Browsing by Subject "Geochronology"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Archaeomagnetism as a Geochronological Tool: Dating a Levantine Iron Age Conflagration
    (2018-06) Stillinger, Michele D.
    Meaningful interpretation of past human culture requires an accurate chronology that can be correlated with our modern calendar. The timing of seminal events during the Levantine Iron Age (~ 1200 to 600 BCE) is hotly debated because conventional dating methods are fraught with subjective interpretations and analytical inaccuracies. This research uses archaeomagnetism, a subfield of paleomagnetism, as an alternative geochronological dating technique. Utilizing traditional archaeomagnetic materials (e.g. pottery) and testing new geologically based materials (ancient bread ovens called tabuns), a new Near East Archaeomagnetic Dating Curve (NEAC) was constructed to date four occupational deposits and a large conflagration at the Iron Age village of Khirbet Summeily, Israel. The results indicate that the destruction was likely associated with the 925 BCE Egyptian military campaign of Sheshonq I (22nd Dynasty). In addition, a new high in geomagnetic field intensity was measured that confirms the recently identified 8th Century BCE intensity spike for the region. This research provides new data that will enable geophysical researchers to improve models of geomagnetic field variability and core processes for the first three millennia BCE.
  • Thumbnail Image
    Item
    New evidence of Proterozoic high P-T metamorphism in East Antarctica from thermobarometry and in-situ U-Pb age dating of monazite in metamorphic glacial clasts, central Transantarctic Mountains, Antarctica
    (2014-11) Nissen, Chelsea I.
    The East Antarctic shield (EAS) is a key component in the study of early crustal evolution due to its ancient geologic history and involvement in the amalgamation and break-up of major supercontinents. The EAS has documented affinities with the cratons of Africa, India, and Australia based on limited coastal outcrop, but an ice cap up to 4 km thick prevents direct access to the interior bedrock of the EAS. Additionally, thermomechanical effects of the Ross Orogeny (~500 Ma) obscure the Precambrian history in rarely-exposed crystalline basement. Metamorphic rock clasts from glacial moraines near the central Transantarctic Mountains were studied for petrologic, geochemical, and isotopic characteristics in order to further understand the geologic history of the EAS. These clasts were presumably eroded from the interior of the EAS and may provide unique natural samples of the ice-covered basement. Metamorphic rock clasts selected in this study are semi-pelitic gneisses with high-pressure mineral assemblages, as well as accessory minerals including monazite and zircon. New in-situ SHRIMP U-Pb analysis of monazite yielded Paleoproterozoic to Neoproterozoic ages in six clasts. Among these, two clasts from Lonewolf Nunataks, at the head of Byrd Glacier, gave previously unrecognized ages of ~1900-1700 Ma, with one having a Mesoproterozoic overprint at ~1200 Ma. Lonewolf Nunataks clasts preserve evidence of high-pressure granulite-facies metamorphic conditions associated with Proterozoic crustal convergence and thickening during orogenic activity. Clasts sampled from moraines near the Miller Range yielded Neoproterozoic U-Pb ages; one clast yielded ages of ~660 and ~590 Ma, whereas other clasts gave ages of ~570- 545 Ma. Neoproterozoic samples conducive to thermobarometric analysis record high P-T conditions comparable to previously documented Ross Orogen activity in reactivated Precambrian crystalline rocks of the Nimrod Group. Together, the clast ages coupled with P-T analysis record previously unknown Paleoproterozoic tectonometamorphic events in central East Antarctica, overprinted by younger Mesoproterozoic metamorphism. Clasts from Lonewolf Nunataks may reflect a Paleoproterozoic event within the EAS related to development of the Nuna supercontinent (~1870-1900 Ma), overprinted by a Mesoproterozoic orogenic event, possibly related to the final amalgamation of Rodinia (~1200 Ma). One clast with a metamorphic age of ~660 Ma may record rift-margin activity associated with supercontinent breakup. Neoproterozoic ages of ~590-545 Ma from some of the clasts are demonstrably older than Ross Orogen ages known from Nimrod Group metamorphic basement; these older ages may therefore provide evidence that Ross orogenic activity was initiated earlier than previously thought, and that its metamorphic overprint extends farther inboard of the Transantarctic Mountains.
  • Thumbnail Image
    Item
    Submerged fossil corals : archives of diagenesis, subsidence and sea level.
    (2008-12) Riker-Coleman, Kristin Emigh
    Corals are well suited as absolute markers of sea-level. Given the modern elevation of the coral, the age of coral, sea level at the coral's time of death, and the paleogrowth position of a fossil corals allow one to compute a vertical tectonic rate of motion; if the tectonic rate is known then one can use the sample's location, paleogrowth position, and age to compute a paleosea level. I studied the uranium-series isotopic composition of fossil corals from three settings: New Britain, Papua New Guinea, Hawaii and the Huon Gulf, Papua New Guinea. These three settings are distinct in their tectonic setting: New Britain corals are emergent as a result of tectonic uplift and the Hawaiian and Huon corals are submergent. Corals samples from uplifted terraces in New Britain are highly altered due high rainfall rates in the area. The measured 230Th ages of corals from the Holocene terrace are between 4.3 ± 0.03 ka to 9.0 ± 0.16 ka. Using these ages, their present elevation and the corresponding paleo sea level, I computed an average uplift rate of 1.6 ±0.4 m/ka. Uranium-series isotopic compositions of submerged corals are not better preserved than their subaerially exposed counterparts, although their diagenetic signatures differ. I report 230Th ages of three fossil reefs from the northwestern coast of Hawaii: the -400 m reef at Mahukona (136.7 ± 0.9 ka to 151.8 ± 1.7 ka), the -1000 m terrace off eastern Kohala (377.2 +13.4/-12.2 ka and 392.5 +20.5/-17.9 ka) and the -1000 m reef off northwestern Kohala (286.5 ± 1.4 ka to 342.8 ± 1.4 ka). From the submerged Hawaiian samples I identify three main trends in uranium-series isotopic composition: (1.) An increase in both 230Th/238U and 234U/238U; (2.) an increase in 230Th/238U with little change in 234U/238U; and (3.) low 230Th/238U with both low and high 234U/238U. Measured 230Th ages of submergent coral samples from the Huon Gulf range from 60 ka to infinite age (>600 ka). The measured 230Th ages of coral samples are older than we expected, but broadly increase in age with depth. Corals from three terraces (-1280 m, -1650 m, and -1950 m) represent material from Stage 11, suggesting that the model of terrace development is likely more complex than the original idea of a distinct sea-level rise event per terrace. Although I am able to identify three main trends in the uranium-series isotopic compositions of the Hawaiian samples, no clear trend emerges in the Huon Gulf setting. The subsidence rate (m/ka) computed from the most reliable ages of the deepest terraces suggests that the subsidence in the Huon Gulf averages 4 m/ka
  • Thumbnail Image
    Item
    Timescales of migmatization, metamorphism, and deformation in a collapsed Orogenic Plateau.
