Atmospheric loading corrections at the observation level in VLBI analysis

This paper investigates whether in very long baseline interferometry (VLBI) analysis atmospheric loading corrections should be applied a priori at the observation level or whether it is sufficient to correct for atmospheric loading effects a posteriori by adding constant values per session to the estimated station coordinates. Simulated observations at single stations corresponding to the precise point positioning approach of global navigation satellite systems show that the atmospheric loading effect can be fully recovered by a posteriori corrections, i.e., the height differences between both approaches stay well below 1 mm. However, real global VLBI network solutions with sessions from 1984 to 2008 reveal that the effect of neglected atmospheric loading corrections at the stations is distributed to the other stations in the network, thus resulting in station height differences between solutions with observation level and with a posteriori corrections which can be as large as 10 mm and a ‘damping’ effect of the corrections. As soon as the terrestrial reference frame and the corresponding coordinate time series are determined, it would be conceptually wrong to apply atmospheric loading corrections at the VLBI stations. We recommend the rigorous application of atmospheric loading corrections at the observation level to all stations of a VLBI network because the seven parameters for translation, rotation, and in particular the network-scale of VLBI networks are significantly affected.     

Petrographic and microthermometrical studies of emeralds in the ‘Garimpo’ of Capoeirana,Nova Era,Minas Gerais State,Brazil

Petrographic investigations in the area of the Capoeirana emerald deposit, Minas Gerais State, revealed two main lithostructural units. The first unit is comprised of gneissic rocks of granitic composition belonging to the basement complex, and the second is composed of a highly weathered metasedimentary-metavolcanic sequence represented by metapelitic schists, amphibolites, schists derived from ultramafic rocks, and quartzites. Quartz and pegmatoid veins appear near the contacts between the gneissic rocks and the mineralization metasedimentary-metavolcanic sequence. The emerald mineralization is dominantly concentrated within the intercalations of meta-ultramafic schists near the contact of the pegmatoid veins. Microthermometric studies of the fluid inclusions of the emerald grains indicate that crystallization occurred in the pressure and temperature ranges of 2000 to 2750 bar and 450 to 650 °C, respectively. These data suggest that the mineralizing solutions have had a late hydrothermal-pneumatolytic origin characterized by low pressures, suggestive of the paragenesis talc + tremolite + carbonate + biotite-phlogopite + chlorite of the emerald wall rocks.     

Avalonia,get bent!–Paleomagnetism from SW Iberia confirms the Greater Cantabrian Orocline

The amalgamation of Pangea formed the contorted Variscan-Alleghanian orogen,suturing Gondwana and Laurussia during the Carboniferous.From all swirls of this orogen,a double curve in Iberia stands out,the coupled Cantabrian Orocline and Central Iberian curve.The Cantabrian Orocline formed at ca.315–290 Ma subsequent to the Variscan orogeny.The formation mechanism of the Cantabrian Orocline is disputed,the most commonly proposed mechanisms include either(1)that south-westernmost Iberia would be an Avalonian(Laurussian)indenter or(2)that the stress field changed,buckling the orogen.In contrast,the geometry and kinematics of the Central Iberian curve are largely unknown.Whereas some authors defend both curvatures are genetically linked,others support they are distinct and formed at different times.Such uncertainty adds an extra layer of complexity to our understanding of the final stages of Pangea’s amalgamation.To solve these issues,we study the late Carboniferous–early Permian vertical-axis rotations of SW Iberia with paleomagnetism.Our results show up to 70
counterclockwise vertical-axis rotations during late Carboniferous times,concurring with the anticipated kinematics if SW Iberia was part of the southern limb of the Cantabrian Orocline.Our results do not allow the necessary penecontemporaneous clockwise rotations in Central Iberia to support a concomitant formation of both Cantabrian and Central Iberian curvature.The coherent rotation of both Gondwanan and Avalonian pieces of SW Iberia discards the Laurussian indenter hypothesis as a formation mechanism of the Cantabrian Orocline and confirms the Greater Cantabrian Orocline hypothesis.The Greater Cantabrian Orocline likely formed as a consequence of a change in the stress field during the late Carboniferous and extended beyond the Rheic Ocean suture affecting the margins of both Laurussia and Gondwana.     

