Extensional Geometries Research Articles

The impact of syn- and post-extension prograding sedimentation on the development of salt-related rift basins and their inversion: Clues from analogue modelling

Various studies have demonstrated the intrinsic interrelationship between tectonics and sedimentation in salt-related rift basins during extension as well as during their inversion by compression. Here, we present seven brittle–ductile analogue models to show that the longitudinal or transverse progradation of sediment filling an elongate extensional basin has a substantial impact on the growth of diapirs and their lateral geometrical variations. We use five extensional models to reveal how these prograding systems triggered diapir growth variations, from proximal to distal areas, relative to the sedimentary source. In the models, continuous passive diapir walls developed, after a short period of reactive–active diapiric activity, during syn-extensional homogeneous deposition. In contrast, non-rectilinear diapir walls grew during longitudinal prograding sedimentation. Both longitudinal and transverse post-extensional progradation triggered well-developed passive diapirs in the proximal domains, whereas incipient reactive–active diapirs, incipient roller-like diapirs, or poorly developed diapirs were generated in the distal domains, depending on the modelled sedimentary pattern. Two models included final phases of 6% and 10% shortening associated with basin inversion by compression, respectively, to discriminate compressional from purely extensional geometries. With the applied shortening, the outward flanks of existing diapir walls steepened their dips from 8°–17° to 30°–50°. Likewise, 6% of shortening narrowed the diapir walls by 32%–72%, with their fully closing (salt welds) with 10% of shortening. We compare our results with the distribution of salt walls and minibasins of the Central High Atlas diapiric basin in Morocco, which was infilled with a longitudinally prograding mixed siliciclastic and carbonatic depositional sequence during the Early–Middle Jurassic with a minimum thicknesses of 2.5–4.0 km.

Fractures in complex fluids: the case of transient networks

We present a comprehensive review of the current state of fracture phenomena in transient networks, a wide class of viscoelastic fluids. We will first define what is a fracture in a complex fluid, and recall the main structural and rheological properties of transient networks. Secondly, we review experimental reports on fractures of transient networks in several configurations: shear-induced fractures, fractures in Hele-Shaw cells and fracture in extensional geometries (filament stretching rheometry and pendant drop experiments), including fracture propagation. The tentative extension of the concepts of brittleness and ductility to the fracture mechanisms in transient networks is also discussed. Finally, the different and apparently contradictory theoretical approaches developed to interpret fracture nucleation will be addressed and confronted to experimental results. Rationalized criteria to discriminate the relevance of these different models will be proposed.

On the formation and evolution of the Pannonian Basin: Constraints derived from the structure of the junction area between the Carpathians and Dinarides

The large number and distribution of rollback systems in Mediterranean orogens infer the possibility of interacting extensional back‐arc deformation driven by different slabs. The formation of the Pannonian back‐arc basin is generally related to the rapid Miocene rollback of a slab attached to the European continent. A key area of the entire system that is neglected by kinematic studies is the connection between the South Carpathians and Dinarides. In order to derive an evolutionary model, we interpreted regional seismic lines traversing the entire Serbian part of the Pannonian Basin. The observed deformation is dominantly expressed by the formation of Miocene extensional detachments and (half) grabens. The extensional geometries and associated synkinematic sedimentation that migrated in time and space allow the definition of a continuous and essentially asymmetric early to late Miocene extensional evolution. This evolution was followed by the formation of few uplifted areas during the subsequent latest Miocene–Quaternary inversion. The present‐day extensional geometry changing the strike across the basin is an effect of the clockwise rotation of the South Carpathians and Apuseni Mountains in respect to the Dinarides. Our study infers that the Carpathian rollback is not the only mechanism responsible for the formation of the Pannonian Basin; an additional middle Miocene rollback of a Dinaridic slab is required to explain the observed structures. Furthermore, the study provides constraints for the pre‐Neogene orogenic evolution of this junction zone, including the affinity of major crustal blocks, obducted ophiolitic sequences and the Sava suture zone.

