Episodes Of Deformation Research Articles

The Role of Orogenic Collapse on Tectonic Inheritance of Passive Continental Margins

AbstractThe break‐up and drift of continents is governed by the reactivation of structures in the lithosphere that occur in preference to formation of new structures during rifting. Numerical analyses indicate that mantle heterogeneities should be a first‐order control on tectonic inheritance; however, the importance of mantle relative to crustal structures has not been tested by observation in the geological record. Seismic anisotropy analysis using shear wave splitting of seismic waves are a useful tool to constrain past episodes of lithospheric deformation in the upper mantle. Analysis of published SKS directions colinear to orogenic trends, geometry of intracratonic faults and rift basins, and prolongation of basement structures into oceanic lithosphere all suggest a role of the mantle in controlling tectonic inheritance and crustal orogenesis. The geometry of the continental margin and temporal constraints point to a process involving lithosphere‐scale structures mimicked in the oceanic lithosphere due to orogenic collapse. The occurrence of fracture zones of the Labrador Sea that correspond with major structures in the Grenville, Appalachian, and older orogens suggest that deformation of the lithosphere and propagation of oceanic transforms are influenced by rheology in the upper mantle that formed during previous episodes of continental collision. If inheritance occurs at a lithospheric scale and is rooted in the mantle, it implies that orogenic collapse is an important mechanism for ocean opening along rifted continental margins. The geometry and morphology of the modern Atlantic Ocean may have been predetermined by Archean plate tectonic processes.

Chronology, structures and salt tectonics in the northern Kuqa Depression, NW China: Implications for the Cenozoic uplift of Tian Shan and foreland deformation

Foreland fold-and-thrust belts (FTBs) are shaped by the coupled influence of surface processes (e.g., erosion, sedimentation) and deep geological processes (e.g., plate rheology, flexure and kinematics) across various timescales. These processes record multiple phases of orogenic evolution and are crucial for understanding the interplay between mountain building and the sedimentary basin filling. In this paper, we conducted an integrated study including magnetostratigraphy of exposed strata, detailed field investigations and interpretations of subsurface seismic profiles in the northern Kuqa Depression. This study aims to elucidate the tectonic uplift of the Tian Shan by examining salt-related structures. The present findings reveal that diapirism in the study area commenced in the early Eocene and continued at least into the late Miocene. Notably, the Tuzimaza diapir has been significantly squeezed due to regional contraction since ∼5.3 Ma. The exposed growth strata were primarily compressional, rather than halokinetic, suggesting the lateral expansion of the Tuzimaza anticline since the latest Miocene (∼5.3 Ma). This expansion was a response to the far-field effects of the Indian-Eurasian collision. We propose that the local activation of the Tian Shan likely initiated in the early Oligocene (∼34 Ma), with three subsequent episodes of deformation occurring ∼24 Ma, ∼10 Ma and ∼ 5.3 Ma. The earlier episodes can be attributed to basement uplift in the northern Kuqa Depression, whereas the later episode starting at ∼5.3 Ma reflects more intensive deformation across the entire foreland, indicative of the steady outward growth of the Tian Shan. Collectively, all these tectonic events have contributed to the formation of the modern Tian Shan.

Modulation of Deformation by Magmatic Tempo, Coast Mountains Batholith, British Columbia, Canada

AbstractThe tectonomagmatic evolution of the southern Coast Mountains batholith, British Columbia reveals the relation of magmatic tempo, deformation, and relative plate motions which inform models for growth of continental crust and convergent plate dynamics. Magmatism in the southern batholith is episodic, derived primarily from the mantle, and reflects lower‐plate dynamics (Cecil et al., 2018, https://doi.org/10.1029/2018gc007874, 2021, https://doi.org/10.1130/ges02361.1). New field, structural, geochronologic, and geobarometric results from the southern batholith document three episodes of deformation, each spatially and temporally coincident with a high magmatic flux event (HFE). Sinistral faulting (<117–103 Ma) and penetrative deformation (110–90 Ma) occurred only within two distinct areas affected by a HFE at 114–102 Ma. Following a magmatic lull, crustal shortening occurred after 90 Ma and before 72 Ma within the region affected by a second HFE (85–70 Ma). After 70 Ma and before 53 Ma, >100 km of dextral slip occurred on the Coast shear zone and minor shortening affected 64–62 Ma plutons, overlapping with the 64–61 Ma HFE. Thus, at the crustal level exposed in the southern batholith, the timing and location of deformation is linked to magmatic tempo and the style of deformation varies through time. These results suggest that magmatic HFE modulate the timing and location of deformation while relative plate motions during HFE influence the style of deformation. Periods of deformation in batholiths may thus record high‐flux magmatic events and coeval plate motion but do not necessarily signal changes in plate motions.

