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How and when sedimentary rocks record Earth's magnetic field is complex. Most studies assume a time-progressive lock-in mechanism during sediment deposition called depositional remanent magnetization (DRM). However, magnetic minerals can also form in situ, recording a chemical remanent magnetization (CRM) that is discontinuous in time. Disentangling the two mechanisms represents a major hurdle, and differences in their recording efficiencies remain unexplored. Here, our theoretical solutions demonstrate that CRM intensities exceed DRM by a factor of six when acquired in the same magnetic field. Novel experiments growing greigite (Fe3S4) in sediments and subsequent redeposition under identical magnetic field conditions confirm the predicted difference in recording efficiency. Thus, if left unrecognized, CRM leads to overestimated paleointensity and deserves more attention when interpreting Earth's magnetic history from sedimentary records. Recognition of fundamental differences between CRM and DRM characteristics provide a way forward to distinguish the recording mechanisms through routine laboratory protocols.

Multipolarity remanence in greigite-bearing sediments has long been recognized, but the cause of this anomalous remanence behavior is not well understood. Here, we use electron microscopic and magnetic analyses to investigate the origin of such multipolarity in Miocene greigite-bearing sediments from the Pannonian Basin (Hungary). We find a magnetic softening and partial transformation of iron sulfides to magnetite and pyrrhotite from “single-polarity” to “multi-polarity” samples. The inward alteration of sulfide grains is topotactic and is size-dependent with higher alteration in smaller grains. We propose a multi-phase self-reversal chemical remanent magnetization (CRM) mechanism in altered greigite: the neoformed magnetite/pyrrhotite shell acquires a CRM coupled in the opposite direction to the primary CRM of the greigite core, likely through magnetostatic interactions or interfacial exchange interactions between the closely contacting core and shell. This new greigite self-reversal model can explain the commonly observed antiparallel polarities and has broad geochronological, tectonic and paleoenvironmental implications.

The Paleocene lavas from Dianzhong Formation (E1d) in Linzhou basin of southern Lhasa terrane are a key target for paleomagnetic investigations into the timing and paleolatitude of the initial India-Asia collision. Controversy exists, however, on whether these rocks preserve a primary remanent magnetization. Here we reanalyze previously published thermal demagnetization data and report detailed rock magnetic results and petrographic observations of these rocks. We find that the original magnetic carrier, a magmatic multidomain Ti-poor titanomagnetite, underwent significant grain size reduction and was variably reacted to single-domain maghemite and nano-hematite. Such strong alteration may have resulted from successive hydrothermal events: a first event related to the ∼52 Ma dike intrusions into the E1d that accompanied a massive ignimbrite eruption deposited above the E1d producing heating up to 300°C; and a secondary event related to the 42–27 Ma southward overthrusting of the basin, heating the E1d up to 130–145°C. Unblocking/inversion temperature spectra of the authigenic maghemite and nano-hematite overlap with those of the titanomagnetite, implying that the primary remanence of the E1d lavas has been contaminated or replaced by thermoviscous and chemical remanent magnetizations. Thus the isolated characteristic remanent magnetization from these rocks, whether slightly or completely altered, cannot be considered primary and should not be used for paleolatitudinal determination. Our study confirms that hydrothermal alteration can seriously jeopardize the remanence carried by titanomagnetite and thus should be tested for paleomagnetic investigations of rock units from tectonically active areas.

This thesis presents research on the paleogeographic data of Early Cretaceous granites and Mid-Late Jurassic limestones in the Zaduo area, Eastern Qiangtang Terrane, Tibetan Plateau, China. The granite study provides further insight into the Lhasa-Qiangtang collision, while the limestones have been remagnetized due to the India-Eurasia collision. The magnetic fabric of the remagnetized limestones documented the NNE-SSW oriented compression during the remagnetization. A comparative study of the remagnetized and unremagnetized rocks, which mainly concentrates on their rock magnetic properties, was also carried out. The combination of gravity and paleomagnetic data reveals a coupled lithosphere-scale oroclinal deformation of the eastern ending of the Qiangtang Terrane. The thesis consists of five chapters. Chapter 1 investigates an Early Cretaceous granite in the Zaduo area and presents a new paleomagnetic investigation on the granite. Chapter 2 discusses the remagnetization of the Jurassic limestones in the Zaduo area and its implications for the India-Eurasia collision. Chapter 3 is about the lithosphere-scale oroclinal deformation of the eastern ending of the Qiangtang Terrane. Chapter 4 focuses on the remagnetization of the limestones and the NNE-SSW oriented compression during the India-Eurasia collision, and Chapter 5 is about the comparative study of the remagnetized and unremagnetized rocks.

