|2 Nov. 2011
GSA Release No. 11-76
Director of Education, Communication, & Outreach
GSA Bulletin Highlights:
New Research Posted Ahead of Print 30 Sept.–21 Oct. 2011
Boulder, CO, USA – Highlights for GSA Bulletin articles published ahead of print between 30 Sept. and 21 Oct. 2011 are provided below. Studies include U-Pb geochronology and major and trace element geochemistry in the Andes; wide-angle reflections generated by five controlled blasts and over 110 timed quarry blasts in the Appalachians; cosmogenic isotope burial dating in Wonderwerk Cave, South Africa; and SHRIMP analysis of zircons in Redoubt Volcano, Alaska.
Keywords: Ligurian accretionary complex, Italy, Eastern Cordillera-Llanos basin boundary, Colombia, ice stream flow, Barents Sea, Fonte dei Pulcini A (FPA) section, Messinian salinity crisis, Mediterranean, Central Appalachians, Asheville Basin. Pliocene, Colorado River, Grand Canyon, Central Anatolian plateau, Turkey, Wonderwerk Cave, South Africa, Kalahari Dunes, SHRIMP, Redoubt volcano, Alaska, Gediz graben, Central Menderes Massif, Alasehir supradetachment, Wallowa arc terrane, Blue Mountains Province, Oregon, Idaho, Canadian Arctic, Geological Survey of Canada, Panamint and Spring Mountains.
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On the tectonic evolution of the Ligurian accretionary complex in southern Italy
S. Ciarcia et al., Dipartimento Scienze della Terra, Universita di Napoli Federico II, Largo San Marcellino 10, 80138 Naples, Italy; doi: 10.1130/B30437.1. Posted online ahead of print 21 Oct. 2011.
The combined role of internal wedge dynamics and continental margin architecture in controlling the tectonic evolution of an accretionary complex during its final emplacement onto the foreland continent is discussed in this study. S. Ciarcia of the University of Naples Federico II and colleagues conduct integrated structural, stratigraphic and low-T thermochronometric analyses on the Ligurian accretionary complex units exposed in the southern Italy and on continental margin successions located in their footwall, as well as on related foredeep and wedge-top basin deposits. Their results point out a series of late early Miocene shortening events, also involving buttressing of the accretionary wedge against the crustal ramp of the foreland continental margin. Emplacement of the over-thickened accretionary complex onto the distal part of the continental margin was followed by horizontal extension and wedge thinning, aiding the development of wedge-top depocenters. Early Miocene NW-SE shortening recorded by Ligurian accretionary complex units is completely unrelated with later NE-directed thrusting in the Apennines, which was coeval with back-arc extension in the Tyrrhenian Sea. These results emphasize the occurrence of a major discontinuity in the Neogene geodynamic evolution of the southern Apennines, whose tectonic history may be clearly subdivided, from a kinematic point of view, into pre- and syn-Tyrrhenian back-arc extension stages.
Clastic deposition, provenance, and sequence of Andean thrusting in the frontal Eastern Cordillera and Llanos foreland basin of Colombia
Alejandro Bande et al., Dept. of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA; doi: 10.1130/B30412.1. Posted online ahead of print 21 Oct. 2011.
Sedimentary deposits at the leading edge of the northern Andes (6 degrees N latitude) provide a long-term record of foreland-basin sedimentation along the Eastern Cordillera-Llanos basin boundary in Colombia. Sedimentological, provenance, and detrital thermochronological (U-Pb and (U-Th)/He) results for an upward-coarsening, ~5-km-thick succession (Nunchia syncline) reveal a systematic shift from (1) craton-derived, shallow-marine distal foreland (backbulge) accumulation, to (2) orogen-sourced, deltaic and coastal-influenced sedimentation of the distal to medial foreland (foredeep), to (3) anastomosing fluvial and distributive braided fluvial megafan systems of the proximal foreland (foredeep to wedge-top) basin. These changes record exhumation of evolving, compartmentalized sediment source areas in the Eastern Cordillera. Alejandro Bande of the Jackson School of Geosciences and colleagues interpret these data in terms of a progressive eastward advance in fold-thrust deformation, with late Eocene-Oligocene deformation in the axial zone of the Eastern Cordillera, early Miocene reactivation (inversion) of the eastern margin of the Mesozoic rift system, and middle-late Miocene propagation of a footwall shortcut fault along the transition from the Eastern Cordillera to Llanos foreland basin. They find that, collectively, the data for the frontal Eastern Cordillera define a general in-sequence pattern of eastward advancing fold-thrust deformation during Cenozoic east-west shortening in the Colombian Andes.
