Extensional Stress Field Research Articles

Heterogeneous outer‐rise seismicity within the Izu–Bonin subduction zone and its tectonic implications

Combining with available focal mechanisms, we chose 13 events (M ≥ 3.5) for waveform inversion to determine the stress field at the outer‐rise region within the Izu–Bonin subduction zone. Indicated by the distribution of the epicentres, seismic energy release, and stress field within the outer‐rise region, the subduction zone is divided into three segments separated by 29°N and 32.5°N. The outer‐rise seismicity presents an extensional stress field in the upper part (0–40 km) of the subducting oceanic plate. Whereas, the maximum focal depth within the outer‐rise region tends to increase northwards, and the deep part (>40 km) of the subducting slab is gradually governed by a general compressional stress regime that has never been observed before, which implies that the subduction of the old Pacific Plate along the Izu–Bonin trench is characterized by a heterogeneous bending deformation. According to modelling the flexural bending of the oceanic lithosphere, it is observed that effective elastic thickness (Te) of the Izu–Bonin subduction zone decreases from 38 to 28 km along the trench strike, indicating a decrease of plate rigidity northwards. The spatial variations of Te exactly explain the heterogeneous outer‐rise seismicity within the Izu–Bonin subduction zone, which is the energy release of the bending formation of the subducting plate. As shown in the results presented here, the varied flexural deformation of the subducting Pacific Plate is closely correlated with the heterogeneous thermal age, the varied morphology, and the coupling degree of the subducting plate.

Permian rifting processes in the NW Junggar Basin, China: Implications for the post-accretionary successor basins

The Junggar Basin, bounded by the West Junggar along late Carboniferous strike-slip faults in the west, is a post- orogenic successor basin within the Neoproterozoic to Paleozoic Central Asian Orogenic Belt (CAOB). The Junggar Basin is a key region for understanding the Permian rifting processes since it is located in an extensional step-over zone between the Irtysh and North Tianshan strike-slip fault zones that were most active in the Permian. Based on new seismic reflection profiles, detrital petrology and zircon U-Pb ages, together with regional geological data and a refraction seismic profile, this study investigates the tectonic evolution and the geodynamic mechanics of the basin. The early Permian phase of syn-rift mechanical subsidence in the western Junggar Basin formed half grabens. Volcanic detritus is abundant, and detrital zircon U-Pb ages show sediment supply was entirely local. During the middle Permian post-rift phase with thermal subsidence, sediment supply exceeded subsidence, and there are increasingly distant sources of sediment. The Permian regional extensional stress field between major post- accretionary strike-slip fault zones triggered the reactivation of the pre-existing late Carboniferous strike-slip faults in West Junggar and produced thinning of the crust beneath the Junggar Basin, with a β crustal stretching factor of 1.74. Seismic refraction data reveals asthenospheric upwelling resulted in underplated mafic lower crust (6–10 km). The location of initial rift axis in the Junggar Basin was controlled by the juxtaposition of thin and weak crust beneath the basin and relatively strengthened thick crust in the West Junggar attributed to the cooling of A-type granites, resulting from localized strain. Similar location of post-orogenic rifting in southwestern CAOB, in areas of high heat flow following collision, suggests that Junggar Basin is an example of a more general process.

Crustal seismic structure and anisotropy of Madagascar and southeastern Africa using receiver function harmonics: interplay of inherited local heterogeneities and current regional stress

