In a recent paper in this journal, Forte et al. [1993] have

proposed a model of global dynamic topography with an

amplitude of three kilometers (six kilometers peak to peak).

If North America, Australia, and other continents were

dynamically depressed by the one to two kilometers

predicted, then these continents would today be below sea level and nearly covered by epeiric seas. Since the continents are nearly entirely exposed and have become progressively exposed since the Late Mesozoic the amplitude of the Forte et al., model is clearly much too large

Bond

Cazenave

Forte

Gurnis

Hager

Hallam

Harrison

Kuenen

McDonough

Michael Gurnis

Sahagian

Schuchert

Wise

Geophysical Research Letters

Gurnis, Michael

Caltech Authors - Main

GEOPHYSICAL RESEARCH LETTERS, VOL. 20, NO. 15, PAGES 1663-1664, AUGUST 6, 1993 
COMMENT ON "DYNAMIC SURFACE TOPOGRAPHY: A NEW INTERPRETATION 
BASED UPON MANTLE FLOW MODELS DERIVED FROM SEISMIC TOMOGRAPHY" 
BY A.M. FORTE, W. R. PELTIER, A.M. DZIEWONSKI AND R. L. WOODWARD 
Michael Gurnis 
Department of Geological Sciences, The University of Michigan 
In a recent paper in this journal, Forte '-ill!. [1993] have 
proposed a model of global dynamic topography with an 
amplitude of three kilometers (six kilometers peak to peak). 
If North America, Australia, and other continents were 
dynamically depressed by the one to two kilometers 
predicted, then these continents would today be below sea-
level and nearly covered by epeiric seas. Since the continents 
are nearly entirely exposed and have become progressively 
exposed since the Late Mesozoic the amplitude of the Forte 
~ ID.. model is clearly much too large. 
Forte et al have predicted dynamic topography of the 
Earth's surface up to and including spherical harmonic degree 
eight in two ways. First, they removed a modeled isostatic 
component of topography from observed topography which 
they termed "dynamic topography". Second, they have 
computed topography on the surface of a viscous mantle with 
driving loads constrained via a tomographic inversion of 
mantle seismic heterogeneity. In their isostatic topography 
model, they have assumed a single crustal column for all 
continental crust and a single column for all oceanic crust; 
these two types of isostatically balanced columns are 
arranged on the Earth's surface using the continent-ocean 
function. Forte .e_t a I. assume there are no crustal thickness 
variations within continents, not necessarily a bad assumption 
at long wavelengths, and that a single global parameter 
characterizes the offset between all continental and all 
oceanic crust. Both predictions of dynamic topography are 
close in terms of pattern and amplitude. 
The dynamic topography presented by Forte, et 1\l. has 
nearly a three kilometer amplitude and all continents are 
located in dynamic topography depressions: North America 
and Australia are in the deepest depressions each of about 
two kilometers maximum and Africa is depressed by up to 
one kilometer. I will show that North America, Australia, and 
Africa cannot possibly have such extremely depressed 
dynamic topographies. First, these continents, like all 
continents, have been repeatedly inundated by shallow seas 
over the Phanerozoic and this inundation can be 
accomplished by global and regional sea-level changes of a 
few hundred meters -- certainly less than one kilometer. 
Second, if the continents were dynamically depressed by two 
kilometers, most of their area would be covered by shallow 
seas. 
Let me review some of the basics of Phanerozoic sea-level 
change and what they imply for Earth dynamics. In major 
platform areas, like North America or Australia, flat lying 
marine rocks of constant age can cover up to about fifty 
percent of the surface and/or sub-surface area of platforms 
Copyright 1993 by the American Geophysical Union. 
Paper Number 93GL01489 
0094-8534/93/93GL-O 1489$03.00 
[Schuchert, 1916]. It has been known since the first few 
