North-South Gravity Lineament Research Articles

Lithospheric structure variations between the eastern and central North China Craton from S- and P-receiver function migration

I imaged the lithospheric structure beneath the Bohai Bay Basin (BBB) in the eastern North China Craton (NCC) and the Taihang Mountains (TM) and areas further to the west in the central NCC by applying the wave equation migration technique to both the S- and P-receiver functions (RFs) collected in this region. The data images and synthetic modeling results indicate that S-RF imaging is more appropriate than P-RF imaging for mapping lithospheric discontinuities, especially the lithosphere-asthenosphere boundary (LAB) in various types of regions, even directly applicable to basin areas that are covered by thick sediments. The constructed S-RF images show that the lithospheric thickness of the study region is highly variable. The imaged LAB is on average ∼80 km beneath the BBB, and displays obvious undulations from ∼120 km under the TM to <90 km further west in the central NCC. A rapid change of ∼30 km in the LAB depth was detected at around the boundary between the BBB and the TM, roughly coincident with the distinct gravity decrease of more than 100 mGal that marks the North–South Gravity Lineament (NSGL) in the region. The S-RF images also revealed a generally thin crust (∼30 km) beneath the basin in the eastern NCC and a thicker one (>40 km) under the mountain areas in the central NCC, consistent with previous P-RF imaging results. The observed structural differences between the eastern and central NCC are likely associated with different lithospheric tectonics in these two regions. While widespread lithosphere reactivation and extension in the eastern NCC during the late Mesozoic time was plausibly responsible for both the thinned crust and lithosphere in the BBB, lithospheric thinning in the central NCC may be equally heterogeneous but less significant, and probably occurred mainly in Cenozoic. Such a process may have given rise to the highly variable present-day lithospheric thickness in this region. However, it probably did not noticeably affect the overlying crust that was reported to have thickened in response to the early magmatic underplating in the Mesozoic time. The different tectonic processes in the eastern and central NCC may have induced the formation of the NSGL, which likely serves as a major tectonic boundary separating the two regions of contrasting lithospheric structures at present.

Integration of geology, geophysics and geochemistry: A key to understanding the North China Craton

Archaean–Proterozoic crust, atop a thick (180 km), cold (40 mW/m 2), melt-depleted mantle keel, characterised the North China Craton (NCC) at the end of the Precambrian. Differing ages for Pre-Cambrian crust and underlying mantle reveal decoupling brought about by repeated orogenic events over the history of the NCC. The most recent thermo-tectonic episode in the Mesozoic–Cenozoic virtually replaced/transformed all of the Precambrian lithosphere. Whether this was caused by “top-down” rapid delamination (< 5 Ma) or “bottom-up” more protracted thermal/chemical erosion (ca. 100 Ma) depends on the interpretation of Jurassic to Cretaceous geological events. The eastern part of the craton east of the North–South Gravity Lineament [lithospheric boundary] has experienced the most re-activation. In the Palaeozoic–Mesozoic this region was intruded by kimberlitic magmas and cross-cut by the Tan-Lu fault zone and in the Mesozoic–Cenozoic, the eastern craton was the site of major mafic igneous activity. By the end of the Cenozoic the eastern craton was characterised by thermal highs [> 64 mW/m 2], thin continental lithosphere (< 50 km) and thin crust (< 32 km) adjacent to the intra-cratonic Bohai Sea.

Diachronous lithospheric thinning of the North China Craton and formation of the Daxin'anling–Taihangshan gravity lineament

The Daxin'anling–Taihangshan or North–South Gravity Lineament (NSGL) is an important geologic zone within the North China Craton (NCC). Crustal elevation, morphology, crustal and lithospheric thickness and gravity anomalies all change considerably across the NSGL. However, the timing of formation and the mechanism are poorly understood. Comparison of on-craton Cenozoic basalts and their included xenoliths reveals that the NSGL is not only a physical boundary but also a chemical “discontinuity” that separates two different mantle domains. Mantle xenoliths from the western NCC have a wider range in isotopes than those from eastern China. In particular, they show the lowest ε Nd and 187Os/ 188Os ratios. Such a lateral heterogeneity may have resulted from lithospheric extension within the NCC, that was diachronously taking place in the east in the Mesozoic and that in the west in the Cenozoic. Such processes could have produced differences in lithospheric thickness underneath the western and eastern NCC during the Cretaceous, eventually giving birth to the NSGL. Formation of the NSGL by diachronous lithospheric thinning is supported by paleogeographic data, which indicates the presence of the NSGL in the early Cretaceous, a period of peak magmatism within the NCC. It is proposed that the NSGL might represent the interaction front between two tectonic regimes, with back-arc extension related to Pacific subduction in the east and Indo-Eurasian collisional extension in the west. The location of the NSGL could be controlled by the western boundary of the vigorous convection induced by stagnant oceanic slabs in the mantle transition zone and old major lithospheric weak zones in the region.

