Publications
129 results found
Magee C, Reeve MT, Jackson CA-L, et al., 2023, Reply to Alves et al. (2022) discussion on "Stratigraphic record of continental breakup, offshore NW Australia" by Reeve et al. (2022), Basin Research, Vol: 35, Pages: 483-486, ISSN: 0950-091X
Wrona T, Whittaker AC, Bell RE, et al., 2022, Rift kinematics preserved in deep-time erosional landscape below the northern North Sea, Basin Research, Pages: 1-18, ISSN: 0950-091X
Our understanding of continental rifting is, in large parts, derived from the stratigraphic record. This record is, however, incomplete as it does not often capture the geomorphic and erosional signal of rifting. New 3D seismic reflection data reveal a Late Permian-Early Triassic landscape incised into the pre-rift basement of the northern North Sea. This landscape, which covers at least 542 km2, preserves a drainage system bound by two major tectonic faults. A quantitative geomorphic analysis of the drainage system reveals 68 catchments, with channel steepness and knickpoint analysis of catchment-hosted palaeo-rivers showing that the landscape preserved a >2 Myr long period of transient tectonics. We interpret that this landscape records a punctuated uplift of the footwall of a major rift-related normal fault (Vette Fault) at the onset of rifting. The landscape was preserved by a combination of relatively rapid subsidence in the hangingwall of a younger fault (Øygarden Fault) and burial by post-incision sediments. As such, we show how and why erosional landscapes are preserved in the stratigraphic record, and how they can help us understand the tectono-stratigraphic evolution of ancient continental rifts.
Joffe A, Jackson CA-L, Steinberg J, et al., 2022, Origin and kinematics of a basin-scale, non-polygonal, layer-bound normal fault system in the Levant Basin, eastern Mediterranean, Basin Research, Pages: 1-30, ISSN: 0950-091X
Polygonal, layer-bound normal faults can extend over very large areas (>2,000,000 km2) of sedimentary basins. Best developed in very fine-grained rocks, these faults are thought to form during early burial in response to a range of diagenetic processes, including compaction and water expulsion. Local deviations from this idealised polygonal pattern are common; however, basin-scale, layer-bound faults with non-polygonal map view are not well-documented and accordingly, their genesis is not well understood. In this study, we use 3D seismic reflection data, biostratigraphy and well logs from the Southern Levant Basin, offshore Israel, to develop an age-constrained seismic-stratigraphic framework and determine the geometry and kinematics of such basin-scale fault system. The faults tip out downwards along an Eocene Unconformity, but unlike layer-bound faults in the Northern Levant Basin, they do not reach the base of the Messinian evaporites, instead tipping out upwards at the top Langhian. On average, the faults in the Southern Levant Basin are 6.3 km long, have an average throw of 120 m, and consistently strike NW-SE. Throw-depth plots, accompanied by thickness changes, indicate that the faults accumulated growth strata during the Late Burdigalian and are spatially and kinematically associated with a WSW-ESE-striking strike-slip fault. Unlike true polygonal faults, these faults propagated through ca. 2 km-thick sandstone-prone Oligocene-Miocene strata. Whereas previous studies from the Northern Levant Basin associate fault nucleation and growth with burial-related diagenesis, the sandstone-prone character of the Oligocene-Miocene suggests that this process cannot be readily applied to the Southern Levant Basin. Instead, we highlight potential tectonic events that occurred during and may have triggered thin-skinned extension at times of fault growth.
El-Yamani MS, John CM, Bell R, 2022, Stratigraphic evolution and karstification of a Cretaceous Mid-Pacific atoll (Resolution Guyot) resolved from core-log-seismic integration and comparison with modern and ancient analogues, Basin Research, Vol: 34, Pages: 1536-1566, ISSN: 0950-091X
Atolls are faithful recorders helping us understand eustatic variations, the evolution of carbonate production through time, and changes in magmatic hotspots activity. Several early Cretaceous Mid-Pacific atolls were previously investigated through ocean drilling, but due to the low quality of vintage seismic data available, few spatial constraints exist on their stratigraphic evolution and large-scale diagenesis. Here, we present results from an integrated core-log-seismic study at Resolution Guyot and comparison with modern and ancient analogues. We identify six seismic-stratigraphic units: (1) platform initiation with aggradation and backstepping through the Hauterivian which ended by platform emersion; (2) reflooding of the platform with progradation and aggradation through the Barremian till the early-Aptian when ocean anoxic event 1a resulted in incipient drowning; (3) platform backstepping till the mid-Aptian when the platform shifted to progradation and aggradation till the mid-Albian; (4) platform emersion; (5) reflooding with backstepping ending at the latest-Albian by platform emersion; and (6) final drowning. The stratigraphic surfaces bounding these units are coeval with some of the Cretaceous eustatic events, which suggest an eustatic control on the evolution of this atoll and confirm that several previously reported sea-level variations in the early Cretaceous are driven by eustasy. Changes in subsidence and carbonate production rates and suspected later magmatism have also impacted the stratigraphic evolution. The suspected later magmatism could lead to environmental perturbations and potentially platform demise. Contrary to previous studies, we identify two emersion events during the mid- and late-Albian which resulted in intensive meteoric dissolution and karstification. The platform margin syndepositional fractures interacted with the subaerial exposure events by focusing the dissolution which formed vertically stacked flank-margin fracture-cave s
