A Model of Palaeozoic Obduction and Exhumation of High-Pressure/Low-Temperature Rocks in the Southern Urals

TECTONOPHYSlCS I

ELSEVIER Tectonophysics 276 (1997) 217-227

A model of Palaeozoic obduction and exhumation of high-pressure/low-temperature rocks in the southern Urals

Alexandre Chemenda a,*, Philippe Matte b, Vadim Sokolov c

o Ggosciences Azur (UMR 6526), Universit~ de Nice-Sophia Antipolis et CNRS, 250 rue Albert Einstein, 06560 Valbonne, France b Laboratoire de Gdochronoloqie, Ggochimie et P~trologie, (UMR 5567), CNRS/Universit~ Montpellier II, Cases 60 and 58, Pl. E. Bataillon - 34095, Montpellier Cedex 05, France c RosComNedra, UraIGeolCom, Bazhenov Geophys. Expedition, Scheelite, Ekaterinenburg, Russia

Received 10 May 1996; accepted 8 November 1996

Abstract

A new evolutionary geodynamic model of Devonian continental subduction and exhumation of high-pressure/low- temperature metamorphic rocks in the southern Urals is proposed based on the results of physical modelling, geological and geophysical data, and on the comparison of this belt with other orogens. The model includes the following principal phases. (1) Closure of the western Uralian ocean by eastward intraoceanic subduction associated with the Magnitogorsk volcanic arc. (2) Subduction of the European continental margin, causing failure of the overriding plate in the arc area along a fault dipping eastward under the arc. (3) Subduction into the mantle of the fore-arc block together with the underlying continental crust and sedimentary layer. The sedimentary cover, as well as the crustal and mantle fragments of this block, are scraped off and accreted in front of and under the arc. The continental crust, shielded from the hot mantle by the fore-arc block, subducts to depths of more than 150 km, remaining at a relatively low temperature. The crust then fails at depths of several tens of kilometres. (4) The subducted crustal slice starts to rapidly rise and intrude the interplate zone, scraping and pushing up the sediments and slivers of the oceanic fore-arc block previously dragged down to different depths. (5) Exhumation of the rising high-pressure/low-temperature rocks within the Uraltau dome, separating the subduction accretionary complex from the Magnitogorsk arc. The eastern border of the Uraltau dome coincides with the Main Uralian Fault. This fault represents a complex suture zone which at the stage of deep crustal subduction was associated with a major thrust (subduction) fault, and at the exhumation stage with a major normal fault corresponding to the upper surface of the rising crustal/sedimentary body.

Keywords: Urals; continental subduction; exhumation; orogenesis; arc--continent collision; obduction

1. Introduction 1991; Dobretsov, 1991; Sobolev et al., 1992; Okay, 1994) has resulted in a new pulse for the investiga- Modern studies of high-pressure/low-temperature tions of collision and obduction, and prompted a re- (HP/LT) rocks in many orogenic belts (Chopin et al., consideration of previous models for these processes. It seems to be generally assumed now (Dobretsov, * Corresponding author. Fax: 33-4-93.65.27.17: E-mail: 1991; Wheeler, 1991; Hodges et al., 1992; Aviad, chem @faille.inice.fr 1992; Michard et al., 1993; Boyle et al., 1994; Ernst

