Contractional Deformation Research Articles

Timing and spatial patterns of Cretaceous and Cenozoic inversion in the Southern Permian Basin

Abstract Mesozoic extensional basins of the Southern Permian Basin (SPB) System became inverted from Late Cretaceous time onwards. Following a first Cretaceous ‘Subhercynian’ pulse of contractional deformation and basin uplift, several distinct inversion events of Cenozoic age were often described. The oldest of these is the ‘Laramide’ event of Paleocene age which coincides with the termination of chalk deposition and widespread regression around the North Sea Basin, whose axial part continued to subside. The spatial extent of these effects is too wide to be compatible with inversion by folding and reverse faulting. The width of the uplifting and subsiding regions was also too large to be consistent with folding of the entire lithosphere under tangential compression. There appears to be no unequivocal evidence of discrete structures formed or reactivated in the Laramide event. By contrast, well-documented younger inversion of approximately Late Eocene to Late Oligocene–(Miocene?) age affected the region from the Celtic Sea to the western Netherlands. The associated deformation is weaker than that of the Late Cretaceous event and spatially overlaps with it only in the Southern North Sea. Structural inversion of the SPB thus comprised only two events separated in time and mostly also in space.

Middle to Late Miocene Contractional Deformation in Costa Rica Triggered by Plate Geodynamics

AbstractContractional deformation in Costa Rica is usually attributed to the subduction of the aseismic Cocos Ridge. In this work, we review the evidences for contraction in the middle to late Miocene, prior to the arrival of the Cocos Ridge at the Middle America Trench. We find that the Miocene phase of contractional deformation is found in all of Costa Rica, probably extending to Nicaragua as well. The widespread distribution of this event requires a regional or plate geodynamic trigger. We analyze the possible mechanisms that could produce the onset of contractional deformation, using the better known case of subduction orogeny, the Andes, as an analog. We propose that a change in the direction of the Cocos plate since ∼19 Ma led to a change from oblique to orthogonal convergence, producing contractional deformation of the upper plate.

Thermal insulation characteristics of a lightweight, porous nanomaterial in high-temperature environments

Thermal-insulating nanomaterials with excellent thermal insulation performance are one type of thermal protection material used in spacecraft. In this study, the high-temperature insulation characteristics of a lightweight, porous aluminum oxide (Al2O3) nanomaterial were studied through experimentation using a self-developed thermal testing system for high-speed spacecraft, and were calculated by numerical simulation. The results showed that in a 1200°C front-surface, high-temperature environment, an Al2O3 nanomaterial sheet with a thickness of only 10mm could reduce the temperature by over 70% while exhibiting stable thermal insulation performance. This demonstrates that the Al2O3 nanomaterial has excellent high-temperature insulation performance. The scanning electron microscopy (SEM) images showed that, after the temperature exceeded 1200°C, the aggregation and growth of the Al2O3 nanoparticles accelerated, and single Al2O3 nanoparticles and voids increased significantly in size; in addition, the fibers inside the material started to melt, and the cracks started to increase considerably in number, depth, and width. Furthermore, a significant contraction and bending deformation occurred at the edges of the Al2O3 nanomaterial sheet; therefore, the Al2O3 nanomaterial is suitable for use in a thermal environment below 1200°C. The results provide an important reference basis for the design of thermal protection systems for spacecraft.

Zircon and allanite U-Pb ID-TIMS ages of vaugnerites from the Calzadilla pluton, Salamanca (Spain): dating mantle-derived magmatism and post-magmatic subsolidus overprint

