Palaeomagnetism of the Ordovician dolerites of the Crozon Peninsula (France)

Summary. In order to obtain a Lower Palaeozoic pole for the Armorican Massif and to test the origin of the Ibero-Armorican arc, the Ordovician dolerites of the Crozon peninsula have been palaeomagnetically studied. The samples show a multicomponent magnetization which has been revealed by AF and thermal demagnetization and thoroughly investigated with rock magnetic experiments, polished section examinations and K/Ar dating. Four groups of directions have been recognized, often superimposed on each other in an individual sample. One component (D) has always the lowest blocking temperatures and coercivities and is considered to be of viscous origin, acquired recently in situ or in the laboratory during storage. Two components (A and B) are interpreted to be of secondary origin and to correspond to the observed K/Ar age distribution between 300 and 190 Myr. These ages represent the time interval between two regional thermo-tectonic events, associated with the Hercynian orogeny and the intrusion of dykes related to the early opening of the Central Atlantic Ocean and the Bay of Biscay. A fourth component (C) could be of Ordovician or younger Palaeozoic age; it is not clear whether the age of the magnetization is pre- or post-folding, but a pre-folding age would yield a direction of magnetization similar to Ordovician results from the Iberian peninsula. The latter interpretation suggests a fairly high palaeolatitude, which is in agreement with a glacio-marine postulated for sediments overlying the dolerite sills.     

Palaeomagnetism of the Lower Ordovician Orthoceras Limestone, St. Petersburg, and a revised drift history for Baltica in the early Palaeozoic

Palaeomagnetic investigation of Lower Ordovician limestone in the vicinity of St. Petersburg yields a pole position at latitude 34.7°N, longitude 59.1°E ( dp / dm =5.7°/6.4°). A probable primary remanence origin is supported by the presence of a field reversal. The limestone carries one other remanent magnetization component associated with a Mesozoic remagnetization event.
An apparent polar wander path is compiled for Baltica including the new result, ranging in age from Vendian to Cretaceous. Ages of the published Lower to mid-Palaeozoic palaeomagnetic pole positions are adjusted in accordance with the timescale of Tucker & McKerrow (1995). The new Arenig result is the oldest of a series of Ordovician and Silurian palaeomagnetic pole positions from limestones in the Baltic region. There are no data to constrain apparent polar wander for the Tremadoc, Cambrian and latest Vendian. If the Fen Complex results, previously taken to be Vendian in age ( c . 565 Ma), are reinterpreted as Permian remagnetizations, an Early Ordovician–Cambrian–Vendian cusp in the polar wander path for Baltica is eliminated. The apparent polar wander curve might then traverse directly from poles for Vendian dykes on the Kola peninsula ( c . 580 Ma) towards our new Arenig pole ( c . 480 Ma). The consequence of this change in terms of the motion of Baltica in Cambrian times is to reduce significantly a rotational component of movement.
The new Arenig pole extends knowledge of Ordovician apparent polar wander an increment back in time and confirms the palaeolatitude and orientation of Baltica in some published palaeogeographies. Exclusion of the Fen Complex result places Baltica in mid- to high southerly latitudes at the dawn of the Palaeozoic, consistent with faunal and sedimentological evidence but at variance with some earlier palaeomagnetic reconstructions.     

The rock magnetic signal of climate change in the maar lake sequence of Lac St Front (France)

Rock magnetic properties of the maar lake sediments of Lac St Front (Massif Central, France) reflect environmental changes during the last climatic cycle. High magnetic concentrations are measured in the sediments deposited under glacial climatic conditions, while lower concentrations correspond with more temperate climatic periods. Low- and high-temperature measurements indicate that the remanence is carried by (titanium-poor) magnetite. However, some maghemite and haematite is present in sediments deposited under temperate conditions.
Normalized intensities and coercivities of the anhysteretic remanent magnetization (ARM) are clearly higher for the sediments deposited during the temperate climatic periods of the Eemian, St Germain I, II and Mid-glacial than for glacial sediments, but other magnetic parameters hardly differ between these groups. Due to slight differences in magnetic composition and possible effects of grain interactions, it is not straightforward to relate this different ARM behaviour to magnetic grain-size variations. For the Holocene sediments, rock magnetic parameters indicate a larger grain size. This trend is also suggested by granulometric experiments with an optical laser granulometer. Dissolution of smaller grains is the most likely explanation for this larger grain size.
Changes in magnetic composition and grain size are extremely limited for the glacial sediments, but magnetic concentration varies considerably. Magnetic concentration maxima in the glacial sediments of Lac St Front correlate with those of the nearby Lac du Bouchet (Thouveny et al. 1994). Correlating the susceptibility records of these sequences with the δ¹⁸O record of the GRIP ice cores (Thouveny et al. 1994) suggests that magnetic concentration maxima may correspond with short cold climatic episodes, associated with Heinrich events.     

