Orientador: Luiz Sérgio Amarante Simões / Banca: Rudolph Allard Johannes Trouw / Banca: Cláudio de Morrisson Valeriano / Banca: Hildor José Seer / Banca: Hans Dirk Ebert / Resumo: O presente trabalho apresenta uma proposta de evolução geológica Neoproterozóica de um segmento da Faixa Brasília Meridional, na região de Tapira (SW de Minas Gerais). A partir de mapeamento geológico detalhado é da caracterização estrutural e metamórfica. As rochas desta região apresentam uma complexa evolução estrutural, onde a principal estrutura reconhecida é uma foliação em baixo ângulo (S4) orientada em média N43W/30SE, à qual associa-se uma lineação de estiramento e/ou mineral orientada N50W/10, atribuídas à fase D4. A foliação S4 normalmente é reconhecida como uma clivagem de crenulação, cuja superfície crenulada é uma foliação S2, sub-paralela ao acamamento sedimentar (S0). Em alguns locais S4 manifesta-se como uma xistosidade ou clivagem contínua. A foliação S4 também afeta dobras normais de escala quilométrica relacionadas à fase D3. Adicionalmente são reconhecidos dois conjuntos de dobras pós-fase principal com eixos de caimentos suaves e planos axiais íngremes, sendo os eixos de D5 de direção NW e os eixos de D6 de direção NNE. Relacionada à fase D5 associam-se três zonas de cisalhamento transcorrentes quilométricas, a partir das quais sub-divididiu-se a área em três domínios tectono-estratigráficos. No domínio oeste (DW) ocorrem duas escamas tectônicas separadas por falha de empurrão. A escama 1 apresenta rochas metapelíticas e pelítico-grafitosas com intercalações psamítica. A associação mineral muscovita + quartzo + granada l clorita l biotita l cloritóide l grafita l albita, permitem situar as rochas dessa escama na fácies xisto verde superior (zona da granada), com condições de T e P estimadas em 540°C e 7,5 kbar. Na escama 2 predominam rochas pelíticas com intercalações psamíticas, adicionalmente ocorrem intercalações de hornblenda-granada-mica xistos e rochas metamáficas e metaultramáficas...(Resumo completo, clicar acesso eletrônico abaixo) / Abstract: This work was done to determine the Neoproterozoic evolution of a southern Brasilia Fold Belt segment, Tapira area (SW Minas Gerais state), using detailed geologic mapping and structural metamorphic characterization. A complex structural evolution is deduced for these rocks and the main structure recognized is a low angle S4 foliation (N43W/30SE), associated with N50W/10 stretching and/or mineral lineation of the D4 phase. The S4 foliation is a crenulation cleavage, where the pre-existing S2 foliation was folded parallel to sedimentary bedding (S0). In some areas, S4 is a schistosity or a continuous cleavage. The S4 foliation also affects kilometric normal folds of the D3 phase. Two post-S4 groups of folds are recognized with low angle axes and high angle axial planes: D5 and D6 with axes in NW and N-S direction, respectively. Three transcurrent shear zones are associated with the D5 phase, dividing the area into three tectonic stratigraphic domains: Western, Eastern and Southern. Two thrust sheets separated by a thrust fault characterize the Western Domain (WD). Metapelitic rocks and graphite-bearing pelites with metapsammitic lenses represent the thrust sheet 1. A muscovite + quartz + garnet l chlorite l biotite l chloritoid l graphite l albite association defines the rocks of thrust sheet 1 as upper greenschist facies (garnet zone), with T = 540 ºC and P = 7.5 kbar conditions. At the top, thrust sheet 2 is characterized by metapelitic rocks and metapsammitic lenses and hornblende-bearing garnet-muscovite schists intercalations, with metamafic and metaultramafic rocks. The muscovite + quartz + garnet l hornblende l chlorite l biotite l oligoclase and the hornblende + oligoclase + biotite associations suggest amphibolite facies with a range of T= 585 to 610 ºC and P= 8 to 10 kbar. The Eastern Domain (ED) comprises three...(Complete abstract click electronic access below) / Doutor

