A long‐lived metamorphic history in the contact aureole of the Mooselookmeguntic pluton revealed by in situ dating of monazite grains preserved as inclusions in staurolite porphyroblasts

Emplacement of the Mooselookmeguntic pluton, located in the western Maine region of the northern Appalachians, was thought to have occurred towards the end of the Acadian deformation at around 370 Ma. Crystallization ages from different parts of the pluton suggest a more sequential emplacement history over a period of c. 20 Myr. Foliation inflection/intersection axes (FIAs) within porphyroblasts from its aureole reveal at least five periods of garnet and staurolite growth. The orientation of FIAs in both garnet and staurolite porphyroblasts trend successively from ESE–WNW, NNW–SSE, E–W, ENE–WSW to NE–SW. Electron probe microanalysis dating of monazite grains included in staurolite porphyroblasts containing one of these five periods of FIA development reveals a succession of apparent ages from 410 Ma to 345 Ma. A similar spread of crystallization ages can be observed for plutons from Maine and adjacent regions. This succession indicates that deformation and metamorphism began well before and continued long after what is classically regarded as the Acadian orogeny. The thermal structure of the orogen progressively evolved to enable pluton emplacement, and it continued to develop afterwards with magmatic fluids still forming at depth.     

Structural Mapping of the Bentong‐Raub Suture Zone Using PALSAR Remote Sensing Data,Peninsular Malaysia: Implications for Sediment‐hosted/Orogenic Gold Mineral Systems Exploration

The Bentong‐Raub Suture Zone (BRSZ) of Peninsular Malaysia is one of the major structural zones in Sundaland, Southeast Asia. It forms the boundary between the Gondwana‐derived Sibumasu terrane in the west and Sukhothai Arc in the east. The BRSZ is genetically related to the sediment‐hosted/orogenic gold deposits associated with the major lineaments in the Central Gold Belt of Peninsular Malaysia. In this investigation, the Phased Array type L‐band Synthetic Aperture Radar (PALSAR) satellite remote sensing data were used to map major geological structures in Peninsular Malaysia and provide detailed characterization of lineaments and curvilinear structures in the BRSZ, as well as their implication for sediment‐hosted/orogenic gold exploration in tropical environments. Major structural lineaments such as the Bentong‐Raub Suture Zone (BRSZ) and Lebir Fault Zone, ductile deformation related to crustal shortening, brittle disjunctive structures (faults and fractures) and collisional mountain range (Main Range granites) were detected and mapped at regional scale using PALSAR ScanSAR data. The major geological structure directions of the BRSZ were N–S, NNE–SSW, NE–SW and NW–SE, which derived from directional filtering analysis to PALSAR fine and polarimetric data. The pervasive array of N–S faults in the Central Gold Belt and surrounding terrain is mainly linked to the N–S trending of the Suture Zone. N–S striking lineaments are often cut by younger NE–SW and NW–SE‐trending lineaments. Gold mineralized trend lineaments are associated with the intersection of N–S, NE–SW, NNW–SSE and ESE–WNW faults and curvilinear features in shearing and alteration zones. Compressional tectonic structures such as the NW–SE trending thrust, ENE–WSW oriented faults in mylonite and phyllite, recumbent folds and asymmetric anticlines in argillite are high potential zones for gold prospecting in the Central Gold Belt. Three generations of folding events in Peninsular Malaysia have been recognized from remote sensing structural interpretation. Consequently, PALSAR satellite remote sensing data is a useful tool for mapping major geological structural features and detailed structural analysis of fault systems and deformation areas with high potential for sediment‐hosted/orogenic gold deposits and polymetallic vein‐type mineralization along margins of Precambrian blocks, especially for inaccessible regions in tropical environments.     

