Steep Normal Faults Research Articles

A new record of the Cenomanian–Turonian transgression preserved in the Ikorfat Fault zone, Nuussuaq Basin, West Greenland

The Ikorfat Fault zone in northern Nuussuaq, Central West Greenland, has preserved a hitherto unknown succession of Upper Cretaceous marine mudstones referred to the Itilli Formation, which have been removed by erosion on the footwall block and buried deeply below sea-level on the hanging-wall block. Outcrops in the fault zone provide new data on sedimentology, palynology, stable carbon isotopes and organic geochemistry in the lower part of the Itilli Formation. A new, detailed geological map of the study area is based on digital photogrammetry. Marine organic walled dinoflagellate cysts (dinocysts) demonstrate a late Cenomanian–Turonian age, coeval with the Oceanic Anoxic Event 2 (OAE2) and link the Nuussuaq Basin with the Baffin Bay and Sverdrup Basins. The Ikorfat Fault zone is part of the eastern boundary fault of the Nuussuaq Basin. Lower Campanian listric faults truncate the footwall block. Subsidence along steep normal faults continued during deposition of the volcanic Vaigat Formation of Danian to Selandian age. The downthrow of the hanging-wall block was around 2.5 km.

Modeling of Continental Normal Fault Earthquakes

AbstractThe magnitude of earthquakes on continental normal faults rarely exceeds 7.0 Mw. However, because of their vicinity to large population centers they can be highly destructive. Long recurrence time, relatively small deformations, and limited observations hinder our understanding of the deformation patterns and mechanisms controlling the magnitude of events. Here, this problem is addressed with 2D thermomechanical modeling of normal fault seismic cycles. The 2020 Samos, Greece Mw7.0 earthquake is used as an example as it is one of the largest and most studied continental normal fault earthquakes. The modeling approach employs visco‐elasto‐plastic rheology, compressibility, free surface, and a rate‐and‐state friction law for the fault. Modeling of the Samos earthquake suggests the pore fluid pressure ratio on the fault ranges from 0 to 0.7. The model demonstrates that most of the deformation during interseismic and coseismic periods, besides on the fault, occurs in the hanging wall and footwall below the seismogenic part of the fault. The largest vertical surface displacement during the earthquake is the subsidence of the hanging wall in the vicinity of the fault, while the uplift of the footwall and remote part of the hanging wall is significantly smaller. Modeling of the seismic cycles on normal faults with different setups shows the dependency of the magnitude on the thermal profile and dipping angle of the fault; low heat flow and low dipping angle are favorable conditions for the largest events, while steep normal faults in the areas of high heat flow tend to have the smallest magnitudes.

Fold formation in Kurdish Zagros (N Iraq)

Analysis of seismic sections and wells near the frontal folds of Iraqi Kurdistan (Low- and Highly folded zone) document the interplay of folds, faults and possible mechanisms of their generation.Serial sections from 3D seismic volume in the Low folded zone are interpreted based on calibration wells. Sections display fold evolution from low amplitude kinked monocline to high amplitude ramp folding. The fold continues towards the exposed Safeen anticline with a subvertical limb. These lowland sections can also be considered as the lateral growth segment of Safeen anticline.Three geoseismic sections through frontal folds were constructed with variable quality seismic and dips, formation tops from surface and from mountains wells. These sections show tight anticlines with thrust faults cutting to near surface on both limbs; remnant normal faults; a subvertical southern limb crossed by flat thrusts; and a Triassic ductile core.Sections suggest that this part of Zagros is built of two ‘rigid’ units: an upper, Mesozoic-Neogene and a lower, Paleozoic one, separated by a ductile detachment along the lower Triassic. The upper unit is prone to folding and faulting, the lower one to faulting. The detachment in-between comprises the Kurrachine C, lower Triassic Beduh and Mirga Mir formations, and potentially the top members of Permian Chia Zairi formation. Detachment is possibly dominated by evaporites. Our detachment is thicker than only Beduh-Mirga Mir detachment proposed by recent reconstructions of others.Folds are initiated as detachment folds above thickened parts of the Lower Triassic, then evolve into ramp folds. Ramp is formed by the merger of flat thrusts with pre-existing steep earlier normal faults that affect the Mesozoic section. Shear and drag beneath these ramps lead to steepening of the southern limb and to fault propagation. A basal flat thrust is the final element of this fault propagation stage. Underlying rigid Paleozoic can form imbricates that push the Triassic detachment upwards, creating passive folds. These Paleozoic imbricates dip to the south and not to the north, as suggested by reconstructions of others. Finally, strike slip shear documented at outcrops also contributes to the final fold form.Own and others' surface geological observations, indications from seismic and radiometric data suggest that fold generation started in Late Cretaceous, continued in the Paleogene in internal Highly folded zone. In our study area only Late Neogene fold growth can be documented, which is in harmony with 5–8 Ma uplift dates of others. Others’ geomorphologic work suggests fold lateral growth is a quick process. We propose that regional folds, such as Safeen anticline grow in distinct short periods separated by longer quiet intervals.

