Introduction: Analysing Orogeny—The Alpine Approach

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Introduction: analysing orogeny—the Alpine approach

S. SIEGESMUND1,B.FU¨ GENSCHUH2 & N. FROITZHEIM3 1Gottinger Zentrum Geowissenschaften, University of Go¨ttingen, Goldschmidtstrasse 03, D-37077, Go¨ttingen (e-mail: [email protected]) 2Institut fu¨r Geologie & Pala¨ontologie, Universita¨t Innsbruck, Innrain 52, A–6020, Innsbruck 3Geologisches Institut, Universita¨t Bonn, Nuballe 8, D–53115, Bonn

The European Alps, the prototype collisional Schulz et al. present a compilation and review orogen and playground of geologists from all over of geochronological, geochemical and structural the world, have been studied by generations of data from the Austroalpine basement south of Earth scientists. The density of data is probably the Tauern Window and reconstruct the evolution matched by no other mountain chain. Still, the of these units from a Neoproterozoic to Ordovi- Alpine chain is far from being over-studied, since cian active margin setting, through a subsequent many fundamental questions have not yet found a passive-margin setting at the northern periphery satisfactory and generally accepted answer, e.g. of Palaeo-Tethys, to Variscan collisional tec- the formation of the Western Alpine arc. In recent tonics, Permian rifting, and Cretaceous collisional years however, tectonic research on the Alpine tectonics, and finally to Tertiary shear-zone mountain chains has made dramatic progress due development and intrusion of the Rieserferner to new findings (e.g. coesite), new methods (e.g. Tonalite. GPS), and new—or newly considered—concepts The Permian part of the history is the subject of (e.g. subduction roll-back). Our picture of the three papers in this volume. During recent years, Alpine orogeny has changed completely. studies in the basement units have shown that Extremely important for Alpine research, the Permian rifting and the related magmatism and opening of borders between western and eastern metamorphism have strongly changed the structure parts of Europe has opened new perspectives: seen and composition of the Alpine basement, leading from the east, the Alps are the result of the junction to voluminous intrusions and widespread high- of the Dinarides and the Carpathians. Parts of the temperature/low-pressure metamorphism. These Alpine evolution, e.g. Jurassic tectonics in the inherited features of the basement had a strong Northern Calcareous Alps, can only be understood influence on Alpine tectonics. Siegesmund et al. in the context of processes in the Internal Dinarides analyse a cross-section of the Ivrea Zone, the and Internal Carpathians. The exchange of infor- most important tract of former lower continental mation and ideas between Alpine, Carpathian, crust exposed in the Alps, by means of structural Pannonian and Dinaride Earth scientists—in which geology and geochronology. In particular, they Stefan Schmid played and still plays a most import- present data from a shear zone (Rosarolo Shear ant role—has been fruitful for all sides. Zone) which formed in an extensional setting The present volume on the Alps, Carpathians during the Permian, contemporaneously with and Dinarides (Fig. 1) includes articles that are magmatic underplating, and was rotated into its related to key aspects of the tectonic evolution of present subvertical orientation during the Alpine these mountain chains. These articles are examples orogeny. The authors present evidence that the of the Alpine approach to orogeny, which combines entire Ivrea Zone was verticalized during the careful fieldwork with a broad variety of laboratory Alpine events, and not during the Permian (as methods, and integrates this into the extensive and suggested by other researchers). detailed knowledge base that has been accumulated Froitzheim et al. present field, microstructural over a long history of geological research. and textural evidence that the western part of the Orobic anticline in the southern Alps represents a Key aspects of Alpine, Carpathian Permian-age metamorphic core complex, and the and Dinaride tectonics basement-cover contact a top-to-the-SE, extensional detachment fault of Permian age as well. Pre-Alpine heritage and Alpine reactivation They demonstrate that normal shearing is coeval with both granitoid intrusion in the footwall of The Variscan continental basement in the Alps, Car- the detachment and volcanism in the hanging pathians and Dinarides is strongly heterogeneous. wall. This is the first description of a Permian

From:SIEGESMUND, S., FU¨ GENSCHUH,B.&FROITZHEIM, N. (eds) Tectonic Aspects of the Alpine-Dinaride- Carpathian System. Geological Society, London, Special Publications, 298,1–4. DOI: 10.1144/SP298.1 0305-8719/08/$15.00 # The Geological Society of London 2008. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

