Multi-phase reactivations and inversions of Paleozoic-Mesozoic extensional basins during the Wilson cycle: case studies from the North Sea (UK) and the Northern Apennines (Italy

Enrico  Tavarnelli; Vittorio Scisciani

The Caledonian and Variscan orogens in northern Europe and the Alpine-age Apennine range in Italy are classic examples of thrust belts that were developed at the expense of formerly rifted, passive continental margins that subsequently experienced various degrees of post-orogenic collapse and extension. The outer zones of orogenic belts, and their adjoining foreland domains and regions, where the effects of superposed deformations are mild to very mild make it possible to recognize and separate structures produced at different times and to correctly establish their chronology and relationships. In this paper we integrate subsurface data (2D and 3D seismic reflection and well logs), mainly from the North Sea, and structural field evidence, mainly from the Apennines, with the aim of reconstructing and refining the structural evolution of these two provinces which, in spite of their different ages and present-day structural framework, share repeated pulses of alternating extension and compression. The main outcome of this investigation is that in both scenarios, during repeated episodes of inversion that are a characteristic feature of the Wilson cycle, inherited basement structures were effective in controlling stress localization along faults affecting younger sedimentary cover rocks.

Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 Multi-phase reactivations and inversions of Paleozoic–Mesozoic extensional basins during the Wilson cycle: case studies from the North Sea (UK) and the Northern Apennines (Italy) VITTORIO SCISCIANI1*, STEFANO PATRUNO2, ENRICO TAVARNELLI3, FERNANDO CALAMITA1, PAOLO PACE1,4 & DAVID IACOPINI5 1 Dipartimento di Scienze della Terra, INGEO, Università degli Studi ‘G. d’Annunzio’ di Chieti-Pescara, Via dei Vestini, 30, Chieti, 65013 Chieti Scalo (CH), Italy 2 PGS Reservoir Ltd, 4 The Heights, Brooklands, Weybridge KT13 0NY, UK 3 Department of Physics, Earth and Environment Sciences, Universita degli Studi di Siena, Via Laterina 8, 53100 Siena, Italy 4 G.E. Plan Consulting – Petroleum Geosciences, Via Ariosto 58, 44121 Ferrara, Italy 5 Department of Geology and Petroleum Geology, School of Geosciences, University of Aberdeen, Meston Building, King’s College, Aberdeen AB24 3UE, UK V.S., 0000-0002-4084-9987; S.P., 0000-0002-6375-995X Present addresses: S.P., Oranje-nassau Energie Ltd, 67-68 Long Acre, London, WC2E 9JD, UK *Correspondence: scisciani@unich.it Abstract: The Caledonian and Variscan orogens in northern Europe and the Alpine-age Apennine range in Italy are classic examples of thrust belts that were developed at the expense of formerly rifted, passive continental margins that subsequently experienced various degrees of post-orogenic collapse and extension. The outer zones of orogenic belts, and their adjoining foreland domains and regions, where the effects of superposed deformations are mild to very mild make it possible to recognize and separate structures produced at different times and to correctly establish their chronology and relationships. In this paper we integrate subsurface data (2D and 3D seismic reﬂection and well logs), mainly from the North Sea, and structural ﬁeld evidence, mainly from the Apennines, with the aim of reconstructing and reﬁning the structural evolution of these two provinces which, in spite of their different ages and present-day structural framework, share repeated pulses of alternating extension and compression. The main outcome of this investigation is that in both scenarios, during repeated episodes of inversion that are a characteristic feature of the Wilson cycle, inherited basement structures were effective in controlling stress localization along faults affecting younger sedimentary cover rocks. In orogenic belts, the coexistence of deformed portions of oceanic crust and passive-margin successions, in turn pulled-apart by subsequent continental break-up, is often ascribed to the Wilson cycle context (Wilson 1966; Vauchez et al. 1997). Within these orogens, formed by continental collision following the closure of ancient oceanic basins, the pre-existing rifting discontinuities are commonly reactivated during subsequent compressional episodes in the context of positive inversion tectonics (e.g. Williams et al. 1989). Similarly, during postcollisional reworking of orogens, a fundamental role has been recognized whereby pre-existing anisotropies, such as reverse faults and compressive fabrics, inﬂuence younger extensional structures (Ring 1994; Vauchez et al. 1997; Korme et al. 2004). Although several separate examples of positive or negative inversion tectonics occurred in disparate times and across diverse structural settings (Harding 1985; Cooper & Williams 1989; Buchanan & Buchanan 1995), few of these testify to a complete Wilson cycle from the initial rift phase to the ﬁnal collisional and post-collisional stages. In his seminal paper, Wilson (1966) ﬁrst proposed the closure of a proto-Atlantic Ocean developed in early Paleozoic times followed by the From: WILSON, R. W., HOUSEMAN, G. A., MCCAFFREY, K. J. W., DORÉ, A. G. & BUITER, S. J. H. (eds) Fifty Years of the Wilson Cycle Concept in Plate Tectonics. Geological Society, London, Special Publications, 470, https://doi.org/10.1144/SP470-2017-232 © 2019 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. assembly of the Laurentia supercontinent (known as the Appalachian–Caledonian Orogeny), its break-up and the re-opening of the Mesozoic–Cenozoic Atlantic Ocean. He also speculated that a similar but more recent event occurred along the present-day Asian and circum-Mediterranean mountain belts following the total and partial closure of the Tethys Ocean. Following the pioneering approach by Wilson, both the Caledonian Orogen in northern Europe and the Apennines fold-and-thrust belt of Italy were recognized as key areas to study the effects of the Wilson cycle with repeated episodes of opening and closing of ocean and extensional basins along similar structural trends (e.g. Butler et al. 2006). In the present work, we have selected two continental margins where regional geology, ﬁeld structural evidence and subsurface data all point out to at least a complete Wilson cycle (Figs 1–3), with longterm preservation of structural grain and reactivation of pre-existing structures within both the suture zones and the foreland domains. The two study areas comprise the East Shetland Platform (ESP) in the UK North Sea and the SE portion of the Northern Apennines of Italy, including the Umbria–Marche Apennine Ridge (UMAR) (Figs 1, 4 & 5). The results of our investigation indicate that the Wilson cycle concept, useful for the description of the tectonic evolution of the North Sea and UMAR, may also be successfully applied in the study of other analogue foreland settings ﬂanking the zones where multiple cyclical events that switch from extension to compression, and vice versa, have occurred. Geological setting of the East Shetland Platform (ESP) and the Northern Apennines The ESP is a large (c. 62 000 km2) Jurassic–Recent offshore platform in the UK North Sea, bounded by the Orkney Islands and Shetland Islands to the west, and by several Jurassic structural lows (e.g. the East Shetland Basin, Viking Graben, Outer Moray Firth, Witch Ground Graben) to the east and south (Figs 1 & 4). The outer half of the ESP hosts an expanded and weakly deformed Tertiary succession (1.0–2.0 s twoway travel time (TWT)), usually resting on a thin horizontal veneer of Upper Cretaceous carbonaterich Chalk Group sediments, which in turn overlies a highly tectonized Devonian-age unit that is 1.0– 3.0 s TWT thick (Platt 1995; Platt & Cartwright 1998; Zanella & Coward 2003; Patruno et al. 2018). An angular unconformity separates the tilted and eroded Devonian unit and the subhorizontal Cretaceous–Tertiary cover. Permo-Triassic intraplatform extensional basin ﬁlls are locally present (e.g. Richardson et al. 2005; Patruno & Reid 2016, 2017). A series of widespread hiatuses and unconformities are partitioned by expanded seismicstratigraphic units, directly linked to a succession of regional inversion tectonic events that have taken place since the Paleozoic (Patruno et al. 2018) (Figs 2 & 3). The Apennines of Italy are a prominent mountain range within the central Mediterranean region (Figs 1 & 5). In particular, the Northern Apennines are an arc-shaped east- to NE-verging fold-and-thrust Fig. 1. Location map of the study area including the ESP in the UK offshore (area a) and the Northern Apennines of Italy (area b). Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS Fig. 2. Synoptic scheme showing the main tectonic, stratigraphic and magmatic events that occurred in the ESP and Northern Apennines (pre-Mid-Permian evolution derived from Sardinia). The ESP and Northern Apennines were subject to two Proterozoic–Paleozoic plate cycles, albeit not completely overlapping and with different chronological duration, namely the Iapetus–Caledonian cycle and the Rheic–Variscan cycle (see the text). Following the Variscan Orogeny, the Northern Apennines underwent a further complete plate cycle, whereas the North Sea has been subject to at least two incomplete Meso-Cenozoic intra-plate extension–compression cycles in an intra-plate setting. Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. Fig. 3. Palaeogeographical reconstructions during (a) the Ordovician, (b) the Late Carboniferous, (c) the Late Jurassic, (d) the Paleocene and (e) the Early Oligocene, compiled from different authors (Decourt et al. 1993, 2000; Stampﬂi & Borel 2002; Coward et al. 2003; Scisciani & Montefalcone 2006; Nance et al. 2010). The ﬁgures illustrate the plate conﬁguration during (a) the Iapetus–Caledonian, (b) the Rheic–Variscan and (c)–(e) Alpine and Apennine plate cycles (see the text for the discussion). Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS Fig. 4. (a) Central and Northern North Sea location map, showing the ESP and the Greater ESP (sensu Patruno & Reid 2016), with the location of the PGS seismic surveys. (b) The ESP location map, including the position of the main hydrocarbon ﬁelds, the wells within the Devonian interval, and the relationships between the Devonian Orcadian Basin and the Late Jurassic platform tectonic elements (modiﬁed after Patruno & Reid 2017). The position of the seismic lines discussed in the present work, as well as the PGS 3D MultiClient GeoStreamer surveys, is shown. (c) Onshore outcrop and offshore Permian sub-crop map of the ESP, West of Shetlands and Moray Firth areas (modiﬁed after Patruno et al. 2018). The main tectonic lineaments, the igneous intrusions and the extent of the Zechstein-age halokinetic salt are also shown. (d) Sketch map of basement units in the Northern North Sea and surrounding areas (modiﬁed from Lundmark et al. 2014); the Dalradian basement of the ESP is delimited to the ENE by the Walls Boundary Fault, corresponding to the offshore prosecution of the Great Glen Fault, and to the SE by the continuation offshore of the Iapetus oceanic units with an interposed volcanic arc basement (e.g. Utsira High and East Shetland Basin Basement). belt developed during Cenozoic times (Malinverno & Ryan 1986; Carmignani & Kligﬁeld 1990). The Northern Apennines continental margin forms a tectonic stack of thick-skinned thrust sheets with a tectonic pile including: (i) fragments of ophiolites derived from the Mesozoic Ligurian Ocean and ﬂysch units; (ii) a metamorphic core (Apuane Units) with basement imprinted by the previous Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. Fig. 5. Structural sketch map of the Central and Northern Apennines; see the upper left inset for the location. UMAR, Umbria–Marche Carbonate Ridge; LAR, Lazio–Abruzzo Carbonate Ridge; OAMS, Olevano–Antrodoco–Mt Sibillini thrust system. Alpine and Variscan orogenies; and (iii) a foreland fold-and-thrust belt mainly composed of pre-Alpine Mesozoic–Paleogene carbonates and synorogenic Miocene ﬂysch, and juxtaposed towards the ENE onto the Adriatic foreland. Starting from the Miocene, the inner sector of the chain was also dissected by post-orogenic extension and magmatism (Carmignani & Kligﬁeld 1990; Carmignani et al. 1994; Calamita et al. 2000; Pizzi & Scisciani 2000; D’Agostino et al. 2001). These thrust sheets crop out in the Northern Apennines mountain belt and are buried underneath Pliocene–Quaternary synorogenic sediments in the Po Plain and Adriatic foreland. The highest topography zone of the Northern Apennines (c. 3000 m) consists of two NW–SE-trending carbonate mountain ridges (i.e. the Umbria–Marche and Lazio– Abruzzo ridges – UMAR and LAR, respectively) separated by the NNE–SSW-trending Olevano– Antrodoco–Mt Sibillini oblique thrust ramp (OAMS: Fig. 5). These ridges are composed of Triassic–Miocene synrift and passive-margin successions of the thinned Adriatic crust, and their prominent structural elevation is testiﬁed to by some of the oldest rocks (Late Triassic–Early Jurassic) exposures in the highest mountains of the entire Apennines belt (e.g. the Monte Vettore and the Corno Grande: Fig. 5). Supra-regional evolution of western Europe The geological evolution of the two study areas is here set within a framework of repeated megaregional Wilson cycles (with both complete and incomplete cycles), which are readily identiﬁable throughout western Europe. The ESP and Northern Apennines study areas were subject to two very similar Proterozoic– Paleozoic plate cycles (albeit not completely overlapping and with different chronological durations), namely the Iapetus–Caledonian cycle and the Rheic– Variscan cycle. Following the Variscan Orogeny, both areas have been subject to cyclical reactivation and inversion of pre-existing structures. These reactivation events took place in an intra-plate setting in the case of the North Sea (with at least two incomplete Meso-Cenozoic intra-plate extension–compression cycles), while in the Northern Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS Apennines a new ‘complete’ orogeny–rifting– drifting–orogeny Wilson cycle took place in Permian–Recent times (Figs 2 & 3). Iapetus–Caledonian (c. 850–405 Ma) and Rheic–Variscan (c. 480–300 Ma) plate cycles In the Late Neoproterozoic and Cambrian, the palaeogeography of western Europe was dominated by the north-trending Iapetus Ocean, opened between the continental blocks of Laurentia, Baltica and Gondwana (Mac Niocaill et al. 1997) (Fig. 3a). Since the Cambrian, Gondwana and Baltica had been, in turn, separated by the west-trending Tornquist seaway (Glennie & Underhill 1998; Toghill 2002). In the Grampian Highlands of Scotland, set on the southern Laurentian continental margins, an early Neoproterozoic–Cambrian rifting event that preceded the opening of the Iapetus Ocean is recorded by up to 25 km of metamorphosed sedimentary rocks belonging to the ‘Dalradian Supergroup’ (Strachan et al. 2002; Toghill 2002; Stephenson et al. 2013). This succession has been interpreted as a subsiding submarine shelf, evolving into a deeper basin and ﬁnally leading to ocean-ﬂoor spreading (Stephenson et al. 2013). The last stage is documented by the late Neoproterozoic submarine Tayvallich basalts cropping out in western Scotland (Stephenson & Gould 1995; Macdonald & Fettes 2007; Fettes et al. 2011). The Dalradian Supergroup is interposed between the Great Glen Fault to the north and the Highland Boundary Fault to the south (Ziegler 1988, 1990; Glennie & Underhill 1998; Coward et al. 1989, 2003). These deep-seated and long-lived NEtrending tectonic lineaments are associated with the complete subduction and suturing of the Iapetus Ocean during the Caledonian Orogeny, and can be expected to continue offshore towards the ESP area (Figs 3a & 4d). Avalonia was a small continental fragment that was detached in the Early Ordovician from the northern Gondwanan margin by a new widening east-trending ocean, known as the Rheic Ocean (Fig. 3a) (Strachan 2000; Nance et al. 2010, 2012). During Ordovician–Silurian times, the fast seaﬂoor spreading of the Rheic Ocean lead to the passive drifting of Avalonia northwards, and initiated the progressive three-plate convergence between Avalonia, Baltica and Laurentia in and around the present-day North Sea and Scottish–Norwegian regions (McKerrow et al. 2000b; Bluck 2001; Coward et al. 2003; Nance et al. 2010, 2012; Mendum 2012) (Fig. 3a). The Iapetus and Tornquist oceans were eventually closed in Silurian times, leading to the Acadian–Grampian–Caledonian Orogeny (Trench & Torsvik 1992; McKerrow et al. 2000b; Mendum 2012) and to the ﬁnal assemblage of a larger continent in the Early Devonian (Laurussia or Old Red Continent: Ziegler 1988, 2012; Scotese & McKerrow 1990; Dalziel et al. 1994; McKerrow et al. 2000a). The assembly of the Caledonian Orogen was quickly followed, during Devonian times, by the extensional collapse of its thickened crust (e.g. McClay et al. 1986; Coward et al. 1989; Seranne 1992; Ziegler 1992; Marshall & Hewett 2003; Wilson et al. 2010) (Figs 1 & 2). This post-orogenic extension produced many quickly subsiding halfgraben throughout Laurussia, with accompanying lower Devonian volcanism and granitic intrusions identiﬁed both onshore Scotland and in the North Sea (e.g. the Utsira High, and the Halibut and Bressay granites). These fast-subsiding Devonian extensional basins hosted the deposition of thick continental clastic successions (Old Red Group: Glennie & Underhill 1998). The Devono-Carboniferous extensional phase was eventually cut short because the Late Carboniferous continental plates switched motion, leading to the subduction of the Rheic Ocean and the subsequent continental collision between Laurussia, Gondwana and several intervening microplates (Variscan Orogeny), with the Early Permian assemblage of the Pangea megacontinent (Fig. 3b). Remnants of the Caledonian orogenic belt span northern Britain and Norway, with Lower Paleozoic crystalline rocks (or ‘Caledonian basement’) thought to underlie the entirety of the present-day North Sea Basin (Coward 1990; Abramovitz & Thybo 2000; Scheck et al. 2002; Coward et al. 2003; Bassett 2003; Zanella & Coward 2003). Conversely, the Variscan metamorphic core belts today straddle the ancient plate margin of Laurussia and Gondwana, from the eastern USA and Newfoundland to Morocco, Iberia, France, Germany and the Ural Mountains. In particular, the northern, approximately east-trending, front of the Variscan orogenic belt spans SW England, Wales and Ireland (Fig. 3b) (Leveridge & Hartley 2006). In southern Europe, the Paleozoic basement of Sardinia, along with scattered outcrops in the Northern Apennines, Western Alps and Calabria, is the result of the Variscan continental collision between the northern Gondwanan margin and the Armorica microplate (Fig. 3b) (Carmignani et al. 1994; Franceschelli et al. 2005; Giacomini et al. 2006; von Raumer & Stampﬂi 2008; Rossi et al. 2009). As a consequence, the Variscan belt is expected to transect underneath the Northern Apennines mountain belt (Doglioni & Flores 1997; Vai 2001). Caledonian and Variscan orogenic belt ‘core complexes’ are therefore present over the greater North Sea area and part of central-southern Europe (including the Northern Apennines), respectively. Conversely, the North Sea and much of southern Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. Europe (including the Northern Apennines) represented part of the foreland of the far-ﬁeld Variscan and Caledonian orogenies, respectively. Nevertheless, large-scale Caledonian-age uplift/erosion and local folding (Sardic deformation) took place in Sardinia and Iberia (Fig. 2) (Carmignani et al. 1994; Romão et al. 2005). Similarly, episodes of Variscan-age intra-plate compression and uplift/ erosion, leading to the partial inversion of Devonian–Carboniferous sedimentary basins, have been identiﬁed in Britain and the North Sea (e.g. Fraser & Gawthorpe 1990; Thomson & Underhill 1993; Corﬁeld et al. 1996; Hay et al. 2005), including the ESP (Patruno et al. 2018) (Figs 3 & 4). The end of the Variscan foreland compressional phase in the North Sea is marked by a regional peneplanation surface, usually referred to as the ‘Base Permian Unconformity’. In the North Sea and northern Britain, the Iapetus–Caledonian plate cycle therefore corresponds to a ‘classical/complete’ Wilson cycle (from likely pre-Iapetus compression to Iapetus extension and ocean spreading, and back to Caledonian compression), while the Rheic–Variscan plate cycle can be described as a ‘foreland/incomplete Wilson cycle’ (from Caledonian Orogeny to a Devonian extension, which did not culminate in ocean spreading, and back to foreland Variscan compression). The opposite is true for the Northern Apennines and southern Europe. Here, the Iapetus–Caledonian cycle is a foreland/incomplete Wilson cycle (no Iapetus-equivalent ocean formation in this region and far-ﬁeld Caledonian compression), while the Rheic–Variscan plate cycle represents a classical/ complete Wilson cycle (from likely pre-Rheic compression