W.; Becker, K.; Klaus, A.; Griffin, Dale W.; Anderson, L.; Haddad, A.G.; Harigane, Y.; Campion, P.L.; Hirayama, H.; Mills, H.J.; Hulme, S.M.; Nakamura, K.; Jorgensen, S.L.; Orcutt, B.; Insua, T.L.; Park, Y.-S.; Rennie, V.; Salas, E.C.; Rouxel, O.; Wang, F.; Russel, J.A.; Wheat, C.G.; Sakata, K.; Brown, M.; Magnusson, J.L.; Ettlinger, Z.

2012-01-01

Integrated Ocean Drilling Program (IODP) Expedition 336 successfully initiated subseafloor observatory science at a young mid-ocean-ridge flank setting. All of the drilled sites are located in the North Pond region of the Atlantic Ocean (22??45'N, 46??05'W) in 4414-4483 m water depth. This area is known from previous ocean drilling and site survey investigations as a site of particularly vigorous circulation of seawater in permeable 8 Ma basaltic basement underlying a <300 m thick sedimentary pile. Understanding how this seawater circulation affects microbial and geochemical processes in the uppermost basement was the primary science objective of Expedition 336. Basement was cored and wireline-logged in Holes U1382A and U1383C. Upper oceanic crust in Hole U1382A, which is only 50 m west of Deep Sea Drilling Project (DSDP) Hole 395A, recovered 32 m of core between 110 and 210 meters below seafloor (mbsf). Core recovery in basement was 32%, yielding a number of volcanic flow units with distinct geochemical and petrographic characteristics. A unit of sedimentary breccia containing clasts of basalt, gabbroic rocks, and mantle peridotite was found intercalated between two volcanic flow units and was interpreted as a rock slide deposit. From Hole U1383C we recovered 50.3 m of core between 69.5 and 331.5 mbsf (19%). The basalts are aphyric to highly plagioclase-olivine-phyric tholeiites that fall on a liquid line of descent controlled by olivine fractionation. They are fresh to moderately altered, with clay minerals (saponite, nontronite, and celadonite), Fe oxyhydroxide, carbonate, and zeolite as secondary phases replacing glass and olivine to variable extents. In addition to traditional downhole logs, we also used a new logging tool for detecting in situ microbial life in ocean floor boreholes-the Deep Exploration Biosphere Investigative tool (DEBI-t). Sediment thickness was ???90 m at Sites U1382 and U1384 and varied between 38 and 53 m at Site U1383. The sediments are predominantly nannofossil ooze with layers of coarse foraminiferal sand and occasional pebble-size clasts of basalt, serpentinite, gabbroic rocks, and bivalve debris. The bottommost meters of sections cored with the advanced piston corer feature brown clay. Extended core barrel coring at the sediment/basement interface recovered <1 m of brecciated basalt with micritic limestone. Sediments were intensely sampled for geochemical pore water analyses and microbiological work. In addition, high-resolution measurements of dissolved oxygen concentration were performed on the whole-round sediment cores. Major strides in ridge-flank studies have been made with subseafloor borehole observatories (CORKs) because they facilitate combined hydrological, geochemical, and microbiological studies and controlled experimentation in the subseafloor. During Expedition 336, two fully functional observatories were installed in two newly drilled holes (U1382A and U1383C) and an instrument and sampling string were placed in an existing hole (395A). Although the CORK wellhead in Hole 395A broke off and Hole U1383B was abandoned after a bit failure, these holes and installations are intended for future observatory science targets. The CORK observatory in Hole U1382A has a packer seal in the bottom of the casing and monitors/samples a single zone in uppermost oceanic crust extending from 90 to 210 mbsf. Hole U1383C was equipped with a three-level CORK observatory that spans a zone of thin basalt flows with intercalated limestone (???70-146 mbsf), a zone of glassy, thin basaltic flows and hyaloclastites (146-200 mbsf), and a lowermost zone (???200-331.5 mbsf) of more massive pillow flows with occasional hyaloclastites in the upper part.

U-Pb dating and isotopic signature of the alkaline ring complexes of Bou Naga (Mauritania): its bearing on late proterozoic plate tectonics around the West African craton

NASA Astrophysics Data System (ADS)

Blanc, A.; Bernard-Griffiths, J.; Caby, R.; Caruba, C.; Caruba, R.; Dars, R.; Fourcade, S.; Peucat, J. J.

1992-04-01

In the West African fold belt of Mauritania, high-grade metamorphic series, similar to those of Amsaga (Reguibat shield-West African Craton), are exposed in a window. At Bou Naga-Mauritania (19° N, 13° 15' W) in the South of this window, an alkaline ring complex has intruded the metamorphic country rocks. This complex consists of two geological formations: the Eastern formation is mainly composed of red rhyolite sills, whereas the Western formation is made up of several kinds of alkaline rocks both saturated and under-saturated which cross cut the earlier saturated units. Three U-Pb zircon age measurements have been made on the alkaline complex, and one on an orthogneiss from the metamorphic country rocks. The syenite and the alkaline granite of the Western block are 676 ± 8 and 687 ± 5 Ma old. The orthogneiss is Archaean with an age of 2709 ± 136 Ma, but the lower intercept of discordia on concordia, shows an age of 756 ± 25 Ma linked with the genesis of the alkaline complex. A major crustal contribution is recorded by Nd and O isotopes in the SiO 2-saturated rocks. These results provide evidence for the correlation of the metamorphic country rocks with the Reguibat Archaean basement and for an early Pan-African continental rifting phase in this area before the tectonometamorphic events in the Mauritanide belt. Furthermore, with regards with previous geodynamic works of the West African Craton, our results leads us to suggest a significant diachronism between late Proterozoic crustal evolution to the West and to the East of the West African Craton. This is a further evidence for modern-type plate tectonics at this time.

Spatial variation of radon and helium in soil gas vis-à-vis geology of area, NW Himalayas, India

NASA Astrophysics Data System (ADS)

Mahajan, S.; Bajwa, B.; Kumar, A.; Singh, S.; Walia, V.; Yang, T. F.

2009-12-01

In an effort to quantify the geological/lithological control on radon, helium soil gas potential and appraise the use of soil gas technique as a geological mapping tool, soil gas measurements were made, in some parts of Himachal Himalayas of NW Himalayan range, using soil gas grab sampling technique. More than 360 soil gas samples were collected from four different geological/lithologic rock units of the area under consideration. The collected soil gas samples were analyzed for radon and helium using RTM-2100 (SARAD) and Helium leak detector (ALCATEL) respectively. The observed values were then correlated with the geology/lithology of the study area. The study area is broadly divided into four different units on the basis of geology/lithology i.e. (A) Upper Shiwaliks (B) Middle & Lower Shiwaliks (C) Lesser Himalayan rocks (D) Higher Himalayan rocks. Significant differences in the soil gas concentrations among the geologic units were observed, where Lesser Himalayan rocks showing maximum concentrations of both radon (254 KBq/m3) and helium (5.46 ppm). Lesser Himalayan zone lies mainly between two major thrusts MBT and MCT running along the Himalayan trend, which still are tectonically active. It can be concluded from the present study that soil gases (radon and helium) can be used as a productive tool for geological mapping. These findings may have very important connation for health risk assessment of the area, since it has been shown that radon soil gas found in soils overlying basement rocks are the main source for indoor radon concentrations. Radioactive isotopes attach rapidly to atmospheric aerosols and can enter into a human body thus constitute significant hazard to human health.

U-Pb isotopic evidence for the accretion of a continental microplate in the Zalm region of the Saudi Arabian Shield.

USGS Publications Warehouse

Stacey, J.S.; Agar, R.A.

1985-01-01

This area includes three of the main tectonic units of the Arabian Shield: the Afif continental terrain, the Nabitah suture with its associated mobile belt, and the Asir ensimatic arc terrain. U/Pb zircon data from a pelitic garnet-sillimanite gneiss show that the Kabib formation in the S of the Afif terrain may be as old as 1770 m.y. Pb and Rb/Sr isotopic data in the Zalm region reveal a change in the nature of the underlying crust, from continental basement in the NE to less radiogenic marginal arc rocks in the SW. Miogeosynclinal continental shelf facies of the Siham group lie unconformably over the Kabid formation. U/Pb zircon age determinations show that this 'Andean' continental margin developed before approx 720 m.y. and the emplacement of calc-alkaline plutonic rocks continued until approx 690 m.y. During the period 685-640 m.y. the continental Afif microplate collided with the Asir terrain as part of the Nabitah orogeny. At approx 640 m.y. age the Najd strike-slip faulting commenced, with a dextral phase that controlled emplacement of granite plutons as well as the development of large pull-apart grabens. Some of the latter were floored by new oceanic crust and filled with volcanosedimentary rocks of the Bani Ghayy group.-R.A.H.

The Pawnee Sequence: Poroelastic Effects from Injection in Osage County, Oklahoma

NASA Astrophysics Data System (ADS)

Barbour, A. J.; Rubinstein, J. L.

2016-12-01

Aggregate multi-year records of wastewater injection in Oklahoma show that the strongest change in injection within 20 km of the 2016 M5.8 Pawnee strike-slip earthquake was in Osage County, where injection rates increased rapidly in late-2012 by nearly a factor of three above previous levels. After this increase, rates there declined steadily over two years to an average rate characteristic of all other injection wells in Pawnee and Noble Counties, remaining relatively constant until the beginning of the earthquake sequence. Here we test if poroelastic effects associated with this injection-rate transient can help explain the relative timing between peak injection rates and the beginning of the Pawnee sequence. Although the alternative hypothesis that regional-scale faults and fractures in critically stressed rock serve as fast-pathways for fluid diffusion cannot be ruled out, it appears to be difficult to reconcile based solely on injection data and space-time patterns for this seismic sequence. We simulate the cylindrically symmetric, transient strain and pore pressure fields for an injection-source time function emulating the injection history in a layered half-space in accordance with linear poroelasticity. In the simulation domain, injection occurs at depths of 1300 - 1900 m, into a homogeneous basal sedimentary reservoir representing the Arbuckle Group, overlying a semi-infinite layer representing granitic basement; we determined the hydraulic, elastic, and poroelastic properties of these layers from published literature. At the mainshock hypocenter, this numerical model predicts a delay between peak injection rates and pore pressure increase that is strongly dependent on hydraulic diffusivity; however, the duration is also controlled by the bulk elastic properties and the undrained Skempton's coefficient of the rock. Furthermore, because of fluid-strain coupling, pore pressures in the basement rock decrease during this delay period, which would tend to stabilize temporarily a critically stressed fault. Even though pore pressure diffusion is the dominant mechanism at play, poroelastic effects do affect the relative timing assuming a reasonable set of material parameters, even though strain rates in the basement are relatively low compared to rates in the Arbuckle layer (and above).

The Proterozoic Mount Isa Fault Zone, northeastern Australia: is it really a ca. 1.9 Ga terrane-bounding suture?

NASA Astrophysics Data System (ADS)

Bierlein, Frank P.; Betts, Peter G.

2004-09-01

In marked contrast to Palaeoproterozoic Laurentia, the location of sutures and boundaries of discrete crustal fragments amalgamated during Palaeoproterozoic formation of the North Australian Craton remain highly speculative. Interpretations of suture locations have relied heavily on the analysis of regional geophysical datasets because of sparse exposure of rocks of the appropriate age. The Mount Isa Fault Zone has been interpreted as one such Palaeoproterozoic terrane-bounding suture. Furthermore, the coincidence of this fault zone with major shale-hosted massive sulphide Pb-Zn-Ag orebodies has led to speculations that trans-lithospheric faults may be an important ingredient for the development of this deposit type. This study has integrated geophysical and geochemical data to test the statute of the Mount Isa Fault as a terrane-bounding suture. Forward modelling of gravity data shows that basement rocks on either side of the Mount Isa Fault have similar densities. These interpretations are consistent with geochemical observations and Sm-Nd data that suggest that basement lithologies on either side of the Mount Isa Fault are geochemically and isotopically indistinguishable from each other, and that the Mount Isa Fault is unlikely to represent a suture zone that separates different Palaeoproterozoic terranes. Our data indicate that the crustal blocks on both sides of the Mount Isa Fault Zone must have been in within close proximity of each other since the Palaeoproterozoic, and that the Western Fold Belt was part of the (ancestral) North Australian Craton well before the ˜1.89-1.87 Ga Barramundi Orogeny. It appears that deep crustal variations in density may be related to the boundary between a shallowly west-dipping high-density mafic to ultramafic plate and low-density basement rocks. This interpretation in turn impacts on crustal-scale models for the development of shale-hosted massive sulphide Pb-Zn mineralisation, which do not require trans-lithospheric faults to tap deep-seated metal reservoirs and/or mantle plumbing systems. The approach applied herein demonstrates the value of multi-disciplinary investigations to the critical assessment of long-lived Proterozoic fault systems which, in the absence of methodical analysis, are commonly assumed to represent terrane-bounding sutures.

Crustal shortening and structural architecture of the Interandean and Subandean zones of southern Bolivia (21°S): Constraints from a new balanced cross section

NASA Astrophysics Data System (ADS)

Anderson, R. B.; Long, S. P.; Horton, B. K.; Calle, A.; Ramirez, V.

2015-12-01

Structural insights obtained from balanced cross sections, including thrust belt geometry, location of footwall ramps, and crustal shortening estimates, provide key information for testing model predictions of orogen dynamics (e.g., Cordilleran cyclicity, critical taper theory). New results from geologic mapping along an east-west transect in the central Andes are integrated with existing geophysical data to construct a balanced cross section across the Interandean (IAZ) and Subandean (SAZ) zones of southern Bolivia at 21°S, in order to define thrust belt geometry and estimate crustal shortening. The IAZ consists of a doubly vergent zone of 2-4 km-thick thrust sheets of mainly Silurian-Devonian rocks, which are structurally elevated ~10 km relative to equivalent SAZ levels to the east. Notably, our proposed IAZ geometry differs from published geometries that lack significant west-directed backthrusts. The SAZ is defined by regional-scale, fault-bend folds (10-20 km wavelength, 4-6 km amplitude) that exhume rocks as deep as Carboniferous above a 10-12 km-deep regional décollement in Silurian rocks. Previous studies have interpreted IAZ and SAZ shortening to be balanced by slip on two separate basement megathrust sheets at depth. We estimate 151 km (44%) of total east-west shortening in the IAZ (71 km) and SAZ (80 km), which is similar to a previous estimate (144 km, 42%). Importantly, our estimate of SAZ shortening restores the leading edge of the basement thrust sheet feeding displacement into the SAZ back to a corresponding footwall ramp that is constrained by a seismic reflection profile 90 km along strike to the south. Our shortening magnitudes are similar to nearby estimates to the north and south, which range between 60-86 km for the SAZ and 43-96 km for the IAZ. Future work will continue the cross section westward into the Eastern Cordillera hinterland, and explore potential variations in the geometry and style of basement deformation.

Along-strike continuity of structure, stratigraphy, and kinematic history in the Himalayan thrust belt: The view from Northeastern India

NASA Astrophysics Data System (ADS)

DeCelles, P. G.; Carrapa, B.; Gehrels, G. E.; Chakraborty, T.; Ghosh, P.

2016-12-01

The Himalaya consists of thrust sheets tectonically shingled together since 58 Ma as India collided with and slid beneath Asia. Major Himalayan structures, including the South Tibetan Detachment (STD), Main Central Thrust (MCT), Lesser Himalayan Duplex (LHD), Main Boundary Thrust (MBT), and Main Frontal Thrust (MFT), persist along strike from northwestern India to Arunachal Pradesh near the eastern end of the orogenic belt. Previous work suggests significant basement involvement and a kinematic history unique to the Arunachal Himalaya. We present new geologic and geochronologic data to support a regional structural cross section and kinematic restoration of the Arunachal Himalaya. Large Paleoproterozoic orthogneiss bodies (Bomdila Gneiss) previously interpreted as Indian basement have ages of 1774-1810 Ma, approximately 50 Ma younger than Lesser Himalayan strata into which their granitic protoliths intruded. Bomdila Gneiss is therefore part of the Lesser Himalayan cover sequence, and no evidence exists for basement involvement in the Arunachal Himalaya. Minimum shortening in rocks structurally beneath the STD is 421 km. The MCT was active during the early Miocene; STD extension overlapped MCT shortening and continued until approximately 15-12 Ma; and growth of the LHD began 11 Ma, followed by slip along the MBT (post-7.5 Ma) and MFT (post-1 Ma) systems. Earlier thrusting events involved long-distance transport of strong, low-taper thrust sheets, whereas events after 12-10 Ma stacked smaller, weaker thrust sheets into a steeply tapered orogenic wedge dominated by duplexing. A coeval kinematic transition is observed in other Himalayan regions, suggesting that orogenic wedge behavior was controlled by rock strength and erodibility.

The Grand St Bernard-Briançonnais Nappe System and the Paleozoic Inheritance of the Western Alps Unraveled by Zircon U-Pb Dating

NASA Astrophysics Data System (ADS)

Bergomi, M. A.; Dal Piaz, G. V.; Malusà, M. G.; Monopoli, B.; Tunesi, A.

2017-12-01

The continental crust involved in the Alpine orogeny was largely shaped by Paleozoic tectono-metamorphic and igneous events during oblique collision between Gondwana and Laurussia. In order to shed light on the pre-Alpine basement puzzle disrupted and reamalgamated during the Tethyan rifting and the Alpine orogeny, we provide sensitive high-resolution ion microprobe U-Pb zircon and geochemical whole rock data from selected basement units of the Grand St Bernard-Briançonnais nappe system in the Western Alps and from the Penninic and Lower Austroalpine units in the Central Alps. Zircon U-Pb ages, ranging from 459.0 ± 2.3 Ma to 279.1 ± 1.1 Ma, provide evidence of a complex evolution along the northern margin of Gondwana including Ordovician transtension, Devonian subduction, and Carboniferous-to-Permian tectonic reorganization. Original zircon U-Pb ages of 371 ± 0.9 Ma and 369.3 ± 1.5 Ma, from calc-alkaline granitoids of the Grand Nomenon and Gneiss del Monte Canale units, provide the first compelling evidence of Late Devonian orogenic magmatism in the Alps. We propose that rocks belonging to these units were originally part of the Moldanubian domain and were displaced toward the SW by Late Carboniferous strike-slip faulting. The resulting assemblage of basement units was disrupted by Permian tectonics and by Mesozoic opening of the Alpine Tethys. Remnants of the Moldanubian domain became either part of the European paleomargin (Grand Nomenon unit) or part of the Adriatic paleomargin (Gneiss del Monte Canale unit), to be finally accreted into the Alpine orogenic wedge during the Cenozoic.

Geological Structure and History of the Arctic Ocean

NASA Astrophysics Data System (ADS)

Petrov, Oleg; Morozov, Andrey; Shokalsky, Sergey; Sobolev, Nikolay; Kashubin, Sergey; Pospelov, Igor; Tolmacheva, Tatiana; Petrov, Eugeny

2016-04-01

New data on geological structure of the deep-water part of the Arctic Basin have been integrated in the joint project of Arctic states - the Atlas of maps of the Circumpolar Arctic. Geological (CGS, 2009) and potential field (NGS, 2009) maps were published as part of the Atlas; tectonic (Russia) and mineral resources (Norway) maps are being completed. The Arctic basement map is one of supplements to the tectonic map. It shows the Eurasian basin with oceanic crust and submerged margins of adjacent continents: the Barents-Kara, Amerasian ("Amerasian basin") and the Canada-Greenland. These margins are characterized by strained and thinned crust with the upper crust layer, almost extinct in places (South Barents and Makarov basins). In the Central Arctic elevations, seismic studies and investigation of seabed rock samples resulted in the identification of a craton with the Early Precambrian crust (near-polar part of the Lomonosov Ridge - Alpha-Mendeleev Rise). Its basement presumably consists of gneiss granite (2.6-2.2 Ga), and the cover is composed of Proterozoic quartzite sandstone and dolomite overlain with unconformity and break in sedimentation by Devonian-Triassic limestone with fauna and terrigenous rocks. The old crust is surrounded by accretion belts of Timanides and Grenvillides. Folded belts with the Late Precambrian crust are reworked by Caledonian-Ellesmerian and the Late Mesozoic movements. Structures of the South Anuy - Angayucham ophiolite suture reworked in the Early Cretaceous are separated from Mesozoides proper of the Pacific - Verkhoyansk-Kolyma and Koryak-Kamchatka belts. The complicated modern ensemble of structures of the basement and the continental frame of the Arctic Ocean was formed as a result of the conjugate evolution and interaction of the three major oceans of the Earth: Paleoasian, Paleoatlantic and Paleopacific.

Mixing from below in hydrothermal ore deposits

NASA Astrophysics Data System (ADS)

Bons, Paul D.; Gomez-Rivas, Enrique; Markl, Gregor; Walter, Bejamin

2014-05-01

Unconformity-related hydrothermal ore deposits typically show indications of mixing of two end-member fluids: (a) hot, deep, rock-buffered basement brines and (b) colder fluids derived from the surface or overlying sediments. The hydromechanics of bringing these fluids together from above and below remain unclear. Classical percolative Darcy-flow models are inconsistent with (1) fluid overpressure indicated by fracturing and brecciation, (2) fast fluid flow indicated by thermal disequilibrium, and (3) strong fluid composition variations on the mm-scale, indicated by fluid inclusion analyses (Bons et al. 2012; Fusswinkel et al. 2013). We propose that fluids first descend, sucked down by desiccation reactions in exhumed basement. Oldest fluids reach greatest depths, where long residence times and elevated temperatures allow them the extensively equilibrate with their host rock, reach high salinity and scavenge metals, if present. Youngest fluids can only penetrate to shallower depths and can (partially) retain signatures from their origin, for example high Cl/Br ratios from the dissolution of evaporitic halite horizons. When fluids are released from all levels of the crustal column, these fluids mix during rapid ascent to form hydrothermal ore deposits. Mixing from below provides a viable hydromechanical mechanism to explain the common phenomenon of mixed shallow and deep fluids in hydrothermal ore deposits. Bons, P.D., Elburg, M.A., Gomez-Rivas, E. 2012. A review of the formation of tectonic veins and their microstructures. J. Struct. Geol. doi:10.1016/j.jsg.2012.07.005 Fusswinkel, T., Wagner, T., Wälle, M., Wenzel, T., Heinrich, C.A., Markl, M. 2013. Fluid mixing forms basement-hosted Pb-Zn deposits: Insight from metal and halogen geochemistry of individual fluid inclusions. Geology. doi:10.1130/G34092.1

Fingerprinting the K/T impact site and determining the time of impact by U-Pb dating of single shocked zircons from distal ejecta

NASA Technical Reports Server (NTRS)

Krogh, T. E.; Kamo, S. L.; Bohor, B. F.

1993-01-01

U-Pb isotopic dating of single 1 - 3 micrograms zircons from K/T distal ejecta from a site in the Raton Basin, Colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site. Data for the least shocked zircons are slightly displaced from the 544 +/- 5 Ma primary age for a component of the target site, while those for highly shocked and granular grains are strongly displaced towards the time of impact at 65.5 +/- 3.0 Ma. Such shocked and granular zircons have never been reported from any source, including explosive volcanic rocks. Zircon is refractory and has one of the highest thermal blocking temperatures; hence, it can record both shock features and primary and secondary ages without modification by post-crystallization processes. Unlike shocked quartz, which can come from almost anywhere on the Earth's crust, shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers. With U-Pb zircon dating, it is now possible to correlate ejecta layers derived from the same target site, test the single versus multiple impact hypothesis, and identify the target source of impact ejecta. The ages obtained in this study indicate that the Manson impact site, Iowa, which has basement rocks that are mid-Proterozoic in age, cannot be the source of K/T distal ejecta. The K/T distal ejecta probably originated from a single impact site because most grains have the same primary age.

Salton Seismic Imaging Project Line 6: San Andreas Fault and Northern Coachella Valley Structure, Riverside and San Bernardino Counties, California

NASA Astrophysics Data System (ADS)

Catchings, R. D.; Fuis, G.; Rymer, M. J.; Goldman, M.; Tarnowski, J. M.; Hole, J. A.; Stock, J. M.; Matti, J. C.

2012-12-01

The Salton Seismic Imaging Project (SSIP) is a large-scale, active- and passive-source seismic project designed to image the San Andreas fault (SAF) and adjacent basins (Imperial and Coachella Valleys) in southernmost California. Data and preliminary results from many of the seismic profiles are reported elsewhere (including Fuis et al., Rymer et al., Goldman et al., Langenheim et al., this meeting). Here, we focus on SSIP Line 6, one of four 2-D seismic profiles that were acquired across the Coachella Valley. The 44-km-long, SSIP-Line-6 seismic profile extended from the east flank of Mt. San Jacinto northwest of Palm Springs to the Little San Bernardino Mountains and crossed the SAF (Mission Creek (MCF), Banning (BF), and Garnet Hill (GHF) strands) roughly normal to strike. Data were generated by 10 downhole explosive sources (most spaced about 3 to 5 km apart) and were recorded by approximately 347 Texan seismographs (average spacing 126 m). We used first-arrival refractions to develop a P-wave refraction tomography velocity image of the upper crust along the seismic profile. The seismic data were also stacked and migrated to develop low-fold reflection images of the crust. From the surface to about 7 km depth, P-wave velocities range from about 2.5 km/s to about 7.2 km/s, with the lowest velocities within an ~2-km-deep, ~20-km-wide basin, and the highest velocities below the transition zone from the Coachella Valley to Mt. San Jacinto and within the Little San Bernardino Mountains. The BF and GHF strands bound a shallow sub-basin on the southwestern side of the Coachella Valley, but the underlying shallow-depth (~4 km) basement rocks are P-wave high in velocity (~7.2 km/s). The lack of a low-velocity zone beneath BF and GHF suggests that both faults dip northeastward. In a similar manner, high-velocity basement rocks beneath the Little San Bernardino Mountains suggest that the MCF dips vertically or southwestward. However, there is a pronounced low-velocity zone in basement rocks between about 2 and 7 km depth beneath and southwest of the MCF, suggesting a vertical or slightly southwest-dipping MCF. The apparent northeast dip of the BF and the apparent vertical or southwest dip of the MCF suggests that the two main strands of the SAF (MCF and BF) merge at about 10 km depth. A plot of double-difference earthquake hypocenters (Hauksson, 2000) along the seismic profile shows events that occurred between 1980-2000 (excluding those in 1992, prior to and after the Joshua Tree and Landers earthquakes) are largely confined to the vicinity of the basement low-velocity zone between the MCF and BF. However, a separate alignment of hypocenters occurs southwest of the BF and projects toward the surface beneath Mt. San Jacinto. Collectively, the velocity images and the seismicity data suggest the BF strand of the SAF dips to the northeast at about 50 degrees in the upper 10 km, and the MCF strand is either vertical or dips southwestward about 80 degrees, with both strands merging at about 10 km depth and forming a near-vertical zone of faults to at least 15 km depth. The SSIP Line 6 data are consistent with structures interpreted by Catchings et al. (2009).

Neoproterozoic complexes of the shelf cover of the Dzabkhan terrane basement in the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Kozakov, I. K.; Kuznetsov, A. B.; Erdenegargal, Ch.; Salnikova, E. B.; Anisimova, I. V.; Plotkina, Ju. V.; Fedoseenko, A. M.

2017-09-01

The formation stages of high-grade metamorphic complexes and the related granitoids of the Dzabkhan terrane basement are considered. The age data (U-Pb method, TIMS) of zircons from the trondhjemite block of the eastern part of the Dzabkhan terrane, which is directly overlain by the dolomite sequence of the Tsagaan Oloom Formation, are given. Trondhjemites yield the U-Pb zircon age of 862 ± 3 Ma. In their structural position, they are assigned to typical postmetamorphic formations that determine the formation and cratonization of rocks of the host block. The geochronological study of trondhjemites gives grounds to distinguish fragments of the continental crust in the Dzabkhan terrane basement, the formation of which occurred at different periods of time: ˜860 and ˜790 Ma. Geological-geochronological and Sm‒Nd isotope-geochemical studies indicate that the Dzabkhan terrane basement is not a single block of the Early Precambrian continental crust, but a composite terrane, comprising Neoproterozoic ensialic and island-arc structural and compositional complexes. Correlation of Sr isotopic characteristics with the 87Sr/86Sr variation curve in the Neoproterozoic and Cambrian seawater shows that carbonate deposits accumulated at the eastern margin of the Dzabkhan terrane near the end of the Neoproterozoic, 700-550 Ma, and in the central part of the terrane in the Early Cambrian, 540-530 Ma.

K-Ar age constrains on chemically weathered granitic basement rocks (saprolites) in Scandinavia

NASA Astrophysics Data System (ADS)

Margreth, Annina; Fredin, Ola; Viola, Giulio; Knies, Jochen; Sørlie, Ronald; Lie, Jan-Erik; Margrethe Grandal, Else; Zwingmann, Horst; Vogt, Christoph

2017-04-01

Remnants of in-situ weathered bedrock, saprolite, are found in several locations in Scandinavia. Saprolites contain important information about past climate conditions and landscape evolution, although their age and genesis are commonly difficult to constrain. It is generally thought that clay-poor, coarse-grained (arêne) saprolites, mostly occurring as thin regolith blankets or in larger outcrops, formed in temperate climate during the Cenozoic, whereas clay-rich (argillic) saprolites, commonly restricted to small, fracture-bounded outcrops, formed in (sub-)tropical climate during the Mesozoic. Recent methodological and conceptual advances in K-Ar dating of illite-bearing fault rocks have been applied to date clay-rich saprolites. To test the K-Ar dating technique for saprolites, we first selected an offshore site in the Viking Graben of the North Sea, where weathered and fractured granitic basement highs have been drilled during petroleum exploration, and an abandoned kaolin mine in Southern Sweden. Both targets provide independent age control through the presence of overlying Mesozoic sedimentary rocks. Clay-rich saprolites occurring in fractured basement rocks were additionally sampled in a joint valley landscape on the southwestern coast of Norway, which can be regarded as the possible onland correlative to the offshore basement high. In order to offer a sound interpretation of the obtained K-Ar ages, the mineralogical and chemical composition of the saprolites requires a thorough characterization. Scanning electron microscopy of thin sections, integrated by XRD and XRF analysis, reveals the progressive transformation of primary granitic rock minerals into secondary clay minerals. The authigenesis of illite is particularly important to understand, since it is the only K-bearing clay mineral that can be dated by the K-Ar method. K-feldspars and mica are the common primary K-bearing minerals, from which illite can be formed. While progressive leaching of interlayer potassium is observed in micas without significant modification of the mineral structure, K-feldspars are gradually dissolved with concomitant precipitation of illite, smectite and kaolinite. Individual illite minerals are difficult to identify, but low-K contents in smectite point to small amounts of illite-interlayers. This finding is supported by XRD patterns (powder analyses on clay size fractions) that lack a clear 10 Å peak indicating the presence of illite/mica, but show a prominent and slight asymmetric 14 Å peak representing smectite with potential low (<10 %) illite-interlayer content. In agreement with previous models of diminishing contamination of protolithic K-bearing phases in the finest grain size fractions, K-Ar ages invariably decrease with grain size suggesting that the finest grain-size is predominantly composed of authigenic, syn-weathering illite, whose age can thus be used to constrain the timing of saprolitization. The obtained Late Permian to Late Triassic ages i) are in accordance with independent age constraints supporting previous hypotheses of intense chemical weathering during the Mesozoic and ii) correlate with similar K-Ar ages obtained from nearby brittle faults suggesting a genetic relationship between weathering and brittle deformation. The combined investigation and K-Ar dating of illite-bearing fractured and weathered bedrock provides new insights into the tectonic and climatic evolution of the Scandinavian landscape prior to the major, and often obliterating, Quaternary glaciations.

Geochemical and isotopic investigations on the thermal and mineral underground waters from the Republic of Moldova

NASA Astrophysics Data System (ADS)

Nisi, Barbara; Bogdevich, Oleg; Vaselli, Orlando; Nicoara, Igor; Tassi, Franco; Culighin, Elena; Mogorici, Cristina; Jeleapov, Victor; Mussi, Mario

2017-04-01

Republic of Moldova (RM) has a large spectrum of underground mineral waters (16 reservoirs) of eight principal aquifers, most of which affected by contaminants originated by natural sources and anthropogenic activities. Inorganic natural tracers and stable isotopes are useful tools to fingerprint the water source and solutes, respectively. The aim of this investigation was to determine the geochemical and isotopic features of the most important thermo- and mineral waters from RM to trace their flow pathways and evaluate the presence of deep fluid sources discharging from fault systems, developed in response to the structural setting of the area. To the best of our knowledge, no systematic geochemical investigations were previously carried out in this area. RM has an area of 33,840 km2 and lies within the East European Precambrian Platform, two structural and/or stratigraphic layers, which are distinguishable in basement and sedimentary cover in the northern and central part of country. The basement rocks include granites, gneisses and gabbros. The sedimentary cover, overlying the crystalline basement, is almost undeformed and consists of Upper Proterozoic, Mesozoic and Cenozoic rocks. The geological structure is like a matrix formed from different layers of rocks consisting of permeable and impermeable strata. The deep aquifers are situated down to 1,000 m depth from the bottom to the top: Vendian (Ediocariam) and crystalline basement rocks, Silurian crystalline limestone, Cretaceous limestone, Baden-Sarmatian limestone and clay-sand deposits, middle Sarmatian limestone and clay-sand layers. Other younger aquifers were not investigated. In this framework, 54 samples from the most important underground reservoirs of RM were collected and analyzed for major, trace species and dissolved gases. An inventory of isotopic (18O/16O and 2H/1H ratios in water and 13C/12C in dissolved CO2) features (including tritium units in selected samples) was also provided. By a geochemical point of view, the Moldavian waters showed neutral to alkaline pH, Total Dissolved Solids between 515 and 75,846 mg/L and mostly negative Eh values. They displayed a relatively high variability in terms of composition, being classified as Ca(Mg)-HCO3(SO4), Na-Cl and Na-HCO3. In the mineralized waters from the Baden-Sarmatian aquifer, trace element distribution revealed significant anomalies for F-, I- and Br- (up to 13.2, 23.7 and 140.5 mg L-1, respectively). Moreover, high values of As and Ni were found in the Dubasari waters (up to 13.8 μg L-1 and 43 μg L-1, respectively). Dissolved gases were mainly dominated by N2 (from 0.16 to 0.78 mmol/L), while CO2 and CH4 were between 0.02 and 0.66 mmol/L and 0.00005 and 0.44 mmol/L, respectively. Oxygen and hydrogen isotopic ratios were ranging from -15.8 to -0.9 ‰ (V-SMOW) and from -104.5 to -32.8 ‰ (V-SMOW), respectively, suggesting a meteoric source slightly modified by prolonged water-rock interactions. Carbon isotopes in dissolved CO2 were very variable (13δ-CO2from -25.2 to +2.8 ), the most positive values being associated with the waters collected from the oil field in the southernmost part of the country. Finally, water ages by using tritium units (presently in progress) will be used to trace the flow of the youngest waters.

Geophysical interpretations of the Libby thrust belt, northwestern Montana

USGS Publications Warehouse

Kleinkopf, M. Dean; with sections by Harrison, Jack Edward; Stanley, W.D.

1997-01-01

Interpretations of gravity and aeromagnetic anomaly data, supplemented by results from two seismic reflection profiles and five magnetotelluric soundings, were used to study buried structure and lithology of the Libby thrust belt of northwestern Montana. The gravity anomaly data show a marked correlation with major structures. The Purcell anticlinorium and the Sylvanite anticline are very likely cored by stacks of thrust slices of dense crystalline basement rocks that account for the large gravity highs across these two structures. Gravity anomaly data for the Cabinet Mountains Wilderness show a string of four broad highs. The principal magnetic anomaly sources are igneous intrusive rocks, major fault zones, and magnetite-bearing sedimentary rocks of the Ravalli Group. The most important magnetic anomalies in the principal study area are five distinct positive anomalies associated with Cretaceous or younger cupolas and stocks.

The role of the thermal convection of fluids in the formation of unconformity-type uranium deposits: the Athabasca Basin, Canada

NASA Astrophysics Data System (ADS)

Pek, A. A.; Malkovsky, V. I.

2017-05-01

In the global production of uranium, 18% belong to the unconformity-type Canadian deposits localized in the Athabasca Basin. These deposits, which are unique in terms of their ore quality, were primarily studied by Canadian and French scientists. They have elaborated the diagenetic-hydrothermal hypothesis of ore formation, which suggests that (1) the deposits were formed within a sedimentary basin near an unconformity surface dividing the folded Archean-Proterozoic metamorphic basement and a gently dipping sedimentary cover, which is not affected by metamorphism; (2) the spatial accommodation of the deposits is controlled by the rejuvenated faults in the basement at their exit into the overlying sedimentary sequence; the ore bodies are localized above and below the unconformity surface; (3) the occurrence of graphite-bearing rocks is an important factor in controlling the local structural mineralization; (4) the ore bodies are the products of uranium precipitation on a reducing barrier. The mechanism that drives the circulation of ore-forming hydrothermal solutions has remained one of the main unclear questions in the general genetic concept. The ore was deposited above the surface of the unconformity due to the upflow discharge of the solution from the fault zones into the overlying conglomerate and sandstone. The ore formation below this surface is a result of the downflow migration of the solutions along the fault zones from sandstone into the basement rocks. A thermal convective system with the conjugated convection cells in the basement and sedimentary fill of the basin may be a possible explanation of why the hydrotherms circulate in the opposite directions. The results of our computations in the model setting of the free thermal convection of fluids are consistent with the conceptual reasoning about the conditions of the formation of unique uranium deposits in the Athabasca Basin. The calculated rates of the focused solution circulation through the fault zones in the upflow and downflow branches of a convection cell allow us to evaluate the time of ore formation up to the first hundreds of thousands years.

Magmatism and Epithermal Gold-Silver Deposits of the Southern Ancestral Cascade Arc, Western Nevada and Eastern California

USGS Publications Warehouse

John, David A.; du Bray, Edward A.; Henry, Christopher D.; Vikre, Peter

2015-01-01

Many epithermal gold-silver deposits are temporally and spatially associated with late Oligocene to Pliocene magmatism of the southern ancestral Cascade arc in western Nevada and eastern California. These deposits, which include both quartz-adularia (low- and intermediate-sulfidation; Comstock Lode, Tonopah, Bodie) and quartz-alunite (high-sulfidation; Goldfield, Paradise Peak) types, were major producers of gold and silver. Ancestral Cascade arc magmatism preceded that of the modern High Cascades arc and reflects subduction of the Farallon plate beneath North America. Ancestral arc magmatism began about 45 Ma, continued until about 3 Ma, and extended from near the Canada-United States border in Washington southward to about 250 km southeast of Reno, Nevada. The ancestral arc was split into northern and southern segments across an inferred tear in the subducting slab between Mount Shasta and Lassen Peak in northern California. The southern segment extends between 42°N in northern California and 37°N in western Nevada and was active from about 30 to 3 Ma. It is bounded on the east by the northeast edge of the Walker Lane. Ancestral arc volcanism represents an abrupt change in composition and style of magmatism relative to that in central Nevada. Large volume, caldera-forming, silicic ignimbrites associated with the 37 to 19 Ma ignimbrite flareup are dominant in central Nevada, whereas volcanic centers of the ancestral arc in western Nevada consist of andesitic stratovolcanoes and dacitic to rhyolitic lava domes that mostly formed between 25 and 4 Ma. Both ancestral arc and ignimbrite flareup magmatism resulted from rollback of the shallowly dipping slab that began about 45 Ma in northeast Nevada and migrated south-southwest with time. Most southern segment ancestral arc rocks have oxidized, high potassium, calc-alkaline compositions with silica contents ranging continuously from about 55 to 77 wt%. Most lavas are porphyritic and contain coarse plagioclase ± hornblende, biotite, and pyroxene phenocrysts. Seven epithermal gold-silver deposits with >1 Moz gold production, several large elemental sulfur deposits, and many large areas (10s to >100 km2) of hydrothermally altered rocks are present in the southern ancestral arc, especially south of latitude 40°N. These deposits are principally hosted by intermediate to silicic lava dome complexes; only a few deposits are associated with mafic- to intermediate-composition stratovolcanoes. Large deposits are most abundant and well developed in volcanic fields whose evolution spanned millions of years. Most deposits are hundreds of thousands to several million years younger than their host rocks, although some quartz-alunite deposits are essentially coeval with their host rocks. Variable composition and thickness of crustal basement is the primary control on mineralization along the length of the southern ancestral arc; most deposits and large alteration zones are localized in basement rock terranes with a strong continental affinity, either along the edge of the North American craton (Goldfield, Tonopah) or in an accreted terrane with continental affinities (Walker Lake terrane; Aurora, Bodie, Comstock Lode, Paradise Peak). Epithermal deposits and quartz-alunite alteration zones are scarce to absent in the northern part of the ancestral arc above an accreted island arc (Black Rock terrane) or unknown basement rocks (Modoc Plateau). Walker Lane structures and areas that underwent large magnitude extension during the Late Cenozoic (areas with Oligocene-early Miocene volcanic rocks dipping >40°) do not provide regional control on mineralization. Instead, these features may have served as local-scale conduits for mineralizing fluids.

33. VIEW OF BASEMENT UNDER EAST BOILER ROOM LOOKING TOWARD ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

33. VIEW OF BASEMENT UNDER EAST BOILER ROOM LOOKING TOWARD WEST BOILER ROOM BASEMENT THROUGH THE ASH TRANSFER TUNNEL. ASH HOPPER FOR BOILER 900 IS ON THE RIGHT. NOTE THE TRACKS ALONG THE FLOOR OF THE TUNNEL. A SMALL ELECTRIC LOCOMOTIVE HAULED CARS FOR TRANSFERRING ASH FROM BOILERS TO DISPOSAL SITES OUTSIDE THE BUILDING. THIS SYSTEM BECAME OBSOLETE IN 1938 WHEN BOILERS IN THE WEST BOILER ROOM WERE REMOVED AND PULVERIZED COAL WAS ADOPTED AS THE FUEL. - New York, New Haven & Hartford Railroad, Cos Cob Power Plant, Sound Shore Drive, Greenwich, Fairfield County, CT

Strain-dependent partial slip on rock fractures under seismic-frequency torsion: Seismic-Frequency Fracture Partial Slip

DOE PAGES

Saltiel, Seth; Bonner, Brian P.; Ajo-Franklin, Jonathan B.

2017-05-05

Measurements of nonlinear modulus and attenuation of fractures provide the opportunity to probe their mechanical state. We have adapted a low-frequency torsional apparatus to explore the seismic signature of fractures under low normal stress, simulating low effective stress environments such as shallow or high pore pressure reservoirs. We report strain-dependent modulus and attenuation for fractured samples of Duperow dolomite (a carbon sequestration target reservoir in Montana), Blue Canyon Dome rhyolite (a geothermal analog reservoir in New Mexico), and Montello granite (a deep basement disposal analog from Wisconsin). We use a simple single effective asperity partial slip model to fit ourmore » measured stress-strain curves, and solve for the friction coefficient, contact radius, and full slip condition. These observations have the potential to develop into new field techniques for measuring differences in frictional properties during reservoir engineering manipulations and estimate the stress conditions where reservoir fractures and faults begin to fully slip.« less

Strain-dependent partial slip on rock fractures under seismic-frequency torsion: Seismic-Frequency Fracture Partial Slip

DOE Office of Scientific and Technical Information (OSTI.GOV)

Saltiel, Seth; Bonner, Brian P.; Ajo-Franklin, Jonathan B.

