Upper mantle inhomogeneity beneath New Zealand: Surface manifestations

Abstract

Volcanism, heat flow, gravity, morphology, and tectonism in the North Island of New Zealand are examined in the light of seismic evidence for inhomogeneity in the upper mantle. The high‐frequency attenuating/transmitting boundary underlies the junction between volcanic and nonvolcanic provinces, and the now active andesite volcanoes lie above this boundary. Heat flow is higher than normal over the attenuating zone and lower than normal over the transmitting zone. Gravity anomalies are positive over the attenuating zone and negative over the transmitting zone; a density model is proposed in which the density variations are envelopes of seismicity in the upper mantle. The Hikurangi trench curves in a similar manner to the attenuating/transmitting boundary and shoals in a similar manner to the seismicity. Similar data are examined for northern Japan, and there is good agreement between the two areas. The special importance of New Zealand in testing the Oliver‐Isacks model of an active margin is demonstrated.

Gravity interpretation profile across Hikurangi subduction zone using seismic constraints — Hawke's Bay to Hikurangi Trench

“…Although the anomaly coincides, in part, with thick deposits of low density Tertiary/Quaternary age sediments, the nature of the anomaly suggests that it is generally deep-seated in origin and related to the Hikurangi subduction system (Robertson and Reilly, 1958). This interpretation is confirmed by the position of the negative anomaly, which is directly above (coincident with) the projection to the surface of the deep Benoiff seismic zone beneath the North Island (Hatherton, 1969).…”

Seismic wave velocities in the uppermost mantle beneath New Zealand

“…The Hikurangi Trench, which crosses region 10, could separate a region to the north-west in which the uppermost mantle is highly disturbed because of the subduction beneath the North Island from an undisturbed region to the south-east (Hatherton 1970). Because of this uncertainty, region 10 is excluded from the discussion in the next section.…”

“…In 1971 very few Sn phases were observed at KRP, GBZ, AVC, ONE, or CRZ from earthquakes below the east coast of the North Island. Hatherton (1970), in a companion paper to that of Mooney, noted than the andesite volcanism and high heat flow in the North Island occur above the zone.…”

“…Many surface phenomena may originate in the upper mantle. For example, the andesitic volcanism and high heat flow in the North Island, New Zealand, occur above a high-frequency attenuating zone in the upper mantle (Hatherton 1970).…”

Features of the Upper Mantle Around Japan as Inferred From Gravity Anomalies

“…The effect of the heterogeneity on the gravity field has also been investigated (OXBURGH and TURCOTTE, 1970;KARIG, 1971;HATHERTON, 1970;TUYEZOV et al, 1968), though some of these studies are open to question in some respects.…”