Upper-plate magma-poor rifted margins: stratigraphic architecture and structural evolution

International audienceAlthough it is generally accepted that many distal, magma-poor rifted margins are asymmetric and can be divided into lower and upper plate margins, little is known about the details of how and when this asymmetry evolves and how upper and lower plate margins can be distinguished. This is due to the fact that most papers focused on the so called lower plate margins, while the upper plate margins remained less well understood, mainly due to the lack of public accessible drill hole data. The aim of this paper is to provide a first order description of the global architecture and stratigraphic evolution of an upper plate, magma-poor rifted margin. In order to provide such a template, we focused on 2 seismic sections, the ION-1000 line (East Indian margin), and the SCREECH 2 line (Newfoundland margin) and describe key, km-scale outcrops from the fossil European margin exposed in the Western/Central Alps, all of which document classical upper plate margins. Based on these data we show that upper plate magma-poor rifted margins can be characterized by a staircase type architecture with terraces (T1, T2, T3) and ramps (R1, R2) that result as a consequence of an evolution through a coupling, exhumation and breakup stage. We also defined key stratigraphic levels that we try to link with the evolution of the margin which enables us to link the tectonic evolution with the creation of accommodation space and formation of the staircase architecture that characterizes the upper plate margin. From these observations we develop a conceptual model for the evolution of upper-plate margins and discuss the applicability of this model for different strain rates, rates of subsidence and sedimentation rates

Transition from orogenic-like to anorogenic magmatism in the Southern Alps during the Early Mesozoic: Evidence from elemental and Nd-Sr-Hf-Pb isotope geochemistry of alkali-rich dykes from the Finero Phlogopite Peridotite, Ivrea–Verbano Zone

The Ivrea-Verbano Zone (IVZ) in the westernmost sector of the Southern Alps is an iconic upper mantle to lower continental crust sequence of the Adriatic Plate and provides a geological window into the tectono-magmatic events that occurred at the Gondwana–Laurussia boundary from Late Paleozoic to Early Mesozoic. In this work, we document new geochemical and Nd-Sr-Hf-Pb isotopic data for Early Mesozoic alkali-rich dyke swarms which intruded the Finero Phlogopite Peridotite (northern IVZ) to provide geological constraints on the nature, origin and evolution of Early Mesozoic magmatism in the Southern Alps. The studied dykes are amphibole-phlogopite-bearing and show geochemical features varying between two end-member groups. A dyke group is characterized by HFSE-poor, Al-rich amphibole (Al2O3 up to 16 wt.%) with high LILE and LREE contents, high radiogenic 87Sr/86Sr(i) (0.704732 to 0.704934) and low radiogenic Nd isotopes (εNd(i) from –0.1 to –0.7), which support the occurrence of significant amounts of recycled continental crust components in the parental mantle melts and impart an overall “orogenic-like” affinity. This dyke group was largely derived from metasomatized lithospheric mantle sources. The second group is HFSE-rich with Al-poorer amphibole enriched in LILE and LREE, low radiogenic 87Sr/86Sr(i) (0.703761–0.704103) and higher radiogenic Nd isotopes (εNd(i) from +3.4 to +5.4) pointing to an “anorogenic” alkaline affinity and asthenospheric to deep lithospheric mantle sources. Some dykes show both orogenic and anorogenic affinities, providing evidence that the orogenic-like magmatism in the IVZ predates the alkaline anorogenic magmatism. The Finero dyke swarms therefore record a geochemical change of the Early Mesozoic magmatism of the Southern Alps from orogenic-like magmatism, typical of post-collisional settings, to anorogenic alkaline magmatism, common in intraplate to extensional settings, and places a temporal correlation of Early Mesozoic magmatism in the IVZ to those in the eastern and central sectors of the Southern Alps

A sequence stratigraphic approach to a Middle Triassic shelf-slope complex of the Ligurian Alps (Ligurian Briançonnais, Monte Carmo-Rialto Unit, Italy)

