Modello geodinamico dell'Area Umbro-Marchigiana e suo significato sismogenetico

Abstract

Starting from the

analisys of the geodynamic evolution of the Umbrian-
Marchean fold belt, we attempt to outline a seismogenetic model of
the area.
This model rests 1) on the assumption that every deformation tends
to reduce the stress by which it has been originated so that the magnitude
of the vectors changes and the stress field results reoriented (Price
1959, Ramsay 1967) and 2) on the observation that Umbrian-Marchean
deformations of both sedimentary mesozoic cover and crystalline ercinic
basement, in spite of their disharmonious behaviour due to the inter-

position ot the incompetent level of the triassic evaporites, show a common
displacement field (Lavecchia & Pialli 1981 a).
At present, as far as the Umbrian-Marchean geodynamic evolution is
concerned, all the available geological and geophysical data are in agreement
with the existence of a displacement field (two axis in a horizontal
plane of apenninic and counterapenninic directions, one axis vertical) whose
orientation has been kep trought space and time, while the magnitude
of its vectors underwent considerable variations. In a given area, the sequence
of events trough the time should be the following. At time T, a
horizontal counterapenninic direction of maximum compression a, shortens
the crystalline basement and causes a crustal thickening. In response
to the consequent increase of the lithostatic load the crust subsides
isostatically. Because the elastic warping of the lithosphere beneath the
load extends beyond the actual limits of the load, a peripheral depression
is created in which the foredeep trough, that migrates northeastward in
advance of the deformation, grows (Price 1973). The continuous increase
of lithostatic load produces, at time T2, a reorientation of the strain
ellipsoid with a vertical intermediate principal direction.
Transcurrent shear zones (left-lateral about E-W and right-lateral N-S)
and anticlockwise rotation are generated in the crust, while an arcuate fold
belt with convexity towards East, formed by periclinal interdigitate folds,
develops in the cover. Such deformation reduces the magnitude of the maximum
counterapenninic complessive vector (a), so that the strain ellipsoid
undergoes a new reorientation (time 3) with a vertical minimum
principal strain direction. Crustal thinning and apenninic horst and graben
structures are so generated. Obviously all these deformational phases
(tensional, transcurrent and compressional) and the associated stress regimes,
should be present at a given time, throughout the space from
SW to NE.
Following Adriatic this scheme, the present seismicity of the Umbrian-
Marchean area should be a consequence of its Adriatic geodynamic
distinction in apenninic fold belt, deformed Pliocene foretrough (B) compressional
zone (C) and should reflect differents stress regimes, with
normal faults to the West (A), transcurrent shear zones in the middle (B)
and reversed in the East (C).
Such seismic zoning is in a good agreement with the distribution ot
the focal mechanisms of this area (Cagnetti et Al. 1978, Lavecchia e Pialli
1981 a) which confirms the existence of a tensional stress regime with
apenninic and counterapenninic direction in the Apennines and a transcurrent
regime in the periadriatic area.
The much less clear evidence of a compressional regime in the Adriatic
aseismic zone (Gasparini, Praturlon 1981) gives a further confirmation of
the proposed model if interpreted as due not to a real absence of a compressional
stress, but due to the existence of aseismic creep on reversed

apenninic shear zones. A much greater concentration of strain energy is
infact required to begin frictional sliding on thrust faults than on normal
or transcurrent faults pore fluid pressure, A, continuous ductile shearing can be stable on
reversed faults, while sudden release of shear strain energy consequently
seismic failure, can realize on normal and transcurrent faults.