The Mars Dichotomy

1. The Mars Dichotomy Evidence for Plate Tectonics or Erosion?

2. 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

3. 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.

4. 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?

5. 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.

7. Utopia Utopia -Inferred shorelines slope down to the North. -However, the slopes appear to be impact related. North facing slopes • 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. Note shoreline reversal Alba Patera South facing slopes (Withers and Neumann, 2001)

8. 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”

9. 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)

10. East 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) Wrinkle Ridges

11. 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)

12. Not to scale

13. Not to scale Plate motion ceases when the rift margin is subducted.

14. Plates and Margins

15. 1 2 -Yellow outline shows the plate margin at the time of the break up. -The singular plate breaks into 2 plates, possibly due to subduction angle and different plate velocities. (Sleep, 1994)

16. 4 3 -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)

17. 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

18. 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.

19. Magnetic Stripes • Magnetic field observations acquired by the MGS suggest Mars possessed a periodically reversing dynamo (Acuna et. al., 1999).

20. 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.

21. 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. Summary

22. 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.....

23. 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.

24. 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.

26. Valles Marineris: Linear shape implies it was formed tectonically.

27. 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)