Passive-roof Duplex Research Articles

Structural style and deformation mechanism of the southern Dabashan foreland fold-and-thrust belt, central China

The Daba Mountains define the southern margin of the East Qinling orogenic belt, and form the boundary of the Sichuan basin in the north and northeast. The Daba Mountains can be divided into two structural belts by the NW-striking Chengkou fault, namely the northern Dabashan thrust-nappe belt and the southern Dabashan foreland fold-and-thrust belt. The southern Dabashan fold-and-thrust belt is a southwestward extruding thin-skinned thrust wedge, showing obvious belted change in deformation style and deformation intensity along the dip direction, and can be divided further into three sub-belts, i.e. the imbricate thrust sub-belt characterized by imbricate stepped-thrust sheets, the thrust-fold sub-belt characterized by the combination of the equally-developed thrusts and related folds, and the detachment-fold sub-belt characterized by box folds and closed overturned-isoclinal folds on the outcrops. Several kinds of structures have been recognized or inferred, including imbricate thrust system, passive-roof duplex (triangle zone), fault-related folds, back-thrust system and pop-up structure. The NE-SW compressive stress from the Qinling orogenic belt and detachment layers in the covering strata are the two most important determinants of deformation style. After the collision between the North China block and Yangtze block at the end of the Middle Triassic, the northward intracontinental subduction along the southern edge of the Qinling orogenic belt was initiated, which led to the corresponding southward thrusting in the upper crust. The thrusting propagated towards the foreland through the Jurassic and extended to the southernmost part of the southern Daba Mountains around the end of the Early Cretaceous, with thrusting deformation to be preferentially developed along major detachment layers and progressing upwards from the Lower Sinian through the Lower Cambrian and Silurian to Middle-Lower Triassic.

Progressive evolution of a late orogenic thrust system, from duplex development to extensional reactivation and disruption: Witwatersrand Basin, South Africa

Abstract This paper examines progressive evolution of fault architectures through late orogenic compression- to post-orogenic extensional deformation in the Witwatersrand Basin, South Africa. The results indicate that rapid extrusion of mafic lavas of the lower Klipriviersberg Group formed a rigid ‘lid’ over the thrust front, changing its mechanical character and thereby driving a change of structural style from fold growth to passive roof duplex. Flexural tightening of folds in the core of the triangle zones at this time may have helped provide the dynamic permeability for distributed ingress of hydrothermal fluids and consequent gold mineralization. Shortly afterwards, the kinematic environment changed to become extensional. However, this study shows sharp lateral partitioning of the duration of kinematic style and structural amplification, such that thrusting and extension coexisted along strike in the upper Klipriviersberg Group. Thus the switch from thrusting to extension was progressive within the region, but locally very rapid. As the local kinematic environment became extensional, the fault system evolved progressively, with the early stages of kinematic changes being dominated by a process of reactivation by architectural scavenging, in which new extensional structures developed by selectively reusing and incorporating geometrical segments of earlier formed thrust and normal faults. Three basic stages can be identified in this evolution: broad extension above underlying detachments, involving reactivation of lateral structures; a period of intensive reactivation and kinematic reworking incorporating frontal structures; and an abandonment stage when the detailed influence of the earlier architecture diminished and the fault system developed larger through-going normal faults. The interaction of the newly developing fault system with the pre-existing architecture constitutes pre-programming of the final geometry, in which individual large faults are composed of a reticulated network of new and inherited segments. The observations are consistent with fault scale being a key control on the fault reactivation involved. This study has involved full integration of a dataset comprising 2D and 3D seismic reflection data, geological mine plans, logging of over 120 km of drill core and underground mapping in deep mine workings that pass 3 km into the seismic volume at 2–3 km depth.

Deformation patterns and structural vergence in brittle–ductile thrust wedges: An additional analogue modelling perspective

Scaled physical models of brittle–ductile thrust wedges investigated the causes leading to the development of the various structural styles observed in fold-and-thrust belts and accretionary prisms. This study focused on some potential controlling factors, such as (1) relative strength between the brittle overburden and the viscous décollement layer (brittle–ductile coupling), (2) the effect of very low basal friction, and (3) pre-shortening rheological stratigraphy, including décollement strength. Type 1 models had a single basal décollement, whereas in Type 2 models the décollement was embedded in brittle material simulating an upper roof sequence and a lower floor sequence. Other models investigated the effects produced by an horizon of mechanical weakness at the base of the viscous décollement (Type 1a), and the role of décollement offset (Type 3). The experiments suggest the occurrence of a genetic relationship between the absolute values of shear stresses and the development of dominating hinterland- or foreland-verging thrust faults, or dually-verging thrusts. By controlling the orientation of the σ 1 axis, basal friction and décollement offset is found to effectively influence the deformation pattern and structural vergence, and could explain the development of dominating backthrusts in Cascadian-type margins. The decoupling of a frontal monocline via the upper décollement (Type 2 models) produced passive-roof duplex structures, termed “composite-roof duplex” when the frontal monocline coexists with outward fold propagation.

