arquitectura corteza urales del sur

8/12/2019 Arquitectura Corteza Urales Del Sur

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The Crustal Architecture of the Southern and Middle Urals Fromthe URSEIS, ESRU, and Alapaev Reflect ion Seismic Surveys

D .B r o w n1 , C. Juhlin 2 , A. Tryggvason 1 , D. Steer 3, P . Ay arza 4 , M. Beckholmen 2, A . Ryb a lka 5and M. Bliznetsov 5The Urals Seismic Experiment and Integrated Studies (URSEIS), Euro-probe's Seismic Reflection Profiling in the Urals (ESRU), and reprocessedRus sian reflection/refraction seismic surveys have shown the know n U ralides

to be bivergent, with a crustal root along the central volcanic axis of theorogen. In the Southern (URSEIS) and Middle (ESRU and Alapaev) Uralsthe East Europe an C raton crust thickens eastward from 40 km to 48 km,and is imaged by sub-horizontal to east-dipping reflectivity that can berelated to its Paleozoic and older evolution. The suture zone between the EastEuropean Craton and the accreted terranes, the Main Uralian fault, is poorlyimaged in the URSEIS section, but in the ESRU and Alapaev sections it isimaged as an abrupt change from a zone of east-dipping reflectivity thatextends from the surface into the middle crust. East of the Main Uralianfault, the Magnitogorsk (Southern Urals) and the Tagil (Middle Urals)volcanic arcs display moderate to weak upper crustal reflectivity, and diffusemiddle to lower crust reflectivity. The Moho beneath both arc complexes ispoorly imaged in the reflection data, but based on refraction data isinterpreted to be at 50 to 55 km depth. East of the arc complexes, the Uralidestructural architecture is dominated by a wide zone of anastomosing strike-slip faulting into which numerous syntectonic Late Carboniferous andPermian granitoids intruded. This area is imaged in the seismic sections asclouds of diffuse reflectivity interspersed with, or cut by sha rp, pred om inan tlywest-dipping reflections. In the Southern and Middle Urals, west-dipping

institute Jaume Almera, Barcelona, Spain2Department of Earth Sciences, Uppsala University, Uppsala,Sweden3University of Akron, A kron, US4Department of Geology, University of Salamanca, Salamanca,Spain5Bazhenov Geophysical Expedition, Scheelite, Russia

Mountain Building in the Uralides: Pangea to the PresentGeophys ica l Monograph 132Copyright 2002 by the American Geophysical Union10.1029/132GM03

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3 4 C R U S T A L A R C H I T E C T U R E O F T H E U R A L I D E Sr e f le c t iv i t y o f t h e T r a n s - U r a l i a n z o n e e x t e n d s f r o m t h e m i d d l e c r u s t i n t o t h el o w e r c r u s t w h e r e it a p p e a r s t o m e r g e w i t h t h e M o h o . T h e b o u n d a r i e s a n di n t e r n a l f a u l t s o f t h e s t r ik e - s l i p f a u l t s y s t e m a r e w e l l m a r k e d b y m a g n e t i ca n o m a l i e s , a l l o w i n g t h e m t o b e c o r r e l a t e d b e t w e e n t h e se i s m i c s e c t i o n s .

1. I N T R O D U C T I O NT h e U r a l i d e o r o g e n d e v e l o p e d d u r i n g t h e L a t e

P a l e o z o i c a s t h e c o n t i n e n t a l m a r g i n o f t h e f o r m e r E a s tE u r o p e a n C r a t o n w a s s u b d u c t e d e a s t w a r d ( c u r r e n tcoord ina te s ) benea th a cha in o f in t ra -ocean ic i s l and a rc s( M a g n i t o g o r s k a n d T a g i l ) d u r i n g t h e L a t e D e v o n i a n a n dE a r l y C a r b o n i f e r o u s [Puchkov, 1997 ; Brown and Spadea,1999] . T h i s was fo l lowed by c los u re o f the pa leo -Ura l i anocean bas in to the ea s t and acc re t ion o f vo lcan ic a rc s andc o n t i n e n t a l c r u s t a l o n g t h e e a s t e r n m a r g i n o f t h e g r o w i n g o r o g e n f r o m t h e L a t e C a r b o n i f e r o u s t h r o u g h t othe L a te Pe rmian-E a r ly T r ia s s i c [e .g . , Zonenshain et al. ,1984, 1990; Puchkov, 1997]. In the s ou th , the Ur a l ide sb e g a n t o f o r m d u r i n g t h e M i d d l e t o L a t e D e v o n i a n a st h e c o n t i n e n t a l m a r g i n o f t h e f o r m e r E a s t E u r o p e a nC r a t o n s u b d u c t e d e a s t w a r d b e n e a t h t h e M a g n i t o g o r s ki s l a n d a r c . A s a c o n s e q u e n c e , a n a c c r e t i o n a r y c o m p l e xw a s d e v e l o p e d a n d e m p l a c e d o v e r t h e s u b d u c t i n g s l a b ,a n d b y t h e E a r l y C a r b o n i f e r o u s t h e a r c w a s s u t u r e d t ot h e c o n t i n e n t a l m a r g i n a l o n g t h e M a i n U r a l i a n f a u l tzone [e .g . , Puchkov, 1997; Brown et al. , 1998;Brown andSpadea, 1999] . A de f o rm a t io n h ia tu s un t i l t he L a teC a r b o n i f e r o u s f o l l o w e d a l o n g t h e e a s t e r n m a r g i n o f t h eE a s t E u r o p e a n C r a t o n . F a r t h e r n o r t h e x p o s u r e i s p o o rand t ec ton ic in te rp re ta t ions a re heav i ly re l i an t ongeophys ica l and boreho le da ta . I t i s gene ra l ly accep ted ,howeve r , tha t the T ag i l vo lcan ic a rc co l l ided wi th thee a s t w a r d s u b d u c t i n g E a s t E u r o p e a n C r a t o n in t h e E a r l yC a r b o n i f e r o u s [ e . g . , Puchkov, 1997] . T h i s in te rp re ta t io ni s ba s e d on the p re s ence o f E a r ly Ca rbo n i fe rou s (Vis ean)f lysh -type depo s i t s w i th an ea s te r n p ro ven anc e ove r ly ingb a t h y a l d e p o s i t s i n t h e u p p e r r e a c h e s o f t h e P e c h o r aR i v e r [Puchkov, th i s bo ok] .

F r o m t h e L a t e C a r b o n i f e r o u s t h r o u g h t o t h e L a t eP e r m i a n - E a r l y T r i a s s i c, t h e p a l e o - U r a l i a n o c e a n b a s i n t othe ea s t c los ed , a s i s l and a rc s and con t inen ta l f ragmentswere acc re ted to the ea s te rn f l ank o f the deve lop ingU r a l i d e s . I t is n o t c l e a r t o w h a t e x t e n t t e r r a n e a m a l g a m a t i o n , d e f o r m a t i o n a n d m e t a m o r p h i s m o c c u r r e d w i t h i nthes e f ragments p r io r to acc re t ion . C los ure o f the pa leo -U r a l i a n o c e a n b a s i n w a s a c c o m p a n i e d b y w e s t w a r dt h r u s t i n g o f t h e E a s t E u r o p e a n C r a t o n P r e c a m b r i a nbas e m ent and i t s l a t e Pa leo zo ic p la t f o rm c ove r to fo rma fo re land th rus t an d fo ld be l t an d a l a rge ly Pe r mia n-ag ef o r e l a n d b a s i n [Kamaletdinov, 1974; Brown et al. , 1997].

Dur ing l a t e s t ages o f the co l l i s ion , ex tens ive wrench ot r a n s p r e s s i v e f a u l t i n g a p p e a r s t o h a v e d o m i n a t e d a l o nthe cen t ra l ax i s o f the o rogen , f ragment ing the T ag i l a ra n d j u x t a p o s i n g m e t a m o r p h i c t e r r a n e s w i t h i n t h e E a sa n d T r a n s - U r a l i a n z o n e s [Ayarza et a l . , 2000 a; Friberet a l . , 2000; Hetzel and Glodny, 2002 ] . Th is s tage oo r o g e n i c d e v e l o p m e n t w a s a c c o m p a n i e d b y w i d e s p r e ac r u s t a l a n d m a n t l e m e l t i n g a n d t h e i n t r u s i o n o f n u m e ro u s p l u t o n s [Bea e t a l . , t h i s bo ok , Gerdes, 2002] (Pla te 1T he Midd le Ura l s was a l s o a f fec ted by Mes ozo ic tT e r t i a r y d e f o r m a t i o n t h a t r e s u l t e d i n f o l d i n g a n d t h r u s ting o f L o we r T r ia s s i c depos i t s in the T ag i l zone [Puchko1997] and the deve lopment o f the Wes t S ibe r i an Bas inT h e e a s t e r n m o s t z o n e s ( E a s t a n d T r a n s - U r a l i a n z o n e sa re wide ly cove red by Mes ozo ic and Cenozo ic s ed im e n t s .