The geometry of fault reactivation and uplift along the central part of the Maacama Fault Zone, Northern California Coast Ranges (USA)

Abstract Fault reactivation of bedrock structures in active fault zones influences stress state and earthquake rupture phenomena through the introduction of weak slip surfaces that impact fault zone geometry and width. Yet, geometric relationships between modern faults and older reactivated faults are difficult to quantify in rocks that have experienced multiple deformation episodes. We used new geologic mapping, geomorphic tools, and structural modeling to quantify rock uplift and subsurface fault geometry of the central part of the Maacama Fault Zone near Ukiah, California, USA, and the surrounding area. Results suggest that the northern Mayacamas Mountains are in a tectonically driven disequilibrium, with differential rock uplift focused on the western side of the range. Steeply east-dipping fault surfaces and splays characterize the geometry of the Maacama Fault Zone. We mapped two newly identified faults to the east of the main Maacama Fault, the Cow Mountain–Mill Creek Fault, and Willow Creek Fault, which align with a moderately east-dipping cluster of microseismicity between 4–10 km depth beneath the Mayacamas Mountains. Static stress modeling on the Maacama Fault Zone and newly identified faults to the east quantify slip tendency values of 0.5–0.4, which suggests that the faults are moderately to poorly suited for slip in the modern stress field and may be weak. We infer that modern uplift is driven by oblique reverse, up-to-the-east, dip-slip motion on the reactivated Cenozoic Cow Mountain–Mill Creek and Willow Creek Faults as material is advected through a restraining bend on the Maacama Fault. This study shows that reactivated bedrock faults increase the fault zone width and introduce fault surfaces that contribute a component of vertical deformation and uplift in major strike-slip fault zones. Deformation is accommodated on an interconnected network of new and reactivated faults that delineate a complex seismic hazard.

The tectonic development of the Central African Plateau: evidence from shear-wave splitting

SUMMARY The Central African Plateau comprises a mosaic of numerous Archean terranes—the Congo, Bangweulu and Kalahari Cratons—sutured in a series of Proterozoic to early Cambrian orogenic events. Major upper-crustal deformation and complex craton margin fault zones reflect the region’s diverse tectonic history: rifting during the Neoproterozoic, collision during the Pan-African orogeny, and more recently, Permo-Triassic Karoo rifting and the Pliocene development of the Southwestern branch of the East African Rift. The tectonic evolution and extent to which the lithospheric mantle has been re-worked by each tectonic event is poorly understood. New seismograph networks across the Plateau provide fresh opportunity to place constraints on the plate-scale Precambrian-to-Phanerozoic processes that have acted across the region. Utilizing data from seismograph deployments across the Central African Plateau, including the new Copper Basin Exploration Science network—a NW–SE-trending, 750-km-long profile of 35 broad-band stations—we explore lithospheric deformation fabrics associated with past and present tectonic events via a shear-wave splitting study of mantle seismic anisotropy. Results reveal short length-scale variations in splitting parameters (fast direction: $\phi$, delay time: $\delta$t), suggestive of a fossil lithospheric fabric cause for the observed anisotropy. A lack of fault-parallel $\phi$ across the Mwembeshi shear zone, suggests it may be too narrow at mantle depths, a thin-skinned, crustal-scale feature, and/or did not experience sufficient fault parallel shear-strain during its last active phase to form a lithospheric deformation fabric discernible via teleseismic shear-wave splitting. In the heart of the Lufilian Arc, we observe abrupt changes in splitting parameters with NE–SW, N–S and NW–SE $\phi$ and 0.5 s $< $$\delta$t$< $ 1.2 s evident at short length-scales: no single, uniform, anisotropic lattice preferred orientation (LPO) fabric defines the entire region. This is consistent with the view that multiple episodes of deformation shaped the Lufilian Arc, or perhaps that pre-existing fabrics, relating to Neoproterozoic Katangan Basin development, have failed to be completely overprinted by the Pan-African orogeny. Near the Domes, where most intense crustal re-working is thought to have taken place during the Pan-African orogeny, there is a cluster of null and low $\delta$t splits which likely reflects the lack of organized LPO fabrics, perhaps due to the presence of depth-dependent anisotropy. The neighbouring Congo Craton margin is marked by consistently weak anisotropy ($\delta$t$\lt $ 0.7 s) indicating a weak horizontal alignment of olivine at mantle lithospheric depths, typical of several Archean terranes worldwide.

