Vincenzo Picotti

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.

Messinian climate change and erosional destruction of the central European Alps

At the end of the Miocene, the European Alps ceased outward expansion, and tectonic uplift and exhumation shifted into the orogen interior. This shift is consistent with a change from orogenic construction to orogenic destruction, reflecting an increase in the ratio of erosional flux to accretionary flux. The coincidence of this change with an increase in sediment yield from the Alps suggests a climate-driven increase in erosional flux. The timing of deformation and sediment release from the southern Alps indicates that the tectonic change occurred synchronous with the last phase of the Messinian salinity crisis. We attribute the increase in erosional flux to a climatic shift to wetter conditions throughout Europe, likely augmented by the base-level fall that occurred during the Mediterranean dessication. This climate change is represented in the stratigraphic record by the Lago Mare deposits of the Mediterranean salinity crisis.

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.

Discriminating between tectonic and sedimentary burial in a foredeep succession, Northern Apennines

An extensive apatite fission‐track survey has been carried out on the Marnoso‐arenacea foredeep succession in the Northern Apennines. The data show a general decrease of the maximum paleotemperature undergone by the sediments toward the foreland areas. The maximum burial calculated by using a geothermal gradient of 20°C km−1 spans from more than 5 km to less than 2.5 km and indicates that the previously assessed total thickness of the Marnoso‐arenacea succession is not enough to justify the determined values. It is concluded that a now eroded Ligurian wedge, up to 5 km thick, was present on top of the Marnoso‐arenacea sediments.

Unraveling tectonic and climatic controls on synorogenic growth strata (Northern Apennines, Italy)

We develop a new high-resolution stratigraphic age model to unravel the contributions of tectonic and climatic processes on early to late Pleistocene synorogenic growth strata. We capitalize on excellent, continuous exposures along the flank of the Po foreland in northern Italy to elucidate hydrologic, geomorphic, and sedimentologic processes that are regularly attributed to, but rarely proven to be caused by, glacial-interglacial climatic changes and unsteady rock uplift. We perform our analysis on the Enza section, a succession of marine and terrestrial strata exposed along the Enza River, between Parma and Reggio Emilia, northern Italy. Bedding in the Enza section displays synorogenic growth strata geometry, with bedding dips that range from 2° to 55°, that becomes progressively shallower upsection. We develop an age model that incorporates biostratigraphy, magnetostratigraphy, rock-magnetic cyclostratigraphy, cosmogenic radionuclide burial dating, and optically stimulated luminescence dating and shows that the Enza section spans the interval between 0.04 and 1.65 Ma. Furthermore, the age model pins the time of deposition for several lithostratigraphic units of regional significance and shows that sediment accumulation was unsteady, ranging from 14–31 cm/k.y. in the marine part of the section to 5–362 cm/k.y. in the overlying littoral and terrestrial part of the section. Unsteady deposition is most pronounced in the terrestrial deposits where thick fluvial gravel packages accumulated in short (∼10–15 k.y.) time periods that coincide with Quaternary glacial intervals. There is direct evidence for a dominant tectonic control in the older, marine part of the section. Here, sediment accumulation rates on the limb of the fold growing along this portion of the Northern Apennine mountain front show that between 1.07 and 1.65 Ma, repetitive progradation of neritic sand units directly followed pulses of rapid, punctuated uplift. In contrast, the cyclic terrestrial facies variations in the Enza section reveal that once the section became emergent at ca. 1 Ma and uplift slowed, climate was the dominant control on sediment production and deposition.

Pliocene sequence stratigraphy, climatic trends and sapropel formation in the Northern Apennines (Italy)

Abstract The Pliocene stratigraphy of the Romagna Northern Apennines foredeep has been reconstructed through mapping and measuring the different lithostratigraphic units. The physically correlated sedimentary bodies have been chronologically calibrated through biostratigraphic analyses. Analysis of facies and fossil content allowed reconstructing the main depositional systems and their evolution. The co-occurrence of shallow- to deep-water environments and carbonate to siliciclastic systems allowed understanding fully the role of eustasy and climate vs. continuous tectonic deformation controlling the accommodation and basin morphology. Following a limited sea-level lowering at 4.2 Ma, associated with the first signal of cooling in the Early Pliocene, a fast growing subtropical type carbonate platform developed, in response to warm and oligotrophic surface waters. Furthermore, a significant drop in sea-level at 3.75 Ma points to an important Pliocene continental glaciation in the Northern Hemisphere. During the subsequent transgression, warm and possibly humid climate favoured the deposition of the first Pliocene sapropel around 3.3 Ma. The growth of a second generation of carbonate platform between 3.3 and 3.042 Ma suggests a renewed warm period and limited runoff. At the top of this highstand, thick sapropels developed, which can easily be correlated across the Mediterranean. The platform abruptly drowned, was overwhelmed by bio-erosion and was covered by iron–phosphate crusts; whereas in the adjacent basins numerous sapropel layers occurred, rich of biosiliceous forms, documenting the shift from Ccarb to Corg deposition. Strong oscillations of climate, due to precessionally controlled seasonality, and increased precipitation and runoff are considered as the main causes of the increase of surface water fertility. After the 2.87 Ma sea-level drop, again very significant, the sapropel rapidly disappears, in conjunction to the development of the lowstand wedge. After this cold period, which ended around 2.6 Ma, carbonate platforms never recovered due to the enduring cooler surface waters and the onset of an active drainage on the Apenninic range. This study documents the role of the eustatic variations on the Pliocene deposition of the actively deforming Northern Apennines foredeep. The abrupt lithostratigraphic changes appear also controlled by palaeoproductivity variations associated with climate and runoff.