0040-1951/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0040-1951(97)00057-7 218 A. Chemenda et al. / Tectonophysics 276 (1997) 217-227 and Liou, 1995) that both the burial of the continen- many poorly constrained parameters such as den- tal crustal/sedimentary rocks and at least part of their sities (and their change due to metamorphism) of rapid exhumation from great depths occurs during the lithospheric layers, strengths and thicknesses of plate convergence and orogenesis. Exhumation of different layers and others. In both regimes the con- HP/LT continental rocks containing coesite implies tinental crust can subduct to a great depth, 150-200 that continental crust can be subducted to depths of km or more. The crust then fails under the buoyancy at least 100 km. Actual depth of continental crust force. The principal difference between the regimes subduction may be considerably greater, because the is the location of the failure. In the HCR the crust metamorphic rocks which reached the surface may fails near the front of the subduction zone, while in represent only the upper part of the uplifted conti- the LCR the failure criterion is fulfilled at a depth nental material. There is now also a near-consensus of several tens of kilometres. Failure is followed that the major force driving the exhumation of high- by a rapid buoyancy-driven uplift of the subducted to ultrahigh-pressure rocks is the buoyancy force ex- crustal slice and exhumation of its upper part. This erted on the low-density crustal material subducted process is different for the two regimes. Both models to great depths (Dobretsov, 1991; Ernst and Liou, were tested in different regions and the conclusion 1995). Analysis of the available data in different was that two regimes of continental subduction prob- collisional belts reveals large structural and meta- ably exist in nature. The HCR corresponds fairly morphic differences between them, although most well to the Miocene evolution of the Himalayas, contain clear evidence of continental subduction. while LCR is representative of the situation in the Subduction and exhumation of continental material Oman Mountains (which resembles very much the appears to be a basic process of orogenesis, although southwestern Urals) and probably can be applied it is still poorly understood. to other orogens where high- to ultrahigh-pressure The way to try to understand the mechanics and rocks outcrop. Based on this similarity Matte and physics of continental subduction is to perform quan- Chemenda (1996) have applied the LCR model of titative modelling (either experimental or mathemat- continental subduction to explain exhumation of the ical) of this process. Modelling can define physically Maksyutovo high-pressure metamorphic complex in possible scenarios (or physical limits on the possi- the Urals. While generally consistent, the proposed ble models), but cannot provide a definitive model model (as all the previous models for the deep because many of the physical parameters characteriz- continental subduction and exhumation) does not ing the prototype are unknown or poorly constrained. take into account the arc-continent collision preced- The possible geodynamic models obtained by mod- ing exhumation. We first believed that arc-continent elling must be tested against observation. One cannot collision does not affect deep subduction and ex- perform a comprehensive test based only on a single humation of the continental crust. However, further orogen, as the available regional information is very analysis of the situation in the Urals, in the view limited. Ideally, the test should involve all the col- of physically possible options for the arc-continent lisional belts corresponding to different evolutionary collision, has shown that this process can play a fun- stages and conditions of continental subduction. In damental role. One of the modes for arc-continent this paper we use the data on the Urals to test and collision, obtained by the physical modelling, in- to further develop the models already tested in other cludes a failure of the overriding plate in the arc area regions. and underthrusting of the fore-arc block under the Based on the results of physical modelling, arc. This block can subduct completely into the man- Chemenda et al. (1995, 1996) have developed a tle together with the underlying continental crust, model for continental subduction and defined two providing thermal insulation of this crust. The conti- principal modes for this process corresponding to a nental crust subducted to great depths and preserved low compressional regime (LCR) and a high com- between the cold fore-arc block and the mantle layer pressional regime (HCR) of continental subduction. of the subducting plate is kept at a relatively low The regime is controlled by the total pull force from temperature. This process is an