Despinning plus global contraction and the orientation of lobate scarps on Mercury: Predictions for MESSENGER

Accommodation of lithospheric shortening on Mercury from altimetric profiles of ridges and lobate scarps measured during MESSENGER flybys 1 and 2

“…Many of these structures, particularly the lobate scarps, are believed to have formed primarily in response to a decrease in planetary radius (Solomon, 1977;Solomon et al, 2008;Watters et al, 2004Watters et al, , 2009) that accompanied cooling of the interior over the course of the planet's evolution (Solomon, 1977;Hauck et al, 2004;Watters et al, 2004;Zuber et al, 2007). Thermal history models (Solomon, 1976(Solomon, , 1977(Solomon, , 1978(Solomon, , 1979Schubert et al, 1988;Hauck et al, 2004;Dombard and Hauck, 2008) are generally consistent with the inference from the distribution of lobate scarps and their stratigraphic relations with plains units that contraction initiated prior to the end of heavy bombardment, with the oldest lobate scarps predating smooth plains emplacement and the youngest postdating the most recent plains material (Solomon et al, 2008). Contraction occurred primarily in response to interior cooling associated with the decay of heat-producing elements, but the thermal state and contractional history were also influenced by the extraction of melt from the mantle, the possible cessation of mantle convection, and the nucleation and growth of a solid inner core .…”

The tectonics of Mercury: The view after MESSENGER's first flyby

“…Models for the lithospheric stresses that produced the observed distribution of tectonic features viewed by Mariner 10 have involved some combination of global contraction accompanying interior cooling, tidal despinning, and stresses related to the formation and modification of the Caloris basin (Strom et al, 1975;Solomon, 1976Solomon, , 1977Solomon, , 1978Solomon, , 1979Cordell and Strom, 1977;Melosh, 1977;Melosh and Dzurisin, 1978;Pechmann and Melosh, 1979;Melosh and McKinnon, 1988;Thomas et al, 1988;Schubert et al, 1988;Phillips and Solomon, 1997;Hauck et al, 2004;Dombard and Hauck, 2008;Matsuyama and Nimmo, 2009). Patterns of lithospheric stress for each of these models predict specific spatial and temporal distributions of contractional faults and their associated lobate scarps.…”

“…Contraction of the planet from interior cooling, absent other influences, would result in global, horizontally isotropic compressive stress and a more or less uniformly distributed population of lobate scarps with no preferred orientation or thrust direction (e.g., Watters et al, 2004). The onset of lithospheric contraction is predicted to occur before the end of heavy bombardment for most thermal history models (Solomon, 1976(Solomon, , 1977(Solomon, , 1978(Solomon, , 1979Schubert et al, 1988;Hauck et al, 2004;Dombard and Hauck, 2008).…”

“…Extension in Mercury's polar regions predicted by tidal despinning has not been observed (Solomon, 1978;Melosh and McKinnon, 1988;Schubert et al, 1988;Watters et al, 2004). Stresses from tidal despinning and contraction may have been coupled if the two processes overlapped in time (Pechmann and Melosh, 1979;Melosh and McKinnon, 1988;Dombard and Hauck, 2008;Matsuyama and Nimmo, 2009;Watters and Nimmo, in press). Dombard and Hauck (2008) drew attention to the possibility that early tidal despinning and global contraction led to a population of northsouth-oriented thrust faults that, although rendered unrecognizable by heavy bombardment, may have been reactivated by subsequent global contraction as the planet continued to cool.…”

“…Stresses from tidal despinning and contraction may have been coupled if the two processes overlapped in time (Pechmann and Melosh, 1979;Melosh and McKinnon, 1988;Dombard and Hauck, 2008;Matsuyama and Nimmo, 2009;Watters and Nimmo, in press). Dombard and Hauck (2008) drew attention to the possibility that early tidal despinning and global contraction led to a population of northsouth-oriented thrust faults that, although rendered unrecognizable by heavy bombardment, may have been reactivated by subsequent global contraction as the planet continued to cool. Matsuyama and Nimmo (2009) emphasized the possible importance of polar wander resulting from the formation and infilling of the Caloris basin and found that the orientations of thrust faults predicted for a combination of such polar wander with contraction and despinning are, under some assumptions, in broad agreement with those observed by Mariner 10 as well as with limited information on the planet's long-wavelength gravity field.…”