Crust–mantle structure difference across the gravity gradient zone in North China Craton: Seismic image of the thinned continental crust

The Archean North China Craton (NCC) has been tectonically modified and lost its thick lithospheric keel during the Mesozoic and Cenozoic. The processes and mechanisms of the lithospheric modification and its appearance in and the relation between different subregions of the NCC are still poorly understood. Seismic data from 45 stations along a 470-km long profile cross the Bohai Bay Basin (BBB) and the Taihangshan Mountain Range (TMR) in the NCC were employed to construct a coherent structural image of the crust and uppermost mantle. An integrated receiver function imaging technique combining the common conversion point stacking approach with waveform inversion and forward modeling was proposed to extract the structural information beneath the study region. Modeling of Bouguer gravity anomalies was also applied to constrain the density distribution. The imaging result reveals distinct structural features between the mountain range and the basin area, and presents a picture of uneven crust thinning within the study region. In the east BBB the crust is significantly thinned due mainly to the reduction in the thickness of the lower crust including the crust–mantle transition zone, by up to ∼12 km. The west TMR, in contrast, is characterized by a relatively thick lower crust of ∼20 km. The teleseismic waveform data and the gravity observation suggest a thicker crust and a buoyant mantle lithosphere beneath the TMR compared with the BBB. The contrasting crustal structural features appear coupled with the lithospheric processes and possibly reflect that different tectonic mechanisms and deformation regimes dominated the evolution of the two regions. The North-South Gravity Lineament, lying between the TMR and BBB, might represent a deep intra-continental boundary separating the NCC into topographically and tectonically different regions.

Late Mesozoic-Eocene Mantle Replacement beneath the Eastern North China Craton: Evidence from the Paleozoic and Cenozoic Peridotite Xenoliths

Xenolith-bearing Paleozoic kimberlites and Cenozoic basalts from the eastern part of the North China craton provide unusual insights into intraplate processes and Phanerozoic lithospheric evolution. Paleozoic peridotite xenoliths represent samples of ancient cratonic mantle; P-T estimates show that a thick (˜230 km), cold (ca 40 mW/m2) lithosphere existed beneath the craton during mid- Ordovician time. However, xenoliths from Tertiary basalts sample a thin (> 90 km), hot (mean geotherm ca 80 mW/m2), compositionally heterogeneous lithosphere beneath the same area in Cenozoic time. Fertile, spinel-facies mantle makes up much of the Cenozoic lithosphere beneath the eastern North China craton, especially in regions along the translithospheric Tanlu fault. However, refractory spinel-facies xenoliths are found locally along the north-south gravity lineament in areas far away from the Tanlu fault. These refractory xenoliths are interpreted as derived from shallow relics of the cratonic mantle embedded in more fertile Cenozoic lithosphere. The increasing incidence of fine-grained, sheared microstructures in xenoliths from the north-south gravity lineament progressively toward the Tanlu fault suggests that the translithospheric fault system played an important role in the Mesozoic-Cenozoic replacement of pre-existing lithospheric mantle by more fertile material. Modification of cratonic mantle beneath the eastern North China craton involved irregular replacement of old lithosphere by cooling products of weakly depleted asthenosphere welling up along major shear systems. This lithosphere replacement was accompanied by an elevated geotherm and a shallower asthenosphere-lithosphere boundary.

Fluid Inclusion and Noble Gas Studies of the Dongping Gold Deposit, HebeiProvince, China: A Mantle Connection for Mineralization?