Tertiary Thrust Systems and Fluid Flow beneath the Beaufort Coastal Plain (1002 Area), Arctic National Wildlife Refuge, Alaska, U.S.A.

Beneath the Arctic coastal plain (commonly referred to as the 1002 area) in the Arctic National Wildlife Refuge, northeastern Alaska, United States, seismic reflection data show that the northernmost and youngest part of the Brookian orogen is preserved as a Paleogene to Neogene system of blind and buried thrust-related structures. These structures involve Proterozoic to Miocene (and younger?) rocks that contain several potential petroleum reservoir facies. Thermal maturity data indicate that the deformed rocks are mature to overmature with respect to hydrocarbon generation. Oil seeps and stains in outcrops and shows in nearby wells indicate that oil has migrated through the region; geochemical studies have identified three potential petroleum systems. Hydrocarbons that were generated from Mesozoic source rocks in the deformed belt were apparently expelled and migrated northward in the Paleogene, before much of the deformation in this part of the orogen. It is also possible that Neogene petroleum, which was generated in Tertiary rocks offshore in the Arctic Ocean, migrated southward into Neogene structural traps at the thrust front. However, the hydrocarbon resource potential of this largely unexplored region of Alaska's North Slope remains poorly known. In the western part of the 1002 area, the dominant style of thin-skinned thrusting is that of a passive-roof duplex, bounded below by a detachment (floor thrust) near the base of Lower Cretaceous and younger foreland basin deposits and bounded above by a north-dipping roof thrust near the base of the Eocene. East–west-trending, basement-involved thrusts produced the Sadlerochit Mountains to the south, and buried, basement-involved thrusts are also present north of the Sadlerochit Mountains, where they appear to feed displacement into the thin-skinned system. Locally, late basement-involved thrusts postdate the thin-skinned thrusting. Both the basement-involved thrusts and the thin-skinned passive-roof duplex were principally active in the Miocene. In the eastern part of the 1002 area, a northward-younging pattern of thin-skinned deformation is apparent. Converging patterns of Paleocene reflectors on the north flank of the Sabbath syncline indicate that the Aichilik high and the Sabbath syncline formed as a passive-roof duplex and piggyback basin, respectively, just behind the Paleocene deformation front. During the Eocene and possibly the Oligocene, thin-skinned thrusting advanced northward over the present location of the Niguanak high. A passive-roof duplex occupied the frontal part of this system. The Kingak and Hue shales exposed above the Niguanak high were transported into their present structural position during the Eocene to Oligocene motion on the long thrust ramps above the present south flank of the Niguanak high. Broad, basement-cored subsurface domes (Niguanak high and Aurora dome) formed near the deformation front in the Oligocene, deforming the overlying thin-skinned structures and feeding a new increment of displacement into thin-skinned structures directly to the north. Deformation continued through the Miocene above a detachment in the basement. Offshore seismicity and Holocene shortening documented by previous workers may indicate that contractional deformation continues to the present day.

Interpretation of the Seattle Uplift, Washington, as a Passive-Roof Duplex

We interpret seismic lines and a wide variety of other geological and geophysical data to suggest that the Seattle uplift is a passive-roof duplex. A passive-roof duplex is bounded top and bottom by thrust faults with opposite senses of vergence that form a triangle zone at the leading edge of the advancing thrust sheet. In passive-roof duplexes the roof thrust slips only when the floor thrust ruptures. The Seattle fault is a south-dipping reverse fault forming the leading edge of the Seattle uplift, a 40-km-wide fold-and-thrust belt. The recently discovered, north-dipping Tacoma reverse fault is interpreted as a back thrust on the trailing edge of the belt, making the belt doubly vergent. Floor thrusts in the Seattle and Tacoma fault zones, imaged as discontinuous reflections, are interpreted as blind faults that flatten updip into bedding plane thrusts. Shallow monoclines in both the Seattle and Tacoma basins are interpreted to overlie the leading edges of thrust-bounded wedge tips advancing into the basins. Across the Seattle uplift, seismic lines image several shallow, short-wavelength folds exhibiting Quaternary or late Quaternary growth. From reflector truncation, several north-dipping thrust faults (splay thrusts) are inferred to core these shallow folds and to splay upward from a shallow roof thrust. Some of these shallow splay thrusts ruptured to the surface in the late Holocene. Ages from offset soils in trenches across the fault scarps and from abruptly raised shorelines indicate that the splay, roof, and floor thrusts of the Seattle and Tacoma faults ruptured about 1100 years ago. Manuscript received 9 September 2003.

