STRAIN PARTITIONING ACROSS A FOLD-AND-THRUST BELT - THE RHENISH MASSIF, MID-EUROPEAN VARISCIDES

Strain data across the entire western Rhenish Massif illustrate the pa ttern of deformation partitioning within a fold and thrust belt from t he foreland on the northern front of the European Variscides to the su ture with the internal zones of higher metamorphic grade. Shortening p erpendicular to cleavage increases from 16 to 27% (constant volume ass umed) in the northern Rhenish Massif to 51% at the southern border. Th ese values correspond to a weak general layer-parallel shortening of t he basin filling in cross-section on the order of about 5-15% and more in the case of volume loss. The geometry of apparent finite strain is nearly plane strain except in the Venn Anticlinorium where strain is apparent prolate and near the southern margin of the Massif where it i s apparent oblate. Net orogenic shortening in the upper crust of the f old and thrust belt is approximately 42%. It is mainly achieved by fol ding and tectonic stacking of the deformed basin filling, which was ap parently detached from the lower crust. Distribution of strain in the orogenic wedge is controlled by different factors at different scales. Heterogeneity is caused by changes in lithology and average grain siz e, by the structural position of samples in folds and their distance f rom thrusts in the hangingwall (at deeper crustal levels), and by the regional distribution of the metamorphic grade, i.e. by the dominant d eformation mechanism. This last factor causes an exponential increase of strain with approach to the ductile lower crust. The superposition of strain partitioning mechanisms on different scales creates a comple x regional strain pattern. The southern part of the thrust belt appare ntly suffered late westward motion (in gently dipping imbricates) or s trike-parallel motion (in steeply dipping imbricates) which is probabl y due to oblique convergence between the Rhenohercynian and Saxothurin gian microplates.