Thrust System Research Articles

A hybrid thrusting system for increasing the endurance time of multirotor unmanned aerial vehicles

One of the most significant disadvantages of electric multirotor unmanned aerial vehicles is their short flight time compared to fuel-powered unmanned aerial vehicles. This is mainly due to the low energy density of electric batteries. Fuel has much more energy density when compared to batteries. Electric-powered motors in multirotor unmanned aerial vehicles cannot be replaced with fuel-based engines because the stability and control of multirotor unmanned aerial vehicles rely on the high response rates of electric motors. One of the possible solutions to overcome this problem of short endurance times is by using hybrid thrusting systems that combine the advantages of both fuel and electrical propulsion systems, where high maneuverability and long endurance flight time could be achieved. In this work, hybrid thrusting and power systems for multirotor unmanned aerial vehicles are studied. Targeted hybrid thrusting systems consist of combustion engines, electric motors, and their power sources. Then a hybrid thrusting system-based quadrotor unmanned aerial vehicle model is developed. The article presents the altitude and attitude control systems of the developed hybrid thrusting system-based unmanned aerial vehicle. The presented hybrid quadcopter model comprises four electric motors and one fuel engine. The fuel engine used in this work is a 4.07 cc internal combustion engine targeting 2–3 kg unmanned aerial vehicles with up to 5 kg maximum takeoff weight. The developed hybrid quadrotor unmanned aerial vehicle achieved a 139% improvement in flight time when compared with traditional electric-based quadrotor unmanned aerial vehicles. The article also reports on other flight time-related issues such as the optimal fuel mass to battery size ratio to maximize the endurance time of the quadrotor unmanned aerial vehicles.

Formation of the Permo-Triassic Ondor Sum thrust system and its constraint on Solonker Ocean closure

ABSTRACT A previously unidentified overthrust system has recently been discovered in the Ondor Sum area at the boundary between the Solonker – Xar Moron Suture Zone (SXSZ) and the Early to Mid – Paleozoic Subduction – accretion Belt to the south. The overthrusting occurred during the middle Permian and was overprinted by late Permian to Middle Triassic dextral shearing. The overthrusting (D1) was characterized by the development of WSW – ENE-trending folds and reverse faults, implying NNW – SSE shortening. The later deformation (D2) involved WSW – ENE dextral strike-slip shearing with a minor reverse component, as indicated by slightly plunging striae on slickensides. A comparison of geochronological and structural data for ophiolites in the SXSZ suggests that the overthrust system formed in response to the closure of the Solonker Ocean and the collision along the SXSZ. The earlier NNW – SSE shortening and later WSW – ENE dextral shearing indicate that a tectonic transformation took place along the SXSZ from the middle Permian to the Late Triassic. Dextral and sinistral strike-slip faults are, respectively, distributed to the south and northwest of the Songliao – Xilinhot Block to the north of the SXSZ, implying eastward extrusion of the Songliao – Xilinhot Block after the closure of Solonker Ocean. The tectonic transformation and extrusion might have been a part of the intracontinental tectonic transformation of the eastern segment of the southern Central Asian Orogenic Belt.

A multi-scale approach to the recent activity of the Stradella thrust in the seismotectonic context of the Emilia Arc (northwestern Italy)

The frontal thrusts and folds of the northern Apennines - Italy - are mainly covered under the alluvial deposits of the Po Plain. Some of these structures show geological evidence of Late Quaternary activity, thus posing the need for an accurate seismic hazard assessment due to widespread housing settlements, industries, lifeline infrastructures, and large towns. We present new morphostructural, geophysical, and seismological data to discuss the recent activity of the Broni-Sarmato fault, an 18 km-long outcropping section of the north-verging Stradella thrust, located 50 km south of Milan, along the Pede-Apennine thrust front (PTF) in the rear of the Emilia Arc thrust system. The new geoelectrical surveys across the fault scarp show deformation of the shallow deposits. The outcropping deformations, with a fault scarp ranging up to 25.8 m, are investigated within the seismotectonic framework of the PTF and the Emilia Arc. The analysis of the associated seismicity and new focal mechanisms highlight two seismogenic contractional volumes dipping at low-angle southwest-ward, at upper (<12 km) and lower crustal depths (~20–30 km). The shallow seismicity partially illuminates the Stradella thrust and its along-strike southeastward prosecution along the extent of the Stradella-Salsomaggiore Arc. Subordinately, it also illuminates some of the Emilia Arc thrust planes. The deeper seismogenic volume shows large patches of the basal thrust of the Emilia Arc fault system. We interpret the above multi-scale data as evidence of ongoing tectonic activity of the outer fronts of the Emilia Arc under a regional NNE-oriented compressional stress field, with some evidence of thrust involvement along the Pede-Apennine front. In our 3D fault-model reconstruction, all the analyzed thrust structures appear as expressions of a thick-skinned deformation that controls earthquake release at different structural levels.

