Compressional Tectonic Regime Research Articles

Geochronological Assessment of the Arabian-Nubian Shield plutonic intrusions in the arc assemblages along the Qift-Quseir transect, Central Eastern Desert of Egypt

The ophiolitic mélange and granitic intrusions along the Qift‒Quseir transect in the Central Eastern Desert (CED) of Egypt are parts of the Egyptian Nubian Shield (ENS), which is the northern segment of the East African Orogeny (EAO). The Arabian-Nubian Shield (ANS) is the longest Neoproterozoic belt on Earth, and it was formed during the East and West Gondwanaland collisions within the framework of the EAO. The ANS basement rocks were developed during three distinct phases of magmatic activity: the island arc and syn-collisional phases, identifying a compressional tectonic regime, and a post-collisional phase that identifies changing the tectonic regime into an extensional type. The geochronological assessment of these magmatic activities is essential for understanding the regional geology and tectonic development of the ANS. In our study, we dated different rock units along the Qift‒Quseir transect and revealed ages ranging from the Late Tonian (820 ± 8 Ma) to the Late Ediacaran (563 ± 4 Ma). These ages were associated with three different magmatic pulses: (1) a seafloor spreading and island arc phase (ophiolite and related rocks), represented by sample QQ05, which was dated from 820 ± 8 Ma; (2) a syn-collisional phase, represented by samples QQ08 and QQ10, dated from 733 ± 10 Ma and 729 ± 10 Ma, respectively; and (3) a post-collisional phase, represented by all the other samples, dated from the Ediacaran at 603 ± 9 Ma to 563 ± 5 Ma. These results showed that the post-collisional phase was dominant, especially in terms of the alkali-feldspar granites, relative to ophiolitic rocks, and the syn-collisional granites in the CED. Initiation of the Dokhan Volcanic eruptions at 639 ± 2 Ma gave us the date of the compressional-to-extensional tectonic transition setting, and the post-collisional tensional regime was activated at 603 ± 9 Ma. Additionally, we identified evidence of local magmatic sources by dating 11 grains of Paleo- to Meso-Proterozoic xenocrysts with ages ranging from 1876 ± 18 to 1070 ± 13 Ma (i.e., components of the pre-Arabian-Nubian Shield).

Fault system dynamics of the Kashmir, NW Himalaya, India using continuous GPS observations and geomorphic evidences

We collected data from the continuous Global Positioning System (cGPS) sites across the Kashmir Valley, situated at latitude 34◦N, spanning from 2008 to 2021. Inter-site velocities define a region of approximately 15,000 km2 with broadly distributed strain accumulation at −7.22×10−8 nano strain/year (compression component) and the maximum shear strain γmax of 1.9051×10−7 nano strain/year. The estimated site velocity in the ITRF14 ranges between 30.5±1–42.85±3 mm/yr. It was observed that the average deformation rate of the GPS sites in the Kashmir region ranges between 2.86±1–15.47±3 mm/yr relative to the India fixed reference frame, suggesting a predominant N-S directed compressional tectonic regime. The focal mechanism solutions of the earthquakes in and around the Kashmir Valley suggest dominant thrust faulting followed by normal faulting. Analysis of the vertical component of the GPS time series shows that the northwest segment of the valley subsides at the rate of −1.71± 0.70 mm/yr, while the southeast segment uplifts at the rate of 5.4 ± 0.5 mm/yr. In addition to vertical component, we observed differential movement of the sites relative to IISC site on the northwest and southeast segments. The rate of baseline change of the GPS sites indicates 7.30 ± 0.75 mm/yr extension in SE-NW direction and −5.32 ± 0.75 mm/yr NE-SW compression across and along the Kashmir Valley. Geodetic observations reveal a transition that aligns with the Magam lineament/fault previously identified by Ganju and Khar (1984) using gravity and magnetic data. The observation was supported by the field investigations and remote sensing techniques, confirming the existence of Magam Fault. During the field investigations, various geomorphic expressions of fault were observed, including fault ruptures, fault scarps, offset ridges, deflected drainages/rivers, linear alignment of springs, linear drainage lines, triangular facets and offset Recent sedimentary deposits (Karewas) were observed. The field evidence suggests exposure of normal faults at Kondabal, Nasrullapora, Biru and Radbugh. These exposed extensional structures, trends in NE-SW direction and dip in NW direction with varying offset and dip amount. GPS observations supplemented by geomorphic evidences infer the presence of normal fault ̴ 80 Km extending from northeast to southwest.

