Regional Extensional Tectonics Research Articles

Mapping paleostress trajectories by means of the clustering of reduced stress tensors determined from homogeneous and heterogeneous data sets

It is easy to draw stress trajectories to investigate the present stress field by interpolating stress orientations determined at control points. However, challenges arise when we deal with the trajectories of paleostresses, because neighboring control points may have the stress orientations of different tectonic phases. We must choose coeval stresses to draw the trajectories. Recent stress inversion techniques can separate stresses from heterogeneous data from fault, dilational fractures, etc. Natural data sets from those structures are often heterogeneous, and age data are usually not enough to classify the stresses by age. As a result, an unsupervised classification problem of the inversion results must be solved to draw the trajectories. Here, we propose a simple and heuristic procedure for this problem. We assume smooth trajectories during each of the phases. The smoothness makes density-based clustering adoptable to solve the problem. The heterogeneity of data sets allows the additional partition of the clusters. As a worked exercise for this technique, the trajectories of minimum horizontal stress orientations were drawn based on the paleostresses determined from the attitudes of felsic dikes and quartz veins formed in mid Cretaceous orogeny in the North Kitakami Terrain, northern Japan. The orogen-parallel and orogen-perpendicular extensional stress fields delineated by the present technique were probably the manifestations, respectively, of the gravitational collapse of the orogen and of regional extensional tectonics in the Far East.

Recently-discovered Bahçecik Au±Ag mineralization in the Eastern Pontides, Gümüşhane-NE Türkiye: geological and geochemical implications on the intermediate sulfidation epithermal deposit

ABSTRACT Intermediate sulfidation epithermal deposits represent significant reservoirs of precious metals, playing a crucial role in global mining operations. The Anatolian Peninsula, predominantly characterized by relatively young (Tertiary) lithologies, hosts numerous occurrences of epithermal base and precious metal mineralization. Within the realm of research in Northeastern Anatolia, several hydrothermal alteration zones have emerged as indicators of concealed epithermal mineralization, with the Bahçecik mineralization standing out as a recent focal point. The Bahçecik mineralization represents an intermediate sulfidation epithermal Au-Ag system, hosted by Late Cretaceous andesitic rocks, as well as Eocene dacitic lava and tuffs. Employing U-Pb LA-ICP-MS zircon dating of the dacitic host rocks, the mineralization is estimated to have occurred during the Lutetian epoch (43.61 ± 0.35 Ma) or later. The genesis of the Bahçecik deposit is believed to be linked to postcollisional regional extensional tectonics, primarily controlling its structural arrangement. Comprising pyrite, chalcopyrite, low-Fe sphalerite, galena, tetrahedrite/tennantite, enargite-luzonite, and gold-silver minerals, the deposit is accompanied by prominent gangue minerals such as Mn- and Ca-carbonates, quartz, and barite. Microprobe measurements have identified a majority of Au-Ag minerals (krennerite, buckhornite, and nagyagite) closely associated with sulphide minerals. Fluid inclusion data reveal that mineralization and fluid transportation occurred at temperatures ranging from 153 to 327°C, with solutions exhibiting salinity levels consistent with those of intermediate sulfidation deposits (1.7–7.3 wt.% NaCl). This study unequivocally classifies the Bahçecik mineralization as an intermediate sulfidation deposit, contrary to the conclusions drawn in previous research. Moreover, our investigation has unveiled the presence of a substantial ore resource, totalling 7.84 million tons, featuring 1.76 g/t Au and 2.24 g/t Ag. As an intermediate-type deposit, it shares characteristics with other subtypes within the epithermal class, bridging the realms of both high and low sulfidation systems.

Geochemical characterization, U–Pb apatite geochronology, and geodynamic significance of olivine minette dykes from the Julian Alps, NE Italy

AbstractWe investigated olivine minette dykes from the Rio Colan Valley of the Julian Alps (NE Italy) to provide new constraints on the mantle geochemistry underneath this extreme sector of the Italian Southern Alps. Petrographic observations, high Mg#s, high Cr and Ni contents, low Dy/YbCN ratios and flat heavy rare earth element (REE) profiles imply these are primary magmas derived from a depleted peridotite with olivine and garnet as possible residual phases. However, high K2O and incompatible trace element contents, coupled with superchondritic Nb/Ta ratios, suggest that the source was modified into a rutile–phlogopite-bearing carbonated peridotite by multiple metasomatic events such as recycling of crustal material and carbonatitic metasomatism related to an old orogenic event and rutile-rich metasomatism linked to the Pangaea break-up. Laser ablation – multi-collector – inductively coupled plasma – mass spectrometer (LA-MC-ICP-MS) U–Pb geochronology of apatites from two dykes yielded differences between unforced and forced discordia ages up to ∼45 Ma, likely due to the initial 207Pb/206Pb correction adopted. Nonetheless, because forced discordia and 206Pb/238U weighted mean ages are overall similar within the uncertainty, our preferred interpretation is that olivine minettes intruded and crystallized at ∼67 Ma. The age and anorogenic signature of these magmas are consistent with the regional extensional tectonics in the Julian Alps due to the advance of the external Dinaric front following Adria plate eastward subduction. Eventually, we show also that Rio Colan magmas are geochemically hybrid products between lamprophyres and lamproites compared with similar Italian and Western Mediterranean volcanics.

