Silicic recharge of multiple rhyolite magmas by basaltic intrusion during the 22.6 ka Okareka Eruption Episode, New Zealand

Deposits of the 22.6 ka Okareka Eruption Episode from Tarawera Volcanic Complex record the sequential and simultaneous eruption of three discrete rhyolite magmas following a silicic recharge event related to basaltic intrusion. The episode started with basaltic eruption ( 0.01 km³ magma), and rapidly changed to a plinian eruption involving a moderate temperature (750 °C), cummingtonite-bearing rhyolite magma (T1) with a volume of  0.3 km³. Hybrid basalt/rhyolite clasts demonstrate direct basaltic intrusion that helped trigger the eruption. Crystals, shards and lapilli of two other rhyolite magmas then joined the eruption sequence. They comprise a cooler (720 °C) crystal-rich biotite–hornblende rhyolite magma (T2) ( 0.3 km³), and a hotter (780 °C), crystal-poor, pyroxene–hornblende rhyolite magma (T3) ( 4.5 km³). All mid to late-stage ash units contain various mixtures of T1, T2 and T3 components with a general increase in abundance of T3 and rapid decline of T1 with time. About 4 km³ of T3 magma was extruded as lavas at the end of the episode. Contrasts in melt composition, crystal and volatile contents, and temperatures influenced viscosity and miscibility, and thus limited pre-eruption mixing of the rhyolite magmas. The eruption sequence and the restricted direct basaltic intrusion into only one magma (T1) is consistent with the rhyolites occupying separate melt pods within a large crystal-mush zone. Melt–crystal equilibria and volatile contents in melt inclusions indicate temporary magma storage depths of < 8 km. Each of the magmas display quartz crystals containing melt inclusions that are compositionally highly evolved relative to the accompanying matrix glass, and thus point to a stage of more complete crystallisation. The matrix glass, enriched in Sr and Ti, represents a re-melting event of underlying the crystal pile induced by basaltic intrusion, presumably part of the same event that erupted scoria at the start of the eruption. This recharge rhyolite melt percolated upward and hybridised with the resident melts in each of the three magma pods. The Okareka episode rhyolites contrast with other well-documented rhyolites that are either continuously or discontinuously zoned, or have been homogenised during re-activation to a uniform composition. Rapid basalt dike intrusion to shallow levels appears to have (prematurely?) triggered the Okareka rhyolites into eruption, so that their early ponding in separate melt pods has been recorded before it could be masked by mixing or stratification had amalgamation into a larger body occurred.     

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.     

Insights into Wasatch fault vertical slip rates using the age of sediments in Timpanogos Cave, Utah

Timpanogos Cave, located near the Wasatch fault, is about 357 m above the American Fork River. Fluvial cave sediments and an interbedded carbonate flowstone yield a paleomagnetic and U–Th depositional age of 350 to 780 ka. Fault vertical slip rates, inferred from calculated river downcutting rates, range between 1.02 and 0.46 mm yr^− 1. These slip rates are in the range of the 0–12 Ma Wasatch Range exhumation rate ( 0.5–0.7 mm yr^− 1), suggesting that the long-term vertical slip rate remained stable through mid-Pleistocene time. However, the late Pleistocene (0–250 ka) decelerated slip rate ( 0.2–0.3 mm yr^− 1) and the accelerated Holocene slip rate ( 1.2 mm yr^− 1) are consistent with episodic fault activity. Assuming that the late Pleistocene vertical slip rate represents an episodic slowing of fault movement and the long-term (0–12 Ma) average vertical slip rate, including the late Pleistocene and Holocene, should be  0.6 mm yr^− 1, there is a net late Pleistocene vertical slip deficit of  50–75 m. The Holocene and late Pleistocene slip rates may be typical for episodes of accelerated and slowed fault movement, respectively. The calculated late Pleistocene slip deficit may mean that the current accelerated Wasatch fault slip rate will extend well into the future.     