[EN] The western Iberian margin underwent complex evolution during the Mesozoic and Cenozoic. It was subjected to successive extensional and compressional phases, where salt tectonics played a significant role in both. The role of onshore structures during basin formation and inversion is not fully understood, and different chronological relationships between structures have been proposed. To better understand the history of salt body emplacement, we present paleomagnetic and magnetic fabric data from Jurassic carbonates, comprising cover rocks of the diapiric system. Magnetic fabrics show two types of behavior. Type 1 fabrics show magnetic foliation parallel to bedding—the magnetic lineation, after bedding correction, is horizontal and parallel to the diapiric contacts. This fabric is a consequence of overpressure during salt migration, whose effect is limited to the area near the diapirs. Type 2 fabric is characterized by a magnetic lineation parallel to the strike of bedding and—the intermediate and minimum magnetic axes are dispersed on a vertical girdle parallel to the shortening direction. This is interpreted as a compressional fabric that developed during basin inversion. Paleomagnetic results reveal a stable paleomagnetic component carry by magnetite with characteristic unblocking temperatures (175–450 °C) and coercivities (30–100 mT). Twelve of the 14 calculated mean directions show homogeneous behavior in their magnetic properties and directions, with positive inclinations, northward directions, with synfolding to postfolding behavior. We calculated the remagnetization direction (Dec: 347.5°, Inc.: 51.3°) and compare it to the APWP. It was likely acquired during the Cretaceous Normal Superchron, following the extensional deformation phase and prior to basin inversion. The paleodips (i.e., the bedding at remagnetization times) indicate that tilting occurred mainly during the extensional stage, meaning that the diapirs had already been developed. This indica

Hematite is a canted antiferromagnet with reddish color that occurs widely on Earth and Mars. Identification and quantification of hematite is conveniently achieved through its magnetic and color properties. Hematite characteristics and content are indispensable ingredients in studies of the iron cycle, paleoenvironmental evolution, paleogeographic reconstructions, and comparative planetology (e.g., Mars). However, the existing magnetic and color reflectance property framework for hematite is based largely on stoichiometric hematite and tends to neglect the effects of cation substitution, which occurs widely in natural hematite and influences the physical properties of hematite. Thus, magnetic parameters for stoichiometric hematite are insufficient for complete analysis of many natural hematite occurrences and can lead to ambiguous geological interpretations. Remagnetization, which occurs pervasively in red beds, is another ticklish problem involving hematite. Understanding red bed remagnetization requires investigation of hematite's formation and remanence recording mechanisms. We elaborate on the influence of cation substitution on the magnetic and color spectral properties of hematite, and on identifying hematite and quantifying its content in soils and sediments. Studies of remagnetization mechanisms are discussed, and we summarize methods to discriminate between primary and secondary remanences carried by hematite in natural samples to aid primary remanence extraction in partially remagnetized red beds. Although there remain unknown properties and unresolved issues that require future work, recognition of the properties of cation-substituted hematite and remagnetization mechanisms for hematite will aid identification and interpretation of the magnetic signals that it carries, which is environmentally important and responsible for magnetic signals on Earth and Mars.

Remarkable and continuous geomagnetic field change, suggesting remagnetization at a shallow depth, was detected through repeated geomagnetic field observation of the summit area of Mt. Usu Volcano from 2008 to 2021. Long-term cooling of the remnant magma, that intruded during the 1977–82 eruption, was considered responsible for the remagnetization. A magnetic dipole parallel to the present geomagnetic field well reproduced the observation. The modeled source was located near the Ginnuma crater on the south side of the previously inferred intrusive body beneath the Usu-Shinzan cryptodome. Meanwhile, no magnetic source was detected on the other side of the intrusion, implying asymmetric heat transport paths around the intrusion. Considering previous studies on seismicity, geodetic modeling, and resistivity structure, the magnetic source region is plausibly a high permeability zone through which heat from the intruded magma has been efficiently transported. In other words, the source region can be a key monitoring target for future eruptions as it may be linked to the subsurface magma system.

Palaeomagnetism is a versatile tool in the Earth sciences: it provides critical input to geological timescales and plate tectonic reconstructions. Despite its undeniable perks, palaeomagnetism is not without complications. Remagnetizations overprinting the original magnetic signature of rocks are frequent, especially in orogens which tend to be the areas with better rock exposure. Unraveling the magnetic history of the rocks is a complicated task, especially in areas that underwent several orogenic pulses. In turn, constraining the timing of remagnetization represents an opportunity to solve post-magnetization structural and tectonic kinematics. Here, we evaluate the magnetization history of Silurian-Devonian carbonates from the Axial Zone of the Pyrenees. The Pyrenees are a multi-orogenic mountain belt where Silurian-Devonian rocks have seen the Variscan collision (late Palaeozoic), the opening of the Atlantic/Bay of Biscay (early Cretaceous) and the Alpine orogeny (late Cretaceous to Miocene). Our results show widespread remagnetization(s) carried by magnetite and pyrrhotite in the Silurian-Devonian series of the Pyrenees. The majority of the samples show a post-folding but pre-alpine tilting magnetization. Considering the equatorial inclinations found in such samples, we suggest that they likely acquired their magnetization during the late Carboniferous and early Permian times. Two of the studied sites (located at the western Axial Zone) were subsequently remagnetized at the end of the Alpine orogeny. The palaeomagnetic results constrained that the Variscan orogeny was responsible for the main folding event affecting Palaeozoic rocks in the Axial Zone, whereas the Alpine orogeny produced the large-scale thrusting and antiformal stacking of these units. In addition, we observed a general clockwise rotational pattern which could be related with the formation of the Cantabrian Orocline and/or rotations associated with the Alpine orogeny. The Silurian-Devonian car