Hughes A, Rood D, DeVecchio DE, et al., 2022, Tectonic controls on Quaternary landscape evolution in the Ventura basin, southern California, quantified using cosmogenic isotopes and topographic analyses, Geological Society of America Bulletin, Vol: 134, Pages: 2245-2266, ISSN: 0016-7606
The quantification of rates for the competing forces of tectonic uplift and erosion has important implications for understanding topographic evolution. Here, we quantify the complex interplay between tectonic uplift, topographic development, and erosion recorded in the hanging walls of several active reverse faults in the Ventura basin, southern California, USA. We use cosmogenic 26Al/10Be isochron burial dating and 10Be surface exposure dating to construct a basin-wide geochronology, which includes burial dating of the Saugus Formation: an important, but poorly dated, regional Quaternary strain marker. Our ages for the top of the exposed Saugus Formation range from 0.36 +0.18/-0.22 Ma to 1.06 +0.23/-0.26 Ma and our burial ages near the base of shallow marine deposits, which underlie the Saugus Formation, increase eastwards from 0.55 +0.08/-0.07 Ma to 3.30 +0.30/-0.42 Ma. Our geochronology is used the calculate a rapid long-term fault throw rate of 4.7–6.3 mm yr-1 since ~1.5 Ma for the San Cayetano fault and a slip rate of 1.3–3.0 mm yr-1 since ~1.5 Ma for the Oak Ridge fault, both of which agree with contemporary reverse slip rates derived from GPS data. We also calculate total cosmogenic nuclide (TCN)-derived catchment-averaged erosion rates that range from 0.18–2.21mm yr-1 and discuss the applicability of TCN-derived catchment-averaged erosion rates in rapidly-uplifting, landslide-prone landscapes. We compare patterns in erosion rates and tectonic rates to fluvial response times and geomorphic landscape parameters to show that in young, rapidly-uplifting mountain belts, catchments may attain a quasi-steady state on timescales <105 years, even if catchment-averaged erosion rates are still 34 adjusting to tectonic forcing.
Morgan JK, Solomon EA, Fagereng A, et al., 2022, Seafloor overthrusting causes ductile fault deformation and fault sealing along the Northern Hikurangi Margin, Earth and Planetary Science Letters, Vol: 593, Pages: 1-13, ISSN: 0012-821X
IODP Site U1518, drilled during IODP Expeditions 372 and 375, penetrated a large-offset (∼6 km) thrust, the Pāpaku fault, rising from a megathrust that hosts recurring slow slip events along the Hikurangi margin. Although drilling intersected the fault zone at only ∼300 m below the seafloor within porous silty mudstone, it exhibits intense tectonic ductile deformation, including finely banded mudstones contorted into decimeter-scale folds; elongate mudstone clasts with grain tail complexes; stacked and truncated silt beds in distorted mudstones; and soft sediment injections. Locally, these ductile features are overprinted by brittle deformation, including normal faults, fracture arrays, and breccias. The more consolidated hanging wall is dominated by brittle structures, whereas the footwall exhibits ductile and brittle deformation that decreases in intensity with depth. The intense tectonic ductile deformation and asymmetric distribution of structures across the fault zone at Site U1518 can be explained by seafloor overthrusting. The emplacement of the hanging wall upon the footwall flat overrode high-porosity, undeformed, and previously unburied sediments, localizing shear deformation within these weak sediments. In contrast, the overconsolidated hanging wall preferentially experienced brittle deformation during folding and displacement. Interstitial pore water geochemical profiles at Site U1518 show a repetition of near-seafloor diagenetic sequences below the fault, consistent with overthrusting of previously unburied strata. The preserved diagenetic profiles in the footwall suggest that overthrusting occurred within the last 50-100 kyr, and indicate little along- or across-fault fluid flow at the location of Site U1518. Thus the Pāpaku fault appears to define a low-permeability seal that restricts footwall consolidation, maintaining locally high pore fluid pressures and low fault strength. If similar low permeability structures occur elsewhere along the m
Leah H, Fagereng A, Bastow I, et al., 2022, The northern Hikurangi margin three-dimensional plate interface in New Zealand remains rough 100 km from the trench, Geology, Vol: 50, Pages: 1256-1260, ISSN: 0091-7613
At the northern Hikurangi margin (North Island, New Zealand), shallow slow slip events (SSEs) frequently accommodate subduction-interface plate motion from landward of the trench to <20 km depth. SSEs may be spatially related to geometrical interface heterogeneity, though kilometer-scale plate-interface roughness imaged by active-source seismic methods is only constrained offshore at <12 km depth. Onshore constraints are comparatively lacking, but we mapped the Hikurangi margin plate interface using receiver functions from data collected by a dense 22 × 10 km array of 49 broadband seismometers. The plate interface manifests as a positive-amplitude conversion (velocity increase with depth) dipping west from 10 to 17 km depth. This interface corroborates relocated earthquake hypocenters, seismic velocity models, and downdip extrapolation of depth-converted two-dimensional active-source lines. Our mapped plate interface has kilometer-amplitude roughness we interpret as oceanic volcanics or seamounts, and is 1–4 km shallower than the regional-scale plate-interface model used in geodetic inversions. Slip during SSEs may thus have different magnitudes and/or distributions than previously thought. We show interface roughness also leads to shear-strength variability, where slip may nucleate in locally weak areas and propagate across areas of low shear-strength gradient. Heterogeneous shear strength throughout the depth range of the northern Hikurangi margin may govern the nature of plate deformation, including the localization of both slow slip and hazardous earthquakes.