A geochemical and geophysical approach to derive a conceptual circulation model of CO2-rich mineral waters: A case study of Vilarelho da Raia, northern Portugal

The Vilarelho da Raia-Chaves region, located in northern Portugal adjacent to the Spanish border, is characterized by both hot and cold CO₂-rich mineral waters issuing from springs and drilled wells. The present paper updates the conceptual circulation model of the Vilarelho da Raia cold CO₂-rich mineral waters. Vilarelho da Raia mineral waters, dominated by Na and HCO₃ ions, have formed mainly by interaction with CO₂ of deep-seated mantle origin. The δ ¹⁸O, δ ²H and ³H values indicate that these waters are the result of meteoric waters infiltrating into Larouco Mountain, NW of Vilarelho da Raia, circulating at shallow depths in granitic rocks and moving into Vilarelho da Raia area. The conceptual geochemical and geophysical circulation model indicates that the hot and cold CO₂-rich mineral waters of Chaves (76 °C) and Vilarelho da Raia (17 °C) should be considered manifestations of similar but not the same geohydrological systems. Electronic Publication     

The impact of El Ni?o on South American summer climate during different phases of the Pacific Decadal Oscillation

In austral summer, the observed El Ni?o (EN) events during warm Pacific Decadal Oscillation (PDO) phases (PDO(+)) exhibited large anomalous upper level wave patterns in response to larger Sea Surface Temperature (SST) anomalies in the Equatorial Pacific and Atlantic Oceans compared with SST anomalies in EN events during cold PDO phases (PDO(?)). The precipitation anomalies in PDO(+) EN are increased over Southeastern South America (SESA) associated with the intensification of the moisture flux convergence in this region. The PDO(?) EN events exhibit positive precipitation anomalies only over southern SESA, while negative anomalies were observed in the north. Downward motion and anomalous divergence over central eastern Brazil may have contributed to the weakening of the northwesterly moisture flux convergence associated with the South American Low Level Jet (SALLJ) over the subtropics. The extratropical cyclones showed higher frequency and lower central pressures in southern Brazil, Uruguay, northeastern Argentina, and Southwest Atlantic Ocean during the PDO(+) EN events compared with the PDO(?) EN events. Such increase in the frequency and intensity of cyclogenesis cases seems to be in accordance with the anomalous moisture flux convergence over the SESA and associated reduction in the Sea Level Pressure observed during PDO(+) EN events. In order to investigate the impact of a canonical El Ni?o event over South America under different PDO phases, two numerical experiments were done with an Atmospheric General Circulation Model. Global SST and ice sea fields average over years characterized by (a) PDO(+) and (b) PDO(?) were considered as climatologically fields, and a composite of anomalies of SST of all El Ni?o events observed in 1950?C1999 was added in the region 20oS?C20oN;120oW?C175oW of both ??climatologies.?? The differences in experiments suggest that a canonical EN may produce significant different anomalous atmospheric patterns associated with distinct PDO climatologies. The more significant differences are simulated over extreme northern and eastern Brazil. Additional numerical experiments isolating the observed variability of SST over several oceanic basins during different PDO phases will be conducted to study their particular role on the South American climate.     

Achievements of the Earth orientation parameters prediction comparison campaign

Precise transformations between the international celestial and terrestrial reference frames are needed for many advanced geodetic and astronomical tasks including positioning and navigation on Earth and in space. To perform this transformation at the time of observation, that is for real-time applications, accurate predictions of the Earth orientation parameters (EOP) are needed. The Earth orientation parameters prediction comparison campaign (EOP PCC) that started in October 2005 was organized for the purpose of assessing the accuracy of EOP predictions. This paper summarizes the results of the EOP PCC after nearly two and a half years of operational activity. The ultra short-term (predictions to 10 days into the future), short-term (30 days), and medium-term (500 days) EOP predictions submitted by the participants were evaluated by the same statistical technique based on the mean absolute prediction error using the IERS EOP 05 C04 series as a reference. A combined series of EOP predictions computed as a weighted mean of all submissions available at a given prediction epoch was also evaluated. The combined series is shown to perform very well, as do some of the individual series, especially those using atmospheric angular momentum forecasts. A main conclusion of the EOP PCC is that no single prediction technique performs the best for all EOP components and all prediction intervals.     

