Subsidiary Faults Research Articles

Hierarchical approach for simulating fluid flow in normal fault zones

Two-phase flow in faults is complex and difficult to predict. To analyse the effect of fault zones on fluid flow, this article presents a hierarchical geological/numerical framework aimed at simulating two-phase flow. The starting point is that fault zones consist of structures at several length scales, with each scale represented by suitable techniques within the same numerical model. A series of two-phase flow simulation experiments was conducted on four geological cases: one reference case with undeformed host rock and three cases with increasingly more complexity added to them. All the structures consist of lower permeability fault rocks in a high permeability host rock. The simulations were performed using an in-house flow simulator. The fault core (large scale) was modelled explicitly through local grid refinement, the subsidiary faults (intermediate scale) were represented in a discrete manner, while an upscaling procedure captured the effect of the deformation bands (fine scale). The simulation results show that each scale has a significant effect on the saturation, pressure drop and oil production, and that capillary pressure and anisotropic permeability are important parameters. The results emphasize the importance of the scale-dependent approach for analysing the effect of faults on two-phase flow.

Deformation of overburden soil induced by thrust fault slip

When the blind thrust fault slips due to earthquakes, overburden soil strata may deform and fail leading to a development of a coseismic fault zone. How overburden soil strata will deform or be faulted is an essential issue in the evaluation on the safety of ground/underground infrastructures located near the potential plastic zone. On the basis of previous work by Cole and Lade [Cole D.A., Lade, P.V., 1984. Influence zones in alluvium over dip-slip faults. Journal of Geotechnical Engineering 110, 599–615], this research further explores the processes of thrust faulting within overburden soil and examines the influences of corresponding factors or parameters under a range of boundary conditions using physical models and numerical analysis for both small-scale and full-scale configurations. Factors explored include uplifting rate, fault dip angle, dilation angle of plastic flow, Young's modulus, Poisson ratio, cohesive strength, frictional angle, scaling, as well as the location of ground loadings applied on ground surface. The experimental results indicate that although one major fault slip surface can be developed, subsidiary faults may also form, which requires special attention when defining stain patterns. Young's modulus and the dilation angle are the most influential parameters in our models. A stiffer E leads to early fault propagation and the occurrence of back-thrusts and causes more significant discrepancies in the deformation behaviors of models with different scales. A greater dilation angle of the overburden soil leads to a wider fault zone. The above-mentioned phenomena highlight that a careful consideration of the applicable Young's modulus and the dilation angle is essential in a proper simulation or prediction of the fault zone development. For small-scale, physical models used to simulate the fault development for a full-scale problem, the stiffness of model soil should be properly scaled down, which still requires further study.

越後平野東縁，庵地断層のトレンチ調査結果―トレース長の短い活断層の評価―

The eastern margin of Echigo Plain, central Japan, is characterized by NNE-SSW trending active faults. The Anchi fault is made up of two segments : the Anchi-nishi fault and the Anchi-higashi fault. Each fault trace is approximately 2-km long. They may be short subsidiary faults on the hanging wall of the master fault in this region (the Tsukioka fault). Trench investigations across the Anchi fault reveal its rupture history as follows : 1) the Anchi-nishi fault and the Anchi-higashi fault are west-dipping reverse faults; 2) the Anchi-nishi fault shows a vertical offset of 1.3-1.9 m after ca. 30 ka; and, 3) the vertical displacement of 3.5 m is associated with two faulting events along the Anchi-higashi fault, one between ca. 30 and 60 ka and the other younger than ca. 30 ka. Geomorphic features indicate that a reverse fault branches into these two faults at a certain depth. Thus, the total vertical offset for the last ca. 30 ky is between 3.1 to 3.7 m. This value is one third of that along the master active fault. If the source fault consists of these active faults, we should evaluate the activity of the master fault taking account of the activity of the Anchi fault. It is very important to know the nature of subsidiary faults, even if they display very short traces.

