Gentle Folds Research Articles

Deformation processes in the shallow levels of an accretionary prism: The Mesozoic Torlesse Terrane, South Island, New Zealand

Abstract The deformation style of the Torlesse Terrane along the southern Kaikoura coast, South Island, New Zealand, records shallow level deformation processes within an accretionary prism during the Early Cretaceous. The beds exhibit complicated structural features resulting from multistage deformations in a lithological unit, that were intimately related with the dewatering and lithification of terrigenous sediments. The earliest phase of deformation throughout the transect studied was the development of pinch‐and‐swell structures and boudinage fabrics due to layer‐parallel extension while the beds were poorly consolidated. This was followed locally by mesoscopic tight to close recumbent folding. The beds are cut locally by two phases of mudstone intrusions. The earlier phase was initiated by ‘in situ’ fluidization of mudstone layers, whereas the later phase represented intrusion of siliceous claystone probably derived from an overpressured decollement. Minor faults at high‐angles to bedding by layer‐normal shortening then disrupted the beds throughout the transect. The deformation was followed by formation of meso‐ and macroscopic scale open to gentle folds by layer‐parallel shortening. Kilometer‐scale differential stratal rotations were produced during the final main tectonic phase that occurred in association with post‐accretion Neogene regional disturbance.

Deformation history of the blueschist-facies sequences from the Villa de Cura unit (Northern Venezuela)

The Serrania del Interior terrane is located in Northern Venezuela, that represents the southern margin of the Caribbean plate. This terrane includes the Villa de Cura unit, which mainly consists of multiple sub-units of metamorphosed and deformed Cretaceous volcano-sedimentary sequences. These sequences are commonly interpreted as generated in a supra-subduction zone setting. The structural history of the Villa de Cura blueschists-facies units includes four main deformational phases, from D1 to D4. The first D1 phase is mainly represented by a relict S1 schistosity developed under HP/LT conditions. The relic S1 schistosity is deformed by isoclinal to subisoclinal F2 folds showing similar geometry. The F2 folds are characterized by a continuous S2 crenulation cleavage developed under greenschist-facies metamorphism. The parallelism between the A2 axes and the related L2 mineral lineations suggests an interpretation of the F2 folds as sheath folds developed during non-coaxial deformation. The kinematic indicators suggest a top-to-W sense of shear for the D2 phase. The D3 phase is distinguished by asymmetric, parallel F3 folds with a S vergence. Finally, the D4 phase consists of F4 open, gentle folds with high-angle to sub-vertical axial planes. The collected data suggest a complex deformation history, characterized by coupling of strike-slip tectonics and shortening during the retrograde evolution of the blueschist-facies sequences.

Structures in tessera terrain, Venus: Issues and answers

Many workers assume that tessera terrain, marked by multiple tectonic lineaments and exposed in crustal plateaus, comprises a global onionskin on Venus. Because tesserae are exposed mostly within crustal plateaus, which exhibit thickened crust, issues of tessera distribution and the mechanism of crustal plateau formation (e.g., mantle downwelling or upwelling) are intimately related. A review of Magellan data indicates that tessera terrain does not form a global onionskin on Venus, although ribbon‐bearing tesserae reflect an ancient time of a globally thin lithosphere. Individual tracts of ribbon‐bearing tessera terrain formed diachronously, punctuating time and space as individual deep mantle plumes imparted a distinctive rheological and structural signature on ancient thin crust across spatially discrete 1600–2500 km diameter regions above hot mantle plumes. Plume‐related magmatic accretion led to crustal thickening at these locations, resulting in crustal plateaus. Crustal plateau surfaces record widespread early extension (ribbon structures) and local, minor perpendicular contraction of a thin, competent layer above a ductile substrate. Within individual evolving crustal plateaus the thickness of the competent layer increased with time, and broad, gentle folds formed along plateau margins and short, variably oriented folds formed in the interior; late complex graben cut folds. Local lava flows accompanied all stages of surface deformation. In contrast to these conclusions, Gilmore et al. [1998] summarized post‐Magellan arguments in favor of downwelling models for crustal plateau formation. In light of this discrepancy, we reexamine the regions investigated by these workers and evaluate their arguments against upwelling models. We show that Gilmore et al. [1998] did not differentiate ribbons from graben and therefore their proposed temporal relations are invalid; they disregarded shear fracture ribbons, thus invalidating their criticism of ribbon models; they misunderstood previous radargrammetric work that constrains ribbon geometry; and they relied solely on geometrical relations to constrain timing, violating kinematic analysis methodology. Their stratigraphic constraints on ribbon‐fold temporal relations are invalid because they (1) misinterpreted implications of map relations; (2) did not isolate radar artifacts due to local radar slope effects from proposed material units; (3) chose a region for analysis that clearly shows the effects of younger tectonism and volcanism; and (4) presented map relations that cannot be reproduced. Their attempts to discount upwelling models of crustal plateau formation fail because they combine fundamentally different pre‐Magellan and post‐Magellan upwelling models. These misconceptions about the upwelling model and processes responsible for global warming [Phillips and Hansen, 1998], lead to serious errors in Gilmore et al.'s [1998] criticism. Furthermore, we show that the data of Gilmore et al. [1998] are actually more consistent with upwelling than downwelling models, consistent with arguments that tessera terrain is not global in spatial distribution.

