A tentative 2D thermal model of central India across the Narmada-Son Lineament (NSL)

This work deals with 2D thermal modeling in order to delineate the crustal thermal structure of central India along two Deep Seismic Sounding (DSS) profiles, namely Khajuriakalan–Pulgaon and Ujjan–Mahan, traversing the Narmada-Son-Lineament (NSL) in an almost north–south direction. Knowledge of the crustal structure and P-wave velocity distribution up to the Moho, obtained from DSS studies, has been used for the development of the thermal model. Numerical results reveal that the Moho temperature in this region of central India varies between 500 and 580 °C. The estimated heat flow density value is found to vary between 46 and 49 mW/m². The Curie depth varies between 40 and 42 km and is in close agreement with the Curie depth (40±4 km) estimated from the analysis of MAGSAT data. Based on the present work and previous work, it is suggested that the major part of peninsular India consisting of the Wardha–Pranhita Godavari graben/basin, Bastar craton and the adjoining region of the Narmada Son Lineament between profiles I and III towards the north and northwest of the Bastar craton are characterized with a similar mantle heat flow density value equal to ∼23 mW/m². Variation in surface heat flow density values in these regions are caused by variation in the radioactive heat production and fluid circulation in the upper crustal layer.     

1891–1883 Ma Southern Bastar–Cuddapah mafic igneous events, India: A newly recognized large igneous province

A newly recognized remnant of a Paleoproterozoic Large Igneous Province has been identified in the southern Bastar craton and nearby Cuddapah basin from the adjacent Dharwar craton, India. High precision U–Pb dates of 1891.1 ± 0.9 Ma (baddeleyite) and 1883.0 ± 1.4 Ma (baddeleyite and zircon) for two SE-trending mafic dykes from the BD2 dyke swarm, southern Bastar craton, and 1885.4 ± 3.1 Ma (baddeleyite) for a mafic sill from the Cuddapah basin, indicate the existence of 1891–1883 Ma mafic magmatism that spans an area of at least 90,000 km² in the south Indian shield.This record of 1.9 Ga mafic/ultramafic magmatism associated with concomitant intracontinental rifting and basin development preserved along much of the south-eastern margin of the south Indian shield is a widespread geologic phenomenon on Earth. Similar periods of intraplate mafic/ultramafic magmatism occur along the margin of the Superior craton in North America (1.88 Ga Molson large igneous province) and in southern Africa along the northern margin of the Kaapvaal craton (1.88–1.87 Ga dolerite sills intruding the Waterberg Group). Existing paleomagnetic data for the Molson and Waterberg 1.88 Ga large igneous provinces indicate that the Superior and Kalahari cratons were at similar paleolatitudes at 1.88 Ga but a paleocontinental reconstruction at this time involving these cratons is impeded by the lack of a robust geological pin such as a Limpopo-like 2.0 Ga deformation zone in the Superior Province. The widespread occurrence of 1.88 Ga intraplate and plate margin mafic magmatism and basin development in numerous Archean cratons worldwide likely reflects a period of global-scale mantle upwelling or enhanced mantle plume activity at this time.     

Crustal velocity inhomogeneities along the Hirapur–Mandla profile, central India and its tectonic implications

Wide-angle seismic and gravity data across the Narmada-Son lineament (NSL) in central India are analyzed to determine crustal structure, velocity inhomogeneities and hence constrain the tectonics of the lineament. We present the 2-D crustal velocity structure from deep wide-angle reflection data by using a ray-trace inverse approach. The main result of the study is the delineation of fault-bounded horst raised to a subsurface depth (1.5 km) and the Moho upwarp beneath the NSL. The crust below the basement consists of three layers with velocities of 6.45–6.7, 6.2–6.5 and 6.7–6.95 km/s and interface depths of about 5.5–8.7, 14–17 and 18–23 km along the profile. The low-velocity (6.2–6.5 km/s) layer goes up to a depth of 5 km and becomes the thickest part (13 km), while the overlying high-velocity (6.45–6.7 km/s) layer becomes the thinnest (3 km) and upper boundary lies at a depth of 1.5 km beneath the NSL. The overall uncertainties of various velocity and boundary nodes are of the order of ±0.12 km/s and ±1.40 km, respectively. The up-lifted crustal block and the up-warping Moho beneath the NSL indicate that the north and south faults bounding the NSL are deeply penetrated through which mafic materials from upper mantle have been intruded into the upper crust. Gravity modeling was also undertaken to assess the seismically derived crustal features and to fill the seismic data gap. The lateral and vertical heterogeneous nature of the structure and velocity inhomogeneities in the crust cause instability to the crustal blocks and played an important role in reactivation of the Narmada south fault during the 1997 Jabalpur earthquake.     

