Abstract

The central Velarde graben is the active subbasin of the Española basin section of the Rio Grande rift in north-central New Mexico. The Velarde graben is bounded on the west, in the Jemez volcanic field, by the Pajarito fault zone. This fault zone has produced a steep fault scarp about 100 m high where it cuts the l.l-m.y.-old Tshirege Member of the Bandelier Tuff. Detailed mapping along the north-trending Pajarito fault zone has revealed a fairly simple geometry. In the Tshirege Member, the faults follow numerous vertical joints. Below this member, fault dips are ∼60° and not listric at shallow depths. This simple geometry allows calculation of a mean rate of extension of ∼0.05 mm/yr across the Pajarito fault zone for the past 1.1 m.y. If extension is not perpendicular to the fault zone, the extension rate could be as great as ∼0.07 mm/yr. Lack of transverse tilt of the Velarde graben wedge implies that the extension rate across the eastern margin is about the same as for the western margin. Comparison with a published extension rate for the northern Albuquerque-Belen basin (just to the south of the Jemez Mountains) of 0.3 mm/yr (both sides) since rifting began 26 m.y. ago indicates a slower opening for the Velarde graben during the past 1.1 m.y. If extension is localized along the margins of the Velarde graben with little activity along other fault zones in the Española basin, then both the mean rate of extension and the width of the actively extending region have decreased with time for this section of the Rio Grande rift.

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