MELTING AND DEFORMATION: INTERACTIONS FROM THE GRAIN- TO THE LITHOSPHERIC
SCALE.
Claudio Rosenberg
Institut für Geologie
Freie Universität Berlin
A review and reinterpretation of previous experimental data on the deformation of partially-melted crustal
rocks reveals that at melt volumes < ~ 7 %, the dependence of aggregate strength on melt fraction is
significantly greater than at melt volumes > ~7 %. This melt volume percentage (~ 7 %) marks the
transition to an interconnected network of melt channels along the grain boundaries, which accommodate
most of the deformation. Thus, deformation is mainly accommodated within the strong grains at melt
volumes < ~ 7%, but along the weak, melt-bearing interfaces at melt volumes > ~ 7%. In the light of the
low melt volume required for the latter transition, it is suggested that melt-induced, crustal-scale
localisation of deformation almost coincides with the onset of melting in most geological settings.
This prediction is confirmed by grain-scale analogue experiments showing the transition from a
homogeneously distributed solid-state deformation to a highly localized mode of deformation upon
melting of the deforming samples. Interestingly, the areas of localized deformation coincide with areas of
melt accumulation, which form the pathways of melt segregation. Similar relationships between melting
and deformation can be observed on the orogen scale, for example in the Tertiary Alpine Chain, where the
ascent of Oligo/Miocene magmas is confined to the Periadriatic Fault System.
Recent numerical models show that the topography and internal geometry of mountain chains are
controlled by melt-induced lower crustal weakening (e.g., Beaumont et al., 2001), which leads to lateral
flow of anatectic crust within horizontal channels usually situated at the base of the continental crust.
Deformation is strongly partitioned into these channels, which contain long-lived melt (tens of millions of
years) and cannot support high gradients of topography. As a consequence, plateaus are expected to form
on top of these channels. A comparison of melt contents inferred from below the Tibetan Plateau and the
Northwestern Himalaya (Unsworth et al., 2005) with strength vs melt-volume diagrams, suggests the
occurrence of a strength drop of ~ 10 times below below Tibet, but only of two to three times below the
Northwestern Himalaya. Numerical models suggest that the formation of a plateau-like topography in
Tibet requires drastic weakening in the lower crust by a factor 10 (Beaumont et al., 2001). This result is in
good agreement with the experimental data presented above, and with the absence of a plateau in the
Northwestern Himalaya.
REFERENCES
Beaumont, C., R.A. Jamieson, M.H. Nguyen, and B. Lee. 2001. Himalayan tectonics explained by
extrusion of a low-viscosity channel coupled to focused surface denudation. Nature, 414: 738–742.
Unsworth, M., A. Jones, W. Wenbo, G. Marquis, S. Gokarn, and INDEPTH team, 2005. Crustal rheology
of the Himalaya and Southern Tibet inferred from magnetotelluric data. Nature 438: 78-81.