Palaeomagnetism of three dyke swarms in Nansen Land,north Greenland (83° N)

Three basic dyke swarms of post-Ellesmerian (post-Early Carboniferous) age in Nansen Land (83° N, 43° W) are still not dated numerically, but cross-cutting relationships show Group 1 to be older than Group 2, while Group 3 is the freshest and likely the youngest. Group 1 (the most northerly swarm) strikes N-S; Group 2 NW-SE, and Group 3 (the most southerly swarm) E-W. From more than 200 dykes 234 specimens from 28 sites were investigated palaeomagnetically. Group 1 dykes show unexpected shallow inclinations with a cleaned mean direction of (Dm, Im) = (151°, –5.8°), N = 7, k = 18.5, ₉₅ = 13.9°. They show hydrothermal alterations, some remagnetization by lightning, and the low inclination indicates a low palaeo latitude. The palaeopole is (Plat, Plon) = (8.9° S, 14.0° W) with (dp, dm) = (7°, 14°), and is close to the North American Early Carboniferous mean pole, suggesting a syn- or early late-tectonic dyke injection. The polarity is reverse. Groups 2 and 3 of presumed Cretaceous or Tertiary age show dominantly normal and reverse polarities, respectively. Their mean directions per polarity are well grouped, with (Dm, Im) = (–30.6°, 76.7°), n = 13, k = 191.4, ₉₅ = 3.9°, and (Dm, Im) = (133.4°, –76.7°), n = 10, k = 87.5, ₉₅ = 5.9°, respectively. They are antipodal within 95% significance, and combining both swarms gives (Dm, Im) = (–37.5°, 76.8°), n = 23, k = 124.3, ₉₅ = 2.7°, corresponding to a mean pole of (Plat, Plon) = (70.0° N, 185.1° E) with (dp, dm) = (4.7°, 5.0°), for which the spline of Late Cretaceous-Tertiary poles for all Greenland indicates a palaeomagnetic age of 57 ± 10 Ma. This pole (in present-day coordinates) is very close to the Late Cretaceous North American pole, in accordance with the fact that Greenland belongs to the North American craton, and that the two younger swarms are essentially postdating the opening of Baffin Bay.     

山东半岛的地体构造及金矿成矿的区域地质背景

本文以不整合面的发现及同位素年龄为依据,重新厘定了本区早前寒武纪的地层,从而搞清了本区的大地构造格架,认为它类似于加拿大地盾的“洋葱构造”。本文将山东半岛划分为４个地体：胶北地体,鲁西地体,胶南地体,胶东南－苏东北－黄海地体。认为胶北地体成矿的大地构造背景与南克拉通成矿背景十分相似。胶南地体,黄海地体对胶北地体的俯冲作用为胶北地体提供了成岩成矿物质。根据胶北群和齐山群的地质及及地球化学特征,认为胶     

Retrograde evolution of quartz segregations from the Dos Picos shear zone in the Nevado-Filabride Complex (Betic chains,Spain). Evidence from fluid inclusions and quartz c-axis fabrics

Synkinematic quartz veins are ubiquitous in the shear zone separating the Veleta unit from the Calar Alto unit in the internal part of the Betic Cordilleras. They have been studied with respect to quartz c-axis fabrics, microstructures and fluid inclusions. Veins were probably generated during syn-metamorphic stacking of the units at P = 500 – 600 MPa and T = 400 – 500°C. Quartz displays two groups of microstructures in the shear zone: (1) older coarse-grained mosaics (CGM) resulting from exaggerated grain growth; and (2) younger fine-grained mosaics (FGM) developed at the expense of the former. The fine-grained mosaics show polygonal granoblastic and elongate mosaic microstructures in general, with ribbon microstructures often found near the boundary of the units. Fluids contained in secondary inclusions vary from high salinity brines to different types of CO₂—brine mixtures and low density CO₂ fluids. Differences in composition and P-T trapping conditions are indicated for the different types of inclusions. Some fluid inclusions are older than the FGM, whereas others are younger, thus constraining the P- T conditions at which the two microstructural events took place. Fluid inclusion evidence suggests conditions of P_fluid > 170 MPa and T 370–430°C for the CGM and P_fluid 20–80 MPa and T > 340°C for the FGM.The quartz c-axis fabrics dealt with here correspond to the second recrystallization event, as little evidence of older fabrics is preserved in the shear zone. C-axis patterns vary across the shear zone from slightly asymmetrical type I crossed girdles in the hanging wall and footwall to more asymmetrical crossed girdles at the boundary of the units. This indicates a correlative increase in the magnitude of the heterogeneous shear strain in the same direction. Most of the deformation is concentrated at the top of the Veleta unit. The sense of movement is top to the west, in agreement with other kinematic markers.The quartz c-axis fabrics resulted from dynamic recrystallization during simple shear. The retrograde P-T path inferred from fluid inclusion analysis, along with other geological and geochronological evidence, indicates that this deformation is coeval with a reduction in the crustal overburden.Geochronological and stratigraphic data show that the proposed Dos Picos extensional detachment, separating the Calar Alto and Veleta units, took place during the early Miocene, synchronous with the intense thinning of the Nevado-Filábride Complex and of the whole continental crust underlying the Alborán Basin.     

