Deriving palaeomagnetic poles from independently assessed inclination and declination data: implications for South American poles since 120 Ma

Palaeomagnetic poles for a stable continental block are typically defined f rom a combination of declination and inclination information from several t emporally constrained studies. Poles from regions that have undergone verti cal-axis rotation as a consequence of tectonics are excluded due to the abs ence of declination data. These poles, however, do contain useful informati on in their inclinations. We develop a simple but statistically rigorous te chnique allowing palaeomagnetic poles to be calculated from a mixture of de clination and inclination data drawn from localities in the stable continen tal block and inclination data from the regions disturbed by vertical-axis rotation. Together this provides a larger data set of high-quality palaeoma gnetic poles from which to calculate reference poles. The technique was use d to define palaeomagnetic poles for South America for the Late Cretaceous- Cenozoic (120-5 Ma) period. Data from stable, cratonic South America, combi ned with data from Africa, rotated into a South America reference frame, an d the Andean margin, yield reference poles, as well as mean poles for the P alaeogene and Neogene. Analysis of the data reveals systematic biases in th e data set, and, in particular, the fit of the inclination data is poor for most time periods. In many cases, this situation is improved if the effect of inclination shallowing due to sedimentary depositional processes and su bsequent compaction is removed. The best-fit poles define an apparent polar wander path for South America that is consistent with the global plate rec onstruction parameters. Use of the new poles in studies of tectonic rotatio n should allow greater temporal and spatial resolution of vertical-axis rot ation and offer the ability to identify smaller rotations in the Andean mar gin.