TECTONOPHYSICS

ELSEVIER Tectonophysics 276 (1997) 253-263

A cross-section through the Zilair Nappe (southern Urals)

F. Bastida a.*, J. Aller a, VN. Puchkov b, Ch. Juhlin c, A. Oslianski d

" Departamento de Geologia, Universidad de Oviedo, Oviedo, Spain h Institute of Geology, Ufimian Scientific Center, Russian Academy of Sciences, Ufa, Russia

C Department of Geophysics, Uppsafa Universit)', S-75235 Uppsala, Sweden d Uralskam Geological Methodic Expedition, Ekaterinburg, Russia

Received 17 May 1996: accepted 2 January 1997

Abstract

A structural transect in the Zilair-Kugarchi area involves the western part of the Suvanyak Complex, the Zilair Nappe and the eastern part of the foreland thrust and fold belt. This section has been analyzed using field, microstructural and seismic data. The cross-section shows the transition from the hinterland to the foreland in the footwall to the suture of the southern Urals. The rocks involved range from early Palaeozoic to Permian in age. A characteristic of the Zilair Nappe is the dominance of a succession of volcanic greywackes and mudrocks of Late Devonian age (Zilair Formation). The metamorphic grade decreases from east to west, from greenschist facies to diagenetic conditions. The structure of the cross-section mainly comprises west-directed thrusts and thrust-related folds with an associated cleavage. Fold vergence changes along the section depending on of the distance to the associated thrust and its geometry. The Zilair thrust which separates the Zilair Nappe from the foreland thrust and fold belt acconunodated ca. 10 km displacement and the characteristics of the deformation are similar on both sides of it. The contact between the Zilair Nappe and Suvanyak Complex is a west-dipping normal fault that does not represent a major tectonic boundary.

Keytmrds: Urals: thrusts: folds: cleavage: seismic reflection

1. Introduction

The part of the southern Urals located to the west of the Main Uralian Fault forms the footwall to the suture of the Uralide orogen (Zonenshain et aI., 1984; Puchkov, 1991; Matte et aI., 1993; Brown et aI., 1996) (Fig. 1). A traverse through the footwall to the suture from east to west encounters in the study section the transition from the hinterland, constituted by the high P metamorphic rocks of the

* Corresponding author. Fax: +34-8-5103103: E-mail: bastida@asturias.geol.uniove.es

Maksutovo Complex, the Suvanyak Complex and the Zilair Nappe, to the foreland thrust and fold belt.

The Zilair Formation consists of an Upper De­vonian-Lower Carboniferous flysch succession, that appears in the core of a large synclinorium (Keller, 1949; Khvorova, 1961; Zonenshain et aL. 1984; Kulagina and Pazukhin, 1986; Rodionov and Rad­chenko, 1988; Puchkov, 1991). The Zilair Formation was considered by Puchkov (1991) as a facies linked to the margin of the East European continent. Other authors have considered the Zilair Nappe as an al­lochthon transported from the east (Matte et aI., 1993; Brown et aI., 1996). However, the internal

0040-1951/97/$17.00 © 1997 Elsevier Science B.Y. All rights reserved. PII S0040-1951 (97)00059-0

U1..'" (b)

Sterlitamak

.~ .~'Zil~

.... c .. . '.1: .'::' .

.'•. ?~dytection

i':f .: ,.

Foreland Basin N(a) and Foreland Thrust and Fold Belt

Permian

~ Carboniferoust o

_ Ordovician to Devonian

o Vendian• :>1o Upper and Middle Riphean Co

S ~ ;:>I::::: :1 Lower Riphean ~

~ Zilair Nappe "­

" ~I:':'· <:::J Mainly Upper Devonian g .g

Suvanyak Complex ::­'~. [:; == Mainly Ordovicia1 to Devonian

~ '" Uzyan Nappe .....

'0o ~

'0 Ordovician-Silurian ~

'"t:; Bashkirian terrane I

0­'" .... ~ Lower Riphean to Devonian

Kraka Allochthon

~ Ophiolites

Maksutovo Complex

a 100 km § High pressure rocks

Fig. I. (a) Sketch showing the location of the area where the Zilair-Kugarchi cross-section is placed in the Ural Mountains. (b) Geological sketch of the wcstem part of the southcrn Urals (sources: Gcology of the USSR 1:500,000 and 1:200,000 South Ural Map Series and Brown ct a!., 1996); the location of Ihe cross-section and the Rl14 seismic profile are shown.