Los niveles salinos depositados antes de la deformación, tanto en márgenes pasivos como en contextos orogénicos, producen una deformación diferencial en los niveles por encima y por debajo del despegue salino. La distribución de los despegues salinos controla el acortamiento diferencial y la formación de arcos orogénicos y el diapirismo puede generar minicuencas asociadas a rotaciones de ejes vertical durante su desarrollo. Nuestro estudio, con 65 estaciones paleomagnéticas, pretende deducir la cinemática de estructuras salinas mediante el estudio de una porción del Arco Vasco, en el margen norte de la Cuenca extensiva Vasco-Cantábrica, donde la cobertera de la sal está compartimentada por varias paredes salinas (diapiros de Bakio, Bermeo, Guernica y Mungia). Los resultados muestran rotaciones antihorarias predominantes y rotaciones horarias y antihorarias en flancos opuestos de los diapiros. Los test del pliegue revelan que algunas estaciones están remagnetizadas mientras otras muestran magnetizaciones ¿prefolding¿. Los experimentos de magnetismo de rocas en parte de las muestras estudiadas confirman estas observaciones. A pesar de que son necesarios más análisis para entender el origen y la distribución de las remagnetizaciones, es posible establecer un modelo cinemático preliminar de la deformación suprasal junto a los materiales despegados infrayacentes.

[EN] Palaeomagnetism is a versatile tool in the Earth sciences: it provides critical input to geological timescales and plate tectonic reconstructions. Despite its undeniable perks, palaeomagnetism is not without complications. Remagnetizations overprinting the original magnetic signature of rocks are frequent, especially in orogens which tend to be the areas with better rock exposure. Unraveling the magnetic history of the rocks is a complicated task, especially in areas that underwent several orogenic pulses. In turn, constraining the timing of remagnetization represents an opportunity to solve post-magnetization structural and tectonic kinematics. Here, we evaluate the magnetization history of Silurian-Devonian carbonates from the Axial Zone of the Pyrenees. The Pyrenees are a multi-orogenic mountain belt where Silurian-Devonian rocks have seen the Variscan collision (late Palaeozoic), the opening of the Atlantic/Bay of Biscay (early Cretaceous) and the Alpine orogeny (late Cretaceous to Miocene). Our results show widespread remagnetization(s) carried by magnetite and pyrrhotite in the Silurian-Devonian series of the Pyrenees. The majority of the samples show a post-folding but pre-alpine tilting magnetization. Considering the equatorial inclinations found in such samples, we suggest that they likely acquired their magnetization during the late Carboniferous and early Permian times. Two of the studied sites (located at the western Axial Zone) were subsequently remagnetized at the end of the Alpine orogeny. The palaeomagnetic results constrained that the Variscan orogeny was responsible for the main folding event affecting Palaeozoic rocks in the Axial Zone, whereas the Alpine orogeny produced the large-scale thrusting and antiformal stacking of these units. In addition, we observed a general clockwise rotational pattern which could be related with the formation of the Cantabrian Orocline and/or rotations associated with the Alpine orogeny. The Silurian-Devonian c

El uso de las remagnetizaciones inter-plegamiento suponen una herramienta idónea para el estudio de cuencas intraplaca invertidas, cuya geometría extensional está modificada por la compresión, y la presencia de este tipo de remagnetizaciones puede ser usada como control geométrico al momento de la adquisición de la remagnetización, siempre que sendos estados de deformación (el previo y el posterior a la remagnetización) sean coaxiales. La aplicación de las técnicas basadas en círculos menores a la remagnetización cretácica (portada por magnetita) presente en el Alto Atlas Central (Marruecos) permite esclarecer algunos aspectos de la evolución de la cuenca, tales como la datación relativa de la foliación tectónica presente en el área, o la importancia de los procesos halocinéticos e ígneos en la generación de las estructuras. Por otro lado, el estudio de las fábricas magnéticas de las calizas remagnetizadas ha permitido esclarecer algunos aspectos sobre el crecimiento de los granos de magnetita., Inter-folding remagnetizations are a useful to analyze inverted basins, whose extensional geometry was overprinted during compression. If both deformation stages are coaxial, this kind of remagnetizations can be used as geometric control at the remagnetization occurrence. The Jurassic carbonates of the Central high Atlas (Morocco) show an inter-folding remagnetization carried by magnetite. Using the small circle tools, the presence of this widespread remagnetization allow to shed light in some geological aspect about the Central High Atlas evolution, as the relative dating of the cleavage present in the area, or the role of halokinesis and igneous intrusion in the development of the characteristic structures present in the Atlas. On the other hand, the analysis of the magnetic fabrics carried by the remagnetized carbonates gives some clues about the growth of the magnetite grains that carry the remagnetization.

Red beds are well-known for recording stable natural remanent magnetization (NRM). However, discriminating primary NRM from secondary remanence in red beds is difficult. The Paleogene Nangqian red beds in eastern Tibetan Plateau variably record an overprint related to nearby magmatism and thus provide a great opportunity to characterize remagnetization in red beds. Through comprehensive rock magnetic, Mössbauer spectroscopic, and petrographic analyses, we find that remagnetization was controlled by temperature. Remagnetized red beds contain abundant authigenic hematite and goethite, with some larger grains unblocking at the Néel temperatures and the remainder (nanoparticles) unblocking at lower temperatures. In contrast, red beds retaining primary NRM are characterized by dominance of detrital hematite and magnetite, presence of fine-grained authigenic hematite, and absence of authigenic goethite and magnetite. High temperature behaviors of NRM/susceptibility are indicative of remagnetization, but the presence of goethite appears to be a more sensitive criterion for diagnosing remagnetization in red beds.