to Rheic extension and ocean spreading, and back to Variscan compression). Permian–Present plate and intraplate cycles (c. 280/260–0 Ma) Following the Variscan Orogeny the North Sea and Northern Apennines areas suffered multiple Permian–Jurassic rifting episodes, associated with the break-up of Pangea (Decourt et al. 1993; Stampﬂi & Borel 2002). In southern Europe these extensional processes resulted in the opening of three distinct oceanic arms (Figs 2 & 3c), including: (i) the Permian–Triassic Ionian–East Mediterranean Ocean (Finetti & Del Ben 2005); (ii) the Middle Triassic NW Tethys (Hallstatt–Meliata and Vardar oceans: Kozur 1991); and (iii) the latest Triassic–Middle Jurassic Alpine Tethys (including the Ligurian– Piedmont Ocean: Stampﬂi & Borel 2002). The two oldest oceans formed the western termination of the Neo-Tethys, whereas the last was an eastwards ramiﬁcation of the central Atlantic opening. Conversely, in the North Sea and ESP, the whole post-Variscan Permian–Recent evolution took place in an intra-plate setting (Figs 2 & 3c–e). In the North Sea, the initial Permo-Triassic rifting is often related to the reactivation and inversion of old Caledonide lines of weakness (Ziegler 1988, 1990, 1992; Glennie & Underhill 1998; Wilson et al. 2010; Patruno et al. 2018). In general, two main PermoTriassic phases of crustal thinning occurred (rift I and rift II in Fig. 2). In the North Sea and ESP, these two synrift units correspond to expanded middle Permian clastics and volcanics (Rotliegend Group) and to lower Triassic clastics, often conﬁned within syndepositional half-graben and graben (Færseth & Ravnås 1998; Stemmerik et al. 2000; Richardson et al. 2005; Patruno & Reid 2017; Patruno & Lampart 2018; Patruno et al. 2018). These two units are separated by upper Permian evaporitic cycles of the Zechstein Group, which were deposited in largely post-rift thermal relaxation basins (Ziegler 1988, 1990; Glennie et al. 2003; Ziegler et al. 2004, 2006; Peryt et al. 2010; Patruno et al. 2018). In the Northern Apennines, there are extensional basins inﬁlled by two distinct sedimentary cycles of Permian–Early Triassic and Middle Triassic–Early Jurassic age, respectively (e.g. Ciarapica & Passeri 2005; Fantoni & Franciosi 2010). After the Permo-Triassic phase, active rifting mainly occurred in Lower and Late Jurassic in the Northern Apennines and in the North Sea and ESP, respectively (Figs 2 & 3c). In the Northern Apennines, this rifting led, in the Early Jurassic, to the partial drowning of the widespread carbonate platforms (Ciarapica & Passeri 2002; Scisciani & Esestime 2017). During the Late Jurassic–Early Cretaceous, the Jurassic Northern Apennines rifting evolved into the onset of oceanﬂoor spreading of the Ligurian–Piedmont Ocean (Alpine Tethys in Fig. 3c) (Stampﬂi & Borel 2002; Bortolotti & Principi 2005). Meanwhile, prior to the Late Jurassic rifting, in the North Sea, Aalenian thermal doming caused a new phase of regional uplift/erosion (Fig. 2) linked to the possible development of a transient mantleplume head, and leading to the extrusion of the Rattray Series and the Forties Igneous Province in the Central North Sea (Ziegler 1992; Underhill & Partington 1993, 1994; Davies et al. 1999; Coward et al. 2003; Hendrie et al. 2003; Husmo et al. 2003). The erosional truncation associated with this uplifting event is known as the Mid-Cimmerian Unconformity (e.g. Patruno & Reid 2016). Renewed active extension in the North Sea began with a diffuse Bajocian–Bathonian proto-rifting which evolved into the Late Jurassic–?Ryazanian main and last rifting event in this region (Ziegler 1990, 1992; Steel 1993; Færseth & Ravnås 1998; Glennie & Underhill 1998; Nøttvedt et al. 2000; Coward Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS et al. 2003; Fraser et al. 2003; Patruno et al. 2015a, b, c; Patruno 2017; Patruno & Helland-Hansen 2018; Turner et al. 2018). The footwall areas of the main rift border faults of the Viking Graben, Moray Firth and Central Graben, including the ESP, became relatively stable Jurassic–Recent platforms (Figs 3c, 4 & 5). As with the Permo-Triassic extension, and unlike the Jurassic rift–drift in the Northern Apennines, the Late Jurassic rifting in the North Sea was aborted before ocean-ﬂoor formation. A new switch in relative plate motion in the ‘MidCretaceous’ triggered the progressive convergence between the Eurasian/Corsica–Sardinian and African–Adriatic continental margins, which led to the closure of the interposed Penninic–Piedmont– Ligurian Ocean and the Eocene–Recent Alpine/ Apennine continental collision (Malinverno & Ryan 1986; Dewey et al. 1989; Carmignani & Kligﬁeld 1990; Calamita et al. 2007) (Figs 2 & 3d, e). The onset of this plate convergence was possibly reﬂected by an abrupt lithostratigraphic transition in the Northern Apennines from typical passivemargin pelagic limestones to Aptian–Albian marlstones (Patruno et al. 2015d; Unida & Patruno 2016). During this Eocene–Recent collisional stage (Fig. 3d, e), the Northern Apennines consisted of an eastwards to northeastwards migration of compressive fronts, coupled by foredeep development ahead of the advancing thrust belt (Ricci Lucchi 1986; Patacca & Scandone 1989; Boccaletti et al. 1990). These Neogene thrust structures have subsequently been affected by arrays of post-orogenic normal faults (Fig. 5) showing maximum fault throws of 600–1500 m and trending roughly parallel to the previous compressive structures (Calamita et al. 2000; Pizzi & Scisciani 2000; Pizzi & Galadini 2009). Several hanging-wall-related intermontane basins inﬁlled with thick ?Pliocene–Quaternary continental deposits (e.g. Galli et al. 2008) are associated with the occurrence of moderate magnitude (M ≤ 7) historical and instrumental earthquakes (e.g. Calamita et al. 2000). In the North Sea and ESP, the propagation of weak and localized intra-plate Alpine-age compression interrupted the Cretaceous–Tertiary regional thermal subsidence (Figs 1, 3d & 4) (Alberts & Underhill 1991; Butler 1998; Patruno et al. 2018). Furthermore, a post-Paleocene easterly tilting affected the whole UK North Sea. As a consequence, on the western portion of the ESP, we observe Paleozoic sediments cropping out to the seaﬂoor (Underhill 1991; Hillis et al. 1994; Patruno et al. 2018; see also BGS 250k 2017 Marine Bedrock Map); while in the eastern portion of the ESP, the Paleozoic is overburdened by 2000 m of Cenozoic clastic sediments. In summary, in the post-Variscan, the two study area shows the following regional geology differences: • The Northern Apennines is constructed through a new ‘classical/complete’ Wilson cycle (from Variscan compression to Permian–Jurassic Western Tethys extension and ocean spreading, and back to Cretaceous–Recent Alpine/Apennine orogenesis). Post-orogenic Plio-Quaternary extensional collapse may, in fact, be interpreted as the early onset of a new cycle. • The North Sea and ESP region instead went through at least two ‘foreland/incomplete’ Wilson cycles in an intra-plate setting: (1) from Variscan compression to Permo-Triassic rifting, and back to Mid-Cimmerian uplift; and (2) from Mid-Cimmerian uplift to Late Jurassic rifting, interrupted by Cretaceous–Paleogene thermal subsidence, and then partly inverted by far-ﬁeld Alpine inversion and, close to the coastlines, by Neogene regional uplift. Seismic interpretation of the ESP The analysis of recently acquired 3D seismic surveys over the outer ESP, coupled with the available stratigraphic well information, revealed signiﬁcant traces of repeated tectonic inversion events. On the basis of well data alone (e.g. the regional well correlation panel shown in Fig. 6), discrete regional unconformities can be inferred, corresponding in age to the regional uplift and erosion events discussed in the previous section. These are the following: (1) Near-Base Tertiary Unconformity (=‘Atlantean Unconformity’) – identiﬁed locally by an erosional top Chalk contact, and possibly related to mantle-plume-related epirogenetic uplift. (2) Base Upper Cretaceous Unconformity – possibly related to early Alpine age uplift, with a direct contact between upper Chalk and Cromer Knoll groups. (3) Base Cretaceous Unconformity (or BCU) = Late Cimmerian Unconformity – situated at the base of the preserved Cromer Knoll Group and representing the end of rift unconformity. (4) Base Jurassic Unconformity (BJU) = MidCimmerian Unconformity (MCU) – hiatus/ truncation at the base of the preserved Jurassic package, highlighted by the direct contact between upper–middle Jurassic units (e.g. Pentland, Heather or Kimmeridge Clay formations) and lower Triassic or, on structural highs, even older units. (5) (5), (6), (7) The top and base of the lower Permian Rotliegend Group, if preserved, corresponds to two discrete erosional surfaces Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. Fig. 6. West to east orientated well correlation panel spanning the southern margins of the ESP, from the Halibut Horst, through the Piper Shelf and the Fladen Ground Spur (UKCS quadrants 14, 15 and 16) to the western edge of the South Viking Graben. Note that the seventh, deepest regional unconformity (i.e. ‘Top crystalline basement = Caledonian Unconformity’) has not been penetrated by these wells and therefore is not shown in this ﬁgure. Modiﬁed after Patruno et al. (2018). Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS associated with the Variscan Orogeny (Top Rotliegend Unconformity and Base Permian Unconformity (or BPU), respectively). However, these and possible deeper Caledonian unconformities (with the most recent one coinciding with the Top crystalline basement) are not well documented, as most wells do not intersect these surfaces. These erosional surfaces are also visible across several seismic lines as time-equivalent angular unconformities. In particular, in Figure 7b, a Devonian–lower Carboniferous package has been tilted and truncated by the BPU. A thin Zechstein–Triassic package is subject to a lesser amount of tilt and is truncated by the BJU. Finally, an upper Jurassic package shows an even lesser amount of tilt and is truncated by the BCU. The stratal packages bounded by these unconformities are characterized by increased tectonic subsidence rates (Fig. 7c), particularly in relation to the Devonian post-orogenic collapse event and the post-Variscan Permo-Triassic rift-related mini-basins (Fig. 7c, d). Discrete unconformity surfaces can be identiﬁed only in intra-platform depocentral areas characterized by maximum stratigraphic preservation of Carboniferous–Cretaceous units, which are normally absent elsewhere on the ESP (e.g. the Piper Shelf in Figs 6 & 7 and the Crawford–Skipper Basin in Fig. 8) (Patruno & Reid 2017; Patruno & Lampart 2018; Patruno et al. 2018). The unconformities instead merge together towards persistent structural highs (e.g. the Kraken High, Fladen Ground Spur and Halibut Horst in Figs 7–9). Intra-platform Carboniferous–Jurassic depocentres and persistent structural highs are also characterized by very different fault densities (Fig. 9). The main ages for fault activity ranges from Devonian, to PermoCarboniferous, to Triassic to Jurassic–Cretaceous, and each fault-age population shows a characteristic strike trend (Fig. 9) (Patruno et al. 2018). In particular, both east-trending well correlation panels and seismic lines across the Piper Shelf–Fladen Ground Spur areas (Figs 6 & 7) reveal several sub-BCU extensional faults, with different timings of activity. Fault 6 in Figure 7a, for example, is associated with a partly inverted Rotliegend synrift wedge, whilst Fault 9 relates to both upper Jurassic and PermoTriassic synrift wedges (see also the backstripped evolution along a similar transect in Patruno 2017). The Rotliegend synrift wedge bounded by Fault 6 and the Jurassic–Early Paleocene reﬂectors lying on it are subject to late inversion, highlighted by gentle anticlinal folding of these reﬂectors, against which later Paleogene reﬂectors onlap (Fig. 7a). An Eo-Alpine and Mid-Cimmerian inversion age was proposed for this structure by Patruno & Reid (2018). This inverted sedimentary wedge corresponds to one of the Carboniferous–Permian mini-basins highlighted by the time–thickness map (Fig. 7d). Further north, relatively minor Late Cretaceous and Paleocene compressional inversion phases are revealed by: (1) the gentle folding of the ?Carboniferous–Paleocene reﬂectors in the Crawford–Skipper Basin (Fig. 8) (see also Patruno & Reid 2017); (2) the larger-scale antiformal folding of the Fladen Ground Spur, with Late Cretaceous strata becoming progressively thinner towards the antiformal culmination of this area and Paleocene reﬂectors onlapping against it (Fig. 8); and (3) the minor reverse reactivation event of a major Devonian master fault on the Kraken High, with an associated fault-related anticline formed by the overlying Paleocene reﬂectors, forming the structural closure for the Kraken Oilﬁeld (Fig. 10). This half-graben is associated with an expanded (up to 2.0 s TWT) hanging-wall Devonian succession, including possible synrift wedge geometries (Figs 8 & 10). This is just one of the several extensional structures developed during the Devonian-age extensional collapse of the Caledonian Orogen. A few kilometres to the north of Kraken, a prominent Devonian syncline is related to the same syndepositional master fault (Fig. 11). Below the Devonian reﬂectors (units 1 and 2 in Fig. 11), a distinctly opaque seismic stratigraphic unit is tentatively identiﬁed as metamorphic basement (units 3 and 4). This unit is about 1.0 s (TWT) thick and does not contain any visible reﬂectors. Below this seismically opaque ‘basement’, a second more highly reﬂective strata (Unit 5) is observed overlying a third more opaque seismic package (Unit 6). Fazlikhani et al. (2017) showed that a very similar tripartite subdivision of the basement seismic facies can be observed throughout the rest of the Northern North Sea. The Devonian successions in this area range from thick shale and sandstone intercalations in the main syncline, to conglomerates and breccias away from it (e.g. wells A and B). The ?Early Devonian, characterized by granite wash grading to granite in well C (‘Bressay Granite’), could either be an intrusion localized in a small area surrounding the well (e.g. Unit 3 in Fig. 11) or could be interpreted as an acoustically transparent ‘basement’ (Unit 4). Unfortunately, the regional extent and thickness of the intrusions is unknown and unconstrained by further data (Stephenson et al. 1999). The highly reﬂective Unit 5 underneath the seismically transparent basement is fortuitously penetrated by well D, drilled across the ESP–Viking Graben border fault. According to the analysis of this well, this unit is composed of gneiss/schists rich in hornblende and biotite, with an Early Devonian radiometric metamorphic cooling (Bassett Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. Fig. 7. Figure showing the complex multi-phase inversion tectonics in the southern ESP (UKCS quadrants 15 and 16), near the transition between the ESP and the Witch Ground Graben (c. 36 km further south) and the South Viking Graben (right-hand side in a). This ﬁgure includes the following parts: (a) interpreted seismic cross-section (PGS GeoStreamer® MC3D-Q15–2014 3D seismic survey) along a similar transect as the correlation panel in Figure 6; (b) close-up of the interpretation of (a); (c) total subsidence curve below present-day sea level and tectonic subsidence rate graphs from well 15/6–1 (backstripping method of Allen & Allen 2005); (d) TWT thickness map of the Permian– Carboniferous interval. LC, Late Cretaceous; EC, Early Cretaceous; LJ, Late Jurassic; MJ, Middle Jurassic (post-Aalenian); Tr, Triassic (mostly Early Triassic); Z, Zechstein Group (latest Permian); Ro, Rotliegend Group (Late Permian); C, Carboniferous. This ﬁgure has been modiﬁed after Reid & Patruno (2015), Patruno & Reid (2018) and Patruno et al. (2018). TVD, true vertical depth. Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS Fig. 8. Regional north–south seismic reﬂection line straddling the western edge of the ESP, parallel to the Viking Graben. The Kraken High and the Fladen Ground Spur are two prominent ?Variscan structural highs, where a thin Chalk/Jurassic veneer is the only stratigraphic unit that separates thick Devonian and Tertiary successions. Between these highs, the Crawford–Skipper Basin (sensu Patruno & Reid 2017) is an area hosting a much more complete Carboniferous–Cretaceous succession. Figure modiﬁed after Patruno & Reid (2017) and Patruno et al. (2018). 2003). As a consequence, Unit 5 possibly represents a transition between different compositions of crystalline rocks that is likely to give rise to a signiﬁcant contrast in acoustic impedance (e.g. acidic granites over basic gneiss?). We interpret Unit 5 as possible pre-Devonian Caledonides or shear zones (e.g. Seismic Facies 2 of Fazlikhani et al. 2017) based on: (1) the lower Devonian metamorphic cooling age; and (2) the presence of likely northwesterly verging compressional compressive structures (at least in the SE half of the line). In the NW half of the line, the presence of likely extensional structures inside the same ‘Caledonide’ package is counterintuitive. A possible explanation is that the NW half of the line may represent a foreland area of the Caledonian Orogeny. The frontal Caledonian thrust was possibly situated in proximity to the previously mentioned Devonian master fault, which can be mapped laterally for several tens of kilometres (thick red fault on Kraken High in Fig. 9). This Devonian master fault and other deeper faults that cut Unit 5 all show a NNE–SSW strike resembling the main lineaments associated with the Caledonian Orogeny (e.g. the Great Glen Fault and its offshore continuation) and the likely orientation of the axis of the Iapetus Ocean (Figs 3a, 4 & 9) (Patruno et al. 2018). This suggests a longterm preservation of the structural grain from the Caledonian compression to the Devonian extension up until the Alpine-age inversion (Figs 9 & 12) (Patruno et al. 2018). Field geology of the Northern Apennines In this section, seismic and outcrop data in the UMAR are integrated, with the aim of reconstructing a balanced crustal geological cross-section of Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. Fig. 9. Map showing 633 different faults on the ESP area constrained by 3D seismic coverage. The faults have been mapped in 3D and classiﬁed according to their kinematic and main age of activity (thick and thin lines indicate major and minor faults, respectively). The predominance of north-striking faults parallel to the Viking Graben and overall east-striking faults parallel to the Witch Ground Graben, despite the wide variation in activity age, suggests a long-term structural inheritance. Figure modiﬁed after Patruno (2017). For a full statistical appraisal of these structures see Patruno et al. (2018). the outer Northern Apennines. Moreover, due to the low resolution of vintage seismics, detailed structural ﬁeldwork has been carried out along selected key regional-scale thrust zones, in order to deﬁne the ﬁeld-scale geometrical structural–stratigraphic relationships. The UMAR comprises a series of east- to NE-verging folds, with 5–8 km wavelength, gently Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS Fig. 10. A NW–SE-trending seismic line across the Paleocene Kraken Oilﬁeld, highlighting an expanded Devonian succession in the hanging wall of a likely lower Devonian master fault, successively subjected to a partial Paleogene-age inversion, leading to the formation of a fault-related anticline in the Paleocene section (i.e. the structural closure of the Kraken Field itself ). Figure modiﬁed after Patruno & Reid (2017) and Patruno et al. (2018). inclined backlimbs and near-vertical to overturned forelimbs (Calamita et al. 2012). Thrust planes commonly truncate the east- to NE limbs of the folds (Fig. 13), with a shortening of c. 1–3 km achieved by each thrust and the related fold. The eastern UMAR comprises four thrust-related anticlines, in which the more continuous corresponds to the Mt Igno–Valnerina Anticline (Fig. 13). Some of these folds are displaced by NW– SE-trending Quaternary normal faults, juxtaposing recent continental deposits in the hanging wall to Jurassic–Miocene carbonates in the footwall, and are responsible for recent destructive earthquakes (Calamita et al. 2000). Detailed structural ﬁeldwork has been carried out in the Mt Igno Anticline area (Fig. 14). This anticline shows an overturned forelimb with Jurassic– Cretaceous strata overlying gently SW-dipping Cretaceous–Miocene beds (Calamita et al. 1993). The anticline is cored by a Jurassic condensed pelagic succession, while the backlimb is affected by a set of WSW-dipping normal faults (Fig. 14). The presence of a thick and complete Mesozoic pelagic succession in the downthrown block indicates a Jurassic synsedimentary activity for these normal faults. Moreover, the absence of clear Quaternary activity indicators (like fault scarps and triangular facets) and stratigraphic evidence of the reactivation of