USING THERMOCHROMETRY TO IMAGE TOPOGRAPHIC EVOLUTION IN THE NORTHERN APENNINES, ITALY

Shallow isotherm surfaces tend to follow the overlying surface topography, with the degree of smoothing increasing with depth. The closure isotherms for (U-Th)/He and fission-track (FT) apatite methods are shallow enough (< ~3 km) that they are strongly affected by surface topography. Thus, these cooling ages should provide information about the surface topography at the time of closure. We exploit this phenomenon using a dense suite of (U-Th)/He and fission-track apatite ages from the northern Apennines, and numerical inverse methods that allow estimation of the evolution of surface topography from the cooling ages. Stratigraphic evidence indicates that the Apennines emerged above sea level at ~5 Ma. Our expectation was that topography and relief would have increased over the last 5 m.y., reaching the maximum elevations observed today of ~2500 m. Age-elevation relationships show a linear increase in age with elevation at the local scale. However, at the regional scale, the cooling ages are inversely correlated with elevation. This relationship is diagnostic of a decrease in relief with time. We have analyzed both the FT and He apatites ages using a finite-element routine (PECUBE) and also a fourier-based routine. The best-fit solution was found by numerical search. We find that the relief of the range has decreased since emergence, by a factor of 2 or 3 starting at ~8 Ma. This result makes sense given that the Apennines started at ~30 Ma as a large submarine subduction wedge. The large relief before 8 Ma reflects the large submarine relief of the wedge at that time and the relatively constant sea floor tempetures in the Mediterranean (~14 C). This wedge was reduced in size when it emerged as it overrode the passive margin of the Adriatic platform.