Interpretation validation on vertically exaggerated reflection seismic sections

Reflection seismic data is usually displayed with a vertical exaggeration factor in the range 2–6 irrespective of whether it is time or depth domain data. This vertical exaggeration gives certain advantages to stratigraphic and structural interpretation, and is sometimes a prerequisite to view long-wavelength structures. However, vertical exaggeration distorts geological structures, affecting dip, curvature, line length, cross-sectional area, volume and angular relationships. Reflection seismic can be assigned a “vertical exaggeration heterogeneity factor” based on the extremes of seismic velocities. In the absence of appreciable poorly-compacted units and bathymetry, and generally on depth domain data, vertical exaggeration is relatively homogeneous. Analytical solutions are derived to quantify the distortion of angular relationships, curvature and bed thickness under homogeneous vertical exaggeration. Modelled extensional and compressional geometries give acceptable area balance restorations using vertical shear under homogeneous vertical exaggeration. These vertically exaggerated restorations are neither admissible nor viable because the deformed and restored state sections are unrealistic. Given low heterogeneity of vertical exaggeration, and relatively simple structure that typifies many extensional geometries and some thrust geometries, aspect ratios of 1:1 are not definitively required for section validation by restoration.

Nature and tectonic significance of co-seismic structures associated with the Mw 8.8 Maule earthquake, central-southern Chile forearc

The Mw 8.8 Maule earthquake on February 27, 2010 affected the central-southern Chilean forearc of the Central Andes. Here we show the results of field investigations of surface deformation associated with this major earthquake. Observations were carried out within three weeks after the seismic event, mostly in the central and northern part of the forearc overlying the rupture zone. We provide a detailed field record of co-seismic surface deformation and examine its implications on active Andean tectonics. Surface rupture consisted primarily of extensional cracks, push-up structures, fissures with minor lateral displacements and a few but impressive extensional geometries similar to those observed in analogical modeling of rift systems. A major group of NW-WNW striking fractures representing co-seismic extensional deformation is found at all localities. These appear to be spatially correlated to long-lived basement fault zones. The NW-striking normal focal mechanism of the Mw 6.9 aftershock occurred on March 11 demonstrates that the basement faults were reactivated by the Mw 8.8 Maule earthquake. The co-seismic surface ruptures show patterns of distributed deformation similar to those observed in mapped basement-involved structures. We propose that co-seismic reactivation of basement structures play a fundamental role in stress release in the upper plate during large subduction earthquakes. The fundamental mechanism that promotes stress relaxation is largely driven by elastic rebound of the upper plate located right above the main rupture zone.

Multiple faults in ductile simple shear: analogue models of flanking structure systems

Abstract Rotational behaviour and deformation around multiple faults was investigated in analogue experiments using a linear viscous matrix material under simple shear boundary conditions. Previous analogue and numerical studies have shown that, for single faults, characteristic deformation geometries are produced in initially straight marker lines parallel to the shear zone boundary (flanking structures). Observations from several natural shear zones suggest that not only single faults, but often several parallel or conjugate fault planes are subjected to progressive shear resulting in distinctive deflection geometries. If the distance between faults is on the order of their length, or less, then the perturbation flow fields interfere and coalescence, and finite deflection structures develop that are distinctly different from those around single fractures. In particular, coeval contractional and extensional geometries may develop across conjugate faults, although for bulk simple shear the total length of marker lines parallel to the shear zone boundary cannot change. This advises caution in inferring shear-zone parallel contraction or extension from secondary slip surfaces. In contrast to single flanking structures, conjugate flanking structure systems occurring in natural shear zones are reliable shear sense indicators due to their triclinic symmetry.

Extensional salt tectonics in a contractional orogen: A newly identified tectonic event in the Spanish Pyrenees

A new Coniacian–early Santonian extensional salt tectonics event has been identified in the Cotiella thrust sheet of the central Spanish Pyrenees. We present a new interpretation of the Cotiella thrust system and describe exceptional outcrop examples of postrift, gravity-driven extensional growth faults. The thrust sheet contains three, kilometer-scale extensional faults with as much as 5.5 km of Coniacian–lower Santonian growth strata in their hanging walls. These postrift listric growth faults detached on the prerift Triassic evaporites on the uplifted and eroded southern flanks of Early Cretaceous rift basins. Early Alpine (late Santonian–Maastrichtian) contractional deformation reactivated and reversed the displacement on both the Early Cretaceous rift faults and some of the postrift-salt detachment faults. Hanging walls of the listric growth faults deformed into major antiforms, but the original extensional geometries are well preserved. Continued Alpine shortening incorporated these reactivated extensional fault systems into the thin-skinned thrust sheets of the southern Pyrenees. These postrift extensional structures account for significant thickness variations in the postrift succession and for along-strike changes in the styles of the thrust structures. The Late Cretaceous extensional salt tectonics is interpreted to result from gravity-induced northward sliding on the Early Cretaceous rift margin, toward the axis of the rift basin. The listric growth faults are similar to those found on the Gulf of Mexico margin, in the southern North Sea, and the South Atlantic margins.

Complex high-strain deformation in the Usagaran Orogen, Tanzania: structural setting of Palaeoproterozoic eclogites

The Palaeoproterozoic Usagaran Orogen of Tanzania contains the Earth's oldest reported examples of subduction-related eclogite facies rocks. Detailed field mapping of gneisses exposed in the high-grade, eclogite-bearing part of the orogen (the Isimani Suite) indicates a complex deformation and thermal history. Deformation in the Isimani Suite can be broadly subdivided into five events. The first of these (D 1), associated with formation of eclogite facies metamorphism, is strongly overprinted by a pervasive deformation (D 2) at amphibolite facies conditions, which resulted in the accumulation of high strains throughout all of the exposed Isimani rocks. The geometry of foliations and lineations developed during D 2 deformation are variable and have different shear directions that enable five D 2 domains to be identified. Analysis of these domains indicates a geometrical and kinematic pattern that is interpreted to have formed by strain and kinematic partitioning during sinistral transpression. U–Pb SHRIMP zircon ages from a post-D 2 granite and previously published geochronological data from the Usagaran eclogites indicate this deformation took place between 2000 ± 1 Ma and 1877 ± 7 Ma (at 1σ error). Subsequent greenschist facies deformation, localised as shear zones on boundaries separating D 2 domains, have both contractional and extensional geometries that indicate post-1877 Ma reactivation of the Isimani Suite. This reactivation may have taken place during Palaeoproterozoic exhumation of the Usagaran Orogen or may be the result of deformation associated with the Neoproterozoic East African Orogen. U–Th–Pb SHRIMP zircon ages from an Isimani gneiss sample and xenocrysts in a “post-tectonic” granite yield ∼2.7 Ga ages and are similar to published Nd model ages from both the Tanzanian Craton and gneiss exposed east of the Usagaran belt in the East African Orogen. These age data indicate that the Isimani Suite of the Usagaran Orogen reflects reworking of Archaean continental crust. The extensive distribution of ∼2.7 Ga crust in both the footwall and hangingwall of the Usagaran Orogen can only be explained by the collision of two continents if the continents fortuitously had the same protolith ages. We propose that a more likely scenario is that the protoliths of the mafic eclogites were erupted in a marginal basin setting as either oceanic crust, or as limited extrusions along the rifted margin of the Tanzanian Craton. The Usagaran Orogen may therefore reflect the mid-Palaeoproterozoic reassembly of a continental ribbon partially or completely rifted off the craton and separated from it by a marginal basin.

