Boulder, Colo., USA - New GSA Bulletin articles cover wind erosion and sediment traps in the Qaidam basin; rain erosion on Kauai; new insights from the Petrified Forest, USA; a forearc sliver in Costa Rica; Quebec's St. Lawrence rift system; a new model for the development of Ries Crater Lake, Germany; bending and buckling mountain belts; a record of 22 large earthquakes in northern Fiordland, New Zealand; and the evolution of the ancient Montana landscape.
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Detailed highlights are provided below.
Climatic and tectonic controls on sedimentation and erosion during the Pliocene-Quaternary in Qaidam Basin (China)
Richard V. Heermance et al., Dept. of Geological Sciences, California State University-Northridge, Northridge, California 91330-8266, USA. Posted online 22 Feb. 2013; http://dx.
The Pliocene-Quaternary boundary, approx. 2.6 million years ago (2.6 Ma), represents a time of rapid global climate change from warm and moist to cool and arid (i.e., glacial) conditions. The influence of this climate change on both sedimentation and tectonics is preserved in strata within the Qaidam Basin, China. Overall, climate-controlled basin aridification initiated 3.1 million years ago and caused the gradual change from more humid lacustrine sedimentation to evaporite conditions by 2.6 Ma. After 2.6 Ma, uplift above active structures combined with wind erosion of the basin sediments produced localized sediment traps that controlled sedimentation. This study provides isotopic (O and C), paleomagnetic, and sedimentologic data that distinguish the climatic versus tectonic controls on sedimentation and erosion within the northeastern Tibetan Plateau at this important time period.
Variation of climate and long-term erosion rates across a steep rainfall gradient on the Hawaiian island of Kauai
Ken L. Ferrier et al., Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, USA. Posted online 22 Feb. 2013; http://dx.
The erosion of volcanic ocean islands creates dramatic landscapes, modulates Earth's carbon cycle, and delivers sediment to coasts and reefs. Despite concerns that modern sediment fluxes to island coasts may exceed long-term fluxes, little is known about how erosion rates and processes vary across island interiors. This study by Ken L. Ferrier and colleagues presents new measurements of erosion rates over five-year to five-million-year time scales on the Hawaiian island of Kauai, which is home to one of Earth's steepest precipitation gradients, with mean annual precipitation ranging from 0.5 to 9.5 m. Eroded rock volumes from basins across Kauai indicate that basin-averaged erosion rates over the past several million years vary by a factor of 40 across the island and increase with modern mean annual precipitation. In the Hanalei basin of Kauai, estimates of sediment fluxes and solute fluxes imply that modern erosion rates are no more than 2.3 plus or minus 0.6 times faster than erosion rates over the past few thousand years, as determined by new measurements of helium-3 in olivines in stream sediment. To the extent that modern precipitation patterns resemble long-term precipitation patterns, these measurements provide new support for a link between precipitation rates and long-term basin-averaged erosion rates.
Sedimentological constraints on the evolution of the Cordilleran arc: New insights from the Sonsela Member, Upper Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA)
Evan R. Howell, Noble Energy, 1625 Broadway, Suite 2200, Denver, CO 80202, USA; and Ronald C. Blakey. Posted online 22 Feb. 2013; http://dx.
The Sonsela Member of the Upper Triassic Chinle Formation in Petrified Forest National Park, Arizona, USA, forms a distinctive part of an extensive ancient river system that once flowed, at least in part, from a major volcanic arc bordering the western margin of North America (Cordilleran magmatic arc). The Sonsela Member is characterized as a relatively coarse-grained unit compared to other mudstone-dominated members of the Chinle Formation, and is therefore thought to reflect a unique period in the evolution of the basin. Modern exposures of the Cordilleran arc are poorly preserved, the result of subsequent tectonic deformation, erosion, and sedimentation in western North America since the Late Triassic. Sedimentary sequences of the Chinle Formation, however (particularly those of the Sonsela Member), preserve the dynamic evolution of the basin as the volcanic arc was established in southwestern North America. The Sonsela Member therefore serves as the most reliable indicator of fluctuations in arc dynamics of a poorly preserved and disjointed portion of the Cordilleran magmatic arc.