Modes of Oblique Inversion: A Case Study From the Cretaceous Fold and Thrust Belt of the Western Transdanubian Range (TR), West Hungary

AbstractInversion of pre‐existing normal faults is a frequent phenomenon in orogenic belts. However, their impact on the evolution of the Cretaceous Eoalpine orogeny is poorly studied, despite the widespread presence of Triassic and Jurassic normal faults. Our study area is the SW part of the Transdanubian Range (TR), which represents the uppermost thick‐skinned nappe of the Eoalpine orogeny in the Alpine realm. The variable trend of Cretaceous contractional structures within the Transdanubian Range has been traditionally explained by poly‐phase shortening. However, based on the combined analyses of field observations and 3D seismic data, we found that the complex geometry of Cretaceous contractional structures can be explained rather by the oblique inversion of segmented Late Triassic grabens. In general, the SW part of the TR can be characterized by N‐S trending contractional structures. NW‐SE striking oblique ramps are nucleated above Late Triassic normal faults, whereas NE‐SW trending folds and thrust with limited strike‐parallel length evolved due to the inversion of pre‐existing breached relay ramps, which connected the segments of Late Triassic normal faults. We determined different types of oblique ramps, incuding normal faults, which suffered transpressional reactivation, oblique hanging wall breakthrough thrusts, oblique footwall short‐cut and hanging wall by‐pass thrusts. Only very steep normal faults, or those that strike at low angle to the regional E‐W shortening direction, were not reactivated.

Structural evolution and sediment depositional system along the transform margin- Palar–Pennar basin, Indian east coast

Continental transform margins are characterized by sub-vertical strike-slip faults and rapid change in thermal gradient across the margin from continental to oceanic crust. The continental crust terminates abruptly along such sheared margins. Tectonic evolution associated with sedimentation in continental slope of the Palar–Pennar transform margin is inadequately understood due to limited seismic data and/or poor imaging with few well controls. Exploratory drilling attempts for hydrocarbon presence have resulted in some promising outcomes in the Palar–Pennar basin.The Palar–Pennar graben in onland is bound by steep normal rift faults, which reactivated by strike-slip movement during late syn-rift time as Antarctica separated along the NNW to ∼ N-trending Coromandal transform margin in the offshore part. The pre-existing Precambrian shear zones and lineaments in the basement have played a governing role in the basin development. The onland drainage pattern is structurally controlled as observed from its rectangular drainage pattern, tectonic index and sinuosity index. In offshore area of the basin, seismic interpretation identifies distinct pull-apart rift basin along the Coromandal transform margin with syn-rift deposits in the lower continental slope portion. Strike-slip movement uplifted the fault blocks in the southern part of the pull-apart basin. This has also reactivated some of the rift faults in the syn-rift section. Such reactivation results in early post-rift seal breach. This tectonic event has implication in the depositional sequences, which has been portrayed by seismic stratigraphic analysis and construction of chronostratigraphic section across the transform margin in this work. Thickening of post-rift deposition towards E -SE, major hiatus, large scale mass transport complexes and erosional events have been identified from such analysis. On this basis, seven distinct tectonic sequences, M1 to M5 in Mesozoic and T1 and T2 in Tertiary, have been identified. The chronostratigraphic section can help in deciphering the relative sea level changes based on the nature of stratal terminations, extent of erosion and non-deposition. This study emphasizes the tectonic-controlled drainage system and geometry as well as sedimentation pattern in onland and offshore deepwater area of Palar – Pennar basin alongside the Coromondal transform margin.

