Arkell.1956.Jurassic..

214 ITALY AND CORSICA
Staub, in a characteristic essay (1932) based on a study of the Apuan
dome, was convinced that the movement had been from east to west,
but nearly all authors, before and since, agree that it was from west to
east (farther north, from SW. to NE.). The source therefore has to be
postulated in the sunken region of the Tyrrhenian Sea. Since in
Corsica the movement is from the east, against the crystalline foreland in
the west (see p. 218), there must be a parting under the sea between
Corsica and Elba. This parting probably runs into the coast near Genoa
and then under the Po basin (Teichmiiller & Schneider, 1935, pp. 46-7).
Originally the site of a sinking trough, in the early Tertiary orogeny it
was uplifted and shed part of its load of sediments as great sheets which
glided off into the fore-deeps on either side, contorting and breaking up
as they travelled. The distances moved (100-200 miles) and the incompetence
of the rocks, however, make an explanation of the Ligurian
'nappe' by means of orthodox nappe tectonics well-nigh inconceivable.
To meet these difficulties Italian geologists have developed the ideas
of gravitational sliding put forward from time to time in various parts of
the world and expounded with special force by Haarmann (1930). The
Italian version, known as the 'successive landslip theory', contains special
and attractive features and merits most serious attention. (See Migliorini,
1945, 1952; Signorini, 1946; Merla, 1948, 1952; Beneo, 1950). The
long distances covered by the incompetent Ligurid materials are accounted
for by supposing that the slipped debris was passed on from time to time
as successive ranges were formed in order from west to east, new materials
being added to the old with each orogenic spasm. The earliest uplift
occurred over the Tyrrhenian Sea in the late Cretaceous; the latest
ended in the Pliocene on the outer margin of the arc, adjoining the Adriatic
coastal plain and the Po basin. The ridges were asymmetric in form, with
the longer slip-slope on the outer (eastern) side. They present internally
a peculiar pattern of 'composite wedges', the ends of which would be liable
to be torn off and incorporated with the slipping cover material (chiefly
Argilli scagliosi), thus providing the exotic blocks. The slips involved
bear close comparison with those proved by intensive drilling in the
oilfield on the Pacific coast of Peru and Equador, as described in 1938
by Baldry and Barrington Brown. The remarkable structures met with
in the Apennine ranges (e.g. Behrmann, 1936, many figures) forcibly
recall Haarmann's book.
The following succession near Perugia, in Umbria, lies near the centre
of the whole Apennine area and may be considered typical (Principi,
1909):—
h. Tithonian grey limestones passing up gradually into Neocomian
limestones (Majolica)
g. Aptychus shales
/. Posidonia alpina beds
e. Limestones, greyish-yellow, siliceous, without fossils
d. Red limestones with Erycites fallax, 'Aalenian'
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