Final Uplift Research Articles

Dating the TIPA shear zone: an Early Devonian terrane boundary between the Famatinian and Pampean systems (NW Argentina)

The TIPA shear zone is a prominent tectonic element of the Las Termas belt of NW Argentina. The belt is squeezed between the rocks of the Pampean system in the east and those of the Famatinian system in the west and is characterized by NNW–SSE-trending ductile shear zones. Granitoid protoliths of the mylonites yield intrusion ages of 487.5±4.3 and 467.0±3.6 Ma (U–Pb on zircon). During mylonitization, an intense fluid and melt influx, correlated with the intrusion of syn- to late-tectonic pegmatoids, led to a significant change in rock composition and the growth of syntectonic garnet. Mylonitization is dated at 402.0±2.0 Ma (Sm–Nd on garnet). U–Pb ages for apatite (meta-granodiorite 342.3±1.8 Ma; mylonite 328±3 Ma), consistent with the age of granites intruding the whole series, document a Carboniferous heating process that affected the whole crust. Final uplift is indicated by homogeneous biotite cooling ages at 300±4 Ma (Rb–Sr). The time span of approximately 40 Ma between apatite crystallization and closure of the Rb–Sr isotopic system in biotite implies a cooling rate of 3–4 °C/Ma, which indicates total uplift of the series of approximately 4 km.

A note on tsunami caused by submarine slides and slumps spreading in one dimension with nonuniform displacement amplitudes

Tsunami created by spreading submarine slides and slumps with spatially variable final uplift are investigated in the near-field using a kinematic model. It is shown that for velocities of spreading comparable to and smaller than the long period tsunami velocity c T = gh ( g is the acceleration due to gravity and h is the ocean depth), the models with spatially uniform final uplift of the accumulation and depletion zones provide good approximation for the tsunami amplitudes in the near-field. For spreading velocities 2–5 times greater than c T, and for applications that use wavelengths of the order of the source dimensions, the spatial variability of the final uplift has to be considered in estimation of the high-frequency tsunami amplitudes in the near-field.

A south‐to‐north biogeographic hypothesis for Andean speciation: evidence from the lizard genus Proctoporus (Reptilia, Gymnophthalmidae)

AbstractAim The lizard genus ProctoporusTschudi, 1845 was used as a model to test the South‐to‐North Speciation Hypothesis (SNSH) for species groups occurring in the Andes Mountains of South America. This hypothesis proposes that speciation of high Andean taxa followed a south‐to‐north pattern, generally coinciding with the progression of final uplift of the Andes. According to SNSH, a phylogenetic hypothesis of relationships of a taxonomic group occurring in the high Andes would show a branching pattern in which the southernmost species diverged first, followed by the more northern species, and so on in a northerly pattern.Location The central and northern Andes Mountains in South America.Methods A phylogenetic hypothesis was reconstructed for all species of the lizard genus Proctoporus by examining the external morphology of 341 individuals. This phylogeny was then examined to determine monophyly of the genus, distribution patterns of species groups, and congruence with SNSH.Results The genus Proctoporus did appear to be monophyletic and, therefore, it was valid to use this group to assess SNSH. The southernmost species were found to be the most basal, which was consistent with SNSH. The species occurring in the northern Andes did not exactly match the SNSH prediction. The Venezuelan and Trinidadian species did appear to be highly derived, as predicted by the hypothesis, but the Ecuadorian and Colombian species did not form a particular pattern in relation to the hypothesis.Main conclusions The SNSH does appear to have predictive power with regard to large‐scale distribution patterns. The finer‐scale patterns of speciation in the Andes, however, appear to be a more complex phenomenon that cannot be fully explained by a simple hypothesis. It is important to have a testable hypothesis in hand with which to compare data from disparate species groups. The incorporation of phylogenetic data of other high Andean taxa with similar distribution patterns is necessary to determine the full utility of SNSH in explaining evolutionary patterns in the Andes of South America.

