Southern Domain Research Articles

Recrystallization of high-pressure marble (Sivrihisar, Turkey)

In the Sivrihisar Massif, Turkey, the southern margin of a blueschist facies terrane was affected by heating and deformation associated with later regional (Barrovian) metamorphism, creating a recrystallization front that can be observed in calcite microstructures. Calcite marble records a progression from high-pressure (H P) textures in a northern domain to completely recrystallized (greenschist to amphibolite facies) textures in a southern domain, with intermediate stages of partial recrystallization of the H P textures. H P marble is characterized by rod-shaped calcite (aragonite pseudomorphs) arranged in series with the long axis of each grain oblique to compositional layering; obliquity varies from 50° to nearly parallel, but high angles are most common. In a narrow (~ 200 m thick) transition zone that contains partially retrogressed blueschist and eclogite, the H P texture is partially overprinted by chains of small, equant, recrystallized grains that preserve the crystallographic orientation of the pre-existing rod-shaped calcite, as revealed by electron backscatter diffraction analysis; rod-shaped crystals occur locally. In a ~ 100 m thick zone south of the transition zone, all calcite grains are equant but have a crystallographic preferred orientation. Further south in the Barrovian sequence, recrystallized marble contains equant calcite that is randomly oriented. The similarity in fabric elements (foliation, lineation) of the two domains further shows the effect of Barrovian-related processes on the H P domain, including the rotation of the calcite rods during shearing along foliation planes. White mica 40Ar/ 39Ar ages for phengite in H P marble are ~ 88 Ma, and for muscovite in Barrovian marble are ~ 59 Ma, indicating ~ 30 Ma time interval between initial exhumation and cooling of the H P rocks and final cooling of the rocks that were deformed and heated during the later metamorphic event.

The Cariris Velhos tectonic event in Northeast Brazil

The Borborema Province in northeastern South America is a typical Brasiliano-Pan-African branching system of Neoproterozoic orogens that forms part of the Western Gondwana assembly. The province is positioned between the São Luis-West Africa craton to the north and the São Francisco (Congo-Kasai) craton to the south. For this province the main characteristics are (a) its subdivision into five major tectonic domains, bounded mostly by long shear zones, as follows: Médio Coreaú, Ceará Central, Rio Grande do Norte, Transversal, and Southern; (b) the alternation of supracrustal belts with reworked basement inliers (Archean nuclei + Paleoproterozoic belts); and (c) the diversity of granitic plutonism , from Neoproterozoic to Early Cambrian ages, that affect supracrustal rocks as well as basement inliers. Recently, orogenic rock assemblages of early Tonian (1000–920 Ma) orogenic evolution have been recognized, which are restricted to the Transversal and Southern domains of the Province. Within the Transversal Zone, the Alto Pajeú terrane locally includes some remnants of oceanic crust along with island arc and continental arc rock assemblages, but the dominant supracrustal rocks are mature and immature pelitic metasedimentary and metavolcaniclastic rocks. Contiguous and parallel to the Alto Pajeú terrane, the Riacho Gravatá subterrane consists mainly of low-grade metamorphic successions of metarhythmites, some of which are clearly turbiditic in origin, metaconglomerates, and sporadic marbles, along with interbedded metarhyolitic and metadacitic volcanic or metavolcaniclastic rocks. Both terrane and subterrane are cut by syn-contractional intrusive sheets of dominantly peraluminous high-K calc-alkaline, granititic to granodioritic metaplutonic rocks. The geochemical patterns of both supracrustal and intrusive rocks show similarities with associations of mature continental arc volcano-sedimentary sequences, but some subordinate intra-plate characteristics are also found. In both the Alto Pajeú and Riacho Gravatá terranes, TIMS and SHRIMP U–Pb isotopic data from zircons from both metavolcanic and metaplutonic rocks yield ages between 1.0 and 0.92 Ga, which define the time span for an event of orogenic character, the Cariris Velhos event. Less extensive occurrences of rocks of Cariris Velhos age are recognized mainly in the southernmost domains of the Province, as for example in the Poço Redondo-Marancó terrane, where arc-affinity migmatite-granitic and meta-volcano-sedimentary rocks show U–Pb ages (SHRIMP data) around 0.98–0.97 Ga. For all these domains, Sm–Nd data exhibit T DM model ages between 1.9 and 1.1 Ga with corresponding slightly negative to slightly positive ε Nd ( t ) values. These domains, along with the Borborema Province as a whole, were significantly affected by tectonic and magmatic events of the Brasiliano Cycle (0.7–0.5 Ga), so that it is possible that there are some other early Tonian rock assemblages which were completely masked and hidden by these later Brasiliano events. Cariris Velhos processes are younger than the majority of orogenic systems at the end of Mesoproterozoic Era and beginning of Neoproterozoic throughout the world, e.g. Irumide belt, Kibaride belt and Namaqua-Natal belt, and considerably younger than those of the youngest orogenic process (Ottawan) in the Grenvillian System. Therefore, they were probably not associated with the proposed assembly of Rodinia . We suggest, instead, that Cariris Velhos magmatism and tectonism could have been related to a continental margin magmatic arc, with possible back-arc associations, and that this margin may have been a short-lived (<100 m.y.) leading edge of the newly assembled Rodinia supercontinent .