decades of this century that sea-level rises and falls of about 
100 meters could explain these observed marine deposits 
[Kuenen, 1939; Hallam, 1992]. Wise [1974] argued that the 
surface of the Earth is in a long-term quasi-equilibrium in 
which freeboard remains approximately constant with the 
oceans periodically flooding the continents. Significantly, 
Phanerozoic transgressions and regressions can be 
accomplished by eustatic and regional sea-level changes and 
although the magnitude of these fluctuations are debated, it 
does not seem that estimates fall below 100 meters [Bond, 
1978] or exceed 700 meters [Hallam, 1984]. Many climatic 
and geologic factors effect sea level, including epeirogeny, 
which has long been known to be an important regional 
factor [eg. Bond, 1978]. Fluctuations in dynamic topography 
are probably a major control on epeirogeny and regional sea-
level change [Gurnis, 1992]. 
Gurnis [1990] showed how continents would become 
flooded and exposed as they moved with respect to a large-
scale, larger than continental scale, pattern of dynamic 
topography -- much like the pattern now predicted by Forte ~ 
1\l. Flooding is dependent on the ratio of geoid to dynamic 
topography, or admittance, but when admittance is as small 
as predicted in the Forte ~ al. model, - 0.03, then dynamic 
topography is by far the most important control on marine 
inundation. When the admittance is small and the continents 
are located in dynamic topography lows, as they are in the 
Forte tl al. predictions, then inundation reaches its 
maximum. Since we know from the stratigraphic record that 
sea-level has oscillated by- 200 meter around a mean, then 
when continents are in dynamic topography lows, continents 
must experience a sea-level rise proportional to the dynamic 
topography. Clearly, the relative sea-level rise would be at 
least one kilometer for North America and Australia in the 
Forte ~ .a...l. model. The actual rise would probably be 
significantly higher since about half of the ocean basins are 
on dynamic topography highs of at least 500 meters. The 
estimate of relative sea level rise of one kilometer (Figure 1) 
is an approximate minimum. The intersection of a relative 
sea-level rise with present day hypsometry (Figure 1) gives 
the fraction of a continent which would be covered by ocean 
water. From the Forte ~ .a...l. predictions we expect N. 
America should be flooded by about 80 percent and Australia 
by about 98 percent. Since North America and Australia, like 
nearly all continents, are mostly exposed, the amplitude of 
dynamic topography and the positioning of continents in lows 
predicted by Forte ~ .!ll. is at odds with these simple 
observations. 
Furthermore, contemporaneous positioning of the 
continents in dynamic topography lows of large amplitude, as 
proposed by Forte ~ 1\l., is at odds with the withdrawal of 
shallow seas from the continents since the end of the 
Mesozoic. Much of this withdrawal of seas from platforms 
was probably eustatic but with some important, regional or 
1663 
1664 Gurnis: Comment 
'§' 
Q) -Q) 
E -c: 
0 
:a 
a; 
w 
2500 
1500 
500 
-500 
-0.4 0 0.4 0.8 1.2 
Fraction of Area 
Fig. 1. Hypsometric curves for North America, NA, and 
Australia, Au, from Harrison ~ill. [1983]. The area has been 
normalized by area presently above sea-level. Shown by light 
shadding overlying hypsometries are the fractional areas 
covered by Cretaceous and younger marine deposits. N. 
American flooding is from Wise [1974] and Australian 
flooding is from Struckmeyer and Brown [1990]. 
epeirogenic motions [Bond, 1978]. In particular, a 93 Ma 
shoreline deposit in Minnesota, USA, shows that sea-level 
has fallen 280m [McDonough and Cross, 1991] but Forte~ 
ill. predict that this mid continental area is dynamically 
depressed by -2 km. The results for Africa are even more 
extreme: Mid-Cretaceous shoreline deposits have been 
elevated to one kilometer, after sediment unloading, above 
present sea-levels [Sahagian, 1988] and this demonstrates 
that Africa has been actively uplifting in addition to the 
general eustatic sea-level fall since the Cretaceous [Bond, 
1978]. If Africa is clearly being uplifted, how can it be in a 
500 m to one kilometer dynamic topography low as predicted 
by Forte ~ ill. ? 
Despite these fundamental problems with the Forte et ill. 
models, other predictions of dynamic topography are more 
consistent with geologic observations. The tomography-
constrained viscous flow model of Hager, tl al. [1985] 
predicts a dynamic topography amplitude of about one 
kilometer, but, first, most continents only straddle dynamic 
topography highs and lows and, second, Africa is on a 
dominant 600 meter dynamic topography high. The former is 
not inconsistent with stratigraphic observations, but the latter 
is very much consistent with geologic observations of the 
Cenozoic uplift of Africa. Significantly, the pattern of 
dynamic topography predicted by Hager~ al. [1985] is in 
phase with the long wavelength residual oceanic topography 
obtained by subtracting the cooling of a simple half-space 
from observed bathymetry [Cazenave and Lago, 1991; 
Pribac, 1991], although there is a factor of two difference 
between them. The low amplitude, - 300 meter, residual 
topography results of Cazenave and Lago [1991] and Pribac 
[1991] imply long wavelength dynamic topographies which 
are most consistent with geologic observations [Gurnis, 
1990]. 
Acknowledgments. Funded by a NSF Presidential Young 
Investigator Award and a Fellowship from the David and 
Lucile Packard Foundation. 
References 
Bond, G., Speculations on real sea-level changes and vertical 
motions of continents at selected times in the Cretaceous 
and Tertiary Periods, Geology, Q, 247-250, 1978. 
Cazenave, A., and Lago, B., Long wavelength topography, 
seafloor subsidence and flattening, Geophys. Res. Lett., 
18, 1257-1260, 1991. 
Forte, A. M., Peltier, W. R., Dzeiwonski, A. M., and 
Woodword, R. L., Dynamic surface topography: A new 
interpretation based upon mantle flow models derived 
from seismic tomography, Geophys. Res. Lett., 20, 225-
228, 1993. 
Gurnis, M., Bounds on global dynamic topography from 
Phanerozoic flooding of continental platforms, Nature, 
.341. 754-756, 1990. 
Gurnis, M., Long-term controls on eustatic and epeirogenic 
motions by mantle convection, GSA Today, 2.. 141-157, 
1992. 
Hager, B. H., Clayton, R. W., Richards, M. A., Comer, R. P., 
and Dziewonski, A. M., Lower mantle heterogeneity, 
dynamic topography and the Geoid, Nature, ill. 541-545, 
1985. 
Hallam, A., Pre-Quaternary sea-level changes, Ann. Rev. 
El!nb.~. ,Sg., 12,205-243, 1984. 
Hallam, A., Phanerozoic Sea-Level Changes, Columbia 
University Press, New York, 266p., 1992. 
Harrison, C. G. A., Miskell, K. 1., Brass, G. W., Saltzman, E. 
S., and Sloan, J. L., Continental hypsography, Tectonics, 
2.. 357-377, 1983. 
Kuenen, P. H., Quantitative estimations relating to eustatic 
movements, Geologie en Mijnbow, R. 194-201, 1939. 
McDonough, K. 1., and Cross, T. A., Late Cretaceous sea 
level from a paleoshoreline, 1. Geophys. ~- 2Q, 6591-
6607, 1991. 
Pribac, F., Superswells due to mantle convection, Ph.D. 
Thesis, Australian National University, Canberra, 174 p., 
1991. 
Sahagian, D., Epeirogenic motions of Africa as inferred from 
Cretaceous shoreline deposits, Tectonics, I. 125-138, 1988. 
Schuchert, C., Correlation and chronology on the basis of 
paleogeography,~- SQs;.. A.m. Bull., 2.1. 491-513, 1916. 
Struckmeyer, H. I. M., and Brown, P. J., Australian sealevel 
curves Part 1: Australian inundation curves, BMR Record 
1990/11, 1990. 
Wise, D. U., Continental margins, freeboard and the volumes 
of continents and oceans through time, in The Geology of 
Continental Margins, ed. by Burk, C. A. and Drake, C. L., 
Springer-Verlag, New York, 45-58, 1974. 
M. Gurnis, Department of Geological Sciences, University 
of Michigan, Ann Arbor, MI 48109-1063. 
(Received: March 9, 1993; 
Accepted: June 3, 1993) 


