bluuebunny/crossref_metadata_2025 huggingface dataset:
Controls of inherited lithospheric heterogeneity on rift linkage: Numerical and analog models of interaction between the Kenyan and Ethiopian rifts across the Turkana depression
Authors: Brune Sascha , Corti Giacomo , Ranalli Giorgio
Published: July 2018

Inherited rheological structures in the lithosphere are expected to have large impact on the architecture of continental rifts. The Turkana depression in the East African Rift connects the Main Ethiopian Rift to the north with the Kenya rift in the south. This region is characterized by a NW-SE trending band of thinned crust inherited from a Mesozoic rifting event, which is cutting the present-day N-S rift trend at high angle. In striking contrast to the narrow rifts in Ethiopia and Kenya, extension in the Turkana region is accommodated in sub-parallel deformation domains that are laterally distributed over several hundred kilometers. We present both analog experiments and numerical models that reproduce the along-axis transition from narrow rifting in Ethiopia and Kenya to a distributed deformation within the Turkana depression. Similarly to natural observations, our models show that the Ethiopian and Kenyan rifts bend away from each other within the Turkana region, thus forming a right-lateral stepover and avoiding a direct link to form a continuous N-S depression. The models reveal five potential types of rift linkage across the pre-existing basin: three types where rifts bend away from the inherited structure connecting via a (1) wide or (2) narrow rift or by (3) forming a rotating microplate, (4) a type where rifts bend towards it, and (5) straight rift linkage. The fact that linkage type 1 is realized in the Turkana region provides new insights on the rheological configuration of the Mesozoic rift system at the onset of the recent rift episode.
Controls on early‐rift geometry: new perspectives from the Bilila‐Mtakataka fault, Malawi
Authors: Hodge Michael , Biggs Juliet , Fagereng Ake , Mdala Hassan
Published: July 2018

This study investigates the controls to fault and rift geometry for a young rift system. Our case study is a large fault in southern Malawi. The geometry of the fault at the surface, the scarp, and structures within the rocks the fault cuts through, are measured. We also calculate the scarp height. The orientation and height of the fault scarp implies that local stresses influence its geometry, not regional stresses. We develop a geometrical model to link the scarp and a fault at depth. The best‐fit model is consistent with the hypothesis that the local stress influences the current fault geometry. We propose that a weak zone at depth may also influence the fault geometry, but weaknesses near the surface only locally influence fault orientation.
Neogene - Quaternary slow coastal uplift of Western Europe through the perspective of sequences of strandlines from the Cotentin Peninsula (Normandy, France)
Authors: Pedoja Kevin , Jara-Muñoz Julius , de Gelder Gino , Robertson Jenni , Meschis Marco , Fernández-Blanco David , NEXER Maëlle , Poprawski Yohann , Olivier Dugue , Delcaillau Bernard
Published: July 2018

The Cotentin Peninsula (Normandy, France) displays sequences of marine terraces and rasas, the latter being wide Late Cenozoic coastal erosion surfaces, that are typical of Western European coasts in Portugal, Spain, France and southern England. Remote sensing imagery and field mapping enabled reappraisal of the Cotentin coastal sequences. From bottom to top, the N Cotentin sequence includes four previously recognized Pleistocene marine terraces (T1 to T4) at elevations < 40 m as well as four higher and older rasas (R1 to R4) reaching 200 ± 5 m in elevation. Low-standing marine terraces are not observed in the central part of the Peninsula and a limited number of terraces are described to the south. The high-standing rasas are widespread all over the peninsula. Such strandline distributions reveal major changes during the Late Cenozoic. Progressive uplift of an irregular sea-floor led to subaerial exposure of bathymetric highs that were carved into rocky platforms, rasas and marine terraces. Eventually, five main islands coalesced and connected to the mainland to the south to form the Cotentin Peninsula. On the basis of previous dating of the last interglacial maximum terrace (i.e. Marine Isotopic Stage, MIS 5e), sequential morphostratigraphy and modelling, we have reappraised uplift rates and derived: (i) mean Upper Pleistocene (i.e. since MIS 5e ~ 44 122 +/- 6 ka, i.e. kilo annum) apparent uplift rates of 0.04 ± 0.01 mm/yr, (ii) mean Middle Pleistocene eustasy corrected uplift rates of 0.09 ± 0.03 mm/yr, and (iii) low mean Pleistocene uplift rates of 0.01 mm/yr. Extrapolations of these slow rates combined with geological evidence implies that the formation of the sequences from the Cotentin Peninsula occurred between 3 Ma (Pliocene) and 15 Ma (Miocene), which cannot be narrowed down further without additional research. Along the coasts of Western Europe, sequences of marine terraces and rasas are widespread (169 preserve the MIS 5e benchmark). In Spain, Portugal, S England and other parts of western France, the sequences morphostratigraphy is very similar to that of Cotentin. The onset of such Western European sequences occurred during the Miocene (e.g. Spain) or Pliocene (e.g. Portugal). We interpret this Neogene - Quaternary coastal uplift as a symptom of the increasing lithospheric compression that accompanies Cenozoic orogenies.
The detection of low magnitude seismic events using array-based waveform correlation
Authors: Gibbons Steven , Ringdal Frode
Published: July 2018

