Data from three refraction profiles of the 1993 LITHOPROBE refraction experiment were used to investigate the structure of the lithosphere of the Trans-Hudson Orogen. Novel digital processing of the wide-angle reflections arrivals with standard type reflection processing techniques revealed significant crustal thickness and Moho reflectivity variations within the orogenic belt. The obtained information was than complemented with the results of near-vertical incidence seismic sections to estimate the crustal thickness variations over the entire study area. The detected crustal thickness and Moho reflectivity changes could not be correlated to the location and extent of geological domains; they appear to reflect the complex deformation and metamorphic history of the orogen. The P-wave velocity image of the crust and upper mantle was established through ray-tracing and inverse modelling of the primary and secondary crustal and mantle arrivals. Successful modelling of the observation required the incorporation of non-standard inversion techniques into the processing sequence. Although the detected crustal velocity variations appear to correlate well with the changes of Moho reflectivity. These variations in the property of the crust are interpreted to be a consequence of differences between the tectonic evolution of orogenic units in the north-western and south-eastern parts of the study area. The transition belt separating these two areas appear to coincide with an anomalous zone located in the upper mantle. This mantle region exhibits strong P-wave velocity anisotropy, determined primarily from modelling of the mantle refraction arrivals. I interpret this anomalous mantle region as a highly deformed zone, a possibly suture, between the two collided Archean plates of the orogen. Additional information on the structure of the mantle was obtained by analyzing the secondary mantle phases, observed at offsets larger than 400 km on the shot records. The broad structure of this deeper mantle region was established by incorporating the results of regional teleseismic studies into the velocity models and refining the models with raytracing. Detailed acoustic properties of the mantle were investigated after introducing random perturbations into the models and comparing the computed finite-difference synthetic-seismic responses to the observations.

No description available.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2015. / In this thesis the crustal structure beneath two areas of Africa is investigated. In Sudan, the new constraints on the crustal structure beneath the northern part of the Khartoum basin have been obtained. In East Africa, the size of the Tanzania Craton, and the differences between the Eastern and Western branches of the East African Rift System (EARS) have been determined. In southern Tanzania, the debate on the secular variation between Proterozoic and Archean crust has been investigated. The approach used in this thesis involves different data sets and methods. In first part of the thesis, the crustal structure of the northern part of the Mesozoic Khartoum basin is investigated by using two modelling methods: H-k stacking of receiver functions, and a joint inversion of receiver functions and Rayleigh wave group velocities. H-k stacking indicated that the crust is 33-37 km thick with an average of 35 km, and the crustal Vp/Vs ratio is 1.74-1.81 with an average of 1.78. Similar results were obtained from the joint inversion for Moho depth, as well as an average shear wave velocity of 3.7 km/s for the crust. These results provide the first seismic estimates of Moho depth for a basin in Sudan. When compared to average crustal thickness for unrifted Proterozoic crust in eastern Africa, our results indicate that only a few kilometers of crustal thinning may have occurred beneath the Khartoum basin. This finding is consistent with estimates of effective elastic plate thickness, which indicate little modification of the Proterozoic lithosphere beneath the basin, and suggests that there may be insufficient topography on the lithosphere-asthenosphere boundary beneath the Sudanese basins to channel plume material westward from Ethiopia. In the second part of the thesis, the uppermost mantle structure beneath East Africa is investigated by using a standard singular value decomposition algorithm model. Results reveal fast Pn velocities beneath the Mozambique Belt to the east of the craton, the Kibaran Belt west of the craton, and beneath the northern half of the Ubendian Belt to the southwest of the craton. These results indicate that the cold, thick lithosphere of the Tanzania Craton extends beneath the Proterozoic mobile belts and the areal extent of the cratonic lithosphere is much larger than is indicated iv by the mapped boundaries of the craton. The results also show that the Pn velocities beneath the volcanic provinces along the Western Branch are not anomalously slow, which indicates little, if any, perturbation of the uppermost mantle beneath them. This is in contrast to the upper mantle structure at depths ≥ 70 km beneath the volcanic regions, which is clearly perturbed. The fast Pn velocities beneath the Western Branch contrast with the slow Pn velocities (7.5-7.8 km/s) beneath the Eastern Branch in Kenya, indicating that the upper mantle beneath the Eastern Branch has been altered more than beneath the Western Branch. In the third part, the crustal structure beneath two Proterozoic mobile belts, the Usagaran and the Ubendian belts, is investigated by using the Non-Dominated Genetic Algorithm method. In the Usagaran belt, results show an average Moho depth of 35 km for station MAFI and 41 and 42 km for stations MOGR and MIKU, respectively. In the Ubendian Belt, results showed an average Moho depth of 43 km beneath the Ufipa sub-terrane compared to 39 km for Wakole sub-terrane. These results indicate localized thickening in the Ufipa sub-terrane, but not beneath the entire Ubendian Belt. These results indicate that is no clear evidence that Paleoproterozoic crust in East Africa is substantially thicker than Archaean crust.

