Cenozoic geometric and kinematic evolution of the tuscan-umbrian tectono-stratigraphic units (northern Apennines) : paleothermometric and paleomagnetic constraints

Abstract

In the past decades, paleothermal, thermochronological and paleomagnetic methodologies have been widely adopted for the reconstruction of orogenic belts evolution. Paleothermal and thermochronological analyses have allowed the reconstruction of the vertical paths (burial and exhumation path) of the sedimentary successions involved in the orogenic belts, providing thermal and time constraints on their evolution. Besides, paleomagnetic analysis are an excellent tool to assess kinematic models of curved orogenic systems because of its great potential to quantify vertical axis rotations. In this framework, a multidisciplinary approach was adopted in this thesis in order to understand the geometric and kinematic evolution of the Tuscan-Umbria-Marche Domain, in the inner portion of the Northern Apennines, during the Cenozoic time. In this sector of the Apennines, some issues on the deformation history are still unexplored. By integrating paleothermal, thermochronological and paleomagnetic results, it was possible to provide new data and constraints to reconstruct the tectonic evolution of this portion of the Apennine chain. Two key areas of Northern Apennines have been investigated. An inner sector, from the Trasimeno Lake to the south to the Garfagnana area to the north, where sedimentary succession of the Tuscan Domain units were analysed. An outer sector, in the Umbria- Marche-domain, where the analyses were concentrated in the area affected by the low angle normal faults Altotiberina system, from Massicci Perugini to east of the Gubbio fault. In chapter 1, the aim of the thesis, the methodology and the main phases of the research are illustrated. The methodological approach is based on the integration of common methods used in basin analysis, such as thermal analysis (organic matter optical analysis -vitrinite reflectance: Ro%, clay mineralogy by means of X-ray analyses) and thermochronological analyses (dating by fission track and (U-Th)/He in apatite). The optical studies of organic matter and X-ray diffraction analysis allowed defining the thermal maturity level reached by sedimentary succession. The thermochronological data define the age and rates of the exhumation. Paleothermal and thermochronological data were used to calibrate 1D thermo-structural models used to reconstruct the burial history and quantify the tectono-stratigraphic loads now removed. The paleomagnetic analyses were conducted in order to reconstruct the rotational history of the internal part of Northern Apennines. Besides paleomagnetic analysis, magnetic mineralogy analysis was carried out in order to identify the carriers of the natural remnant magnetization. The anisotropy of magnetic susceptibility (AMS) analysis was also carried out in the analysed sediments. This method constitutes an unique tool in defining the deformation pattern in poorly deformed sedimentary sequences, whose tectonic setting can’t easily be defined using classical structural methods. Chapter 2 illustrates the paleothermal analyses carried out in the internal sector (Tuscan Domain). Results show two main maturity trends: one perpendicular and the other parallel to the strike of the chain. In the first case, the paleothermal indicators record a decrease in thermal maturity from inner to outer sectors of the chain. In detail, Ro% values of the internal sector range from 0.6 to 0.9% indicating early-mid mature stages of hydrocarbon generation. Moving toward the external areas of the fold-and-thrust belt, values range from 0.3% to 0.5% indicating an immature stage of hydrocarbon generation. These data are in agreement with those obtained by the semi-quantitative analysis of clay fraction which shows a decrease of illite content in mixed layers from 89% (maximum temperature of 120-130 °C) to 38% (temperature below 100 °C) from hinterland towards foreland. Following the method proposed by Hillier (1995), a low heating rate, typical of foredeep basins, was obtained from the comparison between Ro% and I% in I/S mixed layers. A geothermal gradient of about 23 °C/Km is derived combining calculated heating and burial rates and used for thermal modelling. The second trend shows a thermal maturity increase, along the strike of the chain from the SW (Trasimeno lake area) toward the NW (Pratomagno area) where vitrinite reflectance maturity reaches values up to 0.95% and illite 87-89% in agreement with thermal maturity distribution derived from Ro% data. According to the performed one dimensional modelling, maximum burial and thermal maturity of the Tuscan Nappe succession decrease from the inner toward the outer sector with a corresponding reduction of the eroded thicknesses related to a reduction of the allocthonous (Ligurian Unit) which takes place from north to south along the chain, from hinterland towards foreland. Chapter 3 is focused in the area characterized by the low-angle Altoriberina normal fault system. Paleothermal (surface and well water) and thermochronological data were integrated, in order to develop an exhumation history model for this sector of the chain. The main evidence is an increase in the exhumation age from the innermost sector affected by the Altotiberina normal fault system (Massicci Perugini, 3 Ma) to the most external area affected by this system (to the east of the fault of Gubbio, 4.3 Ma). Significant novelty derives also from the rates of exhumation, suggesting quicker exhumation of Massicci Perugini (0.8 mm/yr) when compared to the area to the east of the Gubbio fault (0.5 mm/y). Therefore, these data suggest that most of the exhumation in the Massicci Perugini may be related to the recent activity of the normal faults and not only to the old compression phase and subsequent erosion. Thus the extensional activity of the Altotiberina Fault system may account for the younger exhumation age in the internal sector compared to the external one. Moreover the quicker erosion rate of about 0.8 mm/yr in the internal sector may be related to tectonic activity. In chapter 4, results obtained from paleomagnetic studies carried out in the Tuscan Domain, along the innermost arc, are shown. The rotational pattern recorded in Eocene- Oligocene sediments, shows a decrease of rotation values from lower (area of Trasimeno Lake) towards higher (Garfagnana area) latitude. In fact, mean rotations values recorded in the Trasimeno Lake area are 96° ± 25° counterclockwise, passing through 81° ± 35° counterclockwise for the Mt. Chianti area, up to 37° ± 16° counterclockwise in Garfagnana. These data do not fit into an oroclinal model, proposed for the external sector of the chain (Umbria-Marche domain). In this study I propose a new model that takes into account the contribution of the rotation of the Corsica-Sardinia block to the structural architecture of this sector of chain. This model predicts that the main tectonic phases of the internal sector of the chain occurred between Oligocene and early Miocene times, during the drift of the Corsica-Sardinia block. The Tuscan Domain units recorded rotation of the Corsica-Sardinia block during their incipient deformation. This block rotation occurred around a pole placed at 43.5° N and 9.5° E. At this time, the Northern Apennines and Corsica-Sardinia block were two different sectors of the upper plate of the Central Mediterranean subduction system, whereas the Umbria domain still represented the undeformed foreland. The contribution of the Corsica-Sardinia to the rotation of the Tuscan Domain depended on the position respect to the rotation pole. In fact, the amount of the Corsica-Sardinia block rotation is recorded in the southern areas (Lake Trasimeno) and tends to decrease at higher latitude (Mt. Chianti) until to the area at north of rotation pole where the contribution of Corsica Sardinia block rotation is not recorded (Garfagnana area ). In chapter 5, the AMS carried out on the same deposits analyzed for paleomagnetic reconstructions, are illustrated. AMS data show that the sediments are characterized by a dominant magnetic foliation parallel to the bedding plane, suggesting that the magnetic fabric is due to the compaction process during the diagenetic process that the sediments undergone. Moreover, a distinct magnetic lineation was observed, indicating an incipient deformation related to a compressional deformation, overprinted on to the original magnetic fabric. In most of the cases the lineation is parallel to the fold axes and thrust fronts, in agreement with previous results in the Umbria Marche domain. In chapter 6, a general discussion is developed and final remarks are illustrated.