IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Oral Presentation 2115
Ridge jumps associated with plume-ridge interaction: Weakening of the
lithosphere by upwelling asthenosphere
Mr. Eric Mittelstaedt
Geology and Geophysics University of Hawaii, Manoa IASPEI
Garrett Ito
Interaction of mantle plumes and young lithosphere near mid-ocean ridges can lead to changes in
spreading geometry by shifts of the ridge-axis toward the plume such as seen at Iceland and the
Galapagos. Previous work has shown that, with a sufficient magma flux, magmatism alone may weaken
the plate sufficiently to initiate a ridge jump, but the contribution of upwelling asthenosphere to plate
weakening and ridge-jumps is poorly understood. Using the FLAC (Fast Lagrangian Analysis of
Continua) algorithm, we solve the equations of continuity, momentum and energy to examine
deformation in near-ridge lithosphere associated with relatively hot upwelling asthenosphere and
seafloor spreading. The cold portion of the lithosphere is treated as an elastic-plastic material and
experiences brittle failure while the lower lithosphere and asthenosphere obey a non-Newtonian viscous
rheology. Combined with a freely deformable surface this allows for simulation of gravitational effects
on topography and dynamic faulting. Using a new 2-D dual-grid method, we achieve better than 0.5 km
resolution within the lithosphere over a domain 100 km wide and 50 km deep and 5 km resolution in
the mantle over a domain 1200 km wide and 400 km deep. The upper region of the box is initially set to
a square-root of age thermal profile while a hot patch is placed at the bottom to initiate a single
asthenospheric upwelling. The effect of upwelling asthenosphere on ridge jumps is evaluated by varying
three parameters; the plume excess temperature, the spreading rate and the lateral location of the hot
patch relative to the ridge axis. Preliminary results show plume related thinning (i.e. weakening) of the
lithosphere over a wide area (100s of kms) with the rate of thinning dependent upon the thermal excess
temperature of the plume. Initially thinning occurs as the plume approaches the lithosphere and
asthenospheric material is forced out of the way. As the plume material comes into contact with the
lithosphere, thinning of the boundary layer occurs through thermal weakening and mechanical removal
of material. Thinning of the lithosphere is one of two primary factors in achieving a ridge jump. The
other is high stresses capable of initiating rifting at this weakened location. Model stresses induced by
the buoyant asthenosphere are significant fractions of the lithospheric yield strength near the plume
and reach a maximum at the center of plume upwelling. The stresses decrease with distance from the
plume center and increase with increasing spreading rate. Ridge jumps induced by upwelling
asthenosphere alone are not observed which suggests that additional effects, such as off-axis
magmatism, are important to ridge jump formation.
Keywords: hotspot, rifting, plume ridge