The Mars DichotomyThe Mars DichotomyEvidence for Plate Tectonics or Erosion?Evidence for Plate Tectonics or Erosion?

Theories of the Dichotomy FormationTheories of the Dichotomy Formation Giant ImpactGiant Impact

• One major collision resurfaced the region.One major collision resurfaced the region. Multiple ImpactsMultiple Impacts

• Several large impacts caused resurfacing.Several large impacts caused resurfacing. Ancient OceanAncient Ocean

• An ancient ocean existed in the northern hemisphere. An ancient ocean existed in the northern hemisphere. The ocean eroded away the surface erasing the The ocean eroded away the surface erasing the craters observed in the southern hemisphere.craters observed in the southern hemisphere.

Plate tectonicsPlate tectonics Erosion Erosion

• Out flow channelsOut flow channels

ImpactsImpacts

Wilhems and Squyres Wilhems and Squyres (1984) suggested a (1984) suggested a single impact hypothesis.single impact hypothesis.

The geology of the The geology of the northern lowlands northern lowlands (Vastitas Borealis) is not (Vastitas Borealis) is not consistent with a one consistent with a one impact hypothesis. (Frey impact hypothesis. (Frey et. al., 1986a)et. al., 1986a)

The lowlands are not The lowlands are not radial in shape (Smith et. radial in shape (Smith et. al. 1999), and there is no al. 1999), and there is no evidence of a crater rim.evidence of a crater rim.

Several ImpactsSeveral Impacts The lowland geology could The lowland geology could

have formed through have formed through several large impacts.several large impacts.

Impact basins on mercury Impact basins on mercury and the moon roughly and the moon roughly follow a the D- follow a the D- distribution curves,distribution curves,

Frey and Schultz (1988) Frey and Schultz (1988) concluded that the largest concluded that the largest impact basins on Mars (Fig impact basins on Mars (Fig 2A) roughly follow this 2A) roughly follow this curve, but the proposed curve, but the proposed Borealis impact does not.Borealis impact does not.

But is the 2A curve really But is the 2A curve really a a goodgood fit? fit?

Ocean erosionOcean erosion Topographic profiles across the Mars Topographic profiles across the Mars

dichotomy are not consistent with dichotomy are not consistent with ancient shoreline (Withers and ancient shoreline (Withers and Neumann, 2001).Neumann, 2001).• Possible shoreline slopes are not orientated Possible shoreline slopes are not orientated

in the correct directionin the correct direction Concluded “shorelines” were most likely Concluded “shorelines” were most likely

created by compressive tectonic stress.created by compressive tectonic stress.• Suggested volcanism and impact craters.Suggested volcanism and impact craters.

-Inferred shorelines slope down to the North.-However, the slopes appear to be impact related.

Alba Patera -Inferred shorelinesslope 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

(Withers and Neumann, 2001)

Utopia

Alba Patera

Utopia

North facing slopes

South facing slopes

Plate TectonicsPlate Tectonics The differences between the Northern and The differences between the Northern and

Southern hemisphere's can be interpreted using Southern hemisphere's can be interpreted using analogs to earth’s plate tectonics.analogs to earth’s plate tectonics.• Crustal thickness, volcanoes, and contraction features.Crustal thickness, volcanoes, and contraction features.

““Sea floor spreading” continuously forms new Sea floor spreading” continuously forms new crust at rift margins.crust at rift margins.• New crust will be smother and thinner then old crust.New crust will be smother and thinner then old crust.

Subduction destroys the new crust, which provides Subduction destroys the new crust, which provides the “fuel” for volcanic activity.the “fuel” for volcanic activity.• Tharsis?Tharsis?

Convergence will produce contraction features Convergence will produce contraction features along the “plate boundary”along the “plate boundary”

Contraction FeaturesContraction Features Watters (1993) mapped contraction features on the surface of Mars.Watters (1993) mapped contraction features on the surface of Mars.

• Wrinkle ridges, lobate scarps, and high relief ridges.Wrinkle ridges, lobate scarps, and high relief ridges. Wrinkle ridges are regularly spaced landforms generally caused by Wrinkle ridges are regularly spaced landforms generally caused by

thrust faulting and/or folding.thrust faulting and/or folding.• Wrinkle ridges accounted for 80% of the mapped contraction featuresWrinkle ridges accounted for 80% of the mapped contraction features..

