Imbricate Zone Research Articles

Structural analysis of the Azmir – Goizha anticline, north and northeast of Sulaimani city, Kurdistan Region, Northeast Iraq

The Azmir–Goizha anticline (AGA) is located directly to north and northeast boundary of
 Sulaimani city, Kurdistan Region, NE Iraq. The anticline originally consists of two main connected
 anticlines (Azmir and Goizha anticlines) with many smaller anticlines along southwest and
 northeast limbs. In the studied area, it has length and width of about (10 and 4) kms, respectively,
 with height of 1600 m (a.s.l). It extends from Weladar village, from southeast to Khamza village at
 the northwest and further extends outside of the studied area. The anticline has very complicate
 structural setting and recently few structural analyses were conducted by different workers. The
 present study is an attempt to discover the actual structural setting of the anticline and try to review
 the previous studies critically by using data collected in the field with aid of precise stratigraphic
 analysis. Structurally, the previous pigy-back thrust imbricated fan is changed to nearly isoclinal
 detachment fold, which is detached in the form of either lift-off detachment fold or multi-
 detachment fold or multi-detachment faulted fold. In the core of the anticline, the accommodation
 of the shortening is preserved as second-order folding or second-order conjugate faulting. The
 competent cover (outer layers) of the anticline consists of Kometan and upper part Balambo
 formations. Core Azmir and Qaywan anticlines are occupied plastic incompetent layers of Sarmord
 Formation and lower part of Balambo formations while that of Azmir Bichkola and Naugirdan (or
 Goizha) anticlines are occupied. These units are deformed as disharmonic buckle folds in the cores.
 The main detachment had occurred in the lower part of the Sarmord Formation (Lower
 Cretaceous) and inside Jurassic rocks. All the mapped strike-slip and reverse faults, in the previous
 studies are not recorded in the present study. Furthermore, this study recommends excluding the
 anticline from the Imbricate Zone and including it within the High Folded Zone.

Evaluation of hydrocarbon generation and migration in the Molasse fold and thrust belt(Central Eastern Alps, Austria) using structural and thermal basin models

The Molasse Basin represents the northern foreland basin of the Alps. After decades of exploration, it is considered to be mature in terms of hydrocarbon exploration. However, geological evolution and hydrocarbon potential of its imbricated southernmost part (Molasse fold and thrust belt) are still poorly understood. In this study, structural and petroleum systems models are integrated to explore the hydrocarbon potential of the Perwang imbricates in the western part of the Austrian Molasse Basin. The structural model shows that total tectonic shortening in the modeled north–south section is at least 32.3 km (20.1 mi) and provides a realistic input for the petroleum systems model. Formation temperatures show present-day heat flows decreasing toward the south from 60 to 41 mW/m2. Maturity data indicate very low paleoheat flows decreasing southward from 43 to 28 mW/m2. The higher present-day heat flow probably indicates an increase in heat flow during the Pliocene and Pleistocene. Apart from oil generated below the imbricated zone and captured in autochthonous Molasse rocks in the foreland area, oil stains in the Perwang imbricates and oil-source rock correlations argue for a second migration system based on hydrocarbon generation inside the imbricates. This assumption is supported by the models presented in this study. However, the model-derived low transformation ratios (20%) indicate a charge risk. In addition, the success for future exploration strongly depends on the existence of migration conduits along the thrust planes during charge and on potential traps retaining their integrity during recent basin uplift.

Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

Geochemical and geochronological analyses provide quantitative evidence about the origin, development and motion along ductile faults, where kinematic structures have been overprinted. The Main Central Thrust is a key structure in the Himalaya that accommodated substantial amounts of the India–Asia convergence. This structure juxtaposes two isotopically distinct rock packages across a zone of ductile deformation. Structural analysis, whole-rock Nd isotopes, and U–Pb zircon geochronology reveal that the hanging wall is characterized by detrital zircon peaks at c . 800–1000 Ma, 1500–1700 Ma and 2300–2500 Ma and an ϵ Nd(0) signature of −18.3 to −12.1, and is intruded by c . 800 Ma and c . 500–600 Ma granites. In contrast, the footwall has a prominent detrital zircon peak at c . 1800–1900 Ma, with older populations spanning 1900–3600 Ma, and an ϵ Nd(0) signature of −27.7 to −23.4, intruded by c . 1830 Ma granites. The data reveal a c . 5 km thick zone of tectonic imbrication, where isotopically out-of-sequence packages are interleaved. The rocks became imbricated as the once proximal and distal rocks of the Indian margin were juxtaposed by Cenozoic movement along the Main Central Thrust. Geochronological and isotopic characterization allows for correlation along the Himalayan orogen and could be applied to other cryptic ductile shear zones. Supplementary material: Zircon U–Pb geochronological data, whole-rock Sm–Nd isotopic data, sample locations, photomicrographs of sample thin sections, zircon CL images, and detailed analytical conditions are available at www.geolsoc.org.uk/SUP18704 .

Physical modelling of chemical compaction, overpressure development, hydraulic fracturing and thrust detachments in organic-rich source rock

Geological evidence for overpressure is common worldwide, especially in petroleum-rich sedimentary basins. As a result of an increasing emphasis on unconventional resources, new data are becoming available for source rocks. Abnormally high values of pore fluid pressure are especially common within mature source rock, probably as a result of chemical compaction and increases in volume during hydrocarbon generation. To investigate processes of chemical compaction, overpressure development and hydraulic fracturing, we have developed new techniques of physical modelling in a closed system. During the early stages of our work, we built and deformed models in a small rectangular box (40 × 40 × 10 cm), which rested on an electric flatbed heater; but more recently, in order to accommodate large amounts of horizontal shortening, we used a wider box (77 × 75 × 10 cm). Models consisted of horizontal layers of two materials: (1) a mixture of equal initial volumes of silica powder and beeswax micro-spheres, representing source rock, and (2) pure silica powder, representing overburden. By submerging these materials in water, we avoided the high surface tensions, which otherwise develop within pores containing both air and liquids. Also we were able to measure pore fluid pressure in a model well. During heating, the basal temperature of the model surpassed the melting point of beeswax (∼62 °C), reaching a maximum of 90 °C. To investigate tectonic contexts of compression or extension, we used a piston to apply horizontal displacements.In experiments where the piston was static, rapid melting led to vertical compaction of the source layer, under the weight of overburden, and to high fluid overpressure (lithostatic or greater). Cross-sections of the models, after cooling, revealed that molten wax had migrated through pore space and into open hydraulic fractures (sills). Most of these sills were horizontal and their roofs bulged upwards, as far as the free surface, presumably in response to internal overpressure and loss of strength of the mixture. We also found that sills were less numerous towards the sides of the box, presumably as a result of boundary effects. In other experiments, in which the piston moved inward, causing compression of the model, sills also formed. However, these were thicker than in static models and some of them were subject to folding or faulting. For experiments, in which we imposed some horizontal shortening, before the wax had started to melt, fore-thrusts and back-thrusts developed across all of the layers near the piston, producing a high-angle prism. In contrast, as soon as the wax melted, overpressure developed within the source layer and a basal detachment appeared beneath it. As a result, thin-skinned thrusts propagated further into the model, producing a low-angle prism. In some experiments, bodies of wax formed imbricate zones within the source layer.Thus, in these experiments, it was the transformation, from solid wax to liquid wax, which led to chemical compaction, overpressure development and hydraulic fracturing, all within a closed system. According to the measurements of overpressure, load transfer was the main mechanism, but volume changes also contributed, producing supra-lithostatic overpressure and therefore tensile failure of the mixture.