Metallogenic model of the Lykling ophiolite-hosted lode Au deposit, Scandinavian Caledonides: Insight from fluid inclusions, mineral chemistry and stable isotope geochemistry

The Lykling lode Au deposit represents a unique example of gold mineralization in the Upper Allochthone of the Scandinavian Caledonides. The mineralization is hosted by the Early Ordovician Lykling Ophiolite Complex and the intruding trondhjemite unit and spatially associated with two generations of mafic dykes that crosscut both the ophiolitic complex and trondhjemite. Field observations indicate that the mafic dykes did not play an active role in the emplacement of Au at Lykling, however their contacts with the immediate host rocks (i.e., gabbro and trondhjemite) may have focused ore-forming fluids.Three main stages in the evolution of the hydrothermal system in the Lykling area are documented by two generations of auriferous quartz veins: 1) older quartz-carbonate veins hosted by moderately angled brittle-ductile shear zones and 2) younger quartz-sulfide veins that fill steeply dipping brittle faults. Stage 1 resulted in deposition of barren quartz and gold-free pyrite from moderately saline NaCl-CaCl2-H2O ± CO2 fluids at temperatures between ∼310–330 °C and pressures in the range from 2.7 to 3.5 kbars. Under the given physicochemical conditions Au was mobile in the form of its chloride complexes. Stage 2 is associated with multiple episodes of ductile deformation punctuated by concomitant brecciation reflecting brittle-ductile transition processes. Fluid inclusion data reveals a decrease in temperature and pressure that may result in the formation of stable Au-bisulfide complexes and making the hydrothermal mobility of Au sensitive to changes in pressure. Consequently, the fluctuation in pressure controlled by the development of brittle-ductile structures likely resulted in the precipitation of native gold in quartz veins. Stage 3 represents a pure brittle event which is accompanied by a significant decrease in the fCO2/fS2 ratio. Mixing of higher-temperature and moderate-salinity fluids with cooler fluids has been recognized as the principal trigger for deposition of sulfides and a paragenetically late phase of native gold in the Lykling hydrothermal system.The Lykling ophiolite-hosted lode Au deposit shows numerous similarities with orogenic gold deposits elsewhere, including structural controls during mineralizing events, deposition from moderately to low salinity CO2-bearing aqueous solutions, deposition from a focused fluid flow along trans-crustal fault zones with a mixed brittle-ductile character and the spatial association with regionally metamorphosed terranes. In contrast to the great majority of the known orogenic gold deposits, the Lykling ophiolite-hosted lode Au deposit records a magmatic origin of volatiles. Therefore, taking into consideration field relationships and the regional setting of the Lykling deposit, we argue that its formation is concomitant with emplacement of the Sunnhordland Batholith during post-collisional thinning of the crust and associated localized uplift within the Lykling Ophiolite Complex.

