Structural history of continental volcanic arc rocks, eastern Sierra Nevada, California: A case for extensional tectonics

Mesozoic metavolcanic rocks forming part of the continental volcanic arc along the eastern Sierra Nevada near Mt. Goddard and in the Ritter Range show a complex history related to extensional tectonics. The rocks comprise a thick section of tuffs, breccias, lava flows, sills, and ash-flow tuffs deposited in a subaerial to subaqueous environment, with some subvolcanic sill-like plutons. Pb/U ages of the rocks in the Mt, Goddard area range from ca. 130–160 Ma, while rocks in the Ritter Range have a somewhat wider age range as reported previously. Repetition of the section occurs by faulting, and with the exception of parts of the mid-Cretaceous Minarets Caldera, all the volcanic rocks show a regional slaty cleavage which was subsequently crenulated and/or folded locally. The first cleavage has well-developed stretching lineations, and does not appear to have been associated with significant folding. Finite strain measurements show considerable variation both in magnitude and symmetry. The Mt. Goddard rocks, however, tend to show slightly higher overall strain magnitude and greater constrictional component than the Ritter Range for rocks of comparable age. Calculations based on the strain data suggest the Mt. Goddard section has been thinned by about 50% normal to bedding, much as that documented previously for rocks in the Ritter Range. Deformation within this part of the continental arc was originally thought to have formed by regional compression during the late Jurassic (Nevadan) orogeny. However, our study indicates that (1) parts of the deformed volcanic section are younger than late Jurassic, (2) Nevadan-age breaks in deposition are not present, (3) large-scale folds expected during a regional compression event are not common, and (4) the beds were tilted to a high dip prior to internal deformation. An extensional model is proposed in which beds were rotated to high tilts early in the deformation as a result of listric normal faulting. This normal faulting is thought to have occurred above a regional tumescence related to voluminous magmatism at depth, with preservation of the steeply tilted Goddard and Ritter sections being facilitated by their downward transport along the margins of rising plutons. Flattening and steeply plunging constrictional fabrics superimposed on the tilted sections are related to strain induced by high-level inflation of magma chambers and downward return flow of the keellike pendants. The main tectonic fabric shown by the continental volcanic arc rocks in the eastern Sierra Nevada is largely of Cretaceous age, rather than Jurassic (Nevadan) as originally supposed. In addition, the deformation, both rotation of beds and subsequent tectonite fabric, appears to be genetically related to the dynamic evolution of the magmatic arc, and not the result of an externally imposed tectonic event

Marine Volcaniclastic Record of Early Arc Evolution in the Eastern Ritter Range Pendant, Central Sierra Nevada, California

Marine volcaniclastic rocks in the Sierra Nevada preserve a critical record of silicic magmatism in the early Sierra Nevada volcanic arc, and this magmatic record provides precise minimum age constraints on subduction inception and tectonic evolution of the early Mesozoic Cordilleran convergent margin at this latitude. New zircon Pb/U ages from the Ritter Range pendant and regional correlations indicate arc inception no later than mid‐Triassic time between 37 and 38°N. The regional first‐order felsic magma eruption rate as recorded by marine volcanic arc rocks was episodic, with distinct pulses of ignimbrite emplacement at ca. 221 to 216 Ma and 174 to 167 Ma. Ignimbrites range from dacite to rhyolite in bulk composition, and are petrographically similar to modern arc‐type, monotonous intermediate dacite or phenocryst‐poor, low‐silica rhyolite. Zircon trace element geochemistry indicates that Jurassic silicic melts were consistently Ti‐ and light rare earth‐enriched and U‐depleted in comparison to Triassic melts of the juvenile arc, suggesting Jurassic silicic melts were hotter, drier, and derived from distinct lithospheric sources not tapped in the juvenile stage of arc construction. Pulses of ignimbrite deposition were coeval with granodioritic to granitic components of the underlying early Mesozoic Sierra Nevada batholith, suggesting explosive silicic volcanism and batholith construction were closely coupled at one‐ to two‐million‐year time scales