Plate interactions of Laurussia and Gondwana during the formation of Pangaea — Constraints from U–Pb LA–SF–ICP–MS detrital zircon ages of Devonian and Early Carboniferous siliciclastics of the Rhenohercynian zone, Central European Variscides

The southern Rheinisches Schiefergebirge, which is part of the Rhenohercynian zone of the Central European Variscides, exhibits several allochthonous units: the Gießen-, and the Hörre nappe, and parts of the Frankenbach imbrication zone. These units were thrust over autochthonous and par-autochthonous volcano-sedimentary complexes of the Lahn and Dill–Eder synclines. This paper reports a representative data set of U–Pb LA–SF–ICP–MS ages of 1067 detrital zircon grains from Devonian and Lower Carboniferous siliciclastic sediments of the autochthonous and the allochthonous areas, respectively. The cluster of U–Pb ages from the allochthonous units points to a provenance in the Saxothuringian zone. Zircon populations from the Saxothuringian zone are representative of a Gondwanan hinterland and are characterized by age clusters of ~530–700Ma, ~1.8–2.2Ga, ~2.5–2.7Ga, and ~3.0–3.4Ga. Further samples were taken from the autochthonous and par-autochthonous units of the Lahn–Dill and Kellerwald areas. A Lower Devonian sandstone sample from the Siegen anticline provides a reference for siliciclastic sediments derived from the Old Red Continent. These samples show a provenance representative of Laurussia with debris primarily derived from Baltica and Avalonia. U–Pb zircon age clusters occur at ~400–450Ma, 540–650Ma, 1.0–1.2Ga, ~1.4–1.5Ga, ~1.7–2.2Ga, and 2.3–2.9Ga. Provenance analysis and geochemical data of the Rhenohercynian zone provide new information on the evolution of magmatic arcs in the Mid-Paleozoic. The data set constrains top-SE and top-NW directed subduction of the oceanic crust of the Rheic Ocean. Subduction-related volcanism lasted from the Early Devonian to the Early Carboniferous and thus confirms the existence of the Rheic Ocean until the Early Carboniferous. The tectonic model outlined for the Rhenohercynian zone suggests a wide Rheic Ocean.

TECTONO‐STRATIGRAPHIC EVOLUTION OF THE NW SEGMENT OF THE ZAGROS FOLD‐THRUST BELT, KURDISTAN, NE IRAQ

The Kurdistan (NW) segment of the Zagros fold‐thrust belt, located in the Kurdistan Region of NE Iraq, forms the external part of the Zagros orogen and is bounded by the Zagros suture to the NE. To the SW is the Arabian Plate into which the deformation front has migrated progressively, beginning in the Late Cretaceous and culminating in the Tertiary. Regional compression resulted in obduction of the Mawat ophiolites and emplacement of the Avroman and Qulqula nappes onto the continental margin, and the formation of the Kurdistan foreland basin. In this paper, structural, stratigraphic and palaeontological data together with new field observations are used to investigate the tectono‐stratigraphic evolution of this basin, and to study the propagation of the deformation front from the Zagros Imbricate Zone in the NE towards the Mesopotamian foredeep in the SW. Six unconformities within the Kurdistan foreland basin succession are recognized: Turonian (base‐AP9; 92 Ma); Danian (base‐AP10; 65 Ma); Paleocene–Eocene (intra‐AP10; 55 Ma); late Eocene (top‐AP10; 34 Ma); middle‐upper Miocene (a local unconformity; intra‐AP11; 12 Ma); and Pleistocene. These unconformities can be divided into two groups; obduction‐related (Turonian, Danian, and Paleocene‐Eocene); and collision‐related (late Eocene, middle‐upper Miocene, and Pleistocene).The geographical position of the unconformities is used to determine the rate of propagation of the deformation front, which is estimated at ca. 3 mm/yr. This is in agreement with previous studies which suggested a NW‐ward decrease in the propagation rate. The rate was most rapid (2.95 mm/yr) in the Low Zagros Fold‐Thrust Zone and slower (2.06 mm/yr) in the High Zagros Fold‐Thrust Zone. The more rapid propagation rate in the former area may be attributed to the presence there of the Miocene Lower Fars Formation which acted as a shallow décollement surface.Within the Zagros fold‐thrust belt, the intensity of deformation decreases towards the foreland (SW). Deformation in the High Zagros Fold‐Thrust Zone is characterized by thrust imbricates and high amplitude fault‐propagation folds at the surface separated by narrow synclines. However, the Low Zagros Fold‐Thrust Zone (Simply Folded Belt) is characterised by detachments and low amplitude fault propagation folds separated by broad synclines. In the foredeep area, folds are confined to the subsurface. Deeply buried Jurassic units, together with Upper Cretaceous – Paleocene siliciclastics, and the evaporite‐dominated Lower Fars Formation may have acted as décollement surfaces in the NW segment of the Zagros fold‐thrust belt, and controlled the structural geometry and evolution of the area.

