Rock Magnetic Study Of Fault Rocks From The Laohushan Segment Of The Haiyuan Fault Zone And Their Tectonic Implications

Fault rock,the rock associated with faults,usually refers to tectonic rocks formed by shear deformation within the fault zone,Including fault gouge,fault breccia and fault fractured rock and mylonite.These fault rocks are products of fault activity and an important information carrier of fault activity.They record various information about fault activity and are called "earthquake fossils".In recent years,rock magnetic methods have been widely used in geoscience research for their economic,rapid,and nondestructive measurements.Previous magnetic studies of fault rock have found that some special fault rocks,such as fault gouge and pseudotachylyte,with magnetic anomalies by comparing to the wall rock.These magnetic anomalies are often related to the formation temperature,formation mechanism or fluidity of fault rock.Some researchers have used magnetic anomalies of fault rocks to determine the location of fault planes and to study the distribution of ancient faults.There is great research value in magnetic characteristics of fault rocks.The Haiyuan fault is an important boundary fault on the northeastern margin of the Qinghai-Tibet Plateau.Its activity history is long and it have been pregnant with many large earthquakes.Only in the 1990 s,two major earthquakes of magnitude 8 occurred in this fault.Such a long activity history and huge earthquake energy have created fault zone with a length of tens of meters or even a kilometer in the Haiyuan fault zone.The thickness of the black fault gouge-breccia in the Laohushan section is up to more than ten meters.Is the existence of such a large fault gouge-breccia affecting fault activity,In addition,the Laohushan fault zone contains fault rocks with various colors,including black,orange,purple,gray green,white,and variegated.How were these different colored fault rocks formed? How is the thick black fault gouge-breccia formed? Is there a genetic connection between the black gouge and the Carboniferous coal which located on 30 kilometers east of the Laohushan fault? What is the temperature when the gouge formed by fault shearing?In order to answer the above scientific questions,this paper selects two fault rock profiles of the Laohushan section of the Haiyuan fault zone.The two profiles are located at the east and west ends of the Laohushan fault.The hanging wall of the east profile(the E profile)is the Ordovician Anshan tuff,and the surrounding rock of footwall is the Quaternary conglomerate;The northwest profile(the WN profile)is exposed in the Tertiary light-red and pebbly sandstone.Two profiles with different surrounding rocks were selected for comparative study.The E-profile fault rock has a sampling thickness of 75 meters.The section contains the surrounding rock,fault gouge-breccia belt and fracture zone.At the end of the section,due to the cutting of a stream,there is no direct contacting between fault rock and the the surrounding rock.The samples of the hanging wall were collected from the bedrock within 50 meters away from the end of this profile.A total of 207 samples are collected in E profile.The thickness of the WN profile is about 3 meters,and a total of 38 specimens were sampled.In addition,12 samples of Carboniferous coal and its weathering crusts were sampled in 30 kilometers east of the Laohushan fault.Magnetic susceptibility(?),Anhysteresis remanent magnetization(ARM),saturation isothermal remanent magnetization(SIRM),isothermal remanent magnetization(IRM),hysteresis loop(Loop)and magnetic susceptibility versus temperature curve(?-T curve)and other magnetic parameters combined with particle size,carbon content,XRD and other analyses have conducted on the fault rocks to explore the formation mechanism and tectonic implication of different colored fault rocks in the Jingtai section of the Haiyuan fault zone.Rock magnetic results show that the fault gouge-breccia of the E section shows low magnetic susceptibility compared with the surrounding rock and fracture zone,especially the magnetic susceptibility of the black fault gouge is less than 10×10-8 m3/kg.The hysteresis loops show that most samples exhibit paramagnetic characteristics except for surrounding rock.IRM acquisition curves indicate that the red and variegated fault gouges have higher high coercivity mineral content.The thermomagnetic(?-T)curve analysis shows that most of the fault gouge samples present a large increase in ? VI after heating,indicating that a large amount magnetite neoformed by thermal decomposition of iron-containing silicate or clay minerals during the heating,and the ?-T curve of the Carboniferous coal samples is very similar to black fault gouge.The magnetic results of the WN profile are somewhat different from the E profile,although most black fault gouge samples also show weak magnetic properties and similar thermomagnetic curves to those of the E-profile black fault gouge.However,the magnetic susceptibility of the black fault gouge near the fault plane of the WN profile is significantly higher than surrounding rock and breccia.The magnetic results combined with carbon content and mineral phase analyses indicate that the black fault gouge has a similar mineral composition to the Carboniferous coal near the fault zone.It is inferred that the Carboniferous coal is the parent rock of the black fault gouge.The Carboniferous coal was involved into the fault zone by fault activing,and was continuously grinding under the strong shearing and friction of the fault forming clay minerals such as illite and promoting the chemical transformation of a part of the paramagnetic iron-containing silicate mineral or other iron-containing minerals.This makes the magnetic susceptibility higher in the black fault gouge compared with the diamagnetism of their parent rock,the carbon layer.The temperature is about 420?when the black fault gouge was formed,calculated from the ferri-? of the gouge and the ?-T curve of the coal.Rock magnetic analysis shows that the magnetic susceptibility of the black fault is higher than the parent rock-carboniferous coal,indicating that the heating by shearing and friction causes the decomposition of iron-containing silicate to form ferrimagnetic minerals.The significant increase in the magnetic susceptibility of the gray-green fault rock and the black fault gouge near the fault plane in the WN section also confirms that the frictional heating can lead to the magnetic enhancement of the fault gouge.However,the magnetic susceptibility of the fault gouge at the two profiles near the fault plane is very different,which is related to different mineral composition of these two profiles.The WN profile contains pyrite,which can be converted into ferrimagnetic pyrrhotite at 420 ?,resulting in an abnormally high magnetic susceptibility of the fault gouge near the fault plane.The results show that the main mineral composition of red,gray-green and variegated fault gouge is similar to black fault gouge,but the organic carbon content of colored fault gouge is significantly lower than black fault gouge,while the content of high coercivity magnetic mineral,hematite,is higher in colored gouge.These results indicate that the parent rock of the red,gray-green,and variegated fault gouge are likely to be adjacent to the Carboniferous coal.When the fault was active,the parent rocks were simultaneously involved in the fault and underwent intense shearing and friction to gradually become the fault gouge.Therefore,the various colors of the gouge cannot be used to represent fault activity of different time.