Neogene uplift history of the western Picuris Mountains

The Picuris Mountains are an uplifted, westerly trending block of Precambrian metamorphic rocks in north-central New Mexico, partially overlain by Tertiary sedimentary deposits. The Tertiary deposits are sparse in the Picuris Mountains interior, but are commonly offset across NE-trending strike-slip faults or N-S trending, high angle faults initiated by the strike-slip movement. Field examination of the Tertiary stratigraphic units in the Rio Grande Rift reveals that the only named Tertiary deposit present in the mountain area is the Chama-el-Rito Member (16-12) Ma) of the Tesuque Formation, part of the Santa Fe Group.; The Chama-el-Rito Member is an incomplete sequence in the mountain area and was compared to more complete outcrops. The mountain deposits lie atop a poorly consolidated conglomerate that contains Precambrian lithics, which is only present in local grabens in the two eastern outcrops. The Precambrian clasts indicate that at least part of the Picuris Mountains was a positive feature prior to Chama-el-Rito deposition. The underlying conglomerates are interpreted to be coeval to the Bradley Conglomerate (20 Ma), an alluvial fan deposit that underlies the Chama-el-Rito Member near Pilar, N.M. The lower part of the Chama-el-Rito Member is absent in the local grabens, implying no movement along the N-S faults at that time. The deposition of the Chama-el-Rito Member into the local grabens indicates normal faulting resumed in the mountains. The lack of Precambrian lithics in the Chama-el-Rito deposits indicates that the Picuris Mountains relief had been eroded back to base level. An erosional surface (5-2.8 Ma) scoured the top of the Chama-el-Rito Member, indicating renewed uplift of the Picuris Mountains. Normal offset on the local faults has been ongoing, discontinuously, for greater than 20 m.y. with offset continuing up through the youngest deposits in the Picuris Mountains.