Normal Crustal Thickness Research Articles

Tectono-metamorphic interaction of upper mantle peridotites and lower crustal units during continental rifting in the western Betic Cordillera

Recent geological mapping conducted in the Ronda Peridotites (Betic Cordillera, S Spain) has revealed a systematic field correlation between lower crustal metamorphic units and different tectono-metamorphic domains of the ultramafic massif. Mylonitic and highly tectonized Spl ± Grt peridotites (i.e., Grt-Spl mylonite and Spl tectonite domains), which are considered to be derived from a thick continental lithosphere, are in contact with garnet-bearing gneisses (i.e., kinzigites) of the Jubrique unit along a narrow but continuous mylonitic shear zone. Phase equilibrium calculations indicate that the metamorphic rocks of the Jubrique unit are consistent with an initial continental setting characterized by normal crustal thicknesses, which underwent two melting events. The first melting occurred at 0.9–1.0 GPa / 770–800 °C and resulted in 13 %melt, while the second one took place at shallower crustal conditions (0.4–0.5 GPa and 710–765 °C) and led to more restricted melt production (2–3 %melt). In contrast, the Spl ± Pl peridotites (Pl-tectonite domain), which are stable only at shallowest mantle levels within a highly extended continental lithosphere, are consistently found exposed in contact with heterogeneous granites and migmatites of the Guadaiza unit. According to new thermodynamic modeling, the Guadaiza metamorphic rocks record a single melting event characterized by a theoretical melt production of 6 to 11 % at the base of a very thin continental crust (ca. 0.3 GPa and 675–710 °C). This process was likely facilitated by influx of external water, necessary to generate high melt fractions observed in the diatexites. The systematic correlation observed between crustal metamorphic units, consistently overlaying the mantle rocks, and specific ultramafic domains of the Ronda massif, suggests that their juxtaposition primarily resulted from the severe extension of the continental lithosphere.U-Pb radiometric dating of zircons from gneisses, migmatites, and heterogeneous granites in the middle crustal rocks of the Guadaiza unit indicates that extensional processes, crustal anatexis, and melt stagnation occurred at around 280 Ma. Comparison of these new radiometric ages with previous results from the Jubrique unit suggests that a Permian high-temperature / low- to medium-pressure event uniformly affected the crustal units over the Ronda Peridotites. This event coincided with the formation of characteristic ultramafic mineral assemblages within the Ronda massif and provides evidence for the interaction between upper mantle rocks and lower- to mid-crustal metamorphic rocks during this period.

Petrology and Geochemistry of an Unusual Granulite Facies Xenolith of the Late Oligocene Post-Obduction Koum Granodiorite (New Caledonia, Southwest Pacific): Geodynamic Inferences

Pressure–temperature estimates of a xenolith found within a post-obduction granodiorite in southern New Caledonia provide evidence for subcrustal, granulite facies, peak crystallisation conditions (ca. 850 °C—8.5 ± 1.0 kbar), followed by isobaric cooling to 700 °C, and final decompression with partial rehydration at ca. 650 °C—3.5 kbar. The xenolith, dated at 24.7 Ma (U-Pb zircon), i.e., the same age as the granodiorite host rock, has low SiO2 (35.5 wt%) and high Al2O3 (33.2 wt%) contents, suggesting that it is the restite of a previous melting episode, while the elevated Ca (Ba and Sr) contents suggest mantle metasomatism. Although the concentrations of Rb, K, Ca, Ba, and Sr have been strongly modified, some geochemical (REE patterns and some “immobile” trace element ratios) and isotopic (Sr and Nd isotopic ratios, U-Pb zircon age) characteristics of the granulite facies xenolith are similar to those of the xenoliths found in other Late Oligocene intrusions in southern New Caledonia; therefore, this rock is interpreted to be related to an early magmatic episode. The rock protolith was emplaced and equilibrated at the base of the crust where it underwent ductile deformation. Younger ascending magma picked it up and they eventually crystallised together at a shallow crustal level, near the tectonic sole of the ophiolite. The recrystallisation and ductile deformation at ~8.5 kbar suggest that a rheological discontinuity existed at about 25–28 km, probably representing the Moho. It is concluded that a continental crust of normal thickness must have existed beneath New Caledonia at about 24 Ma, i.e., 10 Ma after obduction.

