| Heat is the driving force of geological events and processes,and records and responds to them.Terrestrial heat flow and the lithospheric thermal structure,including the distribution ratio and component relationships between crustal and mantle heat flow and the temperature field,are key research elements in geothermic,but also important parameters in the study of plate movements and geodynamics.It is a profound reflection of the thermal state and energy balance of the Earth’s interior.The Qinghai-Tibet Plateau is the Earth’s largest,most complex,thickest,and newest plateau.Over the past century,the tectonics,evolutionary processes,and deep dynamical mechanisms of the Plateau have been a topical and challenging issue among the frontier problems of the international community.The study of terrestrial heat flow and lithospheric thermal structure provides an important basis for the tectonic-thermal evolution,thermal lithospheric thickness,and geophysical properties of the Qinghai-Tibet Plateau.The Qinghai-Tibet Plateau is a geothermal anomaly zone formed by plate convergence.Located on its eastern edge,the western Sichuan region is the favorable target area for the high-temperature geothermal system,with abundant geothermal resources and great potential for power generation.However,its thermal mechanism and heat source composition relationship are urgent issues to be resolved.The high geothermal anomaly in western Sichuan is controlled by the thickened crust formed by Indo-Asian collision since Cenozoic,and influenced by several shallow factors,such as high-rate active faults,multi-stage magmatic activity,and densely distributed high temperature springs.This paper studies the current temperature field and lithospheric thermal structure in western Sichuan.It focuses on the differences in the present-day temperature field,the deep thermal mechanisms,and the shallow influences on the eastern margin of the Tibetan Plateau.To address the above problems,this paper started from the geothermal geological survey,the author extensively carried out steady-state temperature measurement and rock thermophysical tests in the blank area of heat flow measurement points,obtaining 4 heat flow data.Moreover,the influences of the thickened crust,fault shear thermal effect,magmatic activity,and groundwater convection on the temperature field were analyzed using finite element numerical simulation together with the geophysical exploration findings in the region.Finally,the thermal contribution of fracture shear friction in the region and the thermal regime difference between western Sichuan and adjacent areas were defined.This paper results in the following accomplishments and understandings:(1)Steady-state temperature logging for borehole was carried out systematically and experiments for core samples thermal properties have done.A total of 5 wells were obtained the steady-state temperature curve and added 4 terrestrial heat flow data,including two class A.A total of 60 groups of thermophysical parameters were obtained,which improve the rock property parameters in this area.According to the temperature logging,the temperature gradient ranges from 32.1 to 37.3℃/km in the western Sichuan,and the terrestrial heat flow ranges from 94.7 to 116.2 m W/m~2 in the songpan—ganzê,which is significantly higher than the mean of global terrestrial heat flow.(2)Although the terrestrial heat flow in the western Sichuan region is significant,the results of the numerical simulation do not indicate an exceptionally high deep heat background.Generally,the lithospheric thermal structure is the hot crust—cold mantle type in western Sichuan,with the temperature field characteristics of the strong tectonic zone.The crustal and mantle heat flow contribution are approximately 70–75 m W/m~2and 30–35 m W/m~2 respectively,the crust-mantle heat flow ratio is close to 2.The average temperature of moho surface is 1000℃in this area.However,the lithospheric thermal structure in the Sichuan Basin is characterized by a warm crust—warm mantle type,with a stable cratonic temperature field characteristics.The crustal heat flow contribution is approximately 30～35 m W/m~2,while the mantle heat flow contribution is approximately 35mW/m~2,the crustal—mantle heat flow ratio is close to 1,and the average moho surface temperature is 650°C.(3)The Longmen mountains is the gradient zone of the regional temperature field.The thickness of the thermal lithosphere and the rheological boundary layer gradually increased from western Sichuan to the Sichuan Basin.In the western Sichuan,the thermal and seismic lithospheric thicknesses are 80～90 km and 80～100 km,respectively.However,the thermal lithospheric thickness is～120 km in the Sichuan Basin east of the Longmen Mountains,and the thickness of the seismic lithosphere is about 120～150 km.(4)The shear heating of the Xianshuihe fault and the Longmenshan fault is a local heat source,with the range and degree of influence controlled by the structure,depth,and activity of the fractures themselves.It is worth noting that the physical qualities of the rocks influence the shear heating process of the fractures.When the friction coefficient is 0.5 and the slip rate is 10 mm/a,the maximum shear thermal effect of the Xianshuihe fault is about 30 m W/m~2,with a diminishing trend from the fracture center to both sides.It should be noted that the influence range of a thermal effect increase greater than 10 m W/m~2 is less than 50 km.(5)Western Sichuan is rich in geothermal resources and a high-quality development target area for high-temperature geothermal systems.The geothermal system is primarily governed by the thicker crust in the complex tectonic background of the Qinghai-Tibet Plateau.The faults in the area are deeply pierced into the crust,creating a favorable channel for deep thermal material to surface.The faults’continual high-speed slip produces a local heat source,while groundwater infiltration at shallow depths through deep fractures increases heat transfer and redistribution to the surface.One of the thermal anomaly variables that cannot be disregarded is the widespread distribution of intermediate-acid magmatic rocks with significant radioactivity.Otherwise,it should also be mentioned that one potential heat source may come from the crust’s broad high-conductivity and low-speed properties. |
