Structural evolution of the southern Sacramento detachment fault system, southeastern California.

摘要

This thesis addresses long-standing controversies surrounding normal faults that initiate and slip at low-angles (30°) and are responsible for extreme crustal extension (>100%). The architecture of a low-angle normal fault (LANF) exposed in southeastern California, provides information about the geometry, mechanics, and evolution of large-scale, gently dipping faults. The throughgoing LANF exposed today is comprised of a linked set of smaller scale structures---ramps and flats---which show very different mechanisms of strain accommodation and evidence of fluid-rock interaction. The magnitude of slip on this fault system is estimated to be about 5 km based on the distance synextensional sedimentary deposits are displaced from their source. Although slip may be considered minor relative to many regional LANFs (10's km), certain structures within this detachment fault may represent the early manifestation of fault zone evolution, preserving a snapshot of the mechanism(s) by which a continuous, gently dipping fault can initiate and move in the brittle regime.;The topography of the fault on the east, downdip side of the domed core complex shows a gently dipping ramp- (10-30°) flat (10°) geometry over a scale of hundreds of meters. Strain and fluid are partitioned within ramp and flat compartments. Ramps have thick (>60 m) damage zones comprised of stacked sequences of tabular fault blocks separated by gently dipping damage zones. Damage zones at fault block boundaries are sites of strain concentration and intense fluid-rock interaction, hosting thick zones of hydrothermal alteration (epidote + quartz + chlorite) indicating that ramps are important fluid conduits in the fault zone. In contrast, flats have thin damage zones (~10 m), showing extreme strain concentration on a single principal slip plane, and display far less evidence of fluid-rock interaction. Flats may not have been important fluid conduits within the fault zone, or fluid flow was more focused along narrow damage zones with evidence of fluid-rock interaction consistently reworked by slip concentrated on the principal slip plane. Field and microstructural observations show that fluid infiltration into the fault zone was episodic, pressurized, and both alternated, and occurred coeval with deformation. Fluid overpressure may have resulted in fault weakening by reducing the effective normal stress on the fault. The ramp-flat geometry of this detachment fault system resembles that of strike-slip fault systems comprised of shear zones linked by dilational jogs. Ramps are characteristically similar to dilational jogs and flats resemble shear components. The interaction and linking of ramp and flat components with progressive slip, resulted in the continuous, gently dipping fault exposed today.;Paleostress analysis performed on reactivated brittle structures preserved in damage zones to the SDF show that this fault slipped under nearly vertical maximum principal stress, indicating no significant vertical rotation of the stress field about the horizontal axis. Paleo-minimum principal stress orientations are discordant with striae recording the most recent slip on the secondary breakaway. However, they are similar to minimum principal stress orientations recorded by the emplacement of dike-like intrusions just prior to fault initiation. These observations suggest rotation of the minimum principal in the horizontal plane during fault slip.