Imaging a Decratonized Lithosphere: A Case of the Eastern North China Craton

During decratonization, the cratonic lithosphere mantle experienced composition transformation and thinning, which have been extensively studied in the North China Craton. Yet, modification from refractory lithospheric mantle to fertile one remains enigmatic due to a shortage of seismically resolvable data on the lithospheric internal structures. New high‐resolution normal incidence seismic reflection transect documents: coexistence of two lithospheric mantles with different reflection characteristics, lithosphere‐scale domes, boudinage upper‐crust, and Cenozoic faulted basins. Reflective and transparent mantles are interpreted to represent the cratonic and the newly formed lithospheric mantles, respectively. The irregular contact boundaries between the ancient and newly formed lithospheric mantles are revealed as channel‐like structures within the mantle, which possibly provide pathways for the rise of melts and/or fluids from the deep mantle. The deep melts/fluids rising through the channel‐like structures are key elements that enable generation of the newly formed lithospheric mantle, and provide means for the modification of cratonic lithospheric mantle.


Geophysical Research Letters
LI ET AL. 10.1029/2022GL099484 2 of 11 normal incidence control-source seismic reflection images across the NCC reveal the internal architecture and geometrical features associated to these processes.
A number of mechanisms related to decratonization were proposed (Foley, 2008;Lee et al., 2011;Wu et al., 2019).The consensus is that alteration of lithospheric mantle is responsible for decratonization.So far indirect geochemical observations provide the major pieces of evidence for inferring the status of lithospheric mantle beneath the cratons (Foley, 2008;Lee et al., 2011).However, these studies are incapable of resolving the fine structures of lithospheric mantle beneath cratons.The characterization of the lithosphere beneath cratons can be illuminated by seismic methods, the resulting images and models will contribute to new ideas, concepts and/or perspectives on the knowledge regrading craton destruction/evolution.
The NCC like the Wyoming and Brazilian cratons is one of the oldest nuclei of continents in the world and a classic example of destructed Archean craton (Carlson et al., 2005;Kusky et al., 2014;Menzies et al., 1993;Wu et al., 2019;Yang et al., 2008).The NCC consists of Eastern and Western Blocks divided by Trans-North China Orogen (central orogen) (Zhao et al., 2005) (Figure 1a).The Eastern Block underwent tectonic reactivation (Griffin et al., 1998;Menzies et al., 1993;Xu, 2001) through regional extension between 120 and 130 Ma (Liu et al., 2021;Zhu et al., 2021).This was accompanied by the replacement of ancient lithospheric mantle of the NCC by the present new lithospheric mantle which is thinner and hotter (Wu et al., 2019).Geophysical studies show that the thickness of the eastern North China lithosphere is less than 80 km (Chen et al., 2008), significantly thinner than that of stable cratons which is typically on the order of 200 km (Chen et al., 2009).Petrological and geochemical data indicate that lithospheric mantle components of the Eastern Block underwent multi-stage modification from the Paleozoic to the Cenozoic (Tang et al., 2021;Zong & Liu, 2018).Models proposed so far for decratonization of the Eastern NCC are different and resulted in complex and diverse structural patterns for the lithosphere.The exact lithospheric architecture resulting from decratonization remains poorly known, partly due to the lack of seismically resolvable images.Deep seismic reflection images are an asset for constraining lithospheric structure (Artemieva & Meissner, 2012;Clowes et al., 1999;Simancas et al., 2013).

Data Acquisition
1,776 small-size shots (8-12-kg) and 83 medium-size shots (24-48-kg) were deployed to obtain the crust and mantle structure.The small-size shots and medium-size shots were spaced 40 and 1,500 m, respectively.The receiver spacing is 20 m.Both were recorded with geophones spreads that had a minimum offset of 10 m and a maximum offset of 10 km.The fold of the survey is 250.The CMP spacing is 10 m.A Sercel 428XL seismic recording system was used.The small-size shots reveal weak surface waves, high S/N (Signal/Noise) ratio revealing upper crustal reflections, and weak Moho event, while the medium size shots provided relative high amplitude surface waves and high amplitude Moho reflection (Figure 2a).Integrated processing of both control source resulted in high S/N ratio data creating in a high-resolution crustal image along the transect.See attachment for more acquisition details.