It has long been accepted that occurrences of a known signal are most effectively detected by cross-correlating the incoming data stream with a waveform template. Such matched signal detectors have received very little attention in the field of detection seismology because there are relatively few instances in which the form of an anticipated seismic signal is known a priori. Repeating events in highly confined geographical regions have been observed to produce very similar waveforms and good signals from events at a given site can be exploited to detect subsequent co-located events at lower magnitudes than would be possible using traditional power detectors. Even greater improvement in signal detectability can be achieved using seismic arrays; running correlation coefficients from single sensors can be stacked over an array or network to result in a network correlation coefficient displaying a significant array gain. If two events are co-located, the time separating the corresponding patterns in the wave train as indicated by the cross-correlation function is identical for all seismic stations and this property means that the correlation coefficient traces are coherent even when the waveforms are not. We illustrate the power of array-based waveform correlation using the 1997 August 16 Kara Sea event. The weak event that occurred 4 hr after the main event was barely detected using an STA/LTA detector on the SPITS array but is readily detected by signal matching on a single channel. The main event was also recorded by the far more distant NORSAR array but no conventional detection can be made for the second event. A clear detection is, however, made when the correlation coefficient traces are beamformed over all sensors of the array. We estimate the reduction in detection threshold of a test signal on a regional seismic array using waveform correlation by scaling down a master signal and immersing it into seismic noise. We show that, for this case, waveform correlation using a single channel detects signals of approximately 0.7 orders of magnitude lower than is possible using an STA/LTA detector on the array beam. Waveform matching on the full array provides an additional improvement of approximately 0.4 magnitude units. We describe a case study in which small seismic events at the Barentsburg coal mine on Spitsbergen were detected using the signals from a major rockburst as master waveforms. Many spurious triggers occurred in this study whereby short sections of signal exhibited coincidental similarity with unrelated incoming wave fronts. We demonstrate how such false alarms can almost always be identified and screened out automatically by performing frequency–wavenumber analysis upon the set of individual correlation coefficient traces.
Electromagnetic torques in the core and resonant excitation of decadal polar motion
Authors: Mound Jonathan
Published: July 2018

Motion of the rotation axis of the Earth contains decadal variations with amplitudes on the order of 10 mas. The origin of these decadal polar motions is unknown. A class of rotational normal modes of the core–mantle system termed torsional oscillations are known to affect the length of day (LOD) at decadal periods and have also been suggested as a possible excitation source for the observed decadal polar motion. Torsional oscillations involve relative motion between the outer core and the surrounding solid bodies, producing electromagnetic torques at the inner-core boundary (ICB) and core–mantle boundary (CMB). It has been proposed that the ICB torque can explain the excitation of the approximately 30-yr-period polar motion termed the Markowitz wobble. This paper uses the results of a torsional oscillation model to calculate the torques generated at Markowitz and other decadal periods and finds, in contrast to previous results, that electromagnetic torques at the ICB can not explain the observed polar motion.This article has been accepted for publication in Geophysical Journal International ©: 2004 RAS Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Environmental signal propagation in sedimentary systems across timescales
Authors: Romans Brian , Castelltort Sébastien , Covault Jacob , Fildani Andrea , Walsh J.P.
Published: July 2018

Review of concepts of environmental signal (climate, tectonics, anthropogenic, etc.) propagation in sedimentary systems from source to sink.
Evaluating models of Coulomb stress transfer: Is variable fault geometry important?
Authors: Mildon Zoe , Toda Shinji , Walker Joanna , Roberts Gerald
Published: July 2018

To determine the importance of receiver fault geometry in Coulomb stress calculations a new methodology is presented to model faults with variable geometry. Although most models use planar faults, it is known that these are inaccurate representations of faults observed in the field. The central Italian Apennines are chosen as a straightforward tectonic system with well‐exposed normal faults to investigate the effect of variable geometry. It is shown that the static Coulomb stress transfer is most sensitive to changes in strike of the receiver faults, rather than changes in dip and rake. Therefore, a novel methodology to generate strike‐variable faults composed of discrete rectangular patches is developed. Using the 2009 LAquila earthquake (Mw = 6.3) the calculated stress transferred to planar versus variable faults is assessed. The stress transferred to variable faults is sufficiently different when compared to planar cases to merit other earthquake sequences being reassessed with available fault geometry.
Combined geophysical measurements provide evidence for unfrozen water in permafrost in the Adventdalen valley in Svalbard
Authors: Keating Kristina , Binley Andrew , Bense Victor , van Dam Remke , Christiansen Hanne
Published: July 2018

