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Mars: First Order Landscapes

Geography 441/541 F/19 Dr. Christine M. Rodrigue. Mars: First Order Landscapes. Explanations for the Crustal Dichotomy. Endogenous explanations Degree-1 convection Planetary accretion Heat accumulation Magma ocean Gravitationally unstable crystal accumulation Mantle overturn

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Mars: First Order Landscapes

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  1. Geography 441/541 F/19 Dr. Christine M. Rodrigue Mars: First Order Landscapes C.M. Rodrigue, 2019 Geography, CSULB

  2. C.M. Rodrigue, 2019 Geography, CSULB Explanations for the Crustal Dichotomy Endogenous explanations • Degree-1 convection • Planetary accretion • Heat accumulation • Magma ocean • Gravitationally unstable crystal accumulation • Mantle overturn • Initiation of upwelling/downwelling plumes • Sinking of cool mantle material intensifies temperature contrast in outer liquid core • This creates a dynamo/planetary magnetic field

  3. C.M. Rodrigue, 2019 Geography, CSULB Explanations for the Crustal Dichotomy Endogenous explanations • Did Mars have plate tectonics? • Upwelling → crustal thinning through tension and ablation (Northern Lowlands?) • Downwelling → compression and thickening (Southern Highlands?) • Cerberus Fossæ a spreading zone rift? • South dipping plate south of Cerberus Fossæ • East dipping plate under Tharsis (volcanic arc?) • Or ... could crustal prominence develop above upwelling plume instead?

  4. C.M. Rodrigue, 2019 Geography, CSULB Explanations for the Crustal Dichotomy Endogenous explanations • Evidence for plate tectonics? • No trenches • Crustal thickening in Terra Cimmeria/Sirenum? • Banded magnetization: Could these symmetrical changes in remanent magnetization be like the bands on Earth's ocean floors in spreading zones, where new lithosphere records the prevailing magnetic field? • Fault systems • Cerberus Fossæ? • Valles Marineris? • Transform fault-like offsets in magnetic bands in Noachis Terra

  5. C.M. Rodrigue, 2019 Geography, CSULB Plate Tectonics: Banded Magnetic Anomalies

  6. C.M. Rodrigue, 2019 Geography, CSULB Explanations for the Crustal Dichotomy Endogenous explanations • Other explanations for the magnetic anomalies • Not the roughly symmetrical polarity reversals seen on Earth's ocean floors near spreading zones • Possibly great basaltic dikes that picked up remanent magnetization during solidification as they ascended through joints in country rock • Maybe the accumulation of terranes with distinct magnetization records due to plate tectonic compression over a downwelling

  7. C.M. Rodrigue, 2019 Geography, CSULB Explanations for the Crustal Dichotomy Endogenous explanations • Stagnant lid convection • Earth's crust is < 10 km thick on the ocean floors but ~40 km thick under continents (up to 70 km thick under continental compression zones, e.g., Tibet) • Mars' crust averages ~ 50 km thick, ranging from ~25 km thick under the Northern Lowlands and Arabia Terra and up to 75 km thick under the Southern Highlands • Could such a thick crust have prevented Mars' lithosphere breaking into plates and enabling vigorous convection? • Interestingly, a stagnant lid would be capable of drifting as a unit, perhaps explaining why Tharsis is centered on the equator.

  8. C.M. Rodrigue, 2019 Geography, CSULB Earth Crustal Thickness: Isopach Map

  9. C.M. Rodrigue, 2019 Geography, CSULB Mars Crustal Thickness: Block Diagram

  10. C.M. Rodrigue, 2019 Geography, CSULB Mars Crustal Thickness: Hypsometric Isopach Map

  11. C.M. Rodrigue, 2019 Geography, CSULB Explanations for the Crustal Dichotomy Exogenous explanations • Impact created the Northern Lowlands basin • Planetary accretion: lots of bombs • Large one may have hit obliquely • But even pretty oblique impacts create spherical craters (impact is a detonation process) • Hard to trace great dichotomy under the Tharsis lump • Andrews-Hanna, Zuber, Banerdt team tried modeling topography, gravimetry, and how the pre-existence of Tharsis would have redistributed crustal thickness • Their results yielded a pretty clear oval of 10,600 km by 8,500 km centered around 67° N and 208° E in Vastitas Borealis (Arcadia Planitia) northwest of Alba Mons

  12. Mars Crustal Thickness • MOLA topography • Crustal thickness • Isostatic root • Andrews-Hanna, Zuber, and Banerdt 2008 C.M. Rodrigue, 2019 Geography, CSULB

  13. Mars Crustal Thickness • MOLA topography • Modelled ellipse • Andrews-Hanna, Zuber, Banerdt 2008 C.M. Rodrigue, 2019 Geography, CSULB

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