Measurements of anisotropy of magnetic susceptibility (AMS) have been carried out on major decollement horizons, following vertical cross sections (30 sites, 500 samples), within "Aalanien" black shales and lower Cretaceous "Valanginien" marls of the Helvetic nappes (SW Switzerland). This study has been realized from a thematic point of view, and offers the occasion to compare AMS fabrics with paleostress, finite strain and other field data taken from the same localities. These results are of fundamental importance to understand the behavior of strain and stress inside thrusts nappes in order to clarify their relationships with thrust transport directions.
Detailed structural analysis of different decollement horizons has revealed that deformation is located within a 3-5 meter thick layer situated along the basal thrust plane, the intensity of deformation decreases rapidly as one moves away from the thrust fault. In light of new set of AMS data, we show that this striking gradient of deformation can directly be correlated with the different types of magnetic fabrics and shapes of susceptibility ellipsoids.
AMS results indicate that the basal thrust is characterized by prolate (X>Y>Z) ellipsoids, which long axes are orientated towards the WNW (305°), these were invariably induced from tectonic shearing. This direction is constant throughout the Helvetic nappes and is in good agreement with Oligocene thrust directions determined from macroscopic paleostress measurements. Oblate ellipsoids (X=Y>Z) are dominant in the rest of the decollement, in this case the short axes are parallel to the schistosity and the long axes are towards the NE (60°), i.e. parallel to the helvetic chain and to the well-known post-nappe emplacement extension. Moreover, the highly deformed layer, presents minimum AMS axes that are subhorizontal and orientated N-S, whereas the maximum AMS axes are orientated WNW-ESE and shallowly inclined. While in the weakly deformed layers, the minimum AMS axes orientations change to become nearly vertical. As the orientations of the AMS axes in deformed sediments are usually parallel to the orientations of the principal strains, the AMS results indicate that the incremental strain state along the basal thrust is one dominated by subhorizontal shortening. In contrast, the AMS axes further away from the basal thrust are consistent with a strain state dominated by vertical shortening (compaction).
The abrupt change in AMS orientations and form within the decollement horizon indicates that true thrust transport directions can only be observed close to the basal thrust, direct comparison with other strain in the decollement or in the entire nappe pile becomes hazardous due the superposition of primary compaction and various tectonic strains.
We present a magnetic fabric study (12 sites) in strongly deformed metapelites from a well exposed detachment shear zone at Tinos Island (Cyclades, Greece). This detachment fault enables the exhumation of the HP rocks (45 Ma) of the Lower Unit under greenschist facies conditions at around 25-20 Ma. We compared the anistropy of magnetic susceptibility (AMS, 150 samples) to the the anisotropy of isothermal remanent magnetization (AIRM, 39 samples) along a N-S orientated transect showing an evolution and a gradient of ductile deformation and metamorphism towards the detachment fault.AMS is largely dominated by phyllosilicates while AIRM is controled by composite mixture of ferromagnetics grains (magnetite, hematite and pyrrhotite). The appearance of the additional ferromagnetic mineral geothite when approching the detachment zone, suggests pervasive fluid flow in this area during the detachment operation. We found general agreement between orientation of AMS and AIRM axes. In contrast, AIRM is more prolate and anisotropic than AMS. The measured AMS magnetic foliations show a good correlation with the foliation measured with a gionometer on the core samples. Magnetic lineations are generally parallel to mineral stretching lineations. However, the occurrence of composite magnetic lineations suggest that petrofabrics contemporaneous of the 40 Ma blueschist episode may be preserved. We also observed a rough increase of the AMS anisotropy parameters which is consistent with the deformation gradient. Unfortunatly, it is not possible to estimate finite deformation using AMS because of the dominant role of phyllosilicates.
