Giovanni Bertotti

From rifting to drifting: tectonic evolution of the South-Alpine upper crust from the Triassic to the Early Cretaceous

Abstract The tectonic evolution of the South-Alpine rifted margin is discussed on the base of four palinspastic upper crustal profiles. Extension, related to the movements between Adria and Europe, began in the Norian after Variscan orogeny, Late Carboniferous to Middle Permian orogenic collapse and continental-scale wrenching. From the Late Triassic to the Early Liassic stretching was mostly limited to the Lombardian basin. During this time, extension was mainly controlled by four major listric faults, symmetrically centred around the Late Carboniferous—Early Permian Collio grabens. Smaller faults which also started in the Norian, were progressively de-activated during the Late Triassic. After the Middle Liassic, faulting in the Lombardian basin gradually ceased and the site of extension shifted westwards, i.e. towards the future site of crustal separation. Extension was then controlled by a set of west-dipping normal faults. Oceanic crust was formed not later than 157 Ma. The overall extension along the profile (which had a final length of 290 km) was 52 km which corresponds to a stretching factor of 1.22 for the whole length of the preserved margin. The strain rate is ca. 1 × 10 −16 / s . The sedimentation history suggests that the extension of the South-Alpine margin was a continuous process from the Norian to the Middle Jurassic. The changes in tectonic pattern are related to progressive hardening of the lithospheric segment undergoing slow extension.

Thermo-mechanical controls on the mode of continental collision in the SE Carpathians (Romania)

Abstract The Carpathians orogenic system, with its along-arc variations in topography developed in the aftermath of continental collision, is associated with unusual foredeep basins, large-scale strain and seismicity concentration and high-velocity mantle bodies. The East Carpathians continental collision was non-cylindrical, leading to large-scale variations in thrust nappe kinematics, orogenic uplift patterns and foredeep subsidence, controlled by the mechanics and geometry of the lower plate. Thermo-mechanical modelling demonstrates that in this low-rate convergence regime, the subducted lithosphere had enough time to interact with the mantle to advance towards a thermal resettlement. This is favored by the low degree of metamorphism, mechanical weakness of the lower plate and the lack of active surface processes at the contact with and in the upper plate. In contrast, low-buoyant, thick lower crust and active surface processes keep the continuity of the slab intact and promote the development of typical foredeep basins. The model explains in a self-consistent manner the unusual geometry of the Vrancea seismogenic slab in the bend zone of the Romanian Carpathians. The model is also consistent with the presence of two high-velocity bodies inferred from seismic tomography studies and explains the depth zonation of seismicity in the Vrancea area. Differences between the northern part of East Carpathians and the southeastern bend of the Carpathians arc are largely controlled by lateral variations in crustal structure, topography emplacement and surface processes along the arc. Mechanical heterogeneity of the Carpathians subduction leads to the development of two end member modes of collision, allowing a study of these states and their transition. Lithospheric configuration and tectonic topography appear to be prime factors controlling variations in slab behavior. In the SE Carpathians, at the terminal phase of continental convergence, slab delamination, roll-back and depocenter migration appear to play a more limited role at shallow and lithospheric levels.

Tertiary tectonic evolution of the external East Carpathians (Romania).

Abstract Paleostress calculation and analysis of mesoscopic structures are integrated with depth interpreted geological profiles based on seismic studies and well correlation to derive a Tertiary tectonic model for the East Carpathians. Following Early Miocene and older orogenic phases, the first tectonic event that affected the studied area is characterised by a WSW–ENE-oriented shortening of Middle Miocene (Late Burdigalian) in age. Resulting deformations induced ENE-ward thrusting of Tarcau and Marginal units, as well as the internal part of the Subcarpathian nappe. A second shortening event with an E–W to WSW–ENE contraction direction took place in the Late Miocene (Sarmatian), characterised by further foreland thrusting of the Subcarpathian nappe and out-of-sequence deformation in the Tarcau and Marginal Folds nappes. Along strike, differences in deformation mechanisms are controlled by the friction coefficients along the main detachment layers, by the lateral variations in the wedge thickness and by the involvement in the northern part of the thrusting system of the thick, competent East European platform. Tear faulting occurred in both tectonic events, the main resulting structure being the triangle zone developed south of the Trotus valley. The Latest Miocene (Latest Sarmatian)–Early Pliocene is characterised by a strike–slip stress field with NNE–SSW compression and WNW–ESE tension axis, left-lateral faults being dominant. The last deformation which affected the studied area is characterised by NNW–SSE shortening during the Pliocene, major deformations taking place mainly in the SW-most bending zone.