Evolution of fluid redox in a fault zone of the Pic de Port Vieux thrust in the Pyrenees Axial Zone (Spain)

Abstract. In mountain ranges, crustal-scale faults localize multiple episodes of deformation. It is therefore common to observe current or past geothermal systems along these structures. Understanding the fluid circulation channelized in fault zones is essential to characterize the thermochemical evolution of associated hydrothermal systems. We present a study of a palaeo-system of the Pic de Port Vieux thrust fault. This fault is a second-order thrust associated with the Gavarnie thrust in the Axial Zone of the Pyrenees. The study focused on phyllosilicates which permit the constraint of the evolution of temperature and redox of fluids at the scale of the fault system. Combined X-ray absorption near-edge structure (XANES) spectroscopy and electron probe microanalysis (EPMA) on synkinematic chlorite, closely linked to microstructural observations, were performed in both the core and damage zones of the fault zone. Regardless of the microstructural position, chlorite from the damage zone contains iron and magnesium (Fetotal / (Fetotal + Mg) about 0.4), with Fe3+ accounting for about 30 % of the total iron. Chlorite in the core zone is enriched in total iron, but individual Fe3+/Fetotal ratios range from 15 % to 40 %, depending on the microstructural position of the grain. Homogeneous temperature conditions about 280–290 °C have been obtained by chlorite thermometry. A scenario is proposed for the evolution of fluid–rock interaction conditions at the scale of the fault zone. It involves the circulation of a single hydrothermal fluid with homogeneous temperature but several redox properties. A highly reducing fluid evolves due to redox reactions involving progressive dissolution of hematite, accompanied by crystallization of Fe2+-rich and Fe3+-rich chlorite in the core zone. This study shows the importance of determining the redox state of iron in chlorite to calculate their temperature of formations and to consider the fluid evolution at the scale of a fault.

Reactivation and propagation of the transbrasiliano strike-slip system in Tocantins province, central Brazil: From Ediacaran to Cretaceous

Strike-slip zones are formed by the accommodation and displacement of crustal blocks, reflecting dynamic behavior within the lithosphere. Repeated deformation in these zones can obscure earlier geological processes by creating new structural frameworks and reactivating ancient weakness zones. In central Brazil, the Transbrasiliano Lineament - a major continental-scale strike-slip system - has significantly reworked the Tocantins Province, altering key geological elements that constrain the final stages of Western Gondwana's assembly. Although several seismic events have been linked to this large structure, establishing a direct correlation remains challenging due to its crustal framework being largely obscured by Phanerozoic sedimentary basins. This study presents a multiscale analysis of geological, geomorphological and geophysical data to characterize the Transbrasiliano Lineament dynamic in central Brazil. Our findings reveal a deep crustal discontinuity associated with the Goiás-Tocantins seismic zone, characterized by low-velocity anomalies and a thinner crust within the Brazilian Platform. The complex configuration of this strike-slip system, due to successive reactivations, has displaced and reoriented previous magnetic elements, affecting the Araguaia, Brasília, and Paraguay Belts. Consequently, deformation of the Transbrasiliano Lineament has propagated from the southern Bananal Basin to the northern Paraná Basin, exhibiting a horsetail splay geometry. Structural geophysical analysis indicates that this ongoing reactivation has recorded deformation episodes from the Ediacaran-Cambrian to the Upper Cretaceous.

Unraveling deformation mechanism of the Fukang fold-and-thrust belt: Insight into intracontinental orogenesis of the Bogda Mountain, NW China

Characterized by the superposition of well-developed thick-skinned and thin-skinned structures, the Fukang fold-and-thrust belt is an ideal place for understanding the intracontinental orogenic process of the Bogda Mountain in the Tianshan orogenic belt, China. This study comprehensively examines the geological architecture, structural evolution, and deformation mechanism of the Fukang fold-and-thrust belt using high-resolution seismic reflection data, apatite-fission track analysis, and discrete element simulation. Seismic profile reveals that the Fukang fold-and-thrust belt exhibits superimposed tectonic styles, including a thrust duplex involving Carboniferous strata and an overlapping thrust imbricate structure. Stratigraphic unconformities and thermochronological results from apatite fission track suggest that this belt experienced five main episodes of compressional deformation in the Late Permian, Middle-Late Triassic, Middle-Late Jurassic, Late Cretaceous-Paleocene, and Late Miocene-present. The discrete element simulation results indicate that the Late Permian initial compressional deformation was triggered by reactivation of the pre-existing extensional faults. Subsequent episodic deformation was primarily controlled by thrusting along the Upper Carboniferous and Middle Permian decollement layers. This study highlights the predominant roles of the pre-existing faults and the decollement layers in the formation and evolution of the piedmont fold-and-thrust belt of the Bogda Mountain at distinct stages, serving as critical factors in determining the extent and amplitude of an intracontinental orogenic belt.

