NW-trending Folds Research Articles

Integrated geological study in an offshore renewable energy test site: a case from the Basque continental shelf (Bay of Biscay, Spain)

Testing and research centres for offshore renewable energy, exemplified by facilities like BIMEP (Biscay Marine Energy Platform) on the Basque coast of Spain, play a crucial role in driving the energy transition. This study utilises pre-existing data at the facility site, such as high-resolution bathymetry and granulometric information from sediment samples, to conduct a comprehensive geological analysis including both sedimentary and rocky seabed. A litho-structural analysis is presented, including a lithological prediction for the continental shelf, the recognition of the main structures, such as NW-trending folds and predominantly NE-SW oriented fractures, and a detailed fracture analysis. Sedimentary seabeds are analysed through a Seabed Sediment Map, illustrating a granulometry-based NE-SW oriented banded distribution. Bedforms are also studied, they are asymmetric and mainly oriented NE-SW. The Seabed Sediment Map and the bedform analysis reveal the effect of an SE-directed bottom current as the main mechanism controlling sediment mobility. This current matches with the predominant swell from the NW and with the direction of the most energetic waves in the area. This approach could serve as a methodological example, offering a cost-effective means for the preliminary geological characterisation of offshore energy sites, and is crucial for establishing a baseline (‘zero state’) before the deployment. This baseline is essential for evaluating and mitigating the impact of new infrastructure on sediment dynamics, which subsequently affects the overall functioning and health of the marine ecosystem.

Tectonic Evolution of the Proto‒Tethys in the Early Devonian: Insights From the Turbidite in the North Qilian Belt, NW China

AbstractThe Devonian tectonic setting is still controversial in the Proto‒Tethyan North Qilian belt. The Hexi Corridor of the North Qilian belt occupies a key position in constraining the tectonic evolution of the Proto‒Tethyan Ocean and the growth of the Asian continent. Turbidites, as an important component of the Jiayuguan mélange in the Hexi Corridor, consist of conglomerate, pebbly sandstone, sandstone, siltstone, calcareous mudstone, and chert that feature typical Bouma sequences of Ba, Bc, Bcd, Babc and/or Bab. They are strongly deformed into NW‒trending folds with NE‒ and SW‒inclined thrust faults. Detrital zircon U‒Pb dating results demonstrate that these sediments successively young to the southwest with maximum depositional ages (MDA) ranging from 560 Ma to 411 Ma, indicating that the youngest block was deposited after 411 Ma. Petrological, geochemical and detrital zircon U‒Pb age data indicate the sedimentary rocks provenanced from weak weathering, recycling, and multiple mixing sources, and that the dominant source was the Andean‒type Alxa arc. Together with the published data, we suggest that double‒sided subduction of the North Qilian Ocean (Proto‒Tethyan Ocean) occurred between the North Qilian arc and the Andean‒type Alxa arc during the early Devonian.

Paleomagnetic constraints on Paleogene-Neogene rotation and paleo-stress in the northern Qaidam Basin

Knowing the rotation and paleo-stress history in Qaidam Basin is a fundamental parameter to quantify the mechanism of intracontinental deformation in Tibetan Plateau. However, few studies have been conducted on tectonic rotation and stress evolution over long timescales in the Qaidam Basin. Here, we report new magnetic declination and the anisotropy of magnetic susceptibility (AMS) from a ∼52–7 Ma sequence of fluvial and lacustrine sediments in the Dahonggou (DHG) section in the northern Qaidam Basin. The magnetic declination revealed that the northern Qaidam Basin underwent a clockwise rotation 25.1° ±8.6° during ∼33–17 Ma, followed by a counterclockwise rotation 16.9°±6.8° during ∼17–13.5 Ma. The AMS results showed that the “earliest deformation” fabrics were interrupted by the “pencil structure” fabrics in the intervals of ∼52–45 and ∼21–15 Ma. The interruption, synchronous with the marked deceleration of the India-Asia convergence rate, indicates pulse of strong tectonic compressive stress. In addition, the AMS results documented a transition in stress direction from S-N to SW-NE at ∼15 Ma, suggesting a kinematic shift in the northeastern TP. Our constraints on the rotation and stress from the northern Qaidam Basin support the two-stage evolution of the Altyn Tagh Fault (ATF). The fast-rate slip motion on the ATF during the early Oligocene caused the clockwise rotation in the northern Qaidam Basin; the second stage with enhanced thrusting since the middle Miocene caused extensive crustal shortening and dispersive NW-trending folds and faults in the Qaidam Basin and the northeastern TP.

