Ancient Orogens Research Articles

Multiple Early Paleozoic granitoids from the southeastern Qilian orogen, NW China: Magma responses to slab roll-back and break-off

The slab dynamic processes, such as roll-back during oceanic subduction and break-off during collision orogenesis, would affect the movement of mantle material and the interaction between crust and mantle, accompanied by significant amounts of magmatism. Providing accurate depictions for these processes are extremely difficult, especially in the case of ancient orogens. Here, we present an integrated study of petrology, geochronology, geochemistry and Sr–Nd–Hf isotopes for Early Paleozoic granitoids from the Duojielaka, Ledu and Binglingsi plutons of the southeastern Qilian orogen. Our LA-ICP-MS UPb zircon data show that these granitoids were emplaced at 476–432 Ma. The petrological and geochemical data suggest that the ~476 Ma Duojielaka porphyritic syenogranites formed by feldspar-dominated fractionation from relatively K-rich basaltic rock-derived adakitic parental magma. The ~454 Ma Duojielaka biotite monzogranites are highly fractionated I-type granites and were derived from basement orthogneisses mixed with minor mafic rocks. The Ledu fine- and medium-grained quartz diorites (~446 and ~452 Ma) are high-K calc-alkaline to shoshonitic and were produced by partial melting of relatively old K-rich mafic crust followed by minor mixing with mantle-derived magmas. The ~432 Ma Binglingsi biotite monzogranites with adakitic characteristics most likely originated by partial melting of thickened mafic lower crust. Based on the new data, in combination with previously published data, we suggest that the roll-back of the South Qilian oceanic slab occurred during the Middle Cambrian to early Middle Ordovician. The Duojielaka porphyritic syenogranites represented crustal anatexis in an arc front associated with slab roll-back. The Late Ordovician–early Middle Silurian intrusive rocks probably resulted from diachronous slab break-off in the post-collisional stage. The rocks record the opening of the break-off window from east to west and the gradual heat propagation from the central part to the southern and northern margins of the eastern Central Qilian due to the widening of break-off window and asthenospheric upwelling. We propose that complex geodynamic processes and their thermal effects can be traced by a comprehensive consideration of the petrogenetic, temporal and spatial evolution of related magmatism.

Active strike-slip faults and an outer frontal thrust in the Himalayan foreland basin

The Himalayan foreland basin formed by flexure of the Indian Plate below the advancing orogen. Motion on major thrusts within the orogen has resulted in damaging historical seismicity, whereas south of the Main Frontal Thrust (MFT), the foreland basin is typically portrayed as undeformed. Using two-dimensional seismic reflection data from eastern Nepal, we present evidence of recent deformation propagating >37 km south of the MFT. A system of tear faults at a high angle to the orogen is spatially localized above the Munger-Saharsa basement ridge. A blind thrust fault is interpreted in the subsurface, above the sub-Cenozoic unconformity, bounded by two tear faults. Deformation zones beneath the Bhadrapur topographic high record an incipient tectonic wedge or triangle zone. The faults record the subsurface propagation of the Main Himalayan Thrust (MHT) into the foreland basin as an outer frontal thrust, and provide a modern snapshot of the development of tectonic wedges and lateral discontinuities preserved in higher thrust sheets of the Himalaya, and in ancient orogens elsewhere. We estimate a cumulative slip of ∼100 m, accumulated in <0.5 Ma, over a minimum slipped area of ∼780 km2 These observations demonstrate that Himalayan ruptures may pass under the present-day trace of the MFT as blind faults inaccessible to trenching, and that paleoseismic studies may underestimate Holocene convergence.

