The Andean orogenic front at Sierra de Las Peñas-Las Higueras,Mendoza, Argentina

The Sierra de Las Peñas-Las Higueras (Mendoza Province, Argentina, 32°15′S–32°45′S) presents one of the clearest and most continuous exposures of the Quaternary thrust front of the Precordilleran fold-and-thrust belt. It is characterized by an east-verging thrust that breaks the surface and causes Neogene sedimentary rocks to override Quaternary alluvial conglomerates. Monoclinal folds and progressive unconformities are characteristic of deformation in the upper part of the alluvial cover, indicating synchronous development of sedimentation and thrusting during the Quaternary. South of this range, ongoing deformation is by gentle warping of the piedmont alluvial plain, hiding blind thrusts at depth.     

Active deformation in the northern Sierra de Valle Fértil,Sierras Pampeanas,Argentina

The Western Sierras Pampeanas region in the San Juan Province is characterized by thick-skinned deformation with approximately N–S trending ranges of average heights of 2500 m and a high frequency occurrence of seismic activity. Its location to the east of the mainly thin-skinned tectonics of the Argentine Precordillera fold-and-thrust belt suggests that at 30°S, deformation is concentrated in a narrow zone involving these two morphostructural units. In this paper, we present new apatite (U–Th)/He results (AHe) across the northern part of the Sierra de Valle Fértil (around 30°S) and analyze them in a framework of thermochronologic available datasets. We found Pliocene AHe results for Carboniferous and Triassic strata in the northern Sierra de Valle Fértil consistent with the hypothesis of recent cooling and inferred erosional denudation concentrated along the northern end of this mountain range. Our analysis shows that this northern region may have evolved under different conditions than the central part of the Sierra de Valle Fértil. Previous studies have observed AHe ages consistent with Permian through Cretaceous cooling, indicating the middle part of the Sierra de Valle Fértil remained near surface before the Pampean slab subduction flattening process. Those studies also obtained ∼5 My cooling ages in the southern part of the Sierra de Valle Fértil, which are similar to our results in the northern end of the range. Taken together, these results suggest a pattern of young deformation in the northern and southern low elevation ends of the Sierra de Valle Fértil consistent with regions of high seismic activity, and Quaternary active faulting along the western-bounding thrust fault of the Sierra de Valle Fértil.     

Lower Carboniferous post-orogenic granites in central-eastern Sierra de Velasco,Sierras Pampeanas,Argentina: U–Pb monazite geochronology,geochemistry and Sr–Nd isotopes

The central-eastern part of the Sierra de Velasco (Sierras Pampeanas, NW Argentina) is formed by the large Huaco (40 × 30 km) and Sanagasta (25 × 15 km) granite massifs and the small La Chinchilla stock (2 × 2 km). The larger granites intrude into Ordovician metagranitoids and crosscut Devonian (?) mylonitic shear zones, whereas the small stock sharply intrudes into the Huaco granite. The two voluminous granites are biotitic-muscovitic and biotitic porphyritic syeno- to monzogranites. They contain small and rounded tonalitic and quartz-dioritic mafic microgranular enclaves. The small stock is an equigranular, zinnwaldite- and fluorite-bearing monzogranite. The studied granites are silica-rich (SiO₂ >70%), potassium-rich (K₂O >4%), ferroan, alkali-calcic to slightly calk-alkalic, and moderately to weakly peraluminous (A/CNK: 1.06–1.18 Huaco granite, 1.01–1.09 Sanagasta granite, 1.05–1.06 La Chinchilla stock). They have moderate to strong enrichments in several LIL (Li, Rb, Cs) and HFS (Nb, Ta, Y, Th, U) elements, and low Sr, Ba and Eu contents. U–Pb monazite age determinations indicate Lower Carboniferous crystallization ages: 350–358 Ma for the Huaco granite, 352.7 ± 1.4 Ma for the Sanagasta granite and 344.5 ± 1.4 Ma for the La Chinchilla stock. The larger granites have similar ?Nd values between ?2.1 and ?4.3, whereas the younger stock has higher ?Nd of ?0.6 to ?1.4, roughly comparable to the values obtained for the Carboniferous San Blas granite (?1.4 to ?1.7), located in the north of the sierra. The Huaco and Sanagasta granites have a mainly crustal source, but with some participation of a more primitive, possibly mantle-derived, component. The main crustal component can be attributed to Ordovician peraluminous metagranitoids. The La Chinchilla stock derives from a more primitive source, suggesting an increase with time in the participation of the primitive component during magma genesis. The studied granites were generated during a post-orogenic period in a within-plate setting, possibly as a response to the collapse of the previous Famatinian orogen, extension of the crust and mantle upwelling. They are part of the group of Middle Devonian–Lower Carboniferous granites of the Sierras Pampeanas. The distribution and U–Pb ages of these granites suggests a northward arc-parallel migration of this mainly post-orogenic magmatism with time.     

