Abstract

The Pajarito fault zone forms the western border of the Velarde graben, the presently active, central subbasin of the Española basin section of the Rio Grande rift in north-central New Mexico. The fault zone is a north-northeast-trending zone of predominantly down-to-the-east faults that cut Miocene to Pliocene volcanic rocks along the eastern flank of the Jemez Mountains. Where the fault zone cuts the 1.1-m.y.-old Tshirege Member of the Bandelier Tuff, it has produced a steep, 50- to 100-m-high fault scarp. The total displacement across the fault zone during its 5-m.y. history is between 200 and 600 m. Rates of displacement for the time periods 0–5, 0–1.1, and 1.1–5 m.y. ago range from 0.02 to 0.136 mm/yr.

Abrupt facies changes between older volcanics and volcaniclastic sediments of the Jemez Mountains appear to have controlled the local position, trend, and character of the Pajarito fault zone. The fault zone bows and/or steps eastward where two large volcanic complexes are present but is found farther west in between and at either end of the volcanic complexes. One complex was sufficiently massive to interfere with the development of the Velarde graben.

Slickensides on mesoscopic faults in the Tshirege Member of the Bandelier Tuff indicate that the Pajarito fault zone has undergone extension in two directions during the past 1.1 m.y., approximately parallel and perpendicular to the local trend of the fault zone. These directions indicate that the Pajarito fault zone has reoriented the regional minimum and intermediate stress directions to perpendicular and parallel, respectively, to the local trend of the fault zone, and that both minimum and intermediate stress directions are tensional.

A tectonic history for the Pajarito fault zone area of the Española basin begins with relatively stable accumulation of prerift and synrift sediments from Eocene to Oligocene time. Sedimentation concomitant with faulting, unrelated to the Pajarito fault zone, filled deep central depressions within the Española basin. This faulting ceased prior to the end of the filling of the basin, around 10 m.y. ago in the local area. Jemez Mountain volcanism began about this time, before movement along the western-margin border faults of the Española basin caused west-tilting of old volcanics and sediments, about 7.5 m.y. ago. Volcanism continued under relatively stable conditions until ∼5 m.y. ago. At this time, the Pajarito fault zone and Velarde graben formed. Faulting has continued to the present, localized along this central subbasin.

First Page Preview

First page PDF preview
You do not currently have access to this article.