STRUCTURES IN SEGMENT BOUNDARY ZONES OF THE LOST RIVER AND LEMHI FAULTS, EAST CENTRAL IDAHO

Geologic mapping and structural analysis of the Lost River and Lemhi normal faults in east central Idaho suggest that many rupture segment boundaries, previously inferred from geomorphic data coincide with two classes of structural complexities: (1) zones of distributed faults and steps or jogs in the range front faults, and (2) intersections with preexisting Eocene to Oligocene(?) cross faults at 10-15 km depth. Segment boundaries contain as many as 13 fault splays and are as much as 13 km long and 5-6 km wide (in plan view). In contrast, simple fault traces characterize the range front faults in the interiors of segments. Thus segment boundaries of normal faults are broad zones of intersecting faults, not discrete points. The structural data further imply that segment boundaries contain large volumes of fractured and faulted rock and that zones of distributed normal faults have influenced rupture propagation and arrest along these two normal faults. Intersections of preexisting cross faults and range front faults may also have influenced the locations of rupture boundaries along the Lost River and Lemhi faults. Downdip projections of preexisting Eocene to Oligocene(?) normal faults in the footwall of the Lost River and Lemhi faults may coincide with four or five segment boundary zones at about 15 km depth, the depth at which large earthquakes nucleate in the Basin and Range province. Structural analyses also show that some cross faults, which coincide with segment boundary zones at the surface, are distant from these zones at depth. Conversely, some potentially significant structures present at depth are far removed from the segment boundary zone at the surface. Thus a close spatial relationship between cross faults and segment boundary zones at the surface cannot prove or disprove a genetic relationship between these features because the near-surface geometry of segment boundary zones may differ dramatically from the subsurface geometry of the zones. Where cross faults define segment boundaries, the enhanced fracture toughness of preexisting fault planes and their damage zones may impede or arrest rupture on younger crosscutting faults.