Formation Of Pull-apart Basins Research Articles

Anatomy of a Neoarchean continental arc-backarc system in the Cross Lake-Pipestone Lake region, northwestern Superior Province, Canada

The Neoarchean Cross Lake-Pipestone Lake terrain, northwestern Superior Province, consists of the Cross Lake greenstone belt, the Pipestone Lake anorthosite complex, and multiple generations of granitoids. The Cross Lake greenstone belt includes an assemblage of basalt, picrite, gabbro, and sedimentary rocks (Pipestone Lake Group) that formed between 2790 Ma and 2760 Ma, and an assemblage of polymictic conglomerate, sandstone, and siltstone (Cross Lake Group) that were deposited between 2704 Ma and 2691 Ma. The Pipestone Lake anorthosite complex is composed of gabbro, megacrystic anorthosite and leucogabbro, and mafic dykes. The Pipestone Lake Group and the Pipestone Lake anorthosite complex are intruded by ca 2760 Ma and younger granitoids. The trace element systematics of the granitoids suggest that they originated in a volcanic-arc to syn-collisional tectonic setting, whereas the trace element signatures of the basalt, picrite, gabbro, anorthosite and leucogabbro association are consistent with a back-arc setting. Whole-rock initial εNd values, zircon initial εHf values and 207Pb/206Pb zircon ages for the granitoids and the anorthosite complex are consistent with long-term depleted mantle sources and variable degrees of crustal contamination during their interaction with Mesoarchean to Paleoarchean continental crust. Zircon trace element data indicate that the granodiorite and the anorthosite complex formed in a continental crust to oceanic crust transition zone at an arc-related setting. On the basis of the field relationships, and trace element and isotopic data, we suggest that the Pipestone Lake anorthosite complex and the spatially and temporally associated Pipestone Lake Group formed in a continental back-arc setting. During and after the closure of the back-arc basin, both the Pipestone Lake Group and the anorthosite complex were intruded by arc-derived granitoids. Accordingly, we interpret the association of the Pipestone Lake Group and Pipestone Lake anorthosite complex as a Neoarchean suture zone. Following the collision and closure of the back-arc basin, the region was dissected by strike-slip faults resulting in the formation of pull-apart basins and deposition of the Cross Lake Group as Timiskaming-type sedimentary rocks.

О тектоно-геодинамических взаимоотношениях Евразийского бассейна и хребта Ломоносова с континентальной окраиной Сибири по новым сейсмическим данным

Basing on the results of the interpretation of new seismic materials, the authors consider the structural features of the southern segments of the Eurasian Basin and the Lomonosov Ridge in the zone of junction with the continental margin of Siberia (the Laptev and East Siberian Seas). Interpretative analysis of the materials shows that the base of the sedimentary cover of the southern segment of the Eurasian Basin, where there are no regular linear magnetic anomalies, is predominantly represented by strongly stretched blocks of the continental basement. The formation the axial zone of spreading of the Gakkel Ridge here took place according to a three-stage development scheme: rifting in the Aptian-Alba and its telescoping development in the Late Cretaceous — Paleocene-Eocene, inherited valley formation in late neotectonic time. The development of the Severny Basin located in the zone of junction of the Lomonosov Ridge with the continental margin is similar to the scenario for the formation of pull-apart basins. Its formation was interconnected with the simultaneously opening adjacent extreme southeastern segment of the Amundsen Basin of the Eurasian Basin.

What processes control the genesis of absarokite to shoshonite-banakite series in an intracontinental setting, as revealed by geochemical and Sr-Nd-Pb isotope data of Karadağ Stratovolcano in Central Anatolia, Turkey

