Multiple origin and regional significance of bedding parallel veins in a fold and thrust belt: The example of a carbonate slice along the Appalachian structural front

The development and evolution of bedding parallel veins (BPV) are investigated in the Saint-Dominique carbonate slice (southern Québec Appalachian structural front), in order to emphasize the regional significance of BPV in a fold and thrust belt and their importance in establishing its structural and diagenetic evolution. Structural and microstructural analyses reveal that most BPV display a laminated structure locally crosscut by a massive one. Both structures show mutual crosscutting relationships with bedding parallel or bedding perpendicular stylolites and with bedding perpendicular veinlets, indicating that BPV cementation and deformation are continuous processes. Opening both sub-parallel and sub-perpendicular to the BPV walls are documented. Early BPV are planes of weakness that focus later reactivation, and evidence of successive and sometimes opposite senses of opening are locally preserved within a single BPV. The bedding parallel orientation of these veins proved particularly helpful in establishing the structural evolution of an area mostly characterized by front parallel structures with little crosscutting relationships otherwise. Petrographic and geochemical (δ¹⁸O _VPDB and δ¹³C) analyses of structural cements suggest a common, locally derived source for the fluids that percolated through BPV, non-bedding parallel veins and faults. The isotopic ratios also reveal a significant enrichment in ¹⁸O and ¹³C through time. This evolution is tentatively correlated with tectonic, syn-convergence exhumation of the Saint-Dominique slice during its imbrication along the Appalachian structural front, after its burial under Taconian thrust sheets.     

Assimilation of continental crust by komatiites in the Precambrian basement of the Carswell structure (Saskatchewan,Canada)

The isotope geochemistry (Sm-Nd, Pb-Pb and Rb-Sr) of mafic gneisses from the basement of the Carswell structure (Saskatchewan, Canada), rich both in Mg and incompatible elements (K, Rb, REE) has been investigated. A good Sm-Nd alignment gives a slope corresponding to an age of 3.7 Ga. However, comparison with major elements data strongly suggests that this alignment is a mixing line between Mg-rich, high CaO/Al₂O₃ magmas and the local felsic crust older than 2.9 Ga. The mafic magmas were probably of komatiitic affinity (MgO > 20 percent) but, nevertheless, were extracted from a source with nearly chondritic to slightly enriched light REE distribution. The age of the komatiite emplacement (1.9–2.9 Ga) is only loosely constrained by the oldest crustal residence age in the series and the subsequent metamorphic events. The granulite facies climax is dated at ca. 1.9 Ga by concordant whole rock Pb-Pb and Sm-Nd garnet-whole rock isochrons. The Rb-Sr systematics have been disturbed by later event(s) younger than 1.5–1.7 Ga, but do not permit a more precise assessment of the perturbation age.     

The influence of fluid unmixing on cation exchange between plagioclases and aqueous chloride solutions at 700° C, 1 kbar

Experimental exchanges between plagioclases (synthesized from gels) and aqueous solutions (0.5N–8N) were carried out according to the reaction $$\begin{gathered} 2NaA1Si_3 O_8 + CaC1_2 \hfill \\ \leftrightarrow CaA1_2 Si_2 O_8 + 4SiO_2 + 2NaC1. \hfill \\ \end{gathered}$$ Distribution coefficients defined by $$K_D = \frac{{X_{An} }}{{(X_{Ab} )^2 }}\frac{{(X_{NaC1} )^2 }}{{X_{CaC1_2 } }}$$ were determined at 700° C and 1 kbar. From previous studies it is known that variations in the concentration of the aqueous solutions have no influence upon K _D if the fluid is a single phase. In this study, variation of K _D with the concentration of the solutions is interpreted as the result of fluid unmixing to vapour and brine phases. This implies boiling of CaCl₂-NaCl-H₂O fluids analogous to that known for the system NaCl-H₂O. Experimental data permit calculation of the compositions of vapours and estimation of those of the brines for fluids in which Ca/Na<0.5. Boiling has an effect upon the exchange between feldspars and solutions (metasomatism) and must be considered when determining the activity coefficients.     

