Formation Of Thrust Faults Research Articles

Geomorphology of contractional salt tectonics along the Kuqa fold-thrust belt, northwestern China: Testing pre-kinematic diapir versus source-fed thrust and detachment fold models

The Kuqa fold-thrust belt arguably hosts Earth's highest density of diverse surface salt structures, including salt-cored anticlines, strike-slip faults that double as salt walls, salt domes, and salt thrusts. Key aspects of the structural development of the range are unresolved. In particular, it is unclear whether the fold-thrust belt developed via forward-propagating salt-cored detachment folds which locally nucleated late thrust faults, or if such detachment folds grew interspersed with either (i) long-lived pre-kinematic salt diapirs that evolved into local salt thrusts and/or (ii) source-fed thrust faults and associated folds. Detachment folds and hanging wall anticlines of source-fed thrusts feature gradual lateral propagation during growth, whereas diapirs either lack lateral propagation or involve irregular propagation and dramatic strain gradients. Therefore, we test the hypotheses by examining potential geomorphological records of lateral ridge propagation along the frontal Kuqa fold-thrust belt. A particular focus is the Quele salt thrust, because it spans one tenth of the length of the frontal belt and it is commonly hypothesized to represent a deformed diapir or a source-fed thrust fault. We find that all ridges along the frontal Kuqa fold-thrust belt feature (1) bow-shaped along-strike topography, with decreasing relief from the topographic crest to the plunging ends of the ridges, (2) curved wind and water gaps, (3) asymmetric fan-shaped and forked-shaped tributaries, and (4) drainage basin maturity decreases towards lateral ridge terminations. These features are consistent with lateral fold propagation. At the Quele salt thrust, such evidence indicates that the hanging wall developed as two distinct anticlines which merged into a composite anticlinorium. Our findings indicate that the frontal Kuqa fold-thrust belt developed either by detachment folding with local late thrust fault formation, or via such structures interspersed with source-fed thrust faults.

Formation of a Composite Albian–Eocene Orogenic Wedge in the Inner Western Carpathians: P–T Estimates and 40Ar/39Ar Geochronology from Structural Units

The composite Albian–Eocene orogenic wedge of the northern part of the Inner Western Carpathians (IWC) comprises the European Variscan basement with the Upper Carboniferous–Triassic cover and the Jurassic to Upper Cretaceous sedimentary successions of a large oceanic–continental Atlantic (Alpine) Tethys basin system. This paper presents an updated evolutionary model for principal structural units of the orogenic wedge (i.e., Fatricum, Tatricum and Infratatricum) based on new and published white mica 40Ar/39Ar geochronology and P–T estimates by Perple_X modeling and geothermobarometry. The north-directed Cretaceous collision led to closure of the Jurassic–Early Cretaceous basins, and incorporation of their sedimentary infill and a thinned basement into the Albian–Cenomanian/Turonian accretionary wedge. During this compressional D1 stage, the subautochthonous Fatric structural units, including the present-day higher Infratatric nappes, achieved the metamorphic conditions of ca. 250–400 °C and 400–700 MPa. The collapse of the Albian–Cenomanian/Turonian wedge and contemporary southward Penninic oceanic subduction enhanced the extensional exhumation of the low-grade metamorphosed structural complexes (D2 stage) and the opening of a fore-arc basin. This basin hemipelagic Coniacian–Campanian Couches-Rouges type marls (C.R.) spread from the northern Tatric edge, throughout the Infratatric Belice Basin, up to the peri-Pieniny Klippen Belt Kysuca Basin, thus tracing the south-Penninic subduction. The ceasing subduction switched to the compressional regime recorded in the trench-like Belice “flysch” trough formation and the lower anchi-metamorphism of the C.R. at ca. 75–65 Ma (D3 stage). The Belice trough closure was followed by the thrusting of the exhumed low-grade metamorphosed higher Infratatric complexes and the anchi-metamorphosed C.R. over the frontal unmetamorphosed to lowest anchi-metamorphosed Upper Campanian–Maastrichtian “flysch” sediments at ca. 65–50 Ma (D4 stage). Phengite from the Infratatric marble sample SRB-1 and meta-marl sample HC-12 produced apparent 40Ar/39Ar step ages clustered around 90 Ma. A mixture interpretation of this age is consistent with the presence of an older metamorphic Ph1 related to the burial (D1) within the Albian–Cenomanian/Turonian accretionary wedge. On the contrary, a younger Ph2 is closely related to the late- to post-Campanian (D3) thrust fault formation over the C.R. Celadonite-enriched muscovite from the subautochthonous Fatric Zobor Nappe meta-quartzite sample ZI-3 yielded a mini-plateau age of 62.21 ± 0.31 Ma which coincides with the closing of the Infratatric foreland Belice “flysch” trough, the accretion of the Infratatricum to the Tatricum, and the formation of the rear subautochthonous Fatricum bivergent structure in the Eocene orogenic wedge.

Effects of faulting on crustal stresses during mountain formation on Io

The vast majority of mountains observed on Io are tectonic, upthrusted blocks – a product of compressional forces. The mechanism behind their formation is generally understood to be related to Io's heat-pipe volcanism and crustal subsidence. Compressional thermoelastic stresses from sustained local or regional shut down of Io's heat-pipe volcanism could be potentially important orogenic drivers as well (Kirchoff and McKinnon, 2009, Formation of mountains on Io: Variable volcanism and thermal stresses, Icarus, 201, 598–614). At the same time, extensional tectonic features (rifts, pull-apart basins, simple graben) are found on Io. Furthermore, results from a two-dimensional finite element model of thrust fault formation in the ionian environment show that natural kinematics of fault motion creates localized regions of near-surface extension, while compressional stresses at depth are greatly reduced (Bland and McKinnon, 2016, Mountain building on Io driven by deep faulting. Nat. Geosci. 9, 429–432). In light of these observations and model results, we refine our previous one-dimensional, advective heat-pipe model of Io's crustal stresses, exploring another horizontal boundary strain end-member, which is the laterally unconfined (or integral stress) condition first suggested by Turcotte (1974, Are Transform Faults Thermal Contraction Cracks? J. Geophys. Res. 79, 2573-2577). We also explore the effects of directly incorporating brittle failure limits and liquid sulfur – through pore pressure – on these failure limits. Horizontally unconfined, vertical boundaries serve as an end-member representation for stress release on preexisting faults, and in the context of Io, predict extensional failure in the upper crust in response to deeper compression of either subsidence and/or thermal origin. Including these modifications greatly decreases the size of the region in compressional failure in the lower crust, unless the effects of pore sulfur are included. As in our previous work, the most favorable condition for mountain formation on Io is suggested to be a combination of subsidence stress, thermal stress from a decrease in volcanic heat piping, and crustal thinning due to basal melting.

Tectonic position of the neoproterozoic gabbro-hyperbasite and gabbroid complexes of the Bayannur block of the Songino ledge, Central Asian orogenic belt

The Early Caledonian folded region of the southern framing of the Siberian platform between Dzavkhan and Tuva-Mongolian terranes contains blocks of Songino ledge crystalline rocks. In the Bayannur block of the southern part of the Songino ledge, Neoproterozoic (890–780 Ma) gneiss-migmatitic Bayannur and metaterrigenous-volcanogenic Kholbonure complexes are selected. The zone of their junction is traced by thrust structures, which are reflected in all structural and real complexes of the Bayannur block. At the junction zone, tectonic plates are presented. Their peculiarity is the presence of rocks formed before the formation of the gabbro-hyperbasite complex and post-thrust gabbroids and gabbro-diorites. The upper age limit of the interval of the formation of thrust faults is defined by Bayannur pluton granitoids with age of 790 ± 3 Ma and gabbroids and anorthosites of Onzula tectonic plate with ages of 785 ± 3 and 784 ± 3 Ma. The lower boundary of the formation of thrust faults is determined by the age of Bayannur complex ultrametamorphic granitoids (802 ± 6 Ma). Bodies of massive (post-thrust) pegmatoid gabbro and rocks of gabbro-hyperbasite complex with clearly manifested structures associated with the thrusts are recognized in the tectonic plate of the ridge overlooking the area of Mount San Node. These rocks are dated at 782 ± 2 and 806 ± 10 Ma, respectively (ID TIMS). The obtained data indicate that the post-thrust and before-thrust formations can not be parts of a single stratified complex. The latter can be considered as fragments of paleooceanic formations in the accretion structure of the Bayannur block of the Songino ledge.

