Problematic Palaeomagnetics and the Position of Palaeogene Eurasia: a Review; Implications for the Tectonic Modeling of East Asia; Efforts to Resolve the Issue

Abstract

Palaeomagnetic data provide key quantitative information for the positioning of crustal blocks, and have proved particularly valuable in modeling theCenozoic tectonic evolution of continental eastern Eurasia and its adjacent marginal basins. In essence "arriving" blocks and arcs and opening basins are positioned relative to a "fixed" (in reality slowly moving) Eurasia. However, the palaeomagnetism community is currently debating the mechanism and effects of what is termed the "stable Asia shallow inclination problem". That is, assuming a coherent Eurasia, palaeomagnetic data obtained from Upper Cretaceous-Miocene red-beds from "stable" central Asia are too shallow with respect to Palaeogene Eurasia's apparent pole, which is based almost entirely on data from the British sector of the North Atlantic Igneous Province (NAIP). Several explanations have been proposed, the most radical being an incoherent Eurasia with tectonic boundaries along the Tornquist-Tesserye Line and/or Ural Mountain belt (Cogne et al., JGR, 1999). With a possible positioning error of ~1600 km for southern and eastern Eurasia, the implications for modeling forthe India Collision are critical and for marginal basin formation in E-SE Asia, extrusion model and arc-arc collision in central Japan etc are quite considerable. The "shallow inclination" issue will be reviewed, followed by a discussion of the various efforts palaeomagnetic workers are currently carrying out to resolve the problem. These include a reinvestigation of NAIP rocks (Rissager et al., EPSL, in press: old NAIP pole too high), and the first datasets from (i) mid-late Palaeogene basalts from central Asia (Bazhenov & Mikolaichuk, EPSL, 2002: old NAIP pole about right) and (ii) Lower Eocene sedimentary rocks from southern England (Ali et al., GJI, in review: old NAIP pole too high).