Abstract

Emplacement of shallow magmatic laccoliths can result in the development of dome-shaped forced folds and fractures in the overburden and at the depositional surface. These folds and fractures have been described from both outcrop studies and seismic interpretation; however, the main controls on fold growth and fracture development remain unclear and, in some cases, controversial. This paper presents a new approach in the analysis of the growth of laccolith intrusions and forced folds. In this study, deep-water growth deposits at an outcrop in the Basque-Cantabrian Basin (north Iberia) record the coeval formation and evolution of a late Albian forced fold and related clastic dike system above the igneous Larragan laccolith. The vertical succession allows field mapping and characterization of the growth deposits, the underlying forced fold, and the forcible Larragan laccolith at depth. The Larragan laccolith is 600 m wide and 40 m thick and intrudes an organic-rich unit at an original (decompacted) depth of ∼450 m. A multidisciplinary study (sedimentology, structure, petrology, and geochemistry) of the growth deposits reveals that the emplacement of the laccolith created a dome-shaped upward-widening fold, which had a final relief of 18–36 m and a width of ∼850 m at the paleoseabed. Simultaneously, in the hinge zone of the fold, local tensional stress induced the formation of a fracture system, which subsequently acted as conduits for overpressured ascending fluids (methane and water) and fluidized sediments originating from the metamorphic aureole of the laccolith. This plumbing system reached the paleoseabed, where extruded sediments were deposited and methane-derived carbonates precipitated. A comprehensive model is proposed in which incremental emplacement of magma sheets induced pulsating contact metamorphism, folding and fracturing of the overburden, and episodic activity of the fluid plumbing system.

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