Geological evolution of the Iraqi Mesopotamia Foredeep, inner platform and near surroundings of the Arabian Plate

Geological evolution of the Iraqi Mesopotamia Foredeep, inner platform and near surroundings of the Arabian Plate

The Manicouagan impact melt sheet: Evidence for isotopic homogenization with limited assimilation

The Rb–Sr, Pb–Pb and Sm–Nd isotope chemistry of impact-generated melt and target rocks has been determined from the 214Ma, ∼90km rim-diameter, Manicouagan impact structure of Canada. Fifty-seven samples were obtained from 8 field sites and 11 drill core sites across the impact melt sheet, located on the 55km-diameter central island (Île René-Levasseur). The results reveal that the impact melt, though locally differentiated via fractional crystallization, was isotopically homogenized during its formation following its derivation primarily from charnockites and mesocratic gneisses. The isotopic signatures of the target rocks indicate that the protolith for the melt was the Proterozoic Manicouagan Imbricate Zone, and that the underlying Archean Gagnon Terrane was not involved in melt production, or its subsequent modification via assimilation, despite impact melt resting on Archean lithologies in the southwest sector of the melt sheet. Limited assimilation of the footwall in this sector is attributed to the relatively rapid cooling of a thinner peripheral melt body (<100m), primarily through contact with cooler footwall and clasts. Assimilation at the base of the thicker (up to 1.4km thick) melt sections, near the center of the structure is attributed to secondary melting of centrally uplifted anorthosite, which locally modified the original isotopic signature.

Nd isotope mapping of Grenvillian crustal terranes in the vicinity of the Manicouagan Impact Structure

Over 70 new Nd isotope analyses are presented for the Manicouagan area of the Grenville Province to estimate the crustal age of target rocks involved in the 214 Ma Manicouagan Impact Structure, and to reconstruct the Precambrian geological evolution of this crustal segment. The rocks fall into two main groups: Samples from the Archean-aged Gagnon Terrane to the north and west of the impact give T DM ages averaging 2.70 Ga. Samples from the Manicouagan Imbricate Zone (MIZ) and other allochthonous lithotectonic domains to the south of the impact yield Paleoproterozoic T DM ages averaging 2.01 Ga for the MIZ and 1.86 Ga for the southern domains. These Paleoproterozoic terranes are correlated with Makkovik-age crust in Labrador that was heavily reworked by Labradorian magmatism that increased in intensity southwards. The target rocks involved in the impact event would have consisted almost entirely of the MIZ, which formed a layer several kilometres thick, overlying Archean crust at depth.

Significance of angular unconformities between Cretaceous and Tertiary strata in the northwestern segment of the Zagros fold–thrust belt, Kurdistan Region, NE Iraq

