Download MolnarAugust29

Peter Molnar, Benson 462C
[email protected]
“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)
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
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
India
(Andean
margin)
Southern Asia
?
Wholesale Underthrusting of India beneath Tibet
[ Argand, Powell & Conaghan, Barazangi & Ni]
India Himalaya
Tibetan Plateau
Kunlun
Tarim Basin
?
Intrusion of Indian Crust into Lower Tibetan Crust by an Indian Piston
[ Goetze, Zhao & Morgan, Zhao & Yuen]
India Himalaya
?
Tibetan Plateau
Kunlun
?
Tarim Basin
Simple shear of Tibetan lithosphere and deblobbing
of the mantle part
India
(Andean
margin)
Southern Asia
?
Thickening of Asian crust [Dewey & Burke, England and Houseman]
India
Tibetan
Plateau
Himalaya
?
Sinking
remnant of
oceanic
lithosphere
Kunlun
Tarim
Basin
?
Convective instability of Asian mantle lithosphere
[England, Houseman, & Molnar]
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.
Tomogram
is here
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
Convergence
between
India and
Eurasia
since
20 Ma:
30%
slowdown in
rate
near 10 Ma
Dependent mostly
on Horner-Johnson
et al. [2005],
Merkouriev and
DeMets [2006], and
Royer et al. [2006],
plus McQuarrie et
al. [2004]
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
environmental
changes
near
Tibet
since
~20 Ma
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:
How could a 1-2-kilometer rise of
the surface of Tibet have such a
great effect on the monsoon?
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
RadiativeConvective
Equilibrium
 = fraction of
surface water
available for
evaporation
Differences
between
calculated air
temperatures
(for radiativeconvective
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?
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 Plate
Fl = 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.