Paleomagnetic Investigation Research Articles

Post-tholeiite rifting and the genesis of ultramafic lamprophyres around 65 Ma in the Deccan large igneous province: Implications for the separation of India and Seychelles

This study examines the mineral and whole-rock compositions and paleomagnetic dating of lamprophyres from the West Coast Alkaline Complex (WCAC) within the Deccan Large Igneous Province. We aim to elucidate their connection with the Réunion plume, Deccan tholeiite magmatism, the onset of rifting and separation of Seychelles, and the role of lithospheric thinning.WCAC lamprophyres contain olivine and phlogopite phenocrysts/macrocrysts set in a groundmass of carbonate, clinopyroxene, nepheline, spinel, and melilite. They are characterized by Ti-Al phlogopite and Al-enriched spinels with high Fe2+/(Fe + Mg) ratios. They are undersaturated in silica and rich in MgO, TiO2, and LREEs, resembling the global ultramafic lamprophyres (UMLs).They originated from an enriched garnet lherzolite mantle, metasomatized by silicate and carbonate veins. Trace-element modeling suggests the derivation of melt by ∼1% partial melting of phlogopite-bearing garnet lherzolite mantle, corresponding to moderate pressure depths (3–4 GPa) and lithospheric mantle thickness of 90–100 km. Paleomagnetic investigations corroborate their intrusions, primarily during chron C29n, approximately at 65 Ma, which coincided with amplified plate velocities in the Indian Ocean.Seismic tomography indicates a current lithospheric thickness of about 50 km under the western Indian margin, suggesting a lithospheric delamination of about 40–50 km following the intrusion of lamprophyres at ca. 65 Ma. The initiation of passive rifts in the region, culminating in the separation of the Indian subcontinent and the Seychelles, prompted the UML magmatism. Lithospheric thinning continues along with continental rifting in response to greater plate-tectonic stresses within regions of persistent lithospheric weakness.

Assessing lunar paleointensity variability during the 3.9 - 3.5 Ga high field epoch

Numerous paleomagnetic studies suggest that a lunar dynamo, with surface field intensities potentially as high as 40-100 µT, existed between ∼3.9 Ga and ∼3.5 Ga. This period is referred to as the High Field Epoch (HFE). However, the debate over the origin of magnetization recorded in lunar rocks still persists. In addition, whether the Moon could have sustained a continuously strong dynamo during the HFE remains unclear. To unravel the origin of magnetization preserved in lunar rocks and to better characterize the evolution of the ancient lunar dynamo, we conducted a comprehensive set of experiments including rock magnetic tests, electron microscopy, and paleomagnetic investigations on four HFE-aged Apollo 11 mare basalt samples: 10003, 10044, 10069, and 10071. Rock magnetic experiments and electron microscopy indicate that the remanence carriers are kamacite grains of varying sizes and domain states. Sample 10003 recorded a paleointensity of 54.10 ± 4.66 µT. Sample 10044, which was shocked (peak pressure >5 GPa), did not preserve a stable high coercivity remanent magnetization. Samples 10069 and 10071 recorded paleointensities of 61.46 ± 26.09 µT and 10.69 ± 2.87 µT, respectively. A series of hydrostatic pressure experiments, isothermal remanent magnetization (IRM) acquisition experiments, and viscous remanent magnetization (VRM) tests preclude the possibility of our samples containing shock remanent magnetization from transient impact-generated fields, IRM acquisition from exposure to spacecraft fields, and VRM acquisition from exposure to the Earth magnetic field. Overall, our study suggests that the source of these magnetizations was likely the lunar dynamo and may indicate nearly order-of-magnitude magnetic field fluctuations during the HFE.