Deformation and structural evolution of metamorphic complexes of the Taldyk antiform in the East Mugodzhar zone of Urals (West Kazakhstan)

The study is focused on mesostructural folded parageneses of the Taldyk antiform (a.k.a. Taldyk block) located in the East Mugodzhar zone. The sequence of their formation is established; the structural evolution of the study area is investigated, and four stages of deformation are identified. The NW-trending folds F1 with SE-vergence formed during the first stage of deformation, DI. The geodynamics and timeline of this stage remain unclear. The W-E-trending folds F2 with E-vergence are related to tectonic movements that took place at stage DII. In the western limb of the antiform, stage DII is evidenced by folds overturned towards the south-east. In the eastern limb, folds plunge to the east and northeast. These fold structures are probably related to the Devonian subduction-obduction processes. At stage DIII, thrusting of the Taldyk antiform over the West Mugodzhar zone and folding F3 with W-vergence is related to the Ural continental collision in the Late Paleozoic, which completed the geodynamic evolution of the Ural paleo-ocean. At stage DIV, postcollisional shearing is evidenced by folds F4 with steeply dipping hinges, which completed the structural evolution of the study area.

Tectonics of the New Siberian Islands archipelago: Structural styles and low-temperature thermochronology

Tectonic evolution of the New Siberian Islands (NSI) has been revealed based on detailed structural investigations and a (U-Th)/He low-temperature thermochronologic study of detrital zircons (ZHe) and apatite (AHe). Our study supports models claiming a non–Siberian affinity of the NSI and furthermore suggests that the study area formed a part of the Arctic-Alaska-Chukotka microcontinent. Seven stages of deformation have been revealed. The earliest stage (Stage 1) involved contractional deformation with transport directions towards the W-to WSW and occurred during the Late Cambrian across the De Long Islands. The next episode of deformation (Stage 2) has been revealed based on the low-temperature thermochronology (ca. 378–414 Ma, ZHe) and structural data. A pre-Frasnian angular unconformity formed as a result of Stage 2 deformation on Kotel’nyi Island, which involved contractional deformation with S-to SW transport directions (modern coordinates) in the mid-Paleozoic. In the latest Early Cretaceous–Late Cretaceous, three deformation stages were initiated by collision between the western part of the Arctic-Alaska-Chukotka microcontinent and Siberia, forming the South Anyui suture zone and overlapping orogenic belt. Stage 3 was characterized by the formation of major NW-trending folds, thrusts, and both transfer dextral and sinistral strike-slip faults with a reverse component. During Stage 4, the Arctic Alaska-Chukotka microcontinent moved westward. On Bolshoy Lyakhovsky Island, sinistral strike-slip faults were formed, whilst E-W compression took place across the Anjou islands. In Stage 5, the Arctic Alaska-Chukotka microcontinent shifted southward, forming a series of N-S-trending dextral strike-slip faults. The ZHe and AHe ages (ca. 93–125 Ma) suggest that these deformation events were associated with significant uplift in the western part of the NSI (Kotel’nyi and Bel’kovsky islands), whilst the eastern part (De Long Islands) was marginally affected by these events without significant uplift. The Cenozoic extension event (Stage 6) corresponds to the opening of the Eurasian Basin. This stage is manifested by the cooling episode (ca. 53 Ma, Early Eocene, AHe) established in the eastern part of the NSI (Jeannette Island). The origins of the late Cenozoic contractional deformations described from the Cenozoic deposits of the Anjou Islands (Stage 7) are unclear, but were possibly caused by movements along the Eurasian and North-American lithospheric plates.

