Jura, Alps and the boundary of the Adria subplate

Tectonophysics 483 (2010) 223–239

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Jura, Alps and the boundary of the Adria subplate Hans Laubscher Wenkenstrasse 26, CH-4125 Riehen, Switzerland

a r t i c l e

i n f o

Article history: Received 18 April 2009 Received in revised form 21 August 2009 Accepted 15 October 2009 Available online 24 October 2009 Keywords: Adria Alps Jura Strain partitioning Nappes Floating orogens

a b s t r a c t The thin-skinned Jura fold/thrust belt is the frontal part of a thick-skinned late Miocene Alpine nappe. Important late Miocene structures in the Alps that help define this nappe are the External Massifs in the north, the Orobic thrusts and the Piemontese Southern Alps in the south, the Giudicarie–Brenner Line in the east and, more vaguely, the south end of the Belledonne Massif in the west. A bottom boundary compatible with the data is the Brittle–Ductile Transition (BDT). This Jura nappe is separated from the (latest Oligocene)–early Miocene Helvetic nappe system most obviously by the late Miocene Orobic–Giudicarie– Brenner fault complex. In contrast, the distinctive boundary of the early Miocene thick-skinned Helvetic nappe was the Iorio–Tonale Line (IT) in the south, merging into the Piemontese Southern Alps at what may be termed the “Locarno Singularity”–the point of minimum width of the thick-skinned Alps; the northern boundary passed through the Gotthard–and Montblanc Massifs. Motion along the southern boundary was dextrally transpressive, associated with complex strain partitioning patterns. One part of these patterns is the confusing entity comprising the Piemontese Southern Alps and the Lepontine “dome”. The slivers composing the Piemontese Southern Alps are dragged dextrally by the Iorio–Tonale strike–slip at the “Locarno Singularity” and thereby exhibit an important characteristic trait of “exotic terranes” — crustal slivers dragged along strike–slip faults. The Lepontine “dome” around the Locarno Singularity on the north side of the Iorio–Tonale Line displays the characteristics of a backstop in thrust systems as modeled in sandbox experiments. The special boundary functioning as a backstop, compatible with the transpressive kinematics, was the Ivrea body, a strong mafic–ultramafic–granulitic complex which in the early Miocene migrated dextrally along the Iorio–Tonale Line, forcing detachment to descend into the hot middle crust and producing an upsurge of the hot masses. In the late Miocene Jura phase this particular impediment had moved to the west, enabling décollement at the Brittle–Ductile Transition to propagate across the Insubric Line into the Lombardic domain. Large-scale strain partitioning manifestations resemble those of the equally dextrally transpressive southern boundary of the Caribbean plate, in spite of a difference in scales. The foremost of these manifestations are regional right-slip faults slicing through a broad band of contraction, accompanied by collapse basins. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The décollement hypothesis of Jura folding (Buxtorf, 1907) celebrated its centenary in 2007. While I got ready to sketch possible commemorative articles, I stumbled on an aspect of the hypothesis that required particular attention. Buxtorf had postulated that the Jura décollement nappe had been pushed by the Alps without, however, explaining how. Although I had broached the problem in several articles (e.g. Laubscher, 1973, 1996, 1997), a number of essential data had not been available then and a further pursuit of the problem had to be postponed. Now, the occasion seemed ripe for an advanced attempt at formulating a scenario if not a model. The project turned out, however, to require a thorough analysis of the kinematics of the entire (latest Oligocene)–Miocene Neoalps (in this article referred to

E-mail address: [email protected]. 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.10.011

as “Miocene” for simplicity). It embodied such far-reaching problems as the thick-skin–thin-skin kinematics of the Neogene motions in the Eurasia–Africa plate boundary zone. This problem had recently been at issue, at least marginally, within attempts at regional integration of the Alpine system (e.g. in Nicolich et al., 2002; Schmid et al., 2004). The authors took pains to link the results of such diverse and up-to-date methods as deep seismic sounding and fission track dating. However, whereas most of these attempts were more ambitious, delving into early stages of the Alpine orogeny from the Mesozoic to the Palaeogene, the Jura problem may be essentially reduced to the Neogene, making it simpler. But even so the confusing wealth of data and opinions is such that their weighting is often difficult. This particular aspect of integrative interpretation of data sets has been the subject of a revealing recent article by Bond et al. (2007). It reported on an experiment designed to quantify “conceptual uncertainty in geoscience interpretation”. Given was a synthetic seismic

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cross-section, and a number of geoscientists were asked to present a tectonic analysis. There was only one correct solution. It turned out, not surprisingly, that the interpretations differed widely according to the interpreter's field of experience and expertise. Most were wrong. Doubtless this insight applies to geological interpretations jointly and severally. Unfortunately, moreover, in nature what is given is not a problem designed in the laboratory but the result of an evolutionary process of unknown design. The correct solution is not known. There is always a range of possible working hypotheses, depending on the choice and weighting of data, and there are preferences due to experience and specialization. In particular, this applies to great regional syntheses of the development of entire mountain systems such as the Alps, even if restricted to the Neogene time interval. 2. A key to the kinematics of the Neoalps: The system of the Periadriatic Line, the Giudicarie Line and the Tauern Window A synopsis of the vast amount of recent work done in the Eastern and Southern Alps has been meritoriously published in Nicolich et al. (2002) in a volume covering a meeting on the deep seismic reflection line “TRANSALP” that passes through the Tauern Window. I shall draw freely on these data. Fig. 1 is a simplified tectonic map of the Alps, including the western part of the Pannonian basin and the Dinaric ranges, and Fig. 2 outlines the major tectonic units around the system of the Periadriatic Line (PL), the Giudicarie Line (GL) and the Tauern Window (TW). The components of the system are well dated. Recently, great efforts have been made to date radiometrically the very fault rocks that are found in the maze of map-scale faults, which are commonly defined to constitute the Periadriatic Line (PL; Mancktelow et al., 2001; Müller et al., 2001). This sub-mapscale information has enormously broadened

the data base of the PL complex. Unfortunately, it also has added new difficulties for its overall kinematics, as evidently local “strain kinematics”, though an important addition, ought to be viewed in the perspective of the more broadbrush picture of regional “system kinematics” such as plate kinematics. Whereas “strain kinematics” often claims that its increasingly sophisticated measurements and their interpretations increasingly add precision to the definition of pieces of a puzzle which eventually fall smoothly into a satisfactory overall picture, “system kinematics” rather starts with a tentative overall picture, chosen from a set of “multiple working hypotheses”, which then is modified as new data seem to demand. In this article I shall argue the overall kinematics of the Periadriatic Line as a part of Miocene plate boundary motions from a system kinematics viewpoint, though always keeping strain kinematics in sight. The main rules of system kinematics turn around manageable simplifications. Though faults tend to be complexly broken zones, and though distributed (or diffuse) deformation is ubiquitous, the crust is treated as a compound of precisely defined fragments whose movements maintain material (~ volume) balance, as done in plate kinematics. Motion is dealt with as occurring in discrete time steps (short “stages” and longer “phases”). Volume balance is approximately maintained by a combination of area balance (estimated) in cross-sections and map view constructions. It sometimes poses difficult problems as it critically depends on the thickness of the deformed unit. Such a problem arises, for instance, in connection with the Lepontine dome in the Central Alps north of Locarno (Fig. 1). Whereas the thickness of the deformed crustal fragments as a rule may be plausibly modeled by assuming lower boundaries at the Brittle–Ductile Transition (BDT) for the thick-skinned parts and incompetent décollement horizons for the thin-skinned ones, the lower boundary for the Lepontine dome had been deeply within the

Fig. 1. The belt of thick-skinned nappes (closely dotted) wrapped around the northern and western boundary of the Adriatic subplate (loosely dotted). The northern boundary of the thick-skinned Alps is the festoon of the External Massifs (dark gray), whereas the southern boundary is somewhat more complex (see text). In particular, the Orobic and Valsugana thrusts, though basement rooted, are within the Adriatic subplate, outside the typical nappe edifice. The Piemontese Southern Alps, although equally outside the nappe belt proper, contain nappe fragments. Black: Peio body (“Tonale nappe”). At the eastern end of the Alps (in the narrow sense) a number of Tertiary basins appear, marking the beginning of the Pannonian depression. Apenninic thrust masses (to the north and east of the Villalvernia–Varzi–Levanto Line) are not shown.