A Non‐Magnetized Chondrite Parent Body Revealed by Paleomagnetic Investigation of LL6 Chondrite NWA 14180

AbstractMagnetic records from meteorites provide valuable information about the formation and evolution of the solar system and planets. The parent planetesimals of chondrites are typically considered to be undifferentiated based on their primary chemical composition and texture. However, recent paleomagnetic investigations of various chondrites indicate that they carry a primary remanence generated by a dynamo, suggesting partial differentiation of their parent planetesimals. The presence of a dynamo within the parent planetesimal of LL chondrites remains uncertain due to the ambiguous origin of the remanent magnetism. Here, we report petrographic, paleomagnetic, and rock magnetic properties for the novel LL6 chondrite NWA 14180. The high metamorphic temperature experienced by NWA 14180 could have removed the pre‐accretionary remanence. The fusion crust baked‐contact test suggests that NWA 14180 preserves primary magnetic information about its parent body. Alternating field demagnetization results from interior subsamples reveal distinct low‐ and medium‐coercivity components that may represent a viscous remanent magnetization acquired in the geomagnetic field. No natural remanent magnetization was unblocked in the high coercivity range, implying that NWA 14180 cooled in zero‐field conditions. Therefore, we suggest that the parent body of NWA 14180 did not have a dynamo. Furthermore, this result suggests that the LL chondrite parent planetesimal accreted later and was smaller in size than other chondrite classes.

Statistical reanalysis of Archean zircon paleointensities: No evidence for stagnant-lid tectonics

The initiation of mobile-lid plate tectonics on Earth represented a critical transition towards a more familiar world in terms of surface temperature stabilization, biogeochemical cycling, topography creation, and other processes. Zircon-based estimates of the geomagnetic field intensity have recently been cited as providing evidence for the lack of mobile-lid motion between 3.9 and 3.4 billion years ago (Ga). We reanalyze the published dataset of 91 zircon paleointensities from the Jack Hills (Australia) and Green Sandstone Bed (GSB; South Africa) localities within this time interval and, using both analytical and bootstrap resampling approaches, show that the small number of samples result in large uncertainties in implied paleolatitude. Specifically, in more likely scenarios that do not assume coherent motion for both localities, all latitudinal displacements on Earth are permitted within the 95 % confidence interval. We also examine the less likely scenario that the two landmasses shared a motion history, which increases the data density and presents the best-case scenario for constraining latitudinal motion. In this case, the 95 % confidence interval of the zircon paleointensity data is compatible with the displacements of between 35 % and 52 % of modern continental localities, all of which experience mobile-lid tectonics. Finally, generating expected paleointensity time series for modern continents undergoing mobile-lid motion shows that about two-thirds of these motions would not be resolved by zircon paleointensities, even in the best-case scenario of combining Jack Hills and GSB datasets. All of these analyses assume that these zircons retain a primary paleomagnetic signal, an assertion which is opposed by a number of published zircon magnetism studies. We conclude that Archean zircon paleointensities do not provide evidence for or against mobile-lid plate tectonics prior to 3.4 Ga. Future paleomagnetic investigation of tectonic regime on the early Earth should therefore focus on magnetization directions in well-preserved, oriented whole rocks.

Hayabusa 2 returned samples reveal a weak to null magnetic field during aqueous alteration of Ryugu's parent body

The JAXA Hayabusa 2 mission returned 5.4 g of material from the C-type asteroid Ryugu. The Mn-Cr ages of dolomite in the returned samples indicate that Ryugu's parent body experienced aqueous alteration sometimes between <1.8 and 6.8 Myr after CAI formation. Because this time range overlaps with the lifetime of the solar nebula, we investigate the possibility that magnetite and pyrrhotite, which are aqueous alteration products found in Ryugu samples, acquired a remanent magnetization reflecting the nebula field intensity. We analyze the intrinsic magnetic properties and paleomagnetic record of three Ryugu samples of 0.82, 0.97 and 21.87 mg. None of the samples exhibit a stable natural remanent magnetization. This indicates that the aqueous alteration of Ryugu's parent body took place either in a field of a few µT, or in a very weak to null field. In the former scenario, the solar nebula field is the most likely magnetizing field, implying that aqueous alteration occurred before its dissipation, i.e., before ∼5 Myr after CAI formation. In the latter scenario, aqueous alteration must have occurred either after the dissipation of the nebula, or at an earlier epoch and a large heliocentric distance (> 5 au). The similarities between Ryugu samples and CI chondrites favor this second hypothesis. Our results contrast with another paleomagnetic study of two Ryugu samples, arguing for a paleofield intensity of 40 to 390 µT. Our interpretation of this discrepancy is that these samples were exposed to artificial magnetic fields (> mT) during preceding experiments. This highlights the importance of conducting, as much as possible, the paleomagnetic investigations of returned samples before any other experiment. We also demonstrate that the ratio of NRM over low-field magnetic susceptibility is a powerful, non-destructive indicator of magnetic contamination. We recommend measuring this ratio routinely before paleomagnetic investigations of meteorites and returned samples.