Lathrop B, Bell R, Jackson C, et al., 2022, Displacement/length scaling relationships for normal faults; a review, critique, and revised compilation, Frontiers in Earth Science, Vol: 10, ISSN: 2296-6463
The relationship between normal fault displacement (D) and length (L) varies due to numerous factors, including fault size, maturity, basin tectonic history, and host rock lithology. Understanding how fault D and L relate is useful, given related scaling laws are often used to help refine interpretations of often incomplete, subsurface datasets, which has implications for hydrocarbon and low-carbon energy applications. Here we provide a review of D/L scaling laws for normal faults, discuss factors that could influence these relationships, including both geological factors and errors in measurement, and provide a critique of previously published D/L databases. We then present our newly assembled database of 4059 normal faults from 66 sources that include explicit information on: 1) faultlength and displacement, 2) host rock lithology, 3) host basin tectonic history, and 4) maturity, as well as fault D and L through time when these data are available. We find an overall scaling law of D = 0.3L0.92, which is similar to previously published scaling equations and that varies in response to the aforementioned geological factors. Our data show thatsmall faults (<1 m length) tend to be over-displaced compared to larger faults, active faults tend to be over-displaced compared to inactive faults, and faults with stiffer host rock lithologies, like igneous and carbonate rocks, tend to be under-displaced with respect to faults within softer, more compliant host rocks, like clastic sedimentary rocks. Our dynamicD/L through time data show that faults follow the hybrid fault growth model, i.e., they initially lengthen, during which time they will appear under-displaced, before accumulating displacement. To the best of our knowledge, this is the first comprehensive, integrated, critical study of D/L scaling laws for normal faults and the factors influencing their growth. These revised relationships can now be utilized for predicting fault length or displacement when only one var
Pan S, Naliboff J, Bell R, et al., 2022, Bridging spatiotemporal scales of normal fault growth during continental extension using high-resolution 3D numerical models, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 23, Pages: 1-16, ISSN: 1525-2027
Continental extension is accommodated by the development of kilometer-scale normal faults, which grow during meter-scale slip events that occur over millions of years. However, reconstructing the entire lifespan of a fault remains challenging due to a lack of observational data with spatiotemporal scales that span the early stage (<106 yrs) of fault growth. Using three-dimensional numerical simulations of continental extension and novel methods for extracting the locations of faults, we quantitatively examine the key factors controlling the growth of rift-scale fault networks over 104–106 yrs. Early formed faults (<100 kyrs from initiation) exhibit scaling ratios consistent with those characterizing individual earthquake ruptures, before evolving to be geometrically and kinematically similar to more mature structures developed in natural fault networks. Whereas finite fault lengths are rapidly established (<100 kyrs), active deformation is transient, migrating both along- and across-strike. Competing stress interactions determine the distribution of active strain, which oscillates between being distributed and localized. Higher rates of extension (10 mm yr−1) lead to more prominent stress redistributions through time, promoting episodic localized slip events. Our findings demonstrate that normal fault growth and the related occurrence of cumulative slip is more complex than that currently inferred from displacement patterns on now-inactive structures, which only provide a space- and time-averaged picture of fault kinematics and related seismic hazard.
Leah H, Fagereng A, Bastow I, et al., 2022, The northern Hikurangi margin 3D plate interface remains rough 100 km from the trench, Geology (Boulder), ISSN: 0091-7613
t the northern Hikurangi margin (North Island, New Zealand), shallow slow slip events (SSEs) frequently accommodate subduction interface plate motion from landward of the trench to <20 km depth. SSEs may be spatially related to geometrical interface heterogeneity, though km-scale plate interface roughness imaged by active-source seismic methods is only constrained offshore at <12 km depth. Onshore constraints are comparatively lacking, but here we map the Hikurangi margin plate interface using receiver functions from data collected by a dense 22 x10 km array of 49 broadband seismometers. The plate interface manifests as a positive-amplitude conversion (velocity increase with depth) dipping west from 10-17 km depth. This interface corroborates relocated earthquake hypocenters, seismic velocity models, and downdip extrapolation of depth-converted 2D active-source lines.