essential element of the subducted lithosphere, which in turn depends on a new model presented in this paper, which explains A. Chemenda et al. / Tectonophysics 276 (1997) 217-227 219 in particular the low peak temperature (550-600°C) ment of the Zilair allochthon itself probably occurred reached by the rocks of the Maksyutovo complex earlier, during the Carboniferous. metamorphosed at a depth of several tens of kilome- One of the most remarkable features of the tres (17-25 kbar; Dobretsov, 1991; Lennykh et al., Urals is a narrow, over 2000 km long belt in 1995). which high-pressure/low-temperature (HP/LT) metamorphic rocks outcrop along the western side of 2. Tectonic overview of the southwestern Urals the Main Uralian suture (Sobolev et al., 1992). In the southwestern Urals, these HP/LT rocks outcrop The Urals are a 3000 km long, N-S-trend- within a narrow window (20 x 200 km), within the ing Palaeozoic mountain belt which separates Eu- Uraltau dome. The dome structure is clearly attested rope from Asia. The western part of the southern at the surface by structural measurements of the Urals, from the Permo-Carboniferous foredeep to metamorphic foliation (Matte et al., 1993; Matte, the Main Uralian Fault, is characterized by west-ver- 1995) and at depth, by the URSEIS Vibroseis and gent thrusting (Fig. 1). Thrusts are well defined by explosion-source reflection profile (Fig. 1), in which drilling in the Permo-Carboniferous foredeep basin the dome appears as an anticlinal stack (Berzin et (Kazantseva and Kamaletdinov, 1986) and by struc- al., 1996; Echtler et al., 1996; Knapp et al., 1996). tural analysis in the Bashkir anticlinorium (Brown On the section in Fig. 1A, mainly low-grade schists, et al., 1996) where variably metamorphosed Riphean probably of early Palaeozoic age (Suvanyak series), sedimentary units are stacked together with Ven- outcrop. It is hard to determine whether the an- dian-lower Palaeozoic sediments. Westward thrust- tiformal bright reflectors visible between 5 and 8 s ing is also clearly revealed in this area by the new TWT in the Uraltau dome are due to the presence URSEIS vibroseis and explosion-source deep reflec- of competent Riphean beds (as is the case in the tion profiling (Echtler et al., 1996; Knapp et al., western foredeep) or, more likely, are due to a meta- 1996) which images the deep structure of the crust morphic banding within the micaschists and dense down to 20 s TWT (approximately 60 km). In the layers like eclogites. Such an alternation is visible Bashkir anticlinorium, east-dipping reflectors clearly in the high-pressure Maksyutovo complex, 150 km cross-cut the bedding of the Vendian and Riphean to the south in the Sakmara River. Here, the high- series. Further to the east, large (500 km long and grade rocks are located along the easternmost margin 20-50 km wide) allochthonous units (ophiolites and of the Uraltau dome, beneath lower-grade Palaeo- flysch) are preserved in the Zilair synform. The root zoic schists comparable to the Suvanyak series. of these oceanic mantle klippes is located 30 km to The Maksyutovo complex consists of metasediments the east along the Main Uralian Fault where various mainly of continental origin (phengite-glaucophane slices of ultramafic rocks and even some large ophi- bearing micaschists, quartzites and meta-arkoses) olitic massifs like the Nurali massif outcrop. The and lenses of metabasalts, glaucophane-lawsonite- lower Zilair unit consists of a thick (up to 5000 m) beating eclogites, and enstatite-bearing metaperi- Devonian and older flysch, characterized by detrital dotites. The whole complex has been metamor- spinels (Kazantseva and Kamaletdinov, 1986). This phosed at 550--600°C and 17-25 kbar (Dobretsov, flysch unit is allochthonous and overrides shallow 1991; Lennykh et al., 1995). This HP/LT metamor- marine Ordovician to Devonian sediments. In the phism has been dated ca. 380 Ma by 39Ar/a°mr on southern section (Fig. 1B), the whole sedimentary phengites of micaschists and eclogites (Matte et al., pile is tightly folded with a steep fan-like cleavage. 1993). In the east, these rocks are separated from the The upper 2000 to 3000 m thick synformal unit low-grade Devonian sediments and volcanics of the (Kraka and Kimpersai ophiolitic klippes) consists Magnitogorsk arc by a complex east-dipping fault mainly of ultramafic rocks (Savelieva and Pertsev, zone, the Main Uralian Fault (MUF), marked by 1995) with a sole of serpentinites and a few basalts various slices of ultramafic rocks (Fig. 1). and gabbros. Folding and cleavage of this allochthon The MUF separates the continental part of the are supposed to be Late Permian, contemporaneous Urals in the west from the island arc complexes with thrusting within more external units. Emplace- in the east, and is a major lithospheric boundary 220 A. Chemenda et al. / Tectonophysics 276 (1997) 217-227