Geochemistry, zircon U-Pb ages and Sr-Nd-Hf isotopes of an Early Triassic appinitic complex in southeastern Inner Mongolia, China: implications for the late tectonic evolution of the Paleo-Asian Ocean

Appinite commonly occurs in convergent plate tectonic settings and thus can constrain the tectonic evolution of ancient orogens. Geochemical and geochronological analyses were carried out on a newly identified Early Triassic appinitic complex in southeastern Inner Mongolia in the eastern segment of the Central Asian Orogenic Belt. Petrographically, the Luotuochang complex can be divided into two zones: an outer zone of intermediate rocks and inner zone dominated by mafic rocks. A monzonite sample from the outer zone yielded weighted mean zircon 206Pb/238U ages of 246 ± 1.6 Ma, whereas the gabbro from the inner zone yielded an age of 243 ± 1.2 Ma. The inner zone mafic rocks have SiO2 contents of 45.56 to 54.27 wt% with high MgO, Cr, Ni and Sr contents, elevated Ba/Nb, Ba/Zr, Rb/Y and Th/Zr, and low TiO2, Nb/Zr and Nb/Y. These features suggest that the metasomatized lithospheric mantle is the primitive magma source. SiO2 contents of the outer zone intermediate rocks range from 57.6 to 63.69 wt% and K2O, Ba and Sr contents are enriched; the eNd(t) (+2.2 to +4.0) and eHf(t) (+8.4 to +13.5) values are also high. These characteristics suggest that the magma of these rocks mainly derived from the mantle with possible juvenile lower crust involvement. Based on these geochemical data and results from regional geological investigations, we propose that the Luotuochang appinitic complex was formed in a post-orogenic extensional setting. Its formation was likely the result of lithospheric delamination, upwelling of new mantle material and partial melting of the overlying lower crust during crustal compression and thickening soon after the closure of the restricted Paleo-Asian Ocean basin.

Reassessing the PT conditions of Neoproterozoic collisional metamorphism and partial melting in southernmost Brazil

Partially equilibrated textures and partial melting products are key features in the study of high-grade metamorphic complexes, but their complexity limits the advancement of knowledge about the tectono-thermal history of ancient orogens. This difficulty is apparent in the wide interval of conditions (720–1000 °C/3–10 kbar) determined by previous studies in a high-grade complex formed during the amalgamation of West Gondwana in southernmost Brazil. In order to understand the orogen, we reanalyse the metamorphic evolution of the Várzea do Capivarita Complex through field data, petrography, whole-rock and mineral chemistry, and thermodynamic modelling. Peak metamorphic conditions (M1) are constrained with diverse mineral assemblages (Bt-Crd-Sil-Grt-Ilm/Bt-Sil-Grt-Ilm/Bt-Grt-Ilm) and overlap at 800 °C/4.5 kbar, while the partially equilibrated domain (Crd-Hc-Sil) indicates post-peak conditions (M2) of 680 °C/2.8 kbar. Models predict leucosome volume up to 20%, in agreement with field and petrographic estimates, attesting the migmatitic character. Leucogranitic veins correlating petrographically and compositionally with leucosome and melt estimates reinforce this character and the thermodynamic modelling, even with low correlations, which are explained by inherited minerals. Thus, M1 and M2 conditions indicate exhumation from depth of 16 to 10 km and high geothermal gradient, which are interpreted as due to anomalous mantle heat flow and rapid exhumation facilitated by large volumes of melt.

Unravelling a Devonian–Triassic seamount chain in the South Tianshan high‐pressure/ultrahigh‐pressure accretionary complex in the Atbashi area (Kyrgyzstan)

