Major Fault Systems Research Articles

Three-dimensional magnetotelluric imaging of the Mérida Andes, Venezuela

The 100km wide Mérida Andes extend from the Colombian/Venezuelan border to the Coastal Cordillera. The mountain chain and its associated major strike-slip fault systems in western Venezuela formed due to oblique convergence of the Caribbean with the South American Plates and the north-eastwards expulsion of the North Andean Block. Due to the limited knowledge of lithospheric structures related to the formation of the Mérida Andes research projects have been developed to illuminate this zone with deep geophysical data. In this study, we present three-dimensional inversion of broadband magnetotelluric data, collected along a 240km long profile crossing the Mérida Andes and the Maracaibo and Barinas–Apure foreland basins. The distribution of the stations limits resolution of the model to off-profile features. Combining 3D inversion of synthetic data sets derived from 3D modelling with 3D inversion of measured data, we could derive a 10 to 15km wide corridor with good lateral resolution to develop hypotheses about the origin of deep-reaching anomalies of high electrical conductivity. The Mérida Andes appear generally as electrically resistive structures, separated by anomalies associated with the most important fault systems of the region, the Boconó and Valera faults. Sensitivity tests suggest that the Valera Fault reaches to depths of up to 12km and the Boconó Fault to more than 35km depth. Both structures are connected to a sizeable conductor located east of the profile at 12–15km depth. We propose that the high conductivity associated with this off-profile conductor may be related to the detachment of the Trujillo Block. We also identified a conductive zone that correlates spatially with the location of a gravity low, possibly representing a SE tilt of the Maracaibo Triangular Block under the mountain chain to great depths (>30km). The relevance of these tectonic blocks in our models at crustal depths seems to be consistent with proposed theories that describe the geodynamics of western Venezuela as dominated by floating blocks or orogens. Our results stress the importance of the Trujillo Block for the current tectonic evolution of western Venezuela and confirm the relevance of the Boconó Fault carrying deformation to the lower crust and upper mantle. The Barinas–Apure and the Maracaibo sedimentary basins are imaged as electrically conductive with depths of 4 to 5km and 5 to 10km, respectively. The Barinas–Apure basin is imaged as a simple 1D structure, in contrast to the Maracaibo Basin, where a series of conductive and resistive bodies could be related to active deformation causing the juxtaposition of older geological formations and younger basin sediments.

Tectonic Geomorphic of Sulawesi Island and Its Implication for Future Large Earthquake

This paper analyzes how resource of past and prospective great earthquake on the Central Sulawesi Arm, adhere on topography analysis from several space-based source. To answer the question, we analysis the tectonic geomorphic, stream pattern, exhumed fault, geological mapping and seismicity data. Detailed tectonic geomorphic studies in Sulawesi still lacking due to tectonic and fault obscures. For instance, Palu Koro Fault (PKF) was unpredictable, because the historical seismic records inevitably remain poorly documented and unrecognized fault strand, which was buried beneath abundant Quaternary alluvium subsequently obscured the fault trace. In other hand, the faults have been active during Quaternary must take into account because potentially dangerous, also the inactive faults during instrumental period must be re-evaluated in order to have awareness for large future large earthquake. Surprisingly, recent seismic activity of PKF generate super shear rupture a Mw 7.5 earthquake on 28th September 2018 with average slip 41 mm/year, which over the past two decade quiet from any seismic activity. The seismic potential for large fault is essential, since it has been silent during the instrumental period. Therefore, our motivation in this study to produce detail tectonic geomorphic map of the region in local scale, which is currently not available to prepare better knowledge and awareness for the large future earthquake. We have use Shuttle Radar Topography Mission (SRTM) with resolution ~30m, which run by ArcGIS software to observed tectonic geomorphic evidence of fault system and supplement with structural, geological and bathymetric data’s as ware available to us. We relate this analysis with seismicity data from Centroid Moment Tensor Solution (CMT) to recognize the seismic source. Our results show the tectonic geomorphic of Central Sulawesi Arm due to nature extension of NNW-SSE left-lateral slip curving to WNW-ESE of Palu-Koro Fault (PKF), then transcript to N-S circular normal fault of Poso Fault (PF). The PF indicate replica of PKF curving, where has not been mapped previously. We have mapped 60 major onshore fault systems, 10 faults showed evidence maximal to rapid rate tectonic activity along instrumental periods. Based on our CMT analysis, Sulawesi Island is greatly dominated by oblique fault.

South Tethyan passive margin paleoslope orientation inferred from soft-sediment deformation and fault kinematic analysis: a case study from the Cretaceous of Borj Cedria area, Tunisia

The Cretaceous outcrops of Borj Cedria-Bou Kournine area belongs to the NE-trending Atlas system of northern Tunisia. This area exposes sub-meridian folds associated with numerous N- to NW-trending major fault systems. This study together with previous surveys reveals that this N-trending folding is believed to be related to the inversion of the Jurassic and Early Cretaceous pre-existing fault zones and generated in response to the late compressive deformations. In addition, the study area provides evidence of soft-sediment deformations by good exposures of Cretaceous-aged slump sheets. Slump folds are usually associated with several meso-scale syn-sedimentary normal faulting together with frequent reworked blocs and occasional conglomeratic horizons. All these features indicate sedimentation on irregular seafloor topography. The aim of the present study is to investigate slump folds by applying techniques to reconstruct the contemporaneous slope gradient which has triggered soft-sediment deformations. Moreover, the brittle deformation is quantified using fault kinematic analysis together with the analysis of lithostratigraphic correlation and syn-sedimentary structures. Considerable thickness variations of Cretaceous deposits are interpreted as controlled by normal faulting activities. Likewise, fault kinematic analysis typifies a regional pure extension that trends NNW during Barremain, NNE during Albian, and probably NW during Cenomanian time. Based on slump folds analysis, the inferred sub-marine paleoslope is believed to have a northward dipping during Barremian and a NNE-dipping during Albian time. On the light of the over-mentioned interpretations, it is believed that Cretaceous sedimentation of the study area is highly controlled by major syn-depositional normal faults associated with intra basin growth faulting. These fault systems seem to be related to the Southern Tethyan expansion of the rifted continental passive margin.

Integrated Sentinel‐1 InSAR and GNSS Time‐Series Along the San Andreas Fault System

AbstractMeasuring crustal strain and seismic moment accumulation, is crucial for understanding the growth and distribution of seismic hazards along major fault systems. Here, we develop a methodology to integrate 4.5 years (2015–2019.5) of Sentinel‐1 Interferometric Synthetic Aperture Radar (InSAR) and continuous Global Navigation Satellite System (GNSS) time series to achieve 6 to 12‐day sampling of surface displacements at ∼500 m spatial resolution over the entire San Andreas fault system. Numerous interesting deformation signals are identified with this product (video link:https://www.youtube.com/watch?v=SxNLQKmHWpY). We decompose the line‐of‐sight InSAR displacements into three dimensions by combining the deformation azimuth from a GNSS‐derived interseismic fault model. We then construct strain rate maps using a smoothing interpolator with constraints from elasticity. The resulting deformation field reveals a wide array of crustal deformation processes including, on‐ and off‐fault secular and transient tectonic deformation, creep rates on all the major faults, and vertical signals associated with hydrological processes. The strain rate maps show significant off‐fault components that were not captured by GNSS‐only models. These results are important in assessing the seismic hazard in the region.

Integration of ASTER satellite imagery and 3D inversion of aeromagnetic data for deep mineral exploration

