 REPORT |
Three-Dimensional Flow, Fabric Development, and Strain in Deformed Rocks and the Significance for Mountain Building Processes: New Approaches
Ascona, Switzerland
18-24 August 2002
Conveners:
- Hermann Lebit
- Department of Geology, Georgia State University, 340 Kell Hall, Atlanta, Georgia 30303, USA
- Catalina Lüneburg
- Department of Geoscience, State University of West Georgia, Carrollton, Georgia 30118, USA
- Peter Hudleston
- Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, Minnesota 55455, USA
- John Ramsay
- Cratoule-Issirac, F-30760 St. Julien de Peyrolas, France
The conference was held August 18-24 at Monte Verita, a conference center of the Swiss Federal Institute of Technology (ETH) situated on a steep hillside overlooking the town of Ascona and Lago Maggiore in the canton of Ticino. This venue was ideally suited for field excursions into the central Penninic nappes and the plate collision zone of the Ivrea-Verbana region. It also allowed participants excellent views of the southern Swiss Alps during the meeting. The 77 participants were a diverse group in age and experience as well as geography: 35 from the United States, eight from Switzerland, eight from Germany, six from Britain, four from Australia, three from Spain, two each from Canada, India, and France, and one each from Bulgaria, Cameroon, Finland, Israel, New Zealand, Norway, and Portugal. Sixteen of the participants were graduate students and five were postdoctoral researchers.
The conference theme was the geometry and mechanics of three-dimensional deformation and crustal tectonics, with insights and contributions coming from applications of the many new techniques and scientific developments that have occurred over the past decade. Various approaches were taken, including field analysis, laboratory experimentation, physical modeling, and computer modeling during several days of presentations and discussion at the Monte Verita Centre interspersed with three days of field excursions in regions particularly relevant to the topic of the conference. The days in Monte Verita were organized around a series of keynote presentations followed by extensive open discussion. The discussion periods were especially productive, allowing everyone a chance to express opinions, ask questions, and challenge one another. The discussion time stimulated wide-ranging, free, and sometimes heated exchanges among audience members and with the speakers. Many participants brought excellent posters; these also were the focus of much informative discussion.
Principal Topics
Strain and what we can do with it.
Keynote speakers: John Ramsay, Sudipta Sengupta, Declan DePaor, John Watkinson, Renee Heilbronner, Ernest Rutter, Bernd Leiss, and Basil Tikoff. The first presentations focused on modeling superposed fold systems, with special reference to the complex three-dimensional geometry and strain states that can arise. This led to a general analysis of stress and strain and the complex mechanics of the folding of inclined layers in three dimensions. Methods of examining cumulative and evolutionary fabrics, both in natural and experimentally deformed rocks-especially those produced using experimental torsion techniques-were shown to have great geological implications. The final presentation involved discussion of how mantle geometry, deduced from an analysis of seismic shear wave splitting, might be related to the structures in the lower crust.
Models and kinematic indicators: What they really tell us.
Keynote speakers: Peter Hudleston, Arthur Snoke, Jordi Carreras, John Dewey, Cees Passchier, Carol Simpson, Rick Law, and John Wheeler. Most of these presentations concentrated on the development of shear zones and shear sense indicators found in and around shear zones. Many shear zones show complex non-plane strains, and speakers showed how these may arise and how strain compatibility may be maintained. Specific applications varied widely from small-scale to regional tectonic features, and a wealth of excellent field data was presented. The regional implications of transpression and transtension were shown to be important in many tectonic zones. Presentations of the results of recent physical and numerical modeling experiments were of particular relevance to field investigations. The last speakers showed how the information these provide might be used to understand the significance of large-scale regional features in the Alps and Himalayas.
Reconstructing regional deformation histories by modeling or by outcrop analysis.
Keynote speakers: Richard Lisle, Alison Ord, Djordje Grujic, Jean-Pierre Burg, James Jackson, Ray Fletcher, and Martin Burkhard. The day began with a novel reinvestigation of the methods that can be used to describe the three-dimensional geometry of many types of folds, both single- and multiple-phase. Pressure solution was the subject of some stimulating new computer modeling done by Australian researchers. Larger-scale general tectonic modeling and specific modeling of Himalayan and Alpine tectonic features showed how approaches based on a geometric analysis of final-state geometry can be used to develop plausible developmental models for such features. The recent use of satellite positioning techniques in currently active tectonic zones in Greece and western North America showed how an analysis of recent deformations can greatly assist an understanding of large finite displacements and strains. It was clear that field geologists can benefit from the insights of those using modeling methods, but that modeling must still be based on accurate and judiciously collected field data.
