Limbs Of Folds Research Articles

Early Precambrian eclogites in the Belomorian Province, eastern Fennoscandian Shield

This study focuses on a boudin of Archean amphibolite that occurs in the Gridino eclogite-bearing mélange (Belomorian Province, eastern Fennoscandian Shield) and contains two varieties of eclogite. Eclogite-1 is banded, retrogressed (omphacite is totally replaced by clinopyroxene-plagioclase symplectite), and deformed by recumbent tight folds identical to those in the TTG-amphibolite host. Zircon groups are dated at 2721 ± 26 Ma (Zrn-I, fir-tree zoning, no or small negative Eu anomalies, flat HREE patterns, 700–730 °C, 14–15 kbar), ca. 2.70 Ga (Zrn-II, rims around Zrn-I, grains, 750–900 °C, 11–14 kbar, high-P granulite facies), and ca. 2.70 Ga (Zrn-III, rims around Zrn-II). Newly formed Paleoproterozoic zircon crystals are lacking. Retrogressed eclogite-1 in which amphibolization and epidotization totally obliterated garnet and plagioclase-clinopyroxene symplectites is intruded by 2646 ± 46 Ma old granodiorite. Eclogite-2 is massive, contains preserved omphacite-garnet-quartz-rutile assemblages, and comprises two patches whose boundaries truncate folds in eclogite-1. A Paleoproterozoic age of eclogite-2 is evidenced by previously published 1.90 Ga ages of zircon rims around Archean zircon and newly formed crystals with omphacite inclusions. The largest patch contains tabular enclaves of three varieties of amphibolites. One variety is identical to some bands of totally retrogressed eclogite-1 changed into monomineral amphibolites in both the Neoarchean and Paleoproterozoic. The orientation and attitude of these tabular enclaves are coherent with those of the banding on limbs of recumbent tight folds in eclogite-1 within the boudin and later drag folds at its margins and form one and the same structural carcass or skeleton. These data along with recent findings of inherited 2.68 Ga old zircon with inclusions of omphacite in eclogite-2 indicate that this eclogite developed after Neoarchean eclogite-1 due to fluid infiltration along reactivated Neoarchean shear zones enveloping the boudin. These results favor the idea that the transition to the modern-style plate tectonics started not later than in the Neoarchean.

Kinematic analysis of the Caçapava do Sul Granitic Complex, southern Brazil: Ediacaran syntectonic magmatism and strain partitioning during inclined ductile extrusion along a transpressional shear zone, Dom Feliciano Belt

The Caçapava do Sul Granitic Complex is a ca. 30 km-long syntectonic batholith emplaced at 580-560 Ma along the N–S to NNE-SSW striking Caçapava do Sul Shear Zone, configuring the core of a double-plunging regional asymmetric antiformal fold, with the Passo Feio Metamorphic Complex metavolcanosedimentary wallrocks occurring at the fold limbs. Structural mapping and kinematic analysis along its whole margins allowed to subdivide the granite complex into seven structural domains, each with its own geometric characteristics and kinematics. The observed microstructures included abundant K-feldspar and plagioclase porphyroclasts with asymmetric recrystallization tails, S–C and S–C–C’ fabrics, and more rarely synmagmatic fractures and asymmetrically developed myrmekites. In general, high temperature microstructures are abundant in the southern half of the granitic complex, while these features are overprinted by low temperature brittle-ductile to brittle microstructures in its northern half. The Southern Domain presents foliations with medium angle dip towards the south, high rake lineation towards the SSW and normal shear sense indicators. The Northwestern and Southwestern domains present mainly subvertical N–S striking foliations with sub-horizontal lineations mostly plunging towards SSW and abundant sinistral shear sense indicators. On the other hand, Southeastern and Santos Ferreira domains presents predominantly NE-SW striking foliations with variably dip and sub-horizontal lineations with abundant dextral shear sense indicators. The Northeastern Domain presents low angle foliation dipping towards the NE, which is also locally described in the Southeastern Domain. Dominantly normal movement with top to NE is described in the Northeastern Domain, also locally reported in the Southeastern Domain. The Northern Domain presents low-angle foliation dipping towards the N, with high rake lineation. No shear sense indicator was observed in the Northern Domain due to brittle deformation overprint, but normal kinematics is inferred. The opposite kinematics of the distinct Caçapava do Sul Shear Zone margins are corroborated by punctual data from the Passo Feio Metamorphic Complex rocks. This is compatibilized by the resultant syntectonic granite flow towards N/NE, which is interpreted in a model of oblique transpression with inclined ductile extrusion. The model suggests that transpression of the middle crust had an important pure shear component which was effectively partitioned into structural domains along the Caçapava do Sul Shear Zone in consequence of post-collisional syntectonic magmatism, and coexisted with extensional tectonics in the upper crust (Camaquã Basin) through a detachment along the brittle-ductile transition.

