Evolution Of Normal Fault Systems Research Articles

Lateral and vertical growth and linkage of normal faults in Bozhong8-4 structure of Western Bozhong Sag, Bohai Bay Basin, China

Many previous studies have shown that normal faults are formed by growth and linkage of shorter fault segments through time and have suggested various fault growth models. Although these models have yielded useful information on the evolution of normal fault systems in map view, the three-dimensional (3D) evolution of normal faults is poorly understood. Here, we employ an integrated 3D seismic reflection and well dataset to investigate the 3D growth process of fault zone F1 in the Bozhong 8–4 (BZ8-4) structure of the Western Bozhong Sag (WBS). We i) constructed 3D structural models of individual fault surfaces, ii) calculated the dip angles and dip azimuth distributions on fault surfaces, iii) produced D-d profiles and throw contour maps of fault surfaces, and iv) investigated the 2D/3D segmentation characteristics of geometry and displacement distribution. We found that Fault F1 exhibits segmentation characteristics of geometry and displacement distribution both along strike and dip oriented directions. Vertically, fault surfaces can be divided into three areas (lower, middle and upper parts) that correspond to three stages of regional tectonic evolution (intense faulting, relatively quiescent faulting and post-rifting). The coalescence zone of the upper and lower fault surfaces shows as a sub-horizontal anomaly of geometry and displacement distribution that is sub-parallel to the traces of the horizons. The lower and upper parts of fault surfaces also show segmentation characteristics along strike, which are identified by vertically anomalous zones of geometry and displacement distribution. However, the origins of the segmentation in this case are different. The segmentation of the lower part of the fault developed during the rifting stage is caused by the intersection of faults with different strikes, whereas the upper part of the fault is the result of the lateral growth and linkage of overlapping fault segments. Therefore, not all fault segmentation characteristics along strike can be attributed to fault growth and linkage. Based on 3D fault segment identification, we therefore establish a 3D evolution model of fault zone F1. During the deposition of the Shahejie Fm up to the member (Mbr) 3 of the Dongying Fm in the Paleogene, the faults in this area were highly active, with the development of the NE-trending Fault F1, the NEE-trending Fault Fa and the NW-trending Fault F2. The intersection of Faults Fa and F2 with Fault F1 results in Fault F1 exhibiting segmentation in geometry and displacement distribution. From the deposition of the Mbr 2 of the Dongying Fm, these faults ceased to be active. Since the Pliocene (5.2 Ma), under the influence of transtensional stress field, an en-echelon fault zone was developed in the Neogene strata above fault zone F1 active in the rifting stage. While these faults grow and link laterally, their tip-lines propagate along dip-oriented directions and activate the lower faults and link vertically with them to form the present fault zone F1.

The role of frictional plasticity in the evolution of normal fault systems

Tectonic extension applied to the upper crust commonly results in the development of sub-parallel arrays of faults and fault belts. A computational model of an extending elastic-viscous-plastic crust is presented, which describes the evolution of stress, strain and displacement in the vicinity of an active normal fault. Within the upper crust discrete episodes of fault slip accommodate up to half the applied extension rate, with the balance accommodated by bands of frictional-plastic shearing representing young proto-faults. Repeated coseismic shear stress reductions in the near-fault vicinity impose a stress-failure shadow where the formation of new faults is prohibited. A near-surface compressional stress regime in the footwall causes the failure shadow to extend disproportionally into this fault block, thereby favoring the formation of new faults in the hanging wall. A continuous shallowing of fault dip is also documented and supports a scenario of migration of faulting into the hanging wall of existing faults, consistent with descriptions of the structural evolution in the western Gulf of Corinth, Greece.

Neogene to Quaternary tectonics and mud diapirism in the Gulf of Squillace (Crotone-Spartivento Basin, Calabrian Arc, Italy)

The Calabrian Arc is an arcuate tectonic feature, formed during the Neogene and Pleistocene, as a consequence of the progressive retreat of the Ionian subduction system. It is a key area for understanding the geodynamic evolution of the Mediterranean Sea. The aim of this paper is to investigate the sedimentary dynamics and the possibly active deformation in the Gulf of Squillace and to highlight their relationships with the fluid escape pathways and mud mobilization. The Squillace embayment is a marine area belonging to the forearc basin of the Calabrian Arc, at the seaward prosecution of the Catanzaro depression. By interpreting public seismic profiles, at different resolutions, calibrated by well logs for hydrocarbon exploration, it was possible to define the structural setting and the Neogene and Quaternary stratigraphic architecture. The geologic setting of the Gulf of Squillace shows the occurrence of normal fault systems cutting through both Neogene and Quaternary deposits. During the Messinian, trans-tensional tectonics led to the formation of a basin whose sediments reached the major thickness in the central portion of the embayment. Since the Early Pliocene extensional tectonics was dominant and the basin depocenter progressively changed its orientation, creating a narrow basin roughly oriented NW–SE. Similar tectonic setting was also present during the Pleistocene extension. A Middle to Late Pliocene contractional event led to development of a thrust zone in the northern Crotone swell. Mud diapirs that occur in the external area of the Gulf of Squillace, are located close to the Pliocene and Pleistocene depocenter. The overpressure provided by the Pliocene and Pleistocene sediment accumulation on Messinian and pre-Messinian sediments and the evolution of normal fault systems, triggered the mud diapirs formation that progressively developed toward the upper slope. Extensional deformation and mud diapirism influence the morphologic evolution of the slope and the basin, resulting in the retrogressive erosion of canyon heads and gravity-driven slope failures.

