Kola Late Proterozoic Inversion Tectonics (Russia)

ACERCA DE LA PUBLICACIÓN “Late proterozoic inversion tectonics in the Northern Margin of the Baltic Shield” por F. López Díaz).

Muchas son las emociones que me embargan al escribir estas palabras. Aquí se muestra, resumido y preparado a la manera académica la publicación que en el año 1993 (más o menos) preparé para las revistas Europeas y ante su rechazo a la Revista de la Sociedad geológica de e$paña, que ante mi sorpresa (después vendrían varias más) también lo rechazó, sin ni siquiera mandar a los árbitros.

Hace poco y por medio del Catedrático D. Enrique Martinez tuve noticia (y tengo las fotocopias) de que alguno de los mencionados en la bibliografía realizó una publicación (año 2002, creo) en la que se defiende un corte aproximado similar al propuesto en el trabajo, pero muy chapucero en mi opinión e incluso a partir de ello deducen unas consideraciones con las que no estoy de acuerdo en absoluto, pero bueno, son cosas del destino.

He de decir que está el texto en Inglés, dado que fue preparado en principio para revistas extranjeras que básicamente lo hacen en ese idioma, después y ante su rechazo, aproveché para publicarlo en inglés en e$paña que siempre queda mejor, pero ni por esas.

Vosotros ya tuvisteis una descripción detallada del viaje a Rusia, y podéis sin esfuerzo imaginaros la emoción que sentí cuando los Rusos me trataban de enseñar algo que era 100% similar a lo que yo había, unos meses antes, expuesto en mi Tesis, solo que allí, en Rusia, en el Ártico se veía infinitamente mejor. Pues ni había sedimentos posteriores que los tapasen ni tampoco deformaciones posteriores que lo complicasen. Esa zona está, desde finales del Precámbrico tal y como era. A pesar de lo que digan unos u otros.

La anécdota fue que al regresar del campo al centro científico. Me preguntó el que me había invitado a ir, que qué me parecia, a lo que contesté: “es lo mismo que en el sur de e$paña” y él, confiado me mostró el Mapa Geológico de la Unión Soviética, diciendo que tenían esa estructura controlada desde el Norte de la Gran Bretaña, hasta donde yo estuve y después hasta Kazajstán. Yo le dije que era imposible, porque es lo que me habían enseñado, y él muy seguró me miró y me preguntó ¿Porqué? A lo que no supe que responder pues lo que había visto con mis propios ojos no lo podía dudar (se ve tan bien, ya que no hay ni siquiera vegetación).

Al llegar a e$paña me dirigí a la Facultad donde hablando con el jefe del departamento le dije que quería dar una conferencia y me contestó, la das y te la pagamos. Lo que sucedió en realidad. Durante un mes preparé los materiales y anuncié la conferencia, que fue un éxito de público, aunque no demasiado de crítica, He de decir que solamente una persona dio muestras de haberse enterado de algo y he de decir su nombre: Joaquina, que curiosamente estaba allí, y me apoyó ante el escepticismo general entre los jefazos de la Facultad. Hasta el día de hoy.

Posteriormente, preparé la publicación y, he de hacer hincapié en que lo que habían dicho en Rusia sobre la extensión lateral de la estructura, hasta Inglaterra (Escocia, más bien) era una verdad como un templo y así, camufladamente estaba esta estructura descrita en diferentes trabajos, que cito en la bibliografía, y en algunos que lo dicen con la boca pequeña, que esos no los cito. De la extensión hasta Kazajstán no tuve bibliografía que estudiar, no probé en la actualidad con Internet, tendría que hacerlo, y quizás lo haga en próximas fechas.

He de decir que las similitudes entre la zona de mi Tesis y lo descrito en este trabajo son sorprendentes y debo hacer constar que yo no sabía nada, cuando realicé la exposición de mi Tesis Doctoral, que por cierto muchas críticas recibió, incluso alguno, no voy a

decir quién, me comentó que me hacía esas críticas para demostrarme que no tenía ni P… Idea, a lo que lo calmé, y no le di mayor importancia. Así que ir a Rusia y ver lo mismo y ver la extensión lateral geográfica enorme de la estructura que yo había descrito en el sur de e$paña, me colmó de alegría y de una satisfacción que no cabe en mí de gozo. He de decir que en la Tesis yo no mostré todo lo que sé de es zona, sino solamente los hechos que me llevaron a proponer el modelo, creyendo que serían más que suficientes para la comprensión general, pero he de decir que algunos doctos personajes, no entendieron nada de nada, una pena y yo con una pérdida de tiempo brutal y una vida que se me complicó sobremanera.

