Showing papers in "Geology in 1976"
TL;DR: A detailed study of the spatial distribution of precisely located hypocenters of South American earthquakes that occurred between lat 0° and 45°S shows that the data can be explained by the simple model of a descending oceanic plate beneath a continental plate.
Abstract: A detailed study of the spatial distribution of precisely located hypocenters of South American earthquakes that occurred between lat 0° and 45°S shows that the data can be explained by the simple model of a descending oceanic plate beneath a continental plate and that the following conditions obtain: (1) The hypocenters clearly define five segments of inclined seismic zones, in each of which the zones have relatively uniform dips. The segments beneath northern and central Peru (about lat 2° to 15° S) and beneath central Chile (about lat 27° to 33° S) have very small dips (about 10°), whereas the three segments beneath southern Ecuador (about lat 0° to 2°S), beneath southern Peru and northern Chile (about lat 15° to 27°S), and beneath southern Chile (about lat 33° to 45°S) have steeper dips (25° to 30°). No clear evidence exists for further segmentation of the descending Nazca plate beneath South America. If the two flat segments are in contact with the lower boundary of the continental plate, the thickness of that plate is less than approximately 130 km. This is in marked contrast to the reports of thicknesses exceeding 300 km for the South American continental plate. (2) There is considerable seismic activity within the upper 50 km of the overriding South American plate. This seismic activity is well separated from the inclined seismic zones and probably occurs in the crustal part of the South American plate. Thus, hypocenters in South America are not evenly distributed through about a 300-km-thick zone as previously described. (3) A remarkable correlation exists between the two flat segments of the subducted Nazca plate and the absence of Quaternary volcanism on the South American plate. (4) The transition from the flat Peru segment to the steeper Chile segment is abrupt and is interpreted as a tear in the descending Nazca plate. The tear is located approximately beneath the northern limit of the Altiplano (a high plateau in the Andes), and about 200 km south of the projection of the oceanic Nazca ridge down the subduction zone. (5) A gap in seismic activity exists between depths of 320 and 525 km.
826 citations
TL;DR: A model for the origin of the andesite-free bimodal trondhjemite-basalt suites that are found in lower Archean gray gneiss complexes is based on a 1968 model of Green and Ringwood as mentioned in this paper.
Abstract: Trondhjemitic and tonalitic liquids may form either by igneous differentiation of less silicic, more mafic liquids or by partial melting of rocks of basaltic composition. Low-Al 2 O 3 trondhjemitic-tonalitic liquids (defined as containing less than 15 percent Al 2 O 3 ) have formed in modern plate-tectonic environments by crystal fractionation of low-potassium ande-sitic liquid and in Precambrian environments by the partial melting of amphibolite and hornblende-bearing gabbro, in which process plagioclase is a residual phase and garnet and (or) hornblende are not. High-Al 2 O 3 trondhjemitic-tonalitic liquids (containing 15 percent or more of Al 2 O 3 ) are generated in both old and modern convergent and tensional tectonic environments, either by hornblende-controlled fractionation of hydrous basaltic liquid or by partial melting of metabasaltic rocks, in which process garnet and (or) hornblende are residual. A model for the origin of the andesite-free bimodal trondhjemite-basalt suites that are found in lower Archean gray gneiss complexes is based on a 1968 model of Green and Ringwood; it proposes (1) mantle upwelling and basaltic vol-canism to form a thick pile, (2) metamorphism of the lower parts of this pile to amphibolite, (3) partial melting of the amphibolite to yield trondhjemitic-tonalitic liquids, (4) ascent and extrusion or intrusion of these liquids into the upper crust before the fraction of melting of the parental amphibolite exceeds about 40 percent, (5) transformation of the residue of partial melting to anhydrous, refractory assemblages, and (6) continuation of mantle upwelling and basaltic volcanism as trondhjemitic-tonalitic liquids are being extruded.
619 citations
TL;DR: In this article, the primary sink for anaerobically generated methane in marine sediments is sulfate reduction, not aerobic oxidation, and the uniformly low methane concentrations in marine, anoxic, sulfate-reducing sea water and sediments represents a balance between production by methanogenic bacteria and consumption by sulfate reducing bacteria.
Abstract: The production of methane in anoxic environments can lead to significant accumulations of this gas in appropriate marine sediments. However, the uniformly low methane concentrations in marine, anoxic, sulfate-reducing sea water and sediments represents a balance between production by methanogenic bacteria and consumption by sulfate-reducing bacteria. The primary sink for anaerobically generated methane in marine sediments is sulfate reduction, not aerobic oxidation.
461 citations
TL;DR: X-radiographs of stony coral slabs reveal two types of annual density bands, and detailed studies of these bands in relation to known variations in air temperatures indicate that sclerochronology is a valid tool for documenting time sequences and changing environmental conditions on a coral reef as mentioned in this paper.
Abstract: X-radiographs of stony coral slabs reveal two types of annual density bands. Detailed studies of these bands in relation to known variations in air temperatures indicate that sclerochronology is a valid tool for documenting time sequences and changing environmental conditions on a coral reef.
265 citations
TL;DR: The average denudation rate of the Central Range of Taiwan Island has been increasing since the Pliocene Epoch as discussed by the authors, which is probably the highest known value in the world.
Abstract: The average denudation rate of the Central Range of Taiwan Island has been increasing since the Pliocene Epoch. Today it is at least 1,365 mg/cm 2 yr, which is probably the highest known value in the world.
236 citations
TL;DR: Debris flow deposits generated by a large sediment slide or slides off the Spanish Sahara have been identified on the lower continental rise west of the Canary Islands as discussed by the authors, covering an area of sea floor of about 30,000 km 2.
Abstract: Clearly defined debris flow deposits generated by a large sediment slide or slides off the Spanish Sahara have been identified on the lower continental rise west of the Canary Islands. The debris flow traveled over a slope as low as 0.1° and a distance of several hundred kilometres. The deposits cover an area of sea floor of about 30,000 km 2 . Recognition of the debris flow deposits is based on a characteristic geometry, a distinctive acoustic character, a pebbly mudstone fabric and sharp angular contacts in piston cores, and an undulating surface morphology revealed by bottom photographs. Similar deposits are observed on the Amazon cone and the North American continental rise and in the Gulf of Mexico and the Mediterranean Sea. Debris flows are probably a much more important depositional process on the sea floor than was previously suspected.
236 citations
TL;DR: New hornblende/groundmass partition coefficients for rare earths in a sodic, high-Al 2 O 3 dacite range from 0.9 to 4.5 as mentioned in this paper.
Abstract: New hornblende/groundmass partition coefficients for rare earths in a sodic, high-Al 2 O 3 dacite range from 0.9 for Ce to 4.5 to 6.2 for heavy rare earths, and the pattern is concave down with a distinct negative Eu anomaly. These coefficients indicate that hornblende may have been an important residual phase in the generation of high-Al 2 O 3 , heavy-rare-earth-depleted trondhjemites, tonalites, and equivalent dacites, implying that the magmas formed at a depth of less than 60 k.
204 citations
TL;DR: In this article, various parameters of deep-sea sediments from the western Gulf of Mexico which are generally accepted as indicators of calcium carbonate dissolution were subjected to a multivariate statistical analysis in order to determine the amount of variance in each parameter down core attributable to dissolution.
Abstract: Various parameters of deep-sea sediments from the western Gulf of Mexico which are generally accepted as indicators of calcium carbonate dissolution were subjected to a multivariate statistical analysis in order to determine the amount of variance in each parameter down core attributable to dissolution. Benthonic foraminiferal abundance and foramini-feral test fragmentation proved to be most indicative of dissolution during the late Quaternary. In contrast to earlier studies, calcium carbonate concentration proved to be the least reliable variable. Ecologic and dynamic processes, such as productivity and lateral transport, also have a significant effect on each of the parameters.
165 citations
TL;DR: In orogenic belts, sedimentary rocks that were deposited on a “basement” of melange and are now tectonically enclosed by melange are hypothesized to be ancient slope-basin deposits as mentioned in this paper.
