Thrust Sequence Research Articles

A complex thrust sequence in western Himalaya: The active Medlicott Wadia Thrust

The recent activity of the Medlicott-Wadia Thrust (MWT) is investigated by geomorphic and tectonic studies in the Riasi zone, south of the Pir Panjal range (India, Jammu-Kasmir state of western Himalaya). In the Riasi area, the MWT forms a splay of five faults that dip northward. The recent activity of the splay is quantified using a set of deformed Quaternary alluvial units. The central branch of the thrust splay moved Precambrian limestones above Quaternary sediments and is sealed by 36 ± 3 ka (youngest OSL age) deposits. The other branches offset the top of a 15 ± 1 ka (youngest OSL age) alluvial fan by 180 to 120 m; the two southernmost branches form 17–34 m high non-cylindrical scarps and the two northernmost branches also offset the fan by ∼8 m and ∼54 m, respectively. A balanced cross-section parallel to the N210°E thrust motion suggests that the sequence of activity is complex: an in-sequence propagation is found for the three southern Tea, Scorpion and Rain faults and an out-of-sequence reactivation for the northern Pillar fault. Furthermore, several thrusts simultaneously activated for intervals of a few thousand years, whereas others are episodically inactive. Nearly 10 mm/yr of India-Eurasia convergence is regularly absorbed by the MWT, a value greater than the one estimated for the western segment of the MWT affected by the 2005 Kashmir earthquake, where the thrust tectonics interacts with the complex kinematics of the syntax. This work indicates that the MWT is very active and might be related to thick-skinned tectonics in the western Himalaya.

Tectonic evolution around the Mont Terri rock laboratory, northwestern Swiss Jura: constraints from kinematic forward modelling

We propose a geometrically, kinematically, and mechanically viable thin-skinned kinematic forward model for a cross section intersecting the Mont Terri rock laboratory in the frontal-most part of the Jura fold-and-thrust belt, Switzerland. In addition to the available tunnel, borehole, and surface data, initial boundary conditions are crucial constraints for the forward modelling scenarios, especially the inherited topography of the basement and any pre-compressional offset within the Mesozoic sediments. Our kinematic analysis suggests an early-stage formation of the Mont Terri anticline located above ENE-trending, Late Paleozoic extensional faults, followed by back-stepping of the deformation developing the Clos du Doubs and Caquerelle anticlines further south. In this model, the thrust sequence was dictated by the inherited basement faults, which acted as nuclei for the ramps, detached along the basal décollement within the Triassic evaporites. The mechanical viability of both the thrust angles and thrust sequence was demonstrated by applying the limit analysis theory. Despite numerous subsurface geological data, extrapolation of structures to depth remains largely under-constrained. We have tested an alternative model for the same cross section, involving an upper detachment at the top of the Staffelegg Formation that leads to duplication of the sub-Opalinus Clay formations, prior to detachment and thrusting on the Triassic evaporites. This model is geometrically and kinematically viable, but raises mechanical questions. A total displacement of 2.9 and 14.2 km are inferred for the classical and the alternative scenarios, respectively. In the latter, forward modelling implies that material was transported 10.8 km along the upper detachment. It is not yet clear where this shortening might have been accommodated. Despite the differences in structural style, both models show that pre-existing basement structures might have interfered in time and space. Both styles may have played a role, with lateral variation dictated by basement inherited structures.

Thrust tectonics in the central part of the External Hellenides, the case of the Gavrovo thrust

Thrust faulting plays an important role in the structural deformation of Gavrovo and Ionian zones in the central part of the ‘External Hellenides’ fold-and-thrust belt. The Skolis mountain in NW Peloponnese as well as the Varassova and Klokova mountains in Etoloakarnania are representative cases of ramp anticlines associated with the Gavrovo thrust. Surface geology, stratigraphic data and interpretation of seismic profiles indicate that it is a crustal-scale thrust acted throughout the Oligocene time. It is characterized by a ramp-flat geometry and significant displacement (greater than 10 km). Out of sequence thrust segmentation is inferred in south Etoloakarnania area. Down flexure and extensional faulting in the Ionian zone facilitated the thrust propagation to the west. The thrust emplacement triggered halokenetic movement of the Triassic evaporites in the Ionian zone as well as diapirisms that were developed in a later stage in the vicinity of the Skolis mountain.