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Fig. 2. The Giudicarie subsystem and the Adige indenter. For the code to the west of the Giudicarie belt see Fig. 1; in addition: EW = Engadin Window, EML = External Massifs Line, Ö TH = Ötztal Thrust, OR = Orobic Thrust, MF = Mölltal Fault, M = Mauls Triple Point, D = Dimaro Triple Point, DA = Deferegger = Antholz fault, RF = Rieserferner pluton, PuL = Pustertal Line. To the east of the belt (vertical ruling), left-slip (~70 km, see text) moved the Adria subplate into the Alpine nappe edifice, creating the Tauern window. “Helvetic” is early Miocene, “Jura” is late Miocene. Horizontally ruled: approximate domain of the Pannonian collapse (PC). Note that when the European plate is kept fixed, in the late Miocene the “Helvetic PL” to the west of the Giudicarie is underthrust, while to the east it is overthrust in the level of the upper crust. For simplicity a pure SN translation is shown. Explanations are in the text.

hot masses of the middle crust and therefore requires particular considerations for estimating material balance (see below). Further uncertainties are met where important rotations about vertical axes and motions in different tectonic levels are involved (for instance in the upper crust vs. the lower crust/upper mantle levels — a problem in the kinematic modelling of the Ivrea zone, see below). Finally, care must be taken to distinguish between units subject to motion in a certain limited time interval and “nappes” in the traditional sense, which may have been moving units (“orogenic lids”, the initial shape of “nappes” such as the “Jura nappe”) during an earlier phase but were mere markers (still called “nappes”) during later deformations (for instance the ~Eocene “Penninic nappes”, which became the partial content of the early Miocene Helvetic units, see below). The sum of these difficulties is such that at this time the system kinematics of the Neoalps is tractable merely in a semiquantitative way. 3. The basis of a system kinematics approach to the Periadriatic Line The timing of motion within the general Periadriatic Line complex is mostly the result of geochronological investigation in the metamorphic (ductile to the Brittle–Ductile Transition) domain, except for the Giudicarie-South Line (GS, see Fig. 2), which is dated stratigraphically as it ties into the late Miocene Lombardic fold/thrust arc, and the Pustertal segment (PuL in Fig. 2), where late Miocene motions are affecting the Tertiary basins to the SE of the Tauern Window (TW). The stratigraphic dating of GS is evidently of paramount importance for the investigation of late Miocene (Jura phase) motions in the Alps, as GS connects with other members of the Periadriatic Line where, at the level of the present surface, late Miocene motions overprint earlier structures by mainly cataclastic deformation, which is difficult to date radiometrically. The tectonic significance of GS therefore needs to be assessed before the kinematics of earlier phases may be contemplated.

An important tie between the stratigraphic and the radiometric methods is zircon fission track dating (ZFD). The reason is that fission tracks are preserved at isotherms close to that of the Brittle–Ductile Transition in quartz-rich rocks (about 200° b T b 250°). ZFD isochrones such as those by Most et al. (2002) therefore date paleo-BDTs and their subsequent uplift and deformation in the brittle mode. For the Tauern Window (TW) they document a cumulative post-15 Ma uplift of ~15 km (in the west) and 30 km (in the center) and attendant (essentially brittle) deformation in the accessible parts, and younger deformations at depth. Consequently, the Tauern Window is a crucial kinematic element of the late Miocene phase. Hence, the Brenner Line (BL) as the western boundary of the Tauern Window, ought to have a late Miocene component too. It has been dated by ZFD (Fügenschuh et al., 1997), documenting both an early and a late Miocene phase. The late Miocene one seems less important at the face of it, but here the interpretation requires careful assessment of the sequence and distribution of late Miocene motions at the western end of the Tauern Window, and this in turn involves the difficult problem of strain partitioning between the right-slip Pustertal Line, the left-slip Giudicarie North-Brenner Line and shortening in the Tauern Window. Laubscher (1996) has proposed a simplified model of the kinematics of this strain partitioning. It shows that because of the simultaneous right-slip at the Pustertal Line, only the latest contributions to the Tauern Window (TW) contraction may be identified at BL, earlier contributions having been displaced by right-slip to the east, where they have been ascertained by Most et al. (2002) (compare Fig. 3, inset a, b). Elements of early Miocene and older motions at BL were merely exhumed by the late Miocene uplift. As dating is extended farther eastward (Fig. 3a), the content of the Tauern Window and, going from W to E, the amount of contraction is increasingly completed by older elements of the late Miocene phase. Partly because of this strain partitioning the Mauls Singularity (M in Figs. 1, 2), where the Alpine “core” (the stack of thick-skinned nappes) is severely constricted and the individual late Miocene elements join in a virtually single point, strain analysis becomes mired

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Fig. 3. Cross-section through the Tauern and the Venetian Southern Alps (location in Figs. 1, 2; based on Schmid et al., 2004 and Doglioni, 2007; simplified). Dotted: putative imbrications of the Tauern window, from Schmid et al. (2004), where they are interpreted as “ductile pop-up”. However, the ZFD of Most et al. (2002), shown in the insets 3a and 3b, rather require an at least partly brittle behavior such as the one modeled for the Aar Massif in (b). The kinematics of folds of stacked brittle to ductile thrusts requires the sequential buildup indicated by the numbers 1–5, where the later thrusts deform all the earlier ones (compare Fig. 9; e.g. Laubscher, 1997). The highly idealized basement slices are composed of fragments of sundry older units, among them early Miocene ones. Loosely dotted: affected by Alpine contraction; dark gray: Mesozoic of the Southern Alps; BDT′: delamination at BDT; Inset (Fig. 3a): zircon fission track ages according to Most et al. (2002). There is some uncertainty in assigning the terms “upper”, “middle” and “lower” crust to the seismic reflections (for seismic refraction data see Cassinis & Scarascia 2002). However, there is general agreement as to the interpretation of a strong reflection band, which appears involved in subduction in the records of the Swiss deep seismic reflection survey (NFP 20, see Pfiffner et al. eds). It is generally termed “lower crust”, although there is no clue as to its chemistry or rheology, characteristics that as a rule are used for defining “lower crust”. “Upper crust” in this article means the seismogenic part of the crust which in the Alps and their foreland varies from 10 to 20 km (Deichmann, 1992). Its base is believed to roughly coincide with BDT. The “middle crust” is the interval in between, which is characterized by generally poor and confused reflection segments.

in a rather chaotic mess (Mancktelow et al., 2001). Previously this situation had been taken as proof that deformations within the Alpine core were strictly separated from those along and south of the Periadriatic Line — an erroneous assumption, as it turns out. The simplified model Fig. 4 summarizes the (thick-skin) late Miocene contractive kinematics from a system-kinematic viewpoint. Note that the southern part of Adria moved uniformly to the north by about 70 km (the amount apparently displacing the Iorio–Tonale Line to the Pustertal Line (PuL) while its northern part was slit at the

Fig. 4. Schematic of the late Miocene Adige indentation. Adria moved north by the same amount on both sides of the Giudicarie, resulting in the same amount of thrusting, but in the western part it was underthrust and partitioned into the External Massifs and Lombardic thrusts. Both Tauern and External Massifs thrusts emerge at the surface in the thin-skinned foreland domain (compare Fig. 3). Ö = Ötztal thrust.

Giudicarie Line, where there was differential contraction on opposite sides of the line. On the eastern side the resulting left-slip of 70 km of the Iorio–Tonale Line (IT) to the Pustertal Line (PuL) is roughly compatible with overall late Miocene contraction in the Tauern Window (Fig. 3a), while in the western flank of the GS–GN–BL Line contraction created both the south-vergent Lombardic Alps (Schönborn, 1992) and the north-vergent External Massifs (Laubscher, 1997). In addition, there was strain partitioning in this transpressive system. Right-slip at the Pustertal Line crosses the Giudicarie Line and the Brenner Line at M, continuing into the External Massifs Line (EML) of the Central and Western Alps (Fig. 2; for strain partitioning in the External Massifs also see Ricou and Siddans, 1986; Laubscher, 1988, for strain partitioning in a transpressive setting in the Jura see Laubscher, 1948). Along EML, late Miocene Brittle–Ductile Transition zones are exposed in the External Massifs. Elsewhere uplift of BDT attendant the continuous right-slip apparently has not reached the surface, except in the Lepontine Domain and its western boundary structures (Fig. 5: SL, C). In a special position is the Engadin Window (Figs. 1, 2). Although it fits in the pattern of the External Massifs Line, no late Miocene ZFD data have been recorded (Hurford et al., 1989): the late Miocene uplift there is comparatively small, apparently less than ~10–20 km, which means that no late Miocene BDT has reached the surface. However, another odd late Miocene contractive feature immediately attached to the western flank of the Brenner fault is the front