Paleomagnetism and geochronology of Early Cretaceous volcanic rocks in the eastern segment of the Lhasa terrane, Tibetan Plateau, and their tectonic implications

Paleogeographic reconstructions of the Lhasa terrane for the Cretaceous provide important constraints on the evolution of the Neo-Tethys Ocean and crustal shortening within Asia. However, the Cretaceous paleogeography of the Lhasa terrane remains contentious. A direct way to study this issue is to conduct paleomagnetic investigations of the Cretaceous rocks of the Lhasa terrane; however, most previous Cretaceous paleomagnetic investigations of the Lhasa terrane were conducted in the middle and western segments of this terrane. Different vertical-axis rotations affected different parts of the Lhasa terrane following the India-Asia collision, and therefore paleomagnetic data from the western and middle segments of the Lhasa terrane cannot necessarily be used to directly constrain the paleolatitudes of its eastern segment. This study presents paleomagnetic data from the Early Cretaceous volcanic rocks with well-constrained ages from the Luolong area in the eastern segment of the Lhasa terrane. Isotope geochronology reveals that these rocks formed at 127−124 Ma. The mean high-temperature direction obtained from 17 paleomagnetic sites is declination/inclination (Ds/Is) = 21.8°/19.0° with κs = 65.8 and α95 = 4.4° (κ—best estimate of the precision parameter; α95—radius of the 95% probability ellipse around the mean direction; s—stratigraphic coordinates). Petrographic investigations, a positive fold test, a reversal test, and a paleosecular variation test indicate the primary origin of this characteristic remanence. A paleomagnetic pole of 60.9°N, 227.2°E with dp/dm = 2.4°/4.6° (dp/dm—semi-axes of the 95% probability ellipse around the mean pole) yields a paleolatitude of 9.2 ± 2.4°N for the eastern segment of the Lhasa terrane. Combined with reliable results from previous paleomagnetic studies, we draw the following conclusions. (1) During the Early Cretaceous, the Lhasa terrane was oriented WNW-ESE as a whole, and the eastern-middle segments may have been oriented nearly E-W. (2) Asia has accommodated 2050 ± 230 km of N-S crustal shortening along 96°E longitude since the Early Cretaceous. (3) The minimum N-S width of the Neo-Tethys Ocean at ca. 125 Ma was 4185 ± 300 km.

Remagnetization Under Hydrothermal Alteration of South Tibetan Paleocene Lavas: Maghemitization, Hematization, and Grain Size Reduction of (Titano)magnetite

AbstractThe 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.

Location and shape of the Lhasa terrane prior to India-Asia collision

The precollisional location and shape of the Lhasa terrane are crucial for constraining the closure of the Neo-Tethys Ocean and the ensuing India-Asia collision; however, estimation of these features of the Lhasa terrane remains highly controversial. Here, we carried out a new paleomagnetic investigation on the Lower Cretaceous Duoni Formation red beds in the central-eastern Lhasa terrane. The tilt-corrected site-mean direction is declination (Ds) = 339.0°, inclination (Is) = 26.8°, ks = 78.4, and α95 = 2.3° (k—precision parameter; α95—the radius that the mean direction lies within 95% confidence; s—stratigraphic coordinates) (N = 50), corresponding to a paleopole at 64.2°N, 324.2°E, with A95 = 1.9° (A95—the radius that the mean pole lies within 95% confidence). These new paleomagnetic data pass a positive fold test and indicate that the studied area was located at 14.3 ± 1.9°N during the Early Cretaceous. No significant inclination shallowing is present in the Lower Cretaceous Duoni Formation red beds. Our new results, combined with previously published reliable Cretaceous paleomagnetic results, show that the Lhasa terrane was located at a paleolatitude of ∼22.9°N to 10.1°N from west to east and was oriented at ∼298°−296° prior to India-Asia collision.

