Evolution of the Sierra Nevada and Walker Lane and puzzling out the ancestral Rockies

(Press-News.org) Boulder, Colo., USA - New Geosphere science covers volcanic activity in the Lake Tahoe-Reno-Carson City area and its tie to abundant seismicity in the region; mapping of the interpreted locations of the Kern Canyon and Breckenridge faults at a level of detail never before published; a study of sediments in Reno-Verdi area, Western Nevada, that provide a record of a warmer, wetter climate featuring large mammal fossils; and answers to some of the puzzles surrounding the Ancestral Rocky Mountains.

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Distinct mantle sources for Pliocene-Quaternary volcanism beneath the modern Sierra Nevada and adjacent Great Basin, northern California and western Nevada, USA Brian Cousens et al., Ottawa-Carleton Geoscience Centre and Isotope Geology and Geochronology Research Facility, Dept. of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada. Posted online 11 April; doi: 10.1130/GS741.1.

A swath of young volcanic activity occurs in the region north of Lake Tahoe, USA, to as far east as Battle Mountain, Nevada. The goal of this work by Brian Cousens and colleagues is to describe how the geochemistry and inferred origin of the lavas changes from west to east. They show that volcanic activity in the Lake Tahoe-Reno-Carson City area is derived largely from melting of the mantle lithosphere underlying the Sierra Nevada. Here, the lithospheric mantle may be melting as a result of hot mantle rising beneath it. However, similar-aged volcanic fields around Fallon and at Battle Mountain are composed of lavas generated largely from the shallow convecting upper mantle beneath the lithosphere. These eastern volcanic fields, situated within the Great Basin, appear to overly lithospheric mantle that has lost its ability to melt, so for volcanism to occur the underlying convecting mantle must be shallow enough to melt. The authors note that the potential for future eruptions remains throughout this broad region, based on the youth of the volcanic fields and the abundant seismicity in the area.



Map of the late Quaternary active Kern Canyon and Breckenridge faults, southern Sierra Nevada, California C.C. Brossy et al., Fugro Consultants, Inc., 1777 Botelho Drive, Walnut Creek, California 94596, USA. Posted online 11 April; doi: 10.1130/GS663.1.

In this paper and the accompanying map, C.C. Brossy and colleagues describe the location and activity of the previously uncharacterized Kern Canyon fault, an approx. 135-kilometer-long (~84 miles) fault located in the southern Sierra Nevada mountain range. The Kern Canyon fault was interpreted to have been inactive for the past several million years until recently completed seismic hazard studies for the Lake Isabella dams demonstrated that the fault has produced multiple surface-rupturing earthquakes within the past several thousand years. The fault map presented by Brossy and colleagues shows the interpreted locations of the Kern Canyon fault and the adjacent Breckenridge fault at a level of detail never before published. They describe how the faults are expressed in the landscape and summarize recent advances in the understanding of the level of activity of the Kern Canyon and Breckenridge faults. The new information in this paper provides a basis characterizing the earthquake potential of the Kern Canyon fault and for assessing earthquake hazard to dams and other engineered infrastructure in the southern Sierra Nevada region.



Constraints on the history and topography of the Northeastern Sierra Nevada from a Neogene sedimentary basin in the Reno-Verdi area, Western Nevada James Trexler et al., Dept. of Geological Sciences and Engineering, MS 172, University of Nevada, Reno, Reno, Nevada 89557, USA. Posted online 11 April; doi: 10.1130/GS735.1.

Neogene (Miocene-Pliocene) sedimentary rocks of the northeastern Sierra Nevada were deposited in small basins that formed in response to volcanic and tectonic activity along the eastern margin of the Sierra. These strata record an early phase (about 10-11 million years ago) of extension and rapid sedimentation of boulder conglomerates and debrites deposited on alluvial fans, followed by fluvio-lacustrine sedimentation and nearby volcanic arc activity but tectonic quiescence, until about 2.6 million years ago. The fossil record in these rocks documents a warmer, wetter climate featuring large mammals and lacking the Sierran orographic rain shadow that dominates climate today on the eastern edge of the Sierra. This record is consistent with evidence that there was no significant topographic barrier between the Pacific Ocean and the interior of the continent east of the Sierra before about 2.6 million years ago. However, these sediments do not record an integrated drainage system either to the east into the Great Basin like the modern Truckee River, or to the west across the Sierra like the ancestral Feather and Yuba rivers. The Neogene Reno-Verdi basin was one of several, scattered endorheic (i.e., internally drained) basins occupying this part of the Cascade intra-arc and back-arc area.



Load-induced subsidence of the Ancestral Rocky Mountains recorded by preservation of Permian landscapes Gerilyn S. Soreghan et al., Conoco-Phillips School of Geology & Geophysics, University of Oklahoma, Norman, Oklahoma 73019, USA. Posted online 11 April; doi: 10.1130/GS681.1.

The Rocky Mountains dominate the landscape of the western United States, but an older system of mountains occupied much the same space roughly 300 million years ago. These "Ancestral Rocky Mountains" have long puzzled geoscientists because they formed far from a plate boundary and have thus eluded simple explanations. Gerilyn S. Soreghan and colleagues present an analysis of both new and old data from this system, which indicates that the mountain evolution was linked to an ancient, dense crustal root that influenced the trend of the old mountains and that ultimately hastened their demise. This root formed during rifting of the North American continent more than half a billion years ago and underlies a region extending from southern Oklahoma to western Colorado. Remarkably, remnants of the approx. 300 million-year-old landscapes formed by these mountains remain relatively intact in parts of Oklahoma and Colorado, forming the most ancient landscapes ever documented in North America -- among the most ancient in the world.

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