Oxyanion Concentrations in Eastern Sierra Nevada Rivers – 3. Boron, Molybdenum, Vanadium, and Tungsten

Water samples were collected from 10 locations along the Truckee River system, 14 locations along the Walker River system, and 12 locations along the Carson River, and analyzed for B, Mo, V, W, Na, Cl, and pH. Boron concentrations ranged from approximately 2 mol/kg in the upper reaches of the Truckee River to almost 1,200 mol/kg in Pyramid Lake. Molybdenum, V, and W had concentrations in the nanomolal range; Mo varied from a low of about 12 nmol/kg to a high of 3,200 nmol/kg (Walker Lake); V ranged from 9 nmol/kg to approximately 470 nmol/kg; and W varied from a low value around 0.8 nmol/kg (West Walker River) to 1,030 nmol/kg. The high concentrations of these oxyanion-forming trace elements in the rivers reflects (1) the relative stability of these oxyanions (e.g., MoO₄ ^2-, HVO₄ ^2-, WO₄ ^2-, B(OH)₃, and/or B(OH)₄ ^-) in the alkaline, well oxygenated river and lake waters, (2) contributions of hydrothermal waters (especially for B), and (3) weathering of rocks/regolith with high concentrations of these elements. In the case of Mo, V, and W, each exhibited relatively conservative behavior in the upper, oxygenated reaches of all three rivers. During the study period the region experienced a prolonged drought such that the lower reaches of each river were typified by no flow or stagnant waters and probably low oxygen and/or anoxic conditions (although not measured). Reductive processes occurring in the low flow to stagnant reaches of each river could have led to removal of Mo, V, and W from solution as coprecipitates with Fe monosulfides, or via sorption to Fe oxides/oxyhydroxides and/or organic matter. Boron, however, exhibited essentially no or minor removal from these rivers, and instead was added to each river via B-rich hydrothermal waters (e.g., Steamboat Creek from Steamboat Hot Springs), or by B-rich groundwaters via base-flow during the extensive drought.     

Permian high pressure rocks—the basement of the Sierra de Limón Verde in Northern Chile

The gneisses and metabasites of the Sierra de Limón Verde were investigated by P–T–t determinations. The rocks are unique in the Central Andes because of their high pressure metamorphic conditions with P≈13±1 kbar at T≈660–720°C. Their age of metamorphism is ≈270 Ma, based on Sm–Nd mineral isochrons. Final uplift of the isolated basement block occurred in the Triassic with a K–Ar age of biotite at ca 235 Ma. In our interpretation, the protolith of the Permian metamorphic rocks is the crust that formed and stabilized during Early Paleozoic. The Sierra de Limón Verde rocks give insight into the lowermost part of the crust in Early Mesozoic. Its Sm–Nd isotopic composition is indistinguishable from the composition of the crust that formed in the Early Paleozoic metamorphic–magmatic cycle (ca 500 Ma) in northern Chile and NW Argentina. The tectonic-geodynamic setting that triggered the high P (∼45 km depth) metamorphism and the locally restricted exhumation of the rocks remains speculative. Continental collision or a subduction related accretionary complex is unlikely considering the regional geological situation. Transpression–transtension in a strike slip system along the continental margin is suggested as a hypothesis for future investigations.     

Über ein dem Greenalit nahestehendes Mineral der Serpentingruppe aus den subvulkanisch-hydrothermalen Erzvorkommen der Sierra de Cartagena in Spanien

Zusammenfassung Die subvulkanisch-hydrothermalen Erzvorkommen der Sierra de Cartagena in Spanien enthalten neben Siderit, Quarz, Magnetit und sulfidischen Erzmineralien in großer Menge eine feinkörnig-dichte, grüne Substanz, die makroskopisch chamo sitischen Erzen sehr ähnlich ist. Durch chemische, röntgenographische und optische Untersuchungen konnte nachgewiesen werden, daß es sich um ein Mineral mit Zweischichtanordnung (diphormische Familie = Gruppe der 7 Å-Chlorite) handelt.Das Mineral hat einen hohen Si- und Fe²⁺-Gehalt bei niedrigem Al-Gehalt. Da die Si-Ionen restlos zur Besetzung der Tetraederplätze ausreichen, muß angenommen werden, daß die Al-Ionen Oktaederplätze besetzen. Damit ergibt sich zwangsläufig eine Zuordnung zur Serpentingruppe (Silikatgruppe der 7 Å-Chlorite). Die Oktaederplätze sind bevorzugt von Fe²⁺, untergeordnet von Fe³⁺, Al; Mg, Mn und Ca besetzt. Die 3/3-Besetzung in den Oktaederschichten ist beinahe erreicht. Die Alkalien haben vermutlich Zwischenschichtbesetzung oder besetzen Oktaederplätze. Das Cartagena-Mineral hat die Formel:Es unterscheidet sich von dem reinen Fe-Serpentin (Greenalit) im wesentlichen durch seinen Al-Gehalt und das Vorhandensein von Alkalien.Die Entstehungsbedingungen sind denen des Mg-Serpentins sehr ähnlich, der durch die Umwandlung von Dolomit durch heiße, SiO₂-haltige Lösungen gebildet werden kann. Der Cartagena-Fe-Serpentin entstand durch hydrothermale Umwandlung von Siderit.

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The subvolcanic-hydrothermal ore deposits of the Sierra de Cartagena in Spain contain a fine grained, green mineral beside siderite, quartz, magnetite and sulphide ore minerals. Macroscopically the mineral looks like chamositic ore. The material was investigated by chemical, X-ray and optical methods. It belonges to the septechlorites.The mineral has a high Si- and Fee+-content, and a low Al-content. It can be supposed, that the Al ions occupy octahedral positions, because the Si ions suffice for the occupation of the tetrahedral positions. Hence the mineral is necessarily allocated to the serpentine group. The octahedral positions are mainly occupied by Fe²⁺, subordinately by Fe³⁺, Al, Mg, Mn and Ca. Na and K ions occupy positions between the layers or octahedral positions. The Cartagena mineral has the following formula:It differs from the pure Fe-serpentine (greenalite) by the presence of Al and alkalies.The genesis of the mineral is similar to the genesis of Mg-serpentine, which can be originated from dolomite hydrothermally by hot SiO₂ containing solutions. The Cartagena Fe-serpentine is formed by hydrothermal transformation of siderite.

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Auszugsweise vorgetragen während der DMG-Tagung 1961 in Tübingen.     

A Middle Paleozoic shear zone in the Sierra de Valle Fértil,Argentina: Records of a continent-arc collision in the Famatinian margin of Gondwana

