Cratonic Foreland Research Articles

Segmentation of the Caledonian orogenic infrastructure and exhumation of the Western Gneiss Region during transtensional collapse

The (ultra)high-pressure Western Gneiss Region of the Norwegian Caledonides represents an archetypical orogenic infrastructure of a continent–continent collision zone. To test established exhumation models, we synthesize the geochronology and structures of major basement windows and provide new ages from poorly dated areas. Migmatite U–Pb zircon samples date melt crystallization at c. 405 Ma in the Øygarden Complex, expanding the spatial extent of Devonian migmatization. Micas from shear zones in the Øygarden and Gulen domes yield 40 Ar/ 39 Ar ages mostly between 405 and 398 Ma, recording the exhumation of metamorphic core complexes. On a larger scale, the youngest ages of various geochronometers in different segments of the Western Gneiss Region show abrupt breaks (10–30 myr) across low-angle detachments and sinistral transfer zones, which also correspond to metamorphic and structural discontinuities. We explain the segmentation of the orogenic infrastructure by partitioned post-orogenic transtension due to lateral and vertical rheological contrasts in the orogenic edifice (strong cratonic foreland and orogenic wedge v. soft infrastructure). Differential crustal stretching dragged out deep levels of the orogenic crust below low-angle detachments and became progressively dominated by sinistral transfer zones. Collapse obliterated the syn-collisional structure of the orogenic root and resulted in the diachronous exhumation of distinct infrastructure segments. Supplementary material: Complete analytical results are available at https://doi.org/10.6084/m9.figshare.c.5241710

Emplacement of Young Island Arc Crust Over Older Mantle Along a Cratonic Foreland: Constraints From Multiple Isotopes and Elemental Geochemistry

AbstractMany studies have revealed that mantle‐derived mafic rocks can be more isotopically enriched than coexisting crustal felsic rocks. However, its origin remains to be elucidated. Here, we report on some Early Cretaceous (ca. 130 Ma) mafic rocks in Gan‐Hang Belt (South China) that are more isotopically enriched than the temporally and spatially associated granitoids. Relatively depleted Nd‐Hf isotopic signatures [εNd(t) = −0.4 to +0.6, εHf(t) = 3.4 to 4.8, εHf(t)zircon = −2.3 to +9.3], coupled with low zircon δ18OSMOW values (5.33–6.84 ‰) and geographical proximity to the exposed island arc rocks, indicate that the isotopically depleted Daixi alkali feldspar granites were dominantly derived from the relatively young Neoproterozoic accreted Shuangxiwu island arc sequences. Yet new and published whole‐rock Sr‐Nd‐Hf‐Pb‐O isotopic data [87Sr/86Sr)i = 0.70788–0.70833; εNd(t) = −7.6 to −3.5; εHf(t) = −6.8 to −1.9; (206Pb/204Pb)i = 18.028–18.108, (207Pb/204Pb)i = 15.565–15.577, (208Pb/204Pb)i = 38.198–38.341; δ18OSMOW = 4.5–5.5 ‰] highlight that the isotopically enriched Siling hornblende gabbros were predominantly sourced from an older, metasomatically enriched SCLM of adjacent Yangtze cratonic affinity. The arc crustal allochthon is assumed to have been tectonically emplaced onto the southeast Yangtze cratonic foreland, resulting in isotope inversion with younger crust on top of the older lithospheric mantle. Our results reveal a juxtaposition geometry of continental material after collision and accretion of island arcs by using isotope methods, which is independently supported by tectonic geological and geophysical studies. It suggests that ancient lithospheres should exist at depth within certain orogens associated with island arc accretion.

Chapter 13 Siliciclastic sedimentation in a foreland basin to the Sveconorwegian orogen and dolerites (0.98–0.95 Ga) related to intracratonic rifting

Abstract Sub-ophitic, equigranular or plagioclase-phyric dolerite dykes, referred to as the Blekinge–Dalarna dolerite (BDD) swarm, were emplaced during the time span 0.98–0.95 Ga and trend NNE–NNW in an arcuate fashion, parallel to and east of the Sveconorwegian orogen. Dolerite sills are locally present. These rocks are subalkaline to alkaline with a monzogabbroic or gabbroic composition and show a predominantly within-plate tectonic affinity. ɛ Nd and ɛ Hf values fall in the range −2 to +4 and +1 to +5, respectively. Siliciclastic sedimentary rocks (Almesåkra Group) in a small outlier in southern Sweden were deposited in an aeolian to fluviatile or lacustrine environment and an arid or semi-arid warm palaeoclimate, coevally with the dolerite sills. Smaller occurrences of sandstone with peperitic field relationships to the BDD dykes are known from other localities. The spatial distribution, orientation and age of the BDD magmatic suite suggest roughly east–west extension in the eastern, cratonic foreland to the Sveconorwegian orogen during the latest phase of this mountain-building event, the age data tentatively suggesting a younging to the east. The siliciclastic sedimentary rocks represent an erosional relict of a larger and spatially much more extensive early Tonian foreland basin to this orogen, as proposed earlier on the basis of fission-track thermochronology.