New Late Cretaceous paleomagnetic and geochronologic results from the southern Qiangtang Terrane: Contributions to the reliability of redbed paleomagnetic datasets from the Tibetan Plateau and shape of the Qiangtang Terrane

To better clarify the reliability of redbed paleomagnetic datasets from the Tibetan Plateau and shape of the Qiangtang Terrane (QT) prior to the India-Asia collision, a combined paleomagnetic and detrital zircon U-Pb geochronologic study is conducted on the Upper Cretaceous Abushan Formation (Fm) redbeds, dated to be within 89.1−83.6 Ma, in the Anduo area. The tilt-corrected grand mean direction for 37 sites is Ds = 0.9°, Is = +36.4°, k = 43.6, α95 = 3.6°, which provides a paleopole at 78.2°N, 266.8°E (A95 = 3.6°), corresponding to a paleolatitude of 20.5° ± 3.6°N for the study area (32.3°N, 91.4°E). Reliable demagnetization curves, positive fold, and conglomerate tests support the interpretation that characteristic remanent magnetization directions recorded primary magnetizations carried by detrital hematite and did not suffer from the influence of distortional strains. Our paleomagnetic results indicate that the mean inclination observed from the southwest dipping limb (Is = +44.2°) is clearly steeper than that from the north dipping limb (Is = +34.2°). The results of syntectonic-sedimentation-correction and the fluvial gravel deposits present in the Abushan Fm redbeds sampled support the contention that the apparent inclination discrepancy is attributed to the syntectonic growth strata and that the paleomagnetic datasets observed from two limbs of folds are still reliable. Furthermore, reliable Late Cretaceous paleomagnetic datasets show that the shape of the QT west of the Eastern Himalaya Syntaxis (EHS) during the Late Cretaceous was similar to its present-day relatively E-W alignment, and that around the EHS, its shape changed from the Late Cretaceous NE-SW alignment to the present-day approximately NW-SE alignment.

Petrological, petrophysical and petrothermal study of a folded sedimentary succession: the Oliana anticline (Southern Pyrenees), outcrop analogue of a geothermal reservoir

The Oliana anticline (Southern Pyrenees) has been characterized as an outcrop analogue of a geothermal reservoir using field data (stratigraphy and fracturing) and petrological, petrophysical and petrothermal analyses. Five lithofacies were established including conglomerates, hybrid arenites, lithic arenites, carbonates and evaporites. Petrophysical measurements indicate widely dispersed values of bulk density, connected porosity, permeability and velocity of compressional acoustic waves. Connected porosity is the factor that mostly influences bulk density, compressional wave velocity and permeability. In turn, diagenetic processes (such as dissolution and cementation) and fracturing, coupled with petrological features such as mineral composition, matrix content and grain size, are the most critical factors controlling rock porosity along the Oliana anticline. Thermal conductivity measures reveal a compositional control on the thermal properties of rocks. Thermal characterization of the structure reveals a low conductive area that matches the carbonate and evaporite succession of the anticline core and a highly conductive zone associated with the detrital succession of the fold limbs. The Oliana anticline has been classified as a petrothermal system due to the low permeability values of the studied sedimentary succession. Despite such classification, this contribution provides a useful exploration tool for future studies of non-conventional geothermal and CO2 storage sites located in folded sedimentary successions in the proximal domain of foreland basins.

Deformation history and processes during accretion of seamounts in subduction zones: The example of the Durkan Complex (Makran, SE Iran)

The Durkan Complex is a tectonic element of the Makran Accretionary Prism (SE Iran) that includes fragments of Late Cretaceous seamounts. In this paper, the results of map- to micro-scale structural studies of the western Durkan Complex are presented with the aim to describe its structural and tectono-metamorphic evolution. The Durkan Complex consists of several tectonic units bordered by mainly NNW-striking thrusts. Three main deformation phases (D1, D2, and D3) are distinguished and likely occurred from the Late Cretaceous to the Miocene–Pliocene. D1 is characterized by sub-isoclinal to close and W-verging folds associated with an axial plane foliation and shear zone along the fold limbs. This phase records the accretion of fragments of the seamount within the Makran at blueschist facies metamorphic conditions (160–300 °C and 0.6 – 1.2 GPa). D2 is characterized by open to close folds with sub-horizontal axial plane that likely developed during the exhumation of previously accreted seamount fragments. An upper Paleocene – Eocene siliciclastic succession unconformably sealed the D1 and D2 structures and is, in turn, deformed by W-verging thrust faults typical of D3. The latter likely testifies for a Miocene – Pliocene tectonic reworking of the accreted seamount fragments with the activation of out of sequence thrusts. Our results shed light on the mechanism of accretion of seamount materials in the accretionary prisms, suggesting that seamount slope successions favour the localization and propagation of the basal décollement. This study further confirms that the physiography of the subducting plates plays a significant role in the tectonic evolution of the subduction complexes.