Fossil metasomatized and newly-accreted fertile lithospheric mantle volumes beneath the Bakony-Balaton Highland Volcanic Field (central Carpathian-Pannonian region)

This study focuses on upper mantle xenoliths from Sabar-hill (Bakony-Balaton Highland Volcanic Field), a newly discovered locality of mantle xenoliths, situated in the central Carpathian-Pannonian region (CPR). The investigated seven peridotite and one pyroxenite xenoliths record two episodes in the geochemical evolution of the local subcontinental lithospheric mantle. The moderately deformed Group I xenoliths reveal modal contents (≥9 vol% orthopyroxene), major element characteristics (depletion in Al2O3 and CaO in orthopyroxenes) and trace element features (e.g., enrichment in U, Pb and Sr in clinopyroxenes) typical in supra-subduction zones (i.e., mantle wedge) xenoliths. All these suggest that the studied xenoliths were formed by silica-rich fluid/melt - peridotitic wall rock interactions. The tectonic background to the formation of Group I xenoliths is likely linked to the subduction of an oceanic slab during the Mesozoic-Paleogene. This may have happened far from the current position of Sabar-hill, to where the lithosphere, including the metasomatized mantle volume, was transferred via plate extrusion. The less deformed, dominantly protogranular and porphyroclastic Group II xenoliths are depleted in light rare earth elements (LREE). In contrast, they are enriched in clinopyroxene (≥11 vol%), incompatible major elements (e.g., Al and Na) and heavy rare earth elements (HREE), by a minimum of four times compared to primitive mantle. All these suggest that Group II xenoliths represent a geochemically fertile mantle partition and the degree of partial melting affected this mantle section was limited (≤7% based on trace elements). Group II xenoliths have higher equilibrium temperatures (1088–1168 °C) compared to Group I xenoliths (1018–1089 °C). The lack of petrographic and geochemical evidence pointing to metasomatism-related annealing in Group II xenoliths suggests that they are derived from a greater depth. It follows that Group II xenoliths could represent prior asthenosphere volumes which recently accreted to the lithosphere. The lithospherization of the upper part of the asthenosphere may have initiated after the tectonic inversion of the region, when compressional tectonic regime beneath the CPR gradually overtook the former extensional one in the last ∼8 Ma. The high bulk structural hydroxyl content in both Group I (22–1699 ppm) and II (55–320 ppm) xenoliths is likely linked to the subduction events that took place in the CPR from the Mesozoic onwards. These events transported significant amounts of fluid (H2O) back to the upper mantle, affecting both the mantle wedge and the asthenosphere.

The Miocene lacustrine carbonate factory of the Ñirihuau Formation, Ñirihuau Basin, North Patagonian Andes, Argentina

AbstractFacies, microfacies and stable isotope analyses of limestone beds in the northernmost Ñirihuau Basin, North Patagonian Andes, Argentina, document and constrain the past hydrological, sedimentological and climate conditions that prevailed during the deposition of a lacustrine system between ca 15 and 13 Ma. This palaeoenvironment is recorded in the middle section of the Ñirihuau Formation, which holds significance because: (1) It was deposited during a transition from an extensional to a compressional tectonic regime; (2) it spans the Middle Miocene Climatic Optimum and the beginning of the Middle Miocene Climatic Transition; and (3) it contains limestone beds interbedded within a 600 m thick interval of mudstones and siltstones, along with intercalated sandstone and volcaniclastic bodies. Two detailed sedimentary logs were surveyed along the Arroyo Las Bayas, at the western and eastern flank of the David Syncline. Limestones from both stratigraphic sections were sampled as well as isolated limestone beds from two other sites. One facies association was defined and interpreted as a perennial lake associated with a deltaic system and dominated by detrital clastic material. It comprises Facies 1 (Marginal lacustrine) and Facies 2 (Lower delta plain); in both, the presence of grainstones and calcimudstones stands out. Through petrography and cathodoluminescence studies of these continental carbonates, nine microfacies were identified: (a) Intraclastic grainstone, (b) Homogeneous calcimudstone, (c) Silty grainstone, (d) Disrupted micrite, (e) Birds eye micrite, (f) Bioclastic mudstone, (g) Calcimudstone with sparse detrital grains, (h) Fenestral micrite, (i) Stromatolitic boundstone. These indicate mainly bio‐induced subaqueous carbonate precipitation and subordinate deposition by tractive flows with short‐distance transport on a littoral lacustrine environment. Most of these microfacies exhibit very early diagenesis (eogenesis) effects. These features, and the geochemistry results, indicate that they were deposited in a palaeolake system under temperate to warm and humid conditions.