The Mesozoic sedimentary cover belonging to the Monte Carmo-Rialto unit of the Ligurian Briançonnais domain is composed of Scythian clastics and Anisian to Carnian carbonate rocks over 300 m thick. This paper focuses on the stratigraphy of this carbonate complex, its environmental significance and its evolution in light of dynamic stratigraphy. Our facies analysis of limestones and dolomites of the Triassic complex allowed us to reconstruct an environmental model. Data support a distally steepened carbonate ramp of Anisian age evolving to a more diversified Ladinian platform with an oolitic sand-bar belt separating the lagoon from the slope. The Monte Carmo-Rialto slope facies are the only witnesses of deep sedimentation in the Triassic terrains of the Ligurian Briançonnais domain, otherwise represented by shallow-water carbonate deposits. On the basis of facies succession, we have identified nine medium-scale cycles (3rd order sequences) in the study area, comparable to those evidenced in the Briançonnais s.s. domain by the French authors. Small-scale cycles analysis evidenced mainly shallowing-upward trends in the examined sequences; although a few evidences of transgression-related deposits (deepening upward cycles) have been found at the base three sequences, they have been mostly obliterated by dolomitisation and masked by local tectonics. For this reason we can undoubtly distinguish only the part of each sequence belonging to HST, while the TST, though present, still remains a partition that cannot be precisely characterised. In the same way, LSTs are not present in the Monte Carmo-Rialto unit, due to the original relative landward position of the examined area. Sequence stratigraphy analysis indicates different long-term dynamics for the two evolutionary stages of the Triassic Ligurian platform: a general landward backstepping to moderate progradation during the Early Anisian and true progradation during the latest Anisian and Ladinian. In addition, a good fit with the sequences proposed by the SEPM chart has been found, indicating a correspondence for the 3rd order sequences of the Middle Triassic

Late Triassic-Early Jurassic Paleokarst from the Ligurian Alps and its geological significance (Siderolitico Auct., Ligurian Briançonnais domain)

Nel domino Brianzonese ligure, le unità carbonatiche medio triassiche, note come Formazione di Costa Losera e Dolomie di San Pietro dei Monti, sono stratigraficamente seguite da una importante lacuna che segna il passaggio ai calcari neritici di Rio di Nava del Batoniano. Nell’ambito di una classica evoluzione di un margine continentale passivo, una stasi nella subsidenza, seguita da un sollevamento portò ad una erosione della piattaforma triassica in misura progressivamente maggiore procedendo verso le unità più interne, cioè verso la Tetide ligure. Tale erosione, che verosimilmente si verificò su di un substrato tettonicamente controllato, portò alla formazione di sequenze sedimentarie assai diversificate, talora mancanti di tutti i terreni triassici o persino dell’intero tegumento permiano. Nell’area studiata (ed in limitate aree esclusivamente appartenenti alla porzione più esterna dell’unità di Ormea), la lacuna mesozoica è ben più di una semplice superficie di erosione; i depositi ad essa associati (“Siderolitico” Auct.) sono costituiti sia da un corpo di peliti rosse interposto fra le sopracitate unità formazionali, sia da una breccia di origine carsica che penetra profondamente le sottostanti dolomie ladiniche. E’ stata condotta una ricerca stratigrafica di dettaglio su questi depositi, unitamente ad un’analisi delle microfacies e petrografico-composizionale, al fine di determinare le caratteristiche dei paleosuoli e del carsismo, di raccogliere maggiori informazioni sulla loro origine e sull’età. Inoltre, è stato discusso il significato regionale e l’importanza di questi depositi grazie ad un confronto a grande scala con il dominio brianzonese classico e con altre località dell’arco alpino occidentale che mostrano una unconformity del tutto simile. Alla luce dei dati raccolti, per l’evento carsico in esame viene proposta un’età compresa fra il Triassico Superiore e il Lias (sino al Baiociano Superiore ?)