Comment on “The geometric and statistical evolution of normal fault systems: an experimental study of the effects of mechanical layer thickness on scaling laws” by R.V. Ackermann, R.W. Schlische and M.O. Withjack

Comment on “The geometric and statistical evolution of normal fault systems: an experimental study of the effects of mechanical layer thickness on scaling laws” by R.V. Ackermann, R.W. Schlische and M.O. Withjack

Reply to the comment by P. Gillespie on “The geometric and statistical evolution of normal fault systems: an experimental study of the effects of mechanical layer thickness on scaling laws”

Reply to the comment by P. Gillespie on “The geometric and statistical evolution of normal fault systems: an experimental study of the effects of mechanical layer thickness on scaling laws”

The geometric and statistical evolution of normal fault systems: an experimental study of the effects of mechanical layer thickness on scaling laws

Detailed analyses of two scaled experimental models of distributed extension that differ only in the thickness of the mechanical layer reveal how normal-fault systems evolve with increasing strain. Faults grow increasingly by linkage and become regularly spaced; the proportion of active structures decreases and converges with that of inactive structures. Large faults contribute increasingly to strain accommodation. The size-frequency distribution of fault lengths changes from power-law (fractal) to exponential, non-linear and dynamic length-displacement scaling arises, and the system becomes less multifractal and more homogeneous. These observations validate many predictions of numerical and geometric models of normal fault growth and system evolution. We propose a generalized three-stage model in which mechanical stratigraphy at times restricts fault growth and the degree of elastic fault interaction. The thickness of the mechanical layer influences the relative timing of stages in this model, as well as the geometry and statistics of the system. As faults encounter and breach multiple mechanical layers, systems may exhibit different scaling attributes at different structural or mechanical levels. Thus, systems may oscillate between different stages of this model, complicating fault-population statistics. We present our data as evidence of the existence of upper and lower bounds for the scale invariant behavior of fault systems, as predicted by Mandelbrot for natural fracture systems.

Geometric controls on the evolution of normal fault systems

Abstract The growth of normal fault arrays is examined in basins where sedimentation rates were higher than fault displacement rates and where fault growth histories are recorded by thickness and displacement variations within syn-faulting sequences. Progressive strain localization is the principal feature of the growth history of normal faults for study areas from the Inner Moray Firth, a sub-basin of the North Sea, and from the Timor Sea, offshore Australia. The kinematics of faulting are similar in both study areas. Fault displacement rates correlate with fault size, where size is measured in terms of either displacement or length. Small faults have higher mortality rates than larger faults throughout the growth of the fault system. Displacement and strain are progressively localized onto the larger faults at the expense of smaller faults at progressively larger scales. Strain localization and the preferential growth of larger faults are attributed to geometric factors, such as size and location, rather than to the mechanical properties of fault rock in individual faults. This conclusion is supported by numerical models that reproduce the main characteristics of fault system growth established from both study areas.

Comment and Reply on "On the role of isostasy in the evolution of normal fault systems"

Comment and Reply on "On the role of isostasy in the evolution of normal fault systems"

On the role of isostasy in the evolution of normal fault systems

The footwalls of west-dipping normal faults that separate the west-central Colorado Plateau from the Basin and Range province record at least 5-7 km, and perhaps as much as 15-20 km, of west-side-up Neogene uplift, with an axis just 10-20 km west of undeformed plateau strata. The uplift is expressed as folding and steep faulting in pre-Tertiary cratonic and disconformably overlying Neogene strata, forming a basement-cored anticline and coincident topographic high on the western margin of the plateau. The authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin. Profound, isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains, calling into question several assumptions fundamental to existing models of the evolution of normal fault systems.

The geometrical evolution of normal fault systems

The purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution. We pay particular attention to seismological and surface data from regions of present day active normal faulting, as the instantaneous three-dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static. Most normal faults are concave upward, or listric. This shape can be produced by geometric constraints, either because the faults reactivate curved thrusts, or because they must be curved to accommodate rotations. Another effect which will produce curved faults is the variation of rheology with depth: brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust. An important geometric feature of normal faulting is the uplift of the footwall. The amount of such uplift is related not only to the elastic properties of the lithosphere, but also to the throw and dip of the fault. A striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction. This behaviour arises because the faults cannot intersect: if they do, one must cease to be active. The rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed. Once a new fault cuts pre-existing faults the earlier faults become locked, and a new set of faults must propagate rapidly across the whole region involved. Many of these geometric constraints also apply to thrust faulting.