Bien, bien está lo que bien acaba, y aunque mi historia no acabó ni mucho menos, he de decir que ahora estoy preparado para lo que sea. Que es un placer haber ido a otra velocidad que la gente común en mi profesión. Que según pasa el tiempo me dan la razón más y más profesionales. Nunca tuve intención de dudar y es algo que quisiera mostrar a gente seleccionada. Pues se trata del conocimiento geológico de rocas de más edad en e$paña.

En fin que es una maravilla mostrároslo a vosotros mis amigos y compañeros de colegio. Me produjo este tema enormes quebraderos de cabeza disgustos y alegrías, pero tengo la satisfacción de haber acertado y ser digno investigador, breve pero intenso, dado que no me han dado muchas oportunidades. Estoy con este trabajo repleto de felicidad. De la geología no os cuento nada porque es un tema que es algo complejo, en realidad, no demasiado, para los profesionales del Precámbrico, pero sí para el resto de geólogos y como no, para los ajenos a esta ciencia. Algún día si me animo haré una comparación entre esta zona y la de mi tesis, punto por punto y entonces haré una publicación explicándosela a ajenos a la geología como vosotros, mientras tanto ahí va lo que se ve en Rusia, en una pequeña parte de su inmenso territorio que es clave para comprender el final de la época precámbrica en este planeta.

Muchas gracias a todos por perder tiempo con este pobre hombre que dedicó muchas horas a patear monte, pese a mi poca habilidad como deportista. Se hizo simplemente lo que se pudo, y simplemente estoy contento con el resultado. Muchas gracias y ánimo, por lo menos las figuras quedaron aparentes.

LATE PROTEROZOIC INVERSION TECTONICS IN THE NORTHERN MARGIN OF THE BALTIC SHIELD F. LĂ“PEZ DĂ?AZ ABSTRACT: Late Proterozoic deposits on the Barents sea coast in the Kola Peninsula show evidence of tectonic inversion during the Late Precambrian. Upper Riphean deposits are controlled by a normal NW-SE fault with a kilometric downward movement of its NE wall. Associated with that fault is a whole series of deposits of turbiditic affinity which, at the base and in the vicinity of the fault, commonly are conglomeratic. Hanging wall are much thicker (up to 3200 m) than foot wall rocks of the same age (up to 1300 m), indicating the fault was active during the deposition.

The inversion of this fault took place during the Vendian period, during a deformation that also produced highly asymmetric folds and thrust faults whose trends are almost parallel to that of the extension fault. Associated with the rise of the NE block there was new deposition of rocks, in the SW block, which lying unconformably on pre- and syn-extension rocks.

INTRODUCTION The Kola Peninsula contains late Proterozoic deposits spread along the periphery that show great similarity with the present coast-line (Fig. 1). Three different palaeogeographic zones can be differentiated (Liubtsov et al. 1990). The Tiersk Zone, in the inner part (SW), consists of terrigenous sediments of continental origin. The Kildin Zone spread all along the Barents sea coast, presents coastal shelf sediments. The third zone, Rybachi, whose boundary with the Kildin consists of an important tectonic lineation, is formed from deeper water sediments than the other two zones and is also much more strongly deformed. The Sriedni and Rybachi peninsulas (Figs. 1, 2) provide the best and most complete rock outcrop of the latter two palaeographic zones, and is the area that is discussed in this paper on the basis of geological data available and a visit to the area conducted by V. Negrutsa in wich the most important localities were studied.

STRATIGRAPHY The present work has a structural emphasis, consequently the following stratigraphic descriptions are very brief, more details regarding stratigraphy can be found in Negrutsa (1971) and Liubtsov et al. (1989 and 1990).

