Tectonophysics, 17 (1973) 61-72. @Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

STUDIES OF MICROTECTONICS, ANISOTROPY OF MAGNETIC SUSCEP- TIBILITY AND PALEOMAGNETISM OF THE PERMIAN DOME DE BARROT (FRANCE): PALEOTECTONIC AND PALEOSEDIMENTOLOGICAL IMPLICATIONS

BERNARDHENRY

Laboratoire de tectonophysique, Paris (France) and Laboratoire de gtomagrkisme, Saint-Maw (France)

(Accepted for publication August 4, 1972)

ABSTRACT

Henry, B., 1973. Studies of microtectonics, anisotropy of magnetic susceptibility and paleomagnetism of the Permian Dome de Barrot (France): paleotectonic and paleosedimentological implications. Tectonophysics, 17: 61-72.

The whole Dome de Barrot area was subjected to Alpine compressions: development of a general schistosity in the northeastern part of the chain and of a magnetic susceptibility anisotropy connected with the tectonics in the entire massif. There is probably a major structural fault, oriented east-north- east-west-southwest, in the southern part of the Dame.

The study of the anisotropy of the magnetic susceptibility and especially of the values of the q- parameter which depends on the principal susceptibilities, allows one to follow the evolution of the in- tensity of deformation throughout the whole chain. This study also reveals the very relative value of the criteria used by Hamilton and Rees in their paleocurrent research.

The coincidence between the natural remanent magnetic direction and the direction of the Alpine compression, which perhaps occurred merely by chance, calls for care in considering earlier paleomag- netic deductions.

INTRODUCTION

The Dome de Barrot is situated in the French Maritime Alps, 5 km southwest of the ex-

ternal crystalline massif of Argentera-Mercantour (Fig. 1). It is composed m basement raised up under de Secondary and Tertiary beds of,the external zone. The Per-

mian, the substratum of which is unknown, is composed from bottom to top of: (1) The Daluis series (minimum thickness 800 m) constituted of massive red pelites in

thick layers.

(2) The Illion-LeouvC series (average thickness 200 m) divided into the Illion facies com- posed of red-layered pelites rich in mud-crack levels, and the L4ouve facies (Bordet, 1950) formed by conglomeration levels, the principal elements of which are rhyolites, sandstones, gneiss and pelites (reworking of the underlying beds). Most often, the passage from one of these three facies to the other is progressive.

The different tectonic studies (Bordet, 1950; Schuiling, 1956; and Vernet, 1958) con- ducted on the Barrot have shown that this brachy-anticline was formed during5 relatively

62 B. HENRY

recent erogenic phase accompanied by an extension of the superficial layers. The first part of this report sums up a microstructural study (microtectonics, analysis of the anisotropy of magnetic susceptibility in parts where no trace of compression can be observed in the field) conducted to determine whether the Alpine compressions which are at the origin of

a very clear schistosity in the Argentera, have also affected the Barrot. The second part concerns the study of the influence of tectonic stresses on the natural remanent magneti-

zation of Permian pelites.

MICROSTRUCTURAL STUDY

Microtectonics

A rough schistosity affects the rocks in the northeastern part of the Dome. The strati-

fication remains, however, quite visible. In the Daluis series, the schistosity planes have a mean dip of 45” towards the northeast in the northern and central parts of the Cians

glens, and towards the north-northeast in the southern part of the glens. In the Illion series, they can only be observed locally, especially in the neighbourhood of overthrusts, contemporaneous with the schistosity, or older faults which were active again during the formation of the schistosity. Around these areas where schistosity appears, the passage to a microfissuration then to apparently undeformed rocks, is progressive. The schistosity planes are, most often, vertical or strongly inclined towards the northeast (orientation in- dependent of those of the faults near which the schistosity appears, except in those parts immediately in contact with these faults).

Three main remarks can be made from these different observations: (1) The normals (poles) to schistosity planes have very similar azimuths (independent

of the orientation of local faults): the schistosity, therefore, appears in the Barrot as a general phenomenon;

(2) In the upper series, true planes of schistosity appear only in zones where the tensions have been stronger (proximity of faults): the northeastern part of the Barrot was thus just at the upper front of schistosity.

(3) The disposition of the schistosity (Fig. 2)*, vertical in the superior series and in-

clined (from 30” to 60’) in the Daluis series, can probably be connected with the develop

ment of shearing stresses, along the Permian basement contact. These stresses could give rise to a separation of the Permian from its substratum. But this last hypothesis is not sup-

ported by any other facts. No element permits of dating this general schistosity in the Dome de Barrot directly,

but there is a general schistosity of the same orientation in the Argentera. In the Roya valley, the schistosity is posterior to the deposit of Cretaceous beds and probably later than that of the Priabonian levels; in the more internal parts (upper Roya valley) this schis- tosity, posterior to the deposit of the Priabonian levels, is earlier than the Figure Oligocene transgression (Guillaume, 1967). It is thus probable that the general schistosity

l The dip correction has been applied to the different diagrams concerning this microstructural study; the dome-shaped deformation of this region is connected, at least for the essential, with an erogenic phase of upthrow posterior to the schistosity.

STUDIES OF MICROTECTONICS 63

50 km

Fig. 1. The Argentera-Dame de Barrot area. 1 = Tertiary and Quarternary beds, 2 = Secondary beds, 3 = Primary beds, 4 = metamorphical rocks, 5 = granite.

of the Barrot is Alpine and dated, here as well as elsewhere, as terminal Eocene-Early

Oligocene age.

Anisotropy of magnetic susceptibility

Structural magnetic analysis has been employed to try to determine whether the Alpine phase had also affected the parts where there is no directly observed trace of deformation. Daly (1970) has shown that, in the deformed rocks, the ellipsoid of anisotropy of mag-

netic susceptibility is generally connected with the last compression phase to which the rocks were submitted, the maximum susceptibility axis being in the direction of the linea-

tion and the minimum axis perpendicular to the schistosity plane. If, on the contrary, the sedimentary rocks have not been tectonized, it is admitted that the minimum suscep- tibility axis approaches the perpendicular to the stratification and that the greatest axis, parallel to the stratification, generally indicates the direction of the paleocurrents.

Consequently, two elements appear essential for the interpretation: the plane including

the maximum and intermediate susceptibility axes (“magnetic foliation”) which generally coincides with a schistosity or stratification plane, and the maximum susceptibility axis (“magnetic lineation”) which often coincides with a tectonic lineation or a paleocurrent direction. In order to show their relative importance, a “foliation” parameter (proportional

64 B. HENRY

Fig. 2. Normals (poles) to schistosity planes (lower hemisphere stereographic projection; dip correction

applied). I = in the Daluis series, 2 = in the Illion series.

to [(Ka t Kb)/2] - Kc) and a “lineation” parameter (proportional to Ku - Kb) were de-

fined, just as the q-parameter which is the ratio of the lineation to the foliation:

q = (Ku - Kb) / (v) -Kc

where Ka, Kb and Kc are the principal susceptibility values, respectively according to the maximum, intermediate and minimum axes (Ku > Kb > Kc). The value of the q-parameter may be varied from 0 (rock affected by foliation without any lineation) to 2 (rock with a very strong lineation). It follows that the q-parameter must have a low value in an un- tectonized sedimentary rock (predominant stratification). The measures performed on such rocks show that this parameter is most often less than 0.5. On this account, Hamilton and Rees (1965) chose this somewhat arbitrary limit (which certainly depends very much on the magnetic mineralogy * to distinguish the undeformed samples.

About one hundred samples have been studied in the Part Saint-Maur laboratory of geomagnetism by means of a torsion magnetometer (Daly and Formont, 1969) which enables determining the principal directions of anisotropy and the q-parameter. The direc-

* The values of the q-parameter are generally, for the same sector, lower in the fine-layered pelites of

the Illion facies (quite noticeable stratification) than in the relatively thick layers of the Daluis series.

STUDIES OF MICROTECTONICS 65

Fig. 3. Principal magnetic susceptibility axes in the Dcme de Barrot (lower hemisphere stereographic projection; dip correction applied). I = maximum axes, 2 = minimum axes, 3 = intermediate axes, A = wer 10 axes in a cone of 5” opening, B = between 10 and 2 axes in a cone of 5” opening.

tion of the main susceptibility axis (after dip correction) is show on Fig. 3. No significant difference appears between samples from the western and the eastern parts of the massif and therefore the measured anisotropy seems to have the same origin in the whole dome. The existence of minimum axes quite near to the vertical shows that the stratification re- mains more important than eventual foliation of tectonic origin. However, the maximum axes, which are well grouped in the neighbourhood of the stratification around a north- west-southeast direction, correspond in the northeastern part of the dome to the intersection of the stratification and schistosity planes. The orientation of maximum and intermediate susceptibility is thus clearly connected with schistosity.

