Development of strike-slip faults in the dolomites of the Sella Group, Northern Italy

The dolomites in the Sella Group, Northern Italy, were intensely jointed and faulted during the Alpine orogeny. Field observation of joints, joint-zones, and faults are the basis for a model for fault development in dolomite. We propose that joints developed parallel to the maximum compressive stress direction and were homogeneously distributed throughout the Sella Group before faults became localized (fault pre-nucleation stage). With ongoing deformation, the joints were localized into en-échelon arrays also called joint-zones. These joint-zones are incipient faults and are characterized by small dilation (≪1 mm) of the individual joints, narrow joint spacing (1–15 mm), a high number of joints in the array (up to thousands), high joint-overlap, and joint-array angles between 10° and 40°. The next stage in fault development involves the break up of the rock bridges in the joint-zones and the localization of shear. Joint-zones with pockets of breccia are the smallest faults encountered and accommodate strike-slip offsets of 5–7 mm, whereas joint-zones with a continuous breccia zone accommodate offsets of 1–3 cm. Faults with offsets of more than 1 m are characterized by a breccia zone 0.5–15 m wide, and high joint densities in the wall-rock. The absence of a distinct granular structure of the dolomites of the Sella Group and the shallow depth of burial during Alpine deformation (less than 1000 m) may have promoted the growth and localization of joints, their linking via cross-joints, and the formation of continuous faults.