Fold and thrust systems in Mass Transport Deposits

Improvements in seismic reflection data from gravity-driven fold and thrust systems developed in offshore Mass Transport Deposits (MTDs) reveal a number of significant features relating to displacement along thrusts. However, the data are still limited by the resolution of the seismic method, and are unable to provide detail of local fold and thrust processes. Investigation of exceptional gravity-driven contractional structures forming part of MTDs in lacustrine deposits of the Dead Sea Basin, enables us to present the first detailed outcrop analysis of fold and thrust systems cutting unlithified ‘soft’ sediments. We employ a range of established geometric techniques to our case study, including dip isogons, fault-propagation fold charts and displacement-distance diagrams previously developed for investigation of thrusts and folds in lithified rocks. Fault-propagation folds in unlithified sediments display tighter interlimb angles compared to models developed for lithified sequences. Values of stretch, which compares the relative thickness of equivalent hangingwall and footwall sequences measured along the fault plane, may be as low as only 0.3, which is significantly less than the minimum 0.5 values reported from thrusts cutting lithified rocks, and reflects the extreme variation in stratigraphic thickness around thrust-related folds. We suggest that the simple shear component of deformation in unlithified sediments may modify the forelimb thickness and interlimb angles to a greater extent than in lithified rocks. The average spacing of thrust ramps and the thickness of the thrust sequence display an approximate 5:1 ratio across a range of scales in MTDs. In general, thicker hangingwall and footwall sequences occur with larger thrust displacements, although displacement patterns on thrusts cutting unlithified (yet cohesive) sediments are more variable than those in lithified rocks. Line-length restoration of thrust systems in MTDs reveals 42% shortening, which reduces to 35% in overlying beds. A 23% reduction in shortening by folding and thrusting along individual thrusts suggests that heterogeneous lateral compaction may increase by ∼10% towards the sediment surface. Thrust systems cutting unlithified sediments display distinct steps in cumulative displacement-distance plots representing increased rates of slip along the floor thrust, while displacement-distance plots along individual thrusts also reveal ‘horizontal steps’ relating to lithological variation. Competent units cut by thrust ramps may display the greatest displacement, which then progressively reduces both upward and sometimes downward along the ramp. This relationship demonstrates that ramps do not necessarily propagate upwards from the underlying flat as in some traditional models, but rather initiate by offset of competent horizons in the hangingwall of the detachment. Critical taper angles in MTDs may be an order of magnitude less than in accretionary complexes or lithified rocks. Overall, thrusts cutting unlithified sediments in MTDs display more variable displacement, and more pronounced displacement gradients toward fault tips, compared to thrusts cutting lithified sequences.

Sociology of a scandal: the emergence of ‘FIFAgate’

This article examines the social forces underlying FIFAgate. Why do corrupt practices, which are often highly consolidated or even institutionalized, suddenly become scandalous? What is a scandal? Why did FIFA fall into crisis in 2015 and not before? To answer these questions, it is necessary to look at the sequence of thrusts and parries between all the parties involved. Our analysis embraces the notion that social processes are based on relationships in order to provide insights into why and how denunciations can lead to long-tolerated corrupt practices suddenly becoming compromising enough to force an organization’s leaders to implement measures that would have previously been unimaginable. We use multiple sources to examine the emergence of FIFAgate and the way FIFA overcame previous critical situations, describing how FIFA neutralized earlier allegations and developed a remarkable ‘resilience’ to scandal. Finally, we analyse the successive mobilizations whose domino effect led to the emergence of FIFAgate and the measures FIFA took to contain the scandal. Our research enabled us to draw up a new theoretical model for analysing corruption scandals.