Watters found that contractional features are generally parallel to Watters found that contractional features are generally parallel to the dichotomy boundary in the eastern hemisphere. the dichotomy boundary in the eastern hemisphere. • Suggests the influence of regional stresses related to dichotomy Suggests the influence of regional stresses related to dichotomy

formation.formation. Whereas the pattern in the western hemisphere reflects the Tharsis Whereas the pattern in the western hemisphere reflects the Tharsis

volcanic province.volcanic province. The geometry of the inferred stress was analyzed by fitting great The geometry of the inferred stress was analyzed by fitting great

circles to each mapped segment, and plotting them on a Schmidt circles to each mapped segment, and plotting them on a Schmidt net to create a Beta diagram (an equal area stereonet projection). net to create a Beta diagram (an equal area stereonet projection). (Watters, 1993)(Watters, 1993)

Above: Beta Diagram showing the concentration of great circle intersections to inferred maximum principle stressdirection.-Note the general E-W trend of the contraction features.-Two clusters dominate at Tharsis and Hesperia Planum.

(Watters, 1993) Wrinkle Ridges

West East

Mars TectonicsMars Tectonics The northern hemisphere of Mars The northern hemisphere of Mars

was formed during “sea floor was formed during “sea floor spreading” along a ridge axis that spreading” along a ridge axis that broke away from Terra Cimmeria.broke away from Terra Cimmeria.• Terra Cimmeria then acted as a passive Terra Cimmeria then acted as a passive

margin.margin. Subduction initiated along Arabia Subduction initiated along Arabia

Terra and the eastern edge of the Terra and the eastern edge of the Tharsis volcanic province.Tharsis volcanic province.

(Sleep, 1994)

Plate motion ceases when the rift margin is subducted.

Plates and MarginsPlates and Margins

-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.

1 2

(Sleep, 1994)

-New plate geometry after the break up. -Note the transform fault betweenthe two plates.

-Plate geometry at the time plate motion is inferred to have ceased.

3 4

(Sleep, 1994)

Fast vs. Slow Spreading RatesFast vs. Slow Spreading Rates On earth a fast spreading center produces On earth a fast spreading center produces

smooth topography, and virtually no vertical smooth topography, and virtually no vertical scarps. scarps.

Slow spreading centers build up topography, and Slow spreading centers build up topography, and have a large number of high angle scarps.have a large number of high angle scarps.

East Pacific Rise

Mid-Atlantic Ridge

Summary of Martian TectonicsSummary of Martian Tectonics(Sleep, 1994)(Sleep, 1994)

Due to the smooth nature of the northern lowlands “sea Due to the smooth nature of the northern lowlands “sea floor spreading” must have occurred relatively fast.floor spreading” must have occurred relatively fast.

A quantitative estimate of a full plate spreading rate is ~80 A quantitative estimate of a full plate spreading rate is ~80 mm/yr. mm/yr. • This is fast, but comparable to places on earth, such as the East This is fast, but comparable to places on earth, such as the East

Pacific Rise.Pacific Rise. The northern lowland crust would have formed rather The northern lowland crust would have formed rather

quickly, and plate tectonics may not have lasted that long.quickly, and plate tectonics may not have lasted that long. Plate tectonics could aid in cooling the interior of Mars.Plate tectonics could aid in cooling the interior of Mars.

Magnetic StripesMagnetic Stripes Magnetic field observations acquired by Magnetic field observations acquired by

the MGS suggest Mars possessed a the MGS suggest Mars possessed a periodically reversing dynamoperiodically reversing dynamo (Acuna et. al., 1999).(Acuna et. al., 1999).

DynamoDynamo On earth the dynamo occurs due to convection in the On earth the dynamo occurs due to convection in the

outer core. Convection in the core occurs because it outer core. Convection in the core occurs because it is being cooled by the mantle.is being cooled by the mantle.• Plate tectonics can drive core convection.Plate tectonics can drive core convection.

Absence of crustal magnetism near large impacts Absence of crustal magnetism near large impacts basins suggests the dynamo was only active early in basins suggests the dynamo was only active early in Mars history during the Naochian epoch ~4 billion Mars history during the Naochian epoch ~4 billion years ago. (Acuna et. al., 1999)years ago. (Acuna et. al., 1999)

It is proposed that cessation of plate tectonics is It is proposed that cessation of plate tectonics is linked to the cessation of the Mars dynamo (Nimmo linked to the cessation of the Mars dynamo (Nimmo and Stevenson, 2000).and Stevenson, 2000).• i.e. Plate tectonics allows the planet to cool from the i.e. Plate tectonics allows the planet to cool from the

inside out.inside out.

