MORPHOLOGY OF PAHOEHOE FLOWS IN THE DECCAN VOLCANIC PROVINCE (INDIA) AND THE BIG PINE VOLCANIC FIELD (CALIFORNIA) AND IMPLICATIONS FOR VOLCANOMORPHOLOGICAL EVOLUTION OF RIFTED CONTINENTAL MARGINS

Pahoehoe flows are observed in a wide range of geologic settings including shield volcanoes, monogenetic cinder cone fields, and continental flood basalts. This study documents pahoehoe flow morphology from two distinctly different geologic settings, the Deccan Volcanic Province (DVP), India, and the Big Pine Volcanic Field (BPVF), California. DVP is a result of extensive tholeiitic basaltic volcanism associated with the rifted continental margin evolution in Gondwana-Land that spanned the K-T boundary. BPVF is part of an embryonic, transtensional rift system east of the Sierra Nevada Mountains and is characterized by Quaternary alkali-subalkali basalt eruptions.

Pahoehoe lavas from DVP are very similar to their Hawaiian counterparts. They are strongly compound on a local scale with hummocky as well as sheet-like morphology. Tumuli and squeeze-ups are common and a typical flow lobe displays a vesicular upper crust, a dense core, and a basal vesicular zone with pipe amygdales. No convincing evidence of major lava tubes has been found in DVP. Preliminary observations suggest that the ubiquitous squeeze-ups developed on inflated sheet lobes acted as ephemeral vents and this aided the long distance transport of lava during sustained eruptions. In contrast to DVP, numerous lava tubes, along with tumuli are associated with the BPVF pahoehoe flows. Most flows are transitional pahoehoe including slabby, toothpaste, and rubbly varieties. Unlike the compound flows in DVP, these flows commonly consist of a single dominant lobe. Highly stretched and elongated vesicles in the crust and locally in the core, as opposed to more equant vesicles in the DVP flows, indicate a higher initial viscosity. Pipe vesicles are conspicuous by their absence. This observation is in agreement with previous studies in Hawaii suggesting that these features develop on low slopes; the relatively higher slopes on the flanks of the cinder cones in BPVF apparently have inhibited their development. We explain these differences in morphology by a combination of a) Initial lava composition, b) Volumetric rate of effusion and actual volumes of lava erupted, c) Palaeotopography and cooling history. We use the changes in these variations to decipher the volcanomorphological evolution of rifted continental margins in different tectonic settings.