Proterozoic fold and thrust belt imaged on reflection seismic in Arabian plate basement

Major structures are imaged on 2D reflection seismic data within the Precambrian basement of the western Rub’ Al-Khali of Saudi Arabia for the first time. In this area the base-Cambrian Angudan Unconformity that defines top basement is at a depth of 1–2 km with a gentle easterly dip. Large antiformal structures are clear on 2D reflection seismic within the basement. The antiforms are separated by zones that are interpreted as low-angle faults. No basal detachment is seen and the faults pass below the base of reflection seismic at a depth of approximately 12 km. Individual folds have dimensions in the order of 20 km wavelength and 2 km amplitude and occur between 3 and 10 km depth. The structural style is interpreted as compressional by comparison with thrust systems and accretionary prisms elsewhere. Regional studies indicate that basement is comprised of a series of north-south trending oceanic Proterozoic terranes that were brought together in the Ediacaran East African Orogeny. Based on current regional tectonic maps the newly-imaged fold belt is on the east margin of the Khida-Abas terrane. Depth conversion shows the thrusts dip at approximately 10° north in the line of section. A novel combination of cross-section to structural trend obliquity estimates combined with matched filtering of potential field data indicate the fold and thrust belt trends 015°-195° with ESE vergence. This compressional belt may be some internal structure of the Khida-Abas terrane, or may be a non-outcropping accretionary prism on the margin of that terrane, or may even represent a set of amalgamated microterranes that do not outcrop along trend. Either way, these observations represent a major, previously undescribed element of this sector of the East African Orogen.

Surface rupture and landscape response in the middle of the great Mw 8.3 1934 earthquake mesoseismal area: Khutti Khola site

Large earthquakes breaking the frontal faults of the Himalayan thrust system produce surface ruptures, quickly altered due to the monsoon conditions. Therefore, the location and existence of the Mw8.3 1934 Bihar–Nepal surface ruptures remain vividly disputed. Even though, previous studies revealed remnants of this surface rupture at the western end of the devastated zone, ruptures extent remains undocumented in its central part. Evidence for recent earthquakes is revealed along the frontal thrust in this region. The Khutti Khola river cuts an 8 m-high fault scarp exposing Siwalik siltstone thrusted over recent alluvial deposits, with faults sealed by a colluvial wedge and undeformed alluvial sediments. Detrital charcoals radiocarbon dating reveals that the last event occurred between the seventeenth century and the post-bomb era, advocating for the 1934 earthquake as the most recent event. In the hanging wall, fluvial terraces associated with fault scarps were abandoned after a penultimate event that happened after the tenth century, a rupture we associate with the historic earthquake of 1255CE. Slips of 11–17 m and 14–22 m for the 1934 and 1255 earthquakes, respectively, compare well with the ~ 10–15 m slip deficit accumulated between the two earthquakes, suggesting that most of the deformation along the front is accommodated by surface-rupturing earthquakes.

