Andean-type Orogen Research Articles

Seismotectonic Setting of the Andes along the Nazca Ridge Subduction Transect: New Insights from Thermal and Finite Element Modelling

The structural evolution of Andean-type orogens is strongly influenced by the geometry of the subducting slab. This study focuses on the flat-slab subduction of the Nazca Ridge and its effects on the South American Plate. The process of flat slab subduction impacts the stress distribution within the overriding plate and increases plate coupling and seismic energy release. Using the finite element method (FEM), we analyse interseismic and coseismic deformation along a 1000 km transect parallel to the ridge. We examine stress distribution, uplift patterns, and the impact of megathrust activity on deformation. To better define the crust’s properties for the model, we developed a new thermal model of the Nazca Ridge subduction zone, reconstructing the thermal structure of the overriding plate. The results show concentrated stress at the upper part of the locked plate interface, extending into the Coastal and Western Cordilleras, with deeper stress zones correlating with seismicity. Uplift patterns align with long-term rates of 0.7–1 mm/yr. Cooling from flat-slab subduction strengthens the overriding plate, allowing far-field stress transmission and deformation. These findings provide insights into the tectonic processes driving stress accumulation, seismicity, and uplift along the Peruvian margin.

Early Cretaceous A-Type Acidic Magmatic Belt in Northern Lhasa Block: Implications for the Evolution of the Bangong–Nujiang Ocean Lithosphere

A-type granites have been the subject of considerable interest due to their distinct anorogenic geological background. The A-type and arc-related granites are crucial in deciphering the evolution of the ocean closure and continental collision in the Tibet Plateau. The demise of the Bangong–Nujiang suture zone (BNSZ) and the Yarlung–Tsangpo suture zone was accompanied by the emplacement of volumes of syn-collisional and post-collisional granites. Controversy has persisted regarding the contribution of the collisional granites within the Lhasa Block to the growth of the Tibetan Plateau. This study provides key evidence about the evolution of the Lhasa Block and Bangong–Nujiang Ocean (BNO) by the newly documented 1200 km long, Early Cretaceous A-type acidic magmatic belt. The resolution was achieved through the utilization of petrology, whole-rock geochemistry, zircon U-Pb geochronology, and in situ zircon Hf isotope analysis of the Burshulaling Granites in the eastern segment and previous existing data in the central and western segment of the Lhasa Block. The Burshulaling Granites are characterized as peraluminous, high-K calc-alkaline series, indicating a post-collision setting with high temperature and low pressure. The zircon grains from two granite samples yield 206Pb/238U ages of 115–113 Ma. In situ zircon Hf analyses with 206Pb/238U ages give εHf(t) of −6.2–0.6, showing prominent characteristics of crust-mantle interaction. Granites from east to west exhibit whole-rock geochemical and geochronological similarities that fall within the well-constrained Early Cretaceous time frame (117–103 Ma) and track post-collisional A-type acidic magmatic belt along BNSZ. We argue that this magmatism resulted from slab break-off or orogenic root detachment, leading to melting and mixing of the lower crust. Meanwhile, this study indicates the existence of the Bangong–Nujiang Ocean southward subduction or a collapse following an Andean-type orogen.

An assessment of dip-slip versus strike-slip faulting modes along the Patagonian Andes (39°-50° S) and their related orogenic models

ABSTRACT The compilation of a database of 225 strain/stress tensors (accounting for ~ 4000 fault-striae data pairs) combined with evidence of syntectonic strata, and a seismotectonic appraisal along the Patagonian Andes (39°-50° S; South America), leads us to assess typical features of Mesozoic to current faulting in this segment of the Andean orogen. At the orogen scale, the fault-slip and focal mechanism database show the prevalence of strike-slip faulting, challenging the classic proposal for the Andean orogen of alternating extensional and contractional deformation phases. However, the wrenching tectonism in Cenozoic times may be explained through a strain partitioning model in the North Patagonian Andes by re-activating a large-scale, inherited anisotropy within the basement (namely the Liquiñe-Ofqui fault system), which is currently the locus of significant seismicity. On the other hand, a non-partitioned context allows explaining the Cenozoic patterns of brittle deformation the South Patagonian Andes. Our results highlight that the classifications of Andean-type orogens should integrate both inherited anisotropies and the heterogeneous distribution of strain across the upper plate in order to address complex patterns of deformation.

