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  • Journal article
    Balco G, Purvance MD, Rood DH, 2011,

    Exposure dating of precariously balanced rocks

    , Quaternary Geochronology, Vol: 6, Pages: 295-303, ISSN: 1871-1014

    Precariously balanced rocks (PBRs) are freestanding boulders that are precarious or fragile in the sense that they could be toppled by relatively low-amplitude earthquake ground motion. They are important in paleoseismology because their continued existence limits the amplitude of ground motion experienced at their location during their lifetime. In order to make quantitative use of PBRs for seismic hazard studies, one must determine when they attained their present state of fragility, that is, the point in time when the contact between the rocks and the pedestals on which they rest was exhumed from surrounding soil and the rock became vulnerable to earthquake ground motions. Cosmogenic-nuclide exposure dating can be used for this purpose, but is complicated because nuclide production occurs throughout exhumation of the PBR, so the apparent exposure age of any part of the rock surface exceeds the time that the rock has actually been precariously balanced. Here we describe a method for determining the length of time that a PBR has been fragile by measuring cosmogenic-nuclide concentrations at several locations on the PBR surface, and linking them together with a forward model that accounts for nuclide production before, during, and after exhumation of the PBR. Fitting model to data yields the rate and timing of rock exhumation and thus the length of time the rock has been fragile. We use this method to show that an example PBR in southern California has been fragile for 18.7 ± 2.8 ka. © 2011 Elsevier B.V.
  • Journal article
    Corbett LB, Young NE, Bierman PR, Briner JP, Neumann TA, Rood DH, Graly JAet al., 2011,

    Paired bedrock and boulder <sup>10</sup>Be concentrations resulting from early Holocene ice retreat near Jakobshavn Isfjord, western Greenland

    , Quaternary Science Reviews, Vol: 30, Pages: 1739-1749, ISSN: 0277-3791

    We measured in situ cosmogenic 10Be in 16 bedrock and 14 boulder samples collected along a 40-km transect outside of and normal to the modern ice margin near Sikuijuitsoq Fjord in central-west Greenland (69°N). We use these data to understand better the efficiency of glacial erosion and to infer the timing, pattern, and rate of ice loss after the last glaciation. In general, the ages of paired bedrock and boulder samples are in close agreement (r2 = 0.72). Eleven of the fourteen paired bedrock and boulder samples are indistinguishable at 1σ; this concordance indicates that subglacial erosion rates are sufficient to remove most or all 10Be accumulated during previous periods of exposure, and that few, if any, nuclides are inherited from pre-Holocene interglaciations. The new data agree well with previously-published landscape chronologies from this area, and suggest that two chronologically-distinct land surfaces exist: one outside the Fjord Stade moraine complex (~10.3 ± 0.4 ka; n = 7) and another inside (~8.0 ± 0.7 ka; n = 21). Six 10Be ages from directly outside the historic (Little Ice Age) moraine show that the ice margin first reached its present-day position ~7.6 ± 0.4 ka. Early Holocene ice margin retreat rates after the deposition of the Fjord Stade moraine complex were ~100-110 m yr-1. Sikuijuitsoq Fjord is a tributary to the much larger Jakobshavn Isfjord and the deglaciation chronologies of these two fjords are similar. This synchronicity suggests that the ice stream in Jakobshavn Isfjord set the timing and pace of early Holocene deglaciation of the surrounding ice margin. © 2011 Elsevier Ltd.
  • Journal article
    West N, Kirby E, Bierman P, Rood Det al., 2011,

    Preliminary estimates of regolith generation and mobility in the Susquehanna Shale Hills Critical Zone Observatory, Pennsylvania, using meteoric <sup>10</sup>Be