The geological links of the ancient Delphic Oracle (Greece): A reappraisal of natural gas occurrence and origin

Recent studies have speculated that the prophetic powers of Pythia, the woman of the Delphic Oracle, at the Temple of Apollo in Greece, were induced by hydrocarbon vapors, specifically ethylene, rising from bedrock fissures at the intersection of the E-W Delphi fault with the NNW-SSE Kerna fault, and producing neurotoxic effects, including trance and delirium. New surveys including gas flux from soil, gas in groundwater, and isotopic analyses of spring scales, provide the experimental confirmation of the gas release in the Delphi area. Presently, methane, ethane, and carbon dioxide are being released from a thermogenic (catagenetic) hydrocarbon-prone environment. This environment is not prone to biogenic production of ethylene in amounts inducing neurotoxic effects (hundreds or thousands of ppmv). A WNW-ESE–trending subsidiary fault within the Delphi fault zone, extending for ∼2 km, passes under the Temple of Apollo and shrine of Athena. The Temple of Apollo, located above this fault, may have been the site of enhanced degassing in the past. If gas-linked neurotoxic effects upon Pythia need to be invoked, they should be sought in the possibility of oxygen depletion due to CO2-CH4 exhalation in the indoor temple. Alternatively, a plausible geological explanation behind the natural presence of sweet scents could be the occurrence of aromatic hydrocarbons, such as benzene, dissolved in the groundwater spring.

Consistent kinematic architecture in the damage zones of intraplate strike-slip fault systems in North Victoria Land, Antarctica and implications for fault zone evolution

Cumulative, polymodal normal (Gaussian distribution) statistics was applied to subsidiary fault data collected from damage zones associated with the Cenozoic Lanterman and Priestley intraplate right-lateral strike-slip fault systems in North Victoria Land, Antarctica. Results show that five Gaussian peaks out of seven in the Lanterman Fault and five out of nine in the Priestley Fault have almost coincident azimuthal values. We named these Gaussian peak pairs as consistent fault sets, arranged in a consistent kinematic architecture that is compatible with the Cenozoic regional strike-slip environment. Angular and kinematic relationships among subsidiary fault sets within the consistent kinematic architecture provide constraints for the inference of the state of stress along the Lanterman and Priestley fault systems. We interpret the fault pattern of the consistent kinematic architecture to be produced by early localisation of the principal displacement zone along pre-existing mechanical discontinuities inherited from the Early Paleozoic Ross Orogeny. Shear localisation was followed by subsidiary faulting at an angle to the principal displacement zone according to the Mohr–Coulomb–Byerlee failure criterion.

Fault zone development and strain partitioning in an extensional strike-slip duplex: A case study from the Mesozoic Atacama fault system, Northern Chile

Upper crustal strike-slip duplexes provide an excellent opportunity to address the fundamental question of fault zone development and strain partitioning in an evolving system. Detailed field mapping of the Mesozoic Atacama fault system in the Coastal Cordillera of Northern Chile documents the progressive development of second- and third-order faults forming a duplex at a dilational jog between two overstepping master faults: the sinistral strike-slip, NNW-striking, Jorgillo and Bolfin faults. These are constituted by a meter-wide core of foliated S-C ultracataclasite and cataclasite, flanked by a damage zone of protocataclasite, splay faults and veins. Lateral separation of markers along master faults is on the order of a few kilometers. Second-order, NW-striking, oblique-slip subsidiary fault zones do not show foliated ultracataclasite; lateral sinistral separations are in the range of ∼ 10 to 200 m with a relatively minor normal dip-slip component. In turn, third-order, east–west striking normal faults exhibit centimetric displacement. Oblique-slip (sinistral–normal) fault zones located at the southern termination of the Bolfin fault form a well-developed imbricate fan structure. They exhibit a relatively simple architecture of extensional and extensional-shear fractures bound by low displacement shear fractures. Kinematic analysis of fault slip data from mesoscopic faults within the duplex area, document that the NW-striking and the EW-striking faults accommodate transtension and extension, respectively. Examination of master and subsidiary faults of the duplex indicates a strong correlation between total displacement and internal fault structure. Faults started from arrays of en echelon extensional/extensional-shear fractures that then coalesced into throughgoing strike-slip faults. Further displacement leads to the formation of discrete bands of cataclasite and ultracataclasite that take up a significant part of the total displacement. We interpret that the duplex formed by progressive linkage of horsetail-like structures at the southern tip of the Bolfin fault that joined splay faults coming from the Jorgillo and Coloso faults. The geometry and kinematics of faults is compared with that observed in analog models to gain an insight into the kinematic processes leading to complex strike-slip fault zones in the upper crust.

