Investigators: Mark Handy, Stefan Schmid
Cooperation partners: Daniel Bernoulli (Basel), Romain Bousquet (Potsdam), Jean-Pierre Brun (Rennes), Frederick Gueydan (Rennes), Eduard Kissling (Zürich), Claudio Rosenberg (Berlin), Marko Vrabec (Lubljana)
Understanding how orogens grow remains an elusive endeavour, especially in Alpine-type mountain belts with their strongly arcuate, non-cylindrical structure. Controversy centers on how shortening, extension and erosion are balanced across orogens, as currently discussed, for example, for the India-Asia collision.
Our research focusses on tectonically active parts of the mountain chains surrounding the Adriatic Sea and the Pannonian basin (Alps, northern Apennines, Dinarides, southern Carpathians). The Alps-Carparthian-Dinarides chains are an excellent natural laboratory for gaining insight into how processes active in Earth’s interior interact with its surface processes. Specifically, we want to relate past and present fault activity in the Periadriatic area to the evolution of lithosphere-scale structure and topography of the overlying mountain belts. Part of this work involves close collaboration with Prof. Stefan Schmid (Univ. Basel) who has just been awarded an Alexander-von-Humboldt Research Prize to spend 2008-2009 with our group (Visit these websites: http://www.uni-protokolle.de/nachrichten/id/150494/, http://www.fu-berlin.de/presse/publikationen/tsp/2008/ts_20081220/ts_20081220_32/index.html).
Handy, M.R. Bousquet, R., Kissling, E., Bernoulli, D. (2009): The fate of Alpine Tethys in light of Jurassic-Tertiary plate motions between Europe and Africa. Geophysical Research Abstracts, X, xxx, EGU 6th General Assembly, 30. April, 2009, Vienna
Investigators: Mark Handy, Claudio Rosenberg
Cooperation partners: Ding Lin (Beijing), Patrick O’Brien (Univ. Potsdam), Matthias Konrad-Schmolke (Univ. Potsdam), Lothar Ratschbacher (Freiberg), Richard Gloagen (Freiberg), Istvan Dunkl (Göttingen), Erwin Appel (Tübungen), Wolfgang Seibel (Tübingen), Jürg Pfänder (Freiburg), Roland Oberhänsli (Univ. Potsdam), Romain Bousquet (Univ. Potsdam)
PhD Candidate: Paola Crudi
The overall goal of this part of the project is to constrain Late Cenozoic rotation and extension of the crust around the Eastern Himalayan Syntaxis in Yunnan/SE-Tibet. Two major questions are addressed: How has crustal material flowed around the syntaxis from the onset of the India-Asia collision to the present? What was the timing of Plateau formation along strike going from SE-Tibet to Western Yunnan to Burma/Thailand? To answer these questions, the tectonic evolution of exhumed upper and middle crustal levels will be traced around the eastern syntax. This will involve a combination of methods, including structural geology, remote sensing, tectonic geomorphology, and thermochronology. The project is part of the DFG Priority Program 1372, Tibetan Plateau: Formation - Climate - Ecosystems (TiP).
Investigators: Mark Handy, Roland Oberhänsli (Potsdam), Claudio Rosenberg, Sebastian Garcia
Partners: Konrad Hammerschmidt (Berlin), Kurt Decker (Vienna), Lothar Ratschbacher (Freiberg), Bernhard Fügenschuh (Insbruck), Ralf Schuster (Vienna), Marko Vrabec (Lubljana)
PhD. Candidates: Andreas Scharf, N.N.
MSc. Students: Daniel Rupp, Mandy Loormis, Marianne Nikolopolous
How does extensional faulting accommodate lateral motion of orogenic crust at the end of orogens? The eastern end of the Tauern Window in the Eastern Alps is a key area for determining how the Alps have grown, both vertically and laterally, during plate indentation. Specifically. we are testing the idea that eastward lateral motion of the Alps in response to Miocene-present indentation of the Adriatic microplate involved two phases of extension and exhumation: (1) Initial low-angle, E- to SE-directed normal faulting along the Katschberg mylonite belt that was coeval with km-scale folding and NW-SE elongation of the Hochalm dome; (2) brittle, high-angle normal faulting that exhumed (and continues to exhume) the southeastern part of the Tauern Window, while the northeastern part of the window underwent folding related to sinistral transpression along the SEMP (Salzach-Ennstal-Mariazell-Puchberg) fault at the northern limit of the window. To date these events and to determine their kinematics, we will integrate textural studies with fission-track campaigns in three areas at the eastern end of the Tauern Window. By obtaining new information on the age, direction, amount and rate of displacement of this boundary, we aim to develop a four-dimensional tectonic model for the evolution of the eastern end of the Alps.