U–Pb zircon and monazite geochronology of post-collisional Hercynian granitoids from the Central Iberian Zone (Northern Portugal)

In the Central Iberian Zone (CIZ) of the Iberian Massif large volumes of granitoids were emplaced during the post-collisional stage of the Hercynian orogeny (syn- to post-D3, the last ductile deformation phase). Twelve granitic units and a quartz monzodiorite were selected for a U–Pb zircon and monazite geochronological study. They represent successive stages of the D3 event. The Ucanha-Vilar, Lamego, Sameiro and Refoios do Lima plutons are coeval (313±2 Ma, 319±4 Ma, 316±2 Ma and 314±2 Ma, respectively) and belong to the earliest stage. Later on the Braga massif was emplaced, its different units yielding the same age: 309±3 Ma for the Braga granite, 309±1 Ma for the Gonça granite and 311±5 Ma for a related quartz monzodiorite. The Braga massif is subcontemporaneous with the Agrela and Celeirós plutons (307±3.5 Ma and 306±2 Ma, respectively), in agreement with field data. The Briteiros granite is younger (300±1 Ma), followed by the emplacement of the Peneda–Gerês massif (Gerês, Paufito, Illa and Carris granites). The Gerês granite, emplaced at 296±2 Ma, seems to represent a first magmatic pulse immediately followed by the intrusion of the Paufito granite at 290±2.5 Ma. For the Carris granite a minimum emplacement age of 280±5 Ma was obtained. Based on these results the following chronology is proposed: (1) syn-D3 biotite granitoids, 313–319 Ma; (2) late-D3 biotite-dominant granitoids, 306–311 Ma; (3) late- to post-D3 granitoids, ca. 300 Ma; (4) post-D3 granitoids, 290–296 Ma. These chronological data indicate that successive granitic intrusions were emplaced in the CIZ during a short time span of about 30 Ma that corresponds to the latest stages of the Hercynian orogeny. A rapid and drastic change occurred at about 300 Ma, between a compressive ductile tectonic regime (D3, ca. 300–320 Ma) associated to calc-alkaline, monzonitic and aluminopotassic plutonism and a fragile phase of deformation (D4) which controlled the emplacement of the subalkaline ferro-potassic plutonism at 290–296 Ma.     

High to low velocity succession in the upper crust related to tectonic emplacement: Tras os Montes-Galicia (Iberia), Brittany and Limousin (France)

Summary. Seismic probing of the upper crust in a catazonal massif in Tras os Montes, Hercynian Iberia, demonstrates the absence of a root towards the lower crust and favours a mechanism of thrust and nappe emplacement. Similar high to low velocity successions in the upper crust are evidenced in particular segments of the Hercynian domain of France. All these occurrences are related to a type region to which a particular meaning has been given in a recent plate tectonics interpretation of the Hercynian orogenesis. This model, implying large-scale nappe displacement and intracrustal thrusts furnishes in turn a general mechanism for the tectonic formation of upper crustal high and low velocity layers of limited extent in orogenic cycles.     

Improved Constraints on Models of Glacial Isostatic Adjustment: A Review of the Contribution of Ground-Based Geodetic Observations

The provision of accurate models of Glacial Isostatic Adjustment (GIA) is presently a priority need in climate studies, largely due to the potential of the Gravity Recovery and Climate Experiment (GRACE) data to be used to determine accurate and continent-wide assessments of ice mass change and hydrology. However, modelled GIA is uncertain due to insufficient constraints on our knowledge of past glacial changes and to large simplifications in the underlying Earth models. Consequently, we show differences between models that exceed several mm/year in terms of surface displacement for the two major ice sheets: Greenland and Antarctica. Geodetic measurements of surface displacement offer the potential for new constraints to be made on GIA models, especially when they are used to improve structural features of the Earth’s interior as to allow for a more realistic reconstruction of the glaciation history. We present the distribution of presently available campaign and continuous geodetic measurements in Greenland and Antarctica and summarise surface velocities published to date, showing substantial disagreement between techniques and GIA models alike. We review the current state-of-the-art in ground-based geodesy (GPS, VLBI, DORIS, SLR) in determining accurate and precise surface velocities. In particular, we focus on known areas of need in GPS observation level models and the terrestrial reference frame in order to advance geodetic observation precision/accuracy toward 0.1 mm/year and therefore further constrain models of GIA and subsequent present-day ice mass change estimates.