Pre-mid-Frasnian angular unconformity on Kotel’ny Island (New Siberian Islands archipelago): evidence of mid-Paleozoic deformation in the Russian High Arctic

We present detailed structural studies which reveal for the first time the existence of an angular unconformity at the base of the Middle Frasnian deposits across the western part of Kotel’ny Island (New Siberian Islands, Russian High Arctic). Pre-Mesozoic convergent structures are characterized by sublatitudinal folds and south-verging thrusts. Based on the age of the rock units above and below the unconformity, the age of the deformation event can be described as post-Givetian but pre-mid-Frasnian. Based on the vergence direction of thrusts deforming pre-Frasnian deposits on Kotel’ny Island, shortening occurred from north to south (in the present day coordinates). The small scale of the structures suggests that this part of the New Siberian Islands formed a distal part of an orogenic belt in the Middle Paleozoic. The angular unconformity described on Kotel’ny Island can be tentatively correlated with the Ellesmerian Orogeny. However, due to a paucity of detailed geological data from the neighboring broad Arctic continental shelves, a precise correlation with known tectonic events of the circum-Arctic cannot be achieved. The subsequent Mesozoic tectonic structures with NW-trending folds and faults were superimposed on pre-existing Paleozoic and older structures. Thus, the data presented here provide additional constraints on the Paleozoic geodynamic affinity of the New Siberian Islands and provide a regional link to other Arctic regions, aiding future tectonic reconstructions of the circum-Arctic.

Structural analysis and implicit 3D modelling of Jwaneng Mine: Insights into deformation of the Transvaal Supergroup in SE Botswana

Country rock at Jwaneng Diamond Mine provides a rare insight into the deformational history of the Transvaal Supergroup in southern Botswana. The ca. 235 Ma kimberlite diatremes intruded into late Archaean to Early Proterozoic, mixed, siliciclastic-carbonate sediments, that were subjected to at least three deformational events. The first deformational event (D1), caused by NW-SE directed compression, is responsible for NE-trending, open folds (F1) with associated diverging, fanning, axial planar cleavage. The second deformational event (D2) is probably progressive, involving a clockwise rotation of the principal stress to NE-SW trends. Early D2, which was N-S directed, involved left-lateral, oblique shearing along cleavage planes that developed around F1 folds, along with the development of antithetic structures. Progressive clockwise rotation of far-field forces saw the development of NW-trending folds (F2) and its associated, weak, axial planar cleavage. D3 is an extensional event in which normal faulting, along pre-existing cleavage planes, created a series of rhomboid-shaped, fault-bounded blocks. Normal faults, which bound these blocks, are the dominant structures at Jwaneng Mine. Combined with block rotation and NW-dipping bedding, a horst-like structure on the northwestern limb of a broad, gentle, NE-trending anticline is indicated. The early compressional and subsequent extensional events are consistent throughout the Jwaneng-Ramotswa-Lobatse-Thabazimbi area, suggesting that a large area records the same fault geometry and, consequently, deformational history. It is proposed that Jwaneng Mine is at or near the northernmost limit of the initial, northwards-directed compressional event.

Three-dimensional structural model of the Qaidam basin: Implications for crustal shortening and growth of the northeast Tibet

AbstractThe Qaidam basin, bounded by the Altyn Tagh fault in the north, is located in the northeast of the Tibet plateau, and it has important implications for understanding the history and mechanism of Tibetan plateau formation during the Cenozoic Indo-Eurasia collision. In this study, we constructed the main geological structures and surfaces in three dimensions through the interpolation of regularly spaced 2D seismic sections, constrained by wells data and surface geology of the Qaidam basin in northeast Tibet. Meanwhile the Cenozoic tectonic history of the Qaidam basin was reconstructed and the uplift mechanism of the Tibetan plateau was discussed. This study presents the subsurface data in conjunction with observations and analysis of the stratigraphic and sedimentary evolution. The Cenozoic deformation history of the Qaidam basin shows geologic synchroneity with uplifting history of the Tibet Plateau. It is therefore proposed that the deformation and uplifting in the south and north edges of the Tibet Plateau was almost synchronous. The total shortening and shortening rate during Cenozoic reached 25.5 km and 11.2% respectively across the Qaidam basin, indicating that the loss of the left-lateral strike slip rates of the Altyn Tagh fault has been structurally transformed into local crustal thickening across NW-trending folds and thrust faults. Meanwhile there is an about 11° vertical component along the strike-slip Altyn Tagh fault, the block oblique slip shows one more growth mechanism of the northeast Tibet.

