Cédric Bulois
email: bulois@geologie.ens.fr

Manuel Pubellier

Nicolas Chamot-Rooke

Louise Watremez

From orogenic collapse to rifting: A case study of the northern Porcupine Basin, offshore Ireland

Keywords:

Offshore Ireland, the North Atlantic opening is generally interpreted as successive, 10-to-15-Myr-long, rifting events during the Mesozoic. However, their interaction is poorly documented in terms of structural inheritance and fault reactivation. From extensive seismic and well data in the northern Porcupine Basin, we show that extension actually evolved over an abnormal period of ca. 220 Myrs with overlapping tectonic stages that, all together, reflect the Irish Atlantic Margin construction from the Palaeozoic.

During the Carboniferous, extension initiated throughout the Variscan and Caledonian thickened crusts and controlled the deposition of generally slightly-tilted, continental clastics. Two subsequent rifting events, during the Triassic-Lower Jurassic and Upper-Jurassic-Lower Cretaceous, controlled shallow-to-deep marine sedimentation in separated sub-basins that progressively widen and connect once extension localised along bounding faults.

We propose that extensional tectonics, first controlled by a general orogenic collapse reflecting an early stage of stretching, is then followed by several rifting periods that ultimately evolved toward the hyper-thinning of the margin and shaped an aborted rift propagator. This evolution is due to pre-existing orogenic-related structures and boundary conditions variations. Such a continuum of deformation, although pulsed, implies varying structural interactions of crustal scale that are often recorded in syn-tectonic sediments.

Introduction

The continental lithosphere deforms according to a number of geological events that successively open and close oceanic domains according to the Wilson Cycle concept [START_REF] Burke | Hot spots and continental breakup: implications for collisional orogeny[END_REF]. From this theoretical view, the divergence of continental masses forms extensional systems on top of old orogenic features within a continuum of deformation during which rift basins progressively evolve to passive margin settings. Also, extension is often defined from the unique recognition of syn-and postrift unconformities that bound characteristic growth-faulting sedimentary sequences [START_REF] Grow | Representative multichannel seismic profiles over the U.S. Atlantic margin, in seismic expression of structural styles[END_REF][START_REF] Badley | Development of rift basins illustrated by the structural evolution of the Oseberg feature, Block 30/6, offshore Norway[END_REF][START_REF] Badley | The structural evolution of the northern Viking Graben and its bearing upon extensional modes of basin formation[END_REF][START_REF] Frostick | Sedimentation in divergent plate-margin basins[END_REF][START_REF] Prosser | Rift-related linked depositional systems and their seismic expression[END_REF][START_REF] Purser | Sedimentation and Tectonics in Rift Basins Red Sea:-Gulf of Aden[END_REF]. Modern data show that deformation is more complex, especially in the case of longstanding extensional history during which basins may encompass several overlapping rifting events. This is partic ularly well observed along the North Atlantic margins (e.g. [START_REF] Ziegler | Evolution of the Arctic-North Atlantic and the Western Tethys[END_REF][START_REF] Ziegler | Geological Atlas of Western and Central Europe[END_REF][START_REF] Tankard | Extensional tectonics and stratigraphy of The north atlantic margin: introduction[END_REF][START_REF] Doré | Principal tectonic events in the evolution of the northwest European Atlantic Margin[END_REF][START_REF] Williams | Comparative Jurassic and cretaceous tectono-stratigraphy and reservoir development in the Jeanne d'Arc and porcupine basins[END_REF][START_REF] Stoker | An overview of the upper palaeozoic-mesozoic stratigraphy of the NE atlantic region[END_REF] where basin geometries and sedimentary infills are influenced by the early basement framework or previous rifted structures (e.g. [START_REF] Bartholomew | Regional structural evolution of the North Sea: oblique slip and the reactivation of basement lineaments[END_REF][START_REF] Doré | Patterns of basement structure and reactivation along the NE Atlantic margin[END_REF][START_REF] Roberts | Palaeozoic to Tertiary rift and basin dynamics : mid-Norway to the Bay of Biscay -a new context for hydrocarbon prospectivity in the deep water frontier[END_REF]. Nevertheless, the tectonic relationships between the various tectonic phases are not fully resolved in terms of nature, significance and timing and their integration within an overall hyper-thinning scenario leading to passive margin setting remains uncertain.

Offshore west of Ireland, the Porcupine Basin (Fig. 1) represents a typical example of such polyphased relationships between basement and shallower rifted structures. The basin is superimposed on top of two Palaeozoic orogenic belts formed during the Pangaea amalgamation and now reveals a highly-stretched continental crust covered by up to 10 km of sediments of Upper Palaeozoic, Mesozoic and Cenozoic sediments and several crosscutting Cretaceous and Cenozoic igneous bodies (Fig. 2, e.g. [START_REF] Naylor | Porcupine-goban Region -a Standard Structural Nomenclature System[END_REF]Naylor andShannon, 2009 &2011;[START_REF] Gagnevin | Geological and geophysical evidence for a mafic igneous origin of the Porcupine Arch, offshore Ireland[END_REF]. It formed during several so-called rifting events that have a variable magnitude at the scale of the basin. The main rifting event, of Upper Jurassic age, formed characteristic rotational fault-blocks sealed by the Base Cretaceous Unconformity (e.g. [START_REF] Macdonald | Geology of oil accumulation in block 26/ 28, Porcupine basin, offshore Ireland[END_REF][START_REF] Sinclair | Tectonic control on sedimentary evolution of three North Atlantic borderland Mesozoic basins[END_REF]Robinson and Canham, 2001;[START_REF] Jones | Structural and stratigraphic evolution of the Connemara discovery, Northern Porcupine Basin: significance for basin development and petroleum prospectivity along the Irish Atlantic Margin[END_REF]. Several other thickening phases have been suggested in the Permo-Triassic [START_REF] Naylor | Porcupine-goban Region -a Standard Structural Nomenclature System[END_REF][START_REF] Štolfová | Permo-Triassic development from Ireland to Norway: basin architecture and regional controls[END_REF] and Cretaceous [START_REF] Naylor | A reflection seismic study of the Porcupine Basin, offshore west Ireland[END_REF][START_REF] Moore | A syn-rift to post-rift transition sequence in the Main Porcupine Basin, offshore western Ireland[END_REF]. This extensional evolution remains however unclear in terms of fault geometries and reactivation mechanisms, due to poor seismic resolution and the full consideration of new crustal extension concepts.

Herein, we specifically investigate the Palaeozoic and Mesozoic extensional history of the northern part of the Porcupine region to highlight tectonic interactions between the various extensional events. Our study is based on original interpretation of 2D and 3D seismic data (19237 km and 324 km 2 respectively) tied to twenty six exploration wells (Fig. 1). The overall dataset was analysed using classical seismic stratigraphic methods [START_REF] Mitchum | Seismic stratigraphy and global changes of sea level; Part 6, Stratigraphic interpretation of seismic reflection patterns in depositional sequences[END_REF] and correlated to lithological information by using traditional gamma-ray and sonic responses [START_REF] Rider | Gamma-ray log shape used as a facies indicator: critical analysis of an oversimplified methodology[END_REF]Serra, 1995;[START_REF] Reading | Sedimentary Environments: Processes, Facies and Stratigraphy[END_REF]. We also further constrained the age and depositional environments by using lithologies and biostratigraphic records provided in well reports, as well as available cores and cuttings. Sedimentary megasequences are mapped regionally and their genetics is discussed according to the formation of the Irish Atlantic Margin. They particularly demonstrate the influence of early structural frameworks on the construction of a wide Atlantic-type passive margin that acquired a certain tectonic independence over time.

Structural framework of the Porcupine region and aims of this study

As part of the Irish Atlantic Margin, the Porcupine Basin opened through the Caledonides and Variscides fold and thrust belts (Fig. 1). Repeated extensional events [START_REF] Doré | Principal tectonic events in the evolution of the northwest European Atlantic Margin[END_REF][START_REF] Stoker | An overview of the upper palaeozoic-mesozoic stratigraphy of the NE atlantic region[END_REF] produced an hyper-stretched, 20-to-7.5-km thick and sometimes thinner, basement, overlain by up to 10 km of Upper Palaeozoic to Cenozoic sediments, and flanked by 30-to-35-km-thick and 25-to-28-km thick continental crusts (e.g. [START_REF] Buckley | A free air gravity anomaly contour map of the Irish continental margin[END_REF][START_REF] Masson | A Free-air Gravity Anomaly Map of the Irish Continental Margin and a New Gravity Model across the Southern Porcupine Seabight[END_REF][START_REF] Makris | Continental crust under the southern Porcupine Seabight west of Ireland[END_REF][START_REF] Jacob | An overview of the results of the RAPIDS seismic project, North Atlantic[END_REF][START_REF] Johnson | Aspects of the structure of the Porcupine and Porcupine Seabight basins as revealed from gravity modelling of regional seismic transects[END_REF][START_REF] O'reilly | Crustal thinning, mantle exhumation and serpentinization in the Porcupine Basin, offshore Ireland: evidence from wide-angle seismic data[END_REF][START_REF] Watremez | Deep structure of the Porcupine Basin from wide-angle seismic data[END_REF][START_REF] Prada | Crustal strain-dependent serpentinization in the Porcupine Basin, offshore Ireland[END_REF]. The rifting led to combined stretching factors increasing southward [START_REF] White | Lithospheric stretching in the Porcupine basin, west of Ireland[END_REF][START_REF] Tate | Lithospheric extention and magmatism in the Porcupine basin west of Ireland[END_REF], and most likely controlled the pronounced v-shape, NS-trending Porcupine indention through the margin. This peculiar orientation, compared to adjacent basins, infers interaction of local basement and extensional faults through time which are still poorly constrained. 2006) and [START_REF] Naylor | Geology of offshore Ireland[END_REF], from which Permo-Triassic depocentres in the study area (black box) are determined but dismissed in our study. Note that old structures (e.g. Iapetus Suture, Great Glen Fault) are not always well-defined onshore and offshore Ireland due to the resolution of subsurface data. B-Schematic cross-section across onshore and offshore Ireland, showing the collage of the different basements involved in the stretching of the Irish margin, and their related structures that likely have controlled subsequent basin formations. The onshore part is based from [START_REF] Cooper | Structural evolution of the Irish Variscides[END_REF] and the offshore parts on deep seismic lines Wire 1 & 2 and SWAT [START_REF] Birps | Deep seismic reflection profiling between England, France and Ireland[END_REF][START_REF] Klemperer | A deep seismic reflection transect across the Irish Caledonides[END_REF]. Abbreviations: AGF Antrim-Galway Fault, DB -Donegal Basin, FHCBL -Fair Head-Clew Bay Line, GGFZ -Great Glen Fault Zone, IS: Iapetus Suture, MB -Mayo Basin, NCSB: North Celtic Sea Basin, VF -Variscan Front.

