Acquiring accurate absolute paleointensities of the geomagnetic field is essential for studying the evolution of the geodynamo in Earth's core and its thermal history. Thermoremanence is a preferred magnetic recording mechanism, but most igneous rocks are dominated by non-single-domain (non-SD) ferromagnetic grains (e.g., magnetite larger than similar to 100 nm), for which there is no strict theory for obtaining unbiased paleointensities. The Repeated Thellier-series Experiment (RESET) method has been recently developed to specifically deal with non-SD effects and thermal alterations so as to provide an accurate paleointensity. To test its capability, we studied a 1915 CE dacitic lava from Mount Lassen (located in northern California, USA, 40 degrees 29 ' N, 121 degrees 30 ' W, elevation similar to 3,100 m) that contains micron-sized titanomagnetites whose thermoremanence was acquired in a known geomagnetic field of 54 mu T. This dacitic lava was previously investigated and found to exhibit concave and S-shaped Arai diagrams along with unsatisfying paleointensities. In this study, out of 34 specimens, we successfully obtained 22 qualified RESET-corrected paleointensity estimates that range from 50.6 to 57.4 mu T with an average of 54.1 +/- 1.6 mu T. Our results show that the thermoremanence recorded in non-SD-dominated igneous rocks can indeed provide accurate paleointensity estimates using the RESET method. We demonstrate that the RESET method provides a promising approach to expanding the range of geological materials suitable for future paleointensity studies, enabling the recovery of accurate paleointensities from samples previously considered non-ideal, which can significantly improve our understanding of the evolution of the geomagnetic field.

Paleomagnetism relies4 on stable remanent magnetizations held in rocks to reconstruct the ancient geomagnetic field direction and intensity. However, rocks may carry secondary overprints that obscure or completely destroy the original signal. To study the stability of the magnetic vector(s), laboratories routinely apply static alternating field demagnetization along three orthogonal axes (AFD₃), which is fast, non-destructive, and easy to automate. Here, we present a multiparticle model that shows AFD₃ can deviate the natural remanent magnetization (NRM). Deviations can be avoided when fulfilling two conditions: (i) the NRM was acquired in a weak field where magnetization intensity varies linearly with field strength, and (ii) the sample is magnetically isotropic. The first condition is generally satisfied for rocks holding thermal or detrital remanent magnetizations, but not those affected by an isothermal remanence (e.g., lightning), even though AFD₃ is often used to remove them. In rocks with an anisotropic particle orientation distribution, stepwise AFD₃ progressively removes different coercivity subpopulations as a function of grain orientation so the effective remanence anisotropy of the surviving carriers changes during demagnetization. The anisotropy-driven deflection therefore evolves with AF step, producing curvilinear demagnetization trajectories. Our theoretical results argue for caution when applying AFD₃ to anisotropic samples or those with isothermal overprints. Undesired NRM rotation can be avoided by tumble demagnetization or mitigated by increasing the number of alternating field axis orientations.

Anhysteretic remanent magnetization (ARM) data serve many applications in the geosciences, such as quantifying the paleomagnetic field intensity, the magnetic grain size distributions in rocks, the degree of particle alignment from flow processes, and the emplacement and strain histories of rocks. However, the use of an alternating field when imparting or removing an ARM may give rise to a gyroremanent magnetization (GRM), which is a spurious signal that can bias ARM data. In this paper, we demonstrate the effect of GRM on ARM acquisition and demagnetization through an example of a relative paleointensity measurement. We describe a method using singular spectrum analysis to remove GRM acquired during static alternating field demagnetization (GRMAF). Another GRM component (GRMARM) is acquired during ARM acquisition. GRMARM is progressively removed during AF demagnetization, which results in a demagnetization spectrum containing two overlapping components (ARM and GRMARM). A second progressive demagnetization of a GRM imparted on the sample using an AF protocol with the same peak field as that used to impart the ARM, should contain the identical GRMARM component as acquired during the original ARM demagnetization. When separately measured, GRMARM can be subtracted from the original ARM demagnetization as a means of isolating a GRM-free ARM. Although we restrict our discussion to relative paleointensity, the methods described herein are applicable to any study involving alternating fields in paleomagnetic laboratory experiments.

ABSTRACT The Searles Lake Formation in Searles Valley, southeastern California, represents deposition of the paleo–Owens River into a Pleistocene and Holocene pluvial terminal lake. A prior 32–10 ka estimated age for the upper part of the Searles Lake Formation relied on uncalibrated, conventional radiocarbon dates. We present accelerator mass spectrometer radiocarbon dates that indicate the base of the Searles Lake Formation at the Poison Canyon type section is 46 ka. That age is consistent with paleomagnetic data at Poison Canyon and the Tire Farm locality, which record high-latitude Southern Hemisphere virtual geomagnetic poles that we assign to the 41 ka Laschamp excursion. The presence of Searles Lake at 46–43 ka also is consistent with a Pacific storm track that extended south of 37.5°N at that time. At the head of Salt Wells Valley–Poison Canyon, sediments that we interpret as a Searles Lake highstand were radiocarbon dated at 14.1 ka.