Reconstruction of petrophysical zoning of the Blagodatnoye gold deposit in the Yenisei Ridge: Geodynamic and physical-chemical aspect

Research subject. Petrophysical zoning of the Blagodatnoye gold-sulfide deposit in the Yenisei Ridge. Aim. To determine indicative petrophysical characteristics of the products of the main occurrence stages and to develop an evolutionary petrophysical model of the investigated deposit.Materials and methods. Physical fields were studied by the methods of magnetic and electrical exploration and gamma-spectrometry. The petromagnetic heterogeneity and mineralogical-geo chemical features of formation of polymetamorphic complexes, metasomatites and ores were studied by a neutron activation analysis of the content of rare earth and radioactive elements, petrochemical x-ray fluorescence analysis, as well as by an electron-probe microanalysis of pyrite.Results. The syncollisional fold-overthrust fault (785 Ma) of the preparatory stage provided structural control over the ore-bearing mineral-forming system. The signs of zone dislocation metamorphism include geophysical anomalies: magnetic and natural electrical anomalies due to pyrrhotite and graphite mineralization of cleavage zones on fold limbs, and specific electrical resistance from silicification zones in fold hinges. The metasomatism of the pre-ore (753 Ma) and ore (698 Ma) stages took place under rifting conditions. Pre-ore quartz-muscovite and chlorite metasomatites with carbon mineralization and supra-background Au concentrations were formed under the action of reducing reaction solutions; they remained unaltered in the non-productive part of the deposit. These formations are characterized by elevated concentrations of radioactive elements and natural electrochemical polarizability. During the ore stage, Au was concentrated by fluids with hydro-carbonate-sulfide composition under the violation of the strike-slip kinematics, which caused significant petrophysical transformations of the productive part of the deposit. Early carbon metasomatites in the sub-ore and root sections of the ore bodies were depleted in terms of U, at the same time as retaining their electrochemical activity. Uranium accumulated in the upper horizons of the productive part, whose rocks lost their polarizability due to scattered carbonate mineralization. Magnetic pyrrhotite crystallized as part of sulfides with a regular increase in its proportion in the root sections of the ore bodies. At the final stage (368 Ma), the mineralized zone was broken into a series of blocks with unequal vertical displacements and levels of erosional truncation by upcasts. This led to the exposure of various-depth sections with contrast petrophysical characteristics.Conclusions. The Blagodatnoye deposit was formed in four stages: preparatory, two ore-generating and final. The petrophysical features of the products of each stage formed the basis for the developed evolutionary petrophysical model, which will be tested on the materials of geophysical studies of the Yenisei Ridge territories.

Mud Diapir or Fault‐Related Fold? On the Development of an Active Mud‐Cored Anticline Offshore Southwestern Taiwan

AbstractThere has been a controversial debate about the roles of diapiric folding versus fault‐related folding in mud‐cored anticlines. This study tackles this debate by looking into the structural developments in southwestern Taiwan. Among all the structures, the Liuchiu anticline is the only active structure exposed above sea level and therefore offers a unique opportunity to integrate land and marine data sets. On Liuchiu Island, we obtained the fold core geometry at the surface and identified three terrace groups with age constraints. In the offshore area, we obtained the fold limb geometry from seismic profiles. Given the tight fold shape and the asymmetric terrace tilting, we found that a hybrid fault tip fold‐fault propagation fold model can best explain the geologic and geomorphic features. In this regard, mud diapirism should be a secondary process, possibly along discrete patches of mobile shales to enhance fold amplification. Our model yields decreasing shortening rates from 0.7 mm/yr during the Plio‐Pleistocene to 0.375 mm/yr since 120 ka. The uplift rate however increases since the mid‐Holocene, which we tentatively attribute to larger mobile shale deformation triggered by earthquakes along the fault after the near‐fault fluids are drained. At the regional scale, we further suggest that the remaining plate convergence is absorbed by a few major structures west of Liuchiu Island.

Laramide contractional folding in the Devils River Uplift, west Texas, U.S.A.

