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Material Balance in Alpine Orogeny CENTENNIAL ARTICLE

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Material Balance in Alpine Orogeny CENTENNIAL ARTICLE Material balance in Alpine orogeny CENTENNIAL ARTICLE HANS LAUBSCHER Geologisch-paläontologisches Institut der Universität Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland ABSTRACT ample, Cornelius and Furlani-Cornelius, 1930). Their geometry suggested three-dimensional kinematics with strike-slip components; it became Recent geophysical data are combined with older information for abundantly clear that the Alps are not just a pile of nappes, although it an updated picture of Alpine kinematics, using semiquantitative con- proved difficult to harmonize the kinematics of overthrust and strike-slip siderations of material balance. In the Alps, a conventional pile of thin tectonics. peel nappes is disrupted by a central longitudinal system of steep faults A number of articles in the Geological Society of America Bulletin and steeply limbed folds, both affecting disharmoniously the entire have been important or even seminal for my own attempts at clarifying edifice of otherwise flat-lying nappes. It is particularly from late Ter- Alpine kinematics. I should like to single out the work of Harry Hess, a tiary phases of deformation that structures of the second type, the man with vision, and his students in the southern Caribbean (for example, Insubric system with the Insubric line and the belt of late Alpine Hess and Maxwell, 1953; Dengo, 1953). I met them in the field more than windows, acquired prominence for the present aspect of the Alps. 30 years ago while employed in Venezuela. Although well aware of trans- These late structures formed as a dextrally transpressive intraconti- pressive tectonics from work in the Jura, I tended to compare what I saw nental branch of the Africa-Europa plate boundary, mostly between with what I had learned about the Alps, which was conditioned by Argan- areas of extension. Early motions in the Late Cretaceous to the late dian cylindrism (Argand, 1916). I thought that I recognized nappes, some Eocene were probably responsible for at least half of the 300-km of them similar in facies and metamorphism to the Penninic nappes of the dextral displacement required for the palinspastic restoration of the Alps. The Hess team, on the other hand, emphasized the maze of steep inner West Carpathians and the Dinarides. During Oligocene exten- young faults that dissect the compressional units of the Caribbean Coast sion, the late Alpine batholiths intruded in a belt roughly along the Range and that belong to what now would be called the "southern dextral Insubric line while deep crustal and lithospheric roots were destroyed. transform boundary," in some cases transpressive, in some transtensive, of In the latest Oligocene to middle Miocene, the Adriatic plate moved the Caribbean plate. It was obvious that both types of structures played a dextrally to the west along the Insubric transfer fault. From its frontal role and that the composite was definitely non-cylindrical. Moreover, an Insubric indenter, a lower crust-upper mantle flake, probably first intriguing analogy turned up. Consider the arc of the western Alps as a obducted in the Cretaceous, was detached and wedged into the Pen- reduced replica of the Antillean arc, and turn it around by 180°. The ninic nappes of the western Alps ("bird's head" of the Ivrea body). central Alps with their steep faults now become a reduced equivalent of Both the northern and the southern transfer faults of the Insubric the Caribbean Coast Range, a dextral transfer (which I prefer to "trans- indenter were disrupted and inactivated by late Miocene to later form") boundary of the Adriatic or Insubric block, which in turn becomes events, the Giudicarie and Neo-North Apennine events. Instead of the the equivalent of the Caribbean block (plate tectonics had not yet been inactivated Insubric fault, the Windows belt of en echelon folds as- born at that time). sumed the role of dextral transpression. All of these successive zones Back in Switzerland, the Alps therefore appeared in a new perspec- of motion are entangled in the "Ligurian knot," largely hidden under tive. In a series of papers, I have attempted, for the past 20 years, to the sea or young sediments. A large part of the pre-Alpine crust and integrate the known facts of Alpine geology into crustal sections and practically the whole mantle were subducted; only in some cases were map-view kinematical schemes. Based as they were on very sketchy in- high-pressure rocks of shallow continental origin re-obducted. The formation in important aspects, they had to be quite schematical. I tried to lithospheric root of the Neo-Alps is largely contained in a vertical slab outline the main kinematic problems and to suggest