Geological, petrological and structural observations were obtained along a 30-km-long traverse across a segment of the Valle Fértil shear zone, central-western Argentina. On a regional scale, the shear zone appears as numerous discontinues belts over 25 km in width and is approximately 140 km in length, extended on the western section of the Sierras Valle Fértil – La Huerta mountain range. The steeply dipping shear zone with a vertical mylonitic lineation is composed of amphibolite facies ribbon mylonites and amphibolite to greenschist facies ultramylonites derived from Early Ordovician plutonic and metasedimentary parent rocks. Locally, syn-kinematic retrogression of mylonites formed greenschist facies phyllonites. During the later stages of deformation, unstrained parent rocks, mylonites, ultramylonites and phyllonites were affected by pervasive cataclasis under low greenschist facies conditions associated with localized faulting. One new ⁴⁰Ar/³⁹Ar age on biotite and published ⁴⁰Ar/³⁹Ar ages on amphibole in the shear zone yield an average cooling rate of 6.2 °C/Ma for a time period that crosses the Silurian–Devonian boundary. Since in metasedimentary rocks the youngest zircon's rims dated at 465 Ma marks the beginning of cooling, nearly continuous uplift of rocks within the shear zone occurred over a minimum time span of 55 Ma. During the period of active deformation, dip-slip movement can explain uplift of several kilometers of the Early Ordovician arc crust. The Valle Fértil shear zone, which was formed near above the inferred suture zone between the Famatinian arc and Cuyania microcontinent, is a major structural boundary nucleated within the Early Ordovician crust. The simplest geodynamic model to explain the evolution of the Valle Fértil shear zone involves the collision of the composite Cuyania/Precodillera microcontinent against the Famatinian arc.     

Kinematics of the Guerrero terrane accretion in the Sierra de Guanajuato,central Mexico: new insights for the structural evolution of arc–continent collisional zones

The Guerrero terrane comprises Middle Jurassic–Early Cretaceous arc successions that were accreted to the North American craton in the late Early Cretaceous, producing closure of the Arperos oceanic basin and the formation of an approximately 100 km-wide fold–thrust belt. Such a suture is key to investigating the structural evolution related to Guerrero terrane accretion and, in general, to arc–continent collisional zones. The Sierra de Guanajuato is an exposure of the Guerrero terrane suture belt and consists of a complex tectonic pile that formed through at least three major shortening phases: D1SG, D2SG, and D3SG (SG, Sierra de Guanajuato). During the D1SG and D2SG phases, the Upper Jurassic–Lower Cretaceous successions of the Arperos Basin piled up, forming a doubly vergent imbricate fan of thrust sheets that accommodated substantial NE–SW shortening. Mylonite microtextures, as well as syntectonic minerals, indicate that the D1SG and D2SG deformation events took place under low greenschist-facies metamorphic conditions. We relate these deformation phases to the progressive NE migration of the Guerrero terrane, which triggered the collapse and closure of the Arperos Basin. During D3SG, the El Paxtle arc assemblage of the Guerrero terrane was tectonically emplaced onto the previously deformed successions of the Arperos Basin. However, D3SG structures indicate that during this deformational stage, the main shortening direction was oriented NW–SE and that contraction was accommodated mostly by SE-vergent ductile thrusts formed under low greenschist-facies metamorphic conditions. We suggest that the top-to-the-SE emplacement of the El Paxtle assemblage may be a result of the tectonic escape of the arc produced by the continuous NE impingement of the Guerrero terrane during its collisional addition to the Mexican mainland.     

Zircon U–Pb geochronology of granitic rocks of the Cordón de Lila and Sierra de Almeida ranges,northern Chile: 30 m.y. of Ordovician plutonism on the western border of Gondwana

In this paper we determined the chronology of Lower Paleozoic arc-related granitic rocks in the Cordón de Lila and Sierra de Almeida ranges, northern Chile, based on new U–Pb ages obtained by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) on single zircons. Plutonism lasted ∼30 m.y., spanning from 490 to 460 Ma (Lower to Mid Ordovician). The obtained ages correspond to the plutonic units' crystallization ages and fit well with the observed contact relationships with their country rocks and mutual intrusion relationships, and also with biostratigraphical data from the sedimentary country rocks. Our geochronological results on the granitic rocks of Cordón de Lila and Sierra de Almeida ranges broadly agree with the known ages of the plutonic rocks in the Argentinian Puna, strengthening the idea that they formed part of the same magmatic arc in the western border of west Gondwana during the Early to Middle Ordovician.     