Crustal development of the Porcupine region

Basement configuration and Irish orogenic cycles

Several Palaeoproterozoic to Palaeozoic basement types constitute structural highs around the Porcupine Basin (e.g. [START_REF] Auffret | Dredged rocks from the armorican and celtic margins[END_REF][START_REF] Auffret | Géologie des Escarpements de Porcupine et de Goban (N.E. Atlantique). Résultats de la campagne de plongée CYAPORC[END_REF][START_REF] Masson | Geology of porcupine bank and goban Spur, northeastern atlantic -preliminary result of the CYAPORC submersible cruise[END_REF] and are geologically or geophysically observed underneath thick Mesozoic-Cenozoic strata or as reworked material in the basin [START_REF] Robeson | The upper palaeozoic geology of the porcupine, erris and donegal basins, offshore Ireland[END_REF]Tate and Dobson, 1989a;[START_REF] Tyrrell | Drainage reorganization during breakup of Pangea revealed by in-situ Pb isotopic analysis of detrital K-feldspar[END_REF]. These basements reflect several orogenic cycles that aggregated the Pangaean continent and formed early discontinuities of crustal importance (e.g. [START_REF] Roberts | Palaeozoic to Tertiary rift and basin dynamics : mid-Norway to the Bay of Biscay -a new context for hydrocarbon prospectivity in the deep water frontier[END_REF].

The Proterozoic framework is relatively poorly constrained in the Porcupine region (Fig. 2). Palaeoproterozoic rocks are known underneath the Finnian's Spur (1.8 Ga, Daly, pers. com.) and belong to a crystalline complex extending through the Slyne-Erris and Rockall region (1.625 Ga;[START_REF] Miller | Precambrian rocks drilled from the Rockall bank[END_REF][START_REF] Morton | Geochemical and isotopic constraints on the nature and age of basement rocks from the Rockall Bank, NE Atlantic[END_REF] and in northernmost Ireland (1.779-1.750 Ga, [START_REF] Daly | Age and crustal evolution of crystalline basement in western Ireland and Rockall[END_REF][START_REF] Daly | Pre-caledonian history of the annagh gneiss complex, north-western Ireland, and correlation with Laurentia-Baltica[END_REF][START_REF] Daly | Precambrian[END_REF]. Younger Mesoproterozoic-Cambrian rocks are part of the Porcupine High (1.31 Ga) and of the NW Atlantic fringes of Ireland (1.2-0.6 Ga). These relics formed during the Laurentian and Grenvillian orogenies that controlled the original NE-SW foundation of the margin (e.g. Great Glen Fault, Fig. 2a).

Next, the Caledonian Orogeny produced a second basement type with the Ordovician-Silurian suturing of the Iapetus Ocean along Laurentian and Grenvillian terranes [START_REF] Lambert | The grampian orogeny[END_REF][START_REF] Soper | Age of the grampian orogeny in Scotland and Ireland[END_REF][START_REF] Mckerrow | The caledonian orogeny revisited[END_REF][START_REF] Chew | Grampian orogeny[END_REF][START_REF] Chew | Late caledonian orogeny and magmatism[END_REF]. This formed pronounced NE-SW thrusts across the north of Ireland and Britain (e.g. Fair Head-Clew Bay Fault Zone, Iapetus Suture; Fig. 2a). In the Porcupine region, Caledonian rocks are mostly evidenced from reworked Lower Ordovician-Devonian pollens [START_REF] Robeson | The upper palaeozoic geology of the porcupine, erris and donegal basins, offshore Ireland[END_REF].

Subsequent orogenic cycle resulted in the Variscides construction following the Devonian-Lower Carboniferous Rheic Ocean subduction (Stampfli et al., 2002;[START_REF] Cocks | European geography in a global context from the Vendian to the end of the Palaeozoic[END_REF]. Characteristic NE-SW-trending thrusts and folds in southern Ireland continue as E-W to NE-SW highs in the Celtic Sea region (Fig. 2; [START_REF] Cooper | Structural style, shortening estimates and the thrust front of the Irish Variscides[END_REF][START_REF] Cooper | Structural evolution of the Irish Variscides[END_REF]. Several N-S features are also known in the southern Porcupine Basin but their relation to the Variscides remains uncertain due to generally-poor seismic imaging and little age constrains, as well as superimposed

Mesozoic extreme marginal thinning [START_REF] Robinson | The Fastnet Basin: an integrated analysis[END_REF][START_REF] De Graciansky | The Goban Spur transect: geologic evolution of a sediment-starved passive continental margin[END_REF][START_REF] Cook | The Goban Spur: exploration in a deepwater frontier basin[END_REF][START_REF] Naylor | Porcupine-goban Region -a Standard Structural Nomenclature System[END_REF]. Lower Carboniferous strata are observed in the rest of the Porcupine region in lateral continuation of Carboniferous sediments across the Clare Basin [START_REF] Gill | Syndepositional Sliding and Slumping in the West Clare Namurian basin, Ireland. GSI Special Paper[END_REF][START_REF] Croker | The Clare Basin: a geological and geophysical outline[END_REF] but internal deformation is poorly constrained.

Late Palaeozoic-Mesozoic tectonostratigraphy and Porcupine Basin formation events

Following these orogenic episodes, the Porcupine region encompassed repeated extension and subsidence events resulting in thick Late Palaeozoic-Mesozoic successions above crystalline basements and with uncertain links with previous orogenic basements. The tectonostratigraphic interpretation of rifted elements mostly depends on data resolution and on the attempt of reconstructing local events in the broad context of the Pangaea breakup.

The prerift basement is subdivided in two megasequences including Mississippian (Tournaisian-Lower Viséan) coarse and fine clastics, shelf carbonates and shales partially preserved in the eastern Porcupine region, followed by Pennsylvanian (Moscovian-Gzhelian) fine and coarse clastics recovered through much of the basin (e.g. [START_REF] Croker | The evolution and hydrocarbon prospectivity of the Porcupine Basin, Offshore Ireland[END_REF][START_REF] Robeson | The upper palaeozoic geology of the porcupine, erris and donegal basins, offshore Ireland[END_REF]Tate and Dobson, 1989a). Although part of the Carboniferous interval is coeval of the Variscides construction, compressional features remain uncertain due to the regional seismic resolution at such stratigraphic level. Similarly, Mississippian normal faulting is observed onshore along reactivated Caledonian trends (Fraser andGawthorpe, 1990, 2003;[START_REF] Worthington | Structure of Lower Carboniferous basins of NW Ireland, and its implications for structural inheritance and Cenozoic faulting[END_REF][START_REF] Davies | Carboniferous sedimentation and volcanism on the laurussian margin[END_REF] but the fault extent in the Porcupine region is also rather vague. Both Mississippian and Pennsylvanian megasequences are separated by a regional unconformity (Late Viséan to Bashkirian) that is alternatively interpreted as a final compressional event in the closure of the Rheic Ocean or as an early effect of orogenic collapse [START_REF] Praeg | Diachronous Variscan late-orogenic collapse as a response to multiple detachments: a view from the internides in France to the foreland in the Irish Sea[END_REF][START_REF] Naylor | Petroleum Geology of Ireland[END_REF].

Overlying Permo-Triassic sediments are directly tested in the North Porcupine Basin together with possible Lower Jurassic synrift strata and are seismically-interpreted in limited fault-bounded clastic depocentres in the rest of the Porcupine region (Fig. 2a; [START_REF] Croker | The evolution and hydrocarbon prospectivity of the Porcupine Basin, Offshore Ireland[END_REF][START_REF] Naylor | A reflection seismic study of the Porcupine Basin, offshore west Ireland[END_REF][START_REF] Johnston | The Mesozoic evolution of the southern North Atlantic region and its relationship to basin development in the south Porcupine Basin, offshore Ireland[END_REF][START_REF] Naylor | Porcupine-goban Region -a Standard Structural Nomenclature System[END_REF][START_REF] Štolfová | Permo-Triassic development from Ireland to Norway: basin architecture and regional controls[END_REF], which formed during the so-called Early Cimmerian rifting event (e.g. [START_REF] Sinclair | Tectonic control on sedimentary evolution of three North Atlantic borderland Mesozoic basins[END_REF]. Following postrift intervals show Lower and Middle Jurassic shallow marine clastics and carbonates of in the North Porcupine Basin and Middle Jurassic fluvial to lacustrine clastics in the Porcupine Basin. These likely reflect the thermal subsidence of the overall margin but may also relate to the "onset warp setting" of [START_REF] Sinclair | Tectonic control on sedimentary evolution of three North Atlantic borderland Mesozoic basins[END_REF].

A second rifting phase, called Late Cimmerian rifting event, produced the most of rotational fault-blocks during the Upper Jurassic to lowermost Cretaceous [START_REF] Masson | Structure and development of the Porcupine Seabight sedimentary basin, offshore Southwest Ireland[END_REF][START_REF] Macdonald | Geology of oil accumulation in block 26/ 28, Porcupine basin, offshore Ireland[END_REF][START_REF] Naylor | A reflection seismic study of the Porcupine Basin, offshore west Ireland[END_REF][START_REF] Croker | Structure and stratigraphy of the Porcupine basin : relationships to deep crustal structure and the opening of The north atlantic[END_REF][START_REF] Sinclair | Tectonic control on sedimentary evolution of three North Atlantic borderland Mesozoic basins[END_REF][START_REF] Williams | Comparative Jurassic and cretaceous tectono-stratigraphy and reservoir development in the Jeanne d'Arc and porcupine basins[END_REF]. In these studies, the rifting onset varies between Bajocian and earliest Tithonian time, controlling a northward migration of transgressive marine facies which submerged fault-blocks by the end of the Kimmeridgian and Tithonian. The rifting termination is generally picked with the Base Cretaceous Unconformity overlain by Berriasian-Barremian shale-prone strata (postrift sediments). Yet, recent 3D seismic interpretation revealed Tithonian-Valanginian 'transitional sequences' prior the development of 'true' postrift conditions [START_REF] Jones | Structural and stratigraphic evolution of the Connemara discovery, Northern Porcupine Basin: significance for basin development and petroleum prospectivity along the Irish Atlantic Margin[END_REF]. Further minor extension in the Aptian-Albian resulted in progradational deltaics sealed by undeformed Late Cretaceous chalk [START_REF] Moore | A syn-rift to post-rift transition sequence in the Main Porcupine Basin, offshore western Ireland[END_REF][START_REF] Shannon | Cretaceous and Tertiary basin development west of Ireland[END_REF]. Therefore, the interval between syn-and postrift strata combines several events including uplift and thermal subsidence, the latter of which implies to reconsider the overall sequences definition.

