HR: 1330h
AN: V12B-0579    [PDF]
TI: Analysis of Gravity in the GLIMPSE study region: evidence for deeper mantle processes in the formation of the Sojourn and Hotu Matua ridge systems.
AU: * Harmon, N
EM: Nicholas_Harmon@brown.edu
AF: Brown University, Department of Geological Sciences, 324 Brook Street, Providence, RI 02912 United States
AU: Forsyth, D W
EM: Donald_Forsyth@brown.edu
AF: Brown University, Department of Geological Sciences, 324 Brook Street, Providence, RI 02912 United States
AU: Scheirer, D S
EM: Daniel_Scheirer@brown.edu
AF: U.S. Geological Survey, Mail Stop 989 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: There are series of intraplate volcanic ridges in the south Pacific that are not the product of hot spot activity. The GLIMPSE (Gravity Lineation Intraplate Melting Petrologic and Seismic Expedition) experiment sought to investigate the formation of these ridges. Using shipboard gravity measurements, we have calculated estimates of effective elastic plate thickness and volcanic edifice density forof the major bathymetric featuresregions in the GLIMPSE study area using a simple elastic flexure model with a grid search method over reasonable values of elastic thickness and load density. The grid search inversion revealed effective elastic thickness that ranged from $\sim$ 2 km beneath the Sojourn Ridge to a maximum of $\sim$ 7 km beneath the Southern Cross Seamount, with the average range of estimates of 2-4 km. The elastic thickness estimates are lower than the thickness predicted for 3-6 Ma seafloor and might be due to the flexural character of the volcanic edifices being "frozen in" after formation near the East Pacific Rise. The seamount densities averaged 2500 kg/m$^3$. Notably, the Matua and Southern Cross seamounts produced anomalously low estimates for load densities, approximately 2050 and 2250 kg/m$^3$ respectively, which could reflect an actual low-density seamount derived from an enricheddepleted mantle source with high volatile content or subsurface loading of lower density mantle material, perhaps underplated crust or a deep magma chamber. Using satellite free air gravity and shipboard bathymetry, we calculated the residual mantle Bouguer Anomaly (RMBA) for the entire GLIMPSE study region. The RMBA reveals a negative gravity anomaliesy ($\sim$ 40 mGals) in the vicinity of regions beneath the volcanic ridges indicating uncompensated downward flexure in the surrounding moats and crustal thickening beneath the ridges and seamounts themselves. After removing an optimal estimate of this flexural component, significant gravity lineations remain that are oriented approximately perpendicular to the East Pacific Rise in the absolute plate motion direction and which extend to seafloor less than 2 Ma old. These residual gravity anomalies may be caused by compositional or thermal anomalies in the underlying mantle at wavelengths that are too short to flex the surface of the plate, thus creating gravity anomalies but no obvious, associated bathymetric anomaly. that there is a significant flexural component to the gravity signature of the study area. An inversion was performed to determine the extent of the flexural component in the MBA. The inversion accounted for a majority of the MBA; however, a clear negative anomaly remained suggesting that more than simple flexure is required to produce the observed MBA. The residual gravity may be due to other processes such as thermally or compositionally low-density mantle underlying the volcanic edifices or subsurface loading of the plate.
DE: 3010 Gravity
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting