A FLUVIAL RECORD OF ACTIVE FAULT-PROPAGATION FOLDING, SALSOMAGGIORE ANTICLINE, NORTHERN APENNINES, ITALY

The Salsomaggiore anticline in the northern Apennines is an actively growing fault-propagation fold that is flanked by a suite of early Middle Pleistocene (~0.8 Ma) to Recent fluvial terraces, which reveal a foreland-migrating wave of coupled incision and aggradation, interpreted to reflect the response of a fluvial system to progressive vertical and lateral fold-propagation. This ~10km-wavelength fold resides ~25 km hinterward of the modern structural front and exhibits a complex growth history extending back to at least the middle Miocene. Langhian-Messinian, marginal- to deep-marine clastics are folded about a NW-SE trending axis, record the majority of fold growth, and are superimposed by the Ligurian nappe and Pliocene-Recent deep-marine, marginal-marine, and fluvial strata with shallowing upward dips. Active growth is documented by fluvial terraces, recent seismicity, deflected stream channels, first-order stream gradients, and long-profile modeling.

1:10,000 scale mapping of terraces in the Stirone, Parola, and Taro River valleys which dissect the anticline, detailed measurements of strath elevations above modern stream channels, and limited radiocarbon and cosmogenic dating of terrace alluvium were undertaken in order to qualitatively and quantitatively characterize the record of recent fold-growth. The amount of incision since early Middle Pleistocene time generally increases from the Stirone (avg. 0.08 mm/yr), east to the Taro River (avg. 0.18 mm/yr), the pace of incision slowed in the middle Middle Pleistocene, and has increased since Late Pleistocene time. Unsteady incision rates may reflect unsteady fold-growth. The increased amount of incision by the Taro River is attributed to a change in thrust-geometry and a corresponding increase in the amount of fault-slip and fold-propagation. Faster incision rates since Late Pleistocene time are probably largely apparent, reflecting a Holocene climate and anthropogenic land-use change that facilitate incision.