Large-Scale Cretaceous Adakitic Magmatism Induced by Water-Fluxed Melting of Continental Crust during the North China Craton Destruction

ABSTRACT The mechanism behind the destabilization of the North China Craton (NCC) remains a contentious topic among researchers. Large-scale Cretaceous adakitic magmatism in the NCC offers insights into the decratonization process. This study focuses on the Huashan and Laoniushan plutons located in the Lesser Qinling on the southern margin of the NCC and compiles published data for coeval adakitic rocks to investigate the role of water in adakitic rock petrogenesis during the peak destruction of NCC. Both the Huashan and Laoniushan plutons exhibit adakitic signatures, including high Sr (193–1080 ppm), low Yb (<14.8 ppm) and Y (<1.24 ppm) concentrations, as well as high Sr/Y (18–100) and La/Yb (24–58) ratios. The zircon Hf–O isotope compositions suggest that the primary source for the Huashan and Laoniushan plutons is the mafic lower crust of NCC. Nevertheless, there are significant differences in trace element characteristics between the two plutons. Specifically, the Huashan pluton displays higher Na2O/K2O ratios, lower levels of Rb, Rb/Sr, Nb, Ta content, and a weak Eu anomaly in comparison to the Laoniushan pluton. These variations in geochemical attributes cannot be accounted for by mechanisms like mantle-derived magma mixing, crustal contamination, or fractional crystallization processes. Instead, these disparities are attributed to distinct modes of crustal anatexis, involving both water-fluxed and dehydration melting. Subsequently, we conducted thermodynamic simulations of the melting process of mafic lower crust under different pressure (0.5–1.5 GPa) and water content conditions (1–3 wt.%). The simulation results suggest that the Huashan pluton is most likely formed through water-fluxed melting in a scenario with normal crustal thickness (1 GPa). On the other hand, the Laoniushan pluton might have originated from dehydration melting under normal crustal thickness and pressure conditions. Notably, high pressure (>1.5 GPa) is not necessary for the formation of intracontinental adakitic rocks. The release of water from metasomatized lithospheric mantle and subsequent hydration of the lower continental crust triggers extensive adakitic magmatism in the NCC. These findings emphasize the significance of deep water cycling in understanding large-scale magmatic events and illuminate the decratonization mechanism.

Deep crustal structure across the Challenger Deep: Tectonic deformation and strongly serpentinized layer

The southern Mariana subduction zone features the Earth’s deepest trench, named the Challenger Deep, and tectonic deformation is obvious in the overriding and subducting plates. We conducted a 230 km-long wide-angle reflection/refraction survey across the Challenger Deep. Based on ocean bottom seismometer (OBS) data, we image the velocity structure using forward and inverse modeling. The velocity model shows that the Pacific plate entering the subduction zone has normal crustal thickness (6.5–7.2 km) but lower velocity than mature Pacific oceanic crust. PnP seismic phases with large offsets and strong seismic reflectivity are observed at eight OBS seismic record sections, which constrain the thickness (4.0–6.5 km) and velocity (7.1–7.5 km/s) of the upper mantle low-velocity layer. We suggest that the observed velocity reduction in the crust and upper mantle of the subducting plate is caused by bending-related normal faults, which not only fracture the crust but also provide pathways for water infiltration and serpentinization of the dry mantle. The upper mantle reflector constrained by PnP seismic phases possibly indicates a rheological and fault-slip interface derived from rapid variations in the mechanical strength of peridotite. The inner trench slope (ITS) near the Challenger Deep is characterized by indented topography, where the upper crust is obviously thinned and the lower crust is uplifted; the isovelocity contour of 6.5 km/s is 1.5 km below the seafloor. This may be caused by the difference in the degree of subducting plate rollback on either side of the western boundary of the diffuse deformation zone.

The spreading rate dependence of the distribution of axial magma lenses along mid-ocean ridges

Seismically imaged axial melt lenses (AMLs) are seen almost everywhere along the axis of fast-spreading ridges but at only a few localized segment centers on slow-spreading ridges. Standard models assuming that AMLs form when melt percolating upward pools where freezing produces an impermeable cap do not explain this fundamental observation. To tackle this long-standing problem, we combine a crustal density model and a thermal model with a recent mechanical model for sill formation. The mechanical model predicts that AMLs form below the axial lithosphere but only if the average density of the axial brittle lithosphere is not greater than the magma density. For standard thermal models, crustal density structures inferred from seismic velocity data and normal crustal thicknesses, AMLs are found to be stable along all of a ridge segment for spreading rates greater than about 50 mm/y. To explain slow-spreading observations, we assume that a share of the melt produced by the mantle upwelling all along a segment is focused to the segment center. Some of this melt partially crystallizes, releasing latent heat, before the evolved magma flows along the axis to build the crust away from the segment center. This "extra" heat, beyond what is supplied by the magma that builds the crust near the segment center, results in the lithosphere thin enough for stable melt lenses at the segment center. Our results are consistent with observations and offer a quantitative explanation of the marked difference in the distribution of AMLs along fast- versus slow-spreading centers.

