GRADIENT OF SHORTENING AND VERTICAL-AXIS ROTATIONS IN THE SOUTHWESTERN PYRENEES (SPAIN)

Balanced and restored cross-sections are the necessary approach to calculate the shortening (SH) in compression regimes and so are paleomagnetic vectors to control vertical axis rotations (VAR). SH varies along-strike in orogenic wedges; this gradient is often connected to the VAR. Both variables have been related in map-view trigonometric models that allow; 1) to correct the effect of out-of-plane movement causing errors on the traditional SH estimation and 2) to calculate the expected difference in shortening between two given sections by using the VAR values.

For the purpose to control VARs a paleomagnetic study was carried out in the Internal Sierras (IS), Western Pyrenees, previous paleomagnetic data were also considered (Pamplona, Jaca and Aínsa Basins and the External Sierras front). The study area is bounded by two well-constrained crustal-scale balanced cross-sections (Ansó-Arzacq and Cotiella sections) that crosscut the range in a NNE-SSW direction. Primary and secondary paleomagnetic components confirm the occurrence of a ≈20° regional clockwise rotations (out of plane movements) in agreement with previous data. Paleomagnetic data were grouped in 37 localities (4-13 sites per group) to achieve statistically robust VARs.

Assuming internal deformation as negligible, SH estimates have been corrected using the rotation values along both balanced sections (model 1). Depending on the rotation confidence angles (α95), the SH error may range between 1% and 23%. On the other hand, the expected gradient of shortening was also calculated from the observed VARs between the sections (model 2): giving an averaged out of 0,34 km of frontal displacement / 1 km of lateral displacement, which varies between 0,10 and 0,72. The independent calculations of SH using the gradient value introduce differences up to 32% greater than the original shortening estimates.