Characteristics And Formation Mechanism Of Tectonic Veins In Dabashan Tectonic Belt

The Dabashan Tectonic Belt lies on the north border of the Sichuan Basin,at the middle south of Qinling Orogenic Belt,also on the boundary line between Qinling Tectonic Belt and Yangtz plate.The Dabashan Tectonic Belt usually can be devided into three secondary tectonic units,namely the northern Dabashan thrust-nappe belt、the southern Dabashan foreland arc thrust fold belt(the Dabashan foreland belt)and the Dabashan foreland depression From north to south with Chengkou-Fangxian fault and Tiexi-Wuxi fault as the boundary.Through field geological survey,a large amount of tectonic veins had been found in the Dabashan Tectonic Belt, and different characteristics of veins are grown in different sub-tectonic units.Three kinds of veins had been marked off by wild macroscopic observation and analysis of microscope.The first stage of tectonic veins were the products of the preindosinian geofluid,they were crumpled veins,which had strongly metamorphic and deformation phenomenon,the vein Mainly distributed in the northern Dabashan thrust-nappe belt.The second phase of veins are X type veins which mainly distribute in the northern Dabashan thrust-nappe belt and the Dabashan foreland belt,the veins were the products of the indosinian geofluid.The third stage of tectonic veins which fill in the extensional fracture were straightness and clean,in all Dabashan Tectonic Belt these veins can be seen,they were products of Yanshanian syntectonic geofluid.Different stages of veins and their surrounding rocks had been sampled,by analyzing the geochemistry of the samples,we can get the following conclusions:three stages of veins formed in a open system which was oxidation environment,the material sources were mainly from wall-carbonate-rock’s recrystalization.The geofluide was mainly rock formation water which developed through water-rock reaction at time of vein formation,the water was meteoric water seeping into the strata through the crack.That,the second stage of veins forming in nearly NS-trending tectonic compression stress at the indosinian,whereas the third stage of veins forming in nearly EW-trending tectonic compression stress at the Yanshanian,had been proved by analyzing the stress field using the tectonic analysis.Geometric features and spatial distribution of the third type veins provided estimation of stress ratio( ? =(σ2-σ3)/(σ1-σ3)) 、 driving stress ratio(R’ =(Pf-σ3)/(σ1-σ3))and fluid overpressure(Δ si=Pf-σ3)at the time of veins formation.The obtained values of ?=0.452 and R’ =0.313 reveal that fluid pressure was lower than the intermediate principal stress,whereas the estimated Δsi can reach to 167 Mpa indicates that the fluid was anomalous pressure fluid at the time of vein formation,and the fluid may be caused by Yanshanian movement.