Synthetic Normal Faults Research Articles

The rock mass as the governing factor for successfully developing deep geothermal systems in Southern Germany

AbstractIn Southern Germany, geothermal wells target Upper Jurassic carbonates, formerly referred to as Malm, below the Northern Alpine Foreland Basin (NAFB). In the northern, shallower carbonates (3000 m depth), e.g., in the Munich area, hydrothermal systems prevail, and wells yield proper flow rates. Deeper carbonates (< 4000 m) towards the south indicate a petrothermal system due to a decline in porosity and permeability. Inexplicably, fractures only contribute poorly to larger flow rates. This sets the goal for the project Dolomitkluft to analyse the rock mass for options countering low flow rates in two geothermal wells at Geretsried, Bavaria. Therefore, 20 m drill cores from 4600 to 4715 m total vertical depth (TVD) depict the initial structure of the rock mass: the rock parameters, the discontinuity network, and the in situ stresses. Alternating homogeneous, micritic, strong, to inhomogeneous, porous, weak limestones and massive, crystalline dolostones define the stratigraphy. Five joint sets from the cores can be distinguished into the bedding, two paleo anti‐ and synthetic normal faults and two recent strike‐slip faults. The latter, in combination with core disking, allows exclusively a strike‐slip (SS) stress regime and delimits the ratio to at least SH : Sv : Sh = 1.7 : 1.0 : 0.7. According to the results, including dilatation‐ and slip‐tendencies, drill trajectories of prosperous geothermal wells should head ESE/WNW, perpendicular to SS‐faults, and avoid the presence of normal faults.

Lower Jurassic deformation in the eastern Huincul High, Argentina

Subsurface characterization of the structure and sedimentary sequences in the Agua del Cajón area was performed to understand the Lower Jurassic deformation across the eastern Huincul High. Integration of seismic, well data, and fossil fauna allowed establishing the Late Triassic rifting and Jurassic compressional evolution underwent for this area. Four Late Triassic - Early Jurassic extensional depocenters were recognized, being the most significant the Agua del Cajón and El Salitral depocenters. In general, all of them present an E-W to WNW-ESE orientation and are flanked by NNE to NE-trend transfer zones. The contractional structural style of the Agua del Cajón area consists of south-vergence reverse faults and folds with two primary trends: (i) E-W to WNW-ESE and (ii) NE-SW to NNE-SSW. According to the slip and uplift amount, the first-order structural feature in the study area corresponds to the S-verging, E-W Huincul fault. Geometries and patterns in the seismic reflectors evidence two compressional episodes from late early Toarcian to Callovian time. The first one is associated with the inversion of the previous master normal fault of the El Salitral and Agua del Cajón depocenters during the late early Toarcian. The inversion of the NE-SW synthetic normal fault associated with the El Salitral depocenter also occurred during the first deformation pulses favored by its perpendicular orientation to NW-SE Jurassic σ1. NE-SW fault associated with the Agua del Cajon inversion shows no evidence of inversion-related deformation mechanisms. Instead, the formation of this structure is related to the previous NE-SW transfer zone. After a tectonic quiescence period in late Toarcian – late Aalenian, the compressional activity continued during the late Aalenian and Bajocian evidenced by progressive unconformities into Jurassic sequences. This activity would have continued until Callovian times. These results are in agreement with previously reported evolution in the eastern-central Huincul High region. However, they indicate that the contractional intraplate deformation in the southern Neuquén basin would have begun during the late early Toarcian; this is older than previously proposed.

Geometry of the inverted Cretaceous Chañarcillo Basin based on 2-D gravity and field data – an approach to the structure of the western Central Andes of northern Chile

Abstract. This paper discusses an integrated approach that provides new ideas about the structural geometry of the NNE-striking, Cretaceous Chañarcillo Basin located along the eastern Coastal Cordillera in the western Central Andes of northern Chile (27–28° S). The results obtained from the integration of two transverse (E–W) gravity profiles with previous geological information show that the architecture of this basin is defined by a large NNE–SSE-trending and east-vergent anticline ("Tierra Amarilla Anticlinorium"), which is related to the positive reactivation of a former Cretaceous normal fault (Elisa de Bordos Master Fault). Moreover, intercalations of high and low gravity anomalies and steep gravity gradients reveal a set of buried, west-tilted half-grabens associated with a synthetic normal fault pattern. These results, together with the uplift and folding style of the Cretaceous synextensional deposits recognized within the basin, suggest that its structure could be explained by an inverted fault system linked to the shortening of pre-existing Cretaceous normal fault systems. Ages of the synorogenic deposits exposed unconformably over the frontal limb of the Tierra Amarilla Anticlinorium confirm a Late Cretaceous age for the Andean deformation and tectonic inversion of the basin.