Tectonic and unroofing history of Neogene Manantiales foreland basin deposits, Cordillera Frontal (32°30′S), San Juan Province, Argentina

The Miocene Manantiales foreland basin is located in Cordillera Frontal of San Juan, between 32°30′ and 33°S. The unroofing study of the synorogenic Miocene deposits provides information about the structural evolution of Cordón de La Ramada fold-and-thrust belt. These Tertiary deposits are represented by the Chinches Formation and comprise seven members (Tc0–Tc6). They are the result of the uplift of Mesozoic sequences that crop out in La Ramada fold-and-thrust belt of the Cordillera Principal. Quaternary deposits unconformably overlying the Chinches Formation are composed of granitic and rhyolitic blocks, and represent the final uplift of the Cordón del Espinacito and a series of out-of-sequence thrusts. The unroofing studies also provide sufficient information to establish the out-of-sequence timing of the deformation at this latitude. Initial deposition of the Tertiary deposits can be dated at about 20Ma, or early Miocene. Andesitic lavas dated in 9.2±0.3, 10.7±0.7, and 12.7±0.7Ma unconformably overlie the structure of La Ramada fold-and-thrust belt. These facts constrain the uplift of the High Andes between 20 and 10Ma at this latitude. The unconformity between Tertiary and Quaternary deposits suggests final uplift during Pliocene–Pleistocene times.

Phylogenetic relationships and historical biogeography of melanotaeniid fishes in Australia and New Guinea

Phylogenetic analysis of melanotaeniid mtDNA cytochrome b and tRNA Pro-control region sequence is broadly consistent with the current taxonomy. However, the molecular phylogeny supports the elevation of M. s. australis to full species status and indicates either that it is a composite species or has introgressed with sympatric Melanotaenia species. Phenotypically cryptic mtDNA diversity in north-eastern Australia possibly represents an undescribed species. Six major monophyletic clades present in the phylogeny were strongly supported by morphological data. The clades represent three biogeographic regions. Fish from northern New Guinea form a monophyletic clade, within which Melanotaenia and Glossolepis are polyphyletic. The divergence of this clade from those in southern New Guinea is consistent with the final uplift of the Central Highlands 5 million years BP. North-western New Guinea and associated islands represent another highly divergent, monophyletic clade of a similar age to that in northern New Guinea. The remaining four clades form a monophyletic assemblage restricted to southern New Guinea and Australia: one in northern Australia, one with a disjunct distribution in north-western and eastern Australia, one widespread throughout Australia and southern New Guinea, and one in southern New Guinea with an outlying species in northern Australia. The phylogenetic relationships between Australia and southern New Guinea are consistent with episodic connection via the freshwater Lake Carpentaria during periods of low sea level.

Erosional landsurfaces of the Campano–Lucano Apennines (S. Italy): genesis, evolution, and tectonic implications

Within the Campano–Lucano Apennines small but widespread relics of gentle erosional landsurfaces (GELs) occur that are elevated several hundred metres above the present valley floors. On the basis of our observations, the old idea that all these relict landforms can be traced back to a single phase of quasi-planation that affected the entire belt after thrust stacking and before a phase of final uplift must be rejected. New field evidence and chronological constraints support the conclusion that the GEL relics belong to various, uncompleted cycles of erosion that occurred just during the construction of the chain (i.e. Late Miocene to Early Pleistocene). As many recent geological studies have pointed out, the Southern Apennines Chain is an arc-shaped accretionary wedge that migrated as a thrusting system towards its retreating foreland by means of addition of thrust units to the front and loss of rifted portions at the rear (Tyrrhenian back-arc basin). Also, the spatial distribution of the various generations of GELs, their ages, and their relations to the main structures and syn-orogenic formations of the belt seem to agree with this geodynamic scenario. Here we propose a ‘first approximation’ conceptual model that considers each generation of Campano–Lucano Apennines GELs as largely formed when the thrust units on which they rest were located at low elevation near the coastline of the foredeep basin, i.e. shortly after the entrance of those thrust units into the tectonic wedge. Then each GEL (as a matter of fact the compartment on which it rested) was taken towards more and more internal parts of the wedge as frontal accretion continued. Owing to this relative migration, the fate of each GEL-carrying compartment was to reach the part of the belt that was affected by extensional block faulting, i.e. the Tyrrhenian side of the chain. Each GEL eventually reaching this domain was tectonically fragmented, deeply cut by waves of fluvial dissection proceeding from the steep tectonic coast toward the NE and finally also rifted under the Tyrrhenian Sea. Other causes of GELs fragmentation and dissection were probably the breaching of the chain's outer flank by out-of-sequence thrust and the up-arching of the roof thrust system caused by the formation of deep anticlinal stacks. On the contrary, when forward propagation of thrusts continued for long periods, the GELs being carried toward the inner zone of the wedge could not have experienced deep dissection because the increase of their elevation a.s.l. was compensated by their increasing distance from the foredeep coastline (maintenance of low gradient along the river valleys thanks to a very low critical taper).

