Tectonic vs. gravitational carbonate breccia within karstified strike-slip faults

We investigate the geometry, distribution, texture and composition of several samples collected along representative sheared breccia dikes and strike-slip faults crosscutting Cretaceous limestones. The study area is located in the Murge Plateau, Italy, where Santonian limestones pertaining to the Calcare di Altamura Formation is nicely exposed along the walls of inactive quarries. During post-Cretaceous tectonics, the 10’s of cm-to-m thick breccias dikes has been sheared, either right- or left-laterally, deforming its original fabric. At the same time, cataclastic deformation took place, at very shallow depths, within the evolving fault zones brecciating the limestone rocks forming tectonic breccia. The former breccia deposits infill sub-vertical open fissures characterized, at the mesoscale, by a constant thickness. These deposits are made up of an uncemented, clast and matrix-supported breccia with reddish matrix, carbonate cements and iron-manganese oxides. Clasts are from cm- to msized, sub-angular to angular and not sorted at all. Tectonic breccia, differently, consists of either isolated pockets (along moffset faults) or continuous, at the mesoscale, volumes (10’s of m-offset faults) made up of cm- to 10’s of cm-sized, sub-angular to sub-rounded clasts embedded within a reddish microcrystalline matrix. Generally, the tectonic breccia is clast-supported. The overall clast size diminishes near the main slip zones. Meso- and microscopic evidences clearly indicates that clasts are due to comminution processes. Ongoing mineralogic analysis will shed lights on the nature of matrix and cements present within the investigated samples. The results of this study will be discussed in terms of the control exerted by pre-existing brecciated limestone on karst evolution. In fact, the aforementioned breccias strongly affected the paleo-fluid pathways within the evolving strike-slip fault zones, as shown by the numerous field evidences documented in this work.