ELSEVIER Earth-Science Reviews 40 (1996) 125-147

The structural architecture of the footwall to the Main Uralian Fault, southern Urals

D. Brown a, * , V. Puchkov b, J. Alvarez-Marron ‘, A. Perez-Estaun ’ ” Institute de Ciencias de la Tierra “Jaume Almera” CSIC, Lluis Sole’ i Sabaris s/n, 08028 Barcelona, Spawn

h Uf rmian Geoscience Center, Russian Academy of Sciences, Ufa, City, Russia ’ Institute de Ciencias de la Tierra “Jaume Almera” CSIC, Lluis SolP i Sabaris s/n, 08028 Barcelona, Spam

Received 19 May 1995; accepted 20 September 1995

Abstract

The Uralide orogen is a linear collisional belt formed during the Upper Paleozoic as a result of convergence between the East European Craton and outboard terranes, and accretion of intervening island arcs and abduction of oceanic crust onto the East European Craton. This paper presents a new structural subdivision of the southern Urals, to the west of the main suture zone, that puts this part of the mountain belt in a context similiar to that of other collisional erogenic belts. This subdivision consists of a west-vergent thrust stack that is made up of an autochthonous to parautochthonous foreland basin, and a parautochthonous foreland thrust and fold belt, flanked to the east by a metamorphic thrust stack. The latter units are structurally overlain by allochthonous rocks of an accretionary complex, slope sediments, oceanic crust and high grade, eclogite-blueschist-bearing, gneiss. Where possible these units are defined on the basis of their internal stratigraphy and structure and on the nature of their bounding faults.

1. Introduction land arc related rocks. Not all of these need to be

The footwall to the suture of collisional oro- genie belts commonly consist of a number of tectonic elements that have been differentiated on the basis of sedimentary facies, structural style and metamorphic grade. These basic elements include a foreland basin, a foreland thrust and fold belt, imbricated platformal rocks and their (reworked and/or crystalline) basement. Com- monly these are overthrust by L7’-H P rocks (eclogites and blueschists), oceanic crust and is-

’ Corresponding author. Tel: 34 3 490 05 52. Fax: 34 3 411 00 12. e-mail: dbrown@ija.csic.es.

present in any one erogenic belt. Recognition of these basic elements, their internal characteristics and the nature of their bounding faults is of fundamental importance for advancement of un- derstanding the geometric, kinematic and me- chanical evolution of an erogenic belt and the processes that formed it (e.g. Cooper, 1981; Boyer and Elliot, 1982; Dahlen et al., 1984; Quinlan and Beaumont, 1984; Hatcher and Williams, 1986; Platt, 1986; Cello and Nur, 1988).

The Uralide Orogen (Fig. 1) is a major linear collisional Paleozoic erogenic belt formed as a result of the convergence of the East European Craton and outboard terranes during the Upper

0012~8252/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0012.8252(95)00051-8

126 D. Brown et ul. /Em&Science Reviews 40 (1996) 125-147

Devonian to Permian times (Hamilton, 1970; Zo- iments of the East European Craton, and Per- nenshain et al., 1984, Zonenshain et al., 1990). mian molasse along its western margin, sutured The orogen is made up of reworked Archean to to the east along the Main Uralian Fault with Vendian rocks and Paleozoic shelf and slope sed- Paleozoic oceanic and island arc rocks and, in the

52”

4” -

Fig .24

* . -. . .

. .

. .

Main

-.

. .

. . . . . . . . . . . . . . . . * * . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......... ........ .........

I Uralian Fault 60”

Ultrabasite-gabbroic rocks

Graben facies

L. Llandoverian transitional oceanic stage

L. SiI.-E. Dev. transitional oceanic stage (eastern subzone) L. Sil.-E. Dev. transitional oceanic stage (eastern and western subzones)

M.-L.-Dev. transitional oceanic stage

Continential sediments

Granitoids

Pre-Cam. and Paleozoic rocks

Riphean and Vendian sediments

Riphean volcanics and sediments

Paleozoic shelf deposits

Paleozoic-Triassic molasse

Russian Platformal sediments

500 km

Fig. 1. Lithologic map of the Uralide orogen (after Ivanov et al., 1975). The Main Uralian Fault marks the suture zone to the orogen. The location of the Uralide orogen is shown in the inset.

D. Brown et al. /Earth-Science Reoiews 40 (1996) 125-147 127

hinterland, by Precambrian massifs, inferred to be parts of microcontinents (Navilkin, 1960; Krapotin, 1967; Ivanov et al., 1975; Zonenshain et al., 1984, 1990; Khain, 1985; Ivanov and Rusin, 1986; Puchkov, 1991). The eastern part of the orogen is buried beneath Mesozoic and Cenozoic sediments of the West Siberian Basin, and is largely unknown. During the accretion and colli- sion processes, the footwall to the suture was imbricated into a number of west-vergent thrust nappes involving the pre-erogenic sequences and their basement, the foredeep sediments and, to- gether with oceanic crust and the accreted ter- ranes derived from the east, were variably tec- tonised and metamorphosed (e.g. Kamaletdinov, 1974; Khain, 1985; Zonenshain et al., 1984, 1990; Seravkin et al., 1992).

standing of this part of the southern Urals and provides a framework for future research.

