Post-collisional exhumation and geotherm pattern in northern Tananao Complex, northeastern Taiwan

Abstract

The northeast region of Taiwan has experienced a stress transition from the compressive collision to the extension induced from the Ryukyu subduction system in a post-collisional orogenic belt. In order to clarify when the extensional stage began and how the cooling and exhumation history varied with the transition of tectonic stress, apatite and zircon fission-track ages on the granitic gneiss in northern Tananao Complex were collected from a ~2700 m high sub-vertical profile. The exhumation rate was estimated as ~4 mm yr-1 since 4.1 - 3.0 Ma using previous biotite 40Ar-39Ar ages with an assumed uniform geothermal gradient. During the period of 1.5 - 0.2 Ma, the exhumation rate remained at 2.3 - 1.6 mm yr-1, evidenced by the similar slopes for apatite and zircon in the age-elevation relationship (AER). The decrease in exhumation occurred in between 3.0 and 1.5 Ma is interpreted as the onset of the extensional impact into NE Taiwan from the Ryukyu subduction zone. After ~0.2 Ma, the intercepts of apatite AER suggest that exhumation accelerated to ~5 mm yr-1 at the mountain range but was steady at the valley bottom. The offset in ages of the vertical profile is supportive of the normal faulting at the shallow crust. The accelerating cooling with the steady exhumation from 1.5 - 0.2 Ma implies the increase in paleogeothermal gradient with time. The paleogeothermal gradients are estimated to be over 100°C km-1 after 1.5 Ma, suggesting that the Hoping area possesses heat sources induced from the complex tectonic configuration. However, the modern geotherm is relatively low and may be influenced by the water circulation near surface.