Kinematics of an arcuate fold–thrust belt: the southern Eastern Carpathians (Romania)

Abstract

Flysch and molasse nappes of the Eastern Carpathians strike mainly N–S and were folded and imbricated since the late-Early Cretaceous. Boundaries of stratigraphic and structural units and calculated fold-trend axes from the southern part of the Eastern Carpathians indicate a curvature of the orogen by 80°. Kinematic data from fault-slip analysis of all nappe systems exposed in the arc document an overall NW–SE contraction. Nappes of the external Moldavides nappe complex underwent deformation since the Late Cretaceous, peaking in the Miocene, and exhibit fanning subhorizontal contraction axes from E–W in the N-striking part of the Eastern Carpathians through WNW–ESE and NW–SE in the northern part of the arc to NNW–SSE in its southwestern part. Geometric relations between faults and folds indicate that this fan pattern reflects an extended interval and was contemporaneous to nappe stacking and subsequent continental collision. Statistical analysis reveals correlation between orientation of the fanning kinematic axes and the structural trend, where fanning of subhorizontal contraction axes is systematically smaller than change of strike of fold–thrust structures. Orogen-parallel extension in the Tarcau nappe of the Moldavides nappe complex is estimated to be ≤20%. This low value of orogen-parallel extension indicates a primary arc in contrast to bending of a previously straight fold–thrust belt. Low orogen-parallel extension and the observed pattern of structural and kinematic axes can be simulated in sand-wedge indentation models where the indenter moves oblique to its margins. In these models, thrust traces and fold axes are curved around the arc but are parallel to the margins of the indenter away from the corner area. The movement directions of the thrust faults in the models are deflected from normal to fault-strike towards the movement direction of the indenter, leading to fanning contraction directions similar to those observed in the Eastern Carpathian arc. Thus, the main controlling factors on arc formation were the plate margin geometry (i.e. the shape of the upper plate) and the movement direction of the upper plate relative to the orientation of its margins. The influence of the geometry of the continental margin in the lower plate was mainly by imposing constraints on the movement direction of the upper plate.