255 F. Bastida et at. /Tectonophysics 276 (1997) 253-263

structure of the Zilair Nappe is not well known. Zo­nenshain et a!. (1984, fig. 27; 1990, fig. 4) described it as a synclinorium composed of west-facing folds and reverse faults in a cross-section across the Kraka Allochthon (Fig. 1). About 20 km to the north of the present section, Kazantseva and Kamaletdinov (1986, fig. I) described the Zilair Nappe as a west­directed imbricate thrust system. A similar geometry has been proposed by Brown et a!. (1996, fig. 9) for a section located ca. 135 km to the north of the section presented in this paper.

The aim of this contribution is to analyse the structure across an E-W-oriented traverse between the villages of Zilair and Kugarchi, in which a com­plete section of the Zilair Nappe appears together with the westernmost part of the Suvanyak Com­plex and the eastern part of the foreland thrust and fold belt (Fig. 2). The structural interpretation has been partially constrained with data provided by a seismic reflection profile (RI14) located ca. 60 km to the north of the geological section (Fig. I). An unmigrated seismic reflection profile (RI16) which approximately follows the geological section has also provided some constraints for the interpretation.

2. Stratigraphy

The westernmost part of the Suvanyak Com­plex consists of phyllites with some interbedded quartzites of probable Lower Palaeozoic age (Rodi­onov and Radchenko, 1988). This succession cul­minates with a Devonian age sandstone unit, that appears below the basal chert that underlies the Zilair Formation. The normal stratigraphic contact between these three rock units can be observed in the westernmost part of the Suvanyak Complex (Fig. 2).

The Zilair Nappe consists predominantly of Up­per Devonian rocks of the Zilair Formation, which consists of a basal chert overlain by a green, monotonous greywacke and mudrock succession, locally interbedded with microconglomerates and limestones. Graded bedding, parallel and cross-lam­ination, and ripple marks are common, and provide criteria for determining the way up. The greywackes and microconglomerates consist mainly of quartz (often with corroded borders indicating a volcanic origin), plagioclase, muscovite, chlorite and minor amounts of K-feldspar and epidote; occasionally de­

trital biotite is also present. Chert, quartzite and volcanic rock fragments are common. The volcanic rock fragments were probably original glass frag­ments that have transformed to chlorite and mus­covite, or fine-grained quartz. In other cases, the volcanic fragments are rich in plagioclase. Some fragments of quartzitic schist have been found in the upper part of the formation. The volcanic contribu­tion in these rocks, though variable, implies that they may be described as volcanic greywackes or micrv­conglomerates. The mudrocks are mainly siltstones composed of quartz, muscovite and chlorite; clay­stones are also common. The precise determination of the thickness of the Zilair Formation is hampered by the presence of thrusts; nevertheless, a thickness of ca. 6000 m is consistent with the geometry of the section.

The foreland thrust and fold belt in the study area consists mainly of a more than 3000 m thick suc­cession of Carboniferous formations with dominant limestones, greywackes and shales (Fig. 2) (Keller, 1949; Sinitsina et a!., 1984; Pazukhin, 1995). In addition, there is a small outcrop of the Zilair For­mation in the easternmost part of the foreland thrust and fold belt.

3. Metamorphism

The metamorphic grade in the Zilair Nappe in­creases towards the east, where greenschist facies (chlorite zone) are found. Preliminary illite crys­tallinity (lC) determinations were made in some mudrocks using a laboratory procedure consistent with that outlined by the IGCP 294 working group (Kisch, 1991). The values of IC obtained were cor­related with the Kubler Neuchatel limits for the anchizone (OAnO.25 .d°2e).

A IC value of 0.21 .d°2e has been obtained in a locality near the village of Zilair (Fig. 2), suggesting that upper chlorite zone conditions were reached in the eastern part of the Zilair Nappe. An IC value of 0.24 .d°2e obtained 1.5 km to the east of the Sosnovka thrust (Fig. 2) indicates that the chlorite zone approximately extends up to this thrust, where the epizone/anchizone boundary probably occurs. An IC of 0.33 .d°2e in the western part of the Zilair Nappe indicates that middle anchizone conditions exist in this part of the nappe. Unfortunately, some

'" c ~ ~ ;:;. '" tv

~ ...... ~ ~ tv

~ I

""­

_

tv V> 0\

N N N N N

:. ~

N=54 N= 73 N= 15 N=62 N= 25 ~ Fold aX8$, foreland Poles to Sl, Central and Fold axes. Zilair Poles 10 Sl, Easlernmost Li. Zilair Nappe thrust and fold bell Western Zilair Nappe Nappe Con:OUTs: Zilair Nappe

1,2,4,8 and 16% 1,2,4,8 and 16% 1,2,4.8 and 16% 1,2,4,8 and 16% 1.2,4,8 and 16% Contours: Contours: Contours: Contours: ~

~

"­-- m",~" rn"" ~O m,,,,,, ZILAIR NAPPE , -----------------~~1"o(r-- SUVANYAK COMPLEX - ~-+-----------------f:..

SOSNOVKA THRUST

KUGARCHI THRUST ZILAIR THRUST _ ~ ~?

~,~GIA;C~'VI~LAG;\ ,.".~~,<~~~~~ --~x; -- --=_"'7'" _, "/'

;t./ "" '___ _ ~\-- "'-...

'----­'----­

tv - 2.; _ Mudstones, sandstones and limestones Permian

Carboniferous _ Limestones, greywackes and shales

Famennian c=J Zilair Fm (Greywackes and mudstones) '<ml~~ Mainly Ordovician c=J Suvanyak Complex (Phyllites and quartzites) o 10 kmto Frasnian

Fig. 2. Geological cross-section between Zilair and Kugarchi with stereographic plots of fold axes, intersection lineation (Li) and poles to S I. See Fig. I for locarion.

257 F. Bastida et al. /Tectollophysics 276 (1997) 253-263

samples collected in the western part of the section could not be used to locate the anchizone/diagenesis boundary because they do not contain illite.

4. Seismic constraints

Bashneftegeofizika collected several reflection seismic lines across the Zilair Nappe in the mid­1980s for petroleum exploration purposes. The lines are on the order of 100 km long and extend down to 6 s two-way time (TWT). They begin near the western margin of the Zilair Nappe and extend east­wards across the Main Uralian Fault. The data were acquired using a 96-channel recording system with a shot spacing of 100 m and a station spacing of 25 m, resulting in 12-fold COP coverage on average.

The western segment of one of these lines, R1l4 (Table 1), has been reprocessed at Uppsala Uni­versity (Table 2). Previous Russian processing has ignored the curvature of the profile, so in the pro­cessing of the western segment of R 114 we gathered the COPs along a slalom line. As a result, the length of the corresponding stacked section is shorter than in earlier Russian displays. Aside from the crooked line binning, other important processing steps were dip filtering, residual statics and Kirchoff migration. The final migrated section is shown in Fig. 3.

The most outstanding feature observed in Fig. 3 is the presence of a reflective band that can be fol­lowed from the surface. where it coincides with the western boundary of the Zilair Nappe, to the eastern border of the seismic profile. This band dips moder-

Table I Acquisition parameters

Line segment RI14a Spread type Split Number of channels 96 Min. offset (m) 12.5 Station spacing (m) 25 Nominal shot spacing (m) 100 Charge type/size (kg) TNT/4.0 Nominal charge depth (m) 5 Nominal fold 12 Recording instrument PROGRESS Sample rate (ms) 2 Record length (s) 6 Profile length (km) 35.8

Table 2 Processing parameters

I Read Russian R4 format tapes 2 Bandpass filter

40-125 Hz. 0 ms 30-90 Hz, 500 ms 25-75 Hz. 2000 111S

20-60 Hz, 4000 ms 3 Trace and shot edit 4 Field statics 5 AGC 400 111S window 6 Mute first arrivals 7 Dip filter 4 ms/trace 8 CDP gather 9 Velocity analyses 10 NMO II Residual statics 12 Stack

oms-4700 m/s 1000 ms-4800 m/s 2000 ms-5500 m/s

13 Dip filter 3 ms/trace 14 Kirchoff migration-velocity 5100 m/s

ately to the east in the western part and it forms a gentle synform and antiform in the eastern part. The maximum depth to the reflective band reaches 2.7 s. This band can be interpreted as the base of the Zilair Nappe (Zilair thrust), and it separates two zones with different seismic facies. Other remarkable reflections in the Zilair Nappe (Fig. 3) probably correspond to east-dipping thrusts which show the imbricate char­acter of this nappe. The previous Russian migration of line RIl4 shows a reflection dipping 70° to the west that coincides with the eastern boundary of the Zilair unit.