2. Geological framework of the southern Uralides

The generally accepted division of the Uralides consists of a number of longitudinal zones, or megazones (e.g. Ivanov et al., 1975; Khain, 1985; Puchkov, 1991) that are largely based on the ages and paleogeography of the dominant rocks within them. This division has little tectonic implication; while recognising that the rocks within these megazones have been deformed and metamor- phosed during the Uralide orogeny and earlier events, it accepts that rocks of very different nature and paleogeographic origin are struc- turally juxtaposed and often included in the same megazone. The major tectonic subdivisions com- mon to other erogenic belts, such as a foreland thrust and fold belt and parautochthonous and allochthonous thrust nappes have not been fully defined in the southern Uralides.

The regional geology of the southern Urals (Fig. 2) is characterised by a roughly linear, N-S to NE-SW structural grain defined by west-ver- gent thrusts. A large, NE-dipping lateral struc- ture, the Karatau fault, offsets the thrust belt. To the west of the Main Uralian Fault, a SW-plung- ing anticlinorium contains Archean and Lower Riphean rocks in its core flanked by Vendian to Upper Riphean strata. The eastern margin of this antiform has been metamorphosed to amphibo- lite facies. Ordovician to Devonian platformal rocks outcrop extensively along its western mar- gin and to the south, but less so to the east. A thin band of Carboniferous rocks fringe its west- ern margin. Locally, ophiolitic units outcrop along the eastern margin of the antiform. West of this antiform, the area is dominated by Permian to Triassic flysh.

The Main Uralian Fault, which forms the main suture zone, is a wide, SE-dipping melange zone decorated by a discontinuous band of mainly De- vonian age rocks. To the east of the suture zone are the Upper Silurian to Upper Devonian island arc volcanics and volcaniclastics and Carbonifer- ous sediments of the Magnitogorsk Zone (Fig. 2), which has been folded into an open synform (Ivanov and Ivanov, 1991; Seravkin ct al., 1992; Sokolov, 1992).

This paper focuses on the southern Urals, west We propose a first order tectonic subdivision of the main suture zone, defined by the Main of the footwall to the Main Uralian Fault that Uralian Fault (Fig. 2). A new tectonic subdivision consists of a west-vergent thrust stack of Archean is proposed for this area in which tectonic units through Paleozoic rocks (Fig. 3). The lowermost, are defined on the basis of their internal stratig- parautochthonous units of the thrust stack (Fig. raphy, structure and metamorphism and, where 3) consist of a foreland basin that, along its east- possible, by the nature of their bounding faults, ern margin, has been incorporated into the fore- thus providing a new and comprehensive struc- land thrust and fold belt (FTFB). The FTFB is tural division for this part of the orogen. How- structurally juxtaposed to the east against the ever, many problems still exist in this subdivision, Bashkirian terrane and the Suvanyak Complex. since much of the detailed structure and, in par- The parautochthonous units are overthrust by ticular, the kinematics of the faults, remains allochthonous units (Fig. 3). The structurally low- poorly understood in the Urals. Nevertheless, we est of the allochthonous units consists of the feel that this new subdivision advances the under- Zilair Nappe, structurally overlain by the thin

128 D. Brown et al. /Earth-Science Reciews 40 (1996) 125-147

Parautochthonous Units Permian

Magnitogorsk Zone

m Carboniferous

Carboniferous

Ordovician, Silurian and Devonian

Vendian

Upper and Middle Riphean

Lower Riphean

Taratash Metamorphic Complex

Allochthonous Units j_ Devonian Zilalr Fm.

@JJ Ordovician-Silurian

Ophiolites

m Maksutovo Complex (High pressure rocks)

m Upper Devonian - Carboniferous

m Upper Devonian

E Middle Devonian

,, Lower Devontan

m Devonian blocks in melange

i S&tan - Devonian

m Granitoids

D. Brown et al. /Earth-Science Reviews 40 (1996) 125-147 129

Uzyan Nappe, which is in turn overthrust by ophiolites of the Kraka Allochthon. The basal thrust of the Kraka Allochthon is a several hun- dred meter-thick melange zone that coincides with the suture zone. Both the parautochthonous and allochthonous units appear to have been backthrust, eastwards, over the Suvanyak Com- plex which has been overthrust from the east by eclogite- and blueschist-bearing rocks of the Maksutovo Complex. This thrust stack is sutured to the Magnitogorsk Zone along the Main Uralian Fault.

3. Parautochthonous units

3.1. Foreland basin - foreland thrust and fold belt

in the southern Urals, the foreland basin con- sists of greater than 3000 m of Upper Carbonifer- ous to mostly Permian-age syn-erogenic sedi- ments that thins and youngs westward for some loo-150 km (Chuvashov and Diupina, 1973; Einor et al., 1984; Chuvashov et al., 1993). The Upper Carboniferous consists of up to 100 m of shales, marls and thin beds of sandstone. The Permian succession, in the eastern part of the foreland basin, is greater than 3000 m thick (Fig. 4) and consists predominantly of Asselian to Artinskian sandstone, conglomerate, shale, marl and lime- stone (Fig. 5A) topped by up to 1000 m of Kun- gurian age anhydrites, gypsum and rocksalt. The central part of the basin consists of approximately 200 m of Asselian to Artinskian age marl and clayey limestone again overlain by Kungurian an- hydrites, gypsum and rocksalt of variable thick- ness. Near Sterlitimak, the Permian consists of up to 600 m of Asselian to Artinskian reefal lime- stones with bedded limestones at the base and overlain by Kungurian anhydrites, gypsum and rocksalt. The Upper Permian consists of up to 3000 m of Ufimian- to Tatarian-age terrigenous sediments developed mostly along the western margin of the foreland basin. Locally these are

overlain by thin Lower Triassic conglomerates and sandstones The morphology and stratigraphy of the foreland basin has been extensively dis- cussed elsewhere (e.g. Chuvashov et al., 1993, and references therein) and is beyond the scope of this paper.

Rocks in the foreland thrust and fold belt (FTFB) (Fig. 3) range in age from Archean to Permian. The oldest rocks that outcrop in the FTFB are Archean crystalline rocks of the Taratash Anticlinorium (Fig. 2). These rocks con- sist predominantly of granulite facies, orthopyrox- ene + clinopyroxene + plagioclase quartzofeld- spatic gneisses, plagiomigmatites, quartzite and gabbro-diorite gneisses and migmatites (Fig. 5B) of the Taratash Metamorphic Complex (Rusin, 1979, 1980; Lennykh, 1980, 1984). The granulite facies metamorphism has been dated at 2.6 to 2.8 Ga using zircon Pb/Pb thermo-isochron methods (Ivanov et al., 1986). Locally, the Taratash Meta- morphic Complex has been retrogressed to am- phibolite facies. This retrogression has been dated at 0.8 to 2.1 Ga by U-Th-Pb, Pb thermo-iso- chron, Rb-Sr and K-Ar methods (Ivanov et al., 1986). The gneisses have been intruded by two sets of diabase dykes, the oldest of which has a K-Ar whole rock age of 1650 Ma (Ivanov et al., 1986).

Lower Proterozoic rocks in the FTFB outcrop southeastern part of the Taratash Anticlinorium. in contact with Lower Riphean greenschist facies carbonates. Here, these rocks consist of garnet + plagioclase + epidote amphibolite facies my- lonites and gneisses of the Alexandrousky Com- plex. The Alexandrousky Complex is bound by shear zones and, locally, retrogressed to green- schist facies.

The Riphean sequence was deposited uncon- formably on top of the Archean rocks. The Riph- ean consists of more than 12,000 m of sediments (Fig. 4) that, near Sterlitimak, thins out westward (Keller and Chumakov, 1983; Alexseev, 1984; Ko- zlov et al., 1989). The Lower Riphean is com- posed of up to, and locally greater than, 5000 m

Fig. 2. Geological map of the western part of the southern Urals (location is shown in Fig. 1). Based on the Geology of the USSR 1500,000 and 1:200,000 South Urals Map Series.

130 D. Brown et al. /Earth-Science Reviews 40 (1996) 125-147

Parautochthonous Units

Foreland Basin and Foreland Thrust and Fold Belt

Arch&m delormation and granulite tacies metamorphism. Greenschist overprint along myionite zones dudng U. Vendian to Cambrian. Oeformed during Uraiiie event.

Deformed and metamwphosed to tower reeschist f&es during U Vendian to Cambrian. Deformed during r&de event. f.7.

I::::::/ Deformed. but unmetamorphosed durmg the Urslide event

Bashkirian terrane

auvanyak L‘omplex Deformed and metamorphosed to middle greenschist facies during Utalide event.

Allochthonous Units . . ’ ..‘.I Zilair Nappe

I to sub - to lower greenschlst

Uzyan Nappe

Deformed and metamorphosed to lower to middle greanschrst facies durina the Uralide event.

. . . - Maksutovo Gomplex

N Deformd and metamorphosed to granulite end blueschist fscies during the Uralide event.

Kraka Allochthon

0 100km Deformed and metamorphosed to lcwer to medla greens&w facies during the Uralide event

Fig. 3. Tectonometamorphic map of the footwall to the Main Uralian Fault showing the distribution of the units in the proposed new subdivision. The ages of deformation and nature of the metamorphic events that have affected the rocks within each sub division are shown. Compiled from the references provided in the text.

D. Brown et al. /Earth-Science Reviews 40 (1996) 125-147 1.3 1

L Rcphean

Fig. 4. Compilation of stratigraphic thicknesses from various locations in the foohvall to the Main Uralian Fault. These compliations were made from the references provided in the text.

132 D. Brown et al. /Earth-Science Reviews 40 (19961 125-147

of conglomerates, alkaline tuffs, shales and car- bonates. The Middle Riphean is composed of up to 5000 m of volcanic and subvolcanic rocks (granites, gabbro-norites and nephelinites), quartzites, arkoses, shales, siltstones and carbon- ates. The Upper Riphean consists of up to 2500

m of quartzites, subarkoses, siltstones. marls, shales and dolomites.

Upper Vendian rocks outcropping in the FTFB, were transgressive over the Riphean, to- wards the platform (Einor et al., 1984) and a slight angular unconformity has been noted. It

Fig. 5. Characteristics of some of the lithological units described in the text. (A) Permian sediments. (B) Folded migmatites from the Taratash Metamorphic Complex. (C) Upper Vendian conglomerate containing red chert and metamorphic clasts. (D) Photomicrograph of the Zilair Fm. showing detrital plagioclase, volcanic and pyroxene clasts. Note also the well-developed pressure solution cleavage. (E) Characteristic of the eclogite-blueschist blocks in the Maksutovo Complex. (F) Aspect of the melange zone developed along the Main Uralian Fault.

D. Brown et al. /Earth-Science Reviews 40 (1596) 125-147 13.3

consists of up to 1500 m (Fig. 4) of feldspar- bearing sandstones overlain by polymictic con- glomerates that contain metamorphic and jasper clasts (Fig. 50. Boreholes in this area intersect middle and upper Riphean rocks beneath the Vendian. The presence of metamorphic and jasper clasts indicates that the Lower Vendian succession was derived from uplifted metamor-

phic terranes and deep oceanic sediments to the east.