Tectono-metamorphic evolution of the Central Bundelkhand Craton, India from geochemistry and bulk composition modelling of amphibolite enclaves

The Bundelkhand Craton (BuC) represents a significant Archean terrane within the northern Indian Craton and yet its tectono-metamorphic evolutionary history remains relatively understudied. Our investigation involves detailed petrography, geochemistry, and bulk composition modeling of both garnet-bearing and garnet-absent amphibolites with a two-fold objective: (i) to constrain the protolithic nature and tectonic settings involved in the genesis of these rocks, and (ii) to propose a tectono-metamorphic evolutionary history for the BuC. The BuC amphibolites originate from basalt and andesitic-basalt protoliths. Their trace element compositions reveal negative anomalies in Nb, Ta, and Ti, while their rare earth element (REE) normalized patterns indicate enrichment in light REEs over heavy REEs. The basaltic protolith is interpreted to have formed during orogeny in a compressional tectonic regime at the active margins of island arcs in a subduction-related setting. This interpretation is supported by various discrimination plots for amphibolites, such as Nb/Th vs Zr/Nb, Zr vs Zr/Y, and Th/Nb vs Ce/Nb, as well as high Th/Yb and low Nb/Yb contents— all of which suggest an island arc setting influenced by subduction. These amphibolites have undergone three distinct phases of metamorphism, as evidenced by petrography, mineral chemistry, and bulk composition modeling. This interpretation is further supported by geochemical discrimination diagrams which indicate that a subduction tectonic setting was active during peak metamorphism. During the pre-peak phase, the garnet-bearing amphibolites experienced pressure and temperature conditions ranging from 6.25 to 6.5 kbar and 580 to 590ºC, while the garnet-absent amphibolites underwent conditions from 5.0 to 5.8 kbar and 400 to 450ºC. Peak metamorphism was observed at pressures ranging from 6.8 to 7.4 kbar and temperatures from 760 to 805ºC for the garnet-bearing amphibolites, and at pressures from 7.0 to 7.4 kbar and temperatures from 785 to 810ºC for the garnet-absent amphibolites. The metamorphic retrograde conditions for the garnet-bearing amphibolites are defined by P-T conditions ranging from 4.45 to 4.75 kbar and 585 to 615ºC, while for the garnet-absent amphibolites, it ranges from 3.1 to 4.0 kbar and 620 to 710ºC. The mineral assemblages and P-T conditions delineate a clockwise P-T path for both, garnet-bearing and garnet-absent amphibolites from the Babina and Mauranipur regions. This suggests that the rocks underwent a burial process amid subduction tectonic settings in an arc-related environment, followed by a decompression stage that brought them to the surface.

Multistage evolution of gold mineralization in the Kibali gold district: Insights from pyrite analyses

The Kibali gold district, located on the Congo Craton, comprises a Neoarchean gold resource exceeding 28.7 Moz. The district is located within a westerly-verging stacked fold-and-thrust belt associated with a compressional tectonic regime active from ca. 2.64–2.60 Ga. Deposits are hosted by a major fault zone that consists of a complex series of smaller thrust faults, shear zones, and folds, which extend over 60 km. Gold deposits are primarily hosted in BIF and clastic sedimentary rocks. The gold and associated sulfides are in veins and disseminated in NE-to NNE-trending ore shoots within the deformed and metamorphosed volcanosedimentary rocks. Pyrite is the dominant sulfide mineral and hosts both solid solution gold and free gold. Because of its strong association with the gold mineralization, our study focusses on unravelling the metallogeny of the area by considering pyrite textural analyses, in situ trace element analyses using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), and S isotopic composition (δ34S) analyses using Secondary Ion Mass Spectrometry (SIMS).Eight distinct pyrite types were identified, including microcrystalline sooty (py-0a), small grains (<40 μm) (py-0b), aggregate recrystallized (py-1), core-rich inclusion (py-2), vein-like (py-3), non-zoned (py-4a), oscillatory As-bands zoned (py-4b), and cubic euhedral (py-5) pyrite. The ore-forming processes occurred over a polyphase evolution. Syn-diagenetic pyrite py-0a in volcaniclastic layers of older basin sediments served as the primary repository of Au (15.6 ppm median) with a near-zero δ34S value. Late diagenetic or early metamorphic pyrite (py-0b) was found in younger basin sediments. Py-1 resulted from solid-state recrystallization of py-0a and py-0b. During the early ore stage, porous py-2 with high Au concentrations (3.1 ppm) and various trace element enrichments precipitated due to fluid-rock interaction. Metal precipitation occurred through sulfidation and carbonation of wall rock with a relatively reduced fluid undergoing 34S-depletion. However, py-3 displayed notable trace element depletion and a complex mineral association. In a second ore stage, py-4a and zoned py-4b deposition occurred, dominated by an over-pressured fluid fracturing brittle rock. Py-4b, rich in Au (5.5 ppm) concentrated in oscillatory zoned As-rich bands, resulted from interactions during pulsed fluid flow. This process involved fluid phase separation, non-equilibrium conditions, 32S-depletion, and precipitation of py-4b with a high δ34S value (6.4 ‰ mean). Subsequently, pyrite (0–4) generations underwent deformation during a later ductile deformation event, leading to the remobilization and concentration of invisible gold into adjacent gold grains within the high-grade shoot plunging NE. This process involved shearing, boudinaging, and folding ore minerals in tightly folded structures. Late-stage py-5 formed under gold-poor fluid conditions or inefficient gold depositional conditions. Brittle fracturing of these grains later provided local sites for remobilized gold. The study contributes insights into the origin and evolution of gold mineralization in the Kibali gold district, supporting a multistage ore-forming fluid system.