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Fig. 5. The Periadriatic Line (PL) in the Oligocene–early Miocene. Several lines of evidence demand latest Eocene–Oligocene extension and attendant magmatic activity transcending the Alps, affecting e.g. the northern foreland (German–Austrian Molasse Basin, MB), the southern “hinterland” (Adamello–Euganei trend, light gray) as well as the western European rifts (Fig. 1, inset). There apparently was sinistral transtension of unknown magnitude along PAL. Transition into (latest Oligocene)–early Miocene contraction may have been slightly earlier in the east. B = Bergell, R = Rieserferner intrusions, EF = Engadin Fault.

of the originally Cretaceous Ötztal nappe (Fig. 2; e.g. Schmid and Froitzheim, 1993). Its young age is inferred from its issuing from the late Miocene Giudicarie–Brenner fault zone and from the fact that it is thrust on younger nappes as well as on the Tertiary (probably Oligocene) Engadin fault. Within the system-kinematic scenario developed in this article it fits the scheme of Fig. 4: it is the first manifestation of the External Massifs Line west of the Brenner Fault. The western segment of the Periadriatic Line (the Iorio–Tonale Line, IT in Fig. 6) was mostly active in the early Miocene (Hurford, 1986), whereas the eastern segment (Pustertal Line) and the Giudicarie Line were moving in the late Miocene or were at least severely overprinted in the late Miocene (e.g. Schönborn, 1992). TW (Tauern Window), according to ZFD data of Most et al. (2002), had acquired its present shape mostly in the late Miocene (Fig. 3a, insets). In brief, in the early Miocene the Periadriatic Line was a right-slip transpressive fault; it was dissected by left-slip of about 70 km at the late Miocene Giudicarie Line (GL) amounting to an indentation of that magnitude by the Adriatic subplate into the Alpine nappe edifice. At the tip of the late Miocene indenter, to the east of the Giudicarie– Brenner Line (GS–GN–BL = Giudicarie Line southern segment, Giudicarie Line northern segment, Brenner Line), the stack of imbrications in the TW was built up. Adopting the cross-section model depicting the kinematics of the External Massifs as proposed by Laubscher (1997), which is characterized by a sequence of staircase thrusts mediating between the thick-skinned core and the thin-skinned foreland thrust belts (Fig. 3b), it turns out that the thick-skinned Tauern buildup touched off the thin-skinned Subalpine Molasse thrust belt in Germany and Austria (Roeder and Bachmann, 1996). This, then, distinguishes the late Miocene “Venetian partial indenter” east of the Giudicarie Line from a “Lombardic partial indenter” to the west of the line. Whereas thick-skinned shortening in the east was concentrated in the Tauern Window (the Valsugana and Montello thrusts playing a minor role), it affected the western part mainly in the Orobic backthrusts and in the north-vergent thrusts of the External Massifs (Fig. 1; compare Soom, 1990; Michalski and Soom, 1990; Seward and Mancktelow, 1994; Laubscher, 1997). Moreover, simultaneous right-slip along the Pustertal Line and the External Massifs Line (Fig. 2) indicates a general right-slip of Adria — apparently associated with continuous sinistral rotation of the subplate according to Laubscher (1988, 1991, 1996); compare the GPS data of Oldow et al., 2002) with respect to stable Europa. As a byproduct it generated, for instance, considerable shortening across the

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left-slip Giudicarie Line, manifest in its attendant thrust belt (e. g. Castellarin and Sartori, 1979; Castellarin et al., 1987). Although this general (system kinematics) conclusion seems inescapable, there are problems in detail, particularly in the Mauls area, where GL (the Giudicarie Line), PL (the Periadriatic Line), BL (the Brenner Line) and TW join. Conventionally, it had been thought that the Giudicarie Line (GL) passed smoothly into the Pustertal Line, being the boundary of the Southern Alps (i.e. the Adria subplate), whereas BL and TW were separate features restricted to the Alpine nappe edifice (“Alpine core”), and this view is still expressed in regional maps as recent as that of Bigi et al. (1990). Based on such unquestioned documentation, the prevailing opinion of most authors was that “a simple offset of the Tonale and Pustertal Lines by the Giudicarie Line could be ruled out because of the non-existence of an extension of the Giudicarie Line into the Central Alps” (Schmid et al., 1987). In the meantime, however, partly as a consequence of the work by Fügenschuh and Stöckli (e.g. in Mancktelow et al., 2001), the extension of the Giudicarie Line into the Brenner Line has been established (compare Laubscher, 1996; Schmid et al., 2004). This jibes with the data of Most et al. (2002) for the Tauern Window. The remaining difficulties concern the kinematics of strain partitioning between strike–slip and contraction and the role of rotation around vertical axes (compare Laubscher, 1996; see below). There is additional confusion as all of these elements contain older components, which have been revealed particularly by painstaking structural and geochronological work (e.g. Mancktelow et al., 2001. Müller et al., 2001). However, within the setting of the general late Miocene kinematics sketched above, these older elements had been formed before the Adria indentation, at least as far away from the present Tauern Window as the position termed “Helvetic Tauern” in Fig. 5. The “Helvetic” Periadriatic Line shown therein is estimated to date the situation in the late Oligocene, when according to Mancktelow et al. (2001) left-slip (probably transtensional, compare Nievergelt et al., 1996; Laubscher, 1976, 1990) gave way to dextral transpression. The present Periadriatic Line, in regional maps conventionally shown as a continuous fault separating the Adriatic subplate from the Alps, therefore turns out to be a kinematically composite feature, and the notion of a clear-cut kinematic separation of the Southern Alps and the Alpine nappe edifice (ANE) turns out to be untenable, at least in the late Miocene. This may be a shocking conclusion to adherents of conventional views on Alpine tectonics; it is not so, however, within the conceptual frame of plate tectonics. At first sight, the fact that the line-up of the External Massifs (EML) is positioned almost exactly on the projected western continuation of the Pustertal Line and the Tauern Window, which converge at M, seems astounding (Fig. 2). However, the transpressive nature of the late Miocene northern boundary of Adria demands some sort of western continuation of the right-slip Pustertal Line, and this role is apparently performed predominantly by EML (Laubscher, 1988). Note that the External Massifs are windows like the Tauern Window, all of them of late Miocene age, due to folding of the preexisting nappe structures, subjecting them to erosion and thereby creating windows. There is only a difference in terms (“Massif” vs. “Window”), not in nature (Fig. 3). Why are the thick-skinned manifestations of shortening along the External Massifs Line discontinuous? Laubscher (1988) opined that this is the effect of strain partitioning in transpressive systems such as EML, the massifs being a sort of en échelon array of thick-skinned brachyanticlines that represent the contractive component, the gaps in between being due to the stretching component arising in transpression because of extensionally linked strike–slip segments and rotational strains. Note that in this view the late Miocene rightslip component splits at M into a southern Pustertal Line and a northern Tauern Window branch, the latter subsequently regaining the Pustertal Line by means of such right-slip faults as the Mölltal and Lavanttal faults (Fig. 2). This is valid for the upper crust level, whereas

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Fig. 6. The Iorio–Tonale (IT) and Canavese (CL) segments of the Insubric Line and their surroundings. Light gray: thick-skinned domain of the Central and Western Alps; vertical dashes: External Massifs (EML = External Massifs Line, P = Pelvoux, B = Belledonne, AR = Aiguilles Rouges, A = Aar Massif); narrow vertical ruling = Gotthard Massif; horizontal ruling: Lepontine “dome”, with C = Camughera, SL = Simplon Line, Fo = Forcola normal fault; Tu = Turba fault; dotted = central Penninic nappes (DM = Dora Maira, GP = Gran Paradiso, MR = Monte Rosa, T = Tambo, S = Suretta); EL = Engadin Line; PL = Peio Line; IML = Internal Massifs Line; TN = Tonale nappe; IT = Iorio–Tonale Line; CL = Canavese Line; Piemontese Southern Alps with Sesia Zone, Lanzo Zone (vertical dashes), Canavese Zone (CA), Ivrea Zone, Verbania Zone, Luino–Lugano domain (oblique ruling); CR = Cremosina Line, VL = Verbano Line; Lombardic Southern Alps (Adria subplate) with the thick-skinned Orobic and the thin-skinned Lombardic domain. The Orobic domain overlaps the Piemonte Southern Alps in the Luino–Lugano domain. GS = Giudicarie-South, GN = Giudicarie-North Line.