An AMS study of 56 sites has been carried out on the Carboniferous porphyritic monzogranite of Pont-de-Montvert-Borne pluton in the French Massif Central. The studies of magnetic mineralogy: thermomagnetic experiments, IRM, NRM, bulk magnetic susceptibility, optic microscopic observation and X-ray diffraction analysis, show that the main magnetic carriers are paramagnetic minerals (i.e. biotite) with very a small proportion of ferromagnetic minerals. Low anisotropy degree (P parameter) and consistent AMS orientations among monzogranite, enclaves and aplitic dykes indicate that the AMS is primary and acquired during the pluton emplacement in the subsolidus stage. Magnetic fabrics demonstrate that linear deformation is prominent in the area where the granite is in direct contact with micaschists and planar deformation becomes more important in the area where the pluton is surrounded by non-porphyritic peraluminous granites. AMS measurements also show that E-W stretching lineation with shallow plunge is the dominant structure over most of the study area. These AMS results fairly agree with fabric inferred from mineral preferred orientation. The AMS data support the E-W extensional tectonic setting of the Pont-de-Montvert-Borne pluton emplacement as the result of the Late Carboniferous crustal thinning of the Hercynian orogen of the French Massif Central.
In the central parts of the Fennoscandian shield a regional shearzone (The Storsjön Edsbyn Deformation Zone, SEDZ) cuts through three 1.70-1.85 Ga old granite-granodiorite bodies. During the emplacement of a c. 1.85 Ga old granodiorite it suffered from a regional high-grade metamorphosis that gave rise to a subhorizontal E-W striking foliation (SGU, 1992). This pattern rotates to N-S strike with a vertical dip approaching the SEDZ. Different generations of dolerite sills and dikes cut, and are cut, by the zone. AMS (Anisotropy of Magnetic Susceptibility) and susceptibility versus temperature measurements, together with AF and thermal demagnetisation, were performed on samples along three trajectories across the zone. Thin sections of selected samples were analysed. The AMS directions correlate well with tectonic fabrics and major structures interpreted from areomagnetic data, with the minimum AMS axis being parallel to the pole of tectonic foliation. In mylonites the AMS axes are tightly grouped with subvertical maximum axes and in less deformed rocks the maximum and intermediate axes form a girdle pattern. In deformed rocks most often only a soft remanence component can be established, which generally lies parallel to the maximum AMS axis. Samples from a metamorphic rock outside of the shearzone-area carry a stable remanence in AF-fields up to 160 mT, and after a correction by use of the AMS tensor, the mean remanence direction better fits the expected paleo-latitude and longitude defined by the APWP for the Fennoscandian shield. A dolerite dike that is cut by a brittle shearzone has a paleomagnetic age of c. 1.6 Ga, indicating that the SEDZ was active after the dikes emplacement, and a dolerite sill that cuts the shearzone-fabric has a paleomagnetic age of c. 1.25 Ga, an age which post dates its movements. Magnetite is generally the main magnetic carrier, but biotite dominates the magnetic properties in one of the granites. Hematite and amphiboles are also present. Examples of good correlation between mean susceptibility and the degree of AMS shows that the degree of AMS cannot be used as a measure of the magnitude of tectonic strain in these rocks.
SGU, Geological Survey of Sweden, Map description, Kårböle 16F SO, (1992).
Although detailed structural studies and abundant geochronological data have elucidated much of the deformational history of the Slave province, the latest parts of its deformational history are still poorly constrained. Recent studies suggest that late leucocratic granites were emplaced during a period of extension, following the main period of compressional deformation in the province. Evidence for deformation in these younger rocks is explained by a second minor compressional event.
The 2588 Ma Ulu pluton and the 2580 Ma Sneezy pluton are located in the High Lake greenstone belt, northern Slave province. A detailed study of fabrics in both plutons reveals a progressive deformation history, which occurred during their emplacement and subsequent cooling to greenschist facies conditions during regional compression. Anisotropy of magnetic susceptibility (AMS) fabric patterns, combined with three-dimensional gravity modelling, indicate that the Ulu pluton was emplaced as a lopolith, with magma ascending in a dyke-like manner along sets of pre-existing fractures. Deflection of wall rock cleavage and bedding patterns around the Ulu pluton, as well as internal solid-state strain, indicate that compression outlasted the cooling of the pluton. In contrast, the younger Sneezy pluton shows negligible solid-state strain related to the regional deformation.