Subsidence analysis and tectonic evolution of the external Carpathian-Moesian Platform region during Neogene times

Abstract The Miocene–Pliocene subsidence of the tectonic platforms in the Romanian Carpathians foreland is analysed using standard 1D backstripping techniques for individual wells, combined in two regional sections and six contour maps. The subsidence patterns were integrated together with previous paleostress and kinematic studies, in order to derive the Tertiary kinematics of the buried faults in the Carpathians lower plate. The study revealed accelerated subsidence during the Early Miocene in the western part of the Moesian Platform/Getic Depression, in direct relationship with the opening of a WSW–ENE trending extensional basin. The largest subsidence recorded in the front of the Carpathians took place during the Late Miocene, due to final E-ward emplacement of the thrust sheets. The Late Miocene subsidence showed anomalous high values between the Intramoesian and Trotus faults as a result of the orogenic collision with the East-European Platform northward and acceleration of the subduction process in the SE Carpathians corner. Further Pliocene subsidence continued only in the latter region, the depocenter being shifted southward near the junction with the South Carpathians foreland.

Extension controls Quaternary tectonics, geomorphology and sedimentation in the N-Apennines foothills and adjacent Po Plain (Italy)

Abstract In the middle Pleistocene, thrusting in the N-Apennine fold-and-thrust belt came to an end and foredeep sedimentation in the adjacent Po Plain basin ceased. A new tectono-sedimentary regime was installed. Extensional faults were activated in the foothills and are limited to the northeast by a major, SW-dipping normal fault, the Bologna fault for which a vertical displacement of >1000 m is estimated. The region northeast of the fault, i.e. its footwall, was progressively tilted towards the Po Plain. The SW sectors of the tilting region were progressively uplifted above sea level, subjected to erosion thereby feeding a deltaic system which prograded towards the northeast in the subsiding regions. Compressional structures without major deformation are recorded in the zone of maximum curvature. All these features are explained by the onset of extensional deformation along the Bologna fault and consequent flexural unloading of its footwall. The evidence, therefore, shows that the transition region between the N Apennine foothills and Po Plain has been under a tensional stress regime since the middle Pleistocene.

Episodic exhumation in the Western Alps

Oligocene to Miocene clastic sediments of the Tertiary Piedmont Basin (northwest Italy) were derived from erosion of Western Alps source rocks. Detrital white micas from dif- ferent stratigraphic units and from sands of three present-day rivers draining the internal Western Alps have been analyzed by 40 Ar/ 39 Ar geochronology. Our data suggest a wide- spread, fast cooling and exhumation event prior to ca. 38 Ma followed by a .30 m.y. period of slower cooling and exhumation combined with erosion of crustal rocks with uniform 40 Ar/ 39 Ar signatures. These processes have resulted in a pattern of regularly increasing lag time up section.

Rifted margin formation in the south Tyrrhenian Sea: A high‐resolution seismic profile across the north Sicily passive continental margin

A new, 150 km long seismic line across the continental margin of north Sicily has been acquired and interpreted. The overall structure of the margin is controlled by extension, which caused crustal thinning and widespread normal faulting. Two main thinned zones are observed in the south in correspondence with the Cefalu basin and farther to the north at the continent-ocean transition. Zones of thinned crust coincide with zones of intense normal faulting. Extension began in late Tortonian times and caused the opening of the Cefalu basin controlled by a northward dipping listric fault. Messinian stretching affected most of the future margin and provoked a widening of the Cefalu basin and normal faulting in the north. Following a phase of relative quiescence in the early Pliocene, renewed extension determined further opening of the Cefalu basin and subordinate normal faulting in the north. Here, however, the record is unclear because of the emplacement of the calc-alkaline Sisifo volcano with associated volcanoclastic deposits. Breakup took place in the late Pliocene and was followed by the deposition of postrift Pleistocene sediments. At the lithospheric scale the sites of extension/thinning did not migrate during rifting. On the smaller scale, on the contrary, the Cefalu basin displays a remarkably systematic pattern of migration toward the foot-wall of the listric fault, which controlled the opening of the basin. The spacing of 4–6 km between faults is also quite systematic. Elongation experienced by the continental part of the margin (presently ∼97 km) has been derived by comparing the present-day and the preextensional lengths and is ∼10 km. The corresponding strain rate is 5×10−16 s−1.

