Aufs/itze

Voo, R. VAN DEn: The rotation of Spain. Paleomagnetic evidence from the spanish Me- seta. - - Paleomagnetism, 8--4, 389--416, Amsterdam 1967.

WELLMAN, H.W.: Wrench (transcurrent) fault systems [in: the Earth's crust and upper mantle, P.J. Hart editor]. - - Am. Geophysics Union, Geophys. Monograph, 18, 545--549, Washington D. C. 1969.

WUNDEBLICr~, H. G.: Zur Frage der Schollenrotation und -translation der Iberischen Halb- insel: Ein Strukturvergleich yon Armorikanischem und Kantabrischem Bogen. - - N. Jb. Geol. Pal/iont. Mh., 1970, 6, 821--884, Stuttgart 1970.

Evidence of progressive deformation in minor structures in Western Asturias (N. W. Spain)

By ALBERTO MARCOS and MARIA L. ARBOLEYA, Oviedo *)

With 10 figures

Zusammenfassung

Beispiele ffir progressive Deformation in Kleinstrukturen sind in der geologischen Literatur nicht h/iufig. In Nordwestspanien k6nnen einige Kleinstrukturen in altpal/io- zoischem, inhomogenem, geschichtetem Material als Ergebnis progressiver Deformation w/ihrend der herzynischen Orogenese gedeutet werden.

Die Beispiele umfassen Strukturen, die durch progressive Kontraktion (gefaltete Keile), Kontraktion mit nachfolgender Verl/ingerung (durch Boudinage unterbrochene FalteR) und Verl/ingerung mit naehfolgender Kontraktion (gefaltete Streekungsspin- deln) entstanden.

Abstract

Examples of progressive deformation provided by minor structures are not frequent in the geological literature, In NW Spain some minor structures in Lower Paleozoic inhomogeneous layered material may be interpreted as the result of progressive defor- mation during the Hercynian Orogeny.

The examples described include struetures generated by progressive contraction (folded wedges), contraction followed by elongation (folds disrupted by boudinage) and elongation followed by contraction (folded boudins).

Resumen

Los ejemplos de estructuras menores que muestran evidencia de haber sido originados pot deformaci6n progresiva no son muy frecuentes en la literatura geol6gica. En el NW de Espafia, determinadas estructuras menores originadas durante la Orog6nesis Herci- niana en rocas inhomogeneas estratificadas del Paleozoico inferior, pueden ser inter- pretadas como el resultado de una deformaci6n progresiva. Los ejemplos que se citan incluyen estructuras originadas pot contracci6n progresiva (cutlas plegadas), contracci6n seguida por elongac~6n (pliegues fuertemente estirados o partidos por boudinage) y elongaci6n seguida por contracci6n (boudins plegados).

*) Authors' address: Dr.A. MARCOS and M.L. ARBOLEY& Depto. de Geotect6nica, Facultad de Ciencias, Universidad, Oviedo, Spain.

278

A. MARC0S, M. L, ARBOLEYA - - Evidence of progressive deformation in minor structures

RpaTEoe co~ep~aHHe

OIIHCaHbI MeJiKHe ,CTpyKTypHbIe TeKTOHHqeCKHe 9JIeMeHTLI ~peBHenaJieo3o~cKo- r o B03pacTa y nopon ~er0MOPeHHLIX H CJI0ttCTI)IX, 05pa30BaBInHecH B pe3y/~bTaTe np0rpeccHBH0i~ ~e~p0pMa~HHa B0 BpeM~ rep~Hc~oro oporeHe3a B ceBepH0t~ I~cIIaHHI~.

OrIHCaH~,I CTpyKTypHbIe 3JIeMeHTI, I, B03HHKalOII4"He HpH Bo3paCTaHHH C~-KaTH51 (CKJ;a~qaT~,Ie ~J~HH],~), CH~aTI, IH C n0c~e~y~0~-~HM paCT:~hxeHHeM (B pe3yJ;~TaTe 5y~HHa~Ka npepBaHI,IX cK~a~oI~) H p a C T ~ e H H H C n0cJIe~[y~0mHM e~KaTHeM (cK~a- ~OqHble <<BepeTeHa>> paCT~eHH:~) .

Introduction

One of the most important contributions of strain theory to the understanding of deformation in rocks has been to envisage the finite state of strain of any substance as an accumulation of successive strain increments. The process of progressive transformation of any material from its initial state to its final state of strain is known as progressive deformation (FLINN, 1962; RAMSAY, 1967 a). Progressive deformation deduced from the syntectonie growth of minerals is well documented in the geological literature (e. g. MISCH, 1969; ELLIOTT, 1972; WmKHAM, 1978) but only ocasionally can the strain history be deduced from minor structures seen in naturally deformed rocks. It is the aim of this paper to present some field examples of minor struetures in which successive stages of the deformation process can be reconstructed.