The Donbas Foldbelt: its relationships with the uninverted Donets segment of the Dniepr–Donets Basin, Ukraine

The Donbas Foldbelt (DF) is a thrust-faulted and folded, inverted, segment of the Dniepr–Donets Basin (DDB). It lies between the Scythian and East European platforms north of the Black Sea and is conventionally thought to have formed in the Early Permian as part of the late Palaeozoic Hercynian–Uralian orogenic frame of cratonic Europe. The DDB itself formed as a result of Late Devonian intracratonic rifting and contains a thick, up to 22 km, late Palaeozoic and Mesozoic sedimentary succession. The deformed strata preserved at the erosional surface of the DF are mainly Carboniferous. The Donets segment of the DDB displays mild inversion structures that are transitional to those in the DF. Recently acquired regional seismic reflection data in the Donets part of the DDB allow these structures to be imaged in the subsurface for the first time. A widespread Early Permian unconformity occurring throughout the DDB is much more pronounced on its southern margin than on its northern one and appears to be related to profound uplift of the southern flank of the basin and the neighbouring Ukrainian Shield rather than to crustal shortening. Faults associated with the Early Permian unconformity are extensional in style. Gentle folds and other local structures in the southern flank and axis of the Donets segment are mainly located in areas associated with Early Permian salt movements. No significant development of compressional structures of Early Permian age can be seen in the seismic data where it is available. In contrast, there is ample evidence of latest Cretaceous–earliest Tertiary compressional deformation in the DDB, including the subsurface expression of structures along strike of the main folds of the DF. A review of geological evidence in the DF also indicates that the main period of shortening that can be definitely constrained in time is latest Cretaceous–earliest Tertiary. The eastern part of the DF records intense Late Triassic-aged (Cimmerian) deformation but none of clearly Permian age. It is tentatively concluded that the main phases of (trans)compressional tectonics forming the DF were Cimmerian and, especially, Alpine rather than Hercynian–Uralian.

Architecture of the tectonically influenced Sobrarbe deltaic complex in the Ainsa Basin, northern Spain