KARABURUN (İZMİR) ÇEVRESİNİN NEOJEN STRATİGRAFİSİ VE PALEOCOĞRAFİK EVRİMİ

The western margin of the Foca Depression (FD) is located in the NE of the KaraburunPeninsula. Terrestrial Neogene sediments in the study area which partly representing thewestern margin of the FD and the mafic volcanics have NW-SE directions towards ‹zmirbay and are separated from the basement rocks by synthetic normal faults. During theMiocene deposition the basin’s boundaries became structurally narrower and two mainsedimentary successions have been defined namely the Karaburun group and the Eflenderegroup which have been separated with angular unconformity in regional scale. TheKaraburun group is represented with dominantly lacustrine deposition in Early-MiddleMiocene period that includes Haseki and Hisarcik formations and Karaburun volcanics.The Lower Miocene Haseki formation is represented by the Salman member developed inthe alluvial fan environment and lacustrine deposits of algal-biostromal Yenilimanlimestone and micritic limestone dominated Aktepe member. NW trending basin marginfaults which formed a boundary between Karaburun high and the FD and caused theeffective second stages of mafic volcanism did not interrupt the lacustrine deposition at thebeggining of the Early Miocene. But it caused relative deepening of the basin, changeddepositional conditions and lacustrine sedimentation continued with the Hisarcikformation. The activity shaping the FD from the western part and partly coinciding withthe Early Miocene basin margin faults has been documented with the unconformityshowing no time gap between Haseki and Hisarcik Formations. Hacihuseyintepe memberhas unconformity with the lavas that represent the second stage of Karaburun volcanicsand laterally passes into the Karaba¤lari member. The Karaba¤lari member consists ofgreen coloured lacustrine shoreface sediments which lie on the Aktepe succession withsublacustrine paraconformity. De¤irmentepe limestone transitionally lies on the top and itis the last member of the Karaburun group. Eflendere group lies on the Hisarcik formationwith regional scale angular uncoformity and presents the Late Miocene-early EarlyPliocene(?) sedimentation grading from alluvial fan deposits of the Saip formation tolacustrine Cukurcak limestone. Calc-alkaline Karaburun volcanism which is representedwith potassium rich andesitic products has three stages that laterally connected with theEarly Miocene-early Middle Miocene deposition The first two stages have dual faciesnamely, pyroclastics at the base and lavas on the top. The first stage products are laterallydiscontinuous reference level that is separating Yeniliman limestone and Aktepe member.Second stage products are in/on the Aktepe member. The third stage lava flows are datedas K/Ar 16.0±1.3 Ma age which is thought to be located at the bottom of the De¤irmentepelimestone that is the last member of the Karaburun group.1988; Cakmako¤lu ve Bilgin, 2006) tektonikyerleflimi ile Erken Miyosen havza oluflumu arasindagecen sure boyunca bolgenin deformasyona u¤radi¤ive aflindi¤i kabul edilmektedir. ‹nceleme alaninda,Cakmako¤lu ve Bilgin (2006)’in tanimladi¤iDikenda¤i formasyonuCamibo¤azi formasyonu (Ladiniyen), Guvercinlikformasyonu (Resiyen-Karniyen) ve ‹zmir fliflinden(Gec Kretase-Erken Tersiyer) oluflan temel kayalari

