Roof Thrust Research Articles

Preliminary appraisal of deformation structures from Great Nicobar Island, Andaman-Sumatra arc system

The Andaman and Nicobar Group of Islands represent a topographic high of forearc sediments in an actively rising accretionary prism on the Andaman subduction margin. Preliminary observations of deformation structures recorded in tuff are reported from Great Nicobar Island. Active compression led to the formation of meso-scale deformation structures such as a schuppen zone with horse units; sole and roof thrusts; fault breccia zone; high angle reverse faults and pop-up structures. Offset of tuff beds up to a few centimetres are observed across these faults.

Structure of the Assynt window, Moine Thrust Zone and relationship of thrusts to alkaline igneous complexes, Caledonian orogeny, NW Scotland

AbstractThe Moine Thrust Zone forms the Caledonian orogenic thrust front where the Moine Supergroup metamorphic rocks have been thrust westward across the Laurentia plate stable foreland, comprising Archean-Proterozoic granulite and amphibolite facies rocks (Lewisian gneisses), with unconformably overlying Mesoproterozoic and Neoproterozoic Torridonian clastic sediments and Cambrian-Ordovician passive margin sedimentary rocks. Four major thrusts beneath the Moine thrust in the Assynt window include the (i) Ben More Thrust, which places the Loch Ailsh syenite intruded into Lewisian basement and Cambrian-Ordovician sedimentary rocks over the Sole thrust sheet, (ii) Glencoul thrust, which places Lewisian basement and folded cover rocks over Cambrian-Ordovician sedimentary rocks, (iii) Borralan thrust, which carries a large alkaline syenite intrusion beneath the Ben More roof thrust and (iv) the Sole thrust sheet, which carries imbricated Cambrian-Ordovician sedimentary rocks and lamprophyre sills over the stable foreland. Three further thrust sheets within the Lewisian basement gneisses are now recognised through restoration of balanced cross-sections, which were responsible for doming of the Assynt window. Although the Moine thrust is mapped as a single line on the map it encompasses, (a) deep ductile shear zone formed of mylonites derived from hangingwall Moine schists, footwall Cambrian quartzites and Ordovician limestones, and basement Lewisian gneisses, (b) roof thrust of the Glencoul and Ben More Thrust sheets and (c) brittle out-of-sequence motion where the Moine schists have been thrust over mylonites, which directly overlie the stable foreland (Knockan Crag).

Structural styles of the Tierra del Fuego fold–thrust belt foothills, Argentina

Regional structural cross-sections based on seismic, borehole and surface data portray the foothills geometry of the Tierra del Fuego fold–thrust belt. Andean contraction was accommodated by two main deformation mechanisms: (1) a lower duplex thrust system that involves mainly Cretaceous deposits, with floor and roof thrusts at the base of the Cretaceous and close to the base of the Cenozoic, respectively; and (2) a series of detachment and faulted detachment anticlines in the Cenozoic, detached in mudstones above the roof thrust of the underlying duplex thrust system. Less common structures include fault-bend and fault-propagation folds. Basement fault reactivation is only locally important, with most Jurassic graben and half-graben preserved without inversion. Shortening along the foothills is modest, with values ranging from ∼8 to 4.6 km. This region of the fold–thrust belt has hydrocarbon exploration interest, but is still largely underexplored. Several large anticlines preserving Cenozoic clastic rocks have not yet been tested. The key risk for these structures is the charge because the thick mudstone package forming their detachment isolates potential Cenozoic reservoirs from the Lower Cretaceous source rocks. Duplex thrust systems at depth may constitute a potential exploration play if the brittle intervals are naturally fractured.

