Wide Fault Zone Research Articles

Boron-bearing skarn-magnetite mineralization of the Mingbulak ore field, Eastern Uzbekistan

Spatial distribution of skarn-magnetite deposits in the Tien Shan region is closely related to gabbroid magmatism that accompanies wide fault zones bordering the South Tien Shan folded system from the north and south and extending as a latitudinal belt towards the Middle and Southwestern Tien Shan.Studies fulfilled during recent decades in the Karzhantau Ridge area have quite clearly demonstrated its gold-copper-bismuth metallogenic specialization with the leading role of gold. The most promising position is characteristic of the Aktash group ore occurrences (Aktam-I, Aktam-II, and Yuzhnoe).Within the studied Mingbulak area of Karzhantau, the most widely distributed are mineral occurrences of iron ore, lead and zinc, gold, and copper, and less developed boron-bearing, tungsten, and fluorite ones. Most of the mineral occurrences in the area formed as a result of magmatic-hydrothermal activity and are confined to fault zones characterized by long duration and complexity of the formation. The mineralization is usually localized in the crush zones and feathering fractures of the faults. The skarn type includes ore occurrences of iron, a metal that is currently strongly deficient for mining enterprises in the Tashkent region.

Long silence on the East Anatolian Fault Zone (Southern Turkey) ends with devastating double earthquakes (6 February 2023) over a seismic gap: implications for the seismic potential in the Eastern Mediterranean region

On 6 February 2023, an earthquake with magnitude M w c . 7.0 on the Narlı Fault, a fault sub-parallel to the East Anatolian Fault Zone (EAFZ), initiated a chain of large earthquakes on the EAFZ. The earthquakes occurred in a seismic gap with low geodetic strain rates and low background seismicity, where deformation is distributed across a wide fault zone and a long recurrence time of historical earthquakes. The c. 50 km long rupture of the Narlı Fault towards Pazarcık led to an M w 7.8 left-lateral strike-slip earthquake breaking a c. 300 km section of the c. 600 km long EAFZ bilaterally with a total duration of more than 80 s. Toward the SW, the rupture propagated on the c. 100 km long Amanos segment with a peak surface offset of 5 m, before diminishing toward the Hatay graben. In the NE direction, the rupture reached a peak surface offset of 7 m before sharply declining at the termination of the 2020 M w 6.8 Sivrice earthquake rupture. A second large earthquake with M w 7.6 occurred 9 h later on the Çardak Fault, located at the western margin of (and sub-parallel to) the EAFZ and breaking the surface with almost 9 m of left-lateral slip (average of c. 4 m). Following these large earthquakes, the increase in the regional stress led to a rapid seismic activation in a broad region from central to eastern Anatolia, loading the faults at various scales and increasing seismic hazard. Two weeks after the initiation of the seismic crisis, a third earthquake with M w 6.4 occurred at the southern boundary of the Hatay graben, near the southwestern termination of the Amanos rupture. The earthquakes caused significant loss of human life, devastating 12 cities. We evaluate the observations prior to the ruptures, and present preliminary seismological results with surface displacements from sub-pixel correlation of optical satellite images and the stress perturbations computed on the nearby faults based on preliminary slip models. The re-evaluation of the seismic potential in light of the recent and historical earthquakes provides some new insight on seismic hazard assessment. The recent series of events on the EAFZ is an important reminder that large faults can generate very large earthquakes on multiple segments. The seismic potential of large earthquakes on these fault zones can be estimated only by considering multiple seismic cycles and moment deficits from very large earthquakes. Supplementary material : Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6567094

Late Paleozoic Evolution of the Anadarko Basin: Implications for Laurentian Tectonics and the Assembly of Pangea

