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Jurassic-Cretaceous evolution of the central Cordilleran foreland-basin system
No full textDuring Jurassic and Cretaceous time deposition in the western interior basin was controlled by a combination of subduction-related dynamic subsidence and thrust-generated flexural subsidence. Changes in the angle of oceanic plate subduction along the western margin of North America and thrust deformation in the Cordillera governed the spatial and temporal influences of these mechanisms throughout basin history. Dynamic subsidence was the primary control on basin deposition during Early-Middle Jurassic and Late Cretaceous time. During these periods, shallow-angle oceanic plate subduction beneath the western margin of North America produced convective mantle circulation and long wavelength subsidence in the western interior. A cessation of dynamic subsidence during Early Cretaceous time, brought on by an increase in the angle of subduction, is partially responsible for the ∼20 m.y. unconformity that separates the Jurassic and Cretaceous sequences in the western interior. During Late Jurassic time, thrusting in the Cordillera resulted in flexural partitioning of the back-arc region. Statal geometries in the Upper Jurassic Morrison Formation in Utah and Colorado indicate deposition in the back-bulge and forebulge depozones of the Late Jurassic foreland basin system and suggest the coeval existence of a flexurally subsiding foredeep to the west. During Early Cretaceous time, >200 km of shortening in the thrust belt resulted in uplift and erosion of the Late Jurassic foredeep and the eastward migration of foreland-basin system flexural components. Areas occupied by the Late Jurassic forebulge were incorporated into the Early Cretaceous foredeep while the Late Jurassic back-bulge depozone became the location of the Early Cretaceous forebulge. In eastern Utah and western Colorado, migration of the forebulge enhanced the regional Early Cretaceous unconformity associated with the cessation of dynamic subsidence. During late Early Cretaceous time sediment accumulation across the entire foreland-basin system may have been facilitated by the reinitiation of dynamic subsidence in the western interior. During the Late Cretaceous, thrusting in the Cordillera resulted in continued flexural subsidence of the foredeep in east-central Utah. However, increased dynamic subsidence throughout Late Cretaceous time allowed thick accumulations of strata to be deposited in the forebulge and back-bulge depozones of the foreland-basin system.This item was digitized from a paper original and/or a microfilm copy. If you need higher-resolution images for any content in this item, please contact us at [email protected] file replaced with corrected file April 2023
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Structural and neodymium-isotopic evidence for the tectonic evolution of the Himalayan fold-thrust belt, western Nepal and the northern Tibetan Plateau
No full textThe Himalayan fold-thrust belt and Tibetan Plateau are the result of the collision between the Indian and Eurasian continents. This dissertation documents the kinematics and tectonic history of the Himalayan fold-thrust belt of western Nepal and the northern Tibetan Plateau. In the Himalayan fold-thrust belt, the Main Central thrust emplaced a hanging wall flat of Greater Himalayan rock over a footwall flat of Lesser Himalayan rock in Early Miocene time. Subsequent growth of the Lesser Himalayan duplex (LHD) uplifted and rotated the Ramgarh thrust sheet, Main Central thrust, and overlying Greater Himalayan rock to the surface. Thus, growth of the LHD is responsible for the northward dips in the Greater Himalaya. New Nd isotopic data from throughout Nepal indicate that Lesser Himalayan rocks consistently have more negative epsilonNd values than Greater and Tibetan Himalayan rocks. Growth of the LHD is documented in the syntectonic sediments of the Neogene Siwalik Group. At ∼10-11 Ma in central and western Nepal, the epsilonNd values of the Siwalik Group shift toward more negative values which indicate detrital input from rocks in the LHD. Regional mapping in western Nepal and three balanced cross sections provide a three-dimensional view of the fold-thrust belt. These cross sections suggest over 900 km of shortening in upper crustal rock from the Indus suture to the Main Frontal thrust. This suggests a corresponding ∼900 km long wedge of lower crustal rock was consumed by the Himalayan-Tibetan orogen. This wedge may have been inserted under the Tibetan Plateau, helping it obtain its anomalously thick crust. If lower crustal rocks have been inserted under the Tibetan Plateau, the Himalayan collision can account for ∼70% of the overthickened crust. This leaves ∼30% to be accounted for by other mechanisms. The Tula uplift documents shortening along the northern edge of the Tibetan Plateau. The lithic composition of its sandstone, deformation, and erosion of strata suggests that significant regional uplift and thickening occurred since Late Jurassic time and is still occurring. These relationships suggest that the northern Tibetan Plateau region was tectonically active, and undergoing shortening, long before the early Tertiary India-Eurasian collision