at the lower crust level there is a suspicion of changes in the vergence of subduction (Fig. 3; Schmid et al., 2004). The Western Alps are represented in Fig. 6. The Figure emphasizes those elements important for the Neoalps. A good starting point for a discussion of the Neogene kinematics in the Central and Western Alps is the Iorio–Tonale (IT) segment of the Periadriatic Line. Its steepness and straight track, cutting obliquely across the southern (“root”) portion of the Alps, had been the main reason for Laubscher (1970) to postulate its being a dextral strike–slip fault flanking the arc of the Western Alps. He originally estimated the total right-slip at 300 to 400 km since the Cretaceous, but the timing of the Lepontine dome adjacent to the Iorio–Tonale Line by Hurford (1986) later induced him

to restrict attempts at Iorio–Tonale Line (IT) kinematics to the Neogene. Looking for correlative features on alternative sides of IT he found that there are two highly metamorphosed terrains, containing elements of lower crust and upper mantle lithologies, which lend themselves for correlation: the Ivrea–Verbano zone on the south-east side and the Peio (or Tonale) body on the north side (compare Laubscher, 1992; Martin et al., 1998a,b). Based on this correlation, Neogene right-slip turns out to be on the order of 100 km, perhaps as much as 150 km, depending on how much of the apparent drag at the fault is taken into account. Is there a corresponding left-slip at the southern flank of the arc of the Western Alps? As early as 1973 Elter and Pertusati (1973) had

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described a regional fault (Villalvernia–Varzi–Levanto Line = VVL) cutting obliquely and zig-zag across Ligury, separating the Ligurian Apennines (Monferrato) from the Ligurian Alps (Fig. 1). Its position fits that of a left-slip fault flanking the arc of the Western Alps on the south side, complementary to the Iorio–Tonale Line. Stratigraphic relations strongly suggest an early Miocene age (compare Schumacher and Laubscher, 1996), and the amount of left-slip estimated from the westward transportation of the Appenninic nappes to the north of the Ligurian Alps is in excess of 100 km, perhaps as much as 140 km. This may be taken to support the estimated right-slip at the Iorio– Tonale Line (Laubscher, 1992). One problem with this solution has been the largely early Miocene Langhe basin, astride VVL and sometimes interpreted in terms of subduction profiles. However, as shown below, small collapse basins along strike–slip faults are a frequent phenomenon. The Langhe basin is compatible with this scenario. The regional importance of strike–slip was questioned by Schmid et al. (1987) and Schmid et al. (1989), partly on the evidence of strain measurements north of the Iorio–Tonale Line (IT). The authors opined that IT was a local fault with dominant north-up reverse or thrust motion. The critical opinion was tempered by Merle et al. (1989), and Schmid et al. (2004) seem to have abandoned their critical stance, adopting a right-slip on the order of 100 km. If the large-scale right-slip nature of IT is accepted, it follows that before the late Miocene left-slip indentation at the Giudicarie Line, the Iorio–Tonale Line ought to have continued somehow to the east of GL (Figs. 2, 5); otherwise the respectable amount of strike–slip would be hard to model. To the west, IT may be followed to Locarno–another special point in the Alps (“Locarno Singularity” = LS)–which in many ways is comparable to the “Mauls Singularity” (M) at the western end of the Pustertal Line. At Locarno, IT splits into a number of branches, which separate the various tectonic slices of the Piemontese Southern Alps (Fig. 6). Conventionally, the southeastern boundary of the narrow Canavese Zone (compare Zingg et al., 1976; Schmid et al., 1987), dubbed the“Canavese Line”, has been considered the actual continuation of the Iorio–Tonale Line, though deflected to the SSW. This view is based on the usual assumption that the Ivrea zone is an upturned part of the Adria subplate. This assumption, however, is doubtful for various reasons. For instance, if the Peio body belongs to the Alpine nappe edifice (“ANE”; compare “Tonale nappe” in Martin et al., 1998a,b), a correlative Ivrea body would be a part of ANE too, rather than a part of Neogene Adria. More important, though not decisive, may be the general geometry of the Piemontese slices revealed by Fig. 6. They are fusiform, and their tapering northern ends are dragged dextrally at the Iorio–Tonale Line. This situation fits a kinematic scheme viewing the slices as a sort of “exotic terranes”, dragged from an original position farther east into their present position by dextral slip: the Piemontese Southern Alps, in this perspective, would be the map view equivalent of imbrications as seen in a cross-section. The interpretation of the Canavese zone as an exotic body is supported by the findings of Ferrando et al. (2004), who demonstrated the close affinity of this zone with the lower Austroalpine nappes of Graubünden, to the north of the Peio body. Are there arguments against that scenario? None, as far as I can see, although for the detailed kinematics no data have yet been pinpointed. The geophysical picture (Bayer et al., 1987; The ECORSCROP Deep Seismic Sounding Group (France/Italy), 1989a,b), is ambiguous. It envisages a number of high-velocity thrust slices, issuing from the Adria upper mantle. True, the striking positive gravity anomaly in the Ivrea zone demands a rather deep-reaching heavy body such as the mantle part of Adria. This body, however, not only has to fit the measured gravity but should also be compatible with the Pustertal- and Iorio–Tonale Line strike–slip kinematics (Fig. 7).

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The gist of the proposed geometry is that not only the Ivrea zone is an exotic terrain transported by right-slip, but the deep-reaching Ivrea body too is an exotic terrain, albeit on a deeper structural “below the Brittle–Ductile Transition” level, separated from the Ivrea zone by the ductile middle crust, and from Adria by a deep-reaching plate boundary fault zone representing the Insubric Line at the mantle level. The Figure emphasizes the late Miocene Brittle–Ductile Transition as a decisive kinematic disharmony. The Ivrea body below the BDT is shown as a composite of lower crust–upper mantle fragments, broken off from Adria (“plate boundary masses”). The tempting aspect of this version of the geophysical Ivrea body is that it fits both the seismic data and the general right-slip scenario because it sets in at the Locarno Singularity together with the shallow slices of the Piemontese Southern Alps. That pieces of mantle may break loose is also reported from other plate margins (e.g. in the Carpathian arc). Otherwise, the shape of the Ivrea body is largely arbitrary, the most important constraint being the gravity anomaly. 4. The geometry of Neogene thick-skinned nappes and a qualitative assessment of the mechanical conditions at their boundaries The discussion of the geometry and underlying kinematics of the Neogene thick-skinned nappes is facilitated by first considering the impact on Alpine tectonics of the dextrally migrating Ivrea body as an important material boundary. It is generally agreed that the Brittle–Ductile Transition zone (BDT), which separates the brittle upper from the ductile middle crust, is an important potential detachment zone. Where not exposed at the surface it may be inferred on the basis of seismic data, for instance as the bottom of the seismogenic upper crust (for the Alps see Deichmann, 1992). Where a fossil equivalent is exposed, it is characterized by the onset of ductile mylonitization in quartz-rich rocks and by pseudotachylites in quartz-poor ones (e.g. Schmutz, 1995; Laubscher, 1983). In a “granitic” crust it coincides approximately with a geotherm of 200 °C to 250 °C, at a depth interval estimated in the present Alps and their northern foreland to fluctuate between about 10 km and 20 km (Deichmann, 1992). The corresponding fossil temperature interval is documented by the onset of zircon fission tracks (ZFD). This means that the fission track ages in essence define fossil Brittle–Ductile Transition positions, subsequently raised to the present surface, and this coincidence renders them particularly useful for the study of Neoalpine tectonic developments. As a plausible working hypothesis, posit that thick-skin detachment indeed proceeded along the Brittle–Ductile Transition and that, as a rule, it ramped to the surface at standard staircase (ramp-flat) thrusts (default assumption). Nucleation of the thrusts would occur at asperities impeding the propagation of décollement. Such asperities would preferably be interruptions in BDT, for instance by mafic bodies, which acquire ductility at higher temperatures. Such a strong obstacle for detachment at BDT is the Ivrea body. As it moved westward in the early Miocene, its influence must have varied during that time interval (the Helvetic phase). Of course, additional detachment within the ductile Middle Crust is also possible and indeed is required in the triangular space between subduction and obduction. Exceptionally it is exposed, for instance in the Lepontine Alps. 5. The Helvetic system Turning to the problem at hand, one therefore faces first the early Miocene dextral transpression between Adria and Eurasia. Transpression implies strain partitioning, which is a complex phenomenon, but for simplicity assume it to be composed of two components, a strike– slip and a contractive one. Examine early Miocene contraction. It is plainly manifest in the thin-skinned Helvetic nappes (e.g. Heim, 1921;

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Fig. 8. The thermal development of the Lepontine complex (after Hurford, 1986). Explanations are in the text.