Sandbox modelling of sequential thrusting in a mechanically two-layered system and its implications in fold-and-thrust belts

Many fold-and-thrust belts display multi-storied thrust sequences, characterizing a composite architecture of the thrust wedges. Despite dramatic progress in sandbox modelling over the last three decades, our understanding of such composite thrust-wedge mechanics is limited and demands a re-visit to the problem of sequential thrusting in mechanically layered systems. This study offers a new approach to sandbox modelling, designed with a two-layered sandpack simulating a mechanically weak Coulomb layer, resting coherently upon a stronger Coulomb layer. Our experimental models reproduce strikingly similar styles of the multi-storied frontal thrust sequences observed in natural fold-and- thrust belts. The upper weak horizon undergoes sequential thrusting at a high spatial frequency, forming numerous, closely spaced frontal thrusts, whereas the lower strong horizon produces widely spaced thrusts with progressive horizontal shortening. This contrasting thrust progression behaviour gives rise to composite thrust architecture in the layered sandpack. We show the evolution of such composite thrust sequences as a function of frictional strength (μb) at the basal detachment and thickness ratio (Tr) between the weak and strong layers. For any given values of Tr and μb, the two thrust sequences progress at different rates; the closely-spaced, upper thrust sequence advances forelandward at a faster rate than the widely-spaced, lower thrust sequence. Basal friction (μb) has little effects on the vergence of thrusts in the upper weak layer; they verge always towards foreland, irrespective of Tr values. But, the lower strong layer develops back-vergent thrusts when μb is low (∼0.36). In our experiments, closely spaced thrusts in the upper sequence experience intense reactivation due to their interaction with widely spaced thrusts in the lower sequence. The interaction eventually affects the wedge topography, leading to two distinct parts: inner and outer wedges, characterised by steep and gentle surface slopes, respectively.

Theorem of Optimal Image Trajectories in the Restricted Problem of Three Bodies

The restricted three-body problem represents the dynamical framework employed for spacecraft mission analysis, in the presence of two attracting bodies, since the 1950s. In this context, orbital motion is often chaotic, although several special solutions (equilibrium points, periodic orbits, and quasiperiodic trajectories) exist, and can be--or have already been--profitably employed in space missions. The theorem of image trajectories, proven five decades ago by Miele, states that for a given path in the restricted problem of three bodies (with primaries in mutual circular orbits), there exists a mirror trajectory (in two dimensions) and three mirror paths (in three dimensions). This theorem regards feasible trajectories and proved extremely useful for investigating the natural dynamics in the restricted problem of three bodies, by identifying special solutions, such as symmetric periodic orbits and free return trajectories. This work extends the theorem of image trajectories to optimal paths, which minimize either the propellant consumption or the time of flight, by determining the relations between the optimal thrust sequence, magnitude, and direction of an outgoing path and a symmetrical returning trajectory. This means that while the theorem of image paths revealed extremely useful for investigating natural dynamics, the theorem of optimal image trajectories can be profitably employed for powered orbital motion, i.e., in the context of impulsive and finite-thrust orbit transfers and rendezvous, as well as for the purpose of analyzing artificial periodic orbits that use very low thrust propulsion.

Deformation structures in the frontal prism near the Japan Trench: Insights from sandbox models

We have used sandbox experiments to explore the mechanics of the frontal prism structures documented by seismic reflection data and new borehole from IODP Expedition 343 (JFAST). This study investigated the effects of down-dip (normal to trench axis) variations in frictional resistance along a decollement on the structural development of the frontal wedges near subduction zones. Interpretation of seismic reflection images indicates that the wedge has been effected by trench-parallel horst-and-graben structures in the subducting plate. We performed sandbox experiments with down-dip patches of relatively high and low friction on the basal decollement to simulate the effect of variable coupling over subducting oceanic plate topography. Our experiments verify that high frictional resistance on the basal fault can produce the internal deformation and fault-and-fold structures observed in the frontal wedge by the JFAST expedition. Subduction of patches of varying friction caused a temporal change in the style of internal deformation within the wedge and gave rise to two distinctive structural domains, separated by a break in the surface slope of the wedge: (i) complexly deformed inner wedge with steep surface slope and (ii) shallow taper outer wedge, with a sequence of imbricate thrusts. Our experiments further demonstrate that the topographic slope-break in the wedge develops when the hinterland part of the wedge essentially stops deforming internally, leading to in-sequence thrusting with the formation of an outer wedge with low taper angle. For a series of alternate high and low frictional conditions on the basal fault the slope of the wedge varies temporally between a topographic slope-break and uniformly sloping wedge.