Analysis of crustal deformation in Luzon, Philippines using geodetic observations and earthquake focal mechanisms

We utilize regional GPS velocities from Luzon, Philippines, with focal mechanism data from the Harvard Centroid Moment Tensor (CMT) Catalog, to constrain tectonic deformation in the complex plate boundary zone between the Philippine Sea Plate and Eurasia (the Sundaland block). Processed satellite imagery and digital elevation models are used with existing gravity anomaly, seismicity, and geologic maps to define a suite of six elastic blocks. Geodetic and focal mechanism data are inverted simultaneously to estimate plate rotations and fault-locking parameters for each of the tectonic blocks and faults comprising Luzon. Major tectonic structures that were found to absorb the plate convergence include the Manila Trench (20–100 mm yr^− 1) and East Luzon Trough ( 9–15 mm yr^− 1)/Philippine Trench ( 29–34 mm yr^− 1), which accommodate eastward and westward subduction beneath Luzon, respectively; the left-lateral strike-slip Philippine Fault ( 20–40 mm yr^− 1), and its northward extensions, the Northern Cordillera Fault ( 17–37 mm yr^− 1 transtension), and the Digdig Fault ( 17–27 mm yr^− 1 transpression). The Macolod Corridor, a zone of active volcanism, crustal thinning, extension, and extensive normal and strike-slip faulting in southwestern Luzon, is associated with left-lateral, transtensional slip of  5–10 mm yr^− 1. The Marikina Fault, which separates the Central Luzon block from the Southwestern Luzon block, reveals  10–12 mm yr^− 1 of left-lateral transpression. Our analysis suggests that much of the Philippine Fault and associated splays are locked to partly coupled, while the Manila and Philippine trenches appear to be poorly coupled. Luzon is best characterized as a tectonically active plate boundary zone, comprising six mobile elastic tectonic blocks between two active subduction zones. The Philippine Fault and associated intra-arc faults accommodate much of the trench-parallel component of relative plate motion.     

A late Quaternary paleotemperature record from Hanging Lake, northern Yukon Territory, eastern Beringia

The late Quaternary paleoclimate of eastern Beringia has primarily been studied by drawing qualitative inferences from vegetation shifts. To quantitatively reconstruct summer temperatures, we analyzed lake sediments for fossil chironomids, and additionally we analyzed the sediments for fossil pollen and organic carbon content. A comparison with the δ¹⁸O record from Greenland indicates that the general climatic development of the region throughout the last glaciation–Holocene transition differed from that of the North Atlantic region. Between  17 and 15 ka, mean July air temperature was on average 5°C colder than modern, albeit a period of near-modern temperature at  16.5 ka. Total pollen accumulation rates ranged between  180 and 1200 grains cm^− 2 yr^− 1. At  15 ka, approximately coeval with the Bølling interstadial, temperatures again reached modern values. At  14 ka, nearly 1000 yr after warming began, Betula pollen percentages increased substantially and mark the transition to shrub-dominated pollen contributors. Chironomid-based inferences suggest no evidence of the Younger Dryas stade and only subtle evidence of an early Holocene thermal maximum, as temperatures from  15 ka to the late Holocene were relatively stable. The most recognizable climatic oscillation of the Holocene occurred from  4.5 to 2 ka.     

Paleomagnetic constraints on the paleogeography and oroclinal bending of the Devonian volcanic arc in Kazakhstan

Numerical modelling, incorporating coupling between surface processes and induced flow in the lower continental crust, is used to address the Quaternary evolution of the Gulf of Corinth region in central Greece. The post-Early Pleistocene marine depocentre beneath this Gulf overlies the northern margin of an older (Early Pleistocene and earlier) lacustrine basin, the Proto Gulf of Corinth Basin or PGCB. In the late Early Pleistocene, relief in this region was minimal but, subsequently, dramatic relief has developed, involving the creation of  900 m of bathymetry within the Gulf and the uplift by many hundreds of metres of the part of the PGCB, south of the modern Gulf, which forms the Gulf's main sediment supply. It is assumed that, as a result of climate change around 0.9 Ma, erosion of this sediment source region and re-deposition of this material within the Gulf began, both processes occurring at spatial average rates of  0.2 mm a^− 1. Modelling of the resulting isostatic response indicates that the local effective viscosity of the lower crust is  4 × 10¹⁹ Pa s, indicating a Moho temperature of  560 °C. It predicts that the  10 mm a^− 1 of extension across this  70 km wide model region, at an extensional strain rate of  0.15 Ma^− 1, is partitioned with  3 mm a^− 1 across the sediment source,  2 mm a^− 1 across the depocentre, and  5 mm a^− 1 across the ‘hinge zone’ in between, the latter value being an estimate of the extension rate on normal faults forming the major topographic escarpment at the southern margin of the Gulf. This modelling confirms the view, suggested previously, that coupling between this depocentre and sediment source by lower-crustal flow can explain the dramatic development in local relief since the late Early Pleistocene. The effective viscosity of the lower crust in this region is not particularly low; the strong coupling interpreted between the sediment source and depocentre results instead from their close proximity. In detail, the effective viscosity of the lower crust is expected to decrease northward across this model region, due to the northward increase in exposure of the base of the continental lithosphere to the asthenosphere; in the south the two are separated by the subducting Hellenic slab. The isostatic consequences of such a lateral variation in viscosity provide a natural explanation for why, since  0.9 Ma, the modern Gulf has developed asymmetrically over the northern part of the PGCB, leaving the rest of the PGCB to act as its sediment source.     