Timing of thermal stabilization of the Zimbabwe Craton deduced from high-precision Rb–Sr chronology, Great Dyke

Dating low temperature events such as magmatic cooling or (hydro-)thermal surges in Archean and Proterozoic terranes is crucial in defining cratonal thermal stabilization after episodic continental growth during the Archean and Early Proterozoic. Rubidium–Sr chronology is potentially a powerful tool in this regard because of its low closure temperature, i.e., <400 °C in most minerals, but has until now been hampered by its relatively low precision compared to high-temperature chronometers. Consequently, Rb–Sr age investigations have so far failed to provide high-precision age constraints on the cooling of rocks older than 2 Ga. Here, it is demonstrated that internal Rb–Sr microchrons can yield important, high-precision age constraints on the cooling history of Archean intrusions. After careful mineral selection and chemical treatment, a Rb–Sr age of 2543.0 ± 4.4 Ma was obtained from the Archean Great Dyke, Zimbabwe Craton, in contrast to the intrusion age of 2575.8 ± 1 Ma, yielding an ambient average cooling of 5 ± 2 °C/Ma. The non-disturbed magmatic Rb–Sr cooling age of the Great Dyke marks the final stage of Zimbabwe craton stabilization and that the greater craton area did not experience any intensive later reheating event during metamorphic or tectonic events.     

Heat flow and lithospheric thermal regime in the Northeast German Basin

New values of surface heat flow are reported for 13 deep borehole locations in the Northeast German Basin (NEGB) ranging from 68 to 91 mW m^− 2 with a mean of 77 ± 3 mW m^− 2. The values are derived from continuous temperature logs, measured thermal conductivity, and log-derived radiogenic heat production. The heat-flow values are supposed free of effects from surface palaeoclimatic temperature variations, from regional as well as local fluid flow and from thermal refraction in the vicinity of salt structures and thus represent unperturbed crustal heat flow. Two-D numerical lithospheric thermal models are developed for a 500 km section along the DEKORP-BASIN 9601 deep seismic line across the basin with a north-eastward extension across the Tornquist Zone. A detailed conceptual model of crustal structure and composition, thermal conductivity, and heat production distribution is developed. Different boundary conditions for the thickness of thermal lithosphere were used to fit surface heat flow. The best fit is achieved with a thickness of thermal lithosphere of about 75 km beneath the NEGB. This estimate is corroborated by seismological studies and somewhat less than typical for stabilized Phanerozoic lithosphere. Modelled Moho temperatures in the basin are about 800 °C; heat flow from the mantle is about 35 to 40 mW m^− 2. In the southernmost part of the section, beneath the Harz Mountains, higher Moho temperatures up to 900 to 1000 °C are shown. While the relatively high level of surface heat flow in the NEGB obviously is of longer wave length and related to lithosphere thickness, changes in crustal structure and composition are responsible for short-wave-length anomalies.     

Constraining models of the tectonic setting of the giant Olympic Dam iron oxide–copper–gold deposit, South Australia, using deep seismic reflection data