It is suggested that the Mafic Units and HS are allochthonous and were emplaced onto the WGR during an early stage of the Caledonian Orogeny. All units, including the Basement Gneisses, have suffered retrogression during a late extensional phase which continued into at least the Middle Devonian.

The Lofoten-Vesterålen province of North Norway consists almost exclusively of Precambrian granulite-facies rocks. The oldest rocks in the province are monzonitic and dioritic migmatitic gneisses, the protoliths of which were formed at 2.7 Ga. The migmatites are overlain by a series of supracrustal gneisses, from 2.1 Ga, largely volcanogenic in origin, but with interbedded marbles and banded ironstones. The first occurrence of marble in western Lofoten is reported. Deposition in a subsiding back-arc basin or in an Andean- type environment on a thin continental margin is inferred. Both gneiss sequences were intruded by basic rocks at 1.8 Ga. The basic rocks could not have been formed simply by extraction from the mantle at 1.8 Ga. The required contribution from 2.7 Ga migmatites could be as much as 37%, but less if contamination took place via anatectic melts. The first report of eclogitic rocks from the Lofoten-Vesterålen province is made in this study; their formation is associated with shear deformation Both gneiss sequences and the basic rocks were intruded by mangeritic rocks at 1.8 - 1.7 Ga. Their chemical compositions can be explained by fractional crystallization from magmas formed from 2.7 Ga and 2.1 Ga gneisses and 1.8 Ga mantle-derived magmas. Parameters derived from Rb-Sr, Sm-Nd and U-Pb systems to express the relative proportions of crust and mantle contributions to the mangerites mutually correlate, supporting the crust-mantle source model for the mangerites. Mixing calculations suggest that the late Archaean contributes in excess of 50% by mass for almost all mangerites. Anatectic veins present, especially in the Moskenesøy supracrustal gneisses, are inferred to represent partial melts which coalesced to form the mangerites at higher structural levels. Anatexis was caused by basaltic underplating associated with limited crustal extension. Later rock-forming events were the emplacement of dolerite dykes; the 1.65 Ga Lødingen Granite; the Leknes Group metasediments and the Caledonian granite pegmatites.

Thesis (M.S.)--University of Wisconsin--Madison, 1974. / Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

The Endeavour Ridge is a segment of the Juan de Fuca Ridge, an active spreading centre which lies off western North America between the Pacific and Juan de Fuca plates. This segment is a bathymetric high and a site of hydrothermal activity—both characteristics suggest an underlying heat source such as an axial magma chamber which is associated with crustal generation. To investigate the creation and evolution of oceanic crust, a detailed refraction survey was carried out over the Endeavour Ridge in the fall of 1985. As a component of this survey, a diamond-shaped array consisting of eight OBS along a 20-km line across the ridge and two OBS placed along it at distances of 10 km on either side of the cross-ridge line was deployed to define the shallow crustal structure near and beneath the ridge, especially the possible existence of an axial magma chamber. Airgun shots at 0.2 km intervals along ~300 km of profiles provide conventional reversed and unreversed refraction lines as well as multiple full azimuthal coverage of the region. Travel-time and amplitude data from fifteen in-line airgun profiles recorded on the inner array were forward modelled using an algorithm based on asymptotic ray theory with a starting model obtained from a concurrent study. Two-dimensional models were constructed and then combined to obtain the three-dimensional structure of the region. These models consist of four layers, with the average model correlating well to the classic model of oceanic crust. Layer 2A averages 0.40 km in thickness and has velocities of 2.6 km/s and 2.8 km/s at the top and bottom of the layer, respectively. To achieve such a low velocity, Layer 2A must consist of highly fractured vesicular basalts. A sharp velocity increase to 4.8 km/s