NEOGENE STRATIGRAPHY AND PALEOGEOGRAPHIC EVOLUTION OF THE KARABURUN AREA, İZMİR, WESTERN TURKEY

The western margin of the Foca Depression (FD) is located in the NE of the Karaburun Peninsula. Terrestrial Neogene sediments in the study area which partly representing the western margin of the FD and the mafic volcanics have NW-SE directions towards ‹zmir bay and are separated from the basement rocks by synthetic normal faults. During the Miocene deposition the basin's boundaries became structurally narrower and two main sedimentary successions have been defined namely the Karaburun group and the Eflendere group which have been separated with angular unconformity in regional scale. The Karaburun group is represented with dominantly lacustrine deposition in Early-Middle Miocene period that includes Haseki and Hisarc›k formations and Karaburun volcanics. The Lower Miocene Haseki formation is represented by the Salman member developed in the alluvial fan environment and lacustrine deposits of algal-biostromal Yeniliman limestone and micritic limestone dominated Aktepe member. NW trending basin margin faults which formed a boundary between Karaburun high and the FD and caused the effective second stages of mafic volcanism did not interrupt the lacustrine deposition at the beggining of the Early Miocene. But it caused relative deepening of the basin, changed depositional conditions and lacustrine sedimentation continued with the Hisarc›k formation. The activity shaping the FD from the western part and partly coinciding with the Early Miocene basin margin faults has been documented with the unconformity showing no time gap between Haseki and Hisarc›k formations. Hac›huseyintepe member has unconformity with the lavas that represent the second stage of Karaburun volcanics and laterally passes into the Karabalar› member. The Karabalar› member consists of green coloured lacustrine shoreface sediments which lie on the Aktepe succession with sublacustrine paraconformity. Deirmentepe limestone transitionally lies on the top and it is the last member of the Karaburun group. Eflendere group lies on the Hisarc›k formation with regional scale angular uncoformity and presents the Late Miocene-early Early Pliocene(?) sedimentation grading from alluvial fan deposits of the Saip formation to lacustrine Cukurcak limestone. Calc-alkaline Karaburun volcanism which is represented with potassium rich andesitic products has three stages that laterally connected with the Early Miocene-early Middle Miocene deposition The first two stages have dual facies namely, pyroclastics at the base and lavas on the top. The first stage products are laterally discontinuous reference level that is separating Yeniliman limestone and Aktepe member. Second stage products are in/on the Aktepe member. The third stage lava flows are dated as K/Ar 16.0±1.3 Ma age which is thought to be located at the bottom of the Deirmentepe limestone that is the last member of the Karaburun group.

Sandbox models of downward-steepening normal faults

Two deformational styles resulted from sandbox experiments examining the hanging-wall deformation above a downward-steepening normal-fault bend comprised of two planar segments dipping at 40 and 60. In the first, hanging-wall layer-parallel extension dominated, with synthetic normal faults forming an upward-widening fault-bound wedge above the fault bend. Early forming, arcuate, vertical-to-reverse faults bound the wedge on its downthrown side and were followed by a planar, large-displacement synthetic normal fault bounding the wedge on the upthrown side. This later fault accommodated almost all of the subsequent displacement that occurred on the underlying 60 fault. Slip variably occurred along the 40 segment of the fault. Good analogs in nature exist for this deformation geometry. In the second deformational style, hanging-wall layer-parallel compression dominated, forming a monocline with antithetic hinge surfaces dipping downward to the fault-bend surface. Antithetic reverse faults formed in the limb of the monocline. The upper hinge of the monocline intersected the footwall at the fault bend, whereas its lower hinge intersected the 60 fault below the fault bend. The lower hinge was older and formed near the location of the fault bend prior to its being translated down the 60 master fault as part of the hanging wall. Elements of this geometry correlate well with the results of some geometric-based numerical models, but strong analogs in nature are elusive.

Analogue and numerical modeling of normal fault patterns produced due to slip along a detachment zone

