Antarctica-New Zealand rifting and Marie Byrd Land lithospheric magmatism linked to ridge subduction and mantle plume activity S. D. Weaver Department of Geology, University of Canterbury, Christchurch, New Zealand R J ' PanWuirst 1 British Antarctic Survey, Natural Environment Research Council, CB3 OET, Cambridge, United Kingdom S. B. Mukasa Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, 48109-1063 V. J. DiVenere Lamont-Doherty Earth Observatory, Palisades, New York, 10964 J. D. Bradshaw Department of Geology, University of Canterbury, Christchurch, New Zealand ABSTRACT Mid-Cretaceous igneous rocks of central Marie Byrd Land, Ant- arctica record a rapid change from subduction-related to rift-related magmatism. This correlates with the final stages of subduction of the Phoenix plate and the subsequent rifting of New Zealand from West Antarctica, prior to the opening of the Southern Ocean. Rift magma- tism produced diverse A-type granitoids and mafic intrusive rocks of continental flood-basalt affinity that were derived ultimately from lithospheric mantle sources. Rifting was caused by changes in plate boundary forces; however, mantle plume activity may have begun in mid-Cretaceous time, triggering melting of the lithosphere and con- trolling the locus of rifting. INTRODUCTION Marine geophysical data and satellite altimetiy indicate that the New Zealand microcontinent separated from Antarctica during the development of the Pacific-Antarctic Ridge (Mayes et al., 1990; Royer et al., 1990), the time of separation being constrained by the oldest oceanic magnetic anomaly adjacent to the Campbell Plateau (Chron 34; 84 Ma). The conjugate margin of the Campbell Plateau and Chatham Rise is found in Marie Byrd Land, West Antarctica (Fig. 1). Marie Byrd Land is well documented as a Cenozoic alkalic volcanic province (LeMasurier and Thomson, 1990), but the pre- Cenozoic geology, which should record the history of continental rifting, is poorly known. The South Pacific Rim International Tectonics Expedition (SPRITE) was planned to clarify the relation of Marie Byrd Land to New Zealand, the East Antarctic craton, and other crustal blocks of West Antarctica. We report here work on the geology of central Marie Byrd Land (Ruppert-Hobbs coast, long 127-140°W). GEOLOGIC SETTING Marie Byrd Land is separated from the Transantarctic Moun- tains by the West Antarctic rift system, a series of subglacial basins filled with Cenozoic and Cretaceous(?) sedimentary rocks (Cooper et al., 1991). On its eastern margin, Marie Byrd Land is separated from the Thurston Island crustal block by Pine Island Bay, a deep trough that appears to continue the trace of the Udintsev Fracture Zone bounding the Chatham Islands margin of New Zealand. The oldest rocks of Marie Byrd Land are those of the turbiditic Swanson Formation which yield late Ordovician metamorphic ages and correlate with the Greenland Group of the Western Province, New Zealand, (Adams et al., 1994). Swanson Formation is intruded by the Devonian-Carboniferous I-type Ford Granodiorite (Weaver et al., 1991). Weaver et al. (1992) described mid-Cretaceous anor- ogenic granites from Edward VII Peninsula, on the Ross Sea margin, and suggested that they formed during continental rifting. K-Ar biotite and zircon-apatite fission-track ages indicate commencement of regional uplift at —100 Ma, immediately after granite emplace- ment (Adams et al., 1994). In the Fosdick Mountains of central Marie Byrd Land, a metamorphic complex was also uplifted and unroofed during rifting (Richard et al., 1994). Throughout Marie Byrd Land, older rocks are unconformably overlain by an extensive Cenozoic volcanic field active since —30 Ma and related by Hole and LeMasurier (1994) to a mantle plume. The Mesozoic geology of Marie Byrd Land contrasts with that of New Zealand, which is dominated by tectonized accretionaiy complex rocks. However, subduction ceased at —105Ma, following collision of the Pacific-Phoenix ridge with the trench, and the tec- tonic regime changed from compressional to extensional (Bradshaw, 1989). Basins containing thick sedimentary sequences formed along Figure 1. Distribution of continental masses around south- west Pacific at present and at 100 Ma. Shape of New Zealand has been modified to account for Neogene deformation on transpresslve Indian-Pacific plate boundary. Reconstruction, based on fitting continental outlines of Campbell Plateau and Marie Byrd Land, calls for - 2 5 % post-100 Ma extension In Ross Sea. However, new paleomagnetic results (DiVenere et al., 1994) suggest that - 5 0 % post-100 Ma extension may have occurred. Stipple Indicates thinned continental crust, dotted lines are selected terrane boundaries within New Zealand. N. NZ and S. NZ = northern, southern New Zealand, CP = Campbell Plateau, CR = Chatham Rise, BT = Bounty Trough (closed), ChP = Challenger Plateau, LHR = Lord Howe Rise, TAS = Tasmania, STR = South Tasman Rise, EP = Edward VII Peninsula, RHC = Ruppert-Hobbs coast, Ti = Thurston Island, MBL = Marie Byrd Land, TAM = Trans- antarctic Mountains. GEOLOGY, v. 22, p. 811-814, September 1994 811