Dynamic Triggering of Earthquakes in Northeast Japan before and after the 2011 M 9.0 Tohoku-Oki Earthquake

ABSTRACT Previous studies documented a relative scarcity of remote dynamic triggering of earthquakes in Japan and suggested that it could be related to Japan’s predominantly compressive tectonic regime or the more frequent occurrence of large earthquakes in Japan. For example, remote triggering in California, characterized by extensional tectonics, occurs at levels of stress change as small as 0.1 kPa, whereas in Japan, transient stresses ≥30 kPa are required. However, the dynamic triggering threshold in Japan, following the 2016 Mw 7.0 Kumamoto earthquake, has been found to be of just a few kilopascals, significantly smaller than reported previously. It was proposed that a decrease in the triggering threshold may have taken place in Japan, in particular at volcanic and geothermal areas, after the 2011 Mw 9.0 Tohoku-Oki earthquake. In this study, we analyze the possible change in dynamic triggering conditions in five areas in northeast Japan, where swarm earthquakes have occurred immediately after the Tohoku-Oki earthquake. The triggering thresholds in these five areas have been estimated based on the analysis of waveform recordings of 49 teleseismic earthquakes that occurred between 2004 and 2020. A decrease of the triggering threshold (or triggering ability) is apparent in all but one region. However, a statistical significance Kolmogorov–Smirnov test could not reject, at a 5% level, the null hypothesis stating that “the distribution of dynamic stress changes for triggering earthquakes that occurred before and after the Tohoku-Oki event is the same.” We interpret the changes in the triggering threshold to be related to the pore pressure increase (and thus a fault strength decrease) in the crust following the Tohoku-Oki earthquake. Our results also indicate that dynamic triggering in Japan is more common than reported previously.

Deciphering Iberian Variscan Orogen Magmatism Using the Anisotropy of Magnetic Susceptibility from Granites

In this paper, we have synthesized the information derived from more than 20 papers and PhD theses on the anisotropy of the magnetic susceptibility (AMS) of 19 Variscan granite plutons, spanning the period between 320 Ma and 296 Ma. The AMS data are obtained from 876 sampling sites with more than 7080 AMS measurements and a re-interpretation is proposed. The studied granites exhibit a magnetic susceptibility (Km) ranging from 30 to 10,436 × 10−6 SI units. Most granites typically exhibit Km values below 1000 × 10−6 SI, indicative of paramagnetic behavior. Biotite serves as the main carrier of iron (Fe), emphasizing the reduced conditions prevalent during the formation of granite melts in the Variscan orogeny. The AMS fabrics of the studied granite plutons record the magma strain, expressing the chronologic evolution of the stress field during the orogeny. This chronologic approach highlights the magmatic events between around 330 and 315 Ma, occurring in an extensional regime, in which the Borralha pluton is an example of a suite that recorded this extensional AMS fabric. Plutons with ages between 315 and 305 Ma show AMS fabrics, pointing out their emplacement in a compressional tectonic regime related to the Variscan collision. The plutons, younger than 305 Ma, record AMS fabrics indicating that the tectonic setting for emplacement changes from a wrench regime to an extensional one at the end of the collision stage. This is evident as there is a chronological overlap between the granites that exhibit AMS fabrics indicating extension and the ones that have AMS fabrics indicating a wrench regime.