Tectonic indentation in the central Alboran Sea (westernmost Mediterranean)

The Alboran Sea constitutes a Neogene–Quaternary basin of the Betic–Rif Cordillera, which has been deformed since the Late Miocene during the collision between the Eurasian and African plates in the westernmost Mediterranean. NNE–SSW sinistral and WNW–ESE dextral conjugate fault sets forming a 75° angle surround a rigid basement spur of the African plate, and are the origin of most of the shallow seismicity of the central Alboran Sea. Northward, the faults decrease their transcurrent slip, becoming normal close to the tip point, while NNW–SSE normal and sparse ENE–WSW reverse to transcurrent faults are developed. The uplifting of the Alboran Ridge ENE–WSW antiform above a detachment level was favoured by the crustal layering. Despite the recent anticlockwise rotation of the Eurasian–African convergence trend in the westernmost Mediterranean, these recent deformations—consistent with indenter tectonics characterised by a N164°E trend of maximum compression—entail the highest seismic hazard of the Alboran Sea.     

The problem, significance and implications for metamorphism of 60 million years of multiple phases of staurolite growth

Microstructural measurements of FIAs in staurolite reveal at least 3 periods of growth in the Proterozoic Colorado Front Range and 5 in the Paleozoic Western Maine. Dated monazite inclusions in staurolite have an absolute age of 1760±12 Ma (FIA 1), 1720±7 Ma (FIA 2), 1682±18 Ma (FIA 3) in Colorado, and 408±10 Ma (FIA 2), 388±8 Ma (FIA 3), 372±6 Ma (FIA 4), 352±4 Ma (FIA 5) in Maine, supporting the multiple periods of deformation and metamorphism indicated by the FIA succession in each region. Multiple phases of growth by similar reactions in the same as well as in diverse adjacent rocks in both regions suggest that PT and X are not the only factors controlling the commencement and cessation of metamorphic reactions. The FIAs preserved by the staurolite porphyroblasts indicate that the local partitioning of deformation at the scale of a porphyroblast was the eventual controlling factor on whether or not the staurolite forming reactions took place.     

The problem,significance and implications for metamorphism of 60 million years of multiple phases of staurolite growth

Microstructural measurements of FIAs in staurolite reveal at least 3 periods of growth in the Proterozoic Colorado Front Range and 5 in the Paleozoic Western Maine. Dated monazite inclusions in staurolite have an absolute age of 1760±12 Ma (FIA 1), 1720±7 Ma (FIA 2), 1682±18 Ma (FIA 3) in Colorado, and 408±10 Ma (FIA 2), 388±8 Ma (FIA 3), 372±6 Ma (FIA 4), 352±4 Ma (FIA 5) in Maine, supporting the multiple periods of deformation and metamorphism indicated by the FIA succession in each region. Multiple phases of growth by similar reactions in the same as well as in diverse adjacent rocks in both regions suggest that PT and X are not the only factors controlling the commencement and cessation of metamorphic reactions. The FIAs preserved by the staurolite porphyroblasts indicate that the local partitioning of deformation at the scale of a porphyroblast was the eventual controlling factor on whether or not the staurolite forming reactions took place.     

Extension structural records in the Qinshui basin (North China) since the Late Mesozoic

Qinshui basin has abundant coal-bed methane resources and has been undergoing intensive intracontinental rifting and extensional tectonics since the Late Mesozoic. Some fractures, which were previously considered as conjugate shear fractures, are interpreted as joint sets with extension characteristics, for the first time in the Qinshui basin. The widely distributed joint sets with stable attitudes can be divided into four sets. This paper presents updated results of fault-slip datasets collected in different zones of the Qinshui basin and addresses the changes in the direction of extensional stresses since the Late Mesozoic. Based on the analysis results of the slickenline of normal faults, joint sets in the field, and focal mechanism solutions data from the Shanxi Province, we identified four main directions of extension since the Late Mesozoic in the Qinshui basin: (1) Early Cenozoic ENE–WSW (85 ± 15°) extension; (2) Palaeogene NNE–SSW (30 ± 5°) extension; (3) Miocene NW–SE (135 ± 15°) extension; and (4) Late Pliocene–quaternary NNW–SSE (170 ± 5°) extension. The principal extension directions in the Qinshui basin seem to have undergone a counterclockwise rotation from the Early Cenozoic to the Miocene. We prefer that the extension deformation events in the Qinshui basin since the Late Mesozoic were mainly related to the back-arc spreading induced by westward subduction of the paleo-Pacific plate under the Eurasian continent.     

