Boulder, Colo., USA - Three main rivers -- the Ganges, Brahmaputra, and Meghna -- meet in the Bengal basin to form the world's largest delta system, which serves as a gateway between the Himalayan mountains and the vast, deep-ocean Bengal Fan. This GSA BULLETIN paper by Stephen Goodbred and colleagues presents a new understanding of how this mega-delta, the Ganges-Brahmaputra-Meghna delta, came together over the past 10,000 years.
To determine the delta's construction during the Holocene, Goodbred and colleagues followed geochemical fingerprints to trace the paths and accumulation of sediment delivered by each of the great rivers. These distinct fingerprints arise because of differences in the rocks that are being eroded in the Himalaya, Tibet, and local regions.
What is revealed is a history of mobile, interactive river systems that crisscross the landscape to build the delta, much like a 3-D printer supplied with sand and mud rather than ink. This history reveals the patterns of river behavior that today defines the way of life for 150 million people living on the delta, who are both sustained by the fertile plains constructed by the rivers but challenged by their regular flooding and erosion.
Piecing together the Ganges-Brahmaputra-Meghna river delta: Use of sediment provenance to reconstruct the history and interaction of multiple fluvial systems during Holocene delta evolution
S.L. Goodbred, Jr., et al., Dept. of Earth and Environmental Sciences, Vanderbilt University, Nashville, Tennessee, USA. Published online 2 June 2014; http://dx.
Other GSA BULLETIN papers, highlighted below, include such topics as
- 1. Study of remnant lake shorelines found in Surprise Valley, California, USA;
2. The long-term exhumation history of the North American Cordillera;
3. The case of the Grand Hogback Monocline of west-central Colorado, USA; and
4. Two studies on the Canadian Cordillera.
GSA BULLETIN articles published ahead of print are online at http://gsabulletin.
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Rise and fall of late Pleistocene pluvial lakes in response to reduced evaporation and precipitation: Evidence from Lake Surprise, California
D.E. Ibarra et al., Dept. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. Published online 2 June 2014; http://dx.
Large lakes in the western United States during the last ice age (14,000 to 29,000 years ago) have long puzzled Earth scientists. Geologic observations indicate that the different climatic and hydrologic conditions filled many now-arid basins with lakes. Reconstruction of the climatic conditions required to maintain large lake systems for thousands of years provides an opportunity to test climate models under vastly altered global boundary conditions. Using the decay of carbon-14 and uranium in shoreline tufa deposits, Daniel Ibarra and colleagues determined the age of the remnant lake shorelines found in Surprise Valley, California, and calculated how lake levels fluctuated in response to the relative contribution of precipitation and evaporation. They found that moderate lake levels during the peak of the last ice age (19,000 to 26,000 years ago) resulted from reduced evaporation, not increased precipitation. These results agreed with an ensemble of climate model simulations for 21,000 years ago. In contrast, 15,000 years ago, during the Lake Surprise highstand, annual precipitation was almost double modern. This multidisciplinary approach explains the conditions driving Pleistocene lake systems, and illustrates the relative importance of seasonal insolation and temperature as long-term drivers of moisture balance in the arid western United States.
Paleozoic siliciclastic rocks from northern Victoria Land (Antarctica): Provenance, timing of deformation, and implications for the Antarctica/Australia connection
G. Di Vincenzo et al., Istituto di Geoscienze e Georisorse, CNR, via Moruzzi 1, I-56124 Pisa, Italy. Published online 2 June 2014; http://dx.
Accretionary orogens are major sites of continental crust growth and world-class ore deposits that have been active on Earth since the early Archean. The Neoproterozoic to late Paleozoic Terra Australis Orogen is an outstanding example of an accretionary orogen. The orogen reached an across-strike width of ~1,600 km and had an along-strike length of ~18,000 km, extending from the Australian-East Antarctic segment of the East Gondwana margin to the South African-Andean segment of the West Gondwana margin. Northern Victoria Land (NVL) plays a key role in many geodynamic reconstructions because it has long been considered the along-strike continuation of Australia in Antarctica. Despite its critical location, several key aspects concerning provenance and deformation of siliciclastic rocks are still under debate. This study reports a detailed investigation on Paleozoic low-grade siliciclastic metasediments in NVL using the 40Ar-39Ar dating method in conjunction with mineral-textural analyses and geochemical data. Results constrain provenance, timing of deposition and deformation of siliciclastic rocks, and strongly support the link between NVL and southeastern Australia. Data also provide for the first time evidence for Darriwilian regional contractional tectonics in NVL, suggesting that literature data are unreliable because affected by the presence of detrital micas.
Exhumation of the North American Cordillera revealed by multi-dating of Upper Jurassic-Upper Cretaceous foreland basin deposits
C.S. Painter et al., Dept. of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA. Published online 2 June 2014; http://dx.
This study deals with the long-term exhumation history of the North American Cordillera, using thermochronology and geochronology of foreland basin material sampled in Utah, Wyoming, South Dakota, and Colorado, USA. With a combination of thermochronology and geochronology, Clayton S. Painter and colleagues measured the lag time of foreland basin deposits, which is the difference in the time between when material cooled in the orogenic belt to when it was exhumed (i.e., uplifted and eroded) and deposited in the foreland basin. These new thermochronologic data suggest exhumation depths of ~5-9 km. Acting as an independent test to biostratigraphic ages, new geochronologic data corroborate existing age controls for North America Cordilleran foreland basin deposits. The combination of both geochronology and thermochronology reveal rapid exhumation rates (faster than one kilometer per million years) throughout the Cretaceous in the North America Cordillera, but such rates are not sustainable over the entire orogenic belt for such a long period of time, and the data presented here suggest total exhumation depths of less than 9 km. Painter and colleagues propose that rapid exhumation was focused on the leading edge of the North American Cordillera, the Sevier fold-thrust belt, and that higher topography to the west experienced lower exhumation rates. This exhumation distribution is similar to what has been recorded in the South American Cordillera.
flexural-slip faults related to evaporite dissolution generate hazardous earthquakes? The case of the Grand Hogback Monocline of west-central Colorado
F. Gutierrez et al., Departamento de Ciencias de la Tierra, Universidad de Zaragoza, C/. Pedro Cerbuna 12, 50009 Zaragoza, Spain. Published online 23 June 2014; http://dx.
