Mesozoic extension in the Basque–Cantabrian basin (N Spain): Contributions from AMS and brittle mesostructures

In this work we analyse and check the results of anisotropy of magnetic susceptibility (AMS) by means of a comparison with palaeostress orientations obtained from the analysis of brittle mesostructures in the Cabuérniga Cretaceous basin, located in the western end of the Basque–Cantabrian basin, North Spain. The AMS data refer to 23 sites including Triassic red beds, Jurassic and Lower Cretaceous limestones, sandstones and shales. These deposits are weakly deformed, and represent the syn-rift sequence linked to basins formed during the Mesozoic and later inverted during the Pyrenean compression. The observed magnetic fabrics are typical of early stages of deformation, and show oblate, triaxial and prolate magnetic ellipsoids. The magnetic fabric seems to be related to a tectonic overprint of an original, compaction, sedimentary fabric. Most sites display a NE–SW magnetic lineation that is interpreted to represent the stretching direction of the Early Cretaceous extensional stage of the basin, without recording of the Tertiary compressional events, except for sites with compression-related cleavage.Brittle mesostructures include normal faults, calcite and quartz tension gashes and joints, related to the extensional stage. The results obtained from joints and tension gashes show a dominant N–S to NE–SW, and secondary NW–SE, extension direction. Paleostresses obtained from fault analysis (Right Dihedra and stress inversion methods) indicate NW–SE to E–W, and N–S extension direction. The results obtained from brittle mesostructures show a complex pattern resulting from the superposition of several tectonic processes during the Mesozoic, linked to the tectonic activity related to the opening of the Bay of Biscay during the Early Cretaceous. This work shows the potential in using AMS analysis in inverted basins to unravel its previous extensional history when the magnetic fabric is not expected to be modified by subsequent deformational events. Brittle mesostructure analysis seems to be more sensitive to far-field stress conditions and record longer time spans, whereas AMS records deformation on the near distance, during shorter intervals of time.     

Mesozoic tectonic evolution of the southwest continental Iberian Margin

An extensional event affected the southwest Margin of Iberia during Late Triassic to Early Cretaceous times, giving place to the Algarve basin. This basin was subjected to tectonic instability and it became infilled with siliciclastic and carbonate sequences with abundant interspersed volcanic rocks. Normal and strike-slip faults accommodated the deformation in the Algarve basin. The presence of a single flat or listric detachment surface is inferred from the study of hanging-wall structures. The dynamic and kinematic analyses of fault systems in the Spanish exposure of the Algarve basin allow us to establish three extensional phases. 1) A Late Triassic to Hettangian NE-SW directed extension associated with the initial breaking of Pangea and the opening of the Tethys in the eastern Mediterranean. 2) NW-SE extension from the Sinemurian to the Callovian, interpreted as a result of the activity as a sinistral fault of the Azores-Gibraltar transform boundary. 3) Finally, E-W extension during the Late Jurassic and Cretaceous, related to the North Atlantic rifting process.     

Late Pliocene to Early Quaternary extensional detachment in the La Paz–El Cabo area (Baja California Sur, Mexico): implications on the opening of the Gulf of California and the mechanics of oblique rifting

We demonstrate that Pliocene to Early Quaternary sedimentary formations in Baja California Sur (Mexico) were deposited syn-tectonically over a major detachment associated with the exhumation of Mesozoic crust. The detachment dips to the ENE and is associated with E–W stretching. This large extensional structure strikes almost parallel to the general trend of the Gulf of California and extension is oblique to the East-Pacific seafloor-spreading direction. Crustal-scale stretching in this area was still active after the beginning of seafloor spreading c.  3.6 Ma ago. The detachment is capped by Late Pleistocene–Holocene alluvial sediments the deposition of which seems to be partly syn-tectonic and controlled by minor stretching subparallel to the present-day North American–Pacific kinematic vector. We discuss the implications of our observations on strain partitioning during opening of the California Gulf as well as on the structure of the Gulf of California margin.     