SummaryThis study investigates the seismic structure and anisotropy in the crust beneath Madagascar and southeastern Africa, using receiver functions. The understanding of seismic anisotropy is essential for imaging past and present deformation in the lithosphere–asthenosphere system. In the upper mantle, seismic anisotropy mainly results from the orientation of olivine, which deforms under tectonic (fossil anisotropy) or flow processes (in the asthenosphere). In the crust, the crystallographic alignment of amphiboles, feldspars (plagioclase) or micas or the alignment of heterogeneities such as fractures, add to a complex geometry, which results in challenges to understanding the Earth's shallow structure. The decomposition of receiver functions into backazimuth harmonics allows to characterize orientations of lithospheric structure responsible for azimuthally varying seismic signals, such as a dipping isotropic velocity contrasts or layers of azimuthal seismic anisotropy. By analysing receiver function harmonics from records of 48 permanent or temporary stations this study reveals significant azimuthally varying signals within the upper crust of Madagascar and southeastern Africa. At 30 stations crustal anisotropy dominates the harmonics while the signature of a dipping isotropic contrast is dominant at the remaining 18 stations. However, all stations’ backazimuth harmonics show complex signals involving both dipping isotropic and shallow anisotropic contrasts or more than one source of anisotropy at shallow depth. Our calculated orientations for the crust are therefore interpreted as reflecting either the average or the interplay of several sources of azimuthally varying signals depending of their strength. However, comparing information between stations allows us to draw the same conclusions regionally: in both southern Africa and Madagascar our measurements reflect the interplay between local, inherited structural heterogeneities and crustal seismic anisotropy generated by the current extensional stress field imposed by the southward propagation of the East-African Rift System. A final comparison of our crustal orientations with SKS orientations attributed to mantle deformation further probes the interplay of crustal and mantle anisotropy on SKS measurements.

Trace element and helium isotope geochemistry of the Cenozoic intraplate volcanism in the East Sea (Sea of Japan): Implications for lithosphere-asthenosphere interaction

Extensive intraplate volcanism in the East Asia regions, including Dokdo and Ulleungdo (DU), occurred due to the extensional stress field during the Cenozoic era. However, the origin of magmatism is still controversial. Here we report new results of helium isotopes in olivine and clinopyroxene phenocrysts as well as major and trace element compositions from the DU basalts in order to determine the origin of the DU magmas and provide insights into Cenozoic mantle dynamics in East Asia. The DU volcanic islands were formed by magmatism after opening of the East Sea (Sea of Japan), a back-arc basin behind the Japanese arc, and are currently located in the Ulleung basin, one of sub-basins of the East Sea. The 3He/4He ratios range from 5.7 to 5.9 Ra for Dokdo and 4.5 to 6.0 Ra for Ulleungdo, respectively, similar to the sub-continental lithospheric mantle (SCLM) range of East Asia (4.6 to 7.7 Ra). Helium isotope compositions indicate that the depleted asthenosphere or the lower mantle plume might not be the direct source of the DU magmas. Also, there is no geochemical evidence that the DU magmas were influenced by subduction-derived components or the HIMU mantle source. Hence, we argue that SCLM provided enriched geochemical features in the DU basalts, contrary to basalts from other back-arc basins composed of the oceanic crust. In this respect, it is likely that the extended SCLM may still remain beneath the Ulleung basin. Our helium isotope and trace element mixing model shows the mixing trend between the metasomatized lithospheric melts and the asthenospheric melts. In addition, the presence of the low-velocity zone beneath the surface of the DU volcanoes in seismic tomography implies that the magmas were formed through the lithospheric melting due to the hot asthenospheric upwelling. Therefore, our study shows that the DU volcanism was mainly contributed by SCLM and that the lithosphere-asthenosphere interaction was the main mechanism that led to the Cenozoic magmatism in East Asia.

The Impact of Complex Volcanic Plumbing on the Nature of Seismicity in the Developing Magmatic Natron Rift, Tanzania