Circulation and multiple-scale variability in the Southern California Bight

The oceanic circulation in the Southern California Bight (SCB) is influenced by the large-scale California Current offshore, tropical remote forcing through the coastal wave guide alongshore, and local atmospheric forcing. The region is characterized by local complexity in the topography and coastline. All these factors engender variability in the circulation on interannual, seasonal, and intraseasonal time scales. This study applies the Regional Oceanic Modeling System (ROMS) to the SCB circulation and its multiple-scale variability. The model is configured in three levels of nested grids with the parent grid covering the whole US West Coast. The first child grid covers a large southern domain, and the third grid zooms in on the SCB region. The three horizontal grid resolutions are 20 km, 6.7 km, and 1 km, respectively. The external forcings are momentum, heat, and freshwater flux at the surface and adaptive nudging to gyre-scale SODA reanalysis fields at the boundaries. The momentum flux is from a three-hourly reanalysis mesoscale MM5 wind with a 6 km resolution for the finest grid in the SCB. The oceanic model starts in an equilibrium state from a multiple-year cyclical climatology run, and then it is integrated from years 1996 through 2003. In this paper, the 8-year simulation at the 1 km resolution is analyzed and assessed against extensive observational data: High-Frequency (HF) radar data, current meters, Acoustic Doppler Current Profilers (ADCP) data, hydrographic measurements, tide gauges, drifters, altimeters, and radiometers. The simulation shows that the domain-scale surface circulation in the SCB is characterized by the Southern California Cyclonic Gyre, comprised of the offshore equatorward California Current System and the onshore poleward Southern California Countercurrent. The simulation also exhibits three subdomain-scale, persistent ( i.e., standing), cyclonic eddies related to the local topography and wind forcing: the Santa Barbara Channel Eddy, the Central-SCB Eddy, and the Catalina-Clemente Eddy. Comparisons with observational data reveal that ROMS reproduces a realistic mean state of the SCB oceanic circulation, as well as its interannual (mainly as a local manifestation of an ENSO event), seasonal, and intraseasonal (eddy-scale) variations. We find high correlations of the wind curl with both the alongshore pressure gradient (APG) and the eddy kinetic energy level in their variations on time scales of seasons and longer. The geostrophic currents are much stronger than the wind-driven Ekman flows at the surface. The model exhibits intrinsic eddy variability with strong topographically related heterogeneity, westward-propagating Rossby waves, and poleward-propagating coastally-trapped waves (albeit with smaller amplitude than observed due to missing high-frequency variations in the southern boundary conditions).