T he Ura l ide s a re p re s e rved , re l a t ive ly in tac t , w i th iE u r a s i a a n d p r o v i d e a r a r e o p p o r t u n i t y t o i n v e s t i g a tthe c rus ta l a rch i t ec tu re o f an en t i re Pa leozo ic o rogenW i th th i s in mind , ne a r ly 1000 km of new deep re f l ect ion s e i s mic da ta have been acqu i red in the Sou the r( U R S E I S ) a n d M i d d l e ( E S R U ) U r a l s s i n c e 1 9 9 3 u n d et h e a u s p ic e s o f E U R O P R O B E [ e. g. , Berzin et al. , 1996Ecthler et al. , 1996;Knapp et al. , 1996;Steer et al. , 1998Juhlin et a l . , 1996, 1998] , an d c . 500 km of s hal loR u s s i a n d a t a ( A l a p a e v , R 1 1 4 a n d R 1 1 5 ) h a v e b e ereproces s ed [e .g . , Steer e t a l , 1995 ; Brown et al. , 1998(Pla te 1 ) . In add i t io n , two re f rac t ion /w ide -ang le re fl ect i o n p r o fi l es ( U W A R S a n d U R S E I S ) h a v e b e e n a c q u i r e[Thouvenot e t a l , 1995 ; Carbonell et al. , 19 96, 200 0T h e s e e x p e r i m e n t s h a v e s h o w n t h e U r a l i d e c r u s t t o bh igh ly re f l ec t ive , to have wha t appea rs to be a b ive rgens t ruc tu ra l a rch i t ec tu re , and to have c lea r re f l ec t ioncha rac te r i s t i c s fo r each t ec ton ic un i t [Echtler et al1996; Knapp et al. , 1996; Steer et a l . , 1998; Juhlin et al1998;Friberg, 2000] . T he Mo ho ap pea rs to have d i s t inccha rac te r i s t i c s f rom wes t to ea s t a c ros s the o rogentha t can be re la t ed to the d i f fe ren t t e c ton ic un i t s andeven t s [Steer et a l . , 1998; Juhlin et a l . , 1998] . De spi ts i m i l a r i t i e s , t h e U R S E I S , E S R U a n d A l a p a e v s e c t i o nd i s p lay c rus ta l re f l ec t iv i ty pa t t e rns tha t s ugges t thUr a l id e c rus t i s d i f feren t in the Sou the rn U ra l s tha n ithe Midd le Ura l s [e .g . , Steer et a l . , 1995, 1998; Juhlie t a l , 1998 ;Ayarza et a l . , 2000] , ind ica t ing that differenla te - o r pos t - t ec ton ic p roces s e s may have been ac t ive ithe two a rea s . T he a im o f th i s pape r i s to in teg ra t


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3 6 C R U S T A L A R C H I T E C T U R E O F T H E U R A L I D E Sthe s e s imic da ta wi th the known s u r face geo logy , p lac ings pec ia l emphas i s on the s t ruc tu re s tha t bound the ma jo rt ec ton ic un i t s , and to deve lop a un i f i ed c rus ta l - s ca lemode l fo r the a rch i t ec tu re o f the Ura l ide s f rom theU R S E I S s e c t i o n i n t h e s o u t h a n d t h e E S R U s e c t i o n i nt h e n o r t h .

2. G E O L O G I C A L F R A M E W O R KA gene ra l ly accep ted d iv i s ion o f the Ura l ide s invo lves

a number o f long i tud ina l zones tha t a re l a rge ly bas edo n t h e a g e s a n d p a l e o g e o g r a p h y o f t h e d o m i n a n t r o c k swi th in them [e .g . , Ivanov et a l . , 1975 ; Khain, 1985;Puchkov, 1991]. In th i s pap e r , how eve r , we us e a modi f i eds ubd iv i s ion tha t inc ludes the recogn i t ion o f t e c ton icun i t s . T h i s s ubd iv i s ion cons i s t s o f a fo re land th rus t andf o l d b e l t c o m p o s e d o f E a s t E u r o p e a n C r a t o n r o c k s , t h ea c c r e t e d M a g n i t o g o r s k a n d T a g i l i s l a n d a r c s, t h e E a s tU r a l i a n z o n e , a n d t h e T r a n s - U r a l i a n z o n e . E a c h o f t h e s etec ton ic un i t s and the i r bound ing fau l t s i s de s c r ibedbe low. Add i t iona l ly , the a rea o f the Ura l ide s d i s cus s ed inth i s pa pe r ha s been t rad i t ion a l ly d iv ided geo graph ica l lyi n t o t h e S o u t h e r n a n d M i d d l e U r a l s a t a b o u t 5 6 N .

2.1. T he Foreland Thrust and Fold BeltS o u t h o f a p p r o x i m a t e l y 5 3 N , t h e f o r e l a n d t h r u s t

and fo ld be l t cons i s t s o f a na r ro w (20 km ) zone o fupr igh t to wes t -ve rg ing fo lds and th rus t s tha t recordo n l y m i n o r a m o u n t s o f s h o r t e n i n g . B e t w e e n 5 3 N a n d56 N i t fo rm s a 120 km w ide wes t -ve rg ing , ba s e me ntinvo lved th rus t s t ack tha t records 20 km of U ra l ides h o r t e n i n g [Brown et a l . , 1997;Perez-Estaun et al. , 1997].Much o f th i s pa r t o f the fo re land th rus t and fo ld be l t i sc o m p o s e d o f t h e g e n t l y s o u t h - p l u n g i n g , P r e c a m b r i a n -c o r e d B a s h k i r i a n A n t i c l i n o r i u m ( P l a t e 1) t h a t w a s d e f o r m e d a n d m e t a m o r p h o s e d d u r i n g t h e L a t e P r o t e r o z o i c[Shatsky, 1963 ; Puchkov, 1993] , and which was on lym i l d l y d e f o r m e d a n d m e t a m o r p h o s e d d u r i n g t h e P a l e o zo ic [Glasmacher et al. , 1999; Geise e t al. , 1999 ]. In theS o u t h e r n U r a l s , a n a c c r e t i o n a r y c o m p l e x , r e l a t e d t o t h eco l l i s ion o f the Magn i togors k i s l and a rc wi th the E as tE u r o p e a n C r a t o n , o v e r l ie s t h e f o r e l a n d t h r u s t a n d f o l dbel t (Pla te 1) [Brown et a l . , 1998; Brown and Spadea,1998; Alvarez-Marron e t a l . , 2000] . N or th o f 56 N , thefo re land th rus t and fo ld be l t i s a na r row, N-S t rend ing ,w e s t - v e r g i n g b a s e m e n t - i n v o l v e d t h r u s t s t a c k m e a s u r i n g50 to 75 km in wid t h (P la te 1 ) . I t ex tends no r th wa rdin to the l inea r th rus t be l t o f the Midd le Ura l s , whe re i t i sc o r e d i n t h e e a s t b y t h e K v a r k u s h A n t i c l i n o r i u m . R o c k si n t h e K v a r k u s h A n t i c l i n o r i u m w e r e d e f o r m e d a n d m e t a m o r p h o s e d d u r i n g th e P r e c a m b r i a n , a n d o n l y m i l d l y

r e w o r k e d d u r i n g t h e P a l e o z o i c . T h e a m o u n t o f s h o r ten ing in the fo re land th rus t and fo ld be l t ha s no t beeca lcu la ted fo r th i s pa r t o f the o rogen .

2 .2 . The Main Uralian Fault ZoneT h e M a i n U r a l i a n f a u l t z o n e ( t h e a r c - c o n t i n e ns u tu re ) in the So u th e rn U ra l s i s an up to 10 km widm e l a n g e c o n t a i n i n g m a t e r i a l t h a t w a s te c t o n i c a l ly e r o d ef r o m t h e v o l c a n i c a r c , i n c l u d i n g a n u m b e r o f m a n t lf ragments [e .g . , Savelieva e t a l . , 1997] . In the nor the rnm o s t p a r t o f t h e S o u t h e r n U r a l s 55N ) , i t wai n t r u d e d b y a n u n d e f o r m e d p h a s e of t h e S y r o s t a n b a t h ol i th (Pl a te 1) , da ted a t 327 2 [Montero et a l . , 2000I n t r u s i o n o f t h e b a t h o l i t h m a r k e d t h e e n d o f t e c t o n iac t iv i ty a long the Ma in Ura l i an fau l t in th i s a rea . Int h e M i d d l e U r a l s , t h e M a i n U r a l i a n f a u l t i s p o o r le x p o s e d . Juhlin et al. [1998] defi ned it as a 10 k m w idz o n e c o n t a i n i n g a n u n d e r l y i n g t h r u s t s t a c k t h a t c o nt a i n s s t r o n g l y d e f o r m e d a n d m e t a m o r p h o s e d s a n d s t o n e sq u a r t z i t e s , a n d q u a r t z - m i c a s c h i s t s o f a p p a r e n t E a sE uropean Cra ton a f f in i ty . He re we de f ine i t t o be thc o n t a c t b e t w e e n t h e E a s t E u r o p e a n C r a t o n a n d t h e T a g ia rc , and the me tamorph ic th rus t s t ack to be in i t s foowall (see a lso Knapp et al. [1998]). Ayarza et al. [2000asugges t tha t there are s ignif icant differences in the evolut i o n o f t h e M a i n U r a l i a n f a u l t b e t w e e n t h e S o u t h e rand Midd le Ura l s , and in the l a t t e r a rea i t may be ex tens ive ly reworked .

