The Other Ductile – Brittle Transition Zone: Syn-Deformational Lithification Within the Shallow Subduction Shear Zone and its Implications for Earthquake Nucleation

Subduction zones are uniquely direct pathways in which originally unconsolidated sediment is conveyed to great depths, all while experiencing continuous shear as it lithifies and metamorphoses. The largest earthquakes on our planet occur within these zones, along with other seismic and aseismic phenomena. The products of these processes are accreted mélanges which provide ‘windows’ into the otherwise inaccessible plate boundary interface at depth. The bulk physical behaviour of these subduction shear zones is controlled by the geometries of the blocks, the proportions of blocks to matrix, and the relative mechanical properties of blocks and matrix. Here we provide a structural and mechanical characterisation of the Chrystalls Beach Mélange, New Zealand, and trace its rheological evolution from the surface to the shallow seismogenic zone. We conducted a detailed 3D macro- and micro-structural investigation coupled with in-situ and laboratory-based rock mechanics to measure sub-block-scale heterogeneities and explain their origins.

The Chrystalls Beach Mélange formed within a Mesozoic Gondwanan–Pacific subduction zone, achieving maximum metamorphic conditions of <600 MPa/<300°C, within the shallow seismogenic zone and below the conditions required for quartz crystal-plasticity. This mélange is composed of subducted seafloor sediments that form decametre- to millimetre-sized blocks of sandstone and chert within a pelitic matrix, mixed with minor exotic blocks of altered basalt. These blocks display overprinting relationships showing a progression from ductile to brittle deformation as they transition from soft sediment to low-grade metamorphosed rock coincident with burial and pervasive shearing.

Four distinct rheological and tectonic regimes were responsible for the structural features we documented:

1) Layer-parallel shortening and fluidisation in the frontal toe of the subduction zone. Unconsolidated interbedded sand, mud, and siliceous ooze experienced ductile deformation producing isoclinal folds and injectites.

2) Layer-parallel extension of poorly consolidated ductile sediments resulted in boudinage and dismemberment in the shallowest subduction channel. This produced blocks with moderate – high aspect ratios, sharp tips, and asymmetric profiles.

3) Continued layer-parallel extension as the blocks lithified and embrittled. Internal stresses transferred from the matrix exceeded the yield stresses of the still-weak blocks, resulting in pervasive brecciation, followed by fragmentation as fluidised matrix injected into these fractures. This produced sub-rounded – sub-angular blocks with low – moderate aspect ratios, blunt tips, and irregular profiles. As blocks continued to indurate to the point that they could no longer be broken by stresses imparted by the matrix, they may still have been broken as they jostled and temporarily jammed the shear zone. At the same time, exotic blocks of basalt entered the mélange as rigid inclusions but underwent progressive weakening during subduction as they experienced brecciation and altered to clay minerals.

4) Localisation of strain previously distributed in the matrix towards more localised shear zones and veins in anastomosing networks.

In-situ Schmidt hammer strength tests show that block margins are systematically weaker than block cores across all lithologies. This is consistent with the increased fracture density towards block margins.  As such, mélange blocks within the shallow seismogenic zone display significant internal heterogeneity and should not be considered as two-phase mixtures.