Ar/
Ar geochronology constraints to aggradational phases and grain size variations show that the two large gravel beds occurring in the sedimentary filling of the Liri fluvial-lacustrine basin ...(central Italy) recorded the occurrence of deglaciation events synchronous within uncertainties with global meltwater pulses at ca. 450 and 350 ka. In particular, we find a precise match between the ages of gravel deposition and the occurrence of moderate sea-level rise events which anticipate those more marked during the glacial termination V and IV in the Red Sea relative sea level curve, as already verified by data from the Tiber River catchment basin. Such correspondence suggests that gravel deposition is facilitated by melting of Apennine mountain range glaciers, which provide the water transport energy and a surplus of clastic input to the rivers draining the mountain regions and flowing into the Tyrrhenian Sea. Therefore, the thick gravel beds intercalated in the sedimentary filling of the catchment basins of the major rivers in central Italy may be regarded as an equivalent proxy of large deglaciation events, similar to the ice-rafted debris in northern Atlantic. Consistent with this hypothesis, we also show the close correspondence between the occurrence of particularly mild (warmer) minima of the mean summer insolation at 65° N and these early aggradational phases, as well as with other anomalous early sea-level rises occurring c. 750 ka and 540 ka at the onset of glacial termination VIII and VI, and 40 ka at the onset of the so-called Heinrich events.
The orogenic belts surrounding the undeformed Adriatic Sea represent the margins of an area known as Adria, the African promontory. We have undertaken a critical appraisal of paleomagnetic data from ...regions of Adria considered parautochthonous relative to Africa, obtained either from biostratigraphically dated sedimentary rocks, corrected for inclination shallowing, or from igneous rocks that are regarded as free from any inclination shallowing bias. Paleomagnetic directions were used to calculate paleomagnetic poles for comparison with coeval, and inclination flattening-free, paleomagnetic poles from stable Africa. Visual coherence of paleopoles for several time slices from the Early Permian to the Eocene supports the construction of a composite apparent polar wander path (APWP) valid for parautochthonous Adria and stable Africa. This composite APWP is compared to previous APWPs, finding good agreement with the global APWP of Kent and Irving (2010). Both APWPs show a remarkable and rapid polar shift of ~40° in the Jurassic that other APWPs tend to underestimate. We interpret this shift to represent a major episode of true polar wander (TPW), from ~183Ma in the Early Jurassic to ~151Ma in the Late Jurassic. Using a simple zonal climate model, the drift motion of Adria attached to Africa appears to be consistent with the distribution of Permian–Cretaceous sedimentary facies on Adria.
•We corrected for inclination bias Permian–Cenozoic paleomagnetic poles from Adria.•We constructed a composite apparent polar wander path (APWP) for Adria and Africa.•We compared our Adria–Africa APWP with global APWPs from the literature.•We found a major true polar wander (TPW) event in the Jurassic.•We found a strong control of paleolatitudes on Tethyan facies deposition.
We present new paleomagnetic results for the early and middle Permian (18 sites and 167 samples) from sedimentary and volcanic rocks from northern and central-southern Sardinia (Italy). ...Characteristic directions magnetization have been retrieved using stepwise thermal demagnetization techniques. The bedding corrected site mean directions for the northern and central-southern Sardinian basins show similar inclinations but differ significantly in declination indicating rotations of up to 55° between the two regions. No indication for inclination shallowing was observed. When corrected for the opening of the Ligurian Sea in the Cenozoic and the Bay of Biscay in the Cretaceous, the resulting paleopoles for northern Sardinia (Latitude 46.6°S, Longitude 46.7°W) and southern Sardinia (Latitude 42.8°S, Longitude 35.0°E), transferred into European coordinates are displaced from the coeval reference poles for stable Europe by ∼30° clockwise and ∼45° counterclockwise, respectively, with a rotation pole located close to the sampling region. Statistical parameters for the inclination-only mean for the 18 sites improve after applying the appropriate tilt corrections, suggesting that the magnetization was acquired before tectonic tilt and therefore allows to date the observed rotations to predate a mid Permian(?) folding event. These new results are in agreement with paleomagnetic data from the Sardinian dyke provinces and support earlier interpretations that the differences in general strike observed there are a secondary feature. Combining the data presented here with published data for the Corso-Sardinian block and the greater Mediterranean realm, we argue that the differential block rotations identified in Permian sediments and volcanic rocks reflect post-Variscan intra-Pangea mobility localized along a wide zone of deformation.