Fore-to-retroarc crustal structure of the north Patagonian margin: How is shortening distributed in Andean-type orogens?

Orogenic belts like the Andes experience several changes during long-term evolution of the subduction margin; however, the degree to which their different components (the fore-to-retroarc domains) are jointly or separately deformed is not well understood. To investigate this problem, we provide new field and seismic data of the Chilean forearc at the Chiloé island latitude (~41-44°S) integrated with the retroarc structure, in order to obtain a clear visualization of the crustal architecture in the north Patagonian margin. Field work in the western Andean flank and Coastal Cordillera along with 2D multichannel lines, basement isobaths and borehole data in the forearc basins are used to build a structural framework and stratigraphic correlations between the offshore and onshore areas. Forearc syntectonic strata associated with basement faults show a change in the tectonic regime from late Oligocene-early Miocene extension to middle-late Miocene contraction, coincidently to the previously determined deformational stages in the main cordillera and retroarc regions. This coupled fore-to-retroarc behavior ceased during the Pliocene when glaciations covered the North Patagonian Andes, providing abundant sedimentary supply to the forearc and promoting the growth of the accretionary wedge. During this period the retroarc fold-thrust belt growth stagnated and a strike-slip regime was activated in the western Andean flank. Different mechanisms controlled the forearc basins during these late Cenozoic changing tectonic conditions; whilst the external depocenter at the oceanic platform is part of a west-vergent wedge affected by a major splay-like thrust beneath the Coastal Cordillera, the internal depocenter next to the main orogen is overthrusted by the steep western Andean slope through a major west-directed fault. We refer to this latter structural system as the “Western Patagonian Thrust” and find that positive tectonic inversion is the main deformation mechanism. These west-directed crustal faults are part of the orogenic prowedge that is associated with considerably less crustal shortening (~4.2 km) than the retroarc fold-thrust belt (~18 km), part of the retrowedge. By placing these values in a global context through a comparison with other segments of the Andes and with type-examples of collisional orogens, we find that shortening along the Andes is preferentially accommodated in the retrowedge, while shortening in collisional orogens is mainly absorbed in the prowedge. These results have implications for models of subduction orogeny and the crustal architecture of the Andes.

Cambrian magmatic flare-up, central Tibet: Magma mixing in proto-Tethyan arc along north Gondwanan margin

AbstractAccompanying Gondwana assembly, widespread but diachronous Ediacaran–early Paleozoic magmatism of uncertain origin occurred along the supercontinent's proto-Tethyan margin. We report new geochemical, isotopic, and geochronological data for Cambrian magmatic rocks (ca. 500 Ma) from the Gondwana-derived North Lhasa terrane, located in the present-day central Tibetan Plateau. The magmatic rocks are composed of basalts, gabbros, quartz monzonites, granitoids (with mafic microgranular enclaves), and rhyolites. Nd-Hf isotopic and whole-rock geochemical data indicate that these rocks were probably generated by mixing of mantle-derived mafic and crust-derived felsic melts. The mantle end-member volumes of mafic, intermediate, and felsic rocks are ∼75%–100%, 50%–60%, and 0–30%, respectively. Integration of our new data with previous studies suggests that the North Lhasa terrane experienced long-term magmatism through the Ediacaran to Ordovician (ca. 572–483 Ma), with a magmatic flare-up at ca. 500 Ma. This magmatism, in combination with other Ediacaran–early Paleozoic magmatism along the proto-Tethyan margin, was related to an Andean-type arc, with the magmatic flare-up event related to detachment of the oceanic slab following collisional accretion of Asian microcontinental fragments to northern Gondwana. Diachroneity of the proto-Tethyan arc system along the northern Gondwanan margin (ca. 581–531 Ma along the Arabian margin and ca. 512–429 Ma along the Indian-Australian margin) may have been linked to orogenesis within Gondwana. The North Lhasa terrane was probably involved in both Arabian and Indian-Australian proto-Tethyan Andean-type orogens, based on its paleogeographic location at the northern end of the East African orogen.