The syn-tectonic Sobrarbe deltaic complex (Eocene of the Ainsa Basin, Spanish Pyrenees) is confined by lateral thrust ramps and influenced by intra-basinal growth anticlines. Six facies associations have been distinguished within the deltaic complex: (i) slope marls and turbidite sandstones; (ii) distal delta front silty and bioturbated sandstones; (iii) proximal delta front and delta plain deposits (tidal inlet, tidal flat, mouth-bar, bay, distributary channel, and floodplain environments); (iv) biogenic deposits at flooding surfaces; (v) collapse zone sediments; and (vi) Nummulite-dominated shallow-marine carbonates. The deltaic complex comprises four composite sequences, which each contain a number of minor sequences of the genetic (maximum flooding surface bounded) type. Regressive unconformities and their correlative surfaces separate the composite sequences (CS), and each CS can be divided into lowstand, transgressive and highstand components. At the base, the Comaron CS is a WNW-prograding deltaic complex containing six minor sequences characterized by basinward-stepping sandstone wedges separated by transgressive mudstones. Double regressive unconformities are often associated with these sandstone wedges proximally in the basin. Growth of the intra-basinal Arcusa anticline caused gentle folding of the Comaron CS, and syn-growth strata at the base of the overlying Las Gorgas CS contain a prominent forced regressive sandstone wedge (the Las Gorgas sandbody) and display thinning of bedsets onto the crest of the anticline. Moreover, it can be demonstrated that relative sea-level rise took place in the growth synclines at the same time as a fall in relative sea level occurred on the growth anticlines. The tectonic deformation caused a shift in the direction of deltaic progradation towards the north-northwest, as demonstrated by the well-developed clinoforms in the highstand sequence set of the Las Gorgas CS. A second phase of tectonic activity, this time triggering large-scale collapse of the delta front, occurred at the transition between the Las Gorgas and Barranco el Solano CS. Tilting of the basin floor led to the development of coeval transgressive and regressive strata separated by a fulcrum (line of zero net-vertical movement of the shoreline). In the post-tilt phase, reduced sediment supply and accelerated subsidence led to widespread transgression and a change from clastic- to carbonate-dominated sedimentation. This phase was terminated by a gradual increase in sediment supply causing progradation of a mixed clastic–carbonate system dominated by large Nummulite banks and bioturbated shoreface marls and sandstones. Finally, a pronounced fall in relative sea level associated with the transition to the Buil CS triggered slope instabilities and eventually established fluvial conditions all over the study area. With the exception of the anticline crests, the relatively high rate of tectonic subsidence in the study area suppressed relative sea-level falls, resulting in development of discontinuous lowstand strata and bounding surfaces. The segmented nature of many regressive unconformities may be related to incremental growth of thrust-related structures, and each segment is separated from neighbouring segments by gaps of limited vertical and lateral displacement. Precursors of progressive unconformities are seen in the upper proximal part of the Comaron CS by downdip `fanning' and updip convergence of minor sequences. These differentially tilted stratal units provide reliable indicators of syn-sedimentary tectonism.

Tectonic controls on the hydrocarbon habitats of the Barito, Kutei, and Tarakan Basins, Eastern Kalimantan, Indonesia: major dissimilarities in adjoining basins

The Barito, Kutei, and Tarakan Basins are located in the eastern half of Kalimantan (Borneo) Island, Indonesia. The basins are distinguished by their different tectonic styles during Tertiary and Pleistocene times. In the Barito Basin, the deformation is a consequence of two distinct, separate, regimes. Firstly, an initial transtensional regime during which sinistral shear resulted in the formation of a series of wrench-related rifts, and secondly, a subsequent transpressional regime involving convergent uplift, reactivating old structures and resulting in wrenching, reverse faulting and folding within the basin. Presently, NNE–SSW and E–W trending structures are concentrated in the northeastern and northern parts of the basin, respectively. In the northeastern part, the structures become increasingly imbricated towards the Meratus Mountains and involve the basement. The western and southern parts of the Barito Basin are only weakly deformed. In the Kutei Basin, the present day dominant structural trend is a series of tightly folded, NNE–SSW trending anticlines and synclines forming the Samarinda Anticlinorium which is dominant in the eastern part of the basin. Deformation is less intense offshore. Middle Miocene to Recent structural growth is suggested by depositional thinning over the structures. The western basin area is uplifted, large structures are evident in several places. The origin of the Kutei structures is still in question and proposed mechanisms include vertical diapirism, gravitational gliding, inversion through regional wrenching, detachment folds over inverted structures, and inverted delta growth-fault system. In the Tarakan Basin, the present structural grain is typified by NNE–SSW normal faults which are mostly developed in the marginal and offshore areas. These structures formed on older NW–SE trending folds and are normal to the direction of the basin sedimentary thickening suggesting that they developed contemporaneously with deposition, as growth-faults, and may be the direct result of sedimentary loading by successive deltaic deposits. Older structures were formed in the onshore basin, characterized by the N–S trending folds resulting from the collision of the Central Range terranes to the west of the basin. Hydrocarbon accumulations in the three basins are strongly controlled by their tectonic styles. In the Barito Basin, all fields are located in west-verging faulted anticlines. The history of tectonic inversion and convergent uplift of the Meratus Mountains, isostatically, have caused the generation, migration, and trapping of hydrocarbons. In the Kutei Basin, the onshore Samarinda Anticlinorium and the offshore Mahakam Foldbelt are prolific petroleum provinces, within which most Indonesian giant fields are located. In the offshore, very gentle folds also play a role as hydrocarbon traps, in association with stratigraphic entrapment. These structures have recently become primary targets for exploratory drilling. In the Tarakan Basin, the prominent NW–SE anticlines, fragmented by NE–SW growth-faults, have proved to be petroleum traps. The main producing pools are located in the downthrown blocks of the faults. Diverse tectonic styles within the producing basins of Kalimantan compel separate exploration approaches to each basin. To discover new opportunities in exploration, it is important to understand the structural evolution of neighbouring basins.