Synrift sedimentation on the northern Tethys margin: an example from the Ligurian Alps (Upper Triassic to Lower Cretaceous, Prepiedmont domain, Italy)

The Prepiedmont domain succession of the Ligurian Alps is formed by a thick Mesozoic sedimentary cover tectonically detached from its substratum. The Arnasco-Castelbianco unit preserves the most complete record of the Ligurian Prepiedmont, although completely overturned and deformed due to Alpine tectonics. It is composed of carbonate and clastics rocks deposited during the Upper Triassic-to-Lower Cretaceous interval. This paper is focused on the stratigraphy of the Jurassic series and its relationships to the Tethyan rifting. Each term of the sedimentary record is seen as a witness of the several phases through which the rifting took place. An early-rifting phase (Late Hettangian-Early Sinemurian) brought to the formation of a normal fault system affecting the carbonate platform, and favoured the development of condensed sedimentation on pelagic highs. The rapid transition from open-platform carbonate to slope-basin cherty limestones, testifies the increased subsidence of the margin in the Late Sinemurian, during which moderate fault activity is recorded (intraformational breccia horizons). Until the Early Pliensbachian, a tectonic pause brought to the sedimentation of a succession of pelagic carbonates, occasionally interrupted by clastic flows. During the Late Pliensbachian (?)-Toarcian, it followed the rifting phase, evidenced by the large amount of clastics generated by the renewed fault activity. Clastics flowed down into the basin as fluxoturbidites at first, and then they passed to breccias during the maximum tectonic pulse. In the Late Toarcian-Aalenian?, the thermal uplift of the Briançonnais shoulder generated a basinal infill of fine clastics. The following thermal subsidence (Aalenian-Tithonian) favoured the restoration of quiet basinal conditions evidenced by the deposition of radiolarites

Architecture of the Distal Piedmont-Ligurian Rifted Margin in NW Italy: Hints for a Flip of the Rift System Polarity

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Structural setting of the easternmost Dauphinois: stratigraphic and tectonic characters from the Ventimiglia area.

Since 2010, a detailed structural and stratigraphic mapping has been performed north of Ventimiglia (IM), between the Roya and Nervia valleys, by the Pavia University surveyors for the Risknat-Interreg project (Regione Liguria). The investigated area is constituted by a thick Dauphinois succession of sedimentary rocks. The post- Cretaceous Dauphinois units near the boundary between Italy and France are formed by a well-known succession: the “Priabonian trilogy”. It is formed by rocks that deposited in the Alpine underfilled peripheral foreland basin. A thick succession of Cretaceous marly limestones (Calcari e calcari marnosi di Trucco) forms the pre-Cenozoic basement, top-bounded by a regional unconformity (mostly an angular unconformity with rarer paraconcordance evidences) sealing up late Cretaceous and Paleocene deposits. The following transgression caused the deposition of marine-continental transitional deposits (conglomerates, sandstones and marls of the Microcodium Formation) during the Early Eocene. The vertical evolution of these deposits in the developing Alpine foreland basin is represented by the nummulitic limestones and calcarenites of the Calcareniti di Capo member, belonging to a shallow platform environment (about few to a hundred meters depth). The deepening trend bring to the sedimentation of marls and pelites of Marne di Olivetta San Michele Formation and finally to the turbidite sandstones of the Ventimiglia Flysch (corresponding to the Italian sub-basin deposit of the Gres d’Annot turbidite system). The field mapping of the study area evidenced the presence of several kilometric thrusts (Abellio thrust system) that juxtaposes two tectonic elements made up by the same, above described Cretaceous-Eocene succession. The units has been informally named “M. Terca unit” (the upper) and “Roja unit” (the lower). The thrusts are roughly NNE-SSW oriented; the amount of westtransport of the M. Terca unit is probably limited but significantly diminishes toward the South. This assumption is supported by observing the amplitude and geometry of the associated drag folds. The thrust is interrupted by a lateral ramp located northward near the M. Gouta area and reaches the coast near Ventimiglia toward the South. The geometry of the thrust has been also reconstructed through digital modelling in order to understand the real implications of its anticlockwise rotation. The Abellio thrust has been tentatively dated as Late Oligocene-Early Miocene, when the north-west Alpine compressive thrust fronts propagated in the Helvetic-Dauphinois zone. Interesting implications may come from its position, as the Abellio thrust could be considered the southermost onland structure affecting the Dauphinois succession. Its transport was probably also influenced by the interactions between the complex geometry of dispersal blocks that followed the rifting of Greater Iberia (causing the anticlockwise rotation of the Corsica- Sardinia) and the Alpine-Apennine orogens