Structures in Cretaceous strata in the Devils River Uplift of west Texas include folds, thrust faults, and bedding-parallel veins indicative of contractional deformation in a thrust faulting stress regime. In this study we analyze a series of small-wavelength (meters to tens of meters) folds in roadcut exposures northwest of Del Rio, Texas. The folds have WNW-ESE dominant trends, gently plunging axes, and are gentle to tight with upright to inclined axial surfaces of variable vergence. These structures are interpreted as detachment folds, with detachments within volcanic ash or mudrock beds. Observed thickening in anticline cores was by ductile flow of volcanic ash or mudrock, small-scale folding, and thrust or reverse faulting. Bed-perpendicular calcite veins are common and in some cases are offset by flexural slip on fold limbs, indicating that folding occurred after brittle failure and extension vein formation in lithified rock. Although rare, bed-parallel calcite veins are also present in the exposure and indicative of a thrust faulting stress regime. Occasional normal and reverse or thrust faults are also present in the exposures. Fluid inclusion analysis of vein cements from extension fractures and faults yielded homogenization temperatures of 71–101 °C, and estimated trapping depths of 2.2–3.5 km prior to and coeval with folding. We interpret these folds as having formed during NNE-SSW directed Laramide shortening (∼80–40 Ma) in a thrust-faulting stress regime with >2 km of overburden, rather than as either penecontemporaneous with deposition or after exhumation by caliche formation, as previously interpreted.

Spatio-temporal variation of fluid flow behavior along a fold: The Bóixols-Sant Corneli anticline (Southern Pyrenees) from U–Pb dating and structural, petrographic and geochemical constraints

This study integrates field structural data, petrographic and geochemical (δ18O, δ13C, Δ47, 87Sr/86Sr, and elemental composition) analyses and U–Pb dating of calcite veins cutting the Bóixols-Sant Corneli anticline (Southern Pyrenees) in order to date and to investigate the spatio-temporal relationships between fluid flow and fold evolution. This E-W trending anticline grew from Late Cretaceous to Paleocene at the front of the Bóixols thrust sheet deforming pre-growth and growth sedimentary sequences. U–Pb dating reveals Late Cretaceous to late Miocene deformation ages, which agree with the age of growth strata deposition and the sequence of deformation interpreted from field and microstructural data. Dates coeval (71.2 ± 6.4 to 56.9 ± 1.4 Ma) and postdating (55.5 ± 1.2 to 27.4 ± 0.9 Ma) Upper Cretaceous to Paleocene growth strata are interpreted to record: (i) the growth of the Bóixols-Sant Corneli anticline during the Bóixols thrust emplacement, and (ii) the tightening of the anticline during the southern tectonic transport of the South-Central Pyrenean Unit. Other ages (20.8 ± 1.2 to 9.0 ± 4.6 Ma) postdate the folding event and have been associated with the collapse of the Bóixols-Sant Corneli anticline. The geochemistry of calcite veins indicates that the fluid flow behavior varied across the Bóixols-Sant Corneli anticline through its growth, showing a compartmentalized fluid system. In the hinge of the anticline and in the upper Santonian to middle Campanian syn-orogenic sequence along the footwall of the Bóixols thrust, the similar petrographic and geochemical features between all calcite cements and host rocks point towards a locally-derived or well-equilibrated fluid system. Contrarily, along large faults such as the Bóixols thrust, and in the anticline limbs, the geochemistry of vein cements indicates a different scenario. Cements in large faults yielded the lightest δ18O values, from −8 to −14 ‰VPDB, and variable enrichment in δ13C, 87Sr/86Sr, elemental composition and δ18Ofluid. This is interpreted as the migration of fluids, through fault zones, that evolved from distinct fluid origins. Cements in the fold limbs exhibit δ18O and δ13C between −8 and −6 ‰VPDB and between −10 and + 2 ‰VPDB, respectively, the lowest Sr contents and the lowest precipitation temperatures, suggesting that the anticline limbs recorded the infiltration and evolution of meteoric waters. The paleohydrological system in the Bóixols-Sant Corneli anticline was restricted to the Bóixols thrust sheet. The Upper Triassic evaporitic basal detachment likely acted as a lower fluid barrier, preventing the input of fluids from deeper parts of the Pyrenean crustal thrust system. This study provides a well-constrained absolute timing of fracturing and fluid flow during basin inversion and folding evolution and highlights the suitability of U–Pb geochronology to refine the age of fractures and veins and their sequential evolution in orogenic belts.

