Thrust Belt Deformation Research Articles

Paleogene stratigraphy in the central area of the Fuegian fold and thrust belt, Austral Basin, Argentina: Implications for the chronology of thrust-belt deformation

We combine extensive field data with calcareous nannofossils and dinocysts analysis and detrital zircon U-Pb ages to identify six Paleocene-Oligocene stratigraphic units in a previously poorly studied area in the external fold and thrust belt of the Fuegian Andes. Detailed petrographic analysis coupled with detrital zircon ages indicate three main provenance peaks, which reflect that 1) the lowermost upper Cretaceous was already exhumed during the Paleocene-early Eocene, 2) the upper Cretaceous rocks were a continuous source at least until the late Eocene, 3) volcanism was coetaneous with the deposition of middle Eocene successions. Three contractional stages known elsewhere in the thrust-belt are identified in the study area in the Danian, the early Eocene-early middle Eocene, and the latest Eocene-early Oligocene. This suggests synchronicity along strike during the evolution of the Fuegian fold and thrust belt.

Heterogeneous deformation in fold and thrust belts induced by anisotropies in the ocean topography; the South Portuguese Variscan Zone example (SW Portugal)

The Variscan fold and thrust belt related with the South Portuguese Zone presents a SW forward propagation and the coeval development of synthetic folds and thrusts pervasive at all scales. Although several models have been proposed for their formation, the estimated strain pattern is more compatible with a push from the rear induced by the north-eastern subduction zone. Although there is a strong consistence concerning the kinematics of these Variscan structures, there is an important contrast of their geometries between the inner and the external sectors, where the very low dips prevails. Although, there are some previous models suggesting that the behaviour of the more external domains could be related with the interference between the undeformed sedimentary sequence of the upper plate with the active margin of the fold and thrust belt, independent evidence of this process is lacking. The realization of scaled analogue experiments concerning the Carboniferous evolution of SW Iberia Variscides, not only show that they could induced the flat lying major thrust of the external domains, but also emphasize their important role in the structural evolution of much of the fold and thrust belt.

Zircon (U-Th)/(He-Pb) double-dating constraints on the interplay between thrust deformation and foreland basin architecture, Sevier foreland basin, Utah

AbstractThe Cretaceous Cordilleran foreland basin strata exposed in the Book Cliffs of eastern Utah and western Colorado have motivated important concepts linking thrust belt deformation and foreland basin evolution largely on the basis of sequence stratigraphy, stratal architecture, and sediment provenance evolution. However, these methods and approaches generally cannot provide critical insights into the temporal or causal linkages between foreland basin architecture and thrust belt deformation. This is in part due to discrepancies in age resolution and lack of evidence with which to directly couple sediment supply and basin-fill evolution to thrust belt unroofing. New detrital zircon (DZ) geothermochronometric data from Upper Cretaceous proximal to distal foreland basin strata in the Book Cliffs provide new quantitative insights into sediment origin and dispersal in relation to thrust belt deformation and exhumation. Detailed DZ U-Pb and (U-Th)/He double dating reveals that the Book Cliffs foredeep detritus was mainly delivered by transverse routing systems from two major sources: (1) Neoproterozoic and Lower Paleozoic strata from the central Utah Sevier thrust belt, and (2) Permian–Jurassic and synorogenic Cretaceous strata recycled from the frontal part of the thrust belt. A dramatic increase in Sierran magmatic arc and Yavapai-Mazatzal DZ U-Pb ages, as well as Paleozoic DZ He ages, in the deeper marine portions of the foreland basin points to axial fluvial and littoral sediment input from the Sierran magmatic arc and Mogollon highland sources. Both transverse and axial transport systems acted contemporaneously during eastward propagation of the Late Cretaceous thrust belt. DZ He depositional lag time estimates reveal three distinct exhumation pulses in the Sevier thrust belt in the Cenomanian and Campanian. The exhumation pulses correlate with shifts in sediment provenance, dispersal style, and progradation rates in the foreland basin. These new data support conceptual models that temporally and causally link accelerated exhumation and unroofing in the thrust belt to increases in sediment supply and rapid clastic progradation in the foreland basin.

Role of erosion in creating thrust recesses in a critical-taper wedge: An example from Eastern Tibet

Mechanical models and analog experiments show that erosion plays an important role in controlling along-strike variations in thrust-belt deformation. Despite this fundamental insight, natural examples of focused erosion causing local variations of thrust-belt geometry, such as the formation of salients and recesses, remain underappreciated. In this study, we address this issue by integrating theoretical model predictions with structural, morphological, and erosion-pattern analyses along the thrust system defining the eastern margin of the Tibetan Plateau. Our work shows a nearly constant surface slope across and an overall cylindrical fault-related fold geometry along the thrust belt. These phenomena can be explained by the development of a critically-tapered thrust wedge along the plateau margin. A prominent recess exists at the intersection between the transverse Minjiang River and the range-bounding thrust. There, the surface trace displays a much higher curvature than the rest of the thrust belt and the local fold geometry is conical rather than cylindrical. We infer a warped passive-roof duplex under the recess. We build a classification diagram for recesses developing in critical-taper wedges, in which six independent variables lead to three morphotectonic scenarios regarding the effects of mass influx, mass efflux, and intrinsic properties of a thrust wedge. Formation of the Dujiangyan recess and its spatial correlation to the Minjiang River can be best attributed to localized fluvial erosion during forward propagation of the thrust wedge in eastern Tibet. This study sets a type example for map-view curves in thrust belts undergoing deformation-erosion interactions.

The influence of syntectonic sedimentation on thrust belt deformation: a kinematic model example from the triangle zone within the Western Kunlun thrust belt

Although syntectonic sedimentation has been shown to be one of the most important factors involved in thrust belt deformation, previous studies have mostly focused on the influence of just one stage in this context. Two geometric sediment packages that are described as progradation and aggradation are known to lead to different thrust belt deformation subsequences, yet the influence of multistage sedimentary strata remains less well-understood. Interpretation of the latest seismic data shows that the Western Kunlun thrust belt is characterized by classical triangle zone. There are two adjacent sections with different geometrical characteristics, a prograding monocline and a spaced-ramp anticline. We restored these two sections and analyzed the potential geological factors leading to their formation in this study. The results of this analysis provide new insights into a kinematic model for triangle zones and suggest that these two distinct styles formed because of two-stage syntectonic sedimentation. We further discuss the kinematic model of tectonic deformation given the presence of two-stage syntectonic sediments and propose four distinct sedimentary superposition patterns. All of these sequences suggest that deformation is controlled by total syntectonic sediment geometry and imply that early sediment shape exerts the greatest influence. The presence of early prograding sediment promotes the development of the spaced thrust belt, while an early aggrading package enhances the structure of the restricted thrust adjacent to the hinterland.

Deformational history and thermochronology of Wrangel Island, East Siberian Shelf and coastal Chukotka, Arctic Russia

Abstract In Arctic Russia, south of Wrangel Island, Jura–Cretaceous fold belt structures are cut by c. 108–100 Ma plutonic rocks and a c. 103 Ma migmatitic complex (U–Pb, zircon) that cooled by c. 96 Ma ( 40 Ar/ 39 Ar biotite); the structures are unconformably overlain by c. 88 Ma and younger (U–Pb, zircon) volcanic rocks. Wrangel Island, with a similar stratigraphy and added exposure of Neoproterozoic basement rocks, was thought to represent the westwards continuation of the Jura–Cretaceous Brookian thrust belt of Alaska. A penetrative, high-strain, S-dipping foliation formed during north–south stretching in Triassic and older rocks, with stretched pebble aspect ratios of c. 2:1:0.5 to 10:1:0.1. Deformation was at greenschist facies (chlorite+white mica; biotite at depth; temperature c. 300–450°C). Microstructures suggest deformation mostly by pure shear and north–south stretching; the quartz textures and lattice preferred orientations suggest temperatures of c. 300–450°C. 40 Ar/ 39 Ar K-feldspar spectra ( n =1) and muscovite ( n =3) (total gas ages c. 611–514 Ma) in Neoproterozoic basement rocks are consistent with a short thermal pulse during deformation at 105–100 Ma. Apatite fission track ages ( n =7) indicate cooling to near-surface conditions at c. 95 Ma. The shared thermal histories of Wrangel Island and Chukotka suggest that Wrangel deformation is related to post-shortening, north–south extension, not to fold–thrust belt deformation. Seismic data (line AR-5) indicate a sharp Moho and strong sub-horizontal reflectivity in the lower and middle crust beneath the region. Wrangel Island probably represents a crustal-scale extensional boudin between the North Chukchi and Longa basins.

Imaging deformation in submarine thrust belts using seismic attributes

Uncertainty exists as to the patterns of deformation that develop within submarine thrust belts. This case study uses a large-scale gravity-driven fold-thrust structure as an analogue for submarine fold thrust systems in general. Seismic attribute analysis and mapping provide ways of identifying complex fault patterns and associated deformation that are otherwise unresolved in conventional amplitude displays. These methods are developed and applied to a 3D dataset and used to investigate the geometry, internal architecture and the nature of the low signal/noise incoherency and discontinuities observed on the km-scale. Semblance (coherency), curvatures and spectral decomposition were all computed and used as attributes. Collectively these define volumes within the seismic data where the signal is greatly reduced — features termed here “disturbance geobodies”. The study shows that thrust faults that, on conventional amplitude displays appear to be simple and continuous, are likely to consist of complex arrays of anastamosing fault strands. Adjacent to these composite fault zones are greater volumes of deformed rocks (disturbance geobodies) across which there are only minor stratal offsets. Similarly volumes of high stratal curvature coincide with disturbance geobodies, again interpreted as zones of weak, distributed deformation. These relationships between narrow thrust faults and broader zones of deformation are broadly comparable to those observed in outcrops within exhumed thrust systems. Application of the seismic imaging techniques developed here will improve the understanding of the localization of deformation in sedimentary successions with important implications for predicting fluid flow within other deep water structures such as subduction accretion complexes.

Gravitational collapse at continental margins: products and processes; an introduction

Gravitational collapse can occur on many scales on submarine slopes. As hydrocarbon exploration has stepped into increasingly deeper water geologists have realized that entire sedimentary prisms on continental margins are deformed by structures that do not penetrate into the underlying crust. In this respect, large-scale gravitational deformation systems at continental margins bear many similarities to deformation in orogenic thrust belts. Also like thrust belts, they exhibit an overall wedge-shaped geometry, suggesting that critical taper theory exerts a first-order control over their formation, and their development is controlled partly by complex feedbacks between the evolving deformation system and the synkinematic (‘syntectonic') stratigraphy. However, unlike orogenic thrust belts, deformation at continental margins is not directly linked to stresses generated at plate margins but rather reflects gravitational instability of the sedimentary pile itself. In effect, submarine slopes are in a state of landslip or flow with generally extensional faulting on the upper slope and continental shelf coupling with contractional structures at the slope toes. These systems can be hundreds of kilometres wide, the scale of many tectonically driven mountain belts (see review by Rowan et al . 2004). As the exploration for oil is revealing thin-skinned gravitational deformation from many sites around the world, it is increasingly seen as a key structural–stratigraphic component of continental margins. Linked extension–contraction deformation systems potentially offer ideal natural laboratories for understanding the accommodation of lateral contraction in poorly lithified sediments (e.g. Butler & Paton 2010). Consequently, there is now substantial interest in the structural evolution of these large-scale systems. Although the products of large-scale gravitational failure, especially deep-water thrust belts, are the focus of extensive exploration efforts by oil and gas companies, there are few descriptions beyond the classic examples of the US Gulf of Mexico …

Comparison of crustal thickening budget and shortening estimates in southern Peru (12-14 S): Implications for mass balance and rotations in the "Bolivian orocline"

Two fundamental questions of interest with regard to the Andean Plateau are the mass balance of material needed to create and sustain a 3-4-km-high plateau and the relationship between the plateau and the bending of the Bolivian orocline. The link be- tween these two questions is the distribution of shortening through the Central Andes. Is crustal shortening suffi cient to support an isostatically compensated 60-70-km-thick crust? Is differential shortening between the hinge of the orocline and its limbs suffi cient to account for both physiographic curvature and measured rotations? Three-dimensional (3-D) models of deformation within curved orogens require, in addition to paleomagnetic data, viable two-dimensional estimates of dis- placements. To this end, we present new map data and use those data to derive estimates of shortening across the northern margin of the Andean Plateau. The cross-section extent, from the eastern edge of the volcanic arc to the foreland basin, is approximately one half of the physiographic width of the Andean Plateau in Peru. Cross-sectional shortening estimates in southern Peru (12-14°S) provide a preferred estimate of 123 km or 40% short- ening, with a minimum estimate of 58 km or 25% shortening and maximum estimate of 333 km or 65% shortening. The largest con- trolling factor on the magnitude of shorten- ing is where basement becomes involved in thrust belt deformation. Using our preferred shortening estimate, the differential shorten- ing between our cross section on the northern margin of the Bolivian orocline (12-14°S) and cross sections across the axis of the Bolivian orocline (17-18°S) is 177 km, corresponding to 23° of rotation. The differential shortening for the young (younger than 15 Ma) defor- mation of the Altiplano and Subandean re- gion between 12-14°S and 17-18°S is 88 km, correlating to 11.5° of rotation. While dif- ferential shortening can account for young (younger than 15 Ma) vertical-axis rotations, it cannot completely account for early 45- 35 Ma rotations (37° ± 15°). This suggests that forearc rotations refl ect both bending due to differential shortening, ~12°, as well as block rotations (~25°-30°), which facilitated defor- mation in a tightening core and transported material toward the center of the orocline. The preferred estimate of shortening is well short of the required 240-300 km of shorten- ing needed to account for a 60-70-km-thick crust under the entire plateau. This suggests that for an isostatically equilibrated crust, either (1) there is a signifi cant amount of shortening (~100-150 km) in the western half of the plateau that is hidden by the volcanic arc; or (2) crustal material is being added to the Peruvian section of the Andean Plateau, most likely through lower-crustal fl ow.

Modelling interactions between fold–thrust belt deformation, foreland flexure and surface mass transport

ABSTRACTInteractions between fold and thrust belt deformation, foreland flexure and surface mass transport are investigated using a newly developed mathematical model incorporating fully dynamic coupling between mechanics and surface processes. The mechanical model is two dimensional (plane strain) and includes an elasto‐visco‐plastic rheology. The evolving model is flexurally compensated using an elastic beam formulation. Erosion and deposition at the surface are treated in a simple manner using a linear diffusion equation. The model is solved with the finite element method using a Lagrangian scheme with marker particles. Because the model is particle based, it enables straightforward tracking of stratigraphy and exhumation paths and it can sustain very large strain. It is thus ideally suited to study deformation, erosion and sedimentation in fold–thrust belts and foreland basins.The model is used to investigate how fold–thrust deformation and foreland basin development is influenced by the non‐dimensional parameter , which can be interpreted as the ratio of the deformation time scale to the time scale for surface processes. Large values of imply that the rate of surface mass transport is significantly greater than the rate of deformation. When , the rates of surface processes are so slow that one observes a classic propagating fold–thrust belt with well‐developed wedge top basins and a largely underfilled foreland flexural depression. Increasing causes (1) deposition to shift progressively from the wedge top into the foredeep, which deepens and may eventually become filled, (2) widespread exhumation of the fold–thrust belt, (3) reduced rates of frontal thrust propagation and possible attainment of a steady‐state orogen width and (4) change in the style and dynamics of deformation. Together, these effects indicate that erosion and sedimentation, rather than passively responding to tectonics, play an active and dynamic role in the development of fold–thrust belts and foreland basins. Results demonstrate that regional differences in the relative rates of surface processes (e.g. because of different climatic settings) may lead to fold–thrust belts and foreland basins with markedly different characteristics. Results also imply that variations in the efficiency of surface processes through time (e.g., because of climate change or the emergence of orogens above sea level) may cause major temporal changes in orogen and basin dynamics.

Miocene tectonic evolution of the southern Apennine thrust front (Italy): stratigraphic and structural constraints from the eastern Calabria-Lucania borderland area

Syn-orogenic sediments provide fundamental information on the timing and modes of deformation in fold and thrust belts. In this study, biostratigraphic and structural analyses on syn-orogenic deposits have been carried out in a key area of the southern Apennines in order to constrain the evolution of the Miocene thrust front and adjacent foreland basin. Our data indicate that, by middle Miocene times, a thin-skinned fold and thrust belt had developed in the study area. This was characterised by a complex tectonic setting, including: (i) a wide foreland basin area to the NE, ahead of the thrust front, where volcaniclastic and then quartz-rich (“Numidian”) sandstones were deposited; and (ii) a series of thrust-top basin depocentres—hosting different siliciclastic and mixed detrital carbonate-siliciclastic successions— SW of the thrust front. Foreland propagation of the deformation produced intense folding of the foredeep succession. Later shortening and refolding around steeply dipping axial surfaces affected all of the tectonic units exposed in the study area. The latter deformation could be associated with Pliocene “en-masse” emplacement of the whole thin-skinned fold and thrust belt — as a major allochthonous detachment sheet—on top of the Apulian foreland sequence which presently underlies the exposed thrust sheets.

Late Cenozoic tectonics of the Kepingtage thrust zone: Interactions of the Tien Shan and Tarim Basin, northwest China

The Kepingtage (Kalpin) thrust zone, northwest China, is an actively deforming part of the India‐Asia collision system. It lies south of the Tien Shan, has an area of ∼16,000 km2, and consists of arcuate, emergent imbricates. Overall vergence is toward the interior of the Tarim Basin to the south. Thrust sheets typically expose Upper Cambrian to Permian platformal strata. Thrusting is largely thin‐skinned; thin Upper Cambrian evaporites are likely to be the main decollement horizon. The Kepingtage thrust zone is the only margin of the Tarim Basin to deform in this style in the Cenozoic. It is replaced to the east and west by thrust zones which have propagated shorter distances into the interior of the Tarim Basin. Thick (up to 10 km) Mesozoic and Cenozoic clastic successions are present and deformed in these regions. Such successions are not present in the Kepingtage thrust zone or the adjacent Bachu Uplift within the Tarim Basin, because of episodic activity on steep, northwest‐southeast trending thrusts which define the margins of the Bachu Uplift. These Mesozoic‐Cenozoic strata may have suppressed the ability of regions along strike from the Kepingtage thrust zone to deform by thin‐skinned thrusting utilizing the Upper Cambrian decollement. The along strike variation in active thrusting at the southern margin of the Tien Shan is an example of syntectonic sedimentation controlling thrust belt deformation style. A balanced section across the thrust zone indicates ∼28% shortening, equivalent to ∼35 km. This is equivalent to an average slip rate of ∼1.8 mm yr−1 and a strain rate of ∼4.4 × 10−16 s−1, assuming deformation began at circa 20 Ma. Active deformation is focused along the frontal thrust, the Kepingtage Fault. The northern boundary of the Kepingtage thrust zone is formed by the South Tien Shan Fault. This major, north dipping thrust is seismically active, with published earthquake focal depths at midcrustal levels (14 and 18 km). It juxtaposes upper Carboniferous sedimentary rocks of different facies and different Paleozoic deformation histories. Speculatively, it represents a reactivation of a late Paleozoic thrust, which originated at the boundary between the platformal interior of the Tarim Block and a deeper‐water foreland basin to its north.

Coeval sedimentation, magmatism, and fold-thrust belt development in the Trans-Hudson Orogen: geochronological evidence from the Wekusko Lake area, Manitoba, Canada

Lithological and structural mapping in the east Wekusko Lake area of the Flin Flon Belt, Trans-Hudson Orogen, suggested an intimate relationship between magmatism, fluvial sedimentation, and initiation of fold and thrust belt deformation. Conventional U-Pb geochronology of volcanic rocks in fault-bounded assemblages provides a minimum age of 1876 ± 2 Ma for McCafferty Liftover back-arc basalts, and ages of between 1833 and 1836 Ma for the Herb Lake volcanic rocks. A rhyolite which unconformably overlies Western Missi Group fluvial sedimentary rocks has complex zircon systematics. This rock may be as old as about 1856 Ma or as young as 1830 Ma. The sedimentary rocks overlying this rhyolite are locally intercalated with 1834 Ma felsic volcanic rocks, and yield sensitive high resolution ion microprobe (SHRIMP) U-Pb and Pb-evaporation detrital zircon ages ranging from 1834 to 2004 Ma. The Eastern Missi Group is cut by an 1826 ± 4 Ma felsic dyke, and contains 1832-1911 Ma detrital zircons. The dominant source for detritus in the Missi Group was the Flin Flon accretionary collage and associated successor arc rocks. The fluvial sedimentary rocks and the Herb Lake volcanic rocks were essentially coeval, and were then incorporated into a southwest-directed fold and thrust belt which was initiated at about 1840 Ma and active until at least peak regional metamorphism.

Geometry and kinematics of vertical-axis rotations in fold and thrust belts

Vertical-axis rotations indicated by palaeomagnetic study are a common and important component of deformation in fold and thrust belts. The kinematics of rotating thrust sheets may be best described as a combination of a far-field rotation (or translation in a planar geometry) and a near-field, or local rotation, so that the instantaneous rotation axis lies outside the body of the sheet. This simplifies the thrust-sheet kinematics significantly, as it means that all the points on the sheet have a component of forward motion, and the bounding fault is a thrust over all its area. The consequences of a simple thrust-sheet rotation for the interpretation of fault lineation data are investigated. The lineations recorded in the footwall and the hangingwall are likely to be different, the hangingwall lineations being skewed towards the direction of rotation. The principal mechanism for local thrust-sheet rotation is differential shortening, either on folds or thrusts, or by a combination of these structures. The forward propagation of thrusting towards the foreland can accumulate large (>120°) rotations on the uppermost sheets. These rotational thrust sheets cannot extend indefinitely along strike; instead they terminate against lateral ramps or transfer faults. Sidewall duplexes may form as a consequence of rotation over a lateral ramp. Examples of rotational geometries from the Subbetic Zone of southern Spain are described. On a larger scale, obliquely convergent orogenic wedges can evolve by rotation of faults with an initial thrust geometry into progressively greater components of strike-slip motion. Reliable palaeomagnetic data are not always available for fold and thrust belts, so independent evidence for rotational deformation is of great importance. Fold and thrust geometries, particularly along-strike variability in fold geometries, highly variable axial traces of folds, evidence for near-synchronous refolding and the deviation of regional trends may all be a consequence of thrust-sheet rotation. Detailed mapping, and particularly the identification of the position of footwall and hangingwall cut-offs can also provide strong evidence for rotation.

Abstract: New Methods for Integrated Modeling of Deformation and Petroleum Generation in Fold and Thrust Belts

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Large‐scale fluid infiltration, metasomatism and re‐equilibration of Archaean basement granulites during Palaeoproterozoic thrust belt construction, Ungava Orogen, Canada

Abstract The metamorphic history of the Archaean Superior Province crystalline basement in the Palaeoproterozoic Ungava Orogen attests to the importance of structural and geohydrological controls on a retrograde amphibolite‐granulite transition. Two distinct metamorphic suites, separated in age by nearly one billion years, are recognized in extensively exposed tonalitic to dioritic metaplutonic gneisses. The older suite comprises c. 2.7‐Ga granulite facies assemblages (orthopyroxene‐clinopyroxene‐hornblende‐plagioclase‐ilmenite ± biotite ± quartz) that record moderate pressures (±5 kbar) and high temperatures (±800° C). A younger, c. 1.8‐Ga suite resulted from amphibolitization of the granulites and is characterized by regionally extensive amphibolite facies mineral zones that broadly parallel the basal décollement of the overlying Proterozoic Cape Smith Thrust Belt. Deformation/mineral growth relationships in the amphibolitized basement indicate that extensive hydration and re‐equilibration of the Archaean granulites occurred during thrust belt deformation. The transition from granulite facies to amphibolite facies assemblages is characterized by the growth of garnet‐hornblende‐quartz ° Cummingtonite coronas between plagioclase and orthopyroxene‐clinopyroxene, as well as titanite coronas on ilmenite. Multi‐equilibrium thermobarometry on the coronitic assemblages documents re‐equilibration of the granulitic gneiss to 7.7 kbar at 644° C in the south and 9.8 kbar at 700° C in the north. The variably deformed, amphibolite facies domain sandwiched between the coronitic garnet zone and the basal décollement is marked by significant metasomatic changes in major element concentrations within tonalite. These changes are compatible with equilibrium flow of an aqueous‐chloride fluid down a temperature gradient. The source of fluids for basement hydration/metasomatism is interpreted to be dehydrating clastic rocks in the overlying thrust belt, with fluid flow probably focused along the basal décollement.

40Ar/39Ar evidence that formation of blueschists in continental crust was synchronous with foreland fold and thrust belt deformation, western Brooks Range, Alaska

Abstract 40Ar/39Ar ages from white mica in rocks of the internal zone of the Brooks Range contractional orogen indicate that the Nanielik antiformal duplex developed at about 120 Ma and was remobilized on its southern boundary at c. 108 Ma. Blueschist facies metamorphism accompanied development of the antiform. The timing of the blueschist facies event and creation of the antiform overlap the period of shallow‐seated deformation in the foreland fold and thrust belt and sedimentation in the foreland basin of the Brooks Range. Blueschist facies P‐T conditions may therefore characterize the thicker parts of orogenic wedges in some orogenic systems; ancient blueschists need not necessarily be interpreted as indicators of active subduction or continent‐continent collision.Microprobe analysis using quantitative wavelength‐dispersive and electron backscattered electron imaging methods was used to characterize the composition of white micas in the dated samples. None of the samples was compositionally homogeneous; many contained 2‐3 populations of white mica, including both potassic and sodic varieties. Samples which had undergone (in sequence) amphibolite, albite‐epidote amphibolite and blueschist facies metamorphic events retained muscovites relict of the amphibolite facies event. Samples that had undergone only the blueschist facies event also contained multiple populations of mica, some probably from detrital sources.

Vertical-axis rotation associated with folding and thrusting: An example from the eastern Subbetic zone of southern Spain

The differential rotation about a vertical axis across a southeast-vergent recumbent fold and thrust fault within a single nappe from the eastern Subbetic zone of southern Spain has been determined using paleomagnetism. The upper limb has rotated clockwise by 68° ±22° relative to the lower limb. Mapping of the structure indicates that the syncline is an isoclinal fold in the northeast and an open fold in the southwest. The differential vertical-axis rotation across this structure appears to have been accommodated by a pro- gressive increase in along-strike horizontal shortening, reaching 8.5 km at 3.7 km from the pivot point. This differential shortening and the rotation are probably the result of increased drag on the underlying thrust close to the pivot point compared to the farthest extent of the structure. The lateral boundary of the rotating sheet is a sinistral strike-slip fault, which is consistent with the rotational geometry. Pinning is probably enhanced by lateral heterogeneities within the stratigraphy. This result ties rotational deformation to early-middle Miocene folding and thrusting, rather than to late Miocene- Holocene strike-slip deformation. Rotation by pinning of thrust sheets may prove to be an important aspect of deformation in many fold and thrust belts.

Geological evidence for a global tectonic polarity

Inferences about plate motions over the past 40 Ma, based on structural and geodetic data, suggest that there is a coherent movement direction on a global scale. Moreover a ‘westward’ drift of the lithosphere relative to the asthenosphere is indicated by plate motions in the hot-spot reference frame. Independent geological observations such as the different styles of deformation in thrust belts associated with subduction following or opposing the relative ‘eastward or northeastward’ mantle flow also support this global tectonic polarity. Unfortunately most hot-spot reference frames are based on assumptions which strongly limit their validity. For example, many combine magmatic sources originating from different depths in the mantle, including hot-spots located along oceanic ridges which are not fixed relative to one another. Using a hot-spot reference frame filtered to exclude shallow hot-spots and anomalous ‘wet-spots’, we would predict a much higher relative ‘westward’ drift of the lithosphere. Plate motions are not random, rather that they are controlled by the relative motion between lithosphere and asthenosphere, and by gradients in lithospheric thickness and composition.Triangular shaped plates may experience rotations resulting from differential drag at the base of the lithosphere with respect to the asthenosphere (e.g. the clockwise rotation of South America). Plates appear to follow an undulatory pattern of motion on a global scale and their poles of rotation fall into a main cone located near the Earth’s rotation poles. Plate rotations induced by triangular shape, differential basal drag or lithospheric anisotropies along subduction zones may generate the scattered distribution of Euler poles. This might have inhibited the recognition of a coherent global tectonic pattern.

Episodicity in foredeep basins

The stratigraphic record contained in several foredeep basins is incompatible with the formation of these basins by flexure of a homogeneously strong foreland lithosphere. For example, the stratigraphy of the Early Proterozoic Kilohigok foredeep basin of northern Canada indicates that the inner part of the basin deepened in time without migration of the flexural bulge toward the foreland, and that the flexural bulge remained fixed in time relative to the downgoing plate. One mechanism that may be responsible for the localization of the flexural bulge in the Kilohigok and similar basins is that of lateral strength variations in the flexing lithosphere. Our results show that lateral strength variations within the foreland lithosphere can produce temporal variations in basin geometry and also serve to localize the position of the flexural bulge relative to the foreland for extended periods of time. This produces basin episodicity on a time scale of millions to tens of millions of years. Perfectly elastic models require a large strength contrast (about 50% in elastic plate thickness), but a brittle-elastic-ductile lithosphere rheology reduces the magnitude of the strength contrast required for episodicity to about 10%-15% of the initial effective elastic plate thickness. We suggest that basin episodicity may induce comparable episodicity in the coeval thrust belt, because theoretical studies of thrust mechanics suggest that the deformation in thrust belts is highly sensitive to the dip of the basal thrust angle. Through mechanisms such as this, the laterally heterogeneous continental lithosphere may provide a natural filter that transforms the presumably continuous and smoothly varying processes of the mantle into the spatially irregular and temporally episodic deformation observed in the crust at time scales of tens of millions of years.