Evidence for mixed-mode failure at shallow crustal levels, Hungaroa Fault Zone, Hikurangi Subduction Margin, New Zealand

Faults and folds exposed along the Hikurangi Subduction Margin (HSM) reflect the onset of subduction in the early Miocene (c. 25 Ma). One such structure, the inactive Hungaroa Fault Zone on the Wairarapa Coast, accommodated large displacements (c. 4-10 km) as a thrust fault developed in Paleogene hemipelagic sediments of the Wanstead Formation. Spectacularly exposed in an uplifted shore platform, the Hungaroa Fault Zone is a mélange comprising deformed calcareous mudstones and marls (27-58% phyllosilicates; 7-56% calcite). The mélange experienced up to four stages of deformation, possibly as part of a continuum evolving towards higher strain: (1) tight to isoclinal, asymmetric to overturned folding of bedding; (2) transposition of the folds along a spaced tectonic foliation, S (or P), that is locally cut by C (or Y) shear bands within a high-strain zone c. 40 m-wide; (3) cross-cutting of the S fabric by C' (or R1) shears; and (4) cross-cutting of these fabrics by discrete brittle faults that are sub-parallel to the shear zone boundaries. Finally, rotation and overturning of the inactive HFZ took place in the younger regional Coastal Anticline. Within the high-strain zone, marls form lozenges embedded in the foliated, calcareous mudstone matrix. The lozenges and matrix contain the following microstructural evidence for fracture, creep, and dissolution-precipitation processes: discrete faults, antitaxial veins containing stretched calcite fibers, shear veins with calcite slickenfibers, calcite foliation-boudinage structures, calcite pressure fringes, dark dissolution seams, stylolites, embayed calcite grains, and an anastomosing (scaly) phyllosilicate foliation. Raman spectroscopy of carbonaceous material (level 1), calcite twin thermometry (twin type 1), and the ubiquitous presence of smectite (montmorillonite) indicate that deformation took place at shallow crustal conditions (