Salt tectonics vs. inversion tectonics: The anticlines of the western Maestrazgo revisited (eastern Iberian Chain, Spain)

AbstractMany works in the last decades underline the role of evaporites, not just as a conditioning factor but as the engine for subsidence and eventually basin inversion. The western Mediterranean alpine ranges are being investigated in this regard because of the presence of discontinuous units of Permian to Triassic evaporites, deposited in the western Tethys basins. This work presents a thorough analysis of two particular structures (Cañada Vellida and Miravete anticlines) in the intraplate Maestrazgo basin (eastern Iberian Chain, Spain) in which evidence to support their reinterpretation as salt‐driven structures have been recently reported. Our analysis includes (i) a comprehensive stratigraphic and structural study of the folds along their entire trace, (ii) the compilation of thickness and distribution of evaporite–bearing and supraevaporite units, paying special attention to changes in the thickness of units in relation to anticlines, and (iii) the study of fault patterns, sometimes in relation to the mechanical stratigraphy. All three aspects are also documented and discussed on a regional scale. The new data and interpretations reported here reinforce the extensional origin of the Late Jurassic–Early Cretaceous basins, and the role of regional extensional tectonics as the responsible for the development of first‐order syn‐sedimentary normal fault zones driving the formation and evolution of sub‐basins. These basins were subsequently inverted and deformed, including the formation of complex, box‐geometry anticlines that, in their turn, controlled deposition in Cenozoic basins. The review of the arguments that support the alternative of salt tectonics for the origin of such anticlines has allowed us to delve into the sedimentary and tectonic evolution of the inverted extensional basins and to propose a specific model for the development of these faulted anticlines. The role of salt levels and other interlayered detachments in the structuring of sedimentary basins and their inversion is also pondered. The observations in the eastern Iberian Chain reported here have implications to assess ongoing reinterpretations in terms of salt tectonics in other alpine basins and ranges of the western Mediterranean.

Lithospheric thinning of the North China craton: Insights from Early Cretaceous intermediate–mafic dyke swarms in Jiaodong peninsula

The coupling/decoupling relationships between shallow crustal deformation and deep mantle tectono-magmatic activities, during lithospheric thinning and destruction of cratons, remain hotly debated in the last decades. Multidisciplinary studies were done on intermediate–mafic dyke swarms diffusively located in southern Jiaodong peninsula, southeastern North China craton (NCC) to explore the crustal and lithospheric mantle responses to regional tectonic extension. Uniform ages (122–121 Ma) of the dykes indicate intensive magma generation at the mantle depth around 122 Ma related to large-scale tectonic extension of regional lithospheric mantle. Systemic anisotropy of magnetic susceptibility (AMS), geochemical, and Sr-Nd isotope analyses suggest that the dykes were originated from enriched lithospheric mantle sources and emplaced along crustal fractures formed in a WNW-ESE oriented extensional stress field. We show that the Jiaodong lithosphere experienced two stages of Early Cretaceous tectonic evolution, i.e., an early stage (135–122 Ma) lithospheric thinning characterized by large-scale tectonic extension of the crust and limited magmatic activities of crustal sources, and a late stage (122–108 Ma) lithospheric root collapse dominated by intensive tectonic extension and magmatism in the lithospheric mantle level. The decoupling crust-mantle evolution during lithospheric thinning and destruction of the NCC is ascribed to continuous tectonic extension in WNW-ESE direction.

Pyroclastic deposition in the Cretaceous Shahezi Formation (Well SK‐2) Songliao Basin, China: Implications for tectonics and volcanism

Well SK‐2 is located in the Songliao Basin. Core samples from this well indicate that the Early Cretaceous Shahezi Formation has a length of 2,359.01 m and is in direct contact with the basin basement of the Middle Triassic, missing the Huoshiling Formation. This paper is focused on the lowest sequence 1 (S1, at the bottom of the Shahezi Formation, interval of 5,450.72–5,695.00 m, a length of 244.28 m) of the Cretaceous basin fills. It was found that the volcanic components contributed important parts in the sequence of basin fills, and the start of the rifting resulted from the combination of regional extensional tectonics and volcanism. Based on the centimetre‐scale description of the continuous core samples, we propose a model to delineate the interaction between tectonics, volcanism, and pyroclastic deposition. The study shows the tectonic‐volcanism in the earliest rifting stages and its control on sedimentary filling. These results also have common sense for the initial rifting process of a continental half‐graben basin. Sequence 1 of the Shahezi Formation in well SK‐2 shows a typical volcanic‐ sedimentary sequence. During the lowstand stage, the weathering‐depositional system was controlled by extension; during the transgressive and highstand stages, the transitional weathering‐depositional system and the mixed volcanic and weathering depositional system were controlled by transtension.