Fracture Pattern Analysis of the Upper Cretaceous-Eocene Carbonates along with the Ghumawan Dome, Hazara Basin

Deformational history of the Hazara basin indicates a primitive collision of the two landmasses that undergoes an episodic deformation with NE-SW structural trend. Panjal Thrust (PT) and Main Boundary Thrust (MBT) demarcate the northern and southern extremities of the basin, respectively. The area bounded between these two thrusts is the core consideration of the present research. Different stratigraphic units juxtapose along the Hazara Kashmir Syntaxes (HKS), while the strike-slip component is indicated by imbrication due to thrusts. The study is amied to analyze the paleo-stresses along with developed fracture patterns. Field data were collected via Circle Inventory Method from various localities of the Ghumawan dome, Hazara basin. The zones of upper Cretaceous to Eocene carbonates were mainly targeted during the data collection. Win-Tensor was the key software that helps to analyze the paleo-stresses and fracture pattern of the study area. NW-trending fracture pattern was observed with a highly non-symmetric to dense fracture pattern. The local thrust system lead to severely de-shape the study area. N-S oriented σ1 indicated the compressional tectonic condition that prevailed during deformation of this area. Some segments also show extensional features i.e. normal faulting.

Salair–Gornaya Shoria Junction (Northwestern Central Asian Orogenic Belt): Deep Structure and Tectonics from Magnetotelluric Data

Abstract —The Salair fold-thrust orogenic belt (Salair orogen, Salair) is located in the northwestern Altai–Sayan fold area within the Central Asian Orogenic Belt. The Salair orogen is an allochthon overriding the Kuznetsk Basin on a system of imbricate thrusts. The southern flank of the Salair thrust system is tectonically juxtaposed against the Gornaya Shoria terrane which differs markedly from Salair in its geological setting. The Salair and Gornaya Shoria terranes are separated by the Nenya-Chumysh Basin, a deep Mesozoic trough. The Salair orogen is composed of Cambrian–Early Ordovician island arc volcanic and sedimentary rocks, widespread garnet amphibolites and gneisses of the Angurep complex in its southern flank, and the Shalap subduction-related melange in the Alambai ophiolite suture. The southern Salair orogen and its junction with Gornaya Shoria have been imaged down to the lower crust by magnetotelluric (MT) soundings, which is an efficient tool for investigating the deep structure and tectonic history of orogenic areas. The MT surveys were performed at 25 stations on a 120 km long profile. MT data revealed an up to 70 km wide low-resistivity zone (a conductor) traceable till a depth of 20 km between the Salair and Gornaya Shoria terranes. The low-resistivity zone has a complex structure with its outer and interior boundaries dipping almost vertically. The conductor lies under several major geological structures: the Shalap melange, the Nenya-Chumysh Basin, and the NE trending Altai–Salair right-lateral strike-slip fault. The Altai–Salair fault, along which the Salair allochthon was displaced relative to Gorny Altai and Gornaya Shoria, joins the Salair system of imbricate thrusts. The Nenya-Chumysh Basin at the Salair–Gornaya Shoria junction is a deep trough having an asymmetric transversal profile with a steep western side and a shallower-dipping stepped eastern side. The southeastern flank of the basin is a wide area of thin sediments over the Paleozoic basement dipping gently in the northwestern direction. The revealed deep structure of the Nenya-Chumysh trough is consistent with its tectonic model implying an Early Cretaceous basin superposed over an early Jurassic pull-apart basin. Early Mesozoic motions on major faults is a regional-scale phenomenon known from many areas of southern West Siberia.

The polyphase evolution of the mafic rocks of the Juiz de Fora Complex: The record of two supercontinent cycles

The Juiz de Fora Complex (JFC) in the Araçuaí-Ribeira orogenic system, southeastern Brazil, is a basement unit displaying a variety of rock types that is tectonically imbricated with the supracrustal rocks of the Raposos Group in the Upper Thrust System of the Occidental Terrane, Ribeira Belt. Here we report new petrological, geochemical, and zircon U–Pb and Lu–Hf isotopic data on mafic garnet-granulites from the JFC with a view to evaluate the timing and P-T conditions of the collisional event as well as protolith formation. This work presents the most comprehensive metamorphic study on the Juiz de Fora Complex rocks to date. Metamorphic P-T estimates for the metamorphic peak conditions of 800–900 °C and 8–9 kbar in basic garnet-bearing granulites were obtained for the JFC, indicating that metamorphism took place at deeper crustal levels. Geochemical, zircon U–Pb geochronology, whole-rock Sm–Nd, Rb–Sr isotopes, and Lu–Hf isotopic data of mafic granulites yielded U–Pb ages of ca. 2201 Ma, and moderately juvenile εNd(t) = −0.5 and were obtained from the JFC marking the timing of volcanic arc magmatism during the Rhyacian period. This arc-related stage was followed by the post-collisional period during the Early Paleoproterozoic with ca. 1.94 Ga E-MORB signature granulites that shows evidence of magma sourced from Neoarchean (∼2.8 Ga) crust with addition of juvenile components. The high-grade metamorphism is possibly related to the final collision of the Congo and São Francisco Cratons at 590–560 Ma, which placed the Neoproterozoic metasediments and the JFC rocks at the same level. The integrated information sheds light on two collisional episodes where the oldest is represented by arc-related granulites during the Rhyacian Orogeny and the youngest is the higher-pressure Neoproterozoic overprint during the Brasiliano Orogeny.

