Geomorphic evidence for an emergent active thrust along the edge of the Po Plain: The Broni‐Stradella fault

We present here geomorphic evidence for an emergent active thrust along the northwestern Apennine front about 50 km south of Milano. Fieldwork combined with SPOT image analysis attests to the presence of a ∼35‐km‐long, en échelon, cumulative fault scarp, that cuts E‐W across Quaternary surface deposits from Casteggio to Sarmato. The scarp offsets vertically alluvial fans and terraces emplaced by tributaries of the Po River that flow northward from the Apennines. Incision by such tributaries occurs only south of the scarp. Valleys of smaller streams stand in hanging position. To the northeast, the buried, east dipping Pavia lateral ramp bounds the San Colombano anticline. We interpret the Montebello‐Sarmato scarp to result from the emergence of an active thrust fault dipping south, the Stradella thrust, which splays eastward from that ramp. Total station profiles leveled perpendicular to the Stradella thrust show variable cumulative surface throws ranging from 2 to 25 m. Scarp degradation analysis indicates fairly recent offset of the terrace surfaces (10–100 kyr) suggesting a minimum uplift rate of 0.3 mm/yr. The underlying fault might thus be one of the most active thrust ramps along this segment of the Apennines. N‐S to N15°W shortening is consistent with both the E‐W strike of this thrust ramp and with coeval oblique sinistral motion on the NNE‐SSW trending Pavia lateral ramp. The Vogherese‐Bobbiese earthquake of 1828 (I ≈ IX) south of Casteggio and Broni may have activated one segment of this lateral ramp rather than the Stradella thrust. Other historical events (I > 7) that caused damage in Pavia, Piacenza, Lodi, and Milano in the last 2000 years (290 A.D., 1249 A.D., 1276 A.D., 1473 A.D.), may have occurred on the Broni‐Stradella thrust or on the blind San Colombano and Lodi thrusts farther north. This makes detailed paleoseismological studies and trenching across its trace mandatory.