ACTIVE TECTONICS, OROGENY, AND EROSION IN THE NORTHERN APENNINES, ITALY; INITIAL RESULTS OF THE RETREAT PROJECT

The Apennines of northern Italy are the exposed portion of a large orogenic wedge built by shortening and crustal thickening associated with the subduction of the Adriatic plate. The conventional notion in this and many mountain ranges is that the emergence of the thickened crust as mountains has occurred in concert with tectonic deformation, namely folding and faulting. We are assembling evidence for the northern Apennines that argues that the rate of lifting of rocks above sea level to create mountains has accelerated over the past 1 m.y. whereas the overall rate of convergence and structural deformation in the orogenic wedge has remained more or less constant. Accordingly, we are able to identify which geomorphic processes have immediate responses and feedbacks with such rapid changes in the rate of rock uplift, and which have considerable lag times. Comparison of some data sets that respond at m.y. or greater time scales such as basin wide erosion rates and thermochronology are in agreement that the northern Apennines are currently being unroofed at approximately 0.6-1 mm/yr. But the preservation of middle to early Pleistocene marine deposits now at over 1 km in elevation and Pleistocene river incision rates of several mm/yr argue for a recent acceleration in the rock uplift rate not reflected in longer-term data. Detailed mapping of river terraces and a differential GPS survey of the long profile of the Reno River that drains towards the Po foredeep reveals a upstream increase in river incision, punctuated by knickpoints associated with active faults. At least one of these faults is interpreted to have thrust-sense motion and is consistent with published focal mechanisms arguing for continued shortening in the prowedge. Nevertheless, the mean elevation and the relief of the range is best explained by embedding these active faults in a much larger, perhaps wedge-scale structure that is upwarping the Apennines and driving subsidence beneath the Po Plain. Our research is now directed to determine if rock deformation, the building of topography, and exhumation of the range are currently decoupled in the Apennines, or if a wedge-scale structure responsible for the current pattern of rock uplift is in fact coupled to orogen-wide changes in erosion or deposition such as recent, rapid burial of the wedge front beneath the Po Plain.