Paleomagnetism of Tarkhanian sediments of the section Skelya (Kerch Peninsula)

With the goal to correlate the magnetostratigraphic data of the Eastern Paratethys obtained by researchers in different years a rockmagnetic and paleomagnetic investigations of the Tarkhanian sediments of the Skelya section was carried out. The main carrier of the natural remanent magnetization is titanomagnetite in the Argunian beds of the Tarkhanian regional stage; in addition to titanomagnetite there is iron sulfide (monoclinic pyrrhotite and/or greigite) in the deposits of the Terskian and Kuvinian beds. The paleomagnetic fold test of natural remanent magnetization directions, recorded in the rocks of the Argunian beds, is positive, which indicates a pre-folding age of the magnetization formation. A comparison with the magnetochronological scale GTS 2020 shows that the normal polarity interval in the lower part of the Argunian beds can presumably be compared with the С5Bn chron. The reversal polarity interval in the upper part of the Argunian beds is with C5ADr. The age of the С5Bn/C5ADr boundary can be estimated as ~14.7 Ma.

Geomagnetic field behaviour during the early Cretaceous Normal Superchron from palaeomagnetic analysis of the Ramon Volcanics, Israel

SUMMARY Unravelling the long-term behaviour of the geomagnetic field is crucial for understanding the dynamics of the deep Earth. Yet, obtaining an accurate measure of geomagnetic palaeosecular variations (PSV) is difficult, partly because of tectonically induced rotations that overprint the original palaeomagnetic signal. We present a detailed palaeomagnetic investigation based on 99 sampling sites collected from the 119 to 112.6 Ma Ramon Volcanics exposed near the dormant Ramon Fault, southern Israel. These basaltic rocks were emplaced at equatorial palaeolatitudes during the beginning of the Cretaceous Normal Superchron (CNS; 123.4–83.6 Ma), during which there were no polarity reversals. Structurally corrected remanence directions consistently vary across the investigated area, whereby the sites found near a sharp bend of the Ramon Fault are clockwise rotated, whereas the other sites show no obvious rotational pattern. Elasto-plastic modelling suggests that the rotations were induced by up to 1.5 km of dextral horizontal slip accommodated by the fault, consistent with previous geological and geophysical estimations. Considering the remanence directions obtained from sites that were not influenced by the fault, we calculated an SB value of $13.3_{ - 1.3}^{ + 1.9 \circ }$ (95 percent confidence, 46 sites), which corresponds to previous SB estimations from pre- and early-CNS volcanic bodies emplaced at low palaeolatitudes (λ &amp;lt; 20°). This observation suggests that the emergence of the superchron was not accompanied by a change in the behaviour of the geomagnetic PSV. Finally, our results, together with previous palaeo-equatorial observations, show that the middle part of the superchron had lower angular dispersion (i.e. lower SB) compared to the scatter that prevailed during the beginning of the superchron. This observation suggests that the geomagnetic field transitioned into a more axial dipole dominance state towards the middle part of the superchron. Altogether, our analysis indicates that the superchron cannot be treated as a period characterized by a steady-state field behaviour.

New 40Ar/39Ar age of the full vector Upper Mammoth geomagnetic polarity transition recorded in the Pu’u Kualakauila volcanic sequence, Hawaii