Neoproterozoic extensional basins and its control on the distribution of hydrocarbon source rocks in the Yangtze Craton, South China

• Neoproterozoic source rocks in the Yangtze Craton. • Structural and sedimentary patterns of the extensional basins evaluated. • The source rocks are related to the extensional basins, climate and life evolution. • Favorable areas for the source rock distribution are predicted. Hydrocarbon exploration in the Neoproterozoic extensional basins in the Yangtze Craton of South China has made prominent progresses in the past decades. However, the spatial distribution of source rocks across the craton still remains unclear. Here, we use outcrop observations, borehole data, and reflective seismic profiles to investigate the prototype, distribution and sediment infilling of these basins in the Yangtze Craton during the periods of source rock deposition. The basin prototype is categorized based on its tectonic origin and structural-sedimentary feature as follows. (1) During the deposition of the Datangpo Formation, rift basins extensively developed, band are characterized by a series of graben or half graben structures that consist of basin-controlling normal faults. The source rocks in the Datangpo Formation were deposited as the inner shelf basin or deep shelf facies, obtaining high thickness (i.e., 200 m) in the Wuling rift basin. (2) During the deposition of the Doushantuo Formation, the intracratonic rift basin and passive continental margin basin formed. These basins were controlled by syn-depositional faults that inherited the existing rift fault structures. The source rocks of the Doushantuo Formation were deposited as shelf lagoon, slope to deep-water basin facies. (3) During the deposition of the Dengying Formation, the intraplatform rift basin and passive continental margin basin developed with influence of regional tectonic extension. Not only are these basins affected by the preexisting faults, but also by the newly developed syn-depositional faults. The argillaceous source rocks of the Dengying Formation deposited as slope and deep-water basin facies. Further in-depth analyses revealed that a range of mechanisms may have contributed to the formation and distribution of source rocks in the Yangtze Craton, such as the break-up of the Rodinia Supercontinent, paleaolatitudes, glacial-interglacial geological events, basin extension and sedimentary environments. By integrating the distribution of the source rocks and the late tectonic reworking, this study predicts several targets for the source rock distribution in the Yangtze Craton, including in the central Sichuan, northeastern Sichuan, southern Shanxi, western Hubei, northeastern Guizhou-western Hunan, and the northeastern Guangxi-central Hunan areas.

Reconstructing environmental signals across the Permian-Triassic boundary in the SE Germanic Basin: A Quantitative Provenance Analysis (QPA) approach

Global tectonic and climatic models for the Permian-Triassic boundary (PTB) are highly debated. One of the most disputed topics is the temperature increase associated with CO 2 emissions generated by the Siberian Trap volcanism and its potential influence on chemical weathering and associated variations in sediment fluxes. By integrating crustal architecture, plate modelling, structural kinematics, and two climatic models, we reconstruct the drainage evolution of Variscan tectonostratigraphic units from the SE portion of the Germanic Basin, from which we also extract the parameters necessary to calculate sediment flux across a time-scale of 28 Ma (Guadalupian-Lower Triassic). We reconstruct the sedimentary response to climatic and tectonic perturbations using Quantitative Provenance Analysis (QPA) and integrate compositional data into a sedimentological framework, paleodrainage and paleoclimatic models. Raman heavy mineral analysis, as well as geochronology and geochemistry of detrital apatite, zircon, and rutile, document variation in drainage lithologies and sediment flux which are controlled by regional extensional tectonics and increasingly humid conditions at the PTB. The sedimentary successions of the SE Germanic Basin record climatic perturbations on a 10 4 years timescale, while the effects of tectonics are visible on a 10 6 years timescale. The interplay of climate, tectonics and lithology, and their effects on sediment production and drainage evolution resulted in changes in sediment flux from 2.3 Mt./yr during the Guadalupian (Capitanian), to 3.80 Mt./yr in the Lopingian (Changhsingian) to 7.44 Mt./yr at the end of the Lower Triassic (Olenekian). The multifaceted workflow provided in this study represents the first step towards more precise reconstructions of sediment routing systems in deep-time and provides the first ground-truthed quantification of sediment flux across the Permian-Triassic Boundary. • Drainage modifications are efficiently identified via Quantitative Provenance Analysis. • Constraining Environmental Signals in deep-time requires paleoclimatic/tectonic models. • Paleodrainage analysis provides the parameters to calculate sediment flux. • Sediment flux increases in the Lower Triassic in response to climate change.