Abstract: In arc systems where an oceanic plate with a thick cover of sediments is subducted, the sediments are scraped off and accreted to the base of the inner trench slope. The accreted sediments form ridges, behind which younger sediments are often ponded in basins. The width of these basins progressively increases from 2 to 3 km at the base of the lower slope to 10 km near the trench slope break. Sediments within the basins increase in thickness from nearly zero in the basins now at greatest depths to several kilometres in the basins on the shallowest part of the slope. Slope basins begin to form at the base of the lower slope, where sediments accumulate between adjacent thrust faults. Addition of more accreted material at the trench causes uplift and rotation of the thrust slices and of overlying slope sediments. As deformation proceeds, motion along some of the thrusts dies out, and the inactive thrusts become buried by slope sediments, thus increasing the size of the slope basins. In orogenic belts, sedimentary rocks that were deposited on a “basement” of melange and are now tectonically enclosed by melange are hypothesized to be ancient slope-basin deposits.
142 citations
TL;DR: In this paper, high pressure experimental melting studies on a pelitic composition with 2 and 5 percent by weight of added water demonstrate that cordierite, quartz, biotite, sillimanite, and plagioclase are important residual phases coexisting with granitic liquid at 4 kb and are joined by garnet at 7 kb.
Abstract: High-pressure experimental melting studies on a pelitic composition with 2 and 5 percent by weight of added water demonstrate that cordierite, quartz, biotite, sillimanite, and plagioclase are important residual phases coexisting with granitic liquid at 4 kb and are joined by garnet at 7 kb. At 10 kb, garnet quartz, biotite, sillimanite, and plagioclase constitute the residuum coexisting with granitic liquid. Compositions of residual phases and their Mg-Fe partition relationships compare closely with phases from cordierite- and garnet-bearing granitic rocks in eastern Australia and strongly support geochemical arguments that this particular granitic suite is derived by partial melting of pelitic sediments. Thus, the cordierite-bearing granitic rocks were probably derived by partial melting of pelite at >25-km depth, whereas the garnet-bearing granites were derived at ≿25-km depth.
140 citations
TL;DR: In this article, the authors show that there are four principal types of levees formed in Etnean lavas: initial, accretionary, rubble, and overflow, and that most levees are hybrids of two or more of the four types.
Abstract: Observations of the 1975 subterminal lava flows and sections through the larger flank lavas on Mount Etna show that there are four principal types of levees formed in Etnean lavas: initial, accretionary, rubble, and overflow. Initial levees are formed because of the yield strength of these non-Newtonian lavas and are thought to determine channel width. The other types of levees are formed subsequently and are built up over the rarely preserved initial levees. Mechanisms of formation of each levee type vary; most levees are hybrids of two or more of the four types. We also discuss the prospects of deducing lava rheology from morphology and the application of this technique to surfaces of other planets.
TL;DR: In this article, a large decrease in the peridotite solidus temperature at a depth of about 90 km, causing melting by intersection with a geotherm, was found in the system CaO-MgO-SiO 2 -CO 2.
Abstract: Recent analyses of fluid inclusions in peridotite minerals suggest that CO 2 is a dominant volatile species in the upper mantle. In a CO 2 -bearing oceanic mantle, the low-velocity zone (LVZ) can be explained by a large decrease in the peridotite solidus temperature at a depth of about 90 km, causing melting by intersection with a geotherm. This decrease in the solidus temperature has been found in the system CaO-MgO-SiO 2 -CO 2 and results from a change in partial melt composition along the solidus from enstatite-normative at pressures less than 26 kb to larnite-normative (melilititic) at greater pressure. Although these liquids dissolve up to 20 wt percent CO 2 , they are silicate liquids containing at least 30 percent SiO 2 . These silica levels are appropriate for kimberlitic liquids, but the liquids are more calcic than typical kimberlites. At depths of less than 90 km in suboceanic mantle, CO 2 may be present in carbonate minerals or in vapor, depending upon the geotherm, but cannot be in solution in silicate peridotite minerals. Beneath continents, CO 2 will be present in carbonate minerals, and the mantle will not melt at least to depths of 120 km.
TL;DR: In this paper, the authors investigate the structural patterns of the late Cenozoic faulting in the Great Basin and delimit two deformational fields, each extensional but contrasting in magnitude and style of extension.
