Conceptual models for fracturing in fault related folds

Fault related folds and fractures, which always improve reservoirs and trap capacity, especially in the oil and gas fields in western China, are of extreme importance for oil and gas exploration. Based on four assumptions and fault related fold theory, we propose four conceptual models for fracturing in fault related folds, i.e., a simple-step parallel fault bend anticline, a simple-step parallel fault propagation anticline, a multiple-bend fault-bend fold and a break-forward imbricate. Fracture conditions depend on structural evolution and specific site conditions. A case study of the Kulongshan Anticline in the Jiuquan Basin shows that our conceptual models match reality data very well.

Late Quaternary Activity of the Huashan Piedmont Fault and Associated Hazards in the Southeastern Weihe Graben,Central China

The Weihe Graben is not only an important Cenozoic fault basin in China but also a significant active seismic zone. The Huashan piedmont fault is an important active fault on the southeast side of the Weihe Graben and has been highly active since the Cenozoic. The well–known Great Huaxian County Earthquake of 1556 occurred on the Huashan piedmont fault. This earthquake, which claimed the lives of approximately 830000 people, is one of the few large earthquakes known to have occurred on a high–angle normal fault. The Huashan piedmont fault is a typical active normal fault that can be used to study tectonic activity and the associated hazards. In this study, the types and characteristics of late Quaternary deformation along this fault are discussed from geological investigations, historical research and comprehensive analysis. On the basis of its characteristics and activity, the fault can be divided into three sections, namely eastern, central and western. The eastern and western sections display normal slip. Intense deformation has occurred along the two sections during the Quaternary; however, no deformation has occurred during the Holocene. The central section has experienced significant high–angle normal fault activity during the Quaternary, including the Holocene. Holocene alluvial fans and loess cut by the fault have been identified at the mouths of many stream valleys of the Huashan Mountains along the central section of the Huashan piedmont fault zone. Of the three sections of the Huashan piedmont fault, the central section is the most active and was very active during the late Quaternary. The rate of normal dip–slip was 1.67–2.71±0.11 mm/a in the Holocene and 0.61±0.15 mm/a during the Mid–Late Pleistocene. As is typical of normal faults, the late Quaternary activity of the Huashan piedmont fault has produced a set of disasters, which include frequent earthquakes, collapses, landslides, mudslides and ground fissures. Ground fissures mainly occur on the hanging–wall of the Huashan piedmont fault, with landslides, collapses and mudslides occurring on the footwall.

A Simple Model for Earthquake Instability

Based on the heterogeneity of fault plane strength,the macro rupture process of a fault plane can be treated as the rupture accumulation process of local micro-elements in the fault surface.Assuming that the strength of the local micro-elements follows the Weibull probability distribution,the macro-fault constitutive relationship of the complete load-deformation process is derived from a statistical mechanics viewpoint.Applying a one-dimensional earthquake mechanics model and using far-field displacement a as the control variable,the problem of earthquake instability is investigated by employing the stability theory.The results show that the system stiffness ratio(stiffness ratio of fault to surroun-ding rock) β is the important parameter that affects the occurrence of earthquakes.Earthquake instability occurs only when β < 1,and the sudden stress jump appears at the displacement turning point of the equilibrium path curve.The expression of three important parameters for earthquakes(fault half-dislocation distance after earthquake,earthquake stress drop and elastic energy release) is also given.When β≥1,the earthquake does not occur and the fault only slips slowly without an earthquake.

Geometry and kinematics of tugerming structural belt in the east of Kuqa Depression,Tarim Basin

Accurate interpretation of clutter seismic reflection is of great significance to make clear the structural geometry and kinematics characteristics.Based on field investigations,boreholes and seismic data,forward modeling is carried out under the trishear model of fault-propagated fold principle,and the activity characteristics of the main faults in the study area are quantitatively analyzed using the balanced restoration technique,which may provide important indication for the potential petroleum exploration and development.The results show that separated by Tuziluoke and Tugerming fault,the structural style of the study area can be divided into the deeply buried forelimb,the kink band and the tiny anticline backlimb section.The Tuziluoke fault has been active with strong intensity since the sedimentary period of the Kuqa Formation and,which controls the formation of the growth strata in the deep buried forelimb of the Tuziluoke anticline.The Tugerming fault has been active intermittently since the end of the Jurassic,causing the strata denudation of the Late Jurassic to Paleogene in varying degrees and the development of series of regulating faults.The multi-stage tectonic activities caused the structural high of the anticline migrated from east to west.