Due to high metal values and raised difficulties in finding superficial deposits, the exploration for mineral resources needs to reach greater depths. To achieve this objective, integrating ASTER and 3D inversion of aeromagnetic can be a useful tool in imaging deep magnetic mineralization hosting-structures. In this study, we present such integrated approach for deep mineral exploration in some promising zones of Gabal (G) Semna region, Eastern Desert (ED) of Egypt. Aeromagnetic data was used to delineate the structures and their relation to ore deposits and ASTER data was utilized for mapping lithological units and the hydrothermal zones. The 3D inversion was employed to imagine the depth and geometry of the magnetized ore bodies in the zones that identified by processing ASTER data. The 3D magnetic inversion revealed that mafic rocks and metavolcanics exhibit high-amplitude magnetic anomalies. High amplitude (positive) magnetic anomalies are joined with the large susceptibilities metavolcanics. Alteration zones in the mafic to basic bedrock and alongside major fault systems are high potential zones that revealed by ASTER image analysis. Aster and aeromagnetic data were used to map and alteration zones surface geology that can host mineralization and the 3D inversion was applied to outline the subsurface extent of these promising areas. This approach can be broadly used for deep mineral exploration in the Egyptian ED and identical areas around the world.

Revealing Geometry and Fault Interaction on a Complex Structural System Based on 3D P-Cable Data: The San Mateo and San Onofre Trends, Offshore Southern California

Deformation observed along the San Mateo (SMT) and San Onofre trends (SOT) in southern California has been explained by two opposing structural models, which have very different hazard implications for the coastal region. One model predicts that the deformation is transpressional in a predominantly right lateral fault system with left lateral step-overs. Conversely in the alternative model, the deformation is predicted to be compressional associated with a regional blind thrust that reactivated detachment faults along the continental margin. State-of-the-art 3D P-Cable seismic data were acquired to characterize the geometry and linkage of faults in the SMT and SOT. The new observations provide evidence that deformation along the slope is more consistent with step-over geometry than a regional blind thrust model. For example, regions in the SOT exhibit small scale compressional structures that deflect canyons along jogs in the fault segments across the slope. The deformation observed in the SMT along northwesterly trending faults has a mounded, bulbous character in the swath bathymetry data with steep slopes ( ∼ 25°) separating the toe of the slope and the basin floor. The faulting and folding in the SMT are very localized and occur where the faults trend more northwesterly (average trend ∼ 285°) with the deformation dying away both towards the north and east. In comparison, the SOT faults trend more northerly (average trend ∼ 345°). The boundary between these fault systems is abrupt and characterized by shorter faults that appear to be recording right lateral displacement and possibly accommodating the deformation between the two larger fault systems. Onlapping undeformed turbidite layers reveal that the deformation associated with both major fault systems may be inactive and radiocarbon dating suggests deformation ceased in the middle to late Pleistocene (between 184 and 368 kyr). In summary, our preferred conceptual model for tectonic deformation along the SMT and SOT is best explained by left lateral step-overs along the predominantly right lateral strike-slip fault systems.

Genetic Environments of the Eunjeok Au–Ag Deposit in the Yeongam District: Implications for Cretaceous Epithermal Au–Ag Mineralization in South Korea

Eunjeok Au–Ag deposits are situated in the Yeongam district, Cheollanamdo-province, South Korea. They are genetically related to the Bulgugsa magmatic event (ca. 110–60 Ma), caused by the transition in the subduction direction and style of the Izanagi Plate. Three gold- and silver-bearing hydrothermal veins filled the fractures of the Cretaceous rhyolitic tuff. The major ore minerals were arsenopyrite (31.47–32.20 at.% As), pyrite, chalcopyrite, sphalerite (8.58–10.71 FeS mole%) and galena with minor amounts of electrum (62.77–78.15 at.% Au), native silver, and argentite. Sericitization was dominant in the alteration zone. The various textures of quartz veins (i.e., breccia, crustiform, comb, and vuggy) may indicate the formation of an epithermal environment. The auriferous fluids with the H2O–NaCl system have homogenization temperatures and salinities of 204 °C to 314 °C, less than 10 wt.% equiv. NaCl, and experienced mixing (dilution and cooling) events during mineralization. Considering the characteristics of the geologic setting, major fault system, and host rock, the Eunjeok Au–Ag deposit within the Yeongam district tends to share the general geologic characteristics of Haenam–Jindo epithermal mineralization episodes. However, the age of gold–silver mineralization (86.0 Ma) is older than that of Haenam–Jindo epithermal mineralization episodes (<70.3 Ma), implying some differences exist in the genetic sequence of extensional characteristics caused by transcurrent Gwangju–Yeongdong faults.

