Sedimentary evolution of the Late Eocene Vernet lacustrine system (South-Central Pyrenees). Tectono-climatic control in an alluvial-lacustrine piggyback basin

A complete third-order tectono-sedimentary cycle filled the Vernet lacustrine piggyback basin during the late Eocene. The depositional sequence can be subdivided into five units or systems tracts with distinct architectural frameworks, which developed a progressive unconformity. Sedimentary infilling of the piggyback basin began with retrogradational alluvial fans and fluvio-alluvial deposits being deposited, arranged in two thinning- and fining-upward cycles during the uplifting and emplacement of the southernmost Pyrenean thrusts, and with three thickening- and coarsening-upward progradational floodplain and lacustrine cycles, during the quiescence of the southernmost thrusts, at the same time as the emplacement of several northernmost thrusts of the southern Pyrenees. The facies associations identified represent different depositional environments in a sheetflow-dominated system, including playa-lake deposits, floodplain deposits, sheetflow fluvial deposits and sheet deltas in ephemeral to shallow lacustrine settings. Fourth and fifth order arid/semi-arid climatic cycles can be recognized, driven either by orbital obliquity and precessional forcing (Milankovitch cyclicity), insolation cycles or possibly subprecessional autogenic mechanisms related to the emplacement of northern thrusts and the consequent changes in the drainage basin. Arid periods are recorded by sheetflood fluvial, ephemeral lacustrine or terminal splay deposits, with abundant medium-to-coarse-grained sandstone packages in red-to-grey mudstones, a product of unconfined or low-confined sheetflows or hyperconcentrated flows, with bedload and suspended load, during flood episodes, at times of lake lowstand. Semi-arid periods are recorded as relative highstand lacustrine deposits, including grey-to-white mudstones deposited in prodelta environments and fine-grained sheet sandstone units sedimented in a delta front (with minor channels and extensive sheet lobes) and prodelta environments in shallow lakes, from sheetflood events.     

The Devonian units of the Marimanya massif and their relationship with the Pyrenean Devonian facies areas

Abstract

The metasedimentary materials of the Marimanya Massif are chiefly made up of Devonian deposits, classically attributed to the Cambro-Ordovician. Their stratigraphic succession has been reconstructed from geological mapping, structural analysis and the study of the limestone conodont content, and subsequently divided into four Devonian units.

The lower part consists of the Moredo-Salau carbonatic unit (braided alternation, crinoidal carbonates, parallel alternation and Campaus limestones). Its age ranges from the Upper Silurian to Lower Emsian. This unit is overlain by lutites with limestone and fine sandstone intercalations from the Beret series (Lower Emsian-Lower Frasnian).

The two units are correlated in the east with the Salau carbonate series and the “Shales Bleutes”, and in the Vielha synclinal with the “Calcaire Basal” and the Entecada, Auba, Sa Fusta and Mont Corbisum units.

The Upper part of the succession is formed by the Montgarri and Can Cabau Detrital units, which are lithologically correlated with the Sa Cal and Riu Nere units in the Vall d’Aran and, in the west, with the Agudes-Cap de Pales and Sia series.

Thus the Devonian Central facies area stretches eastwards to the Pyrenees ariégeoises and westwards to the Vielha syncline, Les Agudes, Sia, Sagette and Laruns.

The new biostratigraphic succession established in the Marimanya Massif represents, to date, the most complete within the Central facies area. The new data obtained enable the correlation between the Devonian units in this facies area with the units in the remaining Devonian facies area in the Pyrenees.     

Middle and Late Carboniferous extension In the Variscan Belt: structural and petrological evidences from the Vosges massif (Eastern France)

Abstract

In the Moldanubian domain of the Vosges massif (INK France) euperimposition of three distinct crustal units has been attributed to Middle to Late Carboniferous thrusting. Л kinematic analysis of mierostuetures within each unit suggests that extension, following the formation of a Stack of nappes, is actually responsible for the bulk structure of this region. In order to estimate the related exhumation, the temperature and pressure evolution of the lowermost unit is investigated. It is characterised by (i) a prograde evolution within the stability field of kyanite, followed by (ii) a syn-kinematic, 3-4 kbar, near-isothermal decompression before (iii) cooling. Thermal modeling shows that the isothermal decompression may be related to rapid exhumation (> I mm.a^-1), which cannot be accounted for by erosion alone. Therefore, exhumation is best explained by extensional processes, possibly related to gravitational collapse of a thickened crust.     