In the Gawler Craton, the completeness of cover concealing the crystalline basement in the region of the giant Olympic Dam Cu–Au deposit has impeded any sufficient understanding of the crustal architecture and tectonic setting of its IOCG mineral-system. To circumvent this problem, deep seismic reflection data were recently acquired from  250 line-km of two intersecting traverses, centered on the Olympic Dam deposit. The data were recorded to 18 s TWT ( 55 km). The crust consists of Neoproterozoic cover, in places more than 5 km thick, over crystalline basement with the Moho at depths of 13–14 s TWT ( 40–42 km). The Olympic Dam deposit lies on the boundary between two distinct pieces of crust, one interpreted as the Archean–Paleoproterozoic core to the craton, the other as a Meso–Neoproterozoic mobile belt. The host to the deposit, a member of the  1590 Ma Hiltaba Suite of granites, is situated above a zone of reduced impedance contrast in the lower crust, which we interpret to be source-region for its  1000 °C magma. The crystalline basement is dominated by thrusts. This contrasts with widely held models for the tectonic setting of Olympic Dam, which predict extension associated with heat from the mantle producing the high temperatures required to generate the Hiltaba Suite granites implicated in mineralization. We use the seismic data to test four hypotheses for this heat-source: mantle underplating, a mantle-plume, lithospheric extension, and radioactive heating in the lower crust. We reject the first three hypotheses. The data cannot be used to reject or confirm the fourth hypothesis.     

2-D Crustal thermal structure along Thuadara-Sindad DSS profile across Narmada-Son lineament,central India

Central India is traversed by a WSW-ENE trending Narmada-Son lineament (NSL) which is characterized by the presence of numerous hot springs, feeder dykes for Deccan Traps and seismicity all along its length. It is divided in two parts by the Barwani-Sukta Fault (BSF). To the west of this fault a graben exists, whereas to the east the basement is uplifted between Narmada North Fault (NNF) and Narmada South Fault (NSF). The present work deals with the 2-D thermal modeling to delineate the crustal thermal structure of the western part of NSL region along the Thuadara-Sindad Deep Seismic Sounding (DSS) profile which runs almost in the N-S direction across the NSL. Numerical results of the model reveal that the conductive surface heat flow value in the region under consideration varies between 45 and 47mW/m². Out of which 23mW/m² is the contribution from the mantle heat flow and the remaining from within the crust. The Curie depth is found to vary between 46 and 47 km and is in close agreement with the earlier reported Curie depth estimated from the analysis of MAGSAT data. The Moho temperature varies between 470 and 500°C. This study suggests that this western part of central Indian region is characterized by low mantle heat flow which in turn makes the lower crust brittle and amenable to the occurrence of deep focused earthquakes such as Satpura (1938) earthquake.     

Moho depth variation beneath southwestern Japan revealed from the velocity structure based on receiver function inversion

The Philippine Sea plate is subducting under the Eurasian plate beneath the Chugoku-Shikoku region, southwestern Japan. We have constructed depth contours for the continental and oceanic Mohos derived from the velocity structure based on receiver function inversion. Receiver functions were calculated using teleseismic waveforms recorded by the high-density seismograph network in southwestern Japan. In order to determine crustal velocity structure, we first improved the linearized time-domain receiver function inversion method. The continental Moho is relatively shallow ( 30 km) at the coastline of the Sea of Japan and at the Seto Inland Sea, and becomes deeper–greater than 40 km–around 35°N and 133.8°E. Near the Seto Inland Sea, a low-velocity layer of thickness 10 km lies under the continental Moho. This low-velocity layer corresponds to the subducting oceanic crust of the Philippine Sea plate. The oceanic Moho continues to descend from south to northwest and exhibits complicated ridge and valley features. The oceanic Moho runs around 25 km beneath the Pacific coast and 45 km beneath the Seto Inland Sea, and it extends to at least to 34.5°N. The depth variation of the Moho discontinuities is in good qualitative agreement with the concept of isostasy. From the configurations of both the continental and oceanic Mohos, we demonstrate that the continental lower crust and the subducting oceanic crust overlap beneath the southern and central part of Shikoku and that a mantle wedge may exist beneath the western and eastern part of Shikoku. The southern edge of the overlapping region coincides with the downdip limit of the slip area of a megathrust earthquake.     