Data Processing
The deep seismic reflection data processing was performed using Omega commercial seismic software package (the acquisition and processing parameters are expanded in the supplementary documentation).Deep seismic reflection in the lower crust is very complex and characterized by laterally discontinuous, low amplitude reflection fabrics, Seismic skeletonization has been use to emphasize the reflection signature and drive toward a better understanding of the complex reflectivity patters and help define the associated crustal structure.A highly improved Skeletonization algorithm was applied to the deep seismic section to further delineated the deep structure and assess its physical properties.See attachment for more details.

Reliability Analysis
The amplitude decay at different frequencies from shot gather (maximum offset 500 m) were extracted to analyze and test the reliability of the seismic image.Tomography static corrections, amplitude recovery, and trace balancing were applied to shot gathers to reduce the impacts of wave propagation and attenuation prior to the analysis.
As shown in Figures 2b-2h.Amplitude varies sharply due to high-impedances between 10.5 and 11.5 s, however, there is no significant seismic impedance variation from the crust to the mantle, as shown in Figure 2e.Some amplitude variations occurred between 11.5 and 16.5 s.The amplitude gradually weakens laterally from section ends to the center of the transect.High frequency attenuation is significantly higher than for low frequency.These reflection characteristics are consistent with the seismic image (Figure 2a).This analysis suggests that the seismic energy reached mantle depth and support that the differences in seismic signature are most probably due to structure alone.

Results
The seismic image was obtained through a conventional processing flow.The seismic fabrics and the prominent events reveal a number of features which can be correlated to significant outcropping geological structures.

Cenozoic Faulted Basins
The nearly E-W deep seismic reflection transect (X-X′ on shown Figures 1 and 2a), goes across three well developed basins bounded by a network of faults.These faulted basins are filled with Paleogene sediments according to the borehole data from the hydrocarbon exploration well (He et al., 2018).It was drilled near CMP 5200 of the seismic transect (Figure 1b).The deposition of these faulted basins was controlled by the normal faults bounding the western margins of the basins, that is, F1, F2, F3 (Figures 3a and 4b).The overlying deposition sequence of the basins and the basement corresponds to a set of nearly horizontal Neogene and Quaternary strata (Figure 4b), forming a regional angular unconformity.Stratigraphic architecture of the Neogene and Quaternary strata deposited during the rift to depression transition stage of the Bohai Bay Basin (He et al., 2018;Qi & Yang, 2010) shows progressive eastward stacking of the sediments (Figure 4b).Beneath the Cenozoic sediments, the eastern block of NCC exhibits strong upper crustal boudinage suggesting well developed extension of the crust.From west to east, the thickness of the upper crust changes from ∼3 s (∼9 km) to ∼1 s (≤∼3 km) in two-ways-travel time (TWT) below the easternmost faulted basin in the seismic image (Figure 4b).