English

Comment on “Dynamic surface topography: A new interpretation based upon mantle flow models derived from seismic tomography” by A. M. Forte, W. R. Peltier, A. M. Dziewonski and R. L. Woodward

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95081/1/grl6870.pd

Deep Blue Documents at the University of Michigan

GEOPHYSICAL RESEARCH LETTERS, VOL. 20, NO. 15, PAGES 1663-1664, AUGUST 6, 1993 
COMMENT ON "DYNAMIC SURFACE TOPOGRAPHY: A NEW INTERPRETATION 
BASED UPON MANTLE FLOW MODELS DERIVED FROM SEISMIC TOMOGRAPHY" 
BY A.M. FORTE, W. R. PELTIER, A.M. DZIEWONSKI AND R. L. WOODWARD 
Michael Gurnis 
Department of Geological Sciences, The University of Michigan 
In a recent paper in this journal, Forte '-ill!. [1993] have 
proposed a model of global dynamic topography with an 
amplitude of three kilometers (six kilometers peak to peak). 
If North America, Australia, and other continents were 
dynamically depressed by the one to two kilometers 
predicted, then these continents would today be below sea-
level and nearly covered by epeiric seas. Since the continents 
are nearly entirely exposed and have become progressively 
exposed since the Late Mesozoic the amplitude of the Forte 
~ ID.. model is clearly much too large. 
Forte et al have predicted dynamic topography of the 
Earth's surface up to and including spherical harmonic degree 
eight in two ways. First, they removed a modeled isostatic 
component of topography from observed topography which 
they termed "dynamic topography". Second, they have 
computed topography on the surface of a viscous mantle with 
driving loads constrained via a tomographic inversion of 
mantle seismic heterogeneity. In their isostatic topography 
model, they have assumed a single crustal column for all 
continental crust and a single column for all oceanic crust; 
these two types of isostatically balanced columns are 
arranged on the Earth's surface using the continent-ocean 
function. Forte .e_t a I. assume there are no crustal thickness 
variations within continents, not necessarily a bad assumption 
at long wavelengths, and that a single global parameter 
characterizes the offset between all continental and all 
oceanic crust. Both predictions of dynamic topography are 
close in terms of pattern and amplitude. 
The dynamic topography presented by Forte, et 1\l. has 
nearly a three kilometer amplitude and all continents are 
located in dynamic topography depressions: North America 
and Australia are in the deepest depressions each of about 
two kilometers maximum and Africa is depressed by up to 
one kilometer. I will show that North America, Australia, and 
Africa cannot possibly have such extremely depressed 
dynamic topographies. First, these continents, like all 
continents, have been repeatedly inundated by shallow seas 
over the Phanerozoic and this inundation can be 
accomplished by global and regional sea-level changes of a 
few hundred meters -- certainly less than one kilometer. 
Second, if the continents were dynamically depressed by two 
kilometers, most of their area would be covered by shallow 
seas. 
Let me review some of the basics of Phanerozoic sea-level 
change and what they imply for Earth dynamics. In major 
platform areas, like North America or Australia, flat lying 
marine rocks of constant age can cover up to about fifty 
percent of the surface and/or sub-surface area of platforms 
Copyright 1993 by the American Geophysical Union. 
Paper Number 93GL01489 
0094-8534/93/93GL-O 1489$03.00 
[Schuchert, 1916]. It has been known since the first few 
decades of this century that sea-level rises and falls of about 
100 meters could explain these observed marine deposits 
[Kuenen, 1939; Hallam, 1992]. Wise [1974] argued that the 
surface of the Earth is in a long-term quasi-equilibrium in 
which freeboard remains approximately constant with the 
oceans periodically flooding the continents. Significantly, 
Phanerozoic transgressions and regressions can be 
