FRACTOGRAPHIC FEATURES ON JOINTS IN KTB DRILL CORES AS INDICATORS OF THE CONTEMPORARY STRESS ORIENTATION

Fractographic features on joints in cores of the superdeep KTB drillin g were used to estimate the orientation of the contemporary maximum ho rizontal stress (S-H) The unique opportunity to investigate cores from a depth down to approximately 8085 m reveals information on in situ s tress orientation with increasing depth. We recognized on the cores th e main normal stress axes which are reflected by common fractographic pattern on coring-induced fractures, present in all sections. The anal ysis is based on the symmetry of joint characteristics: 3D (out-of-pla ne) shape of the joint and surface features of joint planes. The symme try axes of these joint features are related to the orientation of pri ncipal stress? which permits the determination of the orientation of t he contemporary maximum and minimum horizontal stress axes (S-H not eq ual S-h) if the core orientation is determined by other methods. For t hat purpose. the reorientated cores of the pilot borehole (VB) could b e used down to 4000 m. The main borehole (HB) cores are generally not reoriented by several reasons. Some investigations were done by member s of the KTB laboratory to determine the supposedly northward orientat ions of WE core sections by comparison with the foliation and of drill ing-induced fractures at the borehole wall(using FMI/Formation MicroIm ager and FMS/Formation MicroScanner log data) and within the core. Ind ependently, the maximum horizontal in situ stress orientation down to 7800 m was determined from drilling-induced vertical joints, and the s trike and dip of the foliation from borehole measurements down to ca. 8600 m. carried out by the staff members at the KTB. Additionally. roc k fragments from below 9050 m represent a palaeo-tectonic joint system developed at a depth of ca. 9 km.