Dissertations / Theses on the topic 'Appalachian orogen'

Thesis (Ph.D.)--University of Kentucky, 2005.<br>Title from document title page (viewed on August 17, 2005). Document formatted into pages; contains: xi, 214 p. : col. ill. Includes abstract and vita. Includes bibliographical references (p. 181-211).

The Blue Ridge Province of western North Carolina contains a wide variety of metamorphosed igneous and sedimentary rocks that record the tectonic effects of Precambrian and Paleozoic orogenic cycles. Tectonic interpretations of the events that led to the present configuration are varied and often conflicting. This investigation examines metamorphosed mafic rocks that are widely interpreted to have formed during the closure of ocean basins. Metabasites, and specifically eclogites, have a tendency to mark tectonic sutures and frequently preserve pressure (P), temperature (T), and age data (t) that can be gleaned from mineral equilibria and U-Pb isotopic compositions. As such, the examination of the metabasites is considered the key to understanding the orogenic history of the southern Blue Ridge where these metabasites occur. Chapter 2 is an investigation of the retrograde reactions related to the decompression of sodic pyroxenes that react to form diopside-plagioclase-hornblende-quartz symplectites as stability fields are overstepped during isothermal decompression. In Chapter 3 metabasites from the central and eastern Blue Ridge are re-examined and P-T pathways of these lithologies are determined. The argument is made that the Taconic orogeny of the Blue Ridge is the result of a continent-continent collision event that culminated in a mega-mélange that coincides with the Cullowhee terrane and the eastern Blue Ridge mélange of western North Carolina. Chapter 4 contains the results of a geochronological investigation of the Precambrian basement complex of the eastern Great Smoky Mountains. Chapter 5 is a whole rock geochemical study of the same basement complex. In Chapter 6, a potential lithologic correlation between the southern Blue Ridge basement and the Arequipa- Antofalla block of Peru is discussed. The geologic history of western South America from the Mesoproterozoic through Cambrian is summarized, a potential isotope-based lithologic correlation is proposed, and the early tectonic history of the central Blue Ridge is discussed. Chapter 7 contains brief summaries of Chapters 1-6.

Neoproterozoic-Early Cambrian continental rifting related to the breakup of the supercontinent Rodinia framed the continental margin of eastern Laurentia and the departing cratons around the opening Iapetus Ocean. The result of continental extension was the production of a zig-zag set of promontories and embayments on the eastern Laurentian margin defined by northeast-trending rift segments offset by northwesttrending transform faults. The St. Lawrence promontory defines the Laurentian margin in western Newfoundland. There, Neoproterozoic-Carboniferous clastic, volcanic, and carbonate successions record protracted continental rifting and passive-margin thermal subsidence followed by destruction of the margin during the early, middle, and late Paleozoic Appalachian orogenic cycles. Palinspastic restoration of deformed Paleozoic strata by a set of balanced cross sections resolves the structure, stratigraphy, and timing of Paleozoic tectonic events on the St. Lawrence promontory. Synrift and post-rift subsidence profiles, as well as abrupt along-strike variations in the age, thickness, facies, and the palinspastically restored extent of synrift and post-rift stratigraphy, indicate the St. Lawrence promontory was founded upon a low-angle detachment rift system. Upperplate margins, lower-plate margins, and transform faults that bound zones of oppositely dipping low-angle detachments are recognized along specific segments of the promontory. A detailed U-Pb and Lu-Hf isotopic detrital zircon study elucidates the identity of specific cratons conjugate to the St. Lawrence promontory in the pre-rift configuration of Rodinia. Approximately 510 zircons from 9 samples collected from basement and overlying Early Cambrian synrift rocks in Newfoundland were analyzed by LA-ICP-MS for U-Pb ages and Hf isotopic ratios. Synrift samples yielded ages ranging from 3605 Ma to 544 Ma with maximum age frequencies of 1000-1200 Ma (Grenville), 1350-1450 Ma (Pinware), and 2650-2800 Ma (Superior), while two basement samples yielded U-Pb ages of 1044 Ma and 1495 Ma. 177Hf/176Hf isotopic ratios of ca.1000 Ma, 1200 Ma, and 1400- 1600 Ma zircons from Newfoundland basement and synrift rocks are a close match to reported 177Hf/176Hf ratios for Baltican zircons of the same vintage, suggesting that Baltica was conjugate to the St. Lawrence promontory.

