Extending a Continent: Architecture, Rheological Coupling, and Heat Budget
Island of Naxos, Aegean Sea, Greece
8–12 October 2007
- Uwe Ring
- Department of Geological Sciences, Canterbury University, Christchurch 8140, New Zealand
- Brian Wernicke
- Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
- Klaus Regenauer-Lieb
- School of Earth and Geographical Sciences, The University of Western Australia, Crawley 6009 WA, Australia
- Charalampos Fassoulas
- Natural History Museum, University of Crete, Heraklion 71409, Crete, Greece
Continental breakup and the formation of oceanic basins are fundamental processes in earth sciences. Most of our process understanding of the early and intermediate stages of continental extension comes from landborne studies of incipient or failed rifts. Over the past two decades, there has also been a growing appreciation of the role of extensional tectonics in convergent orogens. This trend was initiated early on by the discovery of highly attenuated crustal sections in the Basin and Range province and the recognition that the attenuation was caused by regional-scale horizontal extension, as manifested by low-angle normal faulting.
The Aegean Sea above the retreating Hellenic subduction zone is a well-known example of large-scale continental extension. Horizontal extension in the Aegean occurs directly above the subducting plate, and lithospheric extension is caused by slab rollback. Extension in the Aegean occurs from the forearc through the backarc. Furthermore, it is an onshore-offshore (“amphibian”) setting that allows combining land- and seaborne studies. Within this context, we organized a Penrose Conference to examine all processes that contribute to horizontal extension of continental lithosphere and the origin of oceanic basins.
The conference was held on 8–12 October 2007 at the Naxos Palace Hotel on the Island of Naxos in the Aegean Sea of Greece. It included three days of presentations and two days of field trips. There were 78 participants, 34 from the United States, 13 from Australia, seven from Greece, six from France, five from Switzerland, four from the UK, three from New Zealand, and one each from Canada, Israel, Norway, Finland, Germany, and Spain. The participants included 17 Ph.D. students, and the conference had a relatively large number of women participants: 11 professionals and six students.
Field trips were lead by Olivier Vanderhaeghe from Université de Nancy in France and his colleagues Christian Hibsch, Luc Siebenaller, and Seth Kruckenberg and focused on geologic evidence for Miocene extension and associated high-temperature metamorphism and migmatization. The Island of Naxos is presently in the backarc of the south-facing Hellenic subduction zone, but extension commenced while the island was gradually shifting from a forearc into an intra-arc position. The island has spectacular exposures of high-temperature metamorphic rocks that formed as a consequence of lithospheric extension at depths of ~30 km and were rapidly exhumed in the Miocene.
Session 1: Observations and Models of Narrow Continental Rifts
Brian Wernicke opened the conference with a summary of previous Penrose Conferences on orogenic process and what their major impact on the community was. Cindy Ebinger followed with her keynote on three-dimensional (3-D) aspects of continental breakup using the East Africa Rift. Paul Umhoefer presented new data from the very fast spreading Gulf of California. In contrast to the East African rift, oblique rifting in the Gulf of California initiated above an active continental margin.
Panel discussions focused on differences and common aspects of three narrow rifts, the Taupo Volcanic Zone in New Zealand, the Afar triangle in Ethiopia, and the Gulf of California. The major topic was the role magmas plays in weakening the lithosphere and what mechanism is subsequently driving extension. A key question was how much rifting has to take place before magmatism takes over and controls the breakup processes. Recurring themes throughout the conference were the roles of temperature, magmatism, and differential extension between various layers of the lithosphere in the upper and middle-lower crust and the lithospheric mantle and gravitational potential energy.
Session 2: Local Expression of Extensional Deformation and the Role and Significance of Low-Angle Normal Faulting in Accommodating Large-Scale Extension
This session focused on the old controversy of whether low-angle normal faults exist in nature or not. Gordon Lister’s keynote presented examples from the Basin and Range province arguing for incontrovertible evidence for the existence of low-angle normal faults, a view that Nick Christie-Blick challenged in his subsequent talk. Later on, Daniel Stockli summarized the newest developments in low-temperature thermochronology. Brian Tucholke showed how normal faulting contributes to extension and mantle exhumation in weakly magmatic rifts and mid-oceanic ridges.
The panel debated the low-angle paradox in a lively fashion. It was stressed that there are numerous examples from extensional settings worldwide that appear to prove the existence of low-angle normal faults and that they are capable of achieving large-magnitude extension. Another major issue was the relationship between ductile extensional shear zones and brittle detachments.
