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21.
22.
Carita Augustsson Arne P. Willner Tobias Rüsing Hans Niemeyer Axel Gerdes Christopher J. Adams Hubert Miller 《地学学报》2016,28(2):128-137
Hf‐isotope data of >1100 detrital zircon grains from the Palaeozoic, south‐central Andean Gondwana margin record the complete crustal evolution of South America, which was the predominant source. The oldest grains, with crustal residence ages of 3.8–4.0 Ga, are consistent with complete recycling of existing continental crust around 4 Ga. We confirm three major Archaean, Palaeoproterozoic (Transamazonian) and late Mesoproterozoic to early Neoproterozoic crust‐addition phases as well as six igneous phases during Proterozoic to Palaeozoic time involving mixing of juvenile and crustally reworked material. A late Mesoproterozoic to early Neoproterozoic, Grenville‐age igneous belt can be postulated along the palaeo‐margin of South America. This belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages. Crustal reworking likely dominated over juvenile addition during the Palaeozoic era, and Proterozoic and Archaean zircon was mainly crustally reworked from the eroding, thickened Ordovician Famatinian arc. 相似文献
23.
Cyclones over Fram Strait: impact on sea ice and variability 总被引:1,自引:0,他引:1
Burghard Brümmer Gerd Müller Björn Affeld Rüdiger Gerdes Michael Karcher Frank Kauker 《Polar research》2001,20(2):147-152
The relation between sea ice drift and cyclone activity in the Fram Strait region was studied by both in situ observations and long-term time series. In a 1999 field campaign, the atmospheric forcing and the ice drift were determined using a research aircraft and drifting ice buoys. One cyclone entered the experimental area and caused a temporal increase in ice drift speed. Long-term studies are based on 16 years of cyclone statistics and model, satellite and sonar ice drift estimates. The actual impact of a cyclone depends on its particular track through Fram Strait. On the average, cyclones increase the Arctic ice export through Fram Strait. 相似文献
24.
New gravity and magnetic data from the northern Red Sea reveal the extent of the large gravity anomaly (164 mgal) and the presence of significant magnetic anomalies over St. John's Island. Spectral transformation and three-dimensional potential-field modelling delineate the surface configuration and vertical extent of the causative body and the enormous density contrast required (1.2 g/cm3) suggests that it is composed of unserpentinised peridotite (density 3.4 g/cm3) to a depth of at least 8 km.St. John's Island is uniquely located, not only at a passive continental margin but also within a fracture zone at the transition from plate separation by seafloor spreading to extension by lithospheric attenuation. This precludes several suggested mechanisms for the emplacement of ultramafic bodies in fracture zones.Thermal contraction, serpentinite diapirism and changes in the poles of rotation do not seem possible mechanisms in this tectonic environment and the emplacement is most probably related to the spreading readjustment necessary to create a continent-to-continent fracture zone. A post-Mesozoic age of emplacement, associated with the onset of continental rifting and the rejuvenation of a pre-existing continental fracture, seems most probable. 相似文献
25.
A multi‐system geochronology in the Ad‐3 borehole,Pannonian Basin (Hungary) with implications for dating volcanic rocks by low‐temperature thermochronology and for interpretation of (U–Th)/He data 下载免费PDF全文
Martin Danišík László Fodor István Dunkl Axel Gerdes János Csizmeg Mária Hámor‐Vidó Noreen J. Evans 《地学学报》2015,27(4):258-269
Independent geochronological and thermal modelling approaches are applied to a biostratigraphically exceptionally well‐controlled borehole, Alcsútdoboz‐3 (Ad‐3), in order to constrain the age of Cenozoic geodynamic events in the western Pannonian Basin and to test the efficacy of the methods for dating volcanic rocks. Apatite fission track and zircon U–Pb data show two volcanic phases of Middle Eocene (43.4–39.0 Ma) and Early Oligocene (32.72 ± 0.15 Ma) age respectively. Apatite (U–Th)/He ages (23.8–14.8 Ma) and independent thermal and subsidence history models reveal a brief period of heating to 55–70 °C at ~17 Ma caused by an increased heat‐flow related to crustal thinning and mantle upwelling. Our results demonstrate that, contrary to common perception, the apatite (U–Th)/He method is likely to record ‘apparent’ or ‘mixed’ ages resulting from subsequent thermal events rather than ‘cooling’ or ‘eruption’ ages directly related to distinct geological events. It follows that a direct conversion of ‘apparent’ or ‘mixed’ (U‐Th)/He ages into cooling, exhumation or erosion rates is incorrect. 相似文献
26.