Trümpy, 1969). It is generally agreed that their thick-skinned portion ought to be found in the Gotthard and Montblanc massifs as well as in the lowermost of the “Sub-Penninic” basement nappes exposed in the Lepontine domain (e.g. Schmid et al., 2004; Figs. 3, 7). This “Helvetic system” has peculiar boundaries not easily fitting into the simple scheme outlined above. In the north there were the usual, albeit in detail complex ramps connecting the thick-skinned part with the Helvetic thin-skinned nappes; although these ramps were eroded on top of the External Massifs, they may still be observed at the plunging ends of these Massifs. However, farther to the south, in the Lepontine domain (Fig. 6), décollement descended into the high-T middle crust as evidenced by the high T there recorded for the early Miocene (Hurford, 1986; Fig. 8). In the perspective of the general scenario, propagation of décollement along the Brittle–Ductile Transition must have been impeded by an asperity forcing it to dive into the high-T middle crust. What did this asperity consist of? At present, there is no fitting mafic body at the southern boundary of the Lepontine domain (or “dome”), but there is the Ivrea body immediately to the SW of Locarno, as it were displaced dextrally. Accepting the early Miocene right-slip along the Iorio–Tonale Line, it would have been, during a certain time interval, exactly at the southern boundary of LD. The Ivrea body would have been an important boundary of the Helvetic thick-skinned system, although a migrating rather than a fixed one. It did not permit a smooth passage of the intracrustal Helvetic detachment across IL, acting instead as a backstop forcing an upsurge of middle crust. This middle crust contained, as passive elements, the previously heated, metamorphosed and deformed Penninic nappes. This conclusion is further supported by the subsequent late Miocene (Jura phase) development of the Orobic system, at a time when the impeding Ivrea asperity had moved away into its present position. The new geometry now permitted décollement at the Brittle–Ductile Transition to proceed across the Iorio–Tonale Line and eventually to ramp to the surface, creating the thin-skinned, southvergent Lombardic Alps and their external Milan belt. To concretize this conception, turn to sandbox experiments, e.g. one by Philippe (1994) (Fig. 9a) which serves as a kind of backdrop for Fig. 6. The experiment was designed to explore the dynamics of thin-

skinned nappes, in particular that of Jura décollement, but it is also helpful for obtaining an overview of thick-skinned systems when it is modified to accommodate subduction (Fig. 10). At the backstop the experiment produced an upsurge like that of the Lepontine domain. The experimental upsurge is accomplished by a stack of imbrications, such as that characterizing the relatively cold and brittle Aar Massif (Fig. 3), in contrast to the hot and ductile Lepontine one. This latter was composed in its entirety of hot, ductile masses from below the Brittle–Ductile Transition rather than of discrete imbrications. The ductile upsurge, in this perspective, was the result of contraction (or plate convergence) in a time span of ~ 10 Ma (Fig. 8), rather than an isolated “pop-up” (Merle et al., 1989). To the east of the Lepontine domain (Fig. 9), the upsurge decays, suggesting a reduced influence there of the temporary Ivrea backstop (Fig. 9a). However, thus far the probably irregular right-slip of the Ivrea body during the Helvetic phase and its change of shape in the process, important for appraising its role as an asperity, are not known. 6. IL and the Ivrea body in the Western Alps The present geometry of the (geophysical) Ivrea body to the southwest of Locarno ought to be assessed, as outlined above, within the kinematic framework of strain partitioning between right-slip and contraction at a sub-Brittle–Ductile Transition tectonic level. Its importance is underlined by the exceptional gravity picture. Unfortunately, gravity signals are ambiguous, permitting an infinity of solutions. Some constraints are provided by the ECORS-CROP-1 deep seismic sounding; however, there still remain essential uncertainties. The ECORS-CROP Deep Seismic Sounding Group (1989a,b) as well as Bayer et al. (1987) have identified some high-velocity bodies below the Brittle–Ductile Transition and connected them in their interpretation as northwest-vergent imbrications issuing from the Adriatic mantle, in keeping with the traditional belief that the Ivrea body is an upturned part of Adria. Schmid et al. (2004) prefer to assign some of the high-v bodies to the Northpenninic ophiolite nappe, which they show as thinning and steeply descending in classical root fashion. This interpretation is apparently based on a correlation with the crosssection through the Monte Rosa area (Schmid et al., 2004; Profile b; in

Fig. 7. Cross-sections through the northern margin of Adria, based on Schmid et al. (2004, modified, particularly below BDT). Profile tracks in Fig. 1. As a default procedure, the Brittle–Ductile Transition (BDT) was used to separate the shallow lid (projection from the surface) and the geophysical-kinematic scenarios at depth. BDT is postulated to mark the detachment surface of the brittle lid, distinguishing it from the ductile domain below. In the latter, the European middle crust, thickened by the indentation of marginal plate boundary masses from the south (suggested by seismic data), is merely sketched as a sort of ideogram, keeping them open for particular options, see text. Possible limiting faults of boundary masses within the ductile domain are shown as wavy lines. Vertical ruling: lower crust; spaced dots: European middle crust; dashes: Adriatic middle crust and boundary masses wedged into European middle crust; light gray: European upper crust; heavy lines: latest late Miocene detachment boundaries. Inset (a′): Iv″ is an alternative scenario of a below-BDT Ivrea domain, depicting a dense body in the guise of a flower structure that contributes to the geophysical Ivrea body.

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Fig. 9. Backstop problems. (a) After a sandbox experiment by Philippe (1994): at the backstop a deeper layer is imbricated and uplifted. (b) Early Miocene (Helvetic) backstop (the migrating Ivrea body at the Iorio–Tonale Line, cross-section Fig. 7c). The Ivrea body had moved out of the profile transect by the beginning of the Jura phase, permitting basal décollement to propagate into the Lombardic domain. He (dashed line): putative Helvetic phase basal detachment in the middle crust. A = Aar Massif; O = Orobic basement; H = Helvetic thin-skinned nappes, F = Forcola Fault, E = Engadin Fault.

the present article Fig. 7b), where a similar setup is partially exposed at the surface, particularly the thin, steeply dipping “root” part. However, a different interpretation is possible on the basis of the default assumption that there are distinct kinematic levels, the top one being separated from the deeper ones by the Brittle–Ductile Transition. In the alternative profile Fig. 7b the Monte Rosa nappe is shown as folded above the BDT detachment level by Jura phase contraction as indicated by late Miocene ZFD in the core of the fold (Keller et al., 2005). Projecting these findings into the Gran Paradiso

transect an interpretation such as that in Fig. 7a results. In the lower kinematic level the middle crust is shown as inflated by a wedge of “plate boundary masses”, including the high-velocity slivers of ECORS-CROP Deep Seismic Sounding Group (1989a,b). The exact distribution of densities in this wedge may be chosen such as to satisfy the gravity picture. The “plate boundary masses” are separated from Adria by the deep-level Insubric Line, shown as a wavy curve. Such a shape of the deep-level IL is preferred throughout Fig. 7 to symbolize a sort of interpenetration expected at high T. In this way the conception

Fig. 10. Basal detachment descending at backstop. (a) “Leaky” backstop, depending on the shape of the migrating Ivrea body. Detachment descending merely into the upper part of the middle crust, producing relatively thin imbrications of moderate ductility that rise as External Massifs mode imbrications and reach the surface by in-sequence standard staircase thrusts. In one scenario, the first stages (1–5) thick-skin décollement may have been restricted to the Lepontine Dome and BDT′ (vertical dashes), only the late stages (6, 7) involving the upper crust of the foreland (dotted), creating the Gotthard Massif. (b) “Strong” backstop. Detachment descending all the way to the bottom of the middle crust, causing the ductile upsurge of the Lepontine dome: probable mode of deformation in the central Lepontine Dome. Explanations are in the text.