Age constraints on the formation and emplacement of Neoproterozoic ophiolites along the Allaqi–Heiani Suture, South Eastern Desert of Egypt

Ophiolites are key components of the Neoproterozoic Arabian–Nubian Shield (ANS). Understanding when they formed and were emplaced is crucial for understanding the evolution of the ANS because their ages tell when seafloor spreading and terrane accretion occurred. The Yanbu–Onib–Sol Hamed–Gerf–Allaqi–Heiani (YOSHGAH) suture and ophiolite belt can be traced  600 km across the Nubian and Arabian shields. We report five new SHRIMP U–Pb zircon ages from igneous rocks along the Allaqi segment of the YOSHGAH suture in southernmost Egypt and use these data in conjunction with other age constraints to evaluate YOSHGAH suture evolution. Ophiolitic layered gabbro gave a concordia age of 730 ± 6 Ma, and a metadacite from overlying arc-type metavolcanic rocks yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 733 ± 7 Ma, indicating ophiolite formation at  730 Ma. Ophiolite emplacement is also constrained by intrusive bodies: a gabbro yielded a concordia age of 697 ± 5 Ma, and a quartz-diorite yielded a concordia age of 709 ± 4 Ma. Cessation of deformation is constrained by syn- to post-tectonic granite with a concordia age of 629 ± 5 Ma. These new data, combined with published zircon ages for ophiolites and stitching plutons from the YOSHGAH suture zone, suggest a 2-stage evolution for the YOSHGAH ophiolite belt ( 810–780 Ma and  730–750 Ma) and indicate that accretion between the Gabgaba–Gebeit–Hijaz terranes to the south and the SE Desert–Midyan terranes to the north occurred as early as 730 Ma and no later than 709 ± 4 Ma.     

A review of the thermal regime of the Eastern Alps with respect to the effects of paleoclimate and exhumation

Transient thermal signals such as Pleistocene surface temperature variations or exhumation of great rock volumes are important for the current thermal regime of the Eastern Alpine crust. In this study transient 1-D forward simulations and an analytical approach were used to estimate the order of magnitude of these effects. A comparison with numerical forward simulations and inverse analyses of steady-state heat conduction yields the following main conclusions with respect to the thermal regime of the Eastern Alps along the TRANSALP profile: (1) The change of surface temperatures in the past affects mainly the uppermost part of the Eastern Alpine crust. It results in a maximum thermal signature of more than − 6 K at a depth of 2 km. The deviations from a steady-state temperature gradient and heat flow in the region of the Tauern Window range from 0.3–4 K km^− 1 and 0–6 mW m^− 2, respectively, with maximum values at the surface. (2) Exhumation of the Eastern Alpine lithosphere may result in a thermal signature of up to 4 K at a depth of 1 km. The thermal signature increases further with depth to a maximum of approximately 80 K at a depth of 50 km. As the temperature gradient of the exhumation signal is almost zero at the base of the crust, Moho heat flow appears to be not critically perturbed. (3) The combined effect of exhumation and changing surface temperatures at the Tauern Window amounts to less than 15% of the steady-state temperatures at a depth of  8 km and to less than 10% at the base of Eastern Alpine root. The corresponding perturbation in heat flow is less than 20% at a depth of 4 km, approaching zero below 40 km.     

Paleomagnetism and Detrital Zircon Geochronology of the Upper Vindhyan Sequence, Son Valley and Rajasthan, India: A ca. 1000 Ma Closure age for the Purana Basins?

The utility of paleomagnetic data gleaned from the Bhander and Rewa Groups of the “Purana-aged” Vindhyanchal Basin has been hampered by the poor age control associated with these units. Ages assigned to the Upper Vindhyan sequence range from Cambrian to the Mesoproterozoic and are derived from a variety of sources, including ⁸⁷Sr/⁸⁶Sr and δ ¹³C correlations with the global curves and Ediacara-like fossil finds in the Lakheri–Bhander limestone. New analyses of the available paleomagnetic data collected from this study and previous work on the 1073 Ma Majhgawan kimberlite, as well as detrital zircon geochronology of the Upper Bhander sandstone and sandstones from the Marwar SuperGroup suggest that the Upper Vindhyan sequence may be up to 500 Ma older than is commonly thought. Paleomagnetic analysis generated from the Bhander and Rewa Groups yields a paleomagnetic pole at 44°N, 214.0°E (A95 = 4.3°). This paleomagnetic pole closely resembles the VGP from the well-dated Majhgawan intrusion (36.8°N, 212.5°E, α₉₅ = 15.3°).Detrital zircon analysis of the Upper Bhander sandstone identifies a youngest age population at 1020 Ma. A comparison between the previously correlated Upper Bhander sandstone and the Marwar sandstone detrital suites shows virtually no similarities in the youngest detrital suite sampled. The main 840–920 Ma peak is absent in the Upper Bhander. This supports our assertion that the Upper Bhander is older than the 750–771 Ma Malani sequence, and is likely close to the age of the 1073 Ma Majhgawan kimberlite on the basis of the paleomagnetic similarities. By setting the age of the Upper Vindhyan at 1000–1070 Ma, several intriguing possibilities arise. The Bhander–Rewa paleomagnetic pole allows for a reconstruction of India at 1000–1070 Ma that overlaps with the 1073 ± 13.7 Majhgawan kimberlite VGP. Comparisons between the composite Upper Vindhyan pole (43.9°N, 210.2°E, α₉₅ = 12.2°) and the Australian 1071 ± 8 Ma Bangamall Basin sills and the 1070 Ma Alcurra dykes suggest that Australia and India were not adjacent at this time period.     