Constraining the architecture of complex 3D volcanic plumbing systems within active rifts, and their impact on rift processes, is critical for examining the interplay between faulting, magmatism and magmatic fluids in developing rift segments. The Natron basin of the East African Rift System provides an ideal location to study these processes, owing to its recent magmatic-tectonic activity and ongoing active carbonatite volcanism at Oldoinyo Lengai. Here, we report seismicity and fault plane solutions from a 10 month-long temporary seismic network spanning Oldoinyo Lengai, Naibor Soito volcanic field and Gelai volcano. We locate 6,827 earthquakes with ML−0.85 to 3.6, which are related to previous and ongoing magmatic and volcanic activity in the region, as well as regional tectonic extension. We observe seismicity down to ∼17 km depth north and south of Oldoinyo Lengai and shallow seismicity (3–10 km) beneath Gelai, including two swarms. The deepest seismicity (∼down to 20 km) occurs above a previously imaged magma body below Naibor Soito. These seismicity patterns reveal a detailed image of a complex volcanic plumbing system, supporting potential lateral and vertical connections between shallow- and deep-seated magmas, where fluid and melt transport to the surface is facilitated by intrusion of dikes and sills. Focal mechanisms vary spatially. T-axis trends reveal dominantly WNW-ESE extension near Gelai, while strike-slip mechanisms and a radial trend in P-axes are observed in the vicinity of Oldoinyo Lengai. These data support local variations in the state of stress, resulting from a combination of volcanic edifice loading and magma-driven stress changes imposed on a regional extensional stress field. Our results indicate that the southern Natron basin is a segmented rift system, in which fluids preferentially percolate vertically and laterally in a region where strain transfers from a border fault to a developing magmatic rift segment.

Evolution characteristics of transtensional faults and their impacts on hydrocarbon migration and accumulation: A case study from the Huimin Depression, Bohai Bay Basin, eastern China

The study of transtensional faults is significantly important for analysing migration and accumulation of hydrocarbons in petroliferous basins. The purpose of this study is to present disparities in the temporal and spatial evolution of two boundary transtensional faults (northern Linshang Fault and southern Xiakou Fault) in the Huimin Depression by 2D structural restoration and quantitative calculations of fault activity. This paper also reveals the controls of transtensional faults on the migration and accumulation of hydrocarbons by considering the stress normal to the fault plane (P), the shale gouge ratio (SGR) of the fault zone, and models of migration pathways. The results show that the transtensional systems within the Huimin Depression were generated from the reactivation of basement faults under an oblique extensional stress field. The Linshang Fault linked two initially separated segments and then behaved as a single fault since the Ek period, whereas the Xiakou Fault retained significant three-segment growth properties during the Paleogene. During the hydrocarbon charging period, the weak activity and strong vertical sealing restricted the occurrence of vertical migration around the Xiakou Fault, while the relatively strong movement and weak vertical sealing resulted in multilayer hydrocarbon accumulations around the Linshang Fault. Meanwhile, the vertically closed fault with high SGR (>0.6) acted as barriers and trapped hydrocarbons on the lateral migration pathways, whereas the vertically closed fault with low SGR (<0.6) served as conduits for further lateral migration into structurally higher positions. The discovered hydrocarbons are concentrated around transtensional fault systems, and the divergent E-W-striking brush-shaped secondary faults trapped abundant hydrocarbons within the hanging wall, demonstrating the important controls of the transtensional faults on the migration and accumulation of hydrocarbons in the Huimin Depression.

Early–Middle Jurassic paleogeography reconstruction in the Western Qaidam Basin: Insights from sedimentology and detrital zircon geochronology

