Prediction of a surface state and a related surface insulator-metal transition for the (100) surface of stochiometric EuO

We calculate the temperature and layer-dependent electronic structure of a 20-layer EuO(100)-film using a combination of first-principles and model calculation based on the ferromagnetic Kondo-lattice model. The results suggest the existence of a EuO(100) surface state which can lead to a surface insulator-metal transition.Comment: 9 pages, 5 figures, Phys. Rev. Lett. (in press

Preliminary geophysical interpretation of the McKeand River area, southern Baffin Island, Nunavut: insights from gravity, magnetic and geological data

The recently completed McKeand River and Amittok Lake aeromagnetic surveys on southern Baffin Island, Nunavut pro- vide a new high-resolution magnetic dataset over an area with no previous coverage. Complemented by regional gravity data, newly acquired rock-property information and geological-mapping products, the aeromagnetic dataset yields qualita- tive and quantitative information on the structure and geology of the underlying bedrock. This paper presents a preliminary interpretation of these datasets that delineates three gravimetric and five magnetic domains. The gravity data outline a broad negative anomaly associated with a plutonic-intrusive suite, as well as several isolated gravity highs associated with metasedimentary strata. Magnetic domains are defined on the basis of anomaly amplitude, wavelength and texture, and are correlated to the mapped geology and magnetic properties. Associations between potential-field anomalies, physical prop- erties and mineral occurrences help define the regional distribution of economically significant horizons

Record of modern-style plate tectonics in the Palaeoproterozoic Trans-Hudson orogen

The Trans-Hudson orogen of North America is a circa 1,800 million year old, middle Palaeoproterozoic continental collisional belt. The orogen may represent an ancient analogue to the Himalayan orogen, which began forming 50 million years ago and remains active today. Both mountain belts exhibit similar length scales of deformation and timescales of magmatism and metamorphism. A notable divergence in this correlation has been the absence of high-pressure, low-temperature metamorphic rocks in the Trans-Hudson compared with the Himalaya. It has been debated whether this absence reflects a secular tectonic change, with the requisite cool thermal gradients precluded by warmer ambient mantle temperatures during the Palaeoproterozoic, or a lack of preservation. Here we identify eclogite rocks within the Trans-Hudson orogen. These rocks, which typically form at high pressures and cool temperatures during subduction, fill the gap in the comparative geologic record between the Trans-Hudson and Himalayan orogens. Through the application of phase equilibria modelling and in situ U–Pb monazite dating we show that the pressure–temperature conditions and relative timing of eclogite-facies metamorphism are comparable in both orogenies. The results imply that modern-day plate tectonic processes featuring deep continental subduction occurred at least 1,830 million years ago. This study highlights that the global metamorphic rock record (particularly in older terrains) is skewed by overprinting and erosion

The structure of the Sumatran Fault revealed by local seismicity

[1] The combination of the Sunda megathrust and the (strike-slip) Sumatran Fault (SF) represents a type example of slip-partitioning. However, superimposed on the SF are geometrical irregularities that disrupt the local strain field. The largest such feature is in central Sumatra where the SF splits into two fault strands up to 35 km apart. A dense local network was installed along a 350 km section around this bifurcation, registering 1016 crustal events between April 2008 and February 2009. 528 of these events, with magnitudes between 1.1 and 6.0, were located using the double-difference relative location method. These relative hypocentre locations reveal several new features about the crustal structure of the SF. Northwest and southeast of the bifurcation, where the SF has only one fault strand, seismicity is strongly focused below the surface trace, indicating a vertical fault that is seismogenic to ∼15 km depth. By contrast intense seismicity is observed within the bifurcation, displaying streaks in plan and cross-section that indicate a complex system of faults bisecting the bifurcation. In combination with analysis of topography and focal mechanisms, we propose that the bifurcation is a strike-slip duplex system with complex faulting between the two main fault branches

Completing the bedrock mapping of southern Baffin Island, Nunavut; plutonic suites and regional stratigraphy

This paper summarizes the field observations and initial interpretations following eight weeks of regional and targeted bed- rock mapping on south-central Baffin Island, Nunavut. The 2015 field campaign completes a two-decade mission to update the geoscience knowledge for the whole of Baffin Island south of latitude 70°N. The bedrock in the area is dominated by a Paleoproterozoic metaplutonic suite, ranging in composition from gabbro to syenogranite, with crosscutting relations indi- cating a progression from mafic to silicic magmatism. Phase-equilibria modelling reveals that the prevailing upper-amphi- bolite– to lower-granulite–facies metamorphic conditions overlap the stability limits of magnetite and orthopyroxene for a typical granitoid bulk composition, which is consistent with field observations of the discontinuous presence of both phases throughout the map area. This result is also consistent with regional aeromagnetic data that show complex structures within relatively homogeneous map units, which are primarily attributed to variations in the abundance of magnetite. The granitoid rocks are interpreted as part of the middle Paleoproterozoic Cumberland Batholith. Metasedimentary rocks, including quartzite, pelite, marble and metagreywacke, are present as enclaves and screens within and between plutonic bodies. An examination of the ‘ghost’stratigraphy suggests that the metasedimentary rocks through- out most of the map area can be correlated with the middle Paleoproterozoic Lake Harbour Group, except in the northeast, where the unique presence of greywacke suggests a middle Paleoproterozoic Piling Group affinity. This transition in strata is consistent with the proposal that a middle Paleoproterozoic tectonic suture (the Baffin suture) associated with the Trans- Hudson Orogen runs through Cumberland Sound. Completion of the bedrock mapping in southern Baffin Island indicates that the region offers a world-class exposure of a reworked Paleoproterozoic convergent margin, which affords valuable in- sight into a variety of magmatic and tectonic processes that can be applied to younger collisional belt

