Abstract. We analyse deformation bands related to horizontal contraction with an
intermittent period of horizontal extension in Miocene turbidites of the
Whakataki Formation south of Castlepoint, Wairarapa, North Island, New
Zealand. In the Whakataki Formation, three sets of cataclastic deformation
bands are identified: (1) normal-sense compactional shear bands (CSBs), (2) reverse-sense CSBs, and (3) reverse-sense shear-enhanced compaction bands
(SECBs). During extension, CSBs are associated with normal faults. When
propagating through clay-rich interbeds, extensional bands are characterised
by clay smear and grain size reduction. During contraction,
sandstone-dominated sequences host SECBs, and rare CSBs, that are generally
distributed in pervasive patterns. A quantitative spacing analysis shows
that most outcrops are characterised by mixed spatial distributions of
deformation bands, interpreted as a consequence of overprint due to
progressive deformation or distinct multiple generations of deformation
bands from different deformation phases. As many deformation bands are
parallel to adjacent juvenile normal faults and reverse faults, bands are likely
precursors to faults. With progressive deformation, the linkage of
distributed deformation bands across sedimentary beds occurs to form
through-going faults. During this process, bands associated with the wall-,
tip-, and interaction-damage zones overprint earlier distributions resulting
in complex spatial patterns. Regularly spaced bands are pervasively
distributed when far away from faults. Microstructural analysis shows that
all deformation bands form by inelastic pore collapse and grain crushing
with an absolute reduction in porosity relative to the host rock between 5 %
and 14 %. Hence, deformation bands likely act as fluid flow barriers.
Faults and their associated damage zones exhibit a spacing of 9 m on
the scale of 10 km and are more commonly observed in areas
characterised by higher mudstone-to-sandstone ratios. As a result, extensive
clay smear is common in these faults, enhancing the sealing capacity of
faults. Therefore, the formation of deformation bands and faults leads to
progressive flow compartmentalisation from the scale of 9 m down to
about 10 cm – the typical spacing of distributed, regularly spaced
deformation bands.
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