ALBERT

Description: The eastern escarpment of the Sierra Nevada (USA) forms one of the most prominent topographic and geologic features in the Cordillera, yet the timing and nature of fault displacements along it remain relatively poorly known. The central Sierra Nevada range front is an ideal place to determine the structural evolution of the range front because it has abundant dateable Cenozoic volcanic rocks. The Sonora Pass area of the central Sierra Nevada is particularly good for reconstructing the slip history of range-front faults, because it includes unusually widespread and distinctive high-K volcanic rocks (the ca. 11.5–9 Ma Stanislaus Group) that serve as outstanding strain markers. These include the following, from base to top. (1) The Table Mountain Latite (TML) consists of voluminous trachyandesite, trachybasaltic andesite, and basalt lava flows, erupted from fault-controlled fissures in the Sierra Crest graben-vent system. (2) The Eureka Valley Tuff consists of three trachydacite ignimbrite members erupted from the Little Walker caldera. These ignimbrites are interstratified with lava flows that continued to erupt from the Sierra Crest graben-vent system, and include silicic high-K as well as intermediate to mafic high-K lavas. The graben-vent system consists of a single ~27-km-long, ~8–10-km-wide approximately north-south graben that is along the modern Sierran crest between Sonora Pass and Ebbetts Pass, with a series of approximately north-south half-grabens on its western margin, and an ~24-km-wide northeast transfer zone emanating from the northeast boundary of the graben on the modern range front south of Ebbetts Pass. In this paper we focus on the structural evolution of the Sonora Pass segment of the Sierra Nevada range front, which we do not include in the Sierra Crest graben-vent complex because we have found no vents for high-K lava flows here. However, we show that these faults localized the high-K Little Walker caldera. We demonstrate that the range-front faults at Sonora Pass were active before and during the ca. 11.5–9 Ma high-K volcanism. We show that these faults are dominantly approximately north-south down to the east normal faults, passing northward into a system of approximately northeast-southwest sinistral oblique normal faults that are on the southern end of the ~24-km-wide northeast transfer zone in the Sierra Crest graben-vent complex. At least half the slip on the north-south normal faults on the Sonora Pass range front occurred before and during eruption of the TML, prior to development of the Little Walker caldera. It has previously been suggested that the range-front faults formed a right-stepping transtensional stepover that controlled the siting of the Little Walker caldera; we support that interpretation by showing that synvolcanic throw on the faults increases southward toward the caldera. The Sonora Pass–Little Walker caldera area is shown here to be very similar in structural style and scale to the transtensional stepover at the Quaternary Long Valley field. Furthermore, the broader structural setting of both volcanic fields is similar, because both are associated with a major approximately northeast-southwest sinistral oblique normal fault zone. This structural style is typical of central Walker Lane belt transtension. Previous models have called for westward encroachment of Basin and Range extension into the Sierra Nevada range front after arc volcanism ceased (ca. 6–3.5 Ma); we show instead that Walker Lane transtension is responsible for the formation of the range front, and that it began by ca. 12 Ma. We conclude that Sierra Nevada range-front faulting at Sonora Pass initiated during high-K arc volcanism, under a Walker Lane transtensional strain regime, and that this controlled the siting of the Little Walker caldera.

Description: A recent study of the Matuyama–Brunhes (M-B) geomagnetic field reversal recorded in exposed lacustrine sediments from the Sulmona Basin (Italy) provided a continuous, high-resolution record indicating that the reversal of the field direction at the terminus of the M-B boundary (MBB) occurred in less than a century, about 786 ka ago. In the sediment, thin (4–6 cm) remagnetized horizons were recognized above two distinct tephra layers—SUL2-19 and SUL2-20—that occur ~25 and ~35 cm below the MBB, respectively. Also, a faint, millimetre-thick tephra (SUL2-18) occurs 2–3 cm above the MBB. With the aim of improving the temporal resolution of the previous Sulmona MBB record and understanding the possible influence of cryptotephra on the M-B record in the Sulmona Basin, we performed more detailed sampling and analyses of overlapping standard and smaller samples from a 50 cm-long block that spans the MBB. The new data indicate that (i) the MBB is even sharper than previously reported and occurs ~2.5 cm below tephra SUL2-18, in agreement with the previous study; (ii) the MBB coincides with the rise of an intensity peak of the natural remanent magnetization (NRM) intensity, which extends across SUL2-18; (iii) except for a 2-cm-thick interval just above tephra SUL2-18, the rock magnetic parameters ( k , ARM, M r , M s , B c , B cr ) indicate exactly the same magnetic mineralogy throughout the sampled sequence. We conclude that either SUL2-18 resulted in the remagnetization of an interval of about 6 cm (i.e. during the NRM intensity peak spanning ~260 ± 110 yr, according to the estimated local sedimentation rate), and thus the detailed MBB record is lost because it is overprinted, or the MBB is well recorded, occurred abruptly about 2.5 cm below SUL2-18 and lasted less than 13 ± 6 yr. Both hypotheses challenge our understanding of the geomagnetic field behaviour during a polarity transition and/or of the NRM acquisition process in the Sulmona lacustrine sediment.