Constraints from Moho geometry and crustal thickness on the geodynamic origin of the Vrancea Seismogenic Zone (Romania)

Reprocessing of industry deep seismic reflection data (Ramnicu Sarat and Braila profiles) from the SE Carpathian foreland of Romania provides important new constraints on geodynamic models for the origin of the intermediate depth Vrancea Seismogenic Zone (VSZ). Mantle (70–200 km) earthquakes of the VSZ are characterized by high magnitudes (greater than 6.5), frequent occurrence rates (approximately 25 years), and confinement in a very narrow (30 × 70 × 200 km³) near vertical zone atypical for a Wadati–Benioff plane, located in front of the orogen. These two deep (20 s) seismic reflection profiles (70 km length across the foreland) reveal (1) a high-amplitude, gently east-dipping reflection across most of the section from what we interpret to be the Moho at  15 s (40–42 km) on the Ramnicu Sarat line to  16 s (47–48 km) on the Braila line, (2) a thick sedimentary cover increasing in thickness from east (1 s;  800 m) to west (7.5 s; 14 km), (3) an eastward increase in crustal thickness from 38 km (near VSZ) to  45 km, (4) seismic and topographic evidence for a newly imaged, possibly seismically active basement fault with a surface offset of 30 m observed on the Ramnicu Sarat line, (5) a lack of notable west-dipping structures in the crust and across the Moho, and (6) variable displacements on Peceneaga–Camena Fault of  5 km at Moho and  200 m at the basement–sedimentary cover contact.These observations appear to argue against recent models for west-dipping subduction of oceanic lithosphere at or in the vicinity of the Vrancea Seismogenic Zone given the lack of west-dipping fabrics in the lower crust and across the crust–mantle boundary. Consequently, one possible explanation for the geodynamic origin of VSZ could be partial delamination of the continental lithosphere in an intra-plate setting along a sub-horizontal lithospheric interface in the Carpathian hinterland that likely involves remnant lithospheric coupling between the crust and uppermost mantle in the foreland.     

Heat flow, heat production and fission track data from the Hercynian basement around the Provençal Basin (Western Mediterranean)

In the complex structural framework of the Western Mediterranean. Hercynian areas are expected to be thermally preserved from the recent tectonic evolution. The thermal regime of these areas is studied using heat flow, heat production and fission track data. The surface heat flow is significantly higher in Corsica (76 ± 10 mW m⁻²) than in the Maures and Estérel (58 ± 2 mW m⁻²). Neither heat production nor erosion subsequent to the Alpine orogeny in Corsica can explain such a difference. It is suggested that a deep thermal source related to the asymmetric evolution of the Provençal basin could explain the higher heat flow in Corsica. A model of thermal structure based on the present day thermal regime of the Maures and Estérei is proposed for the stable Hercynian crust in this area. The mantle heat flow is 20–25 mW m⁻² and the temperature at Moho level is 375–500°C, depending on the thermal parameter distribution with depth.     