GEOTHERMAL SYSTEMS IN THE NORTHERN APENNINES, ITALY: MODERN ANALOGUES OF CARLIN-STYLE GOLD DEPOSITS

Abstract Carlin-type gold deposits in northern Nevada are inferred to overlie concealed late Eocene plutons, which are increasingly thought to have provided magmatic input to the meteoric water-dominated fluids from which the gold was precipitated. The Larderello, Monte Amiata, and Latera geothermal systems in the Northern Apennines of southern Tuscany and northern Latium, central Italy, may represent Pliocene to present-day analogues because of their demonstrated association with subsurface plutons and jasperoid-hosted antimony-gold mineralization. The plutons, which at depths of >5–7 km remain at least partially molten, continue to supply heat and magmatic fluids to the meteoric water-dominated geothermal systems. Formerly mined antimony deposits of Pliocene or younger age are exposed on the peripheries of the CO2 ± H2S-emitting geothermal systems, and antimony sulfides are still actively precipitating. Stibnite and submicroscopic gold in disseminated pyrite, along with Au/Ag of <0.5 and anomalous As, Hg, Tl, and Ba values, accompanied jasperoid formation in the Northern Apennines systems. Carlin-type mineralization in northern Nevada and the antimony-gold mineralization in the Northern Apennines are hosted by permeable carbonate rocks, particularly stratabound breccias, where they are intersected by steep normal or oblique-slip faults and confined beneath tectonically emplaced hydrologic seals. The Northern Apennines antimony-gold mineralization formed at shallow, epithermal depths, like that recently recognized in the southern Carlin trend of northern Nevada. Although underexplored, the Northern Apennines gold prospects are unlikely to ever attain the giant status of the Carlin-type deposits in northern Nevada, probably because of lower magmatic fertility (ilmenite-series rather than magnetite-series magmatism) and host-rock receptivity (less reactive iron). Nevertheless, shallow carbonate-rock aquifers within high-temperature, intrusion-related geothermal systems, be they extinct or still active, may be prospective for Carlin-style gold deposits.

Direct dating of brittle extensional deformation contemporaneous of Neogene exhumation of the internal zones of the Rif Chain

The brittle deformation of internal zones of the Rif chain, corresponding to the southern margin of the West Alboran Basin, is not or poorly dated because of the lack of stratigraphic constraints. We provide for the first time a direct dating of brittle deformation of internal zones of the Rif chain, in the Ceuta peninsula located in the westernmost part of the Gibraltar arc. 40Ar/39Ar illite ages from fault gouges in the footwall and hanging wall of the Ceuta Shear Zone range from 11.9 ± 2.5 Ma to 15.7 ± 1.7 Ma and from 14.7 ± 0.1 Ma to 18.9 ± 0.8 Ma, respectively. In addition, (UTh)/He apatite ages from both units have been measured. They are younger in the hangingwall (~20–21 Ma) than in the footwall (14–19 Ma), which highlights slightly different cooling histories. Thus, we interpret the illite ages to date growth of authigenic illite (1M polytype) during exhumation through the top few kilometers of the earth's surface.This study of brittle deformation within the internal zones of the Rif chain reveals that an E-W directed extension occurred between ~18–15 Ma during the low temperature exhumation of Sebtide-Alpujárride units. From 15 to 11 Ma, the direction of extension shifted to NNW-SSE and only minor steep normal faults occurred. Brittle normal faulting occurred during a stage of slow cooling (~1–2 °C/myr) starting at ~19 Ma and coeval with exhumation processes within the uppermost levels of the continental crust. These new data emphasize a continuum of extensional deformation during the Burdigalian–Serravalian interval with an early E-W directed phase and then a NNW-SSE directed phase that match well the main subsidence pulses identified offshore in the neighboring West Alboran Basin.

