1. Gravity anomalies and flexure at the West Taiwan basin:
Implications for the strength of extended continental lithosphere
A. B. Watts and A. T.-S. Lin
Oxford University
Rifted margin and foreland basin sequence
Surface loading
Sub-surface (buried) loading
Elastic thickness, Te
Te, yielding and seismicity

2. Topography and gravity anomaly of the Taiwan region
Passive margin formed by rifting
during the Paleogene
Orogenic belt with
metamorphic core complex
Orogenic loading
+/- couple
Passive margin edge effect
high

3. Seismic reflection Profile AA’ showing the West Taiwan
foreland basin and the underlying East China Sea rifted margin
Rifted margin
Foreland

4. Tectonic subsidence and uplift Backstripping :
Mechanical compaction model (Audet 1995)
Watts & Steckler (1979) sea-level curve
Airy isostasy

5. Depth to the base of the foreland basin sequence

6. Surface loading

7. Gravity, flexure and crustal structure due to surface loading
Yeh et al (1998,
1999)

8. Sub-surface (buried) loading

9. Combined surface and sub-surface (buried) loading

10. Crustal structure

11. Combined surface and sub-surface (buried) loading, Te, and
vertical motions

12. Te and the distribution of seismicity

13. Thickness of Underlying Passive Margin Sequence

14. Te and the seismogenic layer thickness

15. Thermal structure, Yield Strength Envelope, and Tmechanical
Burov & Diament (1995)
h1 = 17 km, hc = 32, h2 = 50 km
Tmechanical = 22 km
h1 = 24 km, hc = 32, h2 = 58 km
Tmechanical = 33 km

16. Brittle-ductile transition, seismicity, Te and Tmechanical
Te ~ ZBDT
Seismicity peaks at the BDT but, extends into ductile lower crust
and brittle mantle.
Tmechanical >> Te

17. Tmechanical, curvature, and Te.
smax = 500 MPa
log10(K) = -6.2
Te/Tmechanical = 0.5
Te = 13 km. Hence, Tmechanical = 26 km
Te ~ Tseismogenic because of (young) initial thermal age of rifted
continental lithosphere and (high) stress/curvature induced by
subsequent orogenic loading.

18. Conclusions
The West Taiwan foreland basin formed by orogenic loading of a rifted continental margin.
Flexure modeling shows that the West Taiwan basin can be explained by combined surface and sub-surface (buried) loading.
The Te that best fits the depth of the base of the foreland sequence is 13 km.
This Te is low when compared to some foreland basins, but is similar
to rift-type basins in extensional settings.
Seismicity peaks at km which corresponds to the depth to
the brittle-ductile transition in the upper crust and to Te
Te, however, is not the same as the seismogenic layer thickness
Yield strength envelope considerations suggest that Te of the West Taiwan basin is determined by the (young) initial thermal age associated with rifting and by the (high) stress/curvature associated with orogenic loading.