Folding Of Rocks Research Articles

Estruturas atectônicas da Bacia do Paraná em Campinas (SP): deformação sin-sedimentar no Subgrupo Itararé

Entre 2000-2001, obras na Rodovia D. Pedro I expuseram dobras atectônicas no Subgrupo Itararé, Bacia do Paraná, no trevo da estrada que une Campinas a Moji-Mirim e Moji-Guaçu. Ocorrem dobras convolutas decimétricas, fechadas a isoclinais, controladas por siltitos laminados. As obras também expuseram, e depois destruíram, ondulações com comprimento de onda da ordem de 20 m. Sintetizam-se critérios distintivos desse tipo de estrutura e descrevem-se feições similares, entre SP e SC. A origem das deformações pode ser associada diretamente à ação do gelo ou ao deslizamento subaqüoso de camadas inconsolidadas. As ocorrências de Campinas parecem resultar de ambos os processos. Localmente, camadas não-confinadas em ambiente subaqüoso sofreram deformações penecontemporâneas à sedimentação, ao passo que as ondulações mais amplas e abertas devem-se à ação direta do gelo.

GEOLOGICAL AND GEOMORPHOLOGICAL CONDITIONS OF OIL BITUMEN DEPOSITS PLACING AND BITUMEN OUTPOURING IN MIDDLE SHESHMA RIVER BASIN, TATARSTAN

In the Volga-Ural bitumen zone, bitumen is concentrated in upper part of terrigenous sandstones of Sheshmian horizon of the Upper Ufimian substage. Sandstones form buried swell-like ridges up to 3-5 km long, 0.5-2.5 km wide, and 25-40 m height; to the top of them bitumen deposits are confined. For a long time these forms were considered as the bars of the end of the Early Permian epoch. New data (parallel orientation of these swells to contemporary river valleys, their location in the Middle Sheshma river basin, distinctive positions in swells, character of bitumen outpouring on the surface) bring us to accept their exotectonic origin. Owing to non-uniform static pressure, swells have been formed in strongly dissected relief at the turn of Miocene and Pliocene. Bitumen diluted high-porous sandstones, which were squeezed out from under watersheds in the direction of river valleys, where overlying rocks pressure was less. Similar subsurface folding in plastic rocks is widespread in the whole Middle Volga region. Distinct from those above geological and geomorphological conditions in neighbor areas restrict territorial distribution of bitumen deposits to Middle Sheshma river basin

Computer modeling of folding in rocks

The results of numerical mathematical modeling of rock deformations under compression are given. The numerical solutions are obtained using discretization of the equations of the solid mechanics with the finite element method. The model of an ideal elastic-plastic material with a Huber-Mises yield surface was used in the calculations. The layered medium structure is taken into account in modeling of the compression of layers of the lower/middle crust on a local scale. The natural folds in strongly deformed metamorphic sequences were reproduced by the mathematical deformation models. It is shown that folding in the lower part of the Earth’s crust is possible, when the yield stresses of the host rocks are approximately two orders of magnitude less than those of the hard layers. The effect of the boundary conditions and thickness of the compressed rocks on the folding is shown.

Newly-formed illite preserves fluid sources during folding of shale and limestone rocks; an example from the Mexican Fold-Thrust Belt

