Abstract

Despite recent interest in transfer zones in regions of continental extension, their detailed geometry and kinematics remain unclear. Four late Palaeogene and Neogene transfer zones within the southern Rio Grande rift, USA, are described, based on geological mapping at the scale of 1:24 000 and analysis of synrift stratigraphy. The Cutter Sag transfer zone is located between the overlapping tips of north-trending, west-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 overlap of north-trending, west-dipping faults, but the structural details of the transfer zone are largely hidden beneath Quaternary lava flows. However, isolated bedrock exposures outside the lava field, gravity data, and the alignment of cinder cones suggest that the West Potrillo transfer zone is a NW-dipping synclinal ramp cut by NE-trending fractures and/or faults. Positioned between north-trending, overlapping border 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 zone separates border faults that do not overlap and has a long history of faulting and sedimentation beginning in latest Oligocene and extending into the Quaternary.

With the possible exception of reactivated early Tertiary compressional structures in the Rincon transfer zone, the transfer zones described here do not appear to be influenced by pre-rift zones of weakness. However, the Cutter Sag and West Potrillo transfer zones are preferred sites of basaltic volcanism, and the Cutter Sag.- Rincon, and Cedar Hill transfer zones influenced the location of the Plio-Pleistocene ancestral Rio Grande.

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