Pre-existing Normal Faults Research Articles

Using U–Pb carbonate dating to constrain the timing of extension and fault reactivation within the Bristol Channel Basin, SW England

The Bristol Channel Basin is a Mesozoic continental rift basin. The basin is an important analogue for offshore reservoirs. Relative cross-cutting relationships and correlation with adjacent sedimentary basins have previously been used to constrain the timing of basin development. In situ U–Pb carbonate geochronology has been used to date calcite slickenfibre development in the cores of normal, thrust and strike-slip faults in the East Quantoxhead and Kilve region of Somerset for the first time. Protracted north–south extension from c. 150 to 120 Ma formed normal faults. Subsequent north–south shortening from c. 50 to 20 Ma was accommodated by (1) mutually cross-cutting strike-slip faults, (2) minor east–west-striking thrust faults and (3) the reactivation of pre-existing normal faults. Throughout Cenozoic contraction, σ 2 and σ 3 remained similar in magnitude and periodically flipped to become vertical; this was probably controlled by local stress permutations and changes in fluid pressure. The timing of inversion is contemporaneous with dominant Pyrenean and later Alpine orogenic events, as well as the opening of the Mid-Atlantic Rift. Early inversion of the Bristol Channel Basin was probably driven by far-field Pyrenean deformation, with later contraction caused by Alpine forces. Ridge push from the Mid-Atlantic Rift exacerbated the reactivation of the basin.

Fold localization at pre-existing normal faults: field observations and analogue modelling of the Achental structure, Northern Calcareous Alps, Austria

Abstract. Within the Northern Calcareous Alps (NCA) fold-and-thrust belt of the Eastern Alps, multiple pre-shortening deformation phases have contributed to the structural grain that controlled localization of deformation at later stages. In particular, Jurassic rifting and opening of the Alpine Tethys led to the formation of extensional basins at the northern margin of the Apulian plate. Subsequent Cretaceous shortening within the Northern Calcareous Alps produced the enigmatic Achental structure, which forms a sigmoidal transition zone between two E–W-striking major synclines. One of the major complexities of the Achental structure is that all structural elements are oblique to the Cretaceous direction of shortening. Its sigmoidal form was, therefore, proposed to be a result of forced folding at the boundaries of the Jurassic Achental basin. This study analyses the structural evolution of the Achental structure through integrating field observations with crustal-scale physical analogue modelling to elucidate the influence of pre-existing crustal heterogeneities on oblique basin inversion. From brittle–ductile models that include a weak basal décollement, we infer that oblique shortening of pre-existing extensional faults can lead to the localization of deformation at the pre-existing structure and predicts thrust and fold structures that are consistent with field observations. Consequently, the Achental low-angle thrust and sigmoidal fold train was able to localize at the former Jurassic basin margin, with a vergence opposite to the controlling normal fault, creating the characteristic sigmoidal morphology during a single phase of NW-directed shortening.

Analogue modelling of basin inversion: implications for the Araripe Basin (Brazil)

Abstract. Basin inversion is a process that takes place when a sedimentary basin is subjected to compressional stresses resulting in the reactivation of pre-existing faults and/or the localization of deformation along new reverse faults. The Araripe Basin (NE Brazil) is an example of a Cretaceous intracontinental aborted rift, with its sedimentary infill found at ca. 1000 m altitude, 500 m above the host basement. Post-rift basin inversion has been proposed by previous authors as the cause of this topographic high, but how inversion affected this basin remains a matter of debate, with the following two end-member scenarios: reactivation of pre-existing normal faults leading to local uplift or regional uplift and differential erosion. Neither end-member fully explains the observations from seismic and field data. In this study, we, therefore, conducted analogue models to explore how basin inversion in the Araripe Basin could have taken place. We present two series of crustal-scale brittle–viscous experiments: (i) extension followed by compression without sedimentation, with a variation in divergence and convergence directions (orthogonal or 45∘ oblique); and (ii) extension with syn-rift sedimentation followed by compression, with the same variation in rifting and inversion directions. We found that orthogonal rifting without sedimentation forms throughgoing graben boundary faults, whereas oblique rifting initially creates en échelon faults that eventually link up, creating large graben boundary faults. Rift basins with syn-rift sedimentation evolved in a similar fashion; however, sedimentary loading resulted in increased subsidence. During both oblique and orthogonal inversion, most shortening was accommodated along new low-angle reverse faults. Significant intra-graben fault reactivation occurred in all models without syn-rift sedimentation. By contrast, orthogonal inversion of models with syn-rift sedimentation did not reactivate rift faults, whereas only a minor reactivation of rift faults took place during oblique inversion since the sediments strengthened the otherwise weakened basin, thus acting as a buffer during convergence. Based on our modelling results, we propose an alternative scenario for the evolution of the Araripe Basin, involving oblique inversion and the development of low-angle reverse faults, which better fits observations from seismic lines and field data from the region.

