The strength of the crust plays a critical role in controlling the size and recurrence time of earthquakes, as well as the evolution of plate tectonics and mountain building over the geological time scale. With a recently developed algorithm, we were able to probe the strength evolution of the rocks in the lower crust beneath the Taiwan orogenic belt, quantified by their viscosity. If rocks have low viscosity, it means they are weak and can be easily deformed; on the contrary, high viscosity means they are strong and stiff.
In this study, we used 14 years of Global Positioning System (GPS) data following the Chi-Chi earthquake that struck central Taiwan with magnitude 7.6 in 1999 to directly image the viscosity in the lower crust. Our approach allowed us to explore the viscosity of rocks in their natural settings and investigate how it distributed and varied over the 14-year time period after the earthquake.
The viscosity of rocks results from the interactions between many physical conditions such as stress, rock composition, grain size, water content, confining pressure, and temperature. In spite of the disparate scales between laboratory and plate tectonics, we found broad agreement between our models and those documented by rock experiments. Comparing the temporal evolution of the viscosity in the lower crust to various theoretical models and laboratory experiments provided insights into the tectonic evolution of Taiwan.
The broad agreement we discovered further encourages us to investigate the plausible range of temperatures and thermal gradients in Taiwan. High thermal gradients indicate the low viscosity of the lower crust along with the absence of earthquakes, reducing the energy budget of mountain building.
By incorporating the laboratory data, our results suggest the temperature beneath the Central Range and Coastal Plain of Taiwan ranges from 437°C to 530°C and 423°C to 557°C, respectively. These results generally agree with the seismicity in Taiwan and indicate an eastward increase of the present-day thermal gradients from 19.5±2.5°C/km in the Coastal Plain to 32±3°C/km in the Central Range.
Our model reconciles geodetic observations, seismicity, and thermal structure in the Taiwan orogenic belt. The high heat flow, thicker crust, and the absence of deep earthquakes is, therefore, a consequence of a weak lower crust beneath the Central Range. This makes it easier to shorten the crust and build the mountain range during the Taiwan orogeny. Our results can provide valuable constraints on the geodynamic study of the mountain building in the Taiwan orogenic belt.
While our models are compatible with laboratory experiments at the steady state, our results emphasize the role of transient deformation during the early postseismic deformation. In the future, the transient behavior of the lower crust may be integrated into geodynamic models of the Taiwan orogeny to incorporate the short-term effect of the seismic cycle.
The 14 years of GPS observations following the 1999 Chi-Chi earthquake illuminates the strength of rocks in the lower crust beneath the Taiwan orogenic belt. Our study demonstrates the potential of geodetic surface observations to access the properties of rocks in their natural tectonic settings and is applicable to any region that experienced a large earthquake with sufficient station coverage.
These findings are described in the article entitled Lower-crustal rheology and thermal gradient in the Taiwan orogenic belt illuminated by the 1999 Chi-Chi earthquake, recently published in the journal Science Advances.