2 S. SIEGESMUND ET AL.

Fig. 1. Topography of the Alps, Carpathians and Dinarides (http://www.marine-geo.org/geomapapp; Carbotte et al. 2004). The study areas of papers in this volume are indicated: (1) Schulz et al.; (2) Siegesmund et al.; (3) Froitzheim et al.; (4) Vesela´ et al.; (5) Dallmeyer et al.; (6) Berger & Bousquet; (7) Tomljenovic et al.; (8) Gro¨ger et al.; (9) Rosenberg & Schneider; (10) Lammerer et al.; (11) Ustaszewski & Pfiffner; (12) Pleuger et al.; (13) Ciulavu et al.; (14) Tischler et al.; (15) Mikes et al.; (16) Nagel; (17) Bousquet et al.; and (18) Molli. detachment fault in the Alpine basement. It empha- by a complex palaeogeography which involved sizes the extensional character of Permian tec- microcontinents and ocean junctions. Changes in tonics, in contrast to some earlier studies which the tectonic style, e.g. from shortening to extension, rather suggested a transpressional setting. Vesela´ may be induced by palaeogeography, e.g. renewed et al. have studied the remains of Post-Variscan oceanic subduction after a continent collision. sedimentary basins, strongly deformed by Alpine Pre-orogenic palaeogeography therefore influences tectonics, in the western part of the Tauern or even controls processes as diverse as the devel- Window of the Eastern Alps. They present two opment of foreland and intramontane basins, oro- interesting new U–Pb zircon data and detailed genic curvature, lateral extrusion, the formation of sediment logs. Important conclusions are that the high- and ultrahigh-pressure metamorphic rocks much-debated Kaserer Series is Upper Permian to and their exhumation. Anisian, and that the Late Carboniferous and Dallmeyer et al. focus on the Meliata zone in Permian clastic sediments were deposited in three the western Carpathians, where some of the earliest elongate, normal-fault-bounded intramontane stages of Alpine tectonics are recorded. The authors basins separated by horsts. They suggest that show that Middle to Late Jurassic subduction- these basins were formed in a basin-and-range-like related tectonics were followed, c. 50 Ma later, by extensional tectonic scenario, very much in accord- Middle Cretaceous thrusting when the Meliata ance with the findings of Froitzheim et al.in complex was emplaced on the Slovako-Carpathian this volume. The arrangement of horsts and units, and this was in turn followed by Late grabens predetermined the localization of major Cretaceous top-to-the-SE extension. This paper Alpine structures. yields new data and interpretations for a still poorly understood part of the common history of Mesozoic palaeogeography and Alpine the Carpathians, Dinarides and Alps. subduction and collision processes Berger & Bousquet review age data for the blueschist- and eclogite-facies metamorphism in Early plate-tectonic models of the Alps and other the Western and Central Alps. They conclude that collisional orogens suggested a rather simple the subduction-related metamorphism took place evolution including a phase of mainly oceanic from 62 to 35 Ma (roughly 100 Ma later than the subduction followed by a phase of continental blueschist metamorphism in the Meliata unit), but collision. However, structural, stratigraphic and at different times in different palaeogeographic/ geochronological studies have shown that in fact tectonic units. They assume that subduction was the Alps, Dinarides and Carpathians evolved in a continuous in the Western and Central Alps but series of subduction and collision events, dictated that the downgoing plate must have become Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