Two Stages of Deformation and Fluid Migration in the West-Central Brooks Range Fold and Thrust Belt, Northern Alaska

The Brooks Range is a north-directed fold and thrust belt that forms the southern boundary of the North Slope petroleum province in northern Alaska. Field-based studies have long recognized that large-magnitude, thin-skinned folding and thrusting in the Brooks Range occurred during arc-continent collision in the Middle Jurassic to Early Cretaceous (Neocomian). Folds and thrusts, however, also deform middle and Upper Cretaceous strata of the Colville foreland basin and thus record a younger phase of deformation that apatite fission-track data have shown to occur primarily during the early Tertiary (60 and 45 Ma). A structural and kinematic model that reconciles these observations is critical to understanding the petroleum system of the Brooks Range fold and thrust belt. New interpretations of outcrop and regional seismic reflection data indicate that from the modern mountain front northward to near the deformation front under the coastal plain, the basal thrust detachment for the orogen is located in the Jurassic and Lower Cretaceous Kingak Shale in the upper part of the regionally extensive, gently south-dipping, north-derived Mississippian to Early Cretaceous Ellesmerian sequence. The frontal part of the orogen lies in middle Cretaceous foreland basin strata and consists of a thin-skinned fold belt at the deformation front and a fully developed passive-roof duplex to the south. Near the mountain front, the orogen is composed of a stacked series of allochthons and thrust duplexes and associated Neocomian syntectonic deposits that are unconformably overlain by proximal foreland basin strata. The foreland basin strata and underlying deformed rocks are truncated by a younger generation of folds and thrusts. Vitrinite reflectance and stable isotope compositions of veins provide evidence of two fluid events in these rocks, including an earlier higher temperature (250–300C) event that was buffered by limestone and a younger, lower temperature (150C) event that had distinctly lower 13C values as a result of oxidation of organic matter and/or methane. Zircon fission-track data from the host rocks of the veins show that the higher temperature fluid event occurred at 160–120 Ma, whereas the lower temperature event probably occurred at about 60–45 Ma. It is proposed that the Brooks Range consists of two superposed contractional orogens that used many of the same mechanically incompetent stratigraphic units (e.g., Kayak Shale, Kingak Shale) as sites of thrust detachment. The older orogen formed in a north-directed arc-continent collisional zone that was active from 160 to 120 Ma. This deformation produced a thin-skinned deformational wedge that is characterized by far-traveled allochthons with relatively low structural relief, because it involved a thin (1–4-km [0.6–2.5-mi]-thick) stratigraphic section. Deeper parts of the deformational wedge are envisioned to have contained relatively high-temperature fluids that presumably migrated from or through limestone-rich source areas in the underlying autochthon or from deeper parts of the orogen. The younger orogen, which formed initially at about 60 Ma and reactivated at 45 Ma, produced a thrust belt and frontal triangle zone with low amounts of shortening and relatively high structural relief, because it involved a structural section 5–10 km (3–6 mi) thick. Fluids associated with this deformation were relatively of lower temperature and suggest that hydrocarbon migration occurred at this time. We conclude that hydrocarbon generation from Triassic and Jurassic source strata and migration into stratigraphic traps occurred primarily by sedimentary burial principally at 100–90 Ma, between the times of the two major episodes of deformation. Subsequent sedimentary burial caused deep stratigraphic traps to become overmature, cracking oil to gas, and initiated some new hydrocarbon generation progressively higher in the section. Structural disruption of the traps in the early Tertiary released sequestered hydrocarbons. The hydrocarbons remigrated into newly formed structural traps, which formed at higher structural levels or were lost to the surface. Because of the generally high maturation of the Colville basin at the time of the deformation and remigration, most of the hydrocarbons available to fill traps were gas.

