Fault Populations Research Articles

Cenozoic structure of the Zhu 3 Depression in the Pearl River Mouth Basin and its response to the dynamic background of the South China Sea

The Zhu 3 Depression is located at the transition between the South China Sea and the South China Block. The Cenozoic structure reflects the dynamic background of the South China Sea. We used 3D seismic and log data to study the structural evolution of the Zhu 3 Depression. We defined the characteristics of Cenozoic rift deformation and the migration of the depocentre based on fault activity and the distribution of sediment thickness. The spatial and temporal differences in the Cenozoic structure of the Zhu 3 Depression are due to the influence of pre-existing faults and regional stresses. Our results show that there are three populations of faults in the Zhu 3 Depression, striking NE, east–west and NW. The NE-striking faults are mainly large-scale boundary faults. The east–west-striking faults are small and were activated in the later stages of deformation. The NW-striking faults are continuous at depth, but are en echelon and parallel to each other at shallow depths. Based on these results, we suggest that the NE- and NW-striking pre-existing faults divide the Zhu 3 Depression into different structural zones and controlled the differences in deformation in the basin. The regional stress direction, which is controlled by regional plate interactions and the Red River Fault Zone, changed clockwise from NW-trending during the Paleocene to NE-trending during the Mid-Miocene. These findings will contribute to a better understanding of the evolution of the entire South China Sea. Thematic collection: This article is part of the Emerging knowledge on the tectonics of the South China Sea collection available at: https://www.lyellcollection.org/topic/collections/south-china-sea

Cenozoic morphotectonic evolution of the eastern edge of Paraná Sedimentary Basin, southern Brazil

The Paraná Sedimentary Basin (PSB) is one of the most important geotectonic structures on the South American continent and its tectono-sedimentary evolution has been widely investigated, especially during the Paleozoic and Mesozoic. However, the post-rift Cenozoic dynamics need further studies to fully understand its evolution. Hence, this research aims to add data and interpretations regarding the morphostructural features inherent in the Cenozoic tectonic framework, especially in the south/southeast region of Brazil. Here, we explore the Cenozoic processes of tectonic reactivation of preexisting structural features, as well as the neoformation of structural features, considering that Mesozoic sedimentary and volcanic strata of the Serra Geral Group in the Paraná Basin were deformed by Cenozoic joints and faults. For this purpose, structural and geomorphological (morphometric) analyses were used in the Itararé and Cinzas rivers catchments, on the eastern edge of the PSB. The study area is part of the Ponta Grossa Arch domain, in the northeast of Paraná state and southwest of São Paulo state. The structural data collected, when associated with the morphometric analyses (qualitative and quantitative), were compatible, showing the influence of tectonic structuring on the geomorphological framework of the investigated watersheds, resulting in the morphotectonic compartmentalization of the surveyed area. Alterations to the topographic surface, asymmetry patterns in the watersheds, disturbances in the longitudinal river profiles and anomalies in the drainage network patterns are the result of reactivation of ancient NE-SW and NW-SE faults in the crystalline basement and present in the units that fill the sedimentary basin, as well as new families of faults, mainly in N–S and E-W direction. The structural analysis supported the determination of the Cenozoic paleostress fields, defined from the analysis of the fault population, spatial arrangement and through statistical and mechanical treatment of the kinematic structural data. The results of the paleostress tensors indicate a landscape evolution associated with at least four tectonic pulses, which compressive SHmax values are oriented in NE-SW, N–S, NW-SE and E-W directions, respectively. These are in agreement with the main regional scientific studies in southern and southeastern Brazil, which involve tectonic events from the Upper Cretaceous and Paleogene, advancing into the Neogene and up to at least the Pleistocene.