The origin and internal structures of submarine-slide deposits in a lower Pleistocene outer-fan succession in the Kazusa forearc basin on the Boso Peninsula of Japan

This study investigates the internal geometry and formation processes of submarine-slide deposits in a lower Pleistocene outer-fan succession in the Kazusa forearc basin on the Boso Peninsula of Japan. The submarine-slide deposits are ~40m thick, with a minimum length of ~900m and a width of ~700m. Both the submarine-slide deposits and host deposits comprise siltstones intercalated with very thin- to medium-bedded, sheet-like turbidites and volcanic ash beds. Based on the sequence-stratigraphic framework of the submarine-fan succession, we conclude that the submarine-slide deposits formed during a glacioeustatic sea-level lowstand at about 1.16Ma.The submarine-slide deposits are characterized by thrust sequences with a ramp anticline in the frontal part. A basal slide plane in the lower part of the deposits is developed at a horizon located 2–4cm below the base of a coarse volcanic ash bed and is associated with sheared deposits. Slide planes are sealed in the upper part of the submarine-slide deposits in association with drag folds and chaotic deposits. Finally, the submarine-slide deposits are transitionally overlain by ~3-m-thick chaotic muddy deposits, and are finally overlain by siltstones intercalated with very thin- to medium-bedded, sheet-like turbidites and volcanic ash beds, which show lithofacies features similar to those of the submarine-slide deposits. The variations in the deformation styles indicate that sliding occurred as a synsedimentary process in the outer-fan environment, and the basal slide plane formed when the porosity of the muddy deposits was reduced to ~55% or less.Based on the empirical relationship between the submarine-fan length and lower-fan slopes from modern examples, the gradient of the outer-fan is estimated at 0.31°–0.46°, which is lower than the threshold gradient of 1.2° for a 40-m-thick submarine slide with the estimated basal porosity. Based on the distribution of deformed deposits within the lower-fan host deposits, the minimum volume and runout distance of the submarine-slide deposits are estimated to be ~3×107m3 and ~5km, respectively. Furthermore, based on data from modern submarine slides, the elevation difference between the failed mass in both the starting and depositional areas is estimated to be 80–170m. Thus, the initiation of a submarine slide in the outer-fan environment was likely influenced by a combination of external forces, such as seismic shaking and increased pore-water pressure in association with the seepage of gas and water from underlying deposits in the Kazusa forearc basin. This study suggests that submarine slides occur in distal and low-gradient deep-water environments in active-margin basins.

Geometry, amount, and sequence of thrusting in the Subalpine Molasse of western Austria and southern Germany, European Alps

In this paper we review the structure of the most external thrust belt of the Alps between the Rhein valley and Salzburg based on a new tectonic map and the (re)interpretation of seismic sections. Specifically we address the correlation between deformation in the Subalpine Molasse and the Alpine thrust belt in general and focus on the control of sedimentary facies on the structural style. A dramatic change in architecture from a ramp-flat structure to buckle folding is related to a change from coarse-grained fans to fine-grained deposits within the Subalpine Molasse. Additionally the interaction of escape tectonics with postcollisional shortening controls the decrease of late Early Miocene and younger shortening within the Subalpine Molasse from 50 km near the Rhine valley to almost zero near Salzburg. Transfer of shortening into the hinterland, which is the zone of lateral escape, ended foreland propagation of the Alpine thrusts and initiated a general break-back sequence of thrusting. Throughout this time the thrusts remained active. In such a scenario, tectonic units on top of the Subalpine Molasse are expected to undergo clockwise rotation around vertical axes. As thrusting in the Subalpine Molasse is closely related to contemporaneous transport and shortening within the tectonically higher Helvetic thrust sheets, amounts of Miocene differential shortening and related clockwise vertical axis rotation are minimum amounts. True clockwise vertical axis rotation is probably larger than the 12° deduced from the Subalpine Molasse thrust belt.