The unmigrated profile R116, shot by Bashnefte­geofizika, shows attenuation of the basal reflective band of the Zilair Nappe and a steeply west-dipping reflection marking the eastern boundary of the Zilair Nappe, which is comparable to the one observed in the Russian migrated line R114.

5. Structure

5.1. Suvanyak Complex

Only the easternmost part of the Suvanyak Com­plex is included in the cross-section. Here, over­turned east-facing folds (Fig. 2) with an associated

258 F Bastida et al. /Tectol1ophysics 276 (1997) 253-263

400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000

COP ~====~ --..,;.;10km

Fig. 3. Western part of the migrated seismic line R114. 21 = Zilair thrust; reflections a and b are probable thrusts within the Zilair Nappe. See Fig. Ib for location.

slaty cleavage to rough cleavage dominate. Small open folds with an associated crenulation cleavage are locally observed. Moreover, there are some kink­bands with moderately dipping axial planes.

5.2. Zilair Nappe

The structure of the Zilair-Kugarchi transect is dominated by N-S trending folds, which mainly verge to the west in the western part of the sec­tion and to the east in the eastern part (Fig. 2). An associated cleavage (S I) is developed in the central and eastern parts of the section. Folds are associ­ated with dominant west-directed thrusts, and east­directed back thrusts. Late extensional faults cut the thrust stack. In the east, one such extensional fault with a minimum displacement of ca. 4 km results in the disappearance at the surface of a kilometric­scale normal limb of the Zilair Nappe, and forms the boundary between the Suvanyak Complex and the Zilair Nappe (Fig. 2). The geometry and kinematics of this fault are not well known, but taking into ac­

count the dip indicated by the seismic data, it has been interpreted as a west-dipping extensional fault.

The west-directed Sosnovka thrust (Fig. 2) has a minimum displacement of 10 km, and divides the Zilair Nappe into eastern and a western subunits. From east to west, the eastern subunit exhibits a close east-facing overturned anticline with abundant minor folds, and a gentle upright syncline in the hangingwall of the Sosnovka thrust. The western subunit includes an anticline and a syncline in the eastern part; both are upright open folds. A polycli­nal geometry can be observed in the anticline, which shows a horizontal crest zone. To the west, a set of open stair-like, east-facing hectometric-scaJe folds appears, that continues in a major upright anticline with abundant close minor folds.

Two west-directed thrusts form the boundary of the Zilair Nappe with the foreland thrust and fold belt (Zilair thrust). The attitude of this thrust has been inferred from the seismic data. This attitude and the structure of the western subunit of the ZiJair Nappe suggest that the Zilair thrust presents a hangingwall

259 F. Bastida et al. /Tectonophysics 276 (1997) 253-263

ramp in the westell1most part and a hangingwall flat to the east (Fig. 2). Assuming that the dip to the east of the western part of the Zilair thrust corresponds to a footwall ramp, a minimum displacement of ca. 10 km can be inferred for this thrust.

All the folds of the Zilair Nappe exhibit a well developed associated cleavage S I. The orientation of minor fold axes and So-S 1 intersection lineations in the Zilair unit are shown in the stereoplots of Fig. 2. In the anticline of the eastern subunit. S1 dips to the west (Fig. 2) and is very well developed. In this area, the cleavage forms an appreciable angle with the reverse limbs of the folds and it is usually subparallel to the normal limbs. However, the cleavage locally dips less than the bedding in the normal limbs, giving rise to transected fold relationships. The cleavage is vertical or dips dominantly to the east in the rest of the Zilair Nappe (Fig. 2).