The Dongping gold deposit (>100 t Au) occurs about 200 km inboard of the northern margin of the North China craton. The deposit is mainly hosted by syenite of a middle Paleozoic alkalic intrusive complex that was emplaced into Late Archean basement rocks. Both groups of rocks are intruded by Late Jurassic to Early Cretaceous crustal-melt granite dikes and stocks, some within a few kilometers of the deposit. The gold ores were deposited during this latter magmatic period at about 150 Ma, a time that was characterized by widespread regional north-south compression that formed the east-west–trending Yanshan deformational belt. The ores include both the telluride mineral-bearing, low sulfide quartz veins and the highly K-feldspar–altered syenite, with most of the resource concentrated in two orebodies (1 and 70). Fluid inclusion microthermometry indicates heterogeneous trapping of low-salinity (e.g., 5–7 wt % NaCl equiv) fluids that varied from a few to 60 mole percent nonaqueous volatile species. Laser Raman spectroscopy confirms that the vapor phase in these inclusions is dominated by CO2 but may be comprised of as much as 9 mole percent H2S and 20 mole percent N2; methane concentrations in the vapor phase are consistently <1 mole percent. The variable phase ratios are consistent with fluid immiscibility during ore formation. Fluid inclusion trapping conditions are estimated to be 250° to 375°C and 0.6 to 1.0 kbar. Helium isotope studies of fluid inclusions in ore-stage pyrites indicate 3He/4He ratios of 2.1 to 5.2 Ra (Ra = 1.4 × 10–6 for air) for orebody 1 and 0.3 to 0.8 Ra for orebody 70. The former data suggest that at least 26 to 65 percent mantle helium occurs in the fluids that deposited the veins in orebody 1. The lower values for orebody 70, which is characterized by a more disseminated style of gold mineralization, are interpreted to reflect an increased interaction of ore fluids with surrounding crustal rocks, which may have contributed additional 4He to the fluids. A mantle source for at least some of the components of the gold-forming fluid is consistent with upwelling of hot asthenosphere and erosion of as much as 100 to 150 km of cool Archean lithosphere beneath the craton during this time. The Dongping deposit is located along the 100-km-wide north-south gravity lineament, which marks the western border of the thinned crust. As both regional metamorphism of Mesoproterozoic and younger cover rocks, and widespread granite magmatism, also occurred at ca. 150 Ma, it is unclear as to whether one or both of these also contributed fluid and/or metals to the hydrothermal system. Importantly, these new data suggest that economically significant gold deposits of similar mineral style and fluid composition, which are scattered along the margins of the craton, may all be products of a fluid originally partly sourced within the mantle.

Relict refractory mantle beneath the eastern North China block: significance for lithosphere evolution

Xenolith-bearing Neogene basalts occur in Hebi county, at the southern end of the Taihangshan–Luliangshan paleo-rift zone in the North China block of the Sino–Korean craton. This locality lies on the North–South Gravity Lineament, which divides the craton into two geophysical zones. The spinel peridotite xenoliths hosted by the basalts can be divided into two groups based on the Mg # values of olivine. The whole-rock compositions of the low-Mg (Fo<91) xenoliths have high Al 2O 3+CaO (average 3.06 wt.%) and Na 2O (average 0.19 wt.%), and low Mg/Si; they are similar to xenoliths from many localities in eastern China and other Phanerozoic volcanic areas. The dominant high-Mg (Fo≥92) group consists of harzburgites (66%) and depleted lherzolites (34%) with coarse-grained (mainly) and porphyroclastic microstructures, and high-Cr spinels (mean Cr #=0.51). The high-Mg xenoliths have low Al 2O 3+CaO (average 1.36 wt.%) and high Mg/Si, are in general strongly depleted in HREE, Ti, Zr and Y, and are compositionally similar to xenoliths in kimberlites from Archean cratons. The Archean lithospheric root beneath the eastern part of the Sino–Korean craton, which was sampled by Paleozoic kimberlites, was largely replaced by fertile Phanerozoic mantle during Mesozoic extension and subduction events. The high-Mg xenoliths are interpreted as relics of the Archean lithosphere, preserved locally at relatively shallow levels, and re-equilibrated to spinel facies in a regime of high heat flow caused by advective heat transport during extension. Their calculated mean room-temperature density (3.36 g/cm 3) and Vp (8.39 km/s) are consistent with this interpretation and with geophysical data for the Hebi area. Regional geophysical data suggest that similar material may be widespread in the uppermost mantle west of the North–South Gravity Lineament, and more locally in the eastern part of the former craton.