Transtensional System Research Articles

Late Jurassic paleogeography of the U.S. Cordillera from detrital zircon age and hafnium analysis of the Galice Formation, Klamath Mountains, Oregon and California, USA

The Upper Jurassic Galice Formation, a metasedimentary unit in the Western Klamath Mountains, formed within an intra-arc basin prior to and during the Nevadan orogeny. New detrital zircon U-Pb age analyses (N = 11; n = 2792) yield maximum depositional ages (MDA) ranging from ca. 160 Ma to 151 Ma, which span Oxfordian to Kimmeridgian time and overlap Nevadan contractional deformation that began by ca. 157 Ma. Zircon ages indicate a significant North American continental provenance component that is consistent with tectonic models placing the Western Klamath terrane on the continental margin in Late Jurassic time. Hf isotopic analysis of Mesozoic detrital zircon (n = 603) from Galice samples reveals wide-ranging εHf values for Jurassic and Triassic grains, many of which cannot be explained by a proximal source in the Klamath Mountains, thus indicating a complex provenance. New U-Pb ages and Hf data from Jurassic plutons within the Klamath Mountains match some of the Galice Formation detrital zircon, but these data cannot account for the most non-radiogenic Jurassic detrital grains. In fact, the in situ Cordilleran arc record does not provide a clear match for the wide-ranging isotopic signature of Triassic and Jurassic grains. When compiled, Galice samples indicate sources in the Sierra Nevada pre-batholithic framework and retroarc region, older Klamath terranes, and possibly overlap strata from the Blue Mountains and the Insular superterrane. Detrital zircon age spectra from strata of the Upper Jurassic Great Valley Group and Mariposa Formation contain similar age modes, which suggests shared sediment sources. Inferred Galice provenance within the Klamath Mountains and more distal sources suggest that the Galice basin received siliciclastic turbidites fed by rivers that traversed the Klamath-Sierran arc from headwaters in the retroarc region. Thus, the Galice Formation contains a record of active Jurassic magmatism in the continental arc, with significant detrital input from continental sediment sources within and east of the active arc. These westward-flowing river systems remained active throughout the shift in Cordilleran arc tectonics from a transtensional system to the Nevadan contractional system, which is characterized by sediment sourced in uplifts within and east of the arc and the thrusting of older Galice sediments beneath older Klamath terranes to the east.

Paleomagnetic Study of the Late Miocene Volcanic Sequence in Sonora, Mexico: Clockwise Vertical Axis Rotation Related to the Oblique Rifting in the Pacific‐North American Plate Boundary

AbstractThe evolution of oblique divergence along the Pacific‐North American plate boundary during the late Miocene is well‐preserved along the eastern margin of the Gulf of California. The early phase of this deformation occurred between ∼12.5 and ∼6 Ma in Sonora, NW Mexico, related to oblique rifting of the Gulf of California. The magmatic sequence for this period is exposed in the Cerro El Vigía, Guaymas region. This sequence is characterized by intrusive rocks, lava flows, and pyroclastic flows ranging in age from 12 to 10 Ma. The Cerro El Vigia has a predominant semicircular geomorphology traditionally ascribed to a volcanic caldera structure, such as large ring faults. Rock magnetic analyses of these magmatic products identify the magnetic minerals and their reliability for tectonic interpretations. The mean paleomagnetic direction from 14 sites from the Cerro El Vigia reveals up to 90° clockwise rotation about an inclined axis (65–85° axis plunge) with respect to stable North America. When interpreted in the context of the semicircular structure, these observations suggest that the structure has a tectonic, not volcanic origin. Paleomagnetic results and tectonics during the late Miocene suggest that this crustal block rotation corresponds to the development of large tension gashes deformed within a dextral transtensional system associated with continental deformation of the proto‐Gulf of California rift. Based on the paleomagnetic data, reported ages, and magmatic stratigraphy, we conclude that the magmatism and associated deformation at Cerro El Vigia was synchronous with transtensional tectonics between ∼11 Ma to ∼8 Ma.

