Sequential limb rotation and kink-band migration recorded by growth strata, Almazán Basin, North Spain

Abstract

Abstract The geometry of growth strata in the Almazan monocline demonstrates that limb rotation and kink-band migration can coexist as folding mechanisms. The Almazan Basin, showing an overall synclinal geometry, was one of the most actively subsiding areas during the Tertiary in the inner part of the Iberian plate, with more than 3500 m of preserved Tertiary non-marine sediments. They are arranged in five tectonosedimentary units, TSU (A1 to A5), bounded by unconformities at the basin borders. The southern basin border is defined by the WNW–ESE-trending Almazan monocline, that can be followed for 25 km along strike, with a maximum structural relief of about 2500 m. The maximum changes in thickness on both limbs of the monocline occur in units A2 and A3, which constrains the maximum deformation period to between the Middle/Upper Eocene (Headonian, top of A1) and Upper Oligocene/Lower Miocene (Agenian, base of A4). The detailed geometry of the Mesozoic, pre-growth, and the Tertiary growth strata of the Almazan monocline was determined from the analysis of six seismic reflection sections, at high angles or perpendicular with the fold axis. The average dip of the monocline changes from 15° in the east to more than 45° in the western sector, being replaced westwards by a north-verging, steeply dipping thrust. In some sections, growth strata in the dipping limb show rotational offlap–onlap syntectonic unconformities, with progressively decreasing dips from bottom to top, indicating that limb rotation took place during the first stages of folding. At the limits of the dipping limb (anticline and syncline), two kink bands appear in some of the sections, the one located on the syncline being more developed. Within each kink-band, beds show a constant dip of 30–40°. The axial surfaces of each kink band converge upwards in the third and fourth tecto-sedimentary units, being parallel in the older units. The fixed axial surface occurs in a down dip position with respect to the active axial surface. The geometry of growth strata indicates that the mechanism of folding followed a sequence of: (i) a first stage of large-scale limb rotation (wavelength larger than 5 km), with minor development of kink-bands in some sections, and (ii) a second stage of more localised kink-band migration, probably favoured by fault propagation in the basement.