Disposition of magmatic eruptions and fault distribution in northwestern Saudi Arabia using pseudo-depth slice magnetic anomaly

AbstractThis study presents the disposition of magmatic eruptions with a fault distribution in northwestern Saudi Arabia, where intensive magma invades the lithosphere. Structural and magmatic features are traced at successive depths through high-resolution magnetic anomaly pseudo-depth slices. The total horizontal gradient technique is applied to pseudo-depth slice magnetic anomalies to enhance the linear trends of faults and related magmatic activity. A comprehensive cross-section constructed from the projection of gradient horizontal maxima relative to pseudo-slices allows the visualization of the vertical behavior of faults and magma sources. Three major fault systems were identified, primarily aligned in the N–S, NE–SW and NW–SE directions. They are characterized by increasing length and width with depth. The N-S fault system is a major non-planar deep system throughout the area, affected by the NW–SE and NE–SW deep discontinuities. The evolution of these discontinuities with depth successfully shows magma uprising zones represented by a circular horizontal gradient, which starts to appear at a depth of 4500 m with a vertical continuity to the surface. They are interpreted as possible locations of ascending magma chambers or vents. The disposition of these magma sources with fault distribution can show a close relationship between the fault systems and the magma eruptions. The interpreted magma vents appear where the NE-trending transform faults intersect the NW or N–S fault zone. These intersections may represent weak zones that act as vertical conduits through which magma discontinuously erupts into the overlying crust, forming major volcanic fields in the eastern Red Sea.

Improved method for quantitative evaluation of fault vertical sealing: A case study from the eastern Pinghu Slope Belt of the Xihu Depression, East China Sea Shelf Basin

Fault vertical sealing evaluation is significantly important for revealing hydrocarbon migration and accumulation processes and reducing exploration risks. Based on the analysis of the stress normal to the fault plane, the pore-fluid pressure and the compressive strength of fault rock, a new parameter, namely, the fault vertical sealing index (FVSI), is proposed for improved quantitative evaluation. In this article, static faults distributed in the eastern Pinghu Slope Belt (EPSB) of the Xihu Depression are selected as examples. The vertical sealing properties of four major fault systems that control hydrocarbon distribution are analyzed and evaluated. The FVSI value is positively correlated with the vertical sealing capacity of the fault zone, and the heterogeneous spatial distribution of the FVSI along the fault planes indicates that a fault cannot be uniformly regarded as a conduit or a barrier. According to the relation between the FVSI and the corresponding natural gas and oil shows, a fault zone with an FVSI value less than 0.4 commonly acts as a vertical conduit and cannot accumulate hydrocarbons, while a fault zone with an FVSI value greater than 0.4 has the ability to seal hydrocarbons. However, whether hydrocarbons can continue to migrate vertically depends on the maximum hydrocarbon column height that the corresponding FVSI can seal. In this case study, fault zones with FVSI values greater than 1 completely act as barriers to vertical hydrocarbon migration and no longer transport hydrocarbons. The changing FVSI of the fault zone controls the process of hydrocarbon migration and accumulation, thereby affecting the distribution of hydrocarbons. In the structurally lower position of the EPSB with sufficient hydrocarbon sources, the weak vertically sealed fault zone commonly causes a vertical multilayered distribution of hydrocarbons. Conversely, in the structurally higher position of the EPSB with insufficient hydrocarbon charging, the strong vertically sealed fault zone results in a more concentrated hydrocarbon distribution in the lower strata. However, there are no natural gas and oil shows in the fault zone that completely act as vertical conduits. Therefore, the FVSI can be an effective quantitative method to analyze vertical hydrocarbon migration pathways and accumulation locations controlled by faults and has great application potential in reducing the risks in petroleum exploration projects.

