Federico Sani

Crustal section based on CROP seismic data across the North Tyrrhenian–Northern Apennines–Adriatic Sea

Abstract Using deep seismic reflection data from the Italian lithospheric exploration project CROP in the Central Mediterranean region, a 400-km-long section, composed of three different profiles crossing the Northern Tyrrhenian Sea (CROP M-12A profile), the Northern Apennines (CROP-03) and the Adriatic Sea (CROP M-16) is reconstructed and discussed. New data allow us to outline a seismically consistent tectono-stratigraphic setting for the crust and upper mantle of the Northern Apennines thrust–belt system and its Adriatic foreland. Time–space analysis of the deformation of the investigated chain and identification of existing macrostratigraphic crustal intervals and tectonic units allow a reasonably controlled interpretation of the geodynamic evolution and of the main orogenic stages. Careful seismic reprocessing and application of advanced techniques to key zones of the explored area (such as the Tuscan Archipelago) were determinants in obtaining fundamental information for understanding of the complex lithospheric structures and their evolution. Profile interpretation supports that the Northern Apennine chain is dominated by a compressive thrust system. Crustal extension, assumed by some authors as the dominating tectonic process for the whole Tuscan Apennine area, represents a subordinate geodynamic event of the last stage (Tyrrhenian). In the Early Cretaceous–Late Jurassic, the paleogeographic framework consisted of the Europe and Adria plates separated by the Alpine Tethys Ocean. During the Late Cretaceous–Early Eocene, Adria–Europe convergence (eo-Alpine stage) and subduction beneath the Adria plate closed the Alpine Tethys Sea, with the Tethyan slab being clearly seismically imaged. The first Apenninic geodynamic stage occurred in the Late Oligocene–Early Miocene with the opening of the Balearic Basin, which generated a first “lithospheric root” of the Apenninic chain in the Tuscan Archipelago area. This root is represented by Adria-verging thrust faults that progressively flatten eastward. Upper parts of the west-verging eo-Alpine thrust blocks were truncated by the east-verging thrust faults of the Balearic stage. A deeper seismic reflector, attributed to the top of the asthenosphere, forms a mantle high below the Elba Island. From the Late Miocene to Present, the Corsica basin and western hinterland area were affected by extensional tectonics related to the Tyrrhenian opening, whereas compressional tectonics continued in the eastern hinterland and mostly on the eastward migrating foreland, with development of a second “lithospheric root” constituted by high-angle thrust faults. These faults give rise to a huge basement culmination below the main Apennines watershed. Impressive E-directed gravity-sliding of sedimentary blocks over their sloping basement occur, generating the Umbria–Marche shallow seismicity. Crustal shortening of the Apennines system amounts to 170 km, 14 km of which are due to the eo-Alpine stage, 71 km to the Balearic and 85 km to the Tyrrhenian one. In the frame of Africa–Europe convergence, the Tyrrhenian–Apennines tectonodynamics were mainly conditioned by the Mesozoic paleogeography.

Cover thrust reactivations related to internal basement involvement during Neogene‐Quaternary evolution of the northern Apennines

Up to recently the Neogene-Quaternary evolution of the northern Apennines (Italy) has been described by the classic model of a migrating eastward, compressive external front, with an extensional regime in the back areas connected with the Tyrrhenian basin formation. However, in the last few years, new structural data have been collected in the internal marine and continental episutural basins, and in the external exposed thrust belt. A complex structural evolution has now been reconstructed, with coeval main tectonic phases that affect both areas with stress field change. Four main tectonic phases have been identified since Late Tortonian times; these are dated as Messinian, late Pliocene, middle, and late Pleistocene. The thrust belt has a complex evolution, detected in a wide external area of the chain after the first emplacement of the main thrust sheets, with reactivations and out of sequence thrusting. These reactivation phases fit very well with the compressive phases affecting the sediments of the internal basins, suggesting a direct relationship within a single evolutive model. The increased knowledge of the deep structure of the northern Apennines through geophysical and subsurface data acquired in the last few years indicates the basement involvement at least for the internal side of the northern Apennines. This basement involvement also played an important role in the tectonic evolution of the external sector of the Apennines. In this paper a new model is proposed that integrates field data, geophysical evidence, and geodynamic constraints. The thrust reactivations and the out-of-sequence structures of the external area are related to internal crustal thrust activity. The deformed sediments of the Neogene-Quaternary basins are dating the thrust activity. All the evidences point to a late Neogene rejuvenation of the tectonic evolution, but some inferences can be drawn on the development of the foredeep.

