Seismic, gravity and petrological evidence for partial melt beneath the thickened Central Andean crust (21–23°S)

On the basis of seismic refraction investigations and gravimetric data we have modelled the crustal structure of the southern Central Andes (21–23°S). A pronounced variation in crustal parameters is seen in N-S- and W-E-crossing seismic profiles over the entire Andean orogene, characterized by a crustal thickness of up to 70 km under the magmatic arc and backarc, strongly reduced seismic velocities and a Bouguer minimum of −450 mGal. Anomalously low velocities of 5.9–6.0 km/s in the deeper crust of the Western Cordillera and Altiplano regions lead to an over-compensation of the Bouguer minima resulting in values of crustal densities higher than estimates based purely on seismic velocity measurements. In an attempt to reconcile these differences, the behavior of crystalline rocks based on published laboratory data was studied under varying pressure and temperature conditions up to the range of partial melting. If the temperature is increased above the melting point, a rapid decrease in seismic velocity is accompanied by a slow decrease in density. For the Central Andes, a good fit of the observed and calculated Bouguer anomalies is obtained if the densities of the rocks from the low-velocity zone (LVZ) beneath the Western Cordillera and the Altiplano are varied. Model calculations lead to a velocity-density relation for partial molten rocks that allows the melt proportions of rocks to be estimated. Model calculations indicate that 15–20 vol.% of basaltic to andesitic melt at depth is necessary to explain the LVZ and Bouguer anomaly beneath the arc and parts of the backarc. High heat flow values (100 mW/m²) support the idea that large areas of the deeper Andean crust are strongly weakened by the presence of partially molten rocks, resulting in reduced seismic velocities, with the Western Cordillera, the active volcanic arc of the Andean mountain range, acting as a ductile buffer between the two more rigid crustal blocks of the forearc and backarc regions.     

Flexural and gravity modelling of the Mérida Andes and Barinas–Apure Basin, Western Venezuela

The kinematic evolution of the Barinas–Apure Basin and the southern Mérida Andes from Lower Miocene to the Present is numerically modelled using flexural isostatic theory and geophysical and geological data. Two published regional transects are used to build up a reference section, which is then used to constrain important parameters (e.g. shortenings and sedimentary thicknesses) for the flexural modelling. To control the location of the main fault system in the flexural model earthquake information is also used. The estimated flexural elastic thickness of the South American lithosphere beneath the Barinas–Apure Basin and the Mérida Andes Range is 25 km. The value for the final total shortening is 60 km. The flexural isostatic model shows that the Andean uplift has caused the South American lithosphere subsidence and the development of the Barinas–Apure Basin.In addition, gravity modelling was used to understand deep crustal features that could not be predicted by flexural theory. Consequently, the best-fit flexural model is used to build a gravity model across the Mérida Andes and the Barinas–Apure Basin preserving the best-controlled structural features from the flexural modelling (e.g. basin wavelength and depth) and slightly changing the main bodies density values and deep crustal structures. The final gravity model is intended to be representative of the major features affecting the gravity field in the study area. The predicted morphology in the lower crustal level of the final gravity model favours the hypothesis of a present delamination or megathrust of the Maracaibo crust over the South American Shield. This process would use the Conrad discontinuity as a main detachment surface within an incipient NW dipping continental subduction.     

Differential Late Paleozoic active margin evolution in South-Central Chile (37°S–40°S) – the Lanalhue Fault Zone

The N–S oriented Coastal Cordillera of South Central Chile shows marked lithological contrasts along strike at 38°S. Here, the sinistral NW–SE-striking Lanalhue Fault Zone (nomen novum) juxtaposes Permo-Carboniferous magmatic arc granitoids and associated, frontally accreted metasediments (Eastern Series) in the northeast with a Late Carboniferous to Triassic basal-accretionary forearc wedge complex (Western Series) in the southwest. The fault is interpreted as an initially ductile deformation zone with divergent character, located in the eastern flank of the basally growing, upwarping, and exhuming Western Series. It was later transformed and reactivated as a semiductile to brittle sinistral transform fault. Rb–Sr data and fluid inclusion studies of late-stage fault-related mineralizations revealed Early Permian ages between 280 and 270 Ma for fault activity, with subsequent minor erosion. Regionally, crystallization of arc intrusives and related metamorphism occurred between 306 and 286 Ma, preceded by early increments of convergence-related deformation. Basal Western Series accretion started at >290 Ma and lasted to 250 Ma. North of the Lanalhue fault, Late Paleozoic magmatic arc granitoids are nearly 100 km closer to the present day Andean trench than further south. We hypothesize that this marked difference in paleo-forearc width is due to an Early Permian period of subduction erosion north of 38°S, contrasting with ongoing accretion further south, which kinematically triggered the evolution of the Lanalhue Fault Zone. Permo-Triassic margin segmentation was due to differential forearc accretion and denudation characteristics, and is now expressed in contrasting lithologies and metamorphic signatures in todays Andean forearc region north and south of the Lanalhue Fault Zone.     