Normal fault damage zone growth in map view from analogue models

Better understanding of stress perturbations, strain propagation and fluid flow in the upper crust require characterisation of fault damage zone evolution. Outcrop studies help understand the different processes operating during fault movement; but capture little about the evolution through time. In this study, we investigate damage zone evolution using high-resolution analogue modelling; simulating the growth of a normal fault population. The incremental strain reveals that early deformation stages occur by strain localisation into corridors of distributed deformation. Active deformation within these corridors becomes narrower as segment linkage occurs, leading to the formation of master faults, encouraging further localisation of incremental strain. This results in wide zones of cumulative strain around the fault, accommodated by different types of fault damage geometry, which formed at very different stages of the fault system growth. We also highlight the new concept of “fault system damage”, that shows similarities with observations in nature. These first descriptions of the fault damage evolution allow an understanding of the composite content of damage zones. This gives support to assess zones of permeability enhancement in naturally fractured reservoirs, and proposes a view of damage distribution for targeting fluid flow, geothermal resources, leaks, micro/macro seismicity and mechanical properties of fault zones.

How Do Rift‐Related Fault Network Distributions Evolve? Quantitative Comparisons Between Natural Fault Observations and 3D Numerical Models of Continental Extension

AbstractContinental extension is primarily accommodated by the evolution of normal fault networks. Rifts are shaped by complex tectonic processes and it has historically been difficult to determine the key rift controls using only observations from natural rifts. Here, we use 3D thermo‐mechanical, high‐resolution (<650 m) forward models of continental extension to investigate how fault network patterns vary as a function of key rift parameters, including extension rate, the magnitude of strain weakening, and the distribution and magnitude of initial crustal damage. We quantitatively compare modeled fault networks with observations of fault patterns in natural rifts, finding key similarities in their along‐strike variability and scaling distributions. We show that fault‐accommodated strain summed across the entire 180 × 180 km study area increases linearly with time. We find that large faults do not abide by power‐law scaling as they are limited by an upper finite characteristic, ω0. Fault weakening, and the spatial distribution of initial plastic strain blocks, exert a key control on fault characteristics. We show that off‐fault (i.e., non‐fault extracted) deformation accounts for 25%–45% of the total extensional strain, depending on the rift parameters. As fault population statistics produce distinct characteristics for our investigated rift parameters, further numerical and observational data may enable the future reconstruction of key rifting parameters through observational data alone.

The significance of volcanic segments and rifts in faults characterization within the Amagmatic graben of the Afar Depression, Ethiopia

The Dobi graben is a northwestern trending, continental rift situated in the East Central Block (ECB) of the Afar Depression (AD), Ethiopia. Ongoing extensional rifting in the graben is evident from the swarm of intermediate magnitude earthquakes (5.7 < Ms < 6.3) in 1989. The graben's extension occurs on steeply dipping faults, where the maximum displacement and traced fault length spans four orders of magnitude. Using a 30 m resolution Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM), we conducted a fault population analysis in the Dobi graben. We traced 953 faults and conducted a size-frequency distribution analysis has been conducted for various spatially heterogeneous structural zones. Our results show that the frequency size distribution revealed a negative exponential fitting trend, indicating strong strain distribution within the Imbrication, the Fault Termination zone, and the active axial graben floor. However, a power law size distribution dominates most first-order border faults, suggesting that the strain is localized mainly at the graben flanks.The fault displacement-length profiles demonstrate that approximately 48% of the total fault traced lengths exhibit increasing slip rates towards the southeast, while about 40% display increasing slip rates towards the northwest. These suggest that ∼88% of the lateral propagation of the 953 faults in the Dobi graben is governed by the regional differential strain transfer of the Red Sea Rift (RSR) and Gulf of Aden Rift (GAR) to the central Afar. Most of the hmax/L aspect ratio of the Dobi graben's faults fits in Category II, which exhibits a constant hmax/L ratio, meaning that the hmax/L ratio increases with fault growth, presumably due to the graben's is in an active tectonic region. This implies that these faults are becoming more efficient at accommodating deformation as they grow. Additionally, the normalized average maximum displacement over maximum length (Dmax/Lmax) ratio for most faults is 0.03, which is in accord with the constant displacement length fault growth model.