The Paleoproterozoic North Hebei Orogen: North China craton's collisional suture with the Columbia supercontinent

Understanding the geologic history and position of the North China craton in the Paleoproterozoic Columbia supercontinent has proven elusive. Paleoproterozoic orogenic episodes (2.00–1.85 Ga) are temporally associated with ultimate stabilization of the North China craton (NCC), followed by the development of extensive craton-wide rift systems at 1.85–1.80 Ga. The age difference between the sedimentary cover and the metamorphic basement is up to 500–700 Ma, suggesting that uplift and doming of cratonic basement occurred in the latest Paleoproterozoic. Mafic dike swarms (1.80–1.77 Ga) and anorogenic magmatism (1.80–1.70 Ga) record the extensional breakup and dispersal of the North China craton during this stage. The late Paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the NCC within the Late Paleoproterozoic supercontinent of Columbia.An east-west striking thousand kilometer long belt of khondalites (granulite facies metapelites) stretches along the northern margin of the North China craton, on the cratonward side of the Northern Hebei orogenic belt. This granulite belt includes Mg–Al (sapphirine bearing) granulites that reached ultrahigh-temperature “peak” metamorphic conditions of  1000 °C at 10 kbars at 1927 ± 11 Ma. Following peak ultrahigh-temperature conditions, the rocks underwent initial isobaric cooling and subsequent isothermal decompression, and these trajectories are interpreted to be part of an overall anti-clockwise P-T evolution indicating that the northern margin of the craton experienced continental collision at 1.93–1.92 Ga. The position of the khondalite belt south of the Northern Hebei orogenic belt makes it analogous to Tibet, a continental collision-related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth. We suggest that the tectonic evolution of the NCC during this period was closely related to the assembly and break-up of the Columbia supercontinent, and that the NCC was adjacent to the Baltic and Amazonian cratons in the period 2.00–1.70 Ga. Craton-wide extension occurred within 100–150 Ma of collision along the northern margin of the craton at 1.93–1.92 Ga. It is concluded that mantle upwellings are chiefly responsible for the breakup of the NCC from the Paleoproterozoic supercontinent.     

Anatolian surface wave evaluated at GEOFON Station ISP Isparta, Turkey

We have studied seismic surface waves of 255 shallow regional earthquakes recently recorded at GEOFON station ISP (Isparta, Turkey) and have selected these 52 recordings with high signal-to-noise ratio for further analysis. An attempt was made by the simultaneous use of the Rayleigh and Love surface wave data to interpret the planar crust and uppermost mantle velocity structure beneath the Anatolian plate using a differential least-square inversion technique. The shear-wave velocities near the surface show a gradational change from approximately 2.2 to 3.6 km s^− 1 in the depth range 0–10 km. The mid-crustal depth range indicating a weakly developed low velocity zone has shear-wave velocities around 3.55 km s^− 1. The Moho discontinuity characterizing the crust–mantle velocity transition appears somewhat gradual between the depth range  25–45 km. The surface waves approaching from the northern Anatolia are estimated to travel a crustal thickness of  33 km whilst those from the southwestern Anatolia and part of east Mediterranean Sea indicate a thicker crust at  37 km. The eastern Anatolia events traveled even thicker crust at  41 km. A low sub-Moho velocity is estimated at  4.27 km s^− 1, although consistent with other similar studies in the region. The current velocities are considerably slower than indicated by the Preliminary Reference Earth Model (PREM) in almost all depth ranges.     

A thermomechanical wedge model of Taiwan constrained by fission-track thermochronometry

