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The Martian Dichotomy

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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.
   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.
   Plate tectonics
   Erosion
    • Out flow channels
                           Impacts
   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
   The lowland geology
    could have formed
    through several large
    impacts.
   Impact basins on mercury
    and the moon roughly
    follow a the D-
    distribution 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                Utopia
-Inferred shorelines
                                      North facing
slope down to the                        slopes
North.
-However, the slopes
appear to be impact
related.



      Alba Patera        Note shoreline reversal
-Inferred shorelines
slope down to the         Alba Patera
south.
-This is opposite of
                                        South facing
the expected slope                        slopes
direction, if formed
by an ocean shore.
-Slopes are most
likely due to volcanic
compression.
                                               (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.
   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                           East




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)            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                                 2




-Yellow outline shows the plate   -The singular plate breaks into 2
margin at the time of the break   plates, possibly due to subduction
up.                               angle and different plate velocities.


                                                           (Sleep, 1994)
3                                   4




-New plate geometry after the
break up.                           -Plate geometry at the time plate
-Note the transform fault between   motion is inferred to have ceased.
the two plates.

                                                           (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
   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
   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
   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
   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)

				
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posted:6/26/2012
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