Re‐Examining Temporal Variations in Intermediate‐Depth Seismicity

AbstractChanges in the frequency of intermediate‐depth (60–300 km) earthquakes in response to static stress transfer can provide insights into the mechanisms of earthquake generation within subducting slabs. In this study, we use the most up‐to‐date global and regional earthquake catalogs to show that both the aftershock productivity of large earthquakes, and the changes in the frequency of intermediate‐depth earthquakes around the timing of major megathrust slip, support the view that faults within the slab are relatively insensitive to static stress transfer on the order of earthquake stress drops. We interpret these results to suggest the population of faults within the slab are much further from their failure stress than is typical for shallow fault systems. We also find that aftershock productivity varies within slabs over small spatial scales, indicating that the mechanism that enables faults to rupture at intermediate depths is likely to be spatially heterogeneous over length‐scales of a few tens of kilometres. We suggest dehydration‐related weakening mechanisms can best account for this heterogeneity.

Using UAV-Based Photogrammetry Coupled with In Situ Fieldwork and U-Pb Geochronology to Decipher Multi-Phase Deformation Processes: A Case Study from Sarclet, Inner Moray Firth Basin, UK

Constraining the age of formation and repeated movements along fault arrays in superimposed rift basins helps us to better unravel the kinematic history as well as the role of inherited structures in basin evolution. The Inner Moray Firth Basin (IMFB, western North Sea) overlies rocks of the Caledonian basement, the pre-existing Devonian–Carboniferous Orcadian Basin, and a regionally developed Permo–Triassic North Sea basin system. IMFB rifting occurred mainly in the Upper Jurassic–Lower Cretaceous. The rift basin then experienced further regional tilting, uplift and fault reactivation during the Cenozoic. The Devonian successions exposed onshore along the northwestern coast of IMFB and the southeastern onshore exposures of the Orcadian Basin at Sarclet preserve a variety of fault orientations and structures. Their timing and relationship to the structural development of the wider Orcadian and IMFB are poorly understood. In this study, drone airborne optical images are used to create high-resolution 3D digital outcrops. Analyses of these images are then coupled with detailed field observations and U-Pb geochronology of syn-faulting mineralised veins in order to constrain the orientations and absolute timing of fault populations and decipher the kinematic history of the area. In addition, the findings help to better identify deformation structures associated with earlier basin-forming events. This holistic approach helped identify and characterise multiple deformation events, including the Late Carboniferous inversion of Devonian rifting structures, Permian minor fracturing, Late Jurassic–Early Cretaceous rifting and Cenozoic reactivation and local inversion. We were also able to isolate characteristic structures, fault kinematics, fault rock developments and associated mineralisation types related to these events

Spatial and temporal distribution of hydrocarbon production on the UK continental shelf

AbstractHydrocarbon production from oil and gas fields is controlled by a variety of interconnected factors with a hierarchy of significance that is, for the most part, difficult to untangle. This article documents and investigates the spatial and temporal distribution of key hydrocarbon field parameters on the UK Continental Shelf. Data have been compiled from publicly available sources for 424 fields. Variables are considered as “descriptive parameters,” “control parameters” and “outcomes”. Descriptive parameters are metadata such as field name, location, etc. Control Parameters include depositional environment, present depth of burial, porosity, permeability, reservoir formation pressure, reservoir temperature, average net‐to‐gross, number of fault populations, hydrocarbon API, field area, bulk rock volume, well density, number of wells (production and injection), well spacing, gas oil ratio, reservoir thickness, fluid saturation, compartmentalization (quantitated by number of observable non‐communicating fault compartments), structural complexity (scaled from 0 to 5), field production strategy, trap type and stratigraphic heterogeneity. Outcomes are used to assess field performance and include final recovery factor (estimated), maximum production rate, and cumulative monthly production. Analysis of the database illustrates a number of empirical observations regarding hydrocarbon production on the UKCS. The Jurassic plays have been the most successful in the region in terms of total volumes produced while the Permian reservoirs of the SNS account for the majority of the gas. Most of the UKCS reservoirs record top depths between 2000 and 4500 m with good reservoir quality. The best reservoir quality is observed in reservoirs that were deposited within deep marine systems. The largest hydrocarbon reserves are found in the Northern North Sea basin in these deep marine (as well as paralic and shallow marine) reservoirs. Using the data from this article and affiliated data, potential exists for extracting insight beyond spatio‐temporal distributions.