In this paper, we investigate normal fault patterns produced by the sliding motion along a gently dipping normal fault by using analogue model tests and numerical modeling. The motivation for this study was offered by microseismic test data that indicate the existence of an active low-angle shear zone at a depth of 9–11 km in the extensional region of high seismic activity of the Gulf of Corinth (Greece). Both modeling techniques seem to support the hypothesis that the system of high-angle normal faults that are responsible for the final asymmetrical graben formation initiate at the tip of the active basal detachment nearest to the free surface. The normal faults propagate upwards with progressive sliding of the inclined basal plane, resulting in a first phase of symmetrical graben configuration that is delimited by a main synthetic fault and an antithetic fault forming a Rankine zone. Subsequent sliding on the inclined base induces a family of secondary antithetic normal faults, which are responsible for the asymmetry of the failure pattern and the diffusive character of deformation in that area. Shear deformation is more intense and localized along the synthetic normal fault than along the antithetic faults. Elaboration on the analogue test results has led to the phenomenological relations among four main parameters that describe the geometry of grabens, namely, (i) the width and (ii) the maximum subsidence of the graben, (iii) the dip angles of the conjugate normal faults, and (iv) the amount of sliding along the low-angle normal fault. However, analogue models do not produce the system of synthetic faults that is observed in the Gulf of Corinth. The effects of both friction angle variation along the detachment base and of the constitutive behavior of the model material on the configuration of the final structural pattern were also studied with a series of numerical continuum models. It was found that (a) the fault pattern of the Gulf of Corinth may be reproduced with either a strain-softening material with low elastic modulus or a constant strength material, and (b) two consecutive grabens, such as those of Gulfs of Corinth and Evia, may also be reproduced by an appropriate combination of variation of dip and frictional properties along the hypothesized detachment zone.

Elastic modeling of the Alto Tiberina normal fault (central Italy): geometry and lithological stratification influences on the local stress field

Two-dimensional, elastic, plane-strain, finite element models (FEMs) are generated to study the state of stress and failure induced by a low-angle normal fault, the Alto Tiberina Fault of the Northern Apennines (central Italy): it is beyond the scope of the present work to show that slip can occur on such a fault. The numerical study was performed to evaluate the influences on the local stress field of the litho-mechanical stratification of formations surrounding the fault, and those induced by the geometry of the fault. The performed models have shown the important role played by evaporites and basement formations of the Umbria-Marche succession as seismogenetic layers. The model results have also underlined that the flat-ramp geometry of the fault induces high relative concentration of stresses in correspondence with the low-angle, east-dipping, synthetic normal faults observed today in shallow depth near the Alto Tiberina surface trace. The stress regime predicted by the final model, in which the behavior of the Alto Tiberina together with the antithetic normal fault of Gubbio was simulated, reconciles available geological and geophysical observable to a greater extent. The numerical results can assist interpretation of the tectonic evolution of the region.

An experimental study of the secondary deformation produced by oblique-slip normal faulting

We have used scaled clay models to define the secondary deformation produced by oblique-slip normal faulting. In the models, the master fault beneath the clay layer dips 45 degrees and strikes at a 45 degrees angle relative to the heave direction (i.e., the horizontal component of the displacement direction). Thus, the master fault has both normal dip-slip and strike-slip components of displacement. The modeling results show the following. (1) The fault patterns produced by oblique-slip normal faulting vary significantly with depth. Secondary faults that strike obliquely to the master-fault trend are more abundant near the top of the clay layer, whereas secondary faults that are subparallel to the master-fault trend are more abundant at depth. (2) A single episode of oblique-slip normal faulting produces two populations of secondary faults that have different trends and ages. Secondary faults that strike obliquely to the master-fault trend are more abundant during the early stages of the experiments, whereas secondary faults that strike subparallel to the master-fault trend are more abundant during the later stages of the experiments. (3) Relay ramps between overlapping secondary synthetic normal faults are wide and temporally persistent in oblique-slip models. The ramps are cut by numerous small-scale normal faults that are subparallel to the ramp-bounding faults. Cross faults are uncommon and begin to develop only during the final stages of the experiments. (4) Both map and cross section data are necessary to distinguish among the deformation patterns produced by strike-slip, oblique-slip, and dip-slip faulting. The map views of oblique-slip models closely resemble those of strike-slip models; in both models, many of the secondary faults strike obliquely to the master-fault trend. These map views, however, differ considerably from those of dip-slip models, in which most of the secondary faults strike subparallel to the master-fault trend. Alternatively, the cross sectional views of oblique-slip models are similar to those of dip-slip models; in both models, a highly faulted extensional forced fold develops. These cross sectional views are dissimilar to those of strike-slip models, which show no appreciable folding and no change of regional level.