The Impact of Complex Volcanic Plumbing on the Nature of Seismicity in the Developing Magmatic Natron Rift, Tanzania

Constraining the architecture of complex 3D volcanic plumbing systems within active rifts, and their impact on rift processes, is critical for examining the interplay between faulting, magmatism and magmatic fluids in developing rift segments. The Natron basin of the East African Rift System provides an ideal location to study these processes, owing to its recent magmatic-tectonic activity and ongoing active carbonatite volcanism at Oldoinyo Lengai. Here, we report seismicity and fault plane solutions from a 10 month-long temporary seismic network spanning Oldoinyo Lengai, Naibor Soito volcanic field and Gelai volcano. We locate 6,827 earthquakes with ML−0.85 to 3.6, which are related to previous and ongoing magmatic and volcanic activity in the region, as well as regional tectonic extension. We observe seismicity down to ∼17 km depth north and south of Oldoinyo Lengai and shallow seismicity (3–10 km) beneath Gelai, including two swarms. The deepest seismicity (∼down to 20 km) occurs above a previously imaged magma body below Naibor Soito. These seismicity patterns reveal a detailed image of a complex volcanic plumbing system, supporting potential lateral and vertical connections between shallow- and deep-seated magmas, where fluid and melt transport to the surface is facilitated by intrusion of dikes and sills. Focal mechanisms vary spatially. T-axis trends reveal dominantly WNW-ESE extension near Gelai, while strike-slip mechanisms and a radial trend in P-axes are observed in the vicinity of Oldoinyo Lengai. These data support local variations in the state of stress, resulting from a combination of volcanic edifice loading and magma-driven stress changes imposed on a regional extensional stress field. Our results indicate that the southern Natron basin is a segmented rift system, in which fluids preferentially percolate vertically and laterally in a region where strain transfers from a border fault to a developing magmatic rift segment.

The Late Jurassic extensional event in the Yanshan fold and thrust belt (North China): New insights from an integrated study of structural geology, geophysics, and geochemistry of the Siganding granitic pluton

The Yanshan fold and thrust belt (YFTB) was built up by multiple contractional events, namely the “Yanshan Movement”, which was traditionally subdivided into the “Yanshan-A” and “Yanshan-B”. In contrast with the better constraint on “Yanshan-B” (ca.140 Ma), the timing of “Yanshan-A” is still in dispute. The Middle Jurassic contraction (172–160 Ma) was widely accepted, however, whether this contraction can last into 150 Ma is still unclear. In this article, to clarify the Late Jurassic tectonic regime in the YFTB, a multidisciplinary study, including structural geology, Anisotropy of Magnetic Susceptibility (AMS), gravity modeling, geochronology, and geochemistry, has been carried out to investigate the emplacement mechanism of the Siganding pluton (ca. 161–157 Ma) in the northern Beijing area. The magma was predominantly derived from the Precambrian continental lower crust. The magnetic foliations are pluton margin-parallel and outward dipping, resembling the fabric pattern of magmatic dome. The NE-SW striking magnetic lineations and NW-SE linear feeder zones revealed by gravity modeling indicate that the emplacement of the Siganding pluton was controlled by a weak NE-SW crustal stretching interpreted as a regional tectonic extension. Considering that the Late Jurassic (164–153 Ma) NE-SW oriented extension structures are not only documented in the YFTB, but also the entire eastern NCC, hereby we propose that the “Yanshan-A” is constrained at 172–164 Ma, instead of a wide range from the Middle Jurassic to Late Jurassic (172–150 Ma) as argued by previous workers. The extension could be ascribed to the oblique subduction of the Izanagi plate.

Extensional tectonic setting of the Triassic metallogenic event in the Xiaoqinling goldfield: Evidence from <sup>40</sup>Ar-<sup>39</sup>Ar chronology of the tectonite