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.     

Linking source and sink: Evaluating the balance between onshore erosion and offshore sediment accumulation since Gondwana break-up, South Africa

The geomorphic origin and evolution of the tectonically unique interior highland of southern Africa, the Kalahari Plateau, and its flanking low-lying coastal planes, remain largely unresolved because of a lack of regional quantitative analyses of its uplift and erosion history. Here we focus on the southern Cape, South Africa and link onshore denudation, based on new apatite fission track thermochronology results, to offshore sediment accumulation, using abundant well data and a seismic reflection profile. We attempt to relate source and sink in order to resolve some first order issues concerning timing of the exhumation and development of the topographic features of southern Africa. The volume of sediment accumulated off South Africa's south coast is calculated using 173 wells and a seismic reflection profile. A total, uncompacted, sediment volume of 268,500 km³ accumulated off South Africa's south coasts since  136 Ma, in the Outeniqua and Southern Outeniqua Basins. Accumulation volumes and rates were highest in the early Cretaceous (48,800 × 10⁴ km³ at  8150 km³/Ma from  136 to 130 Ma, and 57,500 × 10⁴ km³ at 5750 km³/Ma from  130 to 120 Ma) and mid–late Cretaceous (83,700 × 10⁴ km³ at 3200 km³/Ma from  93 to 67 Ma). Volumes and accumulation rates were lowest for the early–mid-Cretaceous (47,400 × 10⁴ km³ at 1750 km³/Ma from  120 to 93 Ma) and the Cenozoic (31,200 × 10⁴ km³ at 450 km³/Ma from  67 to 0 Ma). Although our analysis shows that the accumulated volume of offshore sediments does not match the calculated volume of onshore erosion, as quantified through apatite fission track thermochronology (e.g. Tinker, J.H., de Wit, M.J., Brown, R., 2008. Mesozoic exhumation of the 439 southern Cape, South Africa, quantified using apatite fission track thermochronology. Tectonophysics, doi: 10.1016/j.tecto.2007.10.009), the timing of increased sediment accumulation closely matches the timing of increased onshore denudation. This suggests that the greatest volumes of material were transported from source to sink during two distinct Cretaceous episodes, and that the processes driving onshore denudation decreased by an order of magnitude during the Cenozoic.     

Diagenetic mineralization in Pennsylvanian coals from Indiana, USA: C/C and O/O implications for cleat origin and coalbed methane generation

Cleats and fractures in southwestern Indiana coal seams are often filled with authigenic kaolinite and/or calcite. Carbon- and oxygen-stable isotope ratios of kaolinite, calcite, and coalbed CO₂ were evaluated in combination with measured values and published estimates of δ¹⁸O of coalbed paleowaters that had been present at the time of mineralization. δ¹⁸O_mineral and δ¹⁸O_water values jointly constrain the paleotemperature of mineralization. The isotopic evidence and the thermal and tectonic history of this part of the Illinois Basin led to the conclusion that maximum burial and heat-sterilization of coal seams approximately 272 Ma ago was followed by advective heat redistribution and concurrent precipitation of kaolinite in cleats at a burial depth of < 1600 m at  78 ± 5 °C. Post-Paleozoic uplift, the development of a second generation of cleats, and subsequent precipitation of calcite occurred at shallower burial depth between  500 to  1300 m at a lower temperature of 43 ± 6 °C. The available paleowater in coalbeds was likely ocean water and/or tropical meteoric water with a δ¹⁸O_water  − 1.25‰ versus VSMOW. Inoculation of coalbeds with methanogenic CO₂-reducing microbes occurred at an even later time, because modern microbially influenced ¹³C-enriched coalbed CO₂ (i.e., the isotopically fractionated residue of microbial CO₂ reduction) is out of isotopic equilibrium with ¹³C-depleted calcite in cleats.     

Experimental frictional heating of coal gouge at seismic slip rates: Evidence for devolatilization and thermal pressurization of gouge fluids

High velocity (1 m/s) friction experiments on bituminous coal gouge display several earthquake-related phenomena, including devolatilization by frictional heating, gas pressurization, and slip weakening. Stage I is characterized by sample shortening and reduction in the coefficient of friction (μ) from  1 to 0.6. Stage II is characterized by high frequency ( 5 Hz) oscillations in stress and strain records and by gas emissions. Stage III is marked by rapid weakening (μ  0.1 to 0.35) and sample shortening, together with continued gas emissions. Stage IV produces stable stress records and continued weakness (μ  0.2), but without gas emission. Stage I shortening is due to compaction of the gouge and the weakening is attributed to mechanical or thermal effects. Stage II behavior is interpreted as due to coal gasification and fluctuations in fluid pressure, resulting in high frequency stick-slip type behavior. Dramatic reduction in shear stress in stage III is attributed to gas pressurization by pore collapse and corresponds to a frictional instability, analogous to nucleation of an earthquake. Microstructural observations indicate the deformation was brittle during stages I and II but ductile during stages III and IV. Time dependent finite element frictional heat models indicate the center of the samples became hot ( 900 °C) during stage II, whereas the edge of samples remained relatively cold (< 300 °C). Vitrinite reflectance of coal samples shows an increase in reflectance from  0.5 to  0.8% over the displacement interval 20–40 m (20–40 s), indicating that the reflectance responds to frictional heating on a short time scale. The energy expended per unit area in these low stress, large displacement experiments is similar to that of higher stress ( 50 MPa), short displacement ( 1 m) earthquakes ( 10⁷ J/m²).     

Cretaceous and Cenozoic cooling history across the ultrahigh pressure Tongbai–Dabie belt, central China, from apatite fission-track thermochronology

The crystalline terrane of the Tongbai–Dabie region, central China, comprising the Earth's largest ultrahigh-pressure (UHP) exposure was formed during Triassic collision between the Sino–Korean and Yangtze cratons. New apatite fission-track (AFT) data presented here from the UHP terrane, extends over a significantly greater area than reported in previous studies, and includes the (eastern) Dabie, the Hong'an (northwestern Dabie) and Tongbai regions. The new data yield ages ranging from 44 ± 3 to 142 ± 36 Ma and mean track lengths between 10 and 14.4 μm. Thermal history models based on the AFT data taken together with published ⁴⁰Ar/³⁹Ar, K–Ar, apatite and zircon (U–Th)/He and U–Pb data, exhibit a three-stage cooling pattern that is similar across the study region, commencing with an Early Cretaceous rapid cooling event, followed by a period of relative thermal stability during which rocks remained at temperatures within the AFT partial annealing zone (60–110 °C) and ending with a possible renewed phase of accelerated cooling during Pliocene to Recent time. The first cooling phase followed large-scale transtensional deformation between 140 and 110 Ma and is related to Early Cretaceous eastward tectonic escape and Pacific back arc extension. Between this phase and the subsequent slow cooling phase, a transition period from 120 to 80 Ma (to 70 to 45 Ma along the Tan–Lu fault) was characterised by a relatively low cooling rate (3–5 °C/Ma). This transition is likely related to a tectonic response associated with the mid-Cretaceous subduction of the Izanagi–Pacific plate as well as lithospheric extension and thinning in eastern Asia. The present regional AFT age pattern is therefore basically controlled by the Early Cretaceous rapid cooling event, but finally shaped through active Cenozoic faulting. Following the transition phase the subsequent slow cooling phase pattern implies a net reduction in horizontal compressional stress corresponding to increased extension rates along the continental margin due to the decrease in plate convergence. Modelling of the AFT data suggests a possible Pliocene–Recent cooling episode, which may be supported by increased rates of sedimentation observed in adjacent basins. This cooling phase may be interpreted as a response to the far-field effects of the frontal India–Eurasia collision to the west. Approximate estimates suggest that the total amount of post 120 Ma denudation across the UHP orogen ranged from 2.4 to 13.2 km for different tectonic blocks and ranged from 0.8 to 9.7 km during the Cretaceous to between 1.7 and 3.8 km during the Cenozoic.     