Quantifying the unfrozen water content of permafrost is critical for assessing impacts of surface warming on the reactivation of groundwater flow and release of greenhouse gasses from degrading permafrost. Unfrozen water content was determined along a ~12 km transect in the Adventdalen valley in Svalbard, an area with continuous permafrost, using surface nuclear magnetic resonance and controlled source audio-magnetotelluric data. This combination of measurements allowed for differentiation of saline from fresh, and frozen from unfrozen pore water. Above the limit of Holocene marine transgression no unfrozen water was detected, associated with high electrical resistivity. Below the marine limit, within several kilometers of the coast, up to ~10% unfrozen water content was detected, associated with low resistivity values indicating saline pore water. These results provide evidence for unfrozen water within continuous, thick permafrost in coastal settings, which has implications for groundwater flow and greenhouse gas release in similar Arctic environments.
Empirical relationship between river slope and the elongation of bars in braided rivers: a potential tool for paleoslope analysis from subsurface data
Authors: Castelltort Sébastien
Published: July 2018

Paleoslope of ancient river systems is a fundamental parameter needed to reconstruct paleohydrology and paleoclimate from the fluvial sedimentary record. The shape of braid bars in 22 modern rivers yields a relationship between average bar elongation (length/width) and river slope. Steep rivers display more elongated bars than gently dipping reaches. This relationship has potential application to ancient braided systems preserved in the subsurface and imaged by 3D seismic data. The average elongation of bars identified on a stratal-slice through the DaGang river deposits in North-East China yields a slope prediction which, combined with empirical data on grain size and slope of alluvial rivers, predicts a median grain size of between 0.1 and 1 mm for the considered river sediment, in agreement with grain size data in an adjacent isochronous belt. This study provides an additional approach for paleohydraulics in sedimentary basins and grain size prediction in exploration.
Trend-surfaces analysis of morphometric parameters: A case-study in southearstern Brazil
Authors: Grohmann Carlos
Published: July 2018

Trend-surface analysis was carried out on data from morphometric parameters isobase and hydraulic gradient. The study area, located in the eastern border of Quadrilátero Ferrífero, southeastern Brazil, presents four main geomorphological units, one characterized by fluvial dissection, two of mountainous relief, with a scarp of hundreds of meters of fall between them, and a flat plateau in the central portion of the fluvially-dissected terrains. Morphometric maps were evaluated in GRASS-GIS and statistics were made on R statistical language, using the spatial package. Analysis of Variance (ANOVA) was made to test the significance of each surface and the significance of increasing polynomialdegree. The best results were achieved with 6th-order surface for isobase and 2nd-order surface for hydraulic gradient. Shape and orientation of residuals maps contours for selected trends were compared with structures inferred from several morphometric maps, and a good correlation is present.
Basis functions for the consistent and accurate representation of surface mass loading
Authors: Clarke Peter , Lavallee David , Blewitt Geoffrey , van Dam Tonie
Published: July 2018

Inversion of geodetic site displacement data to infer surface mass loads has previously been demonstrated using a spherical harmonic representation of the load. This method suffers from the continent-rich, ocean-poor distribution of the geodetic data, coupled with the predominance of the continental load (water storage and atmospheric pressure) compared with the ocean bottom pressure (including the inverse barometer response). Finer-scale inversion becomes unstable due to the rapidly increasing number of parameters which are poorly constrained by the data geometry. Several approaches have previously been tried to mitigate this, including the adoption of constraints over the oceanic domain derived from ocean circulation models, the use of smoothness constraints for the oceanic load, and the incorporation of GRACE gravity field data. However, these methods do not provide appropriate treatment of mass conservation and of the ocean’s equilibrium-tide response to the total gravitational field. Instead, we propose a modified set of basis functions as an alternative to standard spherical harmonics. Our basis functions allow variability of the load over continental regions, but impose global mass conservation and equilibrium tidal behaviour of the oceans.We test our basis functions first for the efficiency of fitting to realistic modelled surface loads, and then for accuracy of the estimates of the inferred load compared with the known model load, using synthetic geodetic displacements with real GPS network geometry. Compared to standard spherical harmonics, our basis functions yield a better fit to the model loads over the period 1997-2005, for an equivalent number of parameters, and provide a more accurate and stable fit using the synthetic geodetic displacements. In particular, recovery of the low-degree coefficients is greatly improved. Using a 9-parameter fit we are able to model 58% of the variance in the synthetic degree-1 zonal coefficient time series, 38-41% of the degree-1 non-zonal coefficients, and 80% of the degree-2 zonal coefficient. An equivalent spherical harmonic estimate truncated at degree 2 is able to model the degree-1 zonal coefficient similarly (56% of variance), but only models 59% of the degree-2 zonal coefficient variance and is unable to model the degree-1 non-zonal coefficients.
Constraints on the numerical age of the Paleocene‐Eocene boundary
Authors: Charles Adam , Condon Daniel , Harding Ian , Pälike Heiko , Marshall John , Cui Ying , kump Lee , Croudance Ian
Published: July 2018