Orientador: Luiz Sérgio Amarante Simões / Banca: Rudolph Allard Johannes Trouw / Banca: Antenor Zanardo / Resumo: O estudo de lineações minerais tem fundamental importância na compreensão da evolução tectônica de cinturões orogênicos. Entretanto, em áreas polideformadas, enquanto as orientações das lineações mais novas podem ser resgatadas, as lineações antigas tendem a ser obliteradas pela superposição dos eventos deformacionais mais novos. Esta dissertação lança uma nova metodologia para o resgate da orientação de lineações minerais inclusas em porfiroblastos, utilizando a Platina Universal, além de testar a metodologia na Ilha Elefante, Antártica. Foi realizado um estudo teórico analisando o comportamento de lineação mineral, considerando-se variáveis como diferentes posições iniciais da lineação, diferentes morfologias de porfiroblastos, as possíveis relações temporais com o evento gerador da lineação, variados mecanismos de deformação e rotação (ou não) de porfiroblastos. A metodologia foi aplicada no estudo e leitura direta de lineações minerais em porfiroblastos da Ilha Elefante utilizando a Platina Universal, obtendo-se um padrão de orientação similar às medidas das mesmas estruturas em campo, provando a validade e viabilidade do método. A metodologia apresentada é de grande utilidade em áreas polideformadas, uma vez que contribui para a identificação da orientação dos paleovetores de stress. / Abstract: The study of mineral lineations has been of fundamental importance in the comprehension of tectonic evolution in orogenetic belts. However, in polydeformated areas,, while the newest lineation's orientation can be found, the oldest lineations tend to be obliterated by the newest deformation events. This thesis launches a new methodology to the rescue of mineral lineations' orientation included within porphyroblasts, using the Universal Stage, and testing the methodology in the Elephant Island, Antarctica. It has been made a theorical study, analyzing the behavior of a mineral lineation, considering various hypothesis, like the initial position, different morphologies of porphyroblasts, the possible temporal relationships with the lineation's generator event and various mechanisms of deformation and rotation (or not) of porphyroblasts. The methodology was applied in the study and direct measurement of mineral lineations in porphyroblasts of Elephant Island using the Universal Stage, obtaining an orientation pattern similar to the same structures in the field, proving the validity and viability of the method. The methodology presented is useful in polideformated areas, once that helps to identify the orientation of paleovectors of stress. / Mestre

Studies on the formation of oroclines (bends of originally linear mountain belts) are restricted to Phanerozoic examples. Here I provide a structural, metamorphic and geochronological study of the meta-sedimentary Pohjanmaa Belt, which lies on the most northerly limb of one of the few examples of Paleoproterozoic oroclines, the coupled Bothnian oroclines in Finland. My primary goal is to determine if the structures in the Pohjanmaa belt are consistent with the Svecofennian orogenic belt originating as a linear feature that was subsequently deformed into a pair of coupled oroclines. My detailed structural mapping focused on the geometry of F1, F2, and F3 folds within the Pohjanmaa Belt in west-central Ostrobothnia, Finland. Over 170 measurements were collected including interlimb angles, the attitudes of S1 and S2 foliations, and F2 fold axes. Also, 28 oriented core samples were collected to conduct a U-Pb geochronological study of monazites and an analysis of metamorphic textures in thin section. I first provide a review and comparison of similar structures around the coupled Bothnian oroclines. In my structural analysis I address the relative timing of deformation and metamorphism and use new U-Pb monazite geochronological data to constrain the absolute timing of tectonothermal events. My findings suggest that: 1) D1, D2, D3 deformation stem from a protracted event that records progressive deformation and strain partitioning from pure shear (D1 shortening) to simple shear (D2, D3 sinistral shear) as a result of counter clockwise rotation; 2) D2 folding and coeval garnet + staurolite metamorphism are consistent with early Svecofennian deformation, which occurred 80-90 m. y. prior to late-stage isothermal decompression and staurolite breakdown dated at 1.80 Ga by U-Pb monazite analysis; 3) My analysis, in combination with subsidiary data, provides evidence that sinistral shear during D2-3 is the result of counter clockwise rotation of the Pohjanmaa belt possibly in response to buckling of the coupled Bothnian oroclines during the early Svecofennian Fennia event. / Graduate

Thesis (Ph. D.)--State University of New York at Albany, 2005. / Title from PDF title page. Available through UMI ProQuest Digital Dissertations. Includes bibliographical references (p. 182-194). Also issued in print.

