Strain fabric associated with the Meade thrust sheet: implications for cross-section balancing

Abstract

Emplacement of the Meade thrust sheet, southeastern Idaho, U.S.A., was accompanied by the development of a strain fabric and significant footwall deformation. Characteristics of the strain fabric are based on strain estimates using pressure-solution cleavage and deformed fossils within the Jurassic Twin Creek Formation. The strain fabric formed during early layer-parallel shortening at the thrust front and accommodates from 10 to 35% shortening within the incipient thrust sheet. Footwall strata were later folded above a detachment at the base of the Twin Creek Formation, and the early strain fabric was passively rotated. The structural development of this area, consisting of (1) strain fabric development, (2) folding of the footwall and (3) the truncation of footwall structures by imbricates of the Meade thrust, are placed in a relative time frame to aid in section reconstruction. The passive reorientation of the strain fabric and the truncation and incorporation of footwall structures into the thrust sheet are used as kinematic and geometric constraints for cross-section balancing. We present a sequential retrodeformation of the Meade thrust sheet in which strain is removed from the cross-section at the appropriate geometric stages in the deformation. These sections illustrate some methods by which strain data can be incorporated into cross-sections and also indicate the difficulty involved in balancing cross-sections of internally deformed thrust sheets. Interpretation of these sections suggests that a significant amount of footwall deformation accompanied the emplacement of the Meade thrust sheet and that footwall deformation is strongly controlled by the local stratigraphic package. The sections also illustrate that the earliest deformation within a thrust sheet may be unrelated to motion on the underlying incipient thrust, and that the trace of a thrust fault which cuts previously deformed strata will not restore to a typical ramp and flat geometry.