Pumping-induced reactivation of a pre-existing normal fault: insights from a centrifuge model test

Pumping-induced normal fault reactivation and the resultant ground fracture have been observed in faulted basins worldwide, but the process and mechanism are poorly understood thus far. In this study, we conducted centrifuge model tests to investigate and analyze these issues. Two simplified faulted models, Models 1 and 2, were developed based on an actual event that occurred in the Beijing Plain, China. Our model tests reproduced the pumping-induced normal fault reactivation, characterized by localized hanging wall subsidence with new fault scarp formation in the models. The monitoring results showed that water table decline drastically accelerated uneven subsidence. Although the deformation pattern did not respond to variations in the material properties of the hanging wall and footwall, the magnitude of the reactivated normal faulting was influenced. The maximum vertical offset in Model 2 was much larger than that in Model 1 due to a larger compression modulus in the hanging wall and footwall in Model 2. Furthermore, the reactivation mechanism was revealed based on Anderson’s faulting theory. Normal faulting occurs once the maximum principal stress becomes vertical and the intermediate and minimum principal stresses become horizontal. Groundwater pumping increases the effective stress, leading to the addition of vertical stress and the resultant normal fault reactivation. Our findings provide a better understanding of human interactions with the Earth’s surface and are helpful for mitigating faulting-caused disasters.

Oligocene to Pliocene structural evolution of the frontal Kirthar fold and thrust belt of Pakistan

The spatio-temporal evolution for the frontal part of the Kirthar fold and thrust belt in Pakistan is constrained for the first-time using apatite (U–Th-Sm)/He (AHe) and apatite fission track (AFT) dating of samples from the Oligocene Nari Formation in combination with a published balanced cross-section. The AHe ages appear to be fully reset, allowing us to date the timing of exhumation above ramps. Comparison of partially reset AFT ages (∼22–24 Ma) with previously published zircon fission track (ZFT) ages of the Nari Formation from nearby areas suggest that rocks in the frontal zone were not buried deeper than ∼4 km after deposition. The close range of AFT ages have implications for the timing of hinterland exhumation and Oligocene age of the Nari Formation. The AHe ages suggest that deformation along the major basement ramp was active since ∼7 Ma, forming a major topographic step (∼1.5 km) in the frontal part of the Kirthar fold and thrust belt. Combined analysis of the structural cross-section and thermal modeling of the samples suggest that major cooling occurred between ∼7 Ma and ∼5 Ma. This cooling was due to the erosion of Oligocene to Miocene strata from above the samples when Precambrian to Miocene strata was thrust above the pre-existing normal fault that acted as a basement ramp. The temporal structural evolution suggests that deformation in the frontal part of the Kirthar fold and thrust belt was characterized by faster rates (∼2.5 km/Ma) of orogenic growth and exhumation between ∼7 and 5 Ma, followed by slower shortening rates (<1 km/Ma) since ∼5 Ma.