Gneiss Domes
The lower crust is characterized by: zones of high amplitude and laterally coherent reflection fabric, simulating a laminated lower crust (Figures 2a and 3a).This fabric is interrupted at times by areas that are fairly transparent, they lack relevant seismic events or features of a low amplitude (weak) reflectivity patterns (Figures 3b and 3c).In Figures 3b and 3c, the transparent or low amplitude zones ("A" in Figures 3b and 3c) contain relic reflections ("C" in Figures 3b and 3c) which can correspond to the arched events ("B" in Figures 3b and 3c) around the transparent or low amplitude zones.The specific geometry of the reflections that builds up the laminated lower crust differentiates areas with nearly horizontal events between CMP 6500 and the eastern end of the profile and, another area which is characterized by arched events (between the CMP 6500 and the western end of the profile) (Figure 2a).The arched reflections show concentric patterns (Figures 3b and 3c), simulating dome-shaped structures in the seismic image, that is, the D1, D2, and D3 in Figure 3a.The concentric reflections can also be seen within the mantle (Figure 2a).The lower crustal dome-shaped structures extend upward to the upper crust and downward to the uppermost mantle, forming lithospheric-scale dome-shaped features (Figure 4b).There is a geothermal borehole near the CMP 5,200, which sampled from the top of dome shaped structure (D3) and provided the necessary geological constraints related to the Cenozoic sediments and other strata.The borehole data revealed that the top of the dome (D3) consists of Middle Proterozoic lithologies including: the ash black, gray, gray white dolomite, containing bituminous dolomite.The latter is underlain by Archean gneisses.Except for the largest transparent zone beneath the CMP 4,500, the location of all the remaining transparent zones' correlate with the location of the cores of the dome shaped structures (Figure 2a).Generally, the transparent zones in the deep seismic reflection sections have been interpreted as plutons or intrusive bodies (e.g., granites) (Clowes et al., 1999;Flecha et al., 2006;Ludden et al., 1993;Thybo & Artemieva, 2013).Therefore, combining the geometry revealed by the seismic reflection profile with the lithologic data from the borehole, it may be believed that these domal structures in the profile are gneiss domes (Whitney et al., 2004).The gneiss domes imaged along the profile feature an almost uniform size, and they are nearly evenly spaced.

Moho and Upper Mantle Features
The Moho in the seismic cross-section is well defined by a band of undulating high amplitude reflectivity with finite width (0.250 s), it constitutes a sharp boundary beneath which there is a first marked decrease in reflectivity, it is placed at approximately 11 s in TWT along the transect (Figure 2a).Considering an average P wave velocity of 6 km/s for the crust, the Moho is located at 32-35 km depth delineating a nearly horizontal boundary, and this is consistent with results from wide angle reflection and receiver function analysis (Jia et al., 2014;Zheng et al., 2006).The Moho features high amplitude reflections beneath lower crustal areas with high amplitude reflections reflection fabric and it also features lower amplitudes in areas beneath the low amplitude reflections (Figure 2a).
From east to west, the upper mantle is imaged as alternating reflective and transparent vertical zones.These features are delimited by boundaries with irregular geometries (Figure 2a).The upper mantle zones with relatively high amplitude reflection fabric feature well defined events through out an approximately 9 s (TWT) thick band.Within it the lithospheric mantle features some sort of internal structure (Figure 2a).These laminated mantle with arcuate reflections (arched upwards) underlie similar crustal reflection features (Figure 2a).

