The stratigraphy,structure and regional significance of the Moine Rocks of Mull,Argyllshire, W. Scotland

Two right-way-up Moine lithostratigraphic units—the Shiaba (older) and Assapol (younger) Groups—are distinguished in the Ross of Mull. These were intruded in the west by the post-tectonic, Ross-of-Mull granite complex at 414 ± 3 Ma. Apparently undisturbed inclusions of Moine metasediments within the granite permit the boundary between the lithostratigraphic units to be followed westward almost to the supposed trace of the Moine thrust in the Sound of Iona. The Shiaba and Assapol Groups which have a transitional, albeit attenuated, stratigraphic contact are correlated with the Morar and Glenfinnan Divisions of Inverness-shire, the Sgurr Beag ductile thrust (slide) normally found at the Morar/Glenfinnan Division boundary being absent. This implies that the stratigraphic relationship between these two divisions, which is elsewhere obscured by the thrust, is uniquely preserved on Mull. Within a local, D1-D4 sequence, D2 and D3 structures are dominant. Originally subhorizontal D2 folds are intensely curvilinear on all scales about an originally flat-lying, NNW-SSE trending stretching lineation (L2). In sections parallel to L2, D2 minor folds display a constant sense of vergence throughout the Ross-of-Mull Moine Rocks, overturning generally NW-NNW. The present day structure is dominated by the almost upright, SSW-plunging D3 Assapol synform which overprints all earlier structures. Tentative correlation of the deformation sequence with that of Inverness-shire, suggests that the D1-D2 structures of Mull, with accompanying moderate-to-high-grade metamorphism, may be Precambrian, while the D3 Assapol synform may be Caledonian. The presence of migmatites of kyanite grade means that metamorphic grade, established during MS1-MP2, is anomalously high for Moine rocks lying close to the supposed Caldonian front. This suggests that they may lie within one of the higher Caledonian thrust nappes of the North Highland Moine—possibly the Knoydart nappe, where metamorphic grade is comparable. The greenschist facies metamorphism and single phase of deformation affecting the ‘Torridonian’ rocks of Iona presents a significant contrast to the Moine rocks of the Ross of Mull. A major fault in the Sound of Iona is implied, but the Moine thrust itself probably does not outcrop.     

Resolving the contributing factors to Mississippi Delta subsidence: Past and Present

To date, quantification of individual components that contribute to shallow and deep‐seated subsidence in passive margin deltas worldwide has proven problematic. A new, regional gridded chronostratigraphic dataset for the Lower Mississippi Delta region, derived from 80,928 well reports across the northern Gulf of Mexico (GOM), has bridged the disparity between geodetic mean rates measuring total land surface subsidence across annual‐to‐decadal timescales and the deep‐seated stratigraphic subsidence rates that record isostatic response over timescales of >10⁴ years. Through a quantitative assessment of gridded chronostratigraphic surfaces, sections, and subsidence rates extending from the Middle Pleistocene (0.58 Ma) to the Late Pliocene (3.85 Ma), we identify both temporal and spatial variability in deep‐seated subsidence across the northern GOM. Targeted deep‐seated subsidence data extracted across prior GOM Holocene sea‐level sample locations have revealed more than an order of magnitude greater rates of isostatic compensation in the Mississippi depocentre versus similar GOM sea‐level control sites in Florida and Alabama, casting doubt on efforts towards a representative Holocene sea‐level curve. Spatial variability in subsidence was also assessed locally in both the strike and dip directions to assess the contributions of growth faults. Fault throw displacement magnitude was discovered to decrease with depth, accounting for less than half of the total deep‐seated subsidence record of the Middle Pleistocene. Temporal subsidence complexities were also revealed including a direct, inverse logarithmic relationship between subsidence rate and time indicating variable subsidence component controls across different timescales. Despite the spatial and temporal complexities, this dataset serves as the first regional baseline for deep‐seated subsidence rates across the northern GOM.     