Tectono-geomorphic development of an active transtensional depression: a case study of Mountain Tuwaiq Plateau, south Riyadh City, Saudi Arabia

Central Arabian Mountain Tuwaiq Plateau, south Riyadh City underwent geologic work, including calibrated, filtered, geo-referenced and ortho-rectified synthetic aperture radar (SAR) imageries of the ERS-1/2 and ENVISAT satellites. The Jurassic Mountain Tuwaiq Plateau shows cuestas with flatiron geomorphology dipping gently, which is eroded by an eastward dendritic drainage pattern atop Mountain Tuwaiq, which pours southward running into the Wadi Hanifah valley. The E-W Druma, Nisah, Awsat, Bu’ayja, Rufa’a and Mugarah grabens of Central Arabia are superposed on the plateau’s geologic and geomorphic features and appear to be part of the Sahaba strike-slip fault. The graben system has turned the N-S Mesozoic beds into escarpments, with them dipping inwardly towards the north and south and forming a multiple horst and graben. The overlying Cretaceous rocks were transtensionally deformed, and they crop out next to the Jurassic sequence. The transtensional system has also thrown the upper Cretaceous and Paleogene rocks down atop the lower Cretaceous rocks. The overall drainage pattern is an eastward running centripetal pattern that is overprinted by a longitudinal parallel system. Moreover, the Wadi Hanifah valley was displaced by Wadi Laha sinistral fault. The Central Arabian graben system of Mountain Tuwaiq Plateau may fit in the transtensional horsetail splay tectonic landform and damage zone area of the Wadi Sahaba transform fault at the termination mechanism. This intra-plate setting and continental interior may relate to the collision of Arabian and Eurasian plates, in which the transform fault represents Mountain Zagros’ thrust belt westward escape structures in the Arabian hinterland area.

Unravelling the architecture and evolution of the inverted multi-stage North Iberian-Bay of Biscay rift

Current rift models propose polyphase rift evolutions that develop with the same kinematic framework and accommodate continuous and sequential extension, through the stacking of deformation modes. Several rifted margins show, however, evidence for multiple and out of sequence rift events, which developed within different kinematic frameworks, with a complex spatial and temporal evolution. In this work, we address the problem of multi-stage rifting based on the study of the central North Iberian margin, located at the southern Bay of Biscay triangular oceanic domain. This magma-poor rifted margin recorded three major Mesozoic rift events and a subsequent Alpine compressional reactivation, representing a unique setting to study the architecture of multi-stage rift systems and the exerted control on subsequent reactivation. Relying on a dense set of 2D seismic reflection profiles, boreholes, and published velocity models, we map and describe structural domains and major extensional and compressional structures, and we construct depth and thickness maps of syn-rift units. These new maps display the geometry and spatial distribution of major rift basins and bounding structures. Distinctive scenarios resulting from this tectono-stratigraphic approach led us to define three rift systems. A first, diffuse and widespread Triassic system, with classical fault-bounded half-graben basins; a second, narrow, deep and localised Late Jurassic to Barremian transtensional system, including laterally confined pull-apart basins; and a third, widely distributed, Aptian to Cenomanian hyperextended system. Our results show that each rift system controlled the successive extensional events and the subsequent contractional deformation, resulting in a complex 3D template. Deciphering the tectono-stratigraphic architecture of such, multi-stage systems provide key insights on the spatial, temporal, and thermal evolution of divergent plate boundaries. It is likewise indispensable to propose and test kinematic plate deformable models on conjugate rifted margins and to comprehend the implications for their reactivation.