Abstract: The patterns of late Cenozoic faulting in the Great Basin apparently delimit two deformational fields, each extensional but contrasting in magnitude and style of extension. The field of smaller magnitude, which shows about 10 percent extension, occupies the northern and most of the central part of the Great Basin. It is characterized by steeply dipping normal faults and gently tilted blocks, with a preferred north to northeast trend. Evidence of greater extension occurs in the other deformational field, which lies between Walker Lane and the Sierra Nevada and extends across the narrow southern end of the Great Basin. This field contains most of the complementary strike-slip faults (northwest-striking right-lateral and northeast-striking left-lateral faults), long recognized as major components of the structural framework. It also contains abundant normal faults, most of which strike north to northeast. In certain areas, extension of 50 percent or more is indicated in the association of strike-slip and normal faults and in the palinspastic restoration of fault blocks that have been steeply tilted along gently dipping normal faults. These contrasts in structural pattern and apparent percentage of extension may be related to westward movement of the Sierra Nevada block and southward narrowing of the Great Basin. The faults along which strike-slip displacement occurred in late Cenozoic time appear to have functioned as conjugate shears, the shears and associated normal faults being first-order extensional features. The fault pattern also invites a simplistic interpretation that is based on the orientation of three mutually perpendicular directions of stress. The major normal faults, which strike north-northeast in most parts of the Great Basin, suggest a pervasive horizontal minimum compressive (maximum tensional) stress that is oriented west-northwest. Maximum compressive stress would be oriented perpendicularly where the horst and graben structure predominates and horizontally to the east-northeast where the strike-slip faults are abundant.
TL;DR: In this article, hydraulic fracturing induced by thermal expansion of water is invoked as a common phenomenon by which metamorphic fluid is progressively lost from a sediment pile undergoing metamorphism and subsequent orogeny On linear thermal gradients, water loss may occur by this mechanism for all gradients greater than 12°C/km at depths greater than 5 to 10 km.
Abstract: Hydraulic fracturing, induced by thermal expansion of water, is invoked as a common phenomenon by which metamorphic fluid is progressively lost from a sediment pile undergoing metamorphism and subsequent orogeny On linear thermal gradients, water loss may occur by this mechanism for all gradients greater than 12°C/km at depths greater than 5 to 10 km During burial on lower gradients, water is retained in the pile and may cause widespread metasomatism
TL;DR: The concept that North America was once an interior piece of a much larger continent and that the pieces of this fragmented continent drifted away to form new continents was first proposed by as mentioned in this paper.
Abstract: Approximately 850 m.y. ago, the tectonics of North America changed from a pattern of scattered, locally deep epicratonic troughs to a pattern of encircling marginal miogeoclines. This change can be interpreted as indicating the start of rifting that extended almost continuously around the North American craton. This concept seems to require that North America was once an interior piece of a much larger continent and that the pieces of this fragmented continent drifted away to form new continents. A test of this hypothesis requires an analysis of the world-wide distribution of late Precambrian continents, a subject of current controversy and speculation.
TL;DR: In this paper, the authors show that the folding mechanism combined dissolution of limestone along stylolites subparallel to fold axes with deposition of sparry calcite in extension veins normal to the stylolite, showing that the deformation was brittle, as indicated by extension veins and by foraminifera.
Abstract: Mesozoic pelagic limestones in Umbria (northern Italy) were deformed by Miocene-Pliocene flexural slip folding under 1 to 2 km of cover; there are no signs of metamorphism. Under these conditions the deformation was brittle, as indicated by extension veins and by foraminifera that show no measurable distortional strain. Nevertheless, the limestone is often tightly folded; the folding mechanism combined dissolution of limestone along stylolites subparallel to fold axes with deposition of sparry calcite in extension veins normal to the stylolites. Thus, at the scale of a hand specimen, deformation occurred through rigid-body displacements, but e-twinning in the vein calcite shows a minor component of distributed strain. Spaced cleavage on the limbs of major folds also formed by dissolution. This is shown by (1) the presence of insoluble residues on the cleavage surfaces, (2) imbrication of insoluble chert nodules that indicates tens of percent shortening parallel to bedding, (3) deflection of cleavage away from strain shadows protected by chert nodules and toward places where the nodules have been telescoped, and (4) offsets of bedding at bedding-cleavage intersection that resulted from removal of limestone by dissolution along the cleavage surfaces.