AbstractIn this study, two angular unconformities are found and analysed for the first time in the Mesozoic–Cenozoic succession in the northwestern segment of the Zagros fold–thrust belt (ZFTB) in the Kurdistan Region. The first unconformity exists between Lower Cretaceous and Paleocene–Eocene rocks and the second between the Campanian Shiranish Formation and the Maastrichtian Tanjero Formation. Each of these unconformities is found in two different localities in the Zagros Imbricate Zone (i.e. the highly deformed zone immediately SW of the Zagros Suture) of the ZFTB of the Kurdistan Region near the border with Iran. The study uses recent geological mapping, structural and stratigraphic analyses in addition to using previous biozonation of the stratigraphic units that bound the two unconformities. The first unconformity was initiated with obduction of the ophiolite and Lower Cretaceous radiolarite onto the passive margin of the Arabian plate. This unconformity formed during an early phase of the Zagros orogeny, which is associated with the developing of a foreland basin, and resulted in the folding of the radiolarites and their uplift to form high-relief land. The erosion of this high-relief land resulted in the formation of the Paleocene–Eocene Red Bed Series and their deposition on the folded radiolarite. The timing of the deformation that caused this unconformity is hard to determine; however, its stratigraphic position may suggest that it possibly is related to post-Cenomanian movements. The second unconformity is between the tilted Campanian Shiranish Formation (hemipelagite) and Tanjero Formation (500 m of conglomerate in the more proximal area). These unconformities indicate that deformation and uplift of the sedimentary units was variable during ophiolite obduction in this part of the ZFTB. We argue that deformation, ophiolite obduction and collision are likely to have varied in space and time along thec. 2000 km long ZFTB.

Chapter 58 Evidence of late Neoproterozoic glaciation in the Caledonides of NW Scandinavia

Abstract The northwestern part of the Scandinavian Caledonides, formed by SE- to ESE-directed thrusting through the Neoproterozoic W. Baltica continental shelf, contains numerous small and often isolated outcrops of diamictite and associated strata. No precise biostratigraphic or isotopic data are available to constrain the age of these sediments, but, on the basis of their stratigraphic position, most are correlated with the Mortensnes Formation (Fm.) in E. Finnmark and also presumed to be of glaciogenic origin. The Mortensnes Fm. has been correlated with the 580 Ma Gaskiers glacial event on the basis of δ 13 C isotope studies. Structurally, the deposits occur in the Autochthon (below the Torneträsk Fm.), within an external imbricate zone (Lower Allochthon), within cover successions lying unconformably on allochthonous basement (Window Allochthon) palaeogeographically derived from below or outboard of the Lower Allochthon and, more rarely, within the Middle Allochthon, derived from outboard of the Window Allochthon. Evidence for a glaciogenic origin is typically poor or lacking. Only in the Komagfjord Antiformal Stack (Window Allochthon), where an up to 40-m-thick succession of three fining upwards cycles has been mapped, are the deposits comparable in thickness and complexity to the Mortensnes Fm. Other sequences are sometimes &lt;1 m thick and unconformably overlain by post-‘glacial’ deposits. The Vakkejokk Breccia, a submarine slump in the Torneträsk area of the Autochthon closely underlies the correlative Precambrian–Cambrian lithostratigraphic boundary in E. Finnmark but overlies the first appearance of the boundary marker fossil Treptichnus pedum . Although sometimes interpreted as periglacial, this seems unlikely in view of the 30–50° palaeolatitude during deposition. Calcite nodules (&lt;1 cm size) in the Vakkejokk Breccia have previously been interpreted as glendonite, but the microstructure and palaeolatitude makes this unlikely; they are likely a replacement of gypsum. Diamictites of uncertain origin have also been found in the Ediacaran Lower Siltstone Member of the Torneträsk Fm. and unconformably under the ?Lower Cambrian Lomvatn Fm. in the Komagfjord Antiformal Stack.