Magnetic properties of pseudotachylytes from western Jämtland, central Swedish Caledonides

Abstract. Fault kinematics can provide information on the relationship and assembly of tectonic units in an orogen. Magnetic fabric studies of faults where pseudotachylytes form have recently been used to determine direction and sense of seismic slip in prehistoric earthquakes. Here we apply this methodology to study magnetic fabrics of pseudotachylytes in field structures of the Köli Nappe Complex (central Swedish Caledonides), with the aim to determine fault kinematics and decipher the role of seismic faulting in the assembly of the Caledonian nappe pile. Because the pseudotachylyte veins are thin, we focused on small (ca. 0.2 to 0.03 cm3) samples for measuring the anisotropy of magnetic susceptibility. The small sample size challenges conventional use of magnetic anisotropy and results acquired from such small specimens demand cautious interpretation. Importantly, we find that magnetic fabric results show inverse proportionality among specimen size, degree of magnetic anisotropy and mean magnetic susceptibility, which is most likely an analytical artifact related to instrument sensitivity and small sample dimensions. In general, however, it is shown that the principal axes of magnetic susceptibility correspond to the orientation of foliation and lineation, where the maximum susceptibility (k1) is parallel to the mineral lineation, and the minimum susceptibility (k3) is dominantly oriented normal to schistosity. Furthermore, the studied pseudotachylytes develop distinct magnetic properties. Pristine pseudotachylytes preserve a signal of ferrimagnetic magnetite that likely formed during faulting. In contrast, portions of the pseudotachylytes have altered, with a tendency of magnetite to break down to form chlorite. Despite magnetite breakdown, the altered pseudotachylyte mean magnetic susceptibility is nearly twice that of altered pseudotachylyte, likely originating from the Fe-rich chlorite, as implied by temperature-dependent susceptibility measurements and thin-section observations. Analysis of structural and magnetic fabric data indicates that seismic faulting occurred during exhumation into the upper crust, but these data yield no kinematic information on the direction and sense of seismic slip. Additionally, the combined structural field and magnetic fabric data suggest that seismic faulting was postdated by brittle E–W extensional deformation along steep normal faults. Although the objective of finding kinematic indicators for the faulting was not fully achieved, we believe that the results from this study may help guide future studies of magnetic anisotropy with small specimens (<1 cm3), as well as in the interpretation of magnetic properties of pseudotachylytes.

Distribution Features of the Nanhua‐Sinian Rifts and their Significance to Hydrocarbon Accumulation in the Tarim Basin

AbstractOn the basis of reprocessing 34 new two‐dimensional spliced long sections (20,191 km) in the Tarim Basin, the deep structure features of the Tarim Basin were analyzed through interpreting 30,451 km of two‐dimensional seismic data and compiling basic maps. Seismic interpretation and geological analysis conclude that the Nanhua‐Sinian strata are a set of rift‐depression depositional systems according to their tectonic and depositional features. The rift valley formed in the Nanhua Period, and the transformation became weaker during the late Sinian Period, which eventually turned into depression. From bottom to top, the deposited strata include mafic igneous, tillite, mudstone, and dolomite. Three major depocenters developed inside this basin during the rift stage and are distributed in the eastern Tarim Basin, the Awati area, and the southwestern Tarim Basin. Among them, the rift in the eastern Tarim Basin strikes in the near east‐west direction on the plane and coincides with the aeromagnetic anomaly belt. This represents a strong magnetic zone formed by upwelling basic volcanic rock along high, steep normal faults of the Nanhua Period. Controlled by the tectonic background, two types of sedimentary systems were developed in the rift stage and depression stage, showing two types of sequence features in the Sinian depositional stage. The Nanhua System appears as a wedge‐shaped formation, with its bottom in unconformable contact with the base. The rifting event has a strong influence on the current tectonic units in the Tarim Basin, and affects the distribution of source rock in the Yuertus Formation and reservoir beds in the Xiaoerbulake Formation in Lower Cambrian, as well as the gypseous cap rock in Middle Cambrian. The distribution features of the rifts have important and realistic significance for determining the direction of oil and gas exploration in the deep strata of the Tarim Basin. Comprehensive analysis suggests that the Tazhong region is the most favorable zone, and the Kalpin‐Bachu region is the optimal potential zone for exploring sub‐salt oil and gas in deep Cambrian strata.

Structure of massively dilatant faults in Iceland: lessons learned from high-resolution unmanned aerial vehicle data

Abstract. Normal faults in basalts develop massive dilatancy in the upper few hundred meters below the Earth's surface with corresponding interactions with groundwater and lava flow. These massively dilatant faults (MDFs) are widespread in Iceland and the East African Rift, but the details of their geometry are not well documented, despite their importance for fluid flow in the subsurface, geohazard assessment and geothermal energy. We present a large set of digital elevation models (DEMs) of the surface geometries of MDFs with 5–15 cm resolution, acquired along the Icelandic rift zone using unmanned aerial vehicles (UAVs). Our data present a representative set of outcrops of MDFs in Iceland, formed in basaltic sequences linked to the mid-ocean ridge. UAVs provide a much higher resolution than aerial/satellite imagery and a much better overview than ground-based fieldwork, bridging the gap between outcrop-scale observations and remote sensing. We acquired photosets of overlapping images along about 20 km of MDFs and processed these using photogrammetry to create high-resolution DEMs and orthorectified images. We use this dataset to map the faults and their damage zones to measure length, opening width and vertical offset of the faults and identify surface tilt in the damage zones. Ground truthing of the data was done by field observations. Mapped vertical offsets show typical trends of normal fault growth by segment coalescence. However, opening widths in map view show variations at much higher frequency, caused by segmentation, collapsed relays and tilted blocks. These effects commonly cause a higher-than-expected ratio of vertical offset and opening width for a steep normal fault at depth. Based on field observations and the relationships of opening width and vertical offset, we define three endmember morphologies of MDFs: (i) dilatant faults with opening width and vertical offset, (ii) tilted blocks (TBs) and (iii) opening-mode (mode I) fissures. Field observation of normal faults without visible opening invariably shows that these have an opening filled with recent sediment. TB-dominated normal faults tend to have the largest ratio of opening width and vertical offset. Fissures have opening widths up to 15 m with throw below a 2 m threshold. Plotting opening width versus vertical offset shows that there is a continuous transition between the endmembers. We conclude that for these endmembers, the ratio between opening width and vertical offset R can be reliably used to predict fault structures at depth. However, fractures associated with MDFs belong to one larger continuum and, consequently, where different endmembers coexist, a clear identification of structures solely via the determination of R is impossible.

The influence of faulting style on the size-distribution of global earthquakes

We derive a unifying formulation, reliable at all scales, linking Anderson's faulting theory with the earthquake size-distribution, whose exponent is known as the b-value. Anderson's theory, introduced in 1905, related fault orientation to stress conditions. Independently, laboratory measurements on acoustic emissions have established that the applied differential stress controls their b-value. Our global survey revealed that observed spatial variations of b are controlled by different stress regimes, generally being lower in compressional (subduction trenches and continental collisional systems) and higher in extensional regimes (oceanic ridges). This confirmed previous observations that the b-value depends on the rake angle of focal mechanisms. Using a new plunge/dip-angles-based b-value analysis, we also identified further systematic influences of faulting geometry: steep normal faults (also typical of the oldest subduction zones) experience the highest proportion of smaller events, while low-angle thrust faults (typical of youngest subduction zones) undergo proportionally larger, more hazardous, events, differently from what would be expected by only allowing for rake-angle dependency. To date, however, no physical model has ever been proposed to explain how earthquakes size-distribution, differential stress and faulting styles relate to each other. Here, we propose and analytically derive a unifying formulation for describing how fault orientation and differential stresses determine b-value. Our formulation confirms that b-values decay linearly with increasing differential stress, but it also predicts a different dip-dependent modulation according to the tectonic environment, opening up new ways of assessing a region's seismic hazard.

Intense Seismicity During the 2014–2015 Bárðarbunga‐Holuhraun Rifting Event, Iceland, Reveals the Nature of Dike‐Induced Earthquakes and Caldera Collapse Mechanisms

AbstractOver 2 weeks in August 2014, magma propagated 48 km laterally from Bárðarbunga volcano before erupting at Holuhraun for 6 months, accompanied by collapse of the caldera. A dense seismic network recorded over 47,000 earthquakes before, during, and after the rifting event. More than 30,000 earthquakes delineate the segmented dike intrusion. Earthquake source mechanisms show exclusively strike‐slip faulting, occurring near the base of the dike along preexisting weaknesses aligned with the rift fabric, while the dike widened largely aseismically. The slip sense of faulting is controlled by the orientation of the dike relative to the local rift fabric, demonstrated by an abrupt change from right‐ to left‐lateral faulting as the dike turns to propagate from an easterly to a northerly direction. Around 4,000 earthquakes associated with the caldera collapse delineate an inner caldera fault zone, with good correlation to geodetic observations. Caldera subsidence was largely aseismic, with seismicity accounting for 10% or less of the geodetic moment. Approximately 90% of the seismic moment release occurred on the northern rim, suggesting an asymmetric collapse. Well‐constrained focal mechanisms reveal subvertical arrays of normal faults, with fault planes dipping inward at ∼60° ± 9°, along both the north and south caldera margins. These steep normal faults strike subparallel to the caldera rims, with slip vectors pointing toward the center of subsidence. The maximum depth of seismicity defines the base of the seismogenic crust under Bárðarbunga as 6 km below sea level, in broad agreement with constraints from geodesy and geobarometry for the minimum depth to the melt storage region.

Local seismicity near the actively deforming Corbetti volcano in the Main Ethiopian Rift

Corbetti is currently one of the fastest uplifting volcanoes globally, with strong evidence from geodetic and gravity data for a subsurface inflating magma body. A dense network of 18 stations has been deployed around Corbetti and Hawassa calderas between February 2016 and October 2017, to place seismic constraints on the magmatic, hydrothermal and tectonic processes in the region. We locate 122 events of magnitudes between 0.4 and 4.2 using a new local velocity model. The seismicity is focused in two areas: directly beneath Corbetti caldera and beneath the city of Hawassa. The shallower 0–5 km depth below sea level (b.s.l.) earthquakes beneath Corbetti are mainly focused in EW- to NS-elongated clusters at Urji and Chabbi volcanic centres. This distribution is interpreted to be mainly controlled by a northward propagation of hydrothermal fluids away from a cross-rift pre-existing fault. Source mechanisms are predominantly strike-slip and different to the normal faulting away from the volcano, suggesting a local rotation of the stress-field. These observations, along with a low Vp/Vs ratio, are consistent with the inflation of a gas-rich sill, likely of silicic composition, beneath Corbetti. In contrast, the seismicity beneath Hawassa extends to greater depth (16 km b.s.l.). These earthquakes are focused on 8–10 km long segmented faults, which are active in seismic swarms. One of these swarms, in August 2016, is focused between 5 and 16 km depth b.s.l. along a steep normal fault beneath the city of Hawassa, highlighting the earthquake hazard for the local population.

Initiation of Tectonic Earthquakes Caused by Surface Mining

The influence of surface mining on the initiation of large seismic event is considered. The conditions of shearing-type dynamic events are described. A surface mine with the similar parameters as the Bachatsky open pit mine in Kuzbass is adduced as an example for quantifying the mining-induced change in the stress state in the plane of a future rupture as a result of an induced tectonic earthquake nearby a fault plane is quantified. The calculations are performed for different geometrical parameters of the fault zone: the changes are more observable in the zones of gently dipping thrust faulting and less appreciable in the area of steep normal faulting and strike-slip. In case of large surface mines, the zone of positive change in the Coulomb stresses higher than several tenths mega pascals has a considerable dimension and an area markedly larger than the area of nucleation zone of earthquakes of the magnitude M ≤ 6. In such conditions, even a small variability at the level of first percentage points of the natural stresses can be sufficient for the initiation of seismicity-generating shearing along the high-stress faults. It is found that, as against underground mining, the surface mining activities have no influence on localization of large earthquake sources but can draw the event nearer.

Static Stress Increase in the Outer Forearc Produced by MW 8.2 September 8, 2017 Mexico Earthquake and its Relation to the Gravity Signal