A re-interpretation of the origin of sands and gravels around Banc-y-warren,near Cardigan,west Wales

The sands and gravels form part of a once more continuous succession comprising parts of three delta lobes which advanced into a deepening pro-glacial lake. Normal delta construction was accompanied by sporadic rapid mass movement of coarse sediment. This generated turbidity currents which gouged and filled remarkable channels. The deposits are cut by a very large number of normal faults which bear a strike relationship to the bedding showing they were produced by down-dip extension of the beds as a result of slope instability. The consistently southwesterly current directions, deduced from various sedimentary structures, and the constant strike orientation of faults together show that the mounds cannot be ice-contact features as was previously widely believed. The external form of the deposits is related more to subsequent erosion than to their original mode of formation.     

Decoding temporal and spatial patterns of fault uplift using transient river long profiles

We present detailed observations of rivers crossing active normal faults in the Central Apennines, Italy, where excellent constraints exist on the temporal and spatial history of fault movement. We demonstrate that rivers with drainage areas > 10 km² and crossing faults that have undergone an increase in throw rate within the last 1 My, have significant long-profile convexities. In contrast, channels that cross faults that have had a constant-slip rate for 3 My have concave-up profiles and have similar concavities and steepness indices to rivers that do not cross any active fault structures. This trend is consistent across the Central Apennines and cannot be explained by appeal to lithology or regional base level change. The data challenge the belief that active faulting must always be reflected in river profiles; instead, the long-profile convexities are best explained as a transient response of the river system to a change in tectonic uplift rate. Moreover, for these rivers we demonstrate that the height of the profile convexity, as measured from the fault, scales with the magnitude of the uplift rate increase on the fault; and we establish that this relationship holds for throw rate variation along strike for the same fault segment, as well as between faults. These findings are shown to be consistent with predictions of channel response to changing uplift rate rates using a detachment-limited fluvial erosion model, and they illustrate that analysis of the magnitude of profile convexities has considerable predictive potential for extracting tectonic information. We also demonstrate that the migration rate of the profile convexities varies from 1.5–10 mm/y, and is a function of the slip rate increase as well as the drainage area. This is consistent with n > 1 for the slope exponent in a classical detachment-limited stream-power erosion law, but could potentially be explained by incorporating an erosion threshold or an explicit role for sediment in enhancing erosion rates. Finally, we show that for rivers in extensional settings, where the response times to tectonic perturbation are long (in this case > 1 My), attempts to extract tectonic uplift rates from normalised steepness indices are likely to be flawed because topographic steady state has not yet been achieved.     

Prospects for the use of macro-algae for fuel in Ireland and the UK: An overview of marine management issues

Competing demand for land is driving biofuel and bioenergy research in various directions including macro-algae (seaweed). This paper reviews the main issues for the marine environment of cultivating and harvesting UK and Irish seaweed for biofuels/bioenergy, informed by stakeholder interviews. These showed stakeholders were sceptical of an offshore cultivation industry developing but generally considered inshore cultivation possible, while noting various practical obstacles and conditions. Views on expansion of seaweed harvesting were more divided, with research scientists being relatively cautious.     

Hydrocarbon-derived carbonate buildups of the northern Gulf of Mexico continental slope: A review of submersible investigations

Hydrocarbon-derived and microbially mediated authigenic carbonates occur over the entire depth range of the northern Gulf of Mexico slope. These carbonates consist of nodules and incipient nodules in surface sediments, hardgrounds and isolated slabs, and moundlike buildups of up to 10–20 m relief above the surrounding seafloor. The authigenic carbonates are characterized by ¹³C negative values in the range –18 to –55 (PDB) suggesting mixing of seawater carbon with¹³C-depleted carbon sources ranging from crude oil to biogenic methane. Near the shelf edge, carbonates are diluted with biogenic material produced by reefs—bioherms developed at low sea level stands. Fossil-poor carbonates over salt diapirs of the upper and middle slope formed in the shallow subsurface and have been exhumed by the combined processes of uplift and physical erosion. Middle and lower slope carbonates are generally rich in fossil shells of chemosynthetic organisms. Mg calcite pelloidal matrix and acicular to botryoidal aragonitic void-filling cements are common petrographic features of these hydrocarbonderived carbonates. At two sites carbonates are mixed with barite.     