Field Trips
In a special evening lecture, Stefan Schmid provided an excellent introduction to the three days of excursions, summarizing the regional geology of Switzerland and current thinking on the architecture and tectonics of the Alps. A highlight of his talk was a very new interpretation of seismic data obtained by the Transalp project, which transects the eastern Alps close to the Brenner Pass. In contrast to a similar project in the western Alps that identified a southward-dipping lithospheric slap, the transect in the eastern Alps seems to indicate two oppositely dipping slaps. The south-dipping one may be the continuation of that seen in the western Alps, whereas the north-dipping one may correspond with the Dinaride system.
The Laghetti shear zones of Maggia Nappe of the Central Alps. Conference participants saw, crawled over, and photographed classic examples of shear zones developed during the Alpine deformation of pre-Alpine basement granitic and dioritic rocks. The complex geometry of and interconnections of shear zones are apparent in these examples.
Structures in the main nappe flat zone and steeply dipping "root zone" of the Penninic region. Well-exposed complex folding of crystalline basement and Mesozoic marbles was studied in the walls of the Verzasca hydroelectric dam. Later, the group investigated the spectacular basement outcrops at Lavertezzo in Val Verzasca, where the gneissic banding shows geometrical forms characteristic of superposed folds. These folds are cut by late Alpine aplite and pegmatite dykes.
Ductile mylonitic structures in the Alpine root zone and in the Ivrea-Verbano zone. Complex folding of pegmatite veins in the Monte Rosa root zone north of Arcegno shows that deformation was proceeding as Alpine pegmatites were being intruded into the basement gneisses. A complex history of ductile deformation, mylonite formation, and semi-brittle and brittle shear zones can be deciphered in the Ivrea-Verbana zone in road sections just west of Monte Verita.
A field guide to these superb outcrops, written by Stefan Schmid, Hermann Lebit, Catalina Lüneburg, John Ramsay, Dorothee Dietrich, and Djordje Grujic, is available. Contact Hermann Lebit, hlebit@westga.edu, for details.
Pre-Conference Trip
Prior to the conference, 25 participants undertook a nine-day excursion transecting the Swiss Alps from the external parts to the orogenic core. The trip started with the Glarus thrust, the classic basal thrust of the Helvetic nappes in Eastern Switzerland. The architecture of the Helvetic nappes and the subalpine Molasse was studied around Lake Lucerne. Moving toward the southwest and more internal zones, attention was then focused on basement-cover relations and reactivation of pre-Alpine structures in the Aiguilles Rouges Massif of Western Switzerland. In the overlying Morcles fold nappe, the group examined structures and strain features that indicate changes in transport direction during nappe emplacement. Moving to the internal zones, multiple phases of Alpine deformation and associated structures and fabrics were studied in the Pennine units of the Central Alps. Excellent exposures of tectonic and sedimentary structures were examined along a section through amphibolite-facies Mesozoic metasediments at Nufenen Pass. Fold interference patterns and complex superposed strains are well exposed in the metamorphic cover rocks of the Lepontine nappes. Outcrops of strongly deformed conglomerates in the Lebendun nappe exposed in the Cristallina area provided a wonderful opportunity for discussing the significance of stretching lineations as kinematic indicators.
The three-dimensional geometry and kinematics of late alpine tectonics were studied at the Simplon normal fault, which is a spectacular example of how pre-existing structures from the contractional phase of deformation became modified during late orogenic extension. Pre-existing structures are overprinted by progressively intense brittle deformation in the hangingwall and ductile deformation in the footwall as the shear zone is approached from either side.
The pre-conference field trip benefited greatly from the support and participation of local experts, and we wish to thank David Durney, Martin Burkhard, Neil Mancktelow, Eva Klaper, and Djordje Grujic for sharing their expertise with the group. John Ramsay, Dorothee Dietrich, Flavio Amselmetti, and other colleagues contributed to the field guide, which was compiled by Hermann Lebit and Catalina Lüneburg.
At the close of the conference, participants expressed strong interest in contributing papers to a special publication based on the theme of the conference. With the approval of GSA, the conveners have made arrangements for papers stemming from the conference to be published in a special issue of the Journal of Structural Geology.
Acknowledgments
We are grateful to GSA and the GSA Foundation for sponsoring the meeting as a Penrose Conference. We thank Centro Stefano Franscini at ETH Zürich for professional conference coordination (through the persons of Karin Mellini and Claudia Lafranchi) and we are grateful to ETH for covering fees for the lecture hall and the other facilities at Monte Verita. The Geologisches Institut of ETH, Zürich, generously allowed us to use their facilities for making preparations for the conference and the pre-conference field trip. In all this, Jean Pierre Burg played a key role. The Swiss National Fund is acknowledged for its financial support (SNF 21-68415.02), and the National Science Foundation is acknowledged for a grant to support the attendance of graduate students and young career scientists (EAR 0223797). Finally, we thank the participants for their individual contributions and enthusiastic involvement in all elements of the conference.
Conference participants
Jeff Amato |
Djodje Grujic |
Terry Pavlis |