Investigators: Claudio Rosenberg, Mark Handy, Sebastian Garcia
Partners: Prof. Dr. Bernhard Fügenschuh
PhD. Candidates: Audrey Bertrand
MSc. Students: Nadine Bungies
Tertiary exhumation of the Eastern Alps remained localised within a restricted area (Tauern Window) from the early Tertiary to the Late Miocene. Based on previous and new thermo-chronological investigations the cooling patterns of this area will be constructed, with special attention to the relationship between the 3-D geometry of the isochrones, the 3-D geometry of the extensional fault planes, and the 3-D geometry of the large scale folds. These spatial relationships will be used to constrain the mechanisms of exhumation, i.e. the contribution of extensional unroofing vs. folding and erosion. The comparison of thermal models of end-member-types of exhumation by extensional unroofing and by folding and erosion, in addition to mixed-type models, will allow us to quantify the contribution of each mechanism to the bulk exhumation.
Considering that brittle deformation generally started after the rocks passed through the brittle-ductile transition, whose temperature is approximately defined by the annealing temperature of zircon fission tracks, we will define the maximum age of brittle structures by measuring zircon fission tracks. These ages combined with a structural analysis of brittle deformation will allow us to reconstruct the middle- to late Miocene kinematics and paleostress fields of the axial core of the Eastern Alps, and to constrain the mechanisms of exhumation.
Rosenberg, C.L., and Berger, A., On the causes and modes of lateral growth of the Alps. Tectonics in review (Submitted December 2008).
Rosenberg and Berger, 2009b, EGU general assembly 2009, Geophysical Research Abstracts, Vol. 11, EGU2009-2985
Investigators: Claudio Rosenberg, Konrad Hammerschmidt, Mark Handy
PhD. Candidates: Susanne Schneider
MSc. Students: Olga Frisch, Cornelia Kitzig, Marina Spanka, Monique Wanner, Sandra Wollnik
We aim to develop a temporally and kinematically-constrained model of deformation and exhumation of deep crust in the Tertiary Eastern Alps, on the base of detailed structural and geochronological investigations in the western Tauern Window. Our results obtained in the past 18 months indicate the simultaneous activity of large-scale (50 km length) sinistral shear zones at the southern and northern margins of the western Tauern Window, between 13 and 24 Ma. Both shear zones are transpressive, accommodating north-side-up displacements in the south and south-side-up displacements in the north. Therefore, dating of deformation along these shear zones also dates differential exhumation of the core of the Tauern Window. This conclusion is consistent with the age of rapid cooling in the western Tauern Window, as constrained by biotite and fission track data. The presence of a sinistral transpressive shear zone with N-side-up motion along the southern margin of the Tauern Window is inconsistent with a model of extensional unroofing along the Brenner normal fault, which would require the transfer of extension into dextral displacements. In our view/model, the observed sinistral transpression in this part of the Tauern Window is better explained as part of a system of kinematically linked faults and folds that accommodated Miocene, NNE-directed shortening at the NW corner of the Adriatic Indenter; there, sinistral transpression effecting rapid exhumation and erosion in front of the indenter was transferred to Miocene sinistral displacement along the Giudicarie Fault forming the western boundary of the indenter.
Rosenberg, C.L., and Schneider, S. 2008, The western termination of the SEMP fault (Eastern Alps) and its bearing on the exhumation of the Tauern Window. In: S. Siegesmund, B. Fügenschuh, and N. Froitzheim (eds.), Tectonic aspects of the Alpine-Carpathian-Dinaride System, Geological Society of London, spec. publ., 298, 197-218.
Rosenberg, C.L., and Berger, A., On the causes and modes of lateral growth of the Alps. Tectonics in review (Submitted December 2008).
Schneider et al., 2009, EGU general assembly 2009, Geophysical Research Abstracts Vol. 11, EGU2009-11429.
