The Tonga-Kermadec arc in the SW Pacific comprises a chain of more than 90 volcanic complexes. A continuous 400-km long chain of volcanic activity along the central portion of the Tonga arc has become the focus of intensive research, extending previous studies that have focused on the southern Kermadec chain. Earlier interpretations of the Tonga arc have focused on a perceived lack of volcanism between ~21°S and ~27°S, adjacent to a bend in the trench caused by the collision of the subducting Louisville Seamount Chain (LSC). During swath mapping in 2002, it was revealed that this portion of the arc, including the Louisville and Monowai segments, is in fact one of the most volcanically active parts of the Tonga-Kermadec system. At this location, a combination of oblique convergence of the Pacific Plate and southward compression due to the collision of the LSC has resulted in left-lateral strike-slip faulting and rifting of the arc crust. This has produced a series of left-stepping arc transverse graben and horst structures that localize the voluminous volcanic activity. For this study, a new 1:250,000 scale geological map of the Louisville and Monowai segments has been constructed as a framework for a quantitative analysis of arc volcanism and the eruptive history of these segments. Two types of volcanoes dominate the arc front: deep caldera systems (collapse structures formed due to the evacuation of magma) within the arc rifts, and smaller volcanic cones between the rifts. The cone volcanoes tend to have small summit craters (<10 km3) whereas the large caldera volcanoes have major depressions of up to 50 km3. The cones are relatively undeformed, whereas the larger calderas are affected by multiple stages of collapse, asymmetric subsidence, and distortion caused by regional stresses. Surveys of the crater walls of the cone volcanoes show a predominance of volcaniclastic deposits, whereas the caldera volcanoes contain a high proportion of coherent lava flows. The caldera volcanoes also show a prevalence of basaltic melts compared to the more andesitic and dacitic cones. The largest caldera volcano is the Monowai volcanic complex (25°53’S) occupying a deep depression (Monowai Rift Graben) that crosses the arc front. The volcanic complex consists of a large caldera (12 km wide, 1600 m deep) and an adjacent stratovolcano (Monowai Cone) rising nearly to sea level. We suggest that the different types of volcanoes along the Louisville and Monowai segments reflect the influence of deep structures within the arc crust that have localized strikeslip and normal faulting.

The composition of volcanic products can provide critical information about the source and the conditions of melting. This information is used to highlight differences in melting environments from volcanic regions around the globe. Volcanic lavas and other products from the Rungwe Volcanic Province, in southwest Tanzania (9.13S,33.67E), were collected and studied to test a number of lingering questions about the role of magmatism in a continental rift tectonic environment. The Rungwe Volcanic Province is the only region in this portion of the East African Rift (EAR) system with apparent magmatism. Is magmatism here the product of rifting, like melts generated in oceanic rift tectonic environments (Mid-ocean ridge basalts, MORB), or is melting here facilitated by the upwelling asthenospheric mantle, like melts generated at hotspots or plumes (oceanic intraplate basalts, OIB)? To address this, contributions from the continental lithosphere must also be identified and addressed. Each chapter of this dissertation approaches this fundamental question using different aspects of the comprehensive chemical and isotopic dataset from this study. The second chapter outlines a novel thermobarometer that is then applied to Rungwe samples to estimate the temperatures and depths at which the melts equilibrated. Laboratory melt experiments of garnet peridotite, some containing CO2, create melt with major element characteristics applicable for pressure and temperature estimation of Rungwe samples. The parameterization of Al2O3 and MgO from the experimental melt compositions provides a thermobarometer with a temperature range of 1100-1500C (16C, 1), and a pressure range of 2-5 GPa (0.2 GPa, 1). The maximum potential temperature reached for Rungwe samples is 1372C. Potential temperatures at Rungwe overlap with the ambient asthenospheric mantle, as sampled by the global range of MORB. Potential temperature range for Rungwe is too high for melts to have a derivation from the continental lithosphere, and too low for melts to be derived from the thermally-driven plume. The pressures of melt equilibration for Rungwe span a range from GPa, when converted to depths is 55-101 km. Depth estimates can be compared to the estimated depths of the lithosphere-asthenosphere boundary (LAB) from seismic tomography models. Rungwe melts appear to be derived from the depths at or below the LAB, supporting their derivation from an asthenospheric source. Under the same parameters, other volcanic regions from the Western Branch of the EAR give similar results, while maximum potential temperatures from the Eastern Branch exceed estimates from the ambient asthenospheric mantle, providing more support for a thermally-derived mantle plume there. The third chapter provides a timeline of volcanism at Rungwe including ages from Ar-Ar geochronology performed on samples from this study, as well as dates of two precursor carbonatite bodies in the vicinity of the volcanic province. Most of the Rungwe Volcanic Province was emplaced between present-9Ma, with emerging evidence for eruptions between 9Ma and ~25Ma. A proposed broadening of the age range of each volcanic stage definition helps to include eruptions prior to 9Ma, and encompass eruptions shown to have occurred between the original volcanic stage age ranges. Two carbonatite bodies in the northwest edge of the volcanic province date to 169.0 0.6 Ma and 154.4 0.9 Ma, and show no evidence of Cenozoic reactivation. The emplacement ages of the majority of Rungwe samples coincide with accelerated rifting and basin formation present-9Ma. The updated timeline of Rungwe volcanism suggests that eruptions prior to 9Ma are still tied to tectonic extension, based on comparison to thermochronology cooling ages from the major border faults. The fourth chapter characterizes and provides context about the chemical and isotopic composition of the mantle source of Rungwe melting. Isotopic Sr-Nd-Pb-Hf, as well as major and trace elemental compositions provide a fingerprint for Rungwe melts in which to compare to the range of global OIB and to other EAR melts. The majority of Rungwe melts possess isotopic traits that are consistent with an asthenospheric plume-derived source. Many isotopic and trace element ratio characteristics identified are not shared with any identified OIB-source volcanic region, but are present in other EAR volcanoes. These indicators suggest that some Rungwe melts, together with some EAR volcanoes, share a common source characteristic or melt process that the global OIB does not sample or experience. Homogeneity of plume source or continental lithosphere over the large geographic distances between volcanic provinces in the EAR are not expected. No OIB emplaced on oceanic crust must traverse Archaean or Proterozoic subcontinental lithosphere or crust. The influence of melt interaction with these elements are explored in detail as the main cause of differences between OIB and Rungwe compositions. Metasomatic phases accumulated by melt interaction at the LAB interface over eons create compositions that can influence low-volume melts that traverse them. It appears that no Rungwe melt evaded this overprint from the subcontinental lithospheric mantle, despite large-scale preservation of the plume-derived melt origin.

No description available.

Through geodynamic modeling, I investigate how Mars could have produced the extensive volcanism required to form the Tharsis rise early in its history, as well as continue to produce small amounts of melt up to present-day, in order to account for the evidence of limited geologically recent volcanism. InSight is the first interplanetary mission dedicated primarily to the study of a planet's deep interior, and has provided useful constraints for the present structure and interior temperature of Mars. I use the results from InSight and other spacecraft missions to more accurately model Mars, and evaluate the results of my geodynamic models, so as to constrain the properties that are necessary for or consistent with both the InSight results and the volcanic history reflected on the surface. This modeling has required extensive modification to the CitcomS geodynamic code I use, the bulk of that effort being in implementing and testing the melting calculations. One of the useful constraints that would have been provided by InSight would have been ground truthing the heat flow from the interior at the landing site, and this required determining, among other quantities, the thermal conductivity of the regolith into which the heat flow probe (mole) was placed. While the mole could not penetrate to its designed depth, thus disallowing the complete heat flow measurement, the team were able to obtain the necessary data determine the thermal conductivity, and how it varies seasonally. My rapid analytical method of estimating thermal conductivity produces results that agree surprisingly well with those of the team's complex numerical model, despite the mole not meeting the assumption of a sufficiently high length to width ratio. / Doctor of Philosophy / I investigate how Mars could have produced the extensive volcanism required to form the Tharsis rise early in its history, as well as continue to produce small amounts of melt up to present-day, in order to account for the evidence of limited geologically recent volcanism. I use 3D computer models of the mantle--the solid, but slowly flowing layer that makes up the bulk of rocky planets like Earth and Mars. InSight is the first interplanetary mission dedicated to the study of a planet's deep interior, and has provided useful constraints for the present structure and interior temperature of Mars. I use the results from InSight and other spacecraft missions to more accurately model Mars, and evaluate the results of my models, so as to constrain the properties that are necessary for or consistent with both the InSight results and the volcanic history reflected on the surface. This modeling has required extensive modification to the modeling code I use, the bulk of that effort being in implementing and testing the melting calculations. One of the useful constraints that would have been provided by InSight would have been ground truthing the heat flow from the interior at the landing site, and this required determining, among other quantities, the thermal conductivity of the soil into which the heat flow probe (mole) was placed. While the mole could not penetrate to its designed depth, thus disallowing the complete heat flow measurement, the team were able to obtain the necessary data determine the thermal conductivity, and how it varies seasonally. My rapid analytical method of estimating thermal conductivity produces results that agree surprisingly well with those of the team's complex numerical model, despite the mole not meeting the assumption of a sufficiently high length to width ratio.

