Mario Boccaletti

Late Neogene evolution of the Taza-Guercif Basin (Rifian Corridor, Morocco) and implications for the Messinian salinity crisis

Abstract Magnetostratigraphic and biostratigraphic results are presented from Neogene deposits in the Taza–Guercif Basin, located at the southern margin of the Rifian Corridor in Morocco. This corridor was the main marine passageway which connected the Mediterranean with the Atlantic during Messinian times. Correlation of the biostratigraphy and polarity sequence of the Taza–Guercif composite section to the astronomical time scale, allows an accurate dating of three subsequent events in the Rifian Corridor. (1) The oldest marine sediments marking the opening of the Rifian Corridor were deposited at 8 Ma. At this age, a deep (600 m) marine basin developed in the Taza–Guercif area, marked by deposition of precession-controlled turbidite–marl cycles. (2) Paleodepth reconstructions indicate that a rapid (5 m/ka) shallowing of the marine corridor took place at the Tortonian/Messinian boundary, at an age of 7.2 Ma. This shallowing phase is primarily related to active tectonics, although a small glacio-eustatic sea level lowering also took place. (3) The Taza–Guercif Basin was emergent at an age of 6.0 Ma and, subsequently, continental sedimentation continued well into the Early Pliocene. We suggest that shallowing and restricting the marine passageway through the Rifian Corridor actually initiated the Messinian salinity crisis, well before the deposition of the Messinian evaporites in the Mediterranean.

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.

The Calabrian Arc and the Ionian Sea in the dynamic evolution of the Central Mediterranean

In the Central Mediterranean area it is possible to recognize three main realms: the Tyrrhenian Sea, the Pelagian block and the Ionian block. These domains are limited by important fracture zones that are easily recognizable on land and offshore. These fracture zones can be grouped into four main trends: (1) the NW-SE trend which is characterized by dextral horizontal movements; (2) the NE-SW trend which is generally characterized by sinistral lateral horizontal movements; (3) the E-W trend which also shows dextral shear movements; and (4) the N-S trend which is characterized by normal faults only. The Tyrrhenian Sea realm is characterized by a thinned crust of oceanic type and is connected to the Apenninic chain by means of listric faults. From a geophysical point of view, the area shows a high heat flow which follows the main structural trends, and positive gravimetric anomalies which are compatible with seismic refraction data and can be explained by the proposed model. The Pelagian block shows a continental crust, about 20 km thick, which progressively increases beneath the Sicilian chain. Here an abrupt interruption with the Tyrrhenian block takes place. In the Pelagian realm, rifting processes developed according to the NW-SE and N-S trend. The N-S trend characterizes the westernmost zone of the realm where volcanic processes and a relatively high heat flow can also be observed. The Ionian block is characterized by the following: a thinned crust (17–20 km) which increases (40 km) towards the Southern Calabrian chain; a thick sedimentary sequence; high positive Bouguer anomalies (+300 mGal); and very low heat flow (50 mW m−2). Geophysical and structural analyses allowed us to evaluate the post-Tortonian evolution of the whole area under consideration. This evolution resulted in continental crust formation according to a rigid—plastic deformation model. According to our model, the Ionian block can be considered as a domain, characterized by a thinned continental crust and affected by intrusive material derived from the upper mantle. Furthermore, this domain, of probably Jurassic age, can be considered as a pelagic basin bordered by the Pelagian block to the west and by the Apulian block to the east-northeast. This basin, according to its position with respect to the Apenninic and Sicilian chains, has not been affected by tectogenesis and still represents a site of continuous sedimentation. From a kinematic point of view, the Ionian realm can be linked with the Pelagian block shifting towards the east-northeast, whereas it is limited towards the north by an E-W fracture zone which regulates the opening of the Tyrrhenian Sea and the counterclockwise rotation of the Apenninic chain. According to the proposed model, the opening of the Tyrrhenian Sea can be considered as a megaextension-fracture which developed parallel to the maximum principal stress direction and evolved as a triangular-shaped structural feature. In preparing the model the authors have taken into account a large set of data collected both on land and offshore.

Successive orthogonal and oblique extension episodes in a rift zone: Laboratory experiments with application to the Ethiopian Rift

Small-scale modeling was performed to examine the effects of the superposition of two successive extensional phases from orthogonal to oblique (type 1) and from oblique to orthogonal (type 2). In both the type 1 and type 2 models, faults produced during the first stage strongly control fault development during the second stage. In type 1 models, the oblique faults developed during the second oblique phase are confined within a first-phase graben, whereas in type 2 models the oblique faults, produced during the first phase, continue to develop during orthogonal extension and connect with each other to give sigmoidal fault blocks. Type 1 models are compared with the structural setting of the Ethiopian Rift; the evolution of the rift is related to a recent extensional event, whose principal direction of stretching trends at around 50° to preexisting major normal faults. Type 1 laboratory models are fairly comparable to the northern sector of the Ethiopian Rift, referred to here as MER. They account for both the development of the en echelon oblique faults of the Wonji Fault Belt and the sinistral shear gradient running parallel to the eastern border of the MER, which formed during an oblique rifting extension. The statistical analysis of the whole Ethiopian Rift fault pattern by reference to the experimental data allows the determination of a N100°–N110° mean direction of stretching.

