Bruce Luyendyk, Univ. of California, Santa Barbara
Joann Stock, Cal Tech
The plate tectonic setting in the southwest Pacific during Cretaceous time is largely unknown. Prior MG&G mapping and satellite gravity (Sandwell and Smith, 1997) have identified some key features that may hold clues to the plate tectonic history in this region. These include the Large Igneous Provinces (LIP) such as the Manihiki Plateau east of Samoa and the Hikurangi Plateau east of New Zealand and adjacent to and apparently in the paleotrench along the north side of the Chatham Rise. Other features include linear ridges and scarps that are apparent plate boundaries. Our objectives are to map the crustal grain and marine magnetic anomalies in the deep sea surrounding the Manihiki Plateau in order to determine some key constraints for the plate tectonics in this region.
The region between the Hikurangi Plateau and the Manihiki Plateau is one of the most enigmatic sectors of seafloor in the entire Pacific ocean basin. This region lacks identified magnetic anomalies, except for some isolated regions which have been interpreted individually and do not appear to make a coherent story for the region as a whole. Engebretson et al. (1991) reported M22-M29 east of the Tonga trench and SW of the Manihiki Plateau trending ENE. Sharman and Mammerickx (1990) suggested that the eastern boundary of the Manihiki Plateau (the Eastern Scarp; see track chart) was a propagating rift site, and that anomalies M0 to M3 lie immediately east of the Manihiki Plateau, as part of a sequence that is as old as M10N at 25°S. Pontoise et al. (1986) reported E-W magnetic lineations just east of the Tonga Trench at 25.5° S; they hypothesized that these were M-series anomalies, but did not identify them.
The Manihiki Plateau is considered to be a LIP which formed at or near a ridge crest in Cretaceous time, possibly near a triple junction (Winterer et al., 1974). Its three geomorphic plateaus (High, Western, and Northern) are separated by linear depressions thought to be fault troughs. It is anomalously shallow, about 3 times thicker than normal oceanic crust, and heavily sedimented (Hussong et al., 1979). DSDP site 317 on the Plateau reached Aptian sediments overlying basalt (Winterer et al., 1974; Schlanger et al., 1976); the basalt was later dated at 123±1.5 Ma (R. Duncan, in Mahoney et al., 1993).
Joseph et al. (1993) report that the NE edge of the Manihiki Plateau is rifted, with faults trending NW-SE. In their model, the NE and SE sides, including the prominent Eastern Scarp, are believed faulted by a reorganization of the Pacific-Farallon-Phoenix triple junction (PAC-PHN-FAR), and southward jump of the PAC-PHN ridge, at M0 time. The SW side of the Plateau, the Suvarov Scarp, is an en-echelon NW-striking lineament.
The scarce existing data lead to several different tectonic models for Cretaceous tectonics in the region. Lonsdale (1997) proposed that the Manihiki Plateau rifted from the conjugate Hikurangi Plateau in Early Cretaceous time. The ridge that separated these plateaus died at 105 Ma and is now the Osbourn Trough, a linear gravity feature that follows 25°S.
We will determine the crustal grain and therefore tectonic origin of gravity-bathymetry features near the Manihiki Plateau with MG&G surveys. The plan is to survey during May, 1998. This expedition will conduct MG&G work on the south and east sides of the Manihiki Plateau. Systems we will use are the Seabeam 2112, magnetometer, and single channel seismic reflection. We will also bring a gravimeter rented from NAVOCEANO. The identification of M series anomalies by Engebretson et al. (1991) and Sharman and Mammerickx (1990) will be verified and extended during the Revelle cruise. The primary data that guide the present study are features found by satellite gravimetry. However, oceanic crustal grain formed by spreading is critical to tectonic interpretation and is not detected by satellite.
The Revelle will depart from Pago Pago May 9 and end in Honolulu May 29. We will run MG&G lines orthogonal to the Suvarov and Eastern Scarps. Water depths here are 3 to 5.5 kms and the region is thickly sedimented. Our objectives are to determine the crustal grain here with Seabeam, SCS, and gravity, to ascertain ancient spreading directions, to look for M series anomalies parallel to either scarp, to determine the seismic stratigraphy across the scarps to compare with the Rapuhia and Wishbone scarps, and to model the gravity field to study crustal thickness variations.
Our cruise data should result in:
¥ Determination of crustal grain from bathymetry and basement mapping will constrain the plate tectonic origin of the Eastern and Suvarov scarps of the Manihiki Plateau.
¥ Magnetic surveying will determine whether M series anomalies are associated with any of these features and therefore constrain the timing of their formation and tectonic events.
¥ Geophysical profiles across the Suvarov Scarp can test if it is a conjugate rift feature to the Rapuhia-Udintsev trend forming the NE border of the Hikurangi Plateau. Key data are seismic stratigraphic sections and magnetics.
Our data will in combination, determine the type and ages of the plate boundaries in the region near the Manihiki Plateau.
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