Ordovician, Silurian and Devonian rocks make up the pre-erogenic units that unconformably overly the Precambrian basement. These rocks form a thin veneer on top of the Vendian (Fig. 41, and it is common that Devonian rocks directly overlie the Vendian (Krause and Maslov, 1961;

Fig. 5 (continued).

134 D. Brown et al. /Earth-Science Rwiews 40 (1996) 125-147

Tyazheva, 1961; Einor et al., 1984). The Ordovi- cian and Silurian consist of sandstones, conglom- erates and siltstones and limestones (Einor et al., 1984). The Devonian consists predominantly of carbonates with lesser terrigenous elastics (e.g. Maslov and Abramova, 1983; Bogoyavlenskaya et al., 1983; Chibrikova and Olli, 1991).

Carboniferous rocks of the FTFB consist of Tournasian to Gzelian age platformal facies car-

bonate and carbonate-terrigenous rocks (Chuva- shov and Diupina, 1973; Kochetkova et al., 1981; Einor et al., 1984; Kulagina et al., 1986; Scheko- tova, 1990). Towards the north, the Carbonifer- ous has a maximum thickness of approximately 1350 m (Fig. 4) and consists of moderately to well-bedded detritial and organic limestones with chert lenses and bands, dolomites, dolomitised limestones and lesser marls. Southwards (Fig. 4),

Fig. 5 (continued).

D. Brown et al. /Earth-Science Reciews 40 (1996) 125-147 135

the Carboniferous section consists of up to 5300 m of carbonate-terrigenous rocks composed of mudstones, shales, siltstones and limestones with bands of chert.

Along the eastern margin of the foreland basin, the Upper Carboniferous and Permian rocks have been involved in the FTFB deformation. The transition between the foreland basin and the FTFB is a wide zone of deformation in which the Permian rocks have been folded into ramp anti- clines probably cored by blind thrusts (Figs. 6 and 7).

The foreland thrust and fold belt, can be di- vided into two major units that consist of a nar- row, thin-skinned, west-vergent thrust stack, in- volving Permian rocks from the foreland basin and the pre-erogenic Paleozoic sequence in the west, and a south-plunging, out-of-sequence thrust stack that imbricates the pre-erogenic rocks together with the Vendian and Riphean base- ment (Fig. 61, and possibly the Archean crys- talline rocks (see Fig. 2). A large lateral structure, the Karatau fault (Fig. 3) disrupts the linear nature of the FTFB, and marks the locus of a major bend in the Uralide structural trend that is coincident with a significant narrowing of the orogen northwards (Fig. 1). At present it is diffi- cult to extrapolate the structure of the FTFB to the north of this structure.

Within the thin-skinned unit, thrusts cut listri- tally up-section from a detachment, interpreted from cross section balancing, to be at the base of the Devonian (Fig. 7). Based on the fold-thrust geometry, the dominant deformation mechanism appears to have been fault propagation folding (Fig. 8A) with no axial planar cleavage develop- ment (Fig. 8B). Displacement in this unit is on the order of several kilometres. It is likely that the thin-skinned thrust system also occurs in the thick-skinned unit, but to date it has not been recognised. North of the Karatau fault, the de- tachment level may be in the Middle to Upper Riphean, since these rocks are involved in the thrusting there (Fig. 2). It is possible that there is a lower level of detachment south of the Karatau fault, but it has not yet been recognised.

The thick-skinned unit is characterised by open, upright folds and steeply east-dipping

d Permnn (with Kungurlan)

Carbomferous N

DWOW3l

m Ordovician Silurian

2 Vendfan t i 1 Upper Rlphean

( Thrust

.4 Blind Thrust

’ Fault

Fig. 6. Detailed geological map of a portion of the foreland basin and the ITFB showing the transition from the thin- skinned style of deformation to the thick-skinned (location is shown in Fig. 2). Based in part on the Geology of the USSR 1:200,000 South Urals Map Series, maps N-4fl-XXVII and N-40-Xx1.

thrusts that emplace the basement rocks over the thin-skinned unit to the west (Fig. 7). South of the Karatau lateral structure, the oldest rocks exposed in the thick-skinned part of the FTFB are Middle Riphean, and Russian reflection seis- mic profiles image truncated reflections inter- preted to be Upper to Middle Riphean in age (Skripi and Iunusov, 1989). North of the Karatau

136 D. Brown et al. /Earth-Science Reriews 40 (1996) 125-147

lateral structure, boreholes through the Archean rocks of the Taratash Metamorphic Complex in- tersect Devonian rocks underneath, indicating it was emplaced during the Uralide orogeny.

3.2. Bashkirian terrane

The FTFB is structurally juxtaposed against the Bashkirian terrane (Fig. 3) along the Zurakul fault. The Bashkirian terrane, as defined here, consists of the metamorphic part of the previ- ously defined Bashkirian Anticlinorium (e.g. Zo- nenshain et al., 1984). It consists of a E-vergent thrust stack of poly-deformed, poly-meta- morphosed Riphean metasedimentary rocks, lo- cally folded by kilometer-scale recumbent folds. Lower Vendian metasedimentary rocks outcrop along the eastern margin of the Bashkirian ter- rane (Fig. 2). These rocks consist of up to 2000 m of variably metamorphosed (mostly greenschist facies) shallow-water shale, sandstones, carbon- ates and conglomerate with minor alkaline basalt’s (Parnachev and Kozlov, 1979; Einor et al., 1984). The Riphean and Vendian rocks have a well-de- veloped foliation and are locally metamorphosed to amphibolite facies (Fig. 3). The Proterozoic rocks are locally unconformably overlain by up to 1600 m of unmetamorphosed Ordovician, Sil- urian and Devonian sediments (Fig. 4).

