Transpressional Research Articles

Tectonic evolution of the early Paleozoic intraplate orogen in the South China Block: Insights from Ductile Shear Zones in North Wuyishan

The ductile shear zones within the North Wuyishan domain are key to understand the tectonic evolution of the South China Block (SCB). This study integrates a multifaceted approach, including field observations, thin section analysis, quartz electron backscatter diffraction (EBSD), zircon U-Pb dating, and mica 40Ar/39Ar dating, to elucidate the deformation history of these shear zones in North Wuyishan. The research identifies a two-stage deformation process of early Paleozoic. The initial D1 phase is marked by a top-to-SW thrusting shear, with associated felsic veins dated to the Ordovician period (447 ± 10 Ma to 459 ± 4 Ma), correlating with high-grade metamorphism and partial melting. The later D2 phase is characterized by NE-striking foliation and lineation, indicative of sinistral strike-slip shear, dated to the early Devonian (412 Ma) and early Carboniferous (∼354 Ma). The D1 phase suggests early crustal thickening and melting within the Cathaysia block, while D2 indicates a transpressional regime during post-orogenic adjustment. The geological features of the SCB, notably the absence of arc magmatism, ophiolitic mélange, and high-pressure metamorphism, comply with an intracontinental rift closure model as previously proposed. This model supports the hypothesis that the Early Paleozoic intracontinental orogeny in the SCB was likely a far-field consequence of a continental collision between the SCB-North Vietnam and South Vietnam blocks near the east Gondwana supercontinent.

Neoproterozoic arc-magmatism of the Silet Terrane, Western Hoggar (Algeria), new constraints from zircon U-Pb-Hf isotope analyses and whole rock geochemistry

This study presents results of zircon U-Pb-Hf isotope analyses and whole-rock geochemistry applied to magmatic rocks of the Silet terrane in the Western Hoggar, forming a part of the Neoproterozoic arc exposed along the boundary between the LATEA metacraton and the Paleoproterozoic In Ouzzal terrane. According to zircon U-Pb geochronology four distinct pulses of Neoproterozoic granitoids magmatism are recognized in the Silet terrane: Tonian (c. 870–840 Ma), Late Tonian (c. 770 Ma), Latest Tonian (c. 730 Ma) and Late Cryogenian (c. 675 Ma).Geochemical and Hf isotope data indicate a progressive shift in geotectonic settings. The c. 870–840 Ma Tonian TTG pulses (Tonalite-Trondhjemite-Granodiorite) in the North Silet batholith, with superchondritic εHf values (+5.8 to + 7.2), suggest formation in a subduction zone. The subsequent c. 770 Ma Late Tonian magmatism, characterized by mafic compositions, reflects a back-arc tectonic setting, as evidenced by high Ti/V ratios, low V contents, and εHf values of + 6.12 to + 8.95. The c. 730 Ma Latest Tonian magmatism, with more evolved acidic pulses, indicates hybrid mantle–crustal involvement (εHf + 6.53 to + 8.08). The Late Cryogenian intrusions (c. 675 Ma), featuring the lowest εHf values (+1.84 to + 4.09) and inherited c. 870 Ma zircons, suggest formation from the melting of Tonian TTGs during continental collision.Integrating U-Pb dating with geochemical data provides a comprehensive view of the Silet terrane’s tectonic evolution, supporting a continental arc origin. Disparities in magmatism ages between the northern (c. 870–675 Ma) and southern (c. 740–640 Ma) segments of the Silet arc correlate with the diachronous collision/subduction of a V-shaped “Silet Ocean”, forming an introverted oceanic system (oceans that have formed by rifted continental blocks that were subsequently re‑joined in approximately the same position) within the broader West Gondwana context. Structural analysis indicates that the regional structures of the Silet arc were shaped in a WNW-ESE collisional transpressive regime during the Pan-African orogeny, offering new insights into the Neoproterozoic Silet arc’s geodynamic evolution and its paleogeographical location in the West Gondwana orogeny.

Conjugate Riedel shears in a volcanic caldera: the case of the Laguna del Maule Volcanic Complex, Central Chile

The Andean intra-arc is characterized by large-scale continuous lineaments, often the direct surface expression of tectonic structures. In this study, we analyze the main lineaments around the Laguna del Maule Volcanic Complex, Central Chile (~36° S), to shed light on the main tectonic processes. We show that the lineament directions are consistent with structures based on a theoretical Riedel model and suggest the occurrence of two conjugate Riedel shears crossing Laguna del Maule. This structural arrangement can accommodate deformation related to either a transtensional regime, having a WNW-oriented master fault (Laguna Fea Fault), or a transpressional regime, characterized by NNE-oriented master faults (e.g., Melado Fault). In this context, the NE-striking Troncoso Fault would be a long-lived dextral fault compatible with both regimes. According to the Riedel model, ENE-oriented lineaments are predicted to be tensional structures, whilst NW-oriented structures (e.g., Los Cóndores Fault) would correspond to P-type structures accommodating compressional deformation. This tectonic architecture, characterized by the most intense deformation just at the intersection between shears, has likely promoted magma accumulation and would explain the compositional variability at Laguna del Maule. This study therefore demonstrates the usefulness of oriented lineament maps in interpreting regional tectonics for volcanically active areas.