Chapter 14 Regional context and lithotectonic framework of the 1.1–0.9 Ga Sveconorwegian orogen, southwestern Sweden

Abstract The 1.1–0.9 Ga Sveconorwegian orogen in southwestern Scandinavia belongs to the global system of mountain belts established during the assembly of the supercontinent Rodinia. An overall north–south structural trend and five lithotectonic units bounded by crustal-scale shear zones characterize this orogen. In Sweden, the Eastern Segment abuts the orogen's cratonic foreland eastwards and is separated from the Idefjorden terrane westwards by a ductile shear zone, up to 5 km thick, displaying a sinistral transpressive component. These two lithotectonic units differ on the basis of their pre-Sveconorwegian accretionary tectonic evolution, and the timing of Sveconorwegian high-pressure metamorphism, anatexis and polyphase deformation. High-pressure granulites and migmatites formed at c. 1.05–1.02 Ga in the Idefjorden terrane; eclogites, high-pressure granulites and migmatites at c. 0.99–0.95 Ga in the Eastern Segment. Magmatic activity and crustal extension progressed westwards at c. 0.98–0.92 Ga. Prior to or at 0.93–0.91 Ga, greenschist facies shear deformation with top-to-the-foreland movement affected the frontal part of the orogen. Geodynamic uncertainties concern the affinity of the Idefjorden terrane relative to Fennoscandia (Baltica), the character of the Sveconorwegian orogenesis, and the contiguous or non-contiguous nature of the erosional fronts of the late Mesoproterozoic–early Neoproterozoic orogens in Sweden and Canada.

The Kock Fault Zone as an indicator of tectonic stress regime changes at the margin of the East European Craton (Poland)

Integrated tectonic interpretation of seismic data and core samples from boreholes in the vicinity of the Kock Fault Zone (KFZ) allowed us to identify several tectonic deformation events that were responsible for creating its complex structure. The KFZ is an example of a mechanically weak regional-scale tectonic structure that accumulated deformation over hundreds of millions of years and therefore is a good indicator of stress regime changes in a broader area. The KFZ is here regarded as a combination and superposition of two genetically and temporally different faults: the older Kock Fault, which is an inverted normal fault, and the younger, low-angle Kock Thrust. The first, Silurian stage of KFZ evolution occurred in a tensional stress regime that gave rise to the activation of a deeply rooted normal-slip precursor to the Kock Fault. Subsequently, this fault underwent inversion during the Late Famennian compressive/transpressive event. In the Early Carboniferous, the tectonic stress regime changed into tension/transtension, leading to extrusion of basalt magma and abundant mineralisation in the vicinity of the inverted Kock Fault, followed by tectonically relaxed sedimentation of Carboniferous strata. The deposition was terminated by a compressional event at the end of the Westphalian. Contraction resulted in the formation of the low-angle Kock Thrust decoupled in Silurian shale that cut across the upper part of the Kock Fault and displaced it towards the NE, over the East European Craton foreland.

Significance and dynamics of the Neoproterozoic (810 Ma) Phulad Shear Zone, Rajasthan, NW India

AbstractThe Phulad Shear Zone (PSZ) in northwestern India demarcates the boundary between Grenvillian age para‐metamorphics of the South Delhi Fold Belt (SDFB) to the east and variably deformed Neoproterozoic granitoids of the Marwar Craton to the west. In calcareous and quartzofeldspathic mylonites in and around Phulad, the shear zone is characterized by steep SE dipping mylonitic fabric and a steep oblique stretching lineation. The shear zone is developed in a ductile regime with top‐to‐the‐west reverse sense of movement coupled with pronounced flattening across the PSZ. MnNCKFMASH pressure‐temperature pseudosection analysis of closely spaced specimens of garnet‐bearing mica schists reveals that the stabilization of syn/post‐garnet porphyroblasts in the hanging wall experienced near‐isothermal decompression followed by cooling in the retrograde arm of a clockwise P‐T path. The retrieved P‐T paths are consistent with reverse motion of SDFB mica schist over the cratonic foreland. U‐Th‐Pb (total) monazite age determinations in hanging wall mica schists indicate that reverse motion on the PSZ occurred at 810 ± 6 Ma. The pre‐shearing Grenvillian age (~970 ± 9 Ma) metamorphic monazites of the SDFB occur as embayed cores, in the interiors of younger monazite grains. Intense high‐temperature deformation and the emplacement of mica‐bearing pegmatites during reverse shearing caused recrystallization of monazites by dissolution‐precipitation, mantling older monazites in the hanging wall SDFB schists. It is suggested that the PSZ may represent a terrane boundary shear zone along which the Greater India landmass accreted with the Marwar Craton at ~810 Ma.

