Boulder, CO, USA - The July-August GSA BULLETIN is now online. Highlights include documentation of Holocene rupture on all three major faults in Lake Tahoe basin; evidence for a large paleolake in the Western Desert, Egypt; examination of the southern Colorado Plateau-Arizona Transition Zone groundwater system; a study germane to both the search for natural gas below the Columbia River Basalt Group and to deciphering the seismic hazards of western Washington State; and descriptions of a Peruvian petrified forest.
Late Cretaceous-Paleocene formation of the proto-Zagros foreland basin, Lurestan Province, SW Iran
Stephane Homke et al., Group of Dynamics of the Lithosphere (GDL), Institute of Earth Sciences "Jaume Almera," CSIC, Lluís Sole i Sabaris s/n, Barcelona, 08028, Spain. Pages 963-978.
The formation of the hydrocarbon-rich Zagros mountains is the result of protracted tectonic and sedimentary processes that started by the Late Cretaceous. Homke et al. present an accurate dating of the oldest syntectonic sediments deposited in the northwest Zagros foreland basin. Results are discussed to constrain the early evolution of the Zagros orogen.
Rough crust subduction, forearc kinematics, and Quaternary uplift rates, Costa Rican segment of the Middle American Trench
Peter B. Sak et al., Dept. of Geology, Dickinson College, Carlisle, Pennsylvania 17013, USA. Pages 992-1012.
Sak et al. relate variations in the rates of uplift along the Pacific coast of Costa Rica to the shape of the offshore seafloor. Areas with the most rapid rates of uplift are localized to the portions of the coast that lie opposite seamounts and ridges on the subducting plate. This study also finds that variations in the styles of deformation correlate with the morphology of the offshore seafloor.
A high-resolution seismic CHIRP investigation of active normal faulting across Lake Tahoe Basin, California-Nevada J. Dingler et al., Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, USA. Pages 1089-1107.
Dingler et al. present a comprehensive analysis documenting Holocene rupture on all three major faults in the Lake Tahoe Basin. Seismic CHIRP imaging, multibeam mapping, LiDAR-derived bathymetry, and shallow sediment cores, were all used to measure fault offset, thickness of earthquake-derived colluvial wedges, depth of wave-cut paleoterraces, and other geomorphic features. The results of this study confirm that the Lake Tahoe Basin has been an actively deforming basin at the western most edge of the Walker Lane since at least the Late Pleistocene. Each of the three major faults in the basin shows evidence for 2-plus meter Holocene rupture events, as well as Holocene extension rates that are similar to those measured by previous GPS studies.
Paleoenvironmental reconstruction and water balance of a mid-Pleistocene pluvial lake, Dakhleh Oasis, Egypt
Johanna M. Kieniewicz and Jennifer R. Smith, Dept. of Geosciences, Denison University, Granville, Ohio 43023, USA. Pages 1154-1171.
Mid-Pleistocene carbonate-rich lake sediments from the Dakhleh Oasis, Western Desert, Egypt attest to the presence of a body of water that may have been as large as about 1735 square kilometers. The majority of the sedimentary facies and carbonate stable isotope data are indicative of fairly shallow, freshwater environments that were capable of supporting a savanna fauna requiring perennial fresh water. Water balance modeling estimates the amount of precipitation required to maintain the Dakhleh paleolake to be between 410 and 670 mm/year. The modeled rainfall range is consistent with previous estimates from elsewhere in the Western Desert based upon faunal remains and global circulation model results, suggesting an environment resembling that of semi-Arid Africa today. Kieniewicz and Smith suggest that the currently inhabitable majority of the Western Desert of Egypt may have represented a considerably more favorable environment than that present today, and with the presence of perennial water resources, such as large lakes, active drainages, and springs, would have provided hominids with a favorable route of passage out of Africa into the Mediterranean, Levant, and beyond.
Structure and age of volcanic fissures on Mount Morning: A new constraint on Neogene to contemporary stress in the West Antarctic Rift, southern Victoria Land, Antarctica
Timothy S. Paulsen and Terry J. Wilson, Dept. of Geology, University of Wisconsin Oshkosh, 800 Algoma Boulevard, Oshkosh, Wisconsin 54901, USA. Pages 1071-1088.
Antarctica is characterized by a unique combination of active processes, including active crustal deformation, volcanism, and glacial loading and unloading, but little is known about the Neogene to contemporary geodynamic state of the Antarctic plate. Pauslen and Wilson present new data on the structure and timing of volcanism on the Mount Morning shield volcano, with the purpose of defining Pleistocene stress directions concomitant with volcanism and rifting in the southern portion of the Victoria Land rift basin. Elongate vents and vent alignments indicate parasitic volcanism has predominantly occurred along a primary set of NE fissures. The new contemporary stress datum at Mount Morning is consistent with a neotectonic normal-fault to strike-slip fault regime within the Terror Rift, which was active in Pliocene-Pleistocene times and could remain active today. This stress regime still dominates the contemporary geodynamic state of this sector of the West Antarctic Rift system.
Geologic and taphonomic context of El Bosque Petrificado Piedra Chamana (Cajamarca, Peru)
Deborah Woodcock et al., Marsh Institute of Clark University, 950 Main Street, Worcester, Massachusetts 01610. Pages 1172-1178.
Woodcock et al. present a study on El Bosque Petrificado Piedra Chamana, a fossil forest located near the small village of Sexi in the northern Andes of Peru. Thirty-nine million years ago, an erupting volcano sent up volcanic ash that settled over a forested area, burying the trees to a depth of around a meter. Later, rain falling on the loose volcanic ash created mudflows that moved across the landscape, snapping off the buried trees, carrying them downhill, and forming a wood-rich volcanic mudflow deposit over the lower ash fall. The fossil woods and leaves preserved at the site are evidence of a tropical forest with a diversity of trees including palms. The tropical aspect of the fossil forest contrasts markedly with the montane vegetation now found at the site and shows the extent to which the area was uplifted -- as much as 2600 meters -- when the modern Andes Mountains came into being.
Degassing of mantle-derived CO2 and He from springs in the southern Colorado Plateau region -- Neotectonic connections and implications for groundwater systems
Laura J. Crossey et al., Dept. of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. Pages 1034-1053.
The western U.S., like many arid regions in the world, relies heavily on groundwater for water supply. Crossey et al. examine the groundwater system in the southern Colorado Plateau-Arizona Transition Zone region to better understand quantity and quality of groundwater. They find that groundwaters are a complex mixture of "upper world" waters derived from rain and snowmelt with a newly recognized "lower world" component. Travertine-depositing CO2-rich springs and the spectacular cut-away view of the aquifer system at Grand Canyon were the keys for discovering and characterizing the lower world component. Helium isotope measurements of gases dissolved in spring waters indicate that a significant component of the lower world fluid comes up more than 30 miles along faults from the Earth's mantle to the surface. Also, about one third of the CO2 emitted from the springs is from deep sources. This new integrated view of groundwater links mantle degassing and neotectonics to the geochemistry of the groundwaters we drink, with the important realization for our cities and towns that lower world inputs cause degradation of groundwater quality can be recognized and quantified.
Hydrogen and oxygen isotopic compositions of lake water in the western United States
Anna K. Henderson and Bryan Nolan Shuman, Limnological Research Center, Dept. of Geology, University of Minnesota, 310 Pillsbury Drive, Minneapolis, Minnesota 55455, USA. Pages 1179-1189.
Henderson and Shuman track the importance of monthly precipitation as it contributes to lake water inputs in the western United States. Their findings show that, despite the importance of winter snowpack for stream flow, lake-water inputs, and, presumably, groundwater reflect the weighted mean of precipitation and thus the seasonal distribution of precipitation. Sampling regions in northern New Mexico and southern Colorado are an exception and have input values biased toward warm-season precipitation, which is likely related to the summer monsoon. These results have broad implications for water management, especially if future climate changes influence the monsoon or other seasonal precipitation patterns.