We present 31 new apatite fission-track (AFT) ages for the island of Taiwan that, when combined with existing AFT and zircon fission-track (ZFT) data, provide regional spatial coverage of the island with respect to low-temperature thermochronometry. The overall pattern of ZFT and AFT ages in Taiwan exhibits unreset ages in the southern and western portions of the island and reset ages predominantly in the Central Range and eastern Taiwan. This pattern supports interpretations of the orogen kinematics as reflecting a crustal scale wedge with a southward propagating collision zone. In this model, new material is accreted to the wedge from the west and is transferred to the east with the greatest exhumation occurring along the eastern margin as recorded in the reset ages in the east and unreset ages in the west. The southward propagating collision is consistent with reset ages in the north, where erosional exhumation has been ongoing for longer, and unreset ages in the south, where the younger collision implies less time for erosional exhumation. Despite the variation in the age of the collision along the strike of the island, the widths of the AFT and ZFT reset age zones remain nearly constant between 23° 00′N to  24° 00′N and 23° 20′N to  24° 00′N, respectively, suggesting that the orogen is in an exhumational steady state over these regions with respect to the AFT and ZFT thermochronometers. We use the fission-track data in conjunction with observations of crustal structure, crustal fabric, and heat flow measurements to constrain a time-dependent, two-dimensional, thermomechanical model of orogen evolution. By accounting for the heat transfer, tectonic and erosion processes needed to predict AFT and ZFT ages, we are able to investigate the relationship between the measured ages and the tectonic characteristics of the orogen. With our model we conclude that: (1) roughly half of the material accretion in Taiwan occurs through underplating over an approximately 40 km wide region, (2) current average erosion rates are  3.3 mm/yr in the eastern Central Range and  2.3 mm/yr over the whole island, (3) the collision has been propagating southward at a rate between 20 and 51 km/Ma over the past 2–3 Ma, and (4) central Taiwan is in a topographic, thermal and exhumational steady state.     

Neoproterozoic adakitic plutons in the northern margin of the Yangtze Block, China: Partial melting of a thickened lower crust and implications for secular crustal evolution

Numerous Neoproterozoic felsic and mafic–ultramafic intrusions occur in the Hannan region at the northern margin of the Yangtze Block. Among these, the Wudumen and Erliba plutons consist of granodiorites and have SHRIMP zircon U–Pb ages of  735 Ma. The rocks have high K₂O (0.8–3.6 wt.%) and Na₂O (4.4–6.4 wt.%) and low MgO (0.4–1.7 wt.%). They also have high Sr/Y (32–209) and (La/Yb)_n ratios (4.4–38.6). Their εNd values range from − 0.41 to − 0.92 and zircon initial ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.282353 to 0.282581. These geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust. Our new analytical results, combined with the occurrence of voluminous arc-related mafic–ultramafic intrusions emplaced before 740 Ma, lead us to propose that the crustal evolution in the northern margin of the Yangtze Block during Neoproterozoic involved: (1) rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from  1.0 to  0.74 Ga, and (2) partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window, producing the  735 Ma adakitic Wudumen and Erliba plutons. Our model suggests that the crustal thickness was more than 50 km at the northern margin of the Yangtze Block at  735 Ma, and rule out the possibility of a mantle plume impact causing the > 735 Ma magmatism in the region.     

Seismically constrained two-dimensional crustal thermal structure of the cambay basin

The temperature field within the crust is closely related to tectonic history as well as many other geological processes inside the earth. Therefore, knowledge of the crustal thermal structure of a region is of great importance for its tectonophysical studies. This work deals with the two-dimensional thermal modelling to delineate the crustal thermal structure along a 230 km long Deep Seismic Sounding (DSS) profile in the north Cambay basin. In this work P-wave velocities obtained from the DSS studies have been converted into heat generation values for the computation of temperature distribution. The model result reveals the Curie isotherm at a depth of ≈22 km and Moho temperature at around 900‡C.     

Feldspathic clasts in Yamato-86032: Remnants of the lunar crust with implications for its formation and impact history

