The Thermal Conductivity from Central Cross-lsland Highway and South Cross-Island Highway in Taiwan

Both drilling and down-hole logging data indicate an abnormally high thermal gradient in the Central Range of Taiwan. A common hypothesis is that uplifting rate is faster than the conductive cooling rate, and as a result the heat was kept in hot dry-rock area. A direct way to prove this theory is to collect representative rock samples from two transects across the Central Range (Central and Southern Cross-Island Highway) measuring thermal parameters, including thermal conductivity, diffusivity, and heat capacity. We also utilize thermal profiles from hot-spring wells in the mountain area to obtain the thermal gradient. Armed with these data, we can calculate present heat flow via the equation-1, , where Q is heat flow (W/m²), λ is thermal conductivity (W/mK), T is temperature (K), and z is the depth (m).

Thermal conductivity is a function of both porosity (pressure) and temperature. Our samples are mainly metamorphic rocks with less affected by porosity, and we build up our own temperature model versus thermal conductivity by heating up samples to about 180℃. Combined with experimental equations from other studies, we can extrapolate our model to higher temperature ranges at the depth of the rock formed and presumed paleo-thermal gradient. Basically, thermal conductivity has a negative relationship with temperature. For example, thermal conductivity of marble collected in the Central Highway has an average value of about 3.0 W/mK at 26℃, and decreases to 1.6 W/mK at 450℃.

 With paleo-thermal gradient, we can correlate spatial distribution of the thermal heat with structural cross sections along central and southern cross-island highway and compare the cooling rate obtained from fission track data in the mountain ranges.