Testing erosional and kinematic drivers of exhumation in the central Himalaya

This study investigates the influence of fault geometry, kinematics, and displacement on the exhumation history of the central Himalaya using geologic mapping constraints, a new interpreted cross-section, and a suite of 176 thermochronometer ages through the Marsyangdi valley in central Nepal. Guided by the cross-section, we integrate a forward model of fold-thrust belt evolution with a 2D thermokinematic model. Model-predicted and measured thermochronometer ages were compared to evaluate the sensitivity of thermochronometer ages to the geometry and location of structures, and their rates of deformation. Results indicate 84% of the measured data can be reproduced with a largely in-sequence system of faulting where displacement occurs on the Main Central thrust (MCT) from 23-16 Ma, the Ramgarh-Munsiari thrust (RMT) from 16-7.5 Ma, the Trishuli thrust (TT) from 7.5-6 Ma and the Main Boundary thrust (MBT) from 6-3 Ma. Our cross-section solution shows the development of a duplex that initiates at 4 Ma with the TT and MBT as the roof thrust. The duplex is translated over the Main Himalayan thrust (MHT) ramp, concurrent with forward propagation of faulting in the synorogenic Siwaliks from 3 Ma to present. Notably, the 2-0.5 Ma apatite fission-track ages between the MCT and Tethyan strata 45 km north of the MCT are not reproduced by the wide range of in-sequence deformation scenarios we explored. Instead, these data are consistent with simulations that include significant displacement on two out-of-sequence (OOS) faults in the last ∼1 Ma: 10 km of OOS faulting near the MCT followed by 5 km of displacement on a fault 15 km south of the MCT. Modeled OOS thrusting both builds significant topography in the hinterland and flexurally suppresses topography in the foreland. Consequently, our preferred kinematic solution has ∼10 km of final fault motion in the Siwaliks to rebuild topography at the MBT and the active MFT from ∼0.5 Ma to the present.Modeled exhumation rates are highly variable through time, and are highest during translation of rocks over high (∼10 km) ramps and during significant changes in architecture, such as duplex formation and OOS faulting. Notably high exhumation rates include 10.5 mm/yr during the first 4 Myr of MCT motion, 4-6 mm/yr at ∼7 Ma, due to the translation over an ∼10 km high TT ramp, and 7-12 mm/yr from 4 Ma to present driven by the development of the duplex and its translation over the MHT ramp, followed by OOS thrusting. The high (7-12 mm/yr) exhumation rates documented here from 4 Ma, correlate with a time of distinct climate change, including strengthening of the monsoon, and northern hemisphere glaciation, lending support to potential climate-tectonic feedbacks.

Penetrative Strain and Partitioning of Convergence‐Related Shallow Crustal Shortening, Across Scales, in the Lesser‐ and Sub‐Himalayan Thrusts: Insights From the Eastern Himalaya, Sikkim

AbstractShortening estimates from regional balanced cross‐sections remain lower than the predicted shortening from paleomagnetism and area‐balancing methods in the Himalaya. To reconcile this shortening deficit, we evaluate the shortening contribution from the often‐ignored penetrative strain of quartz grains and integrate these results with two published regional balanced cross‐sections from Sikkim Himalaya. Penetrative strain progressively decreases from the internal to the external thrusts with changing deformation conditions. Strain increases toward the basal thrust zone within each thrust indicating strain localization. Strain ellipsoids are folded along with the thrusts indicating the strain initiated before thrusting and folding. The layer‐normal shortening (LNS) strain dominates the internal thrusts. A combination of LNS and layer‐parallel shortening (LPS) strain exist in the low‐strained external thrust sheets. Approximately 78–91 km of shortening is recorded as penetrative strain in the Lesser‐ and Sub‐Himalayan thrusts. Lateral structural variations in the thrusts are reflected in the thrust‐sheet scale shortening and in the strain ellipsoid orientations, but not in the grain‐scale shortening. Shortening is partitioned differently across scales; the duplex records the highest thrust‐sheet scale shortening, while the roof thrust toward the hinterland records the highest grain‐scale shortening. Due to the dominance of LNS strain, removing strain from thrusts will collectively decrease ∼54–59 km of base lengths, thereby reducing the total minimum shortening of the Sikkim Himalaya by ∼13%. Therefore, the penetrative strain does not contribute to the shortening deficit in the Sikkim Himalaya.