    , Applied Geochemistry, Vol: 26, ISSN: 0883-2927

    This study seeks to quantify the rate and timing of regolith generation in the Critical Zone at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Meteoric 10Be depth profiles were determined using measurements from 30 hillslope soil and bedrock core samples in an effort to constrain 10Be inventories. The SSHO is located in the temperate climate zone of central Pennsylvania and comprises a first-order watershed developed entirely on a Fe-rich, organic-poor, Silurian-aged shale. Two major perturbations to the landscape have occurred at SSHO in the geologically recent past, including significant and sustained periglacial activity until after the retreat of the Laurentide ice sheet (~21ka) and deforestation during early colonial land-use. Bulk soil samples (n=16) were collected at three locations along a planar hillslope on the southern ridge of the catchment, representing the ridge top, mid-slope and valley floor. Rock chip samples (n=14) were also collected from a 24m deep core drilled into the northern ridge top. All meteoric 10Be concentration profiles show a declining trend with depth, with most of the 10Be retained in the uppermost decimeters of the soil. Meteoric 10Be inventories are higher at the mid-slope and valley floor sample sites, at 3.71±0.02×1010at/cm2 and 3.69±0.02×1010at/cm2, than at the ridge top site (1.90±0.01×1010at/cm2). The 10Be inventory at the convex ridge top site implies a minimum residence time of ~10.6ka, or if erosion is steady, an erosion rate of 19.4±0.2m/My. © 2011 Elsevier Ltd.
  • Journal article
    Collins GS, Melosh HJ, Wunnemann K, 2011,

    Improvements to the epsilon-alpha porous compaction model for simulating impacts into high-porosity solar system objects

    , INTERNATIONAL JOURNAL OF IMPACT ENGINEERING, Vol: 38, Pages: 434-439, ISSN: 0734-743X

    We describe improvements to the epsilon-alpha porous compaction model for simulating solar system impacts. To improve the treatment of highly porous materials, we modified the epsilon-alpha model to account for thermal expansion of the matrix during compaction. We validated the improved model by demonstrating good agreement between numerically computed Hugoniot curves for porous iron (up to initial porosities of similar to 80%) using the improved epsilon-alpha model and experimentally-derived Hugoniot data. Moreover, we verified that the model improvements are easily implemented into a hydrocode and preserve the efficiency advantage of a strain-based compaction function. We used the improved epsilon-alpha porous compaction model in the iSALE hydrocode to reproduce 2-km/s porous-target laboratory impact experiments. The simulation results were in qualitative agreement with the experiments but produced craters that were consistently deeper and larger in volume than the experiments. The results of the hydrocode simulations and laboratory experiments show a reduction in crater efficiency with increasing porosity. This reduction is more dramatic if the impactor density and velocity are higher. (C) 2010 Elsevier Ltd. All rights reserved.
  • Journal article
    Glasser NF, Jansson KN, Goodfellow BW, de Angelis H, Rodnight H, Rood DHet al., 2011,

    Cosmogenic nuclide exposure ages for moraines in the Lago San Martin Valley, Argentina

    , Quaternary Research, Vol: 75, Pages: 636-646, ISSN: 0033-5894

    At several times during the Quaternary, a major eastward-flowing outlet glacier of the former Patagonian Ice Sheet occupied the Lago San Martin Valley in Argentina (49°S, 72°W). We present a glacial chronology for the valley based on geomorphological mapping and cosmogenic nuclide (10Be) exposure ages (n=10) of boulders on moraines and lake shorelines. There are five prominent moraine belts in the Lago San Martin Valley, associated with extensive sandar (glaciofluvial outwash plains) and former lake shorelines. Cosmogenic nuclide exposure ages for boulders on these moraines indicate that they formed at 14.3±1.7ka, 22.4±2.3ka, 34.4±3.4ka to 37.6±3.4ka (and possibly 60±3.5ka), and 99±11ka (1ρ). These dated glacier advances differ from published chronologies from the Lago San Martin Valley based on 14C age determinations from organic sediments and molluscs in meltwater channels directly in front of moraines or in kettleholes within end moraine ridges. The moraine boulder ages also point to possible pre-LGM glacial advances during the last glacial cycle and a key observation from our data is that the LGM glaciers were probably less extensive in the Lago San Martin Valley than previously thought. © 2010 University of Washington.
  • Journal article
    Deveugle PEK, Jackson MD, Hampson GJ, Farrell ME, Sprague AR, Stewart J, Calvert CSet al., 2011,

    Characterization of stratigraphic architecture and its impact on fluid flow in a fluvial-dominated deltaic reservoir analog: Upper Cretaceous Ferron Sandstone Member, Utah

    , AAPG Bulletin, Vol: 95, Pages: 693-727, ISSN: 0149-1423
  • Journal article
    Cardozo N, Jackson CAL, Whipp PS, 2011,