Crustal thickening, Barrovian metamorphism, and exhumation of midcrustal rocks during doming and extrusion: Insights from the Himalaya, NW India

AbstractRocks exposed in NW India constrain the burial, partial melting, and exhumation history of the Himalayan crust. New microscale and mesoscale structural analysis, combined with pressure‐temperature estimates from the Haimanta Group exposed in the Sutlej Valley, indicate that the rocks were folded by two generations of NW trending folds. Barrovian metamorphism culminated at ~30 Ma with staurolite and kyanite overgrowths of F2 folds. Crustal thickening created a metamorphic field gradient that increases from the garnet zone (499 ± 99°C and 4.5 ± 1.4 kbar to 571 ± 92°C and 7.8 ± 1.4 kbar) to the staurolite‐kyanite zone (567 ± 105°C and 6.7 ± 1.6 kbar). These data are combined with previous studies to modify a two‐stage conceptual model for the thermal and deformation conditions of the middle and upper crust during the Eocene‐Miocene, excluding the late Miocene to recent. In the Miocene (~23 Ma), Barrovian metamorphism was overprinted by decompression during coeval south directed extrusion of the Greater Himalayan Series beneath the Sangla detachment in the foreland and doming during top‐down‐to‐the‐west displacement along the Leo Pargil shear zone in the hinterland. These data demonstrate that shear zones and detachments, such as the South Tibetan detachment that initially formed during crustal thickening (e.g., Eocene‐Oligocene), contributed to the subsequent distribution of rocks that experienced different pressure‐temperature‐time paths, degrees of partial melting, and exhumation histories during the Miocene.

The Dalradian rocks of the central Grampian Highlands of Scotland

The central Grampian Highlands, as defined here, are bounded to the north-west by the Great Glen Fault, to the south-west by Loch Etive and the Pass of Brander Fault and to the south-east by the main outcrop of the Loch Tay Limestone Formation. The more arbitrary northern boundary runs north-west along the A9 road and westwards to Fort William. The detailed stratigraphy of the Dalradian Supergroup ranges from the uppermost Grampian Group through to the top of the Argyll Group, most notably seen in the two classic areas of Loch Leven–Appin and Schiehallion–Loch Tay; Southern Highland Group strata are preserved only in a small structural inlier south of Glen Lyon.Major F1 and F2 folds are complicated by co-axial northeast-trending F3 and F4 folding, as well as by locally important north- or NW-trending folds. In the Loch Leven area, nappe-like F1 folds verge to the north-west, whereas to the south-east the major recumbent F1/F2 Tay Nappe verges to the south-east. The trace of the upright Loch Awe Syncline lies between the opposing nappes, but in this region a large mass of late-Caledonian granitic rocks obscures their mutual relationship. Three tectonic ‘slides’ are identified that are certainly zones of high strain but which in part could be obscuring stratigraphical variations.The regional metamorphism ranges from greenschist facies on the western seaboard of Argyll to amphibolite facies in most of the remainder of the region. The study of garnets, together with kyanite and staurolite in the Schiehallion area, has enabled a detailed history of the metamorphism and structure to be unravelled.Stratabound mineralization occurs in the Easdale Subgroup, where there is also evidence of changes of sedimentary environment associated with volcanicity and lithospheric stretching. The region is dissected by a series of NE-trending, dominantly left-lateral, faults, subparallel to the Great Glen Fault, whose movement history is illustrated here by that of the Tyndrum Fault.

