Abstract
Deep (>similar to 20 km) crustal seismicity is typically associated with cold Precambrian lithosphere. However, previous studies have reported that the depth above which 90% of seismicity occurs (D90) in the Southland region of New Zealand is 30-35 km, even though it is situated in relatively young Mesozoic crust at the eastern edge of the transpressive Pacific-Australian plate boundary. To examine this unusual lower crustal seismicity further, we deployed 19 short period seismometers between October 2022 and 2023 through the Southland Otago Seismic Array (SOSA). Supplemented by the permanent GeoNet network and an ongoing array to the west in Fiordland, SOSA provided a station spacing of similar to 30 km across this 150 km wide region. In total, SOSA recorded 85 events in and around Southland. These events are not uniformly distributed, with an abrupt decrease in seismicity east of Fiordland. We also identified six spatial-temporal clusters of elevated seismicity across eastern Fiordland and Southland. The distribution of SOSA event depths confirms that seismicity extends into Southland's mid-lower crust and possibly into the upper mantle (D90 = 32 km). This D90 estimate is significantly deeper than suggested by event depths to the north in Otago (D90 = 14 km). Using 1D lithospheric strength profiles, we suggest that the embrittlement of Southland's mid-lower crust reflects the emplacement of mafic terranes in this region during Mesozoic subduction at the Gondwana margin. Our study therefore highlights that a mafic lower crust can lead to deep seismicity within a region undergoing transpression.
Key Points:
• We examine microseismicity in Southland, New Zealand, which lies to the east of the Australian-Pacific plate boundary's principal faults
• We identify six spatial-temporal event clusters, which may reflect small non-tectonic stress transfers in a critically stressed crust
• Focal depths extend to 40 km. Using 1D lithospheric strength profiles, we propose these depths reflect Southland's mafic mid-lower crust