Insights on the Mesoarchean to Neoproterozoic evolution of the western Campo do Meio Shear Belt, SE Brazil, using structural, aerogeophysical and U-Pb zircon data

The Campo do Meio Shear Belt (CMSB) is an approximately 200 × 25 km transcurrent area along the south-southwest margin of the São Francisco Craton. A comprehensive review of geology was performed in the western CMSB, where rocks from the Campos Gerais and Petúnia Complexes are predominant. The Campos Gerais Complex is relatively well positioned in the geotectonic context of the region. It corresponds to an Archean granite-greenstone belt lithological association related to the south-southwest part of the São Francisco Craton. The Petúnia Complex, in contrast, is unraveled in this contribution through the integration of aerogeophysical data from the western Campo do Meio Shear Belt (CMSB) with new fieldwork observations and zircon U–Pb (LA-MC-ICP-MS) dating.The Petúnia Complex has been restricted to two Mesoarchean gneissic units – Petúnia and Ribeirão da Conquista Gneiss, with crystallization ages of 2993 ± 11 Ma and 2870 ± 9 Ma, respectively. Although the ages are broadly coincident with those found in the Campos Gerais Complex, three characteristics suggest an evolutionary independent scenario (pre-S2) during the Archean for the Petúnia and Campos Gerais Complexes: i) the sharp contrast in gamma-ray emissions between these complexes; ii) the absence of komatiite affiliation in the meta-ultramafic rocks from the Petúnia Complex; and, iii) the absence of mafic dykes in the Petúnia Complex. These two complexes also can be distinguished by the frequency of Paleoproterozoic granitoid intrusions. The Petúnia Complex has only one occurrence, the Bom Jesus da Penha Metagranite, with a crystallization age of 2004 ± 11 Ma. On the other hand, several Paleoproterozoic granitoid intrusions (2.0–1.9 Ga) have been found in the Campos Gerais Complex. These Orosirian granitoids are restricted to CMSB and younger than those in the Mineiro Belt. We suggest that these Orosirian granitoids may represent remnants of a collision event that occurred after 2.1 Ga, leading to the integration of the Petúnia and Campo Gerais Complexes into the Columbia paleocontinent. During the Neoproterozoic, the evolution in the CMSBinvolved the cratonic basement and units related to the Southernmost Brasília Orogen. This set configures a tectonic mélange that underwent progressive deformation in a non-coaxial, partitioned transpressional regime.

U–Pb–Hf and morphological evolution of zircon from granites associated with world-class tungsten skarn deposits in the northern Canadian Cordillera

The northern Canadian Cordillera is the most significant tungsten district in North America. Here, high-grade tungsten skarn deposits are associated with small, reduced, high-K calc-alkaline, S-type biotite granite plutons belonging to the 102–96 Ma Tungsten plutonic suite (TPS). A detailed U–Pb–Hf and morphological study of magmatic zircon from plutons in the southern half of the TPS belt was undertaken to better understand magmatic processes leading to the generation of the associated tungsten deposits. Antecrystic zircon from the TPS plutons began crystallizing during a transpressional regime ca. 117 Ma, suggesting the TPS magmas were active for up to 21 Myr prior to their upper crustal emplacement and final crystallization. This prolonged magmatic activity necessitates a magma origin in long-lived, deep crustal magma chambers. Hafnium isotopic compositions in zircon for the southern TPS as a whole form a non-radiogenic, univariate, and relatively wide ranging population (εHfi = −17.6 ± 4.5), but U–Pb–Hf trends become apparent when the data are sub-divided into sample groups with similar age, zircon morphology, and geographic location. These evolutionary trends in magmatic zircon are most simply explained by interactions between the parent melt and dissolving inherited zircon grains. This, along with changing zircon morphology, is consistent with gradual cooling and crystallization pathways exhibited by S-type magmas. Differing evolutionary trends in the U–Pb–Hf isotopic data between sample groups, however, suggest there were multiple magma batches that evolved independently, possibly in separate pockets within large, deep magma chambers. Zircon morphologies also suggest some grains in all sample groups were equilibrated with hotter and more alkaline magmas, although there is no textural or compositional evidence in the zircon for mixing of magmas with widely different compositions. An unconstrained inversion of local aeromagnetic data indicates reduced batholiths could be present 4–6+ km below the surface and that the plutons are apophysies to (or, higher level injections from) these deeper bodies. Although these batholiths can only be short-term holding chambers for magmas ascending from deep crustal levels, they may have been important for the segregation of mineralizing fluids. Since no single magmatic evolutionary pattern in the unaltered TPS plutons can be definitively linked to tungsten mineralization, pulses of mineralizing fluid may have been derived instead from the underlying batholiths. The 20+ Myr duration of deep magmatic activity exhibited by the TPS is similar to timeframes suggested for magmas associated with tungsten deposits in southern China, and may have allowed extended fractionation of a large volume of crustally derived magma to concentrate tungsten into late-stage melts. The emplacement of upper crustal batholiths and plutons in both regions during or following a transition to an extensional tectonic regime suggests the relaxed geodynamic regime may have been important in the ascension of metal-rich magmas and (or) fluids, ultimately resulting globally important tungsten deposits.

