Date of Award
Doctor of Philosophy (PhD)
Paul G. Fitzgerald
apatite fission-track, Appalachian Basin, basin analysis, low-temperature thermochronology, thermal history, (U-Th)/He
Physical Sciences and Mathematics
Since the early development of the fission-track dating method by Price, Walker and Fleischer in the 1960’s and 1970’s, the method has evolved into a robust tool for constraining thermal histories and determining exhumation rates within the upper continental crust. Low-temperature thermochronology, notably apatite fission-track thermochronology because it has a kinetic parameter (i.e. track lengths), is commonly used to constrain the tectonic history of orogenic belts and the thermal history within sedimentary basins for hydrocarbon resource assessment. Collisional tectonics in the Paleozoic which formed of the Appalachian Orogen and the foreland basin has been a subject of considerable study. The objective of this dissertation is to apply low-temperature thermochronology to constrain the post-orogenic thermal history and exhumation of the Northern Appalachian Basin. Previous work has identified that the post-rifting evolution of eastern North America involves a number of periods of rapid cooling and/or exhumation, yet significant debate remains on the temporal and spatial resolution of the periods of rapid cooling within the Mesozoic and Cenozoic, and hence their significance. Chapters one and two of this dissertation are focused on constraining the post-orogenic thermal history and exhumation of the Northern Appalachian Basin, within New York and Pennsylvania. Chapter three is a comprehensive analysis of the thermal history trends and variations synthesized from published low-temperature thermochronology studies across the Appalachian orogen in eastern North America.
Chapter one constrains the post-orogenic thermal history and exhumation of the Northern Appalachian Basin utilizing apatite fission-track (AFT) thermochronology and (U-Th)/He dating integrated with paleothermometers and geologic constraints. Samples were collected from Devonian-aged Catskill delta wedge formations across New York and Pennsylvania (USA). The AFT age of samples decreases from west to east across the Northern Appalachian Basin (~180-120 Ma) a result of synchronous onset of exhumation, with greater amounts of burial and exhumation in the east. Apatite fission-track thermochronology data, apatite (U-Th)/He ages, and vitrinite reflectance maximum paleotemperature information provide inputs for inverse thermal models, which constrain a multi-stage post-orogenic history. Overall cooling rates documented in the Northern Appalachian Basin are slow, however we constrain two periods of “rapid” cooling separated by a long period of slower cooling: (1) Late Triassic to Early Jurassic onset of rapid cooling (1-3 °C/Myr) from maximum temperatures, generally higher than the retentivity of fission-tracks in apatite (i.e. >~110-120 °C); (2) Slow cooling (0.1-0.3 °C/Myr), which is referred to as a period of stabilization within the Northern Appalachian Basin, from the Cretaceous to Late Cenozoic; (3) Rapid cooling (1-2 °C/Myr) from the Mid-Miocene to present. Low-temperature thermochronology, combined with paleothermometers, constrains the thermal and exhumation history of Devonian strata and relates episodes of cooling to contemporaneous tectonic events occurring along the northeastern North American margin.
Chapter two focuses on the thermal history and exhumation of the Catskill Mountains, New York. A ~1 km age-elevation profile of samples collected from Catskill delta wedge units of Slide Mountain were analyzed with apatite fission-track thermochronology and apatite (U-Th)/He dating. The Catskill Mountains formed via exhumation and erosion of the Appalachian Plateau. The slope of the AFT age-elevation profile constrains an apparent exhumation rate of ~50 m/Myr in the Early Cretaceous. Multi-kinetic inverse thermal models reveal “rapid” cooling episodes from the Early Jurassic to the late Early Cretaceous and from the Mid-Miocene to recent. As documented in the Northern Appalachian Basin, these episodes of rapid cooling in the Catskills are separated by a period where samples experienced stabilization and much slower cooling for ~100 Myr. This chapter discusses the debate surrounding the resolution and existence of a Miocene cooling event along eastern North America, which because of its low magnitude makes it challenging to document with low-temperature thermochronology. Miocene cooling constrained in modeling of the Catskills age-elevation profile samples is compared to previous low-temperature thermochronology studies that either rejected or confirmed this cooling. Studies rejecting Miocene cooling used mono-kinetic annealing algorithms in early generation AFT modeling programs, which often produced a spurious late-stage rapid cooling event. However, other low-temperature thermochronology and geomorphological studies in the central and southern Appalachians and a rapid Miocene increase offshore sedimentation rates suggest exhumation and erosion of Appalachian strata was necessary in the Miocene.
Chapter three collates low-temperature thermochronology data and thermal histories from 26 studies utilizing apatite fission-track thermochronology and/or apatite (U-Th)/He dating within the Appalachian physiographic provinces (the Appalachian low-temperature thermochronology database). While there is an abundance of thermochronology data from many studies scattered over the eastern U.S., an orogen-scale synthesis of the low-temperature thermal and exhumation history has not been undertaken. The objective is to determine temporal and spatial trends in the post-orogenic thermal histories, through identifying events which occurred throughout the Appalachian provinces, as well as province-specific events. Challenges exist in interpreting the thermal history trends given that the studies range in publication from the 1980’s to present day and hence vary in style of analysis and objectives, contain synthesis of results from different methods, and utilize different data interpretation and modeling approaches with several generations of modeling programs. Many of the low-temperature thermochronology studies undertaken along the eastern U.S. lie within present day catchments that drain into Mid-Atlantic offshore basins. We examine the onshore record and conclude episodes of rapid cooling and exhumation documented within these catchments correlate well with periods of increased sediment flux into offshore Mid-Atlantic basins. Synthesizing the temporal correlation between onshore exhumation and offshore sediment flux and the provinces’ thermal histories, the post-orogenic low-temperature thermal history across the eastern U.S. passive margin is divided into three extended periods of cooling and exhumation. These periods are related to regional effects of Early Mesozoic continental rifting and post-rift crustal extension, Late Mesozoic variable cooling due to the development of the passive margin and to exogenic mechanisms, and Late Cenozoic (i.e. Miocene) rapid cooling related to the establishment of dynamic topography and drainage reorganization.
Shorten, Chilisa Marie, "Post-Orogenic Thermal History and Exhumation of the Northern Appalachian Basin: Low-Temperature Thermochronologic Constraints" (2018). Dissertations - ALL. 936.
Available for download on Monday, September 14, 2020