Versatile Coordination of Cyclopentadienyl-Arene Ligands and Its Role in Titanium-Catalyzed Ethylene Trimerization

Cationic titanium(IV) complexes with ansa-(η5-cyclopentadienyl,η6-arene) ligands were synthesized and characterized by X-ray crystallography. The strength of the metal-arene interaction in these systems was studied by variable-temperature NMR spectroscopy. Complexes with a C1 bridge between the cyclopentadienyl and arene moieties feature hemilabile coordination behavior of the ligand and consequently are active ethylene trimerization catalysts. Reaction of the titanium(IV) dimethyl cations with CO results in conversion to the analogous cationic titanium(II) dicarbonyl species. Metal-to-ligand backdonation in these formally low-valent complexes gives rise to a strongly bonded, partially reduced arene moiety. In contrast to the η6-arene coordination mode observed for titanium, the more electron-rich vanadium(V) cations [cyclopentadienyl-arene]V(NiPr2)(NC6H4-4-Me)+ feature η1-arene binding, as determined by a crystallographic study. The three different metal-arene coordination modes that we experimentally observed model intermediates in the cycle for titanium-catalyzed ethylene trimerization. The nature of the metal-arene interaction in these systems was studied by DFT calculations.

The effects of stand characteristics on the understory vegetation in Quercus petraea and Q. cerris dominated forests

The shelterwood system used in Hungary has many effects on the composition and structure of the herb layer. The aim of our study was to identify the main variables that affect the occurence of herbs and seedlings in Turkey oak-sessile oak (Quercus cerris and Q. petraea) stands. The study was carried out in the Bükk mountains, Hungary. 122 sampling plots were established in 50-150 year old oak forests, where we studied the species composition and structure of the understorey and overstorey. The occurence of herbs was affected by canopy closure, the heterogenity and patchiness of the stand, the slope and the east-west component of the aspect. The composition of saplings was significantly explained by the ratio of the two major oak species in the stand and the proximity of the adult plants. An important result for forest management was that sessile oaks were able to regenerate almost only where they were dominant in the overstorey

Relationship of insect biomass and richness with land use along a climate gradient

Recently reported insect declines have raised both political and social concern. Although the declines have been attributed to land use and climate change, supporting evidence suffers from low taxonomic resolution, short time series, a focus on local scales, and the collinearity of the identified drivers. In this study, we conducted a systematic assessment of insect populations in southern Germany, which showed that differences in insect biomass and richness are highly context dependent. We found the largest difference in biomass between semi-natural and urban environments (−42%), whereas differences in total richness (−29%) and the richness of threatened species (−56%) were largest from semi-natural to agricultural environments. These results point to urbanization and agriculture as major drivers of decline. We also found that richness and biomass increase monotonously with increasing temperature, independent of habitat. The contrasting patterns of insect biomass and richness question the use of these indicators as mutual surrogates. Our study provides support for the implementation of more comprehensive measures aimed at habitat restoration in order to halt insect declines

Breaking the Lightweight Secure PUF: Understanding the Relation of Input Transformations and Machine Learning Resistance

Physical Unclonable Functions (PUFs) and, in particular, XOR Arbiter PUFs have gained much research interest as an authentication mechanism for embedded systems. One of the biggest problems of (strong) PUFs is their vulnerability to so called machine learning attacks. In this paper we take a closer look at one aspect of machine learning attacks that has not yet gained the needed attention: the generation of the sub-challenges in XOR Arbiter PUFs fed to the individual Arbiter PUFs. Specifically, we look at one of the most popular ways to generate sub-challenges based on a combination of permutations and XORs as it has been described for the Lightweight Secure PUF . Previous research suggested that using such a sub-challenge generation increases the machine learning resistance significantly. Our contribution in the field of sub-challenge generation is three-fold: First, drastically improving attack results by Rührmair et al., we describe a novel attack that can break the Lightweight Secure PUF in time roughly equivalent to an XOR Arbiter PUF without transformation of the challenge input. Second, we give a mathematical model that gives insight into the weakness of the Lightweight Secure PUF and provides a way to study generation of sub-challenges in general. Third, we propose a new, efficient, and cost-effective way for sub-challenge generation that mitigates the attack strategy we used and outperforms the Lightweight Secure PUF in both machine learning resistance and resource overhead