Lithospheric evolution of the Andean fold–thrust belt, Bolivia, and the origin of the central Andean plateau

We combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central Andean plateau between 18° and 20° S from Late Cretaceous to present. By integrating geophysical results of upper mantle structure, crustal thickness, and composition with recently published structural, stratigraphic, and thermochronologic data, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Four key steps in the evolution of the Andean plateau are as follows. 1) Initiation of mountain building by 70 Ma suggested by the associated foreland basin depositional history. 2) Eastward jump of a narrow, early fold–thrust belt at 40 Ma through the eastward propagation of a 200–400-km-long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 to 15 Ma, which thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera–Altiplano boundary during the early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the Eastern Cordillera and Altiplano at 24–23 Ma, and further volcanism recorded by 12–7 Ma crustal ignimbrites. 4) After 20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Long-term rates of shortening in the fold–thrust belt indicate that the average shortening rate has remained fairly constant (8–10 mm/year) through time with possible slowing (5–7 mm/year) in the last 15–20 myr. We suggest that Cenozoic deformation within the mantle lithosphere has been focused at the Eastern Cordillera–Altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination.     

Global 1° × 1° thermal model TC1 for the continental lithosphere: Implications for lithosphere secular evolution

This paper reports a new 1° × 1° global thermal model for the continental lithosphere (TC1). Geotherms for continental terranes of different ages (> 3.6 Ga to present) constrained by reliable data on borehole heat flow measurements (Artemieva, I.M., Mooney, W.D. 2001. Thermal structure and evolution of Precambrian lithosphere: a global study. J. Geophys. Res 106, 16387–16414.), are statistically analyzed as a function of age and are used to estimate lithospheric temperatures in continental regions with no or low-quality heat flow data (ca. 60% of the continents). These data are supplemented by cratonic geotherms based on electromagnetic and xenolith data; the latter indicate the existence of Archean cratons with two characteristic thicknesses, ca. 200 and > 250 km. A map of tectono-thermal ages of lithospheric terranes complied for the continents on a 1° × 1° grid and combined with the statistical age relationship of continental geotherms (z = 0.04  t + 93.6, where z is lithospheric thermal thickness in km and t is age in Ma) formed the basis for a new global thermal model of the continental lithosphere (TC1). The TC1 model is presented by a set of maps, which show significant thermal heterogeneity within continental upper mantle, with the strongest lateral temperature variations (as large as 800 °C) in the shallow mantle. A map of the depth to a 550 °C isotherm (Curie isotherm for magnetite) in continental upper mantle is presented as a proxy to the thickness of the magnetic crust; the same map provides a rough estimate of elastic thickness of old (> 200 Ma) continental lithosphere, in which flexural rigidity is dominated by olivine rheology of the mantle.Statistical analysis of continental geotherms reveals that thick (> 250 km) lithosphere is restricted solely to young Archean terranes (3.0–2.6 Ga), while in old Archean cratons (3.6–3.0 Ga) lithospheric roots do not extend deeper than 200–220 km. It is proposed that the former were formed by tectonic stacking and underplating during paleocollision of continental nuclei; it is likely that such exceptionally thick lithospheric roots have a limited lateral extent and are restricted to paleoterrane boundaries. This conclusion is supported by an analysis of the growth rate of the lithosphere since the Archean, which does not reveal a peak in lithospheric volume at 2.7–2.6 Ga as expected from growth curves for juvenile crust.A pronounced peak in the rate of lithospheric growth (10–18 km³/year) at 2.1–1.7 Ga (as compared to 5–8 km³/year in the Archean) well correlates with a peak in the growth of juvenile crust and with a consequent global extraction of massif-type anorthosites. It is proposed that large-scale variations in lithospheric thickness at cratonic margins and at paleoterrane boundaries controlled anorogenic magmatism. In particular, mid-Proterozoic anorogenic magmatism at the cratonic margins was caused by edge-driven convection triggered by a fast growth of the lithospheric mantle at 2.1–1.7 Ga. Belts of anorogenic magmatism within cratonic interiors can be caused by a deflection of mantle heat by a locally thickened lithosphere at paleosutures and, thus, can be surface manifestations of exceptionally thick lithospheric roots. The present volume of continental lithosphere as estimated from the new global map of lithospheric thermal thickness is 27.8 (± 7.0) × 10⁹ km³ (excluding submerged terranes with continental crust); preserved continental crust comprises ca. 7.7 × 10⁹ km³. About 50% of the present continental lithosphere existed by 1.8 Ga.     