    (2009-01) Gordon, Stacia Michelle
    Migmatites play an important role in the evolution of mountain systems by inducing rheological contrasts and focusing strain. In modern orogenic plateaux, investigations have suggested that a layer of partially molten crust is located in the mid- to lower crust. To understand the role of partially molten crust in orogenic systems, it is important to determine how much of the crust was partially molten for how long, and to link the conditions, timing, and consequences of partial melting to tectonic processes at different crustal levels during construction and collapse of orogens. The Skagit Gneiss, in the highgrade core of the North Cascade continental magmatic arc of Washington, USA and BC, and the Valhalla complex, in the Shuswap metamorphic core complex of southeastern British Columbia, Canada, both contain abundant migmatites and represent the western and eastern margins, respectively, of a proposed orogenic plateau that was once present in western North America during the Late Mesozoic-Early Cenozoic. In the Valhalla complex, samples of migmatite were collected from the dome core to the bounding detachment fault for conventional, in situ, and depth profiling ion microprobe analyses. The conventional and in situ analyses of stromatic migmatites and leucosomes crystallized in boudin necks yield concordant U-Pb zircon ages that cluster near 60 Ma, interpreted as the timing of melt crystallization. Monazite Th-Pb ages range from 57-49 Ma. Patchy zoning and the range of dates suggest that the monazite was recrystallized under fluid-mediated conditions. To better understand the late history recorded in the monazite, depth profiling U-Pb ages were obtained from the outermost rims of zircons and yield a consistent age of 51 Ma. Oxygen isotopic measurements of the unpolished crystal faces systemically yield heavier δ18O (up to 9.0 ‰) relative to interior compositions (down to 5.5 ‰). Furthermore, Ti concentrations of unpolished crystal faces and grain interiors yield temperatures of ~650 ºC. The depth profiling zircon results and the conventional Th-Pb monazite results indicate that deformation- and fluidmediated recrystallization of zircon and monazite occurred at high-T conditions as late as 51 Ma. The Ar cooling ages overlap with the youngest monazite and zircon results and cluster from 51 to 49 Ma. The geochronometric, geochemical and trace element results from the Valhalla complex, combined with field, structural, and petrologic data from this and previous studies of the Omineca domes, show that a large region of orogenic crust in this part of the Cordillera was partially molten in the early Tertiary. Rapid cooling is associated with extension and exhumation of migmatites in the domes. In the Skagit Gneiss, monazite and zircon were dated using the U-Pb TIMS method from migmatites in 3 localities. Zircons from the mesosome of the westernmost locality commonly yield Cretaceous dates, with younger dates clustering at 61 Ma. Leucosomes yield zircon with concordant dates that range from 68 to 47 Ma, interpreted as representing the timing of melt crystallization. In comparison, monazite reveal bimodal results, with one group clustering near 48 Ma and a second set of older dates from 69 to 65 Ma. The latter monazite dates are consistently older than the zircons from the same leucosome, consistent with the possibility that the older monazites record the timing of prograde to possibly peak metamorphism. The Eocene zircon and monazite dates are at the young end of the age spectrum for the North Cascades arc system and overlap with the timing of transtensional basin formation, suggesting that partial melting was an active process during at least the initial stages of extension and exhumation of the high-grade rocks. In addition, in the Skagit Gneiss, a detailed study of part of the eastern bounding strike-slip fault zone suggests that a dynamic system was present between the high-grade Skagit rocks, the fault, and the adjacent basin. A step-over zone in the strikeslip fault may have developed during transpression and allowed part of the basin to be incorporated into the high-grade core and undergo metamorphism and deformation with the Skagit Gneiss. Although ~ 300 km separate the North Cascades from the Shuswap metamorphic core complex today, the two regions share many similarities: 1) both areas expose deformed high-grade gneiss that underwent isothermal decompression; 2) both areas contain abundant deformed migmatites that crystallized at similar times; and 3) Ar cooling ages from the two regions are similar. In both the Skagit Gneiss and the Valhalla complex, the partially molten crust played a significant role in the decompression and exhumation of the terranes. The similarities in P-T-t-d between the two regions strongly indicate that the North Cascades and the Omineca belt were dynamically linked and that the two areas represent the collapsed margins of an orogenic plateau. The migmatites in both areas are evidence of the layer of partially molten crust that once flowed beneath the proposed plateau. The data from the two areas suggest that partial melting must play a major role in the tectonic evolution of orogenic systems that contain abundant melt (e.g., Himalaya-Tibet; Andes-Altiplano-Puna; Cordillera).