Abstract

The east side of the Colorado Plateau follows a north-trending zone of weakness deformed during Proterozoic, Ancestral Rocky Mountain, Laramide, and late Cenozoic tectonism. Northward displacement of the Colorado Plateau during late Laramide flat-plate subduction (late Paleocene-middle Eocene) formed a broad zone of right-lateral faulting accompanied by an echelon series of strike-slip basins. A regional surface of relatively low relief developed across beveled Laramide uplifts and aggraded basins during a prolonged period of erosion and tectonic quiescence in late Eocene. Volcanism spread across the Southern Rocky Mountain-Colorado Plateau boundary as the subducted slab steepened and sank. The earliest calderas (37-33 Ma, central Colorado; 29 Ma, San Juan field; 36-32 Ma, Mogollon-Datil field) formed along the west edge of the incipient Rio Grande rift; the foci of ignimbrite volcanism then migrated westward.

As tensional stresses strengthened, the Rio Grande rift collapsed along the north-trending Colorado Plateau boundary previously weakened by multiple deformations and high heat flow. Several Laramide uplifts collapsed and became basin floors. Structural inversion and basin subsidence began at about 35 Ma, coincident with a change to bimodal volcanism. Local areas of extreme extension, with domino-style block rotation and low-angle normal faults formed above, or proximal to, batholithic intrusions in the Socorro (29-27 Ma) and Questa (26-25 Ma) areas. Apatite fission-track (AFT) cooling ages show strong uplift and denudation of ranges flanking rift basins beginning about 24 Ma. Basin subsidence was particularly rapid between about 16 and 10 Ma. Rifting began more or less concurrently along the Rio Grande rift but proceeded more rapidly in the south; estimates of total extension are: San Luis Basin 8–12%, Albuquerque Basin 17–28%, and Socorro 50%. Isotopic and trace element compositions of basalt flows indicate a change from lithospheric to asthenospheric sources beginning about 10 Ma south of Socorro and progressing northward to the Jemez Mountains by 2 Ma; source regions for magmas north of the Jemez lineament are all in the lithosphere. Changes to a wetter, stormier climate at about 7 Ma lead to integration of closed basins, regional incision of drainages, and development of the Rio Grande.

The physiographic eastern margin of the Colorado Plateau is actually an intra-plateau boundary separating elements of a broad orogenic plateau formed during the Laramide. Mapped and dated remnants of the late Eocene surface and similarities in post-Laramide tectonic, volcanic, and erosion history link together the High Plains, Southern Rocky Mountains, Colorado Plateau, and adjacent portions of the Basin and Range and Wyoming provinces. Key data are: 1) similar Laramide apatite fission-track cooling histories (80-45 Ma) for the Colorado Plateau and Front Range; 2) paleobotanical evidence that the 34.1 Ma Florissant lake beds were deposited at an elevation (2.2–3.3 km) on the Front Range similar to today (2.5 km); 3) lack of differential structural relief between the Front Range and High Plains since late Eocene as evidenced by preservation of old (>100 Ma) AFT cooling ages along the Range margin and lack of significant offset of the 37 Ma Wall Mountain Tuff deposited across the boundary; and 4) similar timing and magnitude of late Miocene-Recent excavation of basin-fill deposits and incision of drainages.