Temporal and spatial variations in tectonic subsidence in the Iberian Basin (eastern Spain): inferences from automated forward modelling of high-resolution stratigraphy (Permian–Mesozoic)

By subsidence analysis on eighteen surface sections and 6 wells, which cover large part of the Iberian Basin (E Spain) and which are marked by high-resolution stratigraphy of the Permian, Triassic, Jurassic and Cretaceous, we quantify the complex Permian and Mesozoic tectonic subsidence history of the basin. Backstripping analysis of the available high resolution and high surface density of the database allows to quantify spatial and temporal patterns of tectonically driven subsidence to a much higher degree than previous studies. The sections and wells have also been forward modelled with a new ‘automated' modelling technique, with unlimited number of stretching phases, in order to quantify variations in timing and magnitude of rifting. It is demonstrated that the tectonic subsidence history in the Iberian Basin is characterized by pulsating periods of stretching intermitted by periods of relative tectonic quiescence and thermal subsidence. The number of stretching phases appears to be much larger than found by earlier studies, showing a close match with stretching phases found in other parts of the Iberian Peninsula and allowing a clear correlation with discrete phases in the opening of the Tethys and Atlantic.     

Structure and tectonic history of the foreland basins of southernmost South America

The common elements and differences of the neighboring Austral (Magallanes), Malvinas and South Malvinas (South Falkland) sedimentary basins are described and analyzed. The tectonic history of these basins involves Triassic to Jurassic crustal stretching, an ensuing Early Cretaceous thermal subsidence in the retroarc, followed by a Late Cretaceous–Paleogene compressional phase, and a Neogene to present-day deactivation of the fold–thrust belt dominated by wrench deformation. A concomitant Late Cretaceous onset of the foreland phase in the three basins and an integrated history during the Late Cretaceous–Cenozoic are proposed. The main lower Paleocene–lower Eocene initial foredeep depocenters were bounding the basement domain and are now deformed into the thin-skinned fold–thrust belts. A few extensional depocenters developed in the Austral and Malvinas basins during late Paleocene–early Eocene times due to a temporary extensional regime resulting from an acceleration in the separation rate between South America and Antarctica preceding the initial opening of the Drake Passage. These extensional depocenters were superimposed to the previous distal foredeep depocenter, postdating the initiation of the foredeep phase and the onset of compressional deformation. Another pervasive set of normal faults of Paleocene to Recent age that can be recognized throughout the basins are interpreted to be a consequence of flexural bending of the lithosphere, in agreement with a previous study from South Malvinas basin. Contractional deformation was replaced by transpressive kinematics during the Oligocene due to a major tectonic plate reorganization. Presently, while the South Malvinas basin is dominated by the transpressive uplift of its active margin with minor sediment supply, the westward basins undergo localized development of pull-apart depocenters and transpressional uplift of previous structures. The effective elastic thickness of the lithosphere for different sections of each basin is calculated using a dynamic finite element numerical model that simulates the lithospheric response to advancing tectonic load with active sedimentation.     

Jurassic extension and Cenozoic inversion tectonics in the Asturian Basin,NW Iberian Peninsula: 3D structural model and kinematic evolution

We constructed a geological map, a 3D model and cross-sections, carried out a structural analysis, determined the stress fields and tectonic transport vectors, restored a cross section and performed a subsidence analysis to unravel the kinematic evolution of the NE emerged portion of the Asturian Basin (NW Iberian Peninsula), where Jurassic rocks crop out. The major folds run NW-SE, normal faults exhibit three dominant orientations: NW-SE, NE-SW and E-W, and thrusts display E-W strikes. After Upper Triassic-Lower Jurassic thermal subsidence, Middle Jurassic doming occurred, accompanied by normal faulting, high heat flow and basin uplift, followed by Upper Jurassic high-rate basin subsidence. Another extensional event, possibly during Late Jurassic-Early Cretaceous, caused an increment in the normal faults displacement. A contractional event, probably of Cenozoic age, led to selective and irregularly distributed buttressing and fault reactivation as reverse or strike-slip faults, and folding and/or offset of some previous faults by new generation folds and thrusts. The Middle Jurassic event could be a precursor of the Bay of Biscay and North Atlantic opening that occurred from Late Jurassic to Early Cretaceous, whereas the Cenozoic event would be responsible for the Pyrenean and Cantabrian ranges and the partial closure of the Bay of Biscay.     

