THE TECTONO-STRATIGRAPHIC EVOLUTION OF BASEMENT HIGHS IN HYPER-EXTENDED DEEP-WATER RIFTED MARGINS: THE EXAMPLE OF THE BRIANÇONNAIS DOMAIN IN THE ALPS AND COMPARISONS WITH MODERN ANALOGUES Proponent: Prof. Gianreto Manatschal; IPG-EOST/UdS-CNRS In collaboration with: Patrick Unternehr (TOTAL) Background and rational of the project The discovery of hydrocarbon systems in hyper-extended deep-water rifted margins in conjunction with technical developments expanded the domain of hydrocarbon exploration into domains that are yet little investigated. The increasing number of high-quality reflection and refraction seismic surveys and drill hole data show that deep-water rifted margins are very different from proximal margins. The new data show evidence for a polyphase rift evolution resulting in complex rift architectures with variable amounts of magmatic activity and local mantle exhumation that cannot be predicted by classical rift models. Thus, understanding the thermal structure, subsidence history, depositional environment and sedimentary architecture is a prerequisite to apply the play elements in these yet little investigated domains, which is essential to evaluate the survivability of syn- to post-rift petroleum systems. In the past 20 years most of my research was focused on the study of deep-water rifted margins. In the last 5 years this research has been done in close collaboration with TOTAL. The research projects included 1 Post-Doc (G. Péron-Pinvidic 2006-08) and 2 PhDs projects (E. Masini 2007-11 and E. Sutra 2009-11) that investigated the tectono-sedimentary evolution of the Alpine, Pyrenean and S-Atlantic domains. The ultimate aim of these projects was to develop observation driven, predictive and quantifiable models to analyse, interpret and predict the thermal, subsidence, tectonic, magmatic and sedimentary evolution of hyper-extended deep-water rifted margins. The results of these projects helped to improve the conceptual understanding of hyper-extended rifted margins and enabled to propose and test new interpretations for yet little investigated margins. Although a big progress was made in the understanding of deep water rifted margins in the last 5 years, there are still many fundamental questions that remain open and ask for further research on this topic. One open scientific question, also relevant for hydrocarbon exploration, is related to the tectono-sedimentary evolution and subsidence history of basement highs in deep water rifted margins. Péron-Pinvidic and Manatschal (2010) showed that different types of basement highs can be distinguished in rifted margins. These highs include micro-continents, continental ribbons, H- blocks and extensional allochthons. Mapping these highs and properly define their stratigraphic and tectonic evolution provide important insights into the tectonic evolution of rifted margins and ultimately enable to propose predictive and quantifiable rift models that are required for the hydrocarbon exploration. In this research project I propose to investigate the stratigraphic and tectonic evolution of basement highs in deep-water rifted margins, also referred to as residual H- Blocks. Examples of such basement highs in deep-water rifted margins can be observed in many present-day rifted margins (for definitions and examples see Péron-Pinvidic and Manatschal, 2010) (Fig. 1), however, these blocks are often at deep-water and sealed by thick post-rift sediments. Therefore access to direct observations requires expensive drillings. An alternative way to study these highs, which is less expensive, is to use field analogues. The most famous and bestinvestigated example is the Briançonnais domain in the Alps (Lemoine et al. 1987) (Fig. 2). The Briançonnais domain was first defined by Lory (1866) based on its peculiar stratigraphy (thick Triassic carbonates and a Jurassic emersion) (Fig. 3). Haug (1909) described in his pioneering work the Briançonnais domain as a paleotectonic unit with a very characteristic geodynamic evolution. 1
However, only the work of Lemoine (1988) was able to link the tectono-stratigraphic evolution of the Briançonnais domain with that of present-day margins. Many regional studies followed (e.g. Borel, 1995; Claudel and Dumont 1999, Decarlis et al. 2008) that resulted in detailed descriptions of the tectonic, stratigraphic and subsidence history of the different parts of the Briançonnais domain. All these studies confirmed the distal position of the Briançonnais domain in the European margin and discussed the prominent and widespread karst that formed at the top of the Triassic to Early Jurassic pre- to early rift carbonates and its sealing by deep-water Upper Jurassic limestones (Decarlis et al. 2008) (Fig. 4). Lemoine et al. (1987) interpreted this karst as being related to an uplift and sub-areal exposure of the Briançonnais domain during rifting followed by a rapid subsidence of the distal European margin at the moment of the continental breakup. Due to the asymmetric evolution of the conjugate Adriatic margin, these authors propose a Wernicke type simple shear model for the Alpine Tethys (Lemoine et al. 1987). More recent investigations show that the overall evolution of the Alpine domain is more complex and cannot be explained by one single lithospheric scale detachment fault. More recent studies (e.g. Mohn et al. 2010) show that the tectonic evolution is more complex and polyphased. However, no compilation of the numerous existing data of the whole Briançonnais domain exists at present, and the overall evolution of this domain has not been compared to those of distal rifted margins within the last 2 decades. The aim of this project is therefore to revisit the Briançonnais domain, to investigate its tectono-sedimentary evolution, as well as to study the transitions from this domain towards more proximal (sub- Briançonnais) and distal (Pre-Piemontais or Ultra-Briançonnais) parts of the margin. Of major importance will be to date the formation and sealing of the karst and to quantify the subsidence history of the Briançonnais domain and of the surrounding