The Latest Cenozoic absarokite to shoshonite-banakite series (ASB) samples were discovered in the Karadağ stratovolcano (KS) which erupted in the Karaman basin in Central Anatolia, Turkey; and has a spatially close association with major strike-slip faults and pull-apart basins. The KS are made up of adakitic andesite and dacite, with minor ASB lava flows. Here, we present integrated data of mineral chemical, whole rock major and trace element, Sr-Nd-Pb isotope and conventional KAr age of the ASB series rocks in Central Anatolia. The rocks were dated by conventional KAr dating method in old of 393 ± 153 ka. The series are comprised of slightly differentiated absarokite and more fractionated shoshonite-banakite lava flows. The Ol-normative absarokites contain phenocrysts of olivine and diopside, whereas the Hyp-normative shoshonites-banakites are made up of olivine, diopside, phlogopite, K-feldspar and plagioclase. Both series have a potassic nature and near-primitive character with high MgO (5–11 wt%) and Mg# (62 to 74). The overall samples were characterized relatively by the enrichment in large ion lithophile elements (LILE) and the depletion in high field strength elements (HFSE), with insignificant negative Eu anomaly. They possess homogeneous and unradiogenic Sr and Nd isotopic compositions, marked by 87Sr/86Sr(t) = 0.70507 to 0.70540 and εNd(t) =0.6 to +1.2. TDM, and Pb isotopic ratios tending towards the EMII mantle end-member.These geochemical features favor the origin of a lithospheric mantle reservoir which was metasomatized by i) sediments of subducting oceanic slab and ii) asthenospheric agents associated with break-off of African slab. The existing geochemical fingerprints and trace element modeling indicate that the Ol-normative absarokites were derived from a parental magma by low degree partial melting (~ %5) of a chemically enriched lowermost subcontinental lithospheric mantle source, composing of a garnet- and phlogopite-bearing lherzolitic lithology. The shoshonites-banakites were formed by fractionation of the absarokitic magmas, with a minor crustal assimilation. We contend that the Quaternary volcanism was probably controlled by the extension of the pull-apart basin system, which was induced by the deep strike-slip faults. Then, the extension caused an upwelling of the hot asthenosphere, triggering partial melting of the mantle source. This data rule out the possibility of the Latest Cenozoic active and steady subducting setting in the region. Instead, our results reveal the period which is likely marked by a transtensional phase and this phase triggered the pull-apart basin formation where the ASB series erupted in an intracontinental setting in Central Anatolia.

2D finite-element elastic models of transtensional pull-apart basins

A set of two-dimensional finite-element elastic models are presented to provide insights on the evolution of transtensional pull-apart basins between two right-stepping, right-lateral fault segments. Three representative fault segment interaction geometries are modelled, showing underlapping, neutral and overlapping segments. Despite the simplifications of the 2D model, overall results are obtained that might help understanding the formation of pull-apart basins. Firstly, the orientations of the local σ1 and σ3 tensional stress directions markedly depend on the segment's position. Secondly, the mean normal stress is extensional in a transtensional basin between segments, while the region outside the step is characterized by more compressive mean normal stresses. Thirdly, the angle of offset between the fault segments is one of the most important parameters controlling the geometries of the transtensional pull-apart basins: connected depocenters with basin high and lozenge shape in the case of underlapping steps, spindle shape or lazy S or Z shape in the case of neutral steps, and broadly elongate rhomboidal to sigmoidal basins in the case of overlapping steps. Generally, en-échelon basin margin system, dual opposing asymmetric depocenter, intrabasin relative structural high, and wide basin width can be used as indicators that a pull-apart basin is developing in transtension zones.

Ambient seismic noise tomography reveals a hidden caldera and its relation to the Tarutung pull-apart basin at the Sumatran Fault Zone, Indonesia

We analyzed the noise recordings of a short-period seismic network to derive a shallow crustal S-wave velocity model at the Sumatra Fault in Northern Sumatra, Indonesia. By correlating the noise of 40 seismic stations' recording for 9months, we could recover Rayleigh waves from vertical component recordings with sufficient signal-to-noise ratio. Group velocities of the Rayleigh waves could be determined in the period range from 0.71 to 4.4s. These group velocities were used to invert for 2D group velocity maps at specific periods. Finally, the derived group velocity maps were inverted for a 3D S-wave velocity model. This model shows a region of a strong velocity decrease off the Great Sumatran Fault Zone, at the northeastern margin of the young Tarutung pull-apart basin. This observed low velocity block coincides with a caldera-like morphological feature which is interpreted as the surface expression of a hidden volcanic caldera. Considering the surface manifestations of geothermal activity around this anomaly, we conclude that the caldera is still acting as a heat source. On the other hand, the weak morphological expression at the surface indicates a certain age of the caldera which might be older than the Tarutung pull-apart basin. The findings provide important constraints on general concepts for the formation of pull-apart basins along the Sumatran fault and their relation to volcanism.