The relationship between mantle potential temperature and oceanic lithosphere buoyancy

The Earth's mantle potential temperature () is thought to have cooled by ∼250 ∘C since the Archean, causing a progressive change in both the structure and composition of oceanic lithosphere. These variables affect the negative buoyancy of subducting slabs, which is known to be an important force in driving plate motions. However, the relationship between and slab buoyancy remains unclear. Here, we model the formation and subduction of oceanic lithosphere as a function of , to investigate how influences the buoyancy of subducting slabs, and by extension how buoyancy forces may have changed through time. First, we simulate isentropic melting of peridotite at mid-ocean ridges over a range of (1300–1550 ∘C) to calculate oceanic lithosphere structure and composition. Second, we model the thermal evolution of oceanic plates undergoing subduction for a variety of scenarios (by varying lithospheric thickness, slab length and subduction velocity). Finally, we integrate the structural, compositional and thermal constraints to forward model subduction metamorphism of oceanic plates to determine down-going slab density structures. When compared with ambient mantle, these models allow us to calculate buoyancy forces acting on subducting slabs. Our results indicate that oceanic lithosphere derived from hotter mantle has a greater negative buoyancy, and therefore subduction potential, than lithosphere derived from cooler mantle for a wide range of subduction scenarios. With respect to the early Earth, this conclusion supports the viability of subduction, and models of subduction zone initiation that invoke the concept of oceanic lithosphere being primed to subduct. However, we also show that decreases to lithosphere thickness and slab length, and reduced crustal hydration, progressively reduce slab negative buoyancy. These results highlight the need for robust estimates of early Earth lithospheric properties when considering whether subduction was operative at this time. Nevertheless, our findings suggest that subduction processes on the early Earth may have been uniformitarian

Ptarmigan Fiord basement-cover thrust imbricates, Baffin Island, Nunavut

The rocks at Ptarmigan Fiord on the Hall Peninsula of Baffin Island underwent midcrustal deformation during the formation of the Paleoproterozoic Trans-Hudson Orogen. The structural style in the region is dominated by imbricate panels of Archean basement orthogneiss and Paleoproterozoic supracrustal strata, interpreted to have been deformed by thick- skinned ductile thrusting. Basement rocks comprise amphibolite-facies metatonalite, metagranodiorite, metaquartz-diorite and metamonzogranite, and cover rocks comprise amphibolite-facies migmatitic pelitic and semipelitic schist, psammitic schist, amphibolite, calcsilicate and quartzite. The S 1a penetrative foliation is variably present in basement rocks and consis- tently present in cover rocks, and is defined by alignment of biotite, sillimanite and leucogranite that formed before and dur- ing the thermal metamorphic peak. The S 1a foliation was deformed by F 1b isoclinal folds with an amplitude of 100 m. These structures are interpreted as forming during a D 1 east-west crustal shortening event. Basement and cover imbrication oc- curred after the thermal metamorphic peak and is interpreted as D 2 thick-skinned ductile thrusting. Ductile thrust faults at the base of seven basement-cover slices are identified on the basis of repetition of units and strain localization, and are inter- preted as predominantly south-to-southeast verging on the basis of shear-sense indicators. There are two structural panels of D 2 thrust imbricates, one in the northwestern part of the map area and one in the eastern part of the map area. Map-scale crosscutting relationships indicate that the northwestern panel overthrusted the eastern panel on a southeasterly T 2c -di- rected thrust fault, following a F 2b folding event that folded the T 2a basement-cover thrust imbricates in the eastern panel. The Ptarmigan Fiord area contains a world-class exposure of thick-skinned structures as they are spectacularly delineated by belts of distinctive grey-weathering Archean basement rocks and brown- to black-weathering Paleoproterozoic supra- crustal rocks

Evolution of density perturbations in double exponential quintessence models

In this work we investigate the evolution of matter density perturbations for quintessence models with a self-interaction potential that is a combination of exponentials. One of the models is based on the Einstein theory of gravity, while the other is based on the Brans-Dicke scalar tensor theory. We constrain the parameter space of the models using the determinations for the growth rate of perturbations derived from data of the 2-degree Field Galaxy Redshift Survey.Comment: 5 pages, 3 eps figure

Kondo-lattice model: Application to the temperature-dependent electronic structure of EuO(100) films

We present calculations for the temperature-dependent electronic structure and magnetic properties of thin ferromagnetic EuO films. The treatment is based on a combination of a multiband-Kondo lattice model with first-principles TB-LMTO band structure calculations. The method avoids the problem of double-counting of relevant interactions and takes into account the correct symmetry of the atomic orbitals. We discuss the temperature-dependent electronic structures of EuO(100) films in terms of quasiparticle densities of states and quasiparticle band structures. The Curie temperature T_C of the EuO films turns out to be strongly thickness-dependent, starting from a very low value = 15K for the monolayer and reaching the bulk value at about 25 layers