The deepest part of the northern Rio Grande rift is just northwest of the Great Sand Dunes, on the eastern side of the San Luis Valley, in south-central Colorado. Approximately 150 km (95 m) of high-quality, 20-fold Common Depth Point seismic data indicate that the basin is filled with approximately 6.4 km (21,000 ft) of Tertiary sediments, mostly Oligocene or younger in age. These data, combined with published data, provide a basis for interpreting the structural geometry of the rift. The internal structural geometry of this part of the rift is surprisingly uncomplicated. Sympathetic and antithetic faulting is not widespread, or major, within the deep part of the basin. The internal geometry of the sedimentary packages, however, indicates a complicated movement history on the faults that are present within the rift. The bounding fault zone between the rift and the Sangre de Cristo Mountains has approximately 9.2 km (30,000 ft) of vertical separation and may be very complicated. Although we were not able to resolve the bounding fault zone with our seismic data, the data constrains the minimum angle of the fault zone to be approximately 45°. Our modeling suggests that a 60° angle is the most likely orientation of the fault zone. The influence of earlier, low-angle extension recognized nearby and elsewhere in the Rio Grande rift is not directly evident in our data. Assuming 60° antithetic shear in the hanging wall, which is supported by the seismic data, a depth of detachment (flattening) is estimated to be approximately 16 km, in the brittle-ductile transition zone estimated from heat-flow data. Cross sections drawn using the constraints of the new seismic data, require only 8 to 12% extension across the rift at this latitude.