Assessing adequacy of spectrum‐matched ground motions for response history analysis

Response spectrum matching is commonly used to generate ground motions with response spectra matching a scenario target spectrum. There is some debate in the literature about whether spectrum‐matched motions lead to biased structural analysis results. Furthermore, there are no objective, quantitative criteria available for deciding whether a ground motion has been manipulated excessively by spectrum matching, and whether large modification may also lead to bias. This study investigates both of these issues by presenting the results of structural analysis using two reinforced concrete moment frame models and two earthquake scenarios, with suites of unmatched and matched ground motions. Through comparison with a robust benchmark, it is shown that no significant bias is introduced by spectrum matching. The period range and target damping values for matching are also investigated, and matching up to three times the fundamental period is shown to be beneficial in reducing dispersion in the results. Finally, these analyses were also used to investigate whether large changes in the ground motion lead to biased analysis results. Several potential measures of change are investigated, including those based on peak absolute ground motion, cumulative squared ground motion (absolute or normalized), and input energy into single‐degree‐of‐freedom systems. Although no systematic, statistically significant correlation is found for the analysis results in terms of any of these measures of change, tentative criteria are proposed, which may be used by analysts to aid in the decision of whether to accept or reject a spectrum‐matched motion. Copyright © 2012 John Wiley & Sons, Ltd.     

Last interglacial western camel (Camelops hesternus) from eastern Beringia

Western camel (C. hesternus) fossils are rare from Eastern Beringia, thus there is little available information on their chronology, paleoecology, and biogeography in this region. In August of 2010, a partial proximal phalanx of a western camel was recovered from a sedimentary exposure along the White River, in the formerly glaciated terrain of southwest Yukon, northwest Canada. The fossil specimen was recovered in situ from sediments that are correlated by stratigraphic, tephra and radiocarbon data to the Marine Isotope Stage (MIS) 5 interglacial period (Sangamonian). Associated paleoenvironmental data indicates that this western camel inhabited a shrub tundra ecosystem that did not include spruce trees or boreal forest during a relatively cold interval between MIS 5e and 5a. This is the oldest reliably dated western camel fossil from Eastern Beringia and supports the model of range expansion for this species to the high latitudes of northwest North America during the last interglacial (sensu lato).     

The Tilia decline: vegetation change in lowland Britain during the mid and late Holocene

Pollen data indicate Tilia was an important component of the primary woodland cover of many lowland areas in northern and central Europe. High values recorded in mid-Holocene pollen diagrams are generally followed by well-defined declines. In this study the palynological, spatial and temporal trends associated with declines in Tilia pollen at sites from lowland Britain are assessed to evaluate the nature and relative importance of the processes responsible. Processes primarily related to the depositional environment (paludification, marine inundation, breaks in sedimentation) and percentage representation of pollen data play the dominant role in 44% of the 164 declines included in the study. Anthropogenic activity can account for the remainder. Tilia rarely recovers from low values after anthropogenic declines, suggesting such declines have the potential to elucidate patterns in the destruction of primary woodland across lowland Britain. Other than declines associated with the mid-Holocene Ulmus decline event, few anthropogenic Tilia declines have been reliably dated prior to c. 5000 cal. BP. Tilia largely disappeared from areas with calcareous and loamy soils as a result of clearance activity in the period 5000–3000 cal. BP (Late Neolithic to Late Bronze Age). Between 3000 and 2500 cal. BP few declines are recorded suggesting a link between clearance activity and climate change. Later declines are concentrated on the sandier lithologies and often associated with heathland formation. The scarcity of declines after 2000 cal. BP is likely to be related to the exhaustion of primary woodland.     

Continuity between eastern and western Bushveld Complex, South Africa, confirmed by xenoliths from kimberlite

The three-dimensional shapes of mafic layered intrusions have to be inferred from surface outcrops, in some cases aided by drilling and/or geophysical data. However, geophysical models are often equivocal. For the 2.06?Ga Bushveld Complex of South Africa, early geological models proposed a shape of a single, gently inward dipping lopolith. Subsequent resistivity and gravity data were interpreted to suggest that the eastern and western limbs were discrete, dipping wedge-shaped intrusions separated by ~150?km. A more recent gravity model that takes crustal flexure into account allows continuity and the reversal to the original model. Distinguishing between these possibilities is difficult from surface-based studies because the central regions of the Complex are obscured by large volumes of younger granites and sedimentary/volcanic cover rocks. Here, we describe xenoliths from the Cretaceous Palmietgat kimberlite pipe, located mid-way between the exposed western and eastern lobes of the Complex. They are chromite-bearing feldspathic pyroxenites considered equivalent to those of the typical outcropping Critical Zone of the Bushveld Complex. This result provides strong support for a regionally interconnected Bushveld Complex, implying its emplacement as a single sill-like body. Confirming the continuity of the Bushveld Complex greatly expands exploration opportunities and implies that other layered mafic intrusions could have similar geometry.     

