Tectonic Evolution Of Orogenic Belt Research Articles

Relative dating of fault activity using the principle of cross-cutting relationships: An automated approach

Fault dating plays an essential role in understanding deformation histories and modeling the tectonic evolution of orogenic belts. However, direct fault dating methods via different isotope geochronological techniques are expensive, and their use is often limited in many cases, making it essential to develop a fast and low-cost fault relative dating method. Therefore, on the basis of knowledge graphs and knowledge reasoning technology, this study proposes an automatic method to relatively date periods of fault activity using the principle of cross-cutting relationships between faults and strata. The method mainly involves (1) generating the knowledge graph based on a digital geological map; (2) using the knowledge reasoning algorithm to interpret the cross-cutting relationships amongst faults and generating the temporal sequence of fault activity; (3) relative dating the faults based on the cross-cutting relationships between faults and strata; and (4) according to the temporal sequence of fault activity, the relationship between faults can be revealed, and relative dating can be optimized. Results for cases in western Nevada and Qixia Hill of Nanjing illustrate the effectiveness of this method for interpreting the period of fault activity. The accuracy rates of the recognition results in the two cases were 90.24% and 80.77%, respectively, which means that the proposed method has the potential to relatively date fault activity across large areas. The algorithm is an effective supplement to the existing direct method of fault dating. The algorithm can efficiently infer the development sequence and the age of fault activity based on geological maps and geological cross-sections, which is of great significance for understanding regional tectonic history, evaluating earthquake disasters, and modeling tectonic evolution processes.

Petrogenesis of granitoids in the southern Anhui Province, China: Implications for the Neoproterozoic tectonic evolution of the eastern Jiangnan orogenic belt

The Jiangnan orogenic belt is a key component of the South China Block, yet the tectonic-magmatic evolution of the orogenic belt remains widely debated. The eastern segment of the Jiangnan orogenic belt experienced Neoproterozoic magmatism evidenced by widespread granitoids, which is key to understanding the tectonic evolution of the orogenic belt. In this study, we conducted detailed field, petrographic, geochemical, and geochronological studies of the Neoproterozoic Xucun pluton and adjacent magmatic rocks in the southern Anhui province of the eastern Jiangnan orogenic belt. Zircon LA-ICP-MS U-Pb dating of two samples yields interpreted crystallization ages of 830.3 ± 2.9 Ma and 831.2 ± 3.3 Ma. The Xuun pluton is an S-type, peraluminous biotite granodiorite that belongs to the calc-alkaline to high-K calc-alkaline series. εNd(t) values for the pluton range from −2.53 to −0.49, and the TDM2 model ages range from 1.66 to 1.83 Ga. The whole-rock, major and trace element and Sm-Nd isotopic compositions of the Xucun pluton suggest petrogenesis in a magmatic arc-related tectonic setting. These new constraints, combined with previous results, indicate that the Xucun pluton formed in a post-collisional tectonic setting. The source for the Xucun pluton was mainly juvenile crust mixed with some Mesoproterozoic continental crust. The juvenile crustal rocks were generated during arc magmatism before ca. 860 Ma. We interpret that the the eastern Jiangnan orogenic belt was generated during the ca. 860–830 Ma collision of the Shuangxiwu arc and a continental terrane. Subsequent collapse of the Jiangnan orogenic belt resulted in erosion and partial melting of juvenile crustal rocks to form S-type granitoids, including the Xucun pluton.

Zircon melt inclusion study on the Miocene Miuchi granitoid pluton, SW Japan: A new approach to estimate crystallization pressures of granitic rocks

Granitic rocks (sensu lato) are a major rock type in orogenic belts. The crystallization pressures of granitic rocks can help to unravel the magmatic processes and tectonic evolution of these regions, particularly in areas where metamorphic rocks are not exposed. However, geobarometric techniques that are suitable for granitoids are quite limited. In this study, we propose a new approach to estimate crystallization pressures of granitoid magmas using melt inclusions in zircon, a ubiquitous accessory mineral in these lithologies. Homogenization experiments of polymineralic inclusions hosted in zircon have been conducted from a biotite granite sample in the Miocene Miuchi pluton, southwest Japan arc, using a piston-cylinder high-pressure–high-temperature apparatus. The homogenized compositions of the inclusions have high SiO2 contents (76–80 wt%) implying that they represent fractionated interstitial melts trapped in growing zircon crystals. The rhyolite-MELTS geobarometer for the homogenized melt inclusions in oscillatory-zoned zircon rim yields pressures ranging from 80 to 114 MPa (with error of ±25 MPa), interpreted as the crystallization pressures of the Miuchi pluton. These results are consistent with field and petrographic observations suggestive of a shallow emplacement level. The approach presented here would be applicable to most granitoids, which could provide fundamental data to better understand granite petrogenesis and the tectonic evolution of orogenic belts.

