Back to Taiwan Main PageEvaluation of the Classic Model
Studies since 1995 have revealed some holes in older models and the critical wedge model of Taiwan orogenesis:
The Lithospheric Model of Collision
- Isotope geochemistry clearly shows the affinity of the Tananao basement complex of Taiwan with the Fujian province of coastal China. Therefore the Tananao basement complex is not an exotic block accreted to Taiwan.
- Interpretations of the Lishan fault and the Lichi Melange in the Longitudinal Valley have changed. Interpretations now favor processes consistent with those presently observed offshore south of Taiwan. The collapse of a forearc basin is clearly shown in this offshore area.
- Deep structure observed from seismic, gravity, and thermal studies contradicts that predicted by a critical wedge model in the following ways:
- The seismic gap observed under the Central Ranges has no explanation under the critical wedge theory.
- The Moho under the deepest part of the critical wedge model is significantly shallower (by 10 km) than the deepest Moho estimated from receiver functions and tomography.
- Tomography shows significant variation in lithospheric P wave velocity in the crust and mantle not accounted for by the critical wedge model.
- Anisotropy shows a correlation between the orientation of deeper lithospheric structure and upper crustal structure. If the critical wedge model is correct, this should not happen, because the upper crust and lithosphere are decoupled.
- Observed surface heat flow does not agree with that predicted by critical wedge heat flow models.
- Leveling and thermochronologic data indicate a possible acceleration uplift in the Central Ranges over the last million or so years. This result would only be reproduced by specific conditions in the critical wedge that result from weather patterns causing increasing erosion off the western slope of Taiwan over the last million years.
The continental subduction of Taiwan orogenesis is best illustrated in figure 9. While a wedge-shaped fold and thrust belt is part of the model, especially in western Taiwan, it differs from the critical wedge model in many ways:
- The buoyancy of the subducted continental crust drives the collapse of the forearc basin and the collision of the arc with the continent, rather than the arc creating a backstop against which to pile continental crustal materials.
- The lower lithosphere of Eurasia is coupled with the overlying upper crust and they deform, in part, as a coherent block.
- Decoupling of the Eurasian continental lithosphere from the down-going oceanic slab is a driving force in the uplift of the Taiwan orogeny.
The continental subduction model fits the data shown above that contradicts the classic critical wedge model:
- The chronology of deformation processes (buildup of wedge, then forearc collapse) matches that seen off the south shore of Taiwan.
- Deep structure and uplift are explained:
- The seismic gap is explained by an increased thermal gradient under the Central Ranges due to decoupling and ensuing rapid uplift.
- Lithospheric involvement (i.e. thickening) could account for the observed Moho depths and anisotropy.
- The tomographic high under the Central Ranges is explained by the exhumation of lower crust and/or mantle due to decoupling and uplift.
- The surface heat flow pattern and known uplift rates match that predicted by thermal modeling of the recent rapid uplift model.
- Metamorphic facies transitions due to a deeply subducted continental crust is not accounted for in any investigated model. Facies changes in the upper and lower crust and the mantle can significantly affect isostasy and uplift after decoupling of the oceanic slab from the Eurasian continental crust.
- Uplift rates should be calculated and compared for both the exhumation and critical wedge theories.
- A comparison of the continental subduction and exhumation model with other orogenic systems where the same thing is supposedly happening (i.e. the Apennines) could be informative.
- Receiver functions could be redone using the slab dip from Benioff zone seismicity and anisotropy from the Rau et al. (2000) study.
- Higher resolution thermochronology across the Central Range could serve as a test of the exhumation model.
- Further study of offshore southern Taiwan could provide further illumination of processes that acted on onshore Taiwan in the past.
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Last edited 5/3/2001 by Megan Anderson firstname.lastname@example.org