Early Miocene (21–23 Ma) high Sr/Y volcanic field in the Gangdese batholith, southern Tibet

A detailed petrographic, geochemical and geochronological study reveals a suite of Miocene (~21–23 Ma) high Sr/Y and La/Yb volcanic rocks of dacite compositions and adjacent Late Cretaceous tonalite and granodiorite (~78–80 Ma) in the Qiewa of the southern Gangdese batholith. The analyses results show that (1) the dacites are characterized by Na-rich, peraluminous, high Sr/Y and La/Yb ratios, showing adakitic affinity, the granitic wall-rocks (tonalite and granodiorite) and enclave have the Na-rich and metaluminous features; and (2) the dacites have relatively radiogenic Sr ( 86 Sr/ 87 Sr(t) = 0.7049–0.7061), unradiogenic Nd (ε Nd (t) = −3.8 - +0.1), and positive Hf (ε Hf (t) = +3.0 − +9.5) isotope compositions, while its associated plutonic rocks have relatively lower Sr ( 86 Sr/ 87 Sr(t) = 0.7035–0.7039), but higher Nd (ε Nd (t) = +3.3 − +4.9) and Hf (ε Hf (t) = +11.1 − +14.7) isotope ratios. Data presented in this study demonstrates that Early Miocene dacites are generated by partial melting of thickened (~70 km) juvenile lower crust, whereas the Late Cretaceous granitic wall-rocks (tonalite and granodiorite) and enclave are derived from partial melting of juvenile crust with a normal crustal thickness (~40–44 km). Deposition of ~23 Ma dacitic volcanic rocks directly upon the ~80 Ma plutonic rocks suggests that the Gangdese batholith experienced an episode of rapid exhumation before ~23 Ma, possibly triggered by the delamination of thickened continental lithosphere. • The Qiewa dacites and the underlying plutonic rocks are formed at ~21–23 Ma and ~ 78–80 Ma, respectively. • The high Sr/Y dacites are derived from partial melting of a thickened (~70 km) enriched lower crust. • The plutonic rocks result from partial melting of a normal thickness (~40–44 km) of juvenile curst. • The Gangdese batholith experienced an episode of rapid exhumation before ~23 Ma.

Reworking of continental crust on northeastern North China Craton: Evidence from geochronology and geochemistry of Early Cretaceous granitic rocks

This paper reports new whole–rock major– and trace–element and zircon UPb and HfO isotopic data for Early Cretaceous granitic rocks on Liaodong Peninsula, NE China, with the aim of constraining continental reworking in the northeastern North China Craton. On the basis of SiO2, MgO, and Sr contents and Sr/Y values, the rocks can be divided into three groups. The low–Mg adakitic rocks show zircon UPb ages of (134–120 Ma), and high SiO2 (67.11–71.14 wt%) and low MgO (0.53–0.62 wt%) contents, and high Sr (531–1108 ppm) contents, Sr/Y (29.3–132) ratios, with enriched Sr–Nd–Hf–O isotopic compositions, indicating an origin of thickened lower mafic crust. The high–Mg adakitic rocks show ages of (126–114 Ma) and are characterized by low SiO2 (53.11–63.88 wt%) and high MgO (2.07–8.93 wt%) contents, and high Sr (391–1041 ppm) contents, Sr/Y (23.9–64.1) ratios, with relatively depleted Sr–Nd–Hf–O isotopic compositions, suggesting that they were derived from partial melting of delaminated lower mafic crust. The normal granitic rocks show ages of (127–121 Ma) and high SiO2 (71.65–77.22 wt%) and low MgO (0.10–0.51 wt%) and Sr (12.1–309 ppm) contents, Sr/Y (0.89–24.5) ratios, with relatively depleted Sr–Nd–Hf–O isotopic compositions, suggesting an origin from lower mafic crust with normal crustal thickness. These constraints, combined with the regional occurrence of A–type granites, bimodal igneous rocks, and metamorphic core complexes, as well as the regional distribution of Mesozoic granitic rocks displaying a younging trend from southeast to northwest for the Jurassic rocks but a reverse trend for the Cretaceous rocks, recorded the roll–back of the Paleo–Pacific plate towards the Eurasian continental margin.

