Gruithuisen domes region: A candidate for an extended nonmare volcanism unit on the Moon

Small lunar areas known as red spots are thought to represent good candidates for nonmare volcanic units predating or contemporaneous with the period of mare volcanism. In this study, we present imaging telescopic and orbital Clementine multispectral surveys of the Gruithuisen region, located in the northwestern border of Mare Imbrium, which contains Imbrian age domical features of likely viscous volcanic origin with spectral characteristics of red spot areas. A rather spectrally homogeneous unit having typical characteristics of red spots is found to coincide with the material within the morphological boundaries of the Gruithuisen Delta, Gamma and Northwest domes. An extended "dome-like" unit is identified spectrally and surrounds and extends to the west and the north of the domes. This unit shows spectral characteristics close to those of the domes, suggesting the presence of a significant amount of dome material extending many kilometers away from the domes themselves. Both the spectral characteristics of the dome and the dome-like unit are clearly different from those of highlands and the surrounding mare basalts. The spectral identification of a widespread dome-like unit suggests that the specific style of eruption that is inferred for the formation of the domes (i.e., viscous flows of possible more silicic composition) might have occurred on a regional scale in this part of the Moon, prior to the Iridum event about 3.8 Gyr ago. This volcanic style appears to be more widespread in the early part of lunar history than previously thought.