The q-parameter values are generally lower than 0.5, especially in the Illion facies pelites (see the note on p. 64). They decrease regularly from the northeast to the south- We.9 (Fig. 4), thus from zones where sehistosity appears towards regions apparently un- tectonized. On the other hand, these values increase noticeably in the vicinity of relatively important faults (Fig. 5). Indeed, the study of their variation makes it possible to follow the development of the intensity of deformation in the whole massif.

Therefore, the structural magnetic analysis shows (Fig. 4) that the whole Dome de Barrot has been affected by Alpine compressions, generally oriented northeast-south- west (perpendicular to the schistosity planes and to the maximum magnetic-susceptibility

B. HENRY

- 1 km

Fig. 4. Directions of compression of the paroxysmal Alpine phase in the Dome de Barrot, reconstituted from the anisotropy of magnetic susceptibility and microtectonic studies. 1 = Permian outcrop bound- ary, 2 = compression directions (dip correction applied), .? = statistical values of the q-parameter in the Daluis series. (In the Illion series, these values change from 0.5 in the east to 0.05 in the west.)

\ \

1

I ’ / I ,’ ii Y I

-- \

/ / / /

\ 1 \I

I 1 km

S

Fig. 5. Values of the q-parameter in the Cians glens (north-south cross-section; samples from the Daluis series). A = parts where traces of compression are very visible, B = intersection of some faults, C = Escouliires glens fault, D = Pra d’ Astier fault.

STUDIES OF MICROTECTONICS 67

axis). In the southern part of the massif, the direction of compression becomes north-north-

east-south-southwest. This could be connected with the existence of pre-Alpine struc- tures, along which shearing stresses would have appeared. It may be an initial phase of the anticlinal axis of the dome (oriented east-west) or most probably of a major structure directed east-northeast-west-southwest, passing along the southern part of the massif, and which might extend into the Argentera.

From a more general point of view, the studies of the anisotropy of magnetic suscep- tibility make it possible to complete usefully the present knowledge of the evolution of a

rock submitted to a field of relatively unimportant stresses. Though no deformation of the rock is apparent in the southern parts of the Barrot, the

minerals already show a preferential orientation (perpendicular to the compression direc- tion). This orientation can be just noticeable (q = 0.05 for some samples). It becomes

more important when the stresses increase (increase of the q-parameter towards the north-

east). The progressive passage to a microfissuration, then to a schistosity, appears (probably

under certain mechanical conditions). The stratification, however, remains the main planar structure in the northeastern part of the Barrot. So there is no connection between the easy magnetization planes (including the maximum and intermediate susceptibility axes)

and the schistosity planes when these last appear. The Barrot rocks show an intermediate phase between the natural susceptibility of

sedimentation and that which results from the tectonic deformation (where all the prin- cipal axes are related to the stress field).

We can also notice that, from the criteria (minimum axis of susceptibility close to the perpendicular to the stratification; q-parameter lower than 0.5) used by Hamilton and Rees (1965) in their studies of paleocurrents, most of the Barrot rocks could be considered

as untectonized sediments. We saw earlier that this is not the case; these results underline the very relative value of these criteria and the care with which the measurements of the anisotropy of magnetic susceptibility must be used in paleosedimentology.

NATURAL REMANENT MAGNETIZATION

Introduction

Van den Ende (1970) made an attempt to study the secular variation of the Permian magnetic pole on samples taken on the western border of the massif (Daluis canyon). In fact, his results are used as a reference by some authors to determine the virtual geomag- netic Permian pole of the European block. So, it seemed interesting during our work to analyse systematically the characteristics of the natural remanent magnetization (N.R.M.) in our samples. The study which is presented here, was carried out with a translation in- ductometer (Daly, 1970) on about one hundred samples taken from the whole massif and from all the Permian series of the Barrot. After dip correction, the average values of the declination and of the inclination of the N.R.M.-vector are respectively 208” (angle counted positively towards the east from the geographical north) and -10” (vector di- rected upward) (Fig. 6 and 7). This magnetization remains almost unchanged when treated

68 B. HENRY

Fig. 6. Remanent magnetization and Alpine compressions in the Dome de Barrot (stereographic pro- jection; dip correction applied). I = vector upward, 2 = vector downward (1 and 2: samples of which the specific magnetization is higher than 7. lop6 e.m.u./g), 3 = compression directions determined from microtectonic studies and analysis of magnetic susceptibility anisotropy.

in an alternating field (up to 600 Oe). During a demagnetization by heating (Van den Ende, 1970) a considerable decrease of magnetization is observed only beyond 500°C. Such a hard magnetization resembles that frequently found (Daly, 1970) in the metamorphic rocks (crystalline magnetization).

