Seismic characteristics and implications of ultramafic rocks from the Red Hills Massif, northern South Island, New Zealand

SUMMARY Olivine, orthopyroxene and clinopyroxene are the most common minerals in ultramafic rocks, their modal contents and crystallographic preferred orientations (CPOs) are main factors determine the total seismic properties (e.g. seismic velocity and anisotropy). Red Hills Massif is the main part of Dun Mountain Ophiolite Belt, and includes all six most typical olivine CPO types (A-E & AG-type) and various ultramafic rock types. In this case study, 6 paired harzburgite and dunite (Series 1 samples) and olivine/clinopyroxene-related ultramafic rocks (3 dunites, 2 wehrlites, 2 olivine clinopyroxenites and 1 clinopyroxenite) are selected to proceed detailed seismic characteristics analysis. Seismic distributions of Series 1 and 2 peridotites are based on their olivine CPO characteristics while the distributions of Series 2 (olivine) clinopyroxenites are based on their clinopyroxene CPO characteristics. Due to the addition of orthopyroxene, almost all harzburgites have slower P-wave velocity and less P/S-wave anisotropy than dunites in the same pair. On the other hand, both olivine & clinopyroxene CPO combinations and modal contents of Series 2 ultramafic rocks are various. With increasing modal proportions of orthopyroxene or clinopyroxene, seismic properties are not necessarily decreasing. When the [001]OL crystallographic axis in harzburgite develops a girdle fabric, or when the [100]OL and [001]CPX orientations exhibit misalignment with respect to the lineation direction, a significant reduction in seismic wave velocities is observed. Concurrently, seismic anisotropy magnitudes demonstrate marked enhancement under orthopyroxene- or clinopyroxene-dominant conditions (>60 per cent modal abundance). The intricate olivine and ortho-/clinopyroxene CPO patterns and related seismic characteristics preserved beneath the Red Hills Massif are thought to pre-date initiation of the Alpine Fault (∼25 Ma). This interpretation is supported by the similarity in olivine grain size between the Red Hills peridotites and the lithospheric mantle beneath West Otago, implying a shared pre-25 Ma mantle domain. Prior to Alpine Fault offset, these two regions were adjacent, and their lithospheric high-velocity seismic signatures remain correspondingly alike. Once the initiation of Alpine Fault kinematics started, the observed anomalous SKS azimuth proximal to the study area may reflect either: (1) the development of protomylonitic textures analogous to those documented in the West Otago lithospheric mantle, or (2) the preservation of pre-existing deformation fabrics. Conversely, lithospheric fast shear wave splitting directions, when combined with gravity data and rock density constraints, can potentially resolve the dominant olivine CPO type(s), the relative proportions of olivine and ortho-/clinopyroxene, and their combined CPO patterns.