HR: 1340h
AN: T33C-0582    [Abstracts]
TI: The Role Of Mantle Flow At The North Fiji Basin: Clues From Anomalous Surface Topography
AU: * Zhang, N
EM: zn@geology.utoronto.ca
AF: Dept. of Geology, Univ. of Toronto, 22 Russell St., Toronto, Ont M5S 3B1 Canada
AU: Pysklywec, R
EM: russ@geology.utoronto.ca
AF: Dept. of Geology, Univ. of Toronto, 22 Russell St., Toronto, Ont M5S 3B1 Canada
AB: The North Fiji Basin (NFB) is a complex back-arc system situated between the opposite-facing underlying Tonga and New Hebrides subduction zones. A unique configuration of ridges, which includes a triple-juction shape of three ridges and a high intersecting angle between the Hazel Holme ridge and the New Hebrides trench, distinguishes the NFB with other back-arc basins (e.g. Japan Sea Basin, Bransfield Basin) which are characterized by approximately parallel trench and back-arc ridges. Lagabrielle et al. (1997) proposed that a buoyant hot upwelling upper mantle beneath the back-arc basin might partially account for the unique configuration of the NFB. Another notable feature of the NFB is its topography. The back-arc region is anomalously high and our calculations of residual topography reveal that it reaches up to 2000 m above what should be isostatically supported. To account for the anomalous topography, our study aims to test whether a hot buoyant upwelling is required beneath the NFB to support the high elevation. The mantle thermal structure beneath the NFB is inferred from the global seismic tomographic model S20RTS, where seismic velocity anomalies are converted to density/temperature anomalies. This thermal structure is input into a 3D mantle convection code, CitcomS, and the flow-induced surface topography is calculated. The predicted dynamic topography is compared with the observed residual topography. The primary focus is to determine what size and magnitude of the buoyant mantle structure can support the anomalous topography. Specifically, we conduct a series of experiments where we eliminate the slow velocity anomalies interpreted as hot regions at different depths in the mantle. This considers an alternative interpretation of the slow seismic anomalies, for example that they may represent chemical heterogeneity in the shallow upper mantle. The experiments suggest that the anomalous topography in the NFB requires the support of hot upper mantle upwelling and the hot anomaly at the top 90 km is a critical component for inducing the observed high residual topography. We also test the predicted topography changes with different imposed lithosphere structures. In a post-processing procedure, we consider the surface response of lithosphere with varying thickness and strength and insert weak breaks in the overlying lithospheric cap to model the configuration of the ridge-fragmented NFB. The experiments demonstrate that the characteristics of the assumed lithosphere will strongly condition the topographic response.
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8199 General or miscellaneous
SC: Tectonophysics [T]
MN: Fall Meeting 2005