Geometry and origin of folding in the Neogene sediments of the Gediz Graben, western Anatolia, Turkey

A close relationship between formation of approximately upright folds with axes normal to the extension direction and ramp/flat extensional geometries is established for well exposed Neogene syn-extensional rocks on the presently low-angle Gediz detachment fault, along the southern margin of the Gediz Graben region of western Anatolia, Turkey. Three unconformity-bounded sedimentary sequences and several metamorphic extensional allochthons were mapped in the upper-plate of the Gediz detachment. The oldest sedimentary sequence consists of deformed and folded strata of sandstones and conglomerates that are regarded as being deposited in a supra-detachment basin during the Miocene-Early Pliocene. This unit rests unconformably on the extensional allochthonous, but directly in fault contact with the lower-plate mylonitic rocks. The younger slightly tilted Late Pliocene-Pleistocene sedimentary sequences are post-detachment units that are controlled by EW-trending high-angle normal faults. The youngest alluvium comprises the undeformed present-day basin fill of the Gediz Graben. The supra-detachment sedimentary rocks contain a number of kilometric-scale longitudinal folds that are nearly parallel to the east-west-trending fault system of the Gediz Graben. The folds have a steeply inclined bisecting surface, an interlimb angle of 130–150°, and a plunge of <10°. These folds may be interpreted to form as a result of bending in the underlying Gediz detachment fault. The bending may have an alternation of ramp and flat geometries on which a hanging-wall syncline and rollover anticline formed, respectively. This study again shows the importance of local geology in understanding of some spectacular structures of the extensional basins. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Geometry and origin of folding in the Neogene sediments of the Gediz Graben, western Anatolia, Turkey

A close relationship between formation of approximately upright folds with axes normal to the extension direction and ramp/flat extensional geometries is established for well exposed Neogene syn-extensional rocks on the presently low-angle Gediz detachment fault, along the southern margin of the Gediz Graben region of western Anatolia, Turkey. Three unconformity-bounded sedimentary sequences and several metamorphic extensional allochthons were mapped in the upper-plate of the Gediz detachment. The oldest sedimentary sequence consists of deformed and folded strata of sandstones and conglomerates that are regarded as being deposited in a supra-detachment basin during the Miocene–Early Pliocene. This unit rests unconformably on the extensional allochthonous, but directly in fault contact with the lower-plate mylonitic rocks. The younger slightly tilted Late Pliocene–Pleistocene sedimentary sequences are post-detachment units that are controlled by EW-trending high-angle normal faults. The youngest alluvium comprises the undeformed present-day basin fill of the Gediz Graben. The supra-detachment sedimentary rocks contain a number of kilometric-scale longitudinal folds that are nearly parallel to the east-west-trending fault system of the Gediz Graben. The folds have a steeply inclined bisecting surface, an interlimb angle of 130–150°, and a plunge of <10°. These folds may be interpreted to form as a result of bending in the underlying Gediz detachment fault. The bending may have an alternation of ramp and flat geometries on which a hanging-wall syncline and rollover anticline formed, respectively. This study again shows the importance of local geology in understanding of some spectacular structures of the extensional basins.

Out-of-Sequence Normal Faults Resulting in Local Contractional Geometry: An Example from the Arava Valley, Southern Israel

We present an outcrop-scale example of a localized contractional fault geometry that developed as part of an imbricate normal fault system in response to regional extension. Although extensional regions are dominated by abundant normal faulting, local thrust duplication may occur during the same phase of deformation, and on a regional scale may concentrate important quantities of hydrocarbons. Characteristics of extension-related fold belts have mostly been derived from seismic sections; thus fault geometries, mechanisms of formation, and kinematics of these structures are not precisely understood. Abundant kinematic indicators and complete exposure of an extended sedimentary sequence within the Dead Sea Transform, however, provide the opportunity to examine these fault geometries and mechanisms in detail. The local contractional geometry developed within an imbricate normal fault system, as a result of an out-of-sequence normal fault that detached at a higher structural level. The out-of-sequence normal fault offset pre-existing faults, but was deflected into a contractional geometry upon encountering an earlier-formed rollover anticline, whose curved bedding surface served as a convenient ramp. Consequently, displacement across the out-of-sequence fault generated coeval extensional and contractional geometries along the same detachment surface. Geometries and kinematics derived from the outcrop structural analysis may serve as important analogs for larger structures identified as potential targets for hydrocarbon exploration.

