GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.1 111 promising sites for paleoseismological investigations. The first of such investigations was performed by means of a trench excavated across the central portion of the Mulazzo Fault, just 500 m west of the Mulazzo village (Fig. 1a). In spite of the thick wood cover, the considered fault segment is here made well evident by prominent triangular facets and a composite fault scarp (Figs. 1b,c). The survey along the streams incisions nearby had already revealed a steep tectonic contact between the bedrock (Macigno Fm.) and Late Quaternary gravel deposits. The trench was dug orthogonal (N50E) to the morphological scarp for a length of about 20 meters and a maximum depth of more than 3 m (Figs. 2, 3). Both wall exposures were logged and samples (charcoal and bulk sediments) were gathered for radiocarbon age determinations. Both trench walls show the footwall made of unaltered, grey, highly fractured sandstone (Macigno Fm) put in sharp contact with Late Pleistocene-Holocene clastic slope and fan deposits through a narrow fault zone, marked by fault breccia and clayey gouge. The striations on the fault plane indicate an almost pure dip-slip kinematics. The shear zone penetrates in the recent deposits resting above, which are cut and deformed (subvertical clasts) up to the bottom of the thin soil top cover. The fault zone plunges underneath the trench floor and the geometry of the top surface of the bedrock is being investigated by geophysics (a 30 m long reflection seismic profile). A couple of meters downslope, within the hangingwall, a second fault zone cuts through mostly stratified fine to coarse fan deposits with some contribution from the slope. The throw here might be in excess of one meter, but further investigation and the results of absolute dating are needed to correlate the cut sediments and properly estimate the actual offset and number of events (Figs. 2, 3). References Bernini M., Papani G.; 2002: La distensione della fossa tettonica della Lunigiana nord-occidentale (con carta geologica alla scala 1:50,000) . Bollettino della Società Geologica Italiana, 121 , 313–341. Bernini M., Papani G., Dall’Asta M., Lasagna S., Haida P.; 1991: The Upper Magra Valley extensional basin: a cross section between Orsaro Mt. and Zeri (Massa Province) . Bollettino della Società Geologica Italiana, 110 , 451-458. Bertoldi R.; 1997: Lineamenti palinostratigrafici di depositi continentali del Pliocene-Pleistocene inferiore iniziale dell’Italia nord-occidentale. Boll. Soc. Paleont. It., 36 , 63-73. Bonini M., Corti G., Delle Donne D., Sani F., Piccardi L., Vannucci G., Genco R., Martelli L., Ripepe M.; 2016: Seismic sources and stress transfer interaction among axial normal faults and external thrust fronts in the Northern Apennines (Italy): A working hypothesis based on the 1916–1920 time–space cluster of earthquakes. Tectonophysics, 680 , 67-89. Fig. 3 - Preliminary draft line-drawing exemplifying the structures in the north wall of the trench. Location of samples for C14 dating is shown.