marks the transition to Layer 2B. This velocity discontinuity is also visible as a reflector on a. multichannel reflection line obtained through the centre of the study region and is caused by an abrupt decrease in porosity. Layer 2B averages 0.67 km in thickness, has a velocity of 5.4 km/s at its base and consists of less fractured pillow basalts and sheet flows. The Layer 2B-Layer 2C interface is a velocity increase to 5.8 km/s and is the pillow basalt-sheeted dike contact. A small velocity increase from 6.3 to 6.5 km/s delineates the base of the 0.95 km-thick Layer 2C which is the boundary between the sheeted dikes and cumulate gabbros in Layer 3. Layer 3 has the lowest velocity gradient (0.30 s⁻¹) and a velocity of 7.3 km/s at 4.65 km below the seafloor, the maximum depth constrained by the modelling. Lateral heterogeneities on the scale of 2-3 km are superimposed on this basic velocity structure. These heterogeneities are effects of porosity changes, differential pressure changes, and alteration caused by hydrothermal circulation. Layer 2A thins and increases in velocity away from the ridge; ridge-parallel cracks create a velocity anisotropy of ~10-25%, the faster direction parallel to the ridge. Velocities within Layers 2B and 2C also increase by 0.1 km/s away from the axis of the ridge. Layer 3 velocities decrease by 0.1 km/s for arrivals travelling under the ridge. Increased Layer 2 velocities at the ridge crest reveal high lateral velocity constrasts in very young crust, but within 0.03 Ma the oceanic crust at the ridge has matured to the off-ridge structure. No firm evidence exists for a large magma chamber under Endeavour Ridge. Although the bathymetric high and high-temperature hydrothermal discharges are evidence for a magma chamber, the lack of recent sheet flows at the ridge crest and the presence of a rift along the crest indicate the magma chamber is waning and must be of a size (<1 km in width) not resolvable by seismic refraction data. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

published_or_final_version / Earth Sciences / Master / Master of Philosophy

Le bassin de Paris est considéré comme un exemple typique de bassin intracratonique (sag) affecté par une subsidence thermique long terme. Le Cénozoïque correspond à une période de faible subsidence (épaisseurs inférieures à 350m) et marque la fin du fonctionnement de ce bassin. C'est en outre une période de forte déformation de la plaque européenne, dans un contexte de convergence Afrique-Eurasie et d'ouverture de l'Atlantique Nord caractérisée par des inversions de grabens dans le Nord et l'Est de l'Europe. Si de nombreux hiatus ont été identifiés, les déformations cénozoïques du bassin de Paris, situées sur une croûte à l'équilibre et leur relation aux contraintes en limite de plaque restent méconnues. Cette thèse a pour objectif de recomposer à haute résolution spatiale et temporelle (de l'ordre du million d'années) les géométries sédimentaires 3D et les paléogéographies successives du Paléocène au début de l'Oligocène. Ce travail qui s'appuie sur de nombreuses données biostratigraphiques consiste en une approche couplée de sédimentologie de faciès et de corrélations diagraphiques (500 puits) selon les principes de la stratigraphie séquentielle. Trois ordres de séquences sont définis. Les cycles d'ordre supérieur (4ème et 3ème ordre) enregistrent les variations climato-eustatiques. Cinq séquences de dépôt (2e ordre), limitées par des discontinuités et/ou des réorganisations paléogéographiques sont identifiées : (1) Maastrichtien-Danien ; (2) Thanétien-Yprésien ; (3) Lutétien-Bartonien ; (4) Bartonien-Priabonien terminal et (5) Priabonien terminal-Chattien. Les architectures des séquences (1) à (4) sont contrôlées par des phases de flexures. Après des émersions lors des paroxysmes de flexuration, la relaxation progressive des flexures se traduit tout d'abord par la mise en place de profils pentés et ouverts, puis par des profils de plus en plus plats et confinés associés à une transgression généralisée. Du Thanétien au Bartonien s'observent des flexures d'axe E-W, dont les âges sont compatibles avec les différentes phases de la convergence Ibérie-Eurasie. En outre, une