The pre-Ordovician is thought to have been widely affected by a tectonothermal event during

B

the late Vendian to early Cambrian (Fig. 31 (Shatsky, 1963; Puchkov, 1993) that imbricated the Lower Vendian and older rocks along E-verg- ing thrusts and related folds. The metamorphism associated with this event reached middle amphi- bolite facies locally, and resulted in widespread foliation development and retrogression of gran- ulite facies Archean rocks along greenschist fa- ties mylonite zones. However, these foliations and shear zones have not been definitively dated using isotopic methods. The main evidence for the metamorphism being of late Vendian to Cam- brian age is the occurrence of metamorphosed Middle Riphean schists and marbles and Lower Vendian greenschist facies schist and quartzites overlain by unmetamorphosed Ordovician sand- stones.

Late Vendian to Cambrian age structures in the Bashkirian terrane are interpreted to be mostly E-vergent. However, during this event the Zarukul fault (Fig. 2) is thought to have been a W-vergent thrust which placed amphibolite facies rocks on top of unmetamorphosed rocks. Paleo- zoic rocks also outcrop in the hangingwall, imme- diately adjacent to the fault (Fig. 21, suggesting it had an extensional component of movement prior to the Ordovician and was possibly reactivated as a thrust during the Uralide event. To the north, well-exposed E-vergent ramp anticlines (Fig. 8C) and a duplex that affect lower greenschist facies Riphean rocks, are interpreted to be related to

8’

Permian

m Carbcnifews

Devonian

5krn m Ordovician / Silurian .

m Vendian

0 IO km m U. Riphean

Fig. 7. Parallel cross sections through the portion of the FTFB shown in Fig. 6. The transition into the foreland basin is a broad zone of low deformation consisting of ramp anticlines cored, to the west, by blind thrusts.

D. Brown et al. /Earth-Science Reviews 40 (1996) 125-147 I.37

the Vendian-Cambrian deformation (Kozlov et al., 1989), and indicate that considerable struc- tural stacking may have taken place at this time.

Faults in the Bashkirian terrane often juxta- pose younger rocks over older rocks, suggesting

that either considerable out-of-sequence thrust- ing, extensional faulting, backthrusting, or reim- brication of Vendian-Cambrian age structures took place. Structural reimbrication of an earlier thrust stack is a possible explanation for some of

Fig. 8. Representative structural characteristics of rocks in the some of the units. (A) Fault propagation fold in C‘arboniferous limestones from the FTFB. (B) Underformed ooids from Bashkirian-age limestones indicating the lack of internal deformation in the thrust sheets in the FTFB. (C) Near-vertical, E-vergent ramp anticline in Lower Riphean rocks. This structure is thought to he Vendian in age. (D) Upright folds in Zilair Fm. rocks from near the eastern margin of the Zilair Nappe. Note the well-developed, vertical axial planar cleavage. (E) W-vergent fold in the Zilair Fm. from near the western margin of the Zilair Nappe. (F) Well-developed chlorite preferred grain-shape lineation in Silurian phyllites from the Suvanyak Complex.

138 D. Brown et al. /Earth-Science Keuiews 40 (1996) 125-147

the apparent out-of-sequence thrusting. However, the internal geometry of the Bashkirian terrane is not well known, and it is clear that a number of important questions related to the timing of the deformation and metamorphic events recorded in the Bashkirian terrane still need to be answered. Determining which features belong to the Ven- dian event and which to the Uralide orogeny is of fundamental importance to the understanding of the tectonic history of the area.

3.3. Suvanyak complex

The Suvanyak Complex (Fig. 3) lies to the east of the Bashkirian terrane and consists of strongly deformed Silurian and Lower to Middle Devo- nian quartzites, schists and phyllites, imbricated along E-verging thrusts. These rocks were previ- ously thought to be Upper Riphean to Vendian in age (e.g. Zonenshain et al., 19841, but have recently been shown to be Silurian and Devonian

Fig. 8 (continued)

D. Brown et al. /Earth-Science Reviews 40 (1996) 125-147 13’)

age slope derived sediments (Rodionov and Rad- chenko, 1988; Chibrikova and Olli, 1994). Within the Suvanyak Complex, a well developed foliation dips moderately to steeply westwards and is lo- cally accompanied by a well-developed, W-plung- ing lineation (Fig. 8F). Metamorphism reaches middle greenschist facies (Fig. 3). Very little in- ternal structure has been determined within the

Suvanyak Complex and its structural boundaries are poorly defined. However, as noted below, it is apparent that along much of its length the Zilair Nappe and part of the Bashkirian terrane have been backthrust over this enigmatic area and it is possible that the E-vergence of the complex may be the result of backthrusting and eastwards transport. The Suvanyak Complex appears to

Fig. 8 (continued)

140 D. Brown et al. /Earth-Science Rerjiews 40 (1996) 125-147

structurally overlie the Maksutovo Complex to the east along the Yantyshevo-Yuluk fault (Zakharov and Puchkov, 1994).

and Denisova, unpublished map) (Fig. 10). Meta- morphism in the Uzyan Nappe reaches lower greenschist facies. At present little is known about the internal characteristics of this nappe.