Paleomagnetic measurements, coupled with 40Ar/39Ar dating, are improving our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the Earth's magnetic field. As part of an investigation of the Wai'anae Volcano, Oahu, and the short-term behavior of the geomagnetic field, we sampled a long volcanic section located on the volcano's collapsed flank at a locality known as Pu’u Kaulakauila. Prior paleomagnetic investigations of the Kamaile'unu Volcanic Series (i.e. Herrero-Bervera and Coe, 1999, Herrero-Bervera and Valet, 1999, 2005) revealed transitional directions. The fresh lava flows, easy access, and close geographical proximity to KAr dated flows made this ~215-m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least eight samples, collected from each of 47 successive flow sites, were stepwise demagnetized by both alternating field and thermal methods. Magnetostratigraphic results indicate the existence of four excursions or aborted reversals occurring at approximately ~36, ~75, ~130, and ~ 151 m above flow#1. The mean directions of magnetization of the entire section sampled indicate a reversed polarity, with ∼10 m of the section characterized by truly excursional/Cryptochron directions (~7 lava flows). Paleointensity (P.I.) determinations indicate a steady decreasing of the PI values from 75 μ-T to ~12.2 μ-T reaching a minimum of 7.1 μ-T. These very low PI values are within the transitional/excursional part of the record. The corresponding VGPs are located on the western part of Australia. 40Ar/39Ar incremental heating experiments on groundmass from transitional flow sites at different stratigraphic levels yield a weighted mean plateau age of 3.201 ± 0.041 Ma, which, combined with the overall reversed polarity and two older polarity reversals, strongly suggests that the transitional lavas correspond to the Upper Mammoth polarity transition.

Paleomagnetic and rock-magnetic study of Los Azufres lava flows (central Mexico)

We report a detailed rock-magnetic and paleomagnetic investigation on seventeen volcanic units (160 standard paleomagnetic cores) belonging to essentially Plio-Pleistocene Azufres Volcanic Field, located in the central part of the Trans-Mexican Volcanic Belt. Titanium-poor titanomagnetite is responsible for thermoremanence in the most lava flows, while unstable titanomaghemites presumably dominate in the less than half of the analyzed lava flows. The mean paleodirections are calculated for 15 sites, discarding apparently transitional flow and poorly defined (scattered) units yielding Dec = 357.23°, Inc = 38.62°, α95 = 4.89°, k = 55. These values are concordant with the expected paleodirections derived from the stable North America reference paleomagnetic poles. Secular variation parameters calculated through the dispersion of the site-mean virtual geomagnetic poles and the inclination anomaly are found slightly different to the available models. Within the 17 paleodirections obtained, four lava flows are normally magnetized between 1.14 and 1.23 Ma. These paleodirections may tentatively be assigned to the worldwide observable Cobb Mountain Geomagnetic event (subchron). This presumably short-lived episode seems to be characterized by apparently major duration than previously reported.

The Early Cretaceous Zaduo Granite, Eastern Qiangtang Terrane (China)—An Attempt to Constrain its Paleolatitude and Tectonic Implications

The Eastern Qiangtang Terrane is an orogenic-like belt around the Eastern Himalayan syntaxis (EHS). The deformation history of this terrane must be known to understand how the EHS region responded to the Lhasa-Qiangtang collision and the closure of the Bangong-Nujiang Ocean (BNO). Here, we present a new paleomagnetic investigation on an Early Cretaceous granite (∼126 Ma) in the Zaduo area, Eastern Qiangtang Terrane. Petrographic observations reflect crystallization from primary melts with only limited subsequent alteration (chloritization of biotite). Magnetite appears to be the dominant carrier of the characteristic remanent magnetization (ChRM) based on stepwise demagnetization of the natural remanent magnetization, supplemented by detailed rock magnetic measurements, including magnetization versus temperature, and acquisition curves of the isothermal and anhysteretic remanent magnetization. End-member modeling of those acquisition curves helped to constrain the paleomagnetic analysis. The inconsistent demagnetization behavior between alternating field (AF) demagnetization at high levels and thermal demagnetization was attributed to the development of gyroremanent magnetization in the AF demagnetization generated by fine-grained single domain magnetite. The ChRM directions from 92 granite samples in geographic coordinates yield an average of declination (Dg) of 2.6° and inclination (Ig) of 38.6° (precision parameter k = 51.4, and 95% confidence cone α95 = 2.1°). The amount of tilting of the granite is poorly constrained which makes proper correction rather tedious. We compared the expected bedding attitudes (Strikeexp = 43.1°, Dipexp = 46.1°) derived from published data (Huang et al., 1992;Tong et al., 2015) with the average observed bedding attitudes (Strikeobs = 54°, Dipobs = 32°) of the Middle-Upper Jurassic sandstones of the Yanshiping Group that was intruded by the Early Cretaceous granite. The discrepancy between the expected and measured bedding attitudes implies that the strata of the Yanshiping Group in the Zaduo area were already tilted prior to the intrusion of the ∼126 Ma Zaduo granite, which was attributed to the Lhasa-Qiangtang collision and the closure of the BNO. The collision led to a series of geological events, such as the tilting of the strata, the ophiolite emplacement, the development of a peripheral foreland basin, and the magmatic activity gap. The tilting/folding of the strata was generally delayed by the layer parallel shortening processes during the early stages of the deformation, thus suggesting an older Lhasa-Qiangtang collision (i.e., &amp;gt;126 Ma).