The Central Asian Orogenic Belt formed by accretion subsequent to the contraction of the Palaeo-Asian Ocean that ultimately disappeared along the Solonker Suture Zone in East Asia. Since typical regional collisional features are absent, the tectonic evolution of the suture remains speculative. Integrated sedimentary provenance analyses across the accretionary collision zone between the Mongolian Arcs and the North China Craton place new constraints on the events that led to final suturing. An investigation on the geochronological and geochemical variability in Permian strata along a southeast-northwest transect revealed distinct differences across the Solonker Suture Zone: northern basins carry a broad Mesoproterozoic to latest Precambrian age signature, and their provenance terranes are of mixed juvenile to crustal magmatic origin. In contrast, southern basins contain detritus from the North China Craton, and their sources are of dominantly crustal contaminated magmatic origin. Provenance analysis suggests, that in the Early Palaeozoic (ca. 429 Ma) the Palaeo-Asian Ocean was consumed along the Uliastai Arc and the North China Craton, initiating the formation of the Northern and Southern Accretionary Orogens, respectively. By the end of the Middle Carboniferous the Mongolian Arcs consolidated after accretion of the Uliastai Arc. In the Late Carboniferous (ca. 314 Ma) the Hegenshan back-arc basin opened, detaching the Northern Accretionary Orogen. While subduction continued there, it may have temporarily ceased along the Southern Accretionary Orogen after accretion of a microcontinent (ca. 300 Ma). During the Middle Permian back-arc basin closure led to the formation and obduction of the Hegenshan supra-subduction zone ophiolite. Eventually, the Palaeo-Asian Ocean closed after wedge-wedge collision, which would not involve continental deep subduction, thus, leading to cryptic suturing from the Late Permian to Early Triassic. Statistical analyses on the heterogeneity and similarity of the age probability density functions require a complex Permian palaeo-geographic setting, involving a variety of arc basins, which received sediments dependent on the contemporary arc geometry. Early stages of the sequence likely resembled a Pacific-type scenario, including Japan-type back-arc basin opening, whereas the late stages were similar to the archipelago-type setting of present-day Southeast Asia. / published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

Surface displacements resulting from upper-crustal intrusion of melt are a paramount concern for communities and facilities located in or near active volcanic areas (e.g. Campi Flegrei, Yucca Mtn.). Study of active intrusions such as Campi Flegrei, Italy west of Mt. Vesuvius, is limited to remote observations through geophysical/geodetic procedures. While the surface displacement due to melt emplacement at depth can easily be determined, the geometries and depth of intrusions are often based on simplified assumptions (e.g. spheres and prolate or oblate ellipsoids). These models benefit from data constraining both the geometries of the individual intrusions, and the kinematics and mechanics of deformation within the superstructure overlying the intrusions. Mount Ellsworth, a partially exposed sub-volcanic system, is an ideal natural laboratory for the study of near surface intrusions. The intrusions of the Henry Mountains are ideal because they were emplaced into relatively flat-lying stratigraphy of the Colorado Plateau, at a time when the stress field was largely isotropic. Previous geologic work done in the Henry Mountains, conducted by C.B. Hunt (1953) and Marie Jackson and Dave Pollard (1988), presents competing emplacement models (i.e. large batch intrusion or incremental sill growth), as well as, differing geologic map data and interpretations. Through a combination of 1:5000 scale field mapping and profile-oriented gravity study, we have produced detailed geologic maps and cross sections of Mt. Ellsworth assess the previous work done on Mt. Ellsworth with new datasets, as well as, evaluate criteria refining various emplacement models. Mapping results demonstrate that several of the assumptions made in models theorized by Hunt (1953) and Jackson and Pollard (1988), were inappropriately applied on Mt. Ellsworth. These assumptions include the thickness and separation of stratigraphic units, the size and distribution of sills and smaller intrusions, structural attitudes of beds and sills, and the presence of exposure of the main body of the intrusion. Gravity data collected on similar intrusions presented in Corry (1988) demonstrates the difficulty of obtaining a gravity anomaly on the wavelength of the assumed size of the intrusion. Forward gravity modeling of various potential geometries beneath Mount Ellsworth suggests that the anomalies are similar in shape with a magnitude between 16 and 20 mGal. Results from the gravity profiles collected for this study fail to predict an anomaly on the wavelength of the Mount Ellsworth intrusion and record a much more complicated anomaly than is presented by the forward models. By combining the stratigraphic data, structural data, and cross sections, it can be determined that the Mount Ellsworth intrusion is a laccolith with a floor 1.5 kilometers beneath the topographic surface, is 1 kilometer thick at its maximum, and has dimensions of 4 kilometers wide by 6 kilometers long.

