This thesis is directed at exploiting information in the coda of seismic phases and the ambient noise field to provide new constraints on the structure of the Australian Continent. ¶ The exploitation of the immediate coda following the onset of P waves from a distant earthquake using radial receiver functions is now a well established method. The 40 sec interval following P contains reverberations and conversions, by deconvolving the radial component trace with the vertical components, the conversions are emphasized by canceling the part of the response that are common to both components. A member of different styles of such deconvolution, are investigated and a variant of the multitaper method is adopted for subsequent applications. The TASMAL experiment 2003-2005 spans the expected location of the transition between Precambrian and Phanerozoic Australia. The 20 portable broadband stations were exploited in receiver function studies to extract S wave crustal structure through the inversion of stacked receiver functions using the Neighbourhood Algorithm. There is no clear crustal transition associated with the presence of Tasman Line. The Precambrian Cratons tend to exhibit crustal thicknesses close to 40 km but such values are also found in some Phanerozoic sites. ¶ The second part of the thesis is directed at the exploitation of ambient noise or seismic coda to gain information on the Green's function between seismic stations. The TASMAL experiment covered a significant fraction of the Australian continent with a simultaneous deployment of portable broadband stations. From these continuous records, it has proved possible to extract very clear Rayleigh wave signals for station separations up to 2000 km, and to demonstrate the frequency dependent variations in group velocity behaviour. The combination of the paths between the 20 stations localize such behaviour, but detailed images needed more data. The entire archive of portable broadband data recorded by RSES was mined, and combined with data from permanent stations to provide more than 1100 estimates of interstation Green's functions within Australia. Group velocity analysis as function of frequency was followed by nonlinear tomography with the Fast Marching Method. The resulting images of group velocity patterns as a function frequency show pronounced regions of lowered group velocities, most of which match regions of thick sediment. The frequency dependence is not consistent with just sedimentary structure and low midcrustal velocities, most likely due to elevated temperatures, are also needed. ¶ The surface wave portion of the interstation Green's function is the most energetic, and is normally all that seen in ambient noise studies. However, in the coda of events record at the broadband Warramunga seismic array in the Northern Territory, the P and S body wave components also emerge. The characteristics of these arrivals match those observed from nearby small earthquakes. The stacked cross-correlation is the normal approach to enhance Green's function information from ambient noise, but a broader spectral band width with the same phase response can be found by spectral division. It appears advantageous to compare both approaches and select the best result, since very little modifications to procedures are needed. ¶ The properties of the ambient noise at a single station have been investigated in the logarithmic spectral domain and a station dependent signal can be extracted by stacking. The signal appears to be related to the local structure beneath the station, and when fully characterized may provide a new means of investigating structure.

Thesis (M.S.)--University of Texas at El Paso, 2008. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

Thesis (Ph.D.)--Memorial University of Newfoundland, 2006. / Bibliography: leaves 309-318. Also available online.

Interpreting the tectonic significance of high strain zones requires detailed knowledge of the P-T-t-D history of rocks on either side and of tectonized rocks within the shear zone. In-situ monazite geochronology is particularly useful because it generates a time-integrated framework of metamorphism and fabric development. This can be achieved by correlating monazite compositional domains with the growth and consumption of major phases. Furthermore, monazite can be a fabric forming mineral, and can be directly linked to structural fabrics and kinematics. The Cora Lake shear zone (CLsz) represents a major lithotectonic discontinuity within the deep crustal Athabasca Granulite terrain, and preserves intense mylonitic to ultramylonitic fabrics. The 3-5 km wide CLsz strikes ~231°, and dips ~62° to the Northwest, has a moderately plunging stretching lineation (SW trend) with abundant sinistral kinematic indicators. These data indicate oblique extension with NW hanging wall down and to the SW relative to the SE footwall. The NW hangingwall is dominated by the ca. 2.6 Ga charnockitic Mary batholith. The southeastern footwall is primarily underlain by the heterogeneous ca. 3.3-3.0 Ga Chipman tonaite straight gneiss. Although both share common Archean (ca. 2.55 Ga) and Paleoproterozoic (ca. 1.9 Ga) deformation events, the style and P-T conditions of deformation are different. The earliest phase of deformation within the NW hangingwall consists of a penetrative subhorizontal flow fabric at 0.9 GPa and ~725°C (2.56 Ga), but folding in the SE footwall associated with the development of a strong upright axially planar fabric at 1.35 GPa and 850°C. Deformation at ca 1.9 Ga was characterized by upright folding, similar in orientation, in both hangingwall (0.9 GPa; 725°C) and footwall (1.17 GPa; 825°C). Deformation related to the CLsz occurred at 1880 Ma (0.9-1.06 GPa; ~775°C), and is responsible for juxtaposing two levels of lower crust. The Cora Lake shear zone is interpreted to be the culmination of a trend of increased strength, localization, strain partitioning, and vertical coupling. Furthermore, the CLsz overprints fabrics from each wall, marks the development of a major lateral lithotectonic discontinuity, and an introduction of major structural and compositional heterogeneity within the lower continental crust.