From extension to tectonic inversion: Mid-Cretaceous onset of Andean-type orogeny in the Lhasa block and early topographic growth of Tibet

AbstractRecent studies have indicated that an Andean-type orogen (Lhasaplano) developed on the Lhasa block in the Cretaceous during northward subduction of Neo-Tethyan oceanic lithosphere. When and how uplift of the Lhasaplano began, however, has remained controversial. This article integrates stratigraphic, sedimentological, tectonic, and provenance data from the latest marine to nonmarine strata in the Linzhou Basin to pinpoint the early topographic growth in southern Tibet. The Takena Formation mainly consists of lagoonal carbonates and mudstones yielding foraminiferal assemblages of Early Aptian age (ca. 123–119.5 Ma). The conformably overlying lower member of the Shexing Formation, mainly deposited in fluvial environments, was fed by volcanic and sedimentary rock fragments from the north Lhasa terrane. Clasts of the Gangdese arc to the south firstly appeared in the middle member and became dominant in the upper member of the Shexing Formation. By contrast, coarse grained, braided river facies occur in the uppermost part of the Shexing Formation, where detritus was mostly recycled from Paleozoic strata of north Lhasa, with minor volcaniclastic contribution from the Gangdese arc. Basin analysis indicates accelerating subsidence and sedimentation rates during deposition of Takena to middle Shexing strata (ca. 125–108 Ma), followed by steady subsidence during deposition of upper Shexing strata (ca. 108–96 Ma). Given this regional tectonic and sedimentary evidence, such an evolution is interpreted to reflect tectonic extension followed by thermal subsidence. Basin inversion and regional compression initiated during deposition of the uppermost Shexing strata (ca. 96 Ma), as indicated by active thrust faults and widespread accumulation of syntectonic conglomerates in the western part of the Lhasa block. This event marked the beginning of the Andean-type orogeny in southern Tibet. Such a paleotectonic evolution, from extension to tectonic inversion, is also documented in the Andes mountain range. It may be typical of the early stage growth of Andean-type active continental margins.

Origin and early evolution of the Lhasa Terrane, South Tibet: Constraints from the Bomi Gneiss Complex

The Precambrian history of the Lhasa Terrane provides important context for understanding the India–Asia continental collision and formation and evolution of the Tibetan Plateau. However, it has not yet been well constrained, due to the scarcity of Precambrian outcrops. In this study, we present geochronological, geochemical, and Lu–Hf isotopic data for granitoid gneisses from the Bomi Complex in the eastern Lhasa Terrane. Our new data, together with those from the literature, reveal three episodes of magmatic activity at ca. 1866–1782 Ma, ca. 1343–1250 Ma, and ca. 824 Ma, and two metamorphic events at ca. 1117 Ma and ca. 625–600 Ma. While the Mesoproterozoic gneiss complexes suggest a transition from early rift-related to late arc signatures, the other two episodes of gneiss formation have arc signatures. We suggest that during the Paleoproterozoic and Middle Neoproterozoic, the Bomi Complex had an affinity with the Lesser and Greater Himalaya terranes along the northern margin of India, respectively. During the Mesoproterozoic and late Neoproterozoic, the Bomi Complex might be correlated with the contact zone between the Indian Archean cratons and the Eastern Ghats Belt in southeastern India. These magmatic and metamorphic events indicate that the Lhasa Terrane was located along the northern margin of the Indian continent during its early evolution. We propose the Precambrian evolution of the Lhasa Terrane was as follows: (1) the Lhasa Terrane and Lesser Himalayan sequence represent a Paleoproterozoic continental arc along the northern margin of the Indian continent; (2) the eastern Lhasa Terrane was affected by the Grenvillian-age Eastern Ghats orogeny during the Mesoproterozoic; (3) the Lhasa Terrane and Greater Himalayan Crystalline Complex formed a middle Neoproterozoic Andean-type orogen along the margin of the Rodinia supercontinent; and (4) the Lhasa Terrane experienced the Pan-African Orogeny, which might have marked the initial assembly of East Gondwana.