小秦岭金矿田位于华北克拉通南缘，同时也是秦岭复合型造山带的北缘组成部分。这里是我国第二大黄金产地，大规模金的成矿作用形成于早白垩世岩石圈大规模减薄的区域伸展构造背景。此外，越来越多的同位素年代学数据显示区内还存在三叠纪的成矿事件，发育钼、铅、铀、铌等与岩浆热液活动密切相关的多金属矿床，主要分布在小秦岭的南北边缘。目前，对于三叠纪多金属成矿作用的构造背景研究尚属薄弱，还没有直接对有关的构造变形开展过专题研究。为此，我们对小秦岭金矿田东端边缘的构造带进行构造观察和测量，并采用⁴⁰Ar-³⁹Ar年代学方法厘定变形时限，揭示三叠纪成矿事件的构造背景，为深入认识区内三叠纪多金属成矿作用特征和规律提供构造证据。研究表明，小秦岭金矿田三叠纪成矿事件发生于晚三叠世的伸展构造背景，可以划分为两个阶段，早期（221~216Ma）是在碰撞向后碰撞转折阶段的重力滑脱构造环境中，发育火成碳酸岩型钼铅、铀铌铅多金属矿床；晚期（214~203Ma）是在后碰撞阶段由俯冲板片断离或岩石圈拆沉作用等深部过程导致的区域伸展构造环境中，发育石英脉型和斑岩型钼矿床。小秦岭中生代晚三叠世和早白垩世两期成矿事件都是在伸展构造背景中的构造-岩浆-流体活动的产物。;The Xiaoqinling goldfield is located in the southern margin of the North China craton, which is also a marginal part of the Qinling complex orogeny. This goldfield is the second largest gold producer in China, where the large-scale gold mineralization developed in the extensional tectonic setting due to regional lithospheric thinning in Early Cretaceous. In addition, a growing number of isotope chronological data indicates that there also developed metallogenic events in Triassic here, which resulted in the formation of polymetallic deposits, such as molybdenum, lead, uranium, and niobium, etc. The Triassic deposits are closely related to magmatic hydrothermal activities, which are mainly distributed in northern and southern margins of the Xiaoqinling goldfield. However, there is little study on the tectonic setting of the Triassic polymetallic mineralization so far, especially the work on tectonic deformation is almost absent. Therefore, we have carried out the observation and measurement on the structure developed within the tectonic belt occurred in the eastern end of Xiaoqinling goldfield, and then employed the ⁴⁰Ar-³⁹Ar chronology method to determine the time limit of deformation, to reveal the tectonic setting of Triassic metallogenic event, providing tectonic evidence for further understanding characteristics and patterns of Triassic polymetallic mineralization in the Xiaoqinling goldfield. Our study shows that the metallogenic event occurred in the extensional tectonic setting in Late Triassic, which can be divided further into two stages. The early stage (221~216Ma) is of the transition stage from collision to post-collision, developing gravitational decollement tectonics, and carbonatite-type molybdenum, lead, uranium, and niobium polymetallic deposits are well developed in this extensional tectonic setting. The late stage (214~203Ma) is of post-collision, developing regional extensional tectonics caused by the break off of subducting slab or lithospheric delamination, and quartz vein-type and porphyry molybdenum deposits are well developed. The two metallogenic events in Late Triassic and Early Cretaceous in the Xiaoqinling goldfield are the products of the tectonic deformation-magmatic activity-fluid flow in extensional tectonic setting.

Inhomogeneous thinning of a cratonic lithospheric keel by tectonic extension: The Early Cretaceous Jiaodong Peninsula–Liaodong Peninsula extensional provinces, eastern North China craton

Abstract The mechanisms of lithospheric thinning and craton destruction have been hotly debated in the last decades. The Early Cretaceous Jiaodong and Liaodong extensional provinces (JEP and LEP, respectively) of the eastern North China craton are typical areas where the cratonic Archean lithosphere has been intensely extended and thinned. Various extensional structures, e.g., metamorphic core complexes (MCCs), low-angle detachment faults, and extensional basins, characterize the Early Cretaceous crustal deformation of the two provinces. However, profound differences exist in structural development and related magmatic activities between the two provinces. Distributed small-scale extensional basins were formed in association with exhumation of the Liaonan and Wanfu MCCs in the LEP, whereas the major Jiaolai Basin was developed coevally with exhumation of the Wulian, Queshan, and Linglong MCCs in the JEP. Sr-Nd isotope compositions of volcanic rocks from the basins of the two provinces are compatible with syntectonic magmatic activities of evolving magma sources in the LEP, but multiple and hybrid magma sources in the JEP. It is shown, from variations in structural styles, plutonic and volcanic activities, and thermal evolution of the two extensional provinces, that two stages (ca. 135–120 Ma and 120–100 Ma) of tectonic extension affected the JEP and LEP in the Early Cretaceous. We demonstrate that regional tectonic extension (parallel extension tectonics, or PET) is responsible for the formation of major extensional structures and the occurrence of the magmatic associations. Progressive wide rifting by coupled crust-mantle detachment faulting of a hot LEP lithosphere was accompanied by evolving magma sources from dominant ancient crust and enriched mantle to juvenile crust. Two stages of narrow rifting of a cold JEP lithosphere led to early crustal detachment faulting transitioning to late crust-mantle faulting, which resulted in intense magmatic activity from hybrid to multiple magma sources. These processes contributed to destruction of the craton, with thinning of its lithospheric keel and local transformation of the nature of the lithospheric mantle. It is expected that such a model is also applicable to interpretation of tectonic extension of contiguous areas of the North China craton and the remobilization of other cratons.

