FINITE STRAIN AND STRAIN VARIATION ANALYSIS IN THE SHEEPROCK THRUST SHEET - AN INTERNAL THRUST SHEET IN THE PROVO SALIENT OF THE SEVIER FOLD-AND-THRUST BELT, CENTRAL UTAH

The Sheeprock thrust sheet in west-central Utah is an internal thrust sheet in the Prove salient of the Sevier fold-and-thrust belt. We have measured finite strain in quartzites (the dominant lithology), sample d along a square grid within the thrust sheet, using the modified norm alized Fry method (McNaught, hi. A. (1994) Modifying the normalized Fr y method for aggregates of non-elliptical grains. Journal of Structura l Geology 16 493-503). The X/Y and X/Z axial ratios from unsampled loc ations within the sample area were estimated using the spatial statist ics approach. The strain ellipsoids exhibit a variable three-dimension al orientation pattern resulting From modification of the initial laye r parallel shortening (LPS) strain ellipsoid by fault parallel shear i n conjunction with vertical flattening and/or horizontal stretching in dicating that the thrust sheet did not undergo plane strain deformatio n in the transport plane. This suggests that the plane strain assumpti on used in drawing restorable balanced cross-sections breaks down for internal thrust sheets with more than one penetrative-strain producing deformation event. The X/Z strain axial ratios decrease away from the thrust towards the middle of the sheet. The X/Y strain axial ratios f rom interpolated image diagrams indicate transport-parallel stretching at the front end of the sheet and strike-parallel stretching at the b ack end of the sheet. The footwall and hanging wall finite strain patt erns are similar indicating that most of the strain in the Sheeprock t hrust sheet developed early in the deformation history of the thrust s heet before and perhaps during the growth of a large fault propagation fold pair. (C) 1998 Elsevier Science Ltd. All rights reserved.