Localization of listric faults at thrust fault ramps beneath the Great Salt Lake Basin, Utah: Evidence from seismic imaging and finite element modeling

Reflection seismic data from the Great Salt Lake Basin, Utah, show that the major basin‐bounding normal faults decrease in dip from ∼60° at the surface to ∼10°–20° at depths as shallow as 4–6 km. This rapid decrease in fault dip at depths shallower than the brittle‐ductile transition zone in the Basin and Range Province suggests an explanation other than a gradual change of rheology and stress orientations with depth. Using a dense grid of seismic data, gravity data, borehole data, and published geologic information from islands in the lake, we constrain the position of the Sevier age Willard thrust and a footwall imbricate and show their reactivation as normal faults during Tertiary extension. In the absence of surface geologic information, we use available subsurface information from the lake to draw an analogy with the Ogden duplex in the Wasatch Front, where Cenozoic normal faulting was superimposed on an earlier Sevier age thrust regime to give rise to listric normal faults. Our interpretations are consistent with finite element modeling results, which demonstrate that extensional slip on preexisting thrust ramps leads to the formation of energetically favored synthetic normal faults, some of which may merge with the thrust ramp and obtain listric geometries. Further slip on these listric faults gives rise to secondary synthetic and antithetic faults resulting in hanging wall grabens.

Gravity-driven thin-skinned extension above Zechstein Group evaporites in the western central North Sea: an application of computer-aided section restoration techniques

Normal faulting and halokinesis have been important controls on the deposition and subsequent deformation of Mesozoic and Tertiary strata in the North Sea. In addition to the previously documented mechanisms of salt withdrawal, dissolution and differential sedimentary loading, it is recognized that gravity-driven thin-skinned extension above inclined salt layers has played an important part in North Sea basin development. Commercial section restoration software has been used to facilitate depth conversion, restoration and decompaction of seismic sections selected from an interpreted regional database in the western central North Sea, allowing validation of the interpretations, and a graphical and highly quantitative description of salt-assisted extension. Results of this work show that Zechstein Group evaporites were deposited in shallow sag basins during the Permian. Triassic sedimentary pods were generated by localized deposition in synclinal basins and grabens above the evaporites. Bedded salt became folded, while mobile salt flowed to fill anticline cores. Since the early Jurassic, regional tectonic tilting related to post-rift subsidence and increasing sedimentary overburden have caused allochthonous Mesozoic and Tertiary strata to extend by gravity spreading above the mobile salt layer, which detaches the allochthon from the underlying autochthonous Late Palaeozoic rocks. Concave-up listric normal faults sole out in the salt layer, propagate into the overlying cover sewuence, and have been active at different geological times causing stratal thickening and folding within the allochthon. Antithetic and synthetic normal faults have developed, producing complex upward branching fault systems. In map view, the listric faults form curvilinear en echelon arrays, the faults linked by relay ramps. Fault blocks are typically 3–7 km wide, 2–3 km thick and 7–10 km long. Salt movement during the Jurassic-Tertiary has been driven by active extension of the cover, causing salt to fill potential voids created by fault block rotation. Thus salt highs occur beneath sites of extension. The listric faults generally dip in the same direction as the sub-salt surface, although there are also some major counter-regional faults. During extension, regional dips have increased up to about 5, which is sufficient for gravity-driven extension above a salt layer. A total extension of about 6% has occurred. The gravity-driven thin-skinned extension documented in the western central North Sea is a phenomenon which can be recognized elsewhere in the North Sea basin, and can be readily compared with similar phenomena already documented in offshore Angola, Brazil, Nova Scotia and the Gulf of Mexico.

Estimating inversion: results from clay models

Abstract Physical models using wet clay show that, although both fold and fault geometries change with increasing amounts of inversion, it is difficult to estimate the magnitude of inversion. During extension, a listric main normal fault and a rollover fold cut by secondary normal faults developed above two diverging, basal plates. With 50 0nversion (4 cm extension, 2 cm shortening), the main normal fault underwent reverse displacement, and an asymmetric syncline formed in the pre-growth layers in the hangingwall. Secondary antithetic and synthetic normal faults passively rotated. Faults on the synclinal limb near the main normal fault rotated away from the fault, whereas those on the opposing limb rotated towards the fault. A broad anticline formed at shallow levels above the original half graben. Compressional reactivation of the main normal fault ceased between 50% and 100 0nversion (4 cm extension, 4 cm shortening). Instead, low-angle thrust faults with small displacements accommodated most shortening. The shallow anticline above the original half graben expanded laterally and vertically. With 200 0nversion (4 cm extension, 8 cm shortening), low-angle thrust faults with large displacements deformed the hangingwall and footwall of the main normal fault. Thrust faults cut the main normal fault and many of the secondary normal faults. The geometry of the hangingwall fold varied with depth. A broad anticline deformed the upper growth layers, changing to a syncline in the lower growth layers and an anticline in the pre-growth layers. Estimates of inversion from null point analysis, amount of uplift above regional datum, or line length balancing, grossly underestimate the amount of shortening in the models. In addition, both the 50% and 100 0nversion models lack the large-scale deformation features typically associated with compression. These results suggest that both quantitative and qualitative techniques used to calculate inversion magnitude can significantly underestimate the amount of shortening. Furthermore, if the inversion uplift is eroded or if many small-scale deformation features are not imaged seismically, then inversion may go unrecognized.