Aim of the present study

The various Proterozoic-Palaeozoic orogenic episodes produced the original structural foundations of the Porcupine region, which likely shaped younger basins during Mesozoic rifting episodes (Fig. 3). Nonetheless, several major uncertainties regarding their definition and structural interactions over time still remain. For instance, the extent of Caledonian and Variscan features, largely determined from deep seismic profiles and gravity or magnetic and data, are partially known in the northernmost Porcupine Basin [START_REF] Young | An interpretation of some magnetic data off the west coast of Ireland[END_REF][START_REF] Max | Compilation Magnetic Map: the Irish Continental Shelf and Adjacent Areas[END_REF][START_REF] Masson | A Free-air Gravity Anomaly Map of the Irish Continental Margin and a New Gravity Model across the Southern Porcupine Seabight[END_REF][START_REF] Klemperer | Seismic reflection evidence for the location of the Iapetus suture west of Ireland[END_REF][START_REF] Klemperer | A deep seismic reflection transect across the Irish Caledonides[END_REF] but are not accurately mapped southward due to Mesozoic and Cenozoic sedimentary thicknesses. Moreover, most studies dealt on small areas with specific hydrocarbon aspects and only a few mapped the rift-related fault zones and associated regional syn-and post-rift unconformities [START_REF] Naylor | A reflection seismic study of the Porcupine Basin, offshore west Ireland[END_REF][START_REF] Mccann | Fault styles in the Porcupine Basin, offshore Ireland: tectonic and sedimentary rocks[END_REF]. In addition, these studies do not necessarily integrate the overall basin evolution in a global framework of hyper-extension [START_REF] Naylor | Porcupine-goban Region -a Standard Structural Nomenclature System[END_REF][START_REF] Pad | Petroleum Systems Analysis of the Rockall and Porcupine Basins Offshore Ireland[END_REF][START_REF] Naylor | Petroleum Geology of Ireland[END_REF][START_REF] Stoker | An overview of the upper palaeozoic-mesozoic stratigraphy of the NE atlantic region[END_REF], even if such scenarii have been debated in the south of the basin (e.g. [START_REF] Reston | Detachment faulting, mantle serpentinization, and serpentinite-mud volcanism beneath the Porcupine Basin, southwest of Ireland[END_REF][START_REF] Calvès | New evidence for the origin of the porcupine median volcanic ridge: early cretaceous volcanism in the Porcupine basin, atlantic margin of Ireland[END_REF][START_REF] Watremez | Deep structure of the Porcupine Basin from wide-angle seismic data[END_REF][START_REF] Prada | Crustal strain-dependent serpentinization in the Porcupine Basin, offshore Ireland[END_REF]. In this context, the synrift elements recognition differs widely in the literature, implying variations of timing and significance for each event.

Our detailed mapping shows well-separated extensional episodes based on an extensive 2D and 3D seismic dataset correlated to all available wells in the study area (Fig. 4). Extension clearly started from the Carboniferous to form small and localised half-graben truncated by the Top Carboniferous Unconformity. It is followed by extension in the Rhaetian-Sinemurian, which controlled tilted blocks in the North Porcupine Basin and did not extend south of the Finnian's Spur. The third episode produced pronounced Oxfordian-Tithonian highly-rotated blocks in the Porcupine Basin and spread in the latest stage across the Finnian's Spur. Reactivation of the main bounding faults occurred during the Neocomian to form poorly-tilted and variably thick depocentres. Thus, it appears that deep crosscutting structures (e.g. Finnian's Spur) intermittently localised subsequent deformation or acted as natural barriers to the deformation. This infers fault interactions through time.

Thus, our study aims to demonstrate the extension initiation and the structural reorganisation and highlights fault reactivation processes during the rifting evolution. We also speculate on the sequences genetics in the broad context of a hyper-extended passive margin.

Palaeozoic structural framework

Pre-Mesozoic rocks, usually described as prerift basement, invariably incorporate sedimentary, metamorphic or magmatic lithologies developed during the Precambrian to the Late Palaeozoic. Therefore, they spanned several orogenic cycles, with related structures having a preponderant role on subsequent depocentres formation. Tectonic controls are, for instance, normal faulting in the Carboniferous or rifting shift across the Finnian's Spur during the Mesozoic (Fig. 4). Herein, we present well and seismic correlations of Proterozoic-Palaeozoic packages to highlight such controls across the study area.

Lithologies

Pre-Mesozoic lithologies are regrouped into four distinct packages made of highly metamorphosed to undeformed sedimentary rocks (Fig. 5). They are bounded by first-order stratigraphic surfaces that link to the various Precambrian and Palaeozoic orogenic cycles. Their distribution reflects a lateral continuum of depocentres, already-known across the surrounding structural highs. Bulois et al. Journal of Structural Geology xxx (2018) xxx-xxx

U N C O R R E C T E D P R O O F C.

Grenvillian-equivalent basement

The oldest known basement rocks underlie structural highs west of the Porcupine Basin, such as the Finnian's Spur. Well 26/26-1 tested fractured metasediments resting on Palaeoproterozoic schists and gneisses (Fig. 5). They are probably equivalent to Dalradian rocks onshore Ireland formed during or slightly later the Grenvillian orogenic cycle.

The so-called Grenvillian Basement (GB) is topped by the Top Dalradian Unconformity (Table 1), of which the genesis is unknown due to the lack of data. Coeval unconformities may correspond to Grenvillian or Caledonian deformation phases described onshore (e.g. [START_REF] Hutton | Evidence of a major neoproterozoic unconformity within the dalradian Supergroup of NW Scotland[END_REF][START_REF] Corfu | New perspectives on the Caledonides of Scandinavia and related areas: introduction[END_REF].

Caledonian-equivalent basement

A second basement type, composed of anchizonal to unmetamorphosed coarse clastic sediments, form a discontinuous package through the northern Porcupine region (Fig. 5). The most complete succession (Well 26/26-1) consists in a 60-to-70 m-thick basal section made of conglomerates and coarse sandstones followed by finer clastics. Top units are correlatable to poorly deformed shales, siltstones and mudstones (Well 26/21-1) and quartzitic sandstones (wells 26/28-1, 26/ 28-3 and 34/5-1). Numerous carbonised spores and lithoclasts of magmatic or metamorphic origins infer a crystalline basement proximity. Basal quartzite in Well 26/28-1 was radiometrically dated as Devonian to earliest Carboniferous (Tournaisian), so that this basement type is referred herein as Caledonian Basement (CB).

It is truncated by the so-called Top Devonian and Base Mississippian unconformities, sometimes observed in dipmeter data but more surely confirmed by sedimentary hiatuses spanning the Tournaisian interval (Fig. 5, Table 1). The surfaces either relate to a late compres sional phase of the Caledonides or to the subsequent Variscides construction.

Variscan-equivalent basement

The Top Devonian and possibly the Top Dalradian unconformities are overlain by unmetamorphosed rocks, deposited during the Mississippian and forming the so-called Variscan Basement (VB). As for other basements, the distribution is unequal due to syn-and post-deposition tectonics (discontinuous depocentres, Fig. 5) but related unconformities may be predicted from well and seismic correlation (Fig. 6). The most complete succession (Well 35/15-1) shows a first section (VB 1 ) composed of calcareous sandstones and micaceous siltstones passing to micritic limestones and siltstones and terminated by mudstones and fine sandstones. It is followed by a second section (VB 2 ) opening on basal conglomerate overlain by limestones with brachiopods and crinoids, dolomites and volcanoclastics. The transition between both sections is genetically unclear (possible sedimentary hiatus) and may reflect localised relative uplift consistent with cyclic terrestrial to shallow-marine conditions expected from the lithology (e.g. carbonates vs shales occurrences). The Variscan Basement is further interpreted in wells 26/26-1 and 26/28-1 based on similarities in lithologies and fossil content. The biostratigraphic information suggests a latest Tournaisian-earliest Namurian age (i.e. Viséan to lower Bashkirian).

Variscan Basement is bounded by the Top Mississippian Unconformity of Namurian-Westphalian A age (i.e. Bashkirian to lowermost Moscovian, Table 1). The surface is consistent with the poor preservation of Mississippian strata in the study area and is coeval to the Variscides construction onshore Ireland.

Post-Variscan Megasequence

Carboniferous sedimentation resumed during the Pennsylvanian resulting in the regional Post Variscan Megasequence deposition (Fig. 5). A first sequence (PVM 1 ) is composed of Westphalian (i.e. ∼Moscovian), fining-upwards clastics alternating between conglomeratic and fine sandstones, mudstones, claystones and coal seams, which together suggest an alluvial-lacustrine environment passing progressively to littoral conditions. This is consistent with the facies distribution suggesting a progressive deepening during Westphalian B/C and getting generalised during Westphalian D (respectively ∼ Duckmantian, ∼lower and ∼upper Bolsovian). A second section (PVM 2 ), composed of mudstones and siltstones containing diverse brackish-water ostracods infers littoral persistence during the Stephanian (∼upper Kasimovian-Gzhelian). A third section (PVM 3 , Well 34/15-1) consists in shales, siltstones and poorly-sorted sandstones interspaced by occasional limestones, evaporates and coal patches in which Autunian, brackish and shallow-marine ostracods are observed.

U N C O R R E C T E D P R O O F
Post Variscan Megasequence is characterised by drastic thickness variations related to the remains of sequence PVM 1 and PVM 2 ., which may be accentuated by the Top Carboniferous Unconformity which locally merges with the Top Autunian Unconformity (Table 1). Both surfaces mark the transition Carboniferous basins with Mesozoic deposition. [START_REF] Aquitaine | Well Elf 34/5-1 Composite Log. Elf Aquitaine Ireland Ltd, prepared[END_REF]& 1981;[START_REF] Deminex | Well Deminex 34/15-1 Composite Log[END_REF]BP, 1979a, 1979b, 1980a, 1980b, 1981a, 1981b, 1981c, 1982a, 1982b& 1982c[START_REF] Gulf | Well Gulf 26/21-1 Composite Log. Gulf Oil (Ireland) Ltd[END_REF]& 1980, Chevron, 1981a& 1981b, Phillips Petroleum Company, 1982& 1983) as well as in [START_REF] Robeson | The upper palaeozoic geology of the porcupine, erris and donegal basins, offshore Ireland[END_REF], Tate and Dobson (1989a) and [START_REF] Croker | The Clare Basin: a geological and geophysical outline[END_REF]. The reader can also refer to [START_REF] Bulois | Marges polyriftées : réactivation et conditions aux limites. Exemples de la Mer de Corail (Papouasie Nouvelle Guinée) et du Bassin de Porcupine[END_REF] for further lithofacies and biostratigraphic information.

Palaeozoic tectonic phases

Well correlation revealed four distinct basement packages, of specific ages and lithologies and separated by regional unconformities (Fig. 5). Their mapping through the seismic volume enables to identify different phases of regional Late Palaeozoic deformation (Fig. 6).

3D seismic data tied to Well 26/28-1 (Fig. 6A Many other rotated fault-blocks are observed through the region (Fig. 6D andE) but remain undrilled, so that it is difficult to determine an accurate age for each basin. Nonetheless, a striking element is the variation in the rotation amount that may directly result from Variscan and Caledonian basement structures reactivated during the extension. In other words, structural directions and basements rheologies have influenced the overall structural style of Late Palaeozoic extensional elements. Bulois et al. Journal of Structural Geology xxx (2018) Basin (north of ∼53°05′N), corresponding to the local Caledonian orientation (Fig. 7). South of 53°N, faults have a predominant N-S orientation that mark the continuation of southernmost structural highs formed during the Variscan compression. Between these two groups, faults turn to an intermediate NE-SW orientation that suggests interaction between both basements.

Late Palaeozoic extensional tectonic patterns and fault reactivation

N C O R R E C T E D P R O O F C.
This implies fault clusters, whose distribution corresponds to key zones with specific deformation pattern that align along crustal basement discontinuities (Fig. 7). In the north, E-W structures are parallel to the Finnian's Spur direction, a structural fault-bounded basement feature made of Dalradian tonalitic gneisses (Grenvillian Basement) drilled in Well 26/26-1 (Figs. 4-6A; e.g. Tate and Dobson, 1989a;[START_REF] Tate | Structural framework and tectono-stratigraphic evolution of the Porcupine Seabight Basin, offshore western Ireland[END_REF][START_REF] Naylor | Porcupine-goban Region -a Standard Structural Nomenclature System[END_REF]. Also, the high and other discrete narrow horsts intermittently separated the Porcupine and North Porcupine basins by localising the deformation. Adjacent Caledonian Basement appears poorly deformed (Fig. 6A andB) and its role on controlling the deformation may be only due to rheological contrasts with the adjacent Grenvillian Basement. To the west, N-S structures are parallel to the Porcupine High that is mostly composed of Precambrian and Palaeozoic rocks [START_REF] Riddihough | A geological framework for the continental margin to the west of Ireland[END_REF][START_REF] Max | Tectonic control of offshore sedimentary basins to the north and west of Ireland[END_REF][START_REF] Naylor | Irish Rockall Basin Region -a Standard Structural Nomenclature System[END_REF]. To the east, they run along the Ruadan High, originally described as a Mesozoic shearing and inversion feature [START_REF] Tate | Structural framework and tectono-stratigraphic evolution of the Porcupine Seabight Basin, offshore western Ireland[END_REF][START_REF] Naylor | Porcupine-goban Region -a Standard Structural Nomenclature System[END_REF] but herein interpreted as part of a buried Variscan nappe gently reactivated afterwards separating the North Mohling and Porcupine basins (Fig. 6C).

Overall, the Palaeozoic fault distribution results in a dog-leg geometry that directly relates to the Caledonian and Variscan basements collage (Fig. 7). This strongly influenced the shape and orientation of subsequent Mesozoic extensional basins.

Rhaetian and Hettangian-Sinemurian rift setting RS 1

Subsequent to Palaeozoic tectonism, the Porcupine region encompassed a period of uplift forming the Top Carboniferous and the Top Autunian unconformities, followed by an early rifting phase during the Permo-Triassic that may have affected the Porcupine region in various places (e.g. [START_REF] Štolfová | Permo-Triassic development from Ireland to Norway: basin architecture and regional controls[END_REF][START_REF] Stoker | An overview of the upper palaeozoic-mesozoic stratigraphy of the NE atlantic region[END_REF]. However, the occurrence of Permo-Triassic strata is regionally debatable as only two wells in the North Porcupine Basin (26/21-1 and 26/22-1A) directly penetrated through Triassic strata and the Top Carboniferous and Top Autunian unconformities mark clearly the contact with Jurassic strata in the Porcupine Basin (Fig. 4; Table 1). This implies to redefine this rift setting, called herein RS 1 (for Rift Setting 1).

Stratigraphy of the Rhaetian and Hettangian-Sinemurian succession

In the North Porcupine Basin, Rift Setting RS 1 controlled Megasequence RS 1 development drilled in wells 26/21-1 and 26/22-1A (Table 2). Herein, we propose to focus on Well 26/22-1A in which sediments and related biostratigraphic content are more complete (Fig. 8A). Two main sequences, called RS 1a and RS 1b , deposited on top of the Top Carboniferous Unconformity (not drilled but recognised from regional mapping around Well 26/21-1).

Sequence RS 1a consists of thick sandstones and intercalations of thin mudstones and siltstones with occasional anhydrites (Fig. 8A). Fossil fragments of undetermined gastropods, enchinoids, palynomorphs and fresh-to brackish-water ostracods increasing topward suggest a littoral to very shallow marine depositional environment. Palynomorphs belong to the Rhaetian-Lower Hettangian interval, assigned to the entire Sequence RS 1a . The boundary with overlying Lower Jurassic Sequence RS 1b , called Top Triassic Unconformity, is picked at the level of the first carbonates occurrence in Well 26/22-1A (Fig. 8A). These usually occur in 20 m-thick limestone beds and dolomites interbedded within mudstones and shales at the base and mudstones and calcareous sandstones upwards. Sequence RS 1b presents abundant palynomorphs and frequent diverse brackish to marine ostracods, and diverse marginal foraminiferas and microplanktonic specimens are getting more important compare to underlying Sequence RS 1a . The age of the sequence is not very well-constrained although definite Sinemurian ostracods and Rhaetian to Hettangian palynofloras infer an overall Hettangian-Sinemurian age.

Paleontological information are different in nature and age from overlying strata deposited during the Middle and Upper Jurassic. This implies an unconformity on top of Sequence RS 1b , called Top Lower Jurassic Unconformity, which merges with younger Mesozoic unconformities on top of the Finnian's Spur (Fig. 4).

Megasequence RS 1a is mapped through the seismic volume based on the recognition of the three above-mentioned bounding unconformities. It extends northward to fill in the entire North Porcupine Basin and is limited by the Finnian's Spur to the south (Table 2,

Rhaetian and Hettangian-Sinemurian normal faults

Several normal faults have controlled the deposition of both sequences RS 1a and RS 1b (Fig. 8B). Sequence RS 1a affects that north-tilted normal faults is most generally limited into small hemi-graben. Faulting did not propagate up to the Top Triassic Unconformity, implying a partially tectonically-driven subsidence. This is consistent with the general base-level rise observed in Well 26/22-1A. Largest faults have been later reactivated during the Hettangian-Sinemurian to control Sequence RS 1b deposition. This lead us to consider two distinct extensional episodes during the Triassic-Lower Jurassic period, whose the transition is marked by the Top Triassic Unconformity.

Fig. 9 displays the subcrop map of fault trends that controlled Sequence RS 1a (Rhaetian). They are oriented along E-W and ENE-WSW directions that are parallel to the Finnian's Spur and therefore correspond to reactivated Late Palaeozoic faults (Fig. 7). In the absence of Triassic-Lower Jurassic strata south of the Finnian's Spur, it is likely that these faults mark the southernmost boundary of the Rift Setting RS 1 . Also, we propose that most of the northern part of the Porcupine Basin was

Upper Jurassic to Lower Cretaceous rift setting RS 2

A second, generally more intense rift episode affected most of the Porcupine region and is here called RS 2 (for Rift Setting 2) for similar reasons than previous Rift Setting RS 1 . Although its definition widely differs in the literature due to internal facies recognition, the synrift unconformity is generally assumed running at the base of the Upper Jurassic and the postrift unconformity at the Base Cretaceous Unconformity (Late Cimmerian rifting phase on Fig. 3). Herein, we redefine this overall synrift setting using all available well and seismic data and reconsider the such surface definitions at basin scale.

Stratigraphy of the Mid-to Upper Jurassic and Lower Cretaceous intervals

To define the synrift interval, we take the example of Well 26/ 28-A1 that shows a complete Middle Jurassic to Lower Cretaceous succession deposited on top of Carboniferous strata (Fig. 10A). The first sediments, contained in Megasequence TS 1 (for Transitional Setting 1, 369 m-thick), are composed of shally, silty and muddy material interbedded with repeated thick (1-to-10 m) clastic levels (coarse or conglomeratic sands) and occasional coal (seams or patches), deposited within a fluvial-lacustrine environment. Numerous plant articles, bisaacate pollens and increasing upward fresh-to brackish-water ostracods, belong to the Bathonian interval. Megasequence TS 1 is toped by the Base Oxfordian Unconformity witnessed by a regional absence of Callovian strata (Table 3).

Subsequent Megasequence RS

2 (for Rift Setting 2, 242 m-thick) is subdivided into three Upper Jurassic sequences (Fig. 10A). Sequence RS 2a (Oxfordian) contains sandstones, mudstones and siltstones and the first occurrence of carbonates (patches or thin beds). Terrestrial and brackish species, including palynofloras, plant articles, dynoflagellates and ostracods, are observed with brackish to shallow-marine foraminiferas, brachiopods, echinoderms and gastropods. Strata are therefore interpreted to have deposited in an estuarine to coastal plain context. Following Sequence RS 2b (Kimmeridgian) is picked from the absence of sands and the first appearance of thick limestone beds (>5 m) interbedded within siltstones and mudstones. A very diverse assemblage of dynophycaea, ostracods, foraminiferas, marine bivalves, brachiopods or echinorderms is observed throughout. The sequence therefore deposited in a marginal marine environment. Overlying Sequence RS 2c (uppermost Kimmeridgian and Tithonian) is mudstone-dominated and incorporates almost exclusively marine species (varied dynoflagellates, foraminiferas and fragmented of echinoderms, gastropods and bivalves). Overall, Megasequence RS 2 shows a progressive base-level rise that most likely terminated with the construction of the Base Cretaceous Unconformity.

