New K-Ar radiometric dates from Miocene volcanic rocks in the northern Española basin and northeastern Jemez Mountains of north-central New Mexico help constrain the late Tertiary depositional and tectonic history of the area. Stratigraphic relationships and radiometric ages from lava flows and dikes show that the Santa Fe Group, which is the dominant fill of the Rio Grande rift in the Española basin, consists of two Miocene alluvial fans separated stratigraphically by eolian deposits. The older (>12.4 Ma) fan, the Chama-EI Rito Member of the Tesuque Formation, was derived mainly from the north and northeast; it consists of volcaniclastic silty sandstone and gravel. The younger (<12 Ma) fan, the Chamita Formation, was built to the southwest and contains rock material from the Sangre de Cristo Mountains and from northern New Mexico in gravel channels cut into sandstone. These fans overlap in the study area and interfinger with the 14 Ma to 12 Ma northeast-transported eolian deposits of the Ojo Caliente Sandstone Member of the Tesuque Formation.

Between about 15 Ma and 14 Ma, during deposition of the Tesuque Formation, Jemez volcanism began with eruption of Lobato Basalt in an area that is now the northern Jemez Mountains. Basaltic volcanism became wide-spread only during a brief period ca. 10 Ma, when flows spread out across much of the study area. This episode of basaltic volcanism largely shut off ca. 9 Ma, approximately coeval with a regional clockwise rotation of the extension direction. This suggests a causal connection between the tectonic activity and basaltic volcanism. Minor Lobato volcanism continued until ca. 7 Ma.

Stratigraphic evidence suggests that the stream that formed Santa Clara Canyon, the largest east-trending drainage in the north-east Jemez Mountains, has been located near its present position since at least middle Miocene time. The channel has meandered and shifted laterally, resulting in markedly different stratigraphic sections on either wall of the canyon in the vicinity of the western boundary (Pajarito) fault of the Española basin.

Two major north-trending faults (the Guaje Mountain and Cañada del Amagre) have significantly larger strike-slip components than dip-slip components. The large horizontal component of displacement on these faults is probably related to counterclockwise rotation of the intra-rift blocks. Approximate slip rates for the faults, calculated using a radiometrically dated basalt dike and dacite dome, are virtually identical, yielding a slip rate of 0.1 mm/yr during the past 5 m.y.

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