Redpath D, Jackson CA, Bell RE, 2022, Mechanical stratigrpahy controls normal fault growth and dimensions, outer Kwanza basin, offshore Angola, Tectonics, Vol: 41, ISSN: 0278-7407
Mechanical stratigraphy controls the growth patterns and dimensions of relatively small normal faults, yet how it influences the development of much larger structures remains unclear. Here, we use 3D seismic reflection data from the Outer Kwanza Basin, offshore Angola to constrain the geometry and kinematics of several normal faults formed in a deep-water clastic succession. The faults are up to 6.3-km long and 1.9-km tall and have up to 44 m of throw. Aspect ratios and lower-tip throw gradients are greater for faults that terminate downward at a c. 100 m thick, mass-transport complex (MTC; up to 5.2 and 0.12) than for those that offset it (up to 2.7 and 0.01). Faults that offset the MTC invariably have >30 m of throw. Based on their geometric properties and throw patterns, we interpret that the faults nucleated above the MTC and propagated down toward it. Upon encountering this unit, which we infer behaved in a more ductile manner than encasing strata, tip propagation was halted until tip stresses were sufficiently high (corresponding to minimum throw of c. 30 m) to breach it. Faults with smaller throw were unable to breach the MTC. We argue that using only geometric criteria to determine fault growth patterns can mask the significant control mechanical stratigraphy has on fault kinematics. Mechanical stratigraphy is therefore a key control on the growth of large, seismic-scale normal faults, in a similar way to that observed for far smaller structures
Reeve MT, Magee C, Jackson CA-L, et al., 2022, Stratigraphic record of continental breakup, offshore NW Australia, BASIN RESEARCH, Vol: 34, Pages: 1220-1243, ISSN: 0950-091X
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Alghuraybi A, Bell R, Jackson C, 2022, Role of normal fault growth in controlling sealing juxtaposition relationships, SW Barents Sea, offshore Norway
Faults prevent fluid migration either by juxtaposing sealing lithologies against non-sealing ones, or by forming fine-grained impermeable fault rocks. Fault juxtaposition geometries, stratigraphic architectures and sedimentological flow properties represent the essential building blocks of subsurface flow models. These components are often associated with significant uncertainty especially in areas of poor seismic quality. Fault growth patterns can considerably influence the development of fault geometries, stratigraphic architectures, and facies distribution within fault-bounded depocenters. Therefore, analyzing fault growth histories and accounting for them in subsurface fluid flow modelling workflows can help mitigate some of the uncertainty in these models and increase their predictability. Here, we use age-constrained 3D seismic reflection and borehole data to analyze two faults from the SW Barents Sea, offshore northern Norway, one of which appears to have grown in accordance with the propagating fault model, whereas the other has geometric and kinematic properties more consistent with the constant-length model. Our study shows that despite having developed in the same basin and having similar tectonic origins, local variation in nucleation time and strain localization can lead to the development of fault systems with diverse kinematic histories, which in turns controls the formation and distribution of favorable sealing juxtaposition relationships.
Alghuraybi A, Bell RE, Jackson CA-L, 2021, The geometric and temporal evolution of fault-related folds constrain normal fault growth patterns, Barents Sea, offshore Norway, Basin Research, Vol: 34, ISSN: 0950-091X
Extensional growth folds form ahead of the tips of propagating normal faults. These folds can accommodate a considerable amount of extensional strain and they may control rift geometry. Fold-related surface deformation may also control the sedimentary evolution of syn-rift depositional systems. Thus, by examining the stratigraphic record, we can constrain the four-dimensional evolution of extensional growth folds, which in turn provides a record of fault growth and broader rift history. Here, we use high-quality 3D seismic reflection and borehole data from the SW Barents Sea, offshore northern Norway to determine the geometric and temporal evolution of extensional growth folds associated with a large, long-lived, basement-rooted fault. We show that the fault grew via the linkage of four segments, and that fault growth was associated with the formation of fault-parallel and fault-perpendicular folds that accommodated a substantial portion (10%–40%) of the total extensional strain. Several periods of fault-propagation folding occurred in response to the periodic burial of the fault, with individual folding events (ca. 25 and 32 Myr) lasting a considered part of the ca. 130 Myr rift period. Our study supports previous suggestions that continuous (i.e. folding) as well as discontinuous (i.e. faulting) deformation must be explicitly considered when assessing total strain in an extensional setting. We also show that changes in the architecture of growth strata record alternating periods of folding and faulting and that the margins of rift-related depocentres may be characterised by basinward-dipping monoclines as opposed to fault-bound scarps. Our findings have broader implications for our understanding of the structural, physiographic and tectonostratigraphic evolution of rift basins.
Davy R, Frahm L, Bell R, et al., 2021, Generating high‐fidelity reflection images directly from full‐waveform inversion: Hikurangi Subduction Zone case study, Geophysical Research Letters, Vol: 48, Pages: 1-10, ISSN: 0094-8276
Full-waveform inversion (FWI) can resolve subsurface physical properties to high resolutions, yet high-performance computing resources have only recently made it practical to invert for high frequencies. A benefit of high-frequency FWI is that recovered velocity models can be differentiated in space to produce high-quality depth images (FWI images) of a comparable resolution to conventional reflection images.Here, we demonstrate the generation of high-fidelity reflection images directly from the FWI process. We applied FWI up to 38 Hz to seismic data across the Hikurangi subduction margin. The resulting velocity models and FWI images reveal a complex faulting system, sediment deformation, and bottom-simulating reflectors within the shallow accretionary prism. Our FWI images agree with conventional reflection images and better resolve horizons around the Pāpaku thrust fault. Thus, FWI imaging has the potential to replace conventional reflection imaging whilst also providing physical property models that assist geological interpretations.