Low concentrations of Th and Fe in the Yamato (Y)-86032 bulk meteorite support earlier suggestions that Y-86032 comes from a region of the moon far distant from the Procellarum KREEP Terrain (PKT), probably from the lunar farside. ³⁹Ar–⁴⁰Ar, Rb–Sr, Sm–Nd, and Sm-isotopic studies characterize the chronology of Y-86032 and its precursors in the mega regolith. One of the rock types present in a light gray breccia lithology is an anorthosite characterized by plagioclase with An 93, i.e., more sodic than lunar FANs, but with very low ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr similar to those of FANs. (FAN stands for Ferroan Anorthosite). This “An93 anorthosite” has Nd-isotopic systematics similar to those of nearside norites. A FAN-like “An97 anorthosite” is present in a second light-colored feldspathic breccia clast and has a more negative ε_Nd value consistent with residence in a LREE-enriched environment as would be provided by an early plagioclase flotation crust on the Lunar Magma Ocean (LMO). This result contrasts with generally positive values of ε_Nd for Apollo 16 FANs suggesting the possibility of assymetric development of the LMO. Other possible explanations for the dichotomy in ε_Nd values are advanced in the text. The Y-86032 protolith formed at least 4.43 ± 0.03 Ga ago as determined from a Sm–Nd isochron for mineral fragments from the breccia clast composed predominantly of An93 anorthosite and a second clast of more varied composition. We interpret the mineral fragments as being predominatly from a cogenetic rock suite. An ³⁹Ar–⁴⁰Ar age of 4.36–4.41 ± 0.035 Ga for a third clast composed predominantly of An97 anorthosite supports an old age for the protolith. Initial ¹⁴³Nd/¹⁴⁴Nd in that clast was −0.64 ± 0.13 ε-units below ¹⁴³Nd/¹⁴⁴Nd in reservoirs having chondritic Sm/Nd ratios, consistent with prior fractionation of mafic cumulates from the LMO. A maximum in the ³⁹Ar–⁴⁰Ar age spectrum of 4.23 ± 0.03 Ga for a second sample of the same feldspathic breccia clast probably reflects some diffusive ⁴⁰Ar loss. Lack of solar wind and lunar atmosphere implanted Ar in the light gray breccia clast allows determination of an ³⁹Ar/⁴⁰Ar age of 4.10 ± 0.02 Ga, which is interpreted as the time of initial brecciation of this litholgy. After correction for implanted lunar atmosphere ⁴⁰Ar, impact melt and dark regolith clasts give Ar ages of 3.8 ± 0.1 Ga implying melt formation and final breccia assembly 3.8 Ga ago. Some breccia lithologies were exposed to thermal neutron fluences of 2 × 10¹⁵ n/cm², only about 1% of the fluence experienced by some other lunar highlands meteorites. Other lithologies experienced neutron fluences of 1 × 10¹⁵ n/cm². Thus, Y-86032 spent most of the time following final brecciation deeply buried in the megaregolith. The neutron fluence data are consistent with cosmogenic ³⁸Ar_cos cosmic ray exposure ages of 10 Ma. Variations among differing lithologies in the amount of several regolith exposure indicators, including cosmogenic noble gas abundances, neutron capture induced variations in Sm isotopic abundances, and Ir contents, are consistent with a period of early (>3.8 Ga ago) lunar regolith exposure, subsequent deep burial at >5 m depth, and ejection from the moon 7–10 Ma ago.     

Paleomagnetism of the Fort Knox Stock, Alaska, and rotation of the Yukon–Tanana terrane after 92.5 Ma

The 92.5 Ma Fort Knox granodiorite stock, near the western end of the Fairbanks Belt in the Yukon–Tanana terrane (YTT) of central Alaska, hosts a world-class gold mine. The stock has been analysed paleomagnetically using thermal and alternating-field step demagnetization and isothermal remanence methods. This pluton retains a primary thermoremanent magnetization at 18 sites (232 specimens) that resides mainly in single-to pseudosingle-domain magnetite with a direction of D = 228.8°, I = 84.3° (N = 18, k = 130, α₉₅ = 3.0°), giving a paleopole at 56.5°N, 197.1°E (dp = 5.9°, dm = 5.8°). The pluton's host rock, the Fairbanks schist, does not retain a stable coherent remanence. Relative to the North American craton, the stock's paleoinclination indicates that the Fairbanks Belt has undergone nonsignificant poleward (northwesterly) translation of 25 ± 750 km only. Analysed in concert with the few available paleoinclinations available for the YTT in Yukon, the paleoinclination suggests further that the YTT has undergone only  250 to 450 km of dextral displacement along the Tintina fault in the past  100 Ma and, therefore, is parautocthonous since the mid-Cretaceous. The stock's paleodeclination records 121 ± 35° of counterclockwise rotation relative to the North American craton. Consideration of models published for Alaska's tectonic evolution suggests that this paleodeclination discordance is caused by rotations associated with the opening of the Canada Basin, with dextral displacement on the Tintina fault, and with development of the western Alaskan orocline. Thus the paleomagnetic results for the Fort Knox stock support a thin-skin tectonic model for the accretion of the YTT and Intermontane Belt terranes to the northern Cordillera.