Heterogeneous mantle associated with asthenosphere and Indian slab metasomatism: Constraints on fertilization of porphyry Cu mineralization in Tibetan orogen

Previous models related to ultrapotassic mantle and post-collisional porphyry Cu mineralization have been challenged by the newly identified ultrapotassic lamprophyres (14.31 ± 0.61 Ma), diorite porphyries (14.82 ± 0.21 Ma) and high Sr/Y monzogranite porphyries (14.19 ± 0.23 Ma) from Dabu deposit in the Gangdese, Tibet. We found that the continental crust and lithospheric mantle beneath Gangdese were highly heterogeneous. The western Gangdese (80°E to 89°E) are mainly characterized by enriched lithospheric mantle with εNd(t) ≤ -10 and 87Sr/86Sr ≥ 0.71, and relatively normal crustal thickness and maturation (∼55–65 km), whereas eastern Gangdese (89°E to 95°E) are dominated by relatively depleted lithospheric mantle with εNd(t) ≥ -10 and 87Sr/86Sr ≤ 0.71, and much more thickened and mature crust (∼62–75 km). Based on three-component modeling, the geodynamic mechanism in the western Gangdese is subducted Indian slab metasomatism induced by its roll-back and subsequent break-off, whereas the eastern Gangdese is upwelling asthenosphere metasomatism resulted from convective delamination of thickened lithospheric mantle root. The lamprophyres at Dabu with εNd(t) values of −11.34 to −10.59 represent the very rare and well-preserved ancient lithospheric mantle without significant metasomatism and modification. Instead of previous model that focused on connection with Indian continental plate, we prefer to the generation of porphyry Cu deposits in Tibetan orogen were optimal alignments and combinations of multiple magmatic and dynamic processes, involving metasomatism of subducted Neo-Tethyan slab, metamorphic dehydration and thickening of lower crust, long-lived melting-assimilation-storage-homogenization (MASH) hot zone in the lower crust, and further fertilization of multiple magma chambers (∼30 km, ∼10 km, 1–6 km) in the upper crust.

Thermo-mechanical destruction of Archean cratonic roots: Insights from the Mesozoic Laiyuan granitoid complex, North China Craton

Igneous complexes with diverse magmatic suites provide essential targets for investigating magma differentiation processes and crust-mantle interaction. The North China Craton (NCC) is a crucial window to gain insights into craton destruction through voluminous magmatism during the Mesozoic involving extensive crust-mantle interaction. Here we investigate the granitoid suite from the Laiyuan complex in the central NCC to constrain the magmatic evolution and petrogenetic linkages and to gain insights into the process of crust-mantle interaction related to craton destruction. The Laiyuan granitoid complex comprises syenogranite, monzogranite, quartz monzonite and monzonite with mafic magmatic enclaves (MMEs). The granitoids were formed during the Early Cretaceous (137–128 Ma) with εHf(t) values and TDMC ages ranging from −21.8 to −16.8 and 2564 to 2255 Ma, respectively. The compositional variations of the different rock types suggest distinct magmatic evolution and petrogenesis. Geochemical and isotopic data indicate that the enriched lithospheric mantle-derived mafic magmas underwent early-stage plagioclase-dominated and late-stage hornblende-dominated fractional crystallization to form the monzonites and quartz monzonites. Geochemical modeling results suggest that the magma of the MMEs originated through mixing between lithospheric mantle-derived mafic magma and crust-derived felsic magma with post-injection hybridization to generate MMEs with variable compositions. The monzogranites are classified as high-K calc-alkaline I-type granites with higher SiO2 contents (69–71 wt%) and lower Mg# (41–44) than intermediate rocks. They were generated by partial melting of the basaltic lower crust, with the addition of mafic magma from an enriched mantle. The highly fractionated syenogranites with lower Mg# (18–27) were evolved from the monzogranitic magma through plagioclase-dominated crystal removal. The adakitic affinities of the monzonites and monzogranites were generated by crystallization of mantle-derived magmas and partial melting of continental crust of normal thickness (~40 km), respectively. The bottom-up crust-mantle interaction accounts for the compositional diversity of the Laiyuan granitoids, and suggests that thermo-mechanical destruction played a dominant role in the destruction of the central NCC as compared with the continental delamination process in the eastern NCC.

