13

Mem. Sci. Geol. v.54 PP.251-260 P^dov^ 2002 ISSN 0191-8602

The rWestern AIps - Eastern Alps transition: tectonics and deep structure

Stefan M. ScHNnD* Befnhafd FtcENscHUH* and Regina LrpprrscH**

*Departnent ofEafth Scie.ces, University ofBasel, Bernoullistr.12, CH.1056 BaEL, Swirze.land**Instilure ofGeophysics, ETH H6nsserbers, CH 8091 ZUNCH, SwiDerland

Key aotds: Alpixe tectonics, collkional orcsen,

I MAJOR TECTONIC UNITS OF THE ALPS

' Fora large-scale overviewthemajor tectonicunirs of theAlps may conveniendy be grouped according to their palmgeographic provenance (Fig. l). As is apparent from fig-ure 1 at a first glance, the eastern Alps arelarge'y",,oc upoftectonic units derived from deApulian plate, the Austroalpine nappes (= " Apulian plare N of PeriadriaticLine"), while thewesternAlps are exclusively made up bymoreexternal, and tectonicallylower units (Europcu urarein, Bia"Conflais teftane, Margna Sesia fragment and inrereningoceanic units, Froitzheim e/rl, 1996). Nore!ha!rhc wes"rn boundary ol rhe Au.rro; lpine nappc,. coinciding with the boundary between western and easternAlps, runs N-S and perpendicular to strikc across easternSwitzerland. This erosional boundary is not far from rhewestern frontal ramp of the Austroalpine nappes, whicbwere internally stacked and sulured to rhe Pienont Ligu riaocean by top WN\X/ shearing durirg a fir* (Cretaceous)ofogenic cycLe \rhich only affected the eastern Alps.

Anorher major change across the boundary benveeneastern and western Alps conccrns the presence or absence o( a Btiaagonrais tezaze. The easternmosr remnantsoi th is rr'azc arc found in rhe Engadine windorv (Fig. 1 ).We inrerprer rhe core of lhe Tauern window ro be madeup bytectooic units anriburedrothe European nargin. Asa consequence, the two occanic domains (Piemoni-Liguriaand Valaisan ) s patially coin cide in the Easrern A1ps. How-ever, rvc still trear Picmonr Liguria and Valais unirs separately from each other for the tollowing reason: the rem.nants of thc Picmont Liguria occan in lheeasrern Alps (egMatrei zone) \ 'ere sutured to the Austroalpine nappesclu.ing C rctaceous or ogeny, s'hile rhe Valais occan (.g rheRbeno.Danubirn f lysch and rhe outcr r in oi rhe Trucrnvindow) did nor closc bclirc thc Late Eocene and rr rheeod ofa second orogenic cycle (Schmid 1/ d1, 1996). ThsPaleogcne cycLc shapccl borh \,estern ind ersre.n Alps.Sracking ol paleogeographic ancltectonic unus (as rop Nto NNV, thc Eurrpcrn phtc ras subJucrecl to rhc sourhand bclos,rhe Apul i rn plate

As far as rhc youngcsr (posr col l is ionalt stages oiAlpineo.oseny a.e concerned, yet rnother rcross srrike bound.

deep structure, Alps, naxtle tonogtaphy.

aryisfound near the transirion betveen western and easrern Alps. The sinisrrally transpressive Giudicarie belrforms the western margin of an indenter situated in theeastern paft of the Southern Alps during the Miocene.This led to massive N S shorteniog in theTauern window.This shorte.ing is conremporaneous vith (1) subsranrialorogen-parallel extension across the Brenner nornal faulr(Fngenschuh eral, 1997), and, (2 ) lateral extrusion of theEastern Alps east of this normal fauft, associated r.ithstrike slip movements along the Periadriatic (dextral) andInnt,l GinGrral) lines (Rarschbacheretal., 1991). Hence,it is obvious that very substantial out ofplane movcmenrsare cxpected across rhe TRANSALP rfansect, situared im-mediately east ofGiudicade belt and Brenner line (Fis.l).

2 TRANSECTS IN THE \SESTERN ALPS

Figures 2a to 2c depicr geological-geophysical ran.sects across rhe \X/eslern Alps s. srr. (ECORS,CROP andNfP-20 \( /E\T), anJ a rrance, r near rhe rran.rrron ,ntothe eastern Alps (NFP,20 EAST) (see drscussron rnSchnid and Kissling,2000). These profiles illustrare rhefollowing major changes which occur along srrike. I rwhen going from thc \0esrern Alps r .r1l . lFiss2a rnd 2b)rowards whar may be refened to as "CenrralAlps" (Fis.2c), ( l ) = Dupl icar ion of European lower crust in rhe\X/eslern Alps,J. wedging oiApulian lower crust inro theEuropean crust; (2) = Apul ian Moho r is ing roqafds rheAlps (lvre,r body) vs. desccnding Apulian Moho ar theb:,c oi-h, lorcr cr rrsral s edee: .