The map in Fig. 2 represents the Sriedni and Rybachi peninsulas and their sedimentary formations. This area exposes the contact between Kildin and Rybachi zones. Description of the rocks in these palaeogeographic zones follows: THE KILDIN ZONE Here the rocks outcrop over all the Sriedni peninsula and part of the Rybachi peninsula, more precisely over the area between the Great Motka and Eyna Bays (Fig. 2). Two different Groups, which will be described from base to top, constitute this zone: The Kildin Group (clastics) and Volokov Group (clastics).

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The Kildin Group: The rocks of this series consist of conglomerates and sandstones in the lower half and sandy and slaty rocks in the upper half. All of them exhibit coastal shelf features.

Six formations have been differentiated which, from base to top, are: Kutov Formation.- This lies unconformably over the archean basement. It consists of around 40 m of arcosic sandstones with small pebbles and cobbles, sometimes of up to 30 cm, and some breccia-like conglomerates from the basement granitics rocks.

Iernovsk Formation.- It is formed by 200 m of clay sandstones, limolites and dark grey shales of varying thickness.

Palvinsk Formation.- It consists of 160 m of coarse grained quartzitic sandstones with some glauconitic sandstones. These rocks show a wide range of colours, wich makes this formation clearly distinct from the previous one. In its lower part, there are some conglomerate levels and some sandstones with carbonate cement.

Poropelonsk Formation.- This is made up of 280 m of turbiditic rocks consisting of thin layers of fine grained sandstones, alternating with shales which sometimes dominate. Brecciated layers of phosphorite on a metric scale also occur here.

Zemliepakhtinsk Formation.- This consists of at least 500 m of fine to middle grained, greenish or yellowish grey sandstones which lie in metric scale beds. Their composition is quite arkosic (20-25% of feldspar), and there are some beds of phosphoritic composition.

Karuyarvinsk Formation.- No more than 250 m thick, it contains sandstones with carbonates, carbonate clay sandstones, limolites and mainly green or red pelites.

Sedimentary structures are extremely common in this Fm., specially mud-cracks and gypsum crystal concretions.

The Volokov Group.- This Group only occurs at the northern edge of the Sriedni peninsula (Fig. 2), and lies unconformably on the Kildin series, since sometimes it is found on the Zemliepakhtinsk Fm., and not on the Karuyarvinsk Fm. The contact between the two series displays abundant iron oxides which indicate a likely subaerial exposure.

This series consists of just two formations: Kuyakansk Formation.- This consists of 170 m of arkosic sandstones and quarzites, quartzitic conglomerate sandstones with clearly distinct horizons of conglomerates and breccias; specially in the lower part. Whereas in the upper part, sub graywackic sandstones with limolite levels are more common. There is also some conglomerate levels of phosphoritic composition.

Pumansk Formation.- This is constituted by 350 m of alternating rhythmites of subgraywackic sandstones and clay shales.

On the whole, both groups show abundant evidence of shallow marine character (differente types of cross-stratifications, etc.) and of synsedimentary deformations (slumps, etc.) indicating a mass transport from the Northeast for the Volokov Group (also palaeostream directions given by erosive channels). On the basis of such structures Liubtsov et al. (1989) created a table including the characteristics of the tectonic conditions, active or passive, and the water depths of the differents deposits (Fig. 3). The table show three main trends: 1.- In the Kildin Group there are two peak periods of synsedimentary tectonism located in the Iernovsk Fm., and in the Poropelonsk-Zemliepakhtinsk transition. These are documented mainly by coarse conglomerates and abundance of slumps.

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2.- There is no continuity in depositional environements between the Kildin and Volokov Groups.

3.- The Volokov Group is characterized by a constant growth of both synsedimentary deformational activity and of deepening of deposition environement.

These trends will be very important in the final consideration of tectonic development in this area.

THE RYBACHI ZONE This zone spreads over the greater part of the peninsula of the same name and over its NW, NE and SE extensions into the sea (Fig. 1). The deposites within it are thicker than in the Kildin zone and the sediments are of a deeper marine environment.

Two different rock groups have been described in this zone (Negrutsa 1971); the Eynov and the Bargout Groups.

The Eynov Group.- the whole Eynov series shows clear facies homogeneity. It begins with pebble conglomerates and basement blocks and becomes gradually finer up the series. It is interpreted to represent the equivalent of the Kildin Group in the Rybachi Zone, and presents common small scale synsedimentary faults and decametric slumps folds.