•We identify primary directions of magnetization in Permian volcanic and sedimentary rocks from Sardinia.•The resulting paleopole positions are significantly displaced from European reference data.•Indications for a major intra-Pangea shear zone active in Permian times
Jurassic paleomagnetic data from North America have long been contentious, generating ambiguities in the shape of the global‐composite apparent polar wander path. Here we show from a restudy of two ...subdivisions of the Late Jurassic Morrison Formation at the classic locality at Norwood on the Colorado Plateau that the derived paleopoles reflect variable overprinting probably in the Cretaceous and are of limited value for apparent polar wander determination. We instead assembled an updated set of Jurassic paleopoles from parauthocthonous Adria, the African promontory, using primary paleomagnetic component directions derived from stratigraphically superposed intervals and corrected for sedimentary inclination error. These paleopoles are found to be in superb agreement with independent igneous paleopoles from the literature across the so‐called Jurassic monster polar shift, which in North American coordinates is a jump of ~30° arc distance from the 190‐ to 160‐Ma stillstand pole at 79.5°N 104.8°E to a 148 ± 3.5‐Ma pole at 60.8°N 200.6°E defined by four Adria sedimentary paleopoles and the published Ithaca, Hinlopenstretet, and Swartsruggens‐Bumbeni igneous paleopoles. The implied high rate of polar motion of ~2.5°/Myr across the monster shift is compatible with maximum theoretical estimates for true polar wander. We include a critique of published Jurassic paleomagnetic data that have been variably used in reference APWPs but that as a result of their low quality muted the real magnitude of the Jurassic monster shift. Finally, we provide paleocontinental reconstructions to describe examples of the bold signature that the monster polar shift left in the distribution of climate‐sensitive sedimentary facies worldwide.
Key Points
We review the status of the Jurassic apparent polar wander path, which has long been controversial
We show with new data that the widely used paleomagnetic poles from the Morrison Formation of the Colorado Plateau are overprinted
We provide paleomagnetic poles from Adria, promontory of Africa, that confirm the Jurassic monster shift, a major candidate for true polar wander
Existing correlations of Early Cretaceous nannofossil events to polarity zones in Italian pelagic limestones are the basis for correlation of polarity chrons to geologic stages, and hence calibration ...of the Early Cretaceous geomagnetic polarity timescale. Here, we present correlations of nannofossil events to polarity chrons for the Late Jurassic and Jurassic/Cretaceous boundary interval from the Southern Alps, making the case for revisions of current geologic timescales in this interval. The Jurassic/Cretaceous and Kimmeridgian/Tithonian boundaries can be usefully defined at the onsets of polarity chrons CM18R and CM22R, respectively. The Oxfordian/Kimmeridgian boundary can be assigned to CM25. An apparent polar wander path (APWP) for the Southern Alps, derived from, and time-calibrated by bio-magnetostratigraphic data, is similar to, and provides a proxy for, the African APWP in the absence of adequate African data of this age. The Southern Alpine APWP is characterized by a cusp (hairpin) at ∼
30°N/270°E at a time close to the Oxfordian/Kimmeridgian boundary (polarity chron CM25, ∼
154
Ma), rapid APWP from high latitude (>
60°N) in the preceding middle Jurassic (Bajocian–Oxfordian) interval coincident with the onset of Central Atlantic/Ligurian oceanic rifting, and a return to high latitudes during the Cretaceous. Early Jurassic poles at high latitudes (>
60°N) are evident for Sinemurian to Toarcian–Aalenian time (175–195
Ma). An APWP for this time interval with this level of temporal calibration cannot be constructed using data from Africa itself, or from other Atlantic-bordering continents by rotation to African coordinates.
Paleomagnetic studies of dyke swarms from the Variscan belt of Europe can be used to reconstruct internal postorogenic rotations within the fold belt. Here we present paleomagnetic data from 13 late ...Variscan dykes from Sardinia ranging in age from 298 ± 5 to 270 ± 10 Ma. The dykes can be grouped on the basis of their different directions in strike in a northern, a central‐eastern and a south‐eastern province. Paleomagnetic component directions have been obtained using thermal and alternating field demagnetization techniques, which give reproducible results. The paleomagnetic mean directions differ significantly between northern Sardinia and south‐eastern and central‐eastern Sardinia, the latter two regions yielding statistically similar paleomagnetic mean directions. These results indicate that Sardinia fragmented into two, arguably three, crustal blocks after emplacement of the dykes, which experienced differential relative rotations, as is also indicated by the differences in overall strike directions. The determination of timing, sense, and magnitude of these rotations has major implications for the reconstruction of the geodynamic evolution of the region in post‐Carboniferous times. We argue that the observed block rotations occurred during the Permian as the result of post‐Variscan intra‐Pangea mobility possibly related to the transformation of an Early Permian Pangea B to a Late Permian Pangea A.