The major Adassil-Medinet fault; a complex evolution of high angle transpressive Variscan shear zones (Western High Atlas, Morocco)

The Western High Atlas is one of the most important sectors of the Variscan foldbelt in Morocco and also of its Alpine intracontinental orogen. Its tectonic evolution is thus essential for the understanding, not only of the Moroccan geology, but also of the interaction between successive orogenic cycles which is one of the focus of this article. Furthermore, as the Moroccan Variscides must be seen in the context of the complex dextral collision between Laurasia and Gondwana, this is also a contribution to the comprehension of the complexity of transpressive regimes and strain partitioning processes.New data show that WNW-ESE sinistral Variscan shear zones (mainly the Addouz-Adassil-Anamrou one - AAAsz), although secondary to the main orogenic scale ENE-WSW dextral shear zones, must be considered in order to understand the Late Paleozoic deformation and its Alpine reactivation. In fact, the concentration of the last stages of the Variscan ductile deformation in the steep and irregular AAAsz played an important role in the initiation and localisation of the minor magmatic events and influenced the geometry and kinematics of the Late Variscan WNW-ESE sinistral shear zones. Furthermore, during the Alpine inversion, the orientation of the AAAsz and related major anisotropies tend to be reactivated, with a major reverse component, giving rise to the irregular Adassil-Medinet fault zone, one of the most important structures of the Western High Atlas.

Fold-thrust belt in the southern front of the central High Atlas (Morocco): Analysis and implications for the tectonic inversion of the atlas system

Thrust folds have developed significantly in the meso-Cenozoic rock units of the Errachidia-Boudnib basin. The study areas correspond to the north part of the Pre-African Trough, which is located between the Anti-Atlas and the Atlas systems. It is characterized by faulted anticlines that are indicative of a foreland orogenic context.The Atlas system's structural evolution underwent several stages, starting with a Triassic extensional event followed by a period of fault reactivation during the Atlas compression. This research focuses on examining the thrust and detachment folds associated with both reverse and strike-slip components. For this purpose, we performed detailed geological mapping and interpretation of folding and fault slip data.As a result of shortening, field observations reveal that the study area exhibited well-developed thrusting geometry. Preexisting blind faults and multiple decollement levels within favorable formations such as Cenomanian evaporitic marls, Cenomanian-Turonian marls rich in organic matter, and Senonian argillites influence the folding patterns, which are not uniform. The findings demonstrate two key points. First, the most significant folds have formed along major blind thrusts. Subsequently, between these major faults, detachment folds were developed within the Jurassic-Cretaceous strata. Currently, due to erosion, certain thrusts have become visible, including the Ta'bbast thrust fault, Ait Atman (from the first stage), and the Timazguit fault zone (from the second stage). The South Atlas fault largely remains a blind fault. Additionally, the study emphasizes the presence of strike-slip components and en-echelon folding, indicating a transpressional regime during the uplift of the Atlas system.

