Bridge structures as sense of displacement criteria in brittle fault zones

Abstract

Fault zones often occur as arrays of en-échelon fractures. There are two main types of en-échelon patterns, involving right- or left-stepping fractures. In such arrays, first-generation fractures define compressive or tensile bridges. The structural features of such natural bridges, at any scale, can be used as criteria to determine the sense of displacement. Small compressive bridges provide criteria based on the increase in linear density of solution seams related to the degree of asymmetry of the fault trace, and to the sequence of formation of the different types of en-échelon fractures. The criteria deduced from large compressive bridges involve upward displacement of the material by folds and hill or mountain ranges, asymmetry of the fault profile and relative chronology of the segments. Tensile bridges yield criteria based upon vein fillings, asymmetry and relative chronolog. In large-scale structures some pull-apart basins are shown to be bounded by normal faults parallel to, but outside, the strike-slip segments. From a comparison of analytical models with well-known large-scale fault zones, a new criterion can be proposed. In fault zones formed through coalescence of en-échelon fractures the bridging segments are wide if facing leeward of the bulk movement sense, and narrow if forward-facing.