ways of solving them. under the Alps, but some parts seem to have been removed and others The European Geotraverse (EGT; Galson and Mfiller, 1985, 1986) disharmonically displaced with respect to the surface structures. with its crucial seismic lines across the Alps is in the final stage of its execution. Already, important new data have been produced and await INTRODUCTION integration with old and new surface geological and geophysical informa- tion. In this article, an effort is made to this end. The meaning of the Classical Alpine kinematics was concerned foremost with cross- combined data for kinematics will be investigated, using material-balance sectional nappism. The nappes, where not deformed subsequently, are considerations. subhorizontal thin peels. In sharp contrast to these peels are the steep, Material balance has played a fundamental role in Alpine tectonics discordant faults, such as the Insubric line, and the steeply limbed folds since its beginning. Mostly, however, its application was essentially quali- affecting the entire pile of nappes, such as the external massifs or the tative, involving rather vague scenarios, and the concept of material Tauern window (Figs. 1 and 2). Attempts at integrating them into cross- balance was not explicitly stated. Before the turn of the century, after the sectional nappe development were rather unconvincing (compare, for ex- discovery of large-scale thrusting, nappism faced the necessity to reconsti- Geological Society of America Bulletin, v. 100, p. 1313-1328,12 figs., September 1988. 1313 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/9/1313/3380301/i0016-7606-100-9-1313.pdf by guest on 01 October 2021 1314 H. LAUBSCHER Figure 1. The tectonic elements of the Alps and their surroundings. Narrow vertical ruling: units derived from the European continental margin of Tethys, including Ligurian Alps and associated units such as Sestri-Voltaggio zone, Antola nappe, Alpine Corsica. Wide ruling: units derived from the African continental margin of Tethys. Wide vertical: Austro-Alpine nappes and internal units of the western Carpathians. Wide horizonal: South Alpine units and Dinarides. With V-patterh added: innermost Dinarides carrying the ophiolites. Horizontal dashes: northern Apennines. Stippled: late Alpine dextral en echelon belt of brachyanticlines (windows, external massifs) = Windows belt (a part of the Tauern window is left blank in order to indicate its polyphase origin). B = Belgrade, Ba = Basel, G = Genoa, K = KoSice, M = Milan, Mo = Monferrato, Mu = Munich, T = Turin, V = Vienna. tute paleogeographic domains now piled on top of each other. These eled early on with roughly balanced sections (Buxtorf, 1907,1916). Much palinspastics were, however, essentially cross sectional and based on the later, for the Alps as a whole, crustal refraction seismic surveys combined ambiguous notion of the "geosyncline." with considerations of isostasy permitted the crude reconstruction of palin- My own views on Alpine tectonics have been conditioned, in addi- spastic sections involving the entire crust. Estimates of water depths at the tion to the Caribbean experience, by the continuous pursuit of material- time of deposition, necessary for guessing the palinspastic position of the balance problems on different scales and with various degrees of Moho, were very speculative, but the earliest palinspastic crustal sections quantitative tolerance. From the outset, it was obvious that palinspastic through the Alps (for example, Laubscher, 1967) have not been dramati- restoration, even if perfectly feasible, would be only a first step for con- cally improved till now. There is still no precise method for deriving depth structing balanced sections. Only the bed length of the most surficial layers of deposition, particularly for pelagic and turbidite sequences, from sedi- would be preserved. Mapping had revealed long ago that only the top few mentology or paleontology. kilometers of the crystalline crust are involved in the Alpine nappes (com- From these crude reconstructions, it was obvious that even for a pare Fig. 2 and Laubscher, 1970a, 1983a). If several hundred kilometers of continental crust close to 30 km thick, only the topmost few kilometers shortening had occurred as palinspastic reconstructions seemed to imply, escaped subduction. It was soon evident that contrary to a commonly held where had the underpinnings of these surficial peels gone? Ampferer belief, large parts of the continental crust, although less dense than the (1906), in prescient cross-sectional material-balance considerations, con- mantle, did not remain afloat. This conclusion was supported by another cluded that underneath the surface structures there must be an equivalent line of evidence; occasionally, subducted continental crust has re-emerged shortening by
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