EARLY MIOCENE TEPHRA IN THE APENNINE PELAGIC SEQUENCE - AN INFERRED SARDINIAN PROVENANCE AND IMPLICATIONS FOR WESTERN MEDITERRANEAN TECTONICS

During the Tertiary Period, the northeastern Apennines of Italy functi oned as a quiet depositional basin for fine-grained, volcanic ashfalls from distant explosive volcanism accompanying the Alpine/Apennine oro genesis. These ashes have been dated by several authors with different radioisotopic methods in the past decade (K/Ar, Ar-40/Ar-39, and Rb/S r on biotite separates; Ar-40/Ar-39 single-crystal laser fusion on pla gioclase and sanidine; and U/Pb on zircon) and provide the means for a precise, accurate age calibration of the Tertiary magnetostratigraphi c and biostratigraphic timescales. However, the provenance of these te phras has remained uncertain. To resolve this problem, we have carried out detailed grain size analysis of the so-called ''Livello Raffaello ,'' a feldspar-bearing bentonite recognized in numerous outcrops throu ghout the region at the very base of the Aquitanian to Burdigalian Bis ciaro formation. The Raffaello, stratigraphically located in the upper part of planktonic foraminiferal Zone N4 and calcareous nannofossil Z one NN1, upper Chron 6A-r (middle Aquitanian), yielded an isochron age of 21.2 +/- 0.5 Ma (2 sigma) from 27 Ar-40/Ar-39 dates by laser fusio n on plagioclase. We measured the mean grain size M Phi of the >63-mu m felsic fraction (i.e., feldspar and quartz) of the Raffaello in 11 r epresentative outcrops in the region and compared the grain size again st distance from two possible volcanic sources; the Venetian province (northern provenance) and the Sardinian province (western provenance). While the Venetian source shows no distinct trend of the M Phi distri bution, the Sardinian plot exhibits a very clear linear grain size dec rease with distance. After having tentatively established a Sardinian source (no other volcanoes of Aquitanian age are known west of the Ape nnine basin), we revised the M Phi distribution through palinspastic r estoration, including the post-Aquitanian anticlockwise rotation of th e Sardinian microplate and consequent orogenic shortening of the north eastern Apennine fold-and-thrust belt. For this we have used the ''thi ck-skinned'' model of Lavecchia et al. (1984), which envisions a short ening for the northeastern Apennines of 3 to 30 km, and the ''thin-ski nned'' model of (Bally et al., 1986), which assumes a variable shorten ing of 50 to 250 km. We have also modeled the distribution of the M Ph i of felsic crystals for modern analogue stratospheric westerly winds blowing in this region from fall to spring at speeds of 10 to 30 m/s a nd altitudes ranging from 25 km to 40 km, which account for distal (>4 00 km) transport of fine tephra (Cornell et al., 1983). In addition, w e have calculated the M Phi of felsic crystals from six representative sites of the Quaternary Y-5 ash, which was produced 38,000 years ago by an explosive volcanic event in the Neapolitan area and was distribu ted downwind throughout the eastern Mediterranean basin. Our results i ndicate that restoration by the thick-skinned model yields a crystal s ize distribution gradient incompatible with that predicted by the comp uted model and overall M Phi values much finer than those observed in the Y-5 sample at a comparable distance from the source. On the other hand, restoration using the thin-skinned structural setting yields goo d agreement with that of the computed model and with the observed M Ph i distribution of the Y-5 ash.