All the structures described are found in inhomogeneous layered materials. Consequently, and for simplicity, the following description is in terms of plane strain. The terminology used agrees with that of RAMSAY (1967 a) and the reader is refered to this paper for further information.

CANTABRIAN S E A I c~------~,....~ CABO VIDIO ",,? @ ~\'~

o ...........

Fig. 1. Locality map.

G e o l o g i c s e t t i n g Good examples of progressive deformation may be seen in the lower Paleozoic

rocks in the Cabo Vidio sector of the Cantabrian coast (Fig.l). During the Hercynian orogeny these rocks were affeeted by two phases of deformation accompanied by low-grade regional metamorphism (MAa~rE, 1968; ARBOLEYA, 1978). The first phase of deformation produced essentially monoclinie folds which plunge to the north and face towards the east. A well developed slaty cleavage ($1) runs parallel to the axial planes of these folds and a stretehing lineation plunges down dip in the cleavage. The second phase gave rise to open upright folds and an associated crenulation cleavage ($2). The folds are nearly

279

Aufs~itze

Fig. 2. Folded wedge in the limb of a first fold at Aguilar beach (loc. 1). The sandstone bed is contained in slates. View looking north.

homoaxial with those of the first set. The second deformation strongly modified the earlier structures and is responsible for the outcrop pattern in the area.

All the examples of progressive deformation refered to in this paper are attributed to the first phase of deformation. They are seen in the Middle Cam- brian to Lower Ordovician "Cabos Series" which is a sequence of alternating competent beds, mainly quartzite, and incompetent slates.

Contraction or elongation of a layer previously contracted

As it is well known, in the simplified ease of an irrotational progressive deformation, lines in some directions within the strain ellipse are progressively contracted while others are extended. The successive stages of the progressive contraction or elongation of a layer are not in general registered in naturally deformed rocks. In layered inhomogeneous rocks folds or boudins will be formed depending on whether contraction or extension takes place, but only in rare circumstances can the history of the deformation be reconstructed. Fig. 9. shows a competent bed in an incompetent matrix which was first shortened by faulting (formation of a wedge) and later by folding with the development of a slaty cleavage. Both these kinds of structures could be generated by shortening during progressive deformation, the change from wedging to folding resulting from changes in the theological properties of the material during the process.

280

A. MAacos, M. L. ARBOLEYA - - Evidence of progressive deformation in minor structures

Fig. 3. Strongly attennuated folded calc-silicate layers contained in marble (first folds). Gaviera beach (locality 2). a) Folds with very elongated limbs; b) folds completely

disrupted.

In other cases, the deformation history of a line is more complex. During irrotational progressive deformation any line (except one parallel to a principal strain axis) which is suffering shortening rotates towards the long axis of the strain ellipse, and some lines are at first contracted and then extended. Thus, in layered inhomogeneous rocks, folds disrupted by boudinage can often be formed, as explained by oK SITT~.R (1958, Fig. 10) in one of the first references to pro- gressive deformation.

Locality 2 (Fig. 1) provides good examples of structures formed by contraction followed by strong extension. The finite strain within the layers depends upon the ductility contrast between competent and incompetent layers. Where there is a low ductility contrast the folds limbs show a strong attenuation (Fig. 8 a); in extreme cases the folds are completely disrupted and in these circumstances reconstruction of the structure is pratically impossible (Fig. 8 b, lower part). In the case of a high ductility contrast, the folds are disrupted by boudinage (Fig. 4). Fig. 5 shows the position of the minor structures described in relation to the major folds, in locality 2. The observed disposition agrees with the pre- dicted distribution of folds and boudins.

Irrotational deformation has been assumed in the above discussion for simpli- city. Nevertheless the geometry of the structures does not exclude the existence of

281

Aufs~itze

Fig. 4. First folds disrupted by boudinage which occur in the sandstone beds. Locality 2. View looking north.

Fig. 5. Position of the minor folds and boudins in relation to a first fold. Locality 2.

