Mesozoic Time Research Articles

Phanerozoic magmatic tempos of North China

Detrital zircons from northeast China record cyclic magmatism along the northern and eastern margins of the North China block during late Paleozoic time and Mesozoic time, respectively. The late Paleozoic zircons record three magmatic flare-ups with a period of ∼60 m.y. that occurred within a magmatic arc constructed along the Paleoasian (northern) margin of North China, and are accompanied by negative εHf(t) excursions representing shortening and increased crustal melting over the duration of each flare-up. The intervening magmatic lulls are accompanied by rapid positive εHf(t) excursions signifying influxes of juvenile magma into the arc, probably during extension and foundering of underlying melt residua. The lack of similar isotopic patterns in zircons derived from contemporaneous intrusions into older continental settings inboard of the arc indicate that this process was restricted to the arc itself. Mesozoic magmatism in North China occurred along the Paleo-Pacific margin following closure of the Paleoasian Ocean, and exhibits a ∼50 m.y. periodicity that is out-of-phase with that of the Paleozoic arc. Although the tectonic setting of North China during Mesozoic time is complex and still controversial, it is possible that this younger periodicity is governed by similar processes as those that dominated the Paleozoic arc. This is a testable hypothesis that warrants further attention. Crustal shortening was widespread in North China during Mesozoic time, and documented lithosphere removal events in eastern North China occurred during the Mesozoic magmatic lulls. Lithospheric thickening/foundering cyclicity, well-documented in Cordilleran arc systems, may be a common process in continental arcs through space and time.

The inheritance of a Mesozoic landscape in western Scandinavia

In-situ weathered bedrock, saprolite, is locally found in Scandinavia, where it is commonly thought to represent pre-Pleistocene weathering possibly associated with landscape formation. The age of weathering, however, remains loosely constrained, which has an impact on existing geological and landscape evolution models and morphotectonic correlations. Here we provide new geochronological evidence that some of the low-altitude basement landforms on- and offshore southwestern Scandinavia are a rejuvenated geomorphological relic from Mesozoic times. K-Ar dating of authigenic, syn-weathering illite from saprolitic remnants constrains original basement exposure in the Late Triassic (221.3±7.0–206.2±4.2 Ma) through deep weathering in a warm climate and subsequent partial mobilization of the saprolitic mantle into the overlying sediment cascade system. The data support the bulk geomorphological development of west Scandinavia coastal basement rocks during the Mesozoic and later, long-lasting relative tectonic stability. Pleistocene glaciations played an additional geomorphological role, selectively stripping the landscape from the Mesozoic overburden and carving glacial landforms down to Plio–Pleistocene times. Saprolite K-Ar dating offers unprecedented possibilities to study past weathering and landscape evolution processes.

Palaeotethys-related sediments of the Karaburun Peninsula, western Turkey: constraints on provenance and stratigraphy from detrital zircon geochronology

Detrital zircon U–Pb geochronology of 15 Late Palaeozoic to Early Mesozoic siliciclastic sandstones from the Karaburun Peninsula in western Turkey determines maximum sedimentation ages, identifies possible source areas, and anchors the study area within the Palaeotethyan realm. Siliciclastic sandstones yielded ages from Triassic to Archean with major input from Palaeozoic to Neoproterozoic sources and very few Mesoproterozoic zircons. The youngest age groups set the new limit of the maximum depositional ages to Late Carboniferous–Early Permian for the Kucukbahce and Dikendagi formations. Detrital zircons from Triassic sandstones are mainly Neoproterozoic and Palaeozoic in age. Zircons from the Scythian–Anisian Gerence Formation are predominantly Devonian and Carboniferous in age, while also Permian and Triassic zircon grains occur in the Carnian–Rhaetian Guvercinlik Formation. According to the zircon age populations and the data available from possible source regions, the Karaburun siliciclastic sediments, with the exception of two samples from the Dikendagi Formation, record sediment supply from units located at the southern margin of Eurasia during Late Palaeozoic and Early Mesozoic times. This interpretation is in agreement with palaeotectonic reconstructions for the closely related Greek islands of Chios and Inousses. The presence of Devonian accompanied by Carboniferous zircons in some of the Karaburun samples reveals similarities with Karakaya Complex sandstones of the Sakarya Zone in NW Turkey.