Geology of El Domo deposit in central Ecuador: a VMS formed on top of an accreted margin

El Domo is a Cu-Au-Zn-Ag volcanogenic massive sulfide deposit (VMS) within the Paleocene–Eocene submarine arc rocks of the Macuchi Formation in central Ecuador. The mineralization includes massive and semi-massive sulfides located at the interface between a submarine rhyodacite dome complex and overlying mafic volcanic and volcaniclastic rocks, including chaotic and unsorted debris flow deposits. Whole rock geochemical analyses show that the felsic rocks belong to a calc–alkaline sequence, whereas the mafic assemblages have a tholeiitic affinity, reflecting a magmatic association typically found in many VMS districts. This geochemical change temporally coincides with the formation of the massive sulfides. New zircon Hf isotopic data collected from a hanging wall rhyodacite suggest that the presence of an over-thickened crust of oceanic plateau affinity may have influenced the composition of the magmatic rocks. The timing of massive sulfide formation is constrained by an 40Ar/39Ar age of 41.49 ± 0.37 Ma (2σ) from the volcaniclastic sequence which hosts the massive sulfides and a U/Pb zircon age date of 42.13 ± 0.54 Ma (2σ) from the footwall rhyodacite; both ages are interpreted to provide minimum and maximum ages for mineralization. The massive sulfides are zoned and contain variable proportions of pyrite, sphalerite, and chalcopyrite. Minor accumulations of galena, bornite, tennantite, stromeyerite, and proustite are also present; the latter two minerals are the major carriers of silver whereas gold typically occurs as minute-free grains or in solid solution with silver. Ore minerals are intergrown with variable proportions of chlorite, barite, quartz, anhydrite, and smectite. The footwall rhyodacite hosts an irregular stringer zone which is rich in pyrite and gypsum and is characterized by intense quartz-sericite-pyrite alteration. The hanging wall volcaniclastic lithologies exhibit pervasive but vertically limited (8–10 m) silicification, chloritization, and argillization. Both felsic and mafic glass-rich rocks located near the contact between rhyodacites and mafic volcaniclastic rocks are replaced by massive to disseminated mineralization, likely controlled by their originally reactive and porous nature. Replacement appears to have taken place at very shallow depths, 10–20 m below the seafloor, as shown by: (1) the presence of massive sulfide clasts within the uppermost volcaniclastic horizons and (2) the absence of mineralization and alteration in the overlying debris flows. No evidence of exhalation on the seafloor has been found, and replacement below the sea floor was probably an efficient mechanism for preserving the VMS within such a highly dynamic volcanic system. Regionally, mineralization occurred within an extensional submarine arc setting whereby massive sulfides were preferentially controlled by the reactivation of NNE-SSW crustal-scale strike slip faults created during an earlier plate collision. Local stress regimes resulted in the formation of pull-apart basins within a regional transpressive setting and provided conduits for magmatic and hydrothermal discharge. The onset of volcanism and local extension coincided with an increase in the oblique convergence of the Farallon and the South American Plate.

A healed strike-slip plate boundary in North Greenland indicated through associated pull-apart basins

Abstract In the North Atlantic, Laurentia–Eurasia break-up commenced in the Late Carboniferous, largely following the structural grain of the Caledonian Fold Belt. However, in the Arctic region, a 45° offset in the plate boundary between North Greenland and Svalbard was determined by a number of pre-Caledonian fundamental faults in North Greenland. As a result, this segment of the plate boundary experienced significant episodes of combined transtension and transpression, in part controlled by the movement of a temporarily independent Greenland Plate. Late Permian–Mesozoic deposits in the North Greenland Wandel Sea Basin record the plate-boundary history along this offset, in our view in a series of at least 20, variously disturbed, pull-apart basins, most of which can be assigned to four major episodes of pull-apart basin formation. The direction of the pre-existing fundamental faults, in combination with the regional variation in rock properties of both the basin floor and basin fill, explains the marked differences in tectonic style recorded along the plate boundary.