    Determining the uniqueness of best-fit trishear models

    , Journal of Structural Geology, Vol: 33, Pages: 1063-1078

    We show the application of a simulated annealing algorithm to trishear inverse modeling. The algorithm traverses the parameter space in search for best-fit models without being trapped in local minima, and thus sampling for more possible solutions globally. Simulated annealing is a robust and efficient technique to determine the uniqueness of best-fit trishear models; the spread of possible trishear models that can fit a structure. We first apply the algorithm to a decameter-size, contractional fault-propagation fold in west-central Taiwan, for which there is an exceptional exposure of pre-growth and growth strata. Simulated annealing shows that even for this complete fold dataset with low uncertainties, there is a range of models and fault slip/uplift histories that can fit the data, with the consequent implications for the assessment of seismic hazard. We then apply the algorithm to a kilometer-size, extensional fault-propagation fold, the Hadahid monocline, Gulf of Suez Rift, Egypt. In this monocline there is only surface coverage in the footwall anticline areas and the algorithm was used to delimit the range of possible models that can fit the data and their uncertainties, thus avoiding biases in the interpretation. Simulated annealing suggests that the along-strike structural variability of the monocline can result from along-strike variability in fault slip, fault propagation to fault slip ratio and depth of fault nucleation. Both examples illustrate the benefits of searching for a possible range of models rather than a precise best-fit model when modeling fault-propagation folds. In an attempt to understand which parameters control fault development, and also how the spread of possible solutions varies with fold growth, we apply the algorithm to four sequential stages of a published, analog clay model of an extensional forced fold. The inversions of the natural examples and the analog model suggest that the spread of the possible models is a manifestation of
  • Journal article
    Choi K, Jackson MD, Hampson GJ, Jones ADW, Reynolds ADet al., 2011,

    Predicting the impact of sedimentological heterogeneity on gas–oil and water–oil displacements: fluvio-deltaic Pereriv Suite Reservoir, Azeri–Chirag–Gunashli Oilfield, South Caspian Basin

    , Petroleum Geoscience, Vol: 17, Pages: 143-163, ISSN: 1354-0793
  • Journal article
    Hwang YK, Ritsema J, Goes S, 2011,

    Global variation of body-wave attenuation in the upper mantle from teleseismic P wave and S wave spectra

    , GEOPHYSICAL RESEARCH LETTERS, Vol: 38, ISSN: 0094-8276
  • Journal article
    Attal M, Cowie PA, Whittaker AC, Hobley D, Tucker GE, Roberts GPet al., 2011,

    Testing fluvial erosion models using the transient response of bedrock rivers to tectonic forcing in the Apennines, Italy

    , JOURNAL OF GEOPHYSICAL RESEARCH-EARTH SURFACE, Vol: 116, ISSN: 2169-9003
  • Journal article
    Jackson CA-L, Huuse M, Barber GP, 2011,

    Geometry of wing-like intrusions adjacent to a deep-water slope channel complex and implications for hydrocarbon exploration and production: a 3D seismic case study from the Maloy Slope, offshore Norway.

    , AAPG Bulletin, Vol: 95, Pages: 559-584

    Because of their potentially large volumes and excellent reservoir properties, winglike clastic intrusion complexes may represent stand-alone exploration targets. However, determining the three-dimensional (3-D) geometry of such complexes is problematic because of limited exposure in the field and insufficient seismic resolution and well coverage in the subsurface.In this study, high-quality 3-D seismic reflection data from offshore Norway are used to determine the 3-D geometry of winglike intrusion complexes adjacent to a deep-water slope channelcomplex. Intrusions form sheets or “wings” that are developed almost continuously along both margins of the 15-km(9-mi)-long channel complex. Intrusions dip up to 20° (more ommonly <10°), crosscut up to 90 m (30 ft) (undecompacted) of the overlying stratigraphy and extend up to 1095m(3593 ft) away from the channel complex. Three styles of intrusion are observed: type 1: dikes lacking sills at their upper tips; type2: dikes that pass upward into sills at their upper tips; and type 3: “stepped sills” that consist of linked sill and dike segments. Thesedifferent styles of intrusions pass laterally into one another, leading to extreme complexity both between and along-strike individualintrusion complexes. Although the mechanisms driving initial overpressure development and injection itself are unclear, this study indicates that 3-D seismic data are a powerful tool in understanding the 3-D geometry of winglike clastic injection complexes and suggests that the true geometry of these featuresismore complex than previously documented. This study also has implications for potential reservoir geometries and hydrocarbon exploitation of winglike clastic intrusion omplexes.
  • Journal article
    Armitage JJ, Duller RA, Whittaker AC, Allen PAet al., 2011,

