Another bend in the orogen? Kinematics of the winding Mexican Fold-and-Trust Belt: Paleomagnetism of the Nazas arc.

The Mexican Fold-and-Trust Belt is a winding belt that formed after a series of protracted tectonic events, which began with the onset of sedimentation in the basins formed during the break-up of Pangea and during the roll-back of the oceanic Kula plate in Jurassic times. Later, the continued subduction of Kula-Farallon constituent plates at the western margin of Mexico triggered arc formation, thrusting, basin inversion, and folding. The kinematics of the fold-and-thrust belt curvature at regional and local scales is still debated. Different hypotheses have been explored to explain the trace of the orogen: 1) curvature is mainly and primarily controlled by the basin architecture; 2) Curvature is progressive and was at least partially acquired during deformation being controlled by the basement geometry, lithologic heterogeneities, and/or the shortening direction, and 3) The curvature is, at least partially, postdating the main orogenic deformation structures being formed as the youngest of the deformation phases. Several regional-scale studies have been performed (Eguiluz et al., 2010; Fitz-Diaz et al., 2017). But most of the previous paleomagnetic studies were focused on studying the configuration of the desegregated fragments of Pangea (Molina-Garza et al., 1992). In this study, we investigate the nature of the curvature in the north and central-eastern part of the Mexican Fold and Trust Belt using paleomagnetic data obtained from the Jurassic rocks of the Nazas Formation, this formation is composed of a volcanic and volcano-sedimentary succession of andesites and dacite flows interbedded with tuff deposited in an intra-arc or back-arc setting. Our results show remagnetizations, some potential primary magnetizations, and significant counterclockwise rotations. This implies a potential oroclinal bending origin for at least part of the Mexican fold-and-thrust-belt curvature. This contribution will discuss the potential mechanisms causing the curvature and the implications for the kinematics, tectonic and geodynamic evolution of the central-western Pacific subduction during the Mesozoic-Paleogene in northeastern Mexico. This work is a posthumous contribution of Dr. Roberto Stanley Molina-Garza and a tribute to his huge contribution to the understanding of the tectonic history of Mexico.

Keywords: Paleomagnetism; Mexican Fold and Trust Belt; Anticlockwise rotation; Remagnetization; Jurassic; Nazas Arc.