Kitzig et al., 2009, EGU general assembly 2009, Geophysical Research Abstracts, Vol. 11, EGU2009-11210
Rosenberg and Berger, 2009b, EGU general assembly 2009, Geophysical Research Abstracts, Vol. 11, EGU2009-2906
Schneider et al., 2007, EGU general assembly 2007, Geophysical Research Abstracts, Schneider and Hammerschmidt, 2009, EGU General Assembly 2009, Geophysical Research Abstracts, Vol. 9, 09136, SRef-ID: 1607-7962/gra/EGU2007-A-09136
Investigators: Sebastian Garcia in collaboration with Nicolas Bellahsen (Paris), Laurent Jolivet (Paris), Frédéric Mouthereau (Paris), Partick Monié (Montpellier), Olivier Lacombe (Paris)
What are the rheological parameters that control deformation at depth? What role do fluids play near the brittle-ductile transition in the crust? This project is a 3D study of the deformation of the Pelvoux massif in the Alps, where we intend to use the contact between pre-Alpine basement and Liasic sedimentary cover as a marker for quantifying strain and fluid flux at an exhumed segment of the ductile-to-brittle transition.
Investigators: Sebastian Garcia in collaboration with Nicolas Arnaud (Montpellier), Rolando Armijo (Paris), Lucilla Benedetti (Aix-Marseille), Didier Bourles (Aix-Marseille), Régis Braucher (Aix-Marseille), Michel Condomines (Montpellier), Laurent Dezileau (Montpellier), Hervé Guillou (Paris), Marc Jolivet (Montpellier), Geoffrey King (Paris), Bertrand Meyer (Paris), Patrick Monié (Montpellier), Jean Louis Poidevin (Clermont-Ferrand), Françoise Roger (Montpellier), Stéphane Scaillet (Paris), Sevket Sen (Paris)
The Anatolian-Aegean plate is currently undergoing translation and extension in response to northward motion of the Saudi Arabian indenter and SE-ward pull from the retreating Hellenic trench. This plate is an excellent laboratory for studying the rheology of the continental lithosphere, but crucal age data are still lacking with which to quantify and refine mechanical models. This project is devoted to obtaining better geochronological constraints on the age of Miocene to the Holocene deformation along the North-Anatolian fault system that forms the northern boundary of this plate. We hope to discern variations in the rate at which displacement has propagated along strike of this plate-boundary fault.
Investigators: Sebastian Garcia in collaboration with Vincent Famin (La Réunion), Franco Barberi (Rome), Nicolas Bellahsen (Paris), Anne-Marie Boullier (Grenoble), Svetlana Byrdina (Clermont-Ferrand), Maria Luisa Carapezza (Rome), Olivier Fabbri (Besançon), Anthony Finizola (La Réunion), Koichiro Fujimoto (Tsukuba), Tetsuro Hirono (Osaka), Laurent Michon (La Réunion), Satoru Nakashima (Osaka), Volker Rath (Aachen), André Revil (Aix-Marseille)
Fluids are ubiquitous during initiation and evolution of a fault zone and can deeply affect the mechanical behavior of an active fault. For instance, they can reduce the static friction coefficient and then increase the recurrence time of EQs or the aseismic slip during the interseismic period. Whereas they can have a dynamic influence on the coseismic sliding, and then increase the magnitude of an EQ. Now we have to determine which of these effects is the preponderant one, and their relative effects on the mechanical behavior of a fault. To this purpose, we need to know the amount of fluids present before, during and after an EQ.
The selected target is the Nojima fault, a very well studied fault since the 1995 Kobe EQ (M7.2). This project has as ambition to quantify the inputs and outputs of fluids during an EQ (petrology and geochemistry), to determine what the fluxes of fluids and heat are during the interseismic period (soil degassing and hydrochemistry), to characterize the hydrothermal system of fault zone (electrical methods), to determine the permeability of the fault and its evolution during the seismic cycle (numerical modeling of fluids and heat transfers), and to constrain the mechanical behavior of the Nojima fault (structural geology and dating of fault-related structures).
Convenor: Mark Handy (FU-Berlin)
Partners: Greg Hirth (Woods Hole, now Brown University), James Rice (Harvard University), Lukas Baumgartner (Lausanne), Walter Mooney (USGS, Menlo Park)
Other participants: XXX
This project culminated in a conference held in January of 2005 on processes that govern the kinematic and dynamic evolution of faults in Earth´s lithosphere. The conference proceedings can be purchased as a book from MIT Press:
Tectonic faults –agents of change on a dynamic Earth
edited by Handy, M.R., Hirth, G. Hovius, N., Dahlem Worshop Report 95, 504 pp., The MIT Press, Cambridge Massachusetts, USA http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=11190).