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is honored that the Ivrea body is separated from Adria and has moved as a separate composite entity into its present position, subjected to strain partitioning between right-slip and contraction. The high-v bodies are interpreted as constituents of that part of the Ivrea body that was indented into the Western Alps mainly by the contractive component. The composition of this Ivrea body in the deep reaches of the Western Alps is largely a matter of speculation. Surface information in the Ivrea zone would predict it to consist mainly of two lithologic components, granulitic to eclogitic schists and mafic to ultramafic igneous bodies, admixed in varying degrees. Geophysically, both components ought to be characterized by high velocities and high densities. Tectonically, they are expected to have been dismembered during transportation and, in the subduction process, retro-metamorphosed into mid-crustal rocks (particularly in tectonized zones) and/or intermingled with mid-crustal rocks. This latter process would be important for the question of boundary conditions discussed above. As a conjecture, it may be assumed that tectonic intermingling influenced strain partitioning, permitting continuous detachment and backthrusting in some places, yet causing asperities in other places (Fig. 10). An acidic admixture may be thin enough to escape geophysical detection yet sufficiently thick to permit detachment. This conjectural scenario has been used to sketch the crosssections (a) and (b) in Fig. 7. In the higher level, the shallow Piemontese Southern Alps are shown as backthrust to the south-east as a consequence of the contractive component. Needless to say that the scant constraints permit all kinds of modifications satisfying both the geophysical data and the kinematic scenario (compare Schumacher and Laubscher, 1996). 7. The Piemontese Southern Alps In the Piemontese Southern Alps, the kinematic sequence may be roughly modeled in two steps. For the first, early Miocene (Helvetic) step, the dating relies on the dextral drag at the early Miocene Iorio– Tonale Line. This step consists in assembling the suite of the exotic terranes. The age of the second, late Miocene (Jura phase) step is inferred from the observation, that the late Miocene Orobic (thickskinned) thrust seems to join and pass into the early Miocene one in the Lugano–Luino area south of Locarno (Figs. 6; 7b, c). Note that this step is implicit in the kinematic scheme Fig. 4. 8. The Lepontine Alps The question now to be investigated is that of the system kinematics position of the Lepontine “pop-up” (Merle et al., 1989). According to Hurford's (1986) dating (Fig. 8), it is evidently an important part of the Miocene scheme of motions, particularly of the early Miocene Helvetic phase. In addition to the Lepontine upsurge, there were transpressional motions of Adria in the early Miocene. They are documented by the thin-skinned Helvetic nappes (contraction) and the thick-skinned Iorio–Tonale Line (right-slip). Evidently, LD was positioned within the thick-skinned part of the Helvetic nappes. We may assess the contractive component of the Lepontine dome (LD), using area balancing (Figs. 9, 10). The data do not permit more than a very crude estimate. Using a steep average geothermal gradient of 20°/km, the 600°-plus metamorphic peak in Fig. 8 had been at a depth of at least 30 km, while the Brittle–Ductile Transition was at an estimated −10 km (compare Deichmann, 1992). Therefore, within the Lepontine domain the Helvetic phase detachment must have descended into the deeper parts of the middle crust, thus locally increasing the thickness of the Helvetic thick-skin. The effect of this increase was enhanced upward escape, forced by the contractive component, which resulted in the Lepontine dome. How much

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contraction was involved? To make a rough estimate of the squeezed-out volume (Fig. 10b, framed inset), consider that today the early Miocene Brittle–Ductile Transition is eroded, but had been raised to at least + 5 km, whereas the present 600° isotherm may still be close to − 30 km. The assumption now is that the gray rectangle (pre-compressional early Miocene thick-skin) in Fig. 10b (inset) was compressed into the putatively corresponding current rectangle (black rectangle). On the basis of these input data contraction was roughly 40 km. In view of the extremely simplified input values, one may infer a contraction in the Lepontine dome of “several tens of kilometers”, and this agrees with the width of the palisinspastically restored Helvetic thin-skinned nappes (compare Trümpy, 1969). The above value of contraction in the LD is probably a minimum. Considerably more contraction would result if LD were to be assembled by piling up a number of thin imbrications such as those of the External Massifs (compare Fig. 3). Laubscher (1996, 1997) argued that the thin imbrications of the External Massifs are due to décollement at the Brittle–Ductile Transition. This argument, however, would be invalid for detachment deep in the middle crust. Moreover, in view of the additional contraction in the Gotthard Massif, thin imbrications would appear to result in excessive values of contraction. For the time being we conclude that the Lepontine dome resulted from a backstop at IL that forced detachment of the thick-skinned Helvetic nappes to descend deeply into the ductile middle crust, compelling hot masses to surge by possibly more than 30 km. Laterally, the Lepontine dome is limited by the broad band of deviating lineations which, around Locarno, swerve to the north as far as the Gotthard Massif (Fig. 11). It suggests overall dextral slip that imparted a dextral rotation to the Lepontine body, showing up in circular strain patterns and extensional domains on both sides of LD (Fig. 11, inset). Therefore, the Lepontine dome may be considered the result of particularly complex strain partitioning near the Locarno Singularity. It developed where the Insubric Line branched, sending one branch to the north well inside the core of the Alps (Fig. 6), in this respect resembling the branching of the Periadriatic Line into Pustertal Line and Tauern Window at the Mauls singularity. Its dome-like prominence with its peak Miocene metamorphism and its transverse “nappes” (Fig. 11) and, as it were, spiralling lineations (e.g. Wenk, 1955; Merle et al., 1989) may be conceived as due to a locally particularly effective asperity at the basal décollement boundary (particularly partial indentation of Ivrea masses, Fig. 7b). The resulting excessive height of the upsurge caused a series of escape mechanisms as described by Merle et al. (1989), including the lateral ones. The upsurging mid-crustal masses, consisting predominantly of thick-skinned Sub-Penninic nappes, were caught in the broad shearing system. They apparently were dextrally rotated, including the strain markers, which rose and cooled simultaneously with the rotational domain, being frozen during the cooling process. Note that the distorted Lepontine “nappes” are not Miocene nappes at all but rather passive markers of Eocene age within the early Miocene Helvetic thick-skinned nappe. The situation is reminiscent of the rotation of porphyroblasts in shear systems (Fig. 11, insets). Because the Lepontine dome is composed of two partial domes, the western one being the younger according to Merle et al. (1989), its shape and development are more complex than the simple schematic in the inset. This scenario is further supported by the occurrence of extensional faults on the two sides of the dome — the Simplon fault in the west and the Forcola fault in the east (compare Spicher et al., 2005). Their location is where in the simple scheme (Fig. 11, inset) there are extensional domains, usually filled by the crystallization of exuded quartz or pegmatite. The Forcola normal fault had not been known to Merle et al. (1989), but Huber and Marquer (1998) called attention to its symmetric relation with regard to the Simplon fault on the western side of the Lepontine domain. The Forcola fault seems to be connected with some traditionally enigmatic structures. For instance, it merges

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Fig. 11. The Lepontine complex (from Spicher et al., 2005, simplified). Vertically ruled: thermal anomaly; gray: Maggia Transverse Zone; dotted: central Penninic Tambo and Suretta basement nappes; loosely dotted: Bergell pluton; black: Chiavenna ophiolites; short lines: lineations (from Merle et al., 1989). Normal faults with barbs on the downthrown side. Insets: comparison of the Lepontine dome with a rotated competent body within a shearing scenario.

into the Chiavenna faults that envelop the puzzling Chiavenna ophiolites (compare Schmid and Froitzheim, 1993; see maps by Spicher et al., 2005; Berger and Mercolli, 2007). Schmid et al. (2004) opine that the Chiavenna ophiolites originated in the Northpenninic sequence, piercing the overlying Penninic nappes due to the contractive component. In a contrasting view, the relation of the ophiolites to the Forcola and the Engadin faults rather suggest a Southpenninic origin, downthrown into a small graben within the extensional area but subsequently squeezed together in the complex development of strain partitioning. Another puzzling feature apparently associated with the Forcola fault is the latest Oligocene Novate granite. Its age contrasts sharply with the early Oligocene ages of the important intrusions along the Periadriatic Line (Figs. 5, 8); it therefore seems to be a manifestation of the Helvetic phase. Within this phase it fits, like the Forcola fault, into the extensional domain east of the LD. In addition to the mappable normal faults, distributed extensional faulting east of the Lepontine dome has been reported by Merle et al. (1989), Schmid and Froitzheim (1993) and Huber and Marquer (1998), possibly accounting for the thinness of the Penninic nappes in the area. The Turba normal fault, which on the face of it belongs to the same system, is rather an Oligocene feature according to Nievergelt et al. (1996). The above scenario may be used to discuss the distribution of strain markers adjacent to the Iorio–Tonale Line (IT). Because of the backstop situation a predominant strain component had been SN contraction (up-north), even close to IT, particularly inside the hot upsurge. Near the contact with IT, where the rocks cooled down rather rapidly, right-slip strain tended to develop in the brittle mode. Consequently, the two distinct rheologies are not a valid argument for postulating two kinematic phases of IT (Schmid et al., 1987, 1989), an

older and more important contractive and a younger, less significant strike–slip one. Nor is it an argument for declaring the steep IT as a “backthrust” (Merle et al., 1989). Backthrusts proper have a different geometry. For instance, the late Miocene Orobic thrust is a backthrust, being moderately inclined and climbing to the surface in staircase fashion. It was enabled when the backstop situation had vanished together with the migrating Ivrea body. And finally, it is no argument either for according IT a mere local significance at the margin of the Lepontine backstop— a stance since abandoned by Schmid et al. (2004). Quite the contrary, the truly local phenomenon is the particularly effective asperity at a fault of regional importance. 9. On Oligocene and Miocene extension Whereas extension on the border of the Lepontine dome may be dated as Miocene, or conjectured to be Miocene for reasons of symmetry, more distant manifestations of Tertiary extension have been attributed an Oligocene age (e.g. Nievergelt et al., 1996). Even earlier, Laubscher (1983) had pleaded for widespread Oligocene extension in the Alpine region and its foreland, attributing, for instance, the Oligocene intrusions along IL to an extensional phase between transpressional emplacement of the Penninic nappes and the renewed onset of transpression in the (latest Oligocene and) Miocene. Thermomechanical arguments seem to support the case, at least in principle, particularly those based on observations in metamorphic core complexes. Such complexes are the result of crustal stretching. Because of ductile thinning of the middle crust the isotherms below the Brittle–Ductile Transition are crowded in the process and the geothermal gradient is increased. Intrusions associated with the extension are often caught in this ductile stretching, acquiring strain markers of subhorizontal shearing, which causes the impression of