Depositional and diagenetic processes in volcanic matrix‐rich sandstones from the Shanxi and Shihezi formations, Ordos Basin, China: Implication for volcano‐sedimentary systems

AbstractVolcanic eruptions can provide large amounts of sedimentary materials and expose fluvial valleys or lakes to catastrophic hyperpycnal flood events, but this process has not been documented in detail. The Ordos Basin Permian fluvio‐deltaic system reveals evidence for abundant volcanism linked to the tectonic evolution of orogenic belt around the basin and provides a ‘natural laboratory’ for investigating hyperpycnal flows associated with volcanic activity. This study analysed volcanic matrix‐rich sandstone (VMS) samples for petrology, mineralogy and geochemistry in order to address current and uncertain volcanogenic material provenance explanations and the lack of systematic investigations into the depositional and diagenetic processes of volcanic related sediments in the southwestern basin. By combining tectonic background surveys, detrital zircon geochronology and spatial distribution of volcanogenic materials, it was found that volcanogenic materials were not derived from the Yinshan‐Yanshan Orogenic Belt (YYOB) as previously thought, but instead evidence a southwestern origin from the North Qinling Orogenic Belt (NQOB). Volcanogenic materials retained in the provenance area during frequent volcanic eruptions were transported to the basin via fluvial systems shortly after the eruption. The associated sediments meet the criterion for hyperpycnites based on lithofacies associations, suggesting the occurrence of hyperpycnal flows during the deposition of VMS. During the subsequent burial stage, the VMS became three distinct types defined by their volcanic matrix content and have similar paragenetic sequences but different diagenetic intensities. The differing content of authigenic minerals in the three types was closely related to the sandstone pore structure characteristics and the evolution of volcanic‐matrix alteration materials. This article proposes a possible explanation for the previously unidentified tectonothermal events in the NQOB during the Permian, validating and reinforcing the theoretical work of hyperpycnal flow. This contribution provides new insights and understanding of the depositional and diagenetic processes of lacustrine basins with similar tectonic settings.

Two orogenic cycles recorded by eclogites in the Yuka–Luofengpo terrane: Implications for the Mesoproterozoic to early Paleozoic tectonic evolution of the North Qaidam orogenic belt, NW China

Eclogites documenting two orogenic cycles not only contain essential information on subduction and exhumation mechanisms during the most recent orogenic cycle, but can also provide insights into the earlier cycle. Such rocks are ideal for deciphering the tectonic evolution of orogenic belts with a multi-orogenic history. This is the case for the North Qaidam orogenic belt (NQOB) in northwest China where, for the first time, we have discovered eclogites that preserve a record of two orogenic cycles. The first orogenic event occurred at ca. 929–916 Ma and the second at ca. 439–430 Ma. The metamorphic P–T conditions of these two events were 5.0–7.0 kbar/616–669 °C and 29.0–32.5 kbar/620–670 °C, respectively. Zircon U–Pb dating yielded protolith ages of 1280–1070 Ma for these eclogites, which are much older than the protolith ages for other eclogites in the NQOB. The eclogites exhibit chondrite-normalized rare earth element patterns that are flat or depleted in light rare earth elements, similar to present-day N- and E-MORB. Magmatic zircon cores from the eclogites have high εHf(t) values of 8.4 ± 4.0 to 18.1 ± 1.1, comparable to depleted mantle within uncertainty, and indicating a juvenile crustal origin. Our data show that the eclogite protoliths were probably pre-Rodinia Mesoproterozoic oceanic crust that was emplaced in an active continental margin during oceanic subduction, and experienced Neoproterozoic metamorphism related to the amalgamation of Rodinia and early Paleozoic continental deep subduction. Using published data for all eclogites and their host gneisses, we reconstructed the Mesoproterozoic to early Paleozoic tectonic evolution of the NQOB. Given the similar Meso–Neoproterozoic evolution of the NQOB and adjacent Tarim Block, we suggest that the Qaidam Block was the eastward extension of the Tarim Block during the Neoproterozoic.