Variscan plate dynamics in the circum-carpathian area

Abstract

Magmatic and metamorphic events, imprinted in the crystalline rocks of the so-called core mountains inside the Alpine structure of the Inner Carpathians, allow the re-construction of the history of the Rheic Ocean opening, its development and its final closure. Intra-Carpathian core-mountains are the remnants of the continents that drifted away from Gondwana and docked, initially, with Baltica as part of Avalonia and later on as parts of the Gondwana-derived Armorica Terrane Group or as a separate micro-continent.

All magmatic suites, mafic and felsic, present in the Carpathians core mountains, show similarities to those found in the European Variscan Belt. All described- and dated metamorphic and magmatic events also have equivalents in the evolution of the Caledonian-Variscan Belts of Europe. The most pronounced feature of all Carpathian core mountains is the syn-collisional, multistage I/S granitoid magmatism (370-340 Ma) related to subduction, mafic-magma influx, extensional decompression and slab melting. That episode marked the Laurussia - Gondwana collision and closure of the Rheic Ocean, as in the whole of Central and Western Europe.

The Carpathian core-mountains, currently dispersed inside the Alpine mountain chain, can be considered the broken fragments of the eastern prolongation of the Variscan orogenic belts – possibly part of the Moldanubian Unit.     

RESTORATION OF SHEEP TRANSHUMANCE IN THE EBRO VALLEY,ARAGON, SPAIN*

For centuries, sheep transhumance has connected the Spanish highlands with the lowlands, creating a livestock system with important social, economic, cultural, and landscape effects. About 5 million sheep were involved in transhumant movements in the mid‐1700s, but transhumance started to decay in the nineteenth century and almost disappeared in the 1970s and 1980s. In the 1980s, transhumant sheep did not number more than 25,000 in the study area; at present the number exceeds 134,000, with herders practicing both descending and ascending transhumance. In this article we provide information not only on current numbers of transhumant sheep but also on the origins and destinations of the flocks. We also confirm that the reputation of transhumance, once regarded as quaint and antiquated, has been enhanced by the ecosystem services it provides and by the cultural and heritage continuities it renders.     

Overview of the subsurface structural pattern of the Paris Basin (France): Insights from the reprocessing and interpretation of regional seismic lines

The study presents the methodology used by the French Geological Survey (BRGM) for the building, reprocessing and interpretation of selected regional seismic lines in the Paris intracratonic basin (France): the 14 constructed E-W and N-S regional transects represent a total of 2,516 km length, and are based on the merge of 240 seismic single profiles recorded by petroleum operators between 1971 and 1995. The regional lines have been selected to cross the main oil fields of the Paris Basin, as well as high potential areas for oil exploration. A first difficulty was to recover the raw data necessary to build-up the regional transects. The signal reprocessing, harmonization and merge of the single seismic lines, constituent of the regional transects, are then described; these operations represent the cornerstone of the study. We put the emphasis on the primary static corrections, as the targeted structures are commonly spatially associated with large seismic velocity variations in the upper Cretaceous chalk and Tertiary sedimentary cover.The interpreted regional transects definitely give complementary information to the existing studies, which generally lack seismic (and therefore structural) data: we give an overview of the main structural and geometrical features of the Paris Basin: inversion structures, major unconformities, as well as Permo-Carboniferous basins. We also describe the structural pattern, and show the close relationships between the faults geometry, the faults density, and the geological evolution of the Paris Basin: we distinguish (1) few large-scale polyphase faults, with a Variscan origin, representing the first order structural frame of the Paris Basin; (2) monophase normal faults, with strike-slip features, representing the subsurface prolongation of Cenozoic grabens cropping out in the neighbourhood; (3) deep normal faults, sealed by the base Calcareous Dogger sequence, related to the Permo-Liassic extensional tectonic regime. This large-scale view of the Paris Basin has highlighted several potential exploration targets.     

中天山北缘华力西期造山作用——变质岩锆石U-Pb年代学限定

笔者对中天山微陆块北缘托克逊干沟地区角闪岩相变质岩中的锆石进行了U-Pb年代学研究,结果证明变质沉积岩中的碎屑锆石记录了从太古宙至元古宙(3320～530 Ma)的源区岩浆热事件,变质火成岩中的岩浆锆石记录了新元古代晚期(550 Ma)的岩浆作用,而变质锆石记录了晚泥盆纪(385～360 Ma)的变质作用。这一定年结果表明,中天山微陆块北缘的造山作用很可能发生在华力西期,中天山微陆块形成于新元古代以前,但并没有经历前寒武纪变质作用,具有与塔里木克拉通明显不同的前寒武纪构造演化历史。因此,中天山微陆块很可能是一个独立的块体,并不支持其是从塔里木板块分离出来的观点。     

Metabasites with eclogite facies relics from Variscides in Sardinia,Italy: a review