Here we present combined radioisotopic dating (U-Pb zircon) and cyclostratigraphic analysis of the carbon isotope excursion at the Paleocene-Eocene (P-E) boundary in Spitsbergen to determine the numerical age of the boundary. Incorporating the total uncertainty from both radioisotopic and cyclostratigraphic data sets gives an age ranging from 55.728 to 55.964 Ma, within error of a recently proposed astronomical age of ∼55.93 Ma. Combined with the assumption that the Paleocene Epoch spans twenty-five 405 kyr cycles, our new age for the boundary suggests an age of ∼66 Ma for the Cretaceous-Paleogene boundary. Furthermore, our P-E boundary age is consistent with the hypothesis that the onset of the Paleocene-Eocene thermal maximum at the boundary occurred on the falling limb of a 405 kyr cycle, suggesting the event was initiated by a different mechanism to that which triggered the other early Eocene hyperthermals.
Temperatures recorded by cosmogenic noble gases since the last glacial maximum in the Maritime Alps
Authors: Tremblay Marissa , Shuster David , Spagnolo Matteo , Renssen Hans , Ribolini Adriano
Published: July 2018

While proxy records have been used to reconstruct late Quaternary climate parameters throughout the European Alps, our knowledge of deglacial climate conditions in the Maritime Alps is limited. Here, we report temperatures recorded by a new and independent geochemical technique—cosmogenic noble gas paleothermometry—in the Maritime Alps since the last glacial maximum. We measured cosmogenic 3He in quartz from boulders in nested moraines in the Gesso Valley, Italy. Paired with cosmogenic 10Be measurements and 3He diffusion experiments on quartz from the same boulders, the cosmogenic 3He abundances record the temperatures these boulders experienced during their exposure. We calculate effective diffusion temperatures (EDTs) over the last ∼22 ka ranging from 8°C to 25°C. These EDTs, which are functionally related to, but greater than, mean ambient temperatures, are consistent with temperatures inferred from other proxies in nearby Alpine regions and those predicted by a transient general circulation model. In detail, however, we also find different EDTs for boulders from the same moraines, thus limiting our ability to interpret these temperatures. We explore possible causes for these intra-moraine discrepancies, including variations in radiative heating, our treatment of complex helium diffusion, uncertainties in our grain size analyses, and unaccounted-for erosion or cosmogenic inheritance.
The 2015 MW 7.1 Earthquake on the Charlie-Gibbs Transform Fault: Repeating Earthquakes and Multi-modal Slip on a Slow Oceanic Transform
Authors: Aderhold Kasey , Abercrombie Rachel
Published: July 2018

The slow spreading rate Charlie-Gibbs Atlantic transform fault slips in large (M~7) quasi-repeating earthquakes. The foreshocks, aftershocks, and unilateral rupture of the 2015 earthquake are similar to a 1974 earthquake on the same transform. The findings of this study are consistent with transform earthquakes starting small near the ridge and large slip asperities nearer to the transform center.
SRTM resample with short distance-low nugget kriging
Authors: Grohmann Carlos
Published: July 2018

The Shuttle Radar Topography Mission (SRTM), was flow on Space Shuttle Endeavour in February 2000, with the objective of acquire a digital elevation model of all land between 60º north latitude and 56º south latitude, using Interferometric Synthetic Aperture Radar (InSAR) techniques. SRTM data is distributed at horizontal resolution of 1 arc-second (aprox. 30m) for areas within the USA and at 3 arc-second (aprox. 90m) resolution for the rest of the world. A resolution of 90m can be considered suitable for small or medium-scale analysis, but it is too coarse for more detailed purposes. One alternative is to interpolate the SRTM data at a finer resolution; it wont increase the level of detail of the original DEM, but it will lead to a surface where there is coherence of angular properties (i.e., slope, aspect) between neighbouring pixels, an important characteristic when dealing with terrain analysis. This work intents to show how the proper adjustment of variogram and kriging parameters, namely the nugget effect and the maximum distance within which values are used in interpolation, can be set to achieve quality results on resampling SRTM data from 3 to 1. We present for a test area in western USA, which include different adjustment schemes (changes in nugget effect value and in the interpolation radius) as well as comparisons with the original 1 model of the area, with the National Elevation Dataset DEMs, and with other interpolation methods (splines and IDW).The basic concepts for using kriging to resample terrain data are: 1) to work only with the immediate neighbourhood of the predicted point, due the high spatial correlation of the topographic surface and omnidirectional behaviour of variogram in short distances; 2) add a very small random variation to the coordinates of the points prior to interpolation, to avoid punctual artifacts generated by predicted points with the same location than original data points and 3) use a small value of nugget effect, to avoid smoothing that can obliterate terrain features.Drainages derived from the surfaces interpolated by kriging and by splines have good agreement with streams derived from the 1 NED, with correct identification of watersheds, even though a few differences occur in the positions of some rivers in flat areas. Although the 1 surfaces resampled by kriging and splines are very similar, we consider the results produced by kriging as superior, since the spline-interpolated surface still presented some noise and linear artifacts, which were removed by kriging.
Quantificação da Deformação Finita nos Metagranitoides Cariris Velhos na Região de Alagoa Grande (PB)
Authors: Grohmann Carlos
Published: July 2018

