Carlo Doglioni

On the post-25 Ma geodynamic evolution of the western Mediterranean

During the Neogene and Quaternary western Mediterranean geodynamics were dominated by the ‘eastward’ migration of the Apenninic arc and associated back-arc basins. The migration was controlled by retreat of the Apenninic slab and was associated with ‘boudinage’ of the lithosphere in the back-arc area. Palaeo-reconstruction of the kinematics of the arc suggests about 775 km of migration from the Late Oligocene to present along a transect from the Gulf of Lions to Calabria. A maximum of 135 km of N‐S converge occurred between Africa and Europe during the same time span. The western Mediterranean was thus mainly shaped by the migration of the slab related to west-directed subduction. It is hypothesized that minor N‐S convergence deformed the arc but was not the cause of its formation.

The Puglia uplift (SE Italy): An anomaly in the foreland of the Apenninic subduction due to buckling of a thick continental lithosphere

The Apenninic foreland shows two distinct structural signatures comparing the central Adriatic Sea and the Puglia region. During the Pliocene-Pleistocene the central Adriatic underwent high subsidence rates due to the eastward rollback of the hinge of the west dipping Apenninic subduction. The Puglia region and the Bradanic foredeep are located southward along strike in the same foreland, but, in contrast with the central Adriatic, after Pliocene-early Pleistocene subsidence they underwent uplift since the middle Pleistocene. The geometry and the kinematics of the frontal accretionary wedge and related foreland changed from that moment on between the two areas. At the front of the central northern Apennines, off scraping and subsidence continued, whereas the foredeep and foreland of the southern Apennines were buckled. Those differences are interpreted as being due to the larger subduction hinge rollback rate since middle Pleistocene of the central Adriatic lithosphere (70 km thick) with respect to the thicker Puglia (110 km). The different thicknesses of the continental crust and lithosphere were inherited from the Mesozoic rifting that disrupted the Adriatic plate. The different thicknesses appear to have controlled the variable degree of flexure of the lithosphere and its asthenospheric penetration rate. The Tremiti E–W alignment is the right-lateral lithospheric transfer zone of those different tectonic regimes. The consequent different dip of the subduction in the two sections (steeper west of Puglia) could also explain the lower elevation of the southern Apennines, compared to their central-northern sector.

Differential Rotation Between Lithosphere and Mantle' A Consequence of Lateral Mantle Viscosity Variations

The description of plate motions in the so-called hotspot reference frame introduces a global rotation of the lithosphere with respect to the mantle. This rotation, called toroidal field of degree 1, is roughly westward. It reaches an amplitude of about 2 cm/yr and has been consistently found in the different generation of plate tectonic models. Various authors have tried to relate this observation to the deceleration of the Earths rotation, to polar wander, or to tidal drag. However, these different physical mechanisms cannot explain the requested amplitude. In this paper, we compare the values of this rotation vector using different relative plate motion models expressed in the hotspot reference frame. In a model Earth with lateral viscosity variations, a differential rotation is predicted. The observed net lithospheric rotation is consistent with the dynamics of a model Earth where the asthenospheric viscosity below the oceans is at least one order of magnitude lower than underneath the continents. This relative westward drift of the lithosphere may account for the significant structural differences between east or west dipping subduction zones.

Orogenic Belts and Orogenic Sediment Provenance

By selecting a limited number of variables (westward vs. eastward subduction polarity; oceanic vs. continental origin of downgoing and overriding plates), we identify eight end‐member scenarios of plate convergence and orogeny. These are characterized by five different types of composite orogenic prisms uplifted above subduction zones to become sources of terrigenous sediments (Indo‐Burman‐type subduction complexes, Apennine‐type thin‐skinned orogens, Oman‐type obduction orogens, Andean‐type cordilleras, and Alpine‐type collision orogens). Each type of composite orogen is envisaged here as the tectonic assembly of subparallel geological domains consisting of genetically associated rock complexes. Five types of such elongated orogenic domains are identified as the primary building blocks of composite orogens: magmatic arcs, obducted or accreted ophiolites, neometamorphic axial belts, accreted paleomargin remnants, and accreted orogenic clastic wedges. Detailed provenance studies on modern convergent‐margin settings from the Mediterranean Sea to the Indian Ocean show that erosion of each single orogenic domain produces peculiar detrital modes, heavy‐mineral assemblages, and unroofing trends that can be predicted and modeled. Five corresponding primary types of sediment provenances (magmatic arc, ophiolite, axial belt, continental block, and clastic wedge provenances) are thus identified, which reproduce, redefine, or integrate provenance types and variants originally recognized by W. R. Dickinson and C. A. Suczek in 1979. These five primary provenances may be variously recombined in order to describe the full complexities of mixed detrital signatures produced by erosion of different types of composite orogenic prisms. Our provenance model represents a flexible and valuable conceptual tool to predict the evolutionary trends of detrital modes and heavy‐mineral assemblages produced by uplift and progressive erosional unroofing of various types of orogenic belts and to interpret petrofacies from arc‐related, foreland‐basin, foredeep, and remnant‐ocean clastic wedges.