Detachment folding, fold amplification, and diapirism in thrust wedge experiments

The relations between detachment folding, fold amplification, and salt diapirism in contractional settings have been investigated by means of scaled analogue models. The viscosity of the silicone layer simulating salt in nature and the shortening rates were combined in order to reproduce weak (type 1 models) and strong (type 2 models) décollements. Deformation patterns in the roof sequence exhibited two contrasting styles, (1) outward propagation of detachment folding along the décollement (OFP mode) and (2) passive roof duplex (PRD mode). In type 2 models, detachment folding propagated away from the most external thrust in the floor sequence, while in type 1 models, long‐lived detachment folds almost invariably localized amplified above a floor thrust tip as a result of strain localization. A silicone wall intruded occasionally into the crestal graben of detachment folds in type 1 and OFP models. Best fitting of transition models data points indicates nonlinear relations with regression curves close to the equilateral hyperbola equation for both OFP‐PRD and amplified detachment folds‐box folds transitions. A quantitative comparison of model results with nature has been attempted by plotting salt‐based fold‐and‐thrust belts data points on the scaled transition curves obtained from the modeling. Such a comparison relates shear stress products and ratios to the conditions favoring the amplification of detachment folds and the potential emplacement of ductile diapirs in their core. By reducing the roof sequence strength, pore fluid pressure λb is inferred to shift the equilibrium of fold‐and‐thrust belts toward the field of OFP and diapirism.

Duplex style and triangle zone formation: insights from physical modeling

Duplexes are a common feature in thrust belts at many scales. Their geometries vary significantly from antiformal stacks with significant forethrusting in the cover (e.g. southern Pyrennes, Spain) to triangle zones (e.g. foreland Canadian Rockies) to low-displacement individually spaced ramp-anticlines (e.g. Sub-Andean thrust belt, Bolivia). We present a series of physical experiments demonstrating that the strength of the décollements relative to that of the intervening and overlying rock layers plays a significant role in controlling the duplex style. The models comprise brittle layers made of dry quartz sand and décollements made of two types of viscous silicone polymers. The strength of the décollements in the models is a function of the shortening rate applied to the model. The relative strength of the décollements and surrounding rocks affects the development of active- or passive-roof duplexes (triangle zones). It also affects the amount of translation of individual thrust blocks and the spacing of thrust ramps, which in turn determine if a duplex evolves into an antiformal stack or into individually spaced ramp-anticlines. Model results indicate that specific associations of structural features form systematically under similar rheological and boundary conditions. The presence of relatively strong décollements promotes local underthrusting of the cover, individual ramp-anticlines, internal deformation of thrust sheets, low early layer-parallel shortening, and sequential towards-the-foreland propagation of structures. Weak décollements promote forethrusting of the cover, antiformal stacks, coeval growth of structures, and low internal strain, with the exception of significant early layer-parallel shortening. No underthrusting at a regional scale occurred in any model.

Geological constraints of giant and medium-sized gas fields in Kuqa Depression

There is a gas-rich and well-charged petroleum system in the Kuqa Depression where Triassic and Jurassic source rocks play important roles. Distributed in an area of more than 10000 km and with a thickness of up to 1000 m, they are composed of dark mudstones, carbonaceous mudstones and coal seams containing 6%, 40% and 90% of TOC, respectively, and are mainly the humic organic matter. As high-quality regional cap rocks, the Neogene and Eogene gypsum rocks and gypseous mudstones matched well with the underlying Neogene and Cretaceous-Eogene sandstones. They have formed the most favorable reservoir-seal assemblages in the Kuqa Depression. Also the Jurassic sandstones and mudstones formed another favorable reservoir-seal assemblage. The traps are shaped late in the fold-thrust belt, mainly fixed in the Tertiary-Quaternary, where ten structure styles have been distinguished. These traps spread as a zone in N-S, are scattered like a segmental line in W-E and show tier-styled vertically. The best traps are gypsum-salt covered fault-bend anticlines related to the passive roof duplex. This petroleum system is characterized by late accumulation. In the early Himalayan Movement, mainly gas condensate and oil accumulated and were distributed in the outer circular region of the kitchen; whereas in the middle and late Himalayan the gas accumulations mainly formed and were distributed in the inner circular region near the kitchen. The overpressure of gas pools is common and is formed by seal capacity of thick gypsum layers, extensive tectonic compression and large uplift. The well-preserved anticline traps underlying the high-quality regional cap rocks of the Tertiary gypsum rocks and gypseous mudstones are the main targets for the discovery of giant and medium-sized gas fields. Above conclusions are important for the petroleum geology theory and the exploration of the fold-thrust belt in foreland basins in central and western China.

