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The Mars Dichotomy
Evidence for Plate Tectonics or Erosion?
Theories of the Dichotomy Formation

Giant Impact
• One major collision resurfaced the region.
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Multiple Impacts
• Several large impacts caused resurfacing.

Ancient Ocean
• An ancient ocean existed in the northern
hemisphere. The ocean eroded away the
surface erasing the craters observed in the
southern hemisphere.
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Plate tectonics
Erosion
• Out flow channels
Impacts
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Wilhems and Squyres
(1984) suggested a
single impact hypothesis.
The geology of the
northern lowlands
(Vastitas Borealis) is not
consistent with a one
impact hypothesis. (Frey
et. al., 1986a)
The lowlands are not
radial in shape (Smith et.
al. 1999), and there is no
evidence of a crater rim.
Several Impacts

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The lowland geology
could have formed
through several large
impacts.
Impact basins on mercury
and the moon roughly
follow a the Ddistribution curves,
Frey and Schultz (1988)
concluded that the largest
impact basins on Mars
(Fig 2A) roughly follow
this curve, but the
proposed Borealis impact
does not.
But is the 2A curve really
a good fit?
Ocean erosion

Topographic profiles across the Mars
dichotomy are not consistent with
ancient shoreline (Withers and
Neumann, 2001).
• Possible shoreline slopes are not
orientated in the correct direction

Concluded “shorelines” were most
likely created by compressive
tectonic stress.
• Suggested volcanism and impact
craters.
Utopia
-Inferred shorelines
slope down to the
North.
-However, the slopes
appear to be impact
related.
Alba Patera
-Inferred shorelines
slope down to the
south.
-This is opposite of
the expected slope
direction, if formed
by an ocean shore.
-Slopes are most
likely due to volcanic
compression.
Utopia
North facing
slopes
Note shoreline reversal
Alba Patera
South facing
slopes
(Withers and Neumann, 2001)
Plate Tectonics

The differences between the Northern and
Southern hemisphere's can be interpreted using
analogs to earth’s plate tectonics.
• Crustal thickness, volcanoes, and contraction features.

“Sea floor spreading” continuously forms new
crust at rift margins.
• New crust will be smother and thinner then old crust.

Subduction destroys the new crust, which
provides the “fuel” for volcanic activity.
• Tharsis?

Convergence will produce contraction features
along the “plate boundary”
Contraction Features

Watters (1993) mapped contraction features on the surface
of Mars.
• Wrinkle ridges, lobate scarps, and high relief ridges.

Wrinkle ridges are regularly spaced landforms generally
caused by thrust faulting and/or folding.
• Wrinkle ridges accounted for 80% of the mapped contraction
features.

Watters found that contractional features are generally
parallel to the dichotomy boundary in the eastern
hemisphere.
• Suggests the influence of regional stresses related to
dichotomy formation.

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Whereas the pattern in the western hemisphere reflects the
Tharsis volcanic province.
The geometry of the inferred stress was analyzed by fitting
great circles to each mapped segment, and plotting them
on a Schmidt net to create a Beta diagram (an equal area
stereonet projection). (Watters, 1993)
West
Above: Beta Diagram showing the
concentration of great circle intersections
to inferred maximum principle stress
direction.
-Note the general E-W trend of the
contraction features.
-Two clusters dominate at Tharsis
and Hesperia Planum.
(Watters, 1993)
East
Wrinkle Ridges
Mars Tectonics

The northern hemisphere of Mars
was formed during “sea floor
spreading” along a ridge axis that
broke away from Terra Cimmeria.
• Terra Cimmeria then acted as a passive
margin.