TECTONIC REGIME, GRANITE EMPLACEMENT AND CRUSTAL STRUCTURE IN THE INNER ZONE OF THE NORTHERN APENNINES (TUSCANY, ITALY) : A NEW HYPOTHESIS

Abstract Geological and geophysical data on the inner part of the Northern Apennines and the deep (4 km) geothermal wells contained therein have been reviewed in order to better characterize the tectonic regime of the region. The area is affected by Neogene–Quaternary igneous activity, reduced crustal thickness, high heat flow and active geothermal systems. The available information leads to the following conclusions: (1) at present, part of the chain, including its inner regions, are undergoing extensional activity, a tectonic situation in southern Tuscany dating back to the Late Tortonian; however, the simple model of solely extensional tectonics cannot exhaustively account for the phenomena observed in the area, as there is ample evidence of compressive events of Pliocene and Pleistocene ages. (2) The geophysical data indicate reduced lithosphere thickness and the uprising of the asthenosphere to shallow crust levels. The upper crust has been intruded by many granite bodies and the brittle–ductile transition is probably very shallow. An integrated geophysical model which accounts for heat-flow, seismic, gravimetric and magnetic data, indicates that a regional strong reflector, rather than coinciding with the beginning of the ductile crust, more likely indicates the presence of deep brines. Rather than a gneiss core complex and a generalized post-collision extensional model ( Cameli et al., 1993 ) we support a more complex geodynamic framework, including compressive phases of the internal zone and re-activation of the thrust of the Apennines as a consequence of crustal thrust emplacements from the Late Tortonian in the frame of a subducting delaminated lithosphere.

Pliocene-Quaternary transpressional evolution of the Anzi-Calvello and Northern S. Arcangelo basins (Basilicata, Southern Apennines, Italy) as a consequence of deep-seated fault reactivation

Abstract In this paper we report the results of a study of the Plio-Pleistocene Anzi-Calvello Basin and the northern sector of the S. Arcangelo Basin (Southern Apennines, Italy). Our work suggests that the evolution of the Anzi-Calvello Basin is related to the activity of a deep-seated, roughly NNE-trending, dextral transpressional fault zone (“Camastra Transcurrent Fault Zone”, CTFZ). Structural control of sedimentation by this structure is indicated by the geometry of the unconformity-bounded stratigraphic units as well as by the deformation of the basin fill. En-echelon N–S-trending thrusts, N- to NE-trending dextral transpressional faults, and NW- to NNW-striking folds affecting the substratum, constitute the surface response to lateral displacement along the CTFZ. The S. Arcangelo Basin is bounded to the north by a deep-seated, roughly WNW/NW-trending, sinistral transpressional structure (“Valsinni Lateral Ramp”, VLR) that terminates northwestward against the CTFZ. At the surface, the VLR is dominated by back thrusts and by transpressional fault zones. The VLR has been interpreted as the sinistral lateral ramp of the large-scale Valsinni thrust anticline and it is inferred to have strongly controlled local deposition within the S. Arcangelo Basin. The deep-seated structures (CTFZ and VLR) probably represent reactivated pre-existing normal and transfer faults affecting the rigid Apulian Platform that were generated during passive margin evolution, as well as during the flexural bending of the Apulian lithosphere. During the Pliocene–Pleistocene, these structures were reactivated as lateral ramps accommodating the thrusting within the Apulian Platform. Transpression along the lateral ramps in the Apulian Platform propagated upwards through the nappe pile, giving rise to a complex surface structural pattern and the local development of flower-like structures.

Evolution, distribution, and characteristics of rifting in southern Ethiopia

Southern Ethiopia is a key region to understand the evolution of the East African rift system, since it is the area of interaction between the main Ethiopian rift (MER) and the Kenyan rift. However, geological data constraining rift evolution in this remote area are still relatively sparse. In this study the timing, distribution, and style of rifting in southern Ethiopia are constrained by new structural, geochronological, and geomorphological data. The border faults in the area are roughly parallel to preexisting basement fabrics and are progressively more oblique with respect to the regional Nubia–Somalia motion proceeding southward. Kinematic indicators along these faults are mainly dip slip, pointing to a progressive rotation of the computed direction of extension toward the south. Radiocarbon data indicate post 30 ka faulting at both western and eastern margins of the MER with limited axial deformation. Similarly, geomorphological data suggest recent fault activity along the western margins of the basins composing the Gofa Province and in the Chew Bahir basin. This supports that interaction between the MER and the Kenyan rift in southern Ethiopia occurs in a 200 km wide zone of ongoing deformation. Fault-related exhumation at ~10–12 Ma in the Gofa Province, as constrained by new apatite fission track data, occurred later than the ~20 Ma basement exhumation of the Chew Bahir basin, thus pointing to a northward propagation of the Kenyan rift-related extension in the area.