Structural and tectonostratigraphic evolution of Matruh Basin, northern Western Desert, Egypt: An example of an inverted rift basin

The Matruh Basin is located in the northwestern part of the Western Desert of Egypt. Being part of the structurally complex northeast African margin, the basin underwent multiple phases of Mesozoic-Cenozoic deformation. Detailed structural analysis of the subsurface structures of this basin, using high-quality 3D seismic and well datasets, has identified two main fault sets: 1) Jurassic- Early Cretaceous (Barremian) NNE-striking normal faults; and 2) Late Cretaceous NW-striking normal faults. The older NNE-striking normal faults were later inverted due to Late Cretaceous compression forming fault-propagation folds (inversion anticlines) in their upper tips. This positive inversion initiated with the “Santonian Event” and continued into the Early Cenozoic. The NW-striking normal faults were not inverted due to their unfavorable orientations.Most inversion structures in the greater Western Desert trend E-W or ENE-WSW and reflect the reactivation of similarly oriented pre-existing normal faults. The NNE orientation of the old, major extensional structures in the Matruh Basin and the resulting control on the later inversion fabric reflects a very different fault architecture. This has dramatically impacted sedimentation patterns and potential hydrocarbon accumulations. The Matruh Basin illustrates the complexities that can be present along the North African Neotethyan margin and underlines the necessity to avoid regional generalizations where gaps in data may be present.

Magnetic fabric analyses of basin inversion: a sandbox modelling approach

Abstract. A magnetic fabric analysis is a useful tool to display deformation in nature and in models. In this study, three sandbox models represent basin inversion above a velocity discontinuity (base plate). After complete deformation of each model, samples were taken in different parts of the models (along faults and areas away from faults) for magnetic fabric analysis. Model I, which simulates basin formation during extension, shows two kinds of magnetic fabric: an “undeformed”/initial fabric in areas away from faults and a normal fault-induced fabric with a magnetic foliation that tends to align with the fault surface. Models II and III were extended to the same stage as Model I but were subsequently shortened/inverted by 1.5 cm (Model II) and 4 cm (Model III). Both inverted models developed “thrusts” during inversion. The thrusts show an alignment of magnetic foliation parallel to the fault surfaces that depends on the maturity of the thrust. Our results highlight that thrusting is more efficient in aligning the magnetic fabric along them compared to normal faults. Moreover, models II and III reveal a magnetic fabric overprint towards a penetrative strain-induced fabric (magnetic lineation perpendicular to shortening direction) with increasing strain in areas away from thrusts. Such overprint shows a gradual transition of a magnetic fabric to a penetrative strain-induced fabric and further into a thrust-induced fabric during shortening/inversion. In contrast, extension (Model I) developed distinct magnetic fabrics without gradual overprint. In addition, pre-existing normal faults are also overprinted to a penetrative strain-induced fabric during model inversion. They define weak zones within the main pop-up imbricate and steepen during model inversion. Steepening influences the magnetic fabric at the faults and, in general, the strain propagation through the model during inversion. The magnetic fabric extracted from the models presented here reflect the different stages of basin development and inversion. This study is a first attempt of applying magnetic fabric analyses on models simulating inverted basins. This study illustrates the possibility of applying a robust tool, i.e. magnetic fabric analyses, to sandbox models, whose initial, intermediate, and final stages are well documented, to understand fabric development in inverted tectonic regimes.

A Reliable Procedure to Estimate the Rupture Propagation Directions from Source Directivity: The 2016–2018 Central Italy Seismic Sequence

Abstract We present a new approach to estimate the predominant direction of rupture propagation during a seismic sequence. A fast estimation of the rupture propagation direction is essential to know the azimuthal distribution of shaking around the seismic source and the associated risks for the earthquake occurrence. The main advantage of the proposed method is that it is conceptually reliable, simple, and fast (near real time). The approach uses the empirical Green’s function technique and can be applied directly to the waveforms without requiring the deconvolution of the instrumental response and without knowing a priori the attenuation model and the orientation of the activated fault system. We apply the method to the 2016–2017 Amatrice-Visso-Norcia high-energy and long-lasting earthquake series in central Italy, which affected a large area up to 80 km along strike, with more than 130,000 events of small-to-moderate magnitude recorded until the end of August 2022. Most of the selected events analyzed in this study have a magnitude greater than 4.4 and only four seismic events have a magnitude in the range of 3.3–3.7. Our results show that the complex activated normal fault system has a rupture direction mainly controlled by the pre-existing normal faults and by the orientation of the reactivated faults. In addition, the preferred direction of rupture propagation is also controlled by the presence of fluid in the pre-existing structural discontinuities. We discuss the possible role of fluids as a cause of bimaterial interface. Another important finding from our analysis is that the spatial evolution of seismicity is controlled by the directivity.