Uplift rate calculation based on the comparison between marine terrace data and river profile analysis: A morphotectonic insight form the Ionian coastal belt of Basilicata, Italy

Geomorphological analyses of a wide coastal belt between northern Calabria and Puglia, where a well-exposed staircase of marine terraces extensively crops out, were performed in order to unravel the uplift history of this key-area. As a matter of fact, the Ionian coast of southern Italy represents a strategic sector for investigating the middle-late Quaternary morphotectonic evolution of the orogenic system of southern Italy.The main goal of our research was the reconstruction of a detailed uplift history using spatial distribution of marine terraces and river profile inversion. This was achieved by producing a new, detailed and complete map of the entire sequence of marine terraces and associated palaeoshorelines along the entire coastal sector. A critical review of the literature related to geochronological data from the study area, integrated with a modeling of the interaction between eustatic changes and tectonic uplift allowed demonstrating that the different terraces are correlated to many highstand peaks, dating the highest/oldest terrace to mid-Pleistocene times. Additional constraints on the spatio-temporal variation of tectonic uplift (or base-level fall) have been derived from the inversion of longitudinal river profiles. Such an approach adopts an independent geomorphic marker than the marine terraces and allowed us to obtain an additional estimation the uplift history of the area. The uplift values from the two methods differ in some cases by an order of magnitude, which could be attributed to the rigorous assumption of the river profile inversion or a limitation related to the chronological scheme of marine terraces.Our results highlight a spatial and temporal variation in the uplift rates, which can be only explained with a morphotectonic framework and likely the whole chain-foredeep-foreland system, remained under a prevailingly compressional tectonic regime during the Quaternary, with significant consequences for the seismic hazard of the region.

Tectonically-determined distribution of monogenetic volcanoes in a compressive tectonic regime: An example from the Pannonian continental back-arc system (Central Europe)

This paper presents the results from a geographic information systems (GIS) workflow, which was used to analyze the spatial distribution and temporal evolution of volcanoes in the Mio-Pleistocene monogenetic Bakony-Balaton Highland Volcanic Field (BBHVF), located in the Pannonian Basin, Hungary. Volcanism occurred during the tectonic inversion in a back-arc setting and a compressive/transpressive tectonic regime on the hottest and thinnest lithosphere of continental Europe. The main goal of this study is to clarify the effect of the pre-existing structure of the upper lithosphere in the distribution of the volcanic centers across the volcanic field using an innovative GIS methodology. Orientation of the volcanic field was compared to the orientation of the faults in the BBHVF, and in its larger vicinity, which resulted in correspondence, suggesting the dominance of the SW-NE direction. The directions of the volcanic lineaments fit well to the two main fault directions. The fault-volcano proximity analysis suggests that the fault plane of a thrust fault was an important structural feature during the lifespan of the volcanism. All results suggest that the fault plane of a regionally significant Cretaceous thrust fault (Litér Fault) might have served as a temporary pathway for the ascending magma, whereby (similarly to other, smaller faults) redirecting the magmas causing clustering of the volcanoes. This highlights the importance of major upper crustal structural heterogeneities for magma transport in a compressive tectonic system, especially in the case of active, monogenetic volcanic fields from a volcanic hazard perspective. The present GIS workflow can be effective in analyzing the spatial patterns of the volcanism and its connection with crustal structures at monogenetic volcanic fields worldwide.

New Constraints on the Late Oligocene‐Miocene Thermo‐Tectonic Evolution of the Southeastern Tibetan Plateau From Low‐Temperature Thermochronology

AbstractThe southeastern (SE) Tibetan Plateau (Yunnan) is characterized by low‐relief uplands that were deeply incised by large rivers. The thermal history of basement rocks in this region remains poorly investigated, while this data is needed to elucidate the complex relationship between tectonics and climate in shaping the surface. To better understand its thermo‐tectonic evolution, we carried out apatite fission track thermochronology on 31 samples collected from a large area that covers different tectonic units, including a vertical profile in the middle Mekong River valley; additional zircon LA‐ICP‐MS U‐Pb dating was performed on four basement rocks. Our results confirm that a large portion of Mesozoic crystalline rocks constitute the basement of the SE Tibetan Plateau. Inverse thermal history modeling of fission track data reveal extensive late Oligocene to Miocene rapid basement cooling and exhumation episodes from both inside and outside the active zones (i.e., ductile shear zone and river valley). These thermal events were coincident with the activities of large‐scale strike‐slip faults that dominate the structural framework. Combined with the published data, we propose that widespread crustal shortening and thickening took place in the SE Tibetan Plateau during the Oligocene‐Miocene in the context of a compressive tectonic regime. Low‐temperature thermochronological data reveal that both tectonic forcing and climate‐driven erosion have played important roles in exhuming the basement rocks in the region. It is also deduced that the present‐day relatively low‐elevation landscape of the Yunnan area resulted from complex interaction between regional tectonic activity and surficial erosion since the late Oligocene.

