An overview of strains in the Sevier thin-skinned thrust belt, Idaho and Wyoming, USA (latitude 42° N)

ABSTRACT Calcite twinning analysis across the central, unbuttressed portion of the Sevier thin-skin thrust belt, using Cambrian–Cretaceous limestones (n = 121) and synorogenic calcite veins (n = 31), records a complex strain history for the Sevier belt, Idaho and Wyoming, USA. Plots of fabric types (layer-parallel shortening, layer-normal shortening, etc.), shortening and extension axes for the Paris thrust (west, oldest, n = 11), Meade thrust (n = 46), Crawford thrust (n = 15), Absaroka thrust (n = 55), Darby thrust (n = 13), Lander Peak klippe (n = 5), eastern Prospect thrust (n = 6), and distal Cretaceous foreland (n = 3) reveal a W-E layer-parallel shortening strain only in the Prospect thrust and distal foreland. Calcite twinning strains in all western, internal thrust sheets are complex mixes of layer-parallel (LPS), layer-normal (LNS), and non-plane strains in limestones and synorogenic calcite veins. This complex strain fabric is best interpreted as the result of oblique convergence to the west and repeated eastward overthrusting by the Paris thrust.

Tectonostratigraphy and major structures of the Georgian Greater Caucasus: Implications for structural architecture, along-strike continuity, and orogen evolution

Although the Greater Caucasus Mountains have played a central role in absorbing late Cenozoic convergence between the Arabian and Eurasian plates, the orogenic architecture and the ways in which it accommodates modern shortening remain debated. Here, we addressed this problem using geologic mapping along two transects across the southern half of the western Greater Caucasus to reveal a suite of regionally coherent stratigraphic packages that are juxtaposed across a series of thrust faults, which we call the North Georgia fault system. From south to north within this system, stratigraphically repeated ~5–10-km-thick thrust sheets show systematically increasing bedding dip angles (<30° in the south to subvertical in the core of the range). Likewise, exhumation depth increases toward the core of the range, based on low-temperature thermochronologic data and metamorphic grade of exposed rocks. In contrast, active shortening in the modern system is accommodated, at least in part, by thrust faults along the southern margin of the orogen. Facilitated by the North Georgia fault system, the western Greater Caucasus Mountains broadly behave as an in-sequence, southward-propagating imbricate thrust fan, with older faults within the range progressively abandoned and new structures forming to accommodate shortening as the thrust propagates southward. We suggest that the single-fault-centric “Main Caucasus thrust” paradigm is no longer appropriate, as it is a system of faults, the North Georgia fault system, that dominates the architecture of the western Greater Caucasus Mountains.

Controls of Deep-Seated Faults and Folds on Hydrocarbon Fluid Migration and Accumulation in Sedimentary Basins: A Case Study from the Northwestern Sichuan Basin, China

Deep-seated faults and folds of foreland basin systems have become important exploration targets in the recent years because they are crucial in controlling fluid migration and hydrocarbon accumulation. In this study, we analyzed the characteristics and formation history of these structures in the northwestern Sichuan Basin using recently acquired two-dimensional (2D) and three-dimensional (3D) seismic data. The seismic interpretation revealed that the thrust sheets, tectonic wedges, and foredeep were well developed in the northwestern Sichuan Basin from the mountain to the basin. Forward thrusts, fault-bend folds, and wedges are the main types of structures in the thrust sheets and tectonic wedges. The deep-seated faults and folds were easily recognized in the high-resolution 3D seismic data. The imbricate thrust faults that merged into detachment layers of the Lower Cambrian are the main types of structures in the foredeep, and they show a prominent strike-slip influence in the horizontal direction. The formation of these structures in the foredeep in the northwestern Sichuan Basin mainly endured two stages of thrusting, including those during the Middle-to-Late Triassic and Cenozoic. Based on the tectonic evolution and seismic data, we infer that these deep-seated faults and folds in the foredeep may have formed earlier than the northern Longmen Shan fold-and-thrust belts and they may have been initially active in the late of Early Triassic and reactive during the Cenozoic. Furthermore, evaporites in the Lower and Middle Triassic were crucial in forming these structures. The petroleum exploration data suggested that the deep-seated faults can facilitate hydrocarbon accumulation. The thrust faults in the foredeep were more likely to act as migration pathways for fluids instead of sealing barriers along the horizontal direction. The interconnected reservoirs of deep-seated folds possess a great potential to allow large-scale hydrocarbon accumulation. Our study provides a good example for evaluating the hydrocarbon exploration potential in the deeply buried area in the sedimentary basin.