déformation de courte durée à l'Yprésien basal est rattachée à l'ouverture de l'Atlantique Nord. Enfin, une réorientation majeure du bassin possiblement liée au début de la collision Apulie-Eurasie est observée au Priabonien. Ce travail fournit un calage à haute résolution pour la compréhension et la modélisation des déformations intraplaques. Différentes tailles de flexure, de l'ordre de 150 à plus 300 km sont observées traduisant une implication d'épaisseurs plus ou moins importantes de la lithosphère. / The Paris basin is currently considered as a typical example of intracratonic basin (sag) affected by long term thermal subsidence. The Cenozoic is a period a low subsidence (less than 300m thick) and correspond to the end of the Paris basin sedimentation. Moreover, it is a period of strong deformation of the European plate related to Africa Europe convergence and North Atlantic opening, well known through numerous grabens inversions in northern and eastern Europe. While hiatus have been highlighted within Paris Basin sedimentation, cenozoic deformations of this thicker crust basin still poorly known. This thesis aims at recompose high resolution temporal and spatial evolution of 3D sedimentary geometries and palaeogeographies from Thanetian to Lower Oligocene. This work is firstly based on available and newly acquired biostratigraphic data. Facies sedimentolgy and well data correlations based on sequence stratigraphy principles allowed to recompose the basin evolution at 1Ma timescale. 2 orders of sequences were identified. Third order sequences (1My duration) seems to be controlled by climate-eustasy. Five main (2nd order) sequences bounded by unconformities and/or palaeogeographic reorganization are highlighted : (1) Maastrichtian-Danian ; (2) Thanetian-Ypresian ; (3) Lutetian-Bartonian ; (4) Bartonian-top Priabonian et (5) Top Priabonian-Chattian. Sequences 1 to 4 correspond to basin scale flexure which control their architecture. Following emersion during the main flexural phases, flooding start with relatively steep depositional profiles. As flexure progressively relax, flatter depositional profiles take place together with overall transgression. This work yield high resolution constraints for the understanding and thermomechanical modelling of intraplate deformations various lenght of flexures form 150 to 300km and more are identified and traduces different thickness of deformed lithosphere. From Thanetian to Bartonian, successives E-W oriented flexures take place which ages are congruent with the main phases of Iberia-Eurasia convergence. A short term deformation in basal Ypresian is attributed to the onset of North Atlantic opening. Finally major basin-scale reorientation during Priabonian could be linked to the onset of Apulia- Eurasia continental collision.

Thesis (MSc)--Stellenbosch University, 2001. / ENGLISH ABSTRACT: This study focussed on the comparison of accessory mineral chemistry and paragenesis in the S-, I- and A-type granites of the Cape Granite Suite. The objective of the study was to use differences in accessory mineral chemistry and petrography to give insight in the evolution, recycling and formation of continental crust as affected by the Cape Granite Suite. Because of the high partition coefficients of the REE and trace elements into accessory minerals these minerals play an important role to explain granite evolution. The accessory mineral features are used as discriminators between barren and mineralized S-, I- and A-type granites in the suite. The petrography of the suite reflects the allanite-monazite dichtonomy with allanite and titanite occurring in the I -type granites while monazite occurs in S-type granites. Monazite becomes unstable in high Ca melts such as I-type granites. Apatite occurs in all the plutons which reflects its stability over a wide range of geological conditions. Rounded crystal habits of apatite and monazite in S-type granites indicate they are relics of sedimentary source rocks. Concentric growth- and sectoral zoning, as observed with CL and SEM, are common features in minerals that crystallized in barren plutons. The overprinting of magmatic textures reflects secondary processes, such as