The role of the Najd Fault System in the tectonic evolution of the Hammamat molasse sediments, Eastern Desert, Egypt

The Hammamat molasse sediments of the Eastern Desert of Egypt were deposited in isolated basins formed during an initial stage of orogen parallel N–S extension (650–580 Ma) in the Neoproterozoic time. Supply of sediments to the molasse basins began after the eruption of Dokhan volcanics (602–593 Ma), exhumation of core complexes (650–550 Ma), and intrusion of late tectonic granites (610–550 Ma). The late Neoproterozoic structures in the molasse sediments include: (1) NNW-directed thrusts due to NNW–SSE shortening (650–640 Ma), indicated by the presence of NE-, ENE-, and WSW-trending folds and NNW-directed thrusts. (2) SW- and NE-directed thrusts due to ENE–WSW constriction during oblique convergence and arc accretion at around 640–620 Ma. Many of the map- and mesoscopic-scale NW-trending folds in the core complexes, the molasse sediments, and the Neoproterozoic nappes in the Eastern Desert are related to this event. Sinistral shearing along the Najd Fault System (650–540 Ma) resulted in the development of subvertical foliation, subhorizontal stretching lineation, and NW-trending tight folds overprinting earlier folds. Stretched pebbles are oriented NW–SE and WNW–ESE in the molasse basins localized within the Najd Fault System, and NE–SW in the basins outside the influence zone of this NW-trending fault system. Strain estimated using pebbles from nine molasse basins indicate that the amount of strain differs from one basin to another and from one place to another within the same basin. Weak tectonic strain (Rs = 2.16–2.24) is obtained from post-orogenic basins; moderate strains are reported from foreland basins (Rs = 2.37–3.18), whereas moderate to high tectonic strains are recorded from the intermontane basins (Rs = 2.40–4.36). The obtained tectonic strain and K values indicate that the flattening strain prevails in the post-orogenic and foreland basins, whereas as both constrictional and flattening strains are recorded in intermontane basins. Strain variation from one basin to another and within the individual basin is attributed to presence of thrust and sinistral shear zones. Away from the deformed zones, the pebbles show no significant stretching. Two phases of thrusting and an episode of transpressional sinistral shearing are the latest structure features of the East African orogeny in the Arabian–Nubian Shield.

The Lewisian terrane model: a review

Synopsis The Lewisian terrane model of Friend and Kinny divides the Lewisian complex into nine separate terranes believed to have amalgamated during the Palaeoproterozoic. This paper analyses the rationale for the terrane model and suggests criteria to evaluate and refine it. Similarities and differences in Palaeoproterozoic structural, metamorphic and igneous features are used to divide the complex into 14 separate blocks of three types: type A (upper-plate) characterized by retrogressed high-pressure granulite-facies Archaean metamorphic rocks and localized Laxfordian deformation; type B (lower plate) characterized by amphibolite-facies Archaean metamorphic rocks originating at higher crustal levels, intense Laxfordian deformation, and swarms of Laxfordian granites and pegmatites; and third, Palaeoproterozoic complexes comprising oceanic and volcanic arc elements. The boundaries between the type A and type B blocks, where seen, are major shear zones believed to represent a deformed and disrupted collisional suture between two large continental plates, the central Greenland craton (CGC) to the NE and the North Atlantic craton (NAC) to the SW. The following sequence of events is proposed. The Palaeoproterozoic complexes were accreted to the upper plate (NAC) at c. 1.9 Ga. This was followed by collision with the CGC at c. 1.87 Ga, causing the intense early Laxfordian deformation and high-grade metamorphism. At c. 1.74 Ga a second collision took place with the 1.80 Ga ‘Malin’ volcanic arc to the south, resulting in reactivation and late Laxfordian deformation, producing NW-trending folds, dextral shear zones, granite sheets and pegmatites. These late Laxfordian movements are considered to have resulted in considerable disruption of the early Laxfordian terranes. They include substantial strike-slip movements that may have led to the creation of further displaced terranes, some of which may prove to be exotic.