A three-dimensional (3D) structural model for an oil-producing basin of the Brazilian equatorial margin

Hydrocarbon discoveries in Equatorial Brazil, Equatorial Africa and French Guiana-Suriname-Guyana have recently confirmed their importance as new exploration frontiers. The Mundaú sub-basin, located on the Brazilian Equatorial Margin, comprises four producing fields in shallow waters: Xaréu, Atum, Espada e Curimã. In order to understand the structural and seismic-stratigraphic frameworks of an oil-producing region in Equatorial Brazil, this work addresses the 3D geometry and spatial distribution of main faults in the Curimã and Espada fields. The occurrence of hydrocarbons in the Mundaú sub-basin is compared with fields in other parts of the Brazilian Equatorial Margin and Equatorial Africa. Data from 12 wells and a 3D post-stack time-migrated multichannel seismic volume are used to define nine (9) seismic-stratigraphic units: the syn-rift Mundaú Formation (Units 1, 2, 3 and 4); the transitional Paracuru Formation (Unit 5); and the drift Ubarana (Uruburetama and Itapagé Members, Units 6 and 7), Tibau and Guamaré Formations (Units 8 and 9). The study area is dominated by NW-SE planar normal faults, basinward-dipping, that form multiple half-grabens, and tilted blocks with small anticlines and synclines genetically related to a transtensional system. Three types of plays are recognised in the Mundaú sub-basins: structural, combined (structural-stratigraphic) and stratigraphic (turbiditic). In the eastern part of the study area, where the basement is shallow, no oil was found. Conversely, oil was discovered in an anticlinal trap formed over a hanging-wall block analogous to fields on the Côte D'Ivoire-Ghana transform margin. This work shows that combined traps on footwall blocks are successful plays near the shelf break of the Mundaú sub-basin, in similarity with the Espoir and Baobab fields in Ivory Coast. Furthermore, turbiditic reservoirs in drift units are analogous to the Stabroek block in Guyana and prospects in the Gulf of Guinea. The structural and petroleum-play analyses in this work are therefore crucial to understand the multiple geological processes leading to the trapping of hydrocarbons in the larger Equatorial Atlantic Ocean.

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

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

The evolution of the Dominga Fe-Cu deposit (northern Chile): Insights from mineral textures and micro-CT analysis

The Fe-Cu Dominga deposit (2082 Mt at 23% Fe, 0.07% Cu), located in the Coastal Cordillera of northern Chile, is hosted by volcanic rocks of the Punta del Cobre Formation (131.5 ± 1.5 Ma zircon U-Pb) and into subvolcanic units (Dioritic Complex, 131.6 ± 1.0 Ma zircon U-Pb). The Fe-Cu mineralization is controlled by three structural systems which developed from a transtensional to a transpressional tectonic regime and can be divided into three groups: Early iron, Late iron and Early copper ores. Early iron ores are comprised of magnetite + pyrite + biotite breccia (1A ore), veins (1B ores), layers (1C ores) and disseminated ores (1D ores). Late iron ores are characterized by two groups of magnetite-apatite-actinolite hydrothermal breccias (2A, 2C ores) and syntaxial/antitaxial veins (2B, 2D, 2E ores). Early copper ores occur as syntaxial K-feldspar and quartz + epidote + chalcopyrite veins (3A ores), and as anhydrite + chalcopyrite-rich matrix breccia and veins (3B ores). This work presents a detailed mineral texture study of veins, hydrothermal breccias and disseminated iron-rich layers of the Dominga deposit which aims to determine fluid flow mechanisms associated with both iron and copper ores. The description of vein and breccia textural and internal structures was conducted in thin/polished sections perpendicular and parallel to the wall using optical and Scanning Electron Microscope techniques. In addition, two oriented surface samples were analyzed by computerized X-ray microtomography and numerical fluid flow simulations through the Lattice-Boltzmann method to obtain (3D) permeability anisotropy associated with early iron ores. Microtextures associated with Dominga iron and copper ores suggest that the main mass transfer fluid flow mechanism corresponds to advection (channelized and pervasive fluid flow), regardless of the tectonic regime. However, early copper ores have more complex mineral textures and internal structure due to the recurrence of crack-seal episodes. We propose that the various mineral textures and structures indicate changes in fluid flow direction over time, controlled by the permeability anisotropy of each tectonic regime. Results from numerical fluid flow simulations of early iron ore (1B) veins show a higher value of structural permeability in the vertical direction (kVz), which is consistent with a transtensional tectonic regime and the formation of vertical veins. Moreover, permeability related to 1C layered ore is higher in horizontal directions (kHx, kHy) rather than vertical (kVz) because of the natural permeability anisotropy of volcaniclastic rocks parallel to bedding. However, the kVz value suggests that the development of such layered ores also exhibits a degree of structural control at several length scales consistent with a transtensional regime. These results indicate that the occurrence of early iron ores as veins and layers may be controlled by both primary permeability anisotropy related to each lithology present at the Dominga Fe-Cu deposit and to the tectonic regime at the time. Finally, the Dominga Fe-Cu deposit attests to long lived hydrothermal activity with a transition from early fluids capable of precipitating iron ores under a transtensional system to later-stage fluids which precipitated copper ores under a transpressional regime that generated multiple crack and seal episodes.

