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

Abstract

Fresh roadside exposures of faults on the western limb of Grey Valley Syncline near Blackball (West Coast, South Island) provide new information on the kinematics of folding since c. 2.5 m.y. ago. These faults are parallel to, or dip within 10° of, steep bedding (60‐>90°) in Pliocene 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°, 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 synchronous. Bedding‐parallel faults in Old Man Group are typically located at the boundaries between mudstone beds and coarser grained sandstone or conglomerate beds. The bedding‐parallel orientations of the faults, fault slickenside striations approximately normal to the hinge of Grey Valley Syncline, and total fault displacement across the fold limb are consistent with these structures being flexural‐slip faults formed during folding. Small amounts of bedding‐parallel shortening and extension within Pliocene strata between faults indicate non‐rigid deformation of the limb and departure of folding from an ideal flexural‐slip model.

Keywords:

• bedding‐parallel faults
• Old Man Group
• Grey Valley Syncline
• flexural‐slip folding