Complex Lesser Himalayan tectonics in the Darjeeling-Sikkim Himalaya: New perspectives from a fold-duplex model

This study provides a comprehensive account of the modes of crustal shortening in the Lesser Himalayan Sequence (LHS) of the Darjeeling-Sikkim Himalaya (DSH). The distributed ductile deformation episodes are integrated with the localized thrusting events in the LHS. The LHS records four major episodes of buckle folding in distributed ductile deformations. From field-based structural correlations, it is demonstrated that multiple orders of third-generation orogen-parallel (F3) and late-stage orogen-perpendicular (F4) folds have resulted in complex interference patterns, varying from plane non-cylindrical (Type 1) to non-plane non-cylindrical (Type 2). Continued N–S shortening in the DSH produced a crustal-scale thrust with ramp-flat geometry, locally known as the Daling Thrust (DT) under the influence of a mechanically weak coal-shale-bearing Gondwana layer, which can be compared with the present-day Main Himalayan Thrust (MHT). The thrust ramp eventually shifted towards the foreland during the India-Asia collision. A fold-duplex model is proposed to explain the potential mechanism of forelandward basal ramp migration. Our model suggests that the southern shallower Main Himalayan Thrust (MHT) flat is susceptible to creeping aseismically due to the influence of coal-shale rheology. In contrast, the mid-crustal ramp and the presently active frontal splay faults (e.g., Main Frontal Thrust, MFT) are potentially seismogenic. These findings have important implications for the interpretation of the MHT seismic cycles.

Slab buckling as a driver for rapid oscillations in Indian plate motion and subduction rate

Plate tectonics is primarily driven by the constant gravitational pull of slabs where oceanic lithosphere sinks into the mantle at subduction zones. Under stable plate boundary configurations, changes in plate motion are then thought to occur gradually. Surprisingly, recent high-resolution Indian plate reconstructions revealed rapid (2–3 Million-year) plate velocity oscillations of ±50 %. We show, through numerical experiments, that the buckling of slabs in the mantle transition zone causes such oscillations. This buckling results from the deceleration of slabs as they sink into the lower mantle. The amplitude and period of buckling-associated oscillations depend on average subduction velocity and the available space in the mantle transition zone. The oscillations also affect the upper plate which may explain enigmatic observations of episodic deformation and fluid flow in subduction-related orogens. We infer that the slab pull that drives plate tectonics is generated in just the top few hundred kilometers of the mantle.

Multiscalar 3D temporal structural characterisation of Smøla island, mid-Norwegian passive margin: an analogue for unravelling the tectonic history of offshore basement highs