The geological framework for Hvideklint, south-east Denmark, using glaciodynamic sequence stratigraphy

Glaciodynamic sequence stratigraphy provides a practical model for grouping and classifying complex geological data to aid interpretation of past climatic and environmental development in Quaternary successions. The principles of glaciodynamic sequence stratigraphy are applied here to summarise the complex glacial geological framework of Hvideklint on the island of Møn, south-east Denmark. The framework of the superimposed deformed Hvideklint is presented in a reconstructed geological cross-section of Hvideklint. For the construction of the architecture of the glaciotectonic complex, the interpretation of structures below sea level was based on a detailed new survey of the cliff section combined with construction of successive approximation balanced cross-sections. The new description is supported by drill hole data from the Jupiter database. Where chalk is not glaciotectonically deformed, the constructed depth to the top-chalk-surface is generally located about 30 m below sea level. In Hvideklint, thrust sheets with chalk are exposed 20 m above sea level, and the balanced cross-section constructions indicate that the décollement surface for a Hvideklint glaciotectonic complex is located about 80 m below sea level. Between the décollement level and the top of the complex, two or more thrust-fault flat-levels and connecting ramps add to the complex architecture of Hvideklint.

Velocity Models for the Crust Hosting the Main Aftershock Cluster of the 2011 Mineral, Virginia, Earthquake

Three-dimensional P- and S-wave velocity (VP and VS) models are determined for the crust containing the main aftershock cluster of the 2011 Mineral, Virginia, earthquake using local earthquake tomography. The inversion uses a total of 5125 arrivals (2465 P- and 2660 S-wave arrivals) for 324 aftershocks recorded by 12 stations. The inversion volume (22 × 20 × 16 km) is completely contained within the Piedmont Chopawamsic metavolcanic terrane. The models are well resolved in the central portion of the inversion volume in the depth range 1–5 km; good resolution does not extend to the hypocenter depth of the mainshock. Most aftershocks are located within a northeast-trending, southeast-dipping region containing negative VP anomalies, positive VS anomalies, and VP/VS ratios as low as 1.53. These velocity results strongly argue for the presence of quartz-rich rocks, which we attribute to either the presence of a giant quartz vein system or metamorphosed orthoquarzite sandstones originally deposited on the Laurentian passive margin and subsequently incorporated into the Chopawamsic thrust sheets during island arc collision in the Taconic orogeny.

Thrust tectonics in the Wetterstein and Mieming mountains, and a new tectonic subdivision of the Northern Calcareous Alps of Western Austria and Southern Germany

We investigate the tectonic evolution of the Wetterstein and Mieming mountains in the western Northern Calcareous Alps (NCA) of the European Eastern Alps. In-sequence NW-directed stacking of thrust sheets in this thin-skinned foreland thrust belt lasted from the Hauterivian to the Cenomanian. In the more internal NCA major E-striking intracontinental transform faults dissected the thrust belt at the Albian–Cenomanian boundary that facilitated ascent of mantle melts feeding basanitic dykes and sills. Afterwards, the NCA basement was subducted, and the NCA were transported piggy-back across the tectonically deeper Penninic units. This process was accompanied by renewed Late Cretaceous NW-directed thrusting, and folding of thrusts. During Paleogene collision, N(NE)-directed out-of-sequence thrusts developed that offset the in-sequence thrust. We use this latter observation to revise the existing tectonic subdivision of the western NCA, in which these out-of-sequence thrusts had been used to delimit nappes, locally with young-on-old contacts at the base. We define new units that represent thrust sheets having exclusively old-on-young contacts at their base. Two large thrust sheets build the western NCA: (1) the tectonically deeper Tannheim thrust sheet and (2) the tectonically higher Karwendel thrust sheet. West of the Wetterstein and Mieming mountains, the Imst part of the Karwendel thrust sheet is stacked by an out-of-sequence thrust onto the main body of the Karwendel thrust sheet, which is, in its southeastern part, in lateral contact with the latter across a tear fault.