In outcrop, S1 is generally a spaced cleavage, and it appears as a pencil cleavage when S1 is perpendic­ular to So in the zones with subhorizontal bedding. In thin section, S I is a rough domainal cleavage. In the sandstones, the cleavage domains are dominantly wiggly or sinuous and they are defined by opaque material in most of the unit and by muscovite, chlo­rite and opaque material in the easternmost part. In the mudrocks. the cleavage is defined by discon­tinuous closely spaced domains. Sj is a result of pressure solution; in addition, oriented recrystalliza­tion of phyllosilicates occurred in the easternmost part of the unit.

5.3. Fore/and thrust andfold belt

The foreland thrust and fold belt in the Zilair­Kugarchi section consists of a west-vergent imbri­cate thrust system, of which only the eastell1most part is shown in Fig. 2. The Kugarchi thrust is a west-directed structure with a displacement of ca. I km near the surface, and it shows a 5 m wide band of intensely brecciated and fractured fault rock. Some small east-directed thrusts (back thrusts) can be seen in the westell1most part of the section.

The folds of this unit (Fig. 2) have the geometry of fault-propagation folds. These folds are upright and open in the eastern part and change to west­vergent open to close folds in the western part. In the hangingwall of the Kugarchi thrust, the interlimb

angle decreases with the proximity to the thrust and overtull1ed folds can be seen near the thrust (Fig. 2). The change of interlimb angle can be observed be­tween the minor folds near the hinge zone of the syncline located 2 km to the east of the Kugarchi thrust and the folds located a few tens of metres from it (Fig. 4a and b, respectively). In the first of these localities, the geometry of the folded layers is mainly class Ie (Fig. 4c), suggesting that they are flattened parallel folds. This allows the determi­nation of the local maximum shortening (Bastida, 1981), that ranges from 30 to 50% (Fig. 4d). An in­cipient cleavage is often observed in the incompetent beds showing a divergent fan relationship to minor folds.

Rarely, small open folds affecting the cleavage are observed in the westell1most part of the section.

5.4. Relationships between .tel/ds and thrusts

In spite of the available seismic data, the interpre­tation of the deep structure of the cross-section and the relationships between the different structures are hampered by the lack of any detailed structural map­ping in the area. Nevertheless, we present a tentative analysis of the relationships between the structures shown in the cross-section.

Folds and west-directed thrusts are the dominant structures. The geometrical relationships between both types of structures are varied (Fig. 2). In some cases, the thrust probably cuts only one limb of the adjacent fold (Kugarchi thrust and Zilair thrust); in other cases the thrust may cut both limbs of the adjoining anticline (Sosnovka thrust); finally, the general structure of the hangingwall of the Zilair thrust forms a gentle synform that is probably paral­lel to the deep trace of this thrust. Although the folds have varied geometries and attitudes, a single cleav­age is associated with folds all along the section. the only exception being local late folds. This cleavage decreases in intensity from east to west. All these features suggest that the folds are generally related to the development of the thrusts. Different examples of thrust-related folds can be tentatively described.

The major anticline in the front of the Zilair Nappe and the folds located near to the Kugarchi thrust can be interpreted as fault-propagation folds developed in the hangingwall of the associated

260 F Bas/ida e/ aL. /Tec/onophysics 276 (1997) 253-263

(a) (b) 60 ~

50 50: N = 13

40

N; 21

% % 30

20

10

o -l----.+--­o· 20· 40· 60· 80· 100· 120· 14 O· O· 20· 40· 60· 80· 100· 120· 140·

Interlimb angle

(c) 3 2 1C 18 1A

20

10

%

-1

N= 33

X= 0.65 s = 0.42

Interlimb angle

(d) 45

40

35 N=20

30 + X=40.28 s= 12.48% 25 :

20 ~ 15 I 10 1

5 ,

o ~­o 1 2 3 o 10 20 30 40 50 60 70 80

Intercept value t'90 Maximum shortening (%)

Fig. 4. Interlimb angle histograms for minor folds developed in Carboniferous limestones and shales of the foreland thrust and fold bell. (a) Locality 2 krn to the east of Kugarchi. (b) Locality near Kugarchi. (c) Intercept values histogram (Bastida, 1993) showing the geometry of the folded layers for the same locality as (a). (d) Histogram of maximum shortening determined from the analysis of flattened parallel folds in the same locality as (a).