2.3. The Magnitogorsk and Tagil Volcanic ArcsI n t h e S o u t h e r n U r a l s , t h e S i l u r ia n t o L a t e D e v o n i aM a g n i t o g o r s k v o l c a n i c a r c i s c o m p o s e d o f a c o m p l e ti s l and a rc vo lcan ic s equence tha t beg ins wi th E ms iabon in i t e -bea r ing a rc - tho le i i t e s in the fo rea rc reg ionfo l lowed by E ms ian to Give t i an a rc - tho le i i t e to ca lca lka l ine vo lcan i s m typ ica l o f a ma tu re a rc [Seravkin et al1992; Brown and Spadea, 1998 ; Spadea et a l . , th is bo okT hes e vo lcan ic un i t s fo rm the bas ement on which up t5 0 0 0 m o f w e s t w a r d - t h i c k e n i n g , F r a s n i a n - t o F a m e n n i a nage fo rea rc bas in vo lcanoc la s t i c s ed imen ts we re depos i t e[Maslov et a l . , 1993;Brown e t a l . , 2001] . L ow er C a rb oni f e ro u s s h a l lo w w a t e r c a r b o n a t e s u n c o n f o r m a b l y o v e r lithe a rc ed i f i ce . L oca l ly , L ower Ca rbon i fe rous g ran i to idi n t r u d e t h e a r c . D e f o r m a t i o n i n t h e M a g n i t o g o r s k v o l c an ic a rc i s low, wi th on ly minor , open fo ld ing and minot h r u s t i n g [Brown e t a l ., 2 0 0 1 ] . T h e m e t a m o r p h i c g r a db a r e l y e x c e e d s s e a f l o o r m e t a m o r p h i s m .

In the Midd le Ura l s , the Midd le S i lu r i an T ag i l a rc haa l s o been in te rp re ted to be an in t ra -ocean ic i s l and a r[Yazeva and Bochkarev, 1996; Bosch et al. , 1997] witp r e d o m i n a n t l y S i l u r i a n a n d e s i t i c m a g m a t i s m i n t h e w e s


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BRO WN E T AL . 3a n d L o w e r D e v o n i a n t r a c h y t e s a n d v o l c a n o c l a s ti c s in t h eeas t . T hes e vo lcan ic and vo lcanoc la s t i c rocks a re ove r l a i n b y 2 0 00 m e t e r s o f L o w e r a n d M i d d l e D e v o n i a nl imes to ne tha t in the ea s t i s in te rc a la ted w i th ca lc -a lka l inev o l c a n i c s [Antsigin et al. , 1994; Yazeva and Bochkarev,1994]. T h e T a g i l a r c f o r m s a n o p e n s y n f o r m a l s t r u c t u r e[e.g., Bashta et al. , 1990; Ayarza et a l . , 2000 b] th a t ha sb e e n m e t a m o r p h o s e d t o l o w e r g r e e n s c h i s t f a c i e s .

2.4. The East Magnitogorsk-Serov-Mauk Fault SystemT h e M a g n i t o g o r s k a r c i s s t r u c t u r a l l y j u x t a p o s e d

a g a i n s t t h e E a s t U r a l i a n z o n e a l o n g a m e l a n g e z o n e t h a th a s b e e n n a m e d t h e E a s t M a g n i t o g o r s k f a u l t [Ayarzae t a l . , 2000a ] . I t ha s t en ta t ive ly b een co r re la ted wi th theSe rov-Mauk fau l t , wh ich s u tu re s the T ag i l a rc to theE a s t U r a l i a n z o n e i n t h e M i d d l e U r a l s . T h e S e r o v - M a u kfaul t is a s t r ike-s l ip faul t zone that can be t raced as ap r o m i n e n t m a g n e t i c a n o m a l y t h r o u g h o u t t h e M i d d l eUr a l s (P la te 1 ) . A lo ng the en t i re l eng th ( > 700 k m) o f theE a s t M a g n i t o g o r s k - S e r o v - M a u k f a u l t s y s t e m t h e r e i s as i gn i f ic a n t j u m p i n m e t a m o r p h i c g r a d e f r o m t h e v o l c a n i ca rc s in the wes t to the E as t Ura l i an zone in the ea s t .

2.5. The East Uralian ZoneT h e E a s t U r a l i a n z o n e is c o m p o s e d o f d e f o r m e d

a n d m e t a m o r p h o s e d P r e c a m b r i a n a n d P a l e o z o i c c o n t i nen ta l - type c rus t and i s l and a rc f ragments [e .g . , Puchkov,1997, 2000; Friberg, 2 0 0 0 ] . I t w a s i n t r u d e d b y n u m e r o u sg r a n i t o i d b a t h o l i t h s a n d s u b o r d i n a t e d i o r i t e a n d g a b b r oi n t r u s i o n s d u r i n g t h e C a r b o n i f e r o u s a n d t h e E a r l yP e r m i a n [Sobolev et al. , 1964 ; Fershtater et al. , 1997;Be a e t a l ., 1 9 9 7, t h i s b o o k ] . T h e r e g i o n a l m e t a m o r p h i cgrade ranges f rom greens ch i s t to g ranu l i t e fac ie s . In then o r t h e r n p a r t o f t h e S o u t h e r n U r a l s a n d i n t h e M i d d l eU r a l s a n u m b e r o f m e t a m o r p h i c c o m p l e x e s h a v e b e e ni d e n t i f i e d . T h e s e i n c l u d e t h e S a l d a , M u r z i n k a - A d u i ,S i s e r t , K r a s n o g v a r d e i s k i i , P e t r o k a m e n s k , a n d A l a p a e v s kcomplexes [e .g . , Antsigin e t a l . , 1994] , wh ich a re jux ta pos ed a long t ec ton ic con tac t s tha t have been ex tens ive lyr e w o r k e d b y a r o u g h l y n o r t h - s o u t h s t r i k i n g s t r i k e - s l i pf a u l t s y s te m . M a n y o f t h e g r a n i t o i d b a t h o l i t h s i n t r u d e ds yn tec ton ica l ly in to th i s fau l t s ys tem.

T he ea s te rn con tac t o f the E as t Ura l i an zone i s on lyk n o w n i n t h e S o u t h e r n U r a l s , w h e r e i t i s a m e l a n g econ ta in ing loca l re l i c s o f ha rzburg i t e . In the a rea c ros s edby the UR S E I S s ec t ion , the me lange i s in t r ud ed by a la t e ,u n d e f o r m e d p h a s e of t h e D z h a b y k g r a n i t e t h a t h a s b e e nda t ed a t 291 + 4 M a [Montero et a l . , 2000 ] . Th e la teo rog en ic , dex t ra l s t rike -s l ip T ro i t s k fau l t l i e s wi th in them e l a n g e .

2.6. The Trans-Uralian ZoneT h e T r a n s - U r a l i a n z o n e i s n o t w e l l k n o w n d u e t o i

p o o r e x p o s u r e ; i t o n l y o u t c r o p s i n t h e S o u t h e r n U r a l sT h e b e s t k n o w n u n i t s a r e D e v o n i a n a n d C a r b o n i f e ro u s c a l c - a l k a l i n e v o l c a n o - p l u t o n i c c o m p l e x e s w h i c h a rc o m p o s e d p r e d o m i n a t e l y o f v o l c a n o c l a s t i c s a n d l a vf lo ws t h a t a r e i n t r u d e d b y c o - m a g m a t i c g a b b r o - d i o r i tand d io r i t e p lu tons [e .g . , Puchkov, 1997 , 2000] . Op h io l iu n i t s a n d h i g h p r e s s u r e r o c k s h a v e a l s o b e e n r e p o r t e[Puchkov, 2 0 0 0 ] . T h e v o l c a n o - p l u t o n i c c o m p l e x e s a ro v e r l a i n b y t e r r i g e n o u s r e d - b e d s a n d e v a p o r i t e s . D e f o rm a t i o n h a s n o t b e e n w e l l s t u d i e d , a l t h o u g h i t a p p e a rt h a t t h e D e v o n i a n a n d L o w e r C a r b o n i f e r o u s u n i t s a raffec ted by open to t ight folds .