Description: 〈p〉We designed and tested a compact deuteron-deuteron fusion neutron generator for application to 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar geochronology. The nearly monoenergetic neutrons produced for sample irradiation are anticipated to provide several advantages compared with conventional fission spectrum neutrons: Reduction of collateral nuclear reactions increases age accuracy and precision. Irradiation parameters within the neutron generator are more controllable compared with fission reactors. Confidence in the prediction of recoil energies is improved, and their likely reduction potentially broadens applicability of the dating method to fine-grained materials without vacuum encapsulation. Resolution of variation in the 〈sup〉39〈/sup〉K(n,p)〈sup〉39〈/sup〉Ar neutron capture cross section at 1.3 to 3.2 MeV and discovery of a strong resonance at ~2.4 MeV illuminate future pathways to improve the technique for 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar dating.〈/p〉

Description: New constraints on the timing of the Cretaceous-Paleogene mass extinction and the Chicxulub impact, together with a particularly voluminous and apparently brief eruptive pulse toward the end of the "main-stage" eruptions of the Deccan continental flood basalt province suggest that these three events may have occurred within less than about a hundred thousand years of each other. Partial melting induced by the Chicxulub event does not provide an energetically plausible explanation for this coincidence, and both geochronologic and magnetic-polarity data show that Deccan volcanism was under way well before Chicxulub/Cretaceous-Paleogene time. However, historical data document that eruptions from existing volcanic systems can be triggered by earthquakes. Seismic modeling of the ground motion due to the Chicxulub impact suggests that the impact could have generated seismic energy densities of order 0.1–1.0 J/m 3 throughout the upper ~200 km of Earth’s mantle, sufficient to trigger volcanic eruptions worldwide based upon comparison with historical examples. Triggering may have been caused by a transient increase in the effective permeability of the existing deep magmatic system beneath the Deccan province, or mantle plume "head." It is therefore reasonable to hypothesize that the Chicxulub impact might have triggered the enormous Poladpur, Ambenali, and Mahabaleshwar (Wai Subgroup) lava flows, which together may account for 〉70% of the Deccan Traps main-stage eruptions. This hypothesis is consistent with independent stratigraphic, geochronologic, geochemical, and tectonic constraints, which combine to indicate that at approximately Chicxulub/Cretaceous-Paleogene time, a huge pulse of mantle plume–derived magma passed through the crust with little interaction and erupted to form the most extensive and voluminous lava flows known on Earth. High-precision radioisotopic dating of the main-phase Deccan flood basalt formations may be able either to confirm or reject this hypothesis, which in turn might help to determine whether this singular outburst within the Deccan Traps (and possibly volcanic eruptions worldwide) contributed significantly to the Cretaceous-Paleogene extinction.

Description: Past orbital analogues to the current interglacial, such as Marine Isotope Stage 19c (MIS 19c, ca. 800 ka), can provide reliable reference intervals for evaluating the timing and the duration of the Holocene and factors inherent in its climatic progression. Here we present the first high-resolution paleoclimatic record for MIS 19 anchored to a high-precision 40 Ar/ 39 Ar chronology, thus fully independent of any a priori assumptions on the orbital mechanisms underlying the climatic changes. It is based on the oxygen isotope compositions of Italian lake sediments showing orbital- to millennial-scale hydrological variability over the Mediterranean between 810 and 750 ka. Our record indicates that the MIS 19c interglacial lasted 10.8 ± 3.7 k.y., comparable to the time elapsed since the onset of the Holocene, and that the orbital configuration at the time of the following glacial inception was very similar to the present one. By analogy, the current interglacial should be close to its end. However, greenhouse gas concentrations at the time of the MIS 19 glacial inception were significantly lower than those of the late Holocene, suggesting that the current interglacial could have already been prolonged by the progressive increase of the greenhouse gases since 8–6 ka, possibly due to early anthropogenic disturbance of vegetation.

Description: We report a palaeomagnetic investigation of the last full geomagnetic field reversal, the Matuyama-Brunhes (M-B) transition, as preserved in a continuous sequence of exposed lacustrine sediments in the Apennines of Central Italy. The palaeomagnetic record provides the most direct evidence for the tempo of transitional field behaviour yet obtained for the M-B transition. 40 Ar/ 39 Ar dating of tephra layers bracketing the M-B transition provides high-accuracy age constraints and indicates a mean sediment accumulation rate of about 0.2 mm yr –1 during the transition. Two relative palaeointensity (RPI) minima are present in the M-B transition. During the terminus of the upper RPI minimum, a directional change of about 180 ° occurred at an extremely fast rate, estimated to be less than 2 ° per year, with no intermediate virtual geomagnetic poles (VGPs) documented during the transit from the southern to northern hemisphere. Thus, the entry into the Brunhes Normal Chron as represented by the palaeomagnetic directions and VGPs developed in a time interval comparable to the duration of an average human life, which is an order of magnitude more rapid than suggested by current models. The reported investigation therefore provides high-resolution integrated palaeomagnetic and radioisotopic data that document the fine details of the anatomy and tempo of the M-B transition in Central Italy that in turn are crucial for a better understanding of Earth's magnetic field, and for the development of more sophisticated models that are able to describe its global structure and behaviour.