About this title - HP–UHP Metamorphism and Tectonic Evolution of Orogenic Belts

High pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks play a key role in understanding the tectonic evolution of orogenic belts. They have typically experienced complex changes during subduction and exhumation processes arising from recrystallization, deformation, fluid–rock interactions and even partial melting, and may therefore carry valuable records of evolving geodynamic systems in an orogenic belt. This special publication addresses the current work on HP–UHP metamorphism and its relation to the tectonic evolution of orogenic belts. This special publication contains fifteen papers covering the important orogenic belts of the Himalaya, Dabie–Sulu, Tian Shan, North Qaidam and others that have been grouped into three parts: (I) new developments in the determination of metamorphic pressure–temperature (PT) conditions and their timing, (II) overview papers of well-known HP–UHP metamorphic belts and (III) research papers for some newly discovered HP–UHP belts.

Late Triassic high Mg diorites of the Wulong pluton in the South Qinling Belt, China: Petrogenesis and implications for crust-mantle interaction

High Mg diorites provide important insights into crust-mantle interaction during tectonic evolution of orogenic belt. An integrated study including zircon U-Pb dating, whole-rock geochemistry and isotopic compositions of Sr and Nd was carried out on high Mg diorites of the Wulong pluton located in the narrowest part of the South Qinling Belt. Crystallized ages of 218-210 Ma are obtained in combination with the published data. These rocks are characterized with relatively high MgO (2.76–7.17%), Cr (68.7-310 ppm) and Ni (21.3–76.3 ppm) contents. Most diorites exhibit high Sr/Y and La/Yb ratios with negligible Eu anomaly. Isotopic compositions of whole-rock show enriched attributes (87Sr/86Sr(i) = 0.7050–0.7071, εNd(t) = −7.56 to −3.45). Geochemical models indicate that these rocks did not result from melting of subducted slab, magma mixing, fractional crystallization or melts derived from delaminated lower crust reacted with mantle. Instead, high Mg diorites were generated through basaltic melts derived from asthenosphere mantle assimilating crustal components with crystallized phase of garnet-bearing amphibolite (AFC). Together with regional geological setting of the South Qinling Belt, we propose that the parental magma of these high Mg diorites was probably originated from upwelled asthenosphere mantle at the base of thinned crust after delamination of thickened crust during Late Triassic. The continental crust of the South Qinling Belt was thickened as a result of collision with the Yangtze Block during Middle Triassic. Then eclogitized lower crust collapsed and foundered into asthenosphere mantle due to partial melting of thickened crust during Late Triassic. In response, the upwelling and underplating of asthenosphere mantle took place at the base of continental crust. Moreover, upwelling of hot mantle may provide the heat source for migmatites and initiate the doming of Foping area.

HP–UHP metamorphism and tectonic evolution of orogenic belts: introduction

AbstractHigh pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks play a key role for understanding the tectonic evolution of orogenic belts (Johnson, M.R.W. & Harley, S.L. 2012. Orogenesis: The Making of Mountains. Cambridge University Press, Cambridge). They have typically experienced complex changes during subduction and exhumation processes, arising from recrystallization, deformation, element redistribution, fluid–rock interactions and even partial melting, and may therefore carry a valuable record of evolving geodynamic systems in an orogenic belt. Until now, more than 20 UHP metamorphic belts, i.e. belts which contain rocks that experienced pressure–temperature (P–T) conditions exceeding the lower limit of the coesite stability field, have been identified all over the world (Carswell, D.A. & Compagnoni, R. 2003. Ultrahigh Pressure Metamorphism. Eotvos University Press, European Mineralogical Union, Notes in Mineralogy, Budapest; Liou, J.G., Ernst, W.G., Zhang, R.Y., Tsujimori, T. & Jahn, B.M. 2009. Ultrahigh-pressure minerals and metamorphic terranes – the view from China. Journal of Asian Earth Sciences,35, 199–231; Zheng, Y.F., Zhang, L.F., McClelland, W.C. & Cuthbert, S. 2012. Processes in continental collision zones: preface. Lithos, 136, 1–9). New and innovative studies from different geoscience disciplines have been invaluable in developing a better understanding of the geodynamic evolution of orogenic belts. This special issue contains 15 papers, most of which were presented as part of the session ‘HP–UHP metamorphism and tectonic evolution of orogenic belts’, held at the 35th International Geological Congress in Cape Town, South Africa during 27 August to 4 September, 2016.

