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“Parade of the Professors” (Most Tuesdays)Purpose: Introduce you to our faculty
Classic Papers (Most Thursdays)
http://www.colorado.edu/GeolSci/courses/GEOL5700-9/
(Aug 31: Pratt and Airy: Isostasy)
Peter Molnar, Benson 462C
molnar@colorado.edu
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?
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.
Proposed Cartoon Histories of Tibetan Crustal Thickening
IndiaHimalaya Tarim Basin
?
KunlunTibetan Plateau
?
Intrusion of Indian Crust into Lower Tibetan Crust by an Indian Piston[Goetze, Zhao & Morgan, Zhao & Yuen]
Tibetan PlateauIndiaHimalaya
?
Tarim BasinKunlun
Wholesale Underthrusting of India beneath Tibet[Argand, Powell & Conaghan, Barazangi & Ni]
Southern AsiaIndia(Andeanmargin)
?
Simple shear of Tibetan lithosphere and deblobbing of the mantle part
TibetanPlateauIndia Himalaya Tarim
Basin
??
Kunlun
Sinkingremnant of
oceaniclithosphere
Thickening of Asian crust [Dewey & Burke, England and Houseman]
Convective instability of Asian mantle lithosphere[England, Houseman, & Molnar]
Southern AsiaIndia(Andeanmargin)
?
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.
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
appliedby 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.
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.
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:
Differences between
calculated air temperatures (for radiative-
convective equilibrium)
above elevated surfaces
from those above a
surface at sea level
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.
Summary1. 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: Advice1. 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?
http://www.colorado.edu/GeolSci/courses/GEOL5700-9/
Compare Force per unit length (Fl) needed to fold Indian lithosphere, Fl = 4.4 (± 1.2) x 1012 N/m,
with Fl applied by Tibet to the Indian PlateFl = 8.4 (± 1.5) x 1012 N/m
Mean Elevation (km) Force/unit length (x 1012 N/m)
5.5 8.4
5.0 7.4
4.5 5.5
4.0 4.6
3.5 3.8
or Fl < 1 x 1012 N/m, if the entire lithosphere thickened.
Removal of mantle lithosphere adds a force per unit length sufficient to fold the Indian lithosphere.
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