Kerstin Drost Axel Gerdes Teresa Jeffries Ulf Linnemann Craig Storey 《Gondwana Research》2011,19(1):213-231
New LA-ICP-MS U–Pb detrital zircon ages from Ediacaran and Paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the Teplá-Barrandian unit (TBU) in the centre of the Bohemian Massif (Central Europe, Czech Republic). The samples taken span the period from ≤ 635 Ma to ~ 385 Ma and permit recognition of provenance changes that reflect changes in geotectonic regime. Detrital zircon age spectra of two Ediacaran, one Lower Cambrian and three Upper Ordovician samples resemble the ages known from the NW African proportion of Gondwana, particularly the Trans-Saharan belt, while three rocks from higher Lower Cambrian to Lowermost Ordovician strata contain detritus that may have been derived exclusively from local sources. The age spectrum of the Devonian rock is a combination of the NW Gondwanan and local features. These new findings in combination with a wide range of published data are in agreement with a Neoproterozoic subduction-related setting at the margin of Gondwana followed by a Cambrian/Early Ordovician rifting stage and an Ordovician passive margin setting. Furthermore the data are in favour of a position of the TBU at the Gondwanan margin throughout pre-Variscan times. 相似文献
27.
We investigate the dependence of surface fresh water fluxes in the Gulf Stream and North Atlantic Current (NAC) area on the
position of the stream axis which is not well represented in most ocean models. To correct this shortcoming, strong unrealistic
surface fresh water fluxes have to be applied that lead to an incorrect salt balance of the current system. The unrealistic
surface fluxes required by the oceanic component may force flux adjustments and may cause fictitious long-term variability
in coupled climate models. To identify the important points in the correct representation of the salt balance of the Gulf
Stream a regional model of the northwestern part of the subtropical gyre has been set up. Sensitivity studies are made where
the westward flow north of the Gulf Stream and its properties are varied. Increasing westward volume transport leads to a
southward migration of the Gulf Stream separation point along the American coast. The salinity of the inflow is essential
for realistic surface fresh water fluxes and the water mass distribution. The subpolar–subtropical connection is important
in two ways: The deep dense flow from the deep water mass formation areas sets up the cyclonic circulation cell north of the
Gulf Stream. The surface and mid depth flow of fresh water collected at high northern latitudes is mixed into the Gulf Stream
and compensates for the net evaporation at the surface.
Received: 19 September 2000 / Accepted: 5 February 2001 相似文献
28.
V. N. Smirnov K. S. Ivanov Yu. L. Ronkin P. A. Serov A. Gerdes 《Geochemistry International》2018,56(6):495-508
This paper reports Rb–Sr and Sm–Nd isotope data on the gabbro–diorite–tonalite rock association of the Reft massif (eastern margin of the Middle Urals) and Lu–Hf isotope data on zircon populations from these rocks. In terms of Nd and Hf isotope composition, the rocks of the studied association are subdivided into two distinctly different groups. The first group consists of gabbros and diorites, as well as plagioclase granites from thin dikes and veins cutting across the gabbros. In terms of 43Nd/144Nd i = 0.512518–0.512573 (εNd(T) = +8.6...+9.7) and 176Hf/177Hf i = 0.282961–0.283019 (εHf(T) = +15.9...+17.9), these rocks are practically identical to depleted mantle. Their Nd and Hf model ages show wide variations, but in general are close to their crystallization time. The second group is represented by tonalites and quartz diorites, which compose a large body occupying over half of the massif area. These rocks are characterized by the lower values of 143Nd/144Nd i = 0.512265–0.512388 (εNd(T) = +3.7...+6.0) and 176Hf/177Hf i = 0.282826–0.282870 (εHf(T) = +11.1...+12.7). The TDM values of the second group are much (two–three times) higher than their geological age (crystallization time), which indicates sufficiently long crustal residence time of their source. The initial 87Sr/86Sr in the rocks of both the groups varies from 0.70348 to 0.70495. This is likely explained by the different saturation of melts with fluid enriched in radiogenic Sr. The source of this fluid could be seawater that was buried in a subduction zone with oceanic sediments and released during slab dehydration. Obtained data make it possible to conclude that the formation of the studied gabbro–diorite–tonalite association is a result of spatially and temporally close magma formation processes in the crust and mantle, with insignificant contribution of differentiation of mantle basite magma. 相似文献
29.
Zieger J. Linnemann U. Hofmann M. Gärtner A. Marko L. Gerdes A. 《International Journal of Earth Sciences》2018,107(3):933-953
International Journal of Earth Sciences - The basement of the Saxo-Thuringian Zone consists of Upper Neoproterozoic (c. 650-570 Ma) Cadomian arc sediments (Lusatian greywackes) and... 相似文献
30.
Luthardt Ludwig Hofmann Mandy Linnemann Ulf Gerdes Axel Marko Linda Rößler Ronny 《International Journal of Earth Sciences》2018,107(7):2465-2489
International Journal of Earth Sciences - The Chemnitz Fossil Forest depicts one of the most completely preserved forest ecosystems in late Paleozoic Northern Hemisphere of tropical Pangaea. Fossil... 相似文献