AbstractInterplay between marginal and intraplate late Paleozoic tectonics in southeastern Laurentia is a subject of debate. In southern Laurentia, intraplate deformation has been attributed to combinations of compressive forces from orogenic belts in the west, southwest, and southeast. While left‐lateral transpression is generally an accepted mechanism for formation of uplifts in the Southern Oklahoma Aulacogen (SOA), to date no studies have explained the driving mechanisms of these kinematics. Here, we analyze the Southern Oklahoma Transpressional System (SOTS), a ∼500 km long ∼50 km wide fault zone that extends from the southeastern Laurentian margin into the plate interior along Cambrian lithospheric weaknesses inherited from the SOA. We use a compilation of published studies to present detailed, time‐integrated sedimentary thickness maps of SOTS‐related basins. We combine these maps with a new interpretation of three 2D seismic reflection lines to constrain timing and kinematics of all major late Paleozoic SOTS fault zones. We show that the SOTS kinematics evolved from contraction to left‐lateral strike‐slip during Mississippian‐Pennsylvanian time. Tectonic activity and fault kinematics in the SOTS are explained by a model in which a remnant ocean basin closed diachronously from northeast to southwest along the Southern Appalachian and Ouachita‐Marathon orogens. Left‐lateral transpression during diachronous closure of the Rheic oceanic basin along the Ouachita‐Marathon margin is related to rotation of the stress field due to changes in slab‐pull and eastward drift of Laurentia toward Gondwana. We speculate that the SOTS acted in part as a STEP (subduction‐transfer edge propagator) fault that followed the pre‐existing SOA.

Seismicity in a weak crust: the transtensional tectonics of the Brawley Seismic Zone section of the Pacific–North America Plate Boundary in Southern California, USA

SUMMARYThe Brawley Seismic Zone (BSZ) is a ∼58-km-long section of the Pacific–North America Plate boundary that connects the southernmost San Andreas Fault (SAF) and the Imperial Fault in southern California. We analyse the BSZ as two segments: a north segment, dominated by SAF tectonics, and a south segment that accommodates a higher level of seismicity. The south segment includes a ∼6 km wide right lateral step-over that includes the Salton Sea geothermal field (SSGF) and Holocene subaerial lava domes at the south end of the Salton Sea, called Salton Buttes. In general, the 40 yr (1981–2021) of BSZ seismicity is characterized by transitory sequences that often are accommodated simultaneously or in quick succession on intricate ladder-like faults. These sequences rarely reactivate faults associated with previous seismicity alignments but rather activate adjacent faults, sometimes located within less than 1 km. They can include several events of similar size as the mainshock, followed by bursts of aftershocks sometimes located away from the mainshock rupture. The seismicity rate and the spatial geometrical complexity varies between the BSZ-north and BSZ-south segments. The 24-km-long BSZ-north accommodates a ∼12-km-long linear trend of seismicity that extends from the SAF terminus into the Salton Sea, where moderate-sized northeast striking sequences form the rungs in a ladder-structure in a weak crust. The seismicity in this area is most likely influenced by the stress state of the SAF. In contrast, the 34 km long BSZ-south segment, which also has a weak crust, has accommodated larger sequences that illuminate irregular ladder-type faulting, with aftershocks defining linear distributions striking either north or northeast. The focal mechanisms exhibit a mostly strike-slip style of faulting with minor dip-slip faulting in the south Salton Sea area as well as the Mesquite basin to the south. The state of stress, as determined from focal mechanisms, consists of almost horizontal σ1, and σ3 with vertical σ2. The activation angle between the trend of σ1 and the preferred nodal plane of the largest nearby event decreases systematically from north to south along the long-axis of the BSZ. In the step-over zone, the Holocene volcanism and the frequent seismicity sequences suggest crustal extension as well as associated reduced crustal strength as compared to other parts of the BSZ. The presence of weak, thin, hot crust and distributed ladder-like or conjugate predominantly strike-slip faulting suggests that the whole BSZ acts as a several kilometre wide fault zone where no prominent principal slip surfaces are required to accommodate moderate-sized (M6+) earthquakes.

Transient conduit permeability controlled by a shift between compactant shear and dilatant rupture at Unzen volcano (Japan)

