柴达木盆地现今大地热流与晚古生代以来构造-热演化

依据钻孔系统稳态测温、静井温度资料与实测热导率数据分析了柴达木盆地地温场分布特征,建立了柴达木盆地热导率柱,新增了17个大地热流数据.柴达木盆地现今地温梯度介于17.1~38.6℃·km-1,平均为28.6±4.6℃·km-1,大地热流介于32.9~70.4mW·m-2,平均55.1±7.9mW·m-2.盆地不同构造单元地温场存在差异,昆北逆冲带、一里坪坳陷属于"高温区",祁南逆冲带属于"中温区",三湖坳陷、德令哈坳陷及欧龙布鲁克隆起属于"低温区",盆地现今地温场分布特征受控于地壳深部结构、盆地构造等因素.以现今地温场为基础,采用磷灰石、锆石裂变径迹年龄分布特征定性分析与径迹长度分布数据定量模拟相结合,研究了柴达木盆地晚古生代以来的沉积埋藏、抬升剥蚀和热演化史,并结合区域构造背景,对柴达木盆地构造演化过程进行了探讨,研究表明柴达木盆地晚古生代以来经历了六期(254.0—199 Ma,177—148.6 Ma,87—62 Ma,41.1—33.6 Ma,9.6—7.1 Ma,2.9—1.8 Ma)构造运动,六期构造事件与研究区构造演化的动力学背景相吻合.其中白垩纪末期(87—62 Ma)的构造事件导致了柴达木盆地东部隆升并遭受剥蚀,欧龙布鲁克隆起形成雏形,柴达木盆地北缘在弱挤压环境下形成坳陷盆地;中新世末的两期构造事件(9.6—7.1 Ma和2.9—1.8 Ma)使柴达木盆地遭受强烈挤压,盆地快速隆升,构造变形强烈,基本形成现今的构造面貌.

Present-day heat flow and tectonic-thermal evolution since the late Paleozoic time of the Qaidam basin

The Qaidam basin is the largest intermountain basin on the northern margin of the Tibetan plateau, and is one of three major petroliferous basins in western China. The characteristics of tectonic-thermal evolution of this basin are important for understanding the uplift mechanism of the Tibetan plateau. This work studies the terrestrial heat flow and thermal history of the basin, which would help research the regional dynamics and tectonic evolution of the basin and can be applied to the petroleum resource assessment in this region. This work employed the advanced temperature continuous acquisition system which measures the deep well steady state temperature at a large scale and high accuracy. Then mass of rock thermal conductivity was tested by the optical scanning method. Borehole temperature measurement data were fitted linearly using the least squares method, yielding 17 borehole profiles of geothermal gradients. Furthermore, original temperature measurement data from previous studies were re-analyzed, resulting in 56 geothermal gradients and the measured heat flow data of boreholes. Based on the present geothermal field, the time-temperature history of the samples was simulated by the combination of analysis of apatite fission track ages with modeling of fission track length distribution. For this simulation, the one-component annealing model, Monte Carlo method, and the software Hefty were used. Apatite samples enclosed track length values and the median age were adopted to simulate the basic parameters, with kinetic parameter (Dpar) as the initial value of 1.65, and the initial track length 16.3 μm. To the zircon fission track samples which did not pass ² test (P(²)<5%), decomposition on single grain fission track age data and frequency statistics analysis were carried out. Based on the analysis of thermochronological dating, we have studied the burial history, uplift and denudation and thermal history of the Qaidam basin. Then we analyzed the tectonic evolution of the Qaidam basin combining with regional tectonic setting. Geothermal gradients of the Qaidam basin vary from 17.1 to 38.6 ℃·km^-1 with an average of 28.6±4.6 ℃·km^-1. The thermal conductivity of the basin varies from 0.601 W/(m·K) to 5.520 W/(m·K) with an average of 2.208 W/(m·K), mostly in the range 1.820 W/(m·K) to 3.020 W/(m·K). Present-day terrestrial heat flow in this area varies from 32.9 to 70.4 mW·m^-2, with an average of 55.1±7.9 mW·m^-2. The test shows that the apatite fission track age varies from 61 Ma to 72 Ma, less than the formation age. The mean closed track length varies from 11.8 to 12.7μm, less than the initial length of the track (16.3±0.9 μm), implying a single-mode distribution. The test results of zircon samples show that the zircon fission track ages concentrate in 155 Ma to 195 Ma, less than the formation age, which have two time periods, i.e. from 254 Ma to 199 Ma and from 177 Ma to 148.6 Ma. There exits the distribution difference of the geothermal field in the Qaidam basin. The Kunbei thrust belt and Yiliping depression are "hot zones", the Qilian thrust belt is a "warm zone", and the Sanhu depression, Oulongbuluke uplift, and Delingha depression are "cold zones". Such distributions of the geothermal field might be controlled by crustal structure in the basin. Based on the present-day geothermal field, the burial history, uplift and denudation and thermal history of Qaidam basin are studied by the combination of analysis of apatite fission track and zircon fission track ages with modeling of fission track length distribution. In terms of the analysis of thermochronological dating, we further explore the tectonic evolution of the Qaidam basin combining with regional tectonic setting. The results show that the Qaidam basin had experienced tectonic movements of six phases (254.0—199 Ma, 177—148.6 Ma, 87—62 Ma, 41.1—33.6 Ma, 9.6—7.1 Ma, 2.9—1.8 Ma); these tectonic events just coincided with the regional tectonic settings of the study area. The eastern Qaidam basin was uplifted and suffered denudation. The prototype of the Oulongbuluke uplift was also formed in the end of Cretaceous (41.1—33.6 Ma), however, the depression basin developed under the weak compressive tectonic regime in the northern Qaidam basin. The present tectonic features of the Qaidam basin appeared after uplift and great structural deformation, which was related with the intense extrusion since the end of Miocene (9.6—7.1 Ma and 2.9—1.8 Ma).