Gravity anomalies and flexure at the West Taiwan basin:

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Presentation transcript:

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

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

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

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

Depth to the base of the foreland basin sequence

Surface loading

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

Sub-surface (buried) loading

Combined surface and sub-surface (buried) loading

Crustal structure

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

Te and the distribution of seismicity

Thickness of Underlying Passive Margin Sequence

Te and the seismogenic layer thickness

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

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

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.

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 10-15 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.