Metabasites with eclogite facies relics occur in northern Sardinia as massive to strongly foliated lenses or boudins embedded within low- to medium-grade rocks (Anglona) and migmatites (NE Sardinia). U–Pb zircon dating yielded 453 ± 14, 457 ± 2 and 460 ± 5 Ma as the protolith ages; 400 ± 10 and 403 ± 4 Ma have been interpreted as the ages of the HP event and 352 ± 3 and 327 ± 7 Ma as the ages of the main Variscan retrograde events. A pre-eclogite stage is documented by the occurrence of tschermakite, zoisite relics within garnet porphyroblasts (Punta de li Tulchi) and an edenite–andesine inclusion within a relict kyanite porphyroblast (Golfo Aranci). Four main metamorphic stages have been distinguished in the eclogite evolution: (1) eclogite stage, revealed by the occurrence of armoured omphacite relics within garnet porphyroblasts. The Golfo Aranci eclogites also include kyanite, Mg-rich garnet and pargasite; (2) granulite stage, producing orthopyroxene and clinopyroxene–plagioclase symplectites replacing omphacite. At Golfo Aranci, the symplectitic rims around relict kyanite consist of sapphirine, anorthite, corundum and spinel; (3) amphibolite stage, leading to the formation of amphibole–plagioclase kelyphites between garnet porphyroblasts and pyroxene–plagioclase symplectites and to the growth of cummingtonite on orthopyroxene. Tschermakite to Mg-hornblende, plagioclase, cummingtonite, ilmenite, titanite and biotite are coexisting phases; (4) greenschist to sub-greenschist stage, defined by the appearance of actinolite, chlorite, epidote ss, titanite, sericite and prehnite. The following P–T ranges have been estimated for the different stages. Eclogite stage 550–700°C; 1.3–1.7 GPa; granulite stage 650–900°C; 0.8–1.2 GPa, clustering in the range 1.0–1.2 GPa; amphibolite stage 550–740°C; 0.3–0.7 GPa; greenschist stage 300–400°C; 0.2–0.3 GPa. Comparable ranges characterise the other Variscan massifs in Europe; eclogite stage: T = 530–800°C; P from 0.7–1.1 to 1.7 ± 0.3 GPa; granulite stage T = 760–870°C and P from 1.1–1.4 to 7.2–9.9 GPa, clustering around 1.0–1.2 GPa. Whole-rock chemistry: Sardinian eclogites are N- to T-MORB; European ones N- to E-MORB or calc-alkaline.     

The Meso-Cenozoic thermo-tectonic evolution of the Eastern Pyrenees: an <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar fission track and (U–Th)/He thermochronological study of the Canigou and Mont-Louis massifs

Vertical displacements on the SW–NE Têt fault (Eastern Pyrenees Axial Zone, France), which separates the Variscan Canigou-Carança and Mont-Louis massifs, were constrained using a thermochronologic multi-method approach. ⁴⁰Ar/³⁹Ar data from the granitic Mont-Louis massif record its Variscan cooling history and reveal no ages younger than Early Cretaceous, while the Canigou-Carança gneiss massif records systematically younger ⁴⁰Ar/³⁹Ar ages. These younger ⁴⁰Ar/³⁹Ar ages in the Canigou-Carança gneiss massif are the result of partial to total rejuvenation of argon isotopic systems related to a thermal flow coeval with the Cretaceous HT-BP metamorphism in the North Pyrenean Zone. Only the deepest rocks from the Canigou-Carança suffered this extensive Mid-Cretaceous thermal overprint probably due to differential burial around 4 km at that time. The post Mid-Cretaceous vertical displacements along the Têt fault are recorded by “low” temperature thermochronology using K-feldspar ⁴⁰Ar/³⁹Ar, zircon and apatite fission track and (U–Th)/He datings. The Mont-Louis granite samples experienced a long period of protracted cooling reflecting a lack of thermo-tectonic activity in this area from Late Palaeozoic to Early Cenozoic, followed by cooling from 55–60 Ma to Late Eocene at a mean rate of 15–20°C/Ma in the final stage. This cooling stage corresponds to Têt fault reactivation with a reversed component, promoting exhumation of the Mont-Louis roof zone contemporaneously with the south-vergent Pyrenean thrusting. In the Canigou-Carança massif, the main cooling event occurred from 32 to 18 Ma at a maximum rate of 30°C/Ma during Early Oligocene followed by a more moderate rate of 3°C/Ma from Late Oligocene to Early Burdigalian, coeval with the normal reactivation of the Têt fault in brittle conditions that accommodated the final exhumation of the massif during the opening of the Gulf of Lion.