PRE-TRIASSIC EVOLUTION

Pre-Alpine basement rocks outcrop in scattered areas throughout the Alps. They are most abundant in the central part where high grade crystalline rocks are exposed due to extensive uplift and erosion in the course ofthe Alpine orogeny. This basement contains a mix ofpre-Variscan basement, Late-ofpre-Variscan granitoids associated with clastic and volcaniclastic rocks, and post -V ariscan volcaniclastic rocks.

The paleogeographic map shown in Figure 6-21 shows the position ofthe major basement blocks containing pre-Triassic rocks in the Alps. It was obtained by a palinspastic reconstruction of the movements suffered during the Alpine orogeny and Mesozoie rifting (for detailed discussion see Pfiffner 1992).

Apart from the Moldanubian zone (Black Forest-Vosges in Figure 6-21) Late-Variscan granitoids are approximately lined up in three E-W trending belts, the northern, central and southern Granite belts. Permo-Carboniferous volcaniclastic sequences were deposited in narrow furrows some which are parallel to these granite belts. The volcaniclastics were in part intruded by these granites and intensively folded as indicated by the angular unconformity with the overlying Trias sie strata. The igneous volcanic and plutonic rocks ofthe calcalkaline series are interpreted as being subduction related (Oberhänsli et al. 1988). The magmatic

I

TECTONIC EVOLUTION OF EUROPE

t

N

t

+ + + + + + +

+ .::::: + Vosges

.?C~/~~~:5

'"

c o N

.;.i.: ...... , ... , ".

~

Black

~

^{+ orest ... :-:-:}

.0:~:~

~

Gran Paradiso Dora Maira

Early Triassie (240 Ma)

Eill

Permo-Carboniferous Volcaniclastics

~ ~ Late- Variscan Granites

o , ' - -_ _ _ _ ^{100 km} ....J'

181

Figure 6-21. Paleogeographic reconstruction ofthe Alpine segment of EGT in Early Triassie time (240 Ma). The names refer to future Alpine nappes or basement blocks.

activity is related to a phase oftherrnal updoming and subsequent stretching ofthe thickened crust (Lorenz and Nicholls 1984). The angular unconforrnities observed within the Perrno-Carboniferous troughs suggest that these furrows forrned in a transpressive regime (Laubscher 1987) which is related to a dextral transforrn zone between the Urals and the Appalachians (Matte 1986, Ziegler 1990, Franke Section 6.3). In Perrnian times, post-Variscan stretching and subsidence broadened the troughs. Typically red beds accumulated in these basins and are conformably (or with a slight unconformity) overlain by Triassic sediments.

6.4.2 MESOZOIC RIFfING PHASE

During the Mesozoic the lithosphere underwent a phase of stretching and thinning associated with sinistral strike slip between the European and the Adriatic-African plate as explained in Chapter 2-4- 13. This rifting phase was particularly active in Jurassic times during the separation of Gondwana and Laurasia and the associated opening of the Tethys and Atlantic ocean_

Figure 6-22 is a paleogeographic map illustrating the situation at the end of the Jurassic

I 182 A CONTINENT REVEALED

I

Vosges

End Jurassie (150 Ma)

1::::lf:::l

New Oceanic Grust with Spreading Ridge ~ Thrust Faults (transpression)

1"-

,,~ Thinned Gontinental Grust ....,...,.. Normal Faults

EUROPE

t

N

t

~

Figure 6-22. Paleogeographic reconstruction at the end ofthe Jurassie (150 Ma) after rifting and opening of the Piemont ocean. In the paleogeographic realm of the future Central Alps, opening is oblique with offset ridges. A-B: trace ofprofile given in Figure 6-23.

(150 Ma). According to plate reconstructions, the Adriatic-African plate moved in an ESE (Dewey et al. 1989) to SE (Savostin et al. 1986) direction generating a basin dominated by transform faults in the future Central Alps (Weissert and Bemoulli 1985, Lemoine et al. 1989, Stampfli and Marthaler 1990).

This Tethyan basin is of a true oceanic nature in its central part (Piemont ocean) and is bordered by the thinned passive margins ofthe Adriatic and European continents (see Figure 6-23).