Kinematic variations across Eastern Cordillera at 24°S (Central Andes): Tectonic and magmatic implications

The Eastern Cordillera (Central Andes,  24°S) consists of a basement-involved thrust system, resulting from Miocene–Quaternary eastward migrating compression, separating the Puna plateau from the Santa Barbara System foreland. The inferred Tertiary strains arising from shortening in the Eastern Cordillera and Santa Barbara System are similar, higher than in the Puna. Slip data collected on the major  N–S trending faults of Eastern Cordillera show a westward progression from dip-slip (contraction) to dextral and sinistral motions. This, consistently with established tectonic models, may result from partitioning due to the oblique Mio-Quaternary underthrusting of the Brazilian Shield north of 24°S. This strain partitioning has three main implications. (1) As the dextral and sinistral shear in the Eastern Cordillera are  62% and 29% of the compressive strain respectively, the Eastern Cordillera results more strained than Santa Barbara System foreland, contrary to previous estimates. (2) The partitioning in the Eastern Cordillera may find its counterpart in that to the west of the Central Andes, giving a possible structural symmetry to the Central Andes. (3) The easternmost N–S strike-slip structures in the Eastern Cordillera coincide with the easternmost Mio-Pliocene magmatic centres in the Central Andes, at  24°S. Provided that, further to the east, the crust is partially molten, the absence of magmatic centres may be explained by the presence of pure compressive structures in this portion of the Eastern Cordillera.     

Mountain building across a lithospheric boundary during arc construction: The Cretaceous Peninsular Ranges batholith in the Sierra San Pedro Martir of Baja California, Mexico

The Jura-Cretaceous Peninsular Ranges batholith (PRB) of Southern and Baja California contains a remarkable example of variation in crustal composition and structure across a batholith-parallel lithospheric-scale discontinuity. This lithospheric boundary between western oceanic-floored and eastern continental-floored crust influenced contractional deformation, arc magmatism, and differential exhumation of western and eastern zones in the batholith during its evolution.In the Sierra San Pedro Martir of Baja California, Mexico, a ca. 20 km wide, doubly vergent fan structure occurs across the PRB basement transition that consists of inward-dipping mylonite thrust sheets on the sides of the fan that gradually transition to a steeply-dipping tectonized zone in the center. A dramatic inverted metamorphic gradient occurs on the western side of this structure where mid-crustal amphibolite metamorphic grade rocks with peak pressures of 5–6 kbar in the center of the fan were thrust over upper-crustal sub-greenschist grade rocks (peak pressures < 2 kbar) in the western zone footwall. An inverted but smaller gradient occurs on the eastern side of the structure where rocks of the fan interior have been thrust eastwards over amphibolite to upper greenschist grade rocks (peak pressures 4–5 kbar).Gradients in cooling ages determined by ⁴⁰Ar/³⁹Ar biotite and K-feldspar and apatite fission track methods coupled with U–Pb zircon ages and Al in hornblende thermobarometry studies on plutons across this zone indicate that structures focused along the transition zone between contrasting lithosphere in the PRB accommodated nearly 15 km of the differential exhumation of western and eastern basement in the orogen. The western zone of the batholith was a major forearc depo-center for thick clastic sequences derived from the uplifting eastern PRB and remained at low average elevation during the Late Cretaceous and Paleogene. In contrast the eastern zone experienced dramatic uplift subsequent to achieving a crustal thickness in excess of 55 km by mid-Cretaceous time. This region had the isostatic potential for 4–5 km surface elevations, and likely formed a topographically high orogenic plateau. Exhumation of the fan structure initiated after 100 Ma and was largely complete by 85 Ma. Eastward-migrating unroofing of the rest of the eastern PRB continued into the Paleogene.A variety of factors were responsible for exhumation in this region. Structural thickening of the eastern zone of the orogen resulted from more than 30 million years of episodic contractional deformation in the fan structure, much of which followed island arc accretion of the western zone along this segment of the batholith. An episode of voluminous magmatism involving the intrusion of the 99–92 Ma La Posta-type magmatic suite across the eastern zone of the PRB triggered exhumation in the fan structure. Denudation in this region appears to have been solely by erosion; no evidence has been found for extensional tectonics during this time. This arc orogen demonstrates the important influence of inherited tectonic boundaries in controlling the spatial distribution of structural thickening and magmatism. It also displays the complex interrelationships among structural thickening, exhumation, and the role of magmatism in triggering exhumation episodes within orogens.     