Basic to intermediate high-K, high-Mg mantle-derived rocks occur throughout the Iberian Massif and are particularly important in the Tormes Dome, where vaugnerites form several stocks and small plutons. One of the largest and geochemically most variable among these plutons is the Calzadilla pluton in the Tormes Dome that crystallized at 318 ± 1.4Ma (Bashkirian; U-Pb TIMS zircon). This age reveals that the vaugnerite pluton was emplaced during the transition from late D2 extensional deformation to early D3 contractional deformation (319 to 317Ma). Large-scale extension in the area resulted, on one hand, in extensive anatexis in the crust due to quasiisothermal decompression and mica-dehydration melting and, on the other hand, in the upwelling of the mantle, which induced partial melting of the enriched domains in the lithospheric mantle. The driving reason why crustal and mantle melts were coeval is extension. The U-Pb ID-TIMS age of allanite is not related to the emplacement nor cooling of the Calzadilla vaugnerite, but it seems to be related to a younger subsolidus overprint ca. 275Ma that, in the scale of the Central Iberian Zone, corresponds to a period of hydrothermal alteration, including episyenite formation and tungsten mineralization.

Closure type effects on the structural pattern of an inverted extensional basin of variable width: Results from analogue models

In order to further understand the controls on the structural configuration of inverted basins, previous research has vastly explored the role of rift-stage stress field variations on the resulting structure produced by basin inversion. Analogue modeling has broadly enabled geoscientists to gain insight into the controls on the structural evolution during basin inversion, such as: the inherited structural array of the basin, sedimentary load or sedimentary thickness, plan-view shape of the basin (length and width), and variations on the governing stress field, among other factors. This work sheds light on the influence exerted by the closure style on inversion of an elongated model basin with a variable width, generated by differential extension. We subsequently induced inversion by modifying the orientation of the post-rift contractional stress field: (i) in a first model, contraction is homogeneous along-strike (parallel to the major axis of the basin); and (ii) in a second model, contraction is heterogeneous along-strike and exerted in the same orientation and amount in which extension was induced (about a pivot point). We focus on the three-dimensional geometry of the structures generated by inversion, their vergence and surficial trace, and where they are prone to concentrate within the basin, to finally analyze and compare our results with natural examples of inverted Andean basin systems. Our results indicate that most of the contractional deformation imposed in the analog models is absorbed in the interiors of the basin in its widest zones by means of inverted normal faults and backthrusts. However, when the amount of shortening is higher than extension, deformation is propagated outside the basin where shortcuts and new oblique reverse faults are dominant, which promote a major uplift.

Late Cretaceous–Cenozoic basin evolution and topographic growth of the Hoh Xil Basin, central Tibetan Plateau

The Hoh Xil Basin in the central Tibetan Plateau was the locus of thick accumulation of terrestrial sediments from the Late Cretaceous to Neogene, providing an archive of regional tectonic activity both before and after initiation of the early Paleogene India-Asia collision. This work focuses on the stratigraphy and provenance of the poorly studied western part of the Hoh Xil Basin. Lithostratigraphy, carbonate oxygen isotopic composition, and deformation features suggest that the braided river and alluvial-fan that dominated the Kangtuo Formation in the southern West Hoh Xil subbasin can be correlated with the Late Cretaceous-early Eocene Fenghuoshan Group deposits of the East Hoh Xil subbasin, whereas deltaic, shallow lacustrine, and evaporite strata of the Suonahu Formation in the central-northern West Hoh Xil subbasin can be correlated to the middle Eocene-early Oligocene-Yaxicuo Formation of the East Hoh Xil subbasin. These observations suggest that the eastern and western parts of Hoh Xil Basin evolved as a unified basin during Late Cretaceous-Oligocene time. Detrital zircon U-Pb ages indicate that the Kangtuo Formation and Feng-huoshan Group mainly received detritus from the Qiangtang terrane to the south, whereas the age probability distributions of the Suonahu Formation and Yaxicuo Formation sandstones are more similar to zircon ages reported for the northern Qiangtang terrane and within the Hoh Xil terrane. We favor a tectonic model with Late Cretaceous-early Eocene contractional deformation and topographic growth of the south-central Qiangtang terrane that caused flexural subsidence of the south-central Hoh Xil region within a retroarc foreland basin, relative to the Tanggula thrusts in central Qiangtang. Following the onset of India-Asia collision, the locus of contractional deformation moved northward, marked by activity of the Fenghuoshan and Hoh Xil thrust systems within the Hoh Xil region, and resulting in transformation of the Hoh Xil foreland basin to a hinter-land basin. We suggest that both middle Eocene to late Oligocene upper-crustal shortening and associated thickening of the lower crust and mantle lithosphere as well as late Oligocene to Miocene loss of dense mantle lithosphere contributed to the topographic growth of the Hoh Xil Basin.

