TRANSFER ZONES IN THE SOUTHERN RIO-GRANDE RIFT

Despite recent interest in transfer zones in regions of continental ex tension, their detailed geometry and kinematics remain unclear. Four l ate Palaeogene and Neogene transfer zones within the southern Rio Gran de rift, USA, are described, based on geological mapping at the scale of 1:24 000 and analysis of synrift stratigraphy. The Cutter Sag trans fer zone is located between the overlapping tips of north-trending, we st-dipping faults, and is characterized by a north-dipping structural ramp segmented by closely spaced, NE-trending normal faults. Similarly , the West Potrillo transfer zone occupies a position in the zone of o verlap of north-trending, west-dipping faults, but the structural deta ils of the transfer zone are largely hidden beneath Quaternary lava Ro ws. However, isolated bedrock exposures outside the lava field, gravit y data, and the alignment of cinder cones suggest that the West Potril lo transfer zone is a NW-dipping synclinal ramp cut by NE-trending fra ctures and/or faults. Positioned between north-trending, overlapping b order faults that dip toward each other, the Cedar Hills transfer zone is a broad, north-trending arch whose summit collapsed into a graben along a series of north-trending normal faults. The Rincon transfer zo ne separates border faults that do not overlap and has a long history of faulting and sedimentation beginning in latest Oligocene and extend ing into the Quaternary. With the possible exception of reactivated ea rly Tertiary compressional structures in the Rincon transfer zone, the transfer zones described here do not appear to be influenced by pre-r ift zones of weakness. However, the Cutter Sag and West Potrillo trans fer zones are preferred sites of basaltic volcanism, and the Cutter Sa g, Rincon, and Cedar Hill transfer zones influenced the location of th e Plio-Pleistocene ancestral Rio Grande.