Coexistence of Two Different Types of Lithospheric Mantle
The contrasting seismic signatures within the lithosphere (mantle) (Figures 2a and 3a, i.e., high amplitude reflective zones, and weak or nearly transparent mantle) most probably reflect notable differences in mantle composition.At a first glance, nearly transparent zones would be consistent with mantle volumes characterized by more homogeneous composition, while areas featuring relatively high amplitude reflection fabrics would be indicative of a relatively high degree of heterogeneity within the mantle (Carbonell, 2004).Herein, two possible interpretations can be proposed for these contrasting signatures.
The base of the crust is represented by the relatively thick (0.25 s) slightly undulating band of high amplitude reflectors (Figures 2a and 3a).Beneath this prominent reflection, a 4 s thick band of relatively high amplitude events can be identified within the upper mantle, reflective upper mantle (beneath CMP 1,250-2,500 and 6,500-8,500).This band extends from beneath the Moho down to, approximately 16.5 s (beneath CMP 1,250-2,500 and 6,500-8,500).This appearance is specially marked in the East and Western ends of the transect.These mantle reflection fabrics consisting of events slightly arched upwards which mimic the overlying lower crustal reflection fabrics, suggest that they, most probably, underwent similar tectonic processes.Evidence from structural geology suggests that the crustal thickness of the eastern NCC reached depths of 47-50 km before the cratonic destruction (Zhang et al., 2011).Therefore, it is possible that these upper mantle reflections represent the remaining thin slices of the Mesozoic lower crust.This thickness of the Mesozoic crust is also supported by geochemical and petrological data (Davis, 2003;Xu et al., 2006).With these constraints the sharp decrease in reflectivity below ∼16.5 s TWT (Figure 2a) might be representative of the bottom of the Mesozoic crust (i.e., Mesozoic Moho).Subtracting the thickness of the Cenozoic strata and, the displacement of the Cenozoic faults, the pre-Cenozoic crustal thickness was estimated to be of about 46.5-48 km which is consistent with the thickness proposed derived by other studies (Zhang et al., 2011).
An alternative interpretation considers that the reflection fabrics in the mantle might be representative of the Archean craton, and that the weakly reflective or transparent zones represents the newly formed (more homogeneous) lithospheric mantle resulting from the decratonization.Generally, Archean lithospheric mantle is mainly composed of refractory harzburgite that feature relatively low-densities (Djomani et al., 2001;Kelemen et al., 1992;Lee, 2003) and, high seismic P wave-velocity (Griffin, O'Reilly et al., 1999).The lithospheric mantle formed in the Phanerozoic is mainly composed of lherzolite with higher densities (Djomani et al., 2001;Lee, 2003) and, lower seismic P-wave velocity (Griffin, O'Reilly et al., 1999) under standard temperature and pressure conditions.Therefore, the contrasts in densities and velocities between slices of ancient and newly formed lithospheric mantles could result in the impedance contrast, which can account for the different seismic fabrics.Most of the geochemical data corresponding to the central region of eastern NCC suggest that there is little Archean cratonic lithospheric mantle (Tang et al., 2021), nevertheless, a few geochemical observations have been found at the boundary (Xiao et al., 2010;Zheng et al., 2001) (i.e., the Hebi in Figure 1a and the Tanlu faults), that suggests that Archean lithospheric mantle is still existing at the boundary between the central orogeny and the Eastern Block (i.e., the Hebi fault in Figure 1a) (Tang et al., 2021 and references therein).The seismic reflection profile samples the transition region from the Eastern Block to the central orogen and, may have a good chance of capturing relics of the Archean cratonic lithospheric mantle beneath the transition region.The shortage of evidence for the Archean lithospheric mantle in the Eastern Block may be attributed to the huge thickness of the Cenozoic sediments covering the block.
Although both models are consistent with the available data, we consider the second scenario more plausible, in the latter model the reflective (heterogeneous) mantle represent the Archean cratonic lithosphere and, the transparent or weakly reflective zones in the seismic section would represent more homogeneous newly formed mantle.The current architecture could be explained by considering: (a) eclogitization of the lower crust.Lower crustal eclogitization would take place when the crust thickened to about 50 km (Kay & Kay, 1993).Numerical modeling results have demonstrated that lower crustal eclogitization can induce delamination (Krystopowicz & Currie, 2013) due to high densities of the modified lower crustal rocks.Therefore, if the reflection fabric within the mantle (down to 50 km depth) would represent the Mesozoic lower crust, the lower crust would have undergone eclogization and then delaminated into the mantle, rather than staying as relics within the mantle.(b) this idea is consistent with the surveyed gravity anomaly.The Bouguer gravity anomaly once corrected for the influence of shallow Cenozoic sediments is consistent with a crustal model that places a density boundary closely coincident with the limit between the two different kinds of reflection fabrics within the mantle (Figure 4).The lower gravity anomalies correspond to the thicker reflection band located in the upper mantle (Figure 4), suggesting a lower density upper mantle.Bouguer gravity anomalies are generally considered to reflect gravity anomalies in the Moho and the crust.In order to show that mantle also influences gravity anomaly, forward simulation of gravity anomaly has been also carried out in this paper.The results show that the simulated results are more consistent with the observations after the addition of mantle (Figure S3 in Supporting Information S1).Compared with the higher density of eclogitized lower crust, the Archean cratonic lithospheric mantle with low density is more in agreement with this feature.Although some scholars disagreed with the delamination as the driving mechanism for the decratonization of NCC in the late Mesozoic (Wu et al., 2019;Zhu et al., 2012), evidence of eclogization in the lower crust during the Mesozoic has been reported (Xu et al., 2006).Therefore, it is plausible that delamination occurred at a local scale rather than a regional scale under the Eastern Block.Potential challenges to the regional scale delamination include two lines of evidence from geophysics and geochemistry.Regional P-wave and S-wave tomography results show that there are no high-velocity anomalies beneath the Eastern Block of NCC (Huang and Zhao, 2006;Liu et al., 2017), which should be representative of the high-density sinking lower crust and underlying lithospheric mantle.The key point for the delamination is that the present lithospheric mantle is relatively young (Wu et al., 2019).If this is the case, the Re depleted age of mantle peridotite xenoliths in Cenozoic basalts of the Eastern Block of NCC would show a consistent age, likely Mesozoic as the lithospheric thinning mainly occurred during this geologic epoch (Zhu et al., 2012).However, the fact is that the Re depleted age of mantle peridotite xenoliths carried out by Cenozoic basalts in the Eastern Block of NCC has a very large span, covering the age range from the Archean to the Cenozoic (Tang et al., 2013).Shortage of intense depleted mantle-derived basaltic magmatism is another geochemical challenge for the regional scale delamination (Wu et al., 2019).
There is no ancient cratonic lithospheric mantle beneath the eastern NCC according to previous results from geochemistry and petrology (Tang et al., 2021).Our results, however, clearly reveal the coexistence of two distinct mantle components in the lithospheric mantle beneath the study area.This result provides constraints on current situation of the lithospheric mantle and new clues to explain the modification of the craton lithospheric mantle, and even decratonization beneath the eastern NCC.

