Hydraulic fracturing as a possible mechanism of dyke-sill transitions and horizontal discordant intrusions in trachytic tabular bodies of Arraial do Cabo, State of Rio de Janeiro, Brazil

Abstract

This paper presents some field descriptions and genetic considerations about dyke-sill transitions and horizontal-discordant intrusions of the early Tertiary felsic alkaline dyke swarm at Arraial do Cabo, State of Rio de Janeiro, Brazil. Two examples show the entire process of dyke-sill transition. Near sea level they are vertical-discordant; at middle height, oblique-concordant; at the top of the cliff, horizontal-discordant. The latter process cannot be explained by a traditional model of magma intrusion along weaknesses in the host body. It can be accounted for by hydraulic fracturing: magma pressure creates a new fracture normal to O3, regardless of whether old fracture systems exist. The dyke-sill transition takes place at a depth such that O3 changes direction from horizontal to vertical. In the specific case described here, the stress change could be caused by intrusion and volume expansion of the Cabo Frio Island syenitic body.