What's news about the Deccan traps emplacement ? An attempt to estimate the eruptive sequence of lava emplacement.

Synchronism between Large Igneous Provinces formations and mass extinctions along the Phanerozoic is clearly evidenced and suggests that LIPs could be deeply involved in such environmental crisis. As example, the Cretaceous-Tertiary (K-T) mass extinction coincides with the emplacement of the Deccan continental flood basalts. However the climatic and environmental impact of the traps emplacement is not well known. One reason is that we don't know the duration of traps emplacement. Actually, time required for the emplacement of large traps of about 1.106 km3 is estimated to 1 Myr. Traps are composed of hundreds of single lava flow. The volume of the single flow can reach about 1000 km3. Such lava flow is not known in recent and historical volcanic eruption. However, in the case of recent largest volcanic eruptions (1783, Laki or 1815, Tambora) a global cooling of 1-2°C could be observed. This temperature change depends on two parameters: the volcanic eruption rate and the amount of sulfur gas injected in the atmosphere. As a consequence, the climatic impact of traps is directly linked to their eruptive sequence. Here we present an estimate of the time sequence of flow emplacement of the Deccan traps constrained by detailed magnetostratigraphy (using geomagnetic secular variation as a century-scale clock) and soil formation (using the development of alteration). This study has been done on the 12 geological formations, which have been chemically defined along the Western Ghats (western flank of the Deccan traps). Altogether, we analysed samples from 168 paleomagnetic sites located into the well exposed lava flows along 11 traverses in the areas of Mahabaleshwar, Nasik and Pune (Western Ghats). Rock magnetism experiments have shown that the formations are mainly characterized by the presence of a single thermally stable ferromagnetic phase compatible with low-Ti titanomagnetite and indicated a pseudo single-domain grain size. Samples were analyzed, mainly using thermal demagnetization, in order to determine site-mean directions of characteristic magnetizations and by this way reconstruct the magnetostratigraphy of the Deccan traps in these regions. Evolution of these directions as a function of stratigraphic position shows successive flows with well-grouped (correlated) directions, which testify to rapid emplacement. The eruptive sequence can therefore be divided into several lava pulses. Therefore, classic geochronological methods have been used in order to correlate dating with results of the paleomagnetic analysis. One may next attempt to estimate the amount of volcanic gases emitted during each pulse, scaling with data from analogous historical or geological fissure eruptions. Reconstruction of the volcanic forcing function (both in amplitude and time) may allow us to estimate the possible climate change induced by the emplacement of the Deccan traps and its potential role on the K-T crisis.