Structural controls on the localization of Cu deposits in the Kerman Cu metallogenic province of Iran using geoinformatic techniques

The Sarcheshmeh area is located in the Kerman Cu metallogenic province (KCMP) in the southeastern part of the Uramia-Dokhtar magmatic arc in Iran. The Sarcheshmeh contains numerous Cu deposits making it ideal for the study on the structural controls in the magmatic arc. The study area is cut by major (first-order) and minor (second-order) faults, and contains different types of Cu occurrences, which have been here studied using geoinformation techniques (GiT), which consist of remote sensing techniques, GIS analyses and careful field investigations. The outcome of this study has confirmed that the Cu mineralization in KCMP is structurally controlled in faults. On a regional scale, the major faults (together with their second-order splays) form a braided fault pattern containing predominantly SE-trending faults. On a local scale, the main faults together with their splays form a rhombohedral pattern, which is due to a transtensional deformation related to the formation of pull-apart basins along the KCMP. This tectonic regime prepared the suitable environment for the ascent and final emplacement of felsic plutons in a broad linear zone trending SE. The Cu mineralization and accompanying alteration zones are hosted by the second-order faults.

Tectonic, sedimentary, and volcanic evolution of a back-arc basin in the East Sea (Sea of Japan)

Abstract This paper focuses on the tectonic, sedimentary, and volcanic evolution of a unique back-arc basin (Ulleung Basin) in the southwestern part of the East Sea (Sea of Japan). The basin consists of thick extended continental (or transitional) crust and an overlying sedimentary succession (4–8 km thick), with interlayered volcanic flows and sills, defining a number of seismic units of variable reflection characteristics. The northern margin is bounded by faulted continental blocks (South Korea Plateau) with isolated basement lows and sub-basins with intruded and extruded volcanics, whereas the southern margin is underlain by deep-seated basement with a thick (> 8 km thick) sedimentary succession. The western margin is bounded by a series of strike-slip and normal faults produced by NNW–SSE-directed dextral movements. These structural features suggest a south–southeastward drift of the southwestern Japanese Arc away from the South Korea Plateau during the Early to Middle Miocene, involving large-scale right-lateral strike-slip deformation along the western margin, akin to pull-apart basin formation. During the back-arc opening, the thinned continental crust was largely modified by intrusive volcanics under tensional stress regime. The volcanics also extruded across the axis of extension as well as along the boundary faults on the west. In the Middle to Late Miocene, major faults in the southern and western margins were inverted, forming partial closure of the basin especially in the southern part, although the plateau in the northern margin experienced continuous subsidence. The Ulleung Basin thus provides an example of an immature back-arc basin which experienced a rather brief episode of rifting and extension followed by closure due to tectonic inversion.

The Cretaceous Duimiangou adakite-like intrusion from the Chifeng region, northern North China Craton: Crustal contamination of basaltic magma in an intracontinental extensional environment

Zircon U–Pb ages, major and trace element and Sr, Nd and Pb isotope compositions of the Duimiangou (DMG) quartz monzonite from the Chifeng region on the northern North China Craton (NCC) were studied to investigate its derivation, evolution and geodynamic significance. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating yields an emplacement age of 128±1Ma for this intrusion, with numerous Mesozoic inherited zircons clustering at 219±12Ma and 161±3Ma, along with some ancient zircons with ages of 2.5Ga, 1.77Ga and 324Ma. Bulk-rock analyses show that this intrusion is characterized by variable SiO2 (63.4–69.4wt.%), Al2O3 (14.5–16.3wt.%), Na2O+K2O (8.01–8.95wt.%), and Mg# (41.3–48.0). They are enriched in large ion lithophile elements and light rare earth elements without significant Eu anomalies (mostly between 0.89–1.10), and depleted in heavy rare earth elements and high field strength elements, with high Sr/Y (63.7–101.7) and (La/Yb)N (20.5–31.0) ratios. The DMG intrusion formed in an intracontinental extensional setting contemporaneous with the formation of pull-apart basins, metamorphic core complexes and intense magmatism, rather than in a convergent margin. It has homogeneous Sr ((87Sr/86Sr)i=0.7059–0.7066), Nd (εNd(t)=−6.2 to −7.2) and Pb ((206Pb/204Pb)i=17.289–17.375, (207Pb/204Pb)i=15.359–15.463, (208Pb/204Pb)i=37.130–37.472) isotope compositions. Sr–Nd isotope modeling results, plus relatively young Nd model ages (1522–1618Ma) and the presence of relict zircons, suggest that this intrusion could have originated from crustal contamination of newly formed basaltic melts derived from asthenospheric mantle, accompanied by fractional crystallization of K-feldspar, biotite, apatite, Fe–Ti oxides and minor hornblende and plagioclase. Thus, the DMG adakite-like intrusion may record the magmatic event associated with underplating of asthenospheric magma in an intracontinental extensional environment, and both asthenospheric magma underplating and lithospheric delamination may have played important roles in the lithospheric thinning of the northern NCC.