The book comprises state-of-the-art background papers and research reports on future avenues of research. PDFs of these papers and reports, including many colour figures are available on request from M.R. Handy.
Dahlem conferences promote cooperation between international researchers who are interested in developing new, interdisciplinary approaches need to resolve controversial issues. More information about the conferences is available by clicking www.fu-berlin.de/dahlem.
Handy, M.R., Hirth, G. and N. Hovius (2007): Tectonic Faults – Agents of Change on a Dynamic Earth. pp. 2-8 in: Tectonic Faults – Agents of Change on a Dynamic Earth, edited by M.R. Handy, G. Hirth, N. Hovius, Dahlem Workshop Report 95, 504 pp., The MIT Press, Cambridge, Mass., USA
Handy, M.R., Hirth, G. and R. Bürgmann (2007): Fault Structure and Rheology from the Frictional-Viscous Transition Downward. pp. 139-181 in: Tectonic Faults – Agents of Change on a Dynamic Earth, edited by M.R. Handy, G. Hirth, N. Hovius, Dahlem Workshop Report 95, 504 pp., The MIT Press, Cambridge, Mass., USA
Furlong, K., Beroza, G.C., Brun, J.-P., Cowie, P.A., Handy, M.R., Mooney, W.D., Taymaz, T., Teyssier, C., Vauchez, A., and B. Wernicke (2007): Nucleation and Growth of Fault Systems. pp. 78-98 in: Tectonic Faults – Agents of Change on a Dynamic Earth, edited by M.R. Handy, G. Hirth, N. Hovius, Dahlem Workshop Report 95, 504 pp., The MIT Press, Cambridge, Mass., USA
Investigators: Mark Handy, Claudio Rosenberg
Cooperation: Jordi Carreras, Elena Druguet, Sergei Medvedev, Bernhard Graseman
PhD. Candidate: Florian Fusseis
MSc students: Roland Christman, Sarah Hauten, Christoph Schrank
What processes govern the length- and time-scales of localization? How does localization affect the bulk mechanical properties of materials? Once strain localization patterns are established, how stable are these patterns with continued strain? The overriding goal of this project was to establish scaling parameters for strain localization patterns in deformed rocks at the brittle-to-viscous transition, and to determine the processes underlying the evolution of these patterns in time and space. To do this, we studied the North Pyreneen fault (NPF), a crustal-scale network of retrograde greenschist-facies shear zones at Cap de Creus in northeastern Spain that exposes the fossil brittle-to-viscous transition (BVT). The coeval nature of fracturing and mylonitic shearing is recognized by mutually overprinting relationships between cracks and greenschist-facies mylonitic foliation. Similar relationships were found in shear zones of the Aar Massif in the Central Alps, Switzerland.
Strain gradients across and along the NPF reveal the localization process on different scales (Fusseis et al. 2006). Shear zones lengthened by a combination of fracturing and mylonitic shearing in front of the shear zone tips. Microstructural observations indicate that fracturing occurred immediately prior to or coevally with a fine-grained mylonite (Fusseis & Handy 2008). The shear zones interconnected along and across their shearing planes and separated rhomb-shaped lozenges of less-deformed rock. The lozenges were subsequently incorporated into the mylonitic shear zones by lateral branching of fractures from the bounding shear zones and by synthetic rotation of the existing foliation between the fractures and shear zones. In this way, the mylonitic shear zones widened and the deformation became homogeneous on the scale of initial fracturing (metre- to decametre-scale). Regarded on larger scales, however, the networking of shear zones represents a localisation of strain.
We developed a new method for quantifying the intensity of strain localization on different scales from maps of natural shear zones (Schrank et al. 2008). Our method employs autocorrelation functions to determine local areal scales of geometric homogeneity. Applied to the shear zones at Cap de Creus, the method reveals strain-dependent changes in the scaling characteristics of the fossil brittle-to-viscous transition (BVT) that are interpreted to reflect a time sequence of localization. We show that shear zone scaling is related to inherited anisotropies (older schistosity, lithological layering, pegmatite bodies) and to the predominant mode of deformation (brittle, viscous). The length-to-width ratio of shear zones increases with their length up to the meter scale and decreases for larger length scales as they evolve from isolated shear fractures to interconnected mylonitic shear zones. Variations in strain localization intensity calculated along a single shear zone indicate that such shear zones weakened from their brittle tips to their mylonitic centers, thus driving their propagation and growth to larger scales. Our results imply that the BVT evolves by ‘‘network widening,’’ a process whereby strain localizes on progressively larger scales until a dense network of weak, mylonitic layers tens to hundreds of meters wide and hundreds to thousands of meters long forms subparallel to the regional shearing plane.
