Plumes, orogenesis, and supercontinental fragmentation

A time-space relationship between large igneous provinces (LIPS), present d ay hot spots, and the fragmentation of Pangea has been documented over seve ral decades, but the cause of fragmentation has remained elusive. LIPS are regarded either as the result of impingement of a mantle plume on the base of the lithosphere, or as the initial products of adiabatic decompression m elting of anomalously hot mantle. Do LIPS therefore constitute evidence of an active role for plumes from the deep mantle in supercontinental fragment ation, or are they merely the first indications of a large-scale but near-s urface tectonic process? Two long recognized and enigmatic orogenic events may offer a solution to this geologically important 'chicken or egg' conund rum. The reconstructed early Mesozoic Gondwanide fold belt of South America , southern Africa, and Antarctica, could have resulted from 'plume-modified orogeny', flattening of a downgoing lithospheric slab due to the buoyancy of a plume rising beneath a continental margin subduction zone. If so, the similar to 180 Ma Karroo and Ferrar LIPS associated with the opening of the ocean basin between East and West Gondwanaland at similar to 165 Ma result ed from impingement of this plume and are unrelated to the thermal insulati on of the shallow mantle beneath Gondwanaland. It would then follow that th e plume itself played an active, possibly critical, role in the initial bre akup of the supercontinent. The Late Paleozoic 'Ancestral Rockies' deformat ion in the southwestern United States could be yet another example of oroge ny driven by a plume that initiated the break-up of Pangea approximately 15 Myr earlier in the Central Atlantic region. (C) 2000 Elsevier Science B.V. All rights reserved.