Following Megasequence TS 2 (for Transitional Setting 2, ca. 40 m-thick) consists in mudstones and limited sandy and carbonate levels (Fig. 10A), sealed by disconformable massive Aptian-Albian sandstones and mudstones. The lowermost part of the sequence is poorly dated and the contact pattern with overlying Aptian-Albian strata, although disconformable in the well, may represent a clear unconformity in other wells (see [START_REF] Bulois | Marges polyriftées : réactivation et conditions aux limites. Exemples de la Mer de Corail (Papouasie Nouvelle Guinée) et du Bassin de Porcupine[END_REF] for further discussion). Megasequence TS 2 contains almost exclusively marine species including foraminiferas, dynophyceae and more minor ostracods that belong to the Berriasian-Barremian interval. It is topped by a regional unconformity, called Top Neocomian Unconformity that produced a condensed succession at well site (Fig. 10B). The thickness and facies nature vary throughout Bulois et al. Journal of Structural Geology xxx (2018) xxx-xxx the study area, as a result of the installation of the Rift Setting RS 2 (see differences between Figs. 8A-10A).

U N C O R R E C T E D P R O O F C.

Seismic expression of the Middle Jurassic-Lower Cretaceous interval

The three previously-described megasequences are correlated to the entire seismic volume (Fig. 10B), together with unconformities of regional extent (Table 3). All of these have specific varying depositional controls.

Above the Top Carboniferous Unconformity, Megasequence TS 1 is characterised by parallel to subparallel, subcontinuous reflectors of medium-to-strong amplitude (Fig. 10B). This geometry implies the absence of faulting but, instead, a strong subsidence as witnessed by the generalised fluvial-alluvial to lacustrine transition. Subsidence relates to thermal cooling following Rift Setting RS 1 in the North Porcupine Basin and clearly predates the installation of the Upper Jurassic-Neocomian Rift Setting (RS 2 ). Megasequence RS 2 , bounded by the Base Oxfordian and the Base Cretaceous unconformities, is characterised by intense growth-faulting. Oxfordian Sequence RS 2a displays subparallel to slightly oblique, subcontinuous reflectors, witnessing of subtle thickening (Fig. 10B). Subsequent Kimmeridgian Sequence RS 2b shows continuous reflectors and much more pronounced thickening toward the bounding faults. Finally, Tithonian Sequence RS 2c shows medium-to strong-amplitude, continuous reflectors that expand beyond the limits of sequences RS 2a and RS 2b . This confirms the overall Upper Jurassic marine transgression as described in the above. Nonetheless, deposition and deformation are not homogeneous as observed on regional seismic lines (Figs. 4 and6). This implies timing or magnitude variations of regional extent that directly link to fault expression.

Upper Jurassic strata are followed by Megasequence TS 2 , sandwiched by the Base Cretaceous and the Top Neocomian unconformities (Fig. 10B). Depocentres are only limited by the main Upper Jurassic faults that progressively die out during the Neocomian. Strata are generally sub-horizontal, implying a rather reduced tectonic control compare to underlying Megasequence RS 2 . Thus, Megasequence TS2 suggests a progressive and uniform sag.

Upper Jurassic-Lower Cretaceous tectonism and relations with earlier structural trends

The analysis of the present 2D and 3D seismic dataset enabled the recognition of several fault orientations to Late Palaeozoic trends (Figs. 7 and11). EW and ENE-WSW faults are dominant in the northwestern part of the Porcupine Basin and in the North Porcupine Basin, where they run parallel to the Finnian's Spur, before turning to NE-SW trend in the northeastern part of the Porcupine Basin. N-S faults appear locally in the north but prevail south of latitude 53°N, in continuation of the Ruadan High. They rotate to a NNE-SSW orientation in the southeastern part of the Porcupine Basin. Such a fault cluster marks the strong v-shape of the overall Porcupine Basin.

Fig. 12 shows the structural relationships existing between Rift Setting RS 2 and underlying Palaeozoic structures. In the southern part of the basin, high-resolution seismic imaging show Carboniferous folds and thrusts, that relate to characteristic thin-skinned Variscan deformation (Fig. 12A). They form buried structural highs, such as the Ruadan High, which clearly controlled the thickness of subsequent Upper Jurassic-Lower Cretaceous strata. To the west (Fig. 12B), extension produced normal faults flattening along Pennsylvanian sedimentary levels interpreted as lithological changes (e.g. higher rate of shale) or variation of structural styles (e.g. change in the fault dips and lesser compressional faults). This produced sliding packages that link onto faults basinward. In the northern part of the Porcupine Basin (Fig. 12C), asymmetrical to slightly overturned folds, locally associated with thrusts, controlled subsequent Pennsylvanian and Upper Jurassic-Lower Cretaceous normal faults that root on their forelimbs (flat part) and progressively upright in the upper part of the sedimentary succession. These structural relations imply a strong control of the Variscan structures onto the RS 2 -related extension, which decreases northward due to the Caledonian Basement predominance.

U N C O R R E C T E D P R O O F

Discussion

Our seismic analysis has shown various extensional megasequences that each reflects specific periods encompassed by the Porcupine region. Herein, we discuss them in terms of margin dynamics in order to constrain the structural response to the faulting propagation at a crustal scale and to conceptualise how inheritance and fault reactivation processes shaped the region in separated extensional frameworks.

Overlapping extensional megacycles

In the study area, several structural trends have been intermittently active since the Late Palaeozoic and relate to the internal basement geometries issued from the Variscan, Caledonian and, to a minor ex Bulois et al. Journal of Structural Geology xxx (2018) tent, Grenvillian compression phases. In the present study, we argue that the deep crustal structures had varying controls on the structural development over a long period of time, resulting in three overlapping extensional systems. These events can be considered as separated extensional megacycles (sensu [START_REF] Bulois | Successive rifting events in marginal basins: the example of the coral Sea region (Papua New Guinea)[END_REF], controlled by either volume or plate dynamics forces (i.e. orogenic collapse or true rifting). They therefore express differently the overall faulting propagation though the region.

U N C O R R E C T E D P R O O F C.
Upper Palaeozoic extension results in slightly-rotated extensional basins formed along E-W and NE-SW Caledonian trends in the north of the basin and N-S Variscan thrusts and folds directions in the south (Fig. 13A). In the Porcupine Basin, Upper Palaeozoic depocentres seem to propagate from east to west and south to north, inferring a progressive collapse of the Variscan Orogen and a poorer control of Caledonian structures at the time of deposition. This type of extension is confirmed by the continental to very-shallow-marine conditions identified throughout Megasequence PVM 2 (our study) and extensional stages identified onshore [START_REF] Praeg | Diachronous Variscan late-orogenic collapse as a response to multiple detachments: a view from the internides in France to the foreland in the Irish Sea[END_REF]. Subsequent Rift Setting RS 1 corresponds to an Upper Triassic-Lower Jurassic rifting phase, restrained to the North Porcupine Basin as it abutted the Finnian's Spur (Fig. 13B). As a result, reactivation of E-W/ENE-WSW Caledonian trends controlled shallow marine depocentres (Megasequence RS 1 ) interpreted as the southern extent of a southwards-propagating rift setting linked to the early Permo-Triassic Arctic rift setting [START_REF] Ziegler | Evolution of the Arctic-North Atlantic and the Western Tethys[END_REF][START_REF] Roberts | Palaeozoic to Tertiary rift and basin dynamics : mid-Norway to the Bay of Biscay -a new context for hydrocarbon prospectivity in the deep water frontier[END_REF]. Final Rift Setting RS 2 took place mostly in the Porcupine Basin during the Upper Jurassic and continued during the Neocomian (Fig. 13C). It resulted in shallow-to-deep-marine depocentres through both basins, with deposition due to the tectonic development of the northward-propagating North Atlantic Ocean [START_REF] Ziegler | Evolution of the Arctic-North Atlantic and the Western Tethys[END_REF][START_REF] Roberts | Palaeozoic to Tertiary rift and basin dynamics : mid-Norway to the Bay of Biscay -a new context for hydrocarbon prospectivity in the deep water frontier[END_REF][START_REF] Doré | Principal tectonic events in the evolution of the northwest European Atlantic Margin[END_REF] and, to a certain extent, of the Central Atlantic Ocean (Stampfli and Borel, 2002;[START_REF] Nirrengarten | Kinematic evolution of the southern North Atlantic: implications for the formation of hyper-extended rift systems: kinematic of hyper-extended rift systems[END_REF]. Further extensional episodes in the Aptian-Albian and the Cenozoic are not taken into account in our study as they have had very minor effects on the overall geometry of the Porcupine Basin (Figs. 6,8 and 10). They are likely to represent deviatoric stresses arising from other plate settings (e.g. opening of the Bay of Biscay or of the North Atlantic Ocean sensu stricto).

In our overall evolutionary concept (Fig. 13), extension progressively migrated from the initial deformation core by reactivating pre-existing orogenic structures and stopped when abutting transversal lineaments. Thus, Caledonian faults initiated and stopped both Triassic-Lower Jurassic and Upper Jurassic-Neocomian extensional phases, while the Variscan Fold Belt, concordant with the Ruadan High, localised Upper Jurassic extension alongside to give the general N-S direction but also stopped the extension further northeast when bending to the more characteristic E-W orientation. Previous palaeogeographic reconstructions showed similar dog-leg shape for the regional Variscan trends (e.g. [START_REF] Ziegler | Evolution of the Arctic-North Atlantic and the Western Tethys[END_REF][START_REF] Ballèvre | Histoire Géologique du Massif Armoricain : actualité de la recherche[END_REF]. Nonetheless, we consider herein that such shape is produced by the collage and internal structures of both Variscan and Caledonian terranes.