Contemporaneous Thick- and Thin-Skinned Seismotectonics in the External Zagros: The Case of the 2021 Fin Doublet, Iran

In this work, we propose a geodetic model for the seismic sequence, with doublet earthquakes, that occurred in Bandar Abbas, Iran, in November 2021. A dataset of Sentinel-1 images, processed using the InSAR (Interferometric Synthetic Aperture Radar) technique, was employed to identify the surface deformation caused by the major events of the sequence and to constrain their geometry and kinematics using seismological constraints. A Coulomb stress transfer analysis was also applied to investigate the sequence’s structural evolution in space and time. A linear inversion of the InSAR data provided a non-uniform distribution of slip over the fault planes. We also performed an accurate relocation of foreshocks and aftershocks recorded by locally established seismographs, thereby allowing us to determine the compressional tectonic stress regime affecting the crustal volume. Despite the very short time span of the sequence, our results clearly suggest that distinct blind structures that were previously unknown or only suspected were the causative faults. The first Mw 6.0 earthquake occurred on an NNE-dipping, intermediate-angle, reverse-oblique plane, while the Mw 6.4 earthquake occurred on almost horizontal or very low-angle (SSE-dipping) reverse segments with top-to-the-south kinematics. The former, which cut through and displaced the Pan-African pre-Palaeozoic basement, indicates a thick-skinned tectonic style, while the latter rupture(s), which occurred within the Palaeozoic–Cenozoic sedimentary succession and likely exploited the stratigraphic mechanical discontinuities, clearly depicts a thin-skinned style.

Geodynamics and tectonic stress model for the Zagros fold–thrust belt and classification of tectonic stress regimes

Stress measurement in rocks is an essential parameter in Earth and petroleum sciences. In hydrocarbon exploration and production from oilfields, a lack of sufficient knowledge on the initial stress field for the fracturing of formations limits proper simulation of the production process.To date, only a limited number of studies have investigated the state of stress in the Zagros Mountains. In this paper, well-established analytical techniques were used to determine the stress state in the Zagros Mountains. First, the Angelier method was used to invert earthquake data in different structural domains. Second, the pattern of crustal stresses was analyzed, and four classes of stress regimes were identified. The stress patterns were reconstructed for five distinct structural domains (zones 1–5). In all five zones, the compressive stress was normal to the main structures (fold axis and fault trace with NW‒SE strike). It can be inferred that the structural pattern in the sedimentary cover of the Zagros fold-and-thrust belt (ZFTB) is controlled by R1 (Regime 1) and R4 for zones 1 and 2; R3 for zone 3; R1 and R2 for zone 4; and zone 5. The reconstructed stress patterns are consistent with the observed strike-slip faulting and compressional tectonic deformation regimes. The patterns indicate that deformation in the ZFTB can be explained by a dextral transpressional tectonic model. First-order NE–SW compression stress controls the main convergence tectonics within the Zagros Mountains. Two subordinate compression directions were also identified: one oriented in a dextral main compression direction (N 241°–243°) and another in a sinistral (N135°–177°) and minor extension (N201°–229°). Classification and modeling of the stress regimes identified both thrusting and strike-slip systems; the former controls the Main Zagros Reverse Fault (hereinafter MZRF), and the latter results in a transpressional tectonic regime.

Tectonic stress of northeastern Indian region derived from seismic focal mechanisms and the effect of focal mechanism on stress drop: a comparative analysis with Kachchh intraplate region of India

SUMMARY In this study, the focal mechanism solutions and source parameters of recent earthquakes that occurred in the northeastern region of India have been determined. The region has very complex tectonics as it is subjected to the compressional forces from all sides, due to the collision of the Indian Plate with the Eurasian, Burma and Tibetan plates. Waveform data from deployed broad-band seismographs (BBS) and strong motion accelerographs (SMA) in the northeastern India are used to determine the focal mechanism solutions and source parameters of moderate earthquakes, respectively. The estimated focal mechanisms are used to understand the existing stress field in the region. It is found that the Shillong-Plateau as well as the Indo-Burma subduction zone is dominated by the compressional tectonic regime, Mikir Hills and Bengal basin are dominated by the trans-tension tectonic regime, and the easternmost Himalayan region is dominated by the strike-slip tectonic regime. The maximum horizontal stress direction Shmax is also determined for above subregions. The direction of Shmax is southeast in the Bengal basin, northeast in Mikir Hills and Indo-Burma subduction zone whereas it is NNE in Shillong Plateau and SSW in the eastern Himalayas. The estimated stress drop value of the earthquakes in the region ranges from 2.11 to 23.89 MPa. The relationship between the source parameters and focal mechanisms is also explored. It is found that the earthquakes with a strike-slip mechanism have the highest average stress drop (7.05 MPa) followed by reverse (6.82 MPa) and normal (5.12 MPa) in the northeastern region of India. According to the examined data set, the stress drop is found to be dependent on the type of focal mechanism, seismic moment and hypocentral depths. The comparison of the results with the Kachchh intraplate region in western India shows earthquakes in Kachchh have larger mean stress drop for all types of mechanisms. In both intraplate and interplate regions of India, the stress drop of earthquakes depends on the type of focal mechanism solution.

