“Parade of the Professors” (Most Tuesdays) Purpose: Introduce you to our faculty Classic Papers (Most Thursdays) courses/GEOL5700-9/ (Aug 31: Pratt and Airy: Isostasy) Peter Molnar, Benson 462C

High Places: (Earth contoured at 4000 m intervals) Only Tibet matters (plus a bit of the Andes)

In Summer, Tibet is the hottest place on earth (at 250 millibars)

Tibet’s upper mantle is weird

Two Big Questions 1.How does mantle dynamics affect mountain building? 2.How does high terrain affect climate, and more specifically what role does a change in high terrain play in climate change?

High Places: (Earth contoured at 4000 m intervals) What about the Andes?

History of elevation change in the Bolivian Altiplano [Garzione et al. 2006; Ghosh et al. 2006]

History of deformation across the Andes [Elger, Oncken, and Glodny, 2005]

Altiplano and Central Andes

Late Cenozoic Nazca-South America Convergence

Lessons from the Altiplano 1.The mean elevation of a plateau can increase rapidly: ~ km/Myr (presumably because mantle lithosphere is removed). 2.Tectonic activity migrates to the flanks of the plateau. 3.Even plate motions may change because of the increased force per unit length associated with the higher plateau.

Shortening, thickening, instability, deblobbing, surface uplift, and extension

Proposed Cartoon Histories of Tibetan Crustal Thickening

Simple shear of Tibetan lithosphere and deblobbing of the mantle part

Tibet’s upper mantle is weird

What does convective removal of mantle lithosphere predict? 1.Convective instability implies large lateral heterogeneity in the upper mantle. 2.Increased elevation and replacement of heavy material imply both: an increased propensity for normal faulting within the plateau. and an increased force per unit length applied by Tibet to surroundings. 3.Removal of heavy material implies that the surface should rise isostatically (Archimedes’s Principal), perhaps rapidly, and without crustal thickening.

What does convective removal of mantle lithosphere predict? 1.Convective instability implies large lateral heterogeneity in the upper mantle. 2.Increased elevation and replacement of heavy material imply both: an increased propensity for normal faulting within the plateau. and an increased force per unit length applied by Tibet to surroundings. 3.Removal of heavy material implies that the surface should rise isostatically (Archimedes’s Principal), perhaps rapidly, and without crustal thickening.

Tilman, Ni and others [2003]

What does convective removal of mantle lithosphere predict? 1.Convective instability implies large lateral heterogeneity in the upper mantle. 2.Increased elevation and replacement of heavy material imply both: an increased propensity for normal faulting within the plateau. and an increased force per unit length applied by Tibet to surroundings. 3.Removal of heavy material implies that the surface should rise isostatically (Archimedes’s Principal), perhaps rapidly, and without crustal thickening.

Fault plane solutions of earthquakes (Harvard CMT and a few more)

What does convective removal of mantle lithosphere predict? 1.Convective instability implies large lateral heterogeneity in the upper mantle. 2.Increased elevation and replacement of heavy material imply both: an increased propensity for normal faulting within the plateau. and an increased force per unit length applied by Tibet to surroundings. 3.Removal of heavy material implies that the surface should rise isostatically (Archimedes’s Principal), perhaps rapidly, and without crustal thickening.

Tectonic events near ~8 Ma in and around the Tibetan Plateau

India-Eurasia Reconstructions Horner-Johnson et al. [2005], McQuarrie et al. [2003], Merkouriev & DeMets [2006], Molnar et al. [1988], Royer et al. [2006], J. M. Stock

Dependent mostly on Horner-Johnson et al. [2005], Merkouriev and DeMets [2006], and Royer et al. [2006], plus McQuarrie et al. [2004] Convergence between India and Eurasia since 20 Ma: 30% slowdown in rate near 10 Ma

What does convective removal of mantle lithosphere predict? 1.Convective instability implies large lateral heterogeneity in the upper mantle. 2.Increased elevation and replacement of heavy material imply both: an increased propensity for normal faulting within the plateau. and an increased force per unit length applied by Tibet to surroundings. 3.Removal of heavy material implies that the surface should rise isostatically (Archimedes’s Principal), perhaps rapidly, and without crustal thickening.

Time series of environ- mental changes near Tibet since ~20 Ma

How could a 1-2-kilometer rise of the surface of Tibet have such a great effect on the monsoon? The facts!(?), 1. Tibet rose 1-2 km some time near 8 Ma (perhaps beginning a few Myr earlier). 2. The monsoon strengthened (dramatically?) at ~8 Ma. pose the scientific question:

Heating near the equator, evaporation, and latent heating lead to meridional circulation

Sensible heating over India & Tibet and latent heating above lead to monsoonal circulation

Radiative- Convective Equilibrium  = fraction of surface water available for evaporation

Differences between calculated air temperatures (for radiative- convective equilibrium) above elevated surfaces from those above a surface at sea level

Effect of a 1000 m increase in elevation on mid-troposphere temperatures

Threshold for Meridional Circulation (Steady State, Zonally Symmetric Flow) Plumb and Hou [1992] (building on work of Held and Hou [1980]; Lindzen and Hou [1988]; and Hou and Lindzen [1992]) Key assumptions: Conservation of angular momentum Negligible viscosity of the atmosphere To induce a meridional circulation, the difference in upper tropospheric temperature between that over Tibet and that farther south should be ~ 5ºC.

Tibet (at 250 millibars) is warmer by ~5ºC than the upper troposphere over the equator and in the equatorial southern hemisphere to its south.

Summary 1. India’s penetration into Eurasia built Tibet. 2. Thickened cold mantle lithosphere became unstable and “deblobbed” at ~10 Ma (and aided by eclogite beneath the Altiplano). 3. The surface of the plateau rose 1-2 km (as did that of the Altiplano ~3 km in ~3 Myr). 4. The increased potential energy was expended by horizontal contraction of the surrounding terrain (as in the Andes too). 5. The higher plateau disrupted atmospheric circulation and (among other phenomena) strengthened the monsoon.

Classic Papers: Purposes 1.Introduction to the foundation of the Earth Sciences. 2.See how people posed questions that mattered. 3.See what they did not do (but maybe you were taught that they did do). 4.Get some experience in reading critically.

Pratt and Airy: Advice 1.Read Airy first to understand what both did. 2.Do not study Pratt, skip the math and tables, but understand what he did. 3.What question did he pose, and answer? 4.What did he assume? (How would different assumptions affect his result?) 5.(What did he not do?) 6.What was Airy’s logic?

Compare Force per unit length (F l ) needed to fold Indian lithosphere, F l = 4.4 (± 1.2) x N/m, with F l applied by Tibet to the Indian Plate F l = 8.4 (± 1.5) x N/m Mean Elevation (km)Force/unit length (x N/m) or F l < 1 x N/m, if the entire lithosphere thickened. Removal of mantle lithosphere adds a force per unit length sufficient to fold the Indian lithosphere.