Measurements of nonlinear modulus and attenuation of fractures provide the opportunity to probe their mechanical state. We have adapted a low-frequency torsional apparatus to explore the seismic signature of fractures under low normal stress, simulating low effective stress environments such as shallow or high pore pressure reservoirs. We report strain-dependent modulus and attenuation for fractured samples of Duperow dolomite (a carbon sequestration target reservoir in Montana), Blue Canyon Dome rhyolite (a geothermal analog reservoir in New Mexico), and Montello granite (a deep basement disposal analog from Wisconsin). We use a simple single effective asperity partial slip model to fit ourmore » measured stress-strain curves, and solve for the friction coefficient, contact radius, and full slip condition. These observations have the potential to develop into new field techniques for measuring differences in frictional properties during reservoir engineering manipulations and estimate the stress conditions where reservoir fractures and faults begin to fully slip.« less

Evidence for a warm ice-free environment on the high latitude Antarctic coast (78°S) during the Middle to Late Eocene

NASA Astrophysics Data System (ADS)

Levy, R. H.; Bohaty, S. M.; Harwood, D. M.; Sangiorgi, F.; Willmott, V.; Talarico, F.; MacLeod, K. G.

2013-12-01

Much of Antarctica's Cenozoic geological record is hidden beneath the thick ice sheets and fringing ice shelves that cover the continent. Glacial erratics of sedimentary rocks present in coastal moraines at Minna Bluff and Mount Discovery, McMurdo Sound, western Ross Sea, Antarctica contain middle and late Eocene plant and marine fossils that were deposited in a range of marine settings along the Antarctic coastline. This suite of sedimentary rocks were likely deposited at the margin of a narrow (c. 100 km wide), relatively deep (up to 1000 m) marine seaway that was bound by the proto-Transantarctic Mountains to the west and a topographic high to the east. Although these Eocene ';';McMurdo Erratics'' lack stratigraphic integrity, they are significant as they offer a rare glimpse into Antarctica's climate during global greenhouse conditions at high latitudes (c. 78°S). Fossils recovered from the rocks are diverse and include marine and terrestrial palynomorphs, diatoms, molluscs, wood, leaves and other macrofauna and flora. Geochemical temperature proxies derived from the sedimentary rocks include organic biomarkers (TEX86) and fish tooth δ18O that indicate coastal sea surface temperatures were at least 15°C in the late Middle Eocene. While rare lonestones occur in several sandstone erratics, we find no conclusive evidence for glaciation at the coast. The fossil-bearing coastal moraines also contain a suite of igneous and metamorphic erratics that are comparable to lithological units exposed in the Transantarctic Mountains between the Skelton and Mulock glaciers. This suggests that the Eocene erratics were eroded from the north-eastern portion of a large sub-glacial basin behind Minna Bluff and/or from grabens in a basement high immediately south-east of Minna Bluff. Importantly, the northeastward extension of this basement high is a target for stratigraphic drilling during the proposed ANDRILL Coulman High Project. Drilling on the Coulman High has an excellent chance of recovering correlative Eocene marine strata, providing stratigraphic context for the fossiliferous erratics and providing new constraints on polar environmental conditions under high atmospheric CO2. Studies on the erratic boulders show that organic biomarkers offer an important paleoenvironmental tool at this high southern latitude site. Biomarker data derived from the new drill cores will be integrated with other environmental proxies to estimate atmospheric and marine temperatures in order to determine the magnitude of polar amplification during high-CO2 'greenhouse' climatic conditions. These new data will improve our understanding of Earth System climate sensitivity in a warmer world.

Possible detachment zone in Precambrian rocks of Kanjamalai Hills, Cauvery Suture Zone, Southern India: Implications to accretionary tectonics

NASA Astrophysics Data System (ADS)

Mohanty, D. P.; Chetty, T. R. K.

2014-07-01

Existence of a possible detachment zone at Elampillai region, NW margin of Kanjamalai Hills, located in the northern part of Cauvery Suture Zone (CSZ), Southern India, is reported here for the first time. Detailed structural mapping provides anatomy of the zone, which are rarely preserved in Precambrian high grade terranes. The detachment surface separates two distinct rock units of contrasting lithological and structural characters: the upper and lower units. The detachment zone is characterized by a variety of fold styles with the predominance of tight isoclinal folds with varied plunge directions, limb rotations and the hinge line variations often leading to lift-off fold like geometries and deformed sheath folds. Presence of parasitic folding and associated penetrative strains seem to be controlled by differences in mechanical stratigraphy, relative thicknesses of the competent and incompetent units, and the structural relief of the underlying basement. Our present study in conjunction with other available geological, geochemical and geochronological data from the region indicates that the structures of the detachment zone are genetically related to thrust tectonics forming a part of subduction-accretion-collision tectonic history of the Neoproterozoic Gondwana suture.

Lithostratigraphic, conodont, and other faunal links between lower Paleozoic strata in northern and central Alaska and northeastern Russia

USGS Publications Warehouse

Dumoulin, Julie A.; Harris, Anita G.; Gagiev, Mussa; Bradley, Dwight C.; Repetski, John E.

2002-01-01

Lower Paleozoic platform carbonate strata in northern Alaska (parts of the Arctic Alaska, York, and Seward terranes; herein called the North Alaska carbonate platform) and central Alaska (Farewell terrane) share distinctive lithologic and faunal features, and may have formed on a single continental fragment situated between Siberia and Laurentia. Sedimentary successions in northern and central Alaska overlie Late Proterozoic metamorphosed basement; contain Late Proterozoic ooid-rich dolostones, Middle Cambrian outer shelf deposits, and Ordovician, Silurian, and Devonian shallow-water platform facies, and include fossils of both Siberian and Laurentian biotic provinces. The presence in the Alaskan terranes of Siberian forms not seen in wellstudied cratonal margin sequences of western Laurentia implies that the Alaskan rocks were not attached to Laurentia during the early Paleozoic.The Siberian cratonal succession includes Archean basement, Ordovician shallow-water siliciclastic rocks, and Upper Silurian–Devonian evaporites, none of which have counterparts in the Alaskan successions, and contains only a few of the Laurentian conodonts that occur in Alaska. Thus we conclude that the lower Paleozoic platform successions of northern and central Alaska were not part of the Siberian craton during their deposition, but may have formed on a crustal fragment rifted away from Siberia during the Late Proterozoic. The Alaskan strata have more similarities to coeval rocks in some peri-Siberian terranes of northeastern Russia (Kotelny, Chukotka, and Omulevka). Lithologic ties between northern Alaska, the Farewell terrane, and the peri-Siberian terranes diminish after the Middle Devonian, but Siberian afµnities in northern and central Alaskan biotas persist into the late Paleozoic.

A new U-Pb LA-ICP-MS age of the Rumburk granite (Lausitz Block, Saxo-Thuringian Zone): constraints for a magmatic event in the Upper Cambrian

NASA Astrophysics Data System (ADS)

Zieger, J.; Linnemann, U.; Hofmann, M.; Gärtner, A.; Marko, L.; Gerdes, A.

2018-04-01

The basement of the Saxo-Thuringian Zone consists of Upper Neoproterozoic (c. 650-570 Ma) Cadomian arc sediments (Lusatian greywackes) and voluminous intrusions of Early Cambrian granitoids with ages of c. 540 Ma (Lausitz Block and Karkonosze-Izera Massif). The latter basement complexes comprise several c. 505 Ma granites, granodiorites, and gneisses emplaced during the change from a collisional tectonic setting to rift-related geotectonics. We present a new age for the Rumburk granite of 504 ± 3 Ma linking Late Cambrian plutonism at the northern margin of Gondwana with the initial phase of a Cambro-Ordovician rift event. Trace element analysis points to a linkage of the Rumburk granite with other Late Cambrian aged rocks of the Karkonosze-Izera Massif. Furthermore, geochemical data also provide evidence of a melting and recycling of Lusatian greywackes by the intrusion of the Rumburk granite. The youngest age peak of the Rumburk granite at c. 504 Ma is considered to be the age of emplacement. Older inherited age populations at c. 540 and c. 610 Ma are present and likely the result of a melting and recycling of Lusatian granitoids and greywackes. The appearance of Neoproterozoic inheritance and Lu-Hf similarities with the Rumburk granite strongly suggest the Lusatian greywackes as source rocks. There is a significant age gap of c. 35 Ma between Cambrian plutonic and volcanic rocks in Saxo-Thuringia. Hence, we consider two distinct pulses of magmatic activity during the transition from the Cadomian orogeny to the opening of the Rheic Ocean.

Pervasive Eclogitization Due to Brittle Deformation and Rehydration of Subducted Basement: Effects on Continental Recycling?

NASA Astrophysics Data System (ADS)

Engi, Martin; Giuntoli, Francesco; Lanari, Pierre; Burn, Marco; Kunz, Barbara; Bouvier, Anne-Sophie

2018-03-01

The buoyancy of continental crust opposes its subduction to mantle depths, except where mineral reactions substantially increase rock density. Sluggish kinetics limit such densification, especially in dry rocks, unless deformation and hydrous fluids intervene. Here we document how hydrous fluids in the subduction channel invaded lower crustal granulites at 50-60 km depth through a dense network of probably seismically induced fractures. We combine analyses of textures and mineral composition with thermodynamic modeling to reconstruct repeated stages of interaction, with pulses of high-pressure (HP) fluid at 650-670°C, rehydrating the initially dry rocks to micaschists. SIMS oxygen isotopic data of quartz indicate fluids of crustal composition. HP growth rims in allanite and zircon show uniform U-Th-Pb ages of ˜65 Ma and indicate that hydration occurred during subduction, at eclogite facies conditions. Based on this case study in the Sesia Zone (Western Italian Alps), we conclude that continental crust, and in particular deep basement fragments, during subduction can behave as substantial fluid sinks, not sources. Density modeling indicates a bifurcation in continental recycling: Chiefly mafic crust, once it is eclogitized to >60%, are prone to end up in a subduction graveyard, such as is tomographically evident beneath the Alps at ˜550 km depth. By contrast, dominantly felsic HP fragments and mafic granulites remain positively buoyant and tend be incorporated into an orogen and be exhumed with it. Felsic and intermediate lithotypes remain positively buoyant even where deformation and fluid percolation allowed them to equilibrate at HP.

Structure of Masuleh Shear Zone: Evidence for Early–Middle Jurassic Dextral Shear Along Paleo-Tethys Suture Zone in the Western Alborz, NW Iran

NASA Astrophysics Data System (ADS)

Moosavi, E.; Rasouli-Jamadi, F.

2018-03-01

The Paleo-Tethys suture zone in northern Iran was formed when the Paleo-Tethys Ocean, (between Gonwana-derived Alborz Microcontinent and the Turan Plate), closed during the Eocimmerian orogeny and after they collided together in the Mid-Late Triassic. The NW-striking Boghrov-Dagh basement Fault Zone that lies in the vicinity of Masuleh village and the southern boundary of Gasht Metamorphic Complex is a part of the Eocimmerian suture zone in the Western Alborz. Along this part of the suture zone, tourmaline leucogranites intruded in metamorphic rocks. We recognize three distinct deformation stages (D1 to D3) in the study area especially in the Masuleh Shear Zone. D1 which was synchronous with formation of the main metamorphic minerals, such as sillimanite and staurolite under medium- to high-grade metamorphic conditions probably during the Hercynian event and a NE-directed shortening. The slaty cleavage in metamorphosed Upper Paleozoic rocks and crenulation cleavage and folds in the older rocks were produced due to D2 deformation during the Eocimmerian event under greenschist facies conditions. The Masuleh Shear Zone formed as a result of a ductile strike-slip shear during the Early-Middle Jurassic Mid-Cimmerian D3 event with a pure dextral to transtension shear sense at low to locally medium-grade conditions. All of the D3 structural features agree with a NNW-directed compression and an ENE-directed extension caused by overall dextral shear parallel to the Masuleh shear zone and the Boghrov-Dagh Fault Zone. Based on the available evidence, especially cross-cutting relationships between structural fabrics and rock units, emplacement of the Gasht-Masuleh leucogranites occurred after the D2 collisional event coeval to the possible slab break-off and before the D3 event, between Eocimmerian and Mid-Cimmerian movements.

A multidisciplinary approach for the characterisation of fault zones in geothermal areas in central Mexico

NASA Astrophysics Data System (ADS)

Comina, Cesare; Ferrero, Anna Maria; Mandrone, Giuseppe; Vinciguerra, Sergio

2017-04-01

There are more than 500 geothermal areas in the Trans-Mexican Volcanic Belt of central Mexico. Of these, two are presently object of a transnational project between EU and Mexico (GEMex): Acoculco, where there is already a commercial exploitation, and Los Humeros, at present not developed yet. The GEMex project aims to improve the resource assessment and the reservoir characterization using novel geophysical and geological methods and interpretations. One of the main issues controlling the geothermal system is the presence of pervasive fracture systems affecting the carbonatic basements underlying the volcanic complex (basalts and andesites). We propose the characterization of rock masses (rock and fractures) using a multiscale analysis, from the field to the outcrop up to the micro scale integrating a number of techniques. In detail, the University of Torino unit will take care of: 1) Technical field studies aimed to the characterization of the mechanical transitions throughout brittle deformation zones, from the intact rock, to the damage zone to the shear/slip zone; moreover, key geophysical parameters (seismic and electrical properties) will be measured; 2) Petrophysical and minero-petrographic detailed studies on representative samples will be performed at room temperature; verification of the mechanical properties of the samples subjected to cycles of heating up to the temperatures of the reservoir (> 400 °C) will be done; measurements of the geophysical properties of the samples will be done in comparison with the measures in place. 3) Numerical modeling to estimate the petrophysical, geophysical and geomechanical properties of the rock mass under the P and T conditions of the reservoir (i.e., using Comsol, VGeST, UDEC, 3DEC, ...). Detailed geological field studies and photogrammetry/laser scanner imaging of studied outcrops are supposed to be available soon: multiscale analysis will benefis from these new data. Results will be shared between EU and Mexican partners to improve the general model of these two geothermal field.

Structural control on geothermal circulation in the Tocomar geothermal volcanic area (Puna plateau, Argentina)

NASA Astrophysics Data System (ADS)

Giordano, Guido

2016-04-01

The reconstruction of the stratigraphical-structural framework and the hydrogeology of geothermal areas is fundamental for understanding the relationships between cap rocks, reservoir and circulation of geothermal fluids and for planning the exploitation of the field. The Tocomar geothermal volcanic area (Puna plateau, Central Andes, NW Argentina) has a high geothermal potential. It is crossed by the active NW-SE trans-Andean tectonic lineament known as the Calama-Olacapato-Toro (COT) fault system, which favours a high secondary permeability testified by the presence of numerous thermal springs. This study presents new stratigraphic, structural, volcanological, geochemical and hydrogeological data on the geothermal field. Our data suggest that the main geothermal reservoir is located within or below the Pre-Palaeozoic-Ordovician basement units, characterised by unevenly distributed secondary permeability. The reservoir is recharged by infiltration in the ridges above 4500 m a.s.l., where basement rocks are in outcrop. Below 4500 m a.s.l., the reservoir is covered by the low permeable Miocene-Quaternary units that allow a poor circulation of shallow groundwater. Geothermal fluids upwell in areas with more intense fracturing, especially where main regional structures, particularly NW-SE COT-parallel lineaments, intersect with secondary structures, such as at the Tocomar field.

2.9-1.9 Ga paleoalterations of Archean granitic basement of the Franceville basin (Gabon)

NASA Astrophysics Data System (ADS)

Mouélé, Idalina Moubiya; Dudoignon, Patrick; El Albani, Abderrazak; Meunier, Alain; Boulvais, Philippe; Gauthier-Lafaye, François; Paquette, Jean-Louis; Martin, Hervé; Cuney, Michel

2014-09-01

The Archean granitoids in the Kiéné area, Gabon, are overlained by the Paleoproterozoic sediments of the Franceville basin (2.1 Ga). The basin is known for its high-grade uranium deposits among which some have been forming natural nuclear fission reactors. Most of the studies were dedicated to the FA-FB Paleoproterozoic sediments hosting these uranium deposits. Little is known on the Archean basement itself and specifically on the hydrous alteration events it experienced before and after the sediment deposition. The present work is focused on their petrographical, mineralogical and geochemical characterization. Dating the successive alteration events has been attempted on altered monazite crystals. Rocks in different alteration states have been sampled from eight drill cores crosscutting the Archean - Paleoproterozoic unconformity. The Archean granitoids observed in the deepest levels exhibit typical petrographical features of a propylitic alteration while they are intensely illitized up to the unconformity. The propylitic alteration is mainly pervasive but the original texture of the granitoïds is conserved in spite of the formation of new minerals: Mg-chlorite, allanite and epidote forming a typical paragenesis. The illitic alteration is much more invasive near the unconformity. The illitization process leads to the replacement of feldspars and the corrosion of quartz crysals by an illitic matrix while the ferromagnesian minerals are pseudomorphosed by a Fe-chlorite + phengite + hematite assemblage. The final fluid-rock interaction step is marked by fissural deposits of calcite and anhydrite. The δ13C isotopic data show that the fissural carbonates precipitated from diagenetic fluids enriched carbon products deriving from the maturation of organic matter. The U-Pb isotopic analyzes performed on monazite crystals have dated three distinct events: 3.0-2.9 Ga (magmatic), 2.6 Ga (propylitic alteration) and 1.9 Ga (diagenetic illitization). The calculation of geochemical mass balances suggests that the water-rock ratio during the propylitic alteration event was weak. On the contrary, it was much higher during the overprinted illitization which is characterized by an intense leaching of Na, Ca, Mg, Sr, REE and an enrichment in K, Rb,Cs. Neither the petrographic features nor the geochemical data militate for an Archean weathering event (paleosol). In the present case, diagenetic fluids have percolated from the unconformity into the basement where they overprinted the illitization processes upon the previously propylitized rocks. These fluids were probably oxidant as they are also responsible of the U mobilization which led to the formation of the ore deposits close to the FA-FB interface.

Evaluation of geothermal energy as a heat source for the oilsands industry in Northern Alberta (Canada)

NASA Astrophysics Data System (ADS)

Majorowicz, J. A.; Unsworth, M.; Gray, A.; Nieuwenhuis, G.; Babadagli, T.; Walsh, N.; Weides, S.; Verveda, R.

2012-12-01

The extraction and processing of bitumen from the oilsands of Northern Alberta requires very large amounts of heat that is obtained by burning natural gas. At current levels, the gas used represents 6% of Canada's natural gas production. Geothermal energy could potentially provide this heat, thereby reducing both the financial costs and environmental impact of the oilsands industry. The Helmholtz Alberta Initiative is evaluating this application of geothermal energy through an integrated program of geology, geophysics, reservoir simulation and calculations of the cost benefit. A first stage in this evaluation is refining estimates of subsurface temperature beneath Northern Alberta. This has involved three stages: (1) Corrected industrial thermal data have been used to revise estimates of the upper crustal temperatures beneath the oilsands regions in Alberta. The geothermal gradient map produced using heat flow and thermal conductivity for the entire Phanerozoic column suggests that the overall gradient of the entire column is less than the gradients calculated directly from industry measurements. (2) Paleoclimatic corrections must be applied , since this region has experienced a significant increase in surface temperatures since the end of the last ice age causing a perturbation of shallow heat flow. For this reason, estimates of geothermal gradient based on shallow data are not necessarily characteristic of the whole sedimentary column and can lead to errors in temperature prediction at depth. (3) Improved measurements have been made of the thermal conductivity of the crystalline basement rocks (average = 2.9±0.8 W/m K). Thermal conductivity exhibits significant spatial variability and to a large degree controls the temperature conditions in the Precambrian crystalline basement rocks and its heat content at given heat flow-heat generation. When these steps are used to calculate subsurface temperatures, it can be shown that the temperatures required for geothermal energy to provide usable heat for oil sands processing can only be found within the crystalline basement rocks beneath the WCSB. Lower temperature geothermal resources can be found in the 2 km thick layer of sedimentary rocks in the Peace River area and beneath urban centres in Northern Alberta. Modeling shows that heat extraction for oilsands processing with a doublet or triplet of 5km wells, operated for 15 years, could be marginally economic when compared to the currently low gas prices. This type of heat extraction would be economically competitive if the system had a life span greater than 20 years or with higher natural gas prices.

Geo-electrical investigation of near surface conductive structures suitable for groundwater accumulation in a resistive crystalline basement environment: A case study of Isuada, southwestern Nigeria

NASA Astrophysics Data System (ADS)

Kayode, J. S.; Adelusi, A. O.; Nawawi, M. N. M.; Bawallah, M.; Olowolafe, T. S.

2016-07-01

This paper presents a geophysical surveying for groundwater identification in a resistive crystalline basement hard rock in Isuada area, Southwestern Nigeria. Very low frequency (VLF) electromagnetic and electrical resistivity geophysical techniques combined with well log were used to characterize the concealed near surface conductive structures suitable for groundwater accumulation. Prior to this work; little was known about the groundwater potential of this area. Qualitative and semi-quantitative interpretations of the data collected along eight traverses at 20 m spacing discovered conductive zones suspected to be fractures, faults, and cracks which were further mapped using Vertical Electrical Sounding (VES) technique. Forty VES stations were utilized using Schlumberger configurations with AB/2 varying from 1 to 100 m. Four layers i.e. the top soil, the weathered layer, the partially weathered/fractured basement and the fresh basement were delineated from the interpreted resistivity curves. The weathered layers constitute the major aquifer unit in the area and are characterized by moderately low resistivity values which ranged between about 52 Ωm and 270 Ωm while the thickness varied from 1 to 35 m. The depth to the basement and the permeable nature of the weathered layer obtained from both the borehole and the hand-dug wells was used to categorize the groundwater potential of the study area into high, medium and low ratings. The groundwater potential map revealed that about 45% of the study area falls within the low groundwater potential rating while about 10% constitutes the medium groundwater potential and the remaining 45% constitutes high groundwater potential. The low resistivity, thick overburden, and fractured bedrock constitute the aquifer units and the series of basement depressions identified from the geoelectric sections as potential conductive zones appropriate for groundwater development.

Application of kinematic vorticity and gold mineralization for the wall rock alterations of shear zone at Dungash gold mining, Central Eastern Desert, Egypt

NASA Astrophysics Data System (ADS)

Kassem, Osama M. K.; Abd El Rahim, Said H.; El Nashar, EL Said R.; AL Kahtany, Kaled M.

