LATE CRETACEOUS-CENOZOIC EVOLUTION OF THE SOUTHERN ROCKY MOUNTAINS

Laramide deformation was heralded by changes in sedimentation (80–75 Ma), the beginning of magmatism (75 Ma), and fission-track apatite cooling ages (80–75 Ma). Crustal thickness increased from ~33 km to 40–55 km as ENE to NE contraction formed elongate anticlinal arches bordered by low-angle thrusts and steeper back-thrusts. Erosional beveling of uplifts, filling of intermontane basins, and aggradation of the east-sloping High Plains piedmont accompanied Eocene waning (43–37 Ma) of Laramide shortening. Westward retreat/disintegration of the Farallon flat slab induced widespread arc-type volcanism (37–23 Ma), including synchronous episodes of bimodal silicic ignimbrite–mafic lava eruptions from Trans-Pecos Texas to central Colorado. Weak regional extension began about 36 Ma and became focused on the Rio Grande rift beginning ~30 Ma. Bolson sediments with interbedded mafic lavas were preserved in several rift basins by 27–26 Ma.

The Ogallala subcrop records progressively deeper erosion southward into New Mexico during late Oligocene–earliest Miocene magmatic/thermal uplift. An early Miocene (21–16 Ma) volcanic lull was followed by major subsidence of rift basins, isostatic uplift of basin margins, and aggradation of the Ogallala fluvial/eolian blanket on the High Plains (18–5 Ma). Late Miocene climate change (~9–6 Ma) increased erosion that exhumed Laramide uplifts and separated the Ogallala from its source regions in the Southern Rocky Mountains (SRM). The Rio Grande was integrating formerly closed basins by 7 Ma and 650–900 m of topographic relief developed along the SRM–High Plains boundary prior to 6 Ma. Incision of drainages isolated remnants of late Eocene/Miocene erosion surfaces on interfluves. Up-to-the-west warping of the pre-Ogallala surface (McMillan, 2002; Leonard, 2002) indicates that increments of isostatic and epeirogenic uplift have added to elevations of the Laramide orogenic plateau. The present condition of the Southern Rocky Mountains with high heat flow, a buoyant upper mantle, and major basaltic volcanism (5–0.003 Ma) along the Jemez lineament suggests interesting times ahead.