3D modeling of the Esker intrusive complex, Ring of Fire intrusive suite, McFaulds Lake greenstone belt, Superior Province: Implications for mineral exploration

A 3D geological model of the Archean Esker ultramafic–mafic intrusive complex of the Ring of Fire intrusive suite of northwestern Ontario (Canada) is presented, providing insight into the nature of its magmatic plumbing system and allowing for speculation on the formation of its valuable Ni-Cu-(PGE) and chromitite deposits. The 3D model was constrained by exploration drillholes and high-resolution aeromagnetic data, which allowed the interpretation of the broad internal structure of the intrusive complex and two shear zones, which define for a major part of their strike extent the contacts between ultramafic and overlying mafic intrusive rocks. Restoring the post-emplacement component of strike-slip displacement and subsequently rotating the intrusive complex back from its tilted to sub-horizontal orientation of emplacement, provides a three-dimensional rendition of a 14 km-long dominantly tabular-shaped intrusive complex. The complex is defined by intact basal intrusive contacts with localized keel-shaped ultramafic promontories parallel to a nearby blade-shaped magmatic conduit. The keels extend in a subparallel orientation to depths of up to 1 km into the underlying tonalitic rocks and coincide with significant offsets of the basal intrusive contact, leading to speculation that their intrusion may have been guided by pre-existing normal faults. Two of these magmatic conduits host Ni-Cu-(PGE) deposits/prospects that formed from gravitational segregation of sulfides. The chromitite deposits, hosted in ultramafic rocks at higher stratigraphic levels, were modeled based on drill hole assay data. Zones of dominantly massive to semi-massive chromitite were defined by the ≥ 35% Cr2O3 threshold, while intercalated sequences of chromitite in dunite/peridotite were defined by the ≥ 15% to ≤ 35% Cr2O3 percentage range. The 3D modeled surfaces fitted to these constraints exhibit ore zones with lateral extents from several hundreds of meters for massive bodies up to a maximum of 2 km for intercalated chromitite ore that are dominantly conformable to the layering of the ultramafic host rocks. The lack of lateral continuity of chromitite ore zones beyond this scale, suggests that the chromitite layers are not continuous along the entire strike extent of the Esker intrusive complex, which is consistent with bifurcations of the 3D modeled ore shells signifying pinch-outs of both the massive chromitite layers and the dunitic interlayers. This discontinuous distribution of chromitite ore resembles that in the stratiform deposits of the Stillwater Complex and may support recent models of mechanical segregation of chromitite from cotectic chromite-olivine slurries. Based on our 3D model, we propose that the Esker intrusive complex evolved as a series of individual intrusions that were emplaced during multiple magmatic pulses that eventually coalesced to form a composite ultramafic–mafic complex prior to being dissected and partly dismembered by post-ore shear zone deformation.

Structural Study of Judaida Subsurface Structure, North of Iraq

This research focuses on the structural study of three folded horizons of Fat'ha, Jeribe, and the Euphrates in Judaida structure, Northern Iraq. Depth contour maps for three horizons using the time and velocity data called from seismic surveys. Seismic interpretation suggests that Judaida structure is a positively inverted structure. It is an anticline on the level of Tertiary and the top and an extensional structure on the level of pre-Tertiary sequence. Judaida structure is an asymmetrical longitudinal anticline; its Northeastern limb is steeper than its Southwestern limb. The axis of the anticline and the major normal faults are both trending toward the Northwest. The normal fault picked influencing only the pre-Late Cretaceous sequence where is the folding affected the whole sequence; of the extension forces, it affected the region during that period. Late Pliocene compressions resulted in folding and reverse faulting of the whole sequence and inversion of movement along the pre-existing normal faults. The resulted faults are called inverted faults.