282

A. MAacos, M. L. ARBOLEYA - - Evidence of progressive deformation in minor structures

Fig. 6. A pre-tectonic pyrite crystal with curved pressure shadow zones which developed during the first deformation, seen on a cleavage plane. Diameter of the pyrite crystal

about I ram. Locality 2.

a rotational component. As it is pointed out by RAMSAY (1969, p. 52) and RAMSAY & GaAHAM (1970, p, 795) irrotational strain is exceptional or not existent in naturally deformed rocks. In locality 2 pre-tectonic pyrite crystals have developed long pressure shadows on the first cleavage (Fig. 6). Quartz and chlorite fibres in the pressure shadows show a slight curvature indicating rotation of the crystal during deformation and therefore a rotational component in the strain.

Contraction of a layer previously elongated

The history of the changes in the length of a line during an irrotational progressive deformation is relatively simple. In rotational progressive deformation some lines can have a more complex history; a line can theoretically be expanded and later contracted, which is not possible in irrotational deformation. The structures to be expected in a competent layer within an incompetent matrix include, for example, folded boudins (RAMSAY, 1967 a, Fig. 3- -57; 1967 b, P1.16).

Folded boudins can be seen in the beach at Gabieiro (locality 3, Fig. 1). The rocks are an alternating series of incompetent shales and competent sandstones and quartzites; in the latter, boudinage is well developed. The boudins vary in size and shape, from simple pinch-and-swell structures to rectangular, lenticular and lozenge shapes. Many of the gaps between the boudins are filled with quartz. The slaty cleavage produced during the first deformation phase ($1) is in general parallel or at a low angle to the bedding. Boudinage was produced during the first phase of Hercynian deformation, as shown by the orientation of boudin axes, approximately normal to the stretching lineation (Fig. 7). In several places, these boudins can be seen to be folded (Figs. 8 and 9). In these examples it is obvious that the boudinaged beds have been shortened along their length and that the folds were initiated at the necks of the boudins. There are obvious differences

283

Aufs~itze

IN

Fig. 7. Stereogram showing (A) the orientation of boudin axes, (B) stretching lineation, (C) axes of folds deforming boudins. First deformation phase. West end of the Gabieiro

beach; locality 8.

Fig. 8. Folded boudins on the beach at Gabieiro (locality 8). Fig. b shows folds in thinly layered material between competent beds and a competent bed with lozenge-shaped

boudins.

284

A. MARcos, M. L. ARBOLEYa - - Evidence of progressive deformation in minor structures

Fig. 9. Folded boudins. Locality 3. The folds develop in the boudin necks.

between these structures and those shown in Fig. 4. These folds are commonly isolated and are clearly asymmetric. Their wave length is usually not more than a few centimetres; these seen in Fig. 9 are exceptionally large. Folds with the same characteristics also occur in the thinly layered materials between the compe- tent boudinaged beds, generally but not exclusively near the boudin necks (Fig. 8 b, left). Folds of the kind described by JoNEs (1959, Fig. 29) in gaps between boudins have not been observed. As shown in Fig. 7, the axes of the folds observed are not parallel to the boudin axes, the angle between the two being about 30 ~ A closely spaced crenulation cleavage (Sta) runs parallel to the axial surfaces of the folds. This crenulation is restricted to a small zone around the folds and has no regional significance.

From these data, it can be deduced that shortening of the beds followed the development of boudins and a slaty cleavage. Further there is no systematic relationship between the folds affecting the boudinaged beds and the folds generated during the second Hercynian phase. The structures referred to above are shown schematically in Fig. 10. Thus it is proposed that the folded boudins were generated by rotational progressive deformation during the first Hercynian phase of deformation.

285

Aufs~itze

folded SE boudins NW

\ ~ \F 1 folds i ~ ,n_/

(,,~'),, t /

F, d , o g ' . ~ , ~-T-- v ~ I~ or parasi t ic~_ Ill , i / '~ ' fo lds ' J ~ n o scule)

Fig. 10. Idealised sketch of the first and second phase structures. Note the orientation of the folds affecting the boudins, in contrast to the second phase folds.

Discussion

In a recent paper ELLIOTT (1972, p. 2681) states that the large angle between the finite-strain ellipses that would generate separately boudins and folds, makes the formation of folded boudins in the course of a single deformation phase very improbable. This is supported by the rarity of this kind of structure. ELL~OTT further states that folded boudins generated during a single deformation phase can be found only in the case of a particular type of rotational deformation. Nevertheless, as indicated by RAMSAY (1967b, p. 189), the structures resulting from an incremental strain in rotational deformation may be attributed not to the geometric form of the two finite strain ellipses before and after the incremental strain, but to the asymmetRr between the two finite-strain ellipses and the incremental ellipse.

