Record of the Late Jurassic true polar wander from the Northern Apennines (Italy)

Recent paleomagnetic studies revealed the presence of a major and rapid plate motion event in the late Jurassic that was undetected or underestimated before due to the smoothing of data intrinsic to the construction of composite Apparent Polar Wander Path (APWP) or to the inclusion of lower quality paleopoles. Such shift in the APWP has been nowadays documented by different Authors both on the basis of original data and compilations from the literature, including sedimentary and igneous rocks collected from different plates.

The late Jurassic shift has a magnitude between 30° and 40° and a temporal duration that is not well defined. In fact, it started between 183 (Pliensbachian-Toarcian boundary) and 160 Ma (Oxfordian) and ended between 151 (early Tithonian) and 145 Ma (Tithonian/Barremian).

We used magnetostratigraphic-derived data from the Pliensbachian-Aptian Salto del Cieco Section (Italy; latitude 42.61°N, longitude 12.86°E) to precise the duration and amplitude of the shift. The mean directions were computed using the magnetozones, or the stage boundaries when magnetostratigraphy was not defined. We evidenced an almost perfect agreement between our new dataset and former discrete paleomagnetic studies from Northern Apennines and Southern Alps. Thus, we computed a composite paleolatitude curve which displays first a slow southward motion of some 15° from 190 to 170 Ma, strongly accelerating between 175 to 160 Ma (a minimum paleolatitude of 11.9° is found at 160 Ma), followed by a slower northward motion up to 130 Ma.

We finally derived an APWP showing first a standstill, followed by a track between 171.8 and 150.4 Ma with a velocity of ca. 20 cm/yr, followed by other standstill between 142 and 150 Ma and finally a hairpin turn with a rather complex direction pattern. Should these data represent True Polar Wander (TPW), the velocity would be nearly twice faster than the presently geodetically measured TPW (10 cm/yr) but well below TPW in excess of 40 cm/yr suggested at the end of Precambrian.