Seamounts are common occurrences on the extant oceanic floors of the earth, and accreted single seamounts are also widely distributed on the modern active margins, but recognition of single seamounts is an uneasy job in the ancient orogens, and the detection of a chain of accreted seamounts is even harder, therefore understanding the multiple accretion of ancient seamounts is nearly negligible. A combined analysis of geochronology and geochemistry was conducted on the basic eclogitic rocks from the Atbashi Accretionary Complex in the Kyrgyz South Tianshan, in a bid to unravel the accretion process of a chain of seamounts in the Paleo‐Asian Ocean in the southern Altaids. A set of reliable U–Pb zircon ages of 397.6 ± 8 Ma age for a basaltic andesite, 353 ± 8 Ma for a gabbro, and 317.1 ± 9 Ma for a diorite were obtained in this study. Both unmetamorphosed basic rocks and eclogites carry geochemical signatures of plume‐derived seamounts that were influenced by their respective mantle source. Taken together with previously published age data, the spatial and temporal features of these Atbashi rocks indicate the timing and sequence of formation, subduction, and extrusion of the South Tianshan oceanic remnants in the Early/Middle Devonian to Late Triassic. The integrated data indicate the existence of a seamount chain composed of Early/Middle Devonian–Late Triassic seamounts in the Atbashi Complex. The paleogeographic pattern and analysis of these fragments derived from the ocean‐type seamounts enable the reconstruction of tectonic models of that portray, magmatism, mantle‐derived plumes, subduction and exhumation of this Devonian–Triassic ocean floor.

Fold and thrust belts: structural style, evolution and exploration – an introduction

Abstract The outer parts of collision mountain belts are commonly represented by fold and thrust belts. Many of the key concepts in the structural geology of fold and thrust belts have origins in ancient orogens such as the Appalachians and Caledonian chains of Europe, together with the Alps. Impetus in thrust belt research then came from the desire to exploit geological resources that reside in the subsurface, especially arising from hydrocarbon exploration in the foothills of the Canadian Cordillera in the 1960s and 1970s. Notwithstanding decades of exploitation, continental fold and thrust belts are still estimated to hold reserves of 700 billion barrels of oil equivalent. But exploration will focus increasingly on small, hard-to-resolve structures. Basic geological understanding remains as important today as it did for the pioneering explorers in the Canadian foothills. It is a theme that runs throughout this Special Publication.

Coupled Zircon-Rutile U-Pb Chronology: LA ICP-MS Dating, Geological Significance and Applications to Sediment Provenance in the Eastern Himalayan-Indo-Burman Region

U-Pb dating by LA ICP-MS is one of the most popular and successful isotopic techniques available to the Earth Sciences to constrain timing and rates of geological processes thanks to its high spatial resolution, good precision (absolute U/Pb age resolution of ca. 2%, 2s), rapidity and relative affordability. The significant and continuous improvement of instrumentation and approaches has opened new fields of applications by extending the range of minerals that can be dated by this method. Following the development and distribution to the community of good quality reference materials in the last decade, rutile U-Pb thermochronology (with a precision only slightly worse than zircon) has become a commonly used method to track cooling of deep-seated rocks. Its sensitivity to mid- to low-crustal temperatures (~450 °C to 650 °C) is ideal to constrain exhumation in active and ancient orogens as well as thermal evolution of slow-cooled terranes. Recrystallization and secondary growth during metamorphism and the presence of grain boundary fluids can also affect the U-Pb isotopic system in rutile. A growing body of research focusing on U-Pb dating of rutile by LA ICP-MS is greatly improving our understanding of the behavior of this mineral with regards to retention of radiogenic Pb. This is key to fully exploit its potential as a tracker of geological processes. The latest developments in this field are reviewed in this contribution. The combined application of U-Pb zircon and rutile chronology in provenance studies, particularly when complemented by lower-T thermochronometry data, allows the isotopic characterization of the sources across a wide range of temperatures. The benefits of applying detrital zircon-rutile U-Pb chronology as a coupled provenance proxy are presented here, with a focus on the Eastern Himalayan-Indo-Burman region, where a growing number of successful studies employs such an approach to help constrain river drainage and basin evolution and to infer feedback relationships between erosion, tectonics and climate.