Within this group, three formations have been differentiated.

Motovsk Formation.- This is at the base of the series. It lies locally unconformable upon rocks which, although more distal, are very similar to those of the Poropelonsk Fm. in the Kildin Zone. This Fm. consists of 350 m of no-differentiated deposits among which there are coarse grained, quartzitic sandstones and graywackic and subgraywackic sandstones. However, the most outstanding feature of this formation is the presence of large granitic blocks (> 100 m) from the archean basement yielding breccias and polymict conglomerates with granitoid pebbles. These occur mainly at the base of the formation and in the vicinity of the contact with the Kildin Group.

Lonsk Formation.- This gradually replaces the previous formation. It consists of 700 m of conglomerates, subgraywackic and graywackic sandstones, limolites, shales and, less frequently, thin layers of argillaceous shales and marls. There is very abundant evidence of synsedimentary movements; the most relevant being large slump folds in sandstone layers of decametric thickness.

Perevalnaya Formation.- This is at the top of the Eynov series. It consists of some metric scale banks of gravellites and specially of ash-gray and yellowish, massive, graywackic sandstones similar to those of the Lonsk and Motovsk formations. There are also some sandstones with carbonate cement. The thickness is not clear, but probably lies between 1500 and 2000 m.

The Bargout Series.- This is separated from the Eynov series, which it overlies, by an overthrust. Overall this Group represents a thrust-repeated more distal equivalent of the Eynov Group. Three successive formations and one lateral equivalent have been distinguished in this Group.

Maisk Formation.- This consists of 250 m of alternating conglomerates, breccias and sandstones. The conglomerates and breccias often show granitoid pebbles from the archean basement and others clearly belonging to the Kildin series. This recalls the Motovsk formation, though here the pebbles are much smaller.

Zubovsk Formation.- This is made up of 500 m of alternating polymict sandstones, gravellites and shales. The beds become gradually thinner up the series,

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forming a positive series, also evidenced by a proportional increase in shales. There are also some centi- and decimetric limestone layers between the shale beds. This formation also shows strong evidence of synsedimentary deformation, especially slumps.

Tsypnavoloksk Formation.- Traditionally (Negrutsa 1971 and Liubtsov et al.

1989, 1990) this formation has been considered as the top of the Bargout Series.

However, its contact with the Zubovsk Fm. is probably tectonic, and thus it is likely to be a distal equivalent of the Bargout Group.

Its minimum thickness is 260 m and it consists of centi- and decimetric alternations of sandstones and shales, with abundant carbonate rocks. Sometimes, there are decametric conglomerate lenses similar to those in the Maisk Fm. As in the rest of the series, slumps are very common.

Skarbeyevsk Formation.- This consists of a minimum thickness of 500 m of alternating sandstones and shales, with some rare conglomerate lenses and frecuent decimetric limestone layers. Its sandstone/shale ratio and the size of the coarse sandstone fraction recall the Lonsk Fm (Negrutsa 1971).

Although this formation resembles a lateral variation of the Rybachi zone rocks, it is no possible to be more precise about it.

AGE With regard to the Kildin Series, there is general agreement that they are of Upper Riphean age, due to its fossils content (Liubtsov et al. 1989, 1990).

The rocks of the Rybachi Zone, regardless of the problems of correlation between formations, are also interpreted to be of the same age. Evidence for this is that the Motovsk Formation already overlies rocks which are almost certainly equivalent to those of the Poropelonsk Fm. in the Kildin Series, and these are no doubt Upper Riphean.

The main sedimentary break in the area under study is erosive, and was thus probably subaerially exposed, and is located between the Kildin and Volokov series.

For this reason, the latter is assumed to be vendian (s.l.). Further support for this assumption will be given in the next chapter, where the origin of this series is explained.

STRUCTURE Late Proterozoic rocks of the Sriedni and Rybachi peninsulas underwent tectonic inversion during the Late Precambrian. Initially there was an extensional stage that controlled the deposition of the rocks in the Rybachi Zone and later a compressive stage that gave rise to the uplift of the rocks of this Zone and of the area as a whole.