Spatially varying crustal stress along the Zagros seismic belt inferred from earthquake focal mechanisms

We determine the stress field of the Zagros fold-and-thrust belt (ZFTB) in the collisional zone between the Arabian and Iranian plates. Using 898 mainshock focal mechanisms, which occurred along the belt between 1956 and 2021, we calculate stress at 32 locations. The results reveal that the crustal stress in the Zagros belt is heterogeneous with variations in the orientation of SHmax as well as in the stress ratio. The interpolation of the Simpson's index Aϕ points to three distinct domains: northern, central and southern, which are characterized by wrench, compressional, and wrench-compressional to compressional tectonic regimes, respectively. Tectonic stress in the study area is characterized by a horizontal to sub-horizontal maximum compression axis in the ∼NE-SW to ∼NNW-SSE direction. The GPS measurements of the strain tensor and the SHmax direction inferred from the stress inversion indicate that both the deformation pattern and the seismicity along the ZFTB zone are primarily affected by convergence of the Arabian and Iranian plates.

Geology and Structural Evolution of the La Huifa Ore Deposit, Central Chile: A Newly Discovered Porphyry Cu-Mo System in the El Teniente District

Abstract La Huifa is a breccia-related Cu-Mo porphyry deposit located in the Andes of central Chile, 3 km northeast of the giant El Teniente porphyry Cu-Mo orebody. It was discovered as part of CODELCO’s (Corporación Nacional del Cobre de Chile) brownfield exploration activities in the El Teniente district. It is the first major discovery in the area, and this work presents its first detailed geologic description. The La Huifa orebody is related to a complex of late Miocene diorite and granodiorite intrusions and hydrothermal breccias, the latter characterized by abundant tourmaline-anhydrite cement, emplaced in middle to late Miocene volcanic and subvolcanic rocks. The structural architecture of La Huifa is dominated by WNW-and NE-striking high angle faults. Usually, slip on the WNW-striking faults involves a sinistral component, whereas slip on the NE-striking faults is predominantly dextral. The main hydrothermal breccia body at La Huifa is located at the intersection of WNW- and NE-striking faults. Four stages are proposed for the tectono-magmatic-hydrothermal evolution of La Huifa:Premineralization, involving tourmaline-albite veins and sodic calcic alteration emplaced under a transpressive tectonic regime with a subhorizontal, ~E-W–trending σ1 (~N-S–trending σ3);Early mineralization, involving a polydirectional vein system associated with poorly developed potassic and transitional alteration zones with a scarce presence of Cu and Mo sulfides;Main mineralization, involving Cu- and Mo-rich anhydrite-tourmaline hydrothermal breccias and polydirectional veins associated with strong chlorite-sericite alteration with a 270° to 290°C emplacement temperature, obtained from chlorite geothermometry. The regional stress field during this period was exceeded by the local fluid pressure. These breccias and veins are followed by the emplacement of NE-striking C and D type veins associated with gray and white sericite alteration, respectively, which show a similar temperature range.Late mineralization, involving a NE-striking, carbonate-rich vein system containing polymetallic sulfides. It is associated with argillic alteration, whose temperature was 200° to 260°C. This stage occurred under a compressive tectonic regime with NE-trending σ1. The relatively low temperature at which mineralization was introduced in this deposit (~270°–290°C) and the predominantly vertical fluid flow pattern typical of hydrothermal breccia systems allow us to infer the presence of more mineralized rock at depth, below the recognized portion of the system. The association of Cu and Mo enrichment with chlorite-sericite alteration, instead of higher-temperature potassic or gray sericite alteration events, makes La Huifa a unique case among the porphyry deposits of central Chile and broadens the scope of mineralization styles that could be targeted in this belt. Finally, the intersection of arc-oblique, high-angle faults and, in particular, the presence of fault systems orthogonal to the main compression direction, favorable for magma storage and differentiation and metal concentration, constitute a valuable tool for exploring porphyry Cu deposits in similar geologic contexts.

