Penrose Conference Report some role. Penetrative deformational fab­ rics are present in most exhumed moun­ tain belts and provide clear evidence that Exhumation Processes: Normal Faulting, ductile flow is an important process that can either increase or decrease the rate of Ductile Flow, and Erosion exhumation, depending upon whether Conveners: the flow causes thinning or thickening in Mark T. Brandon, Department of Geology and Geophysics, Yale University, P.O. Box the vertical direction. Erosion is also an 208109, NewHaven, CT 06520-8109; [email protected] important exhumational process, as indi­ Uwe Ring, Institut fiir Geowissenschaften, Johannes Gutenberg-Universitiit, Postfach 3980, cated by the large volumes of sediment 55099 Mainz, Germany; [email protected] found adjacent to most contractional orogens. Within this context, we organized a Penrose Conference to examine all pro­ Over the past 25 years, there has One of the most difficult questions to cesses that contribute to exhumation of been a growing appreciation of the role answer in most orogenic belts, especially deep-seated rocks in ancient and modern that tectonic processes play in exhuming the older ones, concerns the relative con­ orogens. We started with three broad metamorphic rocks. This trend began with tributions of these different exhumation objectives: (1) to review and synthesize the discovery of highly attenuated crustal processes. Even so, the tectonics commu­ our knowledge about normal faulting, sections in the Basin and Range province nity has moved toward a casual consensus ductile flow, and erosion as exhumation and the recognition that attenuation was that normal faulting is the primary mech­ processes; (2) to examine the geologic caused by regional-scale horizontal exten­ anism for exhumation of deeply seated evidence needed to quantify the relative sion, as manifested by normal faulting. rocks. The most commonly cited evidence contributions of these different processes, This discovery caused many geoscientists is the presence of "younger-over-older" using information from metamorphic to rethink the role that horizontal exten­ relationships, where large faults, with low petrology, isotopic thermochronology, sion or, as it is more commonly called, to moderate dips, have placed younger structural and kinematic analysis, synoro­ tectonic extension, might play in other rocks on older rocks or lower grade rocks genic stratigraphy, geomorphology, and orogenic settings. Of particular interest is on higher grade rocks and, in the process, paleoelevation analysis; and (3) to exam­ emerging evidence that tectonic extension have cut out significant thicknesses of ine relevant geodynamic models and their might be responsible for exhuming meta­ stratigraphic or metamorphic section. But predictions for conditions that might trig­ morphic rocks within convergent orogens, recent papers have shown that this type of ger gravitational collapse. such as onland thrust belts (e.g., Hima­ evidence, by itself, is not diagnostic. Con­ layas, European Alps, Betic Cordillera of tractional faults can also cut out section if THE CONFERENCE southern Spain, Brooks Range of Alaska) the section was tilted back toward the hin­ Theconference was held October 9-13, and subduction-related convergent mar­ terland prior to faulting. A typical contrac­ 1996, at the Orthodox Academy of Greece, gins (e.g., Franciscan of California, Sanba­ tional fault would climb upward toward located near the town of Chania on the gawa of Japan, Hellenic-Aegean conver­ the foreland but would appear to cut · island of Crete in southern Greece. It gent margin of Greece, Hikurangi downward through the tilted section. included three days of presentations accretionary wedge of northeastern Top-to-the-foreland motion would pro­ and two days of field trips. There were 94 New Zealand). duce the appearance of normal offset participants, 42 from the United States, We use the term "exhumation" to where the fault cut through the tilted 15 from England, 13 from Germany, five refer, in a generic way, to all processes section. An additional problem is that from Australia, four each from Greece and that contribute to the unroofing of deeply the total offset and original dip of most Canada, three from Switzerland, and one seated rocks and their rise to the Earth's crustal-scale normal faults, especially those each from France, Israel, Japan, The surface. There are several exhumation pro­ involving deep crustal rocks, are typically Netherlands, New Zealand, Norway, cesses. Normal faulting and extensional poorly resolved. All of these factors make Poland, South Africa, and Spain. The par­ ductile flow operate within the Earth it difficult to quantify the relative contri­ ticipants included 16 Ph.D. students. The and are a direct manifestation of tectonic bution of normal faulting in exhuming conference had a relatively large number deformation, whereas erosion operates at deeply seated rocks. of women participants, 13 professionals the Earth's surface but can be indirectly Despite the current preference for and 9 