Study on the Feature of Surface Rupture Zone of the West of Kunlun Mts. Pass Earthquake (Ms8.1) with High Spatial Resolution Satellite Images

On November 14, 2001, a MS8.1 earthquake occurred on the west of Kunlun Mts. pass. Using high spatial resolution satellite images the earthquake surface rupture zone can be interpreted. By means of 10m spatial resolution SPOT images the main rupture zone can be revealed distinctly, meanwhile, 1m spatial resolution IKONOS images show the fine structures and kinematical characteristics of the surface rupture of the earthquake. The results show that the rupture is about 350km long on the east side of Buka Daban Feng peak and its strike is about 100 degrees. In some section, the rupture consisted of 3 subsidiary faults. The surface rupture zone is mostly located in the valleys on the piedmont diluvial platform or on its back edge along the southern foot of Kunlun Mts. and re-ruptured the pre-existing fault. The water systems, which flow across the fault, are left-laterally wrenched and the average slip rate is 13.4~16.8mm/a. These evidences indicate that the eastern Kunlun fault is an AA class active fault. The maximum left-lateral offset is about 7.8m and the maximum width of rupture zone is about 1250m. The macro-epicenter lies in 93°17'E, 35°47'N, near the surface rupture zone of Yuxi Feng peak.

Neotectonics of the Offshore Oak Ridge Fault near Ventura, Southern California

The Oak Ridge fault is a large-offset, south-dipping reverse fault that forms the south boundary of the Ventura Basin in southern California. Previous research indicates that the Oak Ridge fault south of the town of Ventura has been inactive since 200–400 ka ago and that the fault tip is buried by ∼ 1 km of Quaternary sediment. However, very high-resolution and medium-resolution seismic reflection data presented here show a south-dipping fault, on strike with the Oak Ridge fault, that is truncated at 80 m depth by an unconformity that is probably at the base of late Pleistocene and Holocene sediment. Furthermore, if vertically aligned features in seismic reflection data are eroded remnants of fault scarps, then a subsidiary fault within the Oak Ridge system deforms the shallowest imaged sediment layers. We propose that this subsidiary fault has mainly left-slip offset. These observations of Holocene slip on the Oak Ridge fault system suggest that revision of the earthquake hazard for the densely populated Santa Clara River valley and the Oxnard coastal plain may be needed.

Designer wells to maximize recovery for Otter Field development, Northern North Sea