Post-collisional, Late Neoproterozoic, high-Ba-Sr granitic magmatism from the Dom Feliciano Belt and its cratonic foreland, Uruguay: Petrography, geochemistry, geochronology, and tectonic implications

Post-collisional, granitic magmatism with high-barium-strontium (HiBaSr) content and shoshonitic affinity has been recently recognized both in the southern section of the Dom Feliciano Belt of Uruguay and its cratonic foreland. This group of granitic rocks has a distinctive age, mineralogy, chemistry and field characteristics. New zircon U–Pb LA-ICP-MS data for three of the plutons confirm their Late Neoproterozoic age; 634±7.3 (Sierra de los Caracoles), 604±3.0Ma (Cortez Blanco) and 597±3.6Ma (Guayabo). Similar ages were published previously for the Solis de Mataojo Granitic Complex (584±13Ma) as well as Las Flores (586±2.7Ma) and Sobresaliente plutons (585±2.5Ma). Mineral assemblages of the studied quartz-monzonites, granodiorites and monzogranites comprise quartz, orthoclase and microcline, plagioclase (Ab10–30), hornblende, green biotite, apatite, titanite and allanite. They plot predominantly in the high-K calc-alkaline field with the exception of a few that plot in the shoshonitic field; characteristically they are relatively high in Na2O (normally >4.5%) in acid varieties (SiO2>65%) decreasing to between 3 and 4% for more basic types; K2O normally exceeds 3.5% but can be as low as <2% in the basic varieties. The Uruguayan HiBaSr granitoids show high abundances of Ba (>700ppm), Sr (>500ppm) and light REEs alongside low Nb, Ta and heavy REEs. The Eu anomalies are negligible to slightly positive. Intermediate initial 87Sr/86Sr values (0.7077 to 0.7090) and very low initial epsilon Nd values (−15.8 to −19.3) at 600Ma with high Nd TDM (2.2–2.8Ga) suggest a recycling of ancient Paleoproterozoic to Late Archaean sources. Late Neoproterozoic granitoids of Uruguay have been emplaced within a post-collisional tectonic setting controlled by major shear zones and strike-slip fault zones. Current field and analytical data suggest that these granitoids could have been mostly generated through the partial melting of an intermediate to lower mafic continental crust and/or “enriched” crustal sources (in the presence of garnet±amphibole as stable residual mineral assemblage) with a possible minor mantle component.

Early foreland deformation of the Fuegian Andes (Argentina): Constraints from the strain analysis of Upper Cretaceous-Danian sedimentary rocks

This contribution addresses new structural data from Upper Cretaceous-Danian sedimentary rocks of the Fuegian Andes orogenic front in Argentina. The structures studied, called D′, were formed during the early foreland deformation of the orogen, in the Late Cretaceous-Danian, in times when South America and Antarctica were still connected. The strain analysis of these structures indicates that deformation occurred at upper-crustal depths, and was characterized by flexural folding accompanied by formation of pressure-solution tectonic foliations in zones of higher strain. Deformation intensities increase toward the hinterland and with depth. The history of deformation involved progression from layer-parallel shortening to folding above a detachment, and further formation of a forward-propagating thrust wedge. Layer-parallel shortening and incipient folding recorded in Maastrichtian-Danian rocks indicate the leading edge of D′ deformation. Non-coaxial finite strain orientations were involved during formation of D′ structures along the Fuegian Andes front. These finite strain orientations cannot be explained with the SW-NE regional contraction usually assumed to have driven Andean deformation in this region; alternatively, we consider that N-S contraction combined with buttressing against the cratonic foreland comprise a more suitable interpretation for D′ and younger deformation.

Evolution of reworked Paleoproterozoic basement rocks within the Ribeira belt (Neoproterozoic), SE-Brazil, based on U–Pb geochronology: Implications for paleogeographic reconstructions of the São Francisco-Congo paleocontinent

Geological, geochemical and geochronological data suggest that the basement exposed in the Occidental terrane of the Neoproterozoic-Ordovician Ribeira belt represents the continuation of the São Francisco paleocontinent, which was amalgamated at the end of the Rhyacian (Transamazonian Orogeny). The Mantiqueira complex represents a cordilleran-type active margin of the Archean paleocontinent, while the Juiz de Fora complex represents juvenile Paleoproterozoic material, probably developed within an oceanic setting (primitive arc or plateau?). The latter unit was probably accreted to the active margin of the paleocontinent during the latest stages of the Transamazonian Orogeny, around ca. 2.05 Ma, causing the deformation and metamorphism of the Mantiqueira complex. At this time, to the west and on the cratonic foreland, concurrent deformation of the Minas passive margin and sedimentation of the Sabará basin took place. Subsequent to the Transamazonian Orogeny, the basement associations were intruded by intraplate alkaline and basic rocks related to rifting events between ca. 1.7 and 1.3 Ga that resulted in the development of intracratonic basins. The Paleoproterozoic suture located between the Mantiqueira and Juiz de Fora Paleoproterozoic terranes probably controlled the Neoproterozoic passive margin tectonics and also the development of a major thrust surface during the Neoproterozoic/Cambrian Brasiliano convergence.

