The tectonic and volcanic history of mercury as inferred from studies of scarps, ridges, troughs, and other lineaments

Abstract

This paper consists of separate descriptive and interpretive sections which address the issues of tectonic and volcanic modification of the Mercurian surface. In section 1 a nongenetic classification system based on planimetric form is outlined and applied to scarps, ridges, troughs, and other lineaments on Mercury. Nine landform classes are defined, and type examples of each are presented and discussed. The resulting set of observational constraints provides the basis for the following evolutionary chronology for Mercury developed in section 2. After formation of a solid lithosphere but before the end of heavy meteoritic bombardment, tidal spindown and planetary contraction owing to cooling combined to produce a global pattern of linear fractures and arcuate thrust faults. Throughout this period, intercrater plains formed by extrusion from linear vents or by local lithospheric melting. Near the end of heavy meteoritic bombardment the Caloris basin‐forming event drastically altered the morphology of much of the Mercurian surface. Soon after basin excavation, fluid material covered its floor to form smooth plains. Planimetrically irregular ridges formed inside Caloris in response to compressive stresses induced by regional subsidence. Gravitational readjustment to emplacement of plains material outside Caloris was responsible for scarp and ridge formation there. Uplift of plains inside Caloris produced extensional fractures which transect the previously established pattern of ridges. At roughly the same time, intense modification of preexisting landforms occurred in a region roughly antipodal to Caloris Basin to produce a distinctive terrain unit characterized by scattered hills and linear, open‐ended troughs. By this time, volcanic plains emplacement was largely restricted inside large, fresh craters. Irregular scarps and ridges developed in these plains in response to local stresses, perhaps induced by magma redistribution at depth. The period following the most recent smooth plains formation on Mercury has been characterized by relatively light cratering and only mild landform modification.