The strength of upper mantle peridotite determined from ultramafic pseudotachylytes
Torgeir B. Andersen, Karen Mair, Håkon Austrheim, Yuri Y. Podladchikov and Johannes
C. Vrijmoed, University of Oslo, Physics of Geological Processes, P.O.Box 1048 Blindern,
0316 Oslo, Norway
ABSTRACT
Faulting at high-pressure and low-temperature in the lawsonite-eclogite bearing Alpine
subduction complex of the Cape Region, Corsica generated pseudotachylyte fault veins in
spinel peridotite. We present minimum strength estimates of the upper mantle based on the
energy required to produce fault veins where the shear-strain can be measured. The
pseudotachylytes formed by near complete melting and rapid solidification of the
peridotite. Melting of peridotite at the ambient blueschist- to eclogite facies conditions
requires ~1750oC, corresponding to a temperature increase of ≥1280oC. In the strength
calculation we assume that more than 95% of the work converted to heat along the fault,
hence the strength is proportional to the amount of melting. The results suggest that the
peridotite was able to sustain differential stresses of more than 580 MPa during subduction
towards lawsonite-eclogite facies conditions. This is less than strength calculations from
flow-laws and rheological parameters for cold mantle, but higher than stresses commonly
suggested from seismic data. The abundance of pseudotachylyte on very small faults in the
peridotite suggests that coseismic melting is very common on intermediate- to deep
earthquakes and that the self localized thermal runaway mechanism may be important for
seismic faulting at depth.
Keywords: mantle-strength, peridotite, pseudotachylyte, subduction-earthquakes, Corsica.