Neotectonic faulting and forearc sliver motion along the Atirro-Río Sucio fault system, Costa Rica, Central America
Walter Montero P. et al., Geological Sciences Research Center, University of Costa Rica, San José, Costa Rica; Corresponding author: Sarah Kruse, Dept. of Geology, University of South Florida, Tampa, Florida 33620, USA. Posted online 22 Feb. 2013; http://dx.
Two important questions about the Cocos-Caribbean subduction zone of Costa Rica are how trench-parallel forearc motion is accommodated and what drives forearc sliver motion. This work by Walter Montero P. and colleagues provides critical constraints on the former and lays the foundation for exploring the latter. It documents a network of northwest-striking right-lateral strike slip faults that appears to mark the northern boundary of an upper-plate sliver moving NW relative to the Caribbean plate. Despite high erosion rates and deep weathering, the fault system includes pull-apart basins with preserved normal fault scarps and tilted hanging-wall buttress unconformities, pressure ridges, displaced and beheaded drainages, sag ponds, and fault-controlled upland valleys. Montero and colleagues integrate geomorphic observations with outcrop-scale bedrock fault kinematic and earthquake focal mechanism data to map the active through-going fault zone. The mapping reveals that the fault system traverses the active volcanic arc from NW to SE and connects to an area of high uplift rate in the inactive Talamanca magmatic arc, where the faults are interpreted to originate inboard of the actively colliding Cocos ridge. This suggests that, kinematically, the forearc sliver is rooted in the collision zone.
Mesozoic fault reactivation along the St. Lawrence rift system, eastern Canada: Thermochronologic evidence from apatite fission-track dating
Alain Tremblay et al., Département des Sciences de la Terre et de l'Atmosphère and GEOTOP, Université du Québec. Posted online 22 Feb. 2013; http://dx.
The St. Lawrence rift system is formed by a series of northeast-southwest-trending faults in southeastern Québec that links and includes the northwest-trending Ottawa-Bonnechère and Saguenay River regions. It is an active fault zone where reactivation of Late Precambrian (less than one billion years ago) faults, at times as young as post-Late Devonian (350 million years ago), is believed to occur. Apatite fission-track (AFT) ages, which represent the time that the rocks cooled through 100 degrees Celsius (3-4 km depth) on their way to the surface, have been determined for Late Precambrian rocks from both sides of typical rift faults at different locations in the St. Lawrence and Saguenay river fault systems along the St. Lawrence rift system. Differences in AFT ages were found across all the faults studied, suggesting reactivation of extensional movement approx. 250-200 million years. Along the St. Lawrence River fault system, the AFT ages also suggest a renewal of movement in a compressional sense at ca. 150 Ma. This study provides evidence for extension related to rifting in the Atlantic Ocean followed by compressional deformation in the interior of Canada, more than 500 km west of the Atlantic coastal margin.
Chemical and ecological evolution of the Miocene Ries impact crater lake, Germany: A reinterpretation based on the Enkingen (SUBO 18) drill core
Gernot Arp et al., Georg-August-Universität Göttingen, Geowissenschaftliches Zentrum, Goldschmidtstrasse 3, 37077 Göttingen, Germany. Posted online 22 Feb. 2013; http://dx.
Impact crater lakes potentially form valuable climatologic archives. The lacustrine succession of the 15-million-year-old Ries Crater Lake has previously been interpreted as climate-controlled development from a playa to a highly saline soda lake, which successively decreased in salinity to reach freshwater sedimentation with temporary coal swamps. New multidisciplinary investigations based on a partial section now question this view: The sediments of this new drill core reflect increasing, not decreasing salinities, with brown coals formed by plant debris swept into a hypersaline setting. In addition, the chemical composition of the inflowing waters changed due to the weathering of different ejecta layers in the catchment area. Interpolated to the whole succession, a new model for the Ries Crater Lake is developed: After the development of a brackish soda lake and erosion of the upper ejecta blanket (suevite), an increasing ion influx from the lower ejecta (Bunte Breccia) caused a change to a marine-like, and finally hypersaline salt lake. Therefore, intrinsic factors, such as weathering history in the catchment area, probably dominated over external, climatic factors with respect to the chemical and ecological evolution of this impact crater lake. Moreover, the initial suevite blanket might have been more widespread than previously assumed.
