Research program
LATE PALEOZOIC/MESOZOIC HERITAGE IN THE STRUCTURE OF THE ALPINE OROGEN
University Co-ordinator
Università degli Studi di TORINO -
SCIENZE DELLA TERRA - TORINO(TO)
Research Unit Leader
Giorgio MARTINOTTI
Description
The researches of the Torino unit will be focussed on the Maritime Alps and in particular on the Ligurian Briançonnais domain, limited to the Marguareis and Mongioie areas, close to sectors of the same domain studied by the Pavia unit, and on selected sectors of the Provençal Dauphinois domain as the upper Valle Roja-Valle Argentina and the upper Valle Stura. In these sectors sedimentary successions ranging from Triassic up to Paleogene crop out extensively and testify a complex evolutionary history, starting from the first phases of the continental rifting, going through the opening of the Tethyan ocean and up to the formation of the alpine foreland basin.The Briançonnais domain is characterized by a generally condensed succession with reduced thickness especially in the Jurassic portion and striking thickness variations in the overlying Cretaceous-Paleocene successions. This stratigraphic setting, well known and studied in manysectors of the North Western Alps, has been interpreted as the result of a sedimentation on a structural high and under the influence of a strong Jurassic extensional tectonics (Lemoine et al., 1986; Michard & Henry, 1988; Sartori, 1987; Tricart et al., 1988). In the Maritime Alps (Ligurian Briançonnais domain), instead, and in particular in the sector that will be analysed, the stratigraphic setting has never been studied in great detail: several works of the ‘70s provided a good knowledge of the lithostratigraphy whereas sedimentological features and relationships between tectonics and sedimentation are still poorly understood. Consequently, a full "modern" revision of the stratigraphy and sedimentology of the Jurassic-Paleogene succession will be done, and it will have the following goals:- Check of the extension and characters of the Liassic emersion. In particular the study of the sediments associated to the discontinuity will provide information both on the sedimentary environments and on the level of tectonic denudation due to the rifting and preceding the drowning;- Detailed control of the stratigraphy of the Middle-Upper Jurassic, in order to improve the biostratigraphic knowledges, highlight the facies and thickness variations, and find stratigraphic evidence for synsedimentary tectonics (sedimentary dykes, anomalous stratigraphic characters of pelagic sediments leaning on paleoslopes as it has been recently described in analogous, even if less deformed, settings in the Central Appennine and in Western Sicily: Santantonio, 1993; Martire & Pavia, 2002);- Acquisition of new data and sedimentological reinterpretation of the Cretaceous-Paleogene succession, keeping in mind the existence of an unconformity of fundamental importance at the Cretaceous - Cenozoic boundary in the adjoining Provençal Dauphinois domain. Thereby the relationships between tectonics and sedimentation during the Eocene, only recently evidenced (Michard & Martinotti, 2002), will be explained and some hypotheses on the sandy sediments provenance and the benthic platform foraminifera associations will be formulated.The structural knowledges already acquired (subdivision in tectonic units, deformational styles, distribution of the structural elements etc…)(Gosso et al., 1983; Carminati, 2001; Carminati & Gosso, 2000; Vanossi et al., 1984) will be further improved in the sectors under study. The present setting of the Ligurian Briançonnais domain is the result of multiphase folding under anchi-metamorphic conditions and subsequent extensional events, which on the whole caused a partial trasposition (or at least traslation) of the sedimentary bodies.Nevertheless, thanks to the strain concentration along ductile and brittle shear zones at different scales, the stratigraphic correlations are generally still possible over hectometric or even kilometric areas.In order to define the physical correlability of the stratigraphic units and to realize the relationships between synsedimentary tectonics and alpine tectonics, we will rely on:- The recognition and mapping of the main surfaces of structural correlation (foliations, penetrative cleavages at the regional scale);- The subdivision of the domains in geometric and/or tectono-stratigraphic units and study of the kinematics responsible for their present geometric relationships;- The individuation of the main structural associations and styles at the meso and macro-scale;- The interpretation of the relative chronology of the main deformational events and individuation of phenomena of tectonic reactivation.The Provençal Dauphinois domain is characterized by a quite condensed Jurassic-Cretaceous succession deposited on a structural high sector of the European passive margin, and shows many similarities with the Ligurian Briançonnais. It is overlain, with an angular unconformity, by the so called "Priabonian trilogy" consisting of Nummulitic Limestones, Globigerina Marls, Grès d'Annot of the Middle Eocene-Lower Oligocene. It was deposited during the birth and evolution of the alpine foreland basin and documents the change from a mixed carbonate-silicoclastic ramp to a slope and finally to a turbiditic basin. Also the Nummulitic Limestones show striking lateral facies variations, mainly due to synsedimentary tectonics (Varrone, 2004; Varrone & Clari, 2003). The