Extensional Stage. The motion of the Motovsk Normal Fault provides particulary strong evidence for an extensional episode (Fig. 2). This is a structure with an approximately NW-SE direction that divides the Sriedni and Rybachi peninsulas and which continues further into the Rybachi peninsula between Great Motka and Eyna Bays. Its noral character and the downward movement of the NE block are evident in the following observations: 1.- A series of hectometrically spaced normal faults synthetic with the main one, near to it, occur in the rocks of the Poropelonsk Fm. along the S coast of the Great Motka bay.

2.- There are sedimentary features in the rocks of the Motovsk Fm. along the isthmus between both peninsulas that record extension. These includes: a) Small

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synsediemntary normal faults whose orientation is consistent with the main fault. b) Important slumps affecting not only the Motovsk Fm., but also all the other rocks of the Rybachi Zone. c) Large hectometric blocks from the granitic basement which gradually become conglomerates and microconglomerates in the Motovsk Fm. This implies that the basement was exposed on the slope produced following the motion of the fault or during development of the fault.

3.- The observation of some fault surfaces of the main fault, dipping 70/75, affecting the Poropelonsk Fm. at the SW edge of the Rybachi peninsula. In this exposure slickensides indicates a normal dip-slip movement.

Normal movement of the Motovski Fault, synchronous with deposition of the Poropelonsk and overlying formations within the Kildin Group, is associated with the Rybachi series formations which can today be seen at the surface. However, this fracture could have operated before that. As we have already seen, the highest level of synsedimentary tectonic activity took place during the deposition of the Iernovsk Fm. in the Kildin Group and such activity could reflect earlier motion of the Motovski fracture.

If this were so, under the Poropelonsk Fm., to the NE of the Motovski Fracture, which is at the base of the Motovsk Formation, there could be a series of deposits also associated with the Motovski Fracture (Fig. 4). However, this could only be checked by carrying out sub-surface investigations (drilling or geophysical).

Compressional Stage.- The compressioanl structures of the zone consists mainly of folds and thrusts: Folds.- The better developed folds occur some kilometres from the Motovski Fault, within the rocks along the NE coast of the Rybachi peninsula, and they become gradually weaker towards the SE. They are, therefore, well developed in the Skarbeyevsk, Tsypnavoloksk and Zubovsk Fms. In these formations, they occur as strongly asymmetric SW-verging, metric to hectometric folds, with one limb dipping slighty towards the NE (20-30ยบ) and the other dipping steeply NE (80ยบ) and even dipping markedly towards the NW. Their axial planes are thus incline strongly towards the NE, and their axes are NW/SE oriented and have a subhorizontal plunge. These folds are associated with an axial planar cleavage, well developed in the politic rocks, the nature of which varies from rough slaty cleavage in the central parts of the Rybachi peninsula, disappearing in the southern zone of that peninsula.

These folds form, within the range of a few kilometres, trains (Fig. 5) which occur repeatedly. In them, it is possible to observe a progressive variation of the interlimb angle increasing from the NE to SW, which contrasts with the regional variation, where to the SW folds are open. Also, and associated with this, there is a similar variation, in the plunge of the cleavage (Fig. 5). Due to this, and to the remarkably asymmetric nature of the folds, we think that possibly they are associated with a simple shear process, which would very likely be the same as that in the thrusts which shall be described later on. In this case, each fold train would be probably be associated with a minor thrusts.

In the Sriedni peninsula folds are scarce. The whole outcrop of the Kildin Zone in fact constitutes a large open synform (Fig. 2) whose orientation agrees with that of the folds of the Rybachi Zone. Only the vicinity of the Motovski fault shows decametric folds, commonly open, corresponding, on account of their orientation, to the afore mentioned system.

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The rocks of the Volokov series show very few folds, only in the vicinity of the Motovsk Fault do some irregular flexures occur associated with different minor fractures.

The fact that the Volokov Group rests unconformable upon the open synform that folds the Kildin Group (Fig. 2) indicates a pre-Vendian age for this structure.

Thrusts.- The main thrust surfaces are found in the contact between the Bargout and Eynov series and in the contact between the Tsyppnavoloksk and Zubovsk Fms. Fault Zones, apart from the more or less preserved brittle fault surface, there is a considerable increase in the number of folds. The map trend of these thrusts is approximately parallel to that of the contacts between formations, which provides one more reason to associate folds and thrusts into one single genetic process. The overthrusts block is always the NE one.