Provenance of the late Proterozoic to early Cambrian metaclastic sediments of the Sierra de San Luis (Eastern Sierras Pampeanas) and Cordillera Oriental, Argentina

Provenance studies have been performed utilising major and trace elements, Nd systematics, whole rock Pb–Pb isotopes and zircon U/Pb SHRIMP data on metasedimentary rocks of the Sierra de San Luis (Nogolí Metamorphic Complex, Pringles Metamorphic Complex, Conlara Metamorphic Complex and San Luis Formation) and the Puncoviscana Formation of the Cordillera Oriental. The goal was the characterisation of the different domains in the study area and to give insights to the location of the source rocks. An active continental margin setting with typical composition of the upper continental crust is depicted for all the complexes using major and trace elements. The Pringles Metamorphic Complex shows indications for crustal recycling, pointing to a bimodal provenance. Major volcanic input has to be rejected due to Th/Sc, Y/Ni and Cr/V ratios for all units. The εNd (540 Ma) data is lower for the San Luis Formation and higher for the Conlara Metamorphic Complex, as compared to the other units, in which a good consistency is given. This is similar to the T DM ages, where the metapsammitic samples of the San Luis Formation are slightly older. The spread of data is largest for the Pringles Metamorphic Complex, again implying two different sources. The whole rock 207Pb/ 206Pb isotopic data lies in between the South American and African sources, excluding Laurentian provenances. The whole rock Pb–Pb data is almost indistinguishable in the different investigated domains. Only the PMC shows slightly elevated 208Pb/ 204Pb values. Possible source rocks for the different domains could be the Quebrada Choja in the Central Arequipa–Antofalla domain, the Southern domain of the Arequipa–Antofalla basement, the Brazilian shield or southern Africa. Zircon SHRIMP data point to a connection between the Puncoviscana Formation and the Conlara Metamorphic Complex. Two maxima around 600 Ma and around 1000 Ma have been determined. The Nogolí Metamorphic Complex and the Pringles Metamorphic Complex show one peak of detrital zircons around 550 Ma, and only a few grains are older than 700 Ma. The detrital zircon ages for the San Luis Formation show age ranges between 590 and 550 Ma. A common basin can be assumed for the Conlara Metamorphic Complex and the Puncoviscana Formation, but the available data support different sources for the rest of the Complexes of the Sierra de San Luis. These share the diminished importance or the lack of the Grenvillian detrital peak, a common feature for the late Cambrian–early Ordovician basins of the Eastern Sierras Pampeanas, in contrast to the Sierras de Córdoba, the PVF and the Conlara Metamorphic Complex.

ChemInform Abstract: Stereoselective Synthesis of the Southern Domain of Tubelactomicin A by a Tandem Intramolecular Diels—Alder/Lactonization Reaction.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Transpressional tectonics and nappe stacking along the Southern Variscan Front of Morocco

The Southern Variscan Front in the Tinerhir area involves Palaeozoic allochthonous units (Ouaklim and Tilouine units) thrust onto the northern edge of the West African Craton during late Carboniferous time. Illite crystallinity data highlight an anchizonal grade for the Ouaklim Unit, and a diagenesis-anchizone transition for the Tilouine Unit during deformation phase D1. The tectonic stack is crosscut by major dextral reverse faults bounding E–W trending domains of dominant shortening deformation (central domain) and strike-slip deformation (northern and southern domains), later segmented by a network of post-Variscan faults. This complex deformation pattern is the result of kinematic partitioning of dextral transpression along the Southern Variscan Front, coeval with the Neovariscan (300–290 Ma) oblique convergence observed at the scale of the whole Moroccan Variscides. Partitioning of dextral transpression described in the Tinerhir area is consistent with dextral wrench faulting along the Tizi n’ Test Fault, and with Appalachian-style south-directed thrusting in the Tinerhir and Bechar-Bou Arfa areas.