Linkage and evolution of conjugate strike-slip fault zones in limestones of Somerset and Northumbria

Conjugate strike-slip fault zones that cover metre-scale areas at Beadnell, Northumbria, and Kilve, Somerset, were initiated as conjugate vein arrays. Early conjugate faults are linked by the propagation of one fault that eventually by-passes the other fault. A model for the development of strike-slip faults is presented, using fault and vein geometries and the position of damage zones with respect to the master faults as an indication of the propagation direction. This model includes the evolution of networks from (1) the initial random development of vein arrays, to (2) the isolated development of several unconnected conjugate fault segments that pass into vein arrays, through (3) the intersection of a conjugate set of master faults and linkage with minor antithetic faults, and the formation of new vein arrays with extensional geometries after a linked network of faults is established, to (4) breaching of intersection points by dominant faults, and finally (5) the propagation towards oversteps that are breached to form a through-going fault. The geometry of the active structures simplifies with time, as strain is localised along the master fault, but the complexities are preserved in the fault walls.

Extensional geometries as a result of regional scale thrusting: tectonic slides of the Dunlewy-NW Donegal area, Ireland

The synmetamorphic ductile dislocations, known in the British Caledonian literature as ‘Tectonic Slides’, pose a classical structural problem. That is, despite being associated with synchronous contractional folds and cleavages the low angle dislocations have the effect, in many celebrated cases, of juxtaposing younger over older rocks: a geometry normally associated with extensional rather than contractional deformation. Recent models have attempted to demonstrate that this is the result of thrust reactivation of original, sedimentary, extensional growth faults. The Appin Group Dalradian metasediments of the complex and small Dunlewy area of NW Donegal, Ireland, contain the following geometric elements: (a) an early strike-swing-related stratigraphic facies change; (b) a major inter-deformational dolerite sheet; (c) major regional recumbent folds and slides; (d) major structures related to the 400 Ma sinistral Main Donegal Granite shear zone. This solution to the structural geometry reveals that the early mid-crustal (~11 km depth) D 2 Ardsbeg-Dunlewy Slide is a thrust to the northwest. Its hangingwall contains rocks two-thirds of which are younger than the rocks of the footwall, together with major recumbent folds, coeval with the underlying thrust, which face downwards into the thrust in the direction of transport. Rather than thrust reactivation of an original extensional growth fault, we find that both stratigraphic and structural constraints are satisfied by a double thrusting model, with fault-bend folding onto an upper ramp of an earlier formed but penecontemporaneous and kinematically linked major fold pair. This solution to the geology also allows us to recognize that the regional (pre-granite) structure of the Dalradian of NW Donegal is a series of major D 2 synmetamorphic thrust bounded nappes possibly involving up to 250 km of northwesterly overthrusting.

Extensional origin of ductile fabrics in the Schist Belt, Central Brooks Range, Alaska—II. Microstructural and petrofabric evidence

