Thickness Characteristics of Pahoehoe Lavas in the Deccan Province, Western Ghats, India, and in Continental Flood Basalt Provinces Elsewhere

Constraining the eruption rates of flood basalt lava flows remains a significant challenge despite decades of work. One potential observable proxy for eruption rates is flood basalt lava-flow lobe thicknesses, a topic that we tackle here quantitatively. In this study, we provide the first global compilation of pahoehoe lava-lobe thicknesses from various continental flood basalt provinces (similar to 3,800 measurements) to compare characteristic thicknesses within and between provinces. We refer to thin lobes (similar to <= 5 m), characteristic of compound lavas, as hummocky pahoehoe lava flows or flow-fields. Conversely, we term thicker lobes, characteristic of simple flows, as coming from sheet-lobe-dominated flows. Data from the Deccan Traps and Columbia River flood-basalt provinces are archetypal since they have the most consistent datasets as well as established chemo- and litho-stratigraphies. Examining Deccan lobe thicknesses, we find that previously suggested (and disputed) distinct temporal and regional distributions of hummocky pahoehoe and sheet-lobe-dominated flow fields are not strongly supported by the data and that each geochemically defined formation displays both lobe types in varying amounts. Thin flow-lobes do not appear to indicate proximity to source. The modal lobe thickness of Deccan formations with abundant thin lava-lobes is 8 m, while the mode for sheet-lobe-dominated formations is only 17 m. Sheet-lobes up to 75-80 m are rare in the Deccan and Columbia River Provinces, and ones >100 m are exceptional globally. For other flood basalt provinces, modal thickness plots show a prevalence toward similar lobe thicknesses to Deccan, with many provinces having some or most lobes in the 5-8 m modal range. However, median values are generally thicker, in the 8-12 m range, suggesting that sheet-lobes dominate. By contrast, lobes from non-flood basalt flow-fields (e.g., Hawai'i, Snake River Plain) show distinctly thinner modes, sub-5 m. Our results provide a quantitative basis to ascertain variations in gross lava morphology and, perhaps, this will in future be related to emplacement dynamics of different flood basalt provinces, or parts thereof. We can also systematically distinguish outlier lobes (or regions) from typical lobes in a province, e.g., North American Central Atlantic Magmatic Province lava-lobes are anomalously thick and are closely related to feeder-intrusions, thus enabling a better understanding of conditions required to produce large-volume, thick, flood basalt lava-lobes and flows.

Automated summary

Constraining the eruption rates of flood basalt lava flows poses a significant challenge, with observable proxy being the thickness of lava-flow lobes. A global compilation of pahoehoe lava-lobe thicknesses from various provinces reveals that thin flow-lobes do not indicate proximity to source, and most provinces have similar lobe thicknesses albeit generally thicker. Additionally, non-flood basalt flow-fields show distinctly thinner modes compared to flood basalt provinces.