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Late Eocene to middle Oligocene floras from the area of the present Rio Grande depression represent a time sequence from the upper part of mixed subtropical forest (Bernalillo flora), to an ecotone between broadleaved sclerophyll and mixed conifer forest (Red Rock Ranch flora), to subalpine conifer forest (Hillsboro and Hermosa floras). The implied difference in mean annual temperature of ~ 11°C suggests that altitude increased 2,000 m in 6–8 my. Construction of the Datil-Mogollon volcanic pile, averaging about 1,200 to 1,500 m thick, is thought to be largely responsible for the forest zonation, but regional doming accompanying volcanism may also be involved. The Oligocene subalpine conifer forests now occur in the piñon-juniper belt 900 and 1,200 m below the present subalpine zone, consistent with the subsidence that formed the Rio Grande rift beginning in the late Oligocene/early Miocene and continuing to the present. Later epeirogenic uplift of ~ 1200 m is implied by fossil plants in the Galisteo (~ 40 my bp), Creede (~ 27 my bp) and Tesuque (~ 14 my bp) Formations that border, or are in the rift.

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