Crustal-scale seismic profiles across Taiwan and the western Philippine Sea

We have used combined onshore and offshore wide-angle seismic data sets to model the velocity structure of the Taiwan arc–continent collision along three cross-island transects. Although Taiwan is well known as a collisional orogen, relatively few data have been collected that reveal the deeper structure resulting from this lithospheric-scale process. Our southern transect crosses the Hengchun Peninsula of southernmost Taiwan and demonstrates characteristics of incipient collision. Here, 11-km-thick, transitional crust of the Eurasian plate (EUP) subducts beneath a large, rapidly growing accretionary prism. This prism also overrides the N. Luzon forearc to the east as it grows. Just west of the arc axis there is an abrupt discontinuity in the forearc velocity structure. Because this break is accompanied by intense seismicity, we interpret that the forearc block is being detached from the N. Luzon arc and Philippine Sea plate (PSP) at this point. Our middle transect illustrates the structure of the developing collision. Steep and overturned velocity contours indicate probable large-scale thrust boundaries across the orogen. The leading edge of the coherent PSP appears to extend to beneath the east coast of Taiwan. Deformation of the PSP is largely limited to the remnant N. Luzon arc with no evidence of crustal thickening to the east in the Huatung basin. Our northern transect illustrates slab–continent collision—the continuing collision of the PSP and EUP as the PSP subducts. The collisional contact is below 20 km depths along this transect NE of Hualien. This transect shows elements of the transition from arc–continent collision to Ryukyu arc subduction. Both of our models across the Central Range suggest that the Paleozoic to Mesozoic basement rocks there may have been emplaced as thick, coherent thrust sheets. This suggests a process of partial continental subduction followed by intra-crustal detachment and buoyancy-aided exhumation. Although our models provide previously unknown structural information about the Taiwan orogen, our data do not define the deepest orogenic structure nor the structure of western Taiwan. Additional seismic (active and passive), geologic, and geodynamic modeling work must be done to fully define the structure, the active deformation zones, and the key geodynamic process of the Taiwan arc–continent collision.     

The isostatic state of the southern Urals crust

The Uralide orogen, in Central Russia, is the focus of intense geoscientific investigations during recent years. The international research is motivated by some unusual lithospheric features compared with other collisional belts including the preservation of (a) a collisional architecture with an orogenic root and a crustal thickness of 55–58 km, and (b) large volumes of very low-grade and non-metamorphic oceanic crust and island arc rocks in the upper crust of a low–relief mountain belt. The latter cause anomalous gravity highs along the thickened crust and the isostatic equilibrium inside the Uralides lithosphere as well as the overthrust high-metamorphic rocks. The integrated URSEIS '95 seismic experiment provides fundamentally new data revealing the lithospheric architecture of an intact Paleozoic collisional orogen that allows the construction of density models. In the Urals' lithosphere different velocity structures resolved by wide-angle seismic experiments along both the URSEIS '95- and the Troitsk profile. They can be used to constrain lithospheric density models: a first model consists of a deep subducted continental lower crust which has been highly eclogitized at depths of 60–90 km to a density of 3550 kg/m³. The second model shows a slightly eclogitized lower crust underlying the Uralide orogen with a crustal thickness of 60 km. The eclogitized lower crust causes a too-small impedance contrast to the lithospheric mantle resulting in a lack of reflectors in the area of the largest crustal thickness. Both models fit the measured gravity field. Analyzing the isostatic state of the southern Urals' lithosphere, both density models are in isostatic equilibrium.     

The Western Carpathians—interaction of Hercynian and Alpine processes

The complicated structural and rheologic properties of Western Carpathian lithosphere reflect the complex geodynamic history of the Carpathian orogen. Based on critical analysis of earlier models, new interpolation of existing geophysical data and results of integrated modelling, a new map of the lithosphere thickness for the Carpathian–Pannonian region has been constructed. The map allows for the distinction of a frontal orogen collision zone in the NE (from increased lithosphere thickness) as well as a zone of oblique collision with the Bohemian Massif in the West, where lithosphere is not significantly thickened. The MOHO discontinuity beneath the Western Carpathian hinterland (Danube and East Slovak Basins), as defined by deep reflection seismic profiling, is relatively shallow. This probably reflects recent crustal extension related to oblique collision between the European plate and the ALCAPA block and an increase of the asthenospheric updoming from the Middle Miocene onward.Crustal thickness reflects the combined effects of deep-seated orogenic processes and mantle thermal evolution beneath the Pannonian Basin system. In this study, we focus particularly the structures of: (1) the Late Alpine collision and Neogene back arc basin development, including deep-seated contacts between colliding plates, a zone of slab detachment, the compressional accretionary wedge of the Outer Western Carpathian Flysch Belt, and extensional structures produced by subduction rollback and asthenosphere upwelling; (2) Early Alpine structures related to Cretaceous thrust-stacking, including subhorizontal reflection packages (interpreted as multi-generational extensional structures), the underplated intra-Penninic (Oravic) continental ribbon, and ophiolite traces of the Meliatic oceanic suture; and (3) north-dipping reflectors interpreted as remnant Hercynian lithotectonic fragments with opposed vergency to the subducted Alpine units.     