__. URALTAU DOME ZlLAIR MAGNITOGORSK FOREDEEP BASHKIR ANTICLINAL SYNFOFIM M.U.F. ISLAND ARC I ' i

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Fig. 1. Schematic cross-sections through the southwestern Urals. (A) Section across the Kraka klippe, based on the structural data by Matte (1995), Brown et al. (1996) and the URSEIS seismic data by Echtler et al. (1996) and Knapp et al. (1996). Stars indicate the possible location of HP rocks. (B) Section through the Sakmara River where the Maksyutovo complex outcrops (from Matte and Chemenda, 1996): 1 = inferred Archaean basement (a) and continental mantle (b); 2 = Riphean-Vendian sediments; 3 = shallow-marine Lower Palaeozoic (a) and Suvanyak series (b); 4 = Zilair and fore-arc flysch, mainly Devonian; 5 = Permian molasse: 6 = Magnitogorsk volcanic arc; 7 = oceanic crust and mantle. Stars indicate the HP rocks. at the scale of the whole Urals. The eastward dip section A). Various geological data suggest that the of the MUF is clearly displayed by deep seismic Main Uralian Fault worked as a major thrust dur- profiling both in the central Urals (Sokolov in Zo- ing westward obduction of the oceanic lithosphere nenshain et al., 1990; Juhlin et al., 1995) and in the in Silurian-Devonian times (Matte, 1995; Echtler et southern Urals (Knapp et al., 1996; Echtler et al., al., 1997). On the other hand, a sharp metamorphic 1996) where it appears as a diffuse zone. This suture contrast between both sides of this oceanic suture as fault zone corresponds to the root of the oceanic well as the ductile and brittle shear criteria indicat- klippes, like the Kraka massif, preserved to the west ing down-dip eastward movements, imply a normal- of the Uraltau dome in the Zilair synform (Fig. 1, sense of displacement along the MUE The interplay A. Chemenda et al. / Tectonophysics 276 (1997) 217-227 221 between thrusting and normal faulting as well as 1994). The Main Uralian Fault seems to have the their chronology and succession are still poorly stud- same nature (Matte and Chemenda, 1996). ied and remain unclear. The MUF separates the HP The evolution of the Oman Mountains appears to rocks from the low-grade fore-arc Devonian flysch correspond well to the model of low compressional of the Magnitogorsk volcanic arc. The Magnitogorsk regime of continental subduction depicted in Fig. 3. volcanic arc consists of basalts, andesites and cherts According to this model the continental subduction well dated by conodonts. Beds are gently to tightly follows the subduction of the oceanic lithosphere folded, locally with a steep cleavage, depending on (oceanic subduction) and during initial stages devel- their competence. ops in the same way. This process changes when the continental crust fails upon reaching a depth of 3. Comparison with Oman: low compressional 150-200 km (Fig. 3a). The crustal slice subducted regime of continental subduction into the asthenosphere starts to rise rapidly under the buoyancy force, sliding over the mantle litho- The structure of the southwestern Urals is very spheric layer (over weak and ductile lower crust). similar to that of the Upper Cretaceous orogen in The rising slice intrudes the interplate zone (Fig. 3b). Oman (Fig. 2) which, however, is simpler and better Removal (uplift) of the low-density crust from the studied. The HP/LT rocks of Oman were metamor- mantle layer increases the pull force as this layer is phosed under similar P/T conditions as in the Urals, slightly denser than the asthenosphere. An increased that is, around 20 kbar at 500°(2 (Wills et al., 1991; pull force facilitates the separation of the plates and Michard et al., 1994). These rocks have been ex- the intrusion of the crust between them. This pro- humed within the Saih Hatat window, an analogue cess is thus self-accelerating, with a maxim rate of to the Uraltau dome. Both orogens have a com- crustal upward sliding reaching the usual rates of parable architecture. The HP/LT belts are bordered plate motion (i.e. from a few to several cm/yr). The along their western sides by ophiolitic klippes, the rise of the crustal slice results in the formation of Semail klippe in Oman and the Kraka klippe in the a major syn-obduction normal fault along the upper Urals. On their eastern sides, the metamorphic belts surface of the slice (Fig. 3b). Another consequence are limited by major, very complex ductile faults. of this rising is an uplift and local extension of the The Omanian fault has been studied in great detail frontal part of the overriding plate, which is then fol- (Michard et al., 1993; Searle et al., 1994; Mattauer lowed by separation of the tip of this plate (Fig. 3c), and Ritz, 1996) and was shown to have operated first corresponding to the Semail ophiolite (Fig. 2). The as a major thrust fault and then, during exhumation, previously subducted material is exhumed within the as a normal fault with large (several tens of kilome- formed window (Fig. 3c) which corresponds to the tres) normal-sense of displacement (Michard et al., Saih Hatat window in Oman. The removal of the