Carbonate rocks, widely used for paleomagnetically quantifying the drift history of the Gondwana-derived continental blocks of the Tibetan Plateau and evolution of the Paleo/Meso/Neo-Tethys Oceans, are prone to pervasive remagnetization. Identifying remagnetization is difficult because it is commonly undetectable through the classic paleomagnetic field tests. Here we apply comprehensive paleomagnetic, rock magnetic, and petrographic studies to upper Triassic limestones in the eastern Qiangtang block. Our results reveal that detrital/biogenic magnetite, which may carry the primary natural remanent magnetization (NRM), is rarely preserved in these rocks. In contrast, authigenic magnetite and hematite pseudomorphs after pyrite, and monoclinic pyrrhotite record three episodes of remagnetization. The earliest remagnetization was induced by oxidation of early diagenetic pyrite to magnetite, probably related to the collision between the northeastern Tibetan Plateau and the Qiangtang block after closure of the Paleo-Tethys Ocean in the Late Triassic. The second remagnetization, residing in hematite and minor goethite, which is the further subsurface oxidation product of pyrite/magnetite, is possibly related to the development of the localized Cenozoic basins soon after India-Asia collision in the Paleocene. The youngest remagnetization is a combination of thermoviscous and chemical remanent magnetization carried by authigenic magnetite and pyrrhotite, respectively. Our analyses suggest that a high supply of organic carbon during carbonate deposition, prevailing sulfate reducing conditions during early diagenesis, and widespread orogenic fluid migration related to crustal shortening during later diagenesis, have altered the primary remanence of the shallow-water Tethyan carbonate rocks of the Tibetan Plateau. We emphasize that all paleomagnetic results from these rocks must be carefully examined for remagnetization before being used for paleogeographic reconstructions. Fu

In their comment on our publications on pervasive remagnetization of Jurassic-Paleogene carbonate rocks of the Tibetan Himalaya (Huang et al., 2017, Journal of Geophysical Research: Solid Earth, 122, doi: 10.1002/2016JB013662 and 122, doi: 10.1002/2017JB013987), Yi et al. (2017) questioned our fold tests applied to their published paleomagenetic results from the Paleogene Zongpu and latest Cretaceous Zongshan carbonate rocks (Patzelt et al., 1996, Tectonophysics, 259(4), 259–284; Yi et al., 2011, Earth and Planetary Science Letters, 309(1), 153–165). They argued that authigenic magnetite pseudomorphic after pyrite, which is the dominant magnetic carrier within these carbonate rocks as indicated by our thorough rock magnetic and petrographic investigations, was formed during early diagenesis and that the primary natural remanent magnetization (NRM) is retained by these carbonate rocks. However, their statement for the invalidity of our fold tests is based on unrealistic assumptions that these carbonate rocks carry primary NRM and that the upper Zongpu Formation was deposited on a 10° primary dip. Their argument for immediate oxidization of pyrite to authigenic magnetite after carbonate deposition onto the continental passive margin ignores that sulfate-reducing conditions were prevailing during early diagenesis, it is also inconsistent with the timing of the secondary remanence acquisition in remagnetized carbonate rocks elsewhere. As previously demonstrated, and agreed upon by Yi et al. (2017), the Zongpu and Zongshan carbonate rocks in Gamba are remagnetized; here we argue that the timing of remagnetization cannot be demonstrated to shortly postdate sedimentation. These data should therefore not be used for tectonic reconstructions.

The latitudinal motion of the Tibetan Himalaya—the northernmost continental unit of the Indian plate—is a key component in testing paleogeographic reconstructions of the Indian plate before the India-Asia collision. Paleomagnetic studies of sedimentary rocks (mostly carbonate rocks) from the Tibetan Himalaya are complicated by potentially pervasive yet cryptic remagnetization. Although traditional paleomagnetic field tests reveal some of this remagnetization, secondary remanence acquired prior to folding or tilting easily escapes detection. Here we describe comprehensive rock magnetic and petrographic investigations of Jurassic to Paleocene carbonate and volcaniclastic rocks from Tibetan Himalayan strata (Tingri and Gamba areas). These units have been the focus of several key paleomagnetic studies for Greater Indian paleogeography. Our results reveal that while the dominant magnetic carrier in both carbonate and volcaniclastic rocks is magnetite, their magnetic and petrographic characteristics are distinctly different. Carbonate rocks have “wasp-waisted” hysteresis loops, suppressed Verwey transitions, extremely fine grain sizes (superparamagnetic), and strong frequency-dependent magnetic susceptibility. Volcaniclastic rocks exhibit “pot-bellied” hysteresis loops and distinct Verwey transitions. Electron microscopy reveals that magnetite grains in carbonate rocks are pseudomorphs of early diagenetic pyrite, whereas detrital magnetite is abundant and pyrite is rarely oxidized in the volcaniclastic rocks. We suggest that the volcaniclastic rocks retain a primary remanence, but oxidation of early diagenetic iron sulfide to fine-grained magnetite has likely caused widespread chemical remagnetization of the carbonate units. We recommend that thorough rock magnetic and petrographic investigations are prerequisites for paleomagnetic studies throughout southern Tibet and everywhere in general.