3. T H E S E I S M I C P R O F I L E S

T h e a c q u i s t i o n a n d p r o c e s s i n g p a r a m e t e r s f o r t hU R S E I S s e c t i o n c a n b e f o u n d i n Tryggvason et a[2001], f o r t h e E S R U s e c t i o n i n Juhlin et a l . [1998] anFriberg e t a l . [2000] , an d fo r the Al apa ev s ec t ion in Steee t a l. [1995]. A l l s ec t ions hav e been p r e s en ted in ea r l i ep a p e r s , e x c e p t f o r t h e E S R U 9 9 s e g m e n t ( t h e e a s t e r n m o s70 km s ho wn in P la te 3 ) wh ich i s unp ub l i s h ed u p tn o w . I m p o r t a n t t o n o t e f o r th i s p a p e r i s t h a t t hU R S E I S d a t a w e r e a c q u i r e d w i t h t w o s o u r c e t y p e sv i b r o s e i s [Echtler et a l . , 1996] an d explos ives [Steer et al1998]. T he v ib ros e i s s ec t ions p re s en ted in th i s s tudi m a g e t h e u p p e r c r u s t m u c h b e t t e r t h a n t h e e x p l o s i vones (due to the h ighe r fo ld ) , bu t the s igna l d id nop e n e t r a t e t o t h e M o h o a l o n g th e e n t i r e U R S E I S s e c t i owi th the v ib ros e i s s ource . T he l ine d rawings s hown he r(P la te s 1 to 3 ) we re gene ra ted f ro m coh e ren cy f il te res t a c k s w i t h t h e e v e n t s p i c k e d a u t o m a t i c a l l y a n d t h ed e p t h m i g r a t e d . T h e m i g r a t i o n v e l o c i t i e s u s e d a r e b a s eu p o n r e f r a c t i o n / w i d e - a n g l e d a t a p r e s e n t e d i n Carbonee t a l . [1996] fo r the S ou t he r n U ra l s an d Juhlin et a[1996] fo r the Mid d le Ur a l s .3.1. The Foreland Thrust and Fold Belt

I n t h e U R S E I S s e c t io n ( P l a t e 2 ) , f r o m k i l o m e t e r 0 tab ou t k i lome te r 50 , the fo re land th rus t and fo ld be l t c h a r a c t e r i z e d b y a 2 0 k m t h i c k z o n e o f c o n t i n u o u ss ub hor izo n ta l re f l ec t iv ity tha t i s re l a t ed to the R i ph ea n tP e r m i a n s e d i m e n t s . A t h i n b a n d o f w e a k , o p e n l y c o n c a vup wa rd re f l ec t iv i ty a t 20 km de p th i s in te r p re t ed tm a r k t h e t o p o f t h e A r c h e a n c r y s t a l l i n e b a s e m e n t . B e l ot h i s , t h e c r u s t i s t r a n s p a r e n t a n d t h e M o h o i s n o t i m a g e dF r o m n e a r k i l o m e t e r 5 0 t o th e M a i n U r a l i a n f a u lt aab ou t k i lom e te r 146 , the fo re land th rus t and fo ld be l t cha r ac te r i zed by pa tchy , ea s t and wes t d ipp ing re f l ect iv i ti n t h e u p p e r a n d m i d d l e c r u s t . F r o m k i l o m e t e r 1 1 7 t o t h


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BROW N E T AL . 3M a i n U r a l i a n f a u l t t h e u p p e r m o s t km of the s ec t ionimages the acc re t iona ry complex , the bas e o f which i ss een a s a th in band o f re f l ec t iv i ty tha t d ips mode ra te lyeas tward . Benea th and to the wes t o f the acc re t iona rycom plex th i s re f l ec tiv i ty i s re l a t ed to po ly de fo rm ed an dv a r i a b l y m e t a m o r p h s e d P r o t e r o z o i c r o c k s w i t h i n t h eB a s h k i r i a n A n t i c l i n o r i u m . T h e l o w e r c r u s t i s n e a r l yt r a n s p a r e n t a n d t h e M o h o is n o t i m a g e d .

T he E S R U (P la te 3 ) an d the Al apa ev (P la te 4 ) s ec tions do no t ex tend wes tward ac ros s the en t i re fo re landt h r u s t a n d f o l d b e l t i n t h e M i d d l e U r a l s . F r o m k i l o m e t e r0 to the Ma in Ura l i an fau l t a t abou t k i lome te r 40 theE S R U s e c t i o n m o s t l y c r o s s e s t h e p o l y d e f o r m e d a n dv a r i a b l y m e t a m o r p h o s e d P r o t e r o z o i c r o c k s w i t h i n t h eK v a r k u s h A n t i c l i n o r i u m . A t t h e w e s t e r n e n d o f t h es ec t ion , a band o f s ubhor izon ta l re f l ec t ions a t t o 7 k m d e p t h m a y r e p r e s e n t t h e e a s t e r m o s t e d g e o fu n d e f o r m e d R i p h e a n s e d i m e n t s . B e l o w t h i s , t h e E a s tE u rop ean C ra to n is rep re s en ted by d if fus e re f l ec t iv ity .E a s t w a r d , t h e K v a r k u s h A n t i c l i n o r i u m i n b o t h t h eE S R U a n d t h e A l a p a e v s e c t i o n s i s i m a g e d a s a z o n eof diffuse reflec t ivi ty in the upper crus t , with s t rong,mode ra te ly ea s t -d ipp ing re f l ec t iv i ty in the midd le andlower c rus t (E SRU on ly ) tha t appea rs to be c ros s cu t bymodera te ly wes t -d ipp ing re f l ec t iv i ty in the lower c rus t . Inbo th s ec t ions , in the immedia te foo twa l l to the Ma inU r a l i a n fa u l t, a n a r r o w b a n d o f m o d e r a t e l y e a s t - d i p p i n greflec t ivi ty in the upper crus t is re la ted to s t ronglyd e f o r m e d P r o t e r o z o i c a n d P a l e o z o i c r o c k s i n t h e e a s t e r nl i m b o f t h e K v a r k u s h A n t i c l i n o r i u m . N o t e t h a t i np r e v i o u s i n t e r p r e t a t i o n s [Steer et a l . , 1995; Juhlin et al. ,1998;Friberg, 2000] th i s re f l ec tion pack age was in c ludedin the Ma i n Ur a l i a n fau l t ( see Sec t ion 2 .2 . ) . T h e Mo ho i sn o t c l e a r l y i m a g e d b e n e a t h t h e E a s t E u r o p e a n C r a t o n i nt h e E S R U d a t a , b u t b y p r o j e c t i n g t h e w i d e - a n g l e M o h of r o m t h e U W A R S e x p e r i m e n t s o m e 60 k m t o t h e s o u t h[Thouvenot e t a l . , 1995] on to the E S R U s ec t ion i t appea rstha t the c rus ta l th ickn es s reache s 52 km [see a l s o ,Juhlin et al. , 1998].

3.2. Main Uralian FaultW h e r e c r o s s e d b y t h e U R S E I S s e c t i o n , t h e M a i nU r a l i a n f a u l t i s a p p r o x i m a t e l y 4 k m w i d e , c r o p p i n g

ou t f rom a bo u t k i lom e te r 152 to k i lom e te r 156 (F ig ure 1an d P la te 2 ) . T h e Ma i n Ura l i an fau l t i s nonre f l ec t ivein the UR S E I S s ec t ion , bu t in the upp e r 30 km i t i smarked by a change in re f l ec t iv i ty f rom the E as t E urop e a n C r a t o n t o t h e M a g n i t o g o r s k a r c ( F i g u r e 1 ) . B a s e don th i s , we in te rp re t the Ma in Ura l i an fau l t to d ip s t eep lye a s t w a r d b e n e a t h t h e M a g n i t o g o r s k a r c , a n d p e r h a p s t oe x t e n d t o t h e M o h o a t a p p r o x i m a t e l y k i l o m e t e r 2 1 0 .

T he Ma in Ura l i an fau l t ( a s de f ined in Sec t ion 2 .2 . ) ithe E SRU (a t the s u r face a t abou t k i lome te r 40) and thAlapaev (a t the s u r face a t abou t k i lome te r 17) s ec t ions ii m a g e d a s a s h a r p c h a n g e f r o m m o d e r a t e l y e a s t - d i p p i nre f lec t iv i ty a s s oc ia ted wi th de fo rme d E as t E u ro pe aC r a t o n r o c k s t o g e n t l y w e s t - d i p p i n g t o s u b h o r i z o n t areflec t ivi ty in the Tag i l a rc (Fig ure 1 an d Pla t es 3 an d 4On the bas i s o f the s e obs e rva t ions i t c an be t raced in tthe midd le c rus t in bo th s ec t ions .

3.3. Magnitogorsk and Tagil Volcanic ArcsT h e U R S E I S s e c t i o n c r o s s e s t h e M a g n i t o g o r s k a r

f rom th e M a in U ra l i an fau l t zone a t k i lom e te r 156 to thE a s t M a g n i t o g o r s k f a u l t z o n e a t a b o u t k i l o m e t e255 (P la te 2 ) . T he re f l ec t iv i ty in th i s reg ion i s cha rac te ri zed by pa tchy , noncohe ren t to cohe ren t re f l ec t ions ithe up pe r 10 km . Below th is , ref lec t ivi ty is diffuse ot h e a r c c r u s t is t r a n s p a r e n t . T h e M o h o i s n o t i m a g e d .

T h e E S R U ( f r o m a b o u t k i l o m e t e r 4 1 t o k i lo m e t e r 8 1a n d A l a p a e v ( f r o m n e a r k i l o m e t e r 2 0 t o k i l o m e t e r 4 1s ec t ions (P la te s 3 an d 4 ) ima ge the uppe r c rus t o f thT a g i l a r c a s a n o p e n s y n f o r m a l s t r u c t u r e ( E S R U ) , o r agen t ly wes t -d ipp ing re f l ec t iv i ty (Alapaev) . T he midd lc rus t is cha r ac te r i z ed in bo th s ec t ions by pa t chy , gene ra l ly gen t ly ea s t -d ipp ing re f l ec t iv i ty . From k i lome te r 2to k i lome te r 32 the midd le c rus t i s h igh ly re f l ec t ive it h e E S R U s e c t i o n , b e i n g c h a r a c t e r i z e d b y g e n t l y e a s td ipp ing to s ubhor izon ta l re f l ec t ions .

3.4. East Magnitogorsk-Serov-Mauk Fault SystemT h e E a s t M a g n i t o g o r s k f a u l t z o n e i s c r o s s e d b y t h

U R S E I S s e c t i o n a t a b o u t k i l o m e t e r 2 5 5 ( P l a t e 2 ). I t n o t i m a g e d i n t h e u p p e r 6 k m , b u t a s h a r p c o n t r a s t ire f l ec t iv i ty be tween the a rc and the E as t Ura l i an zonf rom 6 to 23 km de p th i s in te rp re ted t o imag e thloca t ion o f the fau l t zone (F igure 2 ) .

T h e S e r o v - M a u k f a u l t i s c r o s s e d b y t h e E S R Us e c t io n a t a b o u t k i l o m e t e r 8 1 , a n d b y t h e A l a p a es ec t ion a t ab ou t k i lom e te r 41 , ne i the r o f wh ich imag e u n a m b i g u o u s l y ( F i g u r e 2 a n d P l a t e s 3 a n d 4 ) . Juhlie t a l . [1998] in te r p re t s t eep ly wes t d ipp ing re f l ec t ions thap r o j e c t u p t o t h e s u r f a c e l o c a t i o n o f t h e S e r o v - M a ufau l t in the E SRU s ec t ion to ind ica te tha t i t d ips s t eep lw e s t w a r d , t r u n c a t i n g t h e T a g i l a r c i n t h e m i d d l e c r u s(F igu re 2 an d P la te 3 ) . He re , we a l s o p ro po s e an a l t e rn a t i v e i n t e r p r e t a t i o n i n w h i c h t h e S e r o v - M a u k f a u l t is ubve r t i ca l to s t eep ly ea s t -d ipp ing , t runca t ing to the wesa pa tch o f h igh ly cohe ren t midd le and lower c rus tare f lec t iv i ty a t ab ou t k i lom e te r 80 in E S R U , anex ten d ing to the bas e o f the c rus t a t an ap pa ren t o f fs ei n t h e M o h o a t a b o u t k i l o m e t e r 8 5 ( F i g u r e 2 ) .


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[ Tagii arc ~ | Trans UraBan Zone West Siberian Basin _ .Possible location / \ offautt at depthMantle { .

40 60 80 100Distance (km)Plate 3. Uninterprete d and interpreted line drawings of the coherency filtered, d epthmigrated ESR U d ata. See Figure 1 for location. The colors represent the different tectonicunits that make up the Uralide crust. The main suture zones that bind these tectonic unitstogether have been interpreted to end at the Moho. Their exact location at depth cannot beunambiguously interpreted, and have therefore been shown as a zone in which they maypossibly occur. The location of the UWARS wide-angle Moho is from Juhlin et al. [1998].

0 20 40 60 80 100 120Distance (km)

0 20 40 60 80 100 120Distance (km)

East European Craton | East Uralian ZoneTagil arc Possible location/ \ of fault at depth

/ \Plate 4. Uninterpreted and interpreted line drawings of the coherency filtered, depth migrated Alapaev data. SeeFigure 1 for location. The colors represent the different tectonic units that m ake up the U ralide crust. The mainsuture zones that bind these tectonic units together have been interpreted to end at the base of the section.


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140 160 180Distance (km)


Alapaevsur face locat ionx i

X ~ \ volcan icEast Euro pean \c r a t o n .

\20 40

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80

Figure 1. Detailed line drawings of the UR SE IS, ES RU and Alapaev sections showing the interpreted subsurfacelocation of the M ain U ralian fault. The shorter d ash indicates a lesser degree of confidence in the interp retation.

3.5. East Uralian ZoneT h e U R S E I S s e c t i o n c r o s s e s t h e E a s t U r a l i a n z o n e

f r o m a b o u t k i l o m e t e r 2 5 5 t o n e a r k i l o m e t e r 3 3 0 , w h e r et h e en t i r e su r f a c e g e o l o g y c o m p r i s e s t h e D z h a b y k g r a n i t e(P la te 2 ) . In th i s a rea , the up pe r 5 to 6 km of the s ec t ioni s t r a n s p a r e n t , a n d t h e c h a n g e t o m o d e r a t e l y c o h e r e n tre f l ec t ions has been in te rp re ted to image the bas e o ft h e g r a n i t e [Brown and Tryggvason, 2001] . Be low theD zh ab yk g ran i t e , to a dep th o f 20 km , the c rus t isc h a r a c t e r i z e d b y e a s t - d i p p i n g p a t c h e s o f o p e n l y u n d u l a t ing re f l ec t iv i ty tha t , i n the ea s t , become s ha l lowly wes t -d i p p i n g . F r o m n e a r k i l o m e t e r 3 00 t o a b o u t k i l o m e t e r3 2 5 , a 10 km th ick , wes t -d ip p ing ba nd o f re f l ec t ionsbe tw een 12 to 35 km d ep t h fo rms the Ka r ta ly re f lec t ion s equence o f Echtler et a l . [1996] . O n the bas is ofs imi la r i t i e s in re f l ec t ion cha rac te r , the Kar ta ly re f l ec t ion s equence has recen t ly been in te rp re ted to rep re s en tT r a n s - U r a l i a n z o n e c r u s t [Tryggvason et a l . , 2001] .Benea th th i s re f l ec t ion s equence , the midd le and lowerc r u s t a r e c h a r a c t e r i z e d b y m o d e r a t e l y w e s t - d i p p i n gr e f l e c t i o n s t h a t m a y e x t e n d w e s t w a r d t o a b o u t k i l o m e t e r 2 6 0 . T h e M o h o i s m a r k e d b y a s h a r p d e c r e a s ein re f l ec t iv i ty . T he c rus t benea th the Kar ta ly re f l ec t ions e q u e n c e r e a c h e s a m a x i m u m t h i c k n e s s o f 5 3 k m .

T h e E a s t U r a l i a n z o n e i n t h e E S R U ( k i l o m e t e r 8 1 t ok i lome te r 230) and the Alapaev (k i lome te r 41 to the end)s ec t ions i s im age d a s a reg ion o f com plex up pe r c rus ta lre f l ec t iv i ty wi th a h igh ly re f l ec t ive and un i fo rm midd leand lower c rus t (P la te s 3 and 4 ) . T h e Mo h o deep ens

s l igh t ly wes tw ard f ro m 42 to 45 km , wi th a po s s ib lo f fs e t nea r k i lom e te r 85 . T h i s s t ep appea rs in the s ampos i t ion ( i . e . , ben ea t h the s u r face expre s s ion o f thS e r o v - M a u k f a u l t) i n U W A R S w i d e - a n g l e d a t a t o t hs o u t h [Thouvenot e t a l . , 1 9 9 5] . F r o m t h e S e r o v - M a ufau l t a t k i lom e te r 81 to the S i se r t fau l t a t ab ou t k i lom e te132, the uppe r 20 km of the E as t Ura l i an zone c rus t it h e E S R U s e c t i o n i s c h a r a c t e r i z e d b y m o d e r a t e l y ts t eep ly wes t -d ipp ing re f l ec t iv i ty . Wi th in th i s reg ion , the ri s m ino r s ha l lowly ea s t -d ip p ing re f l ec t iv i ty in the w es te r n m o s t u p p e r 1 0 k m a n d i n t h e e a s t e r n m o s t 1 0 k mF r o m k i l o m e t e r 4 1 t o a b o u t k i l o m e t e r 6 1 i n t h e A l a p a es ec t ion the E as t U ra l i an zo ne is ima ged a s d i f fus em o d e r a t e l y e a s t - d i p p i n g r e f l e c t i v i t y , a n d f r o m k i l o m e t e61 to the S i s e r t fau l t a t abou t k i lome te r 90 i t s hows t ron g , mo de r a te l y wes t -d ipp ing re f l ec tiv i ty .

T h e S i s e r t fau l t i s c ros s ed by the E S R U s ec t ion aa b o u t k i l o m e t e r 1 3 2 , a n d b y t h e A l a p a e v s e c t i o n a t a b o uk i lom e te r 90 (F igur e 3 an d P la te s 3 an d 4 ) . In the u pp e10 km of bo th s ec t ions the re a re minor changes in the d io f re f l ec t ions ac ros s the S i s e r t fau l t , bu t the s e changecanno t be t raced in to the midd le and lower c rus t [ s ea l s o , Friberg e t a l . , 2000] . In the Alap aev s ec t ion , t ha rea benea th the s u r face loca t ion o f the S i s e r t fau l t in e a r l y t r a n s p a r e n t , w i t h t h e t r a n s p a r e n t z o n e t r u n c aing p rominen t wes t -d ipp ing re f l ec t iv i ty to the wes t anpa t chy re f l ec t iv i ty in the upp e r 10 km to the ea s t .

E as t o f the S i s e r t fau l t , f rom k i lom e te r 132 to ab ouk i lom e te r 230 , the upp e r c rus t is cha r ac te r i zed in th

10


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4 2 C R U S T A L A R C H I T E C T U R E O F T H E U R A L I D E SURSEIS

0Q

10

20 4

30 H

40 H

50 H

60240

voicartic arc I I East Uralian zone

/1

1 possible location atI depth

260Distance (km)

ESRUT

Alapaev

volpanie arc

i^ middle crustal/^ r ^^ i i f ref lectivi ty?

