Unambiguous SAR signal reconstruction from nonuniform displaced phase center sampling

The displaced phase center (DPC) technique will enable a wide-swath synthetic aperture radar (SAR) with high azimuth resolution. In a classic DPC system, the pulse repetition frequency (PRF) has to be chosen such that the SAR carrier moves just one half of its antenna length between subsequent radar pulses. Any deviation from this PRF will result in a nonuniform sampling of the synthetic aperture. This letter derives an innovative reconstruction algorithm and shows that an unambiguous reconstruction of a SAR signal is possible for nonuniform sampling of the synthetic aperture. This algorithm will also have great potential for multistatic satellite constellations as well as the dual receive antenna mode in Radarsat 2 and TerraSAR-X.     

The structure and the phyllosilicates (chemistry, crystallinity and texture) of Talas Ala-Tau (Tien Shan, Kyrgyz Republic): comparison with more recent subduction complexes

Geological mapping and structural analysis of the Talas Ala Tau (Tien Shan, Kyrgyz Republic) have revealed a complex structure composed of folds with axial-plane cleavage and thrust faults verging towards the NE. The main structures of the range correspond to minor Tertiary and Carboniferous–Permian deformation superimposed on the main deformation event that took place during the Baikalian orogeny. The pervasive axial-plane cleavage diminishes in penetrativity from the hinterland to the foreland in both the Uzunakhmat and Karagoin sheets. The main thrusts developed phyllonitic shear-related rocks on the hangingwall immediately above the thrust planes. A crystal-chemical study of the phyllosilicates growth during the Baikalian deformation event along a cross-section revealed changes in the crystallinity, composition and lattice parameters of them. The phyllosilicates present in the Talas Ala Tau rocks were crystallized in very low-grade metamorphic conditions, that is below 300 °C, as indicated by their Kübler Index (KI), which decreases from SW towards the NE. Detailed TEM study of the phyllosilicates reveals a clear textural difference at the lattice level between samples with higher or lower KI parameters. There is also a clear difference in crystal-chemical parameters (KI and b) and composition between the phyllosilicates growth in relation to the axial-plane cleavage and the ones belonging to the thrust-related phyllonites. The first ones are more affected by the ferrimuscovitic vector than the phyllosilicates of phyllonites, closer to the theoretical phengitic component. Huge ranges of values of phengitic content of micas at sample level are interpreted as the result of a decompression path from at least 8 kbar. We propose a subduction geodynamic environment for the regional deformation and the origin of the phyllosilicates, as they are similar to those obtained in more recent accretionary complexes.     

The circumstellar medium of the peculiar supernova SN1997ab

We report the detection of the slow-moving wind into which the compact supernova remnant SN 1997ab is expanding. Echelle spectroscopy provides clear evidence for a well-resolved narrow (full width at zero intensity, FWZI ∼180 km s⁻¹) P Cygni profile, both in Hα and Hβ, superimposed on the broad emission lines of this compact supernova remnant. From theoretical arguments we know that the broad and strong emission lines imply a circumstellar density ( n  ≥ 10⁷ cm⁻³). This, together with our detection, implies a massive and slow stellar wind experienced by the progenitor star shortly prior to the explosion.     

Seismotectonics of Portugal and its adjacent area in the Atlantic

New elements on the seismicity of Portugal and new focal-mechanism solutions of earthquakes with epicentres situated off the coast of the Portuguese mainland and in the Azores region are presented. Historical seismicity data show that in the territory of the Portuguese mainland there are active faults that are responsible for earthquakes that have caused important damage and many casualties. However, most of the intraplate earthquakes with epicentres situated in the Portuguese mainland or near the shore are normally of small magnitude and this renders difficult their interpretation in the light of focal mechanisms. A solution for one earthquake, with magnitude 5 and epicentre at the Nazaré submarine canyon, is presented.Southwestwards of Cape St. Vincent there is an important seismic zone responsible for high-magnitude earthquakes such as that of 1 November 1755. This zone is situated in the region where the extension of the Messejana fault into the ocean joins with the Azores-Gibraltar fault.The seismicity of the area situated between the western coast of the Portuguese mainland and the Azores increases approximately along the 15°W meridian, from the latitude of the Azores-Gibraltar fault up to 44°N. Focal mechanisms of earthquakes with epicentres situated along this line show very similar solutions.The interpretation of the focal mechanism solutions of the earthquakes with epicentres situated in the studied area shows that the stress field trends approximately NW-SE. It is assumed that this stress field results from the interaction of the Eurasian and African plates; however, this direction is not maintained in the Azores region.     