Another important thrust surface is the Motovsk fault itself, which was reactivated as a reverse fault. This movement is evidenced, on the one hand, by outcrops showing that the Motovsk Fm. overthrusted the Zemliepakhtinsk Fm. and probably the Karuyarvinsk Fm. on the NE coast of the Sriedni Peninsula. On the other hand, in some outcrops of the Motovsk Fault, a set of quartz fibers indicates a normal movement while another one shows the reverse movement.

At the SE edge of the Rybachi peninsula, the Motovsk Fm. shows a splay or subordinate thrust trending approximately parallel to the main fault, although its NW edge displays a lateral ramp orthogonal to the main fault. this lateral ramp shows a slickensides, indicating the upward movement of the fault. On the map, it is possible to see how this thrust situates the rocks of the Motovsk fm. over the contact between the Poropelonsk and Zemliepakhtinsk Fms., which once more proves that it is a reverse fault. Where the fault plane can be observed, it is also possible to see that it dips around 45 to 50Âş towards the NE. The age of these thrusts must be pre-Vendian, according to their probable genetic relationships with the folds previously described.

STRATGRAPHICAL SECTION

EVOLUTION

AND

STRUCTURAL

CROSS-

Fig. 4 summarizes schematically the proposed stratigraphic evolution of the studied zone. For the Rybachi Zone, the similarity between the Motovsk and Maisk Formations was used as a basis to situate them on the same level along with the Tsynavoloksk Fm. On the other hand, the Skarbeyevsk fm. would be a lateral development of these previous ones. This diagram (Fig. 4) represents the stratigraphic situation before the inversion of the Motovsk Fault and includes a pre-Motovsk series comparable to the Kildin Series, which, as we have seen above, cannot be confirmed since there is, perhaps, a thick layer of sediments underlying the Poropelonsk Fm. Fig. 6 shows a structural cross.section of the study zone. As in the previous case, it is not possible to know the exact depth to basement below the Rybachi Zone unless we know whether the Kildin Series presents equivalent extensional rocks in the Rybachi Zone. On account of this, the depth to basement in Fig. 6 is positioned at the minimum possible.

In Fig. 6, a hypothetical dĂŠcollement is assumed at the contact between the Archean basement and sedimentary rocks, based on surface thrusts and the nature of the associated deformation. However, the Motovsk Fault may have had a listric morphology when it moved as extensional fault. In this case, the splay shown by this fault would be

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a short-cut. In actual fact, this is not so, since the rocks of the kildin Zone are not affected. In addition, the Motovski Fault itself, dipping at 75Âş, is reactivated. If it had a listric morphology, logically, with such a steep dip it would be fossilized (do not moves in the reverse event). Accordingly, our view is that the movement of this fracture, only reactivated in its upper part, corresponds to a normal planar morphology affecting also the basement.

LATERAL RELATIONS The nearness of the W part of the zone under study to the Scandianvian Caledonides prevents the direct correlation of these structures. However, all along this chain, frequent changes in facies and thicknesses occurred during the Proterozoic.

The direct extension of the Motovski Fault to the W, finds its equivalent in the Varanger peninsula, where, according to the maps available (Foyn 1985), a fracture with the appropriate position and orientation separates two late Proterozoic domains with very different facies and thicknesses. In fact, while the SW block is 2500 m thick (Vadso, Tanafjord and Vestertana Groups), the NE block has almost 15 000 m (Barents Sea and Lokvikfjell Groups). According to Foyn (1985) this fracture has a strike slip character. However, the NW edge of the Kalak thrust is not clearly displaced.

Therefore, we think that it is a normal Precaledonian fault associated with the Motovsk Fault.

More to the W, in the region of the Repparfjord-Komagfjord tectonic window, there is also a remarkable variation in the thickness of late Proterozoic deposits (Pharaoh 1985), between the rocks of that window and the allochthonous rocks of the Laksefjord Nappe (Laksefjord Group). Whilst the thickness of the former is no more than 160 m; with conglomerates, sandstones and pelites, in the latter, containing the same rock sequence, it ranges between 4600 and 6400 m. Therefore, there must be a fault to account for such strong variation in thickness, though not in materials.