The 2002 to 2003 Otter Field development drilling campaign utilized a combination of detailed trajectory planning and integrated geosteering techniques. The objective of this work was to maximize oil recovery, with a minimal number of wells, from the complexly faulted Otter structure. To achieve this, subhorizontal production wells were planned to track near top reservoir, through the structural culminations, to connect adjacent fault blocks. Otter is the most northwesterly of the Brent Province fields of the Northern North Sea, located in UK blocks 210/15a and 210/20d, 530 km north of Aberdeen, operated by TOTAL with partners Shell U.K. Exploration and Production, ExxonMobil and Dana. The field was discovered by the Phillips 210/15-2 well in 1977 (then called Wendy) and appraised by Fina well 210/15a-5 in 1997, following 3D seismic acquisition in 1994. The decision to proceed with development was confirmed after the success of appraisal well 210/15a-6 drilled by TotalFina in 2000. The Otter structure is an easterly dipping tilted panel that is divided into four major blocks and several minor blocks by a network of subsidiary faults. The reservoir is the Middle Jurassic Brent Group, with the uppermost Tarbert Formation shallow marine sandstones comprising the main producing target. The oil source rock is the Late Jurassic Kimmeridge Clay, present in the off-structure areas, though locally absent over the Otter Field area. Top seal is provided by the Mid- to Late Jurassic Heather Shales. The Otter oil is a medium gravity crude (36.5° API) with a GOR of 79m 3 /m 3 (443 scf/bbl), in a normally pressured reservoir at a crestal depth of 1970m subsea. Otter well planning was conducted using a 3D geocellular model based on interpretation of both conventional and acoustic impedance inversion seismic datasets. Apilot study, prior to development drilling, included geochemical and petrophysical reservoir unit definition and the forward modelling of LWD log response in sub-horizontal wells. The results of these studies were used to aid geosteering, incorporating real time chemostratigraphy and LWD data at the wellsite. In addition, borehole resistivity images while drilling were used to assist in structural interpretation in real time and thus to guide the well trajectory to maximize the pay section. A key component in using these new technologies was the office-based integration of all the data via web-based monitoring of the operations. Three production wells target the culminations at the extremities of the Otter Field, supported by a downdip water injector, all drilled from a centrally located subsea template. Following the successful drilling of the first production well, 210/215a-T1, production start-up was in October 2002, via subsea tieback to the Eider facility.

Gold mineralisation throughout about 45�Ma of Archaean orogenesis: protracted flux of gold in the Golden Mile, Yilgarn craton, Western Australia

The Golden Mile deposit was discovered in 1893 and represents today the largest Archaean orogenic lode gold system in the world (50 M oz produced gold). The Golden Mile deposit comprises three major styles of gold mineralisation: Fimiston, Oroya and Charlotte styles. Fimiston-style lodes formed at 250 to 350 °C and 100 to 200 MPa and are controlled by brittle–ductile fault zones, their subsidiary fault zone and vein networks including breccias and open-cavity-infill textures and hydrothermally altered wall rock. Fimiston lodes were formed late D1, prior to D2 regional upright folding. Hydrothermal alteration haloes comprise a progression toward the lode of diminishing chlorite, an increase in sericite and in Fe content of carbonates. Lodes contain siderite, pyrite, native gold, 17 different telluride minerals (Au–Ag tellurides contain ~25% of total gold), tourmaline, haematite, sericite and V-rich muscovite. Oroya-style lodes formed at similar P–T conditions as the Fimiston lodes and are controlled by brittle–ductile shear zones, associated dilational jogs that are particularly well developed at the contact between Paringa Basalt and black shale interflow sedimentary rocks and altered wall rock. The orebodies are characterised by micro-breccias and zones of intense shear zone foliation, very high gold grades (up to 100,000 g/t Au) and the common association of tellurides and vanadian mica (green leader). Oroya lodes crosscut Fimiston lodes and are interpreted to have formed slightly later than Fimiston lodes as part of one evolving hydrothermal system spanning D1 and D2 deformation (ca. 2,675–2,660 Ma). Charlotte-style lodes, exemplified by the Mt Charlotte deposit, are controlled by a sheeted vein (stockwork) complex of north-dipping quartz veins and hydrothermally altered wall rock. The Mt Charlotte orebody formed at 120 to 440 °C and 150 to 250 MPa during movement along closely spaced D4 (2,625 Ma) and reactivated D2 faults with the quartz granophyre in the Golden Mile Dolerite exerting a strong lithological control on gold mineralisation. Veins consist of quartz–carbonate–minor scheelite, and wall-rock alteration comprises chlorite destruction and growth of ferroan carbonate–sericite–pyrite–native gold. Pyrite–pyrrhotite is zoned on the scale of vein haloes and of the entire mine, giving a vertical temperature gradient of 50–100 °C over 1,000 vertical metres. The structural–hydrothermal model proposed consists of four major stages: (1) D1 thrusting and formation of Fimiston-style lodes, (2) D2 reverse faulting and formation of Oroya-style lodes, (3) D3 faulting and dissecting of Fimiston- and Oroya-style lodes, and (4) D4 faulting and formation of Mt Charlotte-style sheeted quartz vein system. The giant accumulation of gold in the Golden Mile deposit was formed due to protracted gold mineralisation throughout episodes of an Archaean orogeny that spanned about 45 Ma. Fluid conduits formed early in the tectonic history and persisted throughout orogenesis with the plumbing system showing a rare high degree of focussing, efficiency and duration. In addition to the long-lasting fluid plumbing system, the wide variety of transient structural and geochemical traps, multiple fluid sources and precipitation mechanism contributed towards the richest golden mile in the world.