4. Ailochthonous units

4.1. Zilair Nappe 4.3. Kraka allochthon

The Zilair Nappe (Fig. 3) structurally overlies The Kraka Allochthon (Fig. 3) is the struc- the Bashkirian terrane and the FTFB along the turally highest tectonic unit in the thrust stack. It ductile Zilair thrust. Rocks in the Zilair Nappe consists of a 0.3 to 3 km thick package of weakly consist up of 2000-3000 m of Frasnian- to Tour- metamorphosed dunites and lherzolites and lesser nasian-age green-colored, polymictic and gray- garnet peridotites and gabbros, folded into an wacke turbidites and thin beds of chert of the open synform and with a well developed internal Zilair Formation (Keller, 1949; Khvorova, 1961; foliation (Kamaletdinov, 1974; Zonenshain et al., Kulagina and Pazukhin, 1986; Rodionov and 1984; Saveleva, 1987; Seravkin et al., 1992, Fig. Radchenko, 1988). The Zilair Fm. commonly 10). The base of the Kraka Allochthon consists of contains clasts of plagioclase, volcanic fragments, a several hundred metre-thick tectonic melange quartz, detrital chromite and rare pyroxene (Fig. that makes up the basal thrust and coinsides with .5D), suggesting provenance from a volcanic arc. the suture zone.

The Zilair Nappe is an imbricate thrust stack that folds and thrusts the Zilair Fm. (Fig. 9). In the eastern part of the nappe, the Zilair Fm. is folded by tight upright folds (Fig. 8D) with a well-developed, steeply dipping to vertical axial planar pressure solution cleavage (see Fig. 5D). Towards the west the folds become west-vergent and have a shallowly east-dipping axial planar cleavage (Fig. 8E). Along its eastern boundary, a well-developed foliation dips westward and Rus- sian reflection seismic profiles shows steeply west-dipping reflectors penetrating to the middle crust (Sokolov, 1992) and possibly truncating the Zilair thrust, suggesting that the allochthon has been backthrust over the Uraltau Complex (Zakharov and Puchkov, 1994). Metamorphism in the Zilair Nappe reaches lower greenschist facies (Fig. 3).

Zilair Fm.

4.2. Uzyan Nappe ‘i t BeddIng

A A’ 0 20 km

The Uzyan Nappe (Fig. 3) is a small, poorly exposed, thin, W-vergent allochthon of imbri- cated Ordovician and Silurian slope-derived sedi- mentary rocks that has been thrust over the Zilair Nappe, and is preserved at the base of the Kraka Allochthon (Zonenshain et al., 1984; Saveleva

Fig. 9. Detail of part of the Zilair Nappe showing its internal structural style (location is given in Fig. 2). Based in part on a reinterpretation of the Geology of the USSR 1:200,000 South Urals Map Series, map N-40.XXVIII.

D. Brown et al. /Earth-Science Reuiews 40 (1996) 125-147 141

Fig. 10. Schematic map of the Uzyan Nappe and the overlying Kraka Allochthon (redrafted from Saveleva, G.I. and E.A. Den&ova, Geology-structural map of the ultrabasic Kraka Massif, unpublished map). Note the imbricated nature of the Uzyan Nappe. Location is given in Fig. 2.

5. Maksutovo complex

The Maksutovo Complex (Fig. 3) consists of blocks of low T eclogites (Fig. 5E) and blueschists incorporated in metasedimentary schists and gneisses (Sobolov et al., 1986; Dobretsov, 1991) that extends along the southern Urals, in the immediate footwall to the Main Uralian Fault (Fig. 2). The complex consists of several E-di- pping thrust slices of metasediments with a vari- ably oriented, but well-developed schistosity and gneissic banding. A lineation is commonly devel- oped. However, a randomly oriented grainshape preferred orientation of glaucophane crystals is

common on the foliation plane in the blueschists. The Maksutovo Complex is bound to the east by the Main Uralian Fault.

Eclogite and blueschist blocks in the Maksu- tovo Complex record temperature and pressure conditions of 550°C + 700°C and lo-14 kbars, respectively (Dobretsov, 1991). Pseudomorphic coesite as well as relic coesite inclusions in gar- net, jadeite-quartz and jadeite-kyanite-parago- nite assemblages have also been reported (Chesnokov and Popov, 1965; Dobretsov, 1984; Dobretsov and Dobretsova, 1988), indicating pressure conditions exceeding 24 kbars. The age of the metamorphism in the Maksutovo Complex has been suggested to be either Precambrian (Sobolov et al., 1986) or Paleozoic (Puchkov, 1989). Dobretsov (1974) obtained K-Ar ages of 400-500 Ma and, more recently, Matte et al. (1993) reported Ar/Ar cooling ages of 380 Ma for phengite from the blueschist and eclogite. Zakharov and Puchkov (1994) have reported Late Silurian to Early Devonian conodonts in marbles of the Maksutovo Complex.