Abstract. The permeability of magma in volcanic conduits controls the fluid flow and pore pressure development that regulates gas emissions and the style of volcanic eruptions. The architecture of the permeable porous structure is subject to changes as magma deforms and outgasses during ascent. Here, we present a high-resolution study of the permeability distribution across two conduit shear zones (marginal and central) developed in the dacitic spine that extruded towards the closing stages of the 1991–1995 eruption at Unzen volcano, Japan. The marginal shear zone is approximately 3.2 m wide and exhibits a 2 m wide, moderate shear zone with porosity and permeability similar to the conduit core, transitioning into a ∼ 1 m wide, highly sheared region with relatively low porosity and permeability, as well as an outer 20 cm wide cataclastic fault zone. The low-porosity, highly sheared rock further exhibits an anisotropic permeability network, with slightly higher permeability along the shear plane (parallel to the conduit margin), and is locally overprinted by oblique dilational Riedel fractures. The central shear zone is defined by a 3 m long by ∼ 9 cm wide fracture ending bluntly and bordered by a 15–40 cm wide damage zone with permeability enhanced by ∼ 3 orders of magnitude; directional permeability and resultant anisotropy could not be measured from this exposure. We interpret the permeability and porosity of the marginal shear zone to reflect the evolution of compactional (i.e. ductile) shear during ascent up to the point of rupture, which was estimated by Umakoshi et al. (2008) at ∼ 500 m depth. At this point the compactional shear zone would have been locally overprinted by brittle rupture, promoting the development of a shear fault and dilational Riedel fractures during repeating phases of increased magma ascent rate, enhancing anisotropic permeability that channels fluid flow into and along the conduit margin. In contrast, we interpret the central shear zone as a shallow, late-stage dilational structure, which partially tore the core of the spine, leaving a slight permanent displacement. We explore constraints from monitored seismicity and stick-slip behaviour to evaluate the rheological controls, which accompanied the shift from compactional toward dilational shear as magma approached the surface, and discuss their importance in controlling the permeability development of magma evolving from overall ductile to increasingly brittle behaviour during ascent and eruption.

Long‐Term Weakening Processes and Short‐Term Seismic Slip Behavior of an Intraplate Mature Fault Zone: A Case Study of the Yangsan Fault, SE Korea

AbstractAn intraplate tectonic fault evolves with repeated seismic cycles that may exhibit multi‐mode slip behaviors (continuous aseismic sliding, slow slip with silent earthquakes, and earthquakes resulting from stick‐slip frictional instability). Here, we show long‐term (geological time scale) fault zone weakening processes and short‐term (seismic cycle scale) fault behavior of the Yangsan Fault, SE Korea, based on structural and mineralogical observations of an across‐strike fault zone sequence retrieved by inclined borehole drilling. Since the Late Cretaceous, physical and chemical weakening processes have formed a wide and complex fault zone characterized by interconnected clay‐rich fault core strands and fractured lenses. The deformation is more concentrated in the eastern block (Cretaceous sedimentary rock with felsic dikes) of the fault zone than in the western block (Paleogene granite) of the fault zone. The asymmetrical deformation pattern is controlled by the distinct properties of the different host rocks and differential fluid‐assisted reaction softening involving hydrothermal processes. Of the five main strands and numerous subsidiary strands in the wide fault zone, strand S1 between the sedimentary rock and granite has accommodated the largest displacements. In particular, the smectite‐rich, <2‐cm‐wide principal slip zone within S1 has acted as the major pathway for past seismic slip. Our findings consider that slow deformation in the sedimentary rocks formed the asymmetrical wide fault zone over long geological time, whereas that the preferential propagation of repeated seismic slip events propagated preferentially along the narrow principal slip zone of S1.

Application of Automated Throw Backstripping Method to Characterize Recent Faulting Activity Migration in the Al Hoceima Bay (Northeast Morocco): Geodynamic Implications

Automation of the throw backstripping method has proven to be an effective tool for the determination of the evolution of tectonic activity in wide fault zones. This method has been applied to the Al Hoceima Bay (southwesternmost Mediterranean, Alboran Sea) for a time period covering the last 280 kyr on 672 faults imaged on 265 high-resolution seismic reflection profiles. This area was affected by major earthquakes and corresponds to a transtensional basin deformed by growth faults. The automated application of throw backstripping allowed for a faster deciphering of the migration of tectonic activity. Results show a westward migration of the deformation with quickly increasing deformation rates in the most recent time frames near Al Hoceima, one of the most populated cities. This migration is in agreement with the current seismicity, the GPS data, and recent brittle deformation data. Vertical throw rates of up to 0.47 mm/year have been calculated, for the most recent time periods, in segments of the Bokkoya fault zone. The westward migration of the deformation fits with the reconstruction suggested by the westernmost Mediterranean geodynamic models during the Pleistocene epoch, and it might be the consequence of the interaction between the northwest (NW) movement of the South Alboran indenter and the back Rif south-westward displacement. The highly accurate constraints of the evolution of the tectonic activity offered by this automation will substantially improve the seismic hazard assessment.

Fault reactivation and propagation in the northern Adamello pluton: The structure and kinematics of a kilometre-scale seismogenic source

The northern Adamello batholith (European Southern Alps) is crosscut by E-W trending pseudotachylyte-bearing paleoseismic fault zones composed of multiple subparallel fault strands. The main faults are the Gole Strette Fault Zone (GSFZ), here discussed in detail, and the Gole Larghe Fault Zone (GLFZ). The western ending of the GSFZ intersects the propagation of the Gallinera Thrust, a regional structure of Late Cretaceous age, which was truncated and dismembered by the late Eocene - early Oligocene emplacement of the Adamello batholith. Fault slip analysis and paleostress reconstruction suggest that the GSFZ and the Gallinera Thrust were both active during dextral transpression related with Oligocene orogen-parallel shearing along the Periadriatic Fault System. The remnants of the Gallinera Thrust within the Adamello were reactivated as dextral-reverse faults and do not include pseudotachylytes. The pseudotachylyte-bearing GSFZ shows changing structural features from west to east: the fault dip angle decreases from 80° to 45°, the slip vector passes from strike-slip (pitch of 20°) to oblique (pitch of 35° to 45°), and the fault zone thickness increases (> 800 m at the intersection with the GLFZ). The GSFZ is an immature and strong fault as indicated by (i) the local geometry controlled by precursor joints; (ii) the along-strike segmentation; and (iii) the spread of seismogenic faults into wide fault zones.

Creep Along the Central San Andreas Fault From Surface Fractures, Topographic Differencing, and InSAR

AbstractImaging tectonic creep along active faults is critical for measuring strain accumulation and ultimately understanding the physical processes that guide creep and the potential for seismicity. We image tectonic deformation along the central creeping section of the San Andreas Fault at the Dry Lake Valley paleoseismic site (36.468°N, 121.055°W) using three data sets with varying spatial and temporal scales: (1) an Interferometric Synthetic Aperture Radar (InSAR) velocity field with an ~100‐km footprint produced from Sentinel‐1 satellite imagery, (2) light detection and ranging (lidar) and structure‐from‐motion 3‐D topographic differencing that resolves a decade of deformation over a 1‐km aperture, and (3) surface fractures that formed over the 3‐ to 4‐m wide fault zone during a drought from late 2012 to 2014. The InSAR velocity map shows that shallow deformation is localized to the San Andreas Fault. We demonstrate a novel approach for differencing airborne lidar and structure‐from‐motion topography that facilitates resolving deformation along and adjacent to the San Andreas Fault. The 40‐m resolution topographic differencing resolves a 2.5 ± 0.2 cm/yr slip rate localized to the fault. The opening‐mode fractures accommodate cm/yr of fault slip. A 90% ± 30% of the 1‐km aperture deformation is accommodated over the several meter‐wide surface trace of the San Andreas Fault. The extension direction inferred from the opening‐mode fractures and topographic differencing is 40°–48° from the local trend of the San Andreas Fault. The localization of deformation likely reflects the well‐oriented and mature fault.

Geometry and topology of tectonolineaments in the Gorce Mts. (Outer Carpathians) in Poland

The present study was focused on analysis of the geometrical and topological features of the lineament network in the Gorce Mountains, Polish Outer Carpathians. The source data was a 1m high-resolution LiDAR digital elevation model (DEM). Network topology was analysed using the NetworkGT toolset for ArcGIS software. The topological analysis represented the first attempt to apply research of this type in the Polish Carpathians. Five lithostratigraphic units occurring in the research area, characteristic of the Krynica Zone of the Magura Nappe, were selected for analysis. Topology was studied for each of the units separately as well as for the whole area. It was found that the units were differentiated in terms of some topological parameters, such as 2D intensity. Moreover, latitudinal trends were observed due to the presence of wide fault zones. The geometry of the network is consistent with the general Carpathian overview. Sets of conjugate strike slip-faults with splaying ends were identified. In the southern part of the area, the strike of the oldest faults was E-W. This shows that on the scale of the observation (hundreds to thousands of metres), the network was a product of general thrusting processes rather than derivative of a local structural plan. The applicability of both topological analyses and high-resolution DEM as supplementary to classical methods in a study of the Outer Carpathians was confirmed with analysis conducted, despite the identification of some limitations, such as GIS-operator-related bias. The scale of the analyses is linked, in a multi-scale approach, to structural network analyses.