A Large‐Scale Palaeozoic Dextral Ductile Strike‐Slip Zone: the Aqqikkudug‐Weiya Zone along the Northern Margin of the Central Tianshan Belt, Xinjiang, NW China

Abstract The nearly E‐W‐trending Aqqikkudug‐Weiya zone, more than 1000 km long and about 30 km wide, is an important segment in the Central Asian tectonic framework. It is distributed along the northern margin of the Central Tianshan belt in Xinjiang, NW China and is composed of mylonitized Early Palaeozoic greywacke, volcanic rocks, ophiolitic blocks as a mélange complex, HP/LT‐type bleuschist blocks and mylonitized Neoproterozoic schist, gneiss and orthogneiss. Nearly vertical mylonitic foliation and sub‐horizontal stretching lineation define its strike‐slip feature; various kinematic indicators, such as asymmetric folds, non‐coaxial asymmetric macro‐ to micro‐structures and C‐axis fabrics of quartz grains of mylonites, suggest that it is a dextral strike‐slip ductile shear zone oriented in a nearly E‐W direction characterized by “flower” strusture with thrusting or extruding across the zone toward the two sides and upright folds with gently plunging hinges. The Aqqikkudug‐Weiya zone experienced at least two stages of ductile shear tectonic evolution: Early Palaeozoic north vergent thrusting ductile shear and Late Carboniferous‐Early Permian strike‐slip deformation. The strike‐slip ductile shear likely took place during Late Palaeozoic time, dated at 269±5 Ma by the40Ar/39Ar analysis on neo‐muscovites. The strike‐slip deformation was followed by the Hercynian violent S‐type granitic magmatism. Geodynamical analysis suggests that the large‐scale dextral strike‐slip ductile shearing is likely the result of intracontinental adjustment deformation after the collision of the Siberian continental plate towards the northern margin of the Tarim continental plate during the Late Carboniferous. The Himalayan tectonism locally deformed the zone, marked by final uplift, brittle layer‐slip and step‐type thrust faults, transcurrent faults and E‐W‐elongated Mesozoic‐Cenozoic basins.

Permian high pressure rocks—the basement of the Sierra de Limón Verde in Northern Chile

The gneisses and metabasites of the Sierra de Limón Verde were investigated by P–T–t determinations. The rocks are unique in the Central Andes because of their high pressure metamorphic conditions with P≈13±1 kbar at T≈660–720°C. Their age of metamorphism is ≈270 Ma, based on Sm–Nd mineral isochrons. Final uplift of the isolated basement block occurred in the Triassic with a K–Ar age of biotite at ca 235 Ma. In our interpretation, the protolith of the Permian metamorphic rocks is the crust that formed and stabilized during Early Paleozoic. The Sierra de Limón Verde rocks give insight into the lowermost part of the crust in Early Mesozoic. Its Sm–Nd isotopic composition is indistinguishable from the composition of the crust that formed in the Early Paleozoic metamorphic–magmatic cycle ( ca 500 Ma) in northern Chile and NW Argentina. The tectonic-geodynamic setting that triggered the high P (∼45 km depth) metamorphism and the locally restricted exhumation of the rocks remains speculative. Continental collision or a subduction related accretionary complex is unlikely considering the regional geological situation. Transpression–transtension in a strike slip system along the continental margin is suggested as a hypothesis for future investigations.

