Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p41a1595b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P41A-1595
Physics
[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5475] Planetary Sciences: Solid Surface Planets / Tectonics, [6235] Planetary Sciences: Solar System Objects / Mercury
Scientific paper
Images taken by the MESSENGER spacecraft reveal a variety of faulting styles on the floors of major impact basins on Mercury. Two of the best-preserved peak-ring basins, Raditladi and Rachmaninoff (diameter D ~ 265 and 290 km) display predominantly circumferentially oriented graben on the floors within their peak rings. Similar graben are seen in Schrödinger basin on the Moon (D ~ 312 km), though the distribution and orientation of graben differ. This faulting style is not observed within much larger (D > 1000 km) impact basins on the Moon, where faulting is likely related to lithospheric flexure caused by subsidence of volcanic loads within the basin. In the Caloris basin on Mercury (D ~ 1500 km), extensional faulting is generally inferred to be related to uplift in response to different phases of exterior volcanic loading or lateral crustal flow. The smaller sizes of the Raditladi and Rachmaninoff basins combined with the absence of graben outside their peak rings make these mechanism unlikely to explain the observed faulting. Moreover, whereas the smooth plains inside the peak ring of Rachmaninoff basin appear to be volcanic in nature, the plains within Raditladi basin are not resolvably younger than the basin and may consist of solidified impact melt. Thus, any mechanism proposed to explain the origin of graben in these basins must apply to either floor material. Here we use thermo-mechanical finite element models to calculate the stress states resulting from several candidate mechanisms for graben formation. Models of subsidence associated with the emplacement of interior fill as well as those of uplift associated with undercompensated basin topography do not lead to the required stress state. We also explore the idea that porous breccias beneath the smooth plains are heated by impact melt or volcanic fill and undergo viscous compaction, causing subsidence of the fill material. In combination with interior filling by plains that thin outward, this mechanism can produce the required stress state, but resulting differential stresses are too small to induce the observed fault slip. Extension driven by cooling and contraction of an outwardly thinning fill can induce the correct stress state with differential stress magnitudes > 5 GPa, sufficient to induce hundreds of meters of fault slip in agreement with observed strains. This fill geometry is consistent with hydrocode models of peak-ring basin formation that yield thicker impact melt in the basin center. If the basin fill is instead volcanic and emplaced in several phases, a radial thinning of each new layer can result from subsidence of the basin center as a result of cooling or loading.
Blair Matthew D.
Freed Andrew M.
Melosh Henry Jay
Prockter Louise M.
Solomon Stanley C.
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