Tectonics of the Dolomites (southern alps, northern Italy)

Abstract In post-Variscan times the Dolomites underwent a number of tectonic events, which may be summarized as follows: Permian and Triassic rifting phases broke the area into NS trending basins with different degrees of subsidence. A Middle Triassic transpressive event then deformed the region along a N70°E axis, generating flower structures within the basement. Volcano-tectonic domal uplift and subsequent caldera formation occurred at the same time as the Late Ladinian magmatism. Early Jurassic rifting also controlled the subsidence which increased eastward. This long period of deformation was followed by a pre-Neogene (Late Cretaceous-Palaeogene ?) EW (ENE-WSW) compression which generated a W-vergent belt, possibly equivalent to the folded foreland of the Dinaric chain. A 70 km EW section of the Dolomites indicates shortening of at least 10 km. During the Neogene the Dolomites, as far north as the Insubric Lineament, were the innermost part of a S-vergent thrust belt: the basement of the Dolomites was thrust southwards along the Valsugana Line onto the sedimentary cover of the Venetian Prealps for at least 10 km. This caused a regional uplift of 3–5 km. The Valsugana Line and its backthrusts on the northern side of the central Dolomites generated a 60 km wide pop-up in the form of a synclinorium within which the sedimentary cover adapted itself mainly by flexural-slip often forming triangle zones. The shortening linked to this folding is about 5 km with Neogene thrusts faulting and folding pre-existing thrust-planes. On the north-eastern side of the Dolomites, Neogene deformation is apparently more strictly controlled by the transpressive effects of the Insubric Lineament and shortening of the sedimentary cover may be greater than in the central Dolomites. Minor deformation linked to the Giudicarie belt is present in the western Dolomites. The present structure of the Dolomites is thus the result of a number of tectonic events of different significance and different strike. Only a 3-dimensional restoration can unravel the true structure of the Dolomites.

On the origin of west-directed subduction zones and applications to the western Mediterranean

Abstract West-directed subduction zones show east-verging arcs of 1500–3000 km. They are usually younger than 50 Ma and are characterized by a frontal accretionary wedge and a back-arc basin propagating together toward the east. The accretionary wedge scrapes off superficial layers of the downgoing plate (thin-skinned tectonics) whereas the back-arc extension cross-cuts the entire subduction hanging wall (thick-skinned tectonics). The slab of this type of subduction is steep to vertical and the hanging wall of the subduction has a mean elevation of 1000 m below sea level. Trenches and foredeeps are the deepest basins of the Earth and the mean depth is of 5000 m below sea level. West-directed subduction occurs both in case of the highest E-W convergence rates among plates (e.g. W Pacific examples) and no or very low convergence (e.g. Carpathians). Following Atlantic W-directed subduction examples, the W-directed subductions seem to develop along the back-thrust belt of former E-directed subduction zones, where oceanic lithosphere occur in the foreland to the east with the narrowing of the American continents. This could be applied to the onset of the Apennines subduction along the back-thrust belt of the Alpine-Betic orogen where Tethys oceanic crust was present. The Alpine orogen was stretched and scattered in the Apennines back-arc basin. The back-arc extension is internally punctuated by necks (sub-basins) and boudins (horsts of continental lithosphere). Asymmetric extension in the back-arc basin appears controlled by differential drag between the eastward mantle flow and the overlying passively transported crustal remnants. Compression in the accretionary prism may be interpreted as the superficial expression of the shear occurring between the downgoing lithosphere and the horizontally moving mantle which compensates the slab roll-back. The area of the Apennines appears lower than the area of the sedimentary cover before subduction: this favours the idea that not significant crustal slices have been involved in the Apenninic accretionary prism, and the basement thrust sheets included in the western part of the belt are mainly relicts of the Alpine-Betic orogen.