Abstract. Smøla island, situated within the mid-Norwegian passive margin, contains crystalline-basement-hosted intricate fracture and fault arrays formed during a polyphase brittle tectonic evolution. Its detailed study may strengthen correlation attempts between the well-exposed onshore domain and the inaccessible offshore domain, further the understanding of the passive margin evolution, and provide useful constraints on petrophysical properties of fractured basement blocks. A combination of geophysical and remote sensing lineament analysis, field mapping, high-resolution drill hole logging, 3D modelling, petrographic and microstructural studies, and fault gouge K–Ar geochronology made it possible to define five deformation episodes (D1 to D5). These episodes occurred between the post-Caledonian evolution of the regional-scale Møre–Trøndelag Fault Complex (MTFC) and the Late Cretaceous and younger crustal extension preceding the final stages of Greenland–Norway break-up. Each reconstructed deformation stage is associated with different structural features, fault and fracture geometries, and kinematic patterns. Synkinematic mineralisations evolved progressively from epidote–prehnite, sericite–chlorite–calcite, chlorite–hematite, hematite–zeolite–calcite, to quartz–calcite. K–Ar geochronology constrains brittle deformation to discrete localisation events spanning from the Carboniferous to the Late Cretaceous. Multiscalar geometrical modelling at scales of 100, 10, and 1 m helps constrain the extent and size of the deformation zones of each deformation episode, with D2 structures exhibiting the greatest strike continuity and D1 features the most localised. Overall, the approach highlighted here is of great utility for unravelling complex brittle tectonic histories within basement volumes. It is also a prerequisite to constrain the dynamic evolution of the petrophysical properties of basement blocks.

The Neotectonic Deformation of the Eastern Rif Foreland (Morocco): New Insights from Morphostructural Analysis

The Neotectonic Deformation of the Eastern Rif Foreland (Morocco): New Insights from Morphostructural Analysis

2730–2670 Ma rifting triggers sagduction prior to the onset of orogenesis at ca 2650 Ma: implications for gold mineralisation, Eastern Goldfields, Western Australia

The dominant structural fabric in the Eastern Goldfields is the steep north- to north-northwest-trending S2 foliation that is axial planar to upright F2 folds, developed during intense horizontal east–west shortening (D2–4 event). These structures consistently overprint D1 structures, which comprise layer-parallel S1 foliation, extensional shears, thrusts and recumbent F1 folds. The D1 event represents a separate and distinct episode of deformation with a markedly different stress regime (dominant vertical σ1), compared with the stress regime during the D2–4 events (horizontal east–west oriented σ1). There is little evidence for pre-2670 Ma ductile deformation in the Eastern Goldfields, as: (1) the ca 2730–2670 Ma greenstone sequences are deposited conformably on the older ca 2800 Ma greenstone sequences, with the first significant angular unconformities observed at the base of the post greenstone late basins; (2) a lack of schist or gneiss clasts in the late basins suggests that the surrounding uplifted sequence was largely undeformed; (3) the layer-parallel S1 foliation is observed throughout the 2720–2670 Ma greenstone sequence; and (4) the dominant low north- and south-plunges of F2 folds suggest that the sequence was predominantly flat-lying prior to strong east–west shortening (D2–4 events). The ca 2670–2655 Ma D1 event represents a period of sagduction that occurred immediately after cessation of rifting as a result of gravitational instability from deposition of dense, cold, greenstone sequences onto thinned, light, hot felsic crust. Late basins represent depocentres on the sinking greenstones during sagduction. Marked contrasts in the structural style of gold deposits, metallogeny and fluid sources, typically attributed to progressive deformation during orogenesis, could instead reflect temporally distinct mineralising events during changing tectonic regimes. In the East Yilgarn, an early plumbing system dominated by north-northwest-trending basin-controlling structures was likely established during rifting and focused fluids during multiple hydrothermal circulation events.

Geodetic Evidence for Cascading Landslide Motion Triggered by Extreme Rain Events at Joshimath, NW Himalaya

AbstractSlope instability due to tectonic, hydrological and anthropogenic activities cause severe landslides in Himalaya. Joshimath, a densely populated Himalayan town witnessed a catastrophic landslide event during December 2022 and January 2023 causing damages to ∼700 buildings. We use Interferometric synthetic aperture radar, Global Positioning System and rainfall measurements to probe the kinematics of the Joshimath landslide. We separate the seasonal and episodic deformation components using singular spectrum analysis. While the low amplitude annual landslide motions are modulated by seasonal precipitation, acceleration phases are triggered by extreme rain events. Our analysis revealed episodes of cascading motions triggered by extreme rain events resulting an overall increase in landslide velocity from −22 mm/yr during 2004–2010 to −325 mm/yr during 2022–2023. We estimate the landslide depth (∼30 m) and hydraulic diffusivity (∼3 × 10−5 m2/s) using a 1‐D pore‐water pressure diffusion model. Our study reveals the importance of systematic monitoring of ground deformation and weather parameters for landslide hazard mitigation.