2016-11-01

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (Wm) is important for quantifying the relative contributions of pure and simple shear in wall rocks alterations of shear zone at Dungash gold mine. Furthermore, it shows the relationship between the gold mineralization and deformation and also detects the orientation of rigid objects during progressive deformation. The Dungash gold mine area is situated in an EW-trending quartz vein along a shear zone in metavolcanic and metasedimentary host rocks in the Eastern Desert of Egypt. These rocks are associated with the major geologic structures which are attributed to various deformational stages of the Neoproterozoic basement rocks. We conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. The kinematic vorticity number for the metavolcanic and metasedimentary samples in the Dungash area range from 0.80 to 0.92, and together with the strain data suggest deviations from simple shear. It is concluded that nappe stacking occurred early during the underthrusting event probably by brittle imbrication and that ductile strain was superimposed on the nappe structure during thrusting. Furthermore, we conclude that disseminated mineralization, chloritization, carbonatization and silicification of the wall rocks are associated with fluids migrating along shearing, fracturing and foliation of the metamorphosed wall rocks.

Chemical and Physical Characteristics of Basaltic Formation Fluids on a Ridge Flank: Using Drilling Perturbations to Elucidate Water-Rock-Microbial Reactions

NASA Astrophysics Data System (ADS)

Jannasch, H. W.; Wheat, G. C.; Hulme, S.; Becker, K.; Fisher, A. T.; Davis, E. E.

2008-12-01

Holes 1301A and 1301B were drilled, cased, and instrumented with long-term, subseafloor observatories (CORKs) on the eastern flank of the Juan de Fuca Ridge in Summer 2004. These holes penetrate 265 m of sediment and the uppermost 108 to 318 m of 3.5 Ma basaltic basement, in an area of vigorous, warm (64C) hydrothermal circulation. The new boreholes were located 1 km south and 2.4 km southwest of instrumented Holes 1026B and 1027C, respectively, that were emplaced eight years earlier. This network of four instrumented boreholes was established as part of a long-term, cross-hole experiment that will elucidate hydrologic properties and the nature and dynamics of microbial ecosystems within the upper oceanic crust, in a well defined geochemical and physical context. Downhole instrumented OsmoSampler packages in Holes 1301A and 1026B were replaced by submersible in summer 2008, as part of a program of observatory servicing in preparation for the next drilling expedition and the initiation of cross-hole experiments in this area. The borehole instrument package from Hole 1301A sampled borehole fluids within the upper 107.5 m of basaltic crust during a four-year period of drilling disturbance, self-sustaining flow of cold bottom water into basement, and subsequent recovery to near-predrilling chemical and thermal conditions. Because the borehole was incompletely sealed at the time of initial installation, bottom seawater flowed down into the borehole during the first three years following emplacement, driven by the higher density of cold bottom water relative to warm formation fluid. Borehole thermal records during the first 1.5 years show that temperatures in basement were below 10 C, and fluid samples from the borehole have a chemical composition similar to bottom seawater. Temperatures fluctuated for the next 1.5 years between 10 and 30 C, and the fluid composition began to shift towards that seen in regional basement fluids sampled at nearby Baby Bare outcrop and from Hole 1026B. In early September 2007 the natural formation overpressure overcame the excess pressure of cold bottom water and began to vent a mixture of recently-recharged bottom water and warm formation fluid. The present day composition of fluid venting from Hole 1301A is very similar to that sampled from Baby Bare outcrop. The progression from bottom seawater to formation fluid chemistry is not conservative relative to temperature, most likely because of water-rock and microbial reactions within basaltic basement.

How to build the Eiger: Surface expression of litho-tectonic preconditioning

NASA Astrophysics Data System (ADS)

Mair, David; Lechmann, Alessandro; Schlunegger, Fritz

2017-04-01

The north face of the Eiger has exerted a strong attraction on alpinists, but also on geologists during the past decades, mainly because of its triangular, nearly vertical shape. We build on this tradition and investigate the relationship between the shape of this mountain and its underlying lithology, and its history of folding and thrusting. To this extent, we constructed a geometric 3D geological model of the Eiger-Moench-Jungfrau mountain chain in the central Swiss Alps. We proceeded through compilations of geological maps that we combined with new mapping in the field and collection of structural data such as the orientation of lineaments and faults. The model itself was constructed by interpolation of interfaces between geological formations, thrust- and fold-geometries between several NW-SE running, balanced, cross-sections. In addition, new geological data from the Jungfraubahn railway tunnel was used to verify surface data and improve the resulting model in the depth. The analyzed units of the Hercynian crystalline basement of the Aar massif and the Mesozoic cover rocks of the Helvetics form a foliated and thrusted stack. Multiple ductile structure sets bear witness of Alpine deformation and are dominant amid the mark of later brittle deformation across the whole mountain. There are two major outcomes of this analysis. First, the thrust contact between two stacks, which comprise a foliated basement and cover rocks, are responsible for the shape and overall architecture of the Eiger and its famous north face. Second, the high-resolution 3D structural model paired with petrological data shows that second-order, horizontally aligned morphological steps in the north face are related to the foliation within the bedrock. We suspect the inherited fabric significantly modified the susceptibility to erosion mechanisms which in turn further amplified the morphological differences (expressed in e.g. terrain roughness or slope).

Geological Influences on Bedrock Topography and East Antarctic Ice Sheet Dynamics in the Wilkes Subglacial Basin

NASA Astrophysics Data System (ADS)

Ferraccioli, F.; Armadillo, E.; Young, D. A.; Blankenship, D. D.; Jordan, T. A.; Balbi, P.; Bozzo, E.; Siegert, M. J.

2014-12-01

The Wilkes Subglacial Basin (WSB) extends for 1,400 km from George V Land into the interior of East Antarctica and hosts several major glaciers that drain a large sector of the East Antarctic Ice Sheet (EAIS). This region is of key significance for the long-term stability of the ice sheet in East Antarctica, as it lies well below sea level and its bedrock deepens inland, making it potentially prone to marine ice sheet instability, much like areas of the West Antarctic Ice Sheet (WAIS) that are presently experiencing significant mass loss. We present new enhanced potential field images of the WSB combined with existing radar imaging to study geological controls on bedrock topography and ice flow regimes in this key sector of the ice sheet. These images reveal mayor Precambrian and Paleozoic basement faults that exert tectonic controls both on the margins of the basin and its sub-basins. Several major sub-basins can be recognised: the Eastern Basin, the Central Basins and the Western Basins. Using ICECAP aerogeophysical data we show that these tectonically controlled interior basins connect to newly identified basins underlying the Cook Ice Shelf region. This connection implies that any ocean-induced changes at the margin of the EAIS could potentially propagate rapidly further into the interior. With the aid of simple magnetic and gravity models we show that the WSB does not presently include major post Jurassic sedimentary infill. Its bedrock geology is highly variable and includes Proterozoic basement, Neoproterozoic and Cambrian sediments, intruded by Cambrian arc rocks, and cover rocks formed by Beacon sediments intruded by Jurassic Ferrar sills. Enhanced ice flow in this part of the EAIS occurs therefore in a area of mixed and spatially variable bedrock geology. This contrasts with some regions of the WAIS where more extensive sedimentary basins may represent a geological template for the onset and maintenance of fast glacial flow.

Geochronology of Zircon in Eclogite Reveals Imbrication of the Ultrahigh-Pressure Western Gneiss Region of Norway.

NASA Astrophysics Data System (ADS)

Young, D. J.; Kylander-Clark, A. R.; Root, D. B.

2014-12-01

Eclogite provides the only record of kinematic events at the deepest levels of orogens. Integrating the U-Pb geochronology and trace element chemistry of zircon in eclogite reveals the most complete view of the PTt history, yet low concentrations of uranium and zirconium and drier compositions that hinder zircon growth at peak conditions render it a challenging rocktype for this approach. The iconic Western Gneiss Region (WGR) in Norway is one of the largest terranes of deeply subducted continental rocks in the world, and contains many indicators of ultrahigh-pressure metamorphic conditions (P>2.8 GPa) that developed during the Siluro-Devonian Caledonian Orogeny. A metamorphic transition from amphibolite-facies to ultrahigh-pressure eclogite facies broadly coincides with a km-scale shear zone that underlies the majority of the WGR. A critical unknown is the timing of movement on this feature, which emplaced allochthonous units above the Baltica basement, but might also have accommodated late-orogenic exhumation of the WGR from mantle depths. We carried out laser ablation split-stream ICPMS (LASS) and selected multigrain TIMS analyses of zircons from eleven eclogites across the southern WGR, of which eight are located within or above the shear zone. LASS spots on polished grains mostly yield weakly discordant Proterozoic intrusive ages, and often minimal indication of a Caledonian (U)HP metamorphic overprint. Direct ablation into unpolished zircon reveals thin rims of Caledonian age in some cases. Overall, the dataset shows that all samples began zircon growth at approximately the same time (ca. 430-420 Ma). Eclogite from lower levels of the shear zone does not contain any dates younger than ca. 410 Ma, however, while eclogite from higher levels continued growth until ca. 400 Ma. We interpret this to result from thrusting of the WGR above cooler basement after 410 Ma, terminating new zircon crystallization within the shear zone but allowing limited further growth in rocks above.

The stretching amplitude and thermal regime of the lithosphere in the nonvolcanic passive margin of Antarctica in the Mawson Sea region

NASA Astrophysics Data System (ADS)

Galushkin, Yu. I.; Leitchenkov, G. L.; Guseva, Yu. B.; Dubinin, E. P.

2018-01-01

The burial history and thermal evolution of the lithosphere within the passive nonvolcanic Antarctic margin in the region of the Mawson Sea are numerically reconstructed for the margin areas along the seismic profile 5909 with the use of the GALO basin modeling system. The amplitudes of the lithosphere stretching at the different stages of continental rifting which took place from 160 to 90 Ma ago are calculated from the geophysical estimates of the thickness of the consolidated crust and the tectonic analysis of the variations in the thickness of the sedimentary cover and sea depths during the evolution of the basin. It is hypothesized that the formation of the recent sedimentary section sequence in the studied region of the Antarctic margin began 140 Ma ago on a basement that was thinned by a factor of 1.6 to 4.5 during the first episode of margin stretching (160-140 Ma) under a fairly high heat flux. The reconstruction of the thermal regime of the lithosphere has shown that the mantle rocks could occur within the temperature interval of serpentinization and simultaneously within the time interval of lithospheric stretching (-160 < t <-90 Ma) only within separate segments of profile 5909 in the Mawson Sea. The calculations of the rock strength distribution with depth by the example of the section of pseudowell 4 have shown that a significant part of the crust and uppermost mantle fall here in the region of brittle deformations in the most recent period of lithosphere stretching (-104 to-90 Ma ago). The younger basin segments of profile 5909 in the region of pseudowells 5 and 6 are characterized by a high heat flux, and the formation of through-thickness brittle fractures in these zones is less probable. However, serpentinization could take place in these areas as in the other margin segments at the stage of presedimentation ultra slow basement stretching.

Late Proterozoic-Paleozoic evolution of the Arctic Alaska-Chukotka terrane based on U-Pb igneous and detrital zircon ages: Implications for Neoproterozoic paleogeographic reconstructions

USGS Publications Warehouse

Amato, J.M.; Toro, J.; Miller, E.L.; Gehrels, G.E.; Farmer, G.L.; Gottlieb, E.S.; Till, A.B.

2009-01-01

The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska-Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska-Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700-540 Ma, with prominent peaks at 720-660 Ma, 620-590 Ma, 560-510 Ma, 485 Ma, and 440-400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic-Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska-Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0-1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are similar to those in the peri-Gondwanan Avalonian-Cadomian arc system, the Timanide orogen of Baltica, and other circum-Arctic terranes that were proximal to Arctic Alaska prior to the opening of the Amerasian basin in the Early Cretaceous. Our Neoproterozoic reconstruction places the Arctic Alaska-Chukotka terrane in a position near Baltica, northeast of Laurentia, in an arc system along strike with the Avalonian-Cadomian arc terranes. Previously published faunal data indicate that Seward Peninsula had Siberian and Laurentian links by Early Ordovician time. The geologic links between the Arctic Alaska-Chukotka terrane and eastern Laurentia, Baltica, peri-Gondwanan arc terranes, and Siberia from the Paleoproterozoic to the Paleozoic help to constrain paleogeographic models from the Neoproterozoic history of Rodinia to the Mesozoic opening of the Arctic basin. ?? 2009 Geological Society of America.

pre-Mesozoic evolution of the basement of the Catalan Coastal Ranges: implications from geochemical and Sm-Nd isotope data of the Palaeozoic succession of the Collserola Range

NASA Astrophysics Data System (ADS)

Vilà, Miquel; Pin, Christian

2016-04-01

In the whole of the Western Europe and neighbouring areas numerous studies have addressed the provenance of pre-Mesozoic sedimentary rocks and the Palaeozoic geodynamic evolution using the Sm-Nd systematics. However, at present, there are still large areas of the Variscan mountain chain without systematic determinations of their whole - rock Sm-Nd isotope signatures. This is the case of the Palaeozoic blocks of the Catalan Coastal Ranges (NE Iberia). In the context of the Variscan belt many authors interpret the Palaeozoic basement of the Catalan Coastal Ranges as part of the southern foreland basin of the mountain belt. The pre-Mesozoic rocks in the Catalan Coastal Ranges exhibit important stratigraphical affinities with those outcropping in the Eastern Pyrenees, Montagne Noire, Sardinia and Iberian Range. Paleogeographic reconstructions predict that the Catalan Coastal Ranges were located in a transitional area between the northern branch of the Ibero-Armorican arc and the core of the arc. The Collserola Range, located in the metropolitan area of Barcelona, includes a representative Palaeozoic stratigraphic section, from Cambro-Ordovician to Carboniferous, of the central part of the Catalan Coastal Ranges. In this presentation we present an up-to-date review of the stratigraphy and structure of the Palaeozoic of the Collserola Range, and provide geochemical and Sm-Nd isotope data to constrain the Pre-Mesozoic crustal evolution of this sector of the Variscan belt. Geochemical compositions indicate that the Palaeozoic siliciclastic rocks of the Collserola Range were fed by a relative mature heterogeneous source of sediment, comprising from quartz-rich sediments to intermediate igneous rocks. The siliciclastic rocks of the Collserola Range show great geochemical affinity with the turbidites of passive margins. The Sm-Nd signature of the siliciclastic rocks is compatible with those of the Palaeozoic and Late Proterozoic fine grained siliciclastic rocks of the neighbouring terrains of SW Europe. There is a small decrease of the ɛNdT with decreasing age of sedimentation, from the Cambro-Ordovician to the Carboniferous, suggesting an increase of the amount of more 'juvenile' material. The presence of small volumes of alkaline basaltic rocks provides evidence for the input of juvenile material in the Early Palaeozoic basin and suggests that an extensional tectonic regime prevailed during the Cambro-Ordovician sedimentation. From a geodynamic point of view, overall, the analysis of the data evokes that the Palaeozoic rocks of the Catalan Coastal Ranges were part of the Northern Gondwana passive margin before the closure of the Rheic Ocean and the subsequent Variscan orogeny.

Equatorial Pacific gravity lineaments: interpretations with basement topography along seismic reflection lines

NASA Astrophysics Data System (ADS)

Mitchell, Neil C.; Davies, Huw

2018-03-01

The central equatorial Pacific is interesting for studying clues to upper mantle processes, as the region lacks complicating effects of continental remnants or major volcanic plateaus. In particular, the most recently produced maps of the free-air gravity field from satellite altimetry show in greater detail the previously reported lineaments west of the East Pacific Rise (EPR) that are aligned with plate motion over the mantle and originally suggested to have formed from mantle convection rolls. In contrast, the gravity field 600 km or farther west of the EPR reveals lineaments with varied orientations. Some are also parallel with plate motion over the mantle but others are sub-parallel with fracture zones or have other orientations. This region is covered by pelagic sediments reaching 500-600 m thickness so bathymetry is not so useful for seeking evidence for plate deformation across the lineaments. We instead use depth to basement from three seismic reflection cruises. In some segments of these seismic data crossing the lineaments, we find that the co-variation between gravity and basement depth is roughly compatible with typical densities of basement rocks (basalt, gabbro or mantle), as expected for some explanations for the lineaments (e.g., mantle convection rolls, viscous asthenospheric inter-fingering or extensional deformation). However, some other lineaments are associated with major changes in basement depth with only subtle changes in the gravity field, suggesting topography that is locally supported by varied crustal thickness. Overall, the multiple gravity lineament orientations suggest that they have multiple origins. In particular, we propose that a further asthenospheric inter-fingering instability mechanism could occur from pressure variations in the asthenosphere arising from regional topography and such a mechanism may explain some obliquely oriented gravity lineaments that have no other obvious origin.

Petroleum geology and resources of the West Siberian Basin, Russia

USGS Publications Warehouse

Ulmishek, Gregory F.

2003-01-01

The West Siberian basin is the largest petroleum basin in the world covering an area of about 2.2 million km2. The basin occupies a swampy plain between the Ural Mountains and the Yenisey River. On the north, the basin extends offshore into the southern Kara Sea. On the west, north, and east, the basin is surrounded by the Ural, Yenisey Ridge, and Turukhan-Igarka foldbelts that experienced major deformations during the Hercynian tectonic event and the Novaya Zemlya foldbelt that was deformed in early Cimmerian (Triassic) time. On the south, the folded Caledonian structures of the Central Kazakhstan and Altay-Sayan regions dip northward beneath the basin?s sedimentary cover. The basin is a relatively undeformed Mesozoic sag that overlies the Hercynian accreted terrane and the Early Triassic rift system. The basement is composed of foldbelts that were deformed in Late Carboniferous?Permian time during collision of the Siberian and Kazakhstan continents with the Russian craton. The basement also includes several microcontinental blocks with a relatively undeformed Paleozoic sedimentary sequence. The sedimentary succession of the basin is composed of Middle Triassic through Tertiary clastic rocks. The lower part of this succession is present only in the northern part of the basin; southward, progressively younger strata onlap the basement, so that in the southern areas the basement is overlain by Toarcian and younger rocks. The important stage in tectono-stratigraphic development of the basin was formation of a deep-water sea in Volgian?early Berriasian time. The sea covered more than one million km2 in the central basin area. Highly organic-rich siliceous shales of the Bazhenov Formation were deposited during this time in anoxic conditions on the sea bottom. Rocks of this formation have generated more than 80 percent of West Siberian oil reserves and probably a substantial part of its gas reserves. The deep-water basin was filled by prograding clastic clinoforms during Neocomian time. The clastic material was transported by a system of rivers dominantly from the eastern provenance. Sandstones within the Neocomian clinoforms contain the principal oil reservoirs. The thick continental Aptian?Cenomanian Pokur Formation above the Neocomian sequence contains giant gas reserves in the northern part of the basin. Three total petroleum systems are identified in the West Siberian basin. Volumes of discovered hydrocarbons in these systems are 144 billion barrels of oil and more than 1,300 trillion cubic feet of gas. The assessed mean undiscovered resources are 55.2 billion barrels of oil, 642.9 trillion cubic feet of gas, and 20.5 billion barrels of natural gas liquids. The largest known oil reserves are in the Bazhenov-Neocomian total petroleum system that includes Upper Jurassic and younger rocks of the central and southern parts of the basin. Oil reservoirs are mainly in Neocomian and Upper Jurassic clastic strata. Source rocks are organic-rich siliceous shales of the Bazhenov Formation. Most discovered reserves are in structural traps, but stratigraphic traps in the Neocomian clinoform sequence are pro-ductive and are expected to contain much of the undiscovered resources. Two assessment units are identified in this total petroleum system. The first assessment unit includes all conventional reservoirs in the stratigraphic interval from the Upper Jurassic to the Cenomanian. The second unit includes unconventional (or continuous), self-sourced, fractured reservoirs in the Bazhenov Formation. This unit was not assessed quantitatively. The Togur-Tyumen total petroleum system covers the same geographic area as the Bazhenov-Neocomian system, but it includes older, Lower?Middle Jurassic strata and weathered rocks at the top of the pre-Jurassic sequence. A Callovian regional shale seal of the Abalak and lower Vasyugan Formations separates the two systems. The Togur-Tyumen system is oil-prone; gas reserves are insignificant. The principal o

27. EXCAVATION OF EAST (FRONT) BASEMENT WELL AND DRAINAGE SYSTEM, ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

27. EXCAVATION OF EAST (FRONT) BASEMENT WELL AND DRAINAGE SYSTEM, WITH ARCHED ENTRY INTO BASEMENT UNDER FRONT ENTRY IN BACKGROUND, LOOKING NORTH (NOTE GALLETING IN BRICK FOUNDATION) BUT CLOSER RANGE SHOWING BRICK STRUCTURE WHICH CARRIED WATER FROM THE GUTTER DRAIN PIPE INTO THE BRICK DRAIN ALONG THE GROUND AND AWAY FROM THE FOUNDATION OF THE HOUSE - Belair, Tulip Grove Drive, Belair-at-Bowie, Bowie, Prince George's County, MD

Sm-Nd and Rb-Sr isotopic systematics of the Pea Ridge Fe-P deposit and related rocks, southeast Missouri

DOE Office of Scientific and Technical Information (OSTI.GOV)

Marikos, M.A.; Barton, M.D.

1993-03-01

Pea ridge is a discordant Middle Proterozoic Fe-P deposit hosted in rhyolite tuffs and flows of the 1.4--1.5 Ga St. Francois terrane. Host rocks and the deposit are cut by basalt and aplite/pegmatite dikes. The deposit overlies a blind pluton which is partially surrounded by a trachytic ring complex. In the deposit, which is mined for Fe, early Qtz+Amph+Mag+Ap rock is cut by Mag+Ap+Qtz rock. Subsequently, portions of the deposit and host rocks were brecciated, oxidized and silicified to produce a complex suite of rocks enriched in Hem+Qtz+Ksp+Mu. Late breccia pipes/dikes cut the complex and were mineralized with Bar+Ksp+Flu+Chl+Cc+REE-phosphates. Sm/Ndmore » and Rb/Sr isotopic systematics have been studied to: (1) constrain source(s) of igneous rocks and deposit components, (2) refine ages of magmatism, mineralization, and later hydrothermal activity, (3) begin regional comparison of isotopic systematics in SE Missouri Fe deposits, and (4) complement ongoing Missouri DGLS/USGS studies. Fourteen combined Sm-Nd and Rb-Sr analyses were done on materials including two host rhyolites, two nearby trachytes, two gneiss samples representing plausible basement, two intramineral dikes, and six samples of mineralization.« less

Geochemistry and metamorphism of the Paleozoic metasedimentary basement of the Sierra Madre Oriental, NE Mexico. Possible paths from their depositional environment?