INTRODUCTION: ANALYSING OROGENY—THE ALPINE APPROACH 3 fragmented on a large scale in order to explain the on TRANSALP seismics and surface geology. available pressure, temperature and age data. They reconstruct how this cross-section evolved from an early stage of basement/cover duplex Extrusion tectonics: from the Eastern Alps formation, to the uplifting of the duplex by the activity of north-directed thrust ramps at a deeper to the Carpathians and the Pannonian Basin level. The authors emphasize the importance of The concept of eastward extrusion has been N–S shortening in the evolution of the Tauern successful in explaining the Tertiary to Recent Window during Tertiary times. Instead of eastward kinematics of the Eastern Alps. The driving forces tectonic escape, they rather suggest a kinematic of this process, however, as well as the relative framework of dextral transpression. importance of N–S shortening and E–W extension, are still a matter of debate. Four articles in the Orogen-parallel and orogen-perpendicular volume are related to this complex. extension Tomljenovic et al. decipher the tectonic evolution of Mount Medvednica in Croatia. This area Two papers deal with the western Central Alps, and, records several stages of Cretaceous and Early Ter- among other subjects, with synorogenic extensional tiary tectonics. The authors present palaeomagnetic tectonics. Ustaszewski & Pfiffner studied faults evidence that during the Oligocene to earliest in the Penninic and Helvetic units of the Valais Miocene, the area was part of a tectonic block area (western Switzerland). The present-day defor- which rotated clockwise by 1308. This block is mation in this area, as known from earthquakes and bounded to the north by the eastern extension of geodesy, is characterized by orogen-perpendicular the Periadriatic Fault into the Mid-Hungarian extension in the internal zone and shortening in Fault Zone, which was then active as a major the external zone. This strain field very probably dextral strike-slip fault accommodating eastward results from the lateral density anomalies rep- escape of the units to the north of it. Tomljenovic resented by the topography and the crustal root. et al. suggest that the tectonic block broke away The authors distinguish tectonic faults, gravitational from the northern rim of the Dinarides, rotated, faults, and composite faults—that is, tectonic faults and was displaced eastward within the widening with gravitational reactivation. Recent (postglacial) escape corridor. Whereas Tomljenovic et al. tectonic motion could only be documented for two studied the southwestern end of the Mid-Hungarian faults, which suggests that the current strain is Fault Zone, Gro¨ger et al. investigated the north- either predominantly aseismic or, alternatively, eastern end of this fault zone in the Maramures cumulated seismic moment is too low for producing area of northern Romania. Combining structural surface rupture. fieldwork with thermochronology (zircon and Pleuger et al. studied the Alpine deformation of apatite fission-track as well as apatite (U–Th)/He the Monte Rosa Nappe and the Southern Steep Belt dating), they reconstruct a tectonic history includ- southwest and south of the Lepontine Dome. They ing Cretaceous nappe stacking, Late Cretaceous suggest that the tectonic evolution of the area was exhumation, Palaeogene to Early Miocene burial dominated, during the Oligocene and Miocene and heating due to accumulation of sediments on (c. 35 to 15 Ma), by alternating phases of orogen- top of the studied units, Mid-Miocene transpres- parallel extension, orogen-perpendicular extension, sional deformation (16 to 12 Ma), and finally and orogen-perpendicular shortening. In this Mid- to Late Miocene (12 to 10 Ma) exhumation area, the Periadriatic (Insubric) Fault is not only a related to transtension. strike-slip fault and a backthrust, but it represents Rosenberg & Schneider describe an east– the boundary between a northern (Penninic) zone west-striking, ductile, sinistral strike-slip shear which underwent strong extension during the zone in the northern Tauern Window which they Tertiary, and a southern block (Southern Alps) interpret as the westward and originally downward where such extension did not take place. This continuation of the Salzach-Ennstal-Mariazell- resulted in complicated spatial and temporal Puchberg Fault, a 300 km long sinistral strike-slip variations of the kinematics of the Insubric Fault. fault which accommodated eastward escape of the The alternation of orogen-parallel and orogen- units to the south of it. This shear zone is a fine perpendicular extension probably reflects variations example of an exposed transition from brittle to in the relative contributions of gravitational ductile deformation. Moreover, shearing was kine- collapse and across-orogen convergence. matically linked to the formation of upright folds, Like the Lepontine Dome, the Danubian showing that the shear zone developed in a trans- Window in the Southern Carpathians is a meta- pressive framework during, and probably driven morphic complex that was exhumed predominantly by, N–S convergence of Europe and Adria. by orogen-parallel extension, in this case mainly Lammerer et al. present a crustal-scale N–S during the Eocene. The main exhuming normal fault cross-section through the Tauern Window, based is the low-angle Getic detachment. Ciulavu et al. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