Factors controlling foredeep turbidite deposition: the case of Northern Apennines (Oligocene-Miocene, Italy)

Abstract Three major controlling factors affect turbidite deposition in foredeep basins: tectonics in the source area, tectonics in the belt-basin system, and variations of sea-level (local or global). These factors are expected to have different effects on the volume, grain size, provenance and distribution of clastic sediments during the evolution of the basin. The interplay of these factors is investigated for the latest Oligocene-Middle Miocene Northern Appennines Foredeep turbidite wedges by means of turbidite-system-based lithostratigraphy and field mapping, integrated with nannoplankton biostratigraphy and sedimentary petrography. Almost all recognized turbidite systems, unless tectonically truncated, show an overall stacking pattern formed by a lower sand-rich, thickly bedded stage (depocentre stage) passing upward into mud-rich, thinly bedded stages, eventually grading up to mostly mudstone units (abandonment stage). This rhythmically repeated pattern is interpreted as the result of the abrupt switching on and off of coarse-grained input, coupled with an alternating increase/decrease of depositional rate recorded in all detected systems. Biostratigraphy makes it possible to distinguish the switching-off of turbidite systems due to depocentre migration (a new system is switched on basinward) from that due to a regional decrease of clastic input. Sandstone petrography records the compositional variation related to tectonically induced source reorganization. In the latest Oligocene-Middle Miocene NAF foredeep wedges, this integrated dataset allows us to recognize: two different phases of source tectonics in the latest Oligocene and the middle Burdigalian; two major episodes of basin tectonics and related depocentre shift in the latest Oligocene and the Langhian, plus a minor middle Aquitanian phase; and three intervals of reduced regional turbidite deposition during the Late Aquitanian, Middle Burdigalian and Early Serravallian, possibly linked to sea-level rises.

Compression-to-extension record in the Late Pliocene-Pleistocene Upper Valdarno Basin (Northern Apennines, Italy): structural and thermochronological constraints

We use new structural and apatite fission-track data together with apatite fission-track and (U-Th)/He data from literature to examine the tectonic evolution of the continental Upper Valdarno Basin, in the hinterland sector of the Northern Apennines fold-and-thrust belt. This basin is located in-between two structural ridges, the Chianti Mountains to the southwest and the Pratomagno to the northeast. In our interpretation, the Upper Valdarno Basin developed at ca. 3.4-3.3 Ma as pop-down synformal-shaped depression bounded and controlled by oppositely-verging thrust-related structures, namely the thrust system lifting the Chianti Mountains and the southwest-facing backfolds at the base of the Pratomagno. This evolution is compatible with the accelerated exhumation rates at 4-5 Ma documented through apatite fission-track data along both the Pratomagno and Chianti ridges. Shortening suffered by basin deposits is clearly manifested by the outcrop-scale reverse faults and thrust-related folds affecting the Late Pliocene sediments (Castelnuovo dei Sabbioni Synthem), which are well exposed in the Santa Barbara mine. These strongly folded deposits are overlain unconformable by Early Pleistocene sediments (Montevarchi Synthem), which display evidence for syn-depositional normal faulting. This suggests that the Upper Valdarno Basin experienced a phase of normal faulting that started at the base of Pleistocene (ca. 2.6-2.5 Ma) and likely produced the large southwest-dipping “Trappola Fault”, which affects the southwestern margin of the Pratomagno displacing the earlier backthrusts and backfolds. Basin evolution can be thus basically framed into a two-phase history, with extensional tectonics superposed onto compressional structures that were deactivated by ca. 2.7 Ma. Being the Chianti Mountains part of the 250 km-long regional line of thrusts and thrust-related folds (the so-called “Tuscan Nappe front”), the results of this study may also involve regional implications, as they would also hint for the tectonic history of other sectors and basins settled along-strike this regional element.