Several types of triangle zones from the Subandean ranges of Peru: Fish-tails, tectonic wedges and passive-roof duplexes

Surface geology, well data and 2D seismic interpretations reveal the presence of triangle zone structures at the contact of the Subandean Ranges and the Amazonian foreland in Peru. From north to south the Santiago, Marañon, Ucayali and Madre de Dios basins have been studied. Interpretations presented in this study show a great variety of structural styles at this contact area. Simple triangle zones with one back thrust, composite ones with several back thrusts, more complex double vergent ones with pop-up structures, best described as fish-tails, tectonic wedges and passive-roof duplexes are present in the study area. The role of pre-Andean tectonics and paleogeography is key to understanding the Andean evolution, due to the control on the location and geometry of the main detachment levels. The Santiago Basin is characterized by the presence of fish-tail structures with a Permian-Triassic evaporitic basal detachment located at the base of the Pucara Gr. At the Ucayali Basin, several detachment levels interact, the basal ones located within the Ordovician, Devonian, and Carboniferous defining triangle zones or tectonic wedges. At the Madre de Dios Basin, two detachment levels define the base of the triangle zone, Permian shales in the northwest at the Inambari Imbricates area, and Lower Devonian (Cabanillas Gr.) in the southeast at the Candamo area. A roof thrust is located within the Paleogene section at both the Madre de Dios and Ucayali basins, defining passive-roof duplex structures.Several detachment levels are rooted into the Lower Paleozoic; others within the Permian-Triassic section, being locally evaporitic, and finally others are located at the base of the Cretaceous. Paleozoic shales represent the main basal detachment level of the southern Subandean ranges. The role of these detachments and the role of the sedimentation rate are discussed as factors that control the development and geometry of the triangle zones described in the study area.Except for the San Martin-Sagari area of the Ucayali Basin, the triangle zones of the Santiago and Madre de Dios basins remain unexplored even though active petroleum systems are present. Sub-thrust structures at the Azulmayo area of the Madre de Dios Basin remain promising undrilled structures.

The India‐Eurasia convergence system: Late Oligocene to early Miocene passive roof thrusting driven by deep‐rooted duplex stacking

• The India-Eurasia convergence zone hosts a set of post-Eocene tectonic events. • Enhanced duplex stacking from continuous indian underthrusting. • Passive roof thrust system promoted the tectonic exchange. The Late Oligocene-Early Miocene epoch represents a critical stage in the collisional history of the India-Eurasia dominant collision zone. Within this timeframe, a set of tectonic events occurred simultaneously or progressively. However, the key responsible for triggering these nearly coeval events remains unclear. This study used an integrated analysis of both geological and geophysical data to document the tectonic interactions throughout the Himalayan Orogenic Belt from west to east. Deep seismic reflection profiles outline crustal geometry of a decoupled Indian subduction front, and the overlying sheets over the Main Himalayan Thrust (MHT) is evidenced with a sequence of detachment-associated ramp-anticlines. Meanwhile, regional magnetotelluric (MT) profiles document rheologic connections in the form of a high-conductivity anomaly running top-to-the south between the southernmost Tibet and the areas beyond the Yarlung-Zangbo Suture Zone to the south. The overall architecture provides a complete picture of the complex deformation pattern beneath the tectonic convergence system. Together with previous studies in surface geological investigations, we propose that the enhanced duplex stacking of the underthrusting Indian crust increased crustal shortening of the convergence system. The consequent sudden exposure of the northern Himalayan domes released the accumulated stress to trigger the onset of a south-dipping passive roof thrusting through the convergence system to the southernmost Tibetan Plateau. Recognition of this exchange pattern from crustal duplex stacking to passive roof thrusting replenished an understanding of the tectonic interactions of the ongoing India-Eurasia collision.

Structural styles of the Tellian fold-and-thrust belt of Tunisia based on structural transects: Insights on the subsurface oil and gas pre-salt plays

We present several structural transects throughout the internal part of the Tunisian Tell focusing on some representative structures and domains. We used also an interpreted seismic cross-section and a 3-D structural view to highlight the styles within the Tellian units in the Fernana-Adissa allochthon domain. The structural style is mainly represented by active roof-thrust and duplex structures affecting the Mesozoic-Cenozoic sequences belonging respectively to the infra-Kasseb and Kasseb units. The main decollement thrusting layer is located in the Triassic evaporites, while the roof thrust corresponds to the Upper Cretaceous or to the Paleocene shaly levels. The structural transects allow viewing that the Numidian Flysch unit presents different structural configurations and relationships with its subsurface supporting units; (1) it rests on an apparent stratigraphic contact above the underlying Kasseb nappe or even over pre-existing Triassic salt structures while elsewhere (2) it is detached above these infra-Numidian units reworking an originally downlap contact. The area is also characterized by important halokinetic movements controlling the overall architecture of the Tell fold-and-thrust belt that allows the thrusting of the whole Tellian wedge. In addition, originally apical parts of former diapiric structures were remobilized subsequently as sole thrust allowing the displacement of the Numidian Flysch. The Upper Miocene syn-folding sequences sourced by the erosion of the precedent uplifted reliefs are folded and tectonically transported along some active thrusts by the Tell tectonic units suggesting a recent reactivation and rejuvenation of thrust during the Upper Miocene to Quaternary Alpine events. Since the Late Miocene, shortening is still active today, allowing the refolding of older nappes contacts as well as rejuvenation of inherited faults.

Structural and thermal evolution of the eastern Aar Massif: insights from structural field work and Raman thermometry

The thermo-kinematic evolution of the eastern Aar Massif, Swiss Alps, was investigated using peak temperature data estimated from Raman spectroscopy of carbonaceous material and detailed field analyses. New and compiled temperature-time constraints along the deformed and exhumed basement-cover contact allow us to (i) establish the timing of metamorphism and deformation, (ii) track long-term horizontal and vertical orogenic movements and (iii) assess the influence of temperature and structural inheritance on the kinematic evolution. We present a new shear zone map, structural cross sections and a step-wise retrodeformation. From text{ca.;26,Ma} onwards, basement-involved deformation started with the formation of relatively discrete NNW-directed thrusts. Peak metamorphic isograds are weakly deformed by these thrusts, suggesting that they initiated before or during the metamorphic peak under ongoing burial in the footwall to the basal Helvetic roof thrust. Subsequent peak- to post-metamorphic deformation was dominated by steep, mostly NNW-vergent reverse faults (text{ca.} 22–14 Ma). Field investigations demonstrate that these shear zones were steeper than 50^{circ} already at inception. This produced the massif-internal structural relief and was associated with large vertical displacements (7 km shortening vs. up to 11 km exhumation). From 14 Ma onwards, the eastern Aar massif exhumed “en bloc” (i.e., without significant differential massif-internal exhumation) in the hanging wall of frontal thrusts, which is consistent with the transition to strike-slip dominated deformation observed within the massif. Our results indicate 13 km shortening and 9 km exhumation between 14 Ma and present. Inherited normal faults were not significantly reactivated. Instead, new thrusts/reverse faults developed in the basement below syn-rift basins, and can be traced into overturned fold limbs in the overlying sediment, producing tight synclines and broad anticlines along the basement-cover contact. The sediments were not detached from their crystalline substratum and formed disharmonic folds. Our results highlight decreasing rheological contrasts between (i) relatively strong basement and (ii) relatively weak cover units and inherited faults at higher temperature conditions. Both the timing of basement-involved deformation and the structural style (shear zone dip) appear to be controlled by evolving temperature conditions.

Quantifying shortening across the central Appalachian fold-thrust belt, Virginia and West Virginia, USA: Reconciling grain-, outcrop-, and map-scale shortening