Ordovician tectonic shift in the western North China Craton constrained by stratigraphic and geochronological analyses

The western North China Craton (W-NCC) comprises the Alxa Terrane in the west and the Ordos Block in the east; they are separated by the Helanshan Tectonic Belt (HTB). There is an extensive debate regarding the significant Ordovician tectonic setting of the W-NCC. Most paleogeographic reconstructions emphasized the formation and rapid subsidence of an aulacogen along the HTB during the Middle–Late Ordovician, whereas paleomagnetic and geochronologic results suggested that the Alxa Terrane and the Ordos Block were independent blocks separated by the HTB. In this study, stratigraphic and geochronologic methods were used to constrain the Ordovician tectonic processes of the W-NCC. Stratigraphic correlations show that the Early Ordovician strata comprise ~500-m-thick tidal flat and lagoon carbonate successions with a progressive eastward onlap, featuring a west-deepening shallow-water carbonate shelf. In contrast, the Late Ordovician strata are composed of ~3,000-m-thick abyssal turbidites in the west and ~400-m-thick shallow-water carbonates in the east, defining an eastward-tapering basin architecture. Early Ordovician detrital zircons with ages of ~2,800–1,700 Ma were derived from the Ordos Block; the Late Ordovician turbidites were sourced from the western Alxa Terrane, based on zircon ages clustered at ~1,000–900 Ma. The petrographic modal composition and zircon age distribution imply a provenance shift from a stable craton to a recycled orogen in the Middle Ordovician. These shifts define a tectonic conversion from a passive continental margin to a foreland basin at ~467 Ma, resulting in the eastward progradation of the turbidite wedge around the HTB, the eastward backstepping of the carbonate platform in the east and the eastward expansion of orogenic thrusting in the western Alxa Terrane. This tectono-sedimentary shift coincided with the advancing subduction of the southern Paleo-Asian Ocean beneath the Alxa Terrane, generating the western Alxa continental arc and the paired retro-arc foredeep in the east under a compressional tectonic regime.     

Scattering parameters of the lithosphere below the Massif Central, France, from teleseismic wavefield records

Teleseismic P -wave recordings are analysed in the frequency range 0.3–6  Hz to derive structural (statistical) parameters of the lithosphere underneath the French Massif Central. For this we analyse differences in frequency-dependent intensities of the mean wavefield and the fluctuation wavefield. It is possible to discriminate a weak fluctuation regime of the wavefield in the frequency range below 1  Hz and a strong fluctuation regime starting above 1  Hz and continuing to higher frequencies. The observed wavefield fluctuations in the frequency range 0.3–3  Hz can be explained by scattering of the teleseismic P wave front at elastic inhomogeneities in the lithosphere. A statistical distribution of the inhomogeneities is assumed and the concept of random media is applied. The lithospheric structure under the Massif Central can be described as a 70  km thick heterogeneous layer with velocity fluctuations of 3–7 per cent and correlation lengths of the heterogeneities of 1–16  km.     

Palaeomagnetism and K—Ar age of the Upper Ordovician Alcaparrosa Formation, Argentina

Summary. Palaeomagnetic data from 71 hand samples of igneous rocks of Late Ordovician age exposed in western Argentina (31.3°S, 69.4°W, Alcaparrosa Formation) are given. Stable remanent magnetization was isolated in the majority of samples; they yield a palaeomagnetic pole at 56°S 33°E ( N = 8, α₉₅= 16°). Whole rock K-Ar age determinations yield an age of 416 ± 10 Myr for a pillow lava of the Alcaparrosa Formation.
Palaeomagnetic data for South America, Africa, Australia, Antarctica and India suggest that Gondwana was a unit at least as far back as 1000 Myr. The palaeomagnetic data define a rapid polar migration for Gondwana in Ordovician time which is consistent with the widespread occurrences of Late Ordovician glacial deposits across this supercontinent.     