Volcanic rocks of the Elashan Formation in the Dulan-Xiangride Basin, East Kunlun Orogenic Belt, NW China: Petrogenesis and implications for Late Triassic geodynamic evolution

ABSTRACT The volcanic rocks of the Elashan Formation provide important evidence for the Late Triassic post-collisional magmatism in the East Kunlun Orogenic Belt. This study focused on the geochronology, geochemistry, and petrogenesis of volcanic rocks of the Elashan Formation in the Dulan–Xiangride Basin. The 227.9 ± 0.4 Ma dacite exhibits calc-alkaline I-type characteristics, with low Sr/Y and (La/Yb)N ratios and prominent negative Eu anomalies. We suggest that the dacite was derived from the partial melting of lower crust at normal crustal thickness. The crystalline tuff (225.0 ± 0.4 Ma) has negative εHf(t) values, implying that it was likely formed by the partial melting of the ancient crust. The rhyolites (221.3 ± 1.1 and 221.9 ± 0.6 Ma) are peraluminous and calc-alkaline to high-K calc-alkaline. The high SiO2 and Na2O + K2O contents, and high K2O/Na2O and Ga/Al ratios of these rhyolites have an affinity with A-type granite. The rhyolites were formed by the partial melting of Late Paleoproterozoic–Mesoproterozoic lower crust induced by mantle-derived mafic magma underplating. The andesite and basaltic andesites formed at 221.1–217.9 Ma. The zircon Hf isotopic characteristics show that the andesite was likely derived from the partial melting of the Late Paleoproterozoic mafic lower crust. The combination of our new data and previously obtained geological data suggests that the East Kunlun Orogenic Belt underwent three magmatic activities corresponding to three tectono-magmatic events (slab breakoff, the delamination of unstable lower lithospheric mantle, and the intense asthenosphere upwelling with further delamination and continuous crustal thinning) during the Late Triassic.

Mafic microgranular enclaves from the Tieshan batholith, eastern China: Implications for petrogenesis of magma associated with Fe-Cu skarn mineralization

Mineralogy, geochemistry, and geochronology of the mafic microgranular enclaves (MMEs) and their host diorite from the Tieshan batholith, Edong district, eastern China, were studied to constrain their petrogenesis and relationship with Fe-Cu skarn mineralization. The MMEs are diorite in composition and characterized by mixed apatite morphologies (elongated thin needles and stubby prismatic form) and bladed biotite, as well as clots of host diorite and megacrystic alkali-feldspar xenocrysts. The MMEs have similar zircon U-Pb age as their host diorite (~139 Ma). Most of the major and trace elements of the MMEs show a linear trend on Harker diagrams and plot along the low-silica trend of the host diorite. They are also characterized by enrichment of LREE and large ion lithophile elements (LILE), but are depleted in high field strength elements (HFSE). The MMEs display similar initial Sr isotopes and εNd(t), and slightly lower εHf(t) compared to that of host diorite. These results indicate that the Tieshan batholith formed via partial melting of an enriched lithospheric mantle, which is believed to be in equilibrium with a garnet-bearing residue (>50 km), followed by contamination of low crustal materials and fractional crystallization, whereas the MMEs appear to be the products of magma mixing. The presence of Cu- and Ni-bearing minerals in the MMEs probably indicates upper mantle or lower crust accumulation of sulfide which may have played a key role in controlling the formation of the parental magma associated with the skarn-porphyry Cu deposit (148 to 135 Ma) from the Middle-Lower Yangtze River metallogenic belt (MLYRMB). In contrast, the thinner, normal crust (~30 km) at ~ 130 Ma, believed to be in equilibrium with a plagioclase-rich residue, was either unsuitable for producing Cu-bearing accumulations or failed to transfer the deep accumulations to shallower levels to form Cu-bearing skarn systems, resulting in Fe-only mineralization. This may imply that skarn-porphyry Cu deposits prefer to form under > 50 km crustal settings, whereas Fe skarns may preferentially occur under normal (~30 km) crustal thickness.