Symmetry Properties of Optimal Relative Orbit Trajectories

The determination of minimum-fuel or minimum-time relative orbit trajectories represents a classical topic in astrodynamics. This work illustrates some symmetry properties that hold for optimal relative paths and can considerably simplify their determination. The existence of symmetry properties is demonstrated in the presence of certain boundary conditions for the problems of interest, described by the linear Euler-Hill-Clohessy-Wiltshire equations of relative motion. With regard to minimum-fuel paths, the primer vector theory predicts the existence of several powered phases, divided by coast arcs. In general, the optimal thrust sequence and duration depend on the time evolution of the switching function. In contrast, a minimum-time trajectory is composed of a single continuous-thrust phase. The first symmetry property concerns minimum-fuel and minimum-time orbit paths, both in two and in three dimensions. The second symmetry property regards minimum-fuel relative trajectories. Several examples illustrate the usefulness of these properties in determining minimum-time and minimum-fuel relative paths.

Glaciotectonic deformation and reinterpretation of the Worth Point stratigraphic sequence: Banks Island, NT, Canada

Hill-hole pairs, comprising an ice-pushed hill and associated source depression, cluster in a belt along the west coast of Banks Island, NT. Ongoing coastal erosion at Worth Point, southwest Banks Island, has exposed a section (6 km long and ∼30 m high) through an ice-pushed hill that was transported ∼ 2 km from a corresponding source depression to the southeast. The exposed stratigraphic sequence is polydeformed and comprises folded and faulted rafts of Early Cretaceous and Late Tertiary bedrock, a prominent organic raft, Quaternary glacial sediments, and buried glacial ice. Three distinct structural domains can be identified within the stratigraphic sequence that represent proximal to distal deformation in an ice-marginal setting. Complex thrust sequences, interfering fold-sets, brecciated bedrock and widespread shear structures superimposed on this ice-marginally deformed sequence record subsequent deformation in a subglacial shear zone.Analysis of cross-cutting relationships within the stratigraphic sequence combined with OSL dating indicate that the Worth Point hill-hole pair was deformed during two separate glaciotectonic events. Firstly, ice sheet advance constructed the hill-hole pair and glaciotectonized the strata ice-marginally, producing a proximal to distal deformation sequence. A glacioisostatically forced marine transgression resulted in extensive reworking of the strata and the deposition of a glaciomarine diamict. A readvance during this initial stage redeformed the strata in a subglacial shear zone, overprinting complex deformation structures and depositing a glaciotectonite ∼20 m thick. Outwash channels that incise the subglacially deformed strata record a deglacial marine regression, whereas aggradation of glaciofluvial sand and gravel infilling the channels record a subsequent marine transgression. Secondly, a later, largely non-erosive ice margin overrode Worth Point, deforming only the most surficial units in the section and depositing a capping till.The investigation of the Worth Point stratigraphic sequence provides the first detailed description of the internal architecture of a polydeformed hill-hole pair, and as such provides an insight into the formation and evolution of an enigmatic landform. Notably, the stratigraphic sequence documents ice-marginal and subglacial glaciotectonics in permafrost terrain, as well as regional glacial and relative sea level histories. The reinterpreted stratigraphy fundamentally rejects the long-established paleoenvironmental history of Worth Point that assumed a simple ‘layer-cake’ stratigraphy including the type-site for an organically rich, preglacial interval (Worth Point Fm).

