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

Abstract

Summary Palaeomagnetic poles for a stable continental block are typically defined from a combination of declination and inclination information from several temporally constrained studies. Poles from regions that have undergone vertical‐axis rotation as a consequence of tectonics are excluded due to the absence of declination data. These poles, however, do contain useful information in their inclinations. We develop a simple but statistically rigorous technique allowing palaeomagnetic poles to be calculated from a mixture of declination and inclination data drawn from localities in the stable continental block and inclination data from the regions disturbed by vertical‐axis rotation. Together this provides a larger data set of high‐quality palaeomagnetic poles from which to calculate reference poles. The technique was used to define palaeomagnetic poles for South America for the Late Cretaceous–Cenozoic (120–5 Ma) period. Data from stable, cratonic South America, combined with data from Africa, rotated into a South America reference frame, and the Andean margin, yield reference poles, as well as mean poles for the Palaeogene and Neogene. Analysis of the data reveals systematic biases in the 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 subsequent compaction is removed. The best‐fit poles define an apparent polar wander path for South America that is consistent with the global plate reconstruction parameters. Use of the new poles in studies of tectonic rotation should allow greater temporal and spatial resolution of vertical‐axis rotation and offer the ability to identify smaller rotations in the Andean margin.

Paleomagnetism and tectonics of the southern Atacama Desert (25–28°S), northern Chile

“… Besse and Courtillot [2002] have updated several APWPs for most continents. Their new SA APWP (hereafter labeled BC02) is in good agreement with the Lamb and Randall [2001] APWP. The BC02 APWP is the most detailed and reliable APWP and it should henceforth be used to calculate tectonic rotations.…”

Implications for crustal rotation and tectonic evolution in the central Andes fore arc: New paleomagnetic results from the Copiapó region of northern Chile, 26°–28°S

“…The reference poles for South America have been widely discussed in recent years with several sets produced based on differing methodologies and criteria [Beck, 1999;Besse and Courtillot, 2002;Lamb and Randall, 2001;Randall et al, 2001]. We tested all four reference pole data sets plus an unpublished one (R. Somoza, personal communication, 2001) and found that the choice of reference poles does not significantly alter the calculated expected declinations and inclinations nor subsequently derived rotations in this study.…”

Central Andean rotation pattern: Evidence from paleomagnetic rotations of an anomalous domain in the forearc of northern Chile

“…Previously there has been debate over reference pole choice; we found that there is no major difference in the pattern of rotations for the past 100 m.y. when using global data sets (Besse and Courtillot, 2002) or dominantly South American data (Lamb and Randall, 2001 The Neogene data (here Ͻ25 Ma) are almost all located within the present arc to retroarc. Rotations are relatively small, locally variable (kilometer scale), but consistent with identified shortening gradients and local structures that provide an adequate explanation of the variability (Lamb, 2001;Riller and Oncken, 2003).…”