Petrogenesis of ca. 1.95 Ga meta-leucogranites from the Jining Complex in the Khondalite Belt, North China Craton: Water-fluxed melting of metasedimentary rocks

Anatexis occurs in response to changing P-T-fluid conditions during orogenesis, is thus crucial for understanding the tectonic evolution of orogenic belts. Here we report an integrated petrological, geochemical, whole-rock oxygen isotopic and zircon U-Pb-Hf-O isotopic study of meta-leucogranites from the Jining Complex, North China Craton, to unravel its petrogenesis and constrain the tectonic evolution of the Paleoproterozoic Khondalite Belt. The meta-leucogranites are weakly peraluminous with homogeneous minimum-melt compositions, and are characterized by low Rb contents and Rb/Sr ratios, high Ba and Sr contents and Sr/Y ratios, and moderately fractionated REE patterns without obvious Eu anomalies. These trace-element characteristics differ from that of leucogranites derived from fluid-absent melting. Considering their low whole-rock Zr saturation temperatures (746–780°C) and comparable compositions with experimentally generated melts produced by water-fluxed melting, we infer that the meta-leucogranites are low-temperature granites, and were generated by water-fluxed melting of metasedimentary rocks at 6–8kbar. SIMS zircon U-Pb analyses suggest that the meta-leucogranites were emplaced at∼1.95Ga or slightly earlier, and were subsequently metamorphosed and cut by a pegmatite dike at∼1.92Ga. The abundant inherited zircons yield nearly concordant 207Pb/206Pb ages ranging from 2.24 to 1.98Ga, which are consistent with the detrital zircon age spectra of the metasedimentary rocks from the Khondalite Belt. The Hf isotopic compositions of inherited and magmatic zircons (εHf(t)=+0.2 to +7.7) closely overlap with the time evolved εHf range of detrital zircons from metasediments in the Khondalite Belt. Different types of zircons (inherited, magmatic and metamorphic zircons) have similar δ18O values of 7.2–9.2‰, which is lower than that of metamorphic zircons (10.0–13.6‰) from surrounding metasediments. Thus, we propose that the meta-leucogranites were formed by water-fluxed melting of metasedimentary rocks from the Khondalite belt at∼1.95Ga during the prograde upper amphibolite-facies metamorphism.

Oligocene HP metamorphism and anatexis of the Higher Himalayan Crystalline Sequence in Yadong region, east-central Himalaya

The Higher Himalayan Crystalline Sequence (HHCS) provides an excellent natural laboratory to study continental subduction, crustal melting and tectonic evolution of orogenic belt generated through the collision of India with Eurasia. Our petrological study and phase equilibrium modeling reveal that the pelitic migmatites in the HHCS of Yadong region, east-central Himalaya, preserve an early mineral assemblage garnet, kyanite, biotite, quartz, plagioclase, K-feldspar, rutile and ilmenite, and a late sillimanite- and/or cordierite-bearing assemblage, and underwent the high pressure (HP) and high temperature (HT) granulite-facies metamorphism and associated partial melting under P–T conditions of ca. 12kbar and 825–845°C, followed by nearly isothermal decompression and isobaric cooling. The anatexis of the migmatites occurred dominantly through dehydration-melting of both muscovite and biotite during the prograde metamorphism. The melt produced in the peak metamorphic conditions is about 20 to 30vol.% of the rocks, and a significant amount of melt has been extracted from the source leading to the formation of Himalayan leucogranites. The zircon U–Pb dating data shows that the migmatites probably witnessed a prolonged melting episode that began at ca. 30Ma and lasted to ca. 20Ma. These results show that the thickening lower crust of the Himalayan orogen experienced long-lived and continued HP and HT metamorphism and pervasive anatexis, supporting the models on channel flow.