Evolution of the stress and strain fields in the Eastern Cordillera,Colombia

This work integrates stress data from Global Positioning System measurements and earthquake focal mechanism solutions, with new borehole breakout and natural fracture system data to better understand the complex interactions between the major tectonic plates in northwestern South America and to examine how the stress regime in the Eastern Cordillera and the Llanos foothills in Colombia has evolved through time. The dataset was used to generate an integrated stress map of the northern Andes and to propose a model for stress evolution in the Eastern Cordillera. In the Cordillera, the primary present-day maximum principal stress direction is WNW–ESE to NW–SE, and is in the direction of maximum shortening in the mountain range. There is also a secondary maximum principal stress direction that is E–W to ENE–WSW, which is associated with the northeastward “escape” of the North Andean block, relative to stable South America. In the Cupiagua hydrocarbon field, located in the Llanos foothills, the dominant NNE–SSW fractures are produced by the Panama arc–North Andes collision and range-normal compression. However, less well developed asymmetrical fractures oriented E–W to WSW–ENE and NNW–SSE are also present, and may be related to pre-folding stresses in the foreland basin of the Central Cordillera or to present-day shear associated with the northeastward “escape” of the north Andean block. Our study results suggest that an important driver for orogenic deformation and changes in the stress field at obliquely convergent subduction zone boundaries is the arrival of thickened crust, such as island arcs and aseismic ridges, at the trench.     

Neotectonic transpressional intraplate deformation in eastern Eurasia: Insights from active fault systems in the southeastern Korean Peninsula

Transpression occurs in response to oblique convergence across a deformation zone in intraplate regions and plate boundaries. The Korean Peninsula is located at an intraplate region of the eastern Eurasian Plate and has been deformed under the ENE–WSW maximum horizontal compression since the late Pliocene. In this study, we analyzed short-term instrumental seismic (focal mechanism) and long-term paleoseismic (Quaternary fault outcrop) data to decipher the neotectonic crustal deformation pattern in the southeastern Korean Peninsula. Available (paleo-)seismic data acquired from an NNE–SSW trending deformation zone between the Yangsan and Ulleung fault zones indicate spatial partitioning of crustal deformation by NNW–SSE to NNE–SSW striking reverse faults and NNE–SSW striking strike-slip faults, supporting a strike-slip partitioned transpression model. The instantaneous and finite neotectonic strains, estimated from the focal mechanism and Quaternary outcrop data, respectively, show discrepancies in their axes, which can be attributed to the switching between extensional and intermediate axes of finite strain during the accumulation of wrench-dominated transpression. Notably, some major faults, including the Yangsan and Ulsan fault zones, are relatively misoriented to slip under the current stress condition but, paradoxically, have more (paleo-)seismic records indicating their role in accommodating the neotectonic transpressional strain. We propose that fluids, heat flow, and lithospheric structure are potential factors affecting the reactivation of the relatively misoriented major faults. Our findings provide insights into the accommodation pattern of strain associated with the neotectonic crustal extrusion in an intraplate region of the eastern Eurasian Plate in response to the collision of the Indian Plate and the subduction of the Pacific/Philippine Sea Plates.     

Active tectonics,fault patterns,and stress field of Deception Island: A response to oblique convergence between the Pacific and Antarctic plates