Late Paleogene sinistral strike-slip system along east Qinling and in southern North China: Implications for interaction between collision-related block trans-rotation and subduction-related back-arc extension in East China

We present in this paper a compilation of Paleogene rift and strike-slip structures in east Qinling and southern North China based on a synthesis of published stratigraphic, structural and thermochronological data, which aids in delineating a broad (>500-km-wide), kinematically interlinked, Eocene-Oligocene transtensional system. This transtensional system was bifurcated from two discrete strike-slip fault zones in NE Tibetan Plateau. The overall fault patterns indicate a clockwise rotation of South China relative to North China during the Cenozoic, possibly driven by dextral shearing along the eastern Tibetan margin. This collision-related trans-rotation of South China occurred coevally with late Paleogene intracontinental rifting and volcanism in East China in a subduction-related back-arc extensional setting. These two tectonic processes interacted with each other, giving birth to Cenozoic rift systems and contrasting morpho-structural features in East Asia.

Fault kinematic and Plio-Quaternary paleostress evolution of the Bakırçay Basin, Western Turkey

ABSTRACT At the end of the Cenozoic, western Turkey was fragmented by intense intra-continental tectonic deformation resulting in the formation of two extensional areas: a transtensional pull-apart basin systems in the northwest, and graben systems in the central and southwest areas. The question of the connection of this Late Cenozoic extensional tectonics to plate kinematics has long been an issue of discussion. This study presents the results of the fault slip data collected in Bakırçay Basin in the west of Turkey and addresses changes in the direction of extensional stresses over the Plio-Quaternary. Field observations and quantitative analysis show that Bakırçay Basin is not a simple graben basin that has evolved during a single phase. It started as a graben basin with extensional regime in the Pliocene and was transformed into a pull-apart basin under the influence of transtensional forces during the Quaternary. A chronology of two successive extensional episodes has been established and provides reasoning to constrain the timing and location of subduction-related back-arc tectonics along the Aegean region and collision-related extrusion tectonics in Turkey. The first NW–SE trending extension occurred during the Pliocene extensional phase, characterized by slab rollback and progressive steepening of the northward subduction of the African plate under the Anatolian Plate. Western Turkey has been affected, during the Middle Quaternary, by regional subsidence, and the direction of extension changed to N–S, probably in relation with the propagation of the North Anatolian Fault System. Since the Late Quaternary, NE–SW extension dominates northwest Turkey and results in the formation and development of elongated transtensional basin systems. Counterclockwise rotation of Anatolian block which is bounded to the north by the right-lateral strike-slip North Anatolian Fault System, accompanies to this extensional phase.