Structural evolution of the Neoproterozoic Western Allaqi–Heiani suture, southeastern Egypt

The Neoproterozic Allaqi–Heiani suture in southeastern Egypt is the western extension of the Allaqi–Heiani–Onib–Sol Hamed–Yanbu suture that represents one of arc–arc sutures in the Arabian–Nubian Shield. It extends for more than 250 km from the N-trending Hamisana Shear Zone in the east to Lake Nasser in the west. It separates the 750-Ma-old Southeastern Desert terrane in the north from the 830–720-Ma-old Gabgaba terrane in the south. Structural studies supported by remote sensing investigations including Landsat Thematic Mapper (TM) images show that the western part of Allaqi–Heiani suture zone constitutes three S- to SW-verging nappes in the north overriding an autochthonous block to the southwest. SW-verging, low-angle thrust sheets and folds, forming a 10-km wide imbrication fan, dominate the northern upper nappe (northern allochthon). These folds and thrusts deform shelf metasedimentary rocks including psammitic metasediments, marble and subordinate conglomerate. Volcanic rocks including rhyolites and felsic tuffs dominate the upper part of the northern allochthon. The contacts between the metasedimentary rocks on the one hand and the rhyolites and felsic tuffs on the other hand are extrusive. This allochthon overrides an internally deformed nappe (central allochthon) dominated by arc and ophiolitic assemblages now preserved as felsic and mafic schist, talc schist, serpentinites, and metagabbros. This allochthon is characterized by NW-trending, upright folds, which deform the earlier sub-horizontal structures. The structurally lower nappe (southern allochthon) is dominated by NNE-trending folds which deform amphibolite facies schistose metavolcanic and metavolcanoclastic rocks. The NNE-trending folds deform earlier NW-trending folds to produce crescentic dome interference pattern with well-developed NE-trending axial planar cleavage, consistent with ESE–WNW bulk shortening. The southernmost structural unit is an autochthonous block dominated by arc-related volcanic and volcanoclastic rocks. This block has suffered only minor deformation compared to the nappes to its north. The consistent SW-vergence of the structures indicates tectonic transport from northeast to southwest, followed by ESE–WNW shortening similar to that found in the Hamisana Shear Zone, further east. Collision between the Gabgaba–Gebeit terrane and the Southeastern Desert terrane along the Allaqi–Heiani suture, after the consumption of a marginal basin probably over an N-dipping subduction zone, led to the formation of EW- to NW-trending folds and thrusts. This was followed by ESE–WNW tectonic shortening to form NNE-trending folds, which are found to be overprinting the earlier structures. This latest shortening might be due to collision between the Arabian–Nubian Shield and the Saharan Metacraton along an N-trending arc–continent suture represented farther south by the Keraf suture.

Early Proterozoic nappe formation: an example from Sodankylä, Finland, Northern Baltic Shield

The Central Lapland Greenstone belt comprises rift-related metavolcanic and metasedimentary rocks representing one of the largest supracrustal belts in the Baltic Shield. The Sodankylä area in the central part of the belt represents a complex thrust duplex within a nappe overlying Belomorian Archaean basement and autocthonous Luirojoki calc-silicate rocks. Here, an early D1 schistosity is axial planar to at least three coaxial generations of southward-verging, subhorizontal, E–W-plunging D1 folds associated with major southwards thrusting. D2 is represented by broad, map-scale, upright, NE-trending folds in the south and crenulations in the north. Staurolite-grade metamorphism represented by post-tectonic andalusite + staurolite + kyanite assemblages occurred after D2 folding. Later D3 deformation was limited to local NW-trending folds and sinistral faults. The internal nappe-like structure of the Central Lapland Greenstone belt suggests that it represents the foreland of a large collisional complex cored by the Lapland Granulite belt.