The strength of the continental crust at the BVT is inferred to have decreased with time and strain, as fracturing evolved to mylonitic shearing, and as the shear zones coalesced to form a through-going network subparallel to the crustal-scale shearing plane. Crustal strength must therefore be considered as strain- and scale-dependent.
Fusseis, F. and Handy, M.R. (2008): Micromechanisms of shear zone propagation at the brittle to viscous transition, Journal of Structural Geology, doi :10.1016/j.jsg.2008.06.005
Fusseis, F., Handy, M.R., and C. Schrank (2006): Networking of shear zones at the brittle-to-viscous transition (Cap de Creus, NE Spain). Journal of Structural Geology, 28, 1228-1243.
Schrank, C.E., Handy, M.R. and F. Fusseis (2008): Multiscaling of shear zones and the evolution of the brittle-to-viscous transition in continental crust, Journal of Geophysical Research, 113, B01407, doi:10.1029/2006JB004833.
Investigators: Claudio Rosenberg
Partners: Jean-Pierre Brun, Denis Gapais
Experimental models scaled for density and viscosity were performed to investigate the effects of indentation obliquity and rheological stratification on the deformation patterns caused by continental indentation. The shape and orientation of the indenter were inspired by the Dolomites indenter of the southern Europeans Alps. The results of our experimental models showed that small changes in the angle of convergence induce marked differences in the patterns of deformation. The only models whose fault patterns satisfyingly reproduced that of the Eastern Alps were characterized by NNE directed motion of the indenter. In these models, E-W extension formed in front of the leading edge of the indenter, as observed in the Eastern Alps along the Brenner extensional fault. Extensional deformation of the models maintained compatibility between the areas located on both sides of the indenter edge, which shortened at different rates and in different directions. Therefore extension was not caused by gravitational instabilities but by the kinematic and geometrical boundary conditions imposed by the indenter shape and the convergence direction. Lateral escape was always modest in our models, reaching a maximum of 20%. This value is much smaller than previous estimates of lateral escape in the Eastern Alps but very close to the amount inferred by our reassessment of Tertiary E-W extension in the Eastern Alps.
Rosenberg, C. L., Brun, J.-P., Cagnard, F., and Gapais, D. 2004, Coeval shortening, thickening and orogen-parallel extension: laboratory models applied to the Eastern Alps. Bollettino di Geofisica teorica ed applicata, vol. 45, n. 1 supplement, 136-140.
Rosenberg, C. L., Brun, J.-P., and Gapais, D., 2004, An indentation model of the Eastern Alps and the origin of the Tauern Window. Geology, 32, 997-1000.
Rosenberg, C. L., Brun, J.-P., Cagnard, F., and Gapais, D., 2007 Oblique indentation in the eastern Alps: Insights from laboratory experiments. Tectonics, 26, TC2003, doi:10.1029/2006TC001960
Investigators: Mark Handy, Konrad Hammerschmidt, Claudio Rosenberg, Roland Oberhänsli (Univ. Potsdam)
PhD candidates: Jochen Babist, Matthias Konrad
MSc. Students: Rita Häusler, Matthias Janitschke, Sabine Lüdke, Silke Sämann, Bert Sperber
Deeply eroded fault zones in the Western Alps were investigated to establish how faulting exhumed high-pressure rocks and how orogenic indentation affected strain partitioning at the western end of the Periadriatic Line. XXXX
Babist, J., Handy, M.R., Konrad-Schmolke, M., and K. Hammerschmidt (2006). Precollisional, multistage exhumation of subducted continental crust: The Sesia Zone, western Alps. Tectonics, 25, 25 pp., TC6008, doi:10.1029/2005TC001927.
Handy, M.R., Babist, J., Rosenberg, C.L., Wagner, R. and M. Konrad (2005): Decoupling and its relation to strain partitioning in continental lithosphere– Insight from the Periadriatic fault system (European Alps). In: Deformation Mechanisms, Rheology and Tectonics, edited by Gapais, D., Brun, J.P. and P.R. Cobbold, Geological Society, London, Special Publications, 243, 249-276.