Bottom response to a tsunami earthquake: Submersible observations in the epicenter area of the 1993 earthquake off southwestern Hokkaido, Sea of Japan

We conducted a research campaign of nine dives using the manned submersible vehicle Shinkai 6500 in 1995 beneath the central Okushiri Ridge along the eastern margin of the Sea of Japan. The main objective was to investigate coseismic response and bottom disturbance to the tsunamigenic 1993 Offshore Southwestern Hokkaido earthquake (Ms 7.8; 42°47′N, 139°12′E) in the northern aftershock area including the mainshock epicenter. The whole aftershock area occupies the western half of source area of the related tsunami. Between the Okushiri Ridge and the neighboring Shiribeshi Trough to the east, we found a gentle scarplet of the earthquake fault that bounds the first terrace and a zone of open gashes on the flat terrace. A number of sand blows, open cracks, and other slope failures occupied the slope of the Okushiri Ridge on the uplifted hanging wall side of the tsunami source area. Some fractures and holes whose activity had ceased soon after the earthquake began to be buried by the 2‐year accumulation of recent sediments. In contrast, the Shiribeshi Trough, on the footwall of the fault and in the subsided domain of the tsunami source area, suffered no remarkable disturbance other than gentle folding. Our submersible observations elucidated the seafloor response to tsunamigenic uplift of the hanging wall of a seismogenic thrust fault which displays the present time section of back‐thrusting with an embryonic subduction along the eastern margin of the Sea of Japan.

Collision structure in the upper crust beneath the southwestern foot of the Hidaka Mountains, Hokkaido, Japan as derived from explosion seismic observations

The P-wave velocity structure of the upper crust beneath a profile ranging from Niikappu to Samani in the southwestern foot of the Hidaka Mountains, Hokkaido, Japan was obtained through analysis of refraction and wide-angle reflection data. The mountains are characterized by high seismicity and a large gravity anomaly. The present profile crosses the source region of the 1982 Urakawa-oki earthquake ( M s 6.8). The length of the profile is 66 km striking northwest and southeast. Along the profile, 64 vertical geophones were set up and 5 shot points were chosen. For each shot, a 400–600 kg charge of dynamite was detonated. The studied area is composed of four major geological belts: Neogene sedimentary rocks, the Kamuikotan belt, the Yezo Group, and the Hidaka belt. The measurement line crosses these geological trend at an oblique angle. The structure obtained is characterized by remarkable velocity variations in the lateral direction and reflects the surface geological characteristics. A thin, high-velocity layer (HVL) was found between low-velocity materials in the central part of the profile, beneath the Kamuikotan Metamorphic Belt, at a depth ranging from 0.5 to 6 km, overthrusting toward the west on the low-velocity materials consisting of Neogene sedimentary rocks, and forming gentle folds. Outlines of the velocity structure of the Hidaka Mountains yielded by other studies have shown a large-scale overthrust structure associated with the collision of the Outer Kurile and the Outer Northern Honshu Arcs. The shallow velocity structure inferred by the present study showed a similar (although small scale) overthrust structure. The obtained structure shows that the composite tectonic force, comprising westward movement of the Outer Kurile Arc and northward movement of the Outer Northern Honshu Arc, plays an important role in the evolution of the tectonic features of the crust and upper mantle in a wide depth range beneath the Hidaka Mountains.