Ion exchange between tectosilicates with the nepheline-kalsilite framework and molten MNO3 or MO (M = Li,Na, K,Ag)

Natural nepheline, a synthetic Na-rich nepheline, and synthetic kalsilite were ion exchanged in molten MNO₃ or MCl (M = Li, Na, K, Ag) at 220–800° C. Crystalline products were characterized by wet chemical and electron microprobe analysis, single crystal and powder X-ray diffraction, and transmission electron microscopy and diffraction. Two new compounds were obtained: Li-exchanged nepheline with a formula near (Li,K_0.3,□)Li₃[Al₃(Al,Si)Si₄O₁₆] and a monoclinic unit cell with a = 951.0(6) b = 976.1(6) c = 822.9(5)pm γ = 119.15°, and Ag-exchanged nepheline with a formula near (K,Na,□)Ag₃[Al₃(Al,Si)Si₄O₁₆] and a hexagonal unit cell with a = 1007.4(8) c = 838.2(1.0) pm. Both compounds apparently retain the framework topology of the starting material. Ion exchange isotherms and structural data show that immiscibility between the end members is a general feature in the systems Na-Li, Na-Ag, and Na-K. For the system Na-K, a stepwise exchange is observed with (K,D)Na₃[Al₃(Al,Si)Si₄O₁₆] as an intermediate composition which has the nepheline structure and is miscible with the sodian end member (Na,□)Na₃[Al₃(Al,Si)Si₄O₁₆], but not with the potassian end member (K,□)₄[Al₃(Al,Si)Si₄O₁₆] which shows the kalsilite structure; there was no indication for the formation of trior tetrakalsilite (K/(K + Na)≈0.7) at the temperatures studied (350 and 800° C). The exact amount of vacancies □ on the alkali site depends upon the starting material and was found to be conserved during exchange, with ca 0–0.2 and 0.3–0.4 vacancies per 16 oxygen atoms for the synthetic and natural precursors, respectively. Thermodynamic interpretation of the Na-K exchange isotherms shows, as one important result, that the sodian end member is unstable with respect to the intermediate at K/(K+Na)≈0.25 by an amount of ca 45 kJ/mol Na in the large cavity at 800° C (52 kJ/mol at 350° C).     

Metamorphism and exhumation of basement gneiss domes in the Quadrilátero Ferrífero:Two stage dome-and-keel evolution?

