| dc.description.abstract | Spot cores from Phase 1 drilling of the main borehole at the San Andreas Fault
Observatory at Depth (SAFOD) were mapped to characterize the mesoscale structure and
infer paleostress at depth. Cores were oriented by comparing mapped structures with
image logs of the borehole. The upper core (1476-1484 m measured depth, MD) is a
medium-grained, weakly foliated, hornblende-biotite granodiorite containing leucocratic
phenocrysts and lenses. Principal structures are sub-vertical veins, shallow dipping
shears, and natural fractures of unknown kinematics. The features are compatible with
horizontal extension and right-lateral, normal, oblique-slip on faults striking
approximately parallel to the SAF. The lower core (3055.6-3067.2 m MD) has massivebedded,
pebble conglomerates and coarse to fine grained arkosic sandstones grade into
siltstones. Principal structure features are conjugate shears and two minor faults. The
fracture fabric is consistent with strike-slip faulting and a maximum principal
compressive paleostress at ~80° to the SAF plane. This paleostress is essentially parallel
to the current in situ stress measured in the main borehole and to paleostresses inferred
from fracture fabrics in exhumed faults of the San Andreas system to the south. The
similarity between the current state of stress and paleostress states supports the suggestion that the maximum principal compressive stress direction is, on average, at
high angles to the SAF and that the fault has been weak over geologic time. | en |
