No description available.

The Kaapvaal craton of southern Africa and the Pilbara craton of Western Australia, two of the best-preserved Archean cratons in the world, are covered by remarkably similar early Precambrian cover sequences. This has led to the proposal of the so-called Vaalbara hypothesis, which promotes the existence of the two cratons as a single crustal entity, and possibly, Earth’s oldest assembled continent in Neoarchean-early Paleoproterozoic times. Previous studies have failed to prove the existence of Vaalbara conclusively, principally due to a lack of reliable ages or because of uncertainty and gaps in the paleomagnetic record from the Kaapvaal craton. During the present study paleomagnetic samples were collected from selected Neoarchean- Paleoproterozoic cover sequences of the Kaapvaal craton for the establishment of well-defined paleomagnetic poles. In addition, the Hartswater Group of the Ventersdorp Supergroup was sampled for zircon SHRIMP analyses in order to constrain the ages of poles defined from that succession. The paleopoles established here, together with existing paleopoles from the Kaapvaal craton, are used to evaluate the apparent polar wander path of the craton during the Neoarchean-Paleoproterozoic and are compared with poles of similar age from the Pilbara craton as a test of the Vaalbara hypothesis. Regarding the age of the Hartswater Group, zircon SHRIMP ages of 2735 ± 3 Ma and 2724 ± 6 Ma cast doubt on younger ages from the Klipriviersberg Formation, which comprise the base of the Ventersdorp Supergroup. Traditional (younger) age constraints from the Ventersdorp Supergroup do not support the original Vaalbara correlation. A new correlation is suggested here, taking the new ages into account, showing that the Ventersdorp Supergroup overlaps in time with the Fortescue Group of the Pilbara craton. Most importantly, the new ages also provide constraints on the magnetization within the Platberg Group and the Allanridge Formation. Six new paleopoles, of various quality, are added to the existing database from that craton. These poles from the ~2.73 Ga Platberg Group and ~2.7Ga Allanridge Formation of the Ventersdorp Supergroup, the ~2.5Ga lower Transvaal Supergroup, the lower two unconformitybounded sequences of the Waterberg Group (2.05 Ga and ~1.99 Ga) and the upper Soutpansberg Group (~1.76 Ga) have, together with existing poles from the Kaapvaal craton, led to the definition of an APWP for that craton for a period ~2.78 to ~1.76 Ga. Particularly the poles from the Waterberg and Soutpansberg Groups provided the information to identify complexities (looping) in the APWP that have gone unrecognized in the past. The paleomagnetic data gathered and the newly defined APWP could be used in conjunction with geological evidence from the Kaapvaal and Pilbara cratons to evaluate, and validate, the Vaalbara hypothesis. A good match between the APWP’s of the two cratons for the period ~2.78 to ~2.70 Ga and the geological features (lithology and structure) of the two cratons provide the best evidence that Vaalbara existed as a cratonic unit in the late Archean. Paleomagnetic data constrain the position of the Pilbara craton in immediate proximity to the northwest of the Kaapvaal craton (in a Kaapvaal reference frame). The position of the Zimbabwe craton relative to the Pilbara and Kaapvaal cratons is still unresolved, but indications are that it was most likely in a proximal position to the Kaapvaal craton at 2.7 Ga in a configuration not much different from its present day configuration. This would imply that Vaalbara was most probably the Earth’s oldest assembled continent as proposed by earlier workers. The new paleomagnetic data further suggest that Vaalbara did not exist anymore at ~2.0Ga. When evaluated in conjunction with geological evidence a strong argument can be made for the existence of the Vaalbaran continent up until ~2.22 Ga and that the Pilbara and Kaapvaal cratons became separate entities from about ~2.05 Ga. / Prof. NJ Beukes Prof. DAD Evans

No description available.