Reaction Textures of Retrograde Pressure-Temperature-Deformation Paths from Granulites of Schirmacher Hills, East Antarctica

Abstract A retrograde path inferred for the Schirmacher Hills granulites involves near ITD path following post-peak granulite facies metamorphism (M1/D1), preserved only in enclaves, followed by near IBC path, M3, post dating M2/D2 granulites facies metamorphism and deformation. Final uplift and cooling of the terrain occurred in the upper amphibolite facies metamorphic conditions, M4, syntectonic with respect to D3 deformation and emplacement of large volume of syntectonic granitoids. Fluids released during cooling and crystallization of these granitoids probably caused the large scale retrogression of granulite facies rocks to amphibolite facies gneisses.

Formula omitted] geochronology across the Mylonite Zone and the Southwestern Granulite Province in the Sveconorwegian Orogen of S Sweden

The crustal segment between the Mylonite Zone (MZ) and the Protogine Zone in southern Sweden (the Southwestern Granulite Province, SGP) is characterized by numerous occurrences of granulite and upper amphibolite-facies rocks including migmatitic gneisses, mafic granulites, garnet amphibolites and charnockites. The southwest part of the Mylonite Zone (north of Varberg) marks a tectonic upper boundary of the SGP. A 40Ar 39 Ar study was initiated across the Mylonite Zone and within the SGP in order to constrain and compare the ages obtained from within the SGP with those obtained north of and within the Mylonite Zone. 23 homblendes and one muscovite have been analyzed. The results of these analyses indicate the following. (1) Hornblendes from within and near the MZ yield 40Ar 39 Ar hornblende ages of ∼ 915 Ma. These ages are interpreted to date the dynamic crystallization or cooling below 500°C of the deformation associated with the MZ. (2) Away from the MZ there is a distribution of ages between those which yield younger Sveconorwegian ages of 931–934 Ma, and those which yield older Sveconorwegian ages generally between 960 and 1007 Ma. The younger age may be associated with orogenic uplift, while the older one may be associated with an earlier Sveconorwegian crustal thickening event. (3) The one muscovite age of 904 Ma would indicate a late cooling rate of at least 6–15°C/Ma. Cooling rates of this magnitude may suggest that the final uplift of the Sveconorwegian orogen was related to large-scale crustal extension

Eastern Alpine tectono‐metamorphic evolution: Constraints from two‐dimensional P‐T‐t modeling