The characteristic remanent magnetization (ChRM) isolated from Paleogene carbonate rocks of the Zongpu Formation in Gamba (28.3°N, 88.5°E) of southern Tibet has previously been interpreted to be primary. These data are pertinent for estimating the width of Greater India and dating the initiation of India-Asia collision. We have reanalyzed the published ChRM directions and completed thorough rock magnetic tests and petrographic observations on specimens collected throughout the previously investigated sections. Negative nonparametric fold tests demonstrate that the ChRM has a synfolding or postfolding origin. Rock magnetic analyses reveal that the dominant magnetic carrier is magnetite. "Wasp-waisted" hysteresis loops, suppressed Verwey transitions, high frequency-dependent in-phase magnetic susceptibility, and evidence that >70% of the ferrimagnetic material is superparamagnetic at room temperature are consistent with the rock-magnetic fingerprint of remagnetized carbonate rocks. Scanning electron microscopy observations and energy-dispersive X-ray spectrometry analysis confirm that magnetite grains are authigenic. In summary, the carbonate rocks of the Zongpu Formation in Gamba have been chemically remagnetized. Thus, the early Paleogene latitude of the Tibetan Himalaya and size of Greater India have yet to be determined and the initiation of collision cannot yet be precisely dated by paleomagnetism. If collision began at 59±1Ma at ~19°N, as suggested by sedimentary records and paleomagnetic data from the Lhasa terrane, then a huge Greater India, as large as ~3500-3800km, is required in the early Paleogene. This size, in sharp contrast to the few hundred kilometers estimated for the Early Cretaceous, implies an ever greater need for extension within Greater India during the Cretaceous.

The occurrence of a generalized remagnetization at 100 Ma makes the Cameros Basin, an 8 km thick Cretaceous extensional trough inverted during the Cenozoic, a perfect natural frame to apply paleomagnetic vectors to geometrical reconstructions. The widespread remagnetization that occurred between the extensional and compressional episodes, linked to low‐grade metamorphism, provides a tool to reconstruct the attitude of beds at the remagnetization time, thus giving a picture of basin geometry during the Cretaceous. This snapshot is compared with the present‐day geometry to constrain the large‐scale kinematic evolution of folds between these two stages. According to this methodology, a syncline, preinversion geometry of the sedimentary basin was determined and the position of its main axial surface was accurately located. Comparing with the present‐day, postinversion geometry, a northward hinge migration of around 5 km is inferred. This migration is the result of the southward back thrusting in the southern basin border, favored by the detachment level at the base of the Mesozoic cover. Conversely, the main northward directed thrust, which involved both the Paleozoic basement and the Mesozoic cover, did not significantly affect the internal structure of the basin in spite of its overall displacement of more than 20 km.

The occurrence of a generalized remagnetization at 100 Ma makes the Cameros Basin, an 8 km thick Cretaceous extensional trough inverted during the Cenozoic, a perfect natural frame to apply paleomagnetic vectors to geometrical reconstructions. The widespread remagnetization that occurred between the extensional and compressional episodes, linked to low‐grade metamorphism, provides a tool to reconstruct the attitude of beds at the remagnetization time, thus giving a picture of basin geometry during the Cretaceous. This snapshot is compared with the present‐day geometry to constrain the large‐scale kinematic evolution of folds between these two stages. According to this methodology, a syncline, preinversion geometry of the sedimentary basin was determined and the position of its main axial surface was accurately located. Comparing with the present‐day, postinversion geometry, a northward hinge migration of around 5 km is inferred. This migration is the result of the southward back thrusting in the southern basin border, favored by the detachment level at the base of the Mesozoic cover. Conversely, the main northward directed thrust, which involved both the Paleozoic basement and the Mesozoic cover, did not significantly affect the internal structure of the basin in spite of its overall displacement of more than 20 km.

Large directional changes of remanent magnetization within lava flows that cooled during geomagnetic reversals have been reported in several studies. A geomagnetic scenario implies extremely rapid geomagnetic changes of several degrees per day, thus difficult to reconcile with the rate of the earth's core liquid motions. So far, no complete rock magnetic model provides a clear explanation. We revisited lava flows sandwiched between an underlying reverse and an overlying normal polarity flow marking the last reversal in three distinct volcanic sequences of the La Palma Island (Canary archipelago, Spain) that are characterized by a gradual evolution of the direction of their remanent magnetization from bottom to top. Cleaning efficiency of thermal demagnetization was not improved by very rapid heating and cooling rates as well as by continuous demagnetization using a Triaxe magnetometer. We did not observe partial self-reversals and minor changes in magnetic grain sizes are not related to the within-flow directional changes. Microscopic observations indicate poor exsolution, which suggests post-cooling thermochemical remagnetization processes. This scenario is strongly reinforced by laboratory experiments that show large resistance to thermal demagnetization when thermoremanence was acquired over a long time period. We speculate that in the present situation exsolution was reactivated during in field reheating and yielded formation of new magnetite, yet magnetic domain state rearrangements could also play a role. Initial reheating when the overlying flow took place, albeit moderate (less than 200¿300 °C), was enough to produce overlying components with significantly higher unblocking temperatures.