The Cariris Velhos Metagranitoids are considered an important geological unit of the Transversal Zone (Borborema Province – NE Brazil). We evaluated the use of raw and processed digital images to obtain the deformation ellipsoid for the Cariris Velhos Metagranitoids using the Inertia Tensor and Intercepts methods. The ellipsoid symmetry of the deformed fabric was very similar for both methods, the Inertia Tensor and Intercepts, although with different anisotropy values. The orientations of the ellipsoid axes are similar to the orientation of geological structures observed in the eld (principal foliations and mineral lineations). When analyzing the characteristics of the deformation ellipsoids obtained in this study, it is worth pointing out the strong orientation of the C-axis (foliation pole) and the predominance of ellipsoids associated with planar to planar- linear fabrics ( attening eld). The deformation observed in the Cariris Velhos Metagranitoids is associated with the development under transpressive stress of regional Brasiliano shear zones.
Relict topography within the Hangay Mountains in central Mongolia: Quantifying long-term exhumation and relief change in an old landscape
Authors: McDannell Kalin , Zeitler Peter , Idleman Bruce
Published: July 2018

The Hangay Mountains are a high-elevation, low-relief landscape within the greater Mongolian Plateau of central Asia. New bedrock apatite (U-Th)/He single-grain ages from the Hangay span ~70 to 200 Ma, with a mean of 122.7 ± 24.0 Ma (2σ). Detrital apatite samples from the Selenga and Orkhon Rivers, north of the mountains, yield dominant (U-Th)/He age populations of ~115 to 130 Ma, as well as an older population not seen in the Hangay granitic bedrock data. These low-temperature data record regional exhumation of central Mongolia in the Mesozoic followed by limited erosion of <1-2 km since the Jurassic-Cretaceous, ruling out rapid exhumation of this magnitude associated with any late Cenozoic uplift. Apatite (U-Th)/He age-elevation patterns suggest long-term thermal stability of the upper crust, and thermal model inversions require late Mesozoic uplift and spatially variable exhumation driven by isostasy in concert with relief evolution to produce the observed cooling ages in the Hangay region. Alpine cirques and intact moraines are indicative of more recent, climate-driven erosion in the higher peaks of the western Hangay. Regionally, modeling suggests topographic "planation" in the Jurassic followed by rapid relief growth that was completed by the mid-Cretaceous. These results support Mesozoic topograhic evolution and relative stability of the landscape enduring throughout the Cenozoic with very little subsequent exhumation. These data support the notion that in the absence of strong tectonic or climate forcing, erosion is limited and remnant landscapes can persist over 10s-100s of millions of years in a state of disequilibrium.
The role of microbes in snowmelt and radiative forcing on an Alaskan icefield
Authors: Ganey Gerard , Loso Michael , Burgess Annie , Dial Roman
Published: July 2018

A lack of liquid water limits life on glaciers worldwide but specialized microbes still colonize these environments. These microbes reduce surface albedo, which, in turn, could lead to warming and enhanced glacier melt. Here we present results from a replicated, controlled field experiment to quantify the impact of microbes on snowmelt in red-snow communities. Addition ofnitrogen–phosphorous–potassium fertilizer increased alga cell counts nearly fourfold, to levels similar to nitrogen–phosphorusenriched lakes; water alone increased counts by half. The manipulated alga abundance explained a third of the observed variability in snowmelt. Using a normalized-di erence spectral index we estimated alga abundance from satellite imagery and calculated microbial contribution to snowmelt on an icefield of 1,900 km2. The red-snow area extended over about 700 km2, and in this area we determined that microbial communities were responsible for 17% of the total snowmelt there. Our results support hypotheses that snow-dwelling microbes increase glacier melt directly in a bio-geophysical feedback by lowering albedo and indirectly by exposing low-albedo glacier ice. Radiative forcing due to perennial populations of microbes maymatch that of non-living particulates at high latitudes. Their contribution to climate warming is likely to grow with increased melt and nutrient input.
Types of mixing and heterogeneities in siliciclastic-carbonate sediments
Authors: Chiarella Domenico , Longhitano Sergio , Tropeano Marcello
Published: July 2018

Mixed siliciclastic-carbonate deposits consist of a suite of different types of mixing between the two components, from bed (core-plug) to stratigraphic (seismic) scales, producing a high vertical and lateral lithological variability. Mixed deposits result from the interaction of siliciclastic input and coeval carbonate production controlled by temporal and/or spatial factors. Although mixed deposits are very diffuse in the geological record, studies about these deposits are scrappy and not well encoded. Accordingly, mixed deposits represent a labyrinth for researchers who want to investigate them for the first time.In this paper, different types of mixing (compositional versus strata) controlled by different allocyclic (e.g. sea-level, climate) and/or autocyclic (e.g. depositional processes) factors that operate at different scale are documented. Mixing is recognized and described at three main scales of observation: bed/core-plug scale; lithofacies/well-log scale; and stratigraphic/seismic scale. (i) Compositional mixing reflects the contemporaneous accumulation of the two heterolithic fraction in space and time. This type of mixing is observable at lamina to bed scale, locally producing depositional structures diagnostic for particular depositional environments. (ii) Strata mixing results from the alternation of the two heterolithic fraction in time. This type of mixing is observable at lithofacies to stratigraphic scale and can be related to depositional processes, climatic variations and/or relative sea-level changes.A correct identification of these different types of mixing and the scale of their occurrence is crucial in revealing (i) physical processes that control the sedimentation, (ii) environmental factors that influence the carbonate factory related to the siliciclastic dispersal mechanisms, and (iii) internal heterogeneity of the resulting sedimentary deposit. Furthermore, the petroleum industry is interested to unravel new insights about internal properties of mixed siliciclastic-carbonate systems (e.g., porosity, permeability) and to reconstruct predictive 3D models for the related reservoirs. The correct prediction of internal heterogeneity and the recognition of lateral and vertical compartmentalization have an important impact on hydrocarbon exploration and exploitation.
Influence of fault reactivation during multiphase rifting: the Oseberg area, Northern North Sea rift
Authors: Deng Chao , Fossen Haakon , Gawthorpe Robert , Rotevatn Atle , Jackson Christopher , FazliKhani Hamad
Published: July 2018

