Seismic insights into eruption styles and critical zone of the Main Deccan Volcanic Province

Abstract

The Deccan Volcanic Province (DVP) is a large igneous province that originated approximately 65 million years ago and has played a significant role in shaping the Earth’s geological history, contributing to catastrophic events and influencing climate changes. Hence, the study of DVP has fascinated numerous geologists and geophysicists and remains a very active area of research. Geophysicists have the advantage of imaging the deep interior of the subsurface without access to subsurface or rock samples, but the inferences are constructed by indirect observation. Seismic methods are by far the most applied technique for obtaining high-resolution subsurface imaging. Therefore, the current study presents a detailed investigation of the DVP region employing seismic methods with a special focus on lava emplacement style and critical zone processes. The seismic data acquired under the national seismic program over the main DVP area are analyzed for this study. The data was acquired using a symmetric split-spread geometry with a long offset (9 km) and large explosive (12.5 to 15 kg of dynamite) as the source, along with high subsurface sampling (10mcommon-midpoint (CMP) interval). During this study, the data was also reprocessed for four lines using a pre-stack migration workflow to improve subsurface imaging. A key feature of this processing is the estimation of shear wave velocity using ground roll data. The ground roll dispersion curves are estimated for CMP gathers at every 2.5 km interval across four lines of varying orientation and locations. The method provided shear velocity information up to 1.5 km depth, as indicated by sensitivity analysis. The seismic stratigraphy of theDeccan lava flowis prepared based on seismic reflection characteristics. This characterization resulted in four distinct classes of seismic units, and subsequently, thickness maps were generated for each of the four units. The thickness maps helped in deciphering the location of the lava source and its evolution over the time. The transition of the source position agrees well with the hypothesis of the northwardmoving Indian plate over the Reunion plume. The seismic facies analysis reveals the existing topography control over the lava emplacement style. Hence, this study provides a comprehensive understanding of the emplacement style and source of lava in the DVP region. The study of the interaction among the geosphere, hydrosphere, atmosphere, and biosphere is often referred to as critical zone studies. To comprehend the critical zone processes occurring in continental flood basalt provinces, their structure and evolution remain a challenging scientific problem due to lithological complexities and variable weathering patterns. As these processes change the physical properties of the subsurface, geophysical methods such as seismic can provide vital clues about these processes. Although basalt flow stratigraphy in the DVP has been studied in detail, field-scale seismic velocity variations within these flows and their inner structure remain mostly unexplored. This part of the study combines seismic and volcanological data to examine weathering patterns in the topmost 50 m of basalt lava flows around Pune city in the western DVP. Using the multi-channel analysis of surface waves method, we evaluate variations in shear wave velocity across flow units and dykes. By co-analyzing seismic data with morphological variations across outcrops, we devise a field-scale velocity characterization across basalt lavas and dykes. Critical zone facies, identified and validated through outcrop examinations, include soil, weathered bedrock with vesicular basalt, columnar-jointed lava cores, red bole, and intrusive dykes. An investigation of vegetation distribution, landscape morphology, and lithological variability provides insight into the pivotal weathering and erosional processes that shape the critical zone in DVP.