Microstructurally controlled monazite chronology of ultrahigh‐temperature granulites from southern India: Implications for the timing of Gondwana assembly

AbstractUltrahigh‐temperature (UHT) granulite facies rocks from the Achankovil Shear Zone area and the southern domain of the Madurai Granulite Block in South India contain monazite useful for in situ microprobe U–Pb dating. The UHT rocks examined consist of garnet + cordierite (retrograde) + quartz + mesoperthite + biotite + plagioclase + Fe‐Ti oxides ± orthopyroxene ± sillimanite and accessory zircon and monazite. Sillimanite occurs only as inclusions in garnet. Microstructural observations suggest garnet, orthopyroxene, spinel and mesoperthite are products of peak metamorphism. Post‐peak formation of cordierite ± orthopyroxene ± quartz and cordierite + spinel + Fe‐Ti oxides assemblages is also observed. Geothermobarometry on orthopyroxene and garnet‐orthopyroxene bearing assemblages suggest peak UHT conditions of T = 940–1040°C and P = 8.5–9.5 kbar. This was followed by a retrograde stage of 3.5–4.5 kbar and 720 ± 60°C, estimated from garnet‐cordierite assemblages. A small population of rounded, probably detrital, monazites in these rocks yield ages from Meso‐ to Neoproterozoic indicating a heterogeneous source. The youngest associated spot ages are 660–600 Ma suggesting protolith deposition up to ca 600 Ma. In contrast, the vast majority of monazites that crystallized during the latest metamorphic event show late Neoproterozoic to Cambrian ages. Probability‐density plots of monazite age data show a ‘peak’ between 533 and 565 Ma, but this peak need not reflect a particular thermal event. Collating ages from homogenous metamorphic monazites associated with minerals stable at peak P‐T conditions suggests peak metamorphism in these rocks occurred at 580–600 Ma. Together with a re‐evaluation of available data from adjacent granulite blocks in southern India, these data suggest the main metamorphic event coinciding with the suturing of India with the Gondwana amalgam probably occurred 580–600 Ma. The 500–550 Ma ages commonly reported in previous studies might represent post‐peak thermal events.

EMP-monazite age controls on P-T paths of garnet metapelites in the Variscan inverted metamorphic sequence of La Sioule, French Massif Central

AbstractIn the La Sioule region (northern French Massif Central), a Variscan inverted metamorphic sequence is made up by high-grade Upper Gneiss (UGU) and Lower Gneiss Units (LGU) which overlie amphibolite-facies micaschists of a Parautochthonous Unit (PAU). Growth-zoned garnets crystallized in gneisses and micaschists and display different Mg-Fe-Mn-Ca evolution trends in the metamorphic units. Microstructurally-controlled geothermobarometry based on cation exchange and net transfer reactions was used to reconstruct syndeformational prograde P-T paths. The P-T paths do not pass eclogite-facies conditions. Different metamorphic peak temperatures confirm the individual character of the evolution in each of the units. EMP-monazite dating (CHIME method) was used to evaluate the age of the P-T-evolution in the garnet-bearing samples. Monazite ages compiled from numerous matrix grains range between 327 and 343 Ma in single samples. Unimodal distributions of ages signalize only slightly younger (320–330 Ma) monazites in the UGU as in the LGU and PAU (330–340 Ma). The inverted metamorphic stratigraphy can be explained by contractional crustal tectonics. In a northern structural domain this involved a top-to-WNW shearing followed by a first thrusting event at the end of garnet growth in UGU and PAU. In a southern domain, the nappe pile was overprinted by a second event with top-to-SE shearing at low pressures. The bulk of monazites should postdate garnet crystallization and the first thrusting event observed in UGU and PAU. At best the ~340 Ma monazites in the LGU to the south may have crystallized during the second tectonic event. The bulk monazite crystallization appears to be related to a Late-Visean thermal event when the three units were already juxtaposed in a nappe pile. In consequence, metapelite garnet and monazite crystallization in the La Sioule series belong to a Late Devonian – Early Carboniferous prograde-retrograde metamorphism. This metamorphic evolution appears as a second metamorphic cycle subsequent to the Silurian – Early Devonian HP and HT events within the complex frame of the Variscan orogeny in the French Massif Central.

Stereoselective Synthesis of the Southern Domain of Tubelactomicin A by a Tandem Intramolecular Diels-Alder/Lactonization Reaction

This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-070-C00073, KRF-2008-331-C00160) and by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. R01- 2008-000-11094-0) and by KBSI grant (N28043) to G.-S. Hwang.