Multiphase rifts tend to produce fault populations that evolve by the formation of new faults and reactivation of earlier faults. The resulting fault patterns tend to be complex and difficult to decipher. In this work we use seismic reflection data to examine the evolution of a normal fault network in the Oseberg Fault Block in the northern North Sea Rift System – a rift system that experienced Permian – Early Triassic and Middle Jurassic – Early Cretaceous rifting and exhibits N-S, NW-SE and NE-SW oriented faults. Both N-S- and NW-SE-striking faults were established during the Permian – Early Triassic rifting, as indicated by Triassic growth packages in their hanging walls. In contrast, the NE-SW-striking faults are younger, as they show no evidence of Permian – Early Triassic growth, and offset several N-S- and NW-SE-striking faults. Structural analysis show that a new population of NW-SE-striking faults formed in the Lower – Middle Jurassic (inter-rift period) together with reactivation of N-S-striking Permian – Early Triassic faults, indicating a NE-SW inter-rift extension direction. During the Middle Jurassic – Early Cretaceous rifting, faults of all orientations (N-S, NW-SE and NE-SW) were active. However, faults initiated during the Middle Jurassic – Early Cretaceous rifting show mainly N-S orientation, indicating E-W extension during this phase. These observations suggest a reorientation of the stress field from E-W during the Permian – Early Triassic rift phase to NE-SW during inter-rift fault growth and back to E-W during the Middle Jurassic – Early Cretaceous rift phase in the Oseberg area. Hence, the current study demonstrates that rift activity between established rift phases can locally develop faults with new orientations that add to the geometric and kinematic complexity of the final fault population.
Seismotectonics of a diffuse plate boundary: Observations off the Sumatra-Andaman trench
Authors: Aderhold Kasey , Abercrombie Rachel
Published: July 2018

Seismic slip extends from maximum expected depth through the crust in 45-75 Ma oceanic lithosphere in the Indo-Australian intraplate region off the Sumatra-Andaman trench. Large (magnitude greater than 7) strike-slip earthquakes in this region can rupture multiple differently oriented faults. Rupture directivity modeling provides tentative support for active east-west oriented faults.
The Planform Mobility of River Channel Confluences: Insights from Analysis of Remotely Sensed Imagery
Authors: Dixon Simon , Smith Gregory , Best James , Nicholas Andrew , Bull Jon , Vardy Mark , Sarker Mamunul , Goodbred Steven
Published: July 2018

River channel confluences are widely acknowledged as important geomorphological nodes that control the downstream routing of water and sediment, and which are locations for the preservation of thick fluvial deposits overlying a basal scour. Despite their importance, there has been little study of the stratigraphic characteristics of river junctions, or the role of confluence morphodynamics in influencing stratigraphic character and preservation potential. As a result, although it is known that confluences can migrate through time, models of confluence geomorphology and sedimentology are usually presented from the perspective that the confluence remains at a fixed location. This is problematic for a number of reasons, not least of which is the continuing debate over whether it is possible to discriminate between scour that has been generated by autocyclic processes (such as confluence scour) and that driven by allocyclic controls (such as sea-level change). This paper investigates the spatial mobility of river confluences by using the 40-year record of Landsat Imagery to elucidate the styles, rates of change and areal extent over which large river confluence scours may migrate. On the basis of these observations, a new classification of the types of confluence scour is proposed and applied to the Amazon and Ganges-Brahmaputra-Meghna (GBM) basins. This analysis demonstrates that the2drivers of confluence mobility are broadly the same as those that drive channel change more generally. Thus in the GBM basin, a high sediment supply, large variability in monsoonal driven discharge and easily erodible bank materials result in a catchment where over 80 % of large confluences are mobile over this 40-year window; conversely this figure is less than 40 % for the Amazon basin. These results highlight that: i) the potential areal extent of confluence scours is much greater than previously assumed, with the location of some confluences on the Jamuna (Brahmaputra) River migrating over a distance of 20 times the tributary channel width; ii) extensive migration in the confluence location is more common than currently assumed, and iii) confluence mobility is often tied to the lithological and hydrological characteristics of the drainage basins that determine sediment yield.
Geostatistical modelling of cyclic and rhythmic facies architectures
Authors: Le Blévec Thomas , Dubrule Olivier , John Cédric , Hampson Gary
Published: July 2018

