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Numerical Modelling of Arc–Continent Collision: Application to Taiwan

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Numerical Modelling of Arc–Continent Collision: Application to Taiwan Tectonophysics 325 (2000) 23–42 www.elsevier.com/locate/tecto Numerical modelling of arc–continent collision: application to Taiwan J.-C. Tang *, A.I. Chemenda Ge´osciences Azur, UMR 6526, Universite´ de Nice-Sophia Antipolis et CNRS, 250 Rue Albert, Einstein — Sophia Antipolis, 06560 Valbonne, France Received 10 September 1999; received in revised form 14 March 2000; accepted for publication 22 May 2000 Abstract Two-dimensional finite element numerical modelling is applied to study the deformation and failure of the overriding plate during arc–continent collision (continental margin subduction). This plate has elasto-plastic rheology with strain weakening and contains a ‘volcanic arc’ with thinned and weakened lithosphere. The overriding plate deforms due to the normal and tangential stresses applied along the interplate surface. These boundary conditions represent the friction and pressure between the plates. The latter is due to the flexural rigidity of the subducting lithosphere and the buoyancy force generated by the subducting continental crust. The modelling shows that continental margin subduction results in increasing compression and failure of the overriding plate, which occurs along the surface dipping under the arc in either of two possible directions. The failure mode is largely controlled by the two competitive factors: the rigidity of the subducting plate and thickness gradient of the subducted continental crust. A high rigidity favors failure along an ocean-vergent fault, which is followed by a subduction reversal, while a high thickness gradient favors failure in the opposite direction, which is followed by a fore-arc block underthrusting beneath the arc. Both scenarios seem to have natural analogs. We consider one of them, the ongoing arc–continent collision in Taiwan, and argue that this process occurs according to the second scenario corresponding to the fore arc underthrusting. Wavelet statistical analysis of the seismicity distribution to the south of Taiwan has clearly displayed a shallow (0–40 km) zone of high density, coherently distributed seismicity beneath the Luzon Arc. This zone, interpreted as a lithospheric-scale fault, dips from the forearc basin to the east and corresponds to the initiation of the forearc block subduction. A self-consistent, combined mechanical–gravity–topography model is used to see whether failure of the overriding plate to the south of Taiwan can be ‘captured’ by this model. By ‘tuning’ diVerent controlling parameters, we did not succeed in obtaining realistic topography and gravity field in a model where failure of the overriding plate was not allowed. Introduction of this failure and underthrusting of the forearc block under the Luzon Arc allowed us to fit both topography and gravity data. © 2000 Elsevier Science B.V. All rights reserved. Keywords: arc–continent collision; geodynamics; gravity anomalies; numerical modelling; seismicity distribution; Taiwan * Corresponding author. Tel.: +33-4-92-94-26-06; fax: +33- 4-92-64-26-10. E-mail address: [email protected] (J.-C. Tang) 0040-1951/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0040-1951(00)00129-3 24 J.-C. Tang, A.I. Chemenda / Tectonophysics 325 (2000) 23–42 1. Introduction Fh, either extensional or compressional, can be suYcient to cause failure of the overriding plate Experimental (Shemenda, 1993) and numerical in the volcanic arc area, which is a weak zone. (Hassani et al., 1997) modelling of oceanic subduc- Failure under extension results in back arc rifting tion have revealed two principal stress regimes, and spreading. Deformation and failure of the characterised by extension and compression of the overriding plate under compression, corresponding overriding plate, respectively. The regime is defined to the compressional regime of oceanic subduction, by the flexural rigidity of the subducting plate and were studied both experimentally (Shemenda, by the forces acting on this plate, including the 1994) and numerically (Tang et al., in press). It pull force, Fpl, and the force of dynamic interaction was shown that if failure occurs, the resulting between the subducting lithosphere and the sur- lithospheric fault dips under the arc in either of rounding mantle, Fd (Shemenda, 1994). If these two possible directions (Fig. 2). Numerical tests forces and the rigidity were zero, the overriding with diVerent boundary conditions, geometry, and plate would be in hydrostatic equilibrium, which rheologic structure of the overriding plate have means that the interplate pressure Pn (or interplate shown that the failure direction is largely con- normal stress) is equal to the hydrostatic pressure trolled by the distance, L, between the trench and =− Ph rogz and that there is no tectonic stress in the arc axis (Tang et al., in press): when < < the overriding plate (ro is the density of the 180 km L 230 