Soil seed bank of the invasive Robinia pseudoacacia in planted Pinus nigra stands

Pinus nigra and Robinia pseudoacacia are exotic trees used for afforestation in Hungary. Pinus nigra was non-invasive, however R. pseudoacacia escaped from cultivation and invaded several vegetation types including pine plantations. It has recently been planned to cut P. nigra plantations and replace them by native tree stands, especially in nature reserves. The scattered presence of R. pseudoacacia specimens in pine stands might place constraints on planned tree replacement because of their vegetative resprouting and recolonization from an established seed bank. The aim of this study was to investigate the soil seed bank under the canopy of solitary R. pseudoacacia specimens found in P. nigra plantations. Altogether 250 soil samples were collected from the 0–6 and 6–12 cm soil layers under solitary Robinia trees of varying ages (with basal areas between 62.4 and 1089.3 cm2). Seeds were separated by sieving then scarified and germinated. Seed bank density ranged between 640 and 2285 seedsm–2 with an average distribution of 82.7% and 17.3% in the upper and lower soil layer, respectively. Total density of the seed bank and also the seed bank ratio of the lower soil layer increased with tree age. The accumulated seed bank of R. pseudoacacia should be considered in the careful planning of tree replacement operations in Pinus nigra stands

Neoproterozoic to early Paleozoic extensional and compressional history of East Laurentian margin sequences: The Moine Supergroup, Scottish Caledonides

Neoproterozoic siliciclastic-dominated sequences are widespread along the eastern margin of Laurentia and are related to rifting associated with the breakout of Laurentia from the supercontinent Rodinia. Detrital zircons from the Moine Supergroup, NW Scotland, yield Archean to early Neoproterozoic U-Pb ages, consistent with derivation from the Grenville-Sveconorwegian orogen and environs and accumulation post–1000 Ma. U-Pb zircon ages for felsic and associated mafic intrusions confirm a widespread pulse of extension-related magmatism at around 870 Ma. Pegmatites yielding U-Pb zircon ages between 830 Ma and 745 Ma constrain a series of deformation and metamorphic pulses related to Knoydartian orogenesis of the host Moinerocks. Additional U-Pb zircon and monazite data, and 40Ar/39Ar ages for pegmatites and host gneisses indicate high-grade metamorphic events at ca. 458–446 Ma and ca. 426 Maduring the Caledonian orogenic cycle.The presence of early Neoproterozoic silici clastic sedimentation and deformation in the Moine and equivalent successions around the North Atlantic and their absence along strike in eastern North America reflect contrasting Laurentian paleogeography during the breakup of Rodinia. The North Atlantic realm occupied an external location on the margin of Laurentia, and this region acted as a locus for accumulation of detritus (Moine Supergroup and equivalents) derived from the Grenville-Sveconorwegian orogenic welt, which developed as a consequence of collisional assembly of Rodinia. Neoproterozoic orogenic activity corresponds with theinferred development of convergent platemargin activity along the periphery of the supercontinent. In contrast in eastern North America, which lay within the internal parts of Rodinia, sedimentation did not commence until the mid-Neoproterozoic (ca. 760 Ma) during initial stages of supercontinent fragmentation. In the North Atlantic region, this time frame corresponds to a second pulse of extension represented by units such as the Dalradian Supergroup, which unconformably overlies the predeformed Moine succession