SEQUENCE OF DEFORMATIONS RECORDED IN JOINTS AND FAULTS, ARCHES-NATIONAL-PARK, UTAH

Faults and joints in an area of essentially undeformed rocks on a limb of a salt anticline in Utah record a surprisingly complex deformational history. Most of the faults started as widely-spaced zones of deformation bands accommodating a few cm of strike-slip. Some were subsequently opened as joints, then were sheared with a sense opposite that of the original faults. Other faults in the Garden Area are fractures that started as joints, then were subsequently sheared. The sense of shear changes across the area, however, and the pattern of shearing is the pattern that would be produced by bending of joint-bounded slabs about a vertical axis. Slip on the faults and joints produced a total regional strain of about 0.15%. Examination of relations among the structures indicates the following deformational history: first were conjugate, strike-slip faults oriented N30-degree-E or N60-degrees-E, reflecting zero vertical strain (and presumably vertical intermediate compression), maximum compression in the NE direction (normal to the axis of Salt Valley), and maximum extension in the SE direction. The faults are of the deformation-band variety and so, presumably formed when the rocks were several kilometers deep. Deformation bands never again formed in these rocks. Subsequent fracturing was mode I, tension cracking. Second, tension in the SE direction (or minimum compression in the SE direction and pore-water pressure exceeding the minimum compression), parallel to the long axis of the Salt Valley anticline, opened joints along some of the weak deformation-band faults, causing them to become jointed faults. The orientation of minimum compression was unchanged, but the orientations of the maximum and intermediate principal stresses are unknown and may have changed. Third, systematic zones of joints formed, cutting across the band faults without deviating in trend throughout most of the Garden Area, but interacting with the open jointed faults locally. The direction of tension (or minimum compression) at this time was about N75-degrees-E to N90-degrees-E, indicating that the direction of principal extension had rotated about 45-60-degrees clockwise, oblique to the axis of Salt Valley anticline. At the same time, short joint segments formed along the jointed-faults. The jointed faults slipped to become faulted-jointed-faults. Although the amount of slip was less than that on the original deformation bands, the sense of slip was reversed. Finally, the rock slabs bounded by the zones of joints were subjected to flexural slip with different centers of curvature in different parts of the area, converting most of the joints into faulted-joints, with three or four domains of different senses of slip.