Active Deformation along the Southern End of the Tosco-Abreojos Fault System: New Insights from Multibeam Swath Bathymetry

The relative motion of the Pacific plate with respect to the North America plate is partitioned between transcurrent faults located along the western margin of Baja California and transform faults and spreading ridges in the Gulf of California. However, the amount of right lateral offset along the Baja California western margin is still debated. We revisited multibeam swath bathymetry data along the southern end of the Tosco-Abreojos fault system. In this area the depths are less than 1,000 m and allow a finer gridding at 60 m cell spacing. This improved resolution unveils several transcurrent right lateral faults offsetting the seafloor and canyons, which can be used as markers to quantify local offsets. The seafloor of the southern end of the Tosco-Abreojos fault system (south of 24°N) displays NW–SE elongated bathymetric highs and lows, suggesting a transtensional tectonic regime associated with the formation of pull-apart basins. In such an active tectonic context, submarine canyon networks are unstable. Using the deformation rate inferred from kinematic predictions and pull-apart geometry, we suggest a minimum age for the reorganization of the canyon network.

Finite element modelling of the pull-apart formation: implication for tectonics of Bengo Co pull-apart basin, southern Tibet

The tectonic deformation and state of stress are significant parameters to understand the active structure, seismic phenomenon and overall ongoing geodynamic condition of any region. In this paper, we have examined the state of stress and crustal deformation during the formation of the Beng Co pull-apart basins produced by an enéchelon strike-slip fault systems using 2D Finite Element Modelling (FEM) under plane stress condition. The numerical modelling technique used for the experiments is based on FEM which enables us to analyze the static behavior of a real and continues structures. We have used three sets of models to explore how the geometry of model (fault overlap and pre-existing weak shear zone) and applied boundary conditions (pure strike-slip, transpressional and transtensional) influence the development of state of stress and deformation during the formation of pull-apart basins. Modelling results presented here are based on five parameters: 1) distribution, orienttation, and magnitude of maximum (σH max) and minimum (σH max) horizontal compressive stress 2) magnitude and orientation of displacement vectors 3) distribution and concentration of strain 4) distribution of fault type and 5) distribution and concentration of maximum shear stress (σH max) contours. The modelling results demonstrate that the deformation pattern of the en-échelon strike-slip pull-apart formation is mainly dependent on the applied boundary conditions and amount of overlap between two master strike-slip faults. When the amount of overlap of the two master strike-slip faults increases, the surface deformation gets wider and longer but when the overlap between two master strike-slip faults is zero, block rotation observed significantly, and only narrow and small surface deform ation obtained. These results imply that overlap between two master strike-slip faults is a significant factor in controlling the shape, size and morphology of the pull-apart basin formation. Results of numerical modelling further show that the pattern of the distribution of maximum shear stress (τmax) contours are prominently depend on the amount of overlap between two master strike-slip faults and applied boundary conditions. In case of more overlap between two masters strike-slip faults, τ max mainly concentrated at two corners of the master faults and that reduces and finally reaches zero at the centre of the pull-apart basin, whereas in case of no overlap, τmax largely concentrated at two corners and tips of the master strike-slip faults. These results imply that the distribution and concentration of the maximum shear stress is mainly governed by amount of overlap between the master strike-slip faults in the en-échelon pull-apart formation. Numerical results further highlight that the distribution patterns of the displacement vectors are mostly dependent on the amount of overlap and applied boundary conditions in the en-échelon pull-apart formation.