Anatomy of a diffuse cryptic suture zone: An example from the Bohemian Massif, European Variscides: REPLY

The fate of the lower plate during continental collision can be examined in deeply eroded orogens such as the late Paleozoic Variscan belt in continental Europe. In particular, the Bohemian Massif at its eastern extremity preserves well the evolution of an Andean-type orogen involved in continental collision. This process included relamination of subducted light felsic material rich in radioactive elements underneath a dense mafi c lower crust of the upper plate. This led to gravity-driven overturns and overprinting of the original suture by a broad zone of mixed upper and lower plate materials. In the studied example, this zone of interaction repeatedly reappears within the orogen, forming a so-called “diffuse cryptic suture zone.”

Anatomy of a diffuse cryptic suture zone: An example from the Bohemian Massif, European Variscides

The fate of the lower plate during continental collision can be examined in deeply eroded orogens such as the late Paleozoic Variscan belt in continental Europe. In particular, the Bohemian Massif at its eastern extremity preserves well the evolution of an Andean-type orogen involved in continental collision. This process included relamination of subducted light felsic material rich in radioactive elements underneath a dense mafic lower crust of the upper plate. This led to gravity-driven overturns and overprinting of the original suture by a broad zone of mixed upper and lower plate materials. In the studied example, this zone of interaction repeatedly reappears within the orogen, forming a so-called “diffuse cryptic suture zone.”

Geochemistry and zircon U–Pb–Hf isotopic systematics of the Neoarchean Yixian–Fuxin greenstone belt, northern margin of the North China Craton: Implications for petrogenesis and tectonic setting

The Yixian–Fuxin greenstone belt in the Western Liaoning metamorphic terrane is situated in the middle segment of the northern margin of the North China Craton (NCC) and is composed mainly of plagioclase amphibolites, pyroxene amphibolites and hornblende plagioclase gneisses. LA-ICPMS zircon U–Pb isotopic dating of three representative samples of hornblende plagioclase gneisses reveals that the magmatic precursors of the Yixian–Fuxin greenstone belt were formed during 2589 Ma–2534 Ma and subjected to subsequent ~ 2490 Ma metamorphism. Lu–Hf isotopic analyses of the zircons dated from the three samples yield positive εHf(t) values of + 8.3 to + 2.7 and T DM from 2.76 Ga to 2.57 Ga. The youngest T DM value is close to their crystallization ages, suggesting that they were mainly derived from partial melting of the depleted mantle materials. These zircon chronological data and Lu–Hf isotopic features indicate an important ~ 2.5 Ga episode of crustal growth in the Western Liaoning metamorphic terrane. Based on whole rock geochemical characteristics, these meta-volcanic rocks in the Yixian–Fuxin greenstone belt may be subdivided into four groups. Samples in Group #1 show high MgO contents from 7.19% to 7.93% and Cr and Ni contents of 208–395 ppm and 113–210 ppm with Mg numbers (Mg#) of 53.4 to 55.1, and are characterized by slight LREE-depleted to flat REE patterns (La/Sm) N = 0.77–1.05, flat patterns on the primitive mantle normalized multiple trace elements diagram, and Nb/Ta ratios between 12.3 and 15.9, implying that they were derived from partial melting of depleted mantle source. Samples in Group #2 also display higher MgO (7.52%–14.51%), Cr (372–570 ppm) and Ni (240–320 ppm) contents with high Mg# (53.5–70.2), low TiO 2 contents (0.52%–0.64%), high Nb/Ta ratios (15.8–19.9), and have right-declined REE patterns with (La/Sm) N ratios from 1.73 to 3.6, and negative Nb, Ta and Ti anomalies, indicative of their derivation from partial melting of a metasomatized refractory mantle source. Samples in Group #3 show MgO content from 1.81% to 3.56% with Mg# of 35.9 to 43.5 and high Cr (1179–1280 ppm) and Ni (712–721 ppm) contents, lower Y and Yb contents (6.7–9.42 ppm and 0.64–0.88 ppm, respectively) and higher Nb/Ta ratios (18.4–21.8), and are characterized by the highest (La/Yb) N ratios (14.91–22.67) and negative Nb, Ta and Ti anomalies, revealing that they formed from partial melting of garnet-bearing oceanic slab, and the slab melts were contaminated by mantle peridotite and subjected to crustal contamination during their ascent. Samples in Group #4 exhibit high MgO content (5.98%–10.52%) with high Mg numbers (52.4–70.8), Cr (198–376 ppm) and Ni (112–177 ppm) contents and lower Nb/Ta ratios between 12.7 and 14.6, and have (La/Yb) N ratios ranging from 4.12 to 6.78 (lower than those of the Group #3) and strongly negative Nb, Ta and Ti anomalies, suggesting that they were derived from partial melting of mantle peridotite metasomatized by slab-derived fluids. Thus, the Yixian–Fuxin greenstones display geochemical features of the normal-mid-ocean ridge basalts (N-MORBs) (Group #1), boninite-like rocks (Group #2), adakite-like rocks (Group #3) and high magnesium andesites (HMA) (Group #4), suggesting that the protoliths of the greenstone belt might have formed in an active continental margin similar to the Andean-type orogen.