' | , incrci . ,np atrounr

of back-thrurst ing in rhe vic i . i ry of rhe Insubric l iner lal= incrcasing amouna of Miocene shonening within thc

] CHANGE tN SUBDUCTION POLARITYBET\VEEN WES1ERN AND EASTERN ALPS

Rcccnr rc.rr lL, rr-m h.gh r" :olur ion rr lc scr.r r . r , ,mogr.rph), , tocussinc on rhc l i rhosphe.e and uppcr nr.nt le P.rvrvc velociry srLncturc benearh rhc enrire Alps. reverlr changc in subducrion pola rity beru,een Wcstcrn rnclEasrcfn Alps.- fhe Lur. ,pean l i thospheric slab dcscendstowards sourheast unde.ncath rhe Apul ian luhosphc.(

258

(F'e. la) , stcepenirg eastwards and towards the Tauern!v;ndow. East of a point situated undernearh thewesrernpaft ofrhe Ta'rern window, theApulian lithospheric slabis seen to descend underneath rhe Eufopean lithosphereby some 170km (Fig. 3a). This is surprising at first sight,since thefc is no indication for an along strike change inrhe vacking order of rhe major paleoseogr. ,pl" ic unirs inthe Alps (Fig. l). Howcver, two najor orogen-perpen'dicular post'collisional featuf es coincide with rhis changein subduction polarity: Giudicarie belt and the Brennernormalfault. Thissuggests that the chansein subducdongeone,r) was not rnduced belore some 20 Ma ago. , /.rvhen these across+trike features srarted to form. Assuming that subduction polarity was initially identical tothat observed in rhe western Alps, and that polarity indeed changed some 20 Ma ago in the Eastern Alps, thiswould result in a plate conversence rate of 0.85 cm peryear for the last 20 Ma.Inorder to test this working hy,pqrhesis, wc artempted ro rc-interpret the TMNSALPggophysical geological transect (TRANSALP \(/ORKING GROUP,2002)in the light ofthese findings on the, lF"^ . , - , " , , , . .

^ i r l " Al^.

4 TMNSALP TMNSECT AND ALPS DINANDES.CARPATHIANS TRANSITION

The interpretation of.the TMNSALP transect de-picled in ficure 2d closely follows that proposed byTRANSALP W. G. (2002) regarding the .ear,surfacestructures, but differs in terms ofthe interpreration ofthedeep srructure (compare their "nodels A" and "B" intheir figure )). \{/hilc we asree with their 'model A" regarding the conrinuation of S dipping "TaLrern window

reflectors" underneath the Southecn Alps, we do not in-lerprer the Apulian lower crust to wedge inro Europeancrusl (as observed in rhe NFP-20 East profile, see ftgure2c).lnstead we let the Apulian Moho descend northvardsunder the European lithosphere, as is suggested by thelithospheric configuration revealed from tomography anddepicted in figure lb.

This fe-inrcrpreration, although provisional and ofqualitative nature, leads to an easily rerro-deformabletmnsect, ifone assumes a change in subduction polarityand a "wedgc into split apart strucrure " between Apulianlower crust and the frontal thrust of the Miocene ro re-cent thrusr beft of the eastern part ofthe Southern Alps.

This re-interpretation also explains rhe lack ofa clcarseparation belween easlernmost Southern Alps and ex-rernal Dina r ides at the eI rh s sur lace. su.h a.epamrionwould be expected if Alps and Dinarides would still exhibit opposite subduction polarity, as they did during theEocene. Since no separation is visible betv,een SouthernAlps and Dinarides Gee Fis. 1), both are expected topresendy occupy the same, r:e. upper plate, posilion.Instead, a major change occurs across Giudicarie belt andBrenner line. We interpret both these first-ordef tectonic fearures as the surhce expression of a change in sub,duction polarity which initiated at around 20 Ma andwhich had a profound influence on thc style ofposccol-lisionaldeformation. Note that the postulared change insubduction polarity was also accornpanied by very majordexrral strike slip movement of the Sourhern Alps andnorlhefn Dinarides in fespect to Alps and \(resrernCarpathians along the Periadriatic lineament and 'Mid-Hungarian linc" (Fis. 1).

Fig. l : PALEOGEOGRAPHIC l lNlTS lN IHE ALPS

traces of prolites reler lo Figs 2 .nd 3

--;'\..-. . .^ . .

. /- /za:

i **l

Fis. I Map o[ihc Alps and t..ces of prol]les depic'e.l in lrigs 2 and r.

259

NFP-20 WESr b)

NFP-'OEAST C)

. .---{--F oau-,

FlG. 2: geophysical-geological transeclsthrough the AlpsLegend:

E Md**b"""

E h--"*-

El *",",.",rEIHt?;a

I

EEEEM:

f,1

:t

mmrutrl

.- ' , - ; . f f i ' : ' . r . . '

F:s , ) L,uphr. , .d lgeol^s. ! . , . rn.e. . 1ro, ,et , t - - , , t , .

260

N-hlilrd. Id.gr66l

Fig. 3a)

Fis. I ' Secliods rhroush rhe ionogBpbic oodel vith t.msecrs Figs 2c and d superinposed.

