1,721,189 research outputs found
Preliminary results on the paleomag-netism of Upper Permian Verrucano Lombardo sandstones from Val Camonica and of Upper Triassic Val Sabbia sandstones from Val Brembana, Southern Alps
No full textThe Africa-Adria apparent Polar Wander Path and its implications for Pangea paleogeography
No full textPaleomagnetic data of Permian-Cenozoic age from para-autochthonous regions of Adria - the African promontory - are reviewed together with coeval paleomagnetic data from Africa. A total of 75
paleomagnetic pole entries from Africa and Adria have been selected from igneous units or sedimentary units that have been corrected for inclination shallowing. All selected paleopole entries meet fundamental quality criteria such as (1) they use magnetic component directions that have been isolated using appropriate demagnetization techiques, (2) they bear no suspicion of remagnetization, and (3) they are well dated by means of radiometric age techniques or biostratigraphy. As a main outcome, this review shows that paleomagnetic poles from para-autochthonous Adria are statistically undistinguishable (or hardly so) relative to coeval poles from Africa over the extended time span considered in this analysis, i.e. from the Early Permian to the Miocene. This substantial tectonic coherence between Africa and Adria allows the construction of a composite Africa-Adria apparent polar wander path (APWP) that can be used to bolster the Gondwana APWP in the poorly known Permian-Triassic time interval of evolution of Pangea. The Early Permian paleopole of Africa-Adria from radiometrically dated igneous rocks, in conjunction with the coeval Laurasia paleopole again from igneous rocks, support Pangea B. The Late Permian/Early Triassic paleopoles from Africa-Adria and Laurasia support Pangea A, which persisted up to fragmentation in the Jurassic. A final review of the geologic and paleomagnetic evidence in support of a Middle Permian intra-Pangea dextral megashear system will be attempted
The Case for Pangea B: Paleomagnetic Contributions from Adria
No full textThe pre-drift Wegenerian model of Pangea is almost universally accepted, but debate exists on its pre-Jurassic configuration since Ted Irving introduced Pangea B. We review Permian and recently acquired Jurassic-Cretaceous paleomagnetic data from para-autochthonous regions of Adria such as the Southern Alps, which we show to be broadly consistent with "African" APWPs. Paleomagnetic data from paraautochthonous Adria can therefore be used to bolster the Gondwana APWP in the poorly known Late Permian-Triassic time interval. Adria paleopoles are integrated with the Gondwana and Laurasia APWPs and used to generate a tectonic model for the evolution of Pangea. The Early Permian paleopole of Adria from radiometrically dated igneous rocks, in conjunction with the coeval Gondwana and Laurasia paleopoles again from igneous rocks, support Pangea B. The use of paleomagnetic data strictly from igneous rocks excludes artifacts from sedimentary inclination error as a contributing explanation for Pangea B. The ultimate option to reject Pangea B is to introduce a significant zonal octupole component in the Late Paleozoic time-averaged geomagnetic field. Our dataset consisting entirely of paleomagnetic directions with low inclinations from sampling sites confined to one hemisphere show that the effects of a zonal octupole field contribution cannot explain away the paleomagnetic evidence for Pangea B. We therefore regard the paleomagnetic evidence for an Early Permian Pangea B as robust. Because the Late Permian/Early Triassic and the Middle/early Late Triassic paleopoles from Adria and Laurussia support Pangea A, the phase of transcurrent motion between Laurasia and Gondwana that caused the Pangea B to A transition occurred essentially in the Permian. It took place after the cooling of the Variscan megasuture and lasted ~20 m.y., with an average relative plate velocity of approximately 15 cm/yr. Finally, we review geological and paleomagnetic evidence in support of an intra-Pangea dextral megashear system. In particular, we present paleomagnetic data from Corsica and Sardinia that, during the Permian, were
presumably caught into the transcurrent plate boundaries between Gondwana and Laurasia and dissected away in variably rotated crustal blocks
The importance of age control in defining apparent Polar wander paths of fast moving plates: the Jurassic case study