Early Miocene Thrust Tectonics on Raukumara Peninsula, Northeastern New Zealand

<p>Raukumara Peninsula lies at the northeastern end of the East Coast Deformed Belt, a province of deformed Late Mesozoic-Late Cenozoic rocks on the eastern edges of the North Island and northern South Island of New Zealand. Late Cenozoic deformation in this province is associated with westward subduction of the Pacific Plate, which started at about the beginning of the Miocene. Early Miocene tectonism on Raukumara Peninsula took place in a hitherto little-known thrust belt, the East Coast Allochthon. The configuration, evolution and origin of this thrust belt are the subjects of this thesis. The thrust belt extends 110 km from the thrust front in the southwest to the northeastern tip of Raukumara Peninsula. Internal structures strike northwest, perpendicular to the present trend of the continental margin but parallel to the Early Miocene trend suggested by plate reconstructions and paleomagnetic studies. The structure and kinematic evolution of the thrust belt were investigated by detailed mapping of three key areas in its central part and by analysis of previous work throughout the region. Gross differences in structure lead to the division of the belt into three zones: southern, central and northern. Deformation in the southern and central zones (the southwestern two-thirds of the system) was thin-skinned, involving southwestward transport of thrust sheets above a decollement horizon at the top of the Maastrichtian-Paleocene Whangai Formation. The decollement is exposed in the northwest due to southeastward tilting accompanying post-Miocene uplift of the Raukumara Range. Deformation in the northern zone involved reactivations of northeast-directed Cretaceous thrusts as well as southwestward emplacement of allochthonous sheets. Stratigraphic relationships show that thrusting took place during = 6 m.y. in the earliest Miocene. The 18 km wide southern zone is an emergent imbricate fan of rocks detached from above the Whangai Formation in a piggy-back sequence and transported less than about 18 km at rates of 2.6-3.6 mm/yr (plus-minus 20%-100%). The central and northern zones include rocks older than Whangai Formation. The sheets of the central zone and the southwest-directed sheets of the northern zone make up three major allochthonous units: the Waitahaia allochthon, consisting predominantly of mid-Cretaceous flysch above the Waitahaia Fault and equivalent structures, at the bottom of the thrust pile; the Te Rata allochthon, of Late Cretaceous-Early Tertiary continental margin sediments above the Te Rata Thrust, in the middle; and the Matakaoa sheet, an ophiolite body of mid-Cretaceous-Eocene basaltic and pelagic sedimentary rocks, at the top and back of the thrust belt. The Waitahaia allochthon was emplaced first and was subsequently breached by the Te Rata Thrust. The mid-Cretaceous rocks of the Waitahaia allochthon are mostly overturned, a result of the southwest-directed Early Miocene thrusting overprinting a Cretaceous structure of predominantly southwestward dips. The Te Rata allochthon comprises a complex pile of thrust sheets and slices with a general older-on-younger stacking order but with common reversals. Synorogenic sedimentary rocks occur within it. The complexity of internal structure of these two allochthons suggests they have undergone more than the 50% shortening estimated for the southern zone. The minimum southwestward displacement of the Te Rata allochthon is 60 km. The minimum displacements of the Waitahaia and Matakaoa allochthons are 55-195 km and 115-530 km respectively, depending on whether the Te Rata allochthon originally lay in front of the original position of the Waitahaia allochthon or was originally the upper part of the Waitahaia allochthon, and on the amounts of internal shortening of the allochthons. Over the = 6 m.y. period of thrusting, these estimates imply displacement rates for the Matakaoa sheet of 19-88 mm/yr. The average plate convergence rate at East Cape for the period 36-20 Ma is estimated at 25-30 mm/yr; the rate for the Early Miocene-- when subduction was active--may have been faster. Reasonable displacement rates for the Matakaoa sheet would result if the Te Rata allochthon was originally the upper part of the Waitahaia allochthon and if both allochthons have been shortened somewhat less than 50%. The emplacement mechanism of the Matakaoa ophiolite is elucidated by comparison with Northland, northwest along strike from Raukumara Peninsula, onto which correlative rocks were emplaced at the same time. The thinness of the Northland ophiolite bodies, their composition of rocks typical of the uppermost levels of oceanic crust, and the start of andesitic volcanism accompanying their obduction show that they were emplaced as a thin flake of oceanic crust which peeled off the downgoing slab during the inception of southwestward subduction. The reason the ophiolites were initially peeled from the slab is probably that their upper levels prograded southwestward over sediments of the Northland-Raukumara continental margin. In such a situation, initial compression would have led to formation of a northeast-dipping thrust at the volcanic/sediment interface; this thrust would then have propagated back into the downgoing plate with continued convergence, allowing the ophiolites to climb up the continental slope pushing the allochthonous sedimentary sheets ahead of them.</p>