We combine structural, fluid-inclusion microthermometry, illite-crystallinity, X-Ray Diffraction (XRD) and O and H stable isotope analyses of authigenic illite to determine the source of local fluids interacting with rock during folding in anchizonal shales of the Mexican Fold-Thrust Belt (MFTB). A well-exposed train of mesoscopic, asymmetrical folds in a sequence of Cretaceous limestones interbedded with shale was targeted for this study. We test the hypothesis that syn-folding vein minerals and clay minerals were formed from the same fluids by comparing the δ2H composition of inclusion fluids in calcite and quartz from veins, and from illite concentrates from sheared shale layers, and the sources of that fluid.Five clay size-fractions (<0.05, 0.05–0.2, 0.2–1, 1–2, and <2 μm) were separated from eight shale samples. In the 40 clay grain-size fractions analyzed, illite, calcite, kaolinite, smectite, chlorite and minor quartz were identified by XRD analysis. Most samples show different proportions of various clay minerals, except for the finer fractions in two of the samples (BL3 and BL4) were illite is the only clay phase present. The discriminating potential of δ18O values of clay is generally masked by the abundance of calcite in all samples. In contrast, samples containing chlorite and smectite show very low values in δ2H (−75.9 to −53.9‰), while samples containing illite and kaolinite or pure illite show relatively high δ2H values (−33.1 to −50.1‰). The latter fall within the δ2H range (−39 to −49‰) determined in fluid inclusions of syntectonic veins, indicating isotopic equilibrium between water, veins fillings and illitic clay during deformation, according to fractionation factors at these temperatures (220–250 °C). The δ2H values and fluid inclusion salinities in the sampled rocks indicate that water active during folding was partly marine and partly meteoric and that the amount of such pore-water represented a small fraction of the deformed rock volume. Our study demonstrates that the H-isotopic composition of anchizonal illitic clays, commonly present in exhumed fold-thrust belts, can be used to determine the source(s) of fluids that were active during deformation.

Composite mesoscopic and magnetic fabrics of the Paleo-Proterozoic Wangtu Gneissic Complex, Himachal Himalaya, India: Implications for ductile deformation and superposed folding of the Himalayan basement rocks

The present study demonstrates how the Paleo-Proterozoic Wangtu Gneissic Complex (WGC) of the Lesser Himalayan Crystalline sequence experienced superposed folding and doming prior to its exhumation, with the help of integrated field, microstructural, magnetic fabric anisotropy and geochronological studies. The WGC forms the basement of the Lesser Himalaya and is bounded by Vaikrita Thrust (VT) to the northeast and Munsiari Thrust (MT) to the southwest. The regional structure consists of upright large scale early folds (D1) trending NW–SE. The mesoscopic fabric is related to axial plane foliation of the D1 folds and, to a lesser extent, late D2 folds. The axis of maximum compression for D1 and D2 folds are mutually orthogonal. The D1 folds have formed simultaneously with the major Himalayan thrusts whereas the D2 folds have developed during a later deformation event. The magnetic lineation at the hangingwall of the VT is sub-horizontal indicating stretching along the strike of the thrust. In the interior parts of the WGC, the magnetic fabric is of two types: (i) magnetic lineation demarks the intersection of mesoscopic and magnetic foliation indicating superposed deformation and (ii) scattered distribution of magnetic lineations due to D2 folding on initially curved and non-cylindrical D1 surface. 40Ar–39Ar dating of biotite from one site from the core of WGC gives an age of 9.3±0.3 (2σ)Ma. It is inferred that the doming of the WGC took place at ∼9Ma and, instead of large scale thrusting, it is characterized by superposed folding and strike-parallel stretching along the VT zone. It is suggested that the effect of superposed folding and ductile deformation of the Himalayan basement rocks has to be taken into account before cross-section balancing or any estimation of crustal shortening is attempted.

Structural evolution of the Agnew–Wiluna greenstone belt, Eastern Yilgarn Craton and implications for komatiite-hosted Ni sulfide exploration

