HR: 0800h
AN: V21B-0603    [Abstracts]
TI: Effects of Triple Junction Plate Boundary Geometry on Mantle Thermal Structure and Crustal Production
AU: * Georgen, J E
EM: georgen@gly.fsu.edu
AF: Florida State University, Department of Geological Sciences, Tallahassee, FL 32306, United States
AB: Plate boundary geometry likely has an important influence on crustal production at mid-ocean ridges. Many studies have explored the effects of geometrical features such as transform offsets and oblique ridge segments on mantle flow and melting. This investigation calculates how triple junction geometry may influence the generation of oceanic crust. An earlier study (Georgen and Lin 2002) suggested that the effects of a ridge-ridge- ridge configuration are most pronounced under the branch with the slowest spreading rate. Thus, we create a three-dimensional, finite element, variable viscosity numerical model that incorporates thermal buoyancy and focuses on the slowest-diverging ridge. First, we employ a model geometry similar to the Southwest Indian Ridge near the Rodrigues Triple Junction in the central Indian Ocean. Within 100 km of the triple junction, axial temperatures at depths within the partial melting zone are predicted to increase by ~40 deg. C, and crustal thickness is calculated to increase by 1 km. We also explore how varying spreading rate magnitude affects triple junction dynamics. Consistent with Georgen and Lin (2002), variable-viscosity flow models for calculations where all spreading rates are greater than the Rodrigues-like case (i.e., with plate kinematics similar to those observed around the Galapagos Triple Junction) predict little influence of the triple junction geometry on the thermal structure of the slowest-spreading ridge. However, when ridge divergence rates are all relatively slow (i.e., with plate kinematics similar to those observed around the Azores Triple Junction), significant along-axis increases in mantle temperature and crustal thickness are calculated. At depths within the partial melting zone, temperatures are predicted to increase by ~150 deg. C, similar to the excess temperatures associated with mantle plumes. Likewise, crustal thickness is calculated to increase by approximately 6.5 km over the 200 km of ridge closest to the triple junction. These results could imply that some component of the excess volcanism observed in geologic settings such as the Terceira Rift may be attributed to the effects of triple junction geometry, although the important influence of features like nearby hotspots (e.g., the Azores hotspot) cannot be evaluated without additional numerical modeling.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
DE: 3038 Oceanic plateaus and microcontinents
DE: 8150 Plate boundary: general (3040)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting