Large-scale Folding Research Articles

The geology and structural evolution of a Caledonian fold and ductile thrust zone, Kyle of Tongue region, Sutherland, northern Scotland

The Middle to Upper Proterozoic metasedimentary rocks of the northern Highland Moine in Sutherland are disposed in a series of major Caledonian ( c . 470–430 Ma) metamorphic thrust nappes produced by NW/WNW-directed shortening and reworking of parts of a pre-existing Precambrian (c. 1000 Ma) orogen. Lewisian inliers occur throughout Sutherland and represent tectonically interleaved units of the original basement gneisses upon which the Moine cover sequence was deposited. In the western Moine Nappe of the Kyle of Tongue region, the rocks are deformed by two main episodes, D1 and D2, which are thought to represent the effects of Precambrian and Caledonian orogenesis respectively. Little basement-cover interleaving or large-scale folding occurred during the D1 event in the Moine Nappe, but the associated metamorphism (to ut leust garnet grade) produced peak metamorphic mineral assemblages. Subsequent Caledonian reworking (D2) produced a foreland-propagating system of WNW-directed ductile thrusts and related major F2 sheath folds, both of which resulted in much of the observed Moine-Lewisian interleaving. Metamorphism associated with D2 decreases in grade westwards, occurring at similar, or slightly lower grades, than the earlier D1 event; this retrogressive overprint is most obvious at the W margin of the Moine Nappe. The D2 folds and ductile thrusts are geometrically and kinematically similar to the Caledonian structures recognized within the Moine Thrust Zone to the west, the very low (≫5°) ramp angles suggesting that the thrusts must have very shallow-dipping trajectories through the crust. F2 fold formation is attributed to an early phase of WNW-directed overthrusting compression which was then progressively superseded by a regional ductile extensional strain contemporaneous with continued WNW-directed thrusting. The occurrence of internal extension in the orogenic wedge during active thrust-related foreshortening implies that the shear strength of the material close to the basal detatchment(s) was continuously reduced, i.e. wedge motion became supercritical . In this context, strain softening processes within the ductile thrust mylonites were probably of major significance.

Transition from near‐surface thrusting to Intrabasement Decollement, Schlinig Thrust, eastern Alps

West directed thrusting along the Schlinig thrust amounts to a minimum displacement of 45 km. A discrete plane of brittle thrusting is observed in the frontal part, where the Ötztal unit overrides the sediments of the Engadine Dolomites. Both shortening of the more ductile Lower and Middle Triassic and very substantial stretching by domino‐type normal faults in the brittle Upper Triassic dolomites are contemporaneous with, and caused by, thrusting. Extension is related to extrusion of parts of the sediments in the footwall of the Schlinig thrust. A relatively rapid transition into an intrabasement shear zone, which is as much as 2‐km‐thick, occurs farther to the east in a region where a paleotemperature of 300°C is indicated for Cretaceous metamorphism. Still farther to the east, localized shearing is transformed into a wider area affected by large‐scale folding. This second transition corresponds to estimated temperatures in excess of 550°C. Since the changes in structural style and deformation mechanisms toward the hinterland go hand in hand with increasing metamorphic grade, we regard thrusting as contemporaneous with metamorphism. This view is supported by geochronological data. The age of thrusting and metamorphism (90 Ma) indicates very rapid heating after the deposition of the youngest sediments (Cenomanian) involved in Cretaceous deformation. Our data suggest that localized detachment along a discrete thrust plane and within broader mylonite zones cannot always be traced indefinitely to greater depth. The transformation of localized shearing into large scale folding indicates a fundamentally different deformational style for lower crustal levels.

Characterisation of late Cretaceous NE-SW sinistral wrench faults in the Upper Benue Trough (Nigeria) using microtectonic and aeromagnetic data

The northern section of the Cretaceous Benue Trough in Nigeria presents several major faults zones striking N 30°E to N 60°E cross-cutting both sedimentary and Precambrian basement rocks in the area. The Mesozoic deposits in the area are also deformed by the NE-SW to N 80° E trending assymetric large scale folds (Lamurde and Jarawa anticlines, Dadiya syncline). A detailed microtectonic study of the deformed tectonic zones has been carried out, mainly along the major faults zones (Gombe, Burashika, Kaltungo and Teli areas) on the sedimentary basin and on the Pan-African basement (Plateau and Gurukusa areas) surrounding the basin. At each microtectonic station a significant population of striated planes and their associated slickensides were measured. Analysis of the brittle microfaults using a computer-aided method resulted in the definition of the tectonic paleostress tensors responsible for their formation. A first brittle compression characterized by a maximum compressive stress σ 1 predominantly trending N 15° E was mainly identified along the NE-SW striking major fractures. This particular brittle tectonic event is believed to be related to the sinistral Cretaceous reactivation of Pan African NE-SW transcurrent faults. In general all the estimated stress tensors close to strike-slip tensor are characterized by a significant deflection of the direction of the maximum compressive stress σ 1 ranging from N 180° E to N 60°E. By comparing these data with microtectonic and mathematical models on strike-slip faults, the characterization of the NE-SW megafractures as sinistral strike-slip faults is particularly demonstrated. A second N 150° E compressive brittle event is also observed. The two tectonic events are documented as prevailing at the end of the Cretaceous. A structural interpretation of the total magnetic field intensity map of the study area confirms the predominance of NE-SW trends in the area. Many of these trends appear as deep magnetic discontinuities which obviously have their source in the basement. A number of these trends coincide with mapped faults (e.g. Gombe, Teli, Kaltungo and Burashika fractures). In the northeastern part of the investigated area, magnetic elongated zones demonstrate a tectonic control of the Cretaceous sediments by structurates in the basement.

Photointerpretation of Tertiary structures in platform cover strata of interior Oscar II Land, Spitsbergen

A structural map of Oscar II Land compiled using black and white, stereo-pair air photos and other available information sources suggests the existence of three structural zones within the deformed platform cover strata. The western zone consists of basement involved overthrusts of complex geometry in the north large-scale folds of platform cover strata with a stepped, down to the east profile in the south. The central zone consists primarily of folds within Kapp Starostin Fm. and overlying Triassic strata that likely formed above a subhorizontal decollement within Gipshuken Fm. strata. Farther to the east thrusts emplace Permian Kapp Starostin Fm. strata over Triassic strata. This zone probably represents stratigraphic climb of the basal decollement. Conservative estimates of shortening for the central and eastern zones are in the 15-25% range. The contribution of the western zone structures to overall Tertiary shortening depends on the geometry of faults underlying the stepped folds and is uncertain at present. Changes along strike are in part due to differential levels of erosion and changes in transport direction, but also probably important was variable thickness of the platform cover strata involved (less to the north) and the availability of incompetent horizons along which thrust-fiats could form. Two areas with anomalous structural trends may represent oblique ramps.

Early proterozoic continental accretion in the canadian shield: Evidence from U-Pb geochronology of reactivated Archean crust in the labrador trough and Thompson Belt

The Glenelg-Attadale Inlier of the northwest Highlands of Scotland comprises a Western Unit (WU) containing Trondhjemite-Tonalite-Granodiorite (TTG) gneisses and subordinate mafic bodies and an Eastern Unit (EU) containing abundant mafic rocks (eclogite), minor ultrabasic bodies and manganiferous metasedimentary rocks and marbles intercalated with intermediate to acid gneiss. The boundary between these two units is an amphibolite facies ductile shear zone, which contains kinematic criteria indicating a top-to-the-west (contractional) sense of shear. The EU was exhumed from eclogite facies to amphibolite facies conditions of the mid-crust at ca. 1000 Ma via amphibolite facies ductile shearing. A series of shear zones with identical metamorphic grade and kinematics occur at the contacts of the EU with the WU and the surrounding Moine Supergroup.In this study, an attempt is made to directly date the shear zone between the EU and WU, which also contains Moine Supergroup metasediments. A U–Pb titanite age of 669 ± 31 Ma, from syn-kinematic titanite within a mylonite, indicates a minimum age for movement on this shear zone, demonstrating that it must be Late-Proterozoic in age, and not Caledonian as previously supposed. If the shear zone is Grenvillian (ca. 1000 Ma) in age, this would then require that the Moine be deposited before ca. 1000 Ma in order to undergo burial to the mid-crust at this time. However, the youngest detrital zircons in the Moine indicate a maximum deposition age of 1000 Ma and, therefore, preclude this possibility. An alternative possibility is that movement along this shear zone was punctuated, with an initial phase involving uplift of eclogites to the mid-crust at ca. 1000 Ma, followed by a further period of top-to-the-west shearing, coupling of the Moine Supergroup and EU in the mid-crust during a putative Neoproterozoic “Knoydartian” orogenic event. However, since there are now three contractional “orogenic” events affecting the Moine, ca. 820, 740 and 670 Ma, it is suggested here that the term “Knoydartian” is no longer well-defined and requires re-appraisal.A later phase of extensional ductile shearing and large-scale folding, during a period of lower amphibolite facies metamorphism, is shown to be broadly Caledonian, based on a 437 ± 6 Ma U–Pb titanite age from a pegmatite, that is deformed during this event, and prior to movement along the Moine Thrust Zone. This further suggests an Ordovician “Grampian” orogenic event within the Northwest Highlands of Scotland.

On the development of folds in auroral arcs

The development of an auroral arc in the midnight sector, from diffuse to discrete with subsequent large scale folding, is studied with the aid of several ground-based observations, including incoherent scatter radar, and data from a HILAT satellite pass. Ion drift velocities in the F-region, as measured by EISCAT, were consistently eastward throughout and after the whole period of development, whilst the ion temperature showed two large enhancements just prior to the appearance of the main auroral fold. The fold moved eastwards and crossed the EISCAT antenna beam, appearing as a short-lived spike in electron density at altitudes between about 100 km and 400 km. The spike in electron density came progressively later at higher altitudes. The observations are interpreted as the result of enhanced convection in the ionosphere and in the magnetosphere. The auroral arc folding is suggested to be caused by the Kelvin-Helmholtz instability in a velocity shear zone in the magnetosphere.

Evolution of structures and fabrics in the Barbados Accretionary Prism. Insights from leg 110 of the Ocean Drilling Program

Detailed analysis of ODP Leg 110 cores provides evidence for a complex structural evolution of the frontal thrust system in the northern Barbados Accretionary Prism. Initial east-directed in-sequence thrusts are biostratigraphically documented, and are overprinted towards the west by out-of-sequence overthrusts, largescale folds and penetrative deformation of the accreted rocks. This structural progression is interpreted to reflect the need for secondary shape adjustments of the accretionary prism in order to maintain a critical taper. At present the basal décollement is located within Lower Miocene strata. Accreted Eocene sediments suggest either fossil frontal off-scraping at a lower stratigraphic level or duplex formation and underplating. The present localization of the basal detachment is provoked by a zone of initial high porosity in the Lower Miocene radiolarian-rich mudstones. Shearing is accompanied by partial porosity breakdown in a 30–40 m wide zone of intense scaly fabrics and stratal disruption that constitutes the Atlantic-Caribbean plate boundary at present. Large displacements are taken up by a relatively narrow zone of contact strain. Initially the preferred orientation of clay minerals (incipient scaly fabrics) may be a flattening fabric, but with progressive rotational flow the scaly planes may act as discrete microshears and provide small-scale flow partitioning. Alternatively, scaly fabrics may be interpreted as brittle microshears. Mud-filled veins are present in the accreted rocks as well as at the oceanic reference site east of the deformation front. Carbonate-filled veins are restricted to the accreted rocks. This indicates persistence of high fluid pressures, but a fundamental change in the type of fluid-rock interaction during frontal accretion tectonics.

Structural Profiles of Ouachita Mountains, Western Arkansas

The Ouachita Mountains of Oklahoma and Arkansas are the largest exposure of the Pennsylvanian-age orogen rimming the southern margin of North America. The exposure consists of a thick Carboniferous flysch sequence overlying a thin early Paleozoic deep-water sequence and is generally interpreted to have been deformed in a south-dipping subduction zone. Two balanced cross sections (~ 40 km apart) of the Ouachita Mountains in western Arkansas are presented here, illustrating the regional structural style. Major features of the cross sections include (from north to south) (1) triangle zones along the northern border of the frontal thrust zone produced by imbrication at depth, (2) large-scale (~ 10-km wavelength) fault-propagation folds in the frontal thrust zone, formed prima ily above normal faults that offset the basement and act as buttresses at depth, (3) a late-stage basement uplift along the reactivated Johns Valley normal fault system, resulting in the antiformal structure of the Benton uplift and backthrusts in the northern Benton uplift, and (4) small-scale (1-3 km) heavily faulted folds in the early Paleozoic deep-water rocks exposed in the Benton uplift. Greenschist metamorphism in these rocks is attributed to the estimated 13 km of Carboniferous overburden, which was later eroded. Reconstructions of the late Paleozoic continental margin are made from the two cross sections. The reconstructed shelf-to-slope transition is interpreted to underlie the southern flank of the Benton uplift. Using modern analogs for the across-strike width of the shelf-to-slope transition, a minimum regional shortening estimate of 30-50% (110-155 km) is obtained for deep-water rocks currently exposed in the Benton uplift.

The tectonic history of Kohistan and its implications for Himalayan structure

The tectonic history of Kohistan, northern Pakistan, involves two collisional events. Cleavage and folding developed at 90-100 Ma along the northern suture between the Kohistan island arc and the Asian plate. At the same time there was major folding and shearing of the lower part of the Kohistan arc, approximately 100 km south of the suture. This deformation was followed by ocean subduction south of the Kohistan arc, generating the Kohistan calc-alkaline batholith, with subsequent ocean closure during the Eocene and obduction of the Kohistan arc, together with the adjacent part of the Asian plate, over the Indian continental crust. The construction of balanced cross-sections through the imbricated upper part of the Indian continental crust, in the footwall to this southern suture indicates a minimum displacement of 470 km, requiring the western Himalayan hinterland to be underlain by a large wedge of Indian middle to lower crust. There is some shortening of the overriding Kohistan and Asian plates by thrusts and shear zones, but it is insufficient to satisfy the palaeomagnetic data; there must be major crustal shortening, involving thrusts, in the Hindu Kush and Pamirs north of Kohistan. The post-Eocene thrust direction, which for most of Pakistan is towards 160°, is almost perpendicular to that immediately to the east in the Himalayan belt, generating complex refolded thrust patterns in the Hazara syntaxis and large scale folding and rapid uplift with associated brittle faulting and seismic activity adjacent to the Nanga Parbat syntaxis. These different thrust trends indicate that major thrust movement as well as the folds and deformation fabrics, cannot always be related to plate movement vectors, but are modified by, or develop from, complex rotations during place collision or from the gravitational spreading of a thickened crust. A regional approach is required to recognize and correctly attribute the various components in thrust displacements.

Fault rocks of the Kaltungo lineament, NE Nigeria, and their relationship with Benue Trough tectonics

In the upper part of the Benue Trough of Nigeria, several major NE–SW lineaments are essential components in the structure of both the Pan-African basement and the Cretaceous sediments. In order to estimate their influence on the geodynamics of the Trough, detailed structural analyses have been carried out on the fault rocks of one of them, the Kaltungo lineament. It cross-cuts a basement inlier emerging from Lower to Middle Cretaceous sediments. This inlier of granitic rocks exhibits a wide fault zone trending N50E which displays an anastomosing network of both ductile and brittle shear zones. The former are expressed by mylonitic bands generated by a Pan-African dextral wrenching on a regional scale; the latter are represented by fault rocks of the cataclasite series related to a sinistral wrenching which occurred during the Mesozoic. The fault zone extends into the cover where it is seen as a thin crush zone characterized by conjugate strike-slip and reverse faults. From a computer dynamic analysis of microfaults, an earlier compression (N–S shortening axis) was found only within the basement, whereas a second one (N140E shortening axis) appears in both sediments and basement. The first palaeostress field corresponds to cataclasite genesis which preceded or coincided with the formation of pull-apart basins along the sinistral Kaltungo wrench fault, which initiated the Upper Benue Trough during the Lower Cretaceous. Sedimentary basins and uplifted zones (Kaltungo Inlier) were formed on releasing and restraining oversteps of such a fault. The above statement may be extended to other similar faults at the scale of the entire trough. The second palaeostress field corresponds to the Late Cretaceous compression responsible for the large scale folding in the sediments and the formation of fault-breccia along the sinistral Kaltungo lineament.

Kinematics of Austro-Alpine cover nappes: Changing translation path due to transpression

Upper and Middle Austro-Alpine nappes of the Eastern Alps, the structural uppermost subunits of the Alps, suffered two major superposed deformations (D 1, D 2) during the eo-Alpine (Cretaceous) orogeny. D 1 is characterized by NW- to W-directed thrusting and nappe-internal rock flow, D 2 by large-scale folding and imbrication toward the north and northeast. Structures, strain, metamorphic history, microfabrics, and stretching trajectories indicate that the superposition of D 2 over D 1 structures can be interpreted as a progressive deformation. It is concluded that the emplacement of the nappes follows a curved translation path. A model which emphasizes non-coaxial deformation structures explains the detachment of the nappes, the configuration of the separating thrust planes, and the distribution of intense deformation zones. The overall kinematic history of the Austro-Alpine nappes fits a model in which strike-slip shearing, horizontal shortening, and vertical lengthening act together continuously over a finite deformation zone (transpressional model). This model, emphasizing a gradually changing strain regime, eliminates the space problems encountered in models which consider simple south over north imbrication during the Alpine orogeny. The study stresses two interpretations: 1. (1) stretching lineations indicate nappe-transport directions. 2. (2) transpression may be a common feature of orogenic belts.

MACIÇOS INTRUSIVOS DO SUL DO ESPÍRITO SANTO E SEU CONTEXTO REGIONAL

A section of the Brasiliano Mobile Belt (650-450 Ma) was investigated in Espirito Santo from structural, petrological and geochemical points of view. The Archean granulitic rocks of the Juiz de Fora Complex show less recrystallization after formation of large scale folds than the Alegre Complexo Both complexes are separated by a steeply Edipping ductile shear zones (Guacui Lineament). In the Alegre, Complex anatexis and granitisation ocurred in a relatively early stage of deformation. Later gregarious basic to intermediate zoned diapirs intruded all units. Four of these diapirs have been mapped (scale of 1:50.000) and sampled. Petrological and geochemical data indicate a mantle origin magma mixing, differentiation and host rock aasimilation. This group of zoned plutons shows a, complex internal structure with large compositional variation, grading from olivine OPX-hornblende-gabbros, through OPX-CPX-biotite diorites, CPX-biotite monzodiorites up to biotites syenomonzonites (Jacutinga-Torre Complex and to a smaller extent, Venda Nova Complex). OPX-hornblende-biotite monzogabbros and monzodiorites, hornblende-biotite granodiorites and allanite-sphene-biotite granites occur in Santa Angelica and Castelo Complexes. A contemporaneous megmatism of melanocratic and leucocratic rocks produced a complicated pattern of mixed rocks. The presence of mantled feldspars, the different Rb/Sr rations for different lithologies, inherited zircons together with some other petrographic and geochemical characteristics are indicative of fusion of the mantle material which was mixed with different amounts of crustal melting, thus originating hybrid plutons of post-collision types. In this paper, it is analysed the problem of rock contamination, differentiation and production of granitic magma. The showed data was interpreted by a model which involves a large scale thrusting of the Juiz de Fora Complex towards the west over the Alegre Complexo. This also suggests a continent-continent collision which was followed by domic upwarping that exposed the structurally deeper Alegre unit.

Metamorphism and crustal rheology—implications for the structural development of the continental crust during prograde metamorphism

Summary Based on its metamorphic development it can be argued that the orogenic continental crust during prograde metamorphism consists of three layers of contrasting bulk rheology: a lower layer of granulite-facies rocks able to carry relatively high flow stresses; a middle layer of migmatites and amphibolite-facies rocks of very low mechanical strength and an upper layer of low-grade and very low-grade metamorphic rocks, the so-called ‘brittle’ crust in the geophysical sense with higher yield strength possibly increasing upward. There are presumably main décollement horizons between the lithospheric mantle and lower crust, in the roof of a granulite-facies lower crust and along the boundary between the middle and upper crust, giving rise to large-scale displacements and the formation of nappe structures of different metamorphic rocks. Granulite-facies rocks of the lower crust will tend to form large-scale folds which pierce the overlying crust during increasing amplification. Further crustal shortening may produce granulite-facies nappes from strongly amplified large-scale fold structures leading to inverse metamorphism which is generally accompanied by granulite-facies nappe complexes.

Stratigraphic and sedimentological evidence bearing on structural repetition in early Archean rocks of the Barberton greenstone belt, South Africa

The 3.5–3.3 Ga-old Swaziland Supergroup in the Barberton greenstone belt, South Africa, includes a lower, predominantly volcanic unit, the Onverwacht Group, and an upper, largely sedimentary section, the Fig Tree and Moodies Groups. Although Viljoen and Viljoen concluded that the Onverwacht Group exceeds 15 km in stratigraphic thicknesses, more recent workers have suggested that it represents a much thinner section of rocks repeated by folding and faulting. Our mapping indicates that the upper half of the Group includes at least 8 km of unrepeated stratigraphic section. Inferences of structural repetition have arisen in part because a major shear zone is present in the upper half of the Onverwacht Group on the west limb of the Onverwacht anticline. Shearing and the overturning of large blocks of the igneous sequence have combined with the later intrusion of felsic magma to produce a generally stratiform, highly complex zone of deformation up to 2 km wide. The displacement across the zone is unknown, but the presence of unfaulted sections of this part of the stratigraphic sequence indicates that the gross stratigraphic arrangement of units within the Group has not been significantly altered. Structures previously mapped as major fold hinges within the upper part of the Onverwacht Group can be shown to have formed by local deformation and reflect neither large-scale folding nor structural repetition of the rocks.Several kilometers higher in the section, rocks of the uppermost Onverwacht and Fig Tree Groups are stacked and repeated in a series of thrust nappes. Individual nappes extend for up to 15 km along strike and include as much as 900 m of allochthonous volcanic and sedimentary rocks. They were apparently emplaced by faulting or gravity sliding at or near the Earth's surface. Their formation involved the movement of sheets made largely up of sedimentary rocks along shear surfaces within underlying, much weaker, serpentinized ultramafic komatiites. Locally, the sheets were deformed to form fold nappes. The lower limbs of the fold nappes commonly show extreme attenuation and brittle deformation whereas the upper limbs are largely intact. The thrust faults are the loci of small hypabyssal igneous bodies that appear to be co-magmatic with a suite of upper Fig Tree volcanic rocks. This style of faulting has not affected rocks of the overlying Moodies Group. Although estimates of the timing of deformation are as yet imprecise, these features suggest that it occurred during a late Fig Tree phase of tectonism and volcanism.

Lithostratigraphie et structure du Buem unité externe de la chaine panafricaine des Dahomeyides dans la region de Bassar (Togo)

In the area of Bassar (Togo), the most external structural unit of the Pan-African (600 Ma) Dahomeyides fold belt, named the Buem, comprises two well defined zones: (1) A western zone, less affected by deformation and where the following lithostratigraphy can be established: (i) at the bottom, some sandstones, quartzites and mixtites are overlayed by microconglomeratic hematitites. The 1300 m-thick sequence is partially interpreted as a glaciogenic deposit; (ii) at the top, shales and siltstones with intercalations of sandstones and carbonates compound a thick sequence of 1500 m. This unmetamorphosed sequence, which can be correlated with the supergroup 2 in the Volta basin, shows a rather simple structure with a large-scale folding verging to the West. (2) An eastern zone, which displays nearly the same facies than the western one with, however, a finer grained mixtite and a strong cataclasis shown by hard rocks. The Pan-African orogeny in this zone is polyphased with an important thrusting and an anchizonal to epizonal metamorphism. The identification of those two zones leads to propose two possible acceptations of the Buem: (i) the Buem sensu lato, which includes those two zones in the previously defined Buem area; (ii) the Buem sensu stricto, represented only by the eastern zone with the characteristics of the Buem of the northern Benin. According to this second acceptation, the more logical one, the western zone must represent a para-autochtonous unit of the Voltaian Basin.

Early Precambrian crustal evolution at Mount Narryer, Western Australia

The oldest known rocks in Australia occur at Mount Narryer and are associated with quartzite that contains detrital zircons far older than any known terrestrial rocks.The region forms part of the Western Gneiss Terrain of the Archaean Yilgarn Block. The main rock unit is a banded biotite adamellite gneiss which gives a Sm-Nd model age of 3630 Ma. It is intruded by a leucocratic granite gneiss which gives a Sm-Nd model age of 3510 Ma and contains abundant inclusions of a meta-anorthosite—gabbro—ultramafic suite. Quartzites and banded quartz—pyroxene—magnetite rocks form concordant layers within both gneisses.Most rocks are strongly deformed, are folded together into large-scale fold interference patterns, and show mineral assemblages in granulite or retrograded granulite facies. One large body of quartzite has internally escaped intense deformation and contains cross-bedding and graded-bedding structures as well as horizons of both polymict and quartz-pebble conglomerates. The quartzites contain detrital zircons which mostly give U-Pb ages of ∼3750–3500 Ma, but a few zircon grains give ages of 4100–4200 Ma. Samples of the adamellite gneiss give a Rb-Sr whole-rock isochron age of 3348±43 Ma which might date a peak of metamorphism.

Relationship of spaced cleavage to folds and thrusts in the Idaho-Utah-Wyoming thrust belt

The Idaho-Utah-Wyoming thrust belt of the Rocky Mountain Cordillera preserves structures that developed at very shallow levels of the crust ( T < 200°C). The geometry of major structures is well documented and synorogenic conglomerates record the timing of movement on major thrusts. Spaced cleavage and pencil fracturing are developed within micrites of the Jurassic Twin Creek Formation. The cleavage developed dominantly by pressure-solution early in the deformation history of each thrust sheet, aided by elevated temperatures and pressures in areas where increased overburden resulted from emplacement of earlier, overriding sheets. Cleavage within any sheet is well developed in the area covered by the pre-erosional extent of the previous sheet; forelandward from this area cleavage intensity decreases drastically. Thermal modelling of the sheets agrees well with temperature estimates from illite crystallinity studies on material from the cleavage planes. Cleavage is axial planar to small folds and formed concurrently with small-scale buckle folds and contraction faults during the early history of movement on each thrust. However, cleavage strongly fans about large-scale folds formed later during movement, suggesting passive rotation of cleavage with bedding during the later stages of thrust history. The rotated cleavage may also be folded and overprinted by a second weaker cleavage during a second ramping, or during movement on a later, lower thrust. Cross-folding may also develop as a result of lateral ramping. Thus, complex patterns of interfering folds, multiple cleavages, and associated fiber-filled fractures may be produced during motion on a single thrust fault, and so magnify the problem of correlating cleavages and deciphering multiple deformations.

Strain partitioning in the Helvetic thrust belt of eastern Switzerland from the leading edge to the internal zone

The strain and strain partitioning in the Helvetic thrust belt are analysed in order to determine their variation across the belt from the external, thin-skinned portion to the internal, thick-skinned portion, and in order to begin the detailed mechanical analysis of the entire fold-and-thrust belt. Coarse-grained limestones of Jurassic and Cretaceous age were chosen for analysis because they can be traced from the external to the internal zones. The strain within a bed is partitioned into three components: transgranular (pressure solution, microfaults, veins), twinning, granular and intragranular, exclusive of twinning (crystal-plasticity, associated with recrystallization). Transgranular strain was very important in the early history of the Helvetic nappes giving way to twinning at a later stage under tectonic burial. Twinning strain was early in the Infrahelvetic complex, giving way to intracrystalline deformation at the later stage of burial. The transition from transgranular- to intragranular-dominated strain occurred at a vitrinite reflectance value of 3.5%, significantly below the greenschist grade of metamorphism. In a large-scale fold at the leading edge of the Helvetic nappes, transgranular strains were associated with the buckling stage of the fold and twinning occurred during fold-tightening. In more internal parts of the Helvetic nappes twinning strains are also late with regard to transgranular strains, but here their pattern corresponds to the regional finite strain pattern. In the internal zone, the Infrahelvetic complex, twinning seems to have been an early event and records an initial layer-parallel shortening, followed by regional internal deformation with intragranular strains.

Thrust Faults that Violate Classic Thrust Belt Rules: Marathon Basin, Texas: ABSTRACT

One generally accepted rule of thrust belt geometry is that folding does not occur before thrust faulting; thrust faults do not cut across folds in cross-section view. Original mapping in the Marathon basin documents that the Hell's Half Acre thrust sheet was emplaced after an episode of large-scale folding. The Hell's Half Acre thrust fault is a shallow, low-angle fault which separates the northeasterly trending, broad (4-km or 2.5-mi wavelength), open folds on the north from the imbricately thrust faulted and overturned, tightly folded (1-km or 0.6-mi wavelength) Paleozoic rocks of the thrust sheet. The thrust fault trends east and truncates the structures on the north. Within the thrust sheet, folds plunge into and are truncated by thrust faults. The Tesnus Formation (Mississippian) is the principal formation of the sheet, but large blocks of younger Dimple Limestone (Morrowan) and Haymond Formation (Atokan) are lodged within fault planes bounded by Tesnus, thereby documenting folding before thrust faulting. This nonclassic geometry is due to lithology. Classic thrust belt style was developed for a miogeoclinal sequence. The Ouachita stratigraphy is a eugeoclinal sequence composed of lower and middle Paleozoic, thin limestones and cherts with numerous shale interbeds and an upper Paleozoic flysch sequence with a low sand/shale ratio and numerous thick shale horizons. The many thick, incompetent horizons allowed the thrust fault to cut across preexisting structures. End_of_Article - Last_Page 470------------

Late Cretaceous and Paleocene Faulting in Rocky Mountain Foreland and Frontal Thrust Zone, Southeastern Montana: ABSTRACT

The Late Cretaceous and Paleocene structural pattern in southwestern Montana is comprised of three spatially and temporally overlapping sets of structures whose positions strongly reflect the influence of previous basement faults and whose kinematics imply dominantly west-to-east compressive forces. The first of these patterns is a set of steep northwest-trending faults involving the metamorphic basement rocks of the Rocky Mountain foreland. Movement on these faults was oblique (left-reverse) and occurred as a reactivation of faults which developed initially in middle Proterozoic time. Net horizontal shortening by faulting and associated large-scale folding is 10-20 km (6-12 mi). Another fault pattern within the foreland is a set of widely spaced, gently west-dipping thrusts. These thrusts also involve basement rocks and appear to have been controlled by previous zones of weakness within the crust. Movement is principally dip-slip and latest movement appears to involve tearing on the northwest-trending fault set. Shortening by basement thrusting and associated folding is 20-25 km (12-15 mi). The other structural pattern is that of the frontal fold and thrust zone on the western boundary of the foreland and the zone of transverse thrusting on the northern boundary. These zones of impinging thrusts also involve basement rocks principally as slices that were picked off from underlying basement highs during ramping. As the thrusts of the frontal zone advanced to the east they impinged on northwest-trending foreland anticlines and were torn on the northwest-trending faults. The transverse thrust zone on the north involves right-reverse movement with about 20 km (12 mi) of west-east displacement. The zone is fundamentally a transverse ramp produced by deflection along the east-trending Willow Creek fault zone of Proterozoic age. End_of_Article - Last_Page 948------------