Super-eruptions are amongst the most extreme events to affect Earth's surface, but too few examples are known to assess their global role in crustal processes and environmental impact. We demonstrate a robust approach to recognize them at one of the best-preserved intraplate large igneous provinces, leading to the discovery of two new super-eruptions. Each generated huge and unusually hot pyroclastic density currents that sterilized extensive tracts of Idaho and Nevada in the United States. The ca. 8.99 Ma McMullen Creek eruption was magnitude 8.6, larger than the last two major eruptions at Yellowstone (Wyoming). Its volume exceeds 1700 km(3), covering >= 12,000 km(2). The ca. 8.72 Ma Grey's Landing eruption was even larger, at magnitude of 8.8 and volume of >= 2800 km(3). It covers >= 23,000 km(2) and is the largest and hottest documented eruption from the Yellowstone hotspot. The discoveries show the effectiveness of distinguishing and tracing vast deposit sheets by combining trace-element chemistry and mineral compositions with field and paleomagnetic characterization. This approach should lead to more discoveries and size estimates, here and at other provinces. It has increased the number of known super-eruptions from the Yellowstone hotspot, shows that the temporal framework of the magmatic province needs revision, and suggests that the hotspot may be waning.

The measurement of anhysteretic remanent magnetization (ARM) is an easy nondestructive means of evaluating many important rock properties. This involves applying an alternating magnetic field (AF) along with a direct biasing field (DF) to magnetize rock samples. Various techniques employing ARM have been widely used with notable success to help solve problems in tectonics, volcanology, sedimentology, environmental and paleoclimate studies and to better understand geomagnetic field behavior. Often, however, too little attention is paid to the effects of switching field angular dependence, which has important influence on the coercivity and angular distributions of grains that actually carry the ARM. As a result, commonplace methods of ARM measurement are not ideally designed for their intended purposes. For example, differentiation of a progressive ARM acquisition or turning on the direct field within a narrow AF window (i.e. partial ARM) are unable to isolate the ARM contribution from a specified coercivity grain fraction. Instead, the optimal method of measuring ARM in a targeted coercivity grain fraction is to differentiate a progressive tumble demagnetization of a total ARM generated with a peak AF high enough to fully activate the coercivity range of interest. Ignoring the activation property can lead to overestimation of relative paleointensity, underestimation of high coercivity grain concentrations, amplification of ARM anisotropy (e.g. error in natural remanence corrections), and unwanted mixing of anisotropy fabric components held in separate coercivity grain fractions.

Reversal of Earth’s magnetic field polarity every 105 to 106 years is among the most far-reaching, yet enigmatic, geophysical phenomena. The short duration of reversals make precise temporal records of past magnetic field behavior paramount to understanding the processes that produce them. We correlate new 40Ar/39Ar dates from transitionally magnetized lava flows to astronomically dated sediment and ice records to map the evolution of Earth’s last reversal. The final 180° polarity reversal at ~773 ka culminates a complex process beginning at ~795 ka with weakening of the field, succeeded by increased field intensity manifested in sediments and ice, and then by an excursion and weakening of intensity at ~784 ka that heralds a >10 ka period wherein sediments record highly variable directions. The 22 ka evolution of this reversal suggested by our findings is mirrored by a numerical geodynamo simulation that may capture much of the naturally observed reversal process.

Greater India comprises a part of the Indian plate that subducted under Asia to help form the Tibetan Plateau. Defining the size of the Greater India is thus a key constraint to model the India-Asia collision, growth of the plateau, and the tectonic evolution of the Neo-Tethyan realm. We report Early Cretaceous paleomagnetic data from the central and eastern Tethyan Himalaya that yield paleolatitudes consistent with previous Early Cretaceous paleogeographic reconstructions. These data suggest Greater India extended at least 2,675 +/- 720 and 1,950 +/- 970 km farther north from the present northern margin of India at 83.6 degrees E and 92.4 degrees E, respectively. An area of lithosphere >= 4.7 x 10(6) km(2) was consumed through subduction, thereby placing a strict limit on the minimum amount of Indian lithosphere consumed since the breakup of Gondwanaland. Plain Language summary Greater India is part of the Indian plate, subsequently subducted under Asia, that helped create the Tibetan Plateau. The amount of Greater Indian crust therefore plays a critical role to address key problems in continental geodynamics. To what extent can continental crust be subducted? How much crust was derived from horizontal shortening of existing crust? How much of Tibet was created by subducted buoyant, continental crust? We provide paleomagnetic evidence that defines the minimum size of Greater India. Our data show that a lithospheric area of >= 4.7 x 10(6) km(2) was subducted, which supports the notion that the growth of Tibetan Plateau in the Cenozoic occurred by adding buoyant material to its base.