4 S. SIEGESMUND ET AL. present the results of a detailed study of meta- eclogite-facies metamorphism, or were subducted morphism in this area. Low-temperature/low- to together with their country rocks. He comes to the medium-pressure conditions during nappe stacking conclusion that the latter is more likely. He empha- gave way to high-temperature/low-pressure con- sizes that significant internal deformation of the ditions during the extensional exhumation of the Adula Nappe occurred after peak-pressure con- window. Trends in cooling ages and in the thickness ditions and that this deformation contributed to of crust excised by the Getic detachment suggest the exhumation of the high-pressure rocks. Two that the geometry of extension was strongly formerly published evolutionary models are con- asymmetric. In this case, orogen-parallel extension fronted, featuring either one or two subducting probably resulted from the collision of the orogenic slabs in the Penninic zone. wedge with an irregularly shaped continental Bousquet et al. present a review of metamor- margin, including the Moesian Promontory and phism in metasediments throughout the Alps. the Carpathian Embayment, in the downgoing They suggest that the metamorphic evolution of European Plate. the Western Alps is related to subduction followed by exhumation along a cool decompression path, Record of orogeny in foreland basins that the metamorphic evolution of the Central Alps, in contrast, records collisional processes Two articles investigate the sedimentary evolution often with heating during decompression, and that of foreland basins located in two key areas of the the Eastern Alps show different stages and types Carpathians and Dinarides. Tischler et al. studied of metamorphism because of two orogenic cycles the area where the Mid-Hungarian Fault Zone at which affected this part of the Alps. its northeastern termination meets the Carpathian arc. This area represents a major lateral discontinu- ity of the Carpathians, where several tectonic units Relations between Alps, Apennines and of the Western Carpathians end (e.g. the Pieniny Corsica Klippen Belt). Tischler et al. describe the evolution of a foreland basin developing in Oligocene– Molli reviews the tectonics of the Northern Eocene times in front of the southeastward- Apennines and Corsica in relation to the Western advancing thrust system of the Pienide nappes, Alps. He develops a picture including an early from Oligocene deep-marine clastics to Burdigalian (‘Alpine’) stage of subduction of the Iberian Plate molasse-type sediments. They interpret this basin under the Adriatic Plate, which terminated in the as recording the convergence between and rotations Middle Eocene when Corsica, being part of the of two continental blocks—ALCAPA and Iberian Plate, collided with the edge of the upper Tisza-Dacia. plate, leading to the emplacement of oceanic Mikes et al. studied nannoplankton from deep- series on continental crust in northeast Corsica. In marine clastic (flysch) units associated with the the Late Eocene, a new, ‘Apenninic’ subduction thrust front of the Outer Dinarides, with important zone was created, in which the Adriatic Plate was consequence for the tectonic evolution of the subducted under the Iberian Plate, i.e. Corsica. Dinarides. Like in other parts of the Alpine- This subduction zone soon began to retreat east- Mediterranean orogens, nannoplankton evidence ward, creating in its back the young extensional, shows that the flysch units are significantly partly oceanic basins of the Western Mediterranean. younger (up to Mid-Miocene in this case) than pre- viously thought (Eocene). Previous age determi- The editors gratefully acknowledge the following nations were often based on foraminifers which colleagues for their reviews: P. Arkai, A. Berger, G. Bertotti, A. Bistacchi, R. Bousquet, D. Ciulavu, now turn out to be redeposited. Hence, thrusting G. Dal Piaz, K. Decker, C. Doglioni, I. Dunkl, H. von of the Outer Dinarides over Adria continued Eynatten, G. Franz, U. Glasmacher, P. Gleissner, through much of Miocene times. M. Handy, R. Kleinschrodt, J. Konzett, C. Krezsek, J. Kruhl, J. Malavieille, J. Mutterlose, F. Neubauer, Tectonometamorphic evolution H. Ortner, C. Passchier, D. Plasienka, M. Raab, J. von Raumer, M. Rockenschaub, S. Schmid, R. Schuster, Two papers deal with the relations between meta- J. Selverstone, S. Sinigoi, K. Stu¨we, M. Wagreich, morphism and tectonics in the Alps. The paper by J. Wijbrans, W. Winkler. Nagel is a comprehensive review of the structural and metamorphic evolution of the Adula Nappe in the eastern Central Alps. The author discusses the Reference question whether the high- and ultrahigh-pressure CARBOTTE, S. M., ARKO, R., CHAYES,D.N.ET AL. metamorphic rocks of the Adula Nappe were intro- 2004. New Integrated Data Management System for duced into a lower-pressure me´lange after their Ridge2000 and MARGINS Research. EOS, 85, 51.