Late Paleozoic–Early Triassic magmatism on the western margin of Gondwana: Collahuasi area, Northern Chile

The basement in the ‘Altiplano’ high plateau of the Andes of northern Chile mostly consists of late Paleozoic to Early Triassic felsic igneous rocks (Collahuasi Group) that were emplaced and extruded along the western margin of the Gondwana supercontinent. This igneous suite crops out in the Collahuasi area and forms the backbone of most of the high Andes from latitude 20° to 22°S. Rocks of the Collahuasi Group and correlative formations form an extensive belt of volcanic and subvolcanic rocks throughout the main Andes of Chile, the Frontal Cordillera of Argentina (Choiyoi Group or Choiyoi Granite-Rhyolite Province), and the Eastern Cordillera of Peru.Thirteen new SHRIMP U–Pb zircon ages from the Collahuasi area document a bimodal timing for magmatism, with a dominant peak at about 300 Ma and a less significant one at 244 Ma. Copper–Mo porphyry mineralization is related to the younger igneous event.Initial Hf isotopic ratios for the ~ 300 Ma zircons range from about − 2 to + 6 indicating that the magmas incorporated components with a significant crustal residence time. The 244 Ma magmas were derived from a less enriched source, with the initial Hf values ranging from + 2 to + 6, suggestive of a mixture with a more depleted component. Limited whole rock ¹⁴⁴Nd/¹⁴³Nd and ⁸⁷Sr/⁸⁶Sr isotopic ratios further support the likelihood that the Collahuasi Group magmatism incorporated significant older crustal components, or at least a mixture of crustal sources with more and less evolved isotopic signatures.     

Métabasites de la cordillère occidentale d'Équateur,témoins du soubassement océanique des Andes d'Équateur

The building-up of the Andean Range is linked to the subduction of the Pacific lithosphere beneath the South American plate. However, the formation of the Central Andes is marked by continental crustal shortening, whereas accretion and underplating of exotic oceanic terranes occurred in the northern Andes. The study of various magmatic and metamorphic rocks exhumed in the Western Cordillera of Ecuador by Miocene transpressive faults enables us to constrain the nature and thermal evolution of the crustal root of this part of Ecuador. These rocks are geochemically similar to oceanic plateau basalts. The thermobarometric peak conditions of a granulite and an amphibolite indicate temperatures of 800–850?°C and pressures less than 6–9 kbar (lack of garnet). The abnormally high geothermal gradient (≈40?°C?km^?1) is probably due to the activity of the magmatic arc, which developed on the accreted oceanic terranes after Late Eocene times, and may have provoked the re-mobilisation of deeply underplated oceanic material during the genesis of the Neogene to Recent arc. To cite this article: É. Beaudon et al., C. R. Geoscience 337 (2005).     

History of crustal growth and recycling at the Pacific convergent margin of South America at latitudes 29°–36° S revealed by a U–Pb and Lu–Hf isotope study of detrital zircon from late Paleozoic accretionary systems

Detrital zircon provides a powerful archive of continental growth and recycling processes. We have tested this by a combined laser ablation ICP-MS U–Pb and Lu–Hf analysis of homogeneous growth domains in detrital zircon from late Paleozoic coastal accretionary systems in central Chile and the collisional Guarguaráz Complex in W Argentina. Because detritus from a large part of W Gondwana is present here, the data delineate the crustal evolution of southern South America at its Paleopacific margin, consistent with known data in the source regions.Zircon in the Guarguaráz Complex mainly displays an U–Pb age cluster at 0.93–1.46 Ga, similar to zircon in sediments of the adjacent allochthonous Cuyania Terrane. By contrast, zircon from the coastal accretionary systems shows a mixed provenance: Age clusters at 363–722 Ma are typical for zircon grown during the Braziliano, Pampean, Famatinian and post-Famatinian orogenic episodes east of Cuyania. An age spectrum at 1.00–1.39 Ga is interpreted as a mixture of zircon from Cuyania and several sources further east. Minor age clusters between 1.46 and 3.20 Ga suggest recycling of material from cratons within W Gondwana.The youngest age cluster (294–346 Ma) in the coastal accretionary prisms reflects a so far unknown local magmatic event, also represented by rhyolite and leucogranite pebbles. It sets time marks for the accretion history: Maximum depositional ages of most accreted metasediments are Middle to Upper Carboniferous. A change of the accretion mode occurred before 308 Ma, when also a concomitant retrowedge basin formed.Initial Hf-isotope compositions reveal at least three juvenile crust-forming periods in southern South America characterised by three major periods of juvenile magma production at 2.7–3.4 Ga, 1.9–2.3 Ga and 0.8–1.5 Ga. The ¹⁷⁶Hf/¹⁷⁷Hf of Mesoproterozoic zircon from the coastal accretionary systems is consistent with extensive crustal recycling and addition of some juvenile, mantle-derived magma, while that of zircon from the Guarguaráz Complex has a largely juvenile crustal signature. Zircon with Pampean, Famatinian and Braziliano ages (< 660 Ma) originated from recycled crust of variable age, which is, however, mainly Mesoproterozoic. By contrast, the Carboniferous magmatic event shows less variable and more radiogenic ¹⁷⁶Hf/¹⁷⁷Hf, pointing to a mean early Neoproterozoic crustal residence. This zircon is unlikely to have crystallized from melts of metasediments of the accretionary systems, but probably derived from a more juvenile crust in their backstop system.     

