Struntural Analysis Of The Piedmont Fault Zone In Southwestern Depression Of Tarim Basin

The “piedmont fault zone” is a geomorphic term for geological structure,referring to the regional fault zone between the uplift of orogenic belt and basin or plain.In this paper,“piedmont fault zone” was used to describe the tectonic belts between the Southwest Tarim Depression and the orogenic belts on the north or south sides of the depression.It not only refers to the geomorphological location of this tectonic belt in the basin-mountain transition zone,but also attempts to emphasize the intrinsic relationship between the tectonic evolution of basin and mountain.Following the principle of structural analysis,this paper establishes the structural geometry models of different structural positions in the piedmont fault zone of the Southwest Tarim Depression on the basis of geological interpretation of surface geological outcrop data,seismic data and gravity and magnetic exploration data.The structural evolution process of the structural geometry models is analyzed by balanced section principle.Based on the understanding of regional geological background,basin-mountain structure and evolution process,the analogue model was designed to explore the formation mechanism of piedmont fault zone.On the basis of structural analysis,the conditions of hydrocarbon accumulation and distribution of hydrocarbon traps in piedmont zone are analyzed.The paper has obtained the following geological understandings:(1)Based on the comprehensive interpretation results of regional geological background and geological-geophysical data,it is considered that the tectonic deformation between the Southwestern Tarim Depression and the orogenic belts on its north and south sides is two independent fold and thrust systems which rooted in the piedmont deep faults and with the characteristics of vertical stratification,horizontal zonation and segmentation along the strike.Vertically,the Middle Cambrian gypsum rock and Paleogene/Miocene gypsum-mudstone are the slip layer to separate the stratum into three structural deformation layers,with the characteristics of stratified compression and structure superimposition.From the orogen to basin,the fold and thrust systems can be divided into root zone,middle zone and forward zone.Overall,the fold and thrust structures of the middle zone and forward zone extend parallel to the root zone thrusts.Influenced by the character and displacement of the basement-involved faults in root zone,the faults in middle zone and forward zone are characterized by different structural combinations of linear paralled,oblique or arc shape.Along the strike,the fold and thrust system in around the depression can be divided into several segements which are controlled by different root faults deformation or separated by NW-SE or S-N strike-slip faults.From west to east,thrust system of Western Kunlun Mountains can be divided into three sedments: Fusha-Kedong,Kashi-Yecheng and Wuboer.South Tianshan thrust system was divided into eastern Wuqia segment and the western Wuqia segment which are bounded by NW-SE Talas-Fergana fault.(2)According to the the structural unconformity surface,the distribution of sedimentary strata in syntectonic period and the restoration of geological profile,the Cenozoic tectonic evolution in the Southwestern Tarim Depression and the Piedmont areas on both sides can be divided into four stages: basin-mountain flexure in Paleogene,initial compression in Miocene,intense compression in Pliocene and frontal expansion since Pleistocene.In the case of Cenozoic regional compression,the formation and evolution of thrust system are characterized by forward propagation from deep faults along the basin-mountain boundary to the basin,and the structural deformation occurred in stages and progressively.The flexural uplift of basins and mountains in Paleogene laid the foundation of piedmont thrust structure system.Since Miocene,the strong contraction of West Kunlun Mountains and Southern Tianshan Mountains caused the revival of Piedmont deep fault zone and formed two north-south thrust structure systems.In the initial compression stage of Miocene,the basement involvement thrust wedge of basinmountain boundary began to form.Since the intense compression stage of Pliocene,the fold and thrust deformation of multi-layered detachment gradually extended to the foreland direction until the present thrust system was formed.(3)Based on the comprehensive analysis of the temporal and spatial differences of tectonic combinations,structural styles,and crustal structure of basins and mountains,it is considered that the thrust systems on both side of depression conform to the dynamic model of the “root zone thrust system”.That is,the regional compression,caused by the collision between the Indian plate and the Eurasian plate in the Late Cenozoic,lead to the contraction and uplift of the Western Kunlun and South Tianshan orogenic belts.The dynamic reason for the development of thrust system are the process of the orogenic belts upwelling or oblique thrust along the deep faults in root zone.The difference of tectonic deformation combinations in the middle and forward zone of the thrust system are related to the occurrence and displacement of root deep faults,and the distribution of sedimentary strata filled in the basin.(4)Based on the analogue experiments,it is considered that the competent and the structure of strata changing vertically or horizontally have an important influence on the structural deformation pattern.Under the condition of thin weak layer,thrust system is not easy to peopagate forward.The dutile basement is more prone to shortened,which can promote the detachment deformation occurred in the situation of different weak layer thickness,forming a stratified slip,superimposed fold and thrust structure combination.As a result,the thinner thickness of the weak layer in the caprock of Southwest Tarim Depression leads to the less extension width of thrust system,forming a strong deformation zone near the basin margin.The ductile basement of the orogenic belt is favorable for the uplift of thrust wedge in root zone,and forming a wider fold and thrust system with the style of caprock detachment.(5)Comprehensive analysis of regional petroleum geological conditions and geological characteristics of proven oil and gas reservoirs shows that faults are the key factors controlling hydrocarbon accumulation in piedmont thrust belts.The faults near the root zone,which are thrust faults and strike-slip thrust faults involved in the basement or slipped along the basement surface of the basin,are easy to become oil source faults.The key to hydrocarbon accumulation in middle zone of thrust system is whether the late activities of thrust faults destroy the traps formed in the early stage.Most of the faults near the forward zone are thrusting faults that slip along the soft layer of caprock.Steep dip thrust faults and the transeverse faults have the migration ability to connect deep source rocks and shallow reservoirs,are conducive to becoming oil source faults.