NASA Astrophysics Data System (ADS)

Torres Sanchez, Sonia Alejandra; Augustsson, Carita; Alonso Ramirez Fernandez, Juan; Rafael Barboza Gudiño, Jose; Jenchen, Uwe; Abratis, Michael

2013-04-01

We present depositional conditions and possible protholits for Late Paleozoic metasediment in Mexico that were related to the Laurentia-Gondwana collision in Carboniferous time, during Pangea amalgamation. The study aims to reconstruct the depositional and metamorphic evolution of the Granjeno Schist in northeastern Mexico to get a better control on the timing of subduction and collision processes involving the two supercontinents. Remnants of the Mexican Paleozoic continental configuration are present in the Granjeno Schist, the metamorphic basement of the Sierra Madre Oriental in northeastern Mexico. We apply field mapping, petrographic investigations, whole-rock and mineral chemical analysis, as well as U-Pb zircon dating of both metasedimentary and metavolcanic rocks. Field work and petrographic analysis reveal that the Granjeno Schist comprises intercalations of metamorphic rocks with both sedimentary (psammite, pelite, turbidite, conglomerate, black shale) and volcanic (tuff, lava flows, pillow lava and ultramafic bodies) protoliths. The chlorite geothermometer and the presence of phengite in the metasedimentary units as well as U-Pb zircon ages on metapsammite indicate that the Granjeno Schist was metamorphosed under sub-greenschist to greenschist facies with temperatures ranging from 250-345°C during the Carboniferous time (330±30 Ma). The geochemical composition of the metasedimentary rocks is in accordance with iron shale, wacke and quartz arenite protoliths. Some of the variations can be explained by the grain sizes (e. g., 69-74% and 78-96% SiO2 and 10-15% and 3-9% Al2O3 in metapelite and metapsammite, respectively). Our data suggest that the Granjeno Schist metasedimentary units represent a wide variety of clastic sediments derived from mixed felsic basic sources compositions (e. g., Ti/Nb 200-400). Furthermore, the trace element characteristics point to a continental island arc or active continental margin setting due to e. g., Th/Sc and Zr/Sc ratios of 5-8 and 0.3-0.5, respectively, both for metapelite and metapsammite. The metavolcanic rocks are associated with ocean-island basalt (OIB) or mid-ocean ridge basalts (MORB) due to the immobile trace element ratios Zr/Nb and Y/Nb in the ranges 4.91-8.06 and 0.74-1 for the IOB and >9.2 and >1.25 for the MORB, respectively. Detrital zircon ages for three metapsammites reveal that the major sources mainly are Grenvillian (1250-920 Ma) rocks. Such rocks can be found in the ca. 1 Ga Oaxaquia Complex in NE Mexico (Novillo Gneiss). Hence, short transport can be assumed. Maximum depositional ages are Neoproterozoic, Silurian and Devonian. They indicate that the volcanosedimentary deposition probably took place during Devonian time. Based on our results we suggest a plate-tectonic frame for Oaxaquia which is a modification of accepted models. Most models suggest that Oaxaquia was situated between Laurentia and Gondwana during collision in Carboniferous time. The zircon data indicate that the Granjeno Schist was deposited before the collision of Laurentia and Gondwana. The presence of ocean basalt floor, lava flows and serpentinite lenses intercalated with tuff and active continental margin sedimentary rocks necessitates a near-continental environment, such as a back-arc basin. Hence, we present the first evidence of a subduction zone predating the collision of Laurentia and Gondwana.

The inheritance of a Mesozoic landscape in western Scandinavia

PubMed Central

Fredin, Ola; Viola, Giulio; Zwingmann, Horst; Sørlie, Ronald; Brönner, Marco; Lie, Jan-Erik; Grandal, Else Margrethe; Müller, Axel; Margreth, Annina; Vogt, Christoph; Knies, Jochen

2017-01-01

In-situ weathered bedrock, saprolite, is locally found in Scandinavia, where it is commonly thought to represent pre-Pleistocene weathering possibly associated with landscape formation. The age of weathering, however, remains loosely constrained, which has an impact on existing geological and landscape evolution models and morphotectonic correlations. Here we provide new geochronological evidence that some of the low-altitude basement landforms on- and offshore southwestern Scandinavia are a rejuvenated geomorphological relic from Mesozoic times. K-Ar dating of authigenic, syn-weathering illite from saprolitic remnants constrains original basement exposure in the Late Triassic (221.3±7.0–206.2±4.2 Ma) through deep weathering in a warm climate and subsequent partial mobilization of the saprolitic mantle into the overlying sediment cascade system. The data support the bulk geomorphological development of west Scandinavia coastal basement rocks during the Mesozoic and later, long-lasting relative tectonic stability. Pleistocene glaciations played an additional geomorphological role, selectively stripping the landscape from the Mesozoic overburden and carving glacial landforms down to Plio–Pleistocene times. Saprolite K-Ar dating offers unprecedented possibilities to study past weathering and landscape evolution processes. PMID:28452366

Characterization of seismic properties across scales: from the laboratory- to the field scale

NASA Astrophysics Data System (ADS)

Grab, Melchior; Quintal, Beatriz; Caspari, Eva; Maurer, Hansruedi; Greenhalgh, Stewart

2016-04-01

When exploring geothermal systems, the main interest is on factors controlling the efficiency of the heat exchanger. This includes the energy state of the pore fluids and the presence of permeable structures building part of the fluid transport system. Seismic methods are amongst the most common exploration techniques to image the deep subsurface in order to evaluate such a geothermal heat exchanger. They make use of the fact that a seismic wave caries information on the properties of the rocks in the subsurface through which it passes. This enables the derivation of the stiffness and the density of the host rock from the seismic velocities. Moreover, it is well-known that the seismic waveforms are modulated while propagating trough the subsurface by visco-elastic effects due to wave induced fluid flow, hence, delivering information about the fluids in the rock's pore space. To constrain the interpretation of seismic data, that is, to link seismic properties with the fluid state and host rock permeability, it is common practice to measure the rock properties of small rock specimens in the laboratory under in-situ conditions. However, in magmatic geothermal systems or in systems situated in the crystalline basement, the host rock is often highly impermeable and fluid transport predominately takes place in fracture networks, consisting of fractures larger than the rock samples investigated in the laboratory. Therefore, laboratory experiments only provide the properties of relatively intact rock and an up-scaling procedure is required to characterize the seismic properties of large rock volumes containing fractures and fracture networks and to study the effects of fluids in such fractured rock. We present a technique to parameterize fractured rock volumes as typically encountered in Icelandic magmatic geothermal systems, by combining laboratory experiments with effective medium calculations. The resulting models can be used to calculate the frequency-dependent bulk modulus K(ω) and shear modulus G(ω), from which the P- and S-wave velocities V P(ω) and V S(ω) and the quality factors QP(ω) and QS(ω) of fluid saturated fractured rock volumes can be estimated. These volumes are much larger and contain more complex structures than the rock samples investigated in the laboratory. Thus, the derived quantities describe the elastic and anelastic (energy loss due to wave induced fluid flow) short-term deformation induced by seismic waves at scales that are relevant for field-scale seismic exploration projects.

Geologic reconnaissance of the Hot Springs Mountains, Churchill County, Nevada

USGS Publications Warehouse

Voegtly, Nickolas E.

1981-01-01

A geologic reconnaissance of the Hot Springs Mountains and adjacent areas, which include parts of the Brady-Hazen and the Stillwater-Soda Lake Known Geothermal Resource Areas, during June-December 1975, resulted in a reinterpretation of the nature and location of some Basin and Range faults. In addition, the late Cenozoic stratigraphy has been modified, chiefly on the basis of radiometric dates of volcanic rocks by U.S. Geological Survey personnel and others. The Hot Springs Mountains are in the western part of the Basin and Range province, which is characterized by east-west crustal extension and associated normal faulting. In the surrounding Trinity, West Humboldt, Stillwater, and Desert Mountains, Cenozoic rocks overlie ' basement ' rocks of the Paleozoic and Mesozoic age. A similar relation is inferred in the Hot Springs Mountains. Folding and faulting have taken place from the late Tertiary to the present. (USGS)

The influence of joint parameters on normal fault evolution and geometry: a parameter study using analogue modeling

NASA Astrophysics Data System (ADS)

Kettermann, Michael; von Hagke, Christoph; Urai, Janos L.

2017-04-01

Dilatant faults often form in rocks containing pre-existing joints, but the effects of joints on fault segment linkage and fracture connectivity is not well understood. Studying evolution of dilatancy and influence of fractures on fault development provides insights into geometry of fault zones in brittle rocks and will eventually allow for predicting their subsurface appearance. In an earlier study we recognized the effect of different angles between strike direction of vertical joints and a basement fault on the geometry of a developing fault zone. We now systematically extend the results by varying geometric joint parameters such as joint spacing and vertical extent of the joints and measuring fracture density and connectivity. A reproducibility study shows a small error-range for the measurements, allowing for a confident use of the experimental setup. Analogue models were carried out in a manually driven deformation box (30x28x20 cm) with a 60° dipping pre-defined basement fault and 4.5 cm of displacement. To produce open joints prior to faulting, sheets of paper were mounted in the box to a depth of 5 cm at a spacing of 2.5 cm. We varied the vertical extent of the joints from 5 to 50 mm. Powder was then sieved into the box, embedding the paper almost entirely (column height of 19 cm), and the paper was removed. During deformation we captured structural information by time-lapse photography that allows particle imaging velocimetry analyses (PIV) to detect localized deformation at every increment of displacement. Post-mortem photogrammetry preserves the final 3-dimensional structure of the fault zone. A counterintuitive result is that joint depth is of only minor importance for the evolution of the fault zone. Even very shallow joints form weak areas at which the fault starts to form and propagate. More important is joint spacing. Very large joint spacing leads to faults and secondary fractures that form subparallel to the basement fault. In contrast, small joint spacing results in fault strands that only localize at the pre-existing joints, and secondary fractures that are oriented at high angles to the pre-existing joints. With this new set of experiments we can now quantitatively constrain how (i) the angle between joints and basement fault, (ii) the joint depth and (iii) the joint spacing affect fault zone parameters such as (1) the damage zone width, (2) the density of secondary fractures, (3) map-view area of open gaps or (4) the fracture connectivity. We apply these results to predict subsurface geometries of joint-fault networks in cohesive rocks, e.g. basaltic sequences in Iceland and sandstones in the Canyonlands NP, USA.

Origins and Driving Mechanisms for Shallow Methane Accumulations on the Svyatogor Ridge, Fram Strait

NASA Astrophysics Data System (ADS)

Waghorn, K. A.; Bunz, S.; Plaza-Faverola, A. A.; Westvig, I. M.; Johnson, J. E.

2015-12-01

The Svyatogor Ridge, located west of the Knipovich Spreading Ridge (KR) and south of the Molloy Transform Fault (MTF), is hypothesized to have once been the south tip of Vestnesa Ridge; a large sediment drift that was offset during the last 2 Ma along the MTF. The sedimentary cover across Svyatogor Ridge is limited, compared to Vestnesa Ridge, and basement outcrops are identified ~850 mbsf on the apex of the ridge. Despite the limited sedimentation, and its unique location at the intersection between the KR and MTF, Svyatogor Ridge has evidence of shallow gas accumulations; a strong BSR indicating a gas hydrate and underlying free gas system, and fluid flow pathways to the seafloor culminating in pockmarks. Using a high-resolution P-Cable 3D seismic survey, 2D seismic, and multibeam bathymetry data, we investigate how tectonic and sedimentary regimes have influenced the formation of a well-developed gas hydrate system. Sedimentation related with the Vestnesa drift on Svyatogor Ridge is interpreted to have begun ~2-3 Ma. The young age of the underlying oceanic crust, and subsequent synrift sediments below drift strata, suggests gas production from early Miocene aged hydrocarbon source identified in ODP Site 909 to the west, is unlikely in this region. Additionally, given the ultra-slow, magma limited spreading regime of the KR, we do not expect significant thermogenic methane generation from shallow magmatic sources. Therefore, in addition to some microbial gas production, Johnson et al. (2015) hypothesize a contribution from an abiotic source may explain the well-developed gas hydrate system. Large-scale basement faults identified in the seismic data are interpreted as detachment faults, which have exhumed relatively young ultramafic rocks. These detachment faults act as conduits for fluid flow, allowing circulation of seawater to drive serpentinization and subsequently act as pathways for fluids and abiotic methane to reach the shallow subsurface. This work aims to constrain the sedimentary and tectonic history of Svyatogor Ridge to determine 1) the relative interactions between basement detachment faults and overlying faults in the sedimentary cover, 2) the potential role of these faults as gas/fluid conduits and 3) how the underlying structural evolution has influenced the evolution of the gas hydrate system.

Neoarchean crustal growth and Paleoproterozoic reworking in the Borborema Province, NE Brazil: Insights from geochemical and isotopic data of TTG and metagranitic rocks of the Alto Moxotó Terrane

NASA Astrophysics Data System (ADS)

Montefalco de Lira Santos, Lauro Cézar; Dantas, Elton Luiz; Cawood, Peter A.; José dos Santos, Edilton; Fuck, Reinhardt A.

2017-11-01

Pre-Brasiliano rocks in the Borborema Province (NE Brazil) are concentrated in basement blocks, such as the Alto Moxotó Terrane. Petrographic, geochemical, and U-Pb and Sm-Nd isotopic data from two basement metagranitic suites within the terrane provide evidence for Neoarchean (2.6 Ga) and Paleoproterozoic (2.1 Ga) subduction-related events. The Riacho das Lajes Suite is made of medium to coarse-grained hornblende and biotite-bearing metatonalites and metamonzogranites. Whole-rock geochemical data indicate that these rocks represent calcic, magnesian and meta-to peraluminous magmas, and have unequivocal affinities with high-Al low-REE tonalite-trondhjemite-granodiorites (TTG). Zircon U-Pb data from two samples of this suite indicate that they were emplaced at 2.6 Ga, which is the first discovered Archean crust in the central portion of the province. The suite has Neoarchean depleted mantle model ages (TDM) and slightly negative to positive εNd(t), indicating slight crustal contamination. The overall geochemical and isotopic data indicate a Neoarchean intraoceanic setting for genesis of the Riacho das Lajes magma via melting of basaltic oceanic crust submitted to high-pressure eclogite facies conditions. On the other hand, the Floresta Suite comprise metaigneous rocks, which are mostly tonalitic and granodioritic in composition. Geochemical data indicate that this suite shares similarities with calcic to calc-alkalic magmas with magnesian and metaluminous to slightly peraluminous characteristics. Other geochemical features include anomolous Ni, V and Cr contents, as well as high large-ion litophile elements (LILE) values. The suite yields U-Pb zircon ages of approximately 2.1 Ga, Archean to Paleoproterozoic TDM ages, and negative to positive εNd(t) values, suggesting both new crust formation and reworking of Archean crust, in addition to mantle metasomatism, reflecting mixed sources. The most likely tectonic setting for the Floresta Suite magmas involved crustal thickening by terrane accretion, coeval to slab break off. Our results provide new insights on proto-Western Gondwana crustal evolution.

Decrypting the Formation Conditions of the Basement Carbonate-Bearing Rocks at Nili Fossae

NASA Astrophysics Data System (ADS)

Brown, A. J.

2015-12-01

The Nili Fossae region is the site of a number of proposed Landing Sites for the Mars 2020 Rover. A distinguishing feature of many of these sites is the access to large exposures of carbonate (Ehlmann et al. 2008). Serpentinization has been proposed as a formation mechanism of these carbonates, including carbonated (Brown et al. 2010, Viviano, et al. 2013) and low temperature, near surface serpentinization. The potential for carbonated serpentization at Nili Fossae links the region to Earth analogs in terrestrial greenstone belts such as the Pilbara in Western Australia, where talc-carbonate bearing komatiite cumulate units of the Dresser Formation overlie the siliceous, stromatolite-bearing Strelley Pool Chert unit (Van Kranendonk and Pirajno, 2004). If a similar relationship exists on Mars, investigations of rocks stratigraphically beneath the carbonate-bearing units at Nili Fossae ("the basement rocks") may provide the best chance to examine well preserved organic material from the Noachian. This hypothesis is testable by Mars 2020. In preparation for the the Mars 2020 landing site, we are examining the thermodynamic relationships that favor formation of serpentine and talc-carbonate and different pressures and temperatures in the crust (Barnes 2007). This will allow us to constrain the low grade metamorphism required to replicate the proposed models of serpentinisation and help us understand the regional metamophic gradient that is critical to furthering our knowledge of the ancient rocks of Nili Fossae. Refs:Barnes, S. J. "Komatiites: Petrology, Volcanology, Metamorphism, and Geochemistry." S.E.G. 13 (2007): 13. Brown, A. J., et al.. "Hydrothermal Formation of Clay-Carbonate Alteration Assemblages in the Nili Fossae Region of Mars." EPSL 297 (2010): 174-82. Ehlmann, B. L. et al. "Orbital Identification of Carbonate-Bearing Rocks on Mars." Science 322, no. 5909 1828-32. Van Kranendonk, M.J., and F. Pirajno. "Geochemistry of Metabasalts and Hydrothermal Alteration Zones Associated with Ca. 3.45 Ga Chert+/- Barite Deposits" GEEA 4, no. 3 (2004): 253-78. Viviano, C. E., et al. "Implications for Early Hydrothermal Environments on Mars through the Spectral Evidence for Carbonation and Chloritization Reactions in the Nili Fossae Region." JGR 118, no. 9 (2013): 1858-72.

Preserved magnetic fabrics vs. annealed microstructures in the syntectonic recrystallised George granite, South Africa

NASA Astrophysics Data System (ADS)

Ferré, E. C.; Améglio, L.

2000-08-01

The Saldanian basement of the Cape Fold Belt of South Africa outcrops in the Kaaimans inlier with granite plutons intruded in low-grade pelitic and quartzitic metasediments around 535 Ma. New field data support a ubiquitous Saldanian top-to-the-north thrust kinematics coeval with granite emplacement with no substantial Cape tectonic overprint. The granites and their contact aureoles display both synkinematic and post-kinematic fabrics. This and the high strain zone commonly observed all along the contact between the Kaaimans inlier and the Cape Fold Belt, suggest a structural decoupling between the basement and its cover. Microstructures in the Kaaimans inlier and in the George pluton establish a post-kinematic, pervasive and thermal overprint of Saldanian age. Granites and country rocks record a medium-temperature/high-strain deformation phase followed by a strong low-temperature/static recrystallisation. Two sets of andalusite porphyroblasts occur systematically in the contact aureoles of the studied plutons and cannot be explained by successive magmatic pulses. The granites, studied by the Anisotropy of Magnetic Susceptibility (AMS) technique, are paramagnetic (20< Km<300 μSI). Biotite is mostly at the origin of the bulk rock susceptibility although minor contributions of tourmaline or ferromagnetic phases may occur. The contribution of biotite alone to the bulk magnetic susceptibility is supported by two quantitative models based, respectively, on whole rock compositions (Curie-Weiss law) and on intrinsic mineral susceptibilities. The magnetic foliations and lineations are homogeneous throughout the George pluton and are consistent with field structures. The AMS results mainly from the magneto-crystalline anisotropy of biotite and from its lattice preferred orientation (LPO) in the rock. The magnetic fabric reveals the biotite subfabrics that had been acquired before static recrystallisation and which was not modified by the subsequent thermal metamorphic event. The magnetic fabric therefore preserves the emplacement-related deformation fabric.

A Sr-isotopic comparison between thermal waters, rocks, and hydrothermal calcites, Long Valley caldera, California

USGS Publications Warehouse

Goff, F.; Wollenberg, H.A.; Brookins, D.C.; Kistler, R.W.

1991-01-01

The 87Sr/86Sr values of thermal waters and hydrothermal calcites of the Long Valley caldera geothermal system are more radiogenic than those of young intracaldera volcanic rocks. Five thermal waters display 87Sr/86Sr of 0.7081-0.7078 but show systematically lighter values from west to east in the direction of lateral flow. We believe the decrease in ratio from west to east signifies increased interaction of deeply circulating thermal water with relatively fresh volcanic rocks filling the caldera depression. All types of pre-, syn-, and post-caldera volcanic rocks in the west and central caldera have (87Sr/86Sr)m between about 0.7060 and 0.7072 and values for Sierra Nevada granodiorites adjacent to the caldera are similar. Sierran pre-intrusive metavolcanic and metasedimentary rocks can have considerably higher Sr-isotope ratios (0.7061-0.7246 and 0.7090-0.7250, respectively). Hydrothermally altered volcanic rocks inside the caldera have (87Sr/86Sr)m slightly heavier than their fresh volcanic equivalents and hydrothermal calcites (0.7068-0.7105) occupy a midrange of values between the volcanic/plutonic rocks and the Sierran metamorphic rocks. These data indicate that the Long Valley geothermal reservoir is first equilibrated in a basement complex that contains at least some metasedimentary rocks. Reequilibration of Sr-isotope ratios to lower values occurs in thermal waters as convecting geothermal fluids flow through the isotopically lighter volcanic rocks of the caldera fill. ?? 1991.

Mercury- and silver-rich ferromanganese oxides, southern California Borderland: Deposit model and environmental implications

USGS Publications Warehouse

Hein, J.R.; Koschinsky, A.; McIntyre, B.R.