Tectonique superposée de la zone de faille d’Arakapas (Massif de Troodos,Chypre)

Résumén

Une analyse tectonique du couloir faillé d’Arakapas est menée à partir de la mesure de plus de 3 000 plans striés relevés dans les formations ophiolitiques du Troodos et du Kellaki. Des mesures ont également été effectuées dans les formations du Crétacé terminal — Paléogène et du Néogène, afin de préciser l’âge des différents stades de la déformation. Elles ont été traitées par les méthodes des dièdres droits et du calcul du tenseur des contraintes. Il est montré que le couloir d’Arakapas a été le siège de plusieurs déformations successives. Au Néogène règne un régime en extension qui succède à un jeu décrochant dextre contemporain des compressions d’âge Crétacé terminal à Miocène. Un jeu décrochant antérieur peut-être mis en évidence. Il serait compatible avec le fonctionnement en faille transformante océanique proposé par divers auteurs.     

Pillow lavas of the Sierra Leone test site,Mid-Atlantic Ridge, 5°–7°N: Sr-Nd isotope systematics,geochemistry, and petrology

Based on detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial MAR rift zone between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in the basalts and their glasses. It was shown that some samples show significant nonsystematic differences in the ⁸⁷Sr/⁸⁶Sr ratio between the basalts and their chilled glasses and less significant difference in ?_Nd; higher Sr ratios can be observed both in the glasses and basalts of the same lava fragments. No significant correlation is observed between the isotope characteristics of the samples and their geochemistry; it was also shown that seawater did not affect the Sr and Nd isotope composition of the chilled glasses of the studied pillow lavas. It is suggested that such differences in isotope ratios are related to a small-scale heterogeneity of the melts owing to incomplete homogenization during their rapid ascent to the surface. The heterogeneity of the basaltic melts is explained by their partial contamination by the older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the ocean floor surface. The wider scatter of the Sr isotopic ratios relative to Nd is related to the presence of xenocrysts of calcic plagioclase; correspondingly, the absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all of the studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. The causes of this phenomenon remain unclear.     

Isotopic studies on detrital zircons of Silurian�CDevonian siliciclastic sequences from Argentinean North Patagonia and Sierra de la Ventana regions: comparative provenance

The Silurian?CDevonian siliciclastic sedimentary units known as Sierra Grande Formation and the upper part of the Ventana Group crop out in the eastern area of the North Patagonian Massif and in the Ventania system, toward the Atlantic border of Argentina. Both sequences show similar stratigraphical characteristics and were deposited in a shallow marine platform paleoenvironment. Previous contributions have provided evidence of an allochthonous Patagonia terrane that amalgamate to Gondwana during the Permian?CTriassic. However, other lines of research support a crustal continuity southward, where the Pampean and Famatinian events extend into the northern Patagonia. In either case, the detrital input to the Eo?CMesopaleozoic basins generated along the passive margin tectonic setting should reflect the sedimentary sources. In this contribution, new age data on the sedimentary provenance of these units is provided by U?CPb and Lu?CHf isotopic studies on detrital zircons, using LA-ICP-MS and SHRIMP methodologies. The main sedimentary sources of detrital zircons for both regions are of Cambrian?COrdovician and Neoproterozoic age, while a secondary mode is Mesoproterozoic. Zircons from older cratonic sources (Mesoarchean?CPaleoproterozoic ages) are scarcely recorded. The sample from the upper section of the Devonian Lolén Formation (Ventana Group) shows an important change in the sedimentary provenance, with a main mode of Mesoproterozoic detrital zircons. Detrital source areas considering the orogenic cycles known for southwest South America (Famatinian, Pampean?CBrasiliano, Mesoproterozoic?C??Grenvillian?? and Paleoproterozoic?C??Transamazonian??) are proposed.     

Bilan,fonctionnement et protection du système karstique de la Fontaine de Vaucluse (sud-est de la France)

Résumén

L’aquifère karstique de la célèbre Fontaine de Vaucluse a fait l’objet de nouvelles investigations hydrogéologiqurs dont les résultats portent sur la définition et le fonctionnement du système.

Les données géologiques (lithologie, structure) nous ont permis la délimitation d’un impluvium de 1115 km² dont l’altitude moyenne. 870 m, est confirmée par les traceurs naturels physico-chimiques et isotopiques.