Some remarks on the origin of foredeeps

Geological observations show that there are two main types of foredeep basins on the base of their subsidence rate and their relationship with the associated orogen or accretionary wedge. High rates of subsidence up to 1600 m/Myr and a ratio lower than 1 between the area of the elevated belt and the area of the basin characterize the foredeeps associated with W-dipping subduction. Low rates of subsidence up to 300 m/Myr and a ratio higher than 1 between the area of the orogen and the area of the basin characterize the foredeeps associated with E- or NE-dipping subductions. This observation enables us to interpret the slow filling of foredeeps of the first type and the much faster filling of foredeeps of the second type. Moreover, “W-dipping” subductions have a steeper and faster monocline bounding the base of the deep foredeep in contrast to “E- or NE-dipping” subductions which have a shallower monocline. This differentiation supports the notion of the “eastward” mantle flow with respect to the lithosphere detected in the hot-spot reference frame. In this view, the foredeep depth in W-dipping subductions is mainly controlled by the roll-back of the subduction hinge pushed by the relative “eastward” mantle flow while foredeep depth in E- or NE-dipping subductions is instead mainly generated by the load of the thrust sheets and by the roll-back of the subduction hinge due to the advancing upper plate, contrasting the upward push of the mantle. The shape of the foredeeps is regularly arcuate in case of W-dipping subduction, while is linear or following the shape of the inherited continental margin in case of E- or NE-dipping subduction. Fold development in the two opposite foredeeps is significantly different: in W-dipping subduction the folds are transported down in subduction while they are forming, and consequently they are poorly eroded. Syntectonic sediments drape a preserved fold. In the E-NE-dipping subduction, folds and thrust sheets are instead uplifted and deeply eroded. This is also predicted by the envelope of the crests in the two distinct accretionary wedges: in the W-dipping case the envelope may dip toward the subduction, while it is rising toward the hinterland in the other cases. This approach may explain the differences in terms of geometries, amount and rates of subsidence between the foredeeps around the Adriatic plate. For instance the foredeep of the Southern Alps (back-thrust-belt foredeep) is deformed in the western part by the later Apenninic foredeep development, associated with the sinistral oblique ramp of the W-dipping Adriatic plate subduction. The relationship in the frontal parts of a thrust belt between accretion and eustacy has to be analyzed in terms of when, where and how fast thrusts propagation took place with respect to the randomly distributed sea-level third-order fluctuations, and by the type of subduction which is connected to the foredeep and thrust belt or accretionary wedge. Therefore the stratigraphic patterns in foredeep basins are peculiar and casual successions that record the combination of regional tectonic evolution with independent wider-scale sea-level changes.

Geological remarks on the relationships between extension and convergent geodynamic settings

Abstract In convergent geodynamic settings, extensional and compressional tectonics occur contemporaneously or in different times and places, but they are deeply linked to each other in a number of different ways. The main types of extensional regimes here considered are for west-dipping subduction zones: (1) back-arc extension (e.g., Tyrrhenian Sea); (2) uplift and related extension due to asthenospheric wedging (e.g., Apennines); (3) extension at the subduction hinge (e.g., Marianas); (4) extension due to increasing length of arcs (e.g., Apennines). For east-, northeast- or north-northeast-dipping subduction zones: (5) extension induced by collapse of orogens (e.g., Alps, Himalayas); (6) extension induced by differential lithosphere-asthenosphere coupling in the hanging wall of a subduction zone (e.g., Aegean rift); (7) rifting induced by orogenic roots anchored into the mantle (e.g., Atlantic and Tethys). Other types may be considered: (8) inversion of plate motion, from convergent to divergent relative plate vectors due to external velocity fields (e.g., Basin and Range); (9) transtension may occur along oblique ramps of thrust arcs due to differential forward propagation of thrusts planes (e.g., southern arm of the Barbados accretionary prism?); transtension and transpression also occur contemporaneously along an undulate strike-slip setting, or extension can accompany foreland dipping duplexes; (10) apparent extension due to down-section cut of thrust planes along subduction zones. Each type of extension generates different shape, depth and relationships between hanging wall and footwall of the associated normal faults and shear zones.