Implications for the Modification of Lithospheric Mantle
A series of channel-like structures can be recognized from the fine seismic image (Figure 4b).Further evidence for the channel-like structures could also be found in the image revealed by wide angle reflection/refraction data (Wang et al., 2014).The channel-like structures may indicate the modification of the lithospheric mantle.Two possible mechanisms could be used to explain this phenomenon.The first model is that the channel-like structures may be the upwelling channels for the melts/fluids derived from the recycled crust (Huang et al., 2012;Zhang et al., 2003) and/or the asthenosphere (Tang et al., 2013;Xiao et al., 2010).The channels could correspond to fractures and/or weak zones in the lithospheric root (Griffin, Ryan et al., 1999) (Figure 4c), such as the Tan-Lu Fault in the eastern of NCC (Li et al., 2022;Xiao et al., 2010).Along the channels, the melts/fluids rose and reacted with the depleted refractory harzburgites in and around the channels (Figure 4c) to form the fertile lherzolites (Tang et al., 2011).Since the permeability of different parts of the cratonic lithospheric mantle were diverse, the degree of reactions of melts/fluids and peridotite were also varying, resulting in the irregular contact boundaries between the residual cratonic lithospheric mantle and the newly formed lithospheric mantle, as revealed by the seismic profile (Figure 4b).The image is indicative of the mechanism related to the transition of compositions from the cratonic lithospheric mantle to juvenile one.The revealed fine lithospheric mantle structure could be the possible evidence for supporting the melt/fluid-peridotite interaction model which has been proposed as the possible mechanism for modification of lithospheric mantle components (Tang et al., 2013).The second model is that the channel-like structures may result from cooling of upwelling asthenosphere which were converted to the lithosphere mantle.The newly formed lithosphere was then subjected to local melt penetration confined to the vicinity of melt channels.Although both models can explain the observed channel structures, we prefer the first model.The second model is difficult to explain the problem how the old cratonic lithospheric mantle was consumed under the premise that regional scale delamination is difficult to occur.
However, studies indicate that the lithospheric mantle beneath the eastern NCC is highly heterogeneous (Xu et al., 2022;Zheng et al., 2021).Therefore, it is reasonable to believe that the lithospheric mantle modification of cratons is the result of multiple mechanisms.The resulting image revealed by our seismic profile indicates that the deep materials risen through the channel-like structures may play an important role in the decratonization of the eastern NCC.