Tectonics and fluvial dynamism affecting the Tiber River in Prehistoric Rome

Geomorphological investigations in Rome’s river valley are revealing the dynamism of the prehistoric landscape. It is becoming increasingly apparent that paleogeographic conditions that defined Rome in the historical era are the product of changes since the Bronze Age, which may be the result of local fault activity in addition to fluvial dynamism. Through a dedicated borehole chronostratigraphic study, integrated by 14C and archaeological dates, and paleomagnetic investigations, we offer here new evidence for fault displacement since ca. 4500 years/BP. We present the failure of the sedimentary fabric of a clay horizon caused by liquefaction processes commonly linked with seismic shaking, interpreting an (ca. 4 m) offset to signify the existence of a fault line located at the foot of the Capitoline Hill. In addition, we show evidence for another (ca. 1 m) offset affecting a stratigraphic horizon in the river channel, occurring along another hypothesized fault line crossing through the Tiber Valley. Movement along this fault may have contributed to a documented phase of fast overflooding dated to the sixth century BCE which eventually led to the birth of the Tiber Island. The most plausible scenario implies progressive deformation, with an average tectonic rate of 2 mm/year, along these inferred fault lines. This process was likely punctuated with moderate earthquakes, but no large event necessarily occurred. Together, the available evidence suggests that during the early centuries of sedentary habitation at the site of Rome, active fault lines contributed to significant changes to the Tiber River valley, capable of challenging lowland activities.

Low paleolatitude of the Carajás Basin at ∼2.75 Ga: Paleomagnetic evidence from basaltic flows in Amazonia

Establishing the positions of continents during the initial stages of Earth's evolution is one of the most important challenges in geosciences today. This challenge is mainly due to the severe limitations in obtaining geological and/or geophysical data from early Earth time, particularly robust paleomagnetic data. Here, we report the first paleomagnetic data from an Archean block in the Amazonian craton, the Carajás Province, for ∼2.76–2.74 billion years ago (Ga), when extensive dominantly mafic volcanism (Parauapebas Formation) covered an area of ∼18,000 km2. The paleomagnetic investigation was conducted on fresh drill cores drilled into the Carajás iron ore mine and cutting across the Parauapebas Formation. After rotating the drill core segments to geographic coordinates using the viscous magnetic component, two characteristic components, Carajás 1 and 2 (C1 and C2) were identified and further used to calculate paleomagnetic poles: C1 (∼2759 Ma; 40.5°E, −44.6°S, N = 5 A95 = 6.5°, K = 18.5) and C2 (∼2749 Ma; 342.4°E, −54.3°S, N = 28, A95 = 14.8°, K = 27.8). Pole C2 is based on a bigger number of sites, passes a reversal test and is considered robust. A baked contact test was attempted for this component, but it is not conclusive. Our results, integrated with geological evidence reveals that the Carajás block occupied low latitudes at the time, and could have been part of the Superia supercraton during the Neoarchean (∼2.75 Ga) at equatorial latitudes. Finally, a consistent succession of six magnetic reversal events was identified in the lava flow sequence from the Parauapebas Formation, pointing to an already dynamic geodynamo pre-2.7 Ga.

The stepovers of the Central Dead Sea Fault: What can we learn from the confining vertical axis rotations?

