102,326 research outputs found
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
Magneto-biostratigraphy of the 'Buchenstein Beds' at Frotschbach (Western Dolomites, Italy)
No full textIn this short contribution we present preliminary results on the magneto-biostratigraphy of the Anisian/Ladinian boundary interval at Frotschbach in the Dolomites of northern Italy. The interdisciplinary research carried out at this and on other coeval sections such as Aghia Triada and Vlichos on Hydra island, Greece (Muttoni et al., in prep.) is aimed at correlating in detail the well- known Tethyan biozonation to a geomagnetic polarity sequence of reversals for the construction of a standard Triassic time scale
“Wasp‐waisted” hysteresis loops from a pyrrhotite and magnetite‐bearing remagnetized Triassic limestone
No full textRemagnetized samples of the Triassic Prezzo Limestone from northern Italy contain a mixture of pyrrhotite and magnetite, as deduced by thermal unblocking characteristics of triaxial isothermal remanent magnetizations (IRM's) and low‐temperature cycling of saturation IRM's. Hysteresis loops are commonly “wasp‐waisted” and remanent coercivity curves contain a break in slope, as a result of the contrast in coercivity between remanence‐carrying pyrrhotite and magnetite. The relative proportion of the high to low remanent coercivity fractions, as deduced by the study of the remanent coercivity curves, seems to control the degree of “wasp‐waistedness” of the hysteresis loops. Samples that are dominated by one of the two coercivity end members have lower Bcr/Bc values as well as hysteresis loops that have a less pronounced constricted waist compared to samples with higher Bcr/Bc values. Maximum Bcr/Bc values (and thus maximum degrees of “wasp‐waistedness” in the hysteresis loops) are attained when the low remanent coercivity fraction contributes 15–35% to the bulk remanent coercivity curves
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
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 textMagnetostratigraphy of the Milan subsurface
No full textFour cores have been taken from the surroundings of the city of Milan, in the framework of the Milan CARG project. The northernmost drillings (Milano Triulza RL10, Milano Parco Nord RL11) were drilled to 100-m depth; the southernmost drillings (Peschiera Borromeo RL8, Gaggiano RL9) reached a depth of 180 m and 150 m, respectively. A total of 530 m of sediments was recovered.
The overall core lithostratigraphy is composed by three superimposed lithologic sequences, consisting, from the bottom, of alternated silt and medium- to fine-grained sand, arranged in fining-upward cycles, interpreted as meandering alluvial plain; the central sequence develops with coarse-grained sand, pebbly sand and subordinated gravel, interpreted as distal braidplain. Medium- to coarse-grained, poorly sorted, massive sand and pebbly sand, and clast-supported gravels with sandy matrix, interpreted as proximal braidplain, characterize the upper sequence. As a whole, the central and the upper sequence can be regarded as a prograding braidplain, composed by severall small-scale fining-upward cycles.
Paleomagnetic properties were studied on a total of 79 samples collected from cohesive fine-grained sediments with a common average sampling frequency in the order of one sample every 3/4 core-meters. The intensity of the NRM (measured at the Alpine Laboratory of Paleomagnetism) was in the order of 10 -3 - 10-4 A/m and orthogonal projections of demagnetization data typically indicated the existence of a lower unblocking temperature component, superimposed to a higher unblocking temperature component. The higher temperature component was removed to the origin of the demagnetization axes mainly in the magnetite and hematite temperature ranges between ~350 and ~680 °C and it is interpreted as the characteristic component. This characteristic component bears either positive (down-pointing) or negative (up-pointing) inclinations with overall mean values of 60° ± 15 and -54° ± 16, respectively, and is regarded as acquired at or shortly after sediment deposition (DRM or pDRM).
