, Clarke, A.P., Morgan, J.P., Ougier-Simonin, A 2018
The impact of shallow rheological transition along the subduction plate interface on tremor and earthquake nucleation
Abstract
Block-in-matrix rock units typically occur along subduction plate interfaces where accommodation of deformation occurs both by viscous and elastic strain. Usually they are visualised to consist of mechanically strong blocks the within a relatively weak matrix. Triaxial deformation experiments on blocks and matrix from the Osa Mélange in southern Costa Rica reveal that this block-in-matrix assemblage is characterised by blocks which are mechanically weaker than their surrounding matrix.
Triaxial deformation experiments were conducted on samples of both the altered basalt blocks and the indurated volcanoclastic matrix: these revealed that at 60 MPa of confining pressure the volcanoclastic matrix's strength is 7.5 times higher than that of the altered basalt, and 4 times higher at 120 MPa, with the altered basalt experiencing multi-stage failure. Analysis of blocks and matrix in the Osa Mélange reveal that the basalt blocks have been weakened by mechanical brecciation and hydrothermal alteration while their surrounding volcanoclastic matrix has been significantly indurated by compaction, diagenesis, and the development of mechanically strong lenticular fabric. As the Osa Mélange was frontally accreted and not subducted to significant depth, this inversion of the rheological relationship must occur early in the subduction process.
With the rheological relationship inverted, our data show that low block to matrix ratios do not create conditions favourable to aseismic creep, as typically predicted, and high block to matrix ratios promote brittle failure regardless of the rheological relationships. Failure in one block promotes failure in its neighbouring blocks and in the surrounding matrix, which primes the subduction interface for a larger, through-going rupture. Failure of these blocks may be detectable as micro-seismic events analogous to tremors, and would precede the larger seismic rupture when the matrix fails.