Quantifying occurrence of deformation bands in sandstone as a function of structural and petrophysical factors and their impact on reservoir quality: an example from outcrop analog of Productive Series (Pliocene), South Caspian Basin

Deformation bands (DB) are known to influence porosity and permeability in sandstones. This study aims to predict the occurrence of DB and to quantify their impact on reservoir properties based on field measurements in the steeply dipping limb of a kilometer-scale fold in Yasamal Valley, western South Caspian Basin. An integrated approach of characterizing bands and their effect on reservoir properties included measurements of natural gamma radioactivity and permeability using portable tools, along with bed dip and the count of DB across distinct facies. A set of core analyses was performed on outcrop plugs with and without bands to estimate the alteration of rock properties at the pore scale. Interpretation of outcrop gamma-ray data indicates the absence of bands in Balakhany sandstones containing shale volume greater than 18% for unconsolidated and 32% for calcite-rich facies. A high amount of calcite cement appears to increase the number of DB. A poor, positive trend between bed dip and DB concentration was identified. We show that net to gross, defined as the thickness fraction of sandstone bound by mudstones, is among the parameters controlling the occurrence of bands. Samples containing a single DB show a 33% and 3% decrease in permeability and porosity, respectively, relative to the host rock. We reveal a new set of lithological and petrophysical factors influencing DB occurrence. This study offers a direct tool that can be applied in subsurface reservoir analogs to predict the occurrence and concentration of DB and estimate their influence on rock properties.

Palaeomagnetic indication for India–Asia collision at 12°N and maximum 810 km Greater India extent in the western suture zone

SUMMARY Knowing the pre-collisional extent of the northern Indian Plate margin (‘Greater India’) is vital to understanding the tectonic evolution of the India–Asia collision and the formation of the Himalayan–Tibetan orogen. However, suitable geological units for palaeomagnetic investigations along the Himalayan belt are limited, which makes it difficult to reconstruct Greater India during the pre-collisional period in Late Cretaceous to Palaeogene. Often the palaeomagnetic results from the Zongpu Formation at Gamba in southern Tibet (∼88.5°E) were used for estimates of Greater India, but their validity was recently questioned. As a contribution to closing the data gap, we performed a palaeomagnetic study of the Palaeocene/Lower Eocene Dibling limestone (DL) in the western Tethyan Himalaya of Zanskar (34.0°N/76.6°E). The results from 27 sites revealed a well grouping (k = 71.7) syntectonic magnetization with best grouping at 52 per cent unfolding. The remagnetization of the DL was acquired shortly after ∼54 Ma, at the latest at ∼49 Ma, and is probably carried by fine-grained magnetite formed during the early orogenic phase. Assuming proportional tilting of the fold limbs, the corresponding palaeolatitude of 11.8 ± 2.4°N suggests a maximum Greater India extent of 810 ± 420 km and a first continental contact with the southern Eurasian margin at ∼12°N in the western part of the suture zone. The tectonostratigraphic equivalence of the DL with the Zongpu Formation at Gamba and a great similarity in their magnetic properties supports a secondary origin of the Gamba results. Through understanding the mechanism of remagnetization in the DL, an early orogenic remanence acquisition is also indicated for the Zongpu Formation, and thus the Gamba results deserve further credit for Greater India reconstructions. However, we note a large inconsistency of the available Late Cretaceous and Palaeogene palaeolatitude data from the Tethyan Himalaya by up to ∼20°, corresponding to differences of up to ∼2000 km in the size of Greater India. These discrepancies require further palaeomagnetic work in the Tethyan Himalaya, and in particular we recommend comparative studies at same locations and of same units.

Fundamental constraints of lithologically-controlled fault networks on gas migration and accumulation for fractured carbonates in the western Sichuan Basin, China

The structure of subsurface fault networks, which has a significant effect on hydrocarbon migration, is inherently difficult to recognize and map. 3D seismic surveys provide an opportunity to map the fault system within a carbonate reservoir of Xinchang in the western Sichuan Basin, China. We calculated attributes from the seismic reflection data that covers an area of 1330 km2, including variance, edge detection, dip-magnitude and dip azimuth. We focused on the Triassic Leikoupo Formation of dolostone, at ∼6 km depth, adjacent to the Longmen Shan thrusting range. The mapped attributes display (1) a primary network of faults in an orthogonal pattern in the anticline, striking East-West and North-South, and (2) a secondary, conjugate fault set in the fold limb, striking Northeast and Northwest with a length range of 1–10 km. The latter set of conjugate faults are confined to a 300 m-thick layer of dolostone, diminishing into underlaying unit of anhydrite. We envision that the conjugate fault networks and associated fractures enhance gas charging and reflect a model of self-sourced migration and accumulation in the Leikoupo Formation. These results are significant for sweet-spot evaluation of carbonate reservoirs in the Sichuan Basin, and provide insights for understanding the migration of subsurface fluids.