Geochronology of the Precambrian crust in the Mozambique belt in NE Mozambique, and implications for Gondwana assembly

Zircon and monazite U–Pb data document the geochronology of the felsic crust in the Mozambique Belt in NE Mozambique. Immediately E of Lake Niassa and NW of the Karoo-aged Maniamba Graben, the Ponta Messuli Complex preserves Paleoproterozoic gneisses with granulite-facies metamorphism dated at 1950 ± 15 Ma, and intruded by granite at 1056 ± 11 Ma. This complex has only weak evidence for a Pan-African metamorphism. Between the Maniamba Graben and the WSW–ENE-trending Lurio (shear) Belt, the Unango and Marrupa Complexes consist mainly of felsic orthogneisses dated between 1062 ± 13 and 946 ± 11 Ma, and interlayered with minor paragneisses. In these complexes, an amphibolite- to granulite-facies metamorphism is dated at 953 ± 8 Ma and a nepheline syenite pluton is dated at 799 ± 8 Ma. Pan-African deformation and high-grade metamorphism are more intense and penetrative southwards, towards the Lurio Belt. Amphibolite-facies metamorphism is dated at 555 ± 11 Ma in the Marrupa Complex and amphibolite- to granulite-facies metamorphism between 569 ± 9 and 527 ± 8 Ma in the Unango Complex. Post-collisional felsic plutonism, dated between 549 ± 13 and 486 ± 27 Ma, is uncommon in the Marrupa Complex but common in the Unango Complex. To the south of the Lurio Belt, the Nampula Complex consists of felsic orthogneisses which gave ages ranging from 1123 ± 9 to 1042 ± 9 Ma, interlayered with paragneisses. The Nampula Complex underwent amphibolite-facies metamorphism in the period between 543 ± 23 to 493 ± 8 Ma, and was intruded by voluminous post-collisional granitoid plutons between 511 ± 12 and 508 ± 3 Ma. In a larger context, the Ponta Messuli Complex is regarded as part of the Palaeoproterozoic, Usagaran, Congo-Tanzania Craton foreland of the Pan-African orogen. The Unango, Marrupa and Nampula Complexes were probably formed in an active margin setting during the Mesoproterozoic. The Unango and Marrupa Complexes were assembled on the margin of the Congo-Tanzania Craton during the Irumidian orogeny (ca. 1020–950 Ma), together with terranes in the Southern Irumide Belt. The distinctly older Nampula Complex was more probably linked to the Maud Belt of Antarctica, and peripheral to the Kalahari Craton during the Neoproterozoic. During the Pan-African orogeny, the Marrupa Complex was overlain by NW-directed nappes of the Cabo Delgado Nappe Complex before peak metamorphism at ca. 555 Ma. The nappes include evidence for early Pan-African orogenic events older than 610 Ma, typical for the Eastern Granulites in Tanzania. Crustal thickening at 555 ± 11 Ma is coeval with high-pressure granulite-facies metamorphism along the Lurio Belt at 557 ± 16 Ma. Crustal thickening in NE Mozambique is part of the main Pan-African, Kuunga, orogeny peaking between 570 and 530 Ma, during which the Congo-Tanzania, Kalahari, East Antarctica and India Cratons welded to form Gondwana. Voluminous post-collisional magmatism and metamorphism younger than 530 Ma in the Lurio Belt and the Nampula Complex are taken as evidence of gravitational collapse of the extensive orogenic domain south of the Lurio Belt after ca. 530 Ma. The Lurio Belt may represent a Pan-African suture zone between the Kalahari and Congo-Tanzania Craton.

