Rotational Kinematics of Basement Antiformal Stacks: Paleomagnetic Study of the Western Nogueras Zone (Central Pyrenees)

Abstract

The structure of the Pyrenean Axial Zone laterally transitions from an imbricate thrust system to an antiformal stack displaying downward facing structures in its frontal part (the Nogueras zone). We carried out a paleomagnetic study through this structural transition to better constrain thrust kinematics and better define the factors controlling the occurrence of antiformal stacks. Triassic red beds unconformably overlying the Paleozoic basement were sampled across three main thrust sheets (the Nogueras zone, the Bielsa, and the Orri thrust sheets). From 31 new sites, two meaningful magnetizations were defined, both carried by hematite: (1) the primary magnetization, that unblocks at 660-685°C and (2) an intermediate-temperature, syn-orogenic remagnetization. Paleomagnetic data indicate variable, clockwise vertical axis rotations, with mean, thrust-scale values that range from 0 to +38°. They increase both from the base to the top of the stacked units and from west to east and display a thrust-scale directional dispersion that is higher for the lower wavelength thrusts. Results evidence that increasing thrust displacements in the upper thrusts units are a key factor for the formation of antiformal stacks showing downward facing structures. Displacement gradients in the basement are transferred to cover units and can account for important rotations such as those registered by the Mesozoic-Cenozoic units to the south of the study area.

The enigmatic curvature of Central Iberia and its puzzling kinematics

“…These results confirm that the present-day variation in trend of the tectonostratigraphic units, generally attributed to Variscan tectonics (e.g., Weil et al, 2013;Shaw et al, 2012Shaw et al, , 2014, is likely a product of Cenozoic Alpine orogeny. Izquierdo-Llavall et al (2019) confirmed that the interpreted Alpine rotations correspond well with the amount of shortening reconstructed in Mesozoic-Cenozoic basins.…”

“…The Permian and Mesozoic rocks from the Iberian ranges show a consistent ∼ 22 • CW rotation with respect to the apparent polar wander path (APWP) for Iberia (e.g., Pastor-Galán et al, 2018). This rotation likely happened during the Alpine orogeny, in which the northern area of the Iberian Range underwent more shortening than the southern part, resulting in a regional CW vertical-axis rotation (Izquierdo-Llavall et al, 2019). After restoring the Cenozoic rotation, the Devonian rocks show a positive reversal and fold test with inclinations that are steeper than ex-pected from the eP component (Dec. = 85.3 • , Inc. = 12.7 • , λ = −6.4).…”

Triassic stretching directions in Iberia and North Africa inferred from magnetic fabrics

“…In the Central Pyrenees interpretation of AMS results are complicated by the existence of Alpine cleavage and thrust‐scale vertical axis rotations in some of the sampled outcrops (Izquierdo‐Llaval et al, , ) (Table ). In sites with a sedimentary‐type fabric (i.e.…”

“… Vertical axis rotation (VAR) from site data, + and – are clockwise and anticlockwise VARs respectively (from Izquierdo‐Llavall et al, ). …”

Kinematics and strain distribution in an orogen-scale shear zone: Insights from structural analyses and magnetic fabrics in the Gavarnie thrust, Pyrenees

“…basement thrust sequence (see Izquierdo-Llavall et al, 2018, where a moderate clockwise rotation of basement thrust sheets is proposed). Changes in the direction of kinematic indicators (namely AMS related to folding) have been also obtained in other areas of the South-Pyrenean Zone (Pueyo-Anchuela et al, 2012;see also Oliva-Urcia et al, 2009;Pocoví et al, 2014).…”