Pre-orogenic volcanic assemblages and structure in the Western Cordillera of Ecuador between l°40'S and 2°20'S

A major reverse fault with a separation of at least a few thousand meters is exposed in the western Cordillera of Ecuador near 2°S.Ree and other incompatible elements patterns show (i) that the rocks exposed in the upthrust block to the east of the fault areMorb that had not yet been identified and (ii) that a volcanic island-arc assemblage extensively outcrops in the downthrust block west of the fault.The fault therefore bounds a steeply east-dipping thrustsheet comprising the arc and its basement. Thrustsheets of the same type are known farther north in Ecuador. They were stacked in a presently inactive subduction-zone during the early Tertiary when the island-arc, drifting from the west, finally collided with continental south America.The large dextral-slip Guayaquil-Dolores fault system, traditionally but possibly erroneously thought to be the suture-zone between continental south America and the oceanic allochthone blocks collided during the Cretaceous and Tertiary, subsequently disrupted most of that structural arrangement.

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Zusammenfassung Die Gesteine einer bei 2° Südlicher Breite in der Westkordillere Ecuadors zu Tage tretenden und um mehrere tausend Meter versetzten Verwerfung wurden in geochemischer Hinsicht untersucht. Die Verteilung der seltenen Erden und der anderen inkompatiblen Elemente in den Proben zeigte östlich dieser Verwerfung das Vorkommen von Tholeiiten desMorb-Typs auf, deren Auftreten hier bis jetzt unbekannt war. Außerdem konnten die westlich der Verwerfung auftretenden Diabase, Andesite und Rhyolite dem Typ eines Inselbogens zugeordnet werden.Diese Verwerfung begrenzt infolgedessen eine stark nach Osten einfallende Schuppe des Sockels. Andere Schuppen dieser Art treten weiter im Norden Ecuadors auf, wo sie zu Beginn des Tertiärs während des Zusammentreffens des von Westen ankommenden, aus der Kreidezeit stammenden Inselbogens mit dem südamerikanischen Kontinent in eine Paläosubduktionszone eingebettet worden sind.Dieses Strukturgefüge ist anschließend zum größten Teil durch das System der rechtsseitigen Verwerfungen von Guayaquil — Dolores verdeckt worden. Dieses System wird für gewöhnlich — aber wahrscheinlich zu Unrecht — für die Nahtstelle des kontinentalen Südamerikas und der während der Kreidezeit und des Tertiärs gegen den Kontinent sto– Benden und/oder obduzierten allochtonen Blöcke ozeanischen Substrats gehalten.

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Résumé Une faille inverse exposée dans la cordillère occidentale de l'Equateur vers 2°S a un rejet de plusieurs milliers de mètres. L'étude géochimique des terres rares et autres éléments incompatibles permet de reconnaître à l'est de la faille des tholéiites de ride médio-océanique jusqu'alors non identifiées, alors que les diabases, andésites et rhyolites présentes à l'ouest de la faille appartiennent à une formation d'arc insulaire.Cette faille délimite donc une écaille de socle à fort pendage vers l'est. D'autres écailles de ce type sont connues plus au nord en Equateur. Elles se sont imbriquées au début du Tertiaire dans une paléozone de subduction, lors de la collision de l'arc insulaire, venu de l'ouest, avec le continent sud-américain.Ce dispositif structural a été ensuite oblitéré en majeure partie par le systême faillé dextre de Guayaquil-Dolores, qui passe classiquement, mais probablement à tort, pour être la suture entre l'Amérique du sud continentale et les blocs allochtones à substratum océanique collés au continent et/ou obductés durant le Crétacé et le Tertiaire.