New 3D fault model for eastern Flanders (Belgium) providing insights on the major deformation phases in the region since the late Paleozoic

A new 3D fault model was created for the Upper Paleozoic to Cenozoic subsurface in eastern Flanders (Belgium). The model was based on the integration of 2D seismic interpretations, borehole and gravimetric data, as well as coal mining and topographic maps. Four major tectonic phases, taking place during the latest Paleozoic, Jurassic, Late Cretaceous and Cenozoic, shaped the fault network. The latest Paleozoic phase was transpressional and expressed by local folding of the Carboniferous strata underneath the base Permian unconformity. The largest anticlines were asymmetric, with amplitudes of up to 500 m and SSW-NNE oriented axal planes. The subsequent Jurassic phase caused normal faulting across the area, resulting in the development of the northeast-dipping, western border fault systems of the Campine Basin and Roer Valley Graben. Fault strikes were predominantly WNW-ESE to NNW-SSE. Both fault directions were most likely inherited from early Carboniferous and older tectonic phases. Jurassic fault reactivation explains the activity along fault populations with different orientations as well as the linkage of these faults into long (>10 km) systems, despite displaying generally relatively small vertical throws (<500 m). Soft-linked boundaries (accommodation zones) between Jurassic fault domains are located on top of the largest latest Paleozoic transpressional anticlines.The Late Cretaceous major tectonic phase was compressional and involved inversion of the Roer Valley Graben with subsequent subsidence and carbonate deposition in the flanking Campine Block.During the Cenozoic extensional phase, most normal faults that developed were rooted in Jurassic faults and therefore displayed similar WNW-ESE to NNW-SSE strikes. Cenozoic extension was, however, weaker than Jurassic extension; Cenozoic fault numbers, lengths and vertical throws were smaller than during the Jurassic phase. Also during the Cenozoic phase, faulting was more localized onto the pre-existing Roer Valley Graben border fault system compared to the more diffuse Jurassic faulting. The Cenozoic faults were generally planar, whereas some of its Jurassic roots near and in the border fault system of the Roer Valley Graben were listric.

Integrated rupture mechanics for slow slip events and earthquakes

Slow slip events occur worldwide and could trigger devastating earthquakes, yet it is still debated whether their moment-duration scaling is linear or cubic and a fundamental model unifying slow and fast earthquakes is still lacking. Here, we show that the rupture propagation of simulated slow and fast earthquakes can be predicted by a newly-developed three-dimensional theory of dynamic fracture mechanics accounting for finite rupture width, an essential ingredient missing in previous theories. The complete spectrum of rupture speeds is controlled by the ratio of fracture energy to energy release rate. Shear stress heterogeneity can produce a cubic scaling on a single fault while effective normal stress variability produces a linear scaling on a population of faults, which reconciles the debated scaling relations. This model provides a new framework to explain how slow slip might lead to earthquakes and opens new avenues for seismic hazard assessment integrating seismological, laboratory and theoretical developments.

A Coulomb Stress Response Model for Time‐Dependent Earthquake Forecasts

AbstractSeismicity models are probabilistic forecasts of earthquake rates to support seismic hazard assessment. Physics‐based models allow extrapolating previously unsampled parameter ranges and enable conclusions on underlying tectonic or human‐induced processes. The Coulomb Failure (CF) and the rate‐and‐state (RS) models are two widely used physics‐based seismicity models both assuming pre‐existing populations of faults responding to Coulomb stress changes. The CF model depends on the absolute Coulomb stress and assumes instantaneous triggering if stress exceeds a threshold, while the RS model only depends on stress changes. Both models can predict background earthquake rates and time‐dependent stress effects, but the RS model with its three independent parameters can additionally explain delayed aftershock triggering. This study introduces a modified CF model where the instantaneous triggering is replaced by a mean time‐to‐failure depending on the absolute stress value. For the specific choice of an exponential dependence on stress and a stationary initial seismicity rate, we show that the model leads to identical results as the RS model and reproduces the Omori‐Utsu relation for aftershock decays as well stress‐shadowing effects. Thus, both CF and RS models can be seen as special cases of the new model. However, the new stress response model can also account for subcritical initial stress conditions and alternative functions of the mean time‐to‐failure depending on the problem and fracture mode.

