Mechanical and hydraulic properties of fault rocks under multi‑stage cyclic loading and unloading

The rock mass in fault zones is frequently subjected to cyclic loading and unloading during deep resource exploitation and tunnel excavation.Research on the mechanical and hydraulic characteristics of fault rock during the cyclic loading and unloading is of great signifcance for revealing the formation mechanism of water-conducting pathways in fault and preventing water inrush disasters.In this study,the mechanical and seepage tests of fault rock under the multi-stage cyclic loading and unloading of axial compression were carried out by using the fuid–solid coupling triaxial experimental device.The hysteresis loop of the stress–strain curve,peak strain rate,secant Young's modulus,and permeability of fault rock were obtained,and the evolution law of the dissipated energy of fault rock with the cyclic number of load and unloading was discussed.The experimental results show that with an increase in the cyclic number of loading and unloading,several changes occur.The hysteresis loop of the stress–strain curve of the fault rock shifts towards higher levels of strain.Additionally,both the peak strain rate and the secant Young's modulus of the fault rock increase,resulting in an increase in the secant Young's modulus of the fault rock mass.However,the growth rate of the secant Young's modulus gradually slows down with the increase of cyclic number of loading and unloading.The permeability evolution of fault rock under the multi-stage cyclic loading and unloading of axial compression can be divided into three stages:steady increase stage,cyclic decrease stage,and rapid increase stage.Besides,the calculation model of dissipated energy of fault rock considering the efective stress was established.The calculation results show that the relationship between the dissipated energy of fault rock and the cyclic number of loading and unloading conforms to an exponential function.

Patterns of Clay Minerals Transformation in Clay Gouge, with Examples from Revers Fault Rocks in Devonina Niqiuhe Formation in The Dayangshu Basin

The role of authigenic clay growth in clay gouge is increasingly recognized as a key to understanding the mechanics of berittle faulting and fault zone processes,including creep and seismogenesis,and providing new insights into the ongoing debate about the frictional strength of brittle fault(Haines and van der Pluijm,2012).However,neither the conditions nor the processes which

Characteristics of fault rocks and paleo-earthquake source along the Koktokay-Ertai fault zone,Xinjiang,China

The Koktokay Ertai fault zone was developed on the basis of a former ductile shear zone (mylonite zone). The mylonites were formed in the brittle ductile transition zone at 10～15 km depth within the crust. The rocks had experienced multi period deformation processes, including ductile deformation, ductile instability and brittle seismic faulting which had gave rise to the formation of Koktokay Ertai fault zone and the development of pseudotachylytes. The pseudotachylytes are distributed within an area about 60 km in length and 300 m in width, which may represent the scale of the paleo earthquake source. The physical conditions of the paleo earthquake source was similar to those for the formation of the mylonites along this zone.

Internal structure of Longmenshan fault zone at Hongkou outcrop,Sichuan,China,that caused the 2008 Wenchuan earthquake

This paper reports the internal structures of the Beichuan fault zone of Longmenshan fault system that caused the 2008 Wenchuan earthquake, at an outcrop in Hongkou, Sichuan province, China. Present work is a part of comprehensive project of Institute of Geology, China Earthquake Administration, trying to understand deformation processes in Longmenshan fault zones and eventually to reproduce Wenchuan earthquake by modeling based on measured mechanical and transport properties. Outcrop studies could be integrated with those performed on samples recovered from fault zone drilling, during the Wenchuan Earthquake Fault Scientific Drilling （WFSD） Project, to understand along-fault and depth variation of fault zone properties. The hanging wall side of the fault zone consists of weakly-foliated, clayey fault gouge of about 1 m in width and of several fault breccia zones of 30-40 m in total width. We could not find any pseudotachylite at this outcrop. Displacement during the Wenchuan earthquake is highly localized within the fault gouge layer along narrower slipping-zones of about 10 to 20 mm in width. This is an important constraint for analyzing thermal pressurization, an important dynamic weakening mechanism of faults. Overlapping patterns of striations on slickenside surface suggest that seismic slip at a given time occurred in even narrower zone of a few to several millimeters, so that localization of deformation must have occurred within a slipping zone during coseismic fault motion. Fault breccia zones are bounded by thin black gouge layers containing amorphous carbon. Fault gouge contains illite and chlorite minerals, but not smectite. Clayey fault gouge next to coseismic slipping zone also contains amorphous carbon and small amounts of graphite. The structural observations and mineralogical data obtained from outcrop exposures of the fault zone of the Wenchuan earthquake can be compared with those obtained from the WFSD-1 and WFSD-2 boreholes, which have been drilled very close to the Hongkou outcrop. The presence of carbon and graphite, observed next to the slipping-zone, may affect the mechanical properties of the fault and also provide useful information about coseismic chemical changes.