On the morphology and amplitude of 2D and 3D thermal anomalies induced by buoyancy-driven flow within and around fault zones

Abstract. In the first kilometers of the subsurface, temperature anomalies due to heat conduction processes rarely exceed 20–30 ∘C. When fault zones are sufficiently permeable, fluid flow may lead to much larger thermal anomalies, as evidenced by the emergence of thermal springs or by fault-related geothermal reservoirs. Hydrothermal convection triggered by buoyancy effects creates thermal anomalies whose morphology and amplitude are not well known, especially when depth- and time-dependent permeability is considered. Exploitation of shallow thermal anomalies for heat and power production partly depends on the volume and temperature of the hydrothermal reservoir. This study presents a non-exhaustive numerical investigation of fluid flow models within and around simplified fault zones, wherein realistic fluid and rock properties are accounted for, as are appropriate boundary conditions. 2D simplified models point out relevant physical mechanisms for geological problems, such as “thermal inheritance” or pulsating plumes. When permeability is increased, the classic “finger-like” upwellings evolve towards a “bulb-like” geometry, resulting in a large volume of hot fluid at shallow depth. In simplified 3D models wherein the fault zone dip angle and fault zone thickness are varied, the anomalously hot reservoir exhibits a kilometer-sized “hot air balloon” morphology or, when permeability is depth-dependent, a “funnel-shaped” geometry. For thick faults, the number of thermal anomalies increases but not the amplitude. The largest amplitude (up to 80–90 ∘C) is obtained for vertical fault zones. At the top of a vertical, 100 m wide fault zone, temperature anomalies greater than 30 ∘C may extend laterally over more than 1 km from the fault boundary. These preliminary results should motivate further geothermal investigations of more elaborated models wherein topography and fault intersections would be accounted for.

Back-propagating supershear rupture in the 2016 Mw 7.1 Romanche transform fault earthquake

How an earthquake rupture propagates strongly influences the potentially destructive ground shaking. Complex ruptures often involve slip along multiple faults, which masks information on the frictional behaviour of fault zones. Geometrically smooth ocean transform fault plate boundaries offer a favourable environment to study fault dynamics, because strain is accommodated along a single, wide fault zone that offsets the homogeneous geology. Here we present an analysis of the 2016 Mw 7.1 earthquake on the Romanche fracture zone in the equatorial Atlantic, using data from both nearby seafloor seismometers and global seismic networks. We show that this rupture had two phases: (1) upward and eastward propagation towards a weaker region where the transform fault intersects the mid-ocean ridge, and then (2) an unusual back-propagation westwards at a supershear speed towards the centre of the fault. We suggest that deep rupture into weak fault segments facilitated greater seismic slip on shallow locked zones. This highlights that even earthquakes along a single distinct fault zone can be highly dynamic. Observations of back-propagating ruptures are sparse, and the possibility of reverse propagation is largely absent in rupture simulations and unaccounted for in hazard assessments.

An analysis of the factors that control fault zone architecture and the importance of fault orientation relative to regional stress

