阿尔泰—阿尔金地学断面地壳结构

CRUSTAL STRUCTURE ALONG THE GEOSCIENCES TRANSECT FROM ALTAY TO ALTUN TAGH

根据阿尔泰—阿尔金地学断面的地震纵、横波资料 ,建立了地壳速度及泊松比结构 .测区的地壳具有明显的三分结构特征 ,其纵波速度自上而下依次为 6 .0～ 6 .3km s、6 .3～ 6 .6km s及 6 .9～ 7.0km s ;阿尔泰南缘的地壳最厚 ,为 5 6km ,准噶尔盆地的地壳最薄 ,为 4 6km ,大部分地区的地壳厚度为 5 0km左右 .准噶尔盆地与天山之间上地幔顶部的纵波速度为 7.7～ 7.8km s ;阿尔泰南缘及塔里木盆地上地幔顶部的纵波速度较高 ,为 7.9～ 8.0km s.测线南部 ,包括东天山及塔里木东缘 ,自地表至 30km深处的地壳纵波速度低 ,泊松比为 0 .2 5 ,表明上地壳主要为石英及花岗质成分 ;而测线北部 (包括阿尔泰及准噶尔盆地 )的中、上地壳则呈现较高的泊松比 (0 .2 6～ 0 .2 7) ,可能为基性地壳的体现 .厚 15～ 30km的下地壳纵波速度 (6 .9～ 7.0km s)较高 ,泊松比为 0 .2 6～ 0 .2 8,可能以镁铁质的麻粒岩成分为主 .位于天山及其南侧地壳中部的低速层 (VP=5 .9km s,σ=0 .2 5 )则可能为晚古生代的构造热事件中的花岗质侵入岩 .

Based upon the P- and S-wave data acquired along the geoscience transect from Altay to Altun Tagh in Northwest China, the crustal structures of velocities and Poisson's ratio are determined. The crustal velocity structure features an obvious three-layer structure with velocities of 6.0～6.3km/s, 6.3～6.6km/s and 6.9～7.0km/s from surface to depth, respectively. The crustal thickness along the entire profile is mostly 50km with the thickest crust (56km) beneath the Altay and the thinnest (46km) beneath the Junggar basin. The velocities underlying Moho are 7.7 to 7.8km/s between the Tianshan and the Junggar basin, and 7.9 to 8.0km/s below the Altay Mountains and eastern margin of the Tarim basin. The southern half of the profile, including the eastern Tianshan Mountains and eastern margin of the Tarim basin, shows low P-wave velocities and σ=0.25 to a depth of 30km, which suggests a quartz-rich, granitic upper crustal composition. The northern half of the profile below the Altay Mountains and Junggar Accretional Belt has a higher Poisson's ratio of σ=0.26～0.27 to a depth of 30km, indicative of an intermediate crustal composition. The entire profile is underlain by a 15 to 30km thick high-velocity (6.9～7.0km/s; σ=0.26～0.28) lower crustal layer that we interpret to have a bulk composition of mafic granulite. At the southern end of the profile a 5km-thick mid-crustal low-velocity layer (V P=5.9km/s, σ=0.25) underlies the Tianshan and the region to the south, and may be indicative of granitic intrusive in Late Paleozoic.