Analysis of the Wallowa-Baker terrane boundary: Implications for tectonic accretion in the Blue Mountains province, northeastern Oregon

The Baker terrane, exposed in the Blue Mountains province of northeastern Oregon, is a long-lived, ancient (late Paleozoic–early Mesozoic) accretionary complex with an associated forearc. This composite terrane lies between the partially coeval Wallowa and Olds Ferry island-arc terranes. The northern margin of the Baker terrane is a broad zone (>25 km wide) of fault-bounded, imbricated slabs and slices of meta-igneous and metasedimentary rocks faulted into chert-argillite melange of the Elkhorn Ridge Argillite. Metaplutonic rocks within tectonic units in this zone crystallized between 231 and 226 Ma and have low initial 87 Sr/ 86 Sr ratios (0.7033–0.7034) and positive initial e Nd values (+7.7 to +8.5). In contrast, siliceous argillites from the chert-argillite melange have initial 87 Sr/ 86 Sr values ranging from 0.7073 to 0.7094 and initial e Nd values between −4.7 and −7.8. We interpret this broad, imbricate fault zone as a fundamental tectonic boundary that separates the distal, Wallowa island-arc terrane from the Baker accretionary-complex terrane. We propose that this terrane boundary is an example of a broad zone of imbrication made up of slabs and slices of arc crust tectonically mixed within an accretionary complex, providing an on-land, ancient analogue to the actualistic arc-arc collisional zone developed along the margins of the Molucca Sea of the central equatorial Indo-Pacific region.

Seismotectonic Model of the Kangra-Chamba Sector of Northwest Himalaya: Constraints from Joint Hypocenter Determination and Focal Mechanism

The existing seismological network in the Kangra-Chamba sector has been upgraded with 12 three-component digital seismometers to obtain new insight on the nature and sources of continued clustered seismicity in this part of northwest Himalaya. A combination of travel-time-distance plots and travel-time inversion of P and S phases have been used to derive a 1D velocity model for the region. The mini- mum 1D velocity model divides the average 44 km thick crust into four layers. The top ∼10 km thick layer represents the metamorphosed sediments of the Chamba nappe that dominates the surface geology of the study area. Suggestion of a thin low-velocity layer at 15 km depth possibly marks the detachment zone separating the downgoing Indian plate from the overriding wedge. The improved locations of epicenters show close clustering of seismic events immediately northeast of the epicenter of the 1905 Kangra earthquake, while away from this zone the seismicity in the Chamba sector has more even distribution. In the later sector, space-depth distribution of hypocenters suggests that strain resulting from the ongoing collision of the Indian plate with Asia is being consumed by reverse-fault movement on the Chamba thrust. The clustered seismicity in the Kangra sector has three distinct source regions and mechanisms: (1) southward displacement of the thick Chamba nappe sheet over the Panjal imbricate zone along the Panjal thrust accounts for the seismicity at shallow depths of less than 7 km, (2) the nucleation of strains where the northeast dipping main boundary thrust (MBT) merges with the detachment plane produces focused seismicity near this junc- tion, and (3) the seismicity in a small pocket below the plane of detachment appears to be a consequence of stresses generated at the base of the northeast dipping detachment plane by the transverse structure.

Facies and ground‐penetrating radar characteristics of coarse‐grained beach deposits of the uppermost Pleistocene glacial Lake Algonquin, Ontario, Canada

AbstractThe lithofacies of the uppermost Pleistocene (ca 11 800 to 10 400 14C yr bp), cold‐temperate, coarse‐grained beach deposits of Lake Algonquin, the precursor of the present Lake Huron of North America, have been studied and interpreted based on analogous features of modern beaches from the same region. Ice foot and ice‐cementation develop during winter but, unlike Arctic beaches, ice‐related sedimentary features are seldom, if ever, preserved in the Pleistocene and recent deposits of the Great Lakes. Instead, the deposits retain the typical characteristics of wave‐dominated, pure gravel and mixed sand and gravel beaches, there including the classical subdivision of infill zone, swash zone/sand run, imbricated zone, coarse flat‐clast zone and coastal dunes. These zones form a regular succession on the surface of many modern beaches; however, they seldom occur as quasi‐complete vertical successions in older deposits. In the studied uppermost Pleistocene deposits, the various components are separated vertically by erosional contacts (bounding surfaces) readily recognizable on working faces of large sand and gravel pits and mappable in the subsurface by ground‐penetrating radar. The lithofacies are sufficiently diagnostic to allow recognition of depositional settings, and the lithofacies architecture allows the deciphering of important geological events, such as: (i) local input of fluvial material onto the shoreface, where it was partially reworked by waves and moved onto the beachface; (ii) occurrence of major storm events; and (iii) repeated rapid transgressions and regressions typical of the glacial‐lake precursors of the modern Great Lakes.

