GEMOC ARC National Key Centre
Evolution of the lithosphere beneath the Sino-Korean Craton
S.Y. O'Reilly and W. L. Griffin
These notes are a summary of material largely presented in the following reference, with the addition of some unpublished data:
Griffin, W.L., Andi, Z., O'Reilly, S.Y. and Ryan, C.G.. 1997: Phanerozoic evolution of the lithosphere beneath the Sino-Korean Craton. In: Mantle Dynamics and Plate Interactions in East Asia (Flower, M., Chung, S.L., Lo, C.H. and Lee, T. Y. eds) American Geophysical Union Spec. Publ., in press 7/96.
The bibliography contains all the references from this paper and some additional ones.
This is a case history of using "4-D lithosphere mapping methodology" (see section of that title) in tracing the evolution of the Sino-Korean Craton in Northern China from its original state through reworking during several tectonic episodes from at least the Mesozoic to the present day. This illustrates the importance of understanding the time significance of lithosphere information and particularly emphasises that geophysical data generally reveal present-day properties. In places such as the Sino-Korean Craton that have undergone significant tectonothermal events, care must be taken to unravel the timing and nature of these events. For example the Sino-Korean Craton is prospective for diamonds because of the Ordovician kimberlites that traversed Archean-type mantle. However, this study will show that tectonism has altered the lithosphere since the Ordovician so that geophysical data would be misleading as a prospecting tool for area selection based on identifying regions with cratonic physical properties.
Lithosphere Mapping started in Australia, but was constrained by sparse xenolith localities except for the Tertiary basalts along the eastern Australian margin, and also by sparse geophysical coverage. In contrast, Eastern China is constructed of many well-characterised tectonic blocks, has numerous volcanics ranging from kimberlites to basalts containing old and young mantle samples, and most importantly, has excellent geophysical coverage. Eastern China also has a complex terrain structure and recent active tectonism so that it can provide analogues to interpret lithosphere across tectonic boundaries in other regions and in particular, in Australia where data are sparse. This work on the lithosphere of Eastern China is part of a "lithosphere traverse" project that also includes Mongolia and Siberia (Figure 1).
Figure 1: Traverse diagram
Figure 2: tectonic regimes in China.
Figure 2 shows the main large-scale tectonic blocks of China for an overall perspective.
Figure 3: Sino-Korean craton detail
Figure 3 shows the Sino-Korean craton and the location of kimberlites and basalts used in this study. The Sino-Korean craton has two major terranes - Liaolu in the east and Ordos in the west. The Ordos and Liaolu terranes are separated by a N-S belt of middle to late Proterozoic rocks - bounded to the east by the Taihang Fault zone. The Tan-Lu fault zone cuts the eastern part and extends for about 4,000 km.
Figure 4: N-S Gravity Lineament
Figure 4 shows gravity data for China. The very prominent North South Gravity Lineament (NSGL) seen as the steep N-S gravity gradient cuts across major geological boundaries including craton margins and separates two present-day fundamentally different tectonic domains. The NSGL approximately coincides with the large NS paleorift, the Taihang fault zone in the Sino-Korean craton. The region west of this NSGL is characterised by thick crust, large negative Bouguer anomalies, low heat flow, a lithosphere 150 -200 km thick, and major geological structures trend E-W. To the east, the crust is generally thinner, the regional Bouguer anomaly is zero to positive, heat flow is high and base of the lithosphere generally shallow.
The host rocks that provide the mantle samples for this study (locations on figure 3) fall into 2 major age groups: lower Paleozoic and Mesozoic to Tertiary.
As detailed in the Lithosphere Mapping Section, xenoliths in basalts give information on:
Figure 5: Geotherm summary
Figure 5 shows a summary geotherm comparison based on the garnet concentrates and mantle xenoliths, emphasising the difference between the Archean and the younger geotherms. The Ordovician kimberlites contain mantle fragments typical of Archean mantle composition and revealing a low conductive, cratonic geotherm and the lithosphere-asthenosphere boundary (LAB) is deep (around 180 km). Sampled mantle from this time lies in the diamond stability field below about 150 km. The younger mantle samples reveal a high geotherm and shallow LAB at about 80km.
Figure 6: Rock types represented by the Ca and Cr contents of disaggregated garnets
Figure 6 shows the rock types represented by the geochemical signature of the garnets in the kimberlite localities. The Ca and Cr contents define the rock association, ie lherzolite, harzburgite etc from which the mineral grains were derived. The old kimberlites --Menying (Shandong) and Fuxian (Liaoning) have many depleted harzburgites (the blue and green symbols in figure 6) and these correspond to the G10 field of kimberlitic garnets that may indicate high diamond prospectivity). In contrast, the younger kimberlites have no garnets that appear to be derived for depleted mantle rocks such as harzburgites - their garnets are mainly disaggregated from lherzolites, more fertile (Ca, Al and Fe-rich) mantle rock types not usually associated with diamond prospectivity.
Figure 7: Rock type "stratigraphy" in the Ordovician lithosphere
The combination of the geochemical signature that defines rock type and the PT data giving the Garnet Geotherm (Ryan et al., 1996) allow us to put the different rock types into a spatial context, resulting in the construction of the cumulative plot of the rock type sequence through the lithosphere as seen in Figure 7. This shows the amount of lherzolite (yellow), Ca-harzburgite (green) and refractory harzburgite (dark blue) at various depths below the Menying and Fuxian kimberlite fields in the early Paleozoic.
Even though they are both sections of lithosphere of about the same age and on the same tectonic block, it is evident that there is significant lateral variation in the rock type sequences and relative abundances between the two areas. This is significant because these two kimberlite fields are sometimes used to reconstruct the Tan-Lu fault zone on the basis that it was a strike slip fault and these two kimberlite fields were originally adjacent. Our data would indicate that there were very significant differences in the detailed rock type distribution in the mantle for each of these localities.