Mechanical stratigraphy and fault–fold interaction, Absaroka thrust sheet, Salt River Range, Wyoming

A distinct contrast in deformation style from tightly folded and faulted Cambrian through Ordovician strata to broadly folded and faulted Mississippian and younger strata is displayed in the hanging wall of the Absaroka thrust, Salt River Range, Wyoming. Based on style, the Paleozoic section can be divided into two main structural lithic units. Each unit is characterized by a relatively weak, ductile, anisotropic lower section and a relatively strong, brittle, more isotropic upper section. Where the two units were stacked within the thrust sheet, inverted fault–propagation folds formed in the center of each unit, and the overall transition upward from close- to wide-spaced folds and imbricate faults developed in the multilayer. Where the upper unit was isolated, deformation was dominated by imbricate faulting with little associated folding, and inverted fault–propagation folds did not form. Observations illustrate that the mechanical interaction between the two units and boundary conditions imposed on them were significant to defining the deformation response. It is note-worthy that the boundary between the two structural lithic unts, in part defined by the roof thrust of the Stewart passive roof duplex, occurs within a dolostone rather than in overlying shale of the Darby Formation. I propose that the roof thrust and other detachments in dolostone reflect the relatively low frictional strength of dolostone under shallow crustal conditions. Cross-cutting relations suggest that the Absaroka sheet was cut by large imbricate faults during late-stage movement over a major dip ramp. A high deviatoric and low mean compressive stress state just beyond the upper ramp–flat corner promoted break-back imbrication and allowed tectonic thickening of the thrust wedge necessary to reestablish critical taper and for forward translation.

Three-dimensional horizon reconstruction from outcrop structural data, restoration, and strain field of the Baisahi anticline, Western Nepal

Within many fold and thrust belts, preservation of bed thickness is generally assumed during deformation. A three-dimensional (3D) reconstruction method is proposed from outcrop structural data. This technique accounts for along-strike thickness variations. By a series of 3D spatial projections of dip and strike data, the thickness of strata from any point at the ground surface to reference horizons on stratigraphic columns can be calculated across-strike and projected along the poles of the associated strata, providing sets of dots in space, which represent folded horizons. The technique is applied to the Baisahi antiform, located on the frontal Subhimalayan thrust. An along-strike deformation gradient is evidenced by two-dimensional cross-section balancing, which shows a passive-roof duplex structure. The 3D geometry of the roof is reconstructed, allowing other approaches such as unfolding to be done. The residual strain field induced by restoration shows distortion features, which are interpreted as internal strain due to the propagation of the fault.

Passive roof thrusting and forelandward fold propagation in scaled brittle‐ductile physical models of thrust wedges

A set of scaled analogue models was performed to investigate the role of a décollement layer in the structural style of fold‐and‐thrust belts. Silicone putty (SGM 36) has been employed to represent the ductile décollement, while frictional material (quartz sand) has been used to simulate the brittle behavior of roof and floor sequences, situated above and below the décollement, respectively. In order to explore a wide combination of strength profiles, models have been deformed under five distinct rates of compression (0.15, 0.3, 0.45, 0.6, and 1.5 cm h−1) employing four different thicknesses for the silicone layer (0.1, 0.2, 0.4, and 0.8 cm). Experimental results indicate that roof sequences exhibit two distinct styles of deformation: (1) passive roof duplex (PRD) geometry [e.g., Banks and Warburton, 1986] and (2) outward propagation of folding (OFP) along the décollement [e.g., Davis and Engelder, 1985]. In PRD geometry the roof sequence remains relatively “stationary” being underthrust by a wedge‐shaped floor duplex, while in OFP, displacement along the floor thrusts is transferred horizontally along the décollement layer. Experiments suggest the occurrence of a genetic relationship between shear stresses (at the base of the roof sequence τb and within the ductile layer τd) and the development of PRD or OFP geometry. For a given strain rate the deformation pattern of roof sequences depends upon the τb/τd ratio as well as the absolute τb and τd values. The transition field between OFP and PRD styles has been approximated by curvilinear regression of transition‐style experimental data points, with a minimum value of τd, varying between about 112 and 160 Pa, to enter the PRD field. Syntectonic sedimentation in the molasse basin is found to inhibit development of folding into the foreland but to promote PRD style. Experiments simulate many of the principal characteristics of fold‐and‐thrust belts developed above a basal evaporite décollement, as well as the development of passive roof duplexes at mountain fronts. Shear stress values inferred for many fold‐and‐thrust belts correlate well the deformation fields predicted by the present experimental study, allowing to propose a simple model describing the mechanical behavior of natural roof sequences underlain by a décollement layer.