The Vredefort Dome is located southwest of Johannesburg, South Africa, and represents the deeply eroded remnant of the central uplift of the world’s largest known impact structure, with an estimated diameter of ~300 km. The Vredefort impact structure is also the oldest known impact structure on Earth (~2.02 Ga). The Vredefort Dome comprises an ~40 km wide core of Archaean basement gneisses and an ~20 km wide collar of subvertical to overturned Late Archaean to Palaeoproterozoic supracrustal strata. This project presents the results of Landsat-TM and aerial photograph analysis, as well as field mapping of Witwatersrand Supergroup metasedimentary strata in the collar of the Vredefort Dome. The aim of this study was to investigate the structures (such as folds, faults, fractures), at all scales, and other deformation features (such as shatter cones and pseudotachylitic breccias) in the field area, and to establish geometric and temporal relationships between these features with regard to the impact cratering process. This study revealed a highly heterogeneous internal structure of the collar involving folds, faults, fractures and melt breccias that are interpreted as the product of shock deformation and central uplift formation during the Vredefort impact event. Broadly radially-oriented symmetric and asymmetric folds, with wavelengths from tens of metres to kilometres, and conjugate radial to oblique faults with strike-slip displacements of, typically, tens to hundreds of metres accommodated tangential shortening of the collar of the dome that decreased from ~17 %, at a radial distance from the dome centre of 21 km, to <5 % at a radial distance of 29 km. Ubiquitous shear fractures containing pseudotachylitic breccia, particularly in the metapelitic units, display variable local slip senses consistent with either tangential shortening or tangential extension; however, it is uncertain whether they formed at the same time as the larger faults during the rise of the central uplift or earlier, during the shock compression phase of cratering. Contrary to the findings about shatter cones of some earlier workers in the Vredefort structure, the Vredefort cone fractures do not show uniform apex orientations at any given outcrop, nor do small cones show a pattern consistent with the previously postulated “master cone” concept. The model of simple back-rotation of the strata to a horizontal pre-impact position also does not lead to a uniform centripetal-upward orientation of the cone apices. Striation patterns on the cone surfaces are variable, ranging from typically diverging, i.e., branching off the cone apex, to subparallel to parallel on almost flat surfaces. Striation angles on shatter cones do not increase with distance from the crater centre, as suggested previously. Instead, individual outcrops present a range of such striation angles, and a more irregular distribution of striation angle values with regard to the distance from the crater centre suggests localised controls involving the nature and shape of various heterogeneities in the target rock on this aspect of cone morphology. On the basis of the observations made during this study on small-scale structures in the collar of the Vredefort Dome, the relationship of shatter cones with curviplanar fractures (multipli-striated joint sets - MSJS) is confirmed. Pervasive, metre-scale tensile fractures crosscut shatter cones and appear to have formed after the closely-spaced MSJ-type fractures. The results of this study indicate that none of the existing models is able to explain all characteristics of shatter cones fully; therefore, a combination of aspects of the different models may currently be the best possible way to explain the formation and origin of shatter cones, and the formation of the related MSJ and their characteristic aspects (e.g., curviplanar shape, melt formation, etc.). The observed variety of shatter cone orientations, surface morphology and striation geometry in the dome concurs broadly with the results of some previous studies. The abundance of striated surfaces along closely-spaced sets of fractures (MSJ) observed in this study can be reconciled with reflection/scattering of a fast propagating wave at heterogeneities in the target rocks, as proposed by recent studies. This would mean that closely-spaced fractures and shatter cones were not formed during shock compression, as widely postulated in the past, but immediately after the passage of the shock wave, by the interference of the scattered elastic wave and the tensional hoop stress that develops behind the shock front. In addition to shatter cones, quartzite units show two other fracture types – a centimetre-spaced rhomboidal to orthogonal type that may be the product of shock-induced deformation and related to the formation of shatter cones, and later joints accomplishing tangential and radial extension. The occurrence of pseudotachylitic breccia within some of these later joints confirms the general impact timing of these features. Pseudotachylitic breccias in the collar rocks occur as up to several centimetre-wide veins with variable orientations to the bedding and as more voluminous pods and networks in zones of structural complexity, such as the hinges of large-scale folds and along large-scale faults, as well as locally, at lithological interfaces. In places, tension gash arrays along thin veins are observed indicating that movement occurred along these planes. Initial cooling calculations for pseudotachylitic breccias of different widths and compositions (metapelite or quartzite) suggest that thick veins (<10 cm) could have stayed molten over the entire duration of crater development (at least 10 minutes), making it possible for shock-induced melts to intrude dilational sites, such as fold hinges and extensional fractures, during the formation and subsequent collapse of the central uplift. Intrusion of such melts may also have lubricated movements along brittle and ductile structures. Thus, the presence of both shock- and friction-generated melts is likely in the collar of the Vredefort Dome. Based on the spatial and geometric relationship between the structures and other deformation features observed in the collar rocks of the Vredefort Dome, it is possible to establish a temporal sequence of deformation events. Shatter cones and related closely-spaced fractures were formed during the contact/compression phase of the cratering process. The formation of at least some shock-induced pseudotachylitic breccia also belongs into this phase. Large-scale folds and faults and friction-generated melts can be related to the initial formation of the central uplift and extensional joints to the subsequent collapse of the central uplift.