A regional system of S-dipping faults is exposed in the Florence and Fall Creeks area of the south-central Brooks Range. This fault system has previously been mapped as the ·root zone’ of a N-vergent fold-and-thrust belt of Jurassic-Cretaceous age, although individual faults juxtapose lower grade on higher grade metamorphic rocks suggesting apparent extensional geometries. The structurally highest of these faults places rocks of the oceanic Paleozoic-Mesozoic Angayucham terrane, together with unconformably overlying Cretaceous clastic rocks, on Devonian metagreywacke and phyllite. This metagreywacke-phyllite (MP) unit in turn structurally overlies Devonian (?) and older basement rocks of the Brooks Range Schist Belt along a S-dipping structural contact previously mapped as the Florence Creek fault. The Schist Belt and MP units are both characterized by a regionally developed, S-dipping greenschist facies foliation that displays a pronounced N-S-trending, down-dip elongation lineation which earlier workers regarded as being associated with N-vergent thrusting. While only one foliation ( S d) is recognized in the MP unit, in the underlying Schist Belt this foliation ( S 2) overprints an earlier blueschist facies mineral assemblage and associated foliation ( S 1). Mylonites with strong asymmetric crystal fabrics are well developed in quartz stringers in the MP unit and in Schist Belt rocks located at less than 250 m beneath the Florence Creek fault. These asymmetric single- and cross-girdle c-axis fabrics indicate a top down-to-the-south shear sense. This shear sense is confirmed in XZ sections by the presence of elongate, dynamically recrystallized quartz grains oblique to the S d − S 2 mylonitic foliation. Quartzose and quartzo-feldspathic lithologies become coarser grained in the deeper levels of the Schist Belt, and a greater proportion of the quartz grains are equant in outline suggesting the increasing importance of temperature-sensitive recovery processes. These deeper level quartz tectonites display less strongly asymmetric cross-girdle c-axis fabrics and oblique grain shape alignments for at least a further 7.5 km beneath the fault, the asymmetry still indicating a top-down-to-the-south shear sense. Of the 35 quartz-rich samples selected for detailed microstructural and crystal fabric analysis, only two contain c-axis fabrics and oblique grain shape alignments indicating a top-up-to-the-north shear sense. This pervasive top-down-to-the-south shear sense is inconsistent with tectonite fabrics in the Schist Belt being related to N-vergent thrust faulting, but accords well with an extensional model for exhumation of high- P rocks of the Schist Belt. Extension-related high strain fabrics persist for a structural thickness of at least 7.7 km beneath the Florence Creek Fault, indicating that the Florence and Fall Creeks area may be situated over a large-scale (at least 8.0 km thick) zone of extensional shear dipping towards the south. How far this zone of penetrative deformation can be traced along strike remains unclear.

Geometry of the Hanging Wall Above a System of Listric Normal Faults--A Numerical Solution

The process of hanging-wall deformation above a system of listric faults is controlled by the local stress field, rock properties, and fault geometry. Although fault geometry provides a boundary condition, the strained hanging wall can be adequately described by a process of simple shear during hanging-wall collapse. A numerical solution of hanging-wall deformation above a set of listric normal faults clearly demonstrates the importance of inclined simple shear. The true amount of horizontal extension and sense of inclined shear is estimated based on both rollover and fault geometry. This forward modeling technique treats listric faults as continuously curved surfaces, and may incorporate any number of faults, even or unevenly spaced, with variable fault geometry and near surface dip. Basin width is quite sensitive to sense of shear within the hanging wall and depth of detachment, as well as the apparent horizontal extension calculated solely upon the basis of bed-length balance. The model also indicates that antithetic shear is likely to occur near steeply dipping normal faults, whereas geologically reasonable basin geometries are generated under synthetic shear for low-angle normal faults. Ramp and flat listric-fault geometries can be modeled with inclined simple shear to generate basins. Care must be taken when assessing the amount of horizontal extension in extended terranes because the percentage of horizontal extension is quite sensitive to fault geometry and angle of internal shear. Arrays of listric normal faults may develop extensional geometries that resemble domino-style extension. If an apparent planar fault geometry observed near the surface assumes a listric shape downward and distributed deformation occurs within the hanging wall, the resulting estimation of extension based on the angular relationship between faults and beds is likely to be wrong. The model successfully describes the overall geometry of half-graben-type basins, and indicates an important link between fault geometry and sense of shear within the hanging wall of listric normal faults. Near-vertical and synthetic shear are determined near low-angle normal faults, whereas antithetic shear is likely to be observed near steeply dipping normal faults.

Influence of detachment-related passive margin geometry on subsequent active margin dynamics: Applied to the Tasman Fold Belt System

The different upper- and lower-plate settings of detachment-related geometries of passive margins controlled by different extensional geometries can strongly influence the geodynamics and resultant vergence of subsequent active convergent margins during contractional orogenic development. The observed vergence along the Tasman Line and also the foreland-directed vergence of the early Tasmanides (Kanmantoo Fold Belt) and the miogeoclinal belt (Adelaide Fold Belt) support this concept. The less frequently recognized mixed vergence, like that in the Lachlan Fold Belt, has probably been caused by microcontinents and tectonically underplated terrane(s) which acted as structural backstops during the prolonged and complicated orogenic contraction. The geometry of the orogenic float (the mechanically separated surficial rock complexes and crustal units from the underlying continental and oceanic lithosphere) was strongly influenced by the relative displacement of several individual basement domains (some microcontinents) separated by long-lived, (i.e. reactivated), sutures and/or root zones.