Our dataset also suggests that such structural interactions controlled, at least partially, the characteristic v-shaped geometry of the overall Porcupine Basin. This implies a propagator mechanism developing accordingly to two different thickened continental crusts (Caledonian and Variscan) of specific structural patterns. This is confirmed by higher finite extension to the south with lithospheric stretching factors (β) grading from 1.1 to 1.7 to the north to more than 6 to the south [START_REF] Tate | Lithospheric extention and magmatism in the Porcupine basin west of Ireland[END_REF]. In parallel, extreme crustal thinning and serpentinised mantle is observed on deep refraction profiles southward [START_REF] Readman | The deep structure of the Porcupine Basin, offshore Ireland, from gravity and magnetic studies[END_REF][START_REF] O'reilly | Crustal thinning, mantle exhumation and serpentinization in the Porcupine Basin, offshore Ireland: evidence from wide-angle seismic data[END_REF][START_REF] Watremez | Deep structure of the Porcupine Basin from wide-angle seismic data[END_REF][START_REF] Prada | Crustal strain-dependent serpentinization in the Porcupine Basin, offshore Ireland[END_REF] and new seismic refraction models suggest the presence of oceanic crust in the southernmost part of the Porcupine Basin (Chen et al., 2017, submitted). This likely relates to the uppermost Jurassic and Neocomian continental rifting in the Porcupine Basin and coeval uplift in the North Porcupine Basin, resulting in thick strata in the south than thin northward. In that sense, the differential response of the Caledonian and Variscan structures provided, at least partially, the shape of the overall basin within the Irish Atlantic Margin evolution framework.

This global mechanism is similar to the ones considered for other propagator settings such as the Coral, South China or southwestern Barents seas where heterogeneities at crustal and lithospheric scales partly controlled the development of rift propagators in addition of sequent extensional systems [START_REF] Gernigon | Crustal and basin evolution of the southwestern Barents Sea: from Caledonian orogeny to continental breakup[END_REF][START_REF] Savva | Different expressions of rifting on the South China Sea margins[END_REF][START_REF] Bulois | Successive rifting events in marginal basins: the example of the coral Sea region (Papua New Guinea)[END_REF]. Therefore, their evolution combines successive extensional megacycles developing according to the reactivation of early crustal discontinuities and in respect of the overall boundary conditions variations. In the case of the Porcupine Basin, the Aptian-Albian Bay of Biscay opening probably stopped extension in the Porcupine Basin and forced relocation of the deformation in the Rockall and Hatton region (e.g. [START_REF] Srivastava | Evolution of Mesozoic sedimentary basins around the North Central Atlantic: a preliminary plate kinematic solution[END_REF][START_REF] Doré | Principal tectonic events in the evolution of the northwest European Atlantic Margin[END_REF][START_REF] Roberts | Palaeozoic to Tertiary rift and basin dynamics : mid-Norway to the Bay of Biscay -a new context for hydrocarbon prospectivity in the deep water frontier[END_REF][START_REF] Tugend | Spatial and temporal evolution of hyperextended rift systems: implication for the nature, kinematics, and timing of the Iberian-European plate boundary[END_REF].

U N C O R R E C T E D P R O O F

Regional significance of extensional megasequences

Conceptual views of the extensional development within single rifted basins traditionally focus on the fault system evolution and not necessarily on the stratigraphic record at the scale of a basin or of a passive margin (e.g. [START_REF] Leeder | Sedimentary models for extensional tilt-block/ half-graben basins[END_REF][START_REF] Prosser | Rift-related linked depositional systems and their seismic expression[END_REF][START_REF] Ravnås | Architecture of marine rift-basin successions[END_REF][START_REF] Purser | Sedimentation and Tectonics in Rift Basins Red Sea:-Gulf of Aden[END_REF][START_REF] Gawthorpe | Tectono-sedimentary evolution of active extensional basins[END_REF]. The most advanced regional models (e.g. [START_REF] Manatschal | New models for evolution of magma-poor rifted margins based on a review of data and concepts from West Iberia and the Alps[END_REF][START_REF] Péron-Pinvidic | The final rifting evolution at deep magmapoor passive margins from Iberia-Newfoundland: a new point of view[END_REF] suggest discrete stages of evolution of a passive margin that reflects the lateral geometry of fault systems, with block faulting in the stretching domain and highly-rotated half-graben in the hyper-thinning domain oceanward (Fig. 14A). These models usually complete the rifting within 5-20 Myrs [START_REF] Patton | Tectonic evolution and structural setting of the Suez rift[END_REF][START_REF] Gupta | A mechanism to explain rift-basin subsidence and stratigraphic patterns through fault-array evolution[END_REF]Sharp et al., 2000aSharp et al., , 2000b;;[START_REF] Dawers | The role of fault interaction and linkage in controlling synrift stratigraphic sequences: late Jurassic, Statfjord east area, northern north sea[END_REF].

In the Porcupine Basin, deformation is organised in successive extensional episodes that are geographically and temporarily separated regionally. They combine an original orogenic collapse stage passing to classical stretching-thinning episodes. This reflects, on one hand, the large part of the extensional history of the Irish Atlantic Margin and follows, on the other hand, the classical stages of the formation of a passive margin, but on a 220Myr duration. It produces vertically-stacked megasequences bounded by synrift and postrift unconformities, and the relocation of the deformation during the extensional process. From our observation, we therefore discriminate extension driven by the orogenic collapse from the rifting sensu stricto. Collapse-generated extension, directly controlled by folds and thrusts of the Variscan Orogen, produced adjacent, slightly-or highly-rotated fault-blocks (Megasequence PVM, Fig. 6). In comparison, rifting forms numerous fault-blocks before extension focuses along the main bounding faults (Figs. 8 and 10). For both rift settings, this implies a diffuse rifting (high rota tion) responsible of a rift megasequence (RS), followed by a localised rifting (sag), responsible of the deposition of a transitional megasequence (TS). These transitions from orogenic collapse to the rifting and from diffuse to localised rifting produced regional unconformities, which articulate within the extensional megacycle concept in a basin and also into the evolution from stretching to hyper-thinning modes at a margin scale. Deep seismic data in the Porcupine Basin (e.g. [START_REF] Reston | Detachment faulting, mantle serpentinization, and serpentinite-mud volcanism beneath the Porcupine Basin, southwest of Ireland[END_REF][START_REF] Reston | Extreme crustal thinning in the south Porcupine Basin and the nature of the Porcupine Median High: implications for the formation of non-volcanic rifted margins[END_REF][START_REF] O'reilly | Crustal thinning, mantle exhumation and serpentinization in the Porcupine Basin, offshore Ireland: evidence from wide-angle seismic data[END_REF] show a striking crustal asymmetry, expressing the Variscan Front that was likely reactivated as a detachment to produce basin hyper-thinning (Fig. 14B). Similar observation in the adjacent Rockall Basin (e.g. [START_REF] Morewood | The crustal structure and regional development of the Irish Atlantic Margin region[END_REF] are interpreted as an evidence of the Iapetus Suture (i.e. Caledonian crust) while, in the Hatton Basin, the absence of hyper-thinning is interpreted as an effect of the thick Archean crust (i.e. Grenvillian terranes, [START_REF] Smith | Structure of the Hatton Basin and adjacent continental margin[END_REF]. At the same time, a generalised migration of the deformation as proposed in the literature reflects the re-arrangement of the margin from the Triassic to the Upper Cretaceous before Cenozoic breakup west of the Hatton Bank (e.g. [START_REF] Doré | Principal tectonic events in the evolution of the northwest European Atlantic Margin[END_REF][START_REF] Stoker | An overview of the upper palaeozoic-mesozoic stratigraphy of the NE atlantic region[END_REF]. Somehow, this lateral configuration infers a crustal boudinage prior seafloor spreading.

Two different extensional modes are therefore proposed with, on one side, the initiation of the extension (orogenic collapse) and, on the other side, the crustal denudation (diffused rifting and localised hyper-stretching). Both modes are driven by the old crustal architecture and are sensitive to far-field forces. In that sense, extension may be regarded as a long-lived deformation period taking place with discontinuous events, just like in a classical passive margin formation (Fig. 14A), but our study also suggests a strong dependence to continental discontinuities (i.e. ancient orogenic structures) with an oceanward deformation migration at a margin scale (Fig. 14B). In addition, internal basin dynamics arranges in a diffuse and wide rift setting followed by deformation relocation along the main bounding faults towards the ocean. As a result, the stretching and the hyper-thinning stages are typically observed in rifted and transitional megasequences respectively, with a tectonic control along specific faults. In such a way, this succession episode implies extension on a 220Myr-duration with a succession of internal extensional megacycles of 15-20 Myrs, the last one leading to hyper-extension.

Conclusion

In this study, we have documented several extensional megacycles that successively developed in the Porcupine Basin over a long period of time evaluated at 220 Myrs (Carboniferous to Neocomian). Each ex tensional megacycle is thought to represent a specific period encompassed by the Irish Atlantic Margin and may be defined by characteristic extensional megasequences bounded by precise syn-and postrift unconformities. These extensional megacycles are defined as such: 2 and TS 2 deposited in several fault-blocks in a diffuse rift system, followed by more localised faulting along the bounding faults, which controlled a marine transgression at the scale of the basin. We therefore observe that Neocomian extension is a continuation of the Upper Jurassic rifting due to the North Atlantic opening, although faulting is progressively replaced by postrift thermal subsidence. This expresses the ultra-thinning and Moho rise in the south of the Porcupine Basin.

This apparent overlap of extensional settings reflects the formation of a typical Atlantic-type passive margin. It particularly shows a vertical stack of extensional megasequences, separated by characteristic unconformities bounding each extensional megacycles. Also, the structure of the continental crust has a dominant role in the initiation of the rifting (i.e. orogenic collapse) and then decreases as long as extension evolves. In this way, the Porcupine Basin opened as a typical rift propagator, with extension progressively migrating northward and oceanward, as well as differential uplift in the North Porcupine Basin and surrounding highs. This implies unsuccessful continental tears and may explain such a long-standing extensional history along the Irish Atlantic Margin.

Uncited references [START_REF] Dancer | Structural evolution of the Slyne trough[END_REF]Tate and Dobson, 1989b.

Fig. 1 .