Figure 8: Nushan xenoliths
Figure 8 shows a hand specimen of a xenolith from the Nushan locality at the southern edge of the Sino -Korean craton. This basaltic cone and is the youngest of the localities studied at ~ 500,000 years and thus provides samples of recent mantle. There is a remarkable suite of xenoliths (Xu et al, 1997) including garnet pyroxenites, spinel lherzolites, garnet spinel lherzolites and contacts between these rock types. This sample is a spinel lherzolite cut by a garnet pyroxenite vein. Such samples allow geothermobarometry calculations of the mantle equilibration conditions and cross-calibration of geothermometers for different rock types. the garnet lherzolite transition beneath Nushan is determined to be at about 55-65 km.
Figure 9: Nushan geotherm and mantle stratigraphy
The xenoliths from Nushan enable estimation of the depth to the crust-mantle boundary (CMB) and a xenolith geotherm. This geotherm is very high, similar to geotherms associated with regions of basaltic activity worldwide and similar to the results from the garnet concentrates for Teiling. This geotherm would intersect the mantle adiabat at depths of 100±10 km, defining the top of the asthenosphere and the base of the lithosphere. This is equivalent within error to the depth to the regional seismic and magnetotelluric low-velocity zone (LVZ) in this region (Figure 10).
Figure 10: Depth to low velocity zone
The present day depth to the low velocity zone (LVZ) from magnetotelluric and seismic data beneath the Shandong kimberlite province is similar. The identification of the LVZ as the base of the lithosphere in Pleistocene time suggests that the lithosphere beneath the Shandong kimberlites today is 2100 km thick, compared to 180-190 km in Ordovician time. The contrast implies a lithosphere thinning of 80-90 km since the Paleozoic.
Figure 11: Distribution of heat flow in the Sino-Korean craton
Figure 12: Section showing seismic boundaries and isotherms
The position of the Moho and the top of the low velocity zone are shown here with the isotherms extrapolated to depth from surface heat flow measurements assuming a conductive geothermal environment.
These observations on the thermal structure and depth to LAB and LVZ are summarised in the following points:
- The LVZ (eastern Sino-Korean craton) today is 2100 km (geotherm, seismic)
- The Lithosphere Asthenosphere Boundary about 500 Ma ago was at ~180-200 km
- High young and present-day geotherms indicate advective heating by magma movement
- Extensive basin formation occurred in the Jurassic-Cretaceous and Cretaceous-Recent times
- Archean lithosphere was replaced by denser but hotter mantle (starting in the Jurassic)
- Regional subsidence caused basin formation due to thermal relaxation
Figure 13: Modal olivine plotted against mg of olivine
Spinel lherzolites from Nushan provide the geochemical information to characterise the mantle type. They are fertile to mildly depleted lherzolites, and their mg-Mg/Si relations are consistent with an origin as residues from basalt extraction at low P according to Figure 13 (after original plot for Archean mantle composition by Boyd, 1996). They are similar in composition to oceanic mantle but this may only signify that it is young, fertile mantle rather than specifically oceanic in origin. They are distinct in these respects from the peridotite xenoliths in kimberlites from Archean areas, which are strongly depleted and have high orthopyroxene/olivine ratios inconsistent with an origin by basalt extraction. The shallow mantle beneath Nushan therefore cannot be a remnant of thinned Archean lithosphere. If such lithosphere did exist beneath this edge of the North China Craton, then all 180-190 km of it has been displaced by mantle similar in composition to that of oceanic and young active continental areas.
Summary of seismic data
Seismic data for the Sino-Korean craton show 3 important features:
2. Sub-Moho Vp east of the N-S gravity lineament is low ~ 7.6-7.7 km/sec except
3. where lithosphere is thicker in some eastern parts ( ~ 100 km) and then Vp is higher than the general Vp in the east (~8 -8.1 km/sec)
Figure 14: Schematic representation of lithosphere evolution in the north-eastern Sino-Korean craton
The cartoon in Figure 14 is consistent with results from modelling of present day Vp using rock-type data for the xenoliths and mineral concentrates. However, it is our interim view and ideas are being refined as data increase. In summary:
Figure 15: Summary cartoon of lithosphere evolution beneath the Sino-Korean Craton
Figure 16: Location of Tomographic Profiles across Sino-Korean Craton
Figure 17: Tomographic Sections in the Sino-Korean Craton
Figure 16 shows tomographic sections in the Sino-Korean Craton (for location see Figure 15) from Yuan, 1996. Yuan describes this as the "mushroom cloud" model of the lithosphere in which young hot mantle displaces old cratonic mantle in the distinctive pattern.
1. An Archean lithosphere ~200 km thick existed under the eastern Sino-Korean Craton up to Late Ordovician
2. These 2 regions however, had very different distributions of rock types with depth and different types of metasomatic signatures in the Archean to the Ordovician
3. Geophysical data show a present-day thin lithosphere (60 -120 km) and high geothermal gradient today.
4. This implies removal or transformation of 80 -140 km of Archean lithosphere since the Ordovician
5. Replacement lithosphere is fertile, "oceanic" and hot: Vp = 7.6 - 7.7 km/sec
6. Buoyant pockets of "cratonic" lithosphere are preserved sub-Moho in areas with lithosphere 3 ~100 km
7. Thermal erosion of the lithosphere can be linked to 2 episodes of basin development and rifting associated with subduction
8. This case study illustrates the power of the 4-D Lithosphere Mapping methodology in unravelling crust/mantle tectonic evolution
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