Breccias of the Adnet Formation: indicators of a Mid-Liassic tectonic event in the Northern Calcareous Alps (Salzburg/Austria)

Stratigraphy, lithology and depositional structures of Liassic red limestone-breccias of the Adnet Formation, including the Adnet Scheck, were studied at several outcrops of the Northern Calcareous Alps (NCA) south-east of Salzburg. A four-fold lithostratigraphic division is proposed for the Adnet Formation of the Osterhorn Mountains: the hemipelagic Schmiedwirt (Sinemurian) and Kehlbach (Carixian) members are separated from the pelagic Saubach Member (Toarcian) by a layer of amalgamated breccias (Scheck Member, probably Domerian to early Toarcian). Several other breccia beds occur locally from the base of the Kehlbach Member up to the lower Saubach Member. Although the sediments overlying the Scheck Member breccias are of coeval age, the ages of the underlying strata are very different. This can be explained by submarine Liassic erosion during a period of resedimentation from the middle Carixian until the early Toarcian. At least 10–15 m of partly lithified sediments were eroded by gravity flows. The entire Kehlbach Member and up to two-thirds of the Schmiedwirt Member were removed at Adnet. The breccias originated from submarine debris flows. Repeated flows over a long period and the depositional setting exclude a triggering by sea-level fluctuations. Most probably they arose from tectonically triggered slumps and slides of superficial sediments. The Scheck was initiated on the steep upper slope of the drowned Triassic Adnet reef and flowed to the north-east. The Pliensbachian to early Toarcian period of tectonic activity indicated by the breccias was the most important during the Liassic in the Osterhorn Mountains and other parts of the NCA. From the large-scale regional distribution of the breccias and in accord with published data, a roughly northeast trending strike-slip fault zone is proposed, crossing the NCA south of the Osterhorn block, with a peaking activity during the Pliensbachian to early Toarcian as the cause of the tectonic movements.     

Seismogeological acceptance criteria for geological repositories for spent nuclear fuel

Qualitative geological acceptance criteria and quantitative seismological acceptance criteria for radioactive waste disposals are developed. The background material for the initiation of site selection and for its earthquake hazard assessment is discussed. The recent movements of the Earth's surface as well as the other mechanical properties of geological media, hydrological conditions of geological blocks, their groundwater chemistry, geochemical rheology, petrological analyses of rocks, etc., have to be taken into account. A new comprehensive integrated safety analysis of the final underground disposal of spent nuclear fuel has been underway in the Czech Republic since 1991. In accordance with these seismogeological acceptance criteria regions for building underground final radioactive waste disposals are singled out in the Czech Republic.     

Giant Megamullion in the Parece Vela Backarc Basin

We present results of high-resolution bathymetric studies of the extinct intermediate-spreading Parece Vela Basin in the northwestern Pacific, where we have identified an extremely large mullion structure, here termed a giant megamullion. We find that the giant megamullion is nearly an order of magnitude larger than the similar structures in the slow-spreading Mid-Atlantic Ridge (`megamullions'). The giant megamullion has slightly elevated mantle Bouguer anomaly, and yields serpentinized peridotites and gabbros, suggesting that they are exposing oceanic crust and upper mantle. An off-axis rugged `chaotic terrain' was also identified in the Parece Vela Basin. The terrain consists of isolated and elevated blocks capped by corrugated axis-normal lineations, and associated deeps. We thus interpret it as analogues to the Mid-Atlantic Ridge megamullions. We propose that amagmatic tectonics producing the giant megamullion and the chaotic terrain occupied a significant part in crustal construction in the Parece Vela Basin evolution.     