Tectono-magmatic characteristics of post-collisional magmatism: Case study East Carpathians, Călimani-Gurghiu-Harghita volcanic range

This work provides a comprehensive GIS mapping integrated with geological and geophysical data (magnetic and gravimetric) to reveal the interplay between (i) magmatism and volcanic activity, (ii) intra-mountain basin development and (iii) large-scale faulting/tectonic features in the post-collisional setting of the East Carpathians. The geological, geophysical and tectonic data of the Călimani-Gurghiu-Harghita range (CGH) confirm the contemporaneous formation of basins and volcanic activity. The two phenomena propagated southwards in time, parallel to the collisional front, in close relation to the development of the fault systems. The geometry of the faults and alignment of volcanic edifices indicate both strike-slip and normal faulting, with no rotation. The main volcanic range and main tectonic basins evolved along a regional NNW-SSE striking sinistral transtensional fault zone with left stepping stepovers and pull-apart basins in the transfer areas. The fault zones probably acted as least resistance areas that fostered the development of magma ascent paths and plumbing systems. These connected the volcanic centers with numerous magma chambers emplaced at the upper mantle – lower crust boundary and throughout the lower and upper crust. The magma generation is most likely a result of the interplay between large-scale transtensional systems that induced decompression in the previously metasomatised mantle, and asthenosphere upwelling in the post-collisional East Carpathian orogen.

Active degassing across the Maltese Islands (Mediterranean Sea) and implications for its neotectonics

Abstract The Maltese Islands, located in the central Mediterranean Sea, are intersected by two normal fault systems associated with continental rifting to the south. Due to a lack of evidence for offshore displacement and insignificant historical seismicity, the systems are thought to be inactive and the rift-related deformation is believed to have ceased. In this study we integrate aerial, marine and onshore geological, geophysical and geochemical data from the Maltese Islands to demonstrate that the majority of faults offshore the archipelago underwent extensional to transtensional deformation during the last 20 ka. We also document an active fluid flow system responsible for degassing of CH4 and CO2. The gases migrate through carbonate bedrock and overlying sedimentary layers via focused pathways, such as faults and pipe structures, and possibly via diffuse pathways, such as fractures. Where the gases seep offshore, they form pockmarks and rise through the water column into the atmosphere. Gas migration and seepage implies that the onshore and offshore faults systems are permeable and that they were active recently and simultaneously. The latter can be explained by a transtensional system involving two right-stepping, right-lateral NW-SE trending faults, either binding a pull-apart basin between the islands of Malta and Gozo or associated with minor connecting antitethic structures. Such a configuration may be responsible for the generation or reactivation of faults onshore and offshore the Maltese Islands, and fits into the modern divergent strain-stress regime inferred from geodetic data.

Tectonic Evolution of the Pamir Recorded in the Western Tarim Basin (China): Sedimentologic and Magnetostratigraphic Analyses of the Aertashi Section

AbstractThe northward indentation of the Pamir salient into the Tarim basin at the western syntaxis of the India‐Asia collision zone is the focus of controversial models linking lithospheric to surface and atmospheric processes. Here we report on tectonic events recorded in the most complete and best‐dated sedimentary sequences from the western Tarim basin flanking the eastern Pamir (the Aertashi section), based on sedimentologic, provenance, and magnetostratigraphic analyses. Increased tectonic subsidence and a shift from marine to continental fluvio‐deltaic deposition at 41 Ma indicate that far‐field deformation from the south started to affect the Tarim region. A sediment accumulation hiatus from 24.3 to 21.6 Ma followed by deposition of proximal conglomerates is linked to fault propagation into the Tarim basin. From 21.6 to 15.0 Ma, increasing accumulation rates of fining upward clastics is interpreted as the expression of a major dextral transtensional system linking the Kunlun to the Tian Shan ahead of the northward Pamir indentation. At 15.0 Ma, the appearance of North Pamir‐sourced conglomerates followed at 11 Ma by Central Pamir‐sourced volcanics coincides with a shift to E‐W compression, clockwise vertical‐axis rotations and the onset of growth strata associated with the activation of the local east vergent Qimugen thrust wedge. Together, this enables us to interpret that Pamir indentation into Tarim had started by 24.3 Ma, reached the study location by 15.0 Ma and had passed it by 11 Ma, providing kinematic constraints on proposed tectonic models involving intracontinental subduction and delamination.

