No abstract

The hotspot-generated Louisville Ridge is a 4000-km chain of seamounts (typically 2-2.5 km high and 10-40 km in diameter) and an underlying crustal swell (1.5 km high and 100+ km wide) trending NNW across the southwestern Pacific. The northwest end of the Ridge collides with the north trending Tonga Trench (26øS) which, just north of that point, is exceptionally deep (10.8 km) and lacks both a turbidite wedge and a bordering accretionary complex. The collision zone is moving rapidly southward. Multichannel seismic reflection data in the collision zoneshow a west dipping subsurface platform 2-3 km beneath the lower western trench slope, which is interpreted as the flat summit of a subducted guyot, Motuku, of the Louisville chain. Projected eastward, the summit plain passes 1-2 km above the trench floor. Dredging of the nearby inner trench wall recovered uppermost Cretaceous (Maestdchtian) oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted Louisville seamount. The principal effects of hotspot-ridge collision with a sediment-starved trench are (1) the impacting seamounts are subducted rather than accreted, and (2) although some seamount rocks are temporarily accreted, the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km/m.y. Accelerated tectonic erosion is related to (1) fracturing, shearing and general weakening of arc substrate rocks as they are lifted by the swell, penetrated by impacting seamounts, and left to collapse as the ridge moves away, (2) a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell,(3) a possible elevation of the subduction decollement to account for the removal of as much as 30,000 km 3 of material from a 400 km sector of the trench in 1 million years, and (4) a reduction in supply of arc-derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge. "Normal" tectonic erosion in the Tonga Trench is apparently minor, and we conclude that the bulk of the -37,000 km 3 of material which fills subducting grabens each million years is arc-derived volcanic and pelagic sediment. Dupont, J., Morphologie et structures superficielles de l'arc insulaire des Tonga-Kermadec, in Contribution a l'EtudeGeodynamique du Sud-Ouest Pacifique, vol. 147, pp. 263-282, Office de la Recherche Scientifique et Technique Outre-Mer, Paris, 1982. Dupont, J., and R.H. Herzer, Effect of subduction of the Louisville Ridge on the structure and morphology of the Tonga Origins of nonvolcanic seamounts in a forearc environment, in Seamounts, Islands and 93, 3078-3104, 1988. McCann, W.R., and R.E. Haberman, Morphologic and geologic effects of the subduction of bathymetric highs, Pure Appl. Geophys., in press, 1989. McCarthy, J., and D.W. Scholl, Mechanisms of subduction accretion along the central Aleutian Trench, Geol. Soc. Leg 91 Scientific Party, Tectonic evolution of the southwestern tropical Pacific basin (abstract), Eos Trans AGU, 64, 315, 1983. of Leg 91 basalts and...

No abstract

The name Northland Allochthon is proposed for the thick and

Understanding the temporal and spatial development of the early Miocene Northland Volcanic Arc is critical to interpreting the patterns of volcanic activity in northern New Zealand through the late Cenozoic. The northwesterly trending arc is considered to have developed above a southwest-dipping subduction system. The distribution of its constituent eruptive centres is described in terms of an eastern belt that extends along the eastern side of Northland and a complementary broad western belt which includes subaerial and submarine volcanic edifices. Critical examination of all 216 K-Ar ages available, including 180 previously unpublished ages, and their G00012 assessment against tectonic, lithostratigraphic, seismic stratigraphic, and biostratigraphic constraints, leads us to deduce a detailed chronology of periods of activity for the various early (and middle) Miocene arc-type volcanic complexes and centres of northern New Zealand: Waipoua Shield Volcano Complex (19-18 Ma, Altonian); Kaipara Volcanic Complex (23-16 Ma, Waitakian-Altonian); Manukau Volcanic Complex (c. 23-15.5 Ma, WaitakianClifdenian); North Cape Volcanic Centre (23-18 Ma, Waitakian-Altonian); Whangaroa Volcanic Complex (22.5-17.5 Ma, Waitakian-Altonian); Taurikura Volcanic Complex (22-15.5 Ma, Otaian-Clifdenian); Parahaki Dacites (22.5-18 Ma, Waitakian-Altonian); Kuaotunu Volcanic Complex (18.5-11 Ma, Altonian-Waiauan). In general, volcanic activity does not show geographic migration with time, and the western (25-15.5 Ma) and eastern (23-11 Ma) belts appear to have developed concurrently.