There is no evidence that late leucocratic plutons in the northern Slave province were emplaced during extension. On the contrary, the regional compressional deformation responsible for the development of the main cleavage in the High Lake greenstone belt continued during the emplacement and cooling of the Ulu pluton at 2588 Ma, and ended shortly after the emplacement of the Sneezy pluton at 2580 Ma.
For the paleomagnetism, the statistical analysis of the magnetization directions has different possible tools. In many cases, directions cannot be obtained, and only great circles are determined. The single statistical approaches on great circles intersection are based on the combination of directions and great circles. For the magnetic fabrics, the magnetic zone axis corresponds to the best intersection of the magnetic foliation of different samples from a same site. The analysis of this magnetic zone axis is therefore exactly similar to that of great circles intersection. Associated confidence zones for the magnetic zone axis are obtained using the bootstrap method, and the same approach is applied here for the great circles. For N circles, 10000 resamplings with each N poles of remagnetization circle are used. For each resampling, the best intersection is associated with the maximum eigenvalue of the mean inertia tensor for these poles. The sphere is divided in contiguous areas with small size and the number of best intersection within each area is determined. Using these numbers, the confidence zones at 95% and 63% are obtained using density contours having 95 and 63% of the calculated best intersections inside. The most often, the confidence zones are lengthened, roughly elliptical. If these confidence zones are not too large, a parametric confidence zone, based on Fisher statistics or, in a more appropriate manner, on the bivariate form of the Fisher statistics, can be proposed. It is calculated using the maximum angular differences between the mean value from the N circles and the density contour at 95%. The confidence zone is not underestimated because of the use of these maximum values. This can be verified using the analysis of the distribution of the best intersection based on density contours for various probabilities. These confidence zones can be used for the fold test. For example, in the FII formation of the Rodez basin (Diego-Orozco, 1994), during dip correction, the confidence zone strongly increases, giving a negative fold test.
Diego-Orozco A, Thesis, Paris, (1994).
New palaeomagnetic data from the Middle Cambrian Liberty Hills Formation in the Ellsworth Mountains (EM) define a south pole (lat 8.2°N, long 330.2°E, A95=4.8, N=16), interpreted as a primary remanence based on positive fold and reversal tests. Similar directional data are also identified in preliminary analyses of the Middle-Late Cambrian Frazier Ridge Formation. The new pole supports an allochthonous origin for the EM but produces a new and controversial pre-break up position.
The EM form part of the Ellsworth-Whitmore crustal block, one of five displaced terranes that make up West Antarctica. The EM expose a 13 km thick Cambrian to Permian succession. The tectonostratigraphic sequence is most closely correlated with that of South Africa. In particular they are the only localities from the palaeo-Pacific margin that preserve near complete Cambrian-Permian sequences and show little effect of the Cambro-Ordovician Ross Orogeny. This, combined with other geological evidence, supports a pre Gondwana break up position close to South Africa and the Ellsworth-Whitmore mountains block has tended to be placed in or close to the Natal embayment in Gondwana reconstructions (e.g. Curtis and Storey, 1996).
The new palaeomagnetic data indicate a more northerly position for the EM than previously suggested, contradicting the geological correlations that associate the EM with South Africa. Additionally, the pole suggests rotation of the EM by either 50° anticlockwise or 130° clockwise when compared with the Gondwana apparent polar wander (APW) path (Grunow, 1995). This is contrary to previous palaeomagnetic data from the Frazier Ridge Formation that suggested approximately 80° of post-Late Cambrian anticlockwise rotation (Watts and Bramall, 1981).
Interpretations based on palaeomagnetic and geological data are therefore in conflict, which poses a problem in the reconstruction of the EM to their pre break up position. This has significant implications for the reconstruction of Gondwana and models of subsequent break up. Firstly, it suggests that the assembly tectonics of West Antarctica may be more complex that previously thought. Secondly, assuming that the geological correlations and palaeomagnetic pole from the Liberty Hills are correct, the amount of rotation suggested by the Liberty Hills pole is reconcilable with present Gondwana models if a palaeolatitude close to South Africa is allowed, implying a more equatorial position for Gondwana by the Middle Cambrian. Thirdly, the data may indicate a problem with the present Gondwana APW path for the Cambrian.
