The composition of mare-highland boundaries is investigated in the region of southwestern Oceanus Procellarum using multispectral images from the Galileo solid-state imaging system. The data are analyzed using a linear image-based spectral mixture model to map the distribution and abundance of spectral end-members representing mare basalt, mature highland soils, and fresh crater materials. The fraction images that result from the mixture modeling are then integrated into a geographic information system database to analyze the relationship between the compositional gradients and regional geologic and geomorphic units. Within this framework, three basic types of mixing gradients are recognized: narrow mixing zones, broad mixing zones, and complex mixing zones. The narrow zones appear to be the results of simple postformation lateral mass transport of surface materials due to the time-averaged effects of impact cratering and regolith gardening. The broad or moderate mixing zones exhibit evidence for significant incorporation of basalt into the distal basin ejecta of Orientale, and thus are interpreted to indicate that pre-Orientale volcanism and volcanic deposits were widespread in the Procellarum region. This is further corroborated by the mare-rich ejecta blanket observed for the 100-km-diameter impact crater Letronne. The complex mixing zones show evidence for extensive pre-Orientale, and perhaps pre-Imbrium cryptomare, as well as some volcanism extending into the late Imbrian period. These results indicate that mare-highland boundaries frequency contain a convolved geologic record that allows insight into the geologic events prior to the final one apparently recorded by surface deposits. For this region of the Moon, we now recognize that volcanism was well established and widespread in early Imbrian time but had become more focused to a few major volcanic regions (Humorum, Procellarum) by the late Imbrian. ¿ 1996 American Institute of Physics.