Quaternary oblique extensional tectonics in the Ethiopian Rift (Horn of Africa)

Abstract The Ethiopian Rift extends in a northeasterly direction, from Southern Ethiopia to the Afar region. It shows a complex fault pattern, characterised by the interplay of a N30°E—N40°E-trending border fault system with the Quaternary Wonji Fault Belt, which is constituted by right-stepping en-echelon NS to N20°E trending faults. The Wonji Fault Belt affects mainly the rift floor, but it also overlaps with some segments of the margins. Its en-echelon arrangement indicates a left-lateral component of displacement along the rift trend. The general fault pattern of the Ethiopian Rift, as well as mesoscopic fault analyses and structural features of some key areas, indicate the occurrence of a roughly E—W extension, which is compatible with the sinistral shear component of motion along the rift. This paper proposes that oblique rifting related to the E—W direction of extension has been active since the beginning of the Quaternary and that it came after an earlier phase of roughly pure extension orthogonal to the rift trend.

Considerations on the seismotectonics of the Northern Apennines

Abstract The Northern Apennines have been subdivided into homogeneous zones, on the basis of recent structural evolution and crustal structure, in which the earthquake distribution can find a coherent framework. These zones, whose physiography is in strict connection with their structure, are: the Internal Peri-Tyrrhenian Belt; the External or Main Belt; the Buried Belt; and the Pede-Alpine Homocline. Earthquake activity has a tendency to cluster along well-defined bands, particularly in the easternmost border of the Peri-Tyrrhenian Belt, as well as along the zone between the External Belt and the Buried Belt, i.e. along the Padanian margin of the Northern Apennines. A minimum of seismic activity seems to be correlated with some zones of the External Belt, as well as with the Late Tertiary and Quaternary magmatic province of Tyrrhenian Southern Tuscany. The fault-plane solutions are coherent with the structural picture. A tentative seismotectonic model of the Northern Apennines is discussed.

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.

Transtensional tectonics in the Sicily Channel

Abstract Structural, geophysical and volcanological data available for the Sicily Channel demonstrate that the whole area is characterized by the occurrence of transtensional structures activated in a dextral shear zone trending roughly WNW-ESE. These data allowed us to put forward a kinematic model of the area which accounts for both the existence of discrete tectonic depressions and for the localized volcanic activity of this sector of the Pelagian Sea. The proposed model is somewhat different from other rifting mechanisms available for the Sicily Channel in that it may explain the occurrence of extensional features and the associated volcanism in a zone of continental collision through the development of large-scale pull-apart basins involving deep crustal levels.

New data and hypothesis on the development of the Tyrrhenian basin

Abstract Among those basins which developed during the Neogene and Quaternary at the rear of the perimenditerranean mountain chains, the Tyrrhenian basin is the most recently formed. Comparing the available geological and geophysical data, the Tyrrhenian domain and its neighbourhoods can be divided into more or less homogeneous sectors: namely Northern and Southern Tyrrhenian, Apenninic chain, Maghrebides chain, Apulian and Iblean-African foreland, Ionian foreland. For each sector the different phenomena involved and the crustal characteristics are discussed. The evolution of the domain has been explained by the application of a crustal stretching model developed for continental margins. This approach predicts the formation of basinal areas as a result of the Europe/Africa collisional movements within a plastic-rigid deformation model. Taking into account the mass redistribution after collision, the model explains both the distension developed within the collisional system and the penecontemporaneous development of compressive and distensive phases. As a consequence of the intraplate stresses, horizontal shearings affecting the whole lithosphere at different levels, occur, with delaminations, asthenosphere uprising and gradual collapse in the upper crust. E-W trending and strike-slip dextral faults played a primary role in the evolution of the Tyrrhenian basin inducing the largest tearings at the end of the major transcurrent systems. Such an evolution is complicated by the migrating motion of the shear zone from N to S and by the penecontemporaneous stop in the extension from the northern towards the southern domains. Of outstanding importance is the shear zone which borders the Southern Tyrrhenian enabling the opening of the basin and allowing the development of the major group of en-echelon sub-basins with a NW-SE trend.

Plio-Quaternary volcanotectonic activity in the northern sector of the Main Ethiopian Rift: relationships with oblique rifting

Abstract Deformation and magmatism within the ∼90 km wide northern Ethiopian Rift system is concentrated along a narrow zone - the Wonji Fault Belt. Two key areas (the Nazret-Dera and Asela-Ziway areas), located along the eastern margin of the north-northeast to northeast trending Main Ethiopian Rift, have been investigated in order to reconstruct the recent tectonomagmatic evolution of the northern branch of the Main Ethiopian Rift. In these areas, Early Pleistocene volcanic products (Wonji Group) overlie Pliocene volcanic rocks (Eastern Margin Unit). Detailed stratigraphical reconstructions have revealed the presence of several tectonomagmatic units which can be correlated between the two study areas. The stratigraphical and petrological study of these units outlined (1) the bimodal composition (basalts-pantellerites) of the oldest and youngest units and the unimodal character (pantellerites) of the products erupted during the intervening period; (2) the mainly fissural origin of the ignimbrites and oldest basalts; and (3) a mafic/felsic volumetric ratio of 1:5. The geological data suggest that, around the Pliocene-Quaternary boundary, a change in the stress field occurred in this Main Ethiopian Rift sector, passing from a direction of extension roughly orthogonal to the rift shoulders, to oblique rifting related to an east-west trending extension. In this framework the change in the style of volcanism observed in the Nazret-Dera and Asela-Ziway areas can be related to the change of the stress field. A new geodynamic model is presented for the Late Pliocene to Recent evolution of this sector of the Main Ethiopian Rift. According to this model, a large volume of rhyolitic products was erupted during an oblique rifting phase, following a previous period of pure extension. The change in the tectonic regime favoured partial melting of the underplated basalts as a decrease in the pressure and an elevation of isotherms occurred.