Zircon U–Pb ages and Sr–Nd isotope ratios for the Sirstan granitoid body, NE Iraq: Evidence of magmatic activity in the Middle Cretaceous Period

The Sirstan granitoid (SG), comprising diorite and granodiorite, is located in the Shalair Valley area, in the northeastern part of Iraq within the Sanandaj–Sirjan Zone (SSZ) of the Zagros Orogenic Belt. The U–Pb zircon dating of the SG rocks has revealed a concordia age of 110Ma, which is interpreted as the age of crystallization of this granitoid body during the Middle Cretaceous. The whole-rock Rb–Sr isochron data shows an age of 52.4±9.4Ma (MSWD=1.7), which implies the reactivation of the granitoid body in the Early Eocene due to the collision between the Arabian and Iranian plates. These rocks show metaluminous affinity with low values of Nb, Ta and Ti compared to chondrite, suggesting the generation of these rocks over the subduction zone in an active continental margin regime. The SG rocks are hornblende-bearing I-type granitoids with microgranular mafic enclaves. The positive values of ɛNd (t=110Ma) (+0.1 to +2.7) and the low (87Sr/86Sr)i ratios (0.7044 to 0.7057) indicate that the magma source of the SG granitoids is a depleted subcontinental mantle. The chemical and isotope compositions show that the SG body originated from the metasomatic mantle without a major role for continental contamination. Our findings show that the granitoid bodies distributed in the SSZ were derived from the continuous Neo-Tethys subduction beneath the SSZ in Mesozoic times and that the SSZ was an active margin in the Middle Cretaceous.

Late Jurassic, high Ba–Sr Linglong granites in the Jiaodong Peninsula, East China: lower crustal melting products in the eastern North China Craton

AbstractThe Jurassic Linglong granites, intrusive into the North China Craton (NCC) in eastern China, provide a critical record of the first major episode of lithospheric-scale extension and magmatism in NE China during Mesozoic time. Our U–Pb zircon dating reveals that the Linglong granites were emplaced during 161–158 Ma, shortly after the inception of a shallow subduction of the Palaeo-Pacific plate beneath East Asia during Middle Jurassic time. These granites have high alkali contents (K2O + Na2O = 8–9 wt%), low MgO and Mg no. values and variable Cr–Ni abundances. Their relatively high Ba and Sr concentrations, relatively low heavy rare Earth element (HREE) and strongly fractionated REE patterns characterize them as high Ba–Sr granites. The negative whole-rockεNd(t) values ranging from −22.4 to −10.9 and wide-ranging zirconεHf(t) values of −39.1 to −1.5 suggest that magmas of the Linglong granites were produced by partial melting of a garnet-amphibolite-bearing lower crust of the Jiaobei Terrane and by re-melting of the Triassic ultrahigh-pressure (UHP) metamorphic rocks and alkaline suites of the Sulu Terrane. The occurrence in the granitic rocks of inherited zircons of the Neoarchaean, Palaeoproterozoic, Neoproterozoic, Palaeozoic and Triassic ages suggests that magmas of the Linglong granites interacted with the ancient crust in these terranes during their ascent. Asthenospheric upwelling, induced by the steepening and rapid rollback of the Palaeo-Pacific slab during Late Jurassic time, provided the heat source for the inferred lower crustal melting. Trench migration and thermal weakening of the crust caused extensional deformation and thinning in the eastern part of the NCC.

Syntectonic emplacement of the Triassic biotite-syenogranite intrusions in the Taili area, western Liaoning, NE China: Insights from petrogenesis, rheology and geochronology

The North China Craton (NCC) is one of the oldest cratons in the world, and it recently becomes a hot study area because of large volumes of Mesozoic intrusions associated with lithospheric thinning contributing to cratonic destruction in late Mesozoic times. However, the timing of initial thinning and destruction is still controversial. The Taili area, western Liaoning Province, in the northeastern part of the NCC well exposes the Archean basement rocks and the Mesozoic magmatic rocks with variable plastic deformation. This study focuses on the syntectonic emplacement of the Triassic biotite-syenogranite intrusions, in order to understand their petrogenesis, timing as well as the geological significance. Zircon LA-ICP-MS U-Pb ages reveal that the biotite-syenogranites formed between 246 and 191Ma, and contain many ancient (2564–2317Ma) zircon xenocrysts. Geochemical data suggests that the biotite-syenogranites display an adakitic affinity with high Sr/Y=135–167 and (La/Yb)N=48–69, as well as negligible Eu anomalies (δEu=0.87–0.94), high negative zircon εHf(t) values (−15.5 to −21.5) and ancient TDM2 ages (2246–2598Ma). This data suggests that the parent magmas were generated from partial melting of thickened Archean lower crustal rocks probably due to the bidirectional amalgamation of the NCC with the NE China micro-blocks and the Yangtze Craton in its north and south, respectively. In the middle part of the Taili area, magmatic fabrics are well preserved in the biotite-syenogranite intrusion characterized by the strong preferred orientation of biotite and hornblende crystals, which parallel to the intrusion margin and are slightly oblique to the gneissosity of the sheared host Neoarchean granitic gneisses. The quartz grain size piezometer suggests that the paleo-differential stresses weaken toward to the central part of the intrusion, ranging from 21.40–22.22MPa to 16.74–19.34MPa, during quartz crystallization in the emplacement stage. This allow deduce much higher strain rates in the center (1.26×10−11–2.24×10−9s−1) than at the margin (9.07×10−12–1.31×10−9s−1) of the pluton. These observations are interpreted by the rheological behavior of magma during the magmatic “pipe” flow. The adakitic source melts ascended through the conduits along weak NE-trending sinistral shear zones, and emplaced at the shallower depth of ∼16km before Early Jurassic (∼190Ma). The biotite-syenogranites were still in a semisolid state, when garnet-bearing granitic aplites injected at ∼220Ma. This stage records elongate (constrictional) strain under the sinistral shear stresses, particularly in quartz grains occurring in the margin of intrusions. In combination with previous studies, an exhumation rate of the NCC’s Archean basement (from ∼25km to ∼11km in depth) is calculated as initial low exhumation rate of ∼4.0mm/kyr from Neoarchean to Late Triassic, and subsequent a rapid exhumation process of ∼63mm/kyr between Late Triassic to Early Cretaceous. All the results presented here allow us to consider the geodynamic evolution of the eastern NCC and constrain the onset of lithospheric thinning and cratonic destruction of the NCC as early as Middle Triassic (∼240Ma) triggered by the amalgamation of adjacent blocks. It developed prosperously since Late Triassic, due to the oblique subduction of the Paleo-Pacific Plate.

Behavioural studies on the pollen grains of Pinus roxburghii collected from Lucknow, India- A report

We have long recognized that natural climatic shifts influence the development of plants on earth. These slow temperature fluctuations have resulted in either the extinction or evolution of various species. A careful study of the comparative morphology of the living and fossil type suggests that the modern coniferous families originated independently from a group of extinct conifers called the transition conifers by Florin. The conifers reached their climax in the mid–Mesozoic times and have been on the decline ever since. Pinaceae is one such family whose pollen grains have been located as long back as in the Gondwana flora and have demonstrated morphological variations since then due to climatic changes.
 Pinus is the most dominating genus of the family Pinaceae in gymnosperms. Pine grains are large due to their sacs or bladders, which make them one of the easiest pollen grains to identify. They are rich in non–enzymatic anti–oxidants, like provitamin A, B complex, C, D and E plus a host of minerals and amino acids. Apparently pine pollen is also a great defense against radioactive caesium that is appearing in dairy and other foods in the U.S.
 The present study examines the changes occurring in one of the most sensitive phases of plant development in the living genus of Pinus, i.e. the behaviour of pollen, found to be dependent on number of genetic or environmental factors. The major environmental factors contributing to this abnormal behaviour include light intensity and quality, temperature and moisture and level of pollutants in atmosphere.
 During present study, the Pinus cones were collected from five experimental areas of Lucknow that exhibited different level of deformities probably due to different levels of environmental stress or other genetic reasons. The clusters were unique in having deformed cones on the side facing the road while they were near normal on the side away from road indicating that the area around the roadside being more polluted and may be playing a detrimental role in cone morphology. Pollen of normal cones were bilateral, analept, usually bisaccate, spherical, slightly elliptical with verrucate corpus and sacci having pila, normal average size being 60μ–65μ including sacci and sacci attached almost full width of corpus, exine thick in proximal part of reticuloid corpus. While pollen grains of abnormal cones show abnormalities in their size, shape, size and number of wings, size and shape of corpus and development of pollen tube within microsporangium. The abnormalities were observed using scanning electron microscopic studies and its percentage was calculated using a light microscope.