Phase equilibria modelling of kyanite‐bearing anatectic paragneisses from the central Grenville Province

AbstractKyanite‐bearing paragneisses from the Manicouagan Imbricate Zone and its footwall (high‐P belt of the central Grenville Province) preserve evidence of partial melting with development of metamorphic textures involving biotite–garnet ± kyanite ± plagioclase ± K‐feldspar–quartz. Garnet in these rocks displays a variety of zoning patterns with respect to Ca. Pseudosection modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO) system using measured bulk rock compositions accounts for the textural evolution of two aluminous and two sub‐aluminous samples from the presumed thermal peak to conditions at which retained melt solidified. The prograde features are best explained by pseudosections calculated with compositions to account for melt loss. The intersection of isopleths of grossular content and Fe/(Fe + Mg) relating to large porphyroblasts of garnet provide constraints on the P–T conditions of the metamorphic peak. These P–T estimates are considered to be minima because of the potential for diffusional modification of the composition of garnet at high‐T and during the early stages of cooling. However, they are consistent with textural observations and pseudosection topology, with peak assemblages best preserved in rocks for which the calculated pseudosections predict only small changes in mineral proportions in the P–T interval, in which retrograde reactions are inferred to have occurred between the thermal peak and the solidus. Maximum P–T conditions (14.5–15.5 kbar and 840–890 °C) and steep retrograde P–T paths inferred for rocks from the Manicouagan Imbricate Zone are comparable with those determined for mafic rocks from the same area. In contrast, maximum P–T conditions of 12.5–13 kbar and 815–830 °C and flatter P–T paths are inferred for the rocks of the footwall to the Manicouagan Imbricate Zone. The general consistency between textures, mineral compositions and the topologies of the calculated pseudosections suggests that the pseudosection approach is an appropriate tool for inferring the P–T evolution of high‐P anatectic quartzo‐feldspathic rocks.

Numerical simulation of fault development in fold-and-thrust belt of Nepal Himalaya

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 111

A tectonic interpretation of “Eburnean terrane” outliers in the Reguibat Shield, Mauritania

The Reguibat Shield comprises a western “Archaean terrane” and eastern “Eburnean terrane” juxtaposed during the early Palaeoproterozoic Eburnean Orogeny. Metasedimentary rocks of probable Palaeoproterozoic age are preserved as flat-lying klippen (Kediat Ijil and Guelb Zednes) and steep imbricate zones (El Mahaoudat range and Sfariat Belt). These are interpreted to record a phase of thrust tectonics that emplaced a continental margin succession onto a composite Archaean foreland prior to ca. 2.06 Ga sinistral transcurrent deformation. Together, these events reflect partitioned Eburnean transpression.

Microstructural evolution within a partitioned midcrustal transpression zone, northeast Greenland Caledonides