The geometry of stepovers along strike-slip faults dictates the deformational field and topography induced around the faults. Stepover geometry also impacts the ability of a fault rupture to propagate from one fault segment to another and therefore acts as a major controlling factor in the size of earthquakes. However, the geometry of stepovers is often poorly resolved, as they tend to underlie young sedimentary successions. Yet, although mostly ignored, the accumulated long-term relative plate motions accommodated by the stepping faults generate permanent rotational deformation in the crust surrounding them. Here we present the results from a combined mechanical and paleomagnetic investigation of the vertical axis rotational pattern confining stepovers. We focus our investigation on the central Dead Sea Fault, where two prominent stepovers are located at the Kinneret and Hula depressions. To better understand their still somewhat elusive geometric settings, we mapped the crustal rotational field by studying the magnetization of the confining volcanic rocks. Remanent magnetization directions reveal localized zones of anomalously high vertical axis rotations in the vicinity of the stepovers that evolve to negligible rotations elsewhere. We then constructed a series of elastic and elasto-plastic slip models aiming to cover possible plate boundary architectures. Comparable patterns between the observed and predicted rotations were achieved when the Dead Sea Fault motion was concentrated along the eastern margins of both the Kinneret and Hula Basins. Our best-fit model suggests that the along-strike under-lapping distance for the Kinneret Stepover is ~8 km whereas for the Hula Stepover is only ~2 km. Our results, supported by historic and paleoseismic records that show no evidence of an earthquake that ruptured through the Kinneret Stepover, imply an enhanced deformation zone in the center of Lake Kinneret, which acts as a major barrier to rupture propagation. Our combined paleomagnetic and modeling analysis approach provides a new avenue for the study of the geometries of plate boundaries.

Fast Directional Changes during Geomagnetic Transitions: Global Reversals or Local Fluctuations?

Paleomagnetic investigations from sediments in Central and Southern Italy found directional changes of the order of 10∘ per year during the last geomagnetic field reversal (which took place about 780,000 years ago). These values are orders of magnitudes larger than what is expected from the estimated millennial timescales for geomagnetic field reversals. It is yet unclear whether these extreme changes define the timescale of global dipolar change or whether they indicate a rapid, but spatially localised feature that is not indicative of global variations. Here, we address this issue by calculating the minimum amount of kinetic energy that flows at the top of the core required to instantaneously reproduce these two scenarios. We found that optimised flow structures compatible with the global-scale interpretation of directional change require about one order of magnitude more energy than those that reproduce local change. In particular, we found that the most recently reported directional variations from the Sulmona Basin, in Central Italy, can be reproduced by a core-surface flow with rms values comparable to, or significantly lower than, present-day estimates of about 8 to 22 km/y. Conversely, interpreting the observations as global changes requires rms flow values in excess of 77 km/y, with pointwise maximal velocities of 127 km/y, which we deem improbable. We therefore concluded that the extreme variations reported for the Sulmona Basin were likely caused by a local, transient feature during a longer transition.

A full-vector paleomagnetic secular variation record from 55,000 to 33,000 cal. years BP from Río Valdéz glaciolacustrine outcrop (Tierra Del Fuego, Argentina)

High-resolution paleomagnetic investigations were performed on Río Valdez outcrop at the Fagnano Lake, central Isla Grande de Tierra del Fuego, southernmost South America. Our aims were to develop a full-vector paleomagnetic secular variation (PSV) record and establish paleomagnetic chronostratigraphy for the outcrop based on radiocarbon dating and the correlation of the PSV record with other dated PSV records in the region. We detected two distinct anomalous directional intervals at 35,400 and 33,800 cal. years BP and at 41,500 and 39,000 cal. years BP, which could be associated with Mono Lake and Laschamp excursions. Rock magnetic investigations revealed slight changes in concentration, mineralogy, and magnetic grain size along the sedimentary sequence, but the reconstruction of past direction and relative paleointensity variations in the geomagnetic field remained feasible. The record provides new insights into the behavior of the geomagnetic field at high latitudes in the Southern Hemisphere, about which very little is currently known.