2005-01-01

Mercury- and silver-enriched ferromanganese oxide crusts were recovered at water depths of 1,750 tol,300 m from La Victoria knoll, located about 72 km off the coast of northern Baja California. No other ferromanganese precipitate found so far in the modern ocean basins is similarly enriched in Hg and Ag. The precipitates consist of submetallic gray, brecciated, Mn oxide layers overlain by brown earthy, laminated Fe-Mn oxide crusts. Both oxide types are rich in Hg (to 10 ppm) and Ag (to 5.5 ppm). The Mn-rich layers are composed of ??MnO2, with lesser amounts of 10A?? and 7A?? manganates, whereas the Mn phase in the Fe-Mn crusts is solely ??MnO2. The Fe phase in both layers is X-ray amorphous. Established criteria for distinguishing hydrothermal versus hydrogenetic crusts indicate that the Mn-rich layers are predominantly of low-temperature hydrothermal origin, whereas the Fe-Mn crusts are hydrogenetic, although there is some overlap in the source of chemical components in both types. La Victoria knoll is uplifted continental basement rock with basalt, andesite, and schist cropping out at the surface; the knoll may have an intrusive core. The Hg and Ag were derived from leaching by hydrothermal fluids of organic matter-rich sediments in basins adjacent to La Victoria knoll and, to a lesser extent, from continental basement rocks underlying the knoll and adjacent basins. Both rock types are notably enriched in Ag and Hg. Faults were the main fluid transport pathway, and hydrothermal circulation was driven by high heat flow associated with thinned crust. Other elements derived from the hydrothermal fluids include Tl, Cd, Cr, and Li. The main host for Hg and Ag is FeOOH, although MnO2 likely hosts some of the Ag. Minor sulfide and barite also may contain small amounts of these metals. Possible analogs in the geologic record for this deposit type are found in the Basin and Range province of the western United States and Mexico. The discovery highlights the fact that fluids circulating along faults in the offshore California borderland are transporting potentially toxic metals (Hg, Ag, Tl, As, Cd, Cr, Pb, and Ni) and depositing them on and just below the ocean floor. ?? 2005 Society of Economic Geologists, Inc.

Type IV Collagens and Basement Membrane Diseases: Cell Biology and Pathogenic Mechanisms.

PubMed

Mao, Mao; Alavi, Marcel V; Labelle-Dumais, Cassandre; Gould, Douglas B

2015-01-01

Basement membranes are highly specialized extracellular matrices. Once considered inert scaffolds, basement membranes are now viewed as dynamic and versatile environments that modulate cellular behaviors to regulate tissue development, function, and repair. Increasing evidence suggests that, in addition to providing structural support to neighboring cells, basement membranes serve as reservoirs of growth factors that direct and fine-tune cellular functions. Type IV collagens are a major component of all basement membranes. They evolved along with the earliest multicellular organisms and have been integrated into diverse fundamental biological processes as time and evolution shaped the animal kingdom. The roles of basement membranes in humans are as complex and diverse as their distributions and molecular composition. As a result, basement membrane defects result in multisystem disorders with ambiguous and overlapping boundaries that likely reflect the simultaneous interplay and integration of multiple cellular pathways and processes. Consequently, there will be no single treatment for basement membrane disorders, and therapies are likely to be as varied as the phenotypes. Understanding tissue-specific pathology and the underlying molecular mechanism is the present challenge; personalized medicine will rely upon understanding how a given mutation impacts diverse cellular functions. Copyright © 2015 Elsevier Inc. All rights reserved.

Test of Continental Drift by Comparison of Radiometric Ages: A pre-drift reconstruction shows matching geologic age provinces in West Africa and Northern Brazil.

PubMed

Hurley, P M; Rand, J R; Pinson, W H; Fairbairn, H W; de Almeida, F F; Melcher, G C; Cordani, U G; Kawashita, K; Vandoros, P

1967-08-04

1) The distribution of age values obtained by potassium-argon determinations and whole-rock rubidium-strontium determinations appears to be almost identical for West African rocks of the pervasive Eburnean Orogenic Cycle and basement rocks at opposite locations in South America. 2) There is also a close correlation, with respect to potassium-argon age determinations on micas, rubidium-strontium determinations on total-rock samples, and the extent to which these two sets of values differ, between rocks of the Pan-African Orogenic Cycle and rocks of the Caririan Orogenic Cycle in Brazil, where these two groups of rocks lie opposite each other in the two continents. 3) When Africa and South America are "fitted together," the sharply defined boundary between the Eburnean and the Pan-African age provinces in West Africa strikes directly toward the corresponding age boundary in northeast Brazil. 4) The transition from the 550-million-year Pan-African age province to the 2000-million-year age province in the Congo Craton in Cameroun-Gabon is matched in the rocks near the corresponding part of the east coast of Brazil. However the geological and age data are insufficient to do more than suggest the possibility of another age-boundary correlation here. 5) The evidence reported here supports the hypothesis of continental drift.

3D model of Campo de Dalías basement from H/V spectral ratio of ambient seismic noise

NASA Astrophysics Data System (ADS)

García-Jerez, Antonio; Seivane, Helena; Luzón, Francisco; Navarro, Manuel; Molina, Luis; Aranda, Carolina; Piña-Flores, José; Navarro, Francisco; Sánchez-Martos, Francisco; Vidal, Francisco; Posadas, Antonio M.; Sánchez-Sesma, Francisco J.

2017-04-01

Campo de Dalías is a large coastal plain in the southeastern mountain front of the Betic Cordillera (SE of the Iberian Peninsula), being one of the most seismically active regions of Spain. This area has a population of about 213.000 inhabitants, with a high growth rate during the last decades due to the development of intensive agricultural activities. Seismic risk assessment and hydrogeological issues are major topics of interest for this area, relaying on the knowledge of the geophysical properties of the basin. A passive seismic survey has been conducted throughout the basin. We have recorded ambient noise at 340 sites located approximately on the vertexes of a 1000 x 1000 m square grid, as well as around a set of deep boreholes reaching the Triassic basement. These broad-band records, of at least 45 minutes long each, have been analyzed by using the horizontal-to-vertical spectral ratio method (H/V). The spectral analysis shows clear H/V peaks with periods ranging from 0.3 s to 4 s, approximately, associated to relevant contrasts in S-wave velocity (Vs) at depth. Simulations based on the diffuse field approach (Sánchez-Sesma et al. 2011) show that long periods are explained by the effect of several hundred meters of soft sedimentary rocks (mainly Miocene marls). Well-developed high-frequency secondary peaks have been found in some specific zones (e.g. N of Roquetas de Mar town). Then, fundamental frequencies and basement depths at borehole sites have been fitted by means of a power law, which can be applied down to 900 - 970m. Larger depths are estimated by extrapolation. This relationship has been used to map the basement (main Vs contrast) throughout the plain. The prospected basement model describes well the main structural features of this smoothly folded region, namely, the El Ejido Synform and the Guardias Viejas Antiform, with ENE-WSW-trend. These features are shifted toward the south in comparison with Pedrera et al. (2015). The homogeneous resolution provides new insights which cannot be reached from analysis of deep boreholes, due to their very irregular distribution and it complements 2D Vp models derived from seismic reflection surveys. ACKNOWLEDGEMENTS: This research has been supported by the Spanish Ministry of Economy and Competitiveness under grant CGL2014-59908 and by the European Union with ERDF funds.

Plate drift in the Afar and Issas territory (French Somalia) and eastern Ethiopia as seen on space photography

NASA Technical Reports Server (NTRS)

Bannert, D.

1972-01-01

Apollo 9 photographs of the region northwest of the Gulf of Tadjura were studied. The occurrence of a large region of basement rocks, probably crystalline in nature, with a strongly faulted overlayer was recognized and has been named the Tadjura uplift. Study of this region is adding detailed knowledge to the literature on the phenomenon of plate drift in the southern Red Sea region.

Petroleum geology and resources of the Baykit High province, East Siberia, Russia

USGS Publications Warehouse

Ulmishek, Gregory F.

2001-01-01

The Baykit High province consists of two principal structural units?the Baykit regional high in the west, which occupies most of the province, and the Katanga structural saddle in the east. The province is on the western margin of the Siberian craton east of theYenisey Ridge foldbelt. The province is an exploration frontier and only a few prospects have been drilled. The oldest sedimentary rocks of the province, Riphean carbonate and clastic strata of Late Proterozoic age (1,650?650 million years old) that were deposited on the passive margin, cover the Archean?Lower Proterozoic basement. Basal Vendian (uppermost Proterozoic, 650?570 million years old) clastic rocks unconformably overlie various units of the Riphean and locally lie directly on the basement. Younger Vendian and lowermost Cambrian rocks are primarily dolomites. The Vendian/Cambrian boundary is con-formable, and its exact stratigraphic position has not been identified with certainty. The Lower Cambrian section is thick, and it consists of alternating beds of dolomite and evaporites (mostly salt). Middle and Upper Cambrian strata are composed of shale and dolomite. Ordovician-Silurian and upper Paleozoic rocks are thin, and they are present only in the northern areas of the province. Structural pattern of Riphean rocks differs substantially from that of Vendian-Cambrian rocks. A single total petroleum system (TPS) was identified in the Baykit High province. Discovered oil of the system is chiefly concentrated in Riphean carbonate reservoirs of the Yurubchen-Tokhom zone that is currently being explored and that has the Abstract 1 potential to become a giant field (or group of fields). The TPS also contains about 5 trillion cubic feet of discovered recover-able gas in clastic reservoir rocks at the base of the Vendian section. Petroleum source rocks are absent in the stratigraphic succession over most of the TPS area. Riphean organic-rich shales and carbonates that crop out in the Yenisey Ridge foldbelt west of the Baykit high are probable source rocks. Their areal distribution extends from the foldbelt into the foredeep along the province?s western margin. Potential source rocks also are present in platform depressions in eastern areas of the province. Hydrocarbon generation and migration west of the province started as early as Riphean time, before the beginning of the deformation in the Yenisey Ridge foldbelt that occurred about 820?850 million years ago. However, the presently known oil and gas accumulations were formed after deposition of the Lower Cambrian salt seal. Available data allow identification of only one assessment unit, and it covers the entire TPS area. Undiscovered oil and gas resources are moderate, primarily due to the poor quality of reservoir rocks. However, the reserve growth in the Yurubchen-Tokhom zone may be large and may exceed the volume of undiscovered resources in the rest of the province. Most oil and gas resourcesareexpectedtobeinstructuralandstratigraphictrapsin Riphean carbonate reservoirs. Vendian clastic reservoirs are probably gas-prone.

Progressive deformation and superposed fabrics related to Cretaceous crustal underthrusting in western Arizona, U.S.A.

USGS Publications Warehouse

Laubach, S.E.; Reynolds, S.J.; Spencer, J.E.; Marshak, S.

1989-01-01

In the Maria fold and thrust belt, a newly recognized E-trending Cretaceous orogenic belt in the southwestern United States, ductile thrusts, large folds and superposed cleavages record discordant emplacement of crystalline thrust sheets across previously tilted sections of crust. Style of deformation and direction of thrusting are in sharp contrast to those of the foreland fold-thrust belt in adjacent segments of the Cordillera. The net effect of polyphase deformation in the Maria belt was underthrusting of Paleozoic and Mesozoic metasedimentary rocks under the Proterozoic crystalline basement of North America. The structure of the Maria belt is illustrated by the Granite Wash Mountains in west-central Arizona, where at least four non-coaxial deformation events (D1-D4) occurred during the Cretaceous. SSE-facing D1 folds are associated with S-directed thrusts and a low-grade slaty cleavage. D1 structures are truncated by the gently-dipping Hercules thrust zone (D2), a regional SW-vergent shear zone that placed Proterozoic and Jurassic crystalline rocks over upturned Paleozoic and Mesozoic supracrustal rocks. Exposures across the footwall margin of the Hercules thrust zone show the progressive development of folds, cleavage and metamorphism related to thrusting. D3 and D4 structures include open folds and spaced cleavages that refold or transect D1 and D2 folds. The D2 Hercules thrust zone and a D3 shear zone are discordantly crosscut by late Cretaceous plutons. ?? 1989.

DEPOSITIONAL RELATIONS OF UMPQUA AND TYEE FORMATIONS (EOCENE), SOUTHWESTERN OREGON.

USGS Publications Warehouse

Molenaar, C.M.

1985-01-01

The Umpqua Formation (as herein restricted) consists of as much as 10,000 ft of mudstone, sandstone, and conglomerate of nonmarine to deep marine origin. A basaltic basement that underlies the sedimentary rocks in most of the area and was formerly included in the Umpqua is herein considered a separate unit and assigned to the Siletz River Volcanics. A proposal to subdivide the Umpqua into three unconformity-bounded formations in the area west of Roseburg, Oregon, is not recognized in this report because of questionable correlations and limited extent of some units. The Tyee Formation, which conformably overlies the Umpqua, is a predominantly sandstone unit about 6,000 ft thick, deposited in environments ranging from shallow marine and nonmarine deltaic on the south, to slope and deep marine basinal to the north. Deposition across the Umpqua-Tyee boundary contact represents a change in tectonic setting. Refs.

Oceanic crust within the paleozoic Granjeno Schist, northeastern Mexico. Remnants of the Rheic and paleo-Pacific Ocean.

NASA Astrophysics Data System (ADS)

Torres Sanchez, Sonia Alejandra; Augustsson, Carita; Rafael Barboza Gudiño, Jose; Jenchen, Uwe; Torres Sanchez, Dario; Aleman Gallardo, Eduardo; Abratis, Michael

2015-04-01

Late Paleozoic metamorphic rocks in Mexico are related to the Laurentia-Gondwana collision in Carboniferous time, during Pangaea amalgamation. Vestiges of the Mexican Paleozoic continental configuration are present in the Granjeno Schist, the metamorphic basement of the Sierra Madre Oriental. Field work and petrographic analysis reveal that the Granjeno Schist comprises metamorphic rocks with both sedimentary (psammite, pelite, turbidite, conglomerate, black shale) and igneous (tuff, lava flows, pillow lava and ultramafic bodies) protoliths. The chlorite geothermometer and the presence of phengite in the metasedimentary units as well as 40Ar/39Ar ages on metavolcanic and metaultramafic rocks indicate that the Granjeno Schist was metamorphosed under sub-greenschist to greenschist facies with temperatures ranging from 250-345°C with 2.5 kbar during Carboniferous time (330±30 Ma). The presence of metabasalt, metacumulate, serpentinite and talc bodies suggests an oceanic tectonic setting for the evolution of the Granjeno Schist. Serpetinite rocks have mesh, granular and ribbon textures which indicate recrystallization and metasomatic events. The serpentinite rocks are enriched in the very large incompatible elements Cs, U, and Zr and depleted in Ba, Sr, Pb, Zr and Ce. Normalized REE patterns (LaN/YbN = 0.51 - 19.95 and LaN/SmN = 0.72 - 9.08) of the serpentinite and talc/soapstone are characteristic of peridotite from both suprasubduction and mid-ocean ridge zones. Serpentinite from the Granjeno Schist have spinel content which can reveal different stages of evolution in host serpentinite. The composition of chromite indicates that they belong to podiform chromite that may have crystallized from mid-ocean ridge magma. Al-chromite in the serpentinite is characterized by #Cr 0.48 to 0.55, which indicates a depleted mantle source affected by 17 to 18% of partial melting. The ferritchromite has #Cr values of 0.93 to 1.00 which indicates a metamorphic origin. Our study suggests at least two serpentinization stages. The first serpentinization stage is related to an ocean-floor environment. At this stage, mesh-textured serpentinite formed under static conditions under subgreenschist to greenschist conditions. The second serpentinization stage occurred under greenschist to low amphibole conditions. During this stage Cr-spinel progressively was replaced by ferritchromite with magnetite rims due to regional metamorphism. Tectonic contact of the serpentinite with metavolcanic and metasedimentary rocks indicates lithospheric mantle slivers juxtaposed during the metamorphism of the Granjeno Schist during Pennsylvanian time. This metamorphic event occurred in an active continental margin. It represents the last events of the southern closure of the Rheic Ocean and Permo-Carboniferous convergence of Pacific plates on the western margin of Pangea.

K/T age for the popigai impact event

NASA Technical Reports Server (NTRS)

Deino, A. L.; Garvin, J. B.; Montanari, S.

1991-01-01

The multi-ringed POPIGAI structure, with an outer ring diameter of over 100 km, is the largest impact feature currently recognized on Earth with an Phanerozoic age. The target rocks in this relatively unglaciated region consist of upper Proterozoic through Mesozoic platform sediments and igneous rocks overlying Precambrian crystalline basement. The reported absolute age of the Popigai impact event ranges from 30.5 to 39 Ma. With the intent of refining this age estimate, a melt-breccia (suevite) sample from the inner regions of the Popigai structure was prepared for total fusion and step-wise heating Ar-40/Ar-39 analysis. Although the total fusion and step-heating experiments suggest some degree of age heterogeneity, the recurring theme is an age of around 64 to 66 Ma.

Elastic-wave propagation and site amplification in the Salt Lake Valley, Utah, from simulated normal faulting earthquakes

USGS Publications Warehouse

Benz, H.M.; Smith, R.B.

1988-01-01

The two-dimensional seismic response of the Salt Lake valley to near- and far-field earthquakes has been investigated from simulations of vertically incident plane waves and from normal-faulting earthquakes generated on the basin-bounding Wasatch fault. The plane-wave simulations were compared with observed site amplifications in the Salt Lake valley, based on seismic recordings from nuclear explosions in southern Nevada, that show 10 times greater amplification with the basin than measured values on hard-rock sites. Synthetic seismograms suggest that in the frequency band 0.3 to 1.5 Hz at least one-half the site amplitication can be attributed to the impedance contrast between the basin sediments and higher velocity basement rocks. -from Authors

CHARACTER AND REGIONAL SIGNIFICANCE OF GREAT FALLS TECTONIC ZONE, EAST-CENTRAL IDAHO AND WEST-CENTRAL MONTANA.

USGS Publications Warehouse

O'Neill, J. Michael; Lopez, David A.

1985-01-01

The Great Falls tectonic zone, here named, is a belt of diverse northeast-trending geologic features that can be traced from the Idaho batholith in the Cordilleran miogeocline, across thrust-belt structures and basement rocks of west-central and southwestern Montana, through cratonic rocks of central Montana, and into southwestern-most Saskatchewan, Canada. Geologic mapping in east-central Idaho and west-central Montana has outlined a continuous zone of high-angle faults and shear zones. Recurrent fault movement in this zone and strong structural control over igneous intrusion suggest a fundamental tectonic feature that has influenced the tectonic development of the Idaho-Montana area from a least middle Proterozoic time to the present. Refs.

Reconciling deep seismic refraction and reflection data from the grenvillian-appalachian boundary in western New England

USGS Publications Warehouse

Hughes, S.; Luetgert, J.H.; Christensen, N.I.

1993-01-01

The Grenvillian-Appalachian boundary is characterized by pervasive mylonitic deformation and retrograde alteration of a suite of imbricated allochthonous and parautochthonous gneisses that were thrust upon the Grenvillian continental margin during the lower Paleozoic. Seismic reflection profiling across this structural boundary zone reveals prominent dipping reflectors interpreted as overthrust basement slices (parautochthons) of the Green Mountain Anticlinorium. In contrast, a seismic refraction study of the Grenvillian-Appalachian boundary reveals a sub-horizontally layered seismic velocity model that is difficult to reconcile with the pronounced sub-vertical structures observed in the Green mountains. A suite of rock samples was collected from the Green Mountain Anticlinorium and measured at high pressures in the laboratory to determine the seismic properties of these allochthonous and parautochthonous gneisses. The laboratory-measured seismic velocities agree favorably with the modelled velocity structure across the Grenvillian-Appalachian boundary suggesting that the rock samples are reliable indicators of the rock mass as whole. Samples of the parautochthonous Grenvillian basement exposed in the Green Mountains have lower velocities, by about 0.5 km/s, than lithologically equivalent units exposed in the eastern Adirondack Highlands. Velocity reduction in the Green Mountain parautochthons can be accounted for by retrograde metamorphic alteration (hydration) of the paragneisses. Seismic anisotropies, ranging from 2 to 12%, in the mylonitized Green Mountain paragneisses may also contribute to the observation of lower seismic velocities, where the direction of ray propagation is normal to the foliation. The velocity properties of the Green Mountain paragneisses are thus insufficiently different from the mantling Appalachian allochthons to permit their resolution by the Ontario-New York-New England seismic refraction profile. ?? 1993.

Distinguishing Grenvillian basement from pre-Taconian cover rocks in the Northern Appalachians

USGS Publications Warehouse

Karabinos, P.; Aleinikoff, J.N.; Fanning, C.M.

1999-01-01

Distinguishing Grenvillian basement rocks from pre-Taconian cover sequences in the Appalachians is a first-order problem essential for accurate structural interpretations. The Cavendish Formation in southeastern Vermont presents a classic example of this problem. Doll and others (1961) showed the Cavendish Formation as younger than the Middle Proterozoic Mount Holly Complex but older than the lithologically similar Cambrian Tyson and Hoosac Formations. More recently, the name Cavendish Formation has been informally abandoned, and its metasedimentary units have been mapped as the Tyson and Hoosac Formations of Late Proterozoic to Cambrian age. In a radical departure from these interpretations, Ratcliffe and others (1997) reassigned metasedimentary rocks of the Cavendish Formation to the Mount Holly Complex based on an inferred intrusive relationship between them and a 1.42 Ga tonalite. This new age assignment, if correct, requires a completely new structural interpretation of the region. SHRIMP and Pb evaporation ages of detrital zircons extracted from a quartzite layer from Cavendish Gorge near the proposed intrusive contact with the tonalite constrain the time of deposition of the Cavendish Formation. Grain shapes of the zircons vary from euhedral to nearly spherical. Virtually all the grains have pitted surfaces and show at least some rounding of edges and terminations; grains exhibit oscillatory zoning typical of zircons that crystallized from a magma. Single-grain Pb evaporation analyses of ten zircons and SHRIMP analyses of 15 zircons all yield ages less than 1.42 Ga. Seven of the grains are consistent with derivation from the Bull Hill Gneiss that postdates the Grenville orogenic cycle and predates deposition of the Cavendish Formation. Thus, the metasedimentary units of the Cavendish Formation should not be assigned to the Mount Holly Complex.