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Application of growth strata and detrital-zircon geochronology to stratigraphic architecture and kinematic history
No full textGrowth strata analysis and detrital-zircon geochronology are useful applications of stratigraphy to tectonic problems. Whereas both tools can contribute to kinematic analyses of supracrustal rock bodies, growth strata are also useful for analyzing the influence of tectonics on stratigraphic architecture. This study reports: (1) a conceptual model for growth strata development; (2) stratigraphic and kinematic analyses of growth strata architectures from growth structures in southeastern Utah, the Gulf of Mexico, and northeastern Spain; and (3) the detrital-zircon geochronology of the Salinian block of central coastal California. Kinematic sequence stratigraphy subdivides growth strata into kinematic sequences that are separated by kinematic sequence boundaries. Kinematic sequences can be further partitioned into kinematic domains based on the termination patterns of strata within a kinematic sequence. Salt-related fluvial growth strata from the Gulf of Mexico and southeastern Utah contain stratigraphic architectures that are unique to different kinematic domains. Offlap kinematic domains contain fluvial strata indicative of high slopes, low accommodation rates, and strong structural influence on paleocurrent direction. Onlap kinematic domains contain fluvial strata indicative of moderate slopes, high accommodation rates, and decreased structural influence on paleocurrent direction. The stratigraphic architecture of alluvial-fan thrust-belt growth strata in northeastern Spain does not display a marked correlation with kinematic domain, and is most easily interpreted using existing models for autocyclic alluvial-fan evolution. Detrital-zircon (U-Pb) geochronologic data from basement and cover rocks of Salinia suggest that Salinia originated along the southwestern margin of North America, likely in the vicinity of the Mojave Desert. The presence of Neoproterozoic and Late Archean detrital zircons in Salinian basement rocks also suggest that Salinian sediments were recycled from miogeoclinal sediments of the western margin of North America
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Late Cretaceous to recent evolution of the foreland basin system and associated fold-thrust belt in the Central Andes of Bolivia
No full textTemporal-spatial evolution of the Central Andean foreland basin system relates directly to growth of the adjacent Andean orogenic belt during Late Cretaceous to Recent shortening. As the locus of shortening and crustal thickening propagated eastward, so too did the foreland basin system. Eastward growth of the orogenic wedge induced uplift and erosion of large portions of the basin, removing much of the detrital record of mountain building. Analyses of remnants of the Oligocene-Miocene foreland basin system in the Eastern Cordillera help define the kinematic evolution of the thrust belt in southern Bolivia. A series of north-trending depocenters, regarded collectively as a wedge-top depozone, evolved during growth of fold-thrust structures of the then-frontal part of the orogenic wedge. Growth strata and cross-cutting and onlapping relationships between contractional structures and synorogenic strata delineate the chronology of deformation. New 40Ar dates and published K-Ar dates define a minor Oligocene phase of west-vergent backthrusting followed by primarily east-vergent thrusting during Miocene time. These dates, combined with depositional histories, require synchronous and out-of-sequence thrust displacement during the Miocene. Depocenters are composed of alluvial-fan deposits on their flanks and lacustrine and braided-stream deposits in their axes. Most stratigraphic units are confined to individual depocenters, suggesting that streams rarely had sufficient power to cut across growing folds. An arid climate since ∼10-15 Ma may explain the preservation of large parts of the Late Cretaceous-Miocene foreland basin system in southern Bolivia. In contrast, northern Bolivia exhibited a humid climate over this time interval and most parts of any foreland basin were completely eroded. Critical taper theory suggests that rapid erosion in a humid fold-thrust belt may induce subcritical conditions in which thrust-front propagation is inhibited and internal deformation is promoted. An arid thrust belt may be expected to exhibit critical to supercritical conditions that favor thrust-front migration and in-sequence thrusting. Such phenomena are observed in Bolivia, suggesting that climate and erosion exert fundamental controls on the geometry and kinematics of the Andean orogenic belt.This item was digitized from a paper original and/or a microfilm copy. If you need higher-resolution images for any content in this item, please contact us at [email protected] file replaced with corrected file April 202