Monitoring hydrothermal fumaroles in the Azores archipelago - Applications and sources of analytical uncertainties

The Azores archipelago is located in the North Atlantic Ocean and is formed by nine volcanic islands. Present-day volcanic activity in the archipelago comprises not only seismic swarms and episodes of ground deformation in some volcanoes, but also hydrothermal gas manifestations. The main fumaroles are associated with central quiescent volcanoes and have been sampled in the past decade through the Giggenbach methodology. Analysis of the fumarolic fluids from four volcanic systems in São Miguel, Terceira, and Graciosa islands are displayed for the period between July 2015 and February 2023. No significant changes have been observed both on the selected gas ratios (CO2/CH4, H2/CH4, He/CH4, He/CO2, and H2/Ar), and on the equilibrium temperatures estimated for the reservoirs feeding the fumaroles using the H2/Ar gas geothermometer, what is in agreement with the dormant state of activity of the studied volcanic systems. Equilibrium temperatures ranging between 223 °C and 262 °C are estimated for Fogo and Furnas fumarolic fields (São Miguel Island), and an average equilibrium temperature of 254 °C is inferred for Terceira fumaroles. This study also suggests a procedure based on established guidelines to evaluate the analytical uncertainties of the methodologies associated with the Giggenbach sampling strategy. Comparison of the analytical uncertainties with the overall data variation (including additional natural variations and sampling uncertainty), shows that the analytical uncertainties are not the limiting factor for the interpretation of the survey results.

Cretaceous extensional and contractional stages in the Colombian Andes unraveled by a source-to-sink geochronological and thermochronological study in the Upper Magdalena Basin

Extensional and flexural basins can evolve through multiple stages under the same plate tectonic regime over tens of millions of years. The Cretaceous tectonic evolution of the Colombian Andes is characterized by shifts between contractional and extensional tectonics. The upper plate response to these changes is recorded in the Cretaceous sedimentary rocks along the Western, Central, and Eastern cordilleras, and their adjacent basins, including the Upper Magdalena hinterland basin. We integrated field observations, petrography, geochronology, and thermochronology in the Cretaceous sedimentary units in the southern Upper Magdalena Basin and in the adjacent crystalline basement to decipher the provenance of these units and the exhumation patterns of the source areas. These results suggest that the source areas were exhumed between the Late Jurassic to Early Cretaceous, which may have resulted from the combination of Jurassic contraction and the Early Cretaceous extension. Between ∼120 and 100 Ma, a major stage of crustal extension is recorded in the Caballos Formation. Subsequently, between ∼100 and 80 Ma, sedimentation in the Villeta Group marks the end of the extension and the onset of contraction. This contractional phase was characterized by minor rock uplift within the basin. Basin modification continued between 80 and 65 Ma, involving two successive contractional stages that changed the sedimentary patterns, leading to the burial of a previously exhumed horst block and the appearance of new source areas. These major changes in source areas and sedimentary systems are the result of deformation episodes during prolonged extensional and contractional phases. This geologic evolution highlights the stages that characterize extensional to contractional settings.

The Application of Dual Frequency IP Survey to Identify the Concealed Ore Body in Pilbara Craton, Australia: A Case Study from Calvert Gold Deposit

The Mallina Basin in Western Australia, situated in the central part of the Pilbara Craton which is one of the world's oldest cratons, is a structurally deposited basin subjected to multiple episodes of deformation and magmatic intrusion. The Calvert gold deposit in Mallina Basin is heavily covered by Quaternary, thus, it is hardly to identify any distinct mineralization indicators, and there is no any significant progress in exploration so far. The SQ-3C dual frequency induced polarization facility that is developed by China is introduced to carry out the dual frequency IP survey, the author’s team have tested all the types of different rock samples to collect the geophysical parameters in the Calvert area, and the selection of suitable facility and working frequency is based on the geological characters of the Calvert deposit and the geophysical parameters of rocks, and some processes are undertaken to weaken the interference triggered by the electromagnetic coupling, and an IP anomaly zone was delineated. Through the drilling program in the IP anomaly zone, a gold ore body that has the same orientation with the IP anomaly zone was controlled; this result demonstrated the dual frequency IP survey is effective in the local area and provide an efficient technical method for the mineralization prospecting, and this geophysical survey project sets an example for the familiar type of gold deposit in the local area.