Pan S, Bell RE, Jackson CA-L, et al., 2021, Evolution of normal fault displacement and length as continental lithosphere stretches, Basin Research, Vol: 34, Pages: 121-140, ISSN: 0950-091X
Continental rifting is accommodated by the development of normal fault networks. Fault growth patterns control their related seismic hazards, and the tectonostratigraphic evolution and resource and CO2 storage potential of rifts. Our understanding of fault evolution is largely derived by observing the final geometry and displacement (D)-length (L) characteristics of active and inactive fault arrays, and by subsequently inferring their kinematics. We can rarely determine how these geometric properties change through time, and how the growth of individual fault arrays relate to the temporal evolution of their host networks. Here we use 3D seismic reflection and borehole data from the Exmouth Plateau, NW Shelf, Australia to determine the growth of rift-related, crustal-scale fault arrays and networks over geological timescales (>106 Ma). The excellent-quality seismic data allows us to reconstruct the entire Jurassic-to-Early Cretaceous fault network over a relatively large area (ca. 1,200 km2). We find that fault trace lengths were established early, within the first ca. 7.2 Myr of rifting, and that along-strike migration of throw maxima towards the centre of individual fault arrays occurred after ca. 28.5 Myr of rifting. Faults located in stress shadows become inactive and appear under-displaced relative to adjacent larger faults, onto which strain localises as rifting proceeds. This implies that the scatter frequently observed in D-L plots can simply reflect fault growth and network maturity. We show that by studying complete rift-related normal networks, rather than just individual fault arrays, we can better understand how faults grow and more generally how continental lithosphere deforms as it stretches.
Shmela AK, Paton DA, Collier RE, et al., 2021, Normal fault growth in continental rifting: insights from changes in displacement and length fault populations due to increasing extension in the Central Kenya Rift, Tectonophysics, Vol: 814, ISSN: 0040-1951
This study examines the scaling relationship between fault length and displacement for the purpose of gaining a better understanding of the evolution of normal faults within the central Kenya Rift. 620 normal faults were manually mapped from a digital elevation model (DEM), with 30 m2 resolution and an estimated maximum displacement of ~40–~6030 m and fault lengths of 1270 ‐ 60,600 m. To assess the contribution of fault populations to the strain accommodation from south to north, the study area has been divided into three zones of fault populations based upon their average fault orientations; zone 1 in the north is dominated by NNE striking faults, zone 2 in the centre of the rift is characterised by NNW to NNE fault trends, whereas zone 3 in the south is characterised by NNW striking fault systems. Extensional strain was estimated by summing fault heaves across six transects along the rift, which showed a progressive increase of strain from south to north. The fault length and displacement data in the three zones fit to a power law distribution. The cumulative distributions of fault length populations showed similar fractal dimension (D) in the three zones. The cumulative displacement distributions for the three zones showed a decrease in the Power-law fractal dimension with increasing strain, which implies that the strain is increasingly localized onto larger faults as the fault system becomes more evolved from south to north. Increasing displacement with increasing strain while the fault length remains almost constant may indicate that the fault system could be evolving in accordance with a constant length fault growth model, where faults lengthen quickly and then accrue displacement. Results of this study suggest that the process of progressively increasing fault system maturity and strain localization onto large faults can be observed even over a relatively small area (240 × 150 km) within the rift system. It is also suggested that patterns of fault
Merry T, Bastow I, Kounoudis R, et al., 2021, The influence of the North Anatolian Fault and a fragmenting slab architecture on upper mantle seismic anisotropy in the eastern Mediterranean, G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences, Vol: 22, Pages: 1-26, ISSN: 1525-2027
The eastern Mediterranean hosts, within the span of a few hundred kilometers, extensional, strike-slip, and collision tectonics above a set of fragmenting subducting slabs. Slab roll-back, toroidal flow, and lithospheric dripping/delamination processes are also believed to be operating. Associated asthenospheric flow and lithospheric de formation are expected to manifest as seismic anisotropy, measurable via study of SKS shear wave splitting. Surprisingly, previous SKS splitting investigations have resolved only long wavelength patterns of anisotropy in the region, interpreting them as large scale asthenospheric flow; moreover, no anisotropic signature has been associated with the North Anatolian Fault (NAF), unlike other major strike-slip plate boundaries world wide. We present a 29-year record of SKS splitting observations, revealing hitherto unrecognized short-length-scale variations in anisotropy, and backazimuthal variations of splitting parameters that attest to multi-layered anisotropy. Lithospheric anisotropy beneath the NAF exhibits fast directions either fault-parallel or intermediate between the principle extensional strain rate axis and fault strike, diagnostic of a relatively low strained transcurrent mantle shear zone. Elsewhere, anisotropy is consistent with as thenospheric flow through tomographically-imaged slab gaps, and driven by Hellenic trench retreat. Evidence for westward flow of asthenosphere driving Anatolian plate motion is lacking. Shorter splitting delay times and nulls in central Anatolia suggest weaker azimuthal anisotropy in the asthenosphere, supporting models that invoke ver tical mantle flow patterns (lithospheric dripping/asthenospheric upwelling). Thus, we conclude that the signal of mantle anisotropy more closely reflects the lithospheric de formation, complex slab architecture and geodynamic diversity of the region than pre36 viously recognized.
