Dario Bilardello

Position of the Snake River watershed divide as an indicator of geodynamic processes in the greater Yellowstone region, western North America

Tectonic processes, fl exure due to crustal loading, and dynamic mantle fl ow each impart a unique imprint on topography and geomorphic responses over time scales of 104 to 10 6 yr. This paper explores the mobility of regional drainage divides as a key geomorphic metric that can distinguish between the various processes driving crustal deformation in the greater Yellowstone region of the northwestern United States. We propose a new analysis that quantifi es the differences between the location of the presentday drainage divide from divides synthetically generated from fi ltered topography to determine the relative impact of tectonic and dynamic mantle infl uences on landscape development. The greater Yellowstone region is an opportune location for this investigation because contrasting models have been proposed to explain the parabolic shape of elevated topography and active seismicity that outline the imprint of hypothesized hotspot activity. Drainage divides synthesized from topography fi ltered at 50, 100, and 150 km wavelengths within the greater Yellowstone region show that the locations of the actual and synthetic Snake River drainage divides are controlled by both dynamic and fl exural mechanisms in the eastern greater Yellowstone region, but by fl exural mechanisms only in the western greater Yellowstone region. The location of the actual divide deviates from its predicted position in the fi ltered topography where tectonic controls, such as active faults (e.g., Centennial and Teton faults), have uplifted large footwall blocks. Our results are consistent with the notion of a northeastward-propagating greater Yellow stone region topographic and seismic parabola, and suggest that Basin and Range extension follows from, rather than precedes, greater Yellowstone region dynamic topography. Furthermore, our analysis suggests that eastward migration of the Snake River drainage divide lags behind the continued northeastward propagation of high-standing topography associated with the Yellowstone geophysical anomaly by 1–2 m.y.

Numerical simulation of inclination shallowing by rolling and slipping of spherical particles

We report algorithms for two- and three-dimensional numerical simulations of settling spherical magnetic particles with prescribed size-distributions. Particles roll, or roll and slip, on the substrate, which causes their magnetic moments to rotate. These models are applied to the problem of inclination shallowing, which is repeatedly encountered in paleomagnetic studies of sedimentary rocks, where the recorded inclination is less than the expected field inclination. Simulations of equal-sized assemblages of magnetic spheres yield shallowing factors of 0.6, similar to that found in nature and in laboratory redeposition experiments. Comparable results are obtained when varying the size distributions of the spheres. Inclination shallowing is more pronounced when the smaller particles are magnetic and the larger ones are non-magnetic. Our study shows that rolling and/or slip (translation) of spherical particles can significantly contribute to inclination shallowing.

Effect of magnetic anisotropy on the natural remanent magnetization in the MCU IVe’ layer of the Bjerkreim Sokndal Layered Intrusion, Rogaland, Southern Norway

A strong negative magnetic anomaly, caused by an intense natural remanent magnetization (NRM) approximately opposite todays geomagnetic field, is observed above the MCU IVe unit of the Bjerkreim Sokndal Layered Intrusion. The anomaly is strongest in the east, close to Heskestad, and decreases when following the layer toward the north and west. This study investigates how the NRM changes along the layer and how its direction and intensity are affected by magnetic fabrics in the intrusion. NRM, low-field anisotropy of magnetic susceptibility, and anisotropy of anhysteretic remanence have been measured on 371 specimens from 46 sites. The orientation of both the magnetic fabrics and the NRM changes for different locations along the layer, and it appears that the NRM is tilted away from the mean paleofield and toward the direction of maximum susceptibility and maximum anhysteretic remanence. When NRM directions are corrected for magnetic fabrics, the angle between the NRM and mean paleofield direction generally decreases for specimens with a single-component NRM. No correlation was found between the NRM intensity and the directional relationship between NRM, magnetic fabric, and mean paleofield.