Extensional subsidence, contractional folding and thrust inversion of the eastern Cameros basin, northern Spain

Tectonic analysis of the Cameros massif (northern Spain) demonstrates the need to integrate detailed structural analysis with stratigraphic studies in order to understand the development of inverted basins. The Cameros massif underwent three major tectonic events, each characterized by different mechanisms of deformation: The first event (Tithonian–Early Albian) manifested as continental rifting and formation of the Cameros basin and was attended by normal faulting and related folding of pre- and syn-rift sedimentary sequences (up to 9 km thick). It corresponded to the beginning of rifting and opening of the Bay of Biscay to the north of the massif. The second event occurred at the end of the Early Cretaceous. The basin underwent contraction, manifested as gentle folds containing axial-plane cleavage. This contraction was due to the migration of the center of rifting away from the basin and toward the northeast, and correlates with the onset of oceanic spreading in the Bay of Biscay. Deformation happened during a low-grade metamorphism period that reached its thermal peak after folding and cleavage formation (90–100 Ma). The final stage of deformation (Eocene–Miocene), following a structurally quiescent period of approximately 40 Ma, occurred during the convergence/collision between Iberia and Europe. It formed contractional folds related to large E/W-striking overthrusts. The Mesozoic basin was inverted and transported 25 km toward the north. During this stage both unfolding and tightening of the former folds occurred, as determined from analysis of cleavage orientation associated with the previous shortening.

Structural Style of the Warburton Basin and Control in the Cooper and Eromanga Basins, South Australia

Deep-seated structures within the Warburton Basin have been intermittently reactivated during development of the overlying Cooper and Eromanga Basins. Four major deep structures preceding deposition of the Cooper Basin succession are described. 1) A complex deformation zone occurs on the eastern flank of the Birdsville Track Ridge near Kalladeina 1. 2) Simple reverse faults are common and widespread in the Warburton Basin strata. 3) Gentle folds occur in the Coongie area and become increasingly tight towards the Birdsville Track Ridge. 4) Imbricate thrust faults form the cores of structures along the northeast-southwest trending Gidgealpa–Merrimelia–Innamincka Ridge. Warburton Basin structural control in the overlying basins is characterised by reactivation and propagation of thrust faults along the Gidgealpa–Merrimelia–Innamincka Ridge, and by further folding and faulting of the complex deformation zone on eastern flank of the Birdsville Track Ridge. The episodic, compressive reactivation has controlled depositional and erosional patterns of the Cooper and Eromanga Basins, and hence controlled hydrocarbon occurrence in these two basins. Potential footwall traps may be formed by propagation of the thrust faults.

Post-Middle Miocene rotations recorded in the Bourg d'Oisans area (Western Alps, France) by paleomagnetism

In order to test the occurrence of post metamorphic rotations or tiltings, a paleomagnetic study was undertaken in 44 sites from the inverted half graben of Bourg d'Oisans (“Zone Dauphinoise”, Western Alps). The magnetization carried by pyrrhotite was acquired after metamorphic culmination, around the Oligo-Miocene limit. No obvious rotations around a vertical axis, are recorded. Instead, the dispersion of paleomagnetic data is interpreted as due to a variable tilting around a horizontal N20E axis, consistent with structural observations. This gentle folding in the sedimentary cover is probably associated with thrusting of the Belledonne external crystalline massif over the foreland basement. The age of the tiltings reported in this paper is estimated as being younger than 9–13 Ma.

The structural evolution of the Arena Gneisses and its bearing on Proterozoic tectonics of Sri Lanka

The Arena Gneisses (AG), which is considered to form an eastern portion of the Wanni Complex (WC), is exposed at five doubly plunging synforms (arenas) in central Sri Lanka, near the boundary between the WC and the Highland Complex (HC). Structural investigation based on detailed field work on two of the arenas classified the following structural sequences from the earliest to the latest: Dn-2, Dn-1, Dn, Dn + 1, and Dn + 2 deformations. The Dn-2 structures include the major compositional banding, pinch-and-swell and boudinaged structures formed by compression normal to the banding. Mesoscopic intrafolial isoclinal to tight folds (Fn-2) were formed synchronologically with Dn-2 deformation. During the Dn-2 stage, the AG thrusted over the HC with top-to-the-southward movement. The asymmetric structures such as asymmetric foliation boudinaged structure, E-W-trending asymmetric folds (Fn-1b) and asymmetric megacrysts, and N-S-stretching lineations were formed by this simple shear deformation. This transport of the AG caused the N-S shortening and produced macroscopic gentle folds (Fn-1a) with mostly E-W-trending upright axial surfaces. During the Dn-1 stage, the eastward movement of the AG took place. This movement was associated with N-S-trending asymmetric folds (Fn-1c) and microfabrics observed in the AG, as well as the eastward vergences of related folds in the HC. N-S-trending upright linear folds (Fn) and related structures such as parasitic folds and axial surface foliations were formed by E-W shortening, which is normal to the axial surface of the macroscopic Fn fold, during the Dn deformational stage. Dextral shear deformation with vertical shear planes took place during Dn + 1 deformation. ENE-WSW- or NE-SE-trending folds (Fn + 1) were developed during Dn + 2 deformation. The southward thrusting of the AG indicates that the WC has displaced over the HC with top-to-the-southward movement. The eastward transportation of the AG and the HC may be due to the collision of the Vijayan Complex (VC) with the HC. Considering the above-mentioned tectonic elements with Sri Lankan geochronological data (e.g. Hölzl et al., 1994), the first thrusting event might have taken place around 700 Ma and the second displacement probably occurred around 570 Ma.