Palaeostress results derived from brittle mesoscopic structures on Deception Island (Bransfield Trough, Western Antarctica) show a recent stress field characterized by an extensional regime, with local compressional stress states. The maximum horizontal stress (σ_y) shows NW–SE and NNE–SSW to NE–SW orientations and horizontal extension (σ₃) in NE–SW and WNW–ESE to NW–SE directions. Alignments of mesofractures show a maximum of NNE–SSW orientation and several relative maxima striking N030-050E, N060-080E, N110-120E, and N160-170E. Subaerial and submarine macrofaults of Deception Island show six main systems controlling the morphology of the island: N–S, NNE–SSW, NE–SW, ENE–WSW to E–W, WNW–ESE, and NNW–SSE. Geochemical patterns related to submarine hydrothermally influenced fault and fissure pathways also share the same trends. The orientation of these fault systems is compared to Riedel shear fractures. Following this model, we propose two evolutionary stages from geometrical relationships between the location and orientation of joints and faults. These stages imply a counter-clockwise rotation of Deception Island, which may be linked to a regional left-lateral strike-slip. In addition, the simple shear zone could be a response to oblique convergence between the Antarctic and Pacific plates. This stress direction is consistent with the present-day movements between the Antarctic, Scotia, and Pacific plates. Nevertheless, present basalt-andesitic volcanism and deep earthquake focal mechanisms may indicate rollback of the former Phoenix subducted slab, which is presently amalgamated with the Pacific plate. We postulate that both mechanisms could occur simultaneously.     

Sequential granite emplacement: a structural study of the late Variscan Strzelin intrusion, SW Poland

The Strzelin Massif in SW Poland (Central European Variscides) records a protracted igneous evolution, with three main magmatic stages: (1) tonalitic I, (2) granodioritic and (3) tonalitic II/granitic. In the northern part of this Massif, the Strzelin intrusion proper comprises three successively emplaced rock types: a medium-grained biotite granite (303 ± 2 Ma), a fine-grained biotite granite (283 ± 8 Ma) and a fine-grained biotite-muscovite granite; based on field evidence, the third variety postdates both types of the biotite granites. The structural data from the three granites, including their parallel, approximately E–W striking and steeply dipping lithological contacts and ENE–WSW trending subhorizontal magmatic lineations, suggest that the emplacement of all three successive granite varieties was controlled by an active, long-lived strike-slip fault, striking ESE–WNW, with a dextral sense of movement. After the emplacement of the youngest biotite-muscovite granite, the intrusion underwent brittle extension which produced “Q joints” striking NNW–SSE to N–S and dipping at 55–70° WSW to W, and showing evidence of broadly N–S directed sinistral displacements. The structural observations, supported by new geochronological data, indicate that the internal structure of the composite granitoid intrusion, including the faint magmatic foliation and lineation, formed in a long-lived strike-slip setting, different from the subsequent, post-emplacement extensional tectonics that controlled the development of brittle structures.     

Structural control of El Sela granites and associated uranium deposits, Southern Eastern Desert, Egypt

The El Sela area is a part of the basement complex of the Eastern Desert of Egypt and the Pan-African Shield. The area comprises outcrops of dismembered ophiolites thrust over arc volcano-sedimentary sequence and intruded by different syn- to post-tectonic granitoids. Structural analysis of the area enabled the separation and definition of four structural episodes: (E1) folding–thrusting episode associated with the cratonization of the arc/inter-arc rock association and the intrusion of the syntectonic (Older) granites. (E2) Upright folding episode associated with the compression and shortening to the ENE–WSW direction which is different from the NNW–SSE shortening direction during E1; at the end of E2, late tectonic granites were intruded. (E3) Post-tectonic granitic intrusion episode: two mica granite and granitic dikes were intruded during this episode. (E4) Fracturing, faulting, and post-granitic dike extrusion episodes caused different faults that took place after cratonization until the present. There are three generations of folds during ductile deformation (E1 and E2). The F2 folds are nearly coaxial (along ENE–WSW trend) with the F1 folds. The F3 folding is displayed by folds generally trending NNW–SSE. Therefore, the ENE–WSW and NNW–SSE trends can considered as preexisting discontinuities and mechanical anisotropy of the crust in the following structure episodes. Brittle deformation (E3 and E4) reveals the importance of those trends which control the multi-injections and many alteration features in the study area. During reactivation, a simple shear parallel to the inherited ductile fabrics was responsible for the development of mineralized structures along the ENE–WSW and NNW–SSE trends. So they can be considered as paleochannel trends for deep-seated structures and can act as a good trap for uranium and/or other mineral resources. Most of the uranium anomalies are delineated along ENE–WSW and NNW–SSE shear zones where quartz-bearing veins bounded the lamprophyre dike and microgranites and dissected them in relation to the successive fracturation and brecciation corresponding to the repeated rejuvenation of the structures. Therefore, the structural controls of the uranium mineralizations in the El Sela area appear to be related to the interaction between inherited ductile fabrics and overprinting brittle structures.     

