Synclines and Overturned Faulted Slivers in the Footwall of Thrust Faults

John H. Spang, David V. Wiltschko, Robert R. Berg

Asymmetric synclines and overturned faulted slivers of presumably footwall rock have often been observed along thrust faults and form complex compartmentalized structures. These structures develop on a wide range of scales and are typically found in well layered sedimentary strata. Examples of overturned fault slivers can be found in the Pine Mountain thrust in the Valley and Ridge Province of the Appalachian Mountains and in the French thrust in the Sawtooth Mountains in northwestern Montana. Examples of footwall synclines can be found in the Exshaw thrust and on several scales in the Rundle thrust in the Southern Canadian Rocky Mountains west of Calgary, Alberta.

Well documented footwall synclines and overturned footwall slivers are found on several scales beneath basement involved thrusts in the Foreland Province of the Rocky Mountains in Wyoming. Retro-deformation of the well constrained cross section of Sage Creek anticline shows that at lower stratigraphic levels in the sedimentary cover rocks, the syncline was rounded while the box shaped anticline had two angular hinges. Two relatively late mountain flank thrusts formed during the growth of the structure; one was on the overturned limb while the other was along one of the anticlinal axial surfaces. Although only a small overhang developed in the basement rocks, the resulting structure contains both an overturned syncline and an overturned fault sliver in the footwall of the main basement involved thrust.

Possible mechanisms range from a relatively simple model of an early asymmetric fold pair or an early fault propagation fold subsequently cut by a younger thrust (e.g. along the foreland verging anticlinal axial surface) to a more complicated model of fault related folding or imbrication in the footwall. Overturned fault slivers can be derived from overturned limbs of fault bend or fault propagation folds and incorporated into the hanging wall of a structurally younger and higher thrust. Recognition of these features in the footwall reveals much information about the timing and geometry of both the folding and faulting.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995