Early migmatites in the prograde metamorphism zone of gneisses in the northern domain of the Ladoga Region: U-Pb evidence based on monazite

The North Ladoga region is the southeastern fragment of the Svecofennian Belt. The metamorphic rocks of this region were traditionally regarded as constituents of a zonal metamorphosed complex composed largely of the Kalevian (late Paleoproterozoic) turbidite formation [1]. Later data [2] provided evidence for tectonic juxtaposition of the high-temperature core of the complex with less metamorphosed rocks. These relationships made it possible to distinguish the northern (ND) and southern (SD) domains [3]. The relatively low-grade metamorphic rocks of the ND are related to the Sveco-Karelian pericratonic zone; the high-grade metamorphic rocks of the SD, to the Svecofennides that are devoid of the Archean granitic basement (Fig. 1). The geochronological data obtained over recent years furnished convincing evidence that ultrametamorphism of rocks in the SD occurred within a rather narrow time interval (1880‐1870 Ma ago) [4]. The results of geochronological study of migmatites in rocks of the Sveco-Karelian pericratonic zone (ND) are reported in the present communication for the first time. The ND structure is controlled by mantled gneiss domes with the Archean granite‐gneiss basement in their cores. The ND is a domain of zonal metamorphism with southward increasing grade from greenschist to amphibolite facies and poorly developed intrusive magmatism (Fig. 1). The thermobarometric estimates of metamorphic grade in the ND and metamorphic mineral assemblages confirm a decrease in metamorphic grade from south to north. The parameters of peak metamorphism in the amphibolite-facies zone are T = 650‐730 ° C and P = 3.8‐5.0 kbar [5]. The ND rocks also bear indications of retrograde transformations of the andalusite‐muscovite subfacies, which are most intense along the narrow submeridional tectonic zones. The SD is characterized by the predominance of granulite-facies metamorphic rocks. Hence, the SD is a high-temperature granulitic block with numerous gabbro, enderbite, tonalite, and granite intrusions. This block is considered a high-temperature core of the zonal metamorphic complex. The SD is thrust over the ND along the low-angle Meier fault zone [3].

Orogen-parallel tectonic transport: transpression and strain partitioning in the Mauritanides of NE Senegal

Abstract New structural, metamorphic, finite-strain analysis and kinematic data from the southern part of the Mauritanides in Senegal (Bakel area) have revealed a history reflecting different tectonic regimes. The structural framework is expressed in the form of shear-zone systems comprising rectilinear NE–SW trending thrusts and NE–SW-trending ductile sinistral strike-slip faults associated with north–south- to NE–SW-trending transpression belts. The analysis of ductile fabrics reveals the presence of lineation trajectories that show a change from a tangential deformation towards the marginal zones in the Northern and Southern Domains of the belt to sinistral oblique transpression in the Central Domain. The deformation within the belt is strongly partitioned into shear zones distributed across the belt defining structural domains with accompanying large-scale folds and thrusts. This exemplifies wrench-style tectonics dominated by crustal-scale strike-slip shear zones in the orogen core and temporal and spatial progression outwards to the marginal zones. Thus, the Variscan orogeny in the southern part of the Mauritanides belt in Senegal developed through sinistrally oblique transpressive shearing along the borders of the orogen core leading to the exhumation of ultramafic rocks between large shear zones.

Tomographic imaging of the convergent zone in Eastern Taiwan — A subducting forearc sliver revealed?

This paper investigates crustal features of the plate boundary in eastern Taiwan by joint analysis of gravity anomaly and seismic arrival time data. We found two crustal anomalies: (1) two prominent high-velocity/high Poisson's ratio anomalies in the mid- to lower crust beneath the offshore area; (2) several volumes of relatively high-velocity/high Poisson's ratio rocks in the upper- to mid-crust beneath the Central Range. The former is interpreted as the Luzon arc and forearc blocks from east to west. The latter is interpreted as uplift material from the oceanic crust scraped from the Luzon forearc, which forms the core of the Central Range. The features of northward-narrowing in dimension and deepening in depth of the forearc block are revealed in the velocity solution, implying a subducting Luzon forearc fragment in eastern Taiwan. The values of Poisson's ratios and earthquake activity in the northern domain are generally larger than that in the southern domain, suggesting that the crustal accretion process has not been completed in the northern domain of the study area. Our resulting velocity and Poisson's ratio models provide some insight into the tectonic processes presently operating in eastern Taiwan. It's also suggested that the subducting Luzon forarc acts as a backstop against which accreted rocks are thrust, and this contact may have seismogenic potential.