Subsurface structural trends of the offshore Nile Delta area, Egypt: evidences from gravity and magnetic data

Subsurface structural trends and tectonics affecting the offshore Nile Delta area, Egypt, have been studied through the interpretations of gravity and magnetic data. Reduced to the pole, regional–residual separation, Tilt derivative and Euler deconvolution techniques are applied for the processing and interpretations of the magnetic and gravity data. The average depth of the sedimentary cover, estimated from the two-dimensional power spectrum technique ranges between 8 km and 13 km. The interpretation of the gravity and magnetic data indicates that the study area is affected by many subsurface structural trends. The NW–SE is the major trend related to El-Temsah and Misfaq-Bardwil trend. The NE–SW direction is the second dominant trend, related to the Rosetta trend. Other trends defined through the interpretation of gravity and magnetic data include: the N–S direction, related to the Baltim fault trend, the E–W direction, related to the Neogene hinge line and the NNE–SSW related to the Gulf of Aqaba. Accessory trends include the ENE–WSW, WNW–ESE and finally the NNW–SSW.     

Tectonics of the Northern Bresse region (France) during the Alpine cycle

Combining fieldwork and surface data, we have reconstructed the Cenozoic structural and tectonic evolution of the Northern Bresse. Analysis of drainage network geometry allowed to detect three major fault zones trending NE–SW, E–W and NW–SE, and smooth folds with NNE trending axes, all corroborated with shallow well data in the graben and fieldwork on edges. Cenozoic paleostress succession was determined through fault slip and calcite twin inversions, taking into account data of relative chronology. A N–S major compression, attributed to the Pyrenean orogenesis, has activated strike-slip faults trending NNE along the western edge and NE–SW in the graben. After a transitional minor E–W trending extension, the Oligocene WNW extension has structured the graben by a collapse along NNE to NE–SW normal faults. A local NNW extension closes this phase. The Alpine collision has led to an ENE compression at Early Miocene. The following WNW trending major compression has generated shallow deformation in Bresse, but no deformation along the western edge. The calculation of potential reactivation of pre-existing faults enables to propose a structural sketch map for this event, with a NE–SW trending transfer fault zone, inactivity of the NNE edge faults, and possibly large wavelength folding, which could explain the deposit agency and repartition of Miocene to Quaternary deformation.     

A new interpretation for the interference zone between the southern Brasília belt and the central Ribeira belt,SE Brazil