Structural geometry and kinematics of a strike-slip fault zone in an intracontinental thrust system: A case study of the No. 15 fault zone in the Fuling area, eastern Sichuan Basin, southwest China

Structural geometry and kinematics of a strike-slip fault zone in an intracontinental thrust system: A case study of the No. 15 fault zone in the Fuling area, eastern Sichuan Basin, southwest China

The effects of salt flow on the cross-section restoration of salt-bearing fold-and-thrust belts: An example from the Kuqa depression

Thrust sequence analysis using cross-section restoration is critical for understanding the evolution involved in folds and thrusts as well as kinematics, the distribution of strain, and hydrocarbon accumulation. However, the application of cross-section restoration is challenging in salt-bearing fold-and-thrust belts. Salt flow causes uncertainty and difficulty in quantifying the geometry because of salt mobility and poorly constrained deformation kinematics. In this paper, we first analyze the role of salt flow in the cross-section restoration of salt-bearing fold-and-thrust belts using a synthetic model. Then an applicable method has been systematically described to quantify the effect of salt flow on the cross-section restoration. The method is efficient in determining the sub-salt thrust sequence, whereas accurate fault throw can be restored when the history of salt flow is known. Two well-constrained seismic profiles in the western Kuqa salt-bearing fold-and-thrust belts are restored sequentially to analyze the evolutionary process of sub-salt faults. The results show that the sub-salt thrust system propagates forward in sequence in the Bozi segment and out of sequence in the Dabei segment. The differential evolution of the sub-salt thrust between the Bozi and Dabei segments forms an oblique transfer zone. This study shows that understanding salt flow is the key point for cross-section restoration in salt-bearing fold-and-thrust belts, and relationships between supra-salt deformations and sub-salt thrust can be built by clarifying the contribution of salt flow.

A contribution to the quantification of crustal shortening and kinematics of deformation across the Western Andes ( ∼ 20–22° S)

Abstract. The Andes are an emblematic active Cordilleran orogen. Mountain building in the Central Andes (∼20∘ S) started by the Late Cretaceous to early Cenozoic along the subduction margin and propagated eastward. In general, the structures sustaining the uplift of the western flank of the Andes are dismissed, and their contribution to mountain building remains poorly constrained. Here, we focus on two sites along the Western Andes at ∼20–22∘ S in the Atacama desert, where structures are well exposed. We combine mapping from high-resolution satellite images with field observations and numerical trishear forward modeling to provide quantitative constraints on the kinematic evolution of the investigated field sites. When upscaling our local field interpretations to the regional scale, we identify two main structures: (1) the Andean Basement Thrust, a west-vergent thrust system placing Andean Paleozoic basement over Mesozoic strata, and (2) a series of west-vergent thrusts pertaining to the West Andean Thrust System, deforming primarily Mesozoic units. From our interpreted sections, we estimate that both structures together accommodate at least ∼6–9 km of shortening across the sole investigated ∼7–17 km wide field sites. This multi-kilometric shortening represents only a fraction of the total shortening accommodated across the whole Western Andes. The timing of the main deformation recorded in the folded Mesozoic series can be bracketed between ∼68 and ∼29 Ma – and possibly between ∼68 and ∼44 Ma – from dated deformed geological layers, with a subsequent significant slowing-down of shortening rates. Even though the structures forming the Western Andes only absorbed a small fraction of the total shortening across the whole orogen, their contribution was relatively significant at the earliest stages of Andean mountain building before deformation proceeded eastward.

