STRAIN ANALYSIS OF EXTENSION IN VOLCANICALLY FLOODED IMPACT CRATERS ON MERCURY

Observations from the orbital phase of the MESSENGER mission confirm the presence of widespread flood volcanism at high northern latitudes on Mercury. Associated with these deposits, referred to as northern smooth plains, are sets of volcanic and tectonic landforms not resolved in flyby images. Among these landforms are numerous impact structures flooded by plains lavas. At least 21 of these flooded craters and basins, with diameters ranging from ~40 to ~300 km, host extensional troughs of multiple orientations, many of which form polygonal patterns concentrated within the inner part of the buried crater. The localization of the troughs in flooded craters and their commonly polygonal patterns raise the question of whether these troughs originate from contraction of the cooling volcanic fill. Several flooded craters in the same size range are devoid of troughs, however, and measured extensional strains accommodated by troughs within craters that host them are at, or exceed, the upper limit on longitudinal contractional strain for plausible ranges of cooling. These observations suggest that trough formation is of tectonic origin or reflects a combination of tectonic and cooling stresses. This inference is seen best in the 300-km-diameter Goethe basin, which, in its interior, contains two smaller buried impact craters of diameter 41 and 56 km. Across these craters, strain is more than twice as high as the average strain expressed across other buried trough-bearing craters, indicating that an increased thickness of the volcanic cover in these two craters, the result of lower pre-flooding elevations, correlates with amplified strains at the surface. Such fracturing behavior is consistent with an uplift origin for the troughs in the flooded craters, since uplift of thicker layers allows trough-bounding faults to propagate deeper and consequently increase their displacements, leading to higher strains at the surface. Moreover, analysis of the global distribution of these extensional terrains reveals that they occur in clusters in both the northern plains and on the plains exterior to the Caloris basin, implying that there is a spatial correlation to regional geologic setting or long-wavelength geophysical variations. Such relations provide insight into the deformational history of Mercury.