Key Points
We determine relative vertical axis rotations within the island of Sardinia
Breakup of Sardinia in at least two blocks after dyke emplacement
Rotations can be due to Permian large‐scale intra‐Pangea wrench faulting
We present trace metal geochemistry and stable isotope records for the middle Eocene Alano di Piave section, NE Italy, deposited during magnetochron C18n in the marginal Tethys Ocean. We identify a ...∼500 kyr long carbon isotope perturbation event we infer to be the middle Eocene climatic optimum (MECO) confirming the Northern Hemisphere expression and global occurrence of MECO. Interpreted peak climatic conditions are followed by the rapid deposition of two organic rich intervals (≤3% TOC) and contemporaneous positive δ13C excursions. These two intervals are associated with increases in the concentration of sulphur and redox‐sensitive trace metals and low concentrations of Mn, as well as coupled with the occurrence of pyrite. Together these changes imply low, possibly dysoxic, bottom water O2 conditions promoting increased organic carbon burial. We hypothesize that this rapid burial of organic carbon lowered global pCO2 following the peak warming and returned the climate system to the general Eocene cooling trend.
The identification of a massive shear zone separating Gondwana from Laurasia during late Palaeozoic times is one of the prerequisites for the controversial Pangea B to A transition. Here we present ...new paleomagnetic data from Permian and Triassic sediments and volcanic rocks from the Toulon‐Cuers basin, SE France, likely to be situated within this intra‐Pangea shear zone. A total of 150 samples from 14 sites were collected in the field; 108 samples yielded reliable paleomagnetic component directions based on stepwise thermal demagnetization up to maximum temperatures of 690°C. After removal of an initial viscous magnetic component from room temperature up to 200°C, a second component of reverse polarity, oriented to the south‐and‐up, was identified in almost all samples of Permian age. The Triassic samples behave similarly, with the notable difference that here, two polarities of magnetization are present. Positive field tests suggest the primary character of this characteristic magnetization. The latitudes of the resulting Early to Mid Permian paleopoles agree well with the corresponding segment of the apparent polar wander path (APWP) for Europe, whereas the longitudes are strung out along a small circle segment, indicating relative rotations between the sampled regions and stable Europe. The Triassic poles, instead, plot close to the Triassic segment of the European APWP and provide an upper time limit for the observed rotations. These results suggest a wrench faulting event associated with intra‐Pangea crustal instability and transformation during the Permian.
Key Points
Identification of intra‐Pangea mobility through differential block rotations
Activity along the identified block belt during Early to Middle Permian times
Applicability of plate tectonic models for tectonic blocks within shear zones
Several paleomagnetic studies on Carboniferous and Permian sedimentary and volcanic rocks from Sardinia and Corsica have recently demonstrated (1) the tectonic coherence between southern Corsica and ...northern Sardinia and (2) significant rotations between individual crustal blocks within Sardinia itself. The geodynamic significance of these rotations, however, is not clearly understood mainly because of uncertainties in defining their timing and causes. In order to contribute to these issues, a pioneering paleomagnetic study on Jurassic carbonates from the Baronie‐Supramonte region of eastern‐central Sardinia has been extended regionally and stratigraphically. A total of 280 oriented drill cores were taken from 44 sites of Middle and Late Jurassic age in the Nurra, Baronie‐Supramonte, Barbagia‐Sarcidano, and Sulcis regions. Despite generally weak remanent magnetization intensities, on the order of less than 1 mA/m, thermal and alternating field demagnetizations were successfully applied to define a characteristic remanent magnetization component in about 60% of the samples. Site mean directions show rather good agreement after correction for bedding tilt and yield Middle and Late Jurassic overall mean directions of D = 269.7° and I = 45.0° (α95 = 8.0°, k = 14, and n = 25 sites) and D = 275.5° and I = 50.7° (α95 = 7.2°, k = 45.3, and n = 10 sites). Positive regional and local fold and reversal tests demonstrate the primary character of the natural magnetic remanence, which is carried by magnetite. These results indicate only insignificant amounts (±10°) of post‐Jurassic rotations within the island of Sardinia. The resulting Middle and Late Jurassic paleopoles (latitude (Lat) = 16.5°, longitude (Long) = 299.1°, dp = 6.4°, and dm = 10.1° and Lat = 23.4°, Long = 301.2°, dp = 6.5°, and dm = 9.7°), corrected for the opening of (1) the Liguro‐Provençal Basin and (2) the Bay of Biscay using rotation parameters from the literature, fall near the coeval segment of the European apparent polar wander path. These results constrain the timing of large differential block rotations found in Late Carboniferous‐Permian rocks to a pre–Middle Jurassic age and lead us to exclude tectonics related to the Alpine orogeny for such rotations.
Key Points
No differential block rotations occurred within Sardinia since the Jurassic
Sardinia underwent two phases of rotation since the Jurassic
Large rotations within the island from Permian rocks are pre Jurassic in age
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