通过碎屑低温年龄恢复西南天山古高度

目前使用低温年代学来恢复造山带古地形,主要是采用在造山带内部采样来做原地高度的恢复。本文提出用碎屑颗粒低温年代学来恢复造山带平均古高度变化率的方法,即通过山间盆地或山前堆积碎屑物大量的单颗粒年龄,获取蚀源区大面积的、区域性的平均剥露速率,进而通过均衡校正计算出蚀源区的平均古高度变化率,为造山带古地形恢复提供了新的途径。本文以西南天山为例进行尝试,通过已发表的339个碎屑颗粒裂变径迹年龄,获得西南天山的68 Ma(年龄峰期)的剥露速率为0.740.60km/Ma,平均古高度变化率为0.150.23km/Ma、0.120.19km/Ma(降低率)。显示西南天山在68 Ma以来发生了比较快速的剥露,如果不考虑构造抬升等因素,平均古高度也发生了比较快速的降低,如果按8 Ma以来计算,则正好降低了1 0001 500m。

Paleoelevation Reconstruction of Southwestern Tianshan Using Detrital Low-Temperature Thermochronology

At present, paleoelevation reconstructiion of the orogenic belt is mainly based on low-temperature thermochronology of samples collected within the orogenic belt.In this paper, a method is proposed to recover the average height change rate of the orogenic belt by using the low-temperature chronology of the detrital grains, which provides a new way for the restoration of orogenic paleo-terrain.It means that the large-scale and regional average exhumation rate can be calculated through the detrital single grain ages located in intramontane basin or piedmont, and then the average ancient height change rate of the erosion area can be calculated by the isostatic correction.In this paper, the calculation results are, by using the published 339 detrital grain ages of fission track, that the exhumation rate of 6-8 Ma (age peak) of southwestern Tianshan was 0.74-0.60 km/Ma, and the average ancient height change rate was 0.15 to 0.23 km/Ma and 0.12 to 0.19 km/Ma (decreasing rate).This conclusion indicates that the southwestern Tianshan has experienced a rapid exhumation since 6-8 Ma ago, in the case of without considering the structural uplift and other factors, and the average paleo-height has decreased rapidly 1 000-1 500 m since 8 Ma ago.