In order to obtain a kinematic model linking the diapir formation and the geometry of the syn-diapir sediments, a paleomagnetic study has been conducted in selected syn-diapiric sequences of the Basque-Cantabrian Basin. The studied diapirs (Bakio, Bermeo, Guernika and Mungia) developed during the Early Cretaceous in relation to the North Iberian extensional margin which was subsequently reactivated during the Pyrenean contractional deformation (Late Cretaceous-Miocene). They are cored by Triassic red clays and evaporites and they are flanked by synkinematic Albian shelf and slope carbonates and Upper Albian to Cenomanian siliciclastics. The paleomagnetic study has focused on the synkinematic overburden to detect and quantify vertical axis rotations related to the growth of the diapirs. 29 paleomagnetic sites have been analyzed. After obtaining the site mean directions it can be concluded that most of the sites are remagnetized, hindering the kinematics of the diapirs growth to be deduced. The age of this remagnetization is difficult to assess, it could be either an earlier Albian-Maastrichtian remagnetization or a remagnetization linked to the Pyrenean compression.

The Tibetan Himalaya represents the northernmost continental unit of the Indian plate that collided with Asia in the Cenozoic. Paleomagnetic studies on the Tibetan Himalaya can help constrain the dimension and paleogeography of "Greater India," the Indian plate lithosphere that subducted and underthrusted below Asia after initial collision. Here we present a paleomagnetic investigation of a Jurassic (limestones) and Lower Cretaceous (volcaniclastic sandstones) section of the Tibetan Himalaya. The limestones yielded positive fold test, showing a prefolding origin of the isolated remanent magnetizations. Detailed paleomagnetic analyses, rock magnetic tests, end-member modeling of acquisition curves of isothermal remanent magnetization, and petrographic investigation reveal that the magnetic carrier of the Jurassic limestones is authigenic magnetite, whereas the dominant magnetic carrier of the Lower Cretaceous volcaniclastic sandstones is detrital magnetite. Our observations lead us to conclude that the Jurassic limestones record a prefolding remagnetization, whereas the Lower Cretaceous volcaniclastic sandstones retain a primary remanence. The volcaniclastic sandstones yield an Early Cretaceous paleolatitude of 55.5°S [52.5°S, 58.6°S] for the Tibetan Himalaya, suggesting it was part of the Indian continent at that time. The size of "Greater India" during Jurassic time cannot be estimated from these limestones. Instead, a paleolatitude of the Tibetan Himalaya of 23.8°S [21.8°S, 26.1°S] during the remagnetization process is suggested. It is likely that the remagnetization, caused by the oxidation of early diagenetic pyrite to magnetite, was induced during 103-83 or 77-67 Ma. The inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the Tibetan Himalaya.

Paleomagnetically-determined paleolatitudes from the Lhasa terrane, the Yarlung-Zangbo Suture, and the Tibetan Himalaya are key to constraining how, when and where the India-Asia collision happened. This thesis aims at providing robust paleomagnetic determinations of the paleolatitudes from tectonic units in the India-Asia collision zone. Different tectonic reconstruction models are ultimately tested by a compilation of previously reported and newly collected reliable paleomagnetic data. A first focus lies on the Paleogene latitude estimates for the Lhasa terrane, which vary widely from 5°N to 30°N. We investigated Paleogene volcanic and sedimentary sequences of the Linzizong Group from southern Tibet in both the Linzhou basin and the Nanmulin Basin. Sedimentary Eocene upper Linzizong Group in the Linzhou Basin was affected by inclination shallowing. Two independent correction methods yield indistinguishable results of 40.5° [33.1°, 49.5°] (95% confidence limits) and 41.3° ± 3.3°, respectively. In the Nanmulin Basin, the Eocene volcanic rocks were remagnetized due to low-temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously. We tentatively propose a new method to recover a primary remanence with inclination of 38.1° [35.7°, 40.5°] (95% significance). In the Linzhou Basin, our analyses show that volcanic rocks of the lower Linzizong Group are thermally chemically remagnetized, whereas the upper Linzizong volcanic rocks retain a primary remanence. Regardless of the type of the rocks studied, paleomagnetic studies that have tested and corrected for all these potential pitfalls converge on a consistent Paleogene latitude of the Lhasa terrane of 20° ± 4°N. The depositional ages of sedimentary rocks unconformably overlying the Xigaze ophiolite’s mantle units overlap with the magmatic ages of the ophiolites. After correction of the compaction-related inclination ‘shallowing’ with

Paleomagnetic dating of the India-Asia collision hinges on determining the Paleogene latitude of the Lhasa terrane (southern Tibet). Reported latitudes range from 5°N to 30°N, however, leading to contrasting paleogeographic interpretations. Here we report new data from the Eocene Linzizong volcanic rocks in the Nanmulin Basin, which previously yielded data suggesting a low paleolatitude (~10°N). New zircon U-Pb dates indicate an age of ~52Ma. Negative fold tests, however, demonstrate that the isolated characteristic remanent magnetizations, with notably varying inclinations, are not primary. Rock magnetic analyses, end-member modeling of isothermal remanent magnetization acquisition curves, and petrographic observations are consistent with variable degrees of posttilting remagnetization due to low-temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously. Previously reported paleomagnetic data from the Nanmulin Basin implying low paleolatitude should thus not be used to estimate the time and latitude of the India-Asia collision. We show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization. We tentatively propose a new method to recover a primary remanence with inclination of 38.1° (35.7°, 40.5°) (95% significance) and a secondary remanence with inclination of 42.9° (41.5°,44.4°) (95% significance). The paleolatitude defined by the modeled primary remanence-21°N (19.8°N, 23.1°N)-is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper Linzizong Group corrected for inclination shallowing. The start of the Tibetan Himalaya-Asia collision was situated at ~20°N and took place by ~50Ma. Key Points: Linzizong Group volcanic rocks in Nanmulin (Tibet) are variably remagnetized Remagnetization was induced by secondary hematite precipita