The Huqf Supergroup of Oman: Basin development and context for Neoproterozoic glaciation

The Huqf Supergroup of the Sultanate of Oman provides important information on the geological evolution of the Arabian–Persian Gulf region during a protracted period of continental dispersal and reassembly on the periphery of the Gondwanan supercontinent during the Neoproterozoic, and also provides important constraints on the nature of extreme climate swings during this critical period in the evolution of Earth's biosphere. The Huqf Supergroup spans the period ca. 725–540 Ma, and is composed of three groups. The Abu Mahara Group ( ca. 725 to < 645 Ma) hosts two glacial successions separated by an interval of non-glacial, deep to shallow marine sedimentary rocks. The base of the overlying Nafun Group ( ca.< 645–547 Ma) is marked by a transgressive post-glacial carbonate, which initiates an overstepping of basement-cored structural highs and the deposition of an extensive blanket of carbonate and siliciclastic stratigraphy. The Ara Group ( ca. 547–540 Ma), which is known mostly from the subsurface, comprises carbonates, evaporites and organic-rich shales, with interbedded ashes, deposited in a large number of N–S trending troughs and platforms. The three groups of the Huqf Supergroup correspond to three phases of basin development. The Abu Mahara Group was deposited on an eroded crystalline and metasedimentary basement. An early stage of basin formation preserved < 1.5 km of marginal to deeper marine sedimentary rocks, including an older Cryogenian glacial succession infilling erosional palaeovalleys. Renewed tectonic subsidence associated with submarine volcanism allowed the preservation of a > 1 km-thick, cyclical, rift basin-fill of glacial and non-glacial sedimentary rocks representing a younger Cryogenian icehouse epoch. Progressively older source areas were exhumed during the interval ca. 725 to < 645 Ma, with unroofed 800+ Ma granitoid plutons providing the bulk of sediment, supplemented by syn-extension volcanics, and eventually by distant Meso- and Palaeoproterozoic sources. The wide extent of the Nafun Group, basin-wide correlation of major lithostratigraphic units, and its modest thickness (∼ 1 km) suggest a period of thermal contraction following Abu Mahara rifting. The Oman area was probably a region of slightly stretched continental lithosphere (which passed to the NE into a passive continental margin), occupied by a continental margin rim basin during deposition of the Nafun Group. The Nafun Group bears little resemblance to the coeval small basin-fills choked with calc-alkaline volcanic detritus in the Arabian Shield, but the correlation of the Nafun Group with the Jibalah Group indicates that the contiguous continental rim basin extended from Oman across the tectonically deformed eastern fringe of the Arabian–Nubian Shield. Inundation of the Arabian Shield area and incorporation within the Nafun basin was probably facilitated by extensional collapse and tectonic escape. Sources for 600–640 Ma zircons, found in the Nafun Group, can readily be identified in the Arabian–Nubian Shield. Renewed volcanism, compartmentalization of the basin by N–S trending structural highs and troughs, and an increase in sediment accumulation rates, typifies the Ara Group. The Ara Group deposits formed part of an extensive, latitudinal evaporite belt, with a depocentre translated outwards relative to the Nafun basin, suggesting continuing tectonic progradation of the eastern margin of the Arabian–Nubian Shield. Subduction of ocean floor along the former passive continental margin along the periphery of eastern Gondwana is the most likely cause of Ara volcanism and tectonism, in which case the Oman area can be viewed as occupying a retro-arc setting at this stage, between a subducting margin and the East African orogen. Coeval calc-alkaline plutons and rhyolitic to andesitic volcanics are found in the Central Iranian Terrane. Neoproterozoic glaciations are recorded in the rift and passive margin stratigraphy predating the final amalgamation of continental fragments into greater Gondwana. In Oman, glaciation was contemporaneous with the development of an Andean-type orogen in the Arabian–Nubian Shield while passive margin basins continued to subside in northern Iran, India and South China. The elevated topography associated with the Andean-type orogen together with passive margin mega-escarpments may have promoted nucleation of ice caps that sourced marine-terminating valley glaciers and ice streams. However, the triggering of Cryogenian glaciation must ultimately be related to the biogeochemical cycles operating on a planet with a nascent land biota, lack of calcifying plankton and reduced solar luminosity.