Late Miocene surface uplift of the southern margin of the Central Anatolian Plateau, Central Taurides, Turkey
Domenico Cosentino et al., Dipartimento de Scienze Geologiche, Universita degli Studi Roma Tre, Largo San Leonardo Murialdo 1, 00146 Rome, Italy; doi: 10.1130/B30466.1. Posted online ahead of print 21 Oct. 2011.
The timing and pattern of surface uplift of marine sediments capping the southern margin of the Central Anatolian plateau in southern Turkey provide a first-order constraint on possible mechanisms of regional uplift. Using biostratigraphy and paleomagnetic stratigraphy, Domenico Cosentino and colleagues determine an age of 8.35-8.108 Ma for the uppermost marine sediments now at ca. 2-km elevation, yielding a long-term average uplift rate of 0.24-0.25 mm/yr. The minimal deformation of these marine sediments over spatial distances of tens of kilometers implies that uplift mechanisms have originated deep within the crust or mantle. Recently published tomography data and Plio-Quaternary volcanism show evidence for asthenospheric upwelling in a pattern that matches very well the pattern of uplift we observe along the southern margin of the Central Anatolian plateau. This correspondence is consistent with slab break-off beneath the southern plateau margin triggering surface uplift.
U-Pb geochronology and geochemistry of intrusive rocks from the Cougar Creek Complex, Wallowa arc terrane, Blue Mountains Province, Oregon-Idaho
Gene A. Kurz et al., Dept. of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA; doi: 10.1130/B30452.1. Posted online ahead of print 21 Oct. 2011.
This study utilizes high-precision U-Pb geochronology and major and trace element geochemistry to characterize magmatism in the Wallowa arc terrane of the Blue Mountains Province of northeastern Oregon and west-central Idaho. New data presented in this study provide valuable age and compositional context for magmatic rocks for a poorly understood, yet critical section of the North American Cordilleran orogen. U-Pb ages indicate two distinct cycles of magmatism recorded in the Cougar Creek Complex. Geochemical data indicate a switch from older Permian silicic arc-related magmatism to younger Late Triassic igneous activity that appears to have been derived from a depleted mantle source similar to mid-ocean ridge basalt. The apparent switch from normal arc-related magmatism to a depleted MORB-like magma source is inferred to be related to the upwelling of buoyant asthenosphere through an opening slab window beneath the Wallowa arc. In this paper, Gene Kurz of Boise State University and colleagues propose that Late Triassic mafic igneous rocks from the Wallowa terrane and Wrangellia of Alaska and western Canada resulted from the subduction of a spreading ridge and slab window magmatism beneath a late Paleozoic arc, thus linking the Late Triassic geologic evolution of two fundamental components of the Cordilleran orogen.
Orbitally forced paleoenvironmental and paleoclimate changes in the late postevaporitic Messinian of the central Mediterranean Basin
Domenico Cosentino et al., Dipartimento di Scienze Geologiche, Universita degli Studi Roma Tre, Largo San Leonardo Murialdo, 1-00146 Rome, Italy; doi: 10.1130/B30462.1. Posted online ahead of print 14 Oct. 2011.
Palaeoenvironmental and palaeoclimate changes that occurred between 5.394 and 5.336 Ma (late stage of the Messinian salinity crisis) in the Mediterranean Basin offer a new perspective on the relationship between orbital forcing and climate response. The magnetic susceptibility record of the Fonte dei Pulcini A (FPA) section (Maiella Mountains, Italy) allows Domenico Cosentino and colleagues to orbitally tune and to temporally constrain the palaeoenvironmental and palaeoclimate changes evidenced by quantitative palaeontological, stable isotope, and mineralogical analyses. The base of the FPA section is characterized by Paratethyan ostracods and dinocysts, which point to the late Messinian Lago-Mare biofacies (Loxocorniculina djafarovi Zone) of the Mediterranean Messinian stratigraphy. The major changes in terms of palaeodepth, palaeosalinity, evaporation vs. precipitation, aridity vs. humidity, and reworking processes occurred in the upper part of the FPA section during the last Messinian insolation cycle (i-cycle 511/512), which is characterized by high amplitude oscillations. In contrast, the lower part of the FPA section, which was deposited during relatively low amplitude insolation cycles, is characterized by more stable environmental conditions. Comparing summer insolation with the palaeoenvironmental changes at the FPA section, Cosentino and colleagues identify delays of several thousands of years between orbital forcing and climate response.
Isostatic compensation for a portion of the Southern Appalachians: Evidence from a reconnaissance study using wide-angle, three-component seismic soundings
Robert B. Hawman et al., Dept. of Geology, University of Georgia, Athens, Georgia 30602, USA; doi: 10.1130/B30464.1. Posted online ahead of print 14 Oct. 2011.