We use two‐dimensional (2‐D) P‐T‐t modeling to constrain the thermal and rheological aspects of different scenarios for the late Mesozoic and Cenozoic tectonic evolution of the Eastern Alps, inferred from excellent data sets from the Tauern Window (TW). Models invoking subduction of the South Penninic (SP) oceanic lithosphere during thrusting and subsequent erosion of the Austro‐Alpine (AA) upper plate nappe stack are inconsistent with the observed thermal evolution within the AA and Penninic units. In these models, predictions for the AA peak thermal conditions are lower than observed. After exhumation and cooling to midcrustal levels and subduction of the continental Middle Penninic (MP) block, the AA undergoes a phase of renewed heating to almost the previous peak temperatures. Simultaneously, the Penninic units experience a phase of heating upon subduction, followed by cooling after onset of subduction of the North Penninic (NP) basin. The model predictions are inconsistent with the observed nearly isothermal uplift path of the SP after subduction and cannot explain observed inverted metamorphic peak conditions in the deeper AA (amphibolite facies) down to the higher Penninic unit (greenschist facies). A model with the beginning of subduction of the SP occurring after crustal thickening of the AA and subsequent return to normal crustal thicknesses is compatible with the P‐T‐t data. In this model, peak temperature conditions are higher in the AA, followed by a phase of strong cooling in the AA upper plate with the onset of underthrusting. This model also explains successfully nearly isothermal exhumation of the MP and inverted metamorphic peak conditions in the deeper AA. Material accreted to the hanging wall from the oceanic crust (SP) experiences a phase of cooling during ongoing subduction of oceanic lithosphere and begins to heat up to its thermal climax after the subduction of trailing continental lithosphere. The subsequent PT path of the Penninic units strongly depends on the timing and rates of underthrusting by the foreland. Observed PT paths in the MP within the TW require continuous subduction of the NP and the trailing European foreland under the exhuming MP block. Documented rapid cooling in the final uplift phase of the Penninic units in the TW requires exhumation rates up to approximately 4 mm/yr. Predictions of slightly elevated present‐day geothermal gradients in the TW area are consistent with available heat flow data. As a result of Mesozoic rifting followed by late Mesozoic crustal thickening of the AA, paleorheological reconstructions are characterized by a contrast between relatively strong oceanic lithosphere and adjacent weak continental lithosphere. Predicted decoupling of weak continental and oceanic lithosphere during subsequent subduction of the Penninic units can explain observed Late Cretaceous crustal extension in the AA units in terms of gravitational spreading. Ongoing subduction leads to an overall strength increase due to underthrusting of cool oceanic lithosphere, whereas subduction of continental lithosphere causes a strength decrease in the upper levels of the lithosphere. Continuous crustal thickening and relaxation of the depressed isotherms reduce the strength of the lower lithospheric mantle beneath the central orogen, further enhanced by rapid late‐stage uplift. Predictions for the present‐day rheological structure of the Eastern Alps support the existence of a strong upper crustal layer, two wedge‐shaped strong upper mantle layers to the north and the south of the orogen, and a weak upper mantle underlying the central orogen.

Palynology and paleobotany of the Early Pliocene section Río Frío 17 (Cordillera Oriental, Colombia): biostratigraphical and chronostratigraphical implications

Organic-rich sediments of section Río Frío 17 (3165 m altitude, Cordillera Oriental, Colombia) were analyzed for pollen and plant macrofossils. Time control is based on fission-track dating of zircon from an intercalated tephra with an age of 5.3 ± 1.0 Ma. Pollen and macrofossil spectra of several outcrops and cores, including section Río Frío 17, reveal gradually cooler depositional environments during the Neogene. Obvious differences between modern and Pliocene Andean vegetation belts are taken into account. The sequence of outcrops reflects the final uplift of the Cordillera Oriental. Pollen and macrofossils of section Río Frío 17 suggest that lower subandean (lower part of lower montane belt) to tropical lowland conditions prevailed in the area during the Early Pliocene: sediment deposition occurred apparently at c. 1000 m elevation. If Pliocene average temperatures were comparable to present-day values with an amplitude of ± 3°C, an altitudinal shift of 500 m maximum is suggested. Hence sedimentation is estimated to have occurred at 1500 m elevation maximally, based on the fossil plants and the present-day ecological requirements of their extant relatives. The remaining difference in elevation is attributed to tectonic uplift by at least 1700 m. The macrofossil evidence and the presence of Hedyosmum pollen suggest that the sediments of section Río Frío 17 should be placed in Biozone II, instead of Biozone I of the biostratigraphical zonation for the high plain of Bogotá as suggested by Helmens and Kuhry (1990). The arrival of Hedyosmum in the pollen record and the final uplift of the high plain of Bogotá is now estimated at c. 5.3 ± 1.0 Ma. The age of the sediments of sections Salto de Tequendama I and II (Biozone I), estimated as Early Pliocene by Van der Hammen et al. (1973), is earlier than that date. Selected plant macrofossils, such as seeds and fruits, are described and illustrated.

Age of low-grade regional metamorphism in the North Greenland fold belt: mineralogical and Rb–Sr-isotope evidence from pelitic metasediments

Established isotopic sediment-dating techniques have been used to date the time of deformation and metamorphism of the lower Palaeozoic rocks within the North Greenland fold belt. Fine mineral fractions < 2 μm have been separated from pelitic sediments at widely distributed localities and analysed using X-ray diffraction, isotope, and geochemical techniques.The mineralogical study identified two slightly different mineral parageneses of low greenschist facies. One assemblage without the sodium mica paragonite is confined to calcareous lithologies of the Amundsen Land and Vølvedal groups. The second assemblage includes paragonite and is found in noncalcareous lithologies of the Polkorridoren Group.Rb–Sr isotope dating of the mineral fractions yielded isochron ages in the range 318–346 Ma, which shows that final uplift and cooling associated with Ellesmerian tectonism in North Greenland occurred in the Mississippian. The results suggest that, although the metamorphism was of regional scale, recrystallization and isotopic homogenization occurred in closed systems of local extent.