Evaluating the role of stratigraphy and inherited basement structures on the evolution of thick and thin-skinned related folds: Insights from the Salar de Atacama Basin in the Central Andes

We investigated the role of pre-existing basement normal faults, and ductile layers present within post-rift sequences on the final geometry and kinematics of hybrid (thick and thin-skinned) fold and thrust belts that result of tectonic inversion, using as an example the northern region of the Salar de Atacama Basin in the Central Andes. The study was developed integrating field data consisting of regional and local observations and geological mapping, and the seismic and structural interpretation of 2-D industrial seismic profiles to understand the current structure of the basin. A regional-scale balanced cross-section was constructed and restored to validate our interpretations and determine the pre-shortening tectonic configuration of the basin. Our results suggest that low angle normal faults result in favorable structures for the development of thick-skinned thrust ramps from their full tectonic inversion during the growth of fold and thrust belts. In the Salar de Atacama Basin, these structures allowed for the uplift and exhumation of large Paleozoic stratified and crystalline rock on its western section. The existence of ductile strata within post-rift sequences commonly facilitates the transition from thick to thin-skinned structural style during the progressive tectonic inversion of pre-existing half-graben structures. This mechanism allowed for the creation of prominent east-verging basement cored anticlines and doubly verging and narrow, shallow thin-skinned folds and thrusts at the northernmost part of the Salar de Atacama Basin. Pieces of evidence from the basin suggest that the initial areal extension (length) of the post-rift ductile strata exert a fundamental role in the lateral extension of thin-skinned thrusts. Finally, the existence or not of important ductile strata in the overburden stratigraphy conditions the progression of hybrid fold and thrust belts after pre-existing normal faults are reactivated.

Formation mechanism of pumping-induced earth fissures associated with a pre-existing normal fault, Beijing, China

The Shunyi (SY) earth fissures originated due to groundwater pumping and have caused severe damage to the Beijing Capital International Airport (BCIA) and other infrastructure in Beijing, China. They are distributed along the pre-existing NE-SW trending normal Shunyi-Liangxiang (SL) fault zone, with 1–30 cm vertical offsets. Previous studies of this long-term ground displacement have concluded that a subsidence bowl has formed on the footwall (northwestern side) of the SL fault in the BCIA. However, based on our observations, the relative subsidence occurred exclusively on the southeastern side, behaving like the activation of the SL fault. This complex deformation pattern has led to the formation mechanism of the SY earth fissures remaining unclear. In this study, two groups of numerical simulations based on the geological and hydrogeological settings of the SY earth fissures were performed. In Scenario I, the pumping load was on the footwall, whereas in Scenario II, the pumping load was on the hanging wall. Our results show that there are always two areas in the faulted model that have undergone pumping-induced subsidence. The first subsidence area indicates that the actual regional subsidence bowl occurred extensively around the pumping well. This subsidence was blocked by the pre-existing normal fault zone. The second subsidence area is localized and occurred abruptly in the hanging wall adjacent to the fault zone regardless of the effect of the position of the pumping load. This localized differential subsidence, representing the secondary surface faulting, was accompanied by earth fissure formation due to concentration of the surface shear strain. These simulation results agree with the actual land deformation across the SY earth fissures. Furthermore, our results suggest that both the formation location and deformation pattern of the SY earth fissures were dependent upon the characteristics of the pre-existing SL fault zone. This is mainly attributed to the fact that the fault zone has a lower resistance than the surrounding materials. Our findings reveal the formation mechanism of the SY earth fissures and provide a new understanding of pumping-induced earth fissures in other faulted areas.

