No abstract

The history of erosion of southwestern North America and its relationship to surface uplift is a long-standing topic of debate. We use geologic and thermochronometric data to reconstruct the erosion history of southwestern North America. We infer that erosion events occurred mostly in response to surface uplift by contemporaneous tectonism, and were not long-delayed responses to surface uplift caused by later climate change or drainage reorganization. Rock uplift in response to isostatic compensation of exhumation occurred during each erosion event, but has been quantifi ed only for parts of the late Miocene-Holocene erosion episode. We recognize four episodes of erosion and associated tectonic uplift: (1) the Laramide orogeny (ca. 75-45 Ma), during which individual uplifts were deeply eroded as a result of uplift by thrust faults, but Laramide basins and the Great Plains region remained near sea level, as shown by the lack of signifi cant Laramide exhumation in these areas; (2) late middle Eocene erosion (ca. 42-37 Ma) in Wyoming, Montana, and Colorado, which probably occurred in response to epeirogenic uplift from lithospheric rebound that followed the cessation of Laramide dynamic subsidence; (3) late Oligocene-early Miocene deep erosion (ca. 27-15 Ma) in a broad region of the southern Cordillera (including the southern Colorado Plateau, southern Great Plains, trans-Pecos Texas, and northeastern Mexico), which was uplifted in response to increased mantle buoyancy associated with major concurrent volcanism in the Sierra Madre Occidental of Mexico and in the Southern Rocky Mountains; (4) Late Miocene-Holocene erosion (ca. 6-0 Ma) in a broad area of southwestern North America, with loci of deep erosion in the western Colorado-eastern Utah region and in the western Sierra Madre Occidental. Erosion in western Colorado-eastern Utah refl ects mantle-related rock uplift as well as an important isostatic component caused by compensation of deep fl uvial erosion in the upper Colorado River drainage following its integration to the Gulf of California.Erosion in the western Sierra Madre Occidental occurred in response to rift-shoulder uplift and the proximity of oceanic base level following the late Miocene opening of the Gulf of California. We cannot estimate the amount of rock or surface uplift associated with each erosion episode, but the maximum depths of exhumation for each were broadly similar (typically ~1-3 km). Only the most recent erosion episode is temporally correlated with climate change.Paleoaltimetric studies, except for those based on leaf physiognomy, are generally compatible with the uplift chronology we propose here. Physiognomy-based paleo eleva tion data suggest that near-modern elevations were attained during the Paleogene, but are the only data that uniquely support such interpretations. High Paleogene elevations require a complex late Paleogene-Neogene uplift and subsidence history for the Front Range and western Great Plains of Colorado that is not compatible with the regional sedimentation and er...

Great thicknesses of eolian dune deposits of early Oligocene age crop out in the Chuska Mountains of northwestern NewMexico-Arizona (as much as 535 m thick) and in the Mogollon-Datil volcanic fi eld of western New Mexico-Arizona (as much as 300 m thick). 40 Ar/ 39 Ar ages of intercalated volcanic rocks indicate eolian deposition in these areas was approximately synchronous, with eolian accumulation beginning regionally at ca. 33.5 Ma and ending at ca. 27 Ma. Probable eolian sandstone of Oligocene age 483 m thick is also present in the subsurface of the Albuquerque Basin of the Rio Grande rift. The beginning of eolian deposition on the Colorado Plateau corresponds closely to the beginning of eolian (loessic) deposition in the White River Group of the Great Plains and major Oi-1 glaciation in Antarctica, suggesting possible global paleoclimatic control.Successions of Oligocene eolian sandstone on the Colorado Plateau are thicker than all of the better known Upper Paleozoic-Mesozoic eolianites in the region, except the Jurassic Navajo Sandstone. We suggest that the widely separated Oligocene eolianites in the Colorado Plateau region were probably originally continuous, and thus are erosional remnants of an extensive (~140,000 km 2 ), regional sand sea (the Chuska erg). This interpretation is based on: (1) comparison with thickness trends of older eolianites in the Colorado Plateau region, (2) evaluation of regional topographic gradients of modern ergs, and (3) hydrologic modeling of a 300-to 400-mthick zone of saturation that existed during eolian deposition in the Chuska Mountains.The Chuska erg represents the fi nal episode of Paleogene aggradation on the central and southern Colorado Plateau. Aggradation was driven primarily by trapping of fl uvial sediments on the plateau by development of major volcanic fi elds along the eastern plateau margin. These volcanic fi elds blocked earlier Laramide drainages that had previously transported sediments eastward off the plateau. Following a shift to widespread eolian deposition at ca. 33.5 Ma, constructional volcanic topography induced eolian accumulation upwind of developing volcanic fi elds. Stratal accumulation rates (not decompacted) of eolian deposits were ~28-82 m/m.y.The reconstructed top of the Chuska erg would lie at a present-day elevation of ~3000 m or more, and provides a datum for assessing subsequent erosion on the Colorado Plateau. Major exhumation (≥1230 m) occurred during the late Oligocene and early Miocene, following the end of Chuska deposition and prior to the onset of Bidahochi Formation deposition at ca. 16 Ma on the southcentral part of the plateau. The Bidahochi Formation attained a thickness of ~250 m by ca. 6 Ma, followed by ~520 m of late Miocene and younger erosion in the valley of the Little Colorado River. The depth of late Oligoceneearly Miocene (ca. 26-16 Ma) exhumation of the central and southern Colorado Plateau thus was more than twice that of the late Miocene-Holocene (ca. 6-0 Ma). The timing of initial deep erosion in the Colorado Pla...

During middle Eocene to middle Miocene time, development of the Cenozoic icehouse was coincident with a prolonged episode of explosive silicic volcanism, the ignimbrite fl are-up of southwestern North America. We present geochronologic and biogeochemical data suggesting that, prior to the establishment of full glacial conditions with attendant increased eolian dust emission and oceanic upwelling, iron fertilization by great volumes of silicic volcanic ash was an effective climatic forcing mechanism that helped to establish the Cenozoic icehouse. Most Phanerozoic cool-climate episodes were coeval with major explosive volcanism in silicic large igneous provinces, suggesting a common link between these phenomena.

In the southern Rocky Mountains and Rio Grande rift, rock cooling patterns from apatite fission-track (AFT) data spatially correlate with areas of voluminous middle Tertiary caldera-complex magmatism. We use thermochronology and gravity data to explore lithospheric modification by voluminous middle Tertiary magmatism. These data are not traditionally used to constrain magmatic processes, but in our study area they provide first-order constraints on the degree of mantle dedensification by basalt removal. We show that thermal isostatic responses to middle Tertiary magmatism drove spatially variable rock uplift and thermal perturbations that, coupled with variable exhumation, can explain AFT cooling and rock preservation patterns. We further argue that, if rock uplift exceeded exhumation, then surface uplift combined with magmatic weakening of the lithosphere could have influenced subsequent Neogene extension. Figure 1. (A) Topography (with locations of Figs. 1B and 2A) and (B) simplified geology of study area. SRM-Southern Rocky Mountains; SJVF-San Juan volcanic field; SJB-San Juan Basin; LVF-Latir volcanic field; RGR-Rio Grande rift; MDVF-Mogollon-Datil volcanic field; DB-Denver Basin; V-Vaughn, New Mexico. Neogene extension affects regions south of red line in A. Heavy black line in B is apatite fission-track (AFT) profile in Figure 2B. (See footnote one in text for geology and AFT data sources.)