Fault and Fluid Interaction during the 2012 Emilia (Northern Italy) Seismic Sequence

AbstractThe triggering of large earthquakes by anthropic activities is a challenging issue in seismology, invoked also for the ML 5.9 and 5.8 Emilia 2012 destructive earthquakes. The interaction between the two earthquakes that propagated along adjacent thrusts is still an open issue. In this study, we used waveform cross correlation and double-difference (DD) location methods to precisely relocate the aftershock sequence and get insights into fault geometry, structure, and rheology by means of DD seismic tomography. Accurate relocations highlight a complex fault system with small-length fault segments coalescing in the Mirandola and Ferrara thrusts. We observe a broad continuous high-VP/VS anomaly at seismogenic depth (about 6.0 km) that suggests a possible hydraulic connection along the entire fault system. A close look at seismicity indicates a quasi-simultaneous activation of the entire thrust system, with the two mainshocks and large aftershocks occurring within the high-VP/VS, high fluid pressure, and connecting volume.

Rancang Bangun Alat Penyapu Lantai Semi Mekanik untuk Kampus Universitas Muhammadiyah (UM) Sumbar

Sweeping the floor is not a hard job, sweeping activities will be hard and take up a lot of time if the area to be worked on is very large like in UM-West Sumatra. To simplify and save time, it is necessary to innovate a floor sweeper. The push system floor sweeper is a form of innovation that facilitates the process of sweeping the floor. In this study a thrust system floor sweeper was designed and manufactured. The design process begins with identification of needs, concept design, design and analysis using the fabrication method, then this tool is tested in a room of 35.28 meters, 61.2 meters and 122.4 meters. From the trials conducted, it was found that the floor sweeper functions properly and can clean large and spacious rooms with 50% efficiency. Thus it can be concluded that a floor sweeper can simplify the process of sweeping the floor by 50% compared to an ordinary broom.

Large thrusting and late faulting shape the Aiguilles Rouges crystalline massif (Western Alps), structural implications

Although the Helvetic nappes of the Western Alps have been widely studied, their style, amount, and timing of deformation during the Alpine orogeny remain a subject of controversy. The discussion partly arises from various interpretations of the external crystalline massifs geometry and of the cover/basement relationships. Here, we apply structural analysis based on new field data, microtectonics, and 3D geological modeling on the Aiguilles Rouges and Mont-Blanc massifs, to better quantify their tectonic evolution and their relationship with the nappe stacking. Our results show large thrusting of the Helvetic Mesozoic sedimentary cover above the Aiguilles Rouges basement towards the WNW. The basal contact of the Helvetic nappes, which we call the Helvetic Basal thrust (HBT), often corresponds to a décollement level localized above the Triassic unconformity, but that locally also affects the basement. We suggest that most of the subalpine ranges are klippes above the HBT, and estimate the shortening of the thrust from previously published cross-sections to be between 13 and 32 km. According to published ages, we argue that the HBT was active in the Oligo-Miocene, between ∼30 and ∼14 Ma. After ∼14 Ma and until ∼6 Ma, the HBT is offset by late steep out-of-sequence faults, including back-thrusts and the Mont-Blanc shear zone that was previously a ramp of the HBT. The transition of the major thrust system from the HBT rooting above (east) of the Mont-Blanc to the Alpine sole thrust rooting below (west) of the Aiguilles Rouges occurred between 18 and 14 Ma.

Lithospheric double shear zone unveiled by microseismicity in a region of slow deformation

The deformation style of the continental lithosphere is a relevant issue for geodynamics and seismic hazard perspectives. Here we show the first evidence of two well-distinct low-angle and SW-dipping individual reverse shear zones of the Italian Outer Thrust System in Central Italy. One corresponds to the down-dip prosecution of the Adriatic Basal Thrust with its major splay and the other to a hidden independent structure, illuminated at a depth between 25 and 60 km, for an along-strike extent of ~ 150 km. Combining geological information with high-quality seismological data, we unveil this novel configuration and reconstruct a detailed 3D geometric and kinematic fault model of the compressional system, active at upper crust to upper mantle depths. In addition, we report evidence of coexisting deformation volumes undergoing well-distinguished stress fields at different lithospheric depths. These results provide fundamental constraints for a forthcoming discussion on the Apennine fold-and-thrust system's geodynamic context as a shallow subduction zone or an intra-continental lithosphere shear zone.