Le bilan hydrique, faute d’un réseau météorologique représentatif, a été calculé à partir d’un modèle climatique par tranches d’altitude, qui utilise les gradients pluviométriques et thermiques régionaux. On montre alors que les 3/4 des pluies efficaces proviennent des zones situées au-dessus de l’altitude moyenne de l’impluvium.

Les études hydrodynamiques et physico-chimiques nous ont permis de démontrer la faible inertie du système malgré sa taille, donc sa bonne karstification, ce qui confirme à l’échelle de l’ensemble les observations spéléologiques ponctuelles du plateau et de l’émergence.

Malgré leur faible apport au débit total, les précipitations estivales marquent la chimie de l’émergence et interrompent ainsi la phase de tarissement.

La contribution de l’eau des réserves au débit est prépondérante : respectivement les 3/4 et les 2/3 des volumes annuels écoulés en 1981 et 1982.

Compte tenu de l’absence de sols et de terrains poreux de couverture, ainsi que du développement de la karstification, la vulnérabilité du système est grande. Aussi une attention particulière doit-elle être portée à l’aménagement de la zone montagnarde de l’impluvium dont le rôle est prépondérant dans l’alimentation du système.     

Structural and biological control of the Cenozoic epithermal uranium concentrations from the Sierra Pe?a Blanca, Mexico

Epithermal uranium deposits of the Sierra Pe?a Blanca are classic examples of volcanic-hosted deposits and have been used as natural analogs for radionuclide migration in volcanic settings. We present a new genetic model that incorporates both geochemical and tectonic features of these deposits, including one of the few documented cases of a geochemical signature of biogenic reducing conditions favoring uranium mineralization in an epithermal deposit. Four tectono-magmatic faulting events affected the volcanic pile. Uranium occurrences are associated with breccia zones at the intersection of fault systems. Periodic reactivation of these structures associated with Basin and Range and Rio Grande tectonic events resulted in the mobilization of U and other elements by meteoric fluids heated by geothermal activity. Focused along breccia zones, these fluids precipitated under reducing conditions several generations of pyrite and uraninite together with kaolinite. Oxygen isotopic data indicate a low formation temperature of uraninite, 45–55°C for the uraninite from the ore body and ～20°C for late uraninite hosted by the underlying conglomerate. There is geochemical evidence for biological activity being at the origin of these reducing conditions, as shown by low δ³⁴S values (～?24.5‰) in pyrites and the presence of low δ¹³C (～?24‰) values in microbial patches intimately associated with uraninite. These data show that tectonic activity coupled with microbial activity can play a major role in the formation of epithermal uranium deposits in unusual near-surface environments.     

New political violence in Africa – Secular sectarianism in Sierra Leone

Mary Douglas (1993) has indicated some of the shared concerns that keep voluntary enclaves together as ‘social movements’. The present paper describes a rather different kind of enclave politics, being concerned with the ‘new violence’ of groups enclaved by social exclusion and force. The paper focuses on the emergence and subsequent development of dissidents in the civil war in Sierra Leone. The RUF was at the outset a tiny but conventional guerrilla force, modelled on one of the militias in the Liberian civil war, and seeking to overthrow a decayed neo-patrimonial ‘one-party’ regime. Gaining little popular support, its members became marooned in forested isolation on the Liberia-Sierra Leone border. Recruits were mainly school children seized by force and ‘converted’ through having to take part in atrocities against rural civilians. Unable to return to the larger society on pain of summary execution, captives have had little option but to adjust to the political fantasies of a violent and unstable leadership. They were enclaved by force. Indicating the predictive strength of Douglas' arguments about the cultural dynamics of the enclave, the subsequent atrocity-drenched story of the RUF suggests that it is not the content of belief that determines institutional culture but vice versa. To survive, the enforced enclave develops the concerns of the classic ‘sect’ - a doctrinaire ‘rationality’ (expressed in crude acts of ‘subtractive’ violence), rejection of magic, decision making by lots, preoccupation with defection, otiose leadership style. If the rest of the world is to come to terms with violent enclave organizations such as the RUF and Algerian GIA it may have to pay more careful attention to the way enclave institutions think. They may not respond ‘rationally’ to the kinds of incentives offered by mainstream groups organised around hierarchical and/or individualist systems of social accountability. This revised version was published online in August 2006 with corrections to the Cover Date.     