The effect of overburden thickness on deformation mechanisms in the Keping fold-thrust belt, southwestern Chinese Tian Shan Mountains: Insights from analogue modeling

The structural evolution of the Keping fold-thrust belt in the South Chinese Tian Shan, NW China is poorly understood. Here, we present the results of a series of scaled sandbox models based on the structural architecture of the Keping fold-thrust belt in our investigation of the effect of overburden thickness on deformation mechanisms in fold-thrust belts. These models simulated the evolution of the Keping fold-thrust belt, where the western segment was shortened with a thicker overburden relative to the eastern segment. Model results confirm that fewer thrust faults with wider spacing develop in thicker cover units, whereas more thrust faults with narrower spacing develop in thinner cover units. Transfer zones also form as a result of faster and farther propagation in a segment with thicker overburden, consistent with the prediction of the critical taper theory. Our modeling further shows that due to the episodic accretion of thrust wedges and non-simultaneous formation of thrust faults, the deformation front may locally propagate farther in the thinner overburden. Alternatively, there may not be so much difference between two parts of sand layers with different thicknesses, an observation that is inconsistent with the theoretical prediction. The similarities between the model and real geological observations suggest that the variations in the overburden thicknesses across the Piqiang Fault transfer zone strongly influenced the development of the structural architecture of the Keping fold-thrust belt. They further indicate that the growth of fold-thrust belts in nature is episodic, and that the propagation of deformation front cannot be simply predicted by the critical taper theory. The overburden thickness variations across the Piqiang Fault transfer zone were produced by high angle reverse faulting along the pre-existing Piqiang-Selibuya Fault during the late Pliocene and associated crustal uplift and erosion of the hanging wall strata.

Formation of the frontal thrust zone of accretionary wedges

The combination of recent megathrust earthquakes that have ruptured the frontal portions of accretionary prisms and historical evidence for large wedge-front, fold-and-thrust belt events has sparked interest in the development of wedge-front fault systems. Here we explore the formation of these faults using two-dimensional finite-difference models of spontaneous thrust fault formation and wedge accretion. A reference model with uniform material strength parameters predicts wedge-front thrusts to nucleate near the surface of the sedimentary layer and propagate down towards the basal décollement. The formation of a new frontal thrust is preceded by incipient faulting over a zone several kilometers wide forward of the wedge toe, resembling the protothrust zone (PTZ) imaged in seismic reflection data from the front of submarine accretionary prisms. Similar behavior, with a new frontal thrust forming from top to bottom, occurs if the fraction (λ) of pore-fluid pressure relative to the total (dynamic) pressure is uniform and <0.8 throughout the sediment section, or if λ increases with depth and is <0.7 at the base of the décollement. For greater values of λ, models predict more frontal thrusts to form from the bottom to top. These results are reasonably well explained by a simple elastic stress analysis. Models that simulate cohesive brittle strength that is negligible in the shallow sediments and increases with depth also predict new thrust faults to form from top to bottom. Models in which viscous flow at shallow depths represents creep of sediments predict bottom-up thrust nucleation when viscous stresses due to plate convergence remain lower than the brittle yield stress in the top ∼15–50% of the sediment section. Top-down nucleation is otherwise favored when viscous stresses exceed the brittle yield strength throughout most of the sediments. Our study illuminates key controls on the mechanical and temporal links between the PTZ, the forward propagation of slip along the underlying décollement, and the formation of a new frontal thrust.