The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus, here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong Group in the Linzhou basin. The lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes. We report consistent 40Ar/39Ar and U-Pb zircon dates of ∼52 Ma for the upper Linzizong, and 40Ar/39Ar dates (∼51 Ma) from the lower Linzizong are significantly younger than U-Pb zircon dates (64-63 Ma), suggesting that the lower Linzizong was thermally and/or chemically reset. Paleomagnetic results from 24 sites in lower Linzizong confirm a low apparent paleolatitude of ∼5°N, compared to the upper part (∼20°N) and to underlying Cretaceous strata (∼20°N). Detailed rock magnetic analyses, end-member modeling of magnetic components, and petrography from the lower and upper Linzizong indicate widespread secondary hematite in the lower Linzizong, whereas hematite is rare in upper Linzizong. Volcanic rocks of the lower Linzizong have been hydrothermally chemically remagnetized, whereas the upper Linzizong retains a primary remanence. We suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower Linzizong. We estimate that the Paleogene latitude of Lhasa terrane was 20 ± 4°N, consistent with previous results suggesting that India-Asia collision likely took place by ∼52 Ma at ∼20°N.

The Tibetan Himalaya represents the northernmost continental unit of the Indian plate that collided with Asia in the Cenozoic. Paleomagnetic studies on the Tibetan Himalaya can help constrain the dimension and paleogeography of "Greater India," the Indian plate lithosphere that subducted and underthrusted below Asia after initial collision. Here we present a paleomagnetic investigation of a Jurassic (limestones) and Lower Cretaceous (volcaniclastic sandstones) section of the Tibetan Himalaya. The limestones yielded positive fold test, showing a prefolding origin of the isolated remanent magnetizations. Detailed paleomagnetic analyses, rock magnetic tests, end-member modeling of acquisition curves of isothermal remanent magnetization, and petrographic investigation reveal that the magnetic carrier of the Jurassic limestones is authigenic magnetite, whereas the dominant magnetic carrier of the Lower Cretaceous volcaniclastic sandstones is detrital magnetite. Our observations lead us to conclude that the Jurassic limestones record a prefolding remagnetization, whereas the Lower Cretaceous volcaniclastic sandstones retain a primary remanence. The volcaniclastic sandstones yield an Early Cretaceous paleolatitude of 55.5°S [52.5°S, 58.6°S] for the Tibetan Himalaya, suggesting it was part of the Indian continent at that time. The size of "Greater India" during Jurassic time cannot be estimated from these limestones. Instead, a paleolatitude of the Tibetan Himalaya of 23.8°S [21.8°S, 26.1°S] during the remagnetization process is suggested. It is likely that the remagnetization, caused by the oxidation of early diagenetic pyrite to magnetite, was induced during 103-83 or 77-67 Ma. The inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the Tibetan Himalaya.

Inconsistent polarity patterns in sediments are a common problem in magnetostratigraphic and paleomagnetic research. Multiple magnetic mineral generations result in such remanence "haystacks." Here we test whether end-member modeling of isothermal remanent magnetization acquisition curves as a basis for an integrated rock magnetic and microscopic analysis is capable of isolating original magnetic polarity patterns. Uppermost Miocene-Pliocene deep-marine siliciclastics and limestones in East Timor, originally sampled to constrain the uplift history of the young Timor orogeny, serve as case study. An apparently straightforward polarity record was obtained that, however, proved impossible to reconcile with the associated biostratigraphy. Our analysis distinguished two magnetic end-members for each section, which result from various greigite suites and a detrital magnetite suite. The latter yields largely viscous remanence signals and is deemed unsuited. The greigite suites are late diagenetic in the Cailaco River section and early diagenetic, thus reliable, in the Viqueque Type section. By selecting reliable sample levels based on a quality index, a revised polarity pattern of the latter section is obtained: consistent with the biostratigraphy and unequivocally correlatable to the Geomagnetic Polarity Time Scale. Although the Cailaco River section lacks a reliable magnetostratigraphy, it does record a significant postremagnetization tectonic rotation. Our results shows that the application of well-designed rock magnetic research, based on the end-member model and integrated with microscopy and paleomagnetic data, can unravel complex and seemingly inconsistent polarity patterns. We recommend this approach to assess the veracity of the polarity of strata with complex magnetic mineralogy.