Holocene activity of the Scilla Fault, Southern Calabria: Insights from coastal morphological and structural investigations

Integration of on-land and offshore geomorphological and structural investigations coupled to extensive radiometric dating of co-seismically uplifted Holocene beaches allows characterization of the geometry, kinematics and seismotectonics of the Scilla Fault, which borders the eastern side of the Messina Strait in Calabria, Southern Italy. This region has been struck by destructive historical earthquakes, but knowledge of geologically-based source parameters for active faults is relatively poor, particularly for those running mostly offshore, as the Scilla Fault does. The  30 km-long normal fault may be divided into three segments of  10 km individual length, with the central and southern segments split in at least two strands. The central and northern segments are submerged, and in this area marine geophysical data indicate a youthful morphology and locally evidence for active faulting. The on-land strand of the western segment displaces marine terraces of the last interglacial (124 to 83 ka), but seismic reflection profiles suggest a full Quaternary activity. Structural data collected on bedrock faults exposed along the on-land segment provide evidence for normal slip and  NW-SE extension, which is consistent with focal mechanisms of large earthquakes and GPS velocity fields in the region. Detailed mapping of raised Holocene marine deposits exposed at the coastline straddling of the northern and central segments supplies evidence for two co-seismic displacements at  1.9 and  3.5 ka, and a possible previous event at  5 ka. Co-seismic displacements show a consistent site value and pattern of along-strike variation, suggestive of characteristic-type behaviour for the fault. The  1.5–2.0 m average co-seismic slips during these events document M_e  6.9–7.0 earthquakes with  1.6–1.7 ka recurrence time. Because hanging-wall subsidence cannot be included into slip magnitude computation, these slips reflect footwall uplift, and represent minimum average estimates. The palaeoseismological record based on the palaeo-shorelines suggests that the last rupture on the Scilla Fault during the February 6, 1783 M_w = 5.9–6.3 earthquake was at the expected time but it may have not entirely released the loaded stress since the last great event at  1.9 ka. Comparison of the estimated co-seismic extension rate based on the Holocene shoreline record with available GPS velocities indicates that the Scilla Fault accounts for at least  15–20% of the contemporary geodetic extension across the Messina Strait.     

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.     

Continuity of the North Qilian and North Qinling orogenic belts, Central Orogenic System of China: Evidence from newly discovered Paleozoic adakitic rocks

Adakitic intrusive rocks of  430–450 Ma were discovered in the North Qilian orogenic belt, the western section of the Central Orogenic System (COS) in China. These adakitic rocks were lower crust melts rather than slab melts as indicated by their crustal Ce/Pb, Nb/U, Ti/Eu, and Nd/Sm ratios and radiogenically enriched (⁸⁷Sr/⁸⁶Sr)_i of 0.7053–0.7066 and ε_Nd(t) of − 0.9 to − 1.7. While they are all characterized by low Yb (< 1.1 ppm) and Y (< 11.5 ppm) abundances with high Sr/Y (> 65) and (La/Yb)_N (> 13.7) ratios, these adakitic rocks are classified into the low-MgO–Ni–Cr and high-MgO–Ni–Cr groups. The low-MgO samples were derived from partial melting of thickened lower crust, whereas the high-MgO samples were melts from delaminated lower crust, which subsequently interacted with mantle peridotite upon ascent. Adakitic rocks from the adjacent North Qinling orogenic belt also originated from thickened lower crust at  430 Ma. In addition, the North Qilian and North Qinling orogenic belts both consist of lithological assemblages varying from subduction-accretionary complexes at south to central arc assemblages, which include adakitic rocks, then to backarc phases at north. Such a sequence reflects northward subduction of the Qilian and Qinling oceans. In these two orogenic belts, the occurrence of adakitic rocks of common origin and ages together with the similarities in tectonic configurations and lithological assemblages are considered to be the evidence for the continuity between eastern Qilian and western Qinling, forming a > 1000 km Early Paleozoic orogenic belt. In such a tectonic configuration, the Qilian and Qinling oceans that subducted from south possibly represent parts of the large “Proto-Tethyan Ocean”. This inference is supported by the coexistence of Early Paleozoic coral and trilobite specimens from Asia, America and Australia in the North Qilian orogenic belt. Post-400 Ma volcanic rocks occur in the North Qinling orogenic belt but are absent in the North Qilian orogenic belt, indicating that these two orogenic belts underwent distinct evolution history after the closure of the Proto-Tethyan Ocean ( 420 Ma).     