Conjugate shear model for emplacement of mid-crustal plutons during dextral transpression, southern Nova Scotia, Canada: tectonic and petrological consequences

The South Mountain Batholith (SMB) is a syn-tectonic composite batholith emplaced in the upper crust within the Meguma terrane between 380 and 370 Ma during the later stages of the Neoacadian orogeny. Coeval plutons in southern Nova Scotia are surrounded by mid-crustal (∼4 kbar) andalusite–staurolite aureoles and are discordant to northeast-trending, regional Neoacadian folds. Detailed field studies, combined with published results, indicate emplacement within a dextral transpressional regime during the transition from distributed (D1) to focused heterogeneous strain (D2), which provided vertical conduits that facilitated magma ascent. The Port Mouton Pluton intruded along P-orientated crustal-scale fractures as a series of subvertical granitic sheets, which were progressively rotated and folded with ongoing dextral shearing. By contrast, the Barrington Passage Pluton intruded between crustal-scale, antithetic (sinistral) P-shear fractures and spread laterally between them as pulsed increments to form a layered, subhorizontal, and sill-like complex resembling the laccolithic structure of the SMB. The SMB was emplaced below the Meguma Supergroup, with magma derived from underthrust Avalon terrane and Silurian Rockville Notch Group. The lack of mantle components in the SMB suggests transpressional orogenesis facilitated conductive crustal heating without significant mantle addition, consistent with low p-wave velocities of Meguma lower crust. 400–355 Ma zircons, recorded either as inherited grains in granites or in felsic granulite xenoliths, imply the Neoacadian thermal anomaly extended for 45 Ma, but magmatism represented only ∼40% of that perturbation.

Earthquake Relocations Delineate a Discrete Fault Network and Deformation Corridor Throughout Southeast Alaska and Southwest Yukon

AbstractDeformation in southeastern Alaska and southwest Yukon is governed by the subduction and translation of the Pacific‐Yakutat plates relative to the North American plate in the St. Elias region. Despite notable historical seismicity and major regional faults, studies of the region between the Fairweather and Denali faults are complicated by glacial coverage and the remote setting. In the last decade, significant improvements have been made to the density of regional broadband seismometer networks. We relocate more than 5,000 earthquakes between 2010 and 2021 in the region of southeastern Alaska and southwestern Yukon utilizing these improved seismic networks. With reductions in catalog uncertainty, particularly in depth, we quantify the thickness of the seismogenic layer in the crust throughout the region and locate seismicity on a shallow network of upper‐crustal faults. Relocated earthquakes, combined with an updated focal‐mechanism catalog, permit estimating and classifying motion of active faults. This includes mapping the Totschunda‐Fairweather “Connector” fault, which plays an important role in explaining regional deformation, and identifying new faults like the Kathleen Lake fault. We draw similarities between our seismic observations and simplified conceptual models of regional tectonics, which describe a dominant transpressional regime and localized slip partitioning. Our results support a hypothesis where current deformation is taking place on a well‐defined and evolved network of shallow faults in the corridor between the Totschunda‐Fairweather “Connector” and Denali faults.

Genesis of a sediment-hosted vein-type Cu deposit in the Lanping Basin, SW China: New insights from geology and LA–ICP–MS analysis of fluid inclusions

Sediment-hosted Cu deposits, including stratiform and epigenetic vein-type Cu deposits, are the second largest source of Cu worldwide. However, many facets of these vein-type Cu deposits, such as the nature and origin of ore fluids and geodynamic mechanisms that produced these fluids, remain controversial, limiting our interpretations of the ore genesis and hampering the selection of exploration strategies. Numerous sediment-hosted vein-type Cu deposits are present in the Lanping Basin, SW China, of which the Jinman deposit is the largest. Copper orebodies at Jinman consist of Cu-sulfide–quartz–carbonate veins and altered wall-rocks hosted mainly in faults and fracture zones in metamorphosed and deformed clastic rocks. The deposit was formed in three stages, pre-, syn- and post-ore, with the first two stages producing hydrothermal veins. The veins in pre-ore stage consist of quartz + ankerite ± sericite ± pyrite, whereas those in syn-ore stage consist of Cu-sulfides + quartz + carbonate ± sericite. During the syn-ore stage, tennantite, quartz, and ankerite crystallized early, then bornite was precipitated, followed by chalcopyrite and chalcocite and finally calcite. Only minor sericite was formed during the post-ore stage. Two major types of fluid inclusions—CO2-rich and aqueous inclusions—are present in both pre- and syn-ore quartz, whereas only aqueous inclusions are present in syn-ore calcite. In pre- and syn-ore quartz and calcite, CO2-rich inclusions have higher homogenization temperatures (275–323 °C) and pressures (307–3495 bar) and lower salinities (0.22–2.7 wt% NaCl eqv.) than aqueous inclusions (164–299 °C, 6.5–80.4 bar and 3.7–20.9 wt% NaCl eqv.). However, the small variations in Cs/Na and Cs/(Na + K) ratios [log Cs/Na = –3.3 ∼ –1.9, log Cs/(Na + K) = –3.3 ∼ –2.1] of the fluid inclusions suggest a same source for the ore fluids. We propose that during both stages, CO2-rich inclusions resulted from unmixing of initial deep-seated, single-phase CO2-rich fluids, whereas aqueous inclusions likely represent the fluids that evolved from these unmixed fluids by extensive CO2 degassing. This interpretation, combined with microthermometric results of CO2-rich inclusions, suggests a mesothermal, low-salinity H2O–NaCl–CO2 ore fluid system. The fluid inclusions from Jinman have similar compositions to those of metamorphic fluids, suggesting a metamorphic origin for the ore-forming fluids. The evolution of fluid compositions suggests that fluid degassing likely acted as a major mechanism for Cu precipitation. We propose that the ore fluids were generated through metamorphic devolatilization of basement of the Lanping Basin in the Biluoxueshan–Chongshan metamorphic zone under a compressional or transpressional regime caused by Cenozoic India–Asia collision. The Jinman deposit has many similarities to orogenic gold deposits worldwide and thus can be thought of as an orogenic deposit. Based on a compilation of the geological characteristics of global sediment-hosted epigenetic vein-type Cu deposits, we suggest that other sediment-hosted epigenetic vein-type Cu deposits with similar genesis to the Jinman deposit are likely present in other regions worldwide. This highlights the exploration potential of this type of deposit in these regions and the need for further research.