Unroofing, incision, and uplift history of the southwestern Colorado Plateau from apatite (U-Th)/He thermochronometry

The source of buoyancy for the uplift of cratonic plateaus is a fundamental question in continental dynamics. The ~1.9 km uplift of the Colorado Plateau since the Late Cretaceous is a prime example of this problem. We used apatite (U-Th)/He thermochronometry (230 analyses; 36 samples) to provide the fi rst single-system, regional-scale proxy for the unroofi ng history of the southwestern quadrant of the plateau. The results confi rm overall southwest to northeast unroofi ng, from plateau margin to plateau interior. A single phase of unroofi ng along the plateau margin in Late Cretaceous to Early Tertiary (SevierLaramide) time contrasts with multiphase unroofi ng of the southwestern plateau interior in Early and mid- to Late Tertiary time. The Early Cretaceous was characterized by northeastward tilting and regional erosion, followed by aggradation of ≥1500 m of Upper Cretaceous sediments along the eroded plateau margin. Sevier-Laramide denudation affected the entire southwestern plateau, was concentrated along the plateau margin, and migrated from northwest to southeast. Following a period of relative stability of the landscape from ca. 50‐30 Ma, signifi cant unroofi ng of the southwestern plateau interior occurred between ca. 28 and 16 Ma. Additional denudation north of the Grand Canyon took place in latest Tertiary time. Mid-Tertiary dates from the Grand Canyon basement at the bottom of the Upper Granite Gorge limit signifi cant incision of the modern Grand Canyon below the Kaibab surface to 1500 m difference in vertical structural position. If these models are correct, they indicate that a “proto‐Grand Canyon” of kilometer-scale depth had incised post-Paleozoic strata by the Early Eocene. Evidence for kilometerscale mid-Tertiary relief in northeast-fl owing drainages along the plateau margin, as well as the mid-Tertiary episode of plateau interior unroofi ng, imply that the southwestern plateau interior had attained substantial elevation by at least 25‐20 Ma, if not much earlier. These observations are inconsistent with any model calling for exclusively Late Tertiary uplift of the southwestern plateau. Sevier-Laramide plateau surface uplift and incision thus result from one or more processes that enhanced the buoyancy of the plateau lithosphere, expanding the Cordillera’s orogenic highlands into its lowstanding cratonic foreland. The onset of the Laramide slab’s demise at ca. 40 Ma and the major pulse of extension in the Basin and Range from ca. 16‐10 Ma appear to have had little infl uence on the denudation history of the southwestern plateau. In contrast, the post-Laramide unroofi ng episodes may be explained by drainage adjustments induced by rift-related lowering of regions adjacent to the plateau, without the need to otherwise modify the plateau lithosphere. Our data do not preclude a large component of post‐Early Eocene elevation gain (or the geodynamic mechanisms it may imply), but they do point toward Laramide-age buoyancy sources as the initial cause of signifi cant surface uplift, ending more than 500 m.y. of residence near sea level.

Alkaline magmatism along the southeastern margin of the Indian shield: Implications for regional geodynamics and constraints on craton–Eastern Ghats Belt suturing

Deformed alkaline complexes at the contact between the Eastern Ghats Belt (EGB) and the Archaean cratons in southeastern India mark the location of a collisional suture superimposing an earlier Mesoproterozoic (∼1.5–1.3 Ga) rift. This article offers a plausible geodynamic model explaining the alkaline magmatism and its relationship to regional crustal evolution. The Mesoproterozoic rift can be correlated to the breakup of the supercontinent Columbia and may have opened an ocean between eastern India and east Antarctica where the sedimentary sequences of the Eastern Ghats Province (EGP) were deposited between 1.4 and 1.2 Ga. Inversion of the rift basin during the late Mesoproterozoic (∼1.0 Ga) led to the collision of eastern India with east Antarctica during Rodinia assembly. The collision formed the Grenvillian EGP–Rayner complex orogen where the EGP sediments and the rocks of the Rayner complex were deformed, metamorphosed and migmatized at granulite-facies condition. The Mesoproterozoic rifting and Grenvillian basin closure may thus represent a Wilson cycle related to the breakup of Columbia and the assembly of Rodinia. The craton–EGB suture was subsequently modified significantly during Pan-African tectonism (0.5–0.6 Ga) when the EGB granulites were thrust westward over the cratonic foreland along a number of amphibolite-facies shear zones at the contact. The shearing deformed and metamorphosed the cratonic margin and the alkaline complexes it hosts. The thrust stacking in many places has the craton boundary and the alkaline rocks in the footwall and the EGB granulites in the hanging wall. The continuity of the Pan-African shear fabric across the craton–granulite contact and the lack of Grenvillian ages from the alkaline and other craton margin rocks in such regions give the apparent impression that the two crustal segments amalgamated during Pan-African tectonism and not during the earlier Grenvillian collision. Geodynamic constraints however indicate that this was unlikely and the inconsistency arises because the present crustal geometry exposes only the eroded Pan-African thrust contact between the craton and the EGB and not the original Grenvillian suture, which may be lying unexposed under the granulite thrust sheets.