The Black Rock volcanic cluster consists of 30 small volume monogenetic volcanoes. The volcanoes of this cluster have exhibited bimodal volcanism for > 9 Ma. The most recent eruption of Ice Springs volcano ~600 yrs. ago along with ongoing geothermal activity attests to the usefulness of a hazard assessment for this area. The likelihood of a future eruption in this area is estimated to be between a 0.16 and 24% chance over the next 1 Ka (95% confidence). The explosivity and nature of many of these eruptions is not well known. In particular, the physical volcanology of Tabernacle Hill suggests a complicated episodic eruption. Initial phreatomagmatic eruptions at Tabernacle Hill are reported to have begun no later than ~14 Ka. The initial eruptive phase produced a tuff cone approximately 150 m high and 1.5 km in diameter with distinct bedding layers. Recent mapping and sampling of Tabernacle Hill's lava and tuff cone deposits was aimed at better constraining the sequence of events, physical volcanology, and energy associated with this eruption. Blocks located on the rim of the tuff cone of were mapped and analyzed to yield preliminary minimum muzzle velocities of 60-70 m s-1. After the initial phreatomagmatic explosions, the eruption style transitioned to a more effusive phase that partially filled the tuff cone with a semi-steady state lava lake 200 m wide and 15 m deep. Eventually, the tuff cone was breached by the impinging lava resulting in large portions of the cone rafting on top of the lava flows away from the vent. Eruption onto the Lake Bonneville lake bed allowed the Tabernacle Hill lava flows to flow radially from the tuff cone and cover an area of 19.35 km², producing a very uniform high aspect ratio (100:1) flow field. Subsequent eruptive phases cycled several times between effusive and explosive, producing scoria cones and more lava flows, culminating in an almost complete drainage of the lava lake through large lava tubes and drain back.

Graduation date: 1998 / Best scan available for figures. Original is a black and white photocopy.

Thesis (Ph.D.)--Victoria University of Wellington, 2009. / Includes bibliographical references.

Basaltic volcanism in the form of small-volume, subaerial and subaqueous eruptions have occurred in the Wells Cray—Clearwater area of east central British Columbia. These eruptions have been dated by the K-Ar method and by relationships to dated glaciations. The oldest known eruption may be as old as 3.2 Ma, but is more likely 2 Ma or less. The youngest eruptions are less than 7560 ± 110 radiocarbon years. The most extensive basalts are valley-filling and plateau-capping flows of the Clearwater unit, which are Pleistocene in age and greater than 25 km³ in volume. The deposition of flows of the Clearwater unit has overlapped at least three periods of glaciation. The interaction of glacial ice and basaltic magma has been recorded in the form of tuyas, ice ponded valley deposits and subglacial mounds (SUGM). In a few place glacial till has been preserved beneath basalt flows. Flows of Wells Gray—Clearwater suite appear to have erupted from vents that are both spatially and temporally separated. The individual eruptions were of low volume (<1km³) and chemically distinct from one another. Major element composition is variable but the lavas are predominantly alkalic. Olivine is the predominant phenocryst phase. Plagioclase and augitic clinopyroxene rarely occur as phenocrysts, but both minerals are ubiquitous in the groundmass. Orthopyroxene was not seen in any of the samples. Flows appear to have erupted with minimal crystal fractionation or crustal contamination. The range of compositions seen in the suite is best explained by a process of partial melting and the progressive depletion of the mantle source by earlier melts. Progressive depletion of the mantle source was coupled with enrichment of parts of the mantle in K as well as some lithophile and siderophile elements. Increasing alkali content may have triggered the highly enriched eruptions of Holocene age that, despite very low degrees of partial melting, were capable of reaching the surface. Overprinting the effects of partial melting are inherited heterogeneities in the source zone of the magmas. Based on whole-rock chemistry the magma source appears to be a highly depleted region similar to that which produces the most depleted mid-ocean ridge basalts (MORB). The zone is, however, capable of producing large volume (≃ 15%) partial melts and has not been isotopically depleted to the same extent as MORB source regions. Isotope analyses of ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd and whole-rock Pb indicate that the magmas may be derived from a remnant of subducted oceanic lithosphere which has been variously depleted by the prior generation of basaltic melts. Isotopic enrichment above the level seen in MORB's is due in part to crustal contamination. The isotopic results are very different than those obtained from samples erupted through thin, allochthonous crust in the Intermontane Belt and may be explained in part by generation of the magmas in oceanic material which was subducted when allochthonous crust lay against the parautochthonous rocks underlying the Wells Cray—Clearwater area. The alkali olivine basalts of the Wells Cray—Clearwater area have erupted onto a tectonically active surface. A peneplain (erosion surface), formed in Eocene-Miocene time has been uplifted since the Miocene and uplift may be continuing. This uplift is in response to an elevated geothermal gradient which may be due to crustal extension. This crustal extension may be similar to that which occurred in the Eocene. The elevated geothermal gradient and reduced pressures attendant with recent uplift and erosion may have initiated basaltic volcanism in the region, rather than a fixed mantle hot spot as proposed in earlier work. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

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No description available.