Kinematics of subduction in the Ibero-Armorican arc constrained by 3D microstructural analysis of garnet and pseudomorphed lawsonite porphyroblasts from Île de Groix (Variscan belt)

The small island of Groix in southern Brittany, France, is well known for exceptionally well-preserved outcrops of Variscan blueschists, eclogites, and garnetiferous mica schists that mark a Late Devonian suture between Gondwana and Armorica. The kinematics of polyphase deformation in these rocks is reconstructed based on 3D microstructural analysis of inclusion trails within garnet and pseudomorphed lawsonite porphyroblasts using differently oriented thin sections and X-ray tomography. Three sets of inclusion trails striking NE–SW, NNW–SSE, and WNW–ESE are recognized and interpreted to witness a succession of different crustal shortening directions orthogonal to these strikes. The curvature sense of sigmoidal and spiral-shaped inclusion trails of the youngest set is shown to be consistent with northwest and northward subduction of Gondwana under Armorica, provided that these microstructures developed by overgrowth of actively forming crenulations without much porphyroblast rotation. Strongly non-cylindrical folds locally found on the island are reinterpreted as fold-interference structures instead of having formed by progressive shearing and fold-axis reorientation. Six samples of a lower-grade footwall unit of the Groix ophiolitic nappe (Pouldu schists) were also studied. Inclusion trails in these rocks strike E–W, similar to the youngest set recognized on Groix island. They record Carboniferous N–S shortening during continental collision. These new microstructural data from southern Brittany bear a strong resemblance to earlier measured in inclusion-trail orientations in the northwestern Iberia Massif. A best fit between both regions suggests not more than about 15∘ anticlockwise rotation of Iberia during the Cretaceous opening of the Gulf of Biscay.

Chapter 3 Thrusts, extensional faults and fold patterns of the major units

This chapter is concerned with the main faults and folds within the Southeastern Oman Mountains based on available literature. The main, best and most widely exposed thrusts are those related to the SW-directed late Cretaceous obduction of the allochthonous nappes onto the Arabian platform and margin. These thrusts are related to obduction of rocks, which had formed hundreds of kilometres offshore Oman. The thrusts were active from the Cenomanian to the Campanian. Obduction-related thrusts and folds are spectacularly exposed within the rocks of the Arabian platform in the eastern part of the Saih Hatat Dome, including large-scale recumbent cylindrical folds and sheath folds. At least six fold sets can be studied in the Southeastern Oman Mountains. At least two of them had formed prior to obduction and are exposed in the Pre-Permian formations of the Jabal Akhdar Dome. At least three fold sets formed in the course of obduction, while at least one fold set is postobductional in age. Besides the compressional structures, the Oman Mountains expose major post-obductional extensional faults, mostly at the margins of the Jabal Akhdar and Saih Hatat domes. The throw of these faults amounts to a few to several kilometres. Finally, this chapter provides an overview of the enigmatic Batinah Mélange which consists of slivers of Hawasina rocks, resting (unusually) structurally above the Semail Ophiolite.

Kinematics of subduction in the Ibero-Armorican arc constrained by 3D microstructural analysis of garnet and pseudomorphed lawsonite porphyroblasts from Ile de Groix (Variscan belt)

The small island of Groix in southern Brittany, France, is well known for its excellent outcrops of Variscan blueschists, eclogites and garnetiferous micaschists that define a Late-Devonian suture between Gondwana and Armorica. The kinematics of polyphase deformation in these rocks is reconstructed based on 3D microstructural analysis of inclusion trails in garnet- and pseudomorphed lawsonite porphyroblasts using multiple, differently oriented thin sections of single samples and X-ray tomography. Three sets of inclusion trails striking NE-SW, NNW-SSE and WNW-ESE are interpreted to witness a succession of different crustal shortening directions orthogonal to these trends. The curvature sense of sigmoidal- and spiral-shaped inclusion trails of the youngest set is shown to be consistent with southward thrusting or northward subduction of Gondwana under Armorica, provided that these microstructures developed by overgrowth of actively forming crenulations instead of the previously envisaged 'snowball' mechanism. The latter predicts an opposite thrusting direction which is at odds with the regional tectono-metamorphic zonation in the Ibero-Armorican Arc. Strongly non-cylindrical folds locally found on Ile de Groix are reinterpreted as fold-interference structures instead of having formed by progressive shearing. Six additional samples of lower-grade footwall units of the Groix ophiolite were also studied. The oldest inclusion trails in these rocks have similar trends as the youngest one in Ile de Groix. Our new inclusion-trail data for southern Brittany bear a strong resemblance with those documented previously in the north-western Iberian Massif and suggest about 20° anticlockwise rotation of Iberia during the early Cretaceous opening of the Gulf of Biscay.

GMDE: Extracting quantitative information from geologic maps

GMDE is a program, available on the desktop for Apple Macintosh, Microsoft Windows, and Linux platforms and in a mobile version for iOS, that enables geologists to extract quantitative structural information from geologic maps and satellite images. The program facilitates the digitizing of strikes and dips or calculating them from three-point problems, calculation of stratigraphic map thickness, determination of piercing points on faults, and the construction of down-plunge projections and vertical cross sections with projected apparent dips, contacts, and cylindrical folds. The program also enables the automatic plotting of planar contacts across topography based on orientation calculated from three clicked points, which can be carried out in the field for immediate hypothesis testing. Error propagation is built into many of the calculations. Maps and satellite images require no projection or datum information, just four points with known latitude and longitude information. Alternatively, the user can enter the map or image in MBTiles format. Users can easily extract X-Y-Z data for any clicked or calculated point or polygon, enabling them to make their own calculations.

