Constraining the onset of flexural subsidence and the rate of forebulge-foredeep migration in the foreland basin system of the central-southern Apennine belt (Italy) by Sr-isotope stratigraphy

In fold and thrust belts developing at convergent margins, the migration of the advancing wedge is accompanied by bulging of the downgoing plate, followed by the development of a foredeep basin filled by a thick succession of syn-orogenic sediments. The transition from forebulge to foredeep marks a key moment in the evolution of the orogenic system. In deep-water environments, the record of this transition is typically complete and progressive. Conversely, in the shallow-water/continental environment of many collisional systems, the uplift of the forebulge area can imply emersion and erosion, obliterating the stratigraphic record of key steps of the evolution of the orogenic system. The Apennines are a retreating collisional belt where the foreland basin system, in large domains, is floored by a subaerial forebulge unconformity developed due to bulge uplift and erosion. This unconformity is overlain by a diachronous sequence of three lithostratigraphic units made of: (i) shallow-water carbonates, (ii) hemipelagic marls and shales, and (iii) siliciclastic turbidites. Typically, the latter have been interpreted regionally as the onset of syn-orogenic deposition in the foredeep depozone, while little attention has been given to the underlying units. Accordingly, the rate of migration of the southern Apennine foreland basin-belt system has been constrained, so far, exclusively considering the age of the turbidites, which largely postdate the onset of sedimentation in the foredeep depozone. This thesis provides new high-resolution ages obtained by strontium isotope stratigraphy applied to the low-Mg calcite of bivalve shells sampled at the base of the first syn-orogenic deposits overlying the Eocene-Cretaceous pre-orogenic substratum. This new regional dataset of high-resolution ages obtained from the detailed analysis of 203 samples collected from 15 sites (37 sub-sites) across the central-southern Apennines, integrated with previously published data, provide a comprehensive spatial-temporal evolutive model of the Apennine belt and foreland basin system from the early Miocene to the Recent. In particular, this dataset indicates progressive rejuvenation of the strata sealing the forebulge unconformity toward the outer portions of the belt. Plotting the data on a restored section of the pre-orogenic Adria passive margin reveals that the age of the forebulge unconformity linearly scales with the position of the analyzed sites in their pre-orogenic position, pointing to an overall constant velocity of migration of the forebulge wave in the last 25 Myr. A comparative analysis of previously used datasets reveals that dating the base of the post-bulging carbonates represents the best tool to constrain the style and rate of the foreland flexuring

Biostratigraphic investigations assisted by virtual outcrop modeling: a case study from an Eocene shallow-water carbonate succession (Val Rosandra gorge, Trieste, NE Italy)

Virtual outcrop modeling has emerged as a tool for supporting geological field activities such as geological mapping and stratigraphic investigations. Here we show how this technique can be used to support the detailed stratigraphic logging and sampling with a case history from the Eocene carbonate platform succession exposed in the Val Rosandra gorge, in the vicinity of the city of Trieste, NE Italy. The biostratigraphic analysis highlighted the occurrence of Shallow Benthic zones (SBZ) 10 to 12 and the planktonic zones E7/E8. An upwards -deepening trend, from inner platform to a hemipelagic domain, is observed through the studied stratigraphic interval and is in accordance with the vertical evolution recorded in other Eocene successions of the Adriatic Carbonate Platform. Aerial drone imaging was used to produce a virtual outcrop model of the studied succession that provided a highresolution geometrical framework for field measurements, sample geotagging and observations. For instance, the virtual outcrop model assisted in determining the true thickness of beds, a task that can be subject to significant imprecisions when measurements are taken by hand. Ultimately, the integration of virtual outcrop modeling with classical sampling and measuring methods resulted in accurate stratimetry and in the precise spatial positioning of samples that were taken for biostratigraphy and facies characterization

Stratigraphy and facies of the Apennine Carbonate Platform (southern Italy): the record of Mesozoic OAEs and Miocene transgression.

The Apennine Carbonate Platform of southern Italy witnesses nearly 150 Myr (Late Triassic to Late Cretaceous) of shallow-water carbonate sedimentation in the subtropical central Tethys. During this field trip, you will have a look at the stratigraphy and facies across some key intervals of global palaeoenvironmental perturbation and at some important stratigraphic discontinuities in the history of the platform. During the first day, you will visit three localities of the Matese Mts. At San Lorenzello you will look at Milankovitch cyclicity expressed in Lower Cretaceous (Valanginian-Hauterivian) peritidal carbonates. At Pietraroja you will have a look at the foramol facies of the Cusano formation, marking the Early Miocene transgression on top of the eroded Cretaceous substrate. Moving north, at the Regiapiana you will see the abandoned mines that exploited the mid-Cretaceous karst bauxites, which mark a prolonged subaerial exposure. Moreover, you will walk through the Middle Miocene synorogenic history of the platform, from its exposure and erosion in the forebulge to its incipient flexural subsidence, highlighted by the Lower Miocene carbonates of the Cusano formation, to its drowning below the photic zone, marked by a phosphatic hardground overlain by the "Orbulina marls" of the Longano formation. During the second day, you will head south toward Salerno. In a quarry at Mercato San Severino, you will look at the record of the early Toarcian oceanic anoxic event, marked by the abrupt demise of lithiotid bivalves and dasycladalean algae, the major carbonate producers of the Lower Jurassic carbonate platforms. At the base of Mt Tobenna, you will look at the Aptian Orbitolina level: a marker bed whose palaeoenvironmental meaning has been long debated. From Mt Tobenna you will move south toward Monteforte Cilento where you will look at the record of the Cenomanian-Turonian OAE2 in the Apennine Carbonate Platfor