Petrogenesis of the Late Oligocene Takht batholith, Southeastern Iran: Implications for the Diachronous Nature of the Arabia–Eurasia Collision

Zircon U-Pb age, mineral compositional, elemental and Sr-Nd isotopic data are documented for intermediate to felsic rocks in the Takht batholith, a Late Oligocene igneous complex in the southeastern segment of the Urumieh-Dokhtar belt, Iran, to investigate magma genesis in the context of regional tectonics. A large part of the magmatic belt formed by northward subduction of the Neotethys before the Arabia-Eurasia continental collision. Zircon U-Pb age data indicate the batholith crystallized at ~25 Ma, an age consistent with previous results. Geochemical data indicate that the rocks share features typical of calc-alkaline magmas and I-type granitoids. The least evolved magma inferred from the data has basaltic andesite composition, consistent with either of the following origins: (i) a partial melt of the mantle followed by differentiation, or (ii) a partial melt of the lower crust. With increasing SiO2, (87Sr/86Sr)25 Ma increases from 0.7053 to 0.7073, and εNd25 Ma decreases from –0.3 to –2.9, consistent with increasing effects of assimilation–fractional crystallization, although processes such as magma mixing and melting of heterogeneous source rocks cannot be entirely ruled out. Aluminum-in-hornblende thermometry indicates that the batholith might have emplaced at ~6.5 km or shallower paleo-depths. Moho depth proxies Sr/Y and (La/Yb)N yield crustal thickness of ~20–30 km, showing no evidence for magma processing within thick crust. Overall, our results indicate that the Takht batholith is a Cordilleran-type batholith formed beneath a continental or transitional arc of normal crustal thickness. This is consistent with the notion that Arabia and Eurasia have collided in a diachronous manner, propagating from the northwest to the southeast since the Late Eocene–Earlty Oligocene.

Effects of Oceanic Crustal Thickness on Intermediate Depth Seismicity

The occurrence of intermediate depth seismicity (70 – 300 km) is commonly attributed to the dehydration of hydrous phases within the downgoing oceanic plate. While some water is incorporated into the oceanic crust at formation, a significant amount of water is introduced into the plate immediately before subduction along outer-rise faults. These faults have been shown to extend to depths of over 30 km and can channel water to depths of 20 km or more beneath the seafloor. However, the amount of water introduced into the oceanic mantle lithosphere, and the role of that water in the formation of intermediate depth seismicity, has been the topic of ongoing research. Here we compile evidence from areas where the subducted oceanic crust is likely thicker than the penetration depth of water into the downgoing plate. These regions comprise aseismic plateaus and ridges (hot spot tracks) that can be compared directly to adjacent segments of the oceanic plate where oceanic crust of normal thickness is subducted. Regions with thick oceanic crust show little to no seismicity at intermediate depths, whereas adjacent regions with normal oceanic crust (~6-8 km thick) have significant seismicity at similar depths and distances from the trench. We hypothesize that intermediate depth earthquakes observed in regions with thinner oceanic crust are caused by mantle dehydration reactions that are not possible in regions where the oceanic mantle was never hydrated because the thickness of the oceanic crust exceeded the penetration depth of water into the plate. We compare our observations to phase diagrams of hydrous basalt and depleted peridotite to determine pressures and temperatures that would be consistent with our observations. These can provide valuable constraints, not only on the degree of hydration and dehydration in the downgoing plate, but also as ground-truth for thermal models of these regions, all of which have complex, three-dimensional, time-variant subduction geometries and thermal histories.

Geology of the Ionian Basin and Margins: A Key to the East Mediterranean Geodynamics

AbstractWe revisit the nature, structure, and evolution of the Ionian Basin and its surrounding passive margins (Apulia, Eastern Sicily/Malta, and Cyrenaica margins). Relying on geological observations (wells, dredges, and dives) and seismic calibrations from the surrounding platforms and escarpments, we present age correlations for the deepest sedimentary sequences of the Ionian Basin. Two‐ship deep refraction seismic data combined with reprocessed reflection seismic data enable us to identify, characterize, and map these thick sedimentary sequences and to present a consistent seismic stratigraphy across the basin. At a larger scale, we present new geological transects illustrating the tectonostratigraphic relationships between the Ionian Basin and its surrounding rifted margins. On this basis, we suggest that a Late Triassic‐Early Jurassic rifting preceded the late Early Jurassic to Middle Jurassic formation of the Ionian Basin. The combination of geophysical and geological arguments suggests that the entire deep basin is floored with oceanic crust of normal thickness, but with high seismic velocity in the upper part. Upscaling our results in the framework of the eastern Mediterranean, we propose that the Ionian Basin represents the remnant of a short‐lived oceanic basin resulting from the interaction between two propagating oceans: the Central Atlantic and the Neo‐Tethys.

