The Causes and Consequences of the Crustal Dichotomy and Their Implications for the Early Evolution of Mars

Introduction: The two most striking surface features on Mars are the crustal dichotomy and the Tharsis Rise [1]. While it is generally accepted that the Tharsis Rise is formed as a result of plume related volcanism [2], the formation mechanism for the crustal dichotomy is controversial with two main competing proposals: endogenic (mantle convection and flow) [3-5] and exo-genic (mega-impact) mechanisms [6]. Recent studies on modeling mega-impact and reconstructing dichot-omy boundary revive the mega-impact mechanism for the crustal dichotomy [7-9]. However, because the crustal dichotomy, as the oldest feature on Mars (i.e., the pre-Noachian or >4.1 Ga) [10], has been significantly modified by later processes such as the Tharsis formation, it is difficult to constrain the formation mechanism for the crustal dichotomy. The Tharsis Rise is a younger feature and is formed during the Noachian (4.1-3.8 Ga) [11]. Both the crustal dichotomy and Tharsis Rise represent hemispheric asymmetric structrures (i.e., degree-1 feature) in that the crustal dichotomy marks the difference between the northern and southern hemispheres, while the Tharsis Rise repressents the difference between the western and eastern hemispheres. Although the center of Thar-sis is near the dichotomy boundary at the equator, geo-physical and geological observations suggest that Thar-sis volcanisms may have initiated in the southern highlands (e.g., the Thaumasia region at ~40 o S) and subsequently migrated to the current positions [12,13]. Recently, Zhong [14] suggested that the crustal di-chotomy has an important effect on formation and evolution of the Tharsis Rise and volcanism. The key to Zhong's proposal is that the crustal dichotomy repressents lateral variations in lithospheric shell thickness which interacts with one-plume convection to give rise to the differential rotation between the lithospheric shell and the underlying mantle with one-plume con-vection, thus explaining the migration of Tharsis vol-canisms. This indicates that the Tharsis volcanism and tectonism may provide important constraints on the formation mechanism of the crustal dichotomy. The goal of this study is to critically examine both endogenic and exogenic formation mechanisms for the crustal dichotomy. In particular, we seek constraints on the formation mechanism, by further exploring the consequences of the crustal dichotomy from these two different mechanisms on volcanisms and tectonisms and their relation to the Tharsis Rise.