Provenance analysis of Jurassic sandstones from the Otlaltepec Basin, southern Mexico: Implications for the reconstruction of Pangea breakup

The structural evolution that accompanied the breakup of Pangea during Jurassic time has been constrained in Mexico only at the regional scale on the basis of global plate tectonics and geometric considerations. According to available regional-scale reconstructions, the Jurassic tectonic evolution of Mexico was characterized by: (1) anticlockwise rotation of the Yucatan block along NNW-trending dextral faults and (2) sinistral block motions along W- to WNW-trending faults, which are geometrically needed to restore southern and central Mexico to the northwest of its present position during early Mesozoic time and avoid the overlap between North and South America in the reconstruction of Pangea. Reports of W- to WNW-trending sinistral faults that were active in Mexico during Jurassic time are presently few, and the existence, extension, and age of some of these structures have been questioned by many authors. In this work, we present the provenance analysis from a Jurassic clastic succession deposited within the Otlaltepec Basin in southern Mexico. Whole-rock sandstone petrography integrated with chemical analysis of detrital-garnet and U-Pb detrital-zircon geochronology documents that the analyzed stratigraphic record was deposited during rapid exhumation of the Totoltepec pluton along the Matanza fault, which is a W-trending sinistral normal fault that extends along the southern boundary of the Otlaltepec Basin. U-Pb zircon ages and biostratigraphic data bracket the age of the Matanza fault between 163.5 ± 1 and 167.5 ± 4 Ma. This indicates that the Matanza fault was involved in the crustal attenuation that accompanied the breakup of Pangea and that sinistral motion of continental blocks along W-trending structures was taking place in southern Mexico as predicted by global plate tectonic reconstructions.

Late Paleozoic and Early Mesozoic rare-metal granites in Central Mongolia and Baikal region: review of geochemistry, possible magma sources and related mineralization

The Central Asian Orogenic Belt (CAOB) was a scene of intense granitoid magmatism during the Phanerozoic with formation of vast batholiths: Angara-Vitim and Daurian-Khentei. In the Late Paleozoic and Early Mesozoic times, the peripheral zones of batholiths underwent granitic magmatism associated with rare-metal mineralization. Petrological and geochemical studies show that the rare-metal Li–F granites formed, with a gap about 100 My, large igneous provinces of the Mongol-Okhotsk Belt. Late Paleozoic rare-metal granites build a series of multiphase plutons in the Baikal region (e.g. Kharagul 318 ± 7 Ma, Bitu-Dzhida 311 ± 10 Ma and Urugudei 321 ± 5 Ma). The early medium-grained biotite granites and leucogranites were followed by topaz-bearing microclineand amazonite–albite granites and a series of dikes. The Early Mesozoic epoch was marked by the formation of the Daurian-Khentei Batholith (230–190 Ma) in the center of area and rifting zones with alkaline and rare-metal granite plutons on the peripheries. In contrast to the Late Paleozoic, small Early Mesozoic intrusions (e.g., Avdar Pluton ~10 km2, 212–209 Ma) of rare-metal Li–F granites within the Avdar-Khoshutul series of granitoids coexisted with sizable plutons (e.g., Janchivlan Pluton ~70 km2, 227–195.3 Ma). Rare-metal Li–F granites of the Janchivlan Pluton produced small domal intrusions composed of microcline–albite, amazonite–albite and albite–lepidolite granites. The Sn–Ta–Nb mineralization is associated with albite–lepidolite granites. The rare-metal granites of the Baikal region and Central Mongolia of contrasting ages show an identical geochemical signature of Li–F granites. It is expressed by increase in F, Li, Rb, Cs, Sn, Be, Ta and Pb and decrease in Sr, Ba, Zn, Zr, Th and U contents in course of multiphase intrusions formation. The geochemical data confirm the magmatic model for genesis of the studied rare-metal Li–F granites. However, this process of magma differentiation was terminated with formation of albitites, microclinites and muscovite greisens. The whole-rock geochemistry and isotopic composition of the granites points to the Precambrian crust of the Baikal region (T2DМ = 1.0–1.3 Ga) as the most likely source. We propose the formation of the initial granitic melts due to anatexis of the higher levels of the continental crust, with fluids released during granulite-facies metamorphism in the lower crust. The rare-metal Li–F granites of the studied provinces are intraplate formations geochemically different from the Early Paleozoic collisional granitoids. This could be caused by the influence of deep-seated source on the occurrence of rare-metal magmatism.

Geomorphology and topography of relict surfaces: the influence of inherited crustal structure in the northern Scandinavian Mountains