Late Cenozoic transpressional ductile deformation north of the Nazca–South America–Antarctica triple junction

The southern Andes plate boundary zone records a protracted history of bulk transpressional deformation during the Cenozoic, which has been causally related to either oblique subduction or ridge collision. However, few structural and chronological studies of regional deformation are available to support one hypothesis or the other. We address along- and across-strike variations in the nature and timing of plate boundary deformation to better understand the Cenozoic tectonics of the southern Andes.Two east–west structural transects were mapped at Puyuhuapi and Aysén, immediately north of the Nazca–South America–Antarctica triple junction. At Puyuhuapi (44°S), north–south striking, high-angle contractional and strike-slip ductile shear zones developed from plutons coexist with moderately dipping dextral-oblique shear zones in the wallrocks. In Aysén (45–46°), top to the southwest, oblique thrusting predominates to the west of the Cenozoic magmatic arc, whereas dextral strike-slip shear zones develop within it.New ⁴⁰Ar–³⁹Ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike-slip, oblique-slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen. Similar ages were obtained on both high strain pelitic schists with dextral strike-slip kinematics (4.4±0.3 Ma, laser on muscovite–biotite aggregates, Aysén transect, 45°S) and on mylonitic plutonic rocks with contractional deformation (3.8±0.2 to 4.2±0.2 Ma, fine-grained, recrystallized biotite, Puyuhuapi transect). Oblique-slip, dextral reverse kinematics of uncertain age is documented at the Canal Costa shear zone (45°S) and at the Queulat shear zone at 44°S. Published dates for the undeformed protholiths suggest both shear zones are likely Late Miocene or Pliocene, coeval with contractional and strike-slip shear zones farther north. Coeval strike-slip, oblique-slip and contractional deformation on ductile shear zones of the southern Andes suggest different degrees of along- and across-strike deformation partitioning of bulk transpressional deformation.The long-term dextral transpressional regime appears to be driven by oblique subduction. The short-term deformation is in turn controlled by ridge collision from 6 Ma to present day. This is indicated by most deformation ages and by a southward increase in the contractional component of deformation. Oblique-slip to contractional shear zones at both western and eastern margins of the Miocene belt of the Patagonian batholith define a large-scale pop-up structure by which deeper levels of the crust have been differentially exhumed since the Pliocene at a rate in excess of 1.7 mm/year.     

Two-stage collision-related extrusion of the western Dabie HP–UHP metamorphic terranes, central China: Evidence from quartz c-axis fabrics and structures

The western Dabie orogen (also known as the Hong'an block) forms the western part of the Dabie–Sulu HP–UHP belt, central China. Rocks of this orogen have been subjected to pervasive ductile deformation, and include numerous quartz schists and felsic mylonites cropping out in ductile shear zones. Quartz textures in these mylonites contain important clues for understanding the movement sense of late-collisional extrusion and exhumation of high-pressure–ultrahigh-pressure (HP–UHP) rocks from the lower crustal level to the upper crustal level during Middle Triassic and Early Jurassic. The orientation and distribution of quartz crystallographic axes were used to confirm the regional shear sense across the orogen. The asymmetry of c-axis patterns consistently indicates top-to-the-southeast thrusting across the orogen in early structural stages. Later stages of deformation show different senses of movement in northern and southern parts of the orogen, with top-to-the-northwest sinistral shearing recorded in rocks north of the Xinxian HP–UHP eclogite-facies belt, and top-to-the-southeast dextral shearing south of the same unit.Based on our study on quartz c-axis fabrics and marco- to micro-scale structures, simultaneous southeastward shearing within a large part of the orogen and normal faulting north of the Xinxian HP–UHP unit is explained by upward extrusion in early stages of deformation. The extrusion process has been attributed to syn- and late-collisional processes, accounting for some earlier deformation in the western Dabie orogen such as metamorphic sequences around the core of the Xinxian HP–UHP eclogite-facies unit. Much higher pressure of deformation is also indicated in the aligned glaucophane and omphacite from blueschist and eclogite in the field. An orogen-parallel eastward extrusion of the Xinxian HP–UHP eclogite-facies unit, however, occurred diachronously in later stages of deformation. Therefore, a tectonic model combining an early upward extrusion with a later eastward extrusion is presented. Two different stages and types of extrusion for exhumation of HP–UHP rocks are suitable to all of east central China. Geochronological data shows that the first, upward extrusion occurred during Middle Triassic, the second, eastward extrusion occurred during Late Triassic to Early Jurassic. These two extrusions are correlative with two stages of rapid exhumation of the Dabie HP–UHP rocks, respectively. These two-stage late-collisional (Middle Triassic to Early Jurassic) extrusion events bridge the gap between syn-collisional (Early to Middle Triassic) vertical extrusion and post-collisional (Cretaceous) eastward-directed lateral escape and provide vital clues to understanding the more detailed processes of exhumation of HP–UHP rocks.     

