Download I. Abstract II. Geological background III. Thermo

T43B-2669
Ridge subduction as mechanism for initiation of rifting in South China:
Implications from numerical modeling results
(1)
(1)
Xuran ZUO* , Lung Sang CHAN , Manuel PUBELLIER
*Contact email: [email protected]
I. Abstract
The northern margin of South China Sea represents a crustal compression-extension transition of South China Block during the Late Cretaceous-Cenozoic. However, the mechanism
that initiated this transition is still unknown. In this study, we used a numerical thermo-dynamical program to test how varying slab angles, thermal gradient of the upper mantle,
convergence velocity, density at the 670 km discontinuity and ridge subduction may give
rise to crustal extension in a convergent setting. The modelling results reveal the dominance of horizontal compression during the early stage of the subduction, which reverts to
a horizontal extension in the back-arc region. Mantle upwelling appears to be the primary
process leading to crustal extension. The results show that the slab angle is the key factor
controlling the roll-back of the subducted slab, which in turn controls the development of
mantle upwelling. A relatively high convergence velocity is shown to produce a strong coupling of the subducting slab with the overriding plate, and a higher mantle thermal gradient would facilitate the rolling back of the slab. This process probably accounts for the initiation of the extensional regime in the South China Block during the late stage of the
Jurassic magmatism. The extension continued into the Cenozoic and eventually led to the
full opening of the South China Sea. The study also points to a close association between
ridge subduction and back-arc crustal extension.
(2)
(1) Department of Earth Sciences, The University of Hong Kong, Hong Kong
(2) Faculty of Geosciences & Petroleum Engineering, Universiti Teknologi Petronas, Malaysia
III. Thermo-dynamical model of subduction zone
IV. Numerical models of ridge subduction
a. Previous numerical and analogue models
a. Evolution of subduction zone?
Numerical model of subduction process was done to simulate the active-passive transition of South
China during the late Mesozoic-Cenozoic. A thermo-dynamical modelling program basing on Fast
Lagrangian Analysis Continua method (FLAC) was adopted to compute the stress and strain configuration of the trench-backarc region. Lithology and thermal profile were input for oceanic lithosphere,
continental lithosphere and asthenospheric mantle. The model is pushed at a constant convergence
velocity.
Analogue models:
The rigidity of the plate forces the ridge to subduct with the dense oceanic
lithosphere hence precluding significant slab flattening. (Martinod et al., 2005)
Numerical models:
Aseismic ridge doesn’t change slab dynamics or magmatic activity significantly. (Gerya et al., 2009)
Magma production strongly decreases in the case of flat slabs. (van Hunen et al., 2002)
Deficiency of previous numerical models:
1. Ridge is thicker than surrounding oceanic lithosphere, not corresponding to ridge in nature
2. No thermal heterogeneity in ridge area compared to normal oceanic lithosphere
b. Our model on ridge subduction in South China
b. Initiation of crustal extension in a convergent setting?
Ridge with thinner lithosphere
and higher thermal gradient
Ridge with normal lithosphere thickness
and higher thermal gradient
II. Geological background
a. Geological model of South China
Phase 1: Jurassic - Cretaceous
South China
Block
(Compressive)
Phase 2: Late Cretaceous - Cenozoic
South China
Block
(Rifting)
8 My
8 My
South China
Subduction of
Paleo-Pacific
Plate
Rifting of the SC Block
eventually led to the
opening of
South China Sea
General characteristics of geology of South China :
a. Jurassic arc magmatism
An extensive magmatic belt associated with the subduction of Paleo-Pacific plate under
the South China Block.
b. Late Cretaceous - Cenozoic red bed basins
Numerous terrestrial redbed basins formed in an extensional environment during the
Late Cretaceous - Cenozoic.
c. Reactivation and reversal of fault motion
Preexisting fault zones were reactivated or reversed their sense of motion during the
transition of southern South China from active to passive margin.
Mechanism of transition
from active to passive continental margin ?
b. Possible mechanisms for the transition
-- Considerable amount of mantle upwelling that might be triggered by different slab
angles, convergence velocities, thermal gradients of lithosphere and lower boundary
effect.
-- Ridge subduction of Paleo-Pacific plate (Kula / Izanagi plate) under East Asia (Okada,
2009; Hilde, 1977; Sun et al., 2007)
Finding 1: The distribution of vertical principal stress in the continental crust indicates that horizontal
compression during the early stage of the subduction reverts later to a horizontal extension in the backarc region, with vertically directed maximum principal stress.
Stress at 8My
Stress at 8My
No ridge (reference model in Section III)
Finding 3:
Models of slab with subducted ridge could produce more extensive crustal extension than the
reference model without ridge.
Ridge subduction could also be a main factor that
produces mantle upwelling.
c. Mantle upwelling:
Finding 2:
Mantle upwelling appears to be the process
governing crustal extension
The following parameters are found to be crucial to trigger mantle upwelling during the subduction process:
a) Higher dipping angle of the subducting
slab: 50-60 degree
b) Medium thermal gradient of the oceanic
lithosphere: 10-20 degree/km
c) Lower convergence velocity: 2-4 cm/year
d) Higher density contrast between the model
and the lower boundary: 500-1000 kg/m3
Acknowledgement
The current project was supported by Hong Kong General Research Fund HKU700309P.
Prof. E.B. Burov from UPMC (Paris, France) is acknowledged for sharing the modelling program.
References :
- Gerya, T.T., Fossati, D., Cantieni, C., Seward, D., 2009. Dynamic effects of aseismic ridge subduction: numerical modeling. Eur. J. Mineral., 21, 649-661.
- Ling, M.X. et al., 2009. Cretaceous ridge subduction along the Lower Yangtze River Belt, Eastern China. Economic Geology, 104, 303-321.
- Martinod, J., Funiciello, F., Faccenna, C., Labanieh, S., Regard, V., 2005. Dynamical effects of subducting ridges: insights from 3-D laboratory models.
Geophys. J. Int., 163, 1137–1150.
- Okada, H., 1999. Plume-related sedimentary basins in East Asia during the Cretaceous. Palaeogeography, Palaeoclimatology, Palaeoecology, 150,
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- Sun, W.D., Ding, X., Hu, Y.H., and Li, X.H., 2007, The golden transforma- tion of the Cretaceous plate subduction in the west Pacific: Earth and Planetary Science Letters, v. 262, no. 3-4, p. 533–542.
- Van Hunen, J., van den Berg, A.P., Vlaar, N.J., 2002. The impact of the South-American plate motion and the Nazca Ridge subduction on the flat subduction below South Peru. Geophys. Res. Let., 29, 14, 1690, 10.1029/2001GL014004.