The thrust-contact between the Eastern Ghats Belt and the adjoining Bastar craton (Eastern India): Evidence from mafic granulites and tectonic implications

Along its northwestern margin, the Meso-/Neo-proterozoic granulites of the Eastern Ghats Belt are juxtaposed against the Palaeoproterozoic/Archaean lithologies of the Bastar craton along a crustal ductile scale shear zone. In the shear zone hosted garnet-bearing mafic granulites of the Eastern Ghats Belt, garnet porphyroblasts pre-tectonic with respect to the shearing are mantled by coronal orthopyroxene + An-rich plagioclase vermicular intergrowths. The intergrowths typically overgrow the shear zone fabric defined by the shape preferred aggregates of hornblende, orthopyroxene ≫ clinopyroxene in a recrystallized mosaic of plagioclase. Schreinmakers analysis of reaction equilibria in the NCMASH system suggests that the post-shearing orthopyroxene + plagioclase (An-richer) symplectites formed at the expense of hornblende–garnet–An-poorer matrix plagioclase assemblage. The model reaction has high positive d P/d T slope with orthopyroxene + plagioclase stable in the high temperature side of the reaction. The textural relations are inferred to indicate prograde heating T max ∼ 750 °C, P max ∼ 7 kbar post-dating shearing. The gamut of available age data (K–Ar, Rb–Sr, Pb–Pb in zircon, and U–Pb–Th CHIME date in monazites) indicate that the youngest thermal event in and neighboring the shear zone cluster at ∼500–550 Ma, regardless of the vestigal Precambrian dates in rocks from across the shear zone. Since the closure temperature of thermally-driven Pb diffusion in minerals like monazite and zircon is ≥650–700 °C, the Early Palaeozoic age is likely to correspond with the ductile shearing and granulite facies prograde heating at T value higher than the blocking temperature of Pb. Since the shearing is correlated with top-to-the-west thrusting of the Eastern Ghats Belt over the cratonic foreland, the Early Palaeozoic prograde granulite facies metamorphism may relate to suturing of the two disparately evolved crustal blocks. The prograde heating may have been caused by burial of the Eastern Ghats granulite facies gneisses neighboring the shear zone due to emplacement of granulites from the hinterland along west-vergent thrust imbrications.

Petrogenesis of the Kunavaram alkaline complex and the tectonothermal evolution of the neighboring Eastern Ghats Belt granulites, SE India

Geochemical and geochronological study of the Kunavaram alkaline rocks near the craton–Eastern Ghats Belt (EGB) contact in SE India indicates that alkaline magma was emplaced at ∼1384Ma along a NE–SW Mesoproterozoic rift at the cratonic margin. This paleo-rift can be delineated by the several Mesoproterozoic alkaline intrusives at the cratonic margin (e.g. Khariar, Kunavaram, Jojuru, Elchuru and others). The geochemical variations in the Kunavaram alkaline suite can be explained by an assimilation-fractional crystallization (AFC) process dominated by the fractionation of clinopyroxene–amphibole, apatite, titanite, zircon and allanite and the simultaneous assimilation of bulk upper crustal material from the craton. The alkaline complex and other craton margin rocks and the marginal granulites of the EGB were overprinted by an amphibolite facies tectonothermal event during the Neoproterozoic–Paleozoic (611–484Ma) when Pan-African tectonism thrust the EGB granulites westward over the cratonic foreland along a number of northwest-vergent shear zones. The Pan-African thrust stacking has the cratonic boundary and the alkaline rocks in the footwall and the EGB granulites in the hanging wall. The continuous nature of the Pan-African shear fabric across the craton–EGB contact near Kunavaram implies that the two crustal segments shared a common boundary during the Neoproterozoic–Paleozoic. Prior to the Pan-African event, the evolutionary history of the EGB is characterized by the presence of an early Neoproterozoic (1017–1087Ma) granulite-facies tectonometamorphic overprint during the Grenvillian orogeny, evidence for which is lacking in the alkaline and other craton margin rocks. The craton–EGB contact thus represents a rifted Mesoproterozoic margin deformed during Pan-African collisional tectonics.

Intrusion age, geochemistry and metamorphic conditions of a quartz-monzosyenite intrusion at the craton–Eastern Ghats Belt contact near Jojuru, India

The boundary between the Archean cratons and the Eastern Ghats Belt in peninsular India represents a rifted Mesoproterozoic continental margin which was overprinted by a Pan-African collisional event associated with the westward thrusting of the Eastern Ghats granulites over the cratonic foreland. The contact zone contains a number of deformed and metamorphosed nepheline syenite complexes of rift-related geochemical affinities. In addition to the nepheline-bearing rocks, metamorphosed quartz-bearing monzosyenitic bodies can also be identified along the suture in the region between the Godavari-Pranhita graben and the Prakasam Igneous Province. One such occurrence at Jojuru near Kondapalle is geochemically comparable to the nepheline syenites and furnishes a weighted mean concordant U–Th–Pb SHRIMP zircon age of 1263 ± 23 Ma (2 σ), which provides a lower age bracket for the rift-related magmatic activity. The original igneous mineral assemblage in the monzosyenite was partially replaced by the formation of coronitic garnet during the Pan-African metamorphism of the rocks. P– T estimates of garnet corona formation at the interface between clinopyroxene–orthopyroxene–ilmenite clusters and plagioclase indicate mid to upper amphibolite facies condition (5.5–7.0 kbar and 600–700 °C) during the thrust induced deformation and metamorphism associated with the Pan-African collisional tectonics.