Distribution of resistive and conductive structures in Nankai accretionary wedge reveals contrasting stress paths

In this article, we study the characteristics and spatial distribution of the deformation structures along the Kumano transect of the Nankai accretionary wedge, and use this information to interpret the stress path followed by the sediments. Deformation structures are identified from logging while drilling (LWD) resistivity images of the materials surrounding the drill hole and from 3-dimensional X-ray CT-images of cores acquired during the IODP NanTroSEIZE project. The relative resistivity of the structures identified on logs and the strike, dip, and density of structures identified on CT scan images are measured. The analysis of dip and strike of structures indicates that most of the resistive structures identified on logging data correspond to compactive shear bands. Results also indicate that conductive structures predominate at the toe of the prism and above the main out of sequence thrust, in locations where past and recent erosion occurred. We propose several mechanisms that could explain the relation between erosion and the absence of compactive shear bands. We conclude that sediments followed different stress paths depending on their location within the wedge, and that those differences explain the distribution of deformation structures within the wedge. We also show the coexistence of dilatant and compactant structures in fault zones including the frontal thrust and mega splay fault, and we interpret the coexistence of these structures as a possible consequence of a transient fluid pressure.

Geotectonic evolution of Midland Scotland from Cambrian to Silurian: a review

Synopsis During its Lower Palaeozoic history, the Midland Valley of Scotland was subject to compression from the blocks on either side, confirming the views of Kennedy (1958). Cambrian–Llanvirn-aged rocks of the Hebridean craton formed part of a passive margin, which extended to the ESE and is now replaced by the metamorphic complexes of the Moine and Dalradian. Both blocks had undergone metamorphism, uplift and shedding of their covers elsewhere. The ophiolitic part of the Highland Border Complex, which may be a continuation of the structurally foreshortened and rotated Ballantrae Complex, was obducted at c .490 Ma and may have interacted with rocks of the Hebridean passive margin. Ordovician rocks at Girvan are a small remnant of a one-time far larger forearc basin, which formed to the south of a major arc complex, originally situated in the Midland Valley and probably extending from somewhere in the North Sea to offshore Ireland and beyond. The Highland Border Complex includes rocks coeval with the Midland Valley arc at c .470 Ma that formed a back-arc basin. The Southern Uplands have converged on the Midland Valley, with the forearc sequence apparently missing in Scotland. Part of that convergence history is recorded in the Silurian conglomerates of the southern Midland Valley. Silurian sequences in this area contain conglomerates and breccias, derived from the south that record the presence of an igneous and possibly also a metamorphic complex in the region now occupied by the trench sequence of the Southern Uplands. It is suggested that the trench was later thrust over this source block and that the Southern Upland Fault forms part of that thrust sequence. The metamorphic complexes to the north, replacing the Hebridean passive margin, did not interact with the Midland Valley until Lower Devonian times. It follows that there was no mountain-building orogeny in Scotland, other than an arc, a sliver of trench, and numerous examples of crustal contraction by strike-slip and/or thrusting.

Varying frontal thrust spacing in mono-vergent wedges: An insight from analogue models

Sandbox experiments are used to study frontal thrust fault spacing, which is a function of physical properties within the thrust wedge. We consider three styles of thrust progression in mono-vergent wedges: Style I, II and III. In Style I, frontal thrusts progress forelandward, maintaining a constant spacing, whereas Style II and Style III progression show increasing and decreasing spacing, respectively. The three styles are shown as a function of the following factors: basal friction (μ b ), initial surface slope (α) and basal slopes (β), and surface erosion. For high μ b (~0.46), thrust progression occurs in Style II when α 2° and β > 0.5°). Style II transforms to Style I when the wedge undergoes syn-thrusting surface erosion. In contrast, low-basal friction (μ b = 0.36) gives rise to either Style I or III, depending on the magnitudes of α and β. Conditions with α = β = 0 developed Style I, whereas Style III in conditions with any non-zero values of α and β. In this case, surface erosion caused the process of thrust progression unsteady, and prompted out-of-sequence thrusting in the wedge. This study finally presents an analysis of the three styles, taking into account the following two parameters: (1) instantaneous increase of hinterland thickness (ΔH e /H e ) and (2) forelandward gradient of wedge thickness (δH/δx). Experimental data suggest that thrust sequences develop in Style II for low δH/δx and large δH e /H e values and, in Style III as either δH/δx increases or ΔH e /H e drops.