km, the lithosphere fails along overriding plate; g is the acceleration of gravity; z the trenchward dipping fault; at L>230 km, the = = is the depth). When Fpl Fd 0, with a rigidity not failure occurs in the opposite direction. The mecha- V V equal to zero, Pn di ers from Ph. The di erence, nism ‘switching’ the mode of failure with variation = − sr Pn Ph, corresponding to the non-hydrostatic of L is associated with the flexural rigidity of the interplate pressure (normal stress) (Fig. 1), overriding plate and the wavelength of its bending. depends only on the subducting plate rigidity; the The arc/trench distance in subduction zones varies higher the rigidity, the greater the diVerence from ~150 to ~300 km and in most of them is between Pn and Ph. The integration of sr along about 200 km. Therefore, we conclude that the the interplate surface yields a tectonic (non- preferred mode of overriding plate failure during hydrostatic) pressure force, Fp, acting on the over- oceanic subduction is that which results in arc riding plate (Fig. 1). The horizontal component backthrust. of this force, Fh, produces compression of the Subduction of a continental margin can follow overriding plate. An application of the pull force either of the two regimes of oceanic subduction to the subducting plate modifies sr such that the and in both cases results in increasing compression horizontal component Fh of the pressure force can of the overriding plate and in lithosphere failure become extensional (can become oriented in the in the arc area (Chemenda et al., 1997). The force opposite direction) (Shemenda, 1993). causing an increase in compression during subduc- tion of the margin is the buoyancy of a pro- gressively thickened subducted continental crust. The non-isostatic interplate pressure (stress normal to the interplate surface) corresponding to this case roughly represents a superposition of the Fig. 1. Non-hydrostatic interplate normal stress sr due to the subducting plate flexural rigidity: compressional regime of oce- anic subduction. Fp is the force caused by sr; H is the overriding Fig. 2. Two possible modes for the overriding plate failure. L plate thickness; z is the depth (after Shemenda, 1994). is the trench/arc-axis distance. J.-C. Tang, A.I. Chemenda / Tectonophysics 325 (2000) 23–42 25 Fig. 3. Two equivalent settings of numerical experiments: (a) deformation of the overriding plate is caused by the buoyancy of the underthrusted crust of the continental margin. The subducting crust yield limit for normal load is 3.6×107 Pa; (b) the eVect of the subducted crust is simulated by the equivalent interplate normal stress sb calculated for the set-up in (a). = = × 3 3 = = = = rc rv 2.8 10 kg/m ; H 60 km; Lv 70 km; L 200 km; hc 15 km, where rc and rv are the densities of continental crust and the volcanics of the arc, respectively; Lv is the volcanic arc width; L is the arc axis/trench distance; hc is the thickness of continental crust at (under) the trench; h is the thickness of lithosphere in the arc. The volcanic arc is isostatically compensated. The density of the lithosphere is the same as that of the asthenosphere. The water depth is 4.5 km (see text for more explanations). V normal stress sb caused by the buoyancy (Fig. 3b) causes failure of this plate o shore of southern and the stress, sr, defined by the subducting plate Taiwan, along a west-vergent fault dipping under rigidity, as well as the pull force, Fpl, and the force the Luzon Arc. of dynamic interaction, Fd, of the subducted litho- sphere with the surrounding mantle. The forces Fpl and Fd are neglected in this paper. The normal 2. Modelling set-up + stress, sb sr, and the interplate friction stress, tn , are applied to the overriding plate in numerical A one-layer overriding plate containing a vol- models to simulate the deformation and failure of canic arc with a thinned lithosphere floats upon a this plate. We obtained the same failure modes as Winkler (liquid) base with zero viscosity (Fig. 3a). for the compressional oceanic subduction regime. This plate has the same elasto-plastic rheology This time, the failure direction is mainly defined with strain weakening as in the experimental by two opposite torques caused by sb and sr, models of Shemenda (1994) (Fig. 4). The density respectively. Both failure modes are possible in of the plate is 3.3×103 kg/m3, the same as that of nature. The obtained results correspond well with the asthenosphere. The lithosphere is covered by the results from physical modelling of the same 4.5 km of water. The kinematic boundary condi- process (Chemenda et al., 1997) and are applied tion at the right edge (Fig. 3) allows no displace- to the ongoing subduction of the Eurasian conti- ment in the horizontal direction. At the left edge nental margin in Taiwan. Based on a wavelet of the model, the wedge of the elasto-plastic conti- statistical analysis of seismicity and on gravity nental crust is placed beneath the overriding plate modelling, we argue that the subduction of the along the interplate surface (Fig.
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