Deglaciation and neotectonics in SE Raasay, Scottish Inner Hebrides

Changes in the physical landscape of SE Raasay at the end of the last Quaternary glaciation are examined. The area is marked by a major fault system defining the Beinn na Leac Fault Block, and field survey shows this to comprise a rollover anticline in the SW, with extensional movement towards the NE along an oblique transfer fault, the Main Beinn na Leac Fault. The fault system was reactivated after the Last Glacial Maximum. Survey of a distinctive ridge of detached scree along the Main Beinn na Leac fault shows it to have involved a single movement of at least 7.12 m vertical displacement, arguably the greatest fault movement since before the Younger Dryas in Scotland. The present work confirms that the scree became detached during the Younger Dryas, but finds that it overlies a lacustrine deposit of at least 5.6 m of laminated sediments from a lake which had begun to accumulate earlier. Radiocarbon dating of peat overlying the lake sediments gave 10 176–10 315 cal years BP, but morphological and stratigraphical evidence indicates that drainage of the lake occurred earlier and only shortly before movement of the scree. Possible causes of displacement at the fault system are briefly discussed.

Exploiting Thermochronology to Quantify Exhumation Histories and Patterns of Uplift Along the Margins of Tibet

The utilization of thermal-chronological data to constrain mountain building processes exploits the links among rock uplift, exhumation, and cooling during orogenesis. Conceptually, periods of rapid uplift and associated denudation will lead to cooling of rocks as they approach Earth’s surface. The linkage between uplift and exhumation can be complex, but in practice exhumation is often assumed to directly track uplift. The reconstruction of temperature-time histories via thermochronologic systems provides a proxy method to relate the cooling of rock as it is exhumed toward the surface to orogenesis. For the rapid exhumation rates that can occur in active orogenic systems the thermal history will be complex as a result of heat advection, rates of propagation of thermal perturbations, and other processes that affect the cooling behavior. These effects become amplified as exhumation rates increase, and in regions experiencing exhumation rates greater than ∼0.2–0.3 mm/yr (0.2–0.3 km/Ma) simple assumptions of cooling through a constant geotherm will bias the subsequent interpretation. Here we explore, through a suite of generalized models, the impact of exhumation rate and duration on the resulting thermal history and apparent age results. We then apply lessons from these simple exhumation systems to data sets from the high-relief ranges along the eastern margin of the Tibetan Plateau to determine exhumation histories constrained by those data. The resulting exhumation histories provide constraints on the onset of Cenozoic exhumation, the subsequent pace of exhumation, and on the tectonic history of one of the major fault systems in the central Longmen Shan.

Timing and magnitude of progressive exhumation and deformation associated with Eocene arc-continent collision in the NE Caribbean plate

Abstract Puerto Rico and the northern Virgin Islands together preserve a unique archive of island arc construction and plate margin deformation along the northeastern edge of the Caribbean plate. In Eocene times, arc-continent collision of the Caribbean plate and the North American plate led to transpressional deformation along two major fault systems in Puerto Rico, resulting in an island-wide depositional hiatus. Although styles and kinematics of this deformational event are seemingly well understood, the lack of chronologic constraints have left uncertainties related to the timing of inception and activity, the magnitude of crustal exhumation, and the character of deformation (i.e., progressive or polyphase). New zircon and apatite (U-Th)/He ages reveal that deformation associated with arc-continent collision started in the early Eocene (ca. 52 Ma) and ended in the early Oligocene (ca. 29 Ma). Over this 23 m.y. time frame, deformation was not restricted to major faults, instead it propagated gradually eastward, with punctuated episodes of vertical exhumation in the early Eocene (ca. 52–34 Ma) and late Eocene (ca. 36–29 Ma). In contrast, the northern Virgin Islands experienced rapid cooling and exhumation in the early Miocene (ca. 24–21 Ma) associated with the extensional opening of the Anegada Passage. The modeled thermal histories for the central and northeastern part of Puerto Rico indicate collision-related peak transpressional deformation between 36 and 29 Ma and an average exhumation rate 0.9 ± 0.6 km/m.y. These results represent the first direct constraints on the timing and magnitude of collisional exhumation and offer insights into the deformational evolution of the northeastern edge of the Caribbean plate.

