Seismic volcanostratigraphy of large‐volume basaltic extrusive complexes on rifted margins

Abstract

Large‐volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments. The presence and amount of water is of main importance, while local rift basin configuration, erosion, and resedimentation determine the overall geometry of the volcanic constructions. We have developed the concept of seismic volcanostratigraphy, a subset of seismic stratigraphy, to analyze volcanic deposits imaged on seismic reflection data. The method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units. Interpretation of seismic reflection data along the Atlantic and Western Australia rifted margins reveals six characteristic volcanic seismic facies units named (1) Landward Flows, (2) Lava Delta, (3) Inner Flows, (4) Inner Seaward Dipping Reflectors (Inner SDR), (5) Outer High, and (6) Outer SDR. These units are interpreted in terms of a five‐stage tectonomagmatic volcanic margin evolution model comprising (1) explosive volcanism in a wet sediment, broad basin setting, (2) subaerial effusive volcanism forming Gilbert‐type lava deltas along paleoshorelines, (3) subaerial effusive volcanism infilling a fairly narrow rift basin, (4) shallow marine explosive volcanism as the injection axis is submerged below sea level, and finally (5) deep marine sheet flow or pillow‐basalt volcanism. Further, erosion and resedimentation processes are particularly important during the shallow marine stages. Seismic volcanostratigraphy provides important constraints on rifted‐margin development, in particular, on the prevolcanic basin configuration, relative timing of tectonomagmatic events, total amount of volcanic rocks, location of paleoshorelines, and margin subsidence history. These parameters give key boundary conditions for understanding the processes forming volcanic margins and other large‐volume basaltic provinces.