Structural features of mud volcanoes and the fold system of the Mediterranean Ridge, south of Crete

New information on the geometry of the mud volcanoes, folds and faults located in the central part of the Mediterranean Ridge is provided from long range and deep-tow sidescan sonar images, high resolution seismic and low frequency echosounder profiles, and gravity cores obtained on the R/V Gelendzhik during the 1993 UNESCO-ESF Training Through Research cruise. Symmetrical gentle folds, with a mean wavelength of 750 m, deform predominantly Pliocene-Quaternary sediments, but also perhaps as old as Messinian. In most areas of the Mediterranean Ridge, the north-northeast directed subduction of the African plate below the Eurasian plate at the Hellenic arc produces folds with hinge lines subparallel to the trend of the ridge. However, the hinge lines of the folds curve around the southern part of the area with highest concentration of mud volcanoes and produce an irregular U-shape fold belt. Some of the folds show an intrusive nucleus and, in some of them, mud breccia appears to have flowed from the flanks of the folds into the troughs. The mud volcanoes consist of domes of interlayered pelagic sediments and mud breccias containing sediments and rock fragments of Late Aptian to Pleistocene ages. Mud breccia flows are mainly extruded radially from vents, although rare fissure extrusions are also observed. The mud volcanoes have an irregular to elliptical shape, with diameters up to 16 km. The distribution of mud volcanoes in the area is irregular, but they are more frequent in an area called the Olimpi Field. Faults in the uppermost part of the Mediterranean Ridge are rare. Most of the faults are normal and subparallel to fold limbs. In addition, N20°E and N100°E subvertical faults are found controlling the shape of the mud volcanoes.

Structural Geometry of the Rocks of the Southern Part of the Nallamalai Fold Belt, Cuddapah Basin, Andhra Pradesh

Abstract A sequence of quartzite, banded phyllite and slate in the southern part of the Nallamalai fold belt shows a series of macroscopic gently plunging folds (DI) with NW-SE striking axial plane and a pervasively developed axial planar slaty cleavage. A weak later deformation has produced sporadic gentle folds (D2) with axial planes at high angle to the D1 fold axial planes. The rocks along the eastern contact of the belt are mylonitized and the intensity of deformation gradually dies away towards west. A zone of tectonic dislocation is envisaged between the basement granite gneiss towards the east and the supracrustal Nallamalai rocks.

The deformation history of an accreted ophiolite sequence: the Internal Liguride units (Northern apennines, Italy)

The Mt. Gottero, Bracco/Val Graveglia and Colli/Tavarone units from the Northern Apennines, Italy, are remnants of the Ligure-Piemontese oceanic lithosphere that were involved in the tectonic events related to convergence between the European and Adria plates. These events included late Paleocene – middle Eocene subduction followed by late Eocene continental collision.These units, referred to as Internal Liguride units, consist of Jurassic ophiolites covered by a thick sedimentary sequence, which includes late Jurassic-Santonian pelagic deposits (chert, Calpionella limestone and Palombini shale) and Campanian-Early Paleocene trench deposits (Val Lavagna shale and Mt. Gottero sandstone) that are overlain by Early Paleocene lower slope deposits (Bocco shale). This sequence is interpreted as recording trenchward motion of the oceanic lithosphere.The structural analysis of the Internal Liguride units reveals a pre-Oligocene deformation history, which includes the following stages:– stage 1: pre-folding, bedding-parallel veins developed during burial and dewatering of a thick sedimentary sequence;– stage 2: D1 phase distinguished by non cylindrical, isoclinal folds with approximately similar geometries. The associated slaty cleavage is characterized by a mineral assemblage related to P/T metamorphic climax of 0.3–0.4 GPa and 180°-300°C. The asymmetric recrystallized tails around detrital minerals and framboidal pyrites indicate a non-coaxial deformation history characterized by top-to-west sense of shear. Cataclastic shear zones developed in the latest stage of D1;– stage 3: D2 phase marked by overturned, parallel folds associated with a crenulation cleavage. The following D3 phase is characterized by gentle folds with a steep axial plane.The sedimentary records and the structural features support the interpretation of the Internal Liguride units as slices of oceanic lithosphere deformed in a subduction setting. Their deformation history probably developed during underthrusting, cohereht underplating and later exhumation in an accretionary wedge related to an east-dipping, low-rate subduction zone.