Normal Faults and Their Hanging-Wall Deformation: An Experimental Study

We have used clay models to study the effects of fault shape and displacement distribution on deformation patterns in the hanging wall of a master normal fault. The experimental results show that fault shape influences the style of secondary faulting and folding. Mostly antithetic normal faults form above concave-upward fault bends, whereas mostly synthetic normal faults form above low-angle fault segments and convex-upward fault bends. Beds dip toward the master normal fault above concave-upward fault bends and away from the master normal fault above low-angle fault segments and convex-upward fault bends. Generally, secondary faulting and folding are youngest at fault bends and become progressively older past fault bends. Hanging-wall deformation patterns differ significantly when a basal plastic sheet imposes a constant-magnitude displacement distribution on the master normal fault. In models without a plastic sheet, numerous secondary normal faults form in the hanging wall of the master normal fault. Most secondary normal faults propagate upward and, consequently, have greater displacement at depth. In models with a plastic sheet, few visible secondary normal faults develop. Most of these faults propagate downward and, consequently, have less displacement at depth. Hanging-wall folding is wider and bedding dips are gentler in models without a plastic sheet than in identical models with a plastic sheet. The observed particle paths, displacement distributions, bedding dips, and orientations of the principal-strain axes in our physical models with and without a basal plastic sheet are compatible with the assumption that homogeneous, inclined simple shear accommodates the hanging-wall deformation. Not all of our modeling observations, however, are consistent with this assumption. Specifically, the observed variability with depth of the distribution and intensity of deformation is incompatible with homogeneous, inclined simple shear as the hanging-wall deformation mechanism.

Generation and Migration of Heavy Oils from Guarico and Southern Maturin Subbasins

Results from an organic geochemical study from the western half of the Eastern Venezuela Basin (west of the Urica Fault) indicate that Cretaceous intervals from the Querecual and Tigre formations are the major source rocks for the naphtenic type oils. The Tertiary La Pascua, Merecure, and Lower Oficina intervals generated the paraphinic crudes. The mixture of naphtenic-paraphinic oils is interpreted to occur during migration or within the reservoirs. The thermal-maturity modeling from 56 wells, applied to the south of the Serrania del Interior frontal thrusts suggests that the active kitchens shifted gradually from the northwest to the southeast throughout the last 20 ma. To the north-northwest, the Cretaceous source rocks reached peak maturation and crudes were expelled prior to 15 ma. The Paleogene rocks reached similar conditions around 9 ma, and the Miocene intervals around 7 ma. The crudes generated between 15 and 18 ma migrated to the Orinoco Belt along unconformities and synthetic normal faults. Between 8 and 3 ma, the crudes followed a similar pathway, partially modified by the development of compactions faults. This later mechanism developed a series of antithetic faults which prevent the lateral migration and facilitate the vertical segregation. Oil in the Great Oficina areamore » was trapped during this time. Oil from the Anaco Trend was also trapped during this time associated with the inversion of a pre-Cretaceous half-graben.« less

Origin of Rollover (1)