Low-relief surfaces in northern Norway are mapped and analysed to explore (1) whether surfaces were once continuous and possibly correlative, recording an ancient, relict landscape, (2) whether the distribution of (now disrupted) surfaces reveals a tectonic history related to rifting of the North Atlantic and (3) how topography changes across the transition between the North Atlantic and Barents Sea margins. Elevation contours on surfaces, a smoothed fit to mean elevations and histograms of elevation show three distinct, coast-parallel zones onshore northern Norway, and a different pattern in coastal Lofoten–Vesterålen. A sharp transition from continuous surfaces to lower, discretely stepped, discontinuous surfaces is located where the crust thins rapidly to the NW from >39 to <25 km. In margin-parallel transects, elevations remain high for >100 km north of the Senja Fracture Zone and then decrease gradually further NE. The spatial pattern suggests that a continuous, low-relief, relatively low-elevation surface formed in late Mesozoic time and was preserved despite modification by Cenozoic faulting, uplift and slow erosion. Scandinavia's present-day topographic envelope reflects a crustal strength profile set up during earlier hyperextension and maintained by slow Neogene erosion rather than resulting from opening of the North Atlantic or caused by glacial erosion. Supplementary material: A map of polygons of all surface heights within the study area and data on the location of median centres of polygons, assigned attributes and selected zonal statistics are available at https://doi.org/10.6084/m9.figshare.c.3312960

An early Eocene fish fauna from the Bitter Creek area of the Wasatch Formation of southwestern Wyoming, U.S.A.

ABSTRACTEarly Eocene fluvial ichthyofaunas of Wyoming are relatively poorly known compared with the better-preserved lake deposits of the Green River Formation. We describe the teleost fishes from a single floodplain locality in the main body of the Wasatch Formation that immediately predates the formation of Lake Gosiute, in the Washakie Basin of Wyoming. This assemblage was deposited during the Graybullian substage of the Wasatchian Stage, corresponding to the onset of the rapid climatic warming leading to the Early Eocene Climatic Optimum. In addition to a lepisosteiform and an amiid, the locality has a teleost ichthyofauna comprising Diplomystus (Ellimmichthyiformes), a gonorynchiform probably representing Notogoneus, amblyopsid-like percopsiforms that may represent up to three taxa, and perciforms among which are probably an indeterminate centrarchid and ‘Priscacara.’ The fauna demonstrates that many of the Green River Formation fish taxa were already present in fluvial environments prior to the formation of the Green River lakes, indicating that the subsequent climate warming had little effect on these fishes. Some of the elements recognized here as probably representing Notogoneus were previously recovered from Late Cretaceous deposits from Utah to Alberta, suggesting that this gonorynchid was wide-ranging in North American fluvial environments since Mesozoic times, as were several other taxa identified here. Furthermore, the ichthyofauna indicates that the depositional environment was well vegetated and well oxygenated. These waters were shallow, with gentle flow strengths, which, together with the high oxygen content, suggests that the sediments of the locality were deposited in small ponds connected to an active river system.Citation for this article: Divay, J. D., and A. M. Murray. 2016. An early Eocene fish fauna from the Bitter Creek area of the Wasatch Formation of southwestern Wyoming, U.S.A. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1196211.

Joint development and tectonic stress field evolution in the southeastern Mesozoic Ordos Basin, west part of North China

Major joint sets trending E-W (J1), ENE-WSW (J2), NE-SW (J3), N-S (J4), NNW-SSE (J5), NNE-SSW (J6), NW-SE (J7), and WNW-ESE (J8) respectively are recognized in Mesozoic strata within the southeast of Ordos Basin. Among them, the J1, J2 and J3 joint sets are systematic joints, while the other five joint sets (J4, J5, J6, J7, J8) are nonsystematic joints. There are three groups of orthogonal joint systems (i.e. J1 and J4 sets, J2 and J5 sets, and J6 and J8 sets) and two groups of conjugate shear fractures (ENE-WSW and NNE-SSW, ENE-WSW and ESE-WNW) in the study area. Joint spacing analysis indicates that: (1) layer thickness has an effect on the joint spacing, but the correlation of joint spacing and layer thickness is low; (2) joint density of systematic joints is greater than nonsystematic joints, and the joint density of a thin layer is also greater than that of a thick layer; and (3) the joints of Mesozoic strata in the basin are the result of tectonic events affected by multiple stress fields. All these joints in the Mesozoic strata are formed in the two main tectonic events since Late Mesozoic times. One is the westward subduction of the Pacific Plate beneath the Eurasia Plate, which formed the approximately E-W-trending compressive stress field in the China continent. The trends of the J1 joint set (E-W) and the bisector of conjugate shear fractures composed of ENE-WSW and ESE-WNW fractures are all parallel to the trend of maximum compressive stress (E-W). The other stress field is related to the collision of the Indian and Eurasian Plates, which formed the NE-SW-trending compressive stress field in the China continent. The trends of the J3 joint set and bisector of conjugate shear fractures composed of ENE-WSW and NNE-SSW fractures are all parallel to the trend of maximum compressive stress (NE-SW). Finally, we conclude that the J1 and J4 sets are formed in the E-W-trending compressive stress field, and the J2, J3, J5, J6, J7 and J8 sets are formed in the NE-SW-trending stress field.

