Eocene-Early Miocene paleotopography of the Sierra Nevada-Great Basin-Nevadaplano based on widespread ash-flow tuffs and paleovalleys

Abstract

The distribution of Cenozoic ash-fl ow tuffs in the Great Basin and the Sierra Nevada of eastern California (United States) demonstrates that the region, commonly referred to as the Nevadaplano, was an erosional highland that was drained by major westand east-trending rivers, with a north-south paleodivide through eastern Nevada. The 28.9 Ma tuff of Campbell Creek is a voluminous (possibly as much as 3000 km 3 ), petrographically and compositionally distinctive ash-fl ow tuff that erupted from a caldera in north-central Nevada and spread widely through paleovalleys across northern Nevada and the Sierra Nevada. The tuff can be correlated over a modern area of at least 55,000 km 2 , from the western foothills of the Sierra Nevada to the Ruby Mountains in northeastern Nevada, present-day distances of ~280 km west and 300 km northeast of its source caldera. Corrected for later extension, the tuff fl owed ~200 km to the west, downvalley and across what is now the Basin and Range-Sierra Nevada structural and topographic boundary, and ~215 km to the northeast, partly upvalley, across the inferred paleodivide, and downvalley to the east. The tuff also fl owed as much as 100 km to the north and 60 km to the south, crossing several east-west divides between major paleovalleys. The tuff of Campbell Creek fl owed through, and was deposited in, at least fi ve major paleovalleys in western Nevada and the eastern Sierra Nevada. These characteristics are unusual compared to most other ash-fl ow tuffs in Nevada that also fl owed great distances downvalley, but far less east and north-south; most tuffs were restricted to one or two major paleovalleys. Important factors in this greater distribution may be the great volume of erupted tuff and its eruption after ~3 Ma of nearly continuous, major pyroclastic eruptions near its caldera that probably fi lled in nearby topography. Distribution of the tuff of Campbell Creek and other ash-fl ow tuffs and continuity of paleovalleys demonstrates that (1) the Basin and Range-Sierra Nevada structural and topographic boundary did not exist before 23 Ma; (2) the Sierra Nevada was a lower, western ramp to the Nevadaplano; and (3) any faulting before 23 Ma in western Nevada, including in what is now the Walker Lane, and before 29 Ma in northern Nevada as far east as what is now the Ruby Mountains metamorphic core complex, was insuffi cient to disrupt the paleodrainages. These data are further evidence that major extension in Nevada occurred predominantly in the late Cenozoic. Characteristics of paleovalleys and tuff distributions suggest that the valleys resulted from prolonged erosion, probably aided by the warm, wet Eocene climate, but do not resolve the question of the absolute elevation of the Nevadaplano. Paleovalleys existed at least by ca. 50 Ma in the Sierra Nevada and by 46 Ma in northeastern Nevada, based on the age of the oldest paleovalley-fi lling sedimentary or tuff deposits. Paleovalleys were much wider (5-10 km) than they were deep (to 1.2 km; greatest in western Nevada and decreasing toward the paleo-Pacifi c Ocean) and typically had broad, fl at bottoms and low-relief interfl uves. Interfl uves in Nevada had elevations of at least 1.2 km because paleovalleys were that deep. The gradient from the caldera eastward to the inferred paleodivide had to be suffi ciently low so that the tuff could fl ow upstream more than 100 km. Two Quaternary ash-flow tuffs where topography is nearly unchanged since eruption fl owed similar distances as the mid-Cenozoic tuffs at average gradients of ~2.5-8 m/km. Extrapolated 200-300 km (pre-extension) from the Pacifi c Ocean to the central Nevada caldera belt, the lower gradient would require elevations of only 0.5 km for valley fl oors and 1.5 km for interfl uves. The great eastward, upvalley fl ow is consistent with recent stable isotope data that indicate low Oligocene topographic gradients in the Nevadaplano east of the Sierra Nevada, but the minimum elevations required for central Nevada are signifi cantly less than indicated by the same stable isotope data. Although best recognized in the northern and central Sierra Nevada, early to middle Cenozoic paleodrainages may have crossed the southern Sierra Nevada. Similar early to middle Cenozoic paleodrainages existed from central Idaho to northern Sonora, Mexico, and persisted over most of that region until disrupted by major Middle Miocene extension. Therefore, the Nevadaplano was the middle part of an erosional highland that extended along at least this length. The timing of origin and location of this more all-encompassing highland indicates that uplift was predominantly a result of Late Cretaceous (Sevier) contraction in the north and a combination of Late Cretaceous-early Cenozoic (Sevier and Laramide) contraction in the south.