The Western Qaidam Basin (WQB) is situated at the northeastern margin of the Tibetan Plateau. It is adjacent to the Altyn Tagh Range to the west and is bordered at the south and north by the Eastern Kunlun Range and Southern Qilian Range, respectively. Sporadic Jurassic outcrops are exposed along the Altyn Tagh Range. Although subsurface data have revealed that the Jurassic rocks also developed in the basin interior, little is known about their distribution and depositional characteristics. Hydrocarbon exploration has revealed that the Lower–Middle Jurassic strata are important source rocks and reservoirs in the WQB. However, due to post-depositional uplifting, exhumation or deep burial, the sediments provenance, paleogeography and the basin properties during the Early–Middle Jurassic in the whole WQB remain less understood. In this study, we examine the sedimentary records combined with detrital zircon U–Pb geochronology based on an analysis of post-depositional modification, to reveal the source-to-sink relationship between the WQB and surrounding potential source areas, and to further reconstruct the Early–Middle Jurassic paleogeography of the WQB. During the Early Jurassic, there were a series of small-scale, separated rift basins which were characterized by proximal sedimentation and rapid facies shift in the WQB. Specifically, the Lower Jurassic sediments in the southern WQB mainly originated from the Eastern Kunlun Range with minor supply from the Paleozoic intrusions that might have been exposed in the hinterland of the Qaidam Block; the provenance in the northern WQB was dominated by the uplift area to the west. There was a broader depression basin in the southern WQB. The Eastern Kunlun Range and the ancestral Altyn Mountains, served as the two dominant source areas for the clastic material of the southern WQB during the Middle Jurassic. Integrated analysis on sedimentology and provenance reveals that the southeastern Tarim Basin and the WQB were likely to have been interconnected or adjacent during Middle Jurassic period. We thus infer that no large-scale strike-slip displacement along the Altyn Tagh Fault occurred during that period. Combining regional tectonic settings, we tentatively suggest that the extensional setting following the orogenic movement in the eastern part of the Paleo–Tethys Ocean could have been responsible for the formation of the Early Jurassic rift basins. By the early Middle Jurassic, the Bangonghu–Nujiang Tethys Ocean and the Mongol–Okhotsk Ocean were subducted northward beneath the Qiangtang terrane and Siberian continent, respectively. These led to the presence of an extensional stress field and the formation of a broader depression basin in the southern WQB during the Middle Jurassic. Paleogeography reconstruction results indicate that the northern WQB should have much potential for hydrocarbon exploration.

Present-day Tectonic Stress Field and GPS Observations in Hubei Province, Central China

Here, we show the present-day tectonic stress field and regional GPS velocity and strain rate fields in Hubei Province, central China. Our results are calculated based on the digital observation data from 01 January 2010, to 31 December 2017, by using the seis-CAP, P-wave first motion, and grid search methods and the software GAMIT/GLOBK10.4. The results show that the P axis azimuths of focal mechanism solutions, the principal compressional stress field, and the regional velocity and strain rate fields are conformably compressional in a NW–SE direction. The regional stress shape ratio R values are relatively low, and the faults are dominantly compressive-shear or compresso-shear faults. The average velocity modulus value for the GPS observation stations in western Hubei is 6.1 mm/a, which is higher than that in eastern Hubei (5.4 mm/a). The average velocity modulus value in the Jianghan Basin interior is relatively low (4.4 mm/a), while that in the northwestern Jianghan Basin is higher (7.6 mm/a). The strain rate field is characterized by NW–SE compression accompanied by NE–SW tension. The results suggest that the present-day crustal movement in Hubei Province is mainly controlled by collisions with the Indian Plate in the west and the Philippine Plate in the east and the consequent crustal shortening induced by western Hubei wedging into the Jianghan Basin. Further, the resistance by the thrust-and-fold belt in eastern Hubei contributes to the principal compressional movement in the study area. The T axis azimuth of focal mechanism solutions is consistent with the principal extensional stress field direction. In the central and northern Jianghan Basin, the R values are relatively high, the faults are mainly transtensional, and the crustal deformation is mainly extensional, which may be affected by the denudation, thinning and rapid rebound of the Dabie Orogen, resulting in tectonic extrusion and flow in the Jianghan Basin to both the NE and SW sides.