Kinematic model for out-of-sequence thrusting: Motion of two ramp-flat faults and the production of upper plate duplex systems

Kinematic models developed here suggest a bewildering array of structural styles can be generated during out-of-sequence thrusting. Many of these structures would be difficult to distinguish from a normally stacked thrust sequence and the process can produce younger-on-older faults that could easily be misinterpreted as normal faults. This paper considers a small subset of this problem within a large model space by considering structures that develop along a pair of ramp-flat faults that are moving simultaneously, or sequentially. Motion on the lower ramp warps the structurally higher fault due to fault-bend folding and when the fault ruptures through the warp it transfers a horse to the upper hanging wall. Continuity of the process generates what is referred to here as an “upper plate duplex” to distinguish the structure from a conventional duplex. Kinematic parameters are developed for two models within this general problem: 1) a system with a fixed ramp in the lower thrust, overridden by an upper thrust; and 2) a double-duplex system where a conventional duplex develops along the lower fault at the same time as an upper plate duplex is formed along the upper fault. The theory is tested with forward models using 2D Move software and these tests indicate different families of structural styles form in association with relative scaling of ramp systems, slip-ratio between faults, and aspect ratios of horse blocks formed in the upper-plate duplex. A first-order result of the analysis is that an upper plate duplex can be virtually indistinguishable from a conventional duplex unless the trailing branch lines of the horses are exposed or imaged; a condition seldom met in natural exposures. Restoration of an upper-plate duplex produces counterintuitive fault geometry in the restored state, and thus, restorations of upper plate duplexes that erroneously assume a conventional duplex model would produce restored states that are seriously in error. In addition, in most of the models some fault segments place younger rocks on older rocks which could be easily misinterpreted as normal fault systems. In some models younger-on-older juxtapositions are significant and if scaled to crustal scale would produce core-complex style structures that would be difficult to recognize as contractional features. Collectively, these observations imply that many areas where simultaneous contraction and extension are inferred may be entirely contractional with younger-on-older relationships generated by out-of-sequence thrust systems. Examples where this process may have occurred are in southwestern North America and the Moine thrust system and future studies should evaluate these systems in light of these models. Distinguishing upper plate duplex from conventional duplex is potentially important in economic evaluations of thrust systems because fluid migration paths would be very different in the two alternatives. The process may also be important in seismogenic mechanisms, particularly in subduction megathrusts, because faults warping faults could produce fault irregularities that would form transient asperities along the fault.

Fluid channeling along thrust zones: the Lagonegro case history, southern Apennines, Italy

AbstractA regional study on late burial, syntectonic dolomites and syntectonic calcite and quartz veins occurring in the hemipelagic Triassic–Jurassic succession of the Lagonegro Basin (southern Apennines) was carried out by integrating fieldwork with structural, fluid inclusion, and geochemical investigations on both surface and subsurface samples. The main goal was to characterize the different parent fluids, channeled along a major, out of sequence thrust zone. We also investigated calcite veins from the deeper tectonic mélange zone. The results point out that the fluids at peak burial conditions, during the first deformation stage, were characterized by temperatures of 120–140°C and salinities averaging 2 wt% NaCl eq. Fluids channeled along the late, out of sequence thrust zone and responsible for partial dolomitization of the succession had lower temperatures (Th = 95 ± 10°C) and salinities in the range of slightly modified to normal marine seawater (mean at 3.7 wt% NaCl eq). Similar low salinities are encountered in late dolomites from other fold‐and‐thrust belts of Italy. The most probable source for the fluids channeled along the investigated out of sequence thrust zone is represented by Miocene marine pore waters expelled from the tectonic mélange zone. A comparison of the Lagonegro dolomites with similar late stages carbonates formed in other fold‐and‐thrust belts suggests that fluid salinities appear to be controlled by the different lithologies of the fold‐and‐thrust belts décollement surfaces.