Basement composition and basin geometry controls on upper-crustal deformation in the Southern Central Andes (30–36°S)

AbstractDeformation and uplift in the Andes are a result of the subduction of the Nazca plate below South America. The deformation shows variations in structural style and shortening along and across the strike of the orogen, as a result of the dynamics of the subduction system and the features of the upper plate. In this work, we analyse the development of thin-skinned and thick-skinned fold and thrust belts in the Southern Central Andes (30–36°S). The pre-Andean history of the area determined the formation of different basement domains with distinct lithological compositions, as a result of terrane accretions during Palaeozoic time, the development of a widespread Permo-Triassic magmatic province and long-lasting arc activity. Basin development during Palaeozoic and Mesozoic times produced thick sedimentary successions in different parts of the study area. Based on estimations of strength for the different basement and sedimentary rocks, calculated using geophysical estimates of rock physical properties, we propose that the contrast in strength between basement and cover is the main control on structural style (thin- v. thick-skinned) and across-strike localization of shortening in the study area.

Tectono-stratigraphic evolution and crustal architecture of the Orphan Basin during North Atlantic rifting

The Orphan Basin is located in the deep offshore of the Newfoundland margin, and it is bounded by the continental shelf to the west, the Grand Banks to the south, and the continental blocks of Orphan Knoll and Flemish Cap to the east. The Orphan Basin formed in Mesozoic time during the opening of the North Atlantic Ocean between eastern Canada and western Iberia–Europe. This work, based on well data and regional seismic reflection profiles across the basin, indicates that the continental crust was affected by several extensional episodes between the Jurassic and the Early Cretaceous, separated by events of uplift and erosion. The preserved tectono-stratigraphic sequences in the basin reveal that deformation initiated in the eastern part of the Orphan Basin in the Jurassic and spread towards the west in the Early Cretaceous, resulting in numerous rift structures filled with a Jurassic–Lower Cretaceous syn-rift succession and overlain by thick Upper Cretaceous to Cenozoic post-rift sediments. The seismic data show an extremely thinned crust (4–16 km thick) underneath the eastern and western parts of the Orphan Basin, forming two sub-basins separated by a wide structural high with a relatively thick crust (17 km thick). Quantifying the crustal architecture in the basin highlights the large discrepancy between brittle extension localized in the upper crust and the overall crustal thinning. This suggests that continental deformation in the Orphan Basin involved, in addition to the documented Jurassic and Early Cretaceous rifting, an earlier brittle rift phase which is unidentifiable in seismic data and a depth-dependent thinning of the crust driven by localized lower crust ductile flow.

Eustatic and tectonic change effects in the reversion of the transcontinental Amazon River drainage system

ABSTRACT: The development of the transcontinental Amazon River System involved geological events in the Andes Chain; Vaupés, Purus and Gurupá arches; sedimentary basins of the region and sea level changes. The origin and age of this river have been discussed for decades, and many ideas have been proposed, including those pertaining to it having originated in the Holocene, Pleistocene, Pliocene, Late Miocene, or even earlier times. Under this context, the geology of the sedimentary basins of northern Brazil has been analyzed from the Mesozoic time on, and some clarifications are placed on its stratigraphy. Vaupés Arch, in Colombia, was uplifted together with the Andean Mountains in the Middle Miocene time. In the Cenozoic Era, the Purus Arch has not blocked this drainage system westward to marine basins of Western South America or eastward to the Atlantic Ocean. Also the Gurupá Arch remained high up to the end of Middle Miocene, directing this drainage system westward. With the late subsidence and breaching of the Gurupá Arch and a major fall in sea level, at the beginning of the Late Miocene, the Amazon River quickly opened its pathway to the west, from the Marajó Basin, through deep headward erosion, capturing a vast drainage network from cratonic and Andean areas, which had previously been diverted towards the Caribbean Sea. During this time, the large siliciclastic influx to the Amazon Mouth (Foz do Amazonas) Basin and its fan increased, due to erosion of large tracts of South America, linking the Amazon drainage network to that of the Marajó Basin. This extensive exposure originated the Late Miocene (Tortonian) unconformity, which marks the onset of the transcontinental Amazon River flowing into the Atlantic Ocean.