Transition from oceanic subduction to continental collision recorded in the Bangong-Nujiang suture zone: Insights from Early Cretaceous magmatic rocks in the north-central Tibet

The transition from oceanic subduction to continental collision is a key stage in the evolution of ancient orogens. We present new data for Early Cretaceous diorite and granite porphyry from north–central Tibet to constrain the evolution of the Bangong–Nujiang Tethyan Ocean (BNTO). The diorites have moderate SiO2 and high MgO contents, similar to high-Mg andesites. Zircon grains yield U–Pb ages of 128–124 Ma and positive εHf(t) values between +13.2 and + 16.3, corresponding to Hf depleted-mantle model ages (TDM) of 281–131 Ma. The high-Mg diorite was probably formed by partial melting of hydrous mantle wedge fluxed by slab-derived fluids in an oceanic subduction setting. The granite porphyries yield zircon U–Pb ages of 117–115 Ma and zircon εHf(t) values ranging from +0.1 to +4.5. Most samples have high SiO2 and Fe2O3T contents, variable FeOT/MgO and Ga/Al ratios, and are depleted in Ba, Sr, P, and Ti, similar to I- and A-type granites. The granite porphyries were most likely derived from partial melting of juvenile dioritic or granodioritic crust due to break-off of the BNTO lithosphere following collision between the Lhasa and Qiangtang blocks. The Early Cretaceous high-Mg diorite and A-type granite porphyry thus record the Early Cretaceous transition from oceanic subduction to continental collision along the Bangong–Nujiang suture zone (BNSZ).

Tectono‐Metamorphic Evolution of the Central Ribeira Belt, Brazil: A Case of Late Neoproterozoic Intracontinental Orogeny and Flow of Partially Molten Deep Crust During the Assembly of West Gondwana

AbstractRecent advances in understanding the plate tectonics, intracontinental deformation, and flow of partially molten crust have significantly improved our knowledge of collisional tectonics and the way in which we understand complex ancient orogens. The Central Ribeira Belt represents a Neoproterozoic fold‐and‐thrust belt formed in the Brasiliano Orogenic Cycle associated with the assembly of West Gondwana. This fold‐and‐thrust belt is currently interpreted as a result of recurrent collisions and amalgamation of terranes against large cratons. Based on an integrated structural, petrological, and geochronological study in two metamorphic complexes of the Central Ribeira Belt (Embu and Costeiro complexes), we challenge the current model that involves multiple terrane collisions. Our data show for the first time metamorphic ages older than 600 Ma for samples from Costeiro and Embu complexes and suggest that both geological units experienced an intermediate‐P metamorphism (M1) at circa 620 Ma and a low‐P metamorphism (M2) at circa 575 Ma. Our proposed tectonic model is consistent with an M1 event related to an intracontinental orogeny, formed in response to the collision between the São Francisco Craton and the Paranapanema Block. On the other hand, the later M2 metamorphism records extensional and wrench tectonics associated with orogenic collapse, constrained by the decompression paths of the metasedimentary sequences and M2‐related S3 mylonitic foliation. The M2 metamorphism is associated with wide, right‐lateral strike‐slip shear zones and voluminous peraluminous magmatism in the Embu Domain and widespread partial melting of the middle crust forming migmatitic rocks and peraluminous leucogranites in the Costeiro Domain.

Late Oligocene–early Miocene transformation of postcollisional magmatism in Tibet

Abstract Uplift of the Tibetan Plateau is thought to be one of the most important orogenic and climate forcing events of the Cenozoic Era, associated with geodynamic changes related to India-Asia collision and subsequent continental lithosphere subduction. However, the fate and scale of the subducted continental lithosphere segments remain highly controversial. Using a comprehensive compilation of the spatiotemporal distribution of postcollisional magmatic rocks across Tibet, together with new geochemical and Sr-Nd-Pb isotopic data and modeling simulations, we propose a holistic, two-stage evolutionary model to explain the link between genesis of the magmas and continental subduction. The magmatism prior to 25 Ma resulted from continuous upwelling of a carbonate-rich upper-mantle plume induced by northward underthrusting of Indian oceanic and continental lithosphere with its cover of Tethyan platform carbonate sediments, whereas magmatism after 25 Ma was related to opposing north-directed and south-directed continental subduction. Our model indicates a transformation in the distribution and nature of the magmatism in Tibet at ca. 25 Ma, which reflects a significant change in the Himalayan-Tibetan orogen and associated mantle dynamic processes in the early Miocene. Understanding this transformation could have important implications for the utility of the Himalayan-Tibetan system as a modern analogue for ancient orogens.