students, marking a positive trend influenced by tectonically driven changes exhumation by normal faulting, it is clear in topography. that ductile flow and erosion must play Penrose Conference continued on p. 18 PARTICIPANTS Torgeir B. Andersen Jean Crespi Herman Griitter Karen L. Kleinspehn Alan Roberts Stuart Thomson Vaios Avdis George H. Davis Larry Guenther Maarten Krabbendam David Rowley Robert J. Twiss Hans G. John F. Dewey Bradley Hacker Anthi Liati Leigh Royden 0. Vanderhaeghe Ave Lallemant Bill Dinklage Bernard Hallet Gordon S. Lister Elizabeth Schermer Jaume Verges Dov Avigad David Dinter Andreas Henk Lidia Lonergan Fried M. Schwerdtner Lisa A. Verts Gary Axen Theodor Doutsos Ralf Hetzel Neil Mancktelow Jane Selverstone Simon Wallis Suzanne Baldwin W. Gary Ernst Paul Hoffman Stanislaw Mazur Dianne Seward Nicole Wawrzenitz Samantha Barr Hilmar von Eynatten Daniel K. Holm Peter Molnar Eberhard Seidel Laura E. Webb Manfred Brix James Faulds Niels Hovius Patrick O'Brien Sarah Sherlock John Weber Fraukje Brouwer Paul Fitzgerald Simon Inger John S. Oldow Virginia B. Sisson John Wheeler Doug Burbank Marnie Forster Carl Jacobson John P. Platt Colin Stark Donna L. Whitney B. Clark Burchfiel David Foster Rebecca A. Jamieson Raymond A. Price Bernhard Stockhert Sean D. Willett Erin Campbell Wolfgang Franke Barbara E. John Meinert K. Rahn John Tarney Paul F. Williams Christian Chopin Wolfgang Frisch Christopher Johnson Tim Rawling Micheal P. Terry Brian Windley Peter Copeland Dieter Gebauer Sue Keay Steve Reddy Christian Teyssier Meg Coleman Allen F. Glazner Adiamantis Kilias Stephen J. Reynolds GSA TODAY, May 1997 17 Penrose Conference continuedfrom p. 17 exhumation probably operates at very found in the Alps and the Norwegian slow rates. Other evidence is needed to Caledonides, and now known from in the reduction of the gender gap, at least distinguish between tectonic and ero­ numerous places around the world. UHP in the area of exhumation research. sional exhumation. metamorphic rocks are continental or Presentations were divided into six John Platt followed with a keynote oceanic crustal rocks that were metamor­ half-day sessions. Each session had about presentation on synexhumation defor­ phosed within the coesite-eclogite or dia­ two hours of oral presentations, about mation in mountain belts. He reviewed mond-eclogite fades. P-T-t data for these one hour of poster presentations, and geologic evidence for detecting tectonic rocks demonstrate that both oceanic and about one hour for a panel discussion. exhumation, the main clues being the continental crust can be subducted to This report highlights the oral presenta­ excision of metamorphic gradients, the depths >100 km and then returned to the tions and panel discussions. Those who shape of pressure-temperature-time (P-T-t) surface. are interested in further details will find paths, and the kinematics of ductile shear The keynote presentation by Chris­ a complete listing of the conference pro­ zones and brittle faults. tian Chopin provided a general review of gram, including the titles of all presenta­ Jane Selverstone challenged the use the UHP problem. Simon Wallis described tions, at the following Web site: http:// of P-T-t paths to distinguish exhumational some UHP rocks from China and then love.geology.yale.edu/-brandon/exhume. processes. The shape of a P-T-t path is typi­ explored the role that buoyancy might html. cally difficult to resolve, and the youngest play in returning these rocks to the sur­ part of the path, which is most diagnostic face. Paddy O'Brien discussed attempts FIELD TRIPS of process, is the most difficult to con­ to determine the P-T-t path for HP meta­ The field trips were led by Bernard strain. She emphasized her point by show­ morphic rocks in the Bohemian massif Stockhert (Ruhr-Universitat Bochum, ing two sets of published P-T-t paths, one of Germany. Germany) and Eberhard Seidel (Univer­ set attributed to erosion and the other to Dieter Gebauer reviewed his applica­ sitat zu Koln, Germany), and focused on tectonic exhumation. There was no obvi­ tion of SHRIMP U-Pb dating to determine geologic evidence for Miocene exhuma­ ous difference between the sets. The infer­ ages of igneous and metamorphic growth tion in a convergent-margin setting. ence is that steep decompression curves of zircons from HP and UHP metamorphic The island of Crete is.a forearc high that should not be considered diagnostic of rocks of the Alps. His work indicates an formed above the south-facing Hellenic extensional exhumation. Eocene age for metamorphism which subduction zone. The island has spectacu­ Clark Burchfiel reviewed the develop­ would require exhumation rates of -10 to lar exposures of high-pressure-low-tem­ ment of the South Tibetan detachment 30 mm/yr for some of the Pennine nappe. perature (HP) metamorphic rocks that system, a north-dipping system of faults A major problem, however, is that, as of formed at depths of -35 km and were and ductile shear zones with top-north yet, there is no direct tie between exhumed rapidly, starting in the Miocene.