AbstractThe moment magnitude 7.2 El Mayor–Cucapah (EMC) earthquake of 2010 in northern Baja California, Mexico produced a cascading rupture that propagated through a geometrically diverse network of intersecting faults. These faults have been exhumed from depths of 6–10 km since the late Miocene based on low-temperature thermochronology, synkinematic alteration, and deformational fabrics. Coseismic slip of 1–6 m of the EMC event was accommodated by fault zones that displayed the full spectrum of architectural styles, from simple narrow fault zones (< 100 m in width) that have a single high-strain core, to complex wide fault zones (> 100 m in width) that have multiple anastomosing high-strain cores. As fault zone complexity and width increase the full spectrum of observed widths (20–200 m), coseismic slip becomes more broadly distributed on a greater number of scarps that form wider arrays. Thus, the infinitesimal slip of the surface rupture of a single earthquake strongly replicates many of the fabric elements that were developed during the long-term history of slip on the faults at deeper levels of the seismogenic crust. We find that factors such as protolith, normal stress, and displacement, which control gouge production in laboratory experiments, also affect the architectural complexity of natural faults. Fault zones developed in phyllosilicate-rich metasedimentary gneiss are generally wider and more complex than those developed in quartzo-feldspathic granitoid rocks. We hypothesize that the overall weakness and low strength contrast of faults developed in phyllosilicate rich host rocks leads to strain hardening and formation of broad, multi-stranded fault zones. Fault orientation also strongly affects fault zone complexity, which we find to increase with decreasing fault dip. We attribute this to the higher resolved normal stresses on gently dipping faults assuming a uniform stress field compatible with this extensional tectonic setting. The conditions that permit slip on misoriented surfaces with high normal stress should also produce failure of more optimally oriented slip systems in the fault zone, promoting complex branching and development of multiple high-strain cores. Overall, we find that fault zone architecture need not be strongly affected by differences in the amount of cumulative slip and instead is more strongly controlled by protolith and relative normal stress.

On the relation between the rare-earth — rare-metal and gold ore mineralization and fault-block tectonics of the Ukrainian Shield. 1

The paper, which is the first of a series of articles on the relation of metallogeny and fault-block tectonics of the Ukrainian Shield (USh), summarizes data on the chemical-mineralogical composition and the exact location of deposits, ore occurrences and points of increased rare-earth — rare-metal and gold mineralization, which are combined into eight groups: TR, U and Th, W and Mo, Be and Sn, Ta, Nb and other rare metals, zir-con and monazite (Zr), Au, F. They are compared with the scheme of the fault-block structure of the shield constructed from the results of tectonophysical studies. A detailed description of the fault zones of the USh has been given and many of them are shown to be composed of packets of a budinized and cataclasized varicoloured complex: metamorphosed basites and ultrabasites, calciphyres, quartzites, gneisses, micas, skarns, plagioclasites, quartz-feldspar rocks and silicification zones. The formation of such packages of the varicoloured complexes is associated with shear processes in wide (from 7 to 20 km) fault zones, accompanied by tectonic stratification of the basement rocks, dynamometamorphism, diaphthoresis, metasomatism, etc. The age of the fault zones of the USh and correlation with the formation of ore mineralization are considered in this article. It is emphasized that the rock complexes of the Precambrian faults are brought to the surface from considerable depths and are determined by the conditions of deep stress metamorphism and granitization during shifts, therefore they cannot be compared with complexes of modern faults. However, the structural parageneses of Precambrian faults in their geometric relationships are close to those obtained from the results of physical experiments on brittle-plastic and plastic materials. This makes possible to determine the nature of the stress-strain state of the Earth’s crust during the process of fault formation and the parameters of the stress fields. It is established that 94 % of all deposits and ore occurrences are located within fault zones, the area of which occupies about 22 % of the USh area. The obtained data refine the known metallogenic maps of the USh and allow reducing the priority areas in four to five times when planning future mapping and prospecting works for the groups of deposits under consideration.

Laboratory evidence of strength recovery of a healed fault: implications for a mechanism responsible for creating wide fault zones

Fault zones consist of a high-strain fault core and a surrounding damage zone of highly fractured rock. The close, reciprocal relationship between fault zones and earthquake rupture evolution demands better understanding of the processes that create and modify damage zones. This study modeled the evolution of a damage zone in the laboratory by monitoring seismic signals (acoustic emissions) in a specimen of ultramylonite stressed to failure. The result provided evidence supporting the strength recovery of parts of the healed surface. A new fault initiated in an area of heterogeneous structure a short distance from the preexisting fault plane. Repeated cycles of fracture and healing may be one mechanism responsible for wide fault zones with multiple fault cores and damage zones.