U-Pb detrital zircon ages and compositional features of Bifurto quartz-rich sandstones from Southern Apennines (Southern Italy): comparison with Numidian Flysch sandstones to infer source area

Two samples of quartz-rich sandstones belonging to Bifurto Formation in Southern Apennines, were studied for their petrographical, geochemical and geochronological (U-Pb detrital zircon ages) features and compared with those of sandstones from five stratigraphic sections of the Numidian Flysch in SouthernItaly. The purpose of this comparison is to assess the homogeneity and provenance of detritus because quartz-rich sandstones of both formations are coeval having Burdigalian p.p. - Early Langhian age.The depositional domain of the Bifurto formation was located on the sinking Campania-Lucania carbonate platform whereas that of Numidian Flysch was within the Lagonegro-Molise Basin east of the Campania-Lucania Platform. The Bifurto sandstones have sub-arkose - quartzarenitic composition with high Fe2O3 contents (Fe2O3 in the range 2.85 wt% - 4.83 wt%), however, the whole composition varies in the same compositional range of the Numidian Flysch sandstones.The U-Pb detrital zircon ages in Bifurto sandstones range from 2551±40 Ma to 425±9 Ma and 93% of these age data is comprised in the same time lapse of detrital zircon ages of Numidian Flysch sandstones (3047±13 to 516±19 Ma). These ages suggest a provenance area consisting of ancient orogens in which Precambrian and Silurian rocks are present. The absence of Paleozoic - Mesozoic ages excludes provenance from Variscan or Alpine rocks. On this basis, the African Craton represents the exclusive source area of Bifurto sandstones as well as of Numidian Flysch sandstones outcropping in Southern Apennines. Similar deductions have been proposed for Numidian sandstones outcropping along the Betic and Maghrebian chains from Spain to Morocco, Algeria and Sicily.

Tectonic Mode Switches Recorded at the Northern Edge of the Australian Plate During the Pliocene and Pleistocene

AbstractWe report new data from medium‐high grade metamorphic rocks found at the northern margin of the Lengguru Fold Belt in West Papua. The study involved a systematic analysis of cross‐cutting structures to establish the relative timing of deformation, together with isotopic dating to define when these tectono‐thermal events occurred. These data show that the region underwent multiple episodes of deformation within the last six million years. Metamorphic mineral growth was associated with the development of ductile shear zones. This episode occurred during a phase of crustal stretching associated with the formation of a metamorphic core complex. Metamorphic zircon growth at 4.9 to 5.3 Ma was documented in two of the dated samples. These data are interpreted to postdate the peak pressure and temperature conditions of the phase of regional crustal stretching. The shear fabrics associated with the metamorphic core complex were later overprinted by at least two generations of folds. The change in mode from crustal extension to shortening reflects a tectonic mode switch. A subsequent mode switch is documented by numerous brittle extensional faults that cross‐cut the earlier formed ductile fabrics. We interpret ca. 0.75–1.51 Ma (U–Th)/He age data to reflect cooling associated with the later stages of crustal shortening (marked by folds) or the later extensional unroofing of the peninsula. This work demonstrates that an orogen can record multiple tectonic mode switches within several million years. These outcomes should be considered in studies of ancient orogens where analytical uncertainties associated with isotopic dating may mask short‐lived mode switches.