Rodriguez CR, Jackson CA-L, Bell RE, et al., 2021, Deep-water reservoir distribution on a salt-influenced slope, Santos Basin, offshore Brazil, AAPG BULLETIN, Vol: 105, Pages: 1679-1720, ISSN: 0149-1423
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Wrona T, Pan I, Bell RE, et al., 2021, 3D seismic interpretation with deep learning: A brief introduction, The Leading Edge, Vol: 40, Pages: 524-532, ISSN: 1070-485X
Understanding the internal structure of our planet is a fundamental goal of the earth sciences. As direct observations are restricted to surface outcrops and borehole cores, we rely on geophysical data to study the earth's interior. In particular, seismic reflection data showing acoustic images of the subsurface provide us with critical insights into sedimentary, tectonic, and magmatic systems. However, interpretations of these large 2D grids or 3D seismic volumes are time-consuming, even for a well-trained person or team. Here, we demonstrate how to automate and accelerate the analysis of these increasingly large seismic data sets with machine learning. We are able to perform typical seismic interpretation tasks such as mapping tectonic faults, salt bodies, and sedimentary horizons at high accuracy using deep convolutional neural networks. We share our workflows and scripts, encouraging users to apply our methods to similar problems. Our methodology is generic and flexible, allowing an easy adaptation without major changes. Once trained, these models can analyze large volumes of data within seconds, opening a new pathway to study the processes shaping the internal structure of our planet.
Lathrop BA, Jackson CA-L, Bell RE, et al., 2021, Normal Fault Kinematics and the Role of Lateral Tip Retreat: An Example From Offshore NW Australia, TECTONICS, Vol: 40, ISSN: 0278-7407
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Reeve MT, Magee C, Bastow ID, et al., 2021, Nature of the cuvier abyssal plain crust, offshore NW Australia, Journal of the Geological Society, Vol: 178, Pages: 1-17, ISSN: 0016-7649
Magnetic stripes have long been assumed to be indicative of oceanic crust. However, continental crust heavily intruded by magma can also record magnetic stripes. We re-evaluate the nature of the Cuvier Abyssal Plain (CAP), offshore NW Australia, which hosts magnetic stripes and has previously been defined as oceanic crust. We show that chemical data from a basalt within the CAP, previously described as an enriched mid-ocean ridge basalt, could equally be interpreted to contain evidence of contamination by continental material. We also recognize seaward-dipping reflector sequences in seismic reflection data across the CAP. Borehole data from overlying sedimentary rocks suggests that these seaward-dipping reflectors were emplaced in a shallow water (<200 m depth) or subaerial environment. Our results indicate that the CAP may not be unambiguous oceanic crust, but may instead consist of a spectrum of heavily intruded continental crust through to fully oceanic crust. If the CAP represents such a continent–ocean transition zone, then the adjacent unambiguous oceanic crust would be located >500 km further offshore NW Australia than currently thought. This would impact plate tectonic reconstructions, as well as heat flow and basin modelling studies. Our work also supports the growing consensus that magnetic stripes cannot, by themselves, be used to determine crustal affinity.
Elyamani M, John CM, Bell RE, 2021, SENSITIVITY ANALYSIS OF THE AMPLITUDE OF EARLY CRETACEOUS EUSTATIC CHANGES USING FORWARD STRATIGRAPHIC MODELLING (RESOLUTION GUYOT), Pages: 2332-2336
Multiple studies focused on eustatic changes during the Cretaceous as an example of greenhouse world. Most of these studies were performed in local areas. These sea level estimates might be derived from localised effects and therefore reflect relative sea level changes rather than eustasy. Based on that, sensitivity analysis to test the applicability of using the Cretaceous ESL curves of Rohl & ogg (1996), Sahagian et al. (1996), Hardenbol et al. (1998), and Haq (2014) is crucial to validate or refute them. To do that, forward stratigraphic modelling of one of the Mid-Pacific mountain guyots, Resolution Guyot, is performed. The study area is unique as it represents deposition of Cretaceous carbonates (growing at sea-level) on an isolated volcanic island away from the influence of continents and tectonic activity. The initial results show that Haq (2014) ESL curve wasn’t perfectly fitting some of our constraints, and some of the cycles need finer subdivision. The outcomes of this study will constrain the fluctuations of ESL in the Cretaceous and serve as a test to whether the amplitude and timing of regionally-derived eustatic curves are valid for other locations, or whether these curves are too influenced by specific local conditions in the areas.
