GSA PENROSE CONFERENCE
Extending a Continent: Architecture, Rheological Coupling and Heat Budget
A Geological Society of America Penrose Conference, “Extending a Continent: Architecture, Rheological Coupling and Heat Budget” will be held on October 9-13, 2007, on the Island of Naxos in the Aegean Sea of Greece.
Continental breakup and the formation of oceanic basins is a fundamental process 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, such as the East African Rift (especially the new exciting work in the Afar triangle through the EAGLE project), the Taupo Volcanic Zone, the Rio Grande Rift, the Rhine Graben and many others. Over the last 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 Basin-and-Range province is considered an archetypal area for continental extension by low-angle normal faulting. However, the dynamic setting of horizontal extension is complex, given that extension began during mid-Tertiary plate convergence, and continued to evolve through a transition to dextral-oblique shear. The Aegean Sea above the retreating Hellenic subduction zone is another well-known example of large-scale continental extension. Here, the dynamic forcing of crustal extension is better known and horizontal extension there 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 setting that allows combining land- and seaborne studies. Especially the latter offers convenient and fast marine geophysical methods that have an advantage in imaging the present state of the crust.
An important and still not well understood topic is the nature of metamorphic events associated with lithospheric extension. Usually large-scale extension in convergent mountain belts is associated with some sort of `Barrovian-type' metamorphism. In many rift settings, extension is also associated with a temperature-dominated metamorphism but because deep crustal sections of modern rifts are generally not exposed our knowledge mainly stems from xenoliths studies. A fundamental question is what the mechanism driving this metamorphism is: is it mainly radioactive decay from upper crustal rocks now buried at depth? Or does the asthenospheric mantle play an important role? Closely related to this question is the role of magmatic processes and whether magmatism can trigger extension and associated metamorphism or whether it is a consequence of these processes.
New theoretical and observational insights into the mechanics of continental extension just now coming to light through fresh approaches in numerical modeling and measuring active strain. Novel, fully coupled thermo-mechanical approaches are capable of predicting major detachments self-consistently, i.e. without assuming implicitly or explicitly a weak material layer for nucleating the detachment. This may mark a genuinely new direction in understanding the mechanics of continental extension and the rheological coupling in the lithosphere. Space geodetic techniques, in particular continuous GPS, are beginning to reveal the behavior of deeper parts of active rifts, in particular episodic tectonic and magmatic events indicative of non-steady-state rheological behavior below the seismogenic layer.
Another important and controversial aspect of continental extensional tectonics is the possibility that extension may be responsible for exhuming metamorphic rocks within convergent orogens, such as onland thrust belts (i.e., Himalaya, European Alps, Betic Cordillera of southeastern Spain, Brooks Range of Alaska) and subduction-related convergent margins (i.e., Franciscan of California, Sanbagawa of Japan, Hellenic/Aegean convergent margin of western and southern Greece, Hikurangi accretionary wedge of northeastern New Zealand); especially in exposing ultrahigh-pressure metamorphic rocks. We are confronted with one of the most difficult questions in orogenic belts, especially in the older ones: What is the contribution of normal/extensional faulting in the exhumation of deep crustal rocks. There is growing evidence that extrusion wedges can accomplish exhumation of deeply buried rocks from great depth soon after these rocks experienced their maximum metamorphism. The normal fault at the top of the extrusion wedges is a geometric effect - it is not due to lithospheric extension of the region. Although, the tectonics community appears to be moving towards a general consensus that deep exhumation is most often a result of continental extensional processes, unequivocal evidence for this inference is still lacking.
Within this context, we propose a Penrose Conference to examine all processes that contribute to horizontal extension of continental lithosphere and the origin of oceanic basins. We want to look at processes at all scales: normal faulting during early stages of plate convergence and the exhumation of (ultra)high-pressure rocks, lateorogenic extension and core-complex formation, postorogenic extension associated with an extensional boundary condition (i.e. rifting) and processes at continent-ocean transitions. At the broadest scale, the conference will have five distinct goals: (1) to review and synthesize our knowledge about continental extension processes; (2) to examine the geologic and geophysical evidence relevant to resolving a quantitative understanding of the important tectonic processes, as deduced from seismic imaging, metamorphic and magmatic petrology, isotopic thermochronlogy, structural and kinematic analysis, synorogenic stratigraphy, geomorphology, and paleoelevation data; (3) to examine relevant geodynamic models and their predictions for conditions that might trigger the onset of continental extension; (4) to reconcile new geodetic data and computational geodynamics inferences on rheological coupling within the lithosphere with genuine structural observation of these processes; and (5) defining new research frontiers for studying extension.
