Lithospheric Thinning and Gravitational Instability: Insights from Geological Studies

1. After the fall: Lithospheric structure after thinning via gravitational instability Linda T. Elkins-Tanton Brown University

2. Southern Ural Mountains Montana/Alberta Leucite Hills, Wyoming Central Italy Sierra Nevada Colorado Plateau Dabie Shan Tibetan Plateau Western Mexico Uganda/Tanzania Altiplano-Puna Continental magmatism: Lithospheric thinning? Bird, 1979; Turner et al., 1996; Ducea and Saleeby, 1998; Jones and Phinney, 1998; Manley et al., 2000; Lee et al., 2001; Elkins-Tanton and Grove, 2003; Peccerillo and Manetti, 1985; De Astis et al., 2003; Mattie et al., 1997; Lastowa et al., 2001; Kay and Kay, 1993; Righter 2000; Wannamaker et al. 2000, Van Kooten 1980, Conticelli and Peccerillo 1992, Carmichael et al. 1996; Elkins-Tanton and Hager, 2000; Lloyd et al., 2000; Buhlmann et al., 2000; Carlson and Nowell, 2001; Beard and Glazner,1998

3. Gravitational instabilities • Viscosity low enough to allow flow • Lithosphere denser than surroundings Elkins-Tanton (2006) • (e.g. Houseman et al., 1981; Fleitout and Froidevaux, 1982; Conrad and Molnar, 1997; Schott and Schmeling, 1998; Neil and Houseman, 1999; Schott et al., 2000; Elkins-Tanton and Hager, 2000; Morency et al., 2002)

4. Conclusions • In axisymmetric models, surface topographic response is minimal • The ductile process prevents formation of a dome in the lithosphere-asthenosphere boundary • A neck or cusp in the lithosphere-asthenosphere boundary is long-lived and far more likely to be imaged seismically than is the drip itself

5. Numerical model starting conditions Elkins-Tanton (2006)

6. Dense material sinks rapidly • Some material is frozen in place • Note LAB: flat

7. Surface topographic response: Axisymmetric vs. Cartesian Topographic change is caused by removal of dense material and return to isostatic equilibrium, tempered by viscous forces in the lithosphere

8. Ductile, axisymmetric instabilities produce modest topographic signatures • A low-viscosity zone in the lithosphere increases surface topographic response by one to two orders of magnitude • An annulus of higher topography may be formed briefly in the LAB Dense regions may have aspect ratios not best modeled in axisymmetric codes

9. Most extreme topography: Hot lithosphere, low- layer, hot mantle upwelling

10. Lithosphere-asthenosphere boundary after unstable material has sunk 15.8 Myr 6.8 Myr 7.0 Myr Cusps or necks are long-lived and are much more likely to be imaged than actual drips

11. Ductile gravitational instabilities No dome forms in the lithosphere-asthenosphere boundary A shallow annulus of higher topography can persist around the axis of symmetry for between 1 and 10 Myr. Cusps in lithosphere boundaries may be long-lasting Far more likely to see the neck or the cusp than the drip; gravity may be the test of which appears in a seismic image. The lithosphere-asthenosphere boundary will be flat after dense ductile material sinks away, and that thinning would be spread over a wide region rather than isolated at the point of instability