Early Miocene Thrust Tectonics on Raukumara Peninsula, Northeastern New Zealand

<p>Raukumara Peninsula lies at the northeastern end of the East Coast Deformed Belt, a province of deformed Late Mesozoic-Late Cenozoic rocks on the eastern edges of the North Island and northern South Island of New Zealand. Late Cenozoic deformation in this province is associated with westward subduction of the Pacific Plate, which started at about the beginning of the Miocene. Early Miocene tectonism on Raukumara Peninsula took place in a hitherto little-known thrust belt, the East Coast Allochthon. The configuration, evolution and origin of this thrust belt are the subjects of this thesis. The thrust belt extends 110 km from the thrust front in the southwest to the northeastern tip of Raukumara Peninsula. Internal structures strike northwest, perpendicular to the present trend of the continental margin but parallel to the Early Miocene trend suggested by plate reconstructions and paleomagnetic studies. The structure and kinematic evolution of the thrust belt were investigated by detailed mapping of three key areas in its central part and by analysis of previous work throughout the region. Gross differences in structure lead to the division of the belt into three zones: southern, central and northern. Deformation in the southern and central zones (the southwestern two-thirds of the system) was thin-skinned, involving southwestward transport of thrust sheets above a decollement horizon at the top of the Maastrichtian-Paleocene Whangai Formation. The decollement is exposed in the northwest due to southeastward tilting accompanying post-Miocene uplift of the Raukumara Range. Deformation in the northern zone involved reactivations of northeast-directed Cretaceous thrusts as well as southwestward emplacement of allochthonous sheets. Stratigraphic relationships show that thrusting took place during = 6 m.y. in the earliest Miocene. The 18 km wide southern zone is an emergent imbricate fan of rocks detached from above the Whangai Formation in a piggy-back sequence and transported less than about 18 km at rates of 2.6-3.6 mm/yr (plus-minus 20%-100%). The central and northern zones include rocks older than Whangai Formation. The sheets of the central zone and the southwest-directed sheets of the northern zone make up three major allochthonous units: the Waitahaia allochthon, consisting predominantly of mid-Cretaceous flysch above the Waitahaia Fault and equivalent structures, at the bottom of the thrust pile; the Te Rata allochthon, of Late Cretaceous-Early Tertiary continental margin sediments above the Te Rata Thrust, in the middle; and the Matakaoa sheet, an ophiolite body of mid-Cretaceous-Eocene basaltic and pelagic sedimentary rocks, at the top and back of the thrust belt. The Waitahaia allochthon was emplaced first and was subsequently breached by the Te Rata Thrust. The mid-Cretaceous rocks of the Waitahaia allochthon are mostly overturned, a result of the southwest-directed Early Miocene thrusting overprinting a Cretaceous structure of predominantly southwestward dips. The Te Rata allochthon comprises a complex pile of thrust sheets and slices with a general older-on-younger stacking order but with common reversals. Synorogenic sedimentary rocks occur within it. The complexity of internal structure of these two allochthons suggests they have undergone more than the 50% shortening estimated for the southern zone. The minimum southwestward displacement of the Te Rata allochthon is 60 km. The minimum displacements of the Waitahaia and Matakaoa allochthons are 55-195 km and 115-530 km respectively, depending on whether the Te Rata allochthon originally lay in front of the original position of the Waitahaia allochthon or was originally the upper part of the Waitahaia allochthon, and on the amounts of internal shortening of the allochthons. Over the = 6 m.y. period of thrusting, these estimates imply displacement rates for the Matakaoa sheet of 19-88 mm/yr. The average plate convergence rate at East Cape for the period 36-20 Ma is estimated at 25-30 mm/yr; the rate for the Early Miocene-- when subduction was active--may have been faster. Reasonable displacement rates for the Matakaoa sheet would result if the Te Rata allochthon was originally the upper part of the Waitahaia allochthon and if both allochthons have been shortened somewhat less than 50%. The emplacement mechanism of the Matakaoa ophiolite is elucidated by comparison with Northland, northwest along strike from Raukumara Peninsula, onto which correlative rocks were emplaced at the same time. The thinness of the Northland ophiolite bodies, their composition of rocks typical of the uppermost levels of oceanic crust, and the start of andesitic volcanism accompanying their obduction show that they were emplaced as a thin flake of oceanic crust which peeled off the downgoing slab during the inception of southwestward subduction. The reason the ophiolites were initially peeled from the slab is probably that their upper levels prograded southwestward over sediments of the Northland-Raukumara continental margin. In such a situation, initial compression would have led to formation of a northeast-dipping thrust at the volcanic/sediment interface; this thrust would then have propagated back into the downgoing plate with continued convergence, allowing the ophiolites to climb up the continental slope pushing the allochthonous sedimentary sheets ahead of them.</p>