Seismic Reflection Images of Possible Mantle-Fluid Conduits and Basal Erosion in the 2011 Tohoku Earthquake Rupture Area

Multi-channel seismic reflection and sub-bottom profiling data reveal landward-dipping normal faults as potential conduits for mantle-derived fluids in the coseismic slip area of the 2011 Tohoku earthquake (Mw9.0). Normal faults below the helium isotope anomaly sites appear to develop through the forearc crust (i.e., the seafloor sedimentary section and Cretaceous basement) and to evolve to lower dip angles as extension progresses deeper, potentially extending down to the mantle wedge, despite their intermittently continuous reflections. The faults are characterized by high-amplitude, reverse-polarity reflections within the Cretaceous basement. Moreover, deep extension of the faults connecting to a low-velocity region spreading from the Cretaceous basement into the mantle wedge across the forearc Moho suggests that the faults are overpressured by local filling with mantle-derived fluids. The locations of the normal faults are roughly consistent with aftershocks of the 2011 Tohoku earthquake, which show normal-faulting focal mechanisms. The 2011 Tohoku mainshock and subsequent aftershocks can lead the pre-existing normal faults to be reactive and more permeable so that locally trapped mantle fluids can migrate up to the seafloor through fault fracture zones. The reactivated normal faults may be an indicator of shallow coseismic slip to the trench. Locally elevated fluid pressures can decrease the effective normal stress for the fault plane, facilitating easier slip along the fault and local tsunami. The landward-dipping normal faults developing from the seafloor down into the Cretaceous basement are predominant in the middle slope region of the forearc. A possible shear zone with high-amplitude, reverse-polarity reflections above the plate interface, which is almost localized to the middle slope region, suggests more intense basal erosion of the overlying plate in that region.

Variations of thick-skinned deformation along tumuxiuke thrust in Bachu uplift of Tarim Basin, northwestern China

The curved Tumuxiuke thrust is the northeastern boundary of the Bachu uplift in the Tarim Basin. Based on the interpretation of 2D seismic reflection sections, thick-skinned deformation along the Tumuxiuke structural belt is characterized by monoclinal fault-propagation folds that alternate with push-up structures. The localization and patterns of thick-skinned deformation were controlled by pre-existing normal faults. The forward trishear modeling process further revealed the existence of short-cut thrusts branched from pre-existing normal faults and tested their effect on folding deformation. Monoclines decapitated by low-angle short-cut thrusts coexist with anticlines decapitated by high-angle inverted faults due to the selective inversion of pre-existing normal fault segments. Furthermore, the fault propagation to fault slip ratio (P/S) and the combined effect of multiple propagating thrusts determine the variations in the fold amplification rate and the growth-stratal architecture. The coeval activity of the western Kunlun orogen and the Tumuxiuke thrust in the Cenozoic indicates the far-field transmission of orogenic stress into the distal foreland. Under the approximately N–S-directed compressive stress, inverted structures in nearly W-E-striking segments exhibit monoclinal fault-propagation folds controlled by forethrusts; for the oblique NW-SE-striking segments, the transpressional deformation was partitioned into the frontal contractional deformation zone and the backlimb strike-slip zone.

East-vergent thrusts and inversion structures: An updated tectonic model to understand the Domeyko Cordillera and the Salar de Atacama Basin transition in the western Central Andes

The tectonic evolution of the Andean forearc in northern Chile has commonly been related to superimposed deformational events associated with different tectonic settings. The eastern Domeyko Cordillera and the Salar de Atacama Basin on the inner Andean forearc, have been widely investigated, and several alternative tectonic models including extensional and/or contractional deformation are proposed to explain its evolution. Nevertheless, when trying to determine the role of Paleozoic and Mesozoic inherited-basement extensional structures during the Andean orogeny an evolutionary model is still not clear. Therefore, the aim of this research is to improve the understanding of these aspects. Based on the interpretation of a regional 2-D seismic grid and new structural data from outcrops, we propose that the main mechanisms of contractional deformation and uplift are related to: (1) the propagation of east-vergent basement-involved thrust faults, (2) the tectonic inversion of pre-existing normal faults and (3) the activation of shallow thin-skinned thrusts. Additionally, the resultant geometry of the structural styles is frequently controlled by three factors: (i) the initial geometry of inherited major faults, (ii) the rheology of the deformed rocks, and (iii) the rate of accumulation of the Upper Cretaceous-Cenozoic syn-tectonic deposits. Finally, we interpreted that the tectonic transition between the Domeyko Cordillera and the Salar de Atacama Basin is marked by the interaction between east-vergent thick-skinned thrusts and west-vergent inverted normal faults.