The Chiwaukum Structural Low: Cenozoic shortening of the central Cascade Range, Washington State, USA
Eric S. Cheney and N.W. Hayman, Dept. of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA. Pages 1135-1153.
Cheney and Hayman challenge the popular notion that Eocene crustal extension formed the so called Chiwaukum graben, in which the non-marine, arkosic Chumstick Formation reputedly was deposited during faulting. They show that the bounding faults of the "graben" cut the Chumstick Formation and that the southwestern bounding fault really is a composed of northwesterly trending reverse faults and younger northerly strike-slip faults, both of which cut northwesterly striking folds in the Chumstick Formation. The structural low is caused by the superposition of Eocene synclines and gentler regional synclines that fold the Miocene Columbia River Basalt Group. This paper is germane to the search for natural gas below the adjacent Columbia River Basalt Group. It also is relevant to deciphering the seismic hazards of western Washington, which currently is under northeast-southwest compression and where northwesterly trending folds and associated faults dominate the geology.
Reinterpretation of the Greenbrier fault, Great Smoky Mountains: New petrofabric constraints and implications for southern Appalachian tectonics
K.M. Clemons and D.P. Moecher, Dept. of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506 USA. Pages 1108-1122.
The Greenbrier fault is one of the best known geologic features of the Great Smoky Mountains National Park area. First studied over 50 years ago, it has been widely accepted as a major thrust fault with more than 20 km of displacement. However, the fault is rather enigmatic in that its location is inferred merely from other geologic evidence, and some of the assumptions that served as the basis for the original interpretation of the fault were never questioned. Clemons and Moecher describe the first systematic evidence for the nature of the fault at its type locality and other localities in the Great Smoky Mountains region. It is concluded that the fault is younger than originally inferred, involved an order of magnitude less offset, and was probably originally a stratigraphic contact of some kind.
The persistence of waterfalls in fractured rock
Michael P. Lamb and William E. Dietrich, Dept. of Earth and Planetary Science, University of California, Berkeley, California, USA. Pages 1123-1134.
The upstream translation of waterfalls is one of the fundamental drivers of river incision and landscape evolution. Although waterfall propagation is often thought to occur through plunge-pool undermining and collapse (e.g., Niagara Falls), many waterfalls exist that are not undercut. This seems to be especially true in bedrock with intersecting vertical and horizontal joint sets, such as columnar basalt. To explain this observation, Lamb and Dietrich propose that well-jointed rock waterfalls can maintain a vertical form in the absence of undercutting due to block toppling. A torque-balance model and flume experiments confirm that a vertical headwall is expected to be maintained during upstream translation where near vertical joints are tilted in the downstream direction, or slightly tilted in the upstream direction. Such conditions are common in volcanic terrains on the surface of Earth and (likely) Mars, and might explain the persistence of waterfalls and amphitheater-headed valleys there.
Sediment yield from the tectonically active semiarid Western Transverse Ranges of California
Jonathan A. Warrick and Leal A.K. Mertes, USGS Western Coastal and Marine Geology, 400 Natural Bridges Drive, Santa Cruz, California 95060, USA. Pages 1054-1070.
The supply of sediment from the landscape to the ocean directly influences the beaches and coastal systems downstream of these sources. Adequate supply of sediment is necessary for keeping beach "sand-sheds" in balances, thereby reducing the potential for coastal erosion. Warrick and Mertes report on the rates and locations of sediment discharge from the Western Transverse Ranges, a major mountain range immediately northwest of Los Angeles, California. They found that the rates of sediment supply are not only considerably larger than the surrounding southern California region, but that there is considerable variability in the supply rates within the range. Watersheds with the highest rates of sediment supply were shown to originate in areas with relatively young and highly erodable bedrock and in regions of relatively high tectonic uplift. Land use was also found to significantly affect sediment supply rates. Thus, the rate of sediment supply to the southern California coast is directly related to the geological setting and land use history of the source watersheds.
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