The sources of metals in sulfide deposits in the Helgeland nappe complex, north-central Norway; Pb isotope evidence

The Helgeland Nappe Complex of the uppermost Caledonian allochthons of Norway is host to a number of different sulfide deposits. These occur in epicontinental metasedimentary rocks close to, or at, the contact with the granitoid Bindal batholith. Sixty-eight common Pb analyses for six Zn-Pb sulfide deposits and one As-Au deposit make it possible to recognize three distinct episodes of sulfide ore formation, which can be related to successive stages of the tectonic and petrogenetic evolution of the Helgeland Nappe Complex.Prior to peak Caledonian deformation, Zn-Pb sulfides were deposited from ore-forming solutions that derived their lead from a probable Late Proterozoic continental source, as indicated by low 206 Pb/ 204 Pb and 208 Pb/ 204 pb ratios and high 207 Pb/ 204 Pb ratios. Shortly after peak metamorphism and emplacement of the Bindal batholith, skarn related base metal and As-Au vein deposits were formed (lead isotope ratios of the sulfides are indistinguishable from the initial lead isotope ratios of the Bindal batholith), and the metals and ore-forming fluids were probably derived from the granitoid magmas. During the final uplift of the Caledonian nappes after orogenic collapse, structurally controlled base and precious metal sulfide deposits and occurrences were formed. The radiogenic 206 Pb/ 204 Pb ratios and low 208 Pb/ 204 Pb ratios indicate an ore-forming process involving convection of meteoric water and leaching of metals from the rocks in the brittle zone.This lead isotope survey of the sulfide deposits in the Helgeland Nappe Complex has revealed repeated ore-forming activity at previously mineralized sites which involved introduction of new lead when new metal reservoirs became available as a result of changing tectonic settings.

Late Pliocene paleoecology of the Guasca Valley (Cordillera Oriental, Colombia)

Organic-rich layers intercalated in a Late Pliocene fluvial-lacustrine sediment sequence located at 2650 m elevation in the Guasca Valley (high plain of Bogotá, Cordillera Oriental, Colombia) were analysed for plant micro- and macrofossils. Paleoenvironmental reconstructions indicate that a subandean forest (lower montane), at present found from 1000–2300 m alt., was prevalent in the area at that time. This subandean forest showed marked changes in its floristic composition. Several recorded forest taxa, such as Alchornea, Eugenia, Ilex, Myrica, Myrsine, are adapted to disturbance. Alternation of aquatic and paludal environments based on lithology and paleovegetation, shows that local conditions in the marginal areas of the basin were affected by major water level fluctuations. These dynamic conditions were most probably associated with the initial stage of development of the large tectonic-sedimentary basin of Bogotá near the end of the final uplift of the Cordillera Oriental. Sixty-three plant macrofossil types were recognized new for the study area, including leaves, fruits, seeds and bryophytes, and are described and illustrated.

Thermochronologic data from Tobago, West Indies: Constraints on the cooling and accretion history of Mesozoic oceanic‐arc rocks in the southern Caribbean