Generation of continental adakitic rocks: Crystallization modeling with variable bulk partition coefficients

The geochemical signatures (i.e., high Sr/Y and La/Yb ratios) of adakitic rocks in continental settings, which are derived from the continental lower crust rather than from subducted slabs, may reflect high-pressure melting in the lower crust or may be inherited from their sources. The North China Craton (NCC) is an ideal place for investigation of this type of adakites due to its ubiquitous distribution. As an example, we explore the petrogenesis of the Jurassic (~163Ma) adakitic rocks in western Liaoning, in the NE part of the NCC, using elemental and Sr–Nd isotopic analysis and crystallization modeling based on Rhyolite-MELTS. The modeling demonstrates that adakitic signatures can be generated by fractional crystallization of magmas within crust of normal thickness (i.e., 33km). Partial-melting modeling based on the composition of the lower continental crust shows that only the adakitic rocks from orogenic belts require a thickened crust (i.e., 45km). We suggest that continental adakitic rocks are not necessarily linked to high-pressure processes and their use as an indicator of thickened/delaminated continental crust should be regarded with caution.

Rheology of the middle crust under tectonic extension: Insights from the Jinzhou detachment fault zone of the Liaonan metamorphic core complex, eastern North China Craton

Although exhumation of metamorphic core complexes (MCCs) have been traditionally attributed to collapse of orogenically overthickened crust, recent studies reveal that they can also result from extension of lithosphere with normal crustal thickness. For example, there is an ongoing debate about the mechanisms responsible for exhumation of the Liaonan MCC, which occurred during late Mesozoic lithospheric thinning of the North China Craton. This paper attempts to present rheological constraints on middle to upper crustal detachment faulting during the exhumation of the Liaonan MCC and, therefore, on the genesis of the MCC based on a comprehensive study of the microstructural and fabric characteristics of tectonites from the Jinzhou master detachment fault of the Liaonan MCC.The Jinzhou detachment fault zone comprises a thick sequence of fault rocks of middle to shallow crustal depths. Three types of mylonitic rocks characterize middle crustal deformation along the detachment fault during early Cretaceous lithospheric extension. Microstructural and fabric studies reveal that these fault rocks were formed via different mechanisms of crystal plastic deformation and dynamic recrystallization at temperatures from ca. 300 to 650°C. The flow stresses at different crustal depths were calculated using classical paleopiezometers. This study suggests that a pre-heated crust was responsible for the low flow stresses during the Jinzhou detachment faulting, which is somehow different from, e.g., the Whipple Mountain detachment faulting in the North American Cordillera. Based on inferences about the properties of middle to upper crustal flow associated with the Jinzhou detachment faulting, it is suggested that the Liaonan metamorphic core complex was formed by tectonic extension of cratonic lithosphere with a normal crustal thickness, instead of being a typical Cordilleran-type core complex that occurred in a setting with overthickened crust.

Geochemical and geochronological characteristics of Late Cretaceous to Early Paleocene granitoids in the Tengchong Block, Southwestern China: Implications for crustal anatexis and thickness variations along the eastern Neo-Tethys subduction zone

The Tengchong Block of Southwestern China is key to tracing the eastward subduction of Neo-Tethys and collision between Indian and Asian continents. The block contains a magmatic belt that represents the southeastward continuation of the Gangdese belt, produced by the eastward subduction of eastern Neo-Tethyan oceanic lithosphere. In this paper we present geochemical and geochronological data of Late Cretaceous to Early Paleocene (~64Ma) granitic rocks of the Guyong and Husa batholiths in the Tengchong Block. These can be subdivided into high-silica peraluminous granites and low-silica metaluminous granodiorites, and all belong to the high-K calc-alkaline series, are enriched in LILE, and depleted in HFSE. The Guyong granitoids have high initial Sr ratios of 0.706511–0.711753, negative εNd(t) values of −9.2 to −11.6, two-stage model ages of 1.39–1.55Ga, and Pb isotopic compositions that indicate a crustal affinity. The Husa granodiorites also have high initial Sr ratios of 0.716496, negative εNd(t) value of −16.5, two-stage model age of 1.89Ga, variable εHf(t) values of 3.4 to −18.1 and Pb isotopic compositions similar to lower crustal values. These geochemical and isotopic data indicate that the Guyong granitoids were likely derived from partial melting of ancient crustal metapelite or mixed pelite-greywacke sources, while the Husa granodiorites were derived from the partial melting of lower crustal mixed sources involving metasedimentary and metaigneous rocks. To understand the thermal state and architecture of the Late Cretaceous to Early Paleocene magmatic arc crust, the crust-derived intermediate to acidic igneous rocks of the southern-central Lhasa and Tengchong blocks and eastern Himalayan syntaxis are compared. We infer that partial melting of crust occurred at great depth in the southern Lhasa Block, intermediate depths in the eastern Himalayan syntaxis, and shallow depths in the central Lhasa and Tengchong Block. Sr/Y ratios indicate that the Tengchong Block was characterized by thin crust, while the eastern Himalayan syntaxis and northernmost part of southern Lhasa Block contained crust of normal thickness, and the southernmost part of the southern Lhasa Block contained thickened crust.