Le réseau de fracturation,facteur initial de la karstification des craies dans les collines du Perche: l'exemple du site de la Mansonnière (Bellou-sur-Huisne,Orne, France)

Tectonics has been known to be the initial factor of karstification for a long time, but this has never been demonstrated because of the progressive nature of karstification, which destroys evidence of the preliminary phases. Because of its dual hydrological quality, chalk limestone has conserved a great variety of specific forms illustrating various steps in evolution, from the initial primokarst to the classic well-developed drain.In the Perche hills, very recent research has shown specific endokarstic galleries, filled up by sands and clays. Physical, chemical and X-ray analyses have shown that such infillings result not from a classical fluviatile deposit process, but on the contrary, from an alteration process, underlining the progress of a weathering front into the limestone mass down to the watertable. The infillings are not sedimentary deposits, but represent a kind of “shadow rock”, a chemical in situ transformation of limestone, without any transport of the solid fraction.The genesis results from the progression of weathering fronts located on the tectonic pattern into the input karst. When the weathering front crosses the watertable, the resulting water mixing produces a chemical reaction capable of opening the original joint. This results in infilled galleries, similar to classical karst drains, which have never known fluviatile drainage. This illustrates the first step in normal karstic evolution, just before a water flux drains the gallery, resulting in a new karstification step: the drained passage. The specificity of the region of the Perche hills is this karst evolution stopped in the first step, illustrating the “primorkarst” modelling by the researcher, which has never been described before.The conditions for this genesis are a well-developed tectonic pattern, an absence of thick superficial layers, a high soil permeability, and an absence of superficial drainage (lack of sinkholes).     

The Mammoth Peak sheeted complex,Tuolumne batholith,Sierra Nevada,California: a record of initial growth or late thermal contraction in a magma chamber?

The Mammoth Peak sheeted intrusive complex formed in the interior of a ~7–10 km deep magma chamber, specifically in the Half Dome granodiorite of the Tuolumne batholith, central Sierra Nevada, CA (USA). The sheets consist of fractionated melts with accumulated hornblende, biotite, magnetite, titanite, apatite, and zircon. The accumulation, especially of titanite, had a profound effect on minor and trace elements (Nb, Ta, Ti, REE, U, Th, P, Zr, Hf, etc.), increasing their contents up to five to six times. Our thermal–mechanical modeling using the finite element method shows that cooling-generated tensile stresses resulted in the inward propagation of two perpendicular sets of dilational cracks in the host granodiorite. We interpret the sheeted complex to have formed by a crack-seal mechanism in a high strength, crystal-rich mush, whereby outward younging pulses of fractionated magma were injected into these syn-magmatic cracks at the margin of an active magma chamber. Thermal–mechanical instabilities developed after the assembly of the sheeted complex, which was then overprinted by late ~NW–SE magmatic foliation. This case example provides a cautionary note regarding the interpretation that sheeted zones in large granitoid plutons imply a diking mechanism of growth because the sheeted/dike complexes in plutons (1) may display inverse growth directions from the growth of the overall intrusive sequence; (2) need not record initial chamber construction and instead may reflect late pulsing of magma within an already constructed magma chamber; (3) have an overprinting magmatic fabric indicating the continued presence of melt after construction of sheeted complexes and thus a prolonged thermal history as compared to dikes; and (4) because the scale of the observed sheeted complexes may be small (<1%) in comparison to large homogenous parts of plutons, in which there is no evidence for sheeting or diking. Thus, where extensive dike complexes in plutons are absent, such as in much of the Tuolumne batholith, the application of an incremental diking model to explain chamber construction is at best speculative.     