A pluri-Gaussian method is developed for facies variables in three dimensions to model vertical cyclicity, related to facies ordering, and rhythmicity. Cyclicity is generally characterized by shallowing or deepening-upward sequences and rhythmicity by a low range of variability in cycle thicknesses. Both of these aspects are commonly observed in shallow-marine carbonate successions, especially in the vertical direction. A grid-free spectral simulation approach is developed, with a separable covariance allowing a dampened hole-effect to capture rhythmicity in the vertical direction and a different covariance in the lateral plan, along strata as in space-time models. In addition, facies ordering is created by using a spatial shift between two latent Gaussian functions in the pluri-Gaussian approach. Rapid conditioning to data is performed via Gibbs sampling and kriging using the screening properties of separable covariances. The resulting facies transiograms can show complex patterns of cyclicity and rhythmicity. Finally, a three dimensional case study of shallow-marine carbonate deposits at outcrop shows the applicability of the modeling method.
A linear dynamical systems approach to streamflow reconstruction reveals history of regime shifts in northern Thailand
Authors: Nguyen Hung , Galelli Stefano
Published: July 2018

Catchment dynamics is not often modeled in streamflow reconstruction studies; yet, the streamflow generation process depends on both catchment state and climatic inputs. To explicitly account for this interaction, we contribute a linear dynamic model, in which streamflow is a function of both catchment state (i.e., wet/dry) and paleo-climatic proxies. The model is learned using a novel variant of the Expectation-Maximization algorithm, and it is used with a paleo drought record—the Monsoon Asia Drought Atlas—to reconstruct 406 years of streamflow for the Ping River (northern Thailand). Results for the instrumental period show that the dynamic model has higher accuracy than conventional linear regression; all performance scores increase by 45–497%. Furthermore, the reconstructed trajectory of the state variable provides valuable insights about the catchment history—e.g., regime-like behavior—thereby complementing the information contained in the reconstructed streamflow time series. The proposed technique can replace linear regression, since it only requires information on streamflow and climatic proxies (e.g., tree-rings, drought indices); furthermore, it is capable of readily generating stochastic streamflow replicates. With a marginal increase in computational requirements, the dynamic model brings more desirable features and value to streamflow reconstructions.
Crustal strain in the Marmara pull-apart region associated with the propagation process of the North Anatolian Fault
Authors: Karakas Cagil , Armijo Rolando , Lacassin Robin , Suc Jean , Melinte Mihaela
Published: July 2018

Propagation processes of plate-scale faults through continental lithosphere are poorly documented. The North Anatolian fault (NAF) is a continental right-lateral transform with striking evidence for propagation processes in the Marmara Sea pull-apart region. Earlier work [Armijo et al., 1999] suggests that in the Dardanelles, where the principal, northern branch of that fault (NNAF) enters into the Aegean: (1) a fold-thrust system has progressively developed above the NNAF fault tip, at the WSW corner of the Marmara Sea pull-apart. The main anticline formed there was sheared and its SW half laterally offset by ~70 km to the SW; (2) the timing of structure development appears correlated with sea-level changes associated with the Messinian Salinity Crisis (MSC). Our new description of the Dardanelles (or Ganos-Gelibolu) fold-thrust system is based on structural mapping, field observations and calcareous nannoplankton analyses to date key sedimentary units. Our results provide tight constraints on the main pulse of folding associated with propagation of the tip of the NNAF: it took place in the late Miocene to earliest Pliocene (5.60 to 5.04 Ma), before deposition of undeformed Pliocene marine sediments. The folding is mostly coeval with the MSC and accommodated several kilometers of shortening at the fault tip. After full propagation of the NNAF up to the surface, the folded structure was sheared and right-laterally offset, with an average 14 mm/yr of slip-rate during the past ~5 Myrs. A reconstruction of tectonic evolution suggests a flower structure nucleating and taking root at the tip of the fault.
Análise digital de terreno e evolução de longo-termo de relevo do centro-leste brasileiro
Authors: Grohmann Carlos
Published: July 2018

Geomorphological theories for long-term relief evolution postulate the existence of planation surfaces, created by the con- tinuous work of erosion/deposition during periods of tectonic quiescence and recognized as extensive areas of very gentle relief disturbed only locally by residual elevations, or by the apparent leveling of summit heights in a given region. After decades since the publication of the main theories on landform evolution, the validity of these models is still an open dis- cussion. In this paper, we present studies about the compartmentalization of landform elements, on a regional-scale basis, of central-eastern Brazil. The methods involved Digital Terrain Analysis in Geographic Information Systems, aiming the extraction and characterization of topographic variables and the compilation and mathematical analysis of geophysical and thermochronological data. Results were interpreted according to the geological context and the theories for long-term relief evolution. The integration of morphometric, thermochronological and geophysical data does not support the validity of us- ing planation surfaces in regional stratigraphic correlations.
Two Fundamentally Different Types of Submarine Canyons Along the Continental Margin of Equatorial Guinea
Authors: Jobe Zane , Lowe Donald , Uchytil Steve
Published: July 2018