Rotated boudins similar to those described from locality 8, suggest, according to RAMSAY (1967 a, p. 109) an asymmetric orientation of the beds relative to the strain ellipse. This orientation favours a later contraction of the beds without a large angle of rotation of the infinitesimal strain ellipse. The field evidence shows that in the example described here extension was quantitatively more important than the subsequent shortening. The value of this extension can be calculated from the lozenge-shaped boudins. The average value obtained for the quadratic elongation (2) is 2.5 and the maximum value is 4.7. The amount of shortening is more difficult to calculate owing to the isolated character of the folds, but because of this a low contraction can be assumed. This together with the existence of rotated boudins suggests that a large rotational component of de- formation has not been necessary for the folding of the boudins.

Aknowledgements

We wish to thank to Prof. Dr. M. JULIVEItT for numerous stimulating discussions. We also thank to Prof. Dr. R. M. SHACKLETON, Prof. Dr. J. G. RAMSAY, A. C. R~Es and M. CASEY for reviewing the manuscript and suggesting additional changes that have improved the manuscript.

2 8 6

A. MAl~COS, M. L. ARBOLEYA - - Evidence of progressive deformation in minor structures

References

ARBOLEYA, M.L.: La estructura herciniana a lo largo del corte de la costa entre el Cabo Vidrias y Ballota (Asturias, NW Espafia). - - Brev. Geol. Ast6rica, 17, 49 --55, Oviedo 1978.

ELLIOTT, D.: Deformation paths in structural geology. - - Geol. Soc. America Bull., 83, 2621~2688, 1972.

FLINN, D.: On folding during three-dimensional progressive deformation. - - Geol. Soc. London Quart. Jour., 118, 885~488, 1962.

JONES, A.G.: Vernon map-area, British Columbia. - - Mere. Geol. Surv. Canada, 296, 1--186, 1959.

MATrE, PH.: La structure de la virgation hercynienne de Galice (Espagne). - - Geol. Alpine, 44, 1--127, 1968.

MIscn, P.: Paracrystalline microboudinage of zoned grains and other criteria for synkinematic growth of metamorphic minerals. - - Am. Jour. Sci., 267, 48---68, 1969.

RAMSAY, J .G.: Folding and fracturing of rocks. - - 568 p., New York (McGraw-Hill Book Co.) 1967 (a).

- - : A geologist's approach to rock deformation. - - London, Imperial College, Inaugu- ral lectures, 181--148, 1967 (b).

- - The measurements of strain and displacement in orogenic zones. - - In: Time and place in orogeny. Geol. Soc. London, Spec. Publ. 8, 48--79, 1969.

RAMSAY, J.G., & GRAHAM, R.H. : Strain variation in shear belts. - - Canadian Jour. Earth Sci., 7, 786--818, 1970.

DE SITTER, L .U. : Boudins and parasitic folds in relation to cleavage and folding. - - Geol. Mijnbouw, 20, 277--286, 1958.

WICKH~M, J.S.: An estimate of strain increments in a naturally deformed carbonate rock. - - Am. Jour. Sci., 278, 28---47, 1978.

Proterozoic glaciations in eastern Brazil: a review

By REINHARD PFLUG, Freiburg i. Br., and WALTER U. SCHOLL, Diamantina *)

With 7 figures

Zusammenfassung

.~ltere Arbeiten fiber proterozoische Vereisungsspuren in Ostbrasilien werden referiert und im Lichte neuerer Untersuchungen aus der Espinhago-Zone revidiert. Die frfihere ,,Lavras-Serie", hitufig als Beispiel einer infrakambrischen Vereisung zitiert, umfal3t Einheiten verschiedenen Alters. Die ~ilteren Einheiten (Sopa Konglomerat in Minas Gerais; ,,Lavras-Serie" sensu strictu in Bahia) geh6ren der mittel- his altproterozoischen Minas-Supergruppe an und enthahen keine Vereisungsspuren. Die verbreitete und zum Teil glaziale Macaflbas-Gruppe geh6rt der basalen Sfio-Francisco-Supergrnppe an, ffir die aufgrnnd von Stromatolithen ein Mindestalter von 950 Mio. Jahren angegeben wer- den kann. Das tats~ichliche Alter der Macaflbas-Gruppe k6nnte 1650 Mio. Jahre fiber- schreiten. Infrakambrische Vereisungsspuren sind aus Ostbrasilien nicht bekannt.

*) Authors' addresses: Prof. Dr. R. PFLUG, Geologisch-Pal~iontologisches Institut, Uni- versit~it Freiburg, D-78 Freiburg, Hebelstr. 40; Dr. W. U. SCn6LL, a/c Instituto Esch- wege, Bua Silv6rio Lessa 100, 89 100 Diamantina (Minas Gerais), Brasilien.

287