LATE CENOZOIC UPLIFT OF THE SIERRA NEVADA, CALIFORNIA? A CRITICAL ANALYSIS OF THE GEOMORPHIC EVIDENCE

摘要

From the earliest days of California geology, the ramp-like profile of the northern half of the Sierra Nevada mountains and putative signs of recent incision have been interpreted as evidence that the range was formed by the tilting of a rigid block in the late Cenozoic. Over the years, various geomorphic analyses have been used to quantify the magnitude of uplift and to establish its timing, such as analyzing the gradients of ancient channels, examining the tilt of sedimentary beds, and reconstructing the incisional history of rivers. Most studies that have used these methods have supported substantial (>500 m) recent uplift of the Sierra. In contrast, investigations based on other sources of paleotopographic information, such as isotope records, thermochronology, and detrital zircon geochronology, have found that the Sierra have been at high elevations for much of the Cenozoic. This set of contradictory results motivates a re-examination of the geomorphic evidence for late Cenozoic uplift. A critical assessment of these geomorphic studies, based on new topographic analyses and field investigations, reveals that their conclusions are not well supported. For example, several studies based their results on reconstructions of ancient channels that would have flowed up and over bedrock ridges as high as 190 m, a physical impossibility. Other weaknesses include unjustified assumptions regarding the original tilt of fluvial deposits, misinterpretations of stratigraphic relationships, and inadequate recognition of the effect of lithology on channel profiles. The studies supporting recent tilting in the northern Sierra Nevada are inconclusive and rely on observations not unique to tectonic forcing. Indeed, much of the evidence based on the paleogradients of the Tertiary channels is consistent with an early trellis drainage network formed across alternating bands of resistant and weak lithologies. In addition, analyses are presented to demonstrate that deep northern Sierran canyons thought to have been recently incised were, instead, cut as early as the Eocene-Oligocene. Two geomorphic studies from the southern Sierra are consistent with late Cenozoic tilting and uplift although ongoing tectonic activity may be insignificant. Finally, I present a conceptual model of the evolution of the Sierran landscape, applicable primarily to the northern half of the range, illustrating the development of three different drainage networks since the late Jurassic.