Fission‐track thermochronologic studies on mid‐Cretaceous dioritic and tonalitic rocks from Tobago, West Indies, provide important temporal constraints on post magmatic Mesozoic cooling as well as the final phase of accretion of these oceanic‐arc plutonic rocks onto the continental margin of northern South America. Zircon fission track data indicate that the plutonic rocks of Tobago cooled through the zircon closure isotherm (250° – 200°C) at about 103 Ma. Apatite ages and track length analysis suggest rapid cooling through the apatite annealing zone (120° – 60°C) at about 45 Ma. Furthermore, the track lengths in apatites from these plutonic rocks are all greater than 14 microns, which implies that these rocks have not been above 50°C since 45 Ma. If a geothermal gradient of 25°C/km is inferred, the analyzed samples have been at depths of less than 2 km since 45 Ma. Tobago has occupied an upper crustal structural position since the mid‐Cretaceous, and significant final uplift of these Mesozoic oceanic‐arc rocks occurred in the mid‐Eocene. The tectonothermal history of Tobago since the mid‐Eocene has not involved deep burial or a high‐temperature thermal flux. This history apparently has chiefly involved translation of the Tobago terrane along the Caribbean ‐ South America Plate Boundary Zone at upper crustal levels.

Tectonometamorphic evolution of Ghana, Togo and Benin in the light of the Pan-African/Brasiliano orogeny

The South Pan-African belt of West Africa (Ghana, Togo and Benin province) can be divided into three domains: 1. (1) the external nappes thrust over the West African craton, derived from passive-margin sedimentary deposits (Middle to Late Proterozoic) and displaying middle- to high-pressure metamorphism, such as the Kanté and Atacora nappes; 2. (2) the intermediate nappes including the suture-zone rocks with eclogitic assemblages (protoliths ca. 800 Ma), and mainly monocyclic metasedimentary rock and orthogneiss, that are found in central Togo and northwest Benin; 3. (3) the internal nappes composed of high-grade commonly anatectic gneiss (anatexis ca. 600 Ma), including orthogneiss (Eburnian plutons), elongated granulite belts, reworked Archean basement, and an intrusive granite and charnockite association (ca. 600 Ma), which are exposed in Benin and Nigeria. The P- T- t- d paths calculated from rocks in these three domains show a prograde-retrograde evolution: 1. (1) an initial clockwise evolution related to an early burial stage ( HP- HT in the intermediate nappes) followed by oblique thrusting in a SW direction, that caused nappe stacking and subsequent uplift; 2. (2) a second anti-clockwise evolution with late temperature and pressure increases, only recorded in the more internal units. This temperature increase was a consequence of the previous crustal thickening with subsequent doming, extension and anatexis. The pressure increase was related to movement along the Kandi fault, working as a dextral strike-slip fault with a transpressional component under late E-W shortening. The retrograde trend corresponds to the final uplift. The evolution of the belt can thus be divided into three successive stages: 1. (1) a first stage was characterized by an oblique collision inducing SW oblique thrusting with associated syn-foliation reverse metamorphism; 2. (2) during a second stage anatectic doming took place; followed by 3. (3) a third stage at still high temperature with dextral wrench faulting and associated N-S post-foliation folding under late E-W shortening. Comparison between the West African, North Brasiliano and Central African Late Proterozoic belts shows very strong similarities between their lithology, kinematics and metamorphism, and indicates that the Pan-African/Brasiliano chain resulted from collision between three major continental domains: the West African and São-Francisco/Congo cratons and a Late Proterozoic mobile domain.

The Palaeozoic history of an unusual intracratonic thrust belt in central Australia based on 40 Ar- 39 Ar, K-Ar and fission track dating

Intraplate thrusting, involving reactivation of a mid-Proterozoic province boundary within the basement metamorphic rocks of the Arunta block, occurs at the northern margin of the late Proterozoic to mid-Palaeozoic Amadeus Basin in central Australia. Earlier geological and geophysical studies show that the thrust system, referred to as the Alice Springs orogen, is 'thick-skinned' in the central part of the thrust belt and was dominated by the crust-cutting Redbank thrust zone. Several episodes of thrusting, defined stratigraphically, began about 400 Ma ago. Analysis and modelling of 40 Ar- 39 Ar age spectra on K-feldspar from the basement rocks, combined with regional geological constraints, point to cooling rates of 3–10 °C/Ma and moderate overall exhumation rates of 100–400 m/Ma during the Alice Springs orogeny, without appreciably increased heat flow. However, final uplift along the thrust front at 300–320 Ma was rapid, as the time of final closure of small domains in K-feldspar approximates apatite fission track ages. By using 40 Ar- 39 Ar age data as a constraint, together with available geological and geophysical data, crustal shortening across the thrust belt is thought to be approximately 20 to 30%. Overall convergence rates of 0.6–3 km/Ma are slow compared with rates of ≥ 5 km/Ma for continent-continent collision zones.