Early Pleistocene River Terraces of the Gediz River, Turkey: The role of faulting, fracturing, volcanism and travertines in their genesis

Reconstructing and interpreting past hydrological system changes from the Quaternary sediment-landform record presents many challenges especially when trying to establish causality. In such circumstances attempting to isolate individual drivers is perhaps unrealistic and thus inferences are made with respect to the combined influence of multiple controls and constraints. In this paper we seek to interpret the landscape responses recorded in the sediment-landform record of the Early Pleistocene Gediz valley, western Turkey, within the context of interacting geodynamic and hydrodynamic system changes.Our earlier work on the Gediz sequence established the presence of a rich fluvial archive constrained by a comprehensive geochronology from overlying lava flows. Repeated damming of the Gediz valley floor and consequent re-routing of water flow, testifies to a direct connection between fluvial activity and volcanism. This volcanism has been linked to regional tectonic extension with regional uplift promoted as a major driver of the progressive river incision of the Gediz River over the Quaternary. This incision has been punctuated by periods of deposition dictated by sediment and water budgets controlled principally by fluctuating climate and consequent vegetation change. Recently however, the significance of more local fault movements and fracturing has been recognised and here we examine and attempt to quantify these geodynamics in greater detail. In addition, we examine and describe, for the first time, extensive associated travertine deposits. Isotopic analysis of these travertines confirms their thermogenic origin and directly links their creation to the faulting/fracturing, volcanism and the recycling of calcium-rich groundwater. The formation of mounds and sheets extend onto their contemporary valley floors, thus directly connecting the fluvial and travertine archives and allowing linkage between surface and subsurface hydrological change.By utilising both the fluvial and travertine archives a more complex picture of landscape evolution emerges. The youngest terraces (<GT6) appear to be driven by local fault movement associated with the onset of volcanism and thus the link with fluctuating climates, as demonstrated in the higher terraces (GT7 +), is unclear. Detailed observations suggest formation of these lower terraces is also conditional on the local substrates. Here it appears that GT5-GT1 form in response to the creation of a knickpoint where the river intersects the faults in areas of hard basement rock. Hence, this response is highly localized, dependent on the immediate geometry and structure of the underlying strata. The presence of complex basement topography also plays a key role in generating ground water pressure and thus the creation of travertine mounds.The Gediz sequence demonstrates increasing complexity. There is no reason to suggest this is unusual. Although climate and tectonics remain as key drivers the local landscape response is also conditioned by many additional factors. While it is tempting to search for global patterns (“the big picture”), such reduced complexity may be over simplistic. Recognising complexity requires detailed observation and therefore more emphasis on “the smaller picture” which can only be obtained through extensive field observation.

Geochemical, mineralogical and chronological studies of mafic-intermediate dykes in the Jiaodong Peninsula: implications for Late Mesozoic mantle source metasomatism and lithospheric thinning of the eastern North China Craton

ABSTRACT The Late Mesozoic tectonic evolution of the eastern North China Craton (NCC) remains controversial. In this study, a suite of Early Cretaceous mafic-intermediate dyke swarms found in the same section in the Jiaodong Peninsula, eastern NCC, were used to trace the nature of lithospheric mantle and crust–mantle interaction. Zircon U-Pb dating demonstrates that these dykes emplaced into the Jiaodong Peninsula during the same period (112–121 Ma). Diorite dykes have SiO2 contents of 52.7–54.1 wt.%, Na2O+K2O contents of 5.77–6.10 wt.%, a limited range of MgO (4.43–5.13 wt.%), Cr (64.8–85.5 ppm) and Ni (23.6–27.7 ppm), and a narrow range of (87Sr/86Sr)i isotopic ratios (0.709240–0.709246), ƐNd(t) values (−20.0 to −19.1) and ƐHf(t) values (ca. −23.9). Lamprophyre and dolerite dykes have low contents of SiO2 (50.5–55.4 wt.%) and Na2O+K2O (4.88–7.28 wt.%), high contents of Al2O3 (14.6–15.8 wt.%), MgO (5.83–8.60 wt.%), Cr (210–292 ppm) and Ni (114–149 ppm), and a narrow range (87Sr/86Sr)i isotopic ratios (0.709134–0.709788), ƐNd(t) values (−19.9 to −17.4) and ƐHf(t) values (−22.1 to −15.1). Both intermediate and mafic dykes exhibit strong large ion lithophile elements (LILE) enrichment and high field strength element (HFSE) depletion. These geochemical characteristics demonstrate that the magma was primarily derived from relatively low degrees of the partial melting of garnet-lherzolite mantle, which was modified by fluids from a subducting oceanic slab with marine sediments. Both the mafic dykes and intermediate dykes originated from the same magma source and formed in different stages of magma fractionation crystallization. The magma experienced the fractional crystallization of clinopyroxene and more ferromagnesian minerals in a hydrous condition, thus forming mafic dykes. Then, the residual calc-alkaline liquids upwelled into a low-H2O and low-pressure environment, in which more plagioclase crystallized from liquid, thus increasing the degree of differentiation and producing the Ca- and Al-rich diorite dyke. Combined with Late Mesozoic mantle-derived mafic-intermediate dykes and crust-derived volcanic rocks in the region, the eastern NCC has experienced considerable lithospheric thinning along with regional extensional tectonics, which was caused by the prolonged thermo-mechanical-chemical erosion on the lithosphere induced by the rollback of the subducted paleo-Pacific plate.