accomplished by eustatic and regional sea-level changes and 
although the magnitude of these fluctuations are debated, it 
does not seem that estimates fall below 100 meters [Bond, 
1978] or exceed 700 meters [Hallam, 1984]. Many climatic 
and geologic factors effect sea level, including epeirogeny, 
which has long been known to be an important regional 
factor [eg. Bond, 1978]. Fluctuations in dynamic topography 
are probably a major control on epeirogeny and regional sea-
level change [Gurnis, 1992]. 
Gurnis [1990] showed how continents would become 
flooded and exposed as they moved with respect to a large-
scale, larger than continental scale, pattern of dynamic 
topography -- much like the pattern now predicted by Forte ~ 
1\l. Flooding is dependent on the ratio of geoid to dynamic 
topography, or admittance, but when admittance is as small 
as predicted in the Forte ~ al. model, - 0.03, then dynamic 
topography is by far the most important control on marine 
inundation. When the admittance is small and the continents 
are located in dynamic topography lows, as they are in the 
Forte tl al. predictions, then inundation reaches its 
maximum. Since we know from the stratigraphic record that 
sea-level has oscillated by- 200 meter around a mean, then 
when continents are in dynamic topography lows, continents 
must experience a sea-level rise proportional to the dynamic 
topography. Clearly, the relative sea-level rise would be at 
least one kilometer for North America and Australia in the 
Forte ~ .a...l. model. The actual rise would probably be 
significantly higher since about half of the ocean basins are 
on dynamic topography highs of at least 500 meters. The 
estimate of relative sea level rise of one kilometer (Figure 1) 
is an approximate minimum. The intersection of a relative 
sea-level rise with present day hypsometry (Figure 1) gives 
the fraction of a continent which would be covered by ocean 
water. From the Forte ~ .a...l. predictions we expect N. 
America should be flooded by about 80 percent and Australia 
by about 98 percent. Since North America and Australia, like 
nearly all continents, are mostly exposed, the amplitude of 
dynamic topography and the positioning of continents in lows 
predicted by Forte ~ .!ll. is at odds with these simple 
observations. 
Furthermore, contemporaneous positioning of the 
continents in dynamic topography lows of large amplitude, as 
proposed by Forte ~ 1\l., is at odds with the withdrawal of 
shallow seas from the continents since the end of the 
Mesozoic. Much of this withdrawal of seas from platforms 
was probably eustatic but with some important, regional or 
1663 
1664 Gurnis: Comment 
'§' 
Q) 
-
Q) 
E 
-c: 
0 
:a 
a; 
w 
2500 
1500 
500 
-500 
-0.4 0 0.4 0.8 1.2 
Fraction of Area 
Fig. 1. Hypsometric curves for North America, NA, and 
Australia, Au, from Harrison ~ill. [1983]. The area has been 
normalized by area presently above sea-level. Shown by light 
shadding overlying hypsometries are the fractional areas 
covered by Cretaceous and younger marine deposits. N. 
American flooding is from Wise [1974] and Australian 
flooding is from Struckmeyer and Brown [1990]. 
epeirogenic motions [Bond, 1978]. In particular, a 93 Ma 
shoreline deposit in Minnesota, USA, shows that sea-level 
has fallen 280m [McDonough and Cross, 1991] but Forte~ 
ill. predict that this mid continental area is dynamically 
depressed by -2 km. The results for Africa are even more 
extreme: Mid-Cretaceous shoreline deposits have been 
elevated to one kilometer, after sediment unloading, above 
present sea-levels [Sahagian, 1988] and this demonstrates 
that Africa has been actively uplifting in addition to the 
general eustatic sea-level fall since the Cretaceous [Bond, 
1978]. If Africa is clearly being uplifted, how can it be in a 
500 m to one kilometer dynamic topography low as predicted 
by Forte ~ ill. ? 
Despite these fundamental problems with the Forte et ill. 
models, other predictions of dynamic topography are more 