Formation of the Great Bend and Enhanced Quaternary Incision of the Upper Yangtze River: New Insights from Low-Temperature Thermochronology and Tributary Morphology

Abstract The occurrence of a sharp turn along the upper course of the Yangtze River is referred to as the “Great Bend” and represents a large-scale drainage reorganization in response to the surface rise of the Tibetan Plateau. However, the timing and mechanism of the formation of the Great Bend remain disputed. In this paper, we report new (U–Th)/He and apatite fission track thermochronological data from the deep river valley in the Great Bend area of the southeastern margin of the Tibetan Plateau. Compared with the adjacent Jianchuan Basin, two phases of younger rapid cooling for the Great Bend area are identified based on thermal-history modeling, namely, Miocene (ca. 17 to 11–8 Ma) and Quaternary, with the former phase being contemporaneous with the formation of the anticline in the Tiger Leaping Gorge. Progressive increases in the normalized channel steepness (ksn) and the degree of river-valley incision with increasing distance downstream for tributaries of the Yangtze River in the Tiger Leaping Gorge indicate that river rerouting and formation of the Great Bend occurred during the Miocene. Samples located at the bottom of the Tiger Leaping Gorge also reveal a phase of rapid cooling since ca. 1.9 Ma, with an exhumation rate of 1.5 ± 0.2 mm/year. We hypothesize that enhanced Quaternary exhumation in the southeastern margin of the Tibetan Plateau occurred mainly within the narrow region between the Sichuan Basin and the Eastern Himalayan Syntaxis, corresponding to an episode of widespread extensional deformation superimposed above middle- to upper-crustal flexure in this region.

The Zaozigou orogenic gold-antimony deposit, West Qinling Orogen, China: Structural controls on multiple mineralization events

Orogenic gold deposits are generally thought to represent one perhaps protracted event. However, recent research on orogenic gold deposits increasingly offers evidence for some deposits forming through multiple and clearly discreet hydrothermal episodes. The giant Zaozigou orogenic Au-Sb deposit in the Triassic to Cretaceous West Qinling Orogen, central China, includes both steeply dipping and gently dipping orebodies. The two distinct mineralization styles provide a valuable setting for investigating a multiple mineralization model by integrating structural analysis within a robust geochronological framework. Through fieldwork and geochronology, we define a progression of major tectonic events in the area of the Zaozigou deposit. The deposit is hosted within a well-bedded sequence of Early Triassic metasedimentary rocks of the South Qinling Terrane. Pre-mineralization E-W shortening (D1) during subduction of the Mianlue oceanic slab include folding with resulting axial planes striking N-S, emplacement of Triassic ENE-striking and WNW-striking dacite dikes accompanied by Middle Triassic greenschist facies metamorphism. Late Triassic gold-stibnite quartz vein and disseminated mineralization formed along ENE-striking and steeply dipping D2 brittle to ductile sinistral faults. Their orientations suggest a link to the regional NNE-SSW maximum principal stress coinciding with transpression caused by the Late Triassic collision between the South China Block and South Qinling Terrane. Overprinting Early Cretaceous quartz-stibnite veins developed along gently dipping (20° to 40°) brittle D3 normal fault zones, which exhibit a NE-SW minimum principal stress. This younger deformation event is interpreted to be related to the Early Cretaceous tectonic transition from shortening to extension of the West Qinling Orogen. Therefore, the Zaozigou deposit reveals a model of multiple orogenic gold mineralizing events, with migration of hydrothermal fluids during discrete deformation episodes and the resulting formation of a single composite deposit formed along overprinting structures at separate times of orogenesis.