Magmatic activity at the Silurian/Devonian boundary in the Brunovistulia and Małopolska Terranes (S Poland): possible link with the Rheic Ocean closure and the onset of the Rheno-Hercynian Basin

AbstractThe age of granophyric diorite from the Sosnowiec IG-1 borehole (Brunovistulia Terrane) was studied by means of U–Pb single-grain zircon analysis performed on a SHRIMP (sensitive high-resolution ion microprobe) IIe device. The isotope ages and provenance of zircons from the Emsian tuffs cropping out in the southern part of the Holy Cross Mountains (Małopolska Terrane) were also investigated using the same method. The age of the diorite intrusion (420 ± 2 Ma) is comparable with the combined Ar–Ar/magnetostratigraphic age of the Bardo diabase intrusion from the northern part of the Małopolska Terrane. These intrusions were emplaced during the same event of regional tectonic extension associated with the Rheic Ocean closure and the onset of processes creating the Rheno-Hecynian Basin near the Silurian/Devonian boundary. A negative Nb anomaly characteristic of both intrusions could be linked with the subduction of the Rheic oceanic crust under the SE margin of the Old Red Continent. Emsian magmatic activity in the distant Rheno-Hercynian Zone provided several tuff layers in the northern part of the Małopolska Terrane. As can be inferred from zircon ages, these tuffs were derived from mafic eruptions that cut sedimentary rocks containing detrital zircons transported from Baltica. This interpretation fits the existing models of development of the Rheno-Hercynian Basin in the Emsian.

Uplift and syn-orogenic magmatism in the Concepción del Oro Block: A thick-skinned (Laramide style?) contractional structure in the Mexican Fold-Thrust Belt

In this paper we report UPb zircon ages from a syn-tectonic magmatism related to a contractional thick-skinned deformation pulse in the Mexican Fold and Thrust Belt. The study area corresponds to the Concepción del Oro Block (COB), which is located in northern Zacatecas, Mexico. The COB is a NW-SE trending elongated tectonic structure located south of the Parras Transversal Sector in the Mexican Fold Thrust Belt (MFTB) in north-central Mexico. In this area, regional thin-skinned folds were formed between the late Turonian and Campanian and they define the core of a major northeastward-convex orocline around Concepción del Oro mining district, Zacatecas, Mexico. The regional folds were cut by four Eocene-early Oligocene plutons (43-32 Ma; UPb zircon), which in places modified the older thin-skinned structures. Magma was emplaced along thrust faults and modified the attitude of inverted limbs of overturned folds, and the orientation or position of the fold axis at the core of anticlines, causing the formation of complex emplacement-related deformation. Two NW-SE trending lineaments bound the COB, and both cut the periclinal ends of the thin-skinned older folds, at different time. The eastern lineament is here interpreted as a high angle reverse fault, which tilted polymictic conglomeratic continental succession that includes cobbles derived from one of the intrusive bodies and an andesitic syn-tectonic lava flow (40.7 ± 0.6 Ma; UPb zircon). The western lineament is a normal fault generated by later regional tectonic extension related to the Basin and Range province. Altogether suggests that a late Eocene-early Oligocene contractional thick-skinned deformation pulse affected the COB. This pulse was contemporaneous with the Eocene-Oligocene magmatism in the Concepción del Oro, Zacatecas region, and is similar in style and age to some of the structures produced by the Laramide orogeny in the southern Rocky Mountains and in some isolated areas in northern Mexico.

Preservation of high‐P rocks coupled to rock composition and the absence of metamorphic fluids

AbstractEclogites, blueschists and greenschists are found in close proximity to one another along a 1‐km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2–1.9 GPa and 410–530°C followed, at 43–38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–2.1 GPa and 520–580°C. We explain co‐existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10–100‐m and veins on a scale of 1–10‐cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high‐P nappe stack consisting of belts in which high‐P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension.