The presence of dome-and-keel provinces in Archean cratons has been connected with the initiation of plate tectonics on Earth as these features are most commonly observed in Archean rocks.The Quadrilátero Ferrífero in Brazil has been identified as a Paleoproterozoic dome-and-keel province for more than three decades.The prevailing model suggests that it formed during the Rhyacian Transamazonian orogeny,making it unique among dome-and-keel provinces.However,a lack of appropriate lithologies,datable minerals and the metamorphic overprint of later orogenesis has resulted in a cryptic metamorphic record for the formation of this dome-and-keel province.A clinopyroxene-bearing migmatite from the core of the Ba??o dome has peak P-T conditions of 5-7 kbar and 700-750 ℃ and a published age of ca.2730 Ma based on U-Pb ages of zircon from leucosomes,suggesting that this age represents the migmatisation event.A fine-grained epidote-albite-titanite assemblage overprints the coarse-grained clinopyroxene and amphibole,giving P-7 conditions of 8-9 kbar and 550 ℃ with an associated titanite age of ca.2050 Ma.A garnet-bearing amphibolite sample also from the core of the dome has peak P-T conditions of 7-8 kbar and 650-700 ℃,and texturally late titanite from this sample produces an age of ca.2060 Ma.Three additional samples were collected from the edges of the dome.A garnet-gedrite bearing felsic schist produces peak P-T conditions of 8-9 kbar and 650-700℃ on a clockwise P-T evolution.This sample has a U-Pb zircon age of ca.2775 Ma,which could date metamorphism or be the age of its volcaniclastic protolith.Texturally unconstrained titanite from the sample gives an age of ca.2040 Ma.A garnet-bearing amphibolite that occurs as a boudin within the felsic schist gives both zircon and titanite ages of ca.2050 Ma and has peak P-T conditions of 5-6 kbar and 650-700 ℃ on a near isobaric P-T path.An amphibolite dike,observed to cross-cut the felsic schist produces a zircon U-Pb age of ca.2760 Ma.Altogether this data suggests that the samples were metamorphosed in the Archean(ca.2775-2730 Ma)and again during the Transamazonian event.The most plausible explanation for this data is that dome-and-keel formation occurred in the Archean with migmatisation and high-temperature metamorphism occurring at this time.The Paleoproterozoic event is interpreted as a reactivation of the dome-and-keel formation structures,with Paleoproterozoic keels crosscutting Archean keels and producing metamorphic aureoles.The high radiogenic heat production and the presence of dense sedimentary successions in Archean terranes make dome-and-keel provinces a uniquely Archean feature,but they are susceptible to reworking,resulting in an enigmatic record of formation.     

The present-day atmosphere of Mars: Where does it come from?

Recent observations and missions to Mars have provided us with new insight into the past habitability of Mars and its history. At the same time they have raised many questions on the planet evolution. We show that even with the few data available we can propose a scenario for the evolution of the Martian atmosphere in the last three billion years. Our model is obtained with a back integration of the Martian atmosphere, and takes into account the effects of volcanic degassing, which constitutes an input of volatiles, and atmospheric escape into space. We focus on CO₂, the predominant Martian atmospheric gas.Volcanic CO₂ degassing rates are obtained for different models of numerical model crust production rates [Breuer, D., Spohn, T. 2003. Early plate tectonics versus single-plate tectonics on Mars: Evidence from magnetic field history and crust evolution. J. Geophys. Res. - Planets, 108, E7, 5072, Breuer, D., Spohn, T., 2006. Viscosity of the Martian mantle and its initial temperature: Constraints from crust formation history and the evolution of the magnetic field. Planet. Space Sci. 54 (2006) 153–169; Manga, M., Wenzel, M., Zaranek, S.E., 2006. Mantle Plumes and Long-lived Volcanism on Mars as Result of a Layered Mantle. American Geophysical Union Fall Meeting 2006, Abstract #P31C-0149.] and constrained on observation. By estimating the volatile contents of the lavas, the amount of volatiles released in the atmosphere is estimated for different scenarios. Both non-thermal processes (related to the solar activity) and thermal processes are studied and non-thermal processes are incorporated in our modelling of the escape [Chassefière, E., Leblanc, F., Langlais, B., 2006, The combined effects of escape and magnetic field history at Mars. Planet. Space Sci. Volume 55, Issue 3, Pages 343–357.]. We used measurements from ASPERA and Mars Express and these models to estimate the amount of lost atmosphere.An evolution of the CO₂ pressure consistent with its present state is then obtained. A crustal production rate of at least 0.01 km³/year is needed for the atmosphere to be at steady state. Moreover, we show that for most of the scenarios a rapid loss of the primary (and primordial) atmosphere due to atmospheric escape is required in the first 2 Gyr in order to obtain the present-day atmosphere. When CO₂ concentration in the mantle is high enough (i.e. more than 800 ppm), our results imply that present-day atmosphere would have a volcanic origin and would have been created between 1 Gyr and 2 Gyr ago even for models with low volcanic activity. If the volcanic activity and the degassing are intense enough, then the atmosphere can even be entirely secondary and as young as 1 Gyr. However, with low activity and low CO₂ concentration (less than 600 ppm), the present-day atmosphere is likely to be for the major part primordial.