Reconstructing the shape of large synformal folds based on the solution of the 2D non-linear magnetic inverse problem

Abstract

This paper deals with a new approach to reconstructing the shape of a poorly-exposed synform that folds strongly magnetic layers. The approach is based on the solution of the 2D non-linear magnetic inverse problem in areas best suited for 2D modeling. The interpretation model consists of two parts: (i) a geometric model imitating a synform cross-section and (ii) a set of elementary layers of a fold characterized by anisotropic magnetic susceptibility. The main task is to determine the deepest point of magnetic layers and the direction and amount of their dip. It can be solved by a selection of optimal model parameters from a set of random numbers with a priory model constraints. The solution is an iterative process which defines intervals of geometrical model parameters that correspond to minimal values of the misfit between observed and modeled magnetic fields. The proposed approach is characterized by a stability of the determination of reference fold points' coordinates, which is sufficient to confidently answer a number of questions important for the geologist. These include: whether a synformal fold is upright, inclined or overturned, determining the depth of its trough line, which combined with the fold outline on the ground surface allows an evaluation of the hinge line curvature. As an example, the large Paleoproterozoic Serpovidny isoclinal synformal anticline in the Keivy terrane of the northeastern Fennoscandian Shield has been chosen. According to field observations, this synform is a giant sheath fold consisting of the poorly exposed strongly magnetic core and weakly magnetic limbs. The 3D morphology of the Serpovidny fold reconstructed using the new approach is shown to be very close to the geometry determined from field observations. Thereby, this study favors the conclusion based the sheath geometry of the Serpovidny isoclinal synformal anticline that an Alpine-type tectonics operated in the Paleoproterozoic.