Watkins SE, Whittaker AC, Bell RE, et al., 2020, Straight from the source's mouth: Controls on field‐constrained sediment export across the entire active Corinth Rift, central Greece, Basin Research, Vol: 32, Pages: 1600-1625, ISSN: 0950-091X
The volume and grain‐size of sediment supplied from catchments fundamentally control basin stratigraphy. Despite their importance, few studies have constrained sediment budgets and grain‐size exported into an active rift at the basin scale. Here, we used the Corinth Rift as a natural laboratory to quantify the controls on sediment export within an active rift. In the field, we measured the hydraulic geometries, surface grain‐sizes of channel bars and full‐weighted grain‐size distributions of river sediment at the mouths of 47 catchments draining the rift (constituting 83% of the areal extent). Results show that the sediment grain‐size increases westward along the southern coast of the Gulf of Corinth, with the coarse‐fraction grain‐sizes (84th percentile of weighted grain‐size distribution) ranging from approximately 19 to 91 mm. We find that the median and coarse‐fraction of the sieved grain‐size distribution are primarily controlled by bedrock lithology, with late Quaternary uplift rates exerting a secondary control. Our results indicate that grain‐size export is primarily controlled by the input grain‐size within the catchment and subsequent abrasion during fluvial transport, both quantities that are sensitive to catchment lithology. We also demonstrate that the median and coarse‐fraction of the grain‐size distribution are predominantly transported in bedload; however, typical sand‐grade particles are transported as suspended load at bankfull conditions, suggesting disparate source‐to‐sink transit timescales for sand and gravel. Finally, we derive both a full Holocene sediment budget and a grain‐size‐specific bedload discharged into the Gulf of Corinth using the grain‐size measurements and previously published estimates of sediment fluxes and volumes. Results show that the bedload sediment budget is primarily comprised (~79%) of pebble to cobble grade (0.475–16 cm). Our results suggest that the grain‐size of sediment export at the rift scale is particularly
Arai R, Kodaira S, Henrys S, et al., 2020, Three‐dimensional P wave velocity structure of the Northern Hikurangi margin from the NZ3D experiment: evidence for fault‐bound anisotropy, Journal of Geophysical Research: Solid Earth, Vol: 125, Pages: 1-20, ISSN: 2169-9313
We present a high‐resolution three‐dimensional (3‐D) anisotropic P wave velocity (Vp) model in the northern Hikurangi margin offshore Gisborne, New Zealand, constructed by tomographic inversion of over 430,000 first arrivals recorded by a dense grid of ocean bottom seismometers. Since the study area covers a region where shallow slow slip events (SSEs) occur repeatedly and the subduction of a seamount is proposed, it offers an ideal location to link our understanding of structural and hydrogeologic properties at megathrust faults to slip behavior. The Vp model reveals an ~30‐km‐wide, low‐velocity accretionary wedge at the toe of the overriding plate, where previous seismic reflection studies show a series of active thrust faults branching from the plate interface. We find some locations with significant Vp azimuthal anisotropy >5% near the branching faults and the deformation front. This finding suggests that the anisotropy is not ubiquitous and homogeneous within the overriding plate, but more localized in the vicinity of active thrust faults. The fast axes of Vp within the accretionary wedge are mostly oriented to the plate convergence direction, which is interpreted as preferentially oriented cracks in a compressional stress regime associated with plate subduction. We find that the magnitudes of anisotropy are roughly equivalent to values found at oceanic spreading centers, where the extensional stress regime is dominant and the crack density is expected to be higher than subduction zones. This consideration may indicate that additional effects such as fault foliation and clay mineral alignment also contribute to upper plate anisotropy along subduction margins.
Fazlikhani H, Aagotnes SS, Refvem MA, et al., 2020, Strain migration during multiphase extension, Stord Basin, northern North Sea rift, Basin Research, Vol: 33, Pages: 1474-1496, ISSN: 0950-091X
In regions experiencing multiple phases of extension, rift-related strain can vary along and across the basin during and between each phase, and the location of maximum extension can differ between the rift phase. Despite having a general understanding of multiphase rift kinematics, it remains unclear why the rift axis migrates between extension episodes. The role pre-existing structures play in influencing fault and basin geometries during later rifting events is also poorly understood. We study the Stord Basin, northern North Sea, a location characterised by strain migration between two rift episodes. To reveal and quantify the rift kinematics, we interpreted a dense grid of 2D seismic reflection profiles, produced time-structure and isochore (thickness) maps, collected quantitative fault kinematic data and calculated the amount of extension (β-factor). Our results show that the locations of basin-bounding fault systems were controlled by pre-existing crustal-scale shear zones. Within the basin, Permo-Triassic Rift Phase 1 (RP1) faults mainly developed orthogonal to the E-W extension direction. Rift faults control the locus of syn-RP1 deposition, whilst during the inter-rift stage, areas of clastic wedge progradation are more important in controlling sediment thickness trends. The calculated amount of RP1 extension (β-factor) for the Stord Basin is up to β = 1.55 (±10%, 55% extension). During the subsequent Middle Jurassic-Early Cretaceous Rift Phase 2 (RP2), however, strain localised to the west along the present axis of the South Viking Graben, with the Stord Basin being almost completely abandoned. Rift axis migration during RP2 is interpreted to be related to changes in lithospheric strength profile, possibly related to the ultraslow extension (<1 mm/year during RP1), the long period of tectonic quiescence (ca. 50 myr) between RP1 and RP2 and possible underplating. Our results highlight the very heterogeneous nature of temporal and lat
Cook AE, Paganoni M, Clennell MB, et al., 2020, Physical properties and gas hydrate at a near‐seafloor thrust fault, hikurangi margin, New Zealand, Geophysical Research Letters, Vol: 47, Pages: 1-11, ISSN: 0094-8276
The Pāpaku Fault Zone, drilled at International Ocean Discovery Program (IODP) Site U1518, is an active splay fault in the frontal accretionary wedge of the Hikurangi Margin. In logging‐while‐drilling data, the 33‐m‐thick fault zone exhibits mixed modes of deformation associated with a trend of downward decreasing density, P‐wave velocity, and resistivity. Methane hydrate is observed from ~30 to 585 m below seafloor (mbsf), including within and surrounding the fault zone. Hydrate accumulations are vertically discontinuous and occur throughout the entire logged section at low to moderate saturation in silty and sandy centimeter‐thick layers. We argue that the hydrate distribution implies that the methane is not sourced from fluid flow along the fault but instead by local diffusion. This, combined with geophysical observations and geochemical measurements from Site U1518, suggests that the fault is not a focused migration pathway for deeply sourced fluids and that the near‐seafloor Pāpaku Fault Zone has little to no active fluid flow.