High‐Resolution 3‐D Shear Wave Velocity Model of Northern Taiwan via Bayesian Joint Inversion of Rayleigh Wave Ellipticity and Phase Velocity With Formosa Array

AbstractThe Formosa array, with 137 broadband seismometers and ∼5 km station spacing, was deployed recently in Northern Taiwan. Here by using eight months of continuous ambient noise records, we construct the first high‐resolution three‐dimensional (3‐D) shear wave velocity model of the crust in the area. We first calculate multi‐component cross‐correlations to extract robust Rayleigh wave signals. We then determine phase velocity maps between 3 and 10 s periods using Eikonal tomography and measure Rayleigh wave ellipticity at each station location between 2 and 13 s periods. For each location, we jointly invert the two types of Rayleigh wave measurements with a Bayesian‐based inversion method for a one‐dimensional shear wave velocity model. All piecewise continuous one‐dimensional models are then used to construct the final 3‐D model. Our 3‐D model reveals upper crustal structures that correlate well with surface geological features. Near the surface, the model delineates the low‐velocity Taipei and Ilan Basins from the adjacent fast‐velocity mountainous areas, with basin geometries consistent with the results of previous geophysical exploration and geological studies. At a greater depth, low velocity anomalies are observed associated with the Linkou Tableland, Tatun Volcano Group, and a possible dyke intrusion beneath the Southern Ilan Basin. The model also provides new geometrical constraints on the major active fault systems in the area, which are important to understand the basin formation, orogeny dynamics, and regional seismic hazard. The new 3‐D shear wave velocity model allows a comprehensive investigation of shallow geologic structures in the Northern Taiwan.

New tectonic configuration in NW Iran: Intracontinental dextral shear between NW Iran and SE Anatolia

The Main Recent Fault (MRF) is a major active strike-slip fault system at the NE border of the Arabian platform and Central Iran. Both geometry and kinematics of the fault system is rather well known along its central part and at its SE termination, while its possible continuation to the northwest is ambiguous. Moreover, less regard has been paid to possible relationships between this major intracontinental fault system and other strike-slip faults in NW Iran – SE Anatolia. This paper investigates both the structural pattern and kinematics of deformation in the region to the north of latitude 37°N, between the North Tabriz Fault and the MRF. We followed this goal through detailed fault mapping, structural field measurements, and fault-slip analyses, complemented by the reconstruction of cumulative geomorphic offsets along the main fault segments. The collected data sets have led us to propose a new tectonic model for the northwest corner of the Arabia – Central Iran collision zone. Our results reveal that the intracontinental dextral shear between Arabia-Central Iran, which is mainly localized along the MRF, continues to the north of latitude 37°N along a branch of the Neotethyan suture penetrating into SE Anatolia. The dextral fault system abruptly dies out at latitude 37.5° N joining two NNW-striking normal fault zones, which accommodate the dextral translation of Central Iran relative to Arabia through ESE-trending extension. The region in NW Iran and SE Anatolia is affected by a heterogeneous active stress field due to different tectonic processes such as tectonic escape, block translation, and localized extension. The dynamics of these processes is either directly governed by the Arabia – Eurasia oblique convergence or indirectly controlled by frontal collision and pure shear deformation in central Anatolia.

The Andaman Basin Central Fault Zone, Andaman Sea: Characteristics of a major deepwater strike-slip fault system in a polyphase rift

