, Volume 142, Issue 3-4, pp 663-685

Structural analysis of the Brigham City-Weber segment boundary zone, Wasatch normal fault, Utah: Implications for fault growth and structure

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Abstract

We investigate the history, kinematics, principal stress orientations and geometry of deformation at the end of a bent normal fault segment of the Wasatch fault zone, Utah. Three fault types, developed in Archean crystalline rocks, reflect progressive uplift of fault-related footwall rocks. Chlorite-breccias and phyllonites reflect deep-level, reaction-assisted plastic deformation along the north-striking part of the segment. Planar, fretted faults which formed by cataclasis cut the phyllonites and breccias and are developed throughout the footwall of the segment. Youngest faults are hematitecoated, extremely narrow polished surfaces. Slip vectors and kinematic analyses of small faults developed in the footwall indicate oblique normal slip along the north-striking portion of the segment. Slip vectors and fault orientation along the northwest-striking portion of the segment reflect complexly oriented slip on faults which strike subparallel and at high angles to the main fault trace, yet slip is confined to a broad fault-parallel zone. Small faults at the southernmost tip of the segment indicate a strong influence of the north-striking Weber segment to the south. Inversion of fault data for principal stress orientations document complexly oriented principal stresses through the segment boundary zone and suggest that σ3 may have reoriented approximately 60° over the life of the segment. Subsurface structure combined with small fault data indicate the segment boundary is comprised of a southwest-plunging bedrock high which is reflected by a sharp bend in the Brigham City segment. The southern end of the Brigham City segment may have started, as a straight, north-striking fault which has bent due to changes in stress orientations and/or interaction with the adjacent Weber segment.