The Mahelma fault: a secondary structure of the Sahel anticline?

The Mahelma fault, located upon the Sahel anticline at the tip of the major Sahel fault, is often suspected to be a secondary structure. Yet, to date, the Mahelma fault has displayed no definitive evidences for faulting activity and no unambiguous typology. This paper reviews new data, especially geological and subsurface data, acquired through the Algiers west microzoning project. The data indicate that (1) the Mahelma fault is an antithetic high-angle reverse fault, (2) the interpreted dip is around 80° toward the SSE and thus implies a North-verging fault, and (3) the fault zone has been observed in several fault sites and allowed direct observation of fault displacements and fault throws. (4) Displacements directly related to the Mahelma fault affect clearly young sediments including consolidated Quaternary sands. We suggest that the Mahelma fault forms an active backthrust originating from pre-existing normal fault with a possible linkage and interaction with the major Sahel forethrust. We conclude from the foregoing that the Mahelma fault most likely ruptures in concert with earthquakes on the Sahel main fault. The results also point the activity of the Sahel fault-related fold and hence complicate earthquake hazard analyses.

The Predictive Skills of Elastic Coulomb Rate-and-State Aftershock Forecasts during the 2019 Ridgecrest, California, Earthquake Sequence

ABSTRACTOperational earthquake forecasting protocols commonly use statistical models for their recognized ease of implementation and robustness in describing the short-term spatiotemporal patterns of triggered seismicity. However, recent advances on physics-based aftershock forecasting reveal comparable performance to the standard statistical counterparts with significantly improved predictive skills when fault and stress-field heterogeneities are considered. Here, we perform a pseudoprospective forecasting experiment during the first month of the 2019 Ridgecrest (California) earthquake sequence. We develop seven Coulomb rate-and-state models that couple static stress-change estimates with continuum mechanics expressed by the rate-and-state friction laws. Our model parameterization supports a gradually increasing complexity; we start from a preliminary model implementation with simplified slip distributions and spatially homogeneous receiver faults to reach an enhanced one featuring optimized fault constitutive parameters, finite-fault slip models, secondary triggering effects, and spatially heterogenous planes informed by pre-existing ruptures. The data-rich environment of southern California allows us to test whether incorporating data collected in near-real time during an unfolding earthquake sequence boosts our predictive power. We assess the absolute and relative performance of the forecasts by means of statistical tests used within the Collaboratory for the Study of Earthquake Predictability and compare their skills against a standard benchmark epidemic-type aftershock sequence (ETAS) model for the short (24 hr after the two Ridgecrest mainshocks) and intermediate terms (one month). Stress-based forecasts expect heightened rates along the whole near-fault region and increased expected seismicity rates in central Garlock fault. Our comparative model evaluation not only supports that faulting heterogeneities coupled with secondary triggering effects are the most critical success components behind physics-based forecasts, but also underlines the importance of model updates incorporating near-real-time available aftershock data reaching better performance than standard ETAS. We explore the physical basis behind our results by investigating the localized shut down of pre-existing normal faults in the Ridgecrest near-source area.

