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

  • Exhumation rate was constantat~2 mm/yr in1.5-0.2 Ma before athree-fold increase
  • Since at least 1.5 Ma, extension of Ryukyu subduction zonehas influenced NE Taiwan
  • Paleogeothermal gradients increased from 75 to 150 ℃/km inthe last 1.5-0.2 Ma
Abstract

The northeast region of Taiwan has experienced a stress transition from the compressive collision to the extension induced from the Ryukyu subduction systemin a post-collisional orogenic belt. In order to clarify when the extensional stage began and how the cooling and exhumationhistory varied with the transition of tectonic stress, apatite and zircon fission-track ages on the granitic gneissin northern Tananao Complex were collected from a ~2700 m high sub-vertical profile. The exhumation ratewas estimated as ~4 mm/yr since 4.1-3.0 Ma using previous biotite 40Ar-39Ar ageswith 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, 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 interpretedas the onsetof the extensional impact into NE Taiwan fromthe Ryukyu subduction zone. After ~0.2 Ma, the intercepts of apatite AER suggest that exhumation accelerated to ~5 mm/yr 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 1.5to 0.2 Maimplies the increase in paleogeothermal gradientwith time. The paleogeothermal gradientsare estimatedto beover 100℃/kmafter 1.5 Ma, suggestingthat the Hoping area possesses heat sources induced from the complex tectonic configuration. However, the modern geotherm is relatively low and may beinfluenced by the water circulation near surface.

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