Cross-basin Faulting and Extinction of Pull-apart Basins in the Sea of Marmara, NW Turkey

Although offshore multi-channel seismic reflection and multi-beam bathymetric studies were intensively carried out in the Sea of Marmara especially after the August 17, 1999 İzmit earthquake (Mw 7.4), some basic problems remain unsolved, for example whether the seafloor was cut by a double fault system or just by a single fault. The researchers advocating the double fault system assumed that most of the deformation occurred along the margins of the basins and concluded that the classical model of pull-apart basin formation along releasing bends or stepovers within an east−west-trending dextral strike-slip system prevails. An alternative and completely different proposition proposed by some other researchers suggested that the seafloor was cut by a single, continuous strike-slip fault that does not coincide with the basin margins and hence does not fit the pull-apart model. These two apparently opposing ideas in fact, are not in conflict with each other; but instead define successive events. The fault geometry in the Sea of Marmara simply reflects a transition from complex pull-apart basins into single strike-slip faults. All the pull-apart basins in the world, regardless of offset geometry, evolve progressively from narrow grabens bounded by oblique-slip link faults to wider rhombic basins flanked by terraced basin sidewall fault systems. Field observations and analogue models show that in the later stages of this widening, the cross-basin faults cut the floor of the pull-apart basins and link the offset principal displacement zones. The jogs and therefore the pull-apart basins become extinct and strike-slip faults become straight as time passes. This development history of the faults and accompanying basins is valid also for the North Anatolian Fault and the basins in the northern Sea of Marmara as well as those in the Gulf of İzmit and for the other sections of the fault on land displaying two sets of faults, the older being inactive.

The mechanical relationship between strike-slip faults and salt diapirs in the Zagros fold–thrust belt

Analogue models are used to study the mechanical relationship between basement strike-slip faults and salt diapirs. Displacement along a strike-slip fault in 1 g models resulted in extension along pre-existing jogs and the formation of oblique extensional faults, where reactive diapirs were triggered in some models. In the centrifuge models, prescribed cuts, simulating pre-existing structures, were reactivated during simple shear deformation of the models, resulting in formation of pull-apart basins, which were intruded by diapirs. The models show that because of the low ratio of salt to overburden thickness in the Zagros (0.15–0.35), it is unlikely that diapirs have formed solely, if at all, as a result of movement along basement strike-slip faults. Two mechanisms are suggested. First, pre-kinematic thin overburden and continuous movement along a releasing bend in the cover units may have triggered some of the diapirs in the Zagros, which were later downbuilt to their current geometry by additional sedimentation. Second, movement along the strike-slip faults (e.g. the Kazerun and Mangarak faults) induced oblique movement along NW–SE Zagros structures (folds or thrusts) resulting in the formation of pull-apart basins where diapirs were eventually intruded. Fault plane solution of shallow earthquakes supports the second scenario, which is also in agreement with previous interpretations that some of the salt diapirs associated with basement faults are younger than Zagros shortening and young southwestwards.

Metallogenetic epoch and mechanism of the tungsten ore in Yushan, Beishan orogenic belt, Gansu

Using SHRIMP zircon U-Pb dating methods and cathodoluminescence (CL) technique, we have precisely dated ore samples from tungsten ore deposit, ore-bearing volcanic-sedimentary strata and granite in the Yushan region within the east Tianshan-Beishan W-bearing zone. This study reveals Yushan tungsten ore deposit zone formed in 250 ± 9 Ma, and it is closely associated with the Permian granites and felsic volcanic rocks. This new type of tungsten ore deposit in the east Tianshan-Beishang multiple-metallogenetic province may probably be associated with the regionally right-literal shearing after the main Carboniferous collisional orogeny, and the formation of pull-apart basin and the felsic magmatism.

Rapid exhumation and cooling of the Liaonan metamorphic core complex: Inferences from 40Ar/39Ar thermochronology and implications for Late Mesozoic extension in the eastern North China Craton

The Liaonan metamorphic core complex formed during crustal extension in the Liaodong Peninsula, eastern North China craton, and consists of the Jinzhou detachment fault, Proterozoic–Paleozoic sedimentary rocks in its upper plate, and exhumed high-grade Archean metamorphic rocks and Early Cretaceous granitic plutons in the lower plate. Exhumation of its footwall from mid-crustal levels is evidenced in the detachment fault zone by the temporal transition from amphibolite facies mylonitization at depth, through retrograde chloritic shearing and brecciation, to brittle faulting during final uplift. The footwall mylonite zone is 2.5–3.5 km thick and includes Early Cretaceous (128–118 Ma) granitic rocks, together with older metamorphic rocks. The 40Ar/39Ar ages of muscovite, hornblende, biotite, and K-feldspar from the mylonitic rocks record that the core complex cooled between ca. 120 and 107 Ma, from the time of initial crystallization of zircons (122–118 Ma) at 700–800 °C in syntectonic leucocratic dikes and granitic rocks, to closure of argon diffusion in hornblende, micas, and K-feldspar at ∼500 to ∼200 °C. Throughout the eastern North China craton, the synchroneity of cooling and exhumation of metamorphic core complexes, the formation of pull-apart basins, and regional alkaline igneous activity, reflects regional extensional tectonics in the Early Cretaceous. This accompanied lithospheric thinning, possibly resulting from the rollback of the subducted Pacific plate along the eastern Asian margin during the Early Cretaceous.