TL;DR: In this article, the authors consider the primary cause of the discordant coiling-direction changes between the Atlantic and Indo-Pacific Oceans to be the severance of the tropical marine connection across Central America about 35 my ago and the subsequent development of two independent faunal provinces.
Abstract: Systematic changes with time in the coiling direction of the planktonic fora-miniferal genus Pulleniatina provide useful datum levels for interregional correlation of Pliocene-Quaternary marine sediments The pattern of coiling-direction changes was concordant throughout the world oceans from about 6 to 35 my BP From about 35 to 23 my BP, this genus disappeared from the Atlantic and its adjacent seas After the re-establishment of Pulleniatina in the greater Atlantic about 23 my ago, the Atlantic coiling patterns were no longer correlative with the Indo-Pacific patterns I consider the primary cause of the discordant coiling-direction patterns between the Atlantic and Indo-Pacific Oceans to be the severance of the tropical marine connection across Central America about 35 my ago and the subsequent development of two independent faunal provinces
TL;DR: A sequence of intensely sheared, polyphasally deformed turbidites and pillow lava rocks of possible late Paleozoic or Triassic age, exposed along western Alexander Island, is tentatively interpreted as a trench assemblage accreted during an early stage of subduction as discussed by the authors.
Abstract: Three main Upper Jurassic–Cretaceous tectono-stratigraphic units recognized in Palmer Land and Alexander Island are comparable with assemblages formed (1) in an ensialic volcanic arc (Upper Jurassic Volcanic Group), (2) in a forearc or intra-arc marine basin (Fossil Bluff Formation), and (3) in a back-arc marine basin (Crabeater Point sediments and Latady Formation), possibly representing a proto–Weddell Sea. A sequence of intensely sheared, polyphasally deformed turbidites and pillow lava rocks of possible late Paleozoic or Triassic age, exposed along western Alexander Island, is tentatively interpreted as a trench assemblage accreted during an early stage of subduction. There is a close correlation between these units and the approximately coeval island-arc-marginal basin system of the southern Andes and South Georgia.
TL;DR: The collision of irregular continental margins during an Alpine-type orogeny may give rise to a series of deformational events in the forelands, the tectonic style of which may be critically dependent on complete or incomplete suturing of the two colliding continents as mentioned in this paper.
Abstract: The collision of irregular continental margins during an Alpine-type orogeny may give rise to a series of deformational events in the forelands, the tectonic style of which may be critically dependent on complete or incomplete suturing of the two colliding continents. Dilatational structures such as mafic dikes, gravity faults, and grabens may form at those points where the continents first meet. Later, as the suture lengthens, large-scale wrench faults develop in the foreland area. When the compression caused by the orogen ceases, these foreland structures become inactive. The large-scale grabens and wrench faults observed in the forelands of the Alpine and northwestern European Variscide belts may be examples of such collision-related foreland structures. If the model presented in this paper is approximately correct, then it may be used to connect some intraplate phenomena to interplate tectonism.
TL;DR: The authors reconstructs the Pangea during late Paleozoic-early Mesozoic time by comparing the geologic and tectonic features of the segments bordering the Gulf of Mexico with a newly proposed alternative fit of North America, Europe, and Gondwanaland.
Abstract: Previously published reconstructions of Pangea during late Paleozoic-early Mesozoic time have suggested that (1) the Gulf of Mexico has existed as an oceanic basin from at least the late Carboniferous onward or (2) the Gulf of Mexico was created since Permian time by a process of microplate reorganization, macroplate drift, or oceanization (sensu stricto). On the basis of paleomagnetic data, we favor macroplate drift. Major geologic and tectonic features of the Pangea segments bordering the Gulf of Mexico are in agreement with a newly proposed alternative fit of North America, Europe, and Gondwanaland for late Paleozoic time. This reconstruction closes the Gulf of Mexico by juxtaposition of North and South America.
TL;DR: In this paper, the extinction level of Stylatractus universus was compared with the δO 18 record in widespread high and low-latitude deep-sea cores, and it was shown that this level is globally synchronous.