At present, there are a number of high quality studies available for these areas and they have generated a diversity of interpretations and new ideas. While new numerical methods of continental extension are emerging an exciting opportunity is given to test these models against geological and geophysical evidence and to build new consensus on the great diversity of continental extension, understand the different styles of continental extension and find common themes.
The conference will be 5 days long, including 2 days of field trips and 3 days of presentations. The presentations will consist of 6 half-day sessions. Each session will have about 2-hours of oral presentations, including a keynote speaker, a 1-hour discussion session, and a 1-hour poster session. During the discussion session, individuals will be able to get up and show one or two slides to emphasize a point, but no formal presentations will be allowed. We want to avoid the typical meeting format with back-to-back talks, and instead focus on fleshing out old controversies and new ideas.
Tentative session titles are: (1) Local expression of extensional deformation and the role and significance of low-angle normal faulting causing large-scale extension; (2) Tectonic implications of metamorphism associated with extensional deformation; (3) Geodynamic implications of magmatism associated with extensional deformation; (4) The influence of deep-seated phenomena on the geodynamic evolution of extensional provinces: heat input from the mantle, lithospheric delamination, slab rollback, and gravitational collapse; (5) Geodetic data and their implications for the behavior of the deep lithosphere and its coupling to upper crustal extension; and (6) Feedback between the brittle and ductile crust and the Moho on the geodynamic evolution of extensional provinces.
The conference will be held on the Island of Naxos, which sits north of the active Hellenic subduction zone to the south. The island has spectacular exposures of synextensional high-temperature metamorphic rocks which were exhumed from depths as great as 30 km during Miocene extension. The field trips will be led by Olivier Vanderhaeghe (Université Henri Poincaré in Nancy, France). Vanderhaeghe and colleagues have developed a multidisciplinary study of Naxos combining structural geology, metamorphic petrology, geochemistry, and sedimentology in order to decipher the thermal-mechanical evolution of the island. Naxos records a complex geologic history from the genesis of blueschists attesting for burial and accretion under a low geothermal gradient to genesis of granites and exhumation of migmatites in metamorphic core complexes resulting from a drastic change in the geothermal gradient during time. Naxos displays the most complete section from bluschists to migmatites exposed in the core of a kilometer-scale dome.
Our objective in selecting this site for the meeting is to expose participants, especially those from North America, to an extensional setting different from that of the Basin-and-Range, which would be the natural site of choice if the conference were to be held in the United States. The location will mean a slightly more expensive air fare for North American participants, but will otherwise encourage participants from Europe to attend.
The conference will be limited to about 80 persons. Participants will be selected to ensure broad representation by nationality, by occupation (i.e. faculty, graduate students, industry and government scientists), and by research interest (i.e. structural geology, metamorphic petrology, isotope geochronology, sedimentology, geomorphology, and geodynamics). We are particularly encouraging students, early career professionals, women and minority participants to apply.
The registration fee is expected to be about US$950 which will cover all costs, including food and lodging for 6 nights (October 8-14), local travel, and field trip expenses. Airfare is not covered. We hope to be able to partially subsidize the participation of some graduate students, early career professionals, women and minority participants.
Co-conveners of the conference are: Uwe Ring, Department of Geological Sciences, Canterbury University, Christchurch 8004, New Zealand, PHONE: +64-3-364-2987 ext. 7723, FAX: +64-3-364-2769, EMAIL: [log in to unmask]; Klaus Regenauer-Lieb, School of Earth and Geographical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley 6009 WA, Australia, PHONE: + 61-8-64368693, FAX: +61-8-64368690, EMAIL: [log in to unmask]; Brian Wernicke, Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Blvd., Pasadena, California 91125, USA, PHONE: +1-626-395-6192, FAX: +1-626-683-0621, EMAIL: [log in to unmask] and Charalampos Fassoulas, Natural History Museum, University of Crete, Knossou Ave, Heraklion 71409, Crete, Greece, PHONE: +30 2810 393277, FAX: +30 2810 324366, EMAIL: [log in to unmask]
April 1, 2007
Interested persons should send a letter of application to Uwe Ring at the address given above. We would like to encourage people sending their letters by email to: [log in to unmask] The letter should include a brief statement of the applicant's research interests, relevance of those interests to the focus of the conference, and a potential topic that the applicant might want to present. Note that we are planning only a limited number of oral presentations, but we strongly encourage poster presentations and comment presentations in order to ensure an informal and interactive conference.