Relations with the Alpine structures

The whole Dbme de Barrot The average N.R.M.-direction coincides to that of the Alpine compressions (Fig. 6).

For each sample, the angle between these two directions was measured (Fig. 8) the orien- tation of the Alpine compressions being determined by measurements of the anisotropy

STUDIES OF MICROTECTONICS 69

Fig. 7. Map of eastern Asia, with virtual geomagnetic poles derived from Permian rocks of the “stable” European continent. France: Ba = pole from pelites of the Dome de Barrot(Van den Ende, 1970; Henry, 1971b); Es = Es&e1 (Zijderveld, 1967); LO = Loddve (Kruseman, 1962);Mo = Morvan (Nairn, 1957); Ni = Nideck (Roche et al., 1962). Western Germany: Na = Nahe (Irving, 1960; Nijenhuis, 1961). England: Ay = Ayrshire (Armstrong, 1957);Ma = Mauchline (Du Bois, 1957); Ex = Exeter (Creer et al., 1954; Cornwell, 1967; and Zijderveld, 1967). Norway: OS = Oslo (Van Everdingen, 1960). Czechoslowakia: Cz (Krs, 1968).

Fig. 8. Number of samples Cy = percentage) according to the angle (x) between the N.R.M.-vector and the Alpine compression direction.

71) B. HENRY

of magnetic susceptibility *. For 25% of the samples, this angle was found to be less than 5”. Thus, the hypothesis of a relation between N.R.M. and stresses cannot be excluded.

The Tide valley

In the Tinee valley (southwest border of the Argentera), the Permian beds were strong-

ly folded (nearly isoclinal folds with steep-dipping limbs), with formation of a flux schis- tosity during the paroxysmal Alpine phase (compressions of the same northeast -- south- west orientation as in the Barrot). Though no dip correction could be applied (mostly invisible stratification). N.R.M.-measurements made on some samples from this area have given results approaching those obtained in the Barrot.

Conclusion

The coincidence of the directions of Alpine compressions and of N.R.M. can have oc- curred by mere chance, but it may also be explained by the acquisition of significant mag-

netization (connected for example with recrystahisation under stress) during the paroxys- mal Alpine phase. Until it is possible to clear up this problem by the paleomagnetic and structural study of other massifs and by induced anisotropy experiments in the laboratory,

it would be advisable not to use the paleomagnetic results of the Dome de Barrot and, more generally, of the regions having undergone tectonic efforts, to establish a geotectonic hypothesis.

ACKNOWLEDGEMENTS

I would like to thank, particularly Doctors L. Daly and A. Guillaume who encouraged and supported me, as well as Professors L. Glangeaud and E. Thellier who allowed me to use the possibilities of their laboratories for my research, and Professor J. Goguel for

critically reading the manuscript.

l The remanent magnetization could perhaps produce an anisotropy of magnetic susceptibility (Daly, 1971; Violat and Lecaille, 1972). However, we have observed, in the northeast of the Barrot, a strong connection between maximum susceptibility axis and schistosity planes, even when the directions of compression become north-northeast-south-southwest. Moreover, the samples from Ldouvi facies show an anisotropy which makes it possible to reconstitute the same direction of compression as that from Illion facies rocks taken in the vicinity. Contrarily to these latter, the Leouve samples have a N.R.M. of very low intensity which seems to have random orientation. Consequently, in the D6me de Barrot, an anisotropy produced by the N.R.M. would probably have a slight importance compared to one of structural origin.

STUDIES OF MICROTECTONICS 71

REFERENCES

Armstrong, D., 1957. Dating of some minor intrusions of Ayrshire. Nature, 180: 1277.

Birkenmayer, K., and Nairn, A.E.M., 1964. Paleomagnetic study of Polish rocks: The Permian igneous rocks of the Krakow district and some results from the Holy Cross Mountains. Ann. Sot. G&l. Pal., 34: 225-244.

Bordet, P., 1950. Le Dome permien de Barrot et son aureole de terrains secondaires. Bull. Serv. Carte Gkol. France, 228, Tome XLVIII: 5 l-89.

Cornwell, J.D., 1967. Palaeomagnetism of the Exeter lavas, Devonshire. Geophys. J., Vol. 12 (2):

181-196. Creer, K.M., Irving, E. and Runcorn, S.K., 1954. The direction of the geomagnetic field in remote

epochs in Great Britain. Geomagn. Geoelectr., 6: 163-168.

Daly, L., 1970. Etude des prop&t& magnetiques des roches mitamorphiques ou simpiement tectoni- &es. Nature de leur aimantation naturelle. Determination de leur anisotropie magnetique et appli- cation a I’analyse structurale. These. Univ. of Paris, 340 pp.

