Seismic refraction studies in the northern Hauraki Gulf, New Zealand

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Abstract
The seismic velocity structure of sediments and basement rocks underlying the continental shelf between Cape Rodney and the Moko Hinau Islands has been determined by seismic refraction measurements. A penetration of some 5 km was achieved along a 50 km reversed profile using 80 explosions on the sea floor. Jurassic greywacke rocks occur at shallow depths of 100–200 m beneath most of the central part of the profile. These basement rocks are downfaulted by about 2 km to the south-west in the south-western part of the profile; the 13-km-wide fault-angle depression between the coast and the major fault is infilled by a sequence of unconsolidated (?) Quaternary sediments and consolidated, older sediments. The unconsolidated sediments, which are characterised by a seismic velocity of 1.61 km/s, reach a thickness of more than 650 m in the eastern part of the depression and are underlain by a 500–800-m-thick layer of consolidated sediments with a mean velocity of 3.3km/s. The highstanding basement rocks in the central part are dissected by a graben 5 km wide and 250 m deep, infilled with unconsolidated sediments. The volcanic Pleistocene-Pliocene Moko Hinau Islands at the north-eastern end of the profile rest upon a 430-m-thick pedestal of volcanic rocks with a velocity of 3.1–4.0 km/s. The highstanding greywacke rocks in the central part have a rather high velocity of 5.0 km/s which increases to 5.95 km/s at 2.4 km depth. It is likely that the fault-angle depression and the graben are features belonging to the northern extension of the Hauraki Rift.

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