Particle‐Based Modeling of Transtensional Pull‐Apart Basins

AbstractScaled underlapping models based on the discrete element method are used to investigate crack propagation and basin development in transtensional systems with both parallel and nonparallel master faults. The introduction of a small component of oblique divergent motion causes the crack pattern and basin geometry to become quite different. The 5° transtensional model with parallel master faults produces a much wider depression separated by a cross‐basin fault, compared to pure strike slip. A single throughgoing fault that connects the two master faults forms when the obliquity of 10° is reached. For transtensional model with two nonparallel master faults, a minor change of one master fault from parallel to nonparallel with a small component of 5° causes the crack pattern to become much more diffuse. As the obliquity increases from 5° to 15°, the first cracks propagate at a higher angle relative to the master faults, the cross‐basin fault forms at a lower angle clockwise to the motion direction, and the area of the first depression decreases. A single throughgoing fault linking the two master faults initiates when the obliquity of 20° is obtained. The crack patterns and basin geometries of our numerical models show many similarities to natural transtensional basins including the Cinarcik Basin and the Central Basin in the Sea of Marmara. The transtensional models with various initial fault kinematics and different obliquities lead to different fault linkages, which has implications for the possible earthquake hazards in the Sea of Marmara near Istanbul.

Active fault system across the oceanic lithosphere of the Mozambique Channel: Implications for the Nubia–Somalia southern plate boundary

Seismic reflection and multibeam echosounder data were acquired in the Mozambique Channel in 2014 and 2015 during the PTOLEMEE, PAMELA-MOZ02 and -MOZ04 marine surveys aboard the RV Atalante and Pourquoi Pas? These data revealed that an active fault system is deforming the oceanic lithosphere of the Mozambique Basin which has developed during Jurassic-Cretaceous times. The correlation between the fault system and the arrangement of earthquake epicenters suggests that this tectonically active zone directly connects northward with the southern part of the eastern branch of the East African Rift System which corresponds to the seismically active graben system bounding the northern part of the Davie ridge. The fault zone extends southwestward of the Mozambique Ridge along the same trend as the Agulhas-Falkland transform fault zone. The general organization of the fault zone shows the characteristics of an extensional system north of the Mozambique Channel (north of the Europa Island) and a right-lateral transtensional system with coeval normal faults and strike-slip faults south-west of Europa. This tectonic activity is associated with volcanic activity since at least Late Miocene times. Our findings emphasize that the eastern branch of East African Rift System is extending largely toward the south, not only in continental domains but also through the oceanic lithosphere of the Mozambique basin. This fault zone is participating to the complex plate boundary between the main African continent (Nubia Plate) and Madagascar (Somalia Plate).

Evidence of extensional and strike-slip deformation in the offshore Gökova-Kos area affected by the July 2017 Mw6.6 Bodrum-Kos earthquake, eastern Aegean Sea

The interpretation of new multichannel seismic profiles and previously published high-resolution swath and seismic reflection data from the Gokova Gulf and southeast of Kos Island in the eastern Aegean Sea revealed new morphotectonic features related to the July 20, 2017 Mw6.6 Bodrum-Kos earthquake offshore between Kos Island and the Bodrum Peninsula. The seafloor morphology in the northern part of the gulf is characterized by south-dipping E–W-oriented listric normal faults. These faults bend to a ENE–WSW direction towards Kos Island, and then extend parallel to the southern coastline. A left-lateral SW–NE strike-slip fault zone is mapped with segments crossing the Gokova Gulf from its northern part to south of Kos Island. This fault zone intersects and displaces the deep basins in the gulf. The basins are thus interpreted as the youngest deformed features in the study area. The strike-slip faults also produce E–W-oriented ridges between the basin segments, and the ridge-related vertical faults are interpreted as reverse faults. This offshore study reveals that the normal and strike-slip faults are well correlated with the focal mechanism solutions of the recent earthquake and general seismicity of the Gokova Gulf. Although the complex morphotectonic features could suggest that the area is under a transtensional regime, kinematic elements normally associated with a transtensional system are missing. At present, the Gokova Gulf is experiencing strike-slip motion with dominant extensional deformation, rather than transtensional deformation.