6. The Main Uralian Fault

The Main Uralian Fault coincides with the main suture zone in the Southern Uralides, where it is made up of a melange (Fig. 5F) that is approximately 6 km wide and continues for more than 2000 km along most of the length of the Uralides. Near the SG-4 deep borehole, the Main Uralian Fault dips eastward at moderate angles and seismic reflection profiling (Sokolov, 1992; Juhlin et al., 1995) has shown it to be a wide reflective zone that may extend to at least 15 km depth and possibly continues into the lower crust. The melange consists of fragments of andesites, pyroxene andesites, basaltic andesites, tuffs, rodingite, cherts, marbles, marbles in andesite with skarns and porphoritic diorites, all in a ma- trix of serpentinite (e.g. Zonenshain et al., 1984). A large slice of ophiolitic material, the Nurali Ophiolite, consisting of an approximately 4 km section of oceanic crust, from lherzolite-plagioc- lase at the base to lherzolites-harzburgite-dunite and gabbro-norites the top, is incorporated into

142 D. Brown et al. /Earth-Science Reviews 40 (1996) 125-147

(e.g. Zonenshain et al., 19841, or possibly over- thrusts, the melange zone. Most of the material incorporated into the melange zone is of Lower- Middle Devonian age, and represent fragments derived from the Magnitogorsk Zone. Silurian fragments are rare. Metamorphism in the melange is greenschist facies.

The Main Uralian Fault may record a complex compressional and extensional history (Matte et al., 19931, the timing of which is poorly con- strained. In the Southern Urals, the youngest rocks involved in the fault are of Early Carbonif- erous age and, in southernmost areas, it is over- lapped by Jurassic and early Cretaceous sedi- ments, placing an upper limit on the age of defor- mation in this area.

7. Discussion and conclusion

The proposed structural architecture of the southern Urals consists of a W-vergent thrust stack (Fig. 111, complicated by back thrusts, with many of the tectonic elements common to other erogenic belts. Where possible, we have tried to

characterise the various tectonic elements by their internal lithostratigraphy, structural style and the nature of their bounding faults. However, the details of these features are at present incom- pletely known in many areas, but the proposed subdivision provides a tectonic framework that more accurately reflects the geometric, kinematic and mechanical evolution of the orogen. Despite the similarities in the large scale architecture shown by the proposed subdivision, there are significant differences within individual units that make the Uralides unique in many ways.

From west to east (i.e. structurally lowest to highest) the subdivision consists of a foreland basin, a FTFB, a metamorphic thrust stack (formed by the Bashkirian terrane and the Su- vanyak Complex) flanking the eastern side of the FTFB, and the structurally higher allochthons of the Zilair Nappe, the Uzyan Nappe, the Maksu- tovo Complex and, finally, the Kraka Allochthon. The thrust stack measures only 120 km to 150 km in width (Fig. 111, from the Main Uralian Fault to the frontal folds; an anomolously belt when com- pared to contemporaneous orogen such as the Variscides in Northern Spain (ca. 350 km to 700

A Foreland basin - foreland thrust and fold belt

A’ Allochthonous “n,rs

/ .

Parautochthonous Units Permian

Vendian and Paleozoic

EH Paleozoic schists

m Middle and Upper Riphean

m Lower Riphean

m Archean Crystalline rocks

Allochthonous Units Zilair Aliochthon

Suvanyak Complex

m Maksutovo Complex

m Kraka ophiolite

Magnitogorsk Zone

I::::I Devonian

m Carboniferous

0 50 km I

Fig. 11. Schematic cross section of the southern Urals from the foreland basin to the eastern margin of the Magnitogorsk Zone showing the structural relationships between the different units (location is given in Fig. 2). In this section the Bashkirian terrane projects under the allocthonous units and the Suvanyak Complex is a thin unit outcropping to the east of these units.

D. Brown et al. /Earth-Science Reviews 40 (1996) 125-147 141

km) or the Appalachians of the U.S. (ca. 350 km). It is not possible at this point to determine the amount of shortening in the southern Urals, since more data is needed on the significance of the basement involved out-of-sequence thrusting, the kinematics of faults and the role of backthrusting.

dominated load (Kruse and McNutt, 1988) devel- oped in the Uralides, compares well with orogens formed at retreating subduction boundaries (e.g. Royden, 1993).

The Urals foreland basin is comparable in thickness (Table 1) to those developed in orogens with much more shortening and thrust stacking. The development of this broad, thick foreland basin in the southern Urals, which to a degree reflects flexure resulting from the imposed oro- genie load (e.g. Quinlan and Beaumont, 1984; Beaumont et al., 1988; Allen and Allen, 19901, appears to over compensate for the relatively small amount of thrust stacking (i.e. imposed erogenic load) that is apparent in the area. Kruse and McNutt (1988) and McNutt et al. (1988) have concluded that the weight of the topographic load was insufficient to compensate for the ap- parent deflection of the Moho beneath the Uralides imaged by various seismic methods (Sokolov, 1992; Thouvenot et al., 1995). The deep foreland basin, combined with the low topogra- phy, low grade metamorphism and subduction

The foreland thrust and fold belt in the south- ern Urals also appears to be anomalous com- pared to other belts in the world in that there is very little shortening and extensive basement in- volvement and a predominance of a thick-skinned style of deformation. Within this thick-skinned portion of the FTFB, steep out-of-sequence thrusts appear to penetrate to deep in the base- ment, and possibly even into the crystalline rocks beneath. One possible interpretation of these thrusts is that they are reactivated pre-Uralide extensional faults that were developed deep in the crust during the early Paleozoic rifting event (Fig. 11). Several important questions need to be addressed in this part of the orogen. For exam- ple, to what extent does the thin-skinned style of thrusting developed in the western part of the FTFB extend eastward, into the thick-skinned part, thereby accommodating more shortening than is presently realised. Also. the geometry, kinematics and timing of the out-of-sequence

Table 1 Comparison of the Uralides foreland basin with others from around the world