No description available.

The Western U.S. has experienced widespread extension during the past 10’s of millions of years, largely within the Basin and Range and Rio Grande Rift provinces. Tomography results from previous studies revealed narrow fast seismic velocity anomalies in the mantle on either side of the Rio Grande Rift as well as at the western edge of the Colorado Plateau. The fast mantle anomalies have been interpreted as down-welling that is part of small scale mantle convection at the edge of extending provinces. It was also found that crust was thicker than average ab¬¬ove the possible mantle down-welling, indicating that mantle dynamics may influence crustal flow. We present results from P/S conversion receiver functions using SIEDCAR (Seismic Investigation of Edge Driven Convection Associated with the Rio Grande Rift) data to determine crustal and lithospheric structure beneath the east flank of the Rio Grande Rift. Crustal and lithosphere thickness are estimated using P-to-S and S-to-P receiver functions respectively. Receiver function migration methods were applied to produce images of the crust and lithosphere. The results show variable crustal thickness through the region with an average thickness of 45 km. The crust achieves its maximum thickness of 60km at 105W longitude, between 33.5N and 32.2N latitude. This observation confirms previous receiver function results from Wilson et al, 2005. Body wave tomography (Rocket, 2011; Schmandt and Humphreys, 2010) using similar data to what we used for the receiver function analysis, shows mantle downwelling closely associated with the thickened crust. We believe that the thickened crust might be due to lower crustal flow associated with mantle downwelling or mantle delamination at the edge of the Rio Grande Rift. In this model the sinking mantle pulls the crust downward causing a pressure gradient within the crust thus causing the flow. Our S-P images show signal from the lithosphere-asthenosphere boundary (LAB) with an average LAB thickness of 100 km but with a sharp transition at about 1050 W from 75 km to over 100 km. The region with abnormally thick crust overlies a region where the lithosphere appears to have a break. We interpret our results as showing that lower lithosphere has and is delaminating near the edge of the Great Plains accompanied by lower crustal flow in some places determined by lower crustal viscosity. / text

Silicon (Si) isotopes have been extensively studied in low temperature environments but the science of Si isotopes in igneous material has been comparatively ignored. This is because the degree of isotopic fractionation at high temperatures is relatively small, making the accurate measurement of these variations extremely challenging. Using state-of-the-art analytical techniques and instrumentation, which deliver high levels of precision, the objective of this research is to rectify this omission. Specifically, this study aims to investigate whether there are systematic Si isotope variations within, and provide robust Si isotopic compositions for, the major silicate reservoirs on Earth. To this end, a broad range of mantle and crustal lithologies, sourced globally and from various tectonic regimes, have been analysed using high resolution MC-ICP-MS. Analyses indicate that the Si isotopic compositions of mantle-derived mafic and ultramafic material are extremely homogeneous. These data are used to calculate a Bulk Silicate Earth (BSE) average of δ³⁰Si = -0.29 ± 0.08 ‰ (2 s.d.). The degree of Si isotopic fractionation as a result of magmatic differentiation has also been assessed, and found to be small but resolvable between basalt and rhyolitic end-members. Finally, this research shows that, although igneous rocks and sediments derived from the continental crust can be relatively heterogeneous with respect to Si isotopes, bulk averages calculated for the upper, middle and lower continental crust are all very similar to that of BSE. Providing robust estimates for these reservoirs has greatly improved our knowledge of the behaviour of Si isotopes in silicate lithologies and provides a framework for further Si isotopic investigations of such material.