The Malawi Active Fault Database: An Onshore‐Offshore Database for Regional Assessment of Seismic Hazard and Tectonic Evolution

AbstractWe present the Malawi Active Fault Database (MAFD), an open‐access (https://doi.org/10.5281/zenodo.5507190) geospatial database of 113 fault traces in Malawi and neighboring Tanzania and Mozambique. Malawi is located within the East African Rift's (EAR) Western Branch where active fault identification is challenging because chronostratigraphic data are rare, and/or faults are buried and so do not have a surface expression. The MAFD therefore includes any fault that has evidence for displacement during Cenozoic East African rifting or is buried beneath the rift valley and is favorably oriented to the regional stresses. To identify such faults, we consider a multidisciplinary data set: high‐resolution digital elevation models, previous geological mapping, field observations, seismic reflection surveys from offshore Lake Malawi, and aeromagnetic and gravity data. The MAFD includes faults throughout Malawi, where seismic risk is increasing because of population growth and its seismically vulnerable building stock. We also investigate the database as a sample of the normal fault population in an incipient continental rift. We cannot reject the null hypothesis that the distribution of fault lengths in the MAFD is described by a power law, which is consistent with Malawi's relatively thick seismogenic layer (30–40 km), low (&lt;8%) regional extensional strain, and regional deformation localization (50%–75%) across relatively long hard‐linked border faults. Cumulatively, we highlight the importance of integrating onshore and offshore geological and geophysical data to develop active fault databases along the EAR and similar continental settings both to understand the regional seismic hazard and tectonic evolution.

Under-Displaced Normal Faults: Strain Accommodation Along an Early-Stage Rift-Bounding Fault in the Southern Malawi Rift

One of the fundamental problems in continental rift segmentation and propagation is how strain is accommodated along large rift-bounding faults (border faults) since the segmentation of propagating border faults control the expression of rift zones, syn-rift depo-centers, and long-term basin evolution. In the Southern Malawi Rift, where previous studies on the early-stage rifting only assessed border fault structure from surficial and topographic expression, we integrate surface and subsurface data to investigate border fault segmentation, linkage, and growth as proxies for strain accommodation along the Bilila-Mtakataka Fault (BMF) System. We used 30 m-resolution topographic relief maps, electrical resistivity tomography (ERT), and high-resolution aeromagnetic data to characterize the detailed fault geometry and provide a more robust estimate of along-fault displacement distribution. Our results reveal a discrepancy between sub-aerial segmentation of the BMF geometry (six segments), scarp height (five segments) reflecting the most recent episodes of fault offset, and cumulative throw (three composite segments) reflecting the long-term fault offset. We also observe that although the BMF exhibits continuity of sub-aerial scarps along its length, the throw distribution shows a higher estimate at the Northern-to-Central segment relay zone (423 m absolute, 364 m moving median) compared to the Central-to-Southern segment relay zone (371 m absolute, 297 m moving median). The ERT profiles across the relay zones suggest a shallower basement and a possible canyon-mouth alluvial fan stratigraphy at the Central-to-Southern segment relay zone, contrasting the deeper basement and “simpler” electrical stratigraphy at the Northern-to-Central relay. The results suggest a more complex long-term evolution of the BMF than was assumed in previous studies. A comparison of BMF’s maximum displacement-vs-length with those of other Malawi Rift border faults and global normal fault populations suggest that although the BMF has possibly reached its maximum length, it remains largely under-displaced as its 580–837 m maximum displacement is significantly lower than that of faults of equivalent length. We suggest that the BMF may continue to accrue significant strain as tectonic extension progresses in the Southern Malawi Rift, thus posing a major seismic hazard in the region.

Statistical Analysis of Small Faults in Rotated Blocks of Crust Near the Húsavík‐Flatey Transform Fault, Northern Iceland

AbstractThe dextral Húsavík‐Flatey fault, in northern Iceland, provides a unique opportunity to observe an oceanic transform fault on land. Structural and paleomagnetic measurements from lavas and dikes on Flateyjarskagi peninsula indicate that rocks adjacent to the fault have rotated clockwise more than 100°. We examine the effect of this rotation on the orientations of small faults that developed in rocks adjacent to the transform fault. We also use patterns in small‐scale data to build on models of the style of deformation surrounding the fault. We apply statistical tools including spatial regressions to structural data to estimate the amount of distributed shearing around the transform fault, and to predict the orientations of rotated structures. Some small faults are consistent with predicted rotations, while others are consistent with the modern kinematics, suggesting that they postdate block rotation. This protracted temporal history for small faults complicates paleostress interpretations from rocks adjacent to the Húsavík‐Flatey fault. Previous studies assumed the small faults formed in their modern orientations and attributed their variability to stress variations, whereas we identify populations of faults that have likely rotated to their present‐day attitudes.