The 9 km thick Lower Old Red Sandstone succession of the northern part of the Midland Valley Basin, Scotland, ranges from Wenlock to Emsian in age and largely comprises conglomerates in the east passing westwards into sandstones and siltstones. Previously, the depositional and tectonic setting of the basin has been poorly constrained, as has the relationship between sediments of the northern Midland Valley and the Grampian outliers at Aberdeen, Rhynie, Cabrach, Tomintoul and New Aberdour. This study focuses on establishing the stratigraphic framework of the areas and outlining the key controls and source of sedimentation during deposition of the Lower Old Red Sandstone, placing the geological history within the larger Caledonian framework. Sedimentological investigation alongside petrographical point count, heavy mineral and detrital zircon analysis allows the reconstruction of a large distributive fluvial system sourced from the NE within the Caledonian foreland, within which the Lower Old Red Sandstone of the northern Midland Valley Basin was deposited. Sedimentation was continuous across the line of the Highland Boundary Fault. Sedimentation within the Grampian outliers was locally influenced, however facies association development is comparable with the base of the laterally time-equivalent northern Midland Valley Basin stratigraphy. Sedimentary provenance analysis indicates a similar source terrane for the sediments of both areas, with detrital zircon age spectra comparable to those of the Dalradian Supergroup and localised contemporaneous volcanism, with conglomerate clast-size indicating a proximal source. Sedimentation of the Lower Old Red Sandstone of the northern Midland Valley Basin and Grampian outliers is thus attributed to Siluro-Devonian basement uplift in the Caledonian foreland driven by thick-skinned tectonics.

The Littlefield Rhyolite consists of widespread, high-temperature, hotspot-related rhyolitic lavas that erupted in eastern Oregon contemporaneous to late-stage Grande Ronde Basalt lavas. The estimated total volume of erupted rhyolites is ~100 km3 covering ~850 km2. The focus of this study has been to investigate the stratigraphy and petrology of the Littlefield Rhyolite and whether field and geochemical relationships exist to help constrain the timing and storage sites of Grande Ronde Basalt magmas. Although often indistinguishable in the field, our data reveal that the Littlefield Rhyolite consists of two geochemically distinct rhyolite flow packages that are designated here as lower and upper Littlefield Rhyolite, according to stratigraphic relationships in the Malheur River Gorge. Rarely viewed in sequence, these rhyolites are distinguished by Zr, Ba, Nb, TiO2 and FeO contents and 40Ar/39Ar ages (16.12±0.04 and 16.16±0.10 Ma versus 16.01±0.06 and 16.05±0.04 Ma). Rhyolites known either as ‘rhyolite of Cottonwood Mountain’ or ‘rhyolite of Bully Creek Canyon’, and which are exposed around Cottonwood Mountain, northwest of Vale, have compositions similar to samples of lower Littlefield Rhyolite. Additionally, single crystal 40Ar/39Ar ages of two samples (16.12±0.07, 16.20±0.08) are statistically indistinguishable. Among units sandwiched between the lower and upper Littlefield Rhyolite are several lava flows and a one-meter thick agglutinated spatter deposit of Hunter Creek Basalt. The spatter deposit thickens to 10s of meters over a distance of 800 m where the deposit is strongly welded. We recognize this as a venting site of Hunter Creek Basalt, and that Hunter Creek Basalt is geochemically and petrographically similar to, and contemporaneous with, late-stage Grande Ronde Basalt. Ages of Littlefield Rhyolite flow units constrain the timing of eruption of Hunter Creek Basalt to an age span of ~100k years, between 16.05 – 16.12 Ma. One local variant of late-stage Grand Ronde Basalt is icelanditic (~62 wt. % SiO2) and is found at a number of places including a location near the southern extent of the upper Littlefield Rhyolite. Geochemical modeling strongly suggests that icelandite lavas resulted from mixing of Grande Ronde and upper Littlefield Rhyolite magmas, thereby tying a Grande Ronde magma storage site to within the greater Malheur River Gorge area, and indicating contemporaneity of rhyolitic and Grande Ronde magma reservoirs.

Thesis (Ph. D.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1982. / Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 1999. / Vita. Includes bibliographical references (leaves 177-191). Available also in a digital version from Dissertation Abstracts.

Thesis (Honors)--College of William and Mary, 2009. / Includes bibliographical references (leaves 59-61). Also available via the World Wide Web.

Thesis (Ph. D.)--Rutgers University, 2009. / "Graduate Program in Anthropology." Includes bibliographical references (p. 228-258).

Thesis (Ph. D.)--University of Texas at Austin, 1999. / Vita. Includes bibliographical references (leaves 128-134). Available also in a digital version from Dissertation Abstracts.