Abstract
New structural-stratigraphical mapping constrains the three-dimensional kinematics and mechanisms of Eocene-Oligocene growth folding at Sant Llorenç de Morunys (NE Ebro basin, Spain). A 1 km wide sub-vertical panel of syntectonic alluvial gravels passes southwards via a highly asymmetrical growth fold-pair to shallowlydipping strata. The axial surface of the anticline comprises either continuous or en échelon segments while that of the syncline is concave and usually continuous. While converging upwards, the axial surfaces do not define growth triangles. Principal and subsidiary growth unconformities and thickness changes occur across both axial surfaces and the common limb. Dips within the common limb decrease up-stratigraphy and up-dip. Mesostructures indicate that internal deformation was ongoing during folding at all stratigraphical levels, and concentration of cleavage in the syncline indicates that this hinge was essentially fixed. Sequential restoration of three profiles shows that folds amplified principally by limb rotation but incorporated minor passive hinge migration. Particle movement vectors, generated by section restoration, are arcuate about a hinterland pinpoint. A new trishear model of fault propagation folding involving non-rigid limb rotation reproduces the rounded hinge forms, thickening geometries and limb dip variations observed. Simple kink band migration models (fixed axis and constant thickness theories) do not replicate these features.
Highlights
This paper investigates two outstanding problems in the realm of fault-related folding at high levels in the crust:(1) the evolutionary nature of fold amplification and (2)the deformational mechanisms involved
Hinge points for the growth anticline and syncline are shown where possible for each restoration step and demonstrate that limb rotation was accompanied by minor passive hinge migration through time
This is a geometrical phenomenon (Ramsay, 1967) of growth folding, due to the progressive addition and folding of wedge-shaped sedimentary bodies. This is emphasised by the major hinge point jumps associated with periods of offlap (e.g. t5-t7 on profiles 2 and 3). Such passive hinge migration is a unique phenomenon of growth folds and would not occur in folding of non-growth strata
Summary
This paper investigates two outstanding problems in the realm of fault-related folding at high levels in the crust:(1) the evolutionary nature of fold amplification and (2)the deformational mechanisms involved. Geometrical-numerical models of fault-bend and fault-propagation folds have been developed in which limb lengthening due to kink band migration (Fig. la) is the sole formative mechanism (Suppe, 1983; Suppe and Medwedeff, 1990). Similar, models of detachment folds with growth strata (Hardy and Poblet, 1994) and fault-propagation folds (Erslev, 199 1) involve limb rotation (Fig. lb). In the kink-band migration model of fault-propagation folds (Suppe et al, 1992) growth strata within growth triangles of the steep forelimb show no variation in dip or thickness up-section (Fig. la). A limb rotation model (Hardy and Poblet, 1994), for similar sediment accumulation and deformation rates, shows clear changes of growth strata dip and thickness, both updip and up-section on the steep forelimb (Fig. lb)
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