Relative importance of trenchward upper plate motion and friction along the plate interface for the topographic evolution of subduction-related mountain belts

Abstract We present finite-element models that investigate the relative importance of both trenchward motion of the upper plate and interplate coupling for the development of topography at convergent margins. Commonly, the role of a trenchward moving continental plate for the growth of topography is neglected in both modelling and field studies. Instead, forces exerted by the downgoing plate on the continental plate as well as interplate coupling are thought to be responsible for the deformation of the upper plate. Our model set-up includes an oceanic plate, which is in contact with a continental plate along a frictional plate interface and driven by slab pull. Both lithospheres have an elasto-visco-plastic rheology. The models demonstrate that friction along the plate interface can only lead to a high topography if the upper plate is moving toward the trench. Without such a trenchward advance, no high topography is generated, as the upper plate subsides owing to the drag exerted by the subducting plate. Increasing the coefficient of friction only amplifies the drag and increases the amount of subsidence. Our findings imply that trenchward motion of the continental plate plays a key role for the development of mountain belts at convergent margins; subduction of an oceanic plate even with high interplate coupling cannot explain the formation of Andeantype orogens.

Mechanical controls on collision-related compressional intraplate deformation

Intraplate compressional features, such as inverted extensional basins, upthrust basement blocks and whole lithospheric folds, play an important role in the structural framework of many cratons. Although compressional intraplate deformation can occur in a number of dynamic settings, stresses related to collisional plate coupling appear to be responsible for the development of the most important compressional intraplate structures. These can occur at distances of up to ±1600 km from a collision front, both in the fore-arc (foreland) and back-arc (hinterland) positions with respect to the subduction system controlling the evolution of the corresponding orogen. Back-arc compression associated with island arcs and Andean-type orogens occurs during periods of increased convergence rates between the subducting and overriding plates. For the build-up of intraplate compressional stresses in fore-arc and foreland domains, four collision-related scenarios are envisaged: (1) during the initiation of a subduction zone along a passive margin or within an oceanic basin; (2) during subduction impediment caused by the arrival of more buoyant crust, such as an oceanic plateau or a microcontinent at a subduction zone; (3) during the initial collision of an orogenic wedge with a passive margin, depending on the lithospheric and crustal configuration of the latter, the presence or absence of a thick passive margin sedimentary prism, and convergence rates and directions; (4) during post-collisional over-thickening and uplift of an orogenic wedge. The build-up of collision-related compressional intraplate stresses is indicative for mechanical coupling between an orogenic wedge and its fore- and/or hinterland. Crustal-scale intraplate deformation reflects mechanical coupling at crustal levels whereas lithosphere-scale deformation indicates mechanical coupling at the level of the mantle-lithosphere, probably in response to collisional lithospheric over-thickening of the orogen, slab detachment and the development of a mantle back-stop. The intensity of collisional coupling between an orogen and its fore- and hinterland is temporally and spatially variable. This can be a function of oblique collision. However, the build-up of high pore fluid pressures in subducted sediments may also account for mechanical decoupling of an orogen and its fore- and/or hinterland. Processes governing mechanical coupling/decoupling of orogens and fore- and hinterlands are still poorly understood and require further research. Localization of collision-related compressional intraplate deformations is controlled by spatial and temporal strength variations of the lithosphere in which the thermal regime, the crustal thickness, the pattern of pre-existing crustal and mantle discontinuities, as well as sedimentary loads and their thermal blanketing effect play an important role. The stratigraphic record of collision-related intraplate compressional deformation can contribute to dating of orogenic activity affecting the respective plate margin.