Seismicity modulation in a 3-D rate-and-state interacting fault population model

SUMMARY Seismicity rate is sometimes observed to correlate with periodic stress oscillations, such as seasonal changes in hydrological loads, or solid-earth and oceanic tidal stresses. However, seismicity modulation is far from being systematic, often weak and the mechanical control is not yet fully understood. Here, a 3-D interacting fault population loaded by a periodic stress has been studied. For that a new earthquake simulator coupling rate-and-state friction and 3-D quasi-dynamic elasticity has been developed, and used to explore the mechanical conditions leading to seismicity modulation. It is first shown that seismicity modulation is maximized when the amplitude of periodic loading stress approaches aσ, a being the viscous parameter of the rate-and-state friction and σ the normal stress in the earthquake source region. The mode of modulation is frequency dependent: short loading periods lead to a stress control (seismicity rate follows the Coulomb stress) while longer periods lead to stressing rate control (seismicity rate tracks Coulomb stressing rate variations). An analytical expression for the critical period Tc separating the two regimes is derived from Dieterich’s theory, showing a dependence of the transition on Coulomb stress amplitude. Finally, the average seismicity rate either scales exponentially with Coulomb stress (stress control), or linearly with Coulomb stressing rate (stressing rate control), as suggested by Dieterich’s theory. These features indicate that for the range of parameters explored, stress redistribution within the fault population is a second order effect in the seismicity modulation, which is primarily controlled by the properties of remote loading. This paper therefore confirms with numerical simulations the Coulomb stress and stressing rate control of seismicity rate, and the limited importance of stress redistribution in controlling earthquake production anticipated by previous theoretical studies.

Stratigraphically Controlled Stress Variations at the Hydraulic Fracture Test Site-1 in the Midland Basin, TX

We investigated the relationship between stratigraphy, stress, and microseismicity at the Hydraulic Fracture Test Site-1. The site comprises two sets of horizontal wells in the Wolfcamp shale and a deviated well drilled after hydraulic fracturing. Regional stresses indicate normal/strike-slip faulting with E-W compression. Stress measurements in vertical and horizontal wells show that the minimum principal stress varies with depth. Strata with high clay and organic content show high values of the least compressive stress, consistent with the theory of viscous stress relaxation. By integrating data from core, logs, and the hydraulic fracturing stages, we constructed a stress profile for the Wolfcamp sequence, which predicts how much pressure is required for hydraulic fracture growth. We applied the results to fracture orientation data from image logs to determine the population of pre-existing faults that are expected to slip during stimulation. We also determined microseismic focal plane mechanisms and found slip on steeply dipping planes striking NW, consistent with the orientations of potentially active faults predicted by the stress model. This case study represents a general approach for integrating stress measurements and rock properties to predict hydraulic fracture growth and the characteristics of injection-induced microseismicity.

Structural geological complications for tunnel construction in the Kostenets’s section of the Maritsa fault zone in Central Bulgaria

The structural geological features defining the discontinuity network in metamorphic rocks hosting railway tunnels are described. Discussion on the faults, joints, lineation and stresses found from fault population is presented. It is demonstrated that the formation of some brittle joints is related to the ductile stretching lineation. The geotechnical situation for the tunnel construction is complicated and requires reassessment of the design solutions in order to accommodate substantial lateral forces and prevent shear of the tubes.