In southeastern Brazil, the Neoproterozoic NNW–SSE trending southern Brasília belt is apparently truncated by the ENE–WSW central Ribeira belt. Different interpretations in the literature of the transition between these two belts motivated detailed mapping and additional age dating along the contact zone. The result is a new interpretation presented in this paper. The southern Brasília belt resulted from E-W collision between the active margin of the Paranapanema paleocontinent, on the western side, now forming the Socorro-Guaxupé Nappe, with the passive margin of the São Francisco paleocontinent on the eastern side. The collision produced an east vergent nappe stack, the Andrelândia Nappe System, along the suture. At its southern extreme the Brasília belt was thought to be cut off by a shear zone, the “Rio Jaguari mylonites”, at the contact with the Embu terrane, pertaining to the Central Ribeira belt. Our detailed mapping revealed that the transition between the Socorro-Guaxupé Nappe (Brasília belt) and the Embu terrane (Ribeira belt) is not a fault but rather a gradational transition that does not strictly coincide with the Rio Jaguari mylonites. A typical Cordilleran type magmatic arc batholith of the Socorro-Guaxupé Nappe with an age of ca. 640 Ma intrudes biotite schists of the Embu terrane and the age of zircon grains from three samples of metasedimentary rocks, one to the south, one to the north and one along the mylonite zone, show a similar pattern of derivation from a Rhyacian source area with rims of 670–600 Ma interpreted as metamorphic overgrowth. We dated by LA-MC-ICPMS laser ablation (U–Pb) zircon grains from a calc-alkaline granite, the Serra do Quebra-Cangalha Batholith, located within the Embu terrane at a distance of about 40 km south of the contact with the Socorro Nappe, yielding an age of 680 ± 13 Ma. This age indicates that the Embu terrane was part of the upper plate (Socorro-Guaxupé Nappe) by this time. Detailed mapping indicates that the mylonite zone is not a plate boundary because motion along it is maximum a few tens of kilometres and the same litho-stratigraphic units are present on either side. Based on these arguments, the new interpretation is that the Embu terrane is the continuation of the Socorro-Guaxupé Nappe and therefore also part of the active margin of the Paranapanema paleocontinent. The Brasília belt is preserved even further within the central Ribeira belt than previously envisaged.     

鄂尔多斯盆地中生代地层天然裂缝发育特征与构造应力场演化

鄂尔多斯是我国重要的油气盆地,而绝大多数油气储层都会受到天然裂缝的影响。文中通过野外系统观测和室内的统计分析, 详细描述了盆地中生代地层中的天然裂缝发育特征和主要控制因素, 为裂缝性油气藏勘探开发提供基础理论支持。通过研究得知, 盆内中生代地层中主要发育有6组裂缝： E-W向、 ENE-WSW向、 NE-SW向、 N-S向、 NNW-SSE向、 NNE-SSW向。其中E-W向、 ENE-WSW向和NE-SW向为系统裂缝; N-S向、 NNW-SSE向和NNE-SSW向为非系统裂缝。在裂缝组合中, 有两组正交裂缝系统（E-W向和N-S向, ENE-WSW向和NNW-SSE向）, 其中E-W向和N-S向裂缝构成的正交裂缝系统只出露于三叠和侏罗纪地层。同时, 还有两组共轭裂缝（ENE-WSW向和NNE-SSW向, ENE-WSW向和ESE-WNW向）, 其中ENE-WSW向和NNE-SSW向裂缝构成的共轭裂缝出露于整个中生代地层, 而另外一组只出露于上三叠统延长组中。此外, 裂缝间距分析表明： （1）裂缝间距与力学层厚之间的关联性相对较低; （2）E-W向和ENE-WSW向两组系统裂缝的发育强度好于N-S向非系统裂缝组; （3）岩层厚度越小, 对应裂缝密度则越大。（4）除了岩层厚度, 区域应力场对裂缝的发育具有很大的影响。研究区内裂缝主要形成于两期区域应力场： 第一期是晚侏罗世近E-W向的挤压应力场, 由古太平洋板块向欧亚板块的俯冲和碰撞作用所致; 第二期是新生代NE-SW向的挤压应力场, 由印度板块向欧亚板块的俯冲和碰撞作用所致。     

Distinguishing and correlating multiple phases of metamorphism across a multiply deformed region using the axes of spiral, staircase and sigmoidal inclusion trails in garnet