Juxtaposition of allochthonous terranes in the central Korean Peninsula: Evidence from zircon U-Pb ages and O-Hf isotopes in Jurassic granitoids

The eventual docking of an exotic continental mass over the course of a Wilson cycle is marked by intracontinental sites of prograde metamorphism, contractional deformation, and syn- or post-collisional magmatism. This study shows that inherited premagmatic zircon in subsequently emplaced granitoid rocks provides another line of evidence for the collisional orogeny. Following the continental collision between the North and South China Cratons, Jurassic granitoids intruded widely throughout the central Korean Peninsula in association with the subduction of the Paleo-Pacific plate. The latest Early to Middle Jurassic (177–167Ma) granitoids occur throughout the cratonic Gyeonggi Massif, whereas Early Jurassic (194–184Ma) granitoids occur locally along the Okcheon Metamorphic Belt to the southeast of the massif. These two spatiotemporal groups differ profoundly in their age patterns and O-Hf isotopic signatures of inherited zircon. The younger group yields a typically North China-like Archean-Paleoproterozoic (mostly 2.5–1.8Ga) zircon inheritance. In contrast, inherited zircons from the older group exhibit exclusively Neoproterozoic (ca. 0.9–0.7Ga) and Paleozoic (ca. 0.4Ga) cluster along with Archean-Paleoproterozoic populations. The exotic feature of the protolith of the older group is further indicated by significantly lower δ18O (−0.9 to +4.9‰) and positive εHf (+3 to +15) values of the Cryogenian zircon cores. The results of zircon analysis demonstrate that South and North China-like terranes were juxtaposed infracrustally along the southeastern margin of the Gyeonggi Massif during the pre-Jurassic collisional orogeny, and then selectively provided major source material to the older and younger Jurassic granitoids, respectively.

P–T–time evolution of the Mejillones Metamorphic Complex: Insights into Late Triassic to Early Jurassic orogenic processes in northern Chile

Better constrained pressure-temperature (P-T) histories of metamorphic complexes along the Andean continental margin are important for understanding the late Paleozoic and Mesozoic tectonic evolution of the southwestern margin of Gondwana. The Mejillones Metamorphic Complex of the northern Chilean Coastal Cordillera is composed of two tectonic units, the Morro Mejillones and Morro Jorgiño blocks. These units are bound by the NW-trending Caleta Herradura fault and show distinctly metamorphic ages and thermal evolution. The Morro Mejillones block was metamorphosed at low pressure conditions (andalusite-sillimanite series) during the intrusion of tonalitic plutons at ca. 208Ma, as indicated by available geochronological data. In contrast, the Morro Jorgiño block comprises amphibolite-facies schists, gneisses and foliated metabasites with characteristic garnet–bearing mineral assemblages. For garnet–bearing pelitic gneisses, a clockwise P-T path has been determined from pseudosection modelling in the MnNCKFMASHTO system. The proposed evolution is characterized by a pressure increase from 7.5 to 8.5kbar at increasing temperatures from 585 to 615°C. Decompression to 6kbar followed, accompanied by heating to 630–640°C. Electron microprobe Th-U-Pb in-situ dating of high-Y monazite grains yielded a weighted average age of ca. 190±4Ma, which is interpreted as the age of tectonic burial of metamorphic rocks of the Morro Jorgiño tectonic unit. We infer that the block was buried to ~25km depth through contractional deformation of the continental edge in a subduction zone, likely linked to the docking of the Mejillonia terrane. Rapid exhumation followed and the ensuing juxtaposition of both tectonic units was controlled by Jurassic transtensional activity of the Atacama Fault System.