SSW NNE Semail ophiolite S aih Hata_t Gulf of 0 ¢s -<_ --- Oman _0

10 10

20, 20 kin km m4 -Zq7 F -18 Fig. 2. Cross-section trough Oman to the east of Muscat (from Chemenda et al., 1996): 1 = Arabian crust (a, upper strong and brittle layer, and b, lower weak and ductile layer); 2 = continental sedimentarycover (a, Permo-Mesozoic,and b, Proterozoicand Palaeozoic); 3 = oceanic lithosphere;4 = oceanic sediments (Hawasina nappe); 5 = ductile fault; 6 = cleavage and folds; 7 = thrust (a) and normal (b) faults; 8 = eclogitesexhumed from depth of near 60 km (ca. 20 kbar) (the arrows correspondto the stage of exhumation). 222 A. Chemenda et al./Tectonophysics 276 (1997) 217-227

island-arc was built. The continental subduction in the Urals thus underwent a stage of arc-continent collision. Did this stage change conditions in the subsequent deep subduction and exhumation of the continental crust? To answer this question it is neces- sary to understand first what arc-continent collision means in terms of mechanics. We summarize below results of physical modelling of this process reported by Shemenda (1994) and Chemenda et al. (1997).

+ ÷'~.-..~*1~ =.. • . _ . 4. Arc-continent collision

Modelling shows that the presence of a volcanic arc (volcanics) on the overriding plate does not change the process of continental subduction shown in Fig. 3, unless this plate is sufficiently weakened. A weakening indeed occurs in the arc area due to a number of factors such as induced convection in the mantle, magmatic activity (presence of magma chambers and channels), and increasing water con- klippe tent which provides weak wet rheology of the over- + + + + + ÷ ÷'' . . ::,::::: riding plate in the arc area. According to the physical modelling results, subduction of the continental margin progressively increases compression in the overriding plate. Compressive stress reaches the yield limit of this plate in the arc area (weak zone), causing its quasi-plastic shortening. Plastic deformation occurs through shearing along two opposite dipping 2 I\'.13 I systems of slip lines (Fig. 4b). The lithosphere then Fig. 3. Low compressional regime of continental subduction fails in either of these two directions, depending on (from Chemenda et al., 1996): 1 = brittle, strong upper con- two factors: the interplate friction stress, rn (which tinental crust; 2 = ductile, weak lower crust; 3 = overriding is unknown in nature), and the average distance, plate. L, between the interplate zone and the volcanic arc (Fig. 4a). Failure along the fault dipping under the arc from its rear (to the left in Fig. 4) results in crustal layer from the subducted plate results in rup- subduction reversal. If failure occurs along the fault ture and sinking (break-off) of the dense lithospheric dipping under the arc from the fore-arc basin, then mantle which either changes the subduction regime the fore-arc block underthrusts the arc (Fig. 4c). This or causes a jump of the subduction zone (Chemenda block can be subducted into the asthenosphere either et al., 1996). completely (Fig. 4d), if it has a mantle density (or It seems that the same model (Fig. 3) can explain oceanic origin), or partly (Fig. 4e), if the density is the Palaeozoic obduction and exhumation of the HP lower (crustal/sedimentary composition). In the lat- rocks in the southern Urals. There is however an im- ter case, there is the possibility of a failure of the portant difference between the Uralian and Omanian overriding plate at the opposite side of the arc, which orogens: oceanic subduction preceding obduction in would result again in subduction reversal. Oman did not result in the formation of a volcanic Which of these options correspond to the Urals? arc, due probably to a small width of the oceanic It is generally assumed that the overriding plate in basin (Nicolas, 1989), while in the Urals a mature the Urals was oceanic, hence there remain only two A. Chemenda et al. / Tectonophysics 276 (1997) 217-227 223 ® ®