Curtis ML & Storey BC, Geol Soc Spec Pub, 108, 11-30, (1996).
Grunow AM, J Geophys Res, 100, 12589-12603, (1995).
Watts DR & Bramall AM, Nature, 293, 639-641, (1981).
Numerous paleomagnetic data from the European and Apulian units involved in the Alpine orogeny have been acquired since 30 years. Their general interpretation is problematic due to the complexity of the kinematic history since Permian, the uncertainties on references APWPs, the delineation of local and regional rotations, and partly the lack of paleohorizontal. To unravel these difficulties the present review will concentrate on post-50 Ma primary or secondary magnetizations. This allows to refer to the stable Europe reference declinations, quite stable at 6±4 degrees during the 50-20 Ma period (after Besse and Torcq, 1997). The suturing being achieved the debate about African or European references is not pertinent anymore and there is less possibility for successive independant rotations (e.g. in Lias and Cretaceous). The inclusion of recently published data on the Molasse (Kempf) and Northern Apennine (Muttoni) basins as well as unpublished data (ours, Aubourg and Thomas) in Corsica, Cevennes, Ardèche, Provence, Diois and Briançonnais nappes, provides a quite coherent picture around the Western Alps. Stable Europe declinations are found in the subalpine zone North of latitude 45 and in the Massif Central cover. Apart from the 10-15° clockwise rotations observed in the Swiss Molasse and the Aar massif all other areas are characterized by anticlockwise rotations in between Generalized Neogene Anticlockwise rotations in the Western Alpine Realm? 10 and 50°. In particular all internal units together with N. Appenines and Corsica-Sardinia shows an average rotation of 30° just as if these areas were a coherent rotating «plate». The major kinematic problems to solve in this scheme are the age and synchronism of all rotations observed and the way this «plate» is decoupled from stable Europe. A large dextral movement on the Penninic front accounts only for a part of the decoupling. To the South the rotation seems to be partly transferred within the European plate as rotations are recorded also in the Diois nappe, Provence and Corsica-Sardinia. This suggests an interference between the alpine kinematic and the opening of the Provençal oceanic basin in early Miocene. In that scheme Provence would have partially (15° of rotation instead of 35°) followed the detachment of the Corsica-Sardinia block. The partitioning of dextral movement on the Penninic front going to the South may involve a complex transtensionnal zone in between the Pelvoux massif and the Rhone delta. However the data available cannot decide on the synchronism of the different rotations observed troughout the Alpine realm, leaving open the possibility that the Provençal and Alpine story are independent.
In order to test the occurrence of post metamorphic rotations or tiltings, a paleomagnetic study was undertaken in the external alpine area, labelled zone Dauphinoise East of the External Crystalline Massifs (ECM). In this area, a counterclockwise rotation of about 15-20° is expected and associated to a differential shortening in the French "Chaînes Subalpines". This study is a compilation of data from Crouzet et al., 1996 and Ménard et al., 1992 completed by new data. East of the ECM, the zone Dauphinoise is mainly constituted by Liassic marly limestones which has undergone an intense deformation with a slaty cleavage at the same time as a low grade green schist facies metamorphism.The magnetization is clearly carried by pyrrhotite (high resistance to AF, Curie temperature around 320°C) and occurs after the syncleavage deformation. The characteristic remanent magnetization (ChRM) is close to the Miocene paleofield position for the stable Europe. In detail, numerous sites shows significant departures from the expected direction. Orthogonal projection shows a linear behaviour between 150° and the Curie temperature. This implies that possible tiltings or rotations occurred after cooling below 150°C. The fission tracks ages (dating cooling below 110°C) available in the studied area are comprised between 3 and 9 Ma (Sabil, 1995). Taking into account these data, tiltings and/or rotations are estimated younger than 9 - 13 Ma. The stereoplot analysis of the ChRM clearly shows a scatter on both sides of the expected direction interpreted as tilting around a N20 to N40 horizontal axis. In the southern part of the studied area this tilting is consistent with structural data. Nevertheless, the best fitting cone do not always contains the paleofield direction (D=6, I=58) for the stable Europe. Therefore, we must postulate that an other late deformation occurs. Two hypotheses can be considered. First, the above mentioned 15-20° rotation around a vertical axis is possible but not well evidenced. The second hypothesis assume a regional tilting of about 10° toward the N around a N 100 horizontal axis. A comparison between ChRM directions and structural data (mainly S0/S1 lineation) legitimates this interpretation. The relative timing of the different deformations recorded by paleomagnetic data is difficult to establish. The origin of these different deformations will be discussed.