To the SW of these zones, in the south of Norway, a late Proterozoic rift has been identified (Morley 1989). The orientation of this structure is SSW-NNE, but the hanging wall is, as in all other cases, the W block.

Extensional processes of this age are common in this part of the world, as proved by de correlation carried out by Bekker et al. (1971).

CONCLUSIONS It is possible to state that, during the Late Proterozoic, at least what is now the N margin of the Baltic shield was affected by extensional tectonics, specially evidenced by a generalized fault affecting sedimentation on its external (hanging) wall. This fault also underwent, at least in some parts along it (Kola) an inverse tectonic process developing later Proterozoic folds and thrusts lying parallel to the extensional fault.

Acknowledgements: I am indebted to G. CorretgĂŠ and A.Vinogradov, that made possible my trip to Russia and to V. Negrutsa and his team that showed me the Sriedni and Rybachi area and other places in Russia. My thanks also to J. Aller who corrected the English manuscript, and C. Morley, K. Karlstrom and D. Roberts for their constructive criticism. Research was supported by the University od Oviedo (Salamanca-Oviedo-Ekaterimburg joint project).

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REFERENCES Bekker, Y.R.; Solontsov, L.F.; Aksenov, E.M. & Negrutsa, V.Z. (1977).“Correlation of the Upper Precambrian (Riphean) in the Russian platform and its framing”; in Sidorenko, A.V. (Ed) Correlation of the Upper Precambrian Metallogeny of the Precambrian, Nauka, Moscow, p. 190-209.

Foyn, S. (1985).- “The Late Precambrian in the Northern Scandianvia” in D.G. Gee & B.A. Sturt (Eds.) The caledonide Orogen-Scandianvia and related areas. John Wiley & Sons, Ltd. P. 233-245.

Gorbunov, G.I.; Makievski, S.I. and Nikolaeva, K.A. (1978).Mеталлогеническая зональност, связанная с тектоно-магматической активзатцей Балтиского щитаю. Sov. Geol.., Nº 4, p. 15-26.

Liubtsov, V.V. Mijailova, N.S. & Predovski, A.A. (1989).-Литостратиграфия и мигрофоссили познего докембрия Колского полуострова. Apatity 129 p.

Liubtsov, V.V.; Negrutsa, V.Z. & Predovski, A.A. (1990).- Upper Precambrian deposits of the Kola coast of the Barents sea. Geological Institute of the Kola Science Centre. Apatity, 49 p.

Morley, C.K. (1989).- “Basin inversion in the Osen-Roa Thrust Sheet, Southern Norway” in Cooper, M.A. & Williams, G.D. (Eds.) “Inversions Tectonics” Geol. Soc. Special Public. Nº 44, p. 259-273 Negrutsa V.Z. (1971).- “Stratigraphy of the hyperborean deposits in the Sriedni and Rybachi peninsulas and the Kildin island”.- In “problems of Precambrian Geology of the Baltic Shield and the Russian Platform cover” VSEGEI, V. 175, p. 153-186.

Negrutsa V.Z., Basalaev A.A.; Chikiriov I.V. (1994).- Баренцевоморский фосфоритовый Басейн. Российская Академия Наук. Колский научный центр. Геологический Институтю Апатиты Pharaoh, T. (1985).- “The stratigraphy and sedimentology of autochthonous metasediments in the Repparfjord-Komagfjord Tectonic window, west Finnmark” in D.G. Gee & B.A. Sturt (Eds.) “The Caledonide Orogen-Scandinavia and Related areas”.- John Wiley & Sons, p. 347-357.

FIGURE CAPTIONS Fig. 1.- Baltic Shield diagram after Gorbunov et al. 1978: A) Archean; B) Proterozoic belts; C) Caledonian; D) Palaeozoical Platform and E) Mesozoic.

Paleogeographical sketch showing the distribution of the main facies divisions in the Proterozoic of the Kola Peninsula: 1) Lower & Medium Proterozoic; 2) Upper Proterozoic: ; I) Rybachi Zone; II) Kildin Zone; III) Tiersk zone; (Liubtsov et al. 1990): 1.- Rybachi Peninsula; 2.- Sriedni peninsula; 3.- Ivanovsk Bay; 4.- Sviatoi Nos peninsula, and Golovnoi stream; 5.- Kashkarakka and Peschanka rivers; 6.- Ostry, Gubnoi streams; 7.- Rivers Sosnovka, Snerhnitsa, Glubokaya; 8.- Chapoma river; 9.Strelna river; 10.- Varzuga river; 11.- Olenitsa river; 12.- Kurzeka river; 13.- Turi cape.