Sedimentary basins of the atlantic margin of North America

Scismic exploration has identified eight distinct basin structures along the North American Atlantic continental margin forming a chain of elongate depocenters parallel to the continental slope and interrupted by transverse basement arches and impinging oceanic fracture zones. From south to north these are: South Florida—Bahamas Basin bounded on the north by Peninsular Arch and Bahama Escarpment fracture zone; Blake Plateau Basin with Cape Fear Arch and the impinging Great Abaco and Blake Spur fracture zones; Baltimore Canyon Trough bounded by the Long Island Platform and impinging Kelvin fracture zone; Georges Bank Basin with the bounding Yarmouth Arch; Scotian Shelf Basin with Scartarie and Canso Ridges and impinging Newfoundland Ridge fracture zone; Grand Banks Troughs and the intervening horst ridges; and the East Newfoundland Basin separated by Cartwright Arch and the impinging Gibbs fracture zone from the Labrador Shelf Basin.All the basins are characterized by great depths to basement filled with from 7 to 14 km of possible Triassic, Jurassic, Cretaceous and Tertiary sediments. Basement faulting controls the basins' boundaries and the faults have affected the overlying sediments. The major boundary faults of the basins undoubtedly formed during the initial rifting of the Atlantic margin in the Jurassic or perhaps Triassic. However, throughout the Mesozoic and Cenozoic these basement faults have moved in response to different orientations of stress and strain rates produced by continued spreading of the Atlantic Ocean. As a result, the basement faults of the Atlantic Margin were apparently influenced by at least three different local stress systems, spatially overlapping but temporally independent. These are the east—west extensional Atlantic Ocean stress system, the northwest—southeast extensional White Mountain stress system, and the north-south extensional Labrador Sea stress system.Some consequences of this basic tectonic setting were differential cross-strike tilts of the basin blocks with each basin moving somewhat independent of its neighbor. The resulting buildup of the basins' sedimentary geometries reflect these tectonic tilts and varying strain rates. Correlations are found between changes in orientation and rates of Atlantic sea-floor spreading with observed major sedimentary events such as progradations, planar bedding episodes, reef platform development, regressive hiatuses, and transgressions. An understanding of this marginal geosyncline could yield a model with predictability.     

Tectono-stratigraphic evolution of an inverted extensional basin: the Cameros Basin (north of Spain)

The Cameros Basin is a part of the Mesozoic Iberian Rift. It is an extensional basin formed during the late Jurassic and early Cretaceous, in the Mesozoic Iberian Rift context, and it was inverted in the Cenozoic as a result of the Alpine contraction. This work aims to reconstruct the tectono-stratigraphic evolution of the basin during the Mesozoic, using new and revised field, geophysical and subsurface data. The construction of a basin-wide balanced section with partial restorations herein offers new insights into the geometry of the syn-rift deposits. Field data, seismic lines and oil well data were used to identify the main structures of the basin and the basin-forming mechanisms. Mapping and cross-sectional data indicate the marked thickness variation of the depositional sequences across the basin, suggesting that the extension of the depositional area varied during the syn-rift stage and that the depocentres migrated towards the north. From field observation and seismic line interpretation, an onlap of the depositional sequences to the north, over the marine Jurassic substratum, can be deduced. In the last few decades, the structure and geometry of the basin have been strongly debated. The structure and geometry of the basin infill reconstructed herein strongly support the interpretation of the Cameros Basin as an extensional-ramp synclinal basin formed on a blind south-dipping extensional ramp. The gradual hanging-wall displacement to the south shifted the depocentres to the north over time, thus increasing the basin in size northwards, with onlap geometry on the pre-rift substratum. The basin was inverted by means of a main thrust located in a detachment located in the Upper Triassic beds (Keuper), which branched in depth with the Mesozoic extensional fault flat. The reconstruction of the tectono-stratigraphic evolution of the Cameros Basin proposed herein represents a synthesis and an integration of previous studies of the structure and geometry of the basin. This study can be used as the basis for future basin-scale research and for modelling the ancient petroleum system of the basin.     

