Orientador: Giancarlo Scardia / Resumo: O Nordeste Brasileiro apresenta extensa história geológica relacionada a tectonismo divergente e estabelecimento de sequências tipo, responsáveis pela separação dos continentes Sul-americano e Africano durante o Cretáceo. Tais eventos levaram à formação de um conjunto de bacias sedimentares denominadas Bacias Interiores do Nordeste, dentre as quais, o Grupo Iguatu. As bacias abrangem uma área sedimentar rasa de aproximadamente 1135 km2, parcialmente recoberta pelos açudes Orós e Lima Campos e fortemente controladas pela Faixa Cariri-Potiguar e suas zonas de cisalhamento proterozoicas NE-SW. É composto pelas Bacias Iguatu, Malhada Vermelha, Lima Campos e Icó, preenchidas por depósitos fluviais e por sedimentos finos a muito finos de planícies de inundação, além de depósitos de canais distais e de barras cascalhosas distribuídos isoladamente. Indícios de uma interação fluvio-eólica entre períodos chuvosos formadores de elementos arquiteturais fluviais e períodos secos no qual predominam processos de transporte eólico também podem ser observados nos estágios deposicionais finais da evolução das bacias. A Bacia Iguatu, devido ao seu cenário geomorfológico mais favorável, funciona como um bom guia estratigráfico para a aplicação dos estudos. A partir do mapeamento geológico, análise petrográfica, análise de fácies e interpretação de elementos estruturais foram determinadas quatro unidades litoestratigráficas para o Grupo Iguatu: Formação Icó, Malhada Vermelha, Lima Campos e Orós. ... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The northeast of Brazil presents an extensive geological history related to divergent tectonism and establishment of guide sequences through the separation of the South American and African continents during the Cretaceous. These events led to the formation of a set of sedimentary basins called the Interior Basins of Northeast Brazil, among which the group of sedimentary basins of lguatu or lguatu Group. The basins are comprised of a shallow sedimentary area with approximately 1135 km2, partially covered by the Orós and Lima Campos water reservoirs and strongly controlled by the Cariri-Potiguar trend and NE-SW proterozoic shear zones. lt is composed of the sub­basins of lguatu, Malhada Vermelha, Lima Campos and lcó, filled by fluvial deposits and by fine to very fine sediments typical of floodplains, as well as distal channels and gravei bars distributed randomly. Evidence of a fluvial-aeolian interaction between rainy seasons forming architectural elements of fluvial systems and dry periods with dominance of aeolian processes can also be observed in the final stages of deposition. The sub-basin of lguatu, due to its favorable geomorphological scenario, works as a good stratigraphic guide for the application of the studies. Geological mapping, petrographic analysis, facies association and interpretation of structural elements, stablished four lithostratigraphic units for the lguatu Group: the lcó, Malhada Vermelha, Lima Campos and Orós Formations. Thermal demagnetization defi... (Complete abstract click electronic access below) / Mestre

Fault pseudotachylytes commonly form by frictional melting due to seismic slip. These fine-grained clastic rocks result from melt quenching and may show a high concentration of fine ferromagnetic grains. These grains are potentially excellent recorders of the rock natural remanent magnetization (NRM). The magnetization processes of fault pseudotachylytes are complex and may include the following: i) near coseismic thermal remanent magnetization (TRM) acquired upon cooling of the melt; ii) coseismic lightning induced remanent magnetization (LIRM) caused by earthquake lightnings (EQL); iii) post seismic chemical remanent magnetization (CRM) related to both devitrification and alteration. Deciphering these magnetization components is crucial to the interpretation of paleointensities to see if coseismic phenomena such as EQL’s were recorded within these rocks. Hence the paleomagnetic record of fault pseudotachylytes provides an independent set of new constraints on coseismic events. Fault pseudotachylytes from the Santa Rosa Mountains, California host a magnetic assemblage dominated by stoichiometric magnetite, formed from the breakdown of ferromagnesian silicates and melt oxidation at high temperature. Magnetite grain size in these pseudotachylytes compares to that of magnetite formed in friction experiments. Paleomagnetic data on these 59 Ma-old fault rocks reveal not only anomalous magnetization directions, inconsistent with the coseismic geomagnetic field, but also anomalously high magnetization intensities. Here we discuss results of rock magnetism and paleointensity experiments designed to quantify the intensity of coseismic magnetizing fields. The REM’ paleointensity method, previously tested on meteorites, is particularly well suited to investigate NRMs resulting from non-conventional and multiple magnetization processes. Overall findings indicate an isothermal remanent magnetization (IRM) in some, but not all, specimens taken from four different Santa Rosa pseudotachylyte samples. The cause of this IRM may be attributed to an LIRM produced by ground lightning (less likely), LIRM produced by an EQL (more likely), or a VRM imparted during laboratory preparation (not likely). The anomalously high NRM recorded in a few specimens points to LIRM as the most likely explanation for the dominant origin of magnetization.

