Brittle failure at high-pressure conditions: the key role of reactioninduced volume changes
Abstract
Metamorphic reactions can lead to drastic changes in rocks mechanical properties. Indeed, during
such transformations, the nucleation of new phases with different strength, grain size and/or
density compared to the primary phases is enhanced, and transient processes due to the ongoing
reaction are then activated.
Eclogitization of lower crustal rocks during continental subduction constitutes an emblematic
transformation illustrating these processes. In such tectonic context, it has been shown that
eclogitization seems to be closely associated with the occurrence of seismogenic events. However,
the mechanisms that trigger brittle failure in such high pressure environments remain highly
debated. Indeed, whether the change in density or the change in rheology can lead to
embrittlement is still enigmatic.
By using 2D compressible mechanical numerical models we studied the impact of the strong
negative volume change of the eclogitization reaction on the rocks rheological behaviour. We show
that eclogitization-induced density change occurring out of equilibrium can, by itself, generates
sufficient shear stress to fail the rocks at high-pressure conditions.
Rupture initiation at depth in continental subduction zones could therefore be explained by
volume changes, even without considering the modifications of the rheological properties induced
by the transformation. Our results also indicate that the negative volume change associated with
brittle failure can enhance the propagation of the eclogitization process by a runaway mechanism
as long as the reaction is not limited by the lack of reactants.
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