Jurassic basin-boundary faults during the Miocene (Scisciani et al. 2000b) suggest that the last movements along these normal faults occurred during a pre-thrusting episode of foreland ﬂexure-induced extension. The normal-fault pre-thrusting activity, and the thrust hanging-wall ramp onto the footwall ramp relationship observed in the ﬁeld, constrained the balanced geological cross-section for the Mt Igno– Valnerina thrust, allowing an estimated shortening of about 1.7 km (Fig. 14b). Northeastwards from Mt Igno, the Camerino Basin has been interpreted as a Miocene normalfault-controlled basin, ﬁlled by Miocene turbidites (Scisciani et al. 2000b). This area was affected by the 2016 Visso–Norcia–Amatrice normal-faulting seismic sequence (Emergeo Working Group 2016). The eastern margin of the Camerino Basin corresponds to the gently-dipping backlimb of the Sibillini Mountains Anticline (Fig. 13). The regionally extensive Sibillini Mountains Thrust and thrustrelated fold show a curved geometry expressed by two segments trending, respectively, NW–SE (northern sector) and NNE–SSW (Mt Vettore apical Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. Fig. 11. Overall NW–SE arbitrary line to the north of the Kraken Field, again displaying the Devonian syncline (units 1 and 2) within the ESP associated with the partly inverted master fault shown in Figure 10. Below the Devonian reﬂectors, there are opaque seismic–stratigraphic units (units 3 and 4) characterized by no visible reﬂector, followed by another unit with high-amplitude reﬂectors (units 5 and 7), highlighting both compressional (SE sector) and extensional deep-seated structures. Unit 3 is an inferred granitic intrusion. See also Patruno et al. (2017) for further details. central sector, with maximum shortening and structural elevation), that continues southwards into the north-trending Olevano–Antrodoco Thrust (Fig. 13). At least in the central part, the Sibillini Mountains–Olevano–Antrodoco Thrust coincides with an inherited NNE-trending lineament (Ancona– Anzio line in Castellarin et al. 1982), separating the persistent Mesozoic Apulian (Lazio–Abruzzi) Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS Fig. 12. Schematic reconstruction of the structural and stratigraphic evolution of the regional north–south line (Fig. 8). These cartoons were reconstructed based on stratal geometries, inferred depositional thickness trends (seismic and wells), backstripping analyses and facies variations in the wells. See also Patruno & Reid (2017) for further details. Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. Fig. 13. Simpliﬁed geological and structural map of the SE sector of the Northern Apennines (modiﬁed from Scisciani et al. 2014) transected by the arc-shaped UMAR; see the upper left inset for the location. Dotted and continuous lines mark the location of the seismic reﬂection proﬁle A–B of Figure 16 and geological cross-section C– D of Figure 17, respectively. carbonate platform to the east from the timeequivalent Umbria–Marche pelagic basin to the west, and its arcuate shape has been commonly interpreted as being controlled by this structural inheritance (Alberti et al. 1996; Butler et al. 2006; Calamita et al. 2011; Pace et al. 2011; Scisciani & Esestime 2017). The best exposures of the main thrusts crop out along the Fiastrone Valley (along the NW-trending thrust segment), located a few kilometres southwards of the trace of the seismic line (Figs 13 & 15), where the Jurassic–Cretaceous carbonate succession (including the Lower Jurassic Calcare Massiccio Formation) forms a thrust-related anticline emplaced onto Paleogene strata. The steeply dipping to overturned forelimb describes a hanging-wall ramp superimposed onto a footwall ﬂat. A balanced cross-section reconstructed by the collected surface geological data indicates a shortening of c. 4.0 km (Fig. 15). A shortening of c. 2.0 km was estimated more to the north (Mazzoli et al. 2005), and close to the seismic line trace (Fig. 13), documenting a reduction in the orogenic contraction away from the apex of the salient (the Mt Vettore area). Subsurface geology of the outer Northern Apennines In this work, we selected a recently released seismic reﬂection proﬁle transecting the UMAR from the inner Umbria Valley to the outer Laga Basin, through the intervening Camerino and Colﬁorito basins (Figs 5 & 13–16). Key reﬂectors were correlated through: (a) geological ﬁeldwork in proximity to the seismic trace; and (b) direct ties between welllog stratigraphy and seismic data using check-shot or synthetic seismograms of deep wells drilled close to the seismic proﬁle. The reconstructed seismic stratigraphy is consistent with previous studies (Bally et al. 1986; Scisciani & Montefalcone 2005; Mirabella et al. 2008; Scisciani et al. 2014), and consists of eight units Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS Fig. 14. (a) Structural map of the Mt Igno area (location in Fig. 13; modiﬁed from Scisciani et al. 2014). (b) Geological cross-section (trace in a) and restored template across the Mt Igno–Valnerina thrust system. Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. Fig. 15. (a) Simpliﬁed geological map of the Sibillini Mountains Thrust eroded and exposed along the Fiastrone Valley. (b) Exposure of the overturned thrust-related anticline in the hanging-wall block of the Sibillini Mountains Thrust along the northern ﬂank of the Fiastrone Valley. (c) Geological cross-section showing the overturned anticline and the ramp trajectory of the thrust fault across the Lower Jurassic strata (CM, Calcare Massiccio Formation), and the steeply dipping forelimb in the hanging-wall block. (d) Restored template showing the shortcut trajectory of the future Sibillini Mountains Thrust with respect to the pre-thrusting normal faults. with distinctive seismic facies and bounding reﬂectors (Fig. 16). Units 3–7 have been drilled in the Massicci Perugini Ridge–Tiber Valley (10.0 km to the NW of the seismic line) and show a good correlation to the proﬁle (Scisciani et al. 2014). Conversely, in the eastern sector, the older intervals (units 6–8) are unconstrained due to the shallow penetration of the exploration wells. Nevertheless, the results of combined gravimetric and magnetic modelling indicate homogenous characteristics of the deeper intervals along the entire seismic proﬁle (Scisciani et al. 2014). Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS Fig. 16. Geological interpretation of a regional seismic reﬂection proﬁle across the UMAR; trace location in Figures 5 and 13 (modiﬁed after the ViDEPI Project database, http://unmig.sviluppoeconomico.gov.it/ (accessed in March 2009), which contains public information licensed under License CC BY 3.0 Italy). Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. The seismic proﬁle displays an overall westwards thinning of the Liassic–Miocene pre-orogenic succession, which is in agreement with the outcrop geology (Deiana & Pialli 1994; Scisciani et al. 2010). The Pliocene–Quaternary continental deposits (Unit 1) are restricted to the hanging wall of the westdipping normal faults in the Umbria Valley and Colﬁorito Basin (Fig. 16), where they unconformably overlie the older and deformed units. The Miocene siliciclastic deposits (Unit 2) correspond to poorly reﬂective seismic facies with respect to the underlying Unit 3, and are conﬁned to the Umbria Valley, Camerino Basin and Laga Basin (Fig 16). The Marne a Fucoidi reﬂector at the base of Unit 3 (EK in Fig. 16) overlies a very reﬂective package that terminates downwards into a reﬂection-free interval, corresponding to the massive and monotonous Liassic shallow-water carbonates (Calcare Massiccio Formation). The Marne a Fucoidi and Top Calcare Massiccio reﬂectors are therefore distinctive and can be conﬁdently traced regionally, illustrating the deep geometry of outcropping structures. The three large wavelength (5–10 km) folds in the seismic section correspond to the Mt Subasio, Mt Igno–Valnerina and Sibillini Mountains Thrustrelated anticlines. Clear reﬂector terminations against the thrust faults are visible in both the hanging-wall and footwall blocks. The resulting hanging-wall and footwall ramp geometry allow the thrusts to achieve a limited amount of shortening: about 4.0 km for the principal and regional Sibillini Mountains Thrusts, and less for the others. Underneath the core of the large-scale UMAR, the ?Paleozoic–Triassic interval (Units 6–7) is signiﬁcantly thicker (maximum TWT thickness of 3.8 s) than in the areas underneath the Umbria Valley and the footwall of the Sibillini Mountains Thrust (c. 1.2 and 1.8 s, respectively). These lateral thickness variations are also preserved after performing a time-to-depth conversion of the seismic proﬁle, suggesting that they are not artefacts. In particular, Unit 7 has been correlated with the slightly metamorphosed Verrucano facies in adjacent wells (Scisciani & Esestime 2017). This unit shows a wedge-shaped geometry in the hanging walls of normal faults at depth (Fig. 16). Some of these faults show a variation in the offset with depth that is typical of positive inversion tectonics: faults have a normal offset at depth that changes to reverse updip (e.g. Williams et al. 1989), with a mild degree of inversion (sensu Cooper et al. 1989). The geometry of the lower part of Unit 6 (presumably corresponding to the Late Triassic evaporites) clearly depicts the vertical extrusion of the overthickened Mesozoic synrift basin inﬁll. The thick wedge of the lower Unit 6 at depth corresponds at the surface with the long-wavelength exhumed carbonate ridge and with the region of highest topographical elevation (Fig. 16). At shallow depth in the seismic proﬁle, several high-angle faults terminate against low-angle thrusts (e.g. SP1600–1800, 2080–2130 and 2150–2420 in Fig. 16). Some of these high-angle faults show a good correlation with outcropping Jurassic and Miocene normal faults that commonly occur in the core and backlimb of anticlines. This setting corresponds to short-cut geometries that are typical of positive inversion tectonics where a thrust fault truncates across the footwall of a pre-existing inverted normal fault (e.g. Figs 15 & 16) (Cooper et al. 1989; Coward 1994). The west-dipping Quaternary Colﬁorito normal fault system merges downwards into a previous Neogene thrust plane, as suggested by the continuity of the gently-dipping fault-plane reﬂections at 1.0– 2.0 s (TWT) depth (SP1400–1500 in Fig. 16). Conversely, the Quaternary Mt Subasio normal fault system is likely to be associated with a décollement layer within