The Agnew–Wiluna greenstone belt in the Yilgarn Craton of Western Australia is a narrow package of complexly deformed Archean supracrustal rocks that hosts two of the world's largest komatiite-hosted nickel sulfide deposits, the Mt Keith and Perseverance deposits. These deposits and several others in the belt are centred on thick lenses of adcumulate-textured komatiite interpreted to represent areas of channelised magma flow. The large nickel sulfide deposits are located in parts of the belt associated with ca 2720 to 2700 Ma felsic volcanism (e.g. the Leinster and Mt Keith nickel camps). In these areas, felsic to intermediate volcanic rocks are intercalated with syn-volcanic massive sulfides of inferred exhalative origin. While these primary magmatic features are clearly first-order controls on the distribution of Ni sulfide deposits in the belt, several regional-scale deformation events have significantly complicated the interpretation of primary stratigraphic relationships. The earliest recorded deformation events (D1,2,3) resulted in an east–west trending greenstone belt with recumbent isoclinal folds and ductile shear zones. Subsequent west-southwest–east-southeast shortening during the D4 event at ca 2666 Ma involved the refolding of the tectono-stratigraphy to produce belt-scale, north- to north-northwest-trending upright folds, a pervasive axial planar schistosity in all rocks, and the present-day steeply dipping, overturned supracrustal sequences, and emplacement of granitoids in major antiformal fold hinges. Polyphase folding of supracrustal rocks produced Type 2 fold interference patterns with multiple facing reversals at various scales across the belt. West-southwest–east-southeast extension during the D5 event at ca 2665 Ma triggered the development of terrestrial basins (i.e. Scotty Creek and Jones Creek) in areas flanking major antiforms, resulting in the deposition of the Jones Creek Conglomerate. Subsequent west-southwest–east-southeast shortening during the D6 event resulted in the folding of the Jones Creek Conglomerate and formation of gold-bearing veins in the Agnew gold camp. Belt-wide relaxation in east–west shortening during the D7 event caused open, recumbent F7 folding of the steeply dipping stratigraphy. Broadly east–west shortening during the D8 to D10 events resulted in the tightening of existing fold hinges, the dismemberment and displacement of panels of supracrustal rocks by sinistral (e.g. Perseverance shear zone) and then dextral (Waroonga) shear zones. The Agnew–Wiluna belt displays (para)autochthonous associations within the belt, with district-scale heterogeneities caused by primary volcano-sedimentary facies changes combined with polyphase deformation. Importantly, nickel sulfide-bearing sequences identified in nickel camps can potentially be traced to different parts of the belt by unravelling the effects of polyphase deformation.

Folding in power-law viscous multi-layers

We study high-amplitude folding in layered rocks with two-dimensional numerical simulations. We employ the finite-element method to model shortening of an incompressible multi-layer with power-law viscous rheology. The Lagrangian numerical mesh is deformed and re-meshed to accurately follow the layer interfaces. Three settings are considered: (i) pure shearing of a confined multi-layer, (ii) simple shearing of a multi-layer above a detachment, and (iii) slump folding owing to gravity sliding. In our pure shear simulations, finite-amplitude folds always develop despite confinement and thin weak interlayers. The fold shapes can be significantly irregular, resulting from initial geometrical heterogeneities that are perturbations of the layer interfaces and differences in layer thickness. The bulk normal viscosity of the multi-layer decreases significantly with progressive folding. This structural softening decreases the bulk normal viscosities by a factor of 2-20. For simple shear, the multi-layer does not develop asymmetric fold shapes significantly. Fold axial planes in the multi-layer are mostly curved and not parallel. For slump folding, fold shapes can be significantly asymmetric exhibiting strongly curved fold axial planes and overturned fold limbs. The rheology of the competent layers has a major impact on the fold shapes for gravity-driven multi-layer folding.

Structures of the late palaeozoic thrust belt in the Chinese South Tian Shan

Aiming to resolve contradictions in tectonic models and to establish a correlation between Chinese and Kyrgyz sectors of the South Tian Shan we carried out stratigraphic and structural studies in Chinese part of the belt along the Bayinbuluk—Kuqa transect. New data indicate that Chinese South Tian Shan is dominated by top-to-the-south structures, which were formed during the latest Carboniferous and Early Permian. Major allochthons of the Devonian carbonates, thrusted on the Gzhelian and Asselian turbidites, are revealed in the northern part of the belt. Imbricated thrust packages and recumbent folds in deeper marine Devonian and Carboniferous rocks are common in the South. Postkinematic granites yield U-Pb ages of 285–275 Ma, which indicate that thrust deformation ceased by the middle of the Early Permian. The same direction of motion and similar age of deformations in Kyrgyz and Chinese sectors of the South Tian Shan prove, that top-to-the-south structures were formed during the same structural episode, which corresponds to the main collisional stage within entire belt.

Eye and sheath folds in turbidite convolute lamination: Aberystwyth Grits Group, Wales

Eye and sheath folds are described from the turbidites of the Aberystwyth Group, in the Silurian of west Wales. They have been studied at outcrop and on high resolution optical scans of cut surfaces. The folds are not tectonic in origin. They occur as part of the convolute-laminated interval of each sand-mud turbidite bed. The thickness of this interval is most commonly between 20 and 100 mm. Lamination patterns confirm previous interpretations that convolute lamination nucleated on ripples and grew during continued sedimentation of the bed. The folds amplified vertically and were sheared horizontally by continuing turbidity flow, but only to average values of about γ = 1. The strongly curvilinear fold hinges are due not to high shear strains, but to nucleation on sinuous or linguoid ripples. The Aberystwyth Group structures provide a warning that not all eye folds in sedimentary or metasedimentary rocks should be interpreted as sections through high shear strain sheath folds.