Provençal Dauphinois domain succession is deformed by thrusts with associated ramp-folds, branching out from a basal detachment level corresponding to the evaporites of the Upper Triassic. This structural setting is the result of deformational phases linked to the emplacement of the Helmintoid Flysch nappe (Early Oligocene) and the following activation of the more external Digne thrust (Early Miocene), with regional West-South West vergence and local backthrusting toward East-North East (Ford & Lickorish, 2004; Ford et al., 1999). In the sector under study, the Dauphinois thrust belt comprises both the Mesozoic paleoeuropean margin successions and those deposited in the Eocene foreland basin and subsequently (Late Oligocene) incorporated in the chain. The presence of detachment levels and/or successions with different strenghts both laterally and vertically strongly influenced the structural style of Mesozoic-Paleogene successions. In the Provençal Dauphinois domain, generally not deformed by a multiphase folding tectonics, the study of the Mesozoic heritage on the alpine structural setting is easier than in the Ligurian Briançonnais domain. The analysis of the relationships between stratigraphic and structural setting, well exposed in the Dauphinois domain, will provide a rheological model for the interpretation of those portions of the Ligurian Briançonnais successions showing stratigraphic similarities with the Provençal Dauphinois domain. Such extrapolation will be admitted only after the verification of the existence of a common stratigraphic and paleostructural Mesozoic heritage between the two domains that presently may only be hypothesized.The goals so far described will be pursued, in adjoining portions of the Ligurian Briançonnais and Dauphinois-Provence domains, with the following methodologies of study:- Geological and structural detailed mapping (general mapping with a scale 1:25.000 and some selected areas with scales comprised between 1:2.000 and1:10.000), aimed to verify the lateral continuity of the sedimentary bodies and of the structures offsetting them; this approach is necessary both in polyphase deformed settings like the Ligurian Briançonnais domain and in settings involved in a detachment and thrust tectonics such as the Provençal Dauphinois domain. Mapping will be accomplished in a GIS (Geographical Information System) environment with the aid of ArcView v.8 software also with the purpose of attempting bi- and tridimensional reconstructions of architecture of geological bodies;- Selection, on the basis of the results of the structural analysis and of the detailed mapping, of the best sites for a stratigraphic study: here measurements and samplings of stratigraphic sections, facies and microfacies analysis, and micropaleontological studies with thin sections (foraminifera) and smear slides (calcareous nannofossils) will be carried out;- Possibly stable isotope analysis of sedimentary dyke infillings, aimed to highlight subaerial exposure events;- Structural analysis aimed to the comprehension of the distribution of the main structural elements related to the extensional synsedimentary phases, the alpine compressional phases and the recent brittle deformational phases;- Detailed structural analysis in representative sites.The noteworthy charge of field work, that involves geological mapping at different scales, structural and stratigraphic analyses, the complexity of the studied area, the need of analyzing different sectors for comparison goals, and the limited time availability of the university personnel that would not be enough to accomplish the proposed project within the programmed 2 years, result in the need for a collaborator with proved competencies and full time availability (a postDoc position) in order to accomplish the indicated objectives.As to the temporal organization of the program, the objective logistic features of the area under study (scarcity of road and hotel facilities, high mean elevation that constrain the field work to summer months) must be considered. Consequently, geological mapping, structural analysis, stratigraphic section measurement will be concentrated in the first year; the second year will be devoted to map and structural data processing and to laboratory analyses (petrography, micropaleontology, possibly geochemistry) and to control of field data.The main results which are expected from this research are:- Reconstruction of the tectono-sedimentary evolution of the Ligurian Briançonnais and Provençal Dauphinois domains between the Late Triassic and Eocene, with production of original mappings in key sectors; - Comparison between the different evolutions of the two studied domains and interpretation of the paleogeographic relationships both during the Mesozoic structuration of the continental passive margins and during the birth of the alpine foreland basin;- Comparison between the deformational styles of the two studied domains in order to evaluate the role of the preceding stratigraphic and structural heritages;- Re-evaluation of the significance of the "Pennidic Front" concept in this sector of the chain where it divides the Ligurian Briançonnais and Provençal Dauphinois domains on the basis of the similarities/differences between them; - Verification of the validity of a multidisciplinary approach to the study of sedimentary successions involved in orogenic chains and affected by an intense polyphase tectonization.
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