Geodetic exploration of strain along the El Pilar Fault in northeastern Venezuela

AbstractWe use Global Navigation Satellite Systems observations in northeastern Venezuela to constrain the El Pilar Fault (EPF) kinematics and to explore the effects of the variable elastic properties of the surrounding medium and of the fault geometry on inferred slip rates and locking depth. The velocity field exhibits an asymmetric velocity gradient on either side of the EPF. We use five different approaches to explore possible models to explain this asymmetry. First, we infer a 1.6 km locking depth using a classic elastic half‐space dislocation model. Second, we infer a 1.5 km locking depth and a 0.33 asymmetry coefficient using a heterogeneous asymmetric model, including contrasting material properties on either side of a vertical fault, suggesting that the igneous‐metamorphic terranes on the northern side are ~2 times more rigid than the sedimentary southern side. Third, we use a three‐dimensional elastostatic model to evaluate the presence of a compliant zone, suggesting a 30% reduction of rigidity in the upper 3 km at the depth of a 1 to 5 km wide fault zone. Fourth, we evaluate the distribution of fault slip, revealing a widespread partial creep pattern in the eastern upper segment, while the upper western segment exhibits a partially locked area, which coincides with the rupture surface of the 1797 and 1929 earthquakes. To supplement these models, we upgrade the previously published displacement simulation method using nonvertical dislocations with data acquired between 2003 and 2013. The localized aseismic displacement pattern associated with creeping or partially creeping fault segments could explain the low level of historic seismicity.

Internal structures and high-velocity frictional properties of Longmenshan fault zone at Shenxigou activated during the 2008 Wenchuan earthquake

This paper reports internal structures of a wide fault zone at Shenxigou, Dujiangyan, Sichuan province, China, and high-velocity frictional properties of the fault gouge collected near the coseismic slip zone during the 2008 Wenchuan earthquake. Vertical offset and horizontal displacement at the trench site were 2.8 m (NW side up) and 4.8 m (right-lateral), respectively. The fault zone formed in Triassic sandstone, siltstone, and shale about 500 m away from the Yingxiu-Beichuan fault, a major fault in the Longmenshan fault system. A trench survey across the coseismic fault, and observations of outcrops and drill cores down to a depth of 57 m revealed that the fault zone consists of fault gouge and fault breccia of about 0.5 and 250–300 m in widths, respectively, and that the fault strikes N62°E and dips 68° to NW. Quaternary conglomerates were recovered beneath the fault in the drilling, so that the fault moved at least 55 m along the coseismic slip zone, experiencing about 18 events of similar sizes. The fault core is composed of grayish gouge (GG) and blackish gouge (BG) with very complex slip-zone structures. BG contains low-crystalline graphite of about 30 %. High-velocity friction experiments were conducted at normal stresses of 0.6–2.1 MPa and slip rates of 0.1–2.1 m/s. Both GG and BG exhibit dramatic slip weakening at constant high slip rates that can be described as an exponential decay from peak friction coefficient μp to steady-state friction coefficient μss over a slip-weakening distance Dc. Deformation of GG and BG is characterized by overlapped slip-zone structures and development of sharp slickenside surfaces, respectively. Comparison of our data with those reported for other outcrops indicates that the high-velocity frictional properties of the Longmenshan fault zones are quite uniform and the high-velocity weakening must have promoted dynamic rupture propagation during the Wenchuan earthquake.

The Neapolitan Yellow Tuff caldera offshore the Campi Flegrei: Stratal architecture and kinematic reconstruction during the last 15ky

Abstract In this study we integrate high-resolution swath bathymetry, single channel reflection seismic data and gravity core data, to provide new insights into the shallow structure and latest Quaternary to Holocene evolution of the submerged sector of the Neapolitan Yellow Tuff (NYT) caldera (Campi Flegrei) in the Pozzuoli Bay. The new data allow for a reconstruction of the offshore geometry of the NYT caldera collapse–ring fault system, along with the style and timing of deformation of the inner caldera resurgence. Our interpretation shows that the NYT eruption (~ 15 ka BP) was associated with a caldera collapse bounded by an inward-dipping ring fault system. The ring fault system consists in a 1–2 km wide fault zone that encircles an inner caldera region ~ 5 km in diameter and is often marked by the occurrence of pore fluids ascending through the fault zone, up to the seafloor, particularly in the western sector of the bay. A shallow magmatic intrusion along the ring fault zone was also detected offshore Bagnoli in the eastern part of the Pozzuoli Bay. Following the NYT eruption, the inner caldera region underwent significant deformation and resurgence with a maximum cumulative uplift of the offshore structure in the order of 180 m. The net uplift rate of the caldera resurgent dome was ~ 9–12 mm/year during the period 15.0–6.6 ka BP. The style of deformation of the resurgent structure can be described in terms of a broad doming, accompanied by subordinate brittle deformation, mostly concentrated in a small apical graben at the summit of the resurgent dome. Chronostratigraphic calibration of seismic profiles obtained by three tephra layers cored in the Pozzuoli Bay indicates 5 to 25 m of post-Roman differential subsidence and tilting towards ESE of the inner caldera resurgence, as recorded by the drowning of the infralittoral prograding wedge below the present-day storm wave base.

