Joints and faults in Garden Area of Arches National Park, Utah

Guozhu Zhao, Purdue University

Abstract

Faults and joints in an area of essentially undeformed rocks on a limb of a salt anticline in Utah record a surprisingly complex deformation history. Examination of relations among the structures indicates the following deformational history: First were conjugate, strike-slip faults oriented N30E or N60E, reflecting zero vertical strain, 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. Second, tension in the SE direction parallel to the long axis of the Salt Valley anticline, opened joints along some of the weak deformation-band faults, causing some of the faults to become jointed faults. Third, systematic zones of joints formed throughout the Garden Area, cutting across the band faults without deviating in trend. The direction of tension at this time was about N75E to N90E, indicating that the direction of principal extension had rotated about 45 to 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 in different parts of the Garden Area. In places, the pattern of strike-slip band faults consists of a domain of parallel faults, oriented either N30E or N60E. In others, where the pattern is conjugate, age relations are evident. The age relations indicate that conjugate faults do not form simultaneously, but that a few members of one set form and then are offset by a few members of a second. Then, a few new members of the first set form and offset some members of the second set. In this way a conjugate pattern is built up. Band faults involve major shearing and minor normal deformation, so they form primarily as mode II and secondarily as mode I fractures. Theoretical analysis of predominant mode I fracturing by Cottrell and Rice show that paths of mode I fractures are changed by shear and stabilized by compression parallel to the crack path. Extension of the theory to faulting indicates that paths of dilatant, predominant mode II fractures, are changed by compression or tension normal to the fracture. (Abstract shortened with permission of author.)

Degree

Ph.D.

Advisors

Johnson, Purdue University.

Subject Area

Geology|Hydrology|Geophysics

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