Differentiating advancing and retreating subduction zones through regional zircon Hf isotope mapping: A case study from the Eastern Tianshan, NW China

Advancing and retreating subduction zones at convergent margins, primary sites of continental crust growth and loss, are the cornerstones of tectonic evolution of accretionary orogens. Yet, distinguishing the tectonic mode (advancing, retreating, or switching) of subduction zones in fossil orogens is not always straightforward due to the scarcity of strong evidence. As an alternative approach, we utilized zircon Hf isotope mapping of arc magmatism (previously-published data) from different tectonic units in the Eastern Tianshan and the northern Tarim, revealing that (a) an early Paleozoic (≥ca. 450 Ma) southward subduction of the Junggar oceanic plate (subduction zone advance) resulted in compression and crustal thickening in the northern Tarim (upper plate), as evidenced by increasingly more enriched zircon Hf isotopic compositions, and (b) a slab rollback/retreat of the Junggar oceanic plate probably since ca. 450–440 Ma (tectonic switching from subduction zone advance to retreat), as manifested by progressive increases in zircon εHf(t) values, induced crustal extension and opening of the South Tianshan back-arc basin in the northern Tarim, separating the Central Tianshan continental arc as a micro-continent sliver. This case study highlights the great potential of regional zircon Hf isotope mapping, besides traditional investigations, in helping reveal the enigmatic tectonic mode of subduction zones in ancient orogens.

Coupled feedbacks between mountain erosion rate, elevation, crustal temperature, and density

Many ancient (>1 Gyr) mountain belts have crust >50 km thick, in contrast with isostatic and erosion models that predict crustal attenuation to ∼30 km in <100 My. One proposed explanation is that thick crustal roots are preserved through a feedback in which the gradually cooling crust grows progressively denser, floats lower in the mantle, and erodes more slowly, thereby slowing crustal destruction. To explore this feedback, we develop a 1-D numerical model for the thermal and density evolution of an orogen after tectonic growth has stopped. Our simulations show that this densification feedback can indeed slow crustal destruction and help preserve thick crust. These simulations also show that crustal preservation is promoted by erosion rates that result in pressure–temperature conditions favorable for garnet-forming metamorphic reactions, which sharply increase lower crustal density and amplify the densification feedback. We find that the strength of the densification feedback depends nonlinearly on the ratio between advective and conductive cooling, such that crustal preservation is maximized at intermediate values of this ratio. This model predicts secular trends in elevation, erosion rate, and crustal thickness consistent with measured values in orogens up to several hundred My, but not in older orogens, suggesting that processes beyond those in the model (e.g., sedimentation, increasing plate strength, dynamic topography, chemical weathering) may be necessary to generate positive elevations and erosion rates in ancient orogens. This analysis suggests that the feedback between erosion, crustal cooling, and elevation can significantly affect the evolution and longevity of the continental crust.

Age and geochemistry of the Charlestown Group, Ireland: Implications for the Grampian orogeny, its mineral potential and the Ordovician timescale