abstract: ABSTRACT The Sentinel-Arlington Volcanic Field (SAVF) is the Sentinel Plains lava field and associated volcanic edifices of late Cenozoic alkali olivine basaltic lava flows and minor tephra deposits near the Gila Bend and Painted Rock Mountains, 65 km-100km southwest of Phoenix, Arizona. The SAVF covers ~600 km2 and consists of 21+ volcanic centers, primarily low shield volcanoes ranging from 4-6 km in diameter and 30-200 m in height. The SAVF represents plains-style volcanism, an emplacement style and effusion rate intermediate between flood volcanism and large shield-building volcanism. Because of these characteristics, SAVF is a good analogue to small-volume effusive volcanic centers on Mars, such as those seen the southern flank of Pavonis Mons and in the Tempe Terra region of Mars. The eruptive history of the volcanic field is established through detailed geologic map supplemented by geochemical, paleomagnetic, and geochronological analysis. Paleomagnetic analyses were completed on 473 oriented core samples from 58 sites. Mean inclination and declination directions were calculated from 8-12 samples at each site. Fifty sites revealed well-grouped natural remanent magnetization vectors after applying alternating field demagnetization. Thirty-nine sites had reversed polarity, eleven had normal polarity. Fifteen unique paleosecular variation inclination and declination directions were identified, six were represented by more than one site with resultant vectors that correlated within a 95% confidence interval. Four reversed sites were radiometrically dated to the Matuyama Chron with ages ranging from 1.08 ± 0.15 Ma to 2.37 ± 0.02 Ma; and one normal polarity site was dated to the Olduvai normal excursion at 1.91 ± 0.59 Ma. Paleomagnetic correlations within a 95% confidence interval were used to extrapolate radiogenic dates. Results reveal 3-5 eruptive stages over ~1.5 Ma in the early Pleistocene and that the SAVF dammed and possibly diverted the lower Gila River multiple times. Preliminary modeling of the median clast size of the terrace deposits suggests a maximum discharge of ~11300 cms (~400,000 cfs) was necessary to transport observed sediment load, which is larger than the historically recorded discharge of the modern Gila River. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2015

M.Sc. / Please refer to full text to view abstract

M.Sc. / The Late Carboniferous to early Jurassic Karoo Supergroup of South Africa witnessed two of the “big five” Phanerozoic mass extinction events, and the formation and subsequent break-up of the supercontinent Pangea. The closure of the Permian Period witnessed the greatest biotic crisis in the history of life. What is known about the Permian-Triassic boundary (hereafter referred to as the PTB) comes almost exclusively from marine successions in Europe and Asia. Although a major extinction event has been recognized in terrestrial successions, surprisingly little is known about its effects and timing. The exact placement of the PTB in the Karoo basin is not well constrained due to shortcomings of stratigraphic methods employed to date. This has made it extremely difficult to correlate the mass extinction events in the marine and non-marine environments; however, paleomagnetic studies could provide answers to both problems of absolute placement and correlation of the PTB in non-marine and marine successions. The PTB appears to lie within an interval of reversed polarity in many marine successions. A detailed magnetostratigraphic survey across the presumed PTB in the Karoo succession at localities in the north and south of the main Karoo Bain reveal two magnetic chrons, reversed followed by normal (with the boundary close to the reversal), which extends to slightly younger results from a previous study that identified an N/R pattern, thereby identifying a R/N/R pattern. The normal chron might correlate with the long basal Triassic normal polarity interval and the reversed polarity zones above and below it known from marine successions in the Alps, Russia, Pakistan and China. The PTB is thought to be situated coincident with the LAD of Dicynodon and the event bed of Ward et al. (2000), apparently above but not necessarily diachronous with a lithology change from predominantly green- to predominantly red mudstone. This placement falls within a normal polarity interval, but could conceivably have taken place at a time of reverse polarity due to delayed acquisition of magnetic remanence. The idea of an extraterrestrial impact as the cause of the end-Permian mass extinctions is strongly enhanced by a synchronous relationship between them. The configuration of the supercontinent Pangea during this time of earth history has been the matter of debate for the last three decades, with numerous alternative reconstructions to the classic Pangea A1 having been proposed for the time preceding the Jurassic. Paleomagnetic data from the Karoo allow for the definition of a new paleopole for West Gondwanaland, which prove a valuable tool for evaluating these various reconstructions. It is neither consistent with a Pangea B-type not C reconstruction for Pangea during this time interval, because of geological ambiguities. The most likely solution to the problem is that of a persistent non-dipole field contribution to the geomagnetic field during this time. Approximately 50 million years later Pangea was unambiguously in a classic Pangea A1 configuration, and life on earth suffered yet another set back. The end-Triassic mass extinction, which marks the sequence boundary between the Triassic and the Jurassic, has not received as much attention as the other four big Phanerozoic biotic disasters. In the Karoo a pronounced turnover in faunal assemblages from typical Triassic fauna to Jurassic Fauna (dinosaurs) is seen in the Elliot Formation. Magnetostratigraphic study of localities in the north and south of the Karoo Basin provided a magnetic zonation pattern for the Elliot Formation, a tool that has led to the constraining of the sequence boundary to the transition from the lower Elliot Formation to the middle Elliot and added to the hypothesis that the faunal turnover is globally synchronous. The determination of a paleolatitude for the Elliot Formation in combination with characteristically arid lithologies (eolian sandstones) provided the base for the evaluation of the paleoclimate that characterized Pangea during the Late Triassic to Early Jurassic. Key words: Karoo Basin, Magnetostratigraphy, Mass Extinction, Paleoclimate, Paleogeography, Paleomagnetism, Pangea, Permian-Triassic, Triassic-Jurassic

