The role of true polar wander in the Jurassic.

IGLESIA LLANOS, MARÍA P.; PREZZI, CLAUDIA BEATRIZ

BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA

ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA

Año: 2010 vol. 51 p. 141 - 143

0006-6729

For decades, it has always been interpreted that during most of the Mesozoic and even part of the Palaeozoic, Gondwana remained more or less in similar present-day latitudes. This “stationary” geodynamic model is based on the pole. More recent data obtained in South America however, show some Jurassic palaeomagnetic poles positioned far away from the geographic pole, particularly during the Early Jurassic. The revision of the former geodynamical configuration was thus mandatory. Consequently, in this paper we present a substantially different model for Pangea during the Jurassic. We tested in the first place, that the apparent polar wander (APW) path we obtained for South America, was consistent with others from Pangea yielding reliable data such as we find in Eurasia, North America and Africa. All four paths proved to be remarkably similar in terms of shape, length and chronology of the different tracks, showing a cusp -the intersection of two tracks- at ~ 197 Ma, followed by a relatively long track until ~ 185 Ma (Early Jurassic). Therefore, a master path was derived for Pangea by averaging non-overlapping poles with 5 to 10 Myr windows. Subsequently, after testing the well-known palaeogeographical reconstructions for the Jurassic Pangea, curves were rotated to South America present-day coordinates. We chose those which provided the best palaeomagnetic fit, which by the way depended on the age of poles. Thus, we used two different kinematic models to translate the continents to South American coordinates, i.e. one for the palaeopoles bearing ages between ~215 and 193 Ma, and the other model for poles corresponding to the ~ 192-155 Ma interval. In other words according to our palaeomagnetic data, some sort of geodynamic event might have taken place around 192 Ma. We analyse the likely geodynamical phenomena that could have triggered the changes in palaeolatitude and orientation observed during the Early Jurassic, which basically can be attributed to lithospheric motion and/or true polar wander (TPW), defined as the drift of the spin axis relative to the rigid Earth. The latter event was investigated on the basis of palaeolatitude plots, which required the usage of the only grid of hotspots (HS) that goes back to 200 Ma and considers these are fixed to the mantle, in accordance with the methodology used by other authors. In addition, we provide palaeolongitudinal - controlled or “absolute” reconstructions of Pangea throughout the Jurassic. The changes in palaeolatitudes that we observed from our palaeomagnetic data were confronted with palaeoclimatic proxies derived from biogeographical and geological data from both the northern and southern hemispheres. The applicability of completely different disciplines is maybe the most convincing argument to sustain or discard the methodologies used.