An Overview of Paleozoic-Mesozoic Structures Developed in the Central White-Inyo Range, Eastern California

The central White-Inyo Range of eastern California is composed of a 7.5 km thick succession of Neoproterozoic to Upper Cambrian sedimentary rocks that have been deformed into a series of NE and N to NW-trending folds. The map distribution of the sedimentary rocks is primarily controlled by the younger N to NW-trending folds that define the White Mountain-Inyo anticlinorium. Smaller-scale folds and associated cleavage, which strongly fans in a divergent pattern about the fold hinge surfaces, are associated with both periods of fold development. Older NE-trending folds and associated cleavage are developed in a NW-trending belt that crosses the range in a structural saddle between the White Mountain and Inyo anticlines. These NE-trending folds are of smaller wavelength than the older N to NW-trending regional-scale folds, although both fold generations are developed on the cm-km scale. The intensity of both fold and cleavage development generally increases toward the west. Interference between the NE and the N to NW-trending folds has led to the development of a series of structural basins. We correlate formation of the NE-trending structures to the Antler orogeny, and formation of the younger N-to NW-trending structures with translation on the Last Chance thrust, which is exposed in the Inyo Range and presumably underlies the entire central White-Inyo Range. In the eastern and central portions of the range, it is clear that both periods of folding pre-date the emplacement of Jurassic and Cretaceous plutons that are correlatives to the intrusive suites exposed in the Sierra Nevada batholith to the west. On the western margin of the range another set of N to NW-trending structures, which are coaxial with the earlier N to NW-trending set, is probably associated with the East Sierran thrust belt; this thrust belt is better recorded in the southern Inyo Mountains, and developed in the Jurassic period.

Kinematic and dynamic analysis of a low-angle strike-slip fault: The Lake Creek fault of south-central Idaho

A large, low-angle strike-slip fault with significant offset is present in south-central Idaho. The 40 + km long Lake Creek fault is oriented N50°W, 30°SW and has a slip vector 18 km long, plunging 2° to N55°W. Slip was determined by restoring the offset of a map-scale fold trough. Thus, the fault is a dextral-normal fault with predominant dextral strike-slip and minimal normal dip-slip displacement. The present-day gentle dip of the fault appears to be the original dip since older folds and coeval to younger dykes in the area are not tilted. Several hypotheses for the origin of the low-angle fault are tested. Field and kinematic data support the interpretation that the fault formed as a low-angle strike-slip fault, not as a reactivated thrust fault nor in association with other regional fault systems. Based on fault array analysis and published experimental fault studies, the Lake Creek fault is interpreted to have formed as a low-angle strike-slip fault owing to a nearly axial compressive state of stress with a subhorizontal σ 1, and crustal anisotropy induced by NW-trending folds and SW-dipping axial planar cleavage.

Structural development of the King Leopold Orogen, Kimberley region, Western Australia

The King Leopold Orogen consists of two tectonic units: the Hooper Terrane, a complex of Early Proterozoic metasedimentary rocks, migmatites, felsic volcanics, basic sills and granitoid intrusions; and folded rocks of the overlying Early Proterozoic Kimberley Basin succession and a Late Proterozoic glacigene sequence. The oldest deformation ( D 1) affects metasedimentary rocks of the Hooper Terrane and is characterized by layer-parallel structures confined to zones of high strain. D 1 was accompanied by a metamorphic event ( M 1), which was locally high grade. D 2 post-dates the felsic volcanics but pre-dates granitoid intrusion and consists of upright NW-trending folds. D 3 post-dates deposition of the Kimberley Basin succession and, in the Hooper Terrane, takes the form of WNW- to NW-trending shear zones which combine dextral, sinistral and south-sideup movements. In the northwest the Kimberley Basin succession is folded and faulted by D 3 to form the Yampi Fold Belt. Two fold phases are recognized. D 3a, which comprises flat-lying folds and N-directed thrusts, is refolded by D 3b, comprising folds and thrusts linked to shear zones in the Hooper Terrane. D 3 was accompanied by amphibolite-facies metamorphism ( M 2). D 4 post-dates deposition of Late Proterozoic glacigene rocks, and produced SW-directed folds and thrusts to form the Precipice Fold Belt. Throughout the area discussed the contact between the Hooper Terrane and the Kimberley Basin succession is sheared or thrust. D 1, D 2 and M 1 are Early Proterozoic in age with D 1 and M 1 possibly forming in an extensional environment, while D 2 may represent a (?) sinistral strike-slip environment. D 3 and M 2 are the result of Middle to Late Proterozoic intracratonic compression and may be related to a collision located to the southwest. D 4 accompanied Late Proterozoic to Early Phanerozoic sinistral faulting in the Halls Creek Orogen. A major crustal structure of probable D 2 age separates the Hooper Terrane from (?) Archaean basement under the Kimberley Basin. It controls the northeast limit of the Yampi Fold belt ( D 3) and ramping of thrusts in the Precipice Fold Belt ( D 4, and could represent the site of an early Proterozoic terrane boundary.