Changes in the Earth's magnetic field have global significance that reach from the outer core extending out to the uppermost atmosphere. Paleomagnetic records derived from sedimentary and volcanic sequences provide important insights into the geodynamo processes that govern the largest geomagnetic changes (polarity reversals), but dating uncertainties have hindered progress in this understanding. Here, we report a paleomagnetic record from multiple lava flows on Tahiti that bracket the Matuyama-Brunhes (M-B) polarity reversal similar to 771,000 years ago. Our high-precision 40Ar/39Ar ages constrain several rapid and short-lived changes in field orientation up to 33,000 years prior to the M-B reversal. These changes are similar to ones identified in other less well-dated lava flows in Maui, Chile, and La Palma that occurred during an extended period of reduced field strength recorded in sediments. We use a simple stochastic model to show that these rapid polarity changes are highly attenuated in sediment records with low sedimentation rates. This prolonged 33,000 year period of reduced field strength and increased geomagnetic instability supports models that show frequent centennial-to-millennial-scale polarity changes in the presence of a strongly weakened dipole field.

AbstractUnderstanding the dynamics of double‐thickening and uplifting of the Tibetan crust requires constraints on the magnitude and timing of crustal shortening. New elongation/inclination (E/I)‐corrected paleomagnetic data from ~26–22 Ma sediments indicate that the latitude of southern Tibet in the early Miocene was 31.1/−6.8/+5.2°N, not significantly different from today. This implies that the southern margin of Asia, which was at 21–24°N latitude from the Late Cretaceous to the early Eocene, advanced 8–10° northward between the early Eocene and the latest Oligocene. Our results therefore suggest that at least 900–1100 km of continental shortening and significant regional uplift of the plateau occurred between the early Eocene and late Oligocene. Our results suggest that N‐S intra‐Asian convergence was considerably reduced around 26 Ma, corresponding to a transition from compression to extension within the Tibetan Plateau.

In this paper, we present paleomagnetic, geochemical, mineralogical, and geochronologic evidence for correlation of the mid‐Miocene Cougar Point Tuff (CPT) in southwest Snake River Plain (SRP) of Idaho. The new stratigraphy presented here significantly reduces the frequency and increases the scale of known SRP ignimbrite eruptions. The CPT section exposed at the Black Rock Escarpment along the Bruneau River has been correlated eastward to the Brown's Bench escarpment (six common eruption units) and Cassia Mountains (three common eruption units) regions of southern Idaho. The CPT records an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations. Paleomagnetic characterization of eruption units based on geomagnetic field variation has a resolution on the order of a few centuries, providing a strong test of whether two deposits could have been emplaced from the same eruption or from temporally separate events. To obtain reliable paleomagnetic directions, the anisotropy of anhysteretic remanence was measured to correct for magnetic anisotropy, and an efficient new method was used to remove gyroremanence acquired during alternating field demagnetization.

Rogerson Graben, USA, is critically placed at the intersection between the Yellowstone hotspot track and the southern projection of the west Snake River rift. Eleven rhyolitic members of the re-defined, ≥420-m-thick, Rogerson Formation record voluminous high-temperature explosive eruptions, emplacing extensive ashfall and rheomorphic ignimbrite sheets. Yet, each member has subtly distinct field, chemical and palaeomagnetic characteristics. New regional correlations reveal that the Brown’s View ignimbrite covers ≥3300 km2, and the Wooden Shoe ignimbrite covers ≥4400 km2 and extends into Nevada. Between 11.9 and ∼8 Ma, the average frequency of large explosive eruptions in this region was 1 per 354 ky, about twice that at Yellowstone. The chemistry and mineralogy of the early rhyolites show increasing maturity with time possibly by progressive fractional crystallisation. This was followed by a trend towards less-evolved rhyolites that may record melting and hybridisation of a mid-crustal source region. Contemporaneous magmatism-induced crustal subsidence of the central Snake River Basin is recorded by successive ignimbrites offlapping and thinning up the N-facing limb of a regional basin-margin monocline, which developed between 10.59 and 8 Ma. The syn-volcanic basin topography contrasted significantly with the present-day elevated Yellowstone hotspot plateau. Concurrent basin-and-range extension produced the N-trending Rogerson Graben: early uplift of the Shoshone Hills (≥10.34 Ma) was followed by initiation of the Shoshone Fault and an E-sloping half-graben (∼10.3–10.1 Ma). The graben asymmetry then reversed with initiation of the Brown’s Bench Fault (≥8 Ma), which remained intermittently active until the Pliocene.