Fault and fault-rock characteristics associated with Cenozoic extension and core-complex evolution in the Catalina-Rincon region, southeastern Arizona

Cenozoic extensional deformation in southern Arizona included (1) a Neogene phase of Basin and Range deformation recorded by high-angle normal faults and (2) an earlier phase of detachment faulting and brittle-ductile crustal shearing associated with tectonic denudation of metamorphic core complexes. In the Catalina-Rincon region, exposed fault zones produced at different crustal depths during successive extensional episodes display differing fault geometries and types of fault rocks formed during progressive crustal extension. Detachment faults are associated with both mylonites produced by ductile shear and cataclasites produced by brittle shear. Younger faults formed at shallower depths are associated with less intense cataclastic deformation and with brittle fracturing that includes transtensile phenomena at the shallowest crustal levels represented. Qualitative measures of net displacement along individual fault zones are provided by (1) the nature of contrasts among successively overprinted fabrics and internal structures in the footwall and (2) the degree of contrast between fabrics and structures of footwall and hanging-wall rocks. Footwalls of the oldest structures display varied brittle overprints of ductile fabrics and are juxtaposed across gouge zones along detachment surfaces with stratal successions cut by multiple brittle shear surfaces. Footwalls of younger structures formed at shallower depths display multiple generations of cataclastic features, including brecciation of variable intensity and cataclasite dikes, but are juxtaposed against hanging-wall strata that are only moderately deformed by subsidiary faults. The shallowest fault zones lack either structural overprints in their footwalls or any significant contrasts between footwall and hanging-wall deformation. Exposures of mid-crustal rocks within the core complexes reflect successive exhumation and uplift of fault footwalls during sequential episodes of deformation. The present high elevation of mylonitic rocks in the Catalina-Rincon metamorphic core complex reflects dip slip and isostatic footwall flexure during Basin and Range deformation as well as tectonic denudation during detachment faulting. Net uplift of core rocks resulted from multiple phases of deformation.

Pinpointing new potential of mature fields in Mexico: An example from Vernet Field