Structural Geology

Structural geology and continental tectonics were ushered in to the modern quantitative age of geosciences with the arrival of the global plate tectonics paradigm (circa 1968), derived using new data from the oceans' depths, and John Ramsay's 1967 seminal work, Folding and Fracturing of Rocks. Fossen is to be applauded for crafting a unique, high‐caliber, and accessible undergraduate textbook on structural geology that faithfully reflects this advance and the subsequent evolution of the discipline. This well‐written text draws on Fossen's wealth of professional experience, including his broad and diverse academic research and experience in the petroleum industry. This book is beautifully illustrated, with excellent original color diagrams and with impressive color field photographs that are all keyed to locations and placed into geologic context.

Radical changes in bulk shortening directions during orogenesis: Significance for progressive development of regional folds and thrusts

Polyphase folding and metamorphism of rocks in the eastern Mount Isa Inlier resulted from four distinct orogenic cycles. These consist of NE–SW shortening (O 00) from 1672 to ∼1648 Ma, involving a succession of five phases of deformation, which was responsible for the development of the first generation NW–SE striking macroscopic folds, plus cleavages that are mainly only preserved in porphyroblasts. N–S shortening (O 1) followed to 1600 Ma, involving at least three phases of deformation. This period of N–S directed orogenesis rotated large portions of the NW–SE trending regional folds into W–E trends and the dominance of locally well-preserved W–E trending sub-vertical matrix foliations as well as the intensification of foliation parallel to bedding. Subsequent W–E shortening (O 2) from 1600 to ∼1515 Ma was accompanied by the development of N–S trending folds. These dominate the Mount Isa Inlier and where best developed involved the formation of at least three near orthogonal foliations. Where these large-scale folds regionally are weakly developed or not visible, only one sub-vertical foliation has developed. Finally NW–SE shortening (O 3) produced locally developed SW–NE trending folds and weak crenulations until ∼1500 Ma. The asymmetry of steep to gentle pitch changes in inclusion trails preserved in garnet porphyroblasts that developed during O 00 indicates thrusting to the SW. Similar inclusion trail asymmetries in garnet, staurolite and andalusite porphyroblasts that developed in O 1, indicate thrusting to the N confirming a change in thrust direction across the Wonga Duchess belt to the W. Formation of the Meerenurker roof thrust near Mount Isa by thrusting to the SW during O 00 with subsequent development of the Kokkalukkanurker duplex below by thrusting to the S during O 1 reduces the displacement required for the roof thrust from 234 to ∼65 kms. Foliations preserved in O 00 garnets are truncated by foliations associated with the development of O 1 in the matrix. O 1 garnet, staurolite and andalusite porphyroblasts preserve inclusion trails that are commonly truncated against a differentiated crenulation cleavage in the matrix that formed during O 2. The inclusion trails entrapped within O 00 and O 1 generated porphyroblasts reveal that the foliations that progressively developed in the matrix during these orogenies were composite fabrics resulting from the effects of multiple discrete orthogonal increments in each case. Numerous phases of sub-vertical foliation development due to horizontal bulk shortening were followed by phases of gently dipping foliation formation due to gravitational collapse that accompanied thrusting during O 00 and O 1.