A regional structural model for the northern sector of the Calabrian Arc (southern Italy)

A SW–NE-oriented structural crustal section of northern Calabria (Southern Italy) extending from back-arc (SW ‘internal’) up to foreland (NE. ‘external’) areas illustrates the tectonic evolution of this key sector of the Central Mediterranean Calabrian Arc. The outcropping terrains of this part of Calabria show a thrust sheet pile of basement units (‘Calabrian basement Complex’), which are from top to bottom: a Hercynian assemblage of gneisses, granites, and metapelites (‘Calabride’ Units), with a partly detached Jurassic–Paleogene cover and former foredeep sequences, a series of Alpine metamorphic units comprising a Cretaceous to Paleogene metapelitic/ophiolitic/carbonate assemblage (‘Liguride’ Units). This highly tectonized and cataclastic assemblage overlies a Triassic carbonate series that outcrops in a number of tectonic windows along the internal, Tyrrhenian side of the Arc. Our field studies revealed that these can be assigned to the ‘San Donato–Campotenese Unit’ that shows a comprehensive Triassic pelitic to carbonatic platform series, more extensively outcropping in the northern part of the area. Along the external part of the section, well data show that the basement assemblage tectonically overlies a deformed carbonate unit and its Paleogene–Lower Miocene cover. These data confirm the hypothesis that the Calabrian basement was first deformed during Late Eocene (eo-Alpine phase) times and later overthrusted Tethyan carbonate platform areas, already deformed in the Apennine Chain. Highly detailed tectonostratigraphic studies of the Calabrian intra-arc and fore-arc basins, previously published and recently calibrated with a regional interpretation of the available two-dimensional and three-dimensional seismic and well database, showed that the subsequent Late Neogene evolution was characterized by the activity of a number of oblique crustal shear zones that controlled the evolution of a wide range of thrust belt basins. Basin inversion phases during middle Messinian, middle Pliocene and middle Pleistocene times resulted in the widespread occurrence of thin-skinned and thick-skinned oblique back thrusting and the generation of regional passive roof duplex structures, all of these common along the margins and within the Calabrian Element. Our integrated interpretation of field geology, subsurface data and magnetotelluric analysis resulted in a comprehensive crustal section showing the following main features: (1) The superficial Calabrian basement Complex (assembled up to Eocene times) is a relatively thin thrust sheet (ca 1.5–2 km in thickness) overlying all deeper units. (2) Below this, an Early Miocene thrust stack in the subsurface shows a number of opposite (internal, W and external, E) verging tectonic units. The carbonate platform units outcropping in this northern sector of the Calabrian Arc form the top part of this subsurface thrust stack. (3) Below the subsurface thrust stack, a number of antiformal geometries are present, which, in our opinion, are the fundamental crustal terranes of which the Calabrian Arc is composed, juxtaposed by oblique crustal shear zones in Miocene–Recent times. The main branches of these shear zones, which root into a number of deep thick-skinned overthrusts, dissect and deform the Early Miocene thrust sheet pile and are represented by main transpressive fault zones in the surface. Three main subsurface terranes can be recognized: one internal, overlain by an internally verging thrust stack, one central overlain by a number of externally verging thrust sheets, and one externally in continuity with the foreland area. (4) Externally, a regional thin-skinned thrust sheet (‘Metaponte Nappe’) composed of a tectonic assemblage of Eocene and Late Neogene terrains shows a retrovergent internal margin that partly masks the surface trace of one of the major shear zones, and an external flat lying overthrust upon the Gulf of Taranto–Salentino foreland platform unit. Finally, we present an evolutionary model describing the Eocene to Recent evolution of this sector of the Central Mediterranean Mountain Chain.

Piggyback basin development above a thin-skinned thrust belt with two detachment levels as a function of interactions between tectonic and superficial mass transfer: the case of the Subandean Zone (Bolivia)

The Subandean fold and thrust belt of Bolivia is characterised by two major detachment levels and large piggyback basins. ‘Sand-box’ and numerical models have been used to study sedimentation and erosion control on thrust belt evolution and to study the retroactive effects of tectonics on piggyback development in thin-skinned thrust belts with two detachment levels. Analogue models show that surface processes play a dominant role in controlling wedge evolutions: erosion promotes fault reactivation and tectonic delamination (passive roof duplex) while sedimentation promotes forward shifting of the frontal thrust and consequently piggyback basin development. Numerical models were used to understand the development of the Subandean fold and thrust belt of Bolivia. Numerical experiments show that the simultaneity of basement tilting and high sedimentation rates promotes the formation of a stable tectonic wedge. Outer and inner faults are alternately active during the beginning of deformation, a kinematic evolution that favours the development of piggyback basins between. The step-by-step history of the thrust belt predicts that each change in tectonic location is recorded with large unconformities in basins, but these unconformities are not well preserved from progressive erosion in the final geometry.