Paleomagnetism of Devonian dykes in the northern Kola Peninsula and its bearing on the apparent polar wander path of Baltica in the Precambrian

Mafic dykes and large alkaline and carbonatite intrusions of Middle-Late Devonian age are widespread on the Kola Peninsula in NE Fennoscandia. These magmatic rocks are well characterized with petrographic, geochemical and geochronological data but no paleomagnetic results have been reported yet. We studied dolerite dykes from the northern part of the Peninsula and isolated three paleomagnetic components in these rocks. A low-temperature component is aligned along the present-day field, while a major constituent of natural remanent magnetization is an intermediate-temperature component (Decl.=79.6°, Inc.=78.5°, α95=5,9°, N=17 sites) that is present in most Devonian dykes but is found in some baked metamorphic rocks and Proterozoic dykes too. Finally, a primary Devonian component could be reliably isolated from two dykes only. Rock-magnetic studies point to presumably primary low-Ti titanomagnetite and/or pure magnetite as the main remanence carriers but also reveal alteration of the primary minerals and the formation of new magnetic phases.The directions of a major component differ from the Middle Paleozoic reference data for Baltica but closely match those for the 190±10Ma interval recalculated from the apparent polar wander path of the craton. We assume that this Early Jurassic component is a low-temperature overprint of chemical origin. The main impact of the new results is not to mid-Paleozoic or Early Mesozoic times but to much older epochs. Analysis of paleomagnetic data shows that the directionally similar remanences are present in objects with the ages ranging from 500Ma to ~2Ga over entire Fennoscandia. Hence we argue that an Early Jurassic remagnetization is of regional extent but cannot link it to a certain process and a certain tectonic event. If true, this hypothesis necessitates a major revision of the APWP for Baltica over a wide time interval.

Refertilization of lithospheric mantle beneath the Yangtze craton in south-east China: Evidence from noble gases geochemistry

The Yangtze craton (YC), in eastern China, is one of the oldest cratons in the world and is characterized by a complex tectonic and geodynamic evolution. This evolution regards most of the eastern China craton, which since Mesozoic time has undergone significant thinning (>200km) of Archean lithosphere. This thinning favored the refertilization of the old refractory subcontinental lithospheric mantle (SCLM) by the upwelling of younger fertile asthenosphere. Whether this feature is localized only beneath certain areas of eastern China or is a more widespread characteristic of the mantle, including the YC, is a matter of debate.In order to constrain the history of the YC SCLM, we have measured the He- and Ar-isotopic compositions of fluid inclusions hosted in mantle xenoliths in the Lianshan area, which is part of the poorly investigated YC in south-east China. We also report new mineral chemistry and trace element compositions of clinopyroxenes from the same suite of samples, for comparison with noble gases. Two distinct types of xenoliths can be identified: Type 1, characterized by mantle-like He-isotopic (3He/4He) ratios (up to 9.1Ra), represents fragments of a fertile lithospheric mantle; Type 2, showing 3He/4He values in the SCLM range (3He/4He<7Ra), represents shallow relicts of a refractory mantle. The patterns of rare-earth elements as well as the Y and Yb concentrations in the clinopyroxenes normalized to primitive mantle (YN and YbN, respectively) indicate that fractional partial melting might have affected the local mantle by <3% in Type 1 and up to 20% in Type 2 xenoliths from Lianshan, respectively. The range of 4He/40Ar* (40Ar* is corrected for atmospheric contamination) ranges from 4.9×10−4 to 3.6×10−1, which is below the typical production ratio of the mantle (4He/40Ar*=1–5); this range is however compatible with this fractional partial melting. The variable 3He/4He and 4He/40Ar* values in Lianshan xenoliths suggest that the local mantle source was also influenced by kinetic fractionation, possibly triggered by metasomatic melts. Metasomatism associated with carbonatitic melts, together with fluxing by CO2-rich fluids, have permeated the mantle beneath Lianshan, generating the observed decoupling between noble gases and trace elements. The interpretative framework is also applicable for other mantle xenoliths from eastern China, indicating that the refertilization of the SCLM by ascending mantle-like melts is common also to YC, which can be identified using noble gases.