The Origin of Pre- and Synrift, Hypogene Fe-P Mineralization during the Cenozoic along the Dead Sea Transform Fault, Northwest Jordan

The Dead Sea transform fault, a prominent deep-seated suture zone with sinistral displacement, marks the boundary between the Arabian and African plates, sensu lato, with the Dead Sea-Jordan Valley rift as its morphological expression. Hematite-ankerite-goethite deposits in the environs of Warda, an apatite deposit near Suweileh, hot brine sinter terraces with a varied spectrum of Ca-Fe-Mn-Ba mineralization on the banks of the Yarmouk and Jordan River, and strata-bound goethite mineralization on different uplifted blocks northeast of the Dead Sea have been investigated for their mineral assemblages, major and trace element chemistry signatures, and stable isotope (C, 0, S) signatures. The data have been used to interpret the origins of these pre- and synrift hypogene styles of mineralization along the prominent plate boundary. During a Paleogene prerift extensional phase, northeast-to-southwest en echelon extension structures enabled low-temperature hydrothermal solutions to circulate in the eastern block of the Dead Sea transform fault. The strata- and fault-bound orebodies of the Wards Fe ore deposit resulted from low-temperature, epithermal replacement processes. The hydrothermal solutions affecting Late Cretaceous massive limestone at Warda may have tapped a magmatic source of sulfur during the Paleogene. In the neighboring Suweileh P deposit, the Maastrichtian phosphorite beds underwent strong hydrothermal remobilization during the Paleogene, with a moderate contribution of sulfur from underlying Triassic evaporite series. The northeastern shore zone of the Dead Sea and the drainage system of the Yarmouk River had an initial Fe accumulation as early as the Paleogene, with precipitation occurring from hot brines that interacted with Triassic marine evaporites. The Neogene-Quaternary compressional phase gave rise to tight, recumbent folds in Cretaceous rocks along the eastern side of the Dead Sea transform fault and promoted development, at Wards and Suweileh, of southwest to northeast-trending ore anticlines and flexure bulges. Hydrothermal activity lasted until the Quaternary along the fault. Iron mineralization took place during that period near the main fault area of the Dead Sea transform fault. Parasite faults striking almost north to south in the western Yarmouk area enabled hot brines to ascend and cover the banks of the modern river drainage system with sinter terraces. In the study area northeast of the Dead Sea, Fe accumulated in subaquatic lacustrine settings. Neotectonic uplift of these anticlines and the formation of raised platforms caused part of the hanging-wall rocks to be removed from the ore and turned some of these Fe-enriched rocks into desert varnish under arid climatic conditions.

Comparisons of the kinematics and deep structures of the Zagros and Himalaya and of the Iranian and Tibetan plateaus and geodynamic implications

[1] We compare the geologic histories, the deep structures, and the present-day kinematics of deformation of the Himalaya and the adjacent Tibetan Plateau with those of the Zagros and Iranian Plateau to test geodynamic processes of continental collision. Shortly after India and Arabia collided with Eurasia, horizontal shortening manifested itself by folding and thrust faulting of sedimentary rock detached from India's and Arabia's underlying crystalline basement. Subsequently, slip on thrust faults stacked slices of India's basement to build the Himalaya on India's northern margin. Such faulting has not yet developed in the Zagros, where collision is more recent and Arabia penetrates into Eurasia more slowly than India does, so that postcollision convergence with Eurasia is less. The greater elevation, thicker crust, and more marked heterogeneity of the upper mantle beneath the Tibetan than beneath the Iranian Plateau also reflect a more advanced stage of development. Moreover, while thrust or reverse faulting and crustal shortening continue on the margins of both plateaus, normal faulting, suggesting horizontal extension and crustal thinning, occurs within Tibet but not in Iran. Hence, the balance of forces that built the high Tibetan Plateau must have changed, apparently some time since ∼15 Ma. Removal of Tibetan mantle lithosphere could have altered that balance. If mantle lithosphere beneath the Iranian Plateau has been removed, however, the change in force balance has been too small to initiate normal faulting. Low seismic wave speeds in the uppermost mantle just beneath the Moho of both plateaus suggest (to us) that lithosphere beneath both is thin, consistent with late Cenozoic removal of it, but alternative explanations might account for these low speeds. Despite its apparently thin, and hence presumably weak, mantle lithosphere, much of central Iran undergoes little deformation. It illustrates how a crustal block can behave rigidly not necessarily because it is strong but because deviatoric stresses can be small. Whereas differences between the two regions clearly depend on the amount that Arabia and India have penetrated into Eurasia, which scales with both the dates of collision and rates of convergence, we see no differences in the operative processes that depend on the present-day rates of convergence.