Evolution of fracture networks and connectivity during fault–bend folding: Insights from the Sinon Anticline in the southwestern Hongseong–Imjingang Belt, Korea

Crustal shortening in an elastico-frictional regime is mainly accommodated by contractional fault–fold systems with fracture networks. According to recent research, fracture networks in fold–thrust belts express complex internal strain states in response to thrusting and related folding. Furthermore, their connectivity and fluid flow characteristics likely depend on the structural positions and mechanical stratigraphy that control heterogeneous deformation processes. This study provides characteristics of fold-related fracture networks in the Sinon Anticline, which was formed by fault–bend folding in the southwestern Hongseong–Imjingang Belt, Korea. The fracture networks in the metamorphosed turbidites characterized by interbedded competent metasandstone layers and relatively thin incompetent schist layers have evolved through pre-, syn-, and post-folding fracturing events. Their complexity reflects the spatiotemporal variation in the strain pattern related to early layer-parallel shortening and subsequent fault–bend folding. Based on insights from detailed mapping and topological analysis of the fracture network, we conclude that strain partitioning that occurs during flexural folding results in a superposed tangential longitudinal strain expressed by fractures with a high (hydraulic) connectivity in the hinge zones. Strain partitioning is caused by flexural interlayer slip along incompetent schist layers in the fold limbs. Bed-parallel slip localization zones probably have low porosity and permeability and may act as barriers to fluid migration across beds. We suggest that heterogeneous vertical axis rotation, which occurred as the system's hanging wall slid over the footwall ramp, increased the complexity of fracture networks within the Sinon Anticline. Our findings indicate that the evolution, connectivity, and fluid flow properties of fracture networks can be characterized through careful interpretation of folding mechanisms and related strain states during formation of fault–bend fold systems.

Paleomagnetic and geochronological results of the Risong Formation in the western Lhasa Terrane: Insights into the Lhasa-Qiangtang collision and stratal age

To constrain the Lhasa-Qiangtang collisional age, a combined paleomagnetic and geochronologic investigation has been performed on the Risong Formation redbeds and volcanic rocks in the Wuma area of the western Lhasa terrane (LT). A U-Pb dating result from the volcanic interlayer at the bottom of the Risong Formation redbeds yields the youngest weighted mean 206 Pb/ 238 U age of 110.6 ± 1.3 Ma, and the two detrital zircon U-Pb chronological results from the Risong Formation redbeds provide the youngest weighted mean 206 Pb/ 238 U age of 106.9 ± 3.3 Ma (the top strata of section A) and 120.5 ± 3.0 Ma (the top strata of section D), respectively. Our new geochronological results suggest that the studied Risong Formation redbeds formed during the Early Cretaceous, rather than during the Late Jurassic as assigned by 1:250,000 scale Wuma regional geological survey report. Characteristic remanent magnetization (ChRM) directions from two different limbs of folds concentrate in two different groups, and both site-mean directions pass positive fold tests. One group including 32 sites yields a tilt-corrected site-mean direction that is D = 356.1°, I = 33.1° with α 95 = 2.3°, corresponding to a paleopole at 75.2°N, 278.1°E with dp/dm = 1.5°/2.6° and a paleolatitude of 18.1° ± 1.5°N for the study area (32.41°N, 83.39°E). The other group including 24 sites provides a tilt-corrected site-mean direction that is D = 60.5°, I = 48.5° with α 9 5 = 2.0°, corresponding to a paleopole at 38.7°N, 159.6°E with dp/dm = 1.7°/2.6° and a paleolatitude of 29.5° ± 1.7°N for the study area (32.41°N, 83.39°E). After syntectonic-sedimentation-correction, the Fisherian site-mean inclination of 41.7° ± 1.6° and corresponding paleolatitude of 24.0° ± 1.2°N obtained from 56 sites of two different limbs are consistent with the inclination-only mean of 39.6 ± 3.1° and corresponding paleolatitude of 22.5° ± 2.2°N deduced from the ChRM directions of all the 56 paleomagnetic sites, suggesting that the observed inclination discrepancy of the Risong Formation redbeds may result from the syntectonic sedimentation. Comparing the Early Cretaceous paleolatitudes observed from the western LT with those from the western Qiangtang terrane shows insignificant paleolatitude difference, supporting that the Lhasa-Qiangtang collision in the western part occurred at or before the Early Cretaceous. • New dating shows the sampled Risong Formation redbeds formed during the Early Cretaceous rather than the Late Jurassic. • The Wuma sampling area was located at ~22.5° ± 2.2°N during ~120.5–106.9 Ma. • The Lhasa-Qiangtang collision occurred before ~120 Ma in the western part.