Liquid compositions from low-pressure experimental melting of pelitic rock from Morton Pass, Wyoming, USA

Low‐pressure crystal‐liquid equilibria in pelitic compositions are important in the formation of low‐pressure, high‐temperature migmatites and in the crystallization of peraluminous leucogranites and S‐type granites and their volcanic equivalents. This paper provides data from vapour‐present melting of cordierite‐bearing pelitic assemblages and augments published data from vapour‐present and vapour‐absent melting of peraluminous compositions, much of which is at higher pressures. Starting material for the experiments was a pelitic rock from Morton Pass, Wyoming, with the major assemblage quartz‐K feldspar‐biotite‐cordierite, approximately in the system KFMASH. A greater range in starting materials was obtained by addition of quartz and sillimanite to aliquots of this rock. Sixty‐one experiments were carried out in cold‐seal apparatus at pressures of 1–3.5 kbar (particularly 2 kbar) and temperatures from 700 to 840 °C, with and without the addition of water. In the vapour‐present liquidus relations at 2 kbar near the beginning of melting, the sequence of reactions with increasing temperature is: Qtz + Kfs + Crd + Sil + Spl + V = L; Qtz + Kfs + Crd + Spl + Ilm + V = Bt + L; and Qtz + Bt + V = Crd + Opx + Ilm + L. Vapour‐absent melting starts at about 800 °C with a reaction of the form Qtz + Bt = Kfs + Crd + Opx + Ilm + L. Between approximately 1–3 kbar the congruent melting reaction is biotite‐absent, and biotite is produced by incongruent melting, in contrast to higher‐pressure equilibria. Low pressure melts from pelitic compositions are dominated by Qtz‐Kfs‐Crd. Glasses at 820–840 °C have calculated modes of approximately Qtz₄₂Kfs₄₆Crd₁₂. Granites or granitic leucosomes with more than 10–15% cordierite should be suspected of containing residual cordierite. The low‐pressure glasses are quite similar to the higher‐pressure glasses from the literature. However, X_Mg increases from about 0.1–0.3 with increasing pressure from 1 to 10 kbar, and the low‐temperature low‐pressure glasses are the most Fe‐rich of all the experimental glasses from pelitic compositions.     

r-Process abundance universality and actinide cosmochronology

We review recent observational and theoretical results concerning the presence of actinide nuclei on the surfaces of old halo stars and their use as an age determinant. We present model calculations which show that the observed universality of abundances for 56<Z<75 elements in these stars does not necessarily imply a unique astrophysical site for the r-process. Neither does it imply a universality of abundances of nuclei outside of this range. In particular, we show that a variety of astrophysical r-process models can be constructed which reproduce the same observed universal r-process curve for 56<Z<75 nuclei, yet have vastly different abundances for Z≥75 and possibly Z<56 as well. This introduces an uncertainty into the use of the Th/Eu chronometer as a means to estimate the ages of the metal deficient stars. We do find, however, that the U/Th ratio is a robust chronometer. This is because the initial production ratio of U to Th is almost independent of the astrophysical nucleosynthesis environment. The largest remaining uncertainties in the U/Th initial production ratio are due to the input nuclear physics models.     

Did prolonged two-stage fragmentation of the supercontinent Kenorland lead to arrested orogenesis on the southern margin of the Superior province?

Recent geochronological investigations reinforce the early suggestion that the upper part of the Paleoproterozoic Huronian Supergroup of Ontario,Canada is present in the Animikie Basin on the south shore of Lake Superior.These rocks,beginning with the glaciogenic Gowganda Formation,are interpreted as passive margin deposits.The absence of the lower Huronian(rift succession) from the Animikie Basin may be explained by attributing the oldest Paleoroterozoic rocks in the Animikie Basin(Chocolay Group)to deposition on the upper plate of a north-dipping detachment fault,which lacks sediments of the rift phase.Following thermal uplift that led to opening of the Huronian Ocean on the south side of what is now the Superior province,renewed uplift(plume activity) caused large-scale gravitational folding of the Huronian Supergroup accompanied by intrusion of the Nipissing diabase suite and Senneterre dikes at about 2.2 Ga.Termination of passive margin sedimentation is normally followed by ocean closure but in the Huronian and Animikie basins there was a long hiatus- the Great Stratigraphic Gap- which lasted for about 350 Ma.This hiatus is attributed to a second prolonged thermal uplift of part of Kenorland that culminated in complete dismemberment of the supercontinent shortly before 2.0 Ga by opening of the Circum-Superior Ocean.These events caused regional uplift(the Great Stratigraphic Gap) and delayed completion of the Huronian Wilson Cycle until a regional compressional tectonic episode,including the Penokean orogeny,belatedly flooded the southern margin of the Superior province with foreland basin deposits,established the limits of the Superior structural province and played an important role in constructing Laurentia.     

10Be exposure age constraints on the Late Weichselian ice‐sheet geometry and dynamics in inter‐ice‐stream areas,western Svalbard

The Late Weichselian ice sheet of western Svalbard was characterized by ice streams and inter‐ice‐stream areas. To reconstruct its geometry and dynamics we investigated the glacial geology of two areas on the island of Prins Karls Forland and the Mitrahalvøya peninsula. Cosmogenic ¹⁰Be surface exposure dating of glacial erratics and bedrock was used to constrain past ice thickness, providing minimum estimates in both areas. Contrary to previous studies, we found that Prins Karls Forland experienced a westward ice flux from Spitsbergen. Ice thickness reached >470 m a.s.l., and warm‐based conditions occurred periodically. Local deglaciation took place between 16 and 13 ka. At Mitrahalvøya, glacier ice draining the Krossfjorden basin reached >300 m a.s.l., and local deglaciation occurred at c. 13 ka. We propose the following succession of events for the last deglaciation. After the maximum glacier extent, ice streams in the cross‐shelf troughs and fjords retreated, tributary ice streams formed in Forlandsundet and Krossfjorden, and, finally, local ice caps were isolated over both Prins Karls Forland and Mitrahalvøya and their adjacent shelves.