Strong Matrix & Weak Blocks: Evolutionary Inversion of Mélange Rheological Relationships During Subduction and Its Implications for Seismogenesis

Abstract

Subduction zone interface layers are often conceived to be heterogeneous, polyrheological zones analogous to exhumed mélanges. Mélanges typically contain mechanically strong blocks within a weaker matrix. However, our geomechanical study of the Osa Mélange, SW Costa Rica shows that this mélange contains blocks of altered basalt which are now weaker in friction than their surrounding indurated volcanoclastic matrix.

Triaxial deformation experiments were conducted on samples of both the altered basalt blocks and the indurated volcanoclastic matrix at confining pressures of 60 and 120 MPa. These revealed that the volcanoclastic matrix has a strength 7.5 times that of the altered basalt at 60 MPa and 4 times at 120 MPa, with the altered basalt experiencing multi-stage failure.

The inverted strength relationship between weaker blocks and stronger matrix evolved during subduction and diagenesis of the melange unit by dewatering, compaction and diagenesis of the matrix and cataclastic brecciation and hydrothermal alteration of the basalt blocks. During the evolution of this material, the matrix progressively indurated until its plastic yield stress became greater than the brittle yield stress of the blocks. At this point, the typical rheological relationship found within melanges inverts and melange blocks can fail seismically as the weakest links along the subduction plate interface.

The Osa Melange is currently in the forearc of the erosive Middle America Trench and is being incorporated into the subduction zone interface at the updip limit of seismogenesis. The presence of altered basalt blocks acting as weak inclusions within this rock unit weakens the mélange as a whole rock mass. Seismic fractures can nucleate at or within these weak inclusions and the size of the block may limit the size of initial microseismic rock failure. However, when fractures are able to bridge across the matrix between blocks, significantly larger rupture areas may be possible.

While this mechanism is a promising candidate for the updip limit of the unusually shallow seismogenic zone beneath Osa, it remains to be seen whether analogous evolutionary strength-inversions control the updip limit of other subduction seismogenic zones.