|Institution:||California State University – Northridge|
|Keywords:||Initiation radius; Dissertations, Academic ??? CSUN|
|Full text PDF:||http://hdl.handle.net/10211.3/171649|
Regional seismic tomography studies in the Pacific ocean and continental western U.S show linear bands of low velocity anomalies that are aligned with absolute plate motion and occur beneath volcanic lineaments located within the interior of plates far from plate boundaries. But their origin and the formation are still unclear. Small-scale convection provides one possible explanation for these lineations but does not predict age progressive seafloor volcanism nor progressive enrichment trends that opposes plate motion. I propose a new hypothesis where viscous fingering instabilities form due to hot and wet mantle plumes which rise and discharge into the upper mantle asthenosphere and displace higher viscosity depleted mantle. Here, I consider a physical fluid model which studies the viscous fingering in a Hele-Shaw cell using low Reynolds number miscible fluids. I perform laboratory fluid experiments scaled to the Earth's mantle, with stationary and moving surface plates that use glucose-water solutions with viscosities (??) from 0.3 to 326 Pas and viscosity ratios (??2/??1) from 3 to 300. I test the effect of several physical properties including the viscosity ratio, absolute viscosity, gamma, plate spacing, density difference and ultra-slow fluid injection rate. Viscous fingers are observed to form for all viscosity ratios I consider and after an initial growth period, exhibit a constant wavelength that depends on several parameters. Fingering wavelength is strongly dependent on plate spacing (and therefore asthenospheric layer thickness) but shows weak dependence on viscosity ratio and injection rate. For the case with a mobile upper plate, I define the flux ratio, gamma as plume flux to plate velocity which varies from 0.005 to 12700 in our experiments and considers the range expected in the Earth (0.0006 ??? 56). My laboratory results indicate that fingers align with plate motion both upstream and downstream and indicate longer wavelengths in the downstream direction. Experiments scaled to the Earth???s upper mantle show fingers form in the presence of surface plate motion for gamma= 0.5 if asthenospheric thickness is less than 386 km. The initiation radius, Ro, where fingers first form increases with increasing plate spacing. Scaling to study of the south Pacific seafloor shows fingers should develop for Ro ??? 350 km from the plume source. This new geodynamic model for viscous fingering in the asthenosphere links off-axis and rising mantle plumes indirectly to mantle return flow to the spreading centers where they contribute to melting, surface volcanism and the growth and formation of new lithosphere. Advisors/Committee Members: Weeraratne, Dayanthie S (advisor), Simila, Gerald W (committee member).