 1 Fig. 1. Simplified terrane map of the NE European Atlantic margin. Map is adapted from Štolfová and Shannon (2009). Black box shows the location of the study area and red line the approximate position of Fig. 14B. Abbreviations: for basins: CSB -Celtic Sea Basins, HB -Hatton Basin, ISB -Irish Sea Basins, NVG -North Vøring Basin, PB -Porcupine Basin, RB -Rockall Basin, SVB -Southern Vøring Basin, WAB -Western Approaches Basin; Abbreviations for fault zones: ADTZ -Anton Dohm Transfer Zone, GGFZ -Great Glen Fault Zone, HBFZ -Highland Boundary Fault Zone, IS -Iapetus Suture, JML -Jan Mayan Lineament, MT -Moine Thrust, MTFZ -Møre-Trøndelag Fault Zone, OHFZ -Outer Hebrides Fault Zone, ØFZ -Øygarden Fault Zone, SSFZ -Shetland-Spine Fault Zone, SUFZ -Southern Uplands Fault Zone, TFZ -Tørnquist Fault Zone, VF -Variscan Front, WTR -Wyville-Thompson Ridge. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2 .

 2 Fig. 2. Rifted basins around Ireland according to the literature. A-Simplified structural map of Ireland. Map is adapted from many sources, principally GSI (2006) and[START_REF] Naylor | Geology of offshore Ireland[END_REF], from which Permo-Triassic depocentres in the study area (black box) are determined but dismissed in our study. Note that old structures (e.g. Iapetus Suture, Great Glen Fault) are not always well-defined onshore and offshore Ireland due to the resolution of subsurface data. B-Schematic cross-section across onshore and offshore Ireland, showing the collage of the different basements involved in the stretching of the Irish margin, and their related structures that likely have controlled subsequent basin formations. The onshore part is based from[START_REF] Cooper | Structural evolution of the Irish Variscides[END_REF] and the offshore parts on deep seismic lines Wire 1 & 2 and SWAT[START_REF] Birps | Deep seismic reflection profiling between England, France and Ireland[END_REF][START_REF] Klemperer | A deep seismic reflection transect across the Irish Caledonides[END_REF]. Abbreviations: AGF Antrim-Galway Fault, DB -Donegal Basin, FHCBL -Fair Head-Clew Bay Line, GGFZ -Great Glen Fault Zone, IS: Iapetus Suture, MB -Mayo Basin, NCSB: North Celtic Sea Basin, VF -Variscan Front.

Fig. 3 .

 3 Fig. 3. Simplified lithostratigraphy affected by the long-lasting extension in the Porcupine region. The present log, based on lithologies and ages published in well reports and academic publications, shows the two principal Mesozoic rifting episodes (Early Cimmerian and Late Cimmerian) in the Porcupine region on top of orogenic-related terranes (see namely Sinclair et al. (1994), Williams et al. (1999) and Jones and Underhill (2011) for further information).

Fig. 4 .

 4 Fig. 4. 2D seismic cross-section showing three temporarily-and spatially-defined extensional phases in the Porcupine region. Three main extensional phases of Carboniferous, Triassic-Lower Jurassic and Upper Jurassic-Neocomian are recognised together with minor reactivations in Aptian-Albian and computational faults in Upper the Cretaceous and Cenozoic. They shaped the Porcupine and North Porcupine basins differently across the Finnian's Spur. Seismic line location can be seen on Fig. 7 and 11. Basement types as displayed on Fig. 5 may appear slightly different at well site (herein Well 26/28-1 which is located about 1-km south of the line), due to intense faulting and unequal Palaeozoic topography controlling lateral variations.

  Fig. 5. Synthetic correlations of the Late Palaeozoic lithofacies. This simplified lithostratigraphic panel shows megasequences and their relative bounding sequences, notably the Top Devonian Unconformity marking the construction of the Caledonian Orogeny and the Mississippian Unconformity of the Variscan Orogeny. The Top Carboniferous/Top Autunian unconformities characterise the transition of Late Palaeozoic basins generated by orogenic collapse to basins controlled by Middle Jurassic thermal subsidence or by Late Jurassic-Early Cretaceous extensional forces. Ages are based on information published in composite wells and reports (Elf[START_REF] Aquitaine | Well Elf 34/5-1 Composite Log. Elf Aquitaine Ireland Ltd, prepared[END_REF][START_REF] Aquitaine | Well Elf 34/5-1 Completion Report. Elf Aquitaine Ireland Ltd, prepared[END_REF][START_REF] Deminex | Well Deminex 34/15-1 Composite Log[END_REF] BP, 1979a, 1979b, 1980a, 1980b, 1981a, 1981b, 1981c, 1982a, 1982b & 1982c[START_REF] Gulf | Well Gulf 26/21-1 Composite Log. Gulf Oil (Ireland) Ltd[END_REF] & 1980, Chevron, 1981a & 1981b, Phillips Petroleum Company, 1982 & 1983) as well as in[START_REF] Robeson | The upper palaeozoic geology of the porcupine, erris and donegal basins, offshore Ireland[END_REF],Tate and Dobson (1989a) and[START_REF] Croker | The Clare Basin: a geological and geophysical outline[END_REF]. The reader can also refer to[START_REF] Bulois | Marges polyriftées : réactivation et conditions aux limites. Exemples de la Mer de Corail (Papouasie Nouvelle Guinée) et du Bassin de Porcupine[END_REF] for further lithofacies and biostratigraphic information.

  ) show well-developed Caledonian Basement, resting on top of interpreted Grenvillian Basement and unconformably overlain by the Post-Variscan Megasequence. The Top Devonian and Top Mississippian unconformities merge together but their seismic distinction is difficult due to relatively thin remaining Mississippian strata (c. 10 m at well site). This supposes differential erosion of Caledonian Basement during the Variscan uplift. Internally, Devonian and Pennsylvanian sediments show tenuous but discontinuous growth-faulting, implying at least two main phases of extension. Additional syn-tectonic thickening is also observed in the Post-Variscan Megasequence. All together, this suggests succeeding extensional and compressional events liking to the Caledonides and Variscides evolution. Correlation with Well 26/27-1B shows clearer thickening in the Post-Variscan Megasequence, controlled by normal faults rooting in the Caledonian or Grenvillian basements (Fig. 6B). The absence of Mississippian strata witnesses of the Variscan uplift incidence despite clear compression evidence. The Variscan compression is well observed in the southeast where Mississippian strata are arranged in tectonic nappes (Fig. 6C) in lateral continuation of those observed onshore. They probably involve Devon ian rocks, appearing in a chaotic seismic basement with some continuous reflectors that are in turn are overlain by definite Mississippian strata (Well 35/15-1) shaped in open folds controlled by décollement faults. The latter constitutes local highs (e.g. Ruadan High), partially sealed by Pennsylvanian strata of the Post-Variscan Megasequence. Overlying Pennsylvanian succession is affected by numerous normal faults, implying that the Top Mississippian Unconformity marks a transition from thrust-dominated strata (?Devonian and Mississippian) to normal-faulted depocentres (Pennsylvanian). Variscan compression indicators decrease westward, resulting in a global thickening of the Post-Variscan Megasequence along the Porcupine High (i.e. basin tilt to the west). Extension is sealed by the Top Carboniferous Unconformity in most of the central part of the basin and by the Top Autunian Unconformity in limited fault-blocks in the southwest (Well 34/15-1).

  Seismic analysis highlights fault-blocks principally filled in by Pennsylvanian to Autunian sediments (Post-Variscan Megasequence) on top of Caledonian or Variscan basements. Normal faults developed along E-W to ENE-WSW trends in the North Porcupine and northern Porcupine

U

  Fig. 6. 2D and 3D seismic lines highlighting the main Late Palaeozoic growth faulting episodes and their related unconformities. A-3D seismic line calibrated to Well 26/28-1, showing two different episodes of growth in the Devonian (or older) and Pennsylvanian successions, at the scale of a fault-block and within a fault-block. Note that the Variscan Basement, although present therein, is not mapped due to seismic resolution (∼10 m-thick conglomerate in the well). Black box represents the position of the seismic close-up. B-2D seismic line calibrated to Well 26/27-1B showing growth in the Pennsylvanian succession. C-2D seismic line showing the difference between Mississippian, Pennsylvanian-Autunian tectonic extension in the southern part of the study area. The two black boxes represent the position of the seismic close-ups of Fig. 12. D & E-2D seismic lines showing undrilled fault-blocks in which Palaeozoic rocks are differentiated according to the seismic characters and the internal deformation patterns. Seismic lines and wells location can be seen on Fig. 7. Naylor et al., 2002) are most likely Westphalian-to-Stephanian in age according to our seismic interpretations. In the North Porcupine Basin, the Top Carboniferous Unconformity corresponds to truncated Carboniferous reflectors onlaped by Sequence RS 1a . The Top Triassic Unconformity is characterised by medium-to strong-amplitude reflectors correlative to the top of massive sands and the first occurrence of carbonates in Sequence RS 1b . The Top Lower Jurassic Unconformity is determined from localised Middle Jurassic truncations and onlaps. The three surfaces merge the one another north of the North Porcupine Basin (i.e. southward basin tilt).

Fig

  Fig. 6. (Continued)

Fig. 6 .

 6 Fig. 6. (Continued)

Fig. 7 .

 7 Fig. 7. Map showing the fault network affecting Megasequence PVM (Pennsylvanian) in the Porcupine region.

Fig. 8 .

 8 Fig. 8. Rift Setting RS 1 in the North Porcupine Basin. A-Synthetic lithological information of Well 26/22-1A showing the various sequences in the North Porcupine Basin. Well information is adapted from BP (1978a & 1978b). & B-2D seismic line in the North Porcupine Basin, showing the two subsequent faulting episodes affecting RS 1a (diffused, Triassic) and RS 1b

Fig. 9 .