Aspects of the structural evolution of the Lusitanian Basin in Portugal and the shelf and slope area offshore Portugal

The study provides a regional seismic interpretation and mapping of the Mesozoic and Cenozoic succession of the Lusitanian Basin and the shelf and slope area off Portugal. The seismic study is compared with previous studies of the Lusitanian Basin. From the Late Triassic to the Cretaceous the study area experienced four rift phases and intermittent periods of tectonic quiescence. The Triassic rifting was concentrated in the central part of the Lusitanian Basin and in the southernmost part of the study area, both as symmetrical grabens and half-grabens. The evolution of half-grabens was particularly prominent in the south. The Triassic fault-controlled subsidence ceased during the latest Late Triassic and was succeeded by regional subsidence during the early Early Jurassic (Hettangian) when deposition of evaporites took place. A second rift phase was initiated in the Early Jurassic, most likely during the Sinemurian–Pliensbachian. This resulted in minor salt movements along the most prominent faults. The second phase was concentrated to the area south of the Nazare Fault Zone and resulted here in the accumulation of a thick Sinemurian–Callovian succession. Following a major hiatus, probably as a result of the opening of the Central Atlantic, resumed deposition occurred during the Late Jurassic. Evidence for Late Jurassic fault-controlled subsidence is widespread over the whole basin. The pattern of Late Jurassic subsidence appears to change across the Nazare Fault Zone. North of the Nazare Fault, fault-controlled subsidence occurred mainly along NNW–SSE-trending faults and to the south of this fault zone a NNE–SSW fault pattern seems to dominate. The Oxfordian rift phase is testified in onlapping of the Oxfordian succession on salt pillows which formed in association with fault activity. The fourth and final rift phase was in the latest Late Jurassic or earliest Early Cretaceous. The Jurassic extensional tectonism resulted in triggering of salt movement and the development of salt structures along fault zones. However, only salt pillow development can be demonstrated. The extensional tectonics ceased during the Early Cretaceous. During most of the Cretaceous, regional subsidence occurred, resulting in the deposition of a uniform Lower and Upper Cretaceous succession. Marked inversion of former normal faults, particularly along NE–SW-trending faults, and development of salt diapirs occurred during the Middle Miocene, probably followed by tectonic pulses during the Late Miocene to present. The inversion was most prominent in the central and southern parts of the study area. In between these two areas affected by structural inversion, fault-controlled subsidence resulted in the formation of the Cenozoic Lower Tagus Basin. Northwest of the Nazare Fault Zone the effect of the compressional tectonic regime quickly dies out and extensional tectonic environment seems to have prevailed. The Miocene compressional stress was mainly oriented NW–SE shifting to more N–S in the southern part.     

Meso-Cenozoic geodynamic evolution of the Paris Basin: 3D stratigraphic constraints

3D stratigraphic geometries of the intracratonic Meso-Cenozoic Paris Basin were obtained by sequence stratigraphic correlations of around 1 100 wells (well-logs). The basin records the major tectonic events of the western part of the Eurasian Plate, i.e. opening and closure of the Tethys and opening of the Atlantic. From earlier Triassic to Late Jurassic, the Paris Basin was a broad subsiding area in an extensional framework, with a larger size than the present-day basin. During the Aalenian time, the subsidence pattern changes drastically (early stage of the central Atlantic opening). Further steps of the opening of the Ligurian Tethys (base Hettangian, late Pliensbachian;...) and its evolution into an oceanic domain (passive margin, Callovian) are equally recorded in the tectono-sedimentary history. The Lower Cretaceous was characterized by NE–SW compressive medium wavelength unconformities (late Cimmerian–Jurassic/Cretaceous boundary and intra-Berriasian and late Aptian unconformities) coeval with opening of the Bay of Biscay. These unconformities are contemporaneous with a major decrease of the subsidence rate. After an extensional period of subsidence (Albian to Turonian), NE–SW compression started in late Turonian time with major folding during the Late Cretaceous. The Tertiary was a period of very low subsidence in a compressional framework. The second folding stage occurred from the Lutetian to the Lower Oligocene (N–S compression) partly coeval with the E–W extension of the Oligocene rifts. Further compression occurred in the early Burdigalian and the Late Miocene in response to NE–SW shortening. Overall uplift occurred, with erosion, around the Lower/Middle Pleistocene boundary.     

