ABSTRACT

Detailed study of a strip that lies within the Balcones fault zone along the outcrop of the Austin group south from Austin, Texas, reveals that about 40% of the faults and shear joints observed are downthrown to the west although the major faults of the zone are downthrown to the east. Faults and shear joints strike N35-40°E with modal dips of 52° southeast and 60° northwest. The dominant tension joint set strikes parallel to the faults and dips near vertical. Another important vertical tension joint set strikes N45°W.

Except for narrow zones of faulting and associated shear jointing, all fractures in the Austin West quadrangle are nearly vertical tension joints that strike parallel and perpendicular to the open, gentle folds of the Edwards Plateau in spite of the presence of the main member of the Balcones fault zone, the Mount Bonnell fault, traversing the east side of the quadrangle.

It is suggested that the decrease in importance of shear fractures from Austin west onto the Edwards Plateau and south toward the San Marcos Arch reflects the increasing stability of the underlying basement as the Texas Craton is approached from the Ouachita foldbelt.