Rollover is the folding of hanging-wall fault blocks by bending or collapse in response to slip on nonplanar--commonly listric--normal faults. The shapes of rollover folds are controlled by a number of variables, including (1) the shape of the fault, (2) the total fault slip after a bed is deposited, (3) the direction of relative particle motion in hanging-wall collapse, (4) the history of sedimentation rate relative to fault slip rate, and (5) compaction. The importance and role of each of these variables is illustrated by a two-dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments. In this modeling, an active axial surface, oriented parallel to the direction of relative p rticle motion in hanging-wall collapse, emanates from each fault bend and is the instantaneous locus of folding. The quantitative correctness of this theory of rollover is tested by modeling natural structures from the Gulf of Mexico for which both fault shape and fold shape are known from high-quality seismic and well sections. The direction of hanging-wall collapse commonly is in the antithetic or synthetic normal-fault or Coulomb-shear orientations, although sliding along weak bedding planes also is an important collapse mechanism in some regions. Collapse is in the antithetic-shear direction for concave fault bends and in the synthetic-shear direction for convex bends. These collapse directions can be observed directly in some high-quality seismic images as axial surfaces emanating from fault bends. The shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate, as well as the total slip after a bed is deposited. The crests of classic Gulf Coast rollovers are growth axial surfaces, along which are abrupt changes in sedimentation rate within the growth st atigraphic interval. These changes are produced by deformation of the sediment-water interface along active axial surfaces. Compaction can substantially modify the relationship between fault shape and rollover shape; however, under certain common conditions, the history of compaction can be neglected if the folding is modeled in the compacted state.

The Midland Fault Zone, Southern Sacramento Basin, California

Abstract Upper Cretaceous and Paleogene sedimentation in the southern Sacramento Basin has resulted in the formation of a north-south trending growth fault system, the Midland Fault System, along the eastern flank of the depositional basin. The Midland Fault is the dominant growth fault of this syndepositional fault system. Three major growth faults branch off toward the northwest from the Midland Fault in the northern portion of the system. Northeast stepping en echelon faults form the northern termination of the Midland Fault. Numerous synthetic and antithetic normal faults emanate obliquely from, or parallel to, the major growth faults throughout the system’s 55-mile length. Tectonic activity along the Midland Fault System occurred in three phases: 1) Late Cretaceous, 2) middle Paleocene, and 3) late Paleocene through Eocene. Syndepositional fault movement is documented by an abrupt basinward increase in stratigraphic thickness across the fault system, a rollover of beds into the faults, and a counter-regional thickening of strata on the downthrown side of the faults. Average vertical throw of the Upper Cretaceous and Paleogene strata increases with depth, demonstrating progressive growth fault movement through time. Movement along the fault system probably ceased during Oligocene time, as the faults do not appear to offset Oligocene sediments within the Markley submarine canyon.

Mechanics of extensional wedges

The extensional wedges that overlie low‐angle basal detachment faults are characterized by a tapered cross section similar to that exhibited by a fold‐and‐thrust belt, but with an opposite sense of shear on the basal fault. Such wedges are common in zones of crustal extension, such as the Basin and Range province of the North American Cordillera, the North Sea, and the U.S. Gulf Coast. This paper examines the mechanics of these upper crustal extensional wedges, using a modification of the compressional Coulomb critical taper model developed for fold‐and‐thrust belts and accretionary wedges. If a compressional wedge is mechanically analogous to a wedge of soil being pushed up an incline by a moving bulldozer, then an extensional wedge is analogous to the same wedge with the bulldozer moving in reverse gear down the incline. A critically tapered extensional wedge is one that is on the verge of Coulomb failure everywhere. A wedge whose taper is narrower may be slid stably down the same incline without internal deformation; a wedge whose taper is greater will fail by normal faulting and reduce its taper until it is critical. After testing the quantitative predictions of this extensional critical taper theory in a series of laboratory experiments on dry sand wedges, we apply the theory to a pair of active extensional wedges in the Brazos area, offshore Texas. Both wedges are underlain by well‐imaged basal detachment faults dipping at 17° down to depths of 10 km; one wedge exhibits relatively little internal deformation, whereas the other is pervasively faulted along a series of moderately dipping synthetic normal faults. Direct and indirect estimates of the pore fluid pressure distribution within the two wedges suggest that the undeformed wedge is stable whereas the pervasively faulted wedge is unstable but nearly critical. Regional values for the coefficients of basal and internal friction in excess of 0.3 together with vertical cohesion gradients in excess of 1 kPa/m are consistent with the observed pore pressures and the geometry of the wedges, as well as with in situ stress data acquired during drilling and the dips of active synthetic normal faults. Such values are comparable to values inferred from mechanical and thermal models of compressional wedges, and they are consistent with laboratory rock mechanics data.

