Vein distribution in a thrust zone: a case history from the Northern Apennines, Italy

In the Northern Apennines the Cervarola Thrust superposes the Cervarola Unit (Early-Middle Miocene sandstones and shales) on the Marnoso Arenacea Unit (Middle-Late Miocene marly sandstones) and has the geometry of a leading imbricate fan. Each thrust sheet consists of a thick turbiditic sandstones sequence (Mt Cervarola Fm.), which grades downward into shales (Scisti Varicolore Fm.). During thrusting, the Scisti Varicolore Fm. was intensely deformed and widely affected by shear veins, filled with calcite. A regional pattern has been detected in the vein distribution: veins are concentrated some ten metres above the thrust fault, in a band a few metres thick, and can be grouped into two systems. The principal one presents two sets of veins striking in the same direction as the thrust: one set dips at a high angle to the thrust fault (High Angle Set, HAS), whereas the other one lies sub-parallel to the thrust fault (Low Angle Set, LAS). Both of these sets of veins have dip-slip slickenfibres. The secondary system comprises two other sets of veins, which lie obliquely to the thrust fault. Slickenfibres are strike-slip and trend nearly parallel to the thrusting direction and to the intersection lineation between the two sets. Vein development can be explained in the light of the fluid-dynamic studies recently developed for present-day accretionary prisms.