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Tectonic and paleoelevation history of the Thakkhola Graben and implications for the evolution of the southern Tibetan Plateau
No full textSediment accumulation in extensional basins in the Tibetan Plateau records tectonic processes and paleoenvironments on the plateau. It is generally assumed that extension on the plateau took place during uplift of the plateau. Based on this assumption, several studies have been aimed at determining the timing of extensional basin development as a proxy for the timing of uplift of the plateau. This dissertation documents the sedimentology of the N-trending Thakkhola graben in the southern Tibetan Plateau in an attempt to test various models for the timing and mechanisms of uplift of the plateau. Magnetostratigraphic and stable carbon isotopic age constraints indicate that deposition in the Thakkhola graben occurred during the Late Miocene (∼11 Ma) to Pliocene. The oxygen isotopic composition of carbonate rocks deposited in the basin records the isotopic composition of paleometeoric water that fell in the basin and in flanking drainages when the carbonate was precipitating. Carbonate oxygen isotopes indicate high-elevation rainfall in the basin, consistent with modern elevations since the onset of deposition in the basin. This implies that the average elevation in the Thakkhola graben has been >4,500 m since it began to form. Lateral facies changes, conglomerate provenance, and paleocurrent data document significant displacement on the western basin-bounding fault since deposition began in the basin. By Pliocene time, a large, southward axial drainage had developed that was similar in size to the modern Kali Gandaki River, which drains the southern plateau, through the Thakkhola graben and Himalayan fold-thrust belt to the south. Change in environments of deposition in the Thakkhola graben indicates trends toward an increasingly arid climate through time. This climate change is documented throughout south Asia and possibly Tibet between ∼8 and 7 Ma and have been assumed to reflect uplift of the plateau. However, high elevation in the Thakkhola graben since ∼11 Ma challenges these commonly held notions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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Structural evolution of the central Nepal fold-thrust belt and regional tectonic and structural significance of the Ramgarh thrust
No full textTectonic shortening within the Himalayan fold-thrust belt in Nepal has been accommodated by southward displacement of large thrust sheets. Most workers focus on the impact that the Main Central, Main Boundary, and Main Frontal thrusts have had on the orogen's structural, thermal, and geomorphic evolution. However, mapping across Nepal, has revealed the presence of the Ramgarh thrust, which is another orogen-scale thrust. The Ramgarh thrust, which had previously been recognized in India and far-western Nepal, occurs within Lesser Himalayan zone rocks, and accommodates a magnitude of shortening similar to that of the Main Central thrust. This dissertation focuses on the structural and tectonic significance of the Ramgarh thrust. Minor details notwithstanding, the structural characteristics of the Ramgarh thrust remain consistent along the ∼800 km width of the fold-thrust belt in Nepal. At current levels of erosion, the Ramgarh thrust is always exposed in a hanging-wall flat on footwall flat thrust relationship with other Lesser Himalayan zone rocks, and also with overlying rocks carried by the Main Central thrust. Mapping along a north-south transect in central Nepal has permitted the construction of a balanced cross-section, which shows that the fold-thrust belt has accommodated a minimum of 489 km of tectonic shortening. A large proportion of which was accommodated by slip on the Ramgarh thrust. Integrating structural constraints provided by mapping and the cross-section with existing thermochronologic, thermobarometric, and foreland basin provenance datasets yields a kinematic model for the structural evolution of the fold-thrust belt. Recognition of the structural relationship between the Ramgarh and Main Central thrusts also permits new insight into the nature of the Main Central thrust. Structural mapping combined with Nd isotope studies from the vicinity of the Ramgarh and Main Central thrusts in Langtang National Park suggest that the Main Central thrust can be defined as a relatively narrow tectonostratigraphic contact, and not as a broad, poorly defined, shear zone. Additionally, much of the volume of highly strained rocks in the footwall of the Main Central thrust may be genetically related to deformation on faults (including the Ramgarh thrust) that lie structurally below the Main Central thrust
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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