Quantitative assessment of the changes in regional water flow systems caused by the 2016 Kumamoto Earthquake using numerical modeling

Coseismic changes in water levels and discharges are well-known phenomena worldwide. Reliable hypotheses to explain these hydrological changes qualitatively have been proposed using various data and analyses. However, there are few quantitative assessments of these changes, which require hydrological simulators. A dense monitoring network revealed coseismic river and groundwater level changes induced by the 2016 Mw 7.0 Kumamoto crustal earthquake associated with many post-seismic fracture systems. In this study, we reproduced these changes using a physically based, fully distributed, integrated watershed modeling tool (GETFLOWS). In particular, the formation of new fracture systems is key to understanding the hydrological changes observed after the earthquake, which were incorporated into our model by changing parameters. Previous studies suggested that the observed water level drop immediately after the earthquake is explained by the drawdown of surface and aquifer waters to fill open spaces in the upper crust along the fracture systems generated under extensional stress field. After the initial water drop, water levels rose in the long-term because additional water sources were released from surrounding mountains due to the coseismic increase in the permeability of the soils and rocks around the compressional fracture systems. These hypotheses were validated in our model by adjusting the depths of fracture systems. Consequently, we constructed the three-dimensional distribution of these property changes over the study area. Furthermore, the model calculation yielded the volumes of missing and released mountain waters in the studied watershed (ca. 106 and 108 m3, respectively). This study demonstrates the utility of an integrated watershed modeling tool for investigating coseismic hydrological changes in active hydrological systems quantitatively.

Radiogenic trigger and driver for continental rifting and initial ocean spreading

AbstractClosure and opening of oceans on time‐scales of a few hundred million years is a fundamental tectonic process on Earth, typically referred to as a “Wilson cycle”. Subduction of oceanic and continental crust leading up to and during continent–continent collision can refertilize and enrich the orogenic continental lithospheric mantle in heat‐producing elements. The resulting thermal anomaly weakens the lithosphere and, along with structural weaknesses (e.g. sutures), make this orogenic lithosphere more prone to rifting given an extensional stress field. Thermal modelling shows that anomalously hot lithosphere can focus asthenospheric upwellings over time‐scales of a few hundred million years. Processes related to closure of oceans thus provide a mechanism for later localization of rifting and an extensional driving force.

Structural controls on Jurassic gold mineralization, and Cretaceous-Tertiary exhumation in the foreland of the southern Patagonian Andes: New constraints from La Paloma area, Deseado Massif, Argentina

Fluid-rock interaction occurring within high-permeability conduits represented by fault-fracture meshes is well known to control the development of hydrothermal ore deposits. In this study we document the fundamental role of four sets of precursor joints, reactivated as dilational faults displaying variable normal dip-slip and strike-slip components of motion, in the formation of epithermal Au-Ag deposits of economic interest. Paleostress analysis unravels how all the resulting four sets of hybrid extension-shear fractures are kinematically compatible with an extensional stress field characterized by multiple permutations between the σ3 and the σ2 axes. Such permutations are interpreted to be associated with dominantly normal fault reactivation of suitably oriented early joint sets as a result of continued regional extension, and by orthogonal stretching produced by differential downthrow of the hanging-wall blocks of laterally-terminating Jurassic normal faults.Later fault reactivation appears to be associated with far field propagation of Andean stresses, marked by superposed strike-slip and reverse fault slip components recording roughly E-W horizontal compression. This late fault reactivation could be related with a late Early Cretaceous shortening event known to have affected the studied sector of the Deseado Massif. The subsequent tectonic evolution of the study area occurred within the general framework of a slow Cretaceous to Tertiary exhumation recorded by apatite fission track cooling ages. Our new thermochronometric results, recording steady-state cooling at a rate of c. 1 °C/Ma during the last 90 Ma, shed new light onto the post-Jurassic geodynamic evolution of the foreland of the southern Patagonian Andes.

The conversion tectonics from spreading to subduction: Paleostress analysis of dike swarms during the subduction initiation in the Oman Ophiolite