    Transformation of tectonic and climatic signals from source to sedimentary archive

    , NATURE GEOSCIENCE, Vol: 4, Pages: 231-235, ISSN: 1752-0894
  • Journal article
    Tucker GE, McCoy SW, Whittaker AC, Roberts GP, Lancaster ST, Phillips Ret al., 2011,

    Geomorphic significance of postglacial bedrock scarps on normal-fault footwalls

    , JOURNAL OF GEOPHYSICAL RESEARCH-EARTH SURFACE, Vol: 116, ISSN: 2169-9003
  • Journal article
    Rood DH, Burbank DW, Finkel RC, 2011,

    Chronology of glaciations in the Sierra Nevada, California, from <sup>10</sup>Be surface exposure dating

    , Quaternary Science Reviews, Vol: 30, Pages: 646-661, ISSN: 0277-3791

    We use 10Be surface exposure dating to construct a high-resolution chronology of glacial fluctuations in the Sierra Nevada, California. Most previous studies focused on individual glaciated valleys, whereas our study compares chronologies developed throughout the range to identify regional patterns in the timing of glacier response to major climate changes. Sites throughout the range indicate Last Glacial Maximum retreat at 18.8 ± 1.9 ka (2σ) that suggests rather consistent changes in atmospheric variables, e.g., temperature and precipitation, throughout the range. The penultimate glacial retreat occurred at ca 145 ka. Our data suggest that the Sierra Nevada landscape is dominated by glacial features deposited during marine isotope stage (MIS) 2 and MIS 6. Deposits of previously recognized glaciations between circa 25 and 140 ka, e.g., MIS 4, Tenaya, early Tahoe, cannot be unequivocally identified. The timing of Sierra Nevada glacial retreat correlates well with other regional paleoclimate proxies in the Sierra Nevada, but differs significantly from paleoclimate proxies in other regions. Our dating results indicate that the onset of LGM retreat occurred several thousand years earlier in the Sierra Nevada than some glacial records in the western US. © 2010 Elsevier Ltd.
  • Journal article
    Ho TYK, Jardine RJ, Anh-Minh N, 2011,

    Large-displacement interface shear between steel and granular media

    , GEOTECHNIQUE, Vol: 61, Pages: 221-234, ISSN: 0016-8505
  • Journal article
    Styles E, Davies DR, Goes S, 2011,

    Mapping spherical seismic into physical structure: biases from 3-D phase-transition and thermal boundary-layer heterogeneity

    , GEOPHYSICAL JOURNAL INTERNATIONAL, Vol: 184, Pages: 1371-1378, ISSN: 0956-540X
  • Journal article
    Jackson CAL, 2011,

    3D seismic analysis of megaclast deformation within a mass-transport deposit; implications for debris flow kinematics

    , Geology, Vol: 39, Pages: 203-206

    Three-dimensional seismic reflection data are used to investigate the geometry, scale and distribution of structures within large clasts (megaclasts) contained within a Tertiary mass transport deposit (MTD), Santos Basin, offshore Brazil. Normal faults and folds are observed within the megaclasts, the latter typically being best-developed towards either the frontal or lateral margins of the clasts. The highly variable map-view orientation of these structures, their relative ages and their relationship to the geometry of the basal shear surface indicate that the structures developed during both the motion and arrest of the parent flow. This study indicates that megaclasts may be deformed despite the associated flow being cohesive and lacking turbulence. Deformation is related to local differential shear within the viscous body of the flow and mechanical interaction of megaclasts with the basal shear surface.
  • Journal article
    Burberry CM, Jackson CA-L, Cosgrove JC, 2011,

    Late Cretaceous to Recent deformation related to inherited structures and subsequent compression within the Persian Gulf: a 2D seismic case study

