Search

We report geochemical data from (meta-) sedimentary and igneous rocks that crop out in the Ford Ranges of western Marie Byrd Land and discuss the evolution and reworking of the crust in this region during Paleozoic subduction along the former Gondwanan convergent plate margin. Detrital zircon age spectra from the Swanson Formation, a widespread low-grade metaturbidite sequence, define distinct populations in the late Paleoproterozoic, late Mesoproterozoic, and Neoproterozoic-Cambrian. The late Paleoproterozoic group records magmatism derived from a mixed juvenile and crustal source. By contrast, the late Mesoproterozoic group yields Hf isotope values consistent with derivation from a juvenile Mesoproterozoic source inferred to be an unexposed Grenville- age orogenic belt beneath the East Antarctic ice sheet. For the Neoproterozoic- Cambrian population, Hf isotope values indicate reworking of these older materials during Ross-Delamerian orogenesis. New U-Pb ages from the Devonian-Carboniferous Ford Granodiorite suite across the Ford Ranges reveal an extended period of arc magmatism from 375 to 345 Ma. For four younger samples of Ford Granodiorite, Hf and O isotope values in zircon suggest involvement of a larger (meta-)sedimentary component in the petrogenesis than for two older samples. This contrasts with the secular trend toward more juvenile values documented from Silurian to Permian granite suites in the Tasmanides of eastern Australia and Famennian to Tournasian granite suites in New Zealand, pieces of continental crust that were once contiguous with western Marie Byrd Land along the Gondwana margin. The differences may relate to an along-arc change from the typical extensional accretionary mode in eastern Australia to a neutral or an advancing mode in West Antarctica, and to an across-arc difference in distance from the trench between the New Zealand fragments of Zealandia and western Marie Byrd Land. Upper Devonian anatectic granites in the Ford Ranges most l

There are few direct constraints on the timing and style of faulting in the Ross Sea sector of the West Antarctic rift system, although Cretaceous plate reconstructions indicate that Ross Sea extension between East and West Antarctica occurred prior to breakup of the Gondwana margin ca. 80 Ma. Mylonitic gneisses dredged from the eastern Ross Sea indicate shear-zone deformation considerably earlier, at 98-95 Ma. Strain analysis of fabrics indicates 85%-100% extension. Overprinting brittle structures record translation of shear-zone gneisses into the upper crust. Samples yield sensitive high-resolution ion-microprobe U-Pb zircon ages of 102-97 Ma, correlated to Byrd Coast Granite onshore, and concordant 40Ar/39Ar biotite and K-feldspar ages of 98-95 Ma, indicating that granites were mylonitized soon after emplacement and cooled rapidly. Apatite fission-track data corroborate this rapid cooling event, and reveal a second rapid cooling event ca. 80 Ma. Evidence for contemporaneous deformation and a similar thermal evolution at Deep Sea Drilling Project Site 270 on the Ross Sea central high and for a migmatite dome on land attests to the regional extent of intracontinental extension. Extension occurred at a time of complex microplate interactions along the Cretaceous active Gondwana margin, suggesting that distributed deformation in the overriding Antarctic plate may be related to plate boundary dynamics.