In 2002, Compania Mexicana de Geofisica (CMG) conducted an integrated study of Vernet Field. Located in the southern part of Macuspana Basin and discovered in the 1930s, this field has produced oil and gas from 13 intervals in approximately 25 wells. Macuspana, a mature basin in southeast Mexico where hydrocarbon production dates to the early part of the 20th century, is predominantly gas-prone with production of 5.4 trillion ft3 of gas and limited oil from Mio-Pliocene clastics in approximately 35 fields of varying sizes (Figure 1). Figure 1. Map of Macuspana Basin and Vernet Field. Pemex estimates substantial undiscovered hydrocarbons remain in this 160 × 50 km basin. At the end of the study, several recommendations were made for additional exploration opportunities. The first proposed location, drilled by Pemex in December 2002, resulted in a success that will be described in this article. Pemex has subsequently planned additional drilling on this discovery to delineate the reservoir and to produce from other recommended prospects. During the integrated study and after the discovery, a 3D stratigraphic inversion and an AVO analysis were performed. Those results will also be presented. Figure 2 is a line through the field showing our interpreted horizons. It is an extremely complex area consisting of a major growth fault and numerous subsidiary synthetic and antithetic faults that create small traps at various intervals. Production comes from the crestal fault-bounded traps on the downthrown side of the major fault. Most production comes from the Z9-E15 interval on the structure. Production from AS 50 and deeper sands is very limited (40 000 bbls and 1 million ft3) and from only two wells. Initially, it was believed that the sand quality deteriorated with depth; this is no longer the case. The depositional facies varied from nonmarine, channel, near shore to offshore. …

Episodic growth of normal faults as recorded by syntectonic sediments, July oil field, Suez rift, Egypt

The July oil field is a major normal-fault–bounded structural block in the Suez rift basin, Egypt. It is adjacent to a major structural transfer zone, which has controlled sediment influx to the rift basin center for the past 20 m.y. The lower to middle Miocene Upper Rudeis Formation, part of the synrift stratigraphic sequence, records deformation of the July structural block. The formation contains abrupt lateral changes in thickness and facies, which record earlier phases of fault movement and deformation in the July field area that do not conform to the present-day structural configuration.The Upper Rudeis Formation was deposited as turbidites in a submarine-fan system sourced from the western rift shoulder. It was deposited over and around bathymetric highs created by coeval fault displacement in the July field area. By studying thickness and facies patterns, we have determined that the present-day main bounding fault to the July block consisted of a series of unlinked fault segments, which linked after Upper Rudeis deposition. A subsidiary fault west of the block exerted the most control on thickness patterns, not the present-day main bounding fault. Thus, commonly used models of deposition in active half grabens are difficult to apply at July field.

Moment tensor inversion of recent small to moderate sized earthquakes: implications for seismic hazard and active tectonics beneath the Sea of Marmara

SUMMARY We retrieve the moment tensors of 64 small to moderate sized events that occurred mostly beneath the Sea of Marmara using near-field data recorded at strong-motion and broad-band seismic stations. The near-field displacement records are inverted to their sources utilizing Kuge’s method where the best fit between the synthetic and observed seismograms is achieved through searching a centroid moment tensor (CMT) point in a grid scheme. We also analyse the stress fields acting in the eastern and western parts of the Sea of Marmara by inverting the P- and T-axes of the focal mechanisms obtained. Significant biases in the stress tensors are obtained. The nearly horizontal maximum compressive axis σ 1 in the western part is rotated 16 ◦ counter-clockwise compared with σ 1 in the eastern part. The σ 2-axis is close to vertical (shear tectonic regime) in the east and the plunge of σ 2-axis in the west is 36 ◦ (transpressive tectonic regime). Changes in the σ 3-axis are also observed, that is, it is close to horizontal in the east and dips 49 ◦ in the west. The spatial distribution of the focal mechanisms suggests that the stress field in the eastern part of the Sea of Marmara is homogenous compared with the western part, and we identify five distinct subsidiary faults. (1) a WNW‐ESE-striking, right-lateral strike-slip fault located a few kilometres SW of the Princes’ Islands, (2) a WSW‐ENE-striking, right-lateral strike-slip fault named the Yalova‐Hersek fault, (3) an E‐W-striking normal fault located onshore between

Evidence for low-angle, subhorizontal hanging faults in rotated fault blocks, Cabinda, offshore Angola

Extensional structures that include normal faults whose dip decreases with depth are well known in the West African passive margin. The faults detach on the Aptian-aged Loeme salt layer. Rotation and faulting of the hanging-wall blocks has produced hydrocarbon trap geometries that hold several billion barrels of reserves. Single listric fault systems, including subsidiary faults, are responsible for most structures; however, we have mapped a trend of structures formed by two generations of faulting within the block 0 concession offshore Cabinda, Angola. The resulting geometry of cross-cutting faults has introduced additional trap types not recognized before in the region.