Daly, L., 197 1. L’anisotropie de susceptibiliti magnetique appliqude i I’analyse structurale des roches

et autres materiaux. Note presented at the 15th Gen. Assem. of the U.G. G.I., Moscow, 1971. Ab- stract in the LA. G.A. Rep., Nr. 33 1.

Daly, L. and Formont, R., 1969. Determination des faibles anisotropies magndtiques des roches et

autres materiaux par la mdthode du pendule de.$orsion. CR. Acad. Sci., E, 268: 110-l 12.

De Magnee, I. and Nairn, A.E.M., 1963. La mdthode paleomagndtique. Application au poudingue de

Malmedy. Bull. Sot. Beige Geol., 71: 551-565.

Du Bois, P.M., 1957. Comparison of palaeomagnetic results for selected rocks of Great Britain and

North America. Adv. Phys., 6: 509-514. Glangeaud, L., 1944. Le role des failles dans la structure du Jura externe. Bull. Sot. Hist. Nat. Doubs,

51: 17-38. Guillaume, A., 1967. Contribution a l’itude geologique des Alpes liguro-piemontaises. These. Paris,

Dot. Lab. Geol. Fat. Sci. Lyon, 30, 1969, fast. 1 and 2: l-658.

Hamilton, N. and Rees, A.I., 1965. The anisotropy of magnetic susceptibility of the Franciscan rocks

of the Diablo Range, Central California. Scripps Inst. Oceanogr. San Diego, M.P.L. Tech. Memo., 164: 38.

Henry, B., 1971a. Application de l’anisotropie de susceptibilite magndtique a la tectonique du dome

permien de Barrot. CR. Acad. Sci., D, 272: 1586-1589. Henry, B., 1971b. Contribution a l’e’tude structurale du dome de Barrot: Microtectonique, Anisotropie

de susceptibilite magnetique et Pal.&omagnetisme. These, Univ. of Paris, 194 pp.

Henry, B., 1971~. Sur l’aimantation rdmanente naturelle des pdlites permiennes du dome de Barrot.

C.R. Acad. Sci., D, 213: 1560-1562. Irving, E., 1960. Palaeomagnetic directions and pole positions, II. Geophys. J., 3: 444-449.

Krs, M., 1968. Rheological aspects in paleomagnetism. Int. Geol. Congr., 23rd, Prague, 1968, Proc. 5: 87-96.

Kruseman, G.P., 1962. Etude paliomagnetique et sidimentologique du bassin permien de Lodive, Hdrault, France. Geol. Ultraiectina, 9: l-66.

Lecaille, A., 1972. Anisotropie provoquhe sur le sesquioxyde de fer en grains fins. Conserv. Nat. Arts et Metiers Paris, 66 pp.

Nairn, A.E.M., 1957. Observations paleomagndtiques en France: roches permiennes. Bull. Sot. Geoi. France, 6 (7): 721-725.

Nijenhuis, G.H.W., 1961. A palaeomagnetic study of the Permian volcanics in the Nahe region, SW

Germany. Geol. Mijnbouw, 40: 26-38. Roche, A., Saucier, H. and Lacaze, J., 1962. Etude paldomagndtique des roches volcaniques permiennes

de la region Nideck-Donon. Bull. Serv. Carte Geol. Alsace-Lorraine, 15 (2): 59-68. Schuiling, R.D., 1956. Jointing in the Permian dome de Barrot (S. France). Geol. Mijnbouw, 18:

227-234.

Van den Ende, C., 1970. Secular variation in the Permian red beds from the Dome de Barrot (France).

In: SK. Runcorn (Editor), Palaeogeophysics. Academic Press, London, pp. 101-116.

12 B. HENRY

Van Everdingen, R.O., 1960. Palaeomagnetic analysis of the Permian extrusives in the Oslo region,

Norway. Skr. Nor. Vtiensk.-Akad. Oslo, I: Mat.-Naturvidensk. Kl., 1180.

Vernet, J., 1958. Sur la tectonique du socle permo-werfenien du dsme de Barrot. Trav. Lab. Gcol. Fat. Sci. Univ. Grenoble, 34: 219-290.

Violat, C. and Daly, L., 1971. Anisotropie provoquee sur des roches volcaniques par action d’un champ

alternatif. CR. Acad. Sci., B, 273: 158-161. Zijderveld, J.D.A., 1967. The natural remanent magnetization of the Exeter volcanic traps (Permian,

Europe). Tectonophysics, 4 (2): 121-153.