5:15 PM - 7:15 PM
[PPS08-P19] Formation process of the buried graben in Mare Frigoris found by SELENE Lunar Radar Sounder
Keywords:Mare Frigoris, Graben, Imbrium mascon, Thermal expansion of the Moon, SELENE Lurar Radar Sounder (LRS)
Rilles and grabens found on the lunar surface were formed by extensional stress. It is suggested by Solomon and Head (1979) that rilles in Mare Imbrium were formed by extensional stress due to lava loads accumulated in Imbrium basin. Imbrium mascon forms extensional stress not only in Mare Imbrium but also in farther regions such as Mare Frigoris. However, the rilles found on the surface in Mare Frigoris are not considered to be formed by Imbrium mascon but by the thermal expansion of the Moon from their directions (Williams et al., 2019). Using SELENE Lunar Radar Sounder (LRS), the rilles and grabens buried by lava flows can be included into our discussions.
While subsurface reflectors in Frigoris were already found in LRS data by Pommerol et al. (2010), detailed analyses in this study revealed that there were three subsurface reflectors suggesting a large-scale graben in the western-central part of Frigoris, which is below lava flow unit WCF1 (Kramer et al., 2015) whose age is estimated to be 3.5 Ga. Two subsurface reflectors (Reflectors A and B) with dips of 1.1° and 1.5° in the south direction were found in a latitude from 58 to 60°N, and one horizontal reflector (Reflector C) at different depth was found in a less latitude than 58°N. From the subsurface reflectors, the following processes are suggested: (i) Paleo lunar surface (Reflector C) was formed by lave eruption in the western-central part of Frigoris. (ii) Due to the extensional stress, large-scale normal faults were formed at 58°N and 60°N, the surface between 58°N and 60°N (Reflector B) was collapsed, and a large graben was formed. Since the displacements at two faults were different, the surface has a dip of 0.4° in south direction. (iii) Due to the lava eruption, the graben was partially filled and a new horizontal surface (Reflector A) was formed on the floor of the graben. (iv) Due to the activity at the faults, the current and buried floor (Reflectors A and B) of the graben have dips of 1.1° and 1.5° respectively. (v) Due to the lava eruption, the graben and southern plain were covered by lava flow unit WCF1 at 3.5 Ga.
To check whether Imbrium mascon could form the buried graben found in Frigoris, calculations of stress distribution in the lithosphere loaded with lava disks have been performed as in Solomon and Head (1979). Freed et al. (2001) suggested that the equations for derivation of the stress indicated in Solomon and Head (1980) included some errors. They were corrected in the calculations in this study. In Calculations 1 and 2 using the same and similar lava load models with those in Solomon and Head (1980), a total thickness of which is 8 km, the location of the buried graben in Frigoris could not be explained. On the other hand, in Calculation 3 using a lava load model with a load radius of 320 km and load thickness of 1 km estimated from gravity anomaly observation (Gong et al., 2016), the locations of the buried graben in Frigoris, and the rilles in Imbrium could be explained if the lithosphere thickness was larger than 75 km. The calculated stress was, however, up to 0.01 kbar, which is less than the extensional stress 0.2 kbar due to thermal expansion of the Moon at about 3.6 Ga (Solomon and Head, 1979), which suggests that Imbrium mascon could not be main factor of the formation of the rilles in Imbrium and buried graben in Frigoris also.
While subsurface reflectors in Frigoris were already found in LRS data by Pommerol et al. (2010), detailed analyses in this study revealed that there were three subsurface reflectors suggesting a large-scale graben in the western-central part of Frigoris, which is below lava flow unit WCF1 (Kramer et al., 2015) whose age is estimated to be 3.5 Ga. Two subsurface reflectors (Reflectors A and B) with dips of 1.1° and 1.5° in the south direction were found in a latitude from 58 to 60°N, and one horizontal reflector (Reflector C) at different depth was found in a less latitude than 58°N. From the subsurface reflectors, the following processes are suggested: (i) Paleo lunar surface (Reflector C) was formed by lave eruption in the western-central part of Frigoris. (ii) Due to the extensional stress, large-scale normal faults were formed at 58°N and 60°N, the surface between 58°N and 60°N (Reflector B) was collapsed, and a large graben was formed. Since the displacements at two faults were different, the surface has a dip of 0.4° in south direction. (iii) Due to the lava eruption, the graben was partially filled and a new horizontal surface (Reflector A) was formed on the floor of the graben. (iv) Due to the activity at the faults, the current and buried floor (Reflectors A and B) of the graben have dips of 1.1° and 1.5° respectively. (v) Due to the lava eruption, the graben and southern plain were covered by lava flow unit WCF1 at 3.5 Ga.
To check whether Imbrium mascon could form the buried graben found in Frigoris, calculations of stress distribution in the lithosphere loaded with lava disks have been performed as in Solomon and Head (1979). Freed et al. (2001) suggested that the equations for derivation of the stress indicated in Solomon and Head (1980) included some errors. They were corrected in the calculations in this study. In Calculations 1 and 2 using the same and similar lava load models with those in Solomon and Head (1980), a total thickness of which is 8 km, the location of the buried graben in Frigoris could not be explained. On the other hand, in Calculation 3 using a lava load model with a load radius of 320 km and load thickness of 1 km estimated from gravity anomaly observation (Gong et al., 2016), the locations of the buried graben in Frigoris, and the rilles in Imbrium could be explained if the lithosphere thickness was larger than 75 km. The calculated stress was, however, up to 0.01 kbar, which is less than the extensional stress 0.2 kbar due to thermal expansion of the Moon at about 3.6 Ga (Solomon and Head, 1979), which suggests that Imbrium mascon could not be main factor of the formation of the rilles in Imbrium and buried graben in Frigoris also.