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 an d faulted during the Alpine orogeny. Field observation of joints, joint-zon es, and faults are the basis for a model for fault development in dolomite. We propose that joints developed parallel to the maximum compressive stres s direction and were homogeneously distributed throughout the Sella Group b efore faults became localized (fault pre-nucleation stage). With ongoing de formation, the joints were localized into en-echelon arrays also called joi nt-zones. These joint-zones are incipient faults and are characterized by s mall dilation (much less than 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 degrees and 40 degree s. The next stage in fault development involves the break up of the rock br idges in the joint-zones and the localization of shear. Joint-zones with po ckets 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 ch aracterized by a breccia zone 0.5-15 m wide, and high joint densities in th e wall-rock. The absence of a distinct granular structure of the dolomites of the Sella Group and the shallow depth of burial during Alpine deformatio n (less than 1000 m) may have promoted the growth and localization of joint s, their linking via cross-joints, and the formation of continuous faults. (C) 1999 Elsevier Science Ltd. All rights reserved.