An Mw 8.2 earthquake affected on September 8, 2017 the Cocos plate beneath the Caribbean plate next to the transform contact with the North American plate across the Tehuantepec gap, one of the two gaps that exist along the Mexican subduction zone offshore Chiapas. The epicenter occurred at a depth of 47 km and was associated with a highly steep normal fault parallel to the trench. The origin can be related to slab pull forces, dehydration processes and reactivation of outer rise faults. This earthquake produced positive changes in the Coulomb static stress, which would favor the development of thrust displacements along the interplate contact across the Tehuantepec gap instead of liberating tensions accumulated on it for decades. In the case that stresses were not released steadily in the future, a thrusting-related event with a magnitude of Mw 7.9 or higher could be expected after the September 8, 2017 Chiapas earthquake. However, the differential weight of the forearc offshore (that contains a region with a high positive gravity gradient) could inhibit rupture propagation to the coast precluding higher magnitudes. Combined and satellite only earth gravity field models show a highly heterogeneous density structure along the forearc of the North America-Caribbean plates and in particular at the studied zone. Maximum displacements in the rupture zone correlate to minimum gravity-derived anomalies showing a probable relationship between the differential weight of the forearc structure and the extensional faulting affecting the bending of the Cocos plate at depth. In this sense, variable sediment thicknesses over the forearc, and a high gravity gradient zone parallel to the trench in the outer forearc, could be reflecting physical heterogeneities that influence the propagation of the rupture zone. Additionally, to the north, a highly positive gradient signal related to the subducted Tehuantepec Fz projection beneath the North American plate marks the ending of the main slip patch acting as a barrier to the seismic rupture propagation. This event exemplifies (1) how extensional ruptures in the downgoing slab of a subduction zone can increment accumulated elastic strain across a seismic gap instead of liberating tensions, (2) that rupture propagation for this intraplate event was probably controlled (at shallow depths) by the density structure of the forearc, similarly to recent interplate earthquakes along the Chilean margin; (3) how a low gravity gradient signal and low density structures identified from the inversion model in the region of the outer forearc, where the normal stress increased after the main seismic event, suggest a higher risk of occurrence of a great megathrust earthquake.

Crustal Radial Anisotropy and Linkage to Geodynamic Processes: A Study Based on Seismic Ambient Noise in Southern Madagascar

AbstractWe determined radial anisotropy in the crust of southern Madagascar from the differences between the speeds of vertically and horizontally polarized shear waves (VSV and VSH), which we derived from Rayleigh and Love wave dispersion determined from seismic ambient noise correlations. The amalgamated Precambrian units in the east and the Phanerozoic Morondava basin in the west of southern Madagascar were shaped by different geodynamic processes. The crystalline basement was strongly deformed and metamorphosed to varying degrees during the assembly of Gondwana in the Pan‐African Orogeny, whereas the Morondava basin was completed with the separation of Africa and Madagascar. The different developments are reflected in first‐order differences in the radial anisotropy patterns. In the Precambrian domains, positive anisotropy (VSV<VSH) is found in the upper and lower crust, with a layer of negative anisotropy (VSV>VSH) sandwiched in between. The upper crustal anisotropy may reflect shallowly dipping layering within the Archean and adjacent imbricated nappe stacks, whereas the lower crustal anisotropy likely represents fossilized crustal flow during the postorogenic or synorogenic collapse of the Pan‐African Orogen. The negative anisotropy layer may have preserved vertically oriented large shear zones of late Pan‐African age. Within the Morondava basin, negative anisotropy in the uppermost ∼5 km could have been generated by steep normal faults, jointing, and magmatic dike intrusions. The deeper sediments and underlying crustal basement are characterized by positive anisotropy. This is consistent with horizontal bedding in the sediments and with fabric alignment in the basement created by extension during the basin formation.

Surface and shallow subsurface structure of the Middle Kedrovaya paleoseismic rupture zone in the Baikal Mountains from geomorphological and ground-penetrating radar investigations

The 30-km-long Middle Kedrovaya (MK) paleoseismic rupture zone, on the northwestern flank of Lake Baikal, is one of most impressive late Quaternary fault scarps in the Baikal rift. The aim of this study is to reconstruct the near-surface fault geometry of the MK fault zone to clarify its past movement history and seismic potential. We applied ground penetrating radar (GPR), along with high-resolution satellite imagery and field surface observations, to analyze the structure of the upper 12–13 m of the rupture zone and to characterize the complex structural environment in three dimensions. The GPR reveals that the shallow structure of the MK zone is defined by a combination of steep and listric normal faults, which form a surface pattern of subparallel ruptures trending predominantly N30°E. Individual surface scarps range from 5 m to 2.7 km in length and are separated by gaps of a few tens of meters to several kilometers apart. The greatest width of the scarp zone is 1.9 km. The vertical displacements measured independently from surface topographic profiles and from GPR radargrams are linearly related. The height of the main footwall (head) scarp is always 0.5–2 m greater than the throw measured on the radargrams, a result of the scarp face having broadened far up the talus slope after the latest displacement event. Maximum and average vertical normal fault displacements along the main fault trace, inferred from GPR data, are 8.3 and 4.98 m respectively. Paleoearthquake magnitude M estimated from empirical regressions ranges from 6.8 to 7.6, based on equations of M vs. surface rupture length, M vs. displacement, and M vs. landslide volume. Our study of the Middle Kedrovaya paleoseismic rupture zone confirms that listric normal faulting is widespread in the shallow (<13 m) subsurface of the western margin of the North Baikal basin. However, GPR data suggest that listric faulting is confined to the unconsolidated talus deposits and probably represents the effects of downhill gravitational sliding of the talus, and perhaps of underlying shallow bedrock. This downhill (lakeward) sliding of the hanging wall is partly responsible for the large heave component of movement on the MK rupture zone and its resulting unique geomorphic expression.This paper illustrates the practicality of investigating fault surface ruptures in complex environments by integrating data gathered by geophysics (GPR in this case) and by direct geomorphological observations. Because of the loose and mobile nature of the faulted talus deposit, the current surface morphology disguises the true number of subsurface faults, the lateral extent of faulting, and the unique structure types (all visible on radargrams) that developed to accommodate coseismic surface rupture with a large heave component.