Most submarine canyons are erosive conduits cut deeply into the world’s continental shelves through which sediment is transported from areas of high coastal sediment supply onto large submarine fans. However, many submarine canyons in areas of low sediment supply do not have associated submarine fans and show significantly different morphologies and depositional processes from those of ‘classic’ canyons. Using three-dimensional seismic reflection and core data, this study contrasts these two types of submarine canyons and proposes a bipartite classification scheme.The continental margin of Equatorial Guinea, West Africa during the late Cretaceous was dominated by a classic, erosional, sand-rich, submarine canyon system. This system was abandoned during the Paleogene, but the relict topography was re- activated in the Miocene during tectonic uplift. A subsequent decrease in sediment supply resulted in a drastic transformation in canyon morphology and activity, initiating the ‘Benito’ canyon system. This non-typical canyon system is aggradational rather than erosional, does not indent the shelf edge and has no downslope sediment apron. Smooth, draping seismic reflections indicate that hemipelagic deposition is the chief depositional process aggrading the canyons. Intra-canyon lateral accretion deposits indicate that canyon concavity is maintained by thick (> 150 m), dilute, turbidity currents. There is little evidence for erosion, mass wasting, or sand-rich deposition in the Benito canyon system. When a canyon loses flow access, usually due to piracy, it is abandoned and eventually filled. During canyon abandonment, fluid escape causes the successive formation of ‘cross-canyon ridges’ and pockmark trains along buried canyon axes.Based on comparison of canyons in the study area, we recognize two main types of submarine canyons: ‘Type I’ canyons indent the shelf edge and are linked to areas of high coarse-grained sediment supply, generating erosive canyon morphologies, sand- rich fill, and large downslope submarine fans/aprons. ‘Type II’ canyons do not indent the shelf edge and exhibit smooth, aggradational morphologies, mud-rich fill, and a lack of downslope fans/aprons. Type I canyons are dominated by erosive, sandy turbidity currents and mass wasting, whereas hemipelagic deposition and dilute, sluggish turbidity currents are the main depositional processes sculpting Type II canyons. This morphology-based classification scheme can be used to help predict depositional processes, grain size distributions, and petroleum prospectivity of any submarine canyon.
Probability density functions for radial anisotropy: implications for the upper 1200 km of the mantle
Authors: Beghein Caroline , Trampert Jeannot
Published: July 2018

The presence of radial anisotropy in the upper mantle, transition zone and top of the lower mantle is investigated by applying a model space search technique to Rayleigh and Love wave phase velocity models. Probability density functions are obtained independently for S-wave anisotropy, P-wave anisotropy, intermediate parameter η, Vp, Vs and density anomalies. The likelihoods for P-wave and S-wave anisotropy beneath continents cannot be explained by a dry olivine-rich upper mantle at depths larger than 220 km. Indeed, while shear-wave anisotropy tends to disappear below 220 km depth in continental areas, P-wave anisotropy is still present but its sign changes compared to the uppermost mantle. This could be due to an increase with depth of the amount of pyroxene relative to olivine in these regions, although the presence of water, partial melt or a change in the deformation mechanism cannot be ruled out as yet. A similar observation is made for old oceans, but not for young ones where VSH>VSV appears likely down to 670 km depth and VPH>VPV down to 400 km depth. The change of sign in P-wave anisotropy seems to be qualitatively correlated with the presence of the Lehmann discontinuity, generally observed beneath continents and some oceans but not beneath ridges. Parameter η shows a similar age-related depth pattern as shear-wave anisotropy in the uppermost mantle and it undergoes the same change of sign as P-wave anisotropy at 220 km depth. The ratio between dlnVs and dlnVp suggests that a chemical component is needed to explain the anomalies in most places at depths greater than 220 km. More tests are needed to infer the robustness of the results for density, but they do not affect the results for anisotropy.
Insights into agricultural influences and weathering processes from major ion patters
Authors: van Geldern Robert , Schulte Peter , Mader Michael , Baier Alfons , Juhlke Tobias , Barth Johannes , Lee Kern
Published: July 2018

Karst areas and their catchments pose a great challenge for protection because fast conduit flow results in low natural attenuation of anthropogenic contaminants. Studies of the hydrochemistry of karst sources and river solutes are an important tool for securing and managing water resources. A study of the geochemical downriver evolution of the Wiesent River and its tributaries, located in a typical karst terrain, revealed unexpected downstream decreases of nitrate with maximum mean values of 30 mg/L at the source to minimum values of 18 mg/L near the river mouth. This trend persisted over the length of the river even though increased agricultural activities are evident in the downstream section of the catchment. This pattern is caused by fertilizer inputs via diffusive and fast conduits flow from karst lithology in the upstream area that may have reached the river’s source even from beyond the hydrological catchment boundaries. Further downstream, these influences became diluted by tributary inputs that drain subcatchments dominated by claystone and sandstone lithologies that increased potassium and sulfate concentrations. Our findings indicate that bedrock geology remains the dominant control on the major ion chemistry of the Wiesent River, and that agricultural influences are strongest near the headwaters despite increased land-use further downstream, due to long-term storage and accumulation in karst aquifers. This feature may not be unique to the Wiesent River system, as carbonates cover significant portions of the Earth’s surface and subsequent work in other river systems could establish whether such patterns are ubiquitous worldwide.