This thesis presents the results of structural analyses and detailed field mapping from a region near Adams Burn in central Fiordland, New Zealand. The region preserves assemblages of metasedimentary and metaigneous rocks deposited, intruded, and ultimately metamorphosed and deformed during the growth of a Gondwana-margin continental arc from Cambrian-Early Cretaceous. Evidence of arc growth is preserved in the Late Devonian-Early Cretaceous Median Batholith, a belt of intrusive rock whose growth culminated with the emplacement of the Western Fiordland Orthogneiss (WFO) into the middle-lower crust of the margin. Following this magmatic flare-up, the margin experienced Late Cretaceous extensional orogenic collapse and rifting. During the Late Tertiary, the margin records oblique convergence that preceded the Alpine fault. The history of arc growth and record of changing tectonic and deformational regimes makes the area ideal for study of structural reactivation during multiple cycles of magmatism, metamorphism and deformation, including during a mid-lower crust magma flare-up. Structural and lithologic mapping, structural analyses, and cross-cutting relationships between superposed structures and three intrusions were used to bracket the relative timing of four tectonic events (D1-D4), spanning the Paleozoic to the Tertiary. The oldest event (D1) created a composite fabric in the metasedimentary and metaigneous rocks of the Irene Complex and Jaquiery granitoid gneiss prior to emplacement of the Carboniferous Cozette pluton. S1 foliation development, set the stage for structural reactivation during the second phase of deformation (D2), where S1 was folded and reactivated via intra-arc shearing. These second-phase structures were coeval with the emplacement of the Misty pluton, (part of WFO in central Fiordland), and record crustal thickening and deformation involving a kinematically partitioned style of transpression. Arc-normal displacements were localized into the rocks of the Irene Complex. Oblique displacements were localized along the Misty-Cozette plutonic contact, forming a ≥1 km-wide, upper amphibolite-facies gneissic shear zone that records sinistral-reverse offset. Second-phase structures are cross-cut by widespread leucocratic pegmatite dikes. S2 in the Cozette and Misty plutons is reactivated by localized, ≤10 m-thick, greenschist-facies (ultra)mylonitic shear zones that record sinistral-normal offsets. S3/L3 shear zones and lithologic contacts were then reactivated by two episodes of Tertiary, fourth-phase faulting compatible with Alpine faulting, everywhere truncating the pegmatite dikes. Early faults accommodated shortening normal to the Alpine fault, and were obliquely reactivated by a younger population of faults during dextral transpression. My results show that structural reactivation occurred repeatedly after D1, and that structural inheritance played a key role in the geometry, distribution, and kinematics of younger deformation events throughout the arc's history. The sheeted emplacement of the Misty pluton was accompanied, and possibly facilitated, by a system of partitioned transpression during Early Cretaceous crustal thickening and arc magmatism. These results show that transpression helped accommodate and move magma through the middle and lower crust during the flare-up. This conclusion is important for the study of continental arcs globally, as evidence of deformation during high-flux magmatism at lower crustal depths (~40 km) is rarely preserved and exhumed to the surface.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 205-213). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph.D.)--University of Wyoming, 2007. / Title from PDF title page (viewed on June 17, 2009). Includes bibliographical references.