Schists from the Appalachian Orogen in south-east Vermont have undergone multiple phases of garnet growth. These phases can be distinguished by the trend and relative timing of f oliation i nflexion or i ntersection a xes (FIAs) of foliations preserved as inclusion trails in garnet porphyroblasts. The relative timing of different generations of FIAs is determined from samples containing porphyroblasts with two or three differently trending FIAs developed outwards from core to rim (multi-FIA porphyroblasts). Schists from south-east Vermont show a consistent pattern of relative clockwise rotation of FIA trends from oldest to youngest. Four populations or sets of FIAs can be distinguished on the basis of their relative timings and trends. From oldest to youngest, the four sets have modal peaks trending SW–NE, W–E, NNW–SSE and SSW–NNE. These peaks show that each of the four FIA sets has a statistically consistent trend at all scales across a 35×125 km area containing numerous mesoscopic and macroscopic folds. The FIAs of Set 4 are defined by inclusion trails that are continuous with matrix foliations, have trends subparallel to most folds and are inferred to have developed contemporaneously with these structures. Conversely, Sets 1 to 3 are oblique to and pre-date most matrix foliations and folds. All four FIA sets occur in Siluro-Devonian rocks and must have formed in the Acadian Orogeny. The lack of statistically significant differences in the distribution of FIA trends across the study area and their consistent relative timings in multi-FIA porphyroblasts, despite a complex regional deformation history involving numerous phases of folding at all scales, suggest the porphyroblasts have not rotated relative to one another. The change in FIA trend with time resulted from rotation of the kinematic reference frame of bulk flow, possibly as a consequence of the reorganization of lithospheric plates responsible for Acadian orogenesis. Recognition of distinct generations of FIAs provides a means of distinguishing different phases of porphyroblast growth. Four periods of garnet porphyroblast growth occurred in the schists of south-east Vermont. This growth was heterogeneously distributed on the cm²–m² scale. No single porphyroblast records all stages of growth, and adjacent samples from the same or dissimilar rock types commonly contain porphyroblasts that preserve different sequences of growth. Factors that may have been responsible for switching porphyroblast growth on and off at this scale include: (i) subtle differences in bulk chemical composition; (ii) oscillating levels of heat, owing to the buffering effect of endothermic garnet-forming reactions; (iii) channelized infiltration of fluids with localized fluid buffering of bulk composition; and (iv) cyclic controls on the rates of diffusion and material transport of reactants, either by channelized fluid flow or by a changing pattern of microfracturing during foliation development. Consistency in FIA trend and relative timing provide a new method for potentially distinguishing and correlating successive metamorphic events, or even phases of metamorphism within a progressive tectonothermal event, along and across orogens. Using a consistent pattern of core to rim changes in FIA trend, multiple phases of growth of a single porphyroblastic mineral can be quantitatively distinguished, allowing correlation of different phases of growth around and across macroscopic folds. The relative timing of growth of different porphyroblastic minerals can also be quantitatively determined using FIA data and correlated around and across macroscopic folds. Conceptually, the paragenetic history preserved in each generation of porphyroblast growth, in the form of chemical zoning and the minerals in inclusion trails, could be combined to produce a more detailed P–T–t–deformation path than previously determined.     

Geological and archaeological evidence of active faulting on the Martana Fault (Umbria–Marche Apennines,Italy) and its geodynamic implications

This paper examines the morphotectonic and structural–geological characteristics of the Quaternary Martana Fault in the Umbria–Marche Apennines fold‐and‐thrust belt. This structure is more than 30 km long and comprises two segments: a N–NNW‐trending longer segment and a 100°N‐trending segment. After developing as a normal fault in Early Pleistocene times, the N–NNW Martana Fault segment experienced a phase of dextral faulting extending from the Early to Middle Pleistocene boundary until around 0.39 Ma, the absolute age of volcanics erupted in correspondence to releasing bends. The establishment of a stress field with a NE–ENE‐trending σ₃ axis and NW–NNW σ₁ axis in Late Pleistocene to Holocene times resulted in a strong component of sinistral faulting along N–NNW‐trending fault segments and almost pure normal faulting on newly formed NW–SE faults. Fresh fault scarps, the interaction of faulting with drainage systems and displacement of alluvial fan apexes provide evidence of the ongoing activity of this fault. The active left‐lateral kinematic along N–NNW‐trending fault segments is also revealed by the 1.8 m horizontal offset of the E–W‐trending Decumanus road, at the Roman town of Carsulae. We interpret the present‐day kinematics of the Martana Fault as consistent with a model connecting surface structures to the inferred north‐northwest trending lithospheric shear zone marking the western boundary of the Adria Plate. Copyright © 2003 John Wiley & Sons, Ltd.     