Geology of the Piemonte-Ligurian units of the Urtier area (Northwestern Alps – Italy)

ABSTRACT Detailed geological mapping of the Urtier Valley in the Western Alps allowed to realize a new geological map 1:10.000 in scale. In the mapped area, a complete nappe stack of the axial part of the Alpine belt crops out. The study focused on the two units derived from the Piemonte-Ligurian oceanic domain, Broillot and Bardonney units, for which, in addition to the classic petrographic and structural considerations, a lithostratigraphic approach was applied. The lithostratigraphic approach, combined with structural analysis, led to reconstruct a nappe stacking significantly different from that generally proposed for the Western Alps. In the Urtier Valley in fact, the eclogite facies units are at the top of the nappe pile, overlapped on the blueschist facies units. This anomalous stacking order could be explained by interpreting the study area as an inverted limb of a regional fold with the eclogite facies units at the core.

The interplay of structure and metamorphism at Broken Hill, NSW, Australia

Downward younging of stratigraphy in several large F2 fold structures in the Paleoproterozoic Willyama Supergroup in the Broken Hill area has previously been explained by very large-scale F1 nappe folds followed by relatively upright F2 and F3 folds. In three major structures, the Mt Robe–East Eldee Structure, the Paps Synform and the Bijerkerno Synform, the andalusite-sillimanite metamorphic isograd parallels stratigraphy for tens of kilometres. Given the conformability of these segments of isograd in plan view, it is reasonable to suggest that in two of the structures, sillimanite-grade rocks rest on andalusite-grade rocks. In all cases the younger stratigraphic units are at lower metamorphic grade than the older stratigraphic units, whether the stratigraphy is upright, as in the Bijerkerno Synform, or inverted, as in the Mt Robe-East Eldee Structure and the Paps Synform. Major bodies (mini-batholiths) of pegmatite are confined stratigraphically to below the Sundown Group, whether the stratigraphy is upright or overturned. In the D1 nappe model the andalusite-sillimanite isograd and the emplacement of pegmatite mini-batholiths must both predate the D1 nappe folding. There is evidence that andalusite preceded D1, but not sillimanite, which is a defining mineral in the first-generation schistosity (S1). The large-scale nappe model is also not compatible with calculated metamorphic pressures. An alternative model involving extensional D1, followed by D2 sheath folds, explains downward-facing metamorphic facies but is not viable. Large-scale fold structures in the Mt Robe Block can be explained by recumbent folding at a moderate scale, followed by inclined sheath folding. The downward-facing stratigraphy and metamorphic facies in the Mt Robe and Paps synforms are interpreted to lie on the underside of inclined F2 sheath-like folds. KEY POINTS In major fold structures in the Willyama Supergroup, upper stratigraphic units are at a lower metamorphic grade than lower stratigraphic units, whether the fold is upward-facing or downward-facing. In two large F2 folds sillimanite-grade rocks showing melting are situated above andalusite-grade rocks. Pegmatitic mini-batholiths show semi-regional stratigraphic control below the top of the Broken Hill Group, whether they are in upward or downward-facing fold limbs. A new structural model with D1 recumbent folding, followed by D2 sheath-like folding, is based on structures in the Mt Robe Block.

Intracontinental deformation of the western Ordos Basin in North China and sandstone‐type uranium mineralization: Constraints from AFT chronology of the Helan Mountain

Apatite fission track (AFT) sampled from Helan Mountain, of the western Ordos Basin, North China shows that the study area located in the western part of the North China Craton (NCC) has undergone four‐stage exhumation since Mesozoic time, that is, Late Jurassic to Early Cretaceous, Late Cretaceous, Early Cenozoic, and post‐late Miocene. Helan Mountain had slowly uplifted and eroded from the Late Jurassic to the beginning of the Cenozoic. The intensive deformation within NCC is attributable dynamically to the gently angular subduction of the Pacific Plate under the Eurasian Plate (EP). Helan Mountain has undergone sinistral strike‐slipping with a 60‐km maximum displacement caused by the collision of the southeast margin of EP during the Late Cretaceous. The Early Cenozoic tectonic event in the study area is respondent to collision between the Indian Plate and Eurasian Plate, whereas, the post‐late Miocene event is likely related to the eastward extension of Tibet Plateau. Based on the available AFT ages, the partial annealing zone for the palaeo‐Helan Mountain is ascertained to elevation ~1,200 m to ~2,100 m. The tectonic events occurring from Mesozoic to Cenozoic in Helan Mountain is directly related to the plate margin tectonism and representative for the intracontinental deformation of Eurasian Plate convergence. The sandstone‐type uranium deposits in the western margin thrust belt of Ordos Basin are related to the Late Jurassic to Early Cretaceous and Late Cretaceous exhumation events. Immediately after the host rock, the Zhiluo Formation sedimentation during the Middle Jurassic, the Late Jurassic ~ Late Cretaceous exhumations resulted in north–south‐trending anticlines, synclines, and thrust faults; meanwhile, the Zhiluo Formation was uplifted to the surface and infiltrated by uranium‐bearing oxidized fluids originated from the Helan Mountain. Uranium deposits were formed in both east and west limbs of folds. Finally, uranium mineralization ceased since the Cenozoic because the Yinchuan and Hetao faulted grabens have separated the Helan Mountain provenance from the western margin thrust belt.

