PART ONE Introduction Beginning in the 18th century, the Alps were the first mountain belt in the history of European science to attract the interest of Naturalists. In consequence, many key geological principles were developed as a result of the progress of geological exploration in the Alps and, simultaneously, in the oceans. Within mountain belts, deep sea or pelagic sediments were first discov- ered before being clearly recognized as such. Their origin remained enig- matic for a long period before comparison with sediments dredged from the deep sea floor by HMS Challenger which provided, at the end of the 19th century, the necessary insight into their origin. Their presence at the highest elevations of mountain belts allowed recognition of the ephemeral nature of the oceans at geological time scales though cause and effect lay beyond conception at the end of the 19th century. It should be noted that towards the end of the 19th century, the possibility of large scale, 100 km, horizontal displacements of rock units was demonstrated for the first time. This concept was a key generality that directly resulted from the discovery of nappes in 1884. The theory of geosynclines, interpreted as the birthplace of folded belts, emerged in 1859 from the celebrated work of James Hall in the Appalachians. It was a key driver for exploration in folded belts until 1960. By then, however, a steady flow of geophysical results from the oceans had seriously weakened the theory. The advent of palaeomagnetism had also by then confirmed the mobility of the continents thus vindicating Wegener’s visionary hypothesis of 1 2 Introduction continental drift. Quantification of continental drift through the Plate Tectonics hypothesis was achieved in the mid-1960s from marine geological and geophy- sical studies and, especially the dating of oceanic magnetic anomalies, latterly seismic reflection profiles that provided key insights into passive margin struc- ture. These observations provide the geodynamic context and framework for the present geological understanding of Alpine evolution. Chapter 1 of Part I is dedicated to this saga. To understand present-day Alpine structure and the reasons for the nomenclature of the structural units summarised in Chapter 2, it is necessary to recognize the existence of a now missing Ocean (Chapter 3), the Tethys, which evolved into the Alpine fold belt from the end of the Mesozoic and through the Tertiary era to the Present. During the Palaeozoic, the continents comprised a single supercontinent known as Pangaea. During the course of the Mesozoic, Pangaea began to fragment leading to the formation of the Tethys Ocean (Figs. 3.1–3.4). This ocean was bordered to the north by Laurussia, which then included Laur- entia (North America and Eurasia). To the south, it was bordered by the Gondwana supercontinent which included the future continents of South America, Africa, Madagascar, India, Australia and Antarctica. The Alpine fold belts derived from the destruction of the Tethys are collectively oriented in a general east–west sense and extend from Gibraltar to Indonesia via the Himalayas. In this array of folded belts, the Peri-Mediterranean fold belts extend from Gibraltar to Asia Minor. The Alps comprise only a short segment of this complex system (Fig. A). Figure A Location of the Alps in the framework of the Peri-Mediterranean mountain belts (adapted from M. Lemoine (2000), Fig. 5.1, p. 61). CHAPTER ONE Geosynclines, Passive Margins, Foreland Basins and Folded Belts: An Introduction Contents 1. Prologue 3 2. Orogenesis, Rock Deformation and Development of the Thrust Concept 4 3. Mountain Belts and the Geosynclinal Theory (1859–1965) 7 4. Geophysical and Geological Exploration in the Ocean: First Steps and Results 9 5. Continental Drift and Plate Tectonics: Principles 11 5.1 Basin-forming mechanisms 14 5.2 Rifts and passive margins 15 5.3 Foreland basins 17 5.3.1 Seismic imaging of sedimentary basins 17 5.3.2 Structure of passive margins 18 5.3.3 Palaeogeographic domains and passive margins 19 5.3.4 Seismic studies of foreland basins 21 6. Sedimentation in Oceanic Basins and Problems in Palaeodepth Reconstruction 23 7. The Wilson Cycle: Mountain Belts, Passive Margins and Foreland Basin-Folded Belts 24 1. PROLOGUE Although geological sampling of surficial sediments in shallow and deep water dates back to the Challenger expedition in the 19th century, detailed studies of continental margins did not begin until after World War II. While early gravity expeditions led by Meinesz (1941) among others had yielded some insights into the deeper structure of the margins of the Pacific and Atlantic Oceans, the use of sonar and other techniques developed during the war allowed for rapid mapping of the sea floor and also investigation of the deeper structure beneath the shelf continental slope and abyssal plains. It had long been recognized that pelagic sediments identified by the Challenger expedition (Murray and Renard 1891) had their equivalents in folded belts. However, the range of possible interpretations remained large The Western Alps, from Rift to Passive Margin to Orogenic Belt, Volume 14 Ó 2011 Elsevier B.V. ISSN 0928-2025, DOI 10.1016/S0928-2025(11)14001-8 All rights reserved. 