Fluid-solid coupling dynamic equations considering gas desorption con-traction and coal motion deformations

The coals are assumed as isotropic homogeneous saturated porous media, dynamic characteristics of porosity and permeability of coals and coal deformation caused by gas desorption are all considered, gas seepage equations about porosity, permeability and desorption contraction deformation are built based on Darcy’s law, and fluid-solid coupling equations for gas seepage are finally built, which are composed of the above-mentioned dynamic balance differential equations and gas seepage equations. In the end, one horizontal gas drainage hole is taken as a computational example, the gas pressures calculated by the new model built in this paper are compared with those by the normal model that does not include gas adsorption and motion deformation of coals, the new model is proved to be reasonable based on the mechanism of desorption contraction and seepage, and the characteristics of permeability, gas pressure and radial displacements around the hole are analyzed based on the new model.DOI: http://dx.doi.org/10.5755/j01.mech.23.3.18480

S-type granite generation and emplacement during a regional switch from extensional to contractional deformation (Central Iberian Zone, Iberian autochthonous domain, Variscan Orogeny)

Zircon grains extracted from S-type granites of the Meda-Escalhao-Penedono Massif (Central Iberian Zone, Variscan Orogen) constrain the timing of emplacement and provide information about potential magma sources. Simple and composite zircon grains from three samples of S-type granite were analyzed by LA-ICP-MS. New U–Pb data indicate that granites crystallized in the Bashkirian (318.7 ± 4.8 Ma) overlapping the proposed age range of ca. 321–317 Ma of the nearby S-type granitic rocks of the Carrazeda de Anciaes, Lamego and Ucanha-Vilar massifs. The timing of emplacement of such S-type granites seems to coincide with the waning stages of activity of a D2 extensional shear zone (i.e. Pinhel shear zone) developed in metamorphic conditions that reached partial melting and anatexis (ca. 321–317 Ma). Dykes of two-mica granites (resembling diatexite migmatite) are concordant and discordant to the compositional layering and S2 (main) foliation of the high-grade metamorphic rocks of the Pinhel shear zone. Much of the planar fabric in these dykes was formed during magmatic crystallization and subsequent solid-state deformation. Field relationships suggest contemporaneity between the ca. 319–317 Ma old magmatism of the study area and the switch from late D2 extensional deformation to early D3 contractional deformation. Inherited zircon cores are well preserved in these late D2-early D3 S-type granite plutons. U–Pb ages of inherited zircon cores range from ca. 2576 to ca. 421 Ma. The spectra of inherited cores overlap closely the range of detrital and magmatic zircon grains displayed by the Ediacaran to Silurian metasedimentary and metaigneous rocks of the Iberian autochthonous and parautochthonous domains. This is evidence of a genetic relationship between S-type granites and the host metamorphic rocks. There is no substantial evidence for the addition of mantle-derived material in the genesis of these late D2–early D3 S-type granitic rocks. The eNd arrays of heterogeneous crustal anatectic melts may be just inherited from the source, probably reflecting mixing of a range of crustal materials with different ages and primary isotopic signatures. The generation of the Bashkirian S-type granites has been dominated by continental crust recycling, rather than the addition of new material from mantle sources.

The Numerical Study of Steel Plate Forming by Moveable Induction Heating Considering the Plate Edge Shrinkage

In this article, the line heating forming technology by using the moveable induction heating is used to form a concave-type plate. In shipyard, it is necessary to get a bigger contraction deformation at the plate edge for the concave plate. However, the heat input of the metal near the plate edge is less than that of the internal metal along the heating line due to the decrease of heat power near the plate edge, thus causing smaller transverse shrinkage of the plate edge. To solve this problem, the paper investigates the suitable heating way of the inductor for the concave plate forming. An electromagnetic coupling finite element model is built to simulate the moveable induction heating process and reveal different heating and forming characteristic of different inductor heating ways. The simulation results are verified by experiments data. Research results showed that the heating way by which the inductor moves out of plate edge is an effective heating technology which meets the forming requirement of the concave plate.

