GNGTS 2013 - Atti del 32° Convegno Nazionale

Paleoseismological trenches. In the northernmost sector of the Sulmona Plain, at the deeply entrenched mouth of the Malepasso Valley (ca. 400 m a.s.l.), the basal fault splay offsets by 12 m the apex of the Late Pleistocene Santopadre alluvial fan. This is a composite fan which formed at least during four different alluvial phases, each one linked to a different base level, with the younger terraces progressively carved into the older and higher ones. The youngest and still active phase forms a telescopic fan which opens in the present alluvial plain at ca. 250 m a.s.l. (i.e., the same elevation of the neighbour Sulmona Basin outlet, in Popoli), whereas the oldest and highest one has a top surface reaching ca. 400 m a.s.l. in the fault hanging wall. Its base level was reasonably the mentioned upper terrace of the Sulmona Plain (locally at 340 m a.s.l.), whereas the minimum age of its original top-surface can be inferred by the presence of an idiosyncratic tephra outcropping extensively up to 3-4 meters below this. Indeed, on the basis of the chemical, isotopic and textural feature (see also Giaccio et al. , 2007), the tephra can be associated to a well-known volcanic level spread all over the central Apennines, and attributed to the last explosive activity of the Albano Maar (Colli Albani volcanic district, Rome; 36±1 kyr, Freda et al. , 2006; Giaccio et al. , 2009). Along the northern tail of the fan, where this joins and interfingers with the Colle Ferrano slope talus, the fault is revealed by a subdued scarpwhich is deeply reshaped, and retreated uphill by millennial agricultural works. We decided to open here the first three paleoseismological trenches, which have been preceded by one electrical resistivity tomography (ERT), and by several high precision topographic levelling. The ERT survey was performed by means of a Syscal R2 (Iris Instruments) resistivity meter, coupled with a multielectrode acquisition system (48 electrodes), with a constant spacing of 1 m between adjacent electrodes. Along the profile, we applied different array configurations (Wenner-Schlumberger and Dipole-Dipole), obtaining an investigation depths of about 10 m. The apparent resistivity data were inverted using the RES2DINV software (Loke, 2001) to obtain the 2D resistivity images of the subsurface. The best result was obtained from the Wenner-Schlumberger data which have shown a higher signal-to-noise (s/n) ratio, a larger investigation depth and a better sensitivity pattern to both horizontal and vertical changes in the subsurface resistivity. Root Mean Squared (RMS) error was less than 2%, with resistivity values ranging from 20 to more than 508 Ωm (Fig. 2). Around the middle of the NE-SW trending section, the ERT evidences a sharp lateral contact between terrains with different resistivity values. The whole SW side is characterized by a couple of layers with high (bottom, >300 ohm*m), and very low (top, <30 ohm*m) resistivity values, whereas the NE side has intermediate values (>50 ohm*m <400; Fig. 2). Both the sharp lateral contact (presumably the fault) and the presence of low resistivity deposits (reasonably matching with silty colluvia) in the supposed hanging wall, oriented here the opening of the first three trenches. In the footwall, all the trenches unveiled clearly the upper sequence of the oldest Santopadre alluvial fan (left side of Fig. 3) which contains here a brownish pedogenized layer very rich in the volcanic minerals colluviated from the 36 ka Albano tephra layer. This immature paleosol is truncated by other few alluvial gravel layers - also with abundant reworked tephra minerals in the matrix - which are interfingered and progressively substituted upward by slope debris. All these units are faulted against different generation of reddish and brownish colluvia in the hangingwall (right side in Fig. 3), resting over a stiff alluvial sandy unit. The fault plane trends here N110°, dipping 65° southward. The geometry of the faulted succession account for four rupture events, each one evidenced by colluvial wedges, tectonic wedge filled by scarp-derived deposits, secondary splays and fractures and warping of the sedimentary units. The entire sedimentary succession has been chronologically constrained by eleven radiocar- bon dating performed in the Beta Analytic Inc. laboratories through AMS analyses. Leaving aside the footwall deposits, which contain and surely postdate the reworked 36 ka Albano tephra layer, the hanging wall colluvia fall all inside the Holocene, from ca. 9 ka to Modern time. 56 GNGTS 2013 S essione 1.1