Preservation of basement architecture in zircon Hf isotope maps

AbstractZircon Hf isotopic mapping has been proposed to image large‐scale lithospheric architecture. Here we present zircon Hf isotope maps of the Proterozoic Albany–Fraser Orogen (AFO) in Western Australia, which records a protracted geological history in both extensional and compressional tectonic regimes. The spatial variation in Hf isotope composition of AFO granites is nearly orthogonal to the dominant crustal fabric. Although the 176Hf/177Hf compositions are too juvenile to represent direct melts of the Archaean basement, the variability in 176Hf/177Hf in these AFO magmatic rocks closely mimics the structure of the adjoining Yilgarn Craton. This orogen‐parallel heterogeneity is best illustrated during earliest Palaeoproterozoic magmatic events, which are characterised by low Sr/Y ratios, consistent with tectonic models involving relatively uniform extension along the craton margin during the Palaeoproterozoic. These results demonstrate that regional‐scale zircon Hf variability does not always image coeval lithospheric architecture, but may help illuminate deep basement structure in regions with complex deformation histories.

Ductile Shearing and Focussed Rejuvenation: Records of High-P (eo-)Alpine Metamorphism in the Variscan Lower Crust (Serre Massif, Calabria—Southern Italy)

In the present study, we unveil the real significance of mylonitic reworking of the polymetamorphic crystalline basement in the Serre Massif of Calabria (Southern Italy). We use a multidisciplinary approach to comprehend the structural, microstructural and petrologic changes that occurred along a, so far, not much considered shear zone affecting the Variscan lower crustal rocks. It was never before studied in detail, although some late Cretaceous ages were reported for these mylonites, suggesting that this shear zone is of prime importance. Our observations reveal now that the formation of the new structural fabric within the shear zone was accompanied by changes in mineral assemblages, in a dominant compressive tectonic regime. During this tectono-metamorphic event, high-P mylonitic mineral assemblages were stabilized, consisting of chloritoid, kyanite, staurolite, garnet and paragonite, whereas plagioclase became unstable. Average peak P–T conditions of 1.26–1.1 GPa and 572–626 °C were obtained using THERMOCALC software. These data question (i) that the Serre Massif represents an undisturbed continuous section of the Variscan crust, as generally suggested in the literature, and (ii) highlight the role of (eo-)Alpine high-P tectonics in the Serre Massif, recorded within mylonite zones, where the Variscan basement was completely rejuvenated.

Fluid inclusion evidence for extreme overpressure induced by gas generation in sedimentary basins

Abstract Formation and preservation of extremely high-pressure fluid in sedimentary basins is critical to understanding petroleum accumulation and basin evolution; however, this issue remains poorly understood due to a lack of key evidence. We present fluid inclusion evidence from the Paleozoic black shales in the eastern Sichuan Basin (South China block) that suggest that gas generation can form methane-saturated fluids with pressures significantly exceeding the lithostatic pressure. The fluid inclusion internal pressure is so high (e.g., ~77 MPa) that gas hydrate formed at ambient temperature, aqueous-vapor homogenization could not be achieved under ambient pressure, and the homogenization pressure was beyond the scope of the current equation of state for the NaCl-H2O-CH4 system. We infer that the formation of high-pressure (e.g., &amp;gt;230 MPa), methane-saturated fluids induces hydrofracture under weak compressional tectonic regimes, where the excess lithostatic fluids can be accumulated and sustained. Fluid inclusion records with an abnormally high magnitude of overpressure indicate small differential stress and thus can serve as a microscale indicator for tectonic quiescence.

Crustal architecture and evolution of the southwestern South China Sea: Implications to continental breakup

The South China Sea (SCS) is a marginal basin characterized by propagating breakup and subsequent seafloor spreading during the Cenozoic. Our study investigates the crustal architecture and tectonic evolution of the Xisha Islands - Northwest Borneo conjugate margins using a 1000 km long seismic profile and two shorter seismic lines. This system is characterized by wide margins showing a succession of rift basins with thinned continental crust, associated with large extensional detachment faults soling on top of the lower crust. In relation with crustal extension, several syn-rift sequences have been identified. The lower syn-rift unit (syn-rift I) shows well expressed wedge-shape growth strata. The upper one (syn-rift II) presents a more uniform thickness, and is slightly offset by normal faults dipping toward the depocenters. To illustrate the evolution of the southwestern SCS margins during continental breakup, we restore two conjugate sections near the oceanic magnetic anomaly C5En (18.8 Ma). The conjugate COT presents an asymmetrical hyper-extension architecture in cross sections, with the southern margin being the lower plate margin. In map view, the spatial variation of crustal structures illustrates a bayonet shape characterized by an initial N–S extension coeval with the first phase of seafloor spreading in the east sub-basin in the SCS. The extension direction changed shortly later (circa 23Ma) to NW-SE opening in the southwest sub-basin in relation to a ridge jump. We suggest that variable extension direction is linked to changes in the compressional tectonic regime along Borneo and Palawan.