Late Cretaceous extension and exhumation of the Stong and Taku magmatic and metamorphic complexes, NE Peninsular Malaysia

Fragmentation of large continental areas by post-orogenic extension requires favourable geodynamic conditions and frequently occurs along pre-existing suture zones or nappe contacts, as exemplified by the Stong and Taku magmatic and metamorphic complexes of northern Peninsular Malaysia. For this case, we have employed a field and microstructural kinematic study combined with low temperature thermo-chronology to analyse the tectonic and exhumation history. The results show that the Late Palaeozoic - Triassic Indosinian orogeny created successive phases of burial related metamorphism, shearing and contractional deformation. This orogenic structure was subsequently dismembered during a Cretaceous thermal event that culminated in the formation of a large scale Late Santonian - Early Maastrichtian extensional detachment, genetically associated with crustal melting, the emplacement of syn-kinematic plutons and widespread migmatisation. The emplacement of these magmatic rocks led to an array of simultaneously formed structures that document deformation conditions over a wide temperature range, represented by amphibolite- and greenschist- facies mylonites and as well as brittle structures, such as cataclastic zones and normal faults that formed during exhumation in the footwall of the detachment. The formation of this detachment and a first phase of Late Cretaceous cooling was followed by renewed Eocene - Oligocene exhumation, as evidenced from our fission track ages. We infer that an initial Cretaceous thermal anomaly was responsible for the formation of an extensional gneiss dome associated with simple shear and rotation of normal faults. These Cretaceous processes played a critical role in the establishment of the presently observed crustal structure of Peninsular Malaysia.

First records of syn-diagenetic non-tectonic folding in quaternary thermogene travertines caused by hydrothermal incremental veining

This study is the first documentation of syn-diagenetic non-tectonic contractional deformations observed in two Pleistocene thermogene travertine deposits from the late Miocene-Pleistocene Tuscan extensional-hydrothermal province (Italy). The deposits consist of primary porous beds hosting secondary bed-parallel carbonate veins. The porous beds are generally flat-lying, particularly in the upper section of the deposits, whereas the veined beds frequently form undulated structures. These structures are up to a few meters in wavelength, are mostly confined within the lower-middle section of the deposits, and are here mostly interpreted as folds. Field observations, U-Th geochronology, and stable isotope analyses are used to characterize the origin of veins and folds. Radiometrically-determined age inversions, structure overprinting relationships, downward growth of vein crystals, deformation of primary sedimentary structures, and downward increasing frequency of veins and folds show that the undulated travertine beds can be mainly interpreted as the product of syn-diagenetic hydrothermal rejuvenation causing non-tectonic veining and folding. The non-tectonic hypothesis is also supported by the absence of contractional deformation in the travertine-hosting sediments. The folds were generated by complex mechanisms including bending and buckling caused by laterally-confined volume expansion during syn-diagenetic circulation of mineralizing fluids and related incremental veining. Modeling some folds with the Biot-Ramberg's buckling equation shows a vein-to-host travertine viscosity ratio between 1.5 and 4, confirming the syn-diagenetic origin of folds. Veining and folding changed some original properties of travertines including rheology, fabric, porosity, and chronological sequence. The identification of these structures and related changes of rock properties (e.g., age rejuvenation) is relevant for the proper interpretation of thermogene travertines as recorders of many geological processes including paleoclimate oscillations, earthquake occurrences, hydrothermal circulations, and uplift or incision rates. Synthesizing, the studied thermogene travertines appear more like sedimentary-hydrothermal-hybrid zoned deposits rather than pure sedimentary systems uniquely responding to the superposition law.