Accurately reconstructing the growth of continental margins during episodes of ocean closure has important implications for understanding the formation, preservation and location of mineral deposits in ancient orogens. The Charlestown Group of county Mayo, Ireland, forms an important yet understudied link in the Caledonian-Appalachian orogenic belt located between the well documented sectors of western Ireland and Northern Ireland. We have reassessed its role in the Ordovician Grampian orogeny, based on new fieldwork, high-resolution airborne geophysics, graptolite biostratigraphy, U–Pb zircon dating, whole rock geochemistry, and an examination of historic drillcore from across the volcanic inlier. The Charlestown Group can be divided into three formations: Horan, Carracastle, and Tawnyinah. The Horan Formation comprises a mixed sequence of tholeiitic to calc-alkaline basalt, crystal tuff and sedimentary rocks (e.g. black shale, chert), forming within an evolving peri-Laurentian affinity island arc. The presence of graptolites Pseudisograptus of the manubriatus group and the discovery of Exigraptus uniformis and Skiagraptus gnomonicus favour a latest Dapingian (i.e. Yapeenian Ya 2/late Arenig) age for the Horan Formation (equivalent to c. 471.2–470.5Ma according to the timescale of Sadler et al., 2009). Together with three new U–Pb zircon ages of 471.95–470.82Ma from enclosing felsic tuffs and volcanic breccias, this fauna provides an important new constraint for calibrating the Middle Ordovician timescale. Overlying deposits of the Carracastle and Tawnyinah formations are dominated by LILE- and LREE-enriched calc-alkaline andesitic tuffs and flows, coarse volcanic breccias and quartz-feldspar porphyritic intrusive rocks, overlain by more silicic tuffs and volcanic breccias with rare occurrences of sedimentary rocks. The relatively young age for the Charlestown Group in the Grampian orogeny, coupled with high Th/Yb and zircon inheritance (c. 2.7Ga) in intrusive rocks indicate that the arc was founded upon continental crust (either composite Laurentian margin or microcontinental block). Regional correlation is best fitted to an association with the post-subduction flip volcanic/intrusive rocks of the Irish Caledonides, specifically the late-stage development of the Tyrone Igneous Complex, intrusive rocks of Connemara (western Ireland) and the Slishwood Division (Co. Sligo). Examination of breccia textures and mineralization across the volcanic inlier questions the previous porphyry hypothesis for the genesis of the Charlestown Cu deposit, which are more consistent with a volcanogenic massive sulfide (VMS) deposit.

Foliation intersection/inflection axes within porphyroblasts (FIAs): a review of advanced applications and significance

Foliation Intersection/Inflection Axes within porphyroblasts (FIAs) provide a quantitative approach to microstructural and metamorphic analysis that can be used to integrate multiple deformation and metamorphic events and develop lengthy PT path histories during orogenesis. This paper reviews applications of FIAs for determining (1) porphyroblast rotation vs. nonrotation; (2) inclusion trail orientations and orogenic processes involving changes in the direction of bulk horizontal shortening; (3) in situ age determinations for multiple deformation episodes; (4) P-T-t-d paths of poly-metamorphism in an orogenic belt; and (5) constraining relative plate movement paths within ancient orogens. Quantitative structural and metamorphic data correlated directly to relative plate motions, are currently allowing the tectonic reconstruction of multiply deformed and metamorphosed terrains in orogens around the world.

Magnetotelluric Imaging of the Lithosphere Across the Variscan Orogen (Iberian Autochthonous Domain, NW Iberia)

AbstractA new magnetotelluric (MT) survey comprising 17 MT soundings throughout a 30 km long N30°W transect in the Iberian autochthons domain of NW Iberia (Central Iberian Zone) is presented. The 2‐D inversion model shows the resistivity structure of the continental crust up to 10 km depth, heretofore unavailable for this region of the Variscan Orogen. The MT model reveals a wavy structure separating a conductive upper layer underlain by a resistive layer, thus picturing the two main tectonic blocks of a large‐scale D2 extensional shear zone (i.e., Pinhel shear zone). The upper layer represents a lower grade metamorphic domain that includes graphite‐rich rocks. The lower layer consists of high‐grade metamorphic rocks that experienced partial melting and are associated with granites (more resistive) emplaced during crustal thinning. The wavy structure is the result of superimposed crustal shortening responsible for the development of large‐scale D3 folds (e.g., Marofa synform), later deflected and refolded by a D4 strike‐slip shear zone (i.e., Juzbado‐Penalva do Castelo shear zone). The later contribution to the final structure of the crust is marked by the intrusion of postkinematic granitic rocks and the propagation of steeply dipping brittle fault zones. Our study demonstrates that MT imaging is a powerful tool to understand complex crustal structures of ancient orogens in order to design future prospecting surveys for mineral deposits of economic interest.