The control of the smectite–illite transition on passive-roof duplex formation: Canadian Rockies Foothills, Alberta

The roof décollement to the passive-roof duplex and triangle zone in the central Foothills, Rocky Mountain thrust belt is located within a sequence of siliciclastic sediments dominated by shale/mudstone. It is unclear why the décollement is found at a certain level in the stratigraphy, when the whole package appears to have similar lithilogical and hence mechanical properties. Investigation of the mineralogy of the mudstones reveals that the clay fraction preserves a smectite-to-illite transition, which formed during maximum burial conditions. The top of the transition is associated with the top of the roof décollement in the region. In the Gulf of Mexico, the transition correlates with both a zone of overpressure, favorable for detachment, and a corresponding drop in density (rise in porosity). In the Foothills, the transition also records a drop in density and a concomitant drop in sonic velocity. By analogy with the Gulf of Mexico, the drop in density may record a paleo-overpressured horizon that was favorable for detachment and formation of a roof décollement in the Foothills.

Abstract: The Control of the Smectite-Illite Transition on Passive-Roof Duplex Formation: Canadian Rockies Foothills, Alberta

Abstract: The Control of the Smectite-Illite Transition on Passive-Roof Duplex Formation: Canadian Rockies Foothills, Alberta

Thrust geometry controlled by erosion and sedimentation: A view from analogue models

Research Article| May 01, 1997 Thrust geometry controlled by erosion and sedimentation: A view from analogue models J. L. Mugnier; J. L. Mugnier 1Université de Joseph Fourier and Centre National de la Recherche Scientifique, Laboratoire de Géodynamique des Chaînes Alpines, rue M. Gignoux, 38031, Grenoble, France Search for other works by this author on: GSW Google Scholar P. Baby; P. Baby 2Orstom, Apartado 17-11-6596, Quito, Ecuador Search for other works by this author on: GSW Google Scholar B. Colletta; B. Colletta 3Institut Franìais du Pétrole, 1-4 avenue de Bois Préau, Rueil-Malmaison, France Search for other works by this author on: GSW Google Scholar P. Vinour; P. Vinour 1Université de Joseph Fourier and Centre National de la Recherche Scientifique, Laboratoire de Géodynamique des Chaînes Alpines, rue M. Gignoux, 38031, Grenoble, France Search for other works by this author on: GSW Google Scholar P. Bale; P. Bale 3Institut Franìais du Pétrole, 1-4 avenue de Bois Préau, Rueil-Malmaison, France Search for other works by this author on: GSW Google Scholar P. Leturmy P. Leturmy 1Université de Joseph Fourier and Centre National de la Recherche Scientifique, Laboratoire de Géodynamique des Chaînes Alpines, rue M. Gignoux, 38031, Grenoble, France Search for other works by this author on: GSW Google Scholar Author and Article Information J. L. Mugnier 1Université de Joseph Fourier and Centre National de la Recherche Scientifique, Laboratoire de Géodynamique des Chaînes Alpines, rue M. Gignoux, 38031, Grenoble, France P. Baby 2Orstom, Apartado 17-11-6596, Quito, Ecuador B. Colletta 3Institut Franìais du Pétrole, 1-4 avenue de Bois Préau, Rueil-Malmaison, France P. Vinour 1Université de Joseph Fourier and Centre National de la Recherche Scientifique, Laboratoire de Géodynamique des Chaînes Alpines, rue M. Gignoux, 38031, Grenoble, France P. Bale 3Institut Franìais du Pétrole, 1-4 avenue de Bois Préau, Rueil-Malmaison, France P. Leturmy 1Université de Joseph Fourier and Centre National de la Recherche Scientifique, Laboratoire de Géodynamique des Chaînes Alpines, rue M. Gignoux, 38031, Grenoble, France Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1997) 25 (5): 427–430. https://doi.org/10.1130/0091-7613(1997)025<0427:TGCBEA>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. L. Mugnier, P. Baby, B. Colletta, P. Vinour, P. Bale, P. Leturmy; Thrust geometry controlled by erosion and sedimentation: A view from analogue models. Geology 1997;; 25 (5): 427–430. doi: https://doi.org/10.1130/0091-7613(1997)025<0427:TGCBEA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Analogue sandbox models have been set up to study thrust systems formed above two decollements and affected by erosion and sedimentation. The deformation was imaged by X-ray scanner. Three erosion-sedimentation conditions were tested. For all the experiments, a duplex develops between the two decollement levels. For the experiment without erosion and without sedimentation, ramp anticlines form in the upper series. For the model with syntectonic sedimentation and without erosion, the shortening in the upper series is first characterized by fault-related folds and then by a forward propagation of the decollement that induces the development of a piggy-back basin. For the experiment with erosion and sedimentation, a “passive roof duplex” is formed. Relationships between structural style and syntectonic superficial conditions in classical thin-skinned thrust belts are similar to those found in the analogue models: the ramp anticlines of the Jura developed with low erosion and low sedimentation and the large sub-Andean thrust belt is controlled by syntectonic sedimentation, whereas the passive roof duplex of Pakistan is synchronized with intense erosion. This study outlines the crucial effect of the superficial mass-transport phenomena in the tectonic style of thrust belts. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Kinematics of primary contacts between low- and relatively high-pressure rocks in orogens