Geological record of the transition from induced to self-sustained subduction in the Oman Mountains

Along convergent plate boundaries, the negative buoyancy of the lithosphere pulls the slab into subduction. Bending and offscraping of the downgoing plate are processes occurring at subduction zones and acting against plate motions. These localised dissipative processes cause extensional deformation in the bulge-foredeep region and thrusting and folding in the thrust wedge respectively. Within this framework, widespread early subduction-related extensional structures affecting pre-orogenic rocks of the downgoing plate of fossil subduction systems, are commonly interpreted as induced by extension occurring in the forebulge-foredeep zone. Slab pull is to date, rarely considered as a potential causative process when interpreting basin-scale pre-shortening extensional structures. The problem of distinguishing slab-pull and foreland flexuring induced extensional structures relates to the fact that for most belts, slab pull and forebulge-foredeep flexuring are expected to produce extension roughly in the same direction (i.e. parallel to the foredeep-belt system) and, when syn-kinematic strata are not available, discriminating between these two processes is arduous. In this work we present a field investigation of basin-scale extensional faults from the downgoing plate of the Oman Mts. fossil subduction system. Syn-kinematic strata indicate that normal fault development largely predated extension in the bulge-foredeep region. Herein, we argue that such faulting occurred during the transition from induced to self-sustained subduction, when the negative buoyancy of the slab started to exceed the resisting forces and the downgoing plate began to be pulled towards the trench. \ua9 2019 Elsevier Lt

Constraining the Passive to Active Margin Tectonics of the Internal Central Apennines: Insights from Biostratigraphy, Structural, and Seismic Analysis

The polyphase structural evolution of a sector of the internal Central Apennines, where the significance of pelagic deposits atop neritic carbonate platform and active margin sediments has been long debated, is here documented. The results of a new geological survey in the Volsci Range, supported by new stratigraphic constraints from the syn-orogenic deposits, are integrated with the analysis of 2D seismic reflection lines and available wells in the adjacent Latin Valley. Late Cretaceous syn-sedimentary faults are documented and interpreted as steps linking a carbonate platform to the adjacent pelagic basin, located to the west. During Tortonian time, the pelagic deposits were squeezed off and juxtaposed as mélange units on top of the carbonate platform. Subsurface data highlighted stacked thrust sheets that were first involved into an initial in-sequence propagation with top-to-the-ENE, synchronous to late Tortonian foredeep to wedge-top sedimentation. We distinguish up to four groups of thrust faults that occurred during in-sequence shortening (thrusts 1–3; about 55–60 km) and backthrusting (thrust 4). During Pliocene to recent times, the area has been uplifted and subsequently extended by normal faults cross-cutting the accretionary wedge. Beside regional interest, our findings bear implications on the kinematic evolution of an orogenic wedge affected by far-traveled units

Polyphase out-of-sequence thrusting and occurrence of marble detritus within the wedge-top basin deposits in the Mt. Massico (southern Apennines): insights into the late Miocene tectonic evolution of the central Mediterranean

We present a geological survey of the southwestern sector of the Mt. Massico (southern Apennines), which allowed us to reconstruct the stratigraphy and tectonic architecture of this area. The Mt. Massico is a key area to study the out-of-sequence thrusting stage that affected the southern Apennine chain since the late Miocene, which despite the wide occurrence of these structures in the whole chain, is a tectonic process up to now poorly investigated. This area deserves to be analyzed also for the occurrence of marble detritus within a wedge-top basin deposit, representing the only place in the southern Apennines, where such a kind of metamorphic rocks occurs. Historically, this area is known for the extraction of an ornamental stone, named Mondragone Marble. The marble is hosted as olistoliths and clasts within the deposits of the Caiazzo Fm. unconformably covering a Jurassic–middle Miocene succession, mostly made of shallow water-to-deep-basin carbonates. The wedge-top basin deposit mainly consists of a chaotic assemblage of conglomerates, olistostromes, and olistoliths of Meso-Cenozoic limestones, deep-basin rocks, and marbles, embedded in a quartzose matrix. The structural setting results from a polyphase deformation related to the superposition of two thrust systems. The first thrust system includes a duplex verging to east, with the Cenozoic carbonates thrusted onto the Caiazzo Fm. and the roof thrust cut by several breaching thrusts. The late thrust system encompasses ramp-dominated faults verging to the north. Folds, minor thrusts, and S–C structures are associated with the major compressional structures. To date the Caiazzo Fm. and put a temporal constraint to the thrust fault activity, we performed a nannoplankton content analysis that furnished, for the base of this deposit, an age not-older than the upper Tortonian. These out-of-sequence thrusts hence have acted since the late Miocene. Similar structures are widespread in the southern Apennines and are interpreted as the surficial expression of envelopment thrusts formed at deeper structural levels with respect to the thrust front. We envisage that the marble and plutonic supply was provided by the Mesomediterranean continental crust in the late Tortonian when this microcontinent was passing close to the Apennine Platform domain during its orogenic migration toward SE

