1. GEO 5/6690 Geodynamics 12 Nov 2014 © A.R. Lowry 2014 Read for Fri 14 Nov: T&S 226-241 Last Time: Te and Rheology Key point of Willett et al. papers: T e represents a dynamical equilibrium maintained by flow stress (not a static equilibrium) so T e changes (decreases) as stresses are relaxed over time. Rheology (& hence T e ) is sensitive to temperature, rock lithology, water, & strain rate. Ideally, we can measure some of these with geophysics and infer the rest from T e ! Deep temperature from seismic velocity is subject to errors from unknown lithology, attenuation, and depth… but Pn is less perturbed by these than other commonly-used seismic measurements Pn temperature estimates predict much smaller Te variations than we observe… unless we allow variations in quartz & especially water in addition to temperature.

2. Next Journal Article Reading: For Monday Nov 17: Karow & Hampel (2010) Slip rate variations on faults in the Basin-and-Range province caused by regression of Late Pleistocene Lake Bonneville and Lake Lahontan, Int. J. Earth Sci. 99 1941-1953.

3. Anything left in the T e residual probably relates to water! Wet (saturated) end-memberDry end-member

4. So, water appears to be the factor that determines whether U.S. continental lithosphere is stable or deforming!

5. Candidates for control of slab dip include: Slab buoyancy Velocity of upper plate Viscosity  or aperture  of asthenospheric wedge A (Fun) Geodynamical Application: Flat Slab Subduction

6. A (Fun) Geodynamical Application: Flat Slab Subduction Most modelers emphasize buoyancy of the down-going slab and/or velocity of the over-riding upper plate as controls on subduction geometry But these are poorly correlated with slab dip in South America

7. South American T e estimates exhibit correlation of flat slab subduction with high T e near the trench! (Pérez- Gussinyé et al., G 3, 2008)

8. Oceanic lithosphere subducts beneath thin continental lithosphere, the underlying low viscosity asthenosphere allows the slab to detach from the continent’s base and sink into the mantle at normal angles Evolutionary Model

9. When oceanic lithosphere subducts near thick continental lithosphere, the asthenospheric wedge narrows. Wedge flow would entrain mantle from beneath the thick and cold lithosphere. The combined effect of higher wedge viscosity and progressive narrowing of the asthenospheric wedge leads to increase in suction forces Evolutionary Model

10. Either rapid trenchward motion of the continent or high levels of subduction-erosion at the front brings the thick continental lithosphere close enough to the trench for flat subduction ensue. When flat subduction is achieved, the asthenospheric wedge disappears and volcanism ceases. Evolutionary Model i

11. Weakening of the continental lithosphere leads to failure, allowing the slab to decouple from the continental lithosphere and sink into the asthenosphere, and enabling flow of hot, low viscosity asthenosphere back into the space between the slab and the overriding plate. Evolutionary Model

12. This interpretation appears to be corroborated by numerical modeling with thick vs thin upper plate (Patel et al., Eos- FAGU 2008)

13. Manea, Pérez-Gussinyé & Manea (Geology 2011) also did numerical models & found flat slab is favored when lithosphere is thick, distance to thick lithosphere is 600-900 km, and continental lithosphere is over-riding the trench (“trench roll-back”)

14. BUT… These models all simplify slab rheology & assume a relatively low- viscosity (10 21 Pa s) slab… Ask Ravi what happens with realistic rheology!