Primary contacts among low- and relatively high-pressure rocks in convergent orogens have been mainly interpreted as a result of extensional tectonics, or reimbrication of high-pressure thrust units already resting on low-pressure ones and by consequent inversion of their stacking order. Our work in northern Calabria allows us to ascertain the nature and kinematics of several of these contacts, and to work out a general model that considers primary pressure break contacts as resulting from the growth of a deep passive roof duplex within a subduction-accretion system. Estimates of pressure differences across these contacts may be obtained for a simple passive roof duplex geometry as a function of the aspect ratio of the structure and of the angle of subduction. Our results suggest therefore that there may be a further possibility, in addition to extension or reimbrication, which explains pressure break contacts in orogens.

Mechanics of triangle zones and passive-roof duplexes: implications of finite-element models

Finite-element models of passive-roof duplexes simulate the evolution and distribution of fault slip and deformation as functions of boundary conditions, friction on the roof fault, cover strength and amount of overburden. A foreland buttress is essential for backthrust displacement in the models. This foreland buttress is analogous to a pin line between the cover and footwall forelandward of the triangle zone in a balanced cross section, and could be caused by a stratigraphic pinchout of the upper detachment or a strong cover in the foreland. With boundary conditions that prevent the cover from slipping toward the hinterland relative to the wedge, incipient forethrust and backthrust ramps (zones of high equivalent plastic strain) cut through the cover. In the models, there is more backthrust displacement with a lower friction coefficient on the upper detachment and with a stronger cover. Because of pressure dependence, both fault strength and material strength increase with increasing overburden. Therefore, the effect of overburden on backthrust displacement is determined by the nature of the pressure dependencies for fault friction and material strength. In models with multiple imbricate faults, the distribution of equivalent plastic strain adjacent to each fault is similar to that in single-fault models, with differences resulting from bending strains in the wedge and cover. The strain distribution in the cover of the models is similar to that above natural passive-roof duplexes, characterized by layer-parallel shortening above the tip of each horse in the duplex.

Late Neogene kinematics of intra‐arc oblique shear zones: The Petilia‐Rizzuto Fault Zone (Calabrian Arc, Central Mediterranean)

The kinematics of intra‐arc shear zones play a key role in the secondary shaping of orogenic arcs such as the Calabrian Arc (central Mediterranean). Comparison of the Neogene structural development of the Petilia‐Rizzuto Fault Zone and the basement structure of the bordering Sila massif reveals that the fault zone is the surface expression of a deep NW–SE trending sinistral crustal oblique shear zone. This shear zone continues over a length of more than 130 km across the northern segment of the Calabrian Arc and shows a post‐Eocene sinistral displacement of about 50 km. The late Neogene forearc basin development and syndepositional tectonics along the fault zone are reconstructed in great detail by analyzing the middle Miocene‐Recent tectonic sequence stratigraphy. A strike‐slip cycle can be recognized whereby the subsequent activity of Riedel shears, tensional faults, and P shears, positive flower structures and principle displacement wrench faults, can accurately be traced in time. Observed phenomena are discussed in terms of the activity of a conjugate system of oblique thrust zones within the growing accretionary complex. The evolution of special types of thrust belt basins is illustrated. These include oblique thin‐skinned pull‐apart basins, oblique rhomboidal “harmonica” basins, and “detached slab” basins (new terms introduced here), evolving one into the other. A new feature illustrated is the recurrent basin inversion which generated passive roof duplexes through back‐shear motion and out‐of‐sequence thrusting along the wedge. The fault patterns and the style of inversion tectonics imply an E–W directed axis of effective compressive stress in this part of the arc. This resulted from an interaction of (1) local E–W directed compression related to a differential displacement of two parallel segments of the arc (generated by the migration to the southeast of the Calabrian Arc and opening of the Tyrrhenian backarc basin); (2) alternating NW–SE directed compression and extension (related to pulsating thrust wedge dynamics with phases of accretion and underthrusting respectively) and (3) regional, compressive interplate stress (middle Messinian‐middle Pliocene). All structures are overprinted by post middle Pleistocene extensional faulting (related to rapid uplift of intra‐arc massifs) and reversal along thrust planes and transcurrent faults. This extensional collapse reflects isostatic adjustments in response to plate rupture which was provoked by regional compressive stress.