Stretched European margin

Stretching of the European passive margin resulted in a complex maze of basins and swells. The largest basin, the Valais trough, is characterised by intercalations of basaltic material in a thick clastic sequence. This basin might have formed in a transtensional regime as a pull-apart basin. The continental shelf (Helvetic Dauphinois domains) to the NW ofthe Valais trough is marked by slow subsidence increasing basinwards. Stepwise subsidence with characteristics of extension are typical in the Lower Cretaceous sediments (Funk 1985).

The Brian90nnais swell situated to the SE of the Valais trough is bounded by steep scarps with scarp breccias. South of the Adula block the swell is dissected by a transcurrent fault. The sediments associated with this zone are exposed in the Schams nappes of eastem S witzerland. Angular unconformities and sediment transport directions observed within them point to a transpressional scenario (Schmid et al. 1990). The crust underlying the Valais trough is likely to be thinned continental crust; locally it was possibly cut by extensional fractures which

I

TECTONIC EVOLUTION OF EUROPE

STRETCHED EUROPEAN MARGIN WNW

Dauphinois-Helvetic Valais Piemont Ocean

.s:::

Figure 6-23. Cross section illustrating rifting and thinned continental margins ofthe Piemont ocean at the end ofthe Jurassic. Trace of cross section is given in Figure 6-22. Pogallo and Lugano are major syn-sedimentary normal faults extending into the basement.

served as pathways for the ascent of basaltic melts from the mantle. The small volume of ophiolitic material found in the corresponding nappes (e.g. Martegnas melange) speaks for incipient spreading rather than a wide ocean (see also Schmid et al. 1990). Based on a comparison with modern oblique rift systems, Kelts (1981) proposed a two-stage model with a phase of subsiding continental crust segmented by deep vertical faults followed by oblique rifting associated with the formation of a narrow belt of oceanic crust.

Opening of the Piemont ocean

The opening of the Piemont ocean is coeval with the opening of the central Atlantic. The original width of the Piemont ocean is difficult to assess. Estimates range from 100 to 500 km. In any case the opening was oblique in the transect of the Central Alps as opposed to the situation in the Western Alps. The importance of the transform faults is indicated by the occurrence of (a) pelagic sediments in stratigraphie contact with serpentinites (indicating serpentinite protrusions along fracture zones), (b) ophiolite breccias formed by fracturing of oceanic crust coeval with sedimentation in the fractures and (c) pebbles of oceanic and continental provenance (crystalline basement and oolitie limestones) contained within massflow conglomerates in the deep sea sediments (radiolarian cherts) of the Piemont ocean (Lagabrielle et al. 1984, Lemoine 1980, Weissert and Bernoulli 1985). In Figure 6-22 these transform faults are drawn parallel to the movement direction between the European and Adriatic-African continents.

Stretched Adriatic margin

Along the Adriatic margin, stretching and thinning of the continental crust led to a pronounced, rugged morphology with NNE-SSW -trending swells and basins. In the Lombardian basin, for example, the distal continental margin submerged and became increasingly starved. Differential subsidence associated with synsedimentary faulting resulted in a sequence of silicic limestones up to 4 km thick in the basin in contrast to a thin shallow water sequence on the swell to the west. Two normal faults can be followed into the basement; the Pogallo fault, which separates the Ivrea from the Strona-Ceneri basement block (Handy 1987), and the Lugano line that marks the western end of the Lombardian basin (Bertotti 1990). Both indicate crustal attenuation by listric master faults dipping towards the continent, similar to the modern example ofthe Bay ofBiscay margin (Le Pichon and Barbier 1987) where attenuation of the lithosphere is achieved by a ductile zone of decoupling in the

I 184 A CONTINENT REVEALED

I

lower crust situated between brittle upper mantle and brittle upper crust. Allowing for the 30° anticlockwise rotation that the Adriatic micropiate suffered after the Mid-Cretaceous (Gosau) orogenie movements (Mauritsch and Becke 1987), the Lugano and Pogallo faults were striking NNE. It thus seems that the principal stretching direction of the Adriatic margin was parallel to the opening direction of the Piemont ocean.

The cross seetion shown in Figure 6-23 is constructed along the direction of opening and stretching. It displays the horst -graben geometry of the upper crust of both margins and a minimum width of 100 km for the Piemont ocean. The amount of stretching that the margins underwent is difficult to assess. A crude estimate of

ß

between 1.3 and 1.65 was obtained in the Austroalpine nappes (Campo block in Figure 6-22) based on fault geometry (Froitzheim 1988).

Im Dokument It It (Seite 187-191)