Two episodes of Late Paleozoic A-type magmatism in the Qunjisayi area, western Tianshan: Petrogenesis and tectonic implications

A-type magmatism is an effective lithoprobe for constraining the tectonic evolution of orogenic belts. In this study, we have identified two episodes of Late Paleozoic A-type magmatism in Qunjisayi, western Tianshan. Zircon U–Pb geochronology has yielded Late Carboniferous (rhyolites, 306±2Ma)- and Early Permian (granites, 296±3Ma) ages. Both the Qunjisayi rhyolites and granites have depleted Sr–Nd–Hf isotopic compositions, moderate A/CNK ratios (0.91–1) and negative Nb and Ta anomalies. They are also characterized by high K2O+Na2O (8.72–10.6wt.%), FeOT/(FeOT+MgO) (0.86–0.99) and Y/Nb (1.42–2.34) values, similar to those of typical A2-type granitoids. The Qunjisayi rhyolites may have been formed by partial melting of a juvenile calc-alkaline granitic crustal source induced by subduction roll-back in shallow depth. The succeeding Qunjisayi granites were likely to be derived from a lower crustal source that contains depleted mantle-derived components. Asthenospheric mantle upwelling, triggered by post-subduction slab break off, may have been important in the Qunjisayi granites formation. We conclude that the Late Paleozoic Qunjisayi A-type magmatism was genetically linked to the geodynamic transformation from late subduction to collision.

Anatomy of zircon growth in high pressure granulites: SIMS U–Pb geochronology and Lu–Hf isotopes from the Jiaobei Terrane, eastern North China Craton

Granulite facies rocks that witnessed high pressure and high temperature metamorphism preserve important records of the growth and recrystallizaton of zircons during the multiple stages of tectonic evolution of orogenic belts developed in complex subduction–collision settings. Here we investigate zircons in the Paleoproterozoic high pressure (HP) granulites from Jiaobei Terrane in the eastern North China Craton that experienced a multi-stage metamorphic evolution. We present SIMS zircon U–Pb ages for zircons from garnet amphibolites, mafic granulites and pelitic granulites. Our data show four age peaks at 2.5Ga, 1.92Ga, 1.87–1.85Ga and 1.83Ga. Zircons belonging to the latter three age groups show similar characteristics of low Th and U contents, low Th/U ratios (<0.1), sector zoning or unzoned internal structures, corresponding to the features of zircon growth during metamorphism. Using zircon Lu–Hf isotopic compositions combined with a careful analysis of the morphology of the zircons, we evaluate the processes of solid-state recrystallization, intergranular melt growth, coupled dissolution–reprecipitation and subsolidus growth. Zircons formed through solid-state recrystallization during peak metamorphic stage inherited the relatively high 176Lu/177Hf ratios and low 176Hf/177Hf ratios of zircons from the protoliths. The youngest U–Pb age of ca. 1899Ma is interpreted as the maximum estimate of the time of peak metamorphism. Zircons formed through intergranular melt growth during low- to medium-pressure retrogression display homogeneous Lu–Hf isotope compositions, low 176Lu/177Hf and high 176Hf/177Hf ratios. The U–Pb age of 1870–1850Ma yielded by zircons crystallized from intergranular melt is interpreted to represent the time of low- to medium-pressure granulite facies retrogression. Some zircon grains with prismatic morphology and low 176Lu/177Hf and 176Hf/177Hf ratios display heterogeneous Lu–Hf isotopic compositions, possibly related to coupled dissolution–reprecipitation. The U–Pb ages of these zircons are identical to those formed from intergranular melt and these two groups of zircons formed during the same metamorphic stage. Zircons formed through subsolidus growth show large variations in Lu–Hf isotopic compositions and their ca.1833Ma age represents the timing of amphibolite facies retrogression. The HP granulite facies metamorphism in Jiaobei is thus constrained between 1870 and 1899Ma, with low- to medium-pressure granulite facies retrogression at ca. 1850–1870Ma followed by amphibolite facies retrogression at ca. 1833Ma.Our data show that several zircon cores and recrystallized grains were entrained from basement rocks during the emplacement of the mafic magmatic protoliths in the case of mafic granulites. For the pelitic granulites, the older zircons are detrital grains of magmatic origin. The Hf crustal model ages of these zircons that show peaks at ca. 2.7Ga and 2.9Ga are consistent with the timing of mafic magmatism in the region as identified in previous studies. The Lu–Hf isotopes of newly grown metamorphic zircons which have equilibrated chemical compositions trace the nature of the protolith. The zircon Hf crustal model age of ca. 2.5–2.7Ga from the pelitic granulites is consistent with that of the basement rocks in the Jiaobei Terrane and the zircon Hf model ages (TDM) of ca.2.0Ga and 2.2–2.3Ga from the mafic granulites might represent the time of magma generation from the mantle with crustal contamination. The geochronology of the basement rocks as revealed by the old zircon cores and the metamorphic evolution of the Jiaobei Terrane are comparable with the tectonic history of the central and western North China Craton and provide insights into the complex tectonothermal events associated with the Neoarchean–Paleoproterozoic crustal evolution history of the craton. Also, the pervasive ~2.5Ga zircons in a variety of rocks seem to suggest that the Jiaobei Terrane was already part of a coherent tectonic unit at such time.