thrust. The open stair-like, east-facing folds in the the associated cleavage) make with the thrust below, western part of the Zilair Nappe could be detach­ and the distance to this thrust. Axial surfaces of the ment folds above the basal thrust of this unit. The hangingwall folds located far from the thrusts are probable concordance between the bedding of the Zi­ commonly subperpendicular to the thrust, whereas lair Nappe and the deep trace of this thrust suggests the angle becomes acute, with the vertex pointing that the gentle syncline present in the central part of to the east, near to the thrust. This orientation sug­the nappe, below the Sosnovka thrust, is a fault-bend gests that the strain in the hangingwall changes from fold. The major anticline in the eastern part of the a mainly irrotational E-W shortening in places far Zilair Nappe could be a fault-propagation fold of a from the thrust, to a strain that superimposes on blind thrust. According to the strain pattern described this shortening a component of thrust-related rota­below, this fold could have developed, at least in part, tional deformation near to the thrust. Similar models prior to the Sovnoska thrust and be subsequently have been described in other fold and thrusts belts translated by it. Hence, the Sovnoska thrust could be (Mitra, 1978; Rattey and Sanderson, 1982) and the interpreted as an out-of-sequence thrust. corresponding theoretical strain models have been

A relationship exists in the cross-section between developed by Sanderson (1982). This strain pattern the angle that the axial surfaces of the folds (or involves, in addition to the change in attitude of the

261 F Bastida et al. /Tecrol1ophnics 276 (1997) 253-263

principal strain directions, an increase in the amount of maximum shortening when approaching the thrust surface. In the case under study, this strain pat­tern agrees with the fold geometries observed in the hangingwall of the Kugarchi thrust, and it also agrees for the folds above the Zilair thrust, though in this case the folds are of different type. The model does not apply to the hangingwall of the Sosnovka thrust, where tighter folds with better developed cleavage are found to the east, farther from the thrust. This ge­ometry suggests that the eastern folds were initiated prior to the movement of the Sosnovka thrust.

This strain pattern explains the changes in the vergence of the folds along the section (Fig. 2). Folds located above a footwall ramp and far enough from the thrust not to be affected by the thrust related simple shear component verge to the east. Folds located above a footwall flat and far from the thrust below are uptight folds. Folds located near to the thrust below verge to the west or are upright folds in some cases.

6. Discussion and conclusions

The Zilair-Kugarchi cross-section shows an im­bricate thrust system with associated folds. Three major units of the southern Urals (westernmost part of the Suvanyak Complex, Zilair Nappe, and fore­land thrust and fold belt) are present in this sec­tion. The contact between the Zilair Nappe and the Suvanyak Complex is a normal fault that does not represent a major tectonic boundary. In fact, a normal stratigraphic succession including from top to bottom materials of the Zilair Formation, basal cherts and rocks of the Suvanyak Complex has been observed in different localities. Hence, the differenti­ation of the Zilair Nappe and Suvanyak Complex can only be maintained on the basis of their stratigraphic significance. The thrust which separates the Zilair Nappe and the foreland thrust and fold belt exhibits a displacement of ca. 10 km and the characteristics of the deformation are similar on both sides of it. On the other hand, some rocks of the Zilair Formation crop out in the foreland thrust and fold belt with an attitude that suggests a normal stratigraphic contact with the overlaying rocks.

The presence of a thick Upper Devonian textu­rally and mineralogically immature dominantly vol­

canoclastic succession in the Zilair Nappe indicates provenance from a near, partly volcanic source area which, according to Zonenshain et al. (1984), should be located to the east. The presence of fragments of quartzitic schist in the upper part of the succession indicates the erosion of a source area with rocks probably deformed and metamorphosed prior to the Late Devonian.

Greenschist facies metamorphic conditions are found in the eastern part of the section, whereas probable diagenetic conditions exist in the foreland thrust and fold belt. The anchizone/epizone boundary is located within the Zilair Nappe.

A migrated reflection seismic profile located ca. 60 km to the north of the Zilair-Kugarchi section (RI14) shows a strongly reflective band which has been interpreted as the Zilair thrust. On the other hand, an unmigrated profile following the Zilair­Kugarchi section (R 116) suggests that the intensity of this reflective band decreases towards the south.