The Vrica section in Calabria, southern Italy, was the global stratotype for the Pliocene-Pleistocene boundary until this boundary was redefined in 2009. Several paleomagnetic investigations have been carried out at Vrica to determine the age of the formerly defined Pliocene-Pleistocene boundary, which was a key calibration point for the astronomical polarity timescale (APTS). Each study has documented a complex polarity pattern at and above the top of the Olduvai subchron and in relation to the existence of the so-called Vrica subchron. When constructing the APTS, two alternative interpretations for the Vrica section were proposed, neither of which could be conclusively supported. Authigenic growth of magnetic iron sulphide minerals was proposed to explain the complex magnetic polarity record. Availability of a fresh 50-m sediment core enabled us to test this possibility. Our magnetostratigraphic record is similar to that of previous studies, but it is also complex above the Olduvai subchron. We confirm abundant occurrences of authigenic greigite and pyrrhotite, along with detrital titanomagnetite. Authigenic monoclinic pyrrhotite indicates growth significantly later than deposition, and greigite can grow at any time during diagenesis, depending on the availability of dissolved iron and sulphide. The spatially variable magnetic polarity pattern at Vrica is therefore interpreted to have resulted from post-depositional magnetic iron sulphide formation at variable times. Tectonism along the Calabrian arc provides a plausible mechanism for forcing reducing fluids through the sediments, thereby supplying the dissolved ions needed to produce late diagenetic sulphide growth and remagnetization. The complex magnetostratigraphic record at Vrica was taken into account when the APTS was developed, and alternative interpretations result in a maximum age difference of 50 kyr for the upper Olduvai reversal. Our results therefore do not undermine the APTS. Rather, they explain th, authorCount :5

Se ha utilizado el estudio de la reimanación regional cretácica de la Cordillera Ibérica para acotar la edad de formación de un afloramiento de brechas de la base del Jurásico (Fm. Cortes de Tajuña) del norte de la Cordillera Ibérica. Los experimentos de magnetismo de las rocas y el análisis de la NRM demuestran que los cantos de la brecha han registrado la reimanación pero muestran direcciones aleatorias, lo que implica que la formación de la brecha es post-reimanación y por tanto posterior al Cretácico Inferior. La polaridad inversa registrada por la matriz confirma que la brechificación es posterior al Supercrón Normal del Cretácico, es decir post-Santoniense., From the study of remagnetization of Cretaceous age in an outcrop of the widespread breccia unit of the Lower Jurassic sequence (Cortes de Tajuña Fm.) in the northern Iberian Range, the age of formation of this unit can be inferred. Rock magnetism experiments and NRM analyses indicate that breccia clasts are remagnetized and show paleomagnetic vectors with random orientation, what demonstrates that breccia formation post-dates the Early Cretaceous remagnetization stage. The breccia matrix records a reverse paleomagnetic vector thus confirming that brecciation must post-date the Cretaceous Normal Superchron (i.e. they must be post-Santonian in age)., Ministerio de Educación y Ciencia, Depto. de Geodinámica, Estratigrafía y Paleontología, Fac. de Ciencias Geológicas, TRUE, pub

Realignment of magnetic grains below the sediment-water interface is thought to impart a post-depositional remanent magnetization (pDRM). However, there is little convincing evidence in the published literature that pDRM is the dominant mechanism by which sediments become magnetized. We report here preliminary results from two kinds of laboratory experiments designed to investigate whether post-depositional reorientation of magnetic particles is likely to occur in nature. In the first experiment, we monitored changes in the magnetization of natural sediments in response to changing laboratory fields. Our results are inconsistent with post-depositional reorientation of magnetic particles. In a second experiment, we put live worms in a multi-core tube with the original sediment-water interface intact. Remagnetization was only observed in samples taken from a mound of fecal pellets formed at the surface. These pellets had been suspended by a worm, and redeposited in the laboratory field. The other samples, which were not resuspended, but nonetheless bioturbated by the worms, showed no change in magnetization. Our preliminary results do not support the hypothesis that post-depositional reorientation occurs in natural, undisturbed sediments below the sediment-water interface. (C) 2000 Elsevier Science B.V. All rights reserved.

© SGEM2017. All Rights Reserved. We have collected paleomagnetic samples from Late Devonian (Fammennian) basalts and sedimentary redbeds at the Bykovskaya Straight (in the vicinity of Lena River delta). Most studied samples carried interpretable paleomagnetic signal. Three paleomagnetic components have been isolated: A+A’, C, and D. The magnetization of the overwhelmed majority of Late Devonian samples from the Bykovskaya Straight has been reset by modern (or Late Cenozoic) geomagnetic field (component A+A’). The sole site was characterized by the presence of Late Devonian magnetization component it its samples. The virtual pole, calculated from this component, can be used (combined with other virtual poles) to determine the position of the Late Devonian pole for Siberian craton. Studied sedimentary rocks were characterized by the presence of a metachronous syn-folding component. The comparison of the correspondent pole with European reference Apparent Polar Wander Path permits concluding that the folding in the region took place during Middle-Late Jurassic (160-170 Ma).

© SGEM2017. All Rights Reserved. We have collected paleomagnetic samples from Late Devonian (Fammennian) basalts and sedimentary redbeds at the Bykovskaya Straight (in the vicinity of Lena River delta). Most studied samples carried interpretable paleomagnetic signal. Three paleomagnetic components have been isolated: A+A’, C, and D. The magnetization of the overwhelmed majority of Late Devonian samples from the Bykovskaya Straight has been reset by modern (or Late Cenozoic) geomagnetic field (component A+A’). The sole site was characterized by the presence of Late Devonian magnetization component it its samples. The virtual pole, calculated from this component, can be used (combined with other virtual poles) to determine the position of the Late Devonian pole for Siberian craton. Studied sedimentary rocks were characterized by the presence of a metachronous syn-folding component. The comparison of the correspondent pole with European reference Apparent Polar Wander Path permits concluding that the folding in the region took place during Middle-Late Jurassic (160-170 Ma).