2D and 3D seismic reflection data from Thailand waters in the Andaman Sea have revealed a major NNE-SSW trending strike-slip fault (Andaman Basin Central Fault Zone, ABCFZ). This fault developed following a change from E-W to ENE-WSW extension during the Oligocene-earliest Miocene, to NNW-SSE transtension. The ABCFZ is interpreted to have formed subsequent to an extensional detachment translating the upper crust, including the extensional rift basins, to the west, leaving the ABCFZ to be developed on thinned, lower plate, lower continental crust. The ABCFZ was active during the Early and early Middle Miocene. A very extensive belt of ENE-WSW R′ shears up to 60 km wide, hundreds of kilometres in length (NNE-SSW direction), with faults spaced about every kilometer bounds both sides of the ABCFZ. R-shears occur more infrequently and predominantly occur close to the ABCFZ. The distinctive and unusual structural style of regionally dominant R′ shears is interpreted to be a result of NNW-SSE extension related to the northwards movement of the Indian Plate relative to SE Asia. The ABCFZ displays large-scale uplift (~100 km wavelength) on its eastern margin. If the uplift occurred at a restraining bend, then the nearest one requires at least 50 km of dextral displacement. Canyons mapped traversing this high provided pathways for gravity driven sediments to enter the deepwater area. The ABCFZ and the Sagaing Fault to the north are both associated with large basins whose depocentres lie parallel to the fault trace. They represent a different type of strike-slip basin from classic pull-apart basins, and are more laterally extensive than typical pull-apart basins. The East Andaman Basin margin shows regional variations in crustal thickness which have N–S trending contours related to Oligocene-age extension. The ABCFZ follows the location and trend of the necking zone within continental crust, suggesting this was a mechanically favourable location for development of the fault. The ABCFZ illustrates a type of strike-slip deformation and sedimentation pattern that can be expected on other hyper extended continental margins that have evolved from extension to highly oblique transtension.

New ages, morphometric and geochemical data on recent shoshonitic volcanism of the Puna, Central Volcanic Zone of Andes: San Jerónimo and Negro de Chorrillos volcanoes

Shoshonitic volcanic centres are scarce within the back-arc mafic volcanism of the Puna Plateau in the Central Volcanic Zone of the Andes. San Jerónimo and Negro de Chorrillos volcanoes are two well preserved shoshonitic monogenetic edifices that developed mainly Strombolian activity and lava flows, which extended more than 9 and 4 km from the vent, producing a volume DRE (Dense Rock Equivalent) of 0.091 and 0.020 km3, respectively. The new unspiked K–Ar (groundmass) ages of San Jerónimo (144 ± 3 ka) and Negro de Chorrillos (51 ± 2 ka) eruptions presented in this work are both younger than those previously published. Petrographic and compositional features allow distinguishing between potassic and ultrapotassic suites in volcanic products of both eruptions. Phlogopite occurrence and low glass content characterize the ultrapotassic suite. Based on the crustal thickness of this part of the Puna and the experimental constraint, these magmas last equilibrated at Moho depths at around 50 km (the highest depth of spinel lherzolite stability field) for temperatures of 1100–1200 °C. Once in the crust, the K-rich magma underwent a minor interaction with a trachytic-trachydacitic melt, which is evidenced petrographically and geochemically, e.g., mismatch of mineral assemblages (co-occurrence of olivine and quartz), disequilibrium crystal textures (dusty zones in plagioclase, embayments in sanidine), and a wide range of compositional variation in the glasses. Ultrapotassic (i.e., phlogopite-bearing) and potassic lavas in the same eruption are interpreted as pulses of the same magma but with a longer mixing time with the trachytic-trachydacitic melt for the resulting ultrapotassic rocks within an eruptive time frame of hours or days. The singular shoshonitic character of these magmas could be related to their origin along an old terrane boundary. Afterwards, as occurs for other mafic magmas of the Puna, major NW-SE fault systems would favour their ascent to the surface.

Paleoseismologic trenching confirms recent Holocene activity of the major Algeciras fault system in southern Colombia