The folds of the cross-section exhibit an associ­ated cleavage which is well developed in the east and nearly disappears towards the west. The folds are related to the thrusts, and fault-propagation folds, fault-bend folds and detached folds exist in the sec­tion. However, the Sosnovka thrust truncates folds and it is interpreted as an out-of-sequence thrust.

The orientation of the cleavage and the axial surfaces of the folds changes with the dip of the thrust below and the distance to this thrust. These changes and the variation of the interlimb angle are related to an increase of the maximum shortening and the rotational character of the strain towards the basal part of the thrust sheets. The fold vergence changes are related to the dip of the thrusts and the strain pattern described.

Some controversy exists concerning whether or not the Zilair Formation is allochthonous (e.g Puchkov, 1991; Matte et aI., 1993; Brown et aI., 1996). Puchkov (1991) interpreted the Zilair Forma­tion to have been deposited in a basin connected to the margin of the East European continent with a provenance from the Magnitogorsk volcanic arc. However, based on structural data, Matte et al. (1993) and Brown et al. (1996, 1997) considered the Zilair Formation to be an allochthonous unit that was thrust over the Suvanyak Complex and the foreland thrust and fold belt.

262 F Basrida et al. /Tecronophysics 276 (1997) 253-263

The data presented in this paper from the Zilair ­Kugarchi cross-section do not provide clear enough evidence to resolve this question. Field observations point to the existence of a normal stratigraphic con­tact between the Zilair Formation and both the un­derlying rocks of the Suvanyak Complex in the east and the overlying rocks of the foreland thrust and fold belt in the west. On the other hand, in the study section the Zilair thrust has a moderate displacement (ca. 10 km) and the reflection seismic profile R116 shows that it does not have a strong reflective band such as that in profile R114. Moreover, there are no differences in structural style and evolution between the Zilair Nappe and the foreland thrust and fold belt but a gradual increase in the metamorphic grade and the intensity of the cleavage towards the east in the Zilair Formation.

The allochthonous nature of the Zilair Formation is based on several data. The lithology of the Zilair Formation suggests a provenance from the Magnito­gorsk volcanic arc, contrasting sharply with the shal­low-water carbonate rocks that were being deposited contemporaneously on the platform, and which are now in contact with the Zilair Formation in the fore­land thrust and fold belt to the north of the Zilair ­Kugarchi cross-section. Also, in the Belaya River area (ca. 80 k.l11 to the north of the study section), the Zilair thrust exhibits a ca. 500 m calc-mylonite zone that forms the roof thrust to a duplex developed beneath the Zilair Formation (Brown et aI., 1996, 1997). The strongly reflective band imaged at the base of the Zilair Formation in profile R 114 may correspond to this duplex thrust system, suggesting that it is a laterally continuous tectonic boundary and pointing towards an allochthonous nature for the Zilair Formation.

However, in order to make all of the observations compatible, some conditions must be met. The exis­tence of the calc-mylonite at the base of the Zilair Formation in the Belaya River area suggests that the trace of what has been named in this paper as Zilair thrust can be an out-of-sequence thrust cutting the Zilair thrust sensu stricto. On the other hand, if the basal thrust of the Zilair Formation does not outcrop along the present eastern contact with the Suvanyak Complex, it could be further to the east, inside the Suvanyak Complex or beneath it. The age of the Zilair thrust should be Carboniferous. since

no clear stratigraphic discontinuity is shown in this section.

Acknowledgements

We are indebted to the people of the Zilair-95 Expedition, particularly to our colleagues V. Bary­shev and V. Pazukhin, for their help and technical assistance. A. Perez-Estaun, D. Brown, J. Alvarez­Marron, J. Gallastegui, 1.R. Martinez-Catalan and M. Ford are thanked for their comments on previous versions of the manuscript. and C. Brine for the lC determinations. We are also grateful to the Struc­tural Geology group of Oviedo for their valuable discussion about the structure of the study section. We wish to thank A. Makushin, chief geologist, and B. Kiselov, geophysicist, from the Bashkirskaya Geophysical Expedition, who made the R114 seis­mic reflection data available to us. A. Oslianski processed the R114 data in this paper at Uppsala University in 1993 and was partly funded by Upp­sala University's East European Exchange Program. This paper is funded by the INTAS 94-1857 grant, CICYT grant number AMB95-0987-E, and it is a part of the Urals Project, a EUROPROBE program. EUROPROBE publication No. 108.

