Active breaching of a geometric segment boundary in the Sawatch Range normal fault, Colorado, USA

The northwest-trending Sawatch Range normal fault system is the principal r ange-bounding fault of the northern Rio Grande Rift of central Colorado. It displays a geometric segment boundary in the form of a prominent en echelo n step at the Chalk Cliffs, a zone of highly fractured and hydrothermally a ltered quartz monzonite. There, the modern fault geometry suggests that the Sawatch Range fault is presently breaching the boundary. Immediately north of the Chalk Cliffs, the range front fault is active whereas immediately s outh, the range front fault is inactive. Instead, it is replaced by a north -trending break that transects much of the boundary. The Chalk Cliffs also display a four-stage history of faulting and alterati on/mineralization that appears to track the boundary's evolution. First-sta ge chlorite-filled faults and second-stage laumontite-filled faults affect the entire boundary and likely pre-date the breach. They record a change of fluid chemistry during oblique extension associated with the boundary's ea rly history. Third stage, calcite-filled fractures are generally restricted to the eastern half of the boundary, between the breaching fault's project ion into the Chalk Cliffs and the active range front fault. These fractures probably coincide with initiation of the breach. Modern (fourth stage) hyd rothermal activity occurs at, and slightly beyond, the eastern edge. This c oincidence of hydrothermal events with changes in large-scale fault geometr y illustrates how normal fault zones, through breaching, can simplify their geometries and localize associated deformation and hydrothermal activity. (C) 1999 Elsevier Science Ltd. All rights reserved.