An example of the use of veins to establish a cover fold history—irregully formation, western Australia

Structural correlation using fold overprinting and style, plus the timing and orientation of possible syntectonic veins, are used to interpret three minor folding events in part of the Irregully Formation. The formation lies near the base of the Middle Proterozoic Bangemall Group in the Bangemall Basin of Western Australia. It comprises interbedded well-laminated dolostones and orthoquartzites, metamorphosed under lower greenschist-facies conditions. Bedding in the formation is folded about a gently NW-plunging axis. Most mesofolds are consistent with the bedding distribution, exhibiting a fold-axis maximum plunging gently NW and vertical axial planes. Folds with NE-, E- and NNE-trending axial planes are common in the area, but most cannot be explained by reorientation of the dominant NW-trending folds. A deformation history accounting for different fold geometries is established using fold overprinting in conjunction with dilational offsets between fold-related quartz and dolomite veins. A number of approaches to determine the history of folding are possible. Fold overprinting is the most valuable criterion, but in weakly deformed terrains it may not be easily recognised, so that alternative methods should be examined. Fold directions in the Irregully Formation are parallel to trends in the underlying Ashburton Formation, suggesting a degree of basement reactivation. As a fold chronology in the cover has been established accounting for mesofolds and macrofolds, the interpreted reactivation is believed to have involved compression rather than passive drape over earlier structures. In the Irregully Formation differently oriented folds probably developed through a degree of cover shortening, associated with reactivation of basement faults or folds.

Palaeozoic flysch series in the Coastal Cordillera of Northern Chile

A discontinuous outcrop of Palaeozoic rhythmic clastites appears in the North Chilean Coastal Cordillera. The Formacion El Toco (21°15′–22°15′S) whose scarce flora points to a presumably Upper Devonian age forms the northernmost part of the outcrop. It is discordantly overlain by Lower Jurassic volcanics (Fm. La Negra), its base, however, is not exposed. Hercynian heteromorphous deformation led to predominantly NW trending folds strongly inclined mostly to the SW. Chevron-like folds of 0.1 to 30 m occur locally and seem to display properties transitional between synsedimentary-gravitational and early tectonic deformation.

A correlation of Lewisian structures and their displacement across the lower thrusts of the Moine thrust zone, NW Scotland

In the north Assynt district of Scotland the Lewisian rocks of the Moine thrust zone show a similar sequence of structures to the Scourian and Laxfordian structures of the western foreland. Early Scourian phases of deformation produced a flat fabric which, on the foreland, was accompanied by imbrication and repetition of sections of the lower crust. Late Scourian deformation formed NW trending folds whose shape and geometry depended on the pre-existing metamorphic texture. Large kink-band folds formed in pre-existing granulite facies rocks; large sinusoidal folds formed in amphibolite facies gneisses. The Laxford front is a steeply dipping shear zone at Laxford Bridge; a steep fault at Glencoul in the thrust zone. From the change in trend of the linear structures, the shear had a sinistral, north-side-up sense of movement. To the north of the Laxford Front, the Laxfordian foliation and shear zone curve to become flatter but in the same movement direction. Some Laxfordian folds may be explained by differential movement of this shear zone. Using the offset of the Laxford front together with the transport direction suggested by the strike of imbricate faults, the displacement on the lower Caledonian thrusts is 33 km. From the offset of a late Laxfordian fold as well as the Laxford front, the movement is 25 km. These estimates of the displacement are in agreement with others elsewhere along the Moine thrust zone.