The Gondwana Orogeny in northern North Patagonian Massif:Evidences from the Caita Có granite,La Se?a and Pangaré mylonites,Argentina

Structural analyses in the northern part of the North Patagonia Massif,in the foliated Caita Co granite and in La Sena and Pangare mylonites,indicate that the pluton was intruded as a sheet-like body into an opening pull-apart structure during the Gondwana Orogeny.Geochronological studies in the massif indicate a first,lower to middle Permian stage of regional deformation,related to movements during indentation tectonics,with emplacement of foliated granites in the western and central areas of the North Patagonian Massif.Between the upper Permian and lower Triassic,evidence indicates emplacement of undeformed granitic bodies in the central part of the North Patagonian Massif.A second pulse of deformation between the middle and upper Triassic is related to the emplacement of the Caita Co granite,the development of mylonitic belts,and the opening of the Los Menucos Basin.During this pulse of deformation,compression direction was from the eastern quadrant.     

柴达木盆地欧龙布鲁克地区构造演化研究

柴北缘东段古生界构造变形特征、构造演化过程研究较为薄弱,尤其是古构造应力场性质及其转变机制尚不明确。文中对欧龙布鲁克地区野外剖面及应力感构造要素(褶皱、节理、擦痕)进行了系统观测和分析,结果表明：加里东晚期应力场为NE向;晚海西-印支期早期为SN向,晚期NW向两期挤压应力场;燕山早期近EW向拉张,燕山晚期及喜山晚期处于NE向挤压应力场。根据欧南凹陷平衡剖面反演结果,对比不同时代地层收缩速率可知,柴北缘东段寒武纪-新近纪构造演化可以分为4个阶段：(1)加里东早期(C -O1)弧后伸展、晚期(O2-S)弧后挤压,导致柴北缘东段初步形成NW向的背斜凸起;(2)晚海西-印支期(P-T)隆升阶段,欧龙布鲁克地区整体处于水体之上,并没有造成盆内二叠系-三叠系的沉积;(3)燕山早期(J1-J2)陆内伸展断陷、晚期(J3-K)挤压反转,欧龙布鲁克地区为继承性隆起,未完全接受沉积;(4)喜山晚期(N-Q)强烈挤压构造变形,逆断层强烈活动使山体快速隆升,基底卷入型构造样式广泛分布。     

Plate reconstructions in the Arctic region based on joint analysis of gravity,magnetic, and seismic anomalies

Based on the analysis of various geophysical data, namely, free-air gravity anomalies, magnetic anomalies, upper mantle seismic tomography images, and topography/bathymetry maps, we single out the major structural elements in the Circum Arctic and present the reconstruction of their locations during the past 200 million years. The configuration of the magnetic field patterns allows revealing an isometric block, which covers the Alpha–Mendeleev Ridges and surrounding areas. This block of presumably continental origin is the remnant part of the Arctida Plate, which was the major tectonic element in the Arctic region in Mesozoic time. We believe that the subduction along the Anyui suture in the time period from 200 to 120 Ma caused rotation of the Arctida Plate, which, in turn, led to the simultaneous closure of the South Anyui Ocean and opening of the Canadian Basin. The rotation of this plate is responsible for extension processes in West Siberia and the northward displacement of Novaya Zemlya relative to the Urals–Taimyr orogenic belt. The cratonic-type North American, Greenland, and European Plates were united before 130 Ma. At the later stages, first Greenland was detached from North America, which resulted in the Baffin Sea, and then Greenland was separated from the European Plate, which led to the opening of the northern segment of the Atlantic Ocean. The Cenozoic stage of opening of the Eurasian Basin and North Atlantic Ocean is unambiguously reconstructed based on linear magnetic anomalies. The counter-clockwise rotation of North America by an angle of ~ 15° with respect to Eurasia and the right lateral displacement to 200–250 km ensure an almost perfect fit of the contours of the deep water basin in the North Atlantic and Arctic Oceans.     

