Download The thermochemical structure of the lithosphere and upper mantle

The thermochemical structure of the lithosphere and upper mantle beneath
South China: Results from multi-observable probabilistic inversion
B. Shan,1,2,* J. C. Afonso,2 Y. Yang,2 C. J. Grose,2 Y. Zheng,1 X. Xiong,1 L. Zhou3
1. State Key Laboratory of Geodesy and Earth’s Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan, China ( *e-mail: [email protected] )
2. Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, Australia
3. China Earthquake Networks Center, China Earthquake Administration, Beijing, China
Abstract We study the thermal and compositional structure of South China by jointly inverting Rayleigh
1D Forward Model
Figure 2. Discretization scales used in the 1D inversion. The finest
wave dispersion data, geoid height, topography and surface heat flow with a probabilistic (Bayesian) Monte
discretization scale, the vertical grid step of which is set to 2 km, is used
Carlo method. We find that the lithosphere is thin (85-150 km) beneath the South China Fold system and
in the numerical solution of the forward problem (e.g., 1D steady-state
thickens over the Yangtze Craton to maximum thicknesses of up to 280 km beneath the Sichuan Basin. In
heat transfer, geoid and isostasy equations). The intermediate
agreement with geochemical signatures from East China mantle xenoliths, our inversion predicts that the
discretization scale is used in the solution of the Gibbs free energy
lithospheric mantle beneath the South China Fold system and Yangtze Craton is highly fertile (Mg# ~
minimization problem. Here 15 thermodynamic nodes are used in the
88-90). Such fertile compositions, together with the relatively thin lithospheric thickness in the area, point
intermediate discretization. The third discretization scale is represented by
towards a widespread metasomatism/refertilization event. We suggest, as others have, that a flat-subduction
the compositional layers (two compositional layers). Each of these two
layers and the thermodynamic nodes therein is defined by its five major
episode and subsequent slab removal may have triggered both the delamination of the lowermost part of the
oxides within the system CaO-FeO-MgO-Al2O3-SiO2. Once a new
subcontinental lithosphere and the generation of asthenospheric melts that metasomatized (refertilized) the
sample is generated, the relative model parameters are assigned to
remaining lithospheric mantle. Inconsistencies among geophysical observations and anomalously fertile
different nodes as shown in this figure, and then 1D forward problems are
compositions for the Sichuan Basin indicate that this region may be currently affected by small-scale
solved to obtain synthetic observations.
convection or delamination processes. Alternatively, the anomalous observations may be associated with an
Conclusions
eastward push of Tibetan lithosphere beneath the Yangtze Craton.
Keywords
South China; Geophysical-petrological inversion; lithosphere-asthenosphere boundary;
LAB depth
Thermal, Velocity and Density Structure
thermal-compositional structure
Introduction Any model or hypothesis of the evolution of the sub-continental lithospheric mantle (SCLM)
and its interaction with the sub-lithospheric upper mantle (SLUM) requires, first and foremost, knowledge
of the present-day SCLM and SLUM thermochemical structure. Unfortunately, the conversion of seismic
Figure 4. Posterior histograms of LAB depth for the nine
locations studied in this paper, a) columns in eastern
margin of South China Fold system; b) columns in
properties, potential field, surface heat flow (SHF) and topography into estimates of the thermophysical and
central South China Fold system and North China; c)
thermochemical state of the Earth’s interior is not straightforward due to complexities arising from their own
columns in Yangtze Craton.
limitations. Recent advances in the acquisition and modelling of high-pressure, high-temperature
mineral-physics data, together with efficient probabilistic inversion schemes, make it now feasible to jointly
Composition of Lithospheric mantle
invert a multitude of geophysical and (when available) geochemical datasets directly for the thermochemical
structure of the lithosphere and upper mantle.
South China is an ideal natural laboratory to test hypotheses/models about lithospheric structure and
evolution for a number of reasons (Fig. 1): 1. several lines of evidence suggest that this region experienced
Figure 5. Histograms of Mg# from xenolith samples
major changes in the thermochemical structure of its lithospheric mantle throughout the Phanerozoic; 2. a
(transparent) and from the posterior PDFs given by our
number of existing lithospheric models for this region are inconsistent with each other; 3. there is now
inversion (gray). a) 117 xenolith samples from east China
abundant geophysical (topography, geoid, surface heat flow and dense regional seismic arrays) and
(Qilin, Niutoushan, Mingxi, and Nushan). b) 16 xenolith
petrological/geochemical (mantle-derived xenoliths) information available in South China to test for
samples from Ningyuan (close to location SC01).
Acknowledgements All waveform data used were
consistency between different competing lithospheric models.
Thermochemical evolution of South China
In this work, we use the nonlinear, multi-observable, probabilistic inversion method of Afonso et al. [2013a,
obtained from the Data Management Centre of the
China National Seismic Network at the Institute of
Geophysics, China Earthquake Administration. The
b] to constrain the present-day lithospheric thermal and compositional structure of South China based on the
works of BS, JCA, and YY have been supported by an
Australian
joint inversion of Rayleigh wave dispersion curves, topography, geoid height and SHF.
Research
Council
Discovery
Grant
(DP120102372) and the National Natural Science
Foundation of China (Grant No. 41204067). This is
contribution 506 from the ARC Centre of Excellence
Figure 1. Topographic map of South
China with major boundaries of
tectonic units indicated by dashed
lines. Squares identify the locations of
our 1D thermochemical inversion.
Red and yellow squares indicate
locations where xenolith information
is available or absent, respectively.
Color dots display surface heat flow
observations in South China.
for
Figure 7. In the west, part of the Tibetan lithospheric mantle extrudes to the east
Core
to
Crust
Fluid
Systems
(http://www.ccfs.mq.edu.au) and 960 in the GEMOC
along a shear zone, and partly replaces the original material beneath the western
Key Centre (http://www.gemoc.mq.edu.au)
Yangtze Craton. The thick dashed line represents the possible boundary between Tibet
References
Afonso, J. C., J. Fullea, W. L. Griffin, et al. (2013a),
and the Sichuan Basin. In the eastern part, the flat slab subduction model is shown.
The white dashed line indicates the initial location of the oceanic plate during the
3-D multi-observable probabilistic inversion for the
compositional and thermal structure of the lithosphere
and upper mantle. I: a prioripetrological information
episode of flat slab subduction. The violet blocks indicate the post-orogenic state of
subducted oceanic lithosphere, after the processes of slab foundering and arc retreat.
and geophysical observables, J. Geophys. Res. Solid
Earth, 118, 2586-2617.
Afonso, J. C., J. Fullea, Y. Yang, et al. (2013b), 3-D
The original refractory mantle in the southeast was largely removed and replaced by
younger and more fertile material by the upwelling of asthenospheric material.
multi-observable
probabilistic
inversion
for
the
compositional and thermal structure of the lithosphere
and upper mantle. II: General methodology and
Beneath the Sichuan Basin, a hypothesized small-scale downwelling based on the
joint misfits of surface wave data and geoid/elevation data is depicted.
resolution analysis, J. Geophys. Res. Solid Earth, 118,
1650-1676.