ALVIN investigation of an active propagating rift system,Galapagos 95.5° W

ALVIN investigations have defined the fine-scale structural and volcanic patterns produced by active rift and spreading center propagation and failure near 95.5° W on the Galapagos spreading center. Behind the initial lithospheric rifting, which is propagating nearly due west at about 50 km m.y.^–1, a triangular block of preexisting lithosphere is being stretched and fractured, with some recent volcanism along curving fissures. A well-organized seafloor spreading center, an extensively faulted and fissured volcanic ridge, develops ~ 10 km (~ 200,000 years) behind the tectonic rift tip. Regional variations in the chemical compositions of the youngest lavas collected during this program contrast with those encompassing the entire 3 m.y. of propagation history for this region. A maximum in degree of magmatic differentiation occurs about 9 km behind the propagating rift tip, in a region of diffuse rifting. The propagating spreading center shows a gentle gradient in magmatic differentiation culminating at the SW-curving spreading center tip. Except for the doomed rift, which is in a constructional phase, tectonic activity also dominates over volcanic activity along the failing spreading system. In contrast to the propagating rift, failing rift lavas show a highly restricted range of compositions consistent with derivation from a declining upwelling zone accompanying rift failure. The lithosphere transferred from the Cocos to the Nazca plate by this propagator is extensively faulted and characterized by ubiquitous talus in one of the most tectonically disrupted areas of seafloor known. The pseudofault scarps, where the preexisting lithosphere was rifted apart, appear to include both normal and propagator lavas and are thus more lithologically complex than previously thought. Biological communities, probably vestimentiferan tubeworms, occur near the top of the outer pseudofault scarp, although no hydrothermal venting was observed.     

Spatial and temporal variations in minibasin geometry and evolution in salt tectonic provinces: Lower Congo Basin,offshore Angola

In passive margin salt basins, the distinct kinematic domains of thin‐skinned extension, translation and contraction exert important controls on minibasin evolution. However, the relationship between various salt minibasin geometries and kinematic domain evolution is not clear. In this study, we use a semi‐regional 3D seismic reflection dataset from the Lower Congo Basin, offshore Angola, to investigate the evolution of a network of minibasins and intervening salt walls during thin‐skinned, gravity‐driven salt flow. Widespread thin‐skinned extension occurred during the Cenomanian to Coniacian, accommodated by numerous distributed normal faults that are typically 5–10 km long and spaced 1–4 km across strike within the supra‐salt cover. Subsequently, during the Santonian–Paleocene, multiple, 10–25 km long, 5–7 km wide depocentres progressively grew and linked along strike to form elongate minibasins separated by salt walls of comparable lengths. Simultaneous with the development of the minibasins, thin‐skinned contractional deformation occurred in the southwestern downslope part of the study area, forming folds and thrusts that are up to 20 km long and have a wavelength of 2–4 km. The elongate minibasins evolved into turtle structures during the Eocene to Oligocene. From the Miocene onwards, contraction of the supra‐salt cover caused squeezing and uplift of the salt walls, further confining the minibasin depocentres. We find kinematic domains of extension, translation and contraction control the minibasin initiation and subsequent evolution. However, we also observe variations in minibasin geometries associated with along‐strike growth and linkage of depocentres. Neighbouring minibasins may have different subsidence rates and maturity leading to marked variations in their geometry. Additionally, migration of the contractional domain upslope and multiple phases of thin‐skinned salt tectonics further complicates the spatial variations in minibasin geometry and evolution. This study suggests that minibasin growth is more variable and complex than existing domain‐controlled models would suggest.     

Tectonic Imprints of the Hazara Kashmir Syntaxis on the Mesozoic Rocks Exposed in Munda,Mohmand Agency,Northwest Pakistan

Two well-developed mesoscopic folds, D_2 and D_3, which postdate the middle amphibolite metamorphism, were recognized in the western hinterland zone of Pakistan. NW–SE trending D_2 folds developed during NE–SW horizontal bulk shortening followed by NE–SW trending D_3 folds, which developed during SE–NW shortening. Micro- to mesoscopically the NW–SE trending S2 crenulation cleavage, boudins and mineral stretching lineations are overprinted by D_3. The newly established NW–SE trending micro- to mesoscopic structures in Munda termed D_2, which postdated F_1/F_2, is synchronously developed with F3 structures in the western hinterland zone of Pakistan. We interpret that D_2 and D_3 folds are counterclockwise rotated in the tectonic event that has evolved the Hazara Kashmir Syntaxis after the main phase Indian plate and Kohistan Island Arc collision. Chlorite replacement by biotite in the main matrix crenulation cleavages indicates prograde metamorphism related with D_2. The inclusion of muscovite and biotite in garnet porphyroblasts and the presence of staurolite in these rocks indicate that the Barrovian metamorphic conditions predate D_2 and D_3. We interpret that garnet, staurolite and calcite porphyroblasts grew before D_2 because the well developed S2 crenulation cleavage wraps around these porphyroblasts.     

谈塔里木板块开合构造与层次成矿问题——以层控型金属矿床为例

通过塔里木板块的构造开合演化及古地理变迁,去追索层控型金属矿床成矿的时空规律,从而划分出老(元古代,以中元古代为主)、中(晚古生代,以泥盆纪-石炭纪为主)、青(中新生代,以白垩纪-新近纪为主。)三个集中成矿区间(期),并言明其层次成矿构造属性及空间分布格局。