Arkell.1956.Jurassic..

WESTERN AND CENTRAL ALPS
From the point of view of Jurassic stratigraphy the western and central
Alps, reduced to the simplest terms, may be resolved into two parallel
bands: the main geosyncline occupied by the great 'comprehensive
series' of the schistes lustres, and the highly diversified and largely
calcareous deposits accumulated on the outer or foreland edge of the
geosyncline. The line dividing these two bands is vague and often
uncertain, and it does not coincide with the trace of the Pennid thrust
which separates the Pennids from the Helvetids on Staub's beautiful
tectonic map (1923). It is only the inner (southern and eastern) part of
the Pennids that consists of schistes lustres. The outer half comprises
the Brianconnais geanticline and a smaller trough outside it (zone
subbrianconnaise), in which the deposits are transitional to the Helvetids.
The Brianconnais geanticline was comparable with those which now form
the composite crystalline autochthonous massifs (Mercantour, Pelvoux,
Belle Donne, Mt. Blanc, Aiguilles Rouges, Aar), from above and between
which the Helvetid Mesozoic cover has been peeled and squeezed to form
the High Limestone Alps. Moreover, the Jurassic of the Brianconnais
geanticline corresponds so closely with that of the Prealps (Gignoux,
1950, pp. 400-1) that there can hardly be any doubt that the Prealp
nappes rooted on or behind this geanticline just as those of the High
Limestone Alps root between and behind the Mont Blanc-Aiguilles-Aar
geanticlines; in other words, the Prealps did not 'leap-frog' over the
central Alpine geosyncline, but only from an inner zone of its outer margin.
Since the Prealps, themselves a giant klippe, are connected by the klippes
of Lake Lucerne with the Austrid nappes beyond the Rhaeticon, this
stratigraphical observation affects the interpretation of the Eastern Alps
and is against the conclusions of the 'ultranappists'. (See p. 162.)
The southern side of the Alpine geosyncline in the west is buried below
the plain of the Po and begins to come in on the south side of the central
Alps. It will be considered separately (p. 173).
Any palaeogeographical reconstructions have to allow for a narrowing
of the Alps to somewhere around a quarter or a third of the original
width of the ground on which they were formed; for measurement of the
convolutions of marker horizons in the nappes and folds in all parts of the
central Alps shows that on the whole there has been a shortening of
between 66 and 75 per cent. In other words the width of the central
Alps, now about 150 km., was originally about 630 km. (Cadisch, 1953,
p. 287); and, assuming this shortening in the centre to be the maximum,
the shape of the original geosyncline and its marginal furrows would have
been not a strip as at present, but an ellipse almost as wide as long. Uneven
lateral squashing of such a depression can account for the pattern of
sweeping curves at either end of the Alps: the swinging round of the
schistes lustres through the Ligurian Alps to Corsica (see p. 218) and the
inward virgations of the Apennines and Dinaric Alps.
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