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Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1980. / Microfiche copy available in Archives and Science. / Bibliography: leaves 15-16. / by Anna Catarina Shaughnessy. / M.S.

Paleomagnetic directions have been obtained from 190 Middle Miocene (12-20 Ma) mafic volcanic flows in 16 mountain ranges in the Mojave-Sonora desert region of western Arizona and southeastern California. These flows generally postdate Early Miocene tectonic deformation accommodated by low-angle normal faults but predate high-angle normal faulting in the region. After detailed magnetic cleaning experiments, 179 flows yielded characteristic thermal remanent magnetism (TRM) directions. Because of the episodic nature of basaltic volcanism in this region, the 179 flows yield only 65 time-distinct virtual geomagnetic poles (VGPs). The angular dispersion of the VGPs is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation. The paleomagnetic pole calculated from the 65 cooling unit VGPs is located at 85.5°N, 108.9°E within a 4.4° circle of 95% confidence. This pole is statistically indistinguishable (at 95% confidence) from reference poles calculated from similar-age rocks in stable North America and from a paleomagnetic pole calculated from similar-age rocks in Baja and southern California. From the coincidence of paleomagnetic poles from the Mojave-Sonora and adjacent areas, we can conclude that: (1) vertical-axis tectonic rotations have not accompanied high-angle normal faulting in this region; (2) there has been no latitudinal transport of the region since 12-20 Ma; and (3) long-term nondipole components of the Miocene geomagnetic field probably were no larger than those of the recent (0-5 Ma) geomagnetic field. In contrast, paleomagnetic data of other workers indicate vertical-axis rotations of similar-age rocks in the Transverse Ranges, the Eastern Transverse Ranges, and the Mojave Block. We speculate that a major discontinuity in the vicinity of the southeastward projection of the Death Valley Fault Zone separates western areas affected by vertical-axis rotations from eastern areas that have not experienced such rotations.

Heinrich Events (HEs) define intervals of major ice rafting from the Laurentide Ice Sheet (LIS) into the North Atlantic during that last glacial period. The discovery of potential European-sourced precursors to HEs suggest that the smaller, but climactically sensitive, European ice sheets (EIS) may have played a role in the triggering of HEs and their impact on global climates. Environmental magnetism has proved itself to be a useful, rapid and non-destructive tool in the identification and quantification of provenance in sediments from various depositional environments. In this work, environmental magnetic analyses are applied to marine sediment records from the European margin of the NE Atlantic and known to contain ice-rafted debris (IRD) from both LIS and EIS sources. The primary aim in the work of this thesis is to evaluate the methodology as a means of distinguishing IRD provenance. From the data obtained here it is possible to identify several magnetic events that correspond to the HEs and other layers of detrital material and which correlate well to previous standard petrological analyses performed on the same core materials. Magnetic signatures differ within the HEs, suggesting a changing balance of input from multiple sources as opposed to a single LIS source. The data suggest a phasing of these compositional differences through individual HEs. The potential of using environmental magnetic techniques in the identification of IRD provenance within marine sediments is discussed, as is the significance of the observed provenance variations within the cores studied.