Formation of cratonic lithosphere during the initiation of plate tectonics

Earth’s oldest near-surface material, the cratonic crust, is typically underlain by thick lithosphere (>200 km) of Archean age. This cratonic lithosphere likely thickened in a high-compressional-stress environment, potentially linked to the onset of crustal shortening in the Neoarchean. Mantle convection in the hotter Archean Earth would have imparted relatively low stresses on the lithosphere, whether or not plate tectonics was operating, so a high stress signal from the early Earth is paradoxical. We propose that a rapid transition from heat pipe–mode convection to the onset of plate tectonics generated the high stresses required to thicken the cratonic lithosphere. Numerical calculations are used to demonstrate that an existing buoyant and strong layer, representing depleted continental lithosphere, can thicken and stabilize during a lid-breaking event. The peak compressional stress experienced by the lithosphere is 3×–4× higher than for the stagnant-lid or mobile-lid regimes immediately before and after. It is plausible that the cratonic lithosphere has not been subjected to this high stress state since, explaining its long-term stability. The lid-breaking thickening event reproduces features observed in typical Neoarchean cratons, such as lithospheric seismological reflectors and the formation of thrust faults. Paleoarchean “pre-tectonic” structures can also survive the lid-breaking event, acting as strong rafts that are assembled during the compressive event. Together, the results indicate that the signature of a catastrophic switch from a stagnant-lid Earth to the initiation of plate tectonics has been captured and preserved in the characteristics of cratonic crust and lithosphere.

Role of fluids in the tectonic evolution of Titan

Detailed analyses of slopes and arcuate planform morphologies of Titan’s equatorial mountain ridge belts are consistent with formation by contractional tectonism. However, contractional structures in ice require large stresses (4–10MPa), the sources of which are not likely to exist on Titan. Cassini spacecraft imagery reveals a methane-based hydrological cycle on Titan that likely includes movement of fluids through the subsurface. These crustal liquids may enable contractional tectonic features to form as groundwater has for thrust belts on Earth. In this study, we show that liquid hydrocarbons in Titan’s near subsurface can lead to fluid overpressures that facilitate contractional deformation at smaller stresses (<1MPa) by significantly reducing the shear strength of materials. Titan’s crustal conditions with enhanced pore fluid pressures favor the formation of thrust faults and related folds in a contractional stress field. Thus, surface and near-surface hydrocarbon fluids made stable by a thick atmosphere may play a key role in the tectonic evolution of Titan.

Origin of the hydrocarbon gases carbon dioxide and hydrogen sulfide in Dodan Field (SE-Turkey)

Gas occurrences consisting of carbon dioxide (CO2), hydrogen sulfide (H2S), and hydrocarbon (HC) gases and oil within the Dodan Field in southeastern Turkey are located in Cretaceous carbonate reservoir rocks in the Garzan and Mardin Formations. The aim of this study was to determine gas composition and to define the origin of gases in Dodan Field. For this purpose, gas samples were analyzed for their molecular and isotopic composition. The isotopic composition of CO2, with values of −1.5‰ and −2.8‰, suggested abiogenic origin from limestone. δ34S values of H2S ranged from +11.9 to +13.4‰. H2S is most likely formed from thermochemical sulfate reduction (TSR) and bacterial sulfate reduction (BSR) within the Bakuk Formation. The Bakuk Formation is composed of a dolomite dominated carbonate succession also containing anhydrite. TSR may occur within an evaporitic environment at temperatures of approximately 120–145 °C. Basin modeling revealed that these temperatures were reached within the Bakuk Formation at 10 Ma. Furthermore, sulfate reducing bacteria were found in oil–water phase samples from Dodan Field. As a result, the H2S in Dodan Field can be considered to have formed by BSR and TSR.As indicated by their isotopic composition, HC gases are of thermogenic origin and were generated within the Upper Permian Kas and Gomaniibrik Formations. As indicated by the heavier isotopic composition of methane and ethane, HC gases were later altered by TSR. Based on our results, the Dodan gas field may have formed as a result of the interaction of the following processes during the last 7–8 Ma: 1) thermogenic gas generation in Permian source rocks, 2) the formation of thrust faults, 3) the lateral-up dip migration of HC-gases due to thrust faults from the Kas Formation into the Bakuk Formation, 4) the formation of H2S and CO2 by TSR within the Bakuk Formation, 5) the vertical migration of gases into reservoirs through the thrust fault, and 6) lateral-up dip migration within reservoir rocks toward the Dodan structure.