    , Journal of the Geological Society, Vol: 168, Pages: 485-498

    The Persian Gulf is part of an asymmetric foreland basin related to the Zagros Orogen. Few published studies of this basin and associated onshore areas include seismic reflection data. We present a seismic-stratigraphic interpretation based on marine 2D seismic data, which reveals the presence of two types of compressional structures within the basin: (1) faulted domes related to salt movement and the offshore trace of a NNE–SSW-trending dextral basement fault (the Kazerun Fault); (2) long-wavelength (16 km), low-amplitude (60 ms two-way travel time) folds relating to the advancing deformation front associated with the orogen. Thinning of age-constrained stratal units across structures related to the offshore trace of the Kazerun Fault implies a distinct pulse of uplift on this fault during the Maastrichtian. The geometry of growth strata across other intra-basin structures suggests a second, later stage of deformation, which began in the Middle Miocene. Thickening and folding of post-Middle Miocene stratal units towards the NE (i.e. towards the Zagros Orogen) is interpreted to reflect rapid loading, subsidence and compression related to southwestwards advance of the orogen. The results of this study have implications for the interaction between pre-existing structures and later compressional events both within the Persian Gulf and elsewhere.
  • Journal article
    Young NE, Briner JP, Stewart HAM, Axford Y, Csatho B, Rood DH, Finkel RCet al., 2011,

    Response of Jakobshavn Isbræ, Greenland, to Holocene climate change

    , Geology, Vol: 39, Pages: 131-134, ISSN: 0091-7613

    Rapid fluctuations in the velocity of Greenland Ice Sheet (GIS) outlet glaciers over the past decade have made it difficult to extrapolate ice-sheet change into the future. This significant short-term variability highlights the need for geologic records of preinstrumental GIS margin fluctuations in order to better predict future GIS response to climate change. Using 10Be surface exposure ages and radiocarbon-dated lake sediments, we constructed a detailed chronology of ice-margin fluctuations over the past 10 k.y. for Jakobshavn Isbræ, Greenland's largest outlet glacier. In addition, we present new estimates of corresponding local temperature changes using a continuous record of insect (Chironomidae) remains preserved in lake sediments. We find that following an early Holocene advance just prior to 8 ka, Jakobshavn Isbræ retreated rapidly at a rate of ~100 m yr-1, likely in response to increasing regional and local temperatures. Ice remained behind its present margin for ~7 k.y. during a warm period in the middle Holocene with sustained temperatures ~2 °C warmer than today, then the land-based margin advanced at least 2-4 km between A.D. 1500-1640 and A.D. 1850. The ice margin near Jakobshavn thus underwent large and rapid adjustments in response to relatively modest centennial-scale Holocene temperature changes, which may foreshadow GIS response to future warming. © 2011 Geological Society of America.
  • Journal article
    Rood DH, Burbank DW, Finkel RC, 2011,

    Spatiotemporal patterns of fault slip rates across the Central Sierra Nevada frontal fault zone

    , Earth and Planetary Science Letters, Vol: 301, Pages: 457-468, ISSN: 0012-821X

    Patterns in fault slip rates through time and space are examined across the transition from the Sierra Nevada to the Eastern California Shear Zone-Walker Lane belt. At each of four sites along the eastern Sierra Nevada frontal fault zone between 38 and 39° N latitude, geomorphic markers, such as glacial moraines and outwash terraces, are displaced by a suite of range-front normal faults. Using geomorphic mapping, surveying, and 10Be surface exposure dating, mean fault slip rates are defined, and by utilizing markers of different ages (generally, ~20ka and ~150ka), rates through time and interactions among multiple faults are examined over 104-105year timescales. At each site for which data are available for the last ~150ky, mean slip rates across the Sierra Nevada frontal fault zone have probably not varied by more than a factor of two over time spans equal to half of the total time interval (~20ky and ~150ky timescales): 0.3±0.1mm year-1 (mode and 95% CI) at both Buckeye Creek in the Bridgeport basin and Sonora Junction; and 0.4 +0.3/-0.1mm year-1 along the West Fork of the Carson River at Woodfords. Data permit rates that are relatively constant over the time scales examined. In contrast, slip rates are highly variable in space over the last ~20ky. Slip rates decrease by a factor of 3-5 northward over a distance of ~20km between the northern Mono Basin (1.3 +0.6/-0.3mm year-1 at Lundy Canyon site) to the Bridgeport Basin (0.3±0.1mm year-1). The 3-fold decrease in the slip rate on the Sierra Nevada frontal fault zone northward from Mono Basin is indicative of a change in the character of faulting north of the Mina Deflection as extension is transferred eastward onto normal faults between the Sierra Nevada and Walker Lane belt. A compilation of regional deformation rates reveals that the spatial pattern of extension rates changes along strike of the Eastern California Shear Zone-Walker Lane belt. South of the Mina Deflection, extension is accommoda

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