domains during the evolution of the Alpine Tethys rifted margins in Jurassic time. Research plan To achieve our goal we propose to: Review the existing structural, stratigraphic and age data from the whole Briançonnais domain ranging from Liguria/Italy, across the French Alps to Grisons in Switzerland Construct key tectonic sections across the Briançonnais domain that enable to constrain its tectono-stratigraphic evolution (Liguria/S-Alps, Briançonnais/France; Préalpes/Switzerland) Constrain the first order stratigraphic relationships, compare the nature of the internal and external domains and the lateral variations along strike of the Briançonnais domain Constrain the temporal and spatial evolution of the subsidence of the Briançonnais domain (review of existing studies (e.g. Borel 1995) and new studies) Compare the results obtained from the Briançonnais domain with that of seismically imaged basement highs in deep-water rifted margins (e.g. outer high in Campos or Santos) Collaborations already exist with the University of Torino (Dr. M. Beltrando and Dr. Carlo Bertok) and the University of Lausanne (Pr H. Masson). We also plan to establish collaborations with colleagues that worked in the Briançonnais domain in France (mainly the University of Grenoble; Thierry Dumont). Although large data sets already exist, the project will require to revisit most of the areas and to evaluate and re-map some key areas. The choice of the study areas will be guided by the results and their potential to quantify and describe the rift evolution of these domains. Although the goal of this work is of scientific nature, the results will have a direct impact on the exploration of petroleum systems in deep to ultra-deep margins and will result in a better understanding of the origin, characteristics and evolution of the source and depositional environments, migration pathways, the location and nature of hydrocarbon reservoirs, formation of structural traps and trap seals related to the basement highs in deep water hyper-extended rifted margins. 2
3 Project «PhD_TOTAL» Data used in this study The main aim of this project is to study the Briançonnais domain that represents an example of a basement high located in the distal European margin of the Jurassic Alpine Tethys domain, today exposed in the Western and Central Alps. All data from this domain are freely accessible. Although the Briançonnais domain has been locally overprinted by Alpine deformation and metamorphism, in some places, such as in Liguria/NW Italy, the Briançonnais in SE France and the Pré-Alpes in Switzerland, the complete stratigraphic record is well preserved (Borel 1995). This is also true for more internal parts, such as the Tasna nappe that preserve an OCT related to the Briançonnais domain (e.g. Manatschal et al. 2006). The study of the Briançonnais domain will provide a well documented example of a basement high within a distal margin from which we can map the depositional environments and facies distribution of the syn-tectonic sediments over more than 100 kilometres and determine the subsidence history, which can be used to test numerical models. We propose to compare these results with modern examples of basement highs from deep-water hyperextended rifted margins from the Brazilian and West African rifted margins previously studied in our projects with TOTAL (e.g. PostDoc Péron-Pividic and PhD Sutra). Although the results of this comparison may not be directly publishable, comparing ancient with present-day examples is a key to understand how far results from analogue sites (e.g. Briançonnais domain) can be used to understand present-day rifted margins that are yet little explored. Only by such comparisons between ancient and present-day examples it will be possible to bridge observations obtained from drill-hole and seismic scales. Multiplying the number of examples from which direct observations can be made will eventually enable us to better understand and predict basement highs in deep water rifted margins. Benefits for TOTAL This project is in the continuity of a collaboration with TOTAL that initiated in 2006 in which we investigated the architecture of hyper-extended margins and the related depositional evolution of these domains by using the S-Atlantic (Post Doc Péron-Pinvidic 2006-08); the Alpine and Pyrenean examples (PhD Masini 2008-2011) and extension-history (PhD Sutra 2009-2011). The results of these projects enabled to propose conceptual models to explain and interpret the evolution of hyperextended rifted margins and were presented and discussed with groups of TOTAL in Paris, Pau as well as during field trips in the Alps and the Pyrenees and during a workshop in Stavanger (Norway). This new project aims now to study basement highs in deep water rifted margins in order to investigate the processes controlling the sedimentary architecture and basement structures related to their formation. The results of this project will enable to better understand and predict the temporal and spatial evolution of these basement highs during and after rifting. The results will permit to test new models and to develop more predictive numerical models for the tectonic, stratigraphic and thermal evolution of deep, magma-poor margins. The benefits for TOTAL are: The results of the PhD will be presented and discussed in regular meetings at TOTAL Our agreed policy is that the major results from the Alpine margins will be published in peerreviewed journals while the comparisons with present-day margins that are based on confidential data sets can only be published with the agreement of TOTAL; All oral and poster presentations that result from this project are co-authored by TOTAL G. Manatschal will keep TOTAL informed about the development of the project REFERENCES Borel, G. (1995) Préalpes medians romandes: courbes de subsidence et implications glodynamiques: Bull. Soc. Vaud. Sc. Nat. 83.4: 293-315. Claudel, M.-E., and Dumont, T (1999) : A record of multistage continental breaku-up on the Briançonnais marginal plateau (Western Alps) : Early and Middle-Late Jurassic rifting.
Eclogae Geologicae Helvetiae ; 92(1), pp.45-61. Decarlis, A. and Lualdi, A. (2008) : Late Triassic-Early Jurassic paleokarst from the Ligurian Alps and its geological significance (Siderolitico Auct., Ligurian Briançonnais domain). Swiss Journal of Geosciences ; 101 ; pp. 579-593. Haug, E. (1909) : Les géosynclinaux de la chaîne des Alpes pendant les temps secondaires. C.R. Acad. Sci. (Paris) 148, 1637-1639. Lavier L and Manatschal G (2006) A mechanism to thin the continental lithosphere at magma-poor margins. Nature, 440, 324 328. Lemoine, M. (1988) : Des nappes embryonnaires aux blocs basculés : évolution des idées et des modèles sur l histoire mésozoïque des Alpes occidentales. Bull. Soc. géol. France (8) 4/5, 787-797. Lemoine M, Tricart P and Boillot G (1987) Ultramafic and gabbroic ocean floor of the Ligurian Tethys (Alps, Corsica, Apennines): in search of a genetic model. Geology 15(7): 622-625 Lory, C. (1866): Essai sur la structure géologique de la partie des Alpes comprise entre le mont Blanc et le mont Viso. Bull. Soc. Géol. France (2) 23, 482-497. Manatschal G, Engström A, Desmurs L, Schaltegger U, Cosca M, Müntener O and Bernoulli D (2006) What is the tectono-metamorphic evolution of continental break-up: The example of the Tasna Ocean-Continent Transition. Journal of Structural Geology 28(10): 1849-1869 Mohn G., Manatschal G., Müntener O., Beltrando M., Masini E., 2010 Unravelling the interaction between tectonic and sedimentary processes during lithospheric thinning in the Alpine Tethys margins. International Journal of Earth Sciences 99, 75-101. doi: 10.1007/s00531-010-0566-6 Péron-Pinvidic G., Manatschal G., 2010 From microcontinents to extensional allochthons: Witnesses of how continents rift and break apart. Petroleum Geoscience 16 (3), 189-197. doi: 10.1144/1354-079309-903 4
FIGURES Figure 1: Schematic model of rifting evolution illustrating the formation of the different categories of crustal blocks. The genesis, evolution, final shape and position in the margin of each block are related to the distinct modes of deformation affecting the margin during rifting (from Péron-Pinvidic and Manatschal 2010). Figure 2: Tectonic map of the Central and Western Alps and a cross section showing the distribution of major Alpine tectonic units during Late Jurassic time. The cross section through the Alpine Tethys shows the position of the Briançonnais domain. Note that the section across the Adriatic rifted margin (section across the Austroalpine and South 5
Penninic units in Grisons) is offset from the section across the conjugate European rifted margin (section across the Dauphine Briançonnais Valais units along the French- Italian border) (modified after Mohn et al. 2010). Figure 3: Panoramic view of the Lac de l Ascension area showing the Jurassic karst and hiatus from Col du Peyron/Brainçonnais (from Thierry Dumont) 6
(I) (II) (III ) Figure 4: Section across the Alpine Tethys margin (same trace like sections B-B shown in Fig. 2) showing the stratigraphic record documented in the major paleogeographic domains. Zooms show the detailed sediment-architecture of the proximal European margin, the Valais, the Briançonnais and the distal Adriatic margin (for further details see Mohn et al. 2010). The simplified composite stratigraphic logs above show typical logs for the different paleogeographic domains. Abbreviations: M, Mantle, B basement. LT, MT, UT, Lower, Middle, Upper Triassic. LJ, MJ, UJ, Lower, Middle, Upper Jurassic; (H, Hettangian, Si, Sinemurian, Bath, Bathonian, Ox, Oxfordian, Tit, Tithonian), LC, Lower Creatceous. A-A, Aptian-Albian (Ur, Urgonian). The three conceptual 3D model shown below illustrate the temporal and spatial evolution of the Alpine domain from onset of rifting to final rifting for three stages: (I) Early rifting (stretching) phase characterized by distributed fault bounded basins that can be found across the whole future margin. (II) Late rifting (thinning) phase during which the extension is localized in the Proto-Valais and Piedmont-Liguria domains. Note that during this stage the distal margin is undergoing extreme crustal thinning and note also that the Briançonnais domain is delaminated and uplifted during this stage. (III) Onset of mantle exhumation in the Valais and Piedmont-Liguria Basins. Note that the Valais is interpreted to die out laterally, whereas the Piedmont-Liguria basin is evolving into an embryonic oceanic domain. 7
Figure 5: Conceptual model modified from Lavier and Manatschal (2006) showing the evolution of an H-Block from the thinning stage to its tectonic delamination and the final formation of residual and delaminated H-Blocks in the final margin. This model may explain the evolution of the Briançonnais domain in the Alps. 8