Mesoproterozoic rifting and Pan-African continental collision in SE India: evidence from the Khariar alkaline complex

The suture zone between the Bhandara craton and the granulite-facies rocks of the Eastern Ghats Province in SE India contains a number of deformed alkaline and tholeiitic intrusives. The Khariar alkaline complex is one of the several occurrences which intruded in the Mesoproterozoic (1,480±17 Ma, 2σ) and was deformed during the Pan-African tectonothermal event. The geochemical signatures indicate a rift-related setting for the magmatic activity. The nepheline syenite parent magma may have been produced by in-mantle fractionation of clinopyroxene and Ti-rich amphibole from a basanitic primary magma derived from an enriched spinel lherzolite mantle source in the sub-continental lithosphere. Geochemical variations in the Khariar alkaline suite can be modeled by the fractionation of clinopyroxene, amphibole, titanite, zircon, apatite and allanite. The Mesoproterozoic alkaline magmatism at Khariar marks the initiation of a NE-SW rift which formed several craton margin basins and opened an ocean towards the south. The sediments of the cratogenic basins and the Eastern Ghats Province were deposited in these rift-related basins. A K-Ar age of 1,330±53 Ma from glauconites in sandstone suggests that the NW-SE trending Godavari–Pranhita graben formed at approximately the same time as the rift at the craton margin. If the two are related, the Godavari–Pranhita graben may represent the failed arm of a rift system in which the NE-SW arm was the active segment. The granulite-facies deformation and metamorphism of the Eastern Ghats Province sediments may be related to an episode of Grenvillian basin inversion. The Mesoproterozoic rifting and Grenvillian basin closure may thus represent two well-defined parts of a Wilson cycle i.e. the opening and closure of an ocean. The Khariar and other alkaline bodies were, however, deformed during a Pan-African collisional event associated with the westward thrusting of the Eastern Ghats Province granulites over the cratonic foreland.

Molybdenite Re–Os dating constrains gravitational collapse of the Sveconorwegian orogen, SW Scandinavia

Re–Os dating of molybdenite from small deposits is used to define crustal domains exhibiting ductile versus brittle behaviour during gravitational collapse of the Sveconorwegian orogen in SW Scandinavia. A 1019 ± 3 Ma planar quartz vein defines a minimum age for brittle behaviour in central Telemark. In Rogaland–Vest Agder, molybdenite associated with deformed quartz and pegmatite veins formed between 982 ± 3 and 947 ± 3 Ma in the amphibolite-facies domain (three deposits) and between 953 ± 3 and 931 ± 3 Ma west of the clinopyroxene-in isograd (two deposits) in the vicinity of the 0.93–0.92 Ga Rogaland anorthosite complex. The data constrain the last increment of ductile deformation to be younger than 0.95 and 0.93 Ga in these two metamorphic zones, respectively. Molybdenite is the product of an equilibrium between biotite, oxide and sulfide minerals and a fluid or hydrated melt phase, after the peak of 1.03–0.97 Ga regional metamorphism. Molybdenite precipitation is locally episodic. A model for gravitational collapse of the Sveconorwegian orogen controlled by lithospheric extension after 0.97 Ga is proposed. In the west of the orogen, the Rogaland–Vest Agder sector is interpreted as a large shallow gneiss dome, formed slowly in two stages in a warm and structurally weak crust. The first stage at 0.96–0.93 Ga was associated with intrusion of the post-collisional hornblende–biotite granite suite. The second stage at 0.93–0.92 Ga, restricted to the southwesternmost area, was associated with intrusion of the anorthosite–mangerite–charnockite suite. Most of the central part of the orogen was already situated in the brittle upper crust well before 0.97 Ga, and did not undergo significant exhumation during collapse. In the east of the orogen, situated against the colder cratonic foreland, exhumation of high-grade rocks of the Eastern Segment occurred between 0.97 and 0.95 Ga, and included preservation of high-pressure rocks but no plutonism.