the Triassic evaporites, as suggested by: (a) the fault downthrow apparently decreasing downdip; and (b) the top phyllite reﬂector being only moderately offset (SP200–500 in Fig. 16). Integrated ﬁeldwork and seismic interpretation: positive and negative inversions in the Northern Apennines The interpretation and time-to-depth conversion of the seismic line in Figure 16 has been used to constrain a regional balanced and restored geological cross-section across the outer sector of the Northern Apennines (Fig. 17). The basement geometry is characterized by a structural depression located underneath the UMAR salient. The adjoining basement highs are placed beneath the Umbria Valley and Laga Basin. The inferred basement physiography highlights a large pre-existing basin ﬁlled with Triassic and probably late Paleozoic sediments. The resulting structural style appears strictly controlled by the inversion and vertical extrusion of this deep Paleozoic–Triassic extensional basin. The UMAR separates two thick siliciclastic turbiditic wedges inﬁlled during Burdigalian–Serravalian (Marnoso–Areancea Formation) and Messinian (Laga Formation) times. These wedges are 2000– 5000 m thick (e.g. Boccaletti et al. 1990) and their present-day transversal extension is at least of 60 km. In the interposed ridge, the siliciclastic turbidites are generally lacking or extremely reduced in thickness, with the exception of a few NNW–SSEtrending conﬁned basins delimited by normal faults (e.g. the Camerino Basin in Figs 5 & 13) (Scisciani et al. 2001). Analogous extensional basins inﬁlled by siliciclastic foredeep sediments have been found Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS Fig. 17. Balanced crustal geological cross-section (a) across the UMAR reconstructed by integrating the results of surface geology and seismic interpretation (trace in Figs 5 & 13). The restored template (b) illustrates the geometry of the late Paleozoic–Triassic extensional basin that was inverted and extruded during the Miocene. both in the eastern (Belforte Basin and Cingoli area) and western (Gubbio Basin) sectors of the UMAR (e.g. Scisciani et al. 2002a; Mirabella et al. 2008) (Figs 5 & 13). The synsedimentary normal faults trend NNW–SSE to north–south, are both east- and west-dipping, and show a range of throw from about 100 to 800 m. Thick turbiditic successions are concentrated in the basin depocentres, with synsedimentary slumps, slide scars and coarse-grained deposits sourced by steep palaeoscarps created by the synsedimentary normal faults. The evident increase in conglomerates and coarse-grained sandstones close to the palaeoscarps suggest a synsedimentary control exerted by the normal fault in the development of these conﬁned basins (Ricci Lucchi 1986). In contrast, the structural highs are capped by thin and condensed sequences of hemipelagic shales, or are deeply eroded. Detailed stratigraphic studies indicate that the extensional faults were active, and controlled the deposition of Late Miocene foredeep turbidites included in the Marnosa–Arenacea (Burdigallian–Tortonian), Camerino sandstone (Tortonian–early Messinian) and Laga (early Messinian) formations. Moreover, several of these normal faults frequently reactivated pre-existing extensional structures, including Jurassic and Cretaceous preorogenic faults. The Miocene normal faults are commonly deformed or truncated by subsequent folds and thrust faults developed during the Apennines compressive tectonics. Prevailing thrust hanging-wall ramp onto footwall ramp geometrical relationships have been constrained through both ﬁeldwork and seismic interpretation. The total shortening associated with the Neogene compressive deformation is about 7.7 km (i.e. c. 10% of the total section length), with each thrust and related fold realizing a maximum shortening of 4.0 km (e.g. for the Sibillini Mountains Thrust). The common occurrence in the anticline backlimbs of high-angle WSW-dipping normal faults truncated by Neogene thrusts propagated with a Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. short-cut geometry highlights the key role played by positive inversion tectonics in the Apennines Orogeny. Another relevant feature is the negative reactivation of low-angle thrust-fault segments by recent active normal faults, including the WSW-dipping Colﬁorito and Visso-Norcia normal fault systems (indicated with ‘X’ and ‘Y’ in Fig. 16, respectively). These fault systems were responsible for two strong recent seismic sequences with main-shocks of Mw 6.0 (1997 in Colﬁorito) and Mw 6.5 (2016 in Visso–Norcia), respectively. The dense network of seismic stations installed during these recent earthquakes clearly imaged the deep active fault geometry (Barba & Basili 2000; Chiaraluce et al. 2017). Two evident SSW-dipping areas with densely concentrated aftershock locations are consistent with the geometry of the normal fault systems reconstructed by both ﬁeld data and seismic interpretation. All these methodologies suggest that the active normal faults negatively reactivate preceding thrust segments. Discussion Structural reactivation and inversion patterns in the ESP A lateral transition between compressional belt, foredeep and mainly extensional foreland structural domains can be clearly interpreted for a young and relatively well-preserved orogen like the Apennines (e.g. Finetti et al. 2005). It is much more challenging to reconstruct the structural setting for older collisional belts such as the Caledonian Orogen where subsequent high-pressure metamorphic core complex formation and widespread phases of erosional truncation have obliterated the remnants of earlier structural conﬁgurations. However, as the offshore ESP experienced a lower degree of metamorphic overprint of the Caledonian belt conﬁguration than onshore, it is possible to observe and highlight the coexistence of Caledonian-age normal faults and compressional structures (Fig. 11). These offshore structures have been mapped in 3D, revealing an overall north-trending strike orientation that is virtually indistinguishable from the Devonian–Jurassic structural grain of this area, and the hypothesized offshore trend of the Iapetus Suture (Fig. 3a). It can therefore be inferred that these possible Caledonianage extensional structures were formed in the palaeoforeland of the evolving Caledonian Orogen. Moreover, the Devonian normal master fault that underlies the Kraken area might have coincided with the front of the Caledonian belt, as no trace of deep-seated compressional deformation is observed beyond it. Irrespective of kinematics and age of activity, all mapped Caledonian, Devonian, Permo-Triassic and Jurassic–Cretaceous faults share similar trends (Fig. 9). Two areas can be identiﬁed in the outer ESP: one with a structural grain parallel to the northtrending Viking Graben; and the other with conjugate fault trends mirroring the east-trending Witch Ground Graben. This long-term preservation of structural grains and the reconstructed polyphase history of tectonic inversion and reactivation of the same structural zones of weakness highlight the profound impact of structural inheritance in the North Sea and ESP (e.g. Fig. 12) (Johnson & Dingwall 1981; Bartholomew et al. 1993; Glennie & Underhill 1998). During this polyphase post-Devonian inversion history, the multiple intra-plate compressional phases led to the deposition of up to ﬁve main regional erosional unconformities: (1) Saalian (Variscan Orogeny); (2) Mid-Cimmerian (Mid North Sea Doming); (3) Late Cimmerian (BCU); (4) IntraCretaceous (Eo-Alpine); and (5) Base Tertiary and Paleocene Alpine unconformities (Fig. 2). These events correspond to ‘peaks of absence’ of the time-equivalent rocks from >75% of the ESP wells (Patruno et al. 2018). In particular, the Late Carboniferous Variscan compression led to extensive foreland-type uplift and erosion (Figs 6 & 7d), which were signiﬁcant over certain portions of the ESP, such as the Fladen Ground Spur and Kraken High. These areas became permanently inverted into long-lasting positive features, and became associated with erosional processes sourcing sediments to the adjacent relative depocentres (Freer et al. 1996) (Fig. 12). We suggest that, consistent with the main supra-regional Variscan kinematics (Fig. 3b), the local maximum compression vectors were orientated roughly north–south (Fig. 12b). A number of Devonian folds (e.g. Figs 7 & 8) were possibly due to Variscan intra-plate compressional stress propagation. Although the Late Triassic–Aalenian hiatus has been recorded by nearly all of the ESP wells (Patruno & Reid 2017) (Figs 6 & 7d), evidence of signiﬁcant Aalenian uplift is particularly strong in the vicinity of the South Viking Graben ‘triple junction’, where the focus of regional thermal doming and erosion was situated (Underhill & Partington 1993; Coward et al. 2003). In particular, an overall Early–Middle Jurassic tectonic uplift has been highlighted by the burial history modelling of well 15/6–1 (Fig. 7c) (Patruno & Reid 2018). Furthermore, the Devonian–Carboniferous succession between Fault 6 and Anticline A in Figure 7a underwent an initial Variscan tilting and a successive Mid-Cimmerian uplift-related tilting, which had an increasingly greater magnitude towards the proto-Viking Graben. As a consequence, these reﬂectors are now characterized by a greater overall easterly tilt (c. 83 ms km−1) than the overlying Permo-Triassic over the same area Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS (c. 35 ms km−1), which was only affected by the Mid-Cimmerian tilting. Minor Early Alpine compression events are highlighted by the gentle anticlinal folding of the entire ?Carboniferous–Top Cretaceous succession over inverted Permo-Triassic extensional basins, with lower Paleocene strata progressively onlapping against the antiformal culmination (e.g. Figs 7, 8 & 12). These compressional events are probably Late Cretaceous in age, as highlighted by Chalk strata becoming anomalously thin towards the antiformal culmination, which corresponds to the PermoTriassic hanging-wall depocentres (Figs 7a & 8). The inversion of the Devonian extensional master fault on the Kraken High (Figs 10 & 12) highlights a subsequent, Late Paleocene compressional event. The Permo-Triassic and Late Jurassic rifting events left behind a number of syndepositional fault-driven intra-platform tectonic depocentres, such as several half-graben on the Piper Shelf (Figs 7a, d & 9) and the Crawford–Skipper Basin (Figs 8, 9 & 12) (Patruno & Reid 2017). The mini-basins on the Piper Shelf were mostly ﬁlled with Rotliegend