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Static displacement fields due to a seismic source in a nonhomogeneous (dimorphic) medium. Geophysical applications

A new method for finding static displacement fields due to seismic sources (forces or dislocations) in a nonhomogeneous medium (called “dimorphic”) is presented. Previously known results can be easily calculated, as well as new results for sources located at the interface between two media with different elastic properties. The ratio of displacement magnitudes in homogeneous and dimorphic media, which can be as great as 15%, is calculated. Consequences for the seismic moment and some other geophysical applications are discussed.     

Variation in hydrologic connectivity as a result of microtopography explained by discharge to catchment size relationship

In hydrological terms, raised bogs are often approximated by simple models as in the acrotelm–catotelm concept. However, raised bogs are often characterized by a pronounced surface topography, causing large changes in connectivity of contributing areas on the bog. In this study, daily regression of measured discharges versus catchment areas is used to quantify the impact of surface topography on catchment connectivity within a raised bog. The resulting coefficient of determination shows the strength of the relationship between the discharge and catchment area over time under different hydrological conditions. Monitoring of discharge, water table, transmissivity, and basic weather data on a raised bog (1.9 km²) in eastern central Estonia took place from May 2008 to June 2010. Contributing areas, calculated based on the outlet's discharge volume (V _Q ) divided by the net precipitation volume ( ), of the outlet containing the central pool‐ridge system varied between 1×10^?3 and 0.7 km², suggesting significant differences in connectivity between hydrological events. Correlation between discharge and theoretical catchment size was high (R ²>0.75) when the water table was close to the surface (less than 5 cm below peat surface), and consequently, transmissivities were also high (up to 1,030m²d^?1), which led to connectivity of local storage elements, such as pools and hollows. However, a water table below this threshold resulted in large parts of the catchment being disconnected. The importance of water table depths on catchment connectivity suggests the need to reconsider the hydrological concept of raised bogs; to incorporate these shallow flow components and better understand residence time and consequently transport of solutes, such as DOC, from patterned peatlands.     

Structure and organization of submarine basaltic flows: sheet flow transformation into pillow lavas in shallow submarine environments

Distal pillows occur associated with a sheet flow and megapillows in the me?akoz outcrops of the Basque–Cantabrian Basin (N Spain). Basaltic volcanic rocks are interbedded with Turonian sediments and depict typical features of shallow submarine emissions. An exceptional basaltic flow displays four types of morphology: (1) sheet lava with columnar jointing, (2) welded columnar breccia, (3) megapillows, and (4) pillow lavas with sparse megapillows. The field data from me?akoz combined with experimental and field data from the literature for similar volcanic facies can be integrated into a new propagation model for the transition from sheet flows to pillow lavas in underwater environments. At near vent high emission rates, lava flows develop a thin crust immediately after its emplacement and break at the front under the magma pressure allowing for the massive propagation of lava as a sheet flow. Increased cooling promotes thickening of the lava outer crust far from the vent while continuous supply of fresh magma increases the pressure onto the thick crust until its rupture. The lava emitted in small volumes from the flow front promotes the formation of megapillows and pillow lavas that are later on covered by the advancing sheet flow. The lava flow freezes progressively toward more distal parts, gradually increasing its viscosity until it stops. The crust temporarily holds the residual melt pressure increasing the volume of the flow distal section by inflation. Finally, the internal magma pressure breaks the crust and liberates lava at moderate-to-low flow rates producing pillows, while lava drainage inside the inflated sheet flow produces lava tunnels and gravitational collapse of the roofs by hydrostatic pressure to form breccias nurtured by columnar lava fragments.     