Abstract: Through comparison of the extinction level of Stylatractus universus with the δO 18 record in widespread high- and low-latitude deep-sea cores, we show that this level is globally synchronous. It occurs in the lower one-third of the isotopic transition between isotope Stages 12 and 11. Several independent estimates of the date of this extinction converge on 410,000 ± 5,000 yr ago. This biostratigraphic datum is now well dated and is globally synchronous.
TL;DR: Botryoidal aragonite commonly rims and fills voids in Holocene reef-wall limestone of the seaward-facing margins of Belize (British Honduras) barrier and atoll reefs as mentioned in this paper.
Abstract: Botryoidal aragonite commonly rims and fills voids in Holocene reef-wall limestone of the seaward-facing margins of Belize (British Honduras) barrier and atoll reefs. The cavernous, well-cemented reef-wall limestone that occurs between 65 and 120 m on the barrier reef consists of reef corals encased in algal-plate ( Halimeda ) packstone-wackestone; the botryoidal aragonite occurs in cylindrical bivalve borings, in polygonal shelter cavities, and in laminar to amoeboid voids of unknown origin. Mamelons of the fibrous aragonite as much as 5 cm thick often rim void ceilings, occasionally grow from void floors, and frequently fill voids completely. In other cavities the mamelons overlie floors of cemented internal sediment, and in still others they have laminations of internal sediment. The aragonite is clearly marine: (1) it occurs within an unaltered reef limestone less than 13,000 yr old; (2) benthic fora-minifera occur locally within mamelons, and in others there are laminations of marine sediment; and (3) the aragonite has an average Sr 2+ content of 8,300 ppm, an 0 18 average of +1.0 0/00 , and a C 13 average of +4.2 00/0 , all characteristic of marine precipitation. The mamelons of botryoidal aragonite are similar in shape, texture, and fabric to some of the common cavity-filling cements in Paleozoic reef limestones.
TL;DR: The polarity sequence of Heirtzler and his co-workers for Neogene time is modified by compromise solutions between the polarity record in deep-sea sediments and oceanic magnetic anomalies.
Abstract: The polarity sequence of Heirtzler and his co-workers for Neogene time is modified by compromise solutions between the polarity record in deep-sea sediments and oceanic magnetic anomalies. We have also revised the entire chronology by use of more recent isotopic age determinations of the stratigraphic ages of sediments overlying identified anomalies. The main features of the revised time scale are a general reduction of the length of polarity periods during Eocene and later times, and an expansion of Paleocene polarity periods.
TL;DR: In this article, the authors examined the potential role of erosion in the development of the state of stress in the continental crust and found that the stress state induced by erosion may be broken down into three separate components: stresses due to the reduction of overburden pressure, stresses caused by uplift on a sphere, and thermal stresses.
Abstract: In recent years, in situ stress determinations and earthquake source mechanism studies have demonstrated that large deviatoric stresses are common in the upper 10 to 20 km of the continental crust. This study examines the potential role of erosion in the development of the state of stress in the crust. The stress state induced by erosion may be broken down into three separate components: stresses due to the reduction of overburden pressure, stresses due to uplift on a sphere, and thermal stresses. Temperature distributions are calculated as a function of time during an erosional episode; a range of initial temperature distributions and erosion rates are considered. Crustal stress profiles are then obtained assuming elastic behavior at temperatures less than 300°C. During erosion the stress due to removal of overburden pressure is compressional, whereas the stresses due to uplift on a sphere and the thermal stresses are tensional. The latter dominate and we find that erosion leads to predominantly tensional stresses.
TL;DR: On the basis of data from seven cruises in the Amazon River-Atlantic Ocean area, and with 14 anchored stations extending over complete tidal cycles, the currents, suspended sediment concentration, temperature, and salinity were measured at various depths as mentioned in this paper.