Constraints on the geodynamic evolution of the southern Superior Province: U-Pb LA-ICP-MS analysis of detrital zircon in successor basins of the Archean Abitibi and Pontiac subprovinces of Ontario and Quebec, Canada

To investigate the tectonic development of the southern Superior Province, new U-Pb laser ablation inductively-coupled mass-spectrometry analysis of detrital zircon was conducted on graywacke samples from successor basins of the Abitibi and Pontiac subprovinces. The three successor basin successions investigated are the 2690–2685Ma Porcupine assemblage and the 2679–2669Ma Timiskaming assemblage of the Abitibi subprovince, and the ∼2682Ma sedimentary rocks of the Pontiac subprovince. Predominately Neoarchean zircon grains (80–95% of the total zircon population) reflect local sources in the Abitibi and Pontiac subprovinces. The successor basins were deposited within 100Ma of the majority of the detrital zircon grains primary crystallization ages, which is consistent with patterns observed at modern convergent margin settings. Porcupine assemblage samples contain ∼5% Mesoarchean zircon, while Pontiac subprovince and Timiskaming assemblage samples contain ∼18% and ∼13%, respectively. No local Mesoarchean sources exist in the Abitibi or Pontiac subprovinces, suggesting input from an adjacent hinterland. The higher proportion of Mesoarchean zircon in the Timiskaming assemblage relative to the Porcupine assemblage suggests that detritus from the hinterland was more prevalent during the later stages of collision, probably as a result of progressive uplift and denudation of the hinterland. The high proportion of Mesoarchean zircon in the Pontiac subprovince sedimentary rocks may reflect an additional Mesoarchean source. Comparison to previously published geochronology data for the Superior Province indicates that the hinterland was to the northwest and is, in part, composed of the Winnipeg River, Marmion, and Opatica subprovinces. Patterns of contractional deformation, successor basin formation, and felsic plutonism progressively young to the south in the southern Superior Province and are interpreted in terms of accretionary processes. Detrital transport across subprovince or terrane boundaries, progressive hinterland emergence, and foreland-directed propagation of the deformational front recorded by successor basins is similar to that observed in modern-style collisional or accretionary orogens.

Platform-basin transitions and their role in Alpine-style collision systems: a comparative approach

The structure of Alpine-type orogenic belts is widely assumed to have been strongly influenced by the inherited structure of the rifted continental margins from which they were formed. The challenge lies in deciphering these influences when orogenic contraction is strong. Contractional deformation in the Kamchia basin (SW Black Sea), caught between the Balkan orogenic belt and the stable Moesian block, has been arrested, preserving the early development of an Alpine-type orogen. We use 3D seismic and well-data to examine the tectonic and stratigraphic structure of this basin. Significant deformation has occurred underwater, through inversion tectonics within the basin itself. However, the basin margin structures have not reactivated. This margin is marked by a low-angle unconformity across which sediment was routed from the platform into the fledgling syn-orogenic basins. Such pathways may explain non-orogen-derived sediment within parts of the ancestral Apennine foredeep, for example. The role of platform margins to focus contractional deformation may have been over-emphasised elsewhere in the Alpine system. Further, the more strongly subsided portions of rifted margins may have accommodated significant contractional deformation through reactivation of basin faults. In the western Alps the complexity of structural juxapositions across thrust sheets in the Brianconnais may reflect these early-orogenic deformations rather than be the product of deformation that happened after tectonic burial. These early deformations are likely to be recorded in early syn-tectonic depositional sequences.

Midcrustal shearing and doming in aCenozoic compressive setting along theAilaoShan‐RedRiver shear zone