Sodic-calcic alteration and transpressional shear along the Atacama fault system during IOCG mineralization, Copiapó, Chile

The Punta del Cobre district near Copiapó is a center of iron oxide-copper–gold (IOCG) mineralization spatially and temporally associated with regional sodic-calcic hydrothermal alteration, the Atacama fault system (AFS), and two phases of Early Cretaceous magmatism. Here, we investigate the spatiotemporal and geochemical relationships between magmatism, ductile deformation, and hydrothermal alteration along the ~ 200 to 300-m-thick steeply NW-dipping Sierra Chicharra shear zone, interpreted to be the major strand of the AFS. Mylonitic fabrics and oblique sinistral-reverse kinematic indicators together record coaxial flattening in a transpressional regime. Deformation on the AFS took place before, during, and after intrusion of the synkinematic Sierra Chicharra quartz diorite of the Coastal Cordillera arc at ~ 122 Ma and terminated before intrusion of the unstrained ~ 114 Ma Sierra Atacama diorite of the Copiapó batholith. Geochemical data show that the Copiapó batholith was more mafic and more K-rich than the calc-alkaline Coastal Cordillera arc. This time period thus overlaps IOCG mineralization in the Punta del Cobre district (~ 120 to 110 Ma). Multiple phases of sodic-calcic alteration in and around the AFS shear zone are recognized. Textures of altered rock in the shear zone show both synkinematic assemblages and post-kinematic hydrothermal oligoclase. A ~ 775-m-long andradite vein that cuts the shear zone formed broadly at the end of magmatism in the district (~ 95 Ma). Oxygen isotope ratios from the vein indicate that hydrothermal fluids were likely magmatically derived. Together, this work shows the AFS-related shear zone and nearby IOCG mineralization developed in a regional transpressional regime produced by SE-directed oblique convergence across a NE-striking shear zone. IOCG-related magmatic-hydrothermal fluids exploited this transcrustal shear zone to produce multiple episodes of regional sodic-calcic alteration formed from fluids exsolved from magmas or driven by the heat of the Coastal Cordillera arc and Copiapó batholith.

Strain partitioning in the Patagonian Broken Foreland: influence of structural inheritance of early Andean deformation

In the context of Andean-type oblique convergence, strain partitioning between thrust and strike-slip tectonics is well documented in the arc region. However, the impact of this configuration in the retroarc remains poorly understood, especially in broken foreland systems where basement structures compartmentalize the retroarc into small intermontane sedimentary basins. The spatiotemporal analysis of tectonic evolution and the role of inherited weaknesses in this area are often debated. This study addresses these uncertainties by integrating geometric and kinematic analyses of tectonic structures within the Sañicó Massif in the North Patagonian foreland. Results reveal two deformational episodes. The initial Late Cretaceous to Paleocene episode represents a contractional deformation phase, with NW–SE shortening direction causing the tectonic inversion of pre-Andean rift depocentres. The second episode records a Neogene transpressional regime with both contractional and strike-slip deformation, with east–west to NE–SW shortening direction, which progressed under reactivation of pre-existing structures and generated new faults. This work demonstrates the Neogene propagation of the strike-slip regime towards the retroarc, defining strain partitioning at various scales and highlighting its interaction with pre-existing structures. These results enhance our understanding of the complex tectonic history in oblique subduction settings, providing significant evidence of the structural inheritance in retroarc systems.