Crouzet C, Ménard G & Rochette P, Tectonophysics, 263, 137-148, (1996).
Ménard G & Rochette P, Bull. Soc. Géol. Fr, 163-4, 381-392, (1992).
Sabil N, PhD Thesis Univ. Grenoble, (1995).
We present a paleomagnetic study applied to tectonics of Provence (South-East basin of France). Different stratigraphic layers are sampled from lower Jurassic to lower Cretaceous. Magnetic holders are depending on layers but the main magnetization fell down before 350°C in any case. All samples present well defined magnetic component with a systematic normal polarity. Fold tests show a pretectonic to syntectonic remagnetization. Considering the age of pyrenean orogenic phase, we can predict an eocene age for this remagnetization. We propose an inhomogeneous rotation in Provence. On the western boundary, no significative rotation can be observed in the region of the Cevennes' fault when an anticlockwise rotation of 15° is defined in Ventoux-Lure mounts (our paleomagnetic directions are compared with paleomagnetic directions for stable Europe expected in Provence from Besse and Courtillot, 1991). This anticlockwise rotation well agree with paleomagnetic results obtained on Esterelite (Zijderveld, 1975), in the SE of the area.
No interpretation for remagnetization process are done but it converges with many paleomagnetic studies in SE of France. It implies a large process which could be consistent with global change of physico-chemical conditions, probably induced by the beginning of Pyrenean orogenic phase.
There is evidence for an anticlockwise rotation of Provence probably limited by Nîmes' fault on the western boundary. The eastern boundary is not defined for the moment. This rotation could be interpreted like the damping of the rotation of the Corso-Sarde bloc during oligocene to miocene period. The rotation of Ventoux-Lure mounts implies a shortening increasing from West to East of the range and it is consistent with structural data from Gratier (1989).
Besse J & Courtillot V, Geophys. Res., 96, 4029-4050, (1991).
Zijderveld JDA, Paleomagnetism of the Esterel Rocks, Thesis (Utrecht)., (1975).
Gratier JP, Menard G & Arpin R, Spec. Geol. Soc. London, 45, 65-82, (1989).
Plioquaternary volcanics outcrop at several places in the broader Aegean province. They are related to the South Aegean active volcanic arc as well as to the back-arc extensional area. In all cases, radiometric data and detailed mineralogical studies are available.
The present study focuses at four areas, two of which belong to the Aegean volcanic arc (Milos, Methana, Egina) whereas the others are situated further to the North (Thessaly, Almopias). We have performed detailed palaeomagnetic and rock magnetic analysis to approximately 200 samples spread in more than 20 sites. Rock types vary among andesites, dacites, rhyolites and basalts. Ages range from 1-3.5 Ma for Milos, 3-3.9 Ma for Egina,1.8-3 Ma for Methana, 2-4.5 Ma for Almopias and 1-1.5 for Thessaly.
The palaeomagnetic study has shown the presence, in most cases, of a stable component, often reverse, and slightly deviated from the expected direction. Rock magnetic analysis reveals the presence of either single-domain magnetite (Egina, Thessaly) or hematite (Milos).
The anisotropy of magnetic susceptibility reaches significant levels for a part of the samples (Egina: 4.5%-6.5%, Thessaly 1%-5%) whereas the anisotropy ellipsoid could be deffined in only few cases (Egina, Methana).