Abstract Three dimensional finite strain and kinematic data from the Resolution Island Shear Zone, Fiordland, New Zealand record the progressive evolution of a lower crustal metamorphic core complex. The Resolution Island Shear Zone is a mid-Cretaceous (~114-90 Ma) extensional shear zone that juxtaposes high-pressure (P~17-19 kbar) garnet-granulite and eclogite facies orthogneiss from the lower crust against mid-crustal (P~6-8 kbar) orthogneiss and paragneiss along a low-angle upper amphibolite facies ductile normal fault. In the lower plate of the Resolution Island Shear Zone the high-pressure garnetgranulite and eclogite facies gneissic foliations (S1) are attenuated by granulite facies extensional shear zone foliations (S2). Retrograde metamorphism marked by the breakdown of omphacite and garnet to amphibole and feldspar in S2 foliation records the unloading of the lower plate during extension. Continued extension localized strain into weaker amphibole and feldspar-bearing lithologies. Upper amphibolite facies shear zones anastomose around rigid lenses that preserve the S1 and S2 fabric. Upper amphibolite facies shear zone fabrics (S3/L3) that envelop these pods display a regional-scale domeand- basin pattern. These shear zones coalesce and form the Resolution Island Shear Zone. Coeval with the formation of the Resolution Island Shear Zone, a conjugate, southwest dipping, and lesser magnitude shear zone termed the Wet Jacket Shear Zone developed in the upper plate of the Resolution Island Shear Zone. Three-dimensional strain analyses from S3/L3 fabric in the Resolution Island Shear Zone show prolate-shaped strain ellipsoids. Stretching axes (X) from measured finite strain ellipsoids trend northeast and southwest and are subparallel to L3 mineral stretching lineations. Shortening axes (Y, Z) are subhorizontal and subvertical, respectively, and rotate through the YZ plane of the finite strain ellipsoid. This pattern reflects the dome-and-basin geometry displayed by anastomosing S3 foliations and indicates the Resolution Island Shear Zone developed in the field of constriction. Threedimensional kinematic results indicate a coaxial-dominated rotation of stretching lineations toward the X-axis in both the XZ and XY planes of the finite strain ellipsoid. Results suggest that a lower crustal metamorphic core complex developed in a constrictional strain field with components of coaxial-dominated subvertical and subhorizontal shortening. Mid-Cretaceous (~114-90 Ma) extensional structures exposed in Fiordland, including the Resolution Island, Wet Jacket, Mount Irene and Doubtful Sound shear zones and the Paparoa metamorphic core complex allows the reconstruction of a crustal column that describes the geometry of mid-Cretaceous continental rifting of Gondwana. The overall symmetry of crustal-scale structures during continental extension suggests kinematic links between flow in the lower crust and the geometry and mode of continental extension. This result is consistent with numerical models of lithospheric rifting that predict the lower crust has a primary control on the style of continental extension.