+ 4- -- (~0 fore arc block + 4-

f ®

100km

I lOOkm

I Fig. 4. Possible modes of arc-continent collision (from Shemenda, 1994; Chemendaet al., 1997). Two possible scenarios are shown for lithospheric deformation:(a) to (d), and (a), (b), (c) and (e), which depend on the model parameters (see text for explanations). options: subduction reversal or complete subduction along the MUF (Fig. 1), could be what has been left of the fore-arc block into the mantle (Fig. 4d). We from the subducted fore-arc block. prefer the latter option. Indeed, the width of the Deformation of the volcanic arc area, closure (un- fore-arc block (the distance between the accretionary derthrusting under the arc) of the fore-arc basin, prism and arc) in presently active oceanic subduc- and approaching the volcanic arc to the accre- tion zones is 100-200 km. In the southern Urals it tionary prism are currently going on in southern is clearly seen (Fig. 1) that the subduction complex Taiwan (Suppe, 1981; Teng, 1990; Yu et al., 1995; (Zilair allochthon) is located just near the Magni- Chemenda et al., 1997). This region is an example togorsk volcanic arc, which attests to disappearance of active arc-continent collision where the Chinese of a large fore-arc space. Moreover, this complex (Asian) continental margin started (approximately 5 underthrusts the arc, which fits well with the model Ma) to subduct under the oceanic Philippine Sea in Fig. 4d (the accretionary prism is not shown in Plate bearing the Luzon volcanic arc. The subduc- this model). The Kraka ophiolitic klippe, as well as tion of the fore-arc block seems thus to be a general mafic/ultramafic (Nurali) units rooted under the arc phenomenon. Below, we combine the model for the 224 A. Chemenda et al./Tectonophysics 276 (1997) 217-227 arc-continent collision (Fig. 4) with the low com- subduction was associated with a major thrust fault pressional regime model for deep subduction of the (zone), and at the stage of exhumation, with a major continental crust (Fig. 3) to develop an evolution- normal fault (zone) corresponding to the upper sur- ary model of obduction/exhumation in the southern lace of the rising and deforming crustal/sedimentary Urals. body. A removal (uplift) of the crust from the mantle 5. An evolutionary model for obduction in the layer of the subducted continental plate at the stage southern Urals of exhumation (Fig. 5d) increases the pull force and results in rupture and sinking of the lithospheric Stage 1 of the model (Fig. 5a) corresponds to mantle (Fig. 5e). This break-off removes the pull oceanic subduction) and the formation of the associ- force which cause an increase in the horizontal com- ated Magnitogorsk intraoceanic volcanic arc, proba- pression of the lithosphere. Orogenesis drastically bly in Silurian- Devonian time. slows down in this part of the Urals, but continues Stage 2 (Fig. 5b) starts with initiation of sub- to the east of the Magnitogorsk arc. It should be duction of the European continental margin, which noted that in our model the breakage occurs within causes the following sequence of events: the increase the continental lithospheric mantle and not in the in lithospheric compression, the failure of the over- transitional zone from the continental lithosphere to riding oceanic lithosphere in the arc area, and the the oceanic lithosphere as supposed in the models of underthrusting of the fore-arc block under the arc. Sacks and Secor (1990) and Davies and von Blanck- Stage 3 (Fig. 5c) corresponds to subduction into enburg (1995) for the slab break-off. the mantle of the fore-arc block together with the underlying continental crust and sedimentary layer. 6. Conclusions The fragments of the sedimentary cover as well as the crustal and mantle slivers of this block are What is principally new in the proposed model is scraped off and accreted in front and under the arc. incorporation of the stage of arc-continent collision This process continues until the continental crust with a complete subduction into the mantle of the reaches a critical depth (more than 150 kin). The fore-arc block. This process was obtained in labora- subducted crust then fails at depths of several tens of tory (physical modelling) and seems to fit well the kilometres giving start to the next stage: exhumation. geological situation in the southern Urals. There is In stage 4 (Fig. 5d), the subducted crustal slice no space for the fore-arc block on the geological rapidly rises and intrudes the interplate zone be- profiles in Fig. 1. The arc-continent collision was tween subducting and overriding plates. The slice ignored by all previous models for the exhumation slides over the mantle lithospheric layer which keeps of HP/LT rocks. The proposed mechanism, tested subducting. This motion (sliding) first accelerates here in one particular region, may have a funda- and then slows down, with the maximal rate reach- mental significance, providing a possible solution ing a few to several centimetres per year. The rising for the 'low-temperature paradox' of HP metamor- crust scrapes the sediments previously dragged into phism. The exhumed high-pressure rocks are fre- the interplate zone and pushes them up, producing quently (if not always) also low-temperature rocks, uplift and extension of the overlying melange. This which means that they were preserved at relatively process in combination with erosion results in ex- cold temperatures (ca. 700°C) at great depths (around humation of previously subducted material within 100 km), where temperature is normally greater than the Uraltau dome, which separates part of the ac- 1000°C. The temperature of the crust depends on cretionary complex including the ophiolitic klippes the subduction rate and on such poorly constrained (Zilair, Kraka, Kimpersai) from the Magnitogorsk parameters as the exhumation rate and the thickness volcanic arc (stage 5, Fig. 5e). The eastern border of of the overriding plate in the subduction zone. One the dome coincides with the Main Uralian Fault. Ac- can 'play' with these parameters, trying to obtain the cording to the model, this fault represents a complex required low temperatures at the 100 km depth (near suture zone, which at the stage of deep continental the failure location in Fig. 3c or Fig. 5c). However, A. Chemenda et al. / Tectonophysics 276 (1997) 217-227 225