3) Palaeozoic and Mesozoic deposits Fig. 2.- Geological map of the Sriedni and Rybachi peninsulas, after Liubtsov et al, 1989, and Negrutsa et al.1994, and personal observations.

Fig. 3.- Diagramm of the tectonic regime and depositional depth of the Kildin and Volokov series, after Liubtsov et al. 1989

8

Fig. 4.- Stratigraphical sketch of the different formations after the distensive phase.

Fig. 5.- Field cross section in the NW end of the Rybachi peninsula, on Skarbeyevsk Fm. materials.

Fig. 6.- Structural cross section (synoptic) of the studied area.

9

Ba r en ts´

2 1I

Ko

la

II

Pe ni ns u

la

Lower Riphean

10

3

4

III

9 8

16 5 6

7

Ba

en ts´

r

13 1211

S ea

Paleozoic & Mesozoic

S ea

Upper Riphean & Vendian

Wh ite Sea

Sea

Botnian Gu lf

g we

B

a

or

ia n´

s

N

S ea

ltic

lf land Gu n i F

Caledonian

Fig. 1

Archean

Proterozoic

Paleozoic

Mesozoic

Vaida Bay

Great Volokov B. Zemliana C.

Mai-Navolok Cape

Tsyp-Navolok C. Sergueyeva C.

Minor Volokov B.

LEGEND Normal Contact Unconformity Fault Normal Fault Reverse Fault Reactivated Normal Fault

Bargoutni C. Great Motka B. Kutova B.

Eina Bay Moche B.

VOLKOV SERIE

Gorodetski Cape

KILDIN SERIE Kutovsk Fm. BARGOUT SERIE ?

Iernovsk Fm. Archean Basement

Sharapova Cape

Palvinsk Fm.

EYNOV SERIE

Maisk Fm.

Poropelonsk Fm

Motovsk Fm.

Maisk & Zubovsk Fms.

Kuyakansk Fm.

Zemliepakhtinsk Fm.

Lonsk Fm.

Tsyp-Navolok Fm.

Pumansk Fm.

Karuyarvinsk Fm.

Perevalna Fm.

Skarbeyevsk Fm. ?

Fig. 2

FORMATIONS

KUTOVAYA

DEVELOPMENT STAGES

I

Tectonic Regime

KOROVINSKAYA,BEZYMIANAYA IERNOVS- CHERNORECHENSKAYA KAYA (PALVINSKAYA) II

PESTSOVOOZERSKAYA (POROPELONSKAYA)

III

PRIDOROZHNAYA SLANTSEVOOZERSKA- KARUYARYA VINSKAYA (ZIEMLEPAKHTINSKAYA)

IV

V

VI

VII

ACTIVITY STABILITY

Depthness

COASTAL

I Facies

Added Subsidence of the botton of the sedimentary basin (meters)

II III

Un con for mi ty

200 400 600 800 1000 1200 1400 1600

I.- Shore facies II.- Shelf intermediate depth III.- Turbidite y great depth facies

Fig.3

KUYAKANSKAYA

PUMANSKAYA

VIII

IX

125 M

W

E

FIG. 4

VOLOKOV S. KARUYARVINSK ZEMLIEPAKHTINSK

EYNOV S. BARGOUT S. PEREVALNAYA

POROPELONSK PALVINSK IERNOVSK KUTOVSK

LONSKAYA BASEMENT

MOTOVSKAYA Po. Palv.

Iern.

Kut.

FIG. 5

ZUBOVSKAYA

TSYP-NAVOLOK= =SKARBEYEVSK?

MAISKAYA

BASEMENT

VOLOKOV S. SW

KILDIN S.

BASEMENT

Ma isk.

Per ? eva Lons lnaya kaya ?

Mot ovs Kildin S. kaya

Kildin.S? BASEMENT

Sea Level

Mais kay a y Zu Ts bo ypvska Na ya vo lok ? ska ya?

Fig. 6

NE