Miocene extensional detachments in the outcropping basement of the northern Alboran Basin (Betics) and their tectonic implications

Whether or not there are extensional detachment faults in the Alboran basement can be tested directly because a part of the Alboran Basin is now emerged. These detachments, related to crustal thinning beneath the Alboran Basin, occurred from the Aquitanian to Tortonian. The resulting extensional geometries can be described in general terms. During the Serravalian a considerable southwest extension of the basin took place, accompanied by south-southeast extension in the northern Gibraltar Arc. Other detachments affected by Serravalian extension can be found. The spreading of the Alboran was nearly coeval with roughly westward migration of the Gibraltar mountain front.

Extensional Geometries in the Northern Grant Range, East-Central Nevada--Implications for Oil Occurrences in Railroad Valley

Tertiary heterogeneous extension in the northern Grant Range, Nevada, is manifested by a stacked array of curviplanar low-angle attenuation faults that formed concurrent with arching. Attenuation was controlled by lithologic character, structural depth, and geometry of the arch. Extension appears to be greater on the west side of the range than on the east. On the east side of the range, the stacked array of low-angle attenuation faults is subparallel to bedding and attenuation is distributed across many stacked fault zones; except at highest crustal levels, these faults are blind. On the west side of the range, the low-angle attenuation faults of the stacked array merge into a single, major down-to-the-west fault zone across which as much as 19,000 ft of strata are omitted. Arching of the fault array resulted in an extensional culmination. Cross sections incorporating seismic and drill-hole data suggest that the low-angle attenuation faults (particularly the major down-to-the-west attenuation fault) seen in the range extend into Railroad Valley on the west side of the range with no significant offset by high-angle normal faults. Thus, the topographic expression of Grant Range and Railroad Valley may be due to the synchronous arching and low-angle faulting. These data indicate thatmore » both petroleum source and reservoir rocks in Railroad Valley oil fields are located in relatively immature but extensively fractured rocks of the upper plate to the extensional ramp. Lower plate rocks are metamorphosed, illustrate ductile behavior, and lack significant porosity and permeability.« less

Structural evolution of the Moine thrust belt between Loch More and Glendhu, Sutherland

Synopsis Reinvestigation and new mapping confirm that the area contains a number of large thrust sheets and imbricate slices of Cambrian cover and Lewisian basement but emphasise that they are bounded by an interconnecting network of WNW-directed faults. The network demonstrably propagated, piggy-back fashion, into the foreland since higher structures are cut or bulged up by lower imbricates. These new interpretations require a revision of structural units in the Glendhu area. The margins of culminations, which resulted from the continual stacking of imbricate slices, are the sites of extensional faulting which locally post-date thrust structures. However, some of the major thrusts in the area have primary extensional geometries in their footwalls and hanging walls since they cut down stratigraphic section towards the foreland. The proposed explanation is that bedding in the foreland Cambrian shelf was dipping gently to the SE, possibly in response to isostatic loading by the emplacement of the c. 10 km thick Moine thrust sheet. Datum-parallel thrust flats would then cut down stratigraphic section with an extensional sense. Early extensional strains which locally predate imbricate thrusts in those Cambrian quartzites in the immediate footwall to the Moine thrust may reflect a major gravity spreading component in the initial phases of the emplacement of the Moine sheet. The geometry of thrusts in the Glendhu culmination is illustrated on a balanced cross-section. The hanging wall of the Glencoul thrust, which includes a stack of interthrust Cambrian cover and Lewisian basement termed the Aisinnin imbricates, restores to a width of over 15 km. This can be added to the displacement on the Glencoul and lower thrusts as estimated from the offset of foreland structures, of 25–33 km, to give a total restoration for the footwall of the Moine thrust of at least 40 km.