Conclusions
To better image the structure of the lithospheric beneath the eastern NCC, an ∼80-km long deep seismic reflection profile featuring 250-fold and 10 m CMP spacing was acquired in the Eastern Block in 2019.This high-resolution deep seismic reflection data, images the crust and upper mantle down to 20 s in two-way-traveltime (approximately, 60 km depth).The seismic cross-section reveals structures that characterize the modification processes of the upper mantle, thinning and extensional deformation of the crust.The seismic picture documents: the coexistence of slivers of cratonic material interleaved with newly formed lithospheric mantle, the lithospheric-scale gneiss domes, boudinage structures within upper crust, and Cenozoic faulted basins.Irregular boundaries between the interpreted ancient and newly formed lithospheric mantle can be defined as well subvertical channel-like features within the mantle.The latter channel-like structures has been interpreted as possible pathways that enable the rise of the deep materials thus playing a critical role in the modification of cratonic lithospheric mantle.

Figure 1 .
Figure 1.Topographic map of the study region showing simplified tectonic divisions, and location of the deep seismic profile.(a) Division of main tectonic units in the North China Craton and its adjacent areas (Modified from Zhao et al., 2005) compiled on a topographical map (Digital elevation model data download from https://earthexplorer.usgs.gov/).The white solid lines indicate the boundaries of main tectonic units.Insert is the location of Figure 1a within China.The small black rectangle indicates the location of the study area (Figure 1b).CAOB: Central Asian Orogen Belt.QLOB: Qilian Orogen Belt.QDOB: Qinling-Dabie Orogen Belt.YB: Yangtze Block.(b) Detailed geologic map of the study area that shows the most relevant geologic and tectonic features, geologic terranes, faults.The straight black solid line with red dots indicates the location and geometry of seismic transect.The green diamond (close to CMP (Common middle point) 5,000) indicates the location of a geothermal borehole.F1-Baoding-Shijiazhuang fault, F2-Rongcheng fault, F3-Niudong fault, F4-Xushui fault.Faults are all emphasized by red discontinuous lines.

Figure 2 .
Figure 2. Deep seismic reflection profile.(a) The original seismic transect X-X' (East-West black solid line in Figure 1b).The axis label CMP corresponds to the Common Mid-Points.The CMP spacing is 10 m.The axis label two-ways-travel time (TWT) corresponds to TWT in seconds (s).Aspect ratio of the seismic image is 1 to 1. (b-h) Amplitude decay curves at near-vertical incidence in different frequency bands from raw shot gather (after Tomography static, amplitude recovery and trace balancing).FFID represent field file identifier.

Figure 3 .
Figure 3. (a) Key elements of the interpretation of X-X′ seismic cross-section shows the main features of the profile.HAR: High amplitude reflections.LAR: Low amplitude reflections.(b, c) Locally enlarged seismic images for the rectangles indicated in (a).

Figure 4 .
Figure 4. Gravity anomaly and geological interpretation.(a) Gravity anomaly and topography profiles.(b) The geological interpretation based on the line-drawing of the seismic reflection image.Vertical to horizontal scale is ∼1:1.(c) Conceptual sketch cross-section showing the modification of cratonic lithospheric mantle to juvenile lithospheric mantle.