EXTENSION IN THE NORTHERN PLAINS OF MERCURY

Flybys of Mercury by the Mariner 10 and MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft showed that tectonic landforms are dominantly contractional. Identified evidence of extensional deformation was confined almost exclusively to impact basin floors. After the MESSENGER spacecraft was successfully inserted into orbit about Mercury in March 2011, imaging of the heretofore largely unexplored north polar region revealed a large expanse of smooth plains. Although these northern plains are deformed by contractional wrinkle ridges, MESSENGER orbital images show evidence of widespread extension. Narrow, linear, and curvilinear troughs, up to 1-2 km wide and ~15 km in length, are found in the smooth plains and are interpreted to be factures and graben. Many of the graben form polygonal patterns similar to those found on the floors of the Caloris and Rembrandt basins. Graben in other areas of the northern plains tend to have preferred orientations rather than polygonal patterns. The graben are generally confined to plains areas partly or completely ringed by wrinkle ridges, interpreted as marking the rim crests of impact craters buried by lava flows. The diameters of these areas range from <100 km to 300 km in the Goethe basin. Crosscutting relations between graben, ridge rings, and other wrinkle ridges suggest that contraction predated extension of the plains material. Also, crosscutting relations between wrinkle ridges and ridge rings encompassing the buried basins suggests that ridge rings postdate other plains wrinkle ridges. Our interpretation of these relations is as follows. After burial of heavily cratered regional terrain by volcanic plains material, subsidence in response to plains loading led to the formation of wrinkle ridges and ridge rings. Ridge formation was followed by uplift and extension of the plains material, possibly due to continued isostatic response to earlier crater and basin formation or as a result of inward flow of lower crustal material. The crustal flow model adds a component of compressional stresses near the crater or basin rims as well as of extensional stresses in the crater or basin interiors. This later increment of compression could account for the observed crosscutting relations between plains wrinkle ridges and basin-encompassing ridge rings.