Development of volcanic passive margins: Two‐dimensional laboratory models

Abstract

Volcanic margins are inferred to develop during lithosphere extension above mantle plumes. Continental breakup is characterized in such case by (1) thick seaward dipping lava sequences, (2) plutonic complexes associated with dyke swarms parallel to the coast, and (3) zones of high seismic velocity in the lower crust likely attributable to magma underplating. Comparison with classical nonvolcanic passive margins shows that a striking but nonsystematic structural character of volcanic margins is the narrowness of the domain of crustal attenuation (down to 50 km). The existence of a soft magma body at depth may considerably affect the mechanical behavior of the lithosphere during continental breakup. Here we present a series of scaled experiments designed to study the mechanical effects on lithospheric extension of rheological heterogeneities caused by magma emplacement at various levels. Four‐layer models were constructed with sand and silicone putties in order to represent the brittle and ductile layers of both crust and lithospheric mantle. The underplated magma bodies were simulated by low‐viscosity silicone putty with variable geometry and location. The experimental results are compared to interpreted refraction seismic profiles across volcanic margins in the North Atlantic. A narrow zone of necking is obtained only when the high‐strength layer of the sub‐Moho mantle is interrupted by a heterogeneity of low viscosity representing an underplated magma body. As in cross sections of volcanic margins, models show weak deformation in the brittle upper crust. Sequential addition of sand in the rifted area during extension results in the development of layers dipping toward the rift center. This geometrical pattern is directly comparable to the seaward dipping reflectors sequences. However, the normal faults which developed within the seaward dipping wedge have vergence opposite to the one currently observed in volcanic margins. This suggests a possible component of mantle‐generated stresses during extension.