High-resolution magnetostratigraphy of the Upper Nacimiento Formation, San Juan Basin, New Mexico, USA: Implications for basin evolution and mammalian turnover

Lower Paleocene deposits in the San Juan Basin document one of the best records of mammalian change and turnover following the Cretaceous-Paleogene mass extinction and is the type area for the Puercan (Pu) and Torrejonian (To) North America Land Mammal age (NALMA). One of the largest mammalian turnover events in the early Paleocene occurs between the Torrejonian 2 (To2) and Torrejonian 3 (To3) NALMA interval zones. The Nacimiento Formation contains the only deposits in North America where the To2-To3 mammalian turnover can be constrained; however, the precise age and duration of the turnover is poorly understood due to the lack of a precise chronostratigraphic framework. We analyzed paleomagnetic samples, produced a 40Ar/39Ar detrital sanidine age, and developed a detailed lithostratigraphy for four sections of the upper Nacimiento Formation in the San Juan Basin, New Mexico (Kutz Canyon, Escavada Wash, Torreon West and East) to constrain the age and duration of the deposits and the To2-To3 turnover. The polarity stratigraphy for the four sections can be correlated to chrons C27r-C26r of the geomagnetic polarity time scale (GPTS). Using the local polarity stratigraphy for each section, we calculated a mean sediment accumulation rate and developed a precise age model, which allows us to determine the age of important late Torrejonian mammalian localities. Using the assigned ages, we estimate the To2-To3 turnover was relatively rapid and occurred over ∼120 kyr (−60/+50 kyr) between 62.59 and 62.47 Ma. This rapid duration of mammalian turnover suggests that it was driven by external forcing factors, such as environmental change driven by the progradation of the distributive fluvial system across the basin and/or changes in regional or global climate. Additionally, comparisons of the mean sediment accumulation rates among the sections that span from the basin margin to the basin center indicate that sediment accumulation rates equalized across the basin from the end of C27r through the start of C26r, suggesting an accommodation minimum in the basin associated with the progradation of a distributive fluvial system into the basin. This accommodation minimum also likely led to the long hiatus of deposition between the Paleocene Nacimiento Formation and the overlying Eocene San Jose Formation.

Influence of static alternating field demagnetization on anisotropy of magnetic susceptibility: Experiments and implications

Anisotropy of magnetic susceptibility (AMS) indicates the preferred orientation of a rocks constituent minerals. However, other factors can influence the AMS, e.g. domain wall pinning or domain alignment in ferromagnetic minerals. Therefore, it is controversial whether samples should be alternating field (AF) demagnetized prior to AMS characterization. This may remove the influence of natural remanent magnetization (NRM) or domain wall pinning on AMS; however, it may also result in field-induced anisotropy. This study investigates the influence of stepwise AF and low-temperature demagnetization on mean susceptibility, principal susceptibility directions, AMS degree and shape for sedimentary, metamorphic and igneous rocks. Alternating fields up to 200 mT were applied along the sample x, y and z axes, rotating the order for each step, to characterize the relationship between AMS principal directions and the last AF orientation. The changes in anisotropy, defined by the mean deviatoric susceptibility of the difference tensors, are between <2% and 270% of the AMS in NRM-state. Variations in AMS parameters range from small changes in shape to complete reorientation of principal susceptibility axes, with the maximum susceptibility becoming parallel to the last AF direction. This is most prevalent in samples with low degrees of anisotropy in the NRM-state. No clear correlations were found between field-induced anisotropy and hysteresis properties. Therefore, we propose that future studies check any samples whose AMS is carried by ferromagnetic minerals and low anisotropy degrees for AF-induced artifacts. These results highlight the need for understanding the AMS sources and carriers prior to any structural interpretation.

Magnetic Anisotropy: Theory, Instrumentation, and Techniques

Magnetic anisotropy determines the statistical alignment of magnetic minerals in rocks and provides insight into numerous geological processes, from formation to deformation. While anisotropy of magnetic susceptibility (AMS) has become a synonym for magnetic fabrics, a variety of techniques for measuring anisotropy exist that are mostly based on magnetic remanence, though others also exist. Depending on the purpose of the study to be undertaken certain techniques will be more useful than others, and an understanding of what each technique measures is thus a fundamental requirement for a successful anisotropy study. Furthermore, the same types of anisotropy can be measured on very different instruments, with more or less obvious advantages and disadvantages. Choice of instrumentation and measurement protocol should primarily be dictated by the goal of the study, but inevitably instrument availability poses the ultimate control: to this end, this article aims at providing a general background on the study of magnetic anisotropy, describing the available techniques, their applications, and advantages and disadvantages. The attempt is to guide the less-advanced researcher into an informed choice on which is the most valuable technique to be used for a specific study while providing the fundamental theory, instrumentation information, and essential bibliography on the state of the art of magnetic anisotropy research. Applications of the study of magnetic fabrics are not the focus of this article, instead, simple examples are presented together with a few citations from recent studies: from these the interested reader can extract a more in-depth and specific bibliography.