Tectonic and stratigraphic evolution of the central Exmouth Plateau, NW Shelf of Australia

A detailed tectono-stratigraphic analysis of the Mesozoic through Present-day structures and sedimentary sequences has been carried-out on a very large merged 3-D seismic survey that covers the great part of the central Exmouth Plateau, northern Carnarvon Basin, NW Shelf of Australia. Four main megasequences bounded by regional unconformities have been delineated. They are; (1) Megasequence-I - Early-Late Triassic pre-extension, (2) Megasequence-II - Latest Triassic (Rhaetian)-earliest Late Jurassic (Oxfordian) syn-extension; (3) Megasequence-III - Late Jurassic-Early Cretaceous post-extension; and (4) Megasequence-IV - Early Cretaceous-Present-day passive margin strata. Three populations of extensional faults and a previously unrecognised compressional phase of folding have been identified. The main Mesozoic (Rhaetian-Oxfordian) WNW-directed extension produced domino-style planar extensional faults (Fault System 1) with two principle fault sets; (i) large N–S to NNE-SSW-trending, west-dipping faults and (ii) smaller-scale, largely antithetic east-dipping faults. The large faults are possibly reactivated pre-existing deeper and older faults (possibly related to listric growth faults of the large Lower-to Mid-Triassic Mungaroo delta) as well as completely new faults developed in the upper part of the Triassic Mungaroo Formation. Synclinal hangingwall basins of this system are strongly segmented and underfilled with little (<450 m) syn-extensional sedimentation.Fault System 1 was reactivated during the Early Cretaceous-Neogene post-extension stage and strongly influenced the development of the segmented NNE-SSW-trending Fault System 2 in the Cenozoic strata. Two stacked polygonal non-tectonic fault tiers (Fault System 3) separated by little deformed stratigraphy occur within the Berriasian to Pliocene post-extension sequences. In the early post-extension phase, Late Jurassic to Early Cretaceous NNE-SSW-directed minor contraction event produced broad WNW-ESE-striking folds in the Triassic-Early Cretaceous strata. This study documents the development of the extensional fault systems and proposes new 4-D tectono-stratigraphic evolutionary models for the central Exmouth Plateau. The results of this study have wider implications for the tectono-stratigraphy of the northern Carnarvon Basin and future exploration.

Influence of variable decoupling between vertically separated fault populations on structural inheritance – The Laminaria High, NW Shelf of Australia

AbstractWhen extension events are greatly separated in time, older faults may be buried and stratigraphically separated from newly developing faults at shallower depths. During rifting, the buried structures may reactivate and propagate upwards to be expressed within the shallow system. The degree of linkage between structural levels determines the influence that the deeper structures can exert over the geometry and evolution of the incipient fault system. In this study we use 3D seismic reflection data to examine how a deep fault population across the Laminaria High, NW shelf of Australia influences the development of a younger fault system at shallow depths. These fault populations are non‐parallel and decoupled across a mechanically weak interval. The majority of shallow faults are not linked to those at depth. However, the reactivation and upward propagation of some of the deeper faults produce anomalously oriented structures at shallow depths, hard‐linked to the deeper structures. One fault in particular shows along‐strike variability, with the deep segment reactivated and present at shallow depths in the west. To the east, the shallow and deep fault segments become decoupled across the mechanically weak interval, although some soft‐linkage and strain transfer still occurs. We suggest that this switch in the degree of coupling along the fault is due to the geometry of the deeper structure, with the transition corresponding to a prominent relay ramp. We show how the geometry of a deeper fault may affect its propensity to reactivate during subsequent extensional events, ultimately affecting the degree of structural inheritance that is expressed within younger, shallower fault populations.

Mercury's Crustal Thickness and Contractional Strain.

The crust of Mercury has experienced contraction on a global scale. Contractional deformation is expressed by a broadly distributed network of lobate thrust fault scarps. The most likely principal source of stress is global contraction from cooling of Mercury's interior. Global contraction alone would be expected to result in a uniformly distributed population of thrust faults. Mercury's fault scarps, however, often occur in long, linear clusters or bands. An analysis of the contractional strain as a function of crustal thickness, estimated in two crustal thickness models (CT1 and CT2) derived from gravity and topography data obtained during the MESSENGER mission, indicates the greatest contractional strain occurs in crust 50–60 km thick. On Earth, mantle downwelling can thicken and compress overlying crust, regionally concentrating thrust faults. Clusters of lobate scarps collocated with regions of thick crust suggest downward mantle flow contributed to the localization of lithosphere‐penetrating thrust faults.