Modelling cover deformation and decoupling during inversion, using the Mid-Polish Trough as a case study

Seismic sections across the NW part of the Polish Basin show that thrust faults developed in the sedimentary units above the Zechstein evaporite layer during basin inversion. These cover thrust faults have formed above the basement footwall. Based on the evolution of the basin, a series of scaled analogue models was carried out to study interaction between a basement fault and cover sediments during basin extension and inversion. During model extension, a set of normal faults originated in the sand cover above the basement fault area. The distribution and geometry of these faults were dependent on the thickness of a ductile layer and pre-extension sand layer, synkinematic deposition, the amount of model extension, as well as on the presence of a ductile layer between the cover and basement. Footwall cover was faulted away from the basement only in cases where a large amount of model extension and hanging-wall subsidence were not balanced by synkinematic deposition. Model inversion reactivated major cover faults located above the basement fault tip as reverse faults, whereas other extensional faults were either rotated or activated only in their upper segments, evolving into sub-horizontal thrusts. New normal or reverse faults originated in the footwall cover in models which contained a very thin pre-extension sand layer above the ductile layer. This was also the case in the highly extended and shortened model in which synkinematic hanging-wall subsidence was not balanced by sand deposition during model extension. Model results show that inversion along the basement fault results in shortening of the cover units and formation of thrust faults. This scenario happens only when the cover units are decoupled from the basement by a ductile layer. Given this, we argue that the thrusts in the sedimentary infill of the Polish Basin, which are decoupled from the basement tectonics by Zechstein evaporites, developed due to the inversion of the basement faults during the Late Cretaceous-Early Tertiary.

Inter-array and intra-array kinematics of en échelon sigmoidal veins in cross-bedded sandstone, Merimbula, southeastern Australia

The kinematics of en échelon arrays of quartz veins hosted by sandstone of the Upper Devonian Worange Point Formation are strongly influenced by bedding and cross-bedding. Bulk deformation recorded by the vein arrays (inter-array kinematics) was influenced by the anisotropy of the host rock, such that, three types of arrays are observed: (1) east-verging arrays parallel to bedding; (2) east-verging arrays oblique to bedding with veins parallel to cross-bedding; and (3) west-verging arrays parallel to cross-bedding with veins parallel to bedding. These three array types correspond to flats, ramps and backthrusts, respectively and represent the earliest stages of thrust fault formation. Flat and ramp arrays form a staircase geometry and conjugate ramp and backthrust arrays form pop-up structures. The interpretation of the morphology of veins and wall-rock (intra-array kinematics) indicates rotation of bridges was the primary kinematic process in the arrays. The opening of vein space by rotation of bridges which maintain constant thickness is predicted from measurements of the orientations of vein–wall-rock interfaces at vein tips and at the array centre. Comparison of the predictions with direct measurement of the proportion of vein to wall-rock along the centreline of arrays demonstrates the applicability of the bridge-rotation model which has been generalised to include the thickening or thinning of bridges during rotation.

Microfabric of Altered Ash Layers, ODP Leg 131, Nankai Trough

AbstractSamples of ash layers and associated background sediments from Site 808 of ODP Leg 131 in the Nankai Trough accretionary prism were analyzed for changes in mineralogy, porosity and micro-fabric associated with alteration of volcanic ash. Ash layers range from incipient stages of alteration and dissolution to complete alteration to clay minerals and clinoptilolite. Ash layers contain greater abundances of total clay minerals and lower percentages of quartz than do surrounding background hemipelagic sediments. The clay-sized fraction of ash layers is dominated by pure dioctahedral smectite, whereas the background sediments contain primarily illite and chlorite with minor amounts of smectite. Analysis of microfabric revealed dramatic changes in the distributions and abundances of grains and pores during ash alteration. The relative abundances of large pores, grains, and matrix material were quantified on digital back-scattered electron images (BSEI) of ash layer and background sediment samples. During burial, the abundant glass shards of shallow ash layers are initially altered, presumably to smectite. Subsequent dissolution of the glass leaves open, shard-shaped pores, resulting in increased porosities. With greater burial, these pores are filled with clinoptilolite. Although the presence of ash and its alteration products clearly influences sediment physical properties, there is no apparent correlation of the abundance of ash or its alteration products with the formation of thrust faults or other structures within the Nankai Trough accretionary prism.