AbstractWe present a kinematic model for the evolution of the central Appalachian fold-thrust belt (eastern United States) along a transect through the western flank of the Pennsylvania salient. New map and strain data are used to construct a balanced geologic cross section spanning 274 km from the western Great Valley of Virginia northwest across the Burning Spring anticline to the undeformed foreland of the Appalachian Plateau of West Virginia. Forty (40) oriented samples and measurements of >300 joint orientations were collected from the Appalachian Plateau and Valley and Ridge province for grain-scale bulk finite strain analysis and paleo-stress reconstruction, respectively. The central Appalachian fold-thrust belt is characterized by a passive-roof duplex, and as such, the total shortening accommodated by the sequence above the roof thrust must equal the shortening accommodated within duplexes. Earlier attempts at balancing geologic cross sections through the central Appalachians have relied upon unquantified layer-parallel shortening (LPS) to reconcile the discrepancy in restored line lengths of the imbricated carbonate sequence and mainly folded cover strata. Independent measurement of grain-scale bulk finite strain on 40 oriented samples obtained along the transect yield a transect-wide average of 10% LPS with province-wide mean values of 12% and 9% LPS for the Appalachian Plateau and Valley and Ridge, respectively. These values are used to evaluate a balanced cross section, which shows a total shortening of 56 km (18%). Measured magnitudes of LPS are highly variable, as high as 17% in the Valley and Ridge and 23% on the Appalachian Plateau. In the Valley and Ridge province, the structures that accommodate shortening vary through the stratigraphic package. In the lower Paleozoic carbonate sequences, shortening is accommodated by fault repetition (duplexing) of stratigraphic layers. In the interval between the duplex (which repeats Cambrian through Upper Ordovician strata) and Middle Devonian and younger (Permian) strata that shortened through folding and LPS, there is a zone that is both folded and faulted. Across the Appalachian Plateau, slip is transferred from the Valley and Ridge passive-roof duplex to the Appalachian Plateau along the Wills Mountain thrust. This shortening is accommodated through faulting of Upper Ordovician to Lower Devonian strata and LPS and folding within the overlying Middle Devonian through Permian rocks. The significant difference between LPS strain (10%–12%) and cross section shortening estimates (18% shortening) highlights that shortening from major subsurface faults within the central Appalachians of West Virginia is not easily linked to shortening in surface folds. Depending on length scale over which the variability in LPS can be applied, LPS can accommodate 50% to 90% of the observed shortening; other mechanisms, such as outcrop-scale shortening, are required to balance the proposed model.

Role of the Cretaceous normal faults on the formation of the Eocene (Pontide) fold-thrust belt structures in offshore Akcakoca-Amasra area, Western Black Sea basin, Turkey

The Western Black Sea basin formed during therifting of the Moesian Platform in Early Cretaceous. The closure of the Neotethys Ocean in the Middle Eocene resulted in the formation of the Pontide fold and thrust belt in northern Turkey. During this study, eight seismic reflection profiles were interpreted to determine the subsurface structural geometry and tectono-stratigraphic evolution of the offshore Akçakoca-Amasra area. The stratigraphy of the study area is determined based on a composite wireline well log of the Akçakoca-1 wildcat well, which was also used to construct a velocity model based onsonic data. We suggest that a major décollement surface was developed during the Eocene Pontide Orogeny. The decollement is located at the limestone clay-shale intraformational transition within the Late Cretaceous (Maastrichtian) - Paleocene Akveren Formation. Normal faults formed during the Cretaceous rifting in the region are located below the décollement surface. They provide tectonic ramps along the décollement surface and allow the decollement to develop ramp-flat thrust fault geometry. A well-developed duplex structure is also present along the seismic lines. The décollement surface serves as the floor thrust of the duplex structure. The roof thrust of the duplex is in the Pliocene Sarıkum formation, dominantly composed of claystone.

Tracking progressive deformation of an orogenic wedge through two successive internal thrusts: Insights from structure, deformation profile, strain, and vorticity of the Main Central thrust (MCT) and the Pelling-Munsiari thrust (PT), Sikkim Himalayan fold thrust belt

We compare and contrast the structural evolution of two successive internal thrusts, the Main Central thrust (MCT) and the Pelling-Munsiari thrust (PT), from the crystalline core of the Sikkim Himalaya in the context of progressive deformation involving a footwall Lesser Himalayan duplex; the PT is its roof thrust. The mylonitic foliations in both the shear zones are overprinted by successive cleavages. The youngest cleavage from the PT zone continues within the hangingwall MCT sheet that records one additional cleavage possibly indicating time-transgressive cleavage development. The cleavages become steeper while their intensity decreases structurally higher up within individual thrust sheets. The Rs-values and angular shear strain decrease from the mylonite zones to structurally higher up within the sheets. Both the shear zones are Type II with decelerating strain paths. Rs-θ′ relationships and microstructures indicate thrust-parallel stretch is greater than thrust-perpendicular component. Both the shear zones record strain partitioning with lesser competent mylonite domains recording higher Rxz, and a greater proportion of simple-shear than the protomylonite domains. The shear zones recorded the growth of the duplex during progressive deformation that contributed to pure-shear dominated general-shear, higher flattening strain and greater translation on its roof thrust, the PT, than the overlying MCT.