ABSTRACT Doubtful Sound, in SW New Zealand, exposes an exhumed section of lower crust that represents the root of an Early Cretaceous magmatic arc. Here, the lower crust underwent a change from contraction to extension and these tectonic cycles are fundamental to the growth of continental crust. Mafic-intermediate granulite gneisses occur below the extensional Doubtful Sound shear zone (DSSZ) which records the retrogression and transposition of granulite fabrics at the upper amphibolite facies. I compared 3D rock fabrics, microstructures and textures within and below the DSSZ to determine the processes involved in the shift from contraction to extension and to infer the sequential processes of transforming L>S granulites to L=S amphibolites. Below the DSSZ, dehydration zones around felsic veins and leucosome in migmatitic orthogneiss record granulite facies metamorphism. Aggregates of clinopyroxene (cpx) and orthopyroxene (opx) that are rimmed by garnet (grt) and interstitial melt are set in a plagioclase (pl) matrix. Peritectic grt, pl-grt symplectites, beads of pl along grain boundaries, and elongate, inclusion-free pl reflect the anatexis. Pl exhibits a crystal preferred orientation (CPO) and evidence of subgrain rotational recrystallization and grain boundary migration, indicating subsolidus deformation outlasted melting. Mafic aggregates are boudinaged and opx developed subgrains. During peak metamorphism high strain was partitioned to locations enriched in melt, producing L>S fabrics and an upward trajectory in the strain path. A comparison of mineral grain shapes indicates that pl accommodated most of the strain. Granulite-amphibolite transitional rocks inside the DSSZ record a heterogeneous retrogression of the granulites to a polyphase metamorphic assemblage of hornblende (hbl), biotite (bt), and fine pl. Also preserved is the resetting of high strain L>S granulite to low strain, L=S amphibolite. Folia of porphyroblastic hbl + bt progressively penetrate the pl matrix via solution mass transfer. Porphyroblastic pl in the rock matrix becomes increasingly transposed to gneissic layering. A path of decreasing gradient from high strain L>S granulite to low strain L=S amphibolite reflects the development of the DSSZ fabric, growth of new minerals and onset to deformation at the amphibolite facies. Inside the DSSZ, amphibolites show an increasing strain gradient from low strain L=S amphibolite to high strain L=S amphibolite. Pl aggregates lack a CPO and are mostly annealed but preserve grain boundary migration microstructures. Hbl is recrystallized and forms asymmetric fish. Evidence of high fluid activity and reaction softening within the DSSZ include increased hbl + bt and bt beards on pl relative to rocks outside the DSSZ. My observations suggest that magma, heat, and melting initially weakened the lower crust, facilitating the development of high strain zones with L>S fabrics. Partially molten regions deformed by suprasolidus flow and solid portions deformed mostly by dislocation creep in pl and boudinage of cpx + opx. Later, the lower crust was weakened and high strain fabrics were reset from overprinting and transposition as retrogression progressed and low strain L=S fabrics formed. During extension there was an upward trajectory in the strain path to high strain L=S fabrics within the DSSZ, where hbl and bt accommodated more strain. My results illustrate the importance of 1) melting, cooling, and hydration in controlling strain partitioning and the rheological evolution of lower crustal shear zones, and 2) the importance of integrating microstructural and fabric analysis to determine strain paths.

The general aim of this thesis is to investigate heterogeneity in the structure of the crust and uppermost mantle of Archaean and Proterozoic terrains in southern Africa and to use the findings to advance our understanding of Precambrian crustal genesis. Teleseismic, regional and local seismic recordings by the broadband stations of the Southern African Seismic Experiment (SASE), Kimberley array, South African National Seismograph Network (SANSN) and the Global Seismic Network (GSN) are used in the inversion procedures to address the aim of this thesis. In the first part of the thesis, the nature of the lower crust across the southern African shield is investigated by jointly inverting receiver functions and Rayleigh wave group velocities. The resultant Vs models show that much of southern Africa has a lower crust that is mafic in composition, whereas the western parts of the Kaapvaal and Zimbabwe Cratons have a lower crust that is intermediate-to-felsic in composition probably due to rifting. The second part of the thesis evaluates the “dipping-sheet” and “continuous-sheet” models of the Bushveld Complex using better-resolved seismic models derived in a two-step approach, employing high-frequency Rayleigh wave group velocity tomography and the joint inversion of high-frequency receiver functions and 2–60 sec Rayleigh wave group velocities. The resultant seismic models favor a “continuous-sheet” model of the Bushveld Complex, although detailed modelling near the centre of the Complex shows that the subsurface mafic layering could be disrupted. The third part of the thesis, is focused on jointly inverting high-frequency teleseismic receiver functions and 10–60 sec Rayleigh wave group velocities to place shear wave velocity constraints on the source of the Beattie Magnetic Anomaly (BMA) at depth and to evaluate existing geophysical models of the BMA source. The resultant Vs models across the BMA suggest the BMA source to be at upper to middle crustal depths (5–20 km) with high velocity layers (≥ 3.5 km/s). Further to this, is a lower crust that is highly mafic (Vs ≥ 4.0 km/s) and a crust beneath the BMA that is on average thicker than 40 km. Plausible models of the BMA source are massive sulphide ore bodies and/or mineralized granulite-facies mid-crustal rocks and/or mineralized Proterozoic anorthosites. v Overall, the findings in this research project are consistent with the broad features of a previous model of Precambrian lithospheric evolution but allows for refinements of that model.