Finite-element analyses of thrust tectonics: Computer simulation of detachment phase and development of thrust faults

The mechanics of thrust fault formation in rectangular and wedge-shaped, homogeneous thrust blocks are examined using finite-element techniques incorporating elasto-plastic material behaviour. Compression is simulated by applying an incremental end displacement parallel to the top of the horizontal or inclined basement. Shear zones (or faults) can form where the material experiences plastic strain softening. These models simulate that thrusts initiate in sequences that are “pushed from behind”, by some tectonic force, over a decollement horizon. In all models, the formation of thrusts is preceded by the formation of a basal slip plane near the base of the block or in a weak substratum. The front of this slip plane does not control the location of thrusts formed subsequently. In the rectangular block, a thrust forms at the pushed end. In the tapered wedge, drained as well as undrained, a fault system composed of a thrust and a backthrust forms. It is argued that the geometry of the block is the main controlling feature for the formation of thrusts within the tapered wedge. The shape of reverse faults depends on the magnitude of shear stresses acting along the basal slip plane. The higher the shear stress, the more pronounced the curvature. In the drained tapered wedge, the intersection of the backthrust with the basal slip plane appears to control the location of the thrust. Under undrained conditions, the backthrust forms subsequent to the thrust and acts only as an accommodation feature.

Flank tectonics of Martian volcanoes

On the flanks of Olympus Mons is a series of terraces, concentrically distributed around the caldera. Their morphology and location suggest that they could be thrust faults caused by compressional failure of the cone. In an attempt to understand the mechanism of faulting and the possible influences of the interior structure of Olympus Mons, we have constructed a numerical model for elastic stresses within a Martian volcano. In the absence of internal pressurization, the middle slopes of the cone are subjected to compressional stress, appropriate to the formation of thrust faults. These stresses for Olympus Mons are ∼250 MPa. If a vacant magma chamber is contained within the cone, the region of maximum compressional stress is extended toward the base of the cone. If the magma chamber is pressurized, extensional stresses occur at the summit and on the upper slopes of the cone. For a filled but unpressurized magma chamber, the observed positions of the faults agree well with the calculated region of high compressional stress. Three other volcanoes on Mars, Ascraeus Mons, Arsia Mons, and Pavonis Mons, possess similar terraces. Extending our analysis to other Martian volcanoes, we find that only these three and Olympus Mons have flank stresses that exceed the compressional failure strength of basalt, lending support to the view that the terraces on all four are thrust faults.

The effect of carbonate cementation of underthrust sediments on deformation styles during underplating

Fluids expelled during deformation at convergent margins lead to a variety of diagenetic reactions, including carbonate cementation, which alter the rheology of the deforming sediment. The volume of carbonate cement in shallow sediments at modern convergent margins is small, but sandstones in an ancient accretionary complex in Alaska contain significant amounts of carbonate cement that formed before underplating occurred. Other than vein calcite, carbonates are early calcite replacing framework grains or filling pore spaces, followed by ankerite that occurs mainly as pore‐filling cement. Amounts range from 2–16 weight percent in typical samples, averaging about 9 weight percent. Mean porosity filled by carbonate in Kodiak sandstones corresponds to porosities expected at depths of 4–5 km in normal compacting sands underthrust at modem convergent margins. The temperatures of formation are difficult to determine precisely, but were probably between 100°C and 200°C. The bulk of cementation occurred before the formation of thrust faults, folds, and slaty cleavage related to underplating. Lithification by cementation changed the rheology of sandstones by (1) filling pore space and increasing peak strength, and (2) increasing cohesion so that paniculate flow was no longer a viable deformation mechanism. Lithification before significant deformation contributed to underplating of the Kodiak Formation as relatively coherent duplexes.