sediments (e.g. the Fault 6 half-graben in Fig. 7a), suggesting a main Permian age for the post-Variscan rift development over this area, with minor Triassic and Jurassic reactivations. Both Permian and Triassic faults are present in the Crawford–Skipper Basin: they share a mean NNE orientation, which is parallel to the border faults of the nearby Viking Graben (Fig. 9), but show negligible Late Jurassic reactivation; suggesting that in the Jurassic the entire extensional strain had been transferred to the incipient Viking Graben border faults (Figs 9 & 12). Structural reactivation and inversion patterns in the Northern Apennines Following the Variscan Orogeny, the basement of the Northern Apennines underwent repeated episodes of extension during Late Paleozoic–Mesozoic times. Our subsurface interpretation suggests that, presumably starting from the Late Paleozoic and Triassic, a prominent extensional basin developed and was inﬁlled by at least 5 km of syntectonic sediments (Figs 16 & 17). Comparable extensional basins were also reconstructed further east in the Montagna dei Fiori area and in the central Adriatic (Fig. 18). The subsequent Jurassic–Cretaceous extensional event was less severe and achieved an overall reduced amount of downthrow (Fig. 17). The shallower setting and reduced spacing of Jurassic normal faults with respect to previous structures suggest that the thick Triassic evaporitic interval acted as a décollement level during the extension, inﬂuencing vertical fault growth and transversal spacing. The pre-orogenic conﬁguration of basement and the Triassic décollement strongly controlled the style of compressive structures during the Apennines inversion tectonics. The low-wavelength folds are commonly related to minor thrusts propagated from shallow detachments and follow short-cut trajectories with respect to the pre-existing Jurassic normal faults. Conversely, the large-wavelength anticlines forming the prominent mountain fronts and delimited by regional thrusts (e.g. the Sibillini Mountains Thrust) correspond to thick-skinned structures emanating from basement (Figs 18 & 19). Fieldwork and seismic interpretation typically indicate thrusts cross-cutting through hanging-wall and footwall ramps; these geometrical relationships imply a reduced amount of shortening compared to previous thin-skinned reconstructions (e.g. Bally et al. 1986). Normal fault networks have been widely observed in several foreland basins all over the world (e.g. Sinclair 1997), and have also been described in the Northern Apennines and UMAR foredeep basins. Synorogenic normal faults developed in a foreland tectonics context before the onset of contractional deformation, and generated structural highs nearparallel to the ﬂexure axis and to the subsequent compressive structures, therefore controlling the thickness, dispersal and facies distribution of the syntectonic foredeep deposits (Tavarnelli et al. 1998; Tavarnelli & Peacock 1999; Scisciani et al. 2000a, 2001, 2002a, 2002b; Mazzoli et al. 2002; Calamita et al. 2003). Several of these normal faults frequently reactivated pre-existing extensional structures, including Jurassic and Cretaceous preorogenic faults (Calamita et al. 2011), as observed in similar settings where foreland downwarping resulted in the reactivation of inherited passivemargin structures (Frankowicz & McClay 2010; Langhi et al. 2011; Saqab & Bourget 2015). In general, the strong subsidence recorded in foredeep basins is triggered by the ﬂexure of the foreland plate due to the load exerted by the adjacent growing orogenic wedge (Cross 1986). The foreland ﬂexure is locally accommodated by pre-orogenic extension affecting the shallow brittle crust (‘ﬂexural extension’: Turcotte & Schubert 1982; Doglioni 1995). The synorogenic normal faults in the Apennines were also classically interpreted in term of ﬂexural bending extension (e.g. Boccaletti et al. 1990); however, the normal faults within the UMAR and in adjacent Northern Apennines sectors show distinctive architectural characteristics that differ with respect to those of typical ﬂexural bending settings (Fig. 19). In the Northern Apennines, Miocene normal faults are commonly foreland-dipping and are located in the inner edges of foredeep basins (e.g. in the eastern sector of the Sibillini Mountains). In the proposed reconstruction (Fig. 19), the Late Miocene UMAR is interpreted as an embryonic large-wavelength (c. 40 km in width) uplift Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. Fig. 18. Regional geological cross-section (a) and simpliﬁed restoration (b) across the Northern Apennines from the Umbria Valley to the Plio-Quaternary Adriatic Basin (trace in Fig. 5). The transect derives from the present study (C–D segment) and previous reconstructions (E–F–G segment; modiﬁed from Scisciani & Montefalcone 2006; Scisciani 2009; Scisciani & Calamita 2009; Pace et al. 2015). Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS Fig. 19. (a) Sketch map illustrating the reconstructed distribution of the thickness and facies of late Tortonian– Messinian pre-gypsum deposits. (b) Schematic cross-section showing the embryonic growth of the UMAR that was interposed between the thrust front of the Apennines chain and the depocentre of the Messinian Laga Basin. developed in proximity of the advancing Apennines thrust front. This suggests that during the Late Miocene the Adriatic foreland underwent foreland ﬂexure accompanied by normal faulting, prior to large-wavelength (c. 40 km) uplift, with reactivation at shallow levels of the pre-existing normal faults. The large-scale uplifted ‘peripheral bulge’ almost corresponds to the whole UMAR, suggesting a large-scale basin inversion, with embryonic vertical extrusion by positive inversion of the Late Paleozoic–Mesozoic extensional basin at depth, possibly promoted by mechanical weakening of the Northern Apennines foreland lithosphere due to ﬂexural normal faulting. The controlling factor of multi-phase reactivations and inversions Although the ESP and the Northern Apennines underwent different tectonic evolutions within distinct settings, they both show evidence of cyclic reactivation of faults and structural grains, with alternating extension and compression (Fig. 20). Therefore, these areas are expected to be characterized by some of the geological elements that, in past studies, have been suggested to promote this sort of recurring reactivation, at scales ranging from a single fault plane to an entire basin or chain, as predicted by the Wilson cycle concept. The main reported factors include: (1) the nature and composition of the sedimentary cover and crust; (2) thermal conditions; (3) crustal thickness; (4) the amount/rate and direction of deformation; (5) the frequency, orientation and mechanical/petrophysical characteristics of fractures and shear zones; (6) ﬂuid occurrence and relative pressure; and (7) the amount/rate of sedimentation/erosion (Sibson 1985; Williams et al. 1989; Butler et al. 1997; Turner & Williams 2004; Morley et al. 2008; Buiter et al. 2009; Bonini et al. 2012; Lafosse et al. 2016). Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. Fig. 20. Examples of fault reactivation in positive and negative inversion tectonic setting in the ESP and Northern Apennines: (a) positive reactivation during the Alpine foreland tectonics (late Cretaceous–Paleocene) of a Devonian and Triassic normal fault ﬂanking to the west of the Crawford Spur in the ESP; (b) Pliocene–Quaternary positive reactivation of a Triassic–Early Jurassic normal fault in the Adriatic Basin in front of the Northern Apennines (seismic line location given in Fig. 19; modiﬁed from Pace et al. 2015); (c) positive inversion of a synsedimentary Devonian normal fault (note the divergent Du synrift wedge in the hanging wall of the NW-dipping extensional fault) in the ESP (modiﬁed from Platt & Cartwright 1998); the southeastwards thinning of the basal Devonian unit (Dl) towards the inverted fault is also consistent with the negative reactivation of an early reverse fault; and (d) portion of the seismic line in Figure 16 showing the negative inversion of a Neogene thrust fault by Quaternary SW-dipping normal faults (Colﬁorito Basin boundary faults). Normal faults, tick black lines with black arrow; thrust and reverse faults, red lines with red arrow; inverted faults, double arrow (black and red arrow labelled with ‘i’); Tb, Top basement; Dl, Devonian lower unit; Du, Devonian upper unit; Do, onlap surface. Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS Both analysed areas rest on a relatively attenuated continental lithosphere (Zanella et al. 2003; Artemieva 2007; Molinari & Morelli 2011). In particular, in the far undeformed Adriatic foreland, where crustal thickness was preserved by the Cenozoic contractional and extensional tectonics, the Moho is relatively shallow (c. 30–35 km in depth) (Finetti et al. 2005; Miller & Piana Agostinetti 2012). Under present-day onshore Scotland and Greenland Caledonian core complexes, the Moho is also at relatively shallow depths (c. 50 km: Snyder 1991) for a fully collisional orogen. The thickness of the crust in the North Sea was probably reduced to 30– 40 km at the end of the Devonian extensional collapse (Schlindwein & Jokat 1999) and is currently 10–30 km, which are typical values for rift basins (Zanella et al. 2003). Elevated geothermal gradient and inherited basement grain are ‘classical’ drivers for long-term lithosphere weakening, consequently favouring the cyclical reactivation of the former rifted structural grain during the compressional and extensional collapse parts of a Wilson cycle (Krabbendam 2001). Previous numerical modelling studies have demonstrated that a short time interval between riftingrelated crustal thinning and the subsequent compression prevents the basin and the underlying mantle from cooling, favouring positive basin inversion (Burov & Diament 1995; Buiter et al. 2009). This, however, is in direct contrast to our Apennines case study, where the post-rift phase lasted for 140– 190 myr. Instead, in our interpretation the mechanical weakening of the foreland lithosphere due to preinversion ﬂexural normal faulting, tectonomagmatic phases and frequent reactivation of inherited discontinuities, accompanied by possible crustal hydration, is the key in explaining the observed polyphase basin inversions in the two study areas. In relatively old and cold basins, numerical modelling indicates that small amounts of strain softening are required to promote basin inversion by reactivation of inherited fault zones. In