PLUMES, OROGENESIS, AND SUPERCONTINENTAL FRAGMENTATION

Abstract

A time-space relationship between large igneous provinces (LIPS), present day hot spots, and the fragmentation of Pangea has been documented over several 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 melting of anomalously hot mantle. Do LIPS therefore constitute evidence of an active role for plumes from the deep mantle in supercontinental fragmentation, or are they merely the first indications of a large-scale but near-surface tectonic process? Two long recognized and enigmatic orogenic events may offer a solution to this geologically important `chicken or egg' conundrum. 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 ~180 Ma Karroo and Ferrar LIPS associated with the opening of the ocean basin between East and West Gondwanaland at ~165 Ma resulted from impingement of this plume and are unrelated to the thermal insulation of the shallow mantle beneath Gondwanaland. It would then follow that the plume itself played an active, possibly critical, role in the initial breakup of the supercontinent. The Late Paleozoic `Ancestral Rockies' deformation in the southwestern United States could be yet another example of orogeny driven by a plume that initiated the break-up of Pangea approximately 15 Myr earlier in the Central Atlantic region.