Infiltration of meteoric water in the South Tibetan Detachment (Mount Everest, Himalaya): When and why?

AbstractThe South Tibetan Detachment (STD) in the Himalayan orogen juxtaposes low‐grade Tethyan Himalayan sequence sedimentary rocks over high‐grade metamorphic rocks of the Himalayan crystalline core. We document infiltration of meteoric fluids into the STD footwall at ~17–15 Ma, when recrystallized hydrous minerals equilibrated with low‐δD (meteoric) water. Synkinematic biotite collected over 200 m of structural section in the STD mylonitic footwall (Rongbuk Valley, near Mount Everest) record high‐temperature isotopic exchange with D‐depleted water (δDwater = −150 ± 5‰) that infiltrated the ductile segment of the detachment most likely during mylonitic deformation, although later isotopic exchange cannot be definitively excluded. These minerals also reveal a uniform pattern of middle Miocene (15 Ma) 40Ar/39Ar plateau ages. The presence of low‐δD meteoric water in the STD mylonitic footwall is further supported by hornblende and chlorite with very low δD values of −183‰ and −162‰, respectively. The δD values in the STD footwall suggest that surface‐derived fluids were channeled down to the brittle‐ductile transition. Migration of fluids from the Earth's surface to the active mylonitic detachment footwall may have been achieved by fluid flow along steep normal faults that developed during synconvergent extension of the upper Tethyan Himalayan plate. High heat flow helped sustain buoyancy‐driven fluid convection over the timescale of detachment tectonics. Low δD values in synkinematic fluids are indicative of precipitation‐derived fluids sourced at high elevation and document that the ground surface above this section of the STD had already attained similar‐to‐modern topographic elevations in the middle Miocene.

On the Origin of Low-Angle Detachment Faults

Data from California's Whipple Mountains suggest this complex was formed by a succession of steep normal faults, challenging the paradigm that detachments are different types of faults.

The seismogenic fault of the 2010 Efpalion moderate-size seismic sequence (western Corinth gulf, Central Greece)

The 2010 Efpalion seismic sequence with two main moderate earthquake events occurred in the northwestern part of the Corinth Gulf (Central Greece)—a region that has been intensely stretched due to an on-going N–S extensional stress regime. Previous studies assign these two events to activations of (a) two faults dipping to the north with low angles; (b) two faults dipping at high angles, the first dipping to the south, and the second to the north; and (c) two faults dipping at high angles, but the first dips to the north, and the second to the south. The recently proposed TR method for focal mechanisms that identifies the seismogenic fault of an earthquake sequence is applied on the available focal mechanisms of the sequence, and its results are interrelated with the geology of the region, and previous contradictory interpretations. The focal mechanisms constructed with MT inversion define a steep north-dipping normal fault, whereas those constructed with first motions of P-waves define the activation of two adjoining faults that dip with high angles to the SSE and south, respectively, and which are characterized by strain (slip) compatibility. The latter option fits well with the geology of the region that is dominated by a SE to S-dipping horse-tail splay fault zone which exists at the eastern tip of the Nafpaktos Mountain Front. The application of the TR method reveals that the usage itself of different methods for the construction of the focal mechanisms complicates the problem of correctly identifying the seismogenic fault.