--> Criteria and Pitfalls in Identification of Wrench Faults from Exploration Data, by Tod P. Harding; #91024 (1989)

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Criteria and Pitfalls in Identification of Wrench Faults from Exploration Data

Tod P. Harding

The identification of wrench faults from subsurface structural data is problematical and requires detection of combinations of profile and map characteristics, namely (1) basement offset, (2) steeply dipping dislocation at depth, (3) upward-spreading splays (negative and positive flower structures), (4) changes in direction of fault dip and upthrown side, (5) changes of separation sense and merged splays with different separations, (6) a narrow, long, straight, throughgoing, solitary zone of deformation and/or master fault, and (7) coeval, en echelon flanking structures. The criteria that are present must be supported with evidence that refutes alternative styles. Wrench interpretations are negated mainly by zigzag, discontinuous fault trace and absence of basement offset The oblique (en echelon) or parallel structures that flank wrench faults are also essential elements of other styles. The cross occurrences of features and the broad range of structural types--extensional, contractional, or both--have resulted in numerous misidentifications of terrane as wrench faulted or wrench associated. Furthermore, convergent or divergent wrench zones with single fault strands can resemble reverse or normal basement-involved faults, or certain faults with inverted slip. Other profile geometries--narrow contractional horsts or narrow extensional grabens, step faults or splay structures, various anticlines with crestal faults, two-sided orogenes, narrow rift basins, minor disruptions in reflection continuity, and various structures with complex or equivocal relations ips--have been misinterpreted as flow structures. Misidentifications also arise because map patterns have multiple influences, the pattern has been altered by rock anisotropy, the structural components are not coeval, the criteria have been applied at incorrect scales, the zone of deformation has been incorrectly delineated, the evidential structure is incongruent with the proposed tectonics, the primary structure is not recognized, and the observed structures are incongruent with the implied deformation intensity or proposed stress system.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.