Mesozoic plutons of the Yidun Arc, SW China: U/Pb geochronology and Hf isotopic signature

The Yidun Arc is a Triassic volcanic arc located between the Songpan Garzê Fold Belt and the Qiangtang Block, southwest China. To constrain the age of a number of the major granitic plutons from the Yidun Arc, laser ablation ICP-MS U/Pb analysis of zircon was conducted. Hafnium isotope data was also acquired through laser-ablation multicollector ICPMS analysis of zircon, with the aim of gaining insight into the age and nature of the source region of the plutons. Three age groups have been identified from seven granite samples: Early–Middle Triassic ( 245 to 229 Ma), Late Triassic ( 219 to 216 Ma) and Cretaceous ( 105 to 95 Ma). Hafnium analysis shows the Triassic granites to have negative and variable ε_Hf values and Mesoproterozoic ( 1.6 Ga) depleted-mantle model ages, which is interpreted to reflect derivation from an isotopically heterogeneous, largely crustal source. The Cretaceous granite shows higher and less variable ε_Hf values and slightly younger model ages ( 1.3 Ga), and is interpreted to be derived from melting of a more homogeneous crustal source. A depleted-mantle model age of  1.5 Ga is calculated from the pooled Triassic and Cretaceous samples. The source region for these magmas may be tentatively correlated with Mesoproterozoic material of the Yangtze Craton, which has been suggested to underlie the Yidun Arc; however, further work is necessary to demonstrate this suggestion.     

Brittle–ductile–brittle deformation during cooling of tonalite (Adamello, Southern Italian Alps)

Late- to post-magmatic deformation in slightly diachronous contiguous intrusions of the north-western Adamello batholith (Southern Alps, Italy) is recorded as, from oldest to youngest: (i) joints, (ii) solid-state ductile shear zones, (iii) faults associated with epidote-K-feldspar veins and (iv) zeolite veins and faults. Structures (ii) to (iv) are localized on the pervasive precursory network of joints (i), which developed during the earliest stages of pluton cooling. High temperature ( 500 °C), ductile overprinting of joints produced lineations, defined by aligned biotite and hornblende, on the joint surfaces and highly localized mylonites. The main phase of faulting, producing cataclasites and pseudotachylytes, occurred at  250 °C and was associated with extensive fluid infiltration. Cataclasites and pseudotachylytes are clustered along different E–W-striking dextral strike-slip fault zones correlated with the activity of the Tonale fault, a major tectonic structure that bounds the Adamello batholith to the north. Ductile deformation and cataclastic/veining episodes occurred at P = 0.25–0.3 GPa during rapid cooling of the batholith to the ambient temperatures ( 250 °C) that preceded the exhumation of the batholith. Timing of the sequence of deformation can be constrained by ³⁹Ar–⁴⁰Ar ages of  30 Ma on pseudotachylytes and various existing mineral ages. In the whole composite Adamello batholith, multiple magma pulses were intruded over the time span 42–30 Ma and each intrusive body shows the same ductile-to-brittle structural sequence localized on the early joint sets. This deformation sequence of the Adamello might be typical of intrusions undergoing cooling at depths close to the brittle–ductile transition.     