The thrust‐related Imbricate Zone and the sinistral strike‐slip Storstrømmen Shear Zone (SSZ) are two major orogen‐parallel ductile shear zones that mark the western limit of intense Caledonian deformation in northeast Greenland and transect the orogenic hinterland, respectively. Detailed microstructural analysis of associated mylonitic fault rocks from Dronning Louise Land and Hertugen af Orléans Land has been carried out to provide insights into the evolution of these structures and to constrain the relative timing of movement. Amphibolite‐facies mylonites were developed across the entire ca. 8‐km‐wide SSZ and are currently exposed across an area ca. 3–4 km wide within the center of the shear zone. In contrast, retrograde greenschist‐facies mylonites are concentrated within a ca. 2‐ to 4‐km‐wide belt along the western margin of the shear zone. This metamorphic pattern is interpreted to reflect progressive localization of strain associated with exhumation and syntectonic cooling. The ca. 15‐km‐wide Imbricate Zone consists of numerous east‐dipping ductile thrust sheets. At low structural levels, mylonitic fabrics indicate that thrust‐related deformation occurred at middle‐greenschist facies. Toward higher levels, however, microstructures suggest that the metamorphic grade progressively increases. We propose that ductile thrusting across the Imbricate Zone was synchronous with uplift and exhumation of the orogenic hinterland and SSZ, accounting for the observed metamorphic variations. This implies that both structures were active broadly contemporaneously during regional partitioned transpression, an inference supported by scattered 40Ar/39Ar and recent U‐Pb zircon ages. The microstructural observations therefore support models that involve sinistrally oblique Silurian‐Devonian collision between Laurentia and Baltica.

A preliminary study on the lithospheric thermal-rheological structure of the East Qinling orogenic belt

This paper considers the lithospheric rheological structure of the East Qinling orogenic belt to explore its geodynamics. The lithospheric rheological structure was calculated by the constraints of the lithospheric temperature structure. The thermal-rheological stratification structures of the lithosphere in the East Qinling orogenic belt present different features from each other within different tectonic units. The hinterland fault-bounded fold zone (HLZ) and the North Qinling thick-skinned imbricated thrust zone (NQL) in the northern half part of the Qinling orogen, with a temperature of 305°C for the Moho boundary, are characterized by “cold” geotherm, thickened lithosphere and the model C for rheological stratification structure. The South Qinling tectonic zone (SQL), with a mean temperature of 642°C and a high temperature of 826°C for the Moho boundary, has obvious features with the model H of “hot” geotherm, thinned lithosphere and intensive rheological behavior within moderate-lower crust and top of the upper mantle. During post-orogenesis, the NQL, being the convergent frontal region of continental subduction beneath the Qinling orogen by both the North China craton (NC) and Yangtze craton (YZ), is in a coexistence period of a dominantly thickened lithosphere and an initial delamination, and the SQL, probably under pluming, has been developing new delamination and underplating and partial melting within the crust in its axel area and recycling for mass and energy (in the forms of heat transfer and convection) between the crust and mantle.

Oxygen and sulfur isotope geochemistry revealing a significant crustal signature in the genesis of the post-collisional granitoids in central Anatolia, Turkey

Late Cretaceous granitoid rocks from central Anatolia comprise S-I-A-type plutons derived from the collisional stages of the Neo-Tethyan convergence system in central Turkey. These granitoids intrude the tectonic imbrication zone consisting of blocks of supra-subduction zone-type (SSZ-type) central Anatolian ophiolite and crustal metasediments which are present in the İzmir-Ankara-Erzincan suture zone. The plutons are overlain by Late Palaeocene-Early Eocene or younger detrital sediments. Granitoid formation is thought to be related to magma generation processes occurring in a post-collisional lithospheric detachment-related geodynamic setting that resulted from slab break-off or lithospheric delamination. Whole-rock S and quartz/feldspar O isotope data from these plutons yields a broad range of values, and both parameters indicate a nearly exclusively supracrustal origin for the S-type granites, as well as a significant crustal contribution in the genesis of the hybrid I-type and A-type granitoids. The more mafic I-type and A-type granitoid rocks of any given suite have lower S and O values, indicative of their larger degree of mantle component. The combined stable isotope geochemical compositions, when coupled with major, trace and REE geochemistry and regional geology, provide evidence that the significant crustal contribution originated from a metasomatized mantle layer which was affected by earlier SSZ-derived fluids, and then accreted into the subcontinental lithosphere as collision occurred. The partial melting of such a metasomatized mantle layer in a post-collisional extensional geodynamic setting, supplied either by the slab break-off or lithospheric delamination mechanisms, provided the significant crustal signature in the hybrid magmas of the Late Cretaceous I-type and A-type granitoids in central Anatolia, Turkey.