Gravitational dismantling of the Miocene mountain front of the Gibraltar Arc system deduced from the analysis of an olistostromic Complex (Western Betics)

A melange complex seals the internal-external zone boundary of the western part of the Gibraltar Arc orogenic belt and constitutes a key element to establish milestones of the Betic-Rif tectonic evolution. The blocks and olistoliths embedded in this melange provide constraints on the geological history of the main tectonic units involved in the Miocene mountain front. We mapped and analysed the blocks and olistoliths included in this melange in order to understand its age and genesis, which have long been a matter of debate. The relationships of this melange La Joya Olistostromic Complex (LaJOC) with the basement units together with the high variability of the block lithologies suggest a sedimentary origin for this melange. Two large-scale olistoliths retain their original structure prior to their emplacement in the LaJOC Basin. The sedimentological and structural analysis allowed us to correlate these olistoliths with the folded and thrust sequence belonging to the Miocene Betic-Rif accretionary prism (Flysch Trough units), and to constrain the age of deposition of the La Joya Olistostromic Complex. The age of the matrix of the melange deposits is poorly known because of the lack of in-place fauna. Indeed, the formation of the inherited fold-and-thrust structure of these olistoliths is well-known in the western Betics. Accordingly, the LaJOC should have been deposited during middle Miocene times and the blocks and olistoliths included within the melange would derive from the gravitational dismantling of the Gibraltar Arc mountain front. The data presented help us to understand the formation of reliefs and basins in the western part of the Gibraltar Arc orogenic system.

Stratigraphy and deformation history of the Jurassic coal bearing series in the Eastern Mecsek (Hungary)

Abstract Based on integrated interpretation of well-log and core data of 60 fully cored deep drillings and 2D seismic profiles, sequence stratigraphic and structural development models have been elaborated for the “Maza-South–Varalja-South” coal exploration site and for the Northern Imbricates, Eastern Mecsek, SW Hungary. Twenty four mostly coal capped parasequences and 4 sequences have been identified in the deltaic series of the coal bearing formation. Coal seams of high sulfur content are related to early transgressive systems tracts, while those with low sulfur content and great thickness are associated to late transgressive systems tracts. The sequences were traced on seismic profiles and mapped by subsurface methods. The structural reconstruction indicates that an open, almost symmetric, southward plunging anticline was formed in the Maza-South–Varalja-South area striking practically north–south. This anticline has a gradual transition westward, into another open, almost symmetric, westward plunging anticline. Related to the Cretaceous–Paleogene brittle deformation a set of thrusts with northern vergence were formed as the result of an in sequence thrust wedge development causing shortening of 13%. The Northern Imbricates has been re-interpreted as the result of the Neogene activity of this thrust wedge development; its Miocene development represents an in sequence thrust and a series of out of sequence backthrusts causing shortening of 22.5%. This multi-phase compressive deformation history caused the significant (34%) increase of the coal resources in the study area. However, it also resulted in strong structural fragmentation and multiple coal seams in the coal bearing formation increasing the costs of underground mining and the risks of underground coal gasification.

Le front de chevauchement au Jebel Bou El Hanèche-Kalâat Khasba (Tunisie centro-septentrionale). Mise en évidence à partir des donnés géologiques et géophysiques

In Central-northern Tunisia, the septentrional border of Kalâat Khasba trough was considered as a normal fault which has functioned since the Cretaceous to Quaternary periods. A stratigraphic and geophysical recent study of the Kalâat Khasba trough and the Jebel Bou El Haneche fold area, allows us to propose a new tectonic model. The Northern border was reactivated as reverse fault during the Eocene and was considered as a thrust front. A compressive Eocene event resulted in formation of Bou El Haneche fold (fault propagation fold) and in determination of a decollement level situated in the Triassic series. This deformation was controlled by the reactivation of normal faults inherited from a distensive aptien phase. The compressive deformation was amplified by the propagation of this fold and, when a ramp reached the competent carbonates series of late Campanian-lower Maâstrichtien times, the fold was stacked, involving a breakthrough steep limb and creating a second thrust on the North. The order of evolution of these events shows an out of sequence thrust.