Changes in Relative Plate Motion during the Isan Orogeny (1670-1500 Ma) and Implications for Pre-Rodinia Reconstructions

Foliation inflexion/intersection axes(FIAs)preserved within porphyroblasts that grew throughout Isan orogenesis reveal significant anticlockwise changes in the direction of bulk horizontal shortening between 1670 and 1500 Ma from NE-SW,N-S,E-W to NW-SE.This implies an anticlockwise shift in relative plate motion with time during the Isan orogeny.Dating monazite grains amongst the axial planar foliations defining three of the four FIAs enabled an age for the periods of relative plate motion that produced these structures to be determined.Averaging the ages from monazite grains defining each FIA set revealed 1649±12 Ma for NE-SW shortening,1645±7 Ma for N-S shortening,and 1591±10 Ma for that directed E-W.Inclusion trail asymmetries indicate shear senses of top to the SW for NW-SE FIAs and dominantly top to the N for E-W FIAs,reflecting thrusting towards the SW and N.No evidence for tectonism related to early NE-SW bulk horizontal shortening has previously been detected in the Mount Isa Inlier.Amalgamation of the Broken Hill and possibly the Gawler provinces with the Mount Isa province may have taken place during these periods of NE-SW and N-S-directed thrusting as the ages of tectonism are similar.Overlapping dates,tectonic,metamorphic,and metallogenic similarities between eastern Australia(Mount Isa and Broken Hill terranes)and the southwest part of Laurentia imply a most probable connection between both continental masses.Putting Australia in such position with respect to North America during the Late-Paleo-to-Mesoproterozoic time is consistent with the AUSWUS model of the Rodinia supercontinent.     

Brittle tectonism in relation to the Palaeogene evolution of the Thulean/NE Atlantic domain: a study in Ulster

The tectonic effects of the Thulean mantle plume on the opening of the North Atlantic Ocean is still poorly understood. An analysis of the brittle deformation affecting the Late Cretaceous Chalk and Lower Tertiary igneous formations cropping out in Ulster (Northern Ireland), part of the Thulean Province, leads to the recognition of two tectonic phases. Each of these phases is characterized by different stress regimes with similar trends of the horizontal maximum principal stress σ_H. The first phase, syn-magmatic and dominated by NE–SW to ENE–WSW extension, occurred during the Palaeocene. It is followed by a second post-magmatic phase, characterized initially by a probably Eocene strike-slip to compressional palaeo-stress regime with σ₁ (=σ_H) trending NE–SW to NNE–SSW associated with the partial reactivation (as reverse faults) of normal faults formed during the first phase NE–SW extension. This episode is postdated by an Oligocene extension, with σ_H (=σ₂) still striking NNE–SSW/NE–SW, which reactivated Eocene strike-slip faults as nearly vertical dip-slip normal faults. This Palaeogene tectonic evolution is consistent with the tectonic evolution of similar age in western Scotland and in the Faeroe Islands. In particular, the post-magmatic NE–SW compression is here related to the ‘Faeroe compressive event’, which is related to the earliest stages of drift of the Greenland plate.     

Joints,faults and palaeostress evolution in the Campo de Dalias (Betic Cordilleras,southeastern Spain)

The Campo de Dal??as, located between the central and eastern Betic Cordilleras, shows an evolution determined by the overprinting of two main stress fields since Pliocene times. The first of these develops hybrid and tensional joint sets up to Pleistocene (100 000 yr) and is characterized by NNW–SSE horizontal trend of compression and an ENE–WSW horizontal extension. The second stress field has prolate to triaxial extensional ellipsoids, also with ENE–WSW horizontal extension, and continues to be active today. The most recent stresses produce the reactivation of previous joints as faults whose trends are comprised mainly from N120°E to N170°E and have a normal and transtensional regime, with dextral or sinistral components. The palaeostress evolution of this region is similar to that undergone by other basins of the Eastern Betic Cordilleras, although the Pliocene–Pleistocene transcurrent deformations in the Campo de Dal??as only develop joints and not strike-slip faults.