Integrated geological-architectural pilot study of the Biet Gabriel-Rufael rock hewn church in Lalibela, northern Ethiopia

We present a geological and architectural integrated pilot study, aiming at the preservation of the Biet Gabriel-Rufael church, located in Lalibela, the worldwide known Ethiopian rock hewn monumental site protected by UNESCO since 1978. The town developed since the Neolithic up to the medieval age, as inferred from the traces of three distinct architectural phases. Lalibela was built on a geological substratum made of rocks belonging to the Ethiopian Plateau suite, which is mainly composed of basalts of fissural origin or derived from shield volcanoes. The geological units are composed of alternating massive and scoriaceous basalts. The main scoriaceous basalt level, embedded within the massive basalts, is 30–40 m thick and corresponds to the horizon within which the Biet Gabriel-Rufael church all the other monuments of Lalibela have been carved. Therefore, the evolution of the town was strongly conditioned by the occurrence and extent of the softer scoriaceous basalt level. Many fracture systems of both natural (i.e. geological) and anthropic origin (these latter connected to the carving of the church), were recognized. The fracture pattern determined the subdivision of the church into different blocks that can behave independently, thus compromising the stability of the monument. A net of deformometers and fracture gauges was installed for the monitoring of the fracture system and a preliminary Finite Element analysis, following the approach used for underground excavations, was performed, with the aim of elucidate the mechanical behaviour of the rock. The integration between geo-mechanical approach to the rock mass and the architectural study of the critical situation due to the carving and connected to buildings, resulted in the precise individuation of future interventions devoted to the conservation of these monuments.

Tectonosedimentary evolution of the Plio-Pleistocene Sant’Arcangelo Basin (Southern Apennines, Italy)

Abstract This paper describes a new tectonosedimentary model for the evolution of the Plio-Quaternary Sant’Arcangelo Basin, located in the Southern Apennines of Italy. To this purpose, we carried out a new field survey of the basin fill, closely integrating stratigraphy, facies analysis, and structural and tectonic analyses. The geological map at 1:50 000 scale of the whole basin is the first result of this work. We present a new stratigraphic framework for the Sant’Arcangelo Basin succession, which has been subdivided into five major stratigraphic groups, classified as synthems, following the recognition of major basin-wide unconformities. The synthems include smaller-scale stratigraphic units, which are classified as depositional sequences or sub-synthems. These sub-units are composed of different lithofacies assemblages recording cyclic activation of fluvial, deltaic, shallow marine and lacustrine environments throughout the evolution of the basin. Integration of facies analysis and tectonic data led to the definition of a series of palaeogeographical sketches, encompassing the Piacenzian and late Pleistocene, which mark the main steps in the evolution of the Sant’Arcangelo Basin. Basin-scale hinterland-verging thrust faults and folds controlled the development of sub-basins and the progressive isolation of the Sant’Arcangelo Basin from the Ionian foredeep. The new model presented here defines the Sant’Arcangelo Basin as a triangular-shaped basin, bounded by oppositely verging thrusts. The hinterland-verging Valsinni thrust anticline limited its eastern margin and exerted a major control on the basin evolution.

Paleomagnetic evidence for a post-Eocene 90° CCW rotation of internal Apennine units: A linkage with Corsica-Sardinia rotation?

We report on an extensive paleomagnetic study (36 sites) of the Tuscan Nappe succession from the Northern Apennines Arc, aimed to reconstruct the tectonic evolution of the internal sector of this chain. We analyzed Eocene pelagic foreland ramp deposits (Scaglia Toscana Formation) and Oligocene–lower Miocene siliciclastic turbidites (Macigno and Falterona Formations). Paleomagnetic results show that the internal sector of the Northern Apennines underwent large counterclockwise (CCW) rotations with respect to the Adria-Africa foreland. A decrease in the rotation magnitude was observed from the southern to the northern sector of the arc (from 91 to 36°). This trend is opposite to that observed in the more external units of Northern Apennines and demonstrates that the oroclinal bending model, which has been proposed for the external units of the chain, is not appropriate to explain the evolution of the internal sector of the arc. On the basis of the observed paleomagnetic pattern, we propose a new tectonic model in which the Tuscan and Falterona-Cervarola units in the southern area were first rotated CCW along with the Corsica-Sardinia block during its lower Miocene rotational drifting and were later involved in the main phases of rotational emplacement and translation toward the outermost sector (Umbria domain), thus yielding the final curved shape of the Northern Apennines chain. Data from this study represent the first paleomagnetic evidence of the influence of the Corsica-Sardinia CCW rotation in the Apennines orogenic wedge deformation, in the general framework of the geodynamic evolution of the Central Mediterranean subduction system.