The St. Gallen Fault Zone: a long-lived, multiphase structure in the North Alpine Foreland Basin revealed by 3D seismic data

The St. Gallen Fault Zone (SFZ) is a system of major NNE–SSW striking normal faults within the North Alpine Foreland Basin (NAFB), just west of the city of St. Gallen. It used to be only roughly known from 2D seismic data, locally displaying offsets of up to 300 m at the level of the Mesozoic strata. We present a detailed structural interpretation of a recently acquired 3D seismic dataset that reveals the occurrence of multiphase tectonic activity along the SFZ from at least the Late Paleozoic to the early Oligocene, and possibly even later. We can show that the SFZ roots in extensional basement structures that bound small Permo-Carboniferous grabens. Thickness changes in the younger sediments above these Paleozoic grabens indicate several phases of tectonic subsidence during the Triassic and the Jurassic. The Lower Cenozoic units in the northernmost part of the 3D seismic area are also offset by the SFZ. No offsets can be identified in the overlying, shallower part of the Cenozoic units. Most faults constituting the SFZ are favourably oriented in the present-day stress field (SHmax NNW–SSE) to be reactivated in strike-slip mode. The seismic events induced by testing operations at the geothermal exploration borehole “St. Gallen GT-1” (SG GT-1) in July 2013 revealed that, even though the seismicity of northeastern Switzerland is considered to be low and diffuse, parts of the SFZ have to be regarded as critically stressed. Combining the interpretation of geological and seismic data, we conclude that the SFZ represents a reactivated basement-rooted normal fault, which was active during several phases in Permo-Carboniferous and Mesozoic times and that is still active today in strike-slip mode.

Fluid evolution and mineralogy of Mn-Fe-barite-fluorite mineralizations at the contact of the Thuringian Basin, Thüringer Wald and Thüringer Schiefergebirge in Germany

Abstract Numerous small deposits and occurrences of Mn-Fe-fluorite-barite mineralization have developed at the contact of the Thuringian Basin, Thüringer Wald and Thüringer Schiefergebirge in central Germany. The studied mineralizations comprise the assemblages siderite+ankerite-calcite-fluorite-barite and hematite-Mn oxides-calcite-barite, with the precipitation sequence in that order within each assemblage. A structural geological analysis places the origin of the barite veins between the Middle Jurassic and Early Cretaceous. Primary fluid inclusions contain water vapour and an aqueous phase with NaCl and CaCl2 as the main solutes, with salinities mostly between 24–27 mass. % CaCl2 eq. Th measurements range between 85 °C and 160 °C in barite, between 139 °C and 163 °C in siderite, and between 80 °C and 130 °C in fluorite and calcite. Stable isotopes (S, O) point to the evaporitic source of sulphur in the observed mineralizations. The S,C,O isotopic compositions suggest that barite and calcite could not have precipitated from the same fluid. The isotopic composition of the fluid that precipitated barite is close to the sea water in the entire Permo–Mesozoic time span whereas calcite is isotopically distinctly heavier, as if the fluids were affected by evaporation. The fluid evolution in the siliciclastic/volcanic Rotliegend sediments (as determined by a number of earlier petrological and geochemical studies) can be correlated with the deposition sequence of the ore minerals. In particular, the bleaching of the sediments by reduced Rotliegend fluids (basinal brines) could be the event that mobilized Fe and Mn. These elements were deposited as siderite+ankerite within the Zechstein carbonate rocks and as hematite+Mn oxides within the oxidizing environment of the Permian volcanic and volcanoclastic rocks. A Middle-Jurassic illitization event delivered Ca, Na, Ba, and Pb from the feldspars into the basinal brines. Of these elements, Ba was deposited as massive barite veins.

Characterization of the Teeth Skeletal Matrix from &lt;i&gt;Arbacia lixula&lt;/i&gt;

The teeth of sea urchins are highly complex composite structures, composed predominantly of high magnesium calcite, and of a minor heterogeneous assemblage of organic macromolecules that are occluded within the mineral. The organic matrix fulfils important functions in mineralization, in addition to giving the mineral phase peculiar mechanical properties, different from that of purely inorganic calcite. Nevertheless, the composition and function of individual components of the organic matrix still remains largely unknown. Up to now, the detailed protein repertoire of teeth from a single sea urchin species (Strongylocentrotus purpuratus, order Camarodonta) was investigated. In this study, we characterized for the first time the teeth skeletal matrix of another sea urchin, Arbacia lixula (order Arbacioida). The acetic acid soluble and acetic acid insoluble matrices, namely ASM and AIM respectively, were extracted and characterized with different biochemical methods including mono-dimensional SDS-PAGE, FT-IR spectroscopy, HPAE-PAD for monosaccharide analysis, and finally, proteomics. In spite of the paucity of peptide data, several of them displayed a high abundance of hydrophobic residues, i.e., alanine, glycine and valine, and of the apolar proline. We assert that the alanine- and proline-rich domains are important features of some of the matrix proteins associated to the teeth of sea urchins. None of the known skeletal matrix proteins from S. purpuratus teeth were identified in the organic matrix of A. lixula teeth. This might suggest major differences in teeth matrix protein repertoires of these two species belonging to orders that diverged in the Mesozoic times.