东海陆架盆地西湖凹陷岩石圈热流变性质

为了探讨东海陆架盆地西湖凹陷岩石圈热流变性质,本文以实测地温数据为依据,模拟西湖凹陷岩石圈热结构,在此基础上,应用流变学原理模拟确定西湖凹陷岩石圈流变性质。结果表明,西湖凹陷岩石圈为一个冷地壳-热地幔、强地壳-弱地幔的"奶油蛋糕"型岩石圈。西湖凹陷平均地表热流密度为71 m W/m~2,地幔热流密度为40~65 m W/m~2,对地表热流密度的贡献度达73%~79%,地表热流受地幔热流控制,莫霍面温度在700℃左右,热岩石圈平均厚度为66 km。西湖凹陷岩石圈流变分层明显,上、中地壳基本为脆性层,下地壳和岩石圈上地幔为韧性层,岩石圈总流变强度平均约为2.65′10~(12) N/m,其中地壳流变强度为2.12′10~(12) N/m,地幔流变强度为5.29′10~(11) N/m,有效弹性厚度为11.7~14.5 km,地壳的流变性质控制了岩石圈的流变行为。此外,西湖凹陷岩石圈总强度较低,在构造应力作用下易于变形,且存在壳幔解耦现象。西湖凹陷岩石圈热状态及流变性质决定了西湖凹陷东部地区主要以浅部地壳的断层滑动和地层破裂来调节深部的构造应力。

Thermal-rheological Property of Lithosphere Beneath Xihu Sag,East China Sea Shelf Basin

In order to study the thermal-rheological properties of the lithosphere beneath the Xihu Sag,East China Sea Shelf Basin,the lithospheric thermal structure is simulated on the basis of measured geo-temperature and rock thermo-physical parameters with geothermal approaches.The lithospheric rheological profile is determined through rheological modelling based on the thermal structure of lithosphere.The results indicate that:the lithosphere of the Xihu Sag has thermal-rheological characteristics of cold-strong crust and hot-weak mantle,as a typical "crème br(u)lée" model.The average surface heat flow of the Xihu Sag is 71 mW/m2,the mantle heat flow is about 40-65 mW/m2,which is up to 73％-79％ of the surface heat flow.Additionally,the average thickness of thermal lithosphere is 66 km,with the temperature at Moho is about 700 ℃.The above thermal parameters reflect a high thermal state of the lithosphere in the Xihu Sag,which is mainly controlled by the deep mantle.The lithosphere of the Xihu Sag shows obviously rheological stratification properties,the upper and middle curst are almost in the brittle field,and the lithospheric strength is about 2.1× 1012 N/m.In contrast,the lower curst and upper mantle present the characteristics of the ductile field,with the lithospheric strength decreasing to 5.5× 1011 N/m.Meanwhile,the total strength of lithosphere beneath the Xihu Sag is about 2.65× 1012 N/m,the brittle to ductile transition depth is 7.1-7.3 km,the thickness of the competent lithosphere is 30.7-36.2 km,which is significantly thicker than effective elastic thickness,with a depth of 11.7-14.5 km.The rheological structure reveals that the rheological behaviors of the lithosphere are mainly controlled by the rheological properties of the upper crust.Besides,as a low-strength block,the lithosphere of the Xihu Sag is vulnerable to deformation caused by the tectonic stress and thermal state,which commonly results in decoupled deformation between the crust and mantle.The above thermal-rheological properties of lithosphere decide that the fault activities and formation breakup are the main way to adjust the tectonic stress in the east area of the Xihu Sag.