The Montello Thrust and the Active Mountain Front of the Eastern Southern Alps (Northeast Italy)

AbstractNew field and subsurface data are combined to define the geometry and evolution of the mountain front of the eastern Southern Alps. The Montello thrust (MT), the southernmost frontal structure, is reconstructed by recently published well‐located microseismicity and is connected at depth with the larger Bassano‐Valdobbiadene (BV) thrust. The latter started at 10–9 Ma, whereas the MT evolved at 8–6 Ma, as documented by growing unconformities in the foreland deposits and the nature of the natural gas in the anticline culmination. The latter, named Montello anticline, is growing from the upper Miocene to the Quaternary, as shown by surfaces of abrasion and deposition, including the Biadene windgap. The folded bottom of the windgap is a fan unit recently dated to the MIS 3. The MT is presently creeping and its deformation rates are much lower than previous estimates. Most of the late Miocene to Pleistocene deformation at the front of the eastern Southern Alps is accounted for the BV thrust. The reconstructed average shortening rates are in agreement with the geodetic velocity field from literature. The BV and MT are a unique active thrust system with variable slipping and locked patches, whose interactions should be further studied.

An unusually productive microearthquake sequence brings new insights to the buried active thrust system of Montello (Southeastern Alps, Northern Italy)

In August 2021, an extremely productive seismic sequence took place in northeastern Italy. Within 1 month, we identified and localized 407 microearthquakes in the area monitored by the local Collalto Seismic Network. This is about 20 times the average monthly rate of events detected in the last decade; 92% of them are clustered near the village of Refrontolo at about 9 km depth (main event a ML2.5, MW2.4). None of the Refrontolo sequence earthquakes were felt by the population. The earthquakes, mostly identified by automatic procedures, were post-processed by manual picking of P, S-phases and polarities; they were localized by different techniques and define a small SE dipping volume that departs from the brittle surface previously enlightened by microseisms and assigned to the NNW dipping Montello Thrust. We interpret the sequence ruptured pre-stressed patches near to failure of sub-vertical, antithetically oriented faults. The unusual productivity rate and precursory foreshock activity support the hypothesis that the Montello system has a relevant aseismic component. We believe that this episode, of no importance from an energetic point of view, is of greatest interest for deciphering the seismic potential of this area and for strengthening automatic microearthquake detection and location procedures.

Cenozoic kinematic histories of the Tidding and Lohit thrusts in the northern Indo-Burma Ranges: Implications for crustal thickening and exhumation of Gangdese lower arc crust along the Indus-Yarlung suture zone

Crustal thickening has been a key process of collision-induced Cenozoic deformation along the Indus-Yarlung suture zone, yet the timing, geometric relationships, and along-strike continuities of major thrusts, such as the Great Counter thrust and Gangdese thrust, remain inadequately understood. In this study, we present findings of geologic mapping and thermo- and geochronologic, geochemical, microstructural, and geothermobarometric analyses from the easternmost Indus-Yarlung suture zone exposed in the northern Indo-Burma Ranges. Specifically, we investigate the Lohit and Tidding thrust shear zones and their respective hanging wall rocks of the Lohit Plutonic Complex and Tidding and Mayodia mélange complexes. Field observations are consistent with ductile deformation concentrated along the top-to-the-south Tidding thrust shear zone, which is in contrast to the top-to-the-north Great Counter thrust at the same structural position to the west. Upper amphibolite-facies metamorphism of mélange rocks at ∼9−10 kbar (∼34−39 km) occurred prior to ca. 36−30 Ma exhumation during slip along the Tidding thrust shear zone. To the north, the ∼5-km-wide Lohit thrust shear zone has a subvertical geometry and north-side-up kinematics. Cretaceous arc granitoids of the Lohit Plutonic Complex were emplaced at ∼32−40 km depth in crust estimated to be ∼38−52 km thick at that time. These rocks cooled from ca. 25 Ma to 10 Ma due to slip along the Lohit thrust shear zone. We demonstrate that the Lohit thrust shear zone, Gangdese thrust, and Yarlung-Tsangpo Canyon thrust have comparable hanging wall and footwall rocks, structural geometries, kinematics, and timing. Based on these similarities, we interpret that these thrusts formed segments of a laterally continuous thrust system, which served as the preeminent crustal thickening structure along the Neotethys-southern Lhasa terrane margin and exhumed Gangdese lower arc crust in Oligocene−Miocene time.