C t -. change in

from Juhlineta l . , 1998 1 ' * / I

: -V

Moho

280 6030

r

80Distance (km)

30 50Distance (km)

100

60

truncatedlower crustal% reflectivity4 t

Moho Moho step\

M o h o60 90 120

Figure 2. Detailed line drawings of the URSEIS, ESRU and Alapaev sections showing the possible subsurfacelocation of the East Magnitogorsk-Serov-Mauk fault zone. The shorter dash indicates a lesser degree ofconfidence in the interpretation . The enlarged E SR U p anel shows the truncated lower crustal reflectivity and theMoho step that provides the basis for the extrapolation of the Serov-Mauk fault to the Moho.

E SRU s ec t ion by pa tchy , gen t ly ea s t and wes t d ipp ingre f lec t iv ity . F ro m ab ou t k i lom e te r 170 ea s tw ard to theend o f the s ec t ion , the uppe rmos t pa r t o f the s ec t ionimages hor izon ta l re f l ec t iv i ty o f the ea s tward- th icken ingWes t S ibe r i an Bas in . T he midd le and lower c rus t i s cha r a c t e r i z ed b y m o d e r a t e l y e a s t - d i p p i n g t o s u b h o r i z o n t a lre f l ec t iv ity . T he Mo ho b en ea th the E as t Ura l i a n zone isima ged a s a 42 to 44 km deep fea tu r e . In the Ala pae vsect ion, from the Sisert faul t to the end of the sec t ion, theu p p e r 2 5 k m o f t h e c r u s t a p p e a r s a s p a t c h y , p r e d o m i na te ly ea s t -d ip p ing re f l ec t iv i ty .3.6. Trans-Uralian Zone

F r o m a b o u t k i l o m e t e r 3 2 8 t o k i l o m e t e r 3 3 4 , t h eU R S E I S s e c t i o n i m a g e s a z o n e o f n e a r l y t r a n s p a r e n tc r u s t t h a t c o r r e s p o n d s t o t h e b o u n d a r y b e t w e e n t h e E a s t

U r a l i a n a n d t h e T r a n s - U r a l i a n z o n e s i n t h e S o u t h e r nUr a l s (F igure 4 and P la te 2 ) . T h e T ro i t s k fau l t appe a rs ak i lome te r 330 (F igure 4 ) . F rom k i lome te r 334 tk i lom e te r 420 , the uppe r ~ 5 km of the c rus t d i s p layeas t an d wes t d ipp ing re f lec t iv ity . Be tween ~ 5 and 25 k m d e p t h , h o w e v e r , t h e T r a n s - U r a l i a n z o n e i s c h a racterized by a series of eas t and wes t dipping intenselre f l ec t ive zones tha t ex tend wes tward benea th the E asUra l i an zone . T he lower c rus t d i s p lays th in bands owes t -d ipp ing re f l ec t iv i ty tha t appea rs to merge wi th thM o h o , w h i c h is i m a g e d a s a s h a r p t r a n s i t i o n f r o m w e a k lre f l ec t ive lower c rus t to t rans pa ren t uppe r man t l e a- 4 9 k m d e p t h .

T h e b o u n d a r y b e t w e e n t h e E a s t U r a l i a n a n d T r a n sUr a l i a n zones in the E S R U s ec t ion i s in te rp re ted to o ccuin the s ubs ur face a t abou t k i lome te r 233 , co inc iden t wi t


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BRO WN E T AL . 4ESRU

10

20 H

f 30Q

40

50

60

truncatedreflectivity

110 1130Distance (km)

Alapaev

'ltruncated| reflectivity |

_

1l \- I

180 i i100Distance (km)

150

Figure 3. Detailed line drawings of the ESR U and A lapaev sections showing the truncations of upper and middlecrustal reflectivity that indicate the possible subsurface location of the Sisert fault. The shorter dash indicates alesser degree of confidence in the interpretation.

a p r o m i n e n t m a g n e t i c l i n e a m e n t t h a t e x t e n d s t o w h e r e i ti s e x p o s e d n e a r t h e U R S E I S s e c t i o n ( P l a t e 1 ) . T h e u p p e rc r u s t o f t h e T r a n s - U r a l i a n z o n e is n e a r l y t r a n s p a r e n t ,w i th on ly loca l , weak , wes t -d ipp ing re f l ec t ions (F igure 4

an d P la te 3 ) . In con t r a s t , t he midd l e and lowe r c rus t c h a r a c t e r i z e d b y s t r o n g m o d e r a t e l y w e s t - d i p p i n g r e f l e ct i v i t y t h a t e x t e n d s w e s t w a r d b e n e a t h t h e E a s t U r a l i az o n e w h e r e i t a p p e a r s t o m e r g e w i t h t h e M o h o . T h

URSEIS

5060

t runca ted Trans -JUralian reflectivity

M o h o


ESRU

/ _/

360 220

West Siberian BasinEast Ural ian/Trans-Ural ian boun dary

M o h o

240 260Distance (km)

280

Figure 4. Detailed line drawings of the URSEIS and ESRU sections showing the boundary between the EastUralian and Trans-Uralian zones. The location of the boundary in the ESRU profile is determined from themagnetic da ta in Plate 1. The Tro itsk fault is crossed by the U RS EIS section at kilometer 330.


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4 4 C R U S T A L A R C H I T E C T U R E O F T H E U R A L I D E SM o h o is i m a g e d a s a n o p e n l y c o n c a v e u p w a r d s t r u c t u r ea t 46 km dep th .

4. D I S C U S S I O NT h e ove ra l l r e f l ec t ion s e i s mic pa t t e r n o f the U ra l i de

c r u s t a s i m a g e d b y t h e U R S E I S a n d E S R U d a t a isb ive rg en t (P la te s 2 an d 3 ) , pe r ha ps rep re s en t ing th eor ig ina l co l l i s ion - re la ted c rus ta l a rch i t ec tu re [ s ee a l s o ,Echtler e t a l , 1996 ;Knapp et al. , 1996;Steer et al. , 1998;Tryggvason e t a l . , 2001] . Ho we ve r , the Ur a l id e o ro genr e c o r d s a l o n g a n d c o m p l e x s u b d u c t i o n / a c c r e t i o n h i s t o r yprior to the f ina l col l is ion that gave i t i ts bivergenta rch i t ec tu re [e .g . ,Zonenshain et al. , 1984, 1990; Puchkov,1997; Brown and Spadea, 1999; Friberg, 2000] . In add i t ion , i t r e cords a complex l a t e -o rogen ic h i s to ry tha ti n v o l v e d e x t en s i v e w r e n c h f a u l t in g a c c o m p a n i e d b y w i d e s p r e a d m e l t g e n e r a t i o n a n d g r a n i t o i d e m p l a c e m e n t i nthe in te r io r o f the o rogen [Ayarza et a l . , 200 0a; Friberg,2000 ; Echtler et a l . , 1997; Fershtater et al. , 1997 ; Beae t a l . , t h i s book] . He re , we s ugges t tha t th i s l a t e o rogen ica c t iv i t y i s e x p r e s s e d p r e d o m i n a n t l y i n t h e E a s t U r a l i a nzone , whe re i t s ign i f i can t ly ove rp r in ted and /o r reworkedm u c h o f t h e s u b d u c t i o n - a n d a c c r e t i o n - r e l a t e d t e c t o n i cfabr ic , g iv ing th i s zone i t s va r i ed and complex re f l ec t ions e i s mic cha rac te r ( s ee be low) . Be low, we a rgue tha t them a j o r t e c t o n i c b o u n d a r i e s a s s o c i a t e d w i t h t h e w r e n c hfau l t s ys tem can be co r re la ted in the s e i s mic da ta f roms o u t h t o n o r t h . D e s p i t e p r o b l e m s in i m a g i n g t h e s e b o u n d aries in the se ismic data , we interpre t them to affec tt h e e n t i r e c r u s t , a n d p e r h a p s t h e M o h o .

T h e U R S E I S a n d E S R U s e c t i o n s i n d i c a t e t h a t c r u s t a lt h i c k n e s s a n d M o h o t o p o g r a p h y c h a n g e s o m e w h a t b e tween the Sou the rn and Midd le Ura l s (P la te s 2 and 3 ) ,a l though the c rus ta l roo t can be s een to ex tend a longthe wes te rn vo lcan ic ax i s o f the o rogen . Crus ta l th icknes sc a n o n l y b e e s t i m a t e d i n t h e e a s t e r n p a r t o f t h e U R S E I Sv ib ros e i s s ec t ion , wh e re i t r e aches 43 km a nd th icken swes twa rd (P la te 2 ) . Bas ed o n the exp los ion s ource re fl ect i o n d a t a [Steer et a l . , 1998] an d the wid e-an gle reflec t iond a t a [Carbonell et a l . , 1998] the Ur al i de crus t is sho wnto be 41 km th ick in the wes te rn e nd o f the s ec t ion ,th icke n ing to 53 km in the cen t ra l pa r t , be nea th thew e s t e r n m o s t M a g n i t o g o r s k a r c ( P l a t e 2 ) . T h e E S R Us ec t ion (P la te 3 ) imag es a c rus ta l th ickn es s o f 46 kma s s o c i a t e d w i t h t h e T r a n s - U r a l i a n z o n e , t h i n n i n g t o 4 2 k m b e n e a t h t h e E a s t U r a l i a n z o n e . T h e M o h o i s n o tima ged ben ea th the T ag i l a rc , bu t by p ro jec t in g th ere s u l t s o f the UW A R S s ec t ion on to i t t he c rus t appe a rs toreach a th icknes s o f 52 km [Juhlin et al. , 1998].

