Development of volcanic passive margins: Two-dimensional laboratory models

Volcanic margins are inferred to develop during lithosphere extension above mantle plumes. Continental breakup is characterized in such case by (1) th ick seaward dipping lava sequences, (2) plutonic complexes associated with dyke swarms parallel to the coast, and (3) zones of high seismic velocity i n the lower crust likely attributable to magma underplating. Comparison wit h classical nonvolcanic passive margins shows that a striking but nonsystem atic structural character of volcanic margins is the narrowness of the doma in of crustal attenuation (down to 50 km). The existence of a soft magma bo dy at depth may considerably affect the mechanical behavior of the lithosph ere during continental breakup. Here we present a series of scaled experime nts designed to study the mechanical effects on lithospheric extension of r heological heterogeneities caused by magma emplacement at various levels. F our-layer models were constructed with sand and silicone putties in order t o represent the brittle and ductile layers of both crust and lithospheric m antle. The underplated magma bodies were simulated by low-viscosity silicon e putty with variable geometry and location. The experimental results are c ompared to interpreted refraction seismic profiles across volcanic margins in the North Atlantic. A narrow zone of necking is obtained only when the h igh-strength layer of the sub-Moho mantle is interrupted by a heterogeneity of low viscosity representing an underplated magma body. As in cross secti ons of volcanic margins, models show weak deformation in the brittle upper crust. Sequential addition of sand in the rifted area during extension resu lts in the development of layers dipping toward the rift center. This geome trical pattern is directly comparable to the seaward dipping reflectors seq uences. However, the normal faults which developed within the seaward dippi ng wedge have vergence opposite to the one currently observed in volcanic m argins. This suggests a possible component of mantle-generated stresses dur ing extension.