Structural interpretation of frontal folds and hydrocarbon exploration, western sulaiman fold belt, Pakistan

Abstract Seismic reflection profiles and borehole data combined with geological mapping was used to analyze foreland structures with 3-D structural variation, estimates of shortening, and kinematic evolution of western Sulaiman fold belt (SFB), Pakistan. The western SFB is interpreted with passive-roof duplex geometry. The floor thrust (decollement) is interpreted in Triassic strata rather than its inference in Precambrian-Cambrian evaporites. Whereas, the roof thrust (upper detachment) of hinterland vergence is located in Eocene Ghazij shale. Based on seismic data, the folds in the frontal duplex are interpreted as fault-propagation and fault-bend type. They are located over curved and staircase geometry of blind back thrusts. The folds-and-thrusts of EW trend show right-hand en-echelon stepping along NW-SE oriented mountain front due to drag along the western margin of the fold belt. The blind thrusts cut across the hanging-wall flat of the duplex horse with roots in base Triassic at depth of ~4–4.5 km 3D surfaces of reservoir strata show three areas of closed elliptical contours as potential prospects over blind thrusts, with maximum structural relief of ~2200 m. Shortening of 2100 (22%) and 9950 m (47%) is calculated along with thrusts in the frontal duplex with the interpretation of out-of-sequence structures. Passive roof duplex structures between foreland and hinterland vergent back-thrusts are reported from mountain belts around the world. We have identified out-of-sequence deformation in the frontal duplex with stacked structures. Identification of these features adds to our understanding of the development of mountain fronts in the context of Anderson's theory of faulting with the presence of a conjugate set of faults and foreland propagation of mountain fronts with secondary out-of-sequence deformation. Subsurface seismic data appears to be critical in their recognition for analysis of active foreland deformation and assessment of hydrocarbon plays in the fold belts.

Growth of triangle zone fold-thrusts within the NW Borneo deep-water fold belt, offshore Sabah, southern South China Sea

AbstractThe NW Borneo deep-water fold-and-thrust belt, offshore Sabah, southern South China Sea, contains a structurally complex region of three to four seafloor ridges outboard of the shelf-slope break. Previous studies have suggested the seafloor ridges formed either above shale diapirs produced by mass movement of overpressured shales (i.e., mobile shale) or above an imbricate fold-and-thrust array. Here, we performed tectonostratigraphic analyses on a petroleum industry three-dimensional (3-D) seismic volume that imaged the full growth stratal record. We show fold growth history, deformation styles, along-strike structural variabilities, and synkinematic sedimentation during triangle zone–style fold growth. Nine seismic horizons within growth strata were mapped and correlated to petroleum industry seismostratigraphy. Synkinematic sedimentation interactions with growing folds and near-surface strains were analyzed from seismic attribute maps. We interpret that the seafloor structures were formed by imbricate thrusts above multiple detachments. We estimate ∼8 km minimum shortening since the late Miocene ca. 10 Ma. The folds show oversteepened fold forelimbs, back-rotated backlimbs, and forward-vergent (NW to NNW) “blind” thrust ramps that terminate within the growth strata. Fold cores show evidence of internal shear. Immature folds show detachment fold geometries, whereas mature folds show forelimb break thrusts, type I triangle zones, and rotated forward-vergent roof thrusts. Thrust linkages spaced ∼10 km apart were exploited as thrust top synkinematic sedimentation pathways; the linkages also partition near-surface strains. Our comprehensive, three-dimensional documentation of triangle zone fold growth and sedimentation in a deep-water fold belt highlights internal shear, multiple detachments, and opposite thrust vergence; mobile shales are not required to explain the deformation.

