A compilation of high quality post-rift sediment isopach data has been used in conjunction with the observed free-air gravity anomaly to determine segmentation of the long term mechanical properties of the lithosphere at the Eastern USA passive margin. This segmentation is represented by a process-oriented analysis in which the flexural response of the margin to post-rift sediment loading is controlled by spatial variations in effective elastic thickness (T_e) of the underlying lithosphere. Existing Eastern USA margin T_e estimates range from less than 10km to more than 30km. In this study it is shown that high strengths of 10 - 40km T_e are confined to structural arches dividing the broadest marginal basins, while low strengths of less than 10km T_e are typically found in structural embayments and beneath the deep basins. The hinge zone, across which the degree of continental thinning increases rapidly, marks the transition between high and low strength. Yield strength envelope models support an argument that regions of low strength were created by lithospheric thinning during rifting, and sustained by thermal insulation and flexural curvature associated with voluminous post-rift sediment deposition. Along-strike T_e variations - reflected in the alternation of basement platforms and embayments - are attributed to inheritance of lithospheric segmentation from earlier tectonic events. Along-strike segmentation of the margin has previously been observed as a 300 - 500km wavelength spectral energy peak in the shelf break Airy isostatic gravity anomaly (IGA) high. That this segmentation is explained by variations in the underlying lithospheric strength is demonstrated by a flexural IGA high in which the equivalent spectral peak is absent. The spectral energy of the along-strike T_e distribution peaks in the same waveband. Removal of process-oriented components from the observed free-air gravity anomaly reveals other contributions that were not resolved in earlier studies. In particular, the (previously unknown) Carolina Trough Isostatic Gravity Anomaly, has been identified and attributed to an extrusive (syn-rift) volcanic source. Detailed study of this anomaly suggests that the margin is segmented in terms of its volcanic character, and argues against recent estimates of the volume of new igneous material emplaced during rifting.

Acadian (Late Silurian-Early Devonian) metamorphism in the Central Maine Terrane (CMT) in central Massachusetts is characterized by an early low-P, high-T (Buchan-type) metamorphism followed by compression and nearly isobaric cooling to 100-200°C below peak recorded temperatures before eventual unroofing. Evidence for this path includes sillimanite pseudomorphs after early andalusite, abundant cordierite in pelitic lithologies, late replacement of low-P cordierite-bearing assemblages by high-P garnet-bearing assemblages, recrystallization of mylonites associated with late shear zones to form lower-T and higher-P assemblages, and development of high density CO₂ inclusions in the host rock of the mylonite. Peak conditions in the highest grade rocks were 685 - 780°C and 5-6 kbar; the cooling path passed through 550°C at pressures of 6.5-8 kbar. Fluid inclusion data show a range of fluid composition and density during metamorphism. Densities of CO₂ fluid inclusions range from 0.25 g cm⁻³ to 1.04 g cm⁻³. High density fluid inclusions are contained within quartz inclusions in garnets found in partial melt leucosomes, and in quartz grains within migmatites. Fluid inclusions within the quartz inclusions indicate trapping conditions of 650-700°C at pressures below 5 kbar. Other CO₂ fluid inclusions from matrix quartz yield isochores which pass through conditions of 700°C and 5.2 kbar. The higher density inclusions associated with rocks containing the late high-P assemblages have isochores which pass through the estimated P-T conditions for the recrystallization of the mylonite. Fluid inclusions recording late stage conditions include CO₂-rich, N₂-rich and aqueous fluid inclusions. Cathodoluminescence maps of quartz along healed microfractures are used to constrain the late stages of the uplift path. Fluid inclusion evidence suggests an early isobaric heating event followed by cooling and compression with a later isothermal decompression. This evidence is in accordance with previously obtained petrologic evidence and supports a counterclockwise P-T model for the CMT of south-central Massachusetts. / Master of Science

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The major problem addressed by this study was to investigate the relationship between characteristics of the research methods used in selected subject subfields. In order to carry out the investigation a non-experimental design was employed, and an evaluative instrument was developed for assigning a quantitative score to published research based on characteristics of the research methods utilized. Evaluative scores were thus assigned to 244 randomly selected research studies drawn from two scientific subfields manifesting different rates of growth.