 9 Fig. 9. Fault pattern in the North Porcupine Basin at the level of the Top Triassic Unconformity (Sequence RS 1a ).

  1 A late Palaeozoic event (Upper Carboniferous) shows rapid extension with little wedging and reactivates E-W and ENE-WSW trends of the southern boundary of the Caledonian Orogen and NE-SW and N-S faults along the northwestern part of the Variscan Orogen. The localisation of the deformation suggests a strong reactivation of orogenic structures in a general orogenic collapse. In particular, the Variscan crust, with a clear thin-skinned geometry, mostly controlled the formation of extensional fault blocks. 2 An early Triassic to Lower Jurassic extension (RS 1 ) took place in the North Porcupine Basin and did not propagate southward in the Porcupine Basin. It controlled the deposition of Megasequence RS 1 , which shows diffuse deformation during the Rhaetian (Sequence RS 1a ) getting localised along the main boundary faults during the Hettangian-Sinemurian (Sequence RS 1b ). Synchronous to the Slyne-Erris formation, the rifting localised along Caledonian trends in a global NW-SE extensional direction. It corresponds to the southern extent of coeval Arctic rift system. 3 An Upper Jurassic-Neocomian extension (RS 2 ), characterising a strong rifting in the Porcupine Basin, controlled its v-shape development by reactivating NE-SW and N-S Variscan structures. To the north, it abutted the Caledonian crustal root avoiding extensional tectonics to propagate in the North Porcupine Basin. Related megasequences RS

  Fig. 10. Rift Setting RS 2 in the Porcupine region. A-Synthetic lithological information of Well 26/28-A1 showing the main lithological packages TS 1 (prerift) and RS 2a to RS 2C and TS 2 (syn-to postrift). Well information is adapted from Statoil (1997). B-3D seismic line, showing the absence of obvious deformation in the Middle Jurassic and two subsequent faulting episodes affecting megasequences RS 2 (high rotation, possibly diffused, Upper Jurassic) and TS 2 (localised, Neocomian). Seismic line location can be seen on Fig. 11.

  et al. Journal of Structural Geology xxx (2018) xxx-xxx

  Fig. 12. Type of controls of Palaeozoic features during the Upper Jurassic-Lower Cretaceous rifting RS 2 . A-Control of structural highs (Ruadan High complex) related to the Variscan thin-skin deformation onto the thickness of subsequent Upper Jurassic-Lower Cretaceous sequences as a major intrabasinal discontinuity (?Variscan Front). B-Relation of intra-Pennsylvanian discontinuities and subsequent sliding packages developed during the Rift Setting RS 2 . C-Development of extensional faults on the forelimb of Variscan or Caledonian folds (dashed red lines) and their associated thrusts (full green lines). Location is shown on Fig. 11, except for Fig. 12B which relates to zoomed pictures from Fig. 6C. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

  Material and geometric data defining the shell natural configuration.

	C. Bulois et al.	Journal of Structural Geology xxx (2018) xxx-xxx
	Table 1 (Continued)
	Stratigraphic surface Stratigraphic surface Top Autunian Unconformity Top Carboniferous Unconformity Top Mississippian Unconformity Base Mississippian Unconformity (merged with the Top Devonian Unconformity) U N C O R R E C T E D Abbreviated name Well tie Stratigraphic formations involved Bounding seismic units Abbreviated name Well tie Stratigraphic formations involved Bounding seismic units TAU 34/15-1 Top: Middle Jurassic Sequence PVM 3 Bottom: Autunian TCU 26/21-1 Top: Triassic Sequence PVM 1 Bottom: Westphalian B 26/28-1, 26/ 28-2, 26/28-3 Top: Middle Jurassic Sequence PVM 2 Bottom: Stephanian 26/27-1B, 26/ 28-4A, 26/ 29-1 Top: Middle Jurassic Sequence PVM 1 26/30-1 Top: Middle Jurassic Sequence PVM 1 Bottom: Westphalian C 34/5-1 Top: ?Albian Sequence PVM 2 Bottom: Stephanian 34/15-1 Top: Autunian Sequence PVM 2 Bottom: Stephanian 35/15-1 Top: Cenozoic Sequence PVM 2 Bottom: Mississippian, pos. Westphalian A TMU 26/26-1, 26/ 28-1 Top: latest Viséan to lower Bashkirian Sequence VB 2 Bottom: Mississippian Likely under 26/28-4A, 26/ 29-1, 26/30-1 Top: Westphalian B/ C Sequence VB 2 Bottom: Mississippian 35/15-1 Top: Cenozoic Sequence VB 2 Bottom: Mississippian, pos. Westphalian A BMU 26/26-1, 26/ 28-1 Top: Wesphalian B Sequence VB 1 Bottom: ?earliest Tournaisian Possible below 26/28-4A, 26/ 29-1, 26/30-1 Top: Westphalian B Bottom: probably Mississippian 34/15-1 Top: Westphalian B D Bottom: Westphalian P R O O F Seismic definition Seismic definition Moderate-amplitude reflector, tending to lose its signal when approaching faults (see Fig. 5c). Overlying and underlying sediments are usually well imaged away from the surface and sometimes appear as transparent to chaotic facies. Usually moderate-to high-amplitude reflector (especially in the centre of the basin, see Fig. 6a and b). Many truncated reflectors are observed throughout the basin, the surface being often located above high-amplitude, continuous reflectors. Sometimes, it is associated with more transparent to chaotic strata or basement rocks. Equivalent to a moderate-amplitude reflector that generally top folded and/or thrusted Mississippian sediments (see Fig. 6c). Moderate-to locally high-amplitude reflector, with onlaps, toplaps and downlaps locally well observed. The overlying seismic sections are usually well imaged but dating may differ laterally. Bottom: Palaeoproterozoic (untested) Top Devonian Unconformity TDU 26/21-1, 26/ 26-1, 26/28-1 Top: Viséan to Serpukhovian Megasequence CB Most often unclear on seismic due to seismic resolution (see Fig. 6a) but the mergence with the Base Mississippian Unconformity implies similarities in seismic characters. Bottom: Devonian 26/28-3 Top: Westphalian D Bottom: Devonian Granite basement Top Dalradian Unconformity TDaU 26/26-1 Top: Devonian or Westphalian B Megasequence GB Most often unclear on seismic due to seismic resolution. The (see Fig. 6a and b). 34/5-1 Bottom: Palaeoproterozoic Likely under 26/21-1, 26/ 26-1, 26/ 27-1B, 34/ 15-1

xxx-xxx

Table

1

Table 2

 2 xxx-xxx Well and seismic characteristics of unconformities associated with the rift system RS1 in the North Porcupine Basin.

	Stratigraphic surface	Abbreviated name	Well tie	Stratigraphic formations involved	Bounding seismic sequence	Seismic definition
	Top Lower Jurassic Unconformity Top Triassic Unconformity Top Carboniferous Unconformity	TLJU TTU TCU	26/21-1, 26/22-1A 26/21-1, 26/22-1A 26/21-1	Top: Bathonian, locally Neocomian Bottom: Hettangian-Sinemurian (Zone RS Ib ) Top: Hettangian-Sinemurian (Zone RS Ib ) Bottom: Rhaetian (Zone RS Ia ) Top: Triassic Bottom: Westphalian B	Sequence RS 1a Sequence RS 1b Sequence PVM 2	Medium-to strong-amplitude reflector, with many onlap or downlap patterns in overlying Zone TS 1 and/or clear truncational reflectors and erosional features in underlying Zone RS 1b . Medium-to strong-amplitude reflector, without clear onlap, downlap or clear truncational reflectors or erosional features. Medium reflectors, usually continuous, and that locally display internal deformation due to prior Palaeozoic tectonism.

Table 3

 3 Well and seismic characteristics of unconformities associated with the rift system RS2 in the northern Porcupine region.Usually moderate-to high-amplitude reflector (especially in the centre of the basin). Many truncated reflectors are observed throughout the basin, the surface being often located above high-amplitude, continuous reflectors. Sometimes, it is associated with more transparent to chaotic strata and basement.access to the well and processed seismic data, as well as Professor Patrick M. Shannon from University College Dublin (Ireland) and Professor Jacques Déverchère from Université Bretagne Occidentale (France) for thoughtful discussions. We also would like to deeply thank Professor Tim Minshull and Doctor Laurent Gernigon for their constructive comments during the review.

	Stratigraphic surface	Abbreviated name	Well tie	Stratigraphic formations involved	Seismic megasequence	Seismic definition
	Top Neocomian Unconformity Base Cretaceous Unconformity Base Oxfordian Unconformity Top Carboniferous Unconformity	TNU BCU BOU TCU	All wells (except 26/ 21-1) 26/21-1 All wells with Cretaceous (except 26/ 21-1) 26/21-1 All wells with Middle Jurassic (except 26/ 27-1B, 26/ 28-2 & 34/ 15-1) 26/27-1B 26/28-2 & 34/ 15-1 26/21-1	Top: Aptian-Albian Bottom: Neocomian Top: Lower Aptian Bottom: Barremian Top: Lower Cretaceous Bottom: Portlandian (or older) Top: Neocomian Bottom: Lower Jurassic Top: Oxfordian (Zone LC II ) Bottom: Bathonian (Zone LC I ) Top: Oxfordian (Zone LC II ) Bottom: Westphalian D No unconformities Top: Triassic	Megasequence TS 2 Megasequence RS 2 Megasequence TS 1 Megasequence VB 2	High-amplitude reflector generally associated with onlaps of Aptian-Albian reflectors High-amplitude reflector, with overlying Early Cretaceous strata onlapping on truncated Upper Jurassic (or older) sediments. Medium-, locally strong-amplitude reflector, with many onlap and downlap patterns in overlying Zone RS IIb . The truncation of underlying reflectors of Zone RS IIa is uneven. Locally, the unconformity is associated with more transparent strata.
			26/28-1, 26/ 28-2, 26/28-3 26/27-1B, 26/ 28-4A, 26/ 29-1 26/30-1	Bottom: Westphalian B Top: Middle Jurassic Bottom: Stephanian Top: Middle Jurassic Bottom: Westphalian C Top: Middle Jurassic Bottom: Westphalian B

Acknowledgements

This study was co-funded by Ecole Normale Supérieure (Paris, France) and the Irish Shelf Petroleum Studies Group (ISPSG, years 2005-2012) of the Irish Petroleum Infrastructure Programme (PIP). We specifically thank the Petroleum Affairs Division of the Department of Communications, Climate Action and Environment, Ireland, for giving

P R O O F

C. Bulois et al. Journal of Structural Geology xxx (2018) xxx-xxx [START_REF] Bulois | Marges polyriftées : réactivation et conditions aux limites. Exemples de la Mer de Corail (Papouasie Nouvelle Guinée) et du Bassin de Porcupine[END_REF]. The characteristic geometry is based on the asymmetry of the various basins imaged from deep seismic lines [START_REF] Morgan | The Hatton Bank continental margin-III. Structure from wide-angle OBS and multichannel seismic refraction profiles[END_REF][START_REF] Morewood | The crustal structure and regional development of the Irish Atlantic Margin region[END_REF][START_REF] Smith | Structure of the Hatton Basin and adjacent continental margin[END_REF][START_REF] O'reilly | Crustal thinning, mantle exhumation and serpentinization in the Porcupine Basin, offshore Ireland: evidence from wide-angle seismic data[END_REF]. It infers three different basements (Archean, Caledonian and Variscan) that are likely to have controlled younger extension during the rifting of the North Atlantic Ocean. Only main rifting stages are represented for each basin; note their apparent westward migration across the margin.