Development and growth of basement-involved structural wedges in the northwestern Qaidam Basin, China

Structural wedges contain two connected fault segments, a fore thrust and back thrust, that bound a triangular- or wedge-shaped fault block. Coeval displacements on both faults drive the wedge into surrounding rock, causing distinctive patterns of uplift and folding. We identified a series of thick-skinned structural wedges in northwestern Qaidam Basin, China, where syntectonic strata record deformation timing and kinematics. The Qaidam Basin is located at the northern margin of the Tibetan Plateau and contains more than 100 large anticlinal structures, many of which exhibit characteristics of structural wedges. We interpreted and modeled these structures using seismic reflection data, well logs, surface geological exposures, remote sensing images, and digital elevation data. These wedge structures are distinguished by having elevated and deformed strata in the footwalls of the back thrusts and synclinal folds that extend upward from the wedge tips. With the constraints of fold shape, growth strata, structural relief, and shallow fault geometry, we developed a series of forward models that describe the geometry and kinematic evolution of these wedge structures. The analysis suggests that some of the wedge structures reactivate preexisting normal faults. Our results provide a better understanding of how to identify thick-skinned wedge systems and model their geometry and kinematic evolution, which has implications for studying the tectonic history of the basin. Understanding the geometry and kinematics of the wedge structures is also crucial for petroleum exploration because both hanging-wall and footwall traps constitute an important component of current exploration targets.

Late Cretaceous mud volcanism in the southwestern Songliao basin records slab rollback of the subducted paleo-Pacific Plate underneath NE China

We present a new seismic reflection dataset and use it to characterize fossil mud volcanoes in the southwestern Songliao graben basin in northeastern China. The results reveal a link between mud volcanism and slab rollback along the eastern Asian margin. This study focuses on the upper 3 km of the Lujiapu sub-basin, which lies in the southwestern Songliao basin. The base of the sequence consists of Lower Cretaceous siliciclastic deposits that are penetrated by mud volcano fluidization pipes. These deposits are overlain by Upper Cretaceous strata that have been deformed by the mud volcanoes. The sequence is capped by undeformed Neogene–Quaternary sediments. These observations constrain the timing of mud volcanism to the Santonian–Campanian (~87–72 Ma). During Late Cretaceous diapirism, normal faulting at the tops of the mud volcanoes formed horsts and grabens. Surface anticlines and inversion structures near the mud volcano source layer indicate that basin inversion occurred during deposition of the Sifangtai and Mingshui formations (Campanian). Similar structures are also identified in the central Songliao basin. Thus, we propose that slab flattening to deepening during paleo-Pacific Plate subduction led to basin inversion. This process triggered the migration of high-pressure fluids and brecciated plastic rocks along pre-existing normal faults, which in turn produced the mud volcano system.

Basement-cover interaction in the mountain front of the Northern Neuquén fold and thrust belt (37°10’ – 37°40’ S), Argentina

The deformation front of the Neuquén fold and thrust belt (~37°30′S) comprises kilometer-scale basement-involved anticlines, related to development of the Las Yeseras and Pampa Tril thrust systems, connected with a wide frontal syncline. The interpretation of more than 70 seismic lines led to the construction of a time-structure map at top of the Cuyo Group (Middle Jurassic), which shows a structural high east of the Tromen volcano produced by the superposition of thick-skinned structures. Moreover, data from 35 oil wells constrain the along-strike variations in structural reliefs. The structural relief of the Las Yeseras anticline increase from north to south, between ~1000 m and 2000 m, while the structural relief of the Pampa Tril anticline reaches its maximum height of 3700 m in the north and decreases southwards to only 1600 m height and then disappears. Three structural cross-sections based on surface and subsurface information show the overall structural style dominated by north-trending basement-involved anticlines, with subhorizontal backlimbs and maximum structural reliefs around 3–4 km, associated with thick-skinned thrust systems and without significant participation of inverted normal faults. Calculated thick-skinned shortenings are ~8.3 km (14.8%) in the north and ~4 km (7.8%) in the south, which principally reflects the decreasing displacement of the Pampa Tril thrust system. Along-strike structural changes were locally controlled by WNW transfer zones that can be interpreted as SSW-dipping preexisting normal faults with minor positive inversion. Thin-skinned structures that form oil fields at depth are genetically associated with the thick-skinned thrust systems, thus the understanding of basement-cover interaction in the mountain fronts will aid to future hydrocarbon explorations and to comprehend the mechanisms of Andean mountain building.