Mesozoic tectonics in the Eastern Block of the North China Craton: implications for subduction of the Pacific plate beneath the Eurasian plate

Abstract The Jiao-Liao massif is located in the hanging wall of the north-dipping Dabie–Sulu suture zone and is an important part of the Eastern Block of the North China Craton. Several important tectonic models for the tectonic evolution of Eastern Asia rely on critical information from the Jiao-Liao massif. This paper combines new sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon ages of the Dandong Granite in the southern Liaoning Province, China, with extensive field data for the eastern North China Craton, including the Bohai Bay Basin. Combined with other recent SHRIMP dating, we use this information to summarize the Mesozoic tectonic reactivation and evolutionary processes of the Jiao-Liao massif of the Eastern Block of the North China Craton. In this study we identify a c . 160 Ma episode of partial melting of Palaeoproterozoic plutons in the Jiao-Liao massif. Cathode luminescence and backscatter electron imagery reveal c . 167–157 Ma magmatic euhedral single zircons and magmatic zircon rims surrounding c . 2100 Ma cores in the Dandong Granites near the Liaonan Neoarchaean terrane. This partial melting is probably related to in situ remelting of ancient lower continental material, mostly the North China Craton. The Dandong plutons are aligned in a NE–SW direction and are extensively deformed by subhorizontal ductile thrust-related shearing and subsequent NNE–SSW trending folds. Here, we show that for the Dandong area the first deformation occurred between 195 and 193 Ma, based on K–Ar and 40 Ar/ 39 Ar ages of muscovites from east–west-trending shear zones on the Liaodong Peninsula. Based on the field relationships between the plutons and structural fabrics, a range from 153 to 145 Ma is defined as the duration of the second deformation in the Dandong Granites. The third deformation is marked by the formation of NNE–SSW strike-slip faults between 135 and 95 Ma. This deduced age range is similar to an 40 Ar/ 39 Ar age range of 128–132 Ma of initial sinistral strike-slip faulting of the Tan-Lu fault in Anhui Province and to a biotite cooling age of 100±2.3 Ma of the Yilan–Yitong segment of the Tan-Lu fault in the Jilin Province. These faults are transtensive and controlled the formation of pull-apart basins. However, during the third deformation, some metamorphic core complexes in Eastern China formed in the overlapping area between the large-scale sinistral faults. Our SHRIMP data also indicate that the Liaodong basement and its Early Mesozoic magmatism are similar to the Jiaodong basement and its Mesozoic magmatism. Therefore, the Early Mesozoic evolution of the Liaodong area, similar to that of the Jiaodong area, was also closely related to the Sulu orogen in the Early Mesozoic and to the Pacific subduction throughout the Mesozoic.

Lough Neagh: the site of a Cenozoic pull-apart basin

Synopsis The Lough Neagh Basin in Northern Ireland is the site of a Cenozoic depocentre defined by gravity measurements and a thick succession of Paleocene basaltic lavas and Upper Oligocene clays. Much of the Cenozoic outcrop is concealed by Lough Neagh, but the rhombic outline of the lough provides some indication of the underlying structural control of the depocentre. Several authors have inferred that a Cenozoic pull-apart basin lies within the Lough Neagh area and suggest it is one of a number of transtensional basins, including the Bovey and Petrockstow basins in SW England, associated with NW to NNW-trending strike-slip fault zones. Solid geology maps and gravity data show that the structure of the Lough Neagh Basin is dominated by a segmented, orthogonal pattern of offset NNW and NE-trending faults. It is proposed that pull-apart basin formation took place in the Mid-Paleocene by dextral movement on these offset faults. The potential link between strike-slip tectonics and Cenozoic volcanism in the north of Ireland is briefly considered.