The seismic Moho structure of Shatsky Rise oceanic plateau, northwest Pacific Ocean

Abstract Oceanic plateaus are large igneous provinces formed by extraordinary eruptions that create thick oceanic crust, whose structure is poorly known owing to the lack of deep-penetration seismic data. Multichannel seismic (MCS) reflection and wide-angle refraction data allow us to show Moho structure beneath a large part of the Shatsky Rise oceanic plateau in the northwest Pacific Ocean. Moho reflectors in the two data sets can be connected to trace the interface from the adjacent abyssal plain across much of the interior. The reflectors display varied character in continuity, shape, and amplitude, similar to characteristics reported in other locations. Beneath normal crust, the Moho is observed at ∼13 km depth (∼7 km below the seafloor) in MCS data and disappears at ∼20 km depth (∼17 km below the seafloor) beneath the high plateau. Moho at the distal flanks dips downward towards the center with slopes of ∼0.5°–1°, increasing to 3°–5° at the middle flanks. Seismic Moho topography is consistent with Airy isostasy, confirming this widely-applied assumption. Data from this study show that crustal thickness between the massifs in the interior of the plateau is nearly twice normal crustal thickness, despite the fact that this crust records apparently normal seafloor spreading magnetic lineations. The Moho model allows improved estimates of plateau area ( 5.33 × 10 5 km 2 ) and volume ( 6.90 × 10 6 km 3 ), the latter assuming that the entire crust was formed by Shatsky Rise volcanism because the massifs formed at spreading ridges. This study is unique in showing Moho depth and structure over an extraordinarily large area beneath an oceanic plateau, giving insight to plateau structure and formation.

Thin precollision crust can explain aspects of Indo‐Asian convergence

Between 40 and 50 million years ago, the Indian and Eurasian continental plates collided, eventually forming the Himalayas and the Tibetan Plateau. If an average crustal thickness is assumed before the collision occurred, recent research reveals that what is left of the Eurasian and Indian plates after the collision is 30% less than what it should be—a finding that has puzzled scientists because the modern Eurasian plate is twice the normal crustal thickness in Tibet.

Lateral variation of crustal structure in the Ordos block and surrounding regions, North China, and its tectonic implications

Crustal thicknesses and Poissonʼs ratios in the Ordos block and surrounding regions were estimated by the use of the H–k stacking method on teleseismic receiver functions. The data came from 353 temporary and permanent seismic stations in 2006–2011. Results show that the crustal thickness and Poissonʼs ratio gently vary within the Ordos block, with an average of 41.3 km and 0.265, respectively, consistent with a felsic composition of the crust. Crustal thicknesses predicted on the basis of Airy isostasy are consistent with the estimated thicknesses, implying that the topography is approximately compensated. The reactivated portion of the North China Craton that has undergone Mesozoic–Cenzoic lithospheric thinning is also characterized by the thinning crust, while the Ordos block maintains normal crustal thickness and average crustal velocity. Inferred higher densities in the lower crust and the anti-correlation between Poissonʼs ratio and crustal thickness in the Ordos block may be the result from underplating of mafic magmas in the Precambrian. Around the Ordos block, the Paleoproterozoic khondalite zone in the northern edge has higher Poissonʼs ratio and thickened crust, which is consistent with the lower crust being of more mafic composition. The Weihe–Shanxi graben in the southeastern edge has mid-high Poissonʼs ratio, high heat flow and thinning crust, which is consistent with the known transtensional tectonic setting. In the Liupanshan thrust belt, along the southwestern edge of the Ordos plateau, significant variations in crustal thicknesses and Poissonʼs ratios occur on two sides. Besides a thickened crust, a concaved Moho implies horizontal shortening of this edge of the Ordos block due to its collision with the northeastern Tibetan Plateau. The structural differences between the eastern and western edges of the Ordos block reflect that the Ordos block is in tectonic stress environment of the western compression and the eastern extension.