The Early Cretaceous Shangzhuang layered mafic intrusion and its bearing on decratonization of the North China Craton

AbstractThe North China Craton (NCC) is one of the classic examples of decratonization through extensive lithospheric destruction during Mesozoic time. Among the various pulses of magmatism associated with cratonic erosion are the rare mafic intrusions in the Yanshan Belt. Here we investigate the Shangzhuang layered intrusion belonging to this suite, which is characterized by compositional layering with troctolite, noritic gabbro and gabbro/gabbroic anorthosite/gabbrodiorite from the bottom to top. The different lithologies of this intrusion exhibit close field relationships, similar chemical patterns and overall identical Lu–Hf isotopes indicating a co-magmatic nature. The fine-grained gabbros occurring near the margin of the intrusion display U–Pb ages similar to those of the other rocks and are considered to represent the composition of the parent magma, characterized by Fe, Mg and Ti enrichment. The magma was sourced from low-degree partial melting of spinel lherzolite sub-continental lithospheric mantle, which had been enriched by crust–mantle interaction and metasomatic fluids derived from the Mongolian oceanic slab subduction beneath the NCC during Late Palaeozoic time. In addition, limited asthenospheric or deeper-mantle materials were also locally mixed with the enriched mantle as the final source component. Our zircon U–Pb data constrain the emplacement age of this intrusion asc. 128–123 Ma in Early Cretaceous time, and correlates with the regional extensional tectonics betweenc. 135 and 115 Ma in the eastern and central NCC. Mantle upwelling associated with this event resulted in the thermal and chemical erosion of the lithospheric mantle, and emplacement of the parent magma of this layered intrusion.

Crustal radial anisotropy in Northeast China and its implications for the regional tectonic extension

We obtain high-resolution Rayleigh and Love wave phase velocity maps from ambient noise tomography using data recorded by NECESSArray in Northeast China. The resulting radial anisotropic model from the joint inversion of Rayleigh and Love wave dispersion curves reveals strong relationship between the crustal radial anisotropy and tectonic provinces, that is, strong positive anisotropy (Vsh > Vsv) beneath the Songliao Basin and weak radial anisotropy beneath the Xinmeng Belt and Changbaishan Region. The Songliao Basin experienced widespread crustal extension during the late Mesozoic. We interpret the lower crustal anisotropy beneath the Songliao Basin as a result of ductile deformation during the rifting stage, which may lead to the alignment of anisotropic minerals and the observed strong radial anisotropy at present. In the northern Songliao Basin, where thick syn-rift and post-rift sediments (≥4 km) are believed to be present, we observe a broader lateral distribution of anisotropy with stronger amplitude compared with the southern basin. We suggest that the broader distribution of crustal radial anisotropy in the northern basin could be the consequence of outward lower crustal flow driven by the sedimentary loading during the post-rift stage, which is also proposed by previous numerical modeling.

Os and U–Th isotope signatures of arc magmatism near Mount Mazama, Crater Lake, Oregon

Interaction of mantle melts with the continental crust can have significant effects on the composition of the resulting melts as well as on the crust itself, and tracing this interaction is key to our understanding of arc magmatism. Lava flows and pyroclastic deposits erupted from ∼50 to 7.7 ka at Mt. Mazama (Crater Lake, Oregon) were analyzed for their Re/Os and U–Th isotopic compositions. Mafic lavas from monogenetic vents around Mt. Mazama that erupted during the buildup to its climactic eruption have lower 187Os/188Os ratios (0.1394 to 0.1956) and high 230Th excess ((230Th/238U)0 of 1.180 to 1.302), whereas dacites and rhyodacites tend to have higher 187Os/188Os ratios (0.2292 to 0.2788) and significant 238U excess ((230Th/238U)0 of 0.975 to 0.989). The less radiogenic Os isotope compositions of the mafic lavas can be modeled by assimilation of young (∼2.5 to 7 Ma), mafic lower crust that was modified during regional extension, whereas the more radiogenic Os isotope compositions of the dacites and rhyodacites can be attributed to assimilation of older (∼10 to 16 Ma), mid to upper crust that acquired its composition during an earlier period of Cascade magmatism. Production of Th excesses in the lower crust requires very young garnet formation accompanying dehydration melting in the lower crust at less than a few 100 ka by heat from recent basaltic magma injection. The results from this study suggest that the combination of Os and Th isotopes may be used to provide insights into the timescales of evolution of the continental crust in arc settings, as well as the influence of the crust on erupted magmas, and suggest a link between the age and composition of the lower and upper crust to regional tectonic extension and/or earlier Cascade magmatism.