Structural and microtectonic analyses of Barpak of Gorkha district, west-central Nepal

Geological mapping was carried out in the Barpak-Bhachchek area of the Daraudi River valley, Gorkha district, West-Central Nepal for structural analysis. The area comprises rocks of the Higher Himalayan Crystalline and the Lesser Himalayan Sequence. Pelitic and psammitic schist, quartzite, calc-quartzite, dolomitic marble, graphitic schist, gneiss are the main rock types within the Lesser Himalayan Sequence, whereas banded gneiss and quartzite form a significant portion of the Higher Himalayan Crystalline in the study area. The area is affected by poly-phase deformation. Lesser Himalayan Sequence has suffered five deformational phases (DL1-DL2, D3-D5) whereas the Higher Himalayan Crystalline has suffered four deformational events (DH1, D3-D5). The Lesser Himalayan Sequence lying to the northern limb of the Gorkha-Kuncha Anticlinorium is contort into doubly plunging to dome-and-basin-like en echelon type of non-cylindrical folds as Baluwa Dome and Pokharatar Basin (DL2 and D4). The direction of shearing as indicated by shear sense indicators (C' Shear band and Mica fish) is top-to-south coinciding with regional sense of shear related to the MCT propagation. The dynamic recrystallization direction, obtained from rock dominant with phyllosilicate minerals is top-to-south and coincides with mineral lineation and indicate the mineral lineation is contemporary with dynamic recrystallization during the MCT propagation.

Diagrama tangente: útil recurso do programa Ester 2.1 para projeção estereográfica em Geologia

Studies and applied works of structural analysis depend on specialized softwares and resources for 3D modeling. This article presents the results of a research that aimed to evaluate the effectiveness of version 2.1 of the Ester stereographic projection system for Structural Geology. The evaluation of the software involved tests with structural data of different geological contexts, submitted to graphic treatment in packages such as Stereonet, Stereonet9, QuickPlot, Dips, OpenStereo and other softwares accessible at low cost. The study privileged the truly free (open) softwares. One advance was the inclusion of the Tangent Polar Diagram into the Ester package. The tool helps to differentiate, in folded regions, conical folds from cylindrical folds. In the comparative evaluation, it was observed that diagrams produced by Ester 2.1 are similar to those generated by other programs. The advantage relates to resources that are not available in the other softwares analyzed. It is concluded that, in addition to resources heretofore unavailable, the tests showed good functionality. The experience signals new directions in the search for educational alternatives for teaching-learning in Structural Geology.

Spore morphology of the representatives of the subfamily Ceratopteridoideae (J. Sm.) R.M. Tryon from the family Pteridaceae E.D.M. Kirchn. (Pteridophyta)

<p>Scanning electron microscopy (SEM) was used to perform a comparative study for four representatives of the subfamily <em>Ceratopteridoideae</em> (J. Sm.) R.M. Tryon from East Asia: <em>Ceratopteris thalictroides</em> Brongn., <em>C. pteridoides</em> (Hook.) Hieron., <em>Acrostichum aureum</em> Linn., and <em>A.</em> <em>speciosum </em>Willd. The analysis of the external morphology of the representatives of <em>Ceratopteridoideae</em> revealed a strong difference between <em>Ceratopteris</em> and <em>Acrostichum</em>.</p><p>The external morphology of spores of <em>Ceratopteris</em> and <em>Acrostichum</em> exhibited the features characterizing the family Pteridaceae as a whole: tetrahedral spores with a three-ray laesure, from triangular-roundish to roundish, distinct exosporium with a pronounced surface ornamentation and absence of perisporium, the exosporium surface varying from smooth and rough to that covered with large tubercles and roller-like bulges.</p><p>The external morphology of spores of <em>Acrostichum</em> is quite simple, although among other representatives of the family Pteridaceae,<em> </em>the<em> </em>spores of <em>Ceratopteris</em> have distinctive features: very large equatorial diameter (106–124 µm); spores are almost roundish; distinct cylindrical folds running parallel to the corners of the spore across its extensive distal surface.</p><p> </p>

Jurassic–cretaceous deformational phases in the Paraná intracratonic basin, southern Brazil

This paper examines the domes and basins, regional arcs and synclines, and brittle structures of the Paraná Basin flood volcanism to characterize the deformational phases in its Jurassic to Cretaceous history. First-stage fieldwork revealed brittle structures, extensional joints, and strike-slip faults, and second-stage fieldwork investigated the connections of the brittle structures to both open folds and dome-and-basin features. Fault-slip data inversion was performed using two different techniques to distinguish local and remote stress/strain. Geometric and kinematic analyses completed the investigations of the deformation, which characterized two deformational phases for the Jurassic to Cretaceous periods in the Paraná Basin. Both developed under regional bi-directional constrictional (σ1 ≥ σ2 &amp;gg; σ3) stress regimes that produced a number of non-cylindrical folds. A D1 deformational phase produced the N–S and E–W orthogonally oriented domes and basins. The D2 arcs and synclines are oriented towards the NW and NE and indicate a clockwise rotation (35–40°) of both horizontal principal stress tensors. The extensional joints and strike-slip faults characterize the local stress field in the outer rim of the orthogonally buckled single volcanic flow, whereas the inner rim of the buckled single flow supported constriction and thus, developed the local arcuate folds.