Constraints on deformation mechanisms of the Barberton Greenstone Belt from regional stratigraphic and structural data of the synorogenic Moodies Group

Understanding the deformation styles and mechanisms which affect the Barberton Greenstone Belt (BGB) contributes to our ability to interprete the dynamic evolution of Paleoarchean greenstone belts and of the continental lithosphere. A variety of actualistic and non-actualistic, in part contradictory tectonic settings and processes (including gravitational collapse, terrane accretion, foreland basin shortening, continental rifting, doubly-vergent subduction and partial convective overturn) have been proposed to explain BGB deformation at a variety of scales. We compare these models and their regional 3-D predictions with emphasis on the BGB’s youngest deformed stratigraphic unit, the Moodies Group (ca. 3.21 Ga), also adding new structural data.We find that none of the tectonic models accounts for all major structural features of the BGB nor can any model adequately explain the regionally and temporally heterogeneous distribution of deformation styles within the greenstone belt. However, the partial convective overturn model, despite some weaknesses and oversimplifications, appears to be suited best to explain the geometry, orientation and formation of the dominating structural element of the BGB, the vertically-plunging Onverwacht Anticline (OA) with its 10-km-thick fold limbs. The formation and deformation of the adjacent smaller, commonly steeply doubly-plunging synclines filled with synorogenic siliciclastic sediment of the Fig Tree and Moodies groups could be partially explained as spatial adjustments to the rise and tilting of the OA although disparate processes acting at different scales need to be considered. In comparison to this large-scale vertical redistribution of mass by several tens of km, (sub-)horizontal displacements may have played a subordinate role at greenstone-belt scale, except along the regionally highly strained southern BGB margin in Eswatini.

The northern Appalachian terrane wreck model

Ordovician and Siluro-Lower Devonian magmatic rocks in the northern Appalachians south of the Iapetus suture are currently interpreted as distinct belts composed of multiple, small, peri-Gondwanan terranes that amalgamated during the sequential closures of Iapetus (latest Ordovician), the Tetagouche backarc basin (early Silurian), the Acadian seaway (Siluro-Devonian), and the Rheic Ocean (Devono-Carbonferous) (multiple terrane model). Here, the Siluro-Lower Devonian magmatic belts are shown to have slab failure affinities and together with the Ordovician arcs form paired belts parallel to the Iapetus suture, which suggests that they were emplaced along the common, peri-Avalonian margin during pre- and post-collisional processes. The Iapetan suture and the paired belts are inferred to repeat in Atlantic Canada due to dextral, strike-slip processes of mid-Late Devonian or younger age (terrane wreck model). In Newfoundland, the repetition is inferred to be the result of oblique, dextral offset of ca. 250 km. In the Quebec Embayment, the Iapetan paired magmatic belts are repeated twice in the limbs of a Z-shaped orocline related to oblique, dextral offsets of ca. 1200 km of the southern limb. Limited Siluro-Devonian paleomagnetic data indicate no paleolatitudinal differences across the Iapetus suture, however ca. 100° post-mid Silurian clockwise rotation is indicated for the middle fold limb; these data favour the terrane wreck model. The terrane wreck model results in a simple tectonic scenario of southerly subduction of Iapetus beneath a single ribbon continent (Avalonia sensu lato) that was subsequently deformed.

Recumbent folding in the Upper Cretaceous Eaux‐Chaudes massif: A Helvetic‐type nappe in the Pyrenees?

AbstractWe describe a singular structure in Upper Cretaceous rocks of the Eaux‐Chaudes massif of the western Pyrenees, consisting of a kilometre‐scale fold nappe with a sheared overturned limb. High ductile strain attests a deformation style rarely reported for the alpine Pyrenees, and peak temperature in Upper Cretaceous carbonates is estimated by Raman spectroscopy of carbonaceous material palaeothermometry in the lower greenschist facies (>300°C). The normal fold limb retains the original sedimentary textures, while the overturned limb shows calcite crystal‐plastic deformation and dynamic recrystallization, with crystallographic preferred orientation. The observed ductility and metamorphic temperature bear similarities with the lower Helvetic nappes of the Alps, suggesting deep burial and/or possibly high geothermal gradient in this part of the Pyrenees.