Claringbould JS, Bell RE, Jackson CA, et al., 2020, Pre‐breakup extension in the northern North Sea defined by complex strain partitioning and heterogeneous extension rates, Tectonics, Vol: 39, Pages: 1-29, ISSN: 0278-7407
The early stages of continental rifting are accommodated by the growth of upper‐crustal normal fault systems that are distributed relatively evenly across the rift width. Numerous fault systems define fault arrays , the kinematics of which are poorly understood due to a lack of regional studies drawing on high‐quality subsurface data. Here we investigate the long‐term (~150 Myr) growth of a rift‐related fault array in the East Shetland Basin, northern North Sea, using a regionally extensive subsurface dataset comprising 2D and 3D seismic reflection surveys and 107 boreholes. We show that rift‐related strain during the pre‐Triassic‐to‐Middle Triassic was originally distributed across several sub‐basins. The Middle‐to‐Late Triassic saw a decrease in extension rate (~14 m/Myr) as strain localized in the western part of the basin. Early Jurassic strain initially migrated eastwards, before becoming more diffuse during the main, Middle‐to‐Late Jurassic rift phase. The highest extension rates (~89 m/Myr) corresponded with the main rift event in the East Shetland Basin, before focusing of strain within the rift axis and ultimate abandonment of the East Shetland Basin in the Early Cretaceous. We also demonstrate marked spatial variations in timing and magnitude of slip along‐strike of major fault systems during this protracted rift event. Our results imply that strain migration patterns and extension rates during the initial, pre‐breakup phase of continental rifting may be more complex than previously thought; this reflects temporal and spatial changes in both thermal and mechanical properties of the lithosphere, in addition to varying extension rates.
Harold L, Fagereng A, Meneghini F, et al., 2020, Mixed brittle and viscous strain localisation in pelagic sediments seaward of the Hikurangi margin, New Zealand, Tectonics, Vol: 39, ISSN: 0278-7407
Calcareous‐pelagic input sediments are present at several subduction zones and deform differently to their siliciclastic counterparts. We investigate deformation in calcareous‐pelagic sediments drilled ~20 km seaward of the Hikurangi megathrust toe at Site U1520 during IODP Expeditions 372 and 375. Clusters of normal faults and subhorizontal stylolites in the sediments indicate both brittle faulting and viscous pressure solution operated at <850 m below sea floor. Stylolite frequency and vertical shortening estimated using stylolite mass loss, porosity change, and distribution increase with carbonate content. We then use U1520 borehole data to constrain a P‐T‐t history for the sediments, and apply an experimentally‐derived pressure solution model to compare with strains calculated from stylolites. Modelled strains fail to replicate stylolite‐hosted strain distribution or magnitude, but comparison shows porosity, composition, and grain‐scale effects in diffusivity and mass transfer pathway width likely exert a strong influence on pressure solution localisation and strain rate. Stylolite and fault clusters concentrate clay in these sediments, creating weak volumes of clay within carbonates, that may localise slip where the plate interface intersects the carbonates at <5 km depth. Plate interface slip character and rheology will be influenced by the deformation of intermixed phyllosilicates and calcite, occurring by variably‐stable frictional slip and pressure solution of calcite. Pressure solution of calcite is therefore important at the shallow plate interface, waning at the base of the slow‐slipping zone because calcite solubility is low at temperatures > 150°C where frictional (possibly seismic) slip likely predominates.Plain Language SummaryThe type of sediments entering subduction zones will influence the way the plates in the subduction zone slide past one another. We looked at limestones in sediments drilled before they reach the subduction zone an
Hughes A, Bell RE, Mildon ZK, et al., 2020, Three‐dimensional structure, ground rupture hazards, and static stress models for complex non‐planar thrust faults in the Ventura basin, southern California, Journal of Geophysical Research: Solid Earth, Vol: 125, ISSN: 2169-9313
To investigate the subsurface geometry of a recently discovered, seismically‐active fault in the Ventura basin, southern California, USA, we present a series of cross sections and a new three‐dimensional fault model across the Southern San Cayetano fault (SSCF) based on integration of surface data with petroleum industry well‐log data. Additionally, the fault model for the SSCF, along with models of other regional faults extracted from the Southern California Earthquake Center three‐dimensional Community Fault Model, are incorporated in static Coulomb stress modeling to investigate static Coulomb stress transfer between thrust faults with complex geometry and to further our understanding of stress transfer in the Ventura basin. The results of the subsurface well investigation provide evidence for a low‐angle SSCF that dips ~15° north and connects with the western section of the San Cayetano fault around 1.5–3.5 km depth. We interpret the results of static Coulomb stress models to partly explain contrasting geomorphic expression between different sections of the San Cayetano fault and a potential mismatch in timings between large‐magnitude uplift events suggested by paleoseismic studies on the Pitas Point, Ventura, and San Cayetano faults. In addition to new insights into the structure and potential rupture hazard of a recently discovered active reverse fault in a highly populated area of southern California, this study provides a simple method to model static Coulomb stress transfer on complex geometry faults in fold and thrust belts.
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