Age estimates of coastal terraces in the Andaman and Nicobar Islands and their tectonic implications

The great Indian Ocean earthquake of December 26, 2004 caused significant vertical changes in its rupture zone. About 800 km of the rupture is along the Andaman and Nicobar Islands, which forms the outer arc ridge of the subduction zone. Coseismic deformation along the exposed land could be observed as uplift/subsidence. Here we analyze the morphological features along the coast of the Andaman and Nicobar Islands, in an effort to reconstruct the past tectonics, taking cues from the coseismic effects. We obtained radiocarbon dates from coastal terraces of the island belt and used them to compute uplift rates, which vary from 1.33 mm yr^− 1 in the Little Andaman to 2.80 mm yr^− 1 in South Andaman and 2.45 mm yr^− 1 in the North Andaman. Our radiocarbon dates converge on  600 yr and  1000 yr old coastal uplifts, which we attribute to the level changes due to two major previous subduction earthquakes in the region.     

Finite-fault slip models for the 15 April 1979 (Mw 7.1) Montenegro earthquake and its strongest aftershock of 24 May 1979 (Mw 6.2)

We revisit the April 1979 Montenegro earthquake sequence to invert for finite-fault slip models for the mainshock of 15 April 1979 (M_w 7.1) and of the strongest aftershock of 24 May 1979 (M_w 6.2) using P, SH and SV waveforms, retrieved from IRIS data center. We also used body waveform modelling inversion to confirm the focal mechanism of the mainshock as a pure thrust mechanism and rule out the existence of considerable strike slip component in the motion. The mainshock occurred along a shallow (depth 7 km), low angle (14°) thrust fault, parallel to the coastline and dipping to the NE. Our preferred slip distribution model for the mainshock indicates that rupture initiated from SE and propagated towards NW, with a speed of 2.0 km/s. Moment was released in a main slip patch, confined in an area of L  50 km × W  23 km. The maximum slip ( 2.7 m) occurred  30 km to the NW of the hypocenter (location of rupture initiation). The average slip is 49 cm and the total moment release over the fault is 4.38e19 Nm. The slip model adequately fits the distribution of the M_w ≥ 4.3 aftershocks, as most of them are located in the regions of the fault plane that did not slip during the mainshock. The 24 May 1979 (M_w 6.2) strongest aftershock occurred  40 km NW of the mainshock. Our preferred slip model for this event showed a characteristic two-lobe pattern, where each lobe is  7.5 × 7.5 km². Rupture initiated in the NW lobe, where the slip obtained its maximum value of 45 cm, very close to the hypocenter, and propagated towards the south-eastern lobe where it reached another maximum value — for this lobe — of 30 cm, approximately 10 km away from the hypocenter. To indirectly validate our slip models we produced synthetic PGV maps (Shake maps) and we compared our predictions with observations of ground shaking from strong motion records. All comparisons were made for rock soil conditions and in general our slip models adequately fit the observations especially at the closest stations where the shaking was considerably stronger. Through the search of the parameter space for our inversions we obtained an optimum location for the mainshock at 42.04°N and 19.21° E and we also observed that better fit to the observations was obtained when the fault was modeled as a blind thrust fault.     

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.     

SHRIMP zircon U–Pb geochronological and whole-rock geochemical evidence for an early Neoproterozoic Sibaoan magmatic arc along the southeastern margin of the Yangtze Block

It has been generally accepted that the South China Block was formed through amalgamation of the Yangtze and Cathaysia Blocks during the Proterozoic Sibaoan orogenesis, but the timing and kinematics of the Sibao orogeny are still not well constrained. We report here SHRIMP U–Pb zircon geochronological and geochemical data for the Taohong and Xiqiu tonalite–granodiorite stocks from northeastern Zhejiang, southeastern margin of the Yangtze Block. Our data demonstrate that these rocks, dated at 913 ± 15 Ma and 905 ± 14 Ma, are typical amphibole-rich calc-alkaline granitoids formed in an active continental margin. Combined with previously reported isotopic dates for the  1.0 Ga ophiolites and  0.97 Ga adakitic rocks from northeastern Jiangxi, the timing of the Sibao orogenesis is thus believed to be between  1.0 and  0.9 Ga in its eastern segment. It is noted that the Sibao orogeny in South China is in general contemporaneous with some other early Neoproterozoic (1.0–0.9 Ga) orogenic belts such as the Eastern Ghats Belt of India and the Rayner Province in East Antarctica, indicating that the assembly of Rodinia was not finally completed until  0.9 Ga.