Lower crustal melting and the role of open-system processes in the genesis of syn-orogenic quartz diorite–granite–leucogranite associations: constraints from Sr–Nd–O isotopes from the Bandombaai Complex, Namibia

The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous hornblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U–Pb zircon age of 540±3 Ma.

Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (_{Nd (540 Ma)}=−6.3 to −19.8; δ¹⁸O=9.0–11.6‰) but rather low and uniform initial Sr isotope compositions (⁸⁷Sr/⁸⁶Sr_initial=0.70794–0.70982). Two leucogranites and one aplite have higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.70828–0.71559), but similar initial _Nd (−11.9 to −15.8) and oxygen isotope values (10.5–12.9‰). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K₂O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr (<0.6) ratios and are probably generated by partial melting of meta-igneous (intermediate) lower crustal sources by amphibole-dehydration melting. Most of the leucogranites display higher Rb/Sr ratios (>1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material.     

Hot granulite nappes — Tectonic styles and thermal evolution of the Proterozoic granulite belts in East Africa

A section through the Neoproterozoic Mozambique Belt of Tanzania exposes western foreland (Archaean Tanzania Craton and Palaeoproterozoic Usagaran Belt), marginal (Western Granulites) and eastern, internal (Eastern Granulites) portions of the orogen. The assembly of granulite nappes at ca. 620 Ma displays westward emplacement along an eastward deepening basal decollement and forward propagation of thrusts, climbing from the deep crust to the surface. This goes along with eastward increase of syntectonic temperatures, derived from prevalent deformation mechanisms, and eastward decrease of the kinematic vorticity number. Distinctly different pressure - temperature paths with a branch of isothermal decompression (ITD) in Western Granulites and isobaric cooling (IBC) in Eastern Granulites reflect residence times of rocks within lower crustal levels. Western Granulites, exhumed rapidly at the orogen margin, display ITD and non-coaxial fabrics. Eastern Granulites in the internal orogen portions escaped from rapid exhumation and show IBC and co-axial flow fabrics. The vertical variation of structural elements, i.e. basement — cover relations within the Eastern Granulites, shows decoupling between lower and middle crust with horizontal west — east stretching in the basement and horizontal west — east shortening in the cover.A model of hot fold nappes [Beaumont, C., Nguyen, M.H., Jamieson, R.A., Ellis, S., 2006. Crustal flow modes in large hot orogens. In: Law, R.D., Searle, M.P., Godin, L., (eds). Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geological Society, London, Special Publications. vol. 268, 91–145] is adopted to explain flow diversity in the deep crust. The lower crust represented by Eastern Granulite basement flowed coaxially outwards (westward) in response to thickened crust and elevated gravitational forces, supported by a melt-weakened, viscous channel at the crustal base. Horizontal flow with rates faster than thermal equilibration gave rise to isobaric cooling. Simultaneously the mid crust (Eastern Granulite cover) was shortened when hot fold nappes moved along upward climbing thrust planes. Western Granulites preserved isothermal decompression through exhumation by thrusting and coeval erosion at the orogen front.Two different styles define the Neoproterozoic East African Orogen between northern Egypt and southern Mozambique. The Arabian Nubian Shield in the north is classified as small and cold orogen in which thin — skinned thrusting was associated with lateral extrusion. The Central Mozambique Belt in Tanzania/Southern Kenya is classified as large and hot orogen characterized by thick-skinned thrusting and assembly of large granulite nappes.     