Few plutons with a reversed geometry of a felsic rim and mafic core have been described in the geologic literature. The Turtle pluton of S.E. California is an intrusion composed of a granitic rim and granodioritic core and common microgranitoid enclaves. Field observations, mineral textures, and chemistries, major and trace element geochemistry, and isotopic variability support a petrogenetic model of in situ, concomitant, magma mixing, and fractional crystallization of rhyolitic magma progressively mixed with an increasing volume of andesitic magma, all without chemical contribution from entrained basaltic enclaves. Hornblende geobarometry indicates the Turtle pluton crystallized at about 3.5 kb. A crystallization sequence of biotite before hornblende (and lack of pyroxenes) suggests the initial granitic magma contained less than 4 wt% H₂0 at temperatures less than 780°C. U-Pb, Pb-Pb, Rb-Sr and oxygen isotope studies indicate the terrane intruded by the Turtle pluton is 1.8 Ga, that the Turtle pluton crystallized at 130 Ma, that the Target Granite and garnet aplites are about 100 Ma, and that these intrusions were derived from different sources. Models based on isotopic data suggest the rhyolitic end member magma of the Turtle pluton was derived from mafic igneous rocks, and was not derived from sampled Proterozoic country rocks. Similarity of common Sr and Pb isotopic ratios of these rocks to other Mesozoic intrusions in the Colorado River Region suggest the Turtle pluton and Target Granite have affinities like rocks to the east, including the Whipple Mountains and plutons of western Arizona. P-T-t history of the southern Turtle Mountains implies uplift well into the upper crust by Late Cretaceous time so that the heating and deformation events of the Late Cretaceous and Tertiary observed in flanking ranges did not affect the study area. / Ph. D.

The Damara-Katanga orogen in central-southern Africa represents an area of 1.73xl0⁶ sq. km. The region is considered one of the wealthiest metallogenic provinces in the world. Successful exploration for stratabound base-metal deposits has taken place at this particular area since the introduction of more organised methodology in the early 1920s. The genesis, location and distribution of the ore deposits are related to their tectonic settings. Geodynamic evolution of the orogen, which initially formed part of a complex Pan-African rift system, comprises the following stages rifting; downwarping, including spreading on the western portion; syn-orogeny and late-orogeny. Two major tectonic events in the history of the region have been identified: the Katangan (900-750 Ma) and the Damaran episodes (750-500 Ma). Timing of mineralisation of ore deposits has been related to the evolutionary stages of the orogen. Genetic models of the most productive deposits are briefly discussed in this dissertation. The sedimentological, geochemical, paleogeographic and structural features can be employed as geological guidelines for integrated exploration programmes. Discoveries of major deposits and prospects in the orogen are also summarised, focussing on the exploration methods employed. The cost-effective use of the exploration techniques includes the classical copper-lead-zinc soil sampling for residual soils such as those in the Copperbelt area. Airborne magnetics and electromagnetics and follow-up ground geophysics have proved successful in areas where the cover is transported in the search for shallow ore deposits such as the Matchless massive sulphides. Remote sensing, geochemical and geophysical techniques have been tried in covered areas of western Botswana. The lack of geological control makes this interpretation difficult. A detailed geological mapping and the use of geochemical and geophysical techniques has been used to delineate carbonate-hosted base-metal deposits at the Otavi Land. The more expensive traditional methods necessary for the delineation of orebodies, such as pitting, trenching and drilling, are also discussed. Using a sequential approach, a possible exploration strategy is suggested, outlining the cost-effective use of remote sensing, geochemical and geophysical techniques. Standardisation in basic geological information is required for future successful explorations in the Damara-Katanga orogen, as well as attractive mining policies. In the event of their implementation, exploration perspectives are promising, specifically in terms of ore potential.