<p> Foreland and forearc basins are large sediment repositories that form in response to tectonic loading and lithospheric flexure during orogenesis along convergent plate boundaries. In addition to their numerous valuable natural resources, these systems preserve important geologic information regarding the timing and intensity of deformation, uplift and erosion history, and subsidence history along collisional margins, and, in ancient systems, may provide more macroscopic information regarding climate, plate motion, and eustatic sea level fluctuations. This thesis presents two studies focused in the Paleozoic Appalachian foreland basin system along the eastern United States and in the Mesozoic to Cenozoic Matanuska forearc basin system in south-central Alaska. </p><p> Strata of the Appalachian foreland basin system preserve the dynamic history of orogenesis and sediment dispersal along the east Laurentian margin, recording multiple episodes of deformation and basin development during Paleozoic time. A well-exposed, >600 m thick measured stratigraphic section of the Pine Mountain thrust sheet at Pound Gap, Kentucky affords one of the most complete exposures of Upper Devonian through Middle Pennsylvanian strata in the basin. These strata provide a window into which the foreland basin's development during two major collisional events known as the Acadian-Neoacadian and the Alleghanian orogenies can be observed. Lithofacies analysis of four major sedimentary successions observed in hanging wall strata record the upward transition from (1) a submarine deltaic fan complex developed on a distal to proximal prodelta in Late Devonian to Middle Mississippian time, to (2) a Middle to Late Mississippian carbonate bank system developed on a slowly subsiding, distal foreland ramp, which was drowned by (3) Late Mississippian renewed clastic influx to a tidally influenced, coastal deltaic complex to fluvial delta plain system unconformably overlain by (4) a fluvial braided river complex. Four samples of Lower Mississippian to Middle Pennsylvanian sandstone were collected from the hanging wall (n = 3) and footwall (n = 1) of the Pine Mountain thrust sheet at Pound Gap to determine sediment provenance in this long-lived foreland basin system. Paleocurrent indicators considered in the context of the regional foreland basin system suggest transverse regional drainage during the development of Early and Late Mississippian delta complexes. Eustatic fall during the early stages of the Alleghanian orogeny to the east saw a shift in regional drainage with the development of a southwestward-flowing and axial braided river system in Early Pennsylvanian time followed by Middle Mississippian transgression of a fluvio-deltaic complex. Detrital zircon U-Pb age data from Lower Mississippian to Lower Pennsylvanian sandstone support regional interpretations of sediment sourcing from probably recycled foreland basin strata along the east Laurentian margin, whereas compositionally immature Middle Pennsylvanian sediment was sourced by a limited distribution of east Laurentia sources reflecting thrust belt migration into the adjacent foreland basin system during Alleghanian orogenesis. </p><p> In addition, the stratigraphy of the foreland basin system in the central Appalachian basin is significantly different compared to the stratigraphic record that is typified for foreland basin systems and suggests that the Carboniferous Appalachian foreland basin system investigated in this study does not fit the typical foreland basin model that is used widely today for both ancient and modern systems. Possible factors that produce the observed discrepancies between the central Appalachian and typical foreland basin systems may include differences in the timing, type, and frequency of orogenic events leading to foreland basin development, related variations in the rheology of the underlying lithosphere, and whether forebulge migration is mechanically static or mobile. </p><p> The Cordilleran margin of south-central Alaska is an area of active convergence where the Pacific plate is being subducted at a low angle beneath the North American plate. In the Matanuska Valley of south-central Alaska, the geology of the Mesozoic to Cenozoic Matanuska forearc basin system records a complex collisional history along the margin from Cretaceous to Miocene time and provides an opportunity to study how shallow-angle subduction affects upper plate processes. Paleocene-Eocene low-angle subduction of an eastward migrating spreading ridge and Oligocene oceanic plateau subduction caused uplift, deformation, and slab window magmatic intrusion and volcanism in the Matanuska Valley region, thereby modifying the depositional environment and structure of the forearc system. In this study, detailed field mapping in the Matanuska Valley region and structural analysis of Paleocene-Eocene nonmarine sedimentary strata are utilized to better understand the structural response of the forearc basin system to multi-stage flat-slab subduction beneath an accreted continental margin, a process observed along multiple modern convergent margins. Four geologic maps and structural cross-sections from key areas along the peripheries of the Matanuska Valley area and one regional cross-section across the forearc system are presented to delineate its local structural configuration and to contribute to a more complete understanding of how sedimentary and tectonic processes along modern convergent margins may be or have been impacted by shallow-angle type and related subduction processes.</p>