those that occurred in mineralized plutons, by "patchy" zoning and irregular alteration rims (coronas) in the mineralized plutons' accessory minerals. CL and SEM observations revealed that REE are redistributed into these coronas. Mineral chemistry of the accessory minerals reflects mostly the whole rock chemistry and physical conditions of the magmas. (Al~ Fe) substitution in titanite is controlled by P-T conditions, together with Ca, Mn and Mg substitution which is controlled by whole-rock chemistry, are good discriminators in S- and I-type granites. LREE and Sr content in allanite discriminate between the plutons and reflect the whole-rock chemistry. Apatite, because it occurs in all the plutons, is the most useful accessory mineral for discriminating between the plutons. From previous studies it is known that ASI controls the two main substitutions in apatite: Ca+P~Si+REE and Na+REE~2Ca, Fe and Mn content in apatite (0,1 pfu Mn and 0,05 pfu Fe contents are the cut-off between S-and Itype granites) are controlled by oxidation state of the magma and Sr, REE and Mg reflect whole-rock chemistry. The content of these elements in apatite can be used as discriminators between the plutons as their ASI, oxidation states and whole-rock chemistry differ. REE patterns of monazite and allanite are LREE enriched without exception, while apatite and titanite REE patterns are mostly birdwing profiles with occurrences of LREE or HREE enrichment. These patterns are influenced by crystallization of coexisting REE-bearing phases, fractionation history of the pluton and by crystallization sequence of the accessory minerals. Phase relationships were investigated experimentally for monazite and allanite under magmatic conditions (870 °C, 1,8 kbar) in peraluminous to metaluminous granitic melts. Monazite became unstable when aqueous CaCh solutions of 0.7-7 g CaCh/10cc H20 where added to peraluminous melts (ASI> 1 ). Monazite broke down to Cl-apatite and corona textures were observed. Allanite was tested in peraluminous (ASI> 1) and metaluminous (ASI=1) melts with different P20 5 (0.08 - 0.25 wt%) concentrations. Allanite became unstable at high phosphorus and peraluminous melt conditions and broke down to LREE-P± Al, Ca, K phases. Corona (kelyphitic) textures were observed. It is also clear that phosphorus played an important role, with Al, in the melt structure as can be seen from the absence or presence of crystals in the glasses of the different melts. This is possible because adding of phosphorus to the melt results in a depression of the T of the granitic melts' liquidus. Because of an interaction of phosphorus with Si networks and formation of complexes it also depolymerize aluminosilicate melts. The solubility of monazite was also tested in aqueous solutions under atmospheric conditions and low T (100-350 °C) to investigate low TIP alteration. Solutions ofCaCb +NaCl (1:1) chlorides were very reactive and dissolved the monazite completely, while solutions of CaCb were less reactive and only partly dissolved the monazite. These experiments demonstrate the concentrations required in hydrothermal solutions to destabilize monazite and explain textures found in natural rocks. Accessory minerals are useful discriminators between S-, I- and A-type granites and also their mineralized counterparts. Discrimination does depend on what accessory minerals are present and therefore apatite is the best mineral because it occurs in all the plutons. Petrography of these minerals is an indicator of primary or secondary processes. / AFRIKAANSE OPSOMMING: Die fokus van hierdie studie was om die mineraalchemie en paragenese van bykomstige minerale in S-, 1- en A-tipe graniete van die Kaapse Graniet Suite te vergelyk. Die doelwit van hierdie studie was om die verskille in chemie en petrografie van bykomstige minerale te gebruik as insig in die evolusie, herwinning en ontstaan van kontinentale kors soos geaffekteer deur die Kaapse Graniet Suite. Omdat SAE en spoorelemente hoe partisiekoeffisiente het vir bykomstige minerale speel hierdie minerale 'n belangrike rol om graniet evolusie te verklaar. Genoemde kenmerke van bykomstige minerale is ook gebruik om te onderskei tussen ongemineraliseerde