Notes de lecture

Abstract

3D stratigraphic geometries of the intracratonic Meso- Cenozoic Paris Basin were obtained by sequence stratigraphic correlations of around 1 100 wells (well-logs). The basin records the major tectonic events of the western part of the Eurasian Plate, i.e. opening and closure of the Tethys and opening of the Atlantic. From earlier Triassic to Late Jurassic, the Paris Basin was a broad subsiding area in an extensional framework, with a larger size than the present-day basin. During the Aalenian time, the subsidence pattern changes drastically (early stage of the central Atlantic opening). Further steps of the opening of the Ligurian Tethys (base Het- tangian, late Pliensbachian;...) and its evolution into an oceanic domain (passive margin, Callovian) are equally recorded in the tectono-sedimentary history. The Lower Cretaceous was characterized by NE-SW compressive medium wavelength unconformities (late Cimmerian-Jurassic/Cretaceous boundary and intra- Berriasian and late Aptian unconformities) coeval with opening of the Bay of Biscay. These unconformities are contemporaneous with a major decrease of the subsidence rate. After an extensional period of subsidence (Albian to Turanian), NE-SW compression started in late Turanian time with major folding during the Late Cretaceous. The Tertiary was a period of very low subsidence in a com- pressional framework. The second folding stage occurred from the Lutetian to the Lower Oligocene (N-S compression) partly coeval with the E-W extension of the Oligocene rifts. Further compression occurred in the early Burdigalian and the Late Miocene in response to NE-SW shortening. Overall uplift occurred, with erosion, around the Lower/Middle Pleistocene boundary. © 2000 Éditions scientifiques et médicales Elsevier SAS     

贺兰山地区晚三叠世盆地属性研究

本文系统分析了研究区及周边沉积演化历史、深大断裂特征、晚三叠世主要构造事件、现今贺兰山周围地层分布情况,结合地震资料、重磁资料以及最新的物源研究成果,对贺兰山晚三叠世盆地属性做了系统总结。重点探讨了研究区深大断裂的形成及后期多期次活化过程,研究区不同演化阶段的沉积特点以及晚三叠世早期、末期构造事件的表现特征。研究认为贺兰山晚三叠世盆地形成于块体相互作用下的张性环境,主要为叠加在早古生代贺兰山裂谷（凹陷？）盆地和晚古生代稳定盆地之上的拉张断陷盆地,断层系统表现为“地堑式”,盆地形态近似于“梭型”,西部塔尔岭—白芨芨沟一带断陷最为强烈,断距最大,北东延伸至正谊关断裂北部、南部延伸至炭井沟以南一带,整个断裂呈弧形,且两端断距逐渐减少。其中汝箕沟—香棒子沟断层为该时期深大断裂带的浅部延伸,东部以大水沟—鬼头沟断裂和现今银川地堑底部某断裂共同组成该地堑的断层系统。     

Gravity and magnetic joint modeling of the Potiguar Rift Basin (NE Brazil): Basement control during Neocomian extension and deformation

A 2.5D gravity and magnetic investigation was conducted along five transects across the Potiguar Basin in the Borborema Province, NE Brazil. The objective of the study is to model the internal architecture of this intracontinental rift basin, which represents the interaction between the heterogeneous Precambrian basement and the Neocomian extensional tectonics, which led to the South Atlantic opening.Joint modeling of the gravity and magnetic data was constrained by Euler deconvolution results, seismic data, well logs and geologic mapping. This integrated approach allowed to determine the rift architecture that is inserted in a complex tectonic and structural framework. Results from joint modeling show that a series of asymmetric half-grabens is oriented in the NE–SW direction and controlled by a system of normal faults with throw greater than 5.5 km. High-density and low-magnetized material constitutes the footwall and intrarift horsts. These supracrustal heterogeneities in association with preexisting shear zones probably guided the Mesozoic rifting process in NE Brazil. Their composition seems to be related to metamorphic rocks of the Proterozoic basement, as suggested by gravity and magnetic anomalies and the geology of the exposed basement. Our interpretation is supported by geophysical studies carried out in the Benue Trough, the counterpart of the Potiguar Basin in West Africa.     

