HR: 13:40h
AN: S22C-01 INVITED     [PDF]
TI: Metamorphism, Metastability, Mechanical Strength, and the Support of Mountain Belts
AU: * Jackson, J
EM: jackson@esc.cam.ac.uk
AF: Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge, CB3 0EZ United Kingdom
AU: Austrheim, H
EM: hakon.austrheim@geo.uio.no
AF: Department of Geology, University of Oslo, P.O. Box 1047 Blindern, Oslo, N-0316 Norway
AU: McKenzie, D
EM: mckenzie@esc.cam.ac.uk
AF: Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge, CB3 0EZ United Kingdom
AU: Priestley, K
EM: priestley@esc.cam.ac.uk
AF: Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge, CB3 0EZ United Kingdom
AB: We explore links between geophysical observations related to mechanical strength of the lithosphere and geological observations of metamorphism and metastability in the lower continental crust. \hspace*{2em}Our recent studies of elastic thickness ($T_e$), determined from gravity measurements, and seismogenic thickness ($T_e$), determined from teleseismic waveforms, led to the view that everywhere on the continents $T_e < T_s$ and that all significant long-term strength of the continental lithosphere resides in a single seismogenic layer within the crust. We found no significant evidence for teleseismically-recorded earthquakes in the continental mantle, and we suspect their absence is related to small amounts of water in nominally anhydrous mineral phases. The strong, seismically active, lower crust of northern India appears to underthrust much of southern Tibet, where the crust is up to 80~km thick. Several authors have pointed out that the lower crust in this region must consist of granulite, not eclogite. \hspace*{2em}Psuedotachylites that formed in lower continental crust under eclogite conditions in southern Norway are ancient analogues of earthquakes that occur today in the lower crust of southern Tibet at depths of 70-80~km. These psuedotachylites show that the dry granulite host rocks in which they occur were metastable at pressures 10--15~kbar beyond their equilibrium range, and that their partial conversion to eclogite was controlled by water infiltration along fractures. The change from granulite to eclogite was accompanied by a dramatic loss of mechanical strength, and a change in deformation style from localised brittle failure to distributed ductile flow. \hspace*{2em}We suspect the same processes observed in Norway are occurring today beneath S.~Tibet. Argument by analogy suggests that: (1) earthquakes at depths of 70-80~km beneath S.~Tibet occur in the dry, strong, granulite lower crust of the Indian shield; (2) those earthquakes represent the start of a process that will eventually convert, and possibly remove, the lower crust by eclogitization; (3) the sequence of metamorphic and mechanical changes is driven, and limited by, the supply of water --- probably from the underlying mantle of the Indian shield and originating from the breakdown of hydrous mantle phases. \hspace*{2em}In this view, the rheology of the continental lithosphere in S.~Tibet is controlled by composition, rather than by temperature, and suggests that the support of the highest mountains on Earth would not be possible without the metastability of dry continental lower crust.
DE: 8020 Mechanics
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8123 Dynamics, seismotectonics
DE: 8159 Rheology--crust and lithosphere
SC: Seismology [S]
MN: 2003 Fall Meeting