No full textDuring periods of fast plate motion (e.g. Cambrian, Jurassic), plate velocities in excess of ~20 cm/yr (200 km/Myr) relative to the Earth's spin axis have been suggested. Pinning down the position of fast moving plates requires paleomagnetic poles (paleopoles) with age resolution of a few million years. Modern generations of apparent polar wander paths (APWPs) are becoming increasingly sophisticated in handling ever-growing volumes of data, usually by applying moving windows (e.g., 10 Myr) to the available paleopoles. Averaging paleopoles of fast moving plates may however result in loss of resolution whereby abrupt (but real) changes in APWP may appear subdued when a multimillion-year moving window is applied. Episodes of fast motion are better captured by using paleopoles with best age resolution (coupled with good structural control and provided with inclination flattening estimates) grouped within discrete and independent time windows. Best age control is attained when paleopoles are retrieved from laterally reproducible magnetostratigraphic sections calibrated with biostratigraphy and/or radiometric dating and correlated with reference timescales. This approach was recently applied to the construction of the Adria-Africa APWP (Muttoni et al. 2013). Paleopoles from parautochthonous regions of Adria and obtained either from biostratigraphically dated sedimentary rocks, corrected for inclination shallowing, or from radiometrically dated igneous rocks that are regarded as free from inclination shallowing, were compared with coeval, and inclination flattening-free, paleopoles from stable Africa. The resulting composite APWP shows a remarkable agreement with the Kent and Irving (2010) APWP, and displays a rapid polar shift of ~40° during the Jurassic that other APWPs tend to underestimate. This Jurassic monster shift is also predicted for Eurasia. Paleomagnetic data from the Kimmeridgian-Tithonian Garedu Formation of Iran,
which was part of Eurasia since the Triassic, indicate a paleolatitude of deposition that is in excellent agreement with the latitude drop predicted by the monster shift (Mattei et al. 2014), which stands as a major and generalized plate motion event of vast and as yet unexplored paleogeographic implications
Reply to 'Discussion of "Magnetostratigraphic confirmation of a much faster tempo for sea-level change for the Middle Triassic Latemar platform carbonates" by D.V Kent, G. Muttoni and P. Brack [Earth Planet. Sci. Lett. 228 (2004), 369-377]' By L. Hinnov
No full textEcce Homo in Milan
Full text linkCome ogni essere vivente anche l’uomo ha colonizzato gli habitat solo nel momento in cui questi presentavano le condizioni favorevoli alla sua sopravvivenza. La colonizzazione del continente europeo è avvenuta molto probabilmente da Est verso Ovest e l’ingresso in Italia può essere avvenuto solo dal punto più comodo in cui si poteva superare la catena Alpina, ossia passando dai Balcani (Figura 2). Dal record geologico si deduce il susseguirsi di periodi caldi a periodi freddi durante tutto il Pleistocene, tuttavia non ci sono evidenze di un passaggio ad un clima glaciale nel nostro paese prima del Pleistocene Superiore. Questo cambiamento ha come prima conseguenza l’espansione delle calotte glaciali legata all’intrappolamento dell’acqua degli oceani e il conseguente abbassamento del livello del mare che provoca il passaggio di molte zone precedentemente sommerse prima ad un ambiente litorale e inne emerso. Questo è quanto si è vericato in Pianura Padana. Il cambiamento di ambiente ha provocato una variazione nel materiale che si stava depositando: prima si avevano limi e sabbie marine, poi ciottoli legati al trasporto
uviale e materiale ne legato alle esondazioni dei umi. La dierenza nel materiale deposto è stata riconosciuta lungo tutta la Pianura Padana ed è stata nominata Discontinuità R. La Discontinuità R è quindi legata a un cambiamento nel clima, e all’istaurarsi delle grandi glaciazioni nel continente europeo. Mediante lo studio di diverse carote prelevate in Pianura Padana è stato possibile creare un modello di età che ha permesso di associare la Discontinuità R al MIS22, datandola a circa 870.000 anni fa (Figura 1). Il MIS22 corrisponde inoltre alla ne della cosiddetta “Rivoluzione del Pleistocene Medio”, un momento in cui si è assistito ad un cambiamento nella ora con la comparsa delle prime piante legate all’ambiente glaciale, ma anche ad un cambiamento nella fauna legato all’arrivo di specie come il mammuth (Mammuthus meridionalis), l’elefante antico (Elephas antiquus), e il rinoceronte lanoso (Coelodonta antiquitatis). La nostra ipotesi è che l’arrivo dell’uomo in Europa e in Italia sia stato legato alla sua tendenza a seguire le onde migratorie degli animali che costituivano le sue principali prede (Figura 2), e che sia avvenuto in concomitanza con l’instaurarsi delle grandi glaciazioni Pleistocenica, ossia attorno all’età del MIS22
High-resolution magnetostratigraphic and lithostratigraphic correlations in Middle Triassic pelagic carbonates from the Dolomites (northern Italy)
No full textNew magnetostratigraphic and lithostratigraphic data are reported from the Pedraces and Belvedere limestone sections from the Dolomites region of northern Italy. These sections are comprised of the Buchenstein Beds of Late Anisian to Ladinian (Middle Triassic) age. The results from Pedraces and Belvedere are compared with data from the biostratigraphically and isotopically constrained Frotschbach and Seceda sections from the literature. A satisfactory magnetostratigraphic correlation is obtained on laterally traceable limestone and volcaniclastic intervals. These marker beds are then used to extend lithostratigraphic correlations to additional key sections from a palaeogeographically coherent area of around 500 km2. The aim of this study is to unravel the spatial and temporal evolution