Phillipsite formation in nephelinitic rocks in response to hydrothermal alteration at Mount Etinde,Cameroon

Fresh nephelinitic rocks and hydrothermally altered rocks at Mount Etinde (Cameroon Volcanic Line, West Africa) have been studied by combined whole rock geochemistry (ICP-MS), mineralogy and mineral chemistry (SEM-EDS, WDS, XRD) techniques. The nephelinites have feldspathoids, clinopyroxene, perovskite and titanomagnetite as the principal minerals in the mode with subsidiary apatite and sphene. The mineralogy of their hydrothermally altered counterparts includes phillipsite, calcite and analcime which are secondary phases developed in response to hydrothermal fluid events. Correspondingly, the bulk rock geochemical data show elevated SiO₂, CaO, Na₂O and K₂O concentrations with increasing alteration and Al₂O₃ and Fe₂O₃ depletion while MgO, MnO and TiO₂ concentrations are largely unaffected. The nephelinites also have high concentrations of LILLE, HFSE and REE and upon hydrothermal alteration they show an enrichment of LREE and MREE over HREE. Phillipsite is the principal alteration mineral in the rocks and it occurs along cracks, vesicles and also forms alteromorphs after feldspathoids. The Ce content of these categories of phillipsite varies. Phillipsite along cracks is richer in Ce while phillipsite associated with calcite has lower Ce concentration and the phillipsite alteromorphs very little or no Ce. Various stages of fluid circulation are proposed hereby to explain the variations in phillipsite generation and composition.     

Raman spectroscopic properties and Raman identification of CaS‐MgS‐MnS‐FeS‐Cr2FeS4 sulfides in meteorites and reduced sulfur‐rich systems

Raman spectra were acquired on a series of natural and synthetic sulfide minerals, commonly found in enstatite meteorites: oldhamite (CaS), niningerite or keilite ((Mg,Fe)S), alabandite (MnS), troilite (FeS), and daubreelite (Cr₂FeS₄). Natural samples come from three enstatite chondrites, three aubrites, and one anomalous ungrouped enstatite meteorite. Synthetic samples range from pure endmembers (CaS, FeS, MgS) to complex solid solutions (Fe, Mg, Ca)S. The main Raman peaks are localized at 225, 285, 360, and 470 cm^?1 for the Mg‐rich sulfides; at 185, 205, and 285 cm^?1 for the Ca‐rich sulfides; at 250, 370, and 580 cm^?1 for the Mn‐rich sulfides; at 255, 290, and 365 cm^?1 for the Cr‐rich sulfides; and at 290 and 335 cm^?1 for troilite with, occasionally, an extra peak at 240 cm^?1. A peak at 160 cm^?1 is present in all Raman spectra and cannot be used to discriminate between the different sulfide compositions. According to group theory, none of the cubic monosulfides oldhamite, niningerite, or alabandite should present first‐order Raman spectra because of their ideal rocksalt structure. The occurrence of broad Raman peaks is tentatively explained by local breaking of symmetry rules. Measurements compare well with the infrared frequencies calculated from first‐principles calculations. Raman spectra arise from activation of certain vibrational modes due to clustering in the solid solutions or to coupling with electronic transitions in semiconductor sulfides.     

Late Weichselian to Holocene paleoenvironments in the Barents Sea

Paleoceanographic changes since the Late Weichselian have been studied in three sediment cores raised from shelf depressions along a north–south transect across the central Barents Sea. AMS radiocarbon dating offers a resolution of several hundred years for the Holocene. The results of lithological and micropaleontological study reveal the response of the Barents Sea to global climatic changes and Atlantic water inflow. Four evolutionary stages were distinguished. The older sediments are moraine deposits. The destruction of the Barents Sea ice sheet during the beginning of the deglaciation in response to climate warming and sea level rise resulted in proximal glaciomarine sedimentation. Then, the retreat of the glacier front to archipelagoes during the main phase of deglaciation caused meltwater discharge and restricted iceberg calving. Fine-grained distal glaciomarine sediments were deposited from periodic near-bottom nepheloid flows and the area was almost permanently covered with sea ice. The dramatic change in paleoenvironment occurred near the Pleistocene/Holocene boundary when normal marine conditions ultimately established resulting in a sharp increase of biological productivity. This event was diachronous and started prior to 10 ¹⁴C ka BP in the southern and about 9.2 ¹⁴C ka in the northern Barents Sea. Variations in sediment supply, paleoproductivity, sea-ice conditions, and Atlantic water inflow controlled paleoenvironmental changes during the Holocene.