••:,:+:•:•:• i!iiiiill

: <~ x ~ " . . ,,...

100 100 km km

® ® ~:,/..~ I÷÷~.~_::::::::::::::::::::::::::::: ......

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~1 ~2 [Win3 ~4 ~;z~5 ~0 ~-Zq7 FE-]8 Fig. 5. Evolutionary model for obduction and exhumation of HP/LT rocks in the southern Urals: 1 = continental crust (a, upper strong, brittle layer, and b, lower weak, ductile layer); 2 = lower Palaeozoic and Riphean sediments; 3 = Silurian-Devonian flysch; 4 = mantle layer of the oceanic lithosphere; 5 = oceanic crust; 6 = Magnitogorsk volcanic arc; 7 = thrust (a) and normal (b) faults; 8 = marker corresponding to depth of several tens of kilometres (17-25 kbar) at the stage shown in (c). 226 A. Chemenda et al. /Tectonophysics 276 (1997) 217-227

there is another problem: a segment of the crustal Chemenda, A., Mattauer, M., Bokun, A.N., 1996. Continental slice, which is below the failure location, must also subduction and a mechanism for exhumation of high-pressure be cold enough to preserve its rigidity. The rigidity metamorphic rocks: new modelling and field data from Oman. Earth Planet. Sci. Lett. 143, 173-182. is needed to transmit the buoyancy force along the Chemenda, A.I., Yang, R.K., Hsieh, C.-H., Groholsky, A.L., crustal slab and to push its upper parts to the surface. 1997. A possible evolutionary model for the Taiwan collision: If the rigidity is low, then the subducted crust would based on physical modelling. Tectonophysics 274, 253-274. ascend vertically (as a viscous slab) to the base of the Chopin, C., Henry, C., Michard, A., 1991. Geology and petrol- overriding plate, without being able to intrude an in- ogy of the coesite-bearing terrain, Dora Maira massif, Western Alps. Eur. J. Mineral. 3, 263-291. terplate zone. The exhumation mechanism presented Davies, J.H., von Blanckenburg, F., 1995. Slab breakoff: a model in Fig. 3 does not work under these conditions. Thus, of lithosphere detachment and its test in the magmatism and the model of exhumation shown in Fig. 3 lacks some deformation of collisional orogens. Earth Planet. Sci. Lett. important elements which would provide for thermal 129, 85-102. screening of the subducted crust from the hot sur- Dobretsov, N.L., 1991. Blueschists and eclogites: a possible plate tectonic mechanism for their emplacement from the upper rounding mantle. This element could be a subducted mantle. Tectonophysics 186, 253-268. fore-arc block which conserves the continental crust, Echtler, H.P., Stiller, M., Steinhoff, E, Krawczyk, C., keeps it relatively cold, and hence rigid and strong Suleimanov, A., Spiridonov, V., Knapp, J.H., Menshikov, Y., (see Fig. 5). Alvarez-Marron, J., Yunusov, N., 1996. 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