The origin of the 1.73-1.70 Ga anorogenic Ulkan volcano-plutonic complex, Siberian platform, Russia: inferences from geochronological, geochemical and Nd-Sr-Pb isotopic data

USGS Publications Warehouse

Larin, A.M.; Amelin, Yu. V.; Neymark, L.A.; Krymsky, R. Sh

1997-01-01

The Ulkan volcano-plutonic complex, a part of a 750 km Bilyakchian-Ulkan anorogenic belt, is located in the eastern part of the Archean-Paleoproterozoic Aldan shield. The tectonic position and geochemistry indicate that the Ulkan Complex is a typical A-type or intraplate magmatic association. The felsic volcanics of the Uian Group and granitoids of the North Uchur Massif, the major igneous components of the Ulkan Complex, have U-Pb zircon and monazite ages between 1721±1 Ma and 1703±18 Ma. Together with the spatially associated 1736±6 Ma Dzhugdzhur anorthosite massif, the Ulkan Complex forms a typical Proterozoic anorthosite-granite-volcanic association with the minimum duration of formation of 12 m.y. Initial εNd values between 0 and 1.1, similar for the Uian felsic volcanics, early granitoid phases of the North Uchur Massif and high-grade metamorphic basement rocks, indicate, along with geochemical data, that the crustal source of the Ulkan parental magmas may be similar to the basement rocks. The higher εNd(T) values of -0.3 to +1.9 in the later North Uchur granitoids and associated ore-bearing metasomatites, and relatively low time-integrated Rb/Sr, U/Pb, and Th/U estimated for their sources, may demonstrate involvement of variable amounts of a depleted mantle-derived component in the generation of later phases of the North Uchur Massif. The preferred model of formation of magmas parental to the Ulkan Complex involves thermal interaction of an uprising mantle diapir with Paleoproterozoic lower crust, which was accompanied by chemical interaction between a fluid derived from the diapir, with the lower crustal rocks.

Late Jurassic - Early Cretaceous convergent margins of Northeastern Asia with Northwestern Pacific and Proto-Arctic oceans

NASA Astrophysics Data System (ADS)

Sokolov, Sergey; Luchitskaya, Marina; Tuchkova, Marianna; Moiseev, Artem; Ledneva, Galina

2013-04-01

Continental margin of Northeastern Asia includes many island arc terranes that differ in age and tectonic position. Two convergent margins are reconstructed for Late Jurassic - Early Cretaceous time: Uda-Murgal and Alazeya - Oloy island arc systems. A long tectonic zone composed of Upper Jurassic to Lower Cretaceous volcanic and sedimentary rocks is recognized along the Asian continent margin from the Mongol-Okhotsk thrust-fold belt on the south to the Chukotka Peninsula on the north. This belt represents the Uda-Murgal arc, which was developed along the convergent margin between Northeastern Asia and Northwestern Meso-Pacific. Several segments are identified in this arc based upon the volcanic and sedimentary rock assemblages, their respective compositions and basement structures. The southern and central parts of the Uda-Murgal island arc system were a continental margin belt with heterogeneous basement represented by metamorphic rocks of the Siberian craton, the Verkhoyansk terrigenous complex of Siberian passive margin and the Koni-Taigonos late Paleozoic to early Mesozoic island arc with accreted oceanic terranes. At the present day latitude of the Pekulney and Chukotka segments there was an ensimatic island arc with relicts of the South Anyui oceanic basin in backarc basin. Alazeya-Oloy island arc systems consists of Paleozoic and Mesozoic complexes that belong to the convergent margin between Northeastern Asia and Proto-Artic Ocean. It separated structures of the North American and Siberian continents. The Siberian margin was active whereas the North American margin was passive. The Late Jurassic was characterized by termination of a spreading in the Proto-Arctic Ocean and transformation of the latter into the closing South Anyui turbidite basin. In the beginning the oceanic lithosphere and then the Chukotka microcontinent had been subducted beneath the Alazeya-Oloy volcanic belt

Factors influencing the composition of detrital heavy mineral suites in Holocene sands of the Apure River drainage basin, Venezuela

USGS Publications Warehouse

Morton, Andrew C.

1993-01-01

Heavy mineral assemblages in rivers in the Apure River drainage basin of Venezuela and Colombia closely reflect the nature of the source regions, which lie in the Andean orogenic terranes to the west and northwest. The Caribbean Mountains, largely composed of greenschist-facies pelites, phyllites, carbonates, and metavolcanics, supply assemblages dominated by epidote and calcic amphibole. Minor amounts of the high-pressure index minerals glaucophane and lawsonite indicate the presence of blueschistfacies rocks, reflecting the origin of the Caribbean Mountains by subduction-related tectonism. The northern Mérida Andes, which comprise basement gneisses and granites overlain by unmetamorphosed to low-grade metamorphosed clastics, supply two types of assemblage reflecting these two lithological types: garnet-sillimanite-staurolite-amphibole suites from the basement rocks, and epidote-amphibole suites from the overlying cover sequence. The southern Mérida Andes supply stable heavy mineral suites reflecting recycling from the extensive unmetamorphosed sandstones that occur at outcrop. By considering suites from different physiographical provinces, the effects of short-term alluvial storage in the Llanos on heavy mineral assemblages have been evaluated. Weathering during alluvial storage appears to be effective in modifying the apatite-tourmaline ratio, which shows a steady, marked decline with distance from the mountain front, resulting from the removal of apatite during weathering. Clinopyroxene and garnet may also show evidence of loss through weathering, although the trends are poorly constrained statistically. Epidote and amphibole proportions remain essentially constant, possibly through a balance between loss through weathering and continual resupply from the breakdown of rock fragments. In general, the heavy mineral assemblages are less affected than the bulk mineralogy by alluvial storage on the Llanos.

A Triassic to Cretaceous Sundaland-Pacific subduction margin in West Sarawak, Borneo

NASA Astrophysics Data System (ADS)

Breitfeld, H. Tim; Hall, Robert; Galin, Thomson; Forster, Margaret A.; BouDagher-Fadel, Marcelle K.

2017-01-01

Metamorphic rocks in West Sarawak are poorly exposed and studied. They were previously assumed to be pre-Carboniferous basement but had never been dated. New 40Ar/39Ar ages from white mica in quartz-mica schists reveal metamorphism between c. 216 to 220 Ma. The metamorphic rocks are associated with Triassic acid and basic igneous rocks, which indicate widespread magmatism. New U-Pb dating of zircons from the Jagoi Granodiorite indicates Triassic magmatism at c. 208 Ma and c. 240 Ma. U-Pb dating of zircons from volcaniclastic sediments of the Sadong and Kuching Formations confirms contemporaneous volcanism. The magmatic activity is interpreted to represent a Triassic subduction margin in westernmost West Sarawak with sediments deposited in a forearc basin derived from the magmatic arc at the Sundaland-Pacific margin. West Sarawak and NW Kalimantan are underlain by continental crust that was already part of Sundaland or accreted to Sundaland in the Triassic. One metabasite sample, also previously assumed to be pre-Carboniferous basement, yielded Early Cretaceous 40Ar/39Ar ages. They are interpreted to indicate resumption of subduction which led to deposition of volcaniclastic sediments and widespread magmatism. U-Pb ages from detrital zircons in the Cretaceous Pedawan Formation are similar to those from the Schwaner granites of NW Kalimantan, and the Pedawan Formation is interpreted as part of a Cretaceous forearc basin containing material eroded from a magmatic arc that extended from Vietnam to west Borneo. The youngest U-Pb ages from zircons in a tuff layer from the uppermost part of the Pedawan Formation indicate that volcanic activity continued until c. 86 to 88 Ma when subduction terminated.

Geology of the Trenton Prong, west-central New Jersey

DOE Office of Scientific and Technical Information (OSTI.GOV)

Volkert, R.A.; Drake, A.A.Jr.

1993-03-01

The Trenton Prong in New Jersey is underlain by a heterogeneous sequence of rocks that is divisible into northern and southern belts separated by the steeply southeast-dipping Huntingdon Valley fault (HVF). The northern belt contains metagabbro, charnockite, and dacite/tonalite, upon which biotite-bearing quartzofeldspathic gneiss, calc-silicate gneiss, and minor marble may rest unconformably. The mineralogy and geochemistry of these rocks are remarkably similar to those of Middle Proterozoic rocks in the New Jersey Highlands, and the authors interpret them to be correlative. Northern belt rocks are unconformably overlain by the Cambrian Chickies Quartzite, which is cut off to the northeast bymore » the HVF. The southern belt contains felsic to intermediate quartzofeldspathic gneiss and schist and minor amounts of metavolcanic rocks, all of which may be at slightly lower metamorphic grade than those in the northern belt. High TiO[sub 2] metabasalt is chemically identical to diabase dikes that intrude Middle Proterozoic rocks in the New Jersey Highlands; it is interpreted to be Late Proterozoic in age. Rocks in the southern belt have been thrust northwestward over the Chickies and Middle Proterozoic rocks along the HVF. South of the southern belt, biotite schist and gneiss of the Wissahickon Formation are thrust onto both belts of basement rocks on the HVF and a splay from the HVF, the Morrisville thrust fault. Both faults are marked by augen gneiss that shows evidence of dextral shear.« less

Review of buried crystalline rocks of eastern United States in selected hydrogeologic environments potentially suitable for isolating high-level radioactive wastes

USGS Publications Warehouse

Davis, R.W.

1984-01-01

Among the concepts suggested for the deep disposal of high-level radioactive wastes from nuclear power reactors is the excavation of a repository in suitable crystalline rocks overlain by a thick sequence of sedimentary strata in a hydrogeologic environment that would effectively impede waste transport. To determine the occurrence of such environments in the Eastern United States, a review was made of available sources of published or unpublished information, using the following hydrogeologic criteria:The top of the crystalline basement rock is 1,000 to 4,000 feet below land surface.The crystalline rock is overlain by sedimentary rock whose lowermost part, at least, contains ground water with a dissolved-solids concentration of 10,000 milligrams per liter or more.Shale or clay confining beds overlie the saline-water aquifer.The flow system in the saline-water aquifer is known or determinable from presently available data.All of these hydrogeologic conditions occur in two general areas: (1) parts of Indiana, Ohio, and Kentucky, underlain by part of the geologic structure known as the Cincinnati arch, and (2) parts of the Atlantic Coastal Plain from Georgia to New Jersey.

Crustal thinning and exhumation along a fossil magma-poor distal margin preserved in Corsica: A hot rift to drift transition?

NASA Astrophysics Data System (ADS)

Beltrando, Marco; Zibra, Ivan; Montanini, Alessandra; Tribuzio, Riccardo

2013-05-01

Rift-related thinning of continental basement along distal margins is likely achieved through the combined activity of ductile shear zones and brittle faults. While extensional detachments responsible for the latest stages of exhumation are being increasingly recognized, rift-related shear zones have never been sampled in ODP sites and have only rarely been identified in fossil distal margins preserved in orogenic belts. Here we report evidence of the Jurassic multi-stage crustal thinning preserved in the Santa Lucia nappe (Alpine Corsica), where amphibolite facies shearing persisted into the rift to drift transition. In this nappe, Lower Permian meta-gabbros to meta-gabbro-norites of the Mafic Complex are separated from Lower Permian granitoids of the Diorite-Granite Complex by a 100-250 m wide shear zone. Fine-grained syn-kinematic andesine + Mg-hornblende assemblages in meta-tonalites of the Diorite-Granite Complex indicate shearing at T = 710 ± 40 °C at P < 0.5 GPa, followed by deformation at greenschist facies conditions. 40Ar/39Ar step-heating analyses on amphiboles reveal that shearing at amphibolite facies conditions possibly began at the Triassic-Jurassic boundary and persisted until t < 188 Ma, with the Mafic Complex cooling rapidly at the footwall of the Diorite-Granite Complex at ca. 165.4 ± 1.7 Ma. Final exhumation to the basin floor was accommodated by low-angle detachment faulting, responsible for the 1-10 m thick damage zone locally capping the Mafic Complex. The top basement surface is onlapped at a low angle by undeformed Mesozoic sandstone, locally containing clasts of footwall rocks. Existing constraints from the neighboring Corsica ophiolites suggest an age of ca. 165-160 Ma for these final stages of exhumation of the Santa Lucia basement. These results imply that middle to lower crustal rocks can be cooled and exhumed rapidly in the last stages of rifting, when significant crustal thinning is accommodated in less than 5 Myr through the consecutive activity of extensional shear zones and detachment faults. High thermal gradients may delay the switch from ductile shear zone- to detachment-dominated crustal thinning, thus preventing the exhumation of middle and lower crustal rocks until the final stages of rifting.

Detailed petrophysical characterization enhances geological mapping of a buried substratum using aeromagnetic and gravity data; application to the southwestern Paris basin

NASA Astrophysics Data System (ADS)

Baptiste, Julien; Martelet, Guillaume; Faure, Michel; Beccaletto, Laurent; Chen, Yan; Reninger, Pierre-Alexandre

2016-04-01

Mapping the geometries (structure and lithology) of a buried basement is a key for targeting resources and for improving the regional geological knowledge. The Paris basin is a Mesozoic to Cenozoic intraplate basin set up on a Variscan substratum, which crops out in the surrounding massifs. We focus our study on the southwestern part of the Paris basin at its junction with the Aquitaine basin. This Mezo-Cenozoic cover separates the Armorican Massif and the Massif Central which compose of several litho-tectonic units bounded by crustal-scale shear zones. In spite of several lithological and structural correlations between various domains of the two massifs, their geological connection, hidden below the Paris basin sedimentary cover, is still largely debated. Potential field geophysics have proven effective for mapping buried basin/basement interfaces. In order to enhance the cartographic interpretation of these data, we have set up a detailed petrophysical library (field magnetic susceptibility data and density measurements on rock samples) of the Paleozoic rocks outcropping in the Variscan massifs. The combination of aeromagnetic and gravity data supported by the petrophysical signatures and field/borehole geological information, is carried out to propose a new map of the architecture of the Variscan substratum. The new synthetic map of geophysical signature of the Paris basin basement combines: i) the magnetic anomaly reduced to the pole, ii) the vertical gradient of the Bouguer anomaly and iii) the tilt derivative of the magnetic anomaly reduced to the pole. Based on this information, the Eastern extension of the major shear zones below the sedimentary cover is assessed. The petrophysical signatures were classified in three classes of magnetic susceptibility and density: low, intermediate and high. Basic rocks have high magnetization and density values whereas granite, migmatite and orthogneiss show low magnetization and density values, Proterozoic and Paleozoic sediments, micaschists and metagrauwackes have intermediate to low magnetization and density values. Detailed lithological attribution of geophysical anomalies was achieved separately for each geological sub-domain (in between 2 major structures). This methodology will be generalized at the scale of the entire Paris basin in order to propose a tectonic reconstruction of this segment of the Variscan belt, and provide guides for the exploration of hidden resources.

The Fiber Grating Sensors Applied in the Deformation Measurement of Shipborne Antenna Basement

NASA Astrophysics Data System (ADS)

Liu, Yong; Chen, Jiahong; Zhao, Wenhua

2016-02-01

The optical fiber grating sensor is a novel fibre-optical passive device, its reflecting optical spectrum is linearly related with strain. It is broadly applied in the structural monitoring industry. Shipborne antenna basement is the basic supporting structure for the radar tracking movement. The bending deformation of the basement caused by ship attitude changing influences the antenna tracking precision, According to the structure of shipborne antenna basement, a distributed strain testing method based on the fibre grating sensor is approved to measure the bending deformation under the bending force. The strain-angle model is built. The regularity of the strain distribution is obtained. The finite element method is used to analyze the deformation of the antenna basement. The measuring experiment on the contractible basement mould is carried out to verify the availability of the method. The result of the experiment proves that the model is effective to apply in the deformation measurement. It provides an optimized method for the distribution of the fiber grating sensor in the actual measuring process.

Geology of the north end of the Ruby Range, southwestern Montana

USGS Publications Warehouse

Tysdal, Russell G.

1970-01-01

This study consists of two parts: stratigraphy and sedimentation, and structure of rocks in the northern one-third of the Ruby Range of southwestern Montana. Detailed studies of Cambrian marine dolomite rocks in the Red Lion Formation and in the upper part of the Pilgrim Limestone resulted in their division into distinct rock units, termed lithofacies. These lithofacies contain features suggestive of subtidal, intertidal, and supratidal environments similar to those presently forming in the Persian Gulf. Stromatolltic structures occurring in the uppermost part of the Red Lion Formation are similar to those presently forming in Shark Bay, Australia. The Ruby Range within the map area is broken into a series of northwest-plunging basement (Precambrian metamorphic rock) blocks, differentially uplifted during the Cretaceous-Tertiary orogenic period. These blocks are bordered by upthrust faults, which are nearly vertical in their lower segments and are .low-angle in their uppermost parts. Asymmetrical folds in Paleozoic sedimentary rocks formed in response to the differential uplift of the blocks; thus they too plunge to the northwest. Displaced masses of rock border the range on the three sides within the map area and are interpreted as gravity-slide features resulting from uplift of the range. Normal faulting began blocking out the present range margins by Oligocene time.

Geochemical and Nd isotopic constraints for the origin of Late Archean turbidites from the Yellowknife area, Northwest Territories, Canada

NASA Astrophysics Data System (ADS)

Yamashita, Katsuyuki; Creaser, Robert A.

1999-10-01

A detailed geochemical and isotopic study of Late Archean turbidites and volcanic rocks from the Yellowknife area, Slave province, was undertaken to constrain the nature of exposed crust at the time of 2.6 to 2.7 Ga crustal consolidation. The ɛNdT values of the volcanic rocks range from +1.7 to -4.4. This variation can be produced by assimilation of pre-2.8 Ga basement by a depleted mantle-derived magma, possibly followed by fractional crystallization. The turbidites are typically metamorphosed to greenschist to amphibolite facies, and where metamorphosed to greenschist facies, different units of Bouma sequence can be observed. The different units of Bouma sequence were sampled and analyzed separately to evaluate the possible differences in geochemical and isotopic signatures. The geochemical data presented here is in accord with the previously proposed model that argues for a mixture of 20% mafic-intermediate volcanic rocks, +55% felsic volcanic rocks, and +25% granitic rocks as a source of these turbidites. However, our revised calculation with the new data presented here argues for 1 to 2% input from an ultramafic source, as well as somewhat higher input from mafic-intermediate volcanic sources in the upper shale units compared to the lower sand units. The ɛNdT values of the turbidites generally are lower in the upper shale units compared to the lower sand units. Detailed inspection of trace-element data suggest that this is not an artifact of rare earth element-rich heavy minerals concentrating in the lower sand units of the turbidites, but rather is a result of “unmixing” of detritus with different ɛNdT values during sediment transportation and deposition. The upper shale units of the turbidites are isotopically compatible with a derivation mainly from crustally contaminated volcanic rocks, similar to those exposed in the Yellowknife area. The lower sand units contain a higher proportion of westerly derived plutonic rock detritus, characterized by higher ɛNdT, suggesting that there are area(s) west of Yellowknife not underlain by older (2.8-4.0 Ga) basement. The trace-element characteristics of these turbidites (i.e., Cr, Ni, La, Th, Sc, Eu/Eu∗, and GdN/YbN) are distinct from those of typical post-Archean turbidites. This observation is consistent with the models that predict that the chemical composition of the upper continental crust was slightly different in the Archean compared to post-Archean time.

Early Pan-African evolution of the basement around Elat, Israel, and the Sinai Peninsula revealed by single-zircon evaporation dating, and implications for crustal accretion rates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kroener, A.; Eyal, M.; Eyal, Y.

1990-06-01

The authors report {sup 207}Pb/{sup 206}Pb single-zircon evaporation ages for early Pan-African rocks from southern Israel and the northeastern Sinai Peninsula, the northernmost extension of the Arabian-Nubian shield. The oldest rocks are metamorphic schists of presumed island-arc derivation; detrital zircons date the source terrain at ca. 800-820 Ma. A major phase of tonalite-trondhjemite plutonism occurred at ca. 760-780 Ma; more evolved granitic rocks were emplaced at about 745 Ma. A metagabbro-metadiorite complex reflects the youngest igneous phase at ca. 640 Ma. We find no evidence for pre-Pan-African crust, and our data document important crust-forming events that correlate with similar episodesmore » elsewhere in the shield. The widespread presence of early Pan-African juvenile rocks (i.e., ca. 760-850 Ma) in many parts of the Arabian-Nubian shield makes this period the most important in the magmatic history of the shield and supports earlier suggestions for unusually high crust-production rates.« less

Numerical Simulations of Thermo-Mechanical Processes during Thermal Spallation Drilling for Geothermal Reservoirs

NASA Astrophysics Data System (ADS)

Vogler, D.; Walsh, S. D. C.; Rudolf von Rohr, P.; Saar, M. O.

2017-12-01

Drilling expenses constitute a significant share of the upfront capital costs and thereby the associated risks of geothermal energy production. This is especially true for deep boreholes, as drilling costs per meter increase significantly with depth. Thermal spallation drilling is a relatively new drilling technique, particularly suited to the hard crystalline (e.g., basement) rocks in which many deep geothermal resources are located. The method uses a hot jet-flame to rapidly heat the rock surface, which leads to large temperature gradients in the rock. These temperature gradients cause localized thermal stresses that, in combination with the in situ stress field, lead to the formation and ejection of spalls. These spalls are then transported out of the borehole with the drilling mud. Thermal spallation not only in principle enables much faster rates of penetration than traditional rotary drilling, but is also contact-less, which significantly reduces the long tripping times associated with conventional rotary head drilling. We present numerical simulations investigating the influence of rock heterogeneities on the thermal spallation process. Special emphasis is put on different mineral compositions, stress regimes, and heat sources.

