The Ross orogen of Antarctica is one of Earth's great Phanerozoic mountain belts. It is thought from igneous geochemistry, deformation patterns, and sedimentation history to be the result of late Neoproterozoic and early Paleozoic plate-margin convergence between paleo-Pacific oceanic lithosphere and continental lithosphere represented by the composite East Antarctic shield. Convergence along this margin is contemporaneous with, and tectonically linked to, amalgamation of the Gondwana supercontinent following collapse of former ocean basins and collision along the East African orogen. Although there is general agreement about the large-scale tectonic framework of the Ross orogen, there is a great deal of remaining uncertainty regarding crustal province correlation, deformation kinematics, precise timing, and plate-margin paleogeography. Our uncertainty stems from (1) a fragmentary record left by younger tectonic events that have modified and, in some cases, removed parts of the orogen, and (2) extensive ice cover. Because the basement geology of the Ross orogen is composed largely of metamorphic rocks, however, study of the metamorphic roots of the orogen should help to constrain tectonic setting, thermal structure, tectonic displacements, cooling history, and timing. Evidence in the metamorphic domains reflects 60–100 million years of continental-margin subduction, which is characterized by primary magmatic crustal accretion and low- P/T magmatic-arc metamorphism, crustal thickening and high- P/T metamorphism due to convergence and oceanic-arc collision, and high- P/T metamorphism associated with seaward growth of a plate-margin accretionary system.

Siliciclastic rocks of the upper Byrd Group in the Transantarctic Mountains record rapid denudation and molasse deposition during Ross orogenesis along the early Paleozoic convergent margin of Gondwana. These rocks, which stratigraphically overlie Lower Cambrian Byrd carbonate deposits, are dominated by fresh detritus from proximal igneous and metamorphic sources within the Ross Orogen. Biostratigraphic evidence indicates that deposition of the siliciclastic succession is late Botomian or younger (<515 Ma). The largest modes of U-Pb and 40 Ar/ 39 Ar ages from detrital zircons and muscovites respectively in the siliciclastic molasse are Early to Middle Cambrian, but based on ages from crosscutting igneous bodies and neoblastic metamorphic phases, deposition of individual molasse units continued until ∼490–485 Ma (earliest Ordovician). The entire episode of interrelated tectonic, denudational, sedimentary, deformational, and magmatic events is restricted to a time interval of 7–25 m.y. in the late Early Cambrian to earliest Ordovician, within the resolution of these stratigraphic and geochronologic data. Stratigraphic relationships suggest that the detrital zircon and muscovite in the sediments came from the same source terrain, consistent with large volumes of molasse having been shed into forearc and/or marginal basins at this time, primarily due to erosion of igneous rocks and metamorphic basement of the early Ross magmatic arc. Rapid erosion and unroofing in the axial Ross Orogen is consistent with a sharp carbonate-to-clastic stratigraphic transition observed in the upper Byrd Group, reflecting an outpouring of alluvial fan and fluvial-marine clastic detritus. The short time lag between tectonism and sedimentary response is similar to that determined for the corresponding section of the Ross-Delamerian orogen in South Australia and other continental-margin arc systems, such as in the Mesozoic Peninsular Ranges of California. Mineral cooling ages from metamorphic basement adjacent to the orogen yield a syn- to late-orogenic cooling rate of ∼10 °C/m.y., which, combined with a known metamorphic geotherm, indicates a denudation rate of ∼0.5 mm/yr. Such denudation rates are comparable to those in recent convergent or collision orogens and suggest that crustal thickening associated with both magmatic intrusion and structural shortening was balanced by near-synchronous erosional exhumation.

Early Mesozoic blueschist terranes in the Klamath Mountains (the Stuart Fork) and Sierra Nevada (the Red Ant), are lithologically, petrologically and structurally similar. Both contain common crossite-epidote assemblages formed during subduction of oceanic sedimentary and volcanic protoliths that had previously been metamorphosed at an ocean ridge. Both are remnants of a high-P/T metamorphic belt paired with a more easterly magmatic arc along the western margin of North America. These similarities imply that the Mesozoic blueschists in both mountain belts shared a common early history. Important differences, however, indicate that their later postsubduction histories were different: blueschists in the Klamath Mountains were preserved without overprinting by higher T assemblages, whereas blueschists in the Sierra Nevada were overprinted by pumpellyite-actinolite facies metamorphism. From these relations we infer that the Stuart Fork rocks may have been lifted upward more rapidly than the Red Ant terrane, or that the Red Ant terrane was incompletely subducted and not subject to the cooling effect of continuing subduction. The Stuart Fork blueschists were also affected by a contact metamorphic event during widespread Middle to Late Jurassic plutonism, which is not manifest in the northern Sierra Nevada blueschist terrane.