Folding with thermal–mechanical feedback: Discussion

A recent paper in this Journal by Bruce Hobbs, Klaus Regenauer-Lieb and Alison Ord [Hobbs, B., Regenauer-Lieb, K., Ord, A., 2008. Folding with thermal–mechanical feedback. Journal of Structural Geology 30, 1572–1592] presents an alternative theory to the traditional Biot-Ramberg theory for folding of viscous rocks that involves non-equilibrium thermodynamics and thermal–mechanical feedback. The authors convey a strong message throughout their paper that the folds produced by this theoretical and numerical modelling are geologically realistic and provide a better explanation for many natural folds than the traditional theory. They promise the same approach for boudinage, and present this folding paper as part of a “unified framework for rock deformation processes”. Readers of the Journal of Structural Geology might be led to conclude that this paper provides a good alternative model for folding of rocks. Our discussion will disagree, on four counts.

Brittle fracture during folding of rocks: A finite element study

The goal of the present work is the development of a novel computational analysis tool to elaborate folding-induced fracture of geological structures. Discrete failure of brittle rocks is characterised by three sets of governing equations: the bulk problem, the interface problem and the crack problem. The former two sets which define the deformation field are highly nonlinear and strongly coupled. They are solved iteratively within a Hansbo-type finite element setting. The latter set defines the crack kinematics. It is linear and solved in a single post-processing step. To elaborate the features of the computational algorithm, we define a unique benchmark problem of a single, geometrically nonlinear plate, which is subjected to layer-parallel in-plane compression combined with different levels of superposed in-plane shear. The resulting folding, or buckling, induces brittle failure in the tensile regime. By systematically increasing the shear strain at constant compression, we develop crack deviation angle versus shear-to-compression ratio tables. We determine the corresponding damage zones, analyse the folding modes and elaborate the force versus amplification diagrams. The proposed two-field folding-induced fracture algorithm can ultimately be applied to interpret natural folded rocks and understand their evolution, structural development and histology.

Development of the model of the deep structure of Akhtyr flexure-fracture zone and Shugo mud volcano

Most mud volcanoes are located along large tectonic zones within the Alpine and Central Asia folded regions, Pacific mobile belt, and rift zones of the Atlantic and Indian oceans. One of the main conditions of the functioning of mud volcanic processes in mobile belts of the Earth’s crust is dislocation and folding of sedimentary rocks. Precisely such geological conditions exist in the Taman mud volcanic province, where the well-known Shugo Volcano is located.

Geological setting of the 8 October 2005 Kashmir earthquake

The source of the 8 October 2005 earthquake of M 7.6 was the northwest-striking Balakot–Bagh (B–B) fault, which had been mapped by the Geological Survey of Pakistan prior to the earthquake but had not been recognized as active except for a 16-km section near Muzaffarabad. The fault follows the Indus–Kohistan Seismic Zone (IKSZ); both cut across and locally offset the Hazara–Kashmir Syntaxis defined by the Main Boundary and Panjal thrusts. The fault has no expression in facies of the Miocene–Pleistocene Siwalik Group but does offset late Pleistocene terrace surfaces in Pakistan-administered Jammu-Kashmir. Two en-echelon anticlines near Muzaffarabad and Balakot expose Precambrian Muzaffarabad Limestone and are cut by the B–B fault on their southwest sides, suggesting that folding and exposure of Precambrian rocks by erosion accompanied Quaternary displacement along the fault. The B–B fault has reverse separation, northeast side up; uplift of the northeast side accompanied displacement, producing higher topography and steeper stream gradients northeast of the fault. No surface expression of the B–B fault has been found northwest of the syntaxis, although the IKSZ and steeper stream gradients continue at least as far as the Indus River, the site of the Pattan earthquake of M 6.2 in 1974. To the southeast, northwest-striking faults were mapped by the Geological Survey of Pakistan. One of these faults, the Riasi thrust, cuts across the southwest flank of an anticline exposing Precambrian limestone. Farther southeast, in Indian-administered territory, Holocene activity on the Riasi thrust has been described. In the Kangra reentrant still farther southeast, active faulting may follow the Soan thrust, along which Holocene and Pleistocene offsets have been described. The Soan thrust, rather than the south flank of the Janauri anticline, may represent the surface projection of the 1905 Kangra earthquake of M 7.8.

