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New Insights Into the Origin and Evolution of the Hikurangi Oceanic Eos,Vol. 85, No. 41,12 October 2004 VOLUME 85 NUMBER 41 12 OCTOBER 2004 EOS,TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 401-416 The greatest vertical height of a single scarp New Insights Into the Origin was -800 m, which permitted sampling of the interior of the plateau. Recovered igneous samples included non- to slightly vesicular and Evolution of the Hikurangi basalts, dolerites, gabbros, and volcaniclastic rocks with tholeiitic and alkali basaltic to Oceanic Plateau trachybasaltic compositions. Interior Guyot-Type Seamounts PAGES 401,408 Zealand, and the southern boundary may The large seamounts in the interior of the have been partially subducted beneath the plateau rise 1000-1650 m above the plateau Oceanic plateaus and continental flood basalts, Chatham Rise [Davy and Wood, 1994] .The floor, representing a late stage in the evolution collectively referred to as large igneous provinces northeast margin of the plateau is adjacent to of the plateau.The most striking feature of all (LIPs), represent the most voluminous volcanic Cretaceous Pacific sea floor located at a depth the seamounts is their guyot-like form, charac­ events on Earth. In contrast to continental LIPs, of -4.5-5.5 km. terized by circular, steep-sided bases and a relatively little is known about the surface and Geophysical investigations [e.g., Wood and relatively flat top (up to 24 km across with < internal structure, range in age and chemical Davy, 1994] suggest that during its formation, 400 m variation in height from the margin to 3 composition, origin, and evolution of oceanic 3-5 million km of volcanic rocks were erupted the center, Figures lb and lc).Volcanic cones plateaus, which occur throughout the worlds on the seafloor.This volcanism may have been (Figures lb and lc) are common on these oceans [e.g., Mahoney and Coffin, 1997]. even more extensive if the Hikurangi Plateau seamounts; volcanic rift systems, extending up One of the major goals of the R/V Sonne SO 168 was once connected to the Manihiki LIP (now to 20 km from the base of the guyots (Figure lb), ZEALANDIA expedition (depart Wellington, 3 located -3000 km to the north), but separated also occur. Pillow lavas and sheet flows were December 2002, return Christchurch, 15 January in the Cretaceous by seafloor spreading at the clearly recognizable through video observation Osbourn Trough (Figure la),a paleo-spreading 2003) was to investigate the Hikurangi oceanic with a TV grab. Rock samples from the seamounts plateau off the east coast of New Zealand. center [Billen and Stock, 2000]. It has also been range from lavas and volcaniclastic breccias, Detailed multi-beam mapping and sampling suggested that the Manihiki Plateau may have to sandstones and conglomerates. The swath carried out on this expedition yielded one of formed as part of a "greater Ontong Java Plateau mapping and recovered samples are consistent the most detailed bathymetric maps and rock event" [Coffin andEldholm, 1993],which may with the seamounts being the bases of former collections of an oceanic LIP to date.The new have covered nearly 1% of the Earth's surface. island volcanoes, with the flat tops having been maps reveal spectacular views of one of these Whether separate events or a single mega-event, formed by erosion of the island to sea level. massive volcanic events on the seafloor, pro­ the volcanism associated with these three The depths of the erosional platforms beneath viding new insights into the geodynamic evo­ oceanic plateaus must have had a dramatic sea level range from 1600 to 3300 m, indicating lution of the Hikurangi Plateau.The investigations impact on the chemistry temperature, and life provide further support that the Hikurangi and in the Pacific Ocean. that the Hikurangi Plateau underwent substantial subsidence after formation of these seamounts. Manihiki LIPs may have once formed a single During the ZEALANDIA expedition, 20 large plateau, covering an area of -900,000 km2 (similar Small cones located on many of the erosional seamounts and a large portion of the north­ platforms must have formed after the former in size to Germany and France combined),but eastern margin of the Hikurangi Plateau were were subsequently rifted apart. island volcanoes subsided below wave base. mapped using a SIMRAD EM 120 multi-beam As is the case with the depth of the platforms, Large igneous provinces are commonly echo-sounding system and sampled by dredging. believed to have formed over relatively short the depth of the seafloor at the base of the This article discusses the most interesting volcanoes (2600-4200 m) also increases sys­ time scales (several million years) through morphological features of the plateau: (1) tematically to the northeast (Figure Id).The massive, mushroom-shaped mantle upwellings the northeastern margin, including the Rapuhia height of the platform above the surrounding associated with the initial stages of mantle Scarp; (2) large, guyot-type seamounts, which seafloor is relatively constant at -1050 m. plumes [Richards et al, 1989] .Alternative occur primarily within the plateau; and (3) The simplest explanation for the similar hypotheses include formation through plume- ridge-type volcanic seamounts, which occur height of the erosional platforms above the ridge interaction [e.g., Mahoney and Spencer, along the northeastern margin of the plateau. 