A n i m p o r t a n t d if f er e n ce b e t w e e n t h e U R S E I S a n dthe E SRU s ec t ions i s the midd le and lower c rus ta l

reflec t ivi ty . The origin of this ref lec t ivi ty is enigmaticand may be the re s u l t o f the in te rac t ion o f a number oprocesses [e .g . , Juhlin e t a l . , 1998] . I t is no t ew or th y thath i s re f l ec t iv i ty (wi th the excep t ion o f the ea s t -d ipp inre f l ec t iv i ty a s s oc ia ted wi th the E as t E uropean Cra tonoccurs benea th a rea s a f fec ted by the l a t e o rogen ic wrencfau l t s ys tem ( i . e . , t he E as t Ura l i an zone and , to a l e s s eex ten t , the T ag i l a rc ) . T he ma jo r s t ruc tu re s in th i s reg iond o n o t a p p e a r t o p e n e t r a t e b e y o n d t h e t o p o f t h e m i d d lc rus ta l re f l ec t iv i ty . T h i s s ugges t s tha t the midd le anlower c rus ta l re f l ec t iv i ty in the Midd le Ura l s was fo rmeve ry l a t e in the h i s to ry o f the o rogen , ove rp r in t ing evethe wrenc h fau l t s ys tem. T he re is no conc lus ive ev idenctha t the midd le and lower c rus ta l re f l ec t iv i ty i s re l a t ed tl a t e - o r pos t -o rogen ic ex tens ion , a s s ugges ted by Knapet al. [1998]. Friberg et a l . [2000] ident i fy eas t dip pinre f l ec t ions in the E SRU s ec t ion tha t they in te rp re t to br e l a t e d t o e x t e n s i o n . H o w e v e r , d u e t o p o o r e x p o s u r ef ie ld ve r i f i ca tion o f wides p read ex tens io n i s no t av a i l ab leAls o , the wes t -d ipp ing re f l ec t iv i ty o f the T rans -Ura l i az o n e i n t h e E S R U 9 9 d a t a ( P l a t e 3 ) a p p e a r s t o b e u ni n t e r u p t e d b e n e a t h t h e W e s t S i b e r i a n B a s i n , w h e r e t hlocus o f ex tens ion fo r the Wes t S ibe r i an Bas in s hou ld bloca ted , s ugges t ing tha t in th i s key a rea the c rus tas t ruc tu re has no t been a f fec ted by any ma jo r l a t e - opos t -o rogen ic ex tens ion , o r by any o the r p roces s thacou ld ha ve l ed to the midd le and lower c rus ta l re f lec t iv i timaged to the wes t . Recen t ly , Koyiet al . [1999] sugg es tetha t the lower c rus ta l re f l ec t iv i ty in the E SRU s ec t iocou ld be the re s u l t o f roo t rebound and duc t i l e f low inthe lower c rus t . T he a s s oc ia t ion o f the lower c rus tare f l ec t iv i ty and the re l a t ive ly th inne r c rus t a s s oc ia tewi th the E as t Ura l i an zone l ead us to t en ta t ive ly in te rp rethese fea tures to be the resul t of lower crus ta l f low aris inf rom the deve lopment o f the wrench fau l t s ys tem. T habs ence o f lower c rus ta l re f l ec t iv i ty in the URSE ISs ec t ion ( s ee a l s o the exp los ion-s ource da ta [Steer et al1998] ) may be re la t ed to me l t gene ra t ion and emplacem e n t o f t h e D z h a b y k b a t h o l i t h .

T he fo re land th rus t and fo ld be l t a s imaged in thU R S E I S a n d , t o a l e s s e r e x t e n t , t h e E S R U a n d A l a p a es ec t ions i s tha t o f a wes t -v e rgen t , ba s em ent - inv o lveth ru s t s t ack tha t i s f l anked a lon g i ts h in te r l and by bas ement cu lmina t ion . T he re f l ec t ion s e i s mic fab r i c ithe s e bas ement cu lmina t ions rep re s en t the e f fec t s oe x t e ns i v e p r e - U r a l i d e d e f o r m a t i o n a n d m e t a m o r p h i s m .

Along the ea s te rn marg in o f the fo re land th rus t andfo ld be l t , a l l t h ree s e i s mic s ec t ions image ea s t -d ipp inuppe r and midd le c rus ta l re f l ec t iv i ty tha t ex tends benea tthe acc re ted vo lcan ic a rc complexes . T h i s re f l ec t iv i ty ii n t e r p r e t e d t o b e r e l a t e d t o t h e E a s t E u r o p e a n C r a t o nBas ed on i t s t runca t ion , i t i s pos s ib le to t race the Ma in


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BRO WN E T AL . 4Ura l i an fau l t to d ip mode ra te ly ea s tward to a t l e a s t3 0 - 3 5 k m d e p t h . I n t h e S o u t h e r n U r a l s , t h e h i g h p r e s s u r eM a k s u t o v o C o m p l e x ( i n t h e i m m e d i a t e f o o t w a l l o f t h eM a i n U r a l i a n f a u lt , b u t t o t h e s o u t h o f U R S E I S ) a p p e a r st o r e p r e s e n t E a s t E u r o p e a n C r a t o n r o c k s t h a t w e r es u b d u c t e d t o a d e p t h o f 5 0 - 7 0 k m [Schulte and Blumel,1999; Hetzel, 1999; Brown e t a l , 2000] , s ugges t ing t ha tt h e E a s t E u r o p e a n C r a t o n w a s s u b d u c t e d t o a t l e a s tan uppe r man t l e l eve l . S ince the Ma in Ura l i an fau l t r ep r e s e n t s t h e p a l e o s u b d u c t i o n z o n e a l o n g w h i c h t h e E a s tE u r o p e a n C r a t o n a n d t h e v o l c a n i c a r c s ( M a g n i t o g o r s kand T ag i l ) co l l ided , we in te rp re t i t t o ex tend in to thelower c rus t and even to the M o ho a s ind ica ted on P la te s 2a n d 3 . Juhlin et a l . [1998] and Friberg [2000] ha ve sugges ted , howeve r , tha t in the Midd le Ura l s the Ma inUra l i an fau l t does no t ex tend beyond the top o f themidd le c rus ta l re f l ec t iv i ty imaged in the E SRU s ec t ion(a l t e rna t ive I I in P la te 3 ) .T he re f l ec t ion cha rac te r i s t i c s o f the vo lcan ic a rcc o m p l e x e s s h o w s o m e v a r i a t i o n b e t w e e n t h e U R S E I S( M a g n i t o g o r s k ) a n d th e E S R U a n d A l a p a e v ( T a g i l )s ec t ions . T he uppe r c rus ta l re f l ec t iv i ty i s ove ra l l s imi la r ,i m a g i n g m a p p a b l e s t r u c t u r e s i n t h e v o l c a n i c s a n d v o l c a n i -c la s t i c s ed imen ts , whe reas the T ag i l a rc midd le and lowerc rus t i s re l a t ive ly re f l ec t ive compared to tha t o f theM a g n i t o g o r s k a r c .

T h e E a s t M a g n i t o g o r s k f a u l t i n t h e S o u t h e r n a n dt h e S e r o v - M a u k f a u l t i n t h e M i d d l e U r a l s f o r m a m a j o rt ec ton ic bounda ry ac ros s which the re i s a s ign i f i can ti n c r e a s e i n m e t a m o r p h i c g r a d e a n d c h a n g e i n a m o u n tand s ty le o f de fo rma t ion . T h i s fau l t s ys tem can bet raced fo r ab ou t 700 km a lon g s t r ike (P la te 1 ) , an dwhere i t ou tc rops the de fo rma t ion a s s oc ia ted wi th i t i sp redomina te ly s t r ike -s l ip , o r t rans pre s s ive [Echtler et al. ,1997; Friberg, 2 0 0 0 ; Hetzel and Glodny, 2002] . Ne i the rt h e E a s t M a g n i t o g o r s k n o r t h e S e r o v - M a u k f a u l t i sd i rec t ly imaged on the re f l ec t ion s e i s mic da ta , howeve r ,mak ing i t d i f f i cu l t to f ix the i r loca t ion and geome t ry int h e s u b s u r f a c e . I n t h e U R S E I S s e c t i o n , t h e w e s t w a r dt runca t ion o f the E as t Ura l i an zone re f l ec t iv i ty a t abou tk i lome te r 260 (F igure 2 and P la te 2 ) s ugges t s tha t theE a s t M agn i tog ors k fau l t c an be t raced f rom the s u r faceto approx ima te ly 23 km dep th a s a (nea r ) ve r t i c a l s t ruc tu re . I t i s no t c l ea r in e i the r the E S R U o r Alapa ev s ec t i o n s w h a t h a p p e n s t o t h e S e r o v - M a u k f a u l t z o n e a tdep th . Bas ed on re f l ec t ion s e i s mic and boreho le da ta ,Sokolov [1988] s ugges ted tha t the Se rov -M auk fau l t ha sa va r i ab le ea s t -wes t d ip d i rec t ion , wh ich s ugges t s tha tove r i t s l eng th i t may be a s t eep fea tu re . T he E SRU s ec t ion d i s p lays a d i s t inc t chang e in Mo h o re f l ec t iv ity an da s ligh t M oh o o f fs e t a t abo u t k i lom e te r 85 . Thouvenote t a l . [1995] a l s o image a M o ho o f fs et in the U W A R S