of the Buchenstein basin and the surrounding carbonate platforms. Platforms in the northwestern Dolomites show an early stage of aggradation followed by progradation, whereas in the central Dolomites the Cernera platform, after fast initial aggradation, drowned and became a pelagic seamount. In the intervening Buchenstein basin, the differential subsidence between the northwestern and central Dolomites is manifested by the lateral increase of thickness of 'Lower Pietra Verde' volcaniclastic sediments. Accumulation of volcaniclastic material reworked from adjacent carbonate platform slopes characterised the more subsiding and unstable central Dolomites. In the pelagic carbonate intervals, the persistence over tens of kilometres of bedding patterns suggests that carbonate material presumably washed out from the surrounding, still active carbonate platforms was volumetrically small and homogeneously distributed throughout the uniformly subsiding Buchenstein basin. A Milankovitch precessional origin for the platform interior cycles at Latemar as suggested elsewhere implies a net accumulation rate of 2 m/Ma or less for the correlative interval of pelagic Buchenstein Beds. This value differs significantly from the average value of around 10 m/Ma as calculated using high-resolution isotopic age data for the Buchenstein Beds. (C) 2000 Elsevier Science B.V
Paleocene-Eocene magnetostratigraphy and climate-driven rock-magnetism from the Belluno Basin (Italy)
No full textThe magnetostratigraphy and rock-magnetism of the Paleocene-Eocene interval has been studied in the recent years in several Tethyan marine sections of the Belluno Basin of NE Italy (Possagno, Cicogna, South Ardo, Alano). The paleomagnetic results, integrated with calcareous nannofossil biostratigraphy, allowed the recovery of a virtually continuous ~27 Myr-long interval of time spanning from the K/Pg boundary (~65 Ma) at the South Ardo section up to the middle-late Eocene boundary (~38 Ma) at the Alano GSSP candidate, and bracketing some of the most extreme climate conditions of the Cenozoic such the Early Eocene climatic optimum (EECO) and the Paleocene-Eocene thermal maximum (PETM). The rock-magnetic data indicate that the magnetic mineralogy of the sediments generally consists of variable proportions of magnetite-maghemite-hematite, which are iron oxides characterized by different oxidation states and crystal structures. We reconstructed the rock-magnetic variability across the investigated interval, and placed it on a temporal reference frame using a CK95-based age-depth function for comparison with oxygen isotope data from the literature. The rock-magnetic data indicate that relatively warmer climate periods (i.e. the PETM and the early Eocene warming trend leading to EECO) are associated with high contents of detrital hematite relative to magnetite-maghemite, while relatively cooler climates (i.e. the Paleocene) are associated with a relative increase in magnetite-maghemite. We speculate that the increase of detrital hematite observed during warm periods is due to intensified chemical weathering rates of land silicates under warm and humid climates. We therefore show that rock-magnetic properties can be useful proxies to study the efficiency of the silicate weathering negative feedback mechanism to stabilize long-term Earth's surface temperatures
Paleomagnetism of latest Anisian (Middle Triassic) sections of Prezzo Limestone and Buchenstein Formation, Southern Alps, Italy
No full textA paleomagnetic study was carried out at six stratigraphic sections (309 specimens) in the latest Anisian (Middle Triassic) Prezzo Limestone and the overlying Buchenstein Formation. These units outcrop over a wide area in the western Southern Alps, although most of the sampled sections are in the vicinity of the Late Eocene-Early Oligocene Adamello batholith. Three sites suffered a complete remagnetization induced by the Adamello, whereas a characteristic component with a positive fold test has been isolated at the three other sites. The mean pole of the characteristic component (Lat. 63.2-degrees-N, Long. 229.3-degrees-E, N = 3, A95 = 8-degrees, k = 236) is in agreement with the Triassic portion of the West Gondwana apparent polar wander path (APWP), supporting the use of paleopoles from well-dated rocks in the Southern Alps as useful proxies for the African APWP. The characteristic component, where isolated in the sampled sections, is of normal polarity only, corresponding to the latest Anisian on the basis of well-defined ammonoid and conodont biostratigraphy, but the present results suggest that there are good opportunities for extending Middle Triassic magnetostratigraphy in these Southern Alps rock units. The mean pole of the Adamello-induced component (Lat. 74.5-degrees-N, Long. 172.1-degrees-E, N = 4, A95 = 7.6-degrees, K = 145) lies close to the Early Tertiary portion of the APWP for stable Europe. The post-folding, Adamello-induced directions confirm that the 30-42 Ma Adamello batholith intruded after Alpine deformation and that no further deformation apparently occurred in post-emplacement times
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