The state of stress in the limb of the Split Mountain anticline, Utah: constraints placed by transected joints

Transected joints (i.e. systematic joints that strike at an angle to the pr esent fold axis trend) occur on the flanks of Split Mountain, a Laramide an ticline near the eastern end of the Uinta Mountains, Utah. The common orien tation on both flanks for these WNW-striking joints is inconsistent with jo ints driven by a syn-folding stretch normal to the direction of highest cur vature. A smaller dispersion of the poles to these transected joints occurs when they a-re rotated with bedding to their 'pre-fold' orientation. This dispersion of poles is inconsistent with a post-fold genesis in a regional stress field but permits the possibility that these WNW joints propagated a s a systematic set prior to Laramide folding. A pre-fold interpretation is substantiated by a regional WNW-striking joint set within Cretaceous and ol der rocks in the surrounding Piceance, Uinta, and southeastern Sand Wash ba sins. During tilting accompanying the upfolding of Split Mountain, most joi nts of this WNW-striking regional set remain locked without slipping under a shear stress. Fracture toughness and frictional strength are two rock pro perties that serve to lock a joint until a critical resolved shear stress i s achieved. A gravity load caused down-dip slip on some joints that were ti lted to a dip of about 62 degrees. This suggests that a local principal str ess remained roughly vertical during bedding rotation. Assuming fracture st rength and friction prevented slip on most joints during tilting, the ratio of least horizontal, Sh, to vertical stress, S,, at the critical tilt angl e was approximately 0.55. (C) 2001 Elsevier Science Ltd. All rights reserve d.