d a t a w h i c h t h e y a s s o c i a t e d w i t h t h e S e r o v - M a u k f a u l tDoll et a l . [1996] an d Stern and McBride [1998] ha vs hown tha t s t eep ly d ipp ing to ve r t i c a l s t r ike -s l ip fau l ta f fec t the en t i re c rus t in a number o f Pa leozo ic any o u n g e r o r o g e n s . T h e y a l s o s h o w t h a t a M o h o o ff sei s cha r ac te r i s t i c o f the s e fau l t s . O n the bas i s o f the sa r g u m e n t s w e s u g g e s t t h a t t h e S e r o v - M a u k f a u l t ( a na l s o the E as t Magn i togors k fau l t ) may a f fec t the en t i rc rus t an d M oh o (P la te 3 ) . T h i s i s in con t ra s t to Juhlet a l . [1998] and Friberg [2000] wh o in te r p re t the Se rovM a u k f a u l t to b e i m a g e d i n t h e u p p e r c r u s t o f E S R U aa s e t o f wes t -d ipp ing re f l ec to rs ( s ee op t ion 2 in P la te 3In the i r in te rp re ta t ion , th i s s e t o f re f l ec to rs i s a s s oc ia ted wi th a fau l t tha t ou tc rops loca l ly a long the ea s te rn marg in o f the Se rov-Mauk s e rpen t in i t e zone [e .gFriberg, 2000] , wh e reas he re we in te r p re t the s e rpe n tn i t e zone i t s e l f to rep re s en t the de fo rma t ion zone a lont h e b o u n d a r y b e t w e e n t h e T a g i l a r c a n d t h e E a s t U r a l i azon e (op t i on 1 o f P la te 3 ) .

T he S i s e r t fau l t i s a ma jo r in te rna l s t ruc tu re in thE as t Ura l i an zone . I t appea rs to s p lay o f f the E asM a g n i t o g o r s k / S e r o v - M a u k f a u l t s y s t e m a n d c a n bt r a c e d n o r t h w a r d f o r m o r e t h a n 1 50 k m b e f o r e d i s a ppea r ing be nea th the Wes t S ib e r i an Bas in (P la te 1 ) . I t n o t i m a g e d d i r e c t l y i n e i t h e r t h e E S R U o r A l a p a es e c t i o n s , a l t h o u g h i n t h e E S R U s e c t i o n t h e j u x t a p o s i t i oo f re f l ec to rs wi th con t ra s t ing d ips , and pa tchy midd lc rus ta l re f l ec t iv i ty may ind ica te i t s p re s ence . In thAlapaev s ec t ion i t s s u r face expre s s ion co inc ides wi th s eve ra l k i lome te r wide zone o f t rans pa ren t c rus t a c roswhich two zones o f con t ra s t ing re f l ec t iv i ty a re jux tapo s ed (F igure 3 ) . T h i s can be t raced to the bas e o f thA l a p a e v s e c t i o n a t a b o u t 2 3 k m d e p t h , s u g g e s t i n g t h athe S i s e r t fau l t i s a nea r ve r t i c a l s t ruc tu re . Bas ed on tho b s e r v a t i o n s o u t l i n e d a b o v e , w e t e n t a t i v e l y s u g g e s t t h athe S i s e r t fau l t ma y be a nea r ve r t i c a l fea tu re t ha t e x tendin to midd le and pos s ib ly the lower c rus t (P la te 3 ) .

I n t h e S o u t h e r n U r a l s , t h e b o u n d a r y b e t w e e n t hE a s t U r a l i a n a n d t h e T r a n s - U r a l i a n z o n e i s a w i dm e l a n g e z o n e w h o s e m a g n e t i c s i g n a t u r e c a n b e t r a c ef o r n e a r l y 5 00 k m n o r t h w a r d b e n e a t h t h e W e s t S i b e r i aB a s i n ( P l a te 1 ). I n t h e U R S E I S a n d E S R U s e c t i o nt h i s b o u n d a r y i s i m a g e d a s a m o d e r a t e l y t o s t e ep lwes t -d ipp ing change in re f l ec t iv i ty f rom gene ra l ly ea sd ipp ing re f l ec t iv i ty o f the E as t Ura l i an zone to wes td ipp ing re f l ec t iv i ty cha rac te r i s t i c o f the T rans -Ura l i az o n e . I n t h e U R S E I S s e c t i o n , t h e t r a n s p a r e n t c r u sa s s oc ia ted wi th the a rea a f fec ted by the T ro i t s k fau ls ugges t s tha t i t i s a s t eep ly d ipp ing to ve r t i c a l faut h a t c u t s a c r o s s t h e T r a n s - U r a l i a n z o n e , t r u n c a t i nreflec t ivi ty to a t leas t middle crus ta l levels . I t i s noc lea r i f i t ex tends in to the lower c rus t , a l though th i


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4 6 C R U S T A L A R C H I T E C T U R E O F T H E U R A L I D E Spos s ib i l i ty ha s been s hown in P la te 2 . T he T ro i t s k fau l tcanno t be iden t i f i ed in the E SRU s ec t ion . In bo th theU R S E I S a n d E S R U s e c t i o n s , t h e w e s t - d i p p i n g r e f le c t i v i t y i n t h e T r a n s - U r a l i a n z o n e a p p e a r s t o e x t e n d t othe M oh o , s ugges t ing tha t c rus ta l - s ca le , e a s t -ve rge n timbr ica t ion o f the en t i re c rus t occur red . Re f lec t ions int h e T r a n s - U r a l i a n z o n e c a n b e i n t e r p r e t e d t o b e r e l a t e dt o s u b d u c t i o n - a c c r e t i o n p r o c e s s e s d u r i n g a c c r e t i o n a n di m b r i c a t i o n o f v o l c a n i c a r c t e r r a n e s , o c e a n i c a n d c o n t inen ta l c rus t to , and a long , the wes te rn marg in o f theK a z a k h s t a n p l a t e [ e . g . , Tryggvason et a l . , 2001 ] .

5. C O N C L U S I O N ST h e ove ra l l r e f l ec t ion s e i s mic pa t t e r n o f the U ra l id e

c r u s t , a s i m a g e d b y t h e U R S E I S , E S R U a n d A l a p a e vda ta , i s b ive rg en t . T h i s re f l ect ion pa t t e rn c an be in te r p re ted to be the re s u l t o f a p re s e rved , co l l i s ion - re la tedc rus ta l a rch i t ec tu re . In the in te r io r pa r t o f the o rogen ,h o w e v e r , l a t e - o r o g e n i c w r e n c h f a u l t i n g a c c o m p a n i e d b yw i d e s p r e a d m e l t g e n e r a t i o n a n d g r a n i t o i d e m p l a c e m e n th a s s i g n i f i c a n t l y o v e r p r i n t e d a n d / o r r e w o r k e d m u c h o ft h e s u b d u c t i o n / a c c r e t i o n - r e l a t e d t e c t o n i c f a b r i c , g iv i n gth i s zone i t s va r i ed and complex re f l ec t ion s e i s mic cha r a c t e r . T h e m a j o r t e c t o n i c b o u n d a r i e s a s s o c i a t e d w i t h t h ewrench fau l t s ys tem can be co r re la ted f rom s ou th tonor th be tween the s e i s mic s ec t ions , and a re in te rp re ted tohave deve loped on a c rus ta l s ca le . Pe rhaps the mos timpor tan t d i f fe rence be tween Ura l ide c rus t imaged byt h e U R S E I S a n d t h e E S R U s e c t i o n s i s t h e m i d d l e a n dlower c rus ta l re f l ec t iv i ty imaged in E SRU. T he o r ig in o fthis ref lec t ivi ty is enigmatic , but i ts a lmost exclus ived e v e l o p m e n t w i t h i n t h e w r e n c h f a u lt s y s t e m r e p r e s e n t e dla rge ly by the E as t Ura l i an zone s ugges t s tha t i t may bethe re s u l t o f a p roces s , o r p roces s e s re l a t ed to the evo lu t ion o f th i s wr ench fau l t s ys tem. Fo r the f i rs t t ime , theb o u n d a r y b e t w e e n th e E a s t U r a l i a n a n d T r a n s - U r a l i a nzones can be conf iden t ly co r re la ted wi th a s t rong magn e t i c a n o m a l y t h a t is t r a c e d f o r 5 00 k m n o r t h w a r df r o m t h e U R S E I S t o t h e E S R U s e c t i o n . T h e r e a r e s o m ed i ff e re n ce s i n c r u s t a l t h i c k n e s s a n d M o h o t o p o g r a p h yb e t w e e n t h e S o u t h e r n a n d t h e M i d d l e U r a l s , b u t i ngene ra l the s e a re minor . T he c rus ta l roo t can be s een toex tend a long the wes te rn vo lcan ic ax i s o f the o rogenf r o m a t l e a s t t h e l a t i t u d e o f U R S E I S t o t h a t o f E S R U .

Acknowledgments. This work has been funded by the TM RNetwork URO (ERBFMRXCT-960009) of the EuropeanUn ion. C . Juhlin was funded, in part, by the Swedish R esearchCouncil. A. Green and J. Hall are thanked for their commentson the manuscript. This is a EUROPROBE Uralides Projectpublication.

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arquitectura corteza urales del sur

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