Map restoration of an early Pliocene horizon along the Hosgri-Purisima-Lompoc fault system, central California margin: ABSTRACT

The Hosgri fault is the southern part of the San Gregorio-Sur-San Simeon-Hosgri fault system, an important element of the Neogene California transform system. The Hosgri fault proper (top 1-2 km) is located in the hanging-wall of a 10 to 20 km wide zone of mostly E-dipping faults, the Hosgri-Purisima-Lompoc fault system. These faults have undergone Miocene extension, and have been reactivated by post-Miocene contraction or transpression. This wider fault system is truncated against or merges with the E-striking faults in northwestern Santa Barbara Channel. From this starting point of zero displacement, right-lateral displacement on a N- S fault can not be more than the sum of N-S shortening east of it, the N-S extension west of it, and the right-lateral slip fed into it by other faults. We are using a 3-D map restoration technique to quantify the displacements along this fault system. Depth-contoured folded surfaces are flattened using the software UNFOLD, and the restored surfaces are fit back together across faults using an interactive graphic program. Displacements are calculated by comparing the restored surface to the present state with respect to a relatively-fixed block. Our mapping of the early Pliocene top Sisquoc horizon indicates that broad, gentle folds characterizemore » the Point Arguello oil field. Folds located between Point Arguello and Point Sal, are characterized by abrupt changes in amplitude, symmetry, and vergence along strike. The folded surface is unfaulted over wide areas, despite dips up to 45[degrees]. These folds are the result of basin inversion along former rift border faults. Folds are amplified where the basin fill is transpressed against NW-striking restraining bends in a regional NNW-striking system of right-reverse-oblique faults.« less

K-Ar dates from the Altiplano and Cordillera Oriental of Bolivia: implications for Cenozoic stratigraphy and tectonics

New K-Ar dates from tuffs, lavas and granites, collected in the Bolivian Altiplano and Cordillera Oriental, constrain the evolution of the Bolivian Andes. A phlogopite megacryst from a post-cleavage kimberlite dike near Independencia gave an age of ca. 98 Ma. In the central Altiplano, a widespread suite of basaltic to andesitic lavas, sills and rhyolitic tuffs has yielded dates of 25-22 Ma, coeval with the first major influx of conglomerates in the central Altiplano basin. Rapid early Miocene erosion of the Cordillera Oriental is suggested by biotites from the Quimsa Cruz Granite in the Cordillera Oriental, which vary in age between 32-22 Ma, and also by zircon fission track data. Gentle folding in the Cordillera Oriental near Potosi pre-dates the early Miocene Mondragon Formation which contains a ca. 19 Ma ignimbrite near its base. Further folding here occurred prior to the eruption of the flat-lying ca. 7 Ma ignimbrites of the Condor Nasa Meseta and ca. 12 Ma ignimbrites from the central Los Frailes Meseta. Intense folding in the central Altiplano also occurred between 9 Ma and 5 Ma, and younger tuffs are only slightly tilted. In the Cochabamba region, sedimentary infill of the Parotani Basin contains a ca. 20 Ma tuff horizon, and folded tuffs from the nearby Sacaba Basin have been dated at ca. 2.2 Ma. These ages suggest a protracted history for associated basin-margin ESE sinistral strike-slip and normal faults. However, regional folding pre-dates the San Juan del Oro Surface, which formed an extensive flatlying peneplain, preserved at ca. 3000 m in the central and eastern Cordillera Oriental. The age of valley-fill tuffs suggest that dissection of this surface had commenced in the Cochabamba region by 6.5 Ma. However, near Sucre, flat-lying tuffs, dated between 3.5 and 1.4 Ma, mantle the surface and predate the deep Pleistocene dissection of the central Cordillera Oriental. Deformation has been concentrated in the Subandean foreland thrust belt since the Pliocene.

