TECTONIC PATTERNS OF SHORTENING LANDFORMS IN MERCURY’S NORTHERN SMOOTH PLAINS

Mercury’s Northern Smooth Plains (NSPs), an amalgam of volcanic units, cover ~6% of the planet. They are characterized by numerous ghost craters and volcanically buried impact basins, many thrust fault-related landforms (TFLs), and superposing craters. Map patterns and structural interpretations of TFLs in the NSPs have not been analyzed in detail, but those aspects may provide constraints to subsurface architecture of the thrust faults, thickness and geometry of the plains deposits, and details of tectonic processes that led to their formation. We mapped TFLs in the NSPs at a map scale of 1:1,000,000 using MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) global monochrome mosaics (166 m/pixel). We recognized TFLs as curvi-linear asymmetric ridges. We defined the direction of more gradually increasing elevation across the ridge as vergence, which we interpret to represent the direction of tectonic transport. We identified 2175 faults with clear surface breaks and 2849 broad ridges with no clear surface breaks. Regional mosaics (~130 m/pixel) were generated to investigate common landform patterns in greater detail. Like prior studies, we observed thrust faults to follow rims of buried craters. Vergences away from the interiors of ghost crater interiors suggest that faults may propagate along buried crater walls. We found five patterns which had been previously unrecognized in the NSPs: (1) sigmoidal rises bounded by TFLs verging away from the rise, (2) v-shaped rises composed of two landforms terminating at a single sharp point, (3) networks of broad arcuate rises of nearly equal width, (4) parallel, evenly-spaced elevated landforms verging in the same direction, and (5) landforms showing alternation in vergence direction along strike. Respectively, we interpreted these landforms as transpressional uplifts, faults with sharply juxtaposing ramps, pop-up structures, fold and thrust belts, and antithetic fault intersections. We also analyzed the orientations of TFLs, and found that the northernmost landforms (90°-70° N) were predominantly oriented EW while most of the landforms between 50°-30° N were oriented NS. By comparison with map patterns on Earth and patterns produced in models, our results suggest that deformation in the NSPs is thin-skinned.