The dip of the foreland monocline in the Alps and Apennines

The foreland monocline dips underneath thrust belts and accretionary wedges, both in oceanic and continental subduction zones. We present new data on the dip of the monocline in the frontal part of two orogens, the Alps and the Apennines. There is an overall difference between the dip of the relative monoclines, and there is also a strong lateral variation along both arcs. In the Alps, the regional dip varies between 0° in the remote foreland, to an average of 2–3° at the front of the thrust belt below the foredeep, to about 5° beneath the external thrust-sheets within 40 km from the leading edge of the accretionary wedge. The regional dip of the monocline in the Apennines has an average of 4–5° at the front of the thrust belt below the foredeep, to about 10° beneath the external thrust-sheets within 40 km from the leading edge of the accretionary wedge. There are areas where the dip exceeds 20°. The Apennines though topographically lower than the Alps present higher monocline dips and a deeper foredeep. Moreover, there are variations in the dip of the monocline moving along the strike of the two belts: the low values coincide with Permian–Mesozoic inherited horsts, whereas the steeper values correspond to basinal areas, and they usually match the salients of the thrust belt front. Within the salients the distance between thrust ramps increases. Therefore, there are two orders of mean values of the dip of the foreland monocline, the first at the orogen scale (more than 1000 km wavelength), the second at the regional scale (100–200 km wavelength) within the single orogen. Lateral variations in the lithospheric buoyancy due to the inherited Mesozoic stretching may explain the second order variations in foreland dip, but not the first order mean values which seem to be more sensitive to the geographic polarity of the subduction rather than to the lithospheric composition which is rather similar in the Alpine and in the central-northern Apennines slabs.

The global tectonic pattern

Abstract The relative motion vectors between the lithosphere and the underlying mantle appear to follow global flow lines which can be constructed by linking axes of extension and compression over the Earths surface. The flow lines for the last 40 Ma are generally WNW-ESE (E-W), with an undulation of an about 15,000 km wavelength, showing a gradual and progressive variation in orientation. The undulation, which is sharper to the east, may reflect the mantle flow around an unstable rotation axis. The westward motion of the lithospheric plates could be interpreted as a result of differential angular velocity induced by the deceleration of the earths rotation or, in a toroidal field, by the effects induced by lateral heterogeneities both in the lithosphere and in the mantle. In this light, plate tectonics is a consequence of variable decoupling at the base of the lithosphere as a function of mantle anisotropies. Simply stated, when there is compression or transpression between two plates, it is the eastern plate which moves more rapidly westwards relative to the underlying mantle. If there is extension or transtension, it is the western plate that moves faster westwards. Lithospheric subduction, especially if it dips westward, produces an obstacle to the eastward flow of the mantle. This is referred to as the Nail Effect. The eastward roll-back of the subduction hinge due to the mantle push will generate back-arc extension. Subductions following the mantle flow (E or NE-dipping) are associated to thicker thrust belts with huge exposures of basement rocks in the hinterland and shallow foreland basins. The subductions contrasting the mantle flow (W or SW-dipping) are characterized by shallow thrust belts with deep foreland basin and coeval extension in the back. E-dipping subductions are passive responses to actively thrusting plates: the base plate and intra-lithospheric decollements are connected to the surface and can uplift deep rocks. The W-dipping subductions are enhanced by the eastward mantle flow: the base plate detachments are folded, subducted themselves and nerver connected to the surface, and only superficial detachments occur at the plate boundaries. First-order tectonic features appear to be perpendicular or slightly oblique to the global flow lines. Second-order tectonic features are related to localized rotations of plates. The model is applied to the Mediterranean area, where there are several observations that are not easily explicable in terms of standard plate-tectonic processes: the eastward or northeastward relative migration of the underlying mantle with respect to an inhomogeneous disrupted lithosphere could explain the tectonic evolution of this area.