A combined petrologic, structural, and geochronologic approach has been used to characterize a zone in southwestern New England affected by superimposed metamorphisms, and the unroofing history of an area that records only the youngest metamorphism. The area chosen for the study of polymetamorphism, northwestern Connecticut and adjacent New York and Massachusetts, records Taconian (Ordovician) peak metamorphic effects in the west and Acadian (Devonian) peak effects in the east. In between, a complicated zone of overlap contains predominantly Taconian mineral assemblages that have been variably affected by Acadian processes. ⁴⁰Ar/³⁹Ar data presented here indicate that the timing of the Taconian thermal maximum was approximately 445 Ma, and that of the Acadian was approximately 390 Ma. Cooling ages suggest that the highest-grade Taconian assemblages at the present erosion surface remained at elevated metamorphic conditions between the Taconian and Acadian thermal maxima. Rim compositions of the highest-grade porphyroblasts in the Taconian zone generally yield P-T estimates that are inconsistent for subareas within a given thin section. With the onset of Acadian metamorphic overprinting conditions of approximately 500°C, the rim compositions of porphyroblasts yield P-T estimates that are much more consistent, and vary by only a maximum of ±50° C and 1 kb. These results are interpreted to indicate that the rims of preexisting Taconian porphyroblasts reequilibrated pervasively at approximately 500°C and above, whereas reequilibration was incomplete at lower Acadian overprinting temperatures. Garnet developed texturally and chemically distinct rims which appear to reflect prograde Acadian metamorphism in higher-grade parts of the Acadian metamorphic zone. Previous studies of garnet zoning have emphasized volume and surface diffusion as the primary means of changing mineral composition. However, in this polymetamorphic zone changes in the composition of preexisting garnet occurred by metasomatic dissolution and reprecipitation along porphyroblast surfaces in the presence of a metamorphic fluid. Diffusion seems significant for changing the garnet composition during the initial overprint only as an agent to move material in an intergranular fluid. The pressures and temperatures of the Acadian metamorphism a consistent metamorphic field gradient of 16 bar/°C, increasing eastward. In an area mainly affected by Acadian metamorphism, peak. metamorphic temperatures were 8.2 kb and 575°C. Nonlinear unroofing rates from ~10 to less than 1 mm/yr following the high-pressure Acadian metamorphism have been documented using combined petrologic, fluid inclusion, thermochronometric, and thermal modeling techniques. The calculated unroofing path is initially nearly isothermal and is followed by more isobaric cooling. Comparison of the inferred path to those predicted by thermal models suggests unroofing was characterized by initial rapid upward <i>en bloc</i> velocity (~ 1 cm/yr) of brief duration, followed by much slower unroofmg rates (≤ 0.3 mm/yr). This proposed unroofing history is consistent with the Paleozoic sedimentary and igneous record of western New England. / Ph. D.

A Dissertation Submitted to the Faculty of Science University of the Witwatersrand, Johannesburg for the Degree of Master of Science. / Beatrix Mine is located 35 km south of the city of Welkom in the Welkom Goldfield and as such forms the most southerly of the Witwatersrand-type gold mines. The Beatrix Reef overlies an angular unconformity at the base of the Turffontein Subgroup, Central Rand Group Significant, southerly truncation of over 600m of the Johannesburg Subgroup, and the lower formations of the Turffontein Subgroup, occur at this unconformity in the Beatrix area.. characteristics of the Beatrix Reef conglomerates such as the morphology, sorting and packing of clasts, and the arrangement. of the sediments in various sedimentary structures and facies/ sequences, suggest deposition within a braided fluvial environment on a coarse-grained braid-delta. Sedimentation occurred after the fluvial degradation of previously deposited units, and culminated in a marine/ lacustrine transgression. Low aggradation rates led to significant reworking and concentration of placer materials in a depositional model probably typical of ventral Rand Group placer formation. Heavy minerals (and gold) are concentrated in response to hydraulic conditions and show a close association with large and small scale sedimentary features. Transport directions deduced from the sedimentary structures suggest a north to south dispersal of sediment down the braid plain. Sedimentary structures in the finer rained units at the base of the Eldorado Formation are indicative of tidal influences and document the marine transgression as the culmination of the degradational events. The lithologys sedimentary structures and facies sequences of the coarser grained units of the Eldorado Formation well as the overall coarsening upward of these lithologies indicate sedimentation in a braided , fluvial system, on an alluvial fan prograding across the preyiously deposited units" Sedimentary ~tructures and lithologic variations confirm a continued north to south dispersal pattern. In the area south of the Sand over the period of fluvial degradation and transgression after the formation of the Beatrix: Reef was followed by more rapidly aggreding fluvial progradation due to a major change in base level in response to compressional tectonics and uplift along the Western Margin Structure. Only in post-Central Rand Group times did relaxation and extensional tectonics result in the outpourings of the Ventersdorp .supergroup lavas and the cessation of active Witwatersrand Supergroup sedimentation. / Andrew Chakane 2018