Abstract: On the basis of data from seven cruises in the Amazon River–Atlantic Ocean area, and with 14 anchored stations extending over complete tidal cycles, the currents, suspended sediment concentration, temperature, and salinity were measured at various depths. The physical circulation of water shows Amazon River water thrusting out across the continental shelf and over the sea water with some entrainment and mixing with the sea water. The brackish plume from the Amazon is then turned northwestward along the outer shelf by the Guiana current and longshore currents to parallel the coastline for 500 to 700 km. Sea water upwells and flows landward under this plume. The sediments of the Amazon River are thrust out onto the outer shelf, where a gradual depositional process occurs: the sand is deposited first, followed by the silt, and most of the mud is carried shoreward by the landward-moving bottom waters. This combination of processes results in the existing depositional facies pattern; modern mud deposits along the shoreline grade outward into silt deposits and finally into modern sand. The depositional process occurring is contrary to the classic model with sand along the shoreline.
TL;DR: The radiometric dating of the emplacement of the thrust plates by the K-Ar method has been studied in this paper, with ages ranging from 72 to 56 m.p.y.
Abstract: Mesozoic sedimentary rocks overridden by thrust plates in the disturbed belt of northwestern Montana have been metamorphosed by burial beneath these plates. Bentonite in the Cretaceous section has been converted to potash bentonite by this metamorphism, allowing radiometric dating of the emplacement of the thrust plates by the K-Ar method. Ages determined thus far range from 72 to 56 m.y. B.P. The oldest ages coincide with estimates for the beginning of thrusting from stratigraphic and structural evidence. Field evidence does not allow a reliable estimate for the end of thrusting. Thus, the younger ages, which indicate completion of thrusting at the end of Paleocene time, represent additional data of interest to the structural history of the disturbed belt.
TL;DR: The hypothesis of deep erosion of shields by continental ice sheets put forward by W A White is being used by neighboring disciplines Although valuable as a speculative hypothesis, there is field evidence to contradict it as mentioned in this paper.
Abstract: The hypothesis of deep erosion of shields by continental ice sheets put forward by W A White is being used by neighboring disciplines Although valuable as a speculative hypothesis, there is field evidence to contradict it After discussion of the evidence, it is concluded that Pleistocene glacial erosion has removed no more than a few tens of metres of material from the shield areas of the Northern Hemisphere
TL;DR: A detailed bathymetric and photographic investigation of the axial region of the East Pacific Rise (EPR) at lat 21°N indicates that the main extrusion zone is 2.5 km wide as discussed by the authors.
Abstract: A detailed bathymetric and photographic investigation of the axial region of the East Pacific Rise (EPR) at lat 21°N indicates that the main extrusion zone is 2.5 km wide. Local relief is dominated by irregular, steep-sided volcanic extrusive forms. This central zone gives way laterally to sea floor that is characterized by lower local relief and is cut by open fissures and axial-dipping normal faults that trend parallel to the regional bathymetric outline of the rise crest. This lower local relief results from flooding by low-viscosity lavas after the crust has moved away from the central extrusion zone. Although the EPR at this locality is considered to have a spreading rate between rates for slow and fast spreading centers, the axial region reflects many similarities in the sequence of processes that have been proposed for the slow-spreading Mid-Atlantic Ridge in the FAMOUS area.
TL;DR: The combined results from all formations of the Karroo Supergroup of Madagascar are compared with corresponding data obtained in Africa and South America as mentioned in this paper, which provides unequivocal evidence in favor of the reconstructions of du Toit and Smith and Hallam.
Abstract: The combined paleomagnetic results obtained from all formations of the Karroo Supergroup of Madagascar are compared with corresponding data obtained in Africa and South America. This provides unequivocal evidence in favor of the reconstructions of du Toit and Smith and Hallam.
TL;DR: For the first time, the existence of a meandering thalweg and associated point bars has been documented in detail for a mid-ocean channel as discussed by the authors, where the average meander wave length is 50 km between lat 54° and 57°30′N.
Abstract: For the first time, the existence of a meandering thalweg and associated point bars has been documented in detail for a mid-ocean channel. In the world9s largest deep-sea channel, the Northwest Atlantic Mid-Ocean Channel of the Labrador Sea, average meander wave length of the thalweg is 50 km between lat 54° and 57°30′N. The ratio of the meander wave length to channel width is less than that for alluvial channels. Sediments recovered from the channel and natural levees are indicative of sedimentary mass transport in the channel by turbidity currents and related processes, some of which probably extended the entire length of the channel.