Cretaceous epithermal gold–silver mineralization and geodynamic environment, Korea

Epithermal precious-metal mineralization in the Korean Peninsula mainly occurred along NNE-trending major strike-slip fault systems that are associated commonly with formation of pull-apart basins and major volcanic activity during the Cretaceous. Sedimentation in the basins was initiated in the Hauterivian and continued into the Albian, whereas much of the volcanism occurred sporadically from ca. 110 to 50 Ma, with a major episode between ca. 90 and 70 Ma. Epithermal Au–Ag mineralization in Korea took place between approximately 100 and 70 Ma, overlapping with the shallow magmatic activity. Styles of epithermal Au–Ag deposits in Korea include those of the Mugeug-type found in sediment-dominant basins in the central portion, and the Haenam-type in volcanic-dominant basins in the southwest. Epithermal Au–Ag deposits associated with the volcanic-dominant basins in the southern Korea generally formed at very shallow crustal levels (<0.5 kbar) and relatively low temperatures (<300 °C) from fluids containing large components of less-evolved meteoric waters than those associated with sediment-dominant basins. Orthogonal subduction following oblique subduction of the Izanagi Plate along the Pacific continental margin during the Cretaceous probably represents a major control of magmatism and associated Au–Ag mineralization in the Korean Peninsula. In the Early Cretaceous, the left-lateral strike-slip movements due to the northward (oblique) subduction of the plate resulted in the Gongju–Eumseong and Yeongdong–Gwangju fault systems. Late Cretaceous calc-alkaline volcanic activity and associated caldera-related fractures related to an orthogonal convergence that postdates the NNE-trending strike-slip movements may play an important role in the formation of epithermal Au–Ag deposits. Simultaneously with, or soon after heating related to magmatism, continued movement of strike-slip faults may also have been critical to the ore-forming process, leading to relaxation of local compressive forces, enhancement of crust-scale permeability, and promotion of mixing of ore-forming fluids.

Late Mesozoic–Cenozoic sedimentary basins of active continental margin of Southeast Russia: paleogeography, tectonics, and coal–oil–gas presence

Various settings took place during the Late Mesozoic: divergent, convergent, collisional, and transform. After mid-Jurassic collision of the Siberian and Chinese cratons, a latitudinal system of post-collision troughs developed along the Mongol-Okhotsk suture (the Uda, Torom basins and others), filled with terrigenous coal-bearing molasse. The dispersion of Pangea, creation of oceans during the Late Jurassic are correlated to the emergence of the East Asian submeridional rift system with volcano-terrigenous coal-bearing deposits (the Amur-Zeya basin). At that time, to the east there existed an Andean-type continental margin. Foreland (Upper Bureya, Partizansk, and Razdolny) and flexural (Sangjiang-Middle Amur) basins were formed along the margin of the rigid massifs during the Late Jurassic to Berriasian. During the Valanginian-mid-Albian an oblique subduction of the Izanagi plate beneath the Asian continent occurred, producing a transform margin type, considerable sinistral strike slip displacements, and formation of pull-apart basins filled with turbidites (the Sangjiang-Middle Amur basin). The Aptian is characterized by plate reorganization and formation of epioceanic island arcs, fore-arc and back-arc basins in Sakhalin and the Sikhote-Alin (the Alchan and Sangjiang-Middle Amur basins), filled with volcanoclastics. During the mid-Albian a series of terranes accreted to the Asian continental margin. By the end of the Albian, the East Asian marginal volcanic belt began to form due to the subduction of the Kula plate beneath the Asian continent. During the Cenomanian–Coniacian shallow marine coarse clastics accumulated in the fore-arc basins, which were followed by continental deposits in the Santonian–Campanian. From the Coniacian to the Maastrichtian, a thermal subsidence started in rift basins, and continental oil-bearing clastics accumulated (the Amur-Zeya basin). Widespread elevation and denudation were dominant during the Maastrichtian. This is evidenced by thick sediments accumulated in the Western Sakhalin fore-arc basin. During the Cenozoic, an extensive rift belt rmade up of a system of grabens, which were filled with lacustrine–alluvial coal–and oil-bearing deposits, developed along the East Asian margin.