Morphology and Structure of Flow-Lobe Tumuli from Pune and Dhule Areas, Western Deccan Volcanic Province

The dimension, morphology and structure of seventeen flow-lobe tumuli in pahoehoe flows from western Deccan Volcanic Province are collectively reported here for the first time. The flow-lobe tumuli documented here are generally found in hummocky pahoehoe. The presence of flow-lobe tumuli in pahoehoe flows is consistent with tubefed nature of these flows and suggests emplacement by the mechanism of inflation. The flow-lobe tumuli are of various shapes and sizes. In general, the length of flow-lobe tumuli from the present study ranges from 2.8 to 40 m and their widths range from 2.76 to 28.5 m. The ratio of crust width to tumulus width equals unity indicating that uplift did not involve lateral compression. Variations in shape and size of flow-lobe tumuli are attributable to variations in site characteristics such as nature of substrate, degree of slope, interference of other lobes and local rate of lava supply. Similarly, the study of orientation and location of flow-lobe tumuli within flows reveals that locally defined, random factors control the orientation of flow-lobe tumuli. Their presence generally towards the terininal ends of flow fields suggests that they seldom form over the clogged portions of distributory tubes or pathways. Thus, localized inflation due to blockage of lava tube results in formation of flow-lobe tumuli. The three-tiered (crust-core-basal zone) in ternal structure of the flow-lobe tumuli, resembling the typical distribution of vesicles in P-type lobes, confirms emplacement by the mechanism of inflation. Considering the age of the Deccan Volcanic Province (64-65 Ma) it is most probable that the structures described here may be amongst the oldest recognized examples of lava inflation.