References

Bastida. F., 1981. La medida de la deformacion a partir de pliegues paralelos aplastados. Trab. Geol. Univ. Oviedo I I. 15-33.

Bastida, F.. 1993. A new method for the geometrical classifi­cation of large data sets of folds. J. Struct. Geol. 15, 69­78.

Brown. D., Puchkov. v.. Alvarez-Marron. J., perez-Estaun. A., 1996. The structural architecture of the footwall to the Main Uralian Fault. southern Urals. Earth Sci. Rev. 40. 125-147.

Brown, D., Alvarez-Marron, J., perez-Estaun. A.. Gorozhanina. Y. Baryshev, v., Puchkov. v., 1997. Geometric and kinematic evolution of the foreland thrust and fold belt in the southern Urals. Tectonics. in press.

Kazantseva, TT., Kamaletdinov, M.A.. 1986. The geosynclinal development of the Urals. Tectonophysics 127, 371-381 .

Keller. B.M., 1949. The Paleozoic flysch formation in the Zilair synclinorium of the Southern Urals and comparable com­plexes. Tr. Inst. Geol., 104, 165 pp. (in Russian).

Khvorova, IV, 1961. The Flysch and Low Molasse Formation of the Southern Urals. Nauka, Moscow. 352 pp. (in Russian).

Kisch, J.. 1991. Illite crystallinity: recommendations on sam­ple preparation, X-ray diffraction settings and interlaboratory samples. J. Metamorph. Geol. 9. 665-670.

263 F. Bastida et 01. /Tectolloph\'Sics 276 (1997) 253-263

Kulagina. Ye.N .. Pazukhin. VN.. 1986. On the biostratigraphy of the Serpukhovian deposits of the Zilair megasynclinorium. In: The Precambrian and Palaeozoic of the Southern Urals. Inst. Geol.. Ufa. pp. 70-79 (in Russian).

Matte. Ph., Maluski, H.. Caby. R., Nicolas, A.. Kepezhinskas. P, Sobolev. S.. 1993. Geodynamic model and 39Ar/40 Ar dating for the generation and emplacement of the bigh pressure (HP) metamorphic rocks in SW Urals. C.R. Acad. Sci. 317, 1667­1674.

Mitra, S .. 1978. Microscopic deformation mechanisms and flow laws in quartzites within the south Mountain Anticline. .T. Geol. 86. 129-152.

Pazukhin. VN.. 1995. Conodont biostratigraphy of the Lower Carboniferous of the western limh of the Zilair megasynclino­rium. Yezhegodnik-1994. IG UNC RAN, Ufa. pp. 37-38 (in Russian).

Puchkov. VN.. 1991. The Paleozoic of the Ural-Mongolian fold system. Occasional Publications ESRI. University of South Carolina. New Series. N7. Part II. 69 pp.

Rattey. P.R.. Sanderson. D..J .. 1982. Patterns of folding within

nappes and thrust sheets: examples from the Variscan of south­west England. In: G.D. Williams (Editor), Strain within Thrust Belts. Tectonophysics 88. 247- 26 7.

Rodionov, VYu., Radchenko. Y. Y.. 1988. On the stratigraphy of the Paleozoic sediments of the eastern limb of the Zilair megasynclinorium. In: The Biostratigraphy of thc Devonian and Carboniferous of the Urals. Bashkirian Sci. Centre. USSR Acad. Sci .. Ufa. pp. 15-27 (in Russian).

Sanderson. D..J., 1982. Models of strain variation in nappes and thrust sheets: a review. In: G.D. Williams (Editor). Strain within Thrust Belts. Tectonophysics 88. 201-233.

Sinitsina. Z.A.. Sinitsin. LI., Shamov. D.F., 1984. A concise stratigraphic description of the Upper Paleozoic of the South­ern Urals. In: Guidebook to Field Trip 047 k 27 mu MGK. Nauka. Moscow, pp. 9-19 (in Russian).

Zonenshain. L.P.. Korinevsky. VG.. Kazmin, VG., Pechersky. D.M.. Khain, VV. Marveenkov, V.v.. 1984. Plate tectonic model of the south Urals development. Tectonophysics 109, 95-135.