The Miocene Petroleum System of the Northern Apennines in the Central Po Plain (Italy)

We describe the Miocene petroleum system in the context of the geology of the Northern Apennines as a system fed by multiple sources including some potential for deep oil accumulation. The presence of sources deeper than the Miocene reservoir is required by the high thermal maturity of the oils, the thermogenic nature of methane and the high ion content, in the reservoir brines, deriving from decaying organic matter. This is in contrast with the lower thermal maturity measured in the Miocene reservoir coupled with its low organic matter content. A Miocene secondary source, however, is required by the presence of a Tertiary organic marker in the oil. The deeper sources charged reservoirs of different age, geometry and sediment provenance, mostly as a function of stepwise migration of the foredeep and the overlying Ligurian units toward the foreland, which provided rapid overburden. The porosity of the reservoir was preserved in the anticlines mostly because of up-dip migration into early formed structures in the foredeep units. Therefore, the structural evolution of the area, especially the time interval between deposition and deformation of the foredeep units, is crucial for the definition of the quality of the reservoirs. Finally, the Quaternary reactivation of the thrust sheets in the foothills changed the geometry of the reservoirs, inducing new accumulations and/or dismigration from deeper and older traps.

Spontaneous fluid emissions in the Northern Apennines: geochemistry, structures and implications for the petroleum system

Abstract Natural seeps in the Northern Apennines document a variability of fluids and reservoirs in terms of origin, age and evolution. Their spatial distribution appears controlled by the presence or absence of the tectonic overburden provided by the Ligurian nappe. The general trend of deepening of the Mesozoic basement toward the internal part of the thrust belt is reflected by the nature of the seeps, characterized by thermogenic methane and oil at the foothills, whereas the innermost seeps show occurrence of dry thermogenic gas suggesting overcooking of the residual oil. At the front of the Ligurian nappe, or in places never covered by it, the seepages are associated with biogenic methane related to bacterial degradation of the organic-rich intervals occurring in the Pliocene and Pleistocene marine succession. The coupling of geochemical and structural analysis allows reconstructing the tectono-thermal evolution of the belt, improving our knowledge on the processes acting within the reservoir and controlling important parameters of the petroleum system, such as the reservoir porosity and its modifications, and the migration patterns.

Lithospheric weakening during "retroforeland" basin formation: Tectonic evolution of the central South-Alpine foredeep.

Beginning with the Late Cretaceous, convergence between Europe and Adria caused the southward subduction of the European plate beneath Adria. Contractional movements at high crustal levels were partly accommodated by southward thrusting (Southern Alps) causing flexure of the loaded Adriatic plate and the formation of the Late Cretaceous to Late Miocene South Alpine foredeep. With respect to the Alpine subduction system, the South Alpine foredeep is a retroforeland basin. In the Late Paleogene, dip values of the base foredeep varied between 3° and 5°. The shallower dips were found along profiles crossing domains which had undergone extension during previous Late Triassic to Middle Jurassic rifting. With ongoing convergence, the base foredeep along the entire basin steepened and dip values in the Tortonian are of about 7°–8°. The increase in dip goes hand in hand with an increase in the curvature of the loaded plate. This suggests a progressive weakening of the flexed plate. Modeled effective elastic thickness (Te) values derived from the analysis of plate curvatures decrease from 15–20 km in the Late Paleogene to <5 km in the Tortonian. Preconvergence values obtained from modeling studies were even higher, in the 24–27 km range. A practically complete decoupling of the upper/middle crust from its mantle substratum enabled the increase in curvature of the hinge zone. The progressive weakening of the South Alpine lithosphere around the bulge zone is correlated with the its “upper plate” position with respect to the Alpine subduction system.

Late Quaternary uplift and valley evolution in the Northern Apennines: Lamone catchment

Abstract The topographic evolution of mountain chains is fundamentally dictated by the balance of erosional and tectonic processes. In the Northern Apennines, several questions about the late Holocene to present deformation style and uplift rates are still open. We consider a typical NE facing drainage basin, focusing on all the geomorphic markers capable of supplying direct or indirect information regarding tectonic forcing on the landscape. Characteristics of altitude and relief, drainage pattern, river profile and fluvial response expressions were found to be in good agreement, indicating that the most recent tectonic activity consists of a general persistent uplift of the chain, associated with an extensional regime, where different sectors are distinguished through differential uplift. Such differences have been found to act both along and transverse to the chain, causing similar coherent patterns of response in erosional processes. The geomorphology of the catchment displays several features, which together with the long sustained tectonic forcing indicate the near-attainment of a dynamic equilibrium between the rates of tectonic forcing and erosional processes, creating steady-state topography.