Structural-metamorphic evolution of the tia complex, new england fold belt; thermal overprint of an accretion-subduction complex in a compressional back-arc setting

Rocks in the Tia Complex underwent a simple, clockwise P- T- t-deformation path, moving from mid-crustal levels at blueschist facies conditions ( D 1, D 2; 6 kbar, 200°C) to upper-crustal levels at high- T amphibolite facies conditions ( D 4, D 5; 2.5 kbar, 600°C), in a relatively short time (∼ 15 Ma). The rocks remained under these conditions for approximately 40 Ma ( D 6) before they cooled and were brought to the surface ( D 7, D 8). During D 6,Permian rift basins were opening as thermal gradients in the Tia Complex reached a maximum. D 1–2 deformation was associated with accretion-subduction processes and was followed by progressively less intense, mainly E–W oriented compression and uplift during D 3–5, bringing the rocks to within 2 kbars of the surface. D 6 is characterized by limited extensional deformation, and final uplift took place during D 7–8, along shear zones that have orientations similar to D 3–5 thrusts. The shift from subduction-dominated processes to thermally-dominated processes occurred during D 3–4 uplift of ∼4 kbar, around 310-300 Ma. This coincided with a change in tectonic setting of the Tia Complex from an accretionary prism to a back-arc position, as the subduction zone stepped eastward. An increased heat flow, associated with back-arc processes, gradually caused thermal weakening of the lithosphere, which, due to external, subduction-related, compressive forces, resulted in uplift ( D 3). With continued heating of the crust, thermal doming resulted and uplift stopped ( D 4–5), whilst thermal weakening led to limited crustal collapse as Permian basins opened ( D 6). The extremely high heat flow that was maintained for ∼40 Ma. suggests a heat source external to the crust, possibly associated with thermal relaxation, dehydration and detachment of the remnant stab left beneath the New England Orogen after the subduction zone shifted east. A stable crustal, thermal structure was re-established by 260 Ma, as the crust strengthened and could further support subduction-induced, compressional structures ( D 7–8).

Contrasting thermomechanical evolutions in the Southalpine metamorphic basement of the Orobic Alps (Central Alps, Italy)

In the Southern Alps a progressive metamorphic zonation, with an increase in the geothermal gradient from NE to SW, has been widely proposed. However, recent investigations have shown that the greenschist metamorphic imprint of the low‐grade zone corresponds to a metamorphic retrogression following amphibolite facies conditions. On the other hand, in the medium‐grade zone, a later low‐pressure, high‐temperature (LPHT) metamorphic event has also been proposed. In an attempt to resolve these different interpretations, new petrological and partly new structural data have been obtained for two sectors of the Orobic Alps, traditionally attributed to different metamorphic zones. Thermobarometric determinations, supported by microstructural analysis, indicate the following different pressure‐retrograde paths in each sector: (1) in the Val Vedello basement (VVB) rocks, a first metamorphic imprint characterized by P= 7–9 kbar and T= 570–610°C was followed by a greenschist retrogression (P≤ 4 kbar and T≤ 500° C); (2) in the Lario basement (LB) rocks, the first detectable metamorphic stage, characterized by mineral assemblages indicating P= 7–9 kbar and T= 550–630° C, was followed by a LPHT event, synkinematic with F2 extensional deformation. A greenschist retrogression marks the final uplift of these rocks.Reinterpretation of the available geochronological data indicates a diachronism for the two thermomechanical evolutions. In the light of these data, we interpret the retrograde P–T–t path of the VVB rocks as a pre‐Permian post‐thickening uplift and the retrograde P–T–t evolution of the LB rocks as a Permo‐Mesozoic uplift related to the extensional tectonic regime of the Tethyan rifting.