An important number of existing structural data, together with fault plane solutions provide a representative deformation pattern. The stress field for Milos (Pavlides and Simeakis, 1987/1988) implies two extensional phases for the Plioquaternary, whereas normal faults in Thessaly result from a N-S extension (Caputo,1996). In Almopias two extensional phases are also observed (NE-SW and NW-SE, active now. Pavlides, 1998). Comparison of-small-rotation angles and possible rotation of the stress field has been encouraging for the last two cases.
Finally, an attempt has been made to correlate all the above data with geodetic determinations available for the broader area (Le Pichon et al., 1995).
Pavlides S, Simeakis K, Ann.Geol Pays Hell., 33, 161-176, (1987/1988).
Pavlides S., Bulletin of the Geological Society of Greece, XXXII/1, 189-197, (1998).
Caputo R., Annali di Geofisica, 39, 557-574, (1996).
Le Pichon X, Chamot-Rooke N, Lallemant S, Noomen R, Veis G, Journal of Geoph. Research, 100, 12675-12690, (1995).
The Ardennes form part of the Variscan foreland fold and thrust belt in Western Europe. Devonian/Carboniferous carbonate units of low metamorphic grade in the Ardennes have been sampled extensively for paleomagnetic analysis, covering a large area between the North Sea and the German border. The carbonates contain at least two NRM components, also described by Molina Garza and Zijderveld (1996) for the northern Ardennes. A low-temperature component with unblocking temperatures up to 300°C and a high-temperature component with unblocking temperatures up to 580°C. We have studied the spatial and temporal distribution of the two components. The low-temperature component is post-folding in the whole area and shows a higher intensity in the vicinity of Mississippi Valley Type ore deposits, which occur in the easternmost part of the Ardennes. These ore deposits were formed during a post-orogenic extensional event in the Perm/Triassic. The high-temperature component is pre-folding in the northern part of the Ardennes, but shows a pre- to syn-folding relationship in the southern Ardennes. This implies that the high temperature component is a remagnetisation in some way tied to the diachronous Variscan deformation event that occurred during the Late Carboniferous. We will discuss the various possible minerals that carry the two remagnetisations and the mechanisms that may have caused the remagnetisations.
Molina Garza, R.S. & Zijderveld, J.D.A., J. Geophys. Res., 101, 15,799-15,818, (1996).
A preliminary palaeomagnetic study has been carried out on the Early Miocene (22-23 Ma) mafic dyke swarm which intrudes Maláguide and Alpujárride units in the central and western part of the Internal Zone of the Betic Cordilleras. The aim of this study is to ascertain if relative perpendicular orientations (N-S and E-W) shown by different dyke families are of primary or secondary origin. With this purpose, seven sites with dikes showing both orientations were sampled for palaeomagnetic purposes. In addition to palaeomagnetic analysis, we also measured susceptibility versus temperature curves of several representative samples and carried out AMS determinations. Remanence is found to be carried by pyrrothite and magnetite. Low intensities of magnetisation, overlapping of palaeomagnetic components and changes ocurring during the demagnetisation procedure add some dificulties to palaeomagnetic interpretations, but six of the seven studied sites offer reliable results. Mean palaeomagnetic directions show both normal and reversed polarities, and strong clockwise rotations of palaeodeclination (approx. 90-140°) are observed in all cases. Rotations thus seem to be of secondary origin. These rotations are in accord with block rotations observed in the External Zone of the Betics. Since perpendicular dykes provide similar rotations, the perpendicular orientations thus seems to be of primary, intrusive origin.
We present an extensive magnetic fabric study (83 sites) carried out in late Tertiary sandstones from the Zagros-Makran syntaxis. The western part of the syntaxis is constituted by the arc of Fars which contitutes the external part of the Zagros fold belt. Due to a deep décollement within the thick evaporites, folds are mainly detachment folds. These folds present local «Z» shape likely inherited from a complex fault pattern of Cretaceous age. The eastern flank of the syntaxis is made up the arcuated Western Makran which is an active accretionary prism. Folds are tighten and exhibit steep limbs. Magnetic fabrics are essentially oblate and magnetic foliation are parallel to the bedding except in 5% of the sites where magnetic foliations has probably a tectonic significance. We show that magnetic lineations originates from an early parallel shortening and are perpendicular to the shortening direction. In the Fars Arc, we observe a significant ca. 10-20° clockwise deviation of the magnetic linations with respect to the fold axes. This suggests that 1) magnetic lineations are independent of the fold deformation: 2) folds are oblique relatively to the shortening. In the Makran region, a slight anticlockwise obliquity of the magnetic lineation is suggested close to the syntaxis. Further south, we documented magnetic lineations perpendicular and parallel to the expected shortening. We conclude that magnetic fabric formed during the layer parallel shortening and thus behaved passively during the fold event. In this logic, we predict less rotation along vertical axes than the 'V' shape geometry of the syntaxis lets shown.