U–Pb ages of plutonic and metaplutonic rocks in southern Borborema Province (NE Brazil): Timing of Brasiliano deformation and magmatism

The Borborema Province of northeastern Brazil is divided into three main domains: northern, central, and southern. Several U–Pb zircon ages of plutons and orthogneisses became available in the recent years in the central and northern domains, but similar results are scarce in the southern domain. This study reports U–Pb dates for single zircon grains from one orthogneiss (Jupi orthogneiss) and two plutons (Cachoeirinha syenitic pluton and Cabanas granite) south of the East Pernambuco shear zone system (EPSZ). The results provide geochronological constraints on the timing of deformation and magmatism in this part of the southern domain and allow correlations with the central domain. The Jupi orthogneiss was emplaced and deformed during development of the regional flat-lying foliation. A 206Pb/ 238U weighted apparent mean age of 606 ± 8 Ma is interpreted as the crystallization age of the protolith of the orthogneiss and consequently the age of high-grade Brasiliano metamorphism. The NNE-trending Cachoeirinha pluton is only locally affected by strike-slip deformation, whereas the ENE-trending Cabanas granite is intensely affected by deformation related to the EPSZ. The 587 ± 8 Ma and 573 ± 4 Ma ages of the Cachoeirinha pluton and Cabanas granite, respectively, bracket the main period of activity of the EPSZ. Tectonomagmatic activity in the study area is similar to the age of Brasiliano events in the central domain, north of the EPSZ. In addition, xenocrystic zircons in the Jupi orthogneiss and Cabanas granite are interpreted as inherited from Paleoproterozoic source rocks, suggesting the presence of widespread reworked old crust in the southern domain, similar to the central domain. These results support the idea that the central and southern domains belonged to the same crustal block before the onset of the Brasiliano orogeny.

Provenance of late Palaeozoic metasediments of the Patagonian proto-Pacific margin (southernmost Chile and Argentina)

In this provenance study of late Palaeozoic metasediments of the Eastern Andean Metamorphic Complex (EAMC) along the south Patagonian proto-Pacific margin of Gondwana, the palaeogeological setting of the continental margin in Devonian–Carboniferous and Permian times is reconstructed. The study is based on detrital heavy mineral contents, chemical compositions of tourmaline grains, and whole rock element and Nd-Sr isotopic compositions. Element and isotopic compositions reveal that Devonian–Carboniferous metaturbidites deposited before the development of a Late Carboniferous–Permian magmatic arc along the margin were mainly fed from felsic, recycled, old continental rocks. The last recycling phase involved erosion of metasediments that were exposed in Patagonia. Feeder systems to the basin cut either through epidote-rich or garnet-rich metasediments. In Permian time, EAMC metaturbidites were deposited next to the evolving magmatic arc and were derived from felsic, crustal rocks. Two provenance domains are recognised. The metasediments of the northern one are chemically similar to those of the Devonian–Carboniferous metasediments. This domain was fed from the metasedimentary host rocks of the magmatic arc. The southern domain probably was fed from the arc proper, as indicated mainly by the dominance of metaplutonic lithic fragments, abundant detrital biotite, and the major element composition of the metasediments.

