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The Crustal D Ichotom Y on Mars 1991 1 The Crustal Dichotomy On Mars A Thesis submitted for the Degree of Doctor of Philosophy of the University of London by Cordula Astrid Robinson UCL University of London Observatory Annexe Department of Physics & Astronomy University College London University of London 1991 1 ProQuest Number: 10611114 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 10611114 Published by ProQuest LLC(2017). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 Abstract The crastal dichotomy is the term used for the surface division of Mars into a lightly- cratered third of the planet, in the middle to upper latitudes of the northern hemisphere, and a remaining heavily-cratered two-thirds. The lightly-cratered third stands approximately 3 km lower than the heavily-cratered surface. Established explanations for the crustal dichotomy consider it a stable feature that orig­ inated early in the planet’s history either by endogenic means (in the form of catastrophic overturn of the mantle, probably associated with core formation) or by exogenic means (either in the form of a giant single impact or the focusing of several large impacts). A review of the processes acting on the surface of Mars, combined with detailed geological mapping studies, shows that five conditions must be met by any explanation for crustal dichotomy formation. They are: 1. That fretted terrain formation occurred between the late Noachian to early Hesperian, and small-scale structural adjustment occurred subsequently; 2. The subsurface thermal activity leading to surface volcanism between the late Noachi­ an and early Hesperian needs to be accounted for; 3. The existence of a faulted southern-hemisphere-type surface beneath the northern plains needs to be accounted for; 4. The origins of the three highland-lowland boundary provinces (namely the Tharsis Province, the chaotic terrain and the fretted terrain) need to be compatible with the proposed explanation; 5. The topography and isostatic equilibrium of the northern lowlands needs to be ac­ counted for. None of the explanations so far put forward for the crustal dichotomy on Mars are consistent with these conditions. The conditions support an endogenic mechanism for crustal dichotomy formation, and as an alternative I suggest that Mars constitutes a single plate planet that was at the threshold of developing more than one plate between the late Noachian and early Hesperian. This resulted in crustal dichotomy formation whereby the Tharsis Province represents the site of continental rifting, and the northern lowlands a corresponding region of subsidence that formed in order to maintain isostatic equilibrium and to accommodate the stretching involved in the formation of the Tharsis Province and the fretted terrain. * For the man who reminded me ‘you ain’t ever gonna forget’. ‘Ignorance is preferable to error, and he is less remote from the truth who believes nothing, than he who believes what is wrong’ Thomas Jefferson. 3 Acknowledgements My thanks could extend to many, but I would like to express special thanks to the following: The U.K. National Environmental Research Council for funding the project; My mother and Reimer for many things; Claudio Vita-Finzi and Ian Howarth for their academic advice; Dave Rooks for his excellent photographic work; Adrian Fish, John Deacon and Lucy McCrickard for being amazing friends, and also to Adrian for helping me out with computing dilemmas; Valerie Peerless for being lovely company; and of course John Guest for many things, including letting me attempt this project in the first place. 4 Contents Abstract 2 Acknowledgements 4 Contents 5 List of Figures 10 List of Tables 13 1 Introduction 14 1.1 The Planet M a rs .................................................................................................. 14 1.2 The Viking Mission ............................................................................................... 14 1.3 Chapter Review ..................................................................................................... 15 2 Review Of Previous Proposals For Crustal Dichotomy Formation 24 2.1 Introduction ...........................................................................................................24 2.2 Review Of Previous Proposals ............................................................................. 24 2.2.1 Proposal Of Mutch And Saunders, 1975 .............................................. 25 2.2.2 Proposal Of Wise et al., 1979 ............................................................... 29 2.2.3 Proposal Of Wilhelms And Squyres, 1984 ............................................ 34 2.2.4 Proposal Of Frey And Schultz, 1988 ................................................. 39 5 2.2.5 Proposal Of McGill: from personal correspondence; not yet in print 43 2.3 Summary Of Previous Proposals For Crustal Dichotomy Formation ................ 46 3 Review Of Surface Features On Mars 48 3.1 Atmospheric Effects On The Martian Surface ....................................................48 3.1.1 Temperature Effects................................................................................. 49 3.1.2 Pressure Effects.................................... 50 3.1.3 Wind E ffects .............................................................................................50 3.2 Geological Landforms Controlled By Atmospheric E ffe cts ...............................51 3.2.1 Aeolian Features .......................................................................................52 3.2.2 Ice Related Features ................................................................................. 55 3.2.3 Sum m ary ...................................................................................................61 3.3 Origin Of The Martian Plains ..............................................................................61 3.3.1 Features Indicative Of Volcanic Material ................................................. 61 3.3.2 Origin Of The Southern Upland Plains .................................................... 64 3.3.3 Origin Of The Northern Lowland P lains ................................................. 65 3.4 Central Vent Volcanism ....................................................................................... 69 3.4.1 Central Vent Volcanism In The Southern H em isphere ..........................71 3.4.2 Central Vent Volcanism In The Northern H em isphere ..........................72 3.4.3 Sum m ary ...................................................................................................75 3.5 Fluvial Features On M a rs ............................................................................... 75 3.5.1 Origin Of Runoff C hannels ......................................................................78 3.5.2 Origin Of Unconfined Outflow Channels ................................................. 79 3.5.3 Origin Of Confined Outflow C hannels ....................................................81 3.5.4 Origin Of Fretted Channels ......................................................................82 3.5.5 Discussion Of Fluvial F eatures ................................................................83 3.6 The Highland-lowland Boundary On M ars ..........................................................84 6 3.6.1 The Tharsis P rovince .............................................................................84 3.6.2 Chaotic Terrain ........................................................................................ 86 3.6.3 Fretted Terrain ........................................................................................ 87 3.7 Tectonic Processes On M ars ............................................................................... 92 3.7.1 Fault Formation Around T harsis ........................................................... 93 3.7.2 Canyon Formation Around Tharsis ........................................................ 95 3.7.3 Origin Of The Tharsis Province .............................................................. 97 3.7.4 Origin Of The Elysium Province ..........................................................100 3.8 Summary Chapter Three .................................................................................101 4 Geological Mapping Studies 102 4.1 Constructing A Photogeological M ap............................................................... 102 4.2 Selection Of Mapping A reas ..............................................................................103 4.2.1 Construction Of Sketch M aps ................................................................103 4.2.2 Detailed Geological M apping............................................................... 104 4.3 Analysis Of The Geological M a p .....................................................................110 4.4 Fretted T errain .................................................................................................. 112 4.4.1 Nilosyrtis Mensae: N nsm t ....................................................................
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