K‐Ar Dating of Fossil Seismogenic Thrusts in the Shimanto Accretionary Complex, Southwest Japan

AbstractK‐Ar ages of clay‐sized mineral grains are used to determine the timing of activity on fossil seismogenic faults within the Cretaceous‐Paleogene Shimanto accretionary complex, southwest Japan. Samples were collected from three regional faults that separate hanging wall coherent rocks from footwall subduction mélange: the Goshikinohama Fault that caps the Yokonami mélange, the roof thrust of the Okitsu mélange, and the Nobeoka Thrust that caps the Hyuga mélange. The K‐Ar ages of fault rocks decrease with decreasing 2M1 illite polytype component, indicating a mixture of 1Md and 2M1 illite polytypes. Based on illite dating analysis, the extrapolated ages of the pure 2M1 illite polytype from the Goshikinohama Fault, the roof thrust of the Okitsu mélange, and the Nobeoka Thrust are 79.3 ± 5.0, 66.1 ± 8.1, and 46.7 ± 8.2 Ma, respectively, similar to the depositional age of each host rock. Lower intercepts of regression lines, which correspond to samples containing 100% authigenic illite, are calculated as 50.7 ± 1.4, 18.4 ± 1.2, and 24.4 ± 1.4 Ma, respectively. These ages are significantly younger than both the depositional ages and the timing of accretion. These results indicate that authigenic illite associated with fault slip is not related to underthrusting along the subduction interface but rather formed during out‐of‐sequence thrusting in the upper plate. Early Miocene slip along faults of the northern Shimanto belt is coincident with major tectonic events along the convergent margin, including collision with elements of the Izu‐Bonin volcanic arc‐backarc system, and opening of the Japan Sea.

ONE-SIDED rocking analysis of corner mechanisms in masonry structures: Influence of geometry, energy dissipation, boundary conditions

The corner mechanism in masonry structures is one of the out-of-plane modes that may frequently occur under dynamic actions such as earthquakes. The three dimensional motion, in principle complex to treat, can be simplified into a two-dimensional problem, where a prismatic equivalent block is associated to the corner mechanism. This paper provides a method to treat the corner mechanism in two dimensional rocking analysis, taking into account the roof actions – especially the roof thrust that acts as destabilizing force in the preliminary phases of motion – and the boundary conditions such as the transverse walls. A case study is taken as benchmark to perform rocking non-linear analyses and discuss the role of geometry, energy dissipation and boundary conditions. It is shown the relevant influence of the geometry and of the coefficient of restitution on the stability conditions, whenever the oscillation produce horizontal displacement values of some cm. The results of the case study, subjected to the Central Italy earthquake, are compared to the actual response of the corner mechanism, which collapsed during this seismic swarm, showing that the rocking analysis on the equivalent block correctly predicts the collapse occurred.

Polyphase out-of-sequence thrusting and occurrence of marble detritus within the wedge-top basin deposits in the Mt. Massico (southern Apennines): insights into the late Miocene tectonic evolution of the central Mediterranean

We present a geological survey of the southwestern sector of the Mt. Massico (southern Apennines), which allowed us to reconstruct the stratigraphy and tectonic architecture of this area. The Mt. Massico is a key area to study the out-of-sequence thrusting stage that affected the southern Apennine chain since the late Miocene, which despite the wide occurrence of these structures in the whole chain, is a tectonic process up to now poorly investigated. This area deserves to be analyzed also for the occurrence of marble detritus within a wedge-top basin deposit, representing the only place in the southern Apennines, where such a kind of metamorphic rocks occurs. Historically, this area is known for the extraction of an ornamental stone, named Mondragone Marble. The marble is hosted as olistoliths and clasts within the deposits of the Caiazzo Fm. unconformably covering a Jurassic–middle Miocene succession, mostly made of shallow water-to-deep-basin carbonates. The wedge-top basin deposit mainly consists of a chaotic assemblage of conglomerates, olistostromes, and olistoliths of Meso-Cenozoic limestones, deep-basin rocks, and marbles, embedded in a quartzose matrix. The structural setting results from a polyphase deformation related to the superposition of two thrust systems. The first thrust system includes a duplex verging to east, with the Cenozoic carbonates thrusted onto the Caiazzo Fm. and the roof thrust cut by several breaching thrusts. The late thrust system encompasses ramp-dominated faults verging to the north. Folds, minor thrusts, and S–C structures are associated with the major compressional structures. To date the Caiazzo Fm. and put a temporal constraint to the thrust fault activity, we performed a nannoplankton content analysis that furnished, for the base of this deposit, an age not-older than the upper Tortonian. These out-of-sequence thrusts hence have acted since the late Miocene. Similar structures are widespread in the southern Apennines and are interpreted as the surficial expression of envelopment thrusts formed at deeper structural levels with respect to the thrust front. We envisage that the marble and plutonic supply was provided by the Mesomediterranean continental crust in the late Tortonian when this microcontinent was passing close to the Apennine Platform domain during its orogenic migration toward SE.