The Algeciras Fault System (AFS) in Colombia, which is characterized by its predominant dextral strike-slip movement, is part of the major transcurrent fault system known in Ecuador, Colombia and Venezuela as the Eastern Frontal Fault System, that disconnects the North Andean Block (NAB) from the South American plate in a process of tectonic escape. It is responsible for some of the most important seismic events in the seismic history of Colombia that occurred in 1827 with Mw 7.1 that destroyed large part of all settlements in the Huila Department and caused considerable damage and loss of life in the capital of Bogota. More recently another event of Mw 7.0 took place in 1967 that also caused damage in the capital, and in December 2019 an event of Mw 6.0 caused minor damage in various settlements in the Eastern foothills of the Eastern Cordillera. So far, mapping studies based on data obtained from satellite imagery, airphotos and structural field data have outlined the complexity of the AFS and have identified the existence of at least six tectonic basins occurring along the main fault of the system. In 2016, within the framework of activities of the Space Geodesy Research Group of the Geohazards Directorate of the Geological Survey of Colombia (SGC) a program of morphotectonic field studies was carried out in and around the Algeciras pull-apart basin in the Huila Department that led to the opening of two exploratory paleoseismologic trenches. This paper presents the results of the first of these two trenches, located in the center of the Algeciras valley at a place named Lagunillas. It is situated at the rim of a shallow fault-dammed lake formed by an uphill facing scarp generated by the dextral strike-slip displacement of a shortcut fault that traverses the alluvial fill of the basin along its longitudinal axis. The results of the paleoseismologic study prove the occurrence of seven seismic events with magnitudes Mw 7.0 during the Holocene with an average recurrence interval of 1300–1400 years. This constitutes a significant contribution to the study of seismic hazard assessment of the AFS that complements, in a quantitative way, the evidence provided by the data of historic seismicity of this major fault and points out the considerable danger that the fault presents to the country as a whole and in particular to the metropolitan area of Bogota.

Understanding earthquakes using the geological record: an introduction

Understanding earthquakes using the geological record: an introduction

The sediments of Lake Singkarak and Lake Maninjau in West Sumatra reveal their earthquake, volcanic and rainfall history

Natural hazards such as earthquakes, volcanic activity, and heavy rainfall causing floods and debris avalanches are common phenomena in many tropical settings, including the island of Sumatra, located in the Indonesian archipelago. To enhance our understanding of the recurrence of these often destructive events, we studied the sedimentary infill of two lakes in the Padang highlands, West Sumatra. This includes Lake Singkarak, a tectonically-formed lake located on a step-over of a major strike-slip fault system (the Sumatran Fault), and Lake Maninjau, a caldera lake. Both lakes are located ~300 km from the Sunda subduction trench and surrounded by steep slopes and a chain of active volcanoes. Hence, considering their unique tectonic setting, these lakes may potentially record a wide range of natural hazards that affect the region. A combination of seismic-reflection profiles and short sediment cores revealed that both lakes indeed record various types of natural hazards, each with their own sedimentary response to a specific type of event. Lake Singkarak can be used to study past floods and major debris avalanches in addition to high-magnitude megathrust earthquakes, while traces of past intraplate earthquakes have been identified in both lakes. Furthermore, we argue that Lake Singkarak is an ideal recorder of volcanic activity in the region, while Lake Maninjau itself can pose a volcanic hazard to its surroundings as demonstrated by potential activity of the volcano below the lake.

Evolution of slip susceptibility of Quaternary faults in southeastern Korea from Quaternary to present day

We study the geomechanical stability of faults in southeastern Korea that have been active during the Quaternary (mostly Pleistocene), to investigate whether these faults can still be considered active today. We compile fault plane data and slip orientations for 40 Quaternary faults in southeastern Korea distributed along the two major fault systems, Yangsan Fault (group 1) and Ulsan Fault (group 2), and in the younger block to the east of the Ulsan Fault (group 3). Our aim is to determine the representative tectonic stress states in the Quaternary and in the present day, and to compare the slip susceptibility of these faults under the stress fields of these two periods. We estimate Quaternary stress from inversion of Quaternary fault data, and present-day stress from inversion of contemporary earthquake mechanisms. Both stress fields exhibit a maximum principal stress axis that is sub-horizontal and trends ENE-WSW, but the intermediate and minimum principal stress axes are switched, suggesting a change in stress regime from reverse-faulting during the Quaternary to strike-slip-faulting in the present day. We demonstrate that this change in stress regime is due to a marked reduction in the magnitude of horizontal tectonic stress from the Quaternary to the present day. We assess the slip susceptibility of the faults under the Quaternary and present-day stress fields using two indices: slip tendency and fault instability. This analysis shows an increase in slip susceptibility for most of group 1 faults and some of group 3 faults. Some of these faults exhibit a slip susceptibility comparable to the average obtained for contemporary earthquake focal planes, suggesting that these faults can be considered active in the present day.