AbstractThe Cenozoic Xuelong Shan antiformal dome is located along the northern segment of the Ailao Shan‐Red River shear zone in Yunnan, China. Subhorizontal foliation in the gneiss core is recognized, representing a broad top‐to‐NE shear initiated under amphibolite facies conditions and propagating into greenschist facies in the mantling schist and strike‐slip shear zone. Microfabrics of crystallographic‐preferred orientations (CPOs) in quartz suggest that the deformation temperatures increased with increasing structural depth from the upper crust (300–500°C) in the mantling schist to the midcrust (15 km or more, ≥650°C) in the gneissic core. This trend is mirrored by variations in the metamorphic grade of the syn‐kinematic mineral assemblages and microstructures, which range from garnet + amphibole + biotite + sillimanite + rutile + feldspar in the core to garnet + staurolite + biotite + epidote + muscovite within the limb units. The dome experienced the following deformation history: (1) a broad top‐to‐NE shear in the subhorizontal foliation of the gneiss core during the first stage of deformation (D1); (2) opposing reverse‐sense shear along the two schist limbs of the dome during contraction‐related doming (D2–D3); (3) sinistral strike‐slip shearing within the eastern limb (D4); and (4) extensional deformation (D5). The structural‐thermal patterns suggest the antiformal dome formation was roughly coeval with top‐to‐NE ductile shearing in the midcrust of Tibet at 32 Ma or earlier. A major implication is that there was a phase of contractional ductile deformation in the region prior to the initiation of strike‐slip deformation.

A Flexible Wall Permeameter for Investigating Suffusion and Suffosion

A flexible wall permeameter was described for the investigation of seepage-induced internal instability. The device comprises a double-walled triaxial cell, a base pedestal and top cap, a seepage control system, and instrumentation. A novel feature is the measurement of volume change during multistage seepage flow. Tests were reported on gradations of glass beads. Suffusion was observed as a seepage-induced increase in hydraulic conductivity, in the absence of substantial volumetric and axial strains, indicating particle migration. In contrast, suffosion was observed as a change in hydraulic conductivity, accompanied by substantial contractive volumetric strain with or without substantial axial strain, indicating rearrangement of the particle assembly. The test procedure yielded repeatable results. The resolution and accuracy of the measurement technique were sufficient to quantify the onset and progression of deformation in specimens with a volume of approximately 780 cm3. Considering the localized nature of the contractive volumetric deformation associated with suffosion, which can occur in the absence of axial deformation, measurement of volume change during multistage seepage flow proves necessary to avoid any misinterpretation of the phenomenological response to seepage flow.

The thrust contact between the Canastra and Vazante groups in the Southern Brasília Belt: structural evolution, white mica crystallinity and implications for the Brasiliano orogeny

ABSTRACT: Two regional thrust-sheets of Neoproterozoic metasedimentary rocks occur in the Southern Brasília Belt, northwest Minas Gerais. The lower one comprises the Vazante Group, that is formed in the studied area, from base to top, by the Serra do Garrote (metapelites interlayered with carbonaceous phyllite), Serra do Poço Verde (beige to pink stromatolitic metadolomite with interlayered greenish slates), Morro do Calcário (gray stromatolitic metadolomite interlayered with gray slates) and Serra da Lapa (phyllite with dolarenitic lenses interlayered with slates) formations. The upper thrust sheet consists of the Canastra Group (Paracatu formation): laminated sericite phyllites and carbonaceous phyllites interlayered with quartzite. The Braziliano orogeny resulted in four phases of contractional deformation, associated with low-grade metamorphism. The first two (D1 and D2) are ductile, while the third and fourth ones (D3 and D4) are brittle-ductile. D1 developed a slaty S1 cleavage subparallel to the primary layering, with shallow to steep dips to NW. D2 developed a crenulation cleavage (S2) that dips moderately to NW and is associated with tight to isoclinal folds. D3 and D4 phases developed crenulations and open folds and kink bands. S3 dips steeply to NW, while S4 has moderate to steep dips to NE and SW. White mica crystallinity (Kübler index) measurements in metapelites indicate that both the Canastra and Vazante groups reached anchizone/epizone conditions, and metamorphic discontinuities along thrusts indicate that the peak of metamorphism is pre or syn-thrusting.