A possible source of water in seismogenic subduction zones

NASA Astrophysics Data System (ADS)

Kameda, J.; Yamaguchi, A.; Kimura, G.; Iodp Exp. 322 Scientists

2010-12-01

Recent works on the subduction megathrusts have emphasized the mechanical function of fluids contributing dynamic slip-weakening. Basalt-hosting fault zones in on-land accretionary complexes present several textures of seismic slip under fluid-assisted condition such as implosion breccia with carbonate matrix and decrepitation of fluid inclusion. In order to clarify initiation and evolution processes of such fault zones as well as possible source of fluid in the seismogenic subduction zone, we examined a mineralogical/geochemical feature of basaltic basement recovered by IODP Exp. 322 at C0012, that is a reference site for subduction input in the Nankai Trough. A total of 10 samples (about 4 m depth interval from the basement top) were analyzed in this study. XRD analyses indicate that all of the samples contain considerable amount of smectite. The smectite does not appear as a form of interstratified phase with illite or chlorite. Preliminary chemical analyses by EDS in TEM suggest that the smectite is trioctahedral saponite with Ca as a dominant interlayer cation. To determine the saponite content quantitatively, cation exchange capacity (CEC) of bulk samples was measured. The samples show almost similar CEC of around 30 meq/100g, implying that bulk rock contains about 30 wt% of saponite, considering a general CEC of 100 meq/100g for monomineralic saponite. Such abundance of saponite might be a result from intense alteration of oceanic crust due to sea water circulation at low temperature. Previous experimental work suggests that saponite might be highly hydrated (two to three water layer hydration form) at the seismogenic P-T condition. Hence, altered upper oceanic crust is a possible water sink in the seismogenic zone. The water stored in the smectite interlayer region will be expelled via smectite to chlorite transition reaction, that might contribute to the dynamic weakening of the seimogenic plate boundary between the basement basalt and overlying accretionary prism.

Investigations related to scientific deep drilling to study reservoir-triggered earthquakes at Koyna, India

NASA Astrophysics Data System (ADS)

Gupta, Harsh; Purnachandra Rao, N.; Roy, Sukanta; Arora, Kusumita; Tiwari, V. M.; Patro, Prasanta K.; Satyanarayana, H. V. S.; Shashidhar, D.; Mallika, K.; Akkiraju, Vyasulu V.; Goswami, Deepjyoti; Vyas, Digant; Ravi, G.; Srinivas, K. N. S. S. S.; Srihari, M.; Mishra, S.; Dubey, C. P.; Raju, D. Ch. V.; Borah, Ujjal; Chinna Reddy, K.; Babu, Narendra; Rohilla, Sunil; Dhar, Upasana; Sen, Mrinal; Bhaskar Rao, Y. J.; Bansal, B. K.; Nayak, Shailesh

2015-09-01

Artificial water reservoir-triggered earthquakes have continued at Koyna in the Deccan Traps province, India, since the impoundment of the Shivaji Sagar reservoir in 1962. Existing models, to comprehend the genesis of triggered earthquakes, suffer from lack of observations in the near field. To investigate further, scientific deep drilling and setting up a fault zone observatory at depth of 5-7 km is planned in the Koyna area. Prior to undertaking deep drilling, an exploratory phase of investigations has been launched to constrain subsurface geology, structure and heat flow regime in the area that provide critical inputs for the design of the deep borehole observatory. Two core boreholes drilled to depths of 1,522 and 1,196 m have penetrated the Deccan Traps and sampled the granitic basement in the region for the first time. Studies on cores provide new and direct information regarding the thickness of the Deccan Traps, the absence of infra-Trappean sediments and the nature of the underlying basement rocks. Temperatures estimated at a depth of 6 km in the area, made on the basis of heat flow and thermal properties data sets, do not exceed 150 °C. Low-elevation airborne gravity gradient and magnetic data sets covering 5,012 line km, together with high-quality magnetotelluric data at 100 stations, provide both regional information about the thickness of the Deccan Traps and the occurrence of localized density heterogeneities and anomalous conductive zones in the vicinity of the hypocentral zone. Acquisition of airborne LiDAR data to obtain a high-resolution topographic model of the region has been completed over an area of 1,064 km2 centred on the Koyna seismic zone. Seismometers have been deployed in the granitic basement inside two boreholes and are planned in another set of six boreholes to obtain accurate hypocentral locations and constrain the disposition of fault zones.

Role of pre-existing structures in controlling the Cenozoic tectonic evolution of the eastern Tibetan plateau: New insights from analogue experiments

NASA Astrophysics Data System (ADS)

Sun, Ming; Yin, An; Yan, Danping; Ren, Hongyu; Mu, Hongxu; Zhu, Lutao; Qiu, Liang

2018-06-01

Pre-existing weakness due to repeated tectonic, metamorphic, and magmatic events is a fundamental feature of the continental lithosphere on Earth. Because of this, continental deformation results from a combined effect of boundary conditions imposed by plate tectonic processes and heterogeneous and anisotropic mechanical strength inherited from protracted continental evolution. In this study, we assess how this interaction may have controlled the Cenozoic evolution of the eastern Tibetan plateau during the India-Asia collision. Specifically, we use analogue models to evaluate how the pre-Cenozoic structures may have controlled the location, orientation, and kinematics of the northwest-striking Xianshuihe and northeast-striking Longmen Shan fault zones, the two most dominant Cenozoic structures in eastern Tibet. Our best model indicates that the correct location, trend, and kinematics of the two fault systems can only be generated and maintained if the following conditions are met: (1) the northern part of the Songpan-Ganzi terrane in eastern Tibet has a strong basement whereas its southern part has a weak basement, (2) the northern strong basement consists of two pieces bounded by a crustal-scale weak zone that is expressed by the Triassic development of a northwest-trending antiform exposing middle and lower crustal rocks, and (3) the region was under persistent northeast-southwest compression since ∼35 Ma. Our model makes correct prediction on the sequence of deformation in eastern Tibet; the Longmen Shan right-slip transpressional zone was initiated first as an instantaneous response to the northeast-southwest compression, which is followed by the formation of the Xianshuihe fault about a half way after the exertion of northeast-southwest shortening in the model. The success of our model highlights the importance of pre-existing weakness, a key factor that has been largely neglected in the current geodynamic models of continental deformation.

Reduction of nitrogen compounds in oceanic basement and its implications for HCN formation and abiotic organic synthesis

PubMed Central

2009-01-01

Hydrogen cyanide is an excellent organic reagent and is central to most of the reaction pathways leading to abiotic formation of simple organic compounds containing nitrogen, such as amino acids, purines and pyrimidines. Reduced carbon and nitrogen precursor compounds for the synthesis of HCN may be formed under off-axis hydrothermal conditions in oceanic lithosphere in the presence of native Fe and Ni and are adsorbed on authigenic layer silicates and zeolites. The native metals as well as the molecular hydrogen reducing CO2 to CO/CH4 and NO3-/NO2- to NH3/NH4+ are a result of serpentinization of mafic rocks. Oceanic plates are conveyor belts of reduced carbon and nitrogen compounds from the off-axis hydrothermal environments to the subduction zones, where compaction, dehydration, desiccation and diagenetic reactions affect the organic precursors. CO/CH4 and NH3/NH4+ in fluids distilled out of layer silicates and zeolites in the subducting plate at an early stage of subduction will react upon heating and form HCN, which is then available for further organic reactions to, for instance, carbohydrates, nucleosides or even nucleotides, under alkaline conditions in hydrated mantle rocks of the overriding plate. Convergent margins in the initial phase of subduction must, therefore, be considered the most potent sites for prebiotic reactions on Earth. This means that origin of life processes are, perhaps, only possible on planets where some kind of plate tectonics occur. PMID:19849830

Reduction of nitrogen compounds in oceanic basement and its implications for HCN formation and abiotic organic synthesis.

PubMed

Holm, Nils G; Neubeck, Anna

2009-10-22

Hydrogen cyanide is an excellent organic reagent and is central to most of the reaction pathways leading to abiotic formation of simple organic compounds containing nitrogen, such as amino acids, purines and pyrimidines. Reduced carbon and nitrogen precursor compounds for the synthesis of HCN may be formed under off-axis hydrothermal conditions in oceanic lithosphere in the presence of native Fe and Ni and are adsorbed on authigenic layer silicates and zeolites. The native metals as well as the molecular hydrogen reducing CO2 to CO/CH4 and NO3-/NO2- to NH3/NH4+ are a result of serpentinization of mafic rocks. Oceanic plates are conveyor belts of reduced carbon and nitrogen compounds from the off-axis hydrothermal environments to the subduction zones, where compaction, dehydration, desiccation and diagenetic reactions affect the organic precursors. CO/CH4 and NH3/NH4+ in fluids distilled out of layer silicates and zeolites in the subducting plate at an early stage of subduction will react upon heating and form HCN, which is then available for further organic reactions to, for instance, carbohydrates, nucleosides or even nucleotides, under alkaline conditions in hydrated mantle rocks of the overriding plate. Convergent margins in the initial phase of subduction must, therefore, be considered the most potent sites for prebiotic reactions on Earth. This means that origin of life processes are, perhaps, only possible on planets where some kind of plate tectonics occur.

Meso-Cenozoic evolution of the Tuareg Shield (Algeria, Sahara): insights from new thermochronological data

NASA Astrophysics Data System (ADS)

Rougier, Sylvain; Missenard, Yves; Gautheron, Cécile; Barbarand, Jocelyn; Zeyen, Hermann; Pinna, Rosella; Liégeois, Jean-Paul; Bonin, Bernard; Ouabadi, Aziouz; El-Messaoud Derder, Mohammed; Frizon de Lamotte, Dominique; Kettouche, Djouher

2013-04-01

In North Africa, Meso-Cenozoic large scale topographic swells, such as Hoggar, Tibesti or Darfur domes, are superimposed to a Paleozoic arch and basin morphology which characterizes this region. Although these topographic highs are associated to Cenozoic intraplate volcanism, their development remains poorly constrained, both from temporal and spatial points of view. This study is focused on the Tuareg Shield bulge, a topographic high where Precambrian rocks, exposed over 500000 km², can reach 2400 m above sea level (Atakor district, Hoggar, South Algeria). While presumed Cretaceous sedimentary remnants, resting unconformably over the basement, suggest a possible stage of weak topography during the Mesozoic, current high topography is emphasized by <35 Ma volcanic formations, mostly basaltic in composition. In this context, we present first apatite (U-Th)/He thermochronological data acquired across the whole swell (Rougier et al., Geology, in press). Mean ages range from 78 ± 22 Ma to 13 ± 3 Ma. These results demonstrate the existence of a widespread Eocene exhumation of the shield before volcanic activity began. In the northeastern part of the swell, Cretaceous sedimentary remnants unconformably lying on the basement close to our samples evidence that they were near the surface at that time. We show that basement rocks have thus suffered a subsequent heating stage at 60-80 °C. We also present new apatite fission track ages on same samples. Central ages range from 71 ± 6 to 285 ± 29 Ma. When track length measurements were possible, preliminary modelings of the time-temperature history were performed. As previously deduced from apatite (U-Th)/He analyzes, these modelings show that samples underwent a heating to at least 80°C before their Late Eocene exhumation. Moreover, they also indicate that samples underwent another cooling stage during Lower Cretaceous, prior to Upper Cretaceous/Paleogene heating. We interpret these results as an evidence of a large-scale subsidence stage after the Cretaceous and until the Eocene, which allowed the deposition of a 1.5 to 3 km thick sedimentary cover and a heating at ~80°C of the currently outcropping basement. During the Eocene, the establishment of a thermal anomaly beneath the Tuareg Shield lithosphere resulted in erosion of the major part of this cover and, since 35 Ma, the development of intraplate volcanism.

Polyphase tectono-magmatic and fluid history related to mantle exhumation in an ultra-distal rift domain: example of the fossil Platta domain, SE Switzerland

NASA Astrophysics Data System (ADS)

Epin, Marie-Eva; Manatschal, Gianreto; Amann, Méderic; Lescanne, Marc

2017-04-01

Despite the fact that many studies have investigated mantle exhumation at magma-poor rifted margins, there are still numerous questions concerning the 3D architecture, magmatic, fluid and thermal evolution of these ultra-distal domains that remain unexplained. Indeed, it has been observed in seismic data from ultra-distal magma-poor rifted margins that top basement is heavily structured and complex, however, the processes controlling the morpho-tectonic and magmatic evolution of these domains remain unknown. The aim of this study is to describe the 3D top basement morphology of an exhumed mantle domain, exposed over 200 km2 in the fossil Platta domain in SE Switzerland, and to define the timing and processes controlling its evolution. The examined Platta nappe corresponds to a remnant of the former ultra-distal Adriatic margin of the Alpine Tethys. The rift-structures are relatively well preserved due to the weak Alpine tectonic and metamorphic overprint during the emplacement in the Alpine nappe stack. Detailed mapping of parts of the Platta nappe enabled us to document the top basement architecture of an exhumed mantle domain and to investigate its link to later, rift/oceanic structures, magmatic additions and fluids. Our observations show a polyphase and/or complex: 1) deformation history associated with mantle exhumation along low-angle exhumation faults overprinted by later high-angle normal faults, 2) top basement morphology capped by magmato-sedimentary rocks, 3) tectono-magmatic evolution that includes gabbros, emplaced at deeper levels and subsequently exhumed and overlain by younger extrusive magmatic additions, and 4) fluid history including serpentinization, calcification, hydrothermal vent, rodingitization and spilitization affecting exhumed mantle and associated magmatic rocks. The overall observations provide important information on the temporal and spatial evolution of the tectonic, magmatic and fluid systems controlling the formation of ultra-distal magma-poor rifted margins as well as the processes controlling lithospheric breakup. In this context, our field observations can help to better understand the tectono-magmatic processes associated to these, not yet drilled domains that may form in young, narrow rifted margins (e.g. Red Sea, Gulf of Aden) or may represent the Ocean-Continent Transition in more mature, magma-poor Atlantic type systems.

New Zircon U-Pb Age Constrain of the Origin of Devil's River Uplift (SW Texas) and Insights into the Late Proterozoic and Paleozoic Evolution of the Southern Margin of Laurentia

NASA Astrophysics Data System (ADS)

Rodriguez, E.; Dickerson, P. W.; Stockli, D. F.

2017-12-01

The Devils River Uplift (DRU) in SW Texas records the evolution of the southern Laurentian margin from Grenvillian orogenesis and assembly of Rodinia, to its fragmentation by rifting, and to the amalgamation of Pangaea. It was cored by a well (Shell No. 1 Stewart), penetrating Precambrian gneisses and Cambrian metasediments and sandstones. New zircon LA-ICP-MS data from a total of 10 samples elucidate the crystallization and depositional ages, as well as the detrital provenance, of Precambrian and Cambrian rocks from the DRU. Zircons from five Precambrian crystalline basement samples (6000-9693') yield uniform U-Pb crystallization ages of 1230 Ma that are similar to ages for young gneisses of the Valley Spring Domain (Llano uplift) in central Texas, where they mark the cessation of arc magmatism within the Grenville orogenic belt. The 1230 Ma igneous basement is overlain by L.-M. Cambrian metasedimentary rocks ( 4000-6000') with maximum depositional ages of 533-545 Ma. Detrital zircons from Cambrian strata are dominated by a 1070-1080 Ma population, likely derived from basement units exposed in Texas (Llano uplift, Franklin Mts.), with minor contributions from local 1230 Ma Precambrian basement and the 1380-1500 Ma Granite Rhyolite Province. The L.-M. Cambrian interval is dominated (>80%) by Neoproterozoic detrital magmatic zircons with two major distinct age clusters at 570-700 Ma and 780-820 Ma, supporting a two-stage Rodinia rift model and providing strong evidence for major Cryogenian-Eocambrian intraplate magmatism along the southern margin of Rodinia. Moreover, detrital zircon signatures for L.-M. and U. Cambrian strata strongly correlate with those from the Cuyania terrane of W. Argentina - notably the W. Sierras Pampeanas (Sa. Pie de Palo, Sa. de Maz): 1230 Ma from metasandstones (PdP); 1081-1038 Ma from metasiliciclastics (PdP, SdM); Cryogenian-Eocambrian [774 & 570 Ma] plutons (SdM, PdP). In summary, these new zircon U-Pb data from DRU in SW Texas show that it is part of the Grenville orogenic belt, characterized by 1230 Ma magmatism, and that it experienced Cryogenian-Eocambrian intraplate magmatism as well. Significant correlations between DRU and the Cuyania terrane imply that both participated in Rodinia rifting and creation of the southern Laurentian margin.

Subduction of Young Oceanic Lithosphere and Extensional Orogeny in Southwestern North America during Mid-Tertiary Time

NASA Astrophysics Data System (ADS)

Elston, Wolfgang E.

1984-04-01

An "extensional orogeny" deformed the Basin and Range province, probably beginning in the late Eocene (about 40 ± 3 Ma). Its characteristics include partial melting of the continental lithosphere during the "ignimbrite flareup," massive ductile extension (including detachment faulting), and rise of metamorphic core complexes. The affected zone became about 1200 km wide, possibly double its original width. It rose an average of 1-2 km, despite crustal thinning. Locally, some of the highest mountains of North America, up to 4.3 km high, rose through resurgence of ignimbrite cauldrons and isostatic uplift of underlying plutons. The climax of extension occurred prior to the development of the present basin and range topography. Modeling of major and trace elements and Sr and Pb isotopes strongly suggests that mid-Tertiary volcanic magmas equilibrated, and probably originated, in the continental lithosphere. Components attributable to subducted oceanic lithosphere have not yet been identified. The rocks seem to belong to two provinces, separated by the quartz diorite boundary line of Moore (1959), which also marks the western limit of North America at the end of the late Paleozoic Sonoman orogeny. To the west, low-K rocks rest on a basement of predominantly oceanic accreted terranes; to the east, high-K rocks rest on an autochthonous sialic basement. Within the high-K province, potassium variations can be correlated with crustal thickness; there is no need to invoke a K-h relationship. Conventional models of plate convergence and back arc extension which involve subduction of old, rigid, cool, and dense oceanic lithosphere may not apply to the mid-Tertiary Basin and Range province. The overridden Farallon plate is more likely to have been young, hot, ductile, buoyant, and no denser than continental asthenosphere, having been generated in a spreading center close to North America. Under these conditions, motion of the subducting plate slows and slab-pull is likely to approach zero. Even prior to ridge-trench collision, overridden oceanic lithosphere may have become underplated beneath the continental lithosphere and ruptured by rising mantle diapirs. Subducted oceanic lithosphere no longer acted as a heat sink, which could partly account for the great width of the affected zone and the anomalous thermal gradients required for partial melting, extension, and metamorphism. Had these processes not died down, after ridge-trench collision, the western segment of the Cordillera might have separated from North America to form a Japanlike archipelago, while the Basin and Range province foundered into an analog to the Sea of Japan. Instead of rupturing completely, the Basin and Range province fractured into fault blocks.

EBSD characterization of pre-Cambrian deformations in conglomerate pebbles (Sierra de la Demanda, Northern Spain)

NASA Astrophysics Data System (ADS)

Puelles, Pablo; Ábalos, Benito; Fernández-Armas, Sergio

2010-05-01

Pre-Cambrian and unconformable earliest Cambrian rocks from the Sierra de la Demanda (N Spain) exhibit field and microstructural relationships that attest to orogenic events recorded by concealed basement rocks. Neoproterozoic foliated slates ("Anguiano Schists") crop out under up to 300 m thick, unfoliated quartz-rich conglomerates ("Anguiano Conglomerates") and quartzites which are stratigraphically ca. 600 m below the oldest, paleontologically dated, pre-trilobitic Cambrian layers (likely older than 520 Ma). The Anguiano Conglomerates contain mm to cm grainsized well-rounded pebbles of various types including monocrystalline quartz, detrital zircon and tourmaline-bearing sandstones, black cherts and metamorphic poly-crystalline quartz aggregates. The undeformed matrix is made of much smaller (diagenetically overgrown) monocrystaline quartz grains and minor amounts of accesory zircon, tourmaline and mica. Black chert pebbles exhibit microstructural evidence of brittle deformation (microfaults and thin veins of syntaxial fibrous quartz). These and the fine-grained sandstone pebbles can also exhibit ductile deformations (microfolds with thickened hinges and axial planar continuous foliations), too. Polycrystalline quartz pebbles exhibit a variety of microstructures that resulted from syn-metamorphic ductile deformations. These are recognisable under the petrographic microscope and include continuous foliations, quartz shape fabrics, various types of subgrain or recrystallized new grain microtextures, and lattice preferred orientations (LPOs). Conventional characterization of quartz fabrics (after oriented structural sections) is challenged in conglomerate pebble thin sections by the difficulty of unraveling in them the complete structural reference framework provided by foliation (whose trace can be unraveled) and lineation orientation (which cannot be directly identified). Quartz in various metamorphic polycrystalline pebbles was studied with the Electron Back-Scatter Diffraction (EBSD) technique. The identification of quartz c-axis point maxima or girdles and their geometrical relationships with respect to -axis arrangements and pebble foliation traces enabled us to identify the operation of basal and prism- and occasionally prism-[c] intracrystalline slip systems. This points to upper-greenschists and amphibolite facies syn-metamorphic deformations. By contrast, black chert and sandstone pebbles and matrix quartz aggregates lack any LPO. The source area of the conglomerates was likely a pre-Cambrian basement that contained penetratively deformed low- to medium-grade metamorphic rocks. Radiometric dating of this metamorphism has not been accomplished so far though it is known that inherited Precambrian sources in the Iberian Peninsula relate notably to Neoproterozoic (Pan-African and Cadomian) orogens, and to a lesser extent to Paleoproterozoic (1.8-2.1 Ga) or Neoarchean (2.4-2.8 Ga) ones. Neoproterozoic (Cadomian) metamorphism of this grade has only been recognized in SW Iberia. If the fabrics here studied were Cadomian, they might be related to the arc-related igneous suites that have been detected or inferred in other realms of the northern Iberian Massif.

Analysis of photo linear elements, Laramie Mountains, Wyoming

NASA Technical Reports Server (NTRS)

Blackstone, D. L., Jr.

1973-01-01

The author has identified the following significant results. Photo linear features in the Precambrian rocks of the Laramie Mountains are delineated, and the azimuths plotted on rose diagrams. Three strike directions are dominant, two of which are in the northeast quadrant. Laramide folds in the Laramie basin to the west of the mountains appear to have the same trend, and apparently have been controlled by response of the basement along fractures such as have been measured from the imagery.