Abstract We present paleostress analyses of dike swarms intruded during the subduction initiation in the northern Oman Ophiolite to understand the tectonomagmatic environment. Five swarms of subparallel dikes extending WNW-ESE are 1–5 km in width and are spaced every 5 km N-S. Each swarm has a core of 100% sheeted dikes 1–2 km in width, which emanated from the dunite-wherlite-clinopyroxenite-gabbronorite-diorite-tonalite complexes below and intruded through V1 and into V2 extrusive rocks. Individual dike strikes are varied but generally subparallel to the overall trend of the swarm. Paleostress analyses indicate subvertical σ1, ∼σ2, and subhorizontal σ3 with high magma pressures, resulted in the mutually intrusive, extensional shear dikes and abrupt changes in dike strike at high angles. These occurrences suggest intrusions under a more compressive environment compared to the extensional stress field that formed the N-S–striking sheeted dikes of V1 spreading stage. Most E-W–striking dikes possess both boninitic and tholeiitic geochemistry. The latter resemble the V1 flows and dikes with affinities of mid-ocean ridge basalt. Some tholeiitic dikes strike N-S, which are mutually intrusive to E-W–striking dikes. Tholeiitic dikes are more intensely altered than boninite, suggesting their older ages. Conversion of the stress field from a N-S–running spreading axis to inextensional E-W–running rift zones associated with the change in magma geochemistry agree with the relatively compressive V2 arc above a forced subduction zone, which originated from intraoceanic thrusting caused by the clockwise rotation of a microplate including the future northern ophiolite.

Physical and chemical strain-hardening during faulting in poorly lithified sandstone: The role of kinematic stress field and selective cementation

Abstract In this work, we report the results of a multidisciplinary study describing the structural architecture and diagenetic evolution of the Rocca di Neto extensional fault zone developed in poorly lithified sandstones of the Crotone Basin, Southern Italy. The studied fault zone has an estimated displacement of ∼90 m and consists of: (1) a low-deformation zone with subsidiary faults and widely spaced deformation bands; (2) an ∼10-m-wide damage zone, characterized by a dense network of conjugate deformation bands; (3) an ∼3-m-wide mixed zone produced by tectonic mixing of sediments with different grain size; (4) an ∼1-m-wide fault core with bedding transposed into foliation and ultra-comminute black gouge layers. Microstructural investigations indicate that particulate flow was the dominant early-stage deformation mechanism, while cataclasis became predominant after porosity loss, shallow burial, and selective calcite cementation. The combination of tectonic compaction and preferential cementation led to a strain-hardening behavior inducing the formation of “inclined conjugate deformation band sets” inside the damage zone, caused by the kinematic stress field associated with fault activity. Conversely, conjugate deformation band sets with a vertical bisector formed outside the damage zone in response to the regional extensional stress field. Stable isotope analysis helped in constraining the diagenetic environment of deformation, which is characterized by mixed marine-meteoric signature for cements hosted inside the damage zone, while it progressively becomes more meteoric moving outside the fault zone. This evidence supports the outward propagation of fault-related deformation structures in the footwall damage zone.

Shallow necking depth and differential denudation linked to post‐rift continental reactivation: The origin of the Cenozoic basins in southeastern Brazil

AbstractThe southeastern Brazilian margin presents post‐breakup Cenozoic tectonism that created a series of grabens and small sedimentary basins, known as the Continental Rift of Southeastern Brazil. The formation of this rift occurred long after the South Atlantic ocean opening and has been attributed to different mechanisms like regional uplift induced by hotspot activity, pulses of Andean orogeny and reactivation of pre‐existing faults. However, the proposed models lack an analytical or numerical verification from a geodynamic point of view. Based on finite element modelling of the lithospheric stress field evolution we conclude that a shallow necking depth, consistent with the hyperextended southeastern Brazilian margin, combined with differential denudation of the continent, resulted in an extensional stress field in the upper crust that induced the observed Cenozoic tectonism.

