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

Abstract

<p>Starting from the</p><p>analisys of the geodynamic evolution of the Umbrian-<br />Marchean fold belt, we attempt to outline a seismogenetic model of<br />the area.<br />This model rests 1) on the assumption that every deformation tends<br />to reduce the stress by which it has been originated so that the magnitude<br />of the vectors changes and the stress field results reoriented (Price<br />1959, Ramsay 1967) and 2) on the observation that Umbrian-Marchean<br />deformations of both sedimentary mesozoic cover and crystalline ercinic<br />basement, in spite of their disharmonious behaviour due to the inter-</p><p>position ot the incompetent level of the triassic evaporites, show a common<br />displacement field (Lavecchia & Pialli 1981 a).<br />At present, as far as the Umbrian-Marchean geodynamic evolution is<br />concerned, all the available geological and geophysical data are in agreement<br />with the existence of a displacement field (two axis in a horizontal<br />plane of apenninic and counterapenninic directions, one axis vertical) whose<br />orientation has been kep trought space and time, while the magnitude<br />of its vectors underwent considerable variations. In a given area, the sequence<br />of events trough the time should be the following. At time T, a<br />horizontal counterapenninic direction of maximum compression a, shortens<br />the crystalline basement and causes a crustal thickening. In response<br />to the consequent increase of the lithostatic load the crust subsides<br />isostatically. Because the elastic warping of the lithosphere beneath the<br />load extends beyond the actual limits of the load, a peripheral depression<br />is created in which the foredeep trough, that migrates northeastward in<br />advance of the deformation, grows (Price 1973). The continuous increase<br />of lithostatic load produces, at time T2, a reorientation of the strain<br />ellipsoid with a vertical intermediate principal direction.<br />Transcurrent shear zones (left-lateral about E-W and right-lateral N-S)<br />and anticlockwise rotation are generated in the crust, while an arcuate fold<br />belt with convexity towards East, formed by periclinal interdigitate folds,<br />develops in the cover. Such deformation reduces the magnitude of the maximum<br />counterapenninic complessive vector (a), so that the strain ellipsoid<br />undergoes a new reorientation (time 3) with a vertical minimum<br />principal strain direction. Crustal thinning and apenninic horst and graben<br />structures are so generated. Obviously all these deformational phases<br />(tensional, transcurrent and compressional) and the associated stress regimes,<br />should be present at a given time, throughout the space from<br />SW to NE.<br />Following Adriatic this scheme, the present seismicity of the Umbrian-<br />Marchean area should be a consequence of its Adriatic geodynamic<br />distinction in apenninic fold belt, deformed Pliocene foretrough (B) compressional<br />zone (C) and should reflect differents stress regimes, with<br />normal faults to the West (A), transcurrent shear zones in the middle (B)<br />and reversed in the East (C).<br />Such seismic zoning is in a good agreement with the distribution ot<br />the focal mechanisms of this area (Cagnetti et Al. 1978, Lavecchia e Pialli<br />1981 a) which confirms the existence of a tensional stress regime with<br />apenninic and counterapenninic direction in the Apennines and a transcurrent<br />regime in the periadriatic area.<br />The much less clear evidence of a compressional regime in the Adriatic<br />aseismic zone (Gasparini, Praturlon 1981) gives a further confirmation of<br />the proposed model if interpreted as due not to a real absence of a compressional<br />stress, but due to the existence of aseismic creep on reversed</p><p>apenninic shear zones. A much greater concentration of strain energy is<br />infact required to begin frictional sliding on thrust faults than on normal<br />or transcurrent faults <Sibson 1974, 1977), so that at a given depth, z, and<br />pore fluid pressure, A, continuous ductile shearing can be stable on<br />reversed faults, while sudden release of shear strain energy consequently<br />seismic failure, can realize on normal and transcurrent faults.</p>