Mouvement post-messinien sur la faille de Nimes; implications pour la sismotectonique de la Provence

Mouvement post-messinien sur la faille de Nimes; implications pour la sismotectonique de la Provence

Reflection seismic study across the continental shelf of Baba Burnu promontory of Biga Peninsula, northwest Turkey

Seismic and structural characteristics of the continental shelf of Baba Burnu promontory of Biga Peninsula, northwest Turkey were investigated by 67 km of multi-channel seismic reflection survey. Interpretation of the seismic data indicates that the faults may be divided into two groups: (i) in the southern area, almost an E–W trending major listric normal fault zone and subsidiary faults formed first, and later a strike-slip fault on the hanging wall of the major fault formed, (ii) in the northern area, a number of spatially developed parallel faults with major strike-slip components are seen. We propose that the Gulf of Edremit is a fault-bounded basin, which began to form in association with the generation of the E–W trending faulting phase during the late Miocene–early Pliocene.

Paleoseismic event and active faulting: from ground penetrating radar and high-resolution seismic reflection profiles across the Chihshang Fault, eastern Taiwan

Ground penetrating radar (GPR) and high-resolution shallow seismic reflection have been carried out to delineate the subsurface pattern and paleoseismic facies of the active Chihshang Fault, a segment of the Longitudinal Valley Fault, eastern Taiwan. Optimum radar results were obtained along streets in the Chinyuan village. A total of 720m GPR profiles were acquired with 200MHz antenna. Two seismic sections were collected using Mini-Sosie sources, providing a frame to the radar profiles.Three types of structures revealing evidence for a paleoseismic event can be observed in one GPR profile; these are: upward fault termination, colluvial wedge, and sand injection. Other profiles reveal unconformities, reverse-faults, one fault-related fold, and the presence of a colluvial wedge on the downthrown block, all indicating paleoseismic activity.The remarkable feature of the Chihshang Fault at Chinyuan is that the dips of this active fault at surface changes rapidly along strike. Combining the GPR with shallow seismic reflection allows us to reconstruct the three-dimensional fault pattern of the Chihshang Fault and resolve doubts about the abrupt changes of fault dips. These GPR and seismic studies are consistent with the surface observation and reveal the complicated pattern of antithetic subsidiary faults in the near-surface part of the major E-dipping Chihshang thrust fault.

Effects of subsidiary faults on the geometric construction of listric normal fault systems (Geologic Note)

Listric normal fault systems are one of the most important sites for petroleum exploration in extension basins. The geometry of these systems at depth is commonly uncertain because of poor seismic data. Thus several techniques have been developed to construct the shape and position of the master fault at depth using one or more shallower horizons. Antithetic and synthetic faults commonly disrupt the continuity of bedding in the deformed hanging wall and contribute to the overall extension. Current models either neglect or do not adequately account for the effect of these faults in estimating the geometry of the master fault. We suggest that the extension of individual subsidiary faults should be added to the amount of extension on the master fault during the bed-fault construction. The extension on subsidiary faults is supposed to be transferred into the main fault, along the inclination of shear, although the linking mechanism between the small and main faults is unclear. The inclined shear angle can be estimated by orientation of the antithetic faults. Occurrence of synthetic faults may slightly distort the particle motion trajectories (inclined shear) relative to a hanging-wall reference frame but does not change the overall relative motion pattern. Application of our modification of the inclined shear model to unpublished and published seismic data, as well as an analog experiment model, agrees well with observed fault geometry. Negating the amount of heave on subsidiary faults results in an overestimation of depth to detachment.

Hydrothermal minerals and microstructures in the Silangkitang geothermal field along the Great Sumatran fault zone, Sumatra, Indonesia

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