Constraining the onset of flexural subsidence and peripheral bulge extension in the Miocene foreland of the southern Apennines (Italy) by Sr-isotope stratigraphy

in fold and thrust belts developing at convergent margins, the migration of the advancing wedge is accompanied by bulging of the downgoing plate, followed by the development of a foredeep basin filled by a thick succession of syn-orogenic sediments. The transition from forebulge to foredeep marks a key moment in the evolution of the orogenic system. In deep water environments, the record of this transition is typically complete and progressive. Conversely, in the shallow-water/continental environment of many collisional systems, the uplift of the forebulge area can imply emersion and erosion, obliterating the stratigraphic record of key steps of the evolution of the orogenic system. The southern Apennines constitute one of these collisional fold and thrust belts where the development of the forebulge has implied emersion and erosion, with the development of a Miocene forebulge erosional unconformity, accompanied by extensional deformation associated with the bending of the lithosphere during the forebulge stage. In this paper, we use strontium isotope stratigraphy to constrain with unprecedented time-resolution the age of the forebulge unconformity in areas presently incorporated in the northern sector of the southern Apennines fold and thrust belt. Integration of our results and those of previous studies indicates, at the regional scale, a younging toward the foreland of the forebulge unconformity across the belt. Our high-resolution ages also reveal a diachronous onset of the flexural subsidence over short distances, associated with the occurrence of horst and graben structures, possibly resulting from inherited paleotopography along with forebulge extension. This work highlights how high-resolution dating is critical to unravel the evolution of foreland basin systems at different scales

Major Early-Middle Devonian oceanic oxygenation linked to early land plant evolution detected using high-resolution U isotopes of marine limestones

International audienceThe middle Paleozoic (∼420-350 Myr) records a major increase in ocean-atmosphere oxygen levels; however, the timing and pattern of oxygenation are poorly constrained. Two well-dated North American locations in Nevada and Illinois were used to generate a high-resolution U-isotopic profile (δ 238 U) spanning ∼70 Myr of the middle Paleozoic. Stratigraphic and geochemical data support the interpretation that the Nevada profile represents a near-primary record of global-ocean redox variations. First-order δ 238 U trends indicate strongly reducing oceans during the late Silurian and Early Devonian, terminated by a major oxygenation event near the Emsian-Eifelian boundary (∼395 Ma). More oxic seawater conditions persisted for the next 30+ Myr, but were punctuated by multiple Myr-scale anoxic events during the Middle-Late Devonian and Early Mississippian that correlate with known global biotic crises, positive δ 13 C excursions, and widespread organic-rich facies deposition. The timing of the ∼395 Ma oxygenation event suggests that the O 2 rise was the result of increased photosynthesis and organic carbon burial linked to diversification of late Silurian to earliest Middle Devonian terrestrial plants, rather than to subsequent Devonian increases in terrestrial plant root depth, tree height, lignin content, or seed reproduction. These findings demonstrate that early colonization of continents by relatively small, shallowly rooted plants with geographically limited ranges was sufficient to drive long-term oxygenation of the ocean-atmosphere system, paving the way for the evolution of large, mobile animals that have dominated the Earth's surface since the middle Paleozoic

Late Miocene‐Early Pliocene Out‐of‐Sequence Thrusting in the Southern Apennines (Italy)

We present a structural study on late Miocene‐early Pliocene out‐of‐sequence thrusts affecting the southern Apennine orogenic belt. The analyzed structures are exposed in the Campania region (southern Italy). Here, thrusts bound the N‐NE side of the carbonate ridges that form the regional mountain backbone. In several outcrops, the Mesozoic carbonates are superposed onto the unconformable wedge‐top basin deposits of the upper Miocene Castelvetere Group, providing constraints to the age of the activity of this thrusting event. Moreover, a 4‐km‐long N‐S oriented electrical resistivity tomography profile, carried out along the Caserta mountains, sheds light on the structure of this thrust system in an area where it is not exposed. Further information was carried out from a tunnel excavation that allowed us to study some secondary fault splays. The kinematic analysis of out‐of‐sequence major and minor structures hosted both in the hanging wall (Apennine Platform carbonates) and footwall (Castelvetere Group deposits and Lagonegro‐Molise Basin units) indicates the occurrence of two superposed shortening directions, about E‐W and N‐S, respectively. We associated these compressive structures to an out‐of‐sequence thrusting event defined by frontal thrusts verging to the east and lateral ramp thrusts verging to the north and south. We related the out‐of‐sequence thrusting episode to the positive inversion of inherited normal faults located in the Paleozoic basement. These envelopments thrust upward to crosscut the allochthonous wedge, including, in the western zone of the chain, the upper Miocene wedge‐top basin deposits