Rainfall variability in the rainy season of semiarid zone of northeast Brazil (NEB) and its relation to wind regime

AbstractThe rainfall and the 850 hPa wind characteristic of the semiarid interior zone of the northeast Brazil (NEB) are analyzed. The database used are the daily mean u, v, z, T, q and θ, at 850 hPa, 500 hPa and 200 hPa, displayed on a 2.5° × 2.5° grid, from the NCEP/NCAR reanalysis; the daily mean OLR data from the Climate Diagnostics Center, from June 1986 to December 1997; and the gridded daily precipitation (P) analysis data in Brazil (10°S–40°S and 30°W–80°W), produced by the Climate Prediction Center, from 1977 to 1997. Daily time series of the normalized rainfall index (RI) and the normalized 850 hPa wind indexes (i.e. zonal index (ZI), the meridional index (MI) and the u minus v index (UVI)) were calculated for northern (A1: 7°S–10°S and 37°W–44°W) and southern (A2: 10°S–15°S and 40°W–45°W) domains. The RI, ZI, MI and UVI were defined by area‐average of P, u, v, and u minus v, divided by the number of grid points corresponding to each area, respectively. As for regional features this region presents a well‐defined annual cycle of rainfall with rainy seasons from December to April in A1 and from November to March in A2, and a few extreme rainfall events (during a 20‐year period, 26 cases occurred in A1 and 39 cases in A2). Several synoptic scale systems acting alone or simultaneously in the NEB could explain the occurrence of heavy rainfall in this region. The RI is correlated positively with both the ZI and the UVI, and negatively with the MI, which might indicate that when easterly and southerly winds intensify (weaken), the rainfall decreases (increases). From the statistical perspective the UVI appears to be more effective to describe the daily rainfall variability in the NEB semiarid interior region, compared to the ZI or MI. Copyright © 2006 Royal Meteorological Society.

A Pennsylvanian microbial boundstone-dominated carbonate shelf in a distal foreland margin (Picos de Europa Province, NW Spain)

A Pennsylvanian (Bashkirian–Moscovian) high-rising, microbial boundstone-dominated carbonate platform (> 12,000 km 2) developed in the external margin of a Variscan foreland basin. Intensely studied in the northern domain (Cuera area), the stratigraphy and sedimentology of the southern domain (the Picos de Europa Province) is here revisited. Distribution of the Bashkirian facies (Valdeteja Formation) defines a ramp/low-angle platform with an internal zone (central and eastern sectors of the Picos de Europa Province) consisting of huge amounts of massive, cement-rich microbial boundstones, surrounded by an external zone comprising lower-slope boundstone-derived breccias and basinal lime mudstones. From the Bashkirian–Moscovian transition to the early Kasimovian (Picos de Europa Formation), the carbonate platform moderately aggraded acquiring a high-rising and flat-topped geometry and then prograded rapidly into the external zone, where platform-derived carbonate sediment and clay, sourced from the orogen, were deposited. Seismic-scale exposures of the western margin of the shelf show angular-bedding relationship between basin/toe-of-slope, slope and platform-top stratal domains; with slope inclination up to 32° and a topographic relief of 550 m. The platform evolution above described for the Picos de Europa Province significantly differs with respect to the Cuera area, where platform mostly prograded during the Bashkirian and subsequently aggraded during early Moscovian. The evolutionary differences between these two carbonate domains, which are geographically close, are interpreted as changes in tectonic subsidence distribution, acting as the major factor controlling the large-scale platform geometries in the active foreland basin context. Finally, the tectonic overload due to the emplacement of the Cuera thrust sheets during the Moscovian–Kasimovian transition changed the platform growth style from progradational to aggradational and consequently the stacking pattern of the platform-top cycles in the Picos de Europa Province.

Structural analysis of the northern Nagssugtoqidian orogen, West Greenland: an example of complex tectonic patterns in reworked high-grade metamorphic terrains