Tectonic evolution of the High Himalaya in Upper Lahul (NW Himalaya, India)

The Upper Lahul region in the NW Himalaya is located in the transition zone between the High Himalayan Crystalline (HHC) to the SW and the Tethyan Zone sedimentary series to the NE. The tectonic evolution of these domains during the Himalayan Orogeny is the consequence of a succession of five deformation events. An early Dl phase corresponds to synmetamorphic, NE verging folding. This deformation created the Tandi Syncline, which consists of Permian to Jurassic Tethyan metasediments cropping out in the core of a large‐scale synformal fold within the HHC paragneiss. This tectonic event is interpreted as related to a NE directed nappe stacking (Shikar Beh Nappe), probably during the late Eocene to the early Oligocène. A subsequent D2a phase caused SW verging folding in the HHC. This deformation is interpreted as contemporaneous with late Oligocene to early Miocene SW directed thrusting along the Main Central Thrust. In the Tethyan Zone, a D2b phase is marked by a decollement thrust, a system of reverse faults, and gentle folds, associated with SW directed tectonic movements. This deformation is related to an imbricate structure, characteristic of a shallow structural level, and developed in the frontal part of a nappe affecting the Tethyan Zone units of SE Zanskar (Nyimaling‐Tsarap Nappe). A later D3 phase generated the Chandra Dextral Shear Zone (CDSZ), a large‐scale, ductile, dextral strike‐slip shear zone, located in the transition zone between the HHC and the Tethyan Himalaya. The CDSZ most likely represents a part of a system of early Miocene extensional and/or dextral, strike‐slip shear zones observed at the HHC‐Tethyan Zone contact along the entire Himalaya. A final D4 phase induced large‐scale doming and NE verging back folding.

Paradigms for the Pilbara

Abstract The Pilbara Craton is a discrete and coherent body of well preserved and exposed early continental crust in the northwestern part of the Australian continent, with a present surface area of about 180 000 km 2 . It has two tectonostratigraphic components: an older ‘basement’ of granite-greenstone terrane, formed between about 3.5 and 2.9 Ga, unconformably overlain by a supracrustal sequence, the Mount Bruce Supergroup, laid down between about 2.8 and 2.4 Ga in the Hamersley Basin. The main body of the craton has retained its integrity since that time, and has undergone only regional uplift, mild metamorphism, and gentle folding. A tendency to interpret the geology of the craton in terms of paradigms, involving a simplistic comparison with other areas thought to be better understood, has impeded the development of independent hypotheses of crustal evolution based on objective integration of observational evidence from within the Pilbara craton itself.

Dislocated Quaternary deposits in southeastern Kattegat – a glacial or gravitational phenomenon?

The Quaternary deposits in the Store Middelgrund–Rørdebanke area midway between the island of Anholt and Hallandsåsen on the Swedish coast are described on the basis of reflection seismic profiles with a vertical resolution of 5–10 m. The Quaternary rests on Upper Cretaceous limestone, the surface of which is nearly horizontal. Three Quaternary sequences are defined and interpreted as: (1) Late Weichselian marine or lacustrine deposits, (2) Late Weichselian glaciogenic deposits, and (3) Late Saalian–Eemian and Early–Middle Weichselian deposits. Sequence 3 is probably comparable to the upwards‐coarsening sequence known from Skaerumhede in Vendsyssel. The layers in sequence 3 are dislocated in the eastern part of the Store Middelgrund–Rødebanke area mainly by gentle folding, but other types of deformations occur. Folding could be the result of horizontal push from an ice sheet approaching from the east. Alternatively the folding is an effect of vertical, gravitational forces acting on the sediments due to an unstable density profile, as described by the Rayleigh–Taylor instability model. The zone of deformation is located close to the northern flank of the tectonically active Sorgenfrei–Tornquist Zone. It is suggested that the initiation of the folding process was facilitated by tremors from small earthquakes.