Internal structures and high-velocity frictional properties of a bedding-parallel carbonate fault at Xiaojiaqiao outcrop activated by the 2008 Wenchuan earthquake

This paper reports internal structures of a bedding-parallel fault in Permian limestone at Xiaoji-aqiao outcrop that was moved by about 0.5 m during the 2008 MW7.9 Wenchuan earthquake. The fault is located about 3 km to the south from the middle part of Yingxiu-Beichuan fault, a major fault in the Longmenshan fault system that was moved during the earthquake. The outcrop is also located at Anxian transfer zone between the northern and central segments of Yingxiu-Beichuan fault where fault system is complex. Thus the fault is an example of subsidiary faults activated by Wenchuan earthquake. The fault has a strike of 243°or N63°E and a dip of 38°NW and is nearly optimally oriented for thrust motion, in contrast to high-angle coseismic faults at most places. Surface outcrop and two shallow drilling studies reveal that the fault zone is several centimeters wide at most and that the coseismic slip zone during Wenchuan earthquake is about 1 mm thick. Fault zone contains foliated cataclasite, fault breccia, black gouge and yellowish gouge. Many clasts of foliated cataclasite and black gouge contained in fault breccia indicate multiple slip events along this fault. But fossils on both sides of fault do not indicate clear age difference and overall displacement along this fault should not be large. We also report results from high-velocity friction experiments conducted on yellowish gouge from the fault zone using a rotary shear low to high-velocity frictional testing apparatus. Dry experiments at normal stresses of 0.4 to 1.8 MPa and at slip rates of 0.08 to 1.35 m/s reveal dramatic slip weakening from the peak friction coeffcient of around 0.6 to very low steady-state friction coeffcient of 0.1–0.2. Slip weakening parameters of this carbonate fault zone are similar to those of clayey fault gouge from Yingxiu-Beichuan fault at Hongkou outcrop and from Pingxi fault zone. Our experimental result will provide a condition for triggering movement of subsidiary faults or off-fault damage during a large earthquake.

Characteristics of Hydrocarbon Accumulation in the Paleozoic Donghetang Formation in the Southwestern Tahe Area

The ancient structure characteristic,correlation of the oil and the hydrocarbon source rock characteristics,hydrocarbon migration trace,types and conditions of traps,migration passages and characteristic of hydrocarbon accumulation are researched in this paper.It is shown through the analysis that two main large tectonic activities after the Early Hercynian orogeny resulted in different tectonic patterns in the study area.Two main hydrocarbon infills occurred in the Donghetang Formation,the first occurred in the Early Hercynian resulting in the ancient hydrocarbon accumulation in the northern Tahe,the second infill was a large amount that occurred in places beneficial for hydrocarbon accumulation,such as structural traps and structural-stratigraphic traps formed in the Early Himalayan orogeny after migration along the faults through source rocks and other passages.Before the earlier period of the Himalayan orogeny,the petroleum mainly migrated to the north,whereas petroleum migrated to the south and southeast because of the structural reverse in the Himalayan orogeny,so the middle and later period of the Himalayan orogeny is the key period for hydrocarbon accumulation.The model of＂oil generation formed early,hydrocarbon accumulation controlled by the faults through source rocks and structures formed late＂is proposed.It is pointed out that the south of the research area is currently the beneficial district for hydrocarbon accumulation, which provides the basis for future petroleum exploration.