Oroclines: Thick and thin
S.T. Johnston et al., School of Earth & Ocean Sciences, University of Victoria, PO Box 3065 STN CSC, Victoria British Columbia, Canada V8P 4B2. Posted online 22 Feb. 2013; http://dx.
Folded rocks characterize young and old mountain belts the world over, and form some of the most familiar and spectacular of geological structures. But can you fold an entire mountain belt? Stephen Johnston of the University of Victoria and his colleagues Arlo Weil of Bryn Mawr University and Gabriel Gutierrez-Alonso of the University of Salamanca have been studying the geometry of mountain belts, and their findings, summarized in this review paper, suggest that not only can you bend a mountain belt, but that folds of mountain belts, referred to as oroclines, constitute the largest geological structures on Earth. Based on an extensive compilation of geological and geophysical data, they demonstrate that during the development of a mountain chain, minor bends of faults and folds can develop, but that subsequently the entire mountain chain can be buckled into one or more oroclines. For example, the 320-million-year-old Variscan Mountain chain of Iberia, which formed during the continental collisions that gave rise to Pangea, subsequently buckled giving rise to two coupled oroclines. During buckling, a 2,300-km-long, 300-km-wide, linear mountain chain was shortened by more than 1,100 km, giving rise to two of Earth's largest folds and forming the Iberian Peninsula. These findings suggest that the buckling of mountain chains is an important process responsible for the development and growth of continents.
Deriving a long paleoseismic record from a shallow-water Holocene basin next to the Alpine fault, New Zealand
K.J. Clark et al., GNS Science, PO Box 30368, Lower Hutt, New Zealand. Posted online 22 Feb. 2013; http://dx.
Scientists have investigated evidence left by large surface-rupturing earthquakes on the Alpine Fault in New Zealand over an 8,000 year period. The earthquakes left "geological signatures" of alternating peat and silt in the exposed banks of Hokuri Creek, an isolated creek in northern Fiordland. Detailed mapping, sedimentology and microfossil analysis were used to investigate the relationship between sediment deposition and Alpine fault rupture at Hokuri Creek. Repeated fault rupture involving a component of vertical movement is shown to be the most convincing mechanism for explaining the cyclical peat and silt sedimentary sequence. This article by K.J. Clark and colleagues delves into the detail of how a record of 22 earthquakes between circa AD 800 to 6000 BC was obtained. The Hokuri Creek sequence represents one of the longest continuous large earthquake records of any on-land plate boundary fault in the world. The Alpine Fault extends about 600 km along the spine of the South Island between Milford Sound and Marlborough. It last ruptured in 1717 AD producing an earthquake of approx. magnitude 8.
Paleogene postcompressional intermontane basin evolution along the frontal Cordilleran fold-and-thrust belt of southwestern Montana
Theresa M. Schwartz and Robert K. Schwartz, Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Bldg. 320, Stanford, California 94305, USA. Posted online 22 Feb. 2013; http://dx.
This article by Theresa M. Schwartz and Robert K. Schwartz investigates the evolution of the Montana landscape between approximately 50 and 20 million years ago. Detailed examination of the depositional facies and provenance of the Renova Formation in southwestern Montana provides important insight into the physical processes that affected the eastern part of the North American Cordillera after it became inactive. These include the interplay between tectonism (both in the crust and deeper in the lithosphere), erosion on the surface by large river systems, and shifts in climate patterns. Ultimately, this study finds that the complex structure that was generated more than 50 million years ago exerted and continues to exert strong controls on the landscape, dictating areas of erosion and deposition.