Magnetic Memory of Rocks: the Kazakhstan Orocline and Climatic Record of the Indian Monsoon.

The Kazakhstan orocline, a pair of concentric horseshoe-shaped volcanic arcs of Devonian (external) and late Paleozoic (internal) age, is thought to have formed during the amalgamation of Eurasia. Paleomagnetic investigations of several volcanic complexes ranging in age from Silurian to Permian are described in Chapters 2, 3 and 4 of this thesis. These studies have resulted in the construction of a tectonic model for the formation of the Kazakhstan orocline. Our results suggest that in the Middle Devonian, a nearly straight, northwest-southeast trending volcanic arc delineated the northeastern margin of the Kazakhstania landmass. In the Late Devonian, an initial collision with Tarim pinned Kazakhstania’s southern corner, while dextral shear motion and the considerable clockwise rotation of Siberia dragged its northern end. Relative convergence between Siberia and Tarim caused initial buckling of the Kazakhstania continent trapped between them, subdividing the volcanic arc into three (southwestern, middle, and northeastern) segments. Continued subduction under the established limbs of the orocline, with an estimated outward-directed subduction velocity of ~6mm/yr, gradually led to the closure of the intervening Junggar-Balkhash Ocean and tightening of the orocline. By the Late Permian, the Junggar-Balkhash Ocean no longer existed and the Kazakhstan orocline had obtained its present-day strongly curved shape. The ratio of two pedogenic iron oxides, goethite and hematite, has been demonstrated to be a good proxy for precipitation in soil-forming (terrestrial) environments. A similar interpretation of the mineral ratio has been increasingly applied to studies of marine sediments, in which variation in the goethite to hematite ratio is thought to reflect variation in the precipitation regime at the source area of the sediments. The rock magnetic study of Bengal Fan sediments described in Chapter 5 of this thesis suggests that in some intervals of the studied sedimentary section, variation in the ratio reflects a change in the degree of diagenetic alteration of the initial detrital assemblage rather than a climatic signal. Therefore, when assigning climatic interpretation to changes in the relative abundance of goethite and hematite in marine sediments, the possibility of diagenetic modification should be evaluated.Ph.D.GeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/60781/1/alexabra_1.pd

Editorial: Magnetic Records of Extreme Geological Events

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Magnetic Properties and Paleomagnetism of Zebra Rock, Western Australia: Chemical Remanence Acquisition in Hematite Pigment and Ediacaran Geomagnetic Field Behavior

Zebra Rock, a decorative stone remarkable for its unusual pattern of regularly spaced reddish bands and rods with white background, is found within the Neoproterozoic succession in East Kimberley, Western Australia. The unusual pigment distribution suggests that precipitation of hematite, or its precursor phase, occurred in a single episode. Magnetic properties of hematite pigment in Zebra Rock are distinctly different from those of the host shale, with a smaller median particle size and higher degree of structural perfection. The low thermal stability of the Zebra Rock pigment, with onset of thermal alteration at 300°C, suggests that the rocks have not undergone significant metamorphic heating. Stepwise thermal demagnetization reveals multiple magnetization components. Short-range variability in the relative contributions of the components to the total remanence is indicative of the stochastic nature of the hematite pigment growth process. In addition to seven magnetization components with shallow to intermediate inclinations that can be matched to the Paleozoic Australian apparent polar wander path, Zebra Rock samples contain a distinct steeply dipping magnetization that is not observed in the host shales. The steep magnetization appears to be primary, based on its unique association with the Zebra pattern, dissimilarity with younger directions, and evidence for low degree of thermal alteration of the rocks. The steep characteristic remanence contrasts with previous paleomagnetic indications of low Australian Neoproterozoic paleolatitudes. The characteristic Zebra Rock magnetization is the first Australian example of incompatible magnetization directions that have been reported previously from Ediacaran rocks in Laurentia, Baltica, and Africa.We acknowledge the Australian Research Council (ARC) through grant FS100100076 to APR and colleagues that provided a Super Science Fellowship to AA

Hematite (a-Fe2O3) quantification in sedimentary magnetism: limitations of existing proxies and ways forward

Determination of hematite contributions to sedimentary magnetizations is an important but difficult task in quantitative environmental studies. The poorly crystalline and fine-grained nature of hematite nanoparticles makes quantification of their concentrations in natural environments challenging using mineralogical and spectroscopic methods, while the weak magnetization of hematite and often significant superparamagnetic nanoparticle concentrations make quantification difficult using magnetic remanence measurements. We demonstrate here that much-used magnetic parameters, such as the S-ratio and "hard" isothermal remanent magnetization (HIRM), tend to significantly underestimate relative and absolute hematite contents, respectively. Unmixing of isothermal remanent magnetization (IRM) acquisition curves is among the more suitable approaches for defining magnetic mineral contributions, although it has under-appreciated uncertainties that limit hematite quantification. Diffuse reflectance spectroscopy and other methods can enable relative hematite and goethite content quantification under some conditions. Combined use of magnetic, mineralogical, and spectroscopic approaches provides valuable cross-checks on estimated hematite contents; such an integrated approach is recommended here. Further work is also needed to rise to the challenge of developing improved methods for hematite quantification