Evolución de las cuencas Tumaco, Atrato y San Juan sobre una plataforma continua a lo largo del margen pacífico colombiano

Several authors have interpreted the Colombian Pacific margin as a succession of oceanic terrane accretions, with generation of forearc basins under a compressive-transpressive tectonic regime during the Late Cretaceous-Miocene. New data acquired in the last decade, consisting of reprocessed seismic lines, geological cross-sections, stratigraphic data from wells, and new stratigraphic columns, are analyzed in this study. We suggest that the proposed models of successive terrane accretions are not supported by the new data; rather, the onshore basins were likely controlled by a marine platform from the Late Cretaceous to the Middle-Late Eocene, with reef growth to the end of this period. In the Late Cretaceous, contemporary to the sedimentation stage, effusive magmatism, mainly basaltic, also occurred. Since Oligocene time, the basins were affected by three different tectonic processes. First, during the Oligocene-Early Miocene, an extensional-transtensional event occurred in the Tumaco Basin and San Juan Sub-basin, which produced horst and graben structures and domino faults, along with the emplacement of igneous bodies in the eastern flank of the basins. The Atrato Sub-basin was affected by local reverse faulting towards its eastern edge. Second, during the Mid-Late Miocene, the western side of the Tumaco Basin was uplifted, and transpressive deformation occurred in the San Juan Sub-basin as evidenced by the San Juan and Garrapatas fault systems. Finally, in the Atrato Sub-basin, a transpressional regime is evidenced by the Baudó anticline. Our results suggest there is no evidence of oceanic terrane accretions in the abovementioned basins during the Late Cretaceous-Middle Eocene. Furthermore, we do not see evidence of a subduction system during that period. We conclude that subduction in western Colombia could have begun during the Early Oligocene instead.

Late Cretaceous–Early Paleogene tectonic events in the “North–South Axis” of Central Tunisia

In order to better constrain the structural evolution of the North–South Axis (NOSA) running through central Tunisia, a multidisciplinary approach based on geological mapping, field observations and paleostress analysis was used. The geological study of the middle part of the NOSA including the Gadoum, Akrouta, Sidi Khalif, Khechem El Kaleb and Faïd structures, showed the predominance of N–S and E–W fault sets. Movement on the faults of this fault network caused the formation of depositional areas and the collapsed and tilting of fault bounded blocks located in the Southern part of the Gadoum–Akrouta sector. The Gadoum and Akrouta Jebels formed as a result of slip and rotation on a single N–S trending listric fault in the Cenomanian during which time reactivation of both the N–S and E–W fault sets occurred. During Coniacian–Santonian times, when the Aleg Formation was being deposited, the study area was affected by a transtensive regime. This regime led to the division of the area into blocks (e.g., the Gadoum–Akrouta block and the Wadi El Abiod Syncline) and this resulted in the Aleg Formation being deposited with variable thicknesses. During the Campanian–Early Maastrichtian, a N–S transpressive regime was established, and this regime, coupled with the salt tectonics, resulted in the formation of an angular unconformity, subsidence inversion and lateral thickness variations of the Abiod Formation. During the Early Eocene, an E–W fault network affected the sedimentary basin. These faults, arranged in steps, generated accommodation spaces for sediments which increase in thickness along the North–South Axis.

Seismic geomorphology of the deepwater sediment dispersals systems in the drift phase of the Mundaú sub-basin, equatorial margin, Brazil

The Ceará Basin, located in the Brazilian Equatorial Margin, represents an exploratory frontier situated in a complex structural framework. The basin observed a dynamic interplay of extensional, transtensional, and transpressional regimes, registered by several sedimentary intervals and unconformities poorly documented by wells, especially at the deeper portions. In this study, based on 3D seismic and well data from the Mundaú Sub-Basin, we interpreted the main seismic sequences of the drift phase to build a Relative Geological Time (RGT) model, based on a semi-automatic seismic traces correlation matched with the well top formations. A seismic attribute analysis from key seismic surfaces extracted from the RGT, highlights the main sediment dispersal routes revealing a significant change after the Mid-Eocene, once sedimentation overpassed a topographic barrier created by fault reactivations, in a transpressional regime, associated with the Romanche Fracture Zone. The integration of seismic and well data affords a deeper understanding of basin dynamics, revealing how sedimentary processes respond to shifts in tectonic stress in this portion of the basin.

