New Estimates on the Basalt Volume of the Tarim (Not So Large) Igneous Province, NW China

The early Permian basalts of the Tarim Basin of NW China are geographically widespread, covering over 200,000 km(2) and are in places up to 1-km thick and hence frequently referred to represent a Large Igneous Province (LIP). Available volume estimates are based on average lava thickness multiplied by area, using these values results in volumes exceeding 100,000 km(3), and is commonly cited as being evidence of LIP status. Using a database of 58 boreholes, field sections, and seismic data we calculate a range of basalt isopach maps, and use these to estimate new, more refined overall eruption volumes. These volumes are significantly smaller than previously reported estimates. We demonstrate that the thickness distribution of erupted basalts is nonuniform and strongly skewed, indicating that volume estimates using an arithmetically averaged thickness of input data will result in overestimation of total volumes. We find that even with the addition of an estimated nonerupted volume component, representing cumulates and intrusions, that the total volume of the Tarim volcanics is significantly below the minimum threshold of a LIP. Using a simple Monte Carlo model we demonstrate that even while accounting for uncertainty and error in the input data, the probability of the total volume exceeding 100,000 km(3) is very low. The new volume estimates imply a revision of the climate and environmental impact of the Tarim volcanism. We therefore suggest that the term Tarim Igneous Province should be used instead of Tarim Large Igneous Province. Plain Language Summary The early Permian (287-267 million years ago) basaltic rocks of the Tarim Basin in Northwestern China are geographically widespread, and can be up to 1 km in thickness. This has led to the hypothesis that these rocks represent a period of significantly voluminous igneous activity across the area, in what is known as a Large Igneous Province (LIP). LIP are defined by a set of criteria including area, volume, and a short-duration of emplacement. In this study, we have collated all available data to test whether the basalts within the Tarim Basin meet the minimum criteria to be classified as a LIP. In this study, we demonstrate that when integrating a large number of geographically widespread boreholes and outcrops, regional seismic data and published age data that the Tarim basalts represent a significant period of igneous activity, but do not meet the threshold of a LIP. This finding is important as the volume of igneous rock formed during periods of intense igneous activity is used in geological and climate models to infer past climatic effects and extinctions. We also demonstrate that relatively simple calculations of the volume of LIPs will typically overestimate their volume and thus environmental impact, and therefore suggest that all future studies of this nature should aim to integrate all available data and consider uncertainty and probability when estimating eruption volumes.