PALEOLATITUDE OF UPPER PERMIAN TO MID-MESOZOIC VOLCANIC SEQUENCES, NORTHWEST VIETNAM

The Mesozoic dispersal of the Gondwana continents and closure of paleo- and neo-Tethys involved the accretion of several numerous continental fragments to southern Eurasia with attending arc magmatism. Accurate paleo-latitude and paleo-orientation information on these fragments that now comprise Indochina will provide a better understanding of their original configuration relative to the South China Block and timing and style of deformation associated with assembly of Indochina. We have collected well-exposed sequences of uppermost Permian (?) to Triassic volcanic rocks (mainly vesicular basalts and basaltic andesites) exposed near the Song Da lineament (between Song Ma and the Red River) in northwest Vietnam. The sections include numerous distinct lava flows, with clear indications of flow tops and bottoms and consistent orientations of flows at each section. Dip magnitudes and dip directions vary from section to section, affording a regional inclination-only test at the scale of the sampling region. The rocks show excellent response in progressive alternating field and/or thermal demagnetization. For any single section sampled, there is a mix of carriers of the characteristic remanent magnetization (ChRM), with some flows having low Ti magnetite as the principal carrier of the ChRM and other flows having hematite as the ChRM. At the flow (site) level, magnetization directions are consistent and estimates of mean directions have 95% confidence estimates of less than 7^o. Preliminary between-site (flow) data suggest that a reasonable magnitude of paleosecular variation has been recorded. In situ estimated site mean directions do not resemble predicted time-averaged geomagnetic field directions for the study area for Triassic and younger time, using paleomagnetic poles from the South China Block. We argue that these volcanic sections have not been pervasively remagnetized. When corrected for local tilt, based on the orientations of flows at each section, magnetizations are of dual polarity with inclinations generally less than 20^o, implying paleolatitudes within 10^o of the paleoequator. At most localities, overall declinations show little evidence of appreciable vertical axis rotation.