3 4 The Western Alps, from Rift to Passive Margin to Orogenic Belt especially in view of the then established view of the permanence of the ocean basins. Simplicity versus complexity were the two bywords that differentiated the community of marine geoscientists from those concerned with terrestrial geology and geoscience. Suess in his seminal global geology summary (1885) noted that ‘the possibility was recognised of deducing from the uniform strike of the folds of a mountain chain, a mean general direction or trend line: such trend lines were seldom seen to be straight but consisted of arcs or curves, often violently bent curves of accommodation; the trend lines of central Europe were observed to possess a certain regular arrangement and to be traceable in part as far as Asia. It was further recognised that the ocean from the mouth of the Ganges to Alaska and to Cape Horn is bordered by folded mountain chains while in the other hemisphere this is not the case so that Pacific and Atlantic types may be recognised.’ Suess thus recognized, over a hundred years ago, the fundamental differences between the active (Pacific) and passive (Atlantic) continental margins. He noted the con- tinuity of the circum Pacific and Alpine–Himalayan fold belts whose associa- tion with calc alkaline volcanism and deep earthquakes is now very well known and understood. Suess was also well aware of the problems of major marine transgressions especially that of the Late Cretaceous. However, Suess thought that the ocean crust was similar to that of the continents and that the oceans owed their origins to ‘subsidence and collapse’. However, the technological hurdles that had to be overcome to deter- mine the geology of continental margins were matched by the problems imposed by the complex and intense deformation of folded belts exposed on land. 2. OROGENESIS, ROCK DEFORMATION AND DEVELOPMENT OF THE THRUST CONCEPT H.B. de Saussure (1740–1799), the Swiss naturalist from Geneva, was one of the first to express the idea that the torsion of beds observed on the flanks of Alpine valleys might be caused by ‘forcing back’ of rock material. His classic interpretation of the fold of the Arpenaz waterfall (Fig. 1.1) which dominates the Arve valley near the small town of Sallanches (between Chamonix and Geneva) was made, however, without any of the present knowledge of the rheological properties of rocks. The permanence of continents and oceans was considered a basic truth by authors of the first three quarters of the 19th century. In consequence, they found difficulty in conceiving that the rocks forming the mountains had been subjected to horizontal displacements greater than those observed Geosynclines, Passive Margins, Foreland Basins and Folded Belts: An Introduction 5 A B NW SE cmd Figure 1.1 Fold of the Arpenaz waterfall, Haute Savoie (France). H.B. de Saussure (1740–1799), probably the first or among the first Alpine geologists, described the fold of the Arpenaz waterfall between Geneva and Chamonix in the Arve valley. To explain the observed deformation, he proposed that the Jurassic and Cretaceous limestones were soft muds at the time of deformation. Today, one of the interesting aspects of this fold is that it shows the divergence of structure towards the external part of the fold belt, here to the NW. Horace Benedict de Saussure (1790). in associated folded beds. Not until the 1880s would the existence of thrusts be demonstrated clearly. A comparison of concurrent structural interpretations of the Glarus area (Switzerland) classically exemplifies the debate at the end of the 19th century on this issue (Fig. 1.2). 6 The Western Alps, from Rift to Passive Margin to Orogenic Belt A Churfisten Foostock Saurenstock 2259 2528 2491 2610 3054 N 2094 3028 Flims S Wallen 2505 Foo Pass See Rhine Jurassic Trias Ve Ve Cret Cret Verrucano mol Flysch Verrucano Ju Jurassic Trias Trias Basement B PENNINE NAPPES SÄNTIS Trias Ve NAPPE Jurassic Ve Cret Cret Verrucano Flysch molasse Jurassic Verrucano Jurassic Basement Trias 15 km Figure 1.2 Two structural interpretations of the Glarus Alps. (A) Albert Heim sketched two large facing recumbent folds. A fan fold is required to explain the Churfisten structure on the left of section. The interpretation resolves the problem of abnormal superposition with respect to the normal way up succession at the same time minimizing the magnitude of the sub-horizontal tectonic transport of thrust material. The illustration provided by Heim in his 1878 paper is superb. It adopts and recaptures the earlier conclusions of Arnold Escher (1866).