High-resolution imagery of active faulting offshore Al Hoceima, Northern Morocco

Two recent destructive earthquakes in 1994 and 2004 near Al Hoceima highlight that the northern Moroccan margin is one of the most seismically active regions of the Western Mediterranean area. Despite onshore geodetic, seismological and tectonic field studies, the onshore–offshore location and extent of the main active faults remain poorly constrained. Offshore Al Hoceima, high-resolution seismic reflection and swath-bathymetry have been recently acquired during the Marlboro-2 cruise. These data at shallow water depth, close to the coast, allow us to describe the location, continuity and geometry of three active faults bounding the offshore Nekor basin. The well-expressed normal-left-lateral onshore Trougout fault can be followed offshore during several kilometers with a N171°E±3° trend. Westward, the Bousekkour–Aghbal normal-left-lateral onshore fault is expressed offshore with a N020°E±4° trending fault. The N030°E±2° Bokkoya fault corresponds to the western boundary of the Plio-Quaternary offshore Nekor basin in the Al Hoceima bay and seems to define an en échelon tectonic pattern with the Bousekkour–Aghbal fault. We propose that these three faults are part of the complex transtensional system between the Nekor fault and the Al-Idrissi fault zone. Our characterization of the offshore expression of active faulting in the Al Hoceima region is consistent with the geometry and nature of the active fault planes deduced from onshore geomorphological and morphotectonic analyses, as well as seismological, geodetic and geodynamic data.

Pull-apart development at overlapping fault tips: Oblique rifting of a Cenozoic continental margin, northern Mergui Basin, Andaman Sea

The northern Mergui Basin (Andaman Sea) contains east-northeast–west-southwest– to northeast-southwest–striking normal fault–bound basins, and north-northwest–south-southeast–trending strike-slip faults. The two largest strike-slip faults (Manora and Mergui) pass into extensional or transtensional basins at their tips, consistent with dextral offset. The faults provide examples of early stage pull-apart basin development at fault tips instead of the more common model for development at releasing bends. Offset of isochron markers for the Ranong Formation indicate that ∼8 km of dextral offset has occurred along the Mergui fault and 4.5 km of dextral offset has occurred on the Manora fault. The strike-slip faults and associated extensional faults formed relatively late for the history of the entire Mergui Basin during the Early Miocene. The northern part of the Mergui Basin developed after a phase of west-northwest-east-southeast extension during the Oligocene in the Mergui Basin to the south, indicating a rotation in the extension direction toward the north-northwest–south-southeast with time. The basin is part of a major transtensional system involving the Sumatra, West Andaman, and Sagaing faults that accommodated the northern motion of western Myanmar as India moved north relative to Southeast Asia. Fault activity in the northern Mergui Basin decreased significantly when the broad zone of Early Miocene transtension became focused on the Alcock and Sewell Rises during the Middle Miocene, and the West Andaman and Sagaing faults began to develop and interacted in a large pull-apart geometry with the Shan Scarp Fault, and later (Late Miocene or Pliocene) with the Sagaing Fault.