Structural and lithological control on fluid circulation, dilation and ore mineralization (Rio Albano mine, Island of Elba, Italy)

We present the results of geological and structural investigation documenting the interaction between hydrothermal fluids and host rock leading to a vein-type ore mineralization at shallow crustal depths (<7 km) in the mining district of the eastern Island of Elba (Italy). Sulfide- and iron-rich veins and breccia in addition to minor massive iron-ore bodies form the mineralized system. Structural mapping and analysis of vein systems, fractures, faults and associated fault rocks as well as fracture opening modes show that the main factors controlling the formation and distribution of the mineralization are lithology, deformation style and deformation intensity. Their interplay led to a positive feedback between the evolution of pore pressure through time, strain localization and the resulting mineralization. Inversion of fault and vein data defines an E-W extensional stress field at the time of faulting, which favoured fluid ingress and pervasive flow within the porous host sandstone, interstitial sulfide precipitation and reduction of the primary bulk porosity. Subsequently, cyclic channelized fluid flow during repeated fluid ingresses caused extensive veining and numerous episodes of breccia formation.

Oblique extension of pre-existing structures and its control on oil accumulation in eastern Bohai Sea

Abstract Based on 3D seismic data, the evolution mechanism and characteristics of faults were investigated to reveal the structural origin and its control on differential hydrocarbon accumulation through comprehensive analyses, including structure style analysis, fault activity analysis, analogue modelling and comparison among the wells. The complex fault system with differently trending faults resulted from strike-slip and rifting in Paleogene was partly activated, developed successively and stretched obliquely by the near-NS extensional stress field in Neogene. In the area little affected by pre-existing faults, new faults nearly perpendicular to the extension direction developed. The structural development in the study area was not caused by transpressional strike slip. Under the oblique extension effect of pre-existing faults, if the angle between the strike of pre-existing fault and the extensional direction is different, the strike-slip and extensional stresses are different in ratio. The larger the angle between the two is, the stronger the extensional component, the poorer the sealing ability of the fault, and the stronger the oil and gas migration capacity will be. Conversely, the smaller the angle between the two is, the stronger the strike-slip component, the better the sealing ability of the fault, and the poorer the oil and gas migration capacity will be. The accumulation condition analysis results considering the fault trend are in good agreement with the oil and gas shows in wells drilled in this area.

Postglacial deformation history of sackungen on the northern slope of Pic d'Encampadana, Andorra

Slopes undergoing deep-seated gravitational deformation create surface landforms that, if trenched and dated, yield critical data on slope kinematics. The northern slope of Pic d'Encampadana descends steeply 800 m down into the glaciated Valira d'Orient of Andorra and has long been known for its numerous antislope scarps (sackungen), indicative of deep-seated gravitational slope deformation (DSGSD). We excavated three trenches across antislope scarps and their adjacent troughs to date the deformation structures and to infer slope kinematics. Trough deposits have been disrupted by normal and reverse faulting, with the largest fault zone underlying the antislope scarp and dipping into the slope at 40°-80°. Trenches in the troughs expose fining-upward sequences of strata from which we interpret several cycles of: (1) episodic downdropping along graben faults and deepening of the topographic trough, (2) filling the trough with marginal colluvial and axial sag-pond sediments, followed by (3) a hiatus of soil profile development. The average time between deformation events in any one trench in the past 12–15 ka is about 3–3.5 ka. Deformation seems to have begun first in the lower part of the northern slope (1935 m elevation) about 15.3 ka, and had spread to the upper parts of the slope (2320 m) by 11.6 ka. Deformation in the lower slope ceased after 5.5 ka, but continued higher on the slope into the mid-late Holocene. The youngest deformation dated (1.4–1.6 ka) occurred in the highest trench on the slope. This pattern of younger deformation with increasing elevation has been documented at several other sites and has been ascribed to an upslope-migrating extensional stress field which originated at the base of the slope from glacial oversteepening and end-glacial debuttressing.