A laser-probe 40Ar/ 39Ar study of pseudotachylite from the Tambach Fault Zone, Kenya: direct isotopic dating of brittle faults

Understanding the tectonic evolution of orogenic belts and intracratonic areas depends on our ability to determine the age of tectonic features on a variety of scales. This study demonstrates the value of the laser-probe 40Ar/ 39Ar dating technique, which, if applied to fault-derived pseudotachylites, may be used to directly determine the age of brittle faults. The laser-probe technique affords high spatial resolution, enabling a greater opportunity for discriminating between pseudotachylite matrix, host-rock clasts and alteration products that are often present in varying proportions within pseudotachylites. The laser-probe 40Ar/ 39Ar technique has been applied to pseudotachylite samples from the Tambach Fault Zone (TFZ), a major NW–SE trending strike-slip fault within the Kenyan part of the Late Proterozoic/Early Palaeozoic Mozambique Belt. The pseudotachylites of the TFZ were previously thought to have formed either (i) at about 530–430 Ma, or (ii) during the Cenozoic evolution of the Kenya Rift. In the latter case, seismic slip on the rift-bounding normal fault would have generated the pseudotachylites, due to the reactivation of old NW–SE trending structures in the basement. Based on our new data, we interpret the pseudotachylite formation age to be 400 Ma. This rules out the possibility that the pseudotachylites are related to the formation of the Kenya Rift. Although the inherited basement faults may have been locally reactivated as transfer faults, reactivation of these structures during rifting did not occur beyond the margins of the Kenya Rift.

Contrasts in tectonic evolution of orogenic belts in the south-east pacific

Abstract The orogens of southern South America and Antarctic Peninsula, which are of similar age and apparently linked by the Scotia Arc, are shown to have evolved along markedly different patterns of geosynclinal and orogenic history. The Andes of Patagonia and Tierra del Fuego originated from a Late Mesozoic mio-eugeosynclinal couple and volcanic island arc created by subsidence, extension and rifting. The trend of this belt is unrelated to, and across the previously established Gondwanide framework. During the orogenic stage this marginal sea along the borders of an old continent, where apparently new oceanic crust had been formed, was welded to the continental block; the Andes thus constitute an accretionary belt. Orogenic polarity follows geosynclinal polarity; both geosynclinal and orogenic evolution are characterised by progressive outward migration towards the foreland, incorporating successive flysch and molasse basins in the fold belt. Alpino-type tectogenesis is accompanied by high-grade metamorphism and migmatisation. Recent calc-alkaline andesitic-dacitic volcanism in the Andes, their taphrogenic break-down along the Pacific and association with a deep-sea trench indicate a continuing orogenic regime. In the Antarctic Peninsula, no Late Mesozoic geosynclinal orogensis is recognised. I ts history is dominated by Gondwanide post-orogenic consolidation. Subsequent volcanism and localised, partly fault-bounded basins filled with terrestrial to shallowmarine sediments of limited thickness are the characteristic features of JurassicCretaceous time. Mid-Cretaceous large-scale intrusion of granodioritic magmas was accompanied and followed by regional unlift. This orogensis is superimposed upon, and parallel to the Gondwanide orogenic belt. Deformation is germanotype, except at the base of the Peninsula where tight folding occurred prior to the intrusions. Orogenic molasse sequences deposited in external foredeeps remained little disturbed tectonically. Cenozoic rifting and basaltic volcanism of subalkaline composition indicate that no orogenic regime is affecting this region. The Andean orogen of southernmost South America thus is a continental margin feature where accretion occurred, and which is unrelated to Gondwanide tectonics, whereas the Antarctic Peninsula is an intracontinental orogen of essentially Gondwanide inheritance. The contrasting tectonic setting and framework, sedimentary and orogenic history and structure of the two orogenic belts indicate that they have not been an originally continuous and straight mountain belt along the Pacific margin, prior to bending and disruption in the Scotia Arc region. The term “ Antarctandes” therefore is misleading and should not be used. The Scotia Arc appears as a composite feature of much greater complexity, which probably originated by juxtaposition of tectonic elements of entirely different origin and character, and with partial bending only.