Strain partitioning in the Moine Nappe, northernmost Scotland

Abstract Extreme strain in the form of flattening or constriction during noncoaxial shear in ductile shear zones provides a record of ductile thrust system dynamics and the overall tectonic evolution. Within the Moine Nappe in northern Scotland, between the Ben Hope and Moine thrusts, the Strathan Conglomerate displays apparent strain partitioning with extreme flattening (e.g., laterally extensive sheets of deformed pebbles with aspect ratios of 134:113:1 and 88–92% estimated thinning) adjacent to the overlying Ben Hope Thrust and extreme constriction (e.g., rods with aspect ratios of 21:4:1 and estimated extension of 1000%) lower in the nappe package. We demonstrate that partitioning of strain is between its intensity and how deformation is manifested. Field, microstructural, and crystallographic orientation data from this study indicate that both areas were deformed by WNW-directed noncoaxial shear and coaxial flattening under amphibolite-facies conditions. Adjacent to the Ben Hope thrust, flattening was pervasive during nonco-axial shear, whereas beneath and within the Moine Nappe package, polyphase folding dominated. There, early, large-scale folds (F2) rotated into the transport direction. Subsequent transport-parallel (F3) folds and tubular sheath folds formed on the F2 limbs and were dismembered to form rods. No evidence of constriction is observed; instead, pervasive noncoaxial shear was accompanied by minor flattening under decreasing temperature conditions. Thus, these S-tectonites in the Moine Nappe are the result of concentrated flattening of pebbles into sheets during WNW-directed shear, whereas the L-tectonites result from heterogeneously distributed shear and folding, coupled with minor flattening, which produced rods without constriction.

A Thermal Profile across the Idaho-Montana Fold-Thrust Belt Reveals a Low-Relief Orogenic Wedge That Developed atop a Pre-Orogenic Basement High

Abstract Growing orogenic wedges cool rocks during exhumation of thrust hanging walls and heat them during burial of footwalls, leaving behind a resilient thermal record of earlier deformation in fold-thrust belts. In order to investigate early burial of deformed strata within the retroarc Idaho-Montana fold-thrust belt, we use Raman spectroscopy of carbonaceous material to construct a maximum temperature profile that constrains the thicknesses of eroded rocks structurally above the Lemhi arch, a pre-thrusting basement high. In the eastern portion of the study area, a sharp maximum temperature change of ~120°C occurs across the Johnson thrust, signifying that regional burial and heating predated late-stage faulting. West of here, cumulative exhumation is irregular, varying by up to 5 km over large (~75 km) wavelength folds; however, maximum temperatures in this same region are consistently ~200°C higher than correlative stratigraphic units in the adjacent foreland. The pre-thrusting, low-relief unconformity above the Lemhi arch, which served as the early décollement to the fold-thrust belt, was everywhere buried to at least ~6.5 km depth, which is ~1.5-5.0 km deeper than can be explained by stratigraphic burial. We hypothesize that between ~145 and 80 Ma, a combination of Cretaceous deposition and folding and thrusting at higher structural levels buried the décollement of the Medicine Lodge-McKenzie thrust system to this depth. These results suggest that the early orogenic wedge had exceptionally low taper. We propose that thin strata over the low-relief Lemhi arch limited the availability of potential décollements, which restricted the maximum surface slope that could be constructed in a thin-skinned system. Subsequent growth of the orogenic wedge required activation of a much deeper décollement and a switch to a thick-skinned structural style, promoting a shift from burial to exhumation of the former décollement and the underlying Lemhi arch. This suggests that the growth of an orogenic wedge is dependent on the thicknesses of the preexisting strata and the availability of potential décollements, with sedimentation and burial heating potentially playing a key role.