Diachronous episodes of Cenozoic erosion in southwestern North America and their relationship to surface uplift, paleoclimate, paleodrainage, and paleoaltimetry

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...

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“…Evidence for deep, post-6 Ma incision has also been observed throughout the Rocky Mountains and Colorado Plateau, and an enthusiastic debate has developed in the literature regarding the causal mechanisms driving this phenomenon. The debate largely hinges on the relative roles of tectonic rock uplift (Trimble, 1980;McMillan et al, 2002McMillan et al, , 2006Leonard, 2002;Eaton, 1986Eaton, , 2008Karlstrom et al, 2012;Cather et al, 2012) and climatically modulated geomorphic parameters such as changes in water and sediment fl uxes (Molnar and England, 1990;Chase, 1992, 1994;Chapin, 2008;Wobus et al, 2010) and drainage reorganization as drivers of incision. Because the physical effects of tectonic versus climate end-member models are diffi cult to distinguish in the geologic record, workers face the challenge of identifying criteria that differentiate climate-driven patterns of landscape evolution from patterns driven by tectonic rock uplift.…”

The Rocky Mountain Front, southwestern USA

“…6-0 Ma) episode four of Cather et al (2012) resulted in major enhancement of the RMF along the Front Range, Wet Mountains, and Sangre de Cristo Range. The erosion was increased by opening of the Gulf of California ca.…”

“…The third episode of erosion and uplift (discussed in Cather et al, 2012) occurred in late Oligocene-early Miocene time in response to increased mantle buoyancy related to major volcanism in the southern Rocky Mountains and Sierra Madre Occidental of Mexico. Early development of the Rio Grande Rift overlapped with this episode and rifting increased markedly during the interval ca.…”

“…Evidence for four episodes of Cenozoic erosion and uplift in southwestern North America was presented in Cather et al (2012). In episode one, the Laramide orogeny (ca.…”

“…6-0 Ma), which helps explain preservation of the old AFT ages. Today, the middle Eocene erosion surface (Epis and Chapin, 1975) that MacGinitie (1953) estimated was carved at elevations <900 m (see also Cather et al, 2012) rises southward along the crest of the Wet Mountains until it reaches modern elevations as high as 3500 m at Greenhorn Peak. The apparent step to the northeast of the RMF from the Sangre de Cristo Range to the Wet Mountains was simply reactivation of the Apishapa uplift, beveled during and after the Pennsylvanian-Permian ARM orogeny.…”