Geochemical response of magmas to Neogene–Quaternary continental collision in the Carpathian–Pannonian region: A review

The Carpathian–Pannonian Region contains Neogene to Quaternary magmatic rocks of highly diverse composition (calc-alkaline, shoshonitic and mafic alkalic) that were generated in response to complex microplate tectonics including subduction followed by roll-back, collision, subducted slab break-off, rotations and extension. Major element, trace element and isotopic geochemical data of representative parental lavas and mantle xenoliths suggests that subduction components were preserved in the mantle following the cessation of subduction, and were reactivated by asthenosphere uprise via subduction roll-back, slab detachment, slab-break-off or slab-tearing. Changes in the composition of the mantle through time are evident in the geochemistry, supporting established geodynamic models.Magmatism occurred in a back-arc setting in the Western Carpathians and Pannonian Basin (Western Segment), producing felsic volcaniclastic rocks between 21 to 18 Ma ago, followed by younger felsic and intermediate calc-alkaline lavas (18–8 Ma) and finished with alkalic-mafic basaltic volcanism (10–0.1 Ma). Volcanic rocks become younger in this segment towards the north. Geochemical data for the felsic and calc-alkaline rocks suggest a decrease in the subduction component through time and a change in source from a crustal one, through a mixed crustal/mantle source to a mantle source. Block rotation, subducted roll-back and continental collision triggered partial melting by either delamination and/or asthenosphere upwelling that also generated the younger alkalic-mafic magmatism.In the westernmost East Carpathians (Central Segment) calc-alkaline volcanism was simultaneously spread across ca. 100 km in several lineaments, parallel or perpendicular to the plane of continental collision, from 15 to 9 Ma. Geochemical studies indicate a heterogeneous mantle toward the back-arc with a larger degree of fluid-induced metasomatism, source enrichment and assimilation on moving north-eastward toward the presumed trench. Subduction-related roll-back may have triggered melting, although there may have been a role for back-arc extension and asthenosphere uprise related to slab break-off.Calc-alkaline and adakite-like magmas were erupted in the Apuseni Mountains volcanic area (Interior Segment) from15–9 Ma, without any apparent relationship with the coeval roll-back processes in the front of the orogen. Magmatic activity ended with OIB-like alkali basaltic (2.5 Ma) and shoshonitic magmatism (1.6 Ma). Lithosphere breakup may have been an important process during extreme block rotations (60°) between 14 and 12 Ma, leading to decompressional melting of the lithospheric and asthenospheric sources. Eruption of alkali basalts suggests decompressional melting of an OIB-source asthenosphere. Mixing of asthenospheric melts with melts from the metasomatized lithosphere along an east–west reactivated fault-system could be responsible for the generation of shoshonitic magmas during transtension and attenuation of the lithosphere.Voluminous calc-alkaline magmatism occurred in the Cãlimani-Gurghiu-Harghita volcanic area (South-eastern Segment) between 10 and 3.5 Ma. Activity continued south-eastwards into the South Harghita area, in which activity started (ca. 3.0–0.03 Ma, with contemporaneous eruption of calc-alkaline (some with adakite-like characteristics), shoshonitic and alkali basaltic magmas from 2 to 0.3 Ma. Along arc magma generation was related to progressive break-off of the subducted slab and asthenosphere uprise. For South Harghita, decompressional melting of an OIB-like asthenospheric mantle (producing alkali basalt magmas) coupled with fluid-dominated melting close to the subducted slab (generating adakite-like magmas) and mixing between slab-derived melts and asthenospheric melts (generating shoshonites) is suggested. Break-off and tearing of the subducted slab at shallow levels required explaining this situation.     

No flat Wadati–Benioff Zone in the central and southern central Andes

The Wadati–Benioff Zone (WBZ) is an approximate plane defined by earthquakes hypocentres observed in convergent plate boundaries and that usually dips at angles greater than 30°. In some areas of the Andes, where there are gaps in volcanic activity, and where heat flow is abnormally low, this plane in most studies has nearly horizontal dip at a depth of about 75–100 km, and it has been associated to flat subduction of the oceanic lithosphere. This situation has been taken as the present-day analogue of the Laramide orogeny of western North America for which a ‘flat-slab’ episode has been proposed in the past years. In this work, the observed low heat flow in areas of the Andes is assumed to be due to low radiogenic heat generation in geologically old and allochthonous terranes constituting large regions of western South America. On the basis of geotherms obtained for areas of Ecuador, Peru, Chile and Argentina, and of rheological results describing the partition between brittle and ductile regimes, the seismic activity observed both in the lower crust and at depths of about 75–100 km is thoroughly explained. At these depths, earthquakes occur within the subcontinental upper mantle, and then there is no flat WBZ associated to subduction of the oceanic lithosphere. There is evidence from recent seismological observations that the real WBZ lies not horizontally and deeper in the tectonosphere.     

THERMAL MODELLING OF COLLISIONAL OROGENY:IMPLICATIONS FOR THE ULTRA-HIGH PRESSURE METAMORPHISM

The petrological research on the ultra-high pressure metamorphism (UHP) of collisional orogen indicates that the upper-crustal rocks is subducted to depths exceeding 100 km,and returned to the surface rapidly.In this study,we investigate the thermal structure of collisional orogen as a slab of continental lithosphere being subducted beneath an overriding wedge of continental lithosphere by the 2-D finite element method.The advection heat transfer due to the accretion of orogenic wedge is considered.