1991],large-scale melting triggered by mete- seafloor (despite systematically increasing water oritic impacts [e.g.,Rogers, 1982],and accu­ depths of the surface of the plateau toward Northeastern Plateau Margin mulation and amalgamation of multiple the northeastern margin) is that the top of the intra-plate volcanic structures into a LIP through The northeastern edge of the Hikurangi Hikurangi Plateau was roughly horizontal when the subduction process [e.g.,Hoernle et a/., 2004]. Plateau rises up to 1000 m above the Cretaceous the former island volcanoes were eroded to sea The triangular-shaped Hikurangi Plateau covers Pacific abyssal plain.The northernmost 150 km level, and that major differential subsidence 350,000 km2 (similar in size to New Zealand) of this margin, known as the Rapuhia Scarp, occurred after all seamounts were eroded to sea and is located -2500-3500 m below sea level forms a steep (up to 35°) linear slope trending level. Assuming an average sediment thickness (Figure la).The northwest margin is being at 135°.The multi-beam mapping showed that of -400 m on the surface of the plateau, as subducted beneath the North Island of New the Rapuhia Scarp does not form a single scarp suggested by seismic data [Wood and Davy, but generally consists of multiple steps or 1994], the surface of the plateau would have BY K.HOERNLE,FHAUFF R.WERNER, AND N. MORTIMER terraces, interpreted to represent step-faulting. been located at a depth of -1500 m at the Eos,Vol. 85, No. 41,12 October 2004 170°E 180° 170°W 160°W IF 10'S <>• \ 20°S v 30°S 30°S • 40'S if a^ - II 50°S (Q) Oistance from Rapuhia Scarp (km) ^-^^ 600 500 400 300 200 100 0 Depth (m) -<i0 1500 edge of erosional A 2000 O. platform E 170'E 180° 170°W 160°W ^ 2500 §• 3000 m 3500 height: 1 -1050 m 4500 g 4000 Fig. 1. Overview map and typical examples of the guyot-like seamounts in the interior of the Hikurangi Plateau, (a) Bathymetric map of the southwestern Pacific showing the locations of ZEALANDIA (New Zealand micro-continent), the Hikurangi and Manihiki large igneous provinces (LIPs), and the Osbourn Trough (paleo-spreading center) halfway between the two plateaus, (b and c) Bathymetric maps display typical guyot-type seamounts (characterized by steep flanks and a flat top) on the Hikurangi Plateau. In Figure lb, a curvilinear constructional volcanic rift system with abundant cones on its top and flanks extends 20 km southward from the main volcanic shield. The base of the seamount in Figure lc measures 20 by 25 km; post-erosional cones rise up to -250 m above the erosional platform, (d) Plot of the water depth of the edges of the erosional platforms and of the bases of the mapped guyot-type seamounts on the Hikurangi Plateau versus their distance from the northwestern plateau margin. The height of the platforms from the seafloor is roughly constant at ~1 km, whereas the base of the seamounts becomes systematically deeper to the north. Original color image appears at back of this volume. time the volcanic island province was eroded are similar to the elevations of the post-erosional ation with extensional faults.The volcanism to sea level. volcanism (cones and lava fields) on the guyots must have occurred contemporaneous to faulting, (up to 1650 m above the seafloor).Therefore, at since the flanks of the ridge-type seamounts Marginal Ridge-Type Seamounts least the latest stages of volcanism on some of are displaced along some of the margin. In the ridge-type volcanic seamounts, similar to conclusion, the swath mapping suggests that The ridge-type seamounts are morphologically the post-erosional volcanism on the guyots, volcanism was related to the formation of the rifted distinct from the guyot-type seamounts and formed after the interior seamounts were eroded northeastern Hikurangi margin, most likely comprise elongated linear features with sharp to sea level and began to subside. The compo­ resulting from the breakup of the paleo tops (Figure 2).These seamounts occur exclu­ sition of all the seamounts is more Si-undersat- Hikurangi-Manihiki Plateau. sively along-that is, within 70 km of-the north­ urated than the plateau lavas, and range from eastern margin of the Hikurangi Plateau. On alkali basalts through mugearites to basanites, Working Hypothesis the northwestern part of the margin, the mapped and tephrites to nephelinites.The post-erosional ridges trend north to north-northwest, and are and ridge-type seamounts generally have the The swath bathymetry and recovered samples sub-parallel to the plateau margin.
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