Geometry of Rollover: Origin of Complex Arrays of Antithetic and Synthetic Crestal Faults: ABSTRACT

Rollovers form by the collapse or bending of a hanging wall as it slides over bends in a normal fault. Once the authors know the particle motion of the collapse, commonly antithetic Coulomb shear, they can develop a simple relationship between fault shape and rollover geometry that is a function of the relative rates of fault slip and sedimentation. The dip of the rollover is a function of the angle fault bend, which is verified in some structures. A fanning of dips in the rollover is predicted for continuously flattening faults. In contrast, numerous rollovers from the US Gulf Coast, Brunei, and elsewhere do not show significant fanning of dips but show a nearly constant dip that is less than the magnitude predicted from Coulomb shear. These structures show complex arrays of small synthetic and antithetic normal faults on the crest of the rollover, including keystone grabens. High-quality seismic images show that many of these crestal faults suddenly disappear downdip, apparently to run along bedding. These observations indicate that part of the rollover collapse is by bedding slip after a critical rollover angle is reached; the bedding slip extends from the master normal fault up to the crest of themore » rollover where it breaks upward to be the imaged synthetic and antithetic crestal faults. Modeling of this process shows that variation in the relative rates of fault slip and sedimentation predict the different observed styles of complex crestal faulting in offshore Texas, Brunei, and elsewhere.« less

Mechanics of graben formation in crustal rocks: A finite element analysis

Small (1–10 km wide) grabens are ubiquitous tectonic structures that have been observed on the surfaces of all the terrestrial planets and satellites. In spite of their frequent occurrence, there is no fundamental understanding of how they form, other than that they are characteristic of extensional terranes. This paper examines the mechanics of the initial stages of graben formation and demonstrates that the configuration of a pair of antithetically dipping normal faults (a graben) is the energetically most favorable fault configuration in elastic‐brittle rocks subjected to pure extension. A corollary to this hypothesis is that the occurrence of synthetic normal faults is indicative of a stress field other than purely extensional, or of nonhomogeneous characteristics of the rock. The analysis proceeds by computing the stress field in the vicinity of a single initial normal fault using a two‐dimensional finite element model. It is shown that slip on this fault substantially lowers the proximity to failure of the surrounding rocks except for two small regions, one near the surface of the downthrown block and the other near the fault tip. The region on the downthrown block is located a horizontal distance from the initial fault slightly less than the fault's depth. The proximity to failure actually increases here, and a secondary fault develops. An antithetic secondary fault would link this region with the other failure zone near the fault tip. To demonstrate the preference for antithetic secondary faults more rigorously, we show that this fault configuration results in a smaller total strain energy than the synthetic configuration. In addition, a two‐layer model is used to show that the graben's width is only weakly dependent upon the presence of mechanically distinct layers in the crust. Both weak‐over‐strong and strong‐over‐weak configurations are investigated. The depth of the initial fault and the thickness of the top layer are varied, and the width of the resulting graben is evaluated by computing the distance between the initial fault and the region of highest proximity to failure near the surface. This analysis indicates that the depth of the initial fault is the major factor controlling graben width.

Surface expression of an accommodation zone within the Gulf of Suez rift, Egypt

The Gebel Gharamul region in the western Gulf of Suez offers an excellent outcrop example of the structural geometries associated with an accommodation zone termination. The surface expression of the accommodation zone is dominated by a basement promontory, which protrudes from the rift shoulder and underlies the junction of oblique ramps from two adjacent half-grabens, both of which face the Gulf of Suez. The southern half-graben represents the breakaway fault of the southwest-dipping tilt-block domain, and the northern half-graben represents the collapse of the upper plate above the northeast-dipping tilt-block domain. Basement and Nubian rocks in the hanging walls above the opposing low-angle detachments on either side of the accommodation zone are broken by gulf-parallel synthetic normal faults and orthogonal transfer faults, which act as block terminations and incrementally accommodate the large-scale rotation and displacement between adjacent blocks. The prerift and synrift stratigraphic succession is draped over this fault template, resulting in a complex and variable distribution of structures, the product of an inhomogeneous stratigraphic succession within an accommodation zone.