Subduction initiated along Arabia
Terra and the eastern edge of the
Tharsis volcanic province.
(Sleep, 1994)
Plate motion ceases when the rift margin is subducted.
Plates and Margins
1
-Yellow outline shows the plate
margin at the time of the break
up.
2
-The singular plate breaks into 2
plates, possibly due to subduction
angle and different plate velocities.
(Sleep, 1994)
3
4
-New plate geometry after the
break up.
-Note the transform fault between
the two plates.
-Plate geometry at the time plate
motion is inferred to have ceased.
(Sleep, 1994)
Fast vs. Slow Spreading Rates


On earth a fast spreading center produces
smooth topography, and virtually no vertical
scarps.
Slow spreading centers build up topography, and
have a large number of high angle scarps.
East Pacific Rise
Mid-Atlantic Ridge
Summary of Martian Tectonics


(Sleep, 1994)
Due to the smooth nature of the northern
lowlands “sea floor spreading” must have
occurred relatively fast.
A quantitative estimate of a full plate
spreading rate is ~80 mm/yr.
• This is fast, but comparable to places on earth,
such as the East Pacific Rise.


The northern lowland crust would have
formed rather quickly, and plate tectonics
may not have lasted that long.
Plate tectonics could aid in cooling the
interior of Mars.
Magnetic Stripes

Magnetic field observations acquired by
the MGS suggest Mars possessed a
periodically reversing dynamo (Acuna et. al., 1999).
Dynamo

On earth the dynamo occurs due to convection
in the outer core. Convection in the core occurs
because it is being cooled by the mantle.
• Plate tectonics can drive core convection.


Absence of crustal magnetism near large
impacts basins suggests the dynamo was only
active early in Mars history during the Naochian
epoch ~4 billion years ago. (Acuna et. al.,
1999)
It is proposed that cessation of plate tectonics
is linked to the cessation of the Mars dynamo
(Nimmo and Stevenson, 2000).
• i.e. Plate tectonics allows the planet to cool from
the inside out.
Summary

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


It is unlikely a giant impact resurfaced the N
hemisphere of Mars.
It is possible multiple large impacts resurfaced
the N hemisphere.
• However, this is not supported geological or
statistically.
There is no evidence of an ocean shore line,
only tectonic features and impact ridges.
Plate tectonics provides a possible mechanism
for N hemisphere resurfacing.
Magnetic “stripes” are present on Mars,
therefore a dynamo may have existed early in
Mars history.
Future Work


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Photo mapping of the dichotomy
boundary would reveal subtle details
required for an accurate geologic
analysis.
Detailed N-S gravity profiles across
the dichotomy to analyze the details
of the crustal thickness variation.
Send a geologist to Mars to map the
boundary! We’ll figure it out.....
References
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Acuna, M.H. et. al., Global distribution of crustal magnetization discovered
by the Mars Global Surveyor MAG/ER experiment, Science, 284, 790-793,
1999.
Frey et. al., The martian crustal dichotomy: product of accretion and not a
specific event? (abstract) Lunar and Planet. Sci., 27, 241-242, 1986a.
Frey, H. and R.A. Schultz, Large impact basins and the mega-impact origin
for the crustal dichotomy on Mars, Geophys. Res. Lett., 15, 229-232,
1988.
Nimmo F. and D.J. Stevenson, Influence of early plate tectonics on the
thermal evolution and magnetic field of Mars, J. Geophys. Res., 105,
11,969-11,979, 2000.
Sleep, N.H., Martian plate tectonics, J Geophys. Res., 99, 5639-5655,
1994.
Smith, D.E. et. al., The global topography of Mars and implications for
surface evolution, Science, 284, 1495-1502, 1999.
Watters, T. R., Compressional tectonics on Mars, J. Geophys. Res., 98,
17,049-17,060, 1993.
Wilhems D.E. and S.W. Squyres, The martian hemisphere dichotomy may
be due to a giant impact, Letters to Nature, 309, 138-140, 1984.
Withers P. and G.A. Neumann, Enigmatic northern plains of Mars, Nature:
brief communications, 410, p.651, 2001.
Outflow channels
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Images from Mars show distinct
valley networks that flow across the
dichotomy from S to N.
These outflow channels may have
carried water (or even volcanic
materials) into the northern plains
But it would take a large standing
body of water to physically form the
dichotomy.
• Implies episodic flooding.
Valles Marineris: Linear shape implies it was formed tectonically.
Increase in crustal thickness to the south must have
formed by internal processes.
However, the Northern lowlands may have been
resurfaced through water/volcanic outflow.
Arabia Terra
(Zuber, 2001)