Fault slip and identification of the second fault plane in the Varzeghan earthquake doublet

An intraplate earthquake doublet, with 11-min delay between the events, devastated the city of Varzeghan in northwestern Iran on August 11, 2012. The first Mw 6.5 strike-slip earthquake, which occurred after more than 200 years of low seismicity, was followed by an Mw 6.4 oblique thrust event at an epicentral separation of about 6 km. While the first event can be associated with a distinct surface rupture, the absence of a surface fault trace and no clear aftershock signature makes it challenging to identify the fault plane of the second event. We use teleseismic body wave inversion to deduce the slip distribution in the first event. Using both P and SH waves stabilize the inversion and we further constrain the result with the surface rupture extent and the aftershock distribution. The obtained slip pattern shows two distinct slip patches with dissimilar slip directions where aftershocks avoid high-slip areas. Using the estimated slip for the first event, we calculate the induced Coulomb stress change on the nodal planes of the second event and find a preference for higher Coulomb stress on the N-S nodal plane. Assuming a simple slip model for the second event, we estimate the combined Coulomb stress changes from the two events on the focal planes of the largest aftershocks. We find that 90% of the aftershocks show increased Coulomb stress on one of their nodal planes when the N-S plane of the second event is assumed to be the correct fault plane.     

Morphological evolution of a barchan dune migrating past an offshore wind farm foundation

As the number of manmade structures installed on the seafloor is increasing rapidly, we seek to understand the impact of these immobile obstacles on marine geomorphological processes, such as the evolution of bedforms. A 5.8 m diameter monopile foundation was installed at the case study offshore windfarm approximately 30 m ahead of an approaching barchan (crescent-shaped) dune. The impact of the monopile on the dune's evolution was analysed using six multibeam bathymetry surveys spanning 20 years. To substantiate this analysis, coupled three-dimensional numerical modelling of flows and sediment was conducted in which the scour inducing bed shear stresses were calculated from the modelled turbulent kinetic energy. Following the installation of the monopile, the mid-section of the dune accelerated and stretched in the direction of the wake of the monopile. Four years after the monopile's installation the rest of the dune had caught up, flattening out the slip face within half the dune's length downstream of the monopile. Due to the modified flow field, the dune was scoured deeply at the base of the monopile to a depth of 6.8 m (supported by the model results that predicted a scour depth exceeding 2 m over a period of just a few days). The surveyed volume of material scoured amounted to 8% of the total dune volume. Whilst the process of scouring occurs at a timescale of days to weeks, the dune migrated on average by 25 m/yr. The difference in the timescale of the two processes allowed the scouring to occur through the full thickness of the dune. The scoured dune profile recovered rapidly once the dune migrated downstream of the monopile. This article demonstrates how large geomorphological features can intercept and migrate past a monopile foundation without long-lasting impacts on the integrity of the feature or the foundation. © 2020 John Wiley & Sons, Ltd.     

The 1996 Biescas campsite disaster in the Central Spanish Pyrenees,and its temporal and spatial context

On the evening of the 7 August 1996 an intense storm occurred over the Arás catchment near Biescas in the central Pyrenees. Eighty-seven people were killed as a result of the subsequent flood, which hit a campsite located on the alluvial fan at the outlet of the 18·8 km² catchment. This paper presents the main results of a hydromorphological study of the event. The Betés subcatchment received the most intense rainfall, estimated at somewhat in excess of 250 mm, which resulted in a peak flow from this tributary of 300 m³ s⁻¹. Just downstream from the Betés river junction, flow in the main channel reached 400 m³ s⁻¹, increasing to around 500 m³ s⁻¹ further downstream. Rainfall in the larger Aso tributary was less intense, and in the head reaches flow remained within-bank, representing a one in two-year return period event. Flow from this tributary did not exceed 100 m³ s⁻¹, indicating that the Betés subcatchment supplied some 75% of the flow from just 28·7% of the catchment area. The extreme flows caused the collapse of a series of sediment trap dams in the Arás channel downstream of the Betés junction. This resulted in the addition of 68 000 m³ of sediment to an already disastrous event. Data from other rain gauges in the area showed both the extremely local nature of the event, and the problems of return period analysis for such storms, whose peaks are rarely observed at gauges. Together with the high geomorphological risks of the zone, this leads to the conclusion that a new method of spatial and temporal risk analysis is required for infrastructure planning. © 1997 John Wiley & Sons, Ltd.