Detrital zircon U–Pb geochronology of Permian strata in the Galilee Basin,Queensland, Australia

The late Carboniferous to Triassic tectonic history of eastern Australia includes important periods of regional-scale crustal extension and contraction. Evidence for these periods of tectonism is recorded by the extensive Pennsylvanian (late Carboniferous) to Triassic basin system of eastern Australia. In this study, we investigate the use of U–Pb dating of detrital zircons in reconstructing the tectonic development of one of these basins, the eastern Galilee Basin of Queensland. U–Pb detrital zircon ages were obtained from samples of stratigraphically well-constrained Cisuralian and Lopingian (early and late Permian, respectively) sandstone in the Galilee Basin. Detrital zircons in these sandstones are dominated by a population with ages in the range of 300–250 Ma, and ages from the youngest detrital zircons closely approximate depositional ages. We attribute these two fundamental findings to (1) appreciable derivation of detrital zircons in the Galilee Basin from the New England Orogen of easternmost Australia and (2) syndepositional magmatism. Furthermore, Cisuralian sandstone of the Galilee Basin contains significantly more >300 Ma detrital zircons than Lopingian sandstone. The transition in detrital zircon population, which is bracketed between 296 and 252 Ma based on previous high-precision U–Pb zircon ages from Permian ash beds in the Galilee Basin, corresponds with the Hunter–Bowen Orogeny and reflects a change in the Galilee Basin from an earlier extensional setting to a later foreland basin environment. During the Lopingian foreland basin phase, the individual depocentres of the Galilee and Bowen basins were linked to form a single and enormous foreland basin that covered >300 000 km² in central and eastern Queensland.     

胶东半岛乳山地区低角度韧性剪切带几何学、运动学特征:对中国东部岩石圈减薄作用的启示

大规模伸展构造是华北克拉通东部岩石圈减薄的重要表现形式。部分低角度韧性剪切带是地壳伸展变形后所展现的构造形式。本文研究了王格庄韧性剪切带的岩石学、几何学、运动学等特征显示:韧性剪切带走向近南北向,剪切带断层面倾向多变(倾向西、西南、西北方向)。大部分区域面理低角度倾向西,矿物拉伸线理近东西向,不对称旋转碎斑及S-C组构指示顶端指向西的剪切特征。结合研究区西侧与伸展构造相匹配的半地堑伸展盆地证据:本研究认为伸展构造的形成可能与西太平洋板块的后撤相关,即大规模伸展构造作用引发了华北克拉通东部的地壳减薄作用。     

Transition from extensional to compressional magnetic fabrics in the Cretaceous Cabuérniga basin (North Spain)

The transition between extensional and compressional-driven magnetic fabrics in sedimentary rocks is explored in this paper through the study of an example of the Basque–Cantabrian basin. In the area where extensional structures prevail and no superimposed deformation is observed, except for gentle large-scale folds, the magnetic fabric is interpreted as extensional, in consistency with mesostructural (tension gashes) and macrostructural (large-scale faults) data. Compressional tectonic fabrics are unequivocally interpreted in the area with cleavage development related to the buttressing of the syn-rift sequence against faults located near the northern basin margin. In this area, k_max is oriented according either to the intersection lineation or the dip direction of cleavage planes. In the area located in-between, where no macroscopic evidence of either compression or extension exist, there is a transitional fabric between compressional (resulting from the modification during inversion of a previous sedimentary or extensional fabric) and extensional (inherited from the extensional stage) magnetic fabrics that correlate with subtle evidences at the microscopic scale (pressure shadows, deformation and re-orientation of nodules). Therefore, the magnetic fabric is revealed as an exceptionally sensitive marker of deformation in sedimentary rocks. This transition in the magnetic fabric occurs within a length of 6.25 km along the cross-section that correlates with a thickness of 200 m of the stratigraphic pile. These results indicate that even in the absence of clear structural markers of compressional deformation, extensional magnetic fabrics can be only interpreted when there is a minimum separation (in the vertical or the horizontal) to the cleavage front.