USING HIGH-RESOLUTION STRATIGRAPHY TO DATE FOLD-AND-THRUST ACTIVITY -EXAMPLES FROM THE NEOGENE OF SOUTH-CENTRAL SICILY

The integration of structural and stratigraphic data is fundamental fo r determining rates of deformation in the uppermost continental crust. The high temporal resolution provided by Neogene marine sediments is used here to examine deformation rates in part of a thrust belt chosen from the Maghrebian orogen of Sicily. Conventional biozonal stratigra phy, calibrated against the geomagnetic polarity time scale, shows tha t individual thrust-fold structures grew steadily over many millions o f years. Structures across the thrust belt were active at the same tim e and accommodated bulk shortening rates of c. 0.5 mm a(-1). in contra st the basal detachment operated about ten times faster. These results are in broad agreement with some theoretical models for orogenic wedg e kinematics. Shore-line carbonate successions, calibrated with preces sion cycles of sea-lever change, provide a very high-resolution, tempo ral scale with which to chart tilt rates on fold limbs (1 degrees/27.6 ka=0.036 degrees ka(-1)). These show that ford amplification was cont inuous although the rates may have varied with time. Incremental tilti ng of limbs during fold amplification is not predicted by popular mode ls of fault-bend and 'fault-propagation' folding. Geometric modelling suggests that folding occurred by limb rotation, with minor hinge migr ation during budding above buried thrusts. Thus stratigraphic data may be used to examine the kinematic evolution of thrust-fold systems alo ng regional cross-sections, and of local structures. However, the type s of structural models that can be tested using estimates of deformati on rates depend upon the chrono-stratigraphic resolution available for the syn-tectonic sediments.