Tentonic Characteristics And Deformation Mechanism Of The Southern Section Of Longmenshan And Its Frontal Area

On April 20, 2013, the M7.0 earthquake occurred in Lushan County, Ya’an City, the southern section of Longmenshan, it attracted people’s attention to tectonic deformation of the southern section of Longmenshan. Supported by some relevant subjects, this paper will based on the data of typical seismic profiles and strata distribution, combining with historical earthquake data make a tectonic belt division on the southern section of Longmenshan and its frontal area, and calculate the tectonic shortening of each belt in different period since Triassic by balanced cross section, Sand-Box analog models and other methods. Combined with the deep geophysical data and field geological investigation, Lushan earthquake mechanism and deformation data to discuss the tectonic dynamic mechanism on the southern section of Longmenshan and its frontal area. The flowing conclusions mainly got:(1)The characteristics of Longmenshan orogenic belt respectively are complicated strata, multiple segments and multiple zonation on tectonic and demarcated on landscape. The southern segment of Longmenshan fault zone is different from north and middle segment in structure and landscape. There are strongly deformation in late Cenozoic,greater relief amplitude and developing a series of fault related fold.(2) The southern segment of Longmenshan fault zone and its leading edge region can be divided into the Longmenshan thrust zone and Longmenshan frontal propagation deformation belt. The structure characteristic is stratified vertically in Longmenshan southern segment and its leading edge region, where developed at least three detachment layer,which can be divided into shallow,deep and middle detachment layer. There are two completely different structure style and strata deformation pattern in detachment layer up and down. On the detachment layer, development parallel arrangement or diagonal fault related fold from Shuangshi fault east to Longquanshan tectonic belt which based on middle and lower Triassic rich plaster salt layers as bottom slip plane. Under the detachment layer, is well preserved horst-garben tensional tectonic.(3)The displacement generated by the activity of Longmenhan faults transmits to the mountain front through the decollement layer. Then the displacement transmits to the fault- detachment fold belt nearby, when it encounter the resistance of the blind fault ramp. And it threads the anticlinal core and transmit to the mountain front and basin along the slip surface unceasingly. The law of descending of the shortening amount shows the transmission and descending of the tectonic stress.(4)Sand box analog models could show the form and process of the tectonics intuitively in both large and small range. It shows that different buried depth of detachment layer would generates different tectonics, no matter the property of the detachment layer is same or different. The formation of reverse fault in fold-and-thrust belt need experience a process that parallel stratum would shorten, this process on the top of the detachment layer is different to the lower, and the deformation on the top of the detachment layer is more quick and far.And the tectonic styles are discordant between the top and bottom.(5)There is a certain relationship between the Lushan earthquake and the Wenchuan earthquake, Wenchuan earthquake has a triggering effect on earthquake in Lushan. Two earthquake mechanism is different, reflecting the Longmenshan earthquake with different earthquake modes, with different segments.(6) Compared the balanced cross-section restoration results and physical sand box test, we can find out the regular pattern of the temporal and spatial evolution of Sichuan Southwest basin tectonic deformation in intuitive. There are existing a certain tectonic evolution style from early to late: wide flat folding period, thrust structure period, triangle structure, imbricate thrust stage; from the edge to the internal structure of the basin style also has certain change rule: the edge of the basin the imbricate thrust belt, its internal fault triangular structural belt, basin within convex tectonic belt, the wide fold belt.