Tectonic Transport Directions, Shear Senses and Deformation Temperatures Indicated by Quartz c-Axis Fabrics and Microstructures in a NW-SE Transect across the Moine and Sgurr Beag Thrust Sheets, Caledonian Orogen of Northern Scotland

Moine metasedimentary rocks of northern Scotland are characterized by arcuate map patterns of mineral lineations that swing progressively clockwise from orogen-perpendicular E-trending lineations in greenschist facies mylonites above the Moine thrust on the foreland edge of the Caledonian Orogen, to S-trending lineations at higher structural levels and metamorphic grades in the hinterland. Quartz c-axis fabrics measured on a west to east coast transect demonstrate that the lineations developed parallel to the maximum principal extension direction and therefore track the local tectonic transport direction. Microstructures and c-axis fabrics document a progressive change from top to the N shearing in the hinterland to top to the W shearing on the foreland edge. Field relationships indicate that the domain of top to the N shearing was at least 55 km wide before later horizontal shortening on km-scale W-vergent folds that detach on the underlying Moine thrust. Previously published data from the Moine thrust mylonites demonstrate that top to the W shearing had largely ceased by 430 Ma, while preliminary isotopic age data suggest top to the N shearing occurred at ~470–450 Ma. In addition, data from the east coast end of our transect indicate normal-sense top down-SE shearing at close to peak temperatures at ~420 Ma that may be related to the closing stages of Scandian deformation, metamorphism and cooling/exhumation.

Structural controls on earthquake rupture revealed by the 2020 Mw 6.0 Jiashi earthquake (Kepingtag belt, SW Tian Shan, China)

The Kepingtag (Kalpin) fold-and-thrust belt of the southern Chinese Tian Shan is characterized by active shortening and intense seismic activity. Geological cross-sections and seismic reflection profiles suggest thin-skinned, northward-dipping thrust sheets detached in an Upper Cambrian décollement. The January 19 2020 Mw 6.0 Jiashi earthquake provides an opportunity to investigate how coseismic deformation is accommodated in this structural setting. Coseismic surface deformation resolved with Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) is centered on the back limb of the frontal Kepingtag anticline. Elastic dislocation modelling suggests that the causative fault is located at ~7 km depth and dips ~7° northward, consistent with the inferred position of the décollement. The narrow slip pattern (length ~37 km but width only ~9 km) implies that there is a strong structural or lithological control on the rupture extent, with up-dip slip propagation possibly halted by an abrupt change in dip angle where the Kepingtag thrust is inferred to branch off the décollement. A depth discrepancy between mainshock slip constrained by InSAR and teleseismic waveform modelling (~7 km) and well-relocated aftershocks (~10-20 km) may imply that sediments above the décollement are velocity strengthening. We also relocate 148 regional events from 1977 to 2020 to characterize the broader distribution of seismicity across the Kepingtag belt. The calibrated hypocenters combined with previous teleseismic waveform models show that thrust and reverse faulting earthquakes cluster at relatively shallow depths of ~7-15 km but include abundant out-of-sequence events both north and south of the frontal Kepingtag fault.