particular, the weakening mechanism of shear zones or the cyclic reutilization of pre-existing discontinuities are thought to be long-lived driving processes (e.g. Thomson & Underhill 1993; Butler et al. 1995; Imber et al. 1997; Hatcher 2001; Holdsworth et al. 2001; Scisciani 2009; Tavarnelli et al. 2019). Shear zone weakening is enhanced by high ﬂuid pressures, clay-scale gouge and phyllosilicate-rich foliated fault rocks (e.g. Collettini et al. 2009). Several of these key elements have been clearly documented in several reactivated faults in both the Northern Apennines and the North Sea, as further explained below. The main positive inversion stages affecting the North Sea and ESP foreland were preceded by relevant tectonomagmatic phases (Fig. 2) during the Carboniferous–Lower Permian and middle Jurassic, associated with the development of largewavelength erosional unconformities. The North Sea Mid-Cimmerian Unconformity, in particular, causes the Aalenian–upper Triassic interval to be missing in wells, even in relatively structural depocentres (e.g. the Piper Shelf) (Fig. 7) (Patruno & Reid 2017), and is associated with the extrusion of the Rattray Series and the Forties Igneous Province (Ziegler 1992; Underhill & Partington 1993, 1994). This widespread erosion is the result of the impingement of a broad-based (>1250 km diameter) transient plume head at the base of the lithosphere (‘Mid North Sea Doming’ event: sensu Underhill & Partington 1993). Analogously, in the Northern Apennines Adriatic foreland, a Paleogene regional uplift and extensive erosion is associated with the near-complete absence of Paleogene sediments in wellbores and Miocene deposits resting locally unconformably on Lower Cretaceous carbonates (Rusciadelli et al. 2003; Satolli et al. 2014). This regional uplift was also concomitant with an extensive Paleogene anorogenic intra-plate magmatism that affected the Adriatic foreland, associated with intrusive and effusive ultra-maﬁc products sourced by the mantle (Bell et al. 2013). The overall reduction in the mechanical resistance of the rocks in the study areas was thus likely to be favoured by: (a) the magmatic-related increase in heat ﬂow and geothermal ﬂuids; and (b) the diagenetic processes associated with the development of several long-lasting, erosional and compound erosional surfaces. Strain localization subsequently took place within the thermally weakened portions of the crust, and multi-phase reactivations of pre-existing lineaments ensued (Kusznir & Park 1987). In relation to this, it is interesting to note the very quick transition in the Viking Graben from highmagnitude Aalenian uplift to Bajocian–Bathonian proto-rifting and Callovian-Berriasian main rift (Patruno 2017). Also, the locus of maximum Aalenian uplift quickly became the ‘triple junction’ between the axes of the three main Late Jurassic rift systems of the North Sea (the Viking Graben, the Central Graben and the Moray Firth). In addition, the existence of foreland units affected by synorogenic normal faults has several implications on ﬂuid circulation and mechanical characteristics of the foreland plate. Pre-orogenic formations in the foreland are commonly overlain by a veneer of hemipelagic shales and marls which predate the deposition of thick coarse-grained turbidites that act as a ﬂuid barrier (Ricci Lucchi 1986). However, when this conﬁning layer is offset and breached by synorogenic normal faults, the deep reservoir becomes hydraulically interconnected to the shallow one. Consequently, deep and light ﬂuids Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 V. SCISCIANI ET AL. leak-off (hydrocarbons, CO2 and hot water); while cold and dense saltwater may intrude into the deep carbonate reservoir, recharging or enhancing the deep circulation. Analogously, in uplifted forelands the direct exposure of eroded and, in places, karstiﬁed carbonates promotes the meteoric water inﬁltration along the fracture network into the crust and recharges the groundwater circulations (Mindszenty et al. 1995; O’Brien et al. 1999; Allen et al. 2001). In the Northern Apennines foreland, an increase in the salinity of migrating ﬂuids caused diffuse dolomitization of carbonates along Jurassic normal faults reactivated during Miocene synorogenic foreland tectonics (Ronchi et al. 2003). Further evidence of ascending hydrocarbon-charged ﬂuids through a pathway composed of synorogenic fracture and fault networks (Scisciani et al. 2000a) in the Late Miocene Northern Apennines foreland is testiﬁed to by the occurrence of cold-seep carbonate buildups, authigenic patches and carbonate breccias (Iadanza et al. 2013). Analogously, in the Norwegian Northern North Sea, reverse fault slip and gas leakage along sections of previously sealing reservoir-bounding faults occurred in recent times due to a combination of an increase in compressional stress associated with post-glacial rebound and locally elevated pore pressure due to the local presence of natural gas in fault footwalls (Wiprut & Zoback 2000, 2002). It is arguable that similar mechanisms acted on the ESP, where several Alpine-age reverse-fault reactivations have been identiﬁed (e.g. Figs 4d & 7), and where there is evidence of hydrocarbon seepage (Richardson et al. 2005; Patruno & Reid 2016, 2017) and possible ﬂuid leakage (Patruno et al. 2018). Moreover, the inﬁltration of aqueous ﬂuids may lead to the formation of clay minerals, often leading to important changes in the mechanical behaviour of the fault zones, such as local anisotropy enhancement and bulk shear-strength reduction, and increased ﬂuid pressures during shearing (Warr & Cox 2001). Summary and conclusions The Caledonian and Variscan orogens in northerncentral Europe and the Apennines mountain range in Italy are classic examples of thrust belts developed at the expense of former passive margins that underwent multiple events of extension and compression. Both settings represent key regions to study the effects of how a complete Wilson cycle is achieved and preserved in the geological record. In the present work, we have selected two study areas set in the above-mentioned regions, where ﬁeld structural data and subsurface data allow the recognition of at least one ‘classical/complete’ Wilson cycle along with several ‘foreland/incomplete’ Wilson cycles, with long-term preservation of structural grain and polyphase reactivation of pre-existing structures. These two study areas are the offshore East Shetland Platform (ESP) in the UK North Sea and the onshore Northern Apennines of Italy. Regional geology, structural ﬁeld evidence and subsurface data (2D and 3D seismic reﬂection and well logs) have here been integrated with the aim of reconstructing the tectonic evolution and common features that were signiﬁcant in controlling the stress localization along inherited zones of weakness that suffered repeated reactivation. In the ESP, the relatively shallow Paleozoic–Tertiary sedimentary cover, combined with a remarkable penetration depth of recently acquired 3D broadband seismic reﬂection data, revealed several events of compression and extension that have been taking place since the Caledonian Orogeny. We interpret multiple reactivation of the Caledonian basement structures during both negative (e.g. the Devonian post-collision collapse, Permian–Triassic and Middle–Late Jurassic rifting events) and positive inversion tectonics (e.g. from far-ﬁeld Variscan and Alpine orogenic phases). Similarly in the Northern Apennines, due to exceptional outcrop exposures and the availability of abundant subsurface data (e.g. exploration wells and seismic proﬁles), we have been able to recognize repeated rifting and mountain-building processes. This area underwent: (i) compression during the Variscan Orogeny; (ii) extension during the Late Paleozoic–Early Mesozoic rifting and Jurassic Alpine opening of the Tethys Ocean; (iii) Apenninic compression in Cenozoic times; and (iv) late/postorogenic extension in the Miocene–Recent. The outcomes of this study indicate that inherited normal faults related to pre-orogenic rifting phases promoted multiple, deep-rooted, basement-involved positive basin inversion, with different amounts and rates of vertical extrusion of previous wedge-shaped graben. In turn, normal faults produced by negative inversion and post-orogenic extensional collapse were also strongly inﬂuenced by the location of precursor compressional discontinuities (e.g. thrust ramps). In the Northern Apennines, this has resulted in distinct patterns and segmentation of the recent post-orogenic normal faults that are responsible for the present-day seismicity. In the two study areas, pre-inversion thermal softening and fault weakening of the foreland lithosphere, particularly if accompanied by possible crustal hydration and tectonomagmatic phases, are suggested to represent the main controlling factors that promote cyclical structural reactivations during both full Wilson cycles and far-ﬁeld foreland inversions. Downloaded from http://sp.lyellcollection.org/ by guest on May 16, 2019 MULTI-PHASE REACTIVATIONS OF EXTENSIONAL BASINS This study, moreover, illustrates how the structural signature of a complete Wilson cycle is best preserved in foreland domains ﬂanking orogenic systems and is produced at the expenses of pre-orogenic rifted margins. In these settings, earlier lineaments are not completely obscured by later deformations but are still legible in the structural record, thus enabling us to investigate the role, degree and intensity of structural inheritance phenomena. Foreland domains may, therefore, be regarded as ideal settings to study the factors controlling crustal reactivation processes during both positive and negative inversion histories. The authors gratefully acknowledge PGS for the permission to publish the seismic section in Figure 11, from the 3D GeoStreamer North Sea dataset (BBK and NVG surveys). The Geological Society of London is also thanked for permission to reutilize, for parts of Figures 4c & 6–10 of this paper, images and seismic sections that were previously published by Patruno et al. (2018). We thank Dr Stuart Archer, Dr Thomas Phillips, Dr Robert W. Wilson and Prof. Ken McCaffrey for their thorough and helpful comments, which led to a much improved paper. Acknowledgements This work was supported by Chieti-Pescara University funds (to V. Scisciani). Funding References ABRAMOVITZ, T. & THYBO, H. 2000. Seismic images of Caledonian lithosphere-scale collision structures in the southeastern North Sea along Mona Lisa Proﬁle 2. 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