Southwest of Glenwood Springs, Colorado, deep-seated evaporite dissolution causes the unfolding of the Grand Hogback Monocline and the development of more than 40 active bedding-plane faults. These are "flexural-slip faults" with a displacement similar to the slip between the pages of a telephone book when bent. The faults in the Grand Hogback Monocline were thought to be characterized by progressive displacement (creeping faults), and consequently incapable of producing significant earthquakes. Trenches dug across two faults revealed unexpected evidence of multiple sudden surface rupture events. This suggests that those structures, whose length may reach around 25 km, may have the potential produce damaging earthquakes with magnitudes around six. This work presents for the first time compelling data indicating that large gravitational faults related to salt dissolution, frequently obviated in seismic hazard analyses, could generate hazardous seismic events.
Stratigraphy and structure of the Cañas Dulces caldera (Costa Rica)
F. Molina et al., Institute of Earth Sciences Jaume Almera, CSIC, Barcelona, Spain (Marti). Published online 23 June 2014; http://dx.
This paper describes the formation and structure of the Cañas Dulces caldera, which hosts the Rincón de la Vieja-Santa María active volcanic complex and one of the main geothermal reservoirs in Costa Rica. This collapse caldera originated by the catastrophic collapse of the roof of a shallow magma chamber during the massive eruption about 1.43 million years ago of about 200 cubic kilometers of rhyolitic magma that formed the Liberia ignimbrite. Combining field work and a revision of extensive deep boreholes and geophysical data obtained by the Instituto Costarricense de Electricidad (ICE) since 1970, this paper describes the stratigraphy, structure, and volcanic evolution of this collapse caldera and defines a comprehensive model to describe its evolution. The most characteristic aspect of this collapsed caldera is that it formed under strong structural control dominated by a nearly orthogonal set of regional faults. The construction of the Rincón de la Vieja-Santa María volcanic complex on one of the caldera's structural borders facilitated the installation of the highly productive geothermal system inside the caldera.
Late Paleozoic assembly of the Alexander-Wrangellia-Peninsular composite terrane, Canadian and Alaskan Cordillera
L.P. Beranek et al., Department of Earth Sciences, Memorial University of Newfoundland, 300 Prince Philip Drive, St. John's, Newfoundland and Labrador A1B 3X5, Canada. Published online 23 June 2014; http://dx.
The Alexander-Wrangellia-Peninsular (AWP) composite terrane is made up of several crustal blocks that collectively underlie 550,000 square kilometers (212,000 square miles) of the Cordilleran mountain belt in Alaska, southwestern Yukon, and the coastal regions of British Columbia. This paper by Luke Beranek and colleagues reports new field and laboratory data that describe when, where, and how crustal blocks of the AWP composite terrane were assembled prior to their Mesozoic accretion to the Cordilleran continental margin. New data from the Saint Elias Mountains area of Alaska and Yukon show that the AWP composite terrane was constructed in two phases during the late Paleozoic, approx. 310 and approx. 285 million years ago, by plate tectonic processes that are broadly analogous to the modern-day collision between the northern Australian continental margin and the Banda volcanic arc. When combined with published fossil and paleomagnetic information, the new data are consistent with late Paleozoic terrane assembly to have taken place along a convergent margin system near the northwestern side of supercontinent Pangea.
Middle Jurassic to earliest Cretaceous mid-crustal tectono-metamorphism in the northern Canadian Cordillera: Recording foreland-directed migration of an orogenic front R.D. Staples et al., Department of Earth Sciences, Simon Fraser University, 8888 University Dr., Burnaby, British Columbia, Canada V5A 1S6. Published online 23 June 2014; http//dx.doi.org/10.1130/B31037.1.
Pressure-temperature calculations and in situ dating of a metamorphic mineral called monazite have revealed a previously unrecognized mid-crustal deformation and metamorphic event in the eastern portion of the Yukon-Tanana terrane (Finlayson Lake district, southeast Yukon) in the northern Canadian Cordillera. Chemical zoning in the mineral garnet reveals that garnet began to grow at approximately 550 degrees Celsius and 20 km depth, and culminated at approximately 600 degrees Celsius and 25 km depth. Mineral chemistry, rock textures and in situ uranium-lead dating of monazite suggest that the latest stages of garnet growth occurred between 169 and 142 million years ago contemporaneous with the development of ductile deformation fabrics. This event post-dates widespread Early Jurassic (ca. 176 to 197 million years ago) cooling and exhumation of Yukon-Tanana terrane rocks west of the Tintina fault in west-central Yukon, which were previously ductily deformed and metamorphosed in the Permo-Triassic (ca. 240 to 260 million years ago). These data indicate younger, more protracted mid-crustal deformation and metamorphism associated with mountain building in the northern Cordillera than was previously recognized, with events migrating toward the interior of the continent and downwards in the Middle Jurassic to Early Cretaceous (ca. 142 to 169 million years ago).