Ttre poladty of the suture between the Rhenodanu-bien flysch ("Valaisan " ) and the northern rim of the Aus-troalpine nappes ('Apulian plate" in figure lr ooes notchange along strike from wesr to east. This indicates thatthe nonhern rim of the Apulia upper plare remains unaf-fected by the Miocene change in subduction polarity,which only concerns the southern pan of th€ TRANSALPiransect (Fis.2d). Iosread, the northern rim ofthe Alpscan be followed eastwards into the Carpathian loop.There, subducrion roll-back and slab break off initiatedat about 20 Ma ago. Possibly, this roll-back atlowed forthe change in subdr,rction polarity, postulared to haveoc-curred along the TMNSALP profile.

REFERENCES

FRorzsErM N., SCHMTDS.M. ^ndFre\M.,

1996 - Me'ozoicpale-aeeoet"phr aad the tinin. of ecloeite.fa.ie' n.ta doehnw ,a

ksued, February 2003

the Alpt: Auiki"sbypothett. Eclogae geol. Helv., v.89, pp.8l-110.

FocENscHUH B., SEv^rD D. and M^NcKErow N., 1997 . E -tuftatio, h a .onoe/gett orcgen: tbe aeee Taten aiadou.Terra Nova, v.9, pp- 2li.217.

R-arscH!^cHER L., MERLE O., D^vy PH. and CoBsoLD p., t99t .Laklal exttusioh th tbe ealtas Alpt, Pai 1:Borhaartcoaattiant ard expeinerts raledfot Aturit!. "lectonics, !. lO, pp_245.256.

SCHMID S.M., PFlrfNER O,A., FRo.rzHE[{ N., ScHoNBoRN G, andKtssuNcB., r99r . Geopbyical geobgiul traxsect aaa rcctontc@olrtion oftheSoh tal;an, lpr Tectonics,v.lt, pp. tol61064.

ScHitrD S.M and K$sLlNc E.,2000. The aroltheuleneth Atps;a the lisht ol seoplttrcal drta os deep mstal t!tucture.-lec.ronics, v. 19, pp.62 85.

TMNsrp \I/oKrNc crcup, 2002 , Filst daep seistui. rcftectiod in-aeet ofthe Eatten AIps @eal giaat oustal uedser a nansoastal ranps. G<phys.ks.latre6, v. 29llO, pp. 92. I ro 92,1.

Dura

nce

Rhône

Inn

Donau

Drava

Sava

PoPo

Tagliamento

Rhei

n

Basel

Zürich

Genève

Bern

Grenoble

Nice

Genova

Milano

Torino

Venezia

Trieste

Besançon

München

Graz

Zagreb

Wien

Maribor

Passau

PadovaVerona

European margin

Valais ocean (Alpine Tethys)

Briançonnais terrane

Piemont-Liguria ocean (Alpine Tethys)

Margna-Sesia fragment

Apulian plate N of Periadriatic Line

Apulian plate S of Periadriatic Line

Neotethys and its distal passive margin

Tiza unit

N100 km

TRANSALP

NFP-20 EAST

NFP-20 WEST

ECORS-CROP

PALEOGEOGRAPHIC UNITS IN THE ALPS modified after Froitzheim et al. (1996)

7o6o

8o

9o

10o 11o 12o 13o 14o 15o 16o17o

48o

47o

46o

10o

11o 12o 13o 14o 15o 16o

7o6o

8o

9o

5o

48o

47o

46o

45o

44o

"Mid-Hungarian Line"

Periadriatic Line

Tauern window

Inntal line

Giu

dica

rie b

elt

Engadinewindow

Brennerfault

D-D'C-C'

E-E'

Insubric line

Penninicfront

Combin

Sesia

Ivrea mantle

Belledonne M. Blanc

Valaisan

Grivola

Gran Paradiso

chaînes subalpines

zone houillère

NW SE10 km

- 50 km

NW SE 10

-50 km

Préalpes Romandes Valaisan

M. Rosa

Ivrea mantle

subpenninicnappes

Insubric lineDent Blanche

M. Blanc

Combin

SesiaHelvetic nappes

FeeSaas

att-

Zerm

N S 10

-50 km

Aar

Valaisan

Adula

SimanoLucomagno

Gruf

Insubric line

Engadine line

Gotthard

Helvetic nappes

Platta

Avers

Tambo

Suretta

Arosa

Molasse basin

Mesozoic cover

upper crust

lower crust

mantle

Piemont-Liguria

Combin +nappe Supérieure

Brianc. cover

Brianc. upper crust

Valaisan

Bur-Pli cover S-Alps

cover S-Alps in general

upper crust: S-AlpsAustroalpineSesia-D. Blanche

Grauwackenzone

northern Calc. Alps

lower crust

mantle

Bergell

Eu

rop

ea

n m

arg

inA

pu

lian

ma

rgin

FIG. 2: geophysical -geological transectsthrough the AlpsLegend:

Inntal line

N S

Pustertal line

10 km

-50 km

ECORS-CROP

NFP-20 WEST

NFP-20 EAST

TRANSALP