Strain and vorticity analysis using small-scale faults and associated drag folds

Small-scale faults with associated drag folds in brittle-ductile rocks can retain detailed information on the kinematics and amount of deformation the host rock experienced. Measured fault orientation ( α), drag angle ( β) and the ratio of the thickness of deflected layers at the fault ( L) and further away ( T) can be compared with α, β and L/ T values that are calculated with a simple analytical model. Using graphs or a numerical best-fit routine, one can then determine the kinematic vorticity number and initial fault orientation that best fits the data. The proposed method was successfully tested on both analogue experiments and numerical simulations with BASIL. Using this method, a kinematic vorticity number of one (dextral simple shear) and a minimum finite strain of 2.5–3.8 was obtained for a population of antithetic faults with associated drag folds in a case study area at Mas Rabassers de Dalt on Cap de Creus in the Variscan of the easternmost Pyrenees, Spain.

The Monashee reflection: Re-examination of a Lithoprobe crustal-scale seismic reflection in the southern Canadian Cordillera

The Monashee reflection is a major crustal-scale, crosscutting reflection that appears on two mutually perpendicular Lithoprobe seismic profiles in the southern Omineca belt of the Canadian Cordillera. It has previously been interpreted as the down-plunge extension of a regional ductile thrust fault, the Monashee decollement, which is said to separate the Monashee complex from the overlying Selkirk allochthon. We reinterpret the Monashee reflection as a shear zone with modest, normal displacement based on recent mapping, geometric analysis of the seismic profiles, and existing geochronological and metamorphic data. Recent mapping has demonstrated that this boundary is not a shear zone with thrust geometry, but rather it is a gradational boundary that has resulted from mutual folding and transposition of rocks ascribed to the Monashee complex and Selkirk allochthon. Overprinting the transition zone in different areas are three high-strain zones: the Green-bush Lake shear band zone, Slate Mountain shear zone, and a ductile shear zone associated with the Columbia River fault. We interpret these high-strain zones as segments of a single, high-strain zone that wraps around the margins of the Thor-Odin culmination. This marginal zone is a complex, outward-dipping, normal structure, which we name the Thor-Odin high-strain zone. Three alternative three-dimensional geometric models have been developed for the Monashee reflection in order to project the reflection to the surface. We favor a model in which the surface trace of the Monashee reflection coincides with the Thor-Odin high-strain zone. Normal shear sense kinematics are interpreted for the Monashee reflection based on: (1) the overall geometry and fault-drag–like relationship between the Monashee reflection and reflections in the hanging wall and foot-wall; (2) offset of metamorphic and geochronological gradients, which are consistent with an extensional zone rather than with a thrust fault interpretation; and (3) the crosscutting nature of the Monashee reflection, which is consistent with normal structures throughout the region.

Geologic and U–Pb geochronologic evidence for early Paleozoic tectonism in the Dadeldhura thrust sheet, far-west Nepal Himalaya

The Dadeldhura thrust sheet inm western Nepal consists of Proterozoic–Lower Paleozoic sedimentary and plutonic rocks, and their metamorphic equivalents, that rest structurally on Proterozoic strata of the Lesser Himalayan sequence. Although regional metamorphism and ductile deformation were widespread during Tertiary thrust emplacement, relicts of early Paleozoic tectonism are preserved locally. New field and geochronologic studies, together with the findings of previous workers, indicate that this early Paleozoic tectonism included: (1) regional metamorphism to at least garnet grade, (2) regional folding of a thick metamorphic sequence into a broad east–west trending syncline, (3) outcrop-scale folding of metasedimentary rocks, (4) emplacement of Cambro–Ordovician granitic bodies during and after the metamorphism and deformation, (5) uplift and erosion of the metamorphic sequence, with garnet-grade rocks locally exposed at the surface, and (6) derivation of Ordovician conglomeratic sandstones from the early Paleozoic orogen. Similar records of metamorphism, deformation, and uplift/erosion have been found in other regions of the Himalaya, indicating that rocks of the Dadeldhura thrust sheet were originally involved in a regionally extensive orogenic system. Future tectonic models of Himalayan orogenesis must accommodate this early Paleozoic event.