Paleomagnetic testing of the Lachlan Orocline hypothesis: Overprinting of remanence in Paleozoic rocks of the Lachlan Orogen in southeast Australia

Paleomagnetic studies in the Lachlan Orogen have indicated rotation of elements within it, but the evidence for rotation is limited and has significant statistical uncertainty. This study presents a detailed paleomagnetic investigation, assisted by X-ray analysis of magnetic mineralogy of middle Cambrian to lower Silurian sedimentary and igneous rocks in Victoria and New South Wales. Four demagnetisation behaviours (types A, B, C and D) have been recognised. Type A is characterised by a single component, resulting in an approximately linear path over the magnetite unblocking temperature range. Type B specimens exhibit slow progressive demagnetisation over a wide temperature range, with clustering of demagnetised directions above the magnetite Curie temperature, indicating hematite. Behaviour C is also characterised by clustering of directions, but this extends over a wider temperature range. Samples with behaviour D show noisy and non-informative demagnetisation, indicating very low magnetic stability. Hysteresis data confirm that samples with behaviour B and C have high coercivities and remain unsaturated above 300 mT. Mineral liberation analysis of selected samples indicates presence of iron oxides, magnetite, titanomagnetite, botryoidal hematite and iron hydroxides. Only three sites yielded specimens with type A primary magnetisation, with a poorly defined pole from the Dookie Volcanics on the central limb of the orocline, which does match a previous result from the same district. Two paleopoles, of type B and C specimens from the rocks in Tabberabbera Zone and the Rockley Volcanics, plot within the confidence limit of poles defining the late Silurian to Early Devonian Australian apparent polar wander path. These poles match other poles representing Devonian overprints around the margins of the Lachlan Orogen that have been interpreted to be the result of alteration mineralisation produced by fluids driven by tectonic compression during events in the Bindian (420–410 Ma) or early Tabberabberan (405–380 Ma) orogenies. KEY POINTS Paleomagnetic techniques have been used to test the rotation within the elements of the Lachlan Orocline in southeast Australia. Results suggest that the original remanence in the rocks is obscured owing to post-deformation overprint. This overprint is widespread in the Lachlan Orogen and is associated with the deformation during Bindian/Tabberabberan orogenic events.

Meguma terrane orocline: U–Pb age and paleomagnetism of the Silurian Mavillette gabbro, Nova Scotia, Canada

Paleomagnetic results and a U–Pb baddeleyite age from the Silurian Mavillette gabbroic sill in southwest Nova Scotia, Canada, provide new evidence about the Paleozoic tectonic evolution of the Meguma terrane. The Mavillette gabbro sill intruded ca. 440–430 Myr old bimodal rift-related metavolcanic and metasedimentary rocks of the White Rock Formation in the Silurian–Devonian Rockville Notch Group. The 426.4 ± 2.0 Ma Mavillette gabbro age is notably younger than the ca. 440 Ma magmatism, but is part of a geochemically defined suite of within-plate sills and volcanic rocks of the Rockville Notch Group with ages as young as Early Devonian. Paleomagnetic investigation of 13 sites distributed along the Neoacadian (ca. 390 Ma) synclinal limbs of the Mavillette sill reveal magnetization directions that fail a fold test and therefore postdate Silurian emplacement of the gabbro. The post-folding remanence has a mean direction of D = 153.4, I = 17.1°; α 95 = 6.5° (n = 12 sites), with corresponding paleopole 31.9 °S, 325.2 E; dp = 3.5°, dm = 6.7° that resembles a pervasive late Carboniferous Kiaman overprint magnetization in North America, but is rotated significantly 22.2° ± 8.1° counter-clockwise (CCW). Mavillette remanence acquisition likely occurred in concert with fluid mobilization related to Alleghanian deformation, recorded locally by ca. 320 Ma muscovite 40Ar–39Ar ages. Previously published paleomagnetic results from the Meguma terrane also have Carboniferous remanence directions with similar ∼24° CCW discordance. The regional CCW rotation of the southwest Meguma terrane post-dates this ca. 320 Ma tectonothermal remanence acquisition event, likely recording the development of an oroclinal bend of the Meguma terrane during the Alleghanian orogeny.