Ordovician magnetostratigraphy: Llanvirn-Caradoc limestones of the Baltic platform

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Palaeomagnetic sampling has been performed covering 43 stratigraphic levels within the Baltoscandian Ordovician carbonates. After removing a ubiquitous Permo-Carboniferous (287 ± 14 Ma) remagnetization between 200 and 500 C, a Llanvirn-Caradoc reversal stratigraphy is delineated by components with maximum unblocking temperatures up to 550-580 C. Three reversed (SE, down) and three normal (NW. up) antipodal polarity intervals have been recognized. A primary/early diagenetic remanence age is therefore inferred for the stratigraphically linked polarity chrons. Primary magnetizations are resident in detrital/biogenic or early diagenetically formed single- and pseudo-single domain magnetite phases and subordinate early diagenetic pigmentary haematite.
The recognition of a primary remanence within these well-dated Ordovician carbonates has the following important tectonic and magnetostratigraphic consequences.
(1) Accurate time-calibration of the Baltic APW path implies that rapid counterclockwise rotation took place in late Tremadoc and Llandeilo times. The Arenig-Llanvirn epochs are characterized by a 'still stand'. Baltica occupied intermediate to high Southerly latitudes during the early Ordovician (Tremadoc-Llanvirn). Systematic northward drift is recognized in post-Llanvirn times.
(2) A time-calibrated Ordovician reversal stratigraphy is proposed. Presently available data suggest the geomagnetic field was predominantly reversely polarized during Tremadoc and Arenig times. Two normal polarity zones of short duration are identified within mid-Llanvirn and mid-Llandeilo strata. Discontinuities within the succession may mask other short-period events. Late Llandeilo to mid-Caradoc times were then characterized by a normal polarity field.     

Palaeomagnetism of Palaeozoic ultrabasic rocks from the Sudetes Mts (SW Poland): tectonic implications

A complex palaeomagnetic, rock-magnetic and mineralogical study of ultrabasic rocks from the Sowie Góry Block (GSB) and Jordanów–Gogołów Serpentinite Massif (JGSM) revealed the presence of several components of natural remanent magnetization (NRM). The authors found three groups of Palaeozoic as well as Triassic and Recent components of the geomagnetic field. The Palaeozoic components of NRM are carried mainly by magnetite of several generations formed during several serpentinization episodes. Permo-Carboniferous component (A1) present overall the Sudetes was isolated in one JGSM and two GSB exposures, whereas the Late Devonian–Early Carboniferous component (A2) was found in two exposures from the GSB. The corresponding remanent components were already revealed in palaeontologically dated sediments from other West Sudetic units. In the GSB, it was probably acquired during its unroofing dated isotopically for ca. 370–360 Ma. The newly determined group of Palaeozoic directions (A3) was found in three localities from JGSM and in two from GSB is interpreted as the oldest overprint. In JGSM, it was acquired probably shortly after the first oceanic serpentinization phase dated isotopically for ca. 400 Ma. Its acquisition in GSB corresponds to the time of emplacement of ultrabasic xenoliths dated isotopically at ca. 390 Ma. So we suppose that the mean A3 calculated for five exposures corresponds to the 380–400 Ma time span and that at that period both massifs formed one microplate. Mean inclination of A3 places this microplate at 380–400 Ma at the palaeolatitude of 23°S, whereas the West Sudetes were situated during the Early Devonian at 16°S. We suggest that during the Early Devonian the microplate comprising GSB and JGSM massifs was situated to the south from the West Sudetes and accreted them during Middle–Late Devonian.     

Preliminary reconstruction of sediment-source linkages for the past 6000 yr at the Petit Lac d'Annecy, France, based on mineral magnetic data

An 8 m core from the central plain of the Petit Lac d'Annecy, France, two floodplain cores, river bedload sediments and several hundred soil samples from the catchment have been studied using magnetic techniques. The soils, mainly developed on limestones and local glacial tills, show widespread magnetic enhancement with higher ferrimagnetic concentrations and contents of SP grains than found in the lake sediments. Some soils show significant concentrations of canted antiferromagnetic minerals (mainly haematite). Using magnetic quotient parameters the surface soils are classified into four mineralogical types. The lake and floodplain sediment properties over the past 6000 yrs can largely be explained by the erosion and deposition of these sources, with a smaller superimposed biogenic (magnetosomes) signal. Derived sediment-source linkages allow the construction of several hypotheses about geomorphological changes in the catchment system: (i) the long-term erosion of high altitude unweathered substrates has gradually increased towards the present day; (ii) the erosion of high altitude soils has increased within the last 1000 yrs, possibly during the period of the 'Little Ice Age'; (iii) shifts towards an increased erosion of surface lowland soil occurred ~2000 and 1000 yrs ago and may be linked to an accelerated accretion of floodplain overbank deposits; (iv) there has been a significant storage of surface soil within floodplains, which leads to an underestimation of the importance of soil erosion in the lake sediment records; (v) the sediment transported by high magnitude, low frequency flood events has shifted in source from high altitude soils before ~1000 cal. yr BP to lowland and mid-altitude free draining soils after ~1000 cal. yr BP.