Robert B. Hawman of the University of Georgia and colleagues use wide-angle reflections generated by five controlled blasts and over 110 timed quarry blasts to test models for isostatic compensation of topography in the southern Appalachians. The profiles cross the Appalachian gravity gradient and gravity low and sample the highest elevations within the orogen. Migration of P and S reflections suggests an increase in crustal thickness from 35-39 km within the Coastal Plain to 50-52 km along the southeastern flank of the Blue Ridge Mountains. Crustal thickness within the Blue Ridge Mountains ranges from 47-56 km, with a local minimum beneath the Asheville Basin. Analysis of earthquake data for two broadband stations shows a similar pattern in crustal thickness. The greatest Moho depths are associated not with the tallest peaks, but rather with the broadest portions of the mountain chain. This observation combined with the persistence of thick crust beyond the southeastern flank of the mountains suggests that topography is supported by elastic bending of the lithosphere rather than Airy compensation. Estimates for the density contrast between the lowermost crust and uppermost mantle are consistent with models that propose an increase in lower crustal density with age to explain the preservation of roots for old mountain belts.
Dating of Pliocene Colorado River sediments: Implications for cosmogenic burial dating and the evolution of the lower Colorado River
A. Matmon et al., Dept. of Geology, Miami University, Shideler Hall, Oxford, Ohio 45056, USA; doi: 10.1130/B30453.1. Posted online ahead of print 14 Oct. 2011.
A. Matmon of Miami University and colleagues date sediments deposited by the ancestral Colorado using concentrations and ratios of cosmogenic nuclides. At one site, comparison of burial ages to the overlying 4.4 Ma basalt yielded good agreement and suggests that under the most favorable conditions, cosmogenic burial dating can extend back about 5 million years. Results from other sites highlight the complexities inherent in burial dating such as complicated burial histories, insufficient shielding, post burial production of cosmogenic isotopes, and unknown initial 26Al/10Be ratios. In spite of the large range of burial ages, Matmon et al. identify sediment deposition and burial periods at ca. 5.3, 4.7, and 3.6 Ma.
Reconstructing the history of sediment deposition in caves: A case study from Wonderwerk Cave, South Africa
A. Matmon et al., Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel; doi: 10.1130/B30410.1. Posted online ahead of print 14 Oct. 2011.
A. Matmon of Hebrew University and colleagues apply cosmogenic isotope burial dating, magnetostratigraphy, and grain size distribution analysis to elucidate the history of the sedimentary sequence in Wonderwerk Cave, South Africa. The source for the sand is the Kalahari dunes, located ~100 km to the north of the cave. Field observations and grain size analysis suggest aeolian transport of Kalahari sand, temporary storage on hill slopes, and later deposition in the cave. Results presented by Matmon et al. suggest burial ages for sediments that range between 2.63 plus or minus 0.17 Ma and 1.56 plus or minus 0.10 Ma. However, they note that 26Al/10Be ratios measured in sand collected from the surface outside the cave imply an initial burial signal equivalent to 0.78 plus or minus 0.15 Ma, thus reducing the possible age range of the buried samples to between 1.85 plus or minus 0.23 and 0.78 plus or minus 0.18 Ma. Paleomagnetic results for the front of the cave can be correlated with the cosmogenic burial age range. In contrast, such a correlation is not possible at the back of the cave, since the paleomagnetic polarity can be correlated to any Reverse-Normal transition that occurred during the Quaternary. Sedimentological and isotopic similarities between the sediment in the cave and that presently exposed outside the cave suggest that the environmental conditions and rates of geomorphic processes during the late Pliocene and early Pleistocene may have been similar to those currently experienced in the southern Kalahari.
Peninsular terrane basement ages recorded by Paleozoic and Paleoproterozoic zircon in gabbro xenoliths and andesite from Redoubt volcano, Alaska
Charles R. Bacon et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA; doi: 10.1130/B30439.1. Posted online ahead of print 30 Sept. 2011.
Redoubt volcano is a historically active cone located 100 miles (160 km) southwest of Anchorage, Alaska. Plutonic rock fragments (xenoliths) ejected by Redoubt have been thought to be either geologically young or Mesozoic rocks that of the Peninsular terrane, a part of Earth's crust formed elsewhere and transported to its present location by plate tectonic motion. However, new sensitive high-resolution ion microprobe (SHRIMP) ages for zircon from xenoliths, presented here by Charles Bacon of the USGS and colleagues, are dominated by much older ca. 310 million-year-old Pennsylvanian and ca. 1865 million-year-old Paleoproterozoic grains. Textural evidence and ca. 1865 million-year-old zircon in Redoubt lava show that ancient rocks have been digested by Redoubt magma, most likely in the magma accumulation and storage region ~4-10 km below sea level. The zircon ages demonstrate that Pennsylvanian, and probably Paleoproterozoic, igneous rocks exist in or beneath Peninsular terrane basement. Discovery of Pennsylvanian xenoliths similar in age to Skolai arc rocks 310 miles (500 km) northeast of Redoubt indicates that the Peninsular terrane, along with the Wrangellia and Alexander terranes, has been part of the Wrangellia composite terrane since at least Pennsylvanian time. Moreover, the zircon data suggest that a hitherto unknown Paleoproterozoic continental fragment may be present in southern Alaska.
Detrital zircon geochronology and provenance of the Neoproterozoic to Late Devonian Franklinian Basin, Canadian Arctic Islands
Owen A. Anfinson et al., Dept. of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada; doi: 10.1130/B30503.1. Posted online ahead of print 30 Sept. 2011.
The Canadian Arctic Islands remain one of the last relatively unexplored frontiers with respect to geochronologic research. Research presented here by Owen Anfinson of the University of Calgary and colleagues utilizes detrital zircon U/Pb geochronology to help determine the sources of sediment for the northern margin of North America for a period of approximately 200 million years (560-360 Ma). The study utilizes the collection efforts of more than 40 years of expeditions to the Canadian Arctic Islands by Geological Survey of Canada geologists. The samples these geologists collected provide a geographic and stratigraphic distribution of samples that would take millions of dollars and numerous summers of field research to personally collect. In studying these samples, Anfinson and colleagues provide insights into the origins of terranes that collided with the northern margin of North America and provide information on the tectonic evolution of the Canadian Arctic Islands. The study also provides a "fingerprint" of the landmass Crockerland, which likely now lies somewhere in the arctic realm, but may be covered by the sea ice of the Arctic Ocean. By providing detailed geochronologic information on the first sedimentary basin of the Canadian Arctic Islands this paper will help provide a baseline for many future detrital zircon studies in the Canadian Arctic.
The influence of snow sublimation on the isotopic composition of spring and surface waters in the southwestern United States: Implications for stable isotope-based paleoaltimetry and hydrologic studies
Alex R. Lechler and Nathan A. Niemi, Dept. of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA; doi: 10.1130/B30467.1. Posted online ahead of print 30 Sept. 2011.
Stable isotope-based determinations of paleoclimate, paleoelevation and location and timing of groundwater recharge often rely on the fundamental assumption that terrestrial waters (e.g., lakes, streams, springs) are reliable recorders of the isotopic composition (delta-18O and delta-D) of the original precipitation from which they were derived. In cases where substantial post-depositional isotopic modification has taken place, these stable isotope-based reconstructions are prone to considerable errors. Alex Lechler and Nathan Niemi of the University of Michigan show that sublimation of the winter snowpack likely acts as a dominant post-depositional mechanism that influences spring and surface waters and partially erases the original isotopic signatures of precipitation in the Panamint and Spring Mountains located in the arid, continental interior western U.S. These sublimation effects can potentially lead to significant paleoelevation underestimates on the order of 2 km or more. Accounting for sublimation effects will greatly improve future stable isotope-based reconstructions, particularly in light of current climate warming trends in the water-stressed, western U.S. where earlier onset of spring season snowmelt and a greater proportion of annual precipitation falling as rain instead of snow threatens an already scarce seasonal water recharge supply.
Ice-stream flow switching during deglaciation of the southwestern Barents Sea
Monica C.M. Winsborrow et al., Dept. of Geology, University of Tromso, N-9037 Tromso, Norway; doi: 10.1130/B30416.1. Posted online ahead of print 30 Sept. 2011.
The dynamics of ice sheets are strongly influenced by fast-flowing ice streams which discharge the majority of ice, sediment and meltwater. Recent observations from contemporary ice sheets have revealed large variations in the location and velocity of ice streams over relatively short timescales. These "flow-switches" may have significant consequences for the dynamics of the ice sheet as a whole, but the underlying causes of this behavior and the extent to which it is predictable, are poorly known. In this paper, Monica Winsborrow of the University of Tromso, Norway, and colleagues document a major episode of ice stream flow-switching which took place during the final deglaciation of the southwestern Barents Sea. Based on differences in the landforms created by two neighboring ice streams, we reconstruct contrasting retreat histories; with one ice stream switching from fast to slow flow and stabilizing during retreat, whilst the adjacent ice stream rapidly retreated without stabilizing. Winsborrow and colleagues propose that variations in topography over which these ice streams flowed determined how they responded to variations in sea level and that this was the primary factor controlling their retreat behavior. This mechanism is likely an important factor on Greenlandic and Antarctic marine-based ice streams and outlet glaciers.