The Late Proterozoic Bonnemain granodiorite is an elongated E-W pluton of 200 km2 which belongs to the Mancellia domain, part of the Cadomian (Panafrican) block of Brittany. It intrudes Upper Brioverian sedimentary series creating contact metamorphism. It is separated into three different zones which reflect mineralogical changes. These are the Lanhélin's type of biotitic blue granodiorite, the Vire's type of biotite and cordierite grey granodiorite and the Louvgné's type of biotitic white granodiorite. This pluton is characterized by its apparently isotropic structuration. In order to see wether or not it has any secondary anisotropy and therfore to understand how this massif was emplaced, we carried out and anisotropy of magnetic susceptibility investigation. The pluton was sampled at 46 different sites, three to four samples being taken from each site.
The results show a low anisotropy degree, P varying between 1.02 and 1.12 with a mean at 1.04. The foliation is slightly dominant over the lineation. Magnetic lineation directions and magnetic foliation planes were derived from our analyses and they show the pluton has a well defined secondary (second order) structuration.
We will show how these structural results can be interpretated in terms of pluton geometry and emplacement and the relationships between its structuration and the preexisting structuration of the Brioverian sedimentary series.
From the Danish island Bornholm sediment cores from three Lower Cambrian clastic formations (Nexø Sandstone, Balka Sandstone, Grønne Skifre) as well as limestones from the Lower Ordovician Komstad Kalk Fm have been palaeomagnetically analysed by PCA Vector Lines (L), Subtracted vectors (S) and Great-circle tracks (G). By means of Spherical Density plots, 5 to 6 clusters of magnetic components (A-F) have been obtained by each of the methods L, S & G (AL-FL, AS-ES, AG-EG). The clusters of the different methods are confirming each other in most cases, although not all the clusters are significant at the 95% significance level.
Some of the directions (DS, ES, EL, FL) are not yet known from Baltica in the Phanerozoic. Considering the statistical and palaeomagnetic significance, the high blocking temperatures and the bimodal polarity pattern, we suggest that these directions may represent both near-primary and secondary magnetisations of Cambrian age, ordered in a yet unknown sequence. Their common features are 1) shallow inclinations, suggesting a sub-tropical position of Baltica when these components were acquired, and 2) significant changes about 90 degrees of declination in the SE/NW quadrants, indicating quite fast block rotations of Baltica at that time, provided that the age estimates are correct.
A frequently observed relationship between the orientation of anisotropy of magnetic susceptibility (AMS) ellipsoids in folded low grade metamorphic rocks is an alignment of Kmax along the intersection of an axial planar cleavage and primary foliations (e.g. bedding). This relationship is interpreted to represent progressive overprinting of primary depositional/compactional fabrics (Kmin perpendicular to bedding) by a tectonic fabric (Kmin perpendicular to cleavage). Such composite fabrics are usually oblate in shape with Kmax typically aligned perpendicular to regional transport directions.