consistent with geologic observations. The tomography-
constrained viscous flow model of Hager, tl al. [1985] 
predicts a dynamic topography amplitude of about one 
kilometer, but, first, most continents only straddle dynamic 
topography highs and lows and, second, Africa is on a 
dominant 600 meter dynamic topography high. The former is 
not inconsistent with stratigraphic observations, but the latter 
is very much consistent with geologic observations of the 
Cenozoic uplift of Africa. Significantly, the pattern of 
dynamic topography predicted by Hager~ al. [1985] is in 
phase with the long wavelength residual oceanic topography 
obtained by subtracting the cooling of a simple half-space 
from observed bathymetry [Cazenave and Lago, 1991; 
Pribac, 1991], although there is a factor of two difference 
between them. The low amplitude, - 300 meter, residual 
topography results of Cazenave and Lago [1991] and Pribac 
[1991] imply long wavelength dynamic topographies which 
are most consistent with geologic observations [Gurnis, 
1990]. 
Acknowledgments. Funded by a NSF Presidential Young 
Investigator Award and a Fellowship from the David and 
Lucile Packard Foundation. 
References 
Bond, G., Speculations on real sea-level changes and vertical 
motions of continents at selected times in the Cretaceous 
and Tertiary Periods, Geology, Q, 247-250, 1978. 
Cazenave, A., and Lago, B., Long wavelength topography, 
seafloor subsidence and flattening, Geophys. Res. Lett., 
18, 1257-1260, 1991. 
Forte, A. M., Peltier, W. R., Dzeiwonski, A. M., and 
Woodword, R. L., Dynamic surface topography: A new 
interpretation based upon mantle flow models derived 
from seismic tomography, Geophys. Res. Lett., 20, 225-
228, 1993. 
Gurnis, M., Bounds on global dynamic topography from 
Phanerozoic flooding of continental platforms, Nature, 
.341. 754-756, 1990. 
Gurnis, M., Long-term controls on eustatic and epeirogenic 
motions by mantle convection, GSA Today, 2.. 141-157, 
1992. 
Hager, B. H., Clayton, R. W., Richards, M. A., Comer, R. P., 
and Dziewonski, A. M., Lower mantle heterogeneity, 
dynamic topography and the Geoid, Nature, ill. 541-545, 
1985. 
Hallam, A., Pre-Quaternary sea-level changes, Ann. Rev. 
El!nb.~. ,Sg., 12,205-243, 1984. 
Hallam, A., Phanerozoic Sea-Level Changes, Columbia 
University Press, New York, 266p., 1992. 
Harrison, C. G. A., Miskell, K. 1., Brass, G. W., Saltzman, E. 
S., and Sloan, J. L., Continental hypsography, Tectonics, 
2.. 357-377, 1983. 
Kuenen, P. H., Quantitative estimations relating to eustatic 
movements, Geologie en Mijnbow, R. 194-201, 1939. 
McDonough, K. 1., and Cross, T. A., Late Cretaceous sea 
level from a paleoshoreline, 1. Geophys. ~- 2Q, 6591-
6607, 1991. 
Pribac, F., Superswells due to mantle convection, Ph.D. 
Thesis, Australian National University, Canberra, 174 p., 
1991. 
Sahagian, D., Epeirogenic motions of Africa as inferred from 
Cretaceous shoreline deposits, Tectonics, I. 125-138, 1988. 
Schuchert, C., Correlation and chronology on the basis of 
paleogeography,~- SQs;.. A.m. Bull., 2.1. 491-513, 1916. 
Struckmeyer, H. I. M., and Brown, P. J., Australian sealevel 
curves Part 1: Australian inundation curves, BMR Record 
1990/11, 1990. 
Wise, D. U., Continental margins, freeboard and the volumes 
of continents and oceans through time, in The Geology of 
Continental Margins, ed. by Burk, C. A. and Drake, C. L., 
Springer-Verlag, New York, 45-58, 1974. 
M. Gurnis, Department of Geological Sciences, University 
of Michigan, Ann Arbor, MI 48109-1063. 
(Received: March 9, 1993; 
Accepted: June 3, 1993) 


Crossref

http://authors.library.caltech.edu/38271/1/1993_Gurnis_GRL.pdf

Comment on “Dynamic surface topography: A new interpretation based upon mantle flow models derived from seismic tomography” by A. M. Forte, W. R. Peltier, A. M. Dziewonski and R. L. Woodward

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Deep Blue Documents at the University of Michigan

Caltech Authors - Main

Crossref