Session 3: Tectonic Significance of Metamorphism Associated with Extensional Deformation
John Platt started the session off asking, “Can we use P-T-time data to fingerprint the mechanism and cause of late-orogenic extension?” Klaus Gessner followed with a talk on granites, suggesting that radioactive decay causes long-term crustal weakening and continental extension. Geoff Clarke showed the relationship of granulite-facies metamorphism and extension in the Cretaceous magmatic arc of New Zealand.
The panel discussed which critical observations are needed to resolve whether metamorphism, and especially high-temperature weakening of the crust, is facilitating extension or whether metamorphism is a result of extension. Most critical in this context is obtaining precise geochronologic information on the relationship between the structural evolution and the metamorphic history. New developments in geochronology highlight the limited applicability of the closure-temperature concept for dating tectonometamorphic processes. What is required instead of “cooling ages” are precise ages for both ductile deformation processes and for temperature-induced metamorphic reactions.
Session 4: Geodynamic Role of Magmatism
In his keynote presentation, Chris Hawkesworth concentrated on the magmatic record of continental extension and stressed that this record needs to be incorporated in numerical modeling of extension. Bill Collins reviewed granite generations in backarc settings, and Olivier Vanderhaeghe focused on the role of migmatization in the formation and evolution of core complexes.
How magmatism might influence extensional deformation remained the primary topic in the panel discussions. Critical questions are the buoyancy of magmas and their level of emplacement in the crust and how magmatism affects deformation and heat transfer during extension.
Session 5: Geodetic Constraints on Extensional Deformation
This evening session was kicked off by Rick Bennett, who reviewed how horizontal and vertical components of crustal deformation during extension can be resolved by global positioning system (GPS) geodesy. Tim Stern then presented constraints on the short- and long-term evolution of backarc extension in the Taupo Volcanic Zone of New Zealand, followed by Brian Wernicke who presented new results of the BARGAN GPS network in the Basin and Range province and argued for the existence of an active, high-temperature megadetachment beneath the western United States.
The discussion stressed the importance of geodesy for constraining slip rates and crustal rheology. A very important question would be whether or not rifting events are inherently episodic and non–steady-state in nature or not.
Session 6: Influence of Deep-Seated Phenomena: Heat Input from the Mantle and the Role of the Moho and the Brittle-Ductile Transition on the Evolution of Extensional Provinces
Mike Sandiford started the day with an intriguing talk on extensional stress fields, highlighting again the importance of heat and gravitational potential energy on the evolution of the lithosphere. Tim Little discussed the role of eclogite-facies metamorphism and gneiss dome development for the Woodlark Rift. Ritske Huismans stressed the significance of the rheologic stratification of the lithosphere for strain weakening and the formation of passive margins.
Much emphasis in the discussion concerned thermal softening of overthickened crust and delamination of a thickened mantle root. Thermal softening would reduce the strength of the crust and mantle, and delamination would increase the average topography of an extending region.
Session 7: Dynamic Aspects of Extending Lithosphere
The talks by Jean Braun on 3-D mechanical instabilities in extensional tectonic environments and the role of surface processes on the mechanical behavior of the continental crust and by Christian Teyssier on dynamic feedbacks among lithospheric layers during orogenic collapse were followed by Roger Buck’s keynote on the roles of buoyancy and strength in continental rifting. The final two talks ended the conference as it started by stressing the fundamental role that magmatism and heat input plays in the extension of the continents.
The general consensus among the participants was that the conference succeeded in providing a broad overview of the extension problem. It became clear that normal faulting is typically initiated by some major change in the geodynamic system, such as a change in boundary conditions (e.g., plate motions), or a change in the constitutive behavior of the crust (e.g., thermal softening), or a change in the position of rocks relative to a heterogeneous stress field.
New research frontiers are seen in the application of geodesy for addressing steady and non–steady-state behavior of the lithosphere during extension. Ideally, the emerging new results from this approach should be combined with studies on deeply exhumed fossil extensional complexes. A key issue in the latter is to merge our understanding and gain more information on the exact timing of episodic deformation with magmatic, metamorphic, and tectonic microstructures. A general problem is that the new developments and achievements in numerical modeling far outstrip our observations, stressing the importance of more and detailed field-based research in much closer interaction with numerical models.
We acknowledge partial support from the National Science Foundation (EAR-07-29518) and The Geological Society of America.
|Thomas H. Anderson
Jolante van Wijk