The Matrei zone - the southern margin of the Penninic ocean

The nappe theory developed in the Western Alps, but was soon exported to the Eastern Alps, and then to the rest of the globe, by the Swiss geologist Emile Argand (1879 - 1940), who between 1909 to 1916 draw the first profile of the Penninic zone and then represented the structural setting of the entire Western Alps in a 1:500,000 tectonic map and corresponding cross-sections.

Argand's Penninic zone was a stack of six recumbent fold-nappes, made up of a basement core and mantled by Mesozoic sediments. Based on these profiles he developed a formation model who applies to all mountain ranges: the theory of embryotectonics suggests that single nappes are the results of the compression of a chain of sedimentary basins and their infills (he incorporated the existing idea of an Alpine geosyncline), who pushed together became stacked and finally folded.

Fig.1. Argand's generalized view of the Europe-vergent Alpine belt (figure from DalPIAZ 2001).

In this theory the observed amphibolites or serpentine bodies, found intercalated in the Bünder Schists throughout the Alps, where injections and submarine effusions of mafic-ultramafic melts which were supposedly emplaced during orogenic contraction.

Fig.2. Argand's (1916) diagram of the western-Alpine geosyncline during its initial contraction (embryotectonics) with syn-orogenic emplacement of ma?c magmas (black, Piedmont ophiolites) along the sheared lower limb of the Dolin-Dent Blanche geoanticline. Simplifyed legend: (1) rigid foreland, (2) epicontinental basin, (3) Valais foredeep, (4) Gran St. Bernard cordillera, (5) Piedmont basin, (6) Dolin-Dent Blanche cordillera (figure from DalPIAZ 2001).

Argand's theory resolved many geological contradictions (like the to complex reconstruction of double-folds), and seemed supported by the general north to south symmetry of the geological units in the Alps, but his assumption of continuous nappes could not explain some geological abnormalities, who have no symmetrical equivalent on "both sides" of the Alps or show a very complex pattern of different rocks.

At the southern boundary of the Penninic to the Austroalpine units a zone with a variegated rock inventory is situated. Surrounded by mainly calcareous phyllites, typical for the development of the Bündner schists, horizons of breccias, turbiditic sediments and blocks of reworked (Lower) Austroalpine rocks are intercalated. Also quartzites, calcareous and dolomitic marbles, and rauhwackes are accompanied by serpentinites and their associated rock types.

Some of these formations are highly tectonized and fault zones separate the so-called Matrei zone from surrounding tectonic units.

Fig.3. Geological map of the southern border of the Tauerm Window, with the so-called Matrei-Zone.
AUSTRO-ALPIN + PENNINIC
Td Dolomite and limestone - marble (middle and upper Triassic)
Tp White Quartzite (upper Permian - lower Triassic)
Mf Schist and phyllite with Mica (Cima-Dura-Series, Paleozoic)
PENNINIC
a3 Prasinite/Amphibolite with Epidote, Biotite, Chlorite (Matrei Zone)
Cs Calcareous schist (Lias)
from: Giorgo del PIAZ (1928-1930): Carta Geologica delle Tre Venezie, Foglio 4b. Monguelfo.

Fig.4. Tectonized phyllite of the Cima-Dura Series, some meters before the contact to Triassic marbles.

Fig.5. Overview to the north, strong tectonized contact between schist/phyllite of the Cima-Dura Series, part of the Austro-Alpine, to Triassic marble (white escarpment), already part of the Tauern Window. The steep contact plunges directly to the valley floor W-E.

Fig.6. Triassic marble on the valley floor.

Fig.7. Prasinite with layers of epidote and quartz from the Matrei Zone.

With the formulation of the theory of continental drift at the beginning of the 20th century, an his modern modification in form of plate tectonics, today the Matrei zone is explained as a tectonic melange zone, it is not a nappe in the sense proposed by Argand, but an intermingled "plaice", formed during the orogenesis, representing the uppermost transition zone of the Penninic nappes. The slices and blocks of sedimentary rocks can be explained as olistolithes, which glided from the southern continental margin into the Penninic deep-sea sediments. The serpentinites (thought by Argand of syndeformational formation) were scraped off from the ocean floor and brought to the surface. Therefore, the Matrei zone can be interpreted as an expression of the closing processes of the (single) Penninic Ocean in the Cretaceous. Due to the northward drift of the Austroalpine units, the Penninic oceanic crust was subduced into the earth's mantle below the Adriatic microcontinent. In this course parts of the oceanic floor were sliced off and incorporated in an accretionary wedge at the surface. Large blocks of the sedimentary cover of the overthrusted Austroalpine unit broke off and slided through the continental slope to the deep-sea floor. Turbiditiy currents and coarse-grained debris-flows covered these blocks. Once incorporated into the accretionary prism of upthrustet material, the entire rock succession was extremely deformed.

References:

DalPIAZ, G.V. (2001): History of tectonic interpretations of the Alps. Journal of Geodynamics 32: 99-114
STINGL, V. & MAIR, V. (2005): An introduction to the geology of South Tyrol. Provincia Autonoma di Bolzano-Alto Adige - Ufficio geologia e prove materiali: 80