Konrad-Schmolke, M., Babist, J., Handy, M.R., and P.J. O’Brien (2006): The physico-chemical properties of a subducted slab from garnet zonation patterns (Sesia Zone, Western Alps). Journal of Petrology, 47, 11, 2123-2148, doi: 10.1093/petrology/egl039
Konrad, M., Handy, M.R., Babist, J., and P.J. O´Brien (2005): Thermodynamic modeling of diffusion-controlled garnet growth. Contributions to Mineralogy and Petrology, 16, 181-195.
Investigators: Mark Handy, Sergei Medvedev, Stefan Sobolev (GFZ-Potsdam), Robert Trumball (GFP-Potzsdam), Tim Vietor (GFZ-Potsdam)
We examine potential determinates of orogen-parallel variation in Neogene shortening and topography of parts of the South American plate involved in Andean subduction orogenesis. This involved discrete element and finite element modeling. XXXX. Our project was part of the SFB267 (Deformation Processes in the Andes).
Medvedev, S., Podladchikov, Y.Y., Handy, M.R. and E. Scheuber (2006): Controls on the evolution of the Central Andes (10-35°S): Insight from thin-sheet numerical modeling. Chapt. 23 (pp. 475-494) in: The Andes, Active Subduction Orogeny, edited by Onken, O., Chong, G., Franz, G., Giese, P. Götze, H.-J., Ramos, V.A., Strecker, M.R., and P. Wigger, Frontiers in Earth Sciences, 1, 568 pp., Springer-Verlag, Berlin, New York
Rosenberg, C., Medvedev, S., and M.R. Handy (2007): On the effects of melting on faulting and continental deformation. pp. 357-401 in: Tectonic Faults – Agents of Change on a Dynamic Earth, edited by M.R. Handy, G. Hirth, N. Hovius, Dahlem Workshop Report 95, 504 pp., The MIT Press, Cambridge, Mass., USA
Investigators: Mark Handy (FU-Berlin), Roland Oberhänsli (Univ. Potsdam)
Partners: Romain Bousquet (Univ. Potsdam), Bruno Goffé (Paris), Ralf Schuster (Geologische Bundesanstalt, Vienna)
A tectonometamorphic age map on the 1:1.000.000 scale forms an inset to the Metamorphic Structure Map of the Alps sponsored by the Commission for the Geological Map of the World(CGMW). Our age map superposes radiometric ages for subduction-related and thermal-overprinting metamorphism onto a tectonic base map. Available at http://ccgm.free.fr/alp_meta_gb.html.
Handy, M.R. and R. Oberhänsli (2004): Age of the Metamorphic Structure of the Alps – Tectonic Interpretation and Outstanding Problems. In: “Explanatory Notes to the Map: Metamorphic Structure of the Alps.” Edited by R. Oberhänsli. Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 149, 97-121.
Investigators: Claudio Rosenberg, Mark Handy
PhD candidates: Barbara Ziemke
MSc students: Axel Kretschmer
A review and reinterpretation of previous experimental data on the deformation of partially melted crustal rocks reveals that the relationship of aggregate strength to melt fraction is non-linear, even if plotted on a linear ordinate and abscissa. At melt fractions, f < 0.07, the dependence of aggregate strength on U is significantly greater than at f > 0.07. This melt fraction (f = 0.07) marks the transition from a significant increase in the proportion of melt-bearing grain boundaries up to this point to a minor increase thereafter. Therefore, we suggest that it is the increase of melt-interconnectivity that causes the dramatic strength drop between the solidus and a melt fraction of 0.07. We term this drop the melt connectivity transition (MCT). A second, less-pronounced strength drop occurs at higher melt fractions and corresponds to the breakdown of the solid (crystal) framework. This is the solid-to-liquid transition (SLT), corresponding to the well known rheologically critical melt percentage. Although the strength drop at the SLT is about four orders of magnitude, the absolute value of this drop is small compared with the absolute strength of the unmelted aggregate, rendering the SLT invisible in a linear aggregate strength v. melt-fraction diagram. On the other hand, the more important MCT has been overlooked in previous work because experimental data usually are plotted in logarithmic strength v. melt-fraction diagrams, obscuring large strength drops at high absolute strength values. We propose that crustal-scale localization of deformation effectively coincides with the onset of melting, pre-empting attainment of the SLT in most geological settings. The SLT may be restricted to controlling flow localization within magmatic bodies, especially where melt accumulates.
Rosenberg and Handy, 2005