Folding and the formation of bedding-parallel faults on the western limb of Grey Valley Syncline near Blackball, New Zealand

Fresh roadside exposures of faults on the western limb of Grey Valley Syncl ine near Blackball (West Coast, South Island) provide new information on th e kinematics of folding since c. 2.5 m.y. ago. These faults are parallel to , or dip within 10 degrees of, steep bedding (60 --> 90 degrees) in Pliocen e Old Man Group, and offset two gently dipping Quaternary gravel formations . Deformed gravel sequences record reverse-fault displacements of 0.5-c. 80 m and bed dips of 5-50 degrees, which accumulated over the last c. 450 000 yr. The dip of Quaternary gravel beds and fault displacements decrease up- sequence, suggesting that gravel deposition and deformation were synchronou s. Bedding-parallel faults in Old Man Group are typically located at the bo undaries between mudstone beds and coarser grained sandstone or conglomerat e beds. The bedding-parallel orientations of the faults, fault slickenside striations approximately normal to the hinge of Grey Valley Syncline, and t otal fault displacement across the fold limb are consistent with these stru ctures being flexural-slip faults formed during folding. Small amounts of b edding-parallel shortening and extension within Pliocene strata between fau lts indicate non-rigid deformation of the limb and departure of folding fro m an ideal flexural-slip model.