The contribution of Anisotropy of Magnetic Susceptibility (AMS), Anisotropy of P-wave Velocity (APV) and Petrophysic analysis in the strain characterisation at Sheep Mountain Anticline (Wyoming, USA)

Using Magnetic Susceptibility (AMS) and Acoustic Velocity (APV) Anisotropy data, combined with petrophysic analysis and fracture data, we discuss the relationship between fold development and strain distribution. The natural laboratory chosen for this study is the Sheep Mountain Anticline (Wyoming, USA). This fold is asymmetric, basement cored, and was formed during the Laramide orogeny in the Early Tertiary.AMS results suggest the existence of three different directions of maximum susceptibility axis: one is parallel to fold axis, the second one is perpendicular to this axis and the third direction is perpendicular to a small secondary fold. Those directions are carried by the micro-fractures in the limestone formation and by the grain form in the grainstone formations. The high consistency of the strain ellipsoid orientation with respect to the anticline geometry exists.The AMS and APV results show differences of strain ellipsoid between the forelimb, the hinge line and the backlimb of the anticline. The AMS fabrics in the forelimb are less evolved than in the backlimb, in agreement with fracture data which show that there are more sets of fractures in the backlimb than in the forelimb. The P-wave velocity in the backlimb is higher than in the forelimb. The minimum principal axis for APV tensor is perpendicular to the bedding, while its maximum principal axis is perpendicular to the strike. We observe also that there's less anisotropy of P-wave velocity along the hinge. Thus the backlimb apparently suffered less strain, but recorded a more evolved strain pattern, both in term of microstructures and of matrix scale layer parallel shortening. We use these results to discuss the kinematics and fold evolution.