The tectonic frame of the Variscan–Alleghanian orogen in Southern Europe and Northern Africa

By confronting different geological and geophysical data, we attempt to reconstruct the Variscan–Alleghanian orogenic belt, with especial emphasis on the links between Iberia, northwest Africa, and northeast America. The northern Iberia transect corresponds to the rifted margin of Gondwana, inverted during the Variscan orogeny and overthrust in its westernmost sector by a pile of allochthonous units, some of them with oceanic affinity and witnessing an orogenic suture. The southern Iberia section reveals two sutures at both boundaries of a terrane closely tied to Gondwana, namely the Ossa–Morena Zone. The southern boundary of this zone (i.e., the contact with the South Portuguese Zone) is enhanced by amphibolites with oceanic affinity and probably represents the suture of the Rheic Ocean. The Moroccan Variscides can be divided into: (i) a western external zone, namely the Coastal Block and the Central Massif; (ii) an internal zone, namely the Eastern Meseta; (iii) an eastern external zone represented in the Anti Atlas region; and (iv) the African cratonic foreland. Since neither ophiolites nor eclogites crop out along this transect of the orogen, we consider it to be an incomplete transect, lacking the main suture zone (Rheic Ocean). Stratigraphic and faunal affinities between the Moroccan Meseta, on the one hand, and the Central Iberian, Western Asturian–Leonese, and Cantabrian zones of Iberia, on the other hand, suggest that they may well have been part of a common pre-orogenic domain at the margin of Gondwana. On the contrary, there are no counterparts in Morocco for the Ossa–Morena Zone. Thus, the northern Iberia palaeogeographic zones would prolong in the Moroccan Meseta through an eastward arcuate structure, while the suture of the Rheic Ocean would do so offshore Morocco. In our reconstruction, an Avalonian spur (Grand Banks indentor), which included the Caledonian Sehoul block in northern Morocco and the crust of the South Portuguese Zone in SW Iberia, played an important role on the kinematic evolution of the orogen as well as on the final configuration of the suture of the Rheic Ocean. This suture would mould around the Avalonian spur, being laterally translated to southeastern Nova Scotia–Newfoundland.

Structural evolution across the Eastern Ghats Mobile Belt–Bastar craton boundary, India: hot over cold thrusting in an ancient collision zone

Along the western margin of the Eastern Ghats Mobile Belt (EGMB), ultrahigh-temperature granulites are thrust westward over hornblende granites and sedimentary rocks of the Bastar craton. Multiply deformed migmatitic gneisses, and porphyritic charnockite of the EGMB unit preserve thrust-related shear fabrics (S 3M). In the cratonic foreland, undeformed granites show a progressive decrease in grain-size and increase in penetrative foliation to the east, evolving into orthogneiss near the mylonitized contact zone with charnockite. Foreland fabrics with a consistent top-to-the-west shear sense are conformable with S 3M in charnockite and correlate with S 3B in polydeformed orthogneisses of cratonic windows. S 3B in the windows parallels S 3M in overlying EGMB migmatitic gneisses. While S 3M accompanied granulite metamorphism in the EGMB, S 3B temperature estimates vary from T>700 °C in the windows to T<550 °C in the west. Decreasing temperature and later fabric formation in the west are explained by an evolving thermal profile in the cold craton, which is caused by thrusting against hot lower crustal EGMB rocks. Based on lithologic, structural and metamorphic variations across the contact, and resemblances between the EGMB and Rayner Complex, the craton–mobile belt boundary is considered a result of Indo-Antarctic collision, leading to the formation of an ancient supercontinent.

Proposta litoestratigráfica e considerações paleoambientais sobre o Grupo Maricá (Neoproterozóico Iii), Bacia Do Camaquã, Rio Grande Do Sul

The Marica Group (Rio Grande do Sul State, Southern Brazil) contains the first sedimentary successions deposited after the denudation and tectonic subsidence of the basement units of the Gaucho Shield that was formed during the Brasiliano Orogeny. This subsidence event lead to the generation of the Camaqua Basin (Neoproterozoic III / Early Paleozoic), which posteriorly evolved to a rift system characterized by volcanosedimentary successions (Bom Jardim group) and coarse-grained siliciclastics (Santa Barbara and Guaritas groups). The Marica group consists of 2000-2500 m of arkosean sandstones and, in smaller proportion, conglomerates and pelites. Three lithostratigraphic units are here defined: Passo da Promessa formation (fluvial sandstones and conglomerates), Sao Rafael formation (marine sandstones and rhythmites) and Arroio America formation (fluvial sandstones and conglomerates). Stratigraphic and provenance analyses of the alluvial units, as well as paleocurrent data, show that: (i) sedimentary transport was towards the North during the evolution of these units; (ii) source areas were composed of granitic rocks, both foliated and unfoliated; (iii) there was an important contribution of a volcanic source, comprehending mainly rhyolitic and pyroclastic rocks, which frequency of occurrence increase towards the top of the succession, also including rare andesites in the Arroio America formation. The stratigraphic relationship among the Marica formation and the basement units shows that the basin was not related to the Brasiliano Orogeny, as revealed by the fact that the Marica group overlies both the cratonic foreland regions and the collisional suture, represented respectively by paleoproterozoic granulites (Rio the La Plata Craton) and a neoproterozoic subduction complex (Rio Vacacai Terrane).