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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Balanced
Sections
and Seismic Reflection Profiles
Across the Central Apennines, Italy *
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By
Balanced cross
sections
combined with AGIP's magnetic
survey analysis clearly show that the Central Apennines
can be interpreted using experience gained in Western
Canada and Wyoming.
Having stated that the Rocky Mountain model in general works in Italy, we feel we have to emphasize some major differences.
The reflection seismic data in the Apennines and the Adriatic foreland do not permit us to recognize a continuous near basement reflector. Consequently, we have to assume a gently westward dipping decollement surface. This surface, our putative basement, is visually extrapolated from the Adriatic foreland toward the west and underneath the Apennines. The probable reason for the near absence of a "basement" reflector is due to the nature of the basement and possible absorption of seismic energy by Triassic evaporites. In contrast to a number of other folded belts, the structure in the Apennines is dominated by the distribution of Triassic evaporites. Thus, the best analogues for the Apennines are probably the Zagros Mountains and to a lesser extent the Parry Island folded belt of Northern Canada and the Jura Mountains. Seismic data from the Zagros is scarce but seismic data from the Parry Islands and the Jura Mountains clearly demonstrate an evaporitic decollement level.
The geology of Italy is further complicated by Jurassic
extensional tectonics. Jurassic extension has been postulated
on the basis of surface studies in the Southern Alps and in
the Apennines. The arguments favoring extension are
mostly based on stratigraphic observations. We have as yet
to document anything coming near a description of a
Jurassic structural
pattern. We suspect that such a pattern
would reveal a system of listric half-grabens, that are
connected by transform segments. Unfortunately, there is
not a hint of any evidence of Jurassic extensional tectonics
on our reflection seismic profiles or else possibly to the
limitation of widespread extensional tectonics to the outcropping Sibillini thrust sheet and their general absence in
the Adriatic foreland. The situation in the Allochthonous
thrust sheet may also be compared with the reefal buildups
in the central Adriatic Sea and in the Abruzzi, which are not
demonstrably controlled by any pre-existing extensional
tectonics.
Additional widespread decollement levels can be
observed on the surface (base Messinian, Scaglia Cinerea,
Scisti a Fucoidi, Rosso Ammonitico, and many other minor
levels). All these cannot be resolved by conventional reflection
seismic profiles. We choose to ignore these features.
Consequently, the amounts of shortening shown on our
cross
sections
are conservative and much of the shortening
observed in detailed studies should be added to the large
scale regional shortening suggested by our profiles.
The main decollement level descends from about 5 km under the Adriatic Sea to about 15 km under the Central Apennines. Regionally farther west, it is likely that the main decollement level is close to the Moho. This implies that basement and sediments are separated mechanically from the underlying mantle. That mantle would form a lithospheric A-subduction related root, the existence of which still needs to be demonstrated.
The Anzio-Ancona line, or at least the Terminillo-Sibillini segment of that feature is interpreted by us as a lateral ramp.
The Sibillini thrust appears to have been rotated counter clockwise as postulated by paleomagnetic workers. Much of the increased shortening in the south of our area is due to the appearance of N-S striking duplex structures; a)
End_Page 11---------------
the Roccafinadamo structure, b) the Montagna dei Fiori structure, and c) the Laga (Aquasanta) thrust sheet.
Our palinspastic reconstructions are crude first approximations. They suggest that the Sibillini thrust in its reconstruction coincides reasonably well with the present-day position of the leading edge of the Cervarola thrust sheet. With it, the Umbrian domain falls to the present day Toscana and the adjacent Tyrrhenian sea, the Cervarola unit falls in its reconstruction to the northwestern Tyrrhenian sea, while the Tuscan units would position in the area now occupied by Corsica and the Eastern Gulf of Lyon (Provencal Basin).
We speculate the basic structural
style of the Dinarides
to be similar to the style of the Central Apennines to the
extent that a major Triassic evaporitic decollement level
appears evident. Utilizing similar palinspastic restoration
techniques, we suggest that the original Adriatic promontory
was well over 1200 km wide.
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*This project was undertaken in cooperation with C. Cooper, Rice University, L. Burbi and R. Ghelardoni, AGIP, Spa, Sun Donaro, Italy.