Sandstone and shale beds have been sampled around a single upright, open anticline from the sub-greenschist facies Bude Formation (Cornwall, UK) in order to investigate further the kinematic significance these relationships. The mean magnetic susceptibility of these rocks is 0.2 x 10-3 SI, suggesting low concentrations of ferromagnetic phases. The mean corrected anisotropy degree is 1.04 with a mean shape parameter of -0.5 (prolate). Kmin and Kint define a girdle distribution striking sub-parallel to the fold axial plane, with Kmin tending to cluster around the fold axis. Kmax axes from both limbs of the fold define a cluster with a mean azimuth perpendicular to the fold axis. This arrangement of Kmax and Kmin could represent an inverse magnetic fabric of composite primary/tectonic origin. However, this is discounted on the basis of close correlation between the orientation of AMS and anisotropy of isothermal remanence (AIRM) ellipsoids. A further discrepancy with the commonly observed AMS fabrics in folds is the strongly prolate shape of the ellipsoids.
The consistency of Kmax orientations irrespective of position within the fold clearly points to a fabric of tectonic origin. Prolate ellipsoids with long axes perpendicular to the fold hinge line are indicative of superimposed stretching at a late stage or post-dating fold formation. Such a situation is consistent with superimposed simple shear documented by the variation in fold attitude on a regional scale, with Kmax parallel to the regional transport azimuth. For this to be the case, the AMS fabric must represent only the last increments of strain in this area, with earlier primary and fold-related fabrics being entirely obliterated.
Of the various methods proposed for estimating population values and confidence limits from a set of anisotropy measurements, the most fruitful approach is that developed by Hext (1963) and Jelìnek (1978), but their work is not widely applied and their respective contributions often confused. Hext was primarily concerned with problems of measurement of individual specimens. He provided first-order theory for propagating error to principal susceptibilities and their directions via the covariance matrix which derives from an estimate of measurement error. Jelìnek was concerned with estimating population means and their uncertainty from a sample of specimens. He extended Hext's approach, calculating the covariance matrix from the specimen anisotropies.
In magnetic fabric studies, particularly in structural geology, the distribution or scatter of anisotropies within a population can be of as much interest as the mean anisotropy. The covariance matrix conveys information on this distribution, but it is a difficult quantity to visualise.
Based on the work of Hext and Jelìnek, this contribution discusses the use of a projection of the covariance matrix to convey information on fabric variability.
Hext GR, Biometrika, 50, 353, (1963).
Jelìnek V, Studia geoph. et geod, 22, 50, (1978).
Twenty six individual lava flows spanning the last one million years have been sampled for paleomagnetic investigations in the Island of La Guadeloupe, French West Indies (FWI). Because of the lack of continuous sections, dating is mandatory in order to describe the temporal evolution of the geomagnetic field in this latest 1 Myr interval. New K/Ar ages ranging from 50 ka to 1 Ma have been obtained on andesites using the Cassignol-Gillot technique at the UPS-IPGP Orsay laboratory. Additional flows for which K/Ar ages obtained with the same dating technique were available (Blanc, 1983) have also been sampled for paleomagnetic investigations. More than 200 samples have been analyzed using both AF and thermal stepwise demagnetization techniques. Duplicate samplings of two single flows at three different sites allowed to show that the within-flow dispersion is negligible for the andesitic lava flows sampled in this study. Direct comparison with an earlier paleomagnetic study performed in the Island shows that for the three investigated flows, modern demagnetization techniques lead to a much better defined paleomagnetic directions. The Brunhes-Matuyama transition has been recorded in a 3 flow section and dated at 781±18 ka, in good agreement with other recent determinations. The mean paleomagnetic pole calculated from the 23 individual normal polarity flows is indistinguishable from geographic north, implying that no significant persistent axial quadrupole term can be identified at this site for the last one million years. This is in contradiction with earlier results and has strong implications for models of the time average field (TAF). An Occam algorithm was used to construct a TAF model from the global volcanic database from the last 5 Ma. Substitution of the mean direction calculated from the earlier study for the Lesser Antilles by the one we obtained reduces the quadrupole term by more than 30%. This dramatic effect, produced by changing data from a single site, demonstrates that older paleomagnetic sites may need to be re-investigated. Furthermore, it also highlights the limitations in the complexity of the TAF that can be inferred from paleomagnetic databases in their present state.
Index of EUG 10 Volume
Further EUG 10 Information
Index of the Journal of Conference Abstracts
Cambridge Publications Home Page
Last Updated on Wednesday, March 17, 1999.
© 1997 Cambridge Publications