Neoselachians and Chimaeriformes (Chondrichthyes) from the latest Cretaceous–Paleogene of Sierra Baguales,southernmost Chile. Chronostratigraphic,paleobiogeographic and paleoenvironmental implications

This paper discusses a well-represented fossil record of cartilaginous fishes (Chondrichthyes) from southern South America. The recovered samples allow the recognition of three assemblages with chronostratigraphic and paleogeographic value: i) typical Maastrichtian sharks and rays with affinities to eastern Pacific fauna, including the taxa Ischyrhiza chilensis, Serratolamna serrata, Centrophoroides sp. associated to Carcharias sp., and Dasyatidae indet.; ii) a scarce reworked assemblage of Paleocene–Early Eocene age including the taxa Otodus obliquus and Megascyliorhinus cooperi; iii) a rich assemblage with reworked taxa of Early to Middle Eocene age, together with autochthonous deposited Middle to Late Eocene taxa with close affinities to paleoichthyofaunas recovered from the North Atlantic, represented by Carcharias ‘hopei’, Odontaspis winkleri, Carcharoides catticus, Macrorhizodus praecursor, Carcharocles auriculatus, Striatolamia sp., Striatolamia macrota, Hexanchus agassizi, Notorhynchus sp., Myliobatis sp., Abdounia sp., Pristiophorus sp., Squatina sp., cf. Rhizoprionodon sp., Ischyodus sp., and one new species, Jaekelotodus bagualensis sp. nov. The studied samples include for the first time taxa with well established chronostratigraphic resolutions as well as taphonomic information that help clarifying the age of the fossil-bearing units. In addition, they provide relevant information about the evolution of the Magallanes (=Austral) Basin from the Upper Cretaceous to the Paleogene, suggesting a probable connection with the Quiriquina Basin of south-central Chile during the latest Cretaceous. Finally, the studied assemblages indicate a latitudinal pattern of distribution that provides valuable data on the environmental evolution and temperature of southern South America during the Paleogene.     

Le réseau de fracturation,facteur initial de la karstification des craies dans les collines du Perche : l’exemple du site de la Mansonnière (Bellou-sur-Huisne,Orne, France)

Résumé

Le facteur tectonique est reconnu depuis longtemps comme initiateur de la première phase de karstification, mais sans que ceci n’ait jamais été démontré car les témoignages des premières phases sont détruits par le développement de la karstification. Les qualités hydrologiques mixtes de la craie permettent de conserver une grande variété de formes spécifiques, illustrant différents stades d’évolution, depuis l’originel « primokarst » jusqu’au classique drain polyphasé.

Dans les collines du Perche, de très récentes recherches ont mis en évidence des galeries endokarstiques particulières, comblées de sables et d’argiles. Les analyses physiques, chimiques et rayons X ont démontré que ces remplissages résultent non pas d’un processus classique de dépôt fluviatile, mais au contraire d’un processus d’altération soulignant la progression d’un front d’altération dans la craie, jusqu’à la surface piézo- métrique. Les remplissages ne sont pas des dépôts sédimentaires, mais représentent une sorte de « fantôme de roche », résultant d’une transformation in situ de la craie, sans aucun transport de matière solide.

La genèse résulte de la progression de fronts d’altération installés sur le réseau de fissures de la zone d’introduction (de la surface jusqu’au toit de la nappe). À la rencontre de la nappe de la craie, le mélange des eaux produit une activité chimique qui ouvre la fissure originelle. Des galeries comblées, semblables à celles du karst classique, se forment, sans jamais avoir connu de drainage fluviatile. Ces galeries illustrent le premier degré d’une évolution karstique classique, juste avant qu’un flux draine la galerie constituant un nouveau degré de karstification : le conduit drainé. La spécificité de la région du Perche est que la karstification s’y est arrêtée au premier stade, illustrant le « primokarst » modélisé mais jamais décrit jusqu’alors. Les conditions de cette genèse sont acquises lorsque le site offre un réseau de fractures bien développé, une absence de formations superficielles, une perméabilité élevée du sol, une absence de drainage superficiel (pas de perte concentrée), et un encaissant relativement poreux. © Elsevier, Paris