PALEOPROTEROZOIC TECTONICS AND EVOLUTIONARY MODEL OF THE ONEGA SYNCLINORIUM

Consideration is being given to the Onega Paleoproterozoic structure (Onega synclinorium, OS) as a tectonotype of intraplate negative structures, which experience intermittent subsidence over a long period of time. The paper presents a model of the OS and discusses its tectonic evolution. The model is based on the geological and structural data, already published and collected so far by the authors, as well as on the data concerning the OS deep structure, particularly on the interpretation of the 1-EV seismic profile and potential fields. The proposed model illustrates an example of conjectured interaction between different geodynamic factors and explains reasons for the development of the OS throughout the Paleoproterozoic, including the periods of intense subsidence and magmatism, inversions of local basins comprising the Onega trough, and deformations of the Paleoproterozoic strata. An important role in the formation of the OS was played by shear dislocations within an imbricate fan of its controlling Central-Karelian shear zone. The shear dislocations were accompanied by rotation of a large block located to the west of the OS, which led to the rotational-indentational interaction between adjacent blocks and to compensated coexistence among transtensional and transpressional regimes along their separating shear zone. Compensatory dynamic mechanism also manifested itself in crustal layers at the base of the OS. Horizontal flow of the mid-crustal masses and their outflow from the depression were compensated by the development of deep-seated thrust duplexes and uplifts around the depression as well as by the upper crustal extension associated with low-angle dilatant normal faulting. Successive propagation of these faults, dynamically related to shear dislocations within an imbricate fan of the Central Karelia zone, controlled the formation features and southward migration of the OS-contained basins as well as magmatic and syllogenesis-related occurrences. Multilayered subhorizontal flow of low-viscosity rocks at the base and inside the OS section against the background of shear dislocations gave rise to the occurrence of crest-like and diapir-like folding. The processes of OS formation occurred amid the development and localization of active mantle plumes and asthenospheric diapirs. One of the factors of their development and localization were the phenomena of relative decompression within the imbrication fan of the Central Karelian shear zone.

Paleostress analysis and rift kinematics of the petroliferous Barmer rift basin, western Rajasthan, India

The NW-trending Barmer rift in western Rajasthan is one of the major petroliferous Indian basins. The basin rifted twice: (i) NW – SE extension in Early Cretaceous, and (ii) NE – SW extension during Late Maastrichtian to Early Paleogene. We performed structural fieldwork mainly along the rift margins to decipher the rift kinematics through geologic time. We collected ∼500 fault-slip data from limited outcrops along the basin's western and eastern margins. Paleostress analysis was performed for the first time in this basin using Win_Tensor, T-Tecto and FaultKin software. The outcome of this analyses shows that a strike-slip stress regime was prevalent in the western rift margin whereas the eastern margin experienced extension to transtension. Four paleostress regimes, DΔ, Dα, DΦ, and Dβ, were identified by inverting the fault-slip data. The two separate extensional regimes, during the Early Cretaceous and in the Late Maastrichtian to Early Paleogene, are confirmed. These are Dα (NW – SE extension) and Dβ (NE – SW extension). The analysis also presents two strike-slip regimes with varying maximum horizontal stress (SHMax) orientation, viz. DΔ and DΦ. Exact age correlation for these strike-slip regimes remains indeterminate. However, we deduced DΔ and DΦ were associated with the first extensional phase (Dα) during Early Cretaceous linked to India – Madagascar oblique separation. The Barmer rift extended obliquely guided by pre-existing structures. Remote sensing image analysis indicates that part of the western margin is neotectonically active as inferred from the drainage morphometry. The stress inversion analysis of earthquake focal mechanism data depicts a strike-slip transpressive regime prevailing at present in the NW India along with ∼ N-trending SHMax. This is mainly responsible for the neotectonic activities in the basin. We documented several fault gouge zones along moderately-dipping faults in the Cretaceous outcrops at the eastern margin. The lower the fault dips, the thicker the gouge zones associated with the faults. Such thick fault gouge of around 100 cm or more may act as a structural fault bound trap for Cretaceous reservoirs at depth. (Words: 324).

Fluid-Related Features in the Offshore Sector of the Sciacca Geothermal Field (SW Sicily): The Role of the Lithospheric Sciacca Fault System

The Sciacca basin extends in the southwestern part of Sicily and hosts an important geothermal field (the Sciacca Geothermal Field) characterized by hot springs containing mantle gasses. Newly acquired high-resolution seismic profiles (Boomer data) integrated with a multichannel seismic reflection profile in close proximity to the Sciacca Geothermal Field have documented the presence of numerous active and shallow fluid-related features (pipes, bright spots, buried and outcropping mud volcanoes, zones of acoustic blanking, and seafloor fluid seeps) in the nearshore sector between Capo San Marco and Sciacca (NW Sicilian Channel) and revealed its deep tectonic structure. The Sciacca Geothermal Field and the diffuse submarine fluid-related features probably form a single onshore–offshore field covering an area of at least 70 km2. This field has developed in a tectonically active zone dominated by a left-lateral transpressive regime associated with the lithospheric, NNE-striking Sciacca Fault System. This structure probably favored the rising of magma and fluids from the mantle in the offshore area, leading to the formation of a geothermal resource hosted in the Triassic carbonate succession that outcrops onshore at Monte San Calogero. This field has been active since the lower Pleistocene, when fluid emissions were likely greater than today and were associated with greater tectonic activity along the Sciacca Fault System.