At least a magnetic polarity reversal has been recognized in each core, in the depth range of 60-80 m, and it has been interpreted, by means of the available pollen biostratigraphy and the regional framework reported in Carcano & Piccin (2002), Muttoni et al. (2003), Scardia et al. (2006), as the Brunhes/Matuyama boundary; in cores RL 8 and RL9 also the Jaramillo Subchron was recognized.
The major lithologic change observed in each core, produced by a depositional switch from distal meandering alluvial plain to a prograding braidplain, occurs during a reverse polarity period, interpreted as Subchron Late Matuyama, and it is well constrained between the Subchron Jaramillo and the Brunhes/Matuyama boundary in cores RL8 and RL9; the same age constrain can be inferred in cores RL10 and RL11. This episode, already observed by Carcano & Piccin (2002), has been correlated by Muttoni et al. (2003) to an important Pleistocene climatic event, related to the onset of the major glaciations at the southern foothills of the Alps occurred at ~0.87 ka, during the Subchron Late Matuyama.Published86-873.2. Tettonica attivaN/A or not JCRope
Magnetostratigraphy of the Milan subsurface
No full textFour cores have been taken from the surroundings of the city of Milan, in the framework of the Milan CARG project. The northernmost drillings (Milano Triulza RL10, Milano Parco Nord RL11) were drilled to 100-m depth; the southernmost drillings (Peschiera Borromeo RL8, Gaggiano RL9) reached a depth of 180 m and 150 m, respectively. A total of 530 m of sediments was recovered.
The overall core lithostratigraphy is composed by three superimposed lithologic sequences, consisting, from the bottom, of alternated silt and medium- to fine-grained sand, arranged in fining-upward cycles, interpreted as meandering alluvial plain; the central sequence develops with coarse-grained sand, pebbly sand and subordinated gravel, interpreted as distal braidplain. Medium- to coarse-grained, poorly sorted, massive sand and pebbly sand, and clast-supported gravels with sandy matrix, interpreted as proximal braidplain, characterize the upper sequence. As a whole, the central and the upper sequence can be regarded as a prograding braidplain, composed by severall small-scale fining-upward cycles.
Paleomagnetic properties were studied on a total of 79 samples collected from cohesive fine-grained sediments with a common average sampling frequency in the order of one sample every 3/4 core-meters. The intensity of the NRM (measured at the Alpine Laboratory of Paleomagnetism) was in the order of 10 -3 - 10-4 A/m and orthogonal projections of demagnetization data typically indicated the existence of a lower unblocking temperature component, superimposed to a higher unblocking temperature component. The higher temperature component was removed to the origin of the demagnetization axes mainly in the magnetite and hematite temperature ranges between ~350 and ~680 °C and it is interpreted as the characteristic component. This characteristic component bears either positive (down-pointing) or negative (up-pointing) inclinations with overall mean values of 60° ± 15 and -54° ± 16, respectively, and is regarded as acquired at or shortly after sediment deposition (DRM or pDRM).
At least a magnetic polarity reversal has been recognized in each core, in the depth range of 60-80 m, and it has been interpreted, by means of the available pollen biostratigraphy and the regional framework reported in Carcano & Piccin (2002), Muttoni et al. (2003), Scardia et al. (2006), as the Brunhes/Matuyama boundary; in cores RL 8 and RL9 also the Jaramillo Subchron was recognized.
The major lithologic change observed in each core, produced by a depositional switch from distal meandering alluvial plain to a prograding braidplain, occurs during a reverse polarity period, interpreted as Subchron Late Matuyama, and it is well constrained between the Subchron Jaramillo and the Brunhes/Matuyama boundary in cores RL8 and RL9; the same age constrain can be inferred in cores RL10 and RL11. This episode, already observed by Carcano & Piccin (2002), has been correlated by Muttoni et al. (2003) to an important Pleistocene climatic event, related to the onset of the major glaciations at the southern foothills of the Alps occurred at ~0.87 ka, during the Subchron Late Matuyama.Published86-873.2. Tettonica attivaN/A or not JCRope
- …