Ordovician sediments sandwiched between Proterozoic basement slivers: tectonic structures in the Stumsnäs 1 drill core from the Siljan Ring, central Sweden

The Stumsnäs 1 core, drilled in 2011 in the southern part of the Siljan Ring, represents a structurally complex section and provides new insights into the impact-induced local structure in central Sweden. The Siljan Ring was formed around the central uplift of a Late Devonian meteorite crater, the largest known impact structure in Europe. The Stumsnäs 1 core section reveals that about 90 m of Palaeozoic (Lower to Upper Ordovician) sedimentary rocks are sandwiched between Proterozoic igneous basement rocks. The sedimentary contact to the underlying ∼260 m of Proterozoic basement is a prominent unconformity. The contact to the overlying ∼190-m-thick slab of Proterozoic basement is a few metres wide complex fault zone, comprising alternating thin slices of sedimentary and granitic rocks together with fault breccia and gouge. The tectonic emplacement of basement rocks over the Palaeozoic sedimentary succession is apparently impact-related and caused folding and faulting of the underlying sediments, some of which were overturned and cut out. Minor fault zones occur throughout the Stumsnäs 1 core section and have large damage zones with intense fracture networks along which alteration and mineralisation took place, likely also impact-related. Small-scale faults and fractures are common and are critical for fluid migration and hence for ongoing exploration for natural gas and geothermal energy reservoirs in the Siljan impact structure.

Stable carbon isotope behaviour of natural seepage of deep underground 13C-rich methane detected along a fault zone and adsorbed in mudstone: Tokyo Bay area, Japan

Abstract Gas was sampled regionally, including by drilling into faults, in the South Kanto gas-field around Tokyo Bay, Japan. Gas samples were collected from cores in a gas sampling container immediately after drilling. A value of δ13C1 = −44.3‰ was obtained for gas in the container and δ13C1 = −36.3‰ for seeping gas in a fault zone. However, typical CH4 in this dissolved-in-water gas-field is mainly depleted in 13C, and δ13C1 values range from −66‰ to −68‰ owing to microbial degradation of organic material. 13C-rich CH4 is so far uncommon in the South Kanto gas-field. Seepages were observed from the surface along the north–south fault zone. The natural gas is stored below the sandstone layer by impermeable mudstone underlying the boundary at a depth of 30 m. Gas seepages were not observed below a depth of 40 m. Gas rises along the fault zone dissolved-in-groundwater up to the shallow region and then separates from the groundwater. 13C-rich CH4 (adsorbed CH4) was found to have desorbed from drilled mudstone core samples taken at depths of 1400–1900 m in the main gas-production strata. Similarly, 13C-rich CH4 was found in black shale overlying the oceanic crust forming part of a sedimentary accretionary prism underling the Tokyo region. It also appears in the spring-water of spa wells, originating at a depth of 1200–1500 m along a tectonic line. Methane generated by microbial degradation of organic material through CO2 reduction in the South Kanto gas-field mainly originates as biogenic gas mixed with a small amount of 13C-rich CH4, derived from thermogenic gas without oil components in strata. It is assumed that 12C-rich CH4 is easily detached from core or pore water through gas production, whereas 13C-rich CH4 is strongly adsorbed on the surfaces of particles. The 13C-rich CH4 rises along the major tectonic line or up the 50 m wide normal fault zone from relatively deep sources in the Kanto region.