Orogenic belt Age of foreland basin Thickness Facies Elastic Plate Thickness

Uralides

Canadian Cordillera ’

zagros ’

Tien Shan 2 Apennine a

Carpathian ’

Himalaya ’

Alps ’

U. Carboniferous- Lower Triassic

Middle Jurassic- early Tertiary

Late Eocene- Present

Neogene-Recent Miocene-Quaternary

Miocene-Quaternary

Pliocene-Quaternary

Late Miocene

>3OOOm

5000 m

4000-7000 m

> 4500 m 8000 m

up to 7000

Up to 5000 m

5000 m

Shallow marine to non-marine sandstones, conglomerates. marls and evaporites. Shallow marine and non-marine shales, sandstones and conglomerates. Fine-grained elastic

Coarse to fine-grained elastics Deep to shallow marine, fine-grained elastics.

Non-marine to shallow marine. coarse-grained elastics.

Non-marine, predominantly coarse-grained elastics.

Shallow marine to non-marine. predominantly coarse-grained elastics.

75 + 25 km a

ca. 60 km 20 km

40 km

80 km

40-SO km

1 Macqueen and Leckie (1992); 2 Allen et al. (1991); s Royden (1993): a Kruse and McNutt (1988)

144 D. Brown ef al. /Earth-Science Reviews 40 (1996) 125-147

thrusts need to be accurately determined before the cross section can be restored, balanced and the amount of displacement in the FTFB calcu- lated.

The Bashkirian terrane appears to have a pre- Ordovician metamorphic and deformation history that is notably different from the underlying FTFB. Clearly, the Zurakul fault is a major struc- ture that juxtaposes units with different deforma- tion and metamorphic histories, and it is because of this that we have tentatively placed it in a terrane category, although more detailed map- ping and isotopic dating may show this not to be the case. A point to be kept in mind here is that this is a pre-Uralide terrane. The dominant dis- placement across the Zurakul fault is thought to be Vendian in age, at which time amphibolite facies rocks were thrust over unmetamorphosed rocks. The unconformably overlying Paleozoic rocks suggest that it was an extensional feature during the Early Paleozoic rifting event and sub- sequently underwent minor reactivation during the Uralide orogeny. It would seem, therefore, that the Bashkirian terrane and the underlying part of the FTFB that involves the Riphean and older rocks are an inherited feature from the Vendian, and may reflect only minor reworking during the Uralide orogeny. Despite the complex- ities of the Bashkirian terrane_ and the Suvanyak Complex, their structural position *is similar to other thrust belts in the world in which a thrust stack, composed of metamorphosed basement rocks, occur along the hinterland flank of the FTFB (e.g. the Narcea antiform in the Varsican belt of NW Spain (Perez-Estaun et al., 1991); Blue Ridge of the Appalachians (Hatcher and Williams, 1986)).

likely the Magnitogorsk island arc now preserved in the Magnitogorsk Zone to the east. The style and intensity of deformation in the Zilair Nappe is not comparable to that of the underlying FTFB and Bashkirian terrane, indicating that it under- went significant deformation prior to, and/or during emplacement. We suggest that the Zilair Formation may have formed as part of an accre- tionary complex, possibly an accretionary prism, in front of the Magnitogorsk island arc, and it was subsequently thrust over the continental mar- gin. Following its emplacement, out-of-sequence thrusting and backthrusting folded it into an open synform.

Backthrusting of the Zilair Nappe over the Suvanyak Complex, and perhaps the subsequent development of the west-dipping foliation and eastward transport of the rocks in this unit, sug- gests that backthrusting may have played a major role in the cevelopment of the architecture of the southern Urals. A significant amount of shorten- ing in the footwall to the suture may be accom- modated by this mechanism.

Deposition of the Zilair Fm. has been thought to mark the collision of the Magnitogorsk island arc with the East European Craton during Fras- nian to Framenian times and to date the onset of deformation in the southern Urals (Zonenshain et al., 1984, 1990). This age of deformation is also confirmed in the Magnitogorsk Zone, where Frasnian to Framenian-age olistostroms and pro- gressive unconformities (op. crit.) are widely de- veloped (e.g. Maslov, 19801, indicating uplift and erosion during this time. However, to the west, on the platform, passive continental margin facies carbonate sequences were being deposited through the Devonian and up until the Middle to Upper Carboniferous, apparently undisturbed by tectonic activity to the east. The first indication of tectonism and uplift- occurred during the Upper Carboniferous to Lower’Permian, at the onset of the development of the foreland basin. There are two ages of deformation recorded in the rocks of the southern Urals; an early stage in the Zilair Fm., and in similar age rocks in the Magnitogorsk Zone, a younger stage recorded by the sediments of the foreland basin. The older, Upper Devo- nian, deformation in the Magnitogorsk island arc may be the result of collision with a calved frag- ment of the East European Craton (the Bashkirian terrane?).

Acknowledgements

The Zilair Nappe is clearly composed of vol- This project is funded by CICYT grant num- caniclastic sediments derived from an island arc, bers AMB-1225/94-E and AMB950987-E.

D. Brown et al. /Earth-Science Reviews 40 (1996) 125-147 145

Brown is funded by a CICYT post-doctoral fel- lowship. Many Russian colleagues from Ufa have participated in various aspects of our fieldwork and contributed to our understanding of the Urals. F. Bastida, David Gee and an anonymous reviewer are thanked for there comments. This work is part of the Urals Project, a EURO- PROBE program. EUROPROBE publication number 73.

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