Study On Crustal Structure And Lithospheric Thermal State In Mongolia And Its Adjacent Areas

Mongolia and its surrounding areas are located in the central part of the central Asian orogenic belt,which is the core region of the central Asian orogenic belt.The gravity and magnetic data and thermal structure of Mongolia and its surrounding areas are analyzed and studied,and the crustal structure and lithospheric thermal state are revealed by combining with the geological and seismic data of this area,which is of great significance to the in-depth understanding of the geological tectonic evolution of Mongolia and its surrounding areas.In this paper,the free air gravity anomaly and the spatial distribution of bouguer gravity anomaly in Mongolia and surrounding areas are studied according to the sg-ugm-1 model of high order gravity field.In order to study the characteristics of gravity anomalies generated by field sources at different depths of the crust,the wavelet multiscale analysis method was used to decompose the gravity anomalies and to invert the moho surface distribution in the study area.At the same time,the thermal structure of lithosphere is studied by using inversed magnetic anomaly data combined with terrestrial heat flow data.The results show that :(1)there is a certain correspondence between the gravity anomaly in Mongolia and the surrounding areas and the relief of the terrain,and the earthquakes are mostly distributed in the abrupt variation belt of the gravity anomaly,and the earthquakes above magnitude 6 are mainly located in the buguer gravity anomaly gradient belt,indicating that the earthquakes above magnitude 6 are caused by tectonic activities deep in the crust.Altay orogenic belt and the tianshan region distribution is relatively concentrated,high gravity anomaly value and distribution of complex,tianshan and altai is influenced by India-Eurasian plate extrusion collision caused deep material gathered in two focused that slow uplift terrain,also proved that the two mountains in the slow formation and complexity of time scales.(2)the gravity anomalies in Mongolia and surrounding areas reflect the lateral heterogeneity of the distribution of materials in the crust and upper mantle.According to wavelet multi-scale analysis of bouguer gravity anomaly and power spectral analysis,combination of 1-4 order details can be found in the am-east block abnormal value plus or minus alternate with distribution,and there is the phenomenon of increased from south to north,reveals the activities between the Pacific plate and Eurasian plate time scale is larger,and the movement of the plates to the Pacific plate and the Eurasian plate.In the details of the sixth order,the anomalous body at the junction of the xingan block and the Siberian platform is large in size and more accurately located at the inflection point,indicating that the area is caused by the upwelling of more dense material.(3)there is a north-south thickness thinning area between the hangai plateau and the altai mountains,which indicates that the area is the result of uplift and thinning in the middle caused by the two parts of the crust thickening activities squeezing each other.There is a banded crustal thickening zone in the south of the baikal fault zone,which indicates that the baikal fault is caused by the far field effect of subduction and subduction of the Pacific plate.In the sayan block,the altai block,the junggar block and the tarim block,the moho surface distribution is very complex and deep,indicating that the deep tectonic activity in this area is intense due to the compression and collision of the Indian and Eurasian plates along the northwest to southeast direction.(4)the thermal structure model of "hot crust and cold mantle" is mostly used in Mongolia and surrounding areas,and few of them are "cold crust and hot mantle" type.,on the southern margin of the baikal fracture crustal heat flow ratio is reduced,for "cold shell hot mantle" type thermal structure model,speculated that the region is brittle in middle crust and the rheological characteristics of ductile lower crust and mantle,the "strong mantle-weak crustal" the characteristics of lithosphere and the baikal fault zone of the existence of the region and in the southern margin of the phenomenon of high heat value,and pointed out the platform and the sing-an-east Siberia plot and design plot of intersection point for the key of the mantle material upwelling area.(5)the high value region of the moho surface temperature generally corresponds to the region of relatively thin lithospheric thermal thickness.West margin of junggar basin and the southern margin of the altay mountains of moho temperature high value area,and the huge value within the difference in temperature in junggar basin,speculation in the region is being squeezed from India Eurasia plate's collision,the earth's crust tectonic activities strong,but due to the rigidity characteristics of typical basin in junggar basin,deep only after the material is extruded in the western margin of junggar basin and the southern margin of altay mountains,finally rose along the fissures,caused two parts of the temperature rise of moho.