en gemineraliseerde S-, 1- en A-tipe graniete in die suite. Die petrografie van die Kaapse Graniet Suite weerspieel die tweeledigheid van allanietmonasiet deurdat allaniet en titaniet in 1-tipe graniete en monasiet in S-tipe graniete voorkom. Monasiet word dus onstabiel in hoe Ca, 1-tipe, graniete. Apatiet kom in al drie tipes voor wat die mineraal se stabiliteit in verskeie geologiese omgewings weerspieel. Geronde kristalvorme , of reliekteksture, van apatiet en monasiet in S-tipe graniete weerspieel die sedimentere oorsprong van hierdie graniete. Konsentriese groei - en sektorale sonering kom algemeen voor in bykomstige minerale in ongemineraliseerde plutone. Sekondere veranderings rande (koronas) en onreelmatige sonering in gemineraliseerde plutone se bykomstige minerale is 'n aanduiding dat primere teksture gedeeltelik vemietig is deur sekondere prosesse. Katodeluminisensie en skandeerelektron mikroskopie ondersoeke het bewys dat SAE gehermobiliseer word na die koronas. Heelrotschemie en fisiese toestande van die magma word weerspieel in die mineraalchemie van bykomstige minerale. (Al~Fe) substitusie in titaniet word beheer deur P-T toestande en is, saam met Ca, Mn en Mg inhoud wat heelrotschemie weerspieel, goeie diskriminators in S- en 1-tipe graniete. LSAE en Sr inhoud in allaniet onderskei goed tussen plutone omdat hierdie elemente die heelrotschemie weerspieel. Omdat apatiet in al die plutone voorkom is dit die bruikbaarste mineraal om as diskriminant te gebruik. V anuit vorige werk is dit bekend dat die aluminium versadigings indeks die twee hoofsubstitusies: Ca+P~Si+SAE en Na+SAE~2Ca beheer, Fe en Mn inhoud in apatiet (0,1 pfu Mn en 0,05 pfu Fe is die afsnypunt tussen S- en 1-tipe graniete) weerspieel die oksidasietoestand van die magma en Sr, SAE en Mg weerspieel heelrotschemie. Saam kan hierdie elemente dus gebruik word as diskriminatore tussen die verskillende plutone. SAE patrone van allaniet en monasiet is sonder uitsondering verryk in die LSAE, terwyl apatiet en titaniet meestal "birdwing" profiele wys, maar kan ook verryk wees in LSAE of SSAE. Hierdie patrone word beinvloed deur kristallisasie van ander SAE-draende minerale, fraksionering van minerale uit die magma en die kristallisasie volgorde van die mineral e. Faseverwantskappe is eksperimenteel getoets tussen monasiet en allaniet in magmatiese toestande (780 °C en 1,8 kbar). Monasiet word onstabiel in 'n peralumineuse smelt (Aluminium versadigingsindeks >1) as waterig oplossings met konsentrasies van 0.7-7 g CaCh/1 0 cc H20 bygevoeg word. Cl-apatiet vorm as veranderingsproduk om die rande (koronas ). Allaniet is getoets in peralumineuse (Aluminium versadigingsindeks > 1) en metalumineuse smelte (Aluminium versadigingsindeks =1) met wisselende konsentrasies P20s (0.08 - 0.25 wt%). Allaniet het onstabiel geraak in peralumineuse smelte en hoe fosfor konsentrasies en het afgebreek na fases van LSAE+P± Ca, Al, K. Korona (kelifitiese) teksture is waargeneem. Hierdie eksperimente bewys dat fosfor, saam met Al, 'n belangrike rol speel in smeltstruktuur. Dit kan gesien word in die teenwoordig- of afwesigheid van kristalle in die glas. Dit is moontlik deurdat die byvoeging van fosfor 'n verlaging in die graniet likwidus temperatuur veroorsaak. Fosfor depolimeriseer ook aluminiumsilikaat smelte deur interaksie en kompleksvorming tussen fosfor en silika netwerke. Die oplosbaarheid van monasiet is ook getoets in waterige oplossings onder atmosferiese toestande en lae T (100-350 °C) om lae PIT veranderinge te ondersoek. 'n Versadigde oplossing van CaCh en NaCl (1:1) chloried het monasiet heeltemal opgelos terwyl 'n versadigde oplossing van CaCh monasiet net gedeeltelik opgelos het. Hierdie eksperimente dui op die konsentrasies nodig vir hidrotermale vloeistowwe waar bykomstige minerale onstabiel raak en verklaar teksture in natuurlike rotse. Bykomstige minerale kan dus gebruik word as diskriminators tussen ongemineraliseerde en gemineraliseerde plutone en ook tussen S-, I- en A-tipe graniete. Hulle kan egter net gebruik word as hulle teenwoordig is en daarom is apatiet die beste omdat dit in al die plutone aanwesig is. Petrografie is 'n aanduiding van magmatiese of sekondere prosesse.