НОВЫЕ ПАЛЕОМАГНИТНЫЕ ДАННЫЕ ПО СИЛУРИЙСКИМ И ДЕВОНСКИМ ВУЛКАНИТАМ ЧИНГИЗСКОЙ ОСТРОВНОЙ ДУГИ КАЗАХСТАНА И ИХ ВКЛАД В ПРЕДСТАВЛЕНИЯ О ТЕКТОНИЧЕСКОЙ ЭВОЛЮЦИИ УРАЛО-МОНГОЛЬСКОГО ПОЯСА

The tectonic and paleogeographic evolution of the Ural-Mongol belt between the cratons of Baltica, Siberia, and Tarim is the key to the formation of the Eurasian supercontinent during Paleozoic time, but the views on this complicated process remain very disparate and sometimes controversial. Three volcanic formations of the Middle Silurian, LowertoMiddle Devonian and Middle Devonian age from the southwestern boundary of the Chingiz Range (NE Kazakhstan) yields what are interpreted as primary paleomagnetic directions that help clarify the evolution of the belt. A singlepolarity characteristic component in midSilurian andesites yields a positive intraformational conglomerate test, whereas dualpolarity prefolding components are isolated from the two Devonian collections. These new data were evaluated together with previously published paleomagnetic results from Paleozoic rocks in the Chingiz Range, and allow us to establish with confidence the hemisphere in which the area was located at a given time. We conclude that NE Kazakhstan was steadily moving northward crossing the equator in Silurian time. These new paleomagnetic data from the Chingiz range also agree with and reinforce the hypothesis that the strongly curved volcanic belts of Kazakhstan underwent oroclinal bending between Middle Devonian and Late Carboniferous time. A comparison of the Chingiz paleolatitudes with those of Siberia shows similarities between the northward motion and rotational history of the Chingiz unit and those of Siberia, which imposes important constraints on the evolving paleogeography of the Ural-Mongol belt.Тектоническая эволюция Урало-Монгольского подвижного пояса (УМП) многие десятилетия является предметом исследования огромного количества авторов. Однако, несмотря на все усилия, тектонические реконструкции разных авторов различаются самым радикальным образом, а во многом являются взаимоисключающими. Один из способов прояснить ситуацию – получить последовательности разновозрастных палеомагнитных определений и на их основе оценить кинематику ключевых структур УМП. При палеомагнитных исследованиях среднепалеозойских вулканитов Чингизской палеоостровной дуги на северо-востоке Казахстана в андезитах середины силура была выделена первичная компонента намагниченности, что подтверждается положительным тестом галек для внутриформационного конгломерата. В двух среднедевонских объектах также была выделена первичная намагниченность, для которой тест складки и тест обращения положительны. Объединив все имеющиеся данные по этому региону, мы получили последовательность палеомагнитных определений в интервале с позднего кембрия до поздней перми, что позволило уверенно определить, в каком полушарии находилась Чингизская палеодуга. Сделан вывод, что эта структура устойчиво смещалась к северу и пересекла экватор в силуре. Имеющиеся данные так же уверенно указывают на вторичную природу изгиба вулканических поясов Казахстана, имеющих подковообразные очертания. Сравнение этих данных с кривой кажущейся миграции полюса Сибирской платформы позволяет говорить о том, что большую часть палеозоя Чингизская палеодуга двигалась согласованно с Сибирской платформой, что накладывает жесткие ограничения на эволюцию УМП

The ∼270 Ma palaeolatitude of Baltica and its significance for Pangea models

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86901/1/j.1365-246X.2011.05061.x.pd

Paleomagnetism of the Dazhuqu terrane, Yarlung Zangbo suture zone, Southern Tibet

published_or_final_versionEarth SciencesMasterMaster of Philosoph

Magnetic Records of Extreme Geological Events

Recent advances in environmental magnetism offer the opportunity to link the magnetic signature of marine and continental rocks to the paleoenvironmental and paleoclimatic settings that controlled their formation or deposition, as well as to post-depositional events, such as diagenesis, that can alter their primary signature. This Research Topic assembles studies that used state of the art rock magnetic techniques to unravel the causes and effects of catastrophic geological events, including tsunami, meteorite impacts, Archean oxygenation event, geomagnetic reversals, and global climate changes linked to large volcanic eruptions