The Peninsular Ranges orogenic gold belt: Supporting evidence from the San Pedro Mártir mining area (Baja California, Mexico)

The gold deposits in the northern Sierra San Pedro Mártir region of the Peninsular Ranges batholith (PRB) occur within a middle-Cretaceous suture zone that juxtaposes the Alisitos island arc against the continental margin of North America. Mineralization shares affinities with orogenic-type gold deposits and typically consists of shear zone-related gold-bearing quartz-carbonate veins, with minor amounts of sulfides, white mica and native gold. Host rock types encompass varied synorogenic plutonic rocks and greenschist to amphibolite facies metamorphic rocks. In this paper, we present four 40Ar/39Ar ages of hydrothermal white micas for representative gold deposits: 106.2 ± 0.2 Ma (Valladares), 100.5 ± 0.2 Ma (La Fortuna-San José), 94.2 ± 0.2 Ma (El Socorro), and 92.2 ± 0.2 Ma (Corazón de Oro de Jesús). The first two ages correspond to highly accurate determinations of the timing of the orogenic gold event, whereas we attribute the younger ages to thermal resetting caused by the emplacement of a nearby large La Posta-type pluton (∼97‒90 Ma). Additionally, in order to establish the maximum age of mineralization for the granitoid-hosted deposits, three new U–Pb zircon ages were obtained for plutonic rocks (136.7 ± 0.9 Ma, Corazón de Oro de Jesús quartz diorite orthogneiss; 107.0 ± 0.5 Ma, Valladares tonalite pluton; 105.0 ± 2.0 Ma, Santa Cruz diorite pluton) Furthermore, fluid inclusions were analyzed from six gold deposits. Four different types of ore fluids were distinguished, with likely temperatures of trapping between ∼300° and >600 °C, and salinities between ∼1 and 17 wt.% NaCl equiv. Gold mineralization formed subsequent to the initial collisional phase, occurring simultaneously with regional metamorphism and pluton emplacement. At least some synorogenic granitoid-hosted deposits were formed under a transient dextral transpressive regime, intercalated between the predominant regional orthogonal compression. A connection is established between these gold deposits and others found in historical mining districts along the exhumed axis of the PRB (e.g., El Álamo), and we propose that all form part of the ‘Peninsular Ranges orogenic gold belt’.

Kinematic analyses of the asymmetric boudins in the Tutak area within the Zagros hinterland fold-and-thrust belt, Iran

Exhumation of the Tutak gneiss dome occurred in a transpressional regime between the Arabian plate and the Central Iranian microcontinent. The Tutak gneiss dome is located in the eastern part of the basement-involved Zagros hinterland fold-and-thrust belt. This dome consists of exhumed middle crust including the Tutak and Sourian metamorphic complexes, which are lied on its core and edges, respectively. Integrated structural, microstructural, and flow vorticity analyses in quartzo-feldspathic mylonites and well-developed asymmetrical boudins provide significant information regarding the kinematic characteristics coeval with the Tutak gneiss dome development. Asymmetrical boudins have been used as potential shear sense markers throughout the mantle of the Tutak gneiss dome. The deformed quarzitic layers within the slate belt host domino and shear-band boudins, demonstrating both sinistral and dextral shear senses. The mean vorticity number (Wm) was estimated from the quartzo-feldspathic mylonites and asymmetric boudins. Using the hyperbolic distribution (HD) and the RXZ strain ratio/quartz c-axis fabric (RXZ/β) methods. The estimated Wms from the HD method and quartz c-axis patterns are 0.69 to 0.82 and 0.6 to 0.92, respectively, suggesting a combination of 41% to 72% simple- and 28% to 59% pure-shear components. The close match confirms that asymmetric boudins can be a good strain estimator. The estimated deformation temperature (528.33 ± 50 °C) from the opening angles (OAs) of quartz c-axes suggests amphibolite facies conditions. Our results show that the Tutak gneiss dome developed under sub-simple shear conditions, which stretched and shortened the dome parallel and orthogonal to the transpressional boundaries, respectively.