Controllo strutturale e modalità di messa in posto dei Domi Cimini

The 1.3 Ma Cimini trachydacitic dome complex is located in Central Italy along the coast of the Tyrrhenian back arc basin. The dome complex underlies the Pleistocene to recent K-alcaline volcanic products of the Roman volcanic Province. The products mainly lava domes and ignimbrites distributed around the Mt. Cimino polygenic dome, are extensively covered by the k-alcaline ignimbrite and fall deposits from the Vico volcano .
The dome complex area is of key importance for the comprehension of the relationships between regional extensional tectonics and volcanism at the inception of volcanic activity in the region. Our topic has been to reconstruct the evolution of the dome complex and to identify the relative influence of the regional tectonics and the local stress field on the emplacement of the domes.
Structural, geomorphological, geophysical and volcanological data have been processed for the implementation of a dedicated GIS through which the three dimensional model of the area has been constructed.
The morphological shape of each single dome has been identified by quering the GIS system about the median slope of the domes (14°) and the area inscribed inside the 450 m isoline at which the break in slope of most of the domes occurs. For each dome the aspect ratio and the eccentricity have been determined.
The diagram aspect ratio on eccentricity shows that the domes can be grouped in two types: one type, is characterized by high aspect ratio and low eccentricity, whereas the other type has opposite characters. The first group includes the domes just around the Mt. Cimino, the second one occupy an external position. Furthmore, the elongation of the maximum diameter of the external domes converges toward the area where Mt. Cimino is located. We conclude that the domes are tangential and radial respect to a hypothetical center presently occupied by Mt. Cimino.
The high eccentricity of the external domes has been interpreted as due to the emplacement of the domes above an inclined surface induced by the swelling of the area when an intrusive body emplaced.
The deformation caused by the emplacement of the intrusive body has been analyzed reconstructing the top surfaces of the main sedimentary complexes below the volcanics . Geophysical data calibrated on data by shallow and deep wells performed in the area by the ENEL-AGIP company for geothermal purposes have been processed by the GIS system. There is an apparent increasing of thickness in corrispondence of the area of Mt. Cimino that we interpret as a swelling caused by the growing up of a laccolite body intruded at the contact between the two sedimentary complexes. The growing up of the laccolite produced radial and tangential fractures from which lava extruded at the surface forming the dome complex. Some domes show endogenous character and still have on top upper Pliocene thermometamorphosed clay sediments, now outcropping 400m up the regional elevation.
Regional stress on the deformation of the Cimini area has been deduced analyzing the drainage network and the morphotectonic lineaments. Rose diagrams relative to the two maps show a large dispersion of the lineaments that we interpret as due to the presence of radial and tangential fracture systems related to the emplacement of the laccolite. They have regional significance and are responsible of the emplacemnet of the laccolite. NW trending faults acted also after the end of the Cimini volcanism: they lowered the south-western part of the volcanic district, which is now covered by the Vico volcanics.
Ignimbrites erupted after the emplacement of the domes. Stratigraphic sections indicate that almost two ignimbrite eruptions occurred. They are separated by sinerption fluvial sediments suggesting that the two eruptions occurred closely in time. The ignimbrite deposits show lateral facies variations that can be explained in term of induced by the domes presence pre-existing topographic irregularities on which the ignimbrites emplaced.
Trachybasaltic lavas are the final effusive episod, mostly emplacing in the northeastern sector of the Cimini district confined inside the area collapsed after the ignimbrite eruption.
The evolution of the Cimini dome complex is summarized in:
- 1° phase: intrusion of the laccolite, rising along NW and NE trending faults and stopping at the contact between the meso-cenozoic and Plio-Pleistocene sedimentary units
- 2° phase: emplacement of the dome complex following radial and tangential patterns due to the swelling induced by the laccolite growth
- 3° ignimbrite eruptions and effusions of the final trachybasaltic lavas.