An extinct, Late Mesoproterozoic, Sveconorwegian mantle wedge beneath SW Fennoscandia, reflected in seismic tomography and assessed by thermal modelling

AbstractA channel‐like, low‐velocity zone in the lithospheric mantle beneath W Norway coincides spatially with the extension of a recently discovered 200 × 50 km granite batholith, which formed as a result of oceanic subduction beneath the SW Fennoscandian margin between 1.07 and 1.01 Ga. Based on results from numerical modelling, we argue that the low‐velocity zone, at least in part, reflects the thermal (radioactive) effects of the refertilized mantle wedge of this magmatic arc. The geological record in SW Fennoscandia suggests that active‐margin magmatism terminated as a result of rapid slab rollback and trench retreat starting at ca. 1 Ga. The rapid shift from active‐ to passive‐margin processes was probably critical in preserving the mantle wedge, and its identification can therefore shed light on how active‐margin processes terminated in ancient orogens.

Late Devonian-early Permian subduction-accretion of the Zharma-Saur oceanic arc, West Junggar (NW China): Insights from field geology, geochemistry and geochronology

Migration of arc magmatism is of significant importance for accretionary orogens, but few examples have been documented in ancient orogens. In this paper we report late Paleozoic-early Permian, migrating magmatism of the intra-oceanic Zharma-Saur arc in the Tarbagatay Mountains, West Junggar, NW China, in order to address the general role of a growing arc in accretionary orogenesis, and the debates on the formation and timing of the Kazakhstan orocline in the Central Asian Orogenic Belt (CAOB). Our detailed field work suggests that in the Zharma-Saur arc plutons and dikes intruded and lavas erupted in association with accretionary complexes that were dominated by ocean plate stratigraphy (OPS). Mafic to intermediate magmas have arc-type geochemical signatures such as depleted HFSEs and enrichments in LILEs and LREEs, coupled with high εNd(t) (+5.7 to +6.8) to low 87Sr/86Sr initial values (+0.7035 to +0.7037). The Tarbagatay accreted rocks contain early Paleozoic pillow lavas, which have depleted LILEs, high εNd(t) (+7.4 to +8.2) and high 87Sr/86Sr initial values (+0.7041 to +0.7063). These data suggest that the Zharma-Saur arc was an intra-oceanic island arc that developed on accreted OPS material. U-Pb zircon isotopic data (the ZWTB I diorite pluton, 322±3Ma; the JLDK diorite pluton, 318±4Ma; andesitic lavas, 312±2Ma, 310±4Ma, 301±3Ma) demonstrate that the Zharma-Saur arc magmatism occurred in the late Paleozoic. Integration with published geochronological data on the Zharma-Saur arc leads to a complex model of arc growth with a general southward younging of the arc magmatism from 383Ma to 262Ma. These relations imply a southward growth and migration of the magmatic axis of the Zharma-Saur arc that was associated with a short period of ridge subduction. The magmatic activity of the Zharma-Saur arc that probably continued until the early Permian resulted from migrations towards the forearc and backarc, as well as oblique or parallel motion to the trench. These new results not only provide robust evidence for resolving controversies about the Phanerozoic accretionary and continental growth of the Central Asian Orogenic Belt, but also shed light on the migration of magmatism in accretionary orogens in general.

GEODYNAMICS OF THE KAZAKHSTAN OROCLINE, CENTRAL ASIA

Curved mountain belts, commonly referred as to oroclines that result from bending of quasi-linear orogenic belts, have fascinated generations of geologists. Such structures are widely recognized in modern and ancient orogens, and are fundamentally important for understanding geodynamics of convergent plate boundaries. However, how and why orogenic belts become bent has been in debate. Here we investigate the Kazakhstan Orocline in the Central Asian Orogenic Belt with an aim at understanding the geodynamics of oroclinal bending in accretionary orogens.