Structural variation within the Himalayan fold and thrust belt: A case study from the Kohat-Potwar Fold Thrust Belt of Pakistan

The Kohat and Potwar fold thrust belts (KP-FTB) in Pakistan exhibit structural variations over 250 km along strike within the Himalayan fold and thrust system. Our 3D deformation model shows that Kohat surface structures evolved above an active roof thrust in Eocene evaporites. The ramp-forming duplexes in the Kohat were stacked and passively transported toward the foreland above new ramps, resulting in up to 5 km of thickening between the two decollements. Ramps from the Kohat extend into the Potwar as thrust tips of fault propagation folds. The basement slope changes from flat (β < 1°) below the northern part to north-dipping (β > 1°) below the southern part, corresponding to the change in structural style and complexity of the KP-FTB. The Kalabagh Fault Zone, linking the two belts, is interpreted as a zone of complex dextral strike-slip rotational faulting. Salt expulsed from the hanging walls of normal faults and under synclines in the Kalabagh Fault Zone moved toward the footwall of normal faults, accumulated in the cores of anticlines, and formed lobe structures at the deformation front. The fundamental reasons for the variable structural styles are changes in decollement strength, basement slope, preexisting normal faulting, presence of a secondary decollement and spatially-variable salt mobility and accumulation.

Fold duplexes

Fault duplexes were recognized as early as the late nineteenth century and have been identified in most fold - thrust belts. Duplexes appear to provide a mechanical advantage to the emplacement of large thrust sheets by transferring displacement upward from one detachment horizon to another. However, faulting is not the only mechanism for displacement transfer; pressure-solution cleavage and folding may play the same role. Folds or cleavage within a duplex absorb the shortening of the underlying detachment, so that the displacement on the floor thrust decreases toward the foreland. As displacement on the floor decreases, displacement on the upper detachment (roof thrust) increases. In fold duplexes, the geometries of the detachment folds vary from open, concentric and kink-band folds to vertical and recumbent isoclines. A range of duplex styles, from fault duplexes to cleavage and fold duplexes, can be found within a single a single thrust belt, and this variation raises several important questions to guide future study. They include what controls the geometry of the folds within fold duplexes; and, perhaps more importantly, what conditions (lithology, pressure, temperature, strain rate) determine the type of duplex (fault, cleavage or fold) that will develop.

Duplex and along-strike structural variation: A case study from Sikkim Himalayan fold thrust belt

The Sikkim Lesser Himalayan fold thrust belt records along-strike structural variations over ∼15 km. The lower Lesser Himalayan (Rangit) duplex (LHD) is blind, and the overlying Pelling and Ramgarh thrust sheets are more intensely folded and preserved in eastern Sikkim than in western Sikkim. A retrodeformable, regional balanced cross-section reveals a minimum shortening of ∼403 km (∼80%) with an average long-term shortening rate of ∼18 mm/yr in the eastern transect. Comparison with published data indicates that eastern Sikkim records ∼47 km lesser shortening. In western Sikkim, an ∼2 km high footwall Main Himalayan thrust (MHT) ramp below the Rangit duplex caused higher structural culmination exposing the duplex. In contrast, an ∼0.35 km high ramp below the Rangit duplex in eastern Sikkim did not create sufficient culmination to expose the duplex, causing along-strike discontinuous duplex exposure. The LHD with a blind component, and having fewer (11) but gentler dipping (∼40°) horses transferred more displacement to the roof thrust in eastern Sikkim than the twelve, steeper dipping (∼60°) horses in western Sikkim. Lateral variations in location and height of footwall MHT ramp, initial width of the Lesser Himalayan basin, and presence of a lateral ramp explain the structural variation in Sikkim.