SummarySummary It is unlikely a giant impact resurfaced the N It is unlikely a giant impact resurfaced the N

hemisphere of Mars.hemisphere of Mars. It is possible multiple large impacts resurfaced the N It is possible multiple large impacts resurfaced the N

hemisphere.hemisphere.• However, this is not supported geological or However, this is not supported geological or

statistically.statistically. There is no evidence of an ocean shore line, only There is no evidence of an ocean shore line, only

tectonic features and impact ridges.tectonic features and impact ridges. Plate tectonics provides a possible mechanism for N Plate tectonics provides a possible mechanism for N

hemisphere resurfacing.hemisphere resurfacing. Magnetic “stripes” are present on Mars, therefore a Magnetic “stripes” are present on Mars, therefore a

dynamo may have existed early in Mars history.dynamo may have existed early in Mars history.

Future WorkFuture Work Photo mapping of the dichotomy Photo mapping of the dichotomy

boundary would reveal subtle details boundary would reveal subtle details required for an accurate geologic required for an accurate geologic analysis.analysis.

Detailed N-S gravity profiles across Detailed N-S gravity profiles across the dichotomy to analyze the details the dichotomy to analyze the details of the crustal thickness variation.of the crustal thickness variation.

Send a geologist to Mars to map the Send a geologist to Mars to map the boundary! We’ll figure it out..... boundary! We’ll figure it out.....

ReferencesReferences Acuna, M.H. et. al., Global distribution of crustal magnetization discovered by Acuna, M.H. et. al., Global distribution of crustal magnetization discovered by

the Mars Global Surveyor MAG/ER experiment, the Mars Global Surveyor MAG/ER experiment, ScienceScience, 284, 790-793, 1999., 284, 790-793, 1999. Frey et. al., The martian crustal dichotomy: product of accretion and not a Frey et. al., The martian crustal dichotomy: product of accretion and not a

specific event? (abstract) specific event? (abstract) Lunar and Planet. SciLunar and Planet. Sci., 27, 241-242, 1986a. ., 27, 241-242, 1986a. Frey, H. and R.A. Schultz, Large impact basins and the mega-impact origin for Frey, H. and R.A. Schultz, Large impact basins and the mega-impact origin for

the crustal dichotomy on Mars, the crustal dichotomy on Mars, Geophys. Res. LettGeophys. Res. Lett., 15, 229-232, 1988.., 15, 229-232, 1988. Nimmo F. and D.J. Stevenson, Influence of early plate tectonics on the thermal Nimmo F. and D.J. Stevenson, Influence of early plate tectonics on the thermal

evolution and magnetic field of Mars, evolution and magnetic field of Mars, J. Geophys. ResJ. Geophys. Res., 105, 11,969-11,979, ., 105, 11,969-11,979, 2000.2000.

Sleep, N.H., Martian plate tectonics, Sleep, N.H., Martian plate tectonics, J Geophys. ResJ Geophys. Res., 99, 5639-5655, 1994. ., 99, 5639-5655, 1994. Smith, D.E. et. al., The global topography of Mars and implications for surface Smith, D.E. et. al., The global topography of Mars and implications for surface

evolution, evolution, ScienceScience, 284, 1495-1502, 1999., 284, 1495-1502, 1999. Watters, T. R., Compressional tectonics on Mars, Watters, T. R., Compressional tectonics on Mars, J. Geophys. ResJ. Geophys. Res., 98, 17,049-., 98, 17,049-

17,060, 1993.17,060, 1993. Wilhems D.E. and S.W. Squyres, The martian hemisphere dichotomy may be Wilhems D.E. and S.W. Squyres, The martian hemisphere dichotomy may be

due to a giant impact, due to a giant impact, Letters to NatureLetters to Nature, 309, 138-140, 1984., 309, 138-140, 1984. Withers P. and G.A. Neumann, Enigmatic northern plains of Mars, Withers P. and G.A. Neumann, Enigmatic northern plains of Mars, Nature: brief Nature: brief

communicationscommunications, 410, p.651, 2001., 410, p.651, 2001.

Outflow channelsOutflow channels

Images from Mars show distinct valley Images from Mars show distinct valley networks that flow across the dichotomy networks that flow across the dichotomy from S to N.from S to N.

These outflow channels may have carried These outflow channels may have carried water (or even volcanic materials) into water (or even volcanic materials) into the northern plainsthe northern plains

But it would take a large standing body of But it would take a large standing body of water to physically form the dichotomy.water to physically form the dichotomy.• Implies episodic flooding.Implies episodic flooding.

Valles Marineris: Linear shape implies it was formed tectonically.

Increase in crustal thickness to the south must have Increase in crustal thickness to the south must have formed by internal processes.formed by internal processes.

However, the Northern lowlands may have been However, the Northern lowlands may have been resurfaced through water/volcanic outflow.resurfaced through water/volcanic outflow.

Arabia Terra

(Zuber, 2001)