The Kultuk Volcano: spatial-temporal change of magmatic sources at the western terminus of the South Baikal basin between 18 and 12 Ma

The Kultuk volcano erupted at the axial South Baikal basin of the Baikal rift zone (BRZ). Now it exhibits facies of subvolcanic bodies, land-lava eruptions and subaqueous pillow lavas and hyaloclastites. The volcano was controlled by the Obruchev fault that is currently a border of the basin which amplitude of vertical movements is rapidly decreasing in the westward direction. It is found that the Kultuk volcano was active at the beginning and end of the volcanic activity period of the Kamar, Stanovaya, and Bystrinskaya volcanic zones, which took place 18–12 Ma ago. In previous papers, it was assumed that dominant structures in the area under study were major Cenozoic shear displacements along the Main Sayan fault and/or along the Tunka rift valley; however, at the current stage of our study, linear configurations of the volcanic zones do not reveal any of such displacements. Based on analyses of distribution of volcanic rocks in the relief at the western coast of Lake Baikal, distinct vertical crustal movements are revealed; such movements started in the Early Miocene and continue to the present time. It is concluded that volcanism was controlled by the trans-tensional system of volcanic zones. Sources are identified for the shallow lithospheric mantle melt with the substantial admixture of the low-crust component and deeper asthenospheric mantle melts in the Kamar and Stanovaya volcanic zones; for the Bystrinskaya volcanic zone, only components from the deeper source are revealed. The local shallow mantle magmatism occurred only within the lithosphere extension zone beneath the South Baikal basin. The lithosphere thinning is reflected in the change of activity from the sub-lithospheric to lithospheric sources under the Kamar zone. Rifting of the axial structure is recorded at the root of the Slyudyanka lithospheric block that was subjected to the collision-related Early Paleozoic metamorphism. Geochemical characteristics of the collision-type components were inherited by the Miocene basaltic melts.

Kinematic analysis of a transtensional fault system: The Atuel depocenter of the Neuquén basin, southern Central Andes, Argentina

Abstract The Atuel depocenter of the Neuquen basin originated as an Upper Triassic to Lower Jurassic rift system, later inverted during the Andean contractional deformation. In order to study the extensional architecture and the kinematic evolution of this depocenter, we collected a large amount of field and sub-surface data, consisting of slip data from outcrop-scale normal faults, thickness and facies distribution within the synrift deposits, and structural data from angular and progressive unconformities. The Atuel depocenter has a NNW trend, showing a bimodal distribution of NNW and WNW major faults (first and second order faults). On the other hand, from kinematic indicators measured on outcrop-scale faults (third and fourth order faults), we found a mean NE internal extension direction, which is oblique to the general trend of the sub-basin. Taking these particular characteristics into account, we interpreted the Atuel depocenter as an oblique rift system. We evaluated two mechanisms in order to explain the development of this transtensional system: 1) reactivation of upper-crustal NNW-oriented Paleozoic shear zones, and 2) oblique stretching of a previous NNW-oriented lithospheric weakness zone.

Exhumation history of the Red River shear zone in northern Vietnam: New insights from zircon and apatite fission-track analysis

A new set of zircon and apatite fission-track ages from the Ailao Shan and Day Nui Con Voi (DNCV) metamorphic massifs of the Red River shear zone (RRSZ) and neighboring rocks in northern Vietnam is presented. A complex, along-strike diachronous, denudation history is revealed. The southern sector of the DNCV cooled to about 100 °C by the Late Oligocene, whereas its central compartment was affected by the later thermotectonic evolution of the Song Chay dome to the E of the RRSZ, whose final exhumation occurred during the Early Miocene. The northern sector of the RRSZ is characterized by the 35 Ma Phan Si Pang pre- to synkinematic intrusion. Fission-track ages from a vertical section within the Phan Si Pang granite indicate rapid exhumation and cooling. The Paleozoic tectonic block to the west of the RRSZ (fission-track ages between 40 and 30 Ma) was exhumed and cooled earlier than the fault mylonite belt (fission track ages of 30 Ma and younger) and also than the eastern block. Its structural level is consistent with field observations that suggest the RRSZ in northern Vietnam to be a transtensional system, with a regional NE–SW oriented extension component.