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emplacement as a nappe, which in turn (occurring within the Alpine nappe edifice) is interpreted as the result of compression. As a rule, however–basing this opinion on a perusal of worldwide information–intrusions are associated with extension. Because the late (Oligocene) intrusions line up along the IL, a tempting conclusion is that in the Oligocene IL had been an extensional or, more plausibly, a left-slip transtensional fault system. This tentative conclusion is supported by the fault rock dating of Müller et al. (2001) and Mancktelow et al. (2001). They found that fault rock segments along IL were left-slip and extensional in the early Oligocene, but flipped into right-slip and compressional soon after the emplacement of the Periadriatic intrusions. Oligocene left-slip is also conjectured for a variety of other reasons. For one thing, in a regional context, to the west of the arc of the Western Alps there were the extensional Rhone grabens (Fig. 1, inset; Laubscher, 1983), and in the German–Austrian Molasse basin an extensive field of Oligocene normal faults suggests subduction roll-back (Laubscher, 1976). Moreover, faults are found along IL which fit the category of left-slip extensional Riedel faults. One such fault is the Engadin Line. It is accompanied by left-slip markers (e.g. Schmid and Froitzheim, 1993; Schmid et al., 2004), and for this reason has sometimes been considered the complementary shear to the right-slip IL (e.g. Schmid et al., 2004). This would be a reasonable assumption except that the angular relation does not fit. It does fit, however, a left-lateral Riedel fault associated with left-slip transtension. A similar case is that of the left-slip Deferegger–Antholz fault north of the Pustertal Line (e.g. Schmid et al., 2004). Where those Riedels approach IL, the Oligocene Bergell and the Rieserferner intrusions took place: a plausible location for intrusions because of locally enhanced extension (Fig. 5). Unfortunately, information on the quantities involved are sparse (e.g. Schmid and Froitzheim, 1993). The suspected Oligocene extensional Riedels were subsequently, within the Miocene Neoalpine transpressive regime, deformed along with IL. Thus there is a problem with the dating of Tertiary extension within the Neoalps. Fault rock data intimate an early Oligocene extensional phase. Hurford's (1986; Fig. 8) data may be read as favoring an Oligocene collapse phase resembling a metamorphic core complex with extension and rise of the geotherms. The Simplon and

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Forcola faults may be seen as early Miocene extensional features associated with dextral rotation of the Lepontine domain. A coeval extensional feature is the latest Oligocene Novate granite (see maps of Spicher et al., 2005; Berger and Mercolli, 2007 and explanatory notes by Berger et al., 2007). 10. The thick-skinned parts of the Helvetic and Jura nappes Within these regional confines, how can the thick-skinned part of the Miocene Neoalpine nappes be delimited and the Jura nappe be singled out? The thick-skinned part of a nappe may be defined by its boundaries in both cross-section and map view, before it was deformed by ulterior tectonic events. It then had the form of an orogenic lid. An attempt at defining the cross-section view for the Helvetic nappes is presented in Figs. 9 and 10. The frontal boundary in the north is the latest staircase thrust #7 of Fig. 10. The bottom boundary is the Brittle–Ductile Transition except near the southern end, where the Ivrea backstop forced detachment to descend into the middle crust. As a result, the Helvetic nappe system was, as are nappes generally, in the beginning an essentially flat-lying body on the order of 10–20 km thick (an orogenic lid). It contained inherited older structures, particularly older nappes, which as a rule behaved as mere markers. As a new nappe it deformed all the older nappes, those within its thick-skin as well as those overlying it. This general conclusion concerning the Helvetic nappe also holds true for the Jura nappe, although there are important differences. The most important one is the southern boundary at the Orobic thrust, which is a staircase thrust similar the northern frontal thrust in the Jura (e.g. Schumacher, 1990; Schönborn, 1992; Schumacher, 1997; Schumacher et al., 1997). This causes the nappe to appear uplifted rather than contracted, while the underlying masses, those below the Brittle–Ductile Transition are contracted by subduction. In sum, the original Jura phase nappe in cross-section view is essentially the one outlined in Fig. 7, as there have been no major subsequent tectonic events deforming it. In map view, though late Miocene motions affected the entire length of the Africa–Europa plate boundary, there are major lateral changes that justify a narrower definition of the Jura nappe (Fig. 12).

Fig. 12. Map view of the Jura nappe (heavy black line; white dots where most simplified). Black dots identify the thick-skinned core. Heavy dashed outline marks position before 70 km NNE translation of Adria.

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With respect to the thick-skin part, beginning in the east, such a major boundary is the Giudicarie–Brenner Line as shown in Fig. 4. In the south, this line ties into the Orobic backthrusts, and in the north it ends in the External Massifs thrusts. The External Massifs, though in a limited sense continuing the Tauern system, are sufficiently different to permit, as proposed, a definition of the eastern boundary of the Jura nappe: it ends at the Giudicarie–Brenner Line. Indeed, the Tauern thrusts may be perceived as the frontal thick-skinned end of another late Miocene (i.e. “Jura phase”) nappe, whose southern end contains the Pustertal Line and the Valsugana backthrust zone; its northern thin-skinned end are the imbrications of the Subalpine Molasse. The Monticello thrust, however, is younger than the Jura phase, which means it has to be grouped with one of the post-Jura phase events whose vestiges are found in the northern foreland of the Alps, such as that depicted by Bourgeois et al. (2007, Fig. 3h) or the “Sundgau phase” of Laubscher (2001). A suitable southwestern boundary of the Jura nappe is harder to define, and at this point is merely suggested. An acceptable choice may be the southwestern end of the Belledonne Massif, as there one enters the area of interference of the Neoalpine front with the Palaeogene Provence–Pyrenees and even the Cretaceous Dévoluy systems. 11. Adria and the regional significance of the Insubric Line: A comparison with the dextral system of the Caribbean plate boundary (Fig. 13) Thus far the discussion revolved around the significance of IL within the Alpine and the Adria subplate boundary domains, and

particularly with respect to the Helvetic and the Jura nappes. What, however, is the role of right-slip, if any, within the extended Africa– Eurasia plate boundary? In Fig. 13 the Adria subplate is shown as a sort of “promontory” of the African plate, a term that has often been used in the past (recently by Oldow et al., 2002). However, the record of its motions indicates that it moved independently from Africa; in particular, its rotations cannot be tied to those of Africa (e.g. the early Miocene one, compare Laubscher, 1991). The problem is that of identifying its southern boundary, which is below the sea and not obvious on published maps. However, the deep graben system of the Sirt basin (Abdulbaset et al., 2008) document that as early as the early Cretaceous what is now the eastern Mediterranean had been separated from the western one by a deep-reaching fault system passing from the east coast of Sicily through the Malta escarpment into the Sirt basin of Africa. That Sicily even today is fundamental for Mediterranean tectonics, in particular marking the southern boundary of the Tyrrhenian–Adria complex, testifies to both the importance and the longevity of fault systems separating Adria from Africa: Adria obviously was not a promontory of Africa but a crucial, independent crustal block inserted between Africa and Europa. Its size decreased with time. This is implied in the insertion of the Tyrrhenian extension between Corsica–Sardinia and the Adriatic foreland of the Apennines. Indeed the thrusts and folds of the young external Apennines seem to be linked to the Tyrrhenian extension (compare, for instance, Bally et al., 1986). It is therefore hardly surprising that Adria played a fundamental role in the organization of the Alpine system, particularly of the Neoalps. For one thing, it displaced the right-slip component from the

Fig. 13. Dextral transpression in the arcs of the Western Alps (a) and the West Indies (b; south on top). Fault lines: PAL = Periadriatic L., NA = North Anatolian F., Pi = El Pilar F., SS = San Sebastián F., B = Boconó F. Basins (dotted): T = Tyrrhenian, P = Pannonian, A = Aegean, M = Marmara. Explanations are in the text.

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Maghrebides (e.g. Peresan et al., 2002) to the Alps, integrating it in its sinistral rotation (e.g. Bourgeois et al., 2007). IL thereafter gets lost in the Pannonian to Thracian basins, but a very active zone of right-slip re-emerges in the northern Aegean and continues into the North Anatolian fault (e.g. Şen and Yıllar, 2009). In a sense, then, the Insubric Line is a segment of the Africa–Europa right-slip component, displaced by Adria. In this role it is not a local Alpine phenomenon but one of plate-tectonic dimensions. This view differs somewhat from that expressed by Schmid et al. (2008) who opine that the Periadriatic line passes into the Balaton Fault and the Mid-Hungarian fault zone, where it ends (for the controversy over “lateral extrusion” in the Pannonian Basin compare Willingshofer et al., 2002 and Ratschbacher et al., 1991). I prefer to think that the complex Pannonian collapse system, accommodating the roll-back of the Carpathian subduction zone on one side and the roll-back of the Adriatic–Dinaric subduction zone on the other side was dominated by rather local internal kinematics. In contrast, one may argue that the overpowering role in organizing the kinematics of the Africa–Europa plate boundary has been played by Adria, that comparatively rigid subplate extending all across the plate boundary zone from Africa to Europa. It had been subject to sinistral rotation at least as far back as the Helvetic phase and seems to continue this rotation even today, as the GPS data of Oldow et al. (2002) may be interpreted to imply. The plate-dynamic cause for this rotation is obscure. One possible hypothesis that comes to mind when considering the kinematics is that there was (and still is) a deep-reaching anchor of Adria in the Genua area which, combined with relatively easy sliding conditions at the southern border of Adria, forced its sinistral rotation together with the contraction of the Alpine arc. 12. A comparison of Alpine kinematics with that of the southern margin of the Caribbean plate The general kinematic concepts arrived at in this paper would appear to be applicable in other orogens as well. A global survey of extra-alpine mountain ranges may eventually be a worth-while project, which, however, would transcend the subject matter of this paper. On the other hand, it is tempting to test the concepts by comparing the Alps with at least one other mountain range. For such a mountain range I propose, for various reasons, to choose that of the southern margin of the Caribbean plate. One reason is that, when I worked in Venezuela decades ago, I was struck with the similarity of the arc of the Western Alps with the Antilles Island Arc. The Central Alps would, in this scenario, be analogous to the Caribbean–South America plate boundary, with which I had become somewhat familiar. When Harry Hess of Princeton University started his Caribbean Project from the 1950' s on, dextral strike–slip in the Venezuelan coast ranges appeared to be a kinematic necessity in view of the subducting Antilles island arc. His students mapped a superabundance of strike–slip faults, missing, unfortunately, the extensive thrusts, even nappes, which would have completed the identification of transpression and strain partitioning so typical for the Alps. However were there strike–slip faults in the Alps, comparable with those in the Caribbean? A striking candidate was the Insubric Line. For Alpine geologists, introduction of strike–slip along the Insubric Line had been sheer heresay when I first proposed it. In the geology of the Caribbean Coast Range too there had been serious gaps at the time. They have been largely eliminated in the meantime. Fig. 13b portrays an updated version (Gorney et al., 2007) of this plate boundary at the same scale as Fig. 13a, but with south to the top. In spite of the similarities between the Alps and the Caribbean plate there are, obviously, important differences. At the very beginning of the comparison, one is confronted with the realization

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that the width of the Africa–Europa plate boundary zone exceeds that of the entire Caribbean plate: this latter plate may therefore, in a sense, be seen as a mere part of the North America–South America plate boundary zone. The West Indian island arc is slightly shorter than the diameter of the Adria subplate, but has about the dimension of the combined Alps–Apennines arc; it is several times the size of the arc of the Western Alps taken alone. A comparison of the two dextral systems should therefore be done with the necessary caution. But that said, it is still revealing in many points. In both systems, strain partitioning plays a striking role. In Fig. 13b, the contractive component is mainly expressed in the marginal thrust belts, while the right-slip one is evident above all in a family of conspicuously straight fault segments. For a long time the connection of these segments had been in doubt, particularly that of the Oca and San Sebastián faults. These faults seemed to get lost in the Falcón basin, but the new data of Gorney et al. (2007) reveal that there is a connection, albeit a somewhat complex one. One important point of regional comparison with the Insubric (Periadriatic) fault system is the tendency of the strike–slip faults to vanish, apparently, in extensional basins that evolve along their track. In Fig. 13a the Insubric Line gets lost in the Pannonian and Tyrrhenian basins, which may be compared with the Falcón and Bonaire basins in spite of the disparity in size mentioned above. I conjecture that this is inherent in strain partitioning: strike–slip motion engenders rotation around vertical axes, and rotation goes along with a measure of extension. Extension, in turn, tends to destabilize the crust–mantle column; in the dynamic sense it induces the gravity part of the stress system to particularly act on the gravitational instability of the crust– mantle system, permitting the dense mantle to sink e.g. by means of subduction roll-back, while keeping the crust afloat by buoyancy (compare Laubscher, 1976; Peresan et al., 2002). While most of the extensional basins of the “Insubric Line” are quite spacious, being almost of the dimension of the entire width of the Caribbean plate, there are also small ones, such as the Marmara (e.g. Şen and Yıllar, 2009) or the Langhe basins, which are similar in extent to the basins along the Caribbean right-slip system. 13. Concluding remarks For the definition of the thick-skinned part of the late Miocene Jura nappe a system kinematics approach is found to be helpful. It consists of trying to bunch domains that were kinematically active in certain time intervals (in the case at hand ~6 b t b 16 Ma) into coherent bodies bounded by idealized faults. As to the geometry of the nappe, the default choice for the lower boundary is the Brittle–Ductile Transition zone. It divides the lithosphere into an upper (“obducting”) and a lower (“subducting”) level. The arguably best starting point in the endeavor to define the Jura nappe is the system of the Periadriatic Line where it is intersected by the Giudicarie Line (“Mauls Singularity”). There, a late Miocene NNWdirected translational component of the Adria subplate into the Alpine nappe edifice of some 70 km can be ascertained, producing the Tauern Window uplift east of the Giudicarie–Brenner Line. West of this line it resulted in the prominent contractions of the External and Internal Massifs Lines as well as in the Orobic backthrust and its continuation into the Piemontese Southern Alps. There the distinction of tectonic levels is particularly important: at the upper (obducting) layer the mappable units were produced, whereas at the lower (subducting) level the abnormal Ivrea gravity body is its prominent feature. It had been emplaced approximately at its present location by the end of the Helvetic phase. The separation of the Miocene Neoalps into an early Miocene Helvetic and a late Miocene Jura phase is manifest in the Southern and Eastern Alps. There, the late Miocene deformation system connecting the Orobic thrusts and the Tauern contraction by means of the left-slip Giudicarie belt clearly initiates a new, late Miocene phase. The thick-

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skinned part of the Jura nappe (a geometric entity, in contrast to the “Jura phase”, which is a temporal concept) may be defined as that body which is bounded by the External Massifs in front, the Orobic and Piemontese thrusts in the back and the Brittle–Ductile Transition at the bottom. As lateral boundaries, somewhat more arbitrarily, may be chosen the Giudicarie–Brenner left-slip zone, whose existence had been rather apodictically denied in the past, and the zone of interference of the External Massifs with the older Provence and Dévoluy systems at the southern end of the Belledonne Massif. This, then, is that part of the thick-skinned Alps that pushed the thinskinned foreland fold-and-thrust belt in the Jura Mountains. Previous to the late Miocene Jura phase, in the (late Oligocene)– early Miocene Helvetic phase, the thick-skinned part of the Helvetic nappes had followed a similar scenario, distinct, however, mainly in these points: (1) right-slip was concentrated along the Iorio–Tonale segment of the Insubric (or Periadriatic) Line; (2) the Helvetic thickskinned domain extended from the Gotthard–Montblanc Massifs Line in the north to the Iorio–Tonale Line (IT) in the south, where it was bounded by the westward moving Ivrea body that acted as a backstop at IT; (3) this backstop forced basal décollement to descend into the hot middle crust, causing its ductile upsurge in the Lepontine dome. The sinistrally rotating Adriatic subplate penetrated into the Alpine nappe edifice, where it sliced off the fusiform units comprising the Piemontese Southern Alps.

Acknowledgments I am particularly indebted to my colleagues and students at the University of Basel, foremost to the late Peter Bearth and to Daniel Bernoulli, Gregor Schönborn and Markus Schumacher for decades of joint investigations in the Southern Alps; to my Italian colleagues and friends who with their intimate familiarity with the Southern Alps and unflagging enthusiasm enriched my own views, although they may not share all of the conclusions presented in this article. Alberto Bally and Bob Hatcher read an earlier version of this paper. They and Daniel Bernoulli as well as two anonymous reviewers offered very helpful comments.

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