Synaptic physiology and ultrastructure in comatose mutants defines an in vivo role for NSF in neurotransmitter release

The elevation of the Tibetan plateau is a revolutionary event in the Earth history, which resulted in change not only of Cenozoic regional and global climate but also of monsoon intensity. A critical aspect for the development of the Tibetan plateau that remains open to intense debate is whether the collision among the Tibetan continental blocks or the subduction of the Tethys prior to the Indo-Asian collision in the Cenozoic contributed to the plateau growth. We propose here that an Andean-type orogen could have been present in southern Tibet from the Jurassic to Early Cretaceous, but collapsed during the Middle Cretaceous, based on a synthesis of existing structural, sedimentary, geochronological, and geochemical data. The orogeny is marked by north-verging Jurassic–Early Cretaceous thrusting in the Lhasa block, a result of flat subduction of the Yarlung-Zangpo Neo-tethys that is recorded by a narrow belt of 205–174 Ma adakitic rocks along the southernmost margin of the Lhasa block. Denudation of the orogen produced a thick (up to 15 km) sequence of Jurassic turbidites and molasse-type sediments on the northern Lhasa block. Widespread Middle Cretaceous (135–100 Ma) postorogenic, strongly peraluminous to calc-alkaline, magmatism and extensional deformation in the Lhasa block and an extensive marine transgression in Tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling. Meanwhile, the north-dipping subduction of the Bangong Meso-tethys produced a wide Jurassic–Middle Cretaceous magmatic arc along the southern margin of the Qiangtang block. Findings of 132–108 Ma-aged ophiolites rich with Middle Cretaceous radiolarians in central Tibet indicate that the Bangong Meso-tethys did not close until the Late Cretaceous. Repeated flat slab subduction of the Yarlung-Zangpo Neo-tethys occurred during the Late Cretaceous (90–78 Ma) as indicated by the adakites in the southernmost Lhasa block, which, together with the ongoing Qiangtang–Lhasa collision, could have contributed to the growth of the Tibetan plateau.

Tectonic denudation of the Darwin metamorphic core complex in the Andes of Tierra del Fuego, southernmost Chile: Implications for Cordilleran orogenesis

Cordillera Darwin in the southernmost Chilean Andes is a tectonically denuded metamorphic similar to those widely developed in the North American Cordillera. Detailed comparison can be drawn with the Shuswap metamorphic in British Columbia and Washington. Analysis of the geotectonic history of the Darwin reinforces the view that the presence of tectonically thickened continental crust is critical for development of most, if not all, known complexes and also suggests that inversion of a marginal basin during a compressional regime is one way of initiating such a crustal welt. The Cordillera Darwin is unique in the Andes and Antarctandes. This indicates that although low-angle normal faulting occurs at high levels in convergent plate settings such as the Central Andes and the Himalayas, extensional denudation of deep crustal levels in active Andean-type orogens may require special tectonic circumstances. Uplift of the Darwin began at about 70 Ma in a localized extensional setting within the developing transform zone between the South American and Antarctic plates. This strike-slip setting is analogous to some metamorphic complexes in North America and possibly to a recently identified core complex in New Zealand. It is likely to be a common one for complexes in convergent orogens.

