THE CRUSTAL AND DEEP STRUCTURES IN GOLMUD-EJIN QI GGT
CUI ZUO-ZHOU;LI QIU-SHENG;WU CHAO-DONG;YIN ZHOU-XUN;LIU HONG-BING(562 Comprehensive Research Brigade,CAGS,Hebei 101601)(Institute of Geophysics, Academia sinica, Beijing 100101)
Golmud-Ejin Qi Deep Seismic Sounding(DSS) Profile crosses three regions, that is Qaidam basin, Qilian mountain and North mountain. The main profile is about 1000 km long. There are 14 pieces of single branch extended spread and 2 pieces of broadside spread.The wave groups Pg, Pm, P1, P2, P3, P4 and Pn have been identified. According to the time-distance curves for different wave group on the record sections, the inversed and 2-D direct calculation has been completed. On the basis of that, whole profile 2-D velocity structure model has been established.The layered structure is clear as shown by the wave field features. The wave group Pg and P2 are strong, P3 and P4 weaker, the Moho reflection Pm is strong again. According to these features, the crusts is divided into three layers: The upper crusts consists of two layers. One is a gradient layer in which the average velocity is from 5. 5 to 6. 0 km/s along the profile. The other is a constant velocity layer, the velocity varies from 6. 1 to 6. 3 km/s along the profile. The average velocity of upper crust is 5. 7 to 6. 1 km/s, the thickness is 13. 5 to 20 km. The middle crust also consists of two layers. One is a low-velocity layer of 5. 8 to 5. 9 km/s. The other has a constant velocity of 6. 4 km/s. The average velocity of whole middle crust is 6. 1 to 6. 3 km/s. Its thickness is 18 km. The lower crust consists of one layer with a velocity 6. 6 to 7. 4 km/s, the typical value is 6. 8 km/s. Its thickness is 12 to 26 km. The velocity of upper mantle is generally 8. 1 to 8. 2 km/s, but is 7. 8 km/s in some areas. The crust structure varies quite strongly along the profile.The whole profile is roughly divided into three different units: 1. Qaidam block: The thickness of crust is 50 to 53 km. It is thinner in the middle and thicker outwards. The average velocity of crust is 6. 15 to 6. 17 km/s, which is comparatively low. There is a lowvelocity layer in the middle crust. This unit has a steady upper mantle and active crust.2. Qilian mountain tectonic zone: The crust is generally thicker than 60 km, nearly 70 km in the Harhu area. There is a relatively thick low-velocity layer. The average velocity is 6. 35 km/s. The lower crust tends to be thicker. It has an active upper mantle and active crust. 3. North mountain tectonic zone: It features a thinner crust between 40 to 43 km.The average velocity is 6. 5 km/s. There is an inversion in velocity values in the middle crust, but it is not low enough to be called a low-velocity layer. Its value is 6. 0 to 6. 1 km/s. This unit has an active upper mantle and a steady crust.The crust structure of each unit has little relation with the crust movement before Mesozoic Era, but is closely related with geomorphologic unit and the feature of gravity map. For this reason, it can be known that they are a result of Himalaya movement.