Plate-tectonic evolution of the Earth: bottom-up and top-down mantle circulation

Intense devolatilization and chemical-density differentiation attended accretion of planetesimals on the primordial Earth. These processes gradually abated after cooling and solidification of an early magma ocean. By 4.3 or 4.2 Ga, water oceans were present, so surface temperatures had fallen far below low-pressure solidi of dry peridotite, basalt, and granite, ∼1300, ∼1120, and ∼950 °C, respectively. At less than half their T solidi, rocky materials existed as thin lithospheric slabs in the near-surface Hadean Earth. Stagnant-lid convection may have occurred initially but was at least episodically overwhelmed by subduction because effective, massive heat transfer necessitated vigorous mantle overturn in the early, hot planet. Bottom-up mantle convection, including voluminous plume ascent, efficiently rid the Earth of deep-seated heat. It declined over time as cooling and top-down lithospheric sinking increased. Thickening and both lateral extensional + contractional deformation typified the post-Hadean lithosphere. Stages of geologic evolution included (i) 4.5–4.4 Ga, magma ocean overturn involved ephemeral, surficial rocky platelets; (ii) 4.4–2.7 Ga, formation of oceanic and small continental plates were obliterated by return mantle flow prior to ∼4.0 Ga; continental material gradually accumulated as largely sub-sea, sialic crust-capped lithospheric collages; (iii) 2.7–1.0 Ga, progressive suturing of old shields + younger orogenic belts led to cratonal plates typified by emerging continental freeboard, increasing sedimentary differentiation, and episodic glaciation during transpolar drift; onset of temporally limited stagnant-lid mantle convection occurred beneath enlarging supercontinents; (iv) 1.0 Ga–present, laminar-flowing asthenospheric cells are now capped by giant, stately moving plates. Near-restriction of komatiitic lavas to the Archean, and appearance of multicycle sediments, ophiolite complexes ± alkaline igneous rocks, and high-pressure–ultrahigh-pressure (HP–UHP) metamorphic belts in progressively younger Proterozoic and Phanerozoic orogens reflect increasing negative buoyancy of cool oceanic lithosphere, but decreasing subductability of enlarging, more buoyant continental plates. Attending supercontinental assembly, density instabilities of thickening oceanic plates began to control overturn of suboceanic mantle as cold, top-down convection. Over time, the scales and dynamics of hot asthenospheric upwelling versus lithospheric foundering + mantle return flow (bottom-up plume-driven ascent versus top-down plate subduction) evolved gradually, reflecting planetary cooling. These evolving plate-tectonic processes have accompanied the Earth’s thermal history since ∼4.4 Ga.

The transition zone between the Pernambuco-Alagoas Domain and the Sergipano Belt (Borborema Province, NE Brazil): Geochronological constraints on the ages of deposition, tectonic setting and metamorphism of metasedimentary rocks

Metasedimentary rocks in the transition zone between the Pernambuco-Alagoas Domain and the Sergipano Belt (southern Borborema Province, NE Brazil) were studied in order to place constraints on the geological evolution of this portion of West Gondwana. High-grade metamorphic conditions are recorded by the garnet-sillimanite assemblage and common anatexis in pelitic paragneisses. Two samples dated by LA-ICP-MS in the north yielded a predominance of early Neoproterozoic detrital zircons, with age peaks between 990 Ma and 827 Ma, with the youngest grain suggesting deposition after 670 Ma. Low Th/U zircons (Th/U < 0.1) in these samples display a significant spread in ages, from 691 to 568 Ma. One sample in the south is dominated by a homogenous population of Paleoproterozoic grains, with a 2200 Ma peak. The southernmost sample also contains Paleoproterozoic zircons but the most abundant population is Neoproterozoic, and characterized by age peaks at c. 670, 647 and 623 Ma. These results show that deposition of sediments in the southern PEAL Domain and in the northern Sergipano Belt occurred in the Late Neoproterozoic. The differing age spectra between samples are correlated with potential source rocks in the study area or in its proximity, reflecting variable input from local sources. The data are interpreted to indicate that during the course of an extensional event at c. 673-647 Ma, Tonian granitic intrusions and synextensional metamorphic rocks were unroofed and eroded to provide zircons for sediments deposited in the north, whereas Paleoproterozoic and synextensional magmatic rocks were the main sources for sediments in the south. Peak metamorphic conditions and contractional deformation are constrained to c. 630-600 Ma.