Style Zonation in Fold-Thrust Belts: ABSTRACT

All orogens have thick-skinned and thin-skinned-style provinces. Thick-skinned belts form in a variety of crustal settings common in upper plates of Andean-type orogens and in both plates of aging collisional orogens. Thin-skinned belts specifically require transport at one transcrustal sole thrust. In collision belts, the sole thrust is the suture. In belts antithetic to Andean-type subduction, the sole thrust departs upward from the point of the asthenospheric wedge. Arrays of strike belts with different styles signal the thin-skinned scenario and its distortion in thick-skinned settings. A proposed zonation includes the following. (1) Detached fold belt with 1 to 5 km (0.6 to 3 mi) deep sole sloping at 0 to 2°, without topographic slope, and with strain increasing coreward to 80%. (2) Imbricate belt with 3 to 10 km (1.8 to 6 mi) deep sole sloping at 1.5 to 3.5°, and with bulk strain increasing to 100% or more. (3) Allochthonous belt consisting of polyphase (typical: thick-skinned pre-transport strain) crust and cover rocks above a subthrust sediment complex showing 100 to 500% strain. Base of subthrust complex is 5 to 20 km (3 to 12 mi) deep and slopes at 3 to 15°. (4) Root zone with polyphase and progressive strain, steep dips, re rocharriage, and late shallow detachment. (5) Late stage thick-skinned structures including foreland upthrusts, massifs (Helvetic and Penninic), and core complexes (cordilleran) occur anywhere between root zone and foreland. Their detachment, rise, and transport distort the thin-skinned features but cause more thin-skinned detachment in the external belts. Presence and width of strike belts and their style elements (e.g. folds, ramps, imbrications, back thrusts) can be explained as interplay between gravity spreading and thrust loading of an elastic foreland crust. Style-determining factors include (1) time spent within thrust wedge, (2) rigidity of foreland crust, (3) topographic slope, and (4) distribution of competent and incompetent rock units. Polyphase kinematic sequences reflect the numerical divergence between the elastic foreland wedge and the transport requirement of optimum taper; we distinguish polyphase wedge thickening and fold-discordant wedge transport. Seismic work and drilling in all five zones provide the data base for the zonation. Hydrocarbons are produced from all zones except zone 3. Conventional production comes from zones 1, 2, and 5. Most ongoing overthrust plays are made in zone 2. Zone 3 has potential where the allochthon is thin (such as ophiolites or detached shelf sediments), where the foreland sediment series is complete (as in collision belts), and where collision is too recent for thermal stabilization. End_of_Article - Last_Page 542------------

Lead isotopic compositions of Andean igneous rocks, latitudes 26 degrees to 29 degrees S; petrologic and metallogenic implications

Lead isotopic data are reported for 12 whole rock samples of Mesozoic to Quaternary plutonic and volcanic rocks from an intensely mineralized transect of the ensialic Central Andean orogen. In contrast to the initial Sr isotopic ratios and large ion lithophile element abundances of these rocks, the Pb isotope ratios show no trend either with decreasing age or with distance from the continental margin.The ratios 207 Pb/ 204 Pb and 208 Pb/ 204 Pb are significantly higher than those of oceanic tholeiite and alkali basalt from the Nazca plate, and we conclude that the lead cannot have been derived from subducted, metamorphosed basalt. However, there is extensive compositional overlap with the Pb isotopic ratios of ocean floor sediments and manganese nodules from the southeast Pacific.Considered in conjunction with the Sr isotope ratios and rare earth elements (and other large ion lithophile element) abundances of these rocks the new lead data are interpreted in terms of a two-stage magma generation model in which, though lead and strontium were derived from layers 1 and 2, respectively, of subducted oceanic lithosphere, the immediate source of the magmas was in the continental mantle peridotite which had become enriched in large ion lithophile elements through interaction with melts or waters rising from the subduction zone. It should be stressed, however, that the Pb isotopic ratios are not incompatible with a source in the sialic crust, although the wholesale anatexis of crust is here a very improbable mechanism for magma generation. Contributions from the continental crust may have occurred directly, through selective transfer to the invading magmas or, more remotely, following subduction and partial melting of material abraded from the leading edge of the continent.The relatively consistent nature of the rock lead compositions is paralleled by the broadly homogeneous metallogenic aspect of this transect, in which copper deposits almost everywhere predominate, and in which the longitudinal metal zonation, considered a characteristic of Andean-type orogens, is not developed. If rock and ore leads, and the economically important metals (Cu, Mo, Ag, Au), are cogenetic, arguments similar to those advanced to constrain the origin of the parent igneous rocks may be applied to delimit the source of the ore metals. Thus, metalliferous sediments such as those of the Nazca plate are improbable sources, whereas the metal-rich pelagic sediments and Mn nodules are promising candidates. However, the ore metals may have been derived from the deeper continental mantle or the sialic crust.