Earthquake Focal Mechanisms and Stress Field for the Intermediate‐Depth Cauca Cluster, Colombia

AbstractBeneath southwestern Colombia, intermediate‐depth earthquakes in the Cauca cluster locate in the subducting Nazca plate and in two columns extending ~40‐km into the mantle wedge above the slab. To investigate the cluster, we determine focal mechanisms for 69 small‐to‐moderate‐sized (2.3 ≤ Ml ≤ 4.7) earthquakes in the cluster by fitting short‐period body wave waveforms using the cut‐and‐paste method. The focal mechanisms have various faulting types and variably oriented nodal planes. We invert the focal mechanisms of the intraslab earthquakes for the intraslab stress field but cannot fit the region with a homogeneous stress tensor. We find that the principal stress axes rotate with the slab geometry, which has a concave shape and increases in dip angle from north to south. The northern region has slab normal compression and similar‐magnitude maximum and intermediate principal stresses. The minimum stress axis is oriented ~41° counterclockwise from the downdip direction. In the steeper southern region, the intermediate stress axis orients in the downdip direction. Deviation from a typical downdip extensional stress field may result from a buoyant young slab, an eastward mantle flow push, and/or along‐strike compression from the concave shape of the slab. This stress field would allow slip along preexisting faults of various orientations, such as the trench‐perpendicular seafloor features presently observed offshore, and contribute to the apparent heterogeneity of the intraslab stress field. The mantle wedge earthquakes also have various focal mechanisms but tend to have a subvertical nodal plane that aligns with the earthquake locations.

Stress evolution and structural inheritance controlling an intracontinental extensional basin: The central-northern sector of the Neogene Teruel Basin

The Teruel Basin is a NNE-SSW trending intracontinental extensional basin located in central-eastern Iberia. It is asymmetrically bounded to the east by a major fault zone, but intrabasinal faults with diverse orientation (NNE-SSW to NE-SW, E-W, or NW-SE) also appear. Offsets of the successive sedimentary units and of two planation surfaces reveal that tectonic activity initiated at the border faults, while intrabasinal ones mainly developed in a later stage. Fractures on a map scale show a prevailing N-S strike in Neogene synrift rocks, while a dense network made of four main fracture sets (NE-SW, E-W to ESE-WNW, N-S and NNW-SSE), likely inherited from Mesozoic rifting stages, is observed in pre-rift units. The results of palaeostress analyses indicate an overall predominance of σ3 directions around E-W, although two stress episodes have been distinguished during the Late Miocene-Pleistocene: (i) triaxial extension with σ3 E-W; (ii) almost ‘radial’ extension (σ1 vertical, σ2 ≈ σ3) with a somehow prevailing σ3 ENE-WSW. A scenario in which the evolving extensional stress field was able to gradually activate major basement structures with different orientation, inherited from previous tectonic events, is proposed as responsible for the evolution and overall pattern of both the eastern active margin and central parts of the central-northern sector of the Teruel Basin.

Three‐Dimensional Electrical Resistivity Distribution Beneath the Beppu–Shimabara Graben With a Focus on Aso Caldera, Southwest Japan Subduction Zone

AbstractThe Beppu–Shimabara graben, a tectonically active region comprising a N–S extensional stress field, is characterized by volcanically and seismically active regions in the Southwest Japan subduction zone. We determine the three‐dimensional electrical resistivity structure of the graben by inverting network‐magnetotelluric data. Our resistivity structure model indicates that the crust beneath the graben is a discontinuous electrically conductive belt that contains four significant conductive anomalies. The four anomalies in the graben are located from the east to west as the following arrangement: in the Hohi volcanic zone centered on Kuju volcano at depths of 0–39 km, at around the northwestern rim of Aso caldera at depths shallower than 7 km, at around the active Naka‐dake cone in Aso caldera at depths of 0–17 km, and in the northwestern part of the graben at depths shallower than 7 km. These conductive anomalies in the four locations mainly reflect the effects of various fluids: slab‐derived hydrothermal fluid, volcanic thermal fluid, aqueous fluid, and melt. The locations of the anomalies correspond with low seismicity areas in the graben, whereas resistive blocks of the model correspond with high seismicity areas, especially at the southwestern graben, the northeastern graben, and a border between Aso caldera and the Hohi volcanic zone. Our model suggests that the electrical resistivity distribution can contribute to evaluating whether each part of the graben in the regional extensional stress field has a high potential for the earthquake occurrence.