Structural analysis of the deeply eroded northern flank of the Palaeoproterozoic Nagssugtoqidian orogen shows marked regional variations in both the orientation and type of fabrics, as is characteristic of Precambrian high-grade terrains subjected to polyphase deformation. Here we investigate the relationship between strain, metamorphic grade, and the resulting structural patterns. The study area south of Aasiaat in West Greenland consists of amphibolite- to granulite-grade Archaean orthogneisses and relatively thin supracrustal units. The regional foliation displays a WSW–ENE to SW–NE strike associated with steep to moderate dips towards the WNW or SSE. Lineation trends are WSW–ENE and generally plunge gently towards the WSW. Mesoscopic fold hinges are usually colinear with the regional lineation. A systematic change in the plunge of lineations occurs across the south-western part of the study area. Towards the south, the lineation plunge progressively increases, despite the generally uniform strike of foliation. This southward increase of lineation pitch is typically associated with the transition from L &gt; S or L = S shape fabrics in rocks characterised by a low pitch, to S &gt; L or S fabrics in the zone of moderate to high pitch. The structural patterns point to subdivision of the study area into a southern domain mostly characterised by S or S &gt; L shape fabrics and a moderate to high angle of lineation pitch, and a northern domain showing L &gt; S or L = S fabrics and low angles of lineation pitch. This subdivision corresponds well with the map scale boundary between granulite facies rocks in the south and amphibolite facies rocks farther north. The observed structural pattern may be explained by two alternative tectonic models: (1) northward indentation of the previously cooled granulite block into the rheologically weaker amphibolite domain, and (2) strain partitioning within a mid-crustal transpression zone. In model 2 the northern domain represents a localised zone dominated by strike-slip kinematics, whereas the southern domain shows evidence of mostly coaxial shortening. Recent geochronology supports the indentator model in spite of limited available data. Despite the details and structural complexities of the two tectonic models, the granulite and amphibolite facies domains seem to form autochthonous segments of a crustal section linked by a transitional zone that was only reactivated and reworked during indentation or transpression. The Nagssugtoqidian compression was effectively transferred across this zone towards the northern amphibolite domain that suffered penetrative deformation during the Palaeoproterozoic event. The N–S shortening was accommodated through folding, indentation and/or strike-slip displacements, rather than by thrusting and folding as seen south of the study area.

Toward the Total Synthesis of Spirastrellolide A. Part 1. Strategic Considerations and Preparation of the Southern Domain (I).

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Applicability of thin or thick skinned structural models in a region of multiple inversion episodes; southern South Africa

The deformation of the Cape Fold Belt has been attributed to repeated structural reactivation of a mega-detachment from the late Proterozoic to the Mesozoic (650–65 Ma). Through the integration of onshore cross-sections with observations from the offshore Mesozoic extensional system this study evaluates the applicability of the mega-detachment model. Regional scale cross-sections through the Permian-Triassic Cape Fold Belt reveal that it comprises two main structural domains: a northern domain dominated by northward verging and asymmetric folds; and a southern domain comprising a series of approximately 8 km wavelength box folds. The genesis of these box folds is attributed to motion on underlying high angle (>45°) reverse faults. This variation between north and south in the fold belt is reflected by a similar variation in extensional geometry of the Mesozoic normal faults, as revealed by subsurface data. The normal faults demonstrate a progressive increase in dip from 24° in the north to 60° in the south. Features commonly attributed to thin- and thick- skinned tectonic models are observed in both domains, therefore it is not appropriate to describe the observed deformation as one of the two end members. In addition, the structures are inferred to have undergone at least two stages of inversion, irrespective of dip. This is not predicted by either end-member model.

Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence

This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and dunites crop out at Mt. Arpone and Mt. Musinè. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that <1–5% fractional melting of spinel-facies DMM produced the injected melts. Plagioclase peridotites are hybrid rocks resulting from pre-existing spinel peridotites and variable enrichment of plagioclase and micro-gabbroic material by percolating melts. The impregnating melts attained silica-saturation, as testified by widespread orthopyroxene replacement of olivine, during open system migration in the lithosphere. At Mt. Musinè, coarse granular spinel harzburgite and dunite bodies replace the plagioclase peridotites. Most of these replacive, refractory peridotites have interstitial magmatic clinopyroxene with trace element compositions in equilibrium with MORB, while some Cpx have REE-depleted patterns suggesting transient geochemical features of the migrating MORB-type melts, acquired by interaction with the ambient plagioclase peridotite. These replacive spinel harzburgite and dunite bodies are interpreted as channels exploited for focused and reactive migration of silica-undersaturated melts with aggregate MORB compositions. Such melts were unrelated to the silica-saturated melts that refertilized the pre-existing plagioclase peridotites. Finally, MORB melt migration occurred along open fractures, now recorded as gabbroic dikes. Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.