Thermo-mechanical modeling of the Tyrrhenian Sea: Lithospheric necking and kinematics of rifting

We present the results of quantitative forward modeling of the Sardinian rifted margin of the Tyrrhenian Sea. The purpose of this study is to investigate the thermo-mechanical structure that affects the thinning of the lithosphere across the margin. The role of lithospheric necking during basin formation, constrained by basement topography, Moho depth, and gravity anomalies, is modeled for different compensation models. Independent constraints are obtained from the analysis of the predicted thermal structure. A deep level of necking (25 km) is required to explain the observed crustal geometries and gravity anomaly signature. The model predicts spatial and temporal variations in rheology during extension with important implications for kinematics of lithospheric thinning. Prerift lithospheric conditions, strain rate, and temperature during extension appear to be the key controls on the style of lithospheric rifting and necking in the Tyrrhenian Sea. A forward model for basin stratigraphy is presented for the polyphase history of the Tyrrhenian rifting. This model provides quantitative estimates of time-space dependent crustal thinning and rates of extension.

Pre-orogenic tectonics in the Umbria-Marche sector of the Afro-Adriatic continental margin

Abstract The pre-orogenic deformation of the Afro-Adriatic paleomargin during Jurassic–Paleogene times is recorded in the Umbria–Marche regions of Central Italy by facies differences, lateral thickness variations and, most importantly, by faults. Structural data suggest the persistence of normal faulting not only during rifting (Late Trias–Bathonian) but also during drifting and more precisely in Late Cretaceous times. Syn- and post-rift extension caused the development of mainly NNW–SSE- and ENE–WSW-trending structures. Post break-up normal faults are found in several localities of the Apennines and generally coincide with older, syn-rift sites of stretching. One-dimensional numerical modelling of subsidence suggests that thinning during rifting was about 15%, compatible with the stretching factor reconstructed from geological sections. Late Cretaceous thinning was obviously less but still significant and estimated at ca 5%. The acquisition and analysis of the stratigraphic and structural data presented in this study allowed us to correlate the subsidence history of the Umbria–Marche basin with the tectonic events that occurred in pre-orogenic times, in this sector of the Apennines, thus deriving useful indications about the pre-Oligocene evolution of this segment of the perimediterranean mountain belt.

Tertiary tectonic evolution of the external South Carpathians and the adjacent Moesian platform (Romania)

Depth-interpreted seismic sections of the Getic Depression foredeep, paleostress indicator data and analysis of outcrop- to regional-scale structures are integrated to derive the tectonic evolution of the South Carpathians - Moesian platform area. Following Late Cretaceous and older orogenic phases, the South Carpathians - Moesian platform area underwent strike-slip deformation with NE-SW oriented compression and NW-SE tension. In Paleogene to Early Burdigalian times, tensional deformation is recorded which led to the opening of WSW-ENE to E-W trending extensional basins. In the Late Burdigalian, NE-SW oriented contraction took over causing the oblique inversion of preexisting extensional structures. During Sarmatian times, NW-SE and slightly younger N-S trending compression caused the activation of mainly NW-SE dextral strike-slip faults and, in the frontal areas, south directed thrusting. The NW-SE direction of extension determined for Paleogene to Early Burdigalian times is hardly compatible with presently accepted models of substantially continuous dextral wrenching between the Intra-Carpathians units to the north of the South Carpathians and the Moesian platform to the south. In contrast, we have demonstrated dextral transpressive to transtensional movements within an E-W trending corridor from the Late Burdigalian to Late Sarmatian which are compatible with available models.