Controls on magma dynamics and emplacement in shallow faulted sediments: Insights from the Cretaceous Armintza Sill (Basque-Cantabrian rift basin, western Pyrenees)

Magmatic and clastic sills are common elements in rift sedimentary basins worldwide. Many documented magmatic sills are based on seismic interpretations, which, owing to limited resolution and imaging constraints, provide only partial insights into their emplacement mechanisms and magma dynamics. This contribution presents detailed field results from a 1.3-km-long coastal outcrop of a Cretaceous (Albian) igneous sill occurring in rifted sediments of the western Pyrenees. This sheet, previously interpreted as a lava flow, shows many diagnostic features that indicate it intruded in organic-rich wet sediments, constituting a sill (the Armintza Sill). Detailed mapping assisted by drone imagery of the sill, host succession, and preexisting faults shows that the sill is composed of magma fingers arranged in a fault-block pattern. Concordant magma fingers were emplaced by thermal fluidization of host sediment due to flash boiling of pore waters, aided by contact metamorphism-induced fluid generation and overpressuring. We suggest that fluid overpressure also caused transient uplift of the thin overburden, leading to the nucleation of inelastic damage along the preexisting faults, facilitating magma ascent up fault. Progressive up-fault verticalization of the σ3 orientation and reduction of inelastic damage likely created a partial barrier near the upper part of the faults, leading to significant flow thickening, deceleration, and diversion. The similarity between these features of the Armintza Sill and those observed in lava flows that interacted with topographic obstacles is interpreted to be the result of near-zero effective stresses in the host sediments.