Present-day stress indicators from a segment of the African-Eurasian plate boundary in the eastern Mediterranean Sea: Results of formation microscanner data

Abstract

Ocean Drilling Program Leg 160 investigated two segments of the active African-Eurasian plate boundary in the Eastern Mediterranean Sea by shallow drilling: the Eratosthenes Seamount and the Mediterranean Ridge Olimpi mud volcano field. The borehole logging data acquired during Leg 160 were analyzed to obtain stress indicators and a better definition of the present-day stress situation in the Eastern Mediterranean. Both borehole breakout zones (borehole enlargements) and vertical drilling-induced fractures were identified from borehole geometry data recorded using the Formation MicroScanner (FMS) tool and from computer-generated FMS images, respectively. For Holes 966F and 967E at the Eratosthenes Seamount, the enlargements and the drilling-induced fractures show the same orientation. If both were stress induced and representative of the present-day stress orientation, they would be 90°off. A comparison of the strike of the inferred borehole enlargements with the observed paleostress-related fracturing suggests the possibility that borehole enlargements might be aligned with the strike of some of the structures. Thus, we derive the stress orientation at the Eratosthenes Seamount sites